Tellurium-doped lanthanum manganite as catalysts for the oxygen reduction reaction
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Research Letter
Tellurium-doped lanthanum manganite as catalysts for the oxygen reduction reaction V. Celorrio, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK L.J. Morris, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK; EPSRC Centre for Doctoral Training in Catalysis, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK M. Cattelan, N.A. Fox, and D.J. Fermin, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK Address all correspondence to: D.J. Fermin at [email protected] (Received 15 December 2016; accepted 23 March 2017)
Abstract The effect of tellurium (Te) doping on the electrocatalytic activity of La1−xTexMnO3 toward the oxygen reduction reaction is investigated for the first time. La1−xTexMnO3 with x-values up 23% were synthesized from a single ionic liquid-based precursor, yielding nanoparticles with mean diameter in the range of 40–68 nm and rhombohedral unit cell. Electrochemical studies were performed on carbon-supported particles in alkaline environment. The composition dependence activity is discussed in terms of surface density of Mn sites and changes in the effective Mn oxidation state.
Introduction Oxygen electrocatalysis is one of the key processes limiting the efficiency of energy conversion devices such as fuel cells, electrolysers, and metal–air batteries.[1,2] In particular, the oxygen reduction reaction (ORR) is commonly associated with slow kinetics, requiring high overpotentials, and high catalyst loadings. Current research activities are also focused on the development of non-noble metal electrocatalysis such as transition metal oxides.[3,4] A number of studies have shown that Mn-based transition metal oxides are among the most catalytically active for the ORR in alkaline solutions.[5–8] However, the parameters determining the reactivity of Mn sites in this complex systems remain a subject of discussion. In the case of perovskites oxides (ABO3), the key active site is determined by the B-cation. A multiplicity of parameters have been linked to electrocatalytic activity such as the nature of the A-site, d-orbital occupancy in the B-site, oxidation state, A-site surface segregation, oxygen vacancies, particle size, and morphology.[3,4,9,10] Suntivich et al. proposed that single occupancy of eg orbital gives the highest ORR activity,[5] which appears to be supported by DFT studies reported by Calle-Vallejo et al.[9] However, recent studies by Celorrio et al. showed that the B-site orbital occupancy may change under operational conditions, concluding that Mn-sites are uniquely active as they undergo changes in the oxidation state in the region close to the formal ORR potential.[11] This point has also been recognized by other groups.[12,13] The orbital occupancy of Mn can be influenced by the cations occupying the A-site, which might lead to tuning the activity of the catalysts.[7] Celorrio et al. also demonstrated that increasing the
amount of Ca2+ in La1−x
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