**2. Materials for bipolar plates**

Metallic materials used for bipolar plates in fuel cells include non-coated stainless steel, aluminum, titanium, nickel and materials coated with conducting, nitrogen- and carbonbased coatings [5-7]. Metals are very good candidate materials for elements of fuel cells because they exhibit very high thermal conductivity, opportunities for repeated processing and are easy to be machined. Alloy steels are the most common materials used for these components (enhanced corrosion resistance, relatively low prices of steel) [8]. The literature data allow for the classification of the materials used for bipolar plates according to the three basic groups:


The diagram below (Fig. 2) presents the division of materials for bipolar plates according to basic groups. Using the criterion of choice of material for a component, one should decide whether to choose corrosion-resistant materials, which are often more expensive than other available materials or to use cheaper materials. Use of materials which are resistant to corrosion, such as titanium or gold, for bipolar plates substantially improves the costefficiency in manufacturing generators, whereas use of generally available stainless steels can decrease the effectiveness of work of the cell because of their properties (passivation of steel under conditions of operation of fuel cells) [27]. Therefore, searching for materials for bipolar plates should involve optimization of all the parameters. Taking into account multifunctional nature of the plates, this is extremely difficult. The table below presents the materials used (graphite [9-14]), suggested (nickel, titanium, stainless steel [15-24, 28]) or being developed (composites [25, 26]).

Metallic materials used for bipolar plates in fuel cells include non-coated stainless steel, aluminum, titanium, nickel and materials coated with conducting, nitrogen- and carbonbased coatings [5-7]. Metals are very good candidate materials for elements of fuel cells because they exhibit very high thermal conductivity, opportunities for repeated processing and are easy to be machined. Alloy steels are the most common materials used for these components (enhanced corrosion resistance, relatively low prices of steel) [8]. The literature data allow for the classification of the materials used for bipolar plates according to the three

The diagram below (Fig. 2) presents the division of materials for bipolar plates according to basic groups. Using the criterion of choice of material for a component, one should decide whether to choose corrosion-resistant materials, which are often more expensive than other available materials or to use cheaper materials. Use of materials which are resistant to corrosion, such as titanium or gold, for bipolar plates substantially improves the costefficiency in manufacturing generators, whereas use of generally available stainless steels can decrease the effectiveness of work of the cell because of their properties (passivation of steel under conditions of operation of fuel cells) [27]. Therefore, searching for materials for bipolar plates should involve optimization of all the parameters. Taking into account multifunctional nature of the plates, this is extremely difficult. The table below presents the materials used (graphite [9-14]), suggested (nickel, titanium, stainless steel [15-24, 28]) or

Fig. 1. Elements of fuel cell.

basic groups:


being developed (composites [25, 26]).

**2. Materials for bipolar plates** 


Fig. 2. Materials for bipolar plates in fuel cells.

Bipolar plates/interconnectors in fuel cells typically have channels on their surface to allow for the distribution of media to the electrodes [29]. The shape of channels and direction of flow of media might be different for the plate adjacent to anode compared to the plate near cathode. Media which flow in to both electrodes can be supplied by means of parallel channels, where media flow in one direction or channels where media are supplied to fuel cells with opposite directions. Another possible solution is that the media flow in with the direction transverse to the cell. The choice and optimization of the shape of the channels in bipolar plates affect the operation of the cell, particularly the degree of removal of products and distribution of gases to the surface of electrodes. The figure below presents bipolar plates with channels (Fig. 3). The essential effect on operation of the cell is from the depth of the channels, width of the channels, distance between spirals etc.

Review of the types of channels concerns in particular the geometry which depends on the type of fuel cell and demand for media in a particular cell. The list of opportunities for different channel design is obviously not ended and, apart from finding fundamental geometry, one should also consider the number of channels in the surface and distances between the channels. Proper distance between the channels and the number of channels ensure quick diffusion and effective discharge of water, especially in the cathode. **However, it should be emphasized that among a variety of types of channels used for distribution of media in fuel cells, there are no unequivocal research works which would have provided evidence of which type is the best.** 

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 193

Fig. 4 presents the morphology of the powders used for preparation of graphite-steel composites. The values of statistical parameters of the particles of steel and graphite powders are presented in Fig. 5 and Fig. 6 in the form of histograms. Table 3 contains

316LHD graphite

Fig. 5. Histograms of: a) particle surface; b) particle perimeter; c) mean particle diameter; d)

statistical parameters of stereological values of the used powders.

Fig. 4. Powders morphology, magnification x500.

roundness of the particle in 316LHD powder.

Fig. 3. Bipolar plate in fuel cell with channels which supply media.

The subject of the present study is the analysis of opportunities for the use of graphite-steel composites for components of fuel cells. The proposed composites were obtained by means of powder metallurgy. The technology for obtaining the materials used in the study allows for the determination of the effect of compaction and sintering on product properties. Finding the relationships between the technological parameters and properties of sinters allows for obtaining materials with the desired mechanical properties and resistance to corrosion. The investigations of sintered stainless steel confirmed that the use of suitable parameters of compaction pressure and sintering atmosphere ensures obtaining materials with controllable density, pore and grain size, and that suitable chemical composition of powders allows for obtaining sinters with the desired functional properties [30-35].
