Water Uptake and Interfacial Structural Changes of a Perfluorosulfonic acid Ionomer Membranes Measured by Neutron Reflectivity for PEM Fuel Cells
Vivek S. Murthi*, †, [‡], Joseph A. Dura†, §, Sushil Satija† and Charles F. Majkrzak†
* Materials Science and Engineering Department, University of Maryland, College Park, MD-20742
† NCNR, National Institute of Science and Technology, Gaithersburg, MD-20899
Proton exchange membrane fuel cells (PEMFC) are
promising power sources for vehicular transportation, residential and consumer
electronics. The current state-of-the-art membrane material is based on
perfluorinated sulfonic acid chemistry such as Nafionâ. They have good mechanical strength, chemical
stability and high proton conductivity and achieve good performance when
operating at 80-90°C and high relative humidity (>80% RH). However,
these membranes remain expensive and have several limiting factors such as low
conductivity at low relative humidity, high methanol permeability, and a low Tg
(which restricts its application below 100°C). Although
the wealth of prior results seems to indicate that water plays a central role
in determining the O2 permeation in proton exchange membranes, its
exact mechanism remains unclear and deserves further investigation. People have
recognized that gas permeation through a polymer membrane not only is a
function of the chemical structure of the polymer chains but also is determined
by morphology inside the film with typical domain dimensions of several
nanometers. Prior researches have shown that perfluorinated sulfonic acid
membranes such as NafionÒ 117 are phase-separated
materials1-3.
The microstructure model of Nafion suggests the formation of inverted micelles
with 
groups
forming hydrated cluster phase embedded in hydrophobic fluorocarbon phase. The
relative volume of hydrophobic phase is proposed to be responsible for the O2
solubility and the hydrophilic ionic domains are involved to a large extent in
the diffusion process. However,
better understanding of the correlations between the nanostructures of PEMs and
their transport (O2 and proton) properties need further
investigations with techniques such as Neutron Reflectometry (NR) and X-ray
reflectivity.
On the other hand, membrane structural properties and durability continues to be an area of interest that has not seen considerable improvement mainly because of the limited spectroscopic techniques available to study catalyst-MEA interfaces in a working fuel cell. Recently, Neutron Imaging has been used successfully to monitor the water transport gradients within Nafion inside a working fuel cell stack.4, 5
Both NR and X-ray reflectivity allow us to study composition depth profiles. Neutron reflectivity is well suited to probe structures containing water molecules especially because of the vast differences in the scattering length between H and D atoms. This enables us to control the contrast via isotopic substitution. Furthermore, the highly penetrating neutron probe allows for measuring the interfacial structure of complex MEA’s and mapping the water profile within an active fuel cell and yet maintaining its structural composition without any destruction to the assembled device. Thus NR is uniquely suited for in-situ measurements at active electrochemical systems such as DMFC and PEM fuel cells. In our current scope of measurements, NR is used to understand the water uptake and structural changes of the Nafion-water interface on a smooth gold surface vs. that on silicon oxide. Sorption isotherms have also been obtained on Nafion films on gold and compared to the isotherms of bulk films.
In addition, NR is a unique tool to probe film formation and surface roughness as a function of electrolyte, concentration, electrode potential, etc., in complex catalyst formation and degradation. Our preliminary simulations on 50 – 200 Å Pt films show that Ru adlayers as low as 3Å can be measured using Neutron Reflectometry. Measurement of the spontaneous deposition of Ru on Pt from Sulfuric acid containing micro-molar quantities of Ru species will help us gain insight into sample preparation methods and also pave the way for future NR on electrode materials for fuel cell applications and investigating the long term durability of the electrocatalyst layers in regard to leaching and deposition.
References
1. G. Gebel and B. Loppinet, in Ionomers: Characterization, Theory and Applications, S. Schlick Editor, CRC Press, Boca Raton, FL (1996).
2. T. D. Gierke, G. E. Munn and F. C. Wilson, J. Polym. Sci., Polym. Phys., 19, 1687 (1981).
3. F. Wang, M. Hickner, Y. S. Kim, T. A. Zawodzinski and J. E. McGrath, J. Membr. Sci., 197, 231 (2002).
4. R. J. Bellows, M. Y. Lin, M. Arif, A. K. Thompson and D. Jacobson, J. Electrochem. Soc., 146, 1099 (1999).
5. M. A. Hickner, N. P. Siegel, K. S. Chen, D. N. McBrayer, D. S. Hussey, D. L. Jacobson and M. Arif, J. Electrochem. Soc., 153, A902 (2006).