Because of the increasing energy and environmental problems energy conversion and storage systems with high efficiency, low cost and environmental benignity have attracted much attention in recent years. Among these energy systems, supercapacitor, rechargeable metal-air batteries and photoelectrochemistry cells (PEC) are thought to be possible energy storage and hydrogen generation solutions. But they still suffer from some bottlenecks. One of the most important bottlenecks is the design of suitable material for electrode, which can bring high capacitance and excellent catalysis performance at a low cost.

Nanohybridization strategy is a promising method to prepare suitable electrode materials because it can produce materials with characteristics different from individual components. A...[ Read more ]

Because of the increasing energy and environmental problems energy conversion and storage systems with high efficiency, low cost and environmental benignity have attracted much attention in recent years. Among these energy systems, supercapacitor, rechargeable metal-air batteries and photoelectrochemistry cells (PEC) are thought to be possible energy storage and hydrogen generation solutions. But they still suffer from some bottlenecks. One of the most important bottlenecks is the design of suitable material for electrode, which can bring high capacitance and excellent catalysis performance at a low cost.

Nanohybridization strategy is a promising method to prepare suitable electrode materials because it can produce materials with characteristics different from individual components. And it can also bring superior performance to individual materials. Because of these advantages, nanohybridization materials based on carbon, metal, metal oxide or semiconductor nanomaterials would be useful for energy stor age, optoelectronic and photovoltaic applications. My thesis is focused on the nanohybridization approach to electrode materials and oxygen catalysts for energy applications and the mechanism of their performance.

This thesis contains 7 chapters. Chapter 1 mainly introduces nanohybridization, the development of energy devices and the objects in my thesis. Chapter 2 introduces the experiment techniques used in my experiments. Major findings of my study are presented and discussed from chapter 3 to 6. And the conclusions and outlooks are shown in chapter 7.

In chapter 3, to increase the low energy density of supercapacitor, I designed a new flexible solid-state asymmetric supercapacitor which based on MnO₂/ZnO core-shell nanohybridization and reduce graphene oxide (rGO). The design was implemented by successfully preparing a MnO₂/ZnO core-shell nanorod array on carbon cloth which would not only provide a flexible substance but also increase the conductivity. After combining with rGO cathode and electrolyte containing PVA and LiCl, I developed a solid-state asymmetric supercapacitor (ASC) device based on the MnO₂/ZnO (positive electrode)//reduced graphene oxide (negative electrode), and achieved a maximum energy density of 0.234 m Wh cm^-3 which is higher than many previous reports such as the 1D MnO₂ nanomaterials, Au-doped MnO₂, MnO₂/CNT composite and MnO₂/graphene composite.

In chapter 4, to extend the catalytic environment of electrocatalyst for oxygen reduction and evolution reaction (OER/ORR), I prepared cobalt embedded nitrogen doped carbon nanotube using Co-Pc as the cobalt and nitrogen source. This catalyst showed an excellent catalyst performance for OER and ORR in alkaline and neutral media. In OER it showed a low overpotential (200 mV in 0.1 M KOH and 300 mV in neutral media), low Tafel slope (40 mV/Dec in 0.1 M KOH and 50 mV/Dec in neutral media) and prominent electrochemical durability. Moreover, it also exhibited a quasi-­four-electron process in ORR with a comparable performance and higher stability compared with Pt/C in alkaline and neutral media. These outstanding performances are mainly attributed to a combination of the embedded cobalt and doped nitrogen, which can improve the stability and reduce the overpotential.

In chapter 5, to reduce the voltage gap and increase the power density of Zn -air battery, I devised and investigated a novel type of boron/nitrogen co-doped graphene supported Mn doped CoS nanoparticles, which are highly active for both ORR and OER catalysis in 1 M KOH. The catalyst showed comparable ORR performance with Pt/C catalyst. For OER catalysis it showed superior performance than IrO₂ and some other reported catalysts. After ORR process the catalyst showed increased OER performance due to the improved percentage of s^2- and improved electrochemistry active area. The activity and durability of these materials outperformed many non-noble metal electrocatalysts. Based on this catalyst I designed a rechargeable Zn-air battery. The open-circuit voltage and peak power density of our battery is about 1.4 V and 258 mW cm^-2 respectively. Moreover it showed a low charge-discharge voltage gap of ~0.72 V at 20 mA cm^-2 and this value is much lower than some other rechargeable Zn-air batteries.

In chapter 6, to solve the electron-hole recombination which is the bottleneck of PEC electrode, I demonstrated a novel GQDs/BiVO₄ photoanode which exhibited excellent performance in solar water splitting. The ultrathin GQDs layer brings not only higher photo absorption but also lower photoelectron-hole recombination which is the bottleneck for BiVO₄. And we also used the Density functional theory (DFT) calculation to study the mechanism of GQDs in the composite. And I found the GQDs could make the photoelectrons injected faster and then reduce the recombination. With surface modification by an efficient FeNi LDH catalyst, solar water splitting efficiency over 2.2% at lower than 0.6 V (vs. CE) has been achieved for single BiVO₄ based photoanodes.

Chapter 7 gives the summary of my thesis and some outlook about the nanohybridization in the energy field.