A magnesium electrochemical cell having a positive electrode containing as an active ingredient, a material of formula [V.sub.2O.sub.5].sub.c [M.sub.aO.sub.b].sub.d and/or a material of formula [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g in a metastable structural and morphological phase is provided. In the formulas M is an element selected from the group consisting of P, B, Si, Ge and Mo; and X is O or a halide.

According to one embodiment, a magnesium-recycling hydrogen generation system includes: a by-product acquisition unit that separates a by-product from a post-reaction solution, which is a solution after reacting with a hydrogen generation material containing a hydrogen-containing magnesium compound that generates hydrogen via a reaction with the solution, to acquire the by-product including more than one type of oxygen-containing magnesium compound that contains oxygen produced by the reaction, a raw material production unit that reacts the by-product with a halogen-containing substance containing halogen and other atoms than the halogen to produce a raw material containing magnesium halide, a hydrogen generation material production unit that reduces the raw material with plasma containing hydrogen to produce the hydrogen generation material, and a hydrogen generator that reacts the hydrogen generation material with the solution to generate hydrogen.

According to one embodiment, a magnesium-recycling hydrogen generation system includes: a by-product acquisition unit that separates a by-product from a post-reaction solution, which is a solution after reacting with a hydrogen generation material containing a hydrogen-containing magnesium compound that generates hydrogen via a reaction with the solution, to acquire the by-product including more than one type of oxygen-containing magnesium compound that contains oxygen produced by the reaction, a raw material production unit that reacts the by-product with a halogen-containing substance containing halogen and other atoms than the halogen to produce a raw material containing magnesium halide, a hydrogen generation material production unit that reduces the raw material with plasma containing hydrogen to produce the hydrogen generation material, and a hydrogen generator that reacts the hydrogen generation material with the solution to generate hydrogen.