Role of cerium in activating solar fuels production in transition metal-oxides

Full text: Earth abundant and environmentally friendly transition metal oxides are attracting broad research for thermochemical redox cycles. Despite progress, a major challenge is ensuring fast and reversible redox kinetics and large oxygen exchange capacities. We provide insights on the role of Ce dopants in unlocking the redox capacity of manganese oxide. We perform a detailed analysis of the structural evolution and elemental distribution of the material during the redox steps as a function of the cerium content by combining synchrotron-based spectroscopy (NEXAFS, in-depth XPS proling) and in-house microscopy (SEM, TEM). We observe that the cerium atoms migrate in and out the material, and that the presence of cerium in 3+ oxidation state both within the lattice of the metal oxide and on its surface is essential to achieve high and stable reduction kinetics. This is optimally achieved for a cerium content of 3%. Smaller Ce content are insufficient to stabilize the redox reactions resulting in the rapid growth of the manganese oxide grains, while higher content cause the rapid segregation and inactivation of Ce. (author)