Metal-Organic Framework-Based Electrodes for Efficient CO2 Electroreduction to Formate at High Current Densities (up to 1 A cm−2)
Authors: Selva Ochoa Angela Gabriela , Habibzadeh Faezeh , Gyenge Elod Lajos 
Published: August 2024

Achieving efficient CO2 electroreduction for production of valuable chemicals requires affordable, stable, and non-toxic catalysts. One of the most studied and promising products of CO2 reduction is formic acid/formate. The latter species is receiving increased attention as an energy vector [1] or energy storage media (e.g., in CO2 redox flow batteries [2]). At present, the practical application of CO2 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 CO2 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 (FEHCOO
-
= 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 cm2 electrode geometric area, a 100% FEHCOO
- 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 Bi2O2CO3, Bi2O3, and metallic Bi, in selectivity (FE), cell potential, and durability.
These findings hold promise for further scale-up of CO2 reduction to formate using the cost-effective and easily prepared Bi-MOF catalyst.
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