A unit of a polymer electrolyte fuel cell is formed by laminating separators to both sides of a membrane electrode assembly (MEA). Multiple units are then pressed together to form a fuel cell stack. On the anode side, the separator conducts electrons generated by the catalytic reaction of hydrogen gas to an external circuit, whereas the separator at the cathode must have a function for supplying electrons from the external circuit. The separator commonly contains flow channels for feeds into the gas cell and for enabling heat transfer.
A conductive material is required and graphite-based carbon composite polymers or metals have been used in this role. Metals are preferred as they have far superior mechanical strength, allowing a reduction in the size and weight of the stack by using thinner metal plates.
The chosen metal needs to meet requirements of electrical properties and cost. Stainless steel fulfills the cost criteria, but the contact resistance between it and the MEA is greater than that of a graphite separator. This results in a loss of power from the cell. Nickel and chromium can also leech from the steel, poisoning the catalyst. Thus a thin coating of gold is applied to the surface by a method such as plating to reduce the contact resistance to an acceptable level and act as a barrier.
The quantity of gold required needs to strike a balance between cost and performance. A method of easily characterizing the thickness, uniformity and chemical composition of a layer is invaluable. X-ray photoelectron spectroscopy (XPS) has the capabilities to fulfill this requirement. XPS is a rapid and non-destructive technique for the characterization of the surface of materials. It is quantitive, responsive to changes in chemical state, and extremely surface sensitive. For thin films of around 5 nm or less, it is possible to probe both the outer layer and the substrate without removing the overlayer. The attenuation of the substrate signal by the overlayer enables the thickness of the overlayer to be calculated. Alternatively, surface material can be gradually removed by Ar+ ion bombardment to expose the substrate.
Read K-Alpha: XPS Characterization of Thin Gold Layers on Steel Separators for Fuel Cell Applications for more information and examples of depth profiling experiments of gold film on a steel substrate.