Patent Application
20050271915
The invention is directed to direct alcohol fuel cells using solid acid electrolytes.
Alcohols have recently been heavily researched as potential fuels. Alcohols, such as methanol and ethanol, are particularly desirable as fuels because they have energy densities five- to seven-fold greater than that of standard compressed hydrogen. For example, one liter of methanol is energetically equivalent to 5.2 liters of 350 atm-compressed hydrogen. Also, one liter of ethanol is energetically equivalent to 7.2 liters of 350 atm-compressed hydrogen. Such alcohols are also desirable because they are easily handled, stored and transported.
Methanol and ethanol have been the subject of much of the alcohol fuel research. Ethanol can be produced by the fermentation of plants containing sugar and starch. Methanol can be produced by the gasification of wood or wood/cereal waste (straw). Methanol synthesis, however, is more efficient. These alcohols, among others, are renewable resources, and are therefore expected to play an important role both in reducing greenhouse gas emissions and in reducing dependence on fossil fuels.
Fuel cells have been proposed as devices for converting the chemical energy of such alcohols into electric power. In this regard, direct alcohol fuel cells having polymer electrolyte membranes have been heavily researched. Specifically, direct methanol fuel cells and direct ethanol fuel cells have been studied. However, research into direct ethanol fuel cells has been limited due to the relative difficulty in ethanol oxidation compared to methanol oxidation.
Despite these vast research efforts, the performance of direct alcohol fuel cells remains low, primarily due to kinetic limitations imparted by the electrode catalysts. For example, a typical direct methanol fuel cell exhibits a power density of about 50 mW/cm2. Higher power densities, e.g. 335 mW/cm2, have been obtained, but only under extremely severe conditions (Nafion®, 130° C., 5 atm oxygen and 1 M methanol with a flow of 2 cc/min under a pressure of 1.8 atm). Similarly, a direct ethanol fuel cell exhibited a power density of 110 mW/cm2 under similar extremely severe conditions (Nafion®-silica, 140° C., 4 atm anode, 5.5 atm oxygen). Accordingly, a need exists for direct alcohol fuel cells having high power densities in the absence of such extreme conditions.
The present invention is directed to alcohol fuel cells having solid acid electrolytes and using an internal reforming catalyst. The fuel cell generally comprises an anode, a cathode, a solid acid electrolyte, and an internal reformer. The reformer reforms the alcohol fuel into hydrogen. This reforming reaction is driven by the heat generated by the exothermic fuel cell reactions.
The use of solid acid electrolytes in the fuel cell enable the reformer to be placed immediately adjacent to the anode. This was not previously thought possible due to the elevated temperatures required for known reforming materials to function efficiently and the sensitivity of typical polymer electrolyte membranes to heat. However, the solid acid electrolytes can withstand much higher temperatures than the typical polymer electrolyte membranes, enabling the placement of the reformer adjacent the anode and therefore close to the electrolyte. In this configuration, the waste heat generated by the electrolyte is absorbed by the reformer and powers the endothermic reforming reaction.
These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:
The present invention is directed to direct alcohol fuel cells having solid acid electrolytes and utilizing an internal reforming catalyst in physical contact with the membrane-electrode assembly (MEA) for reforming the alcohol fuel into hydrogen. As noted above, the performance of fuel cells that convert the chemical energy in alcohols directly to electric power remains low due to kinetic limitations of the fuel cell electrode catalysts. However, it is well known that these kinetic limitations are greatly reduced when hydrogen fuel is used. Accordingly, the present invention uses a reforming catalyst, or reformer, to reform the alcohol fuel into hydrogen, thereby reducing or eliminating the kinetic limitations associated with the alcohol fuel. Alcohol fuels are steam reformed according to the following exemplary reactions:
Methanol to hydrogen: CH3OH+H2O->3H2+CO2
Ethanol to hydrogen: C2H5OH+3H2O->6H2+2 CO2
The reforming reaction, however, is highly endothermic. Therefore, to drive the reforming reaction, the reformer must be heated. The heat required is typically about 59 kJ per mol methanol (equivalent to combustion of about 0.25 mol hydrogen) and about 190 kJ per mol of ethanol (equivalent to combustion of about 0.78 mol hydrogen).
The passage of current during operation of fuel cells generates waste heat, the efficient removal of which has proven problematic. The generation of this waste heat, however, makes placement of the reformer directly beside the fuel cell a natural choice. Such a configuration enables the reformer to supply the fuel cell with hydrogen and cool the fuel cell, and allows the fuel cell to heat and power the reformer. Molten carbonate fuel cells and methane reforming reactions operating at a temperature of about 650° C. have employed such a configuration. However, alcohol reforming reactions generally take place at temperatures ranging from about 200° C. to about 350° C., and no suitable alcohol reforming fuel cell has yet been developed.
The present invention is directed to such an alcohol reforming fuel cell. As illustrated in
Any alcohol fuel can be used, such as methanol, ethanol and propanol. In addition, dimethyl ether may be used as the fuel.
Historically, this configuration was not thought possible for alcohol fuel cells due to the endothermic nature of the reforming reaction and the heat sensitivity of the electrolyte. Typical alcohol fuel cells use polymer electrolyte membranes which cannot withstand the heat needed to power the reforming catalyst. However, the electrolytes used in the fuel cells of the present invention comprise solid acid electrolytes, such as those described in U.S. Pat. No. 6,468,684, entitled PROTON CONDUCTING MEMBRANE USING A SOLID ACID, the entire contents of which are incorporated herein by reference, and in co-pending U.S. patent application Ser. No. 10/139,043, entitled PROTON CONDUCTING MEMBRANE USING A SOLID ACID, the entire contents of which are also incorporated herein by reference. One nonlimiting example of a suitable solid acid for use as an electrolyte with the present invention is CsH2PO4. The solid acid electrolytes used with the fuel cells of this invention can withstand much higher temperatures, enabling placement of the reforming catalyst immediately adjacent the anode. Moreover, the endothermic reforming reaction consumes the heat produced by the exothermic fuel cell reactions, creating a thermally balanced system.
These solid acids are used in their superprotonic phases and work as proton conducting membranes over a temperature range of from about 100° C. to about 350° C. The upper end of this temperature range is ideal for methanol reformation. To ensure that enough heat is generated to drive the reforming reaction, and to ensure that the solid acid electrolyte conducts protons, the fuel cell of the present invention is preferably operated at temperatures ranging from about 100° C. to about 500° C. More preferably, however, the fuel cell is operated at temperatures ranging from about 200° C. to about 350° C. In addition to significantly improving the performance of alcohol fuel cells, the relatively high operation temperatures of the inventive alcohol fuel cells may enable replacement of precious metal catalysts, such as Pt/Ru and Pt at the anode and cathode, respectively, with less costly catalyst materials.
The following Examples and Comparative Examples illustrate the superior performance of the inventive alcohol fuel cells. However, these Examples are presented for illustrative purposes only, and are not to be construed as limiting the invention to these Examples.
13 mg/cm2 Pt/Ru was used as the anode electrocatalyst. Cu(30 wt %)-Zn(20 wt %)-Al was used as the internal reforming catalyst. 15 mg/cm2 Pt was used as the cathode electrocatalyst. A 160 μm thick membrane of CsH2PO4 was used as the electrolyte. Vaporized methanol and water mixtures were supplied to the anode chamber at a flow rate of 100 μl/min. 30% humidified oxygen was supplied to the cathode at a flow rate of 50 cm3/min (STP). The methanol:water ratio was 25:75. The cell temperature was set at 260° C.
13 mg/cm2 Pt/Ru was used as the anode electrocatalyst. Cu(30 wt %)-Zn(20 wt %)-Al was used as the internal reforming catalyst. 15 mg/cm2 Pt was used as the cathode electrocatalyst. A 160 μm thick membrane of CsH2PO4 was used as the electrolyte. Vaporized ethanol and water mixtures were supplied to the anode chamber at a flow rate of 100 μl/min. 30% humidified oxygen was supplied to the cathode at a flow rate of 50 cm3/min (STP). The ethanol:water ratio was 15:85. The cell temperature was set at 260° C.
13 Mg/cm2 Pt/Ru was used as the anode electrocatalyst. 15 mg/cm2 Pt was used as the cathode electrocatalyst. A 160 μm thick membrane of CsH2PO4 was used as the electrolyte. 3% humidified hydrogen was supplied to the anode chamber at a flow rate of 100 μl/min. 30% humidified oxygen was supplied to the cathode at a flow rate of 50 cm3/min (STP). The cell temperature was set at 260° C.
A fuel cell was fabricated by slurry deposition of CsH2PO4 onto a porous stainless steel support, which served both as a gas diffusion layer and a current collector. The cathode electrocatalyst layer was first deposited onto the gas diffusion layer and then pressed, prior to deposition of the electrolyte layer. The anode electrocatalyst layer was subsequently deposited, followed by placement of the second gas diffusion electrode as the final layer of the structure.
A mixture of CsH2PO4, Pt (50 atomic wt %) Ru, Pt (40 mass %)-Ru (20 mass %) supported on C (40 mass %) and naphthalene was used as the anode electrode. The mixing ratio of CsH2PO4:Pt—Ru:Pt—Ru—C:naphthalene was 3:3:1:0.5 (by mass). A total mixture of 50 mg was used). The Pt and Ru loadings were 5.6 mg/cm2 and 2.9 mg/cm2, respectively. The area of the anode electrode was 1.74 cm2.
A mixture of CsH2PO4, Pt, Pt (50 mass %) supported on C (50 mass %) and naphthalene was used as the cathode electrode. The mixing ratio of CsH2PO4:Pt:Pt—C:naphthalene was 3:3:1:1 (by mass). A total mixture of 50 mg was used. The Pt loadings were 7.7 mg/cm2. The area of the cathode was 2.3-2.9 cm2.
CuO (30 wt %)-ZnO(20 wt %)-Al2O3, i.e. CuO (31 mol %)-ZnO (16 mol %)-Al2O3, was used as the reforming catalyst. The reforming catalyst was prepared by a co-precipitation method using a copper, zinc and aluminum nitrate solution (total metal concentration was 1 mol/L), and an aqueous solution of sodium carbonates (1.1 mol/L). The precipitate was rinsed with deionized water, filtered and dried in air at 120° C. for 12 hours. The dried powder of 1 g was lightly pressed to a thickness of 3.1 mm and a diameter of 15.6 mm, and then calcined at 350° C. for 2 hours.
A 47 μm thick CsH2PO4 membrane was used as the electrolyte.
A methanol-water solution (43 vol % or 37 mass % or 25 mol % or 1.85 M methanol) was fed through a glass vaporizer (200° C.) at a rate of 135 μl/min. The cell temperature was set at 260° C.
A fuel cell was prepared according to Example 3 above except that an ethanol-water mixture (36 vol % or 31 mass % or 15 mol % or 0.98 M ethanol), rather than a methanol-water mixture was fed through the vaporizer (200° C.) at a rate of 114 μl/min.
A fuel cell was prepared according to Example 3 above except that vodka (Absolut Vodka, Sweden)(40 vol % or 34 mass % or 17 mol % ethanol) instead of the methanol-water mixture was fed at a rate of 100 μl/min.
A fuel cell was prepared according to Example 3 above except that dried hydrogen of 100 sccm humidified through hot water (70° C.) was used instead of the methanol-water mixture.
A fuel cell was prepared according to Example 3 above except that no reforming catalyst was used and the cell temperature was set at 240° C.
A fuel cell was prepared according to Comparative Example 2, except that the cell temperature was set at 240° C.
The preceding description has been presented with reference to the presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and modifications may be made to the described embodiments without meaningfully departing from the principal, spirit and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise embodiments described, but rather should be read as consistent with, and as support for, the following claims, which are to have their fullest and fairest scope.
This application claims the benefit of Provisional Application Ser. No. 60/557,522, filed Mar. 30, 2004, entitled DIRECT ALCOHOL FUEL CELLS USING SOLID ACID ELECTROLYTES, the entire disclosure of which is incorporated herein by reference.
The United States government has certain rights in this invention pursuant to Grant No. DMR-9902882, awarded by the National Science Foundation, and Grant No. N00014-02-1-0192, awarded by the Office of Naval Research.
| Number | Date | Country | |
|---|---|---|---|
| 60557522 | Mar 2004 | US |
