The degradation and high cost of electrodes materials used in proton exchange membrane fuel cells (PEMFCs) are major barriers limiting their commercialization in automotive vehicles [1]. The search for more affordable electrode materials has focused on controlling the surface structure and composition of novel multi-metallic catalytic nanoparticles on high surface area support membrane [2]. During fuel cell operation, the catalyst nanoparticles can dissolve, re-deposit and agglomerate, resulting in electrochemical surface area losses, and an associated decrease in catalyst activity [3]. It is therefore critical to understand the degradation mechanism of the nanoparticle catalysts during electrochemical aging in a fuel cell electrodes in order to improve the performance and lifetime of PEMFCs. Here we characterize a newly proposed fuel cell cathode material, comprised of nano-particulate platinum on a NbO_x-carbon hybrid support, using identical location electron tomography. A full HAADF-STEM tomographic tilt series of several representative clusters were obtained before and after acclerated stress tests (30,000 cycles from 0.6 to 1.0 V in an electrochemical cell).

Preliminary results are summarized in Figure 1. A three dimensional (3D) schematic of a simple PEMFC shows the overall structure and location of the cathode electrode catalyst material used in this study. Pt nanoparticles, a few nanometers in diameter, decorate the complex, 3D NbO_x-carbon hybrid support structure, imaged by HAADF STEM under identical conditions in (b) before and after electrochemical cycling and at two tilts separated by 40⁰. The 3D tomographic reconstructions of the structure are aligned and compared in (c). The high fidelity of the reconstructions and the relatively small overall changes observed in the structure enabled the semi-automated matching of over 500 individual Pt nanoparticles before and after cycling (d). The semi-automatized approach was accomplished with the aid of quantitative image analysis techniques including histogram normalization and maximum entropy thresholding, and alignment of the before and after reconstructions via a singular valued decomposition of the reconstructed Pt centroids matrix.  Preliminary results suggest that a net leaching of the Pt into solution has occurred during cycling, indicated by the overall reduction in size of a vast majority of Pt nanoparticles (e). Analysis of 3D reconstructions obtained from Pt-NbO_x-carbon hybrid structures differing in their Pt/NbO_x ratio are underway to better elucidate the role of NbO_x in the degradation of Pt.

Identical location electron tomography before and after electrochemical cycling has provided valuable insight into the degradation mechanism of the PEMFC electrode during cycling, enabling more informed decisions for the design of high-performance durable electrode materials.

References

[1] M. K. Debe, Nature, vol. 486, no. 7401, pp. 43–51, 2012.

[2] Y. Wang, K. S. Chen, J. Mishler, S. C. Cho, and X. C. Adroher, Appl. Energy, vol. 88, no. 4, pp. 981–1007, 2011.

[3] D. Banham, S. Ye, K. Pei, J. Ozaki, T. Kishimoto, and Y. Imashiro, J. Power Sources, vol. 285, pp. 334–348, 2015.

Acknowledgements

GAB is grateful for funding from NSERC under the CaRPE-FC network and to AFCC for partially supporting this work.

Figures:

Figure 1. Tracking the changes of a Pt-NbOx-carbon hybrid support cluster in a PEMFC cathode before and after electrochemical cycling. (a) A 3D schematic of the overall PEMFC. (b) A representative NbOx-carbon hybrid support structure, imaged by HAADF STEM, before (red) and after (green) electrochemical cycling and at two tilts separated by 400. (c) A comparison of the 3D tomographic reconstructions of the structures imaged in (b). (d) The semi-automated matching of over 500 individual Pt nanoparticles in the cluster before (closed circles) and after cycling (open circles, color matched). (e) The reduction in size of individual Pt nanoparticles.

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EMC Abstracts - https://emc-proceedings.com/abstract/understanding-the-degradation-of-pt-nanoparticles-in-a-fuel-cell-electrode-via-identical-location-electron-tomography/