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HighlightsA pilot microbial electrolysis cell (MEC) was tested for H2O2 production.Passive air diffusion to a carbon electrode successfully produced H2O2.The highest H2O2 conversion was only 7.2%.Catholyte pH over 11 can mitigate H2O2 loss in MECs.

This study assessed the conductivity of a Geobacter-enriched biofilm anode in a microbial electrochemical cell (MxC) equipped with two gold anodes (25 mM acetate medium), as different proton gradients were built throughout the biofilm. There was no pH gradient across the biofilm anode at 100 mM phosphate buffer (current density 2.38 A/m) and biofilm conductivity (K) was as high as 0.87 mS/cm. In comparison, an inner biofilm became acidic at 2.5 mM phosphate buffer in which dead cells were accumulated at ∼80 μm of the inner biofilm anode. At this low phosphate buffer, K significantly decreased by 0.27 mS/cm, together with declined current density of 0.64 A/m. This work demonstrates that biofilm conductivity depends on the composition of live and dead cells in the conductive biofilm anode.