Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO4 and α-Fe2O3 as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm−2 at 1.23 VRHE under 1 sun irradiation. A tandem cell composed with the dual photoanodes–silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forward en route to the goal of >10% efficiency required for practical solar hydrogen production.

We report here a facile, one-step precipitating metal nitrate deposition (PMND) method to prepare amorphous metal oxyhydroxide films containing Fe, Co, and Ni as efficient electrocatalysts for water oxidation. The unique synthesis technique allows easy control of the metal composition over a wide range on various substrates. A series of unary and binary metal oxyhydroxides of 30 compositions are synthesized by PMND on fluorine-doped tin oxide (FTO) substrate as water oxidation electrocatalysts. The activity of the metal oxyhydroxide films is represented by a volcano plot as a function of a single experimental descriptor, i.e., the fraction of hydroxide in the surface oxygen species. The optimum compositions for binary metal oxyhydroxide (NiFe, NiCo, and CoFe) are determined on conductive substrates of FTO, nickel foam (NF), nickel mesh (NM), and carbon felt (CF), and the best NiFe (2:8) electrocatalyst on NF exhibits a water oxidation current density of 100 mA/cm2 with only 280 mV of overvoltage, which outperforms conventional noble metal catalysts like IrO x and RuO x in an alkaline medium. Finally, we demonstrate a tandem PV–electrolysis system by using a c-Si PV module with a power conversion efficiency of 13.71% and an electrochemical cell composed of NiFe (2:8)/NF anode and a bare NF cathode with a conversion efficiency of 71.8%, which records a solar-to-hydrogen conversion efficiency of 9.84%.

A bifunctional cobalt phosphide (CoP) electrocatalyst is applied to a doubly promoted BiVO4 photoanode as an oxygen evolution as well as to a cathode as a hydrogen evolution reaction (HER) catalyst to establish a photoelectrochemical (PEC) water splitting cell made of only earth abundant elements without any precious metals.

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO by Mo-doping, heterojunction formation with WO, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (η) of the photoanodes. Thus, the HMA-treated BiVO/WO film achieves the state-of-the art η of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO/WO (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Conversion of sunlight to chemical energy based on photoelectrochemical (PEC) processes has been considered as a promising strategy for solar energy harvesting. Here, we propose a novel platform that converts solar energy into sodium (Na) as a solid-state solar fuel via the PEC oxidation of natural seawater, for which a Na ion-selective ceramic membrane is employed together with photoelectrode (PE)-photovoltaic (PV) tandem cell. Using an elaborately modified bismuth vanadate-based PE in tandem with crystalline silicon PV, we demonstrate unassisted solar-to-Na conversion (equivalent to solar charge of seawater battery) with an unprecedentedly high efficiency of 8% (expected operating point under 1 sun) and measured operation efficiency of 5.7% (0.2 sun) and long-term stability, suggesting a new benchmark for low-cost, efficient, and scalable solid solar fuel production. The sodium turns easily into electricity on demand making the device a nature-friendly, monolithic solar rechargeable seawater battery.

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