The drivability and the power performance of an FCEV may be improved by the addition of a short-term storage system, resulting in a fuel-cell hybrid vehicle (FCHEV) [208]. A promising option consists of using supercapaci- tors, the alternative being represented by electrochemical batteries. Superca- pacitors (see Sect. 4.5) have an extremely high power density and a higher efficiency than batteries for energy charge and discharge. They are used to cover power peaks, typically during accelerations, and for regenerative brak- ing, while the fuel cell is operated almost stationarily. There are many advan- tages associated with this operation. First, the prime mover can be downsized with respect to the peak power and thus it can be a smaller and cheaper unit.
Secondly, the stationary operation increases not only the efficiency of the fuel cell but also its lifetime.
6.1.1 Concepts Realized
In the 1990s and 2000s many prototypes of passenger cars and other types of vehicles equipped with a fuel cell have been demonstrated by research institu- tions and major car manufacturers. Some of these concepts, already realized in small series, e.g., for car sharing, can be considered to be at a pre-commercial stage. Concerning the propulsion architecture, purely fuel-cell vehicles must be distinguished from fuel-cell hybrid-electric vehicles. After pioneering attempts in the preceding decades, the development of FCEVs received a renewed im- pulse in the mid 1990s. Often the same manufacturer explored various hy- drogen storage scenarios, i.e., gaseous, liquid, metal hydride, methanol, or gasoline reforming, in successive prototypes (e.g., DaimlerChrysler Necar se- ries [61], Ford Focus FCV/FC5 [85], Opel HydroGen3 series [181]), although commercial or pre-commercial projects mostly employ compressed gaseous hydrogen (DaimlerChrysler F-Cell [60], GM’s Hy-Wire [93] and Citaro Bus [58]). A recent concept uses sodium borohydride as a hydrogen storage system (DaimlerChrysler Natrium [59]).
Fuel-cell hybrids with a secondary battery as an energy storage system have been similarly investigated with various solutions for hydrogen storage (Toyota FCHV series [249], Ford Focus FCV Hybrid [84], Nissan X-Trail FCV [176], Daihatsu FC-EV series [56]). More recently, hybrid fuel-cell systems us- ing a supercapacitor have been introduced. From being the subject of research and development projects [213, 144], this solution has become an approach adopted by several car manufacturers (Mazda Demio FCEV [165], Honda FCX [114]).
6.2 Fuel Cells
Hydrogen Storage Systems
As mentioned above, one of the main problems for the development of fuel cells as prime movers in passenger cars arises from the usage of hydrogen as a
168 6 Fuel-Cell Propulsion Systems
fuel. The lower energy density of hydrogen in comparison with gasoline makes its storage a difficult task. Various hydrogen storage technologies are the sub- ject of research and testing [255]. Example technologies are pressure vessels, cryogenic accumulators, and metal sponges (chemical adsorption). These tech- nologies lead to a specific energy which is significantly lower than that of gasoline. However, in consideration of the higher system efficiency of fuel-cell power sources, the range of such vehicles is comparable to that of ICE-based passenger cars. Additional disadvantages are generally higher cost and weight, aside from a more complicated fuel management.
Storage under pressures of up to 350 bar is achieved in conventional pres- surized vessels. High-pressure tanks must be periodically tested and inspected to ensure their safety. However, this technology is widely developed, efficiently controllable, and relatively inexpensive. Therefore most of the current fuel-cell vehicle applications use compressed gaseous hydrogen. The specific energy of the fuel stored in a pressurized vessel is
Eht mht
≈ Eht mm
=Hh·ρh·γht, (6.1) wheremht is the mass of the fully charged hydrogen tank, practically coinci- dent with the mass of the vesselmm,Hhis the lower heating value (120 MJ/kg or 33.3 kWh/kg) of hydrogen,ρhits density (depending on the pressure in the vessel), andγht is the storage capacity of the vessel, i.e., the volume stored per unit mass. The energy densityEht/Vhtis given by the productHh·ρh.
Hydrogen can be liquefied, but only at extremely low temperatures. Liquid hydrogen typically has to be stored at 20 K or –253◦C. The storage tanks are insulated to preserve that low temperature and reinforced to store the liquid hydrogen under pressure. The energy density is still evaluated with (6.1).
It is higher than with pressurized vessels, but energy losses of about 1% of the lower heating value are typical during vehicle operation, aside from those occurring in the liquefaction and compression process (typically, 30% of the lower heating value). Moreover, the cryogenic hydrogen must be heated before supplying the fuel cell, which causes additional losses and possibly further problems, especially in transient operation. However, the cryogenic storage has already found some application in fuel-cell vehicles (DaimlerChrysler Necar 4 [61]).
Metal hydride storage systems (metal sponges) represent a relatively new technology, although they have already been used in various fuel-cell vehi- cle prototypes (Toyota FCHV-3 [249], Mazda Demio [165]). The structure of metal hydrides causes hydrogen molecules to be decomposed and hydrogen atoms to be incorporated in the interstices of specific combinations of metallic alloys. During vehicle operation, these hydrogen atoms are liberated through the addition of heat. The energy density is comparable to that of liquid hy- drogen. However, the overall specific energy drops to lower values due to the additional weight of the metal hydride material. The main advantage of metal hydride storage is a relatively simple and safe fuel handling and delivery. This
6.2 Fuel Cells 169 technology avoids the risks due to high pressure or low temperature and, due to chemical bonds, it liberates only very little hydrogen in case of an accident.
A serious drawback is the higher cost that originates on the one hand from the material and on the other hand from the complicated thermal manage- ment. The life of a metal hydride storage system is directly related to the purity of the hydrogen. The specific energy of metal hydride storage systems is calculated differently from (6.1), i.e.,
Eht
mht
≈ Eht
mm
=Hh·ξht, (6.2)
whereξhtis the mass fraction of hydrogen stored in the system. This value is typically 1–2%, though some alloys (e.g., MgH2) are capable of storing up to 8% hydrogen, but only at high temperatures. The energy density of a metal hydride system is obtained from the specific energy by dividing it byγht.
Carbon nanotubes are microscopic tubes of carbon with a diameter of ap- proximately 2 nm that store hydrogen within microscopic pores in the tube structure. Similar to metal hydrides in their mechanism for storing and re- leasing hydrogen, carbon nanotubes are expected to be capable of a storage efficiency of 4–8%. However, this hydrogen storage capacity is still in the re- search and development stage.
Another promising technology is represented by glass microspheres, which are tiny hollow glass spheres warmed and filled by being immersed in high- pressure hydrogen gas. The spheres are then cooled, locking the hydrogen inside. A subsequent increase in temperature releases the hydrogen trapped in the spheres. Microspheres have the potential to be very safe, resistant to contamination, and able to store hydrogen at a low pressure, thus increasing the margin of safety. However, this technology is still at a very early stage of development.
Another technology that has found some application (DaimlerChrysler Natrium [59]) consists of storing hydrogen in sodium borohydride (NaBH4).
When this chemical is combined with a specific catalyst, liquid borax and pure hydrogen gas are produced. The former subsequently can be recycled back into sodium borohydride. This technology is rather expensive due to the costs of the catalyst (ruthenium) and of the processes to produce sodium borohydride and to recycle borax. Moreover, the energy losses associated with the several conversion steps are substantial.
Table 6.2 lists typical1 values of the storage parameters of current storage technologies together with reference values for conventional gasoline storage.
Also shown are the technical targets set by the US Department of Energy (DOE) for the year 2015 [256]. Note that these data do not include the con- version efficiencies as was the case in Fig. 1.7.
1 The research efforts in this field are currently very active such that a reasonable and updated estimation of the average values is very difficult.
170 6 Fuel-Cell Propulsion Systems
Table 6.2.Typical storage parameters of current storage technologies;a: for a steel tank, aluminium 1.5 l/kg, composite 3–4 l/kg,b: see [253],c: the mass of gasoline is not negligible when compared with that of the tank.
γht ρh Eht/mht Eht/Vht ξht
(l/kg) (kg/m3) (kWh/kg) (kWh/l) (%)
Pressure vessel 1a 15 0.5 0.5
Cryogenic storage 1.7 71 4.0 2.4
Metal hydride storageb 0.3 60 0.6 2.0 1.8
Gasoline 1.2c 750 10.8 8.8
DOE target, 2015 3.0 2.7 4.5
Types of Fuel Cells
The classification of fuel cells follows the type of electrolyte. The main types are listed in Table 6.3.
Table 6.3.Types of fuel cells. See below for the definition of the fuel cell efficiency ηf c.
Type Electrolyte ηf c(%) ϑ(◦C) Use
AFC Alkaline (NaOH, KOH) 50–65 80–250 aerospace
PEM Ionic membrane (Nafion) 50–60 40–100 automotive PAFC Phosphoric acid (H3PO4) 35–45 160–220 power MCFC Molten carbonate (KLiCO3) 40–60 600–650 power SOFC Solid oxides (ZrO2, Y2O3) 50 850–1000 power
Alkaline fuel cells (AFC) use an aqueous solution of alkaline (e.g., potas- sium or sodium) hydroxide soaked in a matrix as the electrolyte. The cathode reaction is faster than in other electrolytes, which means a higher performance.
In fact, AFCs yield the highest electrochemical efficiency levels, up to 65%.
Their operating temperature ranges from 80 to 250◦C, although newer, low- temperature designs can operate below 80◦C. They typically have an output of 300 to 5000 W. This technology is widely developed since it was initially used in aerospace applications as far back as the 1960s, to provide not only the power but also the drinking water for the astronauts. Although some exper- imental vehicles (e.g., ZEVCO London hybrid taxi, 1998, Lada Antel, 2001) were powered by AFCs, this technology is not considered to be suitable yet for automotive applications. On the one hand, the caustic electrolyte is highly corrosive and thus high standards are demanded from the material and the safety technology. On the other hand, this type of cell is very sensitive to con- taminations in the supply gases, thus requiring very pure hydrogen to be used.
Moreover, until recently AFCs were too costly for commercial applications.
6.2 Fuel Cells 171 Proton-exchange membrane fuel cells (PEM) use a thin layer of solid or- ganic polymer2 as the electrolyte. This ion-conductive membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum, an expensive material) that are active catalysts. The PEM fuel cell basically requires hydrogen and oxygen as reactants, though the oxidant may also be ambient air, and these gases must be humidified to prevent membrane dehy- dration. Hydrogen must be as pure as possible, since CO poisons platinum catalysts (up to 100 ppm of CO are tolerated). Methane and methanol reform- ing is possible only for low loads. Because of the limitations imposed by the thermal properties of the membrane, PEM fuel cells operate at relatively low temperatures of about 80◦C, which permits a quick start-up. Thanks to this fact and to other advantages such as higher power density and higher safety with the solid electrolyte, PEM fuel cells are particularly suitable for auto- motive applications. Further improvements are required in system efficiency, the goal being an efficiency level of 60%.
Direct methanol fuel cells (DMFC) are similar to the PEM fuel cells in that their electrolyte is also a polymer membrane. However, in the DMFC the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. That is quite an advantage in the automotive area where the storage or generation of hydrogen is one of the main obstacles for the introduction of fuel cells. Another field of application is in the very small power range, e.g., laptops. There are principal problems, including the lower electrochemical activity of the methanol as compared with hydrogen, giving rise to lower cell voltages and efficiency levels. Efficiencies of only about 40% may be expected from the DMFC, at a typical temperature of operation of 50–100◦C. Also, DMFCs use expensive platinum as a catalyst and, since methanol is miscible in water, some of it is liable to cross the water-saturated membrane and cause corrosion and exhaust gas problems on the cathode side.
Phosphoric-acid fuel cells (PAFC) use liquid phosphoric acid soaked in a matrix as the electrolyte. This type of fuel cell is the most commercially de- veloped and is used particularly for power generation. Already various man- ufacturers are represented on the market with complete power plants that generate from 100 to 1000 kW. The efficiency of PAFCs is roughly 40%. The fact that they can use impure hydrogen as a fuel allows the possibility of re- forming methane or alcohol fuels. Nevertheless, due to the higher operating temperatures of 160–220◦C and the associated warm-up times, for automo- tive applications this type is suitable only for large vehicles such as buses.3 Moreover, PAFCs use expensive platinum as a catalyst and their current and power density is small compared with other types of fuel cells.
Molten-carbonate fuel cells (MCFC) use a liquid solution of lithium, sodium, and/or potassium carbonates, soaked in a matrix as the electrolyte.
This cell operates at a temperature of about 650◦C, which is required in or-
2 The most commonly used material is Nafion by DuPont.
3 In 1994 a prototype PAFC bus was demonstrated by Georgetown University.
172 6 Fuel-Cell Propulsion Systems
der to achieve a sufficiently high conductivity of the electrolyte. The higher operating temperature provides the opportunity for achieving higher overall system efficiencies (up to 60%) and greater flexibility in the use of available fuels and inexpensive catalysts, although it imposes constraints on choosing materials suitable for a long lifetime. As of this writing, MCFCs have been operated with various fuels in power plants ranging from 10 kW to 2 MW.
The necessity for large amounts of ancillary equipment would render a small operation, such as an automotive application, uneconomic.
Solid-oxide fuel cells (SOFC) use solid, nonporous metal oxide electrolytes.
The metal electrolyte normally used in manufacturing SOFCs is stabilized zir- conia. This cell operates at a temperature of about 1000◦C, allowing internal reforming and/or producing high-quality heat for cogeneration or bottoming cycles. Thus this type is used in large power plants of up to 100 kW, where it reaches efficiency levels of 60% or even as high as 80% for the combined cycle.
However, high temperatures limit the use of SOFCs to stationary operation and impose severe requirements on the materials used. On the other hand, SOFCs do not need expensive electrode materials. Moreover, various fuels can be used, from pure hydrogen to methane to carbon monoxide. Some developers are testing SOFC auxiliary power units for automotive applications.
Electrochemistry of Hydrogen Fuel Cells
In electrochemical cells, the reaction consists of two semi-reactions, which take place in two spatially separated sections. These two zones are connected by an electrolyte that conducts positive ions but not electrons. The electrons that are released by the semi-reaction of oxidation can arrive at the reduction electrode (cathode) only through an external electric circuit. This process yields an electric current, which is the useful output of the cell. The normal direction of the external current is from the reduction side (cathode, positive electrode) to the oxidation side (anode, negative electrode). In Fig. 6.1 a simple PEM fuel cell is depicted. It consists of a particular membrane that does not conduct electrons but in rather lets ions pass. This membrane, impermeable for neutral gas, serves as an electrolyte. At both sides of the membrane porous electrodes are mounted. The electrodes allow for the gas diffusion, and they accomplish a triple contact gas–electrolyte–electrode. As described below, at the anode and the cathode sides of the membrane hydrogen and oxygen are supplied, respectively.
At the anode, molecular hydrogen is in equilibrium with simple protons and electrons. The anode reaction is written as
H2→2H++ 2e−. (6.3)
Under standard operating conditions, the dissociation rate is small, such that most of the hydrogen is present in the form of electrically neutral molecules.
The equilibrium can be modified with a change in the boundary conditions,
6.2 Fuel Cells 173
RL
anode - cathode +
ion conductive membrane
1/2 O2
e- I
H+
H O2 H2
Fig. 6.1.Principle of operation of a fuel cell.
e.g. the adoption of a catalyst or an increase in temperature. Since the H+ions are formed at the anode, a concentration gradient is established between the two ends of the membrane, generating an ion diffusion toward the cathode.
Such an H+ ion current transports a positive charge from the anode to the cathode. As a consequence, a difference of potential arises across the mem- brane, with the anode as a negative electrode and the cathode as a positive electrode. Since the free electrons cannot follow the electric field through the membrane, they flow through the external circuit.
The electron current dissipates energy across an external resistance. This energy is generated by the chemical reaction that takes place at the cathode, where electrons (from the external path), protons (through the membrane), and oxygen from an external source are combined to yield water. The cathode reaction is
2H++ 2e−+1
2O2→H2O. (6.4)
The water product is at a lower energetic state than the original combination of protons, electrons, and oxygen molecules. The difference is the energy de- livered by the fuel cell. The overall reaction is the combination of (6.3) and (6.4),
H2+1
2O2→H2O. (6.5)
Equations (6.3)–(6.5) only show the chemical transformations. But to- gether with chemical species the reactions also involve energy. In contrast to a combustion process, the energy generated is not accumulated as thermal energy, but it is directly converted into exergy in the form of electric energy.
The limits of such a conversion are described in terms of “free energy” (or Gibbs’ potential), as the following sections will show more clearly.
174 6 Fuel-Cell Propulsion Systems
Thermodynamics of Hydrogen Fuel Cells
When a mixture of O2and H2burns, for every kmole of H2O a defined quantity of heat known asformation energyis liberated. If the reaction takes place at constant pressure, this heat equals the decrease of enthalpy H and is often referred to as “heating value,”
QH=−∆H =−(Hproducts−Hreactants) . (6.6) The heating value is a function of the temperature and the pressure at which the reaction takes place. For a reaction such as that of (6.5),QH also depends on the state of water in the products. If water occurs in the form of vapor, the lower heating value is obtained. Vice versa, condensated water frees an additional amount of energy that leads to the higher heating value. Since the enthalpy of all pure substances in their natural state is zero by definition, the enthalpies of H2and O2are zero, thusHreactants= 0 in (6.6). The enthalpy of water thus corresponds to the heating value of the reaction. For vapor water at 1 atmosphere and 298.15 K (reference temperature and pressure, RTP),
∆HH=−241.8 MJ/kmol. For liquid water at RTP, ∆Hl=−285.9 MJ/kmol.
The difference corresponds to the enthalpy of the vaporization of water.
If the system where the reaction takes place was an isolated system, all the fuel heating value in principle could be converted into useful electrical work.
In fact, in a system without any heat exchange with the surrounding ambient, the work not related to variations of volume equals the variations of enthalpy,
Wid=−∆H =QH. (6.7)
However, it is theoretically impossible to collect all the work Wid from a fuel cell. One limiting condition playing a role similar to that of the Carnot efficiency for thermal systems arises when the entropy is taken into account.
Besides its internal energy each substance is also characterized by a certain entropy level that depends on the particular thermodynamic state. In a closed system and in the ideal case of a reversible reaction, the heat dissipated to the surrounding ambient equals the entropy variation,
QS=−ϑ·∆S , (6.8)
whereQS is positive if released and negative if absorbed. In the general case, (6.8) is transformed into the well-known Clausius inequality. For the sub- stances involved in an H2–O2 fuel cell, the entropy values are listed in Ta- ble 6.4 [73]. If 1 kmol of H2O is formed, 1 kmol of H2 and 0.5 kmol of O2
disappear with their entropy. In the balance, 44.4 kJ/(kmol K) are missing.
Thus the fuel cell releases the heat QS = 298·44.4 = 13.2 MJ/kmol to the ambient.
The fact that in association with the entropy variations there is always a certain amount of heat liberated limits the useful work available. In fact, for