Effect of Reaction Products on the PEFC Catalyst Ink Property and Catalyst Layer Quality
Authors: Uemura Suguru , Sakai Katsunori , Sasabe Takashi , Matsumoto Hidetoshi , Sugimori Hidekazu , Shinohara Kazuhiko , Hirai Shuichiro 
Published: December 2020

The catalyst layer (CL) is a critical component that determines the performance of proton exchange membrane fuel cells, and the production process of CL is important issue. A CL is fabricated from a catalyst ink mixed with platinum-supported carbon (Pt/C), ionomer solution, and a solvent. When the catalyst ink is coated on the sheet and dried, the CL is formed. By transferring the CL 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].
We have reported that alcohol oxidation reaction and condensation reaction, as catalyzed by platinum, occurred in the catalyst ink fabrication process [3]. Several reaction products, aldehydes, acids, and esters, were detected by gas chromatography mass spectrometry (GC/MS) [4]. In this study, concentration of those reaction products were measured quantitatively, and the effect of those reaction products on the catalyst ink property and CL quality was investigated.
Catalyst ink was prepared by using the mixture of water and ethanol as a dispersant. We used platinum catalyst supported on carbon black and Nafion dispersion. The solvent was composed of ethanol and water. The weight ratio between ethanol and water was set to 46:54 or 60:40. Nonvolatile content was 10 wt%. Weight ratio between Nafion and the carbon support was 0.75. As a mixing process, a high-speed rotary type mixer was used, and mixing time was set to 10 min. The quantitative concentration of reaction products in each ink were analyzed by GC/MS. After concentration measurement, CL was fabricated using each ink. The crack formation was observed, and the correlation with the initial composition of the ink and the concentration of the reaction product was investigated.
As a result of GC/MS, several reaction products (acetaldehyde, acetic acid, and ethyl acetate) were detected in the catalyst ink. Table 1 shows quantitative concentration of reaction products. Comparing the both catalyst inks, the concentration of acetaldehyde increased remarkably when the ethanol concentration increased. CLs were fabricated with using those catalyst inks and observed by microscope (Figure 1). There was no crack in the low ethanol concentration ink, and cracks occurred in the high ethanol concentration ink. Those results suggest that acetaldehyde may be a factor in causing cracks in the CL.
To clarify the effect of acetaldehyde, acetic acid, and ethyl acetate on the crack formation in the CL, each substance was artificially added to the low ethanol concentration catalyst ink. In actual catalyst ink, it is considered that ethanol reacts to produce a reaction product. Therefore, in the modified catalyst ink, the amount of ethanol was reduced according to the amount of added substance. The CL was fabricated by using those catalyst ink and the observation results were shown in Figure 2. When the same amount of acetaldehyde contained in the high ethanol concentration catalyst ink was added, many cracks were generated in the CL. Several cracks occurred when ethyl acetate was added, but the required amount was significantly higher than the ethyl acetate concentration detected in the actual catalyst ink. No cracks were generated even if a large amount of acetic acid was added to the catalyst ink. These results indicate that the acetaldehyde is a factor of crack in the CL. It is considered that when the generated acetaldehyde exceeds a certain concentration in the catalyst ink, crack is caused in the CL.

Acknowledgment:

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

References:

T. Suzuki, ECS Transactions, 75(14), 423-434 (2016).

S.J. Shin et al., J. Power Source 106, 146-152 (2002).

S. Uemura et al., J. Electrochem. Soc., 166-2, F89 (2019).

S. Uemura et al., ECS Trans., 92-8, 183-187 (2019).

Figure 1