Patent Application
20080176130
This application claims priority to Korean Patent Application No. 10-2006-0122992, filed on Dec. 6, 2006, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to fuel cell technology, more particularly, to a fuel composition for improving performance of fuel cells and a fuel cell manufactured using the same.
2. Description of the Related Art
A fuel cell is a high efficiency electrochemical system to convert energy generated by chemical reaction into electric energy. Compared with other electric energy resources, the fuel cell has advantages over them such as high efficiency and environmental sustainability. In this regard, it is considered that fuel cell technologies are essential for developments of original advanced technologies in the country.
Although a demand for fuel cell which is an electric power source with high efficiency and environmental sustainability grows, there are still technical difficulties which interfere with use of the fuel cell in wide range of applications.
In general, a fuel cell comprises an anode electrode 11, a cathode electrode 12 and an electrolyte 13 disposed between the electrodes 11 and 12, both of the electrodes being connected to each other by an electric circuit 14. A typical fuel cell uses hydrogen H2 as fuel and oxygen O2 as oxidant Hydrogen generates protons and electrons according to the following chemical reaction 1. Electrons flow to the electric circuit 14 connecting the electrodes 11 and 12, while protons pass through the electrolyte 13 in contact with the anode electrode 11 and the cathode electrode 12.
H2→2H++2e− [Chemical reaction 1]
At the same time, electrons and protons generated from the anode electrode 11 are electrochemically reduced at the cathode electrode 12 by the reductant such as oxygen to produce water as a reactant according to the following chemical reaction 2:
½O2+2H++2e−→H2O [Chemical reaction 2]
As described above, a fuel cell directly using hydrogen fuel is a simple, convenient and clean cell with superior efficiency. But, this has restrictions on use thereof due to high flammability of the fuel. For instance, as hydrogen storage and transportation require high pressure tanks, hydrogen-powered fuel cells inevitably have limitations on use thereof in a variety of applications.
For the reasons described above, hydrogen is usually applied in the form of oxygenated hydrocarbons as fuel rather than direct use of hydrogen element itself. The oxygenated hydrocarbon fuel is a liquid phase fuel, which has higher energy density than hydrogen and is simple and more convenient than gaseous materials for storage, carrying, handling and/or transportation thereof. As a result, there have been proposed applications of oxygenated hydrocarbons including, for example: a direct oxidation process of oxygenated hydrocarbons such as alcohol or methanol; a preparation method of hydrogen by modifying oxygenated hydrocarbons, etc.
A direct (hydrogen) fuel feeding fuel cell is easy and simple to use in view of directly feeding the fuel to the fuel cell. However, if oxygenated hydrocarbon based fuel is directly introduced to a fuel cell, it is difficult for a catalyst to completely oxidize the fuel and carbon monoxide and other carbonaceous intermediates are usually generated according to the following chemical reaction 3:
CH3OH→CO+4H++4e− [Chemical reaction 3]
Even when the intermediates (mostly carbon monoxide) are used in only a small amount, these are securely adsorbed to a platinum catalyst often used in low temperature fuel cells, poison the catalyst (as called “catalyst poisoning”) and reduce activity of the catalyst.
Also, hydrocarbon molecules contained in a biomass material generate plenty of carbon monoxide through catalytic decomposition.
As described above, direct injection type liquid fuels which produce carbon monoxide and other carbonaceous intermediates during electrochemical oxidation have significant problems, particularly, durability of the fuel cells.
In order to solve the above problems, conventional techniques and proposals have focused on developing catalysts with superior poisoning resistance. Particularly, although PtRu/C (platinum-ruthenium/carbon alloy) catalyst has favorable performance, this also has many difficulties and/or limitations in efficiently eliminating carbon monoxide and needs an additional expensive catalyst.
Since most of existing fuel cells work in a strong acidic environment of pH 2 or less, appropriate oxidation assistants are not readily discovered. In case of the low temperature fuel cell, it is very difficult to oxidize carbon monoxide which was strongly adsorbed to a catalyst.
Accordingly, the present invention is directed to solve the problems of conventional arts in regard to fuel cells using hydrocarbon fuels as described above and, an object of the present invention is to provide a fuel composition capable of efficiently oxidizing carbon monoxide and other carbonaceous intermediates.
Another object of the present invention is to provided a fuel cell with improved performance by using the fuel composition according to the present invention.
In order to accomplish the above former object, the inventive fuel composition comprises: at least one of oxygenated hydrocarbons; and at least one polyoxometalate and/or at least one oxidation-reduction reactive anion (hereinafter, referred to as “redox anion”) caused by dissolution of polyoxometalate in an organic medium containing solution and/or an aqueous solution.
Oxygenated hydrocarbons may include compounds having —OH functional groups.
Alternatively, oxygenated hydrocarbons may include at least one selected from CH4O, C2H6O, C4H10O, C5H12O, C6H14O, ethyleneglycol, glycerin and compounds having —COOH and/or —CHO groups.
Preferably, anions contained in the fuel composition according to the present invention may comprise at least one selected from Keggin anions, Wells-Dawson anions, Lindqvist anions and Anderson anions.
Preferably, polyoxometalate and/or at least one redox anion caused by dissolution of polyoxometalate can be comprised in a content ranging from 0.1×10−9M to 0.5M to total 1 liter of the residual ingredients containing at least one of the above mentioned oxygenated hydrocarbons.
In order to accomplish the above latter object, the inventive fuel cell uses the fuel composition according to the present invention as fuel.
Such fuel cell of the present invention uses a catalyst made of noble metals and/or noble metal alloys.
Conventional techniques and/or proposals have substantially concentrated on researches and studies for catalysts with enhanced durability, whereas the present invention suggests an oxidation method of carbonaceous intermediates using a specific additive for fuel, which was substantially not attempted or tried hitherto, as well as applications thereof.
The present invention plays an important role in development of portable fuel cells including, for example, DMFC (direct methanol fuel cell), DAFC (direct alcohol fuel cell), DFAFC (direct formic acid fuel cell) and the like.
Briefly, the present invention utilizes polyoxometalate as an oxidation assistant for low temperature oxidation at less than 100° C. in a fuel anode electrode in addition to oxygenated hydrocarbons (or carbohydrates) such as ethanol, ethyleneglycol, formic acid, etc.
Polyoxometalate used in the present invention is previously known to be stable under a strong acidic environment of not higher than pH 2. or less and, for molybdenum-containing polyoxometalate, this compound exhibits reductive property in the range of capable adsorbing and desorbing carbon monoxide, therefore, can selectively oxidize carbon monoxide. Alternatively, polyoxometalates containing tungsten, vanadium and/or other elements have different redox potentials depending on the elements, which present a preferable solution to selectively oxidize and eliminate undesirable byproducts of fuel cells using various fuels including, for example, ethanol.
It is expected that polyoxometalate described above will be applicable in commonly available catalysts based on noble metals and/or noble metal alloys and, additionally, a variety of alloy catalysts and/or non-noble metal catalysts to be developed in the future.
These and other objects, features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken in conjunction with the accompanying drawings. In the drawings:
Hereafter, the present invention will be described in detail with reference to the accompanying drawings.
Again referring to the chemical reaction 3 and as described above, if the oxygenated hydrocarbon fuel is directly introduced into a fuel cell, the catalyst has a difficulty in complete oxidation of the fuel and, thus, generates carbon monoxide (hereinafter, sometimes referred to as “CO”) and carbonaceous intermediates according to the chemical reaction 3 during use of the fuel cell:
CH3OH→CO+4H++4e− [Chemical reaction 3]
The intermediates (mostly carbon monoxide) become the most common cause to reduce fuel cell efficiency in case of low temperature fuel cell, even when the intermediates are contained in only a small amount. Particularly, for carbon monoxide, it strictly binds to the catalyst used in the fuel cell and causes poisoning of the catalyst.
As such catalyst poisoning reduces active sites of the catalyst and results in decrease in activity of the catalyst, overall efficiency of the fuel cell is lowered. In general, hydrocarbon molecules contained in a biomass also generate plenty of carbon monoxide through catalytic decomposition.
Conventional investigations and studies have been focused on production of high purity hydrogen through fuel modification, however, this requires multiple stage catalyst processes and/or addition processes in modification and/or treatment of fuel, since concentration of carbon monoxide must be controlled to be less than 10 ppm in order to use hydrogen in hydrogen fuel cells for low temperature solid electrolytes.
On the other hand, the present invention proposes addition of polyoxometalate (hereinafter, often referred to as “POM”) as an oxidation assistant to fuel. That is, the fuel composition of the present invention comprises oxygenated hydrocarbons and polyoxometalate. POM can oxidize carbon monoxide according to the following chemical reaction 4, thereby generating hydrogen gas. Carbon monoxide was considered to be one of catalyst poisoning materials to be removed on the basis of traditional knowledge and/or existing techniques. However, in case of the fuel composition according to the present invention, carbon monoxide functions as an appropriate oxidation assistant for active electrochemical oxidation of oxygenated hydrocarbons in an anode electrode.
Polyoxometalate used in the present invention principally means a compound containing polyoxometalate and/or anions which are caused by polyoxometalate and have redox reactivity in an aqueous solution and/or an organic medium containing solution, namely, heteropolyanions (hereinafter often referred to as “HPA”).
Table 1 shows characteristics of anions caused by dissolution of polyoxometalate in an organic solution and/or an aqueous solution.
The anions described above include transitional metals such as molybdenum Mo, vanadium V, tungsten W, etc. as well as oxygen atoms. Based on structures of individuals anions, they have inherent features and are classified into or defined with different names thereof.
The heteropolyanions caused by polyoxometalate applicable in the present invention include, but are not limited to, at least one selected from Keggin anions (PMo12O403−), Well-Dawson anions (P2W18O626−), Lindqvist anions (Mo6O192−) and Anderson anions ([TeMoO6O26)6−) (reference: Chem. Rev. 1998, 98, 273-296).
Based on a principle according to the chemical reaction 4, polyoxometalate improves poisoning resistance of noble metal catalysts while oxidizing carbon monoxide and, additionally, generates hydrogen gas preferably used as fuel.
The chemical reaction 4 is the one using Keggin anions as a preferred embodiment of the present invention:
CO(g)+H2O(l)+PMo12O403−(aq)→CO2(g)+2H+(aq)+PMo12O405−(aq) [Chemical reaction 4]
Oxygenate hydrocarbon used in the present invention is a compound having —OH functional groups, which include, for example, at least one selected from CH4O, C2H6O, C4H10O, C5H12O, C6H14O, ethyleneglycol, glycerin and compounds having —COOH and/or —CHO groups.
Polyoxometalate and/or at least one of anions caused by polyoxometalate is preferably included in a content ranging from 0.1×10−9M to 0.5M to total 1 liter of the residual ingredients containing at least one of the above mentioned oxygenated hydrocarbons.
Particular experimental examples in regard to fuel compositions according to the present invention will be described in detail below. These examples describe a part of experiments carried out with particular ones of oxygenated hydrocarbons and polyoxometalates applicable in the present invention.
In order to evaluate electrochemical performance and characteristics of the fuel composition by addition of specific oxidation assistant according to the present invention, a three (3)-electrode system was prepared as shown in
Referring to
Referring
Firstly, a first solution prepared by adding 0.5M H2SO4 and 2M CH3OH to 40 wt. % Pt/C catalyst (ETEK) and a second solution prepared by adding 0.001M H3PMo12O40 to the first solution were subjected to the cyclic voltammogram (CV) experiment, respectively.
Each of CV experiments was repeated 30 cycles and the catalyst was sufficiently activated before the experiment. Black dot lines 1 shown in
From CV curves of the methanol solution 1 shown in
Furthermore, it was found that Keggin anion type PMO12O403− effectively oxidizes carbonaceous intermediates and byproducts without lowering the current density.
Especially, PtRu/C curves in
It is expected that the present inventive fuel composition which represents high oxidation effect even with a small amount of oxidation assistant, can be used in a wide range of applications, although novel catalysts with a plurality of advantages including, for example, excellent poisoning resistance and/or activity will be developed in the future.
Results of CV and potentiostat experiments for a sample which was prepared by adding H3PMo12O40 to the ethanol solution in the presence of PtRu/C catalyst (red solid lines) as a noble metal catalyst for fuel cell were compared with results of the same experiments for another sample obtained before addition of H3PMo12O40 to the ethanol solution. It was identified that these results are similar to the ones from Experimental example 1.
Results of CV and potentiostat experiments for a sample which was prepared by adding H3PMo12O40 to the ethanol solution in the presence of PtRu/C catalyst (red solid lines) as a noble metal catalyst for fuel cell were compared with results of the same experiments for another sample obtained before addition of H3PMo12O40 to the ethanol solution.
It was identified that these results are similar to the ones from Experimental example 1.
In order to evaluate performance of fuel unit cell, I-V curves were obtained using Fuel Cell Station available from Fuel Cell Technology Company and MEA was measured using MEA for 5 cm2 DMFC available from NANO BEST Corp. The experiments were conducted under a sufficiently stabilized condition by working the unit cell at 0.3V while introducing 1 mole of methanol solution into the cell for 10 hours. The fuel, that is, 1M methanol solution flowed into an anode electrode at a flow rate of 0.63 (ml/min), while 100 sccm of air flowed into a cathode electrode under pressure of 2 atm.
On the other hand, the above procedure was repeated with a solution containing 5×10−5 M POM in 1M methanol in order to obtain CV curves. From the CV curves, it was found that the unit cell has no reduction in performance (
In order to identify durability of the anode electrode, LTSC type CV experiment was conducted to evaluate durability of catalyst.
CV experiment was repeated 1000 times to expose the catalyst to a severe environment and methanol oxidation current densities before and after addition of 0.001M POM were demonstrated in the CV curves, respectively (
As shown in the drawings, it can be clearly understood that there is a great difference between oxidation current densities of the present inventive embodiments and the ones of the comparative controls.
In order to identify influence of concentration on oxidation by a procedure disclosed in detailed description of the present invention, CV experiment was conducted by adding H3PMo12O40 to the methanol solution with different concentrations (0.5×10−5M to 0.01M) in the presence of PtRu/C catalyst as a noble metal catalyst for fuel cell. By this experiment, oxidation effect of carbonaceous intermediates can be identified using H3PMo12O40 even in a small amount thereof. On the other hand, when adding high concentration POM to the methanol solution, it was found that the fuel cell exhibits improved durability although oxidation current density thereof is decreased.
In
Dot line C is a methanol oxidation current curve measured by adding 0.001M POM to 2M methanol solution and dot line D is a methanol oxidation current curve measured by adding 5.0×10−5M POM to 2M methanol solution.
The fuel composition of the present invention is preferably used in manufacturing fuel cells. Thus, a variety of fuel cells including the fuel composition with improved performance as fuel can be fabricated according to the present invention. For example, the present fuel composition is preferably applied to catalysts made of noble metals such as Pt, Au, Ru, Ir, etc. and/or alloys containing the same. Additionally, the present fuel composition is also applied to catalysts made of non-noble metals and alloys thereof.
In consideration of scientific achievement and contribution expected by the present invention, since carbon monoxide is inevitably generated during reaction of an anode electrode on the basis of inherent characteristics of biomass molecules, there are lots of studies about electrochemical interaction between CO and metal electrode catalysts from investigations of CO removal and conversion of CO into electric energy. Results of the studies described above are expected to remarkably contribute in researches on electrode catalysts with poisoning resistance to CO, which is one of most common issues in existing hydrogen fuel cells.
The present invention is very important in terms of scientific and/or practical aspects. More particularly, in view of the scientific aspect, the present invention is preferably applied in existing low temperature solid polymeric fuel cell technology and/or in developments of electrode catalysts that can be obtained from biomass materials and be directly oxidized at low temperature and, thus, is inter-compatible with general fuel cell technology using hydrogen or methanol fuel.
In addition to CO removal according to the present invention, additional fuel can be ensured by generating hydrogen gas. Especially, for polyoxometalate as the oxidation assistant used in the present invention, the assistant is simple and economical because it requires no alternative and/or complex processes.
From the above experimental examples, it was identified that only a small amount of oxidation assistant can exhibit high oxidation of intermediates and, as a result, the present invention is believed to have great commercial merit.
Meanwhile, even though POM is produced in small amounts due to low demand and high cost under existing circumstances, it is obvious that mass production of POM can be realized and, in turn, considerably reduce the price of POM.
Consequently, with technological improvements as described above, it is clear that the present invention advances time for commercial use of portable fuel cells and, furthermore, contributes to a preferable ratio of alternative energy supply.
While the present invention has been described with reference to a limited number of preferred embodiments, it will be understood by those skilled in the art that various modifications, variations and applications may be made therein without departing from the scope of the present invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2006-0122992 | Dec 2006 | KR | national |
