Fuel cell catalyst with high-density active sites and preparation method thereof
阅读说明：本技术 一种具有高密集型活性位点的燃料电池催化剂及其制备方法 (Fuel cell catalyst with high-density active sites and preparation method thereof ) 是由 周嵬 胡斌 张军 袁菁琳 舒清华 邵宗平 于 2021-09-06 设计创作，主要内容包括：本发明涉及一种具有高密集型活性点位的燃料电池氧还原催化剂及其制备方法,所述的催化剂由特殊设计的高密集型的Fe-Nx-C和负载其上的铂纳米颗粒构成,其特征为高密集型的Fe-Nx活性位点没有相互团聚,而是在热耦合的作用下形成了区域密集型的活性位点,构成的具有特殊结构的Fe-Nx-C基底本身就具有非常好的氧还原活性,负载其上的少量纳米铂颗粒因为载体-金属间的协同作用,更充分的发挥出了其活性。该方法一定程度上解决了传统碳基底性能不足的问题,减少了铂用量,在低温氢能燃料电池的实际应用中具有很好的前景。(The invention relates to a fuel cell oxygen reduction catalyst with high-density active sites and a preparation method thereof, wherein the catalyst is composed of specially designed high-density Fe-Nx-C and platinum nanoparticles loaded on the high-density Fe-Nx-C, and is characterized in that the high-density Fe-Nx active sites are not mutually agglomerated but form regional-density active sites under the action of thermal coupling, the formed Fe-Nx-C substrate with a special structure has very good oxygen reduction activity, and a small amount of nano platinum particles loaded on the substrate more fully exert the activity due to the synergistic action between carrier and metal. The method solves the problem of insufficient performance of the traditional carbon substrate to a certain extent, reduces the consumption of platinum, and has good prospect in the practical application of the low-temperature hydrogen energy fuel cell.)
1. A method of preparing a fuel cell catalyst, comprising the steps of:
step 1, calcining a carbon source in an ammonia atmosphere, and taking the calcined carbon source as high specific surface area carbon after grinding; mixing an iron-based nitrogen source, ammonia water, high specific surface area carbon and deionized water, uniformly stirring, and freeze-drying; calcining the freeze-dried powder in an argon atmosphere to obtain a carbon substrate with high-density active sites;
and 2, adding the carbon substrate and the platinum source into a solvent, uniformly stirring, heating, centrifuging, washing and drying a product to obtain the catalyst loaded with the nano Pt particles.
2. The method for preparing a fuel cell catalyst according to claim 1, wherein the carbon source in the step 1 is ketjen black EC-600 high specific surface area carbon.
3. The method of claim 1, wherein the iron-based nitrogen source in step 1 is hemin.
4. The method of preparing a fuel cell catalyst according to claim 1, wherein the calcination under an ammonia atmosphere is performed in a tube furnace at 850-900 ℃ for 0.5-3 h.
5. The method of preparing a fuel cell catalyst according to claim 1, wherein in one embodiment, the source of platinum is H2PtCl6·6H2O。
6. The method for preparing a fuel cell catalyst according to claim 1, wherein in one embodiment, the parameters of the calcination in an argon atmosphere in the step 1 are: the temperature is 600 ℃ and 800 ℃, and the time is 2 h.
7. The method for preparing a fuel cell catalyst according to claim 1, wherein in one embodiment, the solvent is ethylene glycol, the heating temperature is 120 ℃, and the heating time is 2-4h in the step 2.
8. A fuel cell catalyst directly obtained by the production method according to claim 1.
9. The application of the iron-based nitrogen source in preparing Fe-Nx-C fuel cell catalyst materials.
Technical Field
The invention relates to a fuel cell oxygen reduction catalyst with high-density active point positions and a preparation method thereof, belonging to the technical field of fuel cell catalysts.
Background
Among many new energy sources, a fuel cell is a device that directly converts chemical energy stored in a fuel and an oxidant into electric energy, and has advantages of high reliability, environmental friendliness, high energy density, low noise, and the like, and thus has been widely studied by researchers around the world. Fuel cells can be classified into the following by cell electrolyte type: solid Oxide Fuel Cells (SOFC), Proton Exchange Membrane Fuel Cells (PEMFC), Alkaline Fuel Cells (AFC), Molten Carbonate Fuel Cells (MCFC), and Phosphoric Acid Fuel Cells (PAFC). In the meantime, the pem fuel cell has become an ideal power source for large-scale passenger cars, submarines and the like due to its advantages of light weight, low working temperature, fast starting speed, long service life, high specific energy, no loss and corrosion of electrolyte, and the like, and thus has been widely used in various countries in the world to promote the commercial application of PEMFCs. However, PEMFCs, which are one of the ideal energy conversion devices, have not been truly mass-produced and applied, mainly because of the problems of high catalyst cost and low durability. Among them, the catalyst is a key material in PEMFCs, and the development of PEMFCs is seriously hindered by the problems of the catalyst technology. Therefore, the development of low-cost, high-performance, highly durable catalyst materials is expected to solve this critical problem and further advance the development of PEMFCs.
Metal-nitrogen-carbon materials have proven to be the most promising non-noble metal catalysts in PEMFCs, but are still limited by the slow kinetics of the metal-nitrogen active center.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the general metal-nitrogen-carbon material is limited by the slow kinetics of the metal-nitrogen active center, and the traditional commercialized catalyst has the problems of insufficient performance and durability and high cost. The carbon substrate with the high-density Fe-Nx active sites is adopted, the carbon substrate constructed by the area-density Fe-Nx active sites provides high oxygen reduction activity, a small amount of nano platinum particles loaded on the carbon substrate better exert the activity due to the synergistic action between metal and carriers, the cost of the catalyst is effectively reduced, and the performance of the catalyst is greatly optimized.
A method of preparing a fuel cell catalyst comprising the steps of:
step 1, calcining a carbon source in an ammonia atmosphere, and taking the calcined carbon source as high specific surface area carbon after grinding; mixing an iron-based nitrogen source, ammonia water, high specific surface area carbon and deionized water, uniformly stirring, and freeze-drying; calcining the freeze-dried powder in an argon atmosphere to obtain a carbon substrate with high-density active sites;
and 2, adding the carbon substrate and the platinum source into a solvent, uniformly stirring, heating, centrifuging, washing and drying a product to obtain the catalyst loaded with the nano Pt particles.
In one embodiment, the carbon source in step 1 adopts Ketjen black EC-600 high specific surface area carbon.
In one embodiment, the iron-based nitrogen source of step 1 is hemin.
In one embodiment, the calcination under ammonia atmosphere is performed in a tube furnace at 850-.
In one embodiment, the source of platinum is H2PtCl6·6H2O。
In one embodiment, the parameters of the calcination in an argon atmosphere in step 1 are: the temperature is 600 ℃ and 800 ℃, and the time is 2 h.
In one embodiment, in step 2, the solvent is ethylene glycol, the heating temperature is 120 ℃, and the heating time is 2-4 h.
In one embodiment, the mass ratio of the high specific surface area carbon, the iron-based nitrogen source and the ammonia water in the step 1 is 100: 100-; the mass ratio of the carbon substrate to the platinum source in the step 2 is 10: 0.5-1.5.
The application of the iron-based nitrogen source in preparing Fe-Nx-C fuel cell catalyst materials.
In one embodiment, the iron-based nitrogen source is used to increase Fe-NxUniformity of dispersion of the sites in the catalyst material, improved operating life in a fuel cell, improved output or power density in fuel cell operation, electrochemically active area, or half-wave potential.
Advantageous effects
The invention has the advantages that: constructs the special region dense Fe-NxThe carbon substrate of the active site optimizes the traditional metal-nitrogen active site and accelerates the oxygen reduction kinetics, thereby greatly improving the overall ORR activity of the oxidation catalyst. And because of a small amount of Pt nano particles loaded on the catalyst, the interaction force between the metal and the carrier is further improved, the durability of the catalyst is improved, and the cost of the catalyst is reduced.
Drawings
FIG. 1 shows Fe-N preparedxXRD profile of-C.
FIG. 2 is a comparison of LSV performance of the prepared catalyst with commercial Pt/C (Pt relative content 20%).
Fig. 3 is a TEM image of the catalyst prepared.
FIG. 4 is a CV comparison curve of the prepared catalyst and commercial Pt/C (Pt relative content of 20%).
FIG. 5 shows Fe-N preparedxLSV performance of-C carbon substrates was compared to commercial XC-72R carbon substrates.
Fig. 6 is an XPS characterization curve of the prepared catalyst.
FIG. 7 shows Fe-N preparedxSingle electron of-C carbon substrateCell performance curve.
FIG. 8 shows Fe-N preparedx-electrochemical stability curve of carbon substrate C.
FIG. 9 is a BET characterization curve of the prepared catalyst.
Fig. 10 is a graph of the cell stability of the catalyst prepared.
Figure 11 is the preparation of the catalyst STEM characterization results.
Fig. 12 is a graph of the single cell performance of the catalyst prepared.
Detailed Description
The invention provides carbon with highly dense Fe-Nx active sites as a substrate of an oxygen reduction catalyst of a fuel cell, and a regiodense Fe-Nx structure developed by people has a unique oxygen reduction catalytic mechanism, has the synergistic advantages of accelerating catalytic kinetics and highly inhibiting side reactions, and successfully improves the catalytic activity and stability of the catalyst. In addition, the oxygen reduction catalyst loaded with a small amount of nano platinum particles constructed on the carbon substrate shows good electrochemical and battery performances in the PEMFC test due to the interaction force between the metal-support. Our process will provide a new approach to the design of highly active oxygen reduction catalysts.
In one exemplary embodiment, the preparation process of the present invention is detailed as follows:
firstly, 1g of Ketjen black EC-600 is placed in a tube furnace to be calcined for 1h under the atmosphere of ammonia gas at 850-; adding 130-150mg of iron-based nitrogen source hemin, 1-2ml of ammonia water and 100mg of calcined Ketjen black EC-600 high specific surface area carbon into deionized water, performing ultrasonic treatment, and stirring overnight; freeze-drying the stirred solution and collecting; calcining the collected powder for 2h at the temperature of 600-800 ℃ in the argon atmosphere of the tubular furnace, and obtaining the carbon substrate Fe-N with the high-density active sites after the calcination is finishedx-C。
Then, 40mg of carbon substrate Fe-N was takenx-C，4.4mg H2PtCl6·6H2O was added to 20ml Ethylene Glycol (EG) and stirring was continued for 2-4h at 120 deg.C in an oil bath. After the oil bath is finished, the product is usedAnd centrifuging and washing pure water for several times, and drying overnight at the temperature of 60 ℃ in vacuum to obtain the catalyst loaded with the nano Pt particles.
In order to achieve the technical objects of the present invention, the main improvement points in the manufacturing process include:
1. an iron-based nitrogen source compound is adopted, which can be used as an iron source and a nitrogen source at the same time, dense Fe-Nx sites can be generated in the preparation process, and Fe-Nx-C obtained by respectively using the iron source and the nitrogen source in the prior art as raw materials is used as a dispersed active site.
2. After the high-activity substrate is obtained as a carrier, platinum is loaded, so that the high-activity platinum-based catalyst is obtained, the problem of insufficient performance of the traditional commercialized platinum carbon is solved, and the high-activity platinum-based catalyst has a good application prospect.
3. The carbon source adopted in the invention is Ketjen black EC-600 with high specific surface area, and the carbon source has larger specific surface area, so that the uniform dispersion and loading of the material can be realized.
Example 1
Firstly, putting 1g of Ketjen black EC-600 into a tubular furnace to calcine for 1h at 850 ℃ in an ammonia atmosphere, cooling to room temperature after calcination, grinding and collecting; adding 130mg of hemin, 1ml of ammonia water and 100mg of calcined Ketjen black EC-600 carbon with high specific surface area into deionized water, and ultrasonically stirring overnight; freeze-drying the stirred solution and collecting; calcining the collected powder for 2h at 600 ℃ in a tube furnace argon atmosphere, and obtaining the carbon substrate Fe-N with the highly dense active sites after the calcination is finishedx-C. Then, 40mg of carbon substrate Fe-N was takenx-C，4.4mg H2PtCl6·6H2O was added to 20ml Ethylene Glycol (EG) and stirring was continued for 2h at 120 deg.C in an oil bath. And centrifuging and washing the product for several times by using ultrapure water after oil bath is finished, and drying overnight at the temperature of 60 ℃ in vacuum to obtain the catalyst loaded with the nano Pt particles.
Example 2
Firstly, putting 1g of Ketjen black EC-600 in a tube furnace to calcine for 1h at 900 ℃ in an ammonia atmosphere, cooling to room temperature after calcination, grinding and collecting; 130mg of hemin, 2ml of ammonia water, 100mg of calcined Ketjen black EC-600 high specific surface area carbon is added into deionized water for ultrasonic treatment and stirring overnight; freeze-drying the stirred solution and collecting; calcining the collected powder for 2h at 700 ℃ in a tube furnace argon atmosphere, and obtaining the carbon substrate Fe-N with the highly dense active sites after the calcination is finishedx-C. Then, 40mg of carbon substrate Fe-N was takenx-C，4.4mg H2PtCl6·6H2O was added to 20ml Ethylene Glycol (EG) and stirring was continued for 3h at 120 ℃ in an oil bath. And centrifuging and washing the product for several times by using ultrapure water after oil bath is finished, and drying overnight at the temperature of 60 ℃ in vacuum to obtain the catalyst loaded with the nano Pt particles.
Comparative example 1
The difference from example 1 is that: a traditional ZIF-8 molecular sieve and an iron-based compound are pyrolyzed to form an Fe-Nx-C substrate.
The preparation steps of the comparative example are as follows:
1) 0.7065g of zinc nitrate hexahydrate and 0.0348g of ferrous sulfate heptahydrate were dissolved in 50mL of deionized water and designated solution A. Wherein the molar ratio of the zinc nitrate hexahydrate to the ferrous sulfate heptahydrate is 95: 5.
2) 1.642g of 2-methylimidazole and 3mL of triethylamine were dissolved in 50mL of deionized water and identified as solution B. Wherein, the mol ratio of the 2-methylimidazole to the triethylamine is 1: 1. The molar ratio of the total mole number of the metal ions in the step 1) to the mole number of the 2-methylimidazole in the step 2) is 1: 8.
3) The solution A was slowly added to the solution B, stirred for 30min at 500rpm and centrifuged to give a dark yellow solid.
4) Re-dispersing the solid obtained in the step 2) in deionized water for 24h, then centrifugally washing, drying in vacuum at 100 ℃ for 24h, and grinding the obtained solid into powder.
5) Pyrolyzing the powder obtained in the step 3) at the high temperature of 900 ℃ for 3 hours in an inert atmosphere to prepare the Fe-N-C catalyst.
Comparative example 2
The difference from example 1 is that: the carbon source used was XC-72R, which has a lower specific surface area than EC-600 in example 1.
Characterization of XRD
ExamplesXRD characterization of the catalyst prepared in 1 is shown in FIG. 1, in which significant FeN appears at 43.62 ° and 44.66 °0.056The peak indicates that Fe and N form a good coordination effect, the coordination of Fe and N can well improve the ORR activity of the carbon substrate per se, and the overall performance of the catalyst can be greatly improved after the nano Pt particles are loaded.
Testing of electrochemical Performance
The catalyst prepared in example 1 was tested for electrochemical performance and the LSV curve is shown in FIG. 2, Pt/Fe-NxHalf-wave potential of-C is 0.926V, and specific mass activity is 0.922A. mg-1 PtThe half-wave potential of the commercial Pt/C catalyst was 0.888V, and the specific mass activity was 0.152A. mg-1 Pt，Pt/Fe-NxThe mass specific activity of-C is 6 times that of commercial Pt/C, and excellent ORR performance is shown. FIG. 3 is a graph comparing CV test curves of the catalyst and commercial Pt/C, from which it can be calculated that the electrochemically active areas of the catalyst and commercial Pt/C catalyst are 70m, respectively2·g-1，86m2·g-1The catalyst also has a larger electrochemically active area compared to the commercial Pt/C catalyst.
Example 1 in comparison to comparative example 2, FIG. 4 is a carbon substrate Fe-N with highly dense active sitesxComparison of-C with LSV of a commercial carbon substrate XC-72R, from which the curve can be calculated for the carbon substrate Fe-NxThe half-wave potential of-C was 0.801V, and the specific mass activity was 8.77 A.g-1While the commercial carbon substrate XC-72R has almost no ORR activity, in contrast to the carbon substrate Fe-Nxthe-C has excellent ORR activity and can well improve the performance of the whole catalyst loaded with Pt nanoparticles.
TEM characterization
The catalyst is subjected to a TEM characterization test, and a test result graph is shown in FIG. 5, wherein obvious lattice stripes of platinum can be seen, the stripe intervals are 0.226nm and 0.227nm, both represent the (111) crystal face of the platinum, and the activity of the (111) crystal face of the platinum in all crystal face types of the platinum is relatively high, which is one of the expressions that the catalyst activity is high.
XPS characterization
The XPS characterization curve of the catalyst is shown in FIG. 6, the left graph shows that the N content of pyridine is more than 53.8% in all N contents in the N1 s fitting curve of the catalyst obtained by the patent method, the right graph shows that the N content of pyridine in the catalyst prepared by the common method (ZIF-8 and an iron-based compound are pyrolyzed to be used as a substrate in the comparative example 1) is 29.1%, the content of pyridine N in the example is 1.8 times of that in the common method, and the pyridine N has the best coordination effect with a transition metal, so the content proves that the Fe-N content in the catalyst is more than certain degreexThe number of active sites is large, which is also one of the phenomena of high-density active site formation; from the fitting curve of Pt 4f in fig. 7, it can be seen that the platinum valence state prepared by the present patent method is not very different from that prepared by the conventional method, because the platinum-carrying manner of wet reduction is adopted, the content of Pt zero valence state in the figure is large, and Pt zero valence state is one of the evidences that the activity is the highest among all valence states of Pt, which is also the good performance of the catalyst.
Cell performance testing
To further illustrate the carbon substrate Fe-N designed with highly dense active sitesxthe-C has good practical application value, the single cell performance test is carried out on the electrode material prepared in the example 1, and the test result is shown in figure 8, and the test result is Fe-N with dense active sitesxFe-N prepared by common method (ZIF-8 and iron source pyrolysis) after platinum is loaded on-C substrate serving as carrierxComparison of cell application data after platinum loading on the C substrate with commercial platinum carbon (relative platinum content 20% each, loading was accurately measured by ICP). Pt/Fe-Nx-C (prepared by the patent method), Pt/Fe-NxThe power densities of-C (prepared by the conventional method) and commercial Pt/C at 0.6V were 1.17W cm-2、0.65W·cm-2And 0.62 W.cm-2The maximum output power is 1.49 W.cm-2、0.78W·cm-2、0.71W·cm-2From this data, it can be seen that Fe-N has dense active sitesxThe practical battery application performance of the catalyst with the-C substrate as the carrier and the platinum loaded is far higher than that of the catalyst with the substrate loaded by the common methodCommercial Pt/C.
Run durability test
To demonstrate the carbon substrate Fe-N of the designed highly dense active sitesxThe durability of-C, on which 10,000 cycles of accelerated durability cycling test were performed, is shown in fig. 8, and it can be calculated from the left graph that the performance of the substrate prepared by the method of this patent is almost not deteriorated (the activity is deteriorated by < 3%) after 10,000 cycles of stability cycling test, and the right graph shows that the performance of the catalyst (ZIF-8 pyrolyzed with an iron-based compound) substrate prepared in comparative example 1 is deteriorated by 12.5% after 10,000 cycles of stability cycling test.
To test the durability of the catalyst in the actual cell test, it was subjected to a 0.6V constant voltage durability test for 44h in a standard fuel cell system, and the test results are shown in fig. 10, where the current density was almost not attenuated (< 5%) in the durability test for 44h, while the catalyst prepared in comparative example 1 was attenuated by 15.8% and 29.2% from the commercial Pt/C, respectively, illustrating the unique carbon substrate Fe-N prepared by this methodxThe catalyst formed by-C and a small amount of Pt nano particles has excellent durability and has good application value in the practical application of fuel cells.
BET characterization
As shown in the BET characterization curve of FIG. 9, the carbon substrate Fe-N was calculated and found to be present in example 1 in comparison with the catalyst prepared in comparative example 2xSpecific surface area of-C688 m2·g-1Carbon substrate XC-72R used for commercial Pt/C (Pt relative content: 20%) has a specific surface area of 254m2·g-1In contrast, the carbon substrate Fe-N prepared by the method has high dense active sitesxthe-C has larger specific surface area, which can lead the Pt nano particles loaded on the-C to be better dispersed on the substrate, and the activity of each Pt particle can be effectively exerted by the improvement of the dispersing effect.
STEM characterization
STEM diagrams of the materials obtained in example 1 and comparative example 1 are shown in FIG. 10, and the left diagram shows Fe-N formed by pyrolysis of an iron-based nitrogen source compoundxA C substrate, where dense, bright spots can be clearly seen,this is the dense Fe-N that is formedxA site. The right picture is Fe-N formed by pyrolysis of ordinary ZIF-8 and an iron-based compoundxA C substrate, wherein the bright spots are uniformly dispersed on the surface, which indicates Fe-NxThe sites are highly dispersed.