Low-Voltage Electroreduction of CO2: A Lesson from Geoelectrochemistry in the Seabed

To date, most research on heterogeneous CO₂ electroreduction has been aimed at decreasing the large overpotential required for hydrocarbon synthesis. Specifically, most scientists have focused on reducing the external voltage to as low as 1.24 V, as this value represents the thermodynamic limit for producing organic compounds via CO₂ reduction coupled with water oxidation [CO₂ + H₂O to (CH₂O) + O₂, where (CH₂O) represents a carbohydrate].

As an alternative approach, it may be possible to exploit the highly active geoelectrochemical reactions mediated by microorganisms living in deep-sea hydrothermal environments. In such environments, chemolithoautotrophs, as opposed to photoautotrophs, primarily contribute to biomass production through the utilization of geochemical energy sources, such as H₂S and H₂, emitted from hydrothermal vents. Chemolithoautotrophic bacteria are able to utilize these energy-rich compounds for the production of hydrocarbons at rates that are one order of magnitude higher than that of photosynthesis, and several species can synthesize hydrocarbons using Fe²⁺ ions as a sole electron source for CO₂ reduction. It is worth noting that Fe-oxidizing bacteria utilize the proton-motive force (PMF) generated by the reduction of O₂ to H₂O to elevate the energy of electrons obtained from Fe²⁺ oxidation by ~ 1 eV. Thus, although the reversible potential of the Fe³⁺/Fe²⁺ redox couple (0.77 V vs SHE) is more positive than that of NAD(P)/NAD(P)H (−0.32 V at pH 7), Fe-oxidizing bacteria are capable of generating NAD(P)H as a source of reductive energy for CO₂fixation.

By harnessing the ability of Fe-oxidizing bacteria to elevate intracellular electrons into a higher energetic state, it may be possible to construct an integrated bioelectrochemical process in which the bacteria function not only as effective CO₂ reduction catalysts, but also as voltage-multiplier circuits. In this system, CO₂ reduction would occur at an external voltage that is one order of magnitude lower than that of conventional heterogeneous electrocatalytic systems. The thermodynamic limit of the external voltage required for the simultaneous reduction and oxidation of CO₂ and H₂O, respectively, is determined by the difference between the reversible potentials of E(O₂/H₂O) and E(Fe³⁺/Fe²⁺). Thus, the thermodynamic limit of the external voltage is estimated to be 0.035 V at pH 7 if 0.77 V is adopted as the redox potential of the Fe³⁺/Fe²⁺ couple. Based on these estimated values, the integrated bioelectrochemical process mediated by Fe-oxidizing bacteria can potentially proceed at an external voltage lower than 1.24 V, and thus provide an approach for utilizing the low-voltage electricity generated from hydro, solar, wind, and geothermal sources for the electrochemical conversion of CO₂ to chemical fuels.