Strain‐Induced Orbital Contributions to Oxygen Electrocatalysis in Transition‐Metal Perovskites

Abstract

B site have shown high activities and represent a cost-effective alternative to expensive noble metal-based catalysts; however, gas exchange at these perovskite surfaces remains the limiting factor for high efficiency in electrochemical devices. [1][2][3] The development of perovskites with high activity has historically relied on brute-force compositional screening involving the synthesis and testing of numerous bulk-ceramic samples to establish an intuition for chemical trends that can guide researchers to highly active compositions. More recently, however, there has been a significant push to develop electronic descriptors for predicting electrochemical activity. [4][5][6][7] This work has led to the identification of several electronic descriptors that are closely correlated with high activity for ORR and OER catalysts, including the relative position of the oxygen 2p band center, [6] the BO bond covalency, [8] and, in particular, the occupancy of the e g orbitals. [5,9] For example, e g orbital occupation has been shown to be strongly correlated with the B-site-dependent chemical activity trends, and an "ideal" e g occupancy of ≈1.2 electrons has been used as the justification for high activities in LaNiO 3 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) catalysts. [5,9,10] As such, the development of electronic descriptors has already proved fruitful in identifying compositions with high activities and, in turn, developing synthetic control of materials properties to tune the electronic structure toward these ideal values could unlock a new way to optimize materials.In parallel to the development of these electronic descriptors, researchers have leveraged thin-film-based studies to understand how manipulation of the lattice structure of these perovskites can affect the electrochemical activity. Several studies have focused on the role of epitaxial strain in perovskite oxides for controlling electrochemical activity. [11][12][13][14][15][16][17] Reports have noted enhanced electrochemical activities in cobalt-and ironbased perovskites under biaxial-tensile strain driven by straininduced changes to surface chemistry [12] or suggested electronic contributions as the driving force for enhanced electrochemical activity under tensile strain. [13,14,18] Meanwhile, strain-induced changes to e g occupancy have been proposed as the mechanism of enhanced ORR/OER activities in LaNiO 3 films under biaxial compressive strain. [16] This discrepancy suggests that straininduced changes to electronic structure may be dependent on the identity of the B-site cation and calls for more direct studies Epitaxial strain has been shown to produce dramatic changes to the orbital structure in transition metal perovskite oxides and, in turn, the rate of oxygen electrocatalysis therein. Here, epitaxial strain is used to investigate the relationship between surface electronic structure and oxygen electrocatalysis in prototypical fuel cell cathode systems. Combining high-temperature electricalconductivity-relaxation studies and synchrotron-...

Simultaneously Improved Surface and Bulk Participation of Evolved Perovskite Oxide for Boosting Oxygen Evolution Reaction Activity Using a Dynamic Cation Exchange Strategy

“…[2,4,6] To further improve the activity and stability, perovskite oxides with tunable compositions and versatile structures have been researched intensively. [7][8][9][10][11][12][13][14] It has been demonstrated that perovskite oxides possess remarkably high intrinsic activity through the optimization of their electronic structure [10,15] and the participation of lattice oxygen. [16][17][18][19] Other parameters, such as bond strength, [20] charge-transfer energy, [21] and O 2p band center, [22,23] have also been reasonably tuned for promoting activity and stability.…”

Eliminating the Mn 3d Orbital Degeneracy to Suppress the Jahn–Teller Distortion for Stable MnO2 Cathode

“…[57] It should be noted that compared with the moderately increased d x orbital sets are along the z-axis, the energy levels of d xz /d yz increase even more. [58][59][60] Compared to d z 2 and d x 2 -y 2 orbits, the low energy t 2g orbits (d xy , d yz , d xz ) show almost unchanged d-electron occupancy, which can be interpreted by the lowest energy principle and Hund rule: electrons prefer to distribute in low energy orbits carrying spins with the same direction to minimize the energy for the whole atom. Specially, Mn 3+ and Mn 4+ have four and three 3d electrons, and hence three low energy orbits (d xy , d yz , d xz ) can be semi-filled by three electrons.…”

Freestanding complex-oxide membranes

“…The oxygenreduction activity has been the main focus of study on complex oxides and is easily tailorable via chemical doping [250]. Epitaxial thin film synthesis has enabled accurate control over surface structures, atomic termination, and crystal facet orientation [35,129,251,252], thus offering new routes to explore and tune surface reactivity and gas-exchange kinetics, crucial to identify optimal parameters for operation on solid-oxide fuel cells and electrolyzers. Reactions involving water splitting have also been identified on the surface of FE oxides [253][254][255], which hints at their application in hydrogen production.…”