PEM fuel cells offer the advantages of high power density and energy conversion efficiency, simplicity in design and operation, the added environmental benefits such as zero carbon emissions, and the production of benign by-products such as water when using the H₂-O₂ fuel cell.^1,2 Additionally, reversible fuel cells (flow batteries) offer a viable solution to the highly desired need for economical grid energy storage in order to take full advantage of load leveling.³ As the use of intermittent energy sources such as wind and solar power continue to rise throughout the world, the need for reliable, efficient and economical energy storage solutions will grow.³ Excessive liquid water buildup in the fuel cell catalyst layer (CL) at high current densities can lead to electrode flooding, thus restricting transport of gaseous reactants to the catalyst reaction sites. In order to realize the economic viability of fuel cells, the fuel cell catalyst layer (CL) needs to be redesigned to overcome the negative hydration effects common with PEM fuel cells.

Previous studies in our research group addressed the electrode flooding problem by incorporating a hydrophobic phase, such as Teflon, into the cathode CL in order to enable oxygen to be more easily transported to the catalyst reaction site.⁴ With this approach, fuel cell testing demonstrated improved performance when incorporating Teflon into the CL and showed that optimal amounts of Teflon was highly dependent on the Nafion content.⁴ The CL structure with Teflon integration is presented in Figure 1A.⁴ For this research, our hypothesis is based on recent discoveries about the interfacial properties of fluorocarbon-based ionic polymers. Recently, it was discovered that fluorocarbon based polymers exposed to high relative humidity at the air-polymer interface cause the ionic sulfonate groups within Nafion^® to rearrange so that the surface is made up of a larger proportion of ionic sulfonate groups.⁵ Furthermore, when the fluorocarbon-based polymer is exposed to low relative humidity at the air-polymer interface, the ionic sulfonate groups move inward causing the surface to have a higher proportion of hydrophobic Teflon-rich regions.⁵ We verified that the surface structure of Nafion^® and electrospun nanofiber composite membranes depend on the relative humidity (RH) of the gas in contact with the membrane.^6,7 By operating an atomic force microscope (AFM) in both contact and tapping modes, we were able to show the dependence of the membrane’s surface by changing the RH, scanning the surface with an AFM and comparing the AFM images.^5,7,8 Topography, phase contrast, and conductivity images were captured using the AFM to explore the surface structure of Nafion^® and electrospun nanofiber composite membranes.^5,7 Based on these observations, it is hypothesized that by heat treatment of the polymer within the fuel cell CL we can engineer the polymer’s surface inside the pores of the CL to be hydrophobic so that oxygen would be able to access the catalyst reaction sites without having to diffuse through a layer of water. The proposed CL structure for this work is shown in Figure 1B.

References:

1. K. Hongsirikarn, X. Mo, J.G. Goodwin and S. Creager, “Effect of H₂O₂ on Nafion properties and conductivity at fuel cell conditions,” Journal of Power Sources, 196, 3060-3072 (2011).
2. T.V. Nguyen, M.V. Nguyen, K.J. Nordheden and W. He, “Effect of Bulk and Surface Treatments on the Surface Ionic Activity of Nafion Membranes,” Journal of The Electrochemical Society, 154(11), A1073-A1076 (2007).
3. A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick and Q. Liu, “Redox flow batteries: a review,” Journal of Applied Electrochemistry, 41, 1137-1164 (2011).
4. R. Friedmann and T.V. Nguyen, “Optimization of the Microstructure of the Cathode Catalyst Layer of a PEMFC for Two-Phase Flow,” Journal of the Electrochemical Society, 157(2), B260-B265 (2010).
5. T.V. Nguyen, M.V. Nguyen, G. Lin, N. Rao, X. Xie and D. Zhu, “Characterization of Surface Ionic Activity of Proton Conductive Membranes by Conductive Atomic Force Microscope,” Electrochemical and Solid-State Letters, 9(2), A88-A91 (2006).
6. J.B. Ballengee and P.N. Pintauro, “Composite Fuel Cell Membranes from Dual-Nanofiber Electrospun Mats,” Macromolecules, 44, 7307-7314 (2011).
7. R.P. Dowd, T.V. Nguyen, D.S. Moore, P.N. Pintauro and J.W. Park, “Conductive AFM Study to Differentiate Between the Surface Ionic Conductivity of Nafion and Electrospun Membranes,” Electrochemical Society Transactions, 58(1), 607-613 (2013).
8. Y. Wang, R. Song, L. Yingshun and J. Shen, “Understanding tapping-mode atomic force microscopy data on the surface of soft block copolymers,” Surface Science, 530, 136-148 (2003).