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.