Gas-phase electrochemical CO₂ reduction on silver-copper BTC MOF in a zero-gap membrane electrode assembly

Bimetallic AgCu-BTC MOFs, derived from Cu-BTC (HKUST-1), have been synthesized by fast co-precipitation method and evaluated for CO₂ reduction reaction (CO₂RR) using humidified CO₂ gas in a zero-gap configuration. Three compositions with varying Ag content – AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%) – were characterized by SEM-EDS, PXRD, FTIR, and XPS. These structural analyzes confirmed the formation of a low-crystalline AgCu-BTC MOF with Ag⁺¹ ions coordinated to the BTC framework. Electrochemical tests (CV and LSV) revealed that AgCu-3 MOF, with the highest Ag content (16.5 at.%), exhibited a more positive onset potential at -0.65 V vs Ag/AgCl compared to HKUST-1 MOF, indicating enhanced CO₂RR activity, owing to Ag incorporation. Constant potential (CP) experiments showed that AgCu-3 MOF predominantly produced CO and H₂, achieving faradaic efficiency of approximately 65 % for CO production and 5 % for H₂ production. Moreover, lowering the relative humidity (RH%) in the inlet CO₂ gas stream from 80% to 20% increased CO production by 2-fold from 6.2 µmol s^-1 cm^-2 to 13 µmol s^-1 cm^-2, simultaneously suppressing the hydrogen evolution reaction (HER). This reduction in humidity resulted in an increased local concentration of CO₂ at the catalyst site, leading to an enhanced CO₂RR rate. However, substantial morphological changes have been observed in the AgCu-3 MOF after the CP experiment under humid CO₂ reduction conditions.