Physicochemical Analysis of Cu(II)-Driven Electrochemical CO₂ Reduction and its Competition with Proton Reduction

The reduction of CO₂ has become a key role in reducing greenhouse gas emissions in efforts to search for long-term responses to climate change. We report a couple of CO₂-reducing molecular catalysts based on earth-abundant copper complexes. These are [Cu(DPA)(PyNAP)] (1) and [Cu(DPA)(PyQl)] (2) (where, DPA=pyridine-2,6-dicarboxylate, PyNAP=2-(pyridin-2-yl)-1,8-naphthyridine, and PyQl=2-(pyridin-2-yl)quinoline). The copper metal-catalysed 2-electron reduction of CO₂ to CO in the presence of 2-protons is challenging. These catalysts exhibit the production of CO gas in DMF/water mixtures, achieving an impressive Faradaic efficiency of 84 % and 72 % for complex 1 and 2 at -1.7 V vs. SCE, respectively, for selective CO₂ reduction. The production of H₂ due to 2H⁺+2e^- was also observed as a byproduct through the competitive proton reduction reaction. This was cross-verified by online gas and mass analysis. A comprehensive series of electrochemical experiments have substantiated the homogeneous behaviour exhibited by these molecular electrocatalysts. Our investigations confirmed the stability of the electrocatalysts under the electrocatalytic conditions. The mechanistic pathways were proposed to work with the EECC and ECEC (E: electrochemical and C: chemical) mechanisms. A CO₂ insertion into an in-situ generated hydride from the Cu-center generates CO through the favourable path. This critical path kinetically favors excess Faradaic efficiency in 1 than 2, which agrees with the computational investigation.