Chlorine-free high-water-solubility Pt (II) complex and application thereof in preparation of Pt / C electrocatalyst

Abstract

The invention relates to a Pt (II) complex, namely tetrabromo platinous acid ethanol ammonium salt, the molecular formula is (HO-CH2-CH2-NH3) 2 [PtBr4], four Br <-> and Pt (II) ligands form four-coordinated planar square ligand anions, and ethanol ammonium cations are located outside the coordination and play a role in charge neutralization. The synthesis method comprises the following steps: dissolving a commercially available Pt (IV) compound-hexahydroxyplatinic acid serving as an initial raw material in hydrobromic acid, then adding ethanolamine (HO-CH2-CH2-NH2) for quantitative neutralization, and finally adopting hydrazine hydrobromide (N2H4HBr) for controlled reduction to reduce the Pt (IV) into Pt (II), thereby obtaining the target compound. The complex is high in water solubility, does not absorb moisture, does not contain chlorine and other components harmful to the Pt / C electrocatalyst, and replaces chloroplatinic acid to be used for preparing the Pt / C electrocatalyst of a fuel cell, and the durability of the obtained Pt / C electrocatalyst is obviously improved. Therefore, the (HO-CH2-CH2-NH3) 2 [PtBr4] has a good application prospect in the aspect of preparation of the Pt / C electrocatalyst.

Description

Technical Field

[0001] The present invention belongs to the field of chemical catalysis and relates to a chlorine-free, highly water-soluble Pt(II) complex (HO-CH 2 -CH 2 -NH 3 ) 2 [PtBr 4 ] and its application in the preparation of Pt / C electrocatalysts for fuel cells. Background Art

[0002] Fuel cells and electrolytic hydrogen production technologies are key technologies for realizing the mutual conversion of hydrogen energy and electrical energy. Electrocatalysts, especially Pt / C electrocatalysts, are one of the cores of these two key technologies. The development of efficient and highly stable Pt / C electrocatalysts plays an important role in promoting the development of this industry. Pt / C electrocatalysts are high-loaded catalysts with a platinum loading of up to 20-60%. The current mainstream preparation technology is chemical impregnation, and the main steps and processes include modified activated carbon, liquid phase loading, and controlled reduction. Liquid phase loading is to immerse the modified activated carbon in an aqueous solution containing a platinum compound (catalytic precursor) to load the active dispersed onto the activated carbon. Liquid phase loading is an important step in controlling the performance of Pt / C electrocatalysts. Since a high amount of Pt, the active component loaded, is required, highly water-soluble chloroplatinic acid H is commonly used. 2 PtCl 6 Many studies and experiments have shown that the use of chlorine-containing catalytic precursors will result in residual chloride ions in the final catalyst product. Even trace amounts of chloride ions will significantly reduce the durability of the Pt / C catalyst and affect the catalyst life [1. Wang Weiduo, Chen Xintuo. - and SO 4 2-Effect of impurity ions on the performance of fuel cell catalysts. Biochemical Engineering, 2019, 5, 55-58; 2. Ludeck K., Mikkel JL, Ignacio JM, _et al. Synthesis of Pt / C fuel cell electrocatalysts: residual content of chloride and activity in oxygen reduction. Electrocatalysis, 2016, 7, 269-275. 3. Li H, Zhang S, Qian W, Yu Y, et al. Impacts of operating conditions on the effects of chloride contamination on PEM fuel cell performance and durability. J Power Sources, 2012, 218, 375–382].

[0003] To this end, the "Mass Preparation Technology of Automotive Fuel Cell Catalysts" guideline puts forward specific requirements for the low cost, durability, consistency and impurity tolerance of the fuel cell catalyst Pt / C. Among them, the Cl content in the catalyst product is required to be less than 50ppm, the Fe content is less than 50ppm, and the total content of other impurities is less than 500ppm.

[0004] The carrier of Pt / C catalyst generally uses Vulcan-XC72 activated carbon produced by Cobat Company in the United States, which has a very fine particle size of about 30nm. Chloroplatinic acid is an orange-red crystal, generally contains 6 crystal waters, is easily soluble in water, and has a room temperature solubility of more than 50gPt / L, which is the best solubility among known platinum compounds. It is very suitable for the preparation of high-load battery catalyst Pt / C. However, the molecular formula of chloroplatinic acid is H 2 PtCl 6 Platinum is tetravalent, and the ratio of platinum to chlorine is 1 / 6. The number of chlorine atoms is significantly higher than that of platinum atoms. When chloroplatinic acid is used as a catalytic precursor and Pt / C electrocatalyst is prepared by chemical impregnation, it is difficult to elute most of the chloride ions from the activated carbon in the washing process, so that the chloride ion content is less than 50ppm. At the same time, chloroplatinic acid is very easy to absorb moisture and has high acidity, which is very inconvenient to use. Therefore, we believe that developing a platinum catalytic precursor compound that is both highly water-soluble and chlorine-free for the preparation of Pt / C electrocatalyst is one of the most effective methods to control the chlorine content in the catalyst.

[0005] Although a large number of studies and experiments have shown that trace amounts of residual chloride ions in Pt / C electrocatalysts will significantly reduce the durability of the catalyst, for bromine in the same family, in September 2024, the team of Academician Sun Shigang of Xiamen University reported in the authoritative journal [Nature Communications DOI:10.1038 / s41467-024-51858-w] that trace amounts of bromine can help improve the performance of Fe-NC electrocatalysts. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the insufficiency of the existing chloroplatinic acid as an electrocatalyst precursor, and to provide a chlorine-free, highly water-soluble platinum complex to replace chloroplatinic acid for use in the preparation of Pt / C electrocatalysts to improve the durability and life of the catalyst. The compound of the present invention is tetrabromoplatinic acid ethanolammonium salt, and the molecular formula is (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ] Among them, Pt is divalent and coordinates with four bromide ions to form a divalent complex anion [PtBr 4 ] 2- ; ethanolammonium ion HO-CH 2 CH 2 -NH 3 + It is outside the coordination structure and plays a role in charge neutralization.

[0007] The compound of the present invention, ethanolammonium tetrabromoplatinite, is a brown crystalline powder, is stable in the air and has no hygroscopicity, has high water solubility, and reaches about 100 g / L at room temperature (equivalent to 30 gPt / L, which fully meets the requirements of the preparation of Pt / C electrocatalyst for catalytic precursors), and has a pH value of about 3 to 4 in a 10% aqueous solution, thus overcoming the disadvantages of chloroplatinic acid's easy hygroscopicity and high acidity.

[0008] Tetrabromoplatinic acid ethanolammonium salt uses organic ethanolammonium salt as cation, which not only does not contain chlorine, but also does not contain other elements harmful to catalysis (such as P, S, Na, K, Ca, Mg, etc.). P and S elements have a strong bond with platinum, which will cause the catalyst to be poisoned and deactivated; Na + , K + , Ca 2+ Mg 2+At higher voltages, ion migration will occur, causing agglomeration of active metals and affecting the durability of the catalyst [1. Xu Yue. Catalyst Design and Preparation Process. Chemical Industry Press, 2003; 2. Reyes P, Pecchi G, Morales M. Applied Catalysis A General, 1997, 163 (1): 145-152; 3. Uddin M, Wang X, Qi J, et al. Effect of chloride on PEFCs in presence of variouscations. J Electrochem Soc, 2015, 162, 373–379. 4. Anusree U., Vinod MJ, Rajalakshmi N., et al. Chlorine-contaminated anode and cathode PEMFC-recovery perspective. J Solid State Electrochem. 2018, 22, 2107-2113].

[0009] The chemical name of ethanolamine is 2-hydroxyethylamine, which is a cheap organic chemical reagent. In the molecule, the electron induction effect of the hydroxyl group significantly reduces the basicity and coordination ability of the amino group, which is less than that of ammonia NH 3 , will not compete with bromide ions for coordination with platinum, and is an ideal organic base, commonly used as a pH regulator in the preparation process of precious metal catalysts. 2 CH 2 -NH 3 ) 2 [PtBr 4 ] cationic part, which makes the whole compound molecule have high solubility, and its water solubility is significantly higher than that of other Pt(Ⅱ) compounds known so far.

[0010] The common and stable valence states of platinum are 0, +2 and +4, of which +2 and +4 platinum exist in the form of coordination compounds in the solution, and free Pt ions do not exist. Pt(Ⅱ) is generally a 4-coordinate, planar square complex, while Pt(Ⅳ) is a 6-coordinate, octahedral complex. Therefore, the molecular space volume of Pt(Ⅱ) compounds is significantly smaller than that of the corresponding Pt(Ⅳ) complex. For Pt / C electrocatalysts, it mainly adsorbs and activates small molecules of hydrogen and oxygen. The molecular volume of hydrogen and oxygen is very small and can easily enter and exit the pores of activated carbon. Therefore, the active component platinum is not only loaded on the surface of activated carbon, but also preferably loaded on the surface of the pores of activated carbon to increase the active sites. For this reason, we chose divalent platinum (HO-CH 2 CH2 -NH 3 ) 2 [PtBr 4 ] as a precursor for the preparation of Pt / C electrocatalysts in order to make the volume smaller (relative to [PtBr 6 ] 2+ For example, the platinum-containing complex anion [PtBr 4 ] 2+ It can easily penetrate the pores of activated carbon and be loaded on the surface of the pores after reduction.

[0011] The inventors have used (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ] and (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 6 ] was used as the catalytic precursor. Two corresponding Pt / C electrocatalysts with a platinum content of 20% were prepared under the same conditions. The preliminary electrochemical activity was tested on an electrochemical workstation to compare the different valence states of platinum coordination anions [PtBr 4 ] 2+ and [PtBr 6 ] 2+ The application performance of the catalytic precursor is shown in Table 1. The application performance of the divalent platinum complex is significantly better than that of the tetravalent platinum complex.

[0012] Table 1 Preliminary characterization results of catalyst electrochemical performance

[0013]

[0014] Based on the above characteristics, the tetrabromoplatinic acid ethanolammonium salt of the present invention is a good catalytic precursor, suitable for the preparation of Pt / C electrocatalyst. Under the same preparation conditions, the tetrabromoplatinic acid ethanolammonium salt is used to replace chloroplatinic acid to obtain a 60% Pt / C electrocatalyst, and the catalytic activity and durability are measured by standard cyclic voltammetry and linear sweep voltammetry. The results show that the decrease rate of its oxygen reduction mass specific activity and electrochemical active area after 30,000 cycles is significantly reduced, and the durability is significantly improved.

[0015] The compound of the present invention, tetrabromoplatinic acid ethanolammonium salt, can be reduced with a mild organic reducing agent (such as methanol, citric acid) to obtain a highly dispersed Pt / C catalyst, similar to chloroplatinic acid. The inventors have synthesized and tested a series of other highly water-soluble chlorine-free platinum complexes, including Pt(II) complexes [Pt(NH 3 ) 4]X 2 and Pt(IV) compounds [Pt(NH 3 ) 4 (OH) 2 ]X 2 , where X = organic carboxylate. Due to the strong coordination between Pt and ammonia, the resulting complex is very stable and difficult to be reduced by a milder organic reducing agent. In the reduction process of preparing Pt / C catalyst, it is often necessary to use a strong reducing agent such as hydrazine hydrate and sodium borohydride at a higher temperature, resulting in an uncontrollable reduction process. The resulting Pt / C catalyst has a platinum particle size greater than 10nm and low catalytic performance. The initial oxygen reduction mass specific activity MA (mA / mgPt) is less than 60, and the electrochemical active area ECSA (m 2 / g) is less than 40.

[0016] The synthesis of tetrabromoplatinic acid ethanolammonium salt of the present invention is based on the commercially available Pt(IV) compound hexahydroxyplatinic acid (hereinafter referred to as hydroxyplatinic acid, H 2 [Pt(OH) 6 ] or PtO 2 ·4H 2 O) as the starting material, dissolved in hydrobromic acid (HBr), and then added with ethanolamine (HO-CH 2 -CH 2 -NH 2 ) is quantitatively neutralized to generate hexabromoplatinic acid ethanolammonium salt ((HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 6 ]), and finally hydrazine hydrobromide (N 2 H 4 HBr) controlled reduction, Pt(IV) was reduced to Pt(II), and the target compound - tetrabromoplatinic acid ethanolammonium salt (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 The present synthesis method has the characteristics of mild reaction conditions, simple operation, easy control, high yield, and can be suitable for batch synthesis with controllable synthesis cost. The chemical reaction route involved is as follows:

[0017] (1)H 2 [Pt(OH) 6 ]+6HBr→H 2 [PtBr 6 ]+6H 2 O

[0018] (2)H 2 [PtBr6 ]+2HO-CH 2 -CH 2 -NH 2 →(HO-CH 2 -CH 2 -NH 3 ) 2 [PtBr 6 ]

[0019] (3)2(HO-CH 2 -CH 2 -NH 3 ) 2 [PtBr 6 ]+N 2 H 4 HBr→2(HO-CH 2 -CH 2 -NH 3 ) 2 [PtBr 4 ]+5HBr+N 2 .

[0020] Compared with the prior art, the Pt(II) complex (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ] has the following characteristics:

[0021] (1) It does not contain chlorine and other elements and components that are harmful to the catalyst (such as sulfur, phosphorus, sodium, potassium, calcium, and magnesium). The volume of the platinum-containing complex anion is smaller than that of the activated carbon carrier, which is conducive to high dispersion on the activated carbon carrier;

[0022] (2) Using the simple organic compound 2-hydroxyethylammonium as the cation, the entire complex molecule has a high water solubility, with a solubility of up to about 100 g / L (equivalent to 30 g Pt / L) in water at room temperature, which is very suitable for the preparation of high-loading (20-60%) Pt / C electrocatalysts;

[0023] (3) It is non-hygroscopic and non-acidic. The pH of a solution containing 10% platinum is greater than 3, making it easy to use.

[0024] (4) It can replace the Pt / C electrocatalyst currently used in the industry as a catalytic precursor using chloroplatinic acid and prepared by chemical impregnation method, and the durability can be significantly extended. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] Figure 1 : CV and LSV diagrams of Pt / C electrocatalysts of comparative example (solid line) and example (dashed line);

[0027] Figure 2 : CV and LSV diagrams of the catalyst before (solid line) and after (dashed line) cycling of the Pt / C electrocatalyst in the example. DETAILED DESCRIPTION

[0028] Example 1: Pt(II) complex (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 Synthesis of

[0029] Weigh 30 g (101 mmol) of hydroxyplatinic acid H 2 [Pt(OH) 6 ] was added slowly to a 500 mL glass reaction bottle at room temperature. 2 mmol) aqueous hydrobromic acid solution (1 mL contains 0.48 g of HBr), and the reaction was stirred until the hydroxyplatinic acid was completely dissolved to form a dark brown H 2 [PtBr 6 ] solution, add 12.3 g (2 × 10 1 mmol)ethanolamineHO-CH 2 CH 2 -NH 2 Neutralization gives (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 6 ] solution.

[0030] Accurately weigh 5.94 g (51.5 mmol, 2% excess) of hydrazine hydrobromide 2 H 4 HBr (content 98%) was dissolved in 100 mL of water and added dropwise to the above (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 6] The solution was controlled to reduce tetravalent platinum to divalent platinum, and stirred while adding dropwise. After the addition was completed, it was slowly heated to boiling in an oil bath for 60 minutes to decompose the excess hydrazine hydrobromide and the generated hydrobromic acid, cooled to room temperature, and filtered through a membrane to remove a small amount of black insoluble matter (precipitated platinum). The mother liquor was concentrated to dryness under reduced pressure, and finally vacuum dried at 65°C for 120 minutes to obtain 62.7 g of a brown crystalline powder product (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ], yield 97%.

[0031] Analysis and test results: <1> Elemental analysis: measured values ​​Pt 30.1%, C 7.47%, H 2.56%, N 4.32% (calculated values ​​Pt 30.5%, C 7.51%, H 2.50%, N 4.38%); <2> IR(cm -1 , KBr): 3413 (s, ν (OH)), 3061 (s, ν (NH)), 285 (m, ν (Pt-Br); <3> λmax(H 2 O, nm): 267 nm (Pt(II), dd electron transition); <4> 1 HNMR (500 MHz, D 2 O) 3.76–3.70 (m, 2H, CH 2 -NH 3 + ), 3.06 (t, J=5.3Hz, 2H, CH 2 -OH); <5> 13 CNMR (126MHz, D 2 O)δ57.13(s,1C,CH 2 -NH 3 + ), 40.81(s, 1C, CH 2 -OH). The above results are consistent with the composition and structural formula of the compound of the present invention: (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ].

[0032] Example 2: Evaluation of the application performance of the Pt(II) complex of the present invention as a Pt / C catalytic precursor

[0033] (1) Main raw materials, reagents and instruments

[0034] Chloroplatinic acid H 2 PtCl 6The complex of the present invention (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 ], VulconXC-72 carbon black (particle size 30nm), Nafion, electrochemical workstation, rotating ring disk electrode, etc.

[0035] (2) Preparation of Pt / C

[0036] Weigh 0.4 g of carbon black, add 50 mL of water, stir for 60 min, then dropwise add 0.6 g of Pt in H 2 PtCl 6 or (HO-CH 2 CH 2 -NH 3 ) 2 [PtBr 4 After immersion for 4 hours, 50 mL of methanol was added to reduce at 80°C for 3 hours, and finally filtered and collected, washed with 20 mL of ionized water for 3 times, and dried at 105°C for 3 hours to obtain 1 gram of platinum carbon catalyst with a platinum loading of 60%, which were marked as comparative example Pt / C catalyst and example Pt / C catalyst, respectively.

[0037] (3) Catalytic activity test method

[0038] The test was carried out according to the Chinese national standard GB / T 20042.4-2009 Proton Exchange Membrane Fuel Cell Part 4 (Methods in Electrocatalyst Test Methods). That is, the catalyst was subjected to cyclic voltammetry (CV) scanning and linear sweep voltammetry (LSV) scanning using an electrochemical workstation and a rotating disk electrode to evaluate its catalytic oxygen reduction performance. The experimental operation process was carried out using a three-electrode system with N 2 Saturated 0.1M perchloric acid solution was used as electrolyte, the water bath temperature was controlled at 30°C, the scanning range was 0.0~1.1V, the catalyst was first activated at a scanning speed of 20mV / s for 20 cycles, and then the scanning rate was changed to 100mV / s for cyclic voltammetry scanning, and the stable curve was recorded. The catalyst CV line was recorded after the 0.1M perchloric acid solution was passed for more than half an hour, in which the potential scanning range was 0.05~1.05V, and the scanning speed (clockwise) was 50mV / s.

[0039] The catalytic oxygen reduction (ORR) performance of the catalyst was evaluated by linear sweep voltammetry (LSV) using an electrochemical workstation and a rotating disk electrode. 2The ORR polarization curve was tested by CV Linearscan program in saturated 0.1M perchloric acid solution, with a temperature of 30°C, a scanning range of 0.02-1.1V, a scanning rate of 10mV / s, ohmic compensation, an RDE speed of 1600rpm, and a scanning cycle of 2-3 cycles. Then, only the solution was replaced with N 2 The saturated 0.1M perchloric acid solution was used for testing to obtain the background signal. The ORR polarization curve was obtained by subtracting the two obtained curves. The forward scanning curve is usually used as the basis for quantitative analysis of ORR performance.

[0040] (4) Test results and conclusions

[0041] The initial CV and LSV of the comparative example Pt / C catalyst and the example Pt / C catalyst are shown in Figure 1 The CV and LSV changes of the Pt / C catalyst after 30,000 cycles are shown in Figure 2 The relevant oxygen reduction mass specific activity MA and electrochemical active area ECSA are listed in Table 2.

[0042] Table 2. Characterization results of electrochemical performance of catalysts

[0043]

[0044] It can be seen from the test data that the initial electrocatalytic activities of the Pt / C catalyst of the embodiment and the Pt / C catalyst of the comparative example are equivalent, but after 30,000 cycles, the MA and ECSA loss rates of the Pt / C catalyst of the comparative example reach 19% and 15%, respectively, which are significantly greater than those of the Pt / C catalyst of the embodiment, indicating that the durability of the Pt / C electrocatalyst prepared using the compound of the present invention as a precursor is significantly improved, and it can replace chloroplatinic acid for the preparation of Pt / C electrocatalysts.

[0045] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A chlorine-free, highly water-soluble Pt(II) complex, namely tetrabromoplatinic acid ethanolammonium salt, characterized in that: The molecular formula is (HO-CH2-CH2-NH3)2[PtBr4], 4 Br - With Pt(II) ligand, it forms a tetracoordinated planar square anion [PtBr4] 2- , ethanolammonium cation HO-CH2-CH2-NH3 + In the external part of the coordination, it plays a role of charge neutralization.

2. The method for preparing the chlorine-free, highly water-soluble Pt(II) complex according to claim 1, characterized in that: The following steps are involved: (1) dissolving hexahydroxyplatinic acid in hydrobromic acid to obtain H2[PtBr6]; (2) quantitatively neutralizing H2[PtBr6] with ethanolamine to obtain hexabromoplatinic acid ethanolammonium salt, the molecular formula of which is (HO-CH2CH2-NH3)2[PtBr6]; (3) The hexabromoplatinic acid ethanolammonium salt is reduced by hydrazine hydrobromide to obtain the chlorine-free highly water-soluble Pt(II) complex.

3. Use of the chlorine-free, highly water-soluble Pt(II) complex according to claim 1 or the chlorine-free, highly water-soluble Pt(II) complex obtained by the preparation method according to claim 2, characterized in that: As a catalytic precursor, it is used in the preparation of Pt / C electrocatalysts for fuel cells.

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