Improving the Stability\nof Gas Diffusion Electrodes\nfor CO<sub>2</sub> Electroreduction to Formate with Sn and In-Based\nCatalysts at 500 mA cm<sup>–2</sup>: Effect of Electrode Design\nand Operation Mode

The electrochemical carbon dioxide reduction reaction\n(CO<sub>2</sub>RR) using renewable electricity sources could provide\na sustainable\nsolution for generating valuable chemicals, such as formate salt or\nformic acid. However, an efficient, stable, and scalable electrode\ngenerating formate at industrially viable current densities (>100\nmA cm<sup>–2</sup>) is yet to be developed. Sn or In-based\ncatalysts in gas diffusion electrodes (GDE) can efficiently produce\nformate. However, their long-term durability is limited owing to catalyst\ndeactivation, carbonate deposition, and electrode flooding. Herein,\na systematic study of 20 cm<sup>2</sup> GDEs with SnO<sub>2</sub> and\nIn<sub>2</sub>O<sub>3</sub> catalyst layers is presented in conjunction\nwith various electrode operation strategies (i.e., flow-by vs flow\nthrough, dry vs humidified CO<sub>2</sub>, continuous vs reverse polarity\npulse electrolysis). It is demonstrated that the incorporation of\nCeO<sub>2</sub> nanoparticles as a promoter in either SnO<sub>2</sub> or In<sub>2</sub>O<sub>3</sub> catalyst layers coupled with intermittent\nreverse polarity pulse operation dramatically improves the GDE stability\nduring 12 h of tests at 500 mA cm<sup>–2</sup> with over 90%\nformate Faradaic efficiency. Due to its strong oxidizing capacity,\nCeO<sub>2</sub> helps Sn and In regain their valence state of + IV\nand + III, respectively, which are in situ reduced during CO<sub>2</sub>RR, as shown by the surface characterization of the electrodes. The\neffect of the initial particle size of SnO<sub>2</sub> and reverse\npolarity pulse on the catalytic activity, durability, and carbonate\nsalt precipitation in the GDE have also been addressed. Regarding\ntwo-phase flow dynamics, the quasi-convective gas flow through the\nGDE was more beneficial than the gas flow-by mode for enabling stable\noperation at high current densities (up to 500 mA cm<sup>–2</sup>). The synergistic approach of catalyst layer engineering coupled\nwith diverse GDE operation modes explored here is promising for the\nscale-up of efficient and durable reactors for the CO<sub>2</sub>RR\nto formate and CO<sub>2</sub> redox flow batteries.