Electrodes For Oxygen Evolution Reaction Research Articles

Atomically Dispersed Bimetallic FeNi Catalysts as Highly Efficient Bifunctional Catalysts for Reversible Oxygen Evolution and Oxygen Reduction Reactions

AbstractBimetallic atomically dispersed FeNi catalysts anchored on N‐doped carbon nanotube with catalyst loading of 2–7 wt % with different Fe : Ni ratio have been developed as highly active and stable bifunctional catalyst for reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for metal air batteries via a modified one‐pot synthesis method. Compared with atomically dispersed single Fe and Ni catalysts, the bimetallic FeNi catalysts exhibit outstanding performance for reversible OER and ORR, achieving a low potential gap (ΔE) of 0.81 V to deliver an OER current density of 10 mA cm−2 and an ORR current density of 3 mA cm−2. The FeNi electrodes also show a much better stability in the cyclic tests, compared to that of the state‐of‐the‐art Pt/C and Pt/C+Ir/C electrodes for reversible OER and ORR. The high performance is likely due to the significantly enhanced OER activity contributed by the introduction of Ni atoms, forming bridged FeNi bimetallic dual atom active sites for OER. This study provides a new platform for the development of highly active bimetallic atomic catalysts based bifunctional electrocatalysts for metal‐air batteries. The modified one‐pot synthesis methods demonstrated in this study can also be applicable to other atomically dispersed catalysts on CNTs or graphenes.

Coating Electrode for Oxygen Evolution Reaction

Coating Electrode for Oxygen Evolution Reaction

Unique-Structure MoS2 Grow on Nickel Foam As Highly Efficient Self-Supported Electrode for Oxygen Evolution Reaction

Growing demand for sustainable, clean, efficient energy conversion system has derived tremendous interest to develop cost-effective and highly efficient electrocatalysts for oxygen evolution reaction (OER). Sulfides have recently attracted great attention due to their highly efficient electrocatalytic activity. MoS2 has been verified as an efficient catalyst toward OER because of its durability, cost-efficiency and high activity. However, binder and extra conductive materials are required to attach electrocatalyst on a conductive substrate which will result in restricted active surface areas, undesirable interfaces and lots of dead volumes. Binder-free self-supported material can eliminate these problems. Herein, we synthesize self-supported binder-free MoS2 on 3D porous nickel foam (NF) with excellent activity and stability via a facile hydrothermal method. The MoS2@NF electrode is tested in 0.1mol KOH by linear sweep voltammetry (LSV), electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS). Compared with different substrates (Cu foam, NF, Cu paper and carbon paper), MoS2@NF electrode where NF as substrate shows the lowest overpotential at 10mA cm-2 of 359mV for OER. Furthermore, the overpotential at 10mA cm-2 after 10hr is 419mV, which demonstrates high stability and activity. Such high electrochemical performance may attribute to the following reasons: 1) the large ECSA and good conductivity, that improves utilization of electroactive sites and electron transfer; 2) the unique 3D porous structure, that facilitates electrolyte penetration and reactant/product diffusion; and 3) the sulfur vacancy in MoS2, that accelerates the O-H bond breaking.

Se-Doping Activates FeOOH for Cost-Effective and Efficient Electrochemical Water Oxidation.

Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such a Se-doped FeOOH electrode via an FeSe precatalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm-2 is secured at a low overpotential of 348 mV. The application of such an Fe-rich OER electrode in a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.

Fabrication of 3D microporous amorphous metallic phosphides for high-efficiency hydrogen evolution reaction

The design and prediction of composition and structure is critical for the development of robust and efficient oxygen evolution reaction (OER) catalysts for water splitting. To enhance electrochemical catalytic activity of catalyst, a series of 3D microporous amorphous metallic phosphides, such as NiP, FeP, CoP, NiFeP, FeCoP, NiCoP and NiFeCoP, were electrodeposited on nickel mesh (dubbed NM) via hydrogen bubbles electroplating technique. The electrocatalytic performances of all as-prepared phosphides in alkaline solution for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were investigated. The optimal 3D microporous NiFeCoP/NM electrode exhibits remarkable catalytic activity for HER and delivers a current density of 10 mA cm−2 only required an extremely low overpotential (33 mV) and a small Tafel slope (71.1 mV dec−1) in 1.0 M KOH solution. Meanwhile the NiFeCoP/NM electrode also shows outstanding strong durability and robustness in alkaline water electrolysis. Further, the NiFeCoP/NM electrode displays superior catalytic activity towards OER. In a two-electrode water electrolytic cell, the bi-functional NiFeCoP/NM electrodes could obtain current densities of 10 and 50 mA cm−2 at overall cell potentials of only 1.64 and 1.82 V, respectively. The remarkable electrocatalytic activities of the NiFeCoP/NM electrode both for HER and OER are attributed to the synergistic effect of tri-metal elements in metallic phosphide, amorphous phase structure, and the superlarge electrochemical surface area fabricated by the hydrogen bubbles electroplating.

Mesoporous Ultrathin Cobalt Oxides Nanosheets Grown on Carbon Cloth as a High-Performance Electrode for Oxygen Evolution Reaction

Developing high-performance and low-cost electrode for oxygen evolution reaction (OER) is a crucial step to produce clean and sustainable energy sources. Herein, we report mesoporous ultrathin coba...

Synthesis of La0.2Sr0.8CoO3 and its electrocatalytic activity for oxygen evolution reaction in alkaline solution

The perovskite-type oxide La0.2Sr0.8CoO3 was prepared by sol-gel process, which particle was irregular cube with particle size between 50 and 60 nm. A novel air electrode for oxygen evolution reaction prepared by using La0.2Sr0.8CoO3 as a catalyst has a current density of 152 mA cm−2 when the potential is 0.7 V (vs. Hg/HgO) in an alkaline medium (6 mol L−1 KOH) under the air atmosphere. The research results show that n (C6H8O7): n (M) = 1:1, pH = 9, calcination temperature 800 °C, 5 h calcination time is the best preparation process conditions.

Nickel–Cobalt Diselenide Nanosheets Supported on Copper Nanowire Arrays for Synergistic Electrocatalytic Oxygen Evolution

AbstractRational nanoarchitecture design and smart hybridization of bespoke catalysts can greatly accelerate the sluggish kinetics of oxygen evolution reaction (OER) in electrochemical water splitting. Here, hierarchical NixCo1−xSe2 porous nanosheets are synthesized on Cu nanowire arrays (CNW) to fabricate highly efficient core/shell structure integrated OER electrode. Highly conductive CNW arrays are first obtained via the reduction of the pre‐prepared CuO nanowire arrays. NixCo1−x precursor nanosheets are then grown on CNW via hydrothermal route, and the following selenylation led to in situ formation of NixCo1−xSe2 porous nanosheet. In the integrated electrode, the highly conductive CNW core can realize efficient electron transport. The hierarchical core/shell nanoarray structure can provide abundant catalytic active sites and more void space to release gas bubbles, and meanwhile prevent the underneath Cu nanowire core from oxidation. The synergistic effects of optimized components and smart structure of the integrated electrode afford a remarkable OER activity with a low potential of 1.423 V versus reversible hydrogen electrode (RHE) at 10 mA cm−2 current density, a small Tafel of 58 mV dec−1, and excellent stability in alkaline electrolyte. The present fabrication approach offers a direction in the design and synthesis of integrated catalysts on conductive substrates for promising electrocatalytic applications.

Modelling the (Essential) Role of Proton Transport by Electrolyte Bases for Electrochemical Water Oxidation at Near-Neutral pH

The oxygen-evolution reaction (OER) in the near-neutral pH-regime is of high interest, e.g., for coupling of OER and CO2-reduction in the production of non-fossil fuels. A simple model is proposed that assumes equal proton activities in the catalyst film and the near-surface electrolyte. Equations are derived that describe the limitations relating to proton transport mediated by fluxes of molecular “buffer bases” in the electrolyte. The model explains (1) the need for buffer bases in near-neutral OER and (2) the pH dependence of the catalytic current at high overpotentials. The latter is determined by the concentration of unprotonated buffer bases times an effective diffusion constant, which can be estimated for simple cell geometries from tabulated diffusion coefficients. The model predicts (3) a macroscopic region of increased pH close to the OER electrode and at intermediate overpotentials, (4) a Tafel slope that depends on the reciprocal buffer capacity; both predictions are awaiting experimental verification. The suggested first-order model captures and predicts major trends of OER in the near-neutral pH, without accounting for proton-transport limitations at the catalyst–electrolyte interface and within the catalyst material, but the full quantitative agreement may require refinements. The suggested model also may be applicable to further electrocatalytic processes.

Self-supported nickel iron oxide nanospindles with high hydrophilicity for efficient oxygen evolution.

Engineering nanoarray structures with favorable hydrophilicity is proposed to unlock the electrocatalysis of NiFe2O4 for the oxygen evolution reaction (OER). The resulting electrode, consisting of NiFe2O4 nanospindles directly grown on FeNi3 foam, features enlarged active surface area, high hydrophilicity and increased electronic conductivity, which achieves superior OER activity and remarkable durability.

Self-assembled Cu-Ni bimetal oxide 3D in-plane epitaxial structures for highly efficient oxygen evolution reaction

To improve the catalytic activity of copper based electrode for oxygen evolution reaction (OER), double cathodes liquid plasma discharge (DC-LPD) method was developed to fabricate three-dimensional (3D) nest-like structures electrode on the nickel foam (NF). The CuO-NiO/NF electrode obtained through annealing the in-plane epitaxial Cu(OH)2-Ni2O3H/NF in the oven, exhibits excellent electrocatalytic activities for the oxygen evolution reaction at a low overpotential of 319 mV to achieve a current density of 10 mA/cm2. The electrochemically active surface area (ECSA) of CuO-NiO/NF is over 2 times larger than that of Cu(OH)2-Ni2O3H/NF electrode. The synergistic effort between Cu and Ni makes the CuO-NiO/NF electrode a superior electrocatalyst with improved activity and stability as compared to the single NiO/NF electrode.

N-doped carbon-coated Co3O4 nanosheet array/carbon cloth for stable rechargeable Zn-air batteries

Although the application of various nonprecious compounds as the air cathodes of Zn-air batteries has been explored, the construction of highly efficient self-supported Co-based electrodes remains challenging and highly desired given their outstanding electrocatalytic activity and cost-effectiveness. Herein, we fabricated a three-dimensional (3D) self-supported electrode based on N-doped, carbon-coated Co3O4 nanosheets grown on carbon cloth (i.e., NC-Co3O4/CC) through electrochemical deposition and carbonization. When used as a binder-free electrode for oxygen evolution reaction (OER), the NC–Co3O4/CC electrode demonstrated excellent electrocatalytic activity with an overpotential of 210 mV at 10 mA cm−2 and a Tafel slope of 79.6 mV dec−1. In the Zn-air battery test, the electrode delivered a small charge/discharge voltage gap (0.87 V at 10 mA cm−2) and exhibited high durability without degradation after 93 cycles at the large current density of 25 mA cm−2. The durability of our electrode was superior to that of a commercial Pt/C+RuO2 catalyst. The excellent performance of NC–Co3O4/CC could be attributed to the presence of 3D structures that promoted electron/ion transfer. By the absence of a binder, the carbon coating improved electron conductivity and promoted electrochemical stability. Moreover, N doping could be used to adjust the C electron structure and aeffective roccelerate electron transfer. The present study provides a facile and ute for the synthesis of various self-supported electrodes that fulfill the requirements of different energy storage and conversion devices.

Functionalized Phosphorene Quantum Dots as Efficient Electrocatalyst for Oxygen Evolution Reaction.

Phosphorene has attracted great interest in the rapidly emerging field of two-dimensional layered nanomaterials. Recent studies show promising electrocatalytic activity of few-layered phosphorene sheets toward the oxygen evolution reaction (OER). However, controllable synthesis of mono/few-layered phosphorene nanostructures with a large number of electrocatalytically active sites and exposed surface area is important to achieve significant enhancement in OER activity. Here, a novel strategy for controlled synthesis and in situ surface functionalization of phosphorene quantum dots (PQDs) using a single-step electrochemical exfoliation process is demonstrated. Phosphorene quantum dots functionalized with nitrogen-containing groups (FPQDs) exhibit efficient and stable electrocatalytic activity for OER with an overpotential of 1.66 V @ 10 mA cm-2, a low Tafel slope of 48 mV dec-1, and excellent stability. Further, we observe enhanced electron transfer kinetics for FPQDs toward the Fe2+/Fe3+ redox probe in comparison with pristine PQDs. The results demonstrate the promising potential of phosphorene as technologically viable OER electrodes for water-splitting devices.

Charge State Manipulation of Cobalt Selenide Catalyst for Overall Seawater Electrolysis

AbstractFacile and controllable fabrication of highly active and stable bifunctional electrocatalysts for water electrolysis is important for hydrogen production. 3D cobalt selenide electrodes with CoSe and Co9Se8 phases are synthesized by one‐step calcination of Co foil with Se powder in a vacuum‐sealed ampoule. The charge state of Co species and the electrocatalytic performance of the prepared catalysts are manipulated by controlling Co to Se mass ratio. Mechanistic studies show that a high Co charge state favors oxygen evolution reaction performance and a low Co charge state promotes hydrogen evolution reaction activity for as‐prepared cobalt selenide electrocatalysts. The resultant materials can act as free‐standing bifunctional electrocatalytic electrodes for oxygen evolution reaction and hydrogen evolution reaction in alkaline media. Moreover, a 10.3 mA cm−2 current density at 1.8 V is achieved for overall seawater electrolysis through exploiting as‐synthesized cobalt selenide electrodes as both anode and cathode, which is three times higher than that for novel‐metal‐based seawater electrolyzer at the same voltage (2.9 mA cm−2). Experimental results reveal that the cobalt selenide electrodes show significantly higher electrocatalytic performance than that of integrated Ni/Ir–C and Ni/Pt–C electrodes. Consequently, these novel bifunctional electrodes are promising candidates for realistic large‐scale water electrolysis.

Enhanced electrocatalytic performance of NiOx@MnOx@graphene for oxygen reduction and evolution reactions

Oxygen electrodes for oxygen evolution reaction and oxygen reduction reaction were intensively investigated due to high overpotential required to drive the four-electron process. A NiOx@MnOx@G nanostructure supported homogenously on graphene nanosheets through an easy and scalable self-assembly method was studied. The NiOx@MnOx@G exhibited a nanostructured NiOx nanocrystalline with size around 2.3 nm and an amorphous MnOx with controllable thickness. The 25% NiOx@MnOx@G showed remarkable activity for oxygen reduction reaction with a 4-electron process, the half-wave potential was 50 mV, but the stability of 25% NiOx@MnOx@G was better than Pt/C-JM. NiOx@MnOx@G nanostructure exhibited significantly better activity for oxygen evolution reaction compared with MnOx, which can demonstrate that NiOx could tune the activity of surface amorphous MnOx and dramatically increased oxygen evolution reaction activity. NiOx@MnOx@G is demonstrated with superior oxygen catalysts performance for reversible oxygen evolution and oxygen reduction reaction, due to the synergistic effect of NiOx and amorphous MnOx.

Hierarchical Design of NiOOH@Amorphous Ni-P Bilayer on a 3D Mesh Substrate for High-Efficiency Oxygen Evolution Reaction.

Recently, 3d metal phosphide and metal-phosphorus alloy have been intensively studied for the oxygen evolution reaction (OER). Research work has indicated that the presence of phosphorus could lead to the formation of a phosphide/(hydro)-oxide core/shell structure. In this work, we have developed a fabrication technique for a robust NiOOH@amorphous Ni-P bilayer on a zirconia mesh support through the collaboration of electroless deposition and robocasting. During the electroless deposition, a fully amorphous structure can be obtained with a certain phosphorus content (7-8 wt %). Relatively thick films (in the order of 5 μm) had an excellent adhesion on the mesh structure because of the large curvature. A stable Ni oxy/hydroxide surface (∼200 nm) can be formed in bilayer nature (NiOOH/Ni-P) due to preactivation. The combination of catalyst active sites on the surface and high conductivity of metallic body enables good OER performance with an overpotential of 286 mV at a current density of 10 mA cm-2. Together with excellent chemical stability and mechanical strength of the ceramic substrate, this novel combination gives rather excellent adhesion and stability in alkaline solution and provides a different angle for the hierarchical design of corrosion resistant and high-performance OER electrodes for industry.

Precious Metal Oxide Loading Reduction of Dimensionally Stable Electrodes for Oxygen Evolution Reaction

Introduction In dimensionally stable electrodes (DSE® type electrodes) technology, reduction of precious metal loading is significant issue to reduce cost of system. As improvements of the DSE® type electrode, many efforts of preparation conditions have been reported. Higher preparation temperature reduces electrocatalyst consumption, but corrosion of the base substrate/electrocatalyst interface makes limitation of service lifetime [1-4]. Here, intermediate layer should be important to improve durability. As the intermediate layer, 4 and 5th group transition metal oxide or composite with the transition metal oxide and precious metal was proposed, but their property has not be clarified. In addition, these material technology would be important basic for polymer electrolyte membrane water electrolysis, too. In this study, corrosion resistance of low oxygen partial pressure oxidized titanium electrodes and oxygen evolution reaction activity and durability of low IrO2 loaded electrodes of the low oxygen partial pressure oxidized titanium have been investigated as basic study of intermediate layer of DSE® type electrode in industrial electrolysis and electroconductive oxide supported precious metal oxide electrocatalyst for polymer electrolyte membrane water electrolysis. Experimental Titanium electrode was oxidized in H2 – O2 – Ar of estimated p(O2) of 1.84×10-17 atm at 1050°C for 10 or 20 h, H2 – O2 – Ar of the p(O2) of 1.43×10-17 atm at 1050°C for 10 h, air at 500°C for 1 h. For comparison, Titanium electrode was also used. Surface oxide was determined with X-ray diffraction for thin – film of 2.0° of incidence angle. 0.02 mg cm-2 of IrO2, which is ca. 1/50 level of usual industrial electrode, was loaded on the titanium electrodes by thermal decomposition of hydrogen hexachloroiridate(IV) hydrate butanol solution at 400°C for 1 h in air Electrochemical measurement was performed in 0.1 or 1.0 M (= mol dm-3) of H2SO4 at 30°C. Pretreatment was 300 cycles of cyclic voltammetry between 0.05 to 1.2 V vs. RHE with 200 mV s-1 for the all electrodes. The electrochemical stability of the titanium electrodes and the electrocatalytic activity for oxygen evolution reaction for the IrO2 coated titanium electrodes were determined with cyclic voltammetry of 5 mV s-1 of sweep rate. Resistances of the electrolyte and of surface oxide were determined with the electrochemical impedance spectroscopy (EIS). Results and discussion The surface of the titanium electrode oxidized in H2 – O2 – Ar of the p(O2) of 1.84×10-17 atm for 20 h and that of the 1.43×10-17 atm for 10 h at 1050°C was mixture of Ti4O7 and TiO2, and that of the p(O2) of 1.84×10-17 at 1050°C for 10 h and air at 500°C was TiO2. So, the notations of the titanium electrodes are Ti4O7_1050_20h/Ti, Ti4O7_1050_10h/Ti, TiO2_1050_10h/Ti, and TiO2_500/Ti, respectively. Figure 1 shows the surface oxide layer resistance of the titanium electrodes determined by the higher frequency region arc of the EIS measurement as a function of higher limit potential of the slow scan cyclic voltammetry. The resistances of the high temperature (1050°C) oxidized electrodes were constant, while that of the Ti and the 500°C oxidation Ti increased with higher limit potential of the slow scan voltammetry. In addition, the Ti4O7 containing oxide layer showed lower resistance than the TiO2 Therefore, high temperature prepared electroconductive oxidation layer has low resistance with stability as a support material. Figure 2 shows the surface oxide layer resistance of the IrO2 loaded titanium electrodes determined by higher frequency region arc of the EIS. Basically, the surface oxide layer resistance decreased with the IrO2 loading, and especially, the surface oxide layer resistance of the TiO2_1050_10h/Ti significantly decreased rather than others. Figure 3 and 4 shows the IR corrected initial performance and the lifetime test at 150 mA cm-2 for the IrO2 loaded titanium electrodes for OER. The IrO2/Ti, which has no intermediate layer, has the highest initial OER activity, but the shortest lifetime in these electrodes, and TiO2_1050_10h/Ti has relatively high OER activity and the longest lifetime in these electrodes. In the design of the intermediate layer, not only the property of the intermediate material itself but also synergy effect of 4 and 5th group metal oxide and precious metal oxide should be taken into account. This research was supported by the JSPS Grant-in-Aid for Scientific Research (C) JP 16K12345. The authors want to thank the person concern with. Reference K. Nagai, K. Nagasawa, and S. Mitsushima, Electrocatalysis, 7, 441 (2016) http://dx.doi.org/10.1007/s12678-016-0325-y.J. Hu, X. Sun, Y. Hou, J. Zhang, C. Cao, Electrochimica Acta, 53, 3127 (2008).H. Asano, T. Shimamune, USP 4,481,097 (1984).T. Shimamune, Y. Nakajima, USP 6,103,299 (2000). Figure 1

Monolithic nanoporous Ni[sbnd]Fe alloy by dealloying laser processed Ni[sbnd]Fe[sbnd]Al as electrocatalyst toward oxygen evolution reaction

In this paper, monolithic nanoporous NiFe electrocatalyst is developed by dealloying a NiFeAl alloy, fabricated by a laser-based manufacturing method, as the electrocatalyst toward oxygen evolution reaction (OER). The nanoporous NiFe alloy displays an improved electrochemical performance by exhibiting an OER current density of 100 mA/cm2 at 442 mV overpotential in the 1 M KOH aqueous solution, which is better than that of laser processed bulk NiFe alloy (464 mV). The structures, crystallinities, and chemical compositions of the nanoporous NiFe electrocatalyst are characterized by SEM, XRD and EDXS. With an activity comparable to the electrocatalysts reported to date, the monolithic nanoporous NiFe electrodes are also highly flexible in mechanical configurations, due to the combined advantages of both laser and dealloying processes, which provide new options for OER electrode design.

Corrosion engineering towards efficient oxygen evolution electrodes with stable catalytic activity for over 6000 hours

Although a number of nonprecious materials can exhibit catalytic activity approaching (sometimes even outperforming) that of iridium oxide catalysts for the oxygen evolution reaction, their catalytic lifetimes rarely exceed more than several hundred hours under operating conditions. Here we develop an energy-efficient, cost-effective, scaled-up corrosion engineering method for transforming inexpensive iron substrates (e.g., iron plate and iron foam) into highly active and ultrastable electrodes for oxygen evolution reaction. This synthetic method is achieved via a desired corrosion reaction of iron substrates with oxygen in aqueous solutions containing divalent cations (e.g., nickel) at ambient temperature. This process results in the growth on iron substrates of thin film nanosheet arrays that consist of iron-containing layered double hydroxides, instead of rust. This inexpensive and simple manufacturing technique affords iron-substrate-derived electrodes possessing excellent catalytic activities and activity retention for over 6000 hours at 1000 mA cm-2 current densities.

A 3D electrode of core@shell branched nanowire TiN@Ni0.27Co2.73O4 arrays for enhanced oxygen evolution reaction

The combination of highly active materials with judicious nanostructures is crucial to develop high-performance electrocatalysts for oxygen evolution reaction (OER), which involves sluggish multistep electron transfer processes. Herein, TiN nanowire arrays are used as conductive trunks to support active Ni-doped Co3O4 (Ni0.27Co2.73O4) nanosheets to construct a binder-free three-dimensional (3D) electrode for OER. In the core@shell configuration, the TiN nanowire arrays possess a single-crystal-like structure favoring charge migration and the ultrathin Ni0.27Co2.73O4 nanosheets have a mesoporous structure providing more active sites. The hierarchically branched morphology, high conductivity and large surface area can be achieved simultaneously by the optimization in composition and nanostructure, which contributes to excellent electrochemical performances. The TiN@Ni0.27Co2.73O4 electrode shows much better OER activity than the Ni-doped Co3O4 nanosheets deposited on planar Ti substrate and low conductive TiO2 nanowire arrays.