The Swiss Federal Laboratories for Materials Science and Technology (Empa) is leading an international research project on ceramic proton conductors. The team aims to clarify the fundamentals of proton transport in special ceramics using new methods. The Swiss National Science Foundation (SNSF) supports the project under the "External Lead Agency" procedure.
The membranes in fuel cells and electrolysers significantly determine how efficiently the devices operate. Special ceramics are often used for this purpose. According to Empa, how the behaviour of protons in such ceramic membranes is related to their material structure is not yet fully understood. This question is at the centre of the research project, which Artur Braun, group leader in the High-Performance Ceramics Laboratory at Empa, is working on together with international partners.
Two conflicting requirements for proton conductors
Braun and his team have been researching ceramic proton conductors for about 20 years. Good conductors must meet two requirements: They should absorb as many protons as possible, and these should be able to move as freely as possible within the material. Both properties are in conflict. If one is optimised, the other usually deteriorates.
Braun illustrates the problem with an image: "One can imagine the ceramic membrane as a landscape crisscrossed by roads. The protons move on these roads like cars." The more cars there are, the more traffic jams occur. "A multi-lane proton highway, where charges flow unhindered in both directions, is a wishful thinking," says Braun. The currently common proton membranes resemble rather impassable terrain that protons have to traverse via narrow paths.
Dynamic crystal lattices with variable conductivity
Unlike natural landscapes, the crystal lattices of ceramic membranes are dynamic. In a previous work, Braun's team showed that proton conductivity in well-conducting ceramics fluctuates. Sometimes it is lower, sometimes it reaches peak values. "It's as if the mountains and valleys suddenly level out. The protons get, so to speak, free passage for a short moment," explains Braun. Proton transport in ceramic conductors is closely linked to lattice vibrations of the crystal lattice. Embedded protons can change these vibrations themselves.
Poor conductor as a model system
In the current project, the researchers are specifically investigating a poor proton conductor: lanthanum cerium oxide (LCO). The material can absorb many protons but hardly conducts them further. "If a poor proton conductor is comparable to a traffic jam, LCO is a veritable traffic infarction," says Braun. This very disadvantage offers the team the opportunity to specifically investigate the effects of material structure on proton conductivity.
For the analyses, the researchers use high-resolution crystallographic structure elucidation with neutrons at the Paul Scherrer Institute (PSI). Additionally, they use new methods at large research facilities in the USA and Japan to precisely measure lattice vibrations and experimentally verify theoretical models. "If we understand the scientific fundamentals of proton conductivity, we can ideally say how future ceramics can become better proton conductors, thus enabling more efficient membranes for fuel cells and electrolysers," says Braun.
