Structure And Performance Of Nanocrystalline Transition Metal-carbides/nitrides Prepared By Disproportionation In Molten Salt

Proton exchange membrane fuel cell(PEMFC)has been considered as the one with the widest application prospect and the fastest development due to its advantages,such as fast start-up at room temperature,low operating temperature,high specific power and specific energy,and long service life.Metallic bipolar plate,as an important component of PEMFC stacks,suffers the problems of corrosion and passivation in PEMFC environments.An economic and effective way to solve this problem is to apply a high conductive and corrosion-resistant coating on metallic bipolar plate.Moreover,the poor stability of Pt/C catalyst is also one of the major barriers to the commercialization of PEMFC.Therefore,it is significant to develop a new method for preparation of high-quality(compact and defect-free)coating on metallic bipolar plate,as well as new preparation technology for high dispersive Pt based catalyst support so as to improve the corrosion resistance of metallic bipolar plate and the stability of catalyst support,increase the service life and specific power of the stack,reduce the cost of the stack,promote the commercialization of PEMFC and realize the national new energy development strategy.Transition metal nitrides and carbides have advantages of good conductivity and high chemical stability.This paper aims at the practical needs for corrosion-resistant and conductive coating of stainless bipolar plate and the high dispersive nanosized powder of Pt based catalyst support in PEMFC,the dense and defect-free nanocrystalline ?-Nb2N coating and TiC powders with different morphologies as well as NbC nano powder were prepared by disproportionation reaction in the liquid phase of high temperature molten salt.The effects of the ratio of active materials,temperature and time on the preparation of coating and powder were studied by means of phase analysis and microstructure characterization technique.In addition,preparation technology of the transition metal nitride coating and nanocrystalline transition metal carbide powder with controllable morphology were obtained.Moreover,the corrosion behavior and conductivity of the prepared ?-Nb2N coating in simulated PEMFCs environment were also investigated.The main results obtained are as follows:(1)A continuous,dense,uniform,and well-adhered nanocrystalline ?-Nb2N coating with average grain size of?30 nm was prepared on the surface of 430 FSS by disproportionation of Nb(?)ions in NaCl-KCl-NaF molten salt.Higher temperature is more beneficial for the ?-Nb2N coating deposition and the coating with a thickness of?600 nm is obtained after deposition at 850? for 5 h.The mechanism of the coating formation is described as follows.Firstly,Nb(V)ions from the decomposed NbCls react with niobium to form Nb(?)ions.Then,formed Nb(?)ions transport to the surface of the 430 FSS substrate and decompose to Nb(V)ions and Nb(0)atoms by disproportionation.Meanwhile,the obtained Nb(0)atoms in-situ react with[N],which come from the 430 FSS substrate,to form ?-Nb2N coating.(2)The corrosion potentials of 430 FSS substrate are improved and corrosion current densities are reduced both in the simulated PEMFCs cathode and anode environments by ?-Nb2N coating.In addition,the corrosion current densities of?-Nb2N coated 430 FSS in both cathode and anode environments are always maintained at a lower current density during 500 h potentiostatic polarization.Furthermore,the ICRs of ?-Nb2N coated 430 FSS before and after potentiostatic polarization for 500 h in both simulated cathode and anode environments are lower than 10 m?-cm2,which meets the requirement of DOE 2020 for bipolar plate.The good corrosion resistance and conductivity of 430 FSS/?-Nb2N coating is mainly attributed to the characteristics of dense and defect-free microstructure prepared by disproportionation reaction in molten salt,which effectively prevents the penetration of corrosive media into the coating/substrate interface.Therefore,the 430 FSS substrate can be sufficiently protected.(3)The zero-,one-,and two-dimensional nanocrystalline TiC powders are in-situ synthesized by using the disproportionation of Ti(?),which was dissolved from titanium powder,on surface of acetylene black,MWCNTs and graphene,respectively,in NaCl-KCl molten salt.Higher temperature is more beneficial for TiC formation.The TiC powders with high hydrophilicity and high dispersibility and with different morphologies corresponding to the carbon sources are in-situ synthesized by using equimolar ratio of Ti powder and acetylene black,MWCNTs and graphene as starting materials in NaCl-KCl molten salt at 800?900? for 2?3 h.The mechanism TiC formation is described as follows.Firstly,The dissolved Ti(?)ions transport to surface of carbon sources and disproportionate to Ti(0)and Ti(?)ions.Finally,nanocrystalline TiC powders,TiC nanorod and TiC nanosheet are in-situ synthesized by the reaction between Ti(0)and C atoms on surface of carbon sources.(4)Nanosized NbC powder was successfully synthesized by the disproportionation of Nb(?)ions on surface of acetylene black template in molten salt.Effects of NbCl5/Nb molar ratio,reaction temperature and time on the synthesis of NbC were also investigated.Results show that homogenous NbC powder with high hydrophilicity and high dispersibility and particle size of?50 nm is obtained in NaCl-KCl molten salts at 800?900? holding for 0.5 h?5 h by using NbCl5/8Nb/8C raw materials.A higher temperature and higher molar ratio of NbCl5/Nb are more preferable for nanosized NbC powder formation.The mechanism of NbC formation is summarized as follows.Firstly,the Nb powder reacts with Nb(V)ions decomposed by NbCl5,to produce Nb(?)ions.Then,Nb(?)ions diffuse and disproportionate to Nb(0)and Nb(?)ions on the surface of acetylene black.Finally,the NbC nanocrystalline powder is in-situ synthesized via Nb(?)ions and C atoms on the surface of acetylene black.