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

Abstract

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 varies with the NW length. The three different methods are all in agreement, and the current increases with length (or mass) precisely and only because of the surface area effect. The most surprising result is that the OER process occurs with exactly the same intrinsic catalytic activity at the bottom of these deep pores, and gas is evolved without any significant effects of electrolyte resistance, mass transport of dissolved oxygen, or bubble occlusion of the pores. Accordingly, vertically aligned 1D NWs can be used as an effective platform that mitigates the negative effects of gas evolution. Increasing the intrinsic activity by incorporating more active materials will further improve this type of electrode.