Recently, adsorbed oxygen species (adspecies) have been detected at the millisecond scale on different metallic microelectrodes after being exposed to dissolved oxygen ^1,2 . Comparing the experimental current of the oxygen reduction reaction (ORR) with a theoretical diffusion controlled current, it was revealed that both, the adsorbed and diffusion controlled species, were being reduced at the same time. To study the adspecies independently, it is necessary to decouple the contribution of each process. Here, we show that this can be done by controlling the availability of the gas present in solution as well as the potential waveform applied. After the separation, the diffusion controlled current reveals how n_app, the apparent number of electrons for the ORR, depends on k_m, the mass transfer coefficient. On Pt microdiscs, n_app decreased from 4 to 2 as the mass transfer coefficient was increased by decreasing the disc radius or the timescale of the experiment. This effect was previously observed at the steady state with rotating disc and microelectrodes, but its dependency on transient conditions has never been reported ^{3, 4}. Meanwhile, the decoupled desorption current was found to follow a second order process which would be consistent with the production of H₂O₂ via dimerization of OH radicals. These desorption currents were used to estimate the desorption rate constant and maximum coverage of the adspecies as a function of the applied potential. Our results indicate that the amount of adsorbed species left after exposure to dissolved oxygen is below 0.1 of a monolayer, in agreement with previously reported values ^{1, 2}. The high sensitivity of the methodology presented here reveals the subtle effects arising from exposure to dissolved oxygen.

(1) Perry, S. C.; Denuault, G. 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, Chem. Chem. Phys. 2015, 17 (44), 30005-30012.

(2) Perry, S. C.; Denuault, G. The Oxygen Reduction Reaction (ORR) on Reduced Metals: Evidence for a Unique Relationship Between the Coverage of Adsorbed Oxygen Species and Adsorption Energy, Chem. Chem. Phys. 2016, 18, 10218-10223.

(3) Pletcher, D.; Sotiropoulos, S. A Study of Cathodic Oxygen Reduction at Platinum Using Microelectrodes. Electroanal. Chem. 1993, 356 (1-2), 109-119.

(4) Sosna, M.; Denuault, G.; Pascal, R. W.; Prien, R. D.; Mowlem, M. Development of a Reliable Microelectrode Dissolved Oxygen Sensor. Sensors Actuators, B Chem. 2007, 123 (1), 344-351.