Mixed trace metals from microbial growth medium act as hydrogen evolution catalyst during microbial electrosynthesis at high current density
Bio-electrochemical CO2 reduction combines electrochemical and microbial catalysis to convert CO2 into short and medium chain fatty acids. To improve process performance, a metal-based catalyst can be integrated to enhance the formation of electron carriers and thus the energy supply for the biofilm. Here we show the feasibility of forming the hydrogen evolution catalyst formed from the microbial trace elements present in a bio-electrochemical CO2 reduction system. Concentrated trace element mixes with and without EDTA were applied to electrochemical systems with graphite felt cathodes controlled at -1.06 V vs Ag/AgCl. After the addition of the trace metal mixes with and without EDTA, the current and hydrogen production were respectively 10 and 40 times elevated compared to a non-pre-treated system. The high current remained after the system switch to biotic conditions, which allowed for an immediate start-up of microbial activity and formation of acetate. Twenty days after start-up of the biotic phase, n-butyrate was formed and the productivity kept increasing over the whole 35 to 50-day course of the biotic phase of the experiments. Hydrogen was always produced in excess, with higher production rates for the reactors pre-treated with the metal mix without EDTA. The hydrogen decreased slightly over the 35 to 50 days, possibly due to (bio)fouling or poisoning; leaching of XXX was eliminated based on measured catholyte element concentrations. The results show the possibility of electrochemical CO2 reduction to C4-compounds in one integrated system. Based on microbial analysis, a two-step microbial community bioprocess is suggested: acetogenesis (CO2 to acetate) by an unclassified Natranaerobiales strain and chain elongation (C2 to C4) by Clostridium kluyveri (related) species. The proof of principle shown in this study shows promise for use of mixed (microbial trace) metals in saline electrolyte solutions for the in situ formation of hydrogen catalyst(s) for CO2 valorisation with mild, microbial compatible process conditions.