Recent advances in non-precious metal-based carbon materials for enhanced oxygen reduction and evolution reactions in rechargeable zinc-air batteries

Mechanism of ZABs

ZABs operate on the principle of the redox reactions between zinc metal and atmospheric oxygen, making them a promising energy storage system because of the high theoretical energy density, affordability, and environmental sustainability^[24]. The electrochemical processes in ZABs mainly revolve around the ORR during discharge and the OER during charge, both taking place at the air electrode. Understanding the mechanisms of the ORR and OER is fundamental to the development of efficient electrocatalysts for rechargeable metal-air batteries, particularly for ZABs.

Electrode composition of ZABs

Discharge process (ORR): ORR is a complex multi-electron process that involves the reduction of oxygen molecules to water or hydroxide ions in alkaline media. During the discharge phase of a ZAB, the following reactions take place^[30]:

1. Zinc anode oxidation: Metallic zinc at the anode is oxidized to zinc ions (Zn²⁺), releasing electrons in the process.

Zn → Zn²⁺ + 2e^-

2. Oxygen reduction at the air cathode: Oxygen from the air is reduced at the air cathode in the presence of a catalyst, typically absorbing water from the electrolyte to form hydroxide ions (OH^-).

O₂ + 2H₂O + 4e^- → 4OH^-

3. Formation of zinc hydroxide complex: The zinc ions generated at the anode react with the hydroxide ions produced at the cathode to form a zinc hydroxide complex [Zn(OH)₄^2-], which eventually precipitates out as zinc oxide (ZnO) upon saturation in the electrolyte.

Zn²⁺ + 4OH^- → Zn(OH)₄^2-
Zn(OH)₄^2- → ZnO + 2H₂O + 2OH^-

The overall reaction during the discharge phase is:

2Zn + O₂ → 2ZnO

with a theoretical open circuit voltage of approximately 1.65 V.

However, the reaction can proceed via several pathways, including a two-electron and a four-electron process. The two-electron pathway results in the formation of hydrogen peroxide (H₂O₂) as an intermediate, which is undesirable due to its instability and propensity to decompose into water and oxygen, leading to efficiency losses. The four-electron pathway, on the other hand, directly reduces oxygen to water or hydroxide ions, which is more efficient and preferred for battery applications.

The four-electron pathway can be broken down into individual steps, where the oxygen molecule is initially adsorbed onto the active site of the catalyst (M):

(1). Adsorption of O₂: O₂ + M → O₂^*
(2). First protonation and electron transfer: O₂^* + H₂O + 2e^- → OOH^* + OH^-
(3). Second protonation and electron transfer: OOH^* + H₂O + 2e^- → 2OH^-

Charge process (OER): OER is a multistep reaction that proceeds slowly and requires a high overpotential to occur. During the charging phase, the reactions proceed in reverse:

1. Decomposition of zinc oxide: The zinc oxide formed during the discharge phase is decomposed back into zinc and hydroxide ions.

ZnO + 2H₂O + 2e^- → Zn²⁺ + 4OH^-

2. Oxygen evolution at the air cathode: Hydroxide ions are oxidized at the air cathode, releasing oxygen back into the air.

4OH^- → O₂ + 2H₂O + 4e^-

3. Deposition of zinc at the anode: The zinc ions produced during the decomposition of zinc oxide migrate to the anode and deposit as metallic zinc.

Zn²⁺ + 2e^- → Zn

The overall reaction during the charging phase is the reverse of the discharge phase, effectively restoring the original state of the battery components.

The mechanism of OER is not yet fully understood, but it is generally accepted that it involves the following steps:

(1). Dehydration of OH^-: OH^- + M → OH^*
(2). First electron transfer and dehydration: OH^* + OH^- → O^* + H₂O + e^-
(3). Second electron transfer and dehydration: O^* + OH^- → OOH^* + e^-
(4). Final electron transfer and oxygen evolution: OOH^* + OH^- → O₂ + H₂O + 2e^-

N doping

N doping enhances the reactivity of ZABs significantly by altering the electronic properties and structural characteristics of the catalyst. Specifically, N atoms introduce additional electron density, thereby improving the conductivity and oxygen adsorption capacity of the catalyst. These attributes are closely related to the electrochemical activity of the catalyst.

Research indicates that the degree of N doping significantly affects the electrochemical performance of the catalyst^[61]. An appropriate level of N doping can optimize the catalyst’s activity, whereas excessive doping may lead to a decline in performance. The incorporation of N into a carbon framework, exemplified by the CNT-graphene-like architecture found in NiFeCo-NC materials, can enhance both the electronic characteristics and catalytic efficacy of the material. By doping with N, defects can be introduced into the carbon matrix, which are advantageous for increasing the number of active sites involved in ORR and OER. Furthermore, N serves as an electron donor, capable of modifying the electronic structure of nearby atoms, thus improving the material’s capacity to promote these reactions. This is supported by the reduced potential gap observed between the half-wave potential of ORR and the oxidation potential at a current density of 10 mA·cm^-2 during OER, showcasing better performance when compared to other synthesized substances and reference catalysts. Additionally, the NiFeCo-NC₂ catalyst exhibited remarkable tolerance to methanol, indicating its potential for use in direct methanol fuel cell applications. Besides, innovative catalysts such as iron-based activated carbons (FeACs)-vanadium-based nanostructured catalysts (VNNCs) _(1:1)/N-doped carbon framework (NFC) have been developed for ZABs using a molten salt method involving Fe(NO₃)₃·9H₂O and NH₄VO₃ solutions^[68]. N doping, as illustrated in Figure 1A, enhances the functionality of materials utilized as electrocatalysts in ZABs by boosting their conductivity, catalytic efficiency, and structural characteristics. This enhancement occurs by affecting the adsorption behavior of species that contain oxygen, facilitating electron transfer, and modifying the electronic architecture of metallic elements such as iron and vanadium. Moreover, N doping leads to a more organized carbon structure, an increase in surface area, and improved porosity, which are all essential for effective bifunctional catalysis in ZABs. Theoretical and spectroscopic studies suggest that these interfaces alter the electronic structure close to the Fermi level, thereby optimizing the dynamics of adsorption and desorption during ORR and OER [Figure 1B]. Such advancements could lead to more efficient and durable energy storage solutions. Researchers have developed a novel porous carbon catalyst doped with telluride, referred to as ZnCo-Te@NC, which is synthesized through the pyrolysis of ZnCo-MOF-coated Te nanotubes. The ZnCo-Te@NC exhibits exceptional bifunctional electrocatalytic performance, featuring an ORR half-wave potential of 0.873 V and a low OER overpotential of 400 mV at a current density of 10 mA·cm^-2, surpassing the performance of various non-noble metal catalysts. When employed as an air cathode in a ZAB, it shows a reduced charge-discharge voltage gap of 0.932 V at 50 mA·cm^-2 and achieves a higher peak power density of 259.7 mW·cm^-2, along with remarkable long-term stability over 100 h^[69]. As shown in Figure 1C, the excellent performance is attributed to N-rich configurations, which facilitate the adsorption of O₂ and intermediate products during the ORR process, contributing to a decreased overpotential of the catalytic reaction. Researchers developed FeCo@CNTs-60 via one-step pyrolysis using Co-MOF-67, FeCl₃·6H₂O, and dicyandiamide^[46]. This catalyst displays superior ORR performance in alkaline and quasi-neutral solutions, achieving higher power density and greater stability than commercial Pt/C in ZABs. It demonstrates onset potentials of 1.04 V (alkaline) and 1.02 V (quasi-neutral). A low potential difference (DE) of 0.81 V between OER and ORR indicates good bifunctional catalytic activity. Alkaline ZABs with FeCo@CNTs-60 endured over 1,300 h of ultra-long cycling at 5 mA·cm^-2, showing high voltage efficiency and stability [Figure 1D]. N doping significantly enhances the performance of materials by improving their conductivity, catalytic activity, and structural stability, making them highly effective for use as bifunctional electrocatalysts in ZABs. Specifically, as shown in Figure 1E, N-doped CNTs wrapped with Fe and Co nanoparticles facilitate rapid electron transfer and protect active sites, contributing to superior ORR and OER activities. The presence of N in various forms, such as pyridinic-N, pyrrolic-N, and graphitic-N, increases the number of active sites and modifies the electronic structure of the catalyst, further boosting its catalytic performance. Additionally, N doping aids in maintaining the morphological integrity of the catalyst, leading to enhanced stability and durability under prolonged cycling conditions. CoN nanoparticles anchored on ultrathin nitrogen-doped graphene (UNG) serve as a superior ORR electrocatalyst for stable ZABs^[57]. As shown in Figure 1F, synthesized via 1, 10-phenanthroline coordination and polyethyleneimine (PEI) intercalation, preventing graphene stacking, the catalyst of CoN/UNG exhibits ultrathin, evenly dispersed CoN particles. It facilitates a four-electron pathway for oxygen reduction and retains 96% of its initial current density after 12 h, outlasting Pt/C. In ZABs, CoN/UNG showcases enhanced stability and performance, marking a significant stride in ZAB technology. CoFe decorated bougainvillea-like N-doped carbon nanoflowers (CoFe-NCNFs), synthesized via an amine-uracil-assisted pyrolysis method, display a bougainvillea-like structure of interconnected nanosheets^[81]. This material, as shown in Figure 1G, containing ultrasmall CoFe nanoclusters and N-doped graphitic carbon, exhibits superior tri-functional catalytic activity for ORR, OER, and hydrogen evolution reaction (HER). WN-Ni heterostructures were synthesized via an in-situ polyaniline (PANI) nitridation strategy, where PANI acted as both a carbon and N source, facilitating WN phase formation through ammonia generation^[62]. The catalyst’s hierarchical structure, proper crystallinity, and efficient charge/mass transfer contributed to its superior performance, effectively modulating electron distribution at the WN-Ni interface. A novel Fe-N-C catalyst, CH-FeNC, with expanded carbon layer spacing (> 4 Å) was developed to enhance ORR in ZABs^[93]. Using trehalose, ZIF-8, and PANI, the catalyst was synthesized through co-pyrolysis carbonization. Cao et al. innovates a composite material, Fe₃C@NPW, integrating Fe₃C nanoparticles with N-doped wood-based carbon, serving as an efficient, cost-effective air cathode catalyst for reversible ZABs^[94]. The uniform dispersion of Fe₃C on N-doped carbon optimizes mass transfer and increases the electroactive surface area. Fe₃C@NPW exhibits superior ORR and OER performance, with lower overpotentials and higher electrochemical active surface area than Pt/C. In ZABs, Fe₃C@NPW boosts the open circuit voltage, specific capacity, and power density, outperforming Pt/C + RuO₂. As shown in Figure 1H, its success stems from the synergistic effect of catalytic properties of Fe₃C and conductivity of N-doped carbon.

Figure 1. (A) Graphical abstract of FeACs-VNNCs (1:1)/NFC; (B) Calculated d orbital centers of FeNCs/NFC, VNNCs/NFC and FeACs-VNNCs (1:1)/NFC^[68]; (C) XPS survey spectrum of Zn 2p, N 1s of ZnCo-Te@NC, and Co 2p XPS spectrum of ZnCo-Te@NC and ZnCo@NC^[69]; (D) Performance of zinc-air batteries assembled by FeCo@CNTs-60; (E) Stabilized adsorption configurations of ORR/OER intermediates^[46]; (F) TEM and HRTEM (inset) images of CoN/UNG^[57]; (G) HRTEM images of CoFe-NCNFs^[81]; (H) Illustration of Fe₃C@NPW^[94]. NFC: N-doped carbon framework; FeACs: iron-based activated carbons; VNNCs: vanadium-based nanostructured catalysts; XPS: X-ray photoelectron spectroscopy; CNTs: carbon nanotubes; ORR: oxygen reduction reaction; OER: oxygen evolution reaction; TEM: transmission electron microscopy; HRTEM: high-resolution transmission electron microscopy; NCNFs: N-doped carbon nanoflowers.

By incorporating N into carbon-based materials, researchers have successfully tailored the electronic properties of these materials to improve their catalytic performance for ORR and OER in ZABs. The strategic use of N doping offers a promising avenue for enhancing the efficiency and durability of electrocatalysts, which is critical for the advancement of renewable energy technologies.

S doping

S-doped carbon materials, such as S-doped CNFs, through the introduction of S, improve the electron affinity of the catalyst and enhance the activation and desorption of oxygen intermediates, thus boosting catalytic performance^[28,95]. Researchers developed ternary sulfides (MInS/NH₂-CNT, M = Co, Fe, or Mn) based on super-tetrahedral metal sulfide clusters (T4 clusters) as efficient bifunctional electrocatalysts for OER and ORR^[28]. Prepared through a mild self-assembly method using electrostatic interactions, CoInS/NH₂-CNT showed notable electrocatalytic performance. This strategy extends to other cluster-based sulfide materials, offering NPM alternatives for energy applications. The synthesis involved mixing T4 crystals with NH₂-CNT at room temperature, facilitated by ultrasonication and magnetic stirring. CoInS/NH₂-CNT demonstrated excellent OER and ORR activity, highlighting the synergy between T4 clusters and NH₂-CNT. This study provides an effective approach for constructing advanced sulfide materials for various applications. Transition metal sulfides, such as Co₉S₈ and FeNi sulfide nanoalloys, have proven to be efficient bifunctional electrocatalysts due to their unique electronic structures and high conductivity^[96]. Encapsulated in S, N co-doped carbon, Co₉S₈ not only boosts the catalytic performance of ORR and OER but also exhibits good cycling stability and resistance to methanol poisoning. FeNi sulfide nanoalloys, through S modulation, achieve ORR performance comparable to Pt/C while maintaining excellent OER activity, making them ideal electrode materials for ZABs. As shown in Figure 2A, Zhang et al. highlights a cobalt sulfide/multi-heteroatom doped porous carbon composite, created through a single-step sulfidation method using ZIFs, which acts as a highly effective trifunctional electrocatalyst for ORR, OER, and HER^[89]. Its three-dimensional (3D) architecture enhances mass transport and exposes abundant active sites. Performance-wise, it exhibits superior ORR and OER activities, with low overpotentials and high stability, rivaling commercial Pt/C and outperforming other samples. The catalyst’s stability is highlighted by negligible performance decay after thousands of cycles. Applied in ZABs, it delivers high power density and energy efficiency, exceeding commercial Pt/C-IrO₂-based batteries. The achievement is ascribed to the distinct architecture that facilitates effective transport of electrons and ions, the presence of several active sites, and the cooperative influence of heteroatoms along with cobalt sulfide.

Figure 2. (A) Illustration of Co₉S₈/CoNSC^[89]; (B) Illustration of Cu₃P/MoP@C; (C) Structural characterization of Cu₃P/MoP@C, Cu₃P@C, MoP@C^[97]; (D) Illustration and morphology characterization of A-MnO₂/NSPCs; (E) Optimized structures for the adsorbed O atom on A-MnO₂/NSPC and NSPC, DOS and the free energies versus the reaction coordinates toward ORR on NSPC and A-MnO₂/NSPC; (F) High-resolution XPS spectra: N 1s, S 2p, and P 2p of A-MnO₂/NSPC-2^[98]. NSPC: N, S, P co-doped carbon sphere; OER: oxygen evolution reaction; HER: hydrogen evolution reaction; ORR: oxygen reduction reaction; DOS: density of states; XPS: X-ray photoelectron spectroscopy; N: nitrogen; S: sulfur; P: phosphorus.

P doping

P-doped carbon materials, including P-doped CNFs, alter the electronic structure of carbon materials by incorporating P, further optimizing the kinetics of ORR^[97]. For instance, as shown in Figure 2B, a nanostructured composite made of copper phosphide and molybdenum phosphide, doped with P (Cu₃P/MoP), has been developed to serve as an efficient electrocatalyst for the ORR in ZABs^[97]. Fabricated through an impregnation and high-temperature phosphidation process, Cu₃P and MoP are co-anchored onto hollow porous carbon spheres, yielding the Cu₃P/MoP@C catalyst. This distinctive architecture not only furnishes abundant active sites but also facilitates electronic and mass transport through its porosity. The Cu₃P/MoP@C exhibits a half-wave potential of 0.90 V, surpassing the 0.84 V of commercial Pt/C. X-ray photoelectron spectroscopy (XPS) demonstrates that the significant electronic interactions occurring between Cu₃P and MoP species are crucial for its excellent ORR performance. Moreover, the large specific surface area and mesoporous structure of Cu₃P/MoP@C allow for enhanced contact with the electrolyte, accelerating ORR kinetics [Figure 2C]. This triumph is attributed to the synergistic effect between Cu₃P and MoP, coupled with the porous structure that facilitates more efficient electron transfer, thus enhancing overall ZAB performance.

Multi-doping

Multi-doped carbon materials, such as N, S co-doped carbon and N, P co-doped carbon^{[67,77,98-105]}, through synergistic effects, not only increase catalytic activity but also enhance the stability of the catalyst. S, N co-doped porous carbon materials, such as hollow porous carbon spheres with embedded S, N co-doped Fe-N_x species, have exhibited high-efficiency ORR activity^[101]. The modulation of S atoms optimizes the Fe-N_x active sites, improving catalytic efficiency and stability. The high power density and extended cycling stability demonstrated by these materials in ZABs confirm the efficacy of the S, N co-doping approach. A sustainable, one-step method based on tofu was employed to create a scalable, highly effective, and durable catalyst^[77]. This involved creating Fe/Co cross-linked tofu gels followed by thermal processing. The resultant material, enriched with dispersed FeCo phosphide nanoparticles and a N-P co-doped porous carbon network, showed enhanced tri-functional activity for ORR, OER, and HER, outperforming RuO₂ and Pt/C benchmarks in long-term stability. This tofu-inspired catalyst, with its superior tri-functional activity and durability, holds promise for advanced energy conversion systems. As shown in Figure 2D, a crafted structure-amorphous MnO₂ lamellae encapsulated covalent triazine polymer-derived N, S, P co-doped carbon sphere (A-MnO₂/NSPC) was synthesized using self-doping pyrolysis and in-situ coating, yielding a composite with enhanced electronic properties and abundant catalytic sites^[98]. Density functional theory (DFT) calculations and experiments confirmed that surface defects of amorphous MnO₂ and heteroatom sites boost oxygen electrocatalysis [Figure 2E]. Flexible all-solid-state ZABs displayed stability over 140 cycles. As shown in Figure 2F, XPS data showed increasing Mn:O ratios with KMnO₄ addition, verifying MnO₂ formation on NSPC. The surface area of A-MnO₂/NSPC-2 measured 701.5 m²·g^-¹, featuring a hierarchical porous configuration that improved the exposure of active sites and facilitated mass transport. This composite offers significant improvements for ZABs and energy storage systems.

As shown in Figure 3A, an enhanced nanoemulsion method synthesized Co₂P nanoparticles in N- and P-doped carbon nanospheres (Co₂P/NP-C) for ZABs^[99]. Co₂P/NP-C-800 exhibited impressive ORR performance, demonstrated by a half-wave potential of 0.81 V and a limiting current density of 4.54 mA·cm^-2, comparable to commercial Pt/C. As depicted in Figure 3B, a ZAB utilizing Co₂P/NP-C as the catalyst for the air electrode attained a maximum power density of 152.4 mW·cm^-2 along with a significant specific energy density. The exceptional properties of the catalyst can be attributed to the in situ generation of Co₂P nanoparticles, coupled with the elevated surface area of N and P-doped carbon. This combination offers numerous active sites and facilitates effective mass and charge transport for ORR. As shown in Figure 3C, CoFe/Se@CN, a novel bifunctional catalyst for ZABs, enhances ORR and OER^[67]. Prepared through a hydrothermal and calcination process, it exhibits superior electrochemical performance. As depicted in Figure 3D, this catalyst, which is made of NPM, exhibits a minimal DE of 0.625 V, positioning it as a viable option for effective energy storage solutions. As shown in Figure 3E, a new porous carbon material co-doped with N and S, featuring embedded Co₉S₈ nanoparticles (Co₉S₈/NSC), has been created to serve as a bifunctional electrocatalyst for rechargeable ZABs^[106]. Synthesized via a facile one-step pyrolysis process using Co-formic acid framework (Co-FF)-derived Co₃O₄ as a precursor and thiourea as a dopant, the Co₉S₈/NSC-1 variant exhibits superior ORR and OER activity. This catalyst exhibits outstanding catalytic performance, featuring a half-wave potential of 0.83 V for ORR and an overpotential of 300 mV for OER at a current density of 10 mA·cm^-2. The nanoparticles of Co₉S₈ are consistently spread throughout a 3D mesoporous carbon structure, which offers a significant surface area and numerous active sites for electrochemical processes. Co-doping with N and S additionally improves the electronic configuration and wettability of the carbon matrix, promoting quicker mass transfer of oxygen and enhanced electron conduction. As depicted in Figure 3F, the Co₉S₈/NSC-1 catalyst, with its distinct blend of structural characteristics and chemical makeup, achieves a peak power density of 102.0 mW·cm^-2 in ZABs, demonstrating a remarkable cycle life of up to 500 cycles. The remarkable electrochemical efficiency and enduring stability of Co₉S₈/NSC-1 position it as a viable substitute for catalysts based on precious metals.

Figure 3. (A) Illustration of Co₂P/NP-C-800; (B) ZABs performance based on Co₂P/NP-C-800 and Pt/C: Galvanostatic discharging specific capacity at 10 mA·cm^-2. Galvanostatic discharging curves at different current densities. Lighted LED powered by two Co₂P/NP-C-800-based ZABs in series^[99]; (C) Illustration and (D) LSV curve of CoFe/Se@CN in the entire OER/ORR region^[67]; (E) Illustration and (F) ZABs performance of Co₉S₈/NSC-1: galvanostatic discharge and charge cycling curves for ZAB@Co and ZAB@Pt + Ru^[106]. ZABs: Zinc-air batteries; LED: light emitting diode; LSV: linear sweep voltammetry; ORR: oxygen reduction reaction; OER: oxygen evolution reaction.

Figure 4A illustrates the development of the trifunctional electrocatalyst Co₉S₈ encapsulated in S and N co-doped carbon (Co₉S₈@SNC), which shows impressive catalytic performance for ORR, HER and OER^[102]. A strong coordination interaction at the molecular level between Co²⁺ and C@S was utilized to synthesize the catalyst, which was then subjected to a calcination process. This approach guarantees a consistent distribution and close interaction between the active Co₉S₈ nanoparticles and the carbon matrix, thereby improving electron transfer. The Co₉S₈@SNC catalyst demonstrates a half-wave potential of 0.846 V for ORR, along with overpotentials for OER at 320 mV when measured at 10 mA·cm^-2. The carbon shell co-doped with S and N enhances the material’s conductivity, whereas the Co₉S₈ core offers sites that are active in catalysis. The catalyst shows excellent durability, maintaining its activity and structural integrity after thousands of cycles in ORR tests. The improved catalytic performance can be attributed to the collaborative interaction between the Co₉S₈ core and the carbon shell that is co-doped with S and N. As shown in Figure 4B, the co-doping of S and N alters the electronic properties of the carbon matrix, enhancing its wettability and facilitating the transport of reactants.

Figure 4. (A) Graphical abstract and (B) XPS spectrum of Co₉S₈@SNC^[102]; (C) Illustration of CoNi@GO; (D) Specific capacities plots of CoNi@GO and Pd/C + RuO₂. Long-term galvanostatic discharge charge cycling curves of rechargeable CoNi@GO and Pt/C + RuO₂-based ZABs^[56]; (E) Illustration and (F) Galvanostatic discharge plots, galvanostatic discharge-charge cycling curves of ZABs with CoFe-N-CNTs/CNFs-900 or IrO₂/C + Pt/C as catalyst^[107]; (G) Synthesis of Co/BN-CNT/VN-800 derived from Co₆V₁₂B₁₈-POM^[47]. XPS: X-ray photoelectron spectroscopy; GO: graphene oxide; ZABs: zinc-air batteries; CNTs: carbon nanotubes; CNFs: carbon nanofibers; OER: oxygen evolution reaction; HER: hydrogen evolution reaction; ORR: oxygen reduction reaction.

Preparation methods and structural tuning

The preparation methods for NPM-based carbon materials are diverse, each with its own unique advantages and applicability. For example, researchers created a 3D cyano-bridged CoNi complex-derived CoNi@graphene oxide (GO) bifunctional catalyst for oxygen redox reactions in ZABs via pyrolysis [Figure 4C]^[56]. DFT calculations revealed a synergistic coupling between the CoNi alloy and the carbon matrix. As depicted in Figure 4D, the air electrode exhibited a minimal charge-discharge voltage difference of 0.91 V, an impressive energy efficiency of 59.5%, and maintained stable cycling for more than 240 h at a current density of 10 mA·cm^-2. Conversely, continuous CNFs can be generated through electrospinning, whereas the solvothermal and hydrothermal techniques are appropriate for creating nanostructures with particular shapes and dimensions^[28,107,108]. As shown in Figure 4E, a self-catalytic growth method was employed to synthesize 3D nano-forest structured carbon hybrid materials (CoFe-N-CNTs/CNFs-900) using metal-polymer nanofiber precursors and melamine via controlled pyrolysis^[107]. The process involved electrospinning to prepare CoFeZn-polyvinylpyrrolidone (PVP) nanofibers, followed by high-temperature pyrolysis under a hydrogen/argon atmosphere. The engineered substance exhibits distinct structural and compositional benefits, such as a 3D architecture, structures resembling synapses, porous characteristics, a high level of N doping, and bimetallic active elements. These attributes provide the material with stability in structure, efficient mass and electron transport abilities, and an extensive active surface area [Figure 4F]. Templating techniques, using hard or soft templates, guide the ordered growth of materials, whereas laser-induced carbonization and spray pyrolysis can fabricate materials with high specific surface area and rich porosity in a shorter time frame^[106,109]. As shown in Figure 4G, a novel multicomponent catalyst, Co/BN-CNT/VN, was synthesized for rechargeable ZABs, exhibiting superior bifunctional electrocatalytic performance^[47]. Derived from Co-modified boron-rich polyoxometalates, it demonstrates an onset potential for ORR of 0.96 V and an overpotential of 296 mV for OER at a current density of 10 mA·cm^-2. The catalyst attains a maximum power density of 156.3 mW·cm^-2 and a specific capacity of 777 mAh·g, demonstrating impressive cycling stability across 1,000 cycles. Due to its abundant catalytic sites, effective mass transport, and optimal electronic configuration, it presents a viable option to noble metal catalysts in the realm of advanced energy storage systems.

Structural tuning is pivotal to enhancing catalytic performance. Careful manipulation of porosity, specific surface area, surface functional groups, and the arrangement of heteroatoms enhances the catalyst’s electronic structure, improving the adsorption and activation of oxygen molecules, which in turn boosts the catalytic activity for ORR/OER. Adjusting calcination temperature and duration regulates thedegree of graphitization and heteroatom content; employing metal salts and N-containing compounds as precursors introduces metals and N into carbon materials, forming highly efficient active sites.

The superior performance of multi-doped materials in ZABs can be attributed to several key factors: (1) Optimization of electronic structure: The introduction of elements such as S and N modifies the electronic configuration of the materials, enhancing the electron density at active sites and promoting the electron transfer process during the ORR/OER; (2) Enhanced conductivity: The introduction of sulfides and phosphides significantly increases the conductivity of the materials, reducing the internal ohmic resistance of the battery and contributing to improved overall efficiency; (3) Stability and durability: Rational material design leads to good chemical and mechanical stability, ensuring consistent battery performance over extended operational periods. Multi-doped materials, especially composites containing S, N, and transition metal elements, offer efficient, stable, and cost-effective electrocatalyst solutions for ZABs.

While NPM-based carbon materials show tremendous potential in ZABs, they still face the following challenges: (1) Ways to enhance both the activity and long-term stability of catalysts, particularly when subjected to high current densities; (2) The cost and environmental impact of large-scale production must be considered; (3) Further optimization is needed for the overall performance of ZABs, which encompasses energy density, cycle life, and cost-effectiveness.

Utilizing NPM-derived carbon materials in ZABs offers innovative approaches to tackle the kinetic challenges associated with ORR and OER. Through material design, structural tuning, and performance optimization, these materials demonstrate excellent catalytic activity and stability, driving ZABs toward higher energy density, longer cycle life, and lower production costs. Future research directions may include developing novel preparation methods to reduce production costs, exploring more efficient NPM catalyst systems, optimizing the compatibility of catalysts with electrolytes, and refining battery design to enhance overall performance.

Preparation methods and structural tuning of MOF-derived carbon materials

CNTs

Figure 5A illustrates the development of a new technique for producing Co/VN nanoparticles that are encapsulated within a carbon matrix through the carbonization of CoV-MOF-dicyandiamide composites^[116]. The Co/VN nanoparticles (NPs)@C, measuring between 4 and 8 nm, demonstrate an extensive surface area and a conductive structure that enhances electrochemical performance. Its in-situ formed Co and VN nanoparticles in synergy with fast electron transfer through the carbon layer deliver exceptional tri-functional catalytic performance. In rechargeable ZABs, Co/VN NPs@C serves as an electrode material, which demonstrates impressive efficiency, achieving a peak power of 130 mW·cm^-2, a capacity of 757 mAh·g^-1, and enhanced stability in comparison to batteries based on Pt/C and RuO₂ [Figure 5B]. Co/VN NPs@C stands out as a promising candidate for cutting-edge energy storage technologies because of its low-voltage functionality, exceptional efficiency, and sustained reliability in both water splitting and ZABs. CNTs derived from MOFs have shown significant improvements in ORR and OER catalytic activities. As shown in Figure 5C, a novel bifunctional electrocatalyst for ZABs has been developed by leveraging MOFs as precursors^[117]. This groundbreaking catalyst consists of CNTs adorned with multiple active sites, which feature both dispersed Fe-N and encapsulated ultrafine Fe. The catalyst, FeNP@Fe-N-C, was created via a one-step pyrolysis process of MOFs, leading to a distinctive architecture characterized by a high surface area and remarkable porosity. FeNP@Fe-N-C demonstrates outstanding performance in both ORR and OER, featuring a minimal potential gap of 0.73 V, which outperforms commercial Pt/C and RuO₂ electrodes. This remarkable bifunctional activity is attributed to the presence of metallic Fe nanoparticles that enhance oxygen adsorption and activation, aided by the increased O₂ adsorption capability of the Fe-N sites [Figure 5D]. The combined influence of single-atom iron and enclosed nanoparticles, along with the porous structure of CNTs, enhances electron transfer and mass transport, resulting in greater catalytic efficiency. As shown in Figure 5E, a hierarchically structured bifunctional oxygen electrocatalyst, Mo_1-2C/Co-encased CNTs, was synthesized through a hydrothermal-melamine-assisted carbonization method for rechargeable ZABs^[118]. This catalyst exhibits a core-shell architecture, in which the core consists of Mo₂C/MoC, and the shell is made up of Co nanoparticles that are encased within N-doped CNTs [Figure 5F]. The electrocatalytic performance of this catalyst is markedly enhanced due to the multicomponent synergy. Furthermore, the catalyst demonstrates impressive durability, allowing for uninterrupted charging and discharging over 275 h. The cooperative interaction between the external Co@CNTs and the internal Mo₂C/MoC is crucial for enhancing the electrocatalytic efficiency, fine-tuning the adsorption energy of oxygen intermediates, and speeding up the reaction kinetics. The hierarchical structure facilitates mass transfer and electron conduction, enhancing the catalytic efficiency of the material.

Figure 5. (A) Illustration and (B) ZABs performance of Co/VN NPs@C^[116]; (C) Illustration and (D) Graphical abstract of FeNP@Fe-N-C^[117]; (E) Illustration; (F) morphology and structure characterization of Mo₂C/MoC/Co@CNTs^[118]. ZABs: Zinc-air batteries; NPs: nanoparticles; CNTs: carbon nanotubes; ORR: oxygen reduction reaction; OER: oxygen evolution reaction.

Carbon foams

An innovative Co-CoO heterostructure, obtained from nanoscale ZIF-67, is extensively exposed on N-doped porous carbon foam (NPCF) and has been created as a highly effective electrocatalyst for ZABs^[120]. As shown in Figure 6A, the catalyst was created using a self-sacrificial pyrolysis method, resulting in Co-CoO active sites that are both highly dispersed and well-anchored. The distinct architecture of the Co-CoO/NPCF electrocatalyst enhanced mass and electron transfer capabilities. When integrated into rechargeable ZABs, the Co-CoO/NPCF air-cathode displayed significant power densities of 214.1 mW·cm^-2 for liquid-state ZABs and 93.1 mW·cm^-2 for flexible all-solid-state variants, with remarkable cycle stabilities of 600 and 140 cycles, respectively. As demonstrated in Figure 6B, calculations using DFT indicated that a robust electronic interaction exists between Co-CoO and NPCF, enhanced by the presence of numerous C-N_x sites. This interaction altered the electronic structure, playing a significant role in the exceptional electrocatalytic performance.

Figure 6. (A) Illustration and (B) chemical structure of Co-CoO/NPCF^[120]; (C) Graphical abstract of h-Ti₃C₂T_x@Co-NCNT^[121]; (D) Illustration and (E) ZABs performance of Co/CoO@HNC^[122]; (F) Schematic illustrating the preparation of the P-CoSe₂/C@CC catalyst; (G) SEM images of Co-MOF@CC, Co/C@CC, CoSe₂/C@CC, and P-CoSe₂/C@CC^[124]. NPCF: N-doped porous carbon foam; ZABs: zinc-air batteries; ORR: oxygen reduction reaction; OER: oxygen evolution reaction.

Preparation methods and structural tuning of MOF-derived metal oxides/nitrides

As illustrated in Figure 6D, a new engineering method has been devised to construct Co/CoO heterojunctions integrated within mulberry-like open-carbon nanocages for ZABs^[122]. Employing a MOF as a sacrificial template in-situ, this approach generates a distinctive structure comprising numerous N-doped carbon nanosphere subunits and accessible mass transfer channels. The resulting Co/CoO@HNC catalyst demonstrates improved oxygen electrocatalytic performance, attaining a DE of 0.83 V, which notably surpasses that of noble-metal-based Pt/C. The hollow architecture resembling mulberries in the catalyst provides a greater exposed internal volume and more available active sites, thereby promoting effective mass transport and electron transfer. The heterojunction and N-doping further contribute to the electronic properties, improving its catalytic performance. Performance metrics indicate that the Co/CoO@HNC catalyst exhibits both high power density and stability when used in ZABs, highlighting its potential for real-world applications [Figure 6E]. The combined influence of the open architecture, heterojunction formation, and N doping allows the catalyst to sustain significant activity and longevity over prolonged durations, positioning it as a viable option for advanced energy storage systems.

A new electrocatalyst, CoMn₂O₄/C-NH₂-300, was developed through the in-situ formation of cobalt manganate spinel nanodots on carbon black that has been functionalized with amine, using 3-aminopropyltriethoxysilane as a multifunctional agent^[123]. This method not only achieved amine-functionalization of the carbon support but also served as a weak alkali for precipitating metal hydroxides that transform into spinel nanodots upon calcination. The resulting catalyst demonstrates a significant dispersion of CoMn₂O₄ nanoparticles, with an average size of around 3 nm, which provides a substantial surface area for improved electrochemical activity. CoMn₂O₄/C-NH₂-300 exhibits exceptional bifunctional catalytic capabilities for oxygen electrode reaction. It features a minimal overpotential for OER and DE value (E_1/2-E_j10) of 0.75 V, surpassing many advanced catalysts in the field. Furthermore, this catalyst displays remarkable stability and tolerance to methanol, retaining 84.3% of its initial current density after a durability test lasting 40,000 s. The enhanced catalytic performance can be linked to robust interaction between the spinel nanodots and the carbon skeleton, facilitating electron transfer and improving catalytic kinetics.

Preparation methods and structural tuning of MOF-derived P-doped materials

Figure 6F illustrates that a cobalt selenide (CoSe₂) electrocatalyst, which is doped with P, has been created on carbon cloth by employing a solvothermal method, succeeded by processes of selenization and phosphatization^[124]. This cuboid-like formation provides a significant surface area that promotes optimal catalytic activity and stability. The catalyst known as P-CoSe₂/C@CC exhibits exceptional performance in OER, featuring an overpotential of 303.1 mV @ 10 mA·cm^-2, and it shows a distinct reduction peak during ORR evaluations. When incorporated into a ZAB, P-CoSe₂/C@CC displays a peak power density of 124.4 mW·cm^-2, underscoring its promise for energy storage applications. The improved catalytic performance is ascribed to the synergistic influence of P doping, which alters the electronic structure of CoSe₂, facilitating quicker electron transfer and enhancing catalytic efficiency [Figure 6G].

Preparation methods and structural tuning of MOF-derived multifunctional composites

In the field of energy conversion and storage, a novel trilayer MOF comprising iron (Fe), cobalt (Co), and nickel (Ni) has been created to serve as a multifunctional electrocatalyst^[125]. The synthesis of the trilayer MOF utilized a reductive electrosynthesis methodology, which facilitated the layer-by-layer construction of metal cations associated with 2-amino-1, 4-benzenedicarboxylic acid linkers [Figure 7A]. This process led to the creation of a highly porous material displaying exceptional trifunctional electrocatalytic properties for OER and ORR, which is primarily ascribed to the synergistic interaction among the constituent elements and the porous architecture that supports the effective use of active sites and efficient bubble formation. When applied as an air cathode in a ZAB, as illustrated in Figure 7B, the MOF yielded a power density of 161 mW·cm^-2, accompanied by an exceptional specific energy of approximately 945 Wh·kg_Zn^-1. Furthermore, as indicated in Figure 7C, DFT calculations revealed that the initial negative adsorption energy of water on the metal nodes and the extended O–H bond length in the H₂O molecule play significant roles in enhancing the catalytic activity.

Figure 7. (A) Nanoarchitectonics for preparing multifunctional MOFs; (B) ZABs performance and (C) DFT calculations of Fe−Co−Ni MOF structures^[125]; (D) Graphical abstract of FeCo@1D-CNTs/2D-NC; (E) SEM images of Zn-ZIF, ZnFeCo-ZIF and FeCo@1D-CNTs/2D-NC, TEM and HRTEM image of FeCo@1D-CNTs/2D-NC^[111]. ZABs: Zinc-air batteries; DFT: density functional theory; CNTs: carbon nanotubes; SEM: scanning electron microscopy; TEM: transmission electron microscopy; OER: oxygen evolution reaction; HER: hydrogen evolution reaction; ORR: oxygen reduction reaction.

A particular 3D carbon nanomaterial with a hierarchical porous structure was developed through a process derived from MOFs, specifically involving the carbonization of ZIF-8 via a silica-template technique along with the growth of CNTs^[126]. This material, referred to as mFeNC-CNT, contained Fe-based nanoparticles and Fe-N_x sites dispersed at the atomic level within its N-doped graphitic carbon matrix. The 3D porous configuration of mFeNC-CNT greatly diminished charge transport resistance, while the intertwined CNTs further facilitated ion and electron diffusion pathways, thereby improving electrochemical performance. This material exhibited remarkable catalytic activity for ORR, similar to that of commercial Pt/C, with a half-wave potential of 0.908 V. When utilized in ZABs, mFeNC-CNT produced a substantial open-circuit voltage of 1.556 V, demonstrating its high efficiency. The superior ORR activity of mFeNC-CNT can be attributed to its optimized structure, which facilitates faster reaction kinetics.

Porous N-doped carbon composites derived from zinc MOFs have shown outstanding performance in ORR and OER, with enhanced power density and open circuit voltage in assembled ZABs^[127,128]. A novel NPM catalyst, Co NCs/HPNC, was developed for ZAB cathodes via a carboxylate-assisted strategy^[110]. Prepared by pyrolyzing MOFs under inert gas, the catalyst features high surface area and a mesoporous structure, enhancing mass transport and active site exposure. In alkaline conditions, Co NCs/HPNC demonstrated enhanced ORR activity electrochemically, exhibiting a half-wave potential of 0.88 V, which is attributed to cobalt’s nanoscale size, hierarchical porosity, and large surface area, facilitating electrolyte contact and active center utilization. As illustrated in Figure 7D, a new catalyst formed from FeCo alloy nanoparticles is encapsulated within a carbon nanostructure that combines one-dimensional (1D) and two-dimensional (2D) features. This catalyst was synthesized through high-temperature annealing of ZnFeCo-ZIF precursors^[111]. Consequently, this process yields a composite consisting of N-doped CNTs along with a porous carbon matrix [Figure 7E]. The structure of the catalyst offers an extensive surface area, numerous active sites, and effective electron transfer, resulting in elevated catalytic efficiency and robustness. When integrated into batteries, it attains a significant open-circuit voltage, specific capacity, and power density, highlighting its promise for sophisticated energy storage solutions.

As shown in Figure 8A, a high-performance bifunctional catalyst for rechargeable ZABs was developed, engineered through oxygen plasma treatment of MOF-derived Co/FeCo@Fe(Co)₃O₄ heterojunctions^[112]. This treatment optimizes the surface structure, creating oxygen vacancies and additional active sites, thereby boosting both ORR and OER electrocatalytic activity. The optimized catalyst, P-Co₃Fe₁/NC-700-10, exhibits superior ORR and OER performance with reduced Tafel slopes and minimal charge transfer resistance, attributed to enhanced surface defect density facilitating peroxide adsorption. The plasma treatment’s role in surface modification and defect creation underpins the catalyst’s enhanced electrochemical performance [Figure 8B]. Figure 8C demonstrates the development of a high-performance electrocatalyst for ORR by synthesizing a porous N-doped carbon composite derived from a manganese-doped zinc MOF (30-ZnMn-NC)^[129]. As shown in Figure 8D, the composite was synthesized through the pyrolysis of MOF precursors, leading to a material characterized by a distinctive porous architecture and bimetallic active sites of Mn/Zn-N. The 30-ZnMn-NC demonstrated remarkable kinetic current density, which can be ascribed to its porous design, which enhances surface area and promotes electrolyte infiltration, coupled with the robust electronic interactions between the metal centers and N-doped carbon. This interaction optimizes the adsorption energy of oxygen intermediates, accelerating reaction kinetics. The bimetallic active sites further enhance the catalyst’s performance by providing multiple pathways for electron transfer.

Figure 8. (A) Illustration of P-Co₃Fe₁/NC-700; (B) ORR/OER catalytic reaction process and optimized adsorption geometry of O₂, ^*OOH, ^*O, and ^*OH on Co₃Fe₁, Co₃Fe₁/700-5, Co₃Fe₁/700-10, and Co₃Fe₁/700-20^[112]; (C) Illustration of ZnMn-NC^[129]; (D) The preparation process of Co₉S₈/NSCP through polymer hierarchical self-assembly, metal-organic coordination and subsequent space-confined pyrolysis; (E) FESEM, TEM and HRTEM image of Co₉S₈/NSCP^[130]. OER: Oxygen evolution reaction; ORR: oxygen reduction reaction; TEM: transmission electron microscopy; HRTEM: high-resolution transmission electron microscopy; FESEM: field emission scanning electron microscopy.

An innovative bifunctional electrocatalyst for oxygen, Co₉S₈/NSCP, was developed using a controlled MOF-driven technique that incorporates polymer self-assembly, metal-organic coordination, and pyrolysis within a confined space^[130]. As illustrated in Figure 8E, the catalyst comprises ultrafine nanocrystals of Co₉S₈, approximately 6 nm in size, which are embedded in carbon nanoplates that have been multilayer-assembled and co-doped with N and S. The Co₉S₈/NSCP displays remarkable catalytic performance for oxygen electrode reaction, which can be attributed to the synergistic interaction between the highly exposed Co₉S₈ nanocrystals and the 3D carbon superstructure enriched with heteroatoms. Moreover, the NSCP matrix offers a large surface area, protective graphitic layers, and a wealth of active sites, all of which contribute to effective electron transfer and mass transport, enhancing both catalytic activity and stability.

MOFs as electrocatalysts in ZABs have attracted considerable attention due to their unique properties, including high porosity and tunable structures, which can enhance both catalytic activity and stability. However, the application of MOFs in ZAB systems presents several challenges. (1) Stability: MOFs often exhibit poor chemical and mechanical stability under the harsh operating conditions of ZABs, leading to degradation over time; (2) Conductivity: The intrinsically low electrical conductivity of MOFs can impede electron transfer, thereby reducing overall battery performance; (3) Catalytic efficiency: Although promising, MOFs must achieve higher efficiencies for the ORR and OER, which are critical processes in ZABs. The advancement of MOFs in ZABs is marked by trends toward precise structural control and functionalization, aiming to optimize catalytic activity and selectivity through customized pore sizes, shapes, and functional groups. Efforts are also directed toward enhancing the stability and durability of MOFs, with a focus on hydrolytic stability, corrosion resistance, and improved mechanical strength. (1) Structural engineering: Modifying MOF structures through the use of robust linkers or incorporating metal centers that exhibit greater resistance to oxidation can significantly enhance stability; (2) Hybrid materials: The integration of MOFs with conductive materials, such as CNTs or graphene, can improve electrical conductivity while preserving the advantageous properties of MOFs; (3) Surface functionalization: Adjusting the surface chemistry of MOFs can optimize the active sites, thereby enhancing catalytic performance.

The development trend is increasingly oriented toward the design of multifunctional MOFs that can act as both efficient catalysts and stable components within battery systems. This includes: (1) Intelligent design: Employing computational methods to predict optimal MOF designs prior to synthesis; (2) Nanotechnology integration: Merging MOFs with other nanomaterials to develop hybrid systems that exploit synergistic effects; (3) Scalability: Emphasizing scalable synthesis techniques that can produce MOF-based materials in an economical and sustainable manner. These trends collectively position MOFs as key enablers for overcoming the limitations of traditional ZABs, paving the way for more efficient, stable, and economically viable energy storage solutions.

Iron-based oxides and carbon composites

Iron-rich oxides are preferred due to their availability and affordability. Researchers have improved the activities of catalysts related to the ORR and OER by creating interfaces between iron-group metals and manganese dioxide (MnO₂) within porous carbon nanowires, thereby enhancing the efficiency of flexible ZABs. For example, as shown in Figure 10A, a novel fluorine-doped FeWO₄/NC catalyst was developed to improve the oxygen reduction efficiency in ZABs^[149]. A fluorine-doping approach was utilized to construct the catalyst, embedding fluorine-doped FeWO₄ particles within a multi-dimensional NFC. The fabrication process involved the use of ammonium fluoride as the fluorine source, creating defect structures and providing in-situ fluorine doping during high-temperature processing. The FeWO₄/NC catalyst doped with fluorine exhibited enhanced hydrophilicity, attributed to the increased polarity of its chemical bonds. This enhancement promoted the adsorption of reaction intermediates and the diffusion of the electrolyte. Consequently, the catalyst demonstrated exceptional performance in ORR, characterized by a significant half-wave potential and remarkable stability. When utilized in ZABs, this catalyst revealed a high power density and sustained cycle stability, facilitating an effective energy conversion system. As proven in Figure 10B, the enhanced catalytic activity was attributed to the synergistic effects of fluorine doping and the N-doped carbon matrix, which together improved the electronic structure and surface properties of FeWO₄, promoting faster electron transfer and reaction kinetics.

Figure 10. (A) Illustration of F-FeWO₄/NC; (B) Free energy diagram at 1.23 eV for ORR over (111) surfaces during 4e^- ORR and the Fe-O bond length after OOH being absorbed on (111) surfaces of FeWO₄ and F-FeWO₄^[149]; (C) The synthetic scheme of Co₃O₄@ND-CN; (D) The partial density of states of ND-CN and Co₃O₄ in Co₃O₄@ND-CN^[150]; (E) Schematic diagram of preparation procedure for 300NiFe-Mi-C^[151]. ORR: Oxygen reduction reaction.

Manganese-based oxides and carbon composites

Manganese-based oxides, particularly MnO₂, are extensively studied for their high catalytic efficiency in oxygen electrode reaction^[147,148]. Combined with carbon substrates, MnO₂ demonstrates enhanced electronic transport and stability, boosting the power density and energy efficiency of ZABs. Tuning the crystallinity and morphology of MnO₂ can further refine its catalytic performance, for instance, by fabricating hierarchically porous MnO₂ to increase reactant contact area and mass transport efficiency, enhancing the electrochemical performance of ZABs. Iron-doped manganese oxide in the form of nanowires (Fe-MONW)/CNF composites were synthesized through a simple, cost-effective method^[148] Fe-MONW/CNF showed superior electrocatalytic performance over CNF and MONW alone. Iron-doped MnO₂ nanowires combined with CNF significantly improved OER and ORR performance, lowering onset potentials. Defects created by iron doping enhanced oxygen adsorption and catalytic activity. The material 5Fe-MONW/CNF exhibited the lowest ΔE value (922 mV) for both reactions, indicating excellent bifunctionality. The 5Fe-MONW-120/CNF maintained stability during 20 h of cycling, with ΔE decreasing to around 800 mV, showcasing its robustness for practical energy applications.

Cobalt-based oxides and carbon composites

Cobalt-based oxides, such as Co₃O₄, are another vital class of electrocatalysts^[150]. Defect engineering and selection of support materials significantly influence the performance of Co₃O₄. For example, as shown in Figure 10C, by compositing Co₃O₄ with N-defective graphitic carbon nitride, highly efficient bifunctional electrocatalysts with abundant defect sites are produced. As depicted in Figure 10D, these defect sites increase the number of active sites and optimize the electronic structure of the catalyst, facilitating the kinetic processes of ORR and OER. Experimental results show that such composite materials demonstrate high power densities and good cycling stability in ZABs.

Pre-impregnated metal oxides in N-doped carbon

As illustrated in Figure 10E, a novel bifunctional catalyst has been developed by researchers through the pre-implantation of metal oxides into N-doped carbon, enhancing its catalytic performance for oxygen electrode reaction^[151]. The catalyst, designated as 300NiFe-Mi-C, was created by heating a mixture of nickel and iron metal salts to yield metal oxides, which were subsequently combined with melamine and subjected to carbonization at 900 °C. The resulting product features a hierarchical porous architecture, with metal nanoparticles evenly dispersed throughout the N-doped carbon matrix. This specific structure promotes efficient mass transport and electron transfer, resulting in improved performance for oxygen electrode reaction. The exceptional catalytic capability is ascribed to the combined effects of the metal oxides and N-doped carbon, which collectively provide a rich array of active sites and enhance the adsorption and activation of oxygen species.

Perovskite oxide composites

Composites of perovskite oxides demonstrate considerable promise in bifunctional oxygen electrocatalysis, owing to their distinct structure and flexible composition^[152]. By constructing heterostructures of perovskite oxides with other metals, alloys, metal oxides, or N-doped carbon materials, active sites can be strengthened, achieving efficient ORR and OER catalysis. The use of these materials in ZABs greatly enhances the efficiency of energy conversion and the commercial potential of the batteries.

Metal oxides and their composites, serving as efficient and stable electrocatalysts in ZABs, have demonstrated considerable application potential. Through material design and structural optimization, the catalytic performance of ORR and OER can be further enhanced, addressing key challenges in ZABs. Future research should focus on the development of novel metal oxide catalysts, exploration of their synergistic effects with novel electrolytes and zinc anode materials, and optimization of battery design and manufacturing processes to advance the commercialization of ZABs. As material science and electrochemical technologies continue to evolve, the prospects for the application of metal oxides in the field of ZABs will expand.

Significant progress has been made in the application of metal oxides as efficient electrocatalysts in ZABs. Through material design and surface modification techniques, such as the construction of metal oxide-carbon substrate composites, incorporation of defect engineering, and elemental doping, the ORR and OER performances of catalysts can be effectively enhanced, significantly improving the energy efficiency and cycle life of ZABs. Future research should continue to explore innovative methods for synthesizing metal oxide catalysts, optimizing their structure and composition, and developing advanced characterization techniques to gain deeper insights into catalytic mechanisms. Ultimately, this will facilitate the commercial application of ZABs in large-scale energy storage scenarios.

Summary

In conclusion, the development of NPM-based carbon materials has shown significant promise for enhancing the ORR and OER in rechargeable ZABs. These materials, including MOF derivatives, metal-doped carbons, carbon nitrides, heteroatom-doped carbons, and MXenes, offer numerous advantages such as high specific surface areas, tunable morphologies, and the ability to incorporate multiple active sites through doping with elements such as N, S, P, and boron. These attributes facilitate enhanced electron transfer and mass transport, leading to improved catalytic performance for both the ORR and OER. Moreover, the design of NPM-based carbon materials has been enriched by advancements in synthetic methodologies. Techniques such as hydrothermal synthesis, pyrolysis, and electrospinning have enabled the creation of hierarchical porous structures, which are integral to achieving high catalytic activity and stability. Recent studies have demonstrated the successful development of high-performance NPM-based carbon materials for ZABs, such as SACs and heterojunctions, which show significant promise for practical applications.

Challenges and possible further approaches

Despite the significant progress in developing NPM-based carbon materials for the ORR and OER in ZABs, several challenges remain: (1) Achieving the highest possible catalytic activity, particularly in terms of half-wave potential and overpotential, remains a significant challenge. While recent advancements have led to improvements, there is still a need for materials that can match or exceed the performance of PGMs; (2) Ensuring long-term stability under operational conditions, especially at high current densities, is a critical issue. Degradation over time can lead to a decrease in active sites and a decline in catalytic performance; (3) Cost-effectiveness: The cost and environmental impact of large-scale production must be considered. Many NPM-based carbon materials rely on complex synthesis steps and expensive precursors, which can hinder their commercial viability; (4) Improving the overall performance of ZABs, including energy density, cycle life, and cost-effectiveness, requires further optimization. The development of materials that can balance high catalytic activity with durability and cost is essential.

To address these challenges, the following approaches can be pursued: (1) Develop simpler and more cost-effective methods for preparing NPM-based carbon materials. Techniques such as solvothermal synthesis, spray pyrolysis, and microwave-assisted synthesis could offer more efficient routes; (2) Novel support materials: Explore new support materials, such as 2D materials and MOFs, which can provide a more stable anchoring environment for preventing the migration of metal atoms; (3) Combine theoretical calculations with experimental validation to gain a deeper understanding of the mechanisms underlying the catalytic performance of NPM-based carbon materials. This will guide the design and optimization of catalysts; (4) Optimize the structure and composition of NPM-based carbon materials to improve their catalytic performance. This includes the precise control of active site density, the understanding of catalytic mechanisms, and the scalability of synthesis methods; (5) Optimize the compatibility of NPM-based carbon materials with electrolytes and refine battery design to enhance overall performance. This includes the development of quasi-solid-state and all-solid-state electrolytes to improve safety and operational stability; (6) Focus on the application of NPM-based carbon materials in flexible and solid-state ZABs, which represent significant future directions for the commercialization of ZABs in wearable devices and other portable electronics.

By pursuing these approaches, it is anticipated that the challenges faced by NPM-based carbon materials for ZABs can be addressed, paving the way for their widespread adoption in practical energy storage applications.