Abstract:
A fuel cell according to the invention comprises a multilayer proton exchange membrane having high proton conductivity in the temperature range of 300-500° C. This is achieved by the use of very thin (&lt;1 μm) metal oxide polymer films on a metal substrate by electrolytic anodizing of a metal alloy. An exemplary proton exchange membrane according to the invention comprises a Ta 2-x Hf x O 5  film fabricated on a niobium foil. The invention allows for a significant increase in power density and, therefore, a significant reduction in fuel cell cost per unit power.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to fuel cells for vehicular power source applications and, more particularly, to thin film proton exchange membrane fuel cells.  
       BACKGROUND OF THE INVENTION  
       [0002]     Since the chemical energy per mass of hydrogen (124 MJ/kg) is at least three times larger than that of other chemical fuels (for example, the equivalent value for liquid hydrocarbons is 47 MJ/kg), it is considered as the number one candidate for future car fuel. However, the ability to safely and efficiently store hydrogen for vehicular power source application presents a formidable problem.  
         [0003]     Classical high pressure tanks, even tanks made of novel carbon-fiber reinforced composite materials, are considered unsafe. Moreover, these high pressure containers, when full, would contain only approximately 4% hydrogen by mass (4 wt %), while at least 6-7 wt % hydrogen is required by the US Department of Energy (DOE) standards. Keeping hydrogen stored as in liquid form is also considered to be dangerous and impractical.  
         [0004]     There is one safe way of hydrogen storage, based on the metal-to-metal phase transition of some metals (e.g., Pd, RE), or metal compounds (e.g., Ni 5 La, Mg 2 Ni), in which hydrogen is incorporated into metal hydrides such as Ni 5 LaH 6  or Mg 2 NiH 2 . These phase transitions are reversible and exhibit a relatively small enthalpy. The most convenient Ni 5 LaH 6 →Ni 5 La+H 2  phase transition that takes place at room temperature and standard atmospheric pressure. This particular chemistry is widely used for rechargeable batteries. However, all of these systems provide only about 2-3 wt % of hydrogen storage. The only known reversible process providing 7.6 wt % of hydrogen is the metal-to-semiconductor phase transition Mg+H 2 →MgH 2 , which requires a temperature of approximately 300° C. for transition at atmospheric pressure. Recent experiments with vanadium and nickel-doped Mg have demonstrated fast kinetics of hydrogen absorption and desorption, where it takes only a few minutes to load or unload a powder of this material with hydrogen. The problem here is not the temperature of the transition, but its enthalpy. In particular, it takes approximately 78 kJmol H   −1  to melt MgH 2  into H 2  and Mg. This is an unacceptable loss of energy, since a conventional combustion engine efficiency is only about 20% and fuel cells have an efficiency of 40-50%. This problem could be resolved if the wasted energy (the remaining 80-60%) is utilized for the melting of magnesium hydride. However, neither the combustion engine or any other known fuel cells can operate at temperatures in the range of 300-350° C.  
         [0005]     The basic idea behind fuel cells is to place an ion conductor (conducting only H +  or O −  ions) between the anode and cathode electrodes contacting to hydrogen and oxygen, respectively, thus forcing electrons to propagate through an external electrical circuit connected to the electrodes in order to accomplish the chemical reaction between hydrogen and oxygen to create water. In this process, the difference in chemical potential of a hydrogen atom in the H 2  molecule and a molecule of H 2 O is converted into electrical potential appearing between the anode and cathode electrodes. Catalysts such as Pt or Ni are being deposited on electrodes in order to decompose hydrogen and oxygen molecules into atoms. The simplest examples of ion conductors are water-based alkaline and acid electrolytes. Phosphoric acid fuel cells, operating with highly-concentrated acid, can operate at temperatures up to 200° C.  
         [0006]     Solid state electrolytes have also been developed. The polymer protonic conductor Nafion was developed for a proton-exchange membrane (PEM). Proton Exchange Membrane Fuel Cells (PEMFC) are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines. First used in the 1960&#39;s for the NASA Gemini program, PEMFCs are currently being developed and demonstrated for systems ranging from 1 W to 2 kW. PEMFC fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. The fuel for the PEMFC is hydrogen, and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. In particular, the reactions are the electrodes are as follows:  
         [0007]     Anode Reactions: 2H 2           4H + +4e −   
         [0008]     Cathode Reactions: O 2 +4H + +4e −           2H 2 O  
         [0009]     Overall Cell Reactions: 2H 2 +O 2           2H 2 O.  
         [0010]     Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell. Other advantages result from the electrolyte being solid material, compared to a liquid. The sealing of the anode and cathode gases is simpler with a solid electrolyte, and therefore, less expensive to manufacture. The solid electrolyte is also more immune to difficulties with orientation and has less problems with corrosion, as compared to many of the other electrolytes thus leading to a longer cell and stack life.  
         [0011]     One of the disadvantages of the Nafion PEMFC for some applications is that the operation temperature is relatively low (T&lt;100° C.). At this temperature, the rate of cathode chemical reaction and current density is low, even if a significant amount of platinum as the cathode catalyst is used. As a result, the fuel cell cost per unit of power is very high. Moreover, since the electrolyte is required to be saturated with water to operate in an optimal manner, careful control of the moisture of the anode and cathode streams is required.  
         [0012]     The other type of solid state fuel cell is the solid-oxide fuel cell (SOFC), which employs a yttrium-stabilized zirconium oxide ceramic as an electrolyte that conducts oxygen ions at temperatures of approximately 1000° C. This type of fuel cell can use coal in gaseous form as a fuel, with an efficiency of up to 80%, with particularly applicability to power plants. The problem with this type of fuel cell is that the by-products of the reaction dilute the fuel itself.  
         [0013]     In order to accelerate the cathode and anode chemical reactions of PEMFCs, researchers have also been looking into metal oxide protonic conductors, since these conductors can operate at elevated temperatures. As the hydrogen ion is smaller, it is reasonable to believe that it may be more mobile, and spread more quickly than the oxygen ion. Studies have revealed that the protonic conduction properties of a certain number of oxides (based, notably, on rare-earth doped BaCeO 3 , BaZrO 3  and Ba 3 (CaNb 2 )O 9 ). Conductivity levels of approximately 10 −1  S/cm have been measured at 600° C. in perovskites based on BaPrO 3 .  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention relates to an article that comprises a solid oxide fuel cell having features that can result in the reduction of the operation temperature and an increase in the power density of the fuel cell. In particular, the present invention is directed to a thin film solid state fuel cell that is compatible with MgH 2  hydrogen storage and is capable of operating in the temperature range of 300-400° C. In the proposed fuel cell, the thickness of the proton conductor is a fraction of a micron, allowing for a reduction of its resistance by a factor of a thousand which, in turn, allows significant increase of fuel cell power density and efficiency.  
         [0015]     More specifically, the fuel cell of the present invention comprises a multilayer solid state structure on a metal substrate, and contacts that facilitate flowing a current through the multilayer structure. The multilayer structure comprises a metal oxide polymer protonic conductor and metal coatings on both the protonic conductor an the metal substrate. The metal substrate is made of a metal foil having high hydrogen permeability, such as niobium, vanadium or tantalum. The metal oxide polymer protonic conductor is made by electrolytic anodizing of a metal alloy film deposited on the metal substrate, for example, by co-sputtering of transitional metals having different valences. Hafnium and zirconium-doped tantalum are two such examples of such an alloy. A relatively small thickness of the protonic conductor layer (e.g., 0.1-0.5 μm) allows for a significant reduction of its resistance. This results in an increase of power density and efficiency at moderate operating temperatures. Another possibility for a metal oxide thin film protonic conduct is an ReOx film made by electrochemical deposition. The metallic coatings of the protonic conductor and the metal substrate comprise thin films of metals having a large electron work function, such as platinum, palladium or nickel. These films operate as catalysts enhancing decomposition of hydrogen and oxygen molecules into atoms. Encapsulation of the protonic conductor between the metal substrate and the metal coating also preserves high proton concentration in the conductor at elevated temperatures.  
         [0016]     Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Referring now to the drawings,  
         [0018]      FIG. 1  illustrates an exemplary fuel cell formed in accordance with the present invention;  
         [0019]      FIG. 2  is a graph showing the solubility of hydrogen in various metals, with a standard cc of hydrogen per 100 g of the metal; and  
         [0020]      FIG. 3  is a graph showing the permeability of hydrogen in a selected subset of the metals associated with  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 1  illustrates, in schematic form, the relevant aspects of a fuel cell  10  formed in accordance with the present invention. Fuel cell  10  consists of a solid state multilayer structure  20  sandwiched between a pair of gas diffusion layers  31  and  32 , where gas diffusion layers  31 ,  32  comprise porous carbide. In accordance with the present invention, multilayer structure  20  comprises a metal foil layer  21 , with a metal oxide polymer (protonic conductor)  24  disposed over foil layer  21  and in contact with gas diffusion layer  31 . Protonic conductor  24  functions as the solid electrolyte layer in the fuel cell structure. In accordance with the present invention, metal foil layer  21  comprises a metal with a relatively high hydrogen permeability, such as niobium, vanadium or tantalum and generally exhibits a thickness on the order of 25-250 μm. A first metallic coating layer  22  is formed over metal foil layer  21  so as to be disposed between first diffusion layer  31  and metal foil layer  22 . A second metallic coating layer  23  is formed over protonic conductor  24  so as to be in contact with second diffusion layer  32 . First and second metallic coating layers  22 ,  23  may comprise a metal such as platinum, palladium or nickel.  
         [0022]     An external electrical circuit  40  is coupled between first and second diffusion layers  31 ,  32 , where these layers function as the anode and cathode of fuel cell  10 . In general, external electrical circuit  40  may be considered as a resistance disposed between the anode and cathode.  
         [0023]     Hydrogen gas H 2  is supplied, as shown, to multilayer structure  20  through a first input port  42  and directed to first gas diffusion layer  31 . Oxygen gas O 2  (generally, air) is supplied through a second input port  44  to multilayer structure  20  through second gas diffusion layer  32 . An output port  46  is also formed, as shown, to allow for the exit of water resulting from the hydrogen-oxygen reaction.  
         [0024]     Metal foil  21  provides the mechanical support for protonic conductor  24 , while not impeding the propagation of hydrogen atoms from first gas diffusion layer  31  to solid electrolyte (protonic conductor) layer  24 . Since protonic conductor layer  24  will conduct only H +  ions, the interface between protonic conductor  24  and metal foil  21  will divide the hydrogen ions (H + ) and the electrons, thus forcing the electrons to propagate through external electrical circuit  40  to accomplish the chemical reaction between the hydrogen and oxygen and create water. In this process, the difference in chemical potential between hydrogen atoms in the H 2  molecule and a molecule of water is converted into an electrical potential appearing between the anode electrical contact (first diffusion layer  31 ) and the cathode electrical contact (second diffusion layer  32 ).  
         [0025]     Hydrogen permeability of a metal depends on three factors: (1) the hydrogen solubility in the metal; (2) the hydrogen diffusion coefficient in the metal; and (3) the electron work function of the metal (which determines the ability of this metal to separate hydrogen molecules into hydrogen atoms at the metal&#39;s surface).  
         [0026]     The solubility of hydrogen in different metals is illustrated in the graph of  FIG. 2 . It can be seen that the hydrogen solubility of transition metals have a valence number of four (i.e., titanium, zirconium) is greater than that of other metals. However, with particular reference to  FIG. 3 , transition metals having a valence of five are shown to be better candidates for metal foil layer  21  of fuel cell  10  of the present invention. In particular, the high value of hydrogen permeability of these metals results from their low activation energy for hydrogen diffusion. In particular, the diffusion coefficient D H  can be expressed as follows: 
 
 D   H   =D   0 exp(− E   A   /kT ),
 
 where E A  is defined as the diffusion activation energy, k is Boltzman&#39;s constant, T is defined as the absolute temperature and D 0  is defined as approximately av, where “a” is the metal lattice constant and “v” is the hydrogen thermal velocity: v=(3kT/M) 1/2  (M being the mass of a proton). 
 
         [0027]     For vanadium, D 0  is approximately 0.01 cm 2 /sec at T=600° K. and E A =50 meV, resulting in D H  of approximately 0.003 cm 2 /sec (see, for example, S. Hayashi et al, “Hydrogen Absorption and Diffusion in Vanadium-Based Alloys”  Proc. Int. Symp. Mat. Chem. Nucl. Environment,  pp. 505-513, 1996). Having this value of hydrogen diffusion coefficient and a hydrogen concentration (C) of approximately 10% (hydrogen-to-vanadium atomic ratio H/V of approximately 0.1), a vanadium foil of thickness 250 μm does not present any limitation to the fuel cell current density and efficiency.  
         [0028]     Metals such as vanadium, niobium or tantalum dissolve hydrogen atoms, but do not dissolve hydrogen molecules. As was first shown by M. A. Pick et al. (see  Physical Review Letters,  Vol. 43, p. 286 1979), coating of these metals with a thin film of metals having an electron work function exceeding 5 eV (e.g., Pt, Pd, Ni) dramatically enhances the decomposition rate of hydrogen molecules into atoms at the metal surface. Similarly, these materials enhance the decomposition of oxygen molecules, thus accelerating the hydrogen-oxygen reaction to form water. Diffusion layers  31 ,  32  of fuel cell  10  of  FIG. 1  are used to implement these functions.  
         [0029]     Inorganic protonic conductors are oxides of metals with large atomic numbers. Perovskites and some binary metal oxides, such as Nd 2 O 3 , exhibit high proton mobility. Proton solubility of these materials relies on their doping with electron acceptors that provide absorption of water. For perovskites, acceptors are defined as elements of valence three (Sc, Y, Ga or a rare earth) substituting Zr, Ti or Ce. These acceptors from the oxygen under-saturated chemical bond defects that pick up OH −  ions from H 2 O, leaving H +  ions chemically unbound. Positively charged protons are attached to oxygen atoms of the basic material by the polarization force.  
         [0030]     Yttrium-doped BaZrO 3  is an example of protonic conductor which has large proton mobility and concentration. Proton conductivity of BaZr 0.75 Y 0.25 O 3  at a temperature of approximately 400° C. is approximately 0.3 S/cm. Sintered at a temperature of approximately 1700° C., this material is a large grain poly-crystal. This so-called “bulk” material has much higher proton mobility than a ceramic made of a mixture of small micro-crystals. A 1 μm thick film of bulk BaZr 0.75 Y 0.25 O 3  would have a resistance of approximately 0.003Ωcm 2  at a temperature of 400° C.  
         [0031]     Relatively thin (for example, 6 μm) BaZr 0.80 Y 0.20 O 3  films have been made by laser beam evaporation. However, these films have low proton conductivity at temperatures in the range of 300-400° C. because of their small grain structure. Proton conductivity of this material is limited by the grain boundary contribution.  
         [0032]     A way of fabricating of a uniform, thin and continuous metal oxide layer is anodizing of metals. This method is utilized in the electrolytic capacitor industry. In particular, Ta 2 O 5  is made by anodizing Ta, and results in an excellent dielectric material. It is believed that this material is not a crystal, but an amorphous material or a metal oxide polymer (glass). It has been found that at temperatures in the range of 300-400° C. this material has very low electron conductivity (G˜10 −6  1/{fourth root}cm 2  for 0.5 μm thick Ta 2 O 5  layer, and the activation energy for electron conductivity E act =0.55 eV.)  
         [0033]     Hafnium and zirconium are acceptors in Ta 2 O 5  . Hafnium (or zirconium) doped Ta 2 O 5  (Ta 2-x Hf x O 5 , operating as protonic conductor  24  of  FIG. 1 , can be made by anodizing of a hafnium- or zirconium-doped tantalum film sputtered on metal foil  21  of fuel cell  10 . The molar concentration of protons in Ta 2-x Hf x O 5  saturated by water is approximately x. It has been established experimentally that the proton conductivity of metal oxides is correlated with the volume of their unit cells (or volume occupied by a molecule). That is, the higher the volume, the higher the conductivity. The volume of an BaZrO 3  unit cell is 85 cubic Angstroms while that of Ta 2 O 5  is 107 cubic Angstroms. Therefore, the proton conductivity of Ta 2-x Hf x O 5  is expected to be higher than that of BaZr 1-x Y x O 3 .  
         [0034]     The advantage of the proposed invention is that the protonic conductor is much thinner than that of conventional ceramic designs and does not exhibit grains. This allows a low resistance of the proton exchange membrane to be achieved at a relatively low operation temperature. For example, replacing a millimeter thick ceramic by a micron thick metal oxide polymer allows operation at 400° C. instead of 600° C. with resistance reduction by a factor greater than 100. Reduction of protonic conductor resistance results in an increase of power density and efficiency of the fuel cell. Also, encapsulation of the protonic conductor between metal foil  21  and a metal film  23  prevents oxygen depletion (water evaporation) of oxides and preserves a high level of H +  ion concentration at high operation temperatures. Proton concentration in the encapsulated metal oxide protonic conductor does not depend on temperature and water pressure of the ambient.  
         [0035]     The electrical power generated in external electrical circuit  40  of fuel cell  10  results mainly from proton diffusion against a potential built between protonic conductor  24  and metal films  23 ,  25 . In equilibrium, this potential compensates for the difference between proton chemical potentials in protonic conductor  24  and metal films  23 ,  25 , making the electrochemical potential equal across the various layers within the structure. While a proton chemical potential of a protonic conductor is fixed (for a given temperature), the proton chemical potential of a metal depends on the hydrogen processure and the metal electron work function. In particular, the higher the chemical potential difference between metal films  23  and  25 , the higher the voltage of the fuel cell.