ROMANIAN JOURNAL OF PHYSICS

A NEW MATERIAL FOR BIPOLAR PLATES USED IN FUEL CELLS

ALEXIS NEGREA1, ZORICA BACINSCHI1, IOAN ALIN BUCURICA2, SOFIA TEODORESCU2, RALUCA STIRBESCU2 1

“Valahia” University of Targoviste, Faculty of Materials Engineering and Mechanics, 130004 Targoviste, Romania, E-mail: [email protected] 2 “Valahia” University of Targoviste, Multidisciplinary Science and Technology Research Institute, 130004 Targoviste, Romania Received June 15, 2015

The backelite-graphite composite material developed to meet the operating conditions of a proton exchange membrane fuel cells (PEMFC) is obtained from a polymer matrix (Novolac thermo-rigid resin), the hardener – HMTA and the conductive material – graphite powder. The production method requires size conditioning at 90 °C, stiffening thermal treatment at 120–125 °C and the final cross-linking treatment at 280 °C. All of these parameters influence the electrical and mechanical properties of the material and also can be used in recommendation of material to be involved in bipolar plates manufacturing. For the proposed material it was obtained the values for electrical conductivity of 248 Scm-1 and flexural strength of 38 MPa. Key words: bipolar plate, PEMFC, HMTA, resin.

1. INTRODUCTION

The proton exchange membrane fuel cells (PEMFC) are known as alternative energy sources and in the last years have been studied, more and more, for stationary and mobile applications. Due to the low operating temperature (80 °C) and to unpolluting fuels supply, this type of fuel cells can replace (in stationary applications) the actual energy supply sources [1–4]. PEMFC has some high manufacturing costs, 40% influenced by the price of bipolar plates [5–8]. This fact represents the reason to develop new materials capable to fulfill the roughest working conditions of bipolar plates and, also, with lower manufacturing prices. PEMFC bipolar plates used for a system require an ionic transfer surface large enough, good electrical and mechanical property, thermal and chemical stability, and low level of permeability for working gasses [9–14]. Rom. Journ. Phys., Vol. 61, Nos. 3–4, P. 527–535, Bucharest, 2016

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The graphite, in different states (i.e. powder, flakes etc.), has identical properties with above mentioned, but has low mechanic resistance. Therefore it’s been developed different types of matrices which can incorporate the conductive material (graphite). The matrix can be cement, argil, thermo-elastic or thermo-rigid polymers and resins [15–17]. The mixture of those two materials conducts to some properties modification [18–20]. Electrical conductivity is inverse proportional with the matrix quantity. If the conductive material – resin ratio is changing, the chemical stability and the permeability for working gasses also modifying. The aim is to choose the right mixture which satisfies the operating demands of bipolar plates. For this study has been developed a new backelite-graphite composite material obtained from natural graphite powder and thermo-rigid polymeric matrix – Novolac resin. In the end of the study have been tested the electrical and mechanical properties of the new material (conductivity and flexural strength). For a better characterization, the material was investigated using different analytical techniques (Scanning Electron Microscopy – SEM, Raman Spectroscopy, and Energy Dispersive X-ray Spectrometry – EDS). 2. SAMPLES PREPARATION AND EXPERIMENTAL TECHNIQUES 2.1. SAMPLES PREPARATION

The proposed material was obtained from graphite powder, thermo-rigid resin and hexamethylenetetramine (HMTA). The natural colloidal graphite, in powder form, used for obtaining the material comes from the natural deposit and has the following characteristics: carbon (min. 99.8%), sulphur (max. 0.02%), ash (max. 0.5%), humidity (max. 0.5%), and particle size – residue on the 63 μm sieve – (max. 5%). The thermo-rigid resin (Novolac type I and II) is a resin obtained by condensing phenol with formaldehyde in the presence of acids (Fig. 1). To crosslink Novolac is added as hardener the HMTA (also called hexamine).

Fig. 1 – Novolac resin structure.

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The characteristics of Novolac resin (type I and II) are presented in Table 1. Table 1 Characteristics of Novolac resin Characteristics Form Color Free phenol content Viscosity (50% in ethanol at 20 °C) Drop point Water content

Type I flakes yellow brown 6 min. 100 105–115 max. 1.0

– – % cP °C %

Type II flakes yellow brown 7 min. 120 95–105 max. 1.5

Hexamethylenetetramine (HMTA) is an organic heterocyclic compound with chemical formula (CH2)6N4 (Fig. 2). The colloidal graphite requires no preparation prior to mixing with the other two materials if it meets the conditions presented above. Possibly, the quality can be checked by using the 63 μm sieve.

Fig. 2 – HMTA chemical structure.

The resin is grinded using a mill by repeated passes until the entire quantity passing through the 125 μm sieve. This procedure is applied to HMTA, but it is sifted on 200 μm sieve. The ratio of the amounts of graphite and Novolac resin in the composition of the material influence the desired properties of the bipolar plate. For this study were analyzed three materials with different ratios (Table 2) comparative with other material used in bipolar plates (graphite with epoxy resin – EG). Table 2 Percentage ratios of graphite, Novolac resin and HMTA in proposed materials Sample G1 G2 G3

Graphite [%] 60.0 70.0 80.0

Novolac type I [%] 38.4 28.8 19.2

HMTA [%] 1.6 1.2 0.8

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After mixing the powders, the mixture is homogenized in a mixer with heating system. After the mixture cooling, the sample is milled and passed through the 100 μm sieve. The obtained materials are pressed at 1.5 tf/cm2 and heated at 90 °C. 2.2. EXPERIMENTAL TECHNIQUES

The aim of this study is to characterize the proposed material, comparative with the conventional material, from the point of view of: electrical properties (conductivity), mechanical properties (flexural strength), surface morphology and qualitative and quantitative analysis. Electrical conductivity is an important property of the material used in bipolar plates. If the value of electrical conductivity is high this involves the performance and the efficiency of fuel cells. The electrical resistance of the material was measured (in and through the plate plan) using RLC multimeter HM 8118. The resistivity (ρ) and the electrical conductivity (σ) were calculated using the follow equations:  



1





R b h R A  [cm] L L

(1)

L L Scm-1   R b h R A 

(2)

where: R – measured electrical resistance of the material [Ω]; L – sample length [cm]; A – cross-section area [cm]; b – sample width [cm]; h – sample height [cm]. The composite material must satisfy certain requirements in terms of mechanical properties. Bipolar plates have to support the proton-exchange membrane (PEM) and to seal the flow paths of the working gas (gasket). Bipolar plate is required to bend in the corners because the package clamping force. Also, between the plates are positioned the PEM and the gasket.

Fig. 3 – SolidWorks simulation for bipolar plate (flexural strength).

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The flexural strength was tested in three points using the multifunctional system MTS Bionix. The MTS Bionix software calculates the flexural strength using the equation: i 

3 F  L 2  b  h2

 MPa 

(3)

where: F – applied force [N]; L – distance between the support points; h – sample height [mm]; b – sample width [mm]. The surface morphology was studied using a Scanning Electron Microscope (SEM), SU-70 model. It is coupled with Energy Dispersive X-Ray Spectrometer (EDS) UltraDry which was used for quantitative and qualitative analysis (elemental composition and elemental distribution maps). The degree of structural ordering was studied using Raman spectrometer – Xantus 2.

3. RESULTS AND DISCUSSIONS 3.1. ELECTRICAL PROPERTIES

The effect of graphite content on the resistivity (ρ) and on the electrical conductivity (σ) was investigated. The obtained results are presented in Table 3. Table 3 Values of resistivity and electrical conductivity Sample G1 G2 G3 EG

Resistivity (ρ) [Ωcm] 9.02·10-3 6.89 ·10-3 4.03 ·10-3 5.00·10-3

Conductivity (σ) [Scm-1] 110.86 145.14 248.14 200.00

From Table 3 can be observed that the resistivity decreases with increasing graphite content (from 9.02·10-3 at 60 wt.% to 4.03·10-3 at 80 wt.%), but the conductivity increases with increasing graphite content (from 110.86 at 60 wt.% to 248.14 at 80 wt.%). 3.2. MECHANICAL PROPERTIES

The effect of the graphite content on the flexural strength of bipolar plates was investigated using MTS Bionix system. The obtained results were used to draw the diagram presented in Figure 4.

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Fig. 4 – Flexural strength of bipolar plates.

From the Fig. 4 can be observed that the flexural strength of bipolar plates decrease with the increase in graphite content, i.e. from 38 MPa (60 wt.%) to 28 MPa (80 wt.%). To make a correct comparison between the novel material (proposed in this work) and other material (available on the profile market), in the sections 3.3–3.5 will be evaluated the sample G2 (70% graphite) and the sample EG (~ 70% graphite). 3.3. SURFACE MORPHOLOGY

The surface morphology of samples was studied using SU-70 SEM. The acceleration voltage was set at 5 kV and the image was obtained using both secondary electrons detectors (lower and upper). The resolution of SU-70 is 1 nm at 15 kV, the electron guns is a Schottky diode with field emission. The obtained images are presented in Figures 5 and 6.

Fig. 5 – Surface morphology of sample EG.

Fig. 6 – Surface morphology of sample G2.

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From the above images can be concluded that the sample G2 present a better homogeneity comparative with sample EG. The spaces between graphite flakes of sample G2 are little than the spaces observed in sample EG. Also, the dimensions of flakes used in sample G2 are smaller than the flakes used to obtain the sample EG. 3.4. QUANTITATIVE AND QUALITATIVE ANALYSIS

The elemental composition of samples was studied using the energy dispersive X-ray spectrometer (EDS) UltraDry coupled on SU-70 SEM. This EDS allow quantitative and qualitative analysis (from 4Be to 94Pu) on point, line, rectangle, circle or other free choice area. Also, UltraDry (by NSS software) allow X-ray mapping, known as elemental distribution map. The acceleration voltage was set at 20 kV, both apertures were opened to obtain a dead-time value between 20–50 a.u. The obtained results are presented in Figs. 7–8 and Table 4.

Fig. 7 – Elemental distribution map of sample EG.

Fig. 8 – Elemental distribution map of sample G2.

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Table 4 Elemental composition of samples Element C [%] O [%] Al [%] Si [%] S [%] Cl [%] K [%] Ca [%] Fe [%] Na [%]

Sample EG 93.16±0.40 6.28±0.10 0.06±0.01 nd* nd* 0.19±0.01 0.07±0.01 0.06±0.01 nd* 0.18±0.01

Sample G2 89.35±0.39 10.00±0.12 0.07±0.00 0.06±0.00 0.09±0.00 0.03±0.00 0.04±0.00 0.23±0.01 0.13±0.02 nd*

nd* – indeterminate

The elemental analysis shows some differences between the EG and G2 samples. Traces of Si, S and Fe in the obtained material (G2) have been identified and represent the elements adhesion effect during the thermal treatments. The reason for having more than 70% carbon is because the resin and the HMTA have also this element in composition. It is well known that the oxygen affect the electrical conductivity, but in this case the iron compensates the effects. 3.5. RAMAN SPECTROSCOPY

Raman spectroscopy revealed the presence of the structurally disordered phase derived from the resin carbon. The Raman spectra for both samples are presented in Figure 9. The intensity band around 1350 cm-1 correspond to C-C bond (D band) and the intensity around 1580 cm-1 correspond to C = C bond (G band). The ratio between intensity of D band and intensity of G band (ID/IG) represent the degree of structural ordering.

Fig. 9 – Raman spectra of samples EG and G2.

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As can be observed in Fig. 9, sample G2 has both intensity bands (1350 and 1580 cm-1) well defined and the value of ratio ID/IG is 2.056. Sample EG presents the G band with intensity 965 a.u., but the D band isn’t well defined and the intensity is 295 a.u. So, the ratio ID/IG for EG is 0.306. 4. CONCLUSIONS

A bipolar plate composed of backelite-graphite composite material (with Novolac resin and HMTA) has been successfully prepared. The conductivity of the composite bipolar plate can reach 248 Scm−1 and meet the requirement for conductivity of the bipolar plate in a PEMFC when the graphite content is greater than 70 wt.%. The flexural strength of the composite bipolar plate with 70 wt.% graphite content is 28 MPa and is lower than that of a EG graphite bipolar plate (40 MPa). The prepared sample (G2) presents a better homogeneity comparative with sample EG, but it present some impurities (i.e. S, Si and Fe). The electrical conductivity and the flexural strength of this composite bipolar plate can all be improved by optimizing the mould pressing conditions, especially the mould pressing time. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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