Catalyst layers for proton exchange membrane fuel cells (PEMFCs) are typically prepared by first dispersing a catalyst powder, comprised of platinum or platinum alloy nanoparticles supported on carbon blacks, with ionomer in solvent followed by extensive mixing using a variety of methods. The purpose of the ink mixing and processing procedure it to break up large agglomerates of the catalyzed carbon powder (>10 µm) and form a uniform dispersion, However, care must be taken to limit damage to the catalyst particles. Following the dispersing process, the catalyst ink is applied to the surface of the substrate (membrane, diffusion media, or transfer liner) through a variety of coating processes (painting, spraying, screen printing, etc.) [1]. In addition to creating coating defects, large agglomerates resulting from inadequate ink processing can limit catalyst utilization, inhibit mass transport in the catalyst layer, and damage the membrane and possibly also the gas diffusion layer. This can lower beginning-of-life performance by creating transport barriers for oxygen, hydrogen, and protons, and can increase membrane pinhole formation and electrical shorts, thus limiting the cell lifetime. This presentation will describe ink compositions and processing techniques for efficient coating of catalyst layers while optimizing cell performance and lifetime and minimizing ink processing time and cost [2, 3]. Ultra-small angle X-ray scattering (USAXS) was used to measure the agglomerate size distribution during ink mixing/processing. The experimental setup for the USAXS evaluation of inks during sonication is shown in Figure 1. A flask containing catalyst ink was immersed in an ice bath and sonicated using a water bath and/or a horn sonicator and pumped through a thin-walled capillary tube situated in the X-ray beam using a peristaltic pump. The effects of catalyst ink composition and processing variables on catalyst agglomerate size and ink properties were evaluated using different solvents, ionomer-carbon ratios, support types, and sonication time. The proper ink composition and sonication method that facilitate the break-up of carbon agglomerates with the minimum amount of time were determined. The electrochemically-active surface areas of the catalyst after the ink processing were determined using aqueous electrochemical methods to determine the extent of damage to the catalyst after processing (e.g., dislodging of catalyst particles from the support). The correlation of ink processing conditions and agglomerate structure with fuel cell performance will also be described.
Acknowledgements
This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency&Renewable Energy, Advanced Manufacturing Office, Roll-to-Roll Advanced Materials Manufacturing DOE Laboratory Consortium. This research used the resources of the Advanced Photon Source (APS), a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
References
[1] B. Bladergroen, H. Su, S. Pasupathi, V. Linkov, Chapter 3, Electrolysis, INTECH, 2012.
[2] S. A. Mauger, K. C. Neyerlin, A. C. Yang-Neyerlin, M. Ulsh, 232nd ECS Meeting, National Harbor, MD, October 1-5, 201.
[3] I. V. Zenyuk, N. Englund, , G. Bender, A. Z. Weber and M. Ulsh, J. Power Sources 332 (2016) ,372–382.
