Metal‐Organic Frameworks Derived Nanotube of Nickel–Cobalt Bimetal Phosphides as Highly Efficient Electrocatalysts for Overall Water Splitting

The design of highly efficient, stable, and noble‐metal‐free bifunctional electrocatalysts for overall water splitting is critical but challenging. Herein, a facile and controllable synthesis strategy for nickel–cobalt bimetal phosphide nanotubes as highly efficient electrocatalysts for overall water splitting via low‐temperature phosphorization from a bimetallic metal‐organic framework (MOF‐74) precursor is reported. By optimizing the molar ratio of Co/Ni atoms in MOF‐74, a series of Cox Niy P catalysts are synthesized, and the obtained Co4Ni1P has a rare form of nanotubes that possess similar morphology to the MOF precursor and exhibit perfect dispersal of the active sites. The nanotubes show remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance in an alkaline electrolyte, affording a current density of 10 mA cm^?2 at overpotentials of 129 mV for HER and 245 mV for OER, respectively. An electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalyst in alkaline solutions achieves a current density of 10 mA cm^?2 at a voltage of 1.59 V, which is comparable to the integrated Pt/C and RuO₂ counterparts and ranks among the best of the metal‐phosphide electrocatalysts reported to date.     

A High‐Performance Binary Ni–Co Hydroxide‐based Water Oxidation Electrode with Three‐Dimensional Coaxial Nanotube Array Structure

Developing nanostructured Ni and Co oxides with a small overpotential and fast kinetics of the oxygen evolution reaction (OER) have drawn considerable attention recently because their theoretically high efficiency, high abundance, low cost, and environmental benignity in comparison with precious metal oxides, such as RuO₂ and IrO₂. However, how to increase the specific activity area and improve their poor intrinsic conductivity is still challenging, which significantly limits the overall OER rate and largely prevent their utilization. Thus, developing effective OER electrocatalysts with abundant active sites and high electrical conductivity still remains urgent. In this work, a scrupulous design of OER electrode with a unique sandwich‐like coaxial structure of the three‐dimensional Ni@[Ni^(2+/3+)Co₂(OH)_6–7]_x nanotube arrays (3D NNCNTAs) is reported. A Ni nanotube array with open end is homogeneous coated with Ni and Co co‐hydroxide nanosheets ([Ni^(2+/3+)Co₂(OH)_6–7]_x) and is employed as multifunctional interlayer to provide a large surface area and fast electron transport and support the outermost [Ni^(2+/3+)Co₂(OH)_6–7]_x layer. The remarkable features of high surface area, enhanced electron transport, and synergistic effects have greatly assured excellent OER activity with a small overpotential of 0.46 V at the current density of 10 mA cm^?2 and high stability.     

3D Self‐Supported Fe‐Doped Ni2P Nanosheet Arrays as Bifunctional Catalysts for Overall Water Splitting

The development of highly efficient bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial for improving the efficiency of overall water splitting, but still remains challenging issue. Herein, 3D self‐supported Fe‐doped Ni₂P nanosheet arrays are synthesized on Ni foam by hydrothermal method followed by in situ phosphorization, which serve as bifunctional electrocatalysts for overall water splitting. The as‐synthesized (Ni_0.33Fe_0.67)₂P with moderate Fe doping shows an outstanding OER performance, which only requires an overpotential of ≈230 mV to reach 50 mA cm^?2 and is more efficient than the other Fe incorporated Ni₂P electrodes. In addition, the (Ni_0.33Fe_0.67)₂P exhibits excellent activity toward HER with a small overpotential of ≈214 mV to reach 50 mA cm^?2. Furthermore, an alkaline electrolyzer is measured using (Ni_0.33Fe_0.67)₂P electrodes as cathode and anode, respectively, which requires cell voltage of 1.49 V to reach 10 mA cm^?2 as well as shows excellent stability with good nanoarray construction. Such good performance is attributed to the high intrinsic activity and superaerophobic surface property.     

CuCo Hybrid Oxides as Bifunctional Electrocatalyst for Efficient Water Splitting

Solar‐driven water splitting is a promising approach for renewable energy, where the development of efficient and stable bifunctional electrocatalysts for simultaneously producing hydrogen and oxygen is still challenging. Herein, combined with the hydrogen evolution reaction (HER) activity of a copper(I) complex and oxygen evolution reaction (OER) activity of cobalt‐based oxides, a type of 1D copper‐cobalt hybrid oxide nanowires (CuCoO‐NWs) is developed via a facile two‐step growth‐conversion process toward a bifunctional water splitting catalyst. The CuCoO‐NWs exhibit excellent catalytic performances for both HER and OER in the same basic electrolyte, with optimized low onset overpotentials and high current densities. The efficient HER activity is ascribed to the formation of Cu₂O, while the activity for OER is primarily enabled by Co‐based oxides and abundant oxygen vacancies. The CuCoO‐NWs allow for the assembly of a water electrolyzer with strong alkaline media, with a current density of 10 mA cm^?2 at 1.61 V. Further combination with a commercial silicon photovoltaic allows the direct use of solar energy for spontaneous water splitting with excellent stability for over 72 h, suggesting the potential as a promising bifunctional electrocatalyst for efficient solar‐driven water splitting.     

Amorphous Cobalt–Iron Hydroxide Nanosheet Electrocatalyst for Efficient Electrochemical and Photo‐Electrochemical Oxygen Evolution

Finding efficient electrocatalysts for oxygen evolution reaction (OER) that can be effectively integrated with semiconductors is significantly challenging for solar‐driven photo‐electrochemical (PEC) water splitting. Herein, amorphous cobalt–iron hydroxide (CoFe? H) nanosheets are synthesized by facile electrodeposition as an efficient catalyst for both electrochemical and PEC water oxidation. As a result of the high electrochemically active surface area and the amorphous nature, the optimized amorphous CoFe? H nanosheets exhibit superior OER catalytic activity in alkaline environment with a small overpotential (280 mV) to achieve significant oxygen evolution (j = 10 mA cm^?2) and a low Tafel slope (28 mV dec^?1). Furthermore, CoFe? H nanosheets are simply integrated with BiVO₄ semiconductor to construct CoFe? H/BiVO₄ photoanodes that exhibit a significantly enhanced photocurrent density of 2.48 mA cm^?2 (at 1.23 V vs reversible hydrogen electrode (RHE)) and a much lower onset potential of 0.23 V (vs RHE) for PEC‐OER. Careful electrochemical and optical studies reveal that the improved OER kinetics and high‐quality interface at the CoFe? H/BiVO₄ junction, as well as the excellent optical transparency of CoFe? H nanosheets, contribute to the high PEC performance. This study establishes amorphous CoFe? H nanosheets as a highly competitive candidate for electrochemical and PEC water oxidation and provides general guidelines for designing efficient PEC systems.     

Electro‐Oxidation of Ni42 Steel: A Highly Active Bifunctional Electrocatalyst

Janus type water‐splitting catalysts have attracted highest attention as a tool of choice for solar to fuel conversion. AISI Ni42 steel is upon harsh anodization converted into a bifunctional electrocatalyst. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are highly efficiently and steadfast catalyzed at pH 7, 13, 14, 14.6 (OER) and at pH 0, 1, 13, 14, 14.6 (HER), respectively. The current density taken from long‐term OER measurements in pH 7 buffer solution upon the electro‐activated steel at 491 mV overpotential (η) is around four times higher (4 mA cm^?2) in comparison with recently developed OER electrocatalysts. The very strong voltage–current behavior of the catalyst shown in OER polarization experiments at both pH 7 and at pH 13 are even superior to those known for IrO₂‐RuO₂. No degradation of the catalyst is detected even when conditions close to standard industrial operations are applied to the catalyst. A stable Ni‐, Fe‐oxide based passivating layer sufficiently protects the bare metal for further oxidation. Quantitative charge to oxygen (OER) and charge to hydrogen (HER) conversion are confirmed. High‐resolution XPS spectra show that most likely γ?NiO(OH) and FeO(OH) are the catalytic active OER and NiO is the catalytic active HER species.     

Boosting Activity on Co4N Porous Nanosheet by Coupling CeO2 for Efficient Electrochemical Overall Water Splitting at High Current Densities

Developing highly active nonprecious electrocatalysts with superior durability for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to improve the efficiency of overall water splitting but remains challenging. Here, a novel superhydrophilic Co₄N‐CeO₂ hybrid nanosheet array is synthesized on a graphite plate (Co₄N‐CeO₂/GP) by an anion intercalation enhanced electrodeposition method, followed by high‐temperature nitridation. Doping CeO₂ into Co₄N can favor dissociation of H₂O and adsorption of hydrogen, reduce the energy barrier of intermediate reactions of OER, and improve the compositional stability, thereby dramatically boosting the HER performance while simultaneously inducing enhanced OER activity. Furthermore, the superhydrophilic self‐supported electrode with Co₄N‐CeO₂ in situ grown on the conductive substrate expedites electron conduction between substrate and catalyst, promotes the bubble release from electrode timely and impedes catalyst shedding, ensuring a high efficiency and stable working state. Consequently, the Co₄N‐CeO₂/GP electrode shows exceptionally low overpotentials of 24 and 239 mV at 10 mA cm^?2 for HER and OER, respectively. An alkaline electrolyzer by using Co₄N‐CeO₂/GP as both the cathode and anode requires a cell voltage of 1.507 V to drive 10 mA cm^?2, outperforming the Pt/C||RuO₂ electrolyzer (1.540 V@10 mA cm^?2). More significantly, the electrolyzer has extraordinary long‐term durability at a large current density of 500 mA cm^?2 for 50 h, revealing its potential in large‐scale applications.     

Cobalt−Iron Oxide Nanosheets for High‐Efficiency Solar‐Driven CO2−H2O Coupling Electrocatalytic Reactions

Solar‐driven electrochemical overall CO₂ splitting (OCO₂S) offers a promising route to store sustainable energy; however, its extensive implementation is hindered by the sluggish kinetics of two key reactions (i.e., CO₂ reduction reaction and oxygen evolution reaction (CO₂RR and OER, respectively)). Here, as dual‐functional catalysts, Co₂FeO₄ nanosheet arrays having high electrocatalytic activities toward CO₂RR and OER are developed. When the catalyst is applied to a complete OCO₂S system driven by a triple junction GaInP₂/GaAs/Ge photovoltaic cell, it shows a high photocurrent density of ≈13.1 mA cm^?2, corresponding to a remarkably high solar‐to‐CO efficiency of 15.5%. Density functional theory studies suggest that the Co sites in Co₂FeO₄ are favorable to the formation of *COOH and *O intermediates and thus account for its efficient bifunctional activities. The results will facilitate future studies for designing highly effective electrocatalysts and devices for OCO₂S.     

Fabrication of Nickel–Cobalt Bimetal Phosphide Nanocages for Enhanced Oxygen Evolution Catalysis

Replacement of precious metals with earth‐abundant electrocatalysts for oxygen evolution reaction (OER) holds great promise for realizing practically viable water‐splitting systems. It still remains a great challenge to develop low‐cost, highly efficient, and durable OER catalysts. Here, the composition and morphology of Ni–Co bimetal phosphide nanocages are engineered for a highly efficient and durable OER electrocatalyst. The nanocage structure enlarges the effective specific area and facilitates the contact between catalyst and electrolyte. The as‐prepared Ni–Co bimetal phosphide nanocages show superior OER performance compared with Ni₂P and CoP nanocages. By controlling the molar ratio of Ni/Co atoms in Ni–Co bimetal hydroxides, the Ni_0.6Co_1.4P nanocages derived from Ni_0.6Co_1.4(OH)₂ nanocages exhibit remarkable OER catalytic activity (η = 300 mV at 10 mA cm^?2) and long‐term stability (10 h for continuous test). The density‐functional‐theory calculations suggest that the appropriate Co doping concentration increases density of states at the Fermi level and makes the d‐states more close to Fermi level, giving rise to high charge carrier density and low intermedia adsorption energy than those of Ni₂P and CoP. This work also provides a general approach to optimize the catalysis performance of bimetal compounds.     

Fe3O4‐Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction

Cobalt sulfide materials have attracted enormous interest as low‐cost alternatives to noble‐metal catalysts capable of catalyzing both oxygen reduction and oxygen evolution reactions. Although recent advances have been achieved in the development of various cobalt sulfide composites to expedite their oxygen reduction reaction properties, to improve their poor oxygen evolution reaction (OER) activity is still challenging, which significantly limits their utilization. Here, the synthesis of Fe₃O₄‐decorated Co₉S₈ nanoparticles in situ grown on a reduced graphene oxide surface (Fe₃O₄@Co₉S₈/rGO) and the use of it as a remarkably active and stable OER catalyst are first reported. Loading of Fe₃O₄ on cobalt sulfide induces the formation of pure phase Co₉S₈ and highly improves the catalytic activity for OER. The composite exhibits superior OER performance with a small overpotential of 0.34 V at the current density of 10 mA cm^?2 and high stability. It is believed that the electron transfer trend from Fe species to Co₉S₈ promotes the breaking of the Co–O bond in the stable configuration (Co–O–O superoxo group), attributing to the excellent catalytic activity. This development offers a new and effective cobalt sulfide‐based oxygen evolution electrocatalysts to replace the expensive commercial catalysts such as RuO₂ or IrO₂.     

Nanoporous Nitrogen‐Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal‐Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution

Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen‐doped graphene oxide (NGO) and nickel sulfide (Ni₇S₆) derived from a hybrid of a well‐known nickel‐based metal‐organic framework (NiMOF‐74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni₇S₆ exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO₂ under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO₂ and IrO₂.     

Metal–Organic Frameworks Derived Interconnected Bimetallic Metaphosphate Nanoarrays for Efficient Electrocatalytic Oxygen Evolution

The development of low‐cost, high‐performance, and stable electrocatalysts for the sluggish oxygen evolution reaction (OER) in water splitting is essential for renewable and clean energy technologies. Herein, the interconnected nanoarrays consisting of Co–Ni bimetallic metaphosphate nanoparticles embedded in a carbon matrix (Co_2?xNi_xP₄O₁₂‐C) are fabricated through a mild phosphorylating process of cobalt–nickel zeolitic imidazolate frameworks (CoNi‐ZIF). Density functional theory calculations reveal moderate adsorption of oxygenated intermediates on the doping Ni site, and current density simulations imply homogeneous and higher current density due to the morphology integrity of the interconnected metaphosphate nanoarrays. As a consequence, the optimized Co_1.6Ni_0.4P₄O₁₂‐C affords a superior OER activity (η = 230 mV at 10 mA cm^?2) and long‐term stability in alkaline media (1 m KOH) that are comparable to most reported catalysts. The strategy for balancing the doping effect and morphology effect provides a new perspective when designing and developing highly efficient electrocatalysts for energy conversion and storage applications.     

Tuning the Electrochemical Properties of Polymeric Cobalt Phthalocyanines for Efficient Water Splitting

Polymeric metal phthalocyanines have great potential as electrocatalysts, yet their incorporation on a current collector without losing the activity of metal centers remains a challenge. Herein, a new strategy for preparing a series of polymeric cobalt phthalocyanines containing S linkers (pCoPc-1) or SO₂ linkers (pCoPc-2) and their tunable electrochemical properties are reported. The pCoPcs coated on various substrates show favorable electrocatalytic activities toward oxygen and hydrogen evolution reactions (OER and HER). Particularly, the pCoPc-1 layer on Co₃O₄ nanosheet arrays exerts a cooperative effect enhancing both the OER and HER performances, and the subsequent phosphorization (P@pCoPc-1/Co₃O₄|CC) significantly boosts the HER performance with enhanced hydrophilicity and conductivity. The high permeability and stability reinforcement of the pCoPc-1 layer allow the phosphorization of underlying Co₃O₄ to CoP without degradation, which remarkably enhances OER and HER performances as manifested by low overpotentials of 320 and 120 mV at 10 mA cm⁻², respectively. When engaged as a bifunctional electrocatalyst for the overall water splitting, the P@pCoPc-1/Co₃O₄|CC requires a low cell voltage of 1.672 V at 10 mA cm⁻², showing long-term durability and mechanical robustness. This study demonstrates the collaborative catalytic role of polymeric macrocyclic compounds that offers versatile tunability and stability for various electrocatalytic reactions.     

Overall Water Splitting Catalyzed Efficiently by an Ultrathin Nanosheet‐Built,Hollow Ni3S2‐Based Electrocatalyst

Making highly efficient catalysts for an overall ?water splitting reaction is vitally important to bring solar/electrical‐to‐hydrogen energy conversion processes into reality. Herein, the synthesis of ultrathin nanosheet‐based, hollow MoO_x/Ni₃S₂ composite microsphere catalysts on nickel foam, using ammonium molybdate as a precursor and the triblock copolymer pluronic P123 as a structure‐directing agent is reported. It is also shown that the resulting materials can serve as bifunctional, non‐noble metal electrocatalysts with high activity and stability for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). Thanks to their unique structural features, the materials give an impressive water‐splitting current density of 10 mA cm^?2 at ≈1.45 V with remarkable durability for >100 h when used as catalysts both at the cathode and the anode sides of an alkaline electrolyzer. This performance for an overall water splitting reaction is better than even those obtained with an electrolyzer consisting of noble metal‐based Pt/C and IrO_x/C catalytic couple—the benchmark catalysts for HER and OER, respectively.     

Physically Adsorbed Metal Ions in Porous Supports as Electrocatalysts for Oxygen Evolution Reaction

Developing highly efficient and earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is significantly important for water‐splitting. Here, for the first time it is reported that the physically adsorbed metal ions (PAMI) in porous materials can be served as highly efficient OER electrocatalysts, which provides a universal PAMI method to develop electrocatalysts. This PAMI method can be applied to almost all porous supports, including graphene, carbon nanotubes, C₃N₄, CaCO₃, and porous organic polymers and all the systems exhibit excellent OER performance. In particular, the as‐synthesized Co_0.7Fe_0.3CB exhibits a small overpotential of 295 mV and 350 mV at the current density of 10 mA cm^?2 and 100 mA cm^?2, respectively, which exceeds commercial 40 wt% IrO₂/CB and most reported non‐noble metal‐based OER catalysts. Moreover, the mass activity of Co_0.7Fe_0.3CB reaches 643.4 A g^?1 at the overpotential of 320 mV, which is nearly 4.7 times higher than that of 40 wt% IrO₂/CB. In addition, the advanced ex situ and in situ synchrotron X‐ray characterizations are carried out to unravel the PAMI synthetic process. In short, this PAMI method will break the conversional understanding, i.e., the most OER catalysts are synthesized chemically, because the new PAMI method does not require any chemical synthesis, which therefore opens a new avenue for the development of OER electrocatalysts.     

Regulating the Electronic Structure of CoP Nanosheets by O Incorporation for High‐Efficiency Electrochemical Overall Water Splitting

The exploration of earth‐abundant and high‐efficiency bifunctional electrocatalysts for overall water splitting is of vital importance for the future of the hydrogen economy. Regulation of electronic structure through heteroatom doping represents one of the most powerful strategies to boost the electrocatalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a rational design of O‐incorporated CoP (denoted as O‐CoP) nanosheets, which synergistically integrate the favorable thermodynamics through modification of electronic structures with accelerated kinetics through nanostructuring, is reported. Experimental results and density functional theory simulations manifest that the appropriate O incorporation into CoP can dramatically modulate the electronic structure of CoP and alter the adsorption free energies of reaction intermediates, thus promoting the HER and OER activities. Specifically, the optimized O‐CoP nanosheets exhibit efficient bifunctional performance in alkaline electrolyte, requiring overpotentials of 98 and 310 mV to deliver a current density of 10 mA cm^?2 for HER and OER, respectively. When served as bifunctional electrocatalysts for overall water splitting, a low cell voltage of 1.60 V is needed for achieving a current density of 10 mA cm^?2. This proposed anion‐doping strategy will bring new inspiration to boost the electrocatalytic performance of transition metal–based electrocatalysts for energy conversion applications.     

Novel Iron/Cobalt‐Containing Polypyrrole Hydrogel‐Derived Trifunctional Electrocatalyst for Self‐Powered Overall Water Splitting

Development of efficient, low‐cost, and durable electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is of significant importance for many electrochemical devices, such as rechargeable metal–air batteries, fuel cells, and water electrolyzers. Here, a novel approach for the synthesis of a trifunctional electrocatalyst derived from iron/cobalt‐containing polypyrrole (PPy) hydrogel is reported. This strategy relies on the formation of a supramolecularly cross‐linked PPy hydrogel that allows for efficient and homogeneous incorporation of highly active Fe/Co–N–C species. Meanwhile, Co nanoparticles are also formed and embedded into the carbon scaffold during the pyrolysis process, further promoting electrochemical activities. The resultant electrocatalyst exhibits prominent catalytic activities for ORR, OER, and HER, surpassing previously reported trifunctional electrocatalysts. Finally, it is demonstrated that the as‐obtained trifunctional electrocatalyst can be used for electrocatalytic overall water splitting in a self‐powered manner under ambient conditions. This work offers new prospects in developing highly active, nonprecious‐metal‐based electrocatalysts in electrochemical energy devices.     

Searching General Sufficient‐and‐Necessary Conditions for Ultrafast Hydrogen‐Evolving Electrocatalysis

The development of cost‐effective and high‐performance electrocatalysts for the hydrogen evolution reaction (HER) is one critical step toward successful transition into a sustainable green energy era. Different from previous design strategies based on single parameter, here the necessary and sufficient conditions are proposed to develop bulk non‐noble metal oxides which are generally considered inactive toward HER in alkaline solutions: i) multiple active sites for different reaction intermediates and ii) a short reaction path created by ordered distribution and appropriate numbers of these active sites. Computational studies predict that a synergistic interplay between the ordered oxygen vacancies (at pyramidal high‐spin Co³⁺ sites) and the O 2p ligand holes (OLH; at metallic octahedral intermediate‐spin Co⁴⁺ sites) in RBaCo₂O_5.5+δ (δ = 1/4; R = lanthanides) can produce a near‐ideal HER reaction path to adsorb H₂O and release H₂, respectively. Experimentally, the as‐synthesized (Gd_0.5La_0.5)BaCo₂O_5.75 outperforms the state‐of‐the‐art Pt/C catalyst in many aspects. The proof‐of‐concept results reveal that the simultaneous possession of ordered oxygen vacancies and an appropriate number of OLH can realize a near‐optimal synergistic catalytic effect, which is pivotal for rational design of oxygen‐containing materials.     

Metal (Ni,Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) along with hydrogen evolution reaction (HER) have been considered critical processes for electrochemical energy conversion and storage through metal‐air battery, fuel cell, and water electrolyzer technologies. Here, a new class of multifunctional electrocatalysts consisting of dominant metallic Ni or Co with small fraction of their oxides anchored onto nitrogen‐doped reduced graphene oxide (rGO) including Co‐CoO/N‐rGO and Ni‐NiO/N‐rGO are prepared via a pyrolysis of graphene oxide and cobalt or nickel salts. Ni‐NiO/N‐rGO shows the higher electrocatalytic activity for the OER in 0.1 m KOH with a low overpotential of 0.24 V at a current density of 10 mA cm^?2, which is superior to that of the commercial IrO₂. In addition, it exhibits remarkable activity for the HER, demonstrating a low overpotential of 0.16 V at a current density of 20 mA cm^?2 in 1.0 m KOH. Apart from similar HER activity to the Ni‐based catalyst, Co‐CoO/N‐rGO displays the higher activity for the ORR, comparable to Pt/C in zinc‐air batteries. This work provides a new avenue for the development of multifunctional electrocatalysts with optimal catalytic activity by varying transition metals (Ni or Co) for these highly demanded electrochemical energy technologies.     

Rock Salt Ni/Co Oxides with Unusual Nanoscale‐Stabilized Composition as Water Splitting Electrocatalysts

The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert ‐butanol ultrasmall crystalline and highly dispersible Ni _x Co_{1? x} O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non‐equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)‐oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni _x Co_{1? x} O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni _x Co_{1? x} O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting.