A high-temperature tolerance solution for positive electrolyte of vanadium redox flow batteries

Journal of Electroanalytical Chemistry 801 (2017) 92–97

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Short communication

A high-temperature tolerance solution for positive electrolyte of vanadium redox flow batteries

MARK

Donghyeon Kima, Joonhyeon Jeonb,⁎ a b

Department of Energy and Advanced Material Engineering, Dongguk University-Seoul, 30, Pildong-ro 1gil, Jung-gu, Seoul 100-715, Republic of Korea Division of Electronics & Electronical Engineering, Dongguk University-Seoul, 30, Pildong-ro 1gil, Jung-gu, Seoul 100-715, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Energy storage system Redox flow battery Vanadium redox flow battery Sodium formate Thermal stability

Vanadium redox flow battery (VRFB) is attractive for energy storage applications, but there still remains a problem of preventing V-precipitation reaction (i.e., V2O5) to provide the thermal stability of electrolyte employed in VRFB. The V2O5 precipitation is accelerated in the positive electrolyte under high temperature, resulting in decreasing the charge capacity and energy efficiency of VRFB. So far, previous supporting materials, which are used to solve such precipitation problem, provide a disadvantage of decreasing voltage efficiency of VRFBs by increasing electrolyte resistance. This paper describes an advanced vanadium-positive electrolyte with high-temperature tolerance for high-efficiency VRFBs, which uses a sodium formate as a supporting material. The sodium formate plays a role as an agent capable of preventing precipitation reaction in the positive electrolyte and it also provides an effect of decreasing a viscosity of the positive electrolyte. The effectiveness of the proposed electrolyte solution is demonstrated through the following experiments: UV–vis spectrometry, viscosity measurement, cyclic voltammetry (CV), VRFB operation and scanning electron microscopy (SEM) analysis. Then, for the performance comparison of high temperature stability, all experiments are carried out at 60 °C. Experimental results show that using the sodium formate leads to near 4.5 times increase of diffusion coefficient as compared to conventional electrolyte, and also provides 20.2% higher charge capacity (50th cycle) and 2.27% higher average energy efficiency (50 cycles) at the current density of 80 mA cm− 2. In addition, it appears that the precipitation of vanadium species is not observed in the electrolyte during VRFB operation. Therefore, this paper provides that new direction about effect of the additive in the positive electrolyte and the sodium formate can be considered as a promising additive for high-performance electrolyte of VRFBs.

1. Introduction Vanadium redox flow battery (VRFB) is a promising technology as an energy storage system (ESS) for a wide range of applications such as large-scale renewable ESS [1–5]. “The power generation and capacity of VRFB are dependent on the stack design and concentration of electrolytes containing two redox couples of V (II)/(III) and V (IV)/(V) in sulfuric acid solution as a catholyte and anolyte, respectively [6].” In anolyte, Charge

VO 2 + + H2 O‐e‐ ⎯⎯⎯⎯⎯⎯⎯→ VO+2 + 2 H+

(1)

and in catholyte, Charge

V 3 + + e‐ ⎯⎯⎯⎯⎯⎯⎯→ V 2 +

(2)

However, the thermal stability of the vanadium species in the electrolyte solutions is limited [7]. Especially, the V (V) electrolyte

⁎

Corresponding author. E-mail address: [email protected] (J. Jeon).

http://dx.doi.org/10.1016/j.jelechem.2017.07.037 Received 23 February 2017; Received in revised form 15 May 2017; Accepted 19 July 2017 Available online 20 July 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.

suffers from precipitation reaction above high temperature, which leads to decrease the capacity and energy density of VRFB. Several studies have introduced to improve the solubility and stability of the electrolyte solutions, such as increasing the concentration of the aqueous sulfuric acid. Higher concentration of supporting electrolyte can prevent precipitation reaction of VO2+ ions to V2O5 by forming sulfate complexes, such as precipitation reaction through the following reactions [8],

[VO2 (H2 O)3]+ → VO(OH)3 + H3 O+,

(3)

2VO (OH )3 → V2O5⋅H3 O+,

(4)

and achieve increased stability due to de-protonation or dimerization of VO2+ ions to V2O42 + and V2O34 + species [9]. However, it favors the precipitation of V (II, III and IV) and the solubility of V (IV) sulfate decreases with increasing sulfuric acid concentration due to the common ion effect [10,27]. Because of the reduced solubility of the V (IV) ions in the discharged positive half-cell electrolyte, it is not

Journal of Electroanalytical Chemistry 801 (2017) 92–97

D. Kim, J. Jeon

possible to increase the concentration of sulfuric acid above 6.0 M. Several organic additives, which contain –OH, ]O, –NH2 or –SH functional groups, can encapsulate the hydrated penta coordinated vanadate ion and inhibit the precipitation formation, are screened as potential additives [11,12]. Thus the addition of sodium formate is focused on the thermal stability of positive electrolyte. In this paper, sodium formate is used as additives to improve thermal stability and battery performance of positive electrolyte. This supporting material which is the sodium salt of formic acid with the formula NaCO2H, acts as a preventing agent of precipitation reaction and can be added to the positive electrolyte for VRFB. The positive electrolyte without and with the additive are demonstrated through the thermal stability test, cyclic voltammetry (CV) measurement, charge–discharge test, and scanning electron microscopy (SEM) analysis. 2. Experimental 2.1. Preparation of vanadium electrolytes V (IV) electrolyte solution is prepared by dissolving 1.8 M VOSO4 (Sigma Aldrich, USA) + 3.0 M H2SO4 supporting electrolyte (Samchun Chemical, Korea). The V (III and V) electrolyte solutions are prepared by electric charging the V (IV) solutions in an electrolytic cell. The sodium formate of 0.25 wt% (weight percentage of the additive to the solution) is also added into the V (IV) electrolyte for electrochemical measurements [7]. 2.2. UV–vis spectrometry The absorbance of V (IV) electrolyte solutions are measured with a T60 UV–vis spectrophotometer in the range of 400–900 nm using 1.0 cm quartz cell [13]. The measured solutions are as follows: solution I: 0.036 M V (IV) electrolyte (pristine); solution II: 0.005 wt% sodium formate in solution I.

Fig. 1. The 6 cm2 miniature flow cell: (a) operating system and (b) miniature cell components.

discharge cycle operation. The flow cell (active area is about 6 cm2) contains 10 ml anolyte and 10 ml catholyte, electrolyte solution of 1.8 M V (III) and (IV) in 3.0 M H2SO4, respectively. A P/S 1400 pump (Thermo Co., USA) is used to circulate in the unit cell and controls 6 ml min− 1. Fig. 1 shows the operation of VRFB used in the work and the related specification is indicated in Table 1. The cyclic cell operation is carried out for 50 cycles by using WBCS3000 workstation (WonA tech Co., Korea) for a given galvanostatic charge and discharge between 1.7 V and 0.7 V under a current density of 80 mA cm− 2 at 60 °C. To two electrolyte tanks are placed in the oil bath at high temperature and circulated into unit-cell for one hour before charge and discharge cycle operation-i.e., free-cyclic operation proceeds for one hour so that the electrolyte temperature is 60 °C. The coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the cell are also calculated according to following equations: (5–7)

2.3. Viscosity measurement The viscosity of V (IV) solutions (16 ml of 1.8 M VOSO4 + 3.0 M H2SO4) is measured with a DV-E Viscometer (Brookfield, USA) using UL Adapter (Brookfield, USA) and all the viscosity measurements are carried out at 60 °C. 2.4. Thermal stability test The thermal stability test of V (V) electrolyte solutions is carried out in an oil bath at different temperatures (50 and 60 °C). The electrolyte solutions are monitored and recorded the precipitation time when a slight precipitation appeared [14]. 2.5. Electrochemical measurements A pristine electrolyte consists of V (IV) solutions (1.8 M VOSO4 + 3.0 M H2SO4) without and with 0.1, 0.25, 0.50 and 1.0 wt% the sodium formate as thermal stability agent is added into the pristine solution. Specific resistivity and cyclic voltammetry (CV) measurement of the V (IV) solutions are carried out on S470 SevenExcellence™ (Mettler Toledo, Switzerland) and ZIVE SP1 electrochemical workstation (WonA tech. Korea), respectively. The potential scanning range is from 0.4 V to 1.4 V with the scan rate as 10 mV s− 1 at the room temperature, where a platinum wire is used as counter electrode, a saturated calomel electrode (SCE) as reference electrode and a graphite plate (surface area is 0.95 cm2) as working electrode. The VRFB charge & discharge tests are performed in a miniature flow cell, which consists of a Nafion 115 as separator, graphite foil (SGL, USA) and carbon felt (Toyobo, Japan) with flow frame. The Nafion 115 is treated in distilled water at room temperature for 1 h before charge and

discharge capacity ⎞ The coulombic efficiency (CE) = ⎜⎛ ⎟ × 100 ⎝ ch arg e capacity ⎠

(5)

average discharge voltage ⎞ The voltage efficiency (VE) = ⎜⎛ ⎟ × 100 ⎝ average charge voltage ⎠

(6)

Table 1 Data related to Fig. 1.

Cell composition

Parts

Model/company

Value

Remarks

End plate Flow frame Graphite foil Carbon felt

Poly propylene Poly propylene TF6/SGL Co. XF30A/Toyobo Co. Nafion 115/ DuPont Co.

14 × 17 cm2 24 (6)1) cm2 24 (6)1) cm2 (6)1) cm2

t2): t2): t2): t2):

24 (6)1) cm2

t2): 127 μm

Separator

()1): active area, t2): thickness.

93

3 cm 0.3 cm 0.06 cm 0.42 cm

Journal of Electroanalytical Chemistry 801 (2017) 92–97

D. Kim, J. Jeon

5.6

Pristine Sodium formate

1.0

Pristine Sodium formate 5.5

Viscosity (cP)

Absorbance

0.8 0.6 0.4 0.2

5.4

5.3

0.0 400

500

600

700

800

5.2

900

0.0

Wavelength (nm)

CE × VE ⎞ 100 ⎠

1.0

1.5

2.0

Concentration (wt%)

Fig. 2. UV–Vis absorption spectra of V(IV) electrolyte without additive (pristine) and with sodium formate.

The energy efficiency (EE) = ⎛ ⎝

0.5

Fig. 3. Viscosity of 1.8 M V (IV) + 3.0 M H2SO4 with various concentration of the sodium formate at 60 °C.

electrolyte at high temperature as compared to the pristine one, resulting in a precipitation time delayed from 1.80 h to 2.75 h at 60 °C. It is likely to the fact that sodium formate has –O or ]O groups which can helps to the dispersion of V (V) ions by electrostatically repulsing and inhibits precipitation of vanadium species.

(7)

2.6. Analysis of scanning electron microscopy This scanning electron microscopy (SEM) analysis focuses on remaining the precipitation of vanadium species (such as vanadium pentoxide) on the surface of carbon felt electrode after the miniature cell operation. The surface morphology of the felt electrode is observed by using the JSM-7800F (JEOL, Japan) at an acceleration voltage of 10.0 kV.

3.4. Specific resistivity and cyclic voltammetry of the positive electrolyte Table 3 indicates that an excessive concentration of the sodium formate can lead to increase in the specific resistivity of the V (IV) solutions, because the higher specific resistivity of the electrolytes is likely to increase the mass transfer resistance and the kinetic energy loss of the electrolyte. It can reduce the charging and discharging performance of the vanadium redox flow battery [15]. Fig. 4 shows CV curves of positive electrolyte with different amount of the sodium formate, and the related data are also given in Table 4. It can be seen that the potential gaps between oxidization and reduction peaks (ΔEP) of sodium formate is lower than those of the pristine solution and the ratio of anodic peak current to cathodic peak current (IPA/IPC) close to one, indicating improved electrochemical reversibility of V (IV)/V (V) redox couple [15]. Moreover, the anodic and cathodic peak currents (IPA and IPC) of sodium formate are higher than those of the pristine electrolyte, implying improved redox kinetics [17]. This is due to the fact that sodium formate has –O or ]O groups in the molecules, so that the groups can coordinate with VO2+ and react with the surface of graphite electrode and provide more active sites for the reduction and oxidation reaction of V (IV)/V (V) [18–20]. As the additive amount increases from 0.1 wt% to 0.25 wt%, the peak currents can be enhanced. But when the additive amount of the additive is further increased, the peak currents rapidly decrease. The excessive concentration of the sodium formate induces the absorption of the formate ions around vanadium ions, and consequently, this disturbs the mass transfer by the formation of the absorption ions, resulting in the damage of a vanadium ionic hydrated layer. Therefore, 0.25 wt% is selected as the suitable amount of the additive for the positive electrolyte. Fig. 5 shows CV curves of the V (IV) electrolyte without and with 0.25 wt% the sodium formate at different scan rates. As shown in Fig. 6, the plots show a linear relationship between the square root of scan rates and the peak currents. The results indicate a quasi-reversible and an electron process for the redox reaction of V (IV)/(V) redox couple, which agrees with previous reports [21–24], and coefficient of vanadium ion is calculated according to the Randles-Sevcik equation [25,26].

3. Results and discussion 3.1. Effect of the sodium formate on the UV–vis spectra of V (IV) electrolyte Fig. 2 shows the UV–Vis spectra of V (IV) electrolyte (0.036 M V (IV) in 0.06 M H2SO4) without and with the additive. As shown in Fig. 2, a wavelength shift or new absorption peak is not observed compared with the UV–Vis spectra of V (IV) electrolyte without additive as a pristine solution. It indicates that the additives could not affect side reaction with vanadyl or oxovanadium (IV) cation, and change the concentration of VO2 + in the V (IV) electrolyte. 3.2. Viscosity measurement of V (IV) electrolyte The viscosity of the electrolyte, as an important property of electrochemical solutions, can affect the electrochemical performance. Besides, excessive use of additive leads to a decrease in the diffusion coefficient of vanadium ions, which resulted in a decrease in the electrode reaction kinetics [14]. As shown in Fig. 3, with the additive amount increasing from 0.01 wt% to 0.25 wt%, using sodium formate indicates lower viscosity than the pristine (5.525 cP), which means that it enhances the conductivity and electrochemical behavior of the solution [14–16]. It is likely that the additive leads to disperse vanadyl or oxovanadium (IV) cation. But when the additive amount of sodium formate is further increased, the viscosity rises. This might be due to the using excessive concentration of sodium formate. Thus, the 0.25 wt% concentration is selected as the suitable additive dose for the vanadium positive electrolyte. 3.3. Thermal stability The V (V) electrolyte solutions are placed in oil bath and then, their precipitation times are indicated in Table 2. It appears that using the sodium formate potentially leads to the thermal stability of V (V)

1 2

F3 ⎞ iP = 0.4463 ⎛ ⎝ RT ⎠ ⎜

94

⎟

n3 2AD11 2 C0 v1

2

(Reversible reaction)

(8)

Journal of Electroanalytical Chemistry 801 (2017) 92–97

D. Kim, J. Jeon

Table 2 Precipitation time for the V (V) electrolyte without and with the sodium formate at 50 and 60 °C. Additive

Temperature

Precipitation time

Additive

Temperature

Precipitation time

None (Pristine)

50 °C 60 °C

7.09 h 1.80 h

Sodium formate

50 °C 60 °C

16.62 h 2.75 h

Table 3 The specific resistivity of VO2 + (IV) electrolyte without additive (pristine) and with various concentration (0.01, 0.10, 0.25, 0.50 and 1.00 wt%) of the sodium formate.

0.04

Pristine (0)

0.01

0.10

0.25

0.50

1.00

0.03

Specific resistivity, (Ohm·cm)

149.46

151.83

156.18

158.27

160.60

163.55

0.02

0.030

Current (A)

Concentration, (wt%)

0.028

0.025

0.022

-0.03

0.015 0.020 1.0

1.1

1.2

0.4

0.010

0.6

0.8

1.0

1.2

1.4

Potential (V vs. SCE)

0.005 0.000

10 mV/s 20 mV/s 40 mV/s 60 mV/s 80 mV/s 100 mV/s

0.05 -0.004

-0.005

0.04

Pristine 0.1 wt% 0.5 wt%

-0.010 -0.015

0.4

-0.006

0.25 wt% 1.0 wt%

0.6

0.8

0.03

0.25 wt%

-0.008

0.80

1.0

0.85

1.2

0.90

Current (A)

Current (A)

0.00

-0.02

0.024

0.020

0.01

-0.01

0.25 wt%

0.026

(a)

10 mV/s 20 mV/s 40 mV/s 60 mV/s 80 mV/s 100 mV/s

0.05

1.4

Potential (V vs. SCE) Fig. 4. Cyclic voltammograms of the electrolyte (1.8 M V (IV) + 3.0 M H2SO4) without additive (pristine) and with various concentration of the sodium formate; 0.1, 0.25, 0.5, 1.0 wt% at a scan rate of 10 mV s− 1.

Anodic peak

None (pritine) 0.1 wt% 0.25 wt% 0.5 wt% 1.0 wt%

0.01 0.00 -0.01 -0.02 -0.03

Table 4 Cyclic voltammogram data related to Fig. 2. Additive does

0.02

(b)

0.4

Cathodic peak

IPA/IPC

Current (mA)

EPA

Current (mA)

EPC

21.599 26.023 26.309 24.816 23.276

1.094 1.069 1.068 1.078 1.079

14.335 18.662 20.174 17.724 15.474

0.856 0.848 0.857 0.849 0.855

0.6

ΔEP (V)

0.8

1.0

1.2

1.4

Potential (V vs. SCE) Fig. 5. Cyclic voltammograms of the electrolyte (1.8 M V (IV) + 3.0 M H2SO4) (a) without additive (pristine) and (b) with 0.25 wt% sodium formate at different scan rates.

1.51 1.39 1.30 1.40 1.50

0.236 0.221 0.211 0.228 0.224

0.06

Pristine Sodium formate

⎞ iP = 0.4958 ⎛ ⎝ RT ⎠ ⎜

⎟

n3 2AD21 2 C0 v1

2

(Irreversible reaction)

Current (A)

0.05 1 2 F3

(9)

where F, R and T refer to the Faraday constant, the universal gas constant and the Kelvin temperature, respectively, and A, C0, D1, D2 and ν are the surface area of working electrode, the concentration of electroactive species, the diffusion coefficient of a reversible and irreversible reaction and the scan rate, respectively. The value of the diffusion coefficient for a quasi-reversible reaction (D) is between that for a reversible one (D1) and an irreversible one (D2) [17]. For a one-electron reaction at different temperature, it can be deduced according to eqs. (8) and (9). For example, when T = 298.15 K, the eqs. (10) and (11) can be transformed as follow:

iP = (2.69 × 105) AD11 2 C0 v1 iP = (2.99 ×

105)

2

AD21 2 C0 v1 2

0.04

0.03

0.02

0.10

0.15

0.20 1/2

v

0.25 1/2

(V

0.30

0.35

-1/2

s )

Fig. 6. Plots of the anodic peak current versus the square root of scan rates for the electrolyte without and with the sodium formate.

(10)

The diffusion coefficient of V (IV) ion with 0.25 wt% the sodium formate is calculated to be 7.93–9.73 × 10− 8 cm2 s− 1 higher than 1.63–2.00 × 10− 8 cm2 s− 1 of the pristine. Therefore, the additive can

(11) 95

Journal of Electroanalytical Chemistry 801 (2017) 92–97

D. Kim, J. Jeon

100

1.8

b

Voltage efficiency (%)

1.6

Potential (V)

1.4 1.2

b

a

1.0 0.8 0.6

a. Pristine b. Sodium formate 0

80

a 60

a. Pristine b. Sodium formate 40

5

10

15

0

10

20

20

30

40

Time (min)

100

Fig. 7. Charge-discharge curves of all-vanadium redox flow battery employed (a) pristine electrolyte and (b) with 0.25 wt% sodium formate at 50th cycle.

Energy efficiency (%)

b

improve the mass transport and the electrochemical activity of vanadium ion. 3.5. Electrochemical performance of VRFB Fig. 7 shows the charge-discharge curves of the pristine electrolyte and with 0.25 wt% the sodium formate at 50th cycle. This curve using the sodium formate presents a lower charging voltage and higher discharging voltage than the pristine one. Besides, the anolyte with the additive has a higher charge (discharge) capacity than the pristine one as 20.2 (20.7) % at 50th cycle, which means that the additive leads to improve electrochemical reversibility of V (IV)/V (V) redox couple. Fig. 8 shows the discharge capacity of the miniature employing the vanadium electrolyte without and with 0.25 wt% the sodium formate as a function of cycle number at a current density 80 mA cm− 2. The anolyte with the additive indicates a higher initial and final discharge capacity (269.7 mA h, 108.7 mA h) than the pristine one (265.3 mA h, 93.7 mA h). With the cycle increasing, the miniature cell with the additive exhibits less discharge capacity fading compared to the pristine one, which is agreed with the charge-discharge curves. It means that the sodium formate enhances performance of VRFB and inhibits the precipitation reaction. Fig. 9 shows the voltage and energy efficiency of the miniature cell employing the electrolyte without and with 0.25 wt% the sodium formate at high temperature, and average efficiencies are indicated in Table 5. The results present that the voltage efficiency of using the

a 60

a. Pristine b. Sodium formate 0

10

20

30

40

50

Cycle Number Fig. 9. Cycle performance of the miniature cell employing anolyte (a) without additive (pristine) and (b) with the sodium formate during cyclic operation at 60 °C. Table 5 Average efficiencies of the miniature cell using electrolyte without and with sodium formate. Additive

Current efficiency

Voltage efficiency

Energy efficiency

None (Pristine) Sodium formate

95.47% 95.88%

81.05% 83.08%

77.38% 79.65%

additive keeps stable and remains above 82% at high temperature and the energy efficiency maintains above 79% at the current density of 80 mA cm− 2. Moreover, it observes that the average energy efficiency of using the additive is also 2.27% higher than that of the pristine one, which means that the additive leads to inhibit the precipitation formation in V (IV)/V (V) redox couple. Such results are exemplified in Fig. 10, SEM images show the carbon felt electrode in the positive electrolyte after the VRFB operation. It can be seen from Fig. 10(a) that the few precipitate of vanadium species can be observed compared to Fig. 10(b), indicating that these results show that the precipitation reaction could be delayed by the sodium formate as the precipitation preventing agent at high temperature. Overall, the anolyte with the sodium formate presents better electrochemical performance compared with the pristine one.

a. Pristine b. Sodium formate

Discharge capacity (mAh)

80

40

400

300

200

b a

100

50

Cycle Number

4. Conclusion 0

0

10

20

30

40

This paper has focused on an effect of the sodium formate as the precipitation preventing agents of the positive electrolyte on the performance of the VRFB at high temperature. The experimental results have demonstrated that the additive leads to enhance the diffusion coefficient of V (IV) ion from 1.63–2.00 × 10− 8 cm2 s− 1 to

50

Cycle Number Fig. 8. Discharge capacity of the miniature cell employing anolyte (a) without additive (pristine) and (b) with the sodium formate during cyclic operation at 60 °C.

96

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Fig. 10. The SEM images (× 1200) of the carbon felt of a 6 cm2 miniature cell after a 50 charge-discharge cycle: (a) pristine and (b) with the sodium formate.

7.93–9.73 × 10− 8 cm2 s− 1. Besides, the anolyte with the additive has a higher charge (discharge) capacity than the pristine one as 20.2 (20.7) % at 50th cycle and the average voltage (energy) efficiency of using the sodium formate is also 2.03 (2.27) % higher than the pristine one at the current density of 80 mA cm− 2. Moreover, it conducts as preventing precipitation reaction of vanadium species by the dispersion of the hydrated penta coordinated vanadate ion during cyclic operation of the miniature cell at high temperature. Therefore, this paper provides that new direction about effect of sodium formate in the positive electrolyte and the additive can be considered as a promising additive of VRFB. Acknowledgments This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20162020107050, 20174030201520). References [1] C. Fabjana, J. Garcheb, B. Harrera, L. Jörissenb, C. Kolbecka, F. Philippia, G. Tomazicc, F. Wagnerd, The vanadium redox-battery: an efficient storage unit for

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