Role of Fe-Doping in Perovskite Oxides for the Oxygen Evolution Reaction

Abstract

The perovskite oxides with general structure formula of ABO3 have been at the forefront among catalysts for the oxygen evolution reaction (OER) in alkaline media offering a higher degree of freedom in cation arrangement. From its ability to partially accommodate foreign cations (i.e. A’ and B’) of different oxidation states, which then alters the original oxidation state of the B-site cation and the content of oxygen vacancies, correlations can be drawn between physicochemical properties and catalytic performance.1 Particularly, Ba0.5Sr0.5Co0.8Fe0.2O2+ δ (BSCF) has been demonstrated an exceptional activity towards OER,2-6 and recent studies have taken advantages of operando characterization techniques in order to relate changes in its electronic and geometric structure to the OER process.2 These studies have identified the formation of a dynamic self-assembled surface oxy(hydroxide) layer of B-site cations (i.e. Co/Fe for BSCF), which is the result of the lattice oxygen evolution reaction (LOER), as the key feature to obtain a higher OER activity.2, 7-8 In addition, the partial occupancy of Fe at the B-site of BSCF has shown favorable effects in its performance.2 Yet, the functional role of Fe with respect to OER activities and stabilities of Co-based perovskite oxides is still left to be answered. Therefore, in this study, we elucidate the roles of Fe in the OER mechanism of Co-based cubic perovskite oxides; two prospective perovskite oxides – La0.2Sr0.8Co0.8-xFexO3 and Ba0.5Sr0.5Co0.8-xFexO2+ δ with x = 0 and 0.2 – were prepared by flame spray synthesis as nanoparticles. Ex situ and operando X-ray absorption spectroscopy (XAS) was used to study the local electronic and geometric structure under oxygen evolving conditions. In parallel, density function theory (DFT) computational studies were conducted to provide theoretical insights into our findings. Overall, the gathered results highlight the synergetic role of Fe such that Fe would help to stabilize the cobalt in a lower oxidation state leading to allow a greater oxygen vacancy content. In this regard, the dynamic growth of the active (oxy)hydroxide species is inhibited in the absence of Fe for these Co-based perovskites (see Figure 1). Therefore, Fe ultimately contributes to attain a higher OER activity and stability of perovskite catalysts. Overall, information gathered from this study takes another step to understanding the physicochemical properties of perovskite oxides as oxygen evolution reaction catalysts. References Zhu, J.; Li, H.; Zhong, L.; Xiao, P.; Xu, X.; Yang, X.; Zhao, Z.; Li, J., Perovskite Oxides: Preparation, Characterizations, and Applications in Heterogeneous Catalysis. ACS Catalysis 2014, 4 (9), 2917-2940.Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B. J.; Durst, J.; Bozza, F.; Graule, T.; Schaublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J., Dynamic surface self-reconstruction is the key of highly active perovskite nano-electrocatalysts for water splitting. Nat. Mater. 2017, 16 (9), 925-+.Cheng, X.; Fabbri, E.; Kim, B.; Nachtegaal, M.; Schmidt, T. J., Effect of ball milling on the electrocatalytic activity of Ba0.5Sr0.5Co0.8Fe0.2O3 towards the oxygen evolution reaction. J. Mater. Chem. A 2017, 5 (25), 13130-13137.Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334 (6061), 1383-1385.Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J., Superior Bifunctional Electrocatalytic Activity of Ba0.5Sr0.5Co0.8Fe0.2O3-/Carbon Composite Electrodes: Insight into the Local Electronic Structure. Adv. Energy Mater. 2015, 5 (17).Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J., Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 2014, 4 (11), 3800-3821.Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W., Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137 (10), 3638-3648.Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S. H.; Boettcher, S. W., Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27 (22), 7549-7558. Figure 1. Illustration of difference in OER mechanisms of BSC and BSCF; For BSC, the presence of native CoO(OH) perturbs the growth of self-assembled CoO(OH), while BSCF promotes dynamic growth of self-assembled CoO(OH) to attain a high OER activity Figure 1