Electrical energy conversion and storage technologies are crucial to achieve a carbon-neutral energy supply based on renewable energies. Redox-flow batteries (RFBs) are one key technology for the large-scale, stationary and decentralized storage of electrical energy. Compared with conventional secondary batteries (e.g., lithium-ion batteries), RFBs feature low cost and long service life. Their unique function allows decoupling of energy storage capacity and power density, making RFBs highly flexible and scalable. While the energy content of RFBs can easily be increased by increasing the volume of the electrolyte, reaching higher volumetric energy density is still a major challenge: the energy density of RFBs is directly proportional to the molar concentration of the active compound in the electrolyte, the number of electrons which can be reversibly exchanged, as well as the potential range in which the system operates, see equation 1: