Molybdate oxidation/reduction as the low temperature step in the sulfuric acid thermochemical watersplitting cycle

Abstract

SO 2 can be reacted with a solution of molybdic acid in sulfuric acid in the presence of a Pt-catalyst by which SO 3 and a Mo v compound is formed. This compound can be electrochemically reoxidized at high current densities with minimal overpotential. Potentials of 625 mV were found at 200 mA cm −2 for the oxidation of Mo v under simultaneous formation of hydrogen. Hydrogen formation is as calculated from the current passed and no poisoning of the hydrogen evolution catalyst occurs, even if some molybdenum passes the membrane separating the two electrochemical compartments. The liquid at the molybdenum side of the membrane is circulated by a pump, while no liquid is present in the other compartment, hydrogen formation occurring at the catalyst, which is deposited on the membrane. Cell voltage is not very dependent on sulfuric acid percentage between 30 and 55 wt%. At higher concentrations it increases steeply, which is due to the high resistance of the sulfuric acid solution at these concentrations. It is shown that the solution can be cycled between reduction and oxidation cycles. Since, in every reduction step of the molybdenum, SO 2 is oxidized, it is possible to start with a dilute sulfuric acid solution and to go on cycling until the solution becomes too concentrated for the electrochemical reaction to be run economically. In such a way it is possible to save energy in the removal of water, which is necessary before the sulfuric acid can be passed through the high temperature reactor.