The Role of Activation Process on Perovskites-Type Oxides As Electrocatalysts for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is one of the most extensively studied electrochemical reactions. This reaction is of paramount importance in the development of electrochemical technologies such metal-air batteries and electrolyzers. OER is thermodynamically unfavorable and a large overpotential is required to drive the reaction at a rate of practical interest1. Therefore, the development of efficient, durable and low-cost electrocatalysts is necessary. Perovskite-type oxides of the general formula ABO3 are one of the most promising class of materials for this application2. The perovskite structure is able to accommodate a large variety of dopant ions which provides high flexibility in shaping their physical-chemical properties and catalytic activity3,4. The OER activity of perovskite catalysts is usually evaluated in the form of a composite thin film (carbon black + perovskite + binder) on a rotating disk electrode (RDE). Carbon is added to increase the conductivity of the catalyst layer, but it can also promote the OER activity of the oxide through a synergic effect5. However, a wide variety of formulations and oxide loading have been used, making it difficult to compare results between works. The protocol to evaluate the OER activity for an electrocatalyst usually involves: i) conditioning of the electrode by cyclic voltammetry, ii) determination of the electrochemical surface area (ECSA) and iii) recording the OER polarization curves6,7. However, in the literature there is lack of information about the influence of the history of the electrodes on the OER activity of the perovskite oxide catalysts7. Here, we present a specific study on the OER activity of La0.5Sr0.5Co0.8Fe0.2O3- δ perovskite electrocatalyst prepared by solution combustion synthesis, in which the effect of the conditioning of the thin film electrode on the activity towards OER are evaluated. Figure 1 shows the OER polarization curves for a fresh electrode (FE) and for an activated electrode (AE). Compared with FE, the AE shows a better wettability by the electrolyte, higher voltammetric charge under the peaks associated with the Co3+ species and an increase of the OER current at 1.7 V by 117%. The Co3+ ions occupy the B-sites of the perovskite structure which are the active sites for the OER. The conditions and reasons leading to more active sites, resulting in higher current densities will be discussed in detail. Figure 1. Polarization curves of a fresh and activated electrode in 0.1 M KOH at 0.5 mVs-1 and 1600 rpm. Inset: cyclic voltammograms showing the presence of cobalt redox peaks between 1.0 and 1.4 V vs RHE. The enhancement on the OER activity and wettability of the electrode is correlated with a higher voltammetric charge under the redox peaks. References 1. I. Katsounaros, S. Cherevko, A. R. Zeradjanin, and K. J. J. Mayrhofer, Angew. Chemie - Int. Ed., 53, 102–121 (2014). 2. S. Gupta et al., Chem. - An Asian J., 11, 10–21 (2016). 3. J. Suntivich et al., Nat. Chem., 3, 546–550 (2011). 4. J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, and Y. Shao-Horn, Science (80-. )., 334, 1383–1385 (2011). 5. R. Mohamed et al., J. Electrochem. Soc., 162, F579–F586 (2015). 6. F. Deganello et al., ACS Appl. Energy Mater., 1, 2565–2575 (2018). 7. G. Li, L. Anderson, Y. Chen, M. Pan, and P. Y. Abel Chuang, Sustain. Energy Fuels, 2, 237–251 (2018). Figure 1

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