Efficient Generation of H ₂ by Splitting Water with an Isothermal Redox Cycle

Abstract

Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Solar fuel processing efficiency for ceria redox cycling using alternative oxygen partial pressure reduction methods

“…(2) and (3), favor lower temperatures in order to ensure a complete oxidation of ceria [12] with a typical temperature range of around 700 Ke1100 K. The temperature difference between the two steps requires solid phase heat recovery in order to maintain a high solar-to-fuel efficiency [14], which is difficult to realize and increases the system complexity. Therefore, isothermal cycling has been proposed [16,22,23].…”

First-Principles Study of the Adsorption/Dissociation Reactions of Water on a Fe- and Co−Al₂O₄Cluster

“…Recently, Muhich et al demonstrated that the hercynite cycle can be performed at an isothermal condition for M = Co [2] and achieved reducing the energy loss due to the difference between the reduction and oxidation temperatures. However, the microscopic mechanism of these reactions are not clarified yet.…”

Modelling of solar thermochemical reaction systems

“…Co, Ni, Zn, Cu, and Mn substitutions into ferrite spinel structures have recently been used in successful hightemperature H 2 O/CO 2 splitting, suggesting that the M x Fe 3-x O 4 form of these mixed oxides is particularly active (Kodama et al, 2008;Gokon et al, 2011;Kodama et al, 2005;Rydén et al, 2011;Alvani et al, 2005;Tamaura et al, 1998;Hwang et al, 2004;Miller et al, 2008;Fresno et al, 2009;Fresno et al, 2010;Arifin et al, 2012;Gokon et al, 2008b;Agrafiotis et al, 2015;Goikoetxea et al, 2016;Lorentzou et al, 2014;Cha et al, 2007;Kodama et al, 2002). Other spinel structures are also of interest, such as the ''hercynite cycle" (Muhich et al, 2013 (Haussener et al, 2010c). Gokon et al, 2008a;Han et al, 2007;Miller et al, 2008;Nakamura, 1977;Perkins and Weimer, 2004;Scheffe et al, 2010;Stamatiou et al, 2010;Steinfeld et al, 1999).…”