Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H-2 production based on CeO2/Ce2O3 oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO(2) -> Ce2O3 + 0.5O(2); (2) hydrolysis, Ce2O3 + H2O -> 2CeO(2) + H-2. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 degrees C, P = 100-200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400-600 degrees C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO2), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production. (c) 2006 Elsevier Ltd. All rights reserved.