Comparison of in-situ and ex-situ electrolytic H 2 supply for microbial methane production from CO 2 .

Both in-situ and ex-situ electrolytic H supply have been used for biomethane production from CO. 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 H supply, the in-situ electrolytic H bubbles were one order of magnitude smaller, which resulted in improved H mass transfer and biomass growth. Consequently, the methane production rate and the coulombic efficiency of the in-situ H supply (0.51 L·L·d, 96%) were higher than those of the ex-situ H supply (0.30 L·L·d, 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 H supply appears to be more promising, but reducing energy consumption is the key to the success of this technology.