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

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