Other redox flow battery technologies

Redox flow batteries, under development since the early 1970s, are stUl of interest primarily for utility load leveling applications (77). Such a battery is shown schematically in Figure 5. Unlike other batteries, the active materials are not contained within the battery itself but are stored in separate tanks. The reactants each flow into a half-ceU separated one from the other by a selective membrane. An oxidation and reduction electrochemical reaction occurs in each half-ceU to generate current. Examples of this technology include the iron—chromium, Fe—Cr, battery (79) and the vanadium redox cell (80). 

Fundamentally, both MEA-based and membraneless cells require two electrodes with an ionically conductive electrolyte between them. It is therefore proposed that a volumetric power density normalized by the essential volume of the electrochani-cal chamber, including both electrodes and the separating electrolyte, would be the most universally applicable metric for these devices. This metric captures any variations in electrolyte channel separation and electrode thickness with the only assumption being that the inlet/outlet flow field manifolds and other structural support elements are comparable between cells. With this new convention, the key microfluidic electrochemical cell technologies with the highest power densities reported to date were converted where possible and presented in Table 6.1. For comparative purposes, estimates for a typical MEA-based vanadium redox battery (VRB) [17, 18] and a DMFC [19] are also included. 

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