Fig. 1.

Proposed mechanism of pH-driven CO₂ reduction with H₂ across a conducting Fe(Ni)S barrier, and schematic of the reactor. (A) Under alkaline-vent conditions, the oxidation of H₂ (Left) is favored by the alkaline pH due to the presence of hydroxide ions (OH^–) that react exergonically in the production of water. Released electrons would travel across the micrometers- to centimeters-thick Fe(Ni)S network (53) (Center) to the more oxidizing acidic solution on the ocean side. There they meet dissolved CO₂ and a relatively high concentration of protons (H⁺), favoring the production of formic acid (HCOOH) or formate (HCOO^–). This electrochemical system enables the overall reaction between H₂ and CO₂, which is not observed under standard reaction conditions. (B) Diagram of the reactor, with embedded micrograph of a reaction run with precipitate at the interface. Further details are provided in the main text and SI Appendix, Fig. S2.