Effect of Start-Up Strategies and Electrode Materials on Carbon Dioxide Reduction on Biocathodes

Abstract

The enrichment of CO₂-reducing microbial biocathodes is challenging. Previous research has shown that a promising approach could be to first enrich bioanodes and then lower the potential so the electrodes are converted into biocathodes. However, the effect of such a transition on the microbial community on the electrode has not been studied. The goal of this study was thus to compare the start-up of biocathodes from preenriched anodes with direct start-up from bare electrodes and to investigate changes in microbial community composition. The effect of three electrode materials on the long-term performance of the biocathodes was also investigated. In this study, preenrichment of acetate-oxidizing bioanodes did not facilitate the start-up of biocathodes. It took about 170 days for the preenriched electrodes to generate substantial cathodic current, compared to 83 days for the bare electrodes. Graphite foil and carbon felt cathodes produced higher current at the beginning of the experiment than did graphite rods. However, all electrodes produced similar current densities at the end of the over 1-year-long study (2.5 A/m²). Methane was the only product detected during operation of the biocathodes. Acetate was the only product detected after inhibition of the methanogens. Microbial community analysis showed that Geobacter sp. dominated the bioanodes. On the biocathodes, the Geobacter sp. was succeeded by Methanobacterium spp., which made up more than 80% of the population. After inhibition of the methanogens, Acetobacterium sp. became dominant on the electrodes (40% relative abundance). The results suggested that bioelectrochemically generated H₂ acted as an electron donor for CO₂ reduction.IMPORTANCE In microbial electrochemical systems, living microorganisms function as catalysts for reactions on the anode and/or the cathode. There is a variety of potential applications, ranging from wastewater treatment and biogas generation to production of chemicals. Systems with biocathodes could be used to reduce CO₂ to methane, acetate, or other high-value chemicals. The technique can be used to convert solar energy to chemicals. However, enriching biocathodes that are capable of CO₂ reduction is more difficult and less studied than enriching bioanodes. The effect of different start-up strategies and electrode materials on the microbial communities that are enriched on biocathodes has not been studied. The purpose of this study was to investigate two different start-up strategies and three different electrode materials for start-up and long-term operation of biocathodes capable of reducing CO₂ to valuable biochemicals.

Keywords: acetogens; biocathode; cyclic voltammetry; methanogens; microbial community structure; microbial electrolysis cells; start-up strategies.

FIG 1

Current generation with time in MEC1 and MEC2. The positive current in MEC1 represents the current when the anodes were controlled at −0.2 V versus SHE. The negative current in both MECs represents the current when cathodes were controlled at −0.65 V versus SHE. Dashed lines indicate when normal operation was stopped and CV tests were carried out.

FIG 2

Schematic time plan for MECs. Arrows represent different actions that took place during the experiment.

FIG 3

Disconnection tests for measuring current production by the individual electrodes in MEC1 and MEC2. Top, three electrodes that were installed from top of the MECs (graphite foil, carbon felt, and graphite rod from top); bottom, duplicates that were installed near the bottom.

FIG 4

Six different CV tests for MEC1. The first two tests were done before switching the potential from −0.2 to −0.65 V versus SHE, and the other four tests were done after switching the potential. One graphite foil electrode was removed from MEC1 on day 313 due to technical problems.

FIG 5

Six different CV tests for MEC2. Two graphite foil electrodes were removed from the MEC on day 271 due to technical problems.

FIG 6

Methane (top) and acetate (bottom) production over a 2-week period in MEC1 and MEC2. The theoretical production refers to the methane or acetate that should be produced from the current which flowed through the MEC. Experimental production refers to the methane or acetate that was measured in the experiment.

FIG 7

Relative abundances of the 20 most abundant 16S rRNA gene sequences in the bioanodes, biocathodes before and after adding 2-bromoethanesulfunate (BES), medium at the end of the experiment, and inoculum. LA refers to abundances of <0.1%. *, electrodes that were placed in MEC1 after removing bioanodes.