Article Cite This: J. Am. Chem. Soc. 2017, 139, 18552−18557 pubs.acs.org/JACS •− Detection of CO2 in the Electrochemical Reduction of Carbon Dioxide in N,N‑Dimethylformamide by Scanning Electrochemical Microscopy Tianhan Kai, Min Zhou, Zhiyao Duan, Graeme A. Henkelman, and Allen J. Bard* Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States *S Supporting Information ABSTRACT: 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, CO •−. However, the electrochemical detection and •− 2 •− 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 fi •− Hg/Au lm 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 ffi •− SECM substrate (e.g., an Au UME). The collection e ciency (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 CO •− can be evaluated by fitting the collection efficiency vs distance curve. The 2 •− dimerized species of CO2 , oxalate, can also be determined quantitatively. Furthermore, the 0′ ff formal potential (E ) and heterogeneous rate constant (k0) for CO2 reduction were determined with di erent quaternary fi ff ff ammonium electrolytes. The signi cant di erence in k0 is due to a tunneling e ect caused by the adsorption of the electrolytes on the electrode surface at negative potentials. ■ INTRODUCTION copy, in aqueous and aprotic solutions for the detection of •− A recent upsurge in interest in the direct and catalyzed CO2 adsorbed on the metal electrode surface. Unfortunately, electrochemical reduction of CO stems from the possibility of quantitative information and direct electrochemical behavior of 2 − converting CO into fuel for energy storage and conversion.1 3 this species has not been obtained. Fast-scan cyclic 2 voltammetry (FSCV) was used by Saveant,́ Amatore, and co- Although extensive experimental and theoretical studies of the 8 1,4 workers to study CO2 reduction in anhydrous DMF. The heterogeneous electrocatalytic (inner sphere) reactions have 0 •− standard potential (E ) for CO2/CO2 was evaluated to be been reported, the reaction mechanisms are still not clear. A − fi •− −2.21 V vs SCE at a scan rate of 4400 V s 1. The dimerization generally proposed rst step is the initial production of CO2 , 7 −1 −1 through a one electron-transfer reduction at the electrode rate constant was initially calculated to be 10 M s , but this •− value was subsequently corrected to be 5 × 108 M−1 s−1 in their surface. The generated CO2 can then be protonated, 9 dimerize, or be further reduced to the species of interest later work. However, the intrinsic disadvantages of FSCV, even depending on the electrode material and solution composition with ultramicroelectrodes (UMEs), such as interference by (reactions 1−3).4,5 large contributions from double layer capacitance charging and ff −•− adsorbed species a ect the precision of the measurements, as CO22+⇄ e CO (1) seen in earlier experiments and make study of short-lived ffi 10 •− + • species di cult at high scan rates. The above-mentioned CO22+→ H HCO (2) problems can be circumvented by using SECM to evaluate •− •− 2− electrochemically generated CO2 . Since the measurements of 2CO224→ C O (3) SECM are conducted at steady state, it has the advantage over Although CO •− was proposed decades ago as the initial transient electrochemical techniques in that it is not perturbed 2 by capacitive effects and electroactive adsorbed species.10 As a product for CO2 reduction, this species has rarely been observed experimentally, due to its extremely short lifetime in result, it is a powerful tool for the detection of extremely unstable radical species and the study of reaction mecha- either aqueous or nonaqueous solutions. Since a reliable and 10 11 12 quantitative method for its measurement has been absent, the nisms. Acrylonitrile, guanosine, and N,N-dimethylani- reaction mechanism cannot be elucidated. A fair amount of effort has been spent on exploring spectroscopic methods, such Received: August 15, 2017 as Raman6 and Fourier transform infrared (FTIR)7 spectros- Published: December 8, 2017 © 2017 American Chemical Society 18552 DOI: 10.1021/jacs.7b08702 J. Am. Chem. Soc. 2017, 139, 18552−18557 Journal of the American Chemical Society Article line13 radicals in aprotic solution and Sn(III)14 and super- Ag+) and water during SECM experiments, a Pt/PPy reference oxide15 intermediate in aqueous solution were studied using electrode was fabricated by electrochemical deposition of a polypyrrole fi SECM by taking advantage of the micro- or nanogap between (PPy) lm onto a Pt wire in CH3CN containing 0.1 M TBAPF6 and ff 0.01 M pyrrole as described earlier.22 The obtained reference electrode SECM tip and substrate. Moreover, the e ect of tunneling on fi 2+ was placed into a glass tube lled with DMF containing 0.1 M TBAPF6 the electron transfer rates of Ru(bpy)3 oxidation and 16 with a frit at one end. Good potential stability of Pt/PPy reference reduction and the kinetics and mechanism of radical ion electrode was observed, and the potential of Pt/PPy was calibrated to annihilation mode of electrogenerated chemiluminescence be −0.19 V vs aqueous saturated calomel electrode (SCE). 17 (ECL) were also investigated within the nanogap. Here, we Instrumentation. The electrochemical measurements were carried •− propose a method to detect CO2 using the TG/SC mode of out with a CHI 660C potentiostat (CH Instruments, Austin, TX). A SECM in DMF. Taking advantage of the nanogap and CHI 920C SECM workstation (CH Instruments, Austin, TX) for micrometer-sized electrode, we can prove the existence of SECM measurements was placed inside an argon atmosphere drybox •− fi to avoid contamination of solution from water and oxygen. Carbon CO2 electrochemically at steady state. Importantly, by tting the collection efficiency of CO •− at different distances, the dioxide (10%) was introduced to the SECM cell directly through •− 2 bubbling DMF solution for about 5 min before the experiments. The half-life of CO2 and kinetic parameters for dimerization can concentration of CO2 was estimated to be 20 mM in DMF based on be obtained. The high energetic barrier associated with a large an earlier report.23,24 The electrodes and cell configuration are shown structural change, from a linear to a V-shape as shown, for in Figure 1.CO2 was reduced on a hemisphere-shaped Hg/Pt UME (a example, by DFT calculation, explains the relatively small rate of the reduction reaction. Moreover, the significant difference 0 in the heterogeneous rate constant (k ) for CO2 reduction in the presence of different tetra-alkylammonium electrolytes was observed and was attributed to the effect of tunneling, through a layer of the adsorbed electrolyte at the electrode surface at relatively negative potentials. In addition, SECM can show the distribution of the products of CO2 reduction in real-time without the need for additional analytical instruments. The obtained results indicate the possibility of utilizing SECM as an ffi e cient tool to study the mechanism of CO2 reduction in the •− presence of different homogeneous and heterogeneous Figure 1. Schematic depiction of the collection of the CO2 radical in catalysts. TG/SC mode of SECM. ■ EXPERIMENTAL SECTION Chemicals and Materials. Mercurous nitrate (Hg (NO ) ; ≥ =5μm), which was utilized as the SECM tip. The predominant 2 3 2 •− 97%), potassium nitrate (KNO3; trace metal basis, 99%), nitric acid species, oxalate, was generated by CO2 dimerization at the upper (HNO ; trace metal basis), acetonitrile (CH CN; 99.9%), decame- portion of the hemisphere, whereas at lower portion of the 3 3 •− thylferrocene (DMFc; 99%), anhydrous N,N-dimethylformamide hemisphere, CO2 was captured by the SECM substrate (e.g., a a = (DMF; 99.9%) were purchased from Fisher Scientific (Pittsburgh, 12.5 μm Au UME). A Pt/PPy electrode and a Pt wire served as the fl ≥ PA). Tetrabutylammonium hexa uorophosphate (TBAPF6; 99%), reference and counter electrodes, respectively. An accurate alignment fl ≥ tetrapropylammonium hexa uorophosphate (TPAPF6; 98%), tetrae- of the SECM tip and substrate was required before the SECM fl ≥ thylammonium hexa uorophosphate (TEAPF6; 99%), tetrahepty- experiments. The tip and substrate were initially aligned with two lammonium tetraphenylborate (THeATPB; selectophore), tetraocty- digital microscopes (Park, Santa Clara, CA), and then a more accurate lammonium bromide (TOABr; ≥98%), and pyrrole (99%) were alignment was accomplished through a x and y-axis probe scanning acquired from Sigma-Aldrich (St. Louis, MO). All reagents were used curve, combined with the substrate generation/tip collection (SG-TC) without further purification. All aqueous solutions were prepared with mode of SECM.13 DMFc was utilized as the redox-mediator to ultrapure water with the resistivity of 18.3 MΩ/cm from a Milli-Q monitor the tip position when it was in close proximity to the substrate Integral system (EDM Millipore, Berllerica, MA). Argon (5.0 UHP) and to determine the gap spacing (d). As indicated in Figure S2A, well- and carbon dioxide (10%, balance nitrogen, 4.0 UHP) gas were defined steady-state currents of 1 mM DMFc were obtained in μ obtained from Praxair. anhydrous DMF containing 0.1 M TBAPF6 for a a =5 m Hg/Pt Electrode Fabrication.