Pushing the Limits of Rapid Anodic Growth of CuO/Cu(OH)₂Nanoneedles on Cu for the Methanol Oxidation Reaction: Anodization pH Is the Game Changer

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.

The Significance of Properly Reporting Turnover Frequency in Electrocatalysis Research

“…This is the easiest approach and an efficient way of finding out the number of active catalytic sites.T he major limitation is that the redox-peak integration method is applicable to only monometallic catalysts that possess aredox couple within the potential window of the electrolytic process catalyzed on its surface such as Ni, Co,C u, Fe,M n, Pt, Ir, and Ru. [58,60,61,72] Electrocatalytic water oxidation is ar eaction that involves bond-forming and -breaking intermediate steps,requiring the catalytic sites to undergo ac omplete cycle of oxidation and reduction. Hence,i ti ss afe to assume that the number of metal sites undergoing oxidation prior to water oxidation is the exact number of sites participating in the reaction.…”

The Significance of Properly Reporting Turnover Frequency in Electrocatalysis Research

“…Several electrochemical methods are used to determine the exact number of active sites, or the real surface area, of which HUPD, [52] Cu underpotential deposition (CuUPD), [53] Pb underpotential deposition (PbUPD), [54] CO striping, [46,[55][56][57] and redox peak integration [58][59][60][61] are the most noteworthy. Unfortunately, these methods are very materialspecific and cannot be applied universally for all electrocatalysts.…”

“…The major limitation is that the redox-peak integration method is applicable to only monometallic catalysts that possess a redox couple within the potential window of the electrolytic process catalyzed on its surface such as Ni, Co, Cu, Fe, Mn, Pt, Ir, and Ru. [58,60,61,72] Electrocatalytic water oxidation is a reaction that involves bond-forming and -breaking intermediate steps, requiring the catalytic sites to undergo a complete cycle of oxidation and reduction. Hence, it is safe to assume that the number of metal sites undergoing oxidation prior to water oxidation is the exact number of sites participating in the reaction.…”

Fabrication, Characterization and Photocatalytic Activity of Copper Oxide Nanowires Formed by Anodization of Copper Foams

“…When compared to them, anodic copper oxides seem to be unique—these have non-fixed stoichiometry (made simultaneously of cuprous oxide, cupric oxide, and cupric hydroxide), consist of nanowires or nanorods, and are crystalline: cuprous oxide crystallizes as regular cuprite, while cupric oxide crystallizes as a monoclinic tenorite [ 9 , 10 , 11 , 12 , 13 ]. Anantharaj et al recently presented a potentiostatic method of anodization of copper foil for a shorter amount of time in a three-electrode system to form a tall (5–7 µm) nanoneedle array of copper oxide [ 14 , 15 ]. The proposed method showed a better time and energy efficiency and concluded that the anodization time is inversely proportional to the pH of the electrolyte in which anodization is being carried out.…”