Chemical hydrogen storage: 'material' gravimetric capacity versus 'system' gravimetric capacity

Chemical hydrogen storage materials (CHSMs), owing to their high hydrogen content, are presented as having high potential to achieve high gravimetric hydrogen storage capacities in the prospect of technological applications (e.g. vehicle). However, this raises two questions. Is the storage capacity viewed at the material level or at the system level? Do we talk about absolute, excess or net capacities? Being exact in the terms used is all important so the US Department of Energy has set well-defined, precise technical requirements for vehicle application at the system level. This Minireview focuses on the terms that have been used in the open literature to present the storage capacities of the CHSMs. It stands out, through several typical cases, that the terms are generally misused and mislead the reader on the real capacities. For instance, the material level and the system level are often lumped together. Herein, we re-define the much-used terms and expressions, and propose a set of expressions to systematically use in order to avoid any future misuse, misleading terms and misunderstanding.