Reversible valence states with irreversible crystal structures of NiFe layered double hydroxide catalysts: surface stability during the oxygen evolution reaction

Abstract

Stability is crucial in catalysis, and contrasting results have been reported for the most active platinum-group-metal free oxygen evolution reaction (OER) catalyst, nickel–iron layered double hydroxide (NiFe-LDH). In addition, different active valence states have been reported. In this study, stable valence/chemical states on the surface of stable NiFe-LDH were investigated using a combination of in situ visible Raman spectroscopy, ex situ visible/ultraviolet (UV) Raman spectroscopy, and ex situ X-ray photoelectron spectroscopy (XPS). A recently reported coprecipitation method for preparing stable NiFe-LDH without doping or intercalation of specific anions was applied to nickel foam (NF) in this study, and surface stability was investigated using chronopotentiometry (CP) for 24 h. The surface of NiFe-LDH was converted from hydroxide into oxyhydroxide, but its initial stable valence states, Ni²⁺ and Fe³⁺, were retained after a stable operation period.