Electrochemical CO2 reduction on mackinawite [FeSm] and violarite [(Fe,Ni)3S4] electrodes

The discovery of new materials for electrochemical CO2 reduction can take inspiration from biology and geology. Indeed, in the context of the hydrothermal vent theory of the Emergence of Life, alkaline vents represent a system of significant interest. These vents involve an alkaline, H2 and HS− rich fluid that meets the carbonic acidulous primordial ocean, resulting in the precipitation of a mineral barrier composed of iron sulfides and oxyhydroxides. This inorganic membrane separates the two fluids, generating an electrochemical potential difference. This ΔE can be dissipated by coupling CO2 reduction (on the acidic side) with H2 or HS-oxidation (on the alkaline side), potentially leading to the generation of the very first organic molecules on Earth. In this study, mackinawite [FeSm] and violarite [Fe,Ni)3S4] were synthesized through homogeneous precipitation, their structures and electrochemical properties were characterized. Electrodes based on these materials were prepared and tested for CO2 reduction. It was found that FeSm can reduce CO2 to formic acid and methanol, while (Fe,Ni)3S4 preferentially generates formic acid and carbon monoxide at − 0.82 V vs RHE. The results also indicate that very low overpotential is required for HCOOH generation on mackinawite. The proof that CO2 reduction under these conditions occurs electrochemically, rather than purely chemically, is evidenced by the absence of CO2 reduction products without the application of an electrical bias. Although the efficiency of these materials is currently limited, the potential for CO2 reduction using earth-abundant elements such as iron, nickel, and sulfur is promising. Further engineering of these materials could lead to cost-effective technology.