Current in plane

For numerous reasons the local balance of currents in adjacent cells is usually not fulfilled and a significant amount of stack current is transported along the bipolar plate (in-plane current). In-plane current induces a variation of potential over the BP surface. 

Figure 5.9 Magnetic force lines of a direct current in planes perpendicular to the current.

This bistability is favoured by structures, which will adopt n—stacked motifs, i.e. lamellar molecules. In addition the tendency to adopt the planar structure is enforced by the presence of electronegative heteroatoms (currently restricted to N), which lead to a propensity of in-plane electrostatic S- N contacts between rings. These may comprise heterocyclic S- N contacts or S- N contacts to other functional groups such as the pyrazine or cyano nitrogen atoms. 

Davis et. al. (64) have calculated the steady-state thin-layer current component for a series of electrode geometries. In their derivation, these authors have assumed that the flux between the electrodes is one-dimensional (perpendicular to the plane). Particularly relevant to the STM geometry are the equations for the current in a conical electrode/planar electrode TLC, Icon, and those for a hemispherical electrode/planar electrode TLC, Xhsph (64> ... 

Fig. 6.7 (a) The variation of electrical conductivity of PVA-EG hybrid with increasing graphene content. Inset shows the dependence of dielectric constant for the hybrid, (b) The variation of conductivity of the polystyrene-graphene hybrid with filler content. Inset shows the four probe setup for in-plane and transverse measurements and the computed distributions of the current density for in-plane condition (reference [8]). 

The incorporation of nanocarbons in hierarchical composites can also result in large improvements in their electrical conductivity, and to a lesser extent in their thermal conductivity. For ceramic fibers both in-plane and out-of-plane electrical conductivities are increased by several orders of magnitude [41], whereas for CF the improvement is significant only perpendicular to the fiber direction due to the already high conductivity of the fiber itself [46]. The out-of-plane electrical conductivity of CNT/CF/epoxy composites is approaching the requirements for lightning strike protection in aerospace composites, thought to be around 1 10 S/m. Yet further improvements are required, as well as the evaluation of other composite properties relevant for this application, such as maximum current density and thermal conductivity. 

The most typical way to measure the in-plane electrical conductivity of a diffusion layer is through the use of the four-point probe method. A small current is applied across the sample material a separate set of voltage measuring probes that are in touch with the material are used to measure the resulting voltage drop in order to calculate the resistance. With these values, the in-plane resistivity, p, can be calculated with the following equation [9,233] ... 

Nitta et al. [216] designed a method to measure the in-plane conductivity of a DL as a function of the compressed thickness. The sample material was placed on a plate and compressed on both ends by graphite current collectors. Steel gages were located between the graphite blocks and the plate in order to maintain constant thickness of the DL while tests were conducted. A specific current range was applied to the apparatus and the voltage drop was measured in order to calculate the total resistance of the system. 

Accurate modeling of the temperature distribution in a PEFC requires accurate information in four areas heat source, thermal properties of various components, thermal boundary conditions, and experimental temperature-distribution data for model validation. The primary mechanism of heat removal from the catalyst layers is through lateral heat conduction along the in-plane direction to the current collecting land (like a heat sink). Heat removed by gas convection inside the gas channel accounts for less than 5% under typical PEFC operating conditions. 

---