Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Physics

Date of Defense

3-23-2018

Graduate Advisor

Eric H. Majzoub

Co-Advisor

Alexey Yamilov

Committee

Philip Fraundorf

Stephen M. Holmes

Julia E. Medvedeva

Abstract

Recent efforts have demonstrated confinement in porous scaffolds at the nanoscale can alter the hydrogen sorption properties of metal hydrides, though not to an extent feasible for use in onboard hydrogen storage applications, proposing the need for a method allowing further modifications. The work presented here explores how the functionalization of nanoporous carbon scaffold surfaces with heteroatoms can modify the hydrogen sorption properties of confined metal hydrides in relation to non-functionalized scaffolds (FS). Investigations of nanoconfined LiBH4and NaAlH4indicate functionalizing the carbon scaffold surface with nitrogen can shift the activation energy of hydrogen desorption in excess of 20 kJ/mol from the activation energy decrease of greater than or equal to 40 kJ/mol obtained from confinement in non-FS. XPS measurements indicate a significant fraction of the nitrogen contained in the carbon scaffolds is pyridinic, suggesting interactions of the available lone electron pair with the confined hydride and decomposition products strongly influences the hydrogen sorption processes. TPD experiments demonstrate nitrogen-FS can stabilize the release of diborane by ~30 °C, and kinetically stabilize LiBH4against decomposition to higher temperatures. Increased reorientational activation energies measured for the systems with nitrogen-FS using quasielastic neutron scattering support the observed stabilization of LiBH4is connected with the surface chemistry of the scaffold. Peak rates of hydrogen release occur at higher temperature from NaAlH4in nitrogen-FS despite the lower measured activation energy, indicating the existence of a rate-limiting step that may be related to the level of scaffold nitrogen doping and the onset of NaAlH4melting.

Available for download on Thursday, April 18, 2019

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