In the present work, we develop an effective strategy, i.e., carbon nanomaterial-assisted morphological tuning, for both thermodynamic and kinetic destabilization in complex hydrides based on the interaction between the complex anion and the carbon matrix. The NaAlH 4 /carbon nanomaterials of graphene nanosheets (GNs), fullerene (C 60 ) and mesoporous carbon (MC) were selected as model systems for illustrating the positive effect of carbon nanomaterial-assisted morphological tuning. It is demonstrated that through the dissolution-recrystallization process, the morphologies of NaAlH 4 can be altered from the scale-like continuous structure for the GN-assisted sample, to flower-like structures with diameters ranging from 5 to 10 mm for the C 60 -assisted sample and to uniform particles with an average diameter of about 2 mm for the MC-assisted sample. Correspondingly, the onset temperature for dehydrogenation of NaAlH 4 is reduced to about 188, 185 and 160 C for the samples assisted with GNs, C 60 and MC, respectively, much lower than 210 C for the pristine sample. A remarkable reduction in activation energy for three-step dehydrogenation is also obtained in NaAlH 4 /carbon nanomaterial composites relative to the pristine sample, and the improved efficiency of carbon nanomaterials for kinetics is found to be in the order of MC > C 60 > GNs. These positive improvements can be attributed to both the particle refinement and interaction between NaAlH 4 and the carbon nanomaterial that are in intimate contact with each other, which are not only evidenced by FE-SEM observation, but also supported by 27 Al solid-state NMR characterization.
Crystalline Mg-Ti films with a thickness of more than 50 nm are only switched to a highly absorbing state and cannot be further changed to the transparent state after hydrogen loading at room temperature. To solve this problem, 200 nm thick amorphous MgTix (x = 0.11–0.29) films were prepared and their switchable mirror properties upon hydrogen loading and unloading were investigated. The results show that amorphous MgTix films can be reversibly switched between mirror and transparent states without an absorbing state due to the significant acceleration of hydrogen diffusion by amorphization. Moreover, the switching time of amorphous MgTix films are dramatically shortened with increasing Ti content. Using quartz crystal microbalance method plus transmission spectrum, it is experimentally proved that Ti addition shows little influence on hydrogen diffusion but a strong catalytic effect on MgH2 formation and decomposition. Therefore, the quick formation of a blocking MgH2 layer due to the combined effect of slower hydrogen diffusion in crystalline films and rapid MgH2 formation under Ti catalysis is considered as the reason why the crystalline Mg-Ti films cannot be changed to transparent state after hydrogen loading.
Amorphous Mg and MgNix (0.03 ≤ x ≤ 0.30) thin films capped with Pd were prepared by magnetron co-sputtering, and their hydrogen-induced optical transitions were investigated via electrochemical charging and discharging in KOH electrolyte solution. Repetitive transitions, up to dozens of times between the mirror state and transparent state, are achieved in these amorphous Mg and MgNix thin films even though some performance degeneration occurs during cycling. These deteriorations are mainly attributed to the breakdown of the film structure, which is caused by both a large change in film volume during cycling and the corrosive attack of the KOH electrolyte. In addition, calculations based on the electrochemical stripping method indicate that the hydrogen diffusion coefficient is significantly increased by amorphization; however, it is only slightly improved by the addition of Ni. Among the prepared amorphous films, MgNi0.09 film shows the largest hydrogen diffusion coefficient, namely, 2.64 × 10(-13) cm(2) s(-1). More importantly, the optical properties of the amorphous Mg and MgNix films are readily manipulated in the charging process, especially under a small charging current density, where there is a linear correlation between charging capacity and transmittance. The tunable optical properties obtained in the present study will greatly expand the application fields of Mg-based thin films.
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