On the Effects of Halides on the Hydrogen Peroxide Reduction Reaction in Aqueous Electrolytes

The influence of adsorbed species on the kinetics and mechanism of a number of redox reactions has been recognized for decades¹. In particular, the adsorption of both chloride and bromide on Pt electrodes in acidic electrolytes, is known to promote the two-electron reduction of oxygen to peroxide instead of the desired four-electron reduction to water ^2,3. This contribution seeks to gain insights into the reduction of H₂O₂ on a polycrystalline Pt electrode in 0.1 M HClO₄ solutions containing a few ppm of either Cl^- or Br^-. The strategy employed makes use of a chronoamperometric method recently developed in our laboratories to determine independently, the effects of potential, E, and coverage of the adsorbate, q, on the peroxide reduction reaction on a Pt rotating disk electrode, RDE

EXPERIMENTAL

All measurements were performed in an all-glass, three-compartment cell in a 1 mM H₂O₂ in 0.1 M HClO₄ solution either neat or containing 10 µM NaCl or 10 µM KBr.  Experiments were performed with a rotating Pt ring-Pt disk electrode, Pt|Pt RRDE, (Pine Instruments, (A = 0.164 cm²) using a bipotentiostat (Pine Instruments). A Pt foil and a reversible hydrogenelectrode in the same solution (RHE) were used as counter and reference electrodes, respectively.   

RESULTS AND DISCUSSION

Shown in Fig.1 are polarization curves recorded at a scan rate,  ν = 10 mV/s  for a Pt disk electrode  in 0.1 M HClO₄ containing 1 mM H₂O₂in the absence (black curve) and in the presence of either 10 mM NaCl or 10 μM KBr (red curve). In contrast to the rather subtle effects of chloride, the presence of bromide gives rise to a substantial, ca. 0.3 V, shift in the onset for peroxide reduction.

 Chronoamperometric curves, i.e. disk current, i_disk vs time, obtained in upon stepping the potential from a value E_o, at which its coverage q is negligible 0, to a value E_ads at which q attains a finite value are displayed in Fig. 2. The results of these experiments make it possible to correlate the disk current, i, as a function of E_ads at fixed q, or as a function of q for a fixed E_adsas illustrated in Panels A and B in Fig. 3, respectively. Implications of these results on the effects of these impurities on the oxygen reduction reaction (ORR) will be discussed.  

ACKNOWLEDGEMENTS

This work was supported by a grant from NSF (CHE-0911621).

REFERENCES

1. Marković, N. M.; Ross Jr, P. N., Surf. Sci. Reports 2002, 45, 117-229.
2. Stamenkovic, V.; M. Markovic, N.; Ross Jr, P. N., J. Electroanal. Chem. 2001, 500, 44-51.
3. Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J., J. Electroanal. Chem. 2001, 508, 41-47.