Daniel Burnett | Surface Measurement Systems Ltd.

The research of fuel cells as alternative power generators is growing significantly. The humidity of the feed stream is vital to the performance of proton exchange membrane fuel cells. Also, the surface energies of fuel cell components is related to electro-osmotic drag, back diffusion of water, and water retention. In this paper, the surface energies of different fuel cell components have been measured via Inverse Gas Chromatography (IGC SEA) over a range of relative humidities. Additionally, the water sorption isotherms have been determined using Dynamic Vapour Sorption (DVS).

Introduction retention from a fuel cell. These wetting phenomena occur on a nanometre scale for the Proton exchange membrane fuel cells (PEMFC) membrane and are controlled by the type and are an expanding area of research for use as low extent of anionic chemistry (i.e. sulfonate or pollution power generators. The relative phosphonate groups) of the polymer side chains. humidity (RH) of the feed stream is a critical As for GDL wetting, it occurs mainly on a parameter affecting PEMFC performance. If the micrometer (substrate) to sub-micrometer hydration level is too low, the polymers exhibit (bilayer coating) scale and may vary widely greatly reduced ionic conductivity [1] and proton depending on whether water is wetting externally transport will be insufficient. Alternatively, if the (from catalyst layer to GDL or visa versa) or hydration level is too high, excess water can flood internally (along a network of pores formed by the pores in the Gas Diffusion Layers (GDLs) and the carbon fibres). In addition, GDLs are treated catalyst layers, eventually leading to blockage of with a hydrophobic fluoropolymer (PTFE or FEP) reaction sites or reactant transport impediments to create a dual wetting characteristic of within the electrode structure [2,3 ]. Therefore, hydrophobic and hydrophilic regions for the characterizing thermodynamic properties of internal pore surface. The proper balance of wetting, such as internal surface energy, over a hydrophobic and hydrophilic regions, for both the range of RHs is vital to the successful GDL and membrane, must be maintained development of PEMFC components. throughout the life of the PEMFC, or mass- Surface energy determines the wetting affinity of transport performance losses (for the GDL) and liquid water for both the membrane, as related to ionic conductivity losses (for the membrane) will electro-osmotic drag and back diffusion of water, be experienced at long operating times (~1000 hr and GDL, as related to water removal and and longer).

P-SCI-361 v1.0 22JULY2014

- and L are the electron acceptor and donor Method parameters, respectively, of the probe molecule. Unfortunately, in its original form, this equation This study investigates the surface can only be used for relative comparison due to thermodynamic properties of proton exchange inaccurate starting parameters leading to an membranes and GDLs using Inverse Gas overestimation of the basicity [9]. To correct this Chromatography (IGC SEA) and water sorption overestimation, the input parameters have been properties using Dynamic Vapour Sorption (DVS). rescaled for a more accurate determination of IGC SEA is a well-known tool for the acid/base values according to Della Volpe [9,10]. characterization of particulates [4], fibres [5], and With this rescaling, the van Oss concept is useful films [6]. IGC SEA involves the sorption of a for the determination of the specific surface known vapour (probe molecule) onto an energy. The specific surface energy can be adsorbent stationary phase (solid sample) with obtained from the acid and base parameters unknown physico-chemical properties. This according to Equation 2: approach inverts the conventional relationship SP 2 (2) between mobile and stationary phase found in S S S analytical chromatography. The stronger the In order to understand the fundamental interaction, the more energetic the surface and relationship between water wetting of the the longer the retention time. For this reason a GDL/membrane, surface energy, and optimum range of thermodynamic parameters can be water transport in an operating PEMFC over long derived from the retention behaviour. A detailed testing periods, a complete picture of the surface explanation of the theory is given in Reference energy contributions (and changes over time) [4]. must be obtained. One of the most commonly used parameters for The surface energies of two different proton the description of the energetic situation of a exchange membranes, Nafion 1135 and BPSH-30 solid surface is the surface energy, which is [11], and three different GDLs (same fibre analogous to the surface tension of a liquid in substrate with different hydrophobic treatments) contact with air. This is typically divided into two were measured at 30°C and 10, 50, and 90% RH. components: dispersive (i.e. London, van der Waals forces) and specific (i.e. acid-base, For the IGC SEA experiment the samples were cut hydrogen bonding). IGC SEA is advantageous in into thin strips and inserted into silanised glass that it allows for determination of total, columns (30 cm long, 4 mm ID). Prior to dispersive, and specific contributions to the measurement the sample was pre-treated at 303 surface energy of a solid within a single K for 2 hours in situ. IGC SEA measurements were experiment. The dispersive surface energy can be carried out using the SMS-IGC SEA 2000 system directly calculated from the retention times of a (Surface Measurement Systems, UK). The samples series of injected n-alkanes [7]. The specific were measured at 303 K with a helium carrier gas contribution of the surface energy is obtained flow rate of 10 ml/min. Probe molecules were indirectly via the specific free energy ( G), which undecane, decane, nonane, octane, heptane, can be obtained by injecting a range of polar hexane, dichloromethane, acetone, ethyl acetate, probe molecules. By applying the van Oss concept ethanol and acetonitrile. For the membrane [8], the acid and base numbers in the same unit samples and the plain GDL sample, undecane, as the surface energy can be obtained. decane, nonane, and octane were used for the alkanes. For the two treated GDL samples G = N a 2 (( + - )1/2 ( - + )1/2) (1). sp A* m * * L * S + L * S nonane, octane, heptane, and hexane were used + - In this equation S and S are the acid and base for the alkanes due to the stronger vapour + interactions with these samples. All solvents parameters, respectively, of the surface, and L

were supplied by Aldrich and were HPLC grade. (29.5 mJ/m2 at 10% RH to 23.0 mJ/m2 at 90% RH The probe molecules were injected from the for the BPSH-30 sample; 29.1 mJ/m2 at 10% RH to head-space via a loop with 250 µl volume. The 27.7 mJ/m2 at 90% RH for the Nafion 1135 injection concentration was 0.03 p/p0. The sample), suggesting that preferential wetting of deadtime was determined by a methane the hydrophilic side chains occurred. When injection. The experiments were run at least exposed to a sulfonated membrane, water three times for each sample. vapour will preferentially condense at/near DVS is a well-established method for the hydrophilic sulfonate groups on the polymer side determination of vapour sorption isotherms. The chains leaving the ‘hydrophobic’, PTFE-like DVS instrument used for these studies measures backbone of the polymer more exposed to the uptake and loss of vapour gravimetrically vapour. As the RH is increased, a higher fraction using the SMS Ultrabalance with a mass of the vapour-exposed internal pore volume of resolution of at least ±0.1 μg. The high mass the polymer membrane would be that of the resolution and excellent baseline stability allow PTFE-like backbone. This situation would leave a the instrument to measure the adsorption and more overall hydrophobic solid-vapour interface desorption of very small amounts of probe within the membrane and might explain a molecule. The vapour partial pressure around the reduced component to dispersive wetting. In sample is controlled by mixing saturated and dry contrast, the specific surface energies of the carrier gas streams using electronic mass flow membranes (see Figure 2) increased with 2 controllers. The temperature is maintained increasing humidity (5.3 mJ/m at 10% RH to 15.3 2 constant ±0.1 °C, by enclosing the entire system mJ/m at 90% RH for the BPSH-30 sample; 14.3 2 2 in a temperature-controlled incubator. mJ/m at 10% RH to 22.4 mJ/m at 90% RH for the Nafion sample). The increase in specific surface For the DVS experiments, a small (~ 1 cm2) energies may be due to interaction of the polar section of film was placed in a stainless steel probes with water on the PEM surfaces. DVS mesh sample pan. The 400 mesh pan (400 lines studies show the membranes sorb significantly per inch) allowed direct vapour flow to all sides of more water as the humidity increases, (see Figure the film. The sample pan was then placed in the 3). For instance, by 90% RH, both samples sorb DVS at 25 °C and dried at 0% RH to establish a dry over 13% of their dry weights in water vapour. mass. After a stable, dry mass was achieved, the The relatively high mass increases indicate water sample was exposed to the following humidity uptake is dominated by bulk water absorption. profile: 0% to 95% to 0% RH in 5% RH steps. Mass Coupled with the changes in surface energy with equilibrium was reached at each humidity stage humidity, the high water uptakes indicate the by measuring the percentage change in mass with membranes’ properties are greatly affected by respect to time (i.e. slope or dm/dt). Once the the amount of water vapour present. mass slope was below a predetermined threshold value and equilibrium was achieved, the experiment proceeded to the next programmed humidity stage.

Results Figure 1 displays the dispersive surface energy results for the two proton exchange membranes at 10, 50, and 90% RH. As the humidity was increased, the dispersive surface energy of the two proton exchange membranes decreased

Figure 1. Dispersive surface energy values for the Figure 3. Water sorption isotherms for the BPSH- membranes at 30°C and three RHs (10%, 50%, 30 (red) and Nafion 1135 (blue) samples between and 90%). 0 and 95% RH and 25 °C.

Similar surface energy experiments were Comparing the two membrane samples, BPSH-30 performed for three GDLs at the same conditions. sorbs measurably more water than the Nafion The GDLs contained the same fibre type (Toray 1135 sample between 10 and 95% RH. Also, TGP-H 060 substrate) with different hydrophobic Figures 1 and 2 show that increasing the RH to treatments: “plain” (no PTFE, no bilayer), “BA” (5 90% affected the surface energetics of BPSH-30 wt% PTFE substrate, no bilayer), and “bilayer” to a greater extent than Nafion 1135. This finding (5/23 wt% PTFE substrate/bilayer). The could be due to the greater RH dependence of dispersive surface energies are displayed in Figure water vapour absorption of BPSH-30, as clearly 4 and specific surface energies in Figure 5. For all shown in the gravimetric results. A previous three GDL samples the dispersive surface energy study indicated that the water absorption of decreases with increasing humidity. For the BPSH-30 abruptly increased at ~ 80% RH where specific surface energies, the trends are more hydrophilic domains of BPSH-30 became complex. While the BA GDL shows an increase in continuous [12]. Although, the current study specific surface energy with increasing humidity, shows a steady increase in mass across the entire the Plain and Bilayer samples generally decrease humidity range, there is a sharp change in surface with increasing humidity. The decrease in specific energies between 50 and 90% RH. Again, the surface energies may be due to competitive higher water uptakes of the BPSH-30 sample are adsorption between water on the surface and the reflected in the greater RH dependence of the IGC SEA probe molecules. Water molecules may surface energy results. be preferentially adsorbing to higher energy sites, leaving lower energy areas exposed to the probe molecules. The water sorption isotherms in Figure 6 indicate the surfaces are much more hydrophobic than the membranes. The two membranes (BPSH-30 and Nafion 1135) uptake over 14% of their weight in water, while the GDL samples sorb less than 0.12% of their dry weights in water. Comparing the results between GDL samples, the dispersive surface energies obeyed the following Figure 2. Specific surface energy values for the trend: BA > bilayer > plain. The treatments used membranes at 30°C and three RHs (10%, 50%, in the BA and bilayer GDL samples clearly increase and 90%). the dispersive contributions to the surface

energy. Interestingly, the bilayer sample fell greater RH dependences in the surface energy between the BA and plain samples. This finding is values. explained by the fact that the bilayer, although it has a high loading of hydrophobic PTFE, has a high amount of particulate carbon (i.e. 77 wt% with relatively high surface area and mild hydrophilicity). Therefore, application of the state-of-the-art bilayer formulation simultaneously increases both the hydrophobicity and hydrophilicity of the resultant GDL material over the “BA” state. As stated previously, the dispersive surface energy of the GDLs decreased as the humidity increased. The degree of Figure 5. Specific surface energy values for the reduction in dispersive surface energy in going GDLs at 30°C and three RHs (10%, 50%, and 90%). from 10% RH to 90% RH was greatest for the plain and bilayer GDLs, as illustrated by a decrease of almost 33% for the plain sample and only a 20% decrease for the BA sample.

Figure 6. Water sorption isotherms for the Plain GDL (red), BA GDL (blue), and Bilayer GDL (green) samples between 0 and 95% RH and 25 °C.

Figure 4. Dispersive surface energy values for the GDLs at 30°C and three RHs (10%, 50%, and 90%).

The specific surface energies in Figure 5 reveal the following trend at 10% RH: plain > bilayer > BA. This is the opposite relationship from the dispersive surface energy. As with the dispersive surface energy, the specific surface energies for the Bilayer and Plain GDLs show the greatest RH dependence. The Bilayer sample sorbs significantly more water than the other two samples. As stated earlier, the Bilayer sample has a high amount of particulate carbon with a high surface area, which would increase water uptake. The BA GDL sample shows the lowest water uptake, confirming the surface is more hydrophobic. The increased water sorption of the Plain and the Bilayer samples may be causing the

Conclusion Surface energies and water uptakes were References measured for several membranes and GDLs over a range of RHs. For the proton exchange [1]T.V. Nguyen and N. Vanderborgh, J. Membrane Sci. Vol. 143, p. 235 membranes, the dispersive surface energy (1998). [2]W-k Lee, S. Shimpalee, and J.W. Van Zee, J. Electrochem. Soc. Vol. decreased, while the specific free energies 150, p. A341 (2003). [3]T.A. Zawodzinski, M. Neman, L.O. Sillerud, and S. Gottesfeld, J. Phys. increased, with increasing humidification. The Chem. Vol. 95, p. 6040 (1991). water sorption results indicate the BPSH-30 [4] F. Thielmann, J. Chromatography A Vol. 1037, p. 115 (2004). [5] A. van Astem, et al, J. Chromatography A Vol. 888, p. 175 (2000). sample is more hydrophilic than the Nafion 1135 [6] C. Pawlisch, A. Macris, and R. Laurence, Macromolecules Vol. 20, p. sample, causing a greater RH dependence in the 1564 (1987). [7] J. Schultz, et al., J. Adhesion Vol. 23, p. 45 (1987). surface energy results for the BPSH-30 sample. [8] C. Oss, R. Good, and M. Chaudhury, Langmuir Vol. 4, p. 884 (1988). [9] C. Della Volpe and S. Sibioni, J. Coll. Interf. Sci. Vol. 195, p. 121 For the GDL samples, the dispersive surface (1997). energy decreased as the humidity increased. The [10] F. Thielmann and D. Burnett, in preparation. [11] F. Wang, M. Hickner, Y.S. Kim, T.A. Zawodzinski, and J.E. McGrath, humidity effects on the GDL specific surface J. Membrane Sci. Vol. 197, p. 231 (2002). energies were more complex: the Plain and [12]W.L. Harrison, M.A. Hickner, Y.S. Kim, J.E. McGrath, Fuel Cells Vol. 5, p. 201 (2005). Bilayer samples specific surface energies decreased, while the BA specific surface energy increased from 10 to 90% RH. Similar experiments could be applied to other fuel cell components over a wide range of temperature and humidity conditions.

Acknowledgement: Funding for this research was provided in part by the U.S. Department of Energy, Office of Hydrogen, Fuel Cells & Infrastructure Technology (program manager: Nancy Garland). The author would also like to thank SGL Technologies GmbH (Meitingen, Germany) for providing the hydrophobic treatments of the GDLs. SMS also thanks Frank Thielmann, David Wood and Yu Seung Kim for their contributions to the Case study.

Head Office: United States Office: Surface Measurement Systems, Ltd Surface Measurement Systems, Ltd, NA 5 Wharfside, Rosemont Road 2125 28th Street SW, Suite I LondonHA0 4PE, UK Allentown PA, 18103, USA Tel: +44 (0)20 8795 9400 Tel: +1 610 798 8299 Fax: +44 (0)20 8795 9401 Fax: +1 610 798 0334 Email: [email protected]