Conductive mesoporous catalytic films. Current distortion and performance degradation by dual-phase ohmic drop effects. Analysis and remedies

In the active interest aroused by catalysis of electrochemical reactions, particularly molecule activation related to modern energy challenges, mesoporous films deposited on electrodes are often preferred to catalysts homogeneously dispersed in solution. Conduction in the solid portion of the film and in the pores may strongly affect the characteristic catalytic Tafel plots, possibly leading to mechanistic misinterpretation and also degrade the catalytic performances. These ohmic drop effects take place, unlike those classically encountered with a massive electrode immersed in an electrolytic solution, in two different zones of the film, the solid bulk of the film and the pores, that are coupled together by a distributed capacitance and by the faradaic impedance representing the catalytic reaction located at their interface. A transmission line modeling allows the analysis of the capacitance charging responses as a function of only two dimensionless parameters in the framework of linear scan voltammetry: the ratio of the resistances in the two parts of the film and of the time-constant of the film. After validation with an experimental system consisting of an ionic polymer/carbon powder mixture, deposited on a glassy carbon electrode and immersed in a strong electrolyte aqueous solution, a procedure is established that gives access the key-conduction parameters of the film. On these bases, and of the predicted current-potential responses for fast catalytic reactions according to the same transition line model, it is shown how the dual-phase ohmic drop effects can be gauged and compensated. Ensuing consequences on optimization of macroelectrolysis are finally discussed.