Investigation of Bimetallic silver-copper BTC MOF for electrochemical CO2 reduction in Zero-gap Membrane electrode assembly (MEA) configuration

Abstract

Bimetallic Metal-Organic Frameworks (MOFs) of silver, copper and the ligand benzene 1,3,5-tricarboxylate (AgCu-BTC MOFs), derived from Cu-BTC (HKUST-1), have been synthesized by fast co-precipitation method and investigated for CO2 reduction reaction (CO2RR). Three AgCu-BTC MOF variants were synthesized with varying Ag content: AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%). A range of structural characterization techniques, including SEM-EDS, XRD, FTIR, and XPS, were utilized, revealing the formation of low-crystalline AgCu-BTC MOF with Ag+1 in an ionic state coordinated to the BTC framework. The investigation focused on CO2 reduction using humidified CO2 gas with bimetallic AgCu MOFs as electrocatalysts in a zero-gap MEA setup. The setup included a gas diffusion electrode (GDE) with a Sustainion Anion exchange membrane and bicarbonate as the anolyte. Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV) showed that the AgCu-3 MOF, with the highest silver content (16.5 at.%), exhibited a lower onset potential at -0.65 V vs Ag/AgCl compared to pristine Cu-BTC MOF owing to the better activity with Ag inclusion. Constant potential (CP) experiments combined with product analysis indicated that AgCu-3 MOF predominantly produced CO and H2 as the main products, achieving a faradaic efficiency of approximately 60% for CO production and 5% for hydrogen production. Moreover, reducing the humidity level in the inlet CO2 gas stream from 80% to 20% RH increased CO production by 2-fold, simultaneously suppressing the HER. This reduction in humidity resulted in an increased local concentration of CO2 at the catalyst site, leading to an enhanced CO2RR rate. SEM and FTIR investigations after the CP experiment demonstrated the instability of the AgCu-3 MOF, revealing substantial morphological changes under humid CO2 conditions.