A methane-producing biocathode that converts CO2 into methane was studied electrochemically and microbiologically. The biocathode produced methane at a maximum rate of 5.1？L？CH4/m2 projected cathode per day (1.6？A/m2) at ？0.7？V versus NHE cathode potential and 3.0？L？CH4/m2 projected cathode per day (0.9？A/m2) at ？0.6？V versus NHE cathode potential. The microbial community at the biocathode was dominated by three phylotypes of Archaea and six phylotypes of bacteria. The Archaeal phylotypes were most closely related to Methanobacterium palustre and Methanobacterium aarhusense. Besides methanogenic Archaea, bacteria seemed to be associated with methane production, producing hydrogen as an intermediate. Biomass density varied greatly with part of the carbon electrode covered with a dense biofilm, while only clusters of cells were found on other parts. Based on our results, we discuss how inoculum enrichment and changing operational conditions may help to increase biomass density and to select for microorganisms that produce methane. 1. Introduction In bioelectrochemical systems (BES), microorganisms catalyze oxidation and reduction reactions to produce or use electricity. Recently, it has been discovered that microorganisms can accept electrons from an electrode [1] to bioremediate metal and organic contaminants or for microbial electrosynthesis to produce fuels and chemicals. Using microorganisms as catalysts on an electrode instead of chemical catalysts is innovative and sustainable; the microorganisms are self-regenerating and the BES can be operated at ambient conditions (at neutral pH and low temperature), and low-cost carbon electrodes can be used [2, 3]. Microbial electrosynthesis in BES has been described for the production of, for example, hydrogen [4], hydrogen peroxide [5], caustic [6], acetate and 2-oxobutyrate [7, 8], ethanol [9], ammonium [10], butyrate [11], or caproate and caprylate [12]. Another attractive application of microbial electrosynthesis is the conversion of CO2 into methane [13]. Besides producing carbon-neutral methane, BES can convert excess renewable electricity from sun and wind into methane as an energy carrier [13]. Moreover, the infrastructure for transport, storage and consumption of methane is already in place [13]. To improve the performance of a methane-producing BES, focus so far was mainly on BES design [14–17]. However, another key challenge is understanding the methane-producing microbial communities in order to improve the methane production rate and energy efficiency [18]. The microbial consortium (types of
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