Biomass‐Derived Electrocatalysts: Low‐Cost, Robust Materials for Sustainable Electrochemical Energy Conversion

Electrochemical energy conversion is an important strategy for addressing climate change and building a carbon‐neutral society. The use of inexpensive biomass resources to develop high‐performance catalytic materials that reduce the energy barrier of electrochemical reactions and minimize energy consumption has become a research hotspot for energy materials. Previous reviews have often categorized biomass‐derived catalysts by the biomass feedstocks used, but this classification method has major limitations because the roles of the same biomass material in different catalysts can vary. In this review, a new classification approach for biomass‐derived catalytic materials by focusing on the role of bio‐based materials in the overall catalyst system is proposed. The review is divided into three main sections, categorizing bio‐based materials by 1) the active components, 2) the carbon support, and 3) the entire catalyst. Additionally, a comprehensive summary is provided of catalytic materials at different scales, including the nanoscale, molecular scale, and single‐atom scale. It is hoped that this review will guide and inspire the future development of biomass‐derived electrocatalysts.


Advantages of Biomass-Derived Carbon-Supported Catalysts
[49][50][51] For instance, commercial carbon black is typically produced by spraying oil feedstock onto the inner walls of a hightemperature furnace for carbonization, often at temperatures exceeding 1000 °C. [52]On the other hand, the production of commercial carbon nanotubes (CNTs) involves various methods, including arc discharge, laser ablation, chemical vapor deposition, and high-pressure carbon monoxide disproportionation.These methods require expensive equipment and have stringent demands regarding vacuum levels, workspace, and environmental conditions. [53,54]Many synthetic materials heavily depend on finite petrochemical resources.][49][50][51] However, carbon is abundant and widespread in the biosphere, forming the fundamental structure of all living organisms. [17,46,47,51,55,56]][57] These biomass-derived carbon materials preserve the characteristic hierarchical structures and porous nature of the original materials. [47,48,50,51,56,57][57] Biomass often contains large amounts of proteins, amino acids (AAs), and other substances, leading to the generation of self-doped C during biomass pyrolysis, including N, P, S, and so on. [17,44,48,49,58]N-doping is one of the most important and extensive strategies, which can modify the electronic energy-level structure of carbon, increasing the conductivity of carbon carriers and introducing more active sites. [17,48,49,51]urthermore, N dopants can interact synergistically with metal sites on carbon carriers, boosting overall electrocatalytic activity further. [49,51]][59] The synergistic effects among multiple dopant atoms improve the chemical environment and conductivity, leading to superior electrocatalytic performance. [5,17,41,48]onsequently, these self-doped biocarbons are superior to regular carbons as supports and, even as metal-free catalysts, they exhibit high electrocatalytic activity that can rival metal-based catalysts. [17,48,49,56,60]n summary, biomass-derived carbon materials are often chosen for catalysts because they offer the following advantages: 1) affordability and abundant reserves: biomass are not only cost-effective but also widely available, with an abundance of inexpensive biological resources globally each year; 2) renewability: biomass is renewable, thanks to its ability to self-replicate and grow; 3) hierarchical structures and porous morphologies: these materials benefit from the functional properties of biological tissues, characterized by hierarchical structures and porous morphologies; 4) inherent self-doping characteristics: biological materials contain a variety of proteins and AAs, endowing them with inherent self-doping properties; 5) enhancement of electrocatalytic effects when used as carbon supports: when used as carbon supports, they exhibit enhanced electrocatalytic effects due to their rich self-doping properties; and 6) tunable catalytic capabilities when used as active components: many biomass-derived carbon materials possess intrinsic catalytic activity, allowing for the adjustment of catalytic capabilities.Therefore, the use of biomass to synthesize carbon materials for use in electrocatalysis has great potential.

Classification of Biomass-Derived
Carbon-Supported Catalysts

Three Scales of Active Components on Carbon Supports
Generally, carbon-supported materials are mainly consisted of carbon carriers and active components in the whole catalyst system.The active components can be categorized based on the size of active components as catalyst NP on carbon support, catalyst molecule on carbon support, or single atoms on carbon support.Here, we examine the characteristics of these three types of carbon-based catalyst.

Nanoscale Catalysts on Carbon Supports: Trends in the Decreasing Size of the Active Components and the Use of Binding Agents
A large increase in the electrocatalytic activity was observed when metal catalyst particles were first mixed with carbon particles. [9,10,23,24][63][64] However, the components in simple physical mixtures are not strongly bound together.,8,10] However, the addition of binding agents may introduce additional electrical resistance, which can affect the electrocatalytic activity.[1,6,16,33] To combine the active component with the carbon support directly, methods like hydrothermal growth and high-temperature treatment have been used.[65][66][67] Regardless of the method, the function of carbon support remains unchanged.[68][69][70] The main objectives are to increase electrical conductivity, enhance the dispersion of the active component, suppress the aggregation of catalyst particles, increase the catalyst's specific surface area, and expose more electrocatalytically active sites.[2,6,18] Additionally, the gaps between carbon particles provide channels for electrolyte transport, facilitating the transport of reactants and products.24,32] As the catalyst particle size decreases, the interface effect increases.[40,42,43,71] Therefore, the development of smaller NPs, such as nanoclusters composed of two to several dozen metal atoms, has become an important research topic and a general trend in catalyst development.[40,42,43,71] 2.1.2.Molecular Catalysts on Carbon Supports: Controlling Macrocycles and Functional Groups, and Tuning Central Metals Molecular catalysts are macrocyclic compounds, such as porphyrins and porphyrin-like molecules, including porphyrin, phthalocyanine (Pc), and other derivatives.80][81] Pcs are the most widely used and common artificially synthesized macrocyclic catalysts.[72][73][74][75][76]81] The Pc core (H 2 Pc) is formed by linking four isoindole subunits through meso-N atoms, creating a synthetic polymer with 18 π electrons.[72,[75][76][77]79,81,82] The H 2 Pc contains two H atoms and exists as a divalent anion (Pc 2À ) after losing the H atoms. [72][73][74][75]77,78,80] The central cavity of Pc 2À is about 270 pm in diameter, capable of accommodating metal ions to form metal-Pcs (M-Pcs).[78][79]81,82] Functionalized Pcs macrocycles can regulate electrocatalytic activity.Common strategies include substituting groups through pyrrole linking and expanding the π system to broaden the range of macrocycles, such as metal-naphthalocyanines, anthracocyanines, and phenanthrocyanines.[74][75][76]81] Additionally, a common approach to altering catalytic performance is to replace the C atom in the pyrrole ring with N to form azaphthalocyanine. [21,27] Macrocyclic molecules like Pcs exhibit semiconductor properties with poor electrical conductivity, requiring a support material with good electrical conductivity and stability.[2][3][4][5][6][7][8][9][10][11] Carbon supports are the most common materials used for this purpose.Pc macrocycles are typically attached to the carbon support by adsorption onto the graphite surface of the carbon support through π-π interactions, or by anchoring the macrocycle by coordinating it to the central metal of M-Pc using functional groups on the carbon support.[2][3][4][5][6][7][8][9][10][11] Therefore, bio-derived carbon materials rich in N, S, and O functional groups, and exhibiting good electrical conductivity serve as excellent supports for molecular catalysts.

SACs on Carbon Supports: Highly Dispersed, Isolated Active Sites Combined with Different Supports
In 2011, Qiao et al. introduced the concept of SACs and prepared single Pt atoms supported on FeO x NPs for electrocatalytic CO oxidation. [83]Initially, SACs referred to highly active, isolated metal atom sites for electrochemical catalysis. [40,83,84]ubsequently, many metal-free SACs have also been prepared. [22,40]Unlike single-atom cocatalysts, SACs perform electrocatalytic reactions centered around individual atoms. [40,83,84]he hallmark of SACs is the highly dispersed, isolated active sites, with each individual atom possessing a unique coordination environment, which is highly favorable for electrocatalysis. [20,45,71,85]However, single atoms must be dispersed on the supports, which usually include carbon supports and metal oxide supports, among others. [4,40,42,43,85]Figure 1 illustrates the variation in the size of active components in carbon-supported metal catalysts.As the size decreases, the binding strength between the catalytic component and the carbon support gradually increases, and the boundary between the two diminishes. [4,42,43,71,85,86]hen the catalyst reaches the single-atom level, the catalytic sites blend seamlessly onto the surface of the carbon support, achieving the greatest dispersion and electrical conductivity. [4,20,43,45,71,85,86]Consequently, these catalysts often exhibit exceptionally high intrinsic electrocatalytic activity.

Classification of Biomass-Derived Carbon-Supported Catalysts
97] When biomass-derived materials are used as active components, they are typically loaded onto commercial or synthetic carbon substrates, whereas when they are used as carbon support materials, additional loading or anchoring of active electrocatalytic components is often required.We refer to these two types of electrocatalytic materials as semi-bio-based catalysts.However, there are also entirely bio-based catalysts, where both the active components and the carbon support come from biomass-derived materials.Therefore, based on the source of the active component and carbon support, we categorize bio-based catalyst materials into the following three main types.

Synthetic Carbon-Supported Bio-Based Active Component Catalysts
These materials are usually prepared by loading metal-containing biological components, such as ferritin or hemoglobin, onto synthetic carbon materials, such as CNTs or graphene sheets, resulting in well-integrated products after pyrolysis.

Bio-Based Carbon-Supported Synthetic Active Component Catalysts
These biocarbon materials are obtained by pyrolysis and carbonization of biological tissues to form a carbon-based substrate, which is then loaded with metal NPs, single atoms, or other active components.The materials often exhibit abundant doping with elements like N and P, have a large specific surface area, and may have defects, all of which greatly enhance the electrocatalytic performance of the catalytic components.

Entirely Biomass-Derived Carbon-Supported Catalysts
Both the catalytic component and the carbon support material originate from biological materials, making them entirely bio-based.

The Seven Categories of Biomass-Derived Carbon-Supported Catalysts
In this review, considering the three scales of active components, we classify biomass-based catalysts into the following seven categories: 1) synthetic carbon/bio-based nanocatalysts, 2) synthetic carbon/bio-based molecular catalysts; 3) synthetic carbon/ bio-based single-atom catalysts; 4) bio-based carbon/nanocatalysts; 5) bio-based carbon/molecular catalysts; 6) bio-based carbon/single-atom catalysts; and 7) entirely bio-based catalysts.The former six categories are named semi-bio-based catalysts, while the latter one (entirely bio-based catalyst) is usually prepared from one or more types of bio-based materials through pyrolysis and carbonization.Due to the high-temperature decomposition of biological tissues, the active sites are often perfectly integrated with the carbon substrate and are typically at the single-atom level. [17,49,59]These catalyst materials have been developed with the intention of creating low-cost, highly active bio-based materials.Entirely bio-based catalysts have grown rapidly and are the most common type of bio-based catalysts, and this trend is likely to continue.Therefore, biomass-derived catalysts can be divided into seven types.Each type of catalyst material possesses unique properties.In the following three sections, we provide detailed explanations of these catalytic materials.Table 1 outlines the components of these seven catalysts and the sections in which they are discussed in this review.

Synthetic Carbon-Supported Bio-Based Active Component Catalysts
Using synthetic carbons as carriers to support bio-based actives is an important strategy for harnessing biomass resources.A common method involves combining biological feedstocks with carbon materials through processes like high-temperature pyrolysis. [17,47,49,50,56]In this section, we discuss nanoscale, molecular, and single-atom catalytic compositions in detail.The performance of catalytic materials is crucial for developing renewable energy conversion systems.Metallic catalysts with smaller sizes generally exhibit much higher catalytic activity. [40,42,43,71]Thus, exploring small-sized NP catalysts is particularly important.Inspired by nature, biomass templates, such as peptides and proteins, are used to synthesize NPs with unique morphologies on their internal or surface structures. [44,55]100][101] These functional groups interact with metal ions, which can be reduced using a reducing agent.Huang and co-workers synthesized Pt NPs and Pt nanowires (NWs) using the peptide BP7 as a template. [98,99]Figure 2a shows the Pt NP growth pathway of a {111}-bipyramid.Electrochemical tests and comparisons showed that the 1D NWs exhibited superior electrochemical performance.Moreover, the Pt NWs exhibited a higher electrochemical surface The projection shapes and the measured corner angles from the HRTEM images match the geometrical models and calculated values (marked on the geometrical models) well for {111}-bipyramids.Scale bar is 20 nm in (a2,a3) and 2 nm in (a6-a9).Reproduced with permission. [98]opyright 2011, American Chemical Society.b1) Schematic of the synthesis of multifunctional graphene nanosheet-FePt nanohybrids.b2,b3) TEM images of graphene nanosheet-FePt nanohybrids.Reproduced with permission. [103]Copyright 2012, The Royal Society of Chemistry.area (ECSA) compared with commercial Pt/C, and higher mass activity (0.144 vs. 0.091 mA μg À1 ) and specific activity (0.139 vs 0.116 mA cm À2 ) at 0.9 V versus reversible hydrogen electrode (RHE) in O 2 -saturated 0.1 M HClO 4 solution.More importantly, stability tests revealed that after 6000 cycles, the ECSA loss of Pt NWs was 14.2%, which was more stable than Pt/C (ECSA loss of 56.7%).The higher stability of NWs was probably due to their unique structure.Similarly, Wang et al. used Aβ 16-20 peptide to synthesize Pt NPs with (111) facets, which exhibited superior ORR performance compared with commercial Pt/C in 0.5 M H 2 SO 4 electrolyte. [100]The peptide template method provides a new way to prepare NPs, although the method is mainly limited to noble metals, such as Pt.
Another approach involves using protein molecules as nanoreactors for monodisperse ultrafine NPs.Proteins have many surface properties that can contribute to improving and modifying catalyst activity. [44]Ferritin, which has a hollow spherical cage-like structure, is the most common protein template.[104] Channels formed between subunits allow metal ions to enter and exit the cavity.In living organisms, ferritin converts soluble ferrous salts into iron complexes stored inside the cavity.[104] Furthermore, the hollow structure of ferritin can also store other metal ions to prepare nanometal particles.
The nanoscaling of noble metals is the most effective way to reduce their use and enhance their performance.Qiu et al. used apo-ferritin shells as nanoreactors to synthesize Pt NPs in apoferritin cages. [102]The hybrid particles were loaded on the surface of 3D graphene foam, and the ferritin was removed by thermal treatment, resulting in stable Pt NP/graphene oxide (GO) composite materials.Compared with the state-of-the-art Pt/C catalyst, the Pt NPs/GO composite exhibited much greater electrocatalytic activity for methanol oxidation.The electrocatalytic activity was also higher compared with Pt/GO prepared by pulse electrodeposition.This study demonstrates that using protein nanocage templates and assembly for preparing functional composite materials is promising for catalysis and fuel cell applications.
Combining Pt with other nonprecious metals is another major strategy for enhancing electrocatalytic activity by synergistic effects and for decreasing Pt use.Wei et al. formed FePt NPs in apo-ferritin by immobilizing porous apo-ferritin cages in a solution containing Fe and Pt salts (Figure 2b). [103]FePt NPs on reduced graphene oxide (rGO) exhibited high ORR activity, reaching a half-wave potential of 0.828 V versus RHE in 0.5 M H 2 SO 4 solution, whereas Pt/C had a half-wave potential of 0.749 V versus RHE.Therefore, synergistic effects between Fe and Pt substantially increased the ORR activity of FePt NPs.CNTs are also an effective means to improve the dispersion of catalyst particles.Kim et al. achieved high ORR activity by combining CNTs with uniformly distributed Pt NPs generated from ferritin protein recombinants. [104]The Pt NPs/CNTs exhibited good catalytic activity and could be used in biofuel cells and fuel cell applications.Song and co-workers used bovine serum albumin (BSA) as a template to prepare Zn 3 (PO 4 ) 2 @BSA nuclei, which were then grown to produce Zn 3 (PO 4 ) 2 @BSA nanorods.These nanorods were used as carriers to further modify Pt NPs, resulting in the PtNP@Zn 3 (PO 4 ) 2 @BSA composite catalyst.The resulting PtNP@Zn 3 (PO 4 ) 2 @BSA electrocatalyst exhibits a high ECSA and good tolerance to poisoning, thus demonstrating excellent electrocatalytic activity for methanol oxidation. [105]2.Synthetic Carbon/Bio-Based Molecular Catalysts: Porphyrin-Derived Electrocatalysts Natural porphyrins are a class of macrocyclic tetrapyrrole metal complexes widely found in nature and include heme, chlorophyll, VB 12 , and cytochrome c (Cyt c).[72,73,75,78] These molecules have been recognized as promising electrocatalysts in energy conversion materials.Natural porphyrins are typically combined with carbon materials as active components.The general procedure involves mixing natural porphyrins with carbon materials and then pyrolyzing them.[106][107][108][109] Under protective gases or vacuum conditions, organic macrocyclic ligands partially decompose into carbon lattices, while the metal center retains its structure with four N atoms chelated (M-N 4 ).[72,74,76,108,110] Therefore, natural porphyrin-derived electrocatalysts have M-N 4 active centers, where M is a metal center, such as Fe or Co. [72,74,76,108,110] In other words, pyrolysis reanchors the M-N 4 ligands from a π-electron-rich macrocyclic ligand environment to a relatively π-electron-deficient carbon residue environment.[72,74,76,108,110] Despite the change in electron density, this structure retains its effective electron donation, acceptance, and transfer behavior.[72,74,76,108,110] These M-N-C residues and groups provide more chemical adsorption sites for reactants, thereby enhancing the catalytic activity.Furthermore, the high-temperature treatment combines the pyrrole residues with the carbon support, balancing conductivity and stability.
Poor activity and high cost are important challenges to overcome for large-scale electrochemical energy conversion systems, and the key is to develop precious-metal-free catalysts using lowcost biomaterials.8]111] Preliminary studies have shown that the presence of human red blood cell components at the fuel cell cathode leads to a decrease in overpotential and an increase in current, indicating the contribution of heme components to the ORR. [106]However, the reduction in overpotential is relatively small, and the resulting current value is only about several microamperes per square centimeter.To increase the current, hemoglobin has been combined with commercial carbon materials to improve the conductivity of the catalyst and the dispersion of active sites.Franco et al. prepared a catalyst by combining hemoglobin with graphene and pyrolyzing the mixture, resulting in a large increase in ORR catalytic activity, with a maximum current density of À4 mA cm À2 in 0.1 M KOH solution. [112]heng et al. reported the copyrolysis of pig blood with CNTs to design N-doped nanostructured carbon-based (N-doped C) electrocatalysts for the ORR. [108]The N-doped C obtained  [106] Copyright 2017, ScienceDirect-Elsevier. b1) Schematic of the synthesis for pig-blood-derived carbon (PBC) catalysts.b2) TEM images of the PBC catalysts.b3) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showing the iron atoms on the carbon frameworks.b4) Linear sweep voltammetry (LSV) results before and after the accelerated durability test (ADT) of PBC and commercial Pt/C catalysts.Reproduced with permission. [107]Copyright 2021, American Chemical Society.c1) Molecular structure of VB 12 .c 2 ) HAADF elemental mapping images of the constituent elements of the Fe-VB 12 @GR catalyst.c3) HER and OER performance of Fe-VB 12 @GR compared with other catalysts.c4) Photograph demonstrating overall water splitting with the Fe-VB 12 @GR catalysts powered by a 1.5 V battery.Reproduced with permission. [109]Copyright 2022, Elsevier.
at 800 °C exhibited excellent four-electron transfer selective ORR activity in alkaline media, as well as high methanol tolerance and long-term stability.High-temperature pyrolysis facilitated the formation of higher amounts of pyridinic and pyrrolic-N bonding groups, thereby improving the onset potential, half-wave potential, and limiting current density of the ORR.They also proposed that the planar N configuration might be the active site for improving ORR electrocatalytic performance.The simple and cost-effective synthesis of this active, stable electrocatalyst makes it a promising candidate material for electrochemical energy conversion systems, such as fuel cells or metal-air batteries.In addition to containing abundant pyrrole-type N elements as active sites, hemoglobin-derived catalysts also have iron atoms that can form active sites in the catalyst.Vij et al. synthesized CNT-supported Fe 5 C 2 NPs by pyrolyzing a mixture of singlewalled CNTs and freeze-dried hemoglobin at 700 °C. [111]The ORR performance of this catalyst was comparable to that of commercial Pt/C catalysts.In 0.1 M HClO 4 and 0.1 M KOH, the catalyst exhibited onset potentials of 0.92 and 0.55 V versus RHE, and generated current densities of 6.34 and 6.69 mA cm À2 , respectively.The catalyst followed a four-electron mechanism and showed high electrochemical stability in both acidic and alkaline media.Additionally, the catalyst exhibited higher methanol tolerance than commercial Pt/C catalysts.The same group also used a GO/heme-derived catalyst as a cathode material in anion exchange membrane fuel cells and achieved a maximum power density of 658 mW cm À2 , suggesting that heme-derived catalysts are promising for other applications in electrochemical energy conversion (Figure 3b). [107]B 12 , another important natural porphyrin derivative, features Co as the metal center chelated by four N atoms and serves as a crucial natural precursor for Co-N 4 catalyst materials. [72,109,113]ee et al. developed a high-temperature pyrolysis method for preparing graphene-supported Co-N 4 and Fe-N x dual-active sites catalysts using VB 12 , iron nitrate, and graphene flakes (GR) (Figure 3c). [109]The optimized Fe-VB 12 @GR catalyst demonstrated excellent overall water splitting activity in alkaline solution, achieving current densities of 10 mA cm À2 with overpotentials of only 120 and 300 mV while maintaining high stability and durability over 20 h.The cell voltage required was approximately 1.65 V (10 mA cm À2 ) for the double-electrode overall water splitting.Furthermore, in another study, they reported outstanding ORR electrocatalytic activity in acidic solution using the Fe-VB 12 @GR catalyst, with superior durability and methanol tolerance compared with commercial Pt/C catalysts. [113]hlorophyll can also serve as a biological precursor for catalysts, but it requires the replacement of the central Mg atom with other active metal centers, making the process more complex than for other precursors; thus, there are fewer studies using this method.Guo et al. reported a method to replace the central Mg in chlorophyll, which was naturally extracted from spinach, with Co. [110] After pyrolysis, the central metal chelation portion was converted to Co-N-C active sites, whereas the absorbed Co ions from other parts of the chloroplast were converted to CoO x and retained the chloroplast microstructure.The catalyst with a coreshell structure of CoO x encapsulating Co-N-C exhibited unique synergistic effects and an efficient biostructure, resulting in excellent electrocatalytic performance for the ORR.The catalyst showed onset and half-wave potentials of 0.89 and 0.82 V, respectively, with durability and methanol tolerance superior to commercial Pt/C catalysts.Additionally, a Cyt c oxidase model, consisting of a cobalt (II) porphyrin with a copper (I) triazacyclononane macrocycle fastened on the distal face and an imidazole covalently attached to the proximal face, was also found to exhibit ORR catalytic properties in the early exploration. [114]Zhao et al. investigated the electrochemical properties of Cyt c adsorbed on the surface of multiwalled CNTs (MWCNTs).The study revealed that in a phosphate buffer solution (pH 7.0), significant electrocatalytic activity for the reduction of H 2 O 2 was observed for Cyt c/MWCNTs compared to blank MWCNTs. [115]However, there have been relatively few reports on further studies based on this natural porphyrin.

Synthetic Carbon/Bio-Based Single-Atom Catalysts: Protein-or AA-Modified Electrocatalysts
In previous sections, we described the use of ferritin-and hemoglobin-modified carbon electrocatalysts.These two proteins were selected to use their abundant iron elements as active sites for electrocatalytic reactions.However, biomass materials have limited active metal components.Proteins typically contain abundant nonmetal elements, such as N, P, and S, which can generate self-doped carbon materials after pyrolysis. [5,41,48,49,56]Hence, selecting suitable biomass materials rich in proteins and preparing doped carbon materials via high-temperature carbonization is a reasonable technical route.However, directly using proteins as raw materials to modify the synthetic carbon supports has specific requirements because the carbon supports are modified by anchoring the dopant sites into carbon lattice during pyrolysis.Biomaterials have set volumes, and if they cannot make good contact with the carbon support surface, maximum modification cannot be achieved.Therefore, selecting biomaterials rich in proteins imposes stringent conditions for the single-atom doping of synthetic carbon-based materials.Seeds are the unit of reproduction of flowering plants and often contain abundant proteins, such as soybeans.Sun et al. ground soybeans into soybean milk and then uniformly dispersed graphene nanosheets in the soybean milk solution (Figure 4a). [116]This mixture was placed in a sealed autoclave and a hydrothermal reaction was performed under high-temperature and high-pressure conditions.The soybean milk proteins were well combined with GO, and the hydrothermal product was subjected to high-temperature treatment under an NH 3 atmosphere.N-doped rGO with an N-doping level of 9.4% was prepared, which performed better than commercial Pt/C catalysts for the ORR in 0.1 M KOH.The N-doped rGO exhibited an onset potential of 0.96 V versus RHE and superior long-term running durability.Mushrooms also contain abundant proteins.Guo et al. dried and ground enoki mushrooms and mixed the powder with commercial CNTs (Figure 4b). [117]The mixture was subjected to thermal treatment at 900 °C and N-doped CNTs were obtained that exhibited superior ORR electrocatalytic activity compared with commercial Pt/C in acidic electrolytes.
AAs are important building units of proteins, and there are over 300 AAs. [118]Depending on the side chain, AAs can introduce different elemental dopants.The density of doping sites can be controlled by altering the ratio of AAs to carbon, and different types of AAs can be used to replace the doping sites.Therefore, AAs are important for preparing doped carbon materials.Additionally, AAs have reducing properties and can be used to reduce GO to obtain rGO.Rayej et al. mixed Fe salts and GO with arginine, cysteine, and histidine, resulting in modified rGO rich in Fe-N-C active sites. [119]In the presence of AAs, Fe atoms tended to form single atoms, whereas the portion with fewer AAs produced Fe(III)-oxide particles.The N dopant was primarily introduced to the edges and quaternary positions of  [116] Copyright 2019, ScienceDirect-Elsevier. b1) Schematic of the synthesis for the N-C@CNT-900 catalyst from enoki mushroom biomass.b2) TEM images of the N-C@CNT-900 catalyst with dislocation defects.b3) High-resolution N 1s XPS spectra of the N-C@CNT-900 catalysts.b4,b5) LSV results of the N-C@CNT-900 catalysts in O 2 -saturated 0.1 M KOH and 0.5 M H 2 SO 4 solutions.Reproduced with permission. [117]Copyright 2015, the Royal Society of Chemistry.
the graphene layer as pyridine (arginine), pyrrole (cysteine), and quaternary N (histidine).The N-doping order of the three AAs was histidine > cysteine > arginine.The Fe-N-rGO catalyst prepared using histidine exhibited higher double-layer capacitance, conductivity, and ORR activity, and only lagged the commercial Pt/C catalyst by 30 mV in the onset and half-wave potentials.They confirmed that AAs, particularly histidine, can serve as interesting N-doping agents for preparing Fe-N-rGO electrocatalysts with excellent ORR activity.

Bio-Based Carbon-Supported Synthetic Active Component Catalysts
In the previous section, we summarized the different types of biomass-derived catalysts supported by synthetic carbon substrates.In practical applications, using biomass-derived carbon as substrates or carriers to support synthetic catalysts is also a common approach.In this section, we discuss three scenarios of biomass-based carbon-supported synthetic catalysts at different scales.

Bio-Based Carbon/Metal NPs
The development of doped carbon-supported metal NP catalysts has attracted great interest.The dopant atoms not only affect the conductivity of the carbon supports but also interact with the metal active sites, thereby enhancing the catalytic performance. [47,48,51,57,59]Dopants, such as N, P, and S, are often abundant in biomass-based carbon materials due to the presence of substances like proteins, AAs, and alkaloids in biological tissues. [17,47,48,51,57]Moreover, polysaccharides, which are formed by the dehydration polymerization of multiple monosaccharide molecules, are widely distributed in biological materials. [17,47,48,51,57]Plant-based polysaccharides include cellulose, hemicellulose, and starch.Cellulose is a major component of plant cell walls and is widely found in plant materials, in which cellulose and other components, such as hemicellulose and lignin, are distributed along with AAs, proteins, and other substances. [17,47,48,51,57]Plant-based carbon materials often exhibit specific characteristics, such as unique doping, porous structure, and morphology.For instance, Miao et al. synthesized porous Ndoped C materials by pyrolyzing delignified balsa wood, followed by the deposition of γ-Fe 2 O 3 NPs (Figure 5a). [120]The resulting Fe 2 O 3 -CW1000 catalyst showed excellent ORR performance in alkaline environments, with an onset potential of 0.98 V versus RHE, comparable to that of commercial Pt/C catalysts.The superior ORR activity was attributed to the promotion of Fe-N x active sites by the γ-Fe 2 O 3 NPs and the N-rich porous carbon matrix, which facilitated faster electron transfer on the catalyst surface and the full exposure of active sites.
In addition to using naturally occurring biomaterials with hierarchical pore structures, porous carbon materials can also be synthesized artificially, with KOH and ZnCl 2 being common reagents. [17,51,56,57]Furthermore, chemical agents rich in N, such as melamine, are often used to synthesize porous N-doped C materials. [17,47,48,51,57]Zhang et al. used powdered waste nutshells as a precursor and treated them with KOH and heat to obtain porous N-doped C materials. [121]Cu NPs were then grown on the surface of porous N-doped C. The catalyst exhibited excellent catalytic activity for both the HER and OER in a 1 M KOH solution, with overpotentials of 200 mV for the OER and 216 mV for the HER at a current density of 10 mA cm À2 .Additionally, a cell voltage of only 1.65 V was required to achieve a current density of 10 mA cm À2 for overall water splitting.The outstanding electrocatalytic activity was attributed to the synergistic effect of the Cu-N interaction and the large specific surface area provided by the porous morphology.Yun and co-workers have reported a new achievement.They used 3D network carbon derived from aloe waste material (3D-AWC) as a carbon support and successfully loaded three types of molybdenum-based bimetallic oxide NPs (ZnMoO 4 /3D-AWC, Cu 3 Mo 2 O 9 /3D-AWC, and MnMoO 4 /3D-AWC).These catalysts exhibited significantly enhanced catalytic activity in the triiodide reduction reaction and HER due to the synergistic effect between molybdenumbased bimetallic oxide and 3D-AWC.Specifically, the solar cell prepared using the ZnMoO 4 /3D-AWC catalyst achieved an impressive device efficiency of 7.65%, which is comparable to the efficiency obtained with Pt catalysts (6.74%).Furthermore, in a 1.0 M KOH solution, the ZnMoO 4 /3D-AWC catalyst displayed a Tafel slope of 54 mV dec À1 , indicating performance on par with Pt/C.This research outcome holds significant scientific and practical value and serves as a valuable reference for the design of high-performance catalysts across various domains. [64]nimal-based materials are also important sources for doped carbon material precursors and are rich in proteins, with different types of proteins found in different animal parts.For example, collagen is abundant in animal bones, keratin is present in hair and feather fibers, and fibroin is abundant in silk.The morphology of carbon materials generated from different parts of biological materials also varies.Animal bones consist of alternating arrangements of collagen proteins and calcium carbonate, resulting in a natural 3D porous structure in bone-derived carbon materials.Guan et al. prepared N-doped hierarchical porous carbon (NHPC) materials using powdered natural cattle bones as a precursor, followed by treatment with KOH and hightemperature carbonization (Figure 5b). [122]Subsequently, highly dispersed Co 3 O 4 NPs were anchored on the NHPC networks via hydrothermal synthesis, resulting in an efficient bifunctional Co 3 O 4 /NHPC electrocatalyst with excellent catalytic activity for both the ORR and OER in alkaline electrolyte.The catalyst surpassed the performance of commercial Pt/C catalysts for the ORR and was comparable to that of commercial RuO 2 catalysts for the OER.The superior bifunctional catalytic activity was attributed to the large specific surface area (1070 m 2 g À1 ), welldefined hierarchical porous structure, and high N-doping content (4.93 wt%), which synergistically contributed to the uniform dispersion of Co 3 O 4 NPs and increased mass transport capability.Furthermore, a primary Zn-air battery (ZAB) using the Co 3 O 4 /NHPC cathode demonstrated superior performance, including an open-circuit potential of 1.39 V, a specific capacity of 795 mA h g Zn À1 (at 2 mA cm À2 ), and a peak power density of 80 mW cm À2 .
In addition to directly using biomaterials as carbon supports, biomaterials can be combined with commercial carbon materials to produce improved composite carbon materials.For instance, Xu et al. dissolved Bombyx mori (silk worm) cocoons to obtain a liquid regenerated silk fibroin solution, which was then used to coat the surface of GO nanosheets. [123]After thermal treatment, 2D N-doped GO nanosheets with a large surface area were obtained.Au NPs were grown on this N-doped GO support.The catalyst exhibited excellent ORR electrocatalytic activity originating from the interaction between the doped N and Au sites, which increased the activity of Au sites, while the doping atoms improved the conductivity of GO and the regenerated silk fibroin modification increased the specific surface area of carbon support.

Biomass-Based Carbon/Molecular Catalysts
In the previous section, we discussed the development of natural porphyrins as molecular-level electrocatalyst materials.Artificial synthesized porphyrin-like macrocycles have also attracted considerable attention as electrocatalytic materials, including Pcs, porphyrins, corroles, and corrolazines. [72,73,75,81]Pc derivatives have shown the most rapid progress.Porphyrin-like compounds, such as Pcs, are synthetic analogs composed of four isoindole subunits with 18 π electrons, resembling natural porphyrins (Figure 6a). [72,73,75,81]The high catalytic activity of Pcs can be attributed to the unsaturated low-coordination environment formed by the coordination of the metal center with four isoindole subunits. [72,73,75,81]Compared with other porphyrin-like compounds, Pcs are easy to synthesize and cost-effective, making them suitable for large-scale production.Due to the 2D planar structure with four isoindole anchors for metal atoms, Pcs need to be combined with a support material. [72,73,75,79,81]Biomass carbon-supported Pcs have been widely reported as electrocatalytic materials.Generally, carbon-supported Pcs can be used for cathodes, respectively.Reproduced with permission. [122]Copyright 2017, American Chemical Society.
direct loading of Pcs as catalysts, or in high-temperature treatment similar to the heme pyrolysis discussed in Section 3.2 to produce M-N 4 active sites.Both methods can achieve high electrocatalytic activity.
The catalytic centers of Pcs can be precisely tailored, and most metals can form the corresponding metal-Pcs. [72,73,75,81]Zhang et al. used low-cost black locust leaves as C and N precursors for carbonization (Figure 6b) and obtained highly porous carbon materials that were used as supports for Fe-Pcs. [124]The Fe-Pcs catalyst showed a lower half-wave potential (0.91 V) and an increased limiting current density ( J L = 5.03 mA cm À2 ) compared with commercial Pt/C, and exhibited much higher methanol tolerance.ZABs assembled with this catalyst achieved an open-circuit voltage of 1.452 V and a specific capacity of 812.1 mAh g À1 , far surpassing Pt/C (1.435 V, 749.4 mAh g À1 ).Huang et al. synthesized porous N-doped C from the spongy material in sunflower stems as a substrate, and uniformly loaded Cu-Pcs onto the porous carbon. [125]The composite catalyst treated at 800 °C (Cu-Pcs@C-800) exhibited a lower onset potential (38 mV) and higher stability (87.4%) than commercial Pt/C (20 wt%).In addition to catalytic activity for the ORR, peptide cyanide catalysts have also been used for carbon dioxide reduction.Zhao et al. prepared highly porous activated carbon materials from pine bark biomass and then combined them with Figure 6.a1) Molecular structure of Pc, showing possible substitutions and axial positions.Reproduced with permission. [72]Copyright 2021, The Royal Society of Chemistry.b1) Schematic of the fabrication of the porous biomass-derived carbon-supported iron Pcs (FePc 0.5 /PBC) catalyst.b2) LSV curves for the OER of FePc 0.5 /PBC and other catalysts in 0.1 M KOH solution.b3) LSV curves for the ORR of FePc 0.5 /PBC and other catalysts in O 2 -saturated 0.1 M KOH solution.b4) Polarization and power density curves of ZABs based on FePc 0.5 /PBC and Pt/C cathodes, respectively.Reproduced with permission. [124]Copyright 2023, Elsevier.c1) Schematic of the synthesis of single Fe atoms on a hierarchically structured porous carbon support.Reproduced with permission. [127]Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Co-Pcs. [126]After metal optimization, they found that the catalyst prepared at 800 °C showed the best performance, with an onset potential of 1.006 V, higher than that of the commercial Pt/C (20 wt%) catalyst.
Through pyrolysis, the active catalytic centers of Pcs can be anchored on the carbon-based substrate.Zhang et al. mixed cattle-bone carbon powder with KOH and subjected it to thermal treatment to prepare 3D hierarchical porous carbon (3D-HPC). [127]The 3D-HPC was mixed and pyrolyzed with Fe-Pc complexes (Figure 6c).The authors expected that the microporous structure of 3D-HPC support would limit the thermal decomposition of unsubstituted Pcs, thereby enabling uniformly dispersed single Fe atoms to be prepared on 3D-HPC.The single-atom Fe sites coordinated with N ligands showed comparable ORR activity (E 1/2 = 0.81 V) to commercial Pt/C in acidic electrolyte, but exhibited better long-term electrochemical stability (a negative shift of 7 mV after 3000 potential cycles) and fuel selectivity.In alkaline media, the Fe-3D-HPC catalyst outperformed the commercial Pt/C electrode in terms of ORR activity (E 1/2 = 0.89 V), fuel selectivity, and long-term stability (a negative shift of 1 mV after 3000 potential cycles).Zago et al. treated waste tea leaves with urea. [128]The mixture was ball-milled with Fe-Pcs and the catalyst was prepared by pyrolysis under an argon atmosphere at 1500 °C.Compared with commercial Pt/C (20 wt%), the Fe-N-C electrocatalyst exhibited superior ORR activity and methanol tolerance.Zhao et al.'s research reports that they used pine bark, a common material from the northern regions of China, to prepare active porous carbon-based substrates (AC).Subsequently, they mixed cobalt Pc (CoPc) with AC in varying proportions and subjected them to secondary carbonization under N 2 atmosphere.The resulting AC/CoPc composite catalyst exhibited an onset potential of 1.006 V and a stability of 87.8% in 0.1 M KOH, comparable to commercial Pt/C (20%) catalyst. [126]3.Biomass-Based Carbon/Single-Atom Catalysts It should be pointed out that doping is the most common method to load the single-atom sites upon biomass-based carbon supports.The doped atoms are specified into two categories: metal and nonmetal dopants.The field of bio-based carbon-supported metallic SACs has undergone rapid development, garnering much attention.However, research is limited by the selection of metal components and consequently is focusing on developing more bio-based carbon materials and exploring various doping methods.

Nonmetallic Doping
Nonmetal doping with elements such as N, P, S, and B has emerged as the most common technique.For instance, Wang et al. synthesized S and N codoped porous carbon materials by pyrolyzing a mixture of silkworm cocoons, thiourea, and potassium hydroxide. [129]The sample prepared at 800 °C exhibited a unique hierarchical porous structure with a N content of 9.75 at% and a S content of 1.79 at%, offering abundant active sites and facilitating efficient mass transport.This sample had an onset potential of 0.853 V versus RHE, a current density equivalent to commercial Pt/C (4.5 mA cm À2 at 0 V) in alkaline electrolyte, as well as good methanol tolerance and electrochemical stability.Zhao et al. report that they used seaweed fibers as raw materials and processed them with thioacetamide to successfully prepare defective N, S co-doped carbon fiber catalysts (D-CFs).In a 0.1 M KOH electrolyte, D-CFs exhibit impressive performance with an onset potential of 0.92 V versus RHE and a limiting diffusion current density of 5.38 mA cm À2 , comparable to Pt/C catalysts.The outstanding performance can be attributed to the formation of intrinsic carbon defect sites in D-CFs after the removal of doped nitrogen and sulfur atoms.These carbon defect sites encourage O 2 molecules to preferentially adsorb onto defective carbon atoms, providing efficient catalytic active sites for ORR. [130]

Metallic and Nonmetallic Codoping
The codoping of metal and nonmetal elements for incorporating metal active sites has emerged as a prominent research area.The codoping of Fe and N is a common combination.For instance, Wang et al. prepared single-atom-modified 2D porous carbon nanosheets (Fe-N x /C) by using regenerated silk fibroin from silkworm cocoons, FeCl 2 , and ZnCl 2 as precursors through a hydrothermal reaction, followed by high-temperature pyrolysis and atmosphere crackling treatment. [131]The resulting catalyst exhibited a large specific surface area of approximately 2105 m 2 g À1 and demonstrated excellent electrochemical activity for the ORR.The catalyst also exhibited a half-wave potential of 0.853 V and experienced a decrease in E 1/2 of only 11 mV after 30 000 cycles, indicating remarkable catalytic activity for the OER.Similarly, Tang et al. used garlic powder to prepare Ndoped defective carbon, which was then mixed and calcined with Fe-Pcs to obtain a carbon-supported catalyst enriched with Fe-N 4 sites (Figure 7a). [132]Through parameter optimization, they identified that the catalyst with a carbon support-to-Fe-Pcs ratio of 100:1 prepared at 800 °C exhibited optimal performance.In alkaline, acidic, and neutral media, the half-wave potentials were 0.91, 0.70, and 0.83 V versus RHE, respectively.These values surpassed those of state-of-the-art pH-universal ORR catalysts and commercial Pt/C catalysts.Furthermore, they used N-Fe-doped carbon (Fe@G)-800/100 to construct a neutral liquid ZAB cathode with less commonly used phosphate-buffered saline neutral electrolyte.The cathode had a higher open-circuit voltage, excellent charge-discharge cycling behavior, and large specific capacity.
Jiao et al. used discarded corn silk as a biomass feedstock to prepare a Fe-N-C porous structure by thermally treating a mixture of the corn silk with iron nitrate, zinc chloride, and melamine. [133]The resulting material exhibited outstanding electrocatalytic activity for the ORR and OER in alkaline solutions.When the material was used as the air electrode in a flexible ZAB, it exhibited a high peak power density of 101 mW cm À2 and a stable discharge-charge voltage gap (0.73 V) lasting for over 44 h, showcasing the tremendous potential of ZABs.The authors attributed the excellent electrocatalytic activity to the unique hierarchical porous structure and hollow tubular morphology of the material, which exposed a greater number of active sites, allowed easy electronic conductivity, and increased the mass transfer of reactants.Additionally, the Fe-N doping enhanced the intrinsic electrocatalytic activity.The combination of cobalt and N has also been investigated.Wang et al. prepared Co-N-C porous carbon material by heat-treating orange peel powder, cobalt chloride, zinc chloride, and urea. [134]The material achieved a half-wave potential of 0.82 V for the ORR, as well as a high power density of up to 139.89 mW cm À2 , excellent specific capacity, and high cycle stability in ZAB tests. .a1) Schematic of the preparation of Fe@G.a2) Elemental mapping of Fe@G.a3) Atomic-resolution HAADF-STEM images of Fe@G.a4) High-resolution XPS N 1s spectra for Fe@G.a5) The Fourier-transformed extended X-ray absorption fine structure fitting for Fe@G.The inset shows the schematic of the Fe coordination environment in Fe@G.The gray, red, and blue balls correspond to C, N, and Fe atoms, respectively.a6) LSV curves for the ORR of Fe@G and other catalysts in O 2 -saturated 0.1 M KOH.a7) LSV curves for the ORR of Fe@G and other catalysts in O 2 -saturated 0.5 M H 2 SO 4 .a8) Discharge polarization and power density plots of ZABs.a9) Specific capacity testing of the Fe@G-based ZABs at a constant current density of 5 mA cm À2 .Reproduced with permission. [132]Copyright 2023, American Chemical Society.b1) Schematic of the preparation for the Co-N-B-C.b2) Aberration-corrected HAADF-STEM image of the Co-N-B-C.b3) LSV curves for the ORR of Co-N-B-C and other catalysts in O 2 -saturated 0.1 M KOH.b4) Discharge polarization and power density plots of ZABs.b5) One solid-state ZAB powered a small fan.b6) Two solid-state ZABs in series lit up an orange LED bulb brightly.b7) Three solid-state ZABs in series charged a mobile phone.Reproduced with permission. [135]opyright 2022, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Multiple Metallic/Nonmetallic Codoping
Doping with multiple metals or multiple nonmetal elements has also become a focus of research.Combinations of one metal and two nonmetal elements are more common.Xu et al. developed a simple method for synthesizing a Co/N/B triple-doped porous carbon framework (Co-N-B-C) by heat-treating a mixture of chitosan, cobalt salt, and boric acid (Figure 7b). [135]This material not only possessed a hierarchical structure, large specific surface area, and abundant carbon edges but also exhibited a high loading of individual Co atoms (4.2 wt%), resulting in excellent oxygen catalytic performance.As an air cathode catalyst for ZABs, it demonstrated high power density and long-term stability.The authors investigated the role of boric acid further, providing a new method for synthesizing high-performance SACs.Manganese can also be used for doping.Wu et al. synthesized Fe 5 C 2 NPs embedded in Mn, N, and S codoped CNTs (Fe 5 C 2 / Mn-N-S-CNTs) by heat-treating a mixture of lignin, dicyandiamide, iron nitrate, and manganese acetate. [136]The resulting material exhibited an open and tubular structure and demonstrated excellent electrochemical activity for the ORR.When employed as an air electrode in flexible ZABs, this hybrid catalyst demonstrated a remarkable peak power density of 101 mW cm À2 and a consistent discharge-charge voltage differential (0.73 V) sustained for more than 44 h, underscoring the substantial promise of ZABs.The outstanding electrocatalytic performance is attributed to the distinctive hierarchical porous structure, hollow tubular morphology, heightened exposure of active sites, facile electron conduction, and improved reactant mass transfer.
Ternary doping with N, P, and B has been reported.Jiang et al. subjected fungal spores to a hydrothermal reaction with urea, boric acid, and phytic acid, followed by pyrolytic carbonization to prepare N, P, and B ternary-doped porous carbon microspheres (Co@NPBC). [137]Coating these microspheres with Co produced a new OER catalyst.In alkaline conditions, the Co@NPBC material exhibited excellent OER catalytic activity, with an overpotential of 355 mV (at 10 mA cm À2 ) and a Tafel slope of 71.4 mV dec À1 .The synergistic effect of doped heteroatomic carbon microspheres and Co NPs increased the OER catalytic activity, thereby improving water splitting efficiency.For comparison, single-doped catalysts (Co@NC, Co@BC, and Co@PC) and double-doped catalysts (Co@NPC, Co@NBC, and Co@BPC) were also prepared, and Co@NPBC exhibited superior OER catalytic activity compared with these other catalysts.

Entirely Biomass-Based Electrocatalysts
The development of entirely biomass-based materials has great potential and many advantages, making it an important area of research.The preparation of entirely biomass-based materials allows for the efficient use of discarded biological resources, reducing environmental pollution and resource conservation.One of the most common types of entirely biomass-based materials is metal-free biocarbon materials, and their performance improvement is achieved mainly through active atom doping (nonmetal) and the preparation of porous structures.However, unlike previously exogenous doped catalyst, entirely biomass-based materials mainly rely on intrinsic active site doping, making the selection of biomaterials rich in active sites crucial.
Elements like N and P are abundant in animal feces.For instance, Alonso-Lemus et al. dried, ground, and sieved chicken manure, and selected powder particles with a size below 50 μm. [138]After argon-protected calcination at 750 °C, they obtained a P-N codoped carbon material.The P-N-doped C demonstrated superior ORR electrocatalytic activity compared with Pdoped C or N-doped C in 0.5 M KOH.This suggests that N-P codoping improves the dynamic performance of these electrocatalysts.Elements such as N, P, and S are also abundant in plants, making plants ideal candidates for synthesizing doped carbons.Huang et al. dried, ground, and pyrolyzed spinach leaves to prepare a N-P-S tri-doped biocarbon material. [139]The resulting catalyst showed excellent activity in oxygen-saturated 0.1 M KOH solution, with a diffusion current density of 5.19 mA cm À2 at 0.5 V, and better durability and tolerance to CO/methanol than commercial Pt/C (20 wt%).Density functional theory calculations elucidated the enhancement of ORR activity by tri-doping with N, P, and S. Spinach is distributed worldwide; thus, the applications of this spinach-derived carbon material could be expanded to other fields, such as supercapacitors, sensors, and metal-air batteries.
Activity can also be enhanced by the preparation of porous structures, requiring biomaterials with intrinsic porous morphology.Plant leaves are rich in stomatal structures and often contain abundant doping elements, making them ideal candidates for preparing porous doped carbons.Razmjooei et al. cleaned, dried, and carbonized ginkgo leaves to produce a porous carbon codoped with N-P-S, which exhibited superior activity over commercial Pt/C under alkaline conditions (Figure 8a). [140]They also compared the performance of green leaves in summer and yellow leaves in autumn, and yellow leaves with more stomatal structures showed better performance.Moreover, they examined the effect of carbonization temperature on ORR performance.Excessively high temperatures lead to the loss of doped atoms, reducing ORR performance.However, high temperatures also favor the generation of porous structures, enhancing ORR activity.These opposing effects reached equilibrium at 1000 °C, and yellow leaves carbonized at this temperature exhibited optimal ORR performance.Marine plants are also an ideal source for preparing porous doped biocarbons.Zeng et al. purified, cleaned, dried, and ball-milled kelp to obtain kelp powder.High-temperature pyrolysis produced a honeycomb-like N-doped C material. [141]This material was rich in N active sites (approximately 1.51 wt% N content) and possessed a high specific surface area (805.2 m 2 g À1 ), making it an ideal material for the ORR electrocatalysis and gaseous toluene adsorption.
Another approach is using a pore-forming agent to prepare porous structures, requiring biomaterials with intrinsic porous morphology.Initially, Liu and co-workers mixed soybean powder with ZnCl 2 and then subjected it to thermal decomposition, resulting in a porous self-doped carbon material.Compared to unmixed soybean-derived carbon, this catalyst possesses an exceptionally high specific surface area (949 m 2 g À1 ).In 0.1 M KOH, its onset potential and half-wave potential for the ORR reached À0.02 and À0.12 V, respectively (vs Ag/AgCl), nearly comparable to the commercial 20 wt% Pt/C catalyst.This study vividly demonstrates the influence of porous morphology on catalyst performance. [142]Fruits contain abundant doping elements and possess porous structures, making them ideal candidates for preparing porous doped carbons.Murugan et al. purified, cleaned, dried, and ground cedar cones to obtain ultrafine biomass powder. [143]The powder was subjected to precarbonization treatment at 450 °C, mixed with KOH, and then pyrolyzed at a high temperature to prepare a porous carbon material, which exhibited excellent OER electrocatalytic activity in alkaline solutions.The rhizomes of some plants contain fibrous structures, making them ideal templates for porous carbons.He et al. cleaned, dried, and precarbonized the core of juncus rhizomes to obtain a porous carbon. [144]The precarbonized powder was mixed with ZnCl 2 and pyrolyzed at high temperature to obtain a porous N-doped C material that exhibited outstanding ORR catalytic activity and stability owing to its high specific surface area and abundant active sites.The components of fruits, including pectin, fruit acid, fruit sugar, and AAs, are for synthesizing doped carbons.Liu et al. subjected glossy privet to a hydrothermal reaction, drying, and mixing with KHCO 3 , followed by N-protected pyrolysis at 900 °C to obtain a porous N-doped C material with numerous defects. [145]The resulting graphene-like, defect-rich carbon sheets with N doping (GPNCS) exhibited a graphene-like, porous structure, providing high electrical conductivity and specific surface area, contributing to its excellent performance as an electrocatalyst.The synergistic effect between N-doping and topological defects endowed GPNCS with remarkable ORR activity and moderate OER performance.The outstanding catalytic performance of GPNCS was verified in rechargeable ZABs, with a low voltage gap between charging and discharging Figure 8. a1) Schematic of the preparation of heteroatom-doped porous carbon from ginkgo leaves.a2) Effect of pyrolysis temperature on the N content, surface area, and conductivity of heteroatom-doped porous carbon.a3) Half-wave potentials and onset potentials from various catalysts in O 2 -saturated 0.1 M KOH.Reproduced with permission. [140]Copyright 2016, Elsevier.b1-b5) Schematic of the preparation of the heteroatom-doped carbon catalysts from biomass: SEM images of cellulose nanofibers (CNFs) from ascidian tunicates (b1), dried blood meal (b2), and a catalyst (b5).A photograph of a solution of VB 12 and its chemical structure (b3).Structure of heteroatom-doped carbons (b4).b6) LSV curves for the HER of various catalysts in 0.1 M KOH.b7) LSV curves for the ORR of various catalysts in O 2 -saturated 0.1 M KOH.b8) LSV curves for the OER of various catalysts in 0.1 M KOH.Reproduced with permission. [146]Copyright 2022, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
and almost no change after 1340 cycles (approximately 500 h) at 10 mA cm À2 .
Metal-based overall catalyst materials are also essential; however, biomaterials containing electrically catalytic metal components in ecosystems are scarce.Consequently, there have been few reports on entirely bio-based catalytic materials containing metals.Yabu et al. used an innovative approach involving crushing and removing impurities from ascidian tunicates (sea pineapple), preparing a wet paste-like biomass raw material (Figure 8b). [146]They mixed this raw material with pig blood powder containing hemoglobin and subjected the mixture to freezedrying and high-temperature pyrolysis to prepare a biocatalytic material rich in Fe-N-C active sites.Additionally, they added VB 12 to introduce Co-N-C active sites.This catalyst material containing both Fe-N-C and Co-N-C dual-active sites showed enhanced HER, OER, and ORR electrocatalytic properties.By mixing wet biomass raw materials with active and support biomass materials, active components can be uniformly dispersed and integrated into the support, facilitating the uniform distribution of active sites throughout the catalyst system.This approach opens up new possibilities for developing bio-based materials.
In conclusion, the development of bio-based materials is a promising area of research.New strategies for designing and preparing environmentally friendly catalysts can be realized by optimizing the nonmetal active site doping and porous structure preparation for nonmetal biocarbon materials and by exploring metal-containing all-bio-based catalytic materials.We hope that this review provides valuable insights for research on and application of bio-based materials.

Summary and Future Perspectives
There are many more studies focusing on biomass-derived carbon materials as supports than as active components, primarily because biomaterials have often been prepared as self-doped carbon supports that even outperform synthetic carbon.Nevertheless, developing biomaterials as active components remains strategically important.1) Using biomaterials as templates is a promising approach.Peptide template methods have reduced the use of noble metals, but they are mainly applicable to noble metals and are not suitable for all metals.Future work could involve using a combination of Pt-group and non-Pt-group metals as precursors.Using ferritin as a nanoreactor for NP synthesis is a classical biotemplate method and may be used for preparing bi-or multimetallic catalysts of transition metals.Additionally, combining biomaterial templates with bio-based carbon supports is another emerging trend.2) Natural porphyrin materials offer the advantage of being extracted from inexpensive biomaterials, such as chloroplasts and red blood cells.However, the drawbacks include limited diversity compared with synthetic macrocyclic porphyrins, particularly with centrally chelated metal atoms.Therefore, using hemoglobin and chlorophyll as templates to replace the central metals could become a research hotspot.3) The advantage of biomaterials for modifying synthetic carbon by atom doping lies in the abundance of heteroatom carbon sites in protein materials.After pyrolysis, these sites are transferred to the surface lattice of synthetic carbon.However, the availability of such dopants in natural materials is limited.

Figure 3 .
Figure3.a1) Molecular structure of heme.Reproduced with permission.[106]Copyright 2017, ScienceDirect-Elsevier. b1) Schematic of the synthesis for pig-blood-derived carbon (PBC) catalysts.b2) TEM images of the PBC catalysts.b3) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showing the iron atoms on the carbon frameworks.b4) Linear sweep voltammetry (LSV) results before and after the accelerated durability test (ADT) of PBC and commercial Pt/C catalysts.Reproduced with permission.[107]Copyright 2021, American Chemical Society.c1) Molecular structure of VB 12 .c 2 ) HAADF elemental mapping images of the constituent elements of the Fe-VB 12 @GR catalyst.c3) HER and OER performance of Fe-VB 12 @GR compared with other catalysts.c4) Photograph demonstrating overall water splitting with the Fe-VB 12 @GR catalysts powered by a 1.5 V battery.Reproduced with permission.[109]Copyright 2022, Elsevier.

Figure 7
Figure7.a1) Schematic of the preparation of Fe@G.a2) Elemental mapping of Fe@G.a3) Atomic-resolution HAADF-STEM images of Fe@G.a4) High-resolution XPS N 1s spectra for Fe@G.a5) The Fourier-transformed extended X-ray absorption fine structure fitting for Fe@G.The inset shows the schematic of the Fe coordination environment in Fe@G.The gray, red, and blue balls correspond to C, N, and Fe atoms, respectively.a6) LSV curves for the ORR of Fe@G and other catalysts in O 2 -saturated 0.1 M KOH.a7) LSV curves for the ORR of Fe@G and other catalysts in O 2 -saturated 0.5 M H 2 SO 4 .a8) Discharge polarization and power density plots of ZABs.a9) Specific capacity testing of the Fe@G-based ZABs at a constant current density of 5 mA cm À2 .Reproduced with permission.[132]Copyright 2023, American Chemical Society.b1) Schematic of the preparation for the Co-N-B-C.b2) Aberration-corrected HAADF-STEM image of the Co-N-B-C.b3) LSV curves for the ORR of Co-N-B-C and other catalysts in O 2 -saturated 0.1 M KOH.b4) Discharge polarization and power density plots of ZABs.b5) One solid-state ZAB powered a small fan.b6) Two solid-state ZABs in series lit up an orange LED bulb brightly.b7) Three solid-state ZABs in series charged a mobile phone.Reproduced with permission.[135]Copyright 2022, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.