Tuning of Oxygen Electrocatalysis in Perovskite Oxide Nanoparticles by the Cationic Composition
Résumé
Manganese and cobalt perovskite oxides are among the most active precious metal-free electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), respectively. Herein, we question the role of the cationic composition and charge state in manganite, cobaltite, and mixed Mn/Co perovskites in the mechanism of oxygen electrocatalysis for ORR and OER. We synthesize in molten salts a range of perovskite nanoparticles active in ORR (single B-site (LaMn)1−γO3 and (La0.7Sr0.3Mn)1−γO3), in OER (single B-site La0.67Sr0.33CoO3−δ), and in both ORR and OER (mixed B-site (LaMn0.6Co0.4)1−γO3). By using operando X-ray absorption spectroscopy coupled to ex situ electron energy loss spectroscopy, we show that Mn and Co in single B-site perovskites undergo changes in oxidation states at the steady state during electrocatalysis, while their oxidation states remain unchanged in the mixed Mn/Co perovskite during OER and ORR. We relate these distinct behaviors to modifications of the rate-determining steps of both the OER and ORR electrocatalytic cycles, triggered by an increased covalency of B–O bonds in the mixed perovskites. These results highlight how simple cationic substitutions, accompanied by a control of cationic vacancies, offer a pathway to tune oxygen electrocatalysis.
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