Highly reduced and protonated aqueous solutions of [P2W18O62]6- for on-demand hydrogen generation and energy storage.

As our reliance on renewable energy sources grows, so too does our need to store this energy to mitigate against troughs in supply. Energy storage in batteries or by conversion to chemical fuels are the two most flexible and scalable options, but are normally considered mutually exclusive. Energy storage solutions that can act as both batteries and fuel generation devices (depending on the requirements of the user) could therefore revolutionize the uptake and use of renewably generated energy. Here, we present a polyoxoanion, [P2W18O62]6-, that can be reversibly reduced and protonated by 18 electrons/H+ per anion in aqueous solution, and that can act either as a high-performance redox flow battery electrolyte (giving a practical discharged energy density of 225 Wh l-1 with a theoretical energy density of more than 1,000 Wh l-1), or as a mediator in an electrolytic cell for the on-demand generation of hydrogen.