Bismuth vanadate provides a basis for photoelectrochemical electrolysis

For hydrogen to be produced sustainable on an industrial scale, it seems likely that we will need to turn to renewables-powered electrolysis. Photoelectrochemical splitting is one way of achieving this, where solar powered electrodes in water generate a current, splitting the water into its constituent elements.

A variety of materials have been investigated to serve this purpose. Traditional silicon electrodes are corroded by the oxygen they create in the splitting process. To avoid this problem, a team from Stanford University turned to bismuth vanadate, a low-cost, high-stability construction material. On its own, bismuth vanadate is stable but boats a conversion efficiency of less than 2%. The material is not good at capturing sunlight.

However, the Stanford researchers tried layering tiny sheets bismuth vanadate (< 200 nanometres) with silicon nanocones (each about 600 nanometres tall). The cones are able to capture more sunlight, while the bismuth vanadate protects from corrosion. When placed on a perovskite cell and submerged in water, the combined materials demonstrated decent stability (5.8% efficiency decay after ten hours in water), and produce a more respectable solar-to-hydrogen conversion efficiency of 6.2%.

To put that into context, while some other experiments have reported efficiencies of 10%, those have been ‘single junction devices’, which according to the Shockley-Queisser limit have a maximum solar conversion efficiency is ~34%. The Stanford design is a tandem double-junction cell, with a theoretical maximum efficiency of ~45%. So while the current design is a long way short of the best of the competition, the group hopes that their line of research could become the basis for a dominant design in the future.

The details of the experiment can be found at Science Advances (17 June 2016), ‘Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells’.