Breakthrough! Scientists have developed the first room-temperature ultrafast hydrogen anion conductor

Hydrogen anion (H-) has the characteristics of strong reducing property and high redox potential, and is a potential hydrogen carrier and energy carrier, which has broad application prospects in energy and electrochemical conversion devices such as hydrogen anion batteries and fuel cells.

Recently, the team of Chen Ping, researcher Chen Ping and associate researcher Cao Hujun of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences (hereinafter referred to as “Dalian Institute of Chemical Physics”), developed the first room temperature ultrafast hydrogen negative ion conductor, and proposed a new strategy to inhibit the electron conductance in the hybrid conductor.

The team used mechanochemical synthesis to fabricate lattice distortion in the rare earth hydride – lanthanum hydride (LaHx), resulting in a large number of defects and grain boundaries, so that it presents an ultrafast hydrogen negative ion transport state in the temperature range of -40 to 80 °C. The relevant results were published in the journal Nature on April 5, Beijing time, and the co-first authors of the work were Zhang Weijin, a postdoctoral fellow at the Dalian Institute of Chemicals, and Cui Jirong, a doctoral student, and the reviewers commented that the work demonstrated a very interesting and novel research method.

Schematic diagram of hydrogen anion and electron conduction in lattice distortion lanthanum hydride. Photo courtesy of Dalian Chemical Properties

Peculiar hydrogen negative ions

Hydrogen anion is a rather “mysterious” monovalent negative ion.

When a hydrogen atom gets a foreign electron, it transforms into a hydrogen anion. Hydrogen anion has high polarizability, strong reducing property and high redox potential. Hydrides, which are formed by the combination of metal cations and hydrogen negative ions, are promising hydrogen-carrying energy carriers.

Similar to the hot ion conductor materials currently under study, such as lithium ions, sodium ions and proton conductors, hydrogen anion conductors are solid materials that can quickly conduct hydrogen negative ions. Its application potential is huge, and it is expected to achieve a series of technological innovations in secondary ion batteries, fuel cells, electrochemical conversion cells, membrane reactors, etc., and is a cutting-edge topic in the field of energy storage and conversion.

Potential application scenarios for hydrogen negative ion conductors. Photo courtesy of Dalian Chemical Properties

However, unlike the above-mentioned cations, hydrogen negative ions are larger in size and difficult to migrate in the crystal lattice. In addition, the high polarizability of hydrogen negative ions makes it easy to leave electrons in the crystal lattice during transmission, resulting in the unfavorable phenomenon of large electronic conductance of materials, which is extremely challenging to study.

Hydride “Metamorphosis”

The research team has been engaged in the research of hydrides in hydrogen storage and chemical nitrogen fixation. Five years ago, during the analysis of experimental results, it was discovered that our material can undergo H-D isotope exchange under mild conditions, which led to the establishment of the topic of hydrogen negative ion conduction. Chen Ping told reporters, “Dr. Zhang Weijin has carried out a lot of preliminary exploration. Cui Jirong, associate researcher and doctoral student of Cao Hujun, and Wang Shangshang joined one after another, forming a joint force and working together to tackle tough problems. ”

After several setbacks, the research team locked the material system on rare earth metal hydrides. As early as the study of color-changing glass in the last century, scholars found that such substances such as LaHx have rapid hydrogen migration ability, but it is an ion-electron hybrid conductor, and its electron conductance is large, which hinders its development as an ionic conductor. “Recently, researchers have introduced oxygen into the LaHx lattice to form oxygen hydrides, which can effectively inhibit its electron conduction. But the introduction of oxygen also reduces the conductivity of H-. Associate researcher Cao Hujun said.

In response to the difficulties encountered by the H-ion conductor research, the research team pioneered a different strategy – manufacturing lattice distortion to suppress electronic conductance. The mechanical ball milling method is an effective means to achieve this goal.

Mechanical ball milling is a relatively simple experimental method, by placing steel balls of different sizes and materials in a closed container, under high-speed rotation, strong impact, shear and friction between the material, container wall and steel ball, the material can be crushed and even cause lattice distortion. The method is simple, has little environmental pollution and good application prospects.

In this study, the researchers put LaHx particles into a mechanical ball mill for high-speed ball milling. After this high-speed “baptism”, the LaHx particles undergo obvious deformation: lattice distortion and a large number of defects are observed under high-magnification transmission electron microscopy. This distortion and defect disrupts the long-range ordered arrangement of the crystal lattice, “shocks” the electron transport, and reduces its electronic conductivity by more than 5 orders of magnitude compared with the well-crystallized LaHx.

Researchers process the molding of samples prior to conductivity testing. Photo courtesy of Dalian Chemical Properties

More importantly, this lattice distortion does not interfere significantly with H- conduction, and H- can still be quickly transmitted through the co-migration mechanism: the H- ion conductivity of the “deformed” LaHx material at -40 °C is as high as 10-2S/cm, and the activation energy is only 0.12 eV. Materials previously reported in the literature only achieve ultrafast H- conduction at around 300 °C.

“Many known hydride materials are ion-electron hybrid conductors,” Chen said, “and we hope that this lattice distortion strategy can expand the research and development space of hydrogen negative ion conductors.” ”

The application of hydrogen anion conductors “can be expected in the future”

Hydrogen anion conductors can be used as solid electrolyte materials to build new secondary ion batteries, fuel cells and electrochemical conversion cells based on hydrogen anions, which are expected to play an active role in clean energy storage and utilization, and the prospects are very fascinating.

In this research work, after a lot of attempts, the team assembled a solid-state hydrogen anion battery made of “deformed” LaHx as a solid electrolyte and TiH2 and Ti as electrodes, and achieved room temperature discharge for the first time. This encouraging result confirms the feasibility of this new secondary ion battery.

Referring to the future work arrangement, Chen Ping told China Science News: “Further development and optimization of hydrogen negative ion conductor materials and exploration of their applications in secondary ion batteries and electrochemical conversion cells are undoubtedly a top priority.” Our team has been committed to the research of hydrides for more than 20 years, and it is our pursuit to explore and understand the physical properties of this unique material and explore its special functions. ”

The theoretical calculations and neutron diffraction experiments in this work were completed in collaboration with An’an Wu, associate professor of Xiamen University, and Xia Yuanhua, associate researcher of the Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics. (Source: Sun Danning, China Science News)

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