Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu 2 O/Ga 2 O 3-buried p-n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over + 1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm −2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO 2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu 2 O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO 4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency.

Due to its abundance, scalability, and nontoxicity, Cu2O has attracted extensive attention toward solar energy conversion, and it is the best performing metal oxide material. Until now, the high efficiency devices are all planar in structure, and their photocurrent densities still fall well below the theoretical value of 14.5 mA cm(-2) due to the incompatible light absorption and charge carrier diffusion lengths. Nanowire structures have been considered as a rational and promising approach to solve this issue, but due to various challenges, performance improvements through the use of nanowires have rarely been achieved. In this work, we develop a new synthetic method to grow Cu2O nanowire arrays on conductive fluorine-doped tin oxide substrates with well-controlled phase and excellent electronic and photonic properties. Also, we introduce an innovative blocking layer strategy to enable high performance. Further, through material engineering by combining a conformal nanoscale p-n junction, durable protective overlayer, and uniform catalyst decoration, we have successfully fabricated Cu2O nanowire array photocathodes for hydrogen generation from solar water splitting delivering unprecedentedly high photocurrent densities of 10 mA cm(-2) and stable operation beyond 50 h, establishing a new benchmark for metal oxide based photoelectrodes.

The Al2 O3 passivation layer is beneficial for mesoporous TiO2 -based perovskite solar cells when it is deposited selectively on the compact TiO2 surface. Such a passivation layer suppressing surface recombination can be formed by thermal decomposition of the perovskite layer during post-annealing.

Cu 2 S has been regarded as a promising solar energy conversion material because of its favorable visible light absorption and earth abundance. Here, we present an indirect preparation method via a solution-processed ion exchange reaction to synthesize stoichiometric Cu 2 S films with high photoactivity. In addition, we developed a chemical bath deposition method to fabricate CdS buffer layers on Cu 2 S by adding a reducing agent in the precursor solution, avoiding oxidation of Cu 2 S. After being coated with the TiO 2 protection layer and the RuO x hydrogen evolution catalyst, the Cu 2 S photoelectrode delivers a photocurrent density of 7.0 mA cm −2 at −0.3 V vs RHE and an onset potential of 0.48 V vs RHE under AM 1.5G simulated sunlight illumination for solar driven water reduction. To our knowledge, this is the first time that Cu 2 S has been used for solar hydrogen evolution with encouraging performance, which will stimulate further studies on Cu-based photocathodes.