Figure 1. Thermo Scientific™ Nexsa™ XPS System
Proton exchange membrane fuel cells are devices that are often used for the production of electricity followed by an electrochemical reaction between hydrogen and oxygen. Potential applications of these fuel cells are diverse in nature, ranging from powering cars to small electronic devices. In fact, these types of fuel cells are often environmentally friendly during their operation, and are also particularly attractive as a result of their high conversion efficiency.
One of the main components of a proton exchange fuel cell is the Membrane Electrode Assembly (MEA), which typically contains several layers of platinum in carbon black that is responsible for catalyzing the reaction between hydrogen and oxygen. During the manufacturing or developing process of an MEA, the aim is to maximize the surface area of platinum that is electrically connected to the conducting support. Any loss of surface area during these processes can decrease the overall efficiency of the final product of the device. For example, the loss of platinum can sometimes occur when high currents corrode the carbon-black support and release the active metal, thereby allowing its migration from the electrode surface to the adjacent polymer electrolyte to occur.
A useful tool to determine the mobility of hydrogen ions within an electrolyte is Nafion®, which is capable of determining the presence of platinum in a sample. This application note describes how the Nexsa XPS System was used to analyze an MEA and determine the potential migration of platinum from the catalytically active layers into the adjacent Nafion electrolyte.
The MEA is comprised of a series of platinum layers that are tens of microns thick, in which the platinum-containing anode and cathode layers are around the thicker Nafion material of the electrolyte. The Nafion is an electrically insulating material that allows for the transportation of hydrogen ions within the electrolyte. These cathode and anode layers are often too thick for analysis by conventional XPS depth analysis, therefore sectioning is often required for XPS analysis.
For the purposes of the current study, ultra-low angle microtomy (ULAM) was used to cross-section the MEA at an angle of few degrees to allow for effective depth information to be obtained by imaging the cross section. The dimensions of the ULAM-sectioned layers much large as compared to those produced by the X-ray probe area to allow for a greater number of data points to be evaluated on each layer, thereby enabling for the improved detection of the subtle diffusion of platinum in these nanometer scale layers.
The use of XPS is useful for the quantification of both elemental and chemical states within a wide area of the sample. As the concentration of platinum in the catalytically active layer is very low, it is necessary to use a high performance XPS tool to detect it. It is important to note that the detection limit of XPS for elements is 0.5 atomic percent. Even at low concentrations, a high quality and good signal-to-noise spectrum was acquired from the catalyst layer, however, in the middle of the Nafion layer, no detectable platinum concentrations were found.
Figure 2. Principal components phase map of MEA sample
The use of advanced processing procedures implemented in the Advantage Data System allows for a relatively simple and automatic correlation of data. The principal component’s analysis identifies a number of components of the data set that allow the data
to be reconstructed through the use of a subset of the components. The benefit of using this specific procedure is associated with its ability to adequately remove the noise from the data set while simultaneously retaining all spectral information.
Figure 3. Large area XPS map of Pt/Nafion layers and interfaces in ULAM-MEA fuel cell sample overlayed with optical image
Additionally, quantified mapping data acquired by the system can be placed over an optical image of the sample. A cross-section of the mapping data can also be generated to generate an atomic concentration linescan that provides information on the atomic concentration of platinum along a line across the cathode, Nafion and anode layers, thereby demonstrating that no large scale diffusion of the platinum into the Nafion has occurred.
Figure 4. Quantified platinum atomic percent linescan taken from large area XPS map (shown on Figure 3)
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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