Comparison of in-situ and ex-situ electrolytic H₂ supply for microbial methane production from CO₂

Both in-situ and ex-situ electrolytic H₂ supply have been used for biomethane production from CO₂. However, the pros and cons of them have not been systematically compared. The present study makes this comparison using a 20 L continuous stirred-tank reactor equipped with external and internal electrolyzers. Compared to the ex-situ H₂ supply, the in-situ electrolytic H₂ bubbles were one order of magnitude smaller, which resulted in improved H₂ mass transfer and biomass growth. Consequently, the methane production rate and the coulombic efficiency of the in-situ H₂ supply (0.51 L·L^-1·d^-1, 96%) were higher than those of the ex-situ H₂ supply (0.30 L·L^-1·d^-1, 56%). However, due to high internal resistance, the energy consumption for the in-situ electrolysis was 2.54 times higher than the ex-situ electrolysis. Therefore, the in-situ electrolytic H₂ supply appears to be more promising, but reducing energy consumption is the key to the success of this technology.