CN101455970B - Preparation method of carbon supported core-shell Ni-Pt particles for direct methanol fuel cells - Google Patents

Abstract

The invention discloses a method for preparing carbon-loaded core-shell Ni-Pt particles used as catalysts for direct methanol fuel cells, which belongs to the preparation process of catalysts for direct methanol fuel cells. The method is to use sodium citrate as a stabilizer, cationic surfactant CTAB as a dispersant, reduce nickel acetate with sodium hypophosphite, and generate Ni nuclei on the surface of Vulcan XC-72 or mesoporous carbon treated with sodium borohydride; washing After excess sodium hypophosphite, a Pt shell was formed on the surface of the Ni core by chemical replacement. The catalyst has a Ni-core@Pt-shell structure with low Pt loading and high catalytic activity.

Description

Preparation method of carbon-supported core-shell Ni-Pt particles for direct methanol fuel cell catalysts

technical field

The invention relates to a method for preparing carbon-loaded core-shell Ni-Pt particles used as catalysts for direct methanol fuel cells, which belongs to the preparation process of catalysts for direct methanol fuel cells. the

technical background

Direct methanol fuel cells (DMFC) often use carbon-supported platinum as a catalyst for the electrochemical reaction of the electrode. In the early days, the Pt/C catalyst developed by E-TEK in the United States was widely used in commercial production due to its excellent catalytic performance. However, as the resource shortage of precious metal Pt increases and the price is high, the application of Pt in fuel cells has to be reduced; on the other hand, the intermediate products of methanol oxidation, such as CO, are easy to poison Pt catalysts. Therefore, reducing the amount of Pt and researching and developing anode catalysts with high catalytic activity and resistance to CO poisoning is a very important topic. At present, most of the DMFC anode catalysts are Pt-based binary or ternary alloy catalysts. Many research groups have reported Pt-based composite catalysts with comparable performance to Pt/C (E-TEK), such as Pt-Ru, Pt- Mn, Pt-Cu, Pt-Ni, Pt-Co, Pt-Cr and Pt-V, etc., have a good effect on improving the electrocatalytic activity of methanol oxidation and preventing poisoning caused by methanol oxidation intermediate products. However, usually the Pt content in the Pt alloy even exceeds 80%, and the high loading of Pt will also hinder the realization of commercial production. the

Compared with traditional alloy components, core-shell nanoparticles have special electronic structure and surface properties, which not only maintain the physical and chemical properties of the original metal core, but also have excellent metal properties of the cladding layer, so they are widely used in catalysis and other fields. applications are receiving increasing attention. For noble metals such as Pt and Pd, preparing them as shells with non-noble metals as the core can also significantly improve the utilization of noble metals. R.Adzic group (J.Zhang, FHBLima, MHShao, K.Sasaki, JXWang, J.Hanson, RRAdzic, Platinum monolayer on nonnoble metal-noble metal core-shellnanoparticle electrocatalysts for O ₂ reduction.J.Phys.Chem.B, 2005, 109(48): 22701-22704) firstly reported the synthesis of core-shell electrocatalysts by electrochemical method, first UPD-Cu/M was formed by underpotential deposition, and then chemically replaced by Pt, Pd, etc. to form Pt-based Core-shell binary alloy catalyst. However, due to the lack of effective dispersants and stabilizers, the replacement process reacts quickly, the size of Pt particles is not uniform, it is difficult to control its particle size, and it is difficult to prepare highly dispersed nanoparticles as electrocatalysts; The particles use underpotential deposition, which makes the core metal limited. In addition, the electrochemical method has the disadvantage of high energy consumption, so this method is difficult to be used in large-scale commercial production.

Contents of the invention

The purpose of the present invention is to propose a simple method for preparing Pt-based core-shell nanoparticles, and to prepare a catalyst material for direct methanol fuel cells with low Pt loading and high catalytic activity. the

The present invention comprises the following steps: (1), carbon powder and NaBH ₄ are added into distilled water, magnetically stirred, centrifuged, vacuum-dried, and above-mentioned carbon powder is Vulcan XC-72 or mesoporous carbon; (2), pretreatment Add charcoal powder to deionized water and isopropanol, stir magnetically, add nickel acetate, sodium citrate and cetyltrimethylammonium bromide (CTAB) in sequence, stir and heat up to 70-95°C, slowly add Sodium phosphite, react for 1~5h, wherein the molar ratio of nickel acetate, sodium hypophosphite, sodium citrate and CTAB is 1:3~5:0.6~1:0.15~0.2, after cooling to room temperature, centrifuge , fully washed to remove excess sodium hypophosphite; (3), disperse the centrifuged product with water and isopropanol, add 5 mg of CTAB, heat up to 40-60 ° C, and adjust the pH value to 5-6 with HCl (4), add H ₂ PtCl ₆ solution, the ratio of Pt and Ni is 1-5:1, the replacement reaction time is 1-5h, cool to room temperature, wash and centrifuge sufficiently, and vacuum-dry.

The invention seeks a solution to improve the performance of the direct methanol fuel cell catalyst material from three aspects. First, the present invention uses Vulcan XC-72 or mesoporous carbon as the catalyst carrier. This material has a high specific surface area and porosity, and is cheap. Compared with other carbon materials, it is more conducive to meeting the needs of large-scale commercial production. Secondly, the present invention uses the chemical plating method to generate uniform and dispersed Ni nanoparticles on the surface of the carbon carrier as core particles, and then directly generates a Pt shell on the surface of the Ni core by a simple chemical replacement method, in order to significantly While reducing the amount of Pt, through the special electronic characteristics, surface properties and dual-functional synergy between the core and the shell, it has a larger electrochemical active area, thereby improving the catalytic performance of the catalyst. Furthermore, the present invention is easy to screen the core-shell ratio with optimal catalytic performance by adjusting the content of core metal Ni and shell Pt. the

This method prepares a Pt catalyst with a core-shell structure, the experimental process is simple, no additional reducing agent is required, and the shell layer is deposited spontaneously; and the cationic surfactant CTAB is used to effectively control the particle size during the replacement process, and a uniform size is obtained. Small particles are conducive to the improvement of the electrochemical performance of the material; while significantly reducing the amount of noble metal Pt, the catalytic performance is improved. In addition, the present invention combines nanomaterial preparation technology with the development and application of electrode materials to prepare Pt catalyst materials with better dispersion and higher catalytic activity. Research and development have played a positive role in promoting. the

Description of drawings

Fig. 1 is the XRD pattern of the prepared Ni/C (1:0), Ni@Pt/C (3:1) and Pt/C (E-TEK). the

Figure 2 is based on the characteristic diffraction peaks of Pt in Figure 1, showing the average particle size of different particles and the angle of diffraction peaks of different crystal planes. the

Fig. 3 is a TEM photo of Ni@Pt/C (3:1) prepared in the present invention. the

Figure 4 is the UV absorption spectrum of H ₂ PtCl ₆ solution, Ni(Ac) ₂ solution, pure Pt and core-shell Ni@Pt (3:1) sol: (a) Ni ion complexes, (b) Ni@Pt (3:1) sol.

Fig. 5 is the cyclic voltammetry comparison of Ni@Pt/C (3:1), Ni/C (1:0) and Pt/C (0:1) catalysts prepared in the present invention in 0.5mol/ _LNaClO solution Figure, the scan speed is 50mV/s, 25℃.

Figure 6 shows the Ni@Pt/C (1:1), Ni@Pt/C (3:1) and Ni@Pt/C (5:1) catalysts prepared by the present invention and the Pt/ Comparison of cyclic voltammetry curves of catalyst C in 0.5mol/L H ₂ SO ₄ solution, the scan speed is 20mV/s, 25°C.

Fig. 7 shows Ni/C (1:0), Ni@Pt/C (3:1), Pt/C (0:1) and commercial Pt/C (E-TEK) catalyst prepared in the present invention at 0.5mol/ Comparison of cyclic voltammetry curves in L H ₂ SO ₄ solution, with a scan rate of 20mV/s and 25°C.

Figure 8 shows the electrochemically active areas of different catalysts based on the hydrogen absorption and desorption peaks of Pt in Figure 7, after deducting the influence of the electric double layer current. the

Figure 9 shows the cycle of the Ni@Pt/C (3:1) catalyst prepared by the present invention and the commercial E-TEK company's Pt/C catalyst in 0.5mol/L H ₂ SO ₄ +1.0mol/L CH ₃ OH solution Comparison chart of voltammetry curves, the scan speed is 20mV/s, 25℃.

Figure 10 is the characteristic parameters given according to the double peaks of methanol oxidation in Figure 9 . the

Detailed ways

Specific embodiment one:

(1) Take 40mg of XC-72 treated with sodium borohydride, add 20ml of deionized water and 20ml of isopropanol in turn, and sonicate for 10min. the

(2) Under magnetic stirring at 60° C., 0.1 mol/L, 1.5 ml of Ni(Ac) ₂ , 0.1 mol/L, 1.0 ml of sodium citrate and 10 mg of CTAB were sequentially added.

(3) Stir and heat up to 80°C, slowly add 0.1mol/L, 4.5ml of sodium hypophosphite, and react for 1 hour. Cool to room temperature, centrifuge, and wash thoroughly with distilled water and acetone four times to remove excess sodium hypophosphite. The obtained catalyst is denoted as Ni@Pt/C(1:0). the

According to the cyclic voltammetry test, the carbon-supported core-shell platinum catalyst material Ni@Pt/C(1:0) for DMFC of the present invention has almost no catalytic activity for hydrogen oxidation, that is, it exhibits the properties of pure Ni. the

Specific embodiment two:

(1) Take 40mg of XC-72 treated with sodium borohydride, add 20ml of deionized water and 20ml of isopropanol in turn, and sonicate for 10min. the

(2) Under magnetic stirring at 60° C., 0.038 mol/L, 0.6 ml of H ₂ PtCl ₆ , 0.1 mol/L, 1.0 ml of sodium citrate and 10 mg of CTAB were sequentially added.

(3) Stir and heat up to 80°C, slowly add 0.1mol/L, 4.5ml of sodium hypophosphite, and react for 1 hour. Cool to room temperature, centrifuge, and wash thoroughly with distilled water and acetone four times to remove excess sodium hypophosphite. The obtained catalyst is denoted as Ni@Pt/C(0:1). the

Through the test of cyclic voltammetry, the carbon-supported core-shell platinum catalyst material Ni@Pt/C(0:1) for DMFC of the present invention hardly exhibits electrochemical catalytic activity, and only exhibits the capacitive performance of XC-72, indicating that Pt is generated by a reaction of replacing Ni. the

Specific embodiment three:

(1) Take 40mg of XC-72 treated with sodium borohydride, add 20ml of deionized water and 20ml of isopropanol in turn, and sonicate for 10min. the

(2) Under magnetic stirring at 60° C., 0.1 mol/L, 1.5 ml of Ni(Ac) ₂ , 0.1 mol/L, 1.5 ml of sodium citrate and 10 mg of CTAB were sequentially added.

(3) Stir and heat up to 95°C, slowly add 0.1mol/L, 7.5ml sodium hypophosphite, and react for 5 hours. Cool to room temperature, centrifuge, and wash thoroughly with distilled water and acetone four times to remove excess sodium hypophosphite. the

(4) Disperse the centrifuged product with water and isopropanol, add 5 mg of CTAB, heat up to 60° C., and stir for 10 min. the

(5) Adjust the pH value to 5-6 with 0.2mol/L HCl. the

(6) Add 0.038mol/L, 0.35ml H ₂ PtCl ₆ solution for replacement, and the reaction time is 5h. Cool to room temperature, centrifuge, wash with acetone, and dry under vacuum at 80°C for 12h. The obtained catalyst is denoted as Ni@Pt/C(5:1).

Tested by cyclic voltammetry, DMFC of the present invention uses carbon-loaded core-shell type platinum catalyst material Ni@Pt/C (5:1), and its catalytic activity is slightly higher than the Pt/C commercial catalyst of E-TEK company (Natick, MA. 20% Pt supported on Vulcan XC-72, http://www etek-inc com/home.php ).

Specific embodiment four:

(1) Take 40mg of XC-72 treated with sodium borohydride, add 20ml of deionized water and 20ml of isopropanol in turn, and sonicate for 10min. the

(2) Under magnetic stirring at 60° C., 0.1 mol/L, 1.5 ml of Ni(Ac) ₂ , 0.1 mol/L, 1.0 ml of sodium citrate and 10 mg of CTAB were sequentially added.

(3) Stir and heat up to 80°C, slowly add 0.1mol/L, 4.5ml of sodium hypophosphite, and react for 3 hours. Cool to room temperature, centrifuge, and wash thoroughly with distilled water and acetone four times to remove excess sodium hypophosphite. the

(4) Disperse the centrifuged product with water and isopropanol, add 5 mg of CTAB, heat up to 50° C., and stir for 10 min. the

(5) Adjust the pH value to 5-6 with 0.2mol/L HCl. the

(6) Add 0.038 mol/L, 0.6 ml H ₂ PtCl ₆ solution for replacement, and the reaction time is 3 h. Cool to room temperature, centrifuge, wash with acetone, and dry under vacuum at 80°C for 12h. The resulting catalysts were sequentially Ni@Pt/C (3:1).

Through cyclic voltammetry tests, the carbon-supported core-shell platinum catalyst material Ni@Pt/C (3:1) for DMFC of the present invention not only has significantly higher catalytic activity than Ni@Pt/C (5:1), but also has Greater catalytic activity than E-TEK's Pt/C commercial catalyst (Natick, MA. 20% Pt supported on Vulcan XC-72), with an electrochemical active area of 100.1m at a 60% reduction in Pt loading ² g ^-1 , which is 1.2 times that of Pt/C (E-TEK); it has excellent catalytic performance for methanol oxidation, and its catalytic activity is 3.5 times that of Pt/C (E-TEK), which has great application prospects .

Specific embodiment five:

(1) Take 40mg of XC-72 treated with sodium borohydride, add 20ml of deionized water and 20ml of isopropanol in turn, and sonicate for 10min.

(2) Under magnetic stirring at 60° C., 0.1 mol/L, 1.5 ml of Ni(Ac) ₂ , 0.1 mol/L, 1.0 ml of sodium citrate and 8 mg of CTAB were sequentially added.

(3) Stir and heat up to 70°C, slowly add 0.1mol/L, 4.5ml sodium hypophosphite, and react for 1 hour. Cool to room temperature, centrifuge, and wash thoroughly with distilled water and acetone four times to remove excess sodium hypophosphite. the

(4) Disperse the centrifuged product with water and isopropanol, add 5 mg of CTAB, heat up to 40° C., and stir for 10 min. the

(5) Adjust the pH value to 5-6 with 0.2mol/L HCl. the

(6) Add 0.038 mol/L, 0.9 ml of H ₂ PtCl ₆ solution for replacement, and the reaction time is 1 h. Cool to room temperature, centrifuge, wash with acetone, and dry under vacuum at 80°C for 12h. The obtained catalyst is denoted as Ni@Pt/C (1:1).

Tested by cyclic voltammetry, the carbon-supported core-shell platinum catalyst material Ni@Pt/C (1:1) for DMFC of the present invention has a higher catalytic activity than Ni@Pt/C (5:1) and Ni@Pt /C (3:1) decreased significantly, slightly lower than E-TEK's Pt/C commercial catalyst (Natick, MA. 20% Pt supported on Vulcan XC-72), indicating that the ratio between core and shell has reached the upper limit at this time. If the shell is too thick, the special electronic benefit between the core and the shell will be weakened, and the catalytic activity will decrease. the

Figure 1 shows that compared with Pt/C, the (111), (200), (220), and (311) characteristic diffraction peaks of Ni@Pt/C are shifted to the large angle direction, and the degree of shift is shown in Figure 2. This suggests that it forms a binary bimetallic structure because the inner Ni core supports the Pt shell, which induces changes in the Pt lattice parameters. Comparing the diffraction peaks of Ni@Pt/C and Ni/C, it can be seen that there are no obvious Ni or Ni oxide characteristic peaks on Ni@Pt/C, indicating that Pt is completely coated on the surface of Ni core. In Figure 1, the diffraction peak of Ni@Pt/C is broad, which means that the particle size is small. According to the Scherrer formula, the average particle size of Ni@Pt particles calculated from the (220) diffraction peak is 3.2nm, commercial E-TEK's Pt average particle size is 3.4nm. the

It can be seen from Figure 4 that Ni(Ac) ₂ presents an absorption peak at 350nm. After reduction, the absorption peak at 350nm still exists, but shifts to the high-wave band direction. Due to the formed complex, since pure Ni has no absorption peak, it means that there is no excess reducing agent in the solution. H ₂ PtCl ₆ has an obvious absorption peak at around 260nm, but pure Pt has no absorption peak. After adding H ₂ PtCl ₆ , the absorption peak at 260nm disappears, and the surface Pt forms a shell on the surface of the Ni core through a substitution reaction.

As can be seen in Figure 5, compared with Ni/C, the characteristic peaks of Ni on Ni@Pt/C disappear, indicating that the Ni core is completely covered by the Pt shell; Similar oxidation/reduction peaks indicate that Ni@Pt exhibits typical electrochemical characteristics of pure Pt surfaces. However, the characteristic peaks of Ni@Pt/C are still different from those of Pt/C, and shifted to the high potential region respectively, which indicates that the formed Pt shell is different from the pure Pt surface, probably because the core Ni supports Because of the shell Pt. the

It can be seen from Fig. 7 that Ni/C has no catalytic activity, and Pt/C has a small catalytic activity; the forward voltage in the figure is about 0.5V, which shows the reduction peak of the oxide species adsorbed on the Pt nanoparticles. Compared with pure Pt, the core-shell Ni@Pt particles not only have a higher hydrogen oxidation peak, but also a positive shift of the oxide reduction peak by about 50 mV, indicating that the adsorption energy of oxygen-containing species is reduced. Correspondingly, the oxygen reduction on Ni@Pt The activity is significantly enhanced, which may be the nature of the Ni-supported Pt surface monolayer, due to the effect of the Ni core shrinking the shell Pt lattice and so on. the

The ratio of the forward scanning peak current to the reverse scanning peak current ( _if /i _b ) and the starting voltage in FIG. 9 were used to characterize the performance of the catalyst for methanol oxidation. The parameters are shown in FIG. 10 . The start-up voltage of Ni@Pt/C is smaller than that of Pt/C(E-TEK), which is shifted to the low potential region by 5mV, indicating that its reactivity is greatly improved; while Ni@Pt/C has a larger i _f /i _{The b} value indicates that Ni@Pt/C has a stronger ability to resist CO poisoning than Pt/C (E-TEK), and the calculated catalytic activity of Ni@Pt/C per unit loading is that of Pt/C from the commercial E-TEK company. 3.5 times, showing excellent catalytic performance for methanol.

Claims (2)

1. A method for preparing a carbon-supported core-shell type Ni-Pt particle for direct methanol fuel cell catalysts, characterized in that it comprises the following steps: (1), carbon powder and NaBH ₄ were added to distilled water, magnetically stirred, centrifuged, and vacuum-dried, and the above-mentioned carbon powder was Vulcan XC-72 or mesoporous carbon; (2), pre-treated charcoal powder is added to deionized water and isopropanol, magnetically stirred, nickel acetate, sodium citrate and cetyltrimethylammonium bromide (CTAB) are added successively, and the temperature is raised to 70 ~95°C, slowly add sodium hypophosphite, react for 1~5h, wherein the molar ratio of nickel acetate, sodium hypophosphite, sodium citrate and CTAB is 1:3~5:0.6~1:0.15~0.2, After cooling to room temperature, centrifuge and wash thoroughly to remove excess sodium hypophosphite; (3) Disperse the centrifuged product with water and isopropanol, add 5 mg of CTAB, heat up to 40-60°C, and adjust the pH value to 5-6 with HCl; (4) Add H ₂ PtCl ₆ solution, the ratio of the amount of Pt to Ni is 1-5:1, the replacement reaction time is 1-5h, cool to room temperature, fully wash and centrifuge, and then vacuum-dry. 2. the preparation method of the carbon-loaded core-shell type Ni-Pt particle that is used for direct methanol fuel cell catalyst according to claim 1, it is characterized in that: the amount of substance of Pt and Ni described in the (4) step The ratio is 3:1.

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