Possible mechanisms of CO2 reduction by H2 via prebiotic vectorial electrochemistry

Methanogens are putatively ancestral autotrophs that reduce CO ₂ with H ₂ to form biomass using a membrane-bound, proton-motive Fe(Ni)S protein called the energy-converting hydrogenase (Ech). At the origin of life, geologically sustained H+ gradients across inorganic barriers containing Fe(Ni)S minerals could theoretically have driven CO ₂ reduction by H ₂ through vectorial chemistry in a similar way to Ech. pH modulation of the redox potentials of H ₂, CO ₂ and Fe(Ni)S minerals could in principle enable an otherwise endergonic reaction. Here, we analysewhether vectorial electrochemistry can facilitate the reduction of CO ₂ by H ₂ under alkaline hydrothermal conditions using a microfluidic reactor. We present pilot data showing that steep pH gradients of approximately 5 pH units can be sustained over greater than 5 h across Fe(Ni)S barriers, with H+-flux across the barrier about two million-fold faster than OH-flux. This high flux produces a calculated 3-pH unit-gradient (equating to 180 mV) across single approximately 25-nm Fe(Ni)S nanocrystals, which is close to that required to reduce CO ₂. However, the poor solubility of H2 at atmospheric pressure limits CO ₂ reduction by H ₂, explaining why organic synthesis has so far proved elusive in our reactor. Higher H ₂ concentration will be needed in future to facilitate CO ₂ reduction through prebiotic vectorial electrochemistry.