An overview of swelling stress formation and control of metal hydride for high-safety and high-density hydrogen storage applications

The storage of hydrogen is a crucial challenge in the advancement of hydrogen energy due to hydrogen's high diffusivity and flammability. Metal‑hydrogen reaction hydrogen storage (MHHS) realizes efficient and safe hydrogen storage at high hydrogen storage density and low hydrogen storage pressure by reacting metals with hydrogen to form metal hydrides (MHs). However, the expansion effect generated by MHs during hydrogen absorption can exert swelling pressure on the MH hydrogen storage reactor (MHR) equivalent to several times the hydrogen pressure, which may lead to reactor deformation or even cracking. Therefore, identifying the root causes of swelling stress and developing effective mitigation strategies are essential for ensuring the safety and stability of the MHHS system in engineering applications. This paper provides a new insight that non-uniform volume expansion of MHs under constraints is the root cause of swelling stress concentration and identifies the key factors influencing stress through a systematic multidimensional analysis encompassing alloy materials, reaction conditions, cycling number, and reactor orientation. Then, multi-scale numerical models from both microscopic and macroscopic perspectives are established to guide stress reduction based on the proposed formation mechanisms and the corresponding key factors. Furthermore, this paper provides the first comprehensive review of methods for reducing swelling stress at the material, bed, and reactor levels. These methods include enhancing the alloy's resistance to pulverization at the material level, increasing expansion space for MHs within the bed at the bed level, and optimizing MHR's structure at the reactor level, offering valuable technical guidance for safety-oriented large-scale engineering applications of MHHS.