Studying Pt-Based Proton Exchange Membrane Fuel Cell Degradation with Nano-Scale X-Ray Computed Tomography
Authors: Braaten Jonathan , Ogawa Shohei , Yarlagadda Venkata , Kongkanand Anusorn , Litster Shawn 
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are a promising energy conversion technology to help address the world’s need for clean, sustainable power, specifically within the automotive sector. While significant improvements have been made in recent years to improve their efficiency and durability, PEMFCs remain financially non-competitive with the incumbent internal combustion engine. This is largely due to the high cost of the platinum (Pt) used for PEMFCs’ electrocatalysts1. Many efforts have been made to reduce the amount of Pt used in PEMFCs, including the use of higher catalytic activity Pt-based alloys, improving catalyst utilization, and reducing catalyst and support degradation, among numerous others. Catalyst and support degradation is of particular interest, as it has been shown that during voltage cycling, the expensive Pt is stripped from the catalysts and migrates to other, larger catalysts2 or moves to the ionomer membrane where it forms a Pt band near the interface with the cathode catalyst layer (CCL)2,3. The redistribution and loss of this Pt causes significant electrochemically active surface area loss and corresponding performance degradation over time2,4, and thus methods of minimizing this phenomenon are of the utmost importance.

The purpose of this work was to utilize a novel method to visualize, characterize, and compare the loss of Pt for different types of PEMFC membrane electrode assemblies (MEAs). The two types of MEA were 20 wt % Pt on Vulcan (Pt/Vu) and 30 wt % Pt on high surface area carbon (PtCo/HSC), both with PTFE reinforced membranes. We chose these two types of MEA, as it allowed for us to determine the effects of alloying elements in the catalyst (cobalt) as well as the effect of different types of carbon support (Vu versus HSC) on the Pt lost during voltage cycling. For each MEA type, both fresh and cycled (30,000 cycles) MEAs were examined, giving the beginning of life and estimated end of life Pt distributions and large-scale pore morphologies. The analysis method consisted of nano-scale X-ray computed tomography (nano-CT), performed in both Zernike phase contrast (PC) and absorption (ABS) modes. The PC mode allowed for a large-scale pore size analysis for the carbon support to determine carbon/pore degradation, and the ABS mode allowed us to characterize the Pt distribution within each MEA. In this study, we utilized the large field of view (LFOV) feature for nano-CT, giving us a 64x64x64 µm reconstruction volume, with 65 nm voxel size for PC mode, and a 130 nm voxel size for ABS mode. Our PC results showed little to no increase in porosity for both types of MEA, indicating very little carbon degradation (in the CCL). Our ABS results showed significant Pt loss from both types of MEA’s CCLs, as well as an increase in Pt concentration near the CCL/membrane interface. Thus, from this analysis, we concluded that neither the Vu or HSC supports nor the addition of an alloying element to the catalyst (like cobalt) prevents Pt from dissolving and migrating. In addition, this work demonstrates the capabilities of the nano-CT method for analyzing Pt distributions and large-scale pore morphologies with a combination of LFOV ABS and PC modes, respectively.

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

References:

U.S. DOE, DOE Hydrogen and Fuel Cells Program Record, (2015).

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Yasuda, A. Taniguchi, T. Akita, T. Ioroi and Z. Siroma, Phys. Chem. Chem. Phys., 8 (6) 746-752 (2006).

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