Electrochemical Reduction of CO2 on Copper Oxidized By Electrochemical Methods

Abstract

The electrochemical oxidation of copper surface in alkaline solution results in the formation of different products, which are a function of scan rate and concentration of OH- ions. It has been found that, the electro-oxidation results in oxide film on copper surface consist of Cu2O, Cu(OH)2 and CuO. The scanning electron microscopy (SEM) images of copper foils oxidized by linear sweep voltammetry at different scan rate show a systematic growth of oxides on the copper surface. At low scan rate growth of oxide takes place resulting in needle like structures projecting outwards from copper surface. Further, the density of these needles of oxides decreases as the scan rate increases. A similar behaviour was also observed in the chronoamperometry experiments performed at different potential and for different time duration. The length and the density of needles increase with increase in time when the applied potential was in potential region of peak B. On the other hand, at more negative potential and low concentration OH- ions instead of needles, a layer of oxide particles were observed. The x-ray photoelectron spectroscopy (XPS) analysis of oxidized copper surface shows the presence of Cu2O, Cu(OH)2and CuO in the oxide layer and the percentage of CuO increases as the density of needles increases on the surface. It has found that the catalytic activity of the copper obtained by reducing it from its oxides is more than that of the original polycrystalline copper foil for CO2reduction. The oxidation of copper surface can be achieved either by annealing in air or oxygen, or by electrochemical oxidation. Further, electrochemical oxidation of copper foil can be performed either by potential sweep or potential step techniques. Here, the oxidation of copper surface was carried out by chronoamperometry (CA), linear sweep voltammetry (LSV) as well as cyclic voltammetry (successive oxidation and reduction cycles) in Ar saturated 0.5 M KOH solution. The copper foil was oxidized by CA at -0.1 V for 30 minutes and LSV at scan rate of 0.5 mV/s from -1.2 to 0.5 V in Ar saturated 0.5 M KOH. The five cycles of CVs were carried out at a scan rate of 0.5 mV/s in Ar saturated 0.5 M KOH between -1.2 to 0.5 V. The SEM images shows that the needle like growth of oxides gets reduced back to copper in first few minutes of the electrochemical reaction when applied potential is more negative than hydrogen evolution. These in turn have been used for electrochemical reduction of CO2. The main product of CO2 reduction in aqueous phase was found to HCOO- and the gas phase analysis revealed predominantly hydrogen molecule. Faradaic efficiency for formic acid on copper foil oxidized by CV was higher than that for copper foil oxidized by the CA and LSV. The reason this observation may be the amount of active copper produced on the polycrystalline copper from the reduction of the copper oxides. This has been verified from the amount of charge transferred during the oxidation of copper which is highest for the CVs as with each cycle the peak current of both the peaks observed during oxidation of copper increases. During oxidation of copper foil by CV in fifth cycle a total charge of ~0.70 C/cm2 charge was transferred, whereas, during LSV a total of ~0.45 C/cm2 and during CA at total of ~0.30 C/cm2 of charge was transferred. Thus indicating more oxidation during CV than LSV and CA, and hence more amount of active copper will be produced on the copper surface resulting in increased faradaic efficiency of formic acid. Figure 1