Atomistic insights into activation and degradation of La0.6Sr0.4CoO3 ẟ electrocatalysts under oxygen evolution conditions

Abstract

The stability of perovskite oxide catalysts for the oxygen evolution reaction (OER) plays a critical role for their applicability in water splitting concepts. Decomposition of perovskite oxides under applied potential is typically linked to cation leaching and amorphization of the material. However, structural changes and phase transformations at the catalyst surface were also shown to govern the activity of several perovskite electro-catalysts under applied potential. Hence, it is crucial for the rational design of durable perovskite catalysts to understand the interplay between the formation of active surface phases and stability limitations under OER conditions. In the present study, we reveal a surface-dominated activation and deactivation mechanism of the prominent electrocatalyst La0.6Sr0.4CoO3-ẟ under steady-state OER conditions. Using a multi-scale micros-copy and spectroscopy approach, we identify evolving Co-oxyhydroxide as catalytically active surface species and La-hydroxide as inactive species involved in the transient degradation behavior of the catalyst. While the leaching of Sr results in the formation of mixed surface phases, that can be considered as a part of the active surface, the gradual depletion of Co from a self-assembled active CoO(OH) phase and the relative enrichment of passivating La(OH)3 at the electrode surface results in the failure of the perovskite catalyst under applied potential.

Probing Surface Transformations of Lanthanum Nickelate Electrocatalysts during Oxygen Evolution Reaction

“…63 After alkaline treatment and the cycling of the OER, the binding energy shifts to higher energies and the multiplet splitting energy decreases (ΔBE = 3.5 eV), indicative of a more hydroxylated surface. 13 Due to overlap with the Ni 2p 3/2 core region, we did not deconvolute the La 3d 3/2 peak (Figure S21). Overall, the La 3d 5/2 spectra indicate the formation of lanthanum hydroxide species on the surface of the alkaline-treated La 2 NiO 4+δ (i.e., NiO x (OH) y :La and/or La(OH) 3 ).…”

“…However, the reactivity descriptors used are typically applicable to only a restricted set of material families (e.g., perovskites), and importantly, they fail to account for irreversible changes at the catalyst surface under electrocatalytic conditions leading to the formation of active sites different from that of the pristine phase�a phenomenon that is frequently encountered. 12,13 For instance, it was shown that the surface of many oxide materials featuring a high OER activity undergoes a rapid amorphization under catalytic reaction conditions leading to the formation of poorly crystalline metal (oxy)hydroxide phases. 12−18 Surface amorphization is typically accompanied by a loss of redoxinactive elements (e.g., alkaline or alkaline-earth elements, A n+ ) from the structure of the complex oxide owing to the high ionicity of the A n+ −O bonds which imparts a strong driving force for A n+ ions to dissociate and leach out.…”

Orientation-Dependent Oxygen Evolution Activity of Epitaxial Ruddlesden–Popper Pr0.5Ca1.5MnO4 Thin Films

“…Pr 3d 5/2 is found to be roughly at 932.9 eV, and the energy separation between the Pr 3d 3/2 and Pr 3d 5/2 transitions is around 20 eV with a 3:2 intensity ratio. A strong shoulder can be seen on the lower sides of the Pr 3d 5/2 and Pr 3d 3/2 peaks, corresponding to the so-called shake-down satellite, 15 a collective response of the outer electron cloud to the formation of core holes. These satellites are excluded from the calculation of the Pr atomic percentage.…”

“…13 Moreover, the changes in surface chemistry and termination of perovskites have a significant impact on the OER activity. 14,15 For example, OER can be tuned by controlling the as-prepared surface termination of nickelates. 16,5 In particular, comparing epitaxial thin films of different orientations with powders might provide critical new insights into the effects of surfaces, 17 ferroelectric order, 18 and strain 19,20 on the OER.…”