Various noble metal-free electrocatalysts have been explored to enhance the overall water splitting efficiency. Ni-based compounds have attracted substantial attention for achieving efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. Here, we show superior electrocatalysts based on NiFe alloy electroformed by a roll-to-roll process. NiFe (oxy)hydroxide synthesized by an anodization method for the OER catalyst shows an overpotential of 250 mV at 10 mA cm −2 , which is dramatically smaller than that of bare NiFe alloy with an overpotential of 380 mV at 10 mA cm −2 . Electrodeposited NiMo films for the HER catalyst also exhibit a small overpotential of 100 mV at 10 mA cm −2 compared with that of bare NiFe alloy (550 mV at 10 mA cm −2 ). A combined spectroscopic and electrochemical analysis reveals a clear relationship between the surface chemistry of NiFe (oxy)hydroxide and the water splitting properties. These outstanding fully solution-processed catalysts facilitate superb overall water splitting properties due to enlarged active surfaces and highly active catalytic properties. We combined a solution-processed monolithic perovskite/Si tandem solar cell with MAPb(I 0.85 Br 0.15 ) 3 for the direct conversion of solar energy into hydrogen energy, leading to the high solar-to-hydrogen efficiency of 17.52%. Based on the cost-effective solution processes, our photovoltaic−electrocatalysis (PV-EC) system has advantages over latest high-performance solar water splitting systems.

The performance of a BiVO4 photoanode is limited by poor charge transport, especially under front side illumination. Heterojunction of different metal oxides with staggered band configuration is a promising route, as it facilitates charge separation/transport and thereby improves photoactivity. We report a ternary planar heterojunction photoanode with enhanced photoactivity under front side illumination. SnO2/WO3/BiVO4 films were fabricated through electron beam deposition and subsequent wet chemical method. Remarkably high external quantum efficiency of ~80% during back side and ~90% upon front side illumination at a wavelength of 400 nm has been witnessed for SnO2/WO3/BiVO4 at 1.23 V vs. reversible hydrogen electrode (RHE). The intimate contact between the heterojunction films enabled efficient charge separation at the interface and promoted electron transport. This work provides a new paradigm for designing triple heterojunction to improve photoactivity, particularly under front illumination, which would be beneficial for the development of tandem devices.