Advancements in biomass-derived single-metal-doped nanostructured carbon electrocatalysts and electrode materials for rechargeable zinc-air batteries

Abstract

Rechargeable zinc-air batteries (RZABs) are vital for advancing sustainable energy storage technologies. Renewable single-metal-doped nanostructured carbon from biomass offers promising electrocatalysts and electrodes, enabling efficient oxygen reactions for sustainable battery technologies. This review exemplifies the prospective possibility of using carbonized biomass in the advancement of sustainable energy storage, thereby initiating an avenue toward higher efficiency and environmentally friendly RZABs. This review examines various synthetic charring methods, structural features of single-metal-doped materials compared with traditional catalysts, and their electrochemical performance toward secondary zinc-air batteries’ efficiency, stability, cyclability, and durability, as well as general performance, which is directly related to their cost. Finally, it discusses the research gaps and future directions, underlining the research that needs to optimize the synthesis methods and to reveal in detail the structure-activity relationships of biomass carbons toward the proposed electrocatalytic applications.

Graphical abstract

Electrochemical energy storage; electrochemical materials science; materials science; energy materials

Introduction

Rechargeable zinc-air batteries (RZABs) are becoming promising options for energy storage because of their high specific energy density, environmental benefits, safety, and cost-effectiveness. Their versatile design allows them to fit into portable and wearable electronics, as the cathode can directly use atmospheric oxygen. The air electrode that contains oxygen electrocatalysts plays a crucial role in both performance and cost. Materials from biomass, which are rich in heteroatoms and have porous structures, provide renewable ways to develop carbon-based catalysts and porous cathodes in metal-air batteries (MABs).^1,2 These materials are appealing for electrochemical energy storage because of their simple structure, low cost, high energy efficiency, and compatibility with the environment. Compared to lithium-ion batteries (LIBs), MABs have higher specific capacity and energy density, making them ideal for electric vehicle (EV) applications. Typically, these systems include a pure metal anode and an ambient air cathode, using aqueous or aprotic electrolytes. During discharge, the metal anode oxidizes while the air cathode promotes oxygen reduction. However, their progress is slowed by issues with anode stability, catalyst performance, electrolyte compatibility, slow reaction rates, and high overpotentials during charge and discharge cycles.^2,3,4,5 To overcome these limitations, having efficient electrocatalysts and porous cathodes is crucial. Biomass-derived carbon materials (BCMs) are gaining attention due to their excellent electrochemical properties, sustainability, and affordability.^{2,4,5,6,7,8,9} These materials provide a viable alternative to LIBs, with energy densities nearly 10 times higher. Zinc-air batteries (ZABs) and lithium-air batteries (LABs) show particular promise, with ZABs already used in hearing aids because of their safety and energy density. Still, their full potential is limited by poor oxygen mass transport, slow kinetics in oxygen evolution and reduction reactions (OER and ORR), low conductivity, and insufficient nano-microstructural design.^4,7,10 The air cathode is a major challenge, often causing high overpotentials and low round-trip efficiency. RZABs also experience corrosion from the electrolyte and passivation of the zinc anode.^3,7,8,11,12

Noble metal catalysts like Pt, IrO₂, and RuO₂ show excellent activity for ORR and OER. Pt-based catalysts, such as Pt/C, Pt alloys, intermetallic, and single-atom types, are very effective for enhancing ORR.^{13,14,15,16,17} RuO₂ is known for its versatility in pH and catalytic activity, while IrO₂ is recognized for its durability over time.^18,19,20 Despite their strong performance, the high cost of these materials limits their widespread use. As a result, BCMs doped with heteroatoms (like B, N, S, and P) have come up as affordable options, providing stability, conductivity, and potential for large-scale use.^{1,21,22,23,24,25,26} But metal-free biomass carbons often show lower performance and shorter lifespans. This suggests that incorporating transition metals and their oxides could significantly enhance electrochemical activity and durability.

Transition metals, such as Mn, Co, Ni, Cu, and Fe, are increasingly being utilized to replace noble metals due to their availability and cost. When these metals are doped into biomass carbon matrices, they provide unique electronic configurations that enhance ORR and OER activity.^{27,28,29,30,31,32} Their combination with carbon nanostructures like graphene, carbon nanotubes (CNTs), and carbon fibers enhances catalytic efficiency.^29,30 However, typical carbon supports frequently have a restricted surface area and poor interactions with catalyst particles, resulting in instability and decreased efficiency. To address these issues, biomass-derived carbon aerogels (BCAs) have been developed as enhanced supports. BCAs provide multidimensional charge transport pathways, high porosity, and great conductivity, which improve both electron and mass transfer. These features improve both electron and mass transfer.^16,27,33 Their strong structure helps stabilize catalyst particles, enhancing overall performance and durability. The 3D porous design of BCAs allows for better immobilization of active particles and greater exposure of catalytic sites, speeding up electrochemical reactions at the air-electrolyte catalyst interface.^27,34,35,36

Furthermore, while dual- and multi-metal doping strategies (Fe-Co and Mn-Ni) have demonstrated synergistic improvements in ORR/OER, they often require complex synthesis protocols, incur higher material costs, and pose challenges in controlling dopant distribution and interaction, particularly within biomass-derived carbon matrices. These issues can hinder reproducibility and scalability, especially when working with diverse biomass precursors. In contrast, single-metal-doped systems offer a simpler and more tunable platform for catalyst design, enabling clearer insights into active site formation, electronic structure modulation, and overall electrochemical performance. Their simplicity also aligns with low-cost, sustainable manufacturing goals, making them more suitable for large-scale deployment in RZABs. Generally, most research on ZAB electrocatalysts and electrodes has focused on traditional carbon materials such as graphene, CNTs, and activated carbon, often relying on intricate synthesis methods or noble metal additives to achieve bifunctional activity. Besides, studies on non-noble metal-doped biomass carbon chemistry remain limited, particularly in the context of RZABs. Critical aspects such as dopant-carbon interactions, active site generation, and the influence of biomass precursor variability on electrochemical performance require further investigation to unlock their full catalytic potential. This review highlights an emerging class of single-metal-doped nanostructured carbon materials derived from biomass, offering a sustainable, affordable, and adaptable alternative for high-performance RZABs. These materials effectively host transition metal dopants (Fe, Co, Mn) by leveraging the natural porosity, structural diversity, and heteroatom content of biomass precursors, thereby facilitating the formation of highly active ORR/OER sites. The review critically examines the advantages, limitations, and challenges of these systems, while showcasing recent advances in synthesis techniques, structural engineering, and electrochemical performance. Ultimately, this work aims to guide the rational development of functionalized biomass-based electrocatalysts for next-generation RZABs by deepening our understanding of electrolyte-electrode kinetics and promoting innovative design strategies.

Single-metal atom-doped biomass carbon materials for ZABs

Biomass-derived materials are increasingly being used as electrocatalysts and electrodes for RZABs owing to their sustainability, availability, and unique properties. Biomass resources include agricultural and animal waste, cellulose, lignin, chitin, chitosan, and biochar. Plant cellulose, lignin, and crustacean shells containing chitin and chitosan may all be converted into porous carbon compounds with large surface area, superior conductivity, and electrocatalytic characteristics. Cellulose-derived carbon has high mechanical integrity,³⁷ but lignin-based carbon has better electrochemical stability and structural resilience.³⁸ Figure 1 illustrates the common biomass resources used for producing biochar.

Figure 1.

Biomass potential and carbon preparation methods for electrochemical energy storage applications^{1,39,40,41,42,43}

Carbon nanoparticles are renowned for their excellent electrical conductivity, high surface area, porosity, and structural stability, which cumulatively enhance the electrochemical performance of MABs by facilitating charge transfer and effective diffusion of electrolytes and oxygen.^{1,21,25,44,45,46,47} Their chemical and electrochemical performances can be accurately tailored through doping and co-doping methods, which influence their nanostructures and catalytic activity.^{1,22,23,24,48,49,50} Doping with heteroatoms such as N, S, B, and P has been shown to significantly increase the catalytic activity, particularly for the ORR.^51,52,53,54 Consequently, metal-free carbon-based bifunctional electrocatalysts have emerged as promising alternatives to noble metal catalysts for use in energy conversion and storage devices.^55,56 Recent advances in synthesis techniques and structural engineering have enabled the realization of high-performance, metal-free carbon electrocatalysts that are comparable to or even superior to traditional noble metal-based counterparts. However, poor catalytic activity and inadequate long-term stability and durability remain barriers to commercialization.^53,57 More details on the metal-free heteroatom-doped BCMs are available elsewhere.²⁶ ZABs, with their low fabrication cost and high theoretical energy density, have emerged as a topic of renewed interest. Doping with transition metals and transition metal oxides has been an effective strategy for addressing these issues with air cathodes and zinc anodes, such as dendrite formation, electrode passivation, and corrosion, which remain significant hurdles to commercialization^58,59 and enhancing RZAB performance. Specifically, single-atom catalysts (SACs) have shown promise in improving the slow four-electron ORR process through the preference for a more efficient two-electron process, thereby improving zinc peroxide (ZnO₂) chemistry and overall battery performance.^60,61,62,63 The incorporation of SACs in BCMs shows increased synthesis efficiency, stability, and catalytic activity and facilitates the formation of reactive intermediates. Their high atomic utilization, with each metal atom acting as an active site, results in exceptional ORR and OER activities. Furthermore, the strong contact between the single-metal atoms and the carbon matrix prevents agglomeration and degradation, resulting in enhanced endurance. This strategy not only improves performance but also promotes sustainability by utilizing renewable resources and minimizing ecological impact.

Extensive research has focused on transition metal-doped or co-doped BCMs as cost-effective alternatives to synthetic carbon materials for energy conversion and storage devices. For instance, Dong et al. explored atomically dispersed metal-nitrogen-carbon (M-N-C) bifunctional electrocatalysts for RZABs with high metal atom utilization and catalytic activity.⁶⁴ They identified bifunctional performance and long-term stability enhancement strategies as key factors for practical applications. Issues regarding uniform metal dispersion and catalyst integrity over long-term operation still need to be overcome. ZABs with NCA/FeSA+NC cathode catalysts have demonstrated excellent performance even at sub-zero temperatures (-40°C) with an open-circuit voltage (OCV) of 1.47 V, a power density of 49 mW/cm², and durability for over 2300 cycles.^65,66,67,68 The Fe nanoclusters increase the 3d electron density and decrease the magnetic moments, thus improving the ORR activity and oxidation resistance at the FeN₄ sites. These findings demonstrate the potential of single-metal-doped biomass carbon materials for enhancing MAB efficiency and stability. Similarly, Jaimes-Paez et al. investigated the ORR activity of freestanding electrospun carbon fiber catalysts derived from various metal salt/lignin solutions.⁶⁹ Using organosolv lignin as a precursor, they synthesized carbon fibers doped with Co, Fe, Pt, and Pd nanoparticles, which were thermally stabilized and carbonized at 900°C. The incorporation of metal nanoparticles favored the development of porosity, improving the electrolyte accessibility to the active sites. Similarly, He et al. demonstrated a scalable strategy for synthesizing 3D carbon aerogel-supported SACs from biomass hydrogel templates,⁴⁶ providing a viable pathway for the large-scale production of high-performance catalysts. Hydrogel templates were engineered with strategic growth in microporosity and defect concentration for the effective trapping and stabilization of single-metal atoms and prevention of aggregation during pyrolysis. The composites exhibited high catalytic activity toward the ORR in an alkaline medium, with a high potential onset of +1.05 V and half-wave potential of +0.88 V, with the advantage of good long-term stability. A new hypersaline-assisted pyrolysis strategy has also been developed for synthesizing 3D honeycomb-like cobalt and nitrogen co-doped carbon nanosheets (Co/N-CNS) from biomass as a biaxial source of carbon and nitrogen.⁷⁰ Pyridinic nitrogen-abundant biomass facilitates the creation of active Co/N sites, while repeated cycles of salt templating and washing are used to develop an ordered honeycomb porous architecture. The prepared Co/N-CNS catalyst revealed superior ORR activity, outstanding stability, and favorable methanol tolerance in an alkaline environment.

Liu et al. presented a nitrogen and cobalt co-doped porous biocarbon (NCAC-Co) derived from corn Stover as an extremely promising ORR catalyst for aluminum air batteries.⁷¹ The material also has a hierarchical, interconnected pore architecture with a high surface area and beneficial mass transport. Although its half-wave potential is relatively lower than that of commercial Pt/C, NCAC-Co has efficient four-electron ORR mechanisms and high operational stability. Its catalytic activity is attributed to the cooperative effect of cobalt nanoparticles, pyridinic and graphitic nitrogen, and a hierarchical porous structure, making it a low-cost and renewable catalyst. Similarly, Wang et al. investigated cobalt and nitrogen co-doped carbon (CoNC) electrocatalysts produced from chitin biomass that perform well in alkaline environments, with an onset potential of 0.86 V vs. RHE and a high limiting current density of 5.94 mA/cm². CoNC outperforms Pt/C in aluminum-air batteries, achieving a power density of 32.24 mW/cm² and a constant discharge voltage of 1.17 V at 20 mA/cm². This work demonstrates a cost-effective and sustainable approach to producing high-performance ORR catalysts from biomass. W. Zhou et al. in their study prepared a nanohybrid catalyst by immobilizing Mn₃O₄ on cobalt-induced nitrogen-doped CNTs (Mn₃O₄/NCNTs@Co), which was effective for both ORR and OER.⁷² The catalyst displayed half-wave potentials of 0.85 and 1.53 V at 10 mA/cm², an OCV of 1.46 V, and remained stable after 1100 cycles at 5 mA/cm². This study presents a low-cost and scalable procedure for the highly efficient preparation of biomass-derived electrocatalysts. A novel approach for synthesizing uniformly dispersed Co-N_x active sites supported over nitrogen-doped, hyperporous networks of carbon (Co-N-C SAC) from low-cost biomass precursors has also been proposed.⁷³ This method allows for the formation of dense micropores and mesopores, which enhances the density of the Co-N_x moieties and network architecture as a whole. The resulting electrocatalysts exhibited better bifunctional activity in alkaline media with a potential difference of 0.81 V and demonstrated high power density and specific capacity in ZABs. The new synthesis method facilitates the preparation of high-performance SACs for various energy applications. In addition, a low-cost, tri-functional catalyst composed of FeP and Fe₃O₄ nanoparticles dispersed in N- and P-doped microporous carbon nanofibers was prepared via electrospinning.⁷⁴ This catalyst showed excellent performance for the ORR, HER, and OER, owing to the synergistic effects of Fe₃O₄ and N-doped carbon for ORR/OER and FeP for HER. It also exhibits exceptional ORR durability and excellent performance in disposable ZABs, offering a new solution for multifunctional energy systems. A straightforward synthesis process has also been reported for the synthesis of bayberry-like Fe₂P/N, P-doped carbon nanospheres (Fe₂P/NPCs) from polyaniline nanospheres, followed by hydrothermal treatment and pyrolysis.⁷⁵ The resulting catalyst has a homogeneous particle diameter (250 nm), high surface area (523 m²/g) with mesopores (3.46 nm), and shows good ORR activity (half-wave potential of 0.82 V) and good tolerance to methanol. It possesses an OCV of 1.449 V, peak power density of 128.38 mW/cm², and specific capacity of 691.0 mAh/g in ZABs, which are better than those of commercial Pt/C-based systems.

Further, Li et al. synthesized efficient bifunctional oxygen electrocatalysts from cobalt-doped carbon-derived from cotton stalks (Figure 2B). Activated, hydrothermally treated, and pyrolyzed Co₂P nanoparticles were coated on N- and P-doped porous carbon nanosheets (Co₂P@NPAC).⁷⁶ The catalyst exhibited good performance in the ORR and OER under alkaline conditions. ZABs with Co₂P@NPAC have high peak power density, OCV, and narrow charge-discharge voltage gap, making them highly suitable for energy storage. Finally, a green approach to the synthesis of a single-atom Fe catalyst supported on nitrogen-doped hierarchically porous carbon (SA-Fe/NHPC) was explored.⁷⁷ Derived from hemin-adsorbed bioporphyrin carbon, the material contains atomically dispersed Fe sites, which confer improved ORR activity, stability, and methanol tolerance compared to molecular hemin and Fe nanoparticles. Biomass-based methodologies offer a scalable and renewable approach for the preparation of high-performance SACs with potential applications in fuel cells and other electrochemical devices.^78,79,80,81 Shape-tunable carbon foam (Co-NCF) was created with a three-dimensional network of cobalt-carbon nanocages immobilized on nitrogen-doped graphene and fibrous carbon.⁸² Such a design accelerates both mass and charge transport, and the Cobalt-Nitrogen bond (Co-N) bonds at the core-shell interfaces greatly improve the ORR activity, making Co-NCF outperform commercial Pt/C in both catalytic activity and durability. When integrated into ZABs, Co-NCF exhibits a superior discharge capacity and power output, offering a low-cost and scalable option for the creation of sophisticated electrocatalysts. M-N-C hybrids are widely recognized as promising bifunctional catalysts for the ORR and OER. Yet, nanoparticle agglomerations of metals and pore collapse during annealing are issues that affect their functionality. To address this, a CoA@CNC-700 catalyst was prepared using cellulose nanocrystals (CNCs) for the homogeneous dispersion of 9 nm cobalt nanoparticles and the creation of a bimodal porous structure.⁸³ This catalyst possesses excellent bifunctional activity, enabling ZABs to achieve a high power density of 187 mW/cm², specific capacity of 782 mAh/g, and superior cycling stability, outperforming commercial Pt/C + RuO₂ catalysts. To promote the commercialization of MABs, a low-cost bifunctional catalyst was developed by encapsulating FeCox alloy nanoparticles in N-doped porous carbon and CNTs (FCx-NC/CNTs) via a one-step alginate biomass carbonization strategy.⁸⁴ The optimized FCx-NC/CNTs-10 catalyst displayed better ORR and OER activity with a low ΔE of 0.80 V and possessed a narrow voltage gap after 430 cycles (144 h), making it a good candidate for practical energy storage. To tackle the sluggish kinetics of ORR and OER, Fe80-ZIF-67@CN was synthesized as a CoFe alloy and N-doped porous carbon catalyst from eggshell membrane, graphitic carbon nitride (g-C₃N₄), and non-noble metal salts.⁸⁵ This catalyst demonstrated an ORR half-wave potential of 0.86 V, OER overpotentials of 339 mV, and ΔE as low as 0.71 V. It further exhibits a power density of 126.47 mW/cm², an OCV of 1.54 V, and better stability over 176 h when utilized in ZABs, outperforming Pt/C + RuO₂.

Figure 2.

Schematic representation of biomass carbonization and metal-doping processes for RZAB

(A) Scaphium scaphigerum.⁸⁶

(B) Cotton stalk.⁷⁶

Figure 2 depicts the conversion of biomass into nitrogen-doped porous carbon via a stepwise post-treatment procedure.^76,86 The method usually starts with raw biomass precursors that are freeze-dried to maintain internal structure surface area, and then undergoes hydrothermal carbonization (HTC) to produce a hydrochar intermediate. This intermediate undergoes high-temperature activation (typically with KOH or CO₂) to produce hierarchical porosity, followed by nitrogen doping (with urea, melamine, or ammonia) to add active N sites such as pyridinic-N and graphitic-N. The resultant N-doped porous carbon has a well-developed interconnected pore network and evenly distributed nitrogen functions, which improve electron conductivity and active-site density. The figure also describes the creation of M-N-C coordination sites, graphitic domains, and hierarchical porosity, which all improve electrical conductivity, active-site density, and oxygen reaction kinetics.⁷⁶ It also emphasizes structure-function links by demonstrating how controlled activation and doping improve ORR and OER activity, both of which are critical for RZAB charge-discharge efficiency.

Yang and Jin described a Mn and N co-doped cotton biomass aerogel catalyst (Mn-N-C-950) that achieved 114.1 mW/cm² power density and 105.5 h cycle life.⁸⁷ Inexpensive and eco-friendly alternatives to noble-metal catalysts leverage the environmental attractiveness of BCM. Similarly, Lv et al. utilized grapefruit skin to prepare an N-doped carbon carrier that, when combined with γ-MnO₂, delivered a specific capacity of 391.2 mAh/g.⁸⁸ This approach demonstrates a green and efficient method for developing high-performance cathode materials from agricultural waste. Xiaodong Lv et al. reviewed the recent progress of biomass-derived porous cathodes for ZABs, where the influence of different biomass sources on morphology, composition, and electrochemical performance has been discussed.¹ A stability of 92.17% over 3000 cycles was reported, where the inherent porosity and heteroatom content of biomass materials played a crucial role in enhancing the battery performance. C. Jiao et al. prepared SACs via corn silk-derived hierarchical porous carbon with hollow tubular morphology.⁸⁹ The catalysts with high Fe single-atom loadings (4.3 wt %) exhibited improved ORR/OER activity, superior conductivity, and effective mass transport. When used in flexible ZABs, they achieved a maximum power density of 101 mW/cm² and a stable voltage window of 0.73 V. Ultra-high doping (up to 10 wt %) also compensated for the deficiencies of conventional Fe-N-C catalysts. Collectively, these studies illustrate the rewards of BCMs in electrocatalysis, including sustainability, affordability, and improved electrochemical performance. Embedding individual metal atoms in biomass-derived carbon matrices significantly enhances their catalytic activity and stability. The strong interaction between the single-metal atoms and the carbon support inhibits agglomeration and degradation, which are common issues in traditional catalysts. Figure 3 illustrates the working principle of a single-cell RZAB. The battery operates via reversible electrochemical reactions between zinc metal and oxygen in the air. On the left side of the diagram, the zinc anode is oxidized on discharge when the zinc metal loses electrons to form zinc ions. These electrons pass along the external circuit to the air cathode on the right, where oxygen in the air is reduced by water to produce hydroxide ions. These ions pass through the electrolyte and separator, thereby completing the internal circuit. The reactions are reversed during charging. In the air cathode, hydroxide ions are then reconverted back to oxygen and water by the release of electrons. The electrons return to the zinc anode, where the zinc ions are reduced to metallic zinc. The electrolyte facilitates ion transport, and the separator prevents direct contact with the electrode. This reversible process can be used to recharge and recycle batteries, presenting an encouraging solution for renewable energy storage.

Figure 3.

A diagrammatic illustration of how a RZAB works

Discharge reactions: Anode (Zinc): Zn→Zn²⁺+2e⁻; Cathode (Air): O₂+2H₂O+4e⁻→4OH⁻. Charge reactions: Anode (Zinc): Zn²⁺+2e⁻→Zn; Cathode (Air): 4OH⁻→O₂+2H₂O+4e.

Synthesis strategies of single-metal-doped carbon materials from biomass

Biomass-derived single-metal-doped carbon materials provide a renewable and efficient route for the assembly of next-generation functional materials. The materials are synthesized by incorporating metal moieties into carbon skeletons obtained from biomass through performing multistep chemical and thermal processes. Methods such as carbonization (involving pre-carbonization processes), activation, and heteroatom-metal doping or co-doping are generally employed to promote the structural stability and electrochemical activity of the final products.^39,90,91,92 The method is a landmark in carbon material engineering with huge potential in energy storage, catalysis, and environmental remediation. An overview of some recent biomass-to-carbon conversion techniques is graphically illustrated in Figure 4.

Figure 4.

Classifications of biomass carbonization pathways⁹³

Details on the synthesis of biomass-derived single-metal-doped carbon compounds for electrocatalysts, energy storage, and conversion applications are presented later.

Pyrolysis of biomass

Pyrolysis has become a common thermal decomposition method for converting biomass into high-carbon-content materials due to its simplicity, scalability, and effectiveness. Pyrolysis involves the use of biomass at a fairly low temperature of 400°C–800°C in an inert environment (usually nitrogen or argon), leading to the formation of volatile compounds and the production of carbonaceous ashes.^34,91,92 Metal salt treatment as a pretreatment process enables the incorporation of metal dopants and increases the functionality of the produced carbon materials. Among the benefits of pyrolysis are the production of materials with high surface areas and tunable morphological and physicochemical features. These features are controllable through variables like temperature, rate of heating, and stay duration. For this reason, pyrolysis has become an adaptable method used to synthesize high surface-area carbon materials exploitable in electrochemistry uses involving energy conversion and storage.^{94,95,96,97,98,99} Although a common approach of making a porous carbon from biomass involves a simple biomass carbonization at high temperatures, efficiency still heavily rests on intrinsic biomass structural and compositional features. Mostly, the process of carbonization occurs via four heat zones: the first is the drying process(<120°C), followed by the devolatilization (220°C–315°C), intermediate decomposition (315°C–400°C), and ultimate carbonization (500°C–1000°C).^{94,95,100,101}

Self-activation with no external activating agent involved offers a greener and less expensive path, yet one requiring greater temperatures and often yielding fewer quantities of products. Strict control of pyrolysis conditions is crucial in order to optimize pyrolytic carbon quality and performance. For example, Wei et al. demonstrated that cobalt salt doping of coconut shell biomass pyrolysis considerably enhanced ORR activity as a crucial parameter for improved ZAB performance.¹⁰² Ebrahimi et al., similarly, synthesized Fe-doped carbon from barley straw using FeCl₃ and urea activation with high specific capacitance and cycling stability viable as supercapacitor electrodes.¹⁰³ Furthermore, B. Zhou et al. proposed a dual-carbon method comprising wood pyrolysis, followed by activation and heteroatom doping with nitrogen and phosphorus, to design oxygen vacancies and maximize Mn(III) active sites.¹⁰⁴ This technique considerably improves the bifunctional electrocatalytic performance of ZABs, producing a high peak power density of 452 mW/cm² and outstanding cycle stability of 1640 h, providing a stable foundation for lasting, high-efficiency energy storage. Likewise, an activation-doping-assisted interface engineering technique based on wood-derived nitrogen and phosphorus-doped carbon integrated with CoNi layered double hydroxides has been performed, resulting in a freestanding composite (CoNiLDH@NPC) with strong 2D-3D coupling and numerous triple-phase boundaries.¹⁰⁵ This catalyst has high power densities (263 mW/cm² aqueous, 65.8 mW/cm² quasi-solid) and long-term stability (up to 500 h), making it a sustainable option for high-performance air electrodes made from biomass. These experiments illustrate the flexibility of pyrolysis in tailoring biomass-derived carbon toward advanced energy storage applications, particularly with the assistance of strategic metal doping.

Hydrothermal carbonization

HTC is another prominent method for transforming biomass into functional carbon materials, especially for supercapacitor and battery electrode applications.^106,107,108 Unlike pyrolysis, HTC mimics how natural coal forms by treating biomass in water under high pressure and moderate temperatures, usually between 180°C and 250°C, in a sealed autoclave.^{97,98,109,110} This process produces hydrochar, a carbon-rich solid with high oxygen content, making it easy to dope and modify its surface. HTC works well with wet biomass and has several benefits over dry carbonization. Because organic solvents are not used, there is less energy input, softer reaction conditions, and a smaller environmental effect. The procedure allows for complicated processes, including dehydration, aromatization, and polymerization, which depolymerize biomass into low-molecular weight monomers. Key factors such as temperature, pressure, reaction time, and solvent type determine the shape, porosity, and surface chemistry of the end product.^97,98,111 While raising the temperature can improve porosity, it may also lower surface area due to increased aromatization. Still, HTC allows for effective doping and control over the structure, which are crucial for creating bifunctional electrocatalysts for the ORR and OER in MABs. For example, a one-step hydrothermal synthesis of NiCo₂O₄ nanowhiskers supported on biomass-derived porous carbon (NCO@HHPC) produced a catalyst with excellent bifunctional activity (ΔE = 0.78 V) and long cycling stability over 487 h in ZABs.¹¹² Zhang et al. used HTC to fabricate Fe-doped carbon from bamboo biomass, achieving enhanced ORR activity and electrochemical stability suitable for MAB applications.¹¹³ Similarly, Qi et al. generated sucrose-derived carbon spheres with customizable diameters and outstanding electrochemical performance, including a volumetric capacitance of 170 F/cm³ and specific capacitance of 164 F/g in 30% KOH electrolyte.¹¹⁴ These findings illustrate HTC’s ability to develop high-performance carbon materials with specific features for sustainable energy storage systems.

Chemical vapor deposition

Chemical vapor deposition (CVD) is a highly effective method for creating nanostructured carbon materials by breaking down gaseous materials at elevated temperatures, usually above 800°C, to form solid layers on a surface. This process allows for detailed control over the material’s composition, structure, and introduction of impurities, making it suitable for making advanced electrocatalysts from materials obtained from plant sources. CVD helps in forming SACs by incorporating metal components into the carbon framework, which increases the interaction between electrons and enhances the catalytic abilities. It also enables uniform addition of other elements like nitrogen, phosphorus, and sulfur, which can boost the material’s conductivity and exposure of active sites.^115,116 It is still one of the most effective strategies to produce superior electrical materials that contain uniformly doped and high-quality graphene-like carbon.¹¹⁷ Besides, the 3D porous carbon fiber networks enclosed with atomically dispersed Co and N that is produced by the Co-N-C-1050 catalyst synthesized through high temperature ammonia treatment on cobalt and bamboo biomass aerogel, demonstrate relatively high catalytic performance with 135 mW/cm² power density. This is observed through the co-existing ORR and OER activities with prolonged cycle stability of around 138 h, comparable to Pt/C and RuO₂.¹¹⁸ Likewise, Li et al. employed CVD to produce Ni-doped graphene-like carbon from biomass, achieving excellent conductivity and catalytic activity.¹¹⁹ Zhang et al. made manganese-doped carbon nanostructures through CVD, which greatly improved the performance of oxygen reactions for ZAB electrodes.⁷⁶ Campos-Delgado et al. co-doped single-walled CNTs with N, P, and Si by the CVD technique.¹¹⁶ Despite numerous benefits, it has several drawbacks, namely high energy consumption, equipment complexity and costliness, precursor material safety issues, and possible particle agglomeration. Therefore, scalability and substrate compatibility are limited due to the risk of thermal degradation, suggesting further research is required to fully understand and harness the potential of porous carbon-derived sacs in green fine chemical synthesis.

Microwave-assisted synthesis

Microwave-assisted synthesis (MAS) has become a powerful and efficient technique for converting biomass into functional carbon materials, especially for energy storage applications like ZABs. This method uses microwave radiation to quickly and evenly heat biomass mixed with metal precursors.⁴⁷ This process triggers decomposition and carbonization. Key advantages include fast heating rates, uniform temperature distribution, and accurate control over reaction parameters. Together, these factors help produce carbon materials with a high surface area and well-defined nanostructures. The rapid thermal processing from microwave irradiation improves the electrocatalytic properties of the carbon materials, making them effective for ORR and OER. These reactions are crucial for ZAB performance. By utilizing the natural features of biomass, such as nitrogen content, hierarchical porosity, and surface functional groups, MAS aids in creating doped carbon structures that enhance conductivity, catalytic activity, and durability.^47,120

Recent studies have shown the potential of this method for producing single-metal-doped carbon materials. Chen et al. used microwave-assisted pyrolysis to turn peanut shell biomass into copper-doped carbon, resulting in materials with a high surface area and excellent ORR/OER performance.¹²¹ Han et al. developed Fe/N/C catalysts using MAS, leading to the quick formation of high-quality SACs.¹²² These catalysts showed superior ORR activity, with half-wave potentials reaching 0.90 V and excellent stability over thousands of cycles, while significantly lowering energy use compared to traditional methods. Despite its potential, MAS faces challenges with scalability, consistency, and process optimization for industrial use. Differences in biomass composition, microwave absorption, and reactor design can impact product quality and performance. Tackling these issues needs more research into reaction kinetics, precursor choice, and system integration.

Sol-gel method

The sol-gel method involves transforming a system from a liquid “sol” (mostly colloidal) into a solid “gel” phase. This process allows the solution (sol) to turn into a solid (gel), forming a network of interconnected nanostructures. It is used to produce highly uniform and pure carbon materials.^123,124 The method is also useful for doping carbon materials with metals and creating uniform nanostructured carbon. Biomass is dissolved in a solvent to form a sol, then metal precursors are added, and the mixture is allowed to gel. The gel is dried and calcined to produce the final carbon material. This technique provides precise control over the composition and structure of the carbon, making it suitable for the uniform incorporation of metal dopants. These synthesis methods offer various benefits and can be tailored to produce biomass-derived single-metal-doped carbon materials with specific properties for electrocatalysts and electrode materials for batteries.

Activation methods

Synthesizing efficient and stable metal-free catalysts for ORR remains a daunting challenge. Nitrogen-doped microporous carbon is successfully synthesized using waste pine cones with an optimum record high surface area of 1556 m²/g and excellent ORR catalytic activity.¹²⁵ The resulting electrocatalyst demonstrated a high discharge voltage of 1.07 V at 50 mA/cm², with excellent promise for ZAB applications. Metal-free N, S dual-doped carbon materials with 3D hierarchical porous structures are also prepared from biomass waste with the use of ZnCl₂ and Zn(OH)₂ as porogens.¹²⁶ Through pore-size tailoring, especially at 4.6, 12, 17, and 27 nm, the optimized catalyst provided improved ORR activity with a half-wave potential of 0.83 V vs. RHE, together with remarkable stability and methanol tolerance. Remarkably, the catalyst can maintain two Zn-air batteries that drove its 216 LEDs for over 24 h, reflecting its great potential for sustainable energy devices. Activation involves applying activating compounds to carbon materials to enhance their porosity and surface area. Typical agents include KOH, H₃PO₄, and ZnCl₂. During this process, biomass is carbonized, mixed with an activating agent, and heated to high temperatures. The agent reacts with the carbon, forming a porous structure.¹²⁷ The benefits are that activated carbon has a high surface area and porosity, which improve electrochemical performance. The advantages and challenges of biomass carbon synthesis methods for MABs are summarized in Table 1.

Table 1.

Advantages and challenges of biomass carbon preparation approaches for MAB uses

Synthesis methods	Advantages	Challenges
Pyrolysis26,34,91,92,94,95,96,97,98,99,100,101,102,103	High efficiency: Pyrolysis can transform various types of biomass into biochar, bio-oil, and syngas with remarkable efficiency Energy production: The process produces syngas and bio-oil, which can be used as renewable energy sources Carbon sequestration: Pyrolysis-derived biochar has the potential to sequester carbon, playing a role in mitigating climate change Waste reduction: Pyrolysis can process organic waste materials, reducing landfill use and associated environmental issues Versatility: The method can handle various feedstocks, including agricultural residues, forest, and municipal solid waste	High initial costs: Setting up pyrolysis plants can be expensive due to the need for specialized equipment and technology Complex process control: Maintaining optimal pyrolysis conditions (temperature, pressure, and time) can be challenging Environmental concerns: Emissions from the process, such as volatile organic compounds and particulate matter, need to be managed properly Product quality variability: Biochar quality can vary depending on the types of feedstock and process parameters, making consistency a challenge Scalability: Scaling up pyrolysis from pilot to industrial scale can be difficult due to technical and economic issues
HTC97,98,106,107,108,109,110,111,113,114,128	Low energy consumption: Compared to other thermal processes like pyrolysis, HTC operates at relatively lower temperatures (180°C–250°C), resulting in lower energy consumption Environmental benefits: HTC produces fewer emissions and pollutants compared to traditional methods, making it more environmentally friendly High yield: The process can convert wet biomass into carbon-rich materials with high efficiency, even without the need for drying the feedstock Versatility: HTC can process a wide range of biomass feedstocks, including agricultural residues, food waste, and sewage sludge Product quality: The resulting hydrochar has a high carbon content and can be used for various applications, such as soil amendment, adsorbents, and energy storage	High energy consumption: The process requires relatively high temperatures (180°C–250°C) and pressures, leading to significant energy consumption Complex reaction mechanisms: The reactions involved in HTC, such as hydrolysis, dehydration, and decarboxylation, are complex and can be difficult to control Product quality variability: The quality of the resulting hydrochar can vary depending on the feedstock and process conditions, making it challenging to achieve consistent product quality Scalability: Scaling up HTC from lab-scale to industrial-scale operations poses significant challenges, including the need for continuous reactors and effective heat and mass transfer models
CVD115,116	High purity and uniformity: CVD produces materials with high purity and uniformity, which is crucial for applications requiring precise material properties, such as in semiconductor manufacturing Versatility: CVD can be used to deposit a wide range of materials, including metals, ceramics, and semiconductors Complex shapes coating: Due to its non-line-of-sight nature, CVD can coat components with complex shapes uniformly Scalability: The process is easily scalable for batch production, leading to cost savings through economies of scale Formation of alloys: CVD can be used to form alloys tailored to specific applications, enhancing the functionality of the coatings	Hazardous byproducts: Many CVD byproducts are hazardous, including being toxic, explosive, or corrosive, requiring careful handling and disposal Thermal stress on substrates: Thermal CVD processes can cause thermal stress on substrates, potentially leading to stresses and failures High cost of precursor gases: Some precursor gases used in CVD can be very expensive, increasing the overall cost of the process Equipment cost: The cost of CVD equipment can be high, which may be a barrier to entry for some organizations Complex process control: Achieving precise control over the size, shape, and composition of the synthesized materials can be challenging
MAS120,121,122	Rapid heating: MAS provides rapid and uniform heating, which can significantly reduce reaction times Energy efficiency: This method is more energy-efficient compared to conventional heating methods, as it directly couples microwave energy with the reaction mixture Enhanced reaction rates: The rapid and uniform heating can lead to higher reaction rates and improved yields Selective heating: Microwaves can selectively heat specific components in a mixture, leading to more selective reactions and fewer by-products Eco-friendly: MAS often requires fewer solvents and can be performed under solvent-free conditions, making it more environmentally friendly	Equipment cost: The initial cost of microwave equipment can be high, which may be a barrier for some laboratories or industries Scale-up challenges: Scaling up microwave-assisted reactions for industrial applications can be challenging due to the need for specialized equipment and potential issues with uniform heating at larger scales Limited applicability: Not all reactions or materials are suitable for MAS, as some may not absorb microwaves efficiently Safety concerns: Handling microwaves and the associated equipment requires proper safety measures to prevent exposure to high-intensity radiation
Sol-Gel method123,124	Low-temperature process: The Sol-Gel method can be carried out at relatively low temperatures, which minimizes the risk of agglomeration and contamination High purity and homogeneity: It produces materials with high purity and homogeneity, making it suitable for applications requiring precise material properties Versatility: The method can be used to synthesize a wide range of materials, including oxides, nitrides, and carbides Fine powders: It can create very fine powders with controlled particle sizes, which is beneficial for various applications Scalability: The process can be scaled up to produce large quantities of materials Inorganic-organic hybrid materials: It is suitable for producing inorganic-organic hybrid materials, which have unique properties and applications	High cost of raw materials: The chemicals used in the Sol-Gel process, such as metal alkoxides, can be expensive Complex process control: Achieving precise control over the size, shape, and composition of the synthesized materials can be challenging Long processing time: The process can be time-consuming, especially during the aging and drying stages Environmental concerns: Some of the chemicals used in the process may pose environmental and safety risks if not handled properly
Activation methods127	Increased surface area: Activation methods, such as chemical activation with agents like KOH or H3PO4, significantly increase the surface area of carbon materials, enhancing their adsorption capacity Improved pore structure: Activation can create micropores, mesoporous, and macropores, useful for various uses like water purification and gas storage Enhanced adsorption capacity: Activated carbons have higher adsorption capacities for contaminants, making them effective for environmental cleanup and industrial processes Customization: Different activation methods and agents can be used to tailor the properties of activated carbon for specific applications, such as dye removal or heavy metal adsorption	Environmental concerns: Some chemical activation agents, like zinc chloride, can be environmentally hazardous and require careful handling and disposal High cost: The chemicals used for activation can be expensive, increasing the overall cost of producing activated carbon Complex process control: Achieving consistent and optimal activation conditions can be challenging, requiring precise control over temperature, time, and chemical concentrations Potential structural damage: Over-activation can damage the carbon structure, reducing its mechanical strength and stability

When comparing the synthesis methods of biomass-derived single metal-doped carbon materials, there are advantages and drawbacks to each approach. HTC is the favored method because it has mild reaction conditions and high yields, though it is beset by slow reaction times and a lack of scalability. Pyrolysis is described as simple and low-cost, with high carbon content but high temperature requirement, and possible incomplete carbonization with the release of volatile organic compounds. Sol-gel processing is high in purity and homogeneity with rigorous control over composition, but is complex, time-consuming, and employs costly precursors. CVD excels at producing high-quality carbon materials with excellent control over doping levels, but it is costly and needs specialized equipment. The applicability of each method is in the specific application requirements, and compromises among aspects like cost, scalability, and material quality.

Structural characteristics of biomass-derived single-metal-doped carbon materials

Morphology

The nanostructured carbon-based biomass materials’ structure is fundamental to electrocatalyst and electrode performance.¹⁰³ Their use in carbon materials requires well-designed engineering approaches to optimize their properties for specific uses. They are all carbon tubes (1D), carbon nanosheets (2D), carbon aerogel (3D), and HPC, which is highly porous carbon. Each of these categories enjoys distinct preparation methods that add to their structural and functional properties.¹²⁹ A 1D carbon form, such as nanowires, nanoribbons, nanorods, and nanofibers, is under study for its corrosion resistance, strength, and chemical stability. CNTs are prepared through methods such as CVD, through which their diameter and length can be controlled. CNTs have extraordinary electrical conductivity and mechanical strength and can thus be used for electronics and nanocomposites applications.¹³⁰ However, their large-scale synthesis and application are restricted by complex synthesis routes. Organic biomolecules offer a true solution to mitigating energy consumption during electrocatalyst synthesis. Graphene and graphene oxide are produced using methods such as liquid-phase exfoliation and chemical reduction. They have high surface area and decent electrical qualities, and these are useful for applications in sensing and energy storage.¹³¹ 2D porous carbon nanosheets with good electrical conductivity have shown great promise for energy applications. Biomass consists of complex biomolecules that exhibit dissimilar properties under diverse environmental conditions and can be fabricated by the use of self-assembly processes. Chen et al. studied the morphological impact of metal doping on BCMs, with particular focus on Fe-doped carbon. Their research concluded that Fe-doping created a hierarchical porous structure.¹³² The hierarchical structure led to a higher ion flow and expanded the surface area of the carbon material. Fe-doped activated carbons derived from biomass are green supercapacitor electrode materials with higher specific capacitance and energy/power density for a broad range of electrolytes such as aqueous, organic, and ionic liquids.¹³³ Fan et al. evaluated synthesis pathways and doping effects of heteroatoms to maximize the efficiency of biomass-derived porous carbon materials toward electrochemical energy storage through mitigation of pore structure, conductivity, and surface wettability issues.¹³⁴ Further, Zhang et al. prepared a Fe, N co-doped porous carbon material silkworm cocoon (Fe-SilkPNC) by synthesizing and discovered that the Fe-SilkPNC showed high ORR catalytic activity through the use of silk fibroin solubility in concentrated salt aqueous solution and self-assembly of hydrophobic and hydrophilic molecular blocks.¹³⁵ The Fe-SilkPNC material displayed an ordered layer assembly of a lamellar-layered structure that provided superior ORR catalytic activity and long-term stability. In contrast to self-assembled biomolecules, certain biomasses possess a natural flake structure such that they can be directly converted into 2D carbon materials through straightforward pyrolysis activation processes. Zhuo et al. prepared N and S co-doped Fe-N-C carbon materials (Fe-NSDC) with a 2D porous nanosheet structure through the pyrolysis of an abrasive porphyrin and ferric chloride mixture.¹³⁶ The Fe-NSDC-driven Zn-air battery showed huge peak power density and minimal voltage difference between discharge and charge, which reflects its superior feasibility and rechargeability. Researchers have been interested in the synthesis of carbon material with 3D nanostructures as RZAB electrocatalysts. Synthesis of such materials makes use of suitable cross-linking or activating agents. P. Li et al. et al. developed a trifunctional catalyst via incorporating FePx into Fe, N, P, S co-doped porous carbon and exhibiting nanobubble-like and pore-rich morphologies.¹³⁷ The Fe-N-C/FePx/NPSC exhibits excellent electrocatalytic activity. Natural origin lignin and starch utilization also facilitates mass production. HPCs are formed with the help of techniques like pyrolysis and biomass activation, developing a pore structure on multiple scales. Figure 5 highlights the development, structural characteristics, and electrochemical performance of transition metal-doped BCM for RZABs.^1,41 It depicts the synthesis process, which includes biomass carbonization, metal doping (Fe, Co, Ni), and activation, as well as TEM/SEM images of the hierarchical porous morphology and metal sites that improve oxygen reduction and evolution processes. It also shows the electrochemical performance, which includes LSV curves, polarization and power density plots, and discharge-charge cycling curves, highlighting the improved catalytic activity, battery efficiency, and long-term stability achieved by transition metal doping.

Figure 5.

Fabrication, properties, and electrochemical activity of BCMs based RZABs

(A) Transition metals and their derivatives doped carbon composite materials.⁴¹

(B) Electrochemical performance of self-standing biochar cathode materials (A and E), the conversion of natural biomass sources (sugarcane and eggplant) into porous and nanostructured carbon materials (B and F), and the comparison of their electrochemical performance using morphological characterizations (SEM, TEM) and voltage-capacity/current-power density analyses (C, D, G, and H).¹

The 3D structure of the carbon aerogels achieves maximum exposure of active sites and prevents particle loss. Hu et al. et al. prepared a bifunctional catalyst by distributing Co9S8 nanoparticles into chitosan-derived N, P co-doped aerogel carbon matrix (Co9S8/N, P-APC).¹³⁸ In the electrocatalytic system, the 3D porous structure achieves an expanded contact area of the catalyst and the electrolyte, and facilitates mass transport of species involved in the electrochemical reaction. Due to its stable 3D porous structure and homogenous Co9S8 growth on it, the catalyst demonstrates good ORR and OER catalytic activity. Cai et al. explained the effects of the carbonization conditions on the microstructure and morphology of the precursors carbon by abandonment via a reactive template-induced in situ hyper-crosslinking process.¹³⁹ The paper outlined the effects of the carbonization conditions on the microstructure and morphology of HPCs and the adsorption of methylene blue. These new HPCs resulting from carbonization have hierarchical micro-, meso-, and macropore morphology with respective specific surface areas of 2388 m²/g and 1892 m²/g. Similarly, Zhou et al. described the porous structure and morphology of the carbon material through a soft template-assisted hydrothermal process with chitin and chitosan from marine biomass, but do not distinctly classify these materials as one-dimensional, two-dimensional, or three-dimensional.¹³¹ According to their report, there is a rise in specific surface area to 931–1629 m²/g by the optimization of the reaction variable parameters. Although BCMs have high catalytic activity and stability in bifunctional catalysis related to natural heteroatom self-doping and structural defects, there are some problems to be overcome. The pore size and porosity in the case of BCMs heavily affect the electron and mass transport, but their synthesis is challenging without losing their native morphology of raw biomass material. Other than this, the selection of raw material and also synthesis methods considerably contribute to affecting the structure and morphology of the resultant target catalysts. Although some of the surface heteroatoms are being lost during high-temperature pyrolysis, the loss is accompanied by a lower and non-uniform distribution. Efficient methods are therefore needed for reducing the loss of heteroatoms. Characterization is required for determining the structure, chemistry, and catalytic mechanism of the material. Novel characterization method and synthesis approach could provide further use of renewable and low-cost BCMs in energy.

Surface chemistry

Surface material chemistry is also important in defining the activity in electrocatalysts of the materials, namely the functional groups with dopants on the material’s surface. Carbon surface chemistry is responsible for its applications owing to the thermochemical treatment, and carbonaceous materials range from producing multiple functionalities on the surface, which are highly dependent on temperature. Temperature increase reduces the extent of these groups. Biochar surface charge is also related to aqueous phase pH by a mixture of aqueous and biochar, imposing a positive effect on ion transport during electrochemical assessment. Active functional groups on carbon-based electrode materials can be potential catalysts based on their stability, reactivity, and recyclability. Electrochemical features are profoundly affected due to nitrogen doping; hence, it is among the most crucial parameters besides nitrogen content. Decarboxylation leads to a decrease in oxygen content, and decreased nitrogen content is a result of thermochemical conversions and nitrogenous volatile products. Increased carbon content makes the material thermally stable and promotes graphitization, but reduces surface function simultaneously. The work aims to transform low-value biomass feedstock into higher-value carbon materials with increased surface area. The increase in pore diameter within the carbon matrix is supposed to facilitate ion adsorption capability through free approachability for electrolyte ions. Porous carbon of lower mean pore diameter (<2 nm) is best suited for application in electrochemical energy storage since surface porosity would hinder chemical reactions. Increased surface area aids in providing the platform needed to allow for electrochemical reactions, thereby improving the thermodynamics of the adsorption surface-controlled mechanism of electrode-analyte heterogeneous reactions is achieved. This enables electrolyte ions to be adsorbed on the electrode surface at a high rate, which leads to high capacitance. Porosity on the surface might, however, restrict reaction kinetics and how fast chemical reactions take place.

Doping with metal is central to the surface chemistry of carbonaceous biomass materials, as it enhances their applications and quality. This brings not only some new active sites but also a broadened electrical conductivity due to the covalent bond and enhanced chemical stability that make them viable for the conversion and storage of energy. Metal doping also brings new surface functional groups like oxygen, nitrogen, or sulfur that enable this material structure to be aligned with others. Apart from this, metal doping increases the surface area of carbon material, thus developing further adsorption sites and catalysis. Such modification of surface chemistry can result in markedly enhanced performance of BCM in energy storage, catalysis, and environmental remediation. Figure 6 shows the structural and performance plots of a palladium/ionic-liquid-derived carbon composite (Pd/IL/CDC) catalyst used in ZABs. It depicts the creation of Pd nanoparticles distributed over the CDC matrix, which was stabilized via an ionic-liquid-assisted synthesis process. Atomic-scale dispersion and strong metal-support interactions are found to improve catalytic durability and minimize agglomeration while cycling. The corresponding performance plots, which typically include polarization curves, power density plots, and long-term galvanostatic discharge-charge profiles, show higher peak power density, lower charge-discharge overpotential, and longer cycle stability than conventional carbon/Pd/C electrodes.

Figure 6.

Structural and performance plots of a palladium/ionic-liquid-derived carbon composite catalyst for ZABs

(A) Schematic of surface modification.

(B) Discharge polarization profiles and related power density curves.

(C) Long-term discharge stability.¹⁴⁰

Electrochemical performance of RZABs

Biocarbon-metal electrodes and electrocatalysts obtained from the treatment of carbon in a single synthesis step are very promising for RZABs regarding their electrochemical performance.^26,141 Advantages are the improvement of ORR/OER, improved electrical conductance, having a large surface area, being cheap, stable, and durable. Disadvantages come about due to a very unstable synthesis process, possible aggregation of the nanoparticles of metals, the environmental impact of metals and chemicals, and the biomass feedstock and processing condition variability. Biomass as a precursor is cheap and environmentally safe, and the metal-doped carbon materials are more stable and durable. The synthesis process is rather complicated and needs a strict approach to holding its parameters, while the presence of metals and chemicals may have environmental implications if uncontrolled. The ORR is the major reaction in ZABs that has a direct impact on the efficiency and performance of the battery. A study from Yang et al. had more details on the outstanding activity and stability of atomically dispersed Mn-N-C catalysts toward ORR in alkaline media over many conventional catalysts.¹⁴²

Due to the high energy density, low cost, and environmental aspects, RZABs have recently drawn considerable attention.¹⁴³ Substantial progress in performance has been made over recent years, as well as improvements in terms of increased cycling stability and energy efficiency.^144,145 The long-term limitations come from the relatively low power density, short cycle life, and air electrode corrosion. The advantages and disadvantages of RZAB are generally described in Table 2.

Table 2.

Advantages and disadvantages of ZABs

Advantages

Disadvantages

•High energy density, lightweight, flexible, and longer lasting: ZABs provide specific and volumetric energy densities of approximately 500 Wh/kg and 1000 Wh/L, respectively, making them some of the highest among battery systems. The weight of the battery is halved compared to the latest LIBs, and its lifespan exceeds that of LIBs •Environmental friendliness: ZABs have a low environmental impact. •Safety: ZABs are safer than LIBs •Cost-effectiveness: Due to the availability of zinc, ZABs are affordable

•Limited power output: Their power output is limited, mainly because of the performance of air electrodes •Lower voltage: ZABs have a practical cell voltage of 1.35–1.4 V, roughly half of that of lithium-based batteries •Dependence on ambient conditions: The performance and operational lifespan of ZABs are affected by environmental factors like humidity and temperature •Electrolyte challenges: Rechargeable ZABs need careful control of zinc precipitation from water-based electrolytes, and face issues such as dendrite formation, uneven zinc disbanding, limited solubility in electrolytes, decreased power density, reduced lifespan, and corrosion of the air electrode, all impairing performance loss

Despite their promise to be a bifunctional electrocatalyst and electrode material for RZABs, biomass-derived single-metal-doped carbon electrocatalysts struggle to maintain long-term stability and durability. Metal leaching is a crucial degradation mechanism in which transition metal dopants gradually dissolve into the alkaline electrolyte during high potential or extended cycling, resulting in the loss of active catalytic sites and decreased ORR/OER efficiency. This problem is compounded by poor metal-nitrogen coordination, especially in systems without strong anchoring methods, as highlighted by Maliutina et al.⁹⁹ Likewise, Ku et al. found that Fe-N_x sites are sensitive to demetallation in alkaline conditions, which affects long-term ORR performance.¹⁴⁶

Carbon corrosion is another key problem, particularly during OER with high anodic potentials. The defect-rich and heteroatom-rich character of biomass-derived carbons, while helpful for catalytic activity, makes them more sensitive to oxidative degradation, resulting in pore collapse, reduced conductivity, and structural failure. J. Liu et al. discovered that even extremely porous MnO-doped carbon frameworks made from wood exhibit gradual surface oxidation and pore shrinking after prolonged cycling.¹⁴⁷ Likewise, Miao et al. further noted that defect-rich carbon complexes, while catalytically active, require stability to prevent oxidative breakdown carbon.¹⁴⁸ In addition to chemical degradation, physical and interfacial instabilities such as electrolyte flooding, gas crossover, and mechanical stress sturdily contribute to performance degradation,^149,150 particularly in flexible or quasi-solid-state ZAB. These events inhibit active sites, alter triple-phase boundaries, and cause structural fatigue. Cai et al. underlined the need of surface passivation and hydrophobic adjustment in biomass-derived carbon dots in order to reduce flooding and retain catalytic interfaces.¹⁵¹ To address these multifaceted challenges, researchers are investigating strategies such as heteroatom coordination to stabilize metal dopants,^152,153 hierarchical structuring to buffer mechanical stress while maintaining pore accessibility,^154,155 surface passivation and defect engineering to improve corrosion resistance, and electrolyte optimization such as gel-based systems to reduce flooding and improve ion transport.^150,156,157 Furthermore, R. Dubey and Guruviah proposed combining machine learning (ML) models to anticipate degradation-prone configurations and drive the rational design of more robust biomass-derived catalysts.¹⁵⁸

Table 3 compares the performance of several metal-doped biomass-derived carbon composite materials used in MABS systems, with a special emphasis on their usefulness in RZABs. The table displays essential electrochemical measures such as half-wave potential, OCV, peak power density, and cycle life, as well as unique structural and catalytic properties.

Table 3.

Performance comparison of single-metal doped biomass carbon materials for RZABs

Catalyst/material	Biomass resource	Synthesis strategy	Metal dopant(s)	Half-wave potential (V vs. RHE)	OCV (V)	Peak power density (mW cm−2)	Cycle life
NCA/FeSA+NC cathode65	Biomass-derived N-C	Fe single-atom doping	Fe	–	1.47	49	>2300, −40°C
Carbon aerogel-supported SAC46	Biomass hydrogel template	Aerogel trapping	Fe	+0.88	1.81	181.1	–
Co/N-CNS70	Pyridinic-N-rich biomass	Salt-templated pyrolysis	Co	–	–	–	–
NCAC-Co71	Corn stover	Hydrothermal + pyrolysis	Co	0.743	–	–	–
Mn3O4/NCNTs@Co72	Biomass-derived N-doped CNTs	–	Mn, Co	0.85/1.53	1.46	–	1100 at 5 mA/cm2
Co-N-C SAC73	Biomass-derived hyperporous network	SAC synthesis	Co	–	1.49	143.1	–
FeP/Fe3O4-N,P-CNFs74	Electrospun nanofibers	Electrospinning	Fe	–	–	–	–
Fe2P/NPC75	Polyaniline-derived nanospheres	–	Fe	0.82	1.449	128.38	–
Co2P@NPAC76	Cotton stalk biomass	–	Co	0.85	1.38	75	200
Co-NCF118	Bamboo biomass aerogel	–	Co	0.842	–	135	138 h
CoA@CNC-70083	CNCs	–	Co	0.84	1.47	187	50 h
FCx-NC/CNTs-1084	Alginate biomass	–	Fe, Co	0.79	–	156	430 (144 h)
Fe80-ZIF-67@CN85	Eggshell + g-C3N4	–	Fe, Co	0.86	1.54	126.47	176 h
Mn-N-C-95087	Cotton biomass aerogel	–	Mn	0.843	–	114.1	105.5 h
Fe-SAC159	Corn silk biomass	–	Fe	0.86	–	101	–
NCO@HHPC112	Biomass-derived porous carbon	–	Ni, Co	–	–	–	1460 (487 h) at 10 mA/cm2
Fe/N/C122	Biomass precursor	MAS	Fe	0.90	1.56 (aqueous), 1.52 (solid)	235	9000

As shown in Table 3, Fe-doped carbon materials often have the highest ORR and OER activities due to their advantageous electronic structure and abundance of Fe-N-C active sites. Co-doped carbons also exhibit substantial bifunctional catalytic activity, making them appropriate for rechargeable MABs applications. Ni-doped carbons improve OER efficiency by including redox-active Ni species, particularly at edge sites, whereas Mn SACs provide an appealing mix of high ORR activity and long-term stability. In addition to metal doping, structural properties including hierarchical porosity, large surface area, and uniform metal dispersion are important in easing mass movement and improving active site accessibility, which improves overall electrocatalytic performance.

The current limitations and future perspective of ZABs

The current limitations of the RZAB

Despite their inherent advantages, including high safety, high theoretical energy density, low production cost, and environmental sustainability, RZABs continue to face several interconnected limitations that restrict their large-scale practical deployment, particularly when BCMs are employed as electrode components. A central challenge lies in the development of high-performance air cathodes and zinc anodes, as efficient operation of RZABs depends on rapid and reversible ORR and OER. However, the complex biomorph-genetic structures of biomass precursors, together with inefficient bottom-up synthesis strategies, make it difficult to fabricate electrodes with uniformly distributed and highly accessible active sites, leading to sluggish ORR/OER kinetics and reduced catalytic efficiency. On the anode side, zinc metal suffers from intrinsic issues such as dendrite growth, uneven zinc deposition, and surface passivation, which can induce internal short circuits, accelerate capacity fading, and ultimately cause premature battery failure. These challenges are further exacerbated by corrosion and long-term degradation of air electrodes in harsh alkaline electrolytes, significantly compromising cycle life, operational stability, and overall reliability. In addition, the structural instability of BCMs presents a major obstacle, as variations in porosity, defect density, and heteroatom distribution, often dictated by the nature of the biomass source, result in inconsistent electrochemical performance and poor durability, particularly under high current densities and prolonged cycling. The regulation of multi-metal dopants, such as Co, Fe, Mn, and Ni, within porous carbon matrices also remains technically demanding, as achieving controlled spatial distribution and synergistic interactions without sacrificing conductivity or structural integrity is challenging. From a manufacturing and commercialization perspective, scalable and reproducible synthesis routes remain underdeveloped, with batch-to-batch variability and inconsistent material quality arising from the inherent heterogeneity of biomass precursors. Consequently, RZABs currently exhibit lower energy efficiency compared to lithium-based batteries and limited electrochemical reversibility, with cycle life still falling far short of the thousands of cycles required for reliable commercial and grid-scale energy storage applications.

Future perspectives of RZABs

This study provided comprehensive information on the physicochemical properties of RZAB electrodes and electrolytes with metal-doped and co-doped biomass carbon as alternatives to graphite. The development of in situ/operando characterization of ZABs is also closer to the reaction mechanism and dynamic evolution of the electrolyte electrode interface. Therefore, a way forward and development of the application of BCMs in RZABs should be focused toward improvement of electrochemical properties, energy efficiency, stability, sustainable features, scalability, newer synthesis techniques, and improvement of in situ/operando characterization of ZABs. Among some other research directions, at present, it is specifically focused on the improvement of metal doping techniques to improve active sites for improved electrical conductivity. Another key element is the lower environmental impact of manufacturing processes based on the use of cleaner chemicals and energy optimization. Meanwhile, efforts are being made through research to increase the chemical and structural stability of these materials to be able to deliver a longer life cycle in batteries; hence, efforts are aimed at more sophisticated synthesis approaches, where there is more control over material properties, such as MAS and CVD. More promising is the use of biomass-derived carbon in hybridization with other materials, specifically for further performance enhancement. The process of surface modification is also considered to introduce specific functional groups to improve performance toward zinc ions and electrolytes. Interdisciplinary research by a scientist becomes of great relevance in creating new solutions to push technology ahead. Part of the strategies can be agreed upon now to improve the durability of RZAB during actual operation. This can be achieved by increasing the stability and durability of the electrode. A solution can also be proposed concerning the second problem in order to prevent zinc dendrite short circuits and battery failure. The passivation issue of a zinc anode must be studied in order to enhance ion transport as well as cell performance. An excessive focus on long-term performance must be given against corrosion of the air electrode. The above-stated gaps will further help in exploring the performance, lifespan, and reliability of biomass-based RZABs. Briefly, the review identifies a general field about critical gaps in biomass-based RZABs to which future research would be sufficient for upgrading the stage of lower performance, short lifespan, growth of dendrites, passivation, and corrosion of air electrodes. Thus, future research on ZABs should focus on overcoming these limitations through advanced material design, sustainable synthesis strategies, and deeper mechanistic understanding.

• •Anode Restructuring and Reengineering: Recent research emphasizes restructuring the anode to enhance ZAB performance, but further studies are necessary.
• •Multi-Metal Doping Strategies: Incorporating multiple transition metals (e.g., Co, Fe, Mn, and Ni) can synergistically improve ORR/OER by adjusting electronic structures and active site distributions.^1,26,160,161 Yet, regulating the spatial distribution and interaction of dopants inside porous carbon matrices remains problematic, especially when they come from diverse biomass sources.
• •Interface Engineering: Interface engineering can enhance charge transfer, reveal more active sites, and stabilize catalytic interfaces through the rational design of 2D-3D hybrid architectures and heterojunctions (such as LDH-carbon composites).^105,153 Strong coupling between metal species and carbon frameworks without sacrificing mechanical integrity or conductivity is still a challenge, though.
• •Alternative eco-friendly electrolytes: Exploring new electrolytes aims to improve ZABs, with support for developing novel electrolytes and proposals for liquid ionic electrolytes.
• •Bifunctional Oxygen Electrocatalysts: Creating catalysts effective for both ORR and OER is crucial for ZAB advancement, though sustainable bifunctional catalysts remain a challenge. This area is highly active in research to boost ZAB performance.
• •Structural Stability and Durability: Practical ZAB deployment requires improving biomass-derived carbon’s resilience to harsh electrochemical conditions and long-term cycling.^{1,26,160,162,163} Nevertheless, materials with variable porosity and defect profiles that can deteriorate over time or at high current densities are frequently produced from biomass precursors.
• •Scalable and Sustainable Synthesis: Commercialization can be accelerated by developing scalable, low-cost synthesis pathways like green activation techniques, microwave-assisted pyrolysis, or HTC.¹⁵³ However, a significant obstacle to industrial adoption is ensuring uniformity in material quality and electrochemical performance across batches and biomass sources.
• •Integration into Flexible and Solid-State ZABs: BCMs, when engineered with enhanced morphology and electrical conductivity, have significant promise for application in flexible and quasi-solid-state ZAB systems.^153,164 Further work is required to guarantee their appropriate integration with gel-based electrolytes and flexible substrates, while preserving high energy production and long-term operational stability.
• •Battery Reversibility: Reversibility is a critical issue in electrochemical energy storage, with RZABs’ cycle life needing improvement to reach several thousand cycles.
• •Energy Efficiency: ZABs currently have lower energy efficiency than lithium batteries; future goals aim to exceed 70% efficiency.
• •ML-Assisted Design of Biomass-Derived Catalysts: Emerging ML technologies provide effective tools for expediting catalyst identification. ML models can predict optimum doping combinations, pore architectures, and electronic characteristics using vast datasets from experimental and computational investigations. This can considerably minimize trial-and-error in synthesis while guiding logical design.

Finally, integrating BCMs into flexible, quasi-solid-state, and solid-state ZABs represents a promising pathway for next-generation energy storage devices, provided that long-term stability, high energy efficiency, and scalable production can be achieved.

Conclusion

The latest achievements of advanced biomass-produced single-metal-doped nanostructured carbon materials as electrocatalysts and electrodes promise a brighter future for improving the performance of RZABs. High surface area, rich heteroatom content, and high electrical conductivity are what make these materials promise better electrochemical performance. The substitution of synthetic carbon precursors by biomass carbon is very important, but it belongs to another major challenge for the environment, and a higher battery cost. Other setbacks connected to the earlier-mentioned challenges with ZABs include lower performance, shorter lifespan, dendrite formation, passivation, and corrosion of air electrodes. Uniformity, scalability, and optimization are the main drawbacks remaining to the full use of the above material as an electrode and electrocatalyst in this battery. Therefore, future RZAB research should focus on anode restructuring and multi-metal doping to improve electrochemical performance, as well as tackling dopant dispersion issues in biomass-derived carbon matrices. Advances in interface engineering and bifunctional oxygen electrocatalysts are critical for enhancing charge transfer and catalytic stability, but long-term solutions remain challenging. Developing eco-friendly electrolytes and scalable synthesis processes like microwave-assisted pyrolysis and HTC can help with commercialization, as long as material consistency is maintained. Integration into flexible and solid-state ZABs necessitates optimal carbon morphology and compatibility with gel-based systems. Furthermore, enhancing battery reversibility, energy efficiency, and using machine learningto design catalysts will be critical to ensuring long-term operating stability and competitive performance. It should be directed toward how metal doping optimization of these processes, as well as improving the structural stability along with scalable synthesis methods of the materials, can appropriately address the engineering challenges and advanced applications of BCMs in ZABs. Thus, there is an immense untapped potential that, when harvested to its depth, can lead to much more efficient, cost-effective ways of energy storage solutions.

Acknowledgments

The authors sincerely appreciate the financial and institutional assistance from Bahir Dar University via its 2025 Thematic Research initiative on Physical Sciences and STEM, particularly the project entitled “Developing Metal-Air Batteries (Aluminum, Zinc) through Advanced Electrolyte and Electrocatalyst.” This study constitutes the review phase before experimental studies and establishes a theoretical framework for future experimental inquiries. The support from Bahir Dar University has been important in facilitating this research, and the authors genuinely appreciate the resources, direction, and encouragement provided throughout this initial period. Funding: This paper is financed by Bahir Dar University through the 2025 Thematic Research initiative on Physical Sciences and STEM.

Author contributions

Molla Asmare Alemu- conceptualization, drafting, and editing the manuscript; Addisu Alemayehu Assegie- reviewing and editing the revised manuscripts.

Declaration of interests

The authors state that they have no known conflicting financial interests or personal ties that might have influenced the work presented in this study.