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Institute of Energy and Climate Research

# Modelling and Simulation

The main focus of the work lies on characterization of mass transport in channels and in porous matrials and their impact on the electrochemical performance.

• Experimental characterization of fluid flow in channels and porous materials
• Flow simulation in porous materials with the Lattice Boltzmann method
• Fluid flow in fuel cells with Computational Fluid Dynamics

A homogeneous distribution of fluids at the electrodes is an important requirement in order to achieve high efficiency and low degradation. The distribution of fluids is realized by channel structures and porous materials. Depending on the fuel cell type fluids can be liquid, gaseous or a combination of both. The characterization of flow patterns is investigated by experimental methods and modelling techniques. The Lattice-Boltzmann method is used for flow simulation in porous materials with a stochastic inner structure [1,2]. Figure 1 shows the resulting distribution of fluid velocity inside a GDL as obtained from Lattice-Boltzmann simulations.

Figure 1: Fluid transport through a GDL which was built from a stochastic model / Lattice: 1500 x 1500 x 240 for a GDL of 1.25 mm x 1.25 mm x 200 µm

The modelling of complete fuel cells and stacks is accomplished by different model approaches within the classical computational fluid dynamics [3,4]. Figure 2 shows the compuational mesh of a single flow field plate. It consists of 3 parallel serpentine channels.

Figure 2: Computational mesh of a 3-fold serpentine flow field of a typical single cell

This configuration is very common for fuel cells. It ensures a homogeneous supply of fluids to the catalyst layers. At the first step the channels provide a coarse distribution of fluids over the entire geometrical cell area. A porous transport layer (gas diffusion layer, GDL) is placed adjacent to the channels. This layer further homogenizes the fluid distribution. The transport properties of the GDL are determined by the Lattice Boltzmann method as described above. Based on these results effective transport parameters are obtained and are applied to the CFD simulation. Typical questions to be answered are the distribution of fluid velocity within the channels, as shown in Figure 3.

Figure 3: Magnitude of velocity for the fluid flow within the cathode channels

Figure 3 shows at the bottom the thin layer of the GDL. The pressure difference between the single channel sections causes cross flow within the porous layer, which is shown in Figure 4. This effect homogenizes the fluid flow towards the border of the catalyst layer. For PEFC operated below 100 °C this also supports the removal of liquid water from the reaction zone.

Figure 4: Magnitude of velocity within the porous layer (GDL)

Finally, the major goal of the CFD simulation is to calculate the distribution of locla current density from the fluid concentration at the border of the catalyst layer (shown in Figure 5).

Figure 5: Local current density distribution at the border of the cathode catalyst layer

[1] D. Froning, J. Yu, G. Gaiselmann, U. Reimer, I. Manke, V. Schmidt, W. Lehnert; Impact of compression on gas transport in non-woven gas diffusion layers of high temperature polymer electrolyte fuel cells, J. Power Sources 318 (2016) 26-34

[2] D. Froning, G. Gaiselmann, U. Reimer, J. Brinkmann, V. Schmidt, W. Lehnert; Stochastic Aspects of Mass Transport in Gas Diffusion Layers, Transp. Porous Med. 103 (2014) 469–495

[3] Q. Cao, S. B. Beale, U. Reimer, D. Froning and W. Lehnert; The importance of diffusion mechanisms in high temperature polymer electrolyte, fuel cells, ECS Transactions, 69 (17) 1089-1103 (2015)

[4] Kvesic, U. Reimer, D. Froning, L. Lüke, W. Lehnert, D. Stolten, 3D Modeling of a 200 cm² HT-PEFC short stack, Int. J. Hydrogen Energy, 2012, 37 (2012) 2430-2439

[5] C. Tötzke, G. Gaiselmann, M. Osenberg , J. Bohner, T. Arlt, H. Markötter, A. Hilger, F. Wieder, A. Kupsch, B. R. Müller, M. P. Hentschel, J. Banhart , V. Schmidt, W. Lehnert, I. Manke, Three-dimensional studies on compressed gas diffusion layers using synchrotron x-ray imaging. Journal of Power Sources 253 (2014) 123-131