Free-standing vertically aligned carbon nanotubes-graphene foam composite electrode for high power rechargeable Zn-air battery

• Main Author: Cai, Xiaoyi
• Other Authors: Shen Zexiang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Energy materials; Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/138175
• Institution: Nanyang Technological University

As the requirement for energy storage devices’ energy density raises, there is an urgent need to develop batteries with higher energy than the mainstream Li-ion battery. Metal-air batteries are a hopeful candidate for the concept of such post-lithium batteries. ZABs are especially promising because the primary form of Zn-air battery is already commercialized. The most important component that determines a Zn-air battery’s performance is the air-electrode, whose catalytic activity and reactant diffusion properties determines the maximum power the Zn-air battery can achieve. The rechargeability and efficiency also largely depends on the bi-functional activity of the catalyst. The goal of this thesis is to fabricate materials suitable for free-standing electrodes for Zn-air batteries. In the first part of the thesis, a free-standing, nitrogen-doped vertically aligned carbon nanotubes on graphene foam was synthesized. The sample has good catalytic activity for oxygen electrochemical reactions. The rechargeable Zn-air battery assembled by this sample had higher power density than the electrode made of commercial Pt/C electrocatalyst. In the second part of the thesis, the free-standing structure was functionalized with N, P heteroatoms doped carbon. The sample was shown to have good catalytic activity for both oxygen reduction reaction and oxygen evolution reaction. The benefit of the efficient gas diffusion and electron transfer caused by the structure of the material was also discussed. The Zn-air battery assembled with this electrode achieved a high peak power density of 56mW cm-2 In rechargeable Zn-air battery test, the increase of combined overpotential was 0.2V after 75 hours of operation. Overall the Zn-air battery with the free-standing air electrode outperformed cells with commercial Pt/C//IrO2 catalyst. In third part of the thesis, the N,P-doped vertically aligned carbon nanotubes was decorated with Fe and Co sub-nanometer moieties as catalytical active centers for bifunctional oxygen reduction and oxygen evolution reaction. The composite material used as free-standing electrodes for Zn-air batteries achieved a peak power density of 95 W cm-2, higher than the cells made with commercial air diffusion layer and commercial catalyst Pt/C//IrO2, which was 57 W cm-2. After 285 hours of operation the roundtrip efficiency decreased by less than 1%.