A thermodynamic analysis of electron production during syngas fermentation

► The Gibbs free energy of reaction for electron production from CO is thermodynamically favorable for typical conditions. ► The Gibbs free energy of reaction for electron production from H2 can be thermodynamically favorable or unfavorable. ► Electron production from CO is always more thermodynamically favorable compared to electron production from H2. ► It is unlikely that H2 can be utilized in favor of CO for electron production when both species are present. ► CO conversion efficiency will likely be sacrificed during syngas fermentation. Currently, syngas fermentation is being developed as one option towards the production of biofuels from biomass. This process utilizes the acetyl-CoA (Wood–Ljungdahl) metabolic pathway. Along the pathway, CO and CO2 are used as carbon sources. Electrons required for the metabolic process are generated from H2 and/or from CO. This study showed that electron production from CO is always more thermodynamically favorable compared to electron production from H2 and this finding is independent of pH, ionic strength, gas partial pressure, and electron carrier pairs. Additionally, electron production from H2 may be thermodynamically unfavorable in some experimental conditions. Thus, it is unlikely that H2 can be utilized for electron production in favor of CO when both species are present. Therefore, CO conversion efficiency will be sacrificed during syngas fermentation since some of the CO will provide electrons at the expense of product and cell mass formation.