Reversible Adsorption of Outer-Sphere Redox Molecules at Pt Electrodes

ABSTRACT: Adsorption often dominates the response of nanofluidic systems due to their high surface-to-volume ratios. Here we harness this sensitivity to investigate the reversible adsorption of outer-sphere redox species at electrodes, a phenomenon that is easily overlooked in bulk measurements. We find that even though adsorption does not necessarily play a role in the electron-transfer process, such adsorption is nevertheless ubiquitous for the widely used outer-sphere species. We investigate the physical factors driving adsorption and find that this counterintuitive behavior is mediated by the anionic species in the supporting electrolyte, closely following the well-known Hofmeister series. Our results provide foundations both for theoretical studies of the underlying mechanisms and for contriving strategies to control adsorption in micro/nanoscale electrochemical transducers where surface effects are dominant. SECTION: Liquids; Chemical and Dynamical Processes in Solution A n ideal outer-sphere electron-transfer reaction between a molecule in solution and an electrode involves the tunnelling of one or more electrons without significant chemical interactions developing between the two. This is in contrast with inner-sphere reactions that by definition entail some form of linkage between the molecule and the surface − ranging from covalent binding to adsorption at specific sites − before the reaction can proceed. 1 Outer-sphere reactions are often associated with fast electron transfer that is relatively independent of the nature of the substrate. Classification into inner-and outer-sphere reactions, however, pertains only to the electron-transfer mechanism. A reaction proceeding along an outer-sphere pathway does not in itself rule out the possibility of residual interactions between the electrode and the molecule(s) involved, insofar as such interactions do not influence the electron-transfer process. A prototypical illustration is the case of a redox species immobilized at an electrode, an approach that was famously employed to quantitatively study electron tunnelling in outersphere reactions. 2 One can similarly envision that residual interactions between the reduced or oxidized forms of a redox species and an electrode lead to some degree of weak adsorption. While not directly involved in electron transfer, such adsorption can nonetheless affect electrochemical experiments by influencing mass transport. It is widely accepted that small cations and anions in solution can adsorb onto solid electrodes. 3 In general terms, adsorption of a particular species results from a difference in (Gibbs) free energy between the species in bulk solution and at the surface. The microscopic driving mechanism can be enthalpic, entropic, or some combination thereof, and the degree of adsorption is thus sensitive to a broad range of tunable parameters that include temperature, the nature of the solvent, the degree of electrification of the electrode, the presence of other species at the surface and, of course, the identity of the adsorbing species. For example, the degree of specific adsorption of anions on metal electrodes is ion-specific and increases in the order F