Role of Chloride Ions in the Heat Treatment of β-FeOOH(Cl) Catalysts to Enhance the Oxygen Evolution Reaction Activity

Abstract

Fe-based catalysts are ideal for the oxygen evolution reaction (OER) owing to their earth abundance. However, most of these catalysts are usable only in an alkaline solution because they dissolve at low pH. Therefore, to be sufficiently versatile for large-scale energy conversion and storage devices, they must maintain high activity over a wide pH range. Cl^–-stabilized Fe oxyhydroxide, i.e., hollandite-type β-FeOOH(Cl), nanorods have been reported to function in a (nearly) neutral solution with a relatively high activity, which was further improved by heat treatment at ∼200 °C; however, the mechanism underlying this enhancement was poorly understood because of the complicated transformation of β-FeOOH(Cl) to α-Fe₂O₃ during heating. Therefore, we investigated the structural changes in heat-treated β-FeOOH(Cl) using synchrotron-based X-ray diffraction measurements in addition to several spectroscopic methods and compositional analyses. Structural and compositional analyses, including Rietveld methods, revealed that β-FeOOH(Cl) underwent an indirect phase transformation via a Cl^–-enriched β-FeOOH(Cl) intermediate phase at 200 °C before α-Fe₂O₃ formed. The Cl/Fe ratio of the intermediate phase (0.19) was almost twice that of the initial phase (0.08); furthermore, operando X-ray absorption spectroscopy clarified that the Cl^–-enriched β-FeOOH(Cl) phase was stable during the OER even at a high current density of ∼22 mA cm^–2. First-principles calculations using β-FeOOH(Cl) supercell models with Cl/Fe = 0.125 and 0.25 revealed that enrichment with Cl^– ions increased Cl^– diffusivity along the tunnel direction in hollandite, notably increasing the electrical conductivity and diffusion coefficient. Both of these properties could enhance the OER activity of β-FeOOH(Cl) catalysts, in addition to previously proposed structural strain. This finding suggests a structure–activity relationship that can serve as a guideline for broadening the use of β-FeOOH(Cl) OER catalysts.