6.5 Photovoltaic and Electrochemical Equivalent Circuit Model…………………… 240
6.6 Simple Electrochemical Model of Fuel Cells 248
6.7 Continuum Mathematical Model of Fuel Cells 250
6.8 Fluid Dynamics and the Lattice Boltzmann Model………………………………….. 260
6.9 Exercises………………………………………………………………………………………………… 282
Hydrogen is a way in which to store energy, and could have an especially huge impact in automotive or residential applications. The process is typically a two-stage process consisting of first using energy to produce the hydrogen and then, second, of extracting the energy from hydrogen in another form (typically electrical energy using fuel cells).
The production of hydrogen is currently dominated by steam reforming. This involves the direct production of hydrogen from hydrocarbons. The source of these hydrocarbons is mainly fossil fuels. For example, hydrogen can be generated from natural gas with a high degree of efficiency. At high temperatures (generated by burning the natural gas) steam reacts with the natural gas (methane) in an endothermic reaction creating syngas in a process known as steam reforming. In other words, the first stage involves the reaction
CH4 + H2O ^ CO + 3H2 (7.1)
Additional hydrogen is also generated via a water gas shift reaction, where
CO + H2O ^ CO2 + H2 (7.2)
where the important consequence is the generation of carbon dioxide (CO2), a greenhouse gas which we would preferably like to avoid emitting. While it may be possible to sequester the carbon dioxide from the steam reforming process, there are other potentially less polluting methods of generating hydrogen. For example, other (less commonly used at present) methods include electrolysis (using electrical current to drive the reaction), thermolysis (using heat to drive the reaction), or fermentative hydrogen production (using bacteria to convert organic matter to biohydrogen). A particularly interesting extension of the electrolysis method is the use of an “artificial leaf”, which couples the current and voltage from a photovoltaic cell with that of the electrochemical cell, which facilitates water splitting through the introduction of electrical energy.
In this manner, solar energy can be directly converted to hydrogen fuel in a clean and relatively inexpensive manner. Once we have the hydrogen, however, it is necessary to extract the energy for practical use; this is typically achieved using fuel cells.
Fuel cells are, in may ways, similar to batteries; the main difference is that fuel cells work continuously. In particular, fuel cells create an electric current through an electrochemical reaction. Typically, hydrogen is the fuel used by the device, which is taken into the anode of the fuel cell. Meanwhile, oxygen is taken into the cathode of the fuel cell. The chemical reaction produces electricity, heat, and vaporized water. The main types of fuel cells are polymer electrolyte membrane fuel cells (PEMFC), also known as proton exchange membrane fuel cells, where a polymer membrane with high proton conductivity serves as the electrolyte which separates the anode and the cathode. This allows protons to travel from the anode to the cathode within the electrolyte, while the electrons move through an external circuit creating a current.
For typical fuel cells, with a proton conducting electrolyte, hydrogen interacts with the anode, creating two electrons and two protons via
H2 ^ 2H + + 2e – (7.3)
The protons enter the electrolyte and are transported to the cathode. At the cathode the supplied oxygen reacts according to
O2 + 4e – ^ 2O2- (7.4)
creating oxygen ions. During the reactions, the electrons travel from the anode to the cathode via an external circuit. The oxygen ions recombine with the protons which have drifted through the electrolyte to form water
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O – + 2H+ ^ H2 O
This means that the fuel cells are clean and do not pollute the environment, at least while they are in operation.
Polymer electrolyte membrane fuel cells (PEMFCs) are best known for their applications in transportation, with fuel cell vehicles becoming increasingly commercial. The benefits of PEM fuel cells for automotive applications are that they are robust and stable devices that operate at relatively low temperatures (on the order of 80oC) which enables fast start-up times. Furthermore, the fuel cells do not emit carbon monoxide, carbon dioxide, nitrogen oxides or fine particulate matter, pollutants which plague traditional fossil-fuel based automobiles. Fuel cells could also help to reduce America’s dependence on foreign imports of fossil fuels, as the hydrogen fuel could be produced more locally. Of course, the hydrogen production would require energy from some source (hopefully, clean technologies such as nuclear or alternative energies) but the fuel cells are much more efficient than combustion engines (and they operate at peak efficiencies throughout their use, rather than combustion engines which operate more inefficiently in urban or suburban driving conditions). Furthermore, if the energy for hydrogen production were to come from irregular power sources (such as wind or solar), then the need to correlate power availability with demand would no longer be necessary, as hydrogen fuel can be produced at any time. The use of clean technologies would, of course, mean that the fuel cell vehicles would not emit any pollution either during their operation, or during the production of hydrogen fuels to run the vehicles.
The general structure and operation of a PEM fuel cell is depicted in Figure 7.1. The device itself consists of an electrolyte layer in the middle (through which the protons travel), sandwiched between the two porous electrodes. Hydrogen fuel is continuously fed into the cell from one side and interacts with the anode, while simultaneously the oxidant (in general the oxygen in the air) is continuously fed into the other side of the cell and interacts with the cathode. In Figure 7.1 the hydrogen atoms are depicted as solid circles while the oxygen atoms are depicted as white circles. The anode and cathodes are porous to enable them to be permeable to the hydrogen fuel and air, respectively. Meanwhile, the electrolyte would ideally possess a gas permeability as low as possible. However, there remain many challenges (such as reducing costs, while improving efficiencies).
To characterize the performance of a fuel cell it is quite common to look at the polarization curve, otherwise known as the voltage-current (V-I) curve. An example of such a curve is shown in Figure 7.2. The polarization curve can be separated into three different regions, where different mechanisms influence the losses in the system. To the left of the curve, at low currents, the behavior of the fuel cell is dominated by activation or kinetic losses. In other words, the reactions taking place at the surface of the electrodes are not instantaneous and the kinetics of the electrochemical reactions cause a decrease in the voltage of the cell. For example, the slow oxygen reduction kinetics at the cathodes of polymer electrolyte fuel cells can cause such losses. As the current is de-
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FIGURE 7.2 The different regions of the polarization (V-I) curve corresponding to activation, ohmic, and concentration losses. |
creased through the cell, ohmic losses become an increasingly important factor in the lowering of the voltage. This arises simply because of the resistance of the cell electrolyte and electrodes. At high currents, mass-transport limitations become increasingly important. Concentrations of reactants at catalyst sites are consequently reduced, or depleted, faster than the reactants can be transported to the catalyst sites. This results in concentration losses as seen in Figure 7.2 for high current densities. The polarization curve is often used to both experimentally characterize fuel cell performances and gauge the performance of models and simulations, and we will look at a couple of computer models used to capture the behavior and physics of PEM fuel cells. First we will turn our attention to simple electrochemical models which are similar to the equivalent circuit models of solar cells encountered in Chapter 4, before considering a continuum macroscopic model of fuel cell concentrations.
