KR100963152B1 - Pt-Ru-Co-W Quaternary Alloy Catalysts For Direct Methanol Fuel Cell - Google Patents

Abstract

The present invention relates to a four-component alloy catalyst for direct methanol fuel cells, wherein the four-component alloy catalyst according to the present invention comprises platinum (Pt), ruthedium (Ru), cobalt (Co) and tungsten (W). The four-component alloy catalysts according to the present invention show lower starting potential, better stability and higher performance against methanol oxidation than conventional PtRu two-component alloy catalysts. The improvement in performance in the tetracomponent alloy catalyst according to the present invention is explained by the bifunctional mechanism, electronic effect, redox properties and cooperative synergistic effects of other metals added to Pt.

Quad component alloy catalyst, fuel cell

Description

Pt-Ru-Co-W Quaternary Alloy Catalysts For Direct Methanol Fuel Cell}

The present invention relates to a four component alloy catalyst for direct methanol fuel cells. More specifically, Pt-Ru- which is used in direct methanol fuel cells can have excellent catalytic activity against fuel oxidation, thereby improving cell characteristics, and inhibiting catalyst poisoning caused by CO, thereby preventing a decrease in catalyst life. A catalyst is made of a Co-W tetracomponent alloy.

Direct methanol fuel cells (DMFCs) are one of the most promising technologies for energy generation that converts chemical energy of fuels, CH ₃ OH and O ₂ directly into electrical energy.

Because electrode catalysts dominate fuel cell performance, good anode catalysts for methanol oxidation are very necessary for improved DMFC performance. The main research on DMFCs focuses on trying to obtain better anode catalysts typically through alloying Pt with other metals such as Ru, Rh, Co, Ni, Mo, Pd and the like.

To date, alloy catalysts Pt ₅₀ -Ru ₅₀ (where Pt and Ru each comprise the same molar number of 50 mol% each) have been most commonly used as anode catalysts for the oxidation of methanol in DMFCs. However, while Pt ₅₀ -Ru ₅₀ alloy catalysts show high performance and stability in DMFC, their catalytic activity is not high enough to bring fuel cell technology to practical use. The main reason for this can be attributed to the large kinetics (or activation) overvoltage and CO poisoning at the anode catalyst surface.

Identifying a good active catalyst composition through the conventional “one at a time” method is very difficult and time consuming and expensive due to its wide composition range.

At present, a combination method based on multiple sample processing is recognized as a very important technique as an alternative to the conventional method for discovering new materials. Combination chemistry, already widely used in biochemistry and pharmaceuticals, has been used for many years to identify and optimize complex compositions. Combination-based methods involve the rapid and simultaneous synthesis and analysis of a very large number of different chemical compositions to develop samples with specific properties of interest, thereby requiring typical processing time, effort and cost. Minimized compared to the "once at a time" method.

Recently, this approach has been used successfully for the discovery of new catalysts using novel and sophisticated methods for rapid screening of catalyst libraries. An important issue for the successful application of combinatorial analysis is to establish a synthetic and rational and comprehensive analysis method for the sample array.

Science 1998, 280, 1735 by Reddington et al. Reported for the first time an electrochemical analysis based on a combination method using an inkjet printer and a fluorescent acid-base indicator. This method monitors the fluorescence signal caused by the hydrogen ions generated in the electrochemical half-cell reaction, which allows the active electrode catalyst region to be removed from hundreds of samples of a tetracomponent array based on Pt, Ru, Os, Ir and Rh. It has been successfully confirmed and the method has also been applied to the discovery of high efficiency electrocatalysts for oxygen reduction and water oxidation.

Sullivan et al. Anal. Chem . In 1999, 71, 4369, an automated electrochemical analysis system was reported with an electrode array based on a combination method to measure proton concentration and electrochemical current at each electrode of a 64 electrode array.

Jiang et al. J. Eelectroanal . Chem . 2002, 537 and 137 proposed another analytical method for measuring the electrochemical properties of various electrodes in neutral, acidic and basic solutions using a movable electrolyte probe system.

Gorer and his colleagues reported Proc . Electrochem . Soc . (Direct Methanol Fuel Cell), 2001, 2001-4, 191, studied Pt-Ru-Pd powders as methanol oxidation catalysts in multi-electrode arrays.

Strasser and his colleagues used the array electrode method to study the effect of alloying 3d elements and Pt in 'International Symposium on Fuel Cells for Vehicles' O. Yamamoto (Ed), Electrochemical Society of Japan , Nagoya, 2000, 153. . They identified a high activity Pt / Fe binary component and Pt / Co / Cr ternary composition, as expected from initial studies on high surface area catalysts.

Lin et al. J. Electroanal. Chem., 2002, 535, 49 reported the development of high-efficiency screening equipment for fuel cell electrocatalysts, which more closely matches the actual fuel cell usage conditions.

Catalysis Today 2002, 74, 235 by Woo et al. Reported active Pt / Ru / Mo / W tetracomponent electrode catalysts by high-efficiency screening through repeated voltammetry experiments.

Oxidation of methanol on Pt catalysts is known to produce CO as an intermediate, which leads to toxicity on the Pt active surface. Therefore, an auxiliary catalyst which easily oxidizes CO adsorbed on Pt to CO ₂ is very important in order to restore the catalytic activity of the Pt catalyst. Ru in Pt ₅₀ -Ru ₅₀ plays a role in assisting this CO oxidation.

Much effort has been made to identify improved anode catalysts that are less susceptible to CO poisoning, and the inventors have also studied the improved catalysts for DMFCs, particularly anode catalysts, in addition to Ru, third and fourth. As the second metal, for example, when Os, Mo, Ir, Co and W are added to Pt, it helps to form oxygenated species, which can oxidize CO adsorbed on Pt surface at lower potential In particular, Co and W were found to be excellent additions to the PtRu binary system for increasing the catalytic activity performance as well as for reducing the precious metal content. Therefore, the new Pt-Ru-Co-W quadruple anode catalyst developed in the present invention is more active, more stable, and less affected by CO poisoning than the commercially available bicomponent Pt ₅₀ -Ru ₅₀ alloy catalyst. I could confirm it.

Accordingly, the technical problem of the present invention is to provide a multicomponent catalyst for methanol fuel cells having excellent catalytic activity and stability and better resistance to CO surface toxicity.

Still another object of the present invention is to provide a fuel cell membrane-electrode assembly (MEA) comprising the catalyst.

In order to solve the above technical problem, the present invention provides a catalyst for a direct methanol fuel cell of a tetracomponent alloy containing platinum (Pt), ruthedium (Ru), cobalt (Co) and tungsten (W).

In the catalyst for methanol fuel cell according to the present invention, the tetracomponent alloy is Pt 10 to less than 50 mol%; Ru 20-40 mol%; Co 10-25 mole%; And W 10-25 mol%.

In order to solve the above another technical problem, the present invention

An oxidation electrode and a reduction electrode positioned opposite each other, and

A polymer electrolyte membrane positioned between the oxidation electrode and the reduction electrode, wherein at least one of the oxidation electrode and the reduction electrode includes platinum (Pt), ruthedium (Ru), cobalt (Co), and tungsten (W); Provided is a membrane-electrode assembly for a direct methanol fuel cell comprising a component alloy catalyst.

In addition to Pt in the tetracomponent alloys, Ru, Co and W can be used in addition to oxidizing CO adsorbed on the surface of Pt at lower potentials, thereby facilitating the formation of oxygenated species in terms of performance. By reducing the precious metal content it is possible to lower the cost of catalyst production.

Specifically, the improvement in performance in the four-component alloy catalyst according to the present invention can be explained by the bifunctional mechanism, electronic effect, redox characteristic and cooperative synergistic effect of other metals added to Pt.

The bifunctional mechanism is that CO adsorbed on the active Pt surface during methanol oxidation is more effectively removed by the addition of Ru, Co and W metals. That is, CO adsorbed at the Pt active site is easily oxidized to CO ₂ by activated oxygenated species formed on the Ru, Co and W metals in the tetracomponent alloy catalyst.

The electronic effect is that a change in the electronic structure of Pt may result in enhanced activity by the addition of other metals. That is, the role of the alloying metals (Ru, Co and W) alters the electronic structure and chemical properties of the Pt site, resulting in a weaker chemisorption of CO to Pt, thereby reducing the CO electrical conductivity of the PtRuCoW alloy at lower potentials. Promotes oxidation

The redox property is due to the rapid change of the W oxidation state. The rapid change of the W oxidation state has the effect of reducing the binding energy of Pt-CO.

The cooperative synergy is that the redox properties of W and the d-electron density of Co modify the electrical properties of Pt, resulting in weaker chemisorption of CO on Pt.

Therefore, Ru, Co and W added to Pt in the four-component alloy catalyst according to the present invention is Ru 20 to 40 mol%; Co 10-25 mole%; And W 10 to 25 mol% is preferable, and when used below the suggested range, it is not preferable because it increases the use of precious metal Pt, and when used in excess of the above range, it may have the same effect as above. none.

More preferably, Pt is used in the tetracomponent alloy according to the present invention 10 mol% or more and less than 50 mol%. This may also be desirable in terms of reducing the Pt content, which is a precious metal, to lower the cost of catalyst production.

In addition, the catalyst for methanol fuel cell according to the present invention is preferably an anode catalyst.

In addition, the catalyst has an average particle diameter of 2 to 10nm, more preferably 2 to 6nm average particle diameter. If the average particle diameter is less than 2 nm, the alloy catalyst is unstable and easily aggregated to reduce the surface area of the catalyst, which is not preferable. If the average particle diameter is larger than 10 nm, the particles become too large and the utilization efficiency of the catalyst is not preferable.

In addition, the catalyst may be used as the catalyst itself in a black state, or may be supported on a carrier. When supported on a carrier, the size of the catalyst particles can be made small, and thus the reaction surface area of the catalyst can be increased, which is more preferable.

As used herein, the term “black” refers to a state in which a catalyst metal is not supported on a carrier. The carrier may be graphite, denka black, ketjen black, acetylene black, carbon nanotube, carbon nanofiber, carbon. Carbon-based materials such as nanowires, carbon nanoballs, carbon nanohorns, porous carbons having mesopores, or activated carbons may be used, and Zr, Ti, Sn, Zn, Al, Si, W, and combinations thereof It is also possible to use inorganic fine particles which are oxides of an element selected from the group consisting of zirconia or titania, tin oxide, zinc oxide, alumina, silica and the like.

When the catalyst is supported on the carrier as described above, the supported amount is preferably 20 to 90% by weight, more preferably 40 to 80% by weight based on the total weight.

The Pt-Ru-Co-W tetracomponent alloy catalyst according to the present invention has been developed for methanol oxidation electrocatalyst in DMFC by an adapted combined optical screening method using a robotic distribution system.

The Pt-Ru-Co-W tetracomponent alloy catalyst according to the present invention showed a lower onset potential and better catalytic activity against methanol oxidation than the E-TEK bicomponent Pt-Ru catalyst generally used in DMFC. In addition, the four-component alloy catalysts according to the present invention showed better stability, catalytic activity and better resistance to CO toxicity to methanol oxidation than PtRu binary alloys.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

The Pt-based four-component PtRuCoW alloy electrode catalyst according to the present invention was developed based on the "optical multisample analysis method".

For details on the "optical multisample analysis method", see E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, ES Smotkin, TE Mallouk, TE Science 1998 , 280 , 1735; E. Reddington, J.-S. Yu, A. Sapienza, BC Chan, B. Gurau, R. Viswanathan, R. Liu, ES Smokin, S. Sarangapani and TE Mallouk MRS Symp . Proc . 1999 , 549 , 231 .; BC Chan, E. Reddington, A. Sapienza, J.-S. Yu, TE Mallouk, B. Gurau, R. Viswanathan, R. Liu, K. Ley, TJ Lafrenz, ES Smokin and S. Sarangapani "Combinatorial Discovery and Optimization of Anode Electrocatalysts for Direct Methanol Fuel Cells" ◎ Fuel Cells-Clean Energy for Today's World , 1998 , 1 .; ND Morris, TE Mallouk, J. Am. Chem . Soc . 2002 , 124 , 1114. b); And in E. Reddington, J.-S. Yu, A. Sapienza, BC Chan, B. Gurau, R. Viswanathan, R. Liu, ES Smokin, S. Sarangapani and TE Mallouk Combinatorial Chemistry: A Practical Approach , H. Fenniri, Ed., Oxford University Press, Oxford, UK, 2000 , pp 401-420.

In this method a robotic distribution system is used to fabricate a homogeneous electrode array with a number of different chemical compositions on a conductive substrate, where the metal solution is 96 wells to 96 with a resolution of 88 nL / spot precision on toray carbon paper. Printed using the transfer program.

Example  One

Four components  Screening of Catalyst Composition

0.5 M aqueous metal salt solution was prepared from H ₂ PtCl ₆ .xH ₂ O, RuCl ₃ .xH ₂ O, CoCl ₂ .6H ₂ O and Na ₂ WO ₄ 2H ₂ O (Aldrich), respectively. The four-component electrode arrays desired to be obtained with the solution were fabricated by a robotic dispensing system using dispensing software adapted on Teflon-coated Toray carbon paper. Details of the method can be found in E. Reddington, J.-S. Yu, A. Sapienza, BC Chan, B. Gurau, R. Viswanathan, R. Liu, ES Smokin, S. Sarangapani and TE Mallouk Combinatorial Chemistry: A Practical Approach , H. Fenniri, Ed., Oxford University Press, Oxford, UK, 2000 , pp 401-420.

Each composition spot in the four-component electrode array on carbon paper contains varying amounts of each metal solution. Four-component arrays prepared at 11% resolution contain 220 different catalyst compositions, and the total moles of metal are the same for each of these composition spots, which is 1.5 μmol per spot. The array was dried at 80 ° C. for 2 hours in a vacuum drying oven and reduced by adding 5 wt% NaBH ₄ solution (> 10 fold molar) to each spot. The diameter of the spots in these arrays was approximately 2 mm. The distance between each spot was adjusted to 7 mm to prevent mixing of two adjacent spots.

The prepared array of catalysts was immobilized with a home-made cell containing an indicator solution consisting of 6.0 M CH ₃ OH, 0.5 M NaClO ₄ , 30 mM Ni (ClO ₄ ) ₂ and 100 μM PTP dissolved in water. Screening was performed. The pH of the solution was adjusted to 3-4 using HClO ₄ . The cell was connected to a potentiostat (EG & G362) with a Pt counter electrode and an Ag / AgCl (in saturated KCl) reference electrode. Each spot was used as a working electrode in the array. A UV lamp (366 nm) was fixed on the electrode array surface and any observed fluorescence was recorded. The catalyst composition spot that fluoresces preferentially at the lowest overpotential under UV radiation is the most active catalyst composition spot which actively generates hydrogen ions during the methanol oxidation reaction resulting in the largest local pH change. The working electrode potential was set at an initial +200 mV (Ag / AgCl basis), and then the anode potential was increased in steps of 50 mV. The electrode was held at each potential for about 5 minutes. This process was repeated until visible fluorescence was observed, revealing a four-component catalyst composition region with excellent activity.

Four components  Alloy catalyst manufacturers

Subsequently, three quadruple catalysts (Pt (43) Ru (33) Co (14)) W (10) supported by carbon using the four-component catalyst composition found based on the above results) and (Pt (45) Ru (24) Co (10) W (21)), ③ (Pt (45) Ru (23) Co (22) W (10)), respectively, using NaBH using respective metal salt solutions and Vulcan XC 72 carbon black. Prepared by the ₄ reduction method.

That is, H ₂ PtCl ₆ · xH ₂ O, RuCl ₃ · xH ₂ O, CoCl ₂ · 6H ₂ O and Na ₂ WO ₄ · 2H ₂ O prepared according to the three-component alloy composition shown above and deionized water Three kinds of metal salt solutions were prepared, each dissolved in 100 ml and stirred at ambient temperature.

Subsequently, the Vulcan XC carbon black was prepared by appropriately preparing the amount of Vulcan XC carbon black so that the mass percentage of the metal was 60% of the total catalyst amount, and suspended in 200 ml of deionized water to obtain a homogeneous solution by stirring.

Subsequently, the metal salt solution prepared above and the Vulcan XC carbon black solution are mixed with 1500 ml of deionized water and then stirred to form a mixed solution of the homogeneous metal salt and Vulcan XC carbon black. All three tetracomponent alloy solutions are prepared in the same way as a mixed solution of metal salts and Vulcan XC carbon black. NaOH was added dropwise to the three mixtures prepared above to adjust the mixed solution to pH 7-8. Each mixture was rapidly charged with NaBH ₄ solution in an amount of 10 times the total moles of metal (which is prepared by dissolving the appropriate amount of NaBH ₄ in deionized water) and the metal was stirred well for at least 4 hours for complete reduction. The reduced PtRuCoW / support catalyst slurry was settled at the bottom of the reaction vessel, the catalyst slurry was filtered, washed thoroughly with excess ethanol-water (50:50 = v / v) solution, deionized water, and dried at 70 ° C. Dried in a vacuum oven, whereby (Pt (43) Ru (33) Co (14)) W (10)) of 60% by weight metal loading, based on the catalyst total weight, ② (Pt (45) Ru (24) ) Co (10) W (21)) and ③ (Pt (45) Ru (23) Co (22) W (10)) catalysts were prepared, respectively.

The metal loading was 60% by weight as with commercially available E-TEK Pt-Ru catalyst. Also prepared with the same 60% by weight metal loading for comparison with the Vulcan XC-72 carbon supported Pt ₅₀ -Ru ₅₀ alloy catalyst.

Example  2

Unit cell production

In order to evaluate the electrode catalyst activity of the four-component alloy catalyst disclosed in Example 1, a DMFC (direct methanol fuel cell) unit cell having a 2 cm 2 cross-sectional catalyst area was used, and a potentiometer workstation (WMPG-1000) was used. Activity was measured at 30 ° C and 70 ° C.

To make the membrane electrode assembly (MEA), a catalyst ink for the anode was prepared with 0.763 ml of Nafion ionomer solution (Aldrich, 5% by weight) dissolved in isopropyl alcohol and 100 mg of catalyst and the amount of distilled water to be sufficiently wetted with the catalyst. The catalyst ink for the cathode was made by mixing 100 mg of the catalyst, distilled water of sufficient amount to sufficiently wet the catalyst, and 0.275 ml of Nafion ionomer solution (Aldrich, 5% by weight) dissolved in isopropyl alcohol. Here Nafion is required for binding of the electrode material and proton transfer to the electrolyte membrane. The inks were stirred for 24 hours to make a homogeneous slurry.

An appropriate amount of anode or cathode ink was then applied onto teflonized carbon paper (TGPH-090) and dried in an 110 ° C. oven for 30 minutes. The catalyst loadings on the metal-based anode and reduction electrode are 2.5 and 5.0 mg / cm 2 (the reduced electrode is an unsupported Pt catalyst purchased from Johnson Matthey).

The membrane electrode assembly (MEA) was then pretreated with Nafion 115, a polymer electrolyte membrane inserted between an anode catalyst (PtRuCoW (2.5 mg / cm 2)) and a cathode catalyst (Johnson-Matthey Pt Black (5.0 mg / cm 2)). DuPont) membranes were made by hot-pressing (5 min at 135 ° C. under 2000 psi pressure). Nafion 115 membrane was pretreated by first boiling in 3% by weight H ₂ O ₂ for 1 hour and then by boiling in 0.5MH ₂ SO ₄ for 1 hour.

The unit cell is composed of two copper end plates and two graphite plates having a lip-channel pattern allowing a passage of methanol to the anode and a passage of oxygen gas to the cathode. A 2.0 M methanol solution was fed to the anode by a masterplex liquid micro-pump at a flow rate of 1.5 mL / min, while dried O ₂ was fed to the cathode at a rate of 100 cc / min via a flow meter.

TEM, XRD, CV, CA, CO stripping voltammogram, XPS and polarization curves of three tetracomponent alloy catalysts ① to ③ and Pt ₅₀ -Ru ₅₀ alloy catalysts prepared in Example The results are shown in FIGS. 1 to 8.

Transmission electron microscopy (TEM) images are intended to confirm the microstructural characteristics of the samples and were observed with high resolution transmission electron microscopes (HR-TEM, JEM-2100F, JEOL, Japan, FRG-TEM) operating at 2000 kV. Specifically, several ml of the sample dispersed in ethanol under ultrasonic wave is dropped on a carbon coated TEM grid, dried, and observed with an electron microscope.

X-ray diffraction patterns (XRD) were analyzed using a Rigaku diffractomer with Cu Kα radiation at a scan rate of 4 ° / min (2θ). Here, the X-ray gun was operated at 40 kV and 20 mA and the powder sample was placed on a glass slide.

Voltammogram measurements were performed using WMPG-1000 potentiometer (WonA-Tech), and cyclic voltammograms (CV) acted as working electrodes using the same 2 cm 2 active area single cell array. H ₂ O and 2.0 M CH ₃ OH supplied to the anode were performed at 30 ° C. Humidified H ₂ gas was supplied to the Pt cathode at 50 ml min ⁻¹ (zero back pressure), allowing it to act as a counter and quasi-reference electrode (dynamic hydrogen electrode, DHE). All cyclic voltamograms were fed 2.0 M CH ₃ supplied to the anode using a Masterflex liquid micro-pump at a rate of ¹ ml min ⁻¹ in the range of 0.0 V to +1.2 V. Obtained from OH. Baseline curves were obtained under the same conditions using H ₂ O supplied to the anode instead of CH ₃ OH. N ₂ containing 20% CO while maintaining anode potential at 0.1V (DHE) The gas was flowed through an anode at 50 ml min ⁻¹ for 20 minutes to adsorb CO to the PtRu catalyst. The gas was then diverted to argon at a flow rate of 50 ml min ⁻¹ for 2 minutes while still maintaining the potential at 0.1V. Non-adsorbed CO was removed from the gas phase.

CO stripping voltamograms were recorded by scanning potentials from 0.0V to 1.2V at 20 mVs ⁻¹ .

The chromogramgram (CA) shows the current profile of the methanol oxidation process as a function of time at a fixed potential setting. The electrode is first subjected to a number of voltametric cycles (20 mV / s, -0.2 V to +1.0 V based on Ag / AgCl, using a WMPG-1000 potentiometer in 0.5 MH ₂ SO ₄ ) to achieve a reproducible reaction. Wash and oxidization and reduction cycles were then recorded between −0.2 V and +1.0 V at a scan rate of 20 mV / s in an N ₂ degassed mixed solution of room temperature 0.5 MH ₂ SO ₄ and 1.0 M CH ₃ OH. Finally, the chromatogram is obtained by recording the current profile for 1 hour against ambient temperature and 0.45V (Ag / AgCl reference) reference electrode potential. The effective geometric area of the working electrode is 1.0 cm 2, and the electrode has a catalyst loading of 2.0 mg / cm 2 (metal base).

1 shows a TEM image of a commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst (a) and the four-component alloy catalysts ① to ③ (b to d) according to the present invention.

As can be seen from FIG. 1 above, a homogeneous dispersion of small black spots corresponding to Pt-based alloy nanoparticles is seen on the amorphous Vulcan carbon support. The particle size distribution is uniform and it can be seen that there is less aggregation of metal particles in the tetracomponent alloy catalyst according to the present invention as compared to that observed in the E-TEK catalyst.

The particle size measured directly from the TEM photograph in the randomly selected region ranges from 2.0 to 3.5 nm in diameter. Interestingly, the tetracomponent alloy catalyst according to the present invention showed a slightly smaller particle size distribution than the E-TEK catalyst. The average particle size of the Pt-based alloy catalyst was approximately 2.4 nm for Pt (43) Ru (33) Co (14) W (10) and 3.3 nm for E-TEK catalyst.

Figure 2 shows the powder X-ray diffraction (XRD) pattern for the tetracomponent alloy catalyst and the E-TEK Pt ₅₀ -Ru ₅₀ catalyst according to the present invention. The XRD patterns of all Pt alloy catalysts show the fcc structure, which is the XRD pattern of Pt catalysts. The diffraction peak at 2θ = 25 ° observed in all XRD patterns is due to the (002) plane of the hexagonal structure of the Vulcan XC-72 carbon. The elements Ru, Co and centroid cubic W were not observed in the XRD pattern showing uniform Pt alloy formation. The average metal particle size calculated from the (220) X-ray diffraction peak of the Pt fcc grating by the Scherrer equation is in good agreement with that measured directly by TEM.

If a uniform dispersion of the metal particles is assumed on the carbon support, the smaller the catalyst particles will have a higher surface area for the same amount of carbon support used. Therefore, based on the results obtained from the above TEM and XRD, the four-component alloy catalyst according to the present invention has a smaller average particle size compared to the E-TEK catalyst and therefore has a large surface area of the active catalyst nanoparticles, and an E-TEK catalyst. More direct methanol fuel cells (DMFCs) have higher active surface areas for methanol oxidation.

Figure 3 shows the cyclic voltammogram (CV) of the anode catalyst in 2.0M CH ₃ OH at 30 ℃. According to FIG. 3, it is shown that the four-component alloy anode catalyst according to the present invention starts methanol oxidation activity at an onset potential (about 210 to 215 mV (DHE basis)) lower than the E-TEK catalyst (265 mV). Giving. Lower onset potentials indicate good evidence of catalytic activity for methanol oxidation.

In addition, the four-component anode catalyst according to the present invention (Pt (43) Ru (33) Co (14) W (10), Pt (45) Ru (24) Co (10) W (21) and Pt (45) Both Ru (23) Co (22) W (10)) showed higher methanol oxidation activity of 247, 207 and 203 mA compared to 164 mA for the E-TEK catalyst, respectively, at 0.6 V (versus DHE). This shows that the four-component anode catalyst of the present invention exhibits about 24-50% improved activity over E-TEK.

Pt ₅₀ -Ru ₅₀ / Vulcan XC-72 synthesized in this laboratory showed slightly lower methanol oxidation activity compared to commercially available E-TEK catalysts prepared using the same bee XC-72 as carrier (activity data shows transparency Not shown). This shows that the new four-component composition according to the invention increases the catalytic activity of methanol oxidation compared to the two-component Pt-Ru / Vulcan XC-72 catalyst synthesized under the same conditions.

Figure 4 shows the chromoam perogram (CA) of the anode catalyst in a mixed solution of 0.5MH ₂ SO ₄ and 1.0M CH ₃ OH. Chroamperometric (CA) results can provide important information about the electrode dynamics and electrode dispersion stability of catalyst nanoparticles and reagents because they show the current profile of the methanol oxidation process as a function of time at a fixed potential setting. have.

According to FIG. 4, the Pt (43) Ru (33) Co (14) W (10) catalyst according to the present invention showed a higher initial and final current density of methanol oxidation compared to the E-TEK binary catalyst, This is consistent with the CV results of FIG. 3, and also, the slow decrease in current density during the measurement shows the excellent stability of this four component catalyst. After 1 hour, the Pt (43) Ru (33) Co (14) W (10) catalyst maintained 87% of the initial oxidation current density and the E-TEK catalyst maintained only 75% of the initial current density.

This result shows that PtRuCoW is less poisoned by CO than E-TEK catalyst, which is believed to be due to the presence of Co and W metals in addition to the bicomponent Pt-Ru in PtRuCoW.

Alloying Pt with other second and third metals can increase the amount of oxygenated (eg, -OH) species at low potentials, which can facilitate oxidation of CO to CO ₂ and on the catalyst Toxicity will be reduced. CO oxidation activity of the catalyst was investigated using CO stripping voltammetry.

FIG. 5 shows CO stripping voltammograms of PtRu (E-TEK) and Pt (43) Ru (33) Co (14) W (10) catalysts. According to FIG. 5, the four-component alloy catalyst according to the present invention had a low potential of 564 mV compared to 640 mV for the E-TEK catalyst, indicating that oxidation of CO _ads on the Pt surface occurs more readily in PtRuCOW. Thus, the addition of small amounts of Co and W may help to clean the CO poisoned Pt surface more easily and may help maintain an active Pt reaction site for methanol oxidation.

The better performance of the PtRuCoW catalyst according to the invention is explained by the bifunctional mechanism. According to this mechanism, CO adsorbed on the active Pt surface during methanol oxidation is more effectively removed by the addition of the third and fourth metals. That is, CO _ads on the Pt active site are more easily oxidized to CO ₂ by the oxygenated species activated on the Ru, Co and W metals in the tetracomponent alloy catalyst. In addition to this bifunctional mechanism, the electrical effects associated with changes in the electrical structure of Pt by the addition of other metals can also lead to the observed enhanced activity results. The role of the alloy metals (Ru, Co and W) is to alter the electrical structure and chemical properties of the Pt site, resulting in weaker chemisorption of CO on the Pt surface, which leads to CO electrooxidation in the PtRuCoW alloy at lower potentials. Promote

6 shows the 4 f X-ray photoelectron spectra of carbon supported E-TEK PtRu and PtRuCoW catalysts. Referring to Figure 6, the Pt 4 f _7/2 and 4 f _5/2 line exhibits a peak area ratio of the theoretical appear, 4 to 3 in each of ~ 71.2 and ~ 74.3eV. It had a peak for Pt ^{2 +} and Pt ^{4 +} at 73.8 and 74.6 eV is not found, respectively, indicating that the Pt is 0 is present in the form of metal on both PtRuCoW yarn component and the E-TEK Pt-Ru two-component catalyst. PtRuCoW the catalyst Pt 4 f _7/2 It was confirmed that as compared with the PtRu catalyst slightly shifted to a low bonding energy. For example, Pt4 f _7/2 peak of PtRuCoW and PtRu catalyst may in each 70.85eV and 71.1eV. Co is an electrically positive element for Pt, so Pt atoms will tend to attract electrons from neighboring Co atoms. This effect is likely caused the movement resulting in polarization in the combination in Pt-Co alloy and the binding energy of the Pt 4 f _7/2 signal is negative (shift).

Park et al ., Appl . Phys . Lett . 2002 , 81 , 907. and Gochem et al., Electrochem . Acta 1998 , 43 , 3637. reported the enhancement effect of W on Samsung PtRuW catalyst. The ternary catalyst has improved catalytic activity against methanol oxidation. This contributes to the redox reaction of W, which involves a rapid change in W oxidation state. This redox property makes active W easier to oxidize the adsorbed CO generated by dissociative adsorption of water and dehydrogenation of methanol. Thus, the shift to lower binding energy indicates that Co and W modify the electrical properties of Pt, thus reducing the Pt-CO binding energy due to the contribution of the d-electron density of Co and the redox properties of W. . The combined effect of Co and W makes the formation of CO in the active Pt catalyst difficult, but the formation of reactive oxygenated intermediates is much better.

7 to 8 show the voltage and power density of DMFC determined at 30 and 70 ° C. of a Pt (43) Ru (33) Co (14) W (10) tetracomponent alloy catalyst and an E-TEK catalyst according to the present invention. It is a curve. According to FIG. 7, open-circuit voltages (OCV) for Pt (43) Ru (33) Co (14) W (10) tetracomponent alloy catalysts and E-TEK catalysts were 0.635V and 0.654V, respectively. The current density at 92 V for the Pt (43) Ru (33) Co (14) W (10) tetracomponent alloy catalyst was 92 mA / cm 2. Voltage and power density in this region are mainly affected by the activity of the catalyst. This corresponds to a 58% increase compared to the value of 58 mA / cm 2 for the E-TEK catalyst under the same experimental conditions. The maximum power densities for the Pt (43) Ru (33) Co (14) W (10) tetracomponent alloy catalyst and the E-TEK catalyst were 64 mW / cm 2 and 38 mW / cm 2, respectively. This also shows that the Pt (43) Ru (33) Co (14) W (10) catalyst shows 68% higher power density than the E-TEK catalyst. Similar trends were observed at 70 ° C. as shown in FIG. 8. Maximum power densities were 179 mW / cm 2 and 130 mW / cm 2 for the Pt (43) Ru (33) Co (14) W (10) tetracomponent alloy catalyst and the E-TEK catalyst, respectively. This indicates that the Pt (43) Ru (33) Co (14) W (10) catalyst is about 37% better than the E-TEK catalyst.

Based on the above results, it can be seen that the four-component PtRuCoW alloy catalyst according to the present invention is useful as a DMFC anode catalyst.

Figure 1 shows a TEM image of the catalysts according to the invention and commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalysts.

Figure 2 is a graph showing the X-ray diffraction pattern of the catalysts according to the present invention and commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst.

3 is a cyclic voltamogram of catalysts according to the invention and commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalysts.

4 is a chromopergram of a catalyst according to the present invention and a commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst.

5 is a CO stripping voltammogram of the catalyst according to the present invention and commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst.

Figure 6 is an X-ray photoelectron spectra of the catalyst according to the invention and commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst.

7 is a voltage and power density curve of a 30 ° C. direct methanol fuel cell using a catalyst according to the present invention and a commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst as an anode.

8 is a voltage and power density curve of a 70 ° C. direct methanol fuel cell using a catalyst according to the present invention and a commercially available E-TEK Pt ₅₀ -Ru ₅₀ catalyst as an anode.

Claims (8)

Platinum (Pt) of less than 10-50 mol%; Ruthenium (Ru) 20 to 40 mol%; 10 to 25 mole percent cobalt (Co); And A catalyst for a direct methanol fuel cell of a tetracomponent alloy comprising 10 to 25 mol% of tungsten (W). delete The catalyst of claim 1, wherein the catalyst is an anode catalyst. The method of claim 1, The catalyst is a direct methanol fuel cell catalyst having an average particle diameter of 2 to 10nm. The method of claim 1, The catalyst is a direct methanol fuel cell catalyst which is supported by a carrier selected from the group consisting of carbonaceous materials, inorganic fine particles and mixtures thereof. The method of claim 5, The inorganic fine particles are Zr, Ti, Sn, Zn, Al, Si, W and a catalyst for a direct methanol fuel cell that is an oxide of an element selected from the group consisting of a combination thereof. The method of claim 1, The catalyst is a direct methanol fuel cell catalyst that is supported on 20 to 90% by weight based on the total weight. An anode and a cathode positioned opposite to each other, and It includes a polymer electrolyte membrane positioned between the anode and the cathode, Any one or both of the anode and the cathode comprises a catalyst according to any one of claims 1 to 7, wherein the membrane-electrode assembly for direct methanol fuel cell.

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