Electrochemical Application Of The Ultrananocrystalline Diamond/Multilayer Graphene Composite Films

Ultrananocrystalline diamond/multi-layer graphene composite film?UNCD/MLG?is a novel nano-carbon material consisting of a large number of flake graphene and a small amount of sp³ hybrid utrananocrystalline diamond particles.However,the chemical and physical properties of carbon materials will be immensely affected by their microstructures and surface states,which in turn determine their final application performance.The specific embodiment includes in the conventional electrochemical energy storage applications of carbon materials;the capacitive properties are very poor due to the hydrophobicity of the surface.In this thesis,the UNCD/MLG films different microstructure films were prepared by microwave plasma chemical vapor deposition?MPCVD?technology;and their HER catalytic activity and capacitance properties were compared in detail.Then surface modification was carried out by electrochemical anodization processing and thermal trearment for the obtained films.Moreover,the influence of two treatment methods on the surface state of the films was researched,in which the changes in the capacitance properties of the UNCD/MLG films with different surface states were investigated emphatically.The researches indicate that:?1?The test on HER catalytic activity of UNCD/MLG films with different structures showed:the over-potential of the vertically grown UNCD/MLG film is only 75 mV and the Tafel slope is 43 mV dec^-1.However,the planar grown UNCD film has over-potential of 177 mV and a Tafel slope of 143 mV dec^-1,which presents that the UNCD/MLG film with vertical structure of multi-layer flake graphene has better catalytic activity than the planar grown UNCD film.?2?The capacitance performance of UNCD/MLG films with different structures was compared.The planar grown UNCD film has a specific capacitance of only 0.10 mF cm^-2,while the vertically grown has good electric double layer capacitance characteristics with a specific capacitance value of 0.34 mF cm^-2,which may be due to the highly hydrophobic of the primary film and reaction area was limited during the reaction,resulting in poor capacitance.?3?The surface modification of the UNCD/MLG film was carried out by thermal treatment and electrochemical anodization,and subsequently the microstructure and surface composition of the treated UNCD/MLG film were analyzed in detail.The results showed that the surface microstructure of the thermal-treated UNCD/MLG film was destroyed.After3 h of treatment at 500°C,the surface state of the film changed,and the contact angle is reduced from the initial 140°to 67°.Meanwhile,the surface microstructure of the film after electrochemical anodization treatment was not destroyed,and its surface state also changed significantly.The contact angle is gradually reduced from the initial 141°to about 20°,and the surface changes from superhydrophobic to hydrophilic because many oxygen-containing functional groups such as hydroxyl groups and adsorbed water molecules are introduced on the surface.?4?Comparing the influence of two surface treatment techniques on the performance of the film capacitance,the results show that the specific capacitance after thermal treatment increased from 0.34 mF cm^-2 to 10.7 mF cm^-2.The electrochemically anodized film has a specific capacitance value of 113.6 mF cm^-2 and a good EDLC behavior.Obviously,the electrochemical anodization method has a good effect on the capacitance of the UNCD/MLG film relative to the thermal treatment method.?5?The energy storage properties of UNCD/MLG film with different surface states after electrochemical anodization was analyzed in detail.The results show that when a potential of+2.0 V is applied and the anodization time is within 1500 s,the EDLC performance can be greatly improved.The untreatment and after anodized for 1000 s UNCD/MLG films were assembled into a symmetric two-electrode supercapacitor system.The capacitance performance results show that the performance of the anodized UNCD/MLG film is greatly improved compared with the untreated film.The energy density increased from 0.041 W h kg^-1 to 17.5 W h kg^-1,and the power density increased from 42 W kg^-1 to 2400 W kg^-1.According to the results of microstructure and surface state,this is because a large number of oxygen-containing functional groups are adsorbed on the surface of the UNCD/MLG film after anodization,the surface area of the EDLC reaction area and defect sites increaseing.The UNCD/MLG film with special vertical multi-layer flake graphene structure not only has good electrochemical performance,but also has a surface state that is changed significantly from superhydrophobicity to hydrophilicity through proper surface treatment which could result in excellent energy storage properties.Therefore,the film is a potential electrochemical energy storage material.