21st International Symposium on Chemical Reaction Engineering
Sunday June 13th - Wednesday June 16th, 2010
Loews Philadelphia Hotel, Philadelphia, PA, USA
CRE: Addressing resource sustainability, environmental and life science challenges
Fuel cell engineering: towards the design of efficient electrochemical power plants
1 Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstraße 1, 39106 Magdeburg, Germany
2 Process Systems Engineering, Otto-von-Guericke University Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany
Fuel cells are electrochemical reactors which directly convert chemically stored energy into electrical energy at high thermodynamic efficiencies. In the present contribution, the status and the future directions of fuel cell engi¬neering are elucidated from the point of view of chemical reaction engineering. A multiscale approach is applied for discussing and analyzing the physical and chemical mechanisms and phenomena being involved in different compartments of a fuel cell system. Firstly we start with the electrochemical reactions at catalyst surfaces at the nanoscale and then move to the transport phenomena being involved in the different porous layers of a membrane-electrode-assembly (catalytic layers, diffusion layers, membranes). Subsequently, the performance and the operating behavior of single fuel cells and of complete fuel cell power plants will be discussed in terms of material parameters, design variables and operational variables. By this multiscale approach, important types of fuel cells will be covered, i.e. Alkaline Fuel Cell (AFC), Proton Exchange Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Molten Carbonate Fuel Cell (MCFC), and Solid Oxide Fuel Cell (SOFC).
Nanoscale: Electrode Kinetics
With the help of advanced electroanalytical techniques and spectroscopic in-situ tools, one can get access to selected surface species and to elementary steps which dominate the rate of charge transfer between the electrolyte and the electrode, i.e. the local current density. Adsorption of molecular species, formation of radicals, spill-over of radicals between different active sites, and combination of radicals are possible rate determining steps, while the transfer of electrons in many cases does not limit the overall reaction rate. Consequently, the kinetics of charge transfer often does not obey the conventional Butler-Volmer rate equation. Instead more complicated rate expressions are valid if one of the above mentioned non-electrochemical steps takes control over the resulting rate of charge transfer1,2,3. Furthermore, the actual current density and the deactivation kinetics not only depend on the concentration of reactants, but also on the presence of poisonous compounds such as CO and on the size of catalytic particles deposited on the electrolyte or on the electron conductor, e.g. carbon black used in PEMFC and DMFC.
Microscale: Porous Layers
A single fuel cell is a sandwich structure of different porous layers with a thickness between 10 µm and 200 µm, called membrane electrode assembly. Each of the layers has a well-defined functionality: In the so-called catalyst layers, an electron conducting phase, catalytic particles, and an ion conducting material are brought into intimate contact to enable charge transfer reactions. Educts and products of these reactions (non-charged and charged species) are transported from/to the adjacent layers, i.e. diffusion layers and electrolyte layers. Mass, charge and energy transport in these layers are strongly coupled and can be accompanied by phase transition phenomena. In the PEMFC, condensation of the cathodic reaction product water leads to severe performance limitations by flooding of gas pores. The latter phenomenon can be influenced by modifying the hydrophobicity and the pore structure of diffusion layers. Furthermore, the fuel cell performance strongly depends on the ionic conductivity and the permeability of materials which are used to constitute electrolyte layers. In particular, undesired crossover of reactants through the electrolyte can lead to drastic losses of cell voltage due to formation of mixed electrode potentials4,5.
Milli- and Macroscale: From Single Fuel Cells to Fuel Cell Power Plants
The steady state operating characteristics of a single fuel cell, i.e. the current-voltage behavior, are to be analyzed under variation of geometrical parameters (thicknesses, structural parameters of porous layers), material properties (diffusivities, conductivities), and operating variables (e.g. flow rate, composition and temperature of feed gases). As shown for PEMFC and SOFC, depending on these influencing parameters, unique and/or multiple operating states can be obtained at gavanostatic, voltastatic or rheostatic operating modes6,7. For creating electrochemical power plants, several dozens up to hundred single cells have to be stapled to one fuel cells stack. To achieve high energetic efficiencies, heat integration is of paramount importance on the plant level, e.g. the heat being released from the fuel cell stack is supplied to upstream reactors for endothermic steam reforming of hydrocarbons, such as methane, to hydrogen. A very efficient concept is the direct integration of reforming units into the fuel cell stack, as demonstrated in the MCFC Hotmodule concept by MTU8,9,10.
- Christov, M.; Sundmacher, K., Simulation of methanol adsorption on Pt/Ru catalysts, Surface Science 2003, 547 (1-2) 1-8.
- Vidakovic, T.; Christov, M.; Sundmacher, K., Rate expression for electrochemical oxidation of methanol on a direct methanol fuel cell anode, J. Electroanalyt. Chem. 2005, 580 (1) 105-121.
- Krewer, U.; Christov, M.; Vidakovic, T.; Sundmacher, K., Impedance spectroscopic analysis of the electrochemical methanol oxidation kinetics, J. Electroanalyt. Chem. 2006, 589 (1) 148-159.
- Schultz, T.; Sundmacher, K. Mass, charge and energy transport phenomena in a polymer electrolyte membrane (PEM) used in a direct methanol fuel cell (DMFC): Modelling and experimental validation of fluxes, J. Mem. Sci. 2006, 276 (1-2) 272-285.
- Schultz, T.; Sundmacher, K., Rigorous dynamic model of a direct methanol fuel cell based on Maxwell-Stefan mass transport equations and a Flory-Huggins activity model: Formulation and experimental validation, J. Power Sources 2005, 145 (2) 435-462.
- Hanke-Rauschenbach, R.; Mangold, M.; Sundmacher, K., Bistable current-voltage characteristics of PEM fuel cells operated with reduced feed stream humidification J. Electrochem. Soc. 2008, 155 (2) B97-B107.
- Mangold, M.; Krasnyk, M.; Sundmacher, K., Theoretical investigation of steady state multiplicities in solid oxide fuel cells, J. Applied Electrochem. 2006, 36 (3) 265-275.
- Gundermann, M.; Heidebrecht, P.; Sundmacher, K., Validation of a mathematical model using an industrial MCFC plant, J. Fuel Cell Sci. Technol. 2006, 3 (3) 303-307.
- Pfafferodt, M.; Heidebrecht, P.; Sundmacher, K.; Würtenberger, U.; Bednarz, M., Multiscale simulation of the indirect internal reforming unit (IIR) in a molten carbonate fuel cell (MCFC), Ind. Eng. Chem. Res. 2008, 47 (13) 4332-4341.
- Pfafferodt, M.; Heidebrecht, P.; Sundmacher, K., Stack modeling of a Molten Carbonate Fuel Cell (MCFC), Fuel Cells 2009, submitted.
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