Apparent Oxygen Uphill Diffusion in La_0.8Sr_0.2MnO₃ Thin Films upon Cathodic Polarization

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Lanthanum strontium manganese oxide (La_0.8Sr_0.2MnO₃ or LSM) is a key material in solid oxide fuel cells.
• Understanding oxygen transport mechanisms in LSM thin films is crucial for optimizing device performance.
• Cathodic bias can influence ionic transport and surface exchange kinetics in electrode materials.

Purpose of the Study:

• To investigate the effect of cathodic bias on oxygen transport in LSM thin films.
• To analyze the impact of microstructure on bias-induced oxygen transport.
• To elucidate the mechanisms governing oxygen incorporation under applied voltage.

Main Methods:

• Pulsed laser deposition (PLD) for fabricating columnar-grown LSM thin films with varying microstructures.
• ¹⁸O tracer diffusion experiments on thin film microelectrodes under applied cathodic bias (-300 or -450 mV).
• Time-of-flight secondary ion mass spectrometry (TOF-SIMS) for analyzing ¹⁸O concentration profiles.

Main Results:

• Cathodic polarization markedly increased ¹⁸O concentration in LSM microelectrodes compared to unbiased films.
• A distinct ¹⁸O fraction maximum was observed in the center of cathodically polarized films.
• The observed ¹⁸O depth profiles were strongly dependent on the applied bias and LSM microstructure.

Conclusions:

• Cathodic bias significantly alters oxygen transport in LSM thin films, leading to enhanced incorporation.
• The observed ¹⁸O distribution is attributed to a combination of bulk polarization, grain boundary diffusion, and voltage-modified incorporation kinetics.
• Microstructure plays a critical role in mediating the effects of cathodic bias on oxygen transport in LSM.