CN103949251A - Oxygen reduction catalyst as well as preparation method and application of oxygen reduction catalyst - Google Patents

Abstract

The oxygen reduction catalyst provided by the present invention, the catalyst is expressed as: Ag@Pt/MWCNTs-CeO ₂ , which is a catalyst with carbon nanotubes (MWCNTs) as the carrier and CeO ₂ doped core-shell Ag@Pt as the active component , the catalyst is uniformly doped with CeO ₂ in Ag@Pt/MWCNTs; the catalyst is used as a cathode catalyst for proton exchange membrane fuel cells, and its catalytic activity is 50-70% higher than that of Ag@Pt/MWCNTs. The chemical active area reaches 67.1-100.0m ² g ^-1 , and the current density of catalytic oxygen reduction can reach 4.5-7.0mA·cm ^-2 . The preparation method adopted in the present invention is simple and easy to implement and easy to industrial application. The prepared Ag@Pt/MWCNTs-CeO ₂ composite catalyst reduces the loading of platinum and greatly reduces the cost of the catalyst.

Description

A kind of oxygen reduction catalyst and its preparation method and application

technical field

The invention relates to an oxygen reduction catalyst and its preparation, in particular to an Ag@Pt/MWCNTs- _CeO2 catalyst and its preparation method, and the catalyst is used as a proton exchange membrane fuel cell cathode catalyst.

Background technique

Proton exchange membrane fuel cell (PEMFC) has the advantages of long life, high energy density, start-up at room temperature, easy drainage of water, and environmental protection. It has broad application prospects in electric vehicles and mobile power sources. At present, platinum-based electrocatalysts are considered to be the best catalysts for fuel cells due to their high catalytic performance. However, the high cost of platinum-based electrocatalysts limits the application of PEMFCs. Therefore, the development of catalysts with low platinum loading and high performance is the key technology to promote the development of proton exchange membrane fuel cells. Recent studies have found that when Pt-based alloy catalysts are doped with other metal elements such as Ni, Cr, Pd, Ag, Cu, Au and other metals, such binary or multi-element catalysts can not only reduce the amount of Pt metal in the catalyst, but also reduce the The cost will also change the adsorption capacity of the Pt surface for oxygen and the intermediate state of the oxygen reduction process due to the synergistic effect between other metal elements and Pt, thereby improving the oxygen reduction efficiency. Therefore, emerging catalysts such as Pt alloys and core-shell catalysts have attracted widespread attention. Some scholars have found that the doping of metal oxides and platinum-based catalysts will not only reduce the loading of platinum in the catalyst, but also improve the stability and catalytic activity of the catalyst. Due to the low price of CeO ₂ and the existence of lattice defects and oxygen vacancies in the structure, it has a high oxygen storage capacity. Therefore, many scholars have done a lot of work on the performance of the composite catalyst of Pt and CeO ₂ in recent years. .

Literature: Jerzy Chlistunoff et al., Electrochemical Studies of Novel Pt/Ceria/C Oxygen Reduction Catalysts for Fuel Cells.ECS Transactions, 2011.1(41) 2341-2348. In, Chlistunoff et al prepared a Pt-CeO ₂ /C catalyst , through research, it is found that due to the excellent oxygen storage performance of CeO ₂ , the local oxygen partial pressure can be increased under the oxidation voltage of Pt to enhance its catalytic oxygen reduction activity.

Literature: Lim DH et al., Effect of ceria nanoparticles into the Pt/C catalyst as cathode material on the electrocatalytic activity and durability for low-temperature fuel cell. Applied Catalysis B: Environmental, 20101(94), 85-96. Lim et al. found that CeO ₂ itself has the function of storing oxygen, and CeO ₂ distributed near Pt can just act as an oxygen buffer during the oxygen reduction process, supplementing the lack of oxygen in the oxygen reduction process of the catalyst, thereby improving the catalytic performance of the catalyst, rather than providing new active sites

Contents of the invention

The object of the present invention is to provide an oxygen reduction catalyst with low platinum loading and high catalytic activity and a preparation method thereof, and use the catalyst as a cathode catalyst of a proton exchange membrane fuel cell.

The oxygen reduction catalyst provided by the present invention, expressed as: Ag@Pt/MWCNTs-CeO ₂ , is a catalyst with carbon nanotubes (MWCNTs) as the carrier and CeO ₂ doped core-shell Ag@Pt as the active component. The catalyst is CeO ₂ uniformly doped in Ag@Pt/MWCNTs; the catalyst is used as a proton exchange membrane fuel cell cathode catalyst, and its catalytic activity is 50-70% higher than that of Ag@Pt/MWCNTs, and its electrochemical activity The area reaches 67.1-100.0m ² g ^-1 , and the current density of catalytic oxygen reduction can reach 4.5-7.0mA·cm ^-2 .

The specific preparation steps of the oxygen reduction catalyst are as follows:

A disperses silver nitrate, sodium citrate and multi-wall carbon nanotubes in deionized water to prepare suspension A, so that the molar concentration of silver nitrate is 1-6mmol/L, and the molar concentration of sodium citrate is 20-50mmol/L L, the mass concentration of multi-walled carbon nanotubes is 1-3g/L, then dropwise to it and the sodium borohydride/ethanol solution that the volume ratio of suspension A is 1:30-60, filter, dry, obtain Ag/ MWCNTs; the content of sodium borohydride in sodium borohydride/ethanol solution is 100-300mmol/L;

Disperse the prepared Ag/MWCNTs in ethylene glycol solution at 1-3g/L to prepare suspension B, disperse by ultrasonic for 1-4h, add chloroplatinic acid solution accounting for 0.5-1% by volume of suspension B , then use 2-10% KOH/ethylene glycol solution to adjust the pH=5-10, raise the temperature to 60-120°C for 3-6h, filter and dry to obtain Ag@Pt/MWCNTs;

B. Adjust the cerium nitrate solution with a concentration of 0.1-1.0mmol/L to 0.1mol L ^-1 ammonia water or 0.1mol L ^-1 sodium hydroxide solution to adjust pH=7-11, add it to a high-temperature and high-pressure reactor, and React at 90°C for 2-6 hours, then react at 110-160°C for 3-6 hours, filter, wash, and dry to obtain CeO ₂ ;

C. Add Ag@Pt/MWCNTs prepared in step A and CeO ₂ obtained in step B to the ethanol solution at a mass ratio of 4-8:1, ultrasonically disperse for 1-4h, filter, and vacuum at 50-80°C Dry in a drying oven for 5-15 hours to obtain the Ag@Pt/MWCNTs-CeO ₂ composite catalyst.

Figure 1 is the XRD spectrum of CeO ₂ and Ag@Pt/MWCNTs-CeO ₂ prepared in Example 1. In the two curves a and b, the first strong peak corresponding to 2θ=26°C is the characteristic diffraction peak of the carrier multi-walled carbon nanotubes. Comparing the curve b with the standard card (PDF card04-0802), at 2θ=39.8°, 46.2°, 67.4° and 81.3°, the characteristic diffraction peaks of Pt with a face-centered cubic structure, and the corresponding crystal planes are (111), (200), (220), (311). Curve b is the XRD spectrum of the Ag@Pt/MWCNTs-CeO ₂ catalyst. The characteristic diffraction peaks of metal Ag in curve b do not appear, indicating that the metal Ag has become the core and is wrapped in the metal Pt shell. It can be seen from the figure that the characteristic peaks in the composite structure are obvious, and the structure is good.

Figure 2(a) is an electron micrograph of the Ag@Pt/MWCNTs catalyst prepared in Example 2. It can be seen from the figure that Ag@Pt nanoparticles are uniformly distributed on the surface of MWCNTs. Figure 2(b) is the Ag@Pt/MWCNT-CeO ₂ composite structure catalyst prepared in Example 2. In the figure, CeO ₂ is a blocky particle with a diameter of about 600 nm, which is inserted between Ag@Pt/MWCNT to form a uniform composite structure.

The electrochemical performance of fuel cell catalysts was characterized by cyclic voltammetry and linear voltammetry scanning. The results are shown in Figures 3 and 4. It can be seen from Figures 3 and 4 that the catalytic activity of Ag@Pt/MWCNTs-CeO ₂ is significantly Higher than Ag@Pt/MWCNTs, its electrochemical active area can reach 97.2m ² ·g ^-1 , and the catalyst doped with CeO ₂ has a significantly high limiting current, indicating that in the presence of cerium oxide, the local oxygen concentration. During the oxygen reduction process, cerium oxide as the supply source of O maintains a high reaction rate.

Beneficial effects: the _Ag @Pt/ _MWCNTs -CeO ₂ composite catalyst prepared by the present invention not only reduces the loading capacity of platinum, but also reduces the cost of the catalyst _. A cycle of redox reactions between O ₃ occurs, which is easy to take up and release O, and in the process will generate unstable oxygen vacancies, bulk oxygen species have relatively high mobility, and the cycle of oxygen vacancies and Annihilation promotes the flow of oxygen ions, which can promote the adsorption and dissociation of oxygen in the process of oxygen reduction reaction, and promote the occurrence of oxygen reduction reaction, thereby improving the catalytic activity of the catalyst. Its electrochemical active area can reach 97.2m ² g ^-1 , and the current density of catalytic oxygen reduction can reach 4.8mA·cm ^-2 . The adopted preparation method is simple and practicable, and is easy for industrial application.

Description of drawings

Figure 1 is the XRD spectrum of CeO ₂ and Ag@Pt/MWCNTs-CeO ₂ prepared in Example 1. Where a is the XRD curve of CeO ₂ and b is the XRD curve of Ag@Pt/MWCNTs-CeO ₂ .

Fig. 2 is a scanning electron micrograph of Ag@Pt/MWCNTs and Ag@Pt/MWCNTs-CeO ₂ prepared in Example 2. a is the SEM image of Ag@Pt/MWCNTs, b is the SEM image of Ag@Pt/MWCNTs- _CeO2 .

Figure 3 is the cyclic voltammetry curves of Ag@Pt/MWCNTs and Ag@Pt/MWCNTs-CeO ₂ prepared in Example 3. a is the cyclic voltammetry curve of Ag@Pt/MWCNTs, b is the cyclic voltammetry curve of Ag@Pt/MWCNTs-CeO ₂ .

Figure 4 is the polarization curves of Ag@Pt/MWCNTs and Ag@Pt/MWCNTs-CeO ₂ prepared in Example 2. a is the polarization curve of Ag@Pt/MWCNTs, b is the polarization curve of Ag@Pt/MWCNTs-CeO ₂ .

Detailed ways

Example 1

A. Disperse 100 mg of silver nitrate, 1.0 g of sodium citrate and 100 mg of multi-walled carbon nanotubes in deionized water, ultrasonically obtain a uniform black suspension for 1 h, then add 10 ml of 100 mmol/L sodium borohydride/ Ethanol solution to obtain Ag/MWCNTs; disperse the prepared Ag/MWCNTs in 20ml ethylene glycol solution, ultrasonically disperse for 1h, add 2ml chloroplatinic acid solution, adjust pH=5 with 2% KOH/ethylene glycol solution , heated to 60°C for 3h to obtain Ag@Pt/MWCNTs.

B. Adjust the pH of 0.1mmol/L cerium nitrate solution to 7 with 0.1mol L ^-1 ammonia solution, pour the solution into a high temperature and high pressure reactor, and react at 60°C for 2h, then at 110°C for 3h. Then it was filtered, washed and dried to obtain CeO ₂ .

C. Add the Ag@Pt/MWCNTs and CeO ₂ prepared in step A to the ethanol solution at a mass ratio of 4:1, ultrasonically disperse for 1 h, then filter with suction, and dry in a vacuum oven at 50°C for 5 h to obtain Ag@ Pt/MWCNTs-CeO ₂ composite catalyst.

Example 2

A. Disperse 150mg of silver nitrate, 2.0g of sodium citrate and 120mg of multi-walled carbon nanotubes in deionized water, ultrasonically obtain a uniform black suspension for 2h, then add 20ml of 200mmol/L sodium borohydride/ Ethanol solution to obtain Ag/MWCNTs; disperse the prepared Ag/MWCNTs in 60ml ethylene glycol solution, ultrasonically disperse for 2h, add 5ml chloroplatinic acid solution, adjust pH=8 with 7% KOH/ethylene glycol solution , heated to 90°C for 4h to obtain Ag@Pt/MWCNTs.

B. Adjust 0.5mmol/L cerium nitrate solution to pH=9 with 0.1mol L ^-1 ammonia solution, pour the solution into a high-temperature and high-pressure reactor, and react at 70°C for 4h, then at 130°C for 4h. Then it was filtered, washed and dried to obtain CeO ₂ .

C. Add the Ag@Pt/MWCNTs and CeO2 prepared in step A to the ethanol solution at a mass ratio of 5:1, ultrasonically disperse for 2 hours, then filter with suction, and dry in a vacuum oven at 60°C for 10 hours to obtain Ag@Pt /MWCNTs-CeO ₂ composite catalyst.

The Ag/MWCNTs obtained in step A and the Ag@Pt/MWCNTs-CeO ₂ obtained in step C were compared for electrochemical performance by cyclic voltammetry

Glassy carbon electrode pretreatment: 5mgAg/MWCNTs and Ag@Pt/MWCNTs-CeO ₂ were made into solutions with 0.9ml absolute ethanol and 0.1ml5% Nafion solution respectively, and then ultrasonicated in an ultrasonic cleaner for 1h to disperse the catalyst evenly in the In the mixed solution; use a pipette gun to pipette 10 μl of the catalyst solution on the surface of the glassy carbon electrode, and dry it at room temperature.

The test was carried out in a three-electrode system, using the above-mentioned glassy carbon electrode with catalyst on the surface as the working electrode (d=5mm), the reference electrode was Ag/AgCl electrode, the counter electrode was platinum wire, and 0.5mol/L H ₂ SO ₄ solution as electrolyte. By integrating the adsorption peak or desorption peak of hydrogen in the obtained cyclic voltammetry curve, the electrochemically active surface area (ESA) of the catalytic reaction of the catalyst is calculated, and the formula is as follows:

ESA＝ _QH /(2.1×Pt)

In the formula, Q _H (C·m ^-2 ) is the amount of hydrogen desorbed per square meter of platinum surface, and Pt (g·m ^-2 ) is the content of Pt in the catalyst covering the glassy carbon electrode. The unit of ESA is m ² /g, which is one of the important indicators to measure the performance of the catalyst. The results are shown in Figure 3,

It can be seen from Figure 3 that the catalytic activity of Ag@Pt/MWCNTs-CeO ₂ is significantly higher than that of Ag@Pt/MWCNTs, and the electrochemical active area of Ag@Pt/MWCNTs-CeO ₂ can reach 97.2m ² ·g ^-1 . The electrochemically active area of @Pt/MWCNTs is 66.15m ² ·g ^-1

Example 3

A. Disperse 200mg of silver nitrate, 3.6g of sodium citrate and 300mg of multi-walled carbon nanotubes in deionized water, ultrasonically obtain a uniform black suspension for 3h, then add 40ml of 300mmol/L sodium borohydride/ Ethanol solution to obtain Ag/MWCNTs; disperse the prepared Ag/MWCNTs in 100ml ethylene glycol solution, ultrasonically disperse for 4h, add 10ml chloroplatinic acid solution, adjust pH=10 with 10% KOH/ethylene glycol solution , the temperature was raised to 120°C for 6 h to obtain Ag@Pt/MWCNTs.

B. Adjust the 1.0mmol/L cerium nitrate solution to pH=11 with 0.1mol L ^-1 sodium hydroxide solution, pour the solution into a high-temperature and high-pressure reactor, first react at 90°C for 6h, and then react at 160°C for 6h . Then it was filtered, washed and dried to obtain CeO ₂ .

C. Add the Ag@Pt/MWCNTs and _CeO2 prepared in step A to the ethanol solution at a mass ratio of 8:1, ultrasonically disperse for 4 hours, then filter with suction, and dry in a vacuum oven at 80°C for 15 hours to obtain Ag@ Pt/MWCNTs-CeO ₂ composite catalyst.

Adopt the linear voltammetry scanning method to carry out electrochemical performance contrast test to the sample that steps A, C obtain

Glassy carbon electrode pretreatment is the same as cyclic voltammetry.

The test was carried out in a three-electrode system, using the above-mentioned glassy carbon electrode with catalyst on the surface as the working electrode (d=5mm), the reference electrode was an Ag/AgCl electrode, and the counter electrode was a _platinum wire. _4. Continuously inject oxygen into the solution. After the concentration of oxygen in the solution reaches saturation, start the linear voltammetry scanning test, and keep the supply of oxygen during the test. The scanning speed is 5mV/s, the test voltage range is -0.2-0.8V, and the rotation speed of the rotating disk is 1600rpm/min. The test results are shown in Figure 4

It can be seen from Fig. 4 that the oxygen reduction onset potential of Ag@Pt/MWCNTs-CeO ₂ is positively shifted by 40mV relative to Ag@Pt/MWCNTs. The limiting current density of the Ag@Pt/MWCNTs-CeO ₂ curve is significantly higher than that of Ag@Pt/MWCNTs. The catalyst doped with _CeO2 has a significantly high limiting current, indicating that in the presence of ceria, the local oxygen concentration is increased. During the oxygen reduction process, cerium oxide as the supply source of O maintains a high reaction rate.

Claims (3)

1. a preparation method for oxygen reduction catalyst, concrete preparation process is as follows:

A is dispersed in silver nitrate, natrium citricum and multi-walled carbon nano-tubes in deionized water, to prepare suspension A, making the wherein molar concentration of silver nitrate is 1-6mmol/L, the molar concentration of natrium citricum is 20-50mmol/L, the mass concentration of multi-walled carbon nano-tubes is 1-3g/L, sodium borohydride/ethanolic solution that dropping and suspension A volume ratio are 1:30-60 wherein again, filter, dry, obtain Ag/MWCNTs; In sodium borohydride/ethanolic solution, the content of sodium borohydride is 100-300mmol/L;

The Ag/MWCNTs making is dispersed in and in ethylene glycol solution, prepares suspension B by 1-3g/L, ultrasonic dispersion 1-4h, add the platinum acid chloride solution that accounts for suspension B volume parts 0.5-1%, with the KOH/ ethylene glycol solution of 2-10%, regulate pH=5-10 again, be warming up to 60-120 ℃ of reaction 3-6h, filter, dry, obtain Ag@Pt/MWCNTs;

B. the cerous nitrate solution that is 0.1-1.0mmol/L by concentration 0.1mol L ^-1ammoniacal liquor or 0.1mol L ^-1sodium hydroxide solution regulates pH=7-11, adds in high-temperature high-pressure reaction kettle, and first at 60-90 ℃, reaction 2-6h, then at 110-160 ℃ of reaction 3-6h, after filtration, washing, the dry CeO that obtains ₂;

C. the CeO that Ag@Pt/MWCNTs steps A being made and step B obtain ₂mass ratio with 4-8:1 joins in ethanolic solution, and ultrasonic dispersion 1-4h filters, and in the vacuum drying chamber of 50-80 ℃, dry 5-15h, obtains Ag@Pt/MWCNTs-CeO ₂composite catalyst.

2. the oxygen reduction catalyst that prepared by method according to claim 1, is expressed as: Ag Pt/MWCNTs-CeO ₂, be to take CNT as carrier, CeO ₂the catalyst that the hud typed Ag@Pt of doping is active component, this catalyst is CeO ₂uniform Doped is in Ag@Pt/MWCNTs.

3. the application of an oxygen reduction catalyst claimed in claim 2, this catalyst is applied to fuel battery cathode with proton exchange film catalyst, the catalytic activity of its catalyst improves 50-70% than Ag@Pt/MWCNTs, and its electrochemical surface area reaches 67.1-100.0m ²g ^-1, the current density of catalytic oxidation-reduction reaches 4.5-7.0mAcm ^-2.