Fate of H₂ in an upflow single-chamber microbial electrolysis cell using a metal-catalyst-free cathode

With the goal of maximizing the H₂-harvesting efficiency, we designed an upflow single-chamber microbial electrolysis cell (MEC) by placing the cathode on the top of the MEC and carried out a program to track the fate of H₂ and electron equivalents in batch experiments. When the initial acetate concentration was 10 mM in batch-evaluation experiments lasting 32 h, the cathodic conversion efficiency (CCE) from coulombs (i.e., electron equivalents in current from the anode to the cathode) to H₂ was 98 ± 2%, the Coulombic efficiency (CE) was 60 ± 1%, the H₂ yield was 59 ± 2%, and methane production was negligible. However, longer batch reaction time (∼ 7 days) associated with higher initial acetate concentrations (30 or 80 mM) led to significant H₂ loss due to. CH₄ accumulation: up to 14 ± 1% and 16 ± 2% of the biogas at 30 and 80 mM of acetate, respectively. Quantitative PCR proved that no acetoclastic methanogens were present, but that hydrogenotrophic methanogens (i.e., Methanobacteriales) were present on both electrodes. The hydrogenotrophic methanogens decreased the CCE by diverting H₂ generated at the cathode to CH₄ in the upflow single-chamber MEC. In some experiments, the CE was greater than 100%. The cause was anode-respiring bacteria oxidizing H₂ and producing current, which recycled H₂ between the cathode and the anodes, increasing CE to over 100%, but with a concomitant decline in CCE, despite negligible CH₄ formation.