Controllable Synthesis Of Transition Metal Oxide/Carbon Nanocomposites And Studies Of Their Electrochemical Behaviors

Electrochemical capacitors, also called supercapacitors, are a new kind of electrochemical energy storage devices. Due to fast charging/discharging properties (1-30s), high power density (1-10kW/kg), excellent cycle life (100,000-500,000cycles) and high Coulombic efficiency (99%), supercapacitors attract much research attention and exhibit important applications in consumer electronics. However, they are suffering from the poor energy density (1-10Wh/kg). Improving the specific capacitance and enlarging the voltage window would increase the energy density further. Transitional metal oxides (TMO) provide multiple valence states, enable multiple charge transfer reactions and render high pseudocapacitance when TMO were used as the electrode materials. Traditional carbon materials own high electronic conductivity and developed pore channels. Therefore, coupling with carbon nanomaterials to form the TMO/C composite would utilize the electric double layer capacitance derived from the carbon substrate and the pseudocapacitance from TMO. Moreover, the electron transport would be facilitated on the carbon backbone. As a result, both high power density and high energy density would be possibly achieved at the same time. Therefore in this thesis, the morphologies, structures and electrochemical performances of three TMOs/carbon composites, including RuO2, Nb2O5and TiO2/carbon composites, were systematically studied. Main results are as follows:(1) Effects of doped nitrogen species’content and the RuO2loading on the electrochemical performances of the composite. It turned out that nitrogen-free or excess would cause severe RuO2aggregation, form large nanocrystallites and result in much decreased utilization of the redox active sites. Excess RuO2loading also led to the RuO2aggregation. Doping moderate nitrogen content and RuO2amount would maximize the composite’s capacitance, together with capacitive behaviors and high power handling properties of the composite. At0.2A/g, the specific capacitance per RuO2(g) in the composite reached772F/g. Although a remarkable high value of1733F/g per RuO2(g) was revealed at a loading amount of3.8wt.%, the composite’s capacitance was very low and unpractical in a commercial perspective. Such a unique dual effect between the doped nitrogen species and the RuO2amounts could be interpreted as the guiding role of the basic nitrogen sites, which improved the bonding force between the hydrophobic carbon surface and the RuO2nanoparticles, decreased the domain size and increased the utilization of RUO2as a result.(2) Effects of hydrothermal temperature, substrate carbonization temperature and post-annealing temperature on the electrochemical performances of the composite. It turned out that when the hydrothermal reaction was taken place at lower temperature, the formed RuO2was bonded to more water molecules and rendered higher pseudocapacitance, while higher temperature (180℃) resulted in more aggregated RuO2and posed a negative effect on the rate performance. Carbonization temperature affected the surface chemistry, the electric conductivity and ultimately the specific capacitance. For the post-annealed treatment, it was found that amorphous RuO2was partially converted into nanocrystallized RuO2at100℃and enhanced the electron transport, delivering463F/g at0.2A/g. However, when the composite was treated at300℃, almost all of the amorphous RuO2was converted into highly nanocrystalline RuO2, leading to a much decreased capacitance (278F/g at0.2A/g). Therefore, lower temperature annealing is an efficient method to promote the specific capacitance of NMC/RuO2.(3) Electrochemical behaviors of carbide-derived carbon (CDC)/Nb2O5composite. The CDC/Nb2O5was prepared through one-step hydrothermal method. The electrochemical behaviors and factors were studied systematically. It turned out that through adding phenylphosphonic acid (PPA) to the system, the Nb2O5dispersion was much enhanced and the particle size was largely decreased, resulted in the improvement of the specific capacitance. The mechanism could be interpreted as the PPA terminated the CDC with hydrophilic groups and acting as the nucleation sites for the uniform particle distribution. The composite exhibited fast pseudocapacitive responses in the optimized electrolyte (1M LiClO4/EC/DMC). Moreover, the CO2heat-treatment could convert the amorphous Nb2O5into highly crystallized orthorhombic Nb2O5(T-Nb2O5) without severe CDC combustion, forming the T-Nb2O5/CDC (mixed with monoclinic M-Nb2O5) with a highest stored charge of220C/g (1mV/s), and could still deliver157C/g at10mV/s.(4) Preparation of Nb2O5/C and its electrochemical behaviors. Two-dimensional Nb2CTx MXene was successfully converted into highly crystallized T-Nb2O5(mixed with M-Nb2O5) supported on layered carbon/Nb2CTx sheets when it was heated in CO2flow at850℃for1h. It was found that the Li+intercalation/deintercalation was a surface-controlled process at the scan rate below20mV/s. The thermodynamic calculation suggested the oxidation temperature and CO2feed exhibited great effects on the Nb2O5phase and the composition of the as-prepared material. The experimental results also suggested that the formation of orthorhombic Nb2O5occurred at750℃. Pseudohexagonal Nb2O5(TT-Nb2O5) would be formed at600℃while T-Nb2O5/C (mixed with M-Nb2O5) would be formed at850℃. When it was used as the pseudocapacitor electrode material, the oxidized Nb2CTx delivered366F/g with a cycling performance of96%after2000cycles. When it was used as Li-ion battery anode material, the oxidized Nb2CTx delivered a maximum specific capacity of180mAh/g (50mA/g). Highly crystallized T-Nb2O5/C composite could also be prepared from Nb4C3Tx using the CO2oxidation.(5) Preparation of A-TiO2/C composite and its electrochemical behaviors. Layered anatased TiO2(A-TiO2)/C with highly crystallized TiO2was prepared from Ti3C2Tx using CO2oxidation. When the oxidized Ti3C2Tx was used as the pseudocapacitor electrode, it exhibited368C/g (0.5mV/s) and retained85.4%of the initial capacitance after2000cycles. When the oxidized Ti3C2Tx was used as Li-ion battery anode, its Coulombic efficiency reached100%and retained99%of the initial capacitance after400cycles. Further extending the oxidation technique to hydrothermal oxidation, the A-TiO2/C was prepared with highly crystallized TiO2. A maximum capacitance of426C/g (0.5mV/s) was achieved. Therefore through these examples, the oxidation method developed in this thesis could be applied to prepare nitrogen-doped A-TiO2/carbon, V2O5/C and MoO3/C from Ti3CNTx, V2CTX and Mo2CTx MXenes, respectively.