Study On The Preparation And Performances Of Al Alloy Bipolar Plate

The research and application of proton exchange membrane fuel cell (PEMFC) was summarized in detail, as well as the chief restriction of the PEMFC's application . Al alloy was first adopted as basic material of bipolar plate , of which the flow field was prepared by computer carving technics and surface-modified by electroless Ni-P plating ,and Al alloy bipolar plate with good performance has been obtained.1. Flow field based on Al alloy was designed according to the simplest structure of snake shape , which is prepared by computer carving technics and both the width and depth are 1 mm .The computer carving flow field could easily be controlled and the precision is high.2. Immersing zinc is one of the key processes of electroless Ni-P alloy plating . The best solution prescription and parameters of immersing zinc have been determined according to full factorial test: ZnSO₄·7H₂O 30-50 g/L ,NaOH 80-100 g/L , NaF 80-120 g/L , FeCl₃ 0.5-1 g/L, complex reagent 8-20 g/L, 20-40℃, 1-3 min for the first time and 30-40 s for the second . The replaced zinc film is about 50 nm and it is rather uniform and compact.3. Alkaline-acid electroless Ni-P plating technics was adopted and alkaline pre-plating could not only avoid immersing zinc film from over-dissolving ,but also prevent the Zn²⁺ and Al³⁺ dissolved polluting the acid electroless plating solution .Taken deposition rate of the acid plating solution and corrosion resistance of the coating as evaluating standard , the best solution prescription and parameters of electroless plating have been determined according to single factorial test:1) Alkaline pre-plating : NiSO₄·6H₂O 20-30g/L, NaH₂PO₂·H₂O 15～25 g/L ,complex reagent 35-50 g/L , pH9.0～10.0 , 60—70 ℃ and 5 min, then the coating thickness is about 0.5 μm ; 2) Acid plating : NiSO₄·6H₂O 22-38 g/L, NaH₂PO₂·H₂O 28—36 g/L ,HAc 18—35 ml/L, complex reagent 25—35 g/L, rare earths salt 0.5—2 g/L, pH 4.4— 5.0 , 85—92 ℃ .The coating thickness is 30-50 μm and the time is determined by coating thickness .4. The contents of the nickel and phosphorus in the coating was determined by electron probe method , as well as the effect of the temperature and the time of heat treatment on the crystallization structure by X diffraction method and corrosion resistance by volt-ampere curve method .The bonding strength , microhardness , surface appearance and electrical performance of bipolar plate were also detected, which show that:1) The coating in plating form is of obvious non-crystal structure , of which the corrosion resistance is rather high , the bonding strength is about 115 MPa ,and the microhardness is about 520HV .2) The coating starts to crystallize when heat treatment temperature rises to 200 ℃ , and takes on obvious crystal structure as the temperature reaches 600 ℃ , with the crystallized product Ni and Ni₃P . The coating is not obviously crystallized ,but the corrosion resistance and bonding strength could be improved , and the microhardness could reach as high as 1000HV when heated at the temperature 200～250 ℃ for 4～6 h . 3) Resistivity of the coating attained from this technics is rather low ,and the conductivity of the Al alloy bipolar plate prepared with this coating is 1.65 ～1.79×10⁷ S·m^-1,which is higher than the bipolar plate of stainless steel (1.25～1.428×l0⁶ S·m^-1) , pure carbon (6.67～9.09×10⁵ S·m^-1) or graphite composite material (9.1×10⁴ S·m^-1) .Al alloy bipolar plate prepared by this technics is of lower weight, higher conductivity and corrosion resistance ,and easily prepared ,which could satisfy the technic requirement of PEMFC. It could supply cheaper and higher performance bipolar plate for PEMFC by improving its preparation method and parameters.