Study On The Process And Modification Mechanism Of Surface Coating In Mental Bipolar Plates Of Proton Exchange Membrane Fuel Cell

Proton Exchange Membrane Fuel cell（PEMFC）which uses hydrogen as fuel has widely applied on stationary power station,electric vehicle and portable power source cause its features of high conversion efficiency,non-pollution and swift start up at room temperature.But the high cost of key components like bipolar plates and membrane electrode assembly become the stumbling block during the process of PEMFC large-scale application.It should have excellent electrical conductivity,corrosion resistance and hydrophobicity for an ideal bipolar plates,except that,high mechanical strength and air impermeability,low raw material costs and easy machining are indispensable too.Many of studies have shown that the bipolar plates with surface modified coating used 316L stainless steel as substrate has satisfactory comprehensive performances.The C_0.77Cr_0.23.23 amorphous modified coating used 316L stainless steel as substrate that has been prepared by our research group using Pulse bias arc ion plating technology（PBAIP）has outstanding performances.Its corrosion current density is down to0.091μA.cm^-2 in the simulated environment of PEMFC（0.5 M H2SO4+5 ppm F-）.And the contact resistance is just 2.8 mΩ.cm² under 1.2MPa pressure that is lower than electrosilvering’s 3.5 mΩ.cm² which means the C_0.77Cr_0.23.23 amorphous modified coating has a better electrical conductivity.In addition,the water contact angle is up to 105°that is strong hydrophobicity.These values are far better than the technical standards of DOE in 2020.However the question is that serious corrosion occurred on this coating after about 3000h stacking operation.Studies are made to improve the corrosion resistance of C_ˉCr amorphous modified coating in long term operation.We prepared the C-Cr coating with a series of composition on316L stainless steel using the same parameters by new generation equipment which is self-developed production equipment of enhanced magnetic filtering PBAIP system.To make the failure reason clear,we put the best performance sample into a corrosive environment simulated PEMFC and supply various high voltage to accelerate the reacting process.Then the surface morphology after corroded under different periods and voltages are observed.And the reason is acquired by analyzing these results according to the principle of electrochemical corrosion.Base on the failure reason and literature research,a transition layer will have to be deposited.Firstly,the preferred material of transition layer is going to be selected from kinds of corrosion alloy and then bipolar plates with composition coating are prepared and the deposition parameters are optimized.The main achievements are listed:1.The stainless steel bipolar plates with the first generation coating of C-Cr including C_0.76Cr_0.24 and C_0.79Cr_0.21 are put into a simulated environment of PEMFC（0.5mol/L H₂SO₄+5ppm F^-）for electrochemistry test.The corrosion morphology of different periods under the three levels of 1.6V,2.2V and 4V are observed and analyzed.And the results shown as following:The microparticles supply the necessary defects for chemical corrosion and electrochemistry corrosion.But the corrosion mechanism varies with the size of microparticles.Microparticles around 10⁵0μm,which are minority,are always suffered from the simultaneous action of stress corrosion and galvanic corrosion in which coating burst sharply.Microparticles smaller than 10μm drop slowly.After that,a shallow pits come out and deeper gradually until that the electrolyte contact substrate directly under electrochemistry corrosion.Then corrosion cracks extend until coating burst under the simultaneous action of stress corrosion and electrochemistry corrosion.Microparticles are the original of corrosion defects and stress corrosion make the corrosion worse.2.Target at above corrosion mechanism,the second generation coating which add a transition layer are designed to improve the pitting corrosion resistance.The transition layer have been selected and optimized from TA1（commercial pure titanium）,TA9（Ti-0.2Pd）,TA10（Ti-0.3Mo-0.8Ni）and Ti35（Ti-6Ta）guided by principles of corrosion and protection of metals.The contact resistance difference of transition layers between before and after passivation are compared and the result show that TA1 has the biggest D-value and the worst electrical conductivity after passivation;the smallest D-value and the best electrical conductivity are owned Ti35 because the element of Ta mixed into Ti alloy during which conductive oxide film is formed after passivation.The potentiodynamic polarization curve of transition layers are tested and results show that the corrosion resistance of Ti35 is better than others,TA10 is followed under aeration condition;TA1 has the best performance then is Ti35under H₂ condition.Considering all of them,Ti35 has the best corrosion resistance than other transition layers.The surface morphology of transition layers before and after corrosion are observed and results show that TA1 has severed corrosion;TA9 TA10 and Ti35 have no obvious corrosion,but TA9 and TA10 have lots of microparticles on their surface.On the whole,Ti35 is the best transition layer.3.The second generation composition coating of 316L/Ti35/C-Cr is prepared and its preparation parameters are optimized.The results show that high bias–low duty ratio is superior to low bias–high duty in quality and corrosion resistance of coating;arc-current and deposition temperature are the most impacted parameters for quality of coating and the highest quality coating can be acquired at 60A of arc-current and 350℃of deposition temperature,but the deposition time has a little more;rising arc-current up to 80A and decreasing deposition time down to 20min there will be more microparticles but the current density is steady during 200h test of potentiostatic polarization curve that means that its corrosion resistance is excellent.4.The second generation composition coating are tested across the board.The total thickness is 287.2nm after 20 min（Ti35:5min,C-Cr:15min）deposition,within that,transition layer and C-Cr are 157.8nm and 124.9nm respectively.Its contact resistance value is reduced from 3.76 mΩcm² to 2.77mΩcm² that means the electrical conductivity is better than the first generation coating.The same consequence is got at corrosion resistance.And the hydrophobicity is maintained.