From NiMoO₄ to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Water electrolysis using renewable energy is key for sustainable fuel production.
• Catalyst stability and active phase identification are crucial for efficient oxygen evolution reaction (OER).
• Nickel Molybdenum Oxide (NiMoO4) is a promising noble metal-free OER catalyst, but its dynamic transformations require investigation.

Purpose of the Study:

• To investigate the phase transformation of NiMoO4 during water electrolysis.
• To understand the role of molybdenum leaching and nanostructure morphology on catalytic activity.
• To elucidate the active species and structural dynamics of NiMoO4 catalysts under operando conditions.

Main Methods:

• Time-resolved operando Raman spectroscopy to monitor phase transformations.
• Electrolyte elemental analysis to quantify molybdenum leaching.
• Synthesis of NiMoO4 with varying nanostructures (nanoflowers, nanorods) by controlling heating ramps.
• Selective molybdenum etching to assign structural features.

Main Results:

• NiMoO4·H2O transforms into γ-NiOOH under applied bias during water electrolysis.
• Molybdenum leaching increases exposed nickel sites and promotes γ-NiOOH formation, enhancing catalytic activity.
• Different NiMoO4 nanostructures exhibit distinct stabilities in alkaline media.
• Operando Raman spectroscopy successfully tracked potential-dependent phase transformations and structural dynamics.

Conclusions:

• The dynamic transformation of NiMoO4 to γ-NiOOH, coupled with molybdenum leaching, is responsible for high catalytic activity in water electrolysis.
• Controlling nanostructure morphology influences catalyst stability and performance.
• The presented methodology using operando spectroscopy and selective etching is valuable for characterizing other dynamic catalyst systems.