Theoretical and experimental exploration on low-temperature molten carbonate electrolysis processes in a vapor-rich CO2/H2O atmosphere for hydrogen production

As high-temperature electrolyzers based on the Molten Carbonate Fuel Cell (MCFC) technology produce hydrogen mixed with CO (syngas) due to unavoidable CO 2 presence in the cathodic gas atmosphere, this study explores the question of whether suitable reaction conditions may be found to avoid any CO 2 interference during hydrogen production from molten carbonates. Based on previous literature and theoretical considerations, electrolysis experiments were conducted at temperatures approximately 150 °C lower than in typical MCFCs and under a vapor-rich CO 2 /H 2 O gas atmosphere. Such conditions were chosen to promote a molten carbonate chemistry enriched of electroactive bicarbonate ion species in an attempt to exploit their high proton donor ability and thus potentially favor a more selective hydrogen production. Although a minimal CO formation was observed at temperatures in the range 480–520 °C with a Inco X-750 alloy cathode operating under moderate current density regimes in between 29 and 80 mA/cm 2 , however, the objective of a selective hydrogen production was not achieved for the relevant co-formation of hydrocarbons, particularly methane. This indicated that in general CO 2 co-electrolysis effects are not easy to avoid in molten carbonate electrolytes, although our results give evidence that the CO 2 conversion may follow a different reaction path than in MCFCs leading to a prevalent methane formation under low-temperature reaction conditions. In particular, at 480 °C methane evolved as main fuel gas product and with an excellent Faraday's efficiency suggesting that low-temperature molten carbonate electrolysis processes could represent an interesting approach for efficient methane production. Minimal syngas formation was observed at 480 and 500 °C, which was supposed to be due to a rapid chemical conversion of the electrochemically generated syngas to the more thermodynamically stable methane via post-electrolysis Fischer-Tropsch-type reactions. Influence of vapor-rich CO 2 /H 2 O atmospheres and cathode material as key factors to speed up methane formation rate in molten carbonates was also discussed. • A low-T molten carbonate electrolysis study aimed to selective H 2 generation. • Experiments at around 500 °C and under a vapor-rich CO 2 /H 2 O gas to boost H 2 generation. • H 2 with minimal syngas but relevant formation of CH 4 observed. • Syngas conversion to CH 4 via post-electrolysis Fischer-Tropsch reactions suggested. • Effect of vapor-rich CO 2 /H 2 O gas and cathode material on CH 4 formation discussed.