A multilayer gradient coating Nb / NbN / (Nb,Ta)2alc for proton exchange membrane fuel cell metal bipolar plate and a preparation method thereof

Abstract

The application belongs to the field of proton exchange membrane fuel cells, and particularly relates to a Nb / NbN / (Nb, Ta)2AlC multilayer gradient coating for a metal bipolar plate of a proton exchange membrane fuel cell and a preparation method thereof. The metal bipolar plate comprises a metal substrate, a Nb transition layer, a NbN connecting layer and an (Nb, Ta)2AlC outer layer coating. Firstly, the Nb coating is deposited on the metal bipolar plate by using arc ion plating technology with a high deposition rate, so as to improve the adhesion between the substrate and the outer layer. Then, the N2 gas is filled in the process of depositing the Nb, so as to deposit the NbN connecting layer, improve the microstructure of the Nb coating, reduce the columnar crystal and improve the compactness of the coating. Finally, the (Nb, Ta)2AlC coating is deposited by using magnetron sputtering technology, so as to modulate the structure of the multilayer gradient coating and obtain the coating with uniformity, compactness and high adhesion, which can effectively improve the corrosion resistance and the conductive performance after corrosion of the metal substrate. The preparation rate of the overall coating is fast, the process is easy to control, the performance of the bipolar plate can be greatly improved, and the service life of the fuel cell is further improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of proton exchange membrane fuel cell, and particularly relates to a modified Nb / NbN / (Nb, Ta)2AlC multilayer gradient coating for proton exchange membrane fuel cell metal bipolar plate and a preparation method thereof. BACKGROUND

[0002] Proton exchange membrane fuel cell has the advantages of compact structure, small volume, high energy density, high efficiency, fast start, low temperature operation and zero emission, and is considered as an ideal clean power generation energy at present. Bipolar plate, as one of the most important components of PEMFC, connects single cells in series, parallel or mixed connection to form a cell stack, plays a supporting role, can isolate the cathode and anode reaction gas, and discharge the heat and water generated by the cell stack reaction, and is crucial to the performance of PEMFC cell stack. At present, bipolar plates mainly include graphite bipolar plate, metal bipolar plate and composite bipolar plate. The metal bipolar plate has high strength and is easy to process, and the ultra-thin bipolar plate is easy to obtain large-scale production, which can improve the specific power of the fuel cell. However, the bipolar plate working environment has various corrosive ions such as SO4 2- , F-, etc., and the metal bipolar plate material is easily corroded to form a passivation layer, which increases the contact resistance between the bipolar plate and the diffusion layer, greatly affecting the output power and durability of the fuel cell stack. Therefore, through surface coating modification, reducing the surface contact resistance of the metal bipolar plate, improving its conductivity and corrosion resistance is the key to its commercial application. The current bipolar plate coating mainly includes carbon-based coating, noble metal coating, conductive polymer coating, hydrophobic coating and transition metal ceramic compound. Carbon-based coating has excellent corrosion resistance, and excellent conductivity and thermal conductivity, and also has low production cost, and has been widely studied. However, the deposition efficiency of carbon-based coating is low, which affects its large-scale application. The noble metal coating has excellent corrosion resistance and conductivity, but the cost is too high. The conductive polymer coating can provide good protection for the bipolar plate, and has good corrosion resistance and conductivity, among which polyaniline (PANI) and polypyrrole (PPy) are studied more. However, the adhesion between the coating and the substrate is weak. The hydrophobic performance of the hydrophobic coating can greatly affect the corrosion rate of the bipolar plate, but it is difficult to maintain long-term stability. Transition metal ceramic compounds have excellent physical, chemical and mechanical properties, and have excellent corrosion resistance and stability in the working environment of the bipolar plate, and can also maintain high conductivity, which is one of the ideal coating materials for PEMFC bipolar plate and has good development prospects. However, the current transition metal ceramic coating generally has high preparation cost, low efficiency, pinholes, large particles or columnar crystals in the coating, which seriously affects its long-term stability, and the peeling problem caused by poor coating adhesion or poor long-term stability will have a bad impact on the cell stack and accelerate the aging process of the cell stack. SUMMARY

[0003] In order to solve the above problems, the present application provides a multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate and a preparation method thereof, which can obtain a coating with uniformity, compactness and high adhesion, reduce defects such as columnar crystals, pinholes and large particles, and effectively improve the corrosion resistance and corrosion resistance of the metal substrate. The preparation rate of the overall coating is fast, the process is easy to control, and the performance of the bipolar plate can be greatly improved, thereby improving the service life of the fuel cell.

[0004] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0005] A multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate, comprising a metal substrate, a Nb transition layer, a NbN connecting layer and an (Nb, Ta)2AlC outer coating. The coating material is a Nb / NbN / (Nb, Ta)2AlC multilayer gradient coating, the Nb layer is a transition layer, which can reduce the thermal expansion mismatch between layers and improve the element compatibility of each layer, thereby improving the adhesion of the coating.

[0006] Further, the multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate and the preparation method thereof, wherein the coating material is a Nb / NbN / (Nb, Ta)2AlC multilayer gradient coating, the NbN layer is a connecting layer, which can effectively improve the microstructure of the Nb coating, reduce columnar crystals and improve the compactness of the coating.

[0007] Further, the multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate, wherein the outer coating is NbAlC doped with Ta element at Nb position, the doping element accounts for 0.05-25 at.%, and after doping, the corrosion resistance and conductivity of NbAlC are significantly improved, the corrosion current density is reduced by 85-90%, the self-corrosion potential is increased by 0.13-0.18V, and the contact resistance is reduced by 83-90%. As the outer layer, it mainly improves the corrosion resistance and conductivity of the substrate, and its performance is significantly better than that of NbAlC.

[0008] Further, the multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate, wherein the thickness of the Nb transition layer is 50-100 nm, the thickness of the NbN connecting layer is 50-150 nm, and the thickness of the (Nb, Ta)2AlC layer is 200-700 nm.

[0009] Further, the multilayer gradient coating for the metal bipolar plate of the proton exchange membrane fuel cell is characterized in that the outer coating target material is a (Nb, Ta) 2 AlC single-phase target material, the original powder for preparation includes Nb powder, Ta powder, Al powder and graphite powder, and the sintering is prepared in a hot-pressing furnace by using a hot-pressing / solid-liquid phase reaction method, the sintering temperature is 1600-1950 DEG C, the holding time is 40-90 minutes, the hot-pressing pressure is 20-75 MPa, and flowing argon is used as the protective gas.

[0010] Further, the multilayer gradient coating for the metal bipolar plate of the proton exchange membrane fuel cell is characterized in that the Nb transition layer and the NbN connecting layer are deposited by using an arc ion plating device, the Nb transition layer is prepared by using a Nb metal target material, the NbN layer is obtained by heating and filling N2 gas after depositing the Nb coating, and the NbN connecting layer is deposited.

[0011] Further, the multilayer gradient coating for the metal bipolar plate of the proton exchange membrane fuel cell is characterized in that the pretreatment method of the metal bipolar plate is as follows: first, sandpaper is used to polish the metal bipolar plate, i.e., 400#, 600#, 800#, 1000# and 2000# metallographic sandpaper is used for step-by-step polishing, then the polished metal bipolar plate sample is cleaned by ultrasonic cleaning in acetone, alcohol and deionized water for 10-20 minutes, and is dried in air for standby use.

[0012] Further, the multilayer gradient coating for the metal bipolar plate of the proton exchange membrane fuel cell is characterized in that the metal substrate includes but is not limited to SS304, 316L, Ti plate or SS316L bipolar plate metal.

[0013] Further, the multilayer gradient coating for the metal bipolar plate of the proton exchange membrane fuel cell is characterized in that the Nb transition layer and the NbN connecting layer are deposited by using an arc ion plating method, before the coating preparation, the vacuum chamber is pre-evacuated to a background vacuum of 4x10 -3After 500 Pa, the substrate is subjected to reverse sputter cleaning for 5-12 min by applying a negative pulse bias of 500 V to the substrate to remove contaminants and oxide layers on the surface of the substrate. Then the Ar flow valve is opened, the Ar flow rate is 50 ml / min, the working pressure in the vacuum chamber is controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.4 Pa, and the chamber heating temperature is 100-150℃. The Nb target DC power supply is turned on, the arc current is 50-90 A, the bias is -200 to -400 V, and the sputtering time is 2-15 min. Then stop sputtering and cool down. Then the vacuum chamber is pre-evacuated to a background vacuum of 4x10 -3 After 500 Pa, the Ar flow valve is opened, the Ar flow rate is 50 ml / min, the working pressure in the vacuum chamber is controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.45 Pa, the chamber heating temperature is 150-250℃, and the temperature is maintained for 15 min. Then the Nb target DC power supply is turned on again, the arc current is 60-95 A, the bias is -200 to -500 V, and the sputtering time is 5-15 min. Then stop sputtering and cool down.

[0014] Further, the multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate is characterized in that the (Nb, Ta)2AlC outer layer coating is deposited by a magnetron sputtering method, the vacuum chamber is pre-evacuated to a background vacuum of 4x10 -3 After 500 Pa, the Ar flow valve is opened, the Ar flow rate is 50 ml / min, the working pressure in the vacuum chamber is controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.45 Pa, the chamber heating temperature is 150-250℃, and the temperature is maintained for 15 min. Then the Nb target DC power supply is turned on again, the arc current is 60-95 A, the bias is -200 to -500 V, and the sputtering time is 5-15 min. Then stop sputtering and cool down.

[0015] The Nb / NbN / (Nb,Ta)2AlC multilayer gradient coating of the metal bipolar plate has the advantages of uniformity, compactness and high adhesion. The Nb transition layer and the NbN connecting layer are deposited on the metal bipolar plate by using the arc ion plating technology with high deposition rate, and the Nb transition layer and the NbN connecting layer can be obtained by introducing different gases into the cavity during the deposition process, which is efficient, low in cost and convenient to operate. Meanwhile, the Nb transition layer is deposited in the inner layer, which can reduce the thermal expansion mismatch degree between layers and improve the adhesion of the coating. The NbN connecting layer is deposited on the Nb transition layer, which can effectively improve the microstructure of the Nb coating, reduce the columnar crystal and the pinhole structure in the coating, and improve the compactness of the coating. The outer coating is Nb aluminum carbon with Ta element doped in the Nb position, and the proportion of the doped element is 0.05-15at.%. After the doping, the corrosion resistance and the electrical conductivity of the Nb aluminum carbon are significantly improved, the corrosion current density is reduced by 85-90%, the self-corrosion potential is increased by 0.13-0.18V, and the contact resistance is reduced by 83-90%. Meanwhile, the preparation method of the coating is easy to control and efficient, and the gradient coating with compactness, flatness and uniform composition structure can be obtained, which can greatly improve the performance of the bipolar plate and the service life of the fuel cell, and is easy to be popularized in industry. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present application, the figures in the embodiments are briefly introduced. Obviously, the figures in the following description are only some embodiments of the present application, and other figures can be obtained by those skilled in the art or researchers without creative labor on the basis of these figures.

[0017] Figure 1 The scanning electron microscope surface graph of the coating prepared in Example 1 is shown in Figure 1.

[0018] Figure 2 The scanning electron microscope surface graph of the coating prepared in Example 2 is shown in Figure 2. DETAILED DESCRIPTION

[0019] The present application will be further described in detail below in combination with specific embodiments and drawings, but the present application is not limited in any way by the embodiments.

[0020] The metal substrate used in the following examples is a metal bipolar plate. The preparation method of the (Nb,Ta)2AlC target material used for deposition adopts a hot-pressing / solid-liquid phase reaction method to sinter in a hot-pressing furnace, and the raw powder used in the preparation includes Nb powder, Ta powder, Al powder and graphite powder, and the ratio used is determined by the doping content of Ta, (Nb 1-x Ta x)2AlC, the ratio of Nb:Nb:Al:C is configured according to the ratio of 2(1-x):2x:1:1 of the raw material powder, and the wet mixing method is used to mix in the ball mill for 12-48 hours. After that, it is taken out and naturally dried, and then sieved for use. The sintering temperature is 1600-1950℃, the holding time is 40-90 minutes, the hot pressing pressure is 20-75 MPa, and flowing argon is used as the protective gas.

[0021] Comparative Example

[0022] The comparative example prepared a sample of Nb2AlC coating on a metal bipolar plate SS316L.

[0023] First, prepare the Nb2AlC bulk target material, and prepare the SS316L metal bipolar plate sample. That is, use 400#, 600#, 800#, 1000#, 2000# metallographic sandpaper to polish step by step, and then ultrasonically clean the polished metal bipolar plate sample with acetone, alcohol and deionized water for 15 minutes respectively, and then dry in air for standby.

[0024] Then, Nb2AlC outer coating is deposited by magnetron sputtering method. First, adjust the chamber temperature to 300℃ and hold for 15 minutes, then turn on the Nb2AlC target direct current power, the sputtering power is 1kW, and the sputtering time is 30 minutes. After deposition, the temperature is reduced to room temperature at a rate of 10℃ / min under vacuum, and then the vacuum is stopped and the pressure is removed.

[0025] After the experiment, the surface and cross-section microstructure of the deposited coating is observed by scanning electron microscope, and it is found that the obtained coating is flat and well combined with the substrate. In the simulated environment of fuel cell, that is, in 0.5mol / L H2SO4 and 2ppm HF solution, the temperature is 80℃, the potentiostatic test is carried out, the corrosion current density is 3.45μA / cm 2 [Self-corrosion potential 0.02V(vs.SCE)], the contact resistance is 18.9mΩ·cm 2 under the condition of assembly force of 150N / cm 2 .

[0026] Example 1

[0027] First, prepare the (Nb 0.95 Ta 0.05 )2AlC bulk target material, and prepare the commercial Nb target and SS316L stainless steel bipolar plate. That is, use 400#, 600#, 800#, 1000#, 2000# metallographic sandpaper to polish step by step, and then ultrasonically clean the polished metal bipolar plate sample with acetone, alcohol and deionized water for 15 minutes respectively, and then dry in air for standby.

[0028] Nb and NbN layers were deposited using arc ion plating. Before coating preparation, the vacuum chamber was pre-evacuated to a background vacuum of 4 × 10⁻⁶. -3 After Pa, a 500V negative pulse bias voltage was applied to the substrate for backsplash cleaning for 8 minutes to remove contaminants and oxide layers from the substrate surface. Then, the Ar flow valve was opened, and the Ar gas flow rate was 50 ml / min. The working pressure in the vacuum chamber was controlled by adjusting the pumping speed of the molecular pump to maintain it at approximately 0.4 Pa, and the chamber heating temperature was 120℃. The Nb target DC power supply was turned on, with an arc current of 60 A, a bias voltage of -300 V, and a sputtering time of 10 minutes. Sputtering was then stopped, and the chamber was allowed to cool. The vacuum chamber was then pre-evacuated to a background vacuum of 4 × 10⁻⁶. -3 After Pa, open the N2 gas flow valve, set the N2 gas flow rate to 30 ml / min, and control the working pressure in the vacuum chamber by adjusting the pumping speed of the molecular pump to maintain it at approximately 0.45 Pa. The chamber heating temperature is set to 200℃ and held for 15 minutes. Then, turn on the Nb target DC power supply again, setting the arc current to 75 A, the bias voltage to -400 V, and the sputtering time to 6 minutes. Finally, stop sputtering and allow the chamber to cool down.

[0029] Then, Nb was deposited using magnetron sputtering. 0.95 Ta 0.05 )2AlC outer coating, the vacuum chamber is pre-evacuated to a back vacuum of 4×10 -3 After Pa, open the Ar flow valve, set the Ar gas flow rate to 50 ml / min, and control the working pressure inside the vacuum chamber by adjusting the pumping speed of the molecular pump to maintain it at approximately 0.45 Pa. Set the chamber heating temperature to 350℃ and hold for 15 minutes. Then open the (Nb) valve. 0.95 Ta 0.05 The AlC target was powered by a DC power supply with a sputtering power of 1.5 kW and a sputtering time of 45 min. After deposition, the target was cooled to room temperature at a rate of 10 °C / min under the original vacuum conditions, and then the vacuum was stopped and the pressure was released.

[0030] After the experiment, the surface and cross-sectional microstructure of the deposited coating were observed using a scanning electron microscope. The results showed that the coating was smooth, dense, and well-bonded to the substrate, with uniform thickness in all layers. Figure 1 As shown. Potentiodynamic testing was conducted in a fuel cell simulation environment, specifically in a solution with H₂SO₄ concentration of 0.5 mol / L and HF concentration of 2 ppm, at a temperature of 80 °C, with a corrosion current density of 0.52 μA / cm². 2 [Self-corrosion potential 0.18V (vs. SCE)], at an assembly force of 150N / cm 2 Under these conditions, the contact resistance is 2.76 mΩ·cm. 2 .

[0031] Example 2

[0032] Firstly, the bulk target of (Nb 0.9 Ta 0.1 )2AlC was prepared, and the commercial Nb target and SS316L stainless steel bipolar plate were prepared. That is, the bipolar plate sample was polished by 400#, 600#, 800#, 1000# and 2000# metallographic sandpaper in turn, and then the polished metal bipolar plate sample was ultrasonically cleaned with acetone, alcohol and deionized water for 12 min respectively, and then dried in air for standby use.

[0033] The Nb and NbN layers were deposited by arc ion plating method. Before coating preparation, the vacuum chamber was pre-evacuated to a background vacuum of 4×10 -3 Pa, and a 500V negative pulse bias was applied to the substrate for 5min to remove the contaminants and oxide layer on the surface of the substrate. Then the Ar flow valve was opened, the Ar flow rate was 50ml / min, the working gas pressure in the vacuum chamber was controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.4Pa, and the chamber heating temperature was 150℃. The Nb target DC power was turned on, the arc current was 70A, the bias was -200V, and the sputtering time was 5min. Then the sputtering was stopped and the temperature was lowered. Then the vacuum chamber was pre-evacuated to a background vacuum of 4×10 -3 Pa, the N2 flow valve was opened, the N2 flow rate was 30ml / min, the working gas pressure in the vacuum chamber was controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.45Pa, the chamber heating temperature was 250℃, and the temperature was maintained for 15min. Then the Nb target DC power was turned on again, the arc current was 80A, the bias was -300V, and the sputtering time was 8min. Then the sputtering was stopped and the temperature was lowered.

[0034] Then the (Nb 0.9 Ta 0.1 )2AlC outer layer coating was deposited by magnetron sputtering method. The vacuum chamber was pre-evacuated to a background vacuum of 4×10 -3 Pa, then the Ar flow valve was opened, the Ar flow rate was 50ml / min, the working gas pressure in the vacuum chamber was controlled by adjusting the pumping speed of the molecular pump to maintain at about 0.45Pa, the chamber heating temperature was 400℃, and the temperature was maintained for 15min, then the (Nb 0.9 Ta 0.1 )2AlC target DC power was turned on, the sputtering power was 2.0kW, and the sputtering time was 30min. After deposition, the temperature was lowered to room temperature at a rate of 10℃ / min under the original vacuum condition, and then the vacuum and pressure were stopped.

[0035] After the experiment, the surface and cross-section micro-morphology of the deposited coating was observed by scanning electron microscope, and it was found that the obtained coating was flat, dense, and well combined with the substrate, and the thickness of each layer coating was uniform, such as Figure 2The corrosion current density is 0.33 μA / cm 2 [Self-corrosion potential 0.20 V (vs. SCE)], and the contact resistance is 1.98 mΩ·cm 2 under the assembly force of 150 N / cm 2 .

[0036] Example 3

[0037] First, prepare the (Nb 0.8 Ta 0.2 )2AlC bulk target material, and prepare the commercial Nb target material and SS316L stainless steel bipolar plate. That is, polish the bipolar plate sample step by step with 400#, 600#, 800#, 1000# and 2000# metallographic sandpaper, and then ultrasonically clean the polished metal bipolar plate sample in acetone, alcohol and deionized water for 20 min, and dry in air for standby.

[0038] Nb and NbN layers are deposited by arc ion plating method. Before coating preparation, the vacuum chamber is pre-evacuated to a background vacuum of 4×10 -3 Pa, and a 500V negative pulse bias is applied to the substrate for 12 min to remove contaminants and oxide layers on the substrate surface. Then open the Ar flow valve, Ar flow rate 50 ml / min, control the working gas pressure in the vacuum chamber by adjusting the pumping speed of the molecular pump, maintain it at about 0.4 Pa, and the chamber heating temperature is 100°C. Turn on the Nb target DC power supply, arc current 90A, bias -400V, sputtering time 15 min. Then stop sputtering and cool down. Then pre-evacuate the vacuum chamber to a background vacuum of 4×10 -3 Pa, open the N2 flow valve, N2 flow rate 30 ml / min, control the working gas pressure in the vacuum chamber by adjusting the pumping speed of the molecular pump, maintain it at about 0.45 Pa, and the chamber heating temperature is 150°C, and heat for 15 minutes. Then turn on the Nb target DC power supply again, arc current 90A, bias -400V, sputtering time 10 min. Then stop sputtering and cool down.

[0039] Then deposit the (Nb 0.8 Ta 0.2 )2AlC outer layer coating by magnetron sputtering method, pre-evacuate the vacuum chamber to a background vacuum of 4×10 -3 Pa, then open the Ar flow valve, Ar flow rate 50 ml / min, control the working gas pressure in the vacuum chamber by adjusting the pumping speed of the molecular pump, maintain it at about 0.45 Pa, and the chamber heating temperature is 450°C, heat for 15 min, then open the (Nb0.8 Ta 0.2 )2AlC target DC power supply, sputtering power is 2.5kW, sputtering time is 50min. After the end of deposition, under the original vacuum condition, with the rate of 10℃ / min to room temperature, then stop pumping, pressure.

[0040] After the experiment, the surface and cross-section micro-morphology of the deposited coating are observed by scanning electron microscope, it is found that the obtained coating is flat, dense, and well combined with the substrate, the thickness of each layer coating is uniform. In the simulated environment of fuel cell, namely H2SO4 concentration is 0.5mol / L and 2ppm HF solution, temperature is 80℃, the potentiodynamic test is carried out, the corrosion current density is 0.43μA / cm 2 [Self-corrosion potential 0.15V (vs.SCE)], the contact resistance is 3.24mΩ·cm 2 Under the condition of assembling force 150N / cm 2 .

[0041] Of course, the above description is not a limitation of the present application, the present application is not limited to the above examples, the ordinary skilled in the art, within the scope of the present application, the changes, modifications, additions or replacements made, should belong to the protection scope of the present application.

Claims

1. A multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate comprising a substrate and a coating, characterized in that, The material of the coating is a Nb / NbN / (Nb,Ta)2AlC multilayer gradient coating, wherein Nb is a transition layer, NbN is a connecting layer, and (Nb,Ta)2AlC is a modified and optimized material of ternary layered ceramic niobium aluminum carbon; The (Nb,Ta)2AlC outer layer coating is Nb-aluminum carbon with Ta element solid solution doped at the Nb position, and the proportion of the doping element is 0.1-25 at.%; a Nb transition layer is deposited in the inner layer of the coating, and then a NbN connecting layer is deposited on the inner layer Nb transition layer.

2. A multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate according to claim 1, characterized in that, The thickness of the Nb transition layer is 50-100 nm, the thickness of the NbN connecting layer is 50-150 nm, and the thickness of the (Nb,Ta)2AlC layer is 200-700 nm.

3. A method for the production of a multilayer gradient coating for a metallic bipolar plate of a proton exchange membrane fuel cell according to any one of claims 1-2, characterized in that, The method comprises the following steps: Nb transition layer and NbN connecting layer are deposited by arc ion plating method, Nb metal target is used for Nb transition layer, and NbN layer is obtained by pre-vacuumizing the vacuum chamber to a background vacuum of 4x10 -3 Pa, opening N2 flow valve, N2 flow being 30 ml / min, controlling working gas pressure in the vacuum chamber by adjusting pumping speed of the molecular pump, so as to maintain the working gas pressure at 0.45 Pa, chamber heating temperature being 150-250 DEG C, holding for 15 min; then Nb target direct current power is opened again, arc current being 60-95 A, bias voltage being -200--500 V, sputtering time being 5-15 min; then sputtering is stopped, and NbN connecting layer is obtained by cooling. The (Nb,Ta)2AlC coating is deposited on the outer surface by a magnetron sputtering method, the sample is hung on a sample holder of the equipment during deposition of the coating, a stand for hanging the sample can rotate, and the rotating stand can revolve around a rotating table to obtain uniform coating.

4. A method for the production of a multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate according to claim 3, characterized in that, The target material used for the coating is a (Nb,Ta)2AlC single-phase target material, and the original powder in the preparation includes Nb powder, Ta powder, Al powder and graphite powder, which are sintered in a hot-pressing furnace by a hot-pressing method, the sintering temperature is 1600-1950 DEG C, the holding time is 40-90 minutes, the hot-pressing pressure is 20-75 MPa, and flowing argon is used as a protective gas.

5. A method for the preparation of a multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate according to claim 3, characterized in that, The pretreatment method of the metal bipolar plate is as follows: first, sandpaper is used to polish the metal bipolar plate, i.e., 400#, 600#, 800#, 1000# and 2000# metallographic sandpaper is used for step-by-step polishing, then the polished metal bipolar plate sample is ultrasonically cleaned in acetone, alcohol and deionized water for 10-20 minutes, and then dried in air for standby use.

6. A method for the fabrication of a multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate according to claim 3, characterized in that, The arc ion plating method is used to deposit a Nb transition layer and a NbN connecting layer. Before the coating is prepared, the vacuum chamber is pre-evacuated to a background vacuum of 4x10 -3 After the background vacuum is 4x10 Pa, a negative pulse bias of 500 V is applied to the substrate to perform reverse sputter cleaning on the substrate for 5-12 min, so as to remove the contaminants and the oxide layer on the surface of the substrate. Then, the Ar flow valve is opened, the Ar gas flow is 50 ml / min, the working gas pressure in the vacuum chamber is controlled by adjusting the pumping speed of the molecular pump, and the working gas pressure is maintained at 0.4 Pa. The chamber heating temperature is 100-150 DEG C. The Nb target direct current power is turned on, the arc current is 50-90 A, the bias voltage is -200 to -400 V, and the sputtering time is 2-15 min; then the sputtering is stopped, and the temperature is lowered; then the vacuum chamber is pre-evacuated to a background vacuum of 4x10 -3 After 0.45 Pa, the N2 flow valve is opened, the N2 flow is 30 ml / min, the pumping speed of the molecular pump is adjusted to control the working pressure in the vacuum chamber, so that it is maintained at 0.45 Pa, the chamber heating temperature is 150-250 DEG C, and after holding for 15 min; then the Nb target direct current power is turned on again, the arc current is 60-95 A, the bias voltage is -200 to -500 V, and the sputtering time is 5-15 min; then the sputtering is stopped, and the temperature is lowered.

7. A method for the fabrication of a multilayer gradient coating for a proton exchange membrane fuel cell metal bipolar plate according to claim 3, characterized in that, The (Nb, Ta)2AlC outer coating is deposited by magnetron sputtering method, the vacuum chamber is pre-evacuated to a background vacuum of 4×10 -3 After the pressure is stabilized at 0.45 Pa, the chamber heating temperature is 250-450 ℃, and the temperature is maintained for 15 min; then the (Nb, Ta)2AlC target direct current source is turned on, the sputtering power is 0.12-3.0 kW, and the sputtering time is 20 min-1 h; after the deposition is completed, the temperature is reduced to room temperature at a rate of 10 ℃ / min under the original vacuum condition, and then the evacuation and pressure relief are stopped.

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