Abstract
Many researchers have carried out experimental research and theoretical analysis on hydrogen storage in carbon nanotubes (CNTs), but the results are very inconsistent. The present paper reviewed recent progress in improving the hydrogen storage properties of CNTs by various modifications and analyzed the hydrogen storage mechanism of CNTs. It is certain that the hydrogen storage in CNTs is the result of the combined action of physisorption and chemisorption. However, H2 adsorption on metal-functionalized CNTs still lacks a consistent theory. In the future, the research of CNTs for hydrogen adsorption should be developed in the following three directions: (1) A detailed study of the optimum number of metal atoms without aggregation on CNT should be performed, at the same time suitable preparation methods for realizing controllable doping site and doped configurations should be devised; (2) The material synthesis, purification, and activation methods have to be optimized; (3) Active sites, molecular configurations, effectively accessible surface area, pore size, surface topology, chemical composition of the surface, applied pressure and temperature, defects and dopant, which are some of the important factors that strongly affect the hydrogen adsorption in CNTs, should be better understood.
Highlights
At present, the problems of environmental protection and energy storage are increasingly becoming the focus of attention [1,2,3]
The results have shown that isolated single-walled carbon nanotubes (SWCNTs) demonstrate higher hydrogen adsorption capacity at a low temperature, while SWCNT bundles have higher hydrogen adsorption capacity at room temperature, even higher than that of SWCNTs at 80 K, which suggests the significant advantage of SWCNT bundles over isolated nanotubes in terms of on-board hydrogen storage
The investigation reveals that for the given tube diameter and temperature, the hydrogen adsorption capacity of SWCNTs increases with the increase of pressure, which is in agreement with the results presented by [96]; for the given tube diameter and pressure, the hydrogen adsorption capacity of SWCNTs decreases with the increase of temperature; for the given temperature and pressure, the hydrogen adsorption capacity of SWCNTs decreases with the decrease of tube diameter
Summary
The problems of environmental protection and energy storage are increasingly becoming the focus of attention [1,2,3]. The controversy over the hydrogen storage properties is related to whether CNTs are good hydrogen storage materials [42], while the most important controversy over the mechanism of hydrogen storage is whether the nature of hydrogen storage behavior of CNTs is chemisorption or physisorption, or the coexistence of the two kinds of adsorption [43,44,45,46,47]. Nanomaterials 2020, 10, 255 et al [66] attributed hysteresis to the metal impurities formed during CNT synthesis and/or the multilayer structure of the CNTs. the hydrogen storage properties of CNTs have been reported to be very different, there is an extremely great demand for hydrogen energy applications in electric vehicles, batteries, and military fields. Some factors affecting the hydrogen adsorption properties of CNTs were analyzed emphatically, and the key directions of CNT research in the future were pointed out
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