Leveraging the Inductive Effect to Promote Oxygen Evolution on Oxides and Metal Hydroxide-Organic Frameworks

Abstract

Late transition metal oxides are reported to be the most active non-precious catalysts for the oxygen evolution reaction (OER), but the most active oxides based on Ni or Co still have turnover frequencies per metal site at least one order of magnitude lower than that of oxygen-evolving complex (OEC) in biological systems.[1] The OEC contains an intricate manganese-calcium-oxo cluster, and its unparalleled OER activity can be attributed to the unique characteristics of Mn centers regulated by Ca2+ cations. Such electronic structure tuning can be generalized as the inductive effect.[2] Specifically, substituting metal oxides and complexes using foreign metals with higher electronegativity can increase their redox potentials and promote their activity. We have employed this concept to design bismuth-substituted strontium cobaltite, where the strongly electronegative Bi3+ ions give rise to Co centers with record intrinsic OER activity in alkaline solutions.[3] Unfortunately, metal substitution in oxides exhibits a much smaller tunability of electronic structures than that found in metal-oxo clusters. For instance, Ca2+ substitution of Mn4+ in synthetic [CaMn3O4] clusters can modulate the Mn3+/4+ redox potential by ~1 V,[4] whereas negligible changes of ~0.02 V are typically observed for oxides.[5] To tackle this limitation, we have designed metal hydroxide-organic frameworks (MHOFs) that combine the great tunability of enzymatic systems with known oxide-based chemistries.[6] A series of MHOFs were constructed by transforming layered hydroxides into 2D sheets composed of metal-octahedra chains cross-linked with neighboring chains using organic linkers. MHOFs can act as a tunable platform for the OER, where the nature of π-π-interactions between adjacent stacked linkers and the transition metals in the layered metal hydroxides dictate stability and activity, respectively. Substituting MHOF nanosheets with more electron-withdrawing cations increased their OER activity, where Fe-substituted Ni-based MHOFs exhibited three orders of magnitude enhancement in activity per metal site, rivaling those of state-of-the-art OER catalysts. This enhancement was correlated with the MHOF-based modulation of Ni redox potentials and the optimized binding of reaction intermediates. These results represent a step forward toward designing metal centers with ligand fields akin to those in homogenous/enzymatic systems, where MHOFs can act as a versatile platform to develop catalysts with unparalleled tunability.