Electrochemical study: A Solartron 1287 potentiostat was employedfor cyclic voltammetry (CV) experiments. Graphite felt (SGL, Germany) was punched into round discs of 5mm in diameter and sealed onto a SS mesh current collector. Cyclic voltammetry was carried out in a 3-electrode electrochemical cell with a Pt wire as the counter electrode to ensure the area of working electrode is much larger than that of the counter electrode. The scan was carried out at room temperature at a scan rate of 0.5mV/s.
The flow cells consisted of two graphite felt electrodes, two goldcoated
copper current collectors, two polytetrafl uoroethylene (PTFE) gaskets, and a Nafi on 117 membrane. The graphite felt (GFD5, SGL Carbon Group, Germany) was oxidized in air at 400 ° C for 6 h to enhance electrochemical activity and hydrophilicity. The active area of the electrode and the membrane was about 10 cm 2. The details on the flow cell are described in previous work. [19] An Arbin battery tester was used to evaluate the performance of fl ow cells and to control the charging and discharging of the electrolytes. The fl ow rate was fi xed at 20 mL/min, which was controlled by a peristaltic pump. A balanced fl ow cell contained about 50 mL anolyte and 50 mL catholyte. An environmental chamber was used to control the temperatures during fl ow cell tests.
For cell performance evaluation and electrolyte solution preparation,
the cell was normally charged at a current density of 50 mA/cm 2 to 1.7 V and discharged to 0.8 V with a current density of 25 to 100 mA/cm 2. Cell cycling tests were performed at 90% state-of-charge and state-of-discharge at a fixed charging and discharging current density of 50 mA/cm 2.
Electrolyte stability study : The electrolyte solutions of V 2 + , V 3 + , V 4 + ,and V 5 + used in this work were prepared electrochemically in fl ow cells using VOSO 4 (from Alfa Aesar) and VCl 3 as starting chemicals. VCl 3 solutions were prepared by dissolving V 2 O 3 (from Alfa Aesar) in HCl solutions. The electrolyte stability tests were carried out in polypropylene
tubes at −5 ° C, ambient temperature, 40 ° C, 50 ° C, and 60 ° C, using approximately 5 mL solution per sample. During the stability tests, the samples were kept static without any agitation and were monitored daily by naked eye for the formation of precipitation. Selected precipitates were separated out from the solution and analyzed with X-ray diffraction
(XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to check their crystal structures, morphologies, and elemental compositions.
NMR study : The ambient and variable temperature 35 Cl and 51 V NMR measurements were performed using a Varian 500 Inova spectrometer.The 35 Cl and 51 V chemical shifts were externally referenced to H 35 Cl andclear 51 VOCl 3 solutions, respectively ( δiso = 0 ppm). The chemical shifts(error ± 0.5 ppm) and line width (error ± 100 Hz) at each temperature
were obtained from fi tting the line shapes of the resonance lines using
the SpinWorks 3.1 program (K. Marat, University of Manitoba, Manitoba,Canada, 2009).
Others: Solution viscosity was measured using an Ubbelohde
calibrated viscometer tube. Our measurement indicated a viscosity
6.1 cP and a density 1.40 g/mL for the 2.5 M V 4 + , 2.5 M SO 4
2 − , and 6.0 M
Cl − solution at 30 ° C, in comparison with 6.4 cP and 1.45 g/mL for the
2.0 M V 4 + , 5.0 M SO 4
2 − solution at the same temperature.
Thermodynamic calculations were carried out using HSC Chemistry ®
6.1 program from Outotec Research Oy. Quantum chemistry calculations
were carried out using the Amsterdam Density Functional (ADF)
program.