Pushing the Limits of Rapid Anodic Growth of CuO/Cu(OH)<sub>2</sub>Nanoneedles on Cu for the Methanol Oxidation Reaction: Anodization pH Is the Game Changer
Sengeni Anantharaj, Hisashi Sugime, Shohei Yamaoka, Suguru Noda
Abstract
We recently reported the fastest anodization method (just 80 s) of all for accessing a denser array of Cu(OH)2–CuO nanoneedles on a Cu foil substrate by applying a constant potential of 0.864 V vs a reversible hydrogen electrode in 1.0 M KOH that delivered a better activity for the methanol oxidation reaction (MOR). In this study, we show that the strength of the KOH solution used for anodization alters the size, morphology, surface chemistry, electrochemical accessibility of Cu sites, and the subsequent MOR activity trend. Intriguingly, an increase in KOH solution strength shortens the time of anodization from 80 s (1.0 M KOH) to 20 s with 3.0 M KOH, which in turn drastically reduces to just 6 s with 6.0 M KOH. As of now, this is the shortest time ever achieved for the anodic growth of Cu–OH/O nanoneedles on a Cu substrate. A set of detailed and comparative physical and electrochemical characterizations reveal positive relationships between anodization pH and anodization current, the size of Cu–OH/O nanoneedles grown, rate of growth, electrochemical accessibility of Cu sites, and electrocatalytic MOR activity. Thus, this study provides a universal approach to control the size of Cu–OH/O nanoneedles, electrochemical accessibility of Cu sites, and their subsequent MOR activity.