Reactor4-Electrobiome

Electrosynthetic microbiomes use electrodes as a sole electron donor and carbon dioxide as a sole carbon source to produce hydrogen and organic compounds. This system was used to test the hypothesis that a decrease in reactor pH would correlate with increased production of hydrogen due to the thermodynamic dependence of hydrogen formation on pH. Biocathodes were poised at -600 mV vs. SHE and as the pH decreased due to either the application of a salt gradient across the cation exchange membrane, the accumulation of organic acids, or addition of HCl, the hydrogen production rate increased >100-fold. Acetate production ceased when the pH was near 5 or lower. Following repeated exposure to low pH, the microbiome produced acetate (~5-15 mM/day) and high amounts of hydrogen (>1 mol per liter of catholyte volume per day) after a return to near neutral pH (~6.5). Up to 1300 mM/day hydrogen, 16.9 mM/day formate and 51.9 mM/day acetate were produced when the biocathode potential was then lowered to -800 mV. In yield tests with graphite rod electrodes poised at -600 mV, the current density per biomass was 6.5 ± 1.2 A/g of electrode-attached protein. Cyclic voltammetry of the rod biocathodes indicated a 250 mV decrease in the hydrogen overpotential due to the presence and action of the microbiome and a maximum current density normalized to a cathode surface area of up to 12.2 mA/cm2 at -765 mV. The overall result of the repeated exposure to low pH was the development of a microbial community (>90% Acetobacterium plus Desulfovibrio and Sulfurospirillum) on the electrode that produced up to 2.6 kg/m3/day hydrogen with 1 kg H2 ˜ 1 gal gasoline equivalent, and 3.1 kg/m3/day acetate with 1.5 kg of CO2 captured per kg of acetate produced.