Exceptional Performance of Hierarchical Ni–Fe (hydr)oxide@NiCu Electrocatalysts for Water Splitting

Developing low‐cost bifunctional electrocatalysts with superior activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for the widespread application of the water splitting technique. In this work, using earth‐abundant transition metals (i.e., nickel, iron, and copper), 3D hierarchical nanoarchitectures, consisting of ultrathin Ni–Fe layered‐double‐hydroxide (Ni–Fe LDH) nanosheets or porous Ni–Fe oxides (NiFeO_x) assembled to a metallic NiCu alloy, are delicately constructed. In alkaline solution, the as‐prepared Ni–Fe LDH@NiCu possesses outstanding OER activity, achieving a current density of 10 mA cm^?2 at an overpotential of 218 mV, which is smaller than that of RuO₂ catalyst (249 mV). In contrast, the resulting NiFeO_x@NiCu exhibits better HER activity, yielding a current density of 10 mA cm^?2 at an overpotential of 66 mV, which is slightly higher than that of Pt catalyst (53 mV) but superior to all other transition metal (hydr)oxide‐based electrocatalysts. The remarkable activity of the Ni–Fe LDH@NiCu and NiFeO_x@NiCu is further demonstrated by a 1.5 V solar‐panel‐powered electrolyzer, resulting in current densities of 10 and 50 mA cm^?2 at overpotentials of 293 and 506 mV, respectively. Such performance renders the as‐prepared materials as the best bifunctional electrocatalysts so far.     

Synthesis of Bimetallic Conductive 2D Metal–Organic Framework (CoxNiy‐CAT) and Its Mass Production: Enhanced Electrochemical Oxygen Reduction Activity

The development of new electrocatalysts for electrochemical oxygen reduction to replace expensive and rare platinum‐based catalysts is an important issue in energy storage and conversion research. In this context, conductive and porous metal–organic frameworks (MOFs) are considered promising materials for the oxygen reduction reaction (ORR) due to not only their high surface area and well‐developed pores but also versatile structural features and chemical compositions. Herein, the preparation of bimetallic conductive 2D MOFs (Co_xNi_y‐CATs) are reported for use as catalysts in the ORR. The ratio of the two metal ions (Co²⁺ and Ni²⁺) in the bimetallic Co_xNi_y‐CATs is rationally controlled to determine the optimal composition of Co_xNi_y‐CAT for efficient performance in the ORR. Indeed, bimetallic MOFs display enhanced ORR activity compared to their monometallic counterparts (Co‐CAT or Ni‐CAT). During the ORR, bimetallic Co_xNi_y‐CATs retain an advantageous characteristic of Co‐CAT in relation to its high diffusion‐limiting current density, as well as a key advantage of Ni‐CAT in relation to its high onset potential. Moreover, the ORR‐active bimetallic Co_xNi_y‐CAT with excellent ORR activity is prepared at a large scale via a convenient method using a ball‐mill reactor.     

Amorphous Bimetallic Oxide–Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn–Air Batteries

Metal oxides of earth‐abundant elements are promising electrocatalysts to overcome the sluggish oxygen evolution and oxygen reduction reaction (OER/ORR) in many electrochemical energy‐conversion devices. However, it is difficult to control their catalytic activity precisely. Here, a general three‐stage synthesis strategy is described to produce a family of hybrid materials comprising amorphous bimetallic oxide nanoparticles anchored on N‐doped reduced graphene oxide with simultaneous control of nanoparticle elemental composition, size, and crystallinity. Amorphous Fe_0.5Co_0.5O_x is obtained from Prussian blue analog nanocrystals, showing excellent OER activity with a Tafel slope of 30.1 mV dec^?1 and an overpotential of 257 mV for 10 mA cm^?2 and superior ORR activity with a large limiting current density of ?5.25 mA cm^?2 at 0.6 V. A fabricated Zn–air battery delivers a specific capacity of 756 mA h g_Zn^?1 (corresponding to an energy density of 904 W h kg_Zn^?1), a peak power density of 86 mW cm^?2 and can be cycled over 120 h at 10 mA cm^?2. Other two amorphous bimetallic, Ni_0.4Fe_0.6O_x and Ni_0.33Co_0.67O_x , are also produced to demonstrate the general applicability of this method for synthesizing binary metal oxides with controllable structures as electrocatalysts for energy conversion.     

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.     

Co Nanoislands Rooted on Co–N–C Nanosheets as Efficient Oxygen Electrocatalyst for Zn–Air Batteries

Developing non‐precious‐metal bifunctional oxygen reduction and evolution reaction (ORR/OER) catalysts is a major task for promoting the reaction efficiency of Zn–air batteries. Co‐based catalysts have been regarded as promising ORR and OER catalysts owing to the multivalence characteristic of cobalt element. Herein, the synthesis of Co nanoislands rooted on Co–N–C nanosheets supported by carbon felts (Co/Co–N–C) is reported. Co nanosheets rooted on the carbon felt derived from electrodeposition are applied as the self‐template and cobalt source. The synergistic effect of metal Co islands with OER activity and Co–N–C nanosheets with superior ORR performance leads to good bifuctional catalytic performances. Wavelet transform extended X‐ray absorption fine spectroscopy and X‐ray photoelectron spectroscopy certify the formation of Co (mainly Co⁰) and the Co–N–C (mainly Co²⁺ and Co³⁺) structure. As the air‐cathode, the assembled aqueous Zn–air battery exhibits a small charge–discharge voltage gap (0.82 V@10 mA cm^?2) and high power density of 132 mW cm^?2, outperforming the commercial Pt/C catalyst. Additionally, the cable flexible rechargeable Zn–air battery exhibits excellent bendable and durability. Density functional theory calculation is combined with operando X‐ray absorption spectroscopy to further elucidate the active sites of oxygen reactions at the Co/Co–N–C cathode in Zn–air battery.     

Graphene Decorated with Uniform Ultrathin (CoP)x–(FeP)1–x Nanorods: A Robust Non‐Noble‐Metal Catalyst for Hydrogen Evolution

Developing high‐performance but low‐cost hydrogen evolution reaction (HER) electrocatalysts with superior activity and stability for future sustainable energy conversion technologies is highly desired. Tuning of microstructure, configuration, and chemical composition are paramount to developing effective non‐noble electrocatalysts for HER. Herein, a universal “nanocasting” method is reported to construct graphene decorated with uniform ternary (CoP)_x –(FeP)_1?x (0 ≤ x ≤ 1) nanorods hybrids with different chemical compositions [(CoP)_x –(FeP)_1?x –NRs/G] as a highly active and durable nonprecious‐metal electrocatalyst for the HER. The optimized (CoP)_0.54–(FeP)_0.46–NRs/G electrocatalyst exhibits overpotentials of as low as 57 and 97 mV at 10 mA cm^?2, Tafel slopes of 52 and 62 mV dec^?1, exchange current densities of 0.489 and 0.454 mA cm^?2, and Faradaic efficiency of nearly 100% in acidic and alkaline media, respectively. More importantly, this electrocatalyst also exhibits high tolerance and durability in a wide pH range and keeps catalytic activity for at least 3000 cycles and 24 h of sustained hydrogen production. The excellent catalytic performance of the (CoP)_x –(FeP)_1?x –NRs/G electrocatalyst may be ascribed to its unique mesoporous structure and strong synergistic effect between CoP and FeP. Thus, the work provides a feasible way to fabricate cheap and highly efficient electrocatalyst as alternatives for Pt‐based electrocatalysts for HER in electrochemical water splitting.     

Ultrathin Cobalt Oxide Layers as Electrocatalysts for High‐Performance Flexible Zn–Air Batteries

Synergistic improvements in the electrical conductivity and catalytic activity for the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) are of paramount importance for rechargeable metal–air batteries. In this study, one‐nanometer‐scale ultrathin cobalt oxide (CoO_x) layers are fabricated on a conducting substrate (i.e., a metallic Co/N‐doped graphene substrate) to achieve superior bifunctional activity in both the ORR and OER and ultrahigh output power for flexible Zn–air batteries. Specifically, at the atomic scale, the ultrathin CoO_x layers effectively accelerate electron conduction and provide abundant active sites. X‐ray absorption spectroscopy reveals that the metallic Co/N‐doped graphene substrate contributes to electron transfer toward the ultrathin CoO_x layer, which is beneficial for the electrocatalytic process. The as‐obtained electrocatalyst exhibits ultrahigh electrochemical activity with a positive half‐wave potential of 0.896 V for ORR and a low overpotential of 370 mV at 10 mA cm^?2 for OER. The flexible Zn–air battery built with this catalyst exhibits an ultrahigh specific power of 300 W g_cat ^?1, which is essential for portable devices. This work provides a new design pathway for electrocatalysts for high‐performance rechargeable metal–air battery systems.     

Co9S8 Nanoparticles‐Embedded N/S‐Codoped Carbon Nanofibers Derived from Metal–Organic Framework‐Wrapped CdS Nanowires for Efficient Oxygen Evolution Reaction

Metal–organic frameworks (MOFs) with tunable compositions and morphologies are recognized as efficient self‐sacrificial templates to achieve function‐oriented nanostructured materials. Moreover, it is urgently needed to develop highly efficient noble metal‐free oxygen evolution reaction (OER) electrocatalysts to accelerate the development of overall water splitting green energy conversion systems. Herein, a facile and cost‐efficient strategy to synthesize Co₉S₈ nanoparticles‐embedded N/S‐codoped carbon nanofibers (Co₉S₈/NSCNFs) as highly active OER catalyst is developed. The hybrid precursor of core–shell ZIF‐wrapped CdS nanowires is first prepared and then leads to the formation of uniformly dispersed Co₉S₈/N, S‐codoped carbon nanocomposites through a one‐step calcination reaction. The optimal Co₉S₈/NSCNFs‐850 is demonstrated to possess excellent electrocatalytic performance for OER in 1.0 m KOH solution, affording a low overpotential of 302 mV to reach the current density of 10 mA cm^?2, a small Tafel slope of 54 mV dec^?1, and superior long‐term stability for 1000 cyclic voltammetry cycles. The favorable results raise a concept of exploring more MOF‐based nanohybrids as precursors to induce the synthesis of novel porous nanomaterials as non‐noble‐metal electrocatalysts for sustainable energy conversion.     

Aluminum‐Tailored Energy Level and Morphology of Co3−xAlxO4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation

Tuning energy levels plays a crucial role in developing cost‐effective, earth‐abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom‐occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom‐engineered energy levels of spinel Co₃O₄ for highly‐effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co²⁺ ions in the tetrahedral sites and Co³⁺ ions in the octahedral sites (denoted as Co²⁺_Td and Co³⁺_Oh, respectively). Experimental and theoretical simulations demonstrate that Al³⁺ ions substituting Co²⁺_Td and Co³⁺_Oh active sites, especially Al³⁺ ions occupying the Co²⁺_Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co_1.75Al_1.25O₄ nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm^–2, among the best Co‐based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al‐occupied different Co sites in Co_3–xAl_xO₄ composites on OER performance, but also inspire the design of low‐cost, earth‐abundant, and high‐active electrocatalysts toward water oxidation.     

Edge Sites with Unsaturated Coordination on Core–Shell Mn3O4@MnxCo3−xO4 Nanostructures for Electrocatalytic Water Oxidation

Transition‐metal oxides are extensively investigated as efficient electrocatalysts for the oxygen evolution reaction (OER). However, large‐scale applications remain challenging due to their moderate catalytic activity. Optimized regulation of surface states can lead to improvement of catalytic properties. Here, the design of Mn@Co_x Mn_3?x O₄ nanoparticles with abundant edge sites via a simple seed‐mediated growth strategy is described. The unsaturated coordination generated on the edge sites of Co_x Mn_3?x O₄ shells makes a positive contribution to the surface‐structure tailoring. Density functional theory calculations indicate that the edge sites with unsaturated coordination exhibit intense affinity for OH^? in the alkaline electrolyte, which greatly enhances the electrochemical OER performance of the catalysts. The resulting Mn@Co_x Mn_3?x O₄ catalysts yield a current density of 10 mA cm^?2 at an overpotential of 246 mV and a relatively low Tafel slope of 46 mV dec^?1. The successful synthesis of these metal oxides nanoparticles with edge sites may pave a new path for rationally fabricating efficient OER catalysts.     

Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co?O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

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.     

An Oxygen‐Vacancy‐Rich Semiconductor‐Supported Bifunctional Catalyst for Efficient and Stable Zinc–Air Batteries

The highly oxidative operating conditions of rechargeable zinc–air batteries causes significant carbon‐support corrosion of bifunctional oxygen electrocatalysts. Here, a new strategy for the catalyst support design focusing on oxygen vacancy (OV)‐rich, low‐bandgap semiconductor is proposed. The OVs promote the electrical conductivity of the oxide support, and at the same time offer a strong metal–support interaction (SMSI), which enables the catalysts to have small metal size, high catalytic activity, and high stability. The strategy is demonstrated by successfully synthesizing ultrafine Co‐metal‐decorated 3D ordered macroporous titanium oxynitride (3DOM‐Co@TiO_xN_y). The 3DOM‐Co@TiO_xN_y catalyst exhibits comparable activities for oxygen reduction and evolution reactions, but much higher cycling stability than noble metals in alkaline conditions. The zinc–air battery using this catalyst delivers an excellent stability with less than 1% energy efficiency loss over 900 charge–discharge cycles at 20 mA cm^?2. The high stability is attributed to the strong SMSI between Co and 3DOM‐TiO_xN_y which is verified by density functional theory calculations. This work sheds light on using OV‐rich semiconductors as a promising support to design efficient and durable nonprecious electrocatalysts.     

Hollow Co3O4 Nanosphere Embedded in Carbon Arrays for Stable and Flexible Solid‐State Zinc–Air Batteries

Highly active and durable air cathodes to catalyze both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are urgently required for rechargeable metal–air batteries. In this work, an efficient bifunctional oxygen catalyst comprising hollow Co₃O₄ nanospheres embedded in nitrogen‐doped carbon nanowall arrays on flexible carbon cloth (NC‐Co₃O₄/CC) is reported. The hierarchical structure is facilely derived from a metal–organic framework precursor. A carbon onion coating constrains the Kirkendall effect to promote the conversion of the Co nanoparticles into irregular hollow oxide nanospheres with a fine scale nanograin structure, which enables promising catalytic properties toward both OER and ORR. The integrated NC‐Co₃O₄/CC can be used as an additive‐free air cathode for flexible all‐solid‐state zinc–air batteries, which present high open circuit potential (1.44 V), high capacity (387.2 mAh g^?1, based on the total mass of Zn and catalysts), excellent cycling stability and mechanical flexibility, significantly outperforming Pt‐ and Ir‐based zinc–air batteries.     

Active Sites Intercalated Ultrathin Carbon Sheath on Nanowire Arrays as Integrated Core–Shell Architecture: Highly Efficient and Durable Electrocatalysts for Overall Water Splitting

The development of active bifunctional electrocatalysts with low cost and earth‐abundance toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a great challenge for overall water splitting. Herein, metallic Ni₄Mo nanoalloys are firstly implanted on the surface of NiMoO_x nanowires array (NiMo/NiMoO_x ) as metal/metal oxides hybrid. Inspired by the superiority of carbon conductivity, an ultrathin nitrogen‐doped carbon sheath intercalated NiMo/NiMoO_x (NC/NiMo/NiMoO_x ) nanowires as integrated core–shell architecture are constructed. The integrated NC/NiMo/NiMoO_x array exhibits an overpotential of 29 mV at 10 mA cm^?2 and a low Tafel slope of 46 mV dec^?1 for HER due to the abundant active sites, fast electron transport, low charge‐transfer resistance, unique architectural structure and synergistic effect of carbon sheath, nanoalloys, and oxides. Moreover, as OER catalysts, the NC/NiMo/NiMoO_x hybrids require an overpotential of 284 mV at 10 mA cm^?2. More importantly, the NC/NiMo/NiMoO_x array as a highly active and stable electrocatalyst approaches ≈10 mA cm^?2 at a voltage of 1.57 V, opening an avenue to the rational design and fabrication of the promising electrode materials with architecture structures toward the electrochemical energy storage and conversion.     

Ferrocene‐Incorporated Cobalt Sulfide Nanoarchitecture for Superior Oxygen Evolution Reaction

Here, ferrocene(Fc)‐incorporated cobalt sulfide (Co_xS_y) nanostructures directly grown on carbon nanotube (CNT) or carbon fiber (CF) networks for electrochemical oxygen evolution reaction (OER) using a facile one‐step solvothermal method are reported. The strong synergistic interaction between Fc‐Co_xS_y nanostructures and electrically conductive CNTs results in the superior electrocatalytic activity with a very small overpotential of ≈304 mV at 10 mA cm^?2 and a low Tafel slope of 54.2 mV dec^?1 in 1 m KOH electrolyte. Furthermore, the Fc‐incorporated Co_xS_y (FCoS) nanostructures are directly grown on the acid pretreated carbon fiber (ACF), and the resulting fabricated electrode delivers excellent OER performance with a low overpotential of ≈315 mV at 10 mA cm^?2. Such superior OER catalytic activity can be attributed to 3D Fc‐Co_xS_y nanoarchitectures that consist of a high concentration of vertical nanosheets with uniform distribution of nanoparticles that afford a large number of active surface areas and edge sites. Besides, the tight contact interface between ACF substrate and Fc‐Co_xS_y nanostructures could effectively facilitate the electron transfer rate in the OER. This study provides valuable insights for the rational design of energy storage and conversion materials by the incorporation of other transition metal into metal sulfide/oxide nanostructures utilizing metallocene.     

Enhancing Oxygen Evolution Reaction through Modulating Electronic Structure of Trimetallic Electrocatalysts Derived from Metal–Organic Frameworks

The construction of efficient, durable, and non‐noble metal electrocatalysts for oxygen evolution reaction (OER) is of great value but challenging. Herein, a facile method is developed to synthesize a series of trimetallic (W/Co/Fe) metal–organic frameworks (MOFs)‐derived carbon nanoflakes (CNF) with various Fe content, and an Fe‐dependent volcano‐type plot can be drawn out for WCoFe_x ‐CNF. The optimized WCoFe_0.3‐CNF (when the feed ratio of Fe/Co is 0.3) demonstrates superior electrocatalytic performance with a low overpotential of only 254 mV@10 mA cm^?2 and excellent durability of 100 h. Further researches show that appropriate amount of iron doping can regulate the electronic structure, resulting in a favorable synergistic environment. This method may stimulate the exploration of electrocatalysts by utilizing MOFs as precursors while realizing electronic modulation by multimetal doping.     

Enlarged Co?O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co? O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

Bifunctional Transition Metal Hydroxysulfides: Room‐Temperature Sulfurization and Their Applications in Zn–Air Batteries

Bifunctional electrocatalysis for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) constitutes the bottleneck of various sustainable energy devices and systems like rechargeable metal–air batteries. Emerging catalyst materials are strongly requested toward superior electrocatalytic activities and practical applications. In this study, transition metal hydroxysulfides are presented as bifunctional OER/ORR electrocatalysts for Zn–air batteries. By simply immersing Co‐based hydroxide precursor into solution with high‐concentration S^2?, transition metal hydroxides convert to hydroxysulfides with excellent morphology preservation at room temperature. The as‐obtained Co‐based metal hydroxysulfides are with high intrinsic reactivity and electrical conductivity. The electron structure of the active sites is adjusted by anion modulation. The potential for 10 mA cm^?2 OER current density is 1.588 V versus reversible hydrogen electrode (RHE), and the ORR half‐wave potential is 0.721 V versus RHE, with a potential gap of 0.867 V for bifunctional oxygen electrocatalysis. The Co₃FeS_1.5(OH)₆ hydroxysulfides are employed in the air electrode for a rechargeable Zn–air battery with a small overpotential of 0.86 V at 20.0 mA cm^?2, a high specific capacity of 898 mAh g^?1, and a long cycling life, which is much better than Pt and Ir‐based electrocatalyst in Zn–air batteries.     

Hierarchical 3D Oxygenated Cobalt Molybdenum Selenide Nanosheets as Robust Trifunctional Catalyst for Water Splitting and Zinc–Air Batteries

The development of hierarchical nanostructures with highly active and durable multifunctional catalysts has a new significance in the context of new energy technologies of water splitting and metal–air batteries. Herein, a strategy is demonstrated to construct a 3D hierarchical oxygenated cobalt molybdenum selenide (O‐Co_1?xMo_xSe₂) series with attractive nanoarchitectures, which are fabricated by a simple and cost‐effective hydrothermal process followed by an exclusive ion‐exchange process. Owing to its highly electroactive sites with numerous nanoporous networks and plentiful oxygen vacancies, the optimal O‐Co_0.5Mo_0.5Se₂ could catalyze the hydrogen evolution reaction and oxygen evolution reaction effectively with a low overpotential of ≈102 and 189 mV, at a current density of 10 mA cm^?2, respectively, and exceptional durability. Most importantly, the O‐Co_0.5Mo_0.5Se₂||O‐Co_0.5Mo_0.5Se₂ water splitting device only entails a voltage of ≈1.53 V at a current density of 10 mA cm^?2, which is much better than benchmark Pt/C||RuO₂ (≈1.56 V). Furthermore, O‐Co_0.5Mo_0.5Se₂ air cathode‐based zinc–air batteries exhibit an excellent power density of 120.28 mW cm^?2 and exceptional cycling stability for 60 h, superior to those of state‐of‐art Pt/C+RuO₂ pair‐based zinc–air batteries. The present study provides a strategy to design hierarchical 3D oxygenated bimetallic selenide‐based multifunctional catalysts for energy conversion and storage systems.