Metal Oxide-Based Tandem Cells for Self-Biased Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting represents a promising route to convert solar energy into clean hydrogen. Constructing tandem cells has emerged as a feasible approach and attracted tremendous attention for self-biased water splitting, especially using low-cost and stable metal oxides. Herein, a state-of-the-art review of metal oxide-based PEC/photovoltaic (PV) tandem cells and PEC tandem cells is comprehensively presented, with a focus on crucial issues of designing efficient tandem devices from the aspects of the photoanodes, photocathodes, and photovoltaics. Different device configurations and efficiency limitations of tandem cells are introduced, and advances of metal oxide-based PEC/PV and PEC tandem cells in terms of material design and device optimization are discussed. Metal oxide-based tandem cells have shown solar-to-hydrogen efficiencies of ∼8%, which is approaching practicality. Perspectives on remaining challenges and potential strategies are outlined.