Hydrogen-producing cycles using the decomposition of hydrogen bromide

The sulphur-bromide cycle is a promising three-reaction sequence using the decomposition of hydrogen bromide to produce hydrogen. Electrolysis is a possible way to decompose HBr and research is in progress at Ispra to decrease the overvoltage. Thermolysis of HBr is studied as an alternative in this paper. Two-step thermal splitting processes are investigated and can be written: 2 X + 2 HBr→2 X Br + H2; 2 X Br→2 X + Br2 where X may be a metal bromide, oxybromide or oxide. Nine metals are considered: Cr, Cu, Fe, Mo, Pb, Sn, Ti, V, W. Firstly a literature survey is made to establish a complete set of thermochemical data for the metal compounds. When the heat capacity and entropy are unknown, classical correlation rules are used to estimate these thermochemical values. Secondly the free energy of reactions which decompose HBr are calculated as a function of temperature and pressure. Some side reactions are also taken into account to evaluate concentrations of products. Finally three interesting two-reaction sequences (with Cr, V and W) are selected. In each the endothermal step is the bromine evolution. The temperature, heat requirement and corresponding pressure are given and a rough estimation of the water splitting efficiency are indicated for each process