Effects of coating layers on electrochemical properties of Nd-Mg-Ni-based alloys

In order to make Nd-Mg-Ni-based alloys more competitive as a candidate of negative electrode materials for nickel/metal hydride batteries, surface coating Ni and Co was performed on Nd0.7Mg0.3Ni3 surface via an electroless plating method. N2H4 center dot H2SO4 in plating solution cleans the alloy surface and makes the coating layer adhere evenly and tightly to the alloy surface. Scanning electron microscopy and transmission electron microscopy images show that honeycomb-like Ni and flake-like Co deposited at the Ni-coated and the Co-coated alloy surface, respectively. The Ni-coating tends to form a denser and more stable surface layer than the Co-coating, and consequently Ni-coating brings more remarkable improvements in electrochemical properties. At a discharge current density of 1800 mA/g, the high rate dischargeability of the electrodes increases from 55.3% (untreated) to 75.7% (Ni-coated) and 73.6% (Co-coated), respectively. As well as, the initial discharge capacity is improved and the discharge capacity degradation is suppressed by surface coating. The improvement of electrochemical properties is ascribed to the higher catalytic activity and the protective function of the coating layers. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.