| The proton exchange membrane fuel cell（PEMFC）is a clean and efficient fuel cell that is popular in many industries for its fast start-up speed and low operating temperature.As one of the key components of a PEMFC,the bipolar plate plays a role in separating the cathode and anode,collecting heat and current.Metallic bipolar plate is the excellent choice of bipolar plate material due to its excellent mechanical properties,good electrical and thermal conductivity,which makes up for the shortcomings of traditional graphite bipolar plate.However,the operating environment inside the PEMFC(containing SO42-,F-,HSO4-,HCO3-)is prone to dissolution passivation of the metal bipolar plates,and the resulting passivation film greatly reduces the electrical conductivity of the metallic bipolar plates and thus affects the output power of the battery.The preparation of electrically conductive and corrosion resistant coatings on the surface of metallic bipolar plates is one of the most economical and effective ways to address the phenomenon of metal passivation.In this paper,316L stainless steel and pure titanium are used as metal bipolar plate substrates,respectively.PPy-Ti4O7 composite coating and Magnéli phase Ti4O7 coating were prepared on the metal substrate surface using constant current deposition and Ti powder reduction methods.The morphology,contact angle and surface contact resistance of coatings were characterized,and the corrosion resistance of the coatings in a simulated PEMFC cathodic side environment were investigated.The details of this paper are as follows:（1）The conditions for the generation of Magnéli phase substances were investigated using a program-controlled tube furnace.In the Zr reduction method,Ti4O7 powder was successfully prepared when the molar ratio of Zr/Ti O2 was 20:1,the experimental temperature was 1050℃and the calcination time was adjusted to 15 h.In the Ti reduction method,Ti4O7 powder could be obtained by calcination at 850℃for 4 h or 3.5 h when the actual mass ratio of Ti O2/Ti was 3.2:0.5 or3.0:0.5.（2）The Ti4O7 powder was etched by methanesulfonic acid to obtain modified Ti4O7 carrying hydroxyl groups on the surface.Ti4O7 doped polypyrrole composite coatings（Ti4O7/PPy）were prepared on the surface of 316L stainless steel by constant current deposition in a classical electrochemical three-electrode system with 1 m A·cm-2 deposition current density.The addition of Ti4O7 reduces the interface contact resistance of the coating to 12.34 mΩ·cm2 and the surface of the coating exhibits good hydrophobicity（its water contact angle increases to115°）.The corrosion potential of the composite coating was increased by 304m VAg/Ag Cl compared to 316L stainless steel,and the lower Rf values and steadily increasing Rct values of the composite coating samples after 306 h of immersion in the corrosion solution indicate the long-term corrosion resistance of the composite coating.（3）Titanium metal was selected as the substrate and calcined at 700°C in air for 7 h to obtain a rutile Ti O2 coating on the surface of the substrate.The Ti O2coating was converted into Ti4O7 coating by using Ti/Ti O2 mixed powder as reducing agent.This method breaks through the limitations of the conventional preparation method of Ti4O7,and the whole experimental process is realized under the protective atmosphere of pure Ar,and the by-products are non-toxic,which is an efficient and non-polluting method for the preparation of Ti4O7 coatings.The Ti4O7 coating prepared by the Ti powder reduction method has a low surface contact resistance（7.6 mΩ·cm2）;the corrosion potential of the coating is increased by 614 m VAg/Ag Cl compared to the Ti substrate,and its corrosion current density decreases by three orders of magnitude relative to the substrate,which means that the Ti4O7 coating has a good protective effect on the Ti substrate;The Ti4O7 coating meets the performance requirements of bipolar plates and provides a new idea for the surface modification of Ti bipolar plates. |
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