OER Process Research Articles

Triggering efficient reconstructions of Co/Fe dual-metal incorporated Ni hydroxide by phosphate additives for electrochemical hydrogen and oxygen evolutions

Alkaline electrochemical water splitting has been considered as an efficient way for the green hydrogen production in industry, where the electrocatalysts play the critical role for the electricity-to-fuel conversion efficiency. Phosphate salts are widely used as additives in the fabrication of electrocatalysts with improved activity, but their roles on the electrocatalytic performance have not been fully understood. Herein, we fabricate Co, Fe dual-metal incorporated Ni hydroxide on Ni foam using NaH2PO4 ((Co, Fe)NiOxHy-pi) and NaH2PO2 ((Co, Fe)NiOxHy-hp) as additive, respectively. We find that (Co, Fe)NiOxHy-hp with NaH2PO2 in the fabrication shows high activity and stability for both HER and OER (a overpotential of −0.629 V and 0.65 V at 400 mA cm−2 for HER and OER, respectively). Further experiment reveals that the reconstructed structures of electrocatalyst by using NaH2PO2 (hp) endow high electrocatalytic performances: (1) in-situ generated active metal improves the accumulation, transportation and activity of hydrogen species in the HER process; and (2) in-situ generated poor-crystalline hydroxide endows superior charge/mass transportation and kinetics improvements in the OER process. Our study may provide an insightful understanding on the catalytic performance of non-precious metal electrocatalysts by controlling additives and guidance for the design and synthesis of novel electrocatalysts.

Controllable synthesis of crystal-amorphous heterostructures in transition metal phosphide and enhancement mechanism for overall water splitting

The crystal-amorphous (c-a) heterostructure is a promising structure for electrocatalysts in water electrolysis. In this study, stable, efficient, and abundant c-a interfaces were formed by synchronously synthesizing crystalline FeP and amorphous MoP on foam ferrum by controlling the crystalline phase transition temperature. In 1 M KOH, the a-MoP/FeP@FF catalyst showed overpotentials of 75 mV and 253 mV for HER and OER processes, respectively, at a current density of j10. The Tafel slopes were small at 60 mV∙dec−1 and 77 mV∙dec−1 for HER and OER, respectively. Theoretical calculations indicate that the c-a interface optimizes the overall electronic structure, enabling Mo and Fe to have excellent HER and OER activity, respectively. The two types of active sites collaborate fully to promote overall water splitting. This study confirms that the c-a heterostructure can significantly enhance the catalytic performance of transition metal phosphides and explains the mechanism of enhanced catalytic activity. This synthesis method and reaction mechanism not only apply to other transition metal phosphides but also provide reference for other types of catalysts.

Amorphous nanosphere self-supporting electrode based on filter paper for efficient water splitting

Given its superior structure, the abundance of surface functional groups, and low cost, cotton fiber filtration paper can be used as the support for hydrogen evolution reaction (HER) and oxygen evolution reaction catalysts (OER) instead of copper foam (CF), nickel foam (NF), or carbon cloth (CC). Herein, the chemical plating method was used to create thin self-supporting electrodes based on cotton fiber filtration paper (FP). The Co-Ni-P amorphous nanospheres can be uniformly dispersed on the self-supporting FP due to its porous structure and oxygen functional groups. The Co-Ni-P/FP electrode has an overpotential as low as 125 mV for HER and 320 mV for OER at the current density of 10 mA cm−2. Additionally, according to density functional theory (DFT) calculations, the Co atom added to the Ni-P nanosphere can lead to considerably more charge accumulation around the Co and Ni active sites, which greatly encourages the HER and OER processes. The novel porous support, FP, provides a fresh way the preparation of self-supporting electrodes.

Carboxamide Fe(III) complex as the electrocatalyst of water oxidation reaction: WNA and I2M O–O bond formation pathways

A single-site Fe(III) carboxamide complex, [Fe(HbpH) (Cl)2] (1), (HbpH‾ = N,N′-Bis(picolinoyl)hydrazine) anion), was synthesized and used as the electrocatalyst of water oxidation reaction. Complex 1 catalyzes the oxidation of water to O2 at the pH of 8 in phosphate buffer solution with the onset of a catalytic wave at about 0.45 V versus RHE with the turnover frequency (TOF) of 5.8 s−1 at room temperature. The stability of the catalyst in water oxidation processes by electrochemical and UV–vis measurements, demonstrated the realistic molecular electrocatalysis of 1. Differential pulse voltammetry (DPV) of 1 in different pH values showed the dependency of oxidation potential to the pH of the environment, establishing the fact that the catalytic cycle involves the proton-coupled electron transfer (PCET) process. Density functional theory (DFT) calculations on the mechanism of water oxidation process of 1 shows that the oxygen evolution reaction is carried out through the water nucleophilic attack (WNA) to the metal center ion, resulted to the formation of FeIV(O) species. Also, DFT calculations showed that the formation of an O–O bond proceeds through the interaction two molecular mechanism (I2M), which compete with the WNA mechanism following from experimental analysis. The electroactivity of the metal centre and carboxamide ligand and the ability of the ligand to stabilize the high oxidation numbers of the metal-ion centre during the oxidation process, provide different oxidation pathways for an OER process.

Accelerated deprotonation with a hydroxy-silicon alkali solid for rechargeable zinc-air batteries

Transition metal oxides are promising electrocatalysts for zinc-air batteries, yet surface reconstruction caused by the adsorbate evolution mechanism, which induces zinc-ion battery behavior in the oxygen evolution reaction, leads to poor cycling performance. In this study, we propose a lattice oxygen mechanism involving proton acceptors to overcome the poor performance of the battery in the OER process. We introduce a stable solid base, hydroxy BaCaSiO4, onto the surfaces of PrBa0.5Ca0.5Co2O5+δ perovskite nanofibers with a one-step exsolution strategy. The HO-Si sites on the hydroxy BaCaSiO4 significantly accelerate proton transfer from the OH* adsorbed on PrBa0.5Ca0.5Co2O5+δ during the OER process. As a proof of concept, a rechargeable zinc-air battery assembled with this composite electrocatalyst is stable in an alkaline environment for over 150 hours at 5 mA cm–2 during galvanostatic charge/discharge tests. Our findings open new avenues for designing efficient OER electrocatalysts for rechargeable zinc-air batteries.

A 3d-4d-5d High Entropy Alloy as a Bifunctional Oxygen Catalyst for Robust Aqueous Zinc-Air Batteries.

High entropy alloys (HEAs) are highly suitable candidate catalysts for oxygen evolution and reduction reactions (OER/ORR)as they offer numerous parameters for optimizing the electronic structure and catalytic sites. Herein, FeCoNiMoW HEA nanoparticles are synthesized using a solution-based low-temperature approach. Such FeCoNiMoW nanoparticles show high entropy properties, subtle lattice distortions, and modulated electronic structure, leading to superior OER performance with an overpotential of 233mV at 10mAcm-2 and 276mV at 100mAcm-2 . Density functional theory calculations reveal the electronic structures of the FeCoNiMoW active sites with an optimized d-band center position that enables suitable adsorption of OOH* intermediates and reduces the Gibbs free energy barrier in the OER process. Aqueous zinc-air batteries (ZABs) based on this HEA demonstrate a high open circuit potential of 1.59V, a peak power density of 116.9mWcm-2 , a specific capacity of 857mAhgZn -1 , and excellent stability for over 660h of continuous charge-discharge cycles. Flexible and solid ZABs are also assembled and tested, displaying excellent charge-discharge performance at different bending angles. This work shows the significance of 4d/5d metal-modulated electronic structure and optimized adsorption ability to improve the performance of OER/ORR, ZABs, and beyond.

Modulation of perovskite electronic configuration by cobalt doping for efficient catalysts in zinc-air battery electrodes

An in-depth comprehension of the structural characteristics of perovskite and the response mechanisms is crucial for designing materials with excellent properties. Here, the performance of several kinds of Sr0.75Pr0.25Fe1-xCoxO3-δ (x = 0, 0.25, 0.5, 0.75, 1) in an alkaline environment was thoroughly examined. Sr0.75Pr0.25Fe0.25Co0.75O3-δ displayed the lowest overpotential (337 mV @ 10 mA·cm−2) and Tafel slope (69.3 mV·dec−1) in 0.1 M KOH and possessed excellent performance in zinc-air batteries. Cobalt doping encourages the positive shift of the O 2p band center, boosting the number of electrons around the Fermi energy level and delivering an increase in conductivity, according to DFT and experimental studies. Compared to Sr0.75Pr0.25FeO3-δ, the rate-limiting step of Sr0.75Pr0.25Fe0.25Co0.75O3-δ changes from OH*→O* deprotonation to OH* adsorption, which significantly decreases the energy potential barrier during OER process. The A/B site doping method is universal and facile, which is essential to comprehend and develop effective perovskite oxide for zinc-air battery electrodes.

Single-atomic ruthenium coupling with NiFe layered double hydroxide in-situ growth on BiVO4 photoanode for boosting photoelectrochemical water splitting

In the present work, NiFe layered double hydroxide (LDH) supported single Ru atoms in-situ growth on BiVO4 photoanode (BiVO4@NiFe-LDHs/Ru) was fabricated to enhance the photoelectrochemical (PEC) water splitting. Ru atoms anchored to NiFe-LDHs via oxygen coordination to form Ru-O-M bonds, which induced electrons rearrangement to improve charge carriers separation and injection. Combining with the synergistic effect of Ru and NiFe-LDHs, BiVO4@NiFe-LDHs/Ru achieves high photocurrent density of 4.65 mA/cm2 at 1.23 V vs. RHE. Besides, Ru atoms induced formation of V(5−x)+ to stabilize V atoms in the lattice of BiVO4, which avoided V5+ dissolution during PEC water oxidation process. Density functional theory (DFT) calculation indicates that Ru SAs anchored to BiVO4@NiFe-LDHs decrease the reaction energy barrier of rate-limiting step (*O → *OOH), resulting in acceleration of OER process. This work provides an effective pathway to design high efficient and stable photoanodes with single atoms for feasible PEC water splitting application.

Accelerating the energy-saving electrochemical hydrogen production by electrodeposited ultrathin 3-D Ni-Co-S nanosheets on Ni-Co-Cu nano-micro-dendrites

Substituting the OER process with a more thermodynamically favorable process, like the urea oxidation reaction (UOR), leads to energy-saving hydrogen production. Here, the three-dimensional and hierarchical Ni-Co-S@Ni-Co-Cu nanostructure was successfully synthesized and its electrocatalytic behavior was investigated. Investigations of the electrocatalytic behavior showed the unique performance of the Ni-Co-S@Ni-Co-Cu electrode, where in optimized conditions, in the HER process, to achieve 10 mA.cm−2 current density, the overpotential value was 68 mV. Also, when the Ni-Co-S@Ni-Co-Cu electrode was employed in both in HER and UOR processes simultaneously, to achieve the 10 mA.cm−2 current density, 1.38 V was needed. Also, the results of electrocatalytic stability represented insignificant changes in voltage over long periods (100 mA.cm−2, 50 h). The reason for such outstanding electrochemical performance in activity and stability point of view, was the synergistic effect between elements, large active surface area, using a binder-free method, and the regular hierarchical structure.

Targeted Grafting Metal-Organic Frameworks on Polyoxometalates for Efficient Water Oxidation

In recent years, due to the diverse and tunable microenvironment of metal centers, Metal-organic framework materials (MOFs) have shown great advantages in heterogeneous catalysis.[1,2] Studies have found that in some oxidation scenarios, taking water oxidation as an example, the instability of is framework limits its practical application under severe conditions. Therefore, many efforts have focused on improving the catalytic stability of MOFs materials. [3-5]The MOFs-based electro-catalytic systems developed for oxygen evolution under alkaline conditions in our work are discussed in the following three sections. First, based on the loss of active sites faced by MOFs during OER and the high oxidation potential due to insufficient electrical conductivity, a kind of nanoscale inorganic materials with oxygen-enriched surfaces --polyoxometalates (POMs), was selected as a composite modifier. Subsequently, a two-dimensional nanomaterial (MOFs@POMs) with high catalytic activity and ultra-high stability was synthesized on the basis of MOFs using a facile solvothermal method to immobilize the active sites of MOFs through the M-O ionic bond between POM and MOFs. Furthermore, we found that the structure of POMs plays a key role in the formation of this unique superstructure, which leads to a catalytic active site that can be persistently maintained during the OER process. Based on this, a scheme for the immobilization of MOFs active sites by POMs is proposed to synthesize MOFs composites with ultra-high OER stability based on both POMs and MOFs materials. The assembly mechanism of the composites during the synthesis process is proposed, and the intrinsic reason for the modulation of the active site of MOFs by POMs and its long-lasting immobilization is demonstrated, which enables the MOFs materials to maintain excellent catalytic activity throughout the great OER catalytic process. This study successfully demonstrates that the stability challenges faced in the OER process of MOFs can be solved, which may provide a helpful solution for the application of MOFs-related OER catalysts.

Deconvolution of Impedance Spectrums in an Anion Exchange Membrane Electrolyzer with Embedded Voltage Probes

High-efficient hydrogen production requires a minimized electrical work dissipation, which contributed by the resistances of all electrolyzer components during the operation. The electrochemical impedance spectroscopy (EIS) offers the understanding of resistances from different components based on their response to the alternative current frequency. However, the responses are overlapped, and it results in the highly convoluted impedance of the electrodes and limited information of the electrode processes. In this study, voltage probes are embedded in an anion exchange membrane (AEM) electrolyzer, which allow the EIS measurements of individual cell components with a three-electrode or four-electrode approach. Different metal probes made from Pt, Au and Ag are employed to separate the electrolyzer impedance. The summation of the impedances from individual cell components matches well with the total impedance of the entire cell with the error of less than 5%. Therefore, the embedded voltage probes in AEM provides a powerful platform to investigate the kinetics of HER and OER processes separately.

Decoration of Au Nanoparticles over LaFeO3: A High Performance Electrocatalyst for Total Water Splitting.

Electrocatalytic water splitting has emerged as a promising approach for clean and sustainable hydrogen production. The LaFeO3 perovskite structure exhibits intriguing properties such as mixed ionic-electronic conductivity, high stability, and abundant active sites for electrocatalysis. However, its OER and HER activities are limited by the sluggish kinetics of these reactions. To overcome this limitation, Au nanoparticles (NPs) are decorated onto the surface of LaFeO3 through a facile synthesis method. The Au NPs on the LaFeO3 surface provide additional active sites for water splitting reactions, promoting the adsorption and activation of water molecules. The presence of Au enhances the charge transfer kinetics via the heterostructure between Au NPs and LaFeO3 and facilitates electron transport during the OER and HER process. The catalyst requires only 318 and 199 mV as overpotential to attain a 50 mA cm-2 current density in 1 M KOH solution. Our results demonstrate that the Au@LaFeO3 catalyst exhibits significantly improved electrocatalytic activity compared to pure LaFeO3 and other catalysts reported in the literature. The enhanced performance is attributed due to the synergistic effects between Au NPs and LaFeO3, including an increased surface area, improved conductivity, and optimized surface energetics. Overall, the Au-decorated LaFeO3 catalyst presents a promising candidate for efficient electrocatalytic water splitting, providing a pathway for sustainable hydrogen production. The insights gained from this study contribute to the development of advanced catalysts for renewable energy technologies and pave the way for future research in the field of electrochemical water splitting.

Fe3+ induced etching of NiCo-MOF derived transformations into layered ternary hydroxides as efficient oxygen evolution catalyst

Hydrogen energy is the only new energy that meets the requirements of resources, environment and sustainable development due to its advantages such as rich resources, diverse sources, storable, renewable, high calorific value, clean and pollution-free. Electrochemical water splitting technology can achieve large-scale hydrogen production commercially, but the slow kinetics of OER process seriously hinders the development of electrochemical water splitting technology, therefore, high-efficiency and low-cost OER electrocatalyst is urgently needed. Here, we synthesized a layered ternary hydroxide NiCoFe-LTH@NiCo-MOF/NF-30 electrocatalyst with NiCo-MOF as the sacrificial template on conductive substrate nickel foam by a two-step solvothermal method, which requires only low overpotentials of 207 mV and 283 mV to achieve current densities of 10 mA cm−2 and 100 mA cm−2, respectively. Tafel slope is as low as 78.44 mV dec−1, and favorable electrochemical stability can be maintained for at least 50 h. The strategy may provide valuable references for future researchers to prepare better MOF-based electrocatalysts.

Solution combusted ultrathin Co3O4/CoO heterophase electrocatalyst for solar‐energy driven bifunctional water electrolysis

AbstractPotential low‐cost solution combustion synthesis of cobalt oxide (SCSCO: Co3O4/CoO) by using sugar as a fuel for H2 and O2 generation. Co3O4/CoO heterophase provided multiple functional sites and abundant interfaces between the components owing to synergistic effect. The sufficient adsorption sites in SCSCO6 favoured the formation of reactive intermediates which leads to extraordinary OER and HER process. The SCSCO6 exhibits η of 271 mV (102 mV dec−1) for OER and 174 mV (73 mV dec−1) for HER to achieve 10 mA cm−2 (without iR correction). TOF of SCSCO6 (OER: 0.0624 s−1/IrO2‐ 0.0185 s−1; HER: 0.0282 s−1/Pt ‐ 0.0843 s−1) was found to be higher than IrO2 and near to Pt, respectively. The SCSCO6 displays durability of 130 h with potential loss of 3.2 % (OER) and 3.0 % (HER). The disordered oxidation states and high surface area of SCSCO6 (BET ‐ 99 m2 g−1: ECSA ‐ 151.27 m2/g) facilitates the bifunctional activity. The fabricated water electrolyzer consist of SCSCO6//SCSCO6 affords 10 mA cm−2 @1.64 V and exhibits high stability (>130 h: 4.1 % potential loss). The solar‐to‐hydrogen electrolyser (SCSCO6//SCSCO6) afforded non‐stop evolution of H2 and O2 @1.63 V which can be recommended for low‐cost, large‐scale hydrogen production.

Development of Te-CeO2/NF nanofilm with ultralow overpotential for robust oxygen evolution reaction

Electrochemical energy conversion and storage devices rely on the (OER) oxygen evolution reaction, including regenerative metal-air batteries, fuel cells and many other applications. The OER process has intrinsically slow kinetics with a multistep electron transfer via proton-coupled, considering it a bottleneck in numerous electrochemical arrangements. To remove the bottleneck issue, an effective transition metal-based electrocatalyst is vital to lower the required onset and overpotential at a benchmark current density. Due to their durability, plenty of accessibility, and low environmental impact, nonprecious transition metal oxides like doped materials have gained prominence as prospective OER catalysts over the past decade. Therefore, in the present study, Te-CeO2/NF nanofilm is prepared via three steps, less cost alternative OER electrocatalysts via hydrothermal technique. The desired material responds to high activity, i.e., smaller overpotential of 227 mV with a Tafel value of 43 mV dec−1 and long-time stability of 100 h. Noble transition metal oxides doped with telluride are utilized as OER electrocatalysts for the first time and displayed great OER activity. This material provides a stimulating path for innovation and growth in various field such as energy conversion and storage devises, opening up new avenues for accessible and long-term learning.

A self-supported 3D porous high entropy alloy with progressive formation of needle-shaped nanowires during reactions for maintained excellent electrocatalytic oxygen evolution

An efficient 3D porous bulk electrocatalyst is a feasible way to improve the conductivity. Herein, the preparation of a novel self-supporting 3D porous bulk FeCoNiMgB catalyst exhibits excellent electrocatalytic activity and stability properties towards OER. The self-supporting 3D porous framework structure catalyst possesses multiple advantages including large surface area, excellent conductivity, leading to the fast mass and charge transfers, low electrochemical impedance and fast water splitting reaction kinetics. As a result, the 3D porous bulk FeCoNiMgB catalyst requires substantially lower overpotential (234 mV) with a small Tafel slope (36.1 mV dec−1) as compared to pure FeCoNiMgB powders and RuO2 electrodes, as well as stable catalytic properties for over 72 h with no obvious evidence of degradation. Comparative structural characterization and DFT calculations reveal that the needle-shaped nanowires with Fe rich are formed from many open pores during the OER process, which helps to reduce the rate determining step energy barrier (O∗→OOH∗) for OER, and thus has excellent long-term electrochemical stability for continuous oxygen production under alkaline conditions. This work provides a new feasible and promising protocol to realize a novel self-supporting 3D porous bulk high entropy alloy catalyst as an inexpensive catalyst for efficient water splitting.

Robust electrocatalysts decorated three-dimensional laser-induced graphene for selective alkaline OER and HER

Morphological and functional modifications allow graphene to be applied as promising electrode support for improved electrochemical cell performance. Typically, however, modification requires complicated steps with various organic additives, such as binders, which causes low electrochemical stability, especially under harsh pH conditions. Herein, we introduce self-supported and long-term stable water splitting electrodes based on nanoparticles (NPs) decoration on three-dimensional porous laser-induced graphene (3D-LIG). A binder-free, single-step electrochemical process homogeneously decorates the 3D-LIG electrodes with CuO and Pt NPs, referred to CuO-3D-LIG and Pt-3D-LIG electrodes, respectively. These electrodes are individually analyzed for enduring oxygen evolution (OER) and hydrogen evolution (HER) activities. The porous wrinkled morphology of the 3D-LIG offers a large surface area and facilitates electrolyte influx within channels. Whereas strongly anchored CuO and Pt NPs enable highly stable electrochemical performances of the CuO-3D-LIG and the Pt-3D-LIG electrodes. The corresponding OER and HER overpotentials at 10 mA cm−2 (η10) are observed at 251 mV and 455 mV. These electrodes offer excellent electrochemical robustness in operationally challenging alkaline conditions (1 M KOH, pH ∼ 14) for extensive periods. In addition, we confirm the workable electrostatic interactions NPs – graphene via Becke-3-parameter Lee–Yang–Parr (B3LYP) model with 6-31G (d,p) and Stuttgart-Dresden (SDD) basis, which is supposed to be responsible for electrochemical durability and high current density yield of CuO-3D-LIG and Pt-3D-LIG electrodes. This study offers one of a few experiments to utilize decorated 3D-LIG for both OER and HER processes, assuming their substantial industrial potential in overall water splitting.

Heterogeneous bimetallic oxysulfide nanostructure (Ni-Co) as hybrid bifunctional electrocatalyst for sustainable overall alkaline simulated seawater splitting

The design and development of high performance non-precious bifunctional electrocatalytic activity for oxygen/hydrogen evolution reaction (OER/HER) remains a significant challenge in overall seawater splitting process. Herein, we report the Co3S4/Co3O4 hybrid nanostructures catalyst with duel functions is successfully in-situ synthesized through self-templating strategy, whereas, Ninanostructure 3S2/NiO are grown on Ni foam using a facile hydrothermal reaction and post thermal annealing process. The designed self-standing Co3O4(Co3S4)/NiO(Ni3S2)/NF nanostructure (denoted as Co-Ni-S/NF) as a self-standing hybrid electrode demonstrates an excellent bifunctional electrocatalytic activity for OER and HER process in alkaline freshwater/alkaline simulated seawater as electrolyte. Taking advantage of the bimetallic synergistic effect and formation of an efficient interface layer of Ni3S2/NiO in a unique nanostructure, the resultant bifunctional electrocatalytic performance of Co-Ni-S/NF electrode for OER & HER processes are highly performed in both electrolyte conditions. Specifically, Co-Ni-S/NF hybrid electrode requires lower overpotentials of 270 mV (at 20 mV cm−2), 310 mV (at 50 mV cm−2) for OER and lower overpotentials of 239 mV (at 20 mV cm−2), 291 mV (at 50 mV cm−2) for HER in alkaline simulated seawater condition. For two-electrode based electrolyzer analyses, Co-Ni-S/NF electrode based electrolyzer demonstrates low cell voltage of 1.67 V to attain a current density of 10 mA cm−2 with stable activity of overall alkaline simulated seawater splitting process whereas hybrid electrode used as anode and cathode. Therefore, the proposed interface engineering strategy and rationally constructing electrocatalyst nanostructure via a self-templating route could be a promising opportunity to develop an active duel functional catalyst for direct seawater splitting applications.

Cerium doped amorphous Ni(OH)x on Cu(OH)2 nanorods for enhanced oxygen evolution reaction

It is urgent to develop highly efficient anode materials for electrocatalytic oxygen evolution reactions due to their sluggish kinetics. In this work, Ce-doped amorphous Ni(OH)x was constructed on Cu(OH)2 nanorods via the simple electrodeposition method. It was demonstrated that the Ce dopant resulted in the charge transfer from Ni to O, leading to the Ni3+ with a higher valence state, and thus functioning as active sites with more intrinsic activity. Moreover, the Ce dopant could also reduce the charge transfer resistance, leading to faster charge transport during the OER process. The obtained sample exhibited excellent OER performance with an overpotential of 225 mV at 10 mA cm−2 and stable operation at 100 mA cm−2 for 50 h.

Identifying the in situ protection role of MnO2 nanosheets on Co oxide for superior water oxidation

As a limiting step of electrochemical water splitting, the oxygen evolution reaction calls for efficient catalysts to surmount the high energy barrier. In this work, the synthesis of O-deficient Co3O4 nanoparticles with abundant surface area was realized by applying two-dimensional MnO2 nanosheets as a self-protection agent. The strong interaction between Mn and Co was confirmed by our experiments and DFT calculations that the valence state is increased for Co while it is reduced for Mn during the annealing in reducing atmosphere. Mn3+δ with stronger Lewis acidity can protect Co2+ from being reduced when subjected to the H2 treatment. The H2– reduced CoMn oxide under 300 °C (CMO-H300) shows the best OER activity with an overpotential of 280 mV in 1 M KOH and 460 mV in 1 M phosphate buffer (PBS) at 10 mA cm-2 geo, which can be attributed to the optimal chemical state, appropriate oxygen vacancies and highly exposed surface area. Operando Raman spectroscopy combined with ex-situ XPS revealed the manner of active species evolution in CMO-FD (Freezing-drying CoMn oxide) and CMO-H300 under the OER process, which accounted for the dynamic change of chronoamperometry. Our findings provide a different viewpoint for novel electrocatalyst fabrication by introducing a self-sacrifice agent that in situ promotes electronic structure modulation and high surface area preservation.