Effects of surface modification on surface structure and electrochemical properties of Mm(Ni,Co,Mn,Al)5.0 alloy powder

The electrochemical properties of MmNi5-type metal hydride powders produced by gas atomization were improved by acid surface modification. FIB specimen preparation followed by FE-TEM observation was used to analyze the surface structure before/after surface modification. The original oxide layer of the surface was removed and a nickel rich layer was formed by acid treatment. The structure of the nickel rich surface differed with the treatment conditions. For acetic acid treated powder prepared in heated solution, a nanocrystalline nickel layer was formed at the surface. In the absence of any auxiliary conductivity additives, discharge capacity of this powder was improved to 254 mAh/g versus 52 mAh/g for non-treated powder and 62 mAh/g for hydrochloric acid treated powder