CN113659161B - 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 for new energy automobiles has become a necessary 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, the zinc-air battery is particularly emphasized because of its 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 electrocatalytic performance of the metal-nitrogen-carbon composite catalyst is poorer.
Therefore, there is a need to develop an electrocatalyst with economic and efficient electrocatalytic activity of dual-function ORR and OER that can replace precious metals.
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 comprising a nitrogen-doped carbon material and a sulfur-cobalt compound, the nitrogen-doped carbon material encapsulating the sulfur-cobalt compound.
The electrocatalyst is a composite material consisting of sulfur-cobalt compounds wrapped by nitrogen-doped carbon materials, and contains Co-N x A coordination structure, wherein: x is 2 or 4, a thiocobalt compound and Co-N x The 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 for 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 Co 9 S 8 CoS and Co 9 S 8 Both having electrocatalytic activity, wherein: co 9 S 8 Is 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 is increased, and the catalytic activity of the electrocatalyst can be effectively improved. Meanwhile, three-dimensional porous Co @ N-C reacts with thiourea to generate Co 9 S 8 And/or CoS sulfur cobalt compounds, and finally obtain the nitrogen-sulfur doped carbon-based metal sulfide electrocatalyst (Co) x S y @ N-C), wherein: co x S y Is CoS and Co 9 S 8 The sulfur cobalt compound imparting OER electrocatalytic activity to the electrocatalyst, co x S y @ N-C has both ORR and OER dual-functional electrocatalytic activities.
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 SiO 2 Nanospheres of Al 2 O 3 Any one of nanospheres and ZnO nanospheres; further preferably, the inorganic nanospheres are selected from SiO 2 Nanospheres.
Preferably, the average diameter of the inorganic nanospheres is 50-500nm; preferably, the average diameter of the inorganic nanospheres is 100nm.
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 SiO 2 Placing 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 SiO 2 A cobalt nitrogen carbon composite of nanospheres;
s2, removing the wrapped SiO by adopting an alkali solution 2 SiO in cobalt-nitrogen-carbon composite material of nanosphere 2 Washing and drying, then continuously treating with acid solution, washing and drying to obtain the cobalt-nitrogen-carbon composite material;
s3, performing 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 salt 2+ 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 SiO 2 On the nanosphere, wrapping SiO is finally formed 2 Nanosphere metal organic framework composite material (ZIF-67 @ SiO) 2 ). After high-temperature carbonization, is wrapped by SiO 2 The metal organic framework composite material of the nanosphere forms wrapping SiO under the protection of nitrogen 2 Nanosphere cobalt-nitrogen-carbon composites, siO 2 The nanospheres do not react at high temperatures.
In step S2, siO in the cobalt-nitrogen-carbon composite material is removed by alkali solution 2 Nanospheres and washing out unstable impurities with acid solution.
In step S3, thiourea decomposes during the heat treatment: SC (NH) 2 ) 2 →NH 2 CN+H 2 S，H 2 S reacts with simple substance Co and CoO in the cobalt nitrogen carbon composite material at high temperature to form sulfur cobalt compound CoS or Co 9 S 8 。
As a further improvement of the above scheme, in step S1, the pyrolysis reaction conditions are: the reaction is carried out 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 a further improvement of the above scheme, in step S2, the removing of the wrapping SiO 2 The method for preparing the inorganic nanospheres in the cobalt-nitrogen-carbon composite material of the nanospheres comprises the following steps: wrapping the SiO 2 Placing 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 5-30min;
preferably, the concentration of the alkali solution is 4 to 8mol/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 400-800 ℃, the heating rate is 4-6 ℃/min, and the heat preservation time is 1-3 hours; 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 preparation method of the invention adopts the steps of firstly carbonizing and then vulcanizing, ensures the generation of the ORR active site Co-Nx through carbonization, and then generates Co in the composite material through vulcanization 9 S 8 And 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 by the following examples to facilitate the understanding of the present invention by those skilled in the art, and it is necessary to point out that the examples are only used for further illustration of the present invention and should not be construed as limiting the scope of the present invention, and that the non-essential modifications and adjustments of the present invention by those skilled in the art should still fall within the scope of the present invention, and that the raw materials mentioned below are not specified in detail and are all commercially available products, and that the process steps or preparation methods not mentioned in detail are all known to those skilled in the art.
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 SiO 2 SEM scanning electron micrographs of nanospheres and the electrocatalyst of comparative example 1;
FIG. 5 is an SEM scanning electron micrograph of the electrocatalyst of examples 1-3;
figure 6 is an X-ray diffraction pattern of the electrocatalysts of examples 1-3 and comparative example 1.
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.
Example 1
An electrocatalyst prepared from raw materials comprising: siO 2 2 Nanospheres, 2-methylimidazole, cobalt nitrate and thiourea. Wherein: the molar ratio of cobalt nitrate to 2-methylimidazole is 1:15, cobalt nitrate and SiO 2 The mass ratio of the nanospheres is 1:2.2, siO 2 The average diameter of the nanospheres was 100nm.
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 beaker 2 Carrying out ultrasonic treatment on the nanospheres, 2.65g of 2-methylimidazole and 20mL of water for 1 hour to uniformly disperse the nanospheres; adding 4mL of 0.5mol/L cobalt nitrate solution, carrying out ultrasonic reaction for 10 minutes, standing for 6 hours to obtain coated SiO 2 A 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 SiO 2 Cobalt nitrogen carbon composite material of nanosphere (Co-N-C @ SiO) 2 )。
S2, mixing Co-N-C @ SiO 2 Adding A into 6mol/L NaOH solution, performing alkali treatment in 70 deg.C water bath for 6 hr to remove SiO 2 Filtering, washing and drying the nanospheres; and adding 30mL0.5mol/L hydrochloric acid solution, stirring at normal temperature for 15min, washing off 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 porcelain ark, weighing 50mg of thiourea in another porcelain ark, putting the two arks in a tube furnace filled with nitrogen atmosphere, heating to 60 ℃ at a 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 @ x S y @N-C-2-600。
Example 2
EXAMPLE 2 preparation of electrocatalyst the raw material components and preparation method were 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 Co x S y @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 Co x S y @N-C-3-600。
Example 4
EXAMPLE 4 preparation of electrocatalyst feed setThe electrocatalyst prepared in example 4, at a reaction temperature of 400 ℃ in step S3, was labeled as Co x S y @N-C-2-400。
Example 5
Example 5 preparation of electrocatalyst the raw material components and preparation method were substantially the same as in example 1, except that in step S3 of example 5, the reaction temperature was 800 ℃, and the prepared electrocatalyst sample, labeled Co x S y @N-C-2-800。
Comparative example 1
Comparative example 1 preparation of electrocatalyst starting material composition the same as in example 1, comparative example 1 electrocatalyst was prepared as compared to example 1 except that comparative example 1 did not include step S3, i.e. no thiourea sulfidation treatment was performed, and the prepared electrocatalyst sample, labeled co @ n-C.
Comparative example 2
Commercial 20% Pt/C and RuO 2 A 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 -2 The glassy carbon electrode of (1), wherein the loading amount of the electrocatalyst is 0.12mg cm -2 . ORR and OER performance testing of electrocatalysts employed 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 -1 The rotation speed is 1600rpm. 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: e RHE ＝E Hg/HgO +0.098+0.05916pH; FIG. 1a is an ORR linear voltammogram of an electrocatalyst; drawing (A)1b is the corresponding tafel slope of 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 1 x S y @ N-C-2-600 has optimal ORR and OER electrochemical performance. Under alkaline conditions, co is shown in FIG. 1a x S y @ 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 -2 All outperformed other samples of varying degrees of sulfidation and approach 20% pt/C noble metal electrocatalyst. As can be seen from FIG. 1b, co x S y @ N-C-2-600 has the lowest Tafel (Tafel) slope of 57.53mV dec -1 Less than 20% Pt/C and other sulfided catalysts, further illustrating that they have excellent ORR kinetic reaction rates. In addition to that, co x S y @ N-C-2-600 also exhibited excellent OER electrocatalytic activity. At 10mA cm -2 When is Co x S y The electric potential of @ N-C-2-600 is 1.61V, the Tafel slope is 212.38mV dec-1, is superior to other sulfurated electrocatalysts, and is close to the noble metal RuO 2 . The half-wave potential and OER of ORR are usually 10mA cm -2 The charge/discharge reversibility of the electrode reaction can be evaluated. Co x S y @ 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 -2 The 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 -1 The rotation speed is 1600rpm. 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: e RHE ＝E Hg/HgO +0.098+0.05916pH; 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 the current density and the ordinate represents the 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 has x S y @ N-C-2-600 has optimal ORR electrocatalytic activity. As shown in FIG. 2b, co x S y The peak potential of @ N-C-2-600 is 0.91V (vs. RHE), which is greater than that of Co x S y @ N-C-2-800 and Co x S y @ N-C-2-400, the larger the onset potential, the better the catalytic performance. And from the half-wave potential, co x S y Half-wave potential of @ N-C-2-600Then 0.81V, very close to 20% Pt/C. At the limiting current, co can be found x S y The limiting current of @ N-C-2-600 is maximum and 5.16mA cm -2 Is better than 20% of Pt/C, further illustrates Co x S y @ N-C-2-600 has optimal ORR electrochemical performance. From the Tafel slope of FIG. 2c, co can be seen x S y Tafel slope of @ N-C-2-800 is 50.76mV dec -1 Electrocatalysts 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, co x S y The current density of @ N-C-2-600 remained at 97.20% as before, while Pt/C remained at only 89.05%, indicating that Co x S y @ 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 -2 While Co is x S y The potential of @ N-C-2-600 is 1.61V, the Tafel slope is 212.38mV dec -1 (ii) a At 10mA cm -2 When is Co x S y The potential of @ N-C-2-400 is 1.66V, the Tafel slope is 227.96mV dec -1 (ii) a At 10mA cm -2 When is Co x S y The 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 be x S y The 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 obtained in example 1 x S y The application of the present invention to a zinc air cell was further explored by assembling the @ N-C-2-600 as a cathode catalyst into a liquid phase zinc air cell and comparing the 20% Pt/C of comparative example 2 as a comparative sample for discharge performance testing. The performance of the application of the electrocatalysts of example 1 and comparative example 2 to a zinc-air cell is detailed in fig. 3, whichThe method comprises the following steps: fig. 3a is an 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 Co x S y The 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 a cathode catalyst. In addition to this, co can be seen from the discharge curve in FIG. 3b x S y The @ 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-percent pt/C catalyst exhibits concentration polarization in the high current density region, and the slow mass transfer causes a significant loss in current density. Co x S y @ N-C-2-600 cathode catalyst for zinc-air cell at 103.6mA cm -2 The maximum power density is 70.31mW cm -2 Approximately 20% of Pt/C cathode catalyst at 112.6mA cm -2 Maximum power density of 82.91mW cm -2 . Thus, co x S y The @ 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, siO 2 Microstructure testing of nanospheres and comparative example 1 electrocatalyst
FIG. 4 is a graph of SiO with an average diameter of 100nm 2 SEM images of nanospheres and base-treated Co @ N-C, wherein: FIG. 4a is SiO 2 Scanning 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. 4 2 The nanospheres are uniform in size and free of agglomeration, which is favorable for forming regularly and orderly wrapped SiO 2 Precursor of the cobalt nitrogen carbon composite material of the nanosphere. From (C) and (d) Co @ N-C in FIG. 4SEM pictures show that SiO removal by alkali treatment 2 The Co @ N-C catalyst of the nanosphere template has uniform surface pore diameter of 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 exposure of active sites is facilitated, and the electrochemical activity of the catalyst is improved.
FIG. 5 is sample Co of the electrocatalyst prepared in examples 1-3 x S y @N-C-1-600、Co x S y @ N-C-2-600 and Co x S y SEM 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 of example 3. As can be seen from the figure, the microstructure shapes of the three catalysts are not greatly changed compared with the Co @ N-C before vulcanization, the surface pore sizes are 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, co x S y @N-C-1-600、Co x S y @N-C-2-600、Co x S y The 2 theta diffraction peaks of XRD of @ N-C-3-600 and Co @ N-C are both 44.09 degrees, corresponding to the (002) crystal face of Co (PDF # 01-071-4652) of PDF standard card, thereby indicating that part of the simple substance cobalt is remained after vulcanization. Co x S y @N-C-1-600、Co x S y CoS is present throughout @ N-C-3-600 (PDF # 97-062-4857). Wherein, co x S y @N-C-1-600、Co x S y @ N-C-2-600 Presence of Co 9 S 8 (PDF # 01-073-6395). In addition, by comparison, co can also be observed x S y CoO exists at @ N-C-3-600 and Co @ N-C (PDF # 97-017-4027). Thus, it can be seen that: co x S y @ N-C-1-600 contains Co and Co 9 S 8 And CoS; co x S y @ N-C-2-600 contains Co and Co 9 S 8 ；Co x S y @ N-C-3-600 contains Co, coS, coO; and contained in Co @ N-CThere are Co and CoO. At 600 ℃, thiourea will decompose to NH 3 、H 2 S and CO 2 Gas, 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 Co 9 S 8 . When the thiourea is in excess, sulfidation forms Co 9 S 8 And sulfides of CoS. This is probably the reduction of CoO to elemental cobalt, sulfidation leading to the formation of Co 9 S 8 The transition portion CoS was over-sulfurized. The invention converts agglomerated elemental cobalt into active CoS and Co 9 S 8 Greatly improves the catalytic activity of Co @ N-C, and simultaneously CoS and Co 9 S 8 And 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 derived therefrom are intended to be within the scope of the present invention.
Claims (6)
1. An electrocatalyst, characterized in that the electrocatalyst comprises a nitrogen-doped carbon material and a sulphur-cobalt compound, the nitrogen-doped carbon material encapsulating the sulphur-cobalt compound;
the cobalt sulfide compound comprises CoS and/or Co 9 S 8 ；
Raw materials for preparing the electrocatalyst comprise inorganic nanospheres, 2-methylimidazole, cobalt salt and thiourea;
the molar ratio of the cobalt salt to the 2-methylimidazole is 1: (10-20);
the mass ratio of the cobalt salt to the inorganic nanospheres is 1: (1-3);
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;
s3, carrying out heat treatment on the three-dimensional porous cobalt-nitrogen-carbon composite material by using thiourea to prepare the electrocatalyst;
in the step S2, the method for removing the inorganic nanospheres in the inorganic nanosphere-wrapped cobalt-nitrogen-carbon composite material comprises the following steps: 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 ℃; the concentration of the alkali solution is 4-8mol/L.
2. The electrocatalyst according to claim 1, wherein the cobalt salt comprises at least one of cobalt nitrate, cobalt nitrate hydrate, cobalt acetate, cobalt chloride.
3. Electrocatalyst according to claim 2, characterized in that the inorganic nanospheres are selected from SiO 2 Nanosphere, al 2 O 3 Any one of nanospheres and ZnO nanospheres; the average diameter of the inorganic nanospheres is 50-500nm.
4. Electrocatalyst according to claim 1, characterized in that 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;
in step S3, the heat treatment conditions are: the reaction is carried out under the protective atmosphere, the reaction temperature is 400-800 ℃, and the heat preservation time is 1-3 hours; the protective atmosphere is a nitrogen atmosphere.
5. The electrocatalyst according to claim 1, 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).
6. Use of an electrocatalyst according to any one of claims 1 to 5 in a battery.
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