Ultra-small vanadium carbide embedded carbon nanotube material, preparation method and application thereof in aspect of hydrogen production by water splitting
Technical Field
The invention relates to the technical field of synthesis and application of catalysts, in particular to a material of an ultra-small vanadium carbide embedded carbon nanotube, a preparation method and application of the material as a catalyst for producing hydrogen by electrocatalytic cracking of water.
Background
Hydrogen energy is considered a promising energy carrier due to its high energy density. Low cost, efficient hydrogen energy also presents a challenge from a sustainable point of view, requiring scientific and technical support. The water cracking hydrogen production is an economic hydrogen production method, and the process has no emission of carbon dioxide. This process requires a catalyst that reduces the activation energy for hydrogen formation. It is well known that noble metals (Pt, Rh, Pd, etc.) are considered to be excellent hydrogen-producing catalysts due to their low overpotential and fast electron mechanism driving the reaction. However, their high cost and low content limit their wide range of applications. Some non-metallic materials are under extensive study, such as transition metal sulfides, carbides, composites or alloys. Transition metals such as tungsten carbide and molybdenum carbide, among others, are reported to exhibit properties similar to those of the platinum group metals and are good alternatives to the platinum group metals.
Vanadium carbide has high hardness and high melting point, has the general characteristics of transition metal carbide, has good electric conduction, heat conduction and catalytic performance, and has wide application in the fields of physics, chemistry and materials, while the application of vanadium carbide in the field of electrocatalysis is less, so the preparation of the nano vanadium carbide powder has important significance in the field of electrocatalysis. Chinese patent application No. CN103606428A, "a nano vanadium carbide magnetic fluid and a preparation method thereof, is a magnetic fluid prepared by using high energy ball-milled nano magnetic vanadium carbide with a particle size of one as magnetic particles in the magnetic fluid, preparing a precursor by using an aqueous solution dosing method, preparing nano magnetic vanadium carbide by using a vanadium oxide direct carbonization method through high energy ball milling, then pre-dispersing the nano magnetic vanadium carbide particles in a base solution, and performing surface modification to obtain the nano vanadium carbide magnetic fluid. The method has simple steps, but the size of the obtained vanadium carbide is still larger, and the requirement of the electrocatalyst is difficult to meet. Chinese patent No. CN101891193 discloses a method for preparing nano vanadium carbide, which comprises using metavanadate and sucrose as raw materials, hot-melting the raw materials in a corundum-orange pan at 800 ℃ to prepare a gel precursor, drying the precursor in hydrogen atmosphere to obtain sucrose-coated vanadium pentoxide precursor powder, and performing high-temperature heat treatment on the precursor powder at 1000-1200 ℃ to obtain nano powder. The synthesis temperature of the vanadium carbide prepared by the method is too high, which is not beneficial to industrial production.
The method for preparing vanadium carbide powder has the advantages of complex process, higher cost and application in the field of electroless catalysis. Therefore, a preparation method of nano vanadium carbide with low cost and simple process needs to be explored so as to better meet the application of vanadium carbide in the field of electrocatalysts.
Disclosure of Invention
The invention aims to provide a material with ultra-small vanadium carbide embedded carbon nanotubes (VC/CNTs), a preparation method and application thereof in serving as a catalyst for producing hydrogen by electrocatalytic water splitting. The defects of complex preparation scheme, large VC particle size, single appearance and less application in the field of electrocatalysis in the prior art are overcome. The VC/CNTs hydrogen production electrocatalyst with low synthesis temperature, short reaction period, uniform material chemical composition, uniform appearance and size, high electrocatalytic activity in a full pH electrolyte environment and high stability is obtained by a one-step calcination method. The introduction of metal atoms (cobalt, iron and nickel) in the raw materials not only promotes the generation and crystallization of the carbon tube, but also inhibits the growth of VC grains, so that ultra-small VC particles (not more than 3 nm) are generated.
In order to achieve the purpose, the invention adopts the following technical scheme:
a preparation method of a VC/CNTs hydrogen production electrocatalyst comprises the following steps:
the method comprises the following steps: fully mixing dicyandiamide, ammonium metavanadate and a metal catalyst according to a certain proportion, wherein the proportion is (10-20): (2-4): (1-3) fully grinding for 20-50 min to obtain reactant raw materials, wherein the raw materials are ground in a mortar according to the mass ratio;
step two: and (3) placing the reactant raw materials prepared in the first step into a porcelain boat, and reacting in a tubular furnace under a certain atmosphere, wherein the temperature range is 500-1200 ℃, the heat preservation time is 1-5h, and the heating rate is 5-10 ℃/min, so as to obtain black powder.
Step three: to remove the elemental metal from the powder, it was placed at 0.5M H2SO4And (3) neutralizing for 10-24h, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
The metal catalyst in the first step is any one of nickel nitrate hexahydrate, ferric nitrate nonahydrate and cobalt nitrate hexahydrate.
The atmosphere in the second step is any one of argon, nitrogen and vacuum.
The metal simple substance in the third step refers to any one of nickel, iron and cobalt.
The VC/CNTs hydrogen production electrocatalyst prepared by the method is embedded into the wall of a carbon tube, the particle size is less than 3nm, the carbon tubes are mutually wound, the thickness of the crystallized tube wall is about 2-3nm, the appearance of a sample is uniform, and the dispersibility is good.
Compared with the prior art, the invention has the following beneficial technical effects:
1) the one-step sintering method used in the invention has short reaction period and low synthesis temperature (synthesis can be carried out at 700 ℃), and overcomes the defects of complicated steps and high synthesis temperature (at least 1000 ℃) of the traditional method for preparing vanadium carbide;
2) the dicyandiamide contains N element, so that the prepared VC/CNTs are rich in N defect, and the activity of the catalyst is improved;
3) the introduction of metal atoms (iron, cobalt and nickel) in the raw materials is the key for generating the structure (carbon nano tube and ultra-small vanadium carbide), and the catalytic activity of the catalyst is improved to a certain extent;
4) the carbon tube in the VC/CNTs is generated in situ in one step, the one-dimensional carbon tube has good conductivity and thin tube wall, is beneficial to electron transmission, effectively disperses vanadium carbide crystal grains, inhibits the growth of vanadium carbide, enables a sample to expose more active sites, and can protect the vanadium carbide in the catalysis process and prevent the vanadium carbide from being corroded by electrolyte;
5) the VC/CNTs hydrogen production electrocatalyst obtained by the reaction has the VC particle size of less than 3nm, which is far smaller than the VC particle size reported in the literature, and has uniform appearance and good dispersibility;
6) the VC/CNTs hydrogen production electrocatalyst prepared by the method can be used as a water-splitting full-pH hydrogen production electrocatalyst in the field of electrocatalysis.
Drawings
FIG. 1 is an XRD pattern of VC/CNTs prepared in example 1;
FIG. 2 is an SEM photograph of VC/CNTs prepared in example 3;
FIG. 3 is a TEM image of VC/CNTs prepared in example 3;
FIG. 4 is a LSV graph of VC/CNTs prepared in example 4;
FIG. 5 is an i-t plot of VC/CNTs prepared in example 6.
Detailed Description
The present invention is described in further detail below with reference to the attached drawings and examples, which should be understood as illustrative only and not limiting the scope of the present invention. It should be understood that any changes or modifications of the present invention may be made by those skilled in the art after reading the present disclosure, and such equivalents may fall within the scope of the present invention as defined by the appended claims.
Example 1
The method comprises the following steps: weighing 1.5g of dicyandiamide, 0.3g of ammonium metavanadate and 0.2g of cobalt nitrate hexahydrate, and fully mixing and grinding for 40 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tube furnace with vacuum as atmosphere, heating to 500 ℃ at a heating rate of 5 ℃/min, keeping the temperature for 2h, continuously heating to 800 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 10h, which is to remove the metal simple substance in the sampleAnd (3) drying for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
Fig. 1 is an XRD spectrum of the VC/CNTs electrocatalyst prepared in this example, from which it can be seen that the VC/CNTs sample contains graphitized carbon, VC, and cobalt (cobalt embedded in carbon tubes), and the characteristic peaks of the three are obvious, indicating that the crystallinity is good.
Example 2
The method comprises the following steps: weighing 2g of dicyandiamide, 0.2g of ammonium metavanadate and 0.3g of cobalt nitrate hexahydrate, and fully mixing and grinding for 30 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tube furnace with vacuum as atmosphere, heating to 500 ℃ at a heating rate of 10 ℃/min, keeping the temperature for 2h, continuously heating to 700 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 10h to remove metal simple substances in the sample, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
Fig. 2 and 3 are SEM and TEM spectra of the VC/CNTs electrocatalyst prepared in this example, and it can be seen from the SEM images that the carbon nanotubes have a complete morphology and are uniformly dispersed, and it can be seen from the TEM images that cobalt exists (black particle region), the vanadium carbide particle size (gray particle region) is less than 3nm, the carbon nanotube wall thickness is about 2-3nm, and the lattice fringes are obvious, which are indicated as graphitic carbon, and are consistent with the XRD results.
Example 3
The method comprises the following steps: weighing 1g of dicyandiamide, 0.2g of ammonium metavanadate and 0.1g of nickel nitrate hexahydrate, and fully mixing and grinding for 50 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tubular furnace with nitrogen as atmosphere, heating to 500 ℃ at a heating rate of 7 ℃/min, keeping the temperature for 2h, continuously heating to 1000 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 20h to remove the metal simple substance in the sample, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
Example 4
The method comprises the following steps: weighing 1.5g of dicyandiamide, 0.3g of ammonium metavanadate and 0.3g of cobalt nitrate hexahydrate, and fully mixing and grinding for 20 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tubular furnace with nitrogen as atmosphere, heating to 500 ℃ at a heating rate of 8 ℃/min, keeping the temperature for 2h, continuously heating to 900 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 24h to remove the metal simple substance in the sample, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
FIG. 4 is a LSV plot of the VC/CNTs electrocatalyst prepared in this example, showing the current density at 10mA/cm under pH 0 test conditions2And when the scanning speed is 3 mV/s, the overpotential of the sample is 160mV, which shows that the catalytic hydrogen production activity of the sample is excellent.
Example 5
The method comprises the following steps: weighing 1g of dicyandiamide, 0.4g of ammonium metavanadate and 0.1g of ferric nitrate nonahydrate, and fully mixing and grinding for 30 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tubular furnace with argon as atmosphere, heating to 500 ℃ at a heating rate of 10 ℃/min, keeping the temperature for 2h, continuously heating to 1100 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 20h to remove the metal simple substance in the sample, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
Example 6
The method comprises the following steps: weighing 1.5g of dicyandiamide, 0.4g of ammonium metavanadate and 0.2g of ferric nitrate nonahydrate, and fully mixing and grinding for 40 min;
step two: putting the raw materials obtained in the step one into a porcelain boat, reacting in a tubular furnace with argon as atmosphere, heating to 500 ℃ at a heating rate of 6 ℃/min, keeping the temperature for 2h, continuously heating to 1200 ℃, keeping the temperature for 2h, and cooling to room temperature to obtain black powder;
step three: mixing the obtained black powder with 0.5M H2SO4Soaking for 12h to remove metal simple substances in the sample, drying in vacuum for 6h, and grinding to obtain the VC/CNTs hydrogen production electrocatalyst.
FIG. 5 is an i-t plot of the VC/CNTs electrocatalyst prepared in this example, showing a current density of about 10mA/cm at an overpotential of 340mV under the pH 14 test conditions2And the stability is at least 15h, the current density is not obviously attenuated, and the sample stability is excellent.