Immobilising molecular redox mediators for the oxygen evolution reaction using self-assembled monolayers

Abstract

The development of efficient and economical electrocatalysts for the oxygen evolution reaction (OER) that are stable and free of noble metals remains a significant scientific and technological challenge. The use of redox mediators (RMs) offers a promising approach to enhance the efficiency of electrocatalysts for a range of applications. However, the migration of the RM molecules between the electrodes, also known as shuttle effect, leads to undesirable redox side reactions and a reduction of the OER performance. Here, we introduce a novel approach to overcome this limitation by showing how covalently attaching RMs to the electrode surface through self-assembled monolayers (SAMs) is a promising route to immobilize them and prevent their diffusion into the electrolyte. For this purpose, we prepared different SAMs using two types of RMs based on tetrathiafulvalene (TTF) derivatives and using indium tin oxide (ITO) and fluorine doped tin oxide (FTO) as substrates. All electrodes showed efficient electrocatalytic activity under alkaline conditions. In this small proof-of-concept set of systems, we could achieve an OER performance with an overpotential of 400 mV at 0.25 mA cm⁻² and a Tafel slope of 103 mV dec⁻¹. We rationalise these experimental findings with computational chemical modelling, which suggests that further improvements could be achieved through targeted chemical modifications to tune the highest occupied molecular orbital energy in these TTF-based RMs. Thus, this work demonstrates that covalent immobilization of RMs via SAMs offers a robust platform for the rational molecular engineering of electrocatalysts.