γ-FeO(OH) with multiple surface terminations: Intrinsically active for the electrocatalytic oxygen evolution reaction

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Independently prepared iron oxy-hydroxide (FeO(OH)) exhibits poor conductivity and inferior electrocatalytic performance in the oxygen evolution reaction (OER).
• In situ derived FeO(OH) and incorporation of Co/Ni metals enhance OER activity, but phase-pure, active FeO(OH) with good electrochemical performance remains challenging.
• The influence of crystalline phase and surface structure on OER activity is not fully understood.

Purpose of the Study:

• To develop a simple synthetic protocol for phase-pure α-FeO(OH) (goethite) and γ-FeO(OH) (lepidocrocite) materials.
• To investigate the effect of phase and crystallinity on the OER activity of FeO(OH) materials.
• To establish a relationship between surface functionality and electrochemical activity in FeO(OH) for OER.

Main Methods:

• Synthesis of phase-pure α- and γ-FeO(OH) with varied crystallinity by adjusting reaction conditions (iron salt, temperature).
• Deposition of FeO(OH) materials on nickel foam (NF) for alkaline OER studies.
• Electrochemical characterization including overpotential, current density, Tafel slope, and charge transfer resistance (Rct); microscopic characterization.

Main Results:

• Partially crystalline γ-FeO(OH) isolated at room temperature (γ-FeO(OH)@RT) demonstrated the highest activity, with a low overpotential of 260 mV at 10 mA cm⁻² and 12 h stability.
• The γ-FeO(OH)@RT/NF anode achieved high current densities (50-100 mA cm⁻²) and facilitated facile electrokinetics, indicated by small Tafel slope and Rct.
• This active anode was incorporated into a water-splitting electrolyzer operating at 1.68 V, with polycrystallinity and abundant surface active sites (-O, -OH, -Fe) correlating to superior activity.

Conclusions:

• A simple synthesis method yields phase-pure α- and γ-FeO(OH) with tunable crystallinity.
• Partially crystalline γ-FeO(OH)@RT exhibits exceptional OER activity and stability, outperforming previously reported FeO(OH) materials.
• Polycrystallinity, increased defect sites, large exposed surface area, and diverse surface functional groups are key to the enhanced electrochemical performance of γ-FeO(OH)@RT.