Galvanic Replacement of Electrochemically Restructured Copper Electrodes with Gold and Its Electrocatalytic Activity for Nitrate Ion Reduction

Abstract

The electrochemical formation of nanostructured materials is a cost effective route to creating substrates that can be employed in a variety of applications. In this work the surface of a copper electrode was electrochemically restructured in an alkaline solution containing ethanol as an additive to modify the surface morphology, and generate a Cu/Cu₂O surface, which is known to be active for the electrocatalytic reduction of environmentally harmful nitrate ions. To increase the activity of the nanostructured surface it was decorated with gold prisms through a facile galvanic replacement approach to create an active Cu/Cu₂O/Au layer. The surface was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, as well as electrochemical techniques. It was found that the presence of recalcitrant oxides, and Au was beneficial for the increased activity compared to unmodified copper and undecorated restructured copper and was consistent with the incipient hydrous oxide adatom mediator model of electrocatalysis. This approach to generating nanostructured metal/metal oxide surfaces that can be galvanically replaced to create these types of composites may have other applications in the area of electrocatalysis.

Keywords: active sites; electrocatalysis; galvanic replacement; hydrous oxides; nanostructures; nitrate reduction.

Figure 1

Repetitive cyclic voltammograms (20 cycles) recorded at a Cu foil electrode in 1 M NaOH containing 1 M ethanol at 10 mV s⁻¹.

Figure 2

Scanning electron microscopy (SEM) images at (a) low, and (b) higher magnification, for a Cu foil electrode restructured in 1 M NaOH containing 1 M ethanol at 10 mV s⁻¹ for 20 cycles over the potential range of −1.5 to 0.5 V.

Figure 3

X-ray photoelectron spectroscopy (XPS) spectra for (a) Cu 2p_3/2 and (b) O 1s for a Cu foil electrode restructured in 1 M NaOH containing 1 M ethanol at 10 mV s⁻¹ for 20 cycles over the potential range of −1.5 to 0.50 V.

Figure 4

SEM images for a restructured Cu foil (as in Figure 2) galvanically replaced with gold via immersion in an aqueous solution of HAuCl₄ for (a) 1 min, (b) 3 min and (c) 5 min.

Figure 5

XPS spectra for (a) Cu 2p_3/2, (b) O 1s and (c) Au 4f for a restructured Cu foil electrode galvanically replaced with gold via immersion in HAuCl₄ for 3 min (i.e., for the sample shown in Figure 4b).

Figure 6

Linear sweep voltammograms recorded at 100 mV s⁻¹ in 1 M NaOH in the presence of 0.1 M KNO₃ showing (a) the effect of electrochemical restructuring, (b) the effect of galvanic replacement of restructured Cu with gold and (c) the electrochemical behavior of these electrodes in the absence of KNO₃.

Figure 7

(a) Differential pulse voltammetry recorded for restructured Cu galvanically replaced by Au for 5 min in 1 M NaOH containing 0 (dashed line), 1, 5, 10, 25, 50 and 100 mM KNO₃, and (b) a plot of peak height versus concentration for peak 1 and peak 2 identified in (a).