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Poster 04
Abstract
Application of FFC-NMR relaxometry to catalyst layer materials of
proton exchange membrane fuel cells
Makoto Yamaguchi a , Akihiro Ohira a,b
a Fuel Cell Cutting-Edge Research Center, Tokyo, Japan
b National Institute of Advanced Industrial Science <strong>and</strong> Technology (AIST), Osaka, Japan
In proton exchange membrane fuel cells catalyst layers work as either a cathode for oxygen
reduction or an anode for hydrogen oxidation. They consist of platinum catalyst particles
dispersed on carbon black particles <strong>and</strong> coated with ionomer thin films. Application of pulsed
field gradient NMR which made great success to measure self diffusion coefficients of the
membranes would be difficult due to their complex structure in nanoscales. Recently we
reported preliminary results on NMR relaxometry of the catalyst layers which showed NMRD
profiles quite different from the ionomer membranes due to strong interation of protons with
the platinum particles [1]. However, due to their complex structure more detailed studies are
necessary for their components <strong>and</strong> here we report two recent results on application of NMR
relaxometry. NMRD profiles were measured at Larmor frequencies from 0.001 to 40 MHz by
using Spinmaster FFC-2000 (Stelar s.r.l.).
1. Perfluorinated sulfonic acid ionomer membranes at low water content: Perrin et al.
observed changes in NMRD profiles by the type of surface interaction as well as water
content down to =3.4 (=[H 2 O]/[RSO 3 H]) in the case of Nafion membranes [2,3]. As water
content would become lower in the catalyst layers due to diffusion of gaseous reactants, we
measured 1 H NMRD profiles of H-Nafion membranes (DuPont NR212) of =1. The
membranes were evacuated at room temperature <strong>and</strong> sulfonic acid sites are assumed to form
dimers of ion pairs as (RSO 3 - H 3 O + ) 2. The 1 H relaxation rates gradually decreased with
temperature <strong>and</strong> the NMRD profiles became less steeper from power law to logarithmic
frequency depenence. They are fitted by formula corresponding to renormalized Rouse model
[4] which seems to be consistent with the assumption of restricted motions of protons in
bridged side chains.
2. Effect of acidity on proton relaxation by interaction with platinum surface: Proton spin
lattice relaxation rates of aqueous solutions contacting platinum particles were found to be
dependent on their acidity. In acid solutions relaxation rates become gradually larger than
water towards higher frequency <strong>and</strong> NMRD profiles are less steep. No significant effect of
associated anions was identified as surface diffusion of water would not be affected by weak
interaction of diffusion water <strong>and</strong> anions. In contrast, hydronium ions concentrated in the
vicinity of the metal surface would strongly affect surface diffusion of water by hydrogen
bond formation.
This work was financially supported by the Ministry of Economy, Trade <strong>and</strong> Industry
(METI) <strong>and</strong> the New Energy <strong>and</strong> Industrial Technology Development Organization (NEDO),
Japan.
[1] Yamaguchi, M., Ohira, A., MRPM11 (2012).
[2] Perrin J.-C., Lyonnard, S., Guillermo, A., Levitz, P., J. Phys. Chem. B 110 (2006) 5439-5444.
[3] Perrin J.-C., Lyonnard, S., Guillermo, A., Levitz, P., Magn. Reson. Imaging 25 (2007) 501-504.
[4] Fatkullin, N., Kimmich, R., Weber, H. W., Phys. Rev. E 47 (1993) 4600-4603.
<strong>8th</strong> <strong>Conference</strong> on Fast Field Cycling NMR Relaxometry, Turin 23-25 May 2013