**5.5 Investigations of sinter wettability**

In consideration of the degree of wettability, the materials are typically divided into lyophilic materials, which have strong affinity for water (these materials attract water particles) (Fig. 12a) and the materials which repel water particles, termed lyophobic (Fig. 12b). Contact angle Θ provides a measure of wettability, which is an angle between the surface of a solid and tangent going through the point of contact of solid, liquid or gaseous phase determined for the liquid phase. It is conventionally adopted that solid bodies which are characterized by contact angles of Θ < 90° are wettable; these materials show high surface energy (if the liquid is water, these materials are termed hydrophilic). Materials which exhibit contact angle of Θ > 90° are regarded to be non-wettable (lyophobic or, alternatively, hydrophobic = low surface energy).

Fig. 12. Diagram of wetting hydrophobic and hydrophilic materials.

In order to determine the effect of chemical composition of a composite on surface wettability, the analysis of wettability was carried out through evaluation of the value of Θ angle. The investigations concerned 316L steel sinter and sinters with addition of graphite. Fig. 13 presents contact angles evaluated for composite materials. A linear relationship between the proportion of graphite and surface wettability: contact angle increases with proportion of graphite in the composite. The highest contact angle was found for the sinter

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 205

Fig. 14. Effect of porosity on contact angle in graphite-steel composites.

Fig. 15. Behavior of a drop on hydrophobic material depending on surface geometry: (a)

Table 8 contains the values of height and longitudinal parameters for 316L sintered steel and graphite-steel composites. The substantial impact on surface geometry in sinters is from the presence of graphite. In the case of rough surface, an insignificant contribution of contact surface is observed. It is essential for fuel cells that the surface of the material for these parts is smooth, which is obtained through polishing or covering the surface with a coating [59]. Graphite, as a material with high porosity is subjected to polishing in order to obtain the

homogeneous, (b) heterogeneous.

smooth surface.

of 100% graphite (102o). Addition of graphite to steel affects surface energy of the material: hence, composites which contain 50% and more of graphite are numbered among a group of materials which are not hydrophobically wettable. The value of Θ angle evaluated for the materials used in the study are contained in the Table 7.

Fig. 13. Contact angle evaluated for graphite-steel composites.


Table 7. Values of contact angles for graphite-steel composites.

The authors of the study [56] demonstrated that material porosity affects contact angle. Fig. 14 presents the relationship between contact angle and sinter porosity. As can be observed, the relationship of both parameters which characterize the surface is non-linear. The materials whose porosity varies from 10 to 12% are numbered among hydrophobic materials. In the case of graphite-steel composites, with porosity higher than 12%, contact angle is lower than 90o.

#### **5.6 Sinter roughness measurements**

The available literature reports that contact resistance and wettability depend on surface geometry [57-58]. If a material is hydrophobic, a drop covers roughness in the surface and smoothens the unevenness (homogeneously) or it only touches the roughness, leaving a space between the drop and the solid (heterogeneously) (see Fig. 15).

of 100% graphite (102o). Addition of graphite to steel affects surface energy of the material: hence, composites which contain 50% and more of graphite are numbered among a group of materials which are not hydrophobically wettable. The value of Θ angle evaluated for the

materials used in the study are contained in the Table 7.

Fig. 13. Contact angle evaluated for graphite-steel composites.

Table 7. Values of contact angles for graphite-steel composites.

space between the drop and the solid (heterogeneously) (see Fig. 15).

angle is lower than 90o.

**5.6 Sinter roughness measurements** 

Composites Contact angle Θ [o] 100% 316L 75 ± 0.15 80% 316L + 20% grafit 83 ± 0.23 50% 316L + 50% grafit 94 ± 0.28 20% 316L + 80% 434L 97 ± 0.32 100% grafit 102 ± 0.37

The authors of the study [56] demonstrated that material porosity affects contact angle. Fig. 14 presents the relationship between contact angle and sinter porosity. As can be observed, the relationship of both parameters which characterize the surface is non-linear. The materials whose porosity varies from 10 to 12% are numbered among hydrophobic materials. In the case of graphite-steel composites, with porosity higher than 12%, contact

The available literature reports that contact resistance and wettability depend on surface geometry [57-58]. If a material is hydrophobic, a drop covers roughness in the surface and smoothens the unevenness (homogeneously) or it only touches the roughness, leaving a

Fig. 14. Effect of porosity on contact angle in graphite-steel composites.

Fig. 15. Behavior of a drop on hydrophobic material depending on surface geometry: (a) homogeneous, (b) heterogeneous.

Table 8 contains the values of height and longitudinal parameters for 316L sintered steel and graphite-steel composites. The substantial impact on surface geometry in sinters is from the presence of graphite. In the case of rough surface, an insignificant contribution of contact surface is observed. It is essential for fuel cells that the surface of the material for these parts is smooth, which is obtained through polishing or covering the surface with a coating [59]. Graphite, as a material with high porosity is subjected to polishing in order to obtain the smooth surface.

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 207

With regard to application of the used materials, one should determine corrosion resistance in the sinters. Fig. 17 presents the patterns of potentiokinetic curves recorded under conditions of work of fuel cell. The sinter 100% 316L is subjected to passivation both under conditions of cathode operation and under conditions of anode operation. Corrosion potential of the sinter 100% 316L in the analyzed environment does not change whether the solution was saturated with oxygen or hydrogen and amounts to –0.30 V vs. NEK. In the case of composite graphite-steel materials, addition of graphite caused an increase in corrosion resistance of the sinter. As results from the profile of the potentiokinetic curves, value of current density in the anode range is decreased even by two orders of magnitude. The value of corrosion potential in 316L+graphite sinters is insignificantly changed or remains at the same level compared to *Ekor* for the sinter of 100% 316L. It should be noted that the value of corrosion potential for the sinter of 100% graphite is shifted towards positive values and amounts to ca. 0.09 V vs. NEK in the solution saturated with O2, and ca.

**5.8 Assessment of corrosion resistance in sinters** 


Tab. 9.

O2

H2

environment

environment

**Parameters 100%** 

*i* przy 0.6V

*i* przy -0.1V

**316L**

Fig. 17. Potentiokinetic curves recorded for graphite-steel composites.

Table 9. Corrosion parameters of graphite – stainless steel composites.

Values of corrosion parameters estimated based on potentiokinetic curves are contained in

*E*kor [V] -0.336 -0.374 -0.390 -0.379 0.098 *i*kor [A cm-2] 90.7 10-4 22 10-4 13 10-4 8.00 10-4 5.53 10-4

[A cm-2] 2.263 0.128 0.040 0.017 0.005 *R*p [Ω cm2] 30.56 15 784.4 25 615.6 61 244.2 102 404.4

*E*kor [V] -0.303 -0.357 -0.406 -0.314 -0.026 *i*kor [A cm-2] 58.0 10-4 9.42 10-5 7.76 10-5 6.12 10-5 9.42 10-5

[A cm-2] 0.008 0.004 0.030 0.002 1.26 *R*p [Ω cm2] 687.16 285 641.3 450 340.7 533 957.2 638 985.3

**50% 316L + 50% graphite**

**20% 316L + 80% graphite** 

**100% graphite** 

**80% 316L + 20% graphite**

As results from the data contained in Table 8, addition of 20% of graphite to the composite considerably increases surface roughness. Further addition of graphite to the steel insignificantly reduces roughness, but it is still higher than roughness in 100% steel sinter 316L. This fact should be closely associated with the porosity revealed for individual composites.


Table 8. Parameters of surface geometry in graphite-steel composites.

#### **5.7 Measurements of contact resistance in composites**

In order to determine the effect of composition of a composite on contact resistance of interfacial contact, the measurements of contact resistance between graphite-steel composites and the diffusion layer were carried out (Fig. 7). An increase in stress value causes the decrease in contact resistance, whereas at high values of pressure force, contact resistance does not change. The values of contact resistance are the lowest for the system of 316L steel sinter – diffusion layer (Fig. 16). Addition of graphite to steel sinter elevates contact resistance by nearly 40 mΩ cm2 in the case of a composite 80% 316L + 20% graphite.

Fig. 16. Surface contact resistance for graphite-steel composites depending on stress.

As results from the data contained in Table 8, addition of 20% of graphite to the composite considerably increases surface roughness. Further addition of graphite to the steel insignificantly reduces roughness, but it is still higher than roughness in 100% steel sinter 316L. This fact should be closely associated with the porosity revealed for individual

Parameters of surfach geometry

In order to determine the effect of composition of a composite on contact resistance of interfacial contact, the measurements of contact resistance between graphite-steel composites and the diffusion layer were carried out (Fig. 7). An increase in stress value causes the decrease in contact resistance, whereas at high values of pressure force, contact resistance does not change. The values of contact resistance are the lowest for the system of 316L steel sinter – diffusion layer (Fig. 16). Addition of graphite to steel sinter elevates contact resistance by nearly 40 mΩ cm2 in the case of a composite 80% 316L + 20% graphite.

Fig. 16. Surface contact resistance for graphite-steel composites depending on stress.

 100% 316L 4.67 55.7 0.06 25.5 80% 316L + 20% grafit 7.12 76.5 0.06 32.4 50% 316L + 50% grafit 6.67 70.9 0.06 28.6 20% 316L + 80% grafit 6.04 65.4 0.06 24.1 100% grafit 5.46 52.3 0.05 23.6

Table 8. Parameters of surface geometry in graphite-steel composites.

**5.7 Measurements of contact resistance in composites** 

Height feature of profile Lengthwise feature of profile Ra [μm] Rz [μm] Sm [mm] Dp [%]

composites.

Composites

#### **5.8 Assessment of corrosion resistance in sinters**

With regard to application of the used materials, one should determine corrosion resistance in the sinters. Fig. 17 presents the patterns of potentiokinetic curves recorded under conditions of work of fuel cell. The sinter 100% 316L is subjected to passivation both under conditions of cathode operation and under conditions of anode operation. Corrosion potential of the sinter 100% 316L in the analyzed environment does not change whether the solution was saturated with oxygen or hydrogen and amounts to –0.30 V vs. NEK. In the case of composite graphite-steel materials, addition of graphite caused an increase in corrosion resistance of the sinter. As results from the profile of the potentiokinetic curves, value of current density in the anode range is decreased even by two orders of magnitude. The value of corrosion potential in 316L+graphite sinters is insignificantly changed or remains at the same level compared to *Ekor* for the sinter of 100% 316L. It should be noted that the value of corrosion potential for the sinter of 100% graphite is shifted towards positive values and amounts to ca. 0.09 V vs. NEK in the solution saturated with O2, and ca. -0.02 V vs. NEK in the solution saturated with H2.

Fig. 17. Potentiokinetic curves recorded for graphite-steel composites.


Values of corrosion parameters estimated based on potentiokinetic curves are contained in Tab. 9.

Table 9. Corrosion parameters of graphite – stainless steel composites.

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 209







[1] Jayakumar K., Pandiyan S., Rajalakshmi N., Dhathathreyan K.S., Cost-benefit analysis of commercial bipolar plates for PEMFC's, J. Power Sources, 161 (2006) 454-459.

[3] Hermann A., Chaudhuri T., Spagnol P., Bipolar plates for PEM fuel cells: A review, Int. J.

[4] Mehta C., Cooper J.S., 2003, Review and analysis of PEM fuel cell design and

[5] Lee S.-J., Huang C.-H., Lai J.-J., Chen Y.-P., Corrosion- resistance component for PEM

[6] Antepara I., Villarreal I., Rodríguez-Martínez L.M., Lecada N., Castro U., Leresgoiti A.,

[7] Lee S.-J., Lai J.-J., Huang C.-H., 2005, Stainless steel bipolar plates, J. Power Sources, 145

[8] Kumar A., Reddy R. G., Materials and design development for bipolar/end plates in fuel

[9] Mathur R.B., Dhakate S.R., Gupta D.K., Dhami T.L., Aggarwal R.K., Effect of different

[10] Dhakate S.R., Sharma S., Borah M., Mathur R.B., Dhami T.L., Expanded graphite-based

[11] Yasuda E., Enami T., Hoteida N., Lanticse-Diaz L.J., Tanabe Y., Akatsu T., Carbon alloys- multi-functionalization, Materials Sci. Engineering B, 148 (2008) 7-12.

Evaluation of ferritic steels for use as interconnects and porous metal supports in

carbon fillers on the properties of graphite composite bipolar plate, J. Mat. Proc.

electrically conductive composites as bipolar plate for PEM fuel cell, Int. J.

parameters of composites;

wettable (hydrophobic) materials;

surface geometry (the smoothest);

15 784,4 Ω cm2 in O2 environment.

(2005) 362-368.

Tech., 203 (2008) 184-192.

parameters of surface geometry in composites;

[2] U.S. Department of Energy (DOE) www.energy.gov

Hydrogen Energy, 30 (2005) 1297-1302.

cells, J. Power Source, 129 (2004) 62-67.

Hydrogen Energy, 33 (2008) 7146-7152.

manufacturing, J. Power Sources, 114 (2003) 32-53.

fuel cells, J. Power Sources, 131 (2004) 162-168.

IT-SOFCs, J. Power Sources, 151 (2005) 103-107.

porosity;

**7. References** 

Fig. 18 presents the effect of addition of graphite to sintered steel on polarization resistance for the material in the corrosion environment used in the study. The highest corrosion resistance was found for the sinter of 100% graphite, whereas sinter of 100% of 316L steel, compared to graphite, exhibit nearly 1000 time lower polarization resistance in the environment of H2 and several thousand times lower in the environment of O2. The sinters are characterized by higher corrosion resistance in the solution saturated with hydrogen (including the sinter of 100% 316L), compared to the O2 solution.

Fig. 18. Change in polarization resistance depending on proportion of graphite in a composite.
