Active Site Determination of Heterogenized Molecular Electrocatalysts

Abstract

As molecular catalysts are increasingly employed in heterogenized systems such as CO₂ electroreduction, a need arises for more systematic approaches to characterize their preparation, distribution, and activity. Current means of classifying electroactive versus spectator molecules are insufficient, while improvement of ink formulations, depositions, and distributions relies primarily on indirect links to electrochemical performance. In this study, we expand the common utilization of Cyclic Voltammetry (CV) in homogeneous systems toward heterogenized molecular catalyst architectures. We illustrate how, even with porous catalyst layers containing carbon, ionomer, and molecules, a combined redox wave integration and UV-vis analysis can be used as a tool for designing a reproducible deposition procedure. An in-depth CV analysis is then used to study the effect of catalyst aggregation and quantify the number of electroactive sites on carbon supports. We show that FeTPP (Iron(III)meso-tetraphenylporphyrin chloride) gives a non-linear electroactive response when loading is varied, allowing for the identification of distinct loading regions of insufficient, optimal, and excessive coverage. A FeTPP to Vulcan carbon mass ratio of 0.1 provides the highest number of electroactive species, giving the lowest expected aggregation. Overall, the CV approaches are extendable to any redox-active catalysts, providing a versatile means of characterizing porous heterogeneous molecular catalyst systems.