(588g) Surface Energy Characterization of Gdls and Ionomer Membranes Using Inverse Gas Chromatography at Different Relative Humidity Conditions

INTRODUCTION Proton exchange membrane fuel cells (PEMFC) are an expanding area of research for use as low pollution power generators. The humidity of the feed stream is a critical parameter affecting PEMFC performance. If the hydration is too low, the fuel cells can exhibit reduced ionic conductivity within the membrane, and proton transport will be insufficient. If hydration is too high, excess water can flood pores within the Gas Diffusion Layers (GDLs) and catalyst layers, eventually leading to blockage of reaction sites. Therefore, characterizing thermodynamic properties of wetting, such as internal surface energy, over a range of humidities is vital to the successful development of PEMFC components.

Surface energy determines the wetting affinity of liquid water for both the membrane, as related to electro-osmotic drag and back diffusion of water, and GDL, as related to water removal and retention from a fuel cell. These wetting phenomena occur on a nanometer scale for the membrane and are controlled by the type and extent of anionic chemistry (i.e. sulfonate or phosphonate groups) of the polymer side chains. As for GDL wetting, it occurs mainly on a micrometer scale and may vary widely depending on whether water is wetting externally (from catalyst layer to GDL) or internally (along a network of pores formed by the carbon fibers. In addition, GDLs are treated with a hydrophobic fluoropolymer (PTFE or FEP) to create a dual wetting characteristic of hydrophobic and hydrophilic regions for the internal pore surface. The proper balance of hydrophobic and hydrophilic regions, for both the GDL and membrane, must be maintained throughout the life of the PEMFC, or mass-transport performance losses will be experienced at long operating times (~1000 hr and longer).

EXPERIMENTAL This study investigates the surface thermodynamic properties of proton exchange membranes and gas diffusion layers using Inverse Gas Chromatography (IGC). IGC is a well-known tool for the characterization of particulates, fibers, and films. One of the most commonly used parameters for the description of the energetic situation of a solid surface is the surface energy, which is analogous to the surface tension of a liquid in contact with air. This is typically divided into two components: dispersive (i.e. London, van der Waal forces) and specific (i.e. acid-base, polar forces). IGC is advantageous in that it allows for determination of total, dispersive, and specific contributions to the surface energy of a solid within a single experiment. Methods such as Owens-Wendt require a series of Washburn absorption measurements and sophisticated analysis of the resulting liquid uptake data. In order to understand the fundamental relationship between water wetting of the GDL/membrane, surface energy, and optimum water transport in an operating PEMFC over long testing periods, a complete picture of the surface energy contributions (and changes over time) must be obtained.

RESULTS The surface energies of two different proton exchange membranes (Nafion 1135 and BPSH-30) and three different GDLs (same fiber substrate with different hydrophobic treatments) were measured at 30°C and 10, 50, and 90% relative humidity (RH). GDL results are forthcoming. As the humidity was increased, the dispersive surface energy of the two proton exchange membranes decreased (29.5 mJ/m² at 10% RH to 23.0 mJ/m² at 90% RH for the BPSH-30 sample; 29.1 mJ/m² at 10% RH to 27.7 mJ/m² at 90% RH for the Nafion 1135 sample), suggesting that preferential wetting of the hydrophilic side chains occurred. When exposed to a sulfonted membrane, water vapor will preferentially condense at/near hydrophilic sulfonate groups on the polymer side chains leaving the hydrophobic, PTFE-like backbone of the polymer more exposed to vapor. As RH increased, a higher fraction of the vapor-exposed internal pore volume of the polymer membrane would be that of the backbone. This situation would leave a more overall hydrophobic solid-vapor interface within the membrane.

On the contrary, the specific surface energies of the membranes increased with increasing humidity (5.3 mJ/m² at 10% RH to 15.3 mJ/m² at 90% RH for the BPSH-30 sample; 14.3 mJ/m² at 10% RH to 22.4 mJ/m² at 90% RH for the Nafion 1135 sample). Gravimetric studies indicate the membranes sorb significantly more water as the humidity increases, which may also increase the surface polarity. Furthermore, these observations and data agree well with intuition in terms of what is already known about the Nafion nanostructure. As indicated above, increasing RH affected the surface energetics of BPSH-30 to a greater extent than Nafion 1135. This finding could be due to the greater RH dependence of water vapor absorption of BPSH-30.

CONCLUSIONS Surface energies were measured for several membranes and gas diffusion layers over a range of humidities. For the proton exchange membranes, the dispersive surface energy decreased, while the specific free energies increased, with increasing humidification. Similar experiments could be applied to other fuel cell components over a wide range of temperature and humidity conditions.