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

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-tetra­methyl-1-piperi­dinyl­oxy (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.