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EX-SITU CORROSION RESISTANCE ASSESSMENT OF METALLIC BIPOLAR PLATES
Metallic bipolar plates (BPP) are a promising solution for PEM fuel cells due to their superior mechanical strength, excellent electrical and thermal conductivity, and compact design potential. However, suitable metals are often expensive. More affordable alternatives, like 316L stainless steel in a PEMFC environment would suffer from corrosion due to the acidic and humid conditions, leading to metal ion leaching that can poison the membrane and catalyst. Additionally, the formation of passive oxide layers would increase interfacial contact resistance, reducing electrical conductivity and overall fuel cell efficiency.
Therefore, the development of novel coatings is an imperative condition for the mass commercialization of metallic BPP. These shall prevent metal ion leaching, which can degrade the membrane and catalyst, while also maintaining high electrical conductivity and chemical stability in the PEMFC.
Based on three-electrode measurement setup, the typical PEMFC electrochemical environment can be recreated. The ex-situ measurement of the corrosion current is then conducted using electrochemical techniques such as, potentiostatic as well as potentiodynamic tests and Electrochemical Impedance Spectroscopy (EIS).
During potentiodynamic polarization, the electrode potential is swept from cathodic to anodic regions, and the current response is recorded to determine the corrosion potential (E_corr) and the corrosion current density (i_corr) through Tafel extrapolation. The corrosion current density is an indicator of the material’s corrosion rate, with higher values signifying faster degradation. This method helps evaluate the suitability of metallic substrates and coatings for PEMFC applications.
In parallel, it must be guaranteed that the corrosion resistance of the BPP is not traded too much at the cost of the electrical conductivity. This check is performed via a controlled compression setup allowing the ex-situ measurement of Interfacial Contact Resistance (ICR) between GDL and BPP. The methodology involves a two-steps protocol separating the effects of bulk and contact resistances. A DC current is passed through the sample, and the voltage drop across the interface is measured using separate voltage probes. The measurement is repeated at various compression levels to study how contact resistance changes with pressure. Lower ICR values indicate better electrical conductivity and improved performance of the bipolar plate-GDL interface.
Impact and effects
Both developed measurement setups represent a new milestone for HyCentA in the minimization of fuel cell degradation.
Metallic bipolar plates are a mass-scaling capable solution for PEMFCs. Nevertheless, the development of excellent coating for metallic bipolar plates is crucial for ensuring high performance, durability, and efficiency. Additionally, coatings help reduce interfacial contact resistance by preventing the formation of insulating oxide layers, ensuring efficient electron transfer between the bipolar plate and the gas diffusion layer. This improves overall electrical conductivity, enhancing fuel cell efficiency. Furthermore, a well-designed coating improves hydrophobicity, facilitating water management and preventing flooding within the fuel cell. Ultimately, coatings extend the lifespan of metallic bipolar plates and contribute to the reliability of PEMFC systems.