Investigation of Highly Corrosion Resistant and Electrically Conductive Coatings on Metal Bipolar Plates Used in Polymer Electrolyte Membrane Fuel Cell

• Author / Creator

  Wang, Xianzong

• There are apparent technical and economic advantages to replace carbon-based bipolar plates using metal bipolar plates (MBP) for applications in polymer electrolyte membrane fuel cell (PEMFC). However, MBP suffers from corrosion in the acidic environment of PEMFC, and the subsequently formed corrosion products on the metal surface will greatly increase the interfacial contact resistance (ICR), eventually leading to the poor stack performance. Therefore, this thesis work focuses on the development of electrically conductive and highly corrosion-resistant coatings on 304 stainless steel (304 SS) and their applications in PEMFC. The first part of the thesis is the development of a Zr2N2O coating by oxygen plasma treatment on ZrN through plasma enhanced atomic layer deposition (PEALD). During the polarization test, a layer of protective ZrO2 forms on the surface, thus inhibits further corrosion of the underneath coating and subsequently limits the coating oxidation. Zr2N2O thus formed combines the high corrosion resistance of ZrO2 and high conductivity of ZrN to best utilize the individual merits of the two component compounds. Compared with ZrN, Zr2N2O exhibits a significantly enhanced oxidation resistance and just a little increase in ICR after the test. Moreover, multilayer Cr Oxide/Cr coatings were prepared using the remote inductive oxygen plasma (O-ICP) treatment on the Cr coating. O-ICP attributes to the formation of a dense and thick oxide layer (CrO), which decreases the corrosion of the underneath substrate and the coating oxidation during the polarization process and subsequently limits the total thickness increase of the oxide layers. As a result, although the CrO gives the higher ICR than native oxide layer before the polarization test, only a mild increase in ICR after the test is observed for the O-ICP treated coatings. These studies demonstrate that incorporating oxygen atoms or introducing oxide layers into conductive coating proves to be a feasible strategy to improve the corrosion resistance while maintaining considerable electrical conductivity at the same time. This work points to a new direction to prepare the electrically conductive coatings with high corrosion resistance.Additionally, the thesis work also involves the studies on how the Zr2N2O coating improved corrosion resistance against the anodic dissolution induced by cathodic transient potential (CTP). The quite low corrosion current and the mitigated corrosion morphologies of the coated specimens confirm that the Zr2N2O coating can effectively restrain the anodic dissolution caused by the high CTPs. Analysis of the electron energy level diagrams of the oxide layer suggests that a protective coating with a wider gap between Fermi level and energy level of valence band contributes to the improved corrosion resistance towards the transpassive dissolution.Since hydrogen penetrates into the MBP during the operation of PEMFC and the MBP is stressed if the assembly process of stacks is less perfectly performed, the last part of this thesis work involves the investigation of the effects of hydrogen and stress on the corrosion of 304 SS in the simulated environment of PEMFC. Results show that 1) both hydrogen and stress significantly increase the critical current density (icrit) and passive current density (ipass), 2) hydrogen has a more obvious impact on icrit than that on ipass, 3) a high synergistic effect between hydrogen and stress on the current densities is observed in the range of the intermediate hydrogen charging current densities because the stress amplifies the effect of hydrogen alone and vice versa.

• Subjects / Keywords

  • Corrosion
  • Electrical Conductive Coating
  • Metal Bipolar Plates
  • Polymer Electrolyte Membrane Fuel Cell

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-6q1q-9q62

• License

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