The catalyst layer (CL) of polymer electrolyte fuel cell (PEFC) comprises catalyst particles, catalyst support (e.g., carbon particles), and proton-conductive ionomers. The CL plays a critical role in PEFC operation, and also the production process of CL is of significant importance in terms of the cell performance as well as the system cost. The decal method has been used in the commercial production of CL in membrane electrode assemblies, in which the catalyst ink is prepared by mixing catalyst particles and ionomer in the solvent, coated on a decal substrate, dried to form a particulate layer with a specified thickness, and transferred to a membrane via hot pressing [1,2]. Since the three-phase interface (catalyst-void-ionomer) is essential for the mass transport during the electrochemical reaction, it is desirable to control the microstructure of CL [3]. Therefore, the catalyst ink and its use in the decal process should be studied in detail.

We have developed a novel approach to investigate the characteristics of catalyst ink by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy [4]. Although traditional NMR studies of ionomers have been reported [5,6], the detailed discussion regarding the spatial inhomogeneity of particles in catalyst ink and their dynamics during the drying process has been lacking. MRI enables us to visualize the spatial variations in the sample characteristics which cannot be accessed through other spectroscopy techniques [7]. In this study, we applied MRI and ¹H/¹⁹F NMR to the catalyst ink for the decal process for gaining an insight on the dispersion state of catalyst and ionomer.

Using the mixture of water and NPA as a dispersant, we prepared catalyst ink samples. We used platinum catalyst supported on carbon black (Pt/C) and Nafion dispersion D2020 (Chemours). Pre-mixing of catalyst ink was performed using ultrasonicaton to break large Pt/C agglomerates into smaller units. We then used a thin-film spin system high-speed mixer Filmix (Primix), which is designed for the production of slurries of nanometer-sized particles, in the main mixing process. Because Nafion works as a dispersant of Pt/C particles, the dispersion state of Nafion has an impact on the dispersibility of the Pt/C particles and eventually on the stability of catalyst ink. We conducted MRI/NMR measurements for the catalyst ink samples at each stage of preparation process to investigate changes in the dispersion state of Pt/C particles and Nafion.

We found a fully-mixed sample exhibited a MRI image with submillimeter fine contrast because air bubbles were mixed in the sample during the main mixing process. It was difficult to remove air bubbles from the sample only by leaving it at rest for several hours. This observation was supported by the rheology measurement of catalyst ink. Hence we needed to use a magnetic stirrer to effectively perform the degassing of catalyst ink after the main mixing process. The degassed state of catalyst ink was confirmed by MRI image and ¹H NMR spectrum. ¹⁹F NMR spectra showed remarkable changes concerning the peaks associated with the main and side chains of Nafion, which indicated the dispersion state of Nafion varied during the mixing process.

In the present work, we have demonstrated that our experimental approach can provide a unique capability to investigate the dispersion state of catalyst ink. It is expected that the catalyst ink and its drying process will be investigated to optimize the fabrication process of PEFC catalyst layer.

Acknowledgment:

This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References:

[1] H.J. Cho et al., Int. J. Hydrogen Energy 36 (2011) 12465.

[2] X. Liang et al., Fuel 139 (2015) 393.

[3] N. Zamel, J. Power Sources 309 (2016) 141.

[4] Y. Kameya et al., ECS Trans 75 (2016) 275.

[5] S. Ma et al., Solid State Ionics 178 (2007) 1568.

[6] C. Welch et al., ACS Macro Lett. 1 (2012) 1403.

[7] M.H. Levitt, “Spin dynamics” 2nd edition, Wiley (2008).