In situ TEM studies on hydrogen-related issues: hydrogen storage, hydrogen embrittlement, fuel cells and electrolysis.

Hydrogen is attracting attention as an energy carrier for realizing a low-carbon society, because it can directly convert the energy obtained from chemical reactions into electrical energy without carbon dioxide emissions. This paper presents in situ transmission electron microscopy (TEM) observations related to hydrogen storage in metal and metal hydrides, hydrogen embrittlement of metallic materials used for storing and transporting hydrogen in containers and pipes, and fuel cells and water electrolysis using metal catalysts and oxides as electrode materials. All of these processes are important for practical applications of hydrogen. Numerous in situ TEM studies have revealed the microscopic structural changes when hydrogen reacts with the materials, when hydrogen is solidly dissolved in the materials and during the operation of the material. This review is expected to facilitate further development of TEM operando observations of hydrogen-related materials.