ABSTRACT:The controlled surface deposition of a robust and highperformance nanostructured copper-oxide (CuO x -NLs) electrocatalyst for water oxidation is presented. The material exhibits a characteristic leaf-type morphology and self-assembles on a copper substrate by straightforward constant-current anodization. The oxygen onset occurs at about 1.55 V versus RHE (η = 320 mV), which is 400−500 mV less than for amorphous Cu-oxide films. A Tafel slope of 44 mV dec −1 is obtained, which is the lowest observed relative to other copper-based materials. Long-term catalytic performance and stability tests of the electrocatalytic CuO x -NLs sample show a stable current density of >17 mA cm −2 for oxygen evolution, which was sustained for many hours.KEYWORDS: electrocatalyst, water oxidation, copper-oxide, nanoscale leaves, oxygen evolution T here are continuous efforts to develop a strategy to produce renewable and cleaner energy carriers from abundant solar radiation and water.1,2 In this quest, catalytic water oxidation is regarded as a primary process, and many inorganic materials and transition metal-oxides are considered good candidates for this conversion.2,3 Metal-oxide catalysts can be electrodeposited on conducting substrates from carbonate, phosphate, or borate electrolytes in the presence of metal ions. 4−6 To eliminate the possibility for interaction of metal ions with the cathodic sites during electrolysis, membranes or separators are usually employed that make the system more complex and introduce resistance and diffusion limitations in the electrochemical process.7 New preparation methods are required that are easily implemented, and catalytic materials are desired to perform in metal ion free systems for sustained electrochemical operation. 8,9 In addition, control over the catalyst morphology and surface structure is important to induce a high surface area and phase purity, facilitating both charge transfer and mass transport during catalysis reaction.
9,10Stable and efficient metal-oxide derived electrocatalysts can develop in a carbonate/bicarbonate system under mild conditions. 5,11 HCO 3 − /CO 3 2− is suggested to facilitate proton management and prevent the degradation of these catalytic materials under anodic conditions. 12−14 We observed that anodization of a copper electrode via repetitive potentialsweeps or at constant-potential in a carbonate buffer induces the formation of very small copper-oxide particulate materials (CuO x -NPs) having oxygen onset at ∼1.59 V versus RHE (η ≥ 360 mV). In addition, we found that nanoscale leaf-shaped Cuoxide (CuO x -NLs) surface structures ( Figure S1) are formed during controlled anodization of a chemically etched copper substrate at a constant current density of 4.0 mA cm −2 in a carbonate system. The CuO x -NLs exhibit a remarkably low overpotential for water oxidation (η = 320 mV) relative to other Cu-based electrocatalysts 13−15 and many inorganic materials. 16,17 Scanning electron microscopy (SEM) imaging shows a leaves-type copper-oxide matrix (Figur...