Competing acetate consumption and production inside a microbial electrolysis cell

•Consumption of H2 by homoacetogens outweighs formation of new H2.•High H2 partial pressure reduces acetate consumption in MECs.•Homoacetogenesis leads to a decrease in H2 yield.•N2 purging is a promising option to avoid homoacetogenesis.•Deltaproteobacteria most abundant class in acetate fed MECs.