Combined Infrared Thermography, X‑ray Radiography, and Computed Tomography for Ink Drying Studies Raven Fournier, † Andrew D. Shum, † Jiangjin Liu, † Dinesh C. Sabarirajan, † Xianghui Xiao, § and Iryna V. Zenyuk* ,†,‡ † Department of Mechanical Engineering, Tufts University, Medford, Massachusetts 02155, United States ‡ Department of Chemical Engineering and Materials Science and National Fuel Cell Research Center, University of California Irvine, Irvine, California 92617, United States § X-ray Science Division, Argonne National Laboratory, 9700 S. Class Avenue, Lemont, Illinois 60439, United States * S Supporting Information ABSTRACT: This work describes the comparison of drying behavior of ink deposited on two substrates used commonly in polymer electrolyte fuel cells (PEFCs): microporous layer (MPL) and Nafion. For the same drying conditions and ink composition, ink deposited onto the MPL dried faster and also formed cracks faster. During drying, ink deposited on the MPL had more cracks and smaller intercrack spacing compared to ink deposited on Nafion. Smaller intercrack spacing for ink on the MPL substrate is explained by the higher critical stress predicted by the model. A novel combination of infrared (IR) thermography, X-ray radiog- raphy, and computed tomography (CT) is used to character- ize ink drying behavior. X-ray radiography with fast temporal resolution showed the existence of skin on the top of drying film that formed due to slow convective ink transport within the film and fast evaporative drying. From X-ray CT, detailed three-dimensional data was obtained on crack morphology within the drying film and was correlated to temperature profiles obtained with IR-thermography. The formation of a Y-crack in the film deposited on the MPL was correlated to the defect within the MPL substrate. The combination of three techniques is a unique probe to capture the temporal, thermal, and morphological evolution of drying ink. KEYWORDS: IR thermography, X-ray computed tomography, X-ray radiography, evaporation, ink drying, fuel cells 1. INTRODUCTION The quality of coated film is dictated by many factors, some of which are ink processing parameters, ink rheology, and substrate properties. Inks for fuel cell and battery technologies are similar in composition as they typically consist of an active material, a binder (polymer material), and a solvent. 1-3 Fabricating electrodes with precise control of composition is challenging but is much needed as defect-free and uniform films can boost the electrochemical performance of energy- storage and conversion technologies. One of the critical challenges is understanding the transformation of ink into a porous electrode, where ink rheology and processing can be tied to resulting porous morphology. 3 The ink drying process is controlled by solvent evaporation. Drying is also strongly dependent on ink composition, where active material, binder, and solvent local interactions determine the final density of the film, its thickness, and whether the film will be crack-free or remain as a uniform layer. Recent efforts focus on under- standing interactions between ink solvent, active material, and binder by means of rheological studies, X-ray techniques, and electrochemistry. 2-4 For the electrochemical energy-conversion field, the ink drying process is pertinent in fabricating various fuel cell electrodes. A key component of polymer electrolyte fuel cells (PEFCs) is a membrane electrode assemblyMEA), which generally consists of a membrane and porous carbon-supported layers (some of which are catalytically active). When catalyst ink is deposited onto the membrane, a catalyst-coated membrane (CCM) is formed, whereas when ink is deposited onto the microporous layer (MPL) substrate of a gas diffusion layer (GDL), a gas diffusion electrode (GDE) is formed. These two electrode fabrication methods are equally frequent in the PEFC community. Cracks observed for water-based inks are believed to form due to nonuniform removal of solvent, during which hydrophobic interactions between carbon material and solvent dictate local stresses and crack formation and growth. Optimizing ink composition and coating conditions, as well as reducing film thickness can help produce crack-free films. For Received: July 13, 2018 Accepted: October 11, 2018 Published: October 11, 2018 Article www.acsaem.org Cite This: ACS Appl. Energy Mater. 2018, 1, 6101-6114 © 2018 American Chemical Society 6101 DOI: 10.1021/acsaem.8b01147 ACS Appl. Energy Mater. 2018, 1, 6101-6114 ACS Appl. Energy Mater. 2018.1:6101-6114. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/03/18. For personal use only.