Counting hydrogen in porous adsorbents

The ultimate goal of the hydrogen economy is to develop a hydrogen storage system that efficiently meets or exceeds the targeted goal set by the U.S. Department of Energy (DOE) for onboard light-duty vehicles. These targets are defined in terms of usable quantity of hydrogen deliverable to the fuel cell system, including all stored hydrogen, media, reactants, and system components. To achieve reliable system performance, it is essential to precisely and uniformly measure the material property. However, there have been several discrepancies and methodological inconsistencies, particularly in the reporting of volumetric storage capacity of porous framework. In the case of porous systems, where hydrogen is stored through the physisorption process on the pore surface, the concepts of ‘excess’, ‘absolute’, and ‘total’ hydrogen storage appear. Therefore, estimation of storage performance in such systems is complex. Many porous materials have been investigated over the years; however, without the assurance of uniform and proper methodological considerations, the reported performance of these materials remains difficult to assess reliably and does not provide the benefit toward the work in hand. In this perspective, we address these issues and propose certain reporting protocols to ensure valid identification of suitable materials for further development.