Operando Observations of a Manganese Oxide Electrocatalyst for Water Oxidation Using Hard/Tender/Soft X-ray Absorption Spectroscopy

Abstract

A layered manganese oxide catalyst containing K+ cations [K/MnO x catalyst] was developed, and its ability to efficiently decompose water was demonstrated. Operando hard/tender/soft X-ray absorption fine structure (XAFS) techniques were used to investigate the function of the K/MnO x catalyst under working conditions. The Mn valency depended on the potential and the amount of K+ cation adsorption. Mn K-edge (hard X-ray) XAFS measurements for the K/MnO x catalyst suggested that the Mn in the catalyst was an Mn3+ species with an octahedral δ-MnO2 structure at a lower electrode potential, which changed reversibly to an Mn4+ species with a δ-MnO2 structure at higher potential during oxygen evolution. A similar result was obtained from operando O K-edge (soft X-ray) XAFS. The chemical state of K species was analyzed using operando K K-edge (tender X-ray) XAFS, which indicated that K+ cations were intercalated with hydrated states in the δ-MnO2 layers. These operando XAFS results demonstrated that the layered δ-MnO2 containing hydrated K+ cations functioned as efficient oxygen evolution electrocatalysts because of the presence of Mn3+ reaction sites.

Progress of Nonprecious‐Metal‐Based Electrocatalysts for Oxygen Evolution in Acidic Media

“…Moreover, Sargent's group found that higher OER activity was closely related to the readier oxidation transition of 3d metals in NiFe-and FeCo-based oxyhydroxide catalysts by in situ soft and hard X-ray absorption spectroscopies. [130] Recently, Mn 3+ sites in layered d-MnO 2 were identified as the reaction sites for OER using operando X-ray absorption spectroscopy, [131] agreeing with previous conclusion deduced by kinetics analysis. [16] However, most of the current in situ techniques applied focus on the Ir-and Ru-based catalysts in acidic OER condition or nonprecious-metal-based catalysts under alkaline conditions.…”

Activating Lattice Oxygen in Layered Lithium Oxides through Cation Vacancies for Enhanced Urea Electrolysis

“…The magnitude of EXAFS‐FT spectra and Ni K‐edge EXAFS fitting confirm the Ni−O bonds in the first coordination field and the Ni−Ni bonds in the second coordination field (Figure S12–S14). As displayed in Figure 1f, the Ni−O peak moves slightly to the left after delithiation treatment, which suggests that those catalysts of the average Ni−O length decrease compared to the LNO‐0 [15] . In the Fourier transformation (FT) spectra (Figure 1g), the Ni−Ni peak (2.54 Å) for LNO‐0 with increased intensity indicates distorted NiO 6 octahedrons without the Jahn–Teller effect.…”

Selective Catalyst for Oxygen Evolution in Neutral Brine Electrolysis: An Oxygen-Deficient Manganese Oxide Film

“…Analysis by extended X-ray absorption fine structure (EXAFS) is a powerful tool to decide the local structures of Mn catalysts. − In Fourier transforms of k 3 -weighted EXAFS oscillations (Figure ), the peaks of Mn–O and Mn–Mn for Na|MnO 2 , Na|MnO x -400, Mg|MnO 2 , and Mg|MnO x -400 were observed at the same positions as the δ-MnO 2 reference, indicating that the local structure of Mn species remained as birnessite-type even after heating. On the other hand, the heat treatments caused the decrease of Mn–Mn scattering intensity for both Na|MnO 2 and Mg|MnO 2 catalysts, meaning that the long-range structures for both catalysts were distorted by annealing.…”