Solar-power driven photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is one of the most promising approaches for energy conversion, in terms of production of renewable hydrogen with a minimum carbon footprint. There are, however, enormous challenges to be overcome to obtain sufficient solar water-splitting efficiency, such as over-potential losses due to fast recombination rates of charge carriers, electrode photo-degradation, limited light-harvesting capacities, to name a few. In this work, we report an extensive study on Cu₂O based photocathode for hydrogen evolution reaction (HER) in terms of enhancing its solar-to-hydrogen (STH) conversion efficiency, while using low-cost fabrication methods. We have addressed the inefficiency of Cu₂O which mainly arises due to incompatible light absorption and charge carrier diffusion lengths, by fabricating highly reproducible Cu₂O-nanowire arrays on Cu substrate. The reported optimal thickness of Cu₂O for efficient absorption of sunlight is around 2-4 µm, while the minority charge carrier diffusion length is limited to 200-250 nm only. The nanostructured Cu₂O, especially in the form of nanowire arrays, improves its performance through morphology control, by improving the light-harvesting capacity along the full length of wire and providing a shorter diffusion path length corresponding to the radial distance in the wire, towards the electrolyte. We prepared the photocathode by anodizing Cu foil to Cu(OH)₂ nanowires at 10 mA/cm², followed by oxidation to Cu₂O nanowires in an inert atmosphere. Experimental measurements showed that the process was highly reproducible and resulted in an increased current density from 0.5 mA/cm² for planar Cu₂O to 82.3 mA/cm² for Cu₂O nanowires, both at 1 V cathodic bias. The photocurrent density was enhanced from 0.15 mA/cm² for planar Cu₂O to 5.54 mA/cm² for the nanowire morphology, while using visible light source in all cases. All the photoelectrodes were reproducible in the regime of 4 µm thicknesses of Cu₂O nanowires, oxidized over Cu-foil substrates. The STH efficiency calculated for all the fabricated photoelectrodes was as high as 3.54±0.21%, in comparison to 0.24% for the planar Cu₂O photoelectrodes.

Keywords: Cu₂O, nanowires, photocathode, solar water splitting, photoelectrochemistry, STH efficiency, PEC.