Redox flow batteries are an emerging technology for electrochemical energy storage that could help enhance the use of power produced by renewable energy resources. These power resources are inherently irregular in their supply, which doesn’t typically align with demand on the power grid. In principle, redox flow batteries can be designed to have an energy-storage capacity that is independent of its power rating. However, in practice, the ease with which redox-active molecules are transported to electrode surfaces plays an important role in determining their efficiency, the power that is produced or charged and, in some instances, their length of life.