Unassisted solar water splitting with 9.8% efficiency and over 100 h stability based on Si solar cells and photoelectrodes catalyzed by bifunctional Ni–Mo/Ni
Designing a highly efficient and stable photoelectrochemical (PEC) tandem cell for unassisted solar water splitting is considered a promising approach for large-scale solar energy storage. To date, various tandem device configurations have been reported. However, the achievement of a solar-to-hydrogen conversion efficiency (ηSTH) of 10% is still full of challenges, due to the incompatibility between the two photoelectrode materials. Here, we report a highly efficient and stable photovoltaic (PV)–PEC tandem device composed of a dual Si photoelectrode and two series-connected ordinary Si PV cells. The dual-photoelectrode device consists of an n+np+-Si photocathode and p+pn+-Si photoanode, which allow back illumination from the Si substrate, spatially and functionally decoupling the optical absorption and the catalytic activity. By further employing a Ni protective layer together with a bifunctional Ni–Mo catalyst for both the Si photocathode and photoanode, the PV–PEC tandem cell can perform spontaneous water splitting without any applied bias. A high ηSTH of 9.8% with a stability of over 100 h was achieved in alkaline solution under parallel AM 1.5G 1 sun illumination. This work essentially allows a modular independent optimization of each component which can enhance the efficiency and stability of the PV–PEC cells.