Structure,Electrochemical Hydrogen Storage Properties And First-Principles Studies Of Hydrogenated Amorphous Silicon

Hydrogenated amorphous silicon（a-Si:H）thin films have ultrahigh electrochemical discharge capacity of 3635 m Ah g^-1 in ionic liquid electrolytes,which makes them very valuable for research and application in proton batteries.It has been proved that the electrochemical capacity of amorphous silicon（a-Si）thin film electrode can be greatly improved by gas hydrogenation,but the effect of hydrogenation treatment on their structure and the mechanism of correlation between their structure and electrochemical properties are still unclear due to the fact that the short study time of silicon electrodes applied in electrochemical hydrogen storage and the difficulty of accurately determining the structure of a-Si:H.In this dissertation,the a-Si:H structures are predicted based on first-principles technology,and the influence mechanism of structural characteristics on their structural stability and electrical properties is analyzed.Subsequently,based on the electrochemical tests on a-Si:H thin film electrodes,the predicted a-Si:H structures are used and the first-principles calculations are employed to investigate the effect of hydrogenation treatment on the structural and electrical properties of a-Si:H thin film electrodes,predict the mechanism of hydrogen diffusion from the surface to the bulk of the a-Si:H electrode,and reveal the physicochemical properties and electrochemical behavior of a-Si:H thin film electrodes in the electrochemical hydrogen storage/release process.The main conclusions are summarized as follows:1.The a-Si:H nanostructures with different hydrogen contents are predicted at different ambient hydrogen concentrations by first-principles calculation,the structure characteristics of a-Si:H nanostructures at different hydrogen concentrations are explored,and the effect of structure characteristics on their electrical properties is investigated.The a-Si:H generated is mainly three-dimensional（3D）bulk structure under the low ambient hydrogen concentration conditions（≤2.78 wt.%）.The dimension of a-Si:H structures deceases gradually with the increase of hydrogen concentration,the probability of obtaining two-dimensional（2D）layer a-Si:H structures is more than 55%in the range of 3.44～5.08 wt.%,the probability of obtaining one-dimensional（1D）nanowire a-Si:H structures is up to 60%in the range of 6.66～9.67 wt.%,and the probability of obtaining zero-dimensional（0D）nano-cluster a-Si:H structures is as high as 90%when the ambient hydrogen concentration increases to 11.95 wt.%.At high ambient hydrogen concentrations,excess hydrogen is stored in the gaps of 3D structures or in the spacing of 2D,1D and 0D structures in the form of H₂ molecules.The a-Si:H nanostructures dominated by Si-H bonds,Si-H₂ bonds and Si-H₃ bonds are mostly stable,the structures dominated by Si-H-Si bonds are unstable,and the existence of H₂ molecules in the structure gap will affect the structure stability.The a-Si:H nanostructures with low hydrogen content has a certain electrical conductivity.As the hydrogen content of structure increases,the a-Si:H structures change to semiconductor structures,and the band gaps of these structures first increase and then decrease.The band gap of a-Si:H structures can be reduced by adding Si-H₂and Si-H₃ bonds to the structure or by enlarging the void size of structure.Most of a-Si:H structures dominated by Si-H-Si bonds have conductive properties.From a statistical point of view,the average bandgap of a-Si:H structures increases with decreasing structural dimension.2.The a-Si:H thin film electrodes are prepared by magnetron sputtering followed by ex-situ hydrogenation,and the effect of hydrogenation conditions on its structure and electrochemical properties is investigated.Hydrogenation treatment significantly increases the hydrogen content in the electrode,improves the wettability of the ionic liquid electrolyte onto the electrode surface,and significantly increases the redox activity and the electrochemical discharge capacity of the electrode.The electrochemical capacity of a-Si:H thin film electrode hydrogenated at 300 ^oC/5 MPa and 500 ^oC/1 MPa for 2 h increased by about 7～9 times compared with the non-hydrogenated a-Si thin film electrode（180 m Ah g-1）,reaching 1433m Ah g-1 and 1827 m Ah g-1,respectively.Higher electrochemical charging rate and lower discharge rate are helpful to improve the electrochemical discharge capacity of a-Si:H electrode,whereas the decrease of electrode thickness can greatly improve the electrochemical cycle stability of electrode.As the hydrogenation time increases,the activation properties and electrochemical capacity retention rate of a-Si:H thin-film electrodes slightly increase,but the electrochemical capacity of electrodes does not increase much.The activation properties and electrochemical cycle stability of a-Si:H electrode are improved with the increase of hydrogenation pressure.The increase of hydrogenation temperature can significantly reduce the activation times and increase the electrochemical hydrogen storage capacity of a-Si:H electrode.3.The effects of hydrogenation treatment on the electrical properties of a-Si:H structure are investigated by first-principles calculations.As the pressure increases,the band gap of the a-Si:H electrode structure tends to decrease,and at high pressures,its structure changes from a semiconductor to a conductor structure.As the hydrogenation temperature increases,the H atoms stored in the a-Si:H electrode structure collapse the structure into a lower dimensional structure,and the higher the hydrogen content in the electrode structure,the lower the temperature required for structure collapse.The new structures evolved at the high hydrogenation temperature have conductive properties,which explains the fundamental reason why the electrochemical capacity of a-Si:H electrodes can be significantly improved by hydrogenation.When pressure and temperature applied to the a-Si:H structure simultaneously,an increase of hydrogenation pressure is helpful to reduces the probability of the a-Si:H structure evolving to a lower dimensional structure under the action of hydrogenation temperature.4.The mechanism of H atom diffusion on the surface and in the bulk of a-Si:H electrode is investigated by first-principles calculations.Hydrogenation significantly reduces the diffusion barrier of H atoms from the surface to the bulk and from the bulk to the surface of a-Si:H electrode,that is,hydrogenation greatly improves the hydrogen storage properties and the hydrogen storage/release reversibility of a-Si:H electrode.When the hydrogen content in the electrode is low（corresponding to the early stage of electrochemical charging or the late stage of electrochemical discharge）,the H atoms diffuse through the empty dangling bond in the a-Si:H electrode,while when the hydrogen content is high（corresponding to the middle stage of electrochemical charging and discharge）,the H atoms are transported through the silicon-hydrogen bond in a hopping mechanism.H atoms can be diffused between two closely spaced low-dimensional structures,such as two 2D,1D or 0D structures,but the diffusion barrier between two structures increases rapidly with increasing the structure spacing,which indicates that slight delamination and pulverization do not affect the diffusion of H atoms in a-Si:H electrode but severe delamination and pulverization can greatly affect the diffusion of H atoms in the electrodes.5.The physicochemical properties in the electrochemical hydrogen storage process and the electrochemical behavior of a-Si:H electrode during electrochemical charging and discharging processes are investigated by the method of combining the experimental tests and first-principles calculations.The a-Si:H electrode exhibits weak conductivity during the initial charging,but the instability of the electrode electronic structure during the later charging results in a slight fluctuation of the electrochemical charging process.The hydrogen adsorption capability of a-Si:H electrode is weak in the early and final of electrochemical charge/discharge processes,but it is stronger in the intermediate stages of electrochemical charge/discharge processes.The oxygen covering on the electrode surface does not affect the conductivity of the electrode surface,but will affect the hydrogen adsorption capacity and the reversibility of hydrogen storage/discharge of a-Si:H electrode.The electrochemical hydrogen storage process is easier to accomplish than the electrochemical desorption process at the a-Si:H electrodes/electrolyte interface,which is highly advantageous for the application of a-Si:H electrodes in electrochemical hydrogen storage.The interface between a-Si:H electrode and Ni substrate has good conductivity,a-Si:H electrode has better thermal stability on Ni（111）surface than Ni（200）surface,and the different thermal stability of electrode on different Ni substrate surface should be one of the main reasons for the increase of electrode stripping from Ni substrate with the increase of electrochemical cycle times.Through this research,the library of a-Si:H nanostructures is enriched,the correlation mechanism between structure characteristics and electrical properties of a-Si:H is revealed,the effect of hydrogenation condition on electrochemical properties of a-Si:H electrode is clarified,the physicochemical properties and electrochemical behavior of a-Si:H thin film electrodes in the electrochemical hydrogen storage/release process are revealed.We believe that these results have significant references for the research and application of Si materials in chemical,electrochemical hydrogen storage and other fields.