Quest for New Materials for Methane Storage: Gas Adsorption and Neutron Diffraction Measurements
Yang Peng, 1,2 Vaiva Krungleviciute, 1,2 Taner Yildirim 1,2
1NIST Center for Neutron Research, 2University of Pennsylvania
On-board methane storage in fuel cell-powered vehicles is a major component of the national need to achieve energy independence and protect the environment. However, adoption of natural gas vehicles has been hindered by technical and economic barriers imposed by the high pressure required to achieve energy densities for reasonable driving ranges. The compressed natural gas tanks (CNG) used in vehicles today are usually pressurized to 250 bar. Reducing this pressure below 100 bar and at the same time storing the same amount of methane in the tank would be a significant achievement. A possible solution is to use nanoporous metal-organic-framework (MOF) materials that can store natural gas within their pores in high concentration but at relatively low pressures. Since the number of choices for metal centers and the linkers that form the MOFs are practically infinite, it is crucial that we achieve fundamental understanding of the chemical and structural interactions governing the storage and release of methane in a wide spectrum of candidate MOF materials. The main scope of our research is to use neutron scattering methods along with accurate high-pressure isotherm measurements coupled with computer modeling to achieve this fundamental understanding and identify the best MOF material for methane storage.
We have recently started our quest for new methane storage materials by independently validating the previously reported high methane storage material such as PCN-14 and UTSA-20 by synthesizing and fully characterizing them under the same roof using the same gas-sorption apparatus with a well-defined standard protocol. To our surprise, we have discovered that many of the previously reported high methane uptakes involve serious mistakes such as wrong material density, inaccurate real-gas equation of state, and unphysical large methane densities. We also introduce a new concept of working capacity as the storage capacity between two pressures rather than the actual storage uptake as the working capacity ultimately determines the driving range of a CNG-car. With this new definition, some of the MOFs, in particular, those recent ones with surface area exceeding 4000 m2/g and pore volumes more than 2.0 cc/g, become the most promising methane storage materials. Using Fourier difference analysis from neutron diffraction, we identify the methane storage sites and establish the storage mechanism in selected candidate materials. Finally, we point out that in volumetric storage capacity, the packing density (which is about 1/2-2/3 of the actual ideal single crystal density) is the most important parameter and one needs to find a way to compress MOFs with high packing density without collapsing them. Unfortunately, the importance of packing density has not been fully realized yet. We are currently testing various approaches to obtain high-density packed MOF materials, which will increase the volumetric capacities up to 30% while keeping the high gravimetric capacity. Our results will be important in guiding and designing new MOFs with even better methane storage capacities.