Shu-Mao Xu

Nitrogen-doped carbon nets with micro/mesoporous structures as electrodes for high-performance supercapacitors

Carbon-based supercapacitors with high power densities suffer from relatively low energy densities. Nitrogen-doped hierarchical micro/mesoporous carbon nets were successfully fabricated via supramolecular assemblies of block copolymer P123 with the assistance of dicyandiamide and TiO2. The as-obtained carbon nets have a large specific surface area of approximately 2144 m2 g−1, high-level nitrogen doping with a nitrogen content of approximately 8.25 wt%, and a hierarchical porous network composed of micropores and mesopores. The hierarchical porous carbon nets exhibit high specific capacitance (537.3 F g−1 and 306.3 F cm−3 at 0.5 A g−1 in a 0.5 M H2SO4 electrolyte) and outstanding cycling stability (approximately 98.8% of their capacitance was retained after 10 000 cycles at 5 A g−1). The symmetric supercapacitor based on these hierarchical porous carbon nets could deliver a maximum energy density up to 22.6 W h kg−1. The improved electrochemical performance of these carbon nets stems from both high surface area and the hierarchical micro/mesoporous structure, which provides an accessible pathway for electrolyte transport. In addition, the incorporation of nitrogen dopants into the carbon was intended to further enhance the capacitance performance. This research provides a facile and effective method to obtain micro/mesoporous carbon with a high surface area and doping level of heteroatoms for high-performance supercapacitors.

Hydroquinone Resin Induced Carbon Nanotubes on Ni Foam As Binder-Free Cathode for Li-O2 Batteries.

In this work, hydroquinone resin was used to grow carbon nanotubes directly on Ni foam. The composites were obtained via a simple carbonization method, which avoids using the explosive gaseous carbon precursors that are usually applied in the chemical vapor deposition method. When evaluated as cathode for Li-O2 batteries, the binder-free structure showed enhanced ORR/OER activities, thus giving a high rate capability (12690 mAh g(-1) at 200 mA g(-1) and 3999 mAh g(-1) at 2000 mA g(-1)) and outstanding long-term cycling stability (capacity limited 2000 mAh g(-1), 110 cycles at 200 mA g(-1)). The excellent battery performance provides new insights into designing a low-cost and high-efficiency cathode for Li-O2 batteries.

Toward Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium–Oxygen Batteries

To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. Hierarchically porous CoN nanorods were obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 °C. Compared with that of the pristine Co3O4 precursor before nitridation, the overpotential of the obtained CoN cathode was significantly decreased. Moreover, specific capacity and cycling stability of the CoN nanorods were enhanced. It is assumed that the discharged products with different morphologies for Co3O4 and CoN cathodes might be closely associated with the variation in the electronic density induced by occupancy of nitrogen atoms into interstitial sites of metal lattice after nitridation. The nitridation strategy for improved electron density proposed in this work is proved to be a simple but efficient way to improve the electrochemical performance of metal oxide based cathodes for lithium-oxygen batteries.

Free-Standing Air Cathodes Based on 3D Hierarchically Porous Carbon Membranes: Kinetic Overpotential of Continuous Macropores in Li-O2 Batteries

Free-standing macroporous air electrodes with enhanced interfacial contact, rapid mass transport, and tailored deposition space for large amounts of Li2 O2 are essential for improving the rate performance of Li-O2 batteries. An ordered mesoporous carbon membrane with continuous macroporous channels was prepared by inversely topological transformation from ZnO nanorod array. Utilized as a free-standing air cathode for Li-O2 battery, the hierarchically porous carbon membrane shows superior rate performance. However, the increased cross-sectional area of the continuous macropores on the cathode surface leads to a kinetic overpotential with large voltage hysteresis and linear voltage variation against Butler-Volmer behavior. The kinetics were investigated based on the rate-determining step of second electron transfer accompanied by migration of Li+ in solid or quasi-solid intermediates. These discoveries shed light on the design of the air cathode for Li-O2 batteries with high-rate performance.