An efficient toluene barrier membrane for high-performance direct toluene hydrogenation via an electrochemical process

Abstract

Direct electro-hydrogenation of toluene to methyl cyclohexane (MCH) is gaining attention as a green process to store hydrogen in a liquid organic hydrogen carrier (LOHC), ultimately for long-distance hydrogen storage and transportation. A critical challenge in this electrochemical process is preventing toluene crossover to maintain high conversion efficiency and cell performance. This study introduces a novel approach utilizing sulfonated poly(arylene ether sulfone) (SPAES), a hydrocarbon-based proton exchange membrane (PEM) with narrow hydrophilic domains, to significantly reduce toluene diffusivity. Our findings reveal that toluene diffusivity in the SPAES PEM is 19.6-fold lower than in commercially available Nafion, resulting in a 60% reduction in toluene permeability. The enhanced barrier properties of the SPAES PEM substantially improved the Faradaic efficiency of toluene hydrogenation to 72.8%, whereas Nafion achieved 68.4% at a high current density of 600 mA cm⁻². Long-term operation (48 hours) at 150 mA cm⁻² demonstrated the superior performance of the SPAES PEM, with a degradation rate of 728 μV h⁻¹ compared to Nafion's 1270 μV h⁻¹. This research elucidates the effects of toluene crossover in direct electro-hydrogenation electrolyzers and demonstrates the advantages of hydrocarbon-based PEMs for LOHC electro-conversion applications.