Corrosion prevention of graphite collector in vanadium redox flow battery

Journal of Electroanalytical Chemistry 709 (2013) 93–98

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Corrosion prevention of graphite collector in vanadium redox flow battery S. Rudolph a,⇑, U. Schröder b, I.M. Bayanov a,c, G. Pfeiffer a a

Bozankaya BC&C, Carl-Zeiss-Weg 6, 38239 Salzgitter-Watenstedt, Germany Institute of Environmental and Sustainable Chemistry TU-Braunschweig, Hagenring 30, 38106 Braunschweig, Germany c Kazan National Research Technical University, Karl-Marx-str. 10, Kazan 420111, Russian Federation b

a r t i c l e

i n f o

Article history: Received 19 June 2013 Received in revised form 6 September 2013 Accepted 17 September 2013 Available online 24 October 2013 Keywords: Vanadium redox flow battery Pyrolytic graphite Corrosion

a b s t r a c t Results of theoretical and experimental studies of corrosion processes of graphite in vanadium electrolytes and methods for corrosion prevention are presented. Electric contact of positive half-cell electrolyte in the inlet/outlet channels of vanadium redox flow batteries (VRFB) with graphite collectors generates parasitic electric currents, which cause electrochemical oxidation of carbon atoms on the surface of the collectors. This leads to damaging the collectors and flow battery failure. The parasitic currents in the electrolyte channels were calculated and measured. Methods to prevent the corrosion, caused by the parasitic currents, were investigated: the graphite planar collectors were electrically isolated from electrolyte in inlet/outlet channels by o-rings and in inlet/outlet area of half-cell by isolation films. A corrosion resistant battery with collectors, fabricated from pyrolytic graphite, was constructed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Graphite is used as a collector in electrochemical reactions because of its electrical conductivity and chemical resistance. Once placed into the electrolytes, the exchange of electrical charge is carried out on the surface. Usually, the carbon atoms of the collector do not take part in these reactions. But under certain conditions reactions between carbon atoms and electrolyte species can occur. One main concern is the oxidation of carbon atoms. In the past, damage due to carbon oxidation at the cathode in fuel cells has been observed [1]. Similar problems are observed in VRFB [2,3]. Gassing of carbon monoxide and carbon dioxide has been reported [4], indicating the carbon oxidation. Graphite can have different origins: natural graphite, pyrolytic graphite and highly oriented pyrolytic graphite [5]. Depending on its origin it will have different electrochemical properties. Natural graphite is used as a collector material in VRFB due to its high corrosion resistance [6,7]. But natural graphite has some disadvantages such as low flexural strength and relatively high costs. The best substitute would be pyrolytic graphite, but its usage in VRFB is restricted by a low electrochemical resistance. Some property improvements of graphite materials, used in VRFB, were provided

⇑ Corresponding author. Tel.: +49 53318570191; fax: +49 53411899999. E-mail addresses: [email protected] (S. Rudolph), uwe.schroeder@ tu-braunschweig.de (U. Schröder), [email protected] (I.M. Bayanov), [email protected] (G. Pfeiffer). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.09.033

by Scyllas-Kazacos and co-workers [8–12]. The studies were devoted to processability improvement and cost reduction. In this work pyrolytic graphite plates were used as redox flow battery collectors. These are manufactured by proprietary reelto-reel processes using expanded graphite with organic binding compound. 2. Theoretical background 2.1. Electrochemical reactions Vanadium ions in aqueous sulphuric acid solution have four different oxidation states V2+, V3+, V4+, V5+. The ions V2+ and V3+ are mono-atomic vanadium ions, V4+ and V5+ correspond to the complex ions VO2+ and VOþ 2 , respectively. The following electrochemical reactions between the vanadium ions can occur on the surface of electrodes:

V2þ V3þ þ e ; 3þ

V

þ H2 O VO

2þ

VO

þ

ð1Þ 2þ

þ



þ 2H þ e ;

H2 O VOþ2

þ



þ 2H þ e :

ð2Þ ð3Þ

During charging/discharging cycle of VRFB reaction (1) takes place in the negative half-cells and the reaction (3) in the positive half-cells. The potentials of the negative half-cell electrolyte (anolyte) Ea0 = 0.26 V and of the positive half-cell electrolyte (catholyte) Ec0 = 1.00 V vs standard hydrogen electrode (SHG)

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correspond to reactions (1) and (3), respectively [13]. The open circuit voltage between the electrodes is determined by the Nernst equation [14]

RT ½V2þ  RT ½V5þ  RT 2 E ¼ E0 þ ln 3þ þ ln 4þ þ ln ½Hþ  ; F F F ½V  ½V 

A

B

5

ð4Þ

where E0 = Ec0  Ea0 = 1.26 V. Some side reactions, like gassing and graphite corrosion, can occur in redox flow batteries that leads to capacity losses and to damage of battery components [15–17]. For example, in the positive half-cell oxidation reactions of graphite can take place above the critical half-cell voltage of Ecr = 1.6 V [4]:

C þ 2H2 O ! CO2 þ 4Hþ þ 4e ; C þ H2 O ! CO þ 2Hþ þ 2e ;

ð5Þ

6

1

2 3 4

CO þ H2 O ! CO2 þ 2Hþ þ 2e : In contrast to single redox flow cells, in batteries with more than one cell there is not only the electric potential difference between the collector and the electrolyte in half-cell, but also between the bipolar collectors of the different cells. This will be described in Section 2.3. These different collector potentials cannot be avoided in the in-series connected cells, provided by common electrolyte channel. For example, the potential difference between the first and last collector of a N-cells-battery with the common electrolyte channel reaches N  Ecell, which can exceed essentially the critical voltage of carbon oxidation Ecr [4]. 2.2. Electrolyte flow configuration The electrolyte flow configuration in VRFB is formed by a system of channels in PVC frames and of bipolar plates. The battery consists of N in-series connected cells. Each cell has two identically constructed half-cells, which contain graphite planar collectors, graphite felt and PVC frames (Fig. 1). The positive and negative half-cells are separated by an ion exchange membrane to prevent cross-mixing of electrolytes. The electrolyte is pumped through the half-cell from the inlet channel across to the outlet channel, both found in the PVC frame (Fig. 1). Because of the 3-mm-thickness of the PVC-frame, the diameter of the inlet channels is 1 mm and of the outlet channels is 2 mm. The channels of each cell are connected by a common channel, formed by holes in the frames and graphite collectors (Fig. 2). Electric contact of the catholyte with the collectors by dif-

single cell Fig. 2. Electrolyte channel configuration in VRFB. The single cell consists of: PVC frames (1), graphite collectors (2), of graphite felts (3) and membrane (4). The VRFB is assembled with the tension-rods (5) and it has common outlet (6) for electrolyte. Electric contact of planar collectors with catholyte forms the electric circuits in collector hole edges (A) and in cell outlet channels (B).

ferent potentials causes the electric voltages: firstly, at the edges of the holes of the collectors, secondly, through the inlet/outlet channels of the cells (circuits A and B in Fig. 2, respectively). The voltages are more than the critical value Ecr and this leads to corrosion of the graphite on the surface of the collectors. As a result, the parasitic electric currents are generated. The reactions (5), which cause a damage of the collectors, can be named as parasitic also. The configuration of the electric circuits, formed in the electrolyte channels of the battery, is analysed in the next section. 2.3. Electric circuit of electrolyte channels in VRFB The parasitic electric currents, found in the electrolyte channels, can be calculated theoretically with the following approach. In terms of electrodynamics, the electrolyte columns in the channels can be represented as electric resistances Rsl,j, Rl,j, Rt,j, a contact with graphite collector in a hole – as Rgr,j, the cells – as electromotive force Ej with internal resistance Rin,j (Fig. 3). The values of resistances are calculated from the specific resistance of the electrolyte q, the length L and the cross-section S of the channel

L R¼q : S 1

6 2

3

5

4

Fig. 1. Construction of a VRFB half-cell: 1 – graphite planar collector, 2 – PVC frame, 3 – graphite felt, 4 – membrane, 5 – inlet channel and 6 – outlet channel of PVC frame.

ð6Þ

The value of the specific resistance q is changed during charging/discharging cycle operation of the battery due to changing the concentrations of the species. In this model it is accepted to be constant. This approximation describes the corrosion processes with adequate accuracy (see Section 4). The values of the electromotive force and of the internal resistance of the cell were found experimentally. In the electrolyte channel configuration an electric circuit is formed that has N identical units. One unit #j (j = 1, . . . , N) is shown in Fig. 3b. To complete the electric circuit ABCD (Fig. 3) resistor Rgr,j+1 from the next unit is used. A distribution of electric currents and voltages in this circuit can by described by Kirchhoff rules

A : Isl;j þ Igr;j ¼ Il;jþ1 ; K : Isl;j ¼ Il;j þ It;j ; D : Ij ¼ Ijþ1 þ Igr;jþ1 þ It ; ABCD : Isl;j Rsl;j  Il;j Rl;j  Igr;j Rgr;j þ Ij Rin;j þ Igr;jþ1 Rgr;jþ1 ¼ Ej ; AKLM : Isl;j Rsl;j  It;j Rt;j þ Igr;jþ1 Rgrþ1;j ¼ 0;

ð7Þ

S. Rudolph et al. / Journal of Electroanalytical Chemistry 709 (2013) 93–98

95

Fig. 3. Electric circuit of the VRFB electrolyte channels (a), units of the electric circuit (b): Rsl,j, Rl,j, Rt,j are the electric resistances of the electrolyte columns in the channels, Rgr,j is a resistance of contact with graphite collector in a hole the cells, Ej is electromotive force of single cell, Rin,j is internal resistance of single cell.

where j = 1, . . . , N. For j = 1 (the graphite collector #1 in cell #1) following two equations apply:

A : Igr;1 ¼ Il;1 ; D : Iinlet ¼ I1 þ Igr;1 :

ð8Þ

In unit #N Il,N+1 = 0 and IN+1 = Ioutlet. Therefore, 5N + 2 algebraic equations with 5N + 2 variables need to be solve. The solution of this equation system is discussed in Section 4.

3. Test battery construction For the experiments a test battery with N = 10 in-series connected cells was used. The planar current collector (210 mm  148 mm) was made from SIGRACETÒ TF6 material with a thickness of 0.6 mm (SGL, Germany). This bipolar plate is manufactured by proprietary reelto-reel processes using expanded graphite and fluoropolymers as raw materials. The production process leads to a high degree of plane parallelism of the bipolar plate. The final dimensions are conveniently obtained by die-cutting. Using this pyrolytic graphite plate allows an essentially decreasing the total thickness of the battery in comparison to natural graphite plates, which have a thickness of a few millimetres. Additionally, graphite plates provide a seal between the cells, preventing leakage of the electrolyte, and, therefore, eliminating the need of rubber seals between the PVC frames. The porous current collector (160 mm  108 mm) was made of SIGRACELLÒ GFA3 EA material with a thickness of 3 mm (SGL, Germany). Sigracell carbon felts are manufactured by pyrolysis of precursor felts, based on rayon (Sigracell GFA) or polyacrylonitrile (Sigracell GFD) in inert gas atmosphere at temperatures above 2000°. Dedicated raw materials and textile processes are employed to obtain high purity materials with excellent electrochemical stability.

A Fumatech VX 20 membrane was used as a separator between the half-cells. The active area of the membrane in the cell was 173 cm2. Graphite sticks with a diameter of 1 mm were inserted into the PVC frames to detect the voltage distribution in the electrolyte channel. A vanadium electrolyte with species concentrations of 0.8 M VOSO4, 0.8 M V2(SO4)3, 0.05 M H3PO4, 2 M H2SO4 was used (GfE Gesellschaft für Elektrometallurgie mbH, Germany). Data acquisition during the charging/discharging cycle operation was carried out by a flow battery controller (FBC) [18] that allows run-time measurements of battery and cell voltages, open circuit voltages of half-cells, state of charge (SOC) of electrolytes, current, energy and charge of the battery. The operating conditions of the battery were as follows: charging/discharging electric current I = 7 A, corresponding to the current density on the surface of the planar collectors of 40 mA/cm2 and to the current density on the interaction surface of the graphite felt of 0.8 mA/cm2. 4. Results and discussion Direct electric contact of the catholyte with the collectors in the electrolyte channel, marked as a short circuit A in Fig. 2, resulting in the corrosion at the edge of the hole in the collector (Fig. 4a) and in blocking the electrolyte channel by carbon fibres. The structure of graphite layers, shown in Fig. 5, was damaged on the surface of the electrolyte channel. In the SEM image the white traces on the surface indicate the oxidation of carbon atoms – white colour means that the area has a non-conducting polymer material only. Then rubber o-rings were used to isolate the collector from electrolyte (Fig. 4b). The electric resistance Rgr of the contact between graphite collector and electrolyte in the isolated hole edge was much higher than the resistance Rt of the electrolyte in the inlet channel of cell (Rgr and Rt are parallel connected resistors in Fig. 3b). For calculations Rgr was assumed to be  1 MX due to isolation with o-ring. Other parameters of the electric circuit of the electrolyte channels are found in Table 1, which were used to cal-

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(a)

(b) 10 mm

10 mm

Voltage, V

Voltage, V

Fig. 4. Corrosion of the graphite collector in the electrolyte channel (a) and isolation of the electrolyte channel (b).

BBC&C

Fig. 5. SEM image of the structure of a graphite collector in the direction of plane.

culate the current and voltage distributions in the electrolyte channels. The equations in system (7) and (8) were solved numerically using the bisection method, and the values of electric currents Isl,j, Il,j, Igr,j, It,j, Ij (j = 1, . . . , N) were calculated with a relative accuracy of 1010. The voltages in every part of the circuit were calculated as a product of current and of resistance (for example, Ut,j = It,jRt,j). The experimental values for the voltages in the electrolyte channels were measured by means of graphite sticks, placed into the electrolyte channel through the PVC frame in front of the outlet channel of cell. The contact of the graphite stick with the electrolyte corresponded to point K in Fig. 3. The voltages between the graphite sticks were measured during the battery operation. For 10 sticks in 10 cells 9 voltage values were measured. The voltages #0 and #10 were measured between stick #1 and the negative battery pole and between stick #10 and the positive pole, respectively. The results of the calculations were in good agreement with experimental data (Fig. 6). Additionally, the potential of the graphite stick in opposite of the positive graphite collector in the same cell was measured and calculated (Fig. 7). The relative deviation between calculated and measured values was less than 5%. The potential difference in the cells leads to electrochemical oxidation (5), damaging the surface of the positive half-cell graphite collectors. The potential difference between collectors #1 and #10 is the largest. This was also verified by the extent of damage traces found on the collecTable 1 Parameters of battery. Parameter

Symbol

Value

Unit

Number of cells Electric current Electromotive force of cell Internal resistance of cell Specific resistance of electrolyte Lengths of channel parts Cross-sections of channel parts

N I E Rin

10 7 1.41 0.021 0.01 5.2; 1.5; 11.0 7.1; 7.1; 0.8

– A V X Xm mm mm2

q Ll, Lsl, Lt Sl, Ssl, St

6.40 6.20 6.00 5.80 5.60 5.40 5.20 5.00 4.80 4.60

0.30 0.25 0.20 0.15 0.10 0.05 0.00

0

1

2

3

4

5

6

7

8

9

10

11

Cell number Fig. 6. Voltage distribution in the electrolyte channel: connected points – results of calculation, free points – experimental data, the points #0 and #10 are shown in the top part of the diagram.

Cell number Fig. 7. Distribution of voltage between graphite stick and positive graphite collector in cell: line – results of calculation, points – experimental data.

tors. The biggest damage could be seen on collector #10 (Fig. 8a), and the smallest trace of damage on collector #2 (Fig. 8b). The electric current density on the damaged surface of collector #10 was estimated as ratio of calculated electric current in electrolyte channel It,10 to damaged area jelectr = It,10/S = 2 mA/5 mm2 = 40 mA/cm2. The surface of the collector was studied microscopically (Fig. 9). Before the interaction with electrolyte the collector had a non-damaged surface (Fig. 9a). After the interaction the surface was damaged (Fig. 9b). In order to prevent corrosion an isolator film was attached to the surface of positive collector near the inlet and outlet area of the cell. As an isolator the same membrane material was used, which separates the half-cells. In this case the electric current in electrolyte

S. Rudolph et al. / Journal of Electroanalytical Chemistry 709 (2013) 93–98

(a)

97

(b) 5 mm

10 mm

Fig. 8. Electrochemical oxidation trace (marked by white ring) on the surface of the positive graphite collector: maximal by collector #10 (a) and minimal by collector #2 (b).

(a)

5 mm

BBC&C

Fig. 10. Damaged surface (marked by white ring) of a graphite stick by acting of electric current in the electrolyte channel, dashed lines show a position of electrolyte channel in PVC frame.

(b)

5. Conclusions

BBC&C

Fig. 9. SEM images of the surface of the graphite collector before oxidation (a) and after partial oxidation (b).

channel It was distributed on inlet surface of graphite felt Sfelt. This value was estimated as 37.7 mm2, taking into account the size of half-circle radius of the inlet surface, thickness and porosity of the graphite felt. This results in a current density 5 mA/cm2 that is comparable with current density of electrochemical reaction (3) by the battery operation. To proof the ability of the parasitic current It to damage a graphite surface an additional experiment with graphite 1-mm-thickness-sticks, placed in the inlet area of cell, was carried out (Fig. 10). The parasitic current finds the shortest path of the lowest resistance to the contact with graphite on the stick surface area opposite to the channel, resulting in corrosion. As a result, the surface of the graphite collector is not damaged. Therefore, this can be used for the corrosion prevention also. But this method has two disadvantages: firstly, the damaged stick should be changed periodically; secondly, the stick is fragile due to small thickness and, therefore, it is difficult to install in the half-cell. With the modifications the battery was operated continuously a few hundred charging/discharging cycles by constant current 7 A without any signs of corrosion. Therefore, oxidation of collectors, fabricated from pyrolytic graphite, can be prevented and thus allows their use in vanadium redox flow batteries.

In this article the corrosion effects of graphite materials in vanadium solution were studied. In vanadium redox flow battery, containing a number of in-series connected cells, conditions leading to corrosion were found. Inlet/outlet electrolyte channels of the battery were analysed and the causes of graphite collector corrosion were found. The voltage and current distributions in the electrolyte channels were calculated and measured. The results of calculations were in good agreement with experimental data. The divergence is less than 5%. Corrosion protection of collectors, fabricated from pyrolytic graphite, was realized on the basis of an isolation of the collectors from electric contact with electrolytes in the electrolyte channels. A VRFB with corrosion resistant graphite collectors was constructed and successfully tested. Acknowledgements The studies were supported financially by Bozankaya BC&C. The authors thank K. Blenke, D. Hage, and D. Vasilic for assistance in experiments. The authors are grateful to SGL Group (Germany) for provision of graphite materials and to Dr. R. Schweiss and D. Schneider for discussions on graphite corrosion. References [1] S. Maass, F. Finsterwalder, G. Frank, R. Hartmann, C. Merten, J. Power Sources 176 (2008) 444–451. [2] X. Li, K. Huang, S. Liu, N. Tan, L. Chen, Trans. Nonferr. Met. Soc. China 17 (2007) 195–199. [3] H. Liu, Q. Xu, Ch. Yan, Y. Qiao, Electrochim. Acta 56 (2011) 8783–8790. [4] H. Liu, Q. Xu, Ch. Yan, Electrochem. Commun. 28 (2013) 56–62. [5] G.M. Jenkins, K. Kawamura, Polymeric Carbons, Carbon fibre, Glass and Char, Cambridge University Press, Cambridge, UK, 1976. [6] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, J. Electrochem. Soc. 158 (2011) R55–R79. [7] G. Kear, A.A. Shah, F.C. Walsh, Int. J. Energy Res. 36 (2012) 1105–1120.

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