Abstract
ABSTRACTPhotoelectrochemical (PEC) water splitting without an external bias is a potential solution to the growing energy crisis because this method can directly convert solar energy into chemical energy. A tandem cell is a frequently used configuration for unassisted overall water splitting because of the advantages that each component are tied together to form a highly efficient integration. A tandem PEC water splitting device is based on different photoelectrode absorbers, and there are two main models including photoanode/photocathode (PEC/PEC) and photoelectrode/photovoltaic (PEC/PV) tandem cells. In this review, we will focus on the concepts, configurations and recent progress of PEC/PEC and PEC/PV cells. Light absorption and energy band matching are the key points to enhance the solar-to-hydrogen (STH) efficiency. Promoting the performance of a standalone semiconductor material and finding new materials, coupled with an optimized configuration, are future steps for the practical application of tandem PEC cells.
Highlights
The lack of efficient and stable photoelectrode materials for water oxidation has restricted the application of solar water splitting
Using a single semiconductor material, it is difficult to realize highly efficient unassisted overall water splitting. Researchers must turn their attention to PEC-based tandem cells
The high conversion efficiency for water splitting is proportional to the photocurrent of the whole system, and the photocurrent can be optimized by the appropriate selection of semiconductor materials and optimized configuration
Summary
Photoelectrochemical (PEC) water splitting is an important strategy for converting sunlight into a chemical fuel in the form of hydrogen, and has been widely studied for decades [1–5]. Many p-type semiconductors, such as GaP [6], InP [7], GaInP2 [8–10], Si [11–14], SiC [15], WS2 [16], Cu(In,Ga)Se2 [17], Cu2O [18], CuYO2 [19], CaFe2O4 [20], and Mg-doped Fe2O3 [21], have been reported as photocathode materials for water reduction. Some of these photocathodes exhibit considerable solar-to-hydrogen (STH) efficiency. Considering ohmic losses and a kinetic loss due to the overpotentials for oxygen or hydrogen production, the band gap of a semiconductor photoelectrode should be larger than 2.03 eV, corresponding to the light absorption edge of 610 nm [44] This large gap means that photons with long wavelengths cannot be absorbed and wide band gap semiconductor materials are required. Tandem PEC cells for water splitting are discussed including PEC/PEC and PEC/PV systems
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