Effects of Processing Time, Mixing Speed, and Mixer on Agglomerates in Fuel Cell Cathode Inks

Abstract

To prepare a catalyst ink, the large agglomerates of platinum on carbon (Pt/C) powder must be broken down into the primary aggregate particles. This physical process is necessary to form a homogenous mixture that produces uniform and defect-free coatings, as well as maximizing electrochemical performance. Large agglomerates of Pt/C catalyst can limit oxygen diffusion and reduce contact area between ionomer and active Pt area in polymer electrolyte membrane fuel cells (PEMFCs) and electrolyzer catalyst layers. Proper mixing of the catalyst ink before deposition is critical in breaking up these agglomerates, but excessive mixing wastes time and energy and, in some cases, may even damage the catalyst particles.1 There have been several studies of ultrasonication time and energy affecting cathode performance and agglomerate size.1–3 However, there has been considerably less work on mechanical mixing methods, which are more relevant for large-scale manufacturing.This work explores the impact of mixing time and speed on Pt/C agglomerate size using five different mechanical mixers including ball milling and rotor-stator mixers with distinct geometries. Optical micrograph analysis of these rod-coated catalyst layers reveals an exponential decay of particle agglomerates as a function of mixing time, eventually reaching a steady state where there is little to no change in the number of particles over 15 μm in diameter. For a given mixer geometry, mixing speed determines the lowest attainable particle size distribution with higher speeds leading to greater breakup of agglomerate particles (see Figure 1). Rheometry is utilized as a complementary technique to assess the degree of ink mixing completion. Continuous mixing renders higher ink shear viscosity values until plateauing at similar time intervals at which an unvarying particle size distribution is observed. In-situ electrochemical testing of these thin gas diffusion electrodes (GDEs) is employed to correlate the process-driven parameters that govern ink agglomerate size and rheology with fuel cell performance.This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE- AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Advanced Manufacturing Office (AMO).This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Advanced Manufacturing Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.(1) Wang, M.; Park, J. H.; Kabir, S.; Neyerlin, K. C.; Kariuki, N. N.; Lv, H.; Stamenkovic, V. R.; Myers, D. J.; Ulsh, M.; Mauger, S. A. Impact of Catalyst Ink Dispersing Methodology on Fuel Cell Performance Using In-Situ X-Ray Scattering. ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427. https://doi.org/10.1021/acsaem.9b01037.(2) Pollet, B. G. Let’s Not Ignore the Ultrasonic Effects on the Preparation of Fuel Cell Materials. Electrocatalysis 2014, 5 (4), 330–343. https://doi.org/10.1007/s12678-014-0211-4.(3) Pollet, B. G. The Use of Ultrasound for the Fabrication of Fuel Cell Materials. Int. J. Hydrog. Energy 2010, 35 (21), 11986–12004. https://doi.org/10.1016/j.ijhydene.2010.08.021.Caption: Figure 1. Plot of number of catalyst agglomerates particles with equivalent diameter larger than 15 µm as a function of rotor-stator rotational speed and time. Figure 1