Effects of Alkali Metal Ions on O2 and H2O2 Reduction Reactions at Pt Electrodes

Abstract

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 H2O2, which is a strong oxidizer, can be used as an oxidant in fuel cells, and also because H2O2 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 H2O as the final product (reaction 1), whereas the two-electron ORR produces H2O2 as an intermediate (reaction 2), which is further reduced to H2O (reaction 3). O2 + 4H+ + 4e- → 2 H2O (1) O2 + 2H+ + 2e- → H2O2 (2) H2O2 + 2H+ + 2e- → 2H2O (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, OHads-M+(H2O) 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., Na2SO4 and K2SO4, 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 independent on the kind of alkali metal ions, the HPRR was suppressed due to the ions, as shown in Figure 1b. The suppression became stronger in the following order: Li+ < Na+ < K+, which is opposite to the order in the suppression due to the non-covalent interaction observed for alkaline media (see Figure 1a). In the presence of the alkaline metal ions, the decrease in the transport rate of H+ ions also caused an increase in the local pH at the electrode surface during the HPRR. When the concentration of H2O2 was higher than that of H+ ions, the local pH became strongly basic during the HPRR [4]. Note that the local pH remained acidic in the case of Figure 1b because of the lower concentration of H2O2. There arises the question: when the concentration of H2O2 is higher than that of H+ ions, does the local pH increase cause the non-covalent interaction? To answer the question, the effects of alkaline metal ions on the HPRR in strongly acid solutions are studied in this work. Furthermore, the ORR in the presence of the ions is also studied. REFERENCES [1] D. Strmcnik, K. Kodama, D. van der Vliet, J. Greeley, V. R. Stamenkovic and N. M. Marković, Nature Chemistry, 1, 466 (2009). [2] I. Katsounaros and K. J. J. Mayrhofer, Chem. Commun., 48, 6660-6662 (2012). [3] Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi and H. Okamoto, J. Electroanal. Chem., 713, 39 (2014). [4] Y. Mukouyama, H. Kawasaki, D. Hara, Y. Yamada, S. Nakanishi, J. Electrochem. Soc., 164 (2017) H675. FIGURE CAPTION Figure 1. The current (I) – potential (E) curves for a Pt-plate electrode in basic and acidic solutions measured under potential controlled conditions at a scan rate of 0.01 Vs-1. The basic solutions are 0.05 M alkali hydroxide (denoted as MOH) + 0.01 M H2O2, and the acidic ones are 0.05 M H2SO4 + 0.01 M H2O2 with or without 0.05 M alkali sulfate (denoted as M2SO4). Figure 1