KR20070078844A - Solid fuels for fuel cells - Google Patents

Abstract

A solid fuel for use in fuel cells. The solid fuel includes solid oxygenates, and mixtures for generating a gaseous fuel from the solid fuel. The solid fuel can be contained in a cartridge and reacted with a liquid reactant for generating a gaseous fuel used in the fuel cell.

Description

Solid fuels for fuel cells {SOLID FUELS FOR FUEL CELLS}

Fuel cells have been developed as a method of generating electricity from chemicals. Some early developments focused on using hydrogen as a clean fuel source for power generation. Studies on the storage and generation of hydrogen for use in fuel cells have been conducted and are disclosed in US 6,057,051, US 6,267,229, US 6,251,349, US 6,459,231 and US 6,514,478. Hydrogen is a high energy and low pollution fuel, but fuel storage is disadvantageous in terms of energy density and safety.

The difficulty with hydrogen storage is that we need to investigate the generation of hydrogen from more useful fuels. Liquid fuels containing a relatively large amount of hydrogen that can be generated through reforming have received considerable attention. Fuel reforming is expensive and significantly increases the complexity and size of devices that use fuel cells to generate power. As disclosed in US 4,716,859, US 6,238,815 and US 6,277,330, methods and reformers for reforming liquid fuel have been developed. Therefore, there is considerable interest in fuel cells that can use hydrogen enriched fuels that can be processed directly at the fuel cell electrodes. This results in a feed stream to two general classes of fuel cells: indirect or reformer fuel cells (fuel, typically organic fuels are reformed and treated, so that hydrogen is enriched and substantially free of carbon monoxide (CO)). ) And direct oxidation fuel cells (which feed organic fuel directly to the fuel cell and oxidize it without any chemical modification). Direct oxidation fuel cells can use either a liquid supply design or a vapor supply design, and post-oxidation fuel in the fuel cell preferably produces clean combustion products such as water and carbon dioxide (CO ₂ ).

In the early development of direct methanol fuel cells (DMFCs), the use of gaseous methanol requires high heat, which causes decomposition of the fuel cell membrane. This led to the development of DMFCs using methanol in the liquid phase, as shown in US 5,599,638 and US 6,248,460. However, the liquid phase also presents a drawback, the important being the contamination of the cathode and the crossover of the membrane with methanol.

Like gaseous fuel cells, liquid fuel cells also have handling problems. Specific problems include the orientation of a fuel cell or portable device into which liquid fuel can flow out of an opening for releasing waste gas, where a liquid fuel cell is oxidized at the cathode by the spreading of high concentrations of liquid methanol to improve fuel cell efficiency. Has the problem of reducing

Summary of the Invention

The present invention relates to solid fuels used in fuel cells. Solid fuels include solid oxygen products selected from metal oxygenates, gel oxygenates and frozen oxygenates. The present invention particularly includes as a solid fuel a mixture of an oxygenate such as methanol or acetaldehyde and a polymer such as an acrylic polymer in an amount necessary to produce a solid gel.

In one embodiment, the present invention includes the addition of a metal or metal compound selected from the group consisting of alkali metals, alkaline earth metals and mixtures thereof. In particular, preferred metal compounds include magnesium compounds, such as magnesium hydroxide, magnesium oxide, magnesium methoxide, magnesium hydride and mixtures thereof. Preferred metal is magnesium. Metal compounds are provided for materials that enhance the aspect of oxygenates and absorb carbon dioxide generated at the anode.

In another embodiment, the present invention includes the addition of an oxidizing agent. The oxidant is selected from the group consisting of sodium percarbonate, carbamide hydrogen peroxide, organic peroxides, calcium peroxide, magnesium peroxide and mixtures thereof. The addition of oxidant enhances the fuel power density of direct methanol fuel cells.

Other objects, benefits and uses of the present invention will become apparent to those skilled in the art from the following detailed description.

Detailed description of the invention

The present invention includes novel fuels used in fuel cells. The new fuel is a solid fuel and is not limited to the type of fuel cell in which it can be used, including proton exchange membrane (PEM) fuel cells, solid oxide fuel cells (SOFC), phosphoric acid fuel cells (PAFC), direct methanol fuel cells ( DMFC), molten carbonate fuel cell (MCFC) and alkaline fuel cell (AFC).

To overcome the drawbacks of liquid fuel cells, alternative methods of handling liquid fuels have been developed. As shown in US 4,493,878, the method involves combining liquid fuel in a non-flowing state, where the fuel is recovered in flow if necessary. This method still has the drawbacks of liquid fuel in that the liquid fuel spreads to the anode, reducing the cathode efficiency. Methanol crossing through the membrane causes partial shorting of the cell, resulting in lower potential. Membranes are under development to mitigate this diffusion process. On the other hand, the concentration of methanol is generally limited to 1-2 mol / l, that is, 7 wt%. The result is a rapid reduction of the current-voltage (I-V) curve, significant vortex power loss around the pump, and a relatively large number of process steps and fuel cell components for controlling the anode supply.

There is a need for fuels that more easily handle and easily generate gaseous components used in fuel cells. Solid fuels provide higher energy density and ease of handling. Solid fuels can be conveniently loaded, removed and replaced in a fuel cell. Solid fuels reduce the risk of leaks and leaks that can occur using liquid or gaseous fuels. And solid fuels can use lighter containers than with gaseous fuels. In addition, the orientation of the fuel cell as a solid is irrelevant since the fuel will not move independently after loading into the fuel cell and will maintain a fixed position relative to the anode. The fuel may be any solid chemical that generates a suitable fuel, such as oxygenates or hydrogen, for direct oxidation at the fuel cell anode. The fuel comprises a mixture of fuel components, the fuel component being any chemical composition added to the fuel mixture. Oxygenate is a hydrocarbon compound modified by adding one or more oxygen atoms to a hydrocarbon compound. Oxygenates include, but are not limited to, alcohols, diols, triols, aldehydes, ethers, ketones, diketones, esters, carbonates, dicarbonates, oxalates, organic acids, sugars, and mixtures thereof. In the reaction of solid oxygenates, gaseous oxygenates, such as methanol, are produced for reaction in fuel cells.

One preferred group of oxygenates is metal alkoxides which react with water to produce gaseous oxygenates for reaction at the anode of the fuel cell. By generating gaseous oxygenated oxygen, the fuel overcomes the limitations due to the liquid fuel cell overloading the fuel cell and spreading to the cathode. Preferred metal oxygenates include metal alkoxides. Suitable metals include, but are not limited to, alkali and alkaline earth metals, and are selected from lithium (Li), sodium (Na), potassium (K), beryllium (Be), magnesium (Mg) and calcium (Ca). Other suitable metals include rubidium (Rb), cesium (Cs), strontium (Sr), barium (Ba) and aluminum (Al). Oxygen products produced for use in fuel cells preferably have boiling points below 100 ° C. Preferably the oxygenates include low molecular weight alcohols, aldehydes, organic acids and ethers.

Alkali alkoxides, and especially alkali methoxides and ethoxides, are highly reactive, pyrophoric materials. The addition of water produces a violent reaction and heat sufficient to evaporate the alcohol produced during the reaction.

The particular alkoxide studied is lithium methoxide (LiOCH ₃ ). As shown in Scheme 1, lithium methoxide is reacted with water to generate lithium hydroxide and methanol with sufficient heat to generate methanol in the gas phase.

Scheme 1

LiOCH ₃ (s) + H ₂ O (g)-> LiOH (s) + HOCH ₃ (g)

The stability of lithium methoxide has been studied along with the stability of several oxidants. Experiments were performed in a water saturated atmosphere at room temperature. Samples were weighed over time. The sample was found to increase in weight after weight loss and the results are shown in FIG. 1. Without being bound by a particular theory, it is believed that the solid fuel (lithium methoxide) reacted with methanol generated water vapor and then lost weight. The subsequent weight increase is due to the reaction of lithium hydroxide in the air with carbon dioxide to form carbonic acid as in Scheme 2.

Scheme 2

2LiOH (s) + CO ₂ (g)-> Li ₂ CO ₃ (s) + H ₂ O

The fuel should be sealed in a container that is moisture impermeable to prevent fuel consumption through atmospheric exposure. In use, the fuel container is opened but sealed against the anode to form a sealed compartment in the atmosphere. This operation prevents loss of fuel and also prevents excess moisture affecting the fuel. Thus, the consumption of fuel is controlled by partitionable moisture.

A further compositional study yielded results similar to the loss and increase in weight shown in FIG. 1 and listed in Table 1. Part of the test fuel included a small amount of catalyst, MnO ₂ , to facilitate the decomposition of the exothermic reactants. The exothermic reactant generates heat to evaporate the fuel.

Solid fuel 1 Lithium Methoxide (LiOCH ₃ ) Solid fuel 2 LiOCH ₃ + Sodium Percarbonate + MnO ₂ Solid fuel 3 Sodium Percarbonate + MnO ₂ Solid fuel 4 LiOCH ₃ + Carbamide * H ₂ O ₂ + MnO ₂ Solid fuel 5 Carbamide * H ₂ O ₂ + MnO ₂

This leads to the further theory that the solid fuel to be activated is used by exposure to water, included in the vapor phase, generates fuel and heat and then absorbs waste gas to form the solid phase. Other activation means include, but are not limited to, heat supply, application of electrical current, and exposure to carbon dioxide. The fuel reacts at the anode and generates waste gas. For example, methanol reacting at the anode generates carbon dioxide and water in addition to the electricity generated during the reaction according to Scheme 3.

Scheme 3

CH ₃ OH + 1.5O _2- > 2H ₂ O + CO ₂ + Electric

Carbon dioxide is a waste gas that must be treated in some way. According to the invention, carbon dioxide reacts with fuel waste products such as metal hydroxides and forms a solid. Preferred fuels will include components that absorb or react with waste gases from the fuel cell. The fuel component may include, but is not limited to, metal oxides and metal hydroxides. The waste gas reacts to form a solid product or is absorbed into the solid phase. For direct methanol fuel cells, the primary waste gases are carbon dioxide and water. Water will react with the fuel and form more oxygen in the gas phase. If the fuel is a metal alkoxide (where the metal is an alkali or alkaline earth metal), the metal will form a hydroxide that reacts with water to discard the alcohol. The metal hydroxide will then react with the carbon dioxide generated at the anode and the carbon dioxide from the gas phase will be removed to form a carbonate solid product.

Other preferred fuels include gel oxygenates and frozen oxygenates. Gel oxygenates are oxygenates with a polymer added to form a solid. One example of a gel oxygenate comprises a mixture of 5% by weight Carbopol ^™ 981 polymer and 95% by weight methanol. Carbopol 981 is an acrylic polymer manufactured by BF Goodrich, Akron, Ohio. Upon heating, the gel oxygenate releases methanol, which is evaporated and available at the anode. Oxygen products in the solid fuel make up at least 30% by weight, and preferably at least 50% by weight of the fuel. In gels or frozen oxygenates, the fuel contains additional compounds that absorb waste gases from the anode. Additional fuel components for gels and freeze oxygenates include metals, metal oxides, metal hydroxides or metal hydrides. Preferably metals, metal oxides, metal hydroxides and metal hydrides include alkali or alkaline earth metals. Additional components provide components that provide heat to evaporate oxygenates and remove anode waste gases through absorption or reaction to form solid waste products.

Additional materials added to the solid oxygenate include hydrogen reactive materials that add water to generate heat. As a material it is desirable to provide peroxides which absorb further fuels such as hydrogen and / or carbon dioxide. Preferred materials include metal hydrides such as lithium hydride, magnesium hydride, sodium hydride, potassium hydride, aluminum hydride and mixtures thereof.

Solid fuels can be formed by polymerizing organic solutions with selected chemicals to gelatinize organic compounds. The polymerization chemicals make up at least 3% by weight of the solid fuel. Chemicals forming the gel include acrylic / acrylamide polymers, copolymers of polyols, amine emulsifiers and ethylene / acrylic acid copolymers, carboxyl vinyl polymers, polyacrylic acid polymers, olefin-maleic anhydride copolymers, and small OH groups. Including but not limited to copolymers of polymers with formaldehyde. Copolymers of oligomers containing OH groups include high melting point alcohols, ie, alcohols having 12 or more carbons; High melting point glycol; High melting point hydrocarbons; Sugar esters, ie, sorbitan monostearate (S-MAZ 60); And alkali alkoxides. Further polymeric materials are described in US 3,759,674; US 3,148,958; US 3,214,252; US 4,261,700; And US 4,865,971.

Specific gel fuels have been studied to demonstrate the use of gel fuels. Fuels include methanol, calcium oxide (CaO) and Carbopol 981 polymers in ratios of 32: 56: 5, respectively. Fuel was loaded into the DMFC and the fuel cell was turned on. I-V curves of fuel cells were generated for comparison with aqueous methanol fuel at different temperatures, and atmospheric pressure. 2 shows the results of an I-V curve for comparison with a liquid fuel.

The composition of the fuel can be adjusted to compensate for the additional water generated or absorbed by the fuel and the additional heat needed to evaporate the fuel upon exposure to moisture. Additional heat can be generated using water or chemicals that exhibit significant exothermic reactions due to the addition of suitable chemicals that generate heat upon decomposition. Examples of suitable chemicals include, but are not limited to, organic peroxides, and carbamide hydrogen peroxide. The fuel composition can also be adjusted by mixing metal hydrides and metal oxygenates to form a fuel that will generate alcohol and hydrogen for reaction at the fuel cell anode, for example, using a combination of the aforementioned fuels. Alternative mixtures may include additional metal hydroxides for faster reaction of carbon dioxide generated at the anode.

The cost of lithium reduces the usefulness and desirable properties of lithium compounds. Lithium and lithium compounds are much more expensive than other alkali or alkaline earth metals and compounds thereof. Further research to find suitable compounds involves the use of various magnesium compounds. The compositions of some of the mixtures tested are listed in Table 2.

Chemical Composition of Solid Methanol Fuel Using Magnesium Compounds as Additives Fuel I.D. # Mg compound, wt% Solid methanol weight% Explanation One Mg (OH) ₂ , 64.36 35.64 Solid paste 2 MgO, 56 44 Solid paste 3 Mg (OCH ₃ ) ₂ , 100 0 Solid crystals 4 Mg, 42.2 57.6 Solid paste

This test involves the use of magnesium compounds alone or as a mixture with solid methanol in the case of magnesium methoxylated (Mg (OCH ₃ ) ₂ ). Solid methanol comprises a mixture of methanol and Carbopol 981. 3-6 show the results of tests using magnesium or various magnesium compounds with solid methanol in a fuel cell. Magnesium compounds include, but are not limited to, magnesium hydroxide (Mg (OH) ₂ ), magnesium oxide (MgO), magnesium methoxylated (Mg (OCH ₃ ) ₂ ), and magnesium hydride (MgH ₂ ). The figure shows the IV, i.e., current versus potential curve, measured for various compositions. A small amount of water was added in the test to create moisture in the anode chamber. The reaction of the solid fuel begins with a small amount of moisture and can then be generated by itself because of the moisture produced by the reaction at the anode. In addition, the magnesium compound absorbs carbon dioxide (CO ₂ ) generated from the anode when methanol reacts to generate an electric current.

Alternative solid fuels studied are acetaldehyde solid fuels. The fuel contains a mixture of 50 gm of acetaldehyde and 0.5 gm of Carbopol 981. The fuel was gelatinized and the solid fuel mixed with 3.2 gm of magnesium oxide. The I-V curves of MgO and solid acetaldehyde were measured and compared with the results for MgO and solid methanol. The results are shown in FIG. 7, which shows an improvement over solid methanol.

In addition to solid oxygenate fuels, it has been found that the performance of fuels can be improved by the addition of oxidants, in particular oxidants that generate hydrogen during the process of activating the fuel. Direct methanol fuel cells have been studied using oxidants in a pulsed manner. As can be seen in Figure 8, in the case of hydrogen peroxide (H ₂ O ₂ ) after applying the pulse of the oxidant fuel cell showed a dilution effect, but the power density increased. Ultimately, the power density was improved by 14%. 1-3 wt% hydrogen peroxide solution was added to 3 wt% methanol solution and the IV curve was measured. It was found that adding oxidant to methanol as shown in FIG. 9 improves the IV curve. In the case of hydrogen peroxide, the greatest improvement was obtained when 2 wt% solution was added.

Hydrogen peroxide is a liquid, but solid oxidants may be used that are similar in appearance. Alternative oxidants include, but are not limited to, sodium percarbonate, carbamide hydrogen peroxide, organic peroxides such as tert-butyl hydrogen peroxide (TBHP), tert-pentyl hydrogen peroxide, and the like, and alkaline earth metal peroxides such as magnesium peroxide and calcium peroxide.

The addition of strong oxidizers was further studied to determine the effect on power generation for direct methanol fuel cells. Sulfuric acid (H ₂ SO ₄ ) was injected into solid methanol to perform a pulse test. The addition of sulfuric acid generated additional hydrogen from the magnesium metal and improved the performance of the DMFC. The results shown in FIGS. 10-12 show an improvement in power density and an increase in pressure in the anode compartment of the fuel cell. Additional acids that may be used include, but are not limited to hydrochloric acid (HCl) and nitric acid (HNO ₃ ).

Addition of an oxidant improves performance and the oxidant can be added in solid form, where the oxidant reacts with the solid fuel in the presence of moisture. Control of the addition of water to the fuel can be used to regulate the generation of gaseous fuel for the fuel cell and for intermittent power generation. Alternatively, if the fuel is placed in fluid communication with the anode compartment of the fuel cell, the liquid strong oxidant may be kept in a separate sealing compartment for control of addition to the solid fuel.

Another aspect involving the addition of compounds such as peroxides is that they release heat when the peroxides react or decompose. Heat release encourages the evaporation of methanol or other organic compounds that react at the anode of the fuel cell in the gas phase. Peroxides can be easily decomposed by addition of a small amount of catalyst. Catalysts for decomposition of oxidants are calcium (Ca), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), Copper (Cu), Zinc (Zn), Strontium (Sr), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), silver (Ag), cadmium (Cd), barium (Ba), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), It is a compound containing at least one metal selected from iridium (Ir), platinum (Pt), gold (Au) and mercury (Hg). The catalyst may comprise a sol comprising an oxide of a metal, a sulfide of a metal and other sulfur compounds of a metal and a metal. Preferred catalysts include one or more metals from vanadium, iron, cobalt, ruthenium, copper, nickel, manganese, molybdenum, platinum, gold, silver, palladium, rhodium, rhenium, osmium and iridium, more preferred catalysts being iron, Cobalt, nickel and manganese. More preferred compound is manganese oxide (MnO ₂ ).

While the present invention has described preferred embodiments to date, it will be understood that they are not limited to the embodiments disclosed herein. However, arrangements corresponding to various modifications included within the scope of the appended claims are intended to be included in the present invention.

1 shows the stability of some chemical compounds and mixtures;

2 shows a comparison of DMFC liquid and solid fuels;

3 and 4 show the result of comparison of the electric current with respect to the cell potential of solid fuel and liquid methanol having different compositions;

5 and 6 show the result of comparing the current with respect to the cell potential of solid fuels having different compositions;

7 shows a comparison of solid acetaldehyde fuel and solid methanol fuel;

8 shows the effect of additional oxidants added to methanol for a direct methanol fuel cell;

9 shows I-V (current-voltage) curves for hydrogen peroxide different from methanol in a direct methanol fuel cell;

10 shows the voltage and current amounts for magnesium and solid methanol fuel cells using pulses of sulfuric acid;

FIG. 11 shows the voltage, amount of current and power density for magnesium and solid methanol in a fuel cell using a pulse of sulfuric acid; And

12 shows power density and voltage for magnesium and solid methanol in cell fuel using pulses of sulfuric acid.

Claims (10)

A solid fuel for generating a gaseous fuel used in a fuel cell, A solid fuel comprising an oxygen compound selected from the group consisting of metal oxygenates, gel oxygenates, frozen oxygenates and mixtures thereof. The group of claim 1 wherein the oxygenate comprises alcohols, aldehydes, organic acids, ethers, diols, triols, ketones, diketones, esters, carbonates, oxalates, sugars, metal alkoxides, metal aldehydes and mixtures thereof. Solid fuel selected from. The solid fuel according to claim 1 or 2, further comprising a solid oxidant selected from the group consisting of sodium percarbonate, carbamide hydrogen peroxide, organic peroxides and mixtures thereof. The method of claim 1, wherein the gel oxygenate is Oxygenate and Acrylic acid polymers, acrylamide polymers, copolymers of polyols, copolymers of oligomers and formaldehydes containing OH groups, amine emulsifiers and ethylene / acrylic acid copolymers, carbonyl vinyl polymers, polyacrylic acid polymers, olefin-maleic anhydrides Polymer mixtures selected from the group consisting of copolymers and mixtures thereof Solid fuel comprising a. The method of claim 4 wherein the gel oxygenate is At least 30% by weight of an oxygenate selected from the group consisting of methanol, acetaldehyde and mixtures thereof 3% by weight or more of acrylic polymer Solid fuel comprising a. 6. The solid fuel according to claim 1, wherein the gel oxygenate further comprises a metal or metal compound selected from the group consisting of alkali metals, alkaline earth metals and mixtures thereof. 7. The metal or metal compound according to claim 6, wherein the metal or metal compound is magnesium hydroxide (Mg (OH) ₂ ), magnesium oxide (MgO), magnesium methoxide (Mg (OCH ₃ ) ₂ ), magnesium (Mg), magnesium hydride (MgH ₂ ) And a mixture thereof. The solid fuel according to any one of claims 1 to 6, further comprising a solid catalyst selected from the group consisting of transition metals, oxides of transition metals and mixtures thereof. The method of claim 1, wherein the metal oxygenate is lithium (Li), sodium (Na), potassium (K), beryllium (Be), magnesium (Mg), calcium (Ca), rubidium (Rb), cesium (Cs) And a metal selected from the group consisting of strontium (Sr), barium (Ba), aluminum (Al) and mixtures thereof. The solid fuel of claim 9, wherein the metal is selected from the group consisting of lithium, sodium, potassium, magnesium, and mixtures thereof.

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