CN113659161A - Electrocatalyst and preparation method and application thereof - Google Patents
Electrocatalyst and preparation method and application thereof Download PDFInfo
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Abstract
The invention belongs to the technical field of catalysts, and particularly discloses an electrocatalyst, a preparation method and application thereof, wherein the electrocatalyst comprises a nitrogen-doped carbon material and a sulfur-cobalt compound, and the nitrogen-doped carbon material wraps the sulfur-cobalt compound; the preparation method of the electrocatalyst comprises the following steps: s1, mixing inorganic nanospheres, 2-methylimidazole and cobalt salt, and performing pyrolysis reaction to obtain a cobalt-nitrogen-carbon composite material wrapping the inorganic nanospheres; s2, removing the inorganic nanospheres by using an alkali solution to prepare a three-dimensional porous cobalt nitrogen carbon composite material; and S3, carrying out heat treatment on the three-dimensional porous cobalt-nitrogen-carbon composite material by using thiourea to prepare the electrocatalyst. According to the preparation method, the preparation method comprises the steps of firstly carbonizing and then vulcanizing, wherein the carbonization ensures the generation of an ORR active site Co-Nx, the vulcanization generates a sulfur-cobalt compound in the composite material, so that OER electrocatalytic activity is generated, and the dual-function electrocatalytic activity is realized through the synergistic effect of the sulfur-cobalt compound and the Co-Nx.
Description
Technical Field
The invention belongs to the technical field of catalysts, particularly relates to the technical field of battery cathode catalysts, and particularly relates to an electrocatalyst and a preparation method and application thereof.
Background
The exploration of electric energy conversion and energy storage technology which can be used for new energy automobiles becomes an inevitable requirement for pursuing sustainable development and reducing carbon dioxide emission in modern society. The metal-air battery has attracted extensive attention of researchers due to its high theoretical energy density and safety, and is considered as a potential next-generation clean energy device. Among them, zinc-air batteries are particularly valued for their advantages of high energy density, good electrochemical stability, low internal resistance, safe operation, abundant resources, and environmental friendliness. However, slow kinetics of Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) on the electrode have become important factors that restrict the development of rechargeable zinc-air batteries.
Noble metal electrocatalysts (e.g., Pt/C, Ir/C and RuO)2) Has good catalytic activity to ORR or OER. However, the noble metal electrocatalyst has the defects of single electrocatalytic activity, high cost, natural scarcity, poor stability and the like, and influences the large-scale development and application of the zinc-air battery. Although the metal-nitrogen-carbon composite catalyst has better ORR catalytic activity, the OER electro-catalytic performance of the metal-nitrogen-carbon composite catalyst is poorer.
Therefore, there is a need to develop an electrocatalyst with electrocatalytic activity for dual-functional ORR and OER that can replace precious metals, which is cost-effective.
Disclosure of Invention
The invention provides an electrocatalyst, a preparation method and application thereof, which are used for solving one or more technical problems in the prior art and providing at least one beneficial selection or creation condition.
To overcome the above technical problems, a first aspect of the present invention provides an electrocatalyst:
specifically, an electrocatalyst includes a nitrogen-doped carbon material and a sulfur-cobalt compound, the nitrogen-doped carbon material encapsulating the sulfur-cobalt compound.
The electrocatalyst of the invention is coated with nitrogen-doped carbon materialThe electrocatalyst contains Co-NxA coordination structure, wherein: x is 2 or 4, a thiocobalt compound and Co-NxThe coordination structures are synergistic, and excellent ORR and OER dual-functional electrocatalytic activity is realized together. Wherein: cobalt bonding to nitrogen (Co-N)x) The active structure of (2) is beneficial to the absorption and desorption of oxygen intermediates in oxygen reduction, thereby having excellent ORR catalytic activity; the sulfur cobalt compound has proper oxygen precipitation intermediate adsorption, cobalt oxyhydroxide is formed in the oxidation process and is a real active site of OER, and sulfur is further oxidized into sulfate radical and is adsorbed on the surface of the cobalt oxyhydroxide, so that the OER performance is further improved.
As a further improvement of the above, the cobalt sulfide compound comprises CoS and/or Co9S8CoS and Co9S8Both having electrocatalytic activity, wherein: co9S8Is a product with incomplete sulfurization reaction, and has higher catalytic activity due to the lack of one sulfur atom.
As a further improvement of the above scheme, the raw materials for preparing the electrocatalyst include inorganic nanospheres, 2-methylimidazole, cobalt salts and thiourea.
Specifically, the inorganic nanospheres are used as templates and do not participate in specific reactions; reacting 2-methylimidazole with cobalt salt to generate a metal organic framework composite material, combining a template with the metal organic framework composite material (ZIF-67), and pyrolyzing to form a cobalt nitrogen carbon composite material wrapping inorganic nanospheres; after the inorganic nanospheres are removed, the cobalt nitrogen carbon composite material (Co @ N-C) with a three-dimensional porous structure is obtained, the transmission of electrolyte can be promoted, the exposure of active sites can be increased, and the catalytic activity of the electrocatalyst can be effectively improved. Meanwhile, three-dimensional porous Co @ N-C reacts with thiourea to generate Co9S8And/or CoS-CoCo sulfide, and finally obtaining the nitrogen-sulfur doped carbon-based metal sulfide electrocatalyst (Co)xSy@ N-C), wherein: coxSyIs CoS and Co9S8The sulfur cobalt compound imparting OER electrocatalytic activity to the electrocatalyst, CoxSy@ N-C electro-catalysis having both ORR and OER functionsActivating activity.
Preferably, the cobalt salt comprises at least one of cobalt nitrate, cobalt nitrate hydrate, cobalt acetate and cobalt chloride.
As a further improvement of the above scheme, the molar ratio of the cobalt salt to the 2-methylimidazole is 1: (10-20); preferably, the molar ratio of the cobalt salt to the 2-methylimidazole is 1: 15.
preferably, the inorganic nanospheres are selected from SiO2Nanospheres of Al2O3Any one of nanospheres and ZnO nanospheres; further preferably, the inorganic nanospheres are selected from SiO2Nanospheres.
Preferably, the average diameter of the inorganic nanospheres is 50-500 nm; preferably, the average diameter of the inorganic nanospheres is 100 nm.
As a further improvement of the above scheme, the mass ratio of the cobalt salt to the inorganic nanospheres is 1: (1-3); preferably, the mass ratio of the cobalt salt to the inorganic nanospheres is 1: 2.2.
a second aspect of the invention provides a method of preparing an electrocatalyst:
specifically, the preparation method of the electrocatalyst comprises the following steps:
s1, mixing inorganic nanospheres, 2-methylimidazole and cobalt salt, and performing pyrolysis reaction to obtain a cobalt-nitrogen-carbon composite material wrapping the inorganic nanospheres;
s2, removing the inorganic nanospheres in the inorganic nanosphere-wrapped cobalt-nitrogen-carbon composite material by adopting an alkali solution to prepare a three-dimensional porous cobalt-nitrogen-carbon composite material;
and S3, carrying out heat treatment on the three-dimensional porous cobalt-nitrogen-carbon composite material by using thiourea to prepare the electrocatalyst.
As a further improvement of the above scheme, a method for preparing an electrocatalyst comprises the steps of:
s1, mixing SiO2Placing the nanospheres and 2-methylimidazole in water for ultrasonic dispersion, adding cobalt nitrate for stirring to fully react, standing, filtering, drying, and performing high-temperature pyrolysis reaction to obtain wrapped SiO2Nano meterA cobalt nitrogen carbon composite of spheres;
s2, removing the wrapped SiO by adopting an alkali solution2SiO in cobalt-nitrogen-carbon composite material of nanosphere2Washing and drying, then continuously treating with acid solution, washing and drying to obtain the cobalt-nitrogen-carbon composite material;
and S3, carrying out heat treatment on the three-dimensional porous cobalt-nitrogen-carbon composite material by using thiourea to prepare the electrocatalyst.
Specifically, during the standing in step S1, Co in the cobalt salt2+Coordinated and combined with nitrogen in 2-methylimidazole to form a metal organic framework composite material by assembly, and the metal organic framework composite material is wrapped in SiO2On the nanosphere, SiO wrapping is finally formed2Metal organic framework composite material of nanosphere (ZIF-67@ SiO)2). After high-temperature carbonization, wrapping SiO2The metal organic framework composite material of the nanosphere forms wrapping SiO under the protection of nitrogen2Nanosphere cobalt-nitrogen-carbon composites, SiO2The nanospheres do not react at high temperatures.
In step S2, SiO in the cobalt-nitrogen-carbon composite material is removed by an alkali solution2Nanospheres and washing out unstable impurities with acid solution.
In step S3, thiourea decomposes during the heat treatment: SC (NH)2)2→NH2CN+H2S，H2S reacts with simple substance Co and CoO in the cobalt nitrogen carbon composite material at high temperature to form sulfur cobalt compound CoS or Co9S8。
As a further improvement of the above scheme, in step S1, the pyrolysis reaction conditions are: under the protective atmosphere, the reaction temperature is 700-900 ℃, and the heat preservation time is 1-3 hours; preferably, the rate of temperature rise is 2-10 deg.C/min.
Specifically, the reaction is carried out in a protective atmosphere, oxygen is removed, and the metal organic framework composite material is favorably carbonized to form the cobalt nitrogen carbon composite material. If oxygen exists, carbon in the cobalt nitrogen carbon composite material is converted into carbon dioxide, and cobalt is converted into cobalt oxide, so that the sulfur cobalt carbon composite material cannot be obtained.
As at the topIn a further improvement of the above solution, in step S2, the removing of the wrapping SiO2The method for preparing the inorganic nanospheres in the cobalt-nitrogen-carbon composite material of the nanospheres comprises the following steps: wrapping the SiO2Placing the cobalt nitrogen carbon composite material of the nanosphere in an alkaline solution for 5-8 hours, wherein the temperature of the alkaline solution is 60-80 ℃;
preferably, in the acid solution treatment: the concentration of the acid solution is 0.1-1mol/L, and the treatment time is 5-30 min;
preferably, the concentration of the alkali solution is 4 to 8 mol/L.
As a further improvement of the above, in step S3, the heat treatment conditions are: the reaction is carried out under the protective atmosphere, the reaction temperature is 400-; preferably, the rate of temperature rise is 4-6 deg.C/min.
Preferably, in steps S1 and S3, the protective atmosphere is a nitrogen atmosphere.
As a further improvement of the above scheme, in step S3, the mass ratio of the cobalt nitrogen carbon composite material to the thiourea is (1-5): (1-5).
A third aspect of the invention provides the use of an electrocatalyst as described above:
in particular, the use of an electrocatalyst in a battery.
Preferably, the use of an electrocatalyst in a metal air cell.
More preferably, the use of an electrocatalyst in a zinc air cell.
Compared with the prior art, the technical scheme of the invention at least has the following technical effects or advantages:
according to the invention, the inorganic nanospheres are used as templates to be combined with the cobalt-nitrogen-carbon composite material, and the inorganic nanospheres are removed after high-temperature carbonization, so that the formed three-dimensional porous structure can promote the transmission of electrolyte, increase the exposure of active sites, and effectively improve the catalytic activity of the electrocatalyst. Meanwhile, compared with noble metals, the transition metal cobalt has lower cost and higher stability, and is beneficial to the actual popularization and use of the electrocatalyst.
The invention relates to a preparation method by first carbonizing and then vulcanizingThe formation of ORR active sites Co-Nx is ensured by carbonization, and Co is generated in the composite material by vulcanization9S8And CoS and other sulfur-cobalt compounds, so that OER electrocatalytic activity is generated, and finally, excellent ORR and OER dual-functional electrocatalytic activity is realized through the synergistic effect of the sulfur-cobalt compounds and Co-Nx.
Detailed Description
The present invention is described in detail below by way of examples to facilitate understanding of the present invention by those skilled in the art, and it is to be specifically noted that the examples are provided only for the purpose of further illustrating the present invention and are not to be construed as limiting the scope of the present invention.
Drawings
FIG. 1 is a graph of electrochemical performance of electrocatalysts of examples 1-3 and comparative examples 1-2;
FIG. 2 is a graph of the electrochemical performance of the electrocatalysts of examples 1, 4 and 5;
FIG. 3 is a graph of the performance of the electrocatalysts of example 1 and comparative example 2 applied to a zinc-air cell;
FIG. 4 is SiO2SEM scanning electron micrographs of nanospheres and the electrocatalyst of comparative example 1;
FIG. 5 is an SEM scanning electron micrograph of the electrocatalyst for examples 1-3;
figure 6 is an X-ray diffraction pattern of the electrocatalysts of examples 1-3 and comparative example 1.
The present invention is described in detail below by way of examples to facilitate understanding of the present invention by those skilled in the art, and it is to be specifically noted that the examples are provided only for the purpose of further illustrating the present invention and are not to be construed as limiting the scope of the present invention.
Example 1
An electrocatalyst prepared from raw materials comprising: SiO 22Nanospheres, 2-methylimidazole, cobalt nitrate and thiourea. Wherein: the molar ratio of cobalt nitrate to 2-methylimidazole is 1: 15, cobalt nitrate and SiO2The mass ratio of the nanospheres is 1: 2.2, SiO2The average diameter of the nanospheres is 100 nm.
A method of preparing an electrocatalyst: the method comprises the following steps:
s1, taking a 100mL clean beaker, adding 0.80g of SiO into the clean beaker2Carrying out ultrasonic treatment on the nanospheres, 2.65g of 2-methylimidazole and 20mL of water for 1 hour to uniformly disperse the nanospheres; then 4mL of 0.5mol/L cobalt nitrate solution is added, ultrasonic reaction is carried out for 10 minutes, and after standing for 6 hours, SiO coating is prepared2A precursor of the cobalt nitrogen carbon composite material of the nanosphere; heating the prepared precursor to 800 ℃ at the heating rate of 5 ℃/min in a tubular furnace filled with nitrogen atmosphere, and reacting at the temperature of 800 ℃ for 2 hours stably to carry out high-temperature carbonization treatment to obtain the wrapped SiO2Cobalt nitrogen carbon composite material (Co-N-C @ SiO) of nanosphere2)。
S2, mixing Co-N-C @ SiO2Adding A into 6mol/L NaOH solution, performing alkali treatment in 70 deg.C water bath for 6 hr to remove SiO2Filtering, washing and drying the nanospheres; and adding 30mL0.5mol/L hydrochloric acid solution, stirring at normal temperature for 15min, washing out unstable impurities, filtering again, washing, and drying to obtain the cobalt nitrogen carbon composite material (Co @ N-C).
S3, weighing 50mg of Co @ N-C in a ceramic square boat, weighing 50mg of thiourea in another ceramic square boat, putting the two square boats into a tubular furnace filled with nitrogen atmosphere, heating to 60 ℃ at the rate of 5 ℃/min, and maintaining the temperature at 600 ℃ for reaction for 2 hours to obtain an electrocatalyst sample of the nitrogen-sulfur-doped carbon-based metal sulfide, wherein the sample is marked as Co @ N-CxSy@N-C-2-600。
Example 2
EXAMPLE 2 preparation of electrocatalyst the raw material components and preparation method are substantially the same as in example 1 except that the amount of thiourea used in example 2 was 250mg, and a sample of the prepared electrocatalyst, designated CoxSy@N-C-1-600。
Example 3
EXAMPLE 3 preparation of electrocatalyst the raw material components and preparation method are substantially the same as in example 1 except that the amount of thiourea used in example 3 was 10mg, and a sample of the prepared electrocatalyst, designated CoxSy@N-C-3-600。
Example 4
Example 4 preparation of electrocatalyst the raw material components and preparation method are substantially the same as in example 1 except that in step S3 of example 4, the reaction temperature is 400 ℃, and the prepared electrocatalyst sample, labeled Co, is obtainedxSy@N-C-2-400。
Example 5
Example 5 preparation of electrocatalyst the raw material components and preparation method are substantially the same as in example 1 except that in step S3 of example 5, the reaction temperature is 800 ℃, and the prepared electrocatalyst sample, labeled Co, is obtainedxSy@N-C-2-800。
Comparative example 1
Comparative example 1 preparation of electrocatalyst the starting material components were the same as in example 1 and the preparation of the electrocatalyst for comparative example 1 was compared to example 1 except that comparative example 1 did not include step S3, i.e. no thiourea sulfidation treatment was performed and a sample of the electrocatalyst prepared, labeled Co @ N-C.
Comparative example 2
Commercial 20% Pt/C and RuO2A noble metal electrocatalyst.
Performance test 1: electrochemical performance test
(1) ORR and OER performance tests of nitrogen-doped carbon-based sulfur cobalt compound electrocatalysts of different degrees of sulfidation
Electrochemical performance tests were performed on samples of the electrocatalysts prepared in examples 1-3 and comparative examples 1-2, and ORR and OER performance of the samples were analyzed for different degrees of sulfidation.
The working electrode has an electrode area of 0.126cm-2The glassy carbon electrode of (1), wherein the loading amount of the electrocatalyst is 0.12mg cm-2. The ORR and OER performance tests of the electrocatalyst employ a typical three-electrode system. Wherein the counter electrode is graphite rod, the reference electrode is Hg/HgO electrode, all tests are carried out in 0.1M KOH solution, linear volt-ampere test technique is adopted, and scanning speed is 10 mV.s-1The rotation speed is 1600 rpm. The specific test results are detailed in table 1 and fig. 1, wherein: all potentials in fig. 1 are converted to relative hydrogen potentials by the conversion equation: eRHE＝EHg/HgO+0.098+0.05916 pH; FIG. 1a is an ORR linear voltammogram of an electrocatalyst; FIG. 1b is the corresponding Tafel slope for ORR; FIG. 1c is an OER linear voltammogram of an electrocatalyst; FIG. 1d shows the Taphenanthrene slope corresponding to OER. In fig. 1a and 1c, the abscissa represents voltage and the ordinate represents current density; in fig. 1b and 1d, the abscissa represents the current density and the ordinate represents the voltage.
Table 1: comparative table of electrochemical properties of electrocatalyst samples prepared in examples 1-3 and comparative examples 1-2
As can be seen from Table 1 and FIG. 1, the electrocatalyst Co obtained in example 1 is comparable to those obtained in examples 2 to 3 and comparative example 1xSy@ N-C-2-600 has optimal ORR and OER electrochemical performance. Under alkaline conditions, Co is shown in FIG. 1axSy@ N-C-2-600 has an initial potential of 0.91V (vs. RHE), a half-wave potential of 0.81V (vs. RHE) and a limiting current of up to 5.16mA cm-2All outperformed the other samples of varying degrees of sulfidation and approached 20% Pt/C noble metal electrocatalyst. As can be seen from FIG. 1b, CoxSy@ N-C-2-600 has the lowest Tafel (Tafel) slope of 57.53mV dec-1Less than 20% Pt/C and other sulfided catalysts, further illustrating that they have advantagesDifferential ORR kinetic reaction rates. In addition to that, CoxSy@ N-C-2-600 also exhibited excellent OER electrocatalytic activity. At 10mA cm-2When is CoxSyThe @ N-C-2-600 potential is 1.61V, the Tafel slope is 212.38mV dec-1, is superior to other sulfidized electrocatalysts, and is close to the noble metal RuO2. The half-wave potential and OER of ORR are usually 10mA cm-2The charge/discharge reversibility of the electrode reaction can be evaluated. CoxSy@ N-C-2-600 has a minimum voltage differential of 0.8V, further demonstrating excellent ORR and OER electrocatalytic activity simultaneously.
(2) ORR and OER performance tests of nitrogen-doped carbon-based sulfur cobalt compound electrocatalyst with different sulfurization reaction temperatures
Samples of the electrocatalysts prepared in examples 1, 4 and 5 were subjected to electrochemical performance tests and comparative analyses of the ORR and OER performance of the electrocatalysts at different sulfidation reaction temperatures were performed.
The working electrode has an electrode area of 0.126cm-2The glassy carbon electrode of (1), wherein the loading amount of the electrocatalyst is 0.12mg cm-2. The ORR and OER performance tests of the electrocatalyst employ a typical three-electrode system. Wherein the counter electrode is a graphite rod and the reference electrode is an Hg/HgO electrode. All tests were carried out in 0.1M KOH solution using a linear voltammetric technique with a sweep rate of 10 mV. s-1The rotation speed is 1600 rpm. The specific test results are detailed in table 2 and fig. 2, wherein: all potentials in fig. 2 are converted to relative hydrogen potentials by the conversion equation: eRHE＝EHg/HgO+0.098+0.05916 pH; FIG. 2a is a cyclic voltammogram; FIG. 2b is an ORR linear voltammogram of an electrocatalyst; FIG. 2c is the corresponding Tafel slope for ORR; FIG. 2d is an OER linear voltammogram of an electrocatalyst; FIG. 2e is the Taffel slope corresponding to OER; FIG. 2f is a long term stability comparison curve of the ORRs of example 1 and comparative example 2. In fig. 2a, 2b and 2d, the abscissa represents voltage and the ordinate represents current density; in fig. 2c and 2e, the abscissa represents current density and the ordinate represents voltage; in fig. 2f the horizontal ordinate indicates time and the vertical ordinate indicates the relative current density.
Table 2: comparative table of electrochemical properties of electrocatalyst samples prepared in examples 1, 4-5
As shown in FIG. 2a, it can be seen from the oxygen reduction peak positions that the electrocatalysts sulfided at different temperatures all have oxygen reduction performance, wherein the Co prepared in example 1 hasxSy@ N-C-2-600 has optimal ORR electrocatalytic activity. As shown in FIG. 2b, CoxSyThe peak potential of @ N-C-2-600 is 0.91V (vs. RHE), which is greater than that of CoxSy@ N-C-2-800 and CoxSy@ N-C-2-400, the larger the peak potential is, the better the catalytic performance is. And from the half-wave potential, CoxSyThe half-wave potential of @ N-C-2-600 is then 0.81V, very close to 20% Pt/C. At the limiting current, Co can be foundxSyThe limiting current of @ N-C-2-600 is maximum and 5.16mA cm-2Better than 20% Pt/C, further illustrating CoxSy@ N-C-2-600 has optimal ORR electrochemical performance. From the Tafel slope of FIG. 2c, Co can be seenxSyThe Tafel slope of @ N-C-2-800 is 50.76mV dec-1Electrocatalysts with greater than other sulfiding temperatures, demonstrate higher ORR kinetic rates. Furthermore, as can be seen from the ORR stability test results in fig. 2 f: after 20 hours, CoxSyThe current density of @ N-C-2-600 remained at 97.20% as before, while Pt/C remained at only 89.05%, indicating that CoxSy@ N-C-2-600 has greater stability than Pt/C. FIGS. 2d and 2e show the linear voltammograms of OERs and their Tafel slopes at 1600rpm for electrocatalysts of different sulfidation reaction temperatures. It can be seen that at 1600rpm, 10mA cm-2When is CoxSyThe electric potential of @ N-C-2-600 is 1.61V, the Tafel slope is 212.38mV dec-1(ii) a At 10mA cm-2When is CoxSyThe potential of @ N-C-2-400 is 1.66V, and the Tafel slope is 227.96mV dec-1(ii) a At 10mA cm-2When is CoxSyThe potential of @ N-C-2-800 is 1.65V, and the Tafel slope is 169.46mV dec-1. Thus, Co in example 1 was found to bexSyThe OER catalytic activity of @ N-C-2-600 is more excellent in the three samples, probably because the sulfide is further agglomerated due to the fact that the vulcanization temperature is too high, so that the exposure of active sites is reduced, and the vulcanization effect is influenced by too low temperature, so that the performance is further improved.
Performance test 2: zinc-air battery performance testing
The electrocatalyst Co prepared in example 1xSyThe application of the invention in the zinc air battery is further explored by assembling the @ N-C-2-600 serving as a cathode catalyst into the liquid-phase zinc air battery and comparing the discharge performance test by taking 20% Pt/C of the comparative example 2 as a comparative sample. The performance of the electrocatalysts of example 1 and comparative example 2 applied to a zinc-air cell is detailed in fig. 3, wherein: fig. 3a is the open circuit potential of the zinc-air cells of example 1 and comparative example 2; fig. 3b is a discharge polarization curve and a power density curve of the zinc-air batteries of example 1 and comparative example 2. The abscissa of fig. 3a represents time and the ordinate represents voltage; the abscissa of fig. 3b represents the current density and the ordinate represents the voltage and power density.
As shown in FIG. 3a, the cathode catalyst is CoxSyThe open circuit potential of the zinc-air battery of @ N-C-2-600 was 1.45V, which was close to 1.46V, which is the open circuit voltage of the zinc-air battery using 20% Pt/C as the cathode catalyst. In addition to this, Co can be seen from the discharge curve in FIG. 3bxSyThe @ N-C-2-600 catalyst is almost in a linear polarization curve in a high current density range, and shows that the mass transfer effect is good, namely the diffusion/concentration polarization is low. This is mainly due to the fact that the three-dimensional porous structure facilitates mass and charge transport during operation. In contrast, the 20% Pt/C catalyst has concentration polarization in the high current density region, and the slow mass transfer causes a significant loss in current density. CoxSy@ N-C-2-600 cathode catalyst for zinc-air cell at 103.6mA cm-2The maximum power density is 70.31mW cm-2Zinc air cell at 112.6mA cm near 20% Pt/C cathode catalyst-2Maximum power density of 82.91mW cm-2. Thus, CoxSyThe @ N-C-2-600 has excellent discharge performance, can be applied to zinc-air batteries, and has a certain reference value for large-scale development of the zinc-air batteries.
Performance test 3: electrocatalysts of different degrees of vulcanization, SiO2Microstructure testing of nanospheres and comparative example 1 electrocatalyst
FIG. 4 is a graph of SiO with an average diameter of 100nm2SEM images of nanospheres and base treated Co @ N-C, wherein: FIG. 4a is SiO2Scanning electron microscope images of nanospheres; fig. 4b is a scanning electron micrograph of the electrocatalyst in comparative example 1. SiO as a template can be seen from (a) and (b) in FIG. 42The nanospheres are uniform in size and free of agglomeration, which is favorable for forming regularly and orderly wrapped SiO2Precursor of the cobalt nitrogen carbon composite material of the nanosphere. The removal of SiO by base treatment can be seen in the SEM images of (C) and (d) Co @ N-C in FIG. 42The Co @ N-C catalyst of the nanosphere template has uniform surface aperture which is about 100nm, and macropores are mutually connected to form a three-dimensional porous nano structure, so that the specific surface area of the catalyst is effectively increased, a large number of channels are provided for the rapid transmission of oxygen and OH-, the active sites are more favorably exposed, and the electrochemical activity of the catalyst is improved.
FIG. 5 is sample Co of the electrocatalyst prepared in examples 1-3xSy@N-C-1-600、CoxSy@ N-C-2-600 and CoxSySEM picture of @ N-C-3-600, where: FIGS. 5a and 5b are scanning electron micrographs of the electrocatalyst for example 2; FIGS. 5c and 5d are scanning electron micrographs of the electrocatalyst for example 1; fig. 5e and 5f are scanning electron micrographs of the electrocatalyst for example 3. As can be seen from the figure, the microstructure appearance of the three catalysts has no great change compared with Co @ N-C before vulcanization, the surface pore size is uniform, the original three-dimensional porous structure is maintained, namely, the appearance of the catalyst is not greatly influenced by adopting different sulfur contents during vulcanization, and a reactant transmission channel is not blocked.
As shown in FIG. 6, by PDF standard card comparison, CoxSy@N-C-1-600、CoxSy@N-C-2-600、CoxSy@ N-C-3-600 andthe XRD 2 theta diffraction peaks of Co @ N-C are both 44.09 degrees, which correspond to the (002) crystal face of Co (PDF #01-071-4652) of the PDF standard card, thereby indicating that part of the simple cobalt is remained after vulcanization. CoxSy@N-C-1-600、CoxSy@ N-C-3-600 all have CoS (PDF # 97-062-4857). Wherein, CoxSy@N-C-1-600、CoxSy@ N-C-2-600 Presence of Co9S8(PDF # 01-073-6395). In addition, by comparison, Co can also be observedxSyCoO exists at @ N-C-3-600 and Co @ N-C (PDF # 97-017-. Thus, it can be seen that: coxSy@ N-C-1-600 contains Co and Co9S8And CoS; coxSy@ N-C-2-600 contains Co and Co9S8；CoxSy@ N-C-3-600 contains Co, CoS, CoO; co @ N-C contains Co and CoO. At 600 ℃, thiourea will decompose to NH3、H2S and CO2Gas, with some reducibility. When the amount of thiourea is small, the reducibility is weak and the hydrogen sulfide component is small, and only a small part of CoO can be converted into CoS; when the thiourea content is relatively high, CoO is reduced into simple substance cobalt in reducing atmosphere and is vulcanized into Co9S8. When the thiourea is in excess, sulfidation forms Co9S8And sulfides of CoS. This is probably the reduction of CoO to elemental cobalt, sulfidation leading to the formation of Co9S8The transition portion CoS was over-sulfurized. The invention converts agglomerated elemental cobalt into active CoS and Co9S8Greatly improves the catalytic activity of Co @ N-C, and simultaneously CoS and Co9S8And is exposed in a specific three-dimensional porous structure, so that the electrocatalytic activity and stability of the catalyst are improved.
It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are intended to be within the scope of the invention.
Claims (10)
1. An electrocatalyst, comprising a nitrogen-doped carbon material and a sulphur-cobalt compound, the nitrogen-doped carbon material encapsulating the sulphur-cobalt compound.
2. Electrocatalyst according to claim 1, characterized in that the sulphur-cobalt compound comprises CoS and/or Co9S8。
3. Electrocatalyst according to claim 1 or 2, characterized in that the raw materials for preparing the electrocatalyst comprise inorganic nanospheres, 2-methylimidazole, cobalt salts and thiourea;
preferably, the cobalt salt comprises at least one of cobalt nitrate, cobalt nitrate hydrate, cobalt acetate and cobalt chloride.
4. Electrocatalyst according to claim 3, characterized in that the molar ratio of cobalt salt to 2-methylimidazole is 1: (10-20);
preferably, the mass ratio of the cobalt salt to the inorganic nanospheres is 1: (1-3).
5. Electrocatalyst according to claim 3, characterized in that the inorganic nanospheres are selected from SiO2Nanospheres of Al2O3Any one of nanospheres and ZnO nanospheres;
preferably, the inorganic nanospheres have an average diameter of 50-500 nm.
6. A process for preparing an electrocatalyst according to any one of claims 1 to 5, comprising the steps of:
s1, mixing inorganic nanospheres, 2-methylimidazole and cobalt salt, and performing pyrolysis reaction to obtain a cobalt-nitrogen-carbon composite material wrapping the inorganic nanospheres;
s2, removing the inorganic nanospheres in the inorganic nanosphere-wrapped cobalt-nitrogen-carbon composite material by adopting an alkali solution to prepare a three-dimensional porous cobalt-nitrogen-carbon composite material;
and S3, carrying out heat treatment on the three-dimensional porous cobalt-nitrogen-carbon composite material by using thiourea to prepare the electrocatalyst.
7. The method for preparing an electrocatalyst according to claim 6, wherein in step S1, the conditions for the pyrolysis reaction are: under the protective atmosphere, the reaction temperature is 700-900 ℃, and the heat preservation time is 1-3 hours;
preferably, in step S3, the heat treatment conditions are: the reaction is carried out under the protective atmosphere, the reaction temperature is 400-;
preferably, the protective atmosphere is a nitrogen atmosphere.
8. The method for preparing the electrocatalyst according to claim 6, wherein in step S2, the method for removing the inorganic nanospheres in the inorganic nanosphere-wrapped cobalt-nitrogen-carbon composite material is: placing the inorganic nanosphere-coated cobalt-nitrogen-carbon composite material in an alkaline solution for 5-8 hours, wherein the temperature of the alkaline solution is 60-80 ℃;
preferably, the concentration of the alkali solution is 4 to 8 mol/L.
9. The method for preparing the electrocatalyst according to claim 6, wherein in step S3, the mass ratio of the three-dimensional porous cobalt nitrogen carbon composite material to the thiourea is (1-5): (1-5).
10. Use of an electrocatalyst according to any one of claims 1 to 5 in a battery.
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