Hard X-ray Photoelectron Spectroscopy Probing Fe Segregation during the Oxygen Evolution Reaction

Area of Science:

• Electrochemistry
• Materials Science
• Surface Science

Background:

• Nickel-Iron (NiFe) electrocatalysts are highly active for alkaline oxygen evolution reaction (OER) in water splitting.
• Understanding the Ni-Fe interplay and surface phenomena is critical for designing efficient water-splitting electrodes.
• Electrochemical reactions induce complex surface changes like oxide formation and species segregation, hindering rational design.

Purpose of the Study:

• To develop and apply a method for quantifying chemical depth profiling using XPS/HAXPES.
• To investigate the surface reconstruction of NiFe electrodes with varying porosities during OER.
• To elucidate the role of porosity in iron segregation and its impact on electrocatalyst performance.

Main Methods:

• Quantitative chemical depth profiling using X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAXPES).
• Application of the developed method to two NiFe electrodes with distinct porosities.
• In-situ analysis of surface reconstruction during electrochemical oxygen evolution reaction.

Main Results:

• Iron (Fe) segregates to the surface under ambient conditions, forming an inactive FeO phase.
• Catalyst porosity significantly influences the Fe segregation process and electrode performance.
• Higher porosity in nanostructured samples leads to increased Fe diffusion and suppression of the active NiFe-oxyhydroxide phase.

Conclusions:

• Surface chemistry of multielement systems is dynamic and depends on applied potential, electrolyte, and bulk properties.
• Porosity is a critical factor in NiFe electrocatalyst design, affecting Fe surface segregation and OER activity.
• HAXPES provides crucial insights into subsurface properties influencing surface behavior.