Fuel Cells in the Automotive Industry

W H I T E P A P E R O N C H E M I C A L E N G I N E E R I N G Fuel Cells in the Automotive Industry by Ed Fontes EvaNilsson © COPYRIGHT 1994 - 2001 by COMSOL AB All rights reserved C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY Fuel Cells in the Automotive Industry The design of catalyst reactors has made the car engine substantially cleaner during the last two decades Car manufacturers have applied large efforts in reducing the emissions of hazardous gases from the combustion engine However, it has still proven to be difficult to eliminate NOx and SOx emissions from this process The chemical energy of gasoline is converted to mechanical energy via the production of heat in the combustion process of conventional car engines The efficiency of this process is limited by the efficiency formula for the Carnot cycle A more efficient process, and one of the main candidates for power production in future cars, is the new generation of fuel cells These work principally as batteries do, yet while batteries can be considered as batch reactors, fuel cells are continuous reactors In a fuel cell-powered engine, the chemical energy in the fuel is converted to electrical energy, and then to mechanical energy by an electric motor The process by-passes the limitations of the Carnot cycle, and the theoretical efficiency is substantially higher than that for the combustion engine This implies that a fuel cell-powered car will be able to run for longer distances using the same amount of fuel compared to a conventional car Carbon dioxide emissions are consequently lowered, since smaller amounts of fuel are consumed for the same distance traveled The low temperatures in the process practically eliminate the production of NOx and SOx The development of fuel cell powered-vehicles has accelerated during the last five years Competition between the different players is growing and the fight for a share of a potentially huge market has already started Technological development is one of the most important weapons at this early stage, and small companies with technical skills in the field of fuel cell processes have become important partners to the large automotive companies Mathematical modeling is one important tool in the development of fuel cell systems A combination of modeling and experiments has shown to lower costs and accelerate the pace of building prototypes and understanding of these new systems The optimization of the fuel cell, in combination with the auxiliary equipment and the operation of the electrical motor, requires a lot of mathematical puzzling Advancement in the area of computing has implied that simulations that required super computers just a few years ago can today be run on workstations or even PCs This has made computer simulations available to a much larger number of engineers In this paper, we will look at the fuel cell system through a gallery of mathematical models, and particularly at models of the electrochemistry in the fuel cell itself We will C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I NDU STRY look at the processes as they take place in the heart of the fuel cell system, i.e in the electrodes and electrolyte in the fuel cell stack These processes are studied at a micro level, where single catalyst agglomerates are modeled, as well as on the level of a unit cell consisting of an anode, a cathode and the electrolyte in between them We will also look at reactor models of the reformer and the catalytic burner in the fuel processor Finally, we will study the design of the bipolar plates and their influence on the ohmic losses in the fuel cell stack All of the models shown in the figures throughout the paper have been produced by the finite element package, Femlab One of Femlab’s strongest features is that we can define arbitrary nonlinear systems of partial differential equations and fully couple them This makes Femlab extremely powerful in handling the nonlinearities that arise when we model reactors and when we treat the kinetics at the fuel cell electrodes The fuel cell system The fuel cell system can be simply structured into the following components: a fuel processor, an air system, a fuel cell stack and a water and heat management system Figure shows a simplified flow chart of the system Figure Simplified flow chart of the fuel cell system C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY The fuel, methanol in the case of figure 1, enters the fuel processor where it is converted into hydrogen The produced hydrogen reformate is cleaned from by-products that are hazardous for fuel cell catalysts, like carbon monoxide, in a clean-up system The cleaned and moisturized hydrogen-rich reformate is run to the fuel cell’s anode chambers where the hydrogen is oxidized while oxygen is reduced at the cathode Water is used to moisturize hydrogen, since water is transported from the anode to the cathode by electro-osmosis Air is supplied to the cell via a compressor The compressed cathode chamber exhaust is run through an expander, in order to win back some of the energy from the compression step Air is also supplied to the fuel cell processor The DCcurrent produced in the process is transferred to a power conditioner before it is supplied to the electric motor The spent fuel which still contains some hydrogen, is fed to a catalytic burner and the heat produced in this combustion process is used in the fuel processor The Fuel Processor and Auxiliary System The optimal fuel in a fuel cell, from the environmental point of view, is hydrogen produced by means of renewable sources, such as solar power However, hydrogen is still difficult to store in an efficient way, despite extensive research being put into using metal hydrides and nano fibers The storage of hydrogen in alcohols and hydrocarbons is the most effective storage available today For automotive applications, hydrogen can be stored efficiently in methanol Methanol can be reformed into hydrogen in an external reformer, which is in essence a tubular reactor This reformation can be obtained through steam reforming or partial oxidation Partial oxidation offers quick start up and, since it is an exothermic reaction, requires heat dissipation Steam reforming has a higher rate of conversion, but is a slower process and, since it is an endothermic reaction, requires heat being supplied to the system A combination of both reactions is obtained in the auto-thermal reactor, in which the reformation reaction gets its heat from the partial oxidation reaction The design of the reformer is important for the performance and efficiency of the total system It should be able to work at low and high loads, and at high sudden outputs, e.g., when the car is accelerated The weight and space taken up by the reactors should be minimized, and the heat management system optimized for different operating conditions C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I NDU STRY Figure shows the temperature distribution in a tubular reactor for reformation of methanol to hydrogen through the steam reforming reaction We can see from this figure that a jacket heats the reformer while the reactions in the reactor core consume heat The curved surfaces in the core represent isothermal surfaces Different color scales are used for the heating jacket and for the core, since temperature differences are significantly larger in the core The heat is exchanged from the heating channels in the jacket, through the highly conductive jacket material, and into the core We can calculate the temperature profile by defining a heat balance in the reactor, assuming that heat transfer takes place by conduction and convection Figure Simulated temperature profile in the heating jacket and in the core of a steam reforming reactor The hydrogen-rich reformate is supplied to the fuel cell where hydrogen is consumed However, to avoid build up of by-products from the processor, and to optimize the operation of the cell, a surplus of fuel is usually fed to the cell The exhaust from the cell is therefore supplied to a catalytic burner and the heat produced in the combustion process is subsequently used in the fuel processor The advantage of using a catalytic burner is that the combustion process takes place at a low temperature thus minimizing the production of NOx The catalytic burner might consist of a packed bed of sintered palladium catalysts Figure shows the simulated reaction distribution in a catalytic burner We can obtain the flow distribution by combining the mass balance with Darcy’s law for flow through porous media In this case, we assume that one of the burner walls accidentally became too thin in the manufacturing process, which results in a nonuniform flow distribution through the porous catalyst and a non-uniform combustion In figure 3, the red color signifies areas of higher combustion rate due to a larger convective flow of fuel This might eventually lead to a non-uniform temperature distribution, and eventually ignition of the gas stream C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY Figure Dimensionless reaction distribution in a catalytic burner Imperfections in the porous wall of the burner create nonuniformities in the performance of the burner The Electrodes and Electrolyte in the Fuel Cell A fuel cell works through the principle of separating the oxidation of a fuel, e.g hydrogen, and the reduction of oxygen The oxidation and reduction processes take place at the anode and cathode respectively, where electrons are released to an outer circuit at the anode, and received through the same circuit at the cathode Anode reaction: Cathode reaction: The outer circuit is completed through an external load – the power conditioner connected to the electric motor – while the transport of protons in the electrolyte completes the circuit inside the fuel cell, see figure Current is transported through ionic conduction in the electrolyte, while it takes place through electronic conduction via the catalysts and electrode backing at the anode and cathode, and the outer circuit The electrochemical oxidation and reduction reactions at the anode and cathode serve as charge transfers between ionic and electronic conduction The traditional way of classifying fuel cells is through the composition of its electrolyte, e.g., a polymeric proton exchange membrane serves as electrolyte in the proton exchange membrane fuel cell (PEMFC) Figure shows the principle of the fuel C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I NDU STRY cell, in this case exemplified by the unit cell of a proton exchange membrane fuel cell The electrodes are of gas diffusion type and consist of a supporting carbon structure with gas-filled pores, and an active layer, which also contains polymer electrolyte and a solid catalyst The supporting carbon structure carries the electrons to and from the active sites in the catalysts The gas diffusion electrodes are designed to maximize the specific area available for the reaction, and to minimize the transport resistance of oxygen and hydrogen to the active sites as well as the resistance for the proton transport to and from the active sites at the electrodes Figure Schematic drawing of a unit cell in the proton exchange membrane fuel cell The process in the active layer is exemplified for the cathode reaction Oxidation of hydrogen takes place at the active sites of the catalyst and serves as the charge transfer reaction between electronic and ionic current The reaction requires hydrogen, which is transported from the gas-filled pores to the active sites at the catalyst surface The hydrogen is converted into protons and electrons The electrons are transported through the solid electrode material to the external circuit while the protons are carried through the proton exchange membrane towards the cathode At the cathode, the protons react with electrons supplied by the solid catalyst, and oxygen, to produce water Oxygen has to be supplied to the reaction site from the gas-filled pores through the polymer electrolyte incorporated in the active layer We can detect an optimization problem in the design of the cathode Oxygen has to diffuse to the reaction site while, at the same time, polymeric electrolyte is required to transport the protons to the same site In order to minimize the oxygen transport resistance, its path through the polymer has to be as short as possible, while keeping sufficient polymer material in order to reduce resistance for the ionic current transported by protons The oxygen concentration inside and around catalyst agglomerates is shown in figure We can see that substantial concentration gradients arise in the polymer electrolyte at high loads, inside the electrode Modeling can serve as a powerful tool when investigating the distribution of catalyst and polymer electrolyte within the active layer of C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY fuel cell electrodes The simulations give a hint of the degree of utilization of the expensive catalyst in the electrode In figure 5, we can see a projection of two catalyst agglomerates, covered by a thin film of polymer electrolyte The agglomerates form a neck, where the polymer electrolyte forms a meniscus, which additionally increase the oxygen transport resistance The model is highly idealized but the described phenomenon is realistic We treat the model in 2-D by introducing rotational symmetry and cylindrical coordinates Figure Oxygen concentration distribution around and inside two catalyst agglomerates in a fuel cell cathode Potential distribution in a fuel cell Once the protons are produced at the reaction sites, they are transported away from the anode by means of migration in the electrical field The protons are thus transported from the negative electrode to the positive electrode, i.e., apparant in the opposite direction to the electric field However, the electric field inside the fuel cell is reversed during the discharge process This is shown in figure where the potential through the cell is drawn schematically The potential is reversed through the discontinuity in potential at the interface between the solid catalyst and the polymer electrolyte This discontinuity is a part of the total overpotential, and is partly given by the activation energy for the electron transfer between the electrolyte and catalyst Since one of the reactants is the electron, we can change its free energy in the electrode by changing the potential, which also changes the activation energy for the electron transfer reaction We can see that the potential in the active layer shows curvature, which is typical for the presence of sinks or sources C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I NDU STRY Figure Sketch of the potential profile through the fuel cell when a load is applied to the outer circuit The oxygen reduction reaction, which takes place at the active sites of the catalysts, serves as the charge transfer reaction between ionic and electronic current This cathode reaction requires oxygen obtained from the gas-filled pores, protons from the proton exchange membrane electrolyte, and electrons from the solid catalyst The produced water has to be carried away from the electrode, which is done through evaporation The proton exchange membrane needs to be moist in order to conduct ionic current properly The protons drag water molecules on their way from the anode to the cathode by electro-osmosis In order to keep the membrane wet, the hydrogen stream entering the anode chamber is humidified Figure shows the influence of membrane conductivity on the polarization of the unit cell 1.4 Cell Voltage [V] 1.2 Figure Polarization of the unit cell for different membrane conductivities 0.8 0.6 0.4 0.2 0 2000 4000 6000 8000 10000 Current Density [A/m ] 10 12000 C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY One unit cell produces, roughly, kA/m2 at a cell voltage of 0.8 V This implies that we have to couple several cells in series in order to increase the voltage to usable levels This is done in a fuel cell stack, where bipolar plates serve both as separators and current conductors between adjacent anodes and cathodes These bipolar plates also serve as gas suppliers to the electrodes through channels in their structure In addition, the edges of the plates serve as manifolds for the fuel cell stack Figure shows the principle behind a bipolar stack Figure Sketch of a bipolar fuel cell stack The design of the bipolar plate is very important for the performance of the fuel cell stack The plates must be capable of effectively distributing gas to minimize mass transport limitations, while at the same time providing the path for electronic current Figure shows the potential distribution in the contact area between the electrode and the bipolar plate Contact resistance should be minimized and, if the bipolar plate is made of a metallic material, it is important that low-conducting oxide layers are not formed between the electrode and plate The color scales in the figure are different for the plate and electrode due to a large difference in conductivity between the two materials We can use models like the one shown in figure to investigate the influence of bipolar plate design on the cell performance 11 C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I NDU STRY Figure Potential distribution at the contact zone between a bipolar plate and the electrode backing in a fuel cell stack The color scale that goes from green to yellow represents the potential in the metallic plate, while the color scale that goes from blue to red represents the potential in the electrode The Near Future The fuel cell driven car is today a reality for production in small series and concept cars The cost of the materials that are presently used are still too high for mass production However, the large automotive companies have shown a true commitment in solving these problems and they forecast production at an industrial scale within three years This implies that some of the buses and cars in our urban areas will soon be fuel cell driven We believe that modeling will contribute to the success of this mission We also hope that Femlab will be able to contribute in this work 12 C H E M I C A L E N G I N EERI NG I N TH E A U TOMOTI VE I N D U S T RY References Dr Ed Fontes is Product Manager at COMSOL AB He was previously the Manager of the Electrolysis group at Eka Chemicals R&D in Sundsvall, Sweden He made his Ph.D on modeling gas diffusion electrodes in fuel cells Dr Eva Nilsson is a consultant in the field of battery and fuel cell technology at Catella Generics AB, Stockholm, Sweden She made her Ph.D on modeling electrochemical processes in biological systems 13 COMSOL, Inc New England Executive Park Suite 310 Burlington, MA 01803 Tel: 781-273-3322 Fax: 781-273-6603 Email: info@comsol.com Web:www.comsol.com ... RY Fuel Cells in the Automotive Industry The design of catalyst reactors has made the car engine substantially cleaner during the last two decades Car manufacturers have applied large efforts in. .. at the processes as they take place in the heart of the fuel cell system, i.e in the electrodes and electrolyte in the fuel cell stack These processes are studied at a micro level, where single... processor Finally, we will study the design of the bipolar plates and their influence on the ohmic losses in the fuel cell stack All of the models shown in the figures throughout the paper have