Analysis of the Catalyst Layer and the Ionomer for PEFCs of the Different Operating Conditions
Authors: Tsuji Junichi , Akiyama Tsuyoshi , Ueno Yoshihiro , Masuda Akihiro , Kaneko Naoto , Yamaga Yu 
Published: February 2020

1. Introduction

In the durability evaluation of the PEFC (Polymer electrolyte fuel cell), the degradation characteristics are different due to the difference of the potential cycle test method. It is inferred that the influence of the PEFC by the potential cycle test is different every the constructional element. In order to improve the cycle characteristics, it is considered that to clarify the correlation between the degradation acceleration part and the test method is one of the important factors.
In this study, for the MEA (Membrane and electrode assembly) made by the commercial catalyst and electrolyte materials, two potential cycle tests were carried out. The MEAs was analyzed by EPMA (Electron probe micro analyzer), LC/MS (Liquid chromatography–mass spectrometry), et al., and the degradation part was evaluated.

2. Experimental

The electrolyte membrane and the ionomer were composed of Nafion®, and the Pt / C was used as the catalyst powder in both the anode electrode and the cathode electrode. The catalyst ink was applied by the spray method, and the CCMs (Catalyst coated membrane) were created. The durability tests in the two conditions were carried out after the conditioning operation. One of the operating conditions was the Start-Stop cycle test. In that condition, the MEA was operated with the sawtooth-shaped potential cycle of between 1.0 V and 1.5 V (2 s/cycle). With the Load cycle test which is another condition, the MEA was operated with the square-shaped potential cycle of between 0.6V and 1.0V (6 s/cycle). About the CCMs that the durability tests were carried out, the elemental distribution of the cross section was analyzed by EPMA. In addition, the information of the degradation products about the ionomer was obtained by LC/MS.

3. Result and discussion

Figure 1 shows the SEM images (upper side) and the line profiles (lower side). From the SEM image of Initial, each thickness of the anode layer, the electrolyte membrane and the cathode layer is about 7 mm, 60 mm and 7 mm, respectively. Each thickness of the anode layer and the electrolyte membrane in Start-Stop cycle is also the same value as Initial. However, the thickness of the cathode layer is thinner than Initial. It is inferred that the change of the layer thickness is due to the influence of the durability test. On the other hand, there is no significant different about each region in Load cycle. In the line profile of the elements by EPMA, the profile shape of Load cycle is almost the same as Initial. The concentration of Pt in the cathode layer of Start-Stop cycle is higher than the other samples. In the cathode layer of Load cycle, because the layer thickness became thin, it is inferred that Pt concentrated.
As the result of the other analysis, it was revealed by LC/MS that the degradation product quantity for the ionomer of Load cycle was the largest and the degradation product quantity for the electrolyte of Initial was the smallest.
From those results, it is inferred that the influence to the catalyst layer is large on the Start-Stop cycle test and the influence to the ionomer is large on the Load cycle test.

Figure 1