Towards Uncoated Stainless-Steel Bipolar Plates in Automotive PEM Fuel Cells

Abstract

Bipolar plates (BPPs) have been identified as a key component in improving the economic viability of proton exchange membrane fuel cells (PEMFCs), as they contribute approximately 80 % of weight and 30 % of total cost in a fuel cell stack [1]. Stainless steel is considered an excellent material for mass-producing BPPs with complex flow field structures in a cost-effective manner. However, the perceived risk of corrosion and subsequent metal ion contamination as well as the inherently high contact resistance of stainless-steel surfaces have compelled manufacturers to add protective coatings to BPPs.In this study, we show new insights into the actual metal ion dissolution mechanisms in operating fuel cell systems and present experimental results which demonstrate that stainless-steel bipolar plates can be used in real life fuel cell applications without protective coatings. The bipolar plates were evaluated, in an operating fuel cell, using a dynamic load cycling protocol based on the New European Drive Cycle with increased humidity levels which simulates realistic conditions known to facilitate metal ion dissolution mechanisms [2]. The post analysis showed no signs of surface dissolution on any of the tested bipolar plates which had been subjected to the endurance/stress hybrid tests. Furthermore, only trace amounts of critical metals were found in the MEAs after testing, i.e., an order of magnitude below the radical scavenger (Mn) concentration. The trace metal contamination was comparable to a reference test without bipolar plates (graphite current collector), rendering metal input from the balance of plant the most likely source of contamination. The observable changes in cell performance and voltage degradation was unrelated to the presence of bipolar plates. A selection of experimental data is shown in Figure 1.As a theoretical explanation for the apparent stability of bare stainless-steel plates, three inter-linked phenomena were identified, akin to a three line of defence model [3]. The first and arguably most important of these phenomena is ionic decoupling between electrode and bipolar plate surface. The gas diffusion layer (GDL) strongly inhibits transport of ions from the electrode to the bipolar plate, which leads to a largely unpolarized bipolar plate and a stable surface potential even during load changes of the fuel cell. These stable potentials at the anode and cathode bipolar plates are only influenced by the immediate surroundings of the plate surfaces and are generally much milder than the encountered potentials at the electrodes. Secondly, metal dissolution from the surface of stainless steel follows a passive-transpassive-passive pattern. This mechanism describes a dissolution process, where a change in conditions at the surface, which destabilizes a previously established passive film, will only lead to limited amounts of (transpassive) metal dissolution, until a new stable passive film of different structure establishes itself. Finally, an operating regime which mitigates start/stop effects and hydrogen starvation during load changes will ensure very stable conditions at the bipolar plates’ surfaces, which strongly mitigates the risk of transpassive dissolution. As a final demonstration of the industrial feasibility of uncoated stainless-steel plates, an experimental alloy was tested under the same conditions. Results showed similar resistance to metal dissolution paired with low contact resistance at the level of C-coated or graphite plates. These results help to legitimize the efforts to industrialize uncoated stainless-steel plates since only the contact resistance challenge remains.[1] B. Pollet, S. Kocha, I. Staffell, ”Current status of automotive fuel cells for sustainable transport,” Current Opinion in Electrochemistry, vol. 16, pp. 90-95, 2019[2] T. Novalin, B. Eriksson, S. Proch, U. Bexell, C. Moffatt, J. Westlinder, C. Lagergren, G. Lindbergh, R. Wreland Lindström, “Trace-metal contamination in proton exchange membrane fuel cells caused by laser-cutting stains on carbon-coated metallic bipolar plates,” Int. J. Hydrogen Energy, Volume 46, Issue 26, 2021[3] T. Novalin, B. Eriksson, S. Proch, U. Bexell, C. Moffatt, J. Westlinder, C. Lagergren, G. Lindbergh, R. Wreland Lindström, “Concepts for preventing metal dissolution from stainless-steel bipolar plates in PEM fuel cells,” Energy Conversion and Management, 2022 Figure 1