Theoretical Study on the Quantum Capacitance Origin of Graphene Cathodes in Lithium Ion Capacitors

Supercapattery electrode materials by Design: Plasma-induced defect engineering of bimetallic oxyphosphides for energy storage

Although transition metal hydroxides are promising candidates as advanced supercapattery materials, they suffer from poor electrical conductivity. In this regard, previous studies have typically analyzed separately the impacts of defect engineering at the atomic level and the conversion of hydroxides to phosphides on conductivity and the overall electrochemical performance. Meanwhile, this paper uniquely studies the aforementioned methodologies simultaneously inside an all-in-one simple plasma treatment for nickel cobalt carbonate hydroxide, examines the effect of altering the nickel-to-cobalt ratio in the binder-free defect-engineered bimetallic Ni-Co system, and estimates the respective quantum capacitance. Results show that the concurrent defect-engineering and phosphidation of nickel cobalt carbonate hydroxide boost the amount of effective redox and adsorption sites and increase the conductivity and the operating potential window. The electrodes exhibit ultra-high-capacity of 1462 C g^-1, which is among the highest reported for a nickel-cobalt phosphide/phosphate system. Besides, a hybrid supercapacitor device was fabricated that can deliver an energy density of 48 Wh kg^-1 at a power density of 800 W kg^-1, along with an outstanding cycling performance, using the best performing electrode as the positive electrode and graphene hydrogel as the negative electrode. These results outperform most Ni-Co-based materials, demonstrating that plasma-assisted defect-engineered Ni-Co-P/PO_x is a promising material for use to assemble efficient energy storage devices.

Carbon-Based Quantum Dots for Supercapacitors: Recent Advances and Future Challenges

Carbon-based Quantum dots (C-QDs) are carbon-based materials that experience the quantum confinement effect, which results in superior optoelectronic properties. In recent years, C-QDs have attracted attention significantly and have shown great application potential as a high-performance supercapacitor device. C-QDs (either as a bare electrode or composite) give a new way to boost supercapacitor performances in higher specific capacitance, high energy density, and good durability. This review comprehensively summarizes the up-to-date progress in C-QD applications either in a bare condition or as a composite with other materials for supercapacitors. The current state of the three distinct C-QD families used for supercapacitors including carbon quantum dots, carbon dots, and graphene quantum dots is highlighted. Two main properties of C-QDs (structural and electrical properties) are presented and analyzed, with a focus on the contribution to supercapacitor performances. Finally, we discuss and outline the remaining major challenges and future perspectives for this growing field with the hope of stimulating further research progress.

First-principles study of stability, electronic structure and quantum capacitance of B-, N- and O-doped graphynes as supercapacitor electrodes

The structures, stability, electronic properties, and quantum capacitance (C _q) of α-, β-, and γ-graphyne monatomic layer doped with B, N, and O atoms, respectively, were studied using density functional theory. Two different doping sites (i.e. D₁ and D₂) were considered. Upon replacement of C atoms in the three graphynes with B and N atoms, the structure of graphynes was minimally distorted. This change was mainly manifested as a negligible adjustment of bond length around the doped atoms and lattice constant. However, with O atom doping, the structural distortion of graphynes was obvious in the majority of cases. The doping of these atoms significantly improved the electronic state of the original graphyne near the Fermi level, thereby improving graphyne C _q. Pristine graphynes with large pores and specific surface area exhibited better C _q performance than that of pure graphene. C _q of graphynes doped with B, N, and O showed significant advantage over that of doped graphene, especially that of α-, β-, and γ-graphyne with B doping at D₁ and α-graphyne with O doping at D₁. Interestingly, β-graphyne with O doping at D₂ showed a considerably symmetrical C _q. Thus, element-doped graphynes have great potential as electrode materials for supercapacitors.