Letter pubs.acs.org/JPCL
Highly Active Epitaxial La(1−x)SrxMnO3 Surfaces for the Oxygen Reduction Reaction: Role of Charge Transfer Kelsey A. Stoerzinger,*,†,‡ Weiming Lü,∥ Changjian Li,∥,⊥ Ariando,∥,# T. Venkatesan,∥,⊥,#,∇ and Yang Shao-Horn*,†,‡,§ †
Department of Materials Science & Engineering, ‡Electrochemical Energy Laboratory, and §Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ∥ NUSNNI-Nanocore, ⊥National University of Singapore Graduate School for Integrative Sciences and Engineering (NGS), # Department of Physics, and ∇Department of Electrical and Computer Engineering, National University of Singapore, Singapore S Supporting Information *
ABSTRACT: Most studies of oxide catalysts for the oxygen reduction reaction (ORR) use oxide powder, where the heterogeneity of exposed surfaces and the composite nature of electrodes limit fundamental understanding of the reaction mechanism. We present the ORR activity of epitaxially oriented La(1−x)SrxMnO3 surfaces and investigate, by varying Sr substitution, the relationship between the role of charge transfer and catalytic activity in an alkaline environment. The activity is greatest for La(1−x)SrxMnO3 with 33% Sr, containing mixed Mn3+/4+, and the (110) and (111) orientations display comparable activities to that of the (001). Electrochemical measurements using the facile redox couple [Fe(CN)6]3−/4− illustrate that increasing ORR activity trends with faster chargetransfer kinetics, indicating the importance of facile charge transfer at the oxide/water interface and mixed Mn valence in promoting ORR kinetics.
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Ca2+ or Sr2+ has been reported to increase ORR activity, as well as introduce oxygen nonstoichiometry via cation vacancies19 in La(0.6−x)Ca0.4MnO3 and rhombohedral La(1−x)Mn(1−y)O3, referred to historically as LaMnO3+δ.3 In a rotating ring disk electrode experiment by Tulloch et al.,20 La0.4Sr0.6MnO3, characterized by the smallest crystallite size in the series Sr = 0, 0.2, 0.4, 0.6, 0.8, and 1, had the highest ORR activity and smallest production of peroxide in 1 M KOH. Missing, however, are studies which compare the role of valence and charge transport in the absence of grain boundaries21 in the ORR catalysis. The surface electronic conductivity at the AMnO3/electrolyte interface is little understood,8,22−24 which leads to ambiguities in the role of charge transfer on the ORR kinetics. Conventional electrodes consist of oxide particles dispersed with electronically conductive carbon powder, which can dominate electronic conduction in the electrodes25,26 and influence electrode ORR currents, as carbon is highly active to reduce oxygen to peroxide.24 The study of thin-film singlecrystal surfaces can eliminate the use of carbon,23,27 and provide precise control of the surface area and crystallographic orientation, which allows the measurement of charge transfer kinetics at the oxide/electrolyte interface, intrinsic ORR activity, and insights into the mechanism of oxygen electrocatalysis.28
he sluggish kinetics of the oxygen reduction reaction (ORR) limit the efficiency of energy storage in fuel cells1 and metal-air batteries.2 In contrast to proton exchange membrane fuel cells, which catalyze this reaction with costly and scarce Pt, alkaline fuel cells can utilize earth abundant metal oxides with close to comparable efficiencies.3,4 In particular, manganese oxides that are abundant, inexpensive, and nontoxic with rich oxide chemistry can be very attractive for the ORR.5−7 The electronic structure and conductivity of manganese oxides, which may greatly influence the ORR, can be tuned in AMnO3 of the perovskite family. For example, the electronic structure and conductivity can be modified drastically by the substitution of A-site cations, such as altering Mn valence via substituting some portion of A-site commonly from the lanthanide (A3+) group for an alkaline earth (A2+) element8 or changing the ionic radius of the lanthanide element.9,10 The increased conductivity in the case of La(1−x)SrxMnO3 compositions with moderate Sr content has been well studied in the solid state physics community. Incorporation of Sr2+ can lead to oxygen nonstoichiometry,11 but charge compensation also results in the incorporation of ligand holes and Mn4+.12 Oxygen p-metal d coupling causes materials with Sr = 0.2−0.6 to become ferromagnetic metals,12 with compositions of Sr = 0.3−0.4 exhibiting maximum conductivity.13−15 In addition to the conductivity that results from Mn4+ incorporation, ORR studies on MnOx materials have found that the presence of both Mn3+/4+ valence states is beneficial to catalytic activity,16 which has been attributed to their acting as an oxygen acceptor/donor mediator.17,18 In the La(1−x)AxMnO3 system, the inclusion of © XXXX American Chemical Society
Received: March 2, 2015 Accepted: March 31, 2015
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DOI: 10.1021/acs.jpclett.5b00439 J. Phys. Chem. Lett. 2015, 6, 1435−1440
Letter
The Journal of Physical Chemistry Letters In the present work, we consider (001)-oriented epitaxial films of La(1−x)SrxMnO3 (LSMO) to systematically tune the conductivity and valence state of Mn and elucidate the effects in ORR catalysis. We find that the ORR activity trends with the ability to transfer charge, measured both by in-plane conductivity and in situ via a facile redox couple, where metallic-like LSMO with 33% Sr is most active. This observation corroborates findings on binary metal oxides,17,18 which suggest that a mixture of Mn3+/4+ valence states aids in charge transfer to and adsorption of oxygen. For highly active chemistries with 33% and 50% Sr, the ORR activity at pH 13 is comparable for the (001), (110), and (111) pseudocubic (pc) orientations, attributable to either a lack of structural sensitivity in the reaction mechanism or reconstruction of the surface under ORR conditions, meriting further study. Epitaxial LSMO films of seven Sr concentrations, denoted by percent of the A-site from 0 to 100%, were grown by pulsed laser deposition (PLD) on (001)-oriented Nb-doped SrTiO3 (NSTO) substrates (see Experimental Methods, Figure 1).
Figure 2. ORR activity of LSMO films in O2-saturated 0.1 M KOH. (A) CV at a scan rate of 10 mV/s showing the ORR current per surface area (i) versus the ohmic corrected applied voltage (V − iR). (B) Tafel plot obtained from chronoamperometry (constant applied voltage) showing V − iR vs |i| on a logarithmic scale. Lines guide the eye illustrating the shallower slope for highly active Sr = 33% (green) versus less-active Sr = 70% (purple).
the region of oxygen evolution (up to 1.7 V versus the reversible hydrogen electrode (RHE)) for compositions with 10, 20, and 33% Sr, but increased slightly for 50% and 70% Sr (Figure 3, S5). Comparing chemistries, the activity increases with Sr substitution of La to reach a maximum at Sr = 33%, beyond which the ORR activity decreases to an immeasurably low value on SrMnO 3 . In addition, both CV and chronoamperometry measurements show that the Tafel slope decreases with increasing activity (Figure 2B), in the range
Figure 1. Characterization of LSMO films, with the number representing the Sr percentage. (A) X-ray diffraction showing normal 2θ−ω scan of the (001)pc and (002)pc reflections. (B) Exemplary atomic force microscopy of La0.67Sr0.33MnO3 films, with a vertical contrast scale of 7.8 nm. All surfaces showed comparably low rootmean-square roughness and the persistence of terraces from epitaxial growth on the substrate. (C) Photograph of a film sample prepared for electrochemical measurements.
Films have low root-mean-square roughness
