Simulations Identify the Importance of Network Distortions in Ion Conducted Fuel Cell Materials

Ion conduction involves the movement of ions from one place to another inside a material. The ions pass through point defects, which are irregularities in the otherwise coherent arrangement of atoms, called the crystal lattice. This sometimes slow process can limit the performance and efficiency of fuel cells, batteries, and other energy storage technologies.

Before determining which underlying properties of solid materials are essential for improving these applications, researchers need to better understand the factors that control ionic conduction. To deepen this knowledge, a multidisciplinary team from the Oak Ridge National Laboratory (ORNL) of the US Department of Energy (DOE) has developed a computer framework to process and analyze large data sets of ionic conduction solids.

Using a set of data containing more than 80 compositions of different materials called perovskites, researchers have mainly sought to identify and optimize those with promising proton conduction capabilities. These new materials could allow for the production of more reliable and efficient proton-conducting solid oxide fuel cells, energy storage devices that convert chemicals into electricity for practical purposes, such as power supply. vehicles.

The results of this work are published in the Journal of Physical Chemistry and Materials chemistry, and members of the team also presented their findings at the fall meeting of the Materials Research Society in 2018.

"We are looking for better ionic conductive materials because, no matter what solid electrolyte used in fuel cells or batteries, the faster the ions move, the more efficiently the device will work," said L & # 's Principal Investigator Panchapakesan Ganesh, ORNL R & D staff member. for Nanophase Materials Sciences (CNMS). "We now have an understanding that will help us develop new design principles to develop such materials."

The team studied various materials including one of the fastest proton conductors known, a modified version of the barium zirconate compound (BaZrO₃) formed by replacing zirconium (Zr) with yttrium (Y), an element that reduces the overall charge of the compound to facilitate the addition of protons. The elements that exhibit this behavior are called acceptor dopants, and the material in question is often called yttrium-doped BaZrO.₃or Y-BZO.

Systematically reviewing so many candidates from the perovskite data set in a short time would not have been possible without the computing power of Titan, a Cray XK7 supercomputer hosted at the Oak Ridge Leadership Computing Facility (OLCF). With the help of several codes and a computer tool called wraprun, OLCF staff members helped the team to develop an automated workflow optimized for the architecture of Titan.

"We worked closely with OLCF staff to put in place a highly scalable workflow that allowed us to simultaneously use thousands of Titan cores," Ganesh said.

These simulations have shown that the correlations between lattice distortions and the proton binding energy – the amount of energy required to separate a proton from a perovskite material – can increase and slow protons, which inhibits optimal conduction. protons. This revelation could help researchers identify existing materials and develop new ones that can compete with Y-BZO.

"We realized that coupling mobile ions with distortions in the crystal lattice is one of the most important ingredients of ionic conduction," Ganesh said. "Understanding this connection means that we can selectively design solid materials with improved ionic conductivity."

In addition to the practical benefits that these results could have for energy applications, new team knowledge provides fundamental information about scientific concepts.

"During this process to understand what limits the conduction of protons in existing materials, we also hope to discover new physics," Ganesh said. "Everything is related to the underlying atomistic mechanisms."

To validate the computer results, the team members conducted a series of complementary experiments using pulsed laser deposition, transmission scanning electron microscopy, Kelvin probe force microscopy solved in the and atomic probe tomography techniques at the CNMS, as well as Neutron scattering at the Spallation Neutron Source (SNS). The CNMS, SNS and OLCF are all user facilities of the DOE Office of Science located at ORNL.

The researchers plan to extend their efforts beyond protons and perovskites to study the behavior of mobile ions in other categories of materials. Future results could improve the performance of other types of fuel cells, as well as lithium-ion batteries.

"The computer framework developed to study doped perovskites can be applied to other types of crystalline inorganic solids, and the availability of such sets of defect data allows us to leverage the expertise of the ORNL in the advanced techniques of artificial intelligence to accelerate the discovery of materials, "said Ganesh.