Research On Stainless Steel Bipolar Plate Surface Modification For Proton Exchange Membrane Fuel Cell By Niobium-deposition

The bipolar plate is one of the critical components of a proton-exchange-membrane fuel cell (PEMFC). In this study, niobium electrodeposited on stainless steel from air-and water-stable ionic liquids and niobium implanting on stainless steel bipolar plate by metal plasma immersion ion implantation (MePBII) have been researched. The niobium film morphology, electrochemical properties and pitting dynamics course are investigated in the simulated PEMFC environment.Firstly, an air-and water-stable ionic liquid,1-ethyl-3-methylimidazolium trifluoromethane sulfonate ([Emim]OTF), has been applied as electrolytes for niobium electrodeposition for the first time. The electrochemical behavior of bare and niobium coated304stainless steel are evaluated by electrochemical tests in PEMFC environment. The results show that niobium can be electrodeposited on the surface of stainless steel in the ionic liquid electrolyte. The niobium plating roughness is within100nm and a thin niobium film acted as barrier can remarkably improve the corrosion resistance of304stainless steel in the PEMFC environment. The analysis of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic force microscopy (AFM) indicate that the improvement is attributed to the smoothness and strong chemical stability of NbO, Nb2O5film.Subsequently, another water and air-stable choline chloride based ionic liquid, choline chloride-ethylene glycol, has been explored as an electrolyte for niobium electrodeposition on316stainless steel for the first time. A uniform-dense(isolated island-like) niobium film is obtained on316stainless steel surface. The plating roughness is within50nm. The coated niobium form chemically inert compounds of NbO and Nb2O5which behave as an anti-corrosion barrier and improve the passavition ability of surface layer. The results of the electrochemical characteristic indicate that in simulated PEMFC environment, the electrochemical characteristics of316stainless steel are improved greatly after coated with niobium such as corrosion potential increasing, passivation current density decreasing, pitting potential moving toward positive, and reaction resistance enlarging. ionic liquids electrolytes has obvious advantages such as higher character stability, non-pollution, reusable, less side reactions, and wide usage prospects.Finally, niobium implanting on316stainless steel by metal plasma immersion ion implantation (MePBII) has been investigated. The plating roughness of niobium deposited films is within50nm. The electrochemical results reveal that, compared with uncoated substrates, corrosion potential and reaction resistance increases, passivation current density decreases, and pitting potential moves toward positive. The corrosion resistance of the Nb implanted316stainless steel has been improved significantly. The Nb coating was an effective barrier with high resistance and low capacitance which against the inward penetration of corrosive species. This can be attributed to niobium as effectively resistant to pitting corrosion and has more stable chemical properties. the ion implantation niobium316stainless steel corrosion kinetics equations isi0=-556.85.t12and i0=1108.17.t12respectively. It shows that pitting process is controlled by film dissolution.