[Paper Review] Protons in lattice confinement: Static pressure on the Y-substituted, hydrated BaZrO3 ceramic proton conductor decreases proton mobility

Qianli Chen, Artur Braun+4 more|arXiv (Cornell University)|Jun 6, 2011

Advancements in Solid Oxide Fuel CellsMaterials Science18 references20 citations

TL;DR

This study investigates proton transport in Y-doped, hydrated BaZrO3 ceramic proton conductors under high static pressure (1–2 GPa). Using impedance spectroscopy, it finds that increasing pressure reduces proton mobility in the bulk lattice due to lattice compression, while grain boundary resistance decreases at higher pressures due to pressure-induced sintering, suggesting that lattice expansion enhances proton conductivity for applications in thin films.

ABSTRACT

Yttrium substituted BaZrO3, with nominal composition BaZr0.9Y0.1O3, a ceramic proton conductor, was subject to impedance spectroscopy for temperatures 300 K < T < 715 K at mechanical pressures 1 GPa < p < 2 GPa. The activation energies Ea of bulk and grain boundary conductivity from two perovskites synthesized by solid-state reaction and sol-gel method were determined under high pressures. At high temperature, the bulk activation energy increases with pressure by 5% for sol-gel derived sample and by 40% for solid-state derived sample. For the sample prepared by solid-state reaction, there is a large gap of 0.17 eV between the activation energy at 1.0 GPa and > 1.2 GPa. The grain boundary activation energy is around a factor two times as that of the bulk, and it reaches a maximum at 1.25 - 1.5 GPa, and then decrease as the pressure increases, indicating higher proton mobility in the grain boundaries at higher pressure. Since this effect is not reversible, it is suggested that the grain boundary resistance decreases as a result of pressure induced sintering. The steady increase of the bulk resistivity upon pressurizing suggests that the proton mobility depends on the space available in the lattice. In return, an expanded lattice with a/a0 > 1 should thus have a lower activation energy, suggesting that thin films expansive tensile strain could have a larger proton conductivity with desirable properties for applications.

Motivation & Objective

• To understand how static pressure affects proton conductivity in Y-substituted, hydrated BaZrO3 ceramic proton conductors.
• To determine the influence of lattice confinement on proton migration energy barriers in bulk and grain boundary regions.
• To compare the pressure-dependent behavior of proton conductivity in sol-gel and solid-state synthesized BaZrO3 samples.
• To explore the implications of lattice strain on proton conductivity for thin-film applications.

Proposed method

• Impedance spectroscopy was performed on Y-doped BaZrO3 samples across temperatures from 300 K to 715 K under static pressures of 1–2 GPa.
• Two synthesis methods—solid-state reaction and sol-gel—were used to compare microstructural effects on proton transport.
• Activation energies for bulk and grain boundary conductivity were extracted from Arrhenius plots of conductivity data under pressure.
• Pressure-induced structural changes were inferred from shifts in activation energy and resistivity trends.
• The reversibility of resistance changes was tested to assess permanent structural modifications.

Experimental results

Research questions

• RQ1 How does increasing static pressure affect the activation energy of proton conduction in the bulk lattice of Y-doped BaZrO3?
• RQ2 What is the pressure dependence of grain boundary proton conductivity, and does it exhibit reversible or irreversible behavior?
• RQ3 How do different synthesis methods (sol-gel vs. solid-state) influence the pressure response of proton mobility?
• RQ4 Can lattice expansion reduce activation energy and enhance proton conductivity in proton conductors?

Key findings

• The bulk activation energy increased by 5% under pressure for the sol-gel derived sample and by 40% for the solid-state sample, indicating reduced proton mobility with lattice compression.
• For the solid-state sample, a sharp 0.17 eV increase in activation energy occurred between 1.0 GPa and 1.2 GPa, suggesting a critical structural transition.
• Grain boundary activation energy peaked at 1.25–1.5 GPa and then decreased with further pressure, indicating improved proton mobility due to pressure-induced sintering.
• The resistance decrease in grain boundaries was irreversible, confirming permanent microstructural changes from pressure application.
• The steady rise in bulk resistivity with pressure indicates that proton mobility is limited by available lattice space, implying that expanded lattices (e.g., under tensile strain) could enhance conductivity.