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 Pd-based alloy membrane and a NaOH-KOH eutectic electrolyte operating at approximately 250°C. At 250 mA cm^-2 and 250°C, the methane production rate was 290 nmol s^-1 cm^-2 , corresponding to a current efficiency of 91%. At this condition, the current efficiency for hydrogen production was 9%, indicating that the remaining current was consumed for hydrogen generation. 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 crossover. This crossover 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.