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In order to evaluate the technical feasibility of a novel solar hydrogen production process, the molten carbonate electroreduction behavior of water in the range 505-600°C was examined in this paper. Reaction kinetics and mechanism of the water reduction process was elucidated by using galvanostatic polarization methods and cyclic voltammetry. Two different water electrolysis mechanisms were found to be active in this temperature range, involving either bicarbonate or hydroxide anions as intermediate species, much depending on reaction conditions and melt temperature. Due to lack of knowledge, the bicarbonate mechanism was studied in detail. In accordance with a rapid thermodynamic decrease of bicarbonate stability above the 500°C, the voltammetric data showed a transition from bicarbonate to hydroxide mechanism at around 550°C. The results also indicated that water electrolysis in alkali molten carbonates can be conveniently operated in the 500°C area, where the relatively higher bicarbonate stability may enable a more rapid reaction than at higher temperatures. IntroductionHigh temperature electrolysis of water has a tremendous potential in enabling a cost-effective and sustainable production of hydrogen by using renewable heat and electricity from concentrating solar thermal (CSP) plants (1). Most efforts in this research area are currently directed towards investigating solid oxide electrolysis cells (SOECs) based on fuel cell technology (2,3), although this approach is not optimal for thermal integration with CSP plants because of electrolysis temperatures (i.e., above 800°C) significantly exceeding those of most solar heat storage fluids. In order to reduce the temperature gap with current solar thermal fluids, development of electrolysis processes within the 500-600°C range would be a desirable option. In this context, molten salts could be seen as an ideal medium to lower process temperatures with respect to solid oxide electrolyzers since overall ionic conductivity and transport numbers of liquid salts are usually higher than solid-type electrolytes. Recently, alkali 10.1149/06122.0013ecst ©The Electrochemical Society ECS Transactions, 61 (22) 13-25 (2014) 13 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 142.58.129.109 Downloaded on 2014-11-17 to IP molten carbonate salts have gained a return of attention as versatile electrolytes for conducting electrochemical conversion processes of mineral ores and CO 2 gas at moderate temperatures (4,5).In this context, molten carbonate electrolysis (MCE) to produce hydrogen from water is another process that has been recently mentioned in literature (2), although no systematic studies on this process have been so far conducted. Only very recently, the feasibility of a MCE process for hydrogen production has been demonstrated on laboratory scale experiments in the 600-675°C range using a Molten Carbonate Fuel Cell (MCFC) operated in a reverse (electrolysis) mode (6). Interestingly, the a...

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