Two-step hybrid photo-thermochemical looping process, using metallic clusters on metal oxide carriers, for very efficient green hydrogen production†

In this work, we demonstrate a sustainable method for producing high-purity hydrogen through a two-step water-splitting process that leverages reducible oxides to store and release oxygen independently of hydrogen. Unlike conventional solar thermochemical (STCH) water-splitting techniques, which require extremely high temperatures exceeding 1000 °C, our approach operates at significantly lower temperatures—below ≈600 °C—thanks to a sunlight-driven photocatalyst composed of silver metal clusters (Ag₅) supported on ceria and Ce–Zr oxygen storage materials. This lower-temperature operation not only reduces the demand for high-performance materials for the design of the process but also enhances safety, simplifies system design, and improves the long-term stability of both materials and equipment. Overall, this green technology offers an energy-efficient and environmentally responsible pathway for clean hydrogen production. Density-functional-theory calculations show that Ag₅ clusters (1) enhance the photo-absorption, especially in the visible range, by increasing the gap states of the CeO₂ surface, and decrease the oxygen vacancy formation energy (E_VO) in certain positions around the clusters dramatically, and (2) create active sites in the Ag-CeO₂ interface possessing lower reaction energy and activation barrier for the hydrogen evolution reaction. Guided by these studies, we demonstrate a cleaner and more energy-efficient hydrogen production process, achieving an average output of ≈55 mL per cycle (≈435 μmol g⁻¹ h⁻¹). This corresponds to an oxygen vacancy parameter δ ≈ 0.26 per cycle—about 51% of the theoretical maximum—significantly surpassing the performance of traditional high-temperature STCH methods. Notably, our process reaches a solar-to-hydrogen (STH) conversion efficiency of ≈9.7%, placing it at the upper end of the typical STCH range (1–10%) while operating at much lower, intermediate temperatures. These results highlight the strong potential of this greener approach to hydrogen production, offering high efficiency due to the utilization of a broader wavelength range of the solar light and a smaller environmental footprint. Additionally, the use of methane in the reduction cycle promotes the formation of oxygen vacancies, while selectively generating carbon monoxide. The two-step concept has the potential to convert biomethane into a higher-value syngas product, with the added benefit of producing extra hydrogen. This allows for the adjustment of the CO/H₂ ratio, enabling subsequent Fischer–Tropsch processing for liquid fuel production.