Patent Application
20090258272
This application claims the benefit of Korean Patent Application No. 2008-0033520, filed Apr. 11, 2008, the disclosure of which is hereby incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a membrane-electrode assembly for a direct liquid fuel cell and a method of manufacturing the same.
2. Description of the Related Art
Fuel cells are not only environment-friendly but are also expected to be an adequate substitute for conventional energy systems (e.g., secondary batteries and capacitors) in recent times of increasing demand for high-output mobile power sources.
Formic acid, formaldehyde, methanol and ethanol can be used as fuel for a direct liquid fuel cell. Among these fuels, formic acid is oxidized into carbon dioxide via two different pathways—direct oxidation and indirect oxidation. Current research into formic acid fuel batteries focuses on developing an optimal catalyst for direct oxidation of formic acid.
The most common anode for direct formic acid fuel cells (DFAFCs) catalysts are a platinum-ruthenium alloy catalyst and a palladium catalyst which are used as a direct methanol fuel cell, but these catalysts fall below a desired level of performance or long-term stability. Moreover, to achieve a desired level of performance, a fuel cell needs a large amount of catalyst.
Accordingly, there is need for research into improving performance of a formic acid fuel cell to a level required for actual driving conditions, improving stability for long-term operation, and maximizing utilization of a catalyst in order to ensure a competitive production cost for early implementation.
The present invention is directed to a membrane-electrode assembly having improved performance and stability.
The present invention is also directed to a method of manufacturing a membrane-electrode assembly having improved performance and stability.
According to an embodiment of the present invention, a membrane-electrode assembly includes: an anode including a first main catalyst layer, a second main catalyst layer, and a co-catalyst layer disposed between the main catalyst layers; a cathode; and a polymer electrolyte membrane formed between the second main catalyst layer of the anode and the cathode.
According to another embodiment of the present invention, a method of manufacturing a membrane-electrode assembly includes: forming an anode including a first main catalyst layer, a second main catalyst layer, and a co-catalyst layer disposed between the main catalyst layers; forming a cathode; and forming a polymer electrolyte membrane between the second main catalyst layer of the anode and the cathode.
These and/or other objects, aspects and advantages of the invention will become apparent and more readily appreciated from the following description of exemplary embodiments shown in the accompanying drawings, in which:
Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are shown in the accompanying drawings. When the same element appears in different drawings, it will always be denoted by the same reference numeral and it will only be described once.
Referring to
In a direct formic acid fuel cell, formic acid is oxidized by an electrochemical reaction shown in Formula 1 below, thereby producing carbon dioxide, hydrogen ions and electrons. The hydrogen ions produced at the anode 110 move to the cathode 130 through the polymer electrolyte membrane 120, and at the cathode 130, the hydrogen ions react with oxygen and electrons as shown in Formula 2 to produce water. Meanwhile, the electrons produced at the anode 110 move through an external circuit, and convert the changes in free energy due to the chemical reaction into electric energy.
In the overall reaction, shown in Formula 3, formic acid reacts with oxygen to produce water and carbon dioxide, and a difference of electric potential becomes 1.45V.
HCOOH−→CO2+2H++2e− <Formula 1>
0.5O2+2H++2e−−→H2O <Formula 2>
HCOOH+0.5O2−→CO2+H2O <Formula 3>
Referring to
Formic acid has difficulty in controlling diffusion due to its own hydrophilic and hygroscopic properties. The fuel diffusion layer 112 can solve these problems and facilitate diffusion of gas separated from the formic acid fuel.
That is, when high-concentration formic acid is used as fuel, rapid diffusion can occur, but the fuel diffusion layer 112 can prevent such rapid diffusion and also prevent a decrease in ion conductivity. In addition, as penetration through the membrane is minimized, poisoning can be reduced.
When low-concentration formic acid is used as fuel, the fuel diffusion layer 112 may be thermally or electrochemically treated to improve diffusion of formic acid. The fuel diffusion layer 112 may be formed of carbon fiber or carbon paper.
The first main catalyst layer 114 may be formed by mixing a platinum catalyst, an ionomer and a solvent mixture, and the solvent mixture may be a catalyst ink.
The first main catalyst layer 114 may be formed by spraying, painting, doctor-blading, or lithography.
Referring to
The metal formed in the co-catalyst layer 116 may be formed by dissolving a precursor in a solvent, and the Bi, Sb or As precursor may be Bi2O3, Bi(NO3)3, Sb2O3, (Sb)2(SO4)3 or As2O3. The solvent may be a 0.1 to 0.5M perchloric acid solution.
The electrochemical deposition may be OPD or UPD. OPD is electrochemical deposition at a lower electric potential than the standard equilibrium potential, and UPD is electrochemical deposition of metal at a higher electric potential than the standard equilibrium potential and with a lower energy so that the metal can be stably maintained in an environment in which a fuel cell is actually used.
The electrochemical deposition may be performed using an apparatus consisting of a three-electrode system placed in an electrode holder, in which three electrodes are a working electrode, a counter electrode and a reference electrode.
An electrolyte is contained in the electrode holder, and the first main catalyst layer 114 may be placed on the working electrode. The electrolyte may be a solvent in which a metal precursor is dissolved, and an electric potential applied for deposition may depend upon an electric potential of the counter electrode controlled based on the reference electrode.
The electrochemical deposition may be performed at an electric potential ranging from −300 mV to +300 mV, and the potential may be changed according to a degree to which the first main catalyst layer 114 is hydrophobic. The electrochemical deposition may be performed for 5 to 60 minutes.
Referring to
The second main catalyst layer 118 may be formed by spraying, painting, doctor-blading, or lithography.
A polymer electrolyte membrane 120 and a cathode 130 are stacked on the anode 110 including the first main catalyst layer 114, the co-catalyst layer 116 and the second catalyst layer 118 formed as described with reference to
In Comparative Example 1, the fuel diffusion layer 112 was formed of carbon fiber (SGL Technologies, Germany), and the first main catalyst layer 114 was formed by stirring a mixed solution of a carbon-supported catalyst, an ionomer, and a solvent mixture, and by spraying the mixed solution.
The carbon-supported catalyst was formed by mixing carbon (Vulcan XC-72) with a black catalyst (Aldrich). 60% carbon and 40% black catalyst were mixed together to form about 0.5 mg/cm2 of the carbon-supported catalyst. About 10% ionomer (5 wt % Nafion solution) was used for this quantity of the carbon-supported catalyst, an isopropyl alcohol (IPA) solution was used as the solvent mixture.
The membrane-electrode assembly was manufactured by sequentially stacking the anode 110 including the first main catalyst layer 114 formed as described above, the polymer electrolyte membrane 120, and the cathode 130, and pressing them at 10 MPa and 140° C. for 5 minutes.
The polymer electrolyte membrane 120 was formed of Nafion (DuPont), and the cathode 130 was formed by stirring a mixed solution of a carbon-supported catalyst, an ionomer, and a solvent mixture. The carbon-supported catalyst was formed by mixing 40% black catalyst with 60% carbon to have a concentration of 1.5 mg/cm2.
A formic acid solution was used as fuel. A relative humidity was maintained at 80% oxygen.
In Comparative Example 2, a membrane-electrode assembly was manufactured by sequentially forming a first main catalyst layer 114 and a Bi co-catalyst layer 116 using the same method as described in Comparative Example 1.
The co-catalyst layer 116 was formed by dissolving a 5 mM Bi2O3—Bi precursor—in a 0.1M perchloric acid solution through UPD. In order to deposit Bi, the working potential of +100 mV on the basis of equilibrium potential was applied. The deposition was performed for 1 to 15 minutes.
In Comparative Example 3, a membrane-electrode assembly was manufactured by sequentially forming a first main catalyst layer 114 and a Sb co-catalyst layer 116 using the same method as described in Comparative Example 1.
The co-catalyst layer 116 was formed by dissolving 50 mM Sb2O3—Sb precursor—in a 0.1M perchloric solution using UPD. In order to deposit Sb, the working potential of +100 mV on the basis of equilibrium potential was applied. The deposition was performed for 1 to 15 minutes.
In Comparative Example 4, a membrane-electrode assembly was manufactured by forming a first main catalyst layer 114 and a Bi co-catalyst layer 116 using the same method as described in Comparative Example 2, and then forming a second main catalyst layer 118.
As described above, formation of the first main catalyst layer 114 was followed by formation of the co-catalyst layer 116 using a Sb precursor, and then the second main catalyst layer 118 was formed on the co-catalyst layer 116 using the same material and method as the first main catalyst layer 114.
In Comparative Example 5, a membrane-electrode assembly was manufactured by forming a first main catalyst layer 114 and a Sb co-catalyst layer 116 using the same method as described in Comparative Example 3, and then forming a second main catalyst layer 118.
As described above, formation of the first main catalyst layer 114 was followed by formation of the co-catalyst layer 116 using a Sb precursor, and then the second main catalyst layer 118 was formed on the co-catalyst layer 116 using the same material and method as the first main catalyst layer 114.
Analysis 1—Analysis on
Current-voltage (I-V) characteristics of membrane-electrode assemblies of Comparative Examples 1 to 5 were measured at 1st, 5th and 100th cycles, and normalized power densities for a single-layered electrode and a multi-layered electrode are shown in
When the first main catalyst layer 114 and the Bi co-catalyst layer 116 were formed (Comparative Example 2), the power density was relatively high, about 1.0, at the 1st cycle, but dramatically dropped to about 0.7 at the 5th cycle. The power density was dropped again to about 0.6 at the 100th cycle, which was similar to when only the first main catalyst layer 114 was formed.
However, when the first main catalyst layer 114, the Bi co-catalyst layer 116 and the second main catalyst layer 118 were all formed (Comparative Example 4), the power density was about 1.0 at the 1st cycle, which was similar to when the first and second main catalyst layers 114 and 118 were formed, dropped only slightly to about 0.9 at the 5th cycle, and made no significant change from the 5th cycle to the 100th cycle.
Referring to
However, when the first main catalyst layer 114, the co-catalyst layer 116 and the second main catalyst layer 118 were all formed (Comparative Example 5), the power density was about 1.0 at the 1st cycle, which was similar to when the first and second main catalyst layers 114 and 118 were formed, dropped only slightly to about 0.9 at the 5th cycle, and made no significant change from the 5th cycle to the 100th cycle.
From the analysis results in
While, in all Comparative Examples, the co-catalyst layer 116 was formed by UPD, even if formed by OPD, similar results to those shown in
These results show that the co-catalyst layer 116 formed together with the first main catalyst layer 114 is effective in a disposable fuel cell with an improved power density, and the second main catalyst layer 118 formed together with the first main catalyst layer 114 and the co-catalyst layer 116 can be used in a fuel cell continuously maintaining the improved power density.
It can be noted that the deposition of the co-catalyst layer 116 overcame an uneven of the first main catalyst layer 114, and the plating of the second main catalyst layer 118 overcame instability of the co-catalyst layer 116.
Also, the co-catalyst layer 116 may be sandwiched between the first and second main catalyst layers 114 and 118, thereby preventing loss of a catalyst due to repeated use.
Analysis 2—Analysis on
When power density of a fuel cell is in a range from 50 to 100 mW/cm2, it is considered that the fuel cell is possible to come to commercialization. Here, although the co-catalyst layer 116 is formed of one selected from the Group 5B metals, the power density may vary according to the kind of the used metal.
Accordingly, in Analysis 2, each cell voltage was measured at the same current level with respect to each metal of Group 5B to determine the probability of commercialization with respect to each metal. Here, concentrations of precursors for the respective metals were varied during the measurement of cell voltages.
That is, in the membrane-electrode assembly including the Bi co-catalyst layer 116, there is a possibility of commercialization when a concentration of the Bi precursor is in the range from about 1 mM to about 50 mM.
That is, the membrane-electrode assembly including the Sb co-catalyst layer 116 can come to commercialization when a concentration of the Sb precursor is in the range from about 1 mM to about 500 mM.
That is, in the membrane-electrode assembly including the As co-catalyst layer 116, there is a possibility of commercialization when a concentration of the As precursor is in the range from about 5 mM to about 100 mM.
Consequently, this analysis results show a probability of commercializing the membrane-electrode assemblies using Group 5B metals, such as Bi, Sb and As, when applied in fuel cells, and appropriate concentration ranges for respective precursors. However,
Analysis 3: Analysis on
As described above, the co-catalyst layer may be formed by an electrochemical deposition technique such as underpotential deposition (UPD) or overpotential deposition (OPD). However, a variety of results may be obtained according to the ranges of voltages applied for the electrochemical deposition.
Thus, in Analysis 3, variation of cell voltages was measured when the uniform current was applied to the membrane-electrode assemblies including the co-catalyst layers 116 respectively formed by induction of various applied voltages.
To be specific, in the experiments for forming the co-catalyst layers 116 using Bi and Sb, the co-catalyst layers were formed using a 5 mM Bi precursor and a 100 mM Sb precursor with different application voltages, respectively. Afterward, a current of 150 mA/cm2 was uniformly applied to the membrane-electrode assemblies including these co-catalyst layers as formed, and their cell voltages were measured.
When +100 mV was applied in order to form the co-catalyst layer 116 using the Bi precursor by UPD, the cell voltage was about 0.55V and the power density was about 82.5 mW/cm2. Also, when +300 mV was applied, the cell voltage decreased slightly to about 0.5V.
When +100 mV was applied in order to form the co-catalyst layer 116 using the Sb precursor, the cell voltage was about 0.5V and the power density was about 75 mW/cm2. Also, when +300 mV was applied, the cell voltage decreased slightly to about 0.45V.
Also, when −100 mV was applied in order to form the co-catalyst layer 116 using the Bi precursor by OPD, the cell voltage was about 0.45V, and when −300 mV was applied, the cell voltage decreased dramatically to about 0.2V.
However, when −100 mV was applied in order to form the co-catalyst layer 116 using the Sb precursor, the cell voltage was about 0.35V, and when −300 mV was applied, the cell voltage decreased slightly to about 0.3V.
From the measurement result, considering a general Pt catalyst having an average cell voltage of about 0.5V, it can be noted that the formation of the co-catalyst layer 116 formed using the Bi or Sb precursor by UPD or OPD can result in a generally higher cell voltage than the Pt catalyst (except for OPD treatment at −300 mV).
Particularly, it was found that the highest cell voltage can be obtained when the co-catalyst layer 116 is formed using Bi or Sb by UPD at a voltage of 100 mV.
Since these cell voltages are within the power density range of 50 to 100 mW/cm2 for commercialization of a direct liquid fuel cell, it can be seen that the membrane-electrode assemblies according to the present invention performs well enough to be commercialized.
Analysis 4: Analysis on
This is about a durability experiment for the membrane-electrode assembly according to the present invention and a commercialized membrane-electrode assembly.
In Analysis 6, membrane-electrode assemblies having an anode formed using the Bi precursor and the Sb precursor are denoted BiPt and SbPt, respectively, and membrane-electrode assemblies having an anode formed using commercialized catalysts are denoted PtRu, Pd and Pt, respectively. Each cell voltage was measured while a current of 150 mA/cm2 was constantly applied.
Referring to
A Pd that has not been operated exhibited the highest cell voltage of 0.6V, but after 50 hours of operation, it decreased drastically to 0.4V, after 100 hours of operation, it decreased to 0.3V, after 300 hours of operation, it decreased to 0.24V, which is lower than the Pt, and finally after 500 hours of operation, it decreased to the lowest level of 0.2V.
A PtRu that has not been operated exhibited a cell voltage of 0.4V, after 50 hours of operation, the cell voltage was about 0.38V, after 100 hours of operation, it was about 0.35V, and after 300 and 500 hours of operation, it still remained about 0.35V.
A SbPt that has not been operated exhibited a high cell voltage of about 0.54V, and even after 500 hours of operation, the cell voltage remained about 0.5V without a large drop.
A BiPt that has not been operated also exhibited a high cell voltage of about 0.55V, and even after 50 to 500 hours of operation, the cell voltage remained about 0.5V or more.
Consequently, the BiPt and SbPt exhibited higher cell voltages even after a long period of operation than the currently-commercialized membrane-electrode assembly, and thus durability can be significantly increased.
According to the present invention, formation of an anode including a first main catalyst layer, a co-catalyst layer and a second main catalyst layer can result in improvement in performance and stability.
In other words, the first main catalyst layer and the co-catalyst layer can prevent loss of a catalyst due to non-uniform deposition, and a decrease in amount of the catalyst consumed.
Moreover, formation of the second main catalyst layer on the co-catalyst layer can reduce instability due to the co-catalyst layer.
Although a few exemplary embodiments of the present invention have been shown and described in detail, it will be appreciated by those skilled in the art that changes may be made to the described embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2008-0033520 | Apr 2008 | KR | national |
