Improving the Stability of Gas Diffusion Electrodes for CO₂ Electroreduction to Formate with Sn and In-Based Catalysts at 500 mA cm^–2: Effect of Electrode Design and Operation Mode

The electrochemical carbon dioxide reduction reaction (CO₂RR) using renewable electricity sources could provide a sustainable solution for generating valuable chemicals, such as formate salt or formic acid. However, an efficient, stable, and scalable electrode generating formate at industrially viable current densities (>100 mA cm^-2) is yet to be developed. Sn or In-based catalysts in gas diffusion electrodes (GDE) can efficiently produce formate. However, their long-term durability is limited owing to catalyst deactivation, carbonate deposition, and electrode flooding. Herein, a systematic study of 20 cm² GDEs with SnO₂ and In₂O₃ catalyst layers is presented in conjunction with various electrode operation strategies (i.e., flow-by vs flow through, dry vs humidified CO₂, continuous vs reverse polarity pulse electrolysis). It is demonstrated that the incorporation of CeO₂ nanoparticles as a promoter in either SnO₂ or In₂O₃ catalyst layers coupled with intermittent reverse polarity pulse operation dramatically improves the GDE stability during 12 h of tests at 500 mA cm^-2 with over 90% formate Faradaic efficiency. Due to its strong oxidizing capacity, CeO₂ helps Sn and In regain their valence state of + IV and + III, respectively, which are in situ reduced during CO₂RR, as shown by the surface characterization of the electrodes. The effect of the initial particle size of SnO₂ and reverse polarity pulse on the catalytic activity, durability, and carbonate salt precipitation in the GDE have also been addressed. Regarding two-phase flow dynamics, the quasi-convective gas flow through the GDE was more beneficial than the gas flow-by mode for enabling stable operation at high current densities (up to 500 mA cm^-2). The synergistic approach of catalyst layer engineering coupled with diverse GDE operation modes explored here is promising for the scale-up of efficient and durable reactors for the CO₂RR to formate and CO₂ redox flow batteries.