Zirconium‐Regulation‐Induced Bifunctionality in 3D Cobalt–Iron Oxide Nanosheets for Overall Water Splitting

The design of high‐efficiency non‐noble bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting technologies and associated renewable energy systems. Spinel‐structured oxides with rich redox properties can serve as alternative low‐cost OER electrocatalysts but with poor HER performance. Here, zirconium regulation in 3D CoFe₂O₄ (CoFeZr oxides) nanosheets on nickel foam, as a novel strategy inducing bifunctionality toward OER and HER for overall water splitting, is reported. It is found that the incorporation of Zr into CoFe₂O₄ can tune the nanosheet morphology and electronic structure around the Co and Fe sites for optimizing adsorption energies, thus effectively enhancing the intrinsic activity of active sites. The as‐synthesized 3D CoFeZr oxide nanosheet exhibits high OER activity with small overpotential, low Tafel slope, and good stability. Moreover, it shows unprecedented HER activity with a small overpotential of 104 mV at 10 mA cm^?2 in alkaline media, which is better than ever reported counterparts. When employing the CoFeZr oxides nanosheets as both anode and cathode catalysts for overall water splitting, a current density of 10 mA cm^?2 is achieved at the cell voltage of 1.63 V in 1.0 m KOH.     

Engineering of Electronic States on Co3O4 Ultrathin Nanosheets by Cation Substitution and Anion Vacancies for Oxygen Evolution Reaction

Due to the earth abundance and tunable electronic properties, etc., transition metal oxides (TMOs) show attractive attention in oxygen evolution reaction. O‐vacancies (V_o) play important roles in tailoring the local surface and electronic environment to lower the activation barriers. Herein, an effective strategy is shown to enhance the oxygen evolution reduction (OER) performance on Co₃O₄ ultrathin nanosheets via combined cation substitution and anion vacancies. The oxygen‐deficient Fe‐Co‐O nanosheets (3–4 nm thickness) display an overpotential of 260 mV@10 mA cm^?2 and a Tafel slope of 53 mV dec^?1, outperforming those of the benchmark RuO₂ in 1.0 m KOH. Further calculations demonstrate that the combined introduction of Fe cation and V_o with appropriate location and content finely tune the intermediate absorption, consequently lowering the rate‐limiting activation energy from 0.82 to as low as 0.15 eV. The feasibility is also proved by oxygen‐deficient Ni‐Co‐O nanosheets. This work not only establishes a clear atomic‐level correlation between cation substitution, anion vacancies, and OER performance, but also provides valuable insights for the rational design of highly efficient catalysts for OER.     

Ultrathin Iron‐Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction

Electrochemical water splitting is a promising method for storing light/electrical energy in the form of H₂ fuel; however, it is limited by the sluggish anodic oxygen evolution reaction (OER). To improve the accessibility of H₂ production, it is necessary to develop an efficient OER catalyst with large surface area, abundant active sites, and good stability, through a low‐cost fabrication route. Herein, a facile solution reduction method using NaBH₄ as a reductant is developed to prepare iron‐cobalt oxide nanosheets (Fe_x Co_y ‐ONSs) with a large specific surface area (up to 261.1 m² g^?1), ultrathin thickness (1.2 nm), and, importantly, abundant oxygen vacancies. The mass activity of Fe₁Co₁‐ONS measured at an overpotential of 350 mV reaches up to 54.9 A g^?1, while its Tafel slope is 36.8 mV dec^?1; both of which are superior to those of commercial RuO₂, crystalline Fe₁Co₁‐ONP, and most reported OER catalysts. The excellent OER catalytic activity of Fe₁Co₁‐ONS can be attributed to its specific structure, e.g., ultrathin nanosheets that could facilitate mass diffusion/transport of OH^? ions and provide more active sites for OER catalysis, and oxygen vacancies that could improve electronic conductivity and facilitate adsorption of H₂O onto nearby Co³⁺ sites.     

Graphene Composites with Cobalt Sulfide: Efficient Trifunctional Electrocatalysts for Oxygen Reversible Catalysis and Hydrogen Production in the Same Electrolyte

Nitrogen and sulfur‐codoped graphene composites with Co₉S₈ (NS/rGO‐Co) are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere. Significantly, in 0.1 m KOH aqueous solution the best sample exhibits an oxygen evolution reaction (OER) activity that is superior to that of benchmark RuO₂ catalysts, an oxygen reduction reaction (ORR) activity that is comparable to that of commercial Pt/C, and an overpotential of only ?0.193 V to reach 10 mA cm^?2 for hydrogen evolution reaction (HER). With this single catalyst for oxygen reversible electrocatalysis, a potential difference of only 0.700 V is observed in 0.1 m KOH solution between the half‐wave potential in ORR and the potential to reach 10 mA cm^?2 in OER; in addition, an overpotential of only 450 mV is needed to reach 10 mA cm^?2 for full water splitting in the same electrolyte. The present trifunctional catalytic activities are markedly better than leading results reported in recent literature, where the remarkable trifunctional activity is attributed to the synergetic effects between N,S‐codoped rGO, and Co₉S₈ nanoparticles. These results highlight the significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts for versatile electrochemical reactions.     

NiFe (Oxy) Hydroxides Derived from NiFe Disulfides as an Efficient Oxygen Evolution Catalyst for Rechargeable Zn–Air Batteries: The Effect of Surface S Residues

A facile H₂O₂ oxidation treatment to tune the properties of metal disulfides for oxygen evolution reaction (OER) activity enhancement is introduced. With this method, the degree of oxidation can be readily controlled and the effect of surface S residues in the resulted metal (oxy)hydroxides for the OER is revealed for the first time. The developed NiFe (oxy)hydroxide catalyst with residual S demonstrates an extraordinarily low OER overpotential of 190 mV at the current density of 10 mA cm^?2 after coupling with carbon nanotubes, and outstanding performance in Zn–air battery tests. Theoretical calculation suggests that the surface S residues can significantly reduce the adsorption free energy difference between O* and OH* intermediates on the Fe sites, which should account for the high OER activity of NiFe (oxy)hydroxide catalysts. This work provides significant insight regarding the effect of surface heteroatom residues in OER electrocatalysis and offers a new strategy to design high‐performance and cost‐efficient OER catalysts.     

One‐Step Route Synthesized Co2P/Ru/N‐Doped Carbon Nanotube Hybrids as Bifunctional Electrocatalysts for High‐Performance Li–O2 Batteries

The large‐scale commercial application of lithium–oxygen batteries (LOBs) is overwhelmed by the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) associated with insoluble and insulated Li₂O₂. Herein, an elaborate design on a highly catalytic LOBs cathode constructed by N‐doped carbon nanotubes (CNT) with in situ encapsulated Co₂P and Ru nanoparticles is reported. The homogeneously dispersed Co₂P and Ru catalysts can effectively modulate the formation and decomposition behavior of Li₂O₂ during discharge/charge processes, ameliorating the electronically insulating property of Li₂O₂ and constructing a homogenous low‐impedance Li₂O₂/catalyst interface. Compared with Co/CNT and Ru/CNT electrodes, the Co₂P/Ru/CNT electrode delivers much higher oxygen reduction triggering onset potential and higher ORR and OER peak current and integral areas, showing greatly improved ORR/OER kinetics due to the synergistic effects of Co₂P and Ru. Li–O₂ cells based on the Ru/Co₂P/CNT electrode demonstrate improved ORR/OER overpotential of 0.75 V, excellent rate capability of 12 800 mAh g^?1 at 1 A g^?1, and superior cycle stability for more than 185 cycles under a restricted capacity of 1000 mAh g^?1 at 100 mA g^?1. This work paves an exciting avenue for the design and construction of bifunctional catalytic cathodes by coupling metal phosphides with other active components in LOBs.     

Self‐Assembled Ruddlesden–Popper/Perovskite Hybrid with Lattice‐Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

The oxygen evolution reaction (OER) is pivotal in multiple gas‐involved energy conversion technologies, such as water splitting, rechargeable metal–air batteries, and CO₂/N₂ electrolysis. Emerging anion‐redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice‐oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice‐oxygen sites for OER catalysis via constructing a Ruddlesden–Popper/perovskite hybrid, which is prepared by a facile one‐pot self‐assembly method, is developed. As a proof‐of‐concept, the unique hybrid catalyst (RP/P‐LSCF) consists of a dominated Ruddlesden–Popper phase LaSr₃Co_1.5Fe_1.5O_10‐δ (RP‐LSCF) and second perovskite phase La_0.25Sr_0.75Co_0.5Fe_0.5O_3‐δ (P‐LSCF), displaying exceptional OER activity. The RP/P‐LSCF achieves 10 mA cm^?2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO₂ and various state‐of‐the‐art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP‐LSCF oxide. The high catalytic performance for RP/P‐LSCF is attributed to the strong metal–oxygen covalency and high oxygen‐ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice‐oxygen activation to participate in OER. The success of Ruddlesden–Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.     

NiMoFe and NiMoFeP as Complementary Electrocatalysts for Efficient Overall Water Splitting and Their Application in PV‐Electrolysis with STH 12.3%

Complementary water splitting electrocatalysts used simultaneously in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can simplify water splitting systems. Herein, earth‐abundant NiMoFe (NMF) and phosphorized NiMoFeP (NMFP) are synthesized as complementary overall water splitting (OWS) catalysts. First, NMF is tested as both the HER and OER promoter, which exhibits low overpotentials of 68 (HER) and 337 mV (OER). A quaternary NMFP is then prepared by simple phosphorization of NMF, which shows a much lower OER overpotential of 286 mV. The enhanced OER activity is attributed to the unique surface/core structure of NMFP. The surface phosphate acts as a proton transport mediator and expedites the rate‐determining step. With the application of OER potential, the NMFP surface is composed of Ni(OH)₂ and FeOOH, active sites for OER, but the inner core consists of Ni, Mo, and Fe metals, serving as a conductive electron pathway. OWS with NMF‐NMFP requires an applied voltage of 1.452 V to generate 10 mA cm^?2, which is one of the lowest values among OWS results with transition‐metal‐based electrocatalysts. Furthermore, the catalysts are combined with tandem perovskite solar cells for photovoltaic (PV)‐electrolysis, producing a high solar‐to‐hydrogen (STH) conversion efficiency of 12.3%.     

Reducing Oxygen Evolution Reaction Overpotential in Cobalt‐Based Electrocatalysts via Optimizing the “Microparticles‐in‐Spider Web” Electrode Configurations

Sluggish kinetics of the multielectron transfer process is still a bottleneck for efficient oxygen evolution reaction (OER) activity, and the reduction of reaction overpotential is crucial to boost reaction kinetics. Herein, a correlation between the OER overpotential and the cobalt‐based electrode composition in a “Microparticles‐in‐Spider Web” (MSW) superstructure electrode is revealed. The overpotential is dramatically decreased first and then slightly increased with the continuous increase ratio of Co/Co₃O₄ in the cobalt‐based composite electrode, corresponding to the dynamic change of electrochemically active surface area and charge‐transfer resistance with the electrode composition. As a proof‐of‐concept, the optimized electrode displays a low overpotential of 260 mV at 10.0 mA cm^?2 in alkaline conditions with a long‐time stability. This electrochemical performance is comparable and even superior to the most currently reported Co‐based OER electrocatalysts. The remarkable electrocatalytic activity is attributed to the optimization of the electrochemically active sites and electron transfer in the MSW superstructure. Theoretical calculations identify that the metallic Co and Co₃O₄ surface catalytic sites play a vital role in improving electron transport and reaction Gibbs free energies for reducing overpotential, respectively. A general way of boosting OER kinetics via optimizing the electrode configurations to mitigate reaction overpotential is offered in this study.     

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.     

Promoting Oxygen Evolution Reaction of Co‐Based Catalysts (Co3O4, CoS,CoP, and CoN) through Photothermal Effect

The increase of reaction temperature of electrocatalysts is regarded as an efficient method to improve the oxygen evolution reaction (OER) activity. Herein, it is reported that the electrocatalytic performance of dual functional (i.e., electrocatalytic and photothermal functions) Co₃O₄ can be dramatically improved via its photothermal effect. The operating temperature of the Co₃O₄ electrode is elevated in situ under near infrared (NIR) light irradiation, resulting in enhanced oxygen evolution activity due to its accelerated electrical conductivity, reaction kinetics, and desorption rate of O₂ bubbles from the electrode. In addition, photothermal effect can also enhance the electrocatalytic reaction rates of metal‐doped Co₃O₄ electrodes, indicating that it is able to significantly improve the OER activities of electrodes together with other modification strategies. With the assistance of the photothermal effect, the obtained Ni‐doped Co₃O₄ catalyst requires an extremely low overpotential of 208 mV to achieve a benchmark of 10 mA cm^?2 with a small Tafel slope, superior to most reported Co‐based catalysts. Significantly, the electrocatalytic performance of other electrodes with photothermal effect, such as CoN, CoP, and CoS, are also boosted under NIR light irradiation, indicating opportunities for implementing photothermal enhancement in electrocatalytic water splitting.     

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.     

Iridium‐Based Multimetallic Porous Hollow Nanocrystals for Efficient Overall‐Water‐Splitting Catalysis

The development of active and durable bifunctional electrocatalysts for overall water splitting is mandatory for renewable energy conversion. This study reports a general method for controllable synthesis of a class of IrM (M = Co, Ni, CoNi) multimetallic porous hollow nanocrystals (PHNCs), through etching Ir‐based, multimetallic, solid nanocrystals using Fe³⁺ ions, as catalysts for boosting overall water splitting. The Ir‐based multimetallic PHNCs show transition‐metal‐dependent bifunctional electrocatalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic electrolyte, with IrCo and IrCoNi PHNCs being the best for HER and OER, respectively. First‐principles calculations reveal a ligand effect, induced by alloying Ir with 3d transition metals, can weaken the adsorption energy of oxygen intermediates, which is the key to realizing much‐enhanced OER activity. The IrCoNi PHNCs are highly efficient in overall‐water‐splitting catalysis by showing a low cell voltage of only 1.56 V at a current density of 2 mA cm^?2, and only 8 mV of polarization‐curve shift after a 1000‐cycle durability test in 0.5 m H₂SO₄ solution. This work highlights a potentially powerful strategy toward the general synthesis of novel, multimetallic, PHNCs as highly active and durable bifunctional electrocatalysts for high‐performance electrochemical overall‐water‐splitting devices.     

Boosting Neutral Water Oxidation through Surface Oxygen Modulation

Developing efficient electrocatalysts for oxygen evolution reaction (OER) in pH-neutral electrolyte is crucial for microbial electrolysis cells and electrochemical CO₂ reduction. Unfortunately, the OER kinetics in neutral electrolyte is sluggish due to the low concentration of adsorbed reactants, with overpotentials of neutral OER at present much higher than that in acidic or alkaline electrolyte. Here, hydrated metal cations (Ca²⁺) are sought to be incorporated into the state-of-the-art Ru–Ir binary oxide to tailor the surface oxygen environments (lattice-oxygen and adsorbed oxygen species) for efficient neutral OER. Using a sol–gel method, ternary Ru–Ir–Ca oxides are synthesized in atomically homogenous manner, and the obtained catalyst on glassy carbon electrode achieves 10 mA cm⁻² at a low overpotential of 250 mV, with no degradation for 200 h of operation. In situ X-ray absorption spectroscopy, in situ ¹⁸O isotope-labeling differential electrochemical mass spectrometry, and ¹⁸O isotope-labeling secondary ion mass spectroscopy studies are carried out. The results reveal that incorporation of Ca²⁺ can enhance the covalency of metal–oxygen bonds and the electrophilic nature of surface metal-bonded oxygen sites; and simultaneously facilitate the adsorption of water molecules on catalyst surface, which accelerates the lattice-oxygen-involved reaction, thus improving the overall OER performance of RuIrCaO_x catalyst.     

Trifunctional Single‐Atomic Ru Sites Enable Efficient Overall Water Splitting and Oxygen Reduction in Acidic Media

Development of cost‐effective, active trifunctional catalysts for acidic oxygen reduction (ORR) as well as hydrogen and oxygen evolution reactions (HER and OER, respectively) is highly desirable, albeit challenging. Herein, single‐atomic Ru sites anchored onto Ti₃C₂T_x MXene nanosheets are first reported to serve as trifunctional electrocatalysts for simultaneously catalyzing acidic HER, OER, and ORR. A half‐wave potential of 0.80 V for ORR and small overpotentials of 290 and 70 mV for OER and HER, respectively, at 10 mA cm^?2 are achieved. Hence, a low cell voltage of 1.56 V is required for the acidic overall water splitting. The maximum power density of an H₂–O₂ fuel cell using the as‐prepared catalyst can reach as high as 941 mW cm^?2. Theoretical calculations reveal that isolated Ru–O₂ sites can effectively optimize the adsorption of reactants/intermediates and lower the energy barriers for the potential‐determining steps, thereby accelerating the HER, ORR, and OER kinetics.     

Electrochemical Etching Switches Electrocatalytic Oxygen Evolution Pathway of IrOx/Y2O3 from Adsorbate Evolution Mechanism to Lattice-Oxygen-Mediated Mechanism

Oxygen evolution reaction (OER) plays key roles in electrochemical energy conversion devices. Recent advances have demonstrated that OER catalysts through lattice oxygen-mediated mechanism (LOM) can bypass the scaling relation-induced limitations on those catalysts through adsorbate evolution mechanism (AEM). Among various catalysts, IrO_x, the most promising OER catalyst, suffers from low activities for its AEM pathway. Here, it is demonstrated that a pre-electrochemical acidic etching treatments on the hybrids of IrO_x and Y₂O₃ (IrO_x/Y₂O₃) switch the AEM-dominated OER pathway to LOM-dominated one in alkali electrolyte, delivering a high performance with a low overpotential of 223 mV at 10 mA cm⁻² and a long-term stability. Mechanism investigations suggest that the pre-electrochemical etching treatments create more oxygen vacancies in catalysts due to the dissolution of yttrium and then provide highly active surface lattice oxygen for participating OER, thereby enabling the LOM-dominated pathway and resulting in a significantly increased OER activity in basic electrolyte.     

Surface Oxidation of AuNi Heterodimers to Achieve High Activities toward Hydrogen/Oxygen Evolution and Oxygen Reduction Reactions

Although much attention has been paid to the exploration of highly active electrocatalysts, especially catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the development of multifunctional catalysts remains a challenge. Here, we utilize AuNi heterodimers as the starting materials to achieve high activities toward HER, OER and ORR. The HER and ORR activities in an alkali environment are similar to those of Pt catalysts, and the OER activity is very high and better than that of commercial IrO₂. Both the experimental and calculated results suggest that the surface oxidation under oxidative conditions is the main reason for the different activities. The NiO/Ni interface which exists in the as‐synthesized heterodimers contributes to high HER activity, the Ni(OH)₂‐Ni‐Au interface and the surface Ni(OH)₂ obtained in electrochemical conditons gives rise to promising ORR and OER activities, respectively. As a comparison, a Au@Ni core‐shell structure is also synthesized and examined. The core‐shell structure shows lower activities for HER and OER than the heterodimers, and reduces O₂ selectively to H₂O₂. The work here allows for the development of a method to design multifunctional catalysts via the partial oxidation of a metal surface to create different active centers.     

Nickel Nitride Particles Supported on 2D Activated Graphene–Black Phosphorus Heterostructure: An Efficient Electrocatalyst for the Oxygen Evolution Reaction

Hydrogen is regarded as the most promising green clean energy in the 21st century. Developing the highly efficient and low‐cost electrocatalysts for oxygen evolution reaction (OER) is of great concern for the hydrogen industry. In the water electrolyzed reaction, the overpotential and the kinetics are the main hurdles for OER. Therefore, an efficient and durable oxygen evolution reaction electrocatalyst is required. In this study, an activated graphene (AG)–black phosphorus (BP) nanosheets hybrid is fabricated for supporting Ni₃N particles (Ni₃N/BP‐AG) in the application of OER. The Ni₃N particles are combined with the BP‐AG heterostructure via facile mechanical ball milling under argon protection. The synthesized Ni₃N/BP‐AG shows excellent catalytic performance toward the OER, demanding the overpotential of 233 mV for a current density of 10 mA cm^?2 with a Tafel slope of 42 mV dec^?1. The Ni₃N/BP‐AG catalysts also show remarkable stability with a retention rate of the current density of about 86.4% after measuring for 10 000 s in potentiostatic mode.     

Ultrathin Amorphous Nickel Doped Cobalt Phosphates with Highly Ordered Mesoporous Structures as Efficient Electrocatalyst for Oxygen Evolution Reaction

Herein, the facile preparation of ultrathin (≈3.8 nm in thickness) 2D cobalt phosphate (CoPi) nanoflakes through an oil‐phase method is reported. The obtained nanoflakes are composed of highly ordered mesoporous (≈3.74 nm in diameter) structure and exhibit an amorphous nature. Attractively, when doped with nickel, such 2D mesoporous Ni‐doped CoPi nanoflakes display decent electrocatalytic performances in terms of intrinsic activity, and low kinetic barrier toward the oxygen evolution reaction (OER). Particularly, the optimized 10 at% Ni‐doped CoPi nanoflakes (denoted as Ni10‐CoPi) deliver a low overpotential at 10 mA cm^?2 (320 mV), small Tafel slope (44.5 mV dec^?1), and high stability for OER in 1.0 m KOH solution, which is comparable to the state‐of‐the‐art RuO₂ tested in the same condition (overpotential: 327 mV at 10 mA cm^?2, Tafel slope: 73.7 mV dec^?1). The robust framework coupled with good OER performance enables the 2D mesoporous Ni10‐CoPi nanoflakes to be a promising material for energy conversion applications.     

Kinetically Stable Oxide Overlayers on Mo3P Nanoparticles Enabling Lithium–Air Batteries with Low Overpotentials and Long Cycle Life

The main drawbacks of today's state-of-the-art lithium–air (Li–air) batteries are their low energy efficiency and limited cycle life due to the lack of earth-abundant cathode catalysts that can drive both oxygen reduction and evolution reactions (ORR and OER) at high rates at thermodynamic potentials. Here, inexpensive trimolybdenum phosphide (Mo₃P) nanoparticles with an exceptional activity—ORR and OER current densities of 7.21 and 6.85 mA cm⁻² at 2.0 and 4.2 V versus Li/Li⁺, respectively—in an oxygen-saturated non-aqueous electrolyte are reported. The Tafel plots indicate remarkably low charge transfer resistance—Tafel slopes of 35 and 38 mV dec⁻¹ for ORR and OER, respectively—resulting in the lowest ORR overpotential of 4.0 mV and OER overpotential of 5.1 mV reported to date. Using this catalyst, a Li–air battery cell with low discharge and charge overpotentials of 80 and 270 mV, respectively, and high energy efficiency of 90.2% in the first cycle is demonstrated. A long cycle life of 1200 is also achieved for this cell. Density functional theory calculations of ORR and OER on Mo₃P (110) reveal that an oxide overlayer formed on the surface gives rise to the observed high ORR and OER electrocatalytic activity and small discharge/charge overpotentials.