We quantitatively characterized oxygen reduction kinetics at the nanoscale Pt|CsHSO_4 interface
at ~150 °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 Pt|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 Pt|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 Pt|CsHSO_4 experiments, within the Butler-Volmer framework, yielded
exchange coefficients (α) 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 α) 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
