Patent Application
20090130518
This application claims the benefit of Korean Application No. 2007-118522, filed Nov. 20, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
Aspects of the present invention relate to an electrocatalyst for a fuel cell, a method of preparing the same, and a fuel cell including an electrode having the electrocatalyst. More particularly, aspects of the present invention relate to an electrocatalyst for a fuel cell with improved oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) efficiencies, a method of preparing the electrocatalyst, and a fuel cell including the electrocatalyst.
2. Description of the Related Art
Fuel cells obtain electromotive force by a cell reaction that generates water from hydrogen and oxygen. Hydrogen is obtained by reacting raw materials such as methanol and water under the presence of a reformed catalyst. Fuel cells can be categorized into a polymer electrolyte membrane (PEM) type, a phosphate type, a molten carbonate type, and a solid oxide type, depending on the types of electrolytes used. The operating temperatures and properties of the components of fuel cells vary depending on the electrolyte used.
A polymer electrolyte membrane fuel cell (PEMFC), which is a fuel cell using a polymer electrolyte membrane, is conventionally formed of an anode, a cathode, and a membrane-electrode assembly (MEA) including a polymer electrolyte membrane disposed between the anode and the cathode. The anode of a PEMFC includes a catalyst layer to facilitate oxidation of a fuel, and the cathode of a PEMFC includes a catalyst layer to facilitate the reduction of an oxidant.
A catalyst having platinum (Pt) as the active element is typically used as a component of the anode and the cathode, and the activity of the catalyst has a great influence on the electrode performance. Therefore, as shown in Korean Patent Laid-open Publication No. 2000-0063843, research is being actively conducted to develop a fuel cell with high performance by enhancing the activity of platinum supported catalysts.
Aspects of the present invention provide an electrocatalyst for a fuel cell with increased catalytic activity provided by the presence of a cerium oxide, a method of preparing the electrocatalyst, and a fuel cell including an electrode having the electrocatalyst.
According to an aspect of the present invention, there is provided an electrocatalyst for a fuel cell comprising a carbon-based catalyst support; and a ternary metal catalyst comprising Pt, Co and Ce supported on the catalyst support.
According to another aspect of the present invention, the electrocatalyst may include 10 to 60 parts by weight of Pt, 1 to 20 parts by weight of Co, and 0.1 to 30 parts by weight of Ce based on 100 parts by weight of the sum of the catalyst support and the metal catalyst.
According to another aspect of the present invention, the ternary metal catalyst may include a Pt—Co based first metal catalyst and a Ce-based second metal catalyst.
According to another aspect of the present invention, the first metal catalyst and the second metal catalyst may be located adjacent to each other on the carbon based catalyst support.
According to another aspect of the present invention, the first metal catalyst may include a Pt—Co alloy or a Pt—Co—Ce alloy.
According to another aspect of the present invention, the second metal catalyst may include CeO2 and Ce2O3.
According to another aspect of the present invention, the second metal catalyst may include particles having a core including CeO2 and a shell including Ce2O3.
According to another aspect of the present invention, the carbon-based catalyst support may be one of Ketchen black, carbon black, graphite carbon, carbon nanotube, and carbon fiber.
According to another aspect of the present invention, there is provided a method of preparing an electrocatalyst for a fuel cell including obtaining a metal oxide by oxidizing a Pt precursor, a Co precursor, and a Ce precursor; impregnating a carbon-based catalyst support in a mixture including the metal oxides under a hydrogen bubbling condition; and thermally reducing the resulting product at 200 to 350° C. under a hydrogen atmosphere.
According to another aspect of the present invention, there is provided a fuel cell including an electrode including the electrocatalyst for fuel cells previously described and an electrolyte membrane.
According to another aspect of the present invention, the electrode may be a cathode.
According to another aspect of the present invention, there is provided a fuel cell comprising an anode; a cathode; and an electrolyte membrane between the anode and the cathode, wherein at least one of the anode and the cathode comprises the electrocatalyst previously described.
According to another embodiment of the present invention, there is provided an electrocatalyst of a fuel cell comprising oxides of platinum, cobalt and cerium on a solid support.
According to another embodiment of the present invention, there is provided an electrocatalyst of a fuel cell, comprising a solid support; a first metal catalyst on the solid support comprising an alloy of Pt and Co or an alloy of Pt, Co and Ce; and a second metal catalyst on the solid support comprising one or more oxides of Ce.
According to another embodiment of the present invention, there is provided an electrocatalyst formed by the method comprising obtaining a mixture of metal oxides from a Pt precursor, a Co precursor, and a Ce precursor; impregnating the mixture of the metal oxides onto a carbon-based catalyst support under hydrogen bubbling; and heat-treating the resulting product at 200 to 350° C. under a hydrogen atmosphere to provide a ternary metal catalyst comprising Pt, Co and Ce supported on the catalyst support.
Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.
Aspects of the present invention provide an electrocatalyst for a fuel cell including a carbon-based catalyst support and a ternary metal catalyst of Pt—Co—Ce supported on the catalyst support.
Conventional fuel cells include a solid polymer membrane disposed between an anode having a platinum catalytic layer and a cathode also having a platinum catalytic layer. In the anode, the following reaction takes place in the platinum catalytic layer of the anode.
H2→2H++2e−
H+ produced from the reaction diffuses into an electrolyte. Meanwhile, in the cathode, the following reaction takes place in the platinum catalytic layer of the cathode.
2H++2e−+½O2→H2O
The electrocatalyst according to an embodiment of the present invention uses a Pt—Co or Pt—Co—Ce alloy as a first metal catalyst instead of the conventional Pt catalyst, thereby providing a PEMFC or a PAFC with superior electrocatalyst activity for fuel cells. Moreover, the electrocatalyst according to an embodiment of the present invention also uses a second metal catalyst derived from cerium oxide having superior oxygen activity or transferability, providing an electrocatalyst for fuel cells having superior activity, even at operating temperatures under 200° C.
As a non-limiting example, the electrocatalyst for a fuel cell according to an embodiment of the present invention may include 10 to 60 parts by weight of Pt, 1 to 20 parts by weight of Co, and 0.1 to 30 parts by weight of Ce, based on 100 parts by weight of the sum of the catalyst support and the metal catalyst, in view of the electrochemical surface area of the catalyst and oxygen reduction reaction (ORR) and (hydrogen oxidation reaction) HOR.
As non-limiting examples, the carbon-based catalyst support may be one of Ketchen black, carbon black, graphite carbon, carbon nanotube, and carbon fiber, each having high electric conductivity and large surface area.
The electrocatalyst for a fuel cell according to aspects of the present invention may be prepared using a colloidal method.
As a platinum precursor, chloroplatinous acid (H2PtCl4), chloroplatinic acid (H2PtCl6), potassium tetrachloroplatinate (K2PtCl4), potassium hexachloroplatinate (K2PtCl6), diaminedinitroplatinum (Pt(NO2)2(NH3)2), or dihydrogen hexahydroxyplatinum (H2Pt(OH)6) may be used. As a cerium precursor, cerium (III) acetate, cerium (III) bromide, cerium (III) carbonate, cerium (III) chloride, cerium (IV) hydroxide, cerium (III) nitrate, cerium (III) sulfate, or cerium (IV) sulfate may be used. As a cobalt precursor, cobalt (II) chloride (COCl2), cobalt (II) sulfate (COSO4), or cobalt (II) nitrate (Co(NO3)2) may be used.
Under hydrogen bubbling, the carbon-based catalyst support is impregnated into the resulting colloid solution, then, drying is performed to obtain a solid-state intermediate. The resulting product is then washed with water several times, dried, and then thermally reduced to obtain the electrocatalyst. Thermal reduction may be performed under a hydrogen atmosphere at 200 to 350° C. for 0.5 to 4 hours. The thermal reduction provides an electrocatalyst having a superior activity and a significantly increased oxido-reduction current within the range of 0.6 to 0.8V, which is the voltage range typically used by an electrode.
In addition, aspects of the present invention provide a fuel cell including the electrocatalyst described above. The fuel cell according to an embodiment of the present invention includes a cathode, an anode, and an electrolyte membrane disposed between the cathode and the anode, wherein at least one of the cathode and anode contains the electrocatalyst as described above. As a non-limiting example, the supported catalyst according to the present invention may be applied to the cathode. As non-limiting examples, the fuel cell may be a phosphoric acid fuel cell (PAFC), a polymer electrolyte membrane fuel cell (PEMFC), or a direct methanol fuel cell (DMFC). As a specific, non-limiting example, the fuel cell may be a PEMFC.
The fuel cell 1 shown schematically in
Although only two unit cells 11 are shown in
As shown in
The catalytic layers 110 and 110′ function as a fuel electrode and an oxygen electrode each including a catalyst and a binder therein, and may further include a material that can increase the electrochemical surface area of the catalyst. At least one of the catalytic layers comprises an electrocatalyst according to aspects of the present invention.
The first gas diffusion layers 121 and 121′ and the second gas diffusion layers 120 and 120′ may each be formed of a material such as, for example, carbon sheet or carbon paper, The first gas diffusion layers 121 and 121′ and the second gas diffusion layers 120 diffuse oxygen and fuel supplied through the bipolar plates 20 to the entire surface of the catalytic layers 110 and 110′.
The fuel cell 1 including such an MEA 10 typically operates at a temperature of 100 to 300° C. Fuel such as hydrogen is supplied through one of the bipolar plates 20 into a first catalytic layer, and an oxidant such as oxygen is supplied through the other bipolar plate 20 into a second catalytic layer. Then, hydrogen is oxidized in the first catalytic layer producing protons, and conducts the proton to the second catalytic layer, and the conducted protons and oxygen electrochemically react to produce water in the second catalytic layer, and to produce electrical energy. Moreover, hydrogen supplied as a fuel may be hydrogen produced by reforming hydrocarbons or alcohols, and oxygen supplied as an oxidant may be supplied in the form of air. It is to be understood that the structure and operation of the membrane electrode assembly is not limited to what is described herein, and that other structures and modes of operation may be used.
Aspects of the present invention will now be described in detail with reference to the following examples. However the examples are not intended to limit the scope of the present invention.
5 g of NaHSO3 was added to 200 g of a 1 M solution of hydrated chloroplatinic acid (H2PtCl6.xH2O), as a platinum precursor dissolved in water, and stirred thoroughly to produce a solution of H2Pt(SO3)2Cl6.OH. 50 ml of hydrogen peroxide was added to the resulting solution to produce PtO2. Then, 0.5 g of CoCl2.6H2O as a cobalt precursor and 0.5 g of (NH4)2Ce(NO3)6 as a cerium precursor were added and reacted with the hydrogen peroxide remaining in the solution, thereby producing cobalt oxide (CoO) and cerium oxide (CeO2).
To the resulting colloid solution, 0.5 g of Ketchen black was added as a carbon catalytic support while bubbling in hydrogen, and stirring was further performed for 12 hours. The resulting solid was washed several times with water, and then was dried under nitrogen atmosphere at 120° C.
Then, the resulting solid product was thermally reduced at 280° C. in hydrogen gas to produce an electrocatalyst.
The surface of the final electrocatalyst product was analyzed with a transmission electron microscope (TEM) and the results are shown in
The final product prepared above was analyzed with X-ray Photoemission Spectroscopy (XPS) and the result is shown in
5 g of NaHSO3 was added to 200 g of 1 M solution of hydrated chloroplatinic acid (H2PtCl6.xH2O), as a platinum precursor dissolved in water, and stirred thoroughly to produce a solution of H2Pt(SO3)2Cl6.OH. 50 ml of hydrogen peroxide was added to the resulting solution to produce PtO2. Then, 0.5 g of CoCl2.6H2O as a cobalt precursor was added and reacted with the hydrogen peroxide remaining in the solution, thereby producing cobalt oxide (CoO).
To the resulting slurry solution, 0.5 g of Ketchen black was added as a carbon catalytic support while bubbling in hydrogen, and stirring was further performed for 12 hours. The resulting solid was washed several times with water, and dried at 120° C. under nitrogen atmosphere.
The resulting solid product was thermally reduced at 280° C. in hydrogen gas to produce an electrocatalyst.
Comparative Example 2
5 g of NaHSO3 was added to 200 g of 1 M solution of hydrated chloroplatinic acid (H2PtCl6.xH2O), as a platinum precursor dissolved in water, and stirred thoroughly to produce a solution of H2Pt(SO3)2Cl6.OH. 50 ml of hydrogen peroxide was added to the resulting solution to produce PtO2. Then, 0.5 g of CoCl2.6H2O as a cobalt precursor was added and reacted with the hydrogen peroxide remaining in the solution, thereby producing cobalt oxide (CoO).
To the resulting slurry solution, 0.5 g of Ketchen black was added as a carbon catalytic support while bubbling in hydrogen, and stirring was further performed for 12 hours. The resulting solid was washed several times with water, and dried at 120° C. under nitrogen atmosphere.
For each 1 g of the catalyst synthesized in Example 1, 0.1 g of polyvinylidene fluoride (PVDF) and an adequate amount of solvent (n-methylpyrrolidone (NMP)) were mixed to produce a rotating disk electrode (RDE) forming slurry. The slurry was loaded onto a glassy carbon film used as a substrate of the RDE, then a drying process was performed in which the temperature was increased incrementally from room temperature to 150° C. to produce the RDE. Using the produced RDE as a working electrode, the quality of the catalyst was evaluated as described below, with the results shown in
Electrodes were produced using the same method except that the catalysts prepared from Comparative Examples 1 and 2 were used, and the results of quality evaluations of the catalysts according to the comparative examples are also shown in
The oxygen reduction reaction (ORR) activity was evaluated by dissolving oxygen in an electrolyte to saturation, and then scanning the potential in a negative direction of the open circuit voltage (OCV) while recording the corresponding currents (scan rate: 1 mV/s, electrode rotation speed: 1000 rpm). In the I-V (current-voltage) profile below an operating potential (0.6-0.8V) where oxygen reduction reaction of an electrode mainly takes place, material limiting current is reached at a lower potential. A material limiting current is a maximum current upon depletion of reagents, and in the RDE experiment, upon increase of the rotation speed of the electrode, the supply of oxygen dissolved in the electrolyte to the surface of the electrode was increased, thereby increasing the material limiting current, as well as the current in the entire potential region.
Using the electrode prepared by the method detailed above, ORR activities of the catalysts from Example 1 and Comparative Examples 1 and 2 were compared, with the results shown in
The hydrogen oxidation reaction (HOR) activity was evaluated by first dissolving hydrogen in an electrolyte to saturation, and then scanning the potential in a positive direction of the OCV while recording the corresponding currents (scan rate: 1 mV/s, electrode rotation speed: 400 rpm)
Using the electrode prepared by the method detailed above, HOR activities of the catalysts were compared and shown in
For each 1 g of the catalyst synthesized in Example 1, 0.03 g of polyvinylidene fluoride (PVDF) and an adequate amount of solvent (NMP) were mixed to produce a cathode-forming slurry. The cathode-forming slurry was coated with a bar coater on a carbon paper coated with microporous layer. The coated slurry was then subjected to a drying process involving increasing the temperature incrementally from room temperature to 150° C. to produce a cathode.
Separately, an anode was prepared using the same method as above except that a carbon-supported Pt—Co catalyst (Tanaka Jewelry, Pt: 30 wt %, Ru: 23 wt %) was used instead of the catalyst synthesized in Example 1.
A membrane-electrode assembly (MEA) was prepared using poly(2,5-benzimidazole) doped with 85% phosphoric acid as an electrolyte membrane between the cathode and the anode.
Additionally, an MEA was prepared using the catalyst prepared in Comparative Example 1 instead of the catalyst prepared in Example 1.
Then, the MEA properties of the MEA including the catalyst of Example 1 and the MEA including the catalyst of Comparative Example 1 were evaluated at 150° C. using desiccated air for the cathode and desiccated hydrogen for the anode. The results are shown in
Referring to
Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-118522 | Nov 2007 | KR | national |
