Time-Resolved IR Spectroelectrochemistry of a Surface Confined Redox Couple

Time-resolved infrared spectroelectrochemistry is a powerful tool for measuring electrode kinetics. In this work we used diazonium chemistry to graft 2-aminoanthraquinone (AQ) to the surface of a working electrode. Cyclic voltammetry and attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS) were used to study the surface redox of the grafted AQ layer in aqueous solutions and in aprotic solvents to determine the viability for electrochemically mediated carbon capture (EMCC). Electrochemical measurements confirm that at pH 13, the redox reaction does not involve proton transfer between the oxidized and reduced form of the AQ. However, surface sensitive IR reveals that the reduction processes lead to the formation of a strongly hydrogen bonded environment around the anthraquinone dianion. This hydrogen bonding network is shown to shift the apparent formal potential of the redox process resulting in an electrochemically irreversible system. Time resolved SEIRAS reveals that the radical anion coexists with the neutral AQ molecule and its dianion over a narrow range of potentials. A series of kinetic measurements were used to extract rate constants and estimates of the formal potentials for the two, one-electron transfers and indicate that the system exhibits a potential inversion and the extent of the potential inversion depends on the hydrogen bonded network. Aqueous environments are less suitable for EMCC because of the lower solubility of CO2 and for formation of carbonate species. Aprotic environments are more suitable since the CO2 is more soluble and the formation of carbonates is mitigated. In the presence of a larger cation such as tetrabutyl ammonium, the cyclic voltammogram response of the anthraquinone film gives two redox peaks that correspond to two one-electron transfer reactions. In the presence of a smaller cation such as lithium, the cyclic voltammetry response is qualitatively similar to the highly basic aqueous solution. Time-resolved ATR-SEIRAS reveals that, unlike the aqueous system, the lithium in acetonitrile exhibits normal potential ordering. Experimental data was compared to simulated data to extract rate constants and standard potentials for the two, one-electron transfers. Preliminary results show that the lithium in acetonitrile is a good candidate for implementing electrochemically mediated carbon capture.