Atomic‐Level Fe‐N‐C Coupled with Fe3C‐Fe Nanocomposites in Carbon Matrixes as High‐Efficiency Bifunctional Oxygen Catalysts

Highly active and durable bifunctional oxygen electrocatalysts are of pivotal importance for clean and renewable energy conversion devices, but the lack of earth‐abundant electrocatalysts to improve the intrinsic sluggish kinetic process of oxygen reduction/evolution reactions (ORR/OER) is still a challenge. Fe‐N‐C catalysts with abundant natural merits are considered as promising alternatives to noble‐based catalysts, yet further improvements are urgently needed because of their poor stability and unclear catalytic mechanism. Here, an atomic‐level Fe‐N‐C electrocatalyst coupled with low crystalline Fe₃C‐Fe nanocomposite in 3D carbon matrix (Fe‐SAs/Fe₃C‐Fe@NC) is fabricated by a facile and scalable method. Versus atomically FeN_x species and crystallized Fe₃C‐Fe nanoparticles, Fe‐SAs/Fe₃C‐Fe@NC catalyst, abundant in vertical branched carbon nanotubes decorated on intertwined carbon nanofibers, exhibits high electrocatalytic activities and excellent stabilities both in ORR (E_1/2, 0.927 V) and OER (E_J=10, 1.57 V). This performance benefits from the strong synergistic effects of multicomponents and the unique structural advantages. In‐depth X‐ray absorption fine structure analysis and density functional theory calculation further demonstrate that more extra charges derived from modified Fe clusters decisively promote the ORR/OER performance for atomically FeN₄ configurations by enhanced oxygen adsorption energy. These insightful findings inspire new perspectives for the rational design and synthesis of economical–practical bifunctional oxygen electrocatalysts.     

ZIF‐8/ZIF‐67‐Derived Co‐Nx‐Embedded 1D Porous Carbon Nanofibers with Graphitic Carbon‐Encased Co Nanoparticles as an Efficient Bifunctional Electrocatalyst

Herein, an approach is reported for fabrication of Co‐N_x‐embedded 1D porous carbon nanofibers (CNFs) with graphitic carbon‐encased Co nanoparticles originated from metal–organic frameworks (MOFs), which is further explored as a bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Electrochemical results reveal that the electrocatalyst prepared by pyrolysis at 1000 °C (CoNC‐CNF‐1000) exhibits excellent catalytic activity toward ORR that favors the four‐electron ORR process and outstanding long‐term stability with 86% current retention after 40 000 s. Meanwhile, it also shows superior electrocatalytic activity toward OER, reaching a lower potential of 1.68 V at 10 mA cm^?2 and a potential gap of 0.88 V between the OER potential (at 10 mA cm^?2) and the ORR half‐wave potential. The ORR and OER performance of CoNC‐CNF‐1000 have outperformed commercial Pt/C and most nonprecious‐metal catalysts reported to date. The remarkable ORR and OER catalytic performance can be mainly attributable to the unique 1D structure, such as higher graphitization degree beneficial for electronic mobility, hierarchical porosity facilitating the mass transport, and highly dispersed CoN_xC active sites functionalized carbon framework. This strategy will shed light on the development of other MOF‐based carbon nanofibers for energy storage and electrochemical devices.     

Electrocatalytic N‐Doped Graphitic Nanofiber – Metal/Metal Oxide Nanoparticle Composites

Carbon‐based nanocomposites have shown promising results in replacing commercial Pt/C as high‐performance, low cost, nonprecious metal‐based oxygen reduction reaction (ORR) catalysts. Developing unique nanostructures of active components (e.g., metal oxides) and carbon materials is essential for their application in next generation electrode materials for fuel cells and metal–air batteries. Herein, a general approach for the production of 1D porous nitrogen‐doped graphitic carbon fibers embedded with active ORR components, (M/MO_x, i.e., metal or metal oxide nanoparticles) using a facile two‐step electrospinning and annealing process is reported. Metal nanoparticles/nanoclusters nucleate within the polymer nanofibers and subsequently catalyze graphitization of the surrounding polymer matrix and following oxidation, create an interconnected graphite–metal oxide framework with large pore channels, considerable active sites, and high specific surface area. The metal/metal oxide@N‐doped graphitic carbon fibers, especially Co₃O₄, exhibit comparable ORR catalytic activity but superior stability and methanol tolerance versus Pt in alkaline solutions, which can be ascribed to the synergistic chemical coupling effects between Co₃O₄ and robust 1D porous structures composed of interconnected N‐doped graphitic nanocarbon rings. This finding provides a novel insight into the design of functional electrocatalysts using electrospun carbon nanomaterials for their application in energy storage and conversion fields.     

Reduced Graphene Oxide‐Wrapped Co9–xFexS8/Co,Fe‐N‐C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

Searching for highly efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using nonnoble metal‐based catalysts is essential for the development of many energy conversion systems, including rechargeable fuel cells and metal–air batteries. Here, Co_9–xFe_xS₈/Co,Fe‐N‐C hybrids wrapped by reduced graphene oxide (rGO) (abbreviated as S‐Co_9–xFe_xS₈@rGO) are synthesized through a semivulcanization and calcination method using graphene oxide (GO) wrapped bimetallic zeolite imidazolate framework (ZIF) Co,Fe‐ZIF (CoFe‐ZIF@GO) as precursors. Benefiting from the synergistic effect of OER active CoFeS and ORR active Co,Fe‐N‐C in a single component, as well as high dispersity and enhanced conductivity derived from rGO coating and Fe‐doping, the obtained S‐Co_9–xFe_xS₈@rGO‐10 catalyst shows an ultrasmall overpotential of ≈0.29 V at 10 mA cm^?2 in OER and a half‐wave potential of 0.84 V in ORR, combining a superior oxygen electrode activity of ≈0.68 V in 0.1 m KOH.     

Single Fe Atom on Hierarchically Porous S,N‐Codoped Nanocarbon Derived from Porphyra Enable Boosted Oxygen Catalysis for Rechargeable Zn‐Air Batteries

Iron–nitrogen–carbon materials (Fe–N–C) are known for their excellent oxygen reduction reaction (ORR) performance. Unfortunately, they generally show a laggard oxygen evolution reaction (OER) activity, which results in a lethargic charging performance in rechargeable Zn–air batteries. Here porous S‐doped Fe–N–C nanosheets are innovatively synthesized utilizing a scalable FeCl₃‐encapsulated‐porphyra precursor pyrolysis strategy. The obtained electrocatalyst exhibits ultrahigh ORR activity (E_1/2 = 0.84 V vs reversible hydrogen electrode) and impressive OER performance (E_{j = 10} = 1.64 V). The potential gap (ΔE = E_{j = 10} ? E_1/2) is 0.80 V, outperforming that of most highly active bifunctional electrocatalysts reported to date. Furthermore, the key role of S involved in the atomically dispersed Fe–Nx species on the enhanced ORR and OER activities is expounded for the first time by ultrasound‐assisted extraction of the exclusive S source (taurine) from porphyra. Moreover, the assembled rechargeable Zn–air battery comprising this bifunctional electrocatalyst exhibits higher power density (225.1 mW cm^?2) and lower charging–discharging overpotential (1.00 V, 100 mA cm^?2 compared to Pt/C + RuO₂ catalyst). The design strategy can expand the utilization of earth‐abundant biomaterial‐derived catalysts, and the mechanism investigations of S doping on the structure–activity relationship can inspire the progress of other functional electrocatalysts.     

Atomic Fe–Nx Coupled Open‐Mesoporous Carbon Nanofibers for Efficient and Bioadaptable Oxygen Electrode in Mg–Air Batteries

The recently emerging metal–air batteries equipped with advanced oxygen electrodes have provided enormous opportunities to develop the next generation of wearable and bio‐adaptable power sources. Theoretically, neutral electrolyte‐based Mg–air batteries possess potential advantages in electronics and biomedical applications over the other metal–air counterparts, especially the alkaline‐based Zn–air batteries. However, the rational design of advanced oxygen electrode for Mg–air batteries with high discharge voltage and capacity under neutral conditions still remains a major challenge. Inspired by fibrous string structures of bufo‐spawn, it is reported here that the scalable synthesis of atomic Fe–N_x coupled open‐mesoporous N‐doped‐carbon nanofibers (OM‐NCNF‐FeN_x) as advanced oxygen electrode for Mg–air batteries. The fabricated OM‐NCNF‐FeN_x electrodes present manifold advantages, including open‐mesoporous and interconnected structures, 3D hierarchically porous networks, good bio‐adaptability, homogeneously coupled atomic Fe–N_x sites, and high oxygen electrocatalytic performances. Most importantly, the assembled Mg–air batteries with neutral electrolytes reveal high open‐circuit voltage, stable discharge voltage plateaus, high capacity, long operating life, and good flexibility. Overall, the discovery on fabricating atomic OM‐NCNF‐FeN_x electrode will not only create new pathways for achieving flexible, wearable, and bio‐adaptable power sources, but also take a step towards the scale‐up production of advanced nanofibrous carbon electrodes for a broad range of applications.     

A Bifunctional Highly Efficient FeNx/C Electrocatalyst

Herein, a type of Fe, N‐codoped carbon electrocatalyst (FeN_x/C, Fe‐N‐BCNT#BP) containing bamboo carbon nanotubes and displaying bifunctional high catalytic efficiency for both oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) is reported. It shows high electrocatalytic activity and stability for both the ORR process with onset potential of 1.03 V_RHE in alkaline and the CO2RR to CO with high faradic efficiency up to 90% and selectivity of about 100% at low overpotential of 0.49 V. For CO2RR to CO, it is revealed that Fe₃C is active but the activity of FeN_x centers is lower than that of C–N‐based centers, contrary with that observed for ORR. Due to its low cost and high electrocatalytic performance for these two reduction reactions, the obtained catalyst is very promising for extensive application in future. The revealed huge activity difference of the same types of active sites for different reactions can efficiently guide the synthesis of advanced materials with multifunction.     

Defect Engineering toward Atomic Co–Nx–C in Hierarchical Graphene for Rechargeable Flexible Solid Zn‐Air Batteries

Rechargeable flexible solid Zn‐air battery, with a high theoretical energy density of 1086 Wh kg^?1, is among the most attractive energy technologies for future flexible and wearable electronics; nevertheless, the practical application is greatly hindered by the sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics on the air electrode. Precious metal‐free functionalized carbon materials are widely demonstrated as the most promising candidates, while it still lacks effective synthetic methodology to controllably synthesize carbocatalysts with targeted active sites. This work demonstrates the direct utilization of the intrinsic structural defects in nanocarbon to generate atomically dispersed Co–N_x–C active sites via defect engineering. As‐fabricated Co/N/O tri‐doped graphene catalysts with highly active sites and hierarchical porous scaffolds exhibit superior ORR/OER bifunctional activities and impressive applications in rechargeable Zn‐air batteries. Specifically, when integrated into a rechargeable and flexible solid Zn‐air battery, a high open‐circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V, and a high energy efficiency of 63% at 1.0 mA cm^?2 are achieved even under bending. The defect engineering strategy provides a new concept and effective methodology for the full utilization of nanocarbon materials with various structural features and further development of advanced energy materials.     

Silk‐Derived 2D Porous Carbon Nanosheets with Atomically‐Dispersed Fe‐Nx‐C Sites for Highly Efficient Oxygen Reaction Catalysts

Controlled synthesis of highly efficient, stable, and cost‐effective oxygen reaction electrocatalysts with atomically‐dispersed Me–N_x–C active sites through an effective strategy is highly desired for high‐performance energy devices. Herein, based on regenerated silk fibroin dissolved in ferric chloride and zinc chloride aqueous solution, 2D porous carbon nanosheets with atomically‐dispersed Fe–N_x–C active sites and very large specific surface area (≈2105 m² g^?1) are prepared through a simple thermal treatment process. Owing to the 2D porous structure with large surface area and atomic dispersion of Fe–N_x–C active sites, the as‐prepared silk‐derived carbon nanosheets show superior electrochemical activity toward the oxygen reduction reaction with a half‐wave potential (E_1/2) of 0.853 V, remarkable stability with only 11 mV loss in E_1/2 after 30 000 cycles, as well as good catalytic activity toward the oxygen evolution reaction. This work provides a practical and effective approach for the synthesis of high‐performance oxygen reaction catalysts towards advanced energy materials.     

Biomass Derived N‐Doped Porous Carbon Supported Single Fe Atoms as Superior Electrocatalysts for Oxygen Reduction

Exploring sustainable and high‐performance electrocatalysts for the oxygen reduction reaction (ORR) is the crucial issue for the large‐scale application of fuel cell technology. A new strategy is demonstrated to utilize the biomass resource for the synthesis of N‐doped hierarchically porous carbon supported single‐atomic Fe (SA‐Fe/NHPC) electrocatalyst toward the ORR. Based on the confinement effect of porous carbon and high‐coordination natural iron source, SA‐Fe/NHPC, derived from the hemin‐adsorbed bio‐porphyra‐carbon by rapid heat‐treatment up to 800 °C, presents the atomic dispersion of Fe atoms in the N‐doped porous carbon. Compared with the molecular hemin and nanoparticle Fe samples, the as‐prepared SA‐Fe/NHPC exhibits a superior catalytic activity (E _1/2 = 0.87 V and J _k = 4.1 mA cm^?2, at 0.88 V), remarkable catalytic stability (≈1 mV negative shift of E _1/2, after 3000 potential cycles), and outstanding methanol‐tolerance, even much better than the state‐of‐the‐art Pt/C catalyst. The sustainable and effective strategy for utilizing biomass to achieve high‐performance single‐atom catalysts can also provide an opportunity for other catalytic applications in the atomic scale.     

A Hydrangea‐Like Superstructure of Open Carbon Cages with Hierarchical Porosity and Highly Active Metal Sites

Carbon micro‐/nanocages have attracted great attention owing to their wide potential applications. Herein, a self‐templated strategy is presented for the synthesis of a hydrangea‐like superstructure of open carbon cages through morphology‐controlled thermal transformation of core@shell metal–organic frameworks (MOFs). Direct pyrolysis of core@shell zinc (Zn)@cobalt (Co)‐MOFs produces well‐defined open‐wall nitrogen‐doped carbon cages. By introducing guest iron (Fe) ions into the core@shell MOF precursor, the open carbon cages are self‐assembled into a hydrangea‐like 3D superstructure interconnected by carbon nanotubes, which are grown in situ on the Fe–Co alloy nanoparticles formed during the pyrolysis of Fe‐introduced Zn@Co‐MOFs. Taking advantage of such hierarchically porous superstructures with excellent accessibility, synergetic effects between the Fe and the Co, and the presence of catalytically active sites of both metal nanoparticles and metal–N_x species, this superstructure of open carbon cages exhibits efficient bifunctional catalysis for both oxygen evolution reaction and oxygen reduction reaction, achieving a great performance in Zn–air batteries.     

Pyridinic‐N Protected Synthesis of 3D Nitrogen‐Doped Porous Carbon with Increased Mesoporous Defects for Oxygen Reduction

Nitrogen (N)‐doped carbons are potential nonprecious metal catalysts to replace Pt for the oxygen reduction reaction (ORR). Pyridinic‐N‐C is believed to be the most active N group for catalyzing ORR. In this work, using zinc phthalocyanine as a precursor effectively overcomes the serious loss of pyridinic‐N, which is commonly regarded as the biggest obstacle to catalytic performance enhancement upon adopting a second pyrolysis process, for the preparation of a 3D porous N‐doped carbon framework (NDCF). The results show only ≈14% loss in pyridinic‐N proportion in the Zn‐containing sample during the second pyrolysis process. In comparison, a loss of ≈72% pyridinic‐N occurs for the non‐Zn counterpart. The high pyridinic‐N proportion, the porous carbon framework produced upon NaCl removal, and the increased mesoporous defects in the second pyrolysis process make the as‐prepared catalyst an excellent electrocatalyst for ORR, exhibiting a half‐wave potential (E_1/2 = 0.88 V) up to 33 mV superior to state‐of‐the‐art Pt/C and high four‐electron selectivity (n > 3.83) in alkaline solution, which is among the best ORR activities reported for N‐doped carbon catalysts. Furthermore, only ≈18 mV degradation in E_1/2 occurs after an 8000 cycles' accelerating stability test, manifesting the outstanding stability of the as‐prepared catalyst.     

In Situ Exfoliated,Edge‐Rich,Oxygen‐Functionalized Graphene from Carbon Fibers for Oxygen Electrocatalysis

Metal‐free electrocatalysts have been extensively developed to replace noble metal Pt and RuO₂ catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in fuel cells or metal–air batteries. These electrocatalysts are usually deposited on a 3D conductive support (e.g., carbon paper or carbon cloth (CC)) to facilitate mass and electron transport. For practical applications, it is desirable to create in situ catalysts on the carbon fiber support to simplify the fabrication process for catalytic electrodes. In this study, the first example of in situ exfoliated, edge‐rich, oxygen‐functionalized graphene on the surface of carbon fibers using Ar plasma treatment is successfully prepared. Compared to pristine CC, the plasma‐etched carbon cloth (P‐CC) has a higher specific surface area and an increased number of active sites for OER and ORR. P‐CC also displays good intrinsic electron conductivity and excellent mass transport. Theoretical studies show that P‐CC has a low overpotential that is comparable to Pt‐based catalysts, as a result of both defects and oxygen doping. This study provides a simple and effective approach for producing highly active in situ catalysts on a carbon support for OER and ORR.     

Efficient Bifunctional Catalytic Electrodes with Uniformly Distributed NiN2 Active Sites and Channels for Long‐Lasting Rechargeable Zinc–Air Batteries

Freestanding bifunctional electrodes with outstanding oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) properties are of great significance for zinc–air batteries, attributed to the avoided use of organic binder and strong adhesion with substrates. Herein, a strategy is developed to fabricate freestanding bifunctional electrodes from the predeposited nickel nanoparticles (Ni‐NCNT) on carbon fiber paper. The steric effect of monodispersed SiO₂ nanospheres limits the configuration of carbon atoms forming 3D interconnected nanotubes with uniformly distributed NiN₂ active sites. The bifunctional electrodes (Ni‐NCNT) demonstrate ideal ORR and OER properties. The zinc–air batteries assembled with Ni‐NCNT directly exhibit extremely outstanding long term stability (2250 cycles with 10 mA cm^?2 charge/discharge current density) along with high power density of 120 mV cm^?2 and specific capacity of 834.1 mA h g^?1. This work provides a new view to optimize the distribution of active sites and the electrode structure.     

Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas‐foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen‐doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH₄Cl, and Co(NO₃)₂·6H₂O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m² g^?1), and homogeneous distribution of active sites (C–N, Co–N_x, and Co–O moieties), the optimized CoNCF‐1000‐80 (carbonized at 1000 °C, containing 80 mg Co(NO₃)₂·6H₂O in precursors) catalyst exhibits a remarkable bifunctional activity and long‐term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state‐of‐the‐art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn–air batteries, a high energy density (797 Wh kg^?1), a low charge/discharge voltage gap (0.75 V), and a long‐term cycle stability (over 166 h) are achieved at 10 mA cm^?2.     

Preferentially Engineering FeN4 Edge Sites onto Graphitic Nanosheets for Highly Active and Durable Oxygen Electrocatalysis in Rechargeable Zn–Air Batteries

Single-atom FeN₄ sites at the edges of carbon substrates are considered more active for oxygen electrocatalysis than those in plane; however, the conventional high-temperature pyrolysis process does not allow for precisely engineering the location of the active site down to atomic level. Enlightened by theoretical prediction, herein, a self-sacrificed templating approach is developed to obtain edge-enriched FeN₄ sites integrated in the highly graphitic nanosheet architecture. The in situ formed Fe clusters are intentionally introduced to catalyze the growth of graphitic carbon, induce porous structure formation, and most importantly, facilitate the preferential anchoring of FeN₄ to its close approximation. Due to these attributes, the as-resulted catalyst (denoted as Fe/N-G-SAC) demonstrates unprecedented catalytic activity and stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) by showing an impressive half-wave potential of 0.89 V for the ORR and a small overpotential of 370 mV at 10 mA cm⁻² for the OER. Moreover, the Fe/N-G-SAC cathode displays encouraging performance in a rechargeable Zn–air battery prototype with a low charge–discharge voltage gap of 0.78 V and long-term cyclability for over 240 cycles, outperforming the noble metal benchmarks.     

Sustainable Synthesis of Co@NC Core Shell Nanostructures from Metal Organic Frameworks via Mechanochemical Coordination Self‐Assembly: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Herein, a new type of cobalt encapsulated nitrogen‐doped carbon (Co@NC) nanostructure employing Zn_xCo_1?x(C₃H₄N₂) metal–organic framework (MOF) as precursor is developed, by a simple, ecofriendly, solvent‐free approach that utilizes a mechanochemical coordination self‐assembly strategy. Possible evolution of Zn_xCo_1?x(C₃H₄N₂) MOF structures and their conversion to Co@NC nanostructures is established from an X‐ray diffraction technique and transmission electron microscopy analysis, which reveal that MOF‐derived Co@NC core–shell nanostructures are well ordered and highly crystalline in nature. Co@NC–MOF core–shell nanostructures show excellent catalytic activity for the oxygen reduction reaction (ORR), with onset potential of 0.97 V and half‐wave potential of 0.88 V versus relative hydrogen electrode in alkaline electrolyte, and excellent durability with zero degradation after 5000 potential cycles; whereas under similar experimental conditions, the commonly utilized Pt/C electrocatalyst degrades. The Co@NC–MOF electrocatalyst also shows excellent tolerance to methanol, unlike the Pt/C electrocatalyst. X‐ray photoelectron spectroscopy (XPS) analysis shows the presence of ORR active pyridinic‐N and graphitic‐N species, along with CoN_x? C_y and Co? N_x ORR active (M–N–C) sites. Enhanced electron transfer kinetics from nitrogen‐doped carbon shell to core Co nanoparticles, the existence of M–N–C active sites, and protective NC shells are responsible for high ORR activity and durability of the Co@NC–MOF electrocatalyst.     

ZIF‐8 with Ferrocene Encapsulated: A Promising Precursor to Single‐Atom Fe Embedded Nitrogen‐Doped Carbon as Highly Efficient Catalyst for Oxygen Electroreduction

The oxygen reduction reaction (ORR) plays an important role in the fields of energy storage and conversion technologies, including metal–air batteries and fuel cells. The development of nonprecious metal electrocatalysts with both high ORR activity and durability to replace the currently used costly Pt‐based catalyst is critical and still a major challenge. Herein, a facile and scalable method is reported to prepare ZIF‐8 with single ferrocene molecules trapped within its cavities (Fc@ZIF‐8), which is utilized as precursor to porous single‐atom Fe embedded nitrogen‐doped carbon (Fe–N–C) during high temperature pyrolysis. The catalyst shows a half‐wave potential (E_1/2) of 0.904 V, 67 mV higher than commercial Pt/C catalyst (0.837 V), which is among the best compared with reported results for ORR. Significant electrochemical properties are attributed to the special configuration of Fc@ZIF‐8 transforming into a highly dispersed iron–nitrogen coordination moieties embedded carbon matrix.     

Cobalt Phosphide Coupled with Heteroatom‐Doped Nanocarbon Hybrid Electroctalysts for Efficient,Long‐Life Rechargeable Zinc–Air Batteries

Metal phosphides and heteroatom‐doped carbons have been regarded as promising candidates as bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, both have suffered from stability issues during repeated ORR and OER operations in zinc–air batteries (ZABs). Herein, this study reports a versatile cobalt‐based hybrid catalyst with a 1D structure by integrating the metal‐organic framework‐derived conversion approach and an in situ crosslinking method. Among them, the 1D hybrid catalyst composed of ultrasmall cobalt phosphide nanoparticles supported by nitrogen‐, sulfur‐, phosphorus‐doped carbon matrix shows remarkable bifunctional activity close to that of the benchmark precious‐metal catalysts along with an excellent durability in the full potential range covering both the OER and ORR. The overall overpotential of the rechargeable ZABs can be greatly reduced with this bifunctional hybrid catalyst as an air‐electrode, and the cycling stability outperforms the commercial Pt/C catalyst. It is revealed that the cobalt phosphide nanoparticles are in situ converted to cobalt oxide under the accelerated conditions during OER (and/or ORR) of the ZABs and reduces the anodic current applied to the carbon. This contributes to the stability of the carbon material and in maintaining the high initial catalytic properties of the hybrid catalyst.     

Elucidation of Active Sites on S,N Codoped Carbon Cubes Embedding Co–Fe Carbides toward Reversible Oxygen Conversion in High‐Performance Zinc–Air Batteries

The development of high‐performance but low‐cost catalysts for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of central importance for realizing the prevailing application of metal–air batteries. Herein a facile route is devised to synthesize S, N codoped carbon cubes embedding Co–Fe carbides by pyrolyzing the Co–Fe Prussian blue analogues (PBA) coated with methionine. Via the strong metal–sulfur interaction, the methionine coating provides a robust sheath to restrain the cubic morphology of PBA upon pyrolysis, which is proved highly beneficial for promoting the specific surface area and active sites exposure, leading to remarkable bifunctionality of ORR and OER comparable to the benchmarks of Pt/C and RuO₂. Further elaborative investigations on the activity origin and postelectrolytic composition unravel that for ORR the high activity is mainly contributed by the S, N codoped carbon shell with the inactive carbide phase converting into carbonate hydroxides. For OER, the embedded Co–Fe carbides transform in situ into layered (hydr)oxides, serving as the actual active sites for promoting water oxidation. Zn–air batteries employing the developed hollow structure as the air cathode catalyst demonstrate superb rechargeability, energy efficiency, as well as portability.