Polymeric bipolar plates for PEM fuel cells : experimental and modeling approach to assess factors influencing performance

摘要

Fuel cells are widely researched and have applications in residential, automotive, marine craft and space. Their efficiencies are typically 60 % as a result of their electrochemical conversion and due to this they are considered beneficial to the reduction of CO2 which accounts for 77 % of all greenhouse gasses. Polymer electrolyte membrane fuel cells are the most suited to automotive applications for their low operating temperatures, high power densities and fast start up times. Currently there are many problems still to be rectified before commercialisation takes place, one of which is the performance and manufacture of bipolar plates. The elimination of corrosion, reduction of mass and the improvement of mechanical, electrical and thermal conductivity properties are the main aims to progress bipolar plate technology. In addition, the large numbers of bipolar plates required in automotive fuel cell stacks is in the order of 400 plates and so mass production will be necessary to meet future demands as well as reduce costs through cheap production processes. In order to meet these requirements polymeric based bipolar plates with conductive fillers have been pursued. The use of highly conductive, low density, low cost and corrosion resistant materials that can be utilised in production processes such as injection and compression moulding are ideal candidates for bipolar plates. However, balance of electrical/thermal conductivity and mechanical strength becomes the major task as highly conductive composites result in low mechanical strength. Therefore three conductive powders, a carbon black, graphite and magnetite (iron II,III oxide) were used as fillers in a polyethylene matrix to study the balance just mentioned for the two manufacturing processes stated above. The composites were tested for their electrical and thermal conductivities and mechanical properties and compared to the US Department of Energy targets for 2015. The carbon black composites exhibited better electrical conductivity than the other fillers where at 65 wt% the conductivity was ~24 S/cm for through plane conductivity and had a flexural strength of ~32 MPa. Injection moulding produced composites with more material stability and greater mechanical strength than compression mouldings although compression mouldings produced composites with higher thermal conductivities where graphite displayed the highest thermal conductivity of ~2 W/mK. Modeling of the experimental results using Mamunya models for electrical and thermal conductivities and a modified Kerner s equation for mechanical moduli were conducted. Models showed reasonable agreement with the experimental data where parameter tuning and deviations from the model were used to describe microstructural behaviour with regards to electrical tunnelling effects, link, node and blob structures and stress transfer at the filler-matrix interface.