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.