Methane synthesis from CO2 and H2O with concentrated NaOH–KOH electrolyte at 200–250 °C using electrochemical Pd–Ag membrane reactor

Abstract

The synthesis of hydrocarbons from CO₂ and H₂O using electricity derived from renewable energy sources has attracted considerable attention. However, there is currently no direct electrochemical method capable of selectively producing specific hydrocarbons from CO₂ reduction at low overpotentials. By employing an electrochemical membrane reactor that combines water electrolysis with thermocatalytic methanation, methane can be selectively produced at a cell voltage comparable to that of conventional water electrolysis. In this study, methane synthesis from CO₂ and H₂O was investigated using an electrochemical membrane reactor equipped with a 30 wt%-Ru/C catalyst, Pd-based alloy membrane, and a NaOH–KOH eutectic electrolyte operating at approximately 250 °C. At 250 mA cm⁻² based on cathode geometric area and 250 °C, the methane and hydrogen production rate was 290 and 120 nmol s⁻¹ cm⁻², corresponding to a current efficiency of 91 and 9%, respectively. However, when the current density exceeded 250 mA cm⁻², the total current efficiency suddenly deviated significantly from 100%, indicating a decrease in current efficiency due to product cross-leak. At 300 mA cm⁻², significant evolution of H₂ was detected in the anode-side exhaust gas. This cross-leak was also pronounced during operation below 200 °C. Impedance measurements determined that the specific resistance of the electrolyte at 250 °C was 0.50 S cm⁻¹. The current efficiency and related characteristics of this methane synthesis method were discussed in detail.