On the measurement of the electrical conductivity of graphite-polymer composite bipolar plates

We argue that the collinear 4-point probe is unsuitable for measuring in-plane and through-plane electrical conductivity of graphite-polymer composite plates, and strongly recommend the 4-wire Kelvin measurement system with parallel block electrodes. Regarding the latter, we report the effect of applied pressure on the cell resistance and determine the offset resistance in the measurement system at 69 bar by extrapolating the cell resistance for a series of graphite monoliths of different thicknesses to zero thickness. We show that the fringe field effect increases the conductance of the cell, and ipso facto the conductance of the sample plate, when the cross-sectional area of the sample plate exceeds that of the block electrodes. An evaluation of the measurement uncertainty of the through-plane electrical conductivity, shows that the uncertainty of the through-plane resistance of the two gas diffusion layers (carbon fibre mat) and their interfacial contact resistance with the sample and electrodes whilst under compression (collectively referred to here as the 'offset resistance') is the major contributor. This article complements the US Fuel Cell Council's protocol 05-160 and the Keithley White Paper (No. 2399) in establishing an effective measurement procedure to enable a more reliable comparison of conductivity data.