Photoelectrochemical (PEC) water splitting, termed artificial photosynthesis, converts solar energy into hydrogen by harvesting a narrow spectrum of visible light using photovoltaics integrated with water-splitting electrocatalysts. While conceptually attractive, critical materials issues currently challenge technology development(1) and economic viability(2). Despite decades of active research, this approach has not been demonstrated at power levels above a few watts, or for more than a few days of operation.
High-temperature solar thermochemical (STCH) water splitting is an alternative approach that converts solar energy into hydrogen by using the deceptively simple metal oxide-based thermochemical cycle presented in figure 1. The STCH process requires very high temperatures, achieved by collecting and concentrating solar energy. Unlike PEC, two-step metal oxide water-splitting cycles have been demonstrated at the 100kW scale(3), and continuous operation at even higher power levels is nearing pre-commercial demonstration (HYDROSOL-3D). Nonetheless STCH, like PEC, faces critical materials issues that must be addressed for this technology to achieve commercial success.
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