Solar thermal decoupled water electrolysis process I: Proof of concept R. Palumbo a,n , R.B. Diver c , C. Larson a , E.N Coker b , J.E. Miller b , J. Guertin a , J. Schoer a , M. Meyer a , N.P. Siegel d a Valparaiso University, Valparaiso, IN 46383, USA b Sandia National Laboratories, Albuquerque, New Mexico 87123, USA c Diver Solar LLC, Albuquerque, NM 87123, USA d Bucknell University, Lewisburg, PA 17837, USA HIGHLIGHTS c A new solar process for the production of hydrogen from the electrolysis of water. c Theoretical and experimental proof of the functionality of the process concept. c The production of hydrogen with a high temperature windowless solar reactor. c The process’s decomposition step is decoupled from its electrolytic step. c The solar hydrogen process can be considered quasi continuous on a 24 h basis. article info Article history: Received 21 June 2012 Received in revised form 13 August 2012 Accepted 14 August 2012 Available online 23 August 2012 Keywords: Solar Thermal Electrochemistry Hydrogen Thermodynamics Metal oxides abstract A new concept for a solar thermal electrolytic process was developed for the production of H 2 from water. A metal oxide is reduced to a lower oxidation state in air with concentrated solar energy. The reduced oxide is then used either as an anode or solute for the electrolytic production of H 2 in either an aqueous acid or base solution. The presence of the reduced metal oxide as part of the electrolytic cell decreases the potential required for water electrolysis below the ideal 1.23 V required when H 2 and O 2 evolve at 1 bar and 298 K. During electrolysis, H 2 evolves at the cathode at 1 bar while the reduced metal oxide is returned to its original oxidation state, thus completing the H 2 production cycle. Ideal sunlight-to-hydrogen thermal efficiencies were established for three oxide systems: Fe 2 O 3 –Fe 3 O 4 , Co 3 O 4 –CoO, and Mn 2 O 3 –Mn 3 O 4 . The ideal efficiencies that include radiation heat loss are as high or higher than corresponding ideal values reported in the solar thermal chemistry literature. An exploratory experimental study for the iron oxide system confirmed that the electrolytic and thermal reduction steps occur in a laboratory scale environment. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Historical background Efficient and economical production of liquid and gaseous fuels from raw materials with concentrated sunlight is a major goal of those working in the area of high temperature solar thermal chemistry. A sampling of the research effort is found in Coker et al. (2010, 2011, 2012), Furler et al. (2012), Ambrosini et al. (2010), Chueh and Haile (2010), Miller et al. (2008), Allendorf et al. (2008), Diver et al. (1983, 2008), Steinfeld (2005), M¨ uller and Steinfeld (2008), Sturzenegger and Nuesch (1999), Charvin et al. (2007), Diver and Fletcher (1979), Fletcher and Moen (1979), Ehrensberger et al. (1995), Fletcher and Noring (1983), Fletcher et al. (1985), Palumbo and Fletcher (1988), Parks et al. (1988), Schroeder et al. (2011), Kogan et al. (2007). From the early days of the field, the transformative advance has been conceived as the process that converts the two most abundant resources on earth, sunlight and water, into an energy form usable as a transportation fuel. Some of the very earliest experi- ments focused on the single-step thermal reduction of water to H 2 and ½O 2 (Diver and Fletcher, 1979; Diver et al., 1983). The required process temperatures are near 2500 K and system pressures are well below one bar (Fletcher and Moen, 1979). The inherent engineering challenges for this approach, for exam- ple recovering the products while avoiding recombination, are formidable and the focus of research quickly became multiple- step thermal processes and solar thermal electrolytic processes. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2012.08.023 n Corresponding author. E-mail address: robert.palumbo@valpo.edu (R. Palumbo). Chemical Engineering Science 84 (2012) 372–380