Effect of the Specific Adsorption of Sulfonate Group in Ionomer and Adsorbed Oxide Species on the Oxygen Reduction Reaction Activity of PEFC Catalyst

Abstract

Introduction For wide spread of Polymer Electrolyte Fuel Cell (PEFC), It is necessary to improve the oxygen reduction reaction (ORR) activity of cathode catalyst. While the Ionomer (e.g., Nafion®) have been used in commercial PFEC to improve the triple-phase boundary [1]and the effect of side-chain in perfluoro-sulfonic acid ionomers adsorbing on the surface of Pt has been discussed. [2,3] For the terminal sulfonate group is adsorbed on Pt by one or two oxygen atom(s) and the oxygen atom of ether group also interaction with the Pt, it’s shown that the existence of ionomer will block the oxygen reduction reaction. [4]In this study, the weight ratio of Ionomer and carbon which was used in the catalyst (I/C) has been changed to control the degree of specific adsorption in order to analysis the effect of specific activity and construct a model of specific adsorption on the surface of Pt. Experiment Core-shell catalyst (Pd@Pt@Vulcan) is used to eliminate the signal from bulk of Pt catalyst while XAFS measurement. Catalyst ink is made by mixture Catalyst powder(6.446mg) with ultrapure water (7.6ml), isopropanol (2.4ml) and 5wt% Nafion®solution (quantity is changed with I/C) then bath sonicated in ice bath for 20 mins to obtain well-dispersed ink. Thin-film Rotating Disk Electrode (TF-RDE) technique is used to get a homogeneous catalyst layer. Finally, 10μl aliquots of the ink were subsequently pipetted on to the GC disk and get a catalyst layer with ~12μgcatalyst/cm2. For ORR activity measurements, the limiting current is calculated from the raw currents at 0.90V and analyzed to determine the specific activity. Results and Discussion In compare with tiny changes of ECSA, the ORR activity shows evident decrease along with I/C increasing, the more Nafion®is mixed in the ink, the more catalyst will be covered by ionomer which lead to more specific adsorption. For the specific adsorption is only happening at the interface of ionomer and the catalyst, ORR activity of catalyst will not be decreasing continuously with the degree of ionomer increasing. When the amount of Nafion®reaches a certain level the ORR activity is expected to be constant. In this research the I/C increase over about 1.0, there was not any changes have been shown about ORR activity. Reference [1] Kazuma Shinozaki, Yu Morimoto, Bryan S. Pivovar , Shyam S. Kocha, Journal of Power Sources. 325,745(2016) [2]Kazuma Shinozaki, Jason W. Zack,Svitlana Pylypenko, Bryan S. Pivovar,Shyam S. Kocha, Journal of The Electrochemical Society, 162(12) F1384-F1396 (2015) [3]Ram Subbaraman, Dusan Strmcnik, Vojislav Stamenkovic, Nenad M Markovic, J. Phys. Chem. C .114, 8414–8422 (2010) [4] Kensaku Kodama,Kenta Motobayashi, Akihiro Shinohara, Naoki Hasegawa, Kenji Kudo, Ryosuke Jinnouchi, Masatoshi Osawa, Yu Morimoto, ACS Catal. 8, 694(2018) Acknowledgments This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO)