Electrochemical aspects of the H2SO4SO2 thermoelectrochemical cycle for hydrogen production

Oxidation of SO₂ has been studied on electrode surfaces in sulfuric acid solutions as a function of acid concentration and temperature. The majority of work was conducted on platinum electrodes of high surface area. Some of the limitations of the reaction have been determined. The reaction is highly irreversible; however, the final product is sulfuric acid, apparently produced via a dithionate intermediate. The reaction rate at constant potential vs a hydrogen electrode in the same solution is strongly dependent on sulfuric acid concentration. Temperature has little effect on the rate. The reaction is not “catalytic” in the normal electrochemical sense: for example, adsorbing metals (e.g. Pt) show rates similar to those on non-adsorbing metals (e.g. Au). Carbons, graphites and carbides are inactive. In some potential and SO₂ partial pressure ranges, the process has a very low SO₂ reaction order. This can ensure relatively uniform hydrogen current density in a practical system, at high SO₂ conversion rates. The limiting steps in engineered SO₂-depolarized hydrogen production cells are reviewed.