Band Gap Narrowing in a High-Entropy Spinel Oxide Semiconductor for Enhanced Oxygen Evolution Catalysis

Area of Science:

• Materials Science
• Catalysis
• Solid-State Chemistry

Background:

• High-entropy oxides (HEOs) feature five or more randomly mixed metal cations in a crystalline lattice.
• These materials can display unique, enhanced properties due to synergistic effects, particularly in catalysis.
• Spinel oxides are a class of materials with diverse applications.

Purpose of the Study:

• To investigate band gap narrowing in a high-entropy aluminate spinel oxide, (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 (A5Al2O4).
• To explore the catalytic performance of A5Al2O4 for the oxygen evolution reaction (OER).
• To understand the electronic structure origins of the observed properties and catalytic behavior.

Main Methods:

• Synthesis and characterization of the high-entropy aluminate spinel oxide A5Al2O4.
• First-principles calculations to analyze electronic band structure and density of states.
• Electrochemical testing for oxygen evolution reaction (OER) activity and stability.
• Post-reaction analysis using elemental analysis and grazing-incidence X-ray diffraction (GIXRD).

Main Results:

• A5Al2O4 exhibited a significantly narrowed band gap of 0.9 eV, smaller than its parent spinel oxides.
• First-principles calculations attributed band gap narrowing to broadened 3d state energy distributions.
• A5Al2O4 demonstrated superior OER catalytic activity, reaching 10 mA/cm² at 400 mV overpotential.
• Catalyst deactivation after 5 hours was linked to the formation of a passivating surface layer.

Conclusions:

• Band gap narrowing in A5Al2O4 is linked to the high-entropy composition and electronic structure modifications.
• The high-entropy oxide acts as an effective catalyst for the oxygen evolution reaction, surpassing single-component oxides.
• Engineering electronic structure via high-entropy approaches offers a pathway to enhance catalytic properties in multicomponent oxides.