Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.

Photoelectrochemical (PEC) solar water splitting has received extensive attention because it promises to provide an alternative and sustainable source of energy. A key challenge is to achieve a stable PEC system in either acidic or basic electrolyte without degradation of the (photo)­electrodes. We have used a cubic Cu2O film and porous granular bilayer Cu2O/CuO composite with a carbon protection layer as photocathode materials. The films were deposited under different conditions, such as variation of the electrodeposition time, thermal oxidation of the Cu2O films in air versus nitrogen atmosphere, and deposition of the carbon materials, and were investigated structurally and with regard to their PEC performance. The optimized electrodes showed photocurrents up to 6.5 and 7.5 mA/cm–2 at potentials of 0 and −0.1 V vs RHE at pH 5.5, respectively. The stabilities of the Cu2O/C and Cu2O/CuO/C photocathodes, at a low bias of 0.3 V vs RHE, were retained after 50 h. The strongly improved photostability of the photocathodes in comparison to electrodes in the absence of a carbon overlayer is attributed to a more effective charge transfer and a protective role of carbon against photocorrosion.

Large research efforts have been devoted to optimizing the output of earth-abundant photoabsorbers in solar-to-fuel (S2F) devices. Here, we report a Cu2O/Ga2O3 heterojunction/Si microwire photocathode with an underlying buried radial Si p–n junction, which achieves efficient light harvesting across the visible spectrum to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%, with a photocurrent onset above +1.35 V vs RHE, a photocurrent density of ∼10 mA/cm2 at 0 V vs RHE, and an ideal regenerative efficiency of 5.51%. We show step-by-step the effects of every photocathode design element (i.e., Si p–n junction, Cu2O layer thickness, microwire length, microwire pitch, etc.) on the overall efficiency of our final microwire Si/Cu2O photocathode by comparing every addition to a baseline Cu2O photocathode. Lastly, we show a stable operation exceeding 200 h at a bias potential of +1.0 V vs RHE, with an average current density of 4.5 mA/cm2.