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Graphical abstract Highlights • The acetate production rate with fluidized GAC was 2.8 times that of control. • Fluidized GAC provide a high electrode surface and a favorable mass transport. • Fluidized GAC likely promoted the enrichment of electrochemically active bacteria. • Fluidized GAC electrode is a promising strategy to improve MES efficiency. Abstract The electricity-driven bioreduction of carbon dioxide to multi-carbon organic compounds, particularly acetate, has been achieved in microbial electrosynthesis (MES). MES performance can be limited by the amount of cathode surface area available for biofilm formation and slow substrate mass transfer. Here, a fluidized three-dimensional electrode, containing granular activated carbon (GAC) particles, was constructed via MES. The volumetric acetate production rate increased by 2.8 times through MES with 16 g L−1 GAC (0.14 g L−1 d-1) compared with that of the control (no GAC), and the final acetate concentration reached 3.92 g L−1 within 24 days. Electrochemical, scanning electron microscopy, and microbial community analyses suggested that GAC might improve the performance of MES by accelerating direct and indirect (via H 2) electron transfer because GAC could provide a high electrode surface and a favorable mass transport. This study attempted to improve the efficiency of MES and presented promising opportunities for MES scale-up. [ABSTRACT FROM AUTHOR]

Microbial electrosynthesis (MES) is a biocathode-driven process, producing high-value chemicals from CO 2 . Here, an in situ self-assembled graphene oxide (rGO)/biofilm was constructed, in MES, for high efficient acetate production. GO has been successfully reduced by electroautotrophic bacteria for the first time. An increase, of 1.5 times, in the volumetric acetate production rate, was obtained by self-assembling rGO/biofilm, as compared to the control group. In MES with rGO/biofilm, a volumetric acetate production rate of 0.17 g l −1 d −1 has been achieved, 77% of the electrons consumed, were recovered and the final acetate concentration reached 7.1 g l −1 , within 40 days. A three-dimensional rGO/biofilm was constructed enabling highly efficient electron transfer rates within biofilms, and between biofilm and electrode, demonstrating that the development of 3D electroactive biofilms, with higher extracellular electron transfer rates, is an effective approach to improving MES efficiency. [ABSTRACT FROM AUTHOR]