Detection of CO2•- in the Electrochemical Reduction of Carbon Dioxide in N,N-Dimethylformamide by Scanning Electrochemical Microscopy.

The electrocatalytic reduction of CO2 has been studied extensively and produces a number of products. The initial reaction in the CO2 reduction is often taken to be the 1e formation of the radical anion, CO2•-. However, the electrochemical detection and characterization of CO2•- is challenging because of the short lifetime of CO2•-, which can dimerize and react with proton donors and even mild oxidants. Here, we report the generation and quantitative determination of CO2•- in N,N-dimethylformamide (DMF) with the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM). CO2 was reduced at a hemisphere-shaped Hg/Pt ultramicroelectrode (UME) or a Hg/Au film UME, which were utilized as the SECM tips. The CO2•- produced can either dimerize to form oxalate within the nanogap between SECM tip and substrate or collected at SECM substrate (e.g., an Au UME). The collection efficiency (CE) for CO2•- depends on the distance (d) between the tip and substrate. The dimerization rate (6.0 × 108 M-1 s-1) and half-life (10 ns) of CO2•- can be evaluated by fitting the collection efficiency vs distance curve. The dimerized species of CO2•-, oxalate, can also be determined quantitatively. Furthermore, the formal potential (E0') and heterogeneous rate constant (k0) for CO2 reduction were determined with different quaternary ammonium electrolytes. The significant difference in k0 is due to a tunneling effect caused by the adsorption of the electrolytes on the electrode surface at negative potentials.