The development of materials capable of conducting negatively charged hydrogen atoms under ordinary conditions could facilitate the advancement of innovative clean energy storage and electrochemical conversion technologies.

Today, a group of researchers from the Chinese Academy of Sciences’ Dalian Institute of Chemical Physics (DICP) unveiled a new method for creating an all-solid-state hydride cell at room temperature.

The technique involves the introduction and manipulation of defects in the lattice structure of rare earth hydrides. The findings of their study were published in the journal Nature.

Batteries and fuel cells have relied on solid materials capable of conducting lithium, sodium, and hydrogen cations. In specific circumstances, some of these materials can achieve superionic states where ions move at the same speed as they do in liquids by bypassing the solid crystal structure. This property offers significant benefits for chemical and energy conversion processes by enabling ions to travel without the need for a soft membrane or liquid to divide the electrodes. Nevertheless, achieving this state under ambient conditions is challenging, and only a limited number of solid-state materials have been able to achieve this feat.

“Materials that exhibit superionic conduction at ambient conditions would provide huge opportunities for constructing brand new all-solid-state hydride batteries, fuel cells and electrochemical cells for the storage and conversion of clean energy,” explains study author Prof. CHEN Ping.

Hydride ion (H−) conductors, possessing robust reducibility and high redox potential, have emerged as a potential game-changer in energy storage and conversion technologies. In recent years, various H− conductors, such as alkaline earth metal hydrides and oxyhydrides of alkaline earth and rare earth metals, have been developed due to their fast hydrogen migration properties. However, none of these materials have been able to achieve superionic conduction under ambient conditions.

The research team at the DICP took a different approach by focusing on the structure and morphology of trihydrides, which are hydrides containing three hydrogen atoms per molecule, of particular rare earth elements (REHx), including Lanthanum (La). Their innovative approach has led to the development of a new material that can achieve superionic conduction under ambient conditions.

Traditional approaches to increase electronic conductivities typically aim to reduce crystallographic imperfections in metallic nanowire interconnects and nanostructured photovoltaic semiconductors. However, in this study, the research team deliberately created numerous discrete nanosized grains and lattice defects in REHx to disrupt the path of electron transport and decrease electronic conductivity. This approach differs from conventional methods used to engineer materials for ion conduction, which depend on high crystallinity and a consistent structure.

The research team studied the diffusion of H− ions in REHx lattices and observed that they moved effortlessly by hopping between octahedral and tetrahedral sites in the crystal and across interfaces or grain boundaries. However, electrons faced significant obstacles such as scattering at grain boundaries, particle surfaces, and other traps, leading to a reduction of electronic conductivities by three to five orders of magnitude in comparison to their well-crystallized counterparts.

“By creating nano-sized grains, defects and other crystalline mismatched zones in a known ionic-electronic mixed conductor, we demonstrated that the electronic conductivity of LaHx (x » 2.94) can be largely suppressed by five orders of magnitude,” adds CHEN. “Engineering such a material could transform LaHx into a pure hydride ion conductor with record high conductivities in the temperature range of -40 to 80 ℃.”

The researchers were able to effectively diminish electron conduction in LaHx by utilizing high-energy ball milling to decrease particle size and distort the lattice. This process involves subjecting the material to high-energy collisions. The resulting deformed LaHx material displays fast H− conduction and a high ion transfer number, which would make it a suitable material for a hydride ion battery that can operate at room temperature or lower.

“This work demonstrates the effectiveness of lattice deformation in suppressing electron conduction in REHx,” adds CHEN.

The researchers intend to delve deeper into the physics underlying this phenomenon and expand upon the techniques developed in this study to other hydride materials. This approach will expand the range of materials suitable for pure H− conduction.

“Our near-term goal is to demonstrate a brand new all-solid-state hydride ion battery that is of practical potential,” remarks CHEN.

Source: 10.1038/s41586-023-05815-0

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