Molecular Anodes for Electrocatalytic Water Oxidation Based on Self-Assembled Bilayers Driven by Electron Transfer Mediators

Abstract

The generation of solar fuels via water splitting with sunlight requires, among others, robust and efficient electrodes for the water oxidation reaction. For this purpose, the combination of powerful molecular catalysts and graphitic materials has been shown to work outstandingly well. However, in oxide-based materials, that are of enormous importance as conductive or semiconductive materials, the molecular catalysts either do not work or are transformed into the corresponding oxides. Here, we use a supramolecular strategy based on self-assembled bilayers where a silanolate with long alkyl chains is bonded to the electrode surface and acts as a platform to supramolecularly interact with multiple long alkyl chains attached to the water oxidation catalyst. In this manner, the catalyst and the electrolyte assembly are isolated from the oxide surface, conferring great stability, but at the same time, they are sufficiently close so that efficient electron transfer can take place from the catalyst to the electrode. Our best hybrid molecular anode works efficiently as a water oxidation catalyst at pH 7 without practically any activity losses, at current densities of 0.40 mA/cm2 for 15 h, giving more than 33,800 TONs and with a Faradaic efficiency of over 92% while maintaining intact its molecular nature. This work provides a successful proof of concept of the benefit of properly combining molecular catalysts with oxide-based materials to obtain the best of both worlds.