Patent Application
20120077672
The present invention relates to a catalyst for a fuel cell and a method for producing the catalysts thereof. In particular, the present invention relates to a porous catalyst for a fuel cell and a method for producing the porous catalyst thereof.
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material, such as methanol, ethanol, or natural gas. Such a fuel cell is a clean energy source that may replace fossil fuels. It includes a stack composed of unit cells, and produces various ranges of power. Since it has four to ten times higher energy density than a small lithium battery, it has been highlighted as a small portable power source.
Power generation by fuel cells is based on an oxidation reduction reaction generated on electrodes. High active catalysts are essential to improve the performance of the fuel cell. Now, the most popular material for the catalyst contains platinum (Pt), and with the content ratio of Pt increasing the cost of the catalyst also increases. Therefore, there is a need to increase the efficacy of catalyst and to produce a stable catalyst for the catalytic reaction.
Carrier materials, also refer to support, of the catalysts affect the metal particle sizes, electrochemistry and catalytic reaction rate of the catalysts. Many researches focus on the carrier materials of the catalysts. An electro-catalyst is needed to induce the desired electrochemical reactions at the electrodes or, more precisely, at the electrode-electrolyte interfaces. The electro-catalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum supported on carbon. Most popular carrier materials for the catalyst are carbon materials. The characteristics for an ideal carrier comprise: good electron transfer ability or proton transfer ability, increasing transfer effect of the electrons or protons, large surface for metal particles acting uniformly, showing excellent metal particles adsorption ability, stable catalytic ability without being affected by current, chemical stability, porous structure for fuel transfer, and cost etc. As for carbon fibers, a vapor-grown carbon fiber, a carbon nanotube and a PAN type carbon fiber are known. However, in any of the reports which have been made public to date, a technique to produce an electrode comprising a carbon fiber on which fine catalyst particles are uniformly carried with a high density has not been described.
Except carbon materials for the carrier of the catalysts, many metal oxides are also used as the carriers, such as Al, Si, Sn, Ti, Ni, Zr, La and Ce. Electric conductivity of such carriers are worse than carbonfibers, but they show good metal adsorption ability so that metal particles separated on the surface of the carriers uniformly. Further, metal particles will not lose after long term use. Still further, the carriers containing Ru element could provide active oxygen molecules to oxidize Pt—CO to increase the anti-toxin ability of the electrodes.
As described in Jiun-Ming Chen et al., a gel precursor made by TiO2, Pt and Ru which were coated on carbon fibers for redox reactions, and the diameters of above metal particles were reduced 1 to 2 nm when comparing with the catalyst without adding TiO2, and the alloy condition described in Jiun-Ming et al. performed better than conventional ones. Now, it is known that TiO2 increases the well-separation between the catalyst particles.
Huanqiao Song et al. disclosed that TiO2 was applied on the surface of a CNT (carbon-nanotube) for carrying nano metal particles, such as Pt, of the catalyst in ethanol fuel cells. When the ratio of CNT:TiO2 equaled to 1:1, the ethanol fuel cells showed the best performance. When the content of CNT was increased, the electric conductivity reduced and CO desorption ability also reduced. TiO2-CNT complex carriers show excellent CO stripping ability. Huanqiao Song et al. also used TiO2 nanotube as the carriers. The performance of TiNT carriers showed more excellent CO stripping ability than CNT or TiO2 particles. In CO stripping experiments, catalysts contain TNT/Pt/C show the lowest CO stripping electric potentials. Further, different calcination temperatures show different water content and also affect CO stripping ability. When water content in the carriers increased, the electric potentials for CO stripping would be reduced. However, when the calcination temperature was higher than 400° C., the water content in the carrier could not be self-supplemented.
When metal oxides were used as carriers, they provided bi-functional effects including increasing the anti-toxin ability of carbon monoxide and metal oxides carriers also increase the absorption ability of PtRU alloy and also producing smaller particles. However, the electric conductivity was worse. Therefore, there is a need to increase the electric conductivity of the metal oxides carriers to make electron produce from the surface of metal particles smoothly and prevent catalytic efficacy reduction.
The present invention relates to a catalyst for a fuel cell which comprises at least one noble nano-metal particles, an oxide for forming porous structures, and a carrier material for supporting the oxides and the at least one noble nano-metal particles.
Preferably, the porous carrier material is made in a shape of a particle, a tube, a star, or a layer.
Preferably, the oxide is SiO2, Al2O3, TiO2, ZrO2, mesoporous structure metal oxide (MCM) such as MCM-41 and MCm-48, microporous structure metal oxide, such as SBA-1 or a mix of the above oxides. More preferably, the oxide is TiO2 and in a range of 20 to 50 wt %.
Preferably, the porous carrier material is made in a shape of tube and the oxide is made of TiO2, SiO2, Al2O3. More preferably, the oxide and the carrier material are mixed in a range of 1 to 99 wt %.
Preferably, the catalyst further comprises atomic mole ratio of nitrogen in a range of 1 to 25 wt %. More preferably, the catalyst further comprises atomic mole ratio of nitrogen in a range of 6 to 10 wt %.
Preferably, the carrier is XC-72, carbon fiber named Ketjen ECP 300, Ketjen BP-2000, single-walled nanotube, multi-walled nanotube, acetylene black, mesocarbon microbead or mesoporous carbon.
Preferably, the at least one noble nano-metal particle is in a range of 5 to 50 wt %. More preferably, the at least one noble nano-metal particle is about 20 wt %.
Preferably, the diameter of the at least one noble nano-metal particle is less than 20 nanometer.
Preferably, the at least one noble nano-metal particle is a single kind of metal particle, alloy made of two kinds of metal particles or alloy made of multiple metal particles.
The present invention also refers to a method for producing a catalyst composition for a fuel cell, comprising adding a carbon material and a metal oxide into an organic solvent with reductive ability to form a mixture, mixing the mixture well, adding a precursor of a noble nano-metal particle into the mixture, heating the mixture containing the precursor of the noble nano-metal particle to make precursor reduce to noble nano-metal particle, and removing the organic solvent to obtain the catalyst composition.
Preferably, the organic solvent is polyacetals, acids or aldehydes. More preferably, the polyacetal solvent is ethanol, acelene glycol, glycerine or butanediol.
Preferably, heating step is provided as a recurrent flow heating method or a microwaves heating method.
As the skilled artisan will appreciate, any such method may be modified according to the needs of experiments.
As used herein, the term “a precursor of a noble nano-metal particle” means a metal oxide particle in nano size.
a is a photo taken by a scanning electron microscope showing a catalyst made from the sixth comparative example in accordance with the present invention.
b is a photo taken by a scanning electron microscope showing a catalyst made from the fourth embodiment in accordance with the present invention.
c is a photo taken by a scanning electron microscope showing a catalyst made from the third embodiment in accordance with the present invention.
d is a photo taken by a scanning electron microscope showing a catalyst made from the second embodiment in accordance with the present invention.
e is a photo taken by a scanning electron microscope showing a catalyst made from the first embodiment in accordance with the present invention.
a is a photo taken by a transmission electron microscope (TEM) of the catalyst (50 nm) obtained from the fourth embodiment in accordance with the present invention.
b is a photo taken by a transmission electron microscope of the catalyst (100 nm) obtained from the fourth embodiment in accordance with the present invention.
c is a photo taken by a transmission electron microscope of the catalyst (50 nm) obtained from the fifth embodiment in accordance with the present invention.
a to 3c are charts of CO-stripping voltammetry of the first to fourth embodiments in accordance with present invention; the first to fourth embodiments were tested under a constant current for a CO-stripping Experiments with a: Ti2O2 80%; b: TiO2 60%; c: TiO2 40%; TiO2 20% and the fifth comparative example.
a is a photo showing microstructure of the catalyst support doped with nitrogen by TEM in accordance with the present invention;
b is a photo showing microstructure of the catalyst support doped with nitrogen by HRTEM according to the present invention;
a is a chart showing cyclic voltammetries of the seventh embodiment and ninth comparative example in accordance with the present invention;
b is a chart showing cyclic voltammetries of the sixth to the eighth embodiments and the eighth to the ninth comparative examples in accordance with the present invention;
The present invention is described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for the purposes of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. In accordance with the present invention, two systems of catalysts containing TiO2 nanotube and modified TiO2 nanotubes adding with N element are tested and disclosed.
The embodiments of the present invention are presented by the following two systems, the metal oxide nanotubes and the improved metal oxide nanotubes doped with nitrogen atoms, wherein the exemplary metal oxide is TiO2.
The method for preparing the support (as used in the present invention, also refers to carriers), i.e. the TiO2 nanotubes (TNT), is provided as follows. 20 g TiO2 is mixed with 10M NaOH solution and then the mixture is mechanically stirred in an oil bath pan at 110° C. for 72 hours. Subsequently, the mixture is washed with alternate HCl and deionized water to pH=7 by a manner of a filtration. Lastly, the mixture is dried in a dry oven at 80° C.
The method for preparing the catalyst is provided as follows. The compositing ratio of a composed support used for preparing the catalyst is shown in Table 1. 50 mg of the composed support is added into 25 ml ethylene glycol solution, which is served as the solvent and the reductant. After adding adequate 0.05 M chloroplatinic acid solution (H2PtCl6 in ethylene glycol) and 0.05 M ruthenium Chloride solution (RuCl3 in ethylene glycol) into the above solvent and mixing evenly, the reductive reaction is performed with a microwave heating method. After the reduction step, the mixed solution is stirred for cooling and washed repeatedly with deionized water for removing chlorine ions (Cl—), and finally a dry step is performed so as to carry out the preparation of the catalyst. The above solvent, ethylene glycol, can be replaced with other organic solvent, such as an alcohol, an acid or an aldehyde, wherein the alcohol can be an ethanol, an ethylene glycol, a glycerol, or a butylene glycol.
As used in the column of code, E refers to Embodiment, for example, 1E refers to the first embodiment; C refers to Comparative example, for example, 1C refers to the first comparative example.
The weight percents (wt %) of TiO2 and the conductive carbon material (hereafter also refers to XC-72) with high surface area, the electrochemical surface area (ECSA), the on-set potential and the If/Ib ratio representing the tolerance for CO-poisoning effect (hereinafter referred to as “CO-tolerance”) are shown. The composed supports consisting of TiO2 and XC-72 are 1-99% by weight of the catalyst.
In the first, second, third, fourth, and fifth embodiments shown in Table 1, TiO2 is in a shape of nanotubes and is 20%, 40%, 60%, 80% and 50% by weight of the composed supports. In the first, second, third, and fourth comparative examples shown in Table 1, TiO2 is in a shape of nanoparticles and is 20%, 40%, 60% and 80% by weight of the composed supports. The supports in the fifth, sixth and seventh comparative examples are respectively XC-72, TiO2 and XC-72, wherein the seventh comparative example is a commercial catalyst E-TEK. In the sixth comparative example, 50% by weight of the conductive carbon material with high surface area (XC-72) is additionally added to serve as a conduction-assisted agent.
As to the sample name in Table 1, taking the sample name “TX20-PtRu 20-1-mw” for example, “TX” denotes the composition of TiO2 and XC-72, “TX20” denotes TiO2 is 20% by weight of the composed supports, “PtRu 20-1” denotes that the atom ratio of Pt and Ru is 20:1, “mw” denotes a micro-wave heating method is used for preparing the catalyst, and “TP25” denotes that the shape of TiO2 is nanoparticles.
In the first to fourth embodiments, the higher ratio of TiO2 in the composed support creates more pores and consecutive channels in the catalyst. Comparatively, the structure of the catalyst in the first embodiment is more compact. The structure with a large amount of pores and consecutive channels could avoid the aggregation of the catalytic metallic particles and provide channels for transmitting fuels and products, and thus the catalytic metallic particles could be distributed evenly in the catalyst and the catalyst would have a high efficiency. In brief, the exposure of the catalytic metallic particles resulted from the porous and loose structure and the consecutive channels in the composed support would increase the catalytic efficiency of the catalyst.
a)-
Compared
In the embodiments of the present invention, in addition to the Pt—Ru alloy nanoparticles, the catalytic metal further includes a single metal, a dual alloy, a multicomponent alloy and the combination thereof. Further, in addition to the XC-72, the support being a carbon-based material comprises at least one of a conductive carbon black, a single-walled nanotube (SWNT), a multi-walled nanotube (MWNT), an acetylene black, a mesocarbon microbead (MCMB) and a mesoporous carbon (CMK) and a combination thereof. As to the porous oxide, TiO2, used in the above embodiments, based on the actual necessity, one skilled in the art could select one from a group consisting of a silicon oxide, an alumina oxide, a titanium oxide, a zirconium oxide, a carbon mesoporous material and a combination thereof. Depending on different manufacturing methods, the porous oxide could have a shape of a particle, a tube, a star, or a layer.
Compared the electrochemical surface areas (ECSA cm2/g) in the first to fourth embodiments and those of the first to fourth comparative examples in Table 1, it could be found that the increased amount of the TiO2 nanotubes would increase the ECSA, which means that the exposed surface of the catalytic metal is enlarged by the increased TiO2 nanotubes.
a
1)-(a4) are CO-stripping voltammetry of the first to fourth embodiments, respectively.
The above result means the remove the carbon monoxide is relatively easy to occur in the catalysts with TiO2, and thus the catalysts with TiO2 have a better CO-tolerance.
Table 1 shows that the CO-tolerances of the first to third embodiments are all higher than that of the sixth comparative example, especially higher than that of the seventh comparative example, which is a commercial catalyst E-TEK. Based on the above, it could be known that the catalysts for fuel cells disclosed in the present invention have a good CO-tolerance, which would cause the elongated cycle life of the catalysts.
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Under the same ratio of the TiO2 in the composed supports, it could be known from Table 1 that the efficiencies of the second to fourth embodiments are higher than the second to fourth comparative examples. The result indicates that the TiO2 nanotube is effective to increase the activity of the catalysts in comparison with the TiO2 nanoparticle. When the ratio of the TiO2 nanotubes in the composed supports is higher than 40 wt %, there would be a problem of the decreased conductivity as that also exists in the first to fourth comparative examples.
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The manufacturing method of the TiO2 nanotubes doped with nitrogen atoms is described as follows. The TiO2 nanotubes prepared by the foregoing method are put in an aluminum oxide crucible, and the ammonia is introduced into the crucible by heating a tube furnace to 600° C. The flow rate is set for 800 and 400 c.c./min with 12 and 18 hours of the reaction time, which could be modulated based on the experimental requirements.
The manufacturing method of the catalyst with the support including TiO2 nanotubes and the nitrogen atoms are performed by the steps of: using 40 mg TiO2 nanotubes doped with nitrogen atoms and 60 mg XC-72 as the support, adding appropriate 50 ml ethylene glycol to be the solvent and the reductant, adding appropriate 0.05 M H2PtCl6 solution (H2PtCl6 in the ethylene glycol) and 0.05 M RuCl3 solution (RuCl3 in the ethylene glycol), mixing the above solutions evenly, stirring the mixed solution and using a reflux reduction method at a high temperature for performing the reductive reaction for 3 hours, cooling the mixed solution after the reaction is completed, repeatedly performing a wash step by the deionized water for removing the chlorine ions (Cl—), and finally performing a dry step so as to carry out the preparation of the catalyst.
The denominations of the catalysts doped with nitrogen atoms are described as follows. For example, the embodiment under the condition that the flow rate is 400 c.c./min and the reaction time is 18 hours is denominated as N4-18X40-PtRu-rf, wherein N4-18 represents that the flow rate and the reaction time for the TiO2 nanotubes doped with nitrogen atoms are 400 c.c./min and 18 hours, respectively, X is the XC-72, 40 represents TiO2 nanotubes are 40% by weight of the composed support consisting of the TiO2 nanotubes and the XC-72, PtRu represents the nanoparticles of the alloy formed by using 0.05M H2PtCl6 solution and 0.05 M RuCl3 solution, and rf represents the method for reducing the metallic precursor is the reflux reduction method.
Please refer to Table 2, which shows the relevant data of the catalyst doped with nitrogen atoms. The relevant data include the weight ratios of the TiO2 nanotubes and the XC-72, the nitrogen content in the catalyst, the mass current density (A/g Pt), and the on-set potential and peak potential of the fuel cell.
In the sixth embodiment shown in Table 2, TiO2 nanotubes are 40% by weight of the composed supports, wherein nitrogen atoms are not doped into the composed supports. In the seventh and eighth embodiments shown in Table 2, TiO2 nanotubes doped with nitrogen atoms are 40% by weight of the composed supports, wherein in addition to the different nitrogen contents, the difference there between further comprises the reaction time. In the eighth comparative example shown in Table 2, TiO2 nanotubes doped with nitrogen atoms are 40% by weight of the composed supports. The catalyst in the ninth comparative example is the commercial catalyst E-TEK. In the sixth comparative example, 60% wt of the conductive carbon material with high surface area (XC-72) is additionally added for serving as a conduction-assisted agent. In the present invention, the nitrogen atoms are doped into TiO2 in an atomic molar ratio of 0-25%, and the TiO2 contains oxygen atoms substituted by the nitrogen atoms in a substitution ratio of 6-10%.
As used in the column of code, E refers to Embodiment, for example, 6E refers to the sixth embodiment; C refers to Comparative example, for example, 8C refers to the eighth comparative example.
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The above mentioned replacement not only increases the conductivity of the nanotube, but also changes the surface lattice ordering, which results in the new lattice distance and thus generates the phenomenon of the creases. In
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Further, on the surface of TiO2, there are many functional groups helpful to absorb the methanol solution, which property is different from the carbon materials. The force between TiO2 and the metallic particles that results in the particle size ranged between 4-5 nm and the uniform of the particle size is advantageous in the increase of the mass current density. Because of the above reasons, compared with the XC-72, it is better to use the TiO2 nanotube as the support. Moreover, it is much better to use the TiO2 nanotube doped with the nitrogen atoms as the support that increases the conductivity of TiO2 and changes the electronic configuration of the catalytic metal for increasing the efficiency of the catalyst.
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Table 2 further shows that the difference in the reaction time and the flow rate between the seventh and eighth embodiments would affect the catalytic activity of the methanol as well. Based on the XPS test, the nitrogen contents of the seventh and eighth embodiments are 12.15 at. % and 7.52 at. %, respectively, but the seventh embodiment has a better catalytic ability of the methanol than the eighth embodiment according to Table 2 and
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In conclusion, the metallic oxide, such as TiO2 and particularly that doped with nitrogen atoms, could generates a strong force between nano-metal, such as Pt, PtRu and so on, and stabilize the size of the nano-metal. In addition, the OH group on the surface of the TiO2 nanotubes is helpful to increase the CO-tolerance and decreasing the on-set potential. The higher ratio of the TiO2 nanotubes would increase the catalytic performance of the catalyst and maintain the stability of the catalyst after the long-time use.
While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclose embodiments. Therefore, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.
The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.
All patents, applications, publications, test methods, literature, and other materials cited herein are hereby incorporated by reference.
