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

Using chronoamperometry and voltammetry at oxide-free microelectrodes in non-adsorbing electrolyte, KClO4, we investigated the reduction of oxygen containing species which adsorb at potentials below those where oxides are grown electrochemically when the electrode is exposed to dissolved oxygen. Their coverage is found to vary with the metal substrate as follows, Ni > Cu > Pt > Ag, and is negligible on Au microdiscs. This dependence is consistent with the adsorption energy of atomic oxygen for the respective metals predicted by DFT. Furthermore the adsorption energies derived from the stripping peak potentials for the different metals agree very well with those predicted by the DFT calculations. We exploit the oxygen coverage differences between Pt, Pt0.9Rh0.1 and Pt0.9Ir0.1 microdiscs to predict the adsorption energy of these species on the Pt alloys and demonstrate a clear correlation between the metal activity towards the ORR and the charge associated with the reduction of the adsorbed oxygen species. In effect the coverage - adsorption energy relationship turns the charge associated with the reduction of the adsorbed oxygen species into a descriptor of the electrode activity towards the ORR. This study provides new insight into the oxygen reduction reaction and offers a new methodology to investigate low temperature fuel cell ORR catalysts.

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.

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.

Implementation of a pharmacy benefit to a managed Medicaid population.

This paper describes an evaluation of per member per month costs, preauthorization processes, drug utilization, and provider and member educational efforts after one year of managing the pharmaceutical benefit of a former Medicaid population. During TennCare's inaugural year, HMO per member per month pharmacy costs decreased, prior authorization request and call activity decreased, and drug marketshares aligned toward formulary agents. Provider and member acceptance of the restrictive formulary grew as consistent interaction with the HMO became patient-specific.