Researchers Minimize Strain to Maximize Cell Performance

A recent study suggests that materials in electricity-producing fuel cells may be susceptible to strain.

According to scientists from Kyushu University, the strain caused by just a 2% decrease in the space between atoms when deposited on a surface results in a 99.999% reduction in the rate at which the materials undertake hydrogen ions, significantly lowering the effectiveness of solid oxide fuel cells.

The potential availability of high-performance fuel cells for generating clean energy in more homes will be assisted by developing solutions to reduce this strain.

Fuel cells use an electrolyte to move the ions created by severing hydrogen or oxygen molecules from one side of the device to the other. This enables them to produce electricity from hydrogen and oxygen while only discharging water as “waste.”

Even though the word “electrolyte” typically makes people think of liquids or sports drinks, they are also solids. Electrolytes based on ceramics and solid oxides — hard substances made of oxygen and other atoms — that transport positive hydrogen ions or protons are particularly interesting to scientists working on fuel cells.

These proton-conducting solid oxides may work in middle-temperature ranges of 300 to 600 °C, less than their oxygen-ion-conducting counterparts. They are also more robust than liquids and polymer membranes.

One key for good efficiency is to get the protons through the electrolyte to react with oxygen as quickly as possible.

Junji Hyodo, Study Author and Research Assistant Professor, Platform Of Inter/Transdisciplinary Energy Research, Kyushu University

“On paper, we have materials with great properties that should lead to excellent performance when used in solid oxide fuel cells, but the actual performance tends to be much lower,” Hyodo said.

Scientists have investigated what happens when the electrolyte and reaction-inducing electrodes interact.

“Properties of individual materials are often measured in a condition where they are free of influence from surrounding layers—what we call the bulk. However, when an oxide layer is grown on a surface, its atoms often have to readjust to accommodate the properties of the underlying surface, leading to differences from the bulk,” explains Hyodo.

The investigation focused on the barium, zirconium, yttrium, and oxygen atoms that make up the promising oxide known as BZY20. As the oxide develops, BYZ20 crystallizes into a similar cube-shaped structure frequently appearing on the surface.

The atoms on the cube's edges are 2% closer to the oxide-to-surface interface than in layers distant from the surface, according to their analysis of samples of varied thicknesses. This compressive strain causes the proton conductivity to drop to around 1/100,000 of its bulk sample value.

“A change of just 2%—from one meter to 98 cm on a large scale—might sound insignificant, but in a device where interactions happen on an atomic scale, it makes an enormous impact,” observes Yoshihiro Yamazaki, professor at Q-PIT and adviser on the study.

This compressive strain slowly decreases as the layers are added, and the cube finally hits its ideal size far from the interface. Away from the surface, conductivity may be high, but the harm has already been done.

The results of accounting for this decreased conductivity when estimating expected outcomes correspond with actual fuel cell performance results, showing that the strain is probably a factor in lowering performance.

“While we have good individual materials, maintaining their properties when combining them in a device is critical. In this case, we now know that strategies to reduce the strain where the oxide meets the electrode are needed,” says Yamazaki.

Journal Reference:

Hyodo, J., & Yamazaki, Y. (2022) Quantitative evaluation of biaxial compressive strain and its impact on proton conduction and diffusion in yttrium-doped barium zirconate epitaxial thin films. Journal of Physics: Energy. doi.org/10.1088/2515-7655/ac889e.