Tin and Tin Alloys for Low Cost Bipolar Plates in PEM Fuel Cells

• Main Author: McCay, Katie
• Format: Dissertation
• Language: English

Proton Exchange Membrane (PEM) Fuel Cells are one of the most promising futures for green energy, as they provide a sustainable alternative to combustion engines in vehicles. The bipolar plate (BPP) is a crucial component in the PEM fuel cell, providing mechanical support to the stack as well as ensuring the even flow of reactants into the cell and waste products out of the cell. Additionally, it is an important conductive medium for the transfer of electrons through the stack, and any losses occurring in the BPP will reduce the overall performance of the fuel cell. Metallic materials for bipolar plates are nowadays favoured for their high conductivity and mechanical strength. However, given the harsh conditions inside a PEMFC, they will form a protective oxide on the surface, which reduces the cell performance over time due to an increase in interfacial contact resistance (ICR). Dissolution of the metal bipolar plate could also result in leaching of harmful metal ions which cause irreversible damage to the membrane. Therefore, metallic BPPs must be coated with a conductive and stable material to increase the lifetime and performance of the fuel cell. This thesis explores a new concept of soldering the gas diffusion layer (GDL) to the BPP using metallic tin (Sn). A thin layer of Sn is formed on a stainless steel BPP using electrodeposition, and then hot pressed with the GDL at a temperature close to the melting point of Sn. This leads to excellent conduction pathways through the BPP to the GDL, drastically reducing contact resistance and improving fuel cell performance. Additionally, the Sn coating will oxidise under the conditions present inside a fuel cell, leading to a protective SnO2 oxide layer which should prevent further corrosion whilst maintaining conductivity through the soldered GDL fibres. This soldering process produces a BPP/GDL system with an excellent performance and longevity inside a fuel cell, using a simple forming method and low-cost materials. BPPs produced in this way were optimised to give the lowest contact resistance and best corrosion resistance, as judged from electrochemical testing in a simulated PEMFC environment. The optimised Sn/GDL BPP was found to perform much better than stainless steel (SS316) alone, with an ICR of 6.5 mΩ cm2 and 13.2 mΩ cm2 obtained before and after corrosion testing, respectively. The increase in ICR after electrochemical testing was attributed to the dissolution and precipitation of the Sn to form a