Effects of Alkali Metal Ions on H2O2 Reduction Current at Pt Electrodes
Authors: Hara Daisuke , Kawasaki Hirokazu , Kikuchi Mitsunobu , Yamada Yuri , Nakanishi Shuji , Mukouyama Yoshiharu 
Published: February 2020

The hydrogen peroxide reduction reaction (HPRR) has attracted attention because it is involved in the oxygen reduction reaction, which is an important cathode reaction in several fuel cell systems. Thus, it has been widely studied, including effects of electrolyte components and additives on the HPRR at Pt electrodes. Katsounaros and Mayrhofer have reported the influence of the adsorption of (bi)sulfate and halide ions to the HPRR in acidic electrolytes [1]. The strength of the inhibition of the HPRR increases as the degree of the adsorption increases. They also reported [2] that alkali metal cations, e.g., Li+, Na+, and K+, suppress the HPRR in basic electrolytes. The hydrated alkali metal cations and chemisorbed OH species on Pt, denoted as M+(H2O)
x
and OHads respectively, non-covalently stabilize each other, leading to the formation of quasi-specifically adsorbed hydrated metal ion clusters, OHads-M+(H2O)
x

clusters, that blocks electrochemical reactions. The suppression becomes large as the strength of the non-covalent interaction increases (K+ < Na+ < Li+), and as a consequence the rest potential decreases in the following order: K+ < Na+ < Li+, as reproduced in Figure 1a.
On the other hand, inorganic salts which are composed of the alkali metal cations, e.g., Na2SO4 and K2SO4, are often used as supporting electrolytes for electrochemical reactions. However, as we have reported before [3], the salts generally affect the hydrogen evolution reaction (HER) on various kinds of metals, namely, Pt, Au, Rh, Ag, Cu, Fe, Ni, W, Zn, Sn, and In, in strong acidic solutions. The rate of the HER decreased in the presence of the cations because the transport rate of H+ ions to the electrode surface due to electromigration decreased in the presence of the cations. Thus, the reduction current due to the HER decreased as the concentration of the salts increased.
We have reported [4] that the HPRR in acidic electrolytes is also affected when the inorganic salts, Na2SO4 and K2SO4, are added to the electrolytes. The local pH at the electrode surface became basic during the HPRR because of the decrease in the transport rate of H+ in the presence of the alkali metal cations. Furthermore, we have tentatively suggested [5] that the HPRR in acidic electrolytes is affected by the non-covalent interaction between the cations and OHads. Then, in order to verify this, this work focuses on studying how the cations affect the HPRR. Although the rest potential is independent on the kind of alkali metal cations, the HPRR is suppressed due to the presence of the cations. The H2O2 reduction current decreases (in absolute value) in the following order: Li+ < Na+ < K+, as shown in Fig. 1b. This order is in the reverse order of the strength of the non-covalent interaction observed for basic electrolytes. Thus, the suppression cannot be explained on the basis of the non-covalent interaction but rather on the basis of the decrease in the transport rate of H+, which will be discussed in this work.

REFERENCES

[1] I. Katsounaros, W. B. Schneider, J. C. Meier, U. Benedikt, P. U. Biedermann, A. Cuesta, A. A. Auer and K. J. Mayrhofer, Physical Chemistry Chemical Physics, 15, 8058 (2013).
[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, M. Kikuchi, Y. Yamada, S. Nakanishi, ECS Trans., 69 (39), 37-45 (2015).
[5] Y. Mukouyama, D. Hara, H. Kawasaki, M. Kikuchi, Y. Yamada, S. Nakanishi, ECS Trans., 69 (39), 47-57 (2015).

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