Understanding Proton Diffusion in Perovskite Oxides

Proton diffusion is a critical phenomenon in materials science, particularly in the context of fuel-cell applications. Recent studies have uncovered fascinating insights into how protons behave in perovskite-type oxides, revealing that while protons exhibit long-range diffusion, the oxygen atoms remain fixed in their crystalline positions. This disparity plays a significant role in the dynamics of proton transfer reactions, which are essential for the functioning of these materials.

Experiments and quantum molecular dynamics (MD) simulations have consistently pointed to fast rotational diffusion of protons, which occurs with a low activation barrier. This indicates that the process of proton transfer is often the rate-limiting step in many perovskite materials. Interestingly, infrared spectroscopy analyses have shown intense red-shifted OH-stretching vibrations, suggesting strong hydrogen bond interactions that facilitate rapid proton transfers instead of enabling quick reorientation processes.

The structural arrangement of the oxygen atoms in perovskite oxides is crucial for understanding proton mobility. Typically, the distance between oxygen atoms exceeds 290 pm, which limits the formation of strong hydrogen bonds that would otherwise enhance proton transfer. A delicate balance exists between the energy gained from hydrogen-bond formation and the energy required for lattice distortion necessary for these bonds to occur. This interplay affects the overall probability of proton transfer, with both short and longer oxygen separations yielding similar free energies.

A detailed reanalysis of quantum-MD simulations of cubic BaCeO3 has revealed that protonic defects can form short-lived hydrogen bonds with nearby oxygen neighbors, leading to slight reductions in hydroxyl-oxygen separations. This behavior suggests that while the hydrogen bonds stabilize the configuration, they are characterized by “soft” interactions that exhibit significant bond-length fluctuations, influencing the overall transfer dynamics.

Despite the apparent ease of proton diffusion indicated by low activation enthalpy estimates, actual mobility measurements in cubic perovskite oxides show activation enthalpies ranging from 0.4 to 0.6 eV. This discrepancy raises questions about the interactions that dictate the activation energy required for proton transfer, pointing to the complex role of the surrounding lattice and cation structures. Notably, the proton is often found outside the BO6 octahedron in a bent hydrogen bond configuration, which introduces a barrier to proton transfer due to repulsive interactions with the highly charged B-site cation.

As research continues to delve into these intricate mechanisms, the understanding of proton dynamics in perovskite materials offers promising avenues for enhancing the performance of fuel cells and other applications reliant on efficient proton conduction.