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 MFeO (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 (T and O sites). 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.

Core‐Shell Structured NiCo2O4@FeOOH Nanowire Arrays as Bifunctional Electrocatalysts for Efficient Overall Water Splitting

“…The increased surface area, and strong chemical coupling at the NiCo 2 O 4 @FeOOH interface are considered to account for the improved catalytic activity. Moreover, Charge transfer between the catalyst and electrolyte was an important indicator to evaluate the catalytic properties . EIS was utilized to analyze the behavior of charge transfer at the interface or the surface.…”

Metal–Oxygen Hybridization Determined Activity in Spinel-Based Oxygen Evolution Catalysts: A Case Study of ZnFe2–xCrxO4

“…13 Yet, upon water oxidation, transition-metal oxides are under high anodic potential and thus their electronic properties 4 may dynamically change with the applied potential. [14][15][16][17][18] Currently, two popular methods are employed in OER studies. One is the operando investigation of surface change of oxides under OER conditions.…”

“…One is the operando investigation of surface change of oxides under OER conditions. [14][15][16] The other is the correlation between the electronic structures of oxides and their OER activities. 19 The latter has successfully revealed a few descriptors relevant with the electronic properties of TMOs, such as the number of d electrons, transition-metal e g filling, 8 metal-oxygen covalency, 21 O 2p-band center, 7 and etc.…”

π‐Conjugated Organic–Inorganic Hybrid Photoanodes: Revealing the Photochemical Behavior through In Situ X‐Ray Absorption Spectroscopy

“…In this work, several head-to-tail bis-coumarins were synthesized and their photophysical properties characterized. [3] Furthermore, with respectt ot he photoanode,t he light absorber in chargeo fc onverting the solar energy into chemical potential must be also carefully considered. The photoactivity of the photoanodes towards water oxidation was revealed to be governed by both the LUMO energy level and transfer efficiency of the photoinduced charge carriers.…”

“…2O 2 + 2H 2 O + 4e À ). [3] Furthermore, with respectt ot he photoanode,t he light absorber in chargeo fc onverting the solar energy into chemical potential must be also carefully considered. Several stable semiconductors comprised of abundant elements with wide bandgaps such as TiO 2 , [4] Fe 2 O 3 , [5] and BiVO 4 [6,7] have been fabricated as photoanodes for water oxidation.…”