Redox flow batteries (RFBs) are advantageous for energy storage because they are capable of tolerating fluctuating power supplies, repetitive charge/discharge cycles at maximum rates, overcharging, overdischarging, and/or because cycling can be initiated at any state of charge.

However, several disadvantages exist among the many redox couples upon which redox flow batteries are based. For example, many systems utilize redox species that are unstable, are highly oxidative, are difficult to reduce or oxidize, precipitate out of solution, and/or generate volatile gases. One of the main challenges confronting RFB systems is the intrinsically low energy density compared with other reversible energy storage systems, such as lithium-ion batteries. Additionally, many redox flow battery systems use electrolytes containing transition metals, such as the vanadium RFBs, which increases the cost of the electrolyte and thus the overall system. Therefore a need exists for RFB systems having a greater energy density and/or a lower cost electrolyte.

To address this need, PNNL has developed technology that includes embodiments of aqueous soluble organic (ASO)-based catholytes and anolytes for use in an RFB system.  The aqueous catholyte may comprise (i) an optionally substituted thiourea or a nitroxyl radical compound and (ii) a catholyte aqueous supporting solution. The aqueous anolyte may comprise (i) metal cations or a viologen compound and (ii) an anolyte aqueous supporting solution. The catholyte aqueous supporting solution and the anolyte aqueous supporting solution independently may comprise (i) a proton source, (ii) a halide source, or (iii) a proton source and a halide source.