Abstract
The kinetics of the electrochemical Ce(III)/Ce(IV) redox couple in sulfuric acid on polycrystalline gold electrodes reveal an intriguing behavior. With an experimental approach to separately investigate the kinetics of Ce(III)oxidation and Ce(IV) reduction within the same potential range, we found perfect Butler–Volmer behavior with an extreme asymmetry: Accurate linear fits of the kinetic current Tafel plots yielded anodic and cathodic transfer coefficients of αanodic = 0.84 ± 0.02 and αcathodic = 0.157 ± 0.006, respectively, adding up to 1.00 ± 0.02, thus fulfilling the theoretical requirement of αanodic = (1 – αcathodic) at an experimentally unique precision. Several different flavors of Marcus theory could not reproduce the data with the same precision as the phenomenological asymmetric Butler–Volmer equation. Instead, our results suggest that the Ce(III)/Ce(IV) kinetics on gold electrodes in sulfuric acid are determined by a combination of the Frumkin effect and the field effect on the dissociation of cerium–sulfate complexes. The same mechanism could be responsible for a surprising enhancement of the Ce(IV) reduction kinetics observed during the reduction of an electrochemically preoxidized gold surface. Although the reaction proceeds in the outer (diffuse) part of the electrochemical double-layer, its kinetics are found to be sensitive to specific properties of the electrode material and surface, namely the potential of zero charge and the presence of surface adsorbates. Our findings could thus provide an example how electrocatalysis can act in the diffuse layer beyond the outer Helmholtz plane. The electrochemical Ce(III)/Ce(IV) redox couple on gold electrodes in sulfuric acid therefore represents a highly interesting model reaction for further development of fundamental electrocatalysis both from the experimental and theoretical perspective, with possible relevance, for example, for redox flow battery development