Vertically Aligned Ni Nanowires as a Platform for Kinetically Limited Water-Splitting Electrocatalysis

Electrochemical templating through porous membranes is applied to form arrays of micrometer long Ni nanowires (NWs). Detailed structural and electrochemical characterization, including electrochemical impedance spectroscopy (EIS), was conducted to assess the electrocatalytic properties of these Ni NW arrays for the O2 evolution reaction (OER) in 1 M KOH. Detailed structural analysis showed that Ni NWs have a diameter of ca. 350 nm and a mean 80 nm average distance between the NW center. For the longest NWs (20 μm long), the ratio between the pore opening and pore length is 0.4%. From detailed HR-TEM and EELS analysis providing information on the chemical state of atoms from quantitative analysis of the signals, Ni NWs are composed of a Ni metallic core surrounded by a Ni(OH)2 layer that thickens from 10 to 20 nm after extensive electrochemical tests. Three different methods, namely, SEM geometry measurements, the α-Ni(OH)2 charge method, and the capacitance method, were used to assess how the current vari...