Electrocatalytic Activity of Metal Electrodes

Deactivation of metal electrodes in CO2 reduction has been reported by many workers, and regarded as an imavoidable defect irrespective of the potential importance of CO2 reduction. For example, the deactivation was described that the formation of CH4 and C2H4 at Cu electrode decays rapidly and hydrogen evolution prevails in 10 to 30 minutes after the start of the CO2 reduction. However, the features of the deactivation have been various, depending on the research groups. No agreement is found for the reason of the deactivation as well. Bard et al. and Vielstich et al. reported that the surface of the deactivated copper electrode is blackened after electrolysis. Bard et al. analyzed the surface by X-ray photoelectron spectroscopy (XPS), and reported that the 

Nogami et al. and Augustynski et al. showed that the activity of copper electrode, deactivated during the CO2 reduction, is recovered by anodic polarization of the electrode. Periodic anodic pulses are effective to maintain the electrocatalytic activity in prolonged electrolysis of CO2 reduction. 

Hori et al. pointed out that the deactivation takes place due to the presence of heavy metal impurities originally contained in chemical reagents used as the electrolytes. Heavy metal ions in the electrolyte solution are cathodically reduced and deposited on the electrode surface during the CO2 reduction, deteriorating the electrocatalytic properties of metal electrodes. They apphed a classically established technique of preelectrolysis to purification of electrolyte solutions since their early works. Frese also referred to the impurity heavy metals, and mentioned the presence of Fe and Zn on the Cu electrode after electrolysis on the basis of the surface analysis by XPS. The importance of the purity of the electrolyte solution was mentioned in Section I1.2(zz) as well. The mechanism of the deactivation was recently established, and sununarized below.  

The concentration of heavy metals in the electrolyte solutions can be estimated on the basis of tlie electric charge of the anode peaks in the anode stripping voltammogram. The concentration originally contained in the chemical reagents was evaluated below 0.2 ppm both as Fe and Zn, mostly 0.1 ppm or less, far below the standard of the impurity levels guaranteed by the manufacturers. Thus the deactivation of Cu electrode in CO2 reduction is not caused by adsorption of the products or the intermediates produced in CO2 reduction. 

The usefulness of the pre-electrolysis was stressed by Hori et al. They showed that the concentration of Fe in KHCO3 solutions is remarkably decreased by die pre-electrolysis, as evidenced by Electrothermal Atomic Absorption Spectroscopy (ETAAS). Some papers report that deactivation still takes place even with solution treated with pre-electrolysis. Nevertheless, any analytical data has not been presented with regard to the electrolyte solutions. If any preelectrolysis is conducted in a cell, and the solution is transferred to another electrolysis cell, the solution is possibly contaminated during the transfer process. 

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