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a1.8 1.6 1.4 1.2 1.0 0.8 0.6 b With 4 % L-glutamic 4000 3000 X.W. Wu et al.: Electrolytes for energy 665 With 4 % L-glutamic Blank 0 50 0 30 40 50 Blank 100 150 200 250 300 Time (min) 2000 1000 10 20 Cycle number Fig. 3 (a) Charging–discharging curves and (b) discharging capacity fading curves of VRB employing positive electrolyte with and without 4 wt% L-glutamic at the current density of 60 mA cm–2 (modified from ref. [33]). alcohols with -OH groups at the secondary or tertiary carbon atom offer the greatest resistance to the oxida- tion by V(V), which are promising stabilizing agent for VRBs [36]. Inorganic ions which are stable in the electrolyte can also be good additives. For example, Mn2+ ions at an addition range of 0.04–0.13 g L–1 lead to a higher activity and reversibility [37]. However, they present some negative effects on interface impedance, liquid membrane impedance, and electrode reaction imped- ance. The concentration of Cr3+ in the range of 0–0.30 g L–1 could improve the reversibility of the electrode reactions and the diffusion of vanadium ions due to the changed electric field interaction of vanadium ions since Cr3+ ions could hinder the association of vanadium ions. With the increase of [Cr3+], the collision prob- ability between the VO2+ and Cr3+ ions increases. In addition, the electric double-layer at the interface may be altered due to the competition between Cr3+ and VO2+ to adsorb on the electrode surface, which may result in an increased diffusion resistance [38], and further study is needed. The added In3+ affects the hydration state of vanadium ions in the electrolyte and increase the charge transfer process at the electrode/electrolyte interface [39]. The cell using an electrolyte with 10 mM In3+ achieves an increased energy efficiency by 1.9 % compared with the pristine one. Other supporting electrolytes for VRBs From Fig. 3, much work has been done such as optimization of electrolyte composition and additives to increase the stability of vanadium ions in sulfuric acid electrolytes. So far, none of them have achieved a marked improvement. Therefore, new electrolytes which have high solubility of V ions as well as better elec- trochemical properties are needed. Mixed electrolytes from hydrochloric acid and sulfuric acid In the aqueous solution of chloride, vanadium (V) exists as [V2O3Cl2·6H2O]2+ compound at high temperature since it has a higher thermal stability due to its resistance to the de-protonation reaction which is the initial step for the precipitation reaction in vanadium based electrolytes. It is anticipated that the mixed electro- lyte from hydrochloric acid and sulfuric acid could be an effective strategy to increase the energy density of VRB [40]. Recently, scientists from PNNL reported a VRB based on sulfate-chloride mixed electrolytes as the sup- porting electrolyte. Both the positive and negative electrolytes compose of 2.5 M V ions in 2.5 M SO42− and 6 M Cl-. The improved stability of V ions not only leads to a 70 % increase in energy density but also obtains a wider work temperature range from –5 to 50 °C compared with the conventional sulfuric acid electrolyte. This markedly cuts down the operating cost caused by keeping the electrolyte temperature [41]. In addition, the decreased viscosity resulted from the addition of chloride potentially reduces pumping parasitic energy loss. It is also found that the use of chloride solution as the supporting electrolytes gives rise to a higher energy Voltage (V) Discharge capacity (mAh)PDF Image | Electrolytes for vanadium redox flow batteries
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