Recent Progress in Electrochemical Conversion from Biomass Derivatives into High‐Value‐Added Chemicals

Selective and efficient electrochemical conversion of cheap and abundant biomass derivatives into high‐value‐added products can provide a way to store large‐scale renewable intermittent energy such as wind energy, solar energy, etc. Although a large amount of research is devoted to developing highly efficient catalysts or disclosing the reaction processes, there are always challenges of conversion rate and selectivity, and the relevant comprehensive and in‐depth discussion is relatively scarce. This review introduces the basic principles and reaction processes from the charge transfer pathway of biomass electrochemical conversion. Then, for the design of the anode, the representative work is summarized and discussed in detail from the point of view of material adjustment, theoretical calculations, and in situ characterizations. Finally, insights for further optimization of the reaction systems are put forward. It is believed that this review provides guidance for the selection, design, and application of materials for high‐value‐added organic electrochemical synthesis.


Introduction
The transformation of chemicals and the reforming of biomass play pivotal roles in the contemporary chemical industry.Achieving the synthesis of high-value-added chemicals through harmless, environmentally friendly, and gentle strategies is essential for addressing the escalating environmental and energy crises (Figure 1). [1]At present, the biomass derivatives worldwide, including lignocellulose like crop residues, forest residue, and even municipal solid waste in the broader sense, are estimated to reach ≈100 billion metric tons per year. [2]This vast biomass reserve holds immense potential for conversion into fossil fuels and chemical substitutes.In recent decades, some typical biomass derivatives, such as methane, [3] methanol, [4] ethanol, [5] glycerol, [6] glucose, [7] furfural, [8] and 5-hydroxymethylfurfural, [9] have demonstrated their capacity for targeted upgrading through electrochemical oxidation, coupling, or hydrogenation reactions, maintaining their status as focal points in ongoing research.The products derived from electrochemical conversion bear significant implications for industrial production, energy, and the environment, given the process's gentleness, cleanliness, and high selectivity. [10]lectrochemical catalysis, harnessing electricity from clean, renewable intermittent sources such as wind, solar, and tidal energy, is widely regarded as the most ideal and reliable technique, circumventing the high temperatures and pressures associated with traditional thermal catalysis [11] Due to the efficient charge transfer and precise parameter tunability in the electrochemical process, selecting an appropriate potential for the target product becomes feasible, enabling directed oxidation.In short, the advantages of electrochemical catalysis technology include 1) simple operation and mild conditions; 2) high product yield; and 3) environmental friendliness by minimizing waste or pollutant emissions. [12]Crucially, the electrochemical process allows for kinetics control by monitoring the applied potential, regulating the formation rate, and enhancing product selectivity.Based on these considerations, the effective utilization of electrocatalysis for oxidizing organic compounds into high-value-added chemicals holds promise for addressing current energy and environmental problems.
2b,13] Anodic organic oxidations, replacing the OER in an aqueous solution, offer a viable solution to these issues.As shown in Figure 2, the potential required for small-molecular biomass oxidation (such as methane, methanol, ethanol, glycerol, glucose, furfural, HMF, etc.) is significantly lower than that for the OER, presenting a distinct thermodynamic advantage, and the value of the products is much higher than their own value. [14]y comparing the prices, electron transfer numbers, and thermodynamic potentials of reactants and products, it is shown that the oxidation of small-molecule biomass is feasible from both economic and thermodynamic perspectives.Notably, the hydrogen evolution reaction (HER) at the cathode and the synthesis of high-value-added products at the anode enhance the allure of electrochemical organic oxidation. [15]Consequently, over the past decade, the exploration of organic oxidation as an alternative to OER has been recognized as a potentially rewarding strategy for H 2 production.
To our knowledge, although a large amount of research is currently dedicated to enhancing the efficiency of organic molecule oxidation and the selectivity of high-value-added chemicals, and some reviews have explored electrocatalytic types related to organic oxidation, [11,16] there is yet to be a comprehensive discussion on the types and mechanisms of organic matter oxidation leading to various high-value-added chemicals.In this review, we first delve into the research progress of anodes suitable for electrochemical organic oxidation, considering material selection, theoretical calculation, and in situ characterization techniques.Subsequently, we categorize the charge transfer pathways for electrochemical organic oxidation into two types, providing a detailed account of recent advances in the selective oxidation of various representative organic compounds into different high-value-added chemicals.Finally, we present prospects for the development of this field.We firmly believe that, with the continuous progress of research and development, the amalgamation of biomass electro-oxidation and renewable energy holds significant potential for transforming the energy landscape toward a zero-carbon economy.

Design for Anodes: Simulation and Characterization
The electrochemical oxidation of organic matter is a vital chemical reaction that entails the conversion of organic matter into its corresponding product under electrochemical conditions.This reaction holds immense significance in various fields, including energy storage, battery technology, and organic synthesis. [17]electing an appropriate electrocatalyst material is essential to enhance performance and improve the efficiency of electrochemical organic oxidation.Additionally, the utilization of density functional theory (DFT) computations and in-situ characterization methods is necessary for a better understanding of the electrochemical organic oxidation process.This article aims to discuss the current research status and the existing challenges pertaining to these aspects.

Materials Selection
Currently, the selection of materials for organic oxidation is primarily based on electrocatalysts that are effective for OER, as both processes involve nucleophilic reactions.In these reactions, reactants or intermediates tend to attack or bind to the active sites of the catalyst, resulting in similarities in catalytic mechanisms.Moreover, these two reactions share similar processes, involving electron transfer and redox reactions, with identical processes of electron gain and loss, as well as microscopic reconstruction. [18]ue to these commonalities, electrocatalysts that are effective for OER can serve as a model for catalyst materials used in biomass oxidation.This approach aids in the screening of electrocatalysts with higher activity and selectivity, thereby achieving efficient oxidation of biomass.This concept promotes the advancement of biomass oxidation reactions and has demonstrated practical utility.It is worth noting that the functional groups of different organic substrates vary significantly, necessitating the tailored design of catalysts to effectively handle the adsorption of diverse functional groups and complex mass transfer processes.
The choice of electrocatalyst depends on reaction conditions and target products, with a preference for materials that exhibit excellent electrocatalytic activity and stability.Some commonly used noble metals as electrocatalysts include rhodium, palladium, and platinum, as well as certain transition metals and oxides.The chemical composition and microstructure of the catalysts are the key factors that directly influence their electrochemical activity. [19]Noble metal-based catalysts, such as platinum-based and palladium-based catalysts, have been proven to be effective electrocatalysts.Nanostructures have demonstrated the ability to increase the number of active sites, making the preparation of noble metal-based catalysts with nanostructures a reliable choice.By carefully designing catalyst structures, integrating multiple supports, and incorporating multimetal components, large specific surfaces can be achieved, along with excellent mass fractions.This facilitates charge transfer between the active sites within the catalyst structure during the reaction, thereby improving adsorption/dispersion ability, enhancing structural properties, and ultimately enhancing the overall performance of the catalyst.

The Effect of Electrolyte
The choice of electrolyte is also crucial as it can influence the reaction rate and selectivity.Acidic solutions, alkaline solutions, and ionic liquids are commonly used electrolytes.For example, the pH value affects the oxidation pathway of 5-hydroxymethyl-2-furan carboxylic acid (HMF).In strongly alkaline media (pH ≥ 13), HMFCA is primarily formed, while in nonstrong alkaline environments (pH < 13), 2,5-diformylfuran (DFF) is predominantly formed.Gao et al. [20] described the synthesis of a NiSe@NiO x core-shell nanowire that demonstrates high efficiency in converting HMF to FDCA.In this system, both HMFCA and FFCA were detected simultaneously, while DFF was the only detectable intermediate that was not observed, indicating that HMF was more inclined to selective oxidation of aldehyde groups under strongly alkaline conditions.Choi et al. [21] delved into the exploration of the initial intermediates and reaction pathways associated with HMF catalysis on nanostructured nickel oxide nanoparticles (NPs) at near-neutral pH (pH = 7.2).Their findings revealed that the formation rate of DFF was comparatively rapid, suggesting that alcohol oxidation reactions are favored over aldehyde oxidation reactions under these pH conditions.Furthermore, some interesting examples of indirect oxidation pathways have been developed to manipulate the interfacial reaction microenvironment so as to enhance the selectivity of electrochemical organic oxidation.Many reports have found that ionic effects can promote some organic electro-oxidation processes toward specific paths, which have been demonstrated in the oxidation of alkanes, aromatic alcohols, and glycerol.The aggregation of ions in the Helmholtz layer can form noncovalent interactions with the reaction intermediates, leading to polarization or activation.Wu et al. [22] discovered that the introduction of Li þ and K þ ions effectively inhibits the excessive oxidation of glycerol.These cations interact with the aldehyde intermediate, reducing its residence time on the electrode surface and preventing overoxidation.Similarly, in another related study, the introduction of K þ cations disrupts the hydration reaction of intermediates to prevent underlying overoxidation. [23]On the other hand, anions can also participate as reaction mediators by bonding with intermediates.For instance, Cl À ion-mediated oxidation has been demonstrated to lower the barrier for alkane oxidation and achieve quasihomogeneous catalysis. [24]Therefore, selecting an appropriate electrolyte is crucial as it significantly influences the chemical environment of the reaction.

Theoretical Simulation
At the current stage of research on DFT, significant advancements have been made, leading to a deeper understanding of electrochemical organic oxidation.Researchers have achieved this through various means.First, they have optimized computational methods and adopted new material models, enabling more accurate prediction of active sites and catalytic properties in electrochemical organic oxidation reactions.Second, by exploring the electronic structure and interaction between catalysts and substrates, researchers can create new catalysts and enhance their catalytic performance.Additionally, improved DFT methods, such as nonlocal density functionals and modified function approximations, have been employed to better describe electronic structures and reaction kinetics. [25]For instance, Gong et al. [26] utilized DFT to investigate the electrocatalytic oxidation mechanisms and various products on the Pt (111) surface.Their findings demonstrated that the selectivity of products, including furoic acid, succinic acid, maleic acid, and others, could be adjusted by manipulating the electrode potential.This research has significantly advanced our understanding of furfural electrocatalytic oxidation and provides valuable guidance for selecting more effective electrocatalysts.Similarly, Sheng et al. [27] conducted a systematic study on the electrocatalytic oxidation mechanism of ethanol on the Pt(100) electrode using DFT.They explored the thermodynamics and kinetics of partial oxidation to acetic acid as well as complete oxidation to CO 2 (Figure 3a).The study revealed that due to the strong binding of intermediate species of ethanol electro-oxidation with the Pt surface, the surface affinity for the adsorption of intermediates, like CO, was weakened, leading to significant improvement in the electro-oxidation activity of ethanol.These findings highlight the crucial role of theoretical calculations in providing essential data about critical intermediates, reaction rates, main barriers, etc., which effectively guide the design and synthesis of new catalysts.

Use of Descriptors
In electrochemical organic oxidation reactions, common descriptors such as adsorption energy, charge transfer energy, and reaction-free energy of the reactants or intermediates are used.These descriptors provide indirect insights into the important energy changes that occur during the entire oxidation process, especially in the rate-determining step.However, it is important to acknowledge that certain limitations exist due to approximations and computational resources.As a result, some descriptors may lack accuracy or have limited applicability.Consequently, there is a pressing need for further development of new descriptors to enhance the theoretical description capability of electrochemical organic oxidation reactions.Researchers are actively addressing these challenges by exploring alternative methods, such as hybrid functionals, dispersion corrections, and machine learning techniques.These approaches aim to improve the accuracy and broaden the range of descriptors available.Additionally, advancements in computational resources and algorithms enable more extensive calculations and simulations, thereby facilitating a deeper understanding of electrochemical organic oxidation reactions.By developing new descriptors and refining existing ones, researchers strive to enhance the predictive power of theoretical models in studying these reactions.This progress can ultimately contribute to the design and optimization of efficient electrocatalysts and reaction conditions for electrochemical organic oxidation processes.

In situ Characterization Studies
In situ characterization technology plays a crucial role in research on organic electrochemical oxidation.By monitoring the changes in materials and interface properties in real time during the reaction process, in situ characterization techniques can reveal valuable information about reaction mechanisms and kinetics.The literature reports numerous applications of in situ characterization techniques, including electrochemical in situ infrared spectroscopy, in situ Raman spectroscopy, electrochemical mass spectrometry, and in situ X-ray absorption spectroscopy.These techniques provide detailed insights into active sites, intermediates, and products formed during the reaction.For example, Holewinski et al. [8] employed online electrochemical mass spectrometry (OLEMS) and in-situ infrared spectroscopy attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to investigate the oxidation pathway of furfural on platinum catalysts in acidic electrolyte.Their findings, as shown in Figure 3b,c, revealed the mechanism of action of ATR-SEIRAS and OLEMS in the electrocatalytic process.They discovered that decomposition and over-oxidation to CO 2 occurred through ring intermediates derived from decarbonization or decarboxylation, while further oxidation of the formed maleic acid (MA) was not easily achieved.These insights provide guidance for designing more reactive and selective electrocatalysts.Similarly, Zhou et al. [14b] used in-situ SEIRAS (ATR-IR) and online differential electrochemical mass spectrometry (DEMS) techniques to investigate the surface intermediates and product distribution of Pt-SnO 2 /rGO catalysts in the methanol oxidation reaction (MOR).ATR-IR technology enabled the monitoring of absorption peaks related to CO and HCOOB on the catalyst surface using infrared absorption spectroscopy.DEMS technology, on the other hand, monitored gas products generated during the reaction process using online mass spectrometry.These techniques provided information about the activity and selectivity of the catalysts, facilitating the optimization of catalyst design and preparation.Additionally, Li et al. [28] utilized in situ Raman spectroscopy to study the reaction mechanism and catalytic activity of nickel-based catalysts in the MOR.Changes in the Raman spectrum of different samples during the MOR process are depicted in Figure 3d.The study revealed that different nickel-based catalysts exhibited distinct Raman peaks, suggesting structural changes during the reaction process.This research provides valuable insights into the structural changes and catalytic activity of nickel-based catalysts in the MOR.In summary, in situ characterization techniques play a critical role in understanding organic electrochemical oxidation reactions.By providing real-time information on reaction intermediates, products, and catalyst properties, these techniques contribute to the development of more efficient and selective electrocatalysts However, current in situ characterization techniques still face some challenges and limitations.On the one hand, some techniques have poor selectivity for catalysts, making it difficult to distinguish surface-adsorbed species.On the other hand, in situ characterization under certain electrochemical conditions is still difficult.It is imperative, therefore, that in situ techniques undergo further refinement to augment their sensitivity, accuracy, and scope of application.
Electrochemical oxidation of organic matter has significant application value in many fields.This process entails the conversion of organic substances into more stable products, typically accompanied by charge transfer and energy liberation.Although this reaction has been extensively studied, there are still many challenges and opportunities that need to be further explored and studied.Subsequent sections will provide a more comprehensive overview of the latest advancements in this field of research.

Electrochemical Organic Oxidation in Two Charge Transfer Pathways
In the field of electrochemical organic synthesis, there are two main pathways through which charge transfer can occur: mediated electro-oxidation and direct electro-oxidation (as illustrated in Figure 4).Mediated electro-oxidation involves the transfer of charge and protons facilitated by a mediator.The mediator rapidly undergoes oxidation and reduction reactions on the electrode surface, ensuring efficient conversion of the substrate.On the other hand, direct electro-oxidation relies more on the characteristics of the electrode material itself.For direct electro-oxidation to take place, a strong interaction between the electrode surface and the organic substrate is necessary. [18]Both pathways have their own set of advantages and disadvantages.In the case of indirect electro-oxidation, it appears that the oxidation capacity is sufficient for most organic substrates.For instance, hydroxyl radicals (•OH) as a reactive oxygen species (ROS) mediator exhibit exceptional selectivity in oxidizing benzyl alcohol and glycerol.However, it should be noted that the limited oxidation capacity constrains the choice of mediators for indirect oxidation, rendering it unsuitable for certain complex organic substrates.On the other hand, direct electro-oxidation theoretically allows for any desired reaction to take place as long as the oxidation potential is met.Nevertheless, challenges such as excessive oxidation, poor selectivity, and restricted mass transfer processes arising from disparities in catalyst active sites at electrode/ electrolyte interfaces, adsorption/desorption capabilities, and electronic states cannot be overlooked.Both mediated electrooxidation and direct electro-oxidation have been extensively discussed and studied in order to gain a better understanding of the underlying reaction mechanisms and facilitate the design of improved electrodes.

Mediated Electro-Oxidation
Mediated electro-oxidation is a commonly used quasihomogeneous catalysis process in which the mediators employed are molecules that exhibit high stability and sensitivity to charge transfer.Examples of such mediators include TEMPO and NHPI. [29]lternatively, reactive species with strong oxidizing properties can also be used as mediators.In cases where certain complex organic compounds are challenging to oxidize, mediated electrooxidation offers a lower thermodynamic potential, which promotes the rapid progression of the overall reaction.One key advantage of mediated electro-oxidation is its efficacy in the deprotonation process.This is attributed to the mediator's enhanced hydrogen binding ability following facile oxidation and dehydrogenation.The mediators also exhibit strong coordination with the substrate, allowing for efficient capture of hydrogen atoms, including those present in C(sp 3 )─H bonds with higher bond energies. [30]Upon activation of the mediator and subsequent H atom transfer (HAT) process, the substrates undergo deprotonation, resulting in the generation of carbon radicals (•C).These intermediates can then undergo further functionalization to yield novel functional groups or participate in cross-coupling reactions, such as C─O or C─N bond formations through radical-radical coupling. [31]Importantly, mediated electro-oxidation not only facilitates the deprotonation process but also plays a role in C-C cleavage, leading to the formation of new carbon─carbon bonds. [32]2.Direct Electro-Oxidation Direct electro-oxidation is a widely used method in organic conversions and it heavily relies on the characteristics of the electrode materials.Consequently, by modifying the electrode materials, it is possible to design and adjust the performance of the reaction system.This artificial modification allows for greater control over the process.For simple organic substrates and biomass derivatives such as alcohols, aldehydes, and amines, the direct oxidation process involves a nucleophilic oxidation reaction (NOR).This NOR is a crucial activation step that occurs alongside HAT and electron transfer.While there are similarities to the OER process, there are still many complex mechanistic aspects that need to be understood.
In the context of direct electro-oxidation, which is commonly employed in alkane oxidation for C-H functionalization, the first step is dehydrogenation, which takes place on the surface of the catalyst and often acts as the rate-determining step.The alkane molecules diffuse to the electrode surface and interact with the active sites present.Subsequently, C─H bond cleavage occurs, leading to the formation of carbon radicals.These carbon radicals can then react with other reactive radicals, enabling C-H functionalization (as shown in Figure 5a).
2b] The specific pathway followed depends on the types of electrode materials used.For metal and metal oxide electrodes, the process begins with the adsorption of OH-ions, leading to the generation of electrophilic adsorbed oxygen species (OH*) on the electrode surface.These electrophilic groups can react with organic substrates and deprotonate them.In this case, there is a preferential removal of Hα from O-H, resulting in the formation of oxygencarbon radicals.This mechanism primarily occurs when using metal and metal oxide electrodes. [33]In the case of metal hydroxides like NiOOH, alcohol oxidation can proceed through an indirect mechanism.First, alcohols are adsorbed onto the electrode surface and then react with the electrophilic OH* species present in the lattice.33b] Subsequently, the H atom from the O─H bond is removed to achieve complete oxidation.In this indirect mechanism, the cleavage of the C─H bond acts as the rate-determining step. [34]Furthermore, this indirect mechanism allows for selective oxidation of secondary alcohols because the carbon radical intermediates formed are more stable compared to primary alcohols. [6]Additionally, the electrophilic OH* species can also attack C─C bonds, leading to C-C cleavage.Following this, OH* promotes the hydroxylation of the carbon radicals formed by the cleavage. [35]n the direct electro-oxidation process facilitated by active electrophilic OH*, only the adsorption and oxidation of OH À are electrochemical processes.The subsequent desorptions of OH* and HAT are spontaneous nonelectrochemical processes.Therefore, in situ characterization methods can be employed to investigate the structural changes occurring on the electrode surface due to the generation of active electrophilic oxygen.These methods help reveal the mechanism of the electrooxidation process.

Mediated Oxidation in Electrochemical Organic Oxidation
In recent years, there have been significant advancements in the application of indirect oxidation, particularly in the field of simple organic electro-oxidation such as methane oxidation.By integrating indirect oxidation with electrochemical techniques, researchers have successfully achieved efficient and sustainable oxidation of various organic compounds.This combination offers several advantages, including low energy consumption, environmental friendliness, and the ability to handle various organic pollutants.Factors such as current density, voltage, catalyst, electrode material, and electrolyte solution play a crucial role in determining the reaction pathways, efficiency, and selectivity of indirect oxidation processes.In this review, we provide a concise overview of representative cases that involve the conversion of biomass derivatives using indirect electro-oxidation methods.These methods have shown great potential in the transformation of biomass-derived compounds into value-added chemicals and fuels.Overall, the integration of indirect oxidation with electrochemical techniques has opened up new possibilities for the sustainable and efficient conversion of organic compounds, offering promising solutions for addressing important challenges related to environmental pollution and resource utilization.
In the context of indirect methane oxidation, redox intermediates can be employed as mediators for the homogeneous oxidation of methane.These intermediates possess a strong ability to activate C─H bonds and exhibit high turnover frequency (TOF), which helps to avoid excessive oxidation of methane and achieve high conversion rates. [36]One example is the use of platinumbased Pt IV organometallic oxidants, as demonstrated by Kim et al. [37] They introduced Cl À ions into the solution, which facilitated the rapid oxidation of reduced Pt II species to Pt IV at a low potential on a Cl À adsorbed electrode.By adjusting the externally applied current to maintain a dynamic balance of the Pt II/IV ratio, they were able to achieve a highly selective conversion of methane to methanol with a conversion rate of 70%.Rhodium-based catalysts, known for their electron-rich characteristics, have also been utilized for methane activation.However, their incompatibility with oxygen poses challenges to the efficient cycling of redox reactions.To address this, Natinsky et al. [38] employed nanowire array electrodes to promote the separation of incompatible reactions and enable efficient utilization of O 2 molecules in the conversion of methane to methanol (Figure 6b).Kim et al. [39] elaborated a Pd III 2 complex catalyst and proposed a new mechanism for methane activation, that is, Pd III 2 complex saturated by coordination with H x SO 4 (xÀ2) , and then captured H atoms in methane to achieve the HAT process (Figure 6c).Deng et al. [40] used a vanadium (V)-oxo dimer to reduce the methane activation energy in a mild environment and fulfilled a high TOF of 1336 h À1 at 3 bar of CH 4 with a Faradaic efficiency of 90% (Figure 6d).The indirect electro-oxidation strategy effectively avoids competitive reactions such as OER or oxygen reduction reaction (ORR), ensuring the selectivity of methane oxidation.It is important to note that the efficiency of overall organic conversion relies on the contact and reaction between redox mediators and the electrode.Careful consideration should be given to catalyst selection and electrode design, as they play a crucial role in this process.
The indirect electro-oxidation pathway has been demonstrated to be highly effective for the selective oxidation and specific functionalization of alcohols.This pathway relies on the continuous generation of redox mediators and the quasihomogeneous reaction system. [41]It offers advantages such as enhanced reaction rates and improved product selectivity in various catalytic transformations.In recent years, there have been significant advancements in the development of the indirect charge transfer pathway through photoelectrochemical methods.These methods utilize light energy to drive the electro-oxidation of alcohol compounds, further expanding the scope and efficiency of this pathway.By harnessing the power of light, researchers have been able to explore new possibilities for alcohol oxidation reactions.An essential component of the indirect electro-oxidation pathway is the redox medium. [42]The redox medium plays a crucial role in promoting the electro-oxidation reaction and influencing the reaction rate and product selectivity.Chloride ions, for instance, can serve as an effective redox medium due to their significantly lower oxidation potential compared to alcohols.The presence of chloride ions facilitates the transfer of charge, enabling more efficient electro-oxidation processes.The utilization of redox mediators and the optimization of reaction conditions in the indirect electro-oxidation pathway provide valuable tools for achieving selective oxidation and functionalization of alcohols.These advancements have the potential to revolutionize the synthesis of organic compounds and contribute to the development of sustainable and efficient chemical transformations.
Li et al. [43] introduced chloride ions as redox media in alkylammonium chloride electrolyte to prepare 1,1-dimethoxyethane (DEE), with a remarkable Faraday efficiency (FE) exceeding 95%.The oxidation of chlorine leads to the formation of ethyl hypochlorite intermediate, which subsequently undergoes rapid and spontaneous decomposition, yielding acetaldehyde for nucleophilic attack with ethanol (Figure 7a).Lucky et al. [44] employed chloride ions as a catalyst for the electrochemical oxidation of ethanol to 2-Chloroethanol, facilitating the synthesis of ethylene oxide (Figure 7b).The key aspect of this method lies in activating the stable C (sp3) ─H bond in ethanol through Cl⋅ oxidation derived from Cl À , enabling selective activation of specific C─H bonds by adjusting reaction temperature.The existence of this indirect ethanol oxidation intermediate is beneficial for improving the overall atomic economy, while also avoiding the complete oxidation of ethanol and the production of peroxidation products.To prevent excessive oxidation on the electrode surface, controlling the generation of ROS for selective organic oxidation has been a continuous concern.Sheng et al. [45] developed a linear-paired electrochemical valorization strategy for achieving the high-value-added conversion of glycerol.At the cathode, a low potential is required for reducing oxygen to hydrogen peroxide, which in combination with electro-Fenton generates hydroxyl radicals that can be used for highly selective glycerol oxidation to C3 products with a 55% conversion rate (Figure 7c).

Direct Oxidation in Electrochemical Organic Oxidation
Compared to indirect oxidation, direct oxidation at the anode with suitable electrocatalysts offers several advantages.It allows the oxidation reaction to occur directly at the electrode surface, resulting in rapid reaction kinetics, high efficiency, and good selectivity.The investigation of electrochemical methods for converting biomass derivatives, which is a significant application of direct electrochemical oxidation, has been extensively developed.By tailoring electrode modification strategies based on the characteristics of functional groups, we can further enhance the performance of the electrochemical oxidation process.Since the cleavage of chemical bonds in functional groups often acts as a rate-determining step, the oxidation reactions can be categorized into C─O bond cleavage, C─H bond cleavage, and C─C bond cleavage based on the position of the chemical substances.These reactions can be finely regulated according to their thermodynamic and kinetic processes to achieve selective oxidation.During the electrochemical oxidation process, various microphysical processes occur at the interface, including diffusion, adsorption, electron-proton transfer, and desorption of organic substrates.To achieve enhanced efficiency and selectivity Reproduced under terms of the CC-BY license. [37]Copyright 2019, The Authors.Published by American Chemical Society.b) Schematic of the proposed catalytic cycle with the catalyst for CH 3 OH formation.Reproduced under terms of the CC-BY license. [38]Copyright 2019, American Chemical Society.c) Illustration of calculated electrophilic C-H activation pathways for activation of methane.Reproduced with permission. [39]Copyright 2020, American Chemical Society.d) Calculated frontier orbitals involved in the turnover-limiting step and the proposed transition state of the C-H activation step (left) and proposed catalytic cycle (right).Reproduced with permission. [40]opyright 2020, Springer Nature.
based on these characteristics, electrocatalyst materials require abundant active sites and excellent conductivity.The structure of the electrocatalyst can be tailored to possess a porous or surface-controlled configuration, while its conductivity can be improved through element doping and the incorporation of conductive materials.Therefore, in addition to regulating the electrode surface, especially the adsorption and desorption processes of organic molecules, modification methods that aim to achieve higher electrocatalytic performance will also be introduced.

Selective Oxidation of C─H Bond
The functionalization of C─H bonds provides numerous valuable transformations, facilitating the conversion of common and abundant organic molecules into a wide array of functional groups in a single-synthesis operation.These transformations encompass the formation of C─C, C─X (halogen), C─O, C─P, and C─N bonds. [46]While the selective oxidation of inert C─H bonds continues to pose a significant challenge in chemical manufacturing─traditionally leading to the conversion of the C─H bond of alkanes into CO 2 ─electrochemical oxidation holds critical advantages for specific alkane oxidation.Electrochemical reactions typically exhibit kinetics-favoring reactions with fewer proton and electron transfers.Consequently, controlling interfacial charge transfer becomes pivotal in initiating selective oxidation reactions, with the intrinsic properties of the electrode materials and the interface microenvironment playing a crucial role.This aspect has garnered significant research attention in recent years. [47]ethane, being the simplest alkane, has garnered widespread attention due to its affordability, abundant reserves, and potential for conversion into high-value-added chemicals, establishing it as a crucial hydrocarbon raw material.The CH 4 molecule exhibits a symmetrical tetrahedral structure with four identical C─H bonds, each requiring 439 kJ mol À1 of energy for breaking. [48]This is primarily attributed to its low electron affinity (1.9 eV) [49] and high polarizability (2.8 Â 10 À40 C 2 m 2 J À1 ) [50] while exhibiting a high ionization energy (12.6 eV). [51]Once high activation energy is provided for the dissociation of the C─H bond, the target oxygencontaining compounds formed as intermediate products can be easily activated, resulting in more favorable thermodynamically CO x products (Figure 8a). [52]Therefore, the main step in the direct conversion of methane is to find catalysts that can selectively activate the C─H bond and prevent further oxidation to CO x , which makes its chemical utilization extremely challenging.Copyright 2021, American Chemical Society.b) Schematic of the conversion of ethanol to ethylene oxide in an electrochemical cell using a two-step radical-mediated process.Reproduced with permission. [44]Copyright 2022, American Chemical Society.c) Schematic illustration and working principle of the linear paired system in a two-compartment H-cell.Reproduced with permission. [45]Copyright 2019, Springer Nature.
In an aqueous solution, Table 1 displays various possible reactions related to the electrocatalytic conversion of methane, along with the associated redox potentials.The electrochemical oxidation of methane can be achieved through two pathways: direct and indirect electrochemical oxidation, as illustrated in Figure 8b. [53]As previously discussed, the direct electro-oxidation of methane involves its direct adsorption and subsequent activation, while indirect oxidation relies on the utilization of highly reactive species as mediators.
The analysis above highlights the need to address the thermodynamic challenges linked to the full oxidation of methane to CO 2 during direct electro-oxidation.To overcome this, the focus should be on stabilizing the intermediate stage of the reaction.Furthermore, considering the potential occurrence of competitive anodic OERs at elevated potentials, it is crucial to ensure the stabilization of adsorbed oxygen intermediates on the surface. [54]hifting our focus from the discussion on thermodynamic challenges, we now emphasize the importance of stabilizing reaction intermediates.The binding of reaction intermediates to the electrode surface is influenced by factors like electronic structure and active site.For metal catalysts, the rate-determining steps typically include the initial electron transfer and the subsequent activation of methane into adsorbed CH 3 *.Studies have Reproduced with permission. [52]Copyright 2020, Royal Society of Chemistry.b) The electrochemical oxidation of methane by direct and indirect electro-oxidation.Reproduced with permission. [53]Copyright 2021, Elsevier.c) Contour plots for methane oxidation activity as a function of the free energy of adsorption of CO(relative to CO(g)) and OH(relative to H 2 O(l)/H 2 (g)).Reproduced with permission. [55]Copyright 2019, American Chemical Society.d) Schematic illustration of the reaction steps in direct methane-to-methanol oxidation on transition metal oxides.57a] Copyright 2023, American Chemical Society.e) The rates for the OER and methanol production are plotted.Reproduced with permission. [59]Copyright 2018, Royal Society of Chemistry.
Table 1.44a] shown that metals such as Pt possess the capability to activate the C─H bond in methane.However, this ability is constrained by specific crystal planes, highlighting a pronounced structural dependence on methane activation. [55,56]Moreover, on the metal surface, CH3* tends to undergo deprotonation rather than forming CO* or CHO* intermediates.Consequently, theoretical calculations have been conducted to explore the binding of various metals with CO* intermediates and evaluate the influence of OH* species from water oxidation on methane oxidation throughout the process.Catalysts that demonstrate strong CO binding and weak OH binding are deemed optimal for accelerating methane oxidation rates, underscoring the suitability of Pt as a metal catalyst for this process (Figure 8c).However, achieving a higher potential is essential to facilitate the activation process.
Recent reports suggest that metal oxides and MXene possess promising properties for the selective oxidation of methane due to their strong adsorption and binding capabilities toward ROS. [57]Shen et al. [57a] conducted a comprehensive study combining experimental observations and theoretical calculations, revealing that the selectivity of methane oxidation is primarily determined by the adsorption of OH À .This selectivity is manifested through the activation of C─H bonds and the timely desorption of methanol from the active site (Figure 8d).57a] Prajapati et al. [57b] developed t-open circuit potential measurement to further elucidate the crucial reaction intermediates involved in the selective oxidation of methane, demonstrating that enhanced adsorption of OH* can effectively prevent excessive oxidation of methanol products.Arnarson et al. [58] through theoretical calculations, proposed that the ability to bind O* of the catalyst surface could serve as a determining factor for achieving selective methane oxidation (Figure 8e).Consequently, by modulating the binding capability of the catalyst surface toward oxygen atoms, one can attain selective methane oxidation.
As a typical metal oxide catalyst, nickel-based electrocatalysts are considered promising candidates for selective methane conversion due to their effective electroactive sites.Li et al. [3] investigated the CH 4 oxidation reaction on the Fe-Ni-OH nanosheet structure in order to comprehend the reaction mechanism.The synergistic effect of two metallic elements in the Fe-Ni-OH electrode enhances its electrocatalytic activity.Ni III OOH, generated in situ, serves as the active site during this reaction (Figure 9a).Methanol oxidation involves the formation of multistep dehydrogenation intermediates, resulting in a relatively intricate overall mechanism.Aiming at this, Song and his colleagues conducted further research on the formation mechanism of methanol and ethanol using DFT calculations. [59]They discovered that by engineering the NiO/Ni interface, it is possible to directly convert CH 4 into alcohol, particularly ethanol, due to enhanced charge transfer capability and a larger electrochemical active surface area.This facilitates the activation of C─H bonds and the coupling of C─C bonds.The DFT calculations demonstrated that ethanol is preferentially formed over methanol on the NiO/Ni Figure 9. a) Schematic illustration of electrocatalytic methane conversion using Fe-Ni-OH anode.Reproduced with permission. [3]Copyright 2022, Elsevier.b) Calculated reaction energy profiles for CH 4 electro-oxidation to form methanol and ethanol at the NiO(200)/Ni(111) interface.Reproduced with permission. [59]Copyright 2020, Elsevier.c) The reaction mechanism of electrochemical oxidation of methane gas.Reproduced with permission. [60]Copyright 2017, Wiley-VCH.d) The fabrication process of the ZrO 2 :NiCo 2 O 4 quasisolid solution catalyst and its function for CH 4 electrochemical oxidation.Reproduced with permission. [61]Copyright 2019, Elsevier.e) Schematic illustration of electrocatalytic methane conversion using NiO@NiHF anode.47b] Copyright 2020, Elsevier.
interface structure.The rate-determining steps for methanol and ethanol formation involve CH 3 * hydroxylation and CH 3 * dehydrogenation (Figure 9b).Endothermic hydroxylation leads to the formation of CH 3 OH*, while exothermic dehydrogenation results in the production of CH 2 * during the process of ethanol formation.Therefore, it can be concluded that ethanol is more likely to be generated on the NiO/Ni interface.
In addition, the ZrO 2 -based composite catalyst electrodes exhibit potential for direct methane selective activation due to the Lewis acid sites and electron-accepting capabilities of the ZrO 2 surface.These attributes enhance the adsorption of CO 3 2À , which serves as an oxidant for methane activation.Ma et al. [60] developed a Co 3 O 4 /ZrO 2 nanocomposite material for the electrochemical oxidation of methane gas at room temperature using carbonate electrolytes.The remarkable electrooxidation activity of Co 3 O 4 toward methane, combined with the enhanced oxidant absorption capacity of ZrO 2 , synergistically enables a high production efficiency exceeding 60%.Based on experimental findings, acetaldehyde emerges as a pivotal intermediate product.Subsequent nucleophilic addition and free radical addition reactions give rise to the formation of 2-propanol and 1-propanol, respectively (Figure 9c).The discovery of such efficient catalytic pathways for methane oxidation holds significant importance due to the abundance of methane as a potent greenhouse gas and its potential as an alternative energy source.The utilization of Co 3 O 4 /ZrO 2 nanocomposites opens up new possibilities for sustainable energy conversion processes.Further research could focus on optimizing the composition and structure of these nanocomposites to enhance their catalytic performance even more.However, the low selectivity of these two intermediates limits the further implementation of this method, and thus they used ZrO 2 :NiCo 2 O 4 quasisolid solution catalyst as the electrochemical anode of methane partial oxidation of methane to further investigate the selective generation of intermediate products. [61]Due to the high oxygen vacancy density on the Co 3 O 4 surface, nickel ions are well incorporated into the Co 3 O 4 lattice.Therefore, NiCo 2 O 4 , which has a similar spinel structure to Co 3 O 4 , has a high catalytic active site to active CH 4 into CH 3 * promoted by bimetal synergy.The binding energy between Ni ion and CH* radical is so low that stable CH 3 O is formed at the lattice oxygen atom, which allows the selectivity of the intermediate formation of the whole reaction to be regulated.For effective reactions, the microstructure of catalysts has always been considered a key factor in electrocatalytic systems and may directly affect surface area and reaction kinetics (Figure 9d).Oh et al. [62] designed a composite material consisting of Co 3 O 4 supported by ZrO 2 nanotubes, which possess a significantly high specific area and abundant adsorption sites.This unique characteristic contributes to enhanced activity in methane oxidation at a lower onset potential.Furthermore, these ZrO 2 -based composites have been extensively studied with other metal oxides such as NiO/ZrO 2 and CuO x /ZrO 2 , demonstrating the versatility of this composite structure. [63]n the direct electro-oxidation process, oxidation reactions involving gas-liquid-solid heterogeneous phases often suffer from poor mass transfer processes.Therefore, enhancing the three-phase boundary of the system offers a viable approach for achieving rapid and large-scale electrochemical transformations.Guo et al. [47b] demonstrated that using NiO/Ni hollow fiber electrodes (NiO@NiHF), efficient electrochemical oxidation of methane to alcohols can be achieved due to an increased contact area and three-phase boundary (Figure 9e).Methanol and ethanol are efficiently produced at the NiO@NiHF anode with Faradaic efficiencies reaching up to 90% under ambient temperature and pressure conditions.The presence of a porous Ni/NiO interface promotes improved mass transfer processes, thereby facilitating the electro-oxidation reaction.By expanding upon these findings, further advancements can be made in developing efficient electrochemical transformation processes for various applications.The exploration of novel electrode materials or modifications that optimize mass transfer properties will continue to play a crucial role in improving overall performance and enabling large-scale implementation.Overall, this research highlights how enhancing the three-phase boundary through innovative electrode designs can significantly improve mass transfer processes during direct electro-oxidation reactions.Such advancements pave the way for more sustainable and economically viable methods for chemical synthesis through electrochemistry.
In summary, the activation of methane has a strong structural dependence, and enhancing the number of active sites by adjusting the structure of the catalyst to increase the specific surface area and improve the electronic structure can effectively enhance the activity of methane oxidation.Table 2 shows the latest research on the types of catalysts, products, selectivity, FE, and oxidation potential for methane oxidation.However, an efficient and well-controlled CH 4 oxidation process under mild conditions remains a challenge due to the unsatisfactory conversion efficiency and low efficiency.Therefore, designing more efficient catalysts to achieve the desired conversion and selectivity of CH 4 conversion remains a special focus at this stage, which will make significant progress in the future.

Alcohols Oxidation
Among the diverse reactions that can replace OER, alcohol oxidation in biomass conversion emerges as the most attractive option due to its numerous advantages in both thermodynamics and kinetics.First, from a thermodynamic perspective, alcohol oxidation offers higher energy efficiency compared to alternative reactions.This implies that the reaction requires less energy input, leading to cost savings and a reduced environmental impact.Moreover, alcohol oxidation makes use of abundant reactants found in biomass resources such as lignocellulosic materials or waste streams from agricultural and forestry industries.These renewable feedstocks not only provide a sustainable source of raw materials but also contribute to diminishing dependence on fossil fuels.In addition to its favorable thermodynamics and the availability of reactants, alcohol oxidation yields valuable products with significant economic value.For example, the conversion of alcohols can result in the production of high-value chemicals such as aldehydes, ketones, carboxylic acids, or esters, which find widespread use in various industries, including pharmaceuticals, cosmetics, and polymers manufacturing.Moreover, by incorporating alcohol oxidation as a key reaction in biomass conversion processes, it becomes possible to adopt a more integrated approach to achieving sustainability goals.The use of renewable feedstocks in conjunction with efficient catalytic systems facilitates the development of cleaner and greener chemical processes.Overall, considering its advantageous thermodynamics and kinetics, along with abundant reactants and the production of valuable products, alcohol oxidation stands out as a compelling choice among various reactions replacing OER in biomass conversion applications.Its potential economic benefits, coupled with its contribution toward sustainability, make it an attractive avenue for further research and development efforts.Methanol, the simplest saturated alcohol, can be derived from biomass waste through fermentation.It possesses high energy density, as well as ease of storage and transportation.Effectively enhancing the methanol economy involves the selective oxidation of methanol to produce chemicals with higher commercial value.As depicted in Equation (2), under acidic conditions, the selective oxidation of methanol to form formic acid is thermodynamically advantageous compared to OER. [64] CH Table 3 provides a detailed summary of the relevant information on MOR, including the types of catalysts, such as precious metal catalysts and nonprecious metal catalysts, the types of products, the types of electrolytes, selectivity, FE, and oxidation potential.These pieces of information provide researchers with rich research data, which helps to understand the reaction mechanism, optimize reaction conditions, improve the yield of target products, and also provide important reference for the practical application of MOR.
Metals have been proven to be excellent electrocatalysts for methanol oxidation due to their excellent electronic structure and high conductivity. [65]14b,66] Research has shown that the choice of methanol oxidation pathway largely depends on the cleavage of C─H or O─H bonds during the methanol dehydrogenation process, resulting in different oxidation intermediates to achieve selective product generation. [67]Zhou et al. [14b] recently synthesized an interface-rich Pt-SnO 2 nanosheet supported on reduced graphite oxide, which significantly enhanced the selectivity of the methanol oxidation.In situ ATR-SEIRAS measurement revealed that the Pt-SnO 2 electrode exhibited weaker adsorption of CO species and stronger adsorption of HCOO B , respectively, indicating significant electronic structural modification of Pt upon contact with SnO 2 .This modification effectively enhanced the MOR (Figure 10b).Surface oxides can regulate the local electronic structure of internal precious metals and serve as Lewis acids activation reactants to enhance catalyst activity.The synergistic effect between metal oxides and noble metals also has a great impact on the catalytic activity of the catalyst, primarily due to the generation of double active sites at the interface formed by metal oxide edges and noble metal surfaces. [68]Yang et al. [4] synthesized a Fe 2 O 3 /Pd hybrid nanostructure, where the surface of palladium NP was partially covered by an active ultrathin oxide layer of Fe 2 O 3 .This heterostructure exhibited exceptional electrocatalytic performance for MOR.In the Fe 2 O 3 /Pd heterostructure, the charge transfer between Pd and Fe 2 O 3 is beneficial for reducing the d-band center of Pd in Fe 2 O 3 /Pd, thereby promoting the adsorption of CH 3 OH on Pd sites (Figure 10c).At the same time, the catalyst is more energetically advantageous for the adsorption of hydroxyl (OH*) species, making it easier for O─H and C─H in CH 3 OH to undergo cleavage, especially for the cleavage of O─H bonds, thereby promoting the selectivity of methanol electrochemical conversion to formaldehyde, that is, promoting the direct pathway conversion in methanol oxidation.To further improve the economy of catalysts and avoid the possibility of CO poisoning, nonprecious metal based catalysts have been studied in depth, like Ni, Co, Fe based catalysts, which can selectively convert methanol into formate with four times the value than methanol. [69]Li et al. found that the nickel carbide (Ni 3 C) particles have been recognized as a cost-effective catalyst, with an electrochemical conversion rate of nearly 100% from methanol to formate and without the production of detectable CO 2 as a byproduct. [70]Moreover, they developed a bimetallic electrocatalyst (Ni 0.75 Fe 0.25 Se 2 -based electrode) for the conversion of methanol to formate. [71]The catalyst exhibits a high current density of up to 53.5 mA cm À2 in a 1.0 M KOH electrolyte containing 1.0 M methanol at 1.5 V, while simultaneously producing formate at a rate of 0.47 mmol cm À2 h À1 under a current density of 50 mA cm À2 , with an impressive Faradaic conversion efficiency of 99%, and demonstrated continuous operating stability of more than 50000 s (Figure 10d).The incorporation of Fe significantly lowers the starting potential compared to OER, indicating that Fe activates neighboring Ni active centers and even becomes a new active site center.However, when the applied voltage falls within the conventional OER region, there is still competition between MOR and OER.Therefore, effectively suppressing OER while maintaining efficient MOR remains a critical challenge.Jiang et al. [72] synthesized an engineering heterostructure electrocatalyst comprising Ni and Fe, which effectively suppresses OER while promoting MOR.This catalyst exhibits excellent electrocatalytic ability for methanol oxidation to formic acid with close to 100% FE.The transfer of electrons from NiFe-HAB to NiFe-layered double hydroxide (LDH) modifies the electronic structure and elevates the energy barrier for OER intermediates, thereby passivating its catalytic activity (Figure 10e).Consequently, there is a significant enhancement in the conversion rate of methanol to formate.
The most prevalent approach to enhance the electrocatalytic performance of non-noble metal catalysts for MOR is through heteroatom doping, aiming to augment the conductivity of the catalyst or achieve more active sites through morphology design.Notably, incorporating ultralow amounts of precious metals (such as Ru, Ir, etc.) into non-noble metal nanocatalysts effectively enhances their catalytic prowess.This enhancement  [4] Copyright 2021, Elsevier.d) CP profile during 50 000 s operation at a constant current density of 50 mA cm À2 .Reproduced with permission. [71]Copyright 2021, Wiley-VCH.e) Proposed 4e-mechanism of OER on NiFe-LDH/NiFe-HAB for DFT calculation.Reproduced with permission. [72]Copyright 2023, Wiley-VCH.f ) The proposed redox cycle between Ni 2þ and Ni 3þ for the electrocatalytic conversion from methanol to formate.Reproduced with permission. [75]Copyright 2021, Elsevier.
primarily stems from the ability of doped noble metals to modify the electronic structure and local coordination environment of the catalyst, thereby optimizing adsorption capabilities during electrochemical reactions.Additionally, these noble metal dopants can serve as activation centers for electrocatalytic reactions, fostering synergistic effects that elevate overall electrocatalyst performance. [73]Ling et al. [74] in situ grew ruthenium (Ru)-doped iron-based metal-organic framework (MOF) nanoarrays on NF for the efficient production of high-value formates.After adding 4 M methanol to the 1 M KOH electrolyte, the catalyst exhibited a current density of 10 mA cm À2 only at 1.40 V, which is far lower than the current density of the overall water splitting (1.56 V).Xu et al. [75] synthesized Ni-based MOF nanosheet array catalyst doped with Ir, which effectively oxidizes methanol into valuable chemical formates and promotes hydrogen generation on the cathode.Compared with 1.0 M KOH solution without methanol, adding 4 M methanol to the electrolyte can reduce the overpotential by 170 mV at 100 mA cm À2 , indicating that it has ultrahigh energy conversion efficiency with a high FE of almost 100% for the generation of hydrogen and formate.In the NiIr-MOF structure, the catalytic active site for methanol adsorption is located at the Ni site, while the generated Ni 2þ /Ni 3þ species during the reaction act as active intermediates for methanol oxidation.The incorporation of Ir cations further facilitates the formation of Ni 2þ /Ni 3þ species.The electrocatalytic methanol oxidation process on NiIr-MOF/NF is shown in Figure 10f.Throughout the reaction, the Ni 2þ at the coordination center combines with the hydroxide and is oxidized to high-valent Ni 3þ .Subsequently, methanol is adsorbed on active nickel and oxidized to high-valueadded formic acid, while Ni 3þ is reduced to Ni 2þ and reparticipates in the oxidation of methanol.
In brief, methanol is the simplest saturated alcohol, and its oxidation reaction has important application value in the fields of energy and chemistry.By selectively oxidizing methanol into high-value-added chemicals such as formic acid and formaldehyde, the efficiency of the methanol economy can be effectively enhanced.Metal catalysts, especially nonprecious metal catalysts like nickel, cobalt, iron, etc., have garnered extensive attention in this field.These catalysts can efficiently convert methanol into high-value-added chemicals, such as formic acid, which are highly selective and active.In addition, by adjusting the local electronic structure of the catalyst, its performance in the MOR can be optimized.In recent years, there has been notable research progress on catalysts based on nonprecious metals, such as nickel, cobalt, iron, etc., offering a novel research direction for MORs.However, striking a delicate balance between inhibiting OER and preserving the efficacy of MOR poses an ongoing challenge.Consequently, forthcoming research endeavors will persist in exploring and investigating innovative catalysts to attain heightened efficiency in MORs.
Ethanol, which is widely derived from biomass fermentation, is more attractive as a fuel due to its low toxicity, comparable electrochemical activity, and high theoretical mass/energy density (8.0 kWh kg À1 ). [76]In most cases, ethanol undergoes partial oxidation to acetaldehyde or acetic acid with the transfer of only two or four electrons (Figure 11a). [77]The equation demonstrates that on the Pt (111) electrode in a 0.5 M H 2 SO 4 electrolyte, the potential for partial oxidation of ethanol is more favorable than that of OER. [78] Table 4 summarizes various information on ethanol oxidation in the latest research, including key parameters such as catalyst types and performance, product types and yields, FE, etc.In terms of catalysts, research covers precious metal catalysts, nonprecious metal catalysts, and bimetallic catalysts, exploring the effects of different catalysts on ethanol oxidation reaction (EOR).In terms of products, research has focused on products such as acetaldehyde and acetic acid generated by ethanol oxidation and analyzed the selectivity of products under different catalysts.In addition, FE, as an important indicator for evaluating catalyst performance, is also summarized in Table 4, providing FE data for various catalysts to intuitively reflect their activity and efficiency.The summary in Table 4 can provide reference and guidance for the study of ethanol oxidation.
Platinum and other noble metal-based catalysts have been proved to have high activity for selective oxidation of ethanol.To further improve the electronic structure of the catalyst and adjust the adsorption strength of the reaction intermediate, various structures and components are incorporated, including adsorbed atoms, adsorbed layers, intermetallic Pt-M components, bimetallic alloys, metal oxides, and core-shell nanostructures. [79]Zhang et al. [80] reported a convenient strategy for preparing Pt 3 Co using Pt skin-structured bimetallic nanocatalysts directly loaded on porous graphite carbon.The mass activity of the synthesized electrocatalyst is 0.79 mA μg Pt

À1
, which is a 250% enhancement compared with commercial Pt/C (0.32 mA μg Pt

À1
). Via in situ fourier transform ifrared spectroscopy spectra and DFT, ethanol prefers breaking the α À C─H bond producing CH 3 COH* rather than breaking the β À C─H bond yielding CH 2 COH* on Pt 3 Co@Pt/PC (Figure 11b).Qiu et al. [81] developed a Pd-Zn double site that exhibits remarkable enhancement in the efficiency of ethanol electro-oxidation, surpassing the activity of pure Pd sites.The dual Pd-Zn sites demonstrate significantly higher performance for ethanol electro-oxidation compared to both Pd-Pd and individual Pd sites, outperforming commercial Pd/C by ≈24 times.Furthermore, DFT calculations indicate that the Pd-Zn double site is advantageous for adsorbing ethanol and hydroxide ions at lower reaction energies, regulating the reaction pathway of EOR.Unlike ethanol that adsorbs on Pd atoms before EOR and eliminates its methylene C─H bond, the presence of Zn atoms promotes ethanol adsorption instead of Pd atoms and makes ethanol dehydrogenation more likely to initially destroy the O─H bond rather than the methylene C─H.The free energy distribution of these two different pathways is shown in Figure 11c.In recent years, selective oxidation strategies such as Pt-Sn, Pt-Ru, Pt-Rh, Pd-Ni, et al. have been widely developed and innovated to improve the adsorption of intermediates (Figure 11d). [82]lthough Pt and other noble metal electrocatalysts have excellent catalytic performance, their cost and scarcity, as well as the toxicity of CO immediately to noble metal electrocatalysts, severely limit their practical application.Therefore, non-noble metal electrocatalysts, especially nickel-based electrocatalysts, have received great attention to partially or completely replace noble metal electrocatalysts.Nickel helps to adsorb hydroxyl groups on its surface and helps to remove CO, which improve the catalytic activity and stability of the catalyst in the organic oxidation reaction, as well as its resistance to CO.The oxidation reaction of organic substrates by Ni-based electrocatalysis mainly depends on the reversible redox conversion between Ni (II) and Ni (III). [83]When a Ni-based electrocatalyst is immersed in an alkaline electrolyte solution, a hydroxide layer is formed on the surface, and the hydroxide reacts with OH À to generate NiOOH, which is the main active component for the oxidation of organic molecules adsorbed on the surface of the catalyst and can also be reduced to Ni(OH) 2 .For ethanol oxidation, the chemical reaction between Ni (II) and Ni (III) reversible redox conversion and ethanol electro-oxidation is as follows. [84]ðOHÞ Li et al. [85] designed various cation heterostructures based on transition metals (Co-Ni, Fe-Ni, Cu-Ni) to modulate the electronic structure of Ni and enhance its performance in the EOR.The optimized structure obtained through DFT Figure 11.a) Reaction pathways of EOR to C1 and C2 species.Reproduced with permission. [77]Copyright 2021, Elsevier.b) Energy profiles of ethanol electro-oxidation on the stepped Pt 3 Co(211) surface.Reproduced with permission. [80]Copyright 2017, American Chemical Society.c) DFT-calculated free energy profiles of EOR over Pd-Pd sites and Pd-Zn dual sites (pH = 14, U = 0.82 V with respect to the RHE).Reproduced with permission. [81]Copyright 2021, Springer Nature.d) Scheme of the reaction between CH 3 CO radical on Pd and OH radical on Ni.82d] Copyright 2017, Springer Nature.
calculations is depicted in Figure 12a, where NiCoSe demonstrates optimal conversion efficiency for ethanol.Thermodynamically speaking, the conversion from M/CH 3 CH 3 OH to M/OCH 2 CH 3 at the Ni site of NiCoSe offers distinct advantages.Due to its exceptional electronic structure, NiCoSe exhibits selective adsorption capabilities toward reaction intermediates, thereby achieving remarkable electrocatalytic performance.
These findings highlight the excellent potential of NiCoSe as an electrocatalyst for ethanol-to-acetic acid conversion, serving as a substitute for OER in overall water electrolysis.Reproduced with permission. [84]Copyright 2023, Wiley-VCH.b) Transmission electron microscope (TEM) images of Co 3 O 4 .Reproduced under terms of the CC-BY license. [85]Copyright 2016, The Authors.Published by American Chemical Society.c) The charge density in the CoNi-PHNs and d) scheme representation of the electronic coupling between Co and Ni.Reproduced with permission. [86]Copyright 2019, Wiley-VCH.e) Schematic of bond formation of adsorbates (Ads) on the catalyst surfaces.Reproduced with permission. [88]Copyright 2022, RSC Publishing.
Metal oxides and hydroxides, particularly cobalt/nickel-based catalysts, have been developed for the selective electro-oxidation of ethanol.Metal hydroxides hold greater potential for EOR due to their abundant active sites and more flexible electronic structure tunability, especially through bimetal coregulation of electronic structure and local charge density.Co 3 O 4 (111) exhibits enhanced electrocatalytic activity for water and ethanol oxidation in alkaline media due to its plentiful active sites (Figure 12b). [86]herefore, Wang et al. [87] introduced CoNi hydroxide nanosheets to modify the electronic structure of Ni through heteroatom introduction, thereby enhancing ethanol adsorption and achieving selective EOR. Figure 12c illustrates the local charge density distribution calculated using DFT, revealing that Ni atoms migrate toward Co atoms, resulting in an enrichment of more negative charges and a higher electron density compared to Ni atoms alone.This phenomenon promotes increased electrophilicity of Ni atoms and their chemisorption onto nucleophilic hydroxyl groups of ethanol (Figure 12d).Moreover, a twoelectrode system was developed to integrate ethanol oxidation and efficiently couple hydrogen evolution, resulting in substantial hydrogen production of up to 90.5% Faradaic efficiency attributed to the enhanced charge transfer kinetics at the anode.The efficacy of this electron density modulation strategy can also be observed in nanoheterostructures. Li et al. [88] reported Co(OH) 2 @Ni(OH) 2 heterostructures to regulate atomic charge density discreetly, achieving optimal adsorption-desorption performance and lowest thermodynamic energy barrier.The d-band center is considered to be a suitable descriptor for the binding of the adsorbates to the metal since it embodies the antibonding energy state. [89]CoOOH@NiOOH heterostructure formation leads to electrons flowing from Ni to Co, upward shifting the d-state energy at the heterointerface, enhancing adsorption, and promoting EOR (Figure 12e).It is worth noting that similar strategies involving doping or incorporating single-atom cocatalysts can also be explained using these principles.These design approaches aim to manipulate charge density distribution and modify electronic properties at interfaces, ultimately leading to improved catalytic performance. [90]lycerol, a substantial biomass-derived raw material and a representative substance for polyols, is produced as a byproduct in biodiesel production and the oil chemical industry.Its notable functionality arises from its symmetrical chemical structure that incorporates three hydroxyl functional groups (-OH).Notably, the primary and secondary hydroxyl groups exhibit symmetry and are inclined to react with different substances, leading to the creation of a variety of high-value-added products. [91]The oxidation of the primary hydroxyl group results in the formation of glyceric acid (GLA), which can be further oxidized to produce tartronic acid (TTA).Both glyceric acid and tartronic acid are commercially valuable compounds.It's worth noting that glyceric acid is an outcome of the subsequent oxidation of glyceraldehyde.Research has demonstrated that on Pt (100), the oxidation potential of glycerol to glyceraldehyde measures 0.7 V versus RHE. [92]The oxidation of secondary hydroxyl groups produces an important fine chemical substance, dihydroxyacetone (DHA), while the oxidation of all three hydroxyl groups generates highly functionalized molecules of ketomalonic (or mesoxalic) acid. [93]Figure 13a illustrates the established general oxidation pathways, but the properties of different metal catalysts can significantly affect the chemical conversion process.Therefore, comprehending and designing glycerol electrooxidation reaction pathways on various metal catalysts become crucial for controlling the selective direction of glycerol oxidation reactions (GORs), specifically targeting either primary or secondary alcohol functional groups.By studying the behavior of different metal catalysts during glycerol electro-oxidation, researchers aim to gain insights into how to manipulate and direct this chemical transformation effectively.This knowledge allows for precise control over whether primary or secondary alcohol functional groups undergo oxidation.Such control over selective oxidation is vital as it determines not only the yield but also the specific products obtained from glycerol electro-oxidation.By tailoring reaction conditions and selecting appropriate metal catalysts with desired properties, researchers can steer toward targeted outcomes based on their application requirements.
The oxidation of glycerol on platinum catalysts involves multiple sequential steps.DFT calculations reveal that the energy barrier for breaking the C─H/O─H bond is significantly lower compared to the breaking of C─C or C─O bonds (exceeding 0.5 eV).Notably, the rupture of C─C bonds occurs only after substantial dehydrogenation of glycerol. [94]Glycerol oxidation exhibits high sensitivity to structural factors, whereby properties like crystal structure and particle size of Pt can influence both activity and reaction pathways.The surface structure of Pt plays a crucial role in promoting the oxidation process for primary and secondary alcohols under acidic conditions. [92]On the Pt (111) electrode, the dehydroglycerol intermediate binds to the surface through two Pt─C single bonds to produce an ethylene glycol like intermediate, which is a precursor of glycerol aldehyde and dihydroxyacetone.Therefore, glycerol aldehyde, glycerol acid, and dihydroxyacetone are the resulting products of glycerol oxidation.On the Pt (100) electrode, glyceraldehyde emerges as the primary product of this reaction due to the binding of dehydroglycerol intermediate to the surface through Pt═C double bonds (Figure 13b).Consequently, Pt-based bimetallic electrocatalysts are highly regarded as exceptionally active catalysts for selectively oxidizing glycerol.Lee et al. [95] employed carbon-loaded bimetallic PtSb(PtSb/C) as a stable and efficient electrocatalyst in an electrocatalytic reactor to investigate how product distribution and yield depend on electrode potential (Figure 13c).Their findings revealed that elevated electrode potentials lead to decreased glycerol conversion rates and DHA selectivity due to dominance in C─C bond cleavage (Figure 13d).Some nanocrystals with specific structures, such as nanodendrites or excavated structures, can expose a larger surface area and more undercoordinated atoms (e.g., edges, corners, and kinks), thereby promoting the enhancement of catalytic activity.Du et al. [96] synthesized Pt-Co nanocubes with excavated and dendritic structures, demonstrating superior catalytic performance for glycerol electrooxidation (Figure 13e).The synergistic effect between Pt and Co, combined with the high exposure to high-energy surfaces and undercoordinated atoms, contributes to their enhanced performance.This research highlights the importance of using appropriate methods to enhance the exposure number of active sites in electrocatalysts.By exposing the structure or crystal surface of an electrocatalyst, it is possible to increase its efficiency in promoting chemical reactions.This approach proves beneficial in enhancing the overall electrocatalytic performance.
However, the exorbitant cost of precious metal catalysts, which can constitute up to 95% of production expenses for DHA, restricts their potential commercial applications.Nickel and its alloys (such as Ni-Cu and Ni-Co) are the most prevalent nonprecious metal catalysts employed for glycerol oxidation.Prior research has demonstrated that nickel-based catalysts frequently exhibit C─C bond cleavage and low selectivity for DHA under highly alkaline conditions (pH ≥ 13), which is unfavorable for glycerol oxidation. [97]Glycerol oxidizes NiOOH at lower pH levels, with all products being stable, thereby facilitating a more conducive study on the ability of NiOOH to produce C3 and C2 products. [6]The reaction pathway of glycerol oxidation on a Ni electrode can be achieved through both direct and indirect electron transfer processes (Figure 14a), with its rate exhibiting distinct pH and potential dependence.By acknowledging the presence of two dehydrogenation mechanisms and their disparities on NiOOH, it is anticipated that using milder pH and potential conditions could enhance the oxidation of secondary alcohols.A comprehensive and coherent understanding of the intricate GORs occurring on NiOOH lays a solid foundation for further research into electrochemical glycerol valence.
Figure 13.a) The general reaction pathways of glycerol oxidation over metal catalysts in electrochemical processes.2b] Copyright 2021, Wiley-VCH.b) Pt (111) and Pt (100) difference for glycerol oxidation selectivity.Reproduced with permission. [92]Copyright 2016, American Chemical Society.c) Schematic illustration of the dehydrogenative oxidation of biomass-derived glycerol in an electrocatalytic reactor system.d) Performance of the PtSb/C catalysts in the electro-oxidation of glycerol as a function of the applied electrode potential, 0.1 M glycerol, 60 °C, 10 h.Reproduced with permission. [95]Copyright 2016, The Royal Society of Chemistry.e) TEM images of individual Pt-Co EDNCs obtained from [111], [001] and [011] orientation and corresponding 3D models.Reproduced with permission. [96]Copyright 2019, Elsevier.
Cobalt oxide catalysts also exhibit promising activity for the electro-oxidation of glycerol.The unique properties of cobalt oxide make it an attractive choice for catalytic applications.Its high stability under harsh reaction conditions ensures long-term durability and reusability.Additionally, its abundance and relatively low cost compared to other precious metal-based catalysts make it economically viable on a large scale.Vo et al. [98] also reported an effective amorphous cobalt oxide catalyst capable of electro-oxidizing glycerol to DHA at the anode.Initially, CoO x undergoes electrochemical oxidation and transforms into amorphous (oxygen) hydroxides.Subsequently, the ÀOH substance serves as the active site for the oxidation of glycerol or its intermediates.Upon adsorption at the active site, it undergoes incomplete oxidation to C3 species through C-H cleavage.This pathway commences with the initial oxidation of secondary hydroxyl groups to form DHA, which further oxidizes to generate GLA and ultimately furoic acid (FA) (Figure 14b).Using a singleatom doping strategy, the electro-oxidation activity of spinel oxides can be further improved.This approach involves introducing specific atoms into the crystal lattice of spinel oxides, thereby modifying their electronic structure and surface properties.The incorporation of these dopant atoms at the atomic level allows for precise control over the catalytic performance of spinel oxides.Through this single-atom doping strategy, various desirable effects can be achieved.First, it promotes the formation of active sites on the surface of spinel oxide catalysts, which are crucial for facilitating electrochemical reactions.These active sites provide favorable binding sites for reactant molecules and promote efficient charge transfer during oxidation processes.Furthermore, single-atom doping can also enhance the stability and durability of spinel oxide catalysts.By carefully selecting appropriate dopant atoms with suitable electronic configurations and sizes, they can effectively stabilize reactive intermediates formed during electrooxidation reactions.This stabilization prevents undesired side reactions or catalyst deactivation pathways that may occur under harsh reaction conditions.Moreover, this strategy enables fine tuning of key parameters such as redox potentials and adsorption energies on the surface of spinel oxide catalysts.By precisely controlling these factors through single-atom doping, one can optimize their catalytic performance toward specific target reactions or improve overall efficiency in energy conversion devices like fuel cells or metal-air batteries.Therefore, Wang et al. [99] reported a single-atom bismuth (Bi) doping strategy to enhance the activity and selectivity of Co ) (Figure 14c).Mechanism studies have shown that OH* accelerates the oxidation of hydroxyl groups and the cleavage of C─C bonds, achieving GOR activity (400 mA cm À2 with reversible hydrogen electrode [RHE], at 1.446 V vs RHE) and high FE of formate (97.05 AE 2.55%).It has been proven to be effective in customizing glycerol selective oxidation products through the redox of high-valence cobalt species, and the oxidation products include HPA, tartronate, GLC, et al. [100] Transition metal-based nanomaterials rich in the earth have emerged as promising alternatives to expensive precious metal materials and nickel-based materials for various applications.These nanomaterials not only address the issues of high cost and limited availability associated with precious metals but also overcome the limitations of nickel-based materials, such as C─C bond breakage and low selectivity for desired reactions.Han et al. [101] conducted a comprehensive investigation on a series of cobalt-based spinel oxide nanostructures (MCo 2 O 4 , M = Mn, Fe, Co, Ni, Cu, and Zn) as catalysts for the electrochemical oxidation of glycerol.The findings revealed that among these spinel oxides, CuCo 2 O 4 exhibited superior intrinsic catalytic activity for glycerol oxidation in alkaline solution (pH = 13), displaying selective conversion to formic acid.A bulk electrolysis reaction of glycerol oxidation (pH = 13) was carried out with a RHE at a constant potential of 1.30 V.The high selectivity for formic acid production was 80.6%, and the total FE for all value-added products was 89.1%, with a glycerol conversion rate of 79.7%.In summary, transition metal-based nanomaterials enriched with earth elements offer a compelling solution to overcome the limitations associated with traditional catalyst materials.Their ability to simultaneously address challenges related to cost, selectivity, C─C bond breakage, water decomposition, and biomass oxidation positions them as versatile candidates for various industrial applications ranging from energy conversion technologies to sustainable chemical synthesis methods.
As an important biomass-based platform molecule, glucose can be selectively converted into various high-value-added chemicals like gluconic acid (Figure 15a).2b] Therefore, it is considered an excellent substitute for OER.In addition, the abundant proton content of glucose makes cathode HER more energy-efficient. [102]ue to the significant reactivity of precious metals in the oxidation process of glucose under alkaline conditions, extensive research has been conducted on these metals, particularly gold (Au) and platinum (Pt).Glucose can adsorb onto both precious metal surfaces at low potentials; however, there exists a distinction in terms of the adsorbed species. [103]On Pt-NPs, hydrogen atoms, gluconic acid species, and CO are predominantly present, whereas CO adsorption is absent on Au-NPs.Furthermore, the oxidation behavior of adsorbed hydrogen atoms also differs between the two metals.On Pt-NPs, hydrogen atoms undergo oxidation to form protons (H þ ), while on Au-NPs, they are oxidized to molecular hydrogen H 2 instead of H þ .Exploiting the combined characteristics of these two metals and harnessing their synergistic effects offer an effective approach for fabricating glucose electrocatalysts with enhanced performance. [104]Based on this, Lin et al. [7] used nanocarbon carriers to load Pt-Au bimetallic materials and prepare electrocatalysts for glucose oxidation.The remarkable synergistic effect between Pt-Au bimetallic materials led to significant enhancement in electrocatalytic performance.Furthermore, comparative analysis with single-metal Pt and Au revealed that PtAu exhibited superior electrocatalytic activity under alkaline conditions, thereby augmenting the selectivity of glucose oxidation.This can be primarily attributed to the introduction of Au, which induces a shift in the d-band center of Pt toward the Fermi level, consequently reducing the surface energy of Pt and enhancing its electrocatalytic performance and stability (Figure 15b).
Previous studies have primarily focused on catalysts based on precious metals, but their high cost has hindered further advancements.Recently, there has been a growing interest in transition metal-based catalysts that offer adjustable catalytic activity and stability for the electro-oxidation of glucose.Liu et al. [105] synthesized NiFe oxides (NiFeO x ) and NiFe nitride (NiFeN x ) derived from NiFe-LDH nanosheet arrays for the conversion of glucose into gluconic acid (GNA) and glucaric acid (GRA), while also investigating the catalytic active site responsible for glucose oxidation.Despite having lower OER activity compared to NiFeO x , NiFeN x exhibited superior performance in glucose oxidation (Figure 16a), suggesting that the in situ Figure 15.a) Electrochemical reactions of glucose into high-value-added compounds (In aqueous solution, glucose is mainly under its cyclic form: glucopyranose).102b] Copyright 2023, American Chemical Society.b) Schematic diagram of glucose electro-oxidation on Pt, PtAu, and Au catalysis surfaces as a function of the d-band center position.Reproduced with permission. [7]Copyright 2020, Elsevier.
formation of Ni-Fe oxyhydroxides serves as the active center for this reaction.Additionally, the authors developed an innovative two-electrode glucose electrolyzer to achieve efficient production of GNA and GRA with high FE, along with enhanced and stable HER.Given the appealing properties of transition metal oxyhydroxides, it becomes even more crucial to identify their elusive active sites.Therefore, Zhu et al. [106] employed CoOOH as a catalyst for the electro-oxidation of glucose to investigate the active site involved in the reaction.They are committed to using a series of spectroscopic techniques to determine the active source of CoOOH in GOR from the perspective of atomic scale structure.There are two forms of active Co 3þ -oxygen species on CoOOH, as shown in the Figure 16b: 1) hydroxyl adsorbed on Co 3þ ions (μ 1 -OH-Co 3þ ), that is, hydroxyl ions (OH À ) in the electrolyte fill oxygen vacancies (O v ) on CoOOH to form adsorbed OH; 2) lattice oxygen (μ 2 -O-Co 3þ ), that is, the deprotonation of μ 2 -OH-CO 2þ / 3þ to μ 2 -O-Co 3þ ; these two sites are considered as active site of electrocatalytic GOR.The roles of μ 1 -OH-Co 3þ and μ 2 -O-Co 3þ in catalyzing GOR are different, with μ 1 -OH-Co 3þ responsible for oxidation and μ 2 -O-Co 3þ mainly involved in dehydrogenation, both of which are key oxidation steps for glucose oxidation.
Previous studies have shown that glucose can reduce Cu(II) to Cu(I), accompanied by the formation of Gluconic acid. [107]herefore, Zhang et al. [108] constructed a glucose assisted Cu(I)/Cu(II) redox coupling hydrogen production system, where glucose in an alkaline electrolyte can spontaneously reduce Cu(OH) 2 to Cu 2 O to complete the Cu(I)/Cu(II) redox cycle.The system only needs a voltage input of 0.92 V to provide a current density of 100 mA cm À2 , and its FE of producing gluconic acid is 98.7%.The reaction mechanism of glucose on the Cu(OH) 2 electrode is shown in the Figure 16c.Due to the strong interaction between glucose and Cu(OH) 2 electrodes, glucose first adsorbs on the surface of Cu(OH) 2 .Subsequently, Figure 16.a) Linear sweep voltammetry profiles of the NiFeO x -NF and NiFeN x -NF catalysts for glucose oxidation and OER.Reproduced with permission. [105]Copyright 2020, Springer Nature.b) The roles of μ 1 -OH-Co 3þ and μ 2 -O-Co 3þ in the GOR under alkaline conditions.Reproduced with permission. [106]Copyright 2023, Wiley-VCH.c) The reaction pathway for glucose to form gluconic acid with/without Cu(OH) 2 electrode.Reproduced with permission. [107]Copyright 2021, Wiley-VCH.
Cu(OH) 2 can lose two -OH groups to form Cu 2 O structure and the intermediate C 5 H 11 O 5 CO* and -OH group formed by glucose form gluconic acid (C 5 H 11 O 5 COOH).Furthermore, due to the more favorable thermodynamics with lowed onset potential and abundant proton content of glucose, the HER is greatly promoted, and 100% FE of hydrogen production can still be achieved at a high current density of 100 mA cm À2 .
For glycerol and glucose, precious metals such as gold and platinum have been extensively researched for their high catalytic activity in alkaline media.Nevertheless, transition metal-based catalysts, offering adjustable catalytic activity and stability, have demonstrated promising results in glycerol and glucose oxidation reactions.These catalysts present a cost-effective alternative to precious metals.Transition metal oxides, in particular, have emerged as suitable electrocatalysts for glycerol and glucose oxidation due to their unique properties, showcasing excellent stability and cost-effective synthesis compared to precious metals or transition metal-based catalysts.This renders them highly attractive for large-scale applications.To delve deeper into the active sites responsible for catalyzing glycerol and glucose oxidation, research efforts are directed toward exploring transition metal oxide hydroxides.By scrutinizing their structural characteristics and electronic properties, scientists aim to provide a theoretical foundation for developing efficient and stable electrocatalysts.Beyond fundamental research, optimizing the design and preparation methods of these catalysts is crucial for enhancing the efficiency and selectivity of glycerol and glucose oxidation reactions.Therefore, Table 5 summarizes some recently studied catalyst types and related information in the oxidation of glycerol and glucose, which provides a reference for subsequent researchers to explore.

Furanic Oxidation
Furanic compounds, notably furfural (FUR) and 5-hydroxymethylfurfural (HMF), are widely acknowledged as crucial biomass platform chemicals for the synthesis of high-value-added products. [109]Structurally, furanic compounds feature heteroaromatic furan rings containing aldehyde functional groups, endowing them with remarkable chemical reactivity. [110]Furanic compounds are capable of undergoing typical aldehyde reactions, including acetalization, acylation, aldol and Knoevenagel condensation, reduction to alcohol, reductive amination to amine, decarbonylation, oxidation to carboxylic acid, and Grignard reactions.The aromatic furan rings can also participate in alkylation, hydrogenation, oxidation, halogenation, and nitration reactions. [111]Presently, the most extensively researched applications in this field revolve around the hydrogenation of furanic compounds to produce furfuryl alcohol, along with their oxidation to generate furoic acid and furan-2,5-dicarboxylic acid (FDCA).109c] Furfural can produce various high-value-added chemicals through partial oxidation, mainly furoic acid (FA) used as a common preservative, fungicide, and drug precursor. [111]On the Pt electrode, the oxidation potential of furfural is about 0.9 V versus RHE, at which furfural electro-oxidation preferentially generates FA.Although several furfural electro-oxidation products have been identified, the overall pathway and detailed mechanism remain to be fully determined.Recent research has revealed that adjusting the electrode potential during furfural electro-oxidation enables control over product selectivity.Gong et al. [26] determined the adsorption energy of furfural oxidation, relative free energy of key intermediates, and reaction energy by conducting periodic DFT calculations on Pt (111) surfaces.Furthermore, they proposed four different reaction pathways for the electrooxidation of furfural (Figure 17a).From the reaction energy and potential energy of the initial steps calculated by DFT, it was found that Path 1, involving C-H dissociation, has a lower potential battier compared to other steps, suggesting it may be a favorable pathway for furfural oxidation.Furfural undergoes initial oxidation by breaking the C─H bond in its aldehyde group to form FÀCO (F represents the furan ring), which then leads to furoic acid formation.Therefore, DFT analysis can guide the catalyst design of the electro-oxidation process by gaining a deeper understanding of the energy dynamics and reaction pathways involved in the oxidation reaction.However, unresolved mechanistic issues remain due to changes in surface structure at high potentials.On this basis, Roman et al. [8] continued to study the oxidation pathway of furfural in acidic electrolytes on platinum (Pt) catalysts.They used DFT to study the electrochemical oxidation mechanism of furfural on carbon-supported Pt.They found that the electro-oxidation of furfural between 0.9 and 1.1 V versus RHE exhibited high selectivity for FA and 5-hydroxyfuroic acid (HFA).At 1.2 V versus RHE and above, 5-hydroxyfuran-2 (5H)   Reproduced with permission. [26]Copyright 2019, Elsevier.b) Proposed reaction mechanism for the electrooxidation of furfural in acidic environment on Pt.Reproduced with permission. [8]Copyright 2022, American Chemical Society.
ketone (HFN) becomes the main product, and its selectivity to maleic acid (MA) also increases.Notably, this study marks the first identification of HFN as an electrochemical oxidation product derived from furfural.Additionally, a more comprehensive reaction mechanism for the formation of oxygen-containing compounds was proposed (Figure 17b).In summary, furfural undergoes initial dehydrogenation to form aldehydes while releasing H þ and e À .Subsequently, it can either react with OH* on the Pt surface to generate FA or undergo H extraction at position 5 of the ring to produce HFA.The adsorbed furfural or FA species may then proceed through decarbonylation or decarboxylation steps, leading to the formation of adsorbed furfural species and subsequent generation of HEN and MA.In the article, they also studied the possible reaction mechanisms of various products and intermediates using OLEMS and in situ infrared spectroscopy, proposing directions for designing more active and selective electrocatalysts.
Previous studies have demonstrated that noble metal plays a crucial role in the activation and adsorption of C═O and C═C bonds within the furan ring. [112]However, diverse catalysts possessing distinct active sites exhibit varying effects on the adsorption configuration, thereby influencing the selectivity of furfural.To further elucidate the correlation between active sites and adsorption configurations, researchers have conducted pertinent investigations.Zhang et al. [113] used theoretical calculation combined with experiments to verify the effect of various crystal facets of Pd catalyst on the conversion rate and selectivity of furfural hydrogenation.The electrocatalytic hydrogenation (ECH) process of furfural is affected by Langmuir-Hinshelwood mechanism, which is characterized by the difference in adsorption energy between adsorbed hydrogen (*H) and furfural intermediate (*FAL).Pd{111} shows stronger *H adsorption and the weakest *FAL adsorption energy due to the unsaturation of its surface atomic coordination (Figure 18a).Moreover, the thermodynamics of hydrogenation process is better than that of gaseous H 2 evolution, and FE can reach 95.3%.This provides theoretical insights into the crystal facet regulation and hence the control of the yield of the products.
It is worth noting that the hydrogenation products of furfural, such as furfuryl alcohol (FA) and 2-methyl furan (MF), are also significant energy and chemical raw materials.However, traditional industrial hydrogenation approaches have limitations due to the risk of combustible hydrogen and high temperatures.Electrochemical reduction in aqueous solution appears to be an optimal mild conversion pathway.Metal-based catalysts have recently become typical candidates for furfural hydrogenation due to their good surface coverage of adsorbed hydrogen and adsorption to furfural substrates.Jung et al. [114] used a Cu-based cathode to achieve the conversion of furfural to FA and MF and explored the effect of the electrolyte on selectivity.They found that both the pH of the electrolyte and solute ions affect the product selectivity, and the charge transfer of the electrode determines the product conversion rate and yield, which provides a direction for the design of the reaction system.Zhou et al. [115] achieved selective regulation of furfural hydrogenation using a bimetallic alloy based on a copper-based catalyst.The modified CuPd bimetallic electrode exhibits different adsorption selectivity compared with Cu electrode, which on the one hand reduces the formation energy of MF and on the other hand increases the surface coverage of adsorbed hydrogen, with high FE of 75%.
In recent years, there has been a significant interest in the development of cost-effective and sustainable nonprecious metal catalysts for efficiently oxidizing furfural derived from biological sources into valuable chemicals.Zhong et al. [116] successfully Figure 18.a) schematic of the competition between ECH and HER on the Pd facets.Reproduced with permission. [113]Copyright 2022, Elsevier.b) The mechanism proposed for the electro-oxidation of HMF and FUR by the Ni x Se y -NiFe LDH@NF electrode.Reproduced with permission. [116]Copyright synthesized an electrocatalyst, Ni x Se y -NiFe LDH@NF, by combining nickel selenide with NiFe-LDH, which exhibited exceptional catalytic performance for the oxidation of HMF and FUR.The yields of FDCA and furoic acid reached 99.3% and 99.7%, respectively, accompanied by high FE values of 98.9% and 99.5%.This remarkable performance can be attributed to the metalloid conductivity of nickel selenide, the large surface area provided by NiFe-LDH nanosheets, as well as the abundant catalytic active sites exposed on their surfaces that facilitate efficient charge transfer at the nanointerface during HMF and FUR oxidation processes.Additionally, the conversion of Ni 2þ species to high-valence Ni 3þ also plays a crucial role in promoting the oxidation reactions of HMF and FUR (Figure 18b).Yang et al. [117] investigated the correlation between the valence state and the adsorption behavior of the Cu-based catalyst during furfural oxidation.Mixed-valence Cu catalyst (MV Cu) was prepared on copper foam by cyclic voltammetry, which showed 98% conversion rate and 99% selectivity to furoic acid in a flowing electrolytic cell.In situ extended X-ray absorption fine structure spectroscopy confirmed that the valence change of Cu is the origin of the catalytic activity of furfural oxidation reaction (FFOR), however, when the Cu foil is converted to Cu 2 O, it is inactivated, and a reasonable conjecture is related to the adsorption on the surface.Cu species preferentially adsorb hydroxyl groups rather than furfural intermediates under alkaline conditions, and this competitive adsorption results in reduced catalyst activity (Figure 18c).Therefore, the root cause of inactivation in Cu-based catalysts is the unstable change of valence state, and we believe that the regulation of electronic structure and the introduction of new adsorption sites will be important methods to optimize Cu-based catalysts.In an innovative study, Li et al. [118] devised a novel linear paired oxidation strategy to simultaneously convert furfural into furoic acid at both the cathode and anode in an electrochemical cell, thereby enabling cost-effective conversion of furoic acid.The anode employs indirect oxidation facilitated by reactive iodine species, while the cathode similarly shows ORR to generate ROSs, which actively participate in furfural oxidation concurrently.It is noteworthy that hydrogen peroxide exhibits potent oxidative capacity but may lead to excessive oxidation, posing challenges for potential regulation and reaction pathway design due to the relatively low selectivity toward furoic acid at only 27.5%.The optimized electrochemical cell demonstrated efficient conversion of furoic acid with an impressive overall electron efficiency of 125%.
5-hydroxymethylfurfural (HMF) is also an excellent biomassderived chemical with significant applications in the production of fine chemicals, biofuels, drugs, etc.Its oxidation product, 2,5-furandicarboxylic acid (FDCA), holds great potential as a key platform chemical that can potentially replace terephthalic acid in various polyesters such as polyethylene terephthalate (PET). [119]The two possible pathways for forming FDCA are shown in Figure 19a. [17,18,120]One pathway involves oxidizing the alcohol groups in HMF to generate 2,5-diformylfuran (DFF) as the initial intermediate, while the other pathway involves oxidizing the aldehyde groups in HMF to produce 5-hydroxymethyl-2-furan carboxylic acid (HMFCA) as the first intermediate.Subsequently, DFF and HMFCA undergo further oxidation to form 5-formyl-2-furan carboxylic acid (FFCA), which ultimately leading to FDCA formation.It is worth noting that electrocatalytic HMF to FDCA oxidation exhibits a lower oxidation potential compared to OER in aqueous solutions.The direct electrocatalytic HMF to FDCA oxidation through the six-electron reaction mechanism of the anode is shown in the equation [121] HMF Research has demonstrated the pH dependence of the reaction pathway, with pathway II being predominant under strong alkaline media (pH ≥ 13), while pathway I prevails in nonstrong alkaline environments (pH < 13). [120]Kubota et al. [119] conducted electrochemical oxidation of HMF using manganese oxide (MnO x ) anodes in acidic media, resulting in a noteworthy 53.8% yield of FDCA in H 2 SO 4 solution at pH = 1.Throughout the entire reaction process, only a minimal amount of HMFCA was observed, and during the initial stages of electrolysis, there was a significant accumulation of DFF concentration.Choi et al. [21] prepared nanoscale nickel oxide catalysts, which exhibited excellent electrocatalytic performance for the oxidation of HMF under near-neutral conditions (pH = 7.2) and followed the formation pathway of DFF.During the HMF oxidation process, the redox activity cycle between the Ni(OH) 2 species formed on the surface of NiO electrocatalysts and Ni(III)-OOH on the surface of NiO-NP promotes the oxidation of HMF.120a] Therefore, high speed and high yield FDCA production can be easily achieved at pH 14.The HMF oxidation reaction (HMFOR) mechanism in this system is mainly revealed by analyzing the intermediates and reaction pathways in the reaction using DFT.In addition, DFT technology can also be used to evaluate the relative reaction rates of alcohol oxidation and aldehyde oxidation, and the results show that under near-neutral conditions, alcohol oxidation reactions are more preferred.In addition, DFT technology also helps to understand that the final oxidation step from FFCA to FDCA under near-neutral conditions is the reason for the slowest reaction rate, as the lack of OH À ions in the electrolyte solution affects the rate and efficiency of aldehyde oxidation.Therefore, DFT technology has played an important role in revealing the key steps and reaction pathways in the HMFOR mechanism.Liu et al. [120b] prepared NiFe LDH nanosheets loaded on carbon brazing for catalytic oxidation of HMF to FDCA.The catalyst exhibits excellent catalytic activity at pH = 14 (1 M KOH), with FE of up to 99.4% for HMF conversion.Gao et al. [20] reported an electrocatalyst of NiSe@NiO x core-shell nanowires for efficiently upgrading HMF into FDCA.The catalyst shows a high FE of 99%.In this system, both HMFCA and FFCA were detected, while DFF was the only detectable intermediate that was not observed, indicating that HMF mainly passes through the route of HMFCA intermediates on the NiSe@NiO x electrode (Path II), which is consistent with the most reported route in aerobic oxidation systems.
Cobalt-based catalysts are regarded as promising candidates for the electrochemical oxidation of HMF due to the presence of mixed valence (Co 2þ and Co 3þ ) at different atomic positions in cobalt-based spinel oxides.In order to distinguish the individual effects of different sites in Co 3 O 4 on HMF oxidation, [122] Lu et al. [123] investigated the electrochemical properties of HMF on cobalt spinel oxides (Co 3 O 4 , ZnCo 2 O 4 , and CoAl 2 O 4 ) by incorporating Zn 2þ and Al 3þ ions into tetrahedral and octahedral positions respectively (Figure 19b).Tetrahedral Co 2þ Td can chemically adsorb acidic organic molecules, while octahedron Co 3þ Oh plays a pivotal role in the oxidation of HMF, which revealed the influence of different geometric configurations of cobalt in Co 3 O 4 on the catalytic activity of HMF oxidation.Furthermore, they effectively enhanced the activity of HMF oxidation by substituting Cu 2þ ions into octahedral Co 3þ .In addition to the intrinsic catalytic activity, the adsorption at the interface including organic substrates and intermediates also profoundly affects the conversion selectivity and yield of HMF, which can be regulated by adjusting the adsorption energy of the reaction molecules.Therefore, Lu et al. [124] conducted further investigation and optimization of the adsorption behavior of HMF on spinel oxide (Co 3 O 4 ) by introducing single-atom Ir and used some techniques, such as in situ Raman spectroscopy, to reveal reactive sites for better understanding of the reaction process.The results showed that compared with Co 3 O 4 , the Ir-Co 3 O 4 electrocatalysts demonstrate enhanced HMF adsorption through the C═C group (Figure 19c).The optimal HMF adsorption behavior contributes to improved electrocatalytic performance for hydrogenation of HMF.The synergistic effect between Ir and Co atoms in Ir-Co 3 O 4 successfully regulates the interaction between the electrocatalyst and the reactant.Furthermore, they chose Co 3 O 4 and V O -Co 3 O 4 as catalysts to study the role of oxygen vacancies in the HMFOR process and reveal the adsorption behavior of HMF and OH À . [125]ombining the experimental findings with DFT calculations, it was found that nucleophilic OH À tends to fill the V O in the V O -Co 3 O 4 lattice to saturate the Co-O coordination, and further participate in the dehydrogenation and binding of HMF molecules through lattice OH, which can significantly accelerate the rate-determining step of HMFCA dehydrogenation and improve the catalytic activity of HMFOR on spinel oxides (Figure 19d).This study provides valuable insights into the reaction mechanism of HMFOR and provides guidance  [120a] Copyright 2018, American Chemical Society.b) Scheme to show the relative performance of electrochemical HMF oxidation to the targeted product on spinel oxides by building geometric sites of the tetrahedron (Zn 2þ ) or octahedron (Al 3þ ).Reproduced with permission. [123]Copyright 2020, Wiley-VCH.c) The adsorption model of HMF molecules on Ir-Co 3 O 4 .Reproduced with permission. [124]Copyright 2021, Wiley-VCH.d) The scheme of the relationship between structure-activity potential.Reproduced with permission. [125]Copyright 2021, Wiley-VCH.e) Schematic illustration of intermediates evolution over Ni-M/NF during OER.Reproduced with permission. [128]Copyright 2023, Wiley-VCH.
for the design of efficient and advanced electrocatalysts for this process.
Nickel-based electrocatalysts have been widely recognized for their exceptional redox ability in the electrochemical oxidation of 5-hydroxymethylfurfural (HMF).This unique property allows them to efficiently convert HMF into valuable chemical intermediates or fuels.However, to further enhance their catalytic performance, it is crucial to optimize the electronic structure and improve the adsorption properties of intermediates on these catalysts.One effective approach to achieve this optimization is through interfacial engineering.By carefully designing and controlling the interfaces between different materials or phases within the catalyst system, we can manipulate the electronic interactions and create more exposed active sites.This not only promotes efficient charge transfer during electrochemical reactions but also enhances the adsorption capacity for key reaction intermediates.Therefore, Lu et al. [126] investigated the catalytic performance of 3D0graded nanostructured NiO-Co 3 O 4 electrocatalysts for HMF electro-oxidation by augmenting the concentration of interface sites with abundant defects.They used DFT to study the oxidation process of HMF on NiO-Co 3 O 4 electrode, as well as the generation and conversion of intermediates.The results indicate that the interfacial effect engenders a surplus of cationic vacancies, regulates the electronic properties of Co and Ni atoms, and enhances the oxidation state of Ni species.This results in exceptional HMF oxidation activity exhibited by the NiO-Co 3 O 4 catalyst, highlighting its efficacy in improving catalytic performance effectively.Based on this, Zhou et al. [127] proposed an integrated electrode comprising Pt NPs uniformly loaded onto the surface of Ni(OH) 2 as a highly efficient electrocatalyst for HMFOR.They used in situ Raman spectroscopy technology to identify the phase transition between Ni(OH) 2 and Ni(OH)O on the Ni(OH) 2 electrode.In addition, in situ studies were used to demonstrate the optimized redox properties of Pt on Ni(OH) 2 , thereby promoting the formation of the active species Ni(OH)O and accelerating HMFOR.Pt also influences the adsorption behavior of HMF and optimizes its adsorption energy.The introduction of doped metal atoms can effectively improve the electronic structure of the central Ni atom and promote the oxidation activity of HMF.Chen et al. [128] prepared an electrocatalyst (Ni-Cu/NF) for HMF oxidation by supporting Ni-Cu bimetals on foam nickel and carried out DFT calculation for qualitative analysis of this catalytic system.According to the DFT results, the Ni site is the active site on Ni-Cu/NF, while the Fe site is more inclined on Ni-Fe/NF.In summary, the catalyst exhibits excellent catalytic performance, with the FE and yield of the product FDCA exceeding 95%.This is mainly due to the introduction of Cu, which can effectively prevent the charge transfer in the Helmholtz layer formed by the adsorption of OH ions in the electrolyte on the electrode, that is, prevent the deprotonation of OH* to O*, which can greatly passivate the OER process, thereby enhancing the oxidation of HMF (Figure 19e).
Recent studies have made significant advancements in the field of electrocatalysis, particularly regarding nonprecious metal catalysts.Iron, cobalt, nickel, and transition metal oxides have emerged as promising alternatives to precious metals for catalyzing the oxidation of furfural and HMF.These nonprecious metal catalysts have demonstrated excellent catalytic performance that can sometimes rival or even surpass that of their precious metal counterparts.However, despite these remarkable achievements, there are still several challenges that need to be addressed in order to fully harness the potential of these catalysts.One major challenge is the requirement for high-temperature and highpressure conditions during the reaction process.This not only increases energy consumption but also limits the scalability and practicality of these reactions on an industrial scale.Another challenge is the occurrence of potential side reactions during the oxidation process.These side reactions can lead to undesired byproducts or reduced selectivity toward desired products.Therefore, it is crucial for future research to focus on developing strategies to minimize or eliminate these side reactions in order to enhance overall efficiency and yield.Looking ahead, future research efforts should aim at exploring more efficient and sustainable catalytic systems that address these challenges while promoting the oxidation reaction of furfural and HMF.This will not only contribute toward advancing biomass energy conversion technologies but also pave the way for greener chemical production processes with reduced reliance on precious metals.

Selective Electrocatalytic C─C Bond Cleavage
Lignin, as one of the most abundant biopolymers, plays a pivotal role in advancing sustainable biorefineries and serves as a substantial source of chemicals for bioresource regeneration and energy production. [129]Due to its high biomass energy content and numerous functional groups, lignin holds immense potential for reforming.However, its intricate internal structure and challenging-to-break chemical bonds pose obstacles to its depolymerization and selective upgrading.The lignin's structure is shown in Figure 20a, the β-O-4, α-O-4, β-1, β-β, 4-O-5, β-5, and 5-5 0 are the most common linkages in lignin. [130]he C─C bond is the most common and basic chemical bond in lignin, which has huge recalcitrance to split, thus limiting the lignin transforming into various high-value-added chemicals. [131]However, the C─C bond in lignin has a high dissociation energy (307.7 kJ mol À1 ), which makes the selective cleavage of C─C in lignin a challenging problem. [132]Currently, various strategies have been developed to depolymerize lignin, including hydrolysis, pyrolysis, reduction, and oxidation. [133]mong these approaches, selective electrochemical oxidation has garnered considerable attention due to its capability to selectively break C α ─C β bonds without compromising the aromatic ring structure.This process enables the conversion of lignin into valuable products such as phenols, ketones, acids, and acid derivatives with utmost precision.During the electrocatalytic depolymerization of lignin, electrons can directly interact with the C─C bond of the reactant to cleave the reactant and produce free radical intermediates, thereby converting lignin into products.
Lead and lead oxide have been extensively employed in the study of lignin depolymerization owing to their excellent solution stability, electrochemical performance, and high OER potential, among other characteristics.Their exceptional solution stability ensures easy dissolution and dispersion in various solvents or reaction systems without undergoing significant chemical changes.This enables precise control over experimental conditions and facilitates the investigation of lignin depolymerization mechanisms.Furthermore, lead and lead oxide demonstrate remarkable electrochemical performance, rendering them suitable catalysts for lignin depolymerization reactions.They possess a high surface area and abundant active sites, promoting efficient electron transfer during the redox processes involved in lignin degradation.This enhances overall catalytic activity and selectivity toward desired products.Jia et al. [134] used modified PbO 2 to anodize rice straw lignin and subsequently detected and quantified ten oxidation products.Although the electro-oxidation process shows good efficiency, the selectivity and separation of specific products are still limited.In recent years, platinum has been used in the study of lignin depolymerization and upgrading because of its amazing effect on pyrolysis catalysts and its strong catalytic activity on C α -C β cleavage. [135]Ma et al. [136] reported an effective electrocatalytic strategy for C─C bond cleavage of β-O-4 linkage in lignin model compounds at room temperature using platinum as the anode.In this process, tert-butyl hydroperoxides (t-BuOOH in water, 70% aq.) as the oxidation first decompose into tert-butylperoxy radical intermediate and then couples with the C β -centred radical dehydrogenated from lignin on the Pt anode to form peroxide intermediate, and the intermediate continues to undergo electron transfer to produce various aromatic aldehydes by spontaneous C-C cleavage.In order to further regulate the electronic structure and catalytic activity of platinum catalysts, Cui et al. [137] prepared single-atom Pt-based catalyst on the carbon nano tube (Pt 1 /N-CNTs) for selective C α ─C β bond cleavage using a single-atom catalyst strategy, which enhanced the generation of C β -centred radical, resulting in a further increase in the yield of the product (Figure 20b).
Non-noble metal-based electrodes such as nickel-, cobalt-, and copper-based catalysts hold tremendous potential for advancing electrocatalysis due to their abundance, tunability, and ability to replace noble metals effectively.Their pure metal electrodes and NP electrocatalysts have been proven to be effective for lignin depolymerization. [138]Furthermore, to further enhance the catalytic performance of these electrodes, modification and design strategies can be employed to adjust their electronic structure, mass transfer process, and active site characteristics.For instance, introducing S atoms into the electrode material can create sulfide species that provide fast electron transfer channels. [139]This enhancement leads to improved catalytic activity by facilitating efficient charge transport during electrochemical reactions.The incorporation of S not only enhances electron transfer but also influences other important factors such as surface reactivity and stability.By carefully controlling the amount and distribution of S within the electrode material, researchers can optimize its overall performance for specific applications.Based on this, Wang et al. [140] proposed a novel strategy by depositing cobalt sulfide on foam nickel (NF) to prepare catalyst (NF@Co 3 S 4 /(α,β)-NiS) for 2-phenoxy-1-neneneba phenylethanol (PPE) oxidation, which is a typical β-O-4 model.Under 1.414 V versus RHE, the electro-oxidation conversion of PPE was 93.6%, and the yield of benzoate was 83.8%.In the oxidation reaction of PPE (POR), the catalyst provides a reaction active site for it and can effectively adjust the electronic structure of the electrode to increase the adsorption of PPE and electrophilic active oxygen.The presence of C α -OH or C α ═O during the reaction process Copyright 2018, American Chemical Society.b) Proposed mechanism of Pt 1 /N-CNTs-catalyzed conversion.Reproduced with permission. [137]Copyright 2021, American Chemical Society.c) Proposed oxidation pathways of PPE.Reproduced with permission. [140]Copyright 2022, Elsevier.d) The pathways of lignin depolymerization by anode oxidation and ROS.145a] Copyright 2014, RSC Publishing.
can significantly reduce the energy of the C α ─C β bond.Therefore, the free radicals adsorbed on the catalyst can attack the unstable C α -C β , causing the C α ─C β bond to break and form an ideal product (Figure 20c).In addition, the high-valence active substances (Co 3þ and Ni 3þ ) produced by Co 3 S 4 /(α,β)-NiS in the electrochemical process also play an effective catalytic activity for POR.Monatomic catalysts have attracted extensive attention because they can maximize the use of the metal active site, resulting in high catalytic activity.Therefore, Liu et al. [141] used a single-atom Co catalyst (Co-N-C) to oxidize and cleave the β─O─4 bond in lignin, which exhibited excellent activity and stability.Under optimized reaction conditions, the conversion rate of 2-(2-methoxyphenyl)-1-phenylethanol (MPP-ol) can reach 95%, and the yields of guaiacol (G) and benzoic acid (BA) are 59% and 83%, respectively.Co-N-C catalyst can promote further reaction between free radical intermediates and oxygen, thereby promoting the cleavage of C─C bonds in phenylglyoxal and ultimately generating BM.Cobalt sulfide has become a substitute for precious metals due to its high activity, unique electronic characteristics, and low price.
The redox pairs based on metal ions can be used to design more thermally optimized lignin depolymerization reactions.Cu-based oxides can be exploited for lignin depolymerization and upgrading due to their highly active redox pairs.Li et al. [142] proposed a Ce-Cu bimetallic nanosheet catalyst to efficiently convert the lignin into diethyl maleate with 180.9 mg g À1 yield.Cu 2þ /Cu is used for the activation of lignin; subsequently, Ce 4þ /Ce 3þ reduces oxygen to produce superoxide radicals (O 2 •À ), which dominates the C α -C β cleavage.Recognizing the strongly oxidizing properties of redox mediators and the homogeneous process that is more conducive to mass transfer, the electro-oxidation of lignin dominated by free radicals can also be used as an effective strategy.2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) and 4-acetamido-TEMPO (ACT) are effective stoichiometric reagents and catalysts for the oxidation of lignin under mild conditions. [143]Rafiee et al. [144] used TEMPO and ACT as mediators to selectively electro-oxidize primary alcohols in lignin to their corresponding carboxylic acids in an alkaline environment.Zhu et al. [145] developed ROS-assisted electrochemical depolymerization of lignin using H 2 O 2 as a mediator to generate aromatic products with various functional groups such as phenols, aldehydes, ketones, and acids.The phenolic groups of lignin in an alkaline environment can undergo anodic one-electron oxidation or be attacked by hydroxyl radicals (OH • ) to form benzoquinonyl radicals, which can then be depolymerized and oxidized to aldehydes or carboxylic acids by anodic oxidation or ROSs such as O 2 •À or HOO • (Figure 20d).Specific oxidation pathways can be achieved by different electrolytic conditions.

Summary and Outlook
With the continuous advancement of research and development, the integration of biomass electro-oxidation and renewable energy holds the potential to significantly impact the transition toward a zero-carbon economy.In this review, we have discussed the fundamental processes and mechanisms of electrochemical anodic organic conversion, providing detailed insights into the oxidation of various organic substrates from the perspective of material design and microscopic processes.Although extensive research has been conducted on the electro-oxidation of biomass derivatives in recent years, challenges persist in terms of catalyst microstructure reconstruction and reaction kinetics at the interface.As we mentioned above, the rapid development of theoretical calculations and in situ characterization measurements has widened avenues for a more accurate description of dynamic reaction processes, and further microscopic exploration will advance the development of electrochemical oxidation.
It is imperative to acknowledge that, despite advancements in the structure-function relationship between anode material design and selective electrocatalysis, numerous challenges persist and remain intractable at present.The following points can be considered.

Mechanism and Understanding of the Catalytic Sites
Although some electrocatalysts have been proven to be effective for organic oxidation, the specific catalytic sites, species, and the microscopic reconstruction process of the reaction interface are still unclear and controversial.In particular, the thermodynamic processes involved in ROS-mediated indirect oxidation are complex.Operando spectroscopic studies and in situ characterization with fine temporal and spatial resolution have been used to reveal the relationship between structure evolution and catalytic activities.Moreover, more sophisticated interface tailoring has been developed to control the kinetics of organic reactions.Our previous work has demonstrated the relationship between facet edge optimization and ROS generation, and we believe that reasonable microinterface design can benefit the desired organic transformation.

Optimized Mass Transfer Process
Heterogeneous catalytic reactions that involve poorly soluble substrates or products encounter formidable challenges in interfacial mass transfer processes.Moreover, for reactions involving gases, the mass transfer process is often incomplete, and these intermittent reactions can significantly impact reaction efficiency.Our previous research has demonstrated that the regulation of surface hydrophobicity can effectively optimize the three-phase mass transfer process, benefiting from tailoring the adsorptivity of the reaction intermediate at the electrode/electrolyte interface.Furthermore, implementing a closed flow electrolyzer represents a universal strategy for enhancing mass transfer processes, necessitating more stable electrodes to accommodate the system.

Industrialized Application
The shift toward industrial production and large-scale applications marks a pivotal step in extracting economic value from biomass electro-oxidation.The transition from laboratory to mass production necessitates overcoming the limitations of singleblock electrodes, enabling greater integration and scalability within a stable and high-current working conditions.Recently, more compact, integrated, and larger reactor platforms with enhanced fluidity have emerged for organic oxidation reactions at industrial-level currents.These platforms facilitate larger reaction areas for multiphase reactions and are coupled with cathodic reactions, such as large-scale hydrogen production, resulting in a remarkable increase in the yield of organic products with greater energy savings. [146,147]Moreover, intelligent industrial models are beginning to surface, potentially heralding the dawn of industrial applications. [148,149]We believe that with a deeper understanding and further development of reaction systems and highly stable electrodes, electrochemical biomass conversion will offer substantial value at the industrial level.

Figure 1 .
Figure 1.The basic conceptual illustration of electrochemical biomass derivatives conversion.

Figure 2 .
Figure 2. Schematic illustration showing the market prices, thermodynamic potential barriers, and electron transfer numbers of each organic compound discussed in this work.

Figure 4 .
Figure 4. Electrochemical organic oxidation of the two charge transfer pathways: a) mediated electro-oxidation; b) direct electro-oxidation.

Figure 6 .
Figure6.a) Scheme of the catalytic cycle for the functionalization of CH 4 by Pt catalyst.Reproduced under terms of the CC-BY license.[37]Copyright 2019, The Authors.Published by American Chemical Society.b) Schematic of the proposed catalytic cycle with the catalyst for CH 3 OH formation.Reproduced under terms of the CC-BY license.[38]Copyright 2019, American Chemical Society.c) Illustration of calculated electrophilic C-H activation pathways for activation of methane.Reproduced with permission.[39]Copyright 2020, American Chemical Society.d) Calculated frontier orbitals involved in the turnover-limiting step and the proposed transition state of the C-H activation step (left) and proposed catalytic cycle (right).Reproduced with permission.[40]Copyright 2020, Springer Nature.

Figure 7 .
Figure7.a) Electrochemical chloride ions mediate the oxidation of ethanol on the electrode.Reproduced with permission.[43]Copyright 2021, American Chemical Society.b) Schematic of the conversion of ethanol to ethylene oxide in an electrochemical cell using a two-step radical-mediated process.Reproduced with permission.[44]Copyright 2022, American Chemical Society.c) Schematic illustration and working principle of the linear paired system in a two-compartment H-cell.Reproduced with permission.[45]Copyright 2019, Springer Nature.

Figure 8 .
Figure8.a) Illustration of the thermodynamic energy barrier for methane oxidation.Reproduced with permission.[52]Copyright 2020, Royal Society of Chemistry.b) The electrochemical oxidation of methane by direct and indirect electro-oxidation.Reproduced with permission.[53]Copyright 2021, Elsevier.c) Contour plots for methane oxidation activity as a function of the free energy of adsorption of CO(relative to CO(g)) and OH(relative to H 2 O(l)/H 2 (g)).Reproduced with permission.[55]Copyright 2019, American Chemical Society.d) Schematic illustration of the reaction steps in direct methane-to-methanol oxidation on transition metal oxides.Reproduced with permission.[57a]Copyright 2023, American Chemical Society.e) The rates for the OER and methanol production are plotted.Reproduced with permission.[59]Copyright 2018, Royal Society of Chemistry.

Figure 12 .
Figure12.a) Adsorption energy plot for ethanol conversion process on NiCoSe, NiFeSe, and NiCoSe.Reproduced with permission.[84]Copyright 2023, Wiley-VCH.b) Transmission electron microscope (TEM) images of Co 3 O 4 .Reproduced under terms of the CC-BY license.[85]Copyright 2016, The Authors.Published by American Chemical Society.c) The charge density in the CoNi-PHNs and d) scheme representation of the electronic coupling between Co and Ni.Reproduced with permission.[86]Copyright 2019, Wiley-VCH.e) Schematic of bond formation of adsorbates (Ads) on the catalyst surfaces.Reproduced with permission.[88]Copyright 2022, RSC Publishing.
3 O 4 in an electrocatalytic GOR.Single-atom Bi replaces cobalt in the octahedral position of Co 3 O 4 (Co OH 3þ ), promoting the production of reactive hydroxyl species (OH*) at adjacent tetrahedral Co positions (Co Td 2þ

Figure 17 .
Figure 17.a) Calculated elementary step reaction free energies (above arrows) and activation barriers (below arrows) of furfural oxidation in Path 1, Path 2, Path 3, and Path 4 at 0 V versus RHE.Reproduced with permission.[26]Copyright 2019, Elsevier.b) Proposed reaction mechanism for the electrooxidation of furfural in acidic environment on Pt.Reproduced with permission.[8]Copyright 2022, American Chemical Society.

Figure 19 .
Figure19.a) Reaction pathway of HMF oxidation to FDCA.Reproduced with permission.[120a]Copyright 2018, American Chemical Society.b) Scheme to show the relative performance of electrochemical HMF oxidation to the targeted product on spinel oxides by building geometric sites of the tetrahedron (Zn 2þ ) or octahedron (Al 3þ ).Reproduced with permission.[123]Copyright 2020, Wiley-VCH.c) The adsorption model of HMF molecules on Ir-Co 3 O 4 .Reproduced with permission.[124]Copyright 2021, Wiley-VCH.d) The scheme of the relationship between structure-activity potential.Reproduced with permission.[125]Copyright 2021, Wiley-VCH.e) Schematic illustration of intermediates evolution over Ni-M/NF during OER.Reproduced with permission.[128]Copyright 2023, Wiley-VCH.

Figure 20 .
Figure 20.a) A representative lignin structure with typical lignin subunits and linkages encountered.Reproduced under terms of the CC-BY license. [130b]Copyright 2018, American Chemical Society.b) Proposed mechanism of Pt 1 /N-CNTs-catalyzed conversion.Reproduced with permission.[137]Copyright 2021, American Chemical Society.c) Proposed oxidation pathways of PPE.Reproduced with permission.[140]Copyright 2022, Elsevier.d) The pathways of lignin depolymerization by anode oxidation and ROS.Reproduced with permission.[145a]Copyright 2014, RSC Publishing.

Table 2 .
The performance of different catalysts for the electrochemical oxidation of CH 4 .

Table 3 .
The performance of different catalysts for the electrochemical oxidation of CH 3 OH.

Table 4 .
The performance of different catalysts for the electrochemical oxidation of C 2 H 5 OH.

Table 5 .
The performance of different catalysts for the electrochemical oxidation of glycerol and glucose.

Table 6 .
The performance of different catalysts for the electrochemical oxidation of Furanic compounds.