The need for decarbonization, the advantages of electrical-grid decentralization, and the intermittency of wind and solar energy are motivations for the development of grid-scale storage technologies. The aqueous redox flow battery (ARFB) is a promising solution, featuring safety, the decoupling of energy and power capacities, and potential long lifetime, recyclability, and low cost. Redox-active materials have been rapidly developed to approach the requirements for practical grid-scale storage.