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for relatively high-cost membranes are other issues that have plagued the VRB system to date. In comparison, the Fe-Cr flow battery uses relatively low-cost elements, yet its Cr2+/Cr3+ redox couple demonstrates an inferior electrochemical activity that requires a combination of elevated operation temperature (at > 40 °C) and a catalyst-loaded electrode to enhance its redox reaction. Besides, the Fe-Cr system also involves hydrogen gas evolution and is thus complicated by gas management issues, limiting fuel use (~60%) while lowering the coulombic efficiency[30]. New Discoveries at PNNL Researchers at PNNL have recently revisited the solution chemistry of the vanadium electrolytes. It was found that the <1.7M concentration limit of the vanadium sulfate electrolyte is a compromise between the low solubility of VOSO4 at low temperatures and the precipitation of V2O5 at higher temperature[31]. With advanced nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) based computational modeling, the structure and kinetics of vanadium (IV) [32] and vanadium (V) [33] species were further investigated. The scientific study revealed that the vanadium (IV) species in a hydrated vanadyl ion ([VO(H2O)5]2+) form an octahedral coordination with vanadyl oxygen in the axial position, with the remaining positions occupied by water molecules. An increase in the sulfate acid concentration would lead to an increase of sulfate anion in the second-coordination sphere of the vanadyl ion, resulting in an increase in re-orientation activation energy for the vanadyl ion. As such, the VOSO4 solubility decreases with increasing sulfate acid concentration[34]. On the other hand, the vanadium (V) species was identified as a hydrated penta co-ordinated vanadate ion [VO2(H2O3)]1+, which is not stable at elevated temperature and changes into neutral H3VO4 molecules via a deprotonation process, subsequently leading to an irreversible precipitation of the solid V2O5 phase. The advancement in understanding the vanadium species chemistry promoted exploration on the solubility and stability of vanadium ions in different acid support electrolyte systems. The efforts led to the discovery of mixed sulfate and chloride electrolytes that stabilize all four vanadium cations at concentrations ≥2.5M within the temperature range of -5~50oC[31] (Figure 3). The addition of chloride acid achieves two purposes. The solubility of the V (II), V (III) and V (IV) ions were improved because of the decrease in the sulfate ion concentration. On the other hand, the chloride ion improves the stability of V(V) ions at elevated temperatures. This is attributed to the formation of a soluble neutral species VO2Cl(H2O)2, which was first predicted using the Amsterdam Density Functional (ADF) program, and later evidenced in the different chemical shift and width of 51V and 35Cl in pure sulfate acid electrolyte and mixed solution, respectively, by NMR study. The mixed electrolyte VRB system allowed over 70% increase in the energy capacity over the current sulfate system and broadened the operation temperature range from 10 ~ 35°C to -5 ~ 60 °C, which essentially eliminates the need for electrolyte temperature control in most areas of the world. 5PDF Image | Redox Flow Batteries for Stationary Electrical Energy Storage
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