The catalyst layer (CL) is one of critical component for determining the performance of proton exchange membrane fuel cells (PEMFCs), and the production process of CL is important issue. In order to form the CL, catalyst ink mixed with platinum-supported carbon (Pt/C), ionomer solution, and a solvent is necessary. When the catalyst ink is coated on the sheet and dried, the CL is formed. By transferring the catalyst layer to the membrane, catalyst coated membrane can be made [1]. The process from the initial characteristics of the catalyst ink to the drying process determines the structure of CL and dominates the characteristics of the fuel cell. Thus, quality control of the catalyst ink is important to form a high performance and uniform CL [2].

In the previous study, we have reported that oxidation reaction of alcohol, as catalyzed by platinum, resulted in degradation of the catalyst ink [3]. It was found that the oxidation reaction not only occurred in the aging process of the catalyst ink but also occurred through the fabrication process. In order to investigate the degradation process in more detail, the relation between degradation reaction and mixing at catalyst ink fabrication process was analyzed in this study. The composition change of the catalyst ink was investigated using gas chromatography mass spectrometry (GC/MS) and X-ray computed tomography (CT).

Catalyst ink was prepared by using the mixture of water and ethanol (or 1-propanpol) as a dispersant. We used platinum catalyst supported on carbon black and Nafion dispersion D1021 (or D2020) (Chemours). As a mixing process, a high-speed rotary type mixer Filmix (Primix) utilizing shearing force, specialized for producing nanometer slurry was used. The prepared ink was immediately transferred to a sample container and sealed tightly.

Several catalyst inks were prepared using mixing time as a parameter (2–40 min), and the components of each ink were analyzed by GC/MS. The results showed that the concentration of hydrophobic substances (such as ethyl acetate) in the catalyst ink increased as the mixing time increased. Therefore, if the mixing time is increased to improve the dispersibility of the Pt/C and Nafion in the ink, the concentration of the hydrophobic substance is also increased, and phase separation may occur in the catalyst ink. As a result, the risk of cracking is increased when forming the catalyst layer [4].

In order to suppress the production of hydrophobic substances by the oxidation reaction, the catalyst ink was fabricated in an inert atmosphere. However, as a result of GC/MS analysis, it was found that hydrophobic substances were still produced in the ink. Further, as a result of visualizing the inside of the ink by X-ray CT (Figure 1), it was observed that many micro bubbles existing in the catalyst ink grew with time. It is considered that the dehydrogenation reaction of alcohol and water was caused by the catalytic action of Pt even in the absence of oxygen. Since ester and hydrogen are generated even without oxygen, the degradation reaction proceeds even with the catalyst ink stored in the inert gas, and the composition change and the bubble diameter increase were occurred.

Acknowledgment:

This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The authors thank the Suzukakedai Materials Analysis Division for performing sample analysis using GC/MS.

References:

1. T. Suzuki, ECS Transactions, 75(14), 423-434 (2016).
2. S.J. Shin et al., J. Power Source 106, 146-152 (2002).
3. S. Uemura et al., J. Electrochem. Soc., 165-3, F142 (2018).
4. S. Uemura et al., J. Electrochem. Soc., 166-2, F89 (2019).