Electrochemical Hydrogen Pump Using CsH2PO4/SnO2 Composite Proton Conducting Membrane

Abstract

Liquid organic hydrogen carriers (LOHCs) are molecular compounds that have potential to be a promising class of hydrogen storage materials. Upon thermal treatment and in the presence of catalysts, H2-rich LOHCs can give off H2 without phase change. One immediate example is methylcyclohexane (C7H14) (MCH) conversion to toluene (C7H8) (TOL). The rate of H2 production of LOHC’s dehydrogenation reaction is kinetically limited. Therefore, adequate catalysts can help boost kinetics. An alternative approach to overcome the kinetic limitation is to shift the dehydrogenation reaction by removing H2 in the product via either a chemical or electrochemical route. Here we show an electrochemical H2 pump concept to promote the dehydrogenation reaction of MCH to TOL by pumping out H2 from the product line. We selected the super-protonic conductor CsH2PO4 (CDP) as the electrolyte for the pump due to the operating temperature of the dehydrogenation reaction (250-350 oC). However, CDP would not be stable if there is an insufficient amount of H2O (<38%) in the working environment, irreversibly dehydrated into a low-conductivity phase. On the other hand, too much H2O in MCH stream would negatively affect the catalytic performance of MCH-to-TOL conversion. Therefore, it is critical to limit the H2O content in MCH stream without affecting H2 pump performance.Here, we report the use of SnO2 to stabilize the super-protonic conducting phase in CDP. SnO2 is a super-hydrophilic material. We first show the ionic conductivity stabilization over a broad H2O content to demonstrate the effectiveness of SnO2 in stabilizing CDP conductivity. We then show the performance of H2 pump based on the composite CDP-SnO2 protonic conductor. We also show materials characterization results including XRD, SEM and Raman to facilitate our understanding of the observed improvement and degradation. Overall, this study provides a supporting method to accelerate the production of green hydrogen with LOHCs.