Study On Surface Coating Of 316L Stainless Steel Sheet For PEMFC Bipolar Plates

Proton exchange membrane fuel cells?PEMFCs?are very attractive candidates for mobile and stationary power sources because of their high efficiency and near-zero emissions.However,the costs of the main constituents,in particular the bipolar plates,strongly hinders their wide range of application.Commercial graphite bipolar plates have been reported to account for approximately 45%of the total stack cost and for approximately 70%–80%of the total weight of PEMFCs.Therefore,to further eliminate the cost of bipolar plates is a key issue in the case of promoting the commericial application of metallic bipolar plates in PEM fuel cells.316L stainless steel has been demonstrated to be the most promising candidates for replacing machined graphite bipolar plate materials for the proton exchange membrane fuel cell?PEMFC?because it has low gas permeability,high electrical conductivity,ease of machining and shaping into thin sheets,as well as their relatively low material and fabrication costs.A potential disadvantage for stainless steel is its chemical instability in the PEM fuel cell environment.At the cathode side,under typical PEMFC operating conditions,the stainless steel plates tend to form a passive surface layer that leads to poor electrical conductivity.At the anode side,corrosive attack from the strongly acidic environment can free metal ions,such as Fe²⁺/Fe³⁺,Cr³⁺,and Ni²⁺,that contaminate the Nafion membrane and poison the Pt catalyst by restricting proton transfer.The consequent decrease in both proton transfer capability and catalyst activity will result in the deterioration of PEMFC performance.Thus,protective coatings must be deposited onto the surface of stainless steel bipolar plates to avoid corrosion and the consequent deterioration of the PEMFC's performance.At this time,a number of both conductive and anti-corrosion materials,including automic C,metallic Ag,or based on metallic compounds like oxides and nitrides of Cr or Ni,as well as conductive polymers PANI,have been investigated as a thin coating and applied to the metallic bipolar plates.Aimed at the electric conductivity and the resistance to corrosion of 316L stainless steel for bipolar plates in PEM fuel cell,this work describes our development of a method to integrate the advantages of various types of coating materials into SS316L bipolar plates to improve their interfacial conductivity and their corrosion resistance to the levels required for use in PEMFCs.Through this study,we have hoped to expore a novel and low cost surface modification method and to insure SS316L-modified has a sufficiently high corrosion resistance and electroconductivity to enable its use in PEMFC running conditions.The detailed research contents and the resulting outcomes are as follows:Ni–Cr enrichment on stainless steel 316L was achieved by adjusting the concentration and distribution of Cr and Ni from the substrate using in situ oxidation processes by chemical or electrochemical techniques.The super-depth surface configuration measurement and the corresponding energy diffraction spectrometry?EDS?analysis showed that the roughness and porosity at the surface increase resulting from the Ni-Cr enrichment,which result in a number of corrosion pits with dimension of approximately 0.5 to 1?m in width and 2?m in depth;Moreover,the SS316L composition is modified by the surface treatment:the Cr and Ni contents increase from 16.3wt%to 17.8wt%and from 10.5wt%to 11.6wt%,respectively,compared to their contents in the bare SS316L.The combined X-ray photoelectron spectroscopic?XPS?depth profiles and Auger electron spectroxcopic?AES?analysis of the Ni-Cr enrichment layer formed on 316L stainless steel showed that bounder water,hydroxides of Cr and Ni were present in the most outer layer,mixed oxides of Fe,Ni and Cr emerged in the intermediate layer,while the metallic Fe⁰,Ni⁰ and Cr⁰ existed in the most inner layer.Compared to the bare SS316L,the electrical conductivity of the Ni-Cr enriched SS316L in simulated PEMFC environment was remarkable improved,and the interface contact resistance?ICR?between the Ni-Cr enriched SS316L specimen and Toray 060 carbon paper was reduced by over one order of magnitude.These results suggest that the Ni-Cr enriched layer can increase effectively electroconductivity of SS316L.However,the chemical stability of the Ni-Cr enriched SS316L and bare SS316L was nearly identical in either anode or cathode environments of simulated PEMFC,which might result from the dense pits and holes induced by Ni-Cr enriched process.Conducting carbon material like graphite has abundant reserves in the crust,low manufacturing cost,excellent chemical stability in acid condition,and suitable electronic conductivity.Transition metal or ion with empty orbit like Ni,Cr and so on with C,can form weak polarity bond of C-Metal.In this study,we deposited an anti-corrosion and conducting carbon film onto SS316L-modified substrate with PTFE as a binder using a low-cost spraying process in an attempt to develop low-cost bipolar plate materials for PEMFCs.The results observed by scanning electron microscope?SEM?in conjunction with energy-dispersive X-ray spectrometer?EDS?showed that an integrated carbon film with partly flake overlaps and tightly compact structure was obtained,which exhibited a better adhesion of the carbon coating to the stainless steel.Compared to bare SS316L,the C/Ni-Cr enriched/SS316L specimen had greater electrical conductivity and higher chemical stability.A passivation current density?ip?decreased more than 2 orders of magnitude at both anode and cathode sides under the simulated PEMFC static conditions,and the corrosion current densities decreased more than 1² orders of magnitude in the simulated anode and cathode running environments.The ICR results were disappointing in that the ICR values increased significantly with the graphite coating?80.6 m?·cm²?as compared to the Ni-Cr enriched SS316L?26.8 m?·cm²?.Therefore,from the ICR point of view,pure graphite coating is unacceptable to replace the commercial graphite bipolar plates used for PEMFC application.The carbonization of non–conductive ingredients of the PTFE ang the silicon resin that resulted in the degradation of the film`s conductivity,was conducted to increase the carbon content of the above prepared carbon film by chemical strong oxidation treatment.Trace Ag is deposited by in situ immersion and ultra violet?UV?radiation to prepare a very thin silver-doped carbon film on a porous carbon-coated SS316L substrate.The corrosion resistance of this film in mol·L^-1 H₂SO₄ with 5 ppm F^-solution at 80?was investigated using polarization test.Compared to the bare SS316L,the results showed that the corrosion resistance of the Ag-doped graphite-coated SS316L was remarkably improved in both anode and cathode environments of proton exchange membrane fuel cell?PEMFC?.SEM/EDS analysis was performed to characterize the composition and morphology,revealing a dense coverage of Ag and carbon possibly forming a connected network and ensuring a continuous protecting layer on the substrate surface.ICR analysis revealed that the Ag–doped graphite-coated SS316L exhibited greater electrical conductivity compared to bare SS316L,and the interface contact resistance between the specimen and Toray 060 carbon paper was reduced for up to 309.4 m?·cm² at a compaction pressure of 1.2 MPa.Therefore,we believe that the Ag–doped graphite-coated SS316L represents promising material for future PEMFC bipolar plates.Transition metal or ion with empty orbit like Ni,Cr with N with lone electron pair,can combine chemically to form more strong polarity coordinate bond of N-Metal than that of C-Metal.So,the adhesion between the N-bearing monomer and Ni-Cr enriched SS316L is superior to that of the C-bearing monomer.The absorption,polymerization and deposition of aniline monomers with sulphuric acid or oxalic acid as a dopant was realized on Ni-Cr enriched SS316L using potentiostatic method?PM?or Chronopotentiometry?CP?technique.The results showed that a dense nano structure PANI film with highly disordered,different length and overlaps with each other was deposited,which displayed a good adhesion to the substate.Compared to the bare SS316L,the polyaniline coated SS316L bipolar plate was remarkably more stable in both anode and cathode environments of PEMFC.The results of potentiodynamic test exhibited that anodic polarization curves transferred from the state of active-passive region for bare SS316L to the state of self-passivation for polyaniline-coated modified specimen,and with a passivation current density more than 1² orders of magnitude lower than that of the bare steel.The potentiostatic test results showed that the corrosion current density both stayed at a very low value below 10?A·cm^-2.Furthermore,the interface contact resistance between the specimen and Toray 060 carbon paper was reduced up to256.55 m?·cm² at a compaction pressure of 1.4 MPa.Overall,Polyaniline–coated/Ni-Cr modified SS316L had greater electrical conductivity and higher chemical stability compared to bare SS316L,which was mainly depended on the structure,constitution of Ni-Cr enrichment layer and the thickness of PANI film.