title: Valence- and element-dependent water oxidation behaviors: in situ X-ray diffraction, absorption and electrochemical impedance spectroscopies.
abstract: Metal oxides of the spinel family have shown great potential towards the oxygen evolution reaction (OER), but the fundamental OER mechanism of spinel oxides is still far from being completely understood, especially for the role of the metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface conditions (morphology and defects), it is a great challenge to have a fair assessment of the electrocatalytic performance of spinel systems. Herein, we demonstrated a series of MFe2O4(M = Fe, Co, Ni, Zn) with a well-controlled morphology to achieve a comprehensive study of electrocatalytic activity toward OER. By utilizing several in situ analyses, we could conclude a universal rule that the activities for OER in the metal oxide systems were determined by the occurrence of a phase transformation, and this structural transformation could work well in both crystallographic sites (Tdand Ohsites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship, which depended on the atomic arrangement at the interface of spinel and metal oxyhydroxide, while trivalent metal ions remained unchanged as a host lattice. The metal oxyhydroxide was formed during a redox reaction rather than being formed during OER. The occurrence of the redox reaction seems to accompany a remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believe that the approaching strategies and information obtained in the present study can offer a guide to designing a promising electrocatalytic system towards the oxygen evolution reaction and other fields.
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. haomingchen@ntu.edu.tw.
authors.forename: Chia-Shuo
authors.identifier: http://orcid.org/0000-0002-7767-8413
authors.initials: CS
authors.lastname: Hsu
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. haomingchen@ntu.edu.tw.
authors.forename: Nian-Tzu
authors.initials: NT
authors.lastname: Suen
authors.affiliation: Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, Hsinchu 300, Taiwan.
authors.forename: Ying-Ya
authors.initials: YY
authors.lastname: Hsu
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. haomingchen@ntu.edu.tw.
authors.forename: Hsuan-Yu
authors.initials: HY
authors.lastname: Lin
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. haomingchen@ntu.edu.tw.
authors.forename: Ching-Wei
authors.initials: CW
authors.lastname: Tung
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan.
authors.forename: Yen-Fa
authors.initials: YF
authors.lastname: Liao
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan.
authors.forename: Ting-Shan
authors.initials: TS
authors.lastname: Chan
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan.
authors.forename: Hwo-Shuenn
authors.initials: HS
authors.lastname: Sheu
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan.
authors.forename: San-Yuan
authors.initials: SY
authors.lastname: Chen
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. haomingchen@ntu.edu.tw.
authors.forename: Hao Ming
authors.initials: HM
authors.lastname: Chen