Co-Pi Catalyst Research Articles

(Invited) Light-Guided Electrodeposition for Functional Integration of Catalyst Pattern on Semiconductor for Efficient Oxygen Evolution Reaction

Photoelectrochemical cells (PECs) for water splitting typically rely on catalysts deposited on the semiconductor due to its lack of intrinsic electrocatalytic activity. However, the placement of catalysts within the light path between the source and semiconductor in artificial leaf-based architectures can hinder the semiconductor's light absorption, diminishing photoelectrochemical activity, especially at high catalyst loading. To address this challenge, we propose a strategy involving the controlled formation of patterned catalysts on the semiconductor surface. This approach, utilizing light-guided electrodeposition facilitated by a digital micromirror device (DMD), enables the direct, localized deposition of transition metal-based catalysts. The resulting patterned catalysts correspond to the light pattern, allowing for optimized photoelectrochemical performance. We demonstrate the efficacy of this strategy on a hematite (α-Fe2O3)-based photoanode, achieving enhanced performance with grid-patterned CoPi catalysts. This approach not only facilitates high catalyst loading but also ensures sufficient light penetration through the exposed intrinsic surface of α-Fe2O3. Our findings suggest that this technique holds promise for advancing the commercialization of PECs, particularly for self-biased water splitting devices aimed at green H2 production.

Gradient-Doped BiVO4 Dual Photoanodes for Highly Efficient Photoelectrochemical Water Splitting.

Bismuth vanadate (BiVO4) is regarded as a promising photoanode candidate for photoelectrochemical (PEC) water splitting, but is limited by low efficiency of charge carrier transport and short carrier diffusion length. In this work, we report a strategy comprised of the gradient doping of W and back-to-back stacking of transparent photoelectrodes, where the 3-2 wt.% W gradient doping enhances charge carrier transport by optimizing the band bending degree and back-to-back stack configuration shortens carrier diffusion length without much sacrifice of photons. As a result, the photocurrent density of 3-2 % W:BiVO4 photoanode reaches 2.20 mA cm-2 at 1.23 V vs. hydrogen electrode (RHE) with a charge transport efficiency of 76.1 % under AM 1.5 G illumination, and the back-to-back stacked 3-2 % W:BiVO4 photoanodes achieves a photocurrent of 4.63 mA cm-2 after loading Co-Pi catalyst and anti-reflective coating under AM 1.5 G illumination, with long-term stability of 10 hours.

Graphene Quantum Dots as Hole Extraction and Transfer Layer Empowering Solar Water Splitting of Catalyst-Coupled Zinc Ferrite Nanorods.

Despite the narrow band gap energy, the performance of zinc ferrite (ZnFe2O4) as a photoharvester for solar-driven water splitting is significantly hindered due to its sluggish charge transfer and severe charge recombination. This work reports the fabrication of a hybrid nanostructured hydrogenated ZnFe2O4 (ZFO) photoanode with enhanced photoelectrochemical water-oxidation activity through coupling N-doped graphene quantum dots (GQDs) as a hole transfer layer and Co-Pi as a catalyst. The GQDs not only reduce the surface-mediated nonradiative electron-hole pair recombination but also induce a built-in interfacial electric field leading to a favorable band alignment at the ZFO/GQDs interface, helping rapid photogenerated hole separation and serving as a conducting hole transfer highway, improve the hole transportation into the Co-Pi catalyst for enhanced water oxidation reaction kinetics. The optimized ZFO/GQD/Co-Pi hybrid photoanode delivers a 23-fold photocurrent enhancement at 1.23 V versus the reversible hydrogen electrode (RHE) and a significant 360 mV reduction in the onset potential, reaching 0.65 VRHE compared with the ZFO photoanode under 1 sun illumination in a neutral electrolytic environment. This investigation underscores the mechanism of synergistic interplay between the hole transport layer and cocatalyst in boosting the solar-illuminated water-splitting activity of the ZFO photoanode.

3D-Branched Fe2O3 Nanorods on Micropillar Arrays Decorated with Co-Pi for Enhanced PEC Water Splitting

Photoelectrochemical(PEC) water splitting has gained great attention as a route for future hydrogen production owing to its sustainable and cost-effective properties. However, since PEC water splitting has limitations on efficiency and stability, research on materials, structures and catalysts are in progress to solve those problems. In our experiments, the nano-pattern structure is created using direct-printing to increase the photoanode efficiency.In this study, enlarged surface area is accomplished by micropillar structured FTO and Fe2O3 nanorods. We fabricated micropillar structure via direct printing method. The direct-printing method has advantages in not only simple process but also low price to form micro-nano size structures. To obtain micropillar structure, spin coating of hydrogen silsesquioxane(HSQ) was performed on micropillar patterned polydimethylsiloxane(PDMS) mold. Then, we applied glass substrate and pressure on it. Detachment of PDMS mold is followed by deposition of FTO layer. Hydrothermally synthesized Fe2O3 nanorods are formed on micropillar structured FTO and Co-Pi catalyst was electrodeposited on Fe2O3 nanorods to enhance surface water oxidation.Due to micropillar pattern, optical absorption of patterned Fe2O3 photoanode is higher than that of flat Fe2O3 photoanode in the whole range of optical wavelength. Also micropillar patterned Fe2O3 sample showed 2.6 times higher photocurrent at 1.23 VRHE than flat Fe2O3 sample. These results were confirmed that micropillar structure increased the surface area and light scattering effect. Furthermore, applying Co-Pi catalyst increased photocurrent at 1.23 VRHE for 2.0 times higher owing to the enhancement of surface water oxidation. As a result, photocurrent of the micropillar patterned, decorated with Co-Pi sample shows 1.51 mA/cm2, and that of flat, non-decorated sample shows 0.29 mA/cm2 at 1.23 VRHE. Figure 1

A Dual‐Ligand Strategy to Regulate the Nucleation and Growth of Lead Chromate Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting for renewable hydrogen production has been regarded as a promising solution to utilize solar energy. However, most photoelectrodes still suffer from poor film quality and poor charge separation properties, mainly owing to the possible formation of detrimental defects including microcracks and grain boundaries. Herein, a molecular coordination engineering strategy is developed by employing acetylacetone (Acac) and poly(ethylene glycol) (PEG) dual ligands to regulate the nucleation and crystal growth of the lead chromate (PbCrO4 ) photoanode, resulting in the formation of a high-quality film with large grain size, well-stitched grain boundaries, and reduced oxygen vacancies defects. With these efforts, the nonradiative charge recombination is efficiently suppressed, leading to the enhancement of its charge separation efficiency from 47% to 90%. After decorating with Co-Pi cocatalyst, the PbCrO4 photoanode achieves a photocurrent density of 3.15mA cm-2 at 1.23V (vs RHE under simulated AM1.5G) and an applied bias photon-to-current efficiency (ABPE) of 0.82%. This work provides a new strategy to modulate the nucleation and growth of high-quality photoelectrodes for efficient PEC water splitting.

Influence of ZnO Magnetron Sputtering on Controlled Buildout of Zirconium-Doped ZnFe2O4/Fe2O3 Heterojunction Photoanodes for Photoelectrochemical Water Splitting

The development of efficient photoanodes for solar fuel generation via photoelectrochemical (PEC) water splitting is becoming a bottleneck. These limitations necessitate the design of iron-containing metal oxides, like “ferrites-based electrode materials” with improved oxygen evolution kinetics, light absorptivity, and intrinsic stability, yet at a low cost. Herein, we report the in-situ formation of Zr–ZnFe2O4/Fe2O3 heterojunction (ZZFO/HT) photoelectrodes using a facile magnetron sputtering and hydrothermal processes. First, the ZnO is systematically sputtered on in-situ Zr-doped FeOOH electrodes and then the ZnO-sputtered electrodes are quenched at 800 °C, 13 min to form ZZFO/HT. Furthermore, the effect of ZnO sputtering and roles of Zr–ZnFe2O4 and Fe2O3 in the ZZFO/HT heterojunction as well as their structural and photoelectrochemical properties were studied in detail. The optimum biphasic 25.6 nm ZnO-sputtered ZZFO/HT (ZZFO/HT-2) photoelectrode exhibited a photocurrent density of 0.430 mA/cm2 at 1.23 V vs RHE with an appropriate fraction of Zr–ZnFe2O4 and Fe2O3. The enhanced PEC performance is attributed to the optimum fraction of Zr–ZnFe2O4 and Fe2O3 in ZZFO/HT-2 heterojunction, which provides efficient charge transport across the bulk and at heterojunction interface. Lastly, the integration of Al2O3 passivation layer and Co–Pi cocatalyst on the optimized ZZFO/HT-2 photoelectrode exhibited a high photocurrent density of 0.780 mA/cm2 at 1.23 V vs RHE and generated 11.8 and 6.2 μmol/cm2 of hydrogen and oxygen, respectively during PEC water splitting. Further, it is expected that by fine-tuning of Zr–ZnFe2O4 and Fe2O3 NC ratio, the photocurrent density can be improved for establishing a benchmark for ZnFe2O4-based photoelectrodes.

Improving the anode performance of microbial fuel cell with carbon nanotubes supported cobalt phosphate catalyst

Microbial fuel cell (MFC) is a sustainable technology that can convert waste to energy by harnessing the power of exoelectrogenic bacteria. However, the poor biocompatibility and low electrocatalytic activities of surface usually cause weak bacterial adhesion and low electron transfer efficiency, which seriously hampers the development of MFCs. Herein, a novel carbon nanotube supported cobalt phosphate (CNT/Co-Pi) electrode is fabricated by assembling CNTs on carbon cloth, followed by the electrodeposition of Co-Pi catalyst. The deposited amorphous Co-Pi thin film contains phosphate and the cobalt ions of multiple oxidation states. The hydrophilic phosphate can promote the adhesion of microorganisms on electrode. The strong conversion ability of multiple states of cobalt offers excellent electrocatalytic activity for the electron transfer across biotic/abiotic interface. Therefore, the highly conductive CNTs substrate, along with the Co-Pi catalyst, provide an effective electron transfer between the electrogenic bacteria and the electrode, which endows MFC high power densities up to 1200 mW m−2. Our work has demonstrated for the first time that CNT/Co-Pi catalyst can promote the interfacial electron transfer between electrogenic bacteria and electrode, and highlighted the application potentials of Co-Pi as an anode catalyst for the fabrication of high performance MFC anodes.

Uniform Formation of Amorphous Cobalt Phosphate on Carbon Nanotubes for Hydrogen Evolution Reaction†

Main observation and conclusionTremendous efforts have been made to the development of highly active, stable hydrogen evolution reaction (HER) electrocatalysts based on earth‐abundant metal compounds. Recently, cobalt phosphorus (Co‐P) catalysts have received particular attention owing to their good performances for the HER. However, the reported Co‐P catalysts were not uniformly anchored on the substrates. Hence, developing Co‐P catalysts with a uniform surface structure for HER remains a major challenge. Herein, we utilized small molecule tris(4‐fluorophenyl)phosphane (PF) for preliminarily functionalizing carbon nanotubes (PF‐CNTs) via π–π stacking interactions. Using these as the substrate, amorphous cobalt phosphate (CoPi) catalysts could then be uniformly anchored on PF‐CNTs by electrodeposition method. These obtained CoPi/PF‐CNTs catalysts show an onset potential low to 29 mV, a low overpotential (105 mV in 0.5 mol/L H2SO4 at a current density of 10 mA·cm–2 with a Tafel plot of 32 mV·dec–1) and an excellent stability for HER in acidic electrolyte, which is a promising noble‐metal‐free HER catalyst.

Direct growth of hematite film on p+n-silicon micro-pyramid arrays for low-bias water splitting

Solar water splitting based on photoelectrochemical (PEC) cells has been intensively demonstrated to be promising for hydrogen energy, but it is usually difficult for one pure PEC system to achieve overall solar-driven water splitting. To solve this problem, integrating one PEC cell with one photovoltaic (PV) cell is widely employed, however, the preparation of an unabridged PV cell and the electrical connection between them are complicated. Herein, we directly synthesized hematite (α-Fe2O3) film on the as-prepared p+n-Si micro-pyramid arrays with the modifications of SnO2 interlayer and Co-Pi catalyst overlayer, and the turn-on potential achieved a substantial decrease, which is 0.34 V smaller than the Sn-doped hematite film grown on the FTO substrate (FTO/Sn@α-Fe2O3). The influences of interlayer, overlayer and α-Fe2O3 preparation conditions are systemically investigated. Besides, we comparatively studied the FTO/Sn@α-Fe2O3 photoanode in tandem with the complete p+n-Si micro-pyramid PV cell, and observed the turn-on potential as low as 0.32 V vs. RHE. The performance improvement is ascribed to a combination of the additional photovoltage from the embedded p+n-Si junction, the enhancement both in optical absorption and surface area of α-Fe2O3 film, and the promoted collection efficiency of photogenerated carriers from the interface and surface modifications. This work fills the gap of combining α-Fe2O3 photoanode and p+n-Si junction with the aim of overall solar-driven water splitting, and provides a new route to simplify the PEC/PV tandem system.

Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation.

The development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm–2 at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co2+ species to Co3+ species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation.

Intermolecular electron modulation by P/O bridging in an IrO2-CoPi catalyst to enhance the hydrogen evolution reaction

Development and regulation of IrO2-CoPi-CNTs catalyst electronic structure to achieve Pt-like hydrogen evolution reaction (HER) performance.

Photoelectrochemical water oxidation in α-Fe2O3 thin films enhanced by a controllable wet-chemical Ti-doping strategy and Co–Pi co-catalyst modification

The poor electrical conductivity, short hole diffusion length, and slow water oxidation reaction kinetics have severely limited the photoelectrochemical performance of the hematite (α-Fe2O3) photoanodes. In this paper, we report a facile wet-chemical approach to prepare the Ti-doping controllable hematite photoanode with subsequent Co–Pi electrocatalysts modification for solar water splitting. By optimizing the Ti-doping in Fe2O3, the Ti–Fe2O3 photoanodes can retain the primary morphology with the pristine Fe2O3 nanostructures, while simultaneously reducing the photogenerated charge carrier recombination rate in the films. Besides, by further decorating with Co–Pi co-catalysts on Ti–Fe2O3 surface, the formed Ti–Fe2O3/Co–Pi photoanode demonstrated an accelerated electrode/electrolyte kinetics during photoelectrochemical water oxidation reaction. As expected, the Ti–Fe2O3/Co–Pi photoanode produced an improved photocurrent density of 0.76 mA/cm2 at 1.23 V vs RHE, which is much higher than the photocurrent density of individual Fe2O3 (0.25 mA/cm2) and Ti–Fe2O3 photoanode (0.51 mA/cm2). This work provides a good insight for designing the composite photoanode to simultaneously enhance the charge transport and surface water oxidation kinetics for efficient solar fuel production.

Cobalt-Phosphate Catalysts with Reduced Bivalent Co-Ion States and Doped Nitrogen Atoms Playing as Active Sites for Facile Adsorption, Fast Charge Transfer, and Robust Stability in Photoelectrochemical Water Oxidation.

A cobalt-phosphate (Co-Pi) catalyst having octahedral CoO6 molecular units as reaction sites is a key component in photoelectrochemical (PEC) water oxidation systems, but its limited adsorption sites for oxygen-evolving intermediates (*OH, *OOH), slow charge transfer rates, and fast degradation of reaction sites are yet to be overcome. Here, we report that Co-Pi nanoparticles with low-coordinate Co ions and doped nitrogen atoms could be decorated on hematite nanorod arrays to form N-CoPi/hematite composites. Moreover, the local atomic configuration and bond distance studies show that trivalent Co3+ states are partially reduced through nitrogen radicals in the plasma to low-coordinate bivalent Co2+ states playing as the facile adsorption sites of oxygen-evolving intermediates due to the decreased activation barrier for water oxidation. Electron transport is also reinforced by nitrogen species due to the formation of hybridizing N 2p orbitals that give the acceptor levels in the bandgap. As a result, both the incident photon-to-electron conversion efficiency and the charge transfer resistance on N-CoPi/hematite outperform those on a bare hematite by about 3 fold. Furthermore, N-CoPi/hematite gives high activity retention over 90% after the long operation of water oxidation, in support of the reaction sites on N-CoPi not degrading during the successive water oxidation.

ZnO/In2S3/Co–Pi ternary composite photoanodes for enhanced photoelectrochemical properties

The photoelectrochemical (PEC) water splitting properties can be enhanced by broadening the light absorption region and improving the separation of photogenerated carriers. In this paper, a novel ZnO/In2S3/Co–Pi ternary composite photoanode system is provided, by building the ZnO/In2S3 heterojunction to broaden the light absorption region and improve the separation and transfer of photogenerated electron–hole pairs in bulk, and by using the Co–Pi cocatalyst to increase the separation of photogenerated electron–hole pairs between the ZnO/In2S3 heterojunction surface and electrolyte. This ternary composite photoanode system exhibits a negative shifted onset potential and a higher photocurrent density of about 2.4 mA/cm2 at 1.23 V (vs. RHE), which is 3 and 2.18 times compared with bare ZnO nanorod and ZnO/In2S3 heterojunction, respectively. The results show that the ZnO/In2S3/Co–Pi ternary composite photoanode has an excellent potential application for PEC water splitting.

Microporous core-shell Co11(HPO3)8(OH)6/Co11(PO3)8O6 nanowires for highly efficient electrocatalytic oxygen evolution reaction

Exploring high-efficient oxygen evolution reaction (OER) catalysts is critical for producing clean H2 fuel through water splitting. Co and P involved Co-Pi and cobalt phosphide OER catalysts attracted tremendous attention, while limited report on cobalt phosphite as an OER catalyst. Herein, a novel cobalt phosphite nanowires with microporous 1D channel have been succsessfully discovered with remarkable catalytic performance of a low overpotential of ˜340 mV and Tafel slope of 60 mV dec−1 at 10 mA cm−2 at pH 14. Through comprehensive structural characterization, the effective species has been clearly illustrated to be the in-situ formed heterostructural Co11(HPO3)8(OH)6/Co11(PO3)8O6 core-shell structure nanowires. This work fills the gap for exploring high efficient cobalt phosphites OER catalysts under akaline condition and paves the way for exploring novel high efficiency waster splitting catalysts.

Confined growth of Co–Pi co-catalyst by organic semiconductor polymer for boosting the photoelectrochemical performance of BiVO4

The significant recombination of carriers and low OER kinetics depress the solar to chemical energy conversion efficiency over BiVO4.

Electron Diffusion Length and Charge Separation Efficiency in Nanostructured Ternary Metal Vanadate Photoelectrodes

Ternary metal vanadates have recently emerged as promising photoelectrode materials for sunlight-driven water splitting. Here, we show that highly active nanostructured BiVO4 films can be deposited onto fluorine-doped tin oxide (FTO) substrates by a facile sequential dipping method known as successive ionic layer adsorption and reaction (SILAR). After annealing and deposition of a cobalt phosphate (Co-Pi) co-catalyst, the photoelectrodes produce anodic photocurrents (under 100 mW cm-2 broadband illumination, 1.23 V vs. RHE) in pH 7 phosphate buffer that are on par with the highest reported in the literature for similar materials. To gain insight into the reason for the good performance of the deposited films, and to identify factors limiting their performance, incident photon-to-electron conversion efficiency spectra have been analyzed using a simple diffusion–reaction model to quantify the electron diffusion length (Ln; the average distance travelled before recombination) and charge separation efficiency (ηsep) in the films. The results indicate that ηsep approaches unity at sufficiently positive applied potential but the photocurrent is limited by significant charge collection losses due to a short Ln relative to the film thickness. The Co-Pi catalyst is found to improve ηsep at low potentials as well as increase Ln at all potentials studied. These findings help to clarify the role of the Co-Pi co-catalyst and show that there could be room for improvement of BiVO4 photoanodes deposited by SILAR if Ln can be increased.

Three-Dimensional Bicontinuous BiVO4/ZnO Photoanodes for High Solar Water-Splitting Performance at Low Bias Potential.

A photoanode capable of high-efficiency water oxidation at low bias potential is essential for its practical application for photocathode-coupled tandem systems. To address this issue, a photoanode with low turn-on voltage for water oxidation and high charge separation efficiency at low bias potential is essential. In this study, we demonstrate the photoanode of the BiVO4/ZnO three-dimensional (3D) bicontinuous (BC) structure. ZnO has a relatively cathodic flat-band potential, which leads to low turn-on potential; the BiVO4/ZnO 3D BC photoanode shows an onset potential of 0.09 V versus the reversible hydrogen electrode ( VRHE). Moreover, we achieve remarkably high charge separation efficiency at low bias potential (78% at 0.6 VRHE); this is attributed to the application of thin-film BiVO4 shells by high light-scattering properties of the 3D BC structure. As a result, the BiVO4/ZnO 3D BC photoanode generates a high water oxidation photocurrent of up to 3.4 ± 0.2 mA cm-2 (with CoPi catalyst coating). This photocurrent value is reproducible, and the photocurrent-to-O2 conversion efficiency is over 90%. To the best of our knowledge, this is the highest value among the values of the photocurrent at 0.6 VRHE in previous BiVO4-based heterojunction photoanodes.

Designing Co-Pi Modified One-Dimensional n–p TiO2/ZnCo2O4 Nanoheterostructure Photoanode with Reduced Electron–Hole Pair Recombination and Excellent Photoconversion Efficiency (>3%)

The poor visible light absorption, defect-mediated charge carrier recombination, slow water oxidation kinetics, and charge transportation limit the performance of TiO2 photoelectrodes for water oxidation. In order to tackle these issues, here a one-dimensional photoanode is designed by electrodepositing a p-ZnCo2O4 nanolayer on n-TiO2 nanotubes surface and finally electrochemically coupling the TiO2/ZnCo2O4 surface with an ultrathin layer of the cobalt phosphate (Co-Pi) catalyst nanoparticles. These typical TiO2/ZnCo2O4@Co-Pi nanoheterostructures exhibit a remarkably enhanced visible light driven photoelectrochemical property with applied bias photoconversion efficiency (ABPE) ∼ 3% at 0.2 V vs NHE. The TiO2/ZnCo2O4@Co-Pi nanoheterostructures also show enhanced visible light absorption with large photocurrent density ∼440% higher than that of the TiO2 nanotubes electrode at 1.2 V vs Ag/AgCl and significantly low onset potential for water oxidation. Studies on the transient photocurrent and flat-band potent...

An efficient catalyst film fabricated by electrophoretic deposition of cobalt hydroxide for electrochemical water oxidation

The hexagonal-shaped β-Co(OH)2 discs were electrophoretically deposited on an electrode to yield a stably adherent and uniform β-Co(OH)2 layer. The electrocatalytic water oxidation at the β-Co(OH)2 layer-deposited electrode was investigated to seek efficient water oxidation catalyst for artificial photosynthesis as promising energy-providing systems for the future. The β-Co(OH)2 layer did not work as a water oxidation catalyst in a K2SO4 solution at pH 7.0 when applying 1.70V vs. reversible hydrogen electrode (RHE), although the layer was oxidized to the CoO(OH) layer. In a phosphate buffer solution (pH 7.0), the β-Co(OH)2 layer was dissolved and oxidatively re-deposited as the Co-Pi catalyst (Science, 321 (2008) 1072–1075.) when applying the same potential. While in a Na2B4O7 solution at pH 9.4, the β-Co(OH)2 layer was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained when applying 1.7V vs. RHE, and the formed CoO(OH) layer works efficiently for electrocatalytic water oxidation in contrast to no catalytic activity in a K2SO4 solution at pH 9.4. The catalytic performance of the formed CoO(OH) layer was evaluated; the overpotential (η) of 0.42V for current generation of 100μAcm−2 and the Tafel slope of 0.21∼0.29V dec−1 were provided for water oxidation at pH 9.4, which are comparable with efficient cobalt-based catalyst films reported so far.