Functionally Coated Titanium Plate and Its Performance as Bipolar Plate Material for Hydrogen Fuel Cells
A fuel cell is an electrochemical device which converts a hydrogen containing chemical into electricity with less environmental impact. But cost is a major hurdle to commercialisation of fuel cells. Bipolar plate accounts for 80% of weight and 45% of the cost of the Proton Exchange Membrane Fuel cell (PEMFC) stack. Bipolar plate is the multifunctional component responsible for electrical conduction from anode to cathode, distributing reactant gas to electrode, removing heat from the active region and preventing coolant – reactant gas cross-over. Graphite has long been used as Bipolar Plate because it performs most of the required functions but weight, cost, brittleness and poor machinability makes it a poor choice for portable applications.
Metal plates are a better alternative to graphite as these can satisfy most of the qualities of bipolar plate (High electrical and thermal conductivity, easily stamped to get desired shape and good mechanical strength) but are prone to corrosion in the fuel cell environment and need protective coating. While major automakers use noble metal as the coating material, the need for non-noble coating is immense. Even though various researches have been done on non-noble metal coatings, very few reports are available on carbon based coatings and their long term significance for fuel cell performance. The performance of Titanium Nitride (TiN) and amorphous carbon (a-C) coatings are compared along with a novel tungsten carbide carbon (WCC) coating as bipolar coating materials.
The corrosion current density and Interfacial Contact Resistance (ICR) of WCC coating are less than 1 µA/cm2 and 10 mΩ.cm2 respectively. This makes WCC the only coating material that satisfies the US DOE (Department of Energy) target 2025 for bipolar plate. The other coating materials satisfy or are on a par with DOE target either for corrosion resistance or ICR but not both.
A die and punch was designed to fabricate plates using ANSYS software. The fabricated stamped plate showed no cracks and the thickness distributions at the critical spots were comparable with the simulation. The novel coating was extensively tested in the simulated fuel cell environment and coated on the stamped plate to study their long term performance in fuel cell single stack.
The fuel cell performance of the coated plate was analysed and the peak power density of WCC was 0.321 W/cm2 at 0.8 A/cm2, while bare Ti shows peak power of 0.21 W/cm2 at 0.63 A/cm2. The long term performance of the bipolar plate in fuel cell environment was determined by maintaining fuel cell at constant current mode 0.50A/cm2 for 500 hours. The need for non-noble protective coating is immense for bipolar plate application. In this research it is successfully coated and tested in fuel cell environment, achieving all the objectives.