Uncovering the Effect of Lattice Strain and Oxygen Deficiency on Electrocatalytic Activity of Perovskite Cobaltite Thin Films

Abstract

Developing cost effective electrocatalysts with high oxygen evolution reaction (OER) activity is essential for large‐scale application of many electrochemical energy systems. Although the impacts of either lattice strain or oxygen defects on the OER performance of oxide catalysts have been extensively investigated, the effects of both factors are normally treated separately. In this work, the coupled effects of both strain and oxygen deficiency on the electrocatalytic activity of La 0.7 Sr 0.3 CoO 3−δ (LSC) thin films grown on single crystal substrates (LaAlO3 (LAO) and SrTiO3 (STO)) are investigated. Electrochemical tests show that the OER activities of LSC films are higher under compression than under tension, and are diminished as oxygen vacancies are introduced by vacuum annealing. Both experimental and computational results indicate that the LSC films under tension (e.g., LSC/STO) have larger oxygen deficiency than the films under compression (e.g., LSC/LAO), which attribute to smaller oxygen vacancy formation energy. Such strain‐induced excessive oxygen vacancies in the LSC/STO increases the e g state occupancy and enlarges the energy gap between the O 2p and Co 3d band, resulting in lower OER activity. Understanding the critical role of strain–defect coupling is important for achieving the rational design of highly active and durable catalysts for energy devices.

Improving the Activity for Oxygen Evolution Reaction by Tailoring Oxygen Defects in Double Perovskite Oxides

“…It is needed to note that the Co 2p 3/2 and Co 2p 1/2 main peaks overlap with the Ba 3d 5/2 and Ba 3d 3/2 near ≈793 and ≈778 eV, respectively (Figure c) . Despite the complication brought by such overlapping, we can still probe the valence state of Co on the basis of the Co 2p satellite peak position, particularly the satellite peak for Co 2p 3/2 , as demonstrated in previous work . The reason why these satellite peaks were located at higher binding energy than the main peak is that the electrons transferred from the O 2p to the Co 3d orbital can screen the electric field of the holes in the Co 2p orbital created during the photoelectron emission process.…”

“…In this work, XPS and X‐ray diffraction (XRD) were performed to detect changes in the transition metal valence and lattice parameters, respectively. Such changes could be used to estimate the amount of oxygen vacancies generated in PBSCF thin films …”

“…Despite these successful examples, some negative effects of excessive oxygen vacancies were also reported in the literature. The OER activities of La 0.6 Ca 0.4 CoO 3− δ and La 0.7 Sr 0.3 CoO 3− δ were found to be decreased after thermal annealing in a reducing atmosphere. FeCoO x showed the highest OER performance with optimal oxygen vacancy density of 34% after annealing at 200 °C in H 2 of 2.0 MPa.…”

Smart Control of Composition for Double Perovskite Electrocatalysts toward Enhanced Oxygen Evolution Reaction

“…Therefore, previous reports have focused on strategies such as the introduction of OER‐active elements into the B‐site and tailoring the oxygen defect to increase the catalytic performance. Among the transition‐metal‐based perovskite oxides (such as Mn, Fe, Co and Ni), Co‐based oxides have emerged as promising candidates owing to the outstanding OER performance of the CoO 6 octahedron . Co in a perovskite structure usually has more oxidation states (Co 2+ , Co 3+ , Co 4+ ) and spin states than Ni (Ni 2+ ) and Fe (Fe 3+ ) because the formation of high oxidation states of Ni 3+ and Fe 4+ typically require high pressures or oxidation conditions .…”

“…Among the transitionmetal-based perovskite oxides (such as Mn, Fe, Co and Ni), Cobased oxidesh ave emerged as promising candidates owing to the outstanding OER performance of the CoO 6 octahedron. [11][12][13][14][15] Co in ap erovskite structure usually has more oxidation states (Co 2 + ,C o 3 + ,C o 4 + )a nd spin states than Ni (Ni 2 + ) and Fe (Fe 3 + )b ecause the formation of high oxidation states of Ni 3 + and Fe 4 + typically require high pressures or oxidation conditions. [4,16] Co 3 + and Co 4 + cationsare beneficial for the formationo f*OH and *OOH, respectively,ina lkalinec onditions.…”