Atomic Fe Dispersed on N‐Doped Carbon Hollow Nanospheres for High‐Efficiency Electrocatalytic Oxygen Reduction

Exploration of high‐efficiency, economical, and ultrastable electrocatalysts for the oxygen reduction reaction (ORR) to substitute precious Pt is of great significance in electrochemical energy conversion devices. Single‐atom catalysts (SACs) have sparked tremendous interest for their maximum atom‐utilization efficiency and fascinating properties. Therefore, the development of effective synthetic methodology toward SACs becomes highly imperative yet still remains greatly challenging. Herein, a reliable SiO₂‐templated strategy is elaborately designed to synthesize atomically dispersed Fe atoms anchored on N‐doped carbon nanospheres (denoted as Fe–N–C HNSs) using the cheap and sustainable biomaterial of histidine (His) as the N and C precursor. By virtue of the numerous atomically dispersed Fe–N₄ moieties and unique spherical hollow architecture, the as‐fabricated Fe–N–C HNSs exhibit excellent ORR performance in alkaline medium with outstanding activity, high long‐term stability, and superior tolerance to methanol crossover, exceeding the commercial Pt/C catalyst and most previously reported non‐precious‐metal catalysts. This present synthetic strategy will provide new inspiration to the fabrication of various high‐efficiency single‐atom catalysts for diverse applications.     

Co‐N‐Doped Mesoporous Carbon Hollow Spheres as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction

Rational design of cost‐effective, nonprecious metal‐based catalysts with desirable oxygen reduction reaction (ORR) performance is extremely important for future fuel cell commercialization, etc. Herein, a new type of ORR catalyst of Co‐N‐doped mesoporous carbon hollow sphere (Co‐N‐mC) was developed by pyrolysis from elaborately fabricated polystyrene@polydopamine‐Co precursors. The obtained catalysts with active Co sites distributed in highly graphitized mesoporous N‐doped carbon hollow spheres exhibited outstanding ORR activity with an onset potential of 0.940 V, a half‐wave potential of 0.851 V, and a small Tafel slope of 45 mV decade^?1 in 0.1 m KOH solution, which was comparable to that of the Pt/C catalyst (20%, Alfa). More importantly, they showed superior durability with little current decline (less than 4%) in the chronoamperometric evaluation over 60 000 s. These features make the Co‐N‐mC one of the best nonprecious‐metal catalysts to date for ORR in alkaline condition.     

Fe Isolated Single Atoms on S,N Codoped Carbon by Copolymer Pyrolysis Strategy for Highly Efficient Oxygen Reduction Reaction

Heteroatom‐doped Fe‐NC catalyst has emerged as one of the most promising candidates to replace noble metal‐based catalysts for highly efficient oxygen reduction reaction (ORR). However, delicate controls over their structure parameters to optimize the catalytic efficiency and molecular‐level understandings of the catalytic mechanism are still challenging. Herein, a novel pyrrole–thiophene copolymer pyrolysis strategy to synthesize Fe‐isolated single atoms on sulfur and nitrogen‐codoped carbon (Fe‐ISA/SNC) with controllable S, N doping is rationally designed. The catalytic efficiency of Fe‐ISA/SNC shows a volcano‐type curve with the increase of sulfur doping. The optimized Fe‐ISA/SNC exhibits a half‐wave potential of 0.896 V (vs reversible hydrogen electrode (RHE)), which is more positive than those of Fe‐isolated single atoms on nitrogen codoped carbon (Fe‐ISA/NC, 0.839 V), commercial Pt/C (0.841 V), and most reported nonprecious metal catalysts. Fe‐ISA/SNC is methanol tolerable and shows negligible activity decay in alkaline condition during 15 000 voltage cycles. X‐ray absorption fine structure analysis and density functional theory calculations reveal that the incorporated sulfur engineers the charges on N atoms surrounding the Fe reactive center. The enriched charge facilitates the rate‐limiting reductive release of OH* and therefore improved the overall ORR efficiency.     

UIO‐66‐NH2‐Derived Mesoporous Carbon Catalyst Co‐Doped with Fe/N/S as Highly Efficient Cathode Catalyst for PEMFCs

Efficient, low‐cost catalysts are desirable for the sluggish oxygen reduction reaction (ORR). Herein, UIO‐66‐NH₂‐derived multi‐element (Fe, S, N) co‐doped porous carbon catalyst is reported, Fe/N/S‐PC, with an octahedral morphology, a well‐defined mesoporous structure, and highly dispersed doping elements, synthesized by a double‐solvent diffusion‐pyrolysis method (DSDPM). The morphology of the UIO‐66‐NH₂ precursor is perfectly inherited by the derived carbon material, resulting in a high surface area, a well‐defined mesoporous structure, and atomic‐level dispersion of the doping elements. Fe/N/S‐PC demonstrates outstanding catalytic activity and durability for the ORR in both alkaline and acidic solutions. In 0.1 m KOH, its half‐potential reaches 0.87 V (vs reversible hydrogen electrode (RHE)), 30 mV more positive than that of a 20 wt% Pt/C catalyst. In 0.1 m HClO₄, it reaches 0.785 V (vs RHE), only 65 mV less than that of Pt/C. The catalyst also exhibits excellent performance in acidic hydrogen/oxygen proton exchange membrane fuel cells. A membrane electrode assembly (MEA) with the catalyst as the cathode reaches 700 mA·cm^‐2 at 0.6 V and a maximum power density of 553 mW·cm^‐2, ranking it among the best MEAs with a nonprecious metal catalyst as the cathode.     

Metal–Organic Framework‐Derived Bamboo‐like Nitrogen‐Doped Graphene Tubes as an Active Matrix for Hybrid Oxygen‐Reduction Electrocatalysts

In this work, large size (i.e., diameter > 100 nm) graphene tubes with nitrogen‐doping are prepared through a high‐temperature graphitization process of dicyandiamide (DCDA) and Iron(II) acetate templated by a novel metal–organic framework (MIL‐100(Fe)). The nitrogen‐doped graphene tube (N‐GT)‐rich iron‐nitrogen‐carbon (Fe‐N‐C) catalysts exhibit inherently high activity towards the oxygen reduction reaction (ORR) in more challenging acidic media. Furthermore, aiming to improve the activity and stability of conventional Pt catalysts, the ORR active N‐GT is used as a matrix to disperse Pt nanoparticles in order to build a unique hybrid Pt cathode catalyst. This is the first demonstration of the integration of a highly active Fe‐N‐C catalyst with Pt nanoparticles. The synthesized 20% Pt/N‐GT composite catalysts demonstrate significantly enhanced ORR activity and H₂‐air fuel cell performance relative to those of 20% Pt/C, which is mainly attributed to the intrinsically active N‐GT matrix along with possible synergistic effects between the non‐precious metal active sites and the Pt nanoparticles. Unlike traditional Pt/C, the hybrid catalysts exhibit excellent stability during the accelerated durability testing, likely due to the unique highly graphitized graphene tube morphologies, capable of providing strong interaction with Pt nanoparticles and then preventing their agglomeration.     

Nitrogen-doped graphite encapsulated Fe/Fe_3C nanoparticles and carbon black for enhanced performance towards oxygen reduction

Non-noble metal(NNM) catalysts have recently attracted intensive interest for their high catalytic performance towards oxygen reduction reaction(ORR) at low cost.Herein,a novel NNM catalyst was synthesized by the simple pyrolysis of carbon black,urea and a Fe-containing precursor,which exhibits excellent ORR catalytic activity,superior durability and methanol tolerance versus the Pt/C catalyst in both alkaline and acidic solutions.Scanning electron microscopy(SEM),transmission electron microscopy(TEM) and X-ray diffraction(XRD) characterizations demonstrate that the product is a nitrogen-doped hybrid of graphite encapsulated Fe/Fe_3C nanoparticles and carbon black.X-ray photoelectron spectrum(XPS) and electrochemical analyses indicate that the catalytic performance and chemical stability correlate closely with a nitrogen-rich layer on the Fe/Fe_3C nanoparticle after pyrolysis with presence of urea,leading to the same four-electron pathway towards ORR as the Pt/C catalyst.The hybrid is prospective to be an efficient ORR electrocatalyst for direct methanol fuel cells with high catalytic performance at low cost.     

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.     

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.     

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.     

Porous Carbon‐Hosted Atomically Dispersed Iron–Nitrogen Moiety as Enhanced Electrocatalysts for Oxygen Reduction Reaction in a Wide Range of pH

As one of the alternatives to replace precious metal catalysts, transition‐metal–nitrogen–carbon (M–N–C) electrocatalysts have attracted great research interest due to their low cost and good catalytic activities. Despite nanostructured M–N–C catalysts can achieve good electrochemical performances, they are vulnerable to aggregation and insufficient catalytic sites upon continuous catalytic reaction. In this work, metal–organic frameworks derived porous single‐atom electrocatalysts (SAEs) were successfully prepared by simple pyrolysis procedure without any further posttreatment. Combining the X‐ray absorption near‐edge spectroscopy and electrochemical measurements, the SAEs have been identified with superior oxygen reduction reaction (ORR) activity and stability compared with Pt/C catalysts in alkaline condition. More impressively, the SAEs also show excellent ORR electrocatalytic performance in both acid and neutral media. This study of nonprecious catalysts provides new insights on nanoengineering catalytically active sites and porous structures for nonprecious metal ORR catalysis in a wide range of pH.     

Ni‐Fe Nitride Nanoplates on Nitrogen‐Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn‐Air Battery

Obtaining bifunctional electrocatalysts with high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a main hurdle in the application of rechargeable metal‐air batteries. Earth‐abundant 3d transition metal‐based catalysts have been developed for the OER and ORR; however, most of these are based on oxides, whose insulating nature strongly restricts their catalytic performance. This study describes a metallic Ni‐Fe nitride/nitrogen‐doped graphene hybrid in which 2D Ni‐Fe nitride nanoplates are strongly coupled with the graphene support. Electronic structure of the Ni‐Fe nitride is changed by hybridizing with the nitrogen‐doped graphene. The unique heterostructure of this hybrid catalyst results in very high OER activity with the lowest onset overpotential (150 mV) reported, and good ORR activity comparable to that for commercial Pt/C. The high activity and durability of this bifunctional catalyst are also confirmed in rechargeable zinc‐air batteries that are stable for 180 cycles with an overall overpotential of only 0.77 V at 10 mA^?2.     

Nitrogen‐Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt‐free and Fe‐free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high‐performance nitrogen‐coordinated single Co atom catalyst is derived from Co‐doped metal‐organic frameworks (MOFs) through a one‐step thermal activation. Aberration‐corrected electron microscopy combined with X‐ray absorption spectroscopy virtually verifies the CoN₄ coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half‐wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe‐based catalysts and 60 mV lower than Pt/C ‐60 μg Pt cm^?2). Fuel cell tests confirm that catalyst activity and stability can translate to high‐performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well‐dispersed CoN₄ active sites embedded in 3D porous MOF‐derived carbon particles, omitting any inactive Co aggregates.     

金属掺杂聚吡咯碳化物PPY-M的制备及其氧还原反应电催化活性

在Fe³⁺或Co²⁺存在下进行吡咯的聚合反应, 得到金属离子掺杂的聚吡咯, 并在N₂气氛下700℃碳化, 再将该碳化产物在900℃焙烧得到含有不同金属的复合催化剂PPY-M(M为不同的金属)。采用SEM、XRD等对催化剂的结构进行了表征。通过循环伏安和线性电位扫描等电化学手段, 研究了催化剂对氧还原(ORR)的电催化活性及其稳定性。结果表明, 掺杂金属钴的催化剂的活性最好, 在酸性溶液中ORR的起始电位达到0.54 V(vs SCE),电流密度为7.5 mA/mg@-0.3 V(vs SCE); 在碱性溶液中ORR的起始电位为-0.11 V(vs SCE),电流密度为5.7 mA/mg@-0.8 V。Fe或Co掺杂的聚吡咯碳化物对ORR具有较强的电催化活性, 而且制备过程简单、成本低, 有较重要的研究意义。     

Strongly Coupled 3D Hybrids of N‐doped Porous Carbon Nanosheet/CoNi Alloy‐Encapsulated Carbon Nanotubes for Enhanced Electrocatalysis

A novel 3D nanoarchitecture comprising in situ‐formed N‐doped CoNi alloy‐encapsulated carbon nanotubes (CoNi‐NCNTs) grown on N‐doped porous carbon nanosheets (NPCNs) is designed and constructed for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). When evaluated as an electrocatalyst for ORR, the hybrid shows efficient catalytic activity, high selectivity, superior durability, and strong tolerance against methanol crossover compared with the commercial Pt/C catalyst. Such good oxygen reduction reaction performance is comparable to most of the previously reported results and the synergistic effect is found to boost the catalytic performance. Moreover, the constructed hybrid exhibits an excellent ORR activity with a current density of 10 mA cm⁻² at 1.59 V and an onset potential of 1.57 V, even beyond the state‐of‐the‐art Ir/C catalyst in alkaline media. The enhancement in electrochemical performance can be attributed to the unique morphology and defect structures, high porosity, good conductive networks, and strongly interacting CoNi‐NCNT and NPCN in the hybrid. These results suggest the possibility for the development of effective nanocarbon electrocatalysts to replace commercial noble metal catalysts for direct use in fuel cells and water splitting devices.     

Fe/Co/C–N nanocatalysts for oxygen reduction reaction synthesized by directly pyrolyzing Fe/Co-doped polyaniline

Development of precious metal-free catalysts for oxygen reduction reaction (ORR) is of great significance for the practical application of fuel cells. In this article, we report the finding that the C–N composites containing Fe and/or Co (C–PANI, Fe/C–PANI, Co/C–PANI, and FeCo/C–PANI), produced by directly pyrolyzing the Fe- and/or Co-doped polyaniline (PANI) precursors, show efficient electroactivity for ORR both in acidic and in alkaline media. Among the prepared catalysts, Fe/C–PANI and Co/C–PANI present the most positive ORR onset potential that is 0.61 V (vs. SCE) in 0.5 mol L^?1 H₂SO₄ solution or ?0.1 V (vs. SCE) in 1 mol L^?1 NaOH solution. In addition, the Fe/C–PANI catalyst shows the largest limiting-diffusion current density for ORR both in acidic and in alkaline media. A four-electron reaction of ORR on the Fe/C–PANI and Co/C–PANI catalysts is more dominant than a two-electron reaction. Fe/C–PANI catalyst also presents good electrocatalytic activity stability for ORR both in acidic and in alkaline media.     

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.     

碱性溶液中不同微观结构的Fe-N/C催化剂氧还原性能的稳定性对比研究

Fe-N/C催化剂在氧还原反应中的作用机理对于开发高效、可持续使用的非贵金属催化剂在聚合物电解质膜燃料电池中的应用至关重要, 但目前仍存在很多的难以攻克的问题。为了揭示纳米结构与电化学活性的关系, 本研究开发了一种具有高电化学活性的Fe-N/C氧还原催化剂, 该催化剂含有Fe-N_x位点和被氮掺杂的碳纳米管包裹的Fe/Fe₃C纳米晶体两种具有氧还原反应电化学活性的纳米结构。尽管不含贵金属铂, 本研究合成的Fe-N/C催化剂在碱性条件下仍显示出较高的ORR活性, 半波电势为0.86 V(vs RHE), 质量活性为18.84 A/g(0.77 V(vs RHE), 极限电流密度为-4.3 mA·cm ^-2。同时, 电子转移数为3.7(0.2 V(vs RHE), 说明Fe-N/C催化剂中4电子ORR反应的比例较高。石墨烯包覆的金属Fe/Fe₃C纳米晶生长N-CNTs后, 材料的导电性有所提高, 并且Fe-N_x活性位点在Fe/Fe₃C纳米颗粒表面分布均匀, 改善了材料的电化学活性。本研究为非贵金属氧还原电催化剂的继续深入研究以及广泛应用于商业化生产提供了一定的借鉴和依据。     

2D Nanoporous Fe−N/C Nanosheets as Highly Efficient Non‐Platinum Electrocatalysts for Oxygen Reduction Reaction in Zn‐Air Battery

It is an ongoing challenge to fabricate nonprecious oxygen reduction reaction (ORR) catalysts that can be comparable to or exceed the efficiency of platinum. A highly active non‐platinum self‐supporting Fe?N/C catalyst has been developed through the pyrolysis of a new type of precursor of iron coordination complex, in which 1,4‐bis(1H‐1,3,7,8–tetraazacyclopenta(1)phenanthren‐2‐yl)benzene (btcpb) functions as a ligand complexing Fe(II) ions. The optimal catalyst pyrolyzed at 700 °C (Fe?N/C?700) shows the best ORR activity with a half‐wave potential (E_1/2) of 840 mV versus reversible hydrogen electrode (RHE) in 0.1 m KOH, which is more positive than that of commercial Pt/C (E_1/2: 835 mV vs RHE). Additionally, the Fe?N/C?700 catalyst also exhibits high ORR activity in 0.1 m HClO₄ with the onset potential and E_1/2 comparable to those of the Pt/C catalyst. Notably, the Fe?N/C?700 catalyst displays superior durability (9.8 mV loss in 0.1 m KOH and 23.6 mV loss in 0.1 m HClO₄ for E_1/2 after 8000 cycles) and better tolerance to methanol than Pt/C. Furthermore, the Fe?N/C?700 catalyst can be used for fabricating the air electrode in Zn–air battery with a specific capacity of 727 mA hg^?1 at 5 mA cm^?2 and a negligible voltage loss after continuous operation for 110 h.     

Web‐Like Interconnected Carbon Networks from NaCl‐Assisted Pyrolysis of ZIF‐8 for Highly Efficient Oxygen Reduction Catalysis

The oxygen reduction reaction (ORR) is under intense research due to its significance in energy storage and conversion processes. Recent studies show that interconnected and hierarchically porous structures can further enhance ORR kinetics as well as catalyst durability, but their preparation can be quite time and/or chemical consuming. Here, a simple approach is reported to prepare such complex structures by pyrolyzing composites containing NaCl and ZIF‐8. The templating effect of molten NaCl connects ZIF‐8 particles into web‐like carbon networks. During ORR activity measurements, it achieves a 0.964 V onset potential and a 38 mV dec^?1 Tafel slope, which are comparable to those of the benchmark Pt/C (0.979 V and 40 mV dec^?1). Due to the metal‐free feature, this catalyst exhibits a 16 mV shift in half‐wave potential after a 10 000‐cycle durability test, which is only 60% of that of Pt/C. The catalyst is also tested in Zn–air batteries and the assemblies are able to work at above 1.2 V for 140 h, which triples the life held by those with Pt/C. This study demonstrates a facile strategy to prepare metal‐free ORR catalysts with interconnectivity and hierarchical porosity, and proves their great potentials in ORR catalysis and Zn–air batteries.     

Boosting Bifunctional Oxygen Electrocatalysis with 3D Graphene Aerogel‐Supported Ni/MnO Particles

Electrocatalysts for oxygen‐reduction and oxygen‐evolution reactions (ORR and OER) are crucial for metal–air batteries, where more costly Pt‐ and Ir/Ru‐based materials are the benchmark catalysts for ORR and OER, respectively. Herein, for the first time Ni is combined with MnO species, and a 3D porous graphene aerogel‐supported Ni/MnO (Ni–MnO/rGO aerogel) bifunctional catalyst is prepared via a facile and scalable hydrogel route. The synthetic strategy depends on the formation of a graphene oxide (GO) crosslinked poly(vinyl alcohol) hydrogel that allows for the efficient capture of highly active Ni/MnO particles after pyrolysis. Remarkably, the resulting Ni–MnO/rGO aerogels exhibit superior bifunctional catalytic performance for both ORR and OER in an alkaline electrolyte, which can compete with the previously reported bifunctional electrocatalysts. The MnO mainly contributes to the high activity for the ORR, while metallic Ni is responsible for the excellent OER activity. Moreover, such bifunctional catalyst can endow the homemade Zn–air battery with better power density, specific capacity, and cycling stability than mixed Pt/C + RuO₂ catalysts, demonstrating its potential feasibility in practical application of rechargeable metal–air batteries.