Adapting microbial communities to low anode potentials improves performance of MECs at negative potentials
by Alex Lewis, Abhijeet Borole
A unique aspect of microbial electrolysis cells is the use of an applied voltage for H2 production. Applied voltage impacts the conversion efficiency at the anode as well as the efficiency of hydrogen evolution at the cathode, and is thus an important parameter for studying MECs. A variation on this parameter is the use of a controlled anode potential rather than controlled cell voltage, which can result in a more stable redox environment for the anode microbes. In this study, long-term exposure of anode consortia at -400 mV and 0 mV vs Ag/AgCl resulted in a gradual divergence of the respective midpoint potentials by >100 mV over a period of 6 months. Cyclic voltammetry revealed a shift in peak current production to more negative potentials for the reactor poised at -400 mV. Furthermore, chronopotentiometry indicated very different potential profiles, further indicating shifts in their respective redox profiles. At a set current of 15 mA, the stable potential reached for each reactor differed by 500 mV. The reactor poised at -400 mV significantly outperformed the one at 0 mV reactor in maintaining high currents at negative potentials. The substrate used was a bio-oil aqueous phase derived from switchgrass, making this study unique with potential for a biorefinery application of the anodic consortia for production of hydrogen, fuels or chemicals. The results provide evidence for adaptation of complex communities to optimize applied potential and reduce energy input for electrolysis. The study has important implications towards understanding complex community composition-function relationships. The study provides a basis for investigating genomic vs. transcriptomic changes in biomass-fed anode communities as a function of redox potential.