CN111921560B

CN111921560B - Lattice-distorted ultrathin metal organic framework nanosheet catalyst, and preparation method and application thereof

Info

Publication number: CN111921560B
Application number: CN202010831347.6A
Authority: CN
Inventors: 侯阳; 何凡; 杨彬; 张庆华; 雷乐成
Original assignee: Zhejiang University ZJU
Current assignee: Zhejiang University ZJU
Priority date: 2020-08-18
Filing date: 2020-08-18
Publication date: 2021-11-09
Anticipated expiration: 2040-08-18
Also published as: CN111921560A

Abstract

The invention relates to the technical field of energy material synthesis, and discloses a lattice distortion ultrathin metal organic framework nanosheet catalyst, and a preparation method and application thereof. Due to lattice distortion caused by ferrocenecarboxylic acid and an ultra-thin two-dimensional nanosheet structure obtained by ultra-long-time ultrasound, the ferrocenecarboxylic acid nanosheet structure has a larger specific surface area, exposes more active sites, can promote the oxygen evolution reaction, has better catalytic stability, and has the advantages of low cost, high utilization rate, strong pertinence, good effect and the like.

Description

Lattice-distorted ultrathin metal organic framework nanosheet catalyst, and preparation method and application thereof

Technical Field

The invention relates to the technical field of energy material synthesis, in particular to an ultrathin metal organic framework nanosheet catalyst with distorted lattices, and a preparation method and application thereof.

Background

Hydrogen is one of the most recognized clean and high value energy carriers, with zero carbon emissions upon combustion or oxidation of hydrogen in fuel cells, which makes hydrogen play an important role in the face of mitigating the increasing energy demand. Electrochemical water splitting technology can help produce clean and storable hydrogen. However, the overall efficiency of the electrochemical water splitting is limited by the anodic oxygen evolution reaction, and the Oxygen Evolution Reaction (OER) has slower reaction kinetics due to the need for a multi-step proton coupled electron transfer process, needs to overcome a higher reaction energy barrier, and plays an important role as a rate-limiting step in the electrochemical water splitting process.

The research has conducted intensive research on the mechanism of electrochemical water splitting catalytic reaction, and at the same time, the corresponding catalyst has been constructed and modified in all aspects, for example, the preparation of nano materials with various shapes and structures, the doping of different elements to change the energy band position, the loading of conductive matrix to promote the electron transport performance, etc. Currently, Ir/Ru based materials are considered the most advanced electrocatalysts for OER reactions, but the high cost and scarce resources greatly limit their development. There is therefore a need to develop a large number of non-noble metal-based OER electrocatalysts, including transition metal oxides, transition metal hydroxides, transition metal phosphates and carbon-based materials. Among them, Metal Organic Framework (MOF) materials are widely studied by researchers due to their advantages such as high porosity, structural diversity, and surface adjustability.

For example, chinese patent publication No. CN110975937A discloses a preparation method and application of a metal organic framework compound electrocatalyst. Dissolving tartaric acid, ferric chloride hexahydrate and nickel nitrate hexahydrate in methanol respectively, mixing the two solutions, and transferring the mixture into a reaction kettle; vertically putting the foamed nickel into a reaction kettle, heating the reaction kettle for reaction, and naturally cooling; taking out the foamed nickel, washing with ethanol, and finally vacuum drying. However, the three-dimensional nano flower iron-nickel tartrate bimetallic organic framework compound lacks a good space structure and cannot fully expose active sites. Meanwhile, the position of the energy band of the bimetal is not optimized, so that the performance of electrocatalytic decomposition water for oxygen evolution is not ideal.

CN 110655654A discloses preparation and oxygen evolution performance research of a two-dimensional layered cobalt-based metal organic framework (Co-MOF) electrode material, the two-dimensional layered Co-MOF prepared by a hydrothermal method has better stability when used as the electrode material, and the oxygen evolution reaction performance of the two-dimensional layered Co-MOF is better than that of Cu-MOF, but a certain difference still exists compared with a noble metal catalyst with good oxygen evolution performance, and a larger reaction energy barrier and slower reaction kinetics exist.

Therefore, the ultrathin Metal Organic Framework (MOF) nanosheet catalyst prepared by adopting a long-time ultrasonic stripping and lattice distortion induction strategy has wide application prospect and practicability.

Disclosure of Invention

The invention provides a lattice-distorted ultrathin metal organic framework nanosheet catalyst, which can induce a catalyst structure to generate lattice distortion by introducing organic ligands with functional group deletion and optimizing the proportion of different ligands, and is prepared into an ink-shaped sample by ultrasonically stripping the ultrathin nanosheet catalyst for a long time, and the ink-shaped sample is used as a working electrode in a three-electrode system and has excellent electrocatalytic stability.

In order to achieve the purpose, the invention adopts the technical scheme that:

a preparation method of a lattice distortion ultrathin metal organic framework nanosheet catalyst comprises the steps of dissolving terephthalic acid and ferrocenecarboxylic acid in a mixed solution of N, N-Dimethylformamide (DMF), ethanol and water, adding a cobalt salt and an acid-binding agent, carrying out ultrasonic stripping, washing and separating to obtain the lattice distortion ultrathin metal organic framework nanosheet catalyst.

According to the method, cobalt salt is used as a transition metal source, terephthalic acid (BDC) is used as an organic ligand, ferrocenecarboxylic acid (FcCA) is used as a ligand with a functional group missing, the cobalt salt, the terephthalic acid (BDC) and the ferrocenecarboxylic acid (FcCA) are uniformly mixed in an organic phase, and the mixture is subjected to long-time ultrasonic stripping to prepare the ink-shaped ultrathin nanosheet catalyst sample. Compared with a blocky MOF catalyst, the ultrathin two-dimensional nanosheet structure obtained by lattice distortion and ultra-long-time ultrasound caused by ferrocenecarboxylic acid has a larger specific surface area and exposes more active sites, so that the energy barrier of the electrocatalytic water-splitting oxygen evolution reaction is reduced, and the oxygen evolution reaction is promoted to be carried out.

The molar ratio of the terephthalic acid to the ferrocenecarboxylic acid is 1: 0.05-0.3. Too low a ferrocenecarboxylic acid results in insufficient lattice distortion to effectively change the electronic structure of the active site. And excessive addition of ferrocenecarboxylic acid can destroy the original MOF structure, cause the structure collapse and further influence the oxygen evolution reaction performance.

The volume ratio of the N, N-dimethylformamide to the absolute ethyl alcohol to the water in the mixed solution is 12-20: 0.8-1.2. The N, N-dimethylformamide is mainly used for dissolving the terephthalic acid, and the larger proportion is beneficial to the dissolution of the terephthalic acid, but the higher proportion is not beneficial to the dissolution and dispersion of the ferrocenecarboxylic acid and the metal salt. The ligand and the metal salt need to be dispersed in the solvent sufficiently, and the ratio of the ligand to the metal salt exceeds the range, so that the ligand or the metal salt cannot be dissolved sufficiently, the coordination of the ligand and the metal salt is influenced, and the synthesis of the MOF material fails.

The dosage ratio of the terephthalic acid to the N, N-dimethylformamide is 3-7 mg: 1 mL. N, N-dimethylformamide is mainly used for dissolving terephthalic acid, and too little use amount can cause insufficient dissolution of the terephthalic acid, and too much use amount can cause insufficient dissolution of corresponding metal salts, which can cause negative influence on the formation of MOF.

Preferably, the molar ratio of the terephthalic acid to the ferrocenecarboxylic acid is 1: 0.1-0.2; the volume ratio of the N, N-dimethylformamide to the absolute ethyl alcohol to the water in the mixed solution is 14-18: 0.8-1.2; the dosage ratio of the terephthalic acid to the N, N-dimethylformamide is 4-6 mg: 1 mL. The inventor utilizes the ligand with a deficient functional group to introduce to induce the MOF to generate lattice distortion so as to change the energy band position and the electronic structure of the catalyst, can induce the catalyst structure to generate lattice distortion by introducing the organic ligand with a deficient functional group and optimizing the proportion of different ligands, and changes the electron density of the d orbit of the active metal center, thereby changing the binding energy of an active site and an oxygen evolution reaction intermediate, realizing high-efficiency electrochemical oxygen evolution reaction in a preferable range of the proportion between a mixed solution and the ligand, and having better catalytic effect.

In the preparation method, ultra-long-time ultrasonic stripping is crucial to the formation of the nanosheets, and the ultrasonic time is 400-600 min. Too much sonication time results in insufficient MOF exfoliation and still exists in a blocky structure, resulting in insufficient exposure of active sites and a small specific surface area. However, too long an ultrasound treatment period may collapse the MOF structure and prevent the oxygen evolution reaction from proceeding.

The power of the ultrasonic wave is 600-800W. Too low ultrasonic power can lead to insufficient stripping of MOF materials and failure to form ultrathin two-dimensional nanosheet structures. The structural damage of the MOF material can be damaged by the excessive ultrasonic power, and the oxygen evolution reaction is not facilitated.

The temperature of the ultrasound is 15-30 ℃. The ultrasound temperature is mainly room temperature, and the structure of the MOF material can be influenced by too high or too low temperature.

Preferably, the molar ratio of the terephthalic acid to the ferrocenecarboxylic acid is 1: 0.05-0.2; the ultrasonic time is 400-550 min. The addition amount of ferrocenecarboxylic acid determines the ultrasonic of lattice distortion of the catalyst, the ultrasonic time plays a critical role in the sheet structure of the ultrathin metal organic framework, and the performance of the obtained catalyst is catalyzed to the best extent through the experiment of the inventor.

Preferably, the ultrasonic time is 400-550 min, the power is 600-800W, the ultrasonic temperature is 20-25 ℃, and the uniform ultrathin metal organic framework nanosheet catalyst with a large specific surface area can be prepared.

The cobalt salt is soluble inorganic cobalt salt, including cobalt chloride or cobalt nitrate and hydrate thereof. Such as cobalt chloride dihydrate, cobalt chloride hexahydrate, cobalt nitrate hexahydrate, and the like.

The acid-binding agent comprises triethylamine, pyridine and sodium hydroxide. Preferably, the acid scavenger is triethylamine.

The catalyst has an ultrathin nanosheet structure, so that the catalyst has a larger specific surface area, exposes more active sites, can reduce reaction energy barrier, and improves the catalytic effect.

The invention also provides application of the lattice-distorted ultrathin metal organic framework nanosheet catalyst as a working electrode in an electrocatalytic decomposition water-oxygen evolution reaction, the catalyst can rapidly realize electrocatalytic decomposition water-oxygen evolution under the condition of low energy consumption, and meanwhile, the catalyst has good catalytic stability and can support the reaction for a long time.

The ultrathin nanosheet catalyst is used as a working electrode in a three-electrode system, and the over-potential of only 280mV is minimally needed for the electrocatalytic decomposition of water and oxygen precipitation reaction in a 1M KOH solution, so that the over-potential can reach 10mA/cm²The optimum gradient of the Tafel can reach 56mV/dec, and meanwhile, good electrocatalytic stability can be kept at a lower potential.

Compared with the prior art, the invention has the following beneficial effects:

(1) the invention discloses a method for inducing MOF to generate lattice distortion by introducing a ligand with a functional group missing function so as to change the energy band position of a catalyst and related work of an electronic structure.

(2) The catalyst prepared by the invention has an ultrathin two-dimensional nanosheet structure, has a larger specific surface area and exposes more active sites compared with a blocky MOF catalyst, so that the energy barrier of the electrocatalytic water-splitting oxygen-separating reaction is reduced, and the oxygen-separating reaction is promoted to be carried out.

(3) The catalyst provided by the invention has low overpotential and low Tafel slope, the electrochemical performance of the catalyst is even better than that of the existing noble metal Ir-based catalyst with good OER performance, the electrocatalytic decomposition of water and oxygen evolution can be rapidly realized under the condition of low energy consumption, and the catalyst has good catalytic stability, and has the advantages of low cost, high utilization rate, strong pertinence, good effect and the like.

Drawings

Fig. 1 is an XRD spectrum of the catalysts prepared in example 1 and comparative example 1.

FIG. 2 is SEM, TEM and HRTEM images of the catalyst prepared in example 1, wherein A is the SEM image, B is the TEM image, and D is the EDX image of HRTEM; c is TEM of the catalyst prepared in comparative example 1.

FIG. 3 is N for the bulk catalyst and ultra thin catalyst prepared in example 1₂Adsorption-desorption isotherms.

Fig. 4 is an XPS spectrum of the catalysts prepared in example 1 and comparative example 1.

FIG. 5 is a polarization plot of the catalyst prepared in example 1, comparative example 1 and Ir/C.

FIG. 6 is a Tafel slope plot for the catalysts prepared in example 1, comparative example 1, and Ir/C.

FIG. 7 is a polarization curve of catalysts prepared in examples 1 to 4 and comparative example 1.

FIG. 8 is a plot of the polarization curves of the catalysts of examples 1, 5-7 and prepared in bulk.

Figure 9 is a graph of the stability of the catalyst prepared in example 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Those skilled in the art should understand that they can make modifications and equivalents without departing from the spirit and scope of the present invention, and all such modifications and equivalents are intended to be included within the scope of the present invention.

Example 1

Synthesis of the cotdc FcCA catalyst: 166mg of terephthalic acid and 34.50mg of ferrocenecarboxylic acid were added to a mixed solution (32mL:2mL:2mL) of N, N-dimethylformamide, anhydrous ethanol and deionized water, and then 238mg of cobalt chloride hexahydrate was added to the above solution while rapidly adding 1.5mL of triethylamine, and the mixture was magnetically stirred for 10 min. The mixed solution was then transferred to a cell disruptor and ultrasonically peeled for 500min at an intensity of 700W. Then washing with absolute ethanol for several times, and separating out solid samples by a high-speed centrifuge of 10000 r/s. Finally, the sample was dried under vacuum at room temperature for 12 h.

The catalyst after stirring with cobalt chloride was directly dried to obtain a bulk CoBDC FcCA catalyst. The ultra-thin CoBDC FcCA catalyst is finally obtained after the ultrasonic treatment.

Comparative example 1

Following the procedure of example 1, except without adding ferrocenecarboxylic acid, a CoBDC catalyst was obtained.

Example 2

Following the procedure of example 1, except that 11.50mg of ferrocenecarboxylic acid was added, a CoBDC Fc 0.05 catalyst was obtained.

Example 3

Following the procedure of example 1, except that 23.00mg of ferrocenecarboxylic acid was added, a CoBDC Fc 0.1 catalyst was obtained.

Example 4

Following the procedure of example 1, except that 46.00mg of ferrocenecarboxylic acid was added, a CoBDC Fc 0.2 catalyst was obtained.

Example 5

Following the preparation procedure of example 1, except for ultrasonic stripping for 400min, the CoBDC Fc400 catalyst was obtained.

Example 6

Following the preparation procedure of example 1, except for ultrasonic stripping for 450min, the CoBDC Fc450 catalyst was obtained.

Example 7

Following the preparation procedure of example 1, except for ultrasonic stripping for 550min, the CoBDC Fc550 catalyst was obtained.

Performance testing

The catalysts prepared in examples 1 to 7 and comparative example 1 were tested as follows:

the XRD test results are shown in figure 1, and it can be seen from the figure that after the introduction of ferrocenecarboxylic acid, CoBDC FcCA has no obvious change in the types of XRD diffraction peaks compared with CoBDC, which proves that the introduction of ferrocenecarboxylic acid does not damage the crystal structure of the total CoBDC. However, it can be found that the major XRD peak within 10 degrees shifts to the left after the introduction of ferrocenecarboxylic acid, mainly due to lattice distortion of the catalyst caused by the introduction of the ligand lacking functional groups.

The results of surface topography observation by a Scanning Electron Microscope (SEM), a Transmission Electron Microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) are shown in fig. 2, where a is an SEM image of cotc FcCA in example 1, B is a TEM image of cotc FcCA in example 1, and C is a TEM image of cotc in comparative example 1, and it can be seen more clearly that after a long-time ultrasonic stripping treatment, the catalyst exhibits an obvious two-dimensional nanosheet structure, and the nanosheet thickness is very thin. FIG. D is an EDX mapping spectrum of CoBDC FcCA catalyst taken by HRTEM, from which it can be seen that: C. o, Co and Fe are uniformly distributed on the nano-sheet.

Adsorption and desorption testing: the non-ultrasonic bulk catalyst in example 1 and the ultrathin catalyst finally prepared in example are respectively placed in a full-automatic BET specific surface (area) analysis tester by taking 100mg to perform specific surface area test, and the result is shown in fig. 3, from which it can be seen that the ultrathin CoBDC FcCA two-dimensional nanosheet catalyst has a larger specific surface area than the bulk CoBDC FcCA catalyst which is not subjected to ultrasonic stripping, which proves that the ultrasonic has a larger influence on the structure of the catalyst, and the obtained ultrathin two-dimensional structure can expose more active sites.

X-ray photoelectron spectroscopy (XPS) and the results are shown in fig. 4, from which it can be seen that there is a distinct Fe peak in the spectrum of the cotc c hca catalyst, and the XPS peak of Co exhibits a slight shift compared to the cotc catalyst, demonstrating that the introduction of FcCA can change the chemical valence state of the catalyst.

Application example 1

(1) A three-electrode system is used, a working electrode is the catalyst of examples 1-7 and comparative example 1 or Ir/C, a counter electrode is a carbon rod, a reference electrode is a saturated silver/silver chloride electrode, and electrolyte is 1.0M KOH;

(2) CV activation: using electrochemical workstation of CHI 660E, introducing N into electrolyte before testing₂And (3) 30 min. And (3) adopting a CV program, wherein the test interval is 0-0.8V vs. Ag/AgCl, the sweep speed is 50mV/s, and the electrode is circulated for 20 circles to reach a stable state.

3. Linear Sweep Voltammetry (LSV) testing

After the electrode is activated, the switching program is an LSV program, the test interval is 0-0.8V vs. Ag/AgCl, the sweeping speed is 5mV/s, and the overpotential is 1.23V and 10mA cm relative to the reversible hydrogen electrode^-2The potential difference is measured. As shown in FIG. 5, the CoBDC FcCA catalyst has the minimum overpotential of only 280mV to 10mA/cm²The overpotential value of (a) is significantly less than the overpotential of an Ir/C electrode (350mV) and a CoBDC catalyst (410 mV).

FIG. 6 is a plot of the Tafel slopes for different catalysts, from which it can be seen that the CoBDC FcCA catalyst has the smallest value of the Tafel slope, only 56mV/dec, significantly less than the Tafel slopes for the Ir/C electrode (97mV/dec) and CoBDC catalyst (86mV/dec), indicating that the CoBDC FcCA catalyst has faster OER reaction kinetics.

FIG. 7 is a polarization curve of CoBDC FcCA catalysts of examples 1-4 with different terephthalic acid and ferrocenecarboxylic acid ratios, from which it can be seen that the catalysts show the best electrocatalytic oxygen evolution performance when the molar ratio of terephthalic acid to ferrocenecarboxylic acid is 1:0.15 (i.e., example 1).

FIG. 8 is a polarization curve of CoBDC FcCA catalyst ultrasonically stripped in examples 1, 5-7 at different times, and it can be seen from the figure that the catalyst shows the best electrocatalytic oxygen evolution performance when the ultrasonic stripping time is 500min (i.e. example 1).

4. Stability test

After electrode activation, the switching program was an I-t program, the voltage was set to 1.6V vs. rhe, and the time was set to 40000 s. As shown in fig. 9, the current density of the cotc c FcCA catalyst of example 1 did not drop significantly, demonstrating that the catalyst has good stability, with the current density remaining stable for up to 10h or more of the test.

Claims

1. A preparation method of a lattice distortion ultrathin metal organic framework nanosheet catalyst is characterized by dissolving terephthalic acid and ferrocenecarboxylic acid in a mixed solution of N, N-dimethylformamide, ethanol and water, adding a cobalt salt and an acid binding agent, ultrasonically stripping, washing and separating to obtain the lattice distortion ultrathin metal organic framework nanosheet catalyst;

the dosage ratio of the terephthalic acid to the N, N-dimethylformamide is 3-7 mg: 1 mL;

the volume ratio of the N, N-dimethylformamide to the absolute ethyl alcohol to the water in the mixed solution is 12-20: 0.8-1.2;

the molar ratio of the terephthalic acid to the ferrocenecarboxylic acid is 1: 0.05-0.2;

the ultrasonic time is 400-550 min; the power of the ultrasonic wave is 600-800W.

2. The preparation method of lattice distorted ultrathin metal organic framework nanosheet catalyst as recited in claim 1, wherein the cobalt salt is a soluble inorganic cobalt salt comprising cobalt chloride or cobalt nitrate and hydrates thereof.

3. An ultrathin metal organic framework nanosheet catalyst having lattice distortion prepared according to the preparation method of claim 1 or 2.

4. The application of the lattice-distorted ultrathin metal organic framework nanosheet catalyst as a working electrode in electrocatalytic decomposition of water and oxygen evolution reaction.