Membrane Electrolysis of Toluene Hydrogenation with Water Decomposition for Energy Carrier Synthesis

Abstract

Large-scale technology for energy storage and transportation is needed to increase renewable energies which unevenly distribute with fluctuations, such as wind and solar power. The organic hydride of the toluene/methylcyclohexane system is one of the best energy carriers. In order to improve the energy conversion efficiency of the hydrogenation of toluene using electric power, a membrane electrolyzer, which consists of a carbon-supported precious metal-coated porous carbon cathode, perfluorosulfonic acid (PFSA) membrane separator, and DSE^® anode for the oxygen evolution reaction, has been designed and evaluated. The activity of the PtRu/C catalyst layer for the electrohydrogenation of toluene was higher than that of Pt/C. Hydrophilization of the anode side of the PFSA effectively discharged oxygen gas from the anode compartment. The electrohydrogenation of toluene without hydrogen evolution has been successfully up to a 450 mA cm⁻² current density with 100 % toluene feed at ca. 2 V cell voltage. In order to evaluate the conversion ratio for the hydrogenation, the current efficiency as a function of the current density was evaluated at various concentrations of toluene. Significant hydrogen was generated above 100 mA cm⁻² of current density for 10 times the stoichiometry of a 5 % toluene feed. During the electrolysis, by-products were hardly detected except for hydrogen. Although the improvement of the mass transport in the catalyst layer occurred, more than 95 % of the toluene conversion with a high energy efficiency should be feasible based on these experiments.