Inherent N,O-containing carbon frameworks as electrode materials for high-performance supercapacitors

Suppressing Vanadium Dissolution in "Water-in-Salt" Electrolytes for 3.2 V Aqueous Sodium-Ion Pseudocapacitors

Low-cost sodium-ion-based electrochemical energy storage devices, especially vanadium-based sodium-ion pseudocapacitors, are receiving increasing attention. However, the inevitable dissolution of vanadium in aqueous electrolytes usually leads to poor cycling stability and a narrow electrochemical stability window (ESW). In this study, we prepared layered (NH₄)₂V₁₀O₂₅·8H₂O with a hierarchical flower-like structure and an ultralarge layer spacing and evaluated its potential as a sodium-ion pseudocapacitive material. Ex situ X-ray diffraction (XRD) measurement and kinetic analysis demonstrate the reversible intercalation and deintercalation of Na⁺ in (NH₄)₂V₁₀O₂₅·8H₂O in NaClO₄ electrolytes. Significantly improved durability and a large voltage window of 3.2 V are achieved in the high-concentration NaClO₄ electrolyte. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis and molecular dynamics (MD) simulations reveal that the dissolution of vanadium in the high-concentration NaClO₄ electrolyte can be effectively suppressed. An asymmetric sodium-ion capacitor with a wide voltage window of 3.2 V was successfully assembled, and it delivered a high energy density of 53.1 Wh kg^-1 at a power density of 3.2 kW kg^-1.

Bottom-up fabrication of triazine-based frameworks as metal-free materials for supercapacitors and oxygen reduction reaction

Doping porous carbon materials with heteroatoms is an effective approach to enhance the performance in the areas of supercapacitors and the oxygen reduction reaction (ORR). However, most traditional heteroatom-doped metal-free porous carbon materials have random structures and pore distributions with high uncertainty, which is harmful for a deep understanding of supercapacitors and the ORR mechanism. Basing on the molecular design, a series of N, O co-doped porous carbon frameworks (p-PYPZs) has been prepared through the template-free trimerization of cyano groups from our designed and synthesized 2,8-bis(4-isocyanophenyl)-2,3,7,8-tetrahydropyridazino[4,5-g]phthalazine-1,4,6,9-tetraone (PYPZ) monomer and subsequent ionothermal synthesis, which has the advantage that the type, position, content of the heteroatom and the pore structure in the porous carbon material can be regulated. Nitrogen and oxygen atoms introduced via covalent bond and the hierarchically porous structure endow the material with excellent cycling stability, and 110% capacitance retention after 35 000 cycles in 1 M H2SO4. A symmetric supercapacitor was assembled with the material and shows an energy density of 32 W h kg-1. The material can be applied to the area of oxygen reduction reaction as a metal-free catalyst with an onset potential of 0.85 V versus RHE, indicating the good catalytic ability. The material exhibits excellent methanol crossover resistance and a four-electron pathway mechanism. Results also indicate a positive correlation between the N-Q content and the selectivity of the four-electron pathway. In this paper, the electrochemical properties of materials are regulated at the molecular level, which provides a new idea for further understanding the electrochemical mechanism of energy storage devices.

A modelling algorithm for amorphous covalent triazine-based polymers

Rational and purposeful designs of amorphous materials with desirable structures are difficult to implement due to the complex and unordered nature of such materials. In this work, a modelling algorithm was proposed for amorphous covalent triazine-based polymers to construct atomistic representative models that can reproduce the experimentally measured properties of experimental samples. The constructed models were examined through comparisons of simulated and experimental properties, such as surface area, pore volume, and structure factor, and further validated by the good consistency observed among these properties. To assess the predictive capability of the modelling algorithm, we used a new covalent triazine-based polymer and predicted its porosity by constructing a simulated model. The predicted results on the surface area and pore volume of the simulated model were quantitatively consistent with the experimental data derived from the experimentally synthesized sample. This consistency reveals the predictive capacity of the proposed modelling algorithm. The algorithm could be a promising approach to predict and develop advanced covalent triazine-based polymers for multiple applications.

Synergetic Covalent and Spatial Confinement of Sulfur Species by Phthalazinone-Containing Covalent Triazine Frameworks for Ultrahigh Performance of Li-S Batteries

Lithium-sulfur (Li-S) batteries severely suffer from the shuttling of soluble polysulfides intermediates, insulation of sulfur and lithium sulfides, and volumetric expansion of sulfur electrodes, which result in the fast capacity decay and low utilization of active materials. To overcome these issues, a new type of porous phthalazinone-based covalent triazine frameworks (P-CTFs) with inherent N and O atoms has been in situ grown onto conductive reduced graphene oxide (rGO) by the sulfur-mediated cyclization of dinitrile monomers to afford the S/P-CTF@rGO hybrids. The well-designed structure endows the S/P-CTF@rGO composites with several features for enhanced Li-S batteries: (i) the nanoporous structure can spatially trap the sulfur species within the P-CTFs; (ii) the covalent binding of sulfur and polar groups of phthalazinone and triazine in P-CTFs exhibits strong chemical attachment and adsorption with polysulfides and further limits the diffusion of polysulfides; (iii) the conductive rGO and semiconductive P-CTFs help faster electronic transportation and accelerate the electrochemical process. Therefore, the S/P-CTF@rGO cathodes show greatly enhanced electrochemical performances with a high initial specific capacity of 1130 mAh g^-1 at 0.5C and a good capacity retention of 81.4% after 500 cycles, indicating only 0.04% degradation per cycle.

Atomistic structure generation of covalent triazine-based polymers by molecular simulation

The structures of amorphous materials are generally difficult to characterize and comprehend due to their unordered nature and indirect measurement techniques. However, molecular simulation has been considered as an alternative method that can provide molecular-level information supplementary to experimental techniques. In this work, a new approach for modelling the atomistic structures of amorphous covalent triazine-based polymers is proposed and employed on two experimentally synthesized covalent triazine-based polymers. To examine the proposed modelling approach, the properties of the established models, such as surface areas, pore volumes, structure factors and N₂ adsorption isotherms, were calculated and compared with the experimental data. Excellent consistencies were observed between the simulated models and experimental samples, consequently validating the proposed models and the modelling approach. Moreover, the proposed modelling approach can be applied to new covalent triazine-based polymers for predictive purposes and to provide design strategies for future synthesis works.

A well-established modelling approach to construct and predict the structures of amorphous covalent triazine-based polymers is proposed.

Nickel cobaltite@poly(3,4-ethylenedioxypyrrole) and carbon nanofiber interlayer based flexible supercapacitors

Binder free flexible symmetric supercapacitors are developed with nickel cobaltite micro-flower coated poly(3,4-ethylenedioxypyrrole) (NiCo2O4@PEDOP) hybrid electrodes. The free standing films of carbon nano-fibers (CNFs), synthesized by electrospinning, were sandwiched between the NiCo2O4@PEDOP hybrid and the electrolyte coated separators on both sides of the cells. The CNF film conducts both ions and electrons, and confines the charge at the respective electrodes, to result in an improved specific capacitance (SC) and energy density compared to the analogous cell without the CNF interlayers. A high SC of 1775 F g-1 at a low current density of 0.96 A g-1 and an SC of 634 F g-1 achieved at a high current density of 38 A g-1 coupled with an SC retention of ∼95% after 5000 charge-discharge cycles in the NCO@PEDOP/CNF based symmetric supercapacitor, are performance attributes superior to those achieved with NCO and NCO/CNF based symmetric cells. The PEDOP coating serves as a highly conductive matrix for the NCO micro-flowers and also undergoes doping/de-doping during the charge-discharge, thus amplifying the overall supercapacitor response, compared to the individual components. The CNF interlayers show reasonably high ion-diffusion coefficients for K+ and OH- propagation, implying facile pathways available for the movement of ions across the cross-section of the cell, and they also serve as ion reservoirs. The electrode morphologies remain unaffected by cycling in the presence of the CNF interlayer. LED illumination and a largely unaltered charge storage response were achieved in a mutli-cell configuration, proving the potential for this approach in practical applications.

Improved Mechanical Properties of Copoly(Phthalazinone Ether Sulphone)s Composites Reinforced by Multiscale Carbon Fibre/Graphene Oxide Reinforcements: A Step Closer to Industrial Production

The properties of carbon fibre (CF) reinforced composites rely heavily on the fibre-matrix interface. To enhance the interfacial properties of CF/copoly(phthalazinone ether sulfone)s (PPBES) composites, a series of multiscale hybrid carbon fibre/graphene oxide (CF/GO) reinforcements were fabricated by a multistep deposition strategy. The optimal GO loading in hybrid fibres was investigated. Benefiting from the dilute GO aqueous solution and repeated deposition procedures, CF/GO (0.5%) shows a homogeneous distribution of GO on the hybrid fibre surface, which is confirmed by scanning electron microscopy, atomic force microscope, and X-ray photoelectron spectroscopy, thereby ensuring that its PPBES composite possesses the highest interlaminar shear strength (91.5 MPa) and flexural strength (1886 MPa) with 16.0% and 24.1% enhancements, respectively, compared to its non-reinforced counterpart. Moreover, the incorporation of GO into the interface is beneficial for the hydrothermal ageing resistance and thermo-mechanical properties of the hierarchical composite. This means that a mass production strategy for enhancing mechanical properties of CF/PPBES by regulating the fiber-matrix interface was developed.

Building metal-functionalized porous carbons from microporous organic polymers for CO2 capture and conversion under ambient conditions

Metal-functionalized porous carbons derived from microporous organic polymers remain highly desired for their intriguing physical and chemical properties.

Nitrogen-rich hierarchical porous carbon materials with interconnected channels for high stability supercapacitors

Novel nitrogen-rich hierarchical porous carbon (NPC) materials have been successfully synthesized using eggshells as the template and activation agent.

Six Isomorphous Window-Beam MOFs: Explore the Effects of Metal Ions on MOF-Derived Carbon for Supercapacitors

Six isomorphous metal-organic frameworks (MOFs) with a 3D window-beam architecture have been synthesized from solvothermal reactions, and are named Zn, Cd, Ni, Co, Mn and Cu-MOF, respectively. The series of MOFs was utilized as precursors to synthesize MOF-derived carbon with different morphologies. Zn and Cd-MOFs lead to the derivation of porous carbons (PCs), which exhibit remarkable BET specific surface areas. For derivates of Ni, Co and Mn-MOFs, graphitized carbons (GCs) show some carbon graphitization, but their BET specific surface areas are relatively small. C-Cu has the smallest BET specific surface area, and there is no carbon graphitization. Therefore, the metal ion of the parent MOF exerts a crucial effect on the preparation of MOF-derived carbon, such as the pore-forming effect of Zn and Cd species, and catalytic graphitization of Ni, Co, and Mn species. The capacitances of MOF-derived carbon follow the sequence of PCs>GCs>C-Cu, which reveals that the specific surface area plays a dominant role in the capacitive performance of electrical double layer capacitors (EDLCs), and that the graphitization could improve the capacitance. Significantly, PC-Zn exhibits the best specific capacitance (138 F g^-1 at 0.5 Ag^-1 ), and excellent life cycle, which can be applied as an electrode material in supercapacitors.

Nacre-like laminate nitrogen-doped porous carbon/carbon nanotubes/graphene composite for excellent comprehensive performance supercapacitors

A nitrogen-doped porous carbon/carbon nanotubes/graphene (PGMC) composite was prepared through a process of hydrothermal treatments, polymerization of o-phenylenediamine (OPD), and pyrolysis. The as-prepared PGMC composite was found to be of a nacre-like laminate porous structure, constructed with alternatively stacked two-dimensional (2D) graphene sheets and porous carbons, and also interspersed within one-dimensional (1D) multi-walled carbon nanotubes (MWNTs). The MWNTs effectively suppressed agglomeration of graphene sheets during the hydrothermal process and were interspersed in PGMC to help construct more networks with excellent conductivity. The PGMC possessed an enriched nitrogen doping ratio of 15.67 at% and relative high density of 1.39 g cm-3. The electrode composed of PGMC provided high gravimetric capacitance of 562.9 F g-1 and volumetric capacitance of 782.4 F cm-3 at current density of 1 A g-1, as well as excellent rate capability and cycling stability. The symmetric supercapacitors mounted with the as-prepared PGMC electrode were stably operated in a wide potential range of 0-1.3 V and demonstrated a superb gravimetric energy density of 19.79 W h kg-1 at high power density of 650 W kg-1, and a high volumetric energy density of 27.51 W h L-1 with a power density of 904 W L-1. The outstanding electrochemical performance enables this as-prepared nacre-like laminate PGMC composite to be a promising candidate for energy storage application.

Three-dimensional conductive porous organic polymers based on tetrahedral polythiophene for high-performance supercapacitors

Three dimensional porous organic polymers with excellent electrochemical performance and good cyclic stability were constructed by introducing conductive polythiophene units into the frameworks with diamond topology.

Engineered Fabrication of Hierarchical Frameworks with Tuned Pore Structure and N,O-Co-Doping for High-Performance Supercapacitors

A series of multiheteroatom porous carbon frameworks (MPCFs) is prepared successfully from the trimerization of cyano groups of our designed and synthesized 4,4'-(4-oxophthalazine-1,3(4H)-diyl)dibenzonitrile monomers and subsequent ionothermal synthesis. Benefiting from the molecular engineering strategy, the obtained MPCFs framework show a homogeneous distribution of nitrogen and oxygen heteroatoms at the atomic level, confirmed by the transmission electron microscopy mapping intuitively, thereby ensuring the stability of electrical properties. The supercapacitor with the obtained MPCFs@700 as the electrode exhibits a high energy density of 65 Wh kg^-1 at 0.1 A g^-1, with excellent long cycle life and cycle stability (98% capacitance retention for 10 000 cycles in 1-butyl-3-methylimidazolium tetrafluoroborate). Another two electrolyte systems employed also demonstrate the delightful results, showing a 112% capacitance retention for 30 000 cycles in 1 M H₂SO₄ and a 95% capacitance retention for 30 000 cycles in tetraethylammonium tetrafluoroborate in an acetonitrile solution. Moreover, the successful preparation of MPCFs provides new insights into the fabrication of electrode materials intrinsically containing nitrogen and oxygen in the frameworks for readily available components through a facile routine.

Solvent-Induced Cadmium(II) Metal-Organic Frameworks with Adjustable Guest-Evacuated Porosity: Application in the Controllable Assembly of MOF-Derived Porous Carbon Materials for Supercapacitors

In this work, five new cadmium metal-organic frameworks (Cd-MOFs 1-5) have been synthesized from solvothermal reactions of Cd(NO₃ )₂ ⋅4 H₂ O with isophthalic acid and 1,4-bis(imidazol-1-yl)-benzene under different solvent systems of CH₃ OH, C₂ H₅ OH, (CH₃ )₂ CHOH, DMF, and N-methyl-2-pyrrolidone (NMP), respectively. Cd-MOF 1 shows a 3D diamondoid framework with 1D rhombic and hexagonal channels, and the porosity is 12.9 %. Cd-MOF 2 exhibits a 2D (4,4) layer with a 1D parallelogram channel and porosity of 23.6 %. Cd-MOF 3 has an 8-connected dense network with the Schäfli symbol of [4²⁴ ⋅6⁴ ] based on the Cd₆ cluster. Cd-MOFs 4-5 are isomorphous, and display an absolutely double-bridging 2D (4,4) layer with 1D tetragonal channels and porosities of 29.2 and 28.2 %, which are occupied by DMF and NMP molecules, respectively. Followed by the calcination-thermolysis procedure, Cd-MOFs 1-5 are employed as precursors to prepare MOF-derived porous carbon materials (labeled as PC-me, PC-eth, PC-ipr, PC-dmf and PC-nmp), which have the BET specific surface area of 23, 51, 10, 122, and 96 m²  g^-1 , respectively. The results demonstrate that the specific surface area of PCs is tuned by the porosity of Cd-MOFs, where the later is highly dependent on the solvent. Thereby, the specific surface area of PCs could be adjusted by the solvent used in the synthese of MOF precusors. Significantly, PCs have been further activated by KOH to obtain activated carbon materials (APCs), which possess even higher specific surface area and larger porosity. After a series of characterization and electrochemical investigations, the APC-dmf electrode exhibits the best porous properties and largest specific capacitances (153 F g^-1 at 5 mV s^-1 and 156 F g^-1 at 0.5 Ag^-1 ). Meanwhile, the APC-dmf electrode shows excellent cycling stability (ca. 84.2 % after 5000 cycles at 1 Ag^-1 ), which can be applied as a suitable electrode material for supercapacitors.