In order to determine a material's hydrogen storage potential, capacity measurements must be robust, reproducible, and accurate. Commonly, research reports focus on the gravimetric capacity, and often times the volumetric capacity is not reported. Determining volumetric capacities is not as straight‐forward, especially for amorphous materials. This is the first study to compare measurement reproducibility across laboratories for excess and total volumetric hydrogen sorption capacities based on the packing volume. The use of consistent measurement protocols, common analysis, and figure of merits for reporting data in this study, enable the comparison of the results for two different materials. Importantly, the results show good agreement for excess gravimetric capacities amongst the laboratories. Irreproducibility for excess and total volumetric capacities is attributed to real differences in the measured packing volume of the material.

This report is being disseminated by the Department of Energy. As such, the document was prepared in compliance with Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and information quality guidelines issued by the Department of Energy. Though this report does not constitute "influential" information, as that term is defined in DOE's information quality guidelines or the Office of Management and Budget's Information Quality Bulletin for Peer Review (Bulletin), the study was reviewed internally prior to publication.

a b s t r a c tTo enable the wide-spread commercialization of hydrogen fuel cell technologies, the U.S. Department of Energy, through the Office of Energy Efficiency and Renewable Energy's Fuel Cell Technology Office, maintains a comprehensive portfolio of R&D activities to develop advanced hydrogen storage technologies. The primary focus of the Hydrogen Storage Program is development of technologies to meet the challenging onboard storage requirements for hydrogen fuel cell electric vehicles (FCEVs) to meet vehicle performance that consumers have come to expect. Performance targets have also been established for materials handling equipment (e.g., forklifts) and low-power, portable fuel cell applications. With the imminent release of commercial FCEVs by automobile manufacturers in regional markets, a dual strategy is being pursued to (a) lower the cost and improve performance of high-pressure compressed hydrogen storage systems while (b) continuing efforts on advanced storage technologies that have potential to surpass the performance of ambient compressed hydrogen storage.

Hydrogen has many positive attributes that make it a viable choice to augment the current portfolio of combustion-based fuels, especially when considering reducing pollution and greenhouse gas (GHG) emissions. However, conventional methods of storing H 2 via high-pressure or liquid H 2 do not provide long-term economic solutions for many applications, especially emerging applications such as man-portable or stationary power. Hydrogen storage in materials has the potential to meet the performance and cost demands, however, further developments are needed to address the thermodynamics and kinetics of H 2 uptake and release. Therefore, the US Department of Energy (DOE) initiated three Centers of Excellence focused on developing H 2 storage materials that could meet the stringent performance requirements for on-board vehicular applications. In this review, we have summarized the developments that occurred as a result of the efforts of the Metal

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