Determination of the Intrinsic Rates of Heterogeneous Hydrogen Peroxide Decomposition of Selected Materials in Acidic Electrolytes Using a Selenium-Modified Pt Ring of a Rotating Ring-Disk Electrode

Abstract

A rotating ring-disk electrode, RRDE, method is herein described that employs a Se-modified Pt ring electrode to measure the rates of oxygen generation, $r_{O_2,disk}$ derived from the heterogeneous hydrogen peroxide decomposition, het-HPD, on the surface of selected disk materials under open circuit, OC, conditions. This technique was inspired by the earlier work of Shigehara and Anson (J. Phys. Chem. 1982, 86, 2776–2783), who used a high area carbon paste ring electrode modified by an adsorbed layer of a rather unique Co macrocycle to accomplish the same goal. As first discovered by Mo et al. in our laboratory (J. Electrochem. Soc. 2003, 150, E39-E46), a Pt surface bearing 0.83 ML of elemental Se, which we denote herein as Se(0.83)|Pt, catalyzes the oxygen reduction reaction, ORR, in 0.5 M H₂SO₄, yielding Levich plots, i_lim vs ω^1/2, where i_lim is the diffusion limiting current, and ω is the rotation rate of the electrode, consistent with a strict 2e^– process. Such plots, however, displayed a nonzero intercept, signaling complications in the ORR mechanism. As will be shown in this work, this behavior could be accounted for by introducing a first order het-HPD step to the otherwise strict 2e^– ORR. Application of this formalism to data collected for a Se(0.83)|Pt disk polarized at a potential of 0.175 V vs RHE, a value well within the range where the ORR was found to proceed under diffusion limited conditions, yielded an estimated average first order rate constant in peroxide for the het-HPD step, k_D^lim = 2.9 × 10^–4 ± 0.1 cm s^–1. In subsequent experiments, the diffusion limited current flowing through a Se(0.83)|Pt ring of a RRDE polarized at 0.175 V vs RHE, i_ring^lim, was measured for Pt and Se(0.83)|Pt disks held at open circuit, OC, for selected ω and peroxide concentrations in solution. Numerical correlations between i_ring^lim and $r_{O_2,disk}$ were then established using COMSOL, yielding for Se(0.83)|Pt and Pt in [H₂O₂]_B = 1 mM, values of $r_{O_2,disk}$ of 0.215 and 0.84 nmol cm^–2 s^–1, respectively. In stark contrast, a GC disk of a demountable RRDE assembly incorporating a Se(0.83)|Pt ring, displayed no activity for het-HPD at OC, i.e. i_ring^lim was virtually identical to that found for a Teflon disk under the same experimental conditions. Overall, the method described in this work may be regarded as far simpler and more versatile compared to that proposed by Shigehara and Anson, as it avoids the use of chemicals not available from commercial sources.