One key component in the PEMFC which contributes significantly to cost, weight, volume of the stacks and still needs to be improved to ensure cell lifetime is the BiPolar Plate (BPP). Metal based bipolar plates are very attractive, but a protective coating is needed to avoid corrosion and keep the interfacial contact resistance low.
The STAMPEM-consortium was established acknowledging that further development of BPPs require Europe’s best available resources, with respect to both human competence and infrastructure (laboratories). The objective in STAMPEM was to develop a new generation coating for low cost metallic bipolar plates for PEMFCs, with robust and durable properties for assembly and manufacturing, showing high performance after more than 10000 hours of operation.
The concept of STAMPEM was to combine world leading industrial actors capable of volume manufacturing with research institutions with the required generic competence capable of providing breakthrough solutions with respect to a new generation coating for low cost metallic BPPs. By involving an end user of the BPPs developed in the STAMPEM project, the results were thoroughly verified under realistic operating conditions in a PEMFC stack.
The initial phase of the project was used to establish a testing protocol for BPP materials. In order to screen materials basic corrosion experiments will be performed with contact resistance measurements before and after the testing. Promising materials were further tested in fuel cells and even further in stacks. The BPP materials go through a real mass production cycle, and also the real production cost will be analysed. Also the possible detrimental contamination of the membrane will carefully be investigated. The most promising materials will, in the end, be fully integrated into a system, and be produced in series to provide the building blocks in other fuel cell vehicles.
Development of coatings for stainless steel bipolar plates was investigated using four different approaches within this project. These were 1) PVD coatings, investigated by TCL; 2) electrochemically prepared polymer based coatings investigated by UoB; 3) combined GDL-BPP concept and 4) carbon-based coatings investigated by SINTEF. Each of these was developed independently throughout the project, but improvements were achieved in all areas: Great improvements were achieved in corrosion resistance of the new generations PVD coatings. A significant reduction in ICR of the polymer based coatings was achieved by hybridisation with/addition of TiN particles. A reduction in ICR of the carbon composite coatings was obtained due to an improved application process. The concept of a combined GDL and BPP was also validated in small scale in-situ testing.
A metal BPP stack was lifetime tested in a harsh industrial load cycle over 1000 hours and about 1800 start / stop cycles. The industrial load cycle testing was followed by accelerated stress testing over 250 hours and further 1800 start / stop cycles. The gold coated metal BPP reference stack showed a degradation rate of 32 µV/h in the industrial load cycle and 40-60 µV/h during the AST. This result compares to 6 to 9 µV/h degradation rate at typical load cycle testing in the laboratory where no start / stop cycling is applied.
A PVD-coated metal BPP stack showed reduced initial performance, probably due to the higher ICR of the bipolar plates. The degradation rate during industrial load cycling was in the range of 56 and 80µV/h. The AST cycle was not completed due to the poor performance of the stack after the industrial load cycle as the current limitations of the test rig were exceeded. The lifetime testing confirms start / stop cycling is an important parameter for stack degradation and a low ICR is a key parameter for overall stack performance.
The economic assessment delivered 50 000 stacks as a reasonable market entry scenario for fuel cell application in the material handling market worldwide. The critical volume of manufacturing for achieving competitiveness with lead acid batteries (TCO basis) was found to be in the range of 1000 and 5000 units per year. The contribution of coated metal BPPs for fuel cell system cost reduction was analysed in detail and was subsequently found to be considerable compared to moulded graphite BPP originally used. The cost reduction potential at the BPP level was found to be in the range of 62 - 93% for 50 000 stacks per year for changing from current low volume manufactured moulded graphite BPP to coated metal BPP. For the high volume automotive market with 300 000 stacks per year (representing 150 million BPPs per year), a projected 98% cost reduction potential was identified for the PVD coating system developed within the STAMPEM project. The automotive market scenario delivers 5.35 €/kW for low-cost coated metal BPP which is more than twice the current target of 2.5 €/kW.
A further cost reduction of the bipolar plate could be achieved if active area welding is applied in the future. This would eliminate the need for a conductive coating on the inner surfaces of the BPP assembly, thus further reducing the coated area per assembly by a factor of 2. This manufacturing route is not yet fully established, but could be developed in the near future to provide further added value.