Electrochemical characterization of electroplated Ni-Mo and Ni-Mo-P alloy coated stainless steel bipolar plates for PEMFC

Stainless steel bipolar plates (BPPs) are the preferred choice for proton exchange membrane fuel cells (PEMFCs); however, a surface coating is needed to minimize contact resistance and corrosion. In this research, Ni–Mo and Ni–Mo–P coatings were electroplated on stainless steel BPPs and investigated by SEM/EDS, AFM and contact angle measurements. The Ni and Mo alloy elements were selected because they are known to have high corrosion resistance [2]. The performance of the BPPs was studied by corrosion and conduction tests and by measuring their interfacial contact resistances (ICRs) in-situ in a PEMFC set-up. The durability of BPPs was investigated in a PEM single cell, using a commercial Pt/C Nafion membrane electrode assembly (MEA). The effect of the BPP coatings on the electrochemical performance up to 115 h was evaluated from polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy. The results revealed that the applied coatings significantly reduce the ICR and corrosion rate of stainless steel BPP. The Ni–Mo coating showed the lowest and most stable ICR and the smallest effects on MEA performance, including catalyst activity/usability, cathode double layer capacitance, and membrane and ionomer resistance build up with time. After electrochemical evaluation, the BPPs as well as the water effluents from the cell were examined by Scanning Electron Microscopy, Energy Dispersive and Inductively Coupled Plasma spectroscopies. No significant degradation of the coated surface or enhancement in metal release was observed. However, phosphorus addition to the coating did not show to improve its properties, as deterioration of the MEA and consequently fuel cell performance losses was observed.