Bipolar plates are a key component within a PEM fuel cell stack, with roles such as electron transfer, fuel and waste transport, and stack support. However, stainless steel as a bipolar plate material suffers from poor corrosion properties, with increasing interfacial ohmic resistances across the cell and stack due to the formation of non-conductive oxide layers reducing voltage efficiency over time. These increases in resistance can be mitigated by coating the steel with a conductive and corrosion resistant material.

In this work, we electrodeposited a thin layer of Sn onto a stainless steel bipolar plate (BPP) and joined (soldered) it with a gas diffusion layer (GDL) material through hot-pressing at a temperature just below the melting point of tin. This process solders the GDL to the BPP through the Sn, providing an uninhibited conduction pathway and a drastically reduced interfacial contact resistance. In addition, the Sn layer forms a passive oxide upon exposure to a PEM environment, preventing further corrosion whilst maintaining conductivity through GDL fibres that have infiltrated the Sn layer during hot-pressing (see figure 1). We have optimised the electrodeposition and hot-pressing procedure, with promising in-situ results even after 200 hours of operation in a working fuel cell. This confirms that the novel idea is successful in producing a coating with low contact resistance and promising longevity.

However, accelerated ex-situ testing in dilute sulfuric acid solution (pH 5.5) revealed instability of the SnO₂ layer. We propose that introducing alloying elements, such as Bi and In, will increase the stability of the oxide formed and thus impede the degradation of the Sn coating. This work investigates that claim, by depositing Sn alloys containing up to 10 wt % Bi and In onto a stainless steel bipolar plate. Conventional electrochemical measurements such as chronoamperometry and cyclic voltammetry, as well as interfacial contact resistance were performed to investigate the performance of this system.