Stability and Catalytic Performance of Reconstructed Fe₃O₄(001) and Fe₃O₄(110) Surfaces during Oxygen Evolution Reaction

Abstract

Earth-abundant oxides are promising candidates as effective and low-cost catalysts for the oxygen evolution reaction (OER) in alkaline media, which remains one of the bottlenecks in electrolysis and artificial photosynthesis. A fundamental understanding of the atomic-scale reaction mechanism during OER could drive further progress, but a stable model system has yet to be provided. Here we show that Fe3O4 single crystal surfaces, prepared in ultrahigh vacuum (UHV) are stable in alkaline electrolytein the range pH 7–14 and under OER conditions in 1 M NaOH. Fe3O4(001) and Fe3O4(110) surfaces were studied with X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy in UHV, and atomic force microscopy in air. Fe3O4(110) is found to be more reactive for oxidative water splitting than (001)-oriented magnetite samples. Magnetite is electrically conductive, and the structure and properties of its major facets are well understood in UHV. With these newly obtained results, we propose magnetite (Fe3O4) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and under highly oxidizing conditions.

Electrochemical Stability of the Reconstructed Fe3O4(001) Surface

“…The Fe 3 O 4 (001) CTRs exhibit minor changes during the lifting of the reconstruction, suggesting that the latter is a pure surface phase transition and does not involve substantial restructuring or roughening of the surface. This is in agreement with recent ex situ AFM observations of Fe 3 O 4 (001) immersed in aqueous electrolyte solution [4] . However, pronounced irreversible roughening occurs after changing the potential to even more negative values of ≤−1.20 V, where reduction of the Fe oxide to Fe 0 commences (Figure S5) [7]…”

“…While the structure of Fe 3 O 4 (001) is well known in UHV, it is not clear whether the surface changes in aqueous solution. Atomic force microscopy (AFM) studies have shown that the nanoscale morphological appearance of Fe 3 O 4 (001) is maintained in an electrochemical environment, [4] but recent work suggests the reconstruction is lifted by exposure to 6 mbar water vapor, as well as by pure liquid water at pH 7 [5] . This is in line with previous surface X‐ray diffraction measurements, which also found the reconstruction to be lifted by water vapor [3] .…”

Recent developments of nanocarbon based supports for PEMFCs electrocatalysts

“…Nanocarbon materials used as catalyst supports often need to be optimized, such as by adjusting the pore size, doping heteroatoms, constructing defect sites, and others [227]. Additionally, different nanocarbon materials have different characteristics, so a composite carbon material combining two or more nanocarbon materials as a catalyst support [11,[228][229][230] may complement each other with advantages, thereby exhibiting better performance. Compared with the classical metal-supported catalyst model, the metal-organic framework (MOF)-based catalysts have better structure and chemical property controllability and their huge potential cannot be ignored [231].…”

Electrochemical Stability of the Reconstructed Fe3O4(001) Surface

“…[2] While the structure of Fe 3 O 4 (001) is well known in UHV, it is not clear whether the surface changes in aqueous solution. Atomic force microscopy (AFM) studies have shown that the nanoscale morphological appearance of Fe 3 O 4 (001) is maintained in an electrochemical environment, [4] but recent work suggests the reconstruction is lifted by exposure to 6 mbar water vapor, as well as by pure liquid water at pH 7. [5] This is in line with previous surface X-ray diffraction measurements, which also found the reconstruction to be lifted by water vapor.…”

“…This is in agreement with recent ex situ AFM observations of Fe 3 O 4 (001) immersed in aqueous electrolyte solution. [4] However, pronounced irreversible roughening occurs after changing the potential to even more negative values of À1.20 V, where reduction of the Fe oxide to Fe 0 commences (Figure S5). [7] In contrast to the behavior at negative potentials, the reconstruction is remarkably stable at potentials where oxygen evolution commences.…”