The present invention provides a method for storing thermal energy, such as solar energy, as a fuel, by heating a reactive oxide substrate to a first temperature, such that the reactive oxide substrate is reduced, wherein the reactive oxide substrate includes a cerium oxide. The method also includes contacting the reduced reactive oxide substrate at a second temperature with a gas mixture including carbon dioxide, wherein the first temperature is greater than the second temperature, thereby preparing the fuel. The present invention also provides a method for preparing the reactive oxide substrates by heating a mixture including a doped cerium oxide and a pore-forming agent, such that pores are formed in the doped cerium oxide, thereby forming the reactive oxide substrate.