Metal-Organic Framework-Based Electrodes for Efficient CO₂ Electroreduction to Formate at High Current Densities (up to 1 A cm⁻²)

Achieving efficient CO 2 electroreduction for production of valuable chemicals requires affordable, stable, and non-toxic catalysts. One of the most studied and promising products of CO 2 reduction is formic acid/formate. The latter species is receiving increased attention as an energy vector [1] or energy storage media (e.g., in CO 2 redox flow batteries [2]). At present, the practical application of CO 2 reduction to formate still faces challenges due to the lack of electrocatalysts capable of operating at high current densities (> 200 mA cm −2 ) with low degradation over long-duration operation [3]. Traditional metallic catalysts like Bi, Sn, In or Pb when scaled in flow cells typically suffer from low faradaic efficiencies (< 70%) at current densities ≥ 200 mA cm -2 , coupled with inadequate durability [4]. Metal-organic frameworks (MOFs) present promising and thus far largely unexplored attributes as electrocatalysts for CO 2 reduction, including high atom utilization during catalysis due to their porous crystal structure and tunable pore size distribution [5,6]. However, they also face challenges related to high overpotentials and complex synthesis methods [7]. This study elucidates the efficacy of a Bi metal-organic framework (Bi-MOF) synthesized through a rapid and facile method. The Bi-MOF obtained by our proprietary novel method [8], exhibits exceptional catalytic performance. Notably, it demonstrates outstanding faradaic efficiencies towards formate (FE HCOO - = 95–100%) at current densities up to 1 A cm −2 in a gas diffusion electrode, at low catalyst loading (0.5 mg cm −2 ). Moreover, Bi-MOF displays extended stability, operating continuously for over 20 hours at an industrially relevant current density (200 mA cm −2 ) and without electrolyte (1.5 M KOH) replenishment. In a flow reactor with 10 cm 2 electrode geometric area, a 100% FE HCOO - was obtained during 2-hour electrolysis at 100 mA cm −2 across a broad pH range (8–14). The electrochemical testing of the Bi-MOF was supplemented by surface and structural characterizations to correlate the activity with structural features. This analysis unveiled the role of the organic framework and the reason why Bi-MOF surpasses other Bi-based catalysts, including commercial Bi 2 O 2 CO 3 , Bi 2 O 3 , and metallic Bi, in selectivity (FE), cell potential, and durability. These findings hold promise for further scale-up of CO 2 reduction to formate using the cost-effective and easily prepared Bi-MOF catalyst. [1] Bienen, F., Kopljar, D., Löwe, A., Aßmann, P., Stoll, M., Rößner, P., Wagner, N., Friedrich, A., & Klemm, E. (2019). Utilizing Formate as an Energy Carrier by Coupling CO2 Electrolysis with Fuel Cell Devices. Chemie Ingenieur Technik , 91 (6), 872-882. [2] Hosseini-Benhangi, P., Gyenge, C., & Gyenge, E. (2021). The carbon dioxide redox flow battery: Bifunctional CO2 reduction/formate oxidation electrocatalysis on binary and ternary catalysts. Journal of Power Sources , 495 , 229752. [3] Masel, R. I., Liu, Z., Yang, H., Kaczur, J. J., Carrillo, D., Ren, S., Salvatore, D., & Berlinguette, C. P. (2021). An industrial perspective on catalysts for low-temperature CO2 electrolysis. Nature Nanotechnology , 16 (2), 118-128. [4] Zou, J., Liang, G., Lee, C., & Wallace, G. G. (2023). Progress and perspectives for electrochemical CO2 reduction to formate. Materials Today Energy , 38 , 101433. [5] Mazari, S. A., Hossain, N., Basirun, W. J., Mubarak, N. M., Abro, R., Sabzoi, N., & Shah, A. (2021). An overview of catalytic conversion of CO2 into fuels and chemicals using metal organic frameworks. Process Safety and Environmental Protection , 149 , 67-92. [6] Xie, W., Mulina, O. M., O., A., & He, L. (2023). Metal–Organic Frameworks for Electrocatalytic CO2 Reduction into Formic Acid. Catalysts , 13 (7), 1109. [7] Köppen, M., Dhakshinamoorthy, A., Inge, A.K., Cheung, O., Ångström, J., Mayer, P. and Stock, N. (2018), Synthesis, Transformation, Catalysis, and Gas Sorption Investigations on the Bismuth Metal–Organic Framework CAU-17. Eur. J. Inorg. Chem., 2018: 3496-3503. [8] Selva-Ochoa, A.G., Habibzadeh F., Gyenge E.L. (2023). Manuscript in preparation. Department of Chemical & Biological Engineering, UBC.