How Metal/Insulator Interfaces Enable an Enhancement of the Hydrogen Evolution Reaction Kinetics

The nanostructuring of electrodes is a common way of increasing electrocatalytic activity. Yet, the fact that the presence of insulating material in nanostructured composites can have a positive effect on efficiency was an unexpected recent finding. The rate enhancement has been linked to different electric fields at the insulator and metal interfaces, facilitating enhanced transport of reaction products into the bulk electrolyte. In this article, we further uncover the origin of the rate enhancement with parameter studies and simulations. We experimentally investigate various parameter dependencies of the alkaline Hydrogen Evolution Reaction (HER) on well-defined nanometer-sized Au arrays embedded in a silicon nitride insulating layer. We find a significant enhancement of the HER for all experimental conditions and opposite activity trends with pH, electrolyte concentration and the cationic species compared to a continuous Au electrode. Using a mean field model, we quantify the electrostatic interfacial pressure above the Au and the insulator patches. Combining the double layer simulations with rate equations, we demonstrate that all parameter variations can be consistently explained by the fact that the double layer structure above the insulator patches is much less rigid than above the metal islands and is independent of the applied potential.