Investigation of Hot Pressed Polymer Electrolyte Fuel Cell Assemblies via X-ray Computed Tomography

Abstract

The hot pressing process for fabricating membrane electrode assemblies (MEAs) has been widely adopted, yet little is known of its effects on the microstructural properties of the different components of the MEA. In particular, the interaction of the electrolyte, electrode and gas diffusion layer (GDL) due to lamination is difficult to probe as conventional imaging techniques cannot access the internal structure of the MEA. Here, a novel approach is used, which combines characterisation of hot-pressed membrane electrode assemblies using X-ray computed tomography, thermogravimetric analysis, differential scanning calorimetry and atomic force microscopy, with electrochemical performance measurements from polarisation curves and high-frequency impedance spectroscopy. Membrane electrode assemblies hot pressed at 100 o C, 130 o C and 170 o C reveal significant differences in microstructure, which has a consequence for the performance. When hot pressed at 100 o C, which is lower than the glass transition temperature of Nafion (123 o C), the catalyst only partially bonds with the Nafion membrane, leading to increased Ohmic resistance. At 170 o C, the Nafion membrane intrudes into the electrode, forming pinholes, degrading the catalyst layer and filling pores in the GDL. Finally, at 130 o C, the interfacial contact is optimum, with similar roughness factor between the catalyst and Nafion membrane surface, indicating effective lamination of layers.

X‐ray Nano Computed Tomography of Electrospun Fibrous Mats as Flow Battery Electrodes

“…The recent advances in X‐ray computed tomography (CT) have allowed characterisation of the microstructure of many electrochemical devices such as lithium‐ion batteries, solid oxide fuel cells, polymer electrolyte fuel cells, supercapacitors, and, more recently, RFBs . The application of tomographic images in the study of porous media has the benefit of allowing numerical simulations to be conducted directly on the actual structures represented in the images, and therefore allowing prediction of the material's performance based on its microstructural properties.…”

Multi‐Scale Imaging of Polymer Electrolyte Fuel Cells using X‐ray Micro‐ and Nano‐Computed Tomography, Transmission Electron Microscopy and Helium‐Ion Microscopy

“…The area scanned is larger than the representative equivalent area of 0.1 mm 2 ; hence, the observations are representative and do not describe isolated features. Thus, it is possible to calculate the porosity across the fibers in multiple directions, the interfacial contact between the layers (CL/GDL, MPL/CL, Nafion/CL and Nafion/GDL), the CL crack size and the Nafion roughness factor .…”

“…Imaging the Nafion/CL interface with X‐ray nano‐CT also reveals the presence of interfacial features (0.5–2 µm) bonding the phases together. As the catalyst surface is rougher than the Nafion surface (with respective roughness factors of 1.54 and 1.02 reported using micro‐CT and AFM), subsequent deformations and intrusions of both materials into one another occur during the hot‐pressing process. Once the heat and pressure are removed, the two materials remain partially attached to each other (Figure a), with areas fully bonded together; whereas other localised areas highlight no contact between the two surfaces, due to the depth of the cracks in the CL.…”

“…The GDE is hot‐pressed on the Nafion membrane for 3 min with an applied pressure of 2,757 kPa at 130 °C. These conditions are identical to previously published work which showed these conditions resulted in optimal performance . Building a half‐cell for nanoscale work (Figure b) instead of a full cell provides stronger structural integrity, limiting the risk of delamination of the GDL when machined into smaller samples.…”

“…Furthermore, it has been used to characterize the CL surface features and thickness, deposition methods , degradation mechanisms, CL crack length and Pt distribution . Recent work from the authors has linked GDL structure and PTFE content to water distribution using neutron imaging and X‐ray micro‐CT, along with an analysis of the effect of the hot‐pressing temperature on performance and structural changes of CL and Nafion membrane , . However, neither the SEM nor the X‐ray micro‐CT is sufficient to image the carbon and Pt nanoparticles or Nafion agglomerates within the CL and MPL.…”