2020同步年報

060 ACTIVITY REPORT 2020 Fig. 1 : O perando XAFS spectra for the Co K-edge of (a) pure Co 3 O 4 and (b) V O –Co 3 O 4 . The insets show a detailed view of the dotted boxes, respectively. (c) Structural coherence change in the EXAFS coordination number of Co ions under an applied potential relative to the OCP state. [Reproduced from Ref. 1] Reconstructions Revive Active Centers Surface reconstruction can be an effective strategy to improve the performance of catalysts. H ydrogen energy obtained from a water-splitting reaction is a green and renewable source of energy, but the sluggish kinetics of the water-splitting half reactions—the hydrogen and oxygen evolution reactions (HER and OER, respectively) that can be performed under both acidic and alkaline conditions, are a major drawback. The development of efficient and robust catalysts to split water is hence a critical topic for the energy-conversion field. Many recent studies revealed that the presence of defects, such as vacancies, boundaries and stacking faults in nanomaterials can promote significantly their pho- tocatalytic or electrocatalytic activity, but the exact role of a defect structure in catalysts for OER or HER, which is a dynamic process, remains unclear. Study of the structure-activity relation of defective electrocatalysts under operando conditions is thus crucial for an understanding of their intrinsic reaction mechanisms and the dynamic behavior of defect sites. Shuangyin Wang (Hunan University, China) and his coworkers recently constructed pure spinel Co 3 O 4 and V O -rich Co 3 O 4 as catalyst models to study the defect mechanism and to investigate the dynamic behavior of defect sites during the electrocat- alytic OER with various operando characterizations. From operando X-ray-absorption fine-structure (XAFS) spectra recorded at TPS 44A , 1 the results demonstrated that the oxygen vacancies were filled first with OH • for V O –Co 3 O 4 and facilitated pre- oxidation of low-valence Co and promoted reconstruction or deprotonation of intermediate Co–OOH • . For example, Co K-edge data were collected under the critical potential (recorded at OCP to 1.75 V vs . RHE), as plotted in Fig. 1 . The valence state of Co ions in V O –Co 3 O 4 has a more rapid oxidation than the slow rise of the valence state of Co ions in pure Co 3 O 4 . When the anode potential continues to increase and the surface charge cannot oxidize Co(IV) to a higher valence state, electrons are removed from surface oxygen; this effect causes oxygen to escape, accompanied by structural reconstruction. Because of the stronger attraction of the Co sites for the O 2p electron at a high anode potential, the deprotonation of the medium OH – (M–OH • ) becomes facilitated. Because of the enhanced electrophilicity of the exposed cobalt sites, V O is hence likely to pro- mote the adsorption of OH ions on active Co sites to form adsorbed M–OH • species and subsequently prompt the deprotona- tion at a low potential ( i.e ., 1.45 V) to form active oxygen species (Co–OOH • ) on the surface, which accounts for the enhanced OER activity. The surface chemistry of V O –Co 3 O 4 is clearly more easily changed through reconstruction from the oxide into Co–OOH • intermediate species. Xiaoqing Huang (Soochow University, China) and his coworkers have recent- ly made a landmark discovery, success- fully developing a highly efficient and pH-universal Ir-based electrocatalyst for water splitting. It was determined that the reconstruction of IrTe 2 hollow nanoshuttles (HNS) can be regulated on adjusting the potential during electrochemical dealloying, in which mild and high potentials lead to the formation of IrTe 2 HNS with a metal Ir shell (D‐IrTe 2 HNS) and an IrO x surface (DO‐IrTe 2 HNS), respectively. Record- ing operando X-ray absorption spectra (XAS) at TPS 44A , 2 the authors investi- gated the evolution of their electronic structure during the electrocatalysis reaction. The operando XAS measure- ments allowed them to investigate the evolution of electronic structure on the surface of the catalysts through the unique hollow structure of HNS, as shown in Fig. 2 . The results