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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 × 10[sup 8] M[sup -1]> s[sup -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. [ABSTRACT FROM AUTHOR]

Oxalate oxidation in the presence of different oxidized luminophores leads to the emission of light and has been studied extensively in electrogenerated chemiluminescence (ECL). The proposed mechanism involves the initial formation of the oxalate radical anion, C 2 O 4 •- . The ensuing decomposition of C 2 O 4 •- produces a very strong reductant, CO 2 •- , which reacts with the oxidized luminophores to generate excited states that emit light. Although the mechanism has been proposed for decades, the experimental demonstration is still lacking, because of the complexity of the system and the short lifetimes of both radical anions. To address these issues, we studied oxalate oxidation at platinum ultramicroelectrodes (UMEs) in anhydrous N, N-dimethylformamide (DMF) solution by nanoscale scanning electrochemical microscopy (SECM) with the tip generation/substrate collection (TG/SC) mode. A Pt nanoelectrode was utilized as the SECM generator for oxalate oxidation, while another Pt UME served as the SECM collector and was used to capture the generated intermediates. We studied the influence of the gap distance, d, on the substrate current ( i s ). The results indicate that, when 73 nm < d < 500 nm, the species captured by the substrate were primarily CO 2 •- , while C 2 O 4 •- was the predominant intermediate measured when d was below 73 nm. A half-life of 1.3 μs for C 2 O 4 •- was obtained, which indicates a stepwise mechanism for oxalate oxidation. The relevance of these observations to the use of oxalate as the coreactant in ECL systems is also discussed.

The electrocatalytic reduction of CO 2 has been studied extensively and produces a number of products. The initial reaction in the CO 2 reduction is often taken to be the 1e formation of the radical anion, CO 2 •- . However, the electrochemical detection and characterization of CO 2 •- is challenging because of the short lifetime of CO 2 •- , which can dimerize and react with proton donors and even mild oxidants. Here, we report the generation and quantitative determination of CO 2 •- in N,N-dimethylformamide (DMF) with the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM). CO 2 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 2 •- 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 2 •- depends on the distance (d) between the tip and substrate. The dimerization rate (6.0 × 10 8 M -1 s -1 ) and half-life (10 ns) of CO 2 •- can be evaluated by fitting the collection efficiency vs distance curve. The dimerized species of CO 2 •- , oxalate, can also be determined quantitatively. Furthermore, the formal potential (E 0 ') and heterogeneous rate constant (k 0 ) for CO 2 reduction were determined with different quaternary ammonium electrolytes. The significant difference in k 0 is due to a tunneling effect caused by the adsorption of the electrolytes on the electrode surface at negative potentials.