Molecular Electrocatalysis in Confined Spaces: Analysis of the Cyclic Staircase and Cyclic Voltammetry Responses.

Abstract

Molecular electrocatalytic processes in confined environments are becoming relevant processes with many applications in electrosynthesis, electroanalysis, and electrical energy generation and conversion. Nevertheless, the analysis of catalytic responses is mostly carried out with theoretical frameworks developed for semi-infinite linear diffusion conditions, which are not applicable for the adequate understanding of electrochemical processes in confined spaces. To fill the existing gap in the comprehension of these complex reactions, the analysis of a molecular catalytic process under finite diffusive conditions for cyclic staircase voltammetry (CSCV) and cyclic voltammetry (CV) techniques is presented in this work. The proposed model considers a finite diffusive field of thickness L under two configurations: bounded diffusion, where no mass renovation is allowed, and unbounded diffusion, where there is effective mass replenishment at L. Expressions for the current-potential responses under different particular cases have been obtained, leading to a kinetic zone diagram for limiting cases in terms of two key variables related to the thickness of the solution region and the catalytic rate constant. From the general expression of the current, it is observed that the electrochemical response of molecular electrocatalytic processes taking place in confined spaces is strongly dependent on the mass transport conditions. Thus, under bounded diffusion, a decrease of the catalytic current with L is observed, which is more pronounced when the diffusive field is narrower. On the other hand, unbounded conditions give rise to an enhancement of the catalytic current and, eventually, to the loss of the kinetic sensitivity of the response for small enough values of L. An experimental application of the theoretical results is performed for the conversion of isopropyl alcohol (IPA) to acetone mediated by the oxidation of 4-methoxy-2,2,6,6-tetra-methyl-1-piperi-dinyl-oxy (4-methoxy-TEMPO radical) at a glassy carbon electrode for both bound and unbounded configurations. The catalytic rate constant for this process has been obtained from the equations for the current, indicating that the accuracy of the result is strongly dependent on the correct understanding of the mass transport influences at play.