Hydrogen is a sustainable energy carrier. 'Green' hydrogen, produced using renewable energy, can be used as an energy storage and fuel source. To make its production and usage more efficient, durable, and cost-effective, new materials are required. The membranes used in fuel cells and electrolysers are largely responsible for how efficiently the devices operate. These membranes are often made from special ceramics.
The exact relationship between the behaviour of hydrogen ions - protons - in ceramic membranes and their material structure is not yet fully understood. In a project funded by the Swiss National Science Foundation (SNSF), Empa researchers under the direction of Artur Braun, group leader in the Laboratory for High-Performance Ceramics, together with international research partners, aim to explore this question.
Braun and his team have been working with ceramic proton conductors for 20 years. Good proton conductors must meet two main requirements: they must be able to absorb as many protons as possible, and the protons should move as freely as possible within the material. However, these two properties often contradict each other: improving one usually worsens the other.
Uneven terrain instead of free passage
'You can imagine the ceramic membrane as a landscape traversed by roads. The protons move on these roads like cars,' compares Braun. The more cars on the roads, the more traffic congestion there is. 'A multi-lane proton highway, where charges can flow unhindered in both directions, is a utopian vision,' he elaborates. However, current proton membranes resemble uneven terrain, through which protons wearily traverse narrow paths.
Unlike familiar landscapes, the crystal lattices of ceramic membranes are dynamic - in other words, changeable. In a previous study, Braun's team was able to show that the proton conductivity in well-conducting ceramics does not always remain constant but sometimes reaches lower levels, while at other times peaks. 'It's as if the mountains and valleys suddenly level out. The protons get a moment of free passage,' explains Braun. The Empa team was also able to demonstrate that proton transport in ceramic conductors is closely linked to lattice vibrations of the crystal lattice - and that protons embedded in the lattice can themselves alter its vibrations.
The answers in the crystal lattice
To better understand this effect, the researchers in their current project are not focusing on a good proton conductor, but an inferior one. Lanthanum cerium oxide (LCO) can absorb many protons but does not conduct them further. 'If a poor proton conductor is comparable to a traffic jam, LCO represents a genuine traffic gridlock,' says Braun.
However, this apparent disadvantage offers the researchers a unique opportunity to investigate the impacts of material structure on proton conductivity. Alongside high-resolution crystallographic structural elucidation with neutrons at the Paul Scherrer Institute (PSI), the team is using new methods at large research facilities in the USA and Japan to precisely measure the lattice vibrations of crystal lattices and experimentally test the theoretical models. 'If we understand the scientific foundations of proton conductivity, we can ideally specify how future ceramics can become better proton conductors, thereby enabling more efficient membranes for fuel cells and electrolysers,' the physicist explains.
Contact:
Dr. Artur Braun
High-Performance Ceramics
Tel. +41 58 765 48 50
artur.braun@empa.ch