The oxygen reduction reaction (ORR) was studied using Pt based microelectrodes under different mass transfer regimes. Current transients at different potentials were recorded by first applying a cleaning waveform and a rest potential ^1–3 transformed to sampled current voltammograms and finally normalised using the equation proposed by Mahon and Oldham ⁴. In the steady state, the normalised current is akin to an apparent number of electrons n_app, while for shorter times, the presence of an extra current that is attributed to the reduction of oxygen adsorbed species (adspecies) can be seen ^2,3. These adspecies are only observed after exposure of the surface to dissolved oxygen during the rest potential and their reduction was tested with different types of Pt electrodes.

First, Pt nanoparticles were deposited on carbon fibre microdisc electrodes (7 μm Ø). The normalisation gave n_app = 3 at steady state conditions; this value is large for an electrode of this size ⁵ and it is believed that this is due to the effectiveness of the nanoparticles to reduce peroxide towards water due to their close proximity with each other. At shorter times, the normalisation showed an extra current attributed to the reduction of oxygen adspecies ^2,3 but also to the reduction of carbon functionalities. To solve this, carbon nanoelectrodes were electroplated with Pt until sub-micron spheres were obtained and covered the carbon surface completely. At the steady state, n_app = 2. Since no carbon was exposed to the solution, this n_appis attributed to the high rate of mass transfer that can be achieved with these electrodes. At short timescales, the extra current due to the reduction of oxygen adspecies was also visible.

Different sizes of Pt microdiscs were also used (5, 10, 25 and 50 μm Ø). The value of n_app for each electrode agrees with those reported under steady state conditions ⁵. Using the equation proposed by Mahon and Oldham ⁴, a subtraction between the experimental and theoretical current densities was obtained and integrated to calculate the charge density associated to the reduction of the oxygen adspecies. The results from the millisecond to the second scale showed a value of 55 μC cm^-2 in agreement with previous works ^2,3. This value corresponds to 0.1 monolayers and seems to indicate that the coverage does not depend on the electrode size. Further analysis of the current transients assuming a first order process for the rate of reduction of the oxygen adspecies suggests that the maximum coverage that can be obtained corresponds to approximately 0.3 monolayers. This is currently being studied by recording current transients at the microsecond scale. The dependency of the rate of reduction of the oxygen adspecies on the applied potential is also under study. It is expected that this work will help elucidate the lifetime of the processes involved, namely the reduction of oxygen adsorbed species and the reduction of diffusion controlled oxygen that comes from the bulk.

Overall these transient experiments shed new light on the interaction between dissolved oxygen and Pt electrodes and provide new insight on the oxygen reduction reaction.

(1) Perry, S. C.; Al Shandoudi, L. M.; Denuault, G. Sampled-Current Voltammetry at Microdisk Electrodes: Kinetic Information from Pseudo Steady State Voltammograms. Anal. Chem. 2014, 86(19), 9917–9923.

(2) 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. Phys. Chem. Chem. Phys. 2015, 17(44), 30005–30012.

(3) 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. Phys. Chem. Chem. Phys. 2016, 18, 10218–10223.

(4) Mahon, P. J.; Oldham, K. B. Diffusion-Controlled Chronoamperometry at a Disk Electrode. Anal. Chem. 2005, 77(18), 6100–6101.

(5) 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.