Fuel cell reactions exposed


Rebecca Pool

Monday, July 29, 2019 - 18:15
Image: Simulations reveal a previously unreported reaction path for fuel cell reactions.
Using atomic resolution electron microscopy and computer modelling, Japan-based researchers have shed new light onto the reactions that take place in solid-oxide fuel cells.
Realistic atomic-scale models of the active site at the electrode, based on microscope observations, are set to provide new clues to boost performance and durability in future devices.
As Michihisa Koyama, research group leader at Kyushu University’s INAMORI Frontier Research Center highlights, detailed knowledge about the reactions occurring at the electrodes within the devices is vital for further improving the performance and durability of fuel cells.
“Computer simulations have played a powerful role in predicting and understanding reactions that we cannot easily observe on the atomic or molecular scale,” he says. “However, most studies have assumed simplified structures to reduce the computational cost, and these systems cannot reproduce the complex structures and behaviour occurring in the real world.” 
Koyama and colleagues aimed to overcome these shortcomings by applying simulations with refined parameters to realistic models of the key interfaces based on microscopic observations of the actual positions of the atoms at the active site of the electrode.
As the researchers write in Nature Communications Chemistry, the pores of a porous electrode were first infiltrated by epoxy resin under vacuum, so that the bulk cell could be readily handled.
TEM specimen lift-out was then performed using an FIB-SEM (HITACHI MI4000L) instrument followed by polishing.
The researchers then used a  JEOL JEM3200FSK and JEOL JEM-ARM200F in TEM and STEM modes to observe the structure at different scales.
Both instruments were equipped with energy-dispersive X-ray spectroscopy detectors for elemental analysis.
Based on these observations, the researchers reconstructed computer models with the same atomic structures for two representative arrangements that they observed.
Reactions between hydrogen and oxygen in these virtual fuel cells were simulated with a method called Reactive Force Field Molecular Dynamics, which uses a set of parameters to approximate how atoms will interact, even chemically react, with each other.
In this case, the researchers employed an improved set of parameters developed in collaboration with Yoshitaka Umeno’s group at the University of Tokyo.
Looking at the outcome of multiple runs of the simulations on the different model systems, the researchers found that the desired reactions were more likely to occur in the electrode layers with a smaller pore size.
Furthermore, they identified a new reaction pathway in which oxygen migrates through the bulk layers in a way that could potentially degrade performance and durability. 
The initial positions of the atoms in this computer model of a solid-oxide fuel cell were based on observations of the actual atomic configuration using electron microscopy. Simulations using this model revealed a previously unreported reaction (red path) in which an oxygen molecule from the yttria-stabilized zirconia layer (layer of red and light blue balls) moves through the bulk nickel layer (dark blue balls) before forming OH on the nickel surface. [Michihisa Koyama, Kyushu University]
Koyama and colleagues believe strategies to avoid this potential reaction route should be consider as researchers work to design improved fuel cells.
“These are the kinds of insights that we could only get by looking at real-world systems,” highlights Koyama. “In the future, I expect to see more people using real-world atomic structures recreated from microscope observations for the basis of simulations to understand phenomena that we cannot easily measure and observe in the laboratory.”
Research is published in Nature Communications Chemistry.
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