Transient study of the oxygen reduction reaction on reduced Pt and Pt alloys microelectrodes: evidence for the reduction of pre-adsorbed oxygen species linked to dissolved oxygen

Abstract

Using chronoamperometry at preconditioned oxide-free Pt microdisc electrodes in aqueous media, we investigated the oxygen reduction reaction (ORR) on the millisecond timescale and obtained results consistent with the reduction of oxygen species which adsorb on the electrode before the ORR is electrochemically driven. Furthermore these adsorbed species are clearly linked to oxygen in solution. At long times, the amperometric response is solely controlled by the diffusion of dissolved oxygen towards the microelectrode. However, at short times, typically below 50 ms, the reduction of pre-adsorbed oxygen produces a large extra current whose magnitude depends on the oxygen concentration in solution, deliberate electrode poisoning and the rest time before the potential step. Using sampled current voltammetry we show that this extra current affects the entire potential range of the ORR. Using microdisc electrodes made with Pt alloys we find that the amperometric response is sufficiently sensitive to distinguish oxygen coverage differences between Pt, Pt0.9Rh0.1 and Pt0.9Ir0.1 microdiscs. These unexpected and, to our knowledge, never previously reported results provide new insight into the oxygen reduction reaction on Pt. The existence over a wide potential range of irreversibly adsorbed oxygen species arising from dissolved oxygen and different from Pt oxide is particularly relevant to the development of oxygen reduction catalysts for low temperature fuel cells. A. IntroductionThe electroreduction of oxygen in aqueous media is a complex reaction which involves the cleavage of the oxygen bond, adsorption and desorption of intermediates and the transfer of four electrons and four protons. While the overall mechanism is commonly discussed in terms of a series two 2-electron peroxide pathway running in parallel with a direct 4-electron process, 1, 2 the elementary steps and, crucially, the nature of the rate determining step remain uncertain. 3,4 In mechanistic studies the two pathways are discussed in terms of the formation of OOH on the surface (associative mechanism) or of the adsorption of atomic oxygen (dissociative mechanism) 5,6 but the nature of the intermediates is still unclear; while most invoke adsorbed OH species, 7 recent studies support the existence of a soluble intermediate. 3,8,9 The complexity of the reaction is reflected by the dependence of the apparent number of electrons, n app , on experimental conditions including electrode material, 10 surface crystal planes, 11 pH, 12 and even mass transport conditions. [13][14][15] Except for a few studies, 11, 16-21 the reaction has been mostly investigated under steady state mass transport conditions with rotating disc, rotating ring-disc, hanging meniscus rotating disc and microelectrodes.Here we report the unexpected, and to our knowledge never previously reported, results obtained when studying the ORR under transient conditions on the millisecond timescale. To acquire data undistorted by the charging and discharging of the double layer we employ micro...

The oxygen reduction reaction (ORR) on reduced metals: evidence for a unique relationship between the coverage of adsorbed oxygen species and adsorption energy

“…We previously reported that the charge associated with the reduction of the adsorbed oxygen species depends on the metal substrate and that it is sufficiently reproducible to distinguish coverage differences between Pt, Pt 0.9 Rh 0.1 and Pt 0.9 Ir 0.1 . 13 We now provide the results of experiments conducted with Au, Ag, Cu and Ni microdiscs. These metals were specifically chosen to offer a range of adsorption energies towards oxygen since theoretical calculations predict binding strengths increasing in the order: Au < Ag < Pt < Cu < Ni.…”

“…13 We found that the reduction of these adsorbed oxygen species was distinct from that of dissolved oxygen as schematically presented in scheme 1. We also showed that it is possible to control and monitor their coverage and that the experimental protocol was sufficiently sensitive to distinguish between adsorption on Pt, Pt 0.9 Ir 0.1 and Pt 0.9 Rh 0.1 .…”

“…This charge is similar to that determined by chronoamperometry in aerated solution after subtracting the diffusion controlled response for the ORR and we have shown that it increases with the rest time and dissolved oxygen concentration before reaching a plateau. 13 In effect the coverage of adsorbed oxygen species can be controlled through the rest time and the bulk concentration of dissolved oxygen. In our previous study we also showed that this charge is clearly related to the availability of adsorption sites on the electrode since it decreases systematically in the order ClO 4 -> Cl -> Br -> I -when switching to increasingly adsorbing electrolytes.…”

“…13 13 and its diffusion coefficient was determined as reported previously. 19 The working electrodes were systematically preconditioned potentiostatically to obtain reproducible results and to ensure each experiment started with a reduced surface free from oxides.…”

“…19 The working electrodes were systematically preconditioned potentiostatically to obtain reproducible results and to ensure each experiment started with a reduced surface free from oxides. The preconditioning waveform was as previously described 13,20 but the upper cleaning, rest and lower cleaning potentials were adjusted for each metal substrate as listed in table SI-1 (ESI †). Briefly the electrode was cleaned by sweeping between upper and lower cleaning potentials at 500 mV s -1 six times.…”