Spontaneous Unassisted Conversion of CO2 to Multicarbon (≥ C2) Liquid Products on Green Rust Mineral

Efficient low pressure, low temperature electrochemical conversion of CO2 to multicarbon (at least C>2) is a very challenging process due to the involvement of multiple coupled electron and proton transfer steps along with the difficult C-C coupling step between the charged intermediates. Only copper and its alloy show moderate efficiency in converting CO2 to multicarbon gaseous and liquid products but require at least -0.8- -1.0 V overpotentials. Here, we report that green rust, a corrosion product of Fe or steel surfaces formed both naturally and synthetically under anaerobic conditions, is a highly active catalyst capable of reducing CO2 to liquid C2 (ethanol) and C3 (acetone) products both spontaneously (unassisted) and electrocatalytically. A maximum production rate of 2520 μmoles L-1h-1 of C2 and C3 products could be achieved without the application of any external bias, which is 1000 times higher than that reported for other Fe minerals under external bias. Under external bias, GR exhibited a record total faradaic efficiencies (FE) of 60-98% at overpotentials of just 60-290 mV. A maximum ethanol conversion with 95% selectivity and a production rate of 13960 μg L-1h-1 was achieved with the KHCO3 electrolyte at a pH of 9.5 with application of 60 mV overpotential. Under a more acidic pH of 7.3 and an overpotential of 290 mV, the primary conversion product is acetone at a selectivity of 47% along with acetate and formate as the minor additional products (60% total FE). The results reported here represent a new benchmark for the efficient electrocatalytic conversion of CO2 to liquid products. The intrinsically high CO2 activity of the green rust mineral may be related to its unusual band structure with an electron affinity of ~3.2 eV, which is the lowest among all commonly known oxides and sulfides in the aqueous electrolyte.