Controlled Formation of Multilayer Films of Discrete Molecular Catalysts for the Oxygen Reduction Reaction Using a Layer-by-Layer Growth Mechanism Based on Sequential Click Chemistry

Molecular electrocatalysts show promise for energy-relevant multielectron transformations due to their rationally tunable activity and selectivity from systematic ligand modifications. However, surface-immobilized molecular electrocatalytic systems are typically limited by low activity per geometric surface area compared to traditional solid-state analogues because of their lower active site surface coverage. Many existing methods for increasing surface coverage through the formation of multilayer films are based on radical-coupling or electropolymerization strategies that often result in dense, poorly defined films that may inhibit charge or substrate transport and complicate mechanistic studies. We report an alternative controlled layer-by-layer deposition strategy for the formation of multilayer catalyst films on carbon electrode surfaces based on sequential Cu(I)-catalyzed azide-alkyne cycloaddition reactions. As a proof of concept, we explore the growth of multilayer films of Cu(3,8-diethynylphenanthroline) for the oxygen reduction reaction. Double-layer catalyst films operate with increased activity and selectivity for the reduction of O-2 to H2O compared to single-layer catalyst films. We attribute this increased activity and selectivity to the increased coverage of the double-layer films which both increases the number of active sites and facilitates the 4e(-) reduction to H2O, rather than the 2e(-) reduction to H2O2. Unfortunately, growth of triple-layer catalyst films in this system was unsuccessful, possibly due to steric congestion in the double-layer films.