CN108468060A - A kind of preparation and its application of efficient, novel porous nitrogen oxides nanometer sheet catalyst electrode - Google Patents

Abstract

The invention discloses the preparation and application of a highly efficient and novel porous nitrogen oxide nano sheet catalyst electrode (Co ₃ O _3.62 N _0.38 ). Preparation of Porous Co ₃ O _3.62 N _0.38 Nanosheet Catalyst Using cheap graphite sheet as substrate, Co(OH) ₂ precursor was prepared by electrochemical deposition; Medium annealing for 2 hours. DFT theoretical calculations show that the porous Co ₃ O _3.62 N _0.38 nanosheet catalyst has good electrical conductivity and excellent adsorption free energy for intermediates, so the porous Co ₃ O _3.62 N _0.38 nanosheet catalyst exhibits excellent electrochemical catalytic performance. Oxygen performance, with ultra-low electrolyzed water overpotential and low Tafel. At the same time, the synthesis method of the composite electrode is simple and efficient, and the synthesis cost is economical and low, which is suitable for the industrial application of electrochemical water splitting, and has extensive scientific significance.

Description

Preparation of a highly efficient and novel porous nitrogen oxide nanosheet catalyst electrode and its application

technical field

The invention relates to the technical field of electrocatalytic water splitting, in particular to the preparation of a porous nitrogen oxide catalyst and its application in electrochemical water splitting.

Background technique

With the consumption of non-renewable energy such as global fossil energy, the depletion of fossil energy is inevitable, and the energy crisis is a problem that human beings must face. Therefore, it is imminent to develop and utilize sustainable new energy sources (such as solar energy, wind energy, nuclear energy, biomass energy, etc.), and to increase their proportion in the entire energy structure. However, the storage and continuous supply of these new energy sources is a problem that must be solved, and its conversion into chemical energy is an important way, such as electrochemical splitting of water to produce hydrogen and oxygen.

In the electrochemical catalytic water splitting oxygen production system, the oxidation and reduction of water can only occur under a certain electrode potential, so it is particularly important to develop efficient catalysts to reduce the overpotential in the reaction process, so as to effectively reduce energy consumption. . So far, the most effective catalysts for oxygen evolution are RuO ₂ and IrO ₂ , but their industrial application in production is limited due to the high cost and scarcity of noble metals. In recent years, transition metal-based catalysts for electrolysis of oxygen for water have attracted extensive attention of researchers due to their low cost and relatively high catalytic activity. Studies have shown that the activity of the catalyst can be effectively improved and the overpotential of the reaction can be reduced by rationally adjusting the composition of the catalyst and changing the morphology of the catalyst. Nevertheless, the catalytic activity of transition metal-based catalysts still needs to be further improved. Therefore, there is still a lot of work to be done to prepare simple, cheap, and stable materials by rational design to enhance the ability of electrodes to catalyze oxygen production.

Contents of the invention

In view of the existing problems, the problem to be solved by the present invention is to provide an efficient and novel nitrogen oxide catalyst electrode preparation and its application in electrochemical water splitting through rational design, so as to effectively reduce the reaction overpotential.

In order to achieve the above object, the present invention mainly adopts the following technical solutions,

A method for preparing an efficient and novel porous nitrogen oxide nanosheet catalyst electrode, comprising the following steps:

1) Preparation of precursor Co(OH) ₂ nanosheets:

Configure 5-10mM Co(NO ₃ ) ₂ solution as electrolyte, at room temperature, use Graphite as working electrode, stone grinding rod as counter electrode, mercury/mercury oxide electrode as reference electrode, under the condition of -1.42V , electrochemical deposition to obtain Co(OH) ₂ nanosheets, the thickness of the Co(OH) ₂ nanosheets is 10-20nm;

2) Preparation of Co ₃ O _3.62 N _0.38 /Graphite:

The precursor obtained in step 1) was calcined in an NH ₃ atmosphere at 380-420 degrees Celsius for more than 0.5 h, and then naturally cooled to room temperature to obtain a Co ₃ O _3.62 N _0.38 /Graphite electrode.

Preferably, the NH ₃ atmosphere is a NH ₃ /N ₂ mixed gas with an ammonia content of 10%.

Preferably, the calcination temperature is preferably 400°C.

The invention also discloses a highly efficient and novel porous nitrogen oxide nanosheet catalyst electrode prepared by the method; the electrode is a Co ₃ O _3.62 N _0.38 /Graphite electrode; when the current density is 10 mA/cm ² , The required overpotential is only 0.23V, the catalyst has a porous nanosheet structure, and the Co ₃ O _3.62 N _0.38 has a spinel structure.

The invention discloses the application of the porous nitrogen oxide nanosheet catalyst electrode in electrochemically decomposing water to produce oxygen. Electrochemical tests show that in 1.0M KOH solution, Co ₃ O _3.68 N _0.32 catalyst has low overpotential (0.23V, current density is 10.0mA/cm ² ), and the lower Tafel value is 48mV/decade, therefore, Binary catalysts exhibit good electrocatalytic activity, which solves the problems of high cost and high overpotential.

Compared with the prior art, the present invention has the following advantages:

1. The synthesis process is simple, and the catalyst is synthesized by electrodeposition and calcination, which simplifies the process flow.

2. The potential of electrolyzed water is significantly better than that of the existing technology, and the overpotential is only 0.23V, which is obviously the vast majority of other transition metal-based catalysts.

3. The low overpotential results in a larger current at a reduced voltage, resulting in more oxygen and hydrogen at a lower voltage.

4. Electrochemical analysis methods and theoretical calculations prove that the catalyst has excellent electrical conductivity and the ability to adsorb intermediate products, which is also the reason why the catalyst exhibits excellent catalytic performance.

Description of drawings

Fig. 1-1 shows the Co ₃ O ₄ /Graphite morphology observed by scanning electron microscope in Example 1;

Fig. 1-2 shows that embodiment 1 obtains the XPS figure of Co ₃ O ₄ /Graphite by X-ray photoelectron spectroscopy;

Figure 2-1 shows the morphology of Co ₃ O _3.68 N _0.32 /Graphite observed by scanning electron microscope in Example 2;

Fig. 2-2 shows that embodiment 2 obtains the XPS diagram of Co ₃ O _3.68 N _0.32 /Graphite by X-ray photoelectron spectroscopy;

Figure 3-1 shows the morphology of the sample with an annealing temperature of 200°C observed through a scanning electron microscope in Example 3;

Figure 3-2 shows the topography of the sample with an annealing temperature of 300°C observed through a scanning electron microscope in Example 3

Figure 3-3 shows the topography of the sample with an annealing temperature of 500°C observed by scanning electron microscope in Example 3

Fig. 4-1 shows the electrode polarization curve of the sample obtained by different annealing temperatures of embodiment 4;

Fig. 4-2 shows Tafel of catalyst electrode Co ₃ O ₄ /Graphite and Co ₃ O _3.68 N _0.32 /Graphite in Example 4;

Fig. 4-3 shows the electrochemical impedance spectrum of catalyst electrode Co ₃ O ₄ /Graphite and Co ₃ O _3.68 N _0.32 /Graphite in embodiment 4;

Figure 4-4 shows the stability figure of Co ₃ O _3.68 N _0.32 /Graphite electrode in embodiment 4;

Fig. 4-5 shows the oxygen production efficiency curve of Co ₃ O _3.68 N _0.32 /Graphite electrode in embodiment 4;

Figure 5-1 shows the DOS diagrams of Co ₃ O ₄ /Graphite and Co ₃ O _3.68 N _0.32 /Graphite electrodes in Example 5;

Figure 5-2 shows the electron spin resonance spectra of Co ₃ O ₄ /Graphite and Co ₃ O _3.68 N _0.32 /Graphite electrodes in Example 5;

Figure 5-3 shows the adsorption free energy diagrams of Co ₃ O ₄ /Graphite and Co ₃ O _3.68 N _0.32 /Graphite in Example 5.

Detailed ways

Comparative example 1

A graphite sheet (0.6 cm in diameter and 0.1 cm in thickness) with a geometric area of 0.2826 cm ² was ultrasonically cleaned with deionized water, absolute ethanol and acetone in sequence for 30 minutes to remove organic matter on the surface. Place in the oven, dry at 40 degrees for 12 hours, and set aside. The dried graphite sheet is fixed on the bottom of the electrode rod (polytetrafluoroethylene shell with conductive copper rod) with conductive silver glue, as an electrode.

Graphite electrode was used as working electrode, stone grinding rod was used as counter electrode, and mercurous sulfate electrode was used as reference electrode. The electrodeposition solution is a 10mmol/L Co(OH) ₂ solution with a volume of 60mL. Before the start of the electrodeposition reaction, the electrolyte was exposed to high-purity nitrogen for 30 minutes, and N ₂ was also continuously fed during the reaction. The electrodeposition potential is -1.42V (vs.Hg/Hg ₂ SO ₄ ), and the deposition time is 15min. After the deposition, the samples were rinsed with ultrapure water and dried at 60°C for 12 hours.

The Co ₃ O ₄ catalyst electrode was obtained by annealing treatment. The above-mentioned Co(OH) ₂ /Graphite was placed in a tube furnace, and nitrogen gas was introduced for half an hour before the start of the reaction to remove the oxygen in the tube furnace. to room temperature. Then, the temperature was raised to 400° C. at a heating rate of 5° C./min, and the reaction time was 2 hours. After the reaction, the temperature was naturally lowered to room temperature to obtain a Co ₃ O ₄ /Graphite catalyst electrode. The morphology picture of Co ₃ O ₄ /Graphite obtained by scanning electron microscope is shown in Figure 1-1, and the XPS obtained by X-ray photoelectron spectroscopy is shown in Figure 1-2.

Example 1

The pretreatment of graphite sheet and the preparation method of precursor Co(OH) are the same as embodiment ₁ .

The Co ₃ O _3.68 N _0.32 catalyst electrode was obtained by annealing method. Put the above-mentioned Co(OH) ₂ /Graphite in the tube furnace, and pass the NH ₃ /N ₂ mixture gas with an ammonia content of 10% for half an hour before the reaction starts to remove the oxygen in the tube furnace , After the reaction started, the gas was continuously introduced until the temperature dropped to room temperature after the reaction was completed. Then, the temperature was raised to 400° C. at a heating rate of 5° C./min, and the reaction time was 2 hours. After the reaction, the temperature was naturally lowered to room temperature to obtain a Co ₃ O _3.68 N _0.32 /Graphite catalyst electrode. The morphology picture of Co ₃ O _3.68 N _0.32 /Graphite obtained by scanning electron microscope is shown in Figure 2-1, which shows that the morphology of the catalyst is a porous nanosheet array. The formation of the porous structure may be due to the fact that the calcination atmosphere is NH ₃ , active free radicals are generated at a high temperature of 400 degrees, and the Co(OH) ₂ nanosheets are etched, thus forming a porous structure. The XPS obtained by X-ray photoelectron spectroscopy is 2-2, indicating that the catalyst is composed of Co, O, and N, and the element ratio is close to Co:O:N≈3:3.62:0.38. The sum of O and N in the composition formula is 4, probably because a part of N element replaces the position of O element in the reaction process (forming a Co-O bond), and a part of N and O form a polymeric group of -NO-.

Example 2

The experimental process is the same as that of Example 2, except that the annealing temperature is changed to 200°C, 300°C and 500°C. The morphology pictures of the samples obtained by scanning electron microscopy at different calcination temperatures are shown in Figures 3-1 (200°C), 3-2 (300°C) and 3-3 (500°C). When the temperature is 200 °C, the reaction temperature is low and no free radicals are generated, so the morphology of the sample does not change. When the temperature rises to 300°C, free radicals begin to be generated, but the amount of free radicals is small, and the etching reaction is slow, so fewer pores are formed. As the temperature rises to 400°C, the amount of free radicals increases, so the porous structure shown in the figure will appear. When the temperature rises to 500°C, because the temperature is too high, the etching is severe, so the nanosheet structure breaks down. Therefore, the catalyst obtained at 400°C has the best catalytic performance, as shown in Figure 4-1.

Example 3

An electrochemical workstation with a three-electrode system was used, 1M KOH solution was used as the electrolyte, the samples prepared under the above different conditions were used as the working electrode, the counter electrode was a stone mill rod, and the reference electrode was a Hg/HgO electrode to measure the electrocatalytic performance of the catalyst. Before the experiment, the electrolyte was continuously exposed to _N2 for 30 minutes to eliminate the oxygen in the system, and nitrogen gas was continuously introduced during the test. The scan rate is 10mV/s, and the polarization curve of the electrode is shown in Figure 4-1. The sample Co ₃ O _3.68 N _0.32 prepared at the annealing temperature of 400°C has the best catalytic efficiency, and the ^potential is 0.23V. Therefore, Co ₃ O _3.68 N _0.32 and Co ₃ O ₄ were used as the research objects in the follow-up research. As shown in Figure 4-2, the Tafel values of Co ₃ O _3.68 N _0.32 and Co ₃ O ₄ are 48 and 65mV/decade. The electrochemical impedance diagrams of Co ₃ O _3.68 N _0.32 and Co ₃ O ₄ are shown in Figure 4-3. The research shows that the Co ₃ O _3.68 N _0.32 catalyst has smaller impedance. The stability test adopts the current-time curve test, and the test results are shown in Figure 4-4, and the activity of Co ₃ O _3.68 N _0.32 has not decreased significantly after 20 hours of testing. The oxygen production efficiency curve of the electrode is shown in Figure 4-5, and the oxygen production efficiency of the electrode is close to the theoretical calculation value.

Example 5

Taking Co ₃ O _3.68 N _0.32 and Co ₃ O ₄ as the research objects, the electronic structure of the catalysts was studied by first-principles. With CP2K as the calculation program, the PBE function corrected by D3 is used to describe the system. The number of U in the calculation process is 5.9eV. The Gibbs free energy of water splitting oxygen production reaction was calculated using Formula: ΔG=E _DFT +ΔZPE–TΔS. At the same time, the electronic spin resonance method is used to further illustrate the change of the electronic structure of the catalyst. As shown in Figures 5-1 and 5-2, Co ₃ O _3.68 N _0.32 has significantly improved electrical conductivity compared to Co ₃ O ₄ . As shown in Figure 5-3, Co ₃ O _3.68 N _0.32 has better adsorption energy for reaction intermediates.

Claims (5)

1. a kind of preparation method of efficient, novel porous nitrogen oxides nanometer sheet catalyst electrode, it is characterised in that including as follows Step：

1) presoma Co (OH)₂The preparation of nanometer sheet：

Configure 5-10mM Co (NO₃)₂Solution as electrolyte, under room temperature, using Graphite as working electrode, stone mill stick For to electrode, mercury/mercuric oxide electrode is reference electrode, under conditions of -1.42V, electrochemical deposition obtains Co (OH)₂Nanometer Piece, the Co (OH)₂The thickness of nanometer sheet is 10-20nm；

2)Co₃O_3.62N_0.38The preparation of/Graphite：

The presoma that will be obtained in step 1), in 380-420 degrees Celsius of NH₃0.5h or more is calcined under ambient conditions, it is then natural It is cooled to room temperature and obtains Co₃O_3.62N_0.38/ Graphite electrodes.

2. preparation method according to claim 1, it is characterised in that the NH₃Atmosphere is that ammonia level is 10 NH₃/N₂Gaseous mixture.

3. preparation method according to claim 1, it is characterised in that the calcination temperature is preferably 400 DEG C.

4. efficient, the novel porous nitrogen oxides nanometer sheet catalyst electrode that a kind of method as described in claim 1 is prepared, It is characterized in that the electrode is Co₃O_3.62N_0.38/ Graphite electrodes；Its current density is 10mA/cm²When, required mistake Potential is only 0.23V, and the catalyst has porous nano chip architecture, the Co₃O_3.62N_0.38With spinel structure.

5. a kind of porous nitrogen oxides nanometer sheet catalyst electrode answering in electrochemical decomposition aquatic products oxygen as claimed in claim 4 With.