Elucidating the Volcanic-Type Catalytic Behavior in Lithium-Sulfur Batteries via Defect Engineering.

Defects are generally considered to be effective and flexible in the catalytic reactions of lithium-sulfur batteries. However, the influence of the defect concentration on catalysis remains ambiguous. In this work, molybdenum sulfide with different sulfur vacancy concentrations is comprehensively modulated, showing that the defect level and the adsorption-catalytic performance result in a volcano relationship. Moreover, density functional theory and in situ experiments reveal that the optimal level of sulfur defects can effectively increase the binding energy between molybdenum sulfide and lithium polysulfides (LiPSs), lower the energy barrier of the LiPS conversion reaction, and promote the kinetics of Li2S bidirectional catalytic reaction. The lower bidirectional catalytic performance incited by excessive or deficient sulfur defects is mainly due to the deformed geometrical structures and reduced adsorption of key LiPSs on the catalyst surface. This work underscores the imperative of controlling the defect content and provides a potential approach to the commercialization of lithium-sulfur batteries.

Cutting COF-like C4 N to Give Colloidal Quantum Dots: Towards Optical Encryption and Bidirectional Sulfur Chemistry via Functional Group and Edge Effects.

Herein, we report the first synthesis of colloidal C4 N quantum dots (QDs) and their functional composites and explore their optical activities and edge-selective polysulfide adsorption-catalysis. As-obtained C4 NQDs are rich in carbonyl groups and edges, allowing good solution processability and facile assembly with other moieties for creating functionalities. While C4 NQDs show normal fluorescence (FL), the QD/poly(vinyl alcohol) (PVA) composites give FL/room-temperature-phosphorescence (RTP) dual-mode emission, enabling the corresponding solution to be used as an encryption ink. The QDs anchored onto carbon nanotubes can be used as a barrier layer to decorate commercial separators, endowing a Li-S cell with excellent cycling stability, high rate capability, and large areal capacity. Computation and experiment studies show that edge sites in C4 N favor polysulfide adsorption and catalysis and the enriched edges and carbonyl groups in QDs synergically promotecatalytic conversion of sulfur species.

Capturing Visible Light in Low-Band-Gap C4 N-Derived Responsive Bifunctional Air Electrodes for Solar Energy Conversion and Storage.

We report facile synthesis of low-band-gap mesoporous C4 N particles and their use as responsive bifunctional oxygen catalysts for visible-light-sensitive (VLS) rechargeable Zn-air battery (RZAB) and polymer-air battery (RPAB). Compared to widely studied g-C3 N4 , C4 N shows a smaller band gap of 1.99 eV, with a larger photocurrent response, and it can function as visible-light-harvesting antenna and bifunctional oxygen reduction/evolution (ORR/OER) catalysts, enabling effective photocoupling to tune oxygen catalysis. The C4 N-enabled VLS-RZAB displays a low charge voltage of 1.35 V under visible light, which is below the theoretical RZAB voltage of 1.65 V, corresponding to a high energy efficiency of 97.78 %. Pairing a C4 N cathode with a polymer anode also endows an VLS-RPAB with light-boosted charge performance. It is revealed that the ORR and OER active sites in C4 N are separate carbon sites near pyrazine-nitrogen atoms and photogenerated energetic holes can activate OER for improved reaction kinetics.

Redox Donor-Acceptor Conjugated Microporous Polymers as Ultralong-Lived Organic Anodes for Rechargeable Air Batteries.

Herein, we explore a new redox donor-acceptor conjugated microporous polymer (AQ-CMP) by utilizing anthraquinone and benzene as linkers via C-C linkages and demonstrate the first use of CMP as ultralong-lived anodes for rechargeable air batteries. AQ-CMP features an interconnected octupole network, which affords not only favorable electronic structure for enhanced electron transport and n-doping activity compared to linear counterpart, but also high density of active sites for maximizing the formula-weight-based redox capability. This coupled with highly cross-linked and porous structure endows AQ-CMP with a specific capacity of 202 mAh g-1 (96 % of theoretical capacity) at 2 Ag-1 and ≈100 % capacity retention over 60000 charge/discharge cycles. The assembled CMP-air full cell shows a stable and high capacity with full capacity recovery after only refreshing cathodes, while the decoupled electrolyte and cathode design boosts the discharge voltage and voltage efficiency to ≈1 V and 87.5 %.

Versatile, Aqueous Soluble C2N Quantum Dots with Enriched Active Edges and Oxygenated Groups.

C2N has emerged as a new family of promising two-dimensional (2D) layered frameworks in both fundamental studies and potential applications. Transforming bulk C2N into zero-dimensional quantum dots (QDs) could induce unique quantum confinement and edge effects that produce improved or new properties. Despite their appealing potential, C2NQDs remain unexplored, and their intriguing properties and a fundamental understanding of their prominent edge effects are still not well understood. Here, we report the first synthesis of water-soluble C2NQDs via a top-down approach without any foreign stabilizer and exploit their linear/nonlinear optical properties and unique edge-preferential electrocatalytic activity toward polysulfides for versatile applications. The resultant dispersant-free C2NQDs with an average size of less than 5 nm feature rich oxygen-carrying groups and active edges, not only enabling excellent dispersion in water but also creating interesting multifunctionality. They can emit not only blue one-photon luminescence (OPL) under ultraviolet (UV) excitation but also green two-photon luminescence (TPL) with a wide near-infrared (NIR) excitation range of 750-900 nm, enabling their use as a new fluorescent ink. Interestingly, when C2NQDs are introduced to modify commercial separators, they can function as new metal-free catalysts to boost polysulfide redox kinetics and endow Li-S batteries with excellent cycling stability, high rate capability, and large areal capacity (7.0 mA h cm-2) at a high sulfur loading of 8.0 mg cm-2. Detailed theoretical and experimental results indicate that the edge of C2N is more favorable for trapping and catalyzing the polysulfide conversion than the terrace and that the synergy between the active edges and oxygenated groups enriched in C2NQDs remarkably improves polysulfide immobilization and catalytic conversion.