Microbial electrolysis cells with both anode and cathode catalysed by microorganisms

Abstract

Microbial electrolysis cells (MECs), which simultaneously produce hydrogen and treat wastewater, is an innovative technology. This study not only focused on the acquirement of self-sustained biocatalysts but also meant to understand bioelectrodes’ interaction in a single MEC which are crucial to improve the technology and make it economically feasible. For such MECs, further increases in overall performance were successfully done with the information from the studies. Those improvement methods included controlling bioelectrode reactions and cell configuration modification. The bioanodes and biocathodes were enriched separately in half-cell condition at +0.2 and -0.9 V vs. standard hydrogen electrode (SHE) and tested with chronoamperometry to check the best operating potentials. The bioanode achieved a maximum current density of 0.30 ± 0.05 A/m2 between -0.2 and +1.0 V while the biocathode only started to produce hydrogen below -0.8 V vs. SHE. It is preferable to maintain those potentials when both bioelectrodes are utilised in a MEC. The interactions between the bioanode and biocathode were studied in a two-chamber MEC (2cMEC). Both bioelectrodes were enriched simultaneously at a 0.3 V applied voltage. The bioanode grew faster and produced less current than the biocathode required, due to different redox reactions (acetate oxidation vs. proton reduction). Therefore, a fed-batch feeding mode was deployed in order to keep the bioanode active and produce sufficient current output. Three main regions of behaviour were identified under a range of applied voltages: cathode activation (< 0.7 V), maximum production (0.7–1.2 V) and anode limitation (> 1.2 V). The potentials of the biocathode fell from -0.6 to -1.0 V while the bioanode maintained a value of ~ -0.3 V, when the voltage was increased from 0.3 to 0.7 V. Between 0.7 and 1.2 V, the bioanode potential started to increase from -0.3 V to -0.1 V when biocathode potential reached its minimum at -1.1 V. Applied voltages higher than 1.2 V, further increased the current density up to 2.5 ± 0.5 A/m2 and the bioanode potential to +0.5 V. The greatest hydrogen production rate (20.0 ± 5.0 dm3 H2/m2/day) occurred after 0.9 V when an external power supply (increased from 0 to 75 %) took over the bioanode (decreased from 100 to 25 %) energy contribution. Cyclic voltammetry revealed a lower catalytic activity in the bioanode at 2.0 V compared to 0.3 V and the result was opposite for biocathode. Further study involved a three-chamber MEC (3cMEC) where a gas chamber was attached next to the cathodic chamber for the accumulation of CO2 from a gas phase into catholyte. This proof-of-concept MEC showed CO2 can be separated in a single step under specific solubility conditions. The 3cMEC performed almost the same as 2cMEC except it could accumulate a higher concentration of carbonates (550 ± 200 mg/L accumulated vs. 150 ± 50 mg/L pre-added) and alleviate pH increase (10.0 ± 0.5 vs. 11.0 ± 0.5) in the cathode as a result of the CO2 dissolution.