Transport interactions between gas and water in thin hydrophobic porous layers

Project number: C1

Doctoral researcher: Stefan Dwenger

Supervisors: Prof. Dr.-Ing Ulrich Nieken, Prof. Dr.-Ing. Joachim Groß

Motivation and goals

The polymer electrolyte membrane fuel cell (PEMFC) is one of the most promising sources for decentralized electrical power generation. In the last two decades modelling and experimental research about fuel cells has been increasing dramatically. Besides the electrochemical conversion and transport steps, the so-called water management in PEMFCs has more recently been in the focus of detailed experimental and simulation work [Acosta, Gostick, Wang].

Fig. 1 gives a sketch of a PEMFC. Hydrogen as fuel and air as oxidizer are fed through the channels of the flow fields FF on top and bottom of the cell. Both gases diffuse through the respective gas diffusion layers GDL to the catalytic layers CL. At the anode side hydrogen is oxidized into protons and electrons. The protons migrate through a proton conducting membrane M to the cathode CL, where they combine with oxygen and electrons, supplied via FF and GDL, to form water. The external circuit for the electrons provides the electrical power.

A careful water management of the PEMFC is necessary for the following reasons: to be sufficiently proton-conductive, the membrane requires certain water content. Although fluid phase is formed at the cathode, it is also taken up as water vapour by the air that flows through the cathode FF. This means that the water content of the entering air has to be carefully balanced to ensure sufficient water content in the membrane, but prevent the extensive formation of liquid water at the cathode CL and in the attached GDL, since this would impede the required oxygen to diffuse to the cathode CL.

The cathode gas diffusion layer is obviously of prime importance for water release as well as electron and oxygen access to the cathode. Therefore it consists of a complex network of carbon fibres (for electron transport) and of hydrophobic polymer regions to allow both, oxygen access for gas filled pores and water removal towards the air flow field for liquid filled pores. It is the intention of the present project to better understand the complex interplay of gas and water transport in presently applied gas diffusion layers, to provide detailed experimental results for the mathematical modelling of the transport processes under fuel cell conditions and thus to contribute to a further improvement of the PEMFC water management.

Experiments and measurements with gas diffusion layers and well-defined porous materials with different degrees of hydrophobicity and a very ordered porous structure have been performed at the Institute for Chemical Process Engineering. In parallel and close collaboration, numerical investigations within the research training group, performed by LH2, University of Stuttgart [Ochs], help to extend the knowledge about the water management in PEM fuel cells.

Figure 1: Sketch of a PEMFC

Experimental approach

To understand and elucidate the processes and problems mainly occurring at the cathode side of the fuel cell, the experimental approach has been subdivided into two parts: on the one hand, measurement of capillary pressure saturation relationships (pc-Sw) as well as relative permeability saturation relationships (kr-Sw) for commercial GDLs have been performed. These relations are used as constitutive relationships needed for the modelling. On the other hand experiments with co- and counter-current flow of water and gas inside well-defined porous structures have been designed to better understand the basic interactions between the transport processes in a well defined model system similar to the cathode of the PEMFC.

For measuring the pc-Sw curves of gas diffusion layers a new measurement cell was developed, which enables to measure pc-Sw curves at different, well-defined GDL-compression levels [Dwenger]. This is necessary since GDLs mounted in fuel cells are exposed to compression, which strongly influences their two-phase transport properties. In the pc-Sw cell the GDL sample (dark grey in Fig. 2b, A) is sandwiched between a hydrophobic membrane at the top (red) through which it is in contact to air, and a hydrophilic membrane (light blue) at the bottom through which it is in contact with water. A well defined compression can be applied to the membrane/GDL sandwich through a compression stamp carefully adjusted by callipers. During an imbibition experiment water is pressed through the hydrophilic membrane into the GDL, while during drainage air is able to permeate through the hydrophobic membrane. Examples of the resulting imbibition/drainage curves are shown in Fig. 2a. They exhibit characteristic hysteretic behaviour.

For measuring relative permeabilities depending on the saturation and compression of the sample, a new set-up was recently developed and tested. It allows the measurement of kr-Sw relationships by keeping the capillary pressure constant combined with permeation experiments.

Figure 2a: pc-Sw curve of commercial GDL Figure 2b: pc-Sw test station with detail A

A model system, representing the two-phase transport of oxygen and liquid water during the catalytic oxidation of hydrogen at the cathode side of the fuel cell, has also been set up. It consists of a catalyst layer, comparable to the cathode CL, on which a regular porous structure of glass spheres, representing the GDL, is deposited (Fig. 3, top, left). Different from the PEMFC cathode side, hydrogen is fed together with air. At the catalyst layer, which is placed on top of a temperature controlled metal surface, hydrogen is oxidized to water. This setup allows studying counter-current transport of gases and liquid water under isothermal conditions but separated from the electrochemical processes in the PEMFC.

Water as the oxidation product moves as liquid or vapour through the model GDL counter - currently to the oxygen, while liquid filled domains will limit the oxygen access to the catalytic layer. In Fig. 3, bottom, a picture of the set-up, representing a channel of the cathode flow field (flown through from right to left), is shown during a test run. Liquid hold-up inside the packed bed GDL is indicated by the dark areas. In these areas oxygen access and hence the local reaction rate is strongly reduced. Fig. 3, top, right, shows the measured hydrogen conversion rate over time, starting with a dry GDL. The drop in conversion to a final steady state value of about 20% results from the increasing formation of liquid water. To modify the two-phase transport behaviour, the surface properties of the glass beads (hydrophilic vs. hydrophobic) can be changed with chemical coatings (e.g. methyl-groups on the surface gained by reaction with methyltrichlorosilane), allowing to study the influence of wettability on the transport process. The interplay between wetting and drying inside the porous body, the locally distributed reaction as well as flooding are observable and recordable via a camera or a stereo microscope and help to elucidate the occurring processes and the influence of several parameters.

Joined work within NUPUS

The measured pc-Sw curves of commercial gas diffusion layers are used by Andreas Lauser (Dept. of Hydromechanics and Modelling of Hydrosystems, University of Stuttgart) for simulations of two-phase flow within thin, hydrophobic porous media. To design the counter-current flow reactor of Fig. 3, several simulations have been performed. The comparison between experiments and simulations allows validating the chosen two-phase, three-component model, which is only based on the physical properties, partially measured in the test cell of Fig. 2b. The interplay between the partners within the research training group ensured continuous progress and helped to advance with occurring questions.

Moreover, collaboration with Prof. Hans Bruining and Ali Akbar Eftekhari (TU Delft, Section Geoengineering) was established: Ali Akbar Eftekhari visited our institute in 2009

Figure 3: Counter-current set-up

to work together concerning properties of CO2-water mixtures around the supercritical state.

Current and future work

To provide a proper set of constitutive relationships, the precise measurement of relative permeability – saturation relationships has to be continued with the new set-up and possibly modified. Also the determination and parameterization of pc-Sw curves for different materials under well-defined levels of compression has to be advanced to allow predictive modelling of fuel cell processes, depending on chosen materials and operation conditions. With help of pc-Sw respectively kr-Sw relationships, measured for GDLs and packed spheres of different materials, the gap to the counter-current experiments and their modelling shall be bridged.

Already performed experiments in the counter-current reactor will be extended by measurements, where the axial concentration profile is measured. Also the detection of the water content inside the porous structures is one crucial point: a technique for locally resolved measurement of water (e.g. impedance spectroscopy) should be established. Based on this localised information of gas concentration and water content, a comparison between models and simulations will be performed in order to better understand occurring processes in the PEMFC and find appropriate operation strategies to enhance the fuel cell performance.

References

1. M. Acosta, C. Merten, G. Eigenberger, H. Class, R. Helmig, B. Thoben, and H. Müller-Steinhagen: Modelling non-isothermal two-phase multicomponent flow in the cathode of PEM fuel cells. Journal of Power Sources, 159:1123-1141, 2006

2. S. Dwenger, U. Nieken, and G. Eigenberger: Measurement of pc-Sw relationships under well defined compression levels, submitted

3. J. T. Gostick, M. A. Ioannidis, M. W. Fowler, and M. D. Pritzker: Pore network modelling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells. Journal of Power Sources, 173:277-290, 2007

4. S. O. Ochs: Development of a multiphase multicomponent model for PEMFC. Technical report, International Research Training Group NUPUS, 2008

5. C. Y. Wang and P. Cheng: Multiphase flow and heat transfer in porous media. Advances in Heat Transfer, 30:93-182, 1997