Electrochemically Assisted Microbial Production of Hydrogen from Acetate

TLDR

Hydrogen production via bacterial fermentation is limited to 4 mol H₂ / mol glucose, yielding acetate that cannot be further converted to H₂, whereas an anaerobic microbial fuel cell can generate H₂ from any biodegradable dissolved organic matter by complete oxidation. The study demonstrates that a completely anaerobic MFC can bypass the biochemical barrier to produce hydrogen gas from acetate. The authors augmented the bacterial electrochemical potential in the MFC with an extra 250 mV, enabling cathodic hydrogen production directly from oxidized organic matter. More than 90 % of the protons and electrons from acetate oxidation were recovered as hydrogen, yielding a Coulombic efficiency of 60–78 % and an overall yield of 2.9 mol H₂ / mol acetate; when coupled with hydrogen fermentation, the system could produce 8–9 mol H₂ / mol glucose at an energy cost equivalent to 1.2 mol H₂ / mol glucose.

Abstract

Hydrogen production via bacterial fermentation is currently limited to a maximum of 4 moles of hydrogen per mole of glucose, and under these conditions results in a fermentation end product (acetate; 2 mol/mol glucose) that bacteria are unable to further convert to hydrogen. It is shown here that this biochemical barrier can be circumvented by generating hydrogen gas from acetate using a completely anaerobic microbial fuel cell (MFC). By augmenting the electrochemical potential achieved by bacteria in this MFC with an additional voltage of 250 mV or more, it was possible to produce hydrogen at the cathode directly from the oxidized organic matter. More than 90% of the protons and electrons produced by the bacteria from the oxidation of acetate were recovered as hydrogen gas, with an overall Coulombic efficiency (total recovery of electrons from acetate) of 60−78%. This is equivalent to an overall yield of 2.9 mol H2/mol acetate (assuming 78% Coulombic efficiency and 92% recovery of electrons as hydrogen). This bio-electrochemically assisted microbial system, if combined with hydrogen fermentation that produces 2−3 mol H2/mol glucose, has the potential to produce ca. 8−9 mol H2/mol glucose at an energy cost equivalent to 1.2 mol H2/mol glucose. Production of hydrogen by this anaerobic MFC process is not limited to carbohydrates, as in a fermentation process, as any biodegradable dissolved organic matter can theoretically be used in this process to generate hydrogen from the complete oxidation of organic matter.