The impact of externally added hydrogen gas on microbial electrosynthesis from CO2

Abstract

Hydrogen is a key versatile biomolecule in microbial electrosynthesis (MES). It can be directly produced by electrolysis to be used as an intermediate, directly biosynthesize by electroactive microorganisms from protons and electrons, or externally added to drive other bioelectrochemical or biological reactions.

The aim of this study is to bring further understanding on how externally added hydrogen impacts product formation on MES. Two double-chamber microbial electrolysis cells were built in 500mL modified Schott-Duran bottles (Figure 1A). The cathode consisted of a 175 cm2 carbon felt (+1V vs. Ag/AgCl) and a platinum wire was used as counter electrode. The cathode was inoculated according to the procedure detailed in Bajracharya et al. 2017, and following the acclimation period the biocathode was fed with a gas mixture containing 20% H2 / 20% N2 / 60% CO2. After 2 weeks of operation hydrogen was removed from the feed ( 20% N2 / 80% CO2).

When the cell was fed with the hydrogen-containing mixture, acetate and ethanol concentrations (Figure 1B) grew steadily with time (composition ratio around 1:1 (w/w)). This behavior suggested that hydrogen was acting as a reducing agent driving direct production of ethanol, or even its production from acetate. However, when hydrogen was removed from the feed, ethanol concentration declined, while acetate concentration sharply increased showing CO2-acetate selectivities near 100%. These results indicate how ethanol production is highly dependent on externally-added hydrogen, while the synthesis of acetate only requires the cathode as a source of electrons. A.Sotres thanks “Junta de Castilla y Leon” for postdoctoral contract associated to project ref: LE060U16, cofinanced by FEDER funds