Modification Methods Of Graphite And The Influence Of Its Surface Modification On The Electrochemical Hydrogen Storage Performance

Graphite has a layered structure. Surface modification and metal or metal oxide decoration, which enlarge the specific surface area and enhance the electrochemical activity of graphite, can improve the hydrogen storage performance of graphite and are the ways with great potential in the development of electrochemical hydrogen storage materials. In this thesis, we obtained graphite oxide (GO) by the Hummers method, expanded graphite (EG) by two-stage method, and graphene by chemical reduction method through the surface modification of graphite. Pd, TiO2, and Ni nanoparticles are in situ loaded on GO, EG, and graphene through reduction method, sol-gel process, and deposition-precipitation method, respectively. The electrochemical hydrogen storage properties of the samples, as well as the related influence factors, are investigated.The maximum discharge capacity of GO is 1399 mAh/g, equivalent to 4.96 wt.% of hydrogen storage quantity, which is 50 times as much as that of the original graphite, and has good cycle stability. The existence of oxygen functional groups and the enhancement of the specific surface area in GO play important roles in improving the cycle stability and electrochemical activity of GO, and thus, the electrochemical hydrogen storage capacity of GO are improved. After decorated with Pd nanoparticles, Pd nanoparticles with the size of 5-40 nm are uniformly decorated between the graphite layers. The graphite layers are opened and graphene-like structure with a few layers forms. As a result, the specific surface area is enlarged. The discharge capacity of Pd-decorated graphene-like structure is lower than GO, but higher than pure graphite, indicating that Pd nanoparticles could have the catalytic effect on the electrochemical property of graphite.The graphene obtained by reducing the graphite oxidation with hydrazine hydrate are transparent. Ni nanoparticles distribute uniformly on graphene sheets with 5-10 nm in diameter. Ni nanoparticles have catalytic effect on the electrochemical activity of graphene. With the increase of Ni content, the amount of electrochemical hydrogen storage increases. However, when the content of Ni exceeds 10 wt.%, the amount of the electrochemical hydrogen storage is decreased due to the occupancy of the active position of the graphene. Graphene with 5 wt.% content of Ni with well cycle stability has the highest discharge capacity of about 875.9 mAh/g, equivalent to hydrogen storage quantity of 3.16 wt.%.EG prepared by two-stage method has even distribution of pore structure with very thin walls. The discharge capacity of EG is 17.5 mAh/g. TiO2 nanoparticles decorated on EG distribute uniformly on EG layers with 5-10 nm in diameter. It is found that the calcined temperature influences the crystallinity of anatase TiO2 nanoparticles and the electrochemical hydrogen storage behavior of EG. When calcined at 500℃, the discharge capacity of TiO2/EG nanocomposite reaches 373.5 mAh/g. The photocatalysis effect and the strong oxidation reduction ability of TiO2 nanoparticles could account for the enhancement of electrochemical hydrogen storage of EG.Moreover, the effect of the conductive agent on the electrochemical property of the samples is also investigated. In contrast with Ni conductive agent, the working electrodes with Cu powder as conductive agent have higher discharge capacity and better cycle stability. Cu conductive agent could contribute to the improvement of the reaction activity, and the promotion of the conductivity and stability of the working electrode.