gle or bubble point measurements can also provide useful information of two-phase phenomena.
Hakenjos et al. [89] developed a segmented transparent test cell to investigate the water management by the combined spatially resolved measurement of current, tem-perature and water distribution. A special feature was the additional measurement of the local electrochemical impedance spectra [90]. Several research groups use cur-rent mapping in segmented test cells for analyzing the flow field design and operating conditions with respect to the water management, e.g. Mench and Wang [91] or No-ponen et al. [92]. At this point the work of Hartnig et al. [93] and Schneider et al. [94]
has to be mentioned. They combined neutron imaging with the spatially resolved cur-rent measurement and impedance, respectively. The local detection of liquid water blocking parts of the flow channel can be correlated with the current distribution and impedance.
Besides the synchrotron X-ray radiography [86] no imaging diagnostic tool is able to resolve the two-phase flow in the pore structure of the GDL or even the CL. The measurement of the physical/electrochemical properties mentioned above are mainly related to water distribution in the in-plane direction. For developing a two-phase model of a porous medium, knowledge of liquid-phase transport on the pore scale is essential. Thus, applying dynamic electrochemical measurement techniques on a very small fuel cell, where lateral effect can be neglected, combined with inverse modeling is a practicable way to gain more information about two-phase phenomena in PEM fuel cells. In this and further chapters a variety of technological advances is presented to explore water management in PEFC.
6.2 Analysis of Wettability of GDLs by ESEM imaging
Capillary force is the most important transport mechanism for liquid water in the porous structure of a GDL and CL. The pressure difference across the interface be-tween the liquid phase and the gas phase, described by the Young-Laplace equation
∆p= 2σwcos[Θ]
rc
(6.1) is called the capillary pressure and acts as the main driving force for the liquid water in the capillary network. In Eq. 6.1σwis the surface tension,Θis the contact angle1and rc the pore radius. In case of hydrophobic wettability (Θ>90◦C) of the porous media, the Young-Laplace equation describes that the needed pressure difference for filling the pores decreases with increasing pore size. Since the hydrophobic GDL is not a network of capillaries with uniform pore radius but reveals a characteristic pore size distribution, only pores up to a certain size get flooded at a specific water pressure.
1The contact angle is the angle between the liquid/vapor interface and the solid surface.
6 Water Management in Polymer Electrolyte Membrane Fuel Cell
Figure 6.1: Contact angles of131◦C-140◦Cwere measured on the surface of a new untreated Toray TGP-H-090.
The small pores that need a high water pressure for filling remain still free of water and are available for the gaseous reactant transport. Besides the pore radius, the contact angle is the second quantity that strongly influences the capillary pressure and therefore is of major interest for understanding the water transport in GDLs. A standard experimental approach to determine the wettability is measuring the con-tact angle by the Sessile drop technique. As depicted in Fig. 6.1 a defined droplet is placed on the GDL surface and the angle between the solid and the liquid surface is measured.
The wetting properties of two different GDL materials, a Toray TGP-H-090 and a Freudenberg H2315 I3, were investigated. Measurements of a new Toray TGP-H-090 result in an average contact angle of 136.5◦, presented in Alink et al. [7]. The con-tact angle of the Freudenberg GDL with a value of 134◦C is quite similar, indicating at least high hydrophobic properties on the macroscopic GDL surface of both mate-rials. Concerning two-phase transport in the GDL, the question arises whether this measurement technique is more related to the surface structure, roughness, etc. or to the fibers themselves. By taking a look on the distribution of condensed water by means of ESEM imaging (Fig. 6.2), a relationship between the contact angle on the macro scale (GDL surface) and the micro scale (fibers) can not be drawn for the Toray paper. The hydrophobic character on the macro scale is changed to mixed wettability properties on the micro scale. Consequently, a definite qualitative value of the contact angle can not be extracted like it can be done on the macroscopic scale (Fig.6.1).
Figure 6.3 shows a typical sequence of ESEM images during a condensation exper-iment with the Toray paper. The condensation of the first droplets starts on visually unpredictable positions. On several fibers liquid film formation is observable, which causes merging of water droplets extended over many pores. The surface curvature of the liquid phase is low and only weakly constrained by the pore morphology. Only a few parts of the GDL show hydrophobic properties.
A completely different scenario of the water droplet growth is shown in Fig. 6.4, where
6.2 Analysis of Wettability of GDLs by ESEM imaging
Figure 6.2: The contact angle of the Toray paper on the micro scale shows a completely different characteristic than the contact angle on the macro scale in Fig. 6.1.
a GDL from Freudenberg (H2315 I3) is investigated. The whole structure shows a highly hydrophobic behavior. The droplets all show spherical shapes with large con-tact angles. Water film formation around fibers is not observable and therefore the tendency of large water accumulation is suppressed.
The comparison of the ESEM images with the Sessile drop technique of both ma-terials shows that the contact angle on the macro scale (the droplet covers several fibers) is primarily governed by the surface morphology of the GDL structure, not by the fibers themselves. The same conclusion can be drawn with respect to bubble point measurements2, that are also used for characterizing water transport properties of porous media. Similar bubble points were measured for both GDLs (Toray4.7kP a and Freudenberg4.9kP a) showing that this measurement technique does not reflect the different wetting properties on the micro scale of these GDLs too.
For a constructive design improvement of a flow-field concerning flooding issues, the knowledge of local temperature distribution seems to be important due to the depen-dency of vapor saturation pressure on the temperature, which in term determines condensation phenomena. Figure 6.5 highlights that the condensation is dependent on the composition of the material as well. Figure 6.5(a) shows the surface of a used Toray paper that faced the CL before disassembling the fuel cell. Some impurities vis-ible on the fibers could be pieces of the electrode quarried out during decomposition.
As can be seen in Fig. 6.5(b) these impurities act as condensation nuclei and have a strong influence on »when and where« the condensation process begins.
2the hydrostatic pressure of a water column test at which a continuous liquid water path through the media is developed
6 Water Management in Polymer Electrolyte Membrane Fuel Cell
Figure 6.3: A typical sequence of a condensation experiment with a Toray paper. By increas-ing the vapor pressure in the sample chamber from high vacuum to around 750P a conden-sation starts on the sample that has a temperature of 1◦C. Some hydrophilic region, where water film formation is observed, leads to accumulation of large amounts of water.
6.2 Analysis of Wettability of GDLs by ESEM imaging
Figure 6.4:The Freudenberg GDL shows higher hydrophobic behavior than the Toray paper.
The growing water droplets did not cover the fibers, thus the large-area filling of cavities is suppressed.
6 Water Management in Polymer Electrolyte Membrane Fuel Cell
Figure 6.5:Impurities on the GDL fibers can act as condensation nuclei.