Electrocatalytic activity of spinel related cobalties MxCo3−xO4 (M = Li, Ni, Cu) in the oxygen evolution reaction

Investigation of Charge–Discharging Behavior of Metal Oxide–Based Anode Electrocatalysts for Alkaline Water Electrolysis to Suppress Degradation due to Reverse Current

In the bipolar-type alkaline water electrolysis powered by renewable energy, electrocatalysts are degraded by repeated potential change associated with the generation of reverse current. If an electrode has large discharge capacity, the opposite electrode on the same bipolar plate is degraded by the reverse current. In this study, discharge capacity of various transition metal-based electrocatalysts was investigated to clarify the determining factors of electrocatalysts on the reverse current and durability. The discharge capacities from 1.5 to 0.5 V vs. RHE (Qdc,0.5) of electrocatalysts are proportional to the surface area in most cases. The proportionality coefficient, corresponding to the specific capacity, is 1.0 C·m–2 for Co3O4 and 2.3 C·m–2 for manganese-based electrocatalysts. The substitution of Co3+ in Co3O4 with Ni3+ increased Qdc,0.5. The upper limit of theoretical specific capacity for Co3O4 is estimated to be 1.699 C·m–2, meaning the former and latter cases correspond to 2- and 1-electron reactions, respectively, per a cation at the surface. The discharge capacities of the elctrocatalysts increased because of the dissolution and recrystallization of nickel and/or cobalt into metal hydroxides. The increase in the capacities of Co3O4 and NiCo2O4 during ten charge–discharge cycles was below 2–9% and 0.5–38%, respectively. Therefore, if a cathode electrocatalyst with relatively low redox durability is used on the one side of a bipolar plate, it is necessary to control optimum discharge capacity of the anode by changing surface area and constituent metal cations to minimize the generation of reverse current.

Graphical Abstract

Reduction of Transition-Metal Columbite-Tantalite as a Highly Efficient Electrocatalyst for Water Splitting

We successfully report a liquid-liquid chemical reduction and hydrothermal synthesis of a highly stable columbite-tantalite electrocatalyst with remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance in acidic media. The reduced Fe_0.79Mn_0.21Nb_0.16Ta_0.84O₆ (CTr) electrocatalyst shows a low overpotential of 84.23 mV at 10 mA cm^-2 and 103.7 achieved at 20 mA cm^-2 current density in situ for the HER and OER, respectively. The electrocatalyst also exhibited low Tafel slopes of 104.97 mV/dec for the HER and 57.67 mV/dec for the OER, verifying their rapid catalytic kinetics. The electrolyzer maintained a cell voltage of 1.5 V and potential-time stability close to that of Pt/C and RuO₂. Complementary first-principles density functional theory calculations identify the Mn sites as most active sites on the Fe_0.75Mn_0.25Ta_1.875Nb_0.125O₆ (100) surface, predicting a moderate Gibbs free energy of hydrogen adsorption (ΔG_H* ≈ 0.08 eV) and a low overpotential of η = 0.47 V. The |ΔGMn_H*| = 0.08 eV on the Fe_0.75Mn_0.25Ta_1.875Nb_0.125O₆ (100) surface is similar to that of the well-known and highly efficient Pt catalyst (|ΔGPt_H*| ≈ 0.09 eV).

Nickel-Based Electrocatalysts for Water Electrolysis

Currently, hydrogen production is based on the reforming process, leading to the emission of pollutants; therefore, a substitute production method is imminently required. Water electrolysis is an ideal alternative for large-scale hydrogen production, as it does not produce any carbon-based pollutant byproducts. The production of green hydrogen from water electrolysis using intermittent sources (e.g., solar and eolic sources) would facilitate clean energy storage. However, the electrocatalysts currently required for water electrolysis are noble metals, making this potential option expensive and inaccessible for industrial applications. Therefore, there is a need to develop electrocatalysts based on earth-abundant and low-cost metals. Nickel-based electrocatalysts are a fitting alternative because they are economically accessible. Extensive research has focused on developing nickel-based electrocatalysts for hydrogen and oxygen evolution. Theoretical and experimental work have addressed the elucidation of these electrochemical processes and the role of heteroatoms, structure, and morphology. Even though some works tend to be contradictory, they have lit up the path for the development of efficient nickel-based electrocatalysts. For these reasons, a review of recent progress is presented herein.

Research Progress of Oxygen Evolution Reaction Catalysts for Electrochemical Water Splitting

The development of a low-cost and high-efficiency oxygen evolution reaction (OER) catalyst is essential to meet the future industrial demand for hydrogen production by electrochemical water splitting. Given the limited reserves of noble metals and many competitive applications in environmental protection, new energy, and chemical industries, many studies have focused on exploring new and efficient non-noble metal catalytic systems, improving the understanding of the OER mechanism of non-noble metal surfaces, and designing electrocatalysts with higher activity than traditional noble metals. This Review summarizes the research progress of anode OER catalysts for hydrogen production by electrochemical water splitting in recent years, for noble metal and non-noble metal catalysts, where non-noble metal catalysts are highlighted. The categories are as follows: (1) Transition metal-based compounds, including transition metal-based oxides, transition metal-based layered hydroxides, and transition metal-based sulfides, phosphides, selenides, borides, carbides, and nitrides. Transition metal-based oxides can also be divided into perovskite, spinel, amorphous, rock-salt-type, and lithium oxides according to their different structures. (2) Carbonaceous materials and their composite materials with transition metals. (3) Transition metal-based metal-organic frameworks and their derivatives. Finally, the challenges and future development of the OER process of water splitting are discussed.

Design principles of noble metal-free electrocatalysts for hydrogen production in alkaline media: combining theory and experiment

Water electrolysis is a promising solution to convert renewable energy sources to hydrogen as a high-energy-density energy carrier. Although alkaline conditions extend the scope of electrocatalysts beyond precious metal-based materials to earth-abundant materials, the sluggish kinetics of cathodic and anodic reactions (hydrogen and oxygen evolution reactions, respectively) impede the development of practical electrocatalysts that do not use precious metals. This review discusses the rational design of efficient electrocatalysts by exploiting the understanding of alkaline hydrogen evolution reaction and oxygen evolution reaction mechanisms and of the electron structure-activity relationship, as achieved by combining experimental and computational approaches. The enhancement of water splitting not only deals with intrinsic catalytic activity but also includes the aspect of electrical conductivity and stability. Future perspectives to increase the synergy between theory and experiment are also proposed.

Recent Advances in Alkaline Exchange Membrane Water Electrolysis and Electrode Manufacturing

Water electrolysis to obtain hydrogen in combination with intermittent renewable energy resources is an emerging sustainable alternative to fossil fuels. Among the available electrolyzer technologies, anion exchange membrane water electrolysis (AEMWE) has been paid much attention because of its advantageous behavior compared to other more traditional approaches such as solid oxide electrolyzer cells, and alkaline or proton exchange membrane water electrolyzers. Recently, very promising results have been obtained in the AEMWE technology. This review paper is focused on recent advances in membrane electrode assembly components, paying particular attention to the preparation methods for catalyst coated on gas diffusion layers, which has not been previously reported in the literature for this type of electrolyzers. The most successful methodologies utilized for the preparation of catalysts, including co-precipitation, electrodeposition, sol-gel, hydrothermal, chemical vapor deposition, atomic layer deposition, ion beam sputtering, and magnetron sputtering deposition techniques, have been detailed. Besides a description of these procedures, in this review, we also present a critical appraisal of the efficiency of the water electrolysis carried out with cells fitted with electrodes prepared with these procedures. Based on this analysis, a critical comparison of cell performance is carried out, and future prospects and expected developments of the AEMWE are discussed.

Enhancing bifunctional catalytic activity of cobalt-nickel sulfide spinel nanocatalysts through transition metal doping and its application in secondary zinc-air batteries

Developing large-scale and high-performance OER (oxygen evolution reaction) and ORR (oxygen reduction reaction) catalysts have been a challenge for commercializing secondary zinc-air batteries. In this work, transition metal-doped cobalt-nickel sulfide spinels are directly produced via a continuous hydrothermal flow synthesis (CHFS) approach. The nanosized cobalt-nickel sulfides are doped with Ag, Fe, Mn, Cr, V, and Ti and evaluated as bifunctional OER and ORR catalyst for Zn-air battery application. Among the doped spinel catalysts, Mn-doped cobalt-nickel sulfides (Ni1.29Co1.49Mn0.22S4) exhibit the most promising OER and ORR performance, showing an ORR onset potential of 0.9 V vs. RHE and an OER overpotential of 348 mV measured at 10 mA cm-2, which is attributed to their high surface area, electronic structure of the dopant species, and the synergistic coupling of the dopant species with the active host cations. The dopant ions primarily alter the host cation composition, with the Mn(iii) cation linked to the introduction of active sites by its favourable electronic structure. A power density of 75 mW cm-2 is achieved at a current density of 140 mA cm-2 for the zinc-air battery using the manganese-doped catalyst, a 12% improvement over the undoped cobalt-nickel sulfide and superior to that of the battery with a commercial RuO2 catalyst.

Study of the Oxygen Evolution Reaction at Strontium Palladium Perovskite Electrocatalyst in Acidic Medium

The catalytic activity of Sr2PdO3, prepared through the sol-gel citrate-combustion method for the oxygen evolution reaction (OER) in a 0.1 M HClO4 solution, was investigated. The electrocatalytic activity of Sr2PdO3 toward OER was assessed via the anodic potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The glassy carbon modified Sr2PdO3 (GC/Sr2PdO3) electrode exhibited a higher electrocatalytic activity, by about 50 times, in comparison to the unmodified electrode. The order of the reaction was close to unity, which indicates that the adsorption of the hydroxyl groups is a fast step. The calculated activation energy was 21.6 kJ.mol-1, which can be considered a low value in evaluation with those of the reported OER electrocatalysts. The Sr2PdO3 perovskite portrayed a high catalyst stability without any probability of catalyst poisoning. These results encourage the use of Sr2PdO3 as a candidate electrocatalyst for water splitting reactions.

Rational Design of Spinel Oxide Nanocomposites with Tailored Electrochemical Oxygen Evolution and Reduction Reactions for ZincAir Batteries

The unique physical and chemical properties of spinels have made them highly suitable electrocatalysts in oxygen evolution reaction and oxygen reduction reaction (OER & ORR). Zinc–air batteries (ZABs), which are safer and more cost-effective power sources than commercial lithium-ion batteries, hinge on ORR and OER. The slow kinetics of the air electrode reduce its high theoretical energy density and specific capacity, which limits its practical applications. Thus, tuning the performance of the electrocatalyst and cathode architecture is vital for improving the performance of ZABs, which calls for exploring spinel, a material that delivers improved performance. However, the structure–activity relationship of spinel is still unclear because there is a lack of extensive information about it. This study was performed to address the promising potential of spinel as the bifunctional electrocatalyst in ZABs based on an in-depth understanding of spinel structure and active sites at the atomic level.

Bifunctionally active nanosized spinel cobalt nickel sulfides for sustainable secondary zinc–air batteries: examining the effects of compositional tuning on OER and ORR activity

Nanosized cobalt nickel sulfides were prepared via a continuous hydrothermal method and evaluated as electrocatalysts, with the catalytic activity being linked to the cationic composition.

Spiky Nickel Electrodes for Electrochemical Oxygen Evolution Catalysis by Femtosecond Laser Structuring

Micro- and nanostructured Ni/NiO surfaces were generated by femtosecond laser structuring for oxygen evolution reaction in alkaline water electrolysis cells. For two different angles between the laser beam and the nickel surface, two different types of laser-structured electrodes were prepared, characterized, and compared with a plane tempered nickel electrode. Their electrochemical activities for the oxygen evolution reaction were tested by using cyclic and linear sweep voltammetry. The chemical surface composition was investigated by X-ray photoelectron spectroscopy. Laser structuring increased the overall electrochemical performance by more than one order of magnitude. The overpotential of the laser-structured electrodes for the oxygen evolution reaction was decreased by more than 100 mV due to high defect densities of the structures created by the laser ablation process.

Valence- and element-dependent water oxidation behaviors: in situ X-ray diffraction, absorption and electrochemical impedance spectroscopies

Metal oxides of the spinel family have shown great potential towards the oxygen evolution reaction (OER), but the fundamental OER mechanism of spinel oxides is still far from being completely understood, especially for the role of the metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface conditions (morphology and defects), it is a great challenge to have a fair assessment of the electrocatalytic performance of spinel systems. Herein, we demonstrated a series of MFe₂O₄ (M = Fe, Co, Ni, Zn) with a well-controlled morphology to achieve a comprehensive study of electrocatalytic activity toward OER. By utilizing several in situ analyses, we could conclude a universal rule that the activities for OER in the metal oxide systems were determined by the occurrence of a phase transformation, and this structural transformation could work well in both crystallographic sites (T_d and O_h sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship, which depended on the atomic arrangement at the interface of spinel and metal oxyhydroxide, while trivalent metal ions remained unchanged as a host lattice. The metal oxyhydroxide was formed during a redox reaction rather than being formed during OER. The occurrence of the redox reaction seems to accompany a remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believe that the approaching strategies and information obtained in the present study can offer a guide to designing a promising electrocatalytic system towards the oxygen evolution reaction and other fields.

Tailored transition metal-doped nickel phosphide nanoparticles for the electrochemical oxygen evolution reaction (OER)

Foreign transition metals are doped into the nickel phosphide nanoparticles through a simple wet-chemical process to show a high level of doping without significant distortion of crystal structure and morphology, which endows excellent activity in the electrochemical oxygen evolution reaction.

3 D Porous Nickel-Cobalt Nitrides Supported on Nickel Foam as Efficient Electrocatalysts for Overall Water Splitting

Exploring highly efficient and durable bifunctional electrocatalysts from earth-abundant low-cost transition metals is central to obtaining clean hydrogen energy through large-scale electrolytic water splitting. Porous nickel-cobalt nitride nanosheets on macroporous Ni foam (NF) are synthesized through facile electrodeposition followed by a one-step annealing process in a NH₃ atmosphere. The transformation from a metal hydroxide into a metal nitride could efficiently enhance the electrocatalytic performance for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Interestingly, the incorporation of nickel further boosts the catalytic activity of cobalt nitride. When used as bifunctional electrocatalysts, the obtained nickel-cobalt nitride electrocatalyst shows good stability and superior catalytic performance toward both HER and OER with low overpotentials of 0.29 and 0.18 V, respectively, to achieve a current density of 10 mA cm^-2 . The good electrocatalytic performance was also evidenced by the fabrication of an electrolyzer for overall water splitting, which exhibits a high gas generation rate for hydrogen and oxygen with excellent stability during prolonged alkaline water electrolysis. The present work provides an efficient approach to prepare a 3 D interconnected porous nickel-cobalt nitride network with exposed inner active sites for overall water splitting.

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_0.5 Co_0.5 O_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_0.4 Fe_0.6 O_x and Ni_0.33 Co_0.67 O_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.

One-Dimensional Earth-Abundant Nanomaterials for Water-Splitting Electrocatalysts

Hydrogen fuel acquisition based on electrochemical or photoelectrochemical water splitting represents one of the most promising means for the fast increase of global energy need, capable of offering a clean and sustainable energy resource with zero carbon footprints in the environment. The key to the success of this goal is the realization of robust earth-abundant materials and cost-effective reaction processes that can catalyze both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with high efficiency and stability. In the past decade, one-dimensional (1D) nanomaterials and nanostructures have been substantially investigated for their potential in serving as these electrocatalysts for reducing overpotentials and increasing catalytic activity, due to their high electrochemically active surface area, fast charge transport, efficient mass transport of reactant species, and effective release of gas produced. In this review, we summarize the recent progress in developing new 1D nanomaterials as catalysts for HER, OER, as well as bifunctional electrocatalysts for both half reactions. Different categories of earth-abundant materials including metal-based and metal-free catalysts are introduced, with their representative results presented. The challenges and perspectives in this field are also discussed.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

We review the fundamental aspects of metal oxides, metal chalcogenides and metal pnictides as effective electrocatalysts for the oxygen evolution reaction.

Recovered spinel MnCo2O4 from spent lithium-ion batteries for enhanced electrocatalytic oxygen evolution in alkaline medium

The recovery of spinel MnCo₂O₄ from spent LIBs was achieved by a set of physical and chemical treatments, their employment for water oxidation in alkaline conditions was studied, and it was found that the recovered spinel MnCo₂O₄ were more effective than recovered monometallic oxides.

Energy and fuels from electrochemical interfaces

Advances in electrocatalysis at solid-liquid interfaces are vital for driving the technological innovations that are needed to deliver reliable, affordable and environmentally friendly energy. Here, we highlight the key achievements in the development of new materials for efficient hydrogen and oxygen production in electrolysers and, in reverse, their use in fuel cells. A key issue addressed here is the degree to which the fundamental understanding of the synergy between covalent and non-covalent interactions can form the basis for any predictive ability in tailor-making real-world catalysts. Common descriptors such as the substrate-hydroxide binding energy and the interactions in the double layer between hydroxide-oxides and H---OH are found to control individual parts of the hydrogen and oxygen electrochemistry that govern the efficiency of water-based energy conversion and storage systems. Links between aqueous- and organic-based environments are also established, encouraging the 'fuel cell' and 'battery' communities to move forward together.

Self-assembly formation of Bi-functional Co3O4/MnO2-CNTs hybrid catalysts for achieving both high energy/power density and cyclic ability of rechargeable zinc-air battery

α-MnO2 nanotubes-supported Co3O4 (Co3O4/MnO2) and its carbon nanotubes (CNTs)-hybrids (Co3O4/MnO2-CNTs) have been successfully developed through a facile two-pot precipitation reaction and hydrothermal process, which exhibit the superior bi-functional catalytic activity for both ORR and OER. The high performance is believed to be induced by the hybrid effect among MnO2 nanotubes, hollow Co3O4 and CNTs, which can produce a synergetic enhancement. When integrated into the practical primary and electrochemically rechargeable Zn-air batteries, such a hybrid catalyst can give a discharge peak power density as high as 450 mW cm(-2). At 1.0 V of cell voltage, a current density of 324 mA cm(-2) is achieved. This performance is superior to all reported non-precious metal catalysts in literature for zinc-air batteries and significantly outperforms the state-of-the-art platinum-based catalyst. Particularly, the rechargeable Zn-air battery can be fabricated into all-solid-state one through a simple solid-state approach, which exhibits an excellent peak power density of 62 mW cm(-2), and the charge and discharge potentials remain virtually unchanged during the overall cycles, which is comparable to the one with liquid electrolyte.

Strongly Coupled CoCr2 O4 /Carbon Nanosheets as High Performance Electrocatalysts for Oxygen Evolution Reaction

A strongly coupled CoCr2 O4 /carbon nanosheet composite is concurrently grown via a facile one-step molten-salt calcination approach. The strong coupling between carbon and CoCr2 O4 has improved the electrical conductivity and preserved the active sites in catalysts. These results may pave the way to improve the performance of spinel oxides as electrocatalysts for oxygen evolution reactions.

Porous LaCo1-xNixO3-δ Nanostructures as an Efficient Electrocatalyst for Water Oxidation and for a Zinc-Air Battery

Perovskites have emerged as promising earth-abundant alternatives to precious metals for catalyzing the oxygen evolution reaction (OER). Herein, we report the synthesis of a series of porous perovskite nanostructures, LaCo0.97O3-δ, with systematic Ni substitution in Co octahedral sites. Their electrocatalytic activity during the water oxidation reaction was studied in alkaline electrolytes. The electrocatalytic OER activity and stability of the perovskite nanostructure was evaluated using the rotating disk electrode technique. We show that the progressive replacement of Co by Ni in the LaCo0.97O3-δ perovskite structure greatly altered the electrocatalytic activity and that the La(Co0.71Ni0.25)0.96O3-δ composition exhibited the lowest OER overpotential of 324 and 265 mV at 10 mA cm(-2) in 0.1 M KOH and 1 M KOH, respectively. This value was much lower than that of the noble metal catalysts, IrO2, Ru/C, and Pt/C. Furthermore, the La(Co0.71Ni0.25)0.96O3-δ nanostructure showed outstanding electrode stability, with no observable decrease in performance up to 114th cycle in the auxiliary linear sweep voltammetry that lasted for 10 h in chronoamperometry studies. The excellent oxygen evolution activity of the La(Co0.71Ni0.25)0.96O3-δ perovskite nanostructure can be attributed to its intrinsic structure, interconnected particle arrangement, and unique redox characteristics. The enhanced intrinsic electrocatalytic activity of the La(Co0.71Ni0.25)0.96O3-δ catalyst was correlated with several parameters, such as the electrochemical surface area, the roughness factor, and the turnover frequency, with respect to variation in the transition metals of the perovskite structure. Subsequently, La(Co0.71Ni0.25)0.96O3-δ was utilized as the air cathode in a zinc-air battery application.

Influence of annealing temperature on microstructure and lithium storage performance of self-templated CuxCo3−xO4 hollow microspheres

Spinel Cu_xCo_3−xO₄ (x ≤ 0.30) hollow microspheres have been readily prepared via a self-templated solvothermal reaction followed by a thermal annealing step.