Separators are laminated on both sides of a membrane electrode assembly (MEA) to form a unit of a polymer electrolyte fuel cell. A fuel cell stack is then formed by pressing together multiple units. The separator at the anode side conducts electrons formed by the catalytic reaction of hydrogen gas to an external circuit, whereas the separator on the cathode side supplies electrons from the external circuit as shown in Figure 1.
Figure 1. Schematic of a single element of a fuel cell stack. The separators are the outer plates (in yellow) of the cell
Flow channels are typically present in the separator to feed into the gas cell and to facilitate heat transfer. Graphite-based carbon composite metals or polymers are generally used as the conductive material. The superior mechanical strength makes metals the preferred choice over polymer, allowing the use of thinner metal plates to reduce the weight and size of the stack.
However, the metal selected has to satisfy the requirements of cost and electrical properties. In addition, catalyst poisoning can also take place due to leaching of chromium and nickel from the steel poisons. Therefore, the surface is applied with a thin gold coating in order to lower the contact resistance to an acceptable level. The coating will also serve as a barrier.
The amount of gold needed has to achieve a balance between cost and performance. Therefore, it is crucial to have a method that can easily characterize the chemical composition, uniformity and thickness of a layer. This requirement can be handled with X-ray photoelectron spectroscopy (XPS), which is the only analytical technique delivering quantitative elemental and chemical data with unprecedented surface sensitivity.
As a non-destructive method, XPS can perform surface characterization of materials rapidly. It allows probing both the outer layer and the substrate without peeling away the overlayer in the case of thin films of 5nm or less. The overlayer thickness can be calculated due to the attenuation of the substrate signal by the overlayer.
Alternatively, the substrate is exposed by gradually removing the surface material by Ar+ ion bombardment. A Thermo Scientific K-Alpha (Figure 2) was used in this experiment to perform the XPS characterization of thin gold films deposited on steel separators used in fuel cells.
Figure 2. The Thermo Scientific K-Alpha
Au 4f spectra of a stainless steel surface coated with gold are shown in Figure 3. The spectra were collected while performing a depth profiling experiment, which involved the removal of the surface layers by an incident Ar+ ion beam. The calibration to a known standard enables calculating the overlayer thickness from a depth profile (Figure 4).
Figure 3. Au 4f spectra as the gold film is removed from the steel surface
This data can be used to investigate the possibility of material transfer into the film. Figure 4b shows the migration of chromium from the steel surface and formation of a layer between the bulk steel and gold by the chromium.
Figure 4. Examples of depth profiling experiments of gold film on a steel substrate. a) The depth profile of a thick gold film. b) Depth profile of thin gold film.
A non-destructive method can be employed in the case of thin layers having thickness of <10nm. The overlayer thickness that would lead to the observed attenuation of the substrate signal is calculated using a simple model. The automation of the thickness calculation, from data collection to processing, can be done using the Thermo Scientific Avantage Data System. This enables exporting a series of analyses to a spreadsheet and batch processing large sample sets.
Figure 5 presents the thickness calculation results for two different analysis techniques, showing a clear correlation between the destructive and non-destructive depth profiling experiments on those samples where both techniques were applied.
Figure 5. Correlation between depth profiling and a non-destructive calculation method
In this experiment, the Thermo Scientific K-Alpha XPS was used to characterize the steel substrates coated with thin gold films. Two different methods were used to measure the film thickness and their results correlated well with each other. XPS was also found to be useful in the analysis of the film composition.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).
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