Thermochemical cycles based upon sulfur species are among the most promising options being considered for the large scale production of hydrogen from water splitting. These cycles include sequential redox reactions in which sulfur species move between the S(IV) and S(VI) valence states. This separates the overall water splitting process into an oxygen liberation step and a hydrogen production step.
The Hybrid Sulfur Process is one of the best known of the sulfur cycles and uses both thermal and electrical energy. The thermal energy is used to decompose sulfur trioxide at high temperatures to liberate oxygen and form sulfur dioxide. The sulfur dioxide is converted to sulfuric acid by electrolysis, in the process splitting water to produce hydrogen. This process is attractive compared to conventional alkaline electrolysis because the presence of sulfur dioxide substantially reduces the required voltage from around 2 V to 0.6 V.
This paper explores the potential for the use of photovoltaic cells to drive the electrolysis step of the hybrid sulfur process. A well designed system will have good matching of the current/voltage characteristics of the photon capture step (i.e. photovoltaic cell) and the electrochemical step (i.e. electrolyser). This decoupling of photon capture and electrochemistry enables much more efficient use of solar energy than in situ photo-electrochemical methods. Carbon free hydrogen production is possible using such a system if coupled to a concentrating solar facility for the high temperature decomposition step.