Electrochemical behaviors and applications of copper-based nanostructures

Abstract

Copper based nanomaterials (Cu, Cu2O, CuO et al) have been broadly used in electrochemistry related fields due to their favorable performance and the cost effective features. In such practical applications, the electrochemical degradation of Cu-based nanomaterials is much more serious than their counterparts owing to their large surface-to-volume ratio and plentiful surface defects, especially when they are subject to the harsh solutions and high applied potentials. As a consequence, it is of vital significance to study the electrochemical behaviors of Cu-based nanomaterials and use them as references to improve their performance in electrochemical applications.

First, large aspect ratio single crystalline copper nanowires (CuNWs) have been electrochemically deposited by using anodic aluminum oxide (AAO) template. Then, the surface passivation behaviors of the as-obtained CuNWs in both 40 % relative humidity (RH) atmosphere and 0.1 M NaOH aqueous solution have been studied by transmission electron microscope (TEM) observations. In 40 % RH atmosphere, a uniform compact Cu2O passivation layer epitaxially covers the CuNWs surface following a logarithmic law relationship between Cu2O thickness and exposure time, while showing faster growth rate than that on the counterpart bulky Cu surface under the same conditions. The electrochemical passivation behaviors of CuNWs in 0.1 M NaOH solution were investigated by cyclic voltammetry measurements. Initially, a homogeneously compact Cu2O layer is observed to epitaxially coat the CuNWs substrate under solid-state reaction (SSR) mechanism at the first oxidation peak. Subsequently, the previously formed epitaxial compact Cu2O layer will be partially oxidized into a compact CuO inner layer with parallel alignment and an adjacent disordered CuO/Cu(OH)2 outer layer with random orientations under the dissoloution-redeposition (DR) mechanism at the second oxidation peak. When further increasing the applied potential prior to the oxygen evolution, the parallelly orientated Cu2O/CuO inner layer will be completely oxidized into disordered CuO and Cu(OH)2. Finally, a dual-layer structure (CuO inner layer and Cu(OH)2 outer layer) with random orientation forms on CuNW substrate through an enhanced DR mechanism.

Subsequently, hierarchical copper nano-dendrites (CuNDs) are fabricated via electrodeposition method since they can reflect the influence of micro-structure of copper to their electrochemical properties besides the intrinsic ones. It is found that the CuNDs experience a non-equilibrium oxidation process when subject to the cyclic voltammetry (CV) measurement. As expected, the first oxidation peak corresponds to the formation of an epitaxial Cu2O layer over the surface of hierarchical CuNDs. Unusually, a broad oxidation peak including a plateau appears at higher potentials in the CV profile. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations demonstrate that the nucleation and growth of Cu(OH)2 nanoneedles at the localized regions originate and dominate the initial part of the second oxidation peak. Subsequently, further oxidation of the previously formed Cu2O layer take place until it is completely transformed into CuO and Cu(OH)2 to form a dual-layer structure at higher anodic potentials. Meanwhile, a larger amount of Cu(OH)2 nanoneedles sprout and grow in the CuNDs as well.

In order to improve the OER electrocatalytic activity of the CuNDs electrode, a strategy that transforming a metal organic frameworks (MOFs) thin layer into a nanostructured CuO/C hollow shell to coat on the 3D CuNDs was successfully developed. This electrode is claimed to provide an extraordinary electrocatalysis for oxygen evolution reaction (OER) in alkaline media. The hierarchical complex presents fast electronic transmission networks and rich redox sites, leading to the significant enhancement in electrocatalytic OER efficiency. Furthermore, the spherical porous structure and robust architecture facilitate the high-speed diffusion of O2 bubbles in a long-term operation. The results of this study may serve as a reference for the designing of novel class 3D metal/metal oxide hierarchical structures for gas-involved (i.e. O2, H2, CO2 et al) electrocatalytic applications and beyond.