(Invited) Hydrogen Storage Measurements; Protocols and Reproducibility

Abstract

The transport and storage of hydrogen remains one of the critical challenges for hydrogen-based technologies. Specifically, the U.S. Department of Energy (DOE) has set forth hydrogen storage system targets for gravimetric and volumetric capacities in order to make light-duty polymer electrolyte membrane fuel cell vehicles viable alternatives to the current vehicle driving ranges. Hydrogen storage sorbent materials have shown continuous improvement in the past decade and many DOE targets have been met singularly, however, further advancements in materials development are needed to meet all of the performance targets simultaneously. The accuracy of hydrogen storage measurements is fundamental to identifying the potential of novel materials. Previous reports have called into question the reproducibility of hydrogen storage measurements. A round-robin report in 2009,[1] discovered large discrepancies among the different participating laboratories for H2 gravimetric capacity measurements on a standard material. This study brought attention to the challenges of H2 capacity measurements, provided recommendations, and discussed possible sources of errors. In a later study, we demonstrated an improved reproducibility amongst a smaller set of laboratories for the excess gravimetric capacity of standard sorption samples.[2] Both of these studies emphasized H2 gravimetric excess capacity. The volumetric capacity, however, is often not reported in the literature as determining volumetric capacities is not as straight-forward, especially for amorphous materials. Research efforts at the National Renewable Energy Laboratory, as a part of HyMARC, include characterization of thermodynamic and kinetic energetics, and hydrogen adsorption capacity measurements. In this talk, we will discuss the recommended measurement protocols and show the results from a recent comparative study of reproducibility of both volumetric and gravimetric H2 capacities. Common analysis protocols and figure of merits for reporting data[3] allow the materials to be compared on an equal basis, and provide insight into the uncertainty of measurements amongst the research community. We will also discuss the issues of approximating absolute capacities and calculating isosteric heats of adsorption at super critical conditions. [1] C. Zlotea, et.al. Int. J. Hydrog. Energy 34, 3044 (2009), [2] K.E. Hurst et.al. Appl. Phys. A2016, 122, 42.[3] P. Parilla, et.alAppl. Phys. A. 2016, 122, 201.