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Oxygen reducing biocathodes were formed from sludge under constant polarization at − 0.2 and + 0.4 V/SCE. Under chronoamperometry at pH 10.3 ± 0.3, current densities of 0.21 ± 0.03 and 0.12 ± 0.01 A m^− 2 were displayed at − 0.2 V/SCE by the biocathodes formed at − 0.2 and 0.4 V/SCE, respectively. Voltammetry revealed similar general characteristics for all biocathodes and higher diffusion-limited current densities (0.84 ± 0.26 A m^− 2) than chronoamperometry. Up to 3.7 A m^− 2 was reached under air bubbling. A theoretical model was proposed to show the consistency of the chronoamperometric and voltammetric data.

The biocathodes formed at − 0.2 V/ECS that gave the highest electrochemical performance showed a homogeneous selection of Deinococcus–Thermus and Gemmatimonadetes, while the biocathodes formed at 0.4 V/SCE were enriched in different bacteria. The biocathode that led to the …