Electrochemical Investigation on the Reduction of Oxygen at a Perovskite Modified Au Microelectrode
Authors: Rodríguez Oliver , Lima Alan Rogerio Ferreira , Lima Alex Silva , Bertotti Mauro , Denuault Guy 
Published: February 2020

This poster will present recent results obtained for the reduction of dioxygen on perovskite microelectrodes. Briefly, a perovskite powder (La0.58Ag0.4Ni0.6Cu0.4O3) was synthesised using the Pechini method 1 and suspended in a solution consisting of conductive carbon, Nafion, water and ethanol. A 50 μm Ø gold disc electrode was recessed by potentiostatic cycling in a solution of KCl and Ru(NH3)6Cl3, the latter being added to monitor the recess depth in real time. Following the method of Bartlett et al. 2 the depth of the cavity was estimated to be 8.1 μm from the limiting current for the reduction of Ru(NH3)6
3+. The recess was then filled with the perovskite suspension and characterised by voltammetry for the reduction of Ru(NH3)6
3+. The electrode was then used in an oxygen saturated 0.1 M KOH solution to study the oxygen reduction reaction (ORR) under transient and steady state conditions. Experiments were also performed in an argon saturated solution to record the background currents and the subtracted currents were obtained. For both solutions, two pair of peaks around -0.2 V and -0.6 V vs Ag/AgCl were visible. In each case the linear dependency of the peak currents on the scan rate indicates that a surface process is occurring that is related to the material and not to the reduction of oxygen species. This processes are currently under study. Comparison of the voltammograms in argon and oxygen saturated solutions revealed the ORR wave with the typical double crossings seen when metal microelectrodes are used 3. The reason behind the double crossings is still under study but is thought to be linked to the presence of coordinated Ag, Ni and Cu centres.

Chronoamperograms were recorded on the plateau of the ORR wave in both argon and oxygen saturated solutions. A fitting of the experimental currents to the equation proposed by Mahon and Oldham 4, yielded n
app = 1.9 thereby indicating that oxygen is reduced to hydrogen peroxide. Current transients were also recorded at different potentials and the background subtracted transients were converted to sampled current voltammograms and normalised against the Mahon-Oldham equation taking n = 1; this procedure gives information about an apparent number of electrons at the plateau of the sampled current voltammogram if the process is diffusion controlled 3,5,6. It was found that, at steady state conditions, n
app = 2.2, indicating that most of the oxygen is reduced to hydrogen peroxide. At shorter timescales, an extra current that is not diffusion controlled was obtained and attributed to the reduction of pre-adsorbed oxygen species arising from exposure to dissolved oxygen. This is consistent with the results that Perry and Denuault obtained with a range of metallic microelectrodes 6. Since these oxygen species were previously found not to adsorb on gold 6, the results are consistent with their adsorption on metallic clusters at the surface of the perovskite. This appears to be supported by XRD analysis which revealed the presence of metallic Ag and of Ni and Cu oxides.

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(5) Perry, S. C.; Al Shandoudi, L. M.; Denuault, G. Sampled-Current Voltammetry at Microdisk Electrodes: Kinetic Information from Pseudo Steady State Voltammograms. Anal. Chem.
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(6) 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.