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666 X.W. Wu et al.: Electrolytes for energy efficiency compared with traditional sulfuric acid electrolyte based RFB due to the formation of a vanadium dinuclear [V2O3·4H2O]4+ or a dinuclear-chloro complex [V2O3Cl·3H2O]3+ in the solutions over a wide tempera- ture range [42]. In the case of a VRB of 1 kW/1 kWh utilizing mixed acid (sulfate-chloride mixed electrolytes), it produces an output power of more than 1.1 kW with an energy efficiency of 82 % in the operation of 15–85 % state of charge (SOC) at the current density of 80 mA·cm–2. The enhanced performance indicates that the improved stability of electrolyte solution does not sacrifice the electrochemical performance of VRB [43]. Of course, additives such as Bi3+ can also be another choice to promote their electrochemical performance. When 0.01 M Bi3+ is added into the negative electrolytes (2 M VOSO4, 5 M HCl), the electrochemical reversibil- ity of the redox couple V(III)/V(II) is enhanced and a larger energy density and a higher energy efficiency are achieved compared with the VRB without Bi3+ (Fig. 4). It is attributed to the electro-catalytic effects of Bi metal electro deposited on the electrode surface during charging process [44]. Mixed electrolytes from methanesulfonic acid (MSA)-sulfuric acid MSA, CH3SO3H, has excellent thermal stability, water miscibility, low toxicity, lower corrosion and favorable ionic conductivity, and it is utilized as the supporting electrolyte for zinc-vanadium redox battery [45]. VRB with 3 M V(IV)/V(V) in MSA as the positive electrolyte presents an excellent cycling performance, indicat- ing that MSA is an excellent electrolyte solvent for V species. The electrolyte consisting of 2 M V(IV), 1.5 M CH3SO3H and 1.5 M H2SO4 has a lower solution resistance, faster kinetics for electron transference, and enhanced diffusion coefficient compared with the pristine one without MSA. Moreover, the discharge capac- ity and the energy density for the VRB with the MSA-sulfuric acid mixed electrolyte are increased due to the improved stability of vanadium ions (Fig. 5) [46]. The thermal stability of V(V) ion in a single MSA solution a b 4000 3500 3000 2500 2000 a 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.01 M Bi3+ b 100 95 100 mV 90 85 175 mV 80 Without Bi3+ 75 70 0.01 M Bi3+ Without Bi3+ 0 2 4 6 8 10 12 14 16 18 Specific capacity (Ah L-1) 65 0 10 20 Cycle number Fig. 4 (a) Charge-discharge curves at 150 mA cm–2 containing 0.01 M Bi3+ and without Bi3+ (b) and cycling performance VRB with the electrolytes containing 0.01 M Bi3+ and without Bi3+ at 50 mA cm–2 (modified from ref. [44]). 30 40 50 200 76 150 MAS 5 4 3 2 1 The pristine electrolyte 100 0 1234567 50 0 Z′ (Ohm) Equivalent circuit R1 CPE1 Discharge capacity for MAS sample Discharge energy for MAS sample Discharge capacity for pristine sample R2 CPE2 0 20 40 60 80 100120140 Z′ (Ohm) 10000 5 10 15 20 25 30 35 Cycle number 1500 Discharge energy for pristine sample Fig. 5 (a) Electrochemical impedance spectroscopy (EIS) of MSA and the pristine sample and (b) discharge capacity and energy of VRB with MSA and pristine sample (modified from ref. [45]). -Z′′ (Ohm) Voltage (V) -Z′′ (Ohm) Capacity (mAh)/energy (mWh) Energy efficiency (%)PDF Image | Electrolytes for vanadium redox flow batteries
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