Redox-flow batteries and their latest developments : will they be enough to finally establish themselves commercially for mass storage ?

Abstract

Camacho's flow battery, a modification of the dichromate battery invented by Poggendroff in 1842, was the first draft of what would later become a redox flow battery (RFB). The first application of such a battery was by aeronaut and aviation pioneer Charles Renard, who in 1884 developed the airship La France , featuring a propeller driven by an electric motor and powered by a _Zn/Cl2 redox flow battery. The redox flow battery is an electrochemical system capable, like batteries or fuel cells, of transforming chemical energy into electrical energy. However, it differs from batteries in that the solid electrodes are replaced by liquids, within which the redox couple is located. Although based on the use of a liquid flow, this system also differs from fuel cells in that it is a closed, reversible system. As already mentioned, a wide variety of aqueous flow batteries are available. The advantages of redox flow systems lie i) in the possibility of decoupling power and autonomy, the latter being controlled by the size of the tanks containing catholyte and anolyte, and ii) in the fact that they use an aqueous electrolyte and are therefore attractive for sustainable development.

However, this technology has never been able to take on the scale that was initially allocated to it, due to cost issues. Numerous approaches have been pursued in recent years to circumvent this problem by playing on potential and capacity. They have been listed in this lecture. The first is the realization of a non-aqueous Li-based RF system (gain in potential) by a team at MIT, with the novelty of using a catholyte in the form of ink rather than liquid (gain in capacitance). Keeping the ink catholyte concept, aqueous Li-ion batteries were then developed. Finally, two-compartment redox flow systems separated by an inorganic ionic conductor were developed. To reduce the cost of such systems, efforts were made to develop membrane-free systems, such as the lead-acid redox flow battery or the semi-liquid Li/polysulfide membrane-free redox flow battery. Other membrane-free systems take advantage of microfluidics and laminar flow to limit diffusion between catholyte and anolyte. Alternatively, a more reliable method is to use an electrolyte to limit species diffusion.

Finally, current work aimed at giving the electrolyte an active rather than passive role was presented for different battery configurations (conventionalredox flow, semi-solid Li-ion RF, membrane-free RF, multi-membrane RF, redox-mediated RF).