Protonation of Metal Oxides By Charge-Transfer-Protonation
Authors: Furukawa Taichi , Matsumoto Hiroshige 
Published: December 2024

Proton-conducting oxides are typically structured based on a perovskite-type arrangement in the formula of ABO3, which is modified by partially substituting a lower valence cation M than the original, resulting in AB1-xMxO3-δ. Such modification induces oxygen vacancies in the lattice due to electrical neutrality and is accompanied by the hydration of these vacancies to enable proton dissolution in the material.

In this approach, (i) hydration of oxygen vacancies and (ii) ambipolar diffusion of protons and oxide ions must occur simultaneously for complete material protonation. However, (i) necessitates a thermodynamically low temperature, whereas (ii) requires a kinetically high temperature. One reason why proton conductivity has been reported for only a limited class of materials may be attributed to this temperature dilemma. Expanding the working temperature range of proton conductors is crucial for practical applications. Currently, few materials can achieve sufficient proton conductivity in the 300-400°C temperature range. However, there might be examples of materials that fulfill (i) in this temperature range but face kinetic limitations in (ii).

Given the above considerations, we have explored an alternative charge-transfer protonation method for material protonation, which we experimentally investigated.

Charge Transfer Protonation (hereafter abbreviated as CTP) is a mechanism where a metal oxide is doped with a transition metal M, capable of multiple valences, and the material is protonated using the valence change of M. During material baking in air, M undergoes oxidation, transitioning to a higher valence state, corresponding to a stage in the conventional mechanism where (i) oxygen vacancy diffusion is complete. Subsequent exposure of the baked sample to hydrogen results in M reduction, facilitating (ii) proton dissolution. Since the high-temperature process (i) is completed during baking, CTP is anticipated to protonate materials resistant to conventional methods due to thermodynamic constraints and enable existing proton-conducting oxides to function at lower temperatures.

This report presents the results of an investigation utilizing SrZrO3 as a matrix and Mn as a dopant capable of valence change to elucidate the operation principle of the CTP mechanism.

Experimental:

SrZr0.9Mn0.1O3-δ was synthesized using the solid-state reaction method. Samples with varying porosities were prepared to assess the progress of the CTP mechanism, with porosity controlled by firing temperature. Protonation by this mechanism was investigated by measuring conductivity in Ar and 100% H2 at temperatures ranging from 200°C to 900°C and comparing the results using the AC impedance method.

Results and Discussion:

Conductivity measurements were conducted in Ar and H2 to assess whether the nearly dense SrZr0.9Mn0.1O3-δ sintered sample could be protonated via CTP. Higher conductivity was observed in H2 compared to Ar, except at high temperatures, indicating the potential protonation of SrZr0.9Mn0.1O3-δ by CTP. However, the conductivity in H2 was lower than anticipated. In response, we hypothesized that only the material surface was protonated and examined SrZr0.9Mn0.1O3-δ samples with different porosities. Results showed a greater disparity between Ar and H2 conductivity in samples with higher porosity, suggesting broader proton dissolution in porous materials. Proton dissolution primarily occurred at the interface between hydrogen gas and the sample, with significantly increased conductivity observed in porous materials, indicative of complete proton dissolution at the material surface.