Robustness and Thermophysical Properties of MOF-5:A Prototypical Hydrogen Storage Material.

[摘要] MOF-5 has attracted considerable attention due to its ability to store gaseous fuels at low pressure with high densities. However, low thermal conductivity and limited robustness upon exposure to water and other reactive species are two challenges which limit the application of MOF-5. The focus of this dissertation is to understand and overcome these shortcomings through detailed experimental and computational characterization of the prototype compound, MOF-5. Improvements to the thermal conductivity of MOF-5 are demonstrated using densified pellets consisting of a physical mixture of MOF-5 and expanded natural graphite (ENG). We conclude that the low thermal conductivity typical of MOFs can be improved using a judicious combination of second phase additions and processing techniques. Regarding robustness, we first quantify experimentally the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs. pellets). Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs. We subsequently examined the thermodynamics and kinetics of water adsorption/insertion into MOF-5 using van der Waals-augmented DFT calculations and transition state finding techniques. We find that incoming water molecules preferentially adsorb at adjacent sites on Zn-O clusters rather than filling widely separated low energy sites. Our calculations also suggest that the thermodynamics of MOF hydrolysis are coverage dependent, and that hydrolysis is slow at low water coverages and is preceded by an incubation period.The third component in our study of MOF-5 robustness involved cyclic and static exposure to impure hydrogen gas. MOF-5 was exposed to 5 gas mixtures over hundreds of adsorption/desorption pressure cycles and for extended periods of static exposure lasting up to 1 week. Hydrogen chloride was the only impurity that yielded a measurable decrease in hydrogen storage capacity. FTIR and XRD analyses reveal slight changes in the spectra only for those samples exposed to HCl and NH3 impurities.In closing, we briefly examine hydrogen permeation into- and the internal structure of- MOF-5 pellets using neutron and x-ray imaging techniques (tomography and radiography).

[发布机构] University of Michigan

[效力级别] Hydrogen storage

[关键词] Metal organic framework;Hydrogen storage;Physics;Science;Physics