Effects of Alkali Metal Ions on O 2 and H 2 O 2 Reduction Reactions at Pt Electrodes

In the field of electrochemistry, the oxygen reduction reaction (ORR) has been studied extensively for decades, largely because it is one of the most important electrochemical reactions in energy conversion systems such as fuel cells and metal-air batteries. The hydrogen peroxide reduction reaction (HPRR) has attracted attention recently because H 2 O 2 , which is a strong oxidizer, can be used as an oxidant in fuel cells, and also because H 2 O 2 is involved in the ORR. It is generally accepted that the ORR proceeds either via a complete four-electron pathway or via a two-electron pathway. In acidic media, the four-electron ORR produces H 2 O as the final product (reaction 1), whereas the two-electron ORR produces H 2 O 2 as an intermediate (reaction 2), which is further reduced to H 2 O (reaction 3). O 2 + 4H + + 4e - → 2 H 2 O (1) O 2 + 2H + + 2e - → H 2 O 2 (2) H 2 O 2 + 2H + + 2e - → 2H 2 O (3) Pt has a good electrocatalytic activity toward the ORR and HPRR, and hence the reactions have been widely studied using Pt and Pt-based electrodes, including effects of electrolyte components and additives on the reactions. It has been reported that alkali metal ions, e.g., Li + , Na + , and K + , suppress the ORR and HPRR at Pt electrodes in alkaline electrolytes [1, 2]. The hydrated alkali metal ions and chemisorbed OH species on Pt non-covalently stabilize each other, leading to the formation of quasi-specifically adsorbed hydrated metal ion clusters, OH ads -M + (H 2 O) x clusters, that blocks platinum active sites for electrochemical reactions. The suppression becomes stronger as the strength of the non-covalent interaction increases (K + < Na + < Li + ). Consequently, the rest potential decreases in the order (Figure 1a). On the other hand, inorganic salts which are composed of the alkali metal ions, e.g., Na 2 SO 4 and K 2 SO 4 , are often used as supporting electrolytes for electrochemical reactions. However, as we have reported before [3], the salts significantly affect the hydrogen evolution reaction (HER) in strongly acidic solutions. The rate of the HER decreased when the alkaline metal ions, i.e., Na + and K + , are present in the solutions because the transport rate of H + ions to the electrode surface decreased due to the ions. Thus, the HER current decreased as the concentration of the salts increased. We recently found that the HPRR at Pt electrodes in strongly acid solutions was suppressed by the salts. Although the rest potential was independ