Application of In Situ Techniques for the Characterization of NiFe-Based Oxygen Evolution Reaction (OER) Electrocatalysts.

Developing high-efficiency and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a crucial bottleneck on the way to the practical applications of rechargeable energy storage technologies and water splitting for producing clean fuel (H2 ). In recent years, NiFe-based materials have proven to be excellent electrocatalysts for OER. Understanding the characteristics that affect OER activity and determining the OER mechanism are of vital importance for the development of OER electrocatalysts. Therefore, in situ characterization techniques performed under OER conditions are urgently needed to monitor the key intermediates together with identifying the OER active centers and phases. In this Minireview, recent advances regarding in situ techniques for the characterization of NiFe-based electrocatalysts are thoroughly summarized, including Raman spectroscopy, X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy, Mössbauer spectroscopy, Ultraviolet-visible spectroscopy, differential electrochemical mass spectrometry, and surface interrogation scanning electrochemical microscopy. The results from these in situ measurements not only reveal the structural transformation and the progressive oxidation of the catalytic species under OER conditions, but also disclose the crucial role of Ni and Fe during the OER. Finally, the need for developing new in situ techniques and theoretical investigations is discussed to better understand the OER mechanism and design promising OER electrocatalysts.