Purity of the Electrolyte Solution

It is well known that trace amount of impurities will interfere with surface process in electrochemical reactions on the electrodes as well as any heterogeneous interfacial reactions. The electrochemical reduction of CO2 is also a surface process, and is naturally sensitive to the cleanliness of the electrode. Any surface contamination will lead to variation of electrocatalytic property of the electrode. The product selectivity of metal electrode is often severely affected by the presence of extremely small amount of adatoms on the surface. The surface contamination is sometimes a source of many controversial experimental results. 

Many workers studied HER on various metal electrodes since Tafel s work. They attempted to obtain the correlation between the hydrogen overvoltage and the properties of the electrode material. During the course of these works, widely scattered voltage current relations were published by different workers. Kuhn et al. reviewed the previous works, and decided to lay down criteria to select the experimental data for their discussions. They chose the data by only those workers who had used preelectrolysis for the purification of the electrolyte solution. Bockris described the importance of preelectrolysis of the electrolyte solutions in the study 

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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. ... 

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