Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n þ -n homojunction. This concept is demonstrated with a lowcost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-ofthe-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double-or single-junction amorphous Si solar cell in a tandem configuration, stable shortcircuit water-splitting photocurrents of B4 and 3 mA cm À 2 , respectively, are achieved under 1 sun illumination. The 4 mA cm À 2 photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.

Capture and conversion of CO 2 from oceanwater can lead to net-negative emissions and can provide carbon source for synthetic fuels and chemical feedstocks at the gigaton per year scale. Here, we report a direct coupled, proof-of-concept electrochemical system that uses a bipolar membrane electrodialysis (BPMED) cell and a vapor-fed CO 2 reduction (CO 2 R) cell to capture and convert CO 2 from oceanwater. The BPMED cell replaces the commonly used water-splitting reaction with one-electron, reversible redox couples at the electrodes and demonstrates the ability to capture CO 2 at an electrochemical energy consumption of 155.4 kJ mol −1 or 0.98 kWh kg −1 of CO 2 and a CO 2 capture efficiency of 71%. The direct coupled, vapor-fed CO 2 R cell yields a total Faradaic efficiency of up to 95% for electrochemical CO 2 reduction to CO. The proof-of-concept system provides a unique technological pathway for CO 2 capture and conversion from oceanwater with only electrochemical processes.

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water‐splitting device with a benchmark solar‐to‐hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient‐doped tungsten–bismuth vanadate (W:BiVO4) photoanode and a thin‐film silicon solar cell. The improvement with respect to an earlier cell that also used gradient‐doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO4 photoanode, such as an amorphous silicon (a‐Si:H) single junction, an a‐Si:H/a‐Si:H double junction, and an a‐Si:H/nanocrystalline silicon (nc‐Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water‐splitting devices, is reached for a tandem system consisting of the optimized W:BiVO4 photoanode and the micromorph (a‐Si:H/nc‐Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO4 and the nc‐Si:H cell.