Solar thermochemical splitting of water to generate hydrogen

Abstract

Solar photochemical means of splitting water (artificial photosynthesis) to generate hydrogen is emerging as a viable process. The solar thermochemical route also promises to be an attractive means of achieving this objective. In this paper we present different types of thermochemical cycles that one can use for the purpose. These include the low-temperature multistep process as well as the high-temperature two-step process. It is noteworthy that the multistep process based on the Mn(II)/Mn(III) oxide system can be carried out at 700°C or 750°C. The two-step process has been achieved at 1,300°C/900°C by using yttrium-based rare earth manganites. It seems possible to render this high-temperature process as an isothermal process. Thermodynamics and kinetics of H 2 O splitting are largely controlled by the inherent redox properties of the materials. Interestingly, under the conditions of H 2 O splitting in the high-temperature process CO 2 can also be decomposed to CO, providing a feasible method for generating the industrially important syngas (CO+H 2 ). Although carbonate formation can be addressed as a hurdle during CO 2 splitting, the problem can be avoided by a suitable choice of experimental conditions. The choice of the solar reactor holds the key for the commercialization of thermochemical fuel production. The impact of global climate change as well as the likely shortage of fossil fuels demand harvesting of energy using renewable sources. Although solar energy captured by the Earth in an hour is expected to satisfy the energy demand of the world for a year, the high energy density and nonpolluting end product renders hydrogen a viable alternative to fossil fuels (1). In this context, conversion of solar power to H 2 and synfuels with the utilization of renewable H 2 O and CO 2 seems to be a sound option. Artificial photosynthesis and photovoltaic-powered electrolysis of water are promising approaches, although their implementation is somewhat restricted because of the low solar-tofuel conversion efficiency (η solar-to-fuel ) of <5% and <15%, respectively (2, 3). The other strategy would be a solarthermochemical process that provides a high theoretical efficiency and enables large-scale production of H 2 by using the entire solar spectrum (4). Research in thermochemical splitting of H 2 O made a beginning in the early 1980s (5, 6) and several thermochemical cycles have been examined. Thermochemical methods come under two main categories, the low-temperature multistep processes and the high-temperature two-step processes. The two-step process involving the thermal decomposition of metal oxides followed by reoxidation by reacting with H 2 O to yield H 2 is an attractive and viable process that can be rendered to become an isothermal process. Thermochemical splitting of H 2 O at low temperatures (<1,000°C) is accomplished by a minimum of three steps as dictated by thermodynamic energy constraints (7,8). In this paper we present the highlights of recent investigations of H 2 O splitting by the low-temperature m...

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