Kinetic Analysis on the Role of Bicarbonate in Carbon Dioxide Electroreduction at Immobilized Cobalt Phthalocyanine

Abstract

The mechanism for carbon dioxide reduction (CO2RR) to carbon monoxide (CO) at immobilized cobalt phthalocyanine (CoPc) in aqueous electrolytes has been widely debated. In this work, we investigated the mechanism of CO2RR to CO on CoPc via experimental reaction kinetics coupled with model fitting. Unexpectedly, reactant order dependences and Tafel slopes deviate from commonly expected values and change depending on the testing conditions. For example, (1) the effect of bicarbonate deviates from power law kinetics and transitions from inhibitory to promotional with increasingly reductive potential, and (2) the CO2 order dependence deviates from unity at more-reductive potentials. We propose a kinetic model, chosen from more than 15 candidate models, that is able to quantitatively fit all of the experimental data. The model invokes (1) catalyst poisoning via bicarbonate electrosorption, (2) mixed control between concerted proton-electron transfer (CPET) and sequential electron transfer-proton transfer (ET-PT), and (3) both water and bicarbonate as kinetically relevant proton donors. The proposed model also predicts that the relative importance of the above factors changes depending on the reaction conditions, highlighting the potential downfalls of broadly applying reaction mechanisms that were inferred from kinetic data collected in a narrow range of testing conditions. This study emphasizes the importance of cohesively using kinetic data collected over a wide range of operating conditions to test and formulate kinetic models of electrocatalytic reactions.