A mini review of electrocatalytic upgrading of carbohydrate biomass—System, path, and optimization

Upgrading carbohydrate biomass is a promising process to convert low‐value carbohydrate feedstocks into high‐value chemicals and fuels, which is critical for the development of a sustainable and renewable society. Among the various carbohydrate conversion technologies, electrocatalytic methods have gained increasing research interest due to their mild reaction conditions and easy accessibility. Moreover, electrocatalytic methods utilize water as a green and abundant source of hydrogen and oxygen for either reduction reactions on the cathode or oxidation reactions on the anode. In recent years, significant progress has been made in this field, with the establishment of various systems aimed at enhancing the electrocatalytic conversion of carbohydrates. In this review, recent technologies and results of electrocatalytic conversion of carbohydrates are summarized in two parts: anodic electrocatalytic oxidation (cellulose, 5‐hydroxymethylfurfural [HMF], glucose, and 2‐furfural) and cathodic electrocatalytic reduction (glucose, HMF, and 2‐furfural). Furthermore, future outlooks for improving this field are also proposed.


| INTRODUCTION
Biomass upgrading is of growing importance due to the increasing demand for sustainable sources of energy and chemicals, as well as the need to reduce the reliance on finite fossil fuel resources. Carbohydrate biomass, which can be derived from various sources, such as food plants and agricultural waste, is one of the most common biomasses. [1][2][3][4][5][6][7] Meanwhile, as a kind of abundant and renewable feedstocks, carbohydrates biomass can be efficiently converted into various value-added chemicals and fuels through catalytic processes. 8 40 ] at 90°C and 20 bar O 2 pressure in methanolic solution, enabling the formation of formic acid and methyl formate in close to 100% combined selectivity. 18 And our group realized the near stoichiometric conversion of glucose by using 100% H 2 O 2 in the alkali catalytic system (0.6 mol L −1 LiOH), and the yield of formic acid was up to 91.3%. 19 Although the efficiency of converting carbohydrates into valuable chemicals through traditional thermochemistry has already been quite considerable, a large number of expensive oxidants (H 2 O 2 or compressed O 2 ) and/or a high temperature is needed, which result in high requirements for technology and corresponding costs, limiting its practical application to a certain extent. Therefore, it is necessary to find a greener, cleaner, and more efficient technology to convert carbohydrates biomass into high-value-added products.
Electrocatalytic technology is a field of science and technology that involves the use of electricity to drive chemical reactions in the presence of electrocatalysts. By applying an external potential, the current passes through the electrolyte solution or molten electrolyte, inducing the oxidation reaction of the substrate in the electrolyte at the anode and the reduction reaction at the cathode. Compared with the traditional thermochemical method, high temperatures and the addition of extra oxidants/reductants are not required in the electrocatalytic process. Compared with other renewable energy-driven processes such as photo-or photoelectron-catalysis, the electrocatalytic process requires less complex catalysts (since it does not need to consider the photoelectric conversion efficiency) and has greater flexibility in terms of energy source. 20 In addition, the by-products and pollutants are also less emitted for the electrocatalytic process, which means that the electrocatalytic technology is more energy-efficient and environmentally friendly. By utilizing the principles of electrocatalytic technology, substrates including various organic and inorganic substances can be dissolved in the electrolyte solution and be electrically catalyzed to produce target products for oxidation or reduction with only water as the oxidant or reductant.
Therefore, owing to its mild operating conditions, controllable selectivity, and scalability, electrocatalytic conversion of biomass has emerged as a powerful and promising approach for producing diverse high-value chemicals. Recently, Ge et al. have conducted a profound review on the electrochemical oxidation of biomass on various nonnoble metal catalysts, which provides important information in this field. 21 However, for the biomass reduction, especially for the carbohydrates reduction, the related review is limited. Therefore, this review will comprehensively address the current research status of the electrocatalytic conversion of carbohydrates, encompassing anodic electrocatalytic oxidation and cathodic electrocatalytic reduction. The advancements in reaction system design, reaction pathway elucidation, process optimization, and the future challenges will be highlighted.

| ANODIC ELECTROCATALYTIC OXIDATION
One important merit of the electrocatalytic oxidation of carbohydrates in aqueous solutions is that H 2 can be simultaneously produced while the carbohydrate oxidation. Hydrogen is an energy-efficient fuel with a much higher mass energy density than common fuels. As shown in Figure 1A, the mass energy density of hydrogen is estimated to be 33.3 kWh kg −1 , which suggests that 1 kg of hydrogen can effectively substitute for nearly 3 kg of gasoline (11.1 kWh kg −1 ). 25,26 Therefore, hydrogen is viewed as an efficient substitute for fossil fuels in the sustainable energy strategy, especially in the transportation sector. However, current hydrogen production mainly relies on fossil fuel conversion, including brown hydrogen produced from gasification plus water and gray hydrogen from steam methane reforming, which causes significant carbon footprints and emissions. In comparison, the emerging electrolysis of water for hydrogen production is considered to be an economical and efficient solution for the synthesis of clean and renewable green hydrogen ( Figure 1B). 27 However, due to the sluggish oxygen evolution reaction (OER), the cost of hydrogen production technology by electrolysis of water remains too high, which hinders the development of the green hydrogen production market. According to statistics, as of 2019, the hydrogen produced from electrolytic water splitting only accounted for 2% of the total hydrogen energy market. 28 Compared with the anode OER, the theoretical electricity consumption required for hydrogen production from biomass oxidation coupled with electrolytic water is lower, which can essentially reduce the cost of the technology. Therefore, it is an effective alternative strategy for electrolytic hydrogen production to replace slow water oxidation with biomass oxidation which is more favorable in thermodynamics. In fact, the methods of hydrogen production using lignocellulosic biomass and other biomass such as thermochemical processes and biotransformation are currently at the stage of pilot demonstration or commercialization. However, thermochemical processes and biotransformation lag behind electrochemical processes in terms of reaction rate, simplicity, and purity. 29 Thus, the study of electrocatalytic conversion using biomass as a feedstock coupled with cathodic hydrogen production has received great attention.

| Electrocatalytic oxidation of cellulose for hydrogen production
As the most widespread carbohydrate biomass, cellulose is less studied for direct electrocatalytic oxidation in aqueous phase electrolyte solutions due to its complex chemical structure and insolubility in water. In 2018, Takashi Hibino's group reported the direct electrochemical decomposition of bread, sawdust, and straw as feedstock for hydrogen production. After a simple ball milling process, these biomass feedstocks were dissolved in H 3 PO 4 solution and then the electrocatalytic oxidation was performed using a nonprecious metal mesoporous carbon electrode (similar in catalytic activity to a conventional Pt/C anode), while hydrogen was produced at the cathode. The average hydrogen production per biomass in this system was about 0.25 mg H 2 /mg biomass feedstock. 30 Compared with the direct electrocatalytic oxidation pathway of cellulose, the indirect electrochemical oxidation of cellulose has been reported more frequently. These studies focused on finding an intermediate substance that would rapidly and indirectly oxidize cellulose through a redox process, thus releasing a large amount of protons rich in natural biomass such as straw cellulose to obtain hydrogen. 31 In this process, the intermediate substance does not change significantly before and after the reaction, and acts as a catalyst for recycling in the whole system ( Figure 2A). Deng's group first developed a reaction system for the indirect electrochemical conversion of cellulose using polyoxometalate (POM) as an intermediate and catalyst, which can efficiently convert natural biomass to hydrogen at low temperatures and low energy consumption. 26,32 In this study, the water-soluble POM H 3 PMo 12 O 40 was used as the catalyst. Throughout the system, POM was first mixed with biomass feedstock (including cellulose, starch, and glucose), which was gradually oxidized under light or heating conditions while POM was reduced by accepting electrons from the oxidation of biomass, changing from yellow to dark blue. During the electrolysis process, the reduced POM was easily reoxidized at the anode since it has a low standard redox and then changed from dark blue to yellow, completing the whole cycle so that it can be reused ( Figure 2B). Meanwhile, during the reoxidation of POM, water molecules are reduced at the cathode to generate large amounts of hydrogen, and the Faradaic efficiency of this system can reach 96.74% on average. POM, as an electron-coupled proton carrier, was a low-cost self-healing catalyst (theoretically recyclable up to 100,000 times) and was well tolerated to impurities generated by biomass. In this system, the entire reaction step can be achieved without precious metal catalysts, and the energy consumption was very low (0.69 kWh Nm −3 H 2 ) compared with that of electrolytic water for hydrogen production (4.13 kWh Nm −3 H 2 ), which saves about 83.3% of energy consumption.
Due to the large molecular weight of POM, its electrochemical conversion was relatively low, a similar alternative system using Fe 3+ /Fe 2+ redox as the catalyst was proposed by the same group based on the above study. 33 Similar to the system using POM for indirect oxidation of cellulose, hydrogen was produced using feedstock from different biomasses without using any precious metals as electrocatalysts in the electrolysis process ( Figure 3). Fe 3+ in solution first reacts with biomass, and the oxidized biomass was degraded to low molecular weight derivatives, while Fe 3+ was reduced to Fe 2+ . And during the electrolysis, Fe 2+ was reoxidized to Fe 3+ at the anode and water molecules were reduced to release hydrogen gas at the cathode. Fe 3+ /Fe 2+ redox electron pairs, as electron carriers, also have a low redox potential (0.77 V vs. standard hydrogen electrode), which can also effectively reduce the consumption of electrical energy. Besides, laccase (e.g., 2,2,6,6tetramethylpiperidine-N-oxyl [TEMPO]), [34][35][36][37] nitroaromatics such as nitrobenzene or 1,3-dinitrobenzene, 31,38 and halide media including sodium iodide and bromine iodide 39 can effectively operate in systems for the indirect electrochemical oxidation of cellulose.

| Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF)
In the above study, biomass electrocatalytic oxidation reaction was mainly used to replace the sluggish anodic OER in the electrolysis of hydrogen production to reduce F I G U R E 2 (A) Schematic diagram of indirect electrocatalytic oxidation of cellulose and (B) process and phenomenon of hydrogen production from indirect electrocatalytic oxidation of cellulose by polyoxometalate (POM). 26 the reaction potential and improve the efficiency. However, the products of the oxidation of biomass itself have not been carefully studied. Nowadays, the majority of chemicals produced globally are derived from fossil feedstocks, and the industries that produce chemicals are predominantly energy-consuming and CO 2 -intensive. Given to the unsustainability of this production, there is an urgent need for systemic changes to transition to a greener chemical industry that will move the earth's resources toward a better cycle. Therefore, the electrocatalytic conversion of biomass feedstocks represented by cellulose is of high research and application value. As mentioned above, the chemical structure of cellulose is relatively complex and insoluble in water, so the direct electrocatalytic conversion of cellulose and hemicellulose into value-added chemicals has been studied sporadically. Only a few studies have used electrodes containing gold elements for the electrocatalytic oxidation of cellulose 6,40,41 or the indirect oxidation of cellulose as described above. However, the products in these systems were complex and difficult to separate for effective extraction and utilization, and thus their practical application was still difficult. Therefore, the conversion of cellulose and hemicellulose into platform molecules with shorter carbon chains (e.g., HMF and glucose) by biological or thermochemical processes first, and then electrocatalytic conversion of these platform molecules into value-added products is more effective at present.
HMF is a furan compound that can be obtained directly from cellulose hydrolysis, which is typically a three-step reaction that involves acid-catalyzed hydrolysis, isomerization, and dehydration ( Figure 4A). First, cellulose is hydrolyzed to glucose by breaking the glycosidic bonds using an acid catalyst, such as sulfuric acid. Subsequently, glucose is isomerized to fructose by rearranging the carbon atoms. Finally, fructose is dehydrated to HMF by eliminating three water molecules. 42 In addition, some studies mentioned that HMF can also be produced by direct hydrolysis from glucose using solvents such as dimethyl sulfoxide, ionic liquids, or by adding catalysts, such as enzymes and metal salts. [43][44][45] HMF has a simple molecular structure including a furan ring, an aldehyde group, and a hydroxyl group, and is considered as one of the most versatile platform molecules. Because of its specific structure, HMF has fewer electrooxidation selectivity problems F I G U R E 3 Hydrogen production from indirect oxidation of cellulose by Fe 3+ /Fe 2+ redox electric pair. 33 F I G U R E 4 (A) Mechanistic diagram of HMF production from cellulose 42 and (B) electrocatalytic oxidation reaction pathway of HMF. DFF, 2,5-diformylfuran; FDCA, 2,5-furandicarboxylic acid; FFCA, 2-formyl-2-furancarboxylic acid; HMF, 5-hydroxymethylfurfural; HMFCA, 5-hydroxymethyl-2-furancarboxylic acid; MA, maleic acid. than other substrates. With the increasing role of polymers in modern society, the use of bio-based chemicals for producing biopolymers has been of interest. The oxidation of HMF can yield a wide range of important chemicals, such as 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA). Among them, FDCA is considered to be one of the most valuable oxidation products because it is commonly used as a precursor for the production of the biopolymer material polyethylene furanoate. The theoretical oxidation potential of HMF oxidation to FDCA is calculated to be 0.3 V versus normal hydrogen electrode (NHE), 42 which is much less than the potential for OER (1.23 V vs. NHE). Therefore, the electrocatalytic oxidation of HMF has received considerable attention.
The earliest interest in the electrocatalytic conversion of HMF dated back to 1983, when Kawana et al. reported a study on the electrocatalytic oxidation of HMF and its derivatives. 46 Now, the reaction pathway of electrocatalytic oxidation of HMF has been investigated in various conditions, as illustrated in Figure 4B. In various studies, pH was adjusted by changing the composition of the electrolyte. Under mild alkaline (e.g., 0.5 mol L −1 borate buffer solution) or lower pH conditions (e.g., 1.0 mol L −1 phosphate buffer solution), HMF undergoes initial oxidation to form DFF as the first intermediate. 47,48 On the other hand, under strong alkaline conditions (e.g., 1.0 mol L −1 KOH solution), HMF is oxidized to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) as the first intermediate. Both DFF and HMFCA are further oxidized to form 2-formyl-2furancarboxylic acid, which is then ultimately oxidized to form FDCA. 48,49 Moreover, under acidic conditions (e.g., 1.0 mol L −1 H 2 SO 4 solution) and with a high applied potential, HMF can also generate maleic acid (MA) as a product. But the formation of MA is undesirable because it consumes HMF and reduces the yield of FDCA. 50,51 The impacts of HMF concentration on its electrocatalytic oxidation have also been explored. According to the studies of Zhang et al. and Gao et al., when the HMF concentration was relatively low (less than 50 mmol L −1 ), the product yield, as represented by FDCA, increased with ascending HMF concentration. In this range, the charge transfer resistance (R ct ) decreased dramatically, indicating an enhanced interaction between HMF and the electrode surface. In contrast, when the concentration surpassed 50 mmol L −1 , further increase in HMF concentration did not improve the product yield. 52,53 Instead, it tended to cause severe degradation, owing to the insufficient thermal and chemical stability of HMF, especially at higher concentrations. This ultimately undermined the production of the target product. 54,55 Currently, most studies on the electrochemical oxidation of HMF to produce FDCA have focused on the optimization of the electrocatalyst itself with the main objective of reducing the energy consumption of the overall electrochemical system without affecting the yield. Depending on the materials used in the electrocatalyst itself, it can be divided into metallic and nonmetallic electrocatalysts, while metallic electrocatalysts can be subdivided into noble metal and nonnoble metal electrocatalysts.
In the electrocatalytic oxidation system of HMF, noble metal electrocatalysts including platinum, ruthenium, palladium, and gold can easily and rapidly oxidize HMF to intermediate DFF at a low starting potential. [56][57][58][59] In the electrocatalytic oxidation system of HMF, noble metal electrocatalysts including platinum, ruthenium, palladium, and gold can easily and rapidly oxidize HMF to intermediate DFF at a low starting potential. However, the tested current densities were not high and some were less than 10 mA cm −2 in the reported applied potential range. 56 Besides, it was difficult for HMF to be completely oxidized to FDCA on a single noble metal electrode, while noble metal alloy electrodes can effectively solve this problem. Chadderdon et al. synthesized and optimized palladium-gold alloy nanoparticles to selectively oxidize both hydroxyl and aldehyde groups of HMF, achieving 83% FDCA selectivity at an anodic potential of 0.9 V versus reversible hydrogen electrode (RHE). 60 Park et al. further designed a threedimensional palladium-gold alloy electrode from microscopic layer-by-layer assembly by controlling the pairing and composition of bimetallic nanoparticles, and finally obtained a 16.41% FDCA yield using gold nanoparticles wrapped around palladium nanoparticles in 1.0 mol L −1 KOH and at an anodic potential of 0.82 V versus RHE. 61 Considering the cost of noble metals and the poor catalytic activity for the electrooxidation of HMF, although the starting potential of nonprecious metal electrocatalysts was slightly higher, the development of efficient nonprecious metal catalysts is still the mainstream and a large number of research results have been produced due to their convenient source, low price, and high catalytic activity. Among them, nickel-based materials were the most active nonprecious metal catalysts. Various nickel-based compounds exhibit diverse compositions and properties that enable them to be amenable to different applications. Nickel hydroxide electrodes, for example, possess abundant hydroxyl groups that furnish active sites and strong adsorption of reactants, rendering them suitable for prolonged alkaline electrocatalysis. 62 Nickel oxide electrodes feature abundant surface oxygen atoms that provide a high density of active sites for reactant adsorption and catalysis, demonstrating superior catalytic activity compared with nickel hydroxide. 63 Moreover, combining with nonmetal elements can further enhance the performance of nickel-based catalysts. For example, nickel boride, nickel nitride, and nickel selenide ( Figure 5A) contain additional boron, nitrogen, and selenium species, respectively, that supply catalytically active sites and broaden the pH range of applicability. 64,66,67 In short, nickel-based materials have rich 3d electrons and special electron orbitals which provide faster reaction kinetics and can achieve an FDCA Faradaic efficiency close to 100% and yields of 98%. 68,69 However, nickel-based materials have relatively high onset potentials for HMF electrooxidation, usually around 1.30 V versus RHE. 65,[70][71][72][73] In contrast, cobaltbased electrocatalysts typically possess a lower onset potential for the electrocatalytic oxidation of HMF. 74 Besides, cobalt-based catalysts also demonstrate high catalytic activity toward hydrogen evolution reaction (HER). Therefore, the development of bifunctional electrocatalysts to catalyze both the cathodic HER and anodic HMF oxidation can effectively enhance the energy efficiency of the system. For instance, Zhu 75 and Chen et al. 76 78 Other nonprecious metal catalysts such as vanadium, molybdenum, and manganese-based catalysts also have been widely reported for the electrochemical oxidation of HMF.
F I G U R E 5 Schematic diagram of the synthesis of (A) nickel selenide electrode (NiSe@NiO x ) 64 and (B) copper-nickel-cobalt double hydroxide sulfide nanoelectrodes (Cu x S@NiCo-LDH). 65 79 Deng et al. proposed that the introduction of a second metal in Ni-based hydroxides not only increased the number of active sites, but also regulated the redox behavior of Ni ions and negatively shifted their oxidation potentials. For this, as shown in Figure 5B, they prepared copper-nickel-cobalt sulfide double hydroxide nanoelectrodes (Cu x S@NiCo-LDH) by electrodeposition, which negatively shifted the onset potential of HMF oxidation by about 150-200 mV and achieved a current density of 87 mA cm −2 at 1.30 V versus RHE potential and obtained nearly 100% Faradaic efficiency of FDCA. 65 Further, Zhang et al. attempted to add a third metal ion to the bimetallic hydrotalcite structure. The addition of Fe 3+ to NiCo LDHs not only changed the electronic environment of Ni 2+ and Co 2+ , but also formed a thinner layer than NiFe or NiCo LDHs, thus exposing more active sites and greatly improving the catalytic activity. After 60 min of electrolysis, the HMF conversion rate reached 95.5% and the FDCA yield of 84.9% was obtained. 53 In the study of nonmetallic electrocatalytic oxidation of HMF, the catalysts used were mainly homogeneous TEMPO and its derivatives, which belong to the field of the indirect electrochemical oxidation system, and the use of these substances was beneficial to electron transfer and increased the reaction activity and rate, but likewise increased the cost of downstream separation, so the attention is not very high. [80][81][82] However, given the low cost of nonmetallic materials, the exploration and development of other nonmetallic materials such as carbon materials [83][84][85] for more efficient nonmetallic catalysts are also expected, but relatively little research has been conducted. The performance of some of the latest electrocatalysts in HMF electrooxidation is shown in Table 1, and the detailed comparison of selected research is summarized in Table 2 from biomass feedstocks. Typically, 2-furfural can be obtained from lignocellulose biomass through catalytic hydrolysis and dehydration. 117 2-furfural can then be converted into various value-added chemicals via oxidation, such as 2-furancarboxylic acid, MA, and succinic acid, which have wide applications in products, like, polyester resins, surface coatings, solvents, plastics, food additives, and pharmaceuticals. 118, 119 Jiang et al. developed a bifunctional electrocatalyst of nickel phosphide arrays on nickel foam (Ni 2 P/Ni/NF) for 2-furfural oxidation and hydrogen production. It enabled the integrated electrocatalysis of anodic 2furancarboxylic acid production with 94%-98% yield and cathodic hydrogen evolution with nearly 100% Faradaic efficiency. 120 Kubota et al. achieved the ringopening product MA without the use of oxidizing agents, obtaining a yield of 65.1% using a PbO 2 electrode. 121 On the basis of the results of density functional theory calculations, Gong et al. suggested that the product selectivity from the electrooxidation of furfural can be adjusted by modifying the electrode potential. 122 The afore-mentioned theoretical calculations were further validated experimentally by Holewinski et al. They employed differential reactor studies integrated with online electrochemical mass spectrometry as well as in situ infrared spectroscopy attenuated total reflectancesurface-enhanced infrared reflection-absorption spectroscopy to probe the oxidation reaction pathway of furfural over Pt catalysts in acidic electrolytes. Below 1.2 V versus RHE, 2-furancarboxylic acid and 5-hydroxyfuroic acid were identified as the primary products. At higher potentials, selectivity shifted predominantly toward 5-hydroxy-furan-2(5H)-one, accompanied by the appearance of MA. 123 As illustrated in Figure 6, the electrocatalytic oxidation behavior of 2-furfural is largely similar to that of HMF.

| Electrocatalytic oxidation of glucose
Glucose, widely present in food and blood, functions as a fundamental energy source for most organisms. It can also be directly obtained through hydrolysis of cellulose. In addition to the acid-catalyzed hydrolysis method, which has a long history of industrial application, enzyme-catalyzed and alkaline-catalyzed hydrolysis methods are also utilized for glucose production from cellulose ( Figure 7). 124 Acid-catalyzed hydrolysis of cellulose to glucose involves using mineral acids, commonly sulfuric acid, at high temperatures and pressures. 125  | 2953 efficiency, it still suffers from substantial acid consumption and severe environmental pollution. 126 In contrast, enzyme-catalyzed hydrolysis utilizes cellulases, mainly endoglucanase, and β-glucosidase, to efficiently dissociate the cellulose. Although enzyme recovery increases the cost of this method, its milder reaction conditions make it more environmentally friendly than acidcatalyzed hydrolysis. 127 Consequently, the enzymecatalyzed hydrolysis method is gradually replacing and improving the acid method, and is a leading development direction in cellulose hydrolysis. Alkali-catalyzed hydrolysis is another effective method for glucose production from cellulose. Under strong alkaline conditions, the dissociation of cellulose hydroxyl groups into hydroxide ions triggers isomerization and chain breaking, leading to glucose production. However, slower reaction speed, requirement for more alkali, and complicated sodium removal treatment limit its industrial application at present. Despite these drawbacks, the alkali-catalyzed hydrolysis method presents milder conditions and higher product selectivity and deserved for more research in future development. 128,129 Unlike the electrocatalytic oxidation of HMF, there are few studies on the electrocatalytic conversion of glucose-based carbohydrates, and most of the research on glucose is focused on the detection of glucose in the blood and the application in the direction of fuel cells. [130][131][132][133][134] Until recently, researchers have been gradually focusing on the electrocatalytic oxidation of glucose to obtain high-value-added chemical products.
Moggia et al. systematically investigated the catalytic oxidation of glucose in an alkaline environment by metallic copper, platinum, and gold electrodes. Their results pointed out that the gold electrode is the most selective metal electrode, with a selectivity of up to 86.6% for gluconate production at an operating voltage of 0.55 V F I G U R E 6 Electrocatalytic oxidation reaction pathway of 2-furfural. 122 F I G U R E 7 Cellulose hydrolysis for glucose production.
versus RHE, while the platinum electrode only achieves 78.4% selectivity for gluconate at 1.10 V versus RHE. In contrast, the copper electrode can cause partial C─C bond breaking, forming a mixture of gluconic acid, glucaric acid, and formic acid. 135 Ostervold et al. explored the possibility of lactic acid production via electrocatalytic oxidation of glucose and obtained a 23.3% yield of lactic acid using a copper oxide electrode under optimal conditions. 136 Liu et al. achieved efficient oxidation of glucose to produce glucaric acid using nickel-iron oxide/nitrides grown on NF arrays with a Faradaic efficiency of 87% and a yield of 83%. The electrode was also effective in cathodic hydrogen precipitation reactions, achieving a high current of 101.2 mA cm −2 at an overall applied voltage of 1.4 V and stable operation for over 24 h using this bifunctional catalyst. 137 Li et al. prepared a cobalt-based bifunctional electrocatalyst (Fe 0.1 -CoSe 2 /CC) for the catalytic oxidation of glucose and hydrogen production from decomposing water. They designed an alkaline-acidic asymmetric electrolyzer that converts glucose to lactic acid and a small amount of formic acid and produces hydrogen at rates up to 90 µmol H 2 h −1 . 138 Zhu et al. have investigated the electrooxidation of glucose to formate on cobalt oxyhydroxide (CoOOH) electrode, in which, operando spectroscopies and theoretical calculations were combined to reveal two types of reducible Co 3+ -oxo species as the active sites in CoOOH for glucose electrooxidation. This research provides new insights into the study of active sites and reaction mechanisms of metal electrodes in the electrorefining of biomass. 139 In addition, a low-cost and high-efficiency strategy for hydrogen production was proposed by Wang and coworkers, similar to the method proposed in Figure 8, using a system of indirect electrocatalytic oxidation of glucose, in which the anode first electrocatalytically oxidizes Cu (I) to Cu(II), and then Cu(II) oxidizes glucose to gluconic acid, while Cu(II) is reconverted to Cu(I) to achieve cycle. 140,141

| CATHODIC ELECTROCATALYTIC HYDROGENATION
In conventional thermochemical catalysis, sorbitol can be produced by one-step conversion of cellulose using high-pressure hydrogen in the presence of precious metals, such as platinum, rhodium, and iridium. In this process, cellulose is gradually hydrolyzed to soluble oligosaccharides, then to glucose, and finally, glucose is hydrogenated and reduced to sorbitol. 142,143 In contrast to this approach, electrocatalytic hydrogenation does not require an exogenous hydrogen source (e.g., hydrogen gas, formic acid, and other hydrogen suppliers), and the reaction conditions are mild and controllable (room temperature and atmospheric pressure). Therefore, in addition to the anodic catalytic oxidation of biomass, the hydrogenation of carbohydrate biomass using active hydrogen generated at the cathode has received much attention.

| Electrocatalytic hydrogenation of glucose
Sugar is the main source of energy required by living beings and plays an important role in the process of life activities, but excessive sugar intake may induce obesity, poor impact, diabetes, and other adverse effects on the human body. Sorbitol is a promising platform polyol, which has a sweet taste but is not easily absorbed by the body compared with glucose and is now widely used as a sugar substitute (artificial sweetener) by the food industry. Sorbitol, obtained by simple one-step hydrogenation of glucose ( Figure 9A), can satisfy the pursuit of a sweet taste while reducing sugar intake, decreasing the risk of obesity and other diseases.
Actually, the research on electrocatalytic synthesis of sorbitol from glucose was once industrialized in the last century. However, due to the high price of electricity at that time, the lack of development of electrocatalytic technology, and the environmental problems caused by the use of mercury or lead amalgam cathodes, the electrocatalytic route for the hydrogenation of glucose to prepare sorbitol was replaced by the thermocatalytic approach. 145 With the development of new energy technologies and the need for natural environmental protection, electricity is increasingly becoming a green and sustainable form of energy, and research on electrochemical hydrogenation of glucose to produce sorbitol is prosperous again. Bin et al. found that higher Faradaic efficiency can be obtained with zinc electrodes (21%) than with lead amalgam electrodes (15%) due to its F I G U R E 8 Schematic diagram of indirect oxidation of glucose to gluconic acid for hydrogen production using Cu 2+ /Cu + redox electric pair. 140,141 larger active specific surface area of zinc compared with lead. 146 On this basis, Pintauro et al. further improved the Faradaic efficiency of sorbitol to 39% using a zinc amalgam electrode with NaBr as the electrolyte and an elevated operating potential. 147 Lessard et al. investigated the reactivity of Raney nickel for electrocatalytic glucose hydrogenation and obtained 90% and 95% sorbitol selectivity with 40% and 57% Faradaic efficiency in flow-through and intermittent reactors, respectively. A significant decrease in the Faradaic efficiency corresponding to the electrode was found when the electrolyte was changed from H 2 SO 4 to CaBr 2 . 148 Kwon et al., on the other hand, investigated the electrocatalytic hydrogenation of glucose in neutral solution Na 2 SO 4 , using solid monometallic electrodes including nickel, palladium, lead, and gold ( Figure 9B), and confirmed the influence of the electrocatalyst properties on this electrochemical process and achieved a selectivity of 87% for sorbitol production from glucose when using lead electrodes. 144 In summary, sorbitol, the target product of glucose electrocatalytic hydrogenation, obtained up to 87% selectivity over a wide pH range (2-7), and the selectivity and Faradaic efficiency of the product could be modified by using continuous or intermittent reactors. However, a consistent relationship between Faradaic efficiency, reaction configuration, operating parameters of electrocatalytic hydrogenation of glucose, and selectivity of the products has not been observed and still needs to be continued.

| Electrocatalytic hydrogenation of HMF
Analogous to glucose, HMF also possesses an aldehyde functional group and thus can accordingly undergo catalytic hydrogenation reactions. The potential hydrogenation products of HMF encompass 2,5dihydroxymethylfuran (DHMF), 5-methylfurfural, 2,5-dihydroxymethyl-tetrahydrofuran (DHMTHF), 2,5dimethylfuran, 2,5-dimethyl-2,3-dihydrofuran (DMDHF), and 2,5-dimethyltetrahydrofuran (DMTHF) (Figure 10). These compounds boast far-reaching applications in the chemical industry, such as precursors for polyesters, resins, solvents, and so forth. Analogous to the investigation of the electrocatalytic oxidation of HMF, the study of Furthermore, the addition of glucose enhances the selectivity of low-active metals (Zn, Cd, and In) for DHMF. 150 Roylance et al. further optimized the Ag electrode using the electroplating substitution method and sputter coating method, and they reported that DHMF selectivity of 99% and Faradaic efficiency of 99% were obtained at a constant potential of −1.3 V versus Ag/AgCl. 151 Furthermore, the exploration of bimetallic catalyst electrodes in the electrocatalytic hydrogenation of HMF to produce DHMF is gradually gaining prominence. For instance, Sanghez et al. fabricated AgCu bimetallic nanoparticles or dendrites on electrodeposited and displaced catalysts, respectively. Both types of bimetallic AgCu particulates demonstrate a superior electroactive surface area as well as enhanced charge and mass transfer, resulting in ∼100% conversion of HMF and >80% selectivity of DHMF. 152 Piao et al. expounded that the bimetallic BiSn electrocatalyst was also highly active for the hydrogenation of HMF. Prolonged electrolysis afforded a Faradaic efficiency of ∼100% for DHMF production with J > 140 mA cm −2 in a 2 mol L −1 HMF solution at pH ∼ 7. 153 Ji et al. reported a Ru 1 Cu single-atom alloy catalyst with isolated Ru atoms on Cu nanowires, exhibiting a Faradaic efficiency of 85.6% for DHMF at −0.3 V versus RHE. In comparison to other studies, the applied potential required is more anodic and the concentration of HMF employed is higher, indicating that it demands less energy input and offers higher energy efficiency. 154 DMDHF is a subsequent product of the continuous hydrogenation of DHMF, which is a liquid fuel with high energy density and high-octane rating. As an acidic environment can substantially lower the activation barrier for the electrocatalytic hydrogenation of HMF, the production of DMDHF requires a more acidic pH and occurs selectively on particular metal electrodes (Pd, Pt, Al, Zn, In, and Sb). 155 As the pH decreases further, 2,5-hexanedione is produced, which is an important industrial intermediate and reagent, and has wide application in organic synthesis, analytical chemistry, and daily chemical industry. In this process, the furan ring is opened and the aldehyde and alcohol groups are hydrogenated and reduced to alkyl groups. And the lower the pH, the higher the selectivity of 2,5-hexanedione was obtained. 156 For example, a high Faradaic efficiency of 78% and a selectivity of 77% were observed for the conversion of HMF to 2,5-hexanedione on Ag-aerogel-CN x at −1.1 V versus Ag/AgCl in 0.5 mol L −1 H 2 SO 4 . 157 In addition to these, DHMTHF can also be produced through the intermediate of DHMF. Li et al. synthesized the use of a bifunctional Pd/VN (VN-3D vanadium nitride) hollow nanosphere electrode for the simultaneous electrocatalytic oxidation and electrocatalytic hydrogenation of HMF at both F I G U R E 10 Reaction pathway of electrocatalytic hydrogenation of 5-HMF. 149 5-MF, 5-methylfurfural; DHMF, 2,5dihydroxymethylfuran; DHMTHF, 2,5-dihydroxymethyl-tetrahydrofuran; DMDHF, 2,5-dimethyl-2,3-dihydrofuran; DMF, 2,5-dimethylfuran; DMTHF, 2,5-dimethyltetrahydrofuran; HMF, 5-hydroxymethylfurfural. ends of the electrode in an acidic electrolyte of 0.2 mol L −1 HClO 4 . 91 At the cathode, further hydrogenation through the intermediate of DHMF to generate DHMTHF is achieved with a selectivity of more than 88% and Faradaic efficiency of 86%, while the HMF conversion is more than 90%. At the anode, the catalytic oxidation of HMF using a Pd/VN electrode obtained 96% selectivity for FDCA, 98% HMF conversion, and 84% Faradaic efficiency. A comparison of some of electrocatalysts for electrocatalytic hydrogenation of HMF is shown in Table 3.

| Electrocatalytic hydrogenation of 2-furfural
Similar to HMF, 2-furfural can also be used to produce many valuable chemical products through electrochemical hydrogenation. As shown in Figure 11, 2-furfural can be converted into 2-furanol, 2-hydroxytetrahydrofuran, and 2-methyltetrahydrofuran through electrochemical hydrogenation. 158 The main product of the electrochemical hydrogenation of furfural is 2-furanol, which is a valuable platform chemical that can be used to make polymeric resins, coatings, and other compounds with functions as fragrances, fuels, and chemical intermediates.
Studies on the electrocatalytic hydrogenation of 2-furfural are abundant and have been extensively reviewed. 118,149,159 Many parameters such as different electrocatalysts, electrolytes, PH, and reactors were investigated in a quest to achieve maximum yields of respective furfural products. Generally, copper-based catalysts have demonstrated outstanding performance for 2-furfural electroreduction to 2-furanol. 160 For example, Zhang et al. synthesized a nanostructured copper phosphide electrode loaded on commercial carbon fiber cloth (Cu 3 P/CFC), which achieved nearly 100% selectivity for 2-furanol with a high Faradaic efficiency in the range of 92%-98% at an applied potential from −0.05 to −0.55 V versus RHE. 161 The pH value of the electrolyte solution has a significant influence on the selectivity of products. In general, under acidic conditions, 2-methylfuran is the primary product. As the pH gradually increases, 2-furanol becomes the predominant product. 159

| CONCLUSION AND PERSPECTIVE
As far as the prospect of hydrogen production from electrocatalytic resource conversion of carbohydrate biomass is concerned, the current studies have revealed preliminary applicability, but there are still some pressing issues that need to be addressed before practical application. First, as these studies mentioned above mainly focus on hydrogen production, the oxidation products of their biomass feedstock have not received sufficient attention, which is more complex and difficult to be purified for further utilization. Second, considering the effect of photosynthesis on the uptake and consumption of atmospheric carbon dioxide during biomass growth formation, the biomass feedstock is a carbonneutral energy source with zero net greenhouse gas emissions. But carbon dioxide is indeed released during the electrocatalytic oxidation of biomass process for hydrogen production. This carbon dioxide would probably mix with the hydrogen released from the cathode, thus reducing the hydrogen concentration in the output gas. Therefore, additional carbon dioxide capture systems may be required in subsequent studies. 164 In addition, the utilization of biomass in current research is still at the miniaturization stage, and there may be some unknown problems in its scale and process pathway. Therefore, future research should also focus on improving the scale of biomass electrocatalytic oxidation for industrial applications. 30 Although a certain number of reports have studied the electrocatalytic oxidation of cellulose, HMF, 2-furfural, or glucose to produce high-value-added organic compounds, these researches are generally at a nascent stage. In addition to some of the reaction, the mechanism is still unclear, there are also problems of low product electron utilization and selectivity, limiting the overall system efficiency. Thus, from the practical application, the following issues should be addressed. (Ⅰ) Further exploration of the reaction mechanism. Only when the reaction pathway and reaction mechanism of electrocatalytic oxidation of cellulose or glucose are clarified, it is possible to solve various problems in the subsequent industrialization process and obtain the target products in a better and more efficient way. (Ⅱ) Purification of products. Current research mainly focused on the output of target products and the exploration of the reaction path. However, the reaction product is mostly a complex mixture and may require a large number of acidification treatments. Thus, the purification of the product is a significant problem that must be solved on the industrialization path. (Ⅲ) Fabrication of large equipment. Laboratory research is still at the stage of small-scale experiments with mostly modeled and miniaturized equipment, low operating current density, and millimolar level substrate addition, which cannot meet the requirements of industrial production. (Ⅳ) The cyclical and integrated nature of the process. The overall reaction process needs to be better designed and constructed to meet the sustainable recyclability as well as the high value required for industrial production. It is foreseen from the current research that electrocatalytic conversion of carbohydrates has the advantages of being green, environmentally friendly, mild conditions, high efficiency, and profitable conversion rate compared with other technological means, and has a better prospect.
And electrochemical hydrogenation is also a promising technology for the conversion of biomass and its derivatives. Unlike traditional thermochemical hydrogenation, electrocatalytic hydrogenation of carbohydrates is a gentle chemical transfer reaction based on the F I G U R E 11 Reaction pathway of electrocatalytic hydrogenation of 2-furfural. consumption of sustainable electrical energy. The reaction path and process can be effectively controlled by externally applied electric current or voltage. At the same time, the efficiency and selectivity of the reaction can be improved by using different types of electrocatalysts, without considering high-temperature deactivation, only its corrosion resistance in acidic and alkaline environments. However, the competition with cathodic HER and the problems of scaling up electrochemical devices and end-product separation analysis still limit their practical application and require more research and solutions.
In summary, either through electrochemical catalytic direct hydrogen production, anodic catalytic oxidation, or cathodic catalytic hydrogenation, carbohydrate biomass can be effectively utilized. Although there are still many works to be done in optimizing the process for commercial-scale applications, from the current research results, electrocatalytic upgrading of carbohydrates biomass is a very promising strategy that deserves continuous attention and in-depth research.