Novel two-step SnO 2 /SnO water-splitting cycle for solar thermochemical production of hydrogen Ste ´phane Abanades a, *, Patrice Charvin a , Florent Lemont b , Gilles Flamant a a Processes, Materials, and Solar Energy laboratory (PROMES-CNRS, UPR 8521), 7 Rue du Four Solaire, 66120 Font-Romeu, France b Commissariat a ` l’E ´ nergie Atomique (CEA), Rho ˆne Valley Research Center BP17171, 30207 Bagnols-Sur-Ce `ze Cedex, France article info Article history: Received 12 March 2008 Received in revised form 7 May 2008 Accepted 16 May 2008 Available online 26 September 2008 Keywords: Hydrogen Thermochemical cycle Water-splitting Tin oxide Solar energy abstract The production of hydrogen from a novel two-step thermochemical cycle based on SnO 2 / SnO redox reactions is presented. This process targets CO 2 -free hydrogen production by using renewable solar energy and water in a high-temperature water-splitting cycle. The cycle consists of a solar endothermic reduction of SnO 2 into SnO(g) and O 2 followed by a non-solar exothermic hydrolysis of SnO(s) to form H 2 and SnO 2 (s). The objective of this study was to demonstrate this innovative concept for H 2 production and to establish the potential of cycle implementation in an integrated solar chemical process. The reduction and hydrolysis reactions were experimentally tested in order to define optimal operating conditions, chemical conversion and hydrogen yield. The thermal reduction occurs under atmospheric pressure at about 1600  C and over. The solar step encompasses the formation of SnO nanoparticles that can be hydrolysed efficiently in the temperature range 500–600  C with a H 2 yield over 90%. A preliminary process design is also proposed for cycle integration in solar chemical plants. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction The promising energy carrier hydrogen is an environmentally attractive and sustainable transportation fuel, having the potential to displace fossil fuels. Therefore, the development of sustainable, low cost, and efficient technology for the large- scale production of H 2 without greenhouse gas emission currently mobilises strong research efforts worldwide. Hydrogen can be produced from either hydrocarbons (fossil fuels and biomass) or water. Current technological process achievements and economical considerations have mainly directed the industrial production toward steam reforming of natural gas. However, flue gases contain significant amounts of CO and CO 2 , leading to large CO 2 emissions into the atmo- sphere (0.43 mol CO 2 equiv./mol of net hydrogen produced [1]), thereby imposing additional purification treatments for further H 2 processing. This contributes to increased down- stream process costs and complexity. Alternative clean and efficient pathways for the production of pure hydrogen are water electrolysis and thermochemical water-splitting cycles. A thermochemical cycle effects the multi-step decomposition of water into hydrogen and oxygen using only heat. Thus, thermochemical cycles are expected to be more efficient than electrolysis for H 2 production as their energy efficiency is not limited by the conversion of heat to electricity. Cycles with overall energy efficiency above 20% are targeted to compete with electrolysis process (solar heat to electricity conversion 25%  electrolysis efficiency 80%). Consequently, solar-powered thermochemical cycles constitute an attractive option for massive H 2 production, avoiding greenhouse gas emission and allowing complete recycling of chemicals. This sustainable ‘green’ process also * Corresponding author. Tel.: þ33 04 68 30 77 30; fax: þ33 04 68 30 29 40. E-mail address: abanades@promes.cnrs.fr (S. Abanades). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.05.042 international journal of hydrogen energy 33 (2008) 6021–6030