Influence of Cathode Catalyst Layer Structure on PEFC Performance Using Two-Dimensional Carbon Support for Pt Catalyst

Abstract

Introduction Polymer electrolyte fuel cell (PEFC) has been attracting much attention due to its high efficiency at low operating temperature. However, since platinum (Pt), which is used for the electrode catalyst in the cathode, is a metal of low abundance and high cost, reduction of the Pt usage is required. Generally, nano-size Pt particles supported on carbon black are used to increase the mass activity. However, nanoparticles have problems in durability. Moreover, since some Pt particles are located inside the pore of the support, diffusion of supplied oxygen and produced water are suppressed. Recently, nanostructured thin film (NSTF)1) and Pt thin film fabricated by atomic layer deposition (ALD)2) are reported as alternative catalysts. However, they are fabricated on non-conducting materials. Takenaka et al. fabricated metal oxide nanofilms using graphene oxide (GO) as a template to form two-dimensional (2-D) catalysts3). 2-D catalysts are expected to show high mass activity due to the high surface area, and high conductivity during power generation due to the reduced GO and/or metal oxide. In this study, membrane electrode assembly (MEA) was prepared using Pt/GO catalyst, and the relationship between the performance and the structure of the electrode catalyst layer using 2-D carbon support for Pt catalyst was investigated to obtain information towards low Pt loaded PEFC. Experimental Catalyst inks were prepared by mixing Pt /Graphene oxide (Pt/GO), carbon black (CB), n-Propyl alcohol, and distilled water, and then the mixture was sonicated. 5 wt.% Nafion® solution was added to the mixture which was sonicated again. Catalyst ink was mixed by rotation revolution mixer. Then it was coated onto polytetrafluoroethylene (PTFE) sheet using doctor blade method to fabricate catalyst layer (CL). CL and Pt/carbon paper were hot-pressed onto each side of the Nafion® membrane. After the PTFE sheet was peeled off, carbon paper was hot-pressed onto CL to fabricate the MEA. Terminal voltage vs. current density of the MEA was measured at 80 ˚C. Results and Discussion Terminal voltage vs. current density of MEA with various ionomer/carbon ratios (I/C) using Pt/GO are shown in Fig. 1. As it can be seen in the figure, the cell performance in the low current density region was higher when I/C was higher. This indicates that reactive sites have increased due to the coating of catalyst by ionomer. On the other hand, MEA prepared with low I/C showed higher cell performance in the high current density region. It is considered that ohmic resistance was reduced due to the thin coating of catalyst by ionomer. References 1) M. K. Debe, J. Electrochem. Soc, 160 (6), F522-F534 (2013) 2) M. Inaba, T. Suzuki, T. Hatanaka, Y. Morimoto, J. Electrochem. Soc, 162 (7), F634-F638 (2015) 3) S. Takenaka, S. Miyake, S. Uwai, H. Matsune, M. Kishida, J. Phys. Chem. C, 119 (22), 12445-12454 (2015) Figure 1