Bilayer Porous Electrocatalysts for Stable and Selective Electrochemical Reduction of CO₂ to Formate in the Presence of Flue Gas Containing NO and SO₂

The development of stable and selective electrocatalysts for converting CO₂ to value-added chemicals or fuels has gained much interest in terms of their potential to mitigate anthropogenic carbon emissions. Most of the electrocatalysts are tested under pure CO₂; however, industrial outlet flue gas contains numerous impurities, such as NO and SO₂, which poison the electrocatalysts and alter the product selectivity. Developing electrocatalysts that are resistant to such impurities is essential for commercial implementation. Herein, we prepared bilayer porous electrocatalysts, namely, Sn, Bi, and In, on porous Cu foam mesh (Sn/Cu-f, Bi/Cu-f, and In/Cu-f) by a two-step electrodeposition process and employed these electrodes for the electrochemical reduction of CO₂ to formate. It was observed that the bilayer porous electrocatalysts exhibited high CO₂ reduction activity compared to catalysts coated on a Cu mesh. Among bilayer porous electrocatalysts, Sn/Cu-f and Bi/Cu-f electrocatalysts showed more than 80% faradaic efficiency (FE) toward formate production, with a formate partial current density of around -16 and -10.4 mA cm^-2, respectively, at -1.02 V vs RHE. In/Cu-f electrocatalyst showed nearly 40% formate FE with formate partial current density of -15 mA cm^-2 at -1.22 V vs RHE. We investigated the effect of NO and SO₂ impurities (500 ppm of NO, 800 ppm of SO₂, and 500 ppm of NO + 800 ppm of SO₂) on these electrocatalysts' selectivity and stability toward formate. It was observed that the Bi/Cu-f electrocatalyst showed 50 h stability with 80 ± 5% formate FE, and Sn/Cu-f showed 18 h stability with above 80 ± 5% efficiency in the presence of NO and SO₂ mixed with CO₂. Furthermore, we studied the effect of CO₂ concentration with Sn/Cu-f and Bi/Cu-f catalysts in the range of 15-100% CO₂, for which formate FEs of 45-80% were observed.