MRI Investigation of Local Diffusion Phenomena in PEFC Catalyst Ink
Authors: Kameya Yuki , Iriguchi Norio , Uemura Suguru , Sasabe Takashi , Yoshida Toshihiko , Hirai Shuichiro 
Published: February 2020

The catalyst layer (CL) of polymer electrolyte fuels cells (PEFCs) plays a critical role in their 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 a catalyst ink is prepared by mixing catalyst particles and ionomers 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 required for the transport phenomena taking place in the electrochemical reactions, it is desirable to control the microstructure of CL during the decal process [3]. Therefore, the catalyst ink and its decal process should be studied in detail.

Herein, we propose a novel approach to investigate the characteristics of catalyst inks by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy. Although traditional NMR studies of ionomers have been reported [4,5], the detailed discussion regarding the spacial inhomogeneity of particles in catalyst inks and their dynamics during the drying processs has been lacking. MRI enables us to visualize the spacial variations in the sample characteristics which cannot be accessed through other spectroscopy techniques [6,7]. Because the dispersion stability of catalyst particles and the morphology of ionomer depend on the solvent condition, we first examined the solvent dynamics by determining the diffusion coefficients. We developed a technique to acquire NMR spectra from a selected volume of a liquid sample and determine the local diffusion coefficients of component molecules. The proposed technique was verified using the reference values of diffusion coefficients of water and alcohols (D = 2.13×10-9 and 0.57×10-9 m2/s for water and n-propyl alcohol, respectively) [8-10] and then applied to our samples. The diffusion coefficients of water and n-propyl alcohol (NPA or 1-propanol) typically used in the solvent of PEFC catalyst ink were obtained at various mixture compositions. The diffusion coefficients of both water and NPA remarkably changed depending on the mixuture composition, which indicates the change in molecular microstructure [10]. Then, we mixed ionomers with the solvent and examined the effect of ionomer addition on the solvent dynamics. We observed the decrese in the diffusion coefficients of water and NPA due to the existence of mixed ionomers. In addition, we obtained MRI images of catalyst inks and the precipitated bed of carbon particles was visualized. We determined the local diffusion coefficient of water in the particle bed region with a thickness of 2 mm, and the dependence of diffusion coefficient on the diffusion time was investigated. As we increased the diffusion time from 15 to 50 ms, the diffusion coefficient of water existing in the carbon bed monotonically decreased. The result indicated that the molecular diffusion of solvent molecules were distracted by the closely-packed carbon particles. This approach might be useful to investigate the packing condition of CL obtained through the decal method.

We demonstrated that our approach provided a unique capability to investigate the catalyst ink and its decal process. It will be proved that this technique is useful in investigating many samples prepared at various conditions and better understand the decal process.

Acknowledgment

This paper is based on 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] Zamel, J. Power Sources 309 (2016) 141.

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

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

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

[7] W.S. Price, “NMR studies of translational motion”, Cambridge Univ. Press (2009).

[8] M. Holz et al., Phys. Chem. Chem. Phys 2 (2000) 4740.

[9] P.S. Tofts et al., Magn. Reson. Med. 43 (2000) 368.

[10] R. Li et al., J. Phys. Chem. B 118 (2014) 10156.

Figure 1