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Combination of a bioanode and a biocathode in a bioelectrochemical system allows a more than two-fold increase in energetic efficiency of microbial electrosynthesis
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Edité par CCSD -
International audience. Microbial electrosynthesis (MES) is a promising technology to produce organic chemicals or fuels from carbon dioxide. However the prices of the electrobiocommodities produced are logically very sensitive to the price of electricity and the optimization of energy efficiency is thus a crucial step in the development of this new technology. Here we show how the coupling of a bioanode with a biocathode allows a dramatic increase in energy efficiency of MES. We monitored 3 dual-chamber reactors with both biotic anode (carbon cloth) and cathode (stainless steel) separated by a cation-exchange membrane. A specific strategy was developed with successions of potentiostatic and amperometric controls to let the biofilm develop on the anodes at the beginning of the experiment, and then let deliver a stable 5 A/m² current density at the cathodes until anodic substrate exhaustion. After a first substrate injection, acetate was efficiently used at the anodes with a mean coulombic efficiency (CE) of 78%. However 53% of the produced electrons were used at the cathodes for methane production and only traces of VFAs were detected. After a second substrate injection, the CE reached 89% at the anodes. Methanogenesis was inhibited in cathodes using 2-bromo-ethane sulfonate (BES) and VFAs therefore accumulated in the catholytes with a production rate of acetate reaching 11 g.m².d-1 and a CE of 29%. 16S ribotags pyrosequencing showed a dramatic switch of microbial populations on cathodes after BES injection with an inhibition of Methanomicrobiaceae (from 43% of total diversity to less than 1%) and a development of Clostridiaceae (from 2% to 57%). The electrons used for VFAs production were extracted with a potential of -0.24 V (vs. standard hydrogen electrode) thus lowering the required anodic voltage from around 1 V in comparison with water electrolyzing anodes. The combination of a biocathode with a bioanode allowed thus a more than two-fold increase in energetic efficiency compared to systems electrolyzing water at the anode and promise higher energy savings with optimized cathodic potentials.
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