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The sources of capacity fade in VRFBs can be broadly divided into reversible decay due to vanadium crossover that is assumed to be fully recoverable via electrolyte rebalancing, and irreversible materials decay that requires component replacement for performance recovery. Regarding the latter, we assume the tanks, pipes, and pump linings are composed of plastic [20,22,71], and the performance decay of wetted reactor components including electrodes and membranes is minor in a well-engineered system as these processes have been reported slow under typical operating conditions [6,19]. This is further supported by the reported lifetimes of commercial VRFB systems, 25,000 cycles or 20 years with stable energy efficiency [72–74]. We acknowledge the potential of other failure modes, such as capacity loss due to leaks and system degradation due to control strategy faults, but elect to ignore these effects as we were unable to find reports in the peer-reviewed literature that articulate the likelihood of such failures or the magnitude of their effects. Inclusion of such considerations would simply have a scalar effect on the overall fade rate or maintenance costs, which could be easily modified by the user, though we do provide a sensitivity analysis on fade rate to address part of this consideration (Figure II-5). This leaves electrolyte decay resulting from species precipitation due to heat or air exposure and/or side reactions that shift the average valence away from 3.5+. We assume precipitated active species can be recovered via simple maintenance operations such as reversing the cell polarity or partial mixing of the reservoirs to electrochemically dissolve V(V) solids [8]. Thus, in this model we only consider the shift in average oxidation state over time that is caused by coulombic inefficiencies (i.e., side reactions) and accounts for ca. 12.5% of the overall fade (see section S3 of the SI of the published version of this chapter for further details [60]). Specifically, we consider the parasitic hydrogen evolution reaction that occurs at the negative half-cell during charging in place of the desired reduction of V(III) to V(II), which is often cited as the most common and prominent cause of VRFB electrolyte decay [8,45,75]. Hydrogen evolution leaves an imbalance at the end of charging as there is all V(V) in the positive electrolyte but some amount of V(III) remaining in the negative electrolyte, and this results in a capacity loss that cannot be recovered via rebalancing. There are methods for reversing this imbalance, such as the addition of chemical reducing agents that will reduce V(V) to V(IV) and thus balance the positive electrolyte based on the composition of the negative electrolyte [76]. A common reductant is oxalic acid (C2H2O4), which reduces V(V) via the reaction shown in Equation II-4. Note, we assume the electrolyte dilution from the 25PDF Image | Bringing Redox Flow Batteries to the Grid
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