YangHY ECS Trans 2013

Study of the Kinetics of Vanadium (II)/(III) Redox Reaction

Hun-Yun Yanga, Kan-Lin Hsuehb, Chin-Lung Hsiehc, Ju-Shei Hunga

a Department of Chemical Engineering, National United University, Miaoli, Taiwan b Department of Energy Engineering, National United University, Miaoli, Taiwan

c Institute of Nuclear Energy Research, Taoyuan, Taiwan

Vanadium redox flow battery (VRB) is a novel electrical energy storage system. The cell voltage loss is mainly due to the sluggish reaction of these redox reactions, it also affects the overall VRB energy storage efficiency. Rotating disk electrode (RDE) was used to measure the polarization curves of V(IV)/(V) and V(II)/V(III) redox reactions in different electrolyte compositions. On Carbon electrode, the rate constants of V(IV) oxidation is much slower than the rate constant of V(II) oxidation. The mainly voltage loss is due to the V(IV)/(V) in the electrolytes on positive electrode. We also found that the Stoke radius of V(IV)/V(V) ion is larger than the Stoke radius of V(II)/V(III). This provides a reasoning of lower reaction constant of V(IV)/V(V) ion pair.

Background

Today, our daily energy resources are mainly came from fossil fuels. Due to excessive utilization fossil fuels are rapidly depleted. Consumption of fossil fuel also leads to global warming. Development of renewable energy, energy storage, and efficient energy conversion become important. Many energy storages are under development. Vanadium redox flow battery (VRB) is one of the promising batteries for grid scale energy storage application. There are many successful demonstration projects around the world. Table 1 lists some of these demonstration projects of VRB worldwide [1]. Power of VRB energy storage system ranged from kW to MW, and they were used in junction with solar energy, wind turbine, diesel engine, et al.

Table 1 VRB Demonstration Project Year Country Power Rating Applications 1993 Thailand 1 kW/12 kWh Solar/energy storage 1997 Japan 200 kW/800 kWh 1999 Japan 450 kW/1 MWh Peak shaving 2001 Japan 170 kW/1 MWh Wind/energy storage 2002 South Africa 250 kW/520 kWh Emerging backup power 2003 Austria 200 kW Wind/Battery/diesel engine hybrid 2004 Utah, USA 250 kW Load level and peak shaving 2005 Germany 10 kWh Solar/energy storage 2006 Canada 10 kWh Remote power supply 2007 Kenya 5 kW Backup power

10.1149/05045.0087ecst The Electrochemical SocietyECS Transactions, 50 (45) 87-92 (2013)

87 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.1.100.65Downloaded on 2015-03-20 to IP

The high cycle life is one of the advantages in VRB as comparing to other batteries. The V(II)/V(III) and V(V(IV)/V(V) redox couple are the active species in the negative and positive compartment, respectively. Although vanadium ions are the only active species in the electrolyte, the reaction and interaction among these ions are very complicated. There are many forms of vanadium ions depending on electrolyte pH and electrode potential. The battery charge and discharge efficiency and electrolyte stability are heavily affected by the complicated interactions among ions and water. For instance, the V(II) ion is very unstable in the present of air. It will oxidize to V(III) ion when it contact the oxygen. This results a decreasing of battery efficiency.

Solubility of vanadium ion, solution viscosity, and Stokes radius of vanadium ion are depending on the concentration of sulfuric acid and solution temperature [2, 3]. The solubility of vanadium ion is lower in concentrated sulfuric acid than in diluted sulfuric acid. High temperature increases the vanadium ion solubility. Diffusivity of vanadium ion was calculated from the mass transfer limiting current density of RDE (rotating disk electrode) experiments in sulfuric acid solution range from 1 9 M [3]. Vanadium ion is reacted with water molecule to form various complexes depending on solution pH value and electrode potential [4] as given on Pourbaix diagram of vanadium. VO2+, VO+2, V+3 (including VOH+2 and V(OH)2+), and V+2 (including VOH+), are possible species present in the acidic solution. Vanadium oxides precipitation, such as VO(OH)3, V6O13, V2O4 V3O5, V2O3, are formed in neutral solution. Vanadium hydroxides, such as VO2(OH)2-, and VO3OH-2, are present in alkaline solution.

At present, the data of reaction rate constant of vanadium redox couple are rare. To have a better understanding of the redox couple of V(II)/V(III), we study the V(II)/V(III) reaction kinetics in different electrolyte composition by rotating disk electrode (RDE). The RDE experiment was carried out in various vanadium ion concentration and solution pH. The V(II) ion was prepared by adding of Zn powder into VOSO4 solution.

Experiment

In our experiment, the V(IV) was prepared from vanadyl sulfate oxide hydrate (VOSO4, Alfa Aesar, 99%) in sulfuric acid (H2SO4, J.T.Baker, 96.7%). The V(IV) concentrations studied were 0.1 M, 0.5 M, 1.0 M, and 2.0 M and they were in H2SO4 with concentrations of 0.5 M, 1.0 M, and 2.0 M. The V(II) was prepared from reduction of vanadyl sulfate oxide hydrate by zinc metal powder (SHOWA, 90%). Potentiostat (Pine Research Instrument, WaveNow) was used to control the electrode potential and to record the current. A linear scanning voltammetry was used at potential scanning rate of 5 mV s-1. A rotating disk electrode (Pine, AFE3T050GC) was used as the working electrode. Rotating speed of the electrode was controlled by a rotator (Gamry, 710 rotator). Electrode rotating speed was controlled at 100, 400, 900, 1600, 2500, 3600 rpm. Electrolyte pH was measured by a pH meter (Eutech, Cyber scan PH510). Electrolyte viscosity was measured by a viscometer (Ramins Corp., Cannon Fenske Routine type viscometer).

ECS Transactions, 50 (45) 87-92 (2013)


Results and Discussions

During the preparation of V(II) we found that the V(III) ion was reduced to V(II) ion among the pH value 1.08~1.45 as indicated by the changing of solution color from green to purple. Our observations are list on Table 2. If the pH value is greater than 2.12, we found precipitation of vanadium in the solution and the solution color was changed from green to brown.

We use linear sweep voltammetry (LSV) to investigate the kinetics of the V(II)/V(III) oxidation and reduction reactions by a carbon rotating disk electrode (RDE). Fig. 2 shows the redox reaction of V(II)/V(III) ions in 0.1M VOSO4 + 0.5M H2SO4 at electrode rotating speeds from 0 to 3,600 rpm. V(II) is oxidized at potential more positive than -0.5 V vs. Ag/AgCl. There is addition oxidation peak in the potential region between 1.0 and 1.7 V. The reduction current of V(III) in this particular electrolyte is not clear due the the hydrogen evolution reaction. Fig. 3 is the polarization curves of V(II) oxidation in different electrolyte compositions at electrode rotating speed of 400 rpm. With a given V(II) ion concentration (prepared from 0.1 M VOSO4 solution), the mass transfer limiting current is heavily depending on the sulfuric acid concentration.

Table.2 The different concentrations of vanadium solutions by adding zinc powder.

VOSO4 (M)

H2SO4 (M)

Precipitation of Zn Consumption of Zn (g) pH value Color of change No Yes

0.1 0.0 X 1.759 2.533.98 Light green Brown 0.5 0.0 X 2.199 2.133.40 Light green Brown 1.0 0.0 X 2.873 1.903.31 Deep green Brown 2.0 0.0 X 4.043 1.712.96 Deep green Brown 0.1 0.5 X 1.028 1.451.38 Green Purple 0.5 0.5 X 2.001 1.401.45 Green Purple 1.0 0.5 X 1.86 1.352.24 Deep green Brown 2.0 0.5 X 3.11 1.282.51 Deep green Brown 0.1 1.0 X 1.937 1.281.19 Green Purple 0.5 1.0 X 3.284 1.201.00 Deep green Deep purple 1.0 1.0 X 3.342 1.081.02 Deep green Deep purple 2.0 1.0 X 3.711 0.982.12 Deep green Brown 0.1 2.0 X 1.798 1.261.15 Green Purple 0.5 2.0 X 2.721 1.160.99 Deep green Deep purple 1.0 2.0 X 3.255 1.081.07 Deep green Deep purple 2.0 2.0 X 3.937 0.982.35 Deep green Brown



-0.005

0

0.005

0.01

-1 -0.5 0 0.5 1 1.5 2

Electrode Potential, V

Curr

ent,

mA

Electroderotatingspeed, rpm 3,600 2,500 1,600 900 400 100 0

Fig. 2 The LSV, scan rate is 5 mV/s, rotating speed from 0~3600 rpm in 0.1M VOSO4 + 0.5M H2SO4.

Fig. 3 The LSV, scan rate is 5 mV/s, rotating speed is 400 rpm in different electrolyte compositions.

We study the kinetic of V(IV)/(V) and V(II)/V(III) redox reactions, respectively. We use rotating disk electrode (RDE) to measure the polarization curves of V(IV)/(V) and V(II)/V(III) redox reactions in different electrolyte compositions. The linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are used during the measurement. The reaction rate constants of these redox reactions are calculated from measured data and limiting current equations. Calculated value of kinetic parameter of V(VI) oxidation in various electrolyte concentrations is listed on Table 3. The Calculated value of kinetic parameter of V(II) oxidation in various electrolyte concentrations is listed on Table 4. On Carbon electrode, the rate constants of V(IV) oxidation is much slower than the rate constant of V(II) oxidation, they are 7.89 10-14 ~4.38 10-7 cm s-1 for V(IV) oxidation and are 5.52 10-3 ~ 2.81 101 cm s-1 for V(II) oxidation. The mainly voltage loss is due to the V(IV)/(V) in the electrolytes on positive electrode, and the reactions on Pt electrode is faster than the reactions on carbon electrode. We also found that the Stoke



radius of V(IV)/V(V) ion is larger than the Stoke radius of V(II)/V(III). This provides a reasoning of lower reaction constant of V(IV)/V(V) ion pair (Fig.4).

V+2

V+4

Electrode

Fig. 4 The charge transfer of V(II) and V(IV) oxidation reaction

Table 3 Calculated value of kinetic parameter of V(VI) oxidation in various electrolyte concentrations

VOSO4 H2SO4 Number pHvalue

(g/cm3) (cp) (cm2/s)

10-3 D(cm2/s)

10-6 k0(cm/s) R() Tafel slope

(mV/decade) 0.1 M 0.5 M 1 1.02 1.00 0.78 7.75 13.07 0.345 1.68E-08 2.19 11.04 0.5 M 0.5 M 2 0.98 1.05 0.89 8.53 3.25 0.183 1.39E-08 7.66 8.50 1.0 M 0.5 M 3 0.9 1.09 1.10 10.10 3.78 0.294 4.74E-12 5.34 13.95 2.0 M 0.5 M 4 0.82 1.18 1.77 15.01 2.75 0.197 4.38E-07 4.56 7.08 0.1 M 1.0 M 5 1 1.03 0.86 8.34 1.32 0.200 6.94E-08 19.52 7.97 0.5 M 1.0 M 6 0.95 1.07 1.00 9.32 3.67 0.582 2.32E-11 6.07 18.61 1.0 M 1.0 M 7 0.83 1.12 1.23 10.98 3.31 0.569 1.96E-11 5.47 18.21 2.0 M 1.0 M 8 0.7 1.20 1.88 15.59 2.63 0.586 9.78E-12 4.50 18.75 0.1 M 2.0 M 9 0.84 1.06 1.00 9.42 5.55 0.410 1.08E-10 3.99 14.72 0.5 M 2.0 M 10 0.72 1.10 1.15 10.40 3.79 0.429 2.15E-10 5.11 15.39 1.0 M 2.0 M 11 0.62 1.15 1.40 12.16 3.32 0.768 7.89E-14 4.79 24.55 2.0 M 2.0 M 12 0.55 1.23 2.11 17.16 1.81 0.742 7.53E-14 5.82 23.74 0.1 M - 13 2.34 1.12 0.77 6.87 5.18 0.211 4.09E-06 5.56 6.76 0.5 M - 14 2 1.16 0.84 7.26 4.24 0.112 1.70E-05 6.22 4.43 1.0 M - 15 1.8 1.20 1.15 9.61 2.47 0.109 9.05E-06 7.81 4.34 2.0 M - 16 1.52 1.26 1.77 14.02 1.07 0.189 7.48E-07 11.66 6.78



Table 4 Calculated value of kinetic parameter of V(II) oxidation in various electrolyte concentrations

VOSO4 H2SO4 Number pHvalue (g/cm3) (cp) (cm2/s)

10-3 D(cm2/s)

10-6 k0(cm/s) R() Tafel slope

(mV/decade)

0.1 M - 2.533.98 0.5 M - 2.133.40 1.0 M - 1.903.31 2.0 M - 1.712.96 0.1 M 0.5 M 17 1.451.38 1.05 0.84 7.94 1.13 -1.724 2.81E+01 23.39 28.04

0.5 M 0.5 M 18 1.401.45 1.09 1.00 9.22 2.25 -0.750 2.68E-02 9.86 12.32

1.0 M 0.5 M 1.352.24 2.0 M 0.5 M 1.282.51 0.1 M 1.0 M 19 1.281.19 1.10 0.98 8.92 6.23 -1.256 5.81E+00 3.62 22.56

0.5 M 1.0 M 20 1.201.00 1.16 1.34 11.57 3.65 -1.112 1.41E+00 4.52 19.96

1.0 M 1.0 M 21 1.081.02 1.20 1.81 15.10 2.98 -0.806 3.19E-02 4.11 12.89

2.0 M 1.0 M 0.982.12 0.1 M 2.0 M 22 1.261.15 1.10 1.00 9.02 5.86 -1.159 2.12E+00 3.80 20.18

0.5 M 2.0 M 23 1.160.99 1.18 1.45 12.36 4.16 -0.879 2.13E-01 3.67 15.79

1.0 M 2.0 M 24 1.081.07 1.22 1.84 15.10 3.25 -0.702 5.52E-03 3.71 9.37

2.0 M 2.0 M 0.982.35

Acknowledgement

Authors wish to thank the financial support from Institute of Nuclear Energy Research (INER), Atomic Energy Council, Taiwan. Author (KLH) also wishes to thanks financial support from National Science Council, Taiwan, R.O.C.

References

1. B. Guo, X. Li, and S. Yang, Chemical Sources: Principle of battery and manufacture technology, Central South University Press (2009) (in Chinese)

2. F. Rahman, M. Skyllas-Kazacos, Solubility of vanadyl sulfate in concentrated sulfuric acid solutions , Journal of Power Sources 72 (1998) 105-110

3. G. Oriji, Y. Katayama, T. Miura , Investigation on V(IV)/V(V) species in a vanadium redox flow battery , Electrochimica Acta 49 (2004) 30913095

4. X. Zhou, C.Wei , M. Li, S. Qiu, X. Li, Thermodynamics of vanadiumsulfurwater systems at 298 K, Hydrometallurgy 106 (2011) 104112



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YangHY ECS Trans 2013