Electrochemical Quartz Crystal Microbalance Studies On Ruthenium Supercapacitors

The supercapacitors is a kind of new energy-storing devices of high current density, high power density, multicycling tolerance, environment friendship and maintenance-free nature, which has high potential in relevant applications. At present, there are two problems in supercapacitor researches, namely, exploring of new inexpensive and high-performance capacitance materials, and improving the reaction efficiencies of currently available capacitance materials. Nowadays, the supercapacitors of the highest supercapacitive performance seem to be ruthenium (Ru)-based ones, however, the ruthenium supercapacitors are mainly used in military and aerospace owing to the high price of ruthenium. Even so, the mechanisms of capacitance reactions of ruthenium supercapacitor are still ambiguous and under dispute, which may largely impact the use and optimization of ruthenium supercapacitors.Currently, the mechanisms of many pseudocapacitors are not very clearly understood or still lacking of a reasonable universal explanation, which affects the applications of currently available available supercapacitors and the exploring of new high-performance and inexpensive capacitance materials.Graphene as a two-dimensional material (single atom layer carbon slice) exhibits unique properties of good conductivity, high structural strength, high surface area, and so on. The use of graphene to load ruthenium for developing supercapacitor can improve the supercapacitive performance, e.g. the reaction efficiency and mass specific capacitance.In this thesis, we briefly review the supercapacitor and graphene, and the electrochemical reactions of ruthenium is studied in detail by electrochemical quartz crystal microbalance (EQCM). The mechanisms of ruthenium electrochemical reactions and capacitance reaction of RuO2 are discussed, and the undetermined coefficients in the widely accepted supercapacitive reactions of RuO2 are quantified for the first time. The graphene and porous Au loaded ruthenium materials for supercapacitor applications are investigated. The main contents are as follows.1. The EQCM was used to study the capacitance, stability and electrochemical reactions of Ru in aqueous H2SO4, which had been electrodeposited in different aqueous acids (0.2 M H2SO4,0.1 M HCl,0.2 M HClO3 or HNO3). The result shows the capacitance and stability of ruthenium electrodeposited in aqueous H2SO4 is the best, and the capacitance decrease with the thickness of Ru increase. The capacitance of ruthenium electrodeposited on multiwalled carbon nanotubes (MWCNTs) modified Au electrode is larger than that on bare Au electrode. We used the EQCM data to discuss the ruthenium electrochemical reactions, including the pseudocapacitance reaction of RuO2, and the undetermined coefficients in the widely accepted supercapacitive reactions of RuO2 are quantified. The EQCM conclusion is supported by experiments of scanning electron microscope (SEM) and the X-ray diffraction (XRD). We conclude that the morphology and crystal structure of RuO2 change negligibly during the supercapacitive reaction, and the supercapacitive reaction has the smallest activation energy, so the capacitance current is highly reversible. This conclusion may hold for many other supercapacitance reactions of other pseudocapacitance materials, which may be helpful in explaining the reaction mechanisms of many other supercapacitors and in searching new high-performance and inexpensive pseudocapacitive materials.2. We prepared a series of composites of electrodeposited Ru and graphene and investigated their supercapacitive performance. Briefly, adding RuCl3 to a dispersion of sulfuric acid and carboxyl graphene (CGRA) followed by potentiostatic deposition yielded a composite of electrodeposited Ru and graphene. The characterizations of SEM and X-ray energy dispersive spectroscopy (EDS) prove the successful loading of the composite on the electrode and the uniform dispersion of graphene and ruthenium in the composite. The specific capacitance reaches 737 F g-1, the power density is 2.5 kW kg-1 (5 A g-1) and the energy density is 100 Wh kg-1. The such-prepared ruthenium-graphene composite exhibits good supercapacitive performance, and the new preparation method is expected to find wide applications for studies of other nanocomposites and supercapacitors.3. We prepared porous Au by Au electrodeposition in the presence of electroactive glucose template, and the porous Au has a 3.92-fold increased surface area (vs. bare gold). Then, Ru was electrodeposited on porous Au, and the reaction efficiency and specific capacitance of RuO2 were efficiently increased owing to the increased surface area (vs. bare gold). The specific capacitance reaches 1138 F g-1, the power density is 5 kW kg-1 (10 A g-1) and the energy density is 165 Wh kg-1.The new method is simple and highly effective, which is recommended for wide applications in film electrochemistry.