The University of Michigan - Shanghai Jiao Tong University Joint Institute (UM-SJTU JI,) Professor Qianli Chen and her collaborators are proposing a new design principle for perovskite materials with high proton conductivity for use as electrolyte materials in solid oxide fuel cells. A paper on their work is published in the journal Advanced Energy Materials.

Solid oxide fuel cells are electrochemical devices that directly converts fuels (e.g. hydrogen, natural gas) from chemical energy into electricity, with advantages of high energy conversion efficiency, cleanliness and environmental friendliness. However, the current working temperature of solid oxide fuel cells is generally high (700-1000°C), imposing strict requirements on the high-temperature durability of all component materials for the device.

When proton-conducting ceramics are used as fuel cell electrolyte material, the operating temperature can be reduced to 450-700°C, thus significantly reducing the production cost. However, their proton conductivity needs to be further improved for the commercialization of such intermediate-temperature fuel cells.

The researchers suggested that the enhancement of proton conductivity at low temperatures can be achieved by tuning lattice vibration frequency toward a desired isokinetic temperature.

The diffusion of protons needs to overcome an energy barrier, called activation energy. In general, to increase the proton conductivity, the activation energy is expected to be reduced. The authors found that the proton conductivity follows the Meyer-Neldel rule (MNR) on the dynamics of atomic diffusion in condensed matter. When the activation energy decreases, the pre-factor in the conductivity equation decreases correspondingly, thus suppressing the enhancement of conductivity.

The authors discovered that when tuning the activation energy by changing the material structure, the conductivities with different activation energies intersect at an isokinetic temperature, where the proton conductivity is independent of activation energy, but only depends on the intrinsic properties of the material.

From the relationship between isokinetic temperature and material structure, the authors proposed that enhancement of proton conductivity at lower temperatures can be well achieved by modulating material structure toward a desired isokinetic temperature.

Perovskite-type metal oxides such as Y-doped BaMO₃ (M = Zr/Ce) have drawn considerable attention as proton-conducting electrolytes for intermediate temperature ceramic electrochemical cells. Improving the proton conductivity at lower temperatures requires a comprehensive understanding of the proton conduction mechanism. By applying high pressure or varying the Ce content of Y-doped BaMO₃, it is demonstrated that the proton conductivity follows the Meyer–Neldel rule (MNR) well. In the Arrhenius plot, the conductivities intersect at an isokinetic temperature, where the proton conductivity is independent of activation energy. Considering the relationship between isokinetic temperature and lattice vibration frequency, a high isokinetic temperature is observed in materials with stiff lattices, consisting of light atoms and small M—O bond length. Based on consideration of the MNR, it is suggested that the enhancement of proton conductivity at low temperature can be well achieved by tuning lattice vibration frequency toward a desired isokinetic temperature.

—Du et al.

By revealing the relationship between lattice vibration and proton conductivity, the authors suggest a design principle for new perovskite materials with high proton conductivity.

Resources

• Peng Du, Nana Li, Xiao Ling, Zhijun Fan, Artur Braun, Wenge Yang, Qianli Chen, Arthur Yelon (2021) “Optimizing the Proton Conductivity with the Isokinetic Temperature in Perovskite-Type Proton Conductors According to Meyer–Neldel Rule” Advanced Energy Materials doi: 10.1002/aenm.202102939