Metal Doping and Ligand Engineering as Tools for Tailoring the Electronic Structure of Coordination Polymers and their Oxygen Evolution Electrocatalytic Activity

Electrocatalytic production of fuels [green hydrogen from water, C1-high energy molecules from CO2, NAD(P)H from NAD(P)(+)], utilizing renewable energy sources, has in the oxygen evolution reaction (OER) its Achilles' heel. Among the catalysts developed for facilitating OER, those based on coordination polymers (CPs) are attracting considerable interest due to their simplicity of preparation and modularity. In this review, after having recalled the key-parameters that have to be considered when evaluating and benchmarking electrocatalysts, it is shown how the performance of CP-based OER catalysts can be rationally improved by a) the suitable selection of the metal ion combination and b) engineering CPs with appropriate organic linkers. It is inferred that the catalytic effect of adding a different metal ion, than that of the CP structure (metal doping), occurs through an indirect alteration of the electronic features of the active metal center, which might be described by considering the energy of the d-band with respect to that of the Fermi level (E-d), the e(g) occupancy or the metal ionic electronegativity (MIE). In rare cases, it is assumed that the added metal center can directly and synergistically participate at the catalytic process. The much less explored strategy b) to improve the performance of OER electrocatalysts, by properly engineering organic linkers, is related not only to an alteration of the electron density (acidity) of the metal center but also to a modification of the electron localization and conductivity of CPs.