The influence of phosphorus content on the microstructure and specific capacitance of etched electroless Ni-P coatings

Abstract

Acidic and alkaline electroless bath Ni-P coatings with varying P contents (4.7–8.1 wt%) were deposited on Ti substrate. These as-deposited Ni-P coatings had no apparent difference in specific capacitance. The deposition rate of coating increased with the increase in NaH2PO2·H2O content regardless of the bath type, while the alkaline bath yielded a lower phosphorus content. After the Ni-P coatings were etched by a HNO3 solution, dealloying first occurred in the boundaries between the nodules and then, in the inside of the nodules. Finally, the nodules were etched into numerous nanoparticles with a size of 25–35 nm. An increased specific surface area, due to porous structures, is the major contribution for high specific capacitance. The phosphorus content significantly affects the corrosion resistance of Ni-P coatings and, thus, plays an important role in deciding the microstructure and specific capacitance of etched Ni-P coatings. The specific capacitance of etched Ni-P coatings is a function of phosphorus content and etching time at a fixed etching condition, and an optimum value exists. The highest value achieved is dependent on phosphorus content. The mechanism of the etched Ni-P coatings used as the electrode of the supercapacitor also was discussed further.