Oxygen Evolution on Mechanically Strained TiO₂/NiTi: Implications of Compositional Heterogeneity at...

Area of Science:

• Materials Science
• Electrochemistry
• Surface Science

Background:

• Catalyst performance in (photo)electrochemical applications depends on surface properties.
• Interpreting performance is challenging without interface-sensitive compositional characterization.
• Near-surface compositional heterogeneity can lead to misinterpretations of bulk or near-surface material properties.

Purpose of the Study:

• Investigate the role of near-surface compositional heterogeneity in alkaline oxygen evolution reaction (OER) activity.
• Highlight challenges in correlating composition from surface- and near-surface-sensitive probes.
• Understand how mechanical strain influences Ni site density and OER kinetics in TiO2/NiTi.

Main Methods:

• Fabrication of TiO2 thin films on Nitinol (NiTi) via air-annealing.
• Mechanical straining of TiO2/NiTi films to induce cracking and expose Ni sites.
• (Photo)electrochemical measurements to assess OER activity, overpotentials, Tafel slopes, and fill factors.
• Surface- and near-surface-sensitive techniques (XPS, TOF-SIMS, STEM-EDS) for compositional analysis.

Main Results:

• Tensile strain on TiO2/NiTi causes cracking, increasing electrochemically active Ni site density.
• (Photo)electrochemical OER kinetics (overpotential, Tafel slope) improve with increasing Ni site density below 10^13 Ni/cm^2.
• Near-surface Ni site densities measured by XPS, TOF-SIMS, and STEM-EDS are two orders of magnitude higher than those from electrochemical measurements on unstrained samples.
• Photoelectrochemical fill factors show similar trends with Ni site density.

Conclusions:

• Near-surface compositional heterogeneity significantly complicates the correlation between measured composition and (photo)electrochemical performance.
• Accurate interpretation requires self-consistent analysis of multiple characterization techniques.
• Fundamental studies with controlled composition and structure are crucial for understanding electrochemical interfaces.