Redox flow batteries (RFBs) are particularly well suited for storing the intermittent excess supply of renewable electricity, so-called "junk" electricity. Conventional RFBs are charged and discharged electrochemically, with electricity stored as chemical energy in the electrolytes. In the RFB system reported here, the electrolytes are conventionally charged but are then chemically discharged over catalytic beds in separate external circuits. The catalytic reaction of particular interest generates hydrogen gas as secondary energy storage. For demonstration, indirect water electrolysis was performed generating hydrogen and oxygen in separate catalytic reactions. The electrolyte containing V(II) was chemically discharged through proton reduction to hydrogen on a molybdenum carbide catalyst, whereas the electrolyte comprising Ce(IV) was similarly discharged in the oxidation of water to oxygen on a ruthenium dioxide catalyst. This approach is designed to complement electrochemical energy storage and may circumvent the low energy density of RFBs especially as hydrogen can be produced continuously whilst the RFB is charging. Broader contextRenewable energy technologies have evolved to deliver hundreds of terawatt hours of electricity, yet without its direct utilization in the grid part of that energy could be lost. In order to establish a thriving renewable energy economy it is of paramount importance that intermediate energy storage systems be developed. Mediating electricity production and usage will overcome the issues relating to intermittency, which presently limits widespread dependence on wind and photovoltaic power. Various approaches are under development, but no single approach is liable to address the issue as a whole. Combining technologies and hybridizing storage systems to adapt to a multifaceted energy future is the more viable option. This paper discusses one such hybrid system, in which electrochemical energy storage is combined with renewable hydrogen production, delivering a dual platform for energy storage as an electrochemical and chemical medium. † Electronic supplementary information (ESI) available: Charging and discharging curves, cyclic voltammetry, UV-vis spectra of V(II) and V(III), a picture of the cell, and details on the kinetics measurement and calculations. See

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