TECHNICAL FIELD

The present invention relates to a field of fuel cells, and is a low-platinum catalyst based on nitride nanoparticles applicable to a fuel cell, and a preparation method thereof.

BACKGROUND

Energy shortages and environmental problems caused by burning large amounts of fossil fuels are aggravating, forcing people to pay more and more attention to exploring new sources of energy and new energy conversion technologies. A low temperature fuel cell, due to its advantages such as high energy conversion efficiency, quick start and non-polluting is known as a class of fuel cell having the most large-scale industrialization prospects. The low temperature fuel cell includes a proton exchange membrane fuel cell, a direct-methanol fuel cell and a direct-acid fuel cell, etc. Catalysts for these fuel cells all use precious metal platinum that is expensive and of scarcity as a main active component, and the resulting high cost of the fuel cell has become a key factor restricting the commercialization of the fuel cell. Meanwhile, as for a commercial Pt/C catalyst that is currently widely used, since only a weak absorption exists between loaded metal particles and a carbon support and the carbon support is easily corroded under an operating condition of the fuel cell, resulting in exfoliation and migration of active metal nanoparticles, seriously limiting a service life of the fuel cell. Therefore, preparation and study of a low-cost, high-activity and high-stability catalyst is of great significance for development and promotion of the fuel cell.

Transition metal nitrides have characteristics such as high melting point, high hardness, corrosion resistance and high electrical conductivity. At the same time, a large number of studies show that nitrides exhibit relatively high activity in some processes of hydrogenation reduction and oxygen reduction and are called “platinum-like catalyst”. Chen et al. used carbon nitride as a hard template to synthesize carbon-supported titanium nitride particles and directly used titanium nitride to catalyze a redox reaction (Ji Chen, Kazuhiro Takanabe, Ryohji Ohnishi, et al. Chem. Commun., 2010, 46, 7492-7494). Although the carbon-supported titanium nitride particles prepared by this technique show certain oxygen reduction activity, its activity is still far away from practical application.

DiSalvo et al. first treated a mixture of zinc oxide and titanium dioxide at 1350° C. for 3 hours, and a sample was taken after cooling and ammonia gas was introduced at 800-900° C. to obtain titanium nitride particles having a particle size of 25-45 nm. By using the titanium nitride particles instead of a catalyst prepared by carbon powder supporting Pt, it was found to have better methanol oxidation performance and stability than the commercial Pt/C catalyst (Minghui Yang, Zhimin Cui and Francis J. DiSalvo, Phys. Chem. Chem. Phys., 2013, 15, 1088). Meanwhile, this research group made use of the same synthesis method to obtain chromium nitride particles with a particle size of 30-50 nm. The Pt catalyst supported by chromium nitride particles showed higher stability than the conventional Pt/C catalyst, and at the same time, an oxygen reduction activity and a methanol oxidation activity were 1.8 times and 1.4 times that of the Pt/C, respectively (Minghui Yang, Rohiverth Guarecuco, and Francis J. DiSalvo, Chem. Mater., 2013, 25, 1783-1787; Minghui Yang, Zhimin g Cui and Franc is J. DiSalvo, Phys. Chem. Chem. Phys., 2013, 15, 7041). However, this method has high requirements on equipment, preparation of samples requires extremely high temperature, and energy consumption is high, while a loading amount of Pt fails to be effectively reduced.

Thotiyl et al. first deposited a layer of titanium nitride on a stainless steel wire by cathodic arc deposition technique to prepare a working electrode, then deposited Pd or Pt metal by a DC deposition method, and thereby prepared a Pt or Pd catalyst supported by the titanium nitride. In an ethanol oxidation experiment, this catalyst showed better catalytic activity and stability than the Pt/C (M.M.O. Thotiyl, T. Ravi Kumar, and S. Sampath, J. Phys. Chem. C, 2010, 114, 17934-17941; M.M.O. Thotiyl, S. Sampath, Electrochim. Acta, 2011, 56, 3549-3554). A method for preparing the catalyst is complicated, an average particle size of the precious metal prepared is 200 nm, and amplitude of increase of activity per unit mass of platinum of the catalyst is very limited.

Chinese patent application No. 200610027287 discloses a preparation technique of an anti-corrosion fuel cell catalyst, wherein Pt was supported on a surface of nitride-supported metal oxide by hydrolysis or vapor-phase thermal decomposition, but morphology of the prepared catalyst and performances of the catalyst are not clearly described and provided with relevant proof materials.

In summary, although there have been many efforts to utilize a nitride directly as a fuel cell catalyst and as a carrier for the fuel cell catalyst, one or more disadvantages remain. In the prior art, a related report that a pulse electrodeposition method is used to directly deposit a thin active metal layer on a surface of inexpensive transition-metal nitride nanoparticles to prepare a catalyst suitable for fuel cell with a particle size within 10 nm has not been found.

SUMMARY OF THE INVENTION

The present invention discloses a low-platinum catalyst based on nitride nanoparticles for a fuel cell and a pulse electrodeposition preparation method thereof. The novel catalyst may be used for a proton exchange membrane fuel cell and other processes that require a precious metal catalyst. For current deficiencies such as high loading amount of Pt, complicated preparation technique of the low-platinum catalyst and easy corrosion of a carbon support, it is provided a method of efficiently and inexpensively preparing a high-performance and high-stability low-platinum loading catalyst.

A non-aqueous ammonia gas complexation method is used for first synthesizing transition-metal ammonia complex nanoparticles, then preparing transition-metal nitride nanoparticles by means of high temperature nitriding; using a salt solution of an active metal as an electrodeposition precursor, by a method pf constant current pulse electrodeposition, using different Ton/Toffvalues (0.1-100), uniformly depositing the active metal with an atomic layer level thickness on a surface of the transition-metal nitride nanoparticles, to obtain the low-platinum catalyst having high activity and high stability.

An active component of the catalyst is the nitride nanoparticles cladded with a platinum atomic layer, the active metal directly cladding on the surface of the nitride nanoparticles or the surface of the nitride nanoparticles supported on the carbon support in an ultrathin atomic layer form. The catalyst uses a transition metal nitride or a carbon-supported transition metal nitride as a substrate, and uses one or more of Pt, Ir, Pd, Ru or Au element as a covering layer. The preparation method includes: preparing the transition metal nitride or the carbon-supported transition metal nitride that serves as a substrate material, fabricating a working electrode for pulse electrodeposition, and finally obtaining the catalyst by means of pulse electrodeposition. The catalyst may be used as an anode or a cathode catalyst of a low temperature fuel cell, and the catalyst has advantages of high activity and good stability. The preparation technique has advantages such as easy operation, free of protection of inert atmosphere, and controllable amount of deposition, and is suitable for large-scale production. The catalyst may significantly decrease a precious metal usage amount of the fuel cell, greatly reduces the cost of the fuel cell, and has great significance for accelerating a commercialization process of the fuel cell.

A preparation method of the low-platinum catalyst based on nitride nanoparticles, includes several steps as follows:

(1) a preparation of the nitride nanoparticles: dissolving one or more transition metal salts in a non-aqueous solvent, then introducing ammonia gas for 0.5-1 hour, evaporating the solvent at 50-90° C. in a vacuum drying oven to obtain a transition-metal ammonia complex; high temperature nitriding the transition-metal ammonia complex in ammonia gas atmosphere for 3-5 hours to prepare transition-metal nitride nanoparticles; the transition-metal ammonia complex includes an ammonia complex formed by any one or a binary or ternary ammonia complex formed by two or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo or Ta; a temperature of the high temperature nitriding is 500-900° C.; the prepared transition-metal nitride nanoparticle has a diameter of 5-15 nm; the non-aqueous solvent is alcohols, or a mixture formed by the alcohols with a ketone or an ester; the transition metal salt includes titanium tetrachloride, tetrabutyl titanate, chromium acetate, manganese chloride, ferric nitrate, cobaltous acetate, copper chloride, niobium chloride, molybdenum chloride and tantalum chloride; and

a preparation of carbon-supported transition-metal nitride nanoparticles: except that the transition metal salts and a pretreated carbon support are added into the solvent simultaneously, other steps are the same as the preparation of the nitride nanoparticles; the carbon support includes an XC-72R carbon black, a carbon nanotube, a carbon nanofiber or graphene; a loading amount of the nitride nanoparticles on the carbon support is 10 wt %-40 wt %;

(2) a fabrication of a working electrode for a pulse electrodeposition, utilizing a method a or a method b as follows:

the method a: weighing an appropriate amount of the nitride nanoparticles or the carbon-supported transition-metal nitride nanoparticles to add into 1-5 mL of an alcoholic solution containing an adhesive, ultrasonically dispersing to make into a slurry, using a micropipette to take an appropriate amount of the slurry to uniformly coat a surface of a working electrode substrate used, a final loading amount of a substrate material is 0.1-0.5 mg/cm2, and the working electrode for the pulse electrodeposition is obtained after drying; the adhesive includes a polytetrafluoroethylene emulsion, a fluorocarbon resin emulsion or a perfluorosulfonate resin emulsion, a mass percent of a usage amount of the adhesive accounts for 0.5%-20% of a total amount of the catalyst substrate material based on a dry polymer resin; the alcohols include ethanol, isopropanol or ethylene glycol; the working electrode substrate includes a glassy carbon, a nickel foam, a titanium sheet, a platinum plated titanium sheet or a platinum sheet; and a way of the drying includes drying by natural air-drying, radiation drying under infrared light or drying by putting into an oven; and

the method b: directly adding the nitride nanoparticles or the carbon-supported transition-metal nitride nanoparticles that are used as the substrate material into a cathode electrolyte solution containing a required active metal for the pulse electrodeposition, stirring, forming the working electrode by a continuous contact of the particles with a cathode conductor; and a catholyte and an anolyte are isolated using a microporous medium; and

(3) the pulse electrodeposition: placing the fabricated working electrode into 0.1-0.5 M H2SO4solution saturated with nitrogen, scanning from an open-circuit voltage to −0.25˜0 V at a scan speed of 5-50 mV/s, with a number of scanning laps of 10-50 laps, achieving cleanness and an activating treatment of the substrate material; then transferring the electrode into a nitrogen saturated electrodepositing solution containing a salt of the required active metal, a complexing agent (or stabilizer) and a conductive aid under an atmosphere of nitrogen, connecting an auxiliary electrode with a reference electrode; setting a pulse frequency, a number of times of pulse deposition, a conduction time and a disconnection time, then opening a pulse electrodeposition program, washing the catalyst from the surface of the electrode when the electrodeposition is completed to obtain the low-platinum catalyst based on the nitride nanoparticles.

In the above-described preparation method, specific method for pretreatment of the carbon support is as follows: weighing 5-20 g of the carbon support, adding into a 200-1000 mL beaker, injecting acetone of ⅗ volume of the beaker, stirring at room temperature for 2-12 hours, filtering, washing, then vacuum drying at 50-80° C.; calcining the dried carbon support at 200-500° C. under an atmosphere of high purity argon for 2-3 hours, then heating and refluxing in a mixed solution of HNO3and H2SO4(a molar ratio is 1:1-1:5, and a solution concentration is 2-5 mol/L) for 6-10 hours, keeping a temperature at 70-80° C., after finally filtering the carbon support and washing the carbon support to neutral with double-distilled water, vacuum drying in the oven at 50-90° C. for 8-24 hours to obtain the pretreated carbon support.

In the above-described preparation method, a specific preparation method of the transition-metal ammonia complex in a non-aqueous solution is as follows: adding a transition metal precursor into a beaker containing the non-aqueous solvent, transferring into a Meng washing bottle after dissolving of the transition metal precursor is completed, introducing ammonia gas for complexation, with a gas flow of ammonia gas being controlled as 30-100 ml/min, and an introducing time of 0.5-1 hour; transferring an obtained mixture containing a complex into a crucible, vacuum drying in the oven at 50-90° C. for 8-24 hours to obtain a nitride ammonia complex; wherein the precursor includes one or two or three of titanium tetrachloride, iron acetate, cobalt acetate, nickel acetate, vanadium chloride, chromic acetate, manganese chloride and niobium chloride; and a concentration range of the precursor in a reaction system solution is 0.1-3 mg/mL.

In the above-described preparation method, a component of the active metal contained in the electrodepositing solution in the step (3) includes: one or more of Pt, Au, Pd, Ru and Ir; a salt of the active metal includes one or more of tetraammineplatinum chloride monohydrate, chloroplatinic acid, chloroauric acid, palladium dichloride, ruthenium trichloride and iridous chloride; the complexing agent includes citric acid, EDTA or polyvinylpyrrolidone; the conductive aid is sodium sulfate or potassium sulfate; and a concentration of the component of the active metal is 5-100 mM.

In the above-described preparation method, a way of deposition of the active metal in the step (3) is the pulse electrodeposition, the pulse frequency is 100-10000 s-1, each pulse contains a turn-on time and a turn-off time, the turn-on time (ton) is 0.00003 s to 0.001 s, the turn-off time (toff) is 0.00015-0.01 s, a ratio of the turn-on time to the turn-off time (ton/toff) varies depending on a molar concentration of the active metal in an electrolyte and the loading amount of the active metal required, with a value between 0.1 and 100; and a total pulse number is 500-20000.

In the above-described preparation method, a pulse current density of the pulse electrodeposition in the step (3) is 1-50 mA/cm2.

In the above-described preparation method, an active component of the prepared catalyst is the transition-metal nitride nanoparticles with an atomic layer cladding of an active metal, the active metal cladding on a surface of a transition metal nitride served as a substrate or of transition-metal nitride nanoparticles supported on a carbon support in an ultrathin atomic layer form; wherein the transition metal nitride served as the substrate includes a unary, a binary or a ternary transition metal nitride, an average particle size of the nanoparticles being 5-15 nm. The active metal used for deposition includes one or an alloy formed by two or more of Pt, Ir, Pd, Ru or Au. The carbon support includes a carbon black particle, a carbon nanotube fiber, a carbon nanotube or graphene. A mass composition of the catalyst is as follows: the carbon support is 50%-80%; the transition metal nitride is 10%-40%, and the component of the active metal is 4%-10%.

In the above-described preparation method, the ultrathin atomic layer is composed of 1-5 atomic layers; and the transition metal nitride includes binary and ternary transition metal nitrides consisting of one or two or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo or Ta.

In the above-described preparation method, the catalyst shows good activity for a cathodic oxygen reduction, a methanol oxidation and a formic acid oxidation reaction, and can be used as anode and cathode catalysts for a hydrogen-oxygen fuel cell, a direct methanol fuel cell and a direct formic acid fuel cell. Its precious metal usage amount is 4-5 times less than that of a commercial Pt/C catalyst, while its mass activity and specific activity can be increased by 3-10 times compared with that of the commercial Pt/C catalyst. In addition, the catalyst is also applicable to a hydrogenation catalyst in chemical industry.

The preparation technique of the present invention can also directly use commercially available nitrides, carbides, oxides, sulfides, phosphides, etc. as substrates for supporting active metals.

Compared with the prior art, the low-platinum catalyst based on nitride nanoparticles and the preparation method thereof according to the present invention have following advantages:

(1) the present invention proposes a preparation method of preparing small particle size nitride nanoparticles with controllable particle size, and the prepared nitride nanoparticles have a particle size of less than 10 nm, making it suitable for preparing the fuel cell catalyst;

(2) compared with the usual carbon support, nitrides have better stability for electrochemical corrosion resistance; it can effectively solve the problem that a carbon corrosion of the traditional carbon-supported catalyst leads to an activity decay of the fuel cell;

(3) the present invention utilizing the pulse electrodeposition technique, may realize an atomic layer level deposition (which is usually a thickness of 3-5 atomic layers) of the active component on the surface of the nitride nanoparticles; compared with a traditional core-shell structure catalyst, it is unnecessary to use a precious metal or an alloy of the precious metal as a core of the core-shell structure, ensuring the high performance of the active component while effectively reducing the cost of the catalyst;

(4) compared with the conventional under-potential deposition technique for preparing the core-shell structure catalyst, the activity of the catalyst prepared by the pulse electrodeposition technique proposed by the present invention is comparable to that of the catalyst prepared by the under-potential deposition; meanwhile, the pulse electrodeposition technique has a controllable depositing amount, and is simple, free of protection of inert gas; and industrial production and application are easy to achieve; and

(5) the catalyst prepared by the present invention has relatively high catalytic performance for the methanol oxidation, the anodic oxidation of formic acid and the cathodic reduction reaction of oxygen, the activity per unit mass of platinum is 2-10 times that of the commercial Pt/C catalyst, and simultaneously the catalyst has extremely high stability.

