Tapered-Nanoflakes-Array of Cupric Oxide for Bias-Free Tandem Solar Water-Splitting

Photoelectrochemical (PEC) hydrogen production is perceived to be a potential game-changer, prompting a transition from carbon-based to hydrogen-fueled technologies. Tandem PEC configuration, in particular, has received much attention as it allows multi-absorption and physicochemical reactions at both photocathode and photoanode, realizing external bias-free PEC water-splitting. Nevertheless, there are still technical challenges associated with practical photocathode developments, which require sufficiently fast photocatalytic conversion, small bandgap, high solar spectrum utilization, and natural abundance. Cupric oxide (CuO) is known to be one of the most promising photocathode materials because it has a small bandgap (1.2 – 1.5 eV), high absorption coefficient, suitable band-edge position, and favorable characteristics such as earth-abundancy, low cost, and non-toxicity. Here, we report an exciting nanostructure of CuO photocathode, i.e., tapered nanoflake array (TNA) that shows a record photoelectrochemical performance. The unique tapered nanoflake morphology that formed by self-assembly of CuO nanoribbons greatly reduces light reflection losses in the visible-to-near infrared region. Moreover, the CuO TNA exhibits enhanced charge transport and transfer properties compared with its porous and compact nanoparticle-film counterparts. Our optimized CuO TNA photocathode exhibits a record photocurrent density of ~6.3 mA/cm² at 0.0 V vs. the reversible hydrogen electrode (RHE) without any scavenger addition under simulated sunlight illumination (AM1.5G, 100 mW/cm²), which is the highest performance among all the CuO photocathodes. Finally, we demonstrated the practical feasibility and potential scale-up by constructing a tandem PEC cell with a TiO₂ branched nanorods array (BNA) photoanode and obtained a bias-free solar water-splitting (STH efficiency: ~1.0%).