Patent Application
20070087234
1. Field
This invention relates generally to electrochemical fuel cells that generate electricity for portable power.
2. Discussion of the Background
A direct oxidation fuel cell (DOFC) is an electrochemical device that generates electricity from complete electro-oxidation of a liquid fuel. The liquid fuel of interest typically includes methanol, formic acid, dimethyl ether (DME), and their aqueous solutions. The oxidant may be substantially pure oxygen or a dilute stream such as that in air. Significant advantages of employing a DOFC in portable and mobile applications (e.g. notebook computers, mobile phones, PDAs, etc.) include easy storage/handling and high energy density of the liquid fuel.
One example of a DOFC system is a direct methanol fuel cell or DMFC. A DMFC generally employs a membrane-electrode assembly (hereinafter, “MEA”) having an anode, a cathode, and a proton-conducting membrane electrolyte put therebetween. A typical example of the membrane electrolyte is Nafion® (Nafion(® is a registered trademark of E.I. Dupont de Nemours and Company). Methanol/water solution is directly supplied to the anode as the fuel and air is supplied to the cathode as the oxidant. On the anode, methanol reacts with water in the presence of typically Pt-Ru catalysts to produce carbon dioxide, protons and electrons. That is,
CH3 l OH+H2O→CO2+6H++6e− (1)
The protons migrate to the cathode through the proton conducting membrane electrolyte, which is non-conductive to electrons. The electrons travel to the cathode through an external circuit where electric power is delivered. On the cathode, the protons, electrons and oxygen molecules from air are combined to form water, namely
3/2O2 +6H++6e−→3H2 O (2)
These two electrochemical reactions form an overall cell reaction as:
CH3OH+3/202→CO2+2H2O (3)
In general, in a DMFC the methanol partly permeates the membrane electrolyte from the anode to the cathode and such methanol is called “crossover methanol”. The crossover methanol reacts with oxygen at the cathode, causing reduction in fuel utilization efficiency and cathode potential so that power generation of the fuel cell is suppressed. In addition, there exists large water crossover through the membrane driven by electroosmotic drag and diffusion, resulting in significant water loss from the anode. It is thus conventional for DMFC systems to use excessively dilute (3-6% by vol.) methanol solution in the anode in order to: (1) limit methanol crossover and hence its detrimental consequences, and (2) supply sufficient water to sustain excessive water crossover to the cathode through the membrane. However, the problem with such a DMFC system is that it requires a significant amount of water to be carried in a portable system, thus sacrificing the system energy density.
Some conventional systems recover water from the cathode exhaust and recycle it to the anode (U.S. Pat. No. 5,599,638). Liquid water transport in gas diffusion layers of polymer electrolyte fuel cells is discussed in U. Pasaogullari and C. Y. Wang, J. Electrochem. Soc., Vol. 151, pp.A399-A406, March 2004.
A direct oxidation fuel cell dual pump anode system includes an anode; a circulation loop and a circulation pump for circulating liquid in the circulation loop; a fuel cartridge; and a fuel pump for injecting a fuel from the fuel cartridge into the circulating liquid. The anode system is configured to accept no water from a cathode exhaust.
DMFC technology is competing with advanced batteries such as lithium-ion batteries. The inventors have found that ability to use high concentration fuel at the anode is desirable for portable power sources, such as DMFCs.
One embodiment provides a dual-pump anode system that enables a direct oxidation fuel cell to operate directly on high concentration fuel from a cartridge, including neat methanol, without the recovery of water from the cathode exhaust. Another embodiment provides a direct oxidation fuel cell that retains optimal performance using high concentration fuel in a cartridge and elevated cell temperature.
One embodiment provides an electrochemical fuel cell that generates electricity for portable power. Another embodiment provides a direct methanol fuel cell that operates on high concentration fuel without the recovery or recycling or reuse of water from the cathode exhaust. Another embodiment includes a fuel cell having a dual-pump anode system in which water produced at the cathode is not recovered or recycled but rather is discarded from the cathode exhaust.
When water is not recovered from the cathode exhaust, the maximum allowable concentration of fuel from a fuel cartridge is determined by water and methanol losses from the anode compartment. The molar rate of methanol loss from the anode is represented by:
where β is the ratio of crossover methanol to methanol consumed for power generation, and F is Faraday's constant. “1” on the right hand side of Equation (4) represents one mole of methanol consumed in the anode catalyst layer for power generation, i.e. to produce the current density I. Similarly, the molar rate of water loss from the anode is given by:
where α is a number of water molecules per proton penetrating the electrolyte membrane or commonly known as the net water transport coefficient through the membrane. “1” described in the bracket corresponds to one mole of water consumed in the anodic reaction (1). The molar ratio of methanol to water supplied to the anode is thus equal to:
NCH3OH: NH2O=(1β):(1+6α) (6)
In one embodiment, β is controlled to be less than 0.25 in order to maintain the fuel efficiency higher than 80%. Therefore, the fuel concentration equivalently given by the molar ratio is solely depending upon the water crossover coefficient α, according to Equation (6). For example, for β=0.25 (80% fuel efficiency) and α=0.4, Equation (6) yields a molarity of 11.2M in the fuel cartridge. Table 1 lists a one-to-one correspondence between the maximum allowable concentration (in M) in the fuel cartridge and the membrane water crossover coefficient α assuming the membrane methanol crossover coefficient β=0.25. Thus, achieving α low a is one key to using high concentration fuel in DMFCs.
βmay range from 0 to 1, which includes 0,0.1,0.15,0.2,0.25, 0.3,0.35,0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, or 1, any non-integer value therebetween, or any combination thereof. Over this range of β, fuel efficiency may range from 50 to 100%, which includes 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 99, substantially 100, or 100%, any value, including non-integer value, therebetween, or any combination thereof. In one embodiment, β is controlled to be 0.25 or less in order to maintain the fuel efficiency of 80% or higher. In one embodiment, the fuel efficiency is 80% or higher.
Table 1 below shows te relationship between fuel molarity (moles of methanol in a methanol/water solution) and water crossover coeffiecient α through the membrane for β=0.25:
Recently, low-α MEAs have been made possible by two principal methods. One is to utilize liquid water backflow through a thin Nafion® membrane as described in U.S. patent application Ser. No. 11/013,922, filed Dec. 17, 2004, the entire contents of which are hereby incorporated by reference. In this approach α=0.4 at 60° C. has been demonstrated by using a highly hydrophobic microporous layer in an air-circulating cathode and a thin membrane (e.g. Nafion® 112) (C. Lim and C. Y. Wang, “High Performance Electrode Fabrication for Direct Methanol Fuel Cells,” Paper No.200 presented at 201st Electrochemical Society Meeting, May 12-17, 2002, Philadelphia; and C. Lim and C. Y. Wang, “Development of High-Power Electrodes for a Liquid-Feed Direct Methanol Fuel Cell,” Journal of Power Sources 113, pp.145-150. January 2003; the entire contents of each of which are hereby incorporated by reference). The other possibility to obtain low-α is to use hydrocarbon membranes. For example, α=1.3 was demonstrated for sulfonated poly(arylene ether benzonitrile) membranes (Y. S. Kim, M. J. Sumner, W. L. Harrison, J. S. Riffle, J. E. McGrath, and B. S. Pivovar, Direct Methanol Fuel Cell Performance of Disulfonated Poly(arylene ether benzonitrile) Copolymers, J. Electrochem. Soc., Vol. 151, pp. A2157-A2172, Dec 2004, the entire contents of which are hereby incorporated by reference).
Unfortunately, having a low-α MEA without more does not permit direct feed of high concentration fuel into the anode, as described by equation (6). This is because direct feed of high concentration fuel in the anode, while satisfying the methanol and water balances, results in two detrimental consequences: (1) increased methanol crossover due to a larger methanol concentration gradient across the membrane, and (2) excessive methanol vapor loss through the CO2 emission due to much higher methanol vapor saturation pressure from a highly concentrated liquid fuel. One embodiment provides a dual-pump anode with a circulating fluid to solve these problems and thus enable the direct use of high concentration fuel as specified by equation (6).
One embodiment provides a dual-pump anode with a circulating fluid, wherein the total methanol and water rates injected into the circulation loop have a ratio ranging from 0.02 to 1.25 for β=0.25, as seen in Table 1, which includes 0.02, 0.03, 0.04, 0,05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, and 1.25, and any non-integer value therebetween. In one embodiment, the ratio is greater than 0.3.
In one embodiment, the fuel is a high concentration fuel. The high concentration fuel may have a concentration ranging from about 10 to 24 M, which range includes 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 M, or any value, including any non-integer value, therebetween. In one embodiment, the fuel is 30% by weight of methanol in an aqueous methanol solution. In another embodiment, the fuel is pure or substantially pure methanol.
In one embodiment, as shown in
In one embodiment, a small amount of a liquid is initially stored in the circulation loop, referred to as the circulating fluid. The circulating fluid serves as a carrier liquid and, in one embodiment, is not created or consumed by the operation of the fuel cell. In one embodiment, the circulating fluid does not take part in the anode reaction. In one embodiment, the circulating fluid does not permeate through the membrane. In one embodiment, the circulating fluid does take part in the anode reaction. In one embodiment, the circulating fluid permeates through the membrane. In one embodiment, the amount of circulating fluid is maintained at a constant or substantially constant level. In one embodiment, the circulating fluid takes part in the anode reaction, and the amount of circulating fluid is maintained at a constant or substantially constant level. In one embodiment, the circulating fluid permeates through the membrane, and the amount of circulating fluid is maintained at a constant or substantially constant level. The circulating fluid can be pure water, or dilute water-methanol solution, or an aqueous methanol solution containing a third liquid. In one embodiment, the third liquid is miscible with methanol and water. In another embodiment, the third liquid is less volatile and electrochemically less active than methanol on the anode catalysts. In another embodiment, the third liquid does not absorb on or poison the anode catalysts. In another embodiment, the third liquid has large molecules to be difficult to permeate through the membrane electrolyte. Suitable non-limiting examples of the third liquid include dimethyl sulfoxide, ethanol, sulfuric acid, triflic acid, acetic acid, 5% Nafion® solution, polar solvent, highly polar solvent with high boiling point, or a combination thereof.
In one embodiment, high concentration fuel from the fuel cartridge is injected into the circulation loop near the anode inlet by a fuel pump. The rate of the fuel pump is controlled such that the methanol and water from the fuel cartridge match with their rates of loss in the anode either by the anodic reaction or crossover through the membrane to the cathode side according to equations (4) and (5). Hence, the anode exhaust returns to the original compositions of the circulating fluid. Throughout the circulation loop, the circulating fluid remains at its initial compositions except for the portion inside the anode where it is enriched with fuel by injection of high concentration fuel at the anode inlet and then depleted in fuel by anode consumption. In one embodiment, the circulating liquid provides a background fluid to dilute the fuel from the fuel cartridge such that fuel crossover in the fuel cell and fuel evaporation into the anode gas emissions are minimized. The anode exhaust is returned to a gas separator where CO2 gas is vented and the liquid re-enters the circulation pump. To prevent small gas bubbles present in the gas separator from being entrained in the circulation pump and entering in the anode to cause spontaneous fuel starvation and hence cell voltage fluctuation, a porous thin-film filter may optionally be used at the intake of the circulation pump.
One advantage of the present dual-pump anode system with a circulating liquid is that the fuel pump is controlled to inject the precise amount of methanol and water required by equations (4) and (5), while the circulation pump can be independently controlled to facilitate CO2 removal from the anode and hence optimize the cell performance. Therefore, both needs of a DMFC for use of high concentration fuel and optimized cell performance can be met simultaneously by this system with the two pumps actively and independently controlled.
The functional relationship between the fuel injection pumping rate and circulation pumping rate can be mathematically expressed as:
F1+F2=F3
C1F1+C2F2=C3F3 (7)
where F1, F2, and F3 are the flow rates of circulating liquid, fuel from the fuel cartridge, and total liquid feed into the anode of a DMFC, respectively, as shown in
In one embodiment, shown in
Alternatively, high concentration fuel from the fuel cartridge can be injected by the fuel pump perpendicularly to the face of anode plate like a shower through a porous structure. Such a face feed creates a uniform methanol concentration throughout the anode flow path and may provide advantages in certain applications.
At the start of operating the fuel cell, a small initial charge of circulating liquid with little water may be contained inside the gas separator. Alternatively, there can be no initial charge of water. Upon starting the fuel cell from a temperature lower than the design point or operating temperature (e.g. 45-60° C.), the water crossover coefficient through the membrane is initially lower due to lower cell temperatures. Hence, there will be water surplus in the anode exhaust initially and this surplus will be kept in the gas separator to form the circulating liquid. Once the DMFC temperature reaches the design point, there is no more surplus water in the anode exhaust and the liquid amount in the circulation loop ceases to increase.
In one embodiment, the gas separator can be housed with fuel cartridge in an inflatable bag, for example, a plastic bag, as shown in
The proton-conducting electrolyte membrane in the system shown in
The cathode side can be either air circulating or air breathing. No water recovery from the cathode side is necessary.
In one embodiment, the fuel cell provides from 100 mW to 100 W of power, which range includes 100, 200, 300, 400, 500, 600, 700, 800, or 900 mW, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 W, or any value, including any non-integer value, therebetween or any combination thereof.
The fuel cell is suitable as a power source for any electronic device, for example, as a battery replacement, supplement, or back up. Some non-limiting examples of electronic devices include computers, personal digital assistants, cell phones, cameras, digital cameras, portable music devices, handheld game devices, and the like.
The following examples are provided for illustration purposes only and are not intended to be limiting unless otherwise specified.
An experiment was conducted in which a cell having a 12 cm2 active area was used, the cathode gas diffusion layer (hereinafter, “GDL”) included a carbon cloth GDL of 300 μm in thickness and a 30 μm thick microporous layer (MPL). The anode backing layer was a Toray carbon paper TGPH 090 with the thickness of 260 μm. The MEA was made by hot-pressing the anode backing layer and cathode GDL onto a catalyst coated Nafion® 112 membrane. The loading of Pt—Ru (Pt—Ru black HiSPEC 6000, Pt:Ru=1:1 atomic ratio, Alfa Aesar, a Johnson Matthey Company) on the anode and Pt (supported on carbon 40% Pt/Vulcan XC72 from E-TEK) was 4.6 mg/cm2 and 1.2 mg/cm2, respectively. The MEA was installed in a conventional cell fixture with two-pass serpentine flowfield, and the cell was operated in the same setup as shown in
Several embodiments discussed herein make it possible to use concentrated fuel with no recovery of water from the cathode exhaust and match the optimized cell performance with diluted methanol. It should be noted that the fuel feeding rate and air stoichiometry used may vary with cell operating conditions.
