Designing Redox-Stable Cobalt-Polypyridyl Complexes for Redox Flow Batteries: Spin-Crossover Delocalizes Excess Charge

Abstract

Redox-active organometallic molecules offer a promising avenue for increasing the energy density and cycling stability of redox flow batteries. The molecular properties change dramatically as the ligands are functionalized and these variations allow for improving the solubility and controlling the redox potentials to optimize their performance when used as electrolytes. Unfortunately, it has been difficult to predict and design the stability of redoxactive molecules to enhance cyclability in a rational manner, in part because the relationship between electronic structure and redox behavior has been neither fully understood nor systematically explored. In this work, rational strategies for exploiting two common principles in organometallic chemistry for enhancing the robustness of pseudo-octahedral cobalt-polypyridyl complexes are developed. Namely, the spin-crossover between low and highspin states and the chelation effect emerging from replacing three bidentate ligands with two tridentate analogues. Quantum chemical models are used to conceptualize the approach and make predictions that are tested against experiments by preparing prototype Co-complexes and profiling them as catholytes and anolytes. In good agreement with the conceptual predictions, very stable cycling performance over 600 cycles is found.