Thus, the description of the Pt/ionomer interface is essential in understanding how these two are transported to the reaction sites because ORR is ultimately tied to interfacial O2 transport resistance and catalyst activity. Neutron Reflectometry (NR) has been a heavily exploited as a non-invasive technique to indirectly visualize the interface and material phase distribution within a thin film. Dura et. Al applied NR to measure a 42 nm thick Nafion film coated on planar Pt [3]. Their findings revealed a 1.1 nm thick water rich region near the interface and uniform water containing bulk film at 92% RH. However, in a similar condition, based on a NR study Wood et. al, claims differed greatly and they identified a water scarce region near the interface [4]. So far only these two NR studies have elucidated the Pt/ionomer interface in wet conditions.
We have investigated water sorption on bare Pt and in ~15 nm thick ionomer films coated on Pt. Three different ionomers of varying equivalent weight have been examined: Nafion (EW1100), 3M (EW725), and PFIA (EW 625). The presentation will share the results on the effect of ionomer molecular structure on interfacial morphology as well as water distribution within the each ionomer film. Some of the key findings emerging from our work are: (i) the presence of few monolayers equivalent water rich interfacial layer at Pt/ionomer interface (see Figure 1 below) but varying with ionomer EW; (ii) varying water content and water distribution for the different ionomers; (iii) temperature affects water distribution not merely content.
References
[1] K. Kudo, R. Jinnouchi, Y. Morimoto. Electrochimica Acta, 209 (2016) 682.
[2] K. Kodama, K. Motobayashi, A. Shinohara, N. Hasegawa, K. Kudo, R. Jinnouchi, M. Osawa, and Y. Morimoto. ACS Catal., 8, (2017) 694.
[3] J.A. Dura, V.S. Murthi, M. Hartman, S.K. Satija, C.F. Majkrzak. Macromolecules, 42, (2009) 4769.
[4] D. L. Wood III, J. Chlistunoff, J. Majewski, and R.L. Borup. J. of American Chemical Society, 131, (2009) 18096.