**4. Research methodology**

**4.1 Phase analysis** of graphite-steel composites was carried out with X-ray XRD Seifert 3003 T-T diffractometer. The investigations were carried out using cobalt lamp with the wavelength of radiation of λCoKα = 0.17902 nm. The diffractometer operated with the following parameters:


**4.2 Microstructural analysis** of the obtained composites were carried out using Axiovert optical microscope.

**4.3 Hardness tests** for the graphite-steel composites were carried out by means of Rockwell method in B and F scale.

**4.4 Mean grain size** for the composites was determined based on comparison of microscopic photographs with the pattern scale (comparative method) according to PN-EN ISO 643 standard [36]. The investigations were also supported by the results obtained based on the research using mercury porosimeter PoroMaster 33.

**4.5 Analysis of porosity** of graphite-steel composites was carried out using mercury porosimeter PoroMaster 33 equipped in Quantachrome Instruments software for Windows.

**4.6 Analysis of wettability** of composites was carried out in a following manner: 3μl of water was dropped on the surface of material which had been previously polished with a set of abrasive papers with the finishing paper with grit designation of 2500. Before the examination, the material was degreased and left in the air until dry. The images of the material with a water drop were analyzed by a MicroCapture micro-camera which features software for image analysis. The functionality of angle analysis allowed for the determination of Θ angle.

**4.7 Analysis of roughness.** In order to determine surface topography and parameters of surface geometry in the composites, the examinations using Hommel T1000 profilometer were carried out. The examinations of sinter geometry were carried out using measurement needle with the ball tip with the radius of 2.5 μm. Using the profilometer allowed for the determination of the parameters which describe height and longitudinal characteristics of the profile [37-38].

**4.8 Analysis of contact resistance.** Techniques of measurement of interfacial contact resistance have been broadly discussed in the studies [39-40]. Measurements of electrical contact resistance between the surfaces of diffusion layer (GDL, usually carbon composite) and bipolar plates (BP) were carried out according to the methodology used by Wang

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 197

In order to determine the effect of stress on contact resistance between sintered materials and carbon paper, the analysis of contact resistance was carried out with the following stress

**4.9 Analysis of corrosion resistance in sinters under operating conditions of fuel cell.** Key impact on operation of fuel cell is from the processes which occur simultaneously during reaction on cell electrodes. In the case of the use of metal components for building of individual parts, one should take into consideration the possibility of solubilisation of these

The process of destruction of metals or metal alloys is intensified by acidification of the environment as a consequence of reactions which occur in the electrodes. Moreover, the

the material which is the most often used for electrolyte in PEM cells is Nafion®. Ions from the membrane intensify the corrosion processes in metal elements, whereas the cations which are created as a result of solubilisation of these components are the cause of 'poisoning' of the membrane. In consideration of the fact that virtually all the cations show higher affinity with sulphonic groups present in the membrane, compared to ion affinity H+, cations of metals react with polymer and reduce ion conductivity of the membrane. Mechanisms of degradation of the membrane have been broadly discussed in studies by [43- 45]. Both phenomena, i.e. 'poisoning' of the membrane and metal corrosion, do not only damage individual parts in the cell but they also impact on reduction in efficiency of the

During operation of the fuel cell, one should additionally consider indirect reactions between the products of corrosion in metallic components with oxidizers and with the fuel. According to the literature data, corrosion products can react with oxygen at the cathode side of the cell, creating oxide layers at the electrode surface [46-48]. This effect results in blocking pores on the electrode, which leads to reduction in the efficiency of the fuel cell. Similarly, on the anode side, hydrogen can reduce metal cations to metallic form. The created metal, which is deposited on the anode, blocks electrochemical processes. Both phenomena (on the cathode and anode) may lead to a reduction in active surface area of catalyst, and, in consequence, to impeding electrochemical processes in fuel cell. Shores and Deluga [49] demonstrated in their study that the environment in the initial phase of operation of H2/air (*PEMFC*) cell is acid (pH = 1 - 4), whereas after a certain period of time, the environment changes to pH = 6 - 7 [50]. In consideration of the phenomena which occur in fuel cell, the solution of 0.1 mol dm-3 H2SO4 + 2 ppm F-was proposed in order to evaluate corrosion resistance in materials [51-52]. Since the operating temperature of PEM fuel cell amounts to ca. 80 oC (this cell belongs to low-temperature cells), the corrosion investigations were carried out at the temperature of 80 oC ± 2oC. Thermostat system allowed for maintaining constant temperature of the solution. The proposed corrosion environment allowed for a rough simulation of operating conditions in fuel cell and the evaluation of corrosion parameters in metal components of the cell was

During potentiokinetic measurements, the working electrode was provided by the sintered steel, whereas the reference electrode was saturated calomel electrode, whereas platinum


, because

values: 20N cm-2, 40N cm-2, 60N cm-2, 100N cm-2, 140N cm-2, 160N cm-2.

process of corrosion is activated through ions from the membrane i.e. F-, SO3

components in working environment of the fuel cell.

generator.

possible.

discussed in the studies [41-42]. For the purposes of the present study, the device for experimental determination of the relationship between contact resistance and unit pressure for a set of pairs of GDL+BP samples of the analyzed materials was designed.

The pressure acting on the sets of samples was generated by pneumatic press with adjustment of pressure force. The pressure force was measured by means of digital force gauge (KMM20 + ADT1U-PC *(Wobit))* with the following metrological parameters:


Resistance in the samples was measured by means of 34401A *(Hewlett Packard)* device connected with the samples by means of a measurement system in Kelvin (four-point) configuration. The samples were in the form of the stack composed of two layers of carbon composite (carbon paper) which performs the role of a diffusion layer (GDL) in the cell. A plate made of composite material was placed between the carbon paper in order to ensure even distribution of reactants to the electrodes. The set of studied layers were connected with the resistance meter by means of the electrodes made of polished cuprum. The sample was electrically isolated from the press components by means of the plates made of nonconducting PTFE (polytetrafluoroethylene, Teflon). The diagram which illustrates the method of measurement is presented in Fig. 7.

Fig. 7. a) Measurement of contact resistance in the samples which modeled cell components b) measurement of 'inclusion' resistance with cuprum electrodes and diffusion layer.

discussed in the studies [41-42]. For the purposes of the present study, the device for experimental determination of the relationship between contact resistance and unit pressure

The pressure acting on the sets of samples was generated by pneumatic press with adjustment of pressure force. The pressure force was measured by means of digital force

Resistance in the samples was measured by means of 34401A *(Hewlett Packard)* device connected with the samples by means of a measurement system in Kelvin (four-point) configuration. The samples were in the form of the stack composed of two layers of carbon composite (carbon paper) which performs the role of a diffusion layer (GDL) in the cell. A plate made of composite material was placed between the carbon paper in order to ensure even distribution of reactants to the electrodes. The set of studied layers were connected with the resistance meter by means of the electrodes made of polished cuprum. The sample was electrically isolated from the press components by means of the plates made of nonconducting PTFE (polytetrafluoroethylene, Teflon). The diagram which illustrates the

Fig. 7. a) Measurement of contact resistance in the samples which modeled cell components b) measurement of 'inclusion' resistance with cuprum electrodes and diffusion layer.

for a set of pairs of GDL+BP samples of the analyzed materials was designed.



method of measurement is presented in Fig. 7.


gauge (KMM20 + ADT1U-PC *(Wobit))* with the following metrological parameters:

In order to determine the effect of stress on contact resistance between sintered materials and carbon paper, the analysis of contact resistance was carried out with the following stress values: 20N cm-2, 40N cm-2, 60N cm-2, 100N cm-2, 140N cm-2, 160N cm-2.

**4.9 Analysis of corrosion resistance in sinters under operating conditions of fuel cell.** Key impact on operation of fuel cell is from the processes which occur simultaneously during reaction on cell electrodes. In the case of the use of metal components for building of individual parts, one should take into consideration the possibility of solubilisation of these components in working environment of the fuel cell.

The process of destruction of metals or metal alloys is intensified by acidification of the environment as a consequence of reactions which occur in the electrodes. Moreover, the process of corrosion is activated through ions from the membrane i.e. F-, SO3 - , SO4 -, because the material which is the most often used for electrolyte in PEM cells is Nafion®. Ions from the membrane intensify the corrosion processes in metal elements, whereas the cations which are created as a result of solubilisation of these components are the cause of 'poisoning' of the membrane. In consideration of the fact that virtually all the cations show higher affinity with sulphonic groups present in the membrane, compared to ion affinity H+, cations of metals react with polymer and reduce ion conductivity of the membrane. Mechanisms of degradation of the membrane have been broadly discussed in studies by [43- 45]. Both phenomena, i.e. 'poisoning' of the membrane and metal corrosion, do not only damage individual parts in the cell but they also impact on reduction in efficiency of the generator.

During operation of the fuel cell, one should additionally consider indirect reactions between the products of corrosion in metallic components with oxidizers and with the fuel. According to the literature data, corrosion products can react with oxygen at the cathode side of the cell, creating oxide layers at the electrode surface [46-48]. This effect results in blocking pores on the electrode, which leads to reduction in the efficiency of the fuel cell. Similarly, on the anode side, hydrogen can reduce metal cations to metallic form. The created metal, which is deposited on the anode, blocks electrochemical processes. Both phenomena (on the cathode and anode) may lead to a reduction in active surface area of catalyst, and, in consequence, to impeding electrochemical processes in fuel cell. Shores and Deluga [49] demonstrated in their study that the environment in the initial phase of operation of H2/air (*PEMFC*) cell is acid (pH = 1 - 4), whereas after a certain period of time, the environment changes to pH = 6 - 7 [50]. In consideration of the phenomena which occur in fuel cell, the solution of 0.1 mol dm-3 H2SO4 + 2 ppm F-was proposed in order to evaluate corrosion resistance in materials [51-52]. Since the operating temperature of PEM fuel cell amounts to ca. 80 oC (this cell belongs to low-temperature cells), the corrosion investigations were carried out at the temperature of 80 oC ± 2oC. Thermostat system allowed for maintaining constant temperature of the solution. The proposed corrosion environment allowed for a rough simulation of operating conditions in fuel cell and the evaluation of corrosion parameters in metal components of the cell was possible.

During potentiokinetic measurements, the working electrode was provided by the sintered steel, whereas the reference electrode was saturated calomel electrode, whereas platinum

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 199

Fig. 8. Diffractograms of graphite-steel composites.

**5.2 Microstructural examinations, density and hardness of sinters** 

**100% 316L 80% 316L + 20% graphite**

Fig. 9 presents microstructures in graphite-steel composites.

wire was used as auxiliary electrode. The sintered samples had been previously polished with a set of abrasive papers with grit of 60, 80, 100, 180, 400, 800, 1000, with the finishing paper with grit designation of 2500. During electrochemical measurements, corrosion solution was saturated with oxygen or hydrogen. Both gases were obtained by means of an electrolyzer. Before and during measurements, the solution was saturated with a respective gas (ca. 1 hour). Potentiokinetic testing was carried out at a scan rate of 5 mV s-1. This scanning rate prevented too deep etching of the material during a single potentiometric measurement and was sufficient for registration of only Faraday processes in the electrode. Potentiokinetic curves were recorded after 10 seconds from the moment of putting the sample into the solution. The range of potential varied from the cathode values (-0.8 V vs. SCE) to anode values (1.8 V vs. SCE). Polarization curves were recorded by means of electrochemical measurement station CHI 1140 (CH Instruments, USA) connected to the computer. Polarization curves were used for determination or evaluation of the following corrosion parameters:


Determination of the polarization resistance *Rp* allows for the evaluation of the corrosion rate. After the determination of corrosion potential, the sample was subjected to the potential from the range of *Ekor* ± 20 mV. This means the range where the Stern-Hoar relationship is valid: density of external current is linear function of potential. Tangent of slope angle for the relationships of *E* = f(*i*) is reversely proportional to the corrosion rate. It should be emphasized that the corrosion rate determined by means of polarization resistance method might differ even by several times from the value of corrosion rate determined through extrapolation of Tafel sections, which, on the other hand, differ from stationary gravimetric measurements. For this reason, in order for the results to be comparable, research station, methodology and conditions of the research was defined in details as above.

Corrosion current density was obtained from extrapolation of tangents to anode potentiokinetic curves with the slope of 0.04 V/decade (it was adopted that the process of anode solubilisation of the sintered steels occurs according to Bockris mechanism [53-54]). The extrapolation method allowed for evaluation of the corrosion rate in composites.

### **5. Results and discussion**

#### **5.1 X-ray examinations**

Fig. 8. presents the diffractograms of graphite-steel composites. As results from X-ray examinations, the sinter 316L exhibits austenitic structure (CrFeNi phase). Steel sinters modified with graphite revealed the presence of hexagonal graphite and rhombohedral graphite (unstable thermally), made of deformed hexagonal graphite [55].

wire was used as auxiliary electrode. The sintered samples had been previously polished with a set of abrasive papers with grit of 60, 80, 100, 180, 400, 800, 1000, with the finishing paper with grit designation of 2500. During electrochemical measurements, corrosion solution was saturated with oxygen or hydrogen. Both gases were obtained by means of an electrolyzer. Before and during measurements, the solution was saturated with a respective gas (ca. 1 hour). Potentiokinetic testing was carried out at a scan rate of 5 mV s-1. This scanning rate prevented too deep etching of the material during a single potentiometric measurement and was sufficient for registration of only Faraday processes in the electrode. Potentiokinetic curves were recorded after 10 seconds from the moment of putting the sample into the solution. The range of potential varied from the cathode values (-0.8 V vs. SCE) to anode values (1.8 V vs. SCE). Polarization curves were recorded by means of electrochemical measurement station CHI 1140 (CH Instruments, USA) connected to the computer. Polarization curves were used for determination or evaluation of the following


Determination of the polarization resistance *Rp* allows for the evaluation of the corrosion rate. After the determination of corrosion potential, the sample was subjected to the potential from the range of *Ekor* ± 20 mV. This means the range where the Stern-Hoar relationship is valid: density of external current is linear function of potential. Tangent of slope angle for the relationships of *E* = f(*i*) is reversely proportional to the corrosion rate. It should be emphasized that the corrosion rate determined by means of polarization resistance method might differ even by several times from the value of corrosion rate determined through extrapolation of Tafel sections, which, on the other hand, differ from stationary gravimetric measurements. For this reason, in order for the results to be comparable, research station, methodology and conditions of the research was defined in

Corrosion current density was obtained from extrapolation of tangents to anode potentiokinetic curves with the slope of 0.04 V/decade (it was adopted that the process of anode solubilisation of the sintered steels occurs according to Bockris mechanism [53-54]). The extrapolation method allowed for evaluation of the corrosion rate in

Fig. 8. presents the diffractograms of graphite-steel composites. As results from X-ray examinations, the sinter 316L exhibits austenitic structure (CrFeNi phase). Steel sinters modified with graphite revealed the presence of hexagonal graphite and rhombohedral

graphite (unstable thermally), made of deformed hexagonal graphite [55].

corrosion parameters:

vs. SCE [A cm-2];

details as above.

composites.

**5. Results and discussion** 

**5.1 X-ray examinations** 




Fig. 8. Diffractograms of graphite-steel composites.
