Induced Codeposition of Mo and W from Aqueous Electrolytes

Mo and W alloys are recognized for their outstanding corrosion, wear resistance and catalytic properties towards the generation of hydrogen. The electrochemical codeposition behavior of Mo or W with iron-group elements (i.e., Ni, Co, Fe) exhibits induced codeposition, since they cannot be reduced alone, but only with iron group elements. Although Mo and W have been intensively investigated, their reaction mechanisms remain not well understood. In work presented here, Mo induced codeposition with Ni was explored when the concentration of nickel ions in the electrolyte was much lower than molybdate. Their composition and deposit thickness were characterized by X-ray fluorescence. At this limit, the nickel species is expected to limit the reaction rate of molybdate reduction. Mo-rich alloys were obtained, setting a new record, with > 80 wt. % Mo content. Mo-rich MoNi deposits' morphology was examined by scanning electron microscopy. The MoNi alloys had a nodular morphology and contained micro-cracks, attributed to the large hydrogen evolution side reaction rate. Catalytic properties were investigated and exhibited Tafel behavior consistent with their hallmark feature as a catalyst for electrolytic hydrogen evolution. MoWNi alloys were also electrodeposited from aqueous electrolytes. The nickel preferentially induced the deposition of molybdenum over that of tungsten. The tungsten partial current density, hence reaction rate, was inhibited when deposition was compared to a molybdenum free electrolyte.