Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

Abstract

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li 2 Co 2 O 4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and timedependent valence state transition from Co 3+ to Co 4+ accompanied by spontaneous delithiation, whereas the edge-shared CoO network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co 4+ site, rather than the Co 3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity.

Boosting oxygen reduction activity and enhancing stability through structural transformation of layered lithium manganese oxide

“…The partial high oxidation state gives rise to high electrochemical performance 40 . Further oxidation of the lattice oxygen leads to the formation of O-O bonds, which was directly observed in the RIXS and XAS spectra at the O-K edge for the stable O 2p holes 58 .…”

“…However, the sharp lower-energy peak (641.6 eV) at the Mn-L3 edge of Mn 4+ oxide is a sensitive fingerprint for the Mn 4+ content. The difference between HLM and Li 2 MnO 3 is most likely related to the removal of oxygen from the lattice upon the replacement of Li by less metallic H ions, which creates Jahn-Teller Mn 3+ ions40 . Figure2bpresents the normalized O Kedge XAS spectra of Li 2 MnO 3 and HLM.…”

Fast operando spectroscopy tracking in situ generation of rich defects in silver nanocrystals for highly selective electrochemical CO2 reduction

“…Meanwhile, extended X-ray absorption fine structure (EXAFS) spectra offer clues on the local coordinated environments from their oscillating intensity 28 , 34 , 35 . Although the operando XAS technique has exhibited advantages in studying catalysts for some electrocatalytic reactions, such as the oxygen evolution reaction 36 , 37 and hydrogen evolution reaction (HER) 38 , relevant reports on ECR are still limited. In addition, considering the fast transition process of catalysts during electrochemical reactions, which usually finish within several minutes 39 , the conventional XAS technique hardly meets the needs of operando investigation.…”

“…The conventional XAS may take tens of minutes to obtain a single spectrum, thus giving rise to an incorrect information on the crystal and electronic structure, namely an average of many different states of the evolving catalysts, and the reliability depending on the evolving times from materials to materials. Fast XAS technique with excellent time − resolution should be pursued to understand the mechanism and the formation process of active sites in catalysts during the electrochemical reaction, which is vital for the rational design of new catalysts 36 , 40 , 41 .…”

Tuning Reconstruction Level of Precatalysts to Design Advanced Oxygen Evolution Electrocatalysts

“…However, surface reconstruction engineering is a comprehensive and complex process, which requires the rational design of the precatalyst and precise control of the surface change process [ 7 , 63 , 88 ]. The parameters of phase/electronic/geometrical structures, oxidation/spin states, and the morphology should be systemically considered [ 53 , 79 , 89 , 90 , 91 , 92 , 93 ]. Furthermore, in situ/operando spectroscopic techniques are crucial to identify the actual active species, providing greater insights into the relationship between the structure and activity of the studied catalysts [ 45 , 75 , 82 , 87 , 94 , 95 , 96 , 97 , 98 ].…”