XPS was used for the study of the membrane electrode assembly of a proton exchange fuel cell for determining the platinum distribution in the component. Also, layer uniformity was studied with large area imaging. The preparation of the samples was performed with ultra-low angle microtomy before the analysis.
Proton exchange membrane fuel cells are devices used for electricity generation from the electrochemical reaction of hydrogen and oxygen, with potential applications ranging from operating cars to the powering of tiny electronic devices. As the fuel cells have excellent conversion efficiency, they are appealing and are eco-friendly at the point of use.
The membrane electrode assembly is one of the components of a proton exchange fuel cell. The MEA has platinum layers in carbon black that catalyze the hydrogen-oxygen reaction. While developing or manufacturing an MEA, the objective is to increase the surface area of the platinum, which is electrically coupled to the conducting support.
The device efficiency is reduced by any surface area loss. Platinum loss may take place sometimes when high current corrode the carbon-black support causing release of the active metal enabling its migration from the electrode surface to the adjacent polymer electrolyte. Nafion® is a typical electrolyte material.
Platinum presence in the Nafion will stop hydrogen ion mobility in the electrolyte. This article enumerates how XPS is used for MEA analysis and to determine if platinum migration has taken place from the catalytically active layers into the adjacent Nafion electrolyte.
The MEA comprises layers that are tens of microns thick. The anode and cathode layers containing platinum surround the thicker Nafion electrolyte (Figure 1). The Nafion is electrically insulating, however, enables hydrogen ion transport. These layers are very thick for conventional XPS depth profiling. Hence, for XPS analysis, sectioning is needed.
For performing a cross-section of the MEA at an angle of a few degrees, ultra-low angle microtomy (ULAM) was used, enabling appropriate depth information to be obtained by cross-section imaging.
In comparison with the X-ray probe area, the ULAM-sectioned layer dimensions are large enough so it is possible to have a number of data points per layer. This makes it possible to identify the slightest platinum diffusion in these nanometer scale layers.
Figure 1. Thermo Scientific K-Alpha optical image of ULAM-prepared MEA fuel cell sample
XPS can be used for the quantification of chemical and elemental stress across a broad sample area. As the platinum concentration in the catalytically active layer is very low, it is important to use a high performance XPS tool for detection.
The XPS detection limit for elements is 0.5 atomic percent. Even at a low concentration, a good, superior quality signal-to-noise spectrum was obtained from the catalyst layer.
There was no detectable platinum in the center of the Nafion layer. XPS can be used for the quantification of chemical and elemental states across a broad sample area. An epoxy versus Nafion versus platinum (Figure 2) map was produced by obtaining the complete spectral datasets at each mapping pixel. Automatic data correlation is relatively simple while using sophisticated processing procedures implemented in the Avantage Data System.
Figure 2. Principal components phase map of MEA sample
Several components of the data set are identified in the analysis of the principal component. It also enables reconstruction of the data using a component subset. This procedure is advantageous as noise is removed from the data set but all spectral data is retained. Consequently, the signal-to-noise ratio is enhanced.
The quantified mapping data can also be taken and overlaid onto the sample’s optical image (Figure 3) or a cross-section of the mapping data can be obtained to create an atomic concentration linescan (Figure 4). Platinum’s atomic concentration along a line across the cathode, anode and Nafion layers is shown in the linescan and it proves that there is no large-scale platinum diffusion into the Nafion.
Figure 3. Large area XPS map of Pt/Nafion layers and interfaces in ULAM-MEA fuel cell sample overlayed with optical image
Figure 4. Quantified platinum atomic percent linescan taken from large area XPS map (shown on Figure 3)
From XPS investigation of membrane electrode assembly, it was concluded that the catalytic effect at the anode and cathode on this sample is not detrimentally affected by platinum loss. It was also observed that platinum did not migrate from the catalytically active layers into the adjacent Nafion electrolyte.
This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – X-Ray Photoelectron Spectroscopy.
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