Nanoscale electrodes by conducting atomic force microscopy: oxygen reduction kinetics at the Ptmid R:CsHSO4 interface

We quantitatively characterized oxygen reduction kinetics at the nanoscale Ptmid R:CsHSO(4) interface at approximately 150 degrees C in humidified air using conducting atomic force microscopy (AFM) in conjunction with AC impedance spectroscopy and cyclic voltammetry. From the impedance measurements, oxygen reduction at Ptmid R:CsHSO(4) was found to comprise two processes, one displaying an exponential dependence on overpotential and the other only weakly dependent on overpotential. Both interfacial processes displayed near-ideal capacitive behavior, indicating a minimal distribution in the associated relaxation time. Such a feature is taken to be characteristic of a nanoscale interface in which spatial averaging effects are absent and, furthermore, allows for the rigorous separation of multiple processes that would otherwise be convoluted in measurements using conventional macroscale electrode geometries. The complete current-voltage characteristics of the Ptmid R:CsHSO(4) interface were measured at various points across the electrolyte surface and reveal a variation of the oxygen reduction kinetics with position. The overpotential-activated process, which dominates at voltages below -1 V, was interpreted as a charge-transfer reaction. Analysis of six different sets of Ptmid R:CsHSO(4) experiments, within the Butler-Volmer framework, yielded exchange coefficients (alpha) for charge transfer ranging from 0.1 to 0.6 and exchange currents (i(0)) spanning 5 orders of magnitude. The observed counter-correlation between the exchange current and exchange coefficient indicates that the extent to which the activation barrier decreases under bias (as reflected in the value of alpha) depends on the initial magnitude of that barrier under open circuit conditions (as reflected in the value of i(0)). The clear correlation across six independent sets of measurements further indicates the suitability of conducting AFM approaches for careful and comprehensive study of electrochemical reactions at electrolyte-metal-gas boundaries.