CN110575842B - Preparation method of adjustable and controllable yolk-shell structure nitrogen-carbon-doped cobalt molybdenum sulfide counter electrode catalyst - Google Patents

Preparation method of adjustable and controllable yolk-shell structure nitrogen-carbon-doped cobalt molybdenum sulfide counter electrode catalyst Download PDF

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CN110575842B
CN110575842B CN201910969478.8A CN201910969478A CN110575842B CN 110575842 B CN110575842 B CN 110575842B CN 201910969478 A CN201910969478 A CN 201910969478A CN 110575842 B CN110575842 B CN 110575842B
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钱兴
杨家辉
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Fuzhou University
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Abstract

The invention discloses a nitrogen-carbon doped cobalt molybdenum sulfide NC-CoS with a controllable yolk-shell structure2@Co‑MoS2A method for preparing the catalyst. Dissolving ammonium sulfide and ammonium molybdate in water, adding ammonia water, carrying out oil bath reaction, adding the prepared solution into ZIF-67 polyhedron dispersion liquid, mixing, and stirring to obtain an intermediate ZIF-67@ Co-MoS2(ii) a Then the intermediate is further vulcanized at high temperature to obtain NC-CoS with a yolk-shell structure2@Co‑MoS2A catalyst. The method controls the shell thickness, the inner core size and the yolk-shell spacing by regulating and controlling specific reaction time and reaction mass ratio, thereby forming catalysts with different morphological structures to achieve different catalytic effects. The prepared catalyst has high specific surface area, high porosity and good electrocatalytic performance, is used for dye-sensitized solar cells, and has the photoelectric conversion efficiency of 9.38%.

Description

Preparation method of adjustable and controllable yolk-shell structure nitrogen-carbon-doped cobalt molybdenum sulfide counter electrode catalyst
Technical Field
The invention belongs to the field of material preparation, and particularly relates to a nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with a controllable yolk-shell structure2@Co-MoS2The preparation method of (1).
Background
Based on the excessive development and consumption of fossil energy in the last century and the gradual deterioration of the environment, the development of a green energy conversion device has become a hot topic in recent decades. Among all renewable clean energy sources, solar energy is the most utilized. The solar energy has universality, and the utilization of the solar energy can be carried out in any place in the world where the sunlight can be irradiated; and the utilization of solar energy does not cause any secondary pollution, which also makes it one of the most environmentally-friendly available energy sources today. The high-efficiency utilization of solar energy can thoroughly change the existing energy utilization mode, and the society of people can enter a new era of pollution-free energy conservation.
In 1991, Gr ä tzel et al reported a new type of solar energy conversion device known as Dye Sensitized Solar Cell (DSSC). In the next few years, the photoelectric conversion efficiency of DSSCs has increased and it is likely to become the leading solar cell in place of silicon-based cells. Compared with the previous generation silicon-based solar cell, the solar cell has attracted wide attention in the field of clean energy due to the outstanding advantages of high photoelectric conversion efficiency, low manufacturing cost, environmental protection and the like.
The DSSC consists of three parts of a sandwich-like structure, namely a counter electrode, an electrolyte and TiO loaded with dye2And a photo-anode. The counter electrode is an important component of the DSSC, and the noble metal platinum (Pt) is widely applied to the DSSC as an industrial counter electrode material. Pt has good charge transfer and electrocatalytic capability and is used for catalyzing I3 –/I–Redox reaction of ion pair. However, large-scale commercial application of DSSCs is limited due to the scarce and expensive Pt reserves. Therefore, it has become the research direction to find a non-noble metal catalyst with higher catalytic activity and lower price to replace the noble metal Pt.
In recent years, various non-noble metal catalysts have been found to be useful for electrocatalysis of counter electrodes, such as carbon materials, alloy materials, composite materials and conductive polymer materials, each of which has advantages and disadvantages. For example, carbon materials have excellent catalytic activity and good corrosion resistance. But also has certain drawbacks: firstly, the carbon material is black and opaque, so that light is blocked from entering the battery, and the efficiency of the battery is greatly reduced; secondly, the carbon material has poor adhesion and is easy to fall off from the conductive glass to cause short circuit of the DSSC cell.
Among a plurality of materials, the ZIF-67 polyhedron has better chemical stability and thermal stability, can be used as a template agent, can also provide Co, N and C elements, and still keeps the shape of the polyhedron after being heated. In addition, the ZIF-67 polyhedron also has the advantages of high porosity, large specific surface area and the like, and is widely applied to preparation of an electrocatalytic material of DSSC. Additionally, transition metal chalcogenides such as MoS2、CoS2、CoS、FeS、NiS2The electrochemical performance is more prominent due to the cheap price and excellent electrocatalytic activity, particularly those transition metal sulfides with yolk-shell structures. The catalyst nanoparticles are small in size, and the yolk-shell structure causes gaps in the nanoparticles, so that the specific surface area is increased, more active sites are exposed, more ion channels are provided, and the catalyst nanoparticles are expected to be a substitute for Pt in a DSSC counter electrode.
Disclosure of Invention
The invention aims to provide a controllable yolk-shell structure nitrogen-carbon doped cobalt molybdenum sulfide catalyst (NC-CoS) with simple process and low cost2@Co-MoS2) The preparation method is used for replacing a noble metal Pt catalyst in the DSSC. The method has the advantages of simple synthesis process and easy operation, and the synthesized catalyst nanoparticles have small size, large specific surface area, controllable morphology and high catalytic activity.
In order to achieve the purpose, the invention adopts the following technical scheme:
a preparation method of a controllable yolk-shell structure nitrogen-carbon doped cobalt molybdenum sulfide catalyst is characterized by comprising the following steps:
1) respectively dissolving cobalt nitrate hexahydrate and 2-methylimidazole in methanol at room temperature under normal pressure, mixing the two solutions, stirring uniformly, standing for 24 hours, and centrifugally drying to obtain a purple precipitate ZIF-67 polyhedron;
2) dissolving ammonium molybdate and 20wt% ammonium sulfide solution in water, adding a certain amount of ammonia water, and reacting for 0.5-1 h under the condition of 50-80 ℃ oil bath to obtain ammonium thiomolybdate solution;
3) ultrasonically dispersing ZIF-67 polyhedron in BMixing the solution in alcohol with the ammonium thiomolybdate solution obtained in the step 2), stirring and reacting for a period of time, and centrifugally drying to obtain ZIF-67@ Co-MoS2An intermediate;
4) the resulting ZIF-67@ Co-MoS2Putting the intermediate and a certain amount of sulfur powder into a porcelain boat, calcining in a tubular furnace at 300-500 ℃ to carry out secondary vulcanization on the catalyst, and keeping the temperature for reaction for 1.5-3 h to obtain the NC-CoS with the yolk-shell structure2@Co-MoS2A catalyst.
The mass ratio of the cobalt nitrate hexahydrate and the 2-methylimidazole in the step 1) is 1: 1-1.5, the volume of the added methanol is 100 ml of methanol corresponding to each gram of cobalt nitrate hexahydrate, and 100 ml of methanol corresponding to each gram of 2-methylimidazole.
The volume ratio of the ammonia water to the 20wt% ammonium sulfide solution in the step 2) is 1: 10-15, 10-15 mg of ammonium molybdate is added correspondingly to each 100 mu L of 20% ammonium sulfide solution, and the volume of water is 5mL of water corresponding to each 45mg of ammonium molybdate.
And 3) the mass ratio of the ZIF-67 polyhedron to ammonium molybdate is 1: 0.15-1.5, the volume of the added ethanol is 100 ml of ethanol corresponding to each 300mg of the ZIF-67 polyhedron, and the reaction time is 0.5-2 h.
Step 4) the ZIF-67@ Co-MoS2The mass ratio of the intermediate to the sulfur powder is 1: 2-4.
The obtained NC-CoS with the yolk-shell structure2@Co-MoS2The catalyst can be used for preparing a counter electrode of a dye-sensitized solar cell (DSSC).
NC-CoS2@Co-MoS2The mechanism of catalyst formation is explained as an etch/ion exchange process. The precursor ZIF-67 polyhedron is subjected to etching reaction with ammonium molybdate and ammonium thiomolybdate generated by ammonium sulfide under the condition of ammonia water, and Co in the ZIF-67 polyhedron2+Ions are gradually diffused to the edge, and are etched on the outer surface to generate Co-MoS with sulfur ions2Shell to form intermediate ZIF-67@ Co-MoS2. Secondary sulfurization through a tube furnace, intermediate ZIF-67@ Co-MoS2By formation of nitrogen-carbon doped CoS2A part of Co2+Continued CoS Generation at the Shell2And finally forming the NC-CoS of the yolk-shell structure2@Co-MoS2A catalyst. As the reaction time increased, the interior of ZIF-67 lost Co2+The more ions, the smaller the core and the larger the gap with the shell. Wherein, the shape of the obtained catalyst can be regulated and controlled by accurately controlling the reaction time and the mass ratio of reactants. The reaction degree can be deepened by increasing the amount of ammonium molybdate and ammonium sulfide or prolonging the reaction time, the prepared catalyst has smaller kernel and larger space between the yolk and the shell, and even a hollow shell structure (Co-MoS) can be obtained2). On the contrary, the amount of ammonium molybdate and ammonium sulfide is reduced or the reaction time is shortened, the obtained catalyst core is larger, the core is tightly attached to the shell, the gap is smaller, but the structure is more stable and firmer, so that the service life of the catalyst is prolonged. If the reaction is excessive, serious morphology breakage and shell collapse and fragmentation can be caused; if the reaction time is too short, the ZIF-67 polyhedron is not enough to be etched into an egg yolk-shell structure, and a formed sulfide shell is easy to fall off, so that the specific surface area and the active sites of the catalyst are seriously influenced, and the electrochemical catalytic performance is greatly reduced. Therefore, the specific mass ratio of the reactants and the reaction time regulate the morphology structure of the catalyst, and further control the catalytic activity of the catalyst.
The shape of the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst with the adjustable and controllable yolk-shell structure, synthesized by the invention, keeps the shape of a ZIF-67 polyhedron, a spherical core of the nitrogen-carbon-doped cobalt disulfide is arranged inside the catalyst, a certain gap is arranged between the core and a shell, and the shell is cobalt disulfide/molybdenum disulfide. The shell and the inner core are distributed with a large number of nano particles, which greatly enhances the specific surface area and also exposes more active sites. The catalyst nano particles are small in size, about 600nm, the shell thickness can be regulated and controlled between 10 nm and 90 nm, and the inner core can be regulated and controlled between 0nm and 500 nm, so that the catalyst nano particles have the advantages of large particle materials and solid materials. The yolk-shell structure provides more ion exchange channels, so that the yolk-shell structure is more beneficial to the transmission and exchange of electrolyte ions, and has more excellent electro-catalytic performance. Meanwhile, the inner core is doped with carbon and nitrogen, so that the chemical stability of the catalyst is improved, and the synthetic raw materials are cheap and easy to obtain, so that the catalyst has greater advantages compared with a Pt catalyst.
Drawings
FIG. 1 is a ZIF-67@ Co-MoS polyhedral from example 22Intermediates and NC-CoS2@Co-MoS2SEM image of catalyst. (a) (b) (c) is ZIF-67 polyhedron, (d) (e) (f) is ZIF-67@ Co-MoS2Intermediate, (g) (h) (i) is NC-CoS2@Co-MoS2SEM image of catalyst.
FIG. 2 shows NC-CoS obtained in example 2 and example 32@Co-MoS2TEM images of the catalyst. Wherein (a) and (b) are the NC-CoS prepared in example 22@Co-MoS2Catalyst, (c) (d) is NC-CoS prepared in example 32@Co-MoS2TEM images of the catalyst.
FIG. 3 shows the hollow Co-MoS obtained in example 62TEM images of the catalyst.
FIG. 4 shows NC-CoS obtained in example 22@Co-MoS2XRD pattern of catalyst.
FIG. 5 shows NC-CoS obtained in example 22@Co-MoS2Catalyst and hollow Co-MoS prepared in example 62Aperture distribution map of and N2Adsorption and desorption curves.
FIG. 6 shows the results obtained using NC-CoS prepared in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And Pt counter electrode to form DSSCJ-VCurve and photovoltaic parameters of the counter electrode prepared from the three materials.
FIG. 7 shows the NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And a Pt counter electrode are assembled into a cyclic voltammogram of the DSSC.
FIG. 8 shows the NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And the Pt counter electrode are assembled into a polarization curve of the DSSC.
FIG. 9 shows NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62DSSC assembled with Pt counter electrodeA wire.
Detailed Description
The present invention will be described in detail with reference to specific examples, but the use and purpose of these examples are merely to illustrate the present invention, and the present invention is not limited to the actual scope of the present invention in any form, and the present invention is not limited to these.
Example 1:
respectively dissolving 4.5 g of cobalt nitrate hexahydrate and 4.5 g of 2-methylimidazole in 450 mL of methanol, uniformly mixing the two solutions after complete dissolution, stirring for 10 min, standing for 24 h, and centrifugally drying to obtain the purple ZIF-67 polyhedron. And dissolving 45mg of ammonium molybdate and 300 mu L of 20% ammonium sulfide solution in 5mL of water, adding 20 mu L of ammonia water, and performing oil bath at 50 ℃ for 0.5h to obtain an ammonium thiomolybdate solution. 300mg of ZIF-67 polyhedron is dispersed in 100 mL of absolute ethyl alcohol, mixed with ammonium thiomolybdate solution and stirred for 0.5h at normal temperature, centrifuged, washed and dried to obtain a purple black intermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by cobalt molybdenum sulfide2. Then 100 mg ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 200 mg of sulfur powder, placing the mixture into a porcelain boat, heating the mixture to 300 ℃ in a tube furnace, preserving the heat for 1.5 h at the heating rate of 1.5 ℃/min, and finally obtaining the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with the yolk-shell structure2@Co-MoS2. The prepared catalyst has a shell thickness of about 30 nm, an average internal void of about 10 nm and an inner core diameter of about 550 nm.
Example 2:
dissolving 4.64 g of cobalt nitrate hexahydrate in 464 mL of methanol, dissolving 5.24 g of 2-methylimidazole in 524mL of methanol, mixing the two solutions uniformly after complete dissolution, stirring for 10 min, standing for 24 h, and performing centrifugal drying to obtain the purple ZIF-67 polyhedron. 70 mg of ammonium molybdate and 500 muL of 20% ammonium sulfide solution are dissolved in 7.8 mL of water, and after 36 muL of ammonia water is added, the solution is subjected to oil bath for 0.5h at 60 ℃ to obtain ammonium thiomolybdate solution. 300mg of ZIF-67 polyhedron is dispersed in 100 mL of absolute ethyl alcohol, mixed with ammonium thiomolybdate solution and stirred for 0.5h at normal temperature, centrifuged, washed and dried to obtain a purple black intermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by cobalt molybdenum sulfide2. 100 mg of the above ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 200 mg of sulfur powder, placing the mixture into a porcelain boat, heating the mixture to 350 ℃ in a tube furnace, preserving the heat for 2 hours at the heating rate of 2 ℃/min, and finally obtaining the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with the yolk-shell structure2@Co-MoS2. The prepared catalyst has a shell thickness of about 45 nm, an average internal void of about 30 nm and a core diameter of about 500 nm.
Example 3:
dissolving 4.64 g of cobalt nitrate hexahydrate in 464 mL of methanol, dissolving 5.24 g of 2-methylimidazole in 524mL of methanol, mixing the two solutions uniformly after complete dissolution, stirring for 10 min, standing for 24 h, and performing centrifugal drying to obtain the purple ZIF-67 polyhedron. Dissolving 140 mg of ammonium molybdate and 1077 muL of 20% ammonium sulfide solution in 15.6 mL of water, adding 83 muL of ammonia water, and carrying out oil bath at 60 ℃ for 0.5h to obtain the ammonium thiomolybdate solution. 300mg of ZIF-67 polyhedron is dispersed in 100 mL of absolute ethyl alcohol, mixed with ammonium thiomolybdate solution and stirred for 1 h at normal temperature, centrifuged, washed and dried to obtain intermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by purple black cobalt molybdenum sulfide2. 100 mg of the above ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 200 mg of sulfur powder, placing the mixture into a porcelain boat, heating the mixture to 400 ℃ in a tube furnace, preserving the heat for 2 hours at the heating rate of 2 ℃/min, and finally obtaining the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with the yolk-shell structure2@Co-MoS2. The prepared catalyst has a shell thickness of about 50 nm, an average internal void of about 65 nm and a core diameter of about 400 nm.
Example 4:
dissolving 4.64 g of cobalt nitrate hexahydrate in 464 mL of methanol, dissolving 5.24 g of 2-methylimidazole in 524mL of methanol, mixing the two solutions uniformly after complete dissolution, stirring for 10 min, standing for 24 h, and performing centrifugal drying to obtain the purple ZIF-67 polyhedron. Dissolving 180 mg of ammonium molybdate and 1500 muL of 20% ammonium sulfide solution in 20 mL of water, adding 125 muL of ammonia water, and performing oil bath at 60 ℃ for 0.5h to obtain an ammonium thiomolybdate solution. Dispersing 300mg of ZIF-67 polyhedron in 100 mL of absolute ethanol, mixing with ammonium thiomolybdate solution, stirring at normal temperature for 0.5h, centrifuging, washing and drying to obtain the productIntermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by purple black cobalt molybdenum sulfide2. 100 mg of the above ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 200 mg of sulfur powder, placing the mixture into a porcelain boat, heating the mixture to 500 ℃ in a tube furnace, preserving the heat for 2 hours at the heating rate of 3 ℃/min, and finally obtaining the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with the yolk-shell structure2@Co-MoS2. The prepared catalyst has a shell thickness of about 60 nm, an average internal void of about 90 nm and a core diameter of about 300 nm.
Example 5:
dissolving 4.64 g of cobalt nitrate hexahydrate in 464 mL of methanol, dissolving 5.24 g of 2-methylimidazole in 524mL of methanol, mixing the two solutions uniformly after complete dissolution, stirring for 10 min, standing for 24 h, and performing centrifugal drying to obtain the purple ZIF-67 polyhedron. Dissolving 225 mg of ammonium molybdate and 2045 muL of 20% ammonium sulfide solution in 25 mL of water, adding 186 muL of ammonia water, and performing oil bath at 70 ℃ for 0.5h to obtain an ammonium thiomolybdate solution. 300mg of ZIF-67 polyhedron is dispersed in 100 mL of absolute ethyl alcohol, mixed with ammonium thiomolybdate solution and stirred for 1 h at normal temperature, centrifuged, washed and dried to obtain intermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by purple black cobalt molybdenum sulfide2. 100 mg of the above ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 300mg of sulfur powder, placing the mixture into a porcelain boat, heating the mixture to 400 ℃ in a tube furnace, preserving the heat for 3 hours at the heating rate of 2 ℃/min, and finally obtaining the nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with the yolk-shell structure2@Co-MoS2. The prepared catalyst has a shell thickness of about 65 nm, an average internal void of about 110 nm and an inner core diameter of about 250 nm.
Example 6:
dissolving 4.64 g of cobalt nitrate hexahydrate in 464 mL of methanol, dissolving 6.96 g of 2-methylimidazole in 696mL of methanol, mixing the two solutions uniformly after complete dissolution, stirring for 10 min, standing for 24 h, and performing centrifugal drying to obtain the purple ZIF-67 polyhedron. Dissolving 450 mg of ammonium molybdate and 4500 muL of 20% ammonium sulfide solution in 50 mL of water, adding 450 muL of ammonia water, and performing oil bath for 1 h at 80 ℃ to obtain ammonium thiomolybdate solution. 300mg of ZIF-67 polyhedron was dispersed in 100 mL of anhydrous ethanol, and thiomolybdate was addedMixing the ammonium solutions, stirring for 2 h at normal temperature, centrifuging, washing and drying to obtain intermediate ZIF-67@ Co-MoS of ZIF-67 wrapped by purple black cobalt molybdenum sulfide2. 100 mg of the above ZIF-67@ Co-MoS2Grinding and uniformly mixing the intermediate and 400 mg of sulfur powder, placing the mixture in a porcelain boat, heating the mixture to 350 ℃ in a tube furnace, preserving the heat for 3 hours at the heating rate of 2 ℃/min, and finally obtaining Co-MoS with a hollow structure2A catalyst. The prepared catalyst has a shell thickness of about 70 nm and no core inside.
Assembling:
the photo-anode is prepared by adopting a screen printing technology. Coating five layers of 20 nm granular titanium dioxide layer with the thickness of 12 mu M and two layers of 200 nm granular titanium dioxide layer with the thickness of 4 mu M on FTO glass by a screen printing technology, then putting the prepared FTO glass into a muffle furnace to be calcined for 1 h at the temperature of 500 ℃, taking out the FTO glass and soaking the FTO glass in 0.04M TiCl4The aqueous solution is put into a muffle furnace to be calcined for 0.5h at 500 ℃ after 1 h. The prepared photo-anode is divided into small blocks with proper size, soaked in 0.3 mM N719 dye ethanol solution for standby, and placed for 12 h in dark for sensitization treatment. The electrolyte was 0.1M LiI, 0.05 MI20.3M DMPII (1, 2-dimethyl-3-propylimidazolium iodide) and 0.5M tert-butylpyridine in acetonitrile.
The electrode is prepared by adopting a spin coating technology. 10 mg of NC-CoS prepared in example 2 were taken2@Co-MoS2Adding 1 mL of absolute ethyl alcohol into the catalyst, performing ultrasonic treatment for 30 min, and then spin-coating the obtained catalyst suspension on pretreated FTO glass at a rotating speed of 600-650 revolutions per minute for 8 seconds each time for 3-4 times. The loading of catalyst on each FTO glass was about 0.45 mg cm–2. For comparison, a Pt counter electrode was also fabricated. And (3) spin-coating 20 mM ethanol solution of chloroplatinic acid on FTO glass under the same conditions, and putting the FTO glass into a muffle furnace to be calcined for 0.5h at 450 ℃ to obtain the Pt counter electrode.
And finally, packaging the counter electrode and the photo-anode by using a Shalin heat-sealing film, then injecting electrolyte between the photo-anode and the counter electrode, fixing and clamping, and assembling the battery with the structure of the three-component Mingming. The cell was tested under standard simulated solar conditions (AM 1.5G, 100 mW cm)–2）。
The following analysis is made in conjunction with the accompanying drawings
FIG. 1 is ZIF-67 polyhedron, ZIF-67@ Co-MoS2Intermediates and NC-CoS prepared under the conditions of example 22@Co-MoS2SEM image of catalyst. As can be seen from FIGS. (a) to (c), ZIF-67 is a regular dodecahedron having a uniform size of about 600 nm. FIGS. (d) - (f) show the ZIF-67@ Co-MoS formed after the etching reaction2The SEM image of the intermediate compared with the precursor ZIF-67 still maintains the framework structure of a polyhedron, and a plurality of nano-particles grow on the surface. FIG. f is ZIF-67@ Co-MoS after sonication2The SEM image of the intermediate shows that the inside of the intermediate is a ZIF-67 inner core which is tightly wrapped by a shell. FIGS. (g) - (i) are diagrams of NC-CoS formed after calcination and vulcanization2@Co-MoS2SEM images of the catalyst, which had a rougher outer surface with a slightly concave outer wall to which the larger nanoparticles were attached, still maintained the polyhedral shape. As can be seen from the broken morphology in the graph (i), the catalyst has a spherical inner core inside, the surface of the inner core is rough, and obvious gaps are formed between the inner core and the outer shell, so that the specific surface area of the catalyst is greatly increased, and the catalyst has higher electrocatalytic activity. The element proportion of the alloy is 4.92 percent of Mo, 8.01 percent of C, 10.8 percent of N, 24.6 percent of Co and 51.6 percent of S according to an EDS (Energy-Dispersive X-ray Spectroscopy) test.
FIG. 2 shows NC-CoS obtained in example 2 and example 32@Co-MoS2TEM images of the catalyst. Wherein (a) and (b) are the NC-CoS prepared in example 22@Co-MoS2Catalyst, (c) (d) is NC-CoS prepared in example 32@Co-MoS2TEM images of the catalyst. As is apparent from the figure, NC-CoS obtained in example 22@Co-MoS2The thickness of the catalyst shell is about 45 nm, the average internal void is about 30 nm, and the diameter of the inner core is about 500 nm; on the other hand, in the catalyst of example 3, the reaction degree becomes higher, and the inner core is further reduced in size as compared with the former two figures, and the gap between the inner core and the outer shell is slightly increased.
FIG. 3 shows the hollow Co-MoS obtained in example 62TEM images of the catalyst. As can be seen from the figure, under the reaction conditionsThe resulting catalyst core had completely disappeared, leaving only the outer shell, which was about 70 nm thick.
FIG. 4 shows NC-CoS obtained in example 22@Co-MoS2XRD pattern of catalyst. As can be seen from the figure, NC-CoS2@Co-MoS2The catalyst can be reacted with CoS2And MoS2Was verified to contain CoS2And MoS2。
FIG. 5 shows NC-CoS obtained in example 22@Co-MoS2Catalyst and hollow Co-MoS prepared in example 62Aperture distribution map of and N2Adsorption and desorption curves. Wherein NC-CoS2@Co-MoS2Catalyst and hollow Co-MoS2Respectively has a specific surface area of 142 m2g–1And 116 m2g–1. The pore size calculated by Barrett Joyner Halenda (BJH) is mainly distributed around 4.3 nm. Such a structure with a large specific surface area and a significant pore structure can provide sufficient adsorption sites and active area to improve the electrocatalytic performance of the material.
FIG. 6 shows NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And Pt counter electrode to form DSSCJ-VCurve and photovoltaic parameters of the counter electrode prepared from the three materials. As can be seen from the figure, the hollow Co-MoS is used2Open circuit voltage of DSSC prepared as counter electrodeV oc801 mV, current densityJ scIs 16.4 mA cm–2Fill factor FF of 63.6%, photoelectric conversion efficiencyη8.36%, which is higher than the cell efficiency of the Pt counter electrode composition under the same conditions (Pt 8.19%). Under the same conditions, the NC-CoS of the invention2@Co-MoS2Open circuit voltage of catalyst prepared DSSCV oc811 mV, current densityJ scIs 17.7 mA cm–2Fill factor FF is 65.4%, photoelectric conversion efficiencyη9.38%, i.e., more hollow Co-MoS2And Pt has higher conductivity and catalytic efficiency. This indicates that NC-CoS2@Co-MoS2The yolk-shell structure of the catalyst can provide more active sitesThereby obtaining higher catalytic efficiency.
FIG. 7 shows NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And a Pt counter electrode. As can be seen from FIG. 7, the CV curve has two distinct redox peaks, the potential difference between the first oxidation peak and the first reduction peakE ppCurrent density of the first reduction peakJ Red-1Are two vital pieces of data. Potential differenceE ppThe current density of the first reduction peak is related to the reversibility of the redox reactionJ Red-1With catalysis I3 –/I–The speed of the pair is related. As can be seen in the figure, NC-CoS2@Co-MoS2Redox pair I in the circulation curve of the catalyst3 –/I–Is higher than the other two and the area enclosed by the CV curve is larger, indicating NC-CoS2@Co-MoS2The catalytic activity of the catalyst towards the electrode is better than the other two. NC-CoS2@Co-MoS2Potential difference between first oxidation peak and first reduction peak of catalystE ppAt about 261 mV compared with hollow Co-MoS2The 371 mV of Pt is small compared with 392 mV of Pt. Thus NC-CoS2@Co-MoS2The catalyst has stronger electrocatalytic activity.
FIG. 8 shows NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And a polarization curve of the DSSC assembled with the Pt counter electrode. In the polarization curve, exchange current densityJ 0Slope of cathode or anode, limiting diffusion current densityJ limIs the intercept value of the anode curve on the y coordinate axis, which is two important parameters for measuring the electrochemical performance. As can be seen from fig. 8, the order of the exchange current densities is as follows: NC-CoS2@Co-MoS2Catalyst (2.03 log (mAcm)–2)）>Hollow Co-MoS2（1.90 log (mA cm–2)）>Pt（1.78 log (mA cm–2) ); the limiting diffusion current density arrangement order is as follows: NC-CoS2@Co-MoS2Catalyst (0.701 log (mA cm)–2)）>Hollow Co-MoS2（0.540 log(mA cm–2)）>Pt（0.438 log (mA cm–2) NC-CoS)2@Co-MoS2The catalyst has the highest exchange current densityJ 0And ultimate diffusion current densityJ limThis indicates NC-CoS2@Co-MoS2The catalyst possesses the highest electrocatalytic activity.
FIG. 9 shows NC-CoS obtained in example 22@Co-MoS2Catalyst, hollow Co-MoS prepared in example 62And Pt counter electrode. All electrode materials exhibit two semicircles, where the semicircle located in the high frequency region (left) reflects the charge transport between the electrode material and the electrolyte interface, and the first intersection with the x-axis represents the series impedance: (R s ) Generally including FTO substrate impedance, active material impedance, and contact impedance therebetween; the diameter of which reflects the interfacial charge transfer impedance between the surface of the electrode material and the electrolyte solutionR ct ). The semicircle (right) located in the low frequency region reflects the charge transfer condition in the electrolyte, and the corresponding equivalent circuit diagram is simulated by Z-view software. The results show that all samples show approximationsR s Due to the several electrode materialsR s The value is determined primarily by the impedance of the FTO substrate and the FTO/electrode material interfacial impedance, and thus its reference to the electrocatalytic performance of the electrode material is negligible. Of the prepared samplesR ct Are arranged in order: pt (2.63 omega)>Hollow Co-MoS2(1.85 Ω)>NC-CoS2@Co-MoS2Catalyst (1.09 Ω). In general, the catalytic activity of the catalyst is dependent onR ct Is increased, it can be seen that the catalytic activity of the catalyst is in order from small to large: pt, hollow Co-MoS2、NC-CoS2@Co-MoS2A catalyst.
The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.

Claims (7)

1. Nitrogen-carbon-doped cobalt molybdenum sulfide catalyst NC-CoS with adjustable yolk-shell structure2@Co-MoS2The preparation method is characterized by comprising the following steps:
1) respectively dissolving cobalt nitrate hexahydrate and 2-methylimidazole in methanol at room temperature under normal pressure, mixing the two solutions, stirring uniformly, standing for 24 hours, and centrifugally drying to obtain a purple precipitate ZIF-67 polyhedron;
2) dissolving ammonium molybdate and 20wt% ammonium sulfide solution in water, adding a certain amount of ammonia water, and reacting for a period of time under the condition of oil bath to obtain ammonium thiomolybdate solution;
3) ultrasonically dispersing ZIF-67 polyhedron in ethanol, mixing with the ammonium thiomolybdate solution obtained in the step 2), stirring for reacting for a period of time, and centrifugally drying to obtain ZIF-67@ Co-MoS2An intermediate;
4) the resulting ZIF-67@ Co-MoS2Putting the intermediate and a certain amount of sulfur powder into a porcelain boat, and carrying out high-temperature calcination in a tube furnace to carry out secondary vulcanization on the catalyst to obtain the NC-CoS with the yolk-shell structure2@Co-MoS2A catalyst;
and 3) the mass ratio of the ZIF-67 polyhedron to ammonium molybdate is 1: 0.15-0.75, the volume of the added ethanol is 100 ml of ethanol corresponding to each 300mg of the ZIF-67 polyhedron, and the reaction time is 0.5-1 h.
2. The preparation method according to claim 1, wherein the mass ratio of the cobalt nitrate hexahydrate and the 2-methylimidazole in the step 1) is 1: 1-1.5, the volume of the added methanol is 100 ml of methanol for each gram of the cobalt nitrate hexahydrate, and 100 ml of methanol for each gram of the 2-methylimidazole.
3. The preparation method according to claim 1, wherein the volume ratio of the ammonia water in the step 2) to the 20wt% ammonium sulfide solution is 1: 10-15, and the mass of ammonium molybdate is 10-15 mg of ammonium molybdate added to each 100 μ L of the 20wt% ammonium sulfide solution.
4. The preparation method according to claim 1, wherein the volume of the water in the step 2) is 5mL per 45mg of ammonium molybdate, the temperature of the oil bath is 50-80 ℃, and the reaction time is 0.5-1 h.
5. The method of claim 1, wherein step 4) is performed using ZIF-67@ Co-MoS2The mass ratio of the intermediate to the sulfur powder is 1: 2-4.
6. The preparation method of claim 1, wherein the reaction temperature of the tube furnace in the step 4) is 300-500 ℃, the reaction time is 1.5-3 h, and the heating rate is 1.5-3 ℃/min.
7. A controlled yolk-shell structured NC-CoS product prepared by the method of claim 12@Co-MoS2The catalyst is applied to a counter electrode of a dye-sensitized solar cell.
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