| Bipolar plates?BPPs?are important components of proton exchange membrane fuel cells?PEMFC?which have functions of supporting cells,collecting and conducting electricity,separating oxidant and reductant,and directing the flow of oxidant and reductant on the inner surface of the electrode.However,metal bipolar plates have the problem of poor corrosion resistance and excessive contact resistance in the internal environment of PEMFC,which restricts the application of metal bipolar plates in PEMFC.The method of metal surface modification is one of the effective methods for improving corrosion resistance and reducing contact resistance.The research group had prepared Cr-C amorphous modified coating on316L stainless steel obtaining good electrical and corrosion resistance,but it failed after 3000hours of operation.The cause of failure was found that lied in the arc ion plating technology.The coating prepared by the plating method had large particle contamination which was easy to cause pinhole penetration,and insufficient corrosion resistance in the strong acid corrosion environment of stainless steel.In this paper,titanium alloy with better corrosion resistance in acidic environment was selected as the research object.Six different compositions of Cr-C modified films were prepared on TA1 and TC4 substrate by a new generation of magnetic filter enhanced pulsed bias arc ion plating technique.The morphology,structural composition and properties were analyzed by scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,X-ray diffraction instrumentation?XRD?and electrochemistry.The results showed that the Cr-C modified films prepared in this paper were carbon-based amorphous films mainly composed of sp2 and sp3 bond carbon atoms,and the average thickness of the film was about100 nm.The contact resistance of the modified bipolar plates was greatly reduced.Since the TA1 substrate was pure metal,the conductivity was better than that of the TC4 titanium alloy,but the corrosion resistance was weaker than that.At a pressure of 0.6 MPa,the TA1-2#sample had the lowest contact resistance of 0.62 m?·cm2 among series of TA1 samples.The contact resistance values of each sample of TC4 were less than 1 m?·cm2,and the lowest value was 0.42 m?·cm2.In general,the TC4-6#sample(Cr0.21C0.79)the best comprehensive performance in conductivity and corrosion resistance that exhibited the characteristic of cathodic protection at–0.1 V anode operating voltage,corrosion current density of 0.10?A/cm2 at 0.6 V cathodic operating voltage and 0.45 m?·cm2 at 0.6 MPa.Indicating that the Cr-C coating modified bipolar plate based on titanium alloy had excellent performance in terms of corrosion resistance and electrical conductivity.Compared with stainless steel,titanium alloy has higher cost and poor plastic working property,and the residual stress is larger when the flow field is formed by extrusion molding.Therefore,it is considered to utilize the excellent pitting resistance of the titanium alloy as the transition layer material.Preparation of TA1?industrial pure titanium?,TA9?Ti-0.2Pd?,TA10?Ti-0.3Mo-0.8Ni?and Ti35?Ti-6Ta?coatings on a 316L stainless steel substrate for transition layer screening,followed by preparation of 316L/Ti/Cr0.23C0.77 composite bipolar plate and its corrosion mechanism was studied by electrochemical impedance spectroscopy.The results showed that the corrosion resistance of the four titanium alloys was ranked from large to small by electrochemical test:Ti35>TA9>TA10>TA1,so Ti35 was finally selected as the transition layer material of the multilayer composite bipolar plates.The 316L/Ti35/Cr0.23C0.77composite bipolar plate and the control group 316L/Cr0.23C0.77 were prepared.The corrosion mechanism was analyzed by electrochemical impedance spectroscopy?EIS?.Two samples from high frequency to low frequency were observated that only a single capacitive arc occured,and the surface corrosion process was controlled by the charge transfer rate,and there was no material transfer process.At the same time,after increasing the Ti35 transition layer,the charge transfer resistance increased from 5.5×104?·cm2 to 8.1×104?·cm2,which meant that the charge transfer difficulty increasing and the corrosion resistance increasing.The 316L/Ti35/Cr0.23C0.77.77 bipolar plate is more excellent in corrosion resistance benefit from the addition of Ta element which can inhibits the anodic polarization process and increases the open circuit potential.On the other hand,the internal corrosion environment of PEMFC has less F-ion content,and the thermodynamic properties of Ti and Ta are active.The affinity between oxygen atoms and these two elements is greater,and it is easy to form a dense passivation film of TiO2 and Ta2O5.When corrosion solution encounters the Ti35 coating through Cr0.23C0.77 defects,Ti and Ta rapidly form a passivation film,so that the electrode potential is raised at this place,which can effectively inhibit the pitting corrosion and protect the substrate from corrosion. |
PDF Full Text Request