Ternary nanocomposites of conductive polymer/functionalized GO/MOFs: Synthesis, characterization and electrochemical performance as effective electrode materials in pseudocapacitors

Approaches to Enhancing Electrical Conductivity of Pristine Metal-Organic Frameworks for Supercapacitor Applications

Metal-organic frameworks (MOFs), known as porous coordination polymers, have attracted intense interest as electrode materials for supercapacitors (SCs) owing to their advantageous features including high surface area, tunable porous structure, structural diversity, etc. However, the insulating nature of most MOFs has impeded their further electrochemical applications. A common solution for this issue is to transform pristine MOFs into more stable and conductive metal compounds/porous carbon materials through pyrolysis, which however losses the inherent merits of MOFs. To find a consummate solution, recently a surge of research devoted to improving the electrical conductivity of pristine MOFs for SCs has been carried out. In this review, the most related research work on pristine MOF-based materials is reviewed and three effective strategies (chemical structure design of conductive MOFs (c-MOFs), composite design, and binder-free structure design) which can significantly increase their conductivity and consequently the electrochemical performance in SCs are proposed. The conductivity enhancement mechanism in each approach is well analyzed. The representative research works on using pristine MOFs for SCs are also critically discussed. It is hoped that the new insights can provide guidance for developing high-performance electrode materials based on pristine MOFs with high conductivity for SCs in the future.

Efficient Photocatalytic Degradation of Gaseous Benzene and Toluene over Novel Hybrid PIL@TiO2/m-GO Composites

In this work, the PIL (poly ionic liquid)@TiO2 composite was designed with two polymerized ionic liquid concentrations (low and high) and evaluated for pollutant degradation activity for benzene and toluene. The results showed that PIL (low)@TiO2 composite was more active than PIL (high)@TiO2 composites. The photodegradation rate of benzene and toluene pollutants by PIL (low)@TiO2 and PIL (high)@TiO2 composites was obtained as 86% and 74%, and 59% and 46%, respectively, under optimized conditions. The bandgap of TiO2 was markedly lowered (3.2 eV to 2.2 eV) due to the formation of PIL (low)@TiO2 composite. Besides, graphene oxide (GO) was used to grow the nano-photocatalysts’ specific surface area. The as-synthesized PIL (low)@TiO2@GO composite showed higher efficiency for benzene and toluene degradation which corresponds to 91% and 83%, respectively. The resultant novel hybrid photocatalyst (PIL@TiO2/m-GO) was prepared and appropriately characterized for their microstructural, morphology, and catalytic properties. Among the studied photocatalysts, the PIL (low)@TiO2@m-GO composite exhibits the highest activity in the degradation of benzene (97%) and toluene (97%). The ultimate bandgap of the composite reached 2.1 eV. Our results showed that the as-prepared composites hold an essential role for future considerations over organic pollutants.

Stable Forward Osmosis Nanocomposite Membrane Doped with Sulfonated Graphene Oxide@Metal-Organic Frameworks for Heavy Metal Removal

A sulfonated graphene oxide@metal-organic framework-modified forward osmosis nanocomposite (SGO@UiO-66-TFN) membrane was developed to improve stability and heavy metal removal performance. An in situ growth method was applied to uniformly distribute UiO-66 nanomaterial with a frame structure on SGO nanosheets to form SGO@UiO-66 composite nanomaterial. This nanomaterial was then added to a polyamide layer using interfacial polymerization. The cross-linking between SGO@UiO-66 and m-phenylenediamine improved the stability of the nanomaterial in the membrane. Additionally, the water permeability was improved because of additional water channels introduced by SGO@UiO-66. SGO, with its lamellar structure, and UiO-66, with its frame structure, made the diffusion path of the solute more circuitous, which improved the heavy metal removal and salt rejection performances. Moreover, the hydrophilic layer of the SGO@UiO-66-TFN membrane could block contaminants and loosen the structure of the pollution layer, ensuring that the membrane maintained a high removal rate. The water flux and reverse solute flux of the SGO@UiO-66-TFN membrane reached 14.77 LMH and 2.95 gMH, and compared with the thin-film composite membrane, these values were increased by 41 and 64%, respectively. The membrane also demonstrated a good heavy metal ion removal performance. In 2 h, the heavy metal ion removal rate (2000 ppm Cu²⁺ and Pb²⁺) was greater than 99.4%, and in 10 h the removal rate was greater than 97.5%.

Insight into the Effect of ZIF-8 Particle Size on the Performance in Nanocarbon-Based Supercapacitors

Carbon materials derived from zeolitic imidazolate framework-8 (ZIF-8) and composites thereof have been intensively investigated in supercapacitors. The particle size of the used ZIF-8 ranges from dozens of nanometers to several microns. However, the influence of the particle size of ZIF-8 on the capacitive performances is still not clear. A series of ZIF-8 with different particle sizes (from 25 to 296 nm) has been synthesized and carbonized for supercapacitors. Based on TEM, EDX mapping, XRD, Raman, nitrogen adsorption-desorption, XPS, and the results of electrochemical tests, the optimal particle size (≈70 nm) for superior supercapacitor performances in both acidic and alkaline electrolytes has been obtained. This important result provides a significant reference to guide future ZIF-8 related research to achieve the best electrochemical performance.

Simultaneous improvement of mechanical and conductive properties of poly(amide-imide) composites using carbon nano-materials with different morphologies

Two conductive carbon materials, one with a beaded-like structure (carbon black, ECP) and another with tube-like structure (functionalized multi-walled carbon nanotubes, FMWCNTs), were added into a poly(amide-imide) (PAI) matrix. Combining the advantages of ECP (good compatibility) and FMWCNT (high conductivity), the conductivity was improved from 3.7 S m−1 for PAI/FMWCNT polymer composites to 100 S m−1 for PAI/FMWCNT/ECP ternary conductive polymer composites, much higher than that of the sum of PAI/ECP and PAI/FMWCNT. The tensile strength increased from 40 to 70 MPa. The improved conductive and mechanical properties were mainly due to much more intensive conductive network produced in the PAI/FMWCNT/ECP ternary composites, which is useful for electron flow and stress spread. The number of hydrogen bond was increased by adding ECP into PAI/FMWCNT binary composites, and played an important role in forming the unique morphology as evident by Fourier transform infrared spectrometry (FTIR) and X-ray diffraction (XRD) measurements. These conductive composites have potential for flexible electronic applications.

Construction of mesoporous carbon microsphere/polyaniline composites as high performance pseudocapacitive electrodes

Mesoporous carbon microspheres (MCMs), as a supercapacitor electrode material, have good gravimetric capacitance and rate performance; however, their low volumetric capacitance, which results from their low density, restricts their application as a micro power source. Herein, polyaniline was introduced into the channels of MCMs to achieve a synergistic effect and significantly increase the volumetric capacitance. MCMs with a high surface area and pore volume allowed the uniform dispersion of PANI within their channels in nanoscale dimensions. The interconnected carbon framework could provide excellent electrical conductivity and alleviate the structural collapse of PANI. Moreover, PANI could function not only as an active pseudocapacitive material that facilitated energy storage but also as a proton transport media that promoted a rapid protonation/deprotonation process during the redox reaction in the internal channels. As a result, PANI/MCM composites, even with a poor pore structure, delivered a high volumetric capacitance of 539F cm^-3 at 1 A g^-1 and an excellent rate performance of 83% at current densities ranging from 0.5 to 20 A g^-1. In addition, PANI/MCM composites exhibited good cycling stability, retaining 84% of the capacitance after 1000 charge/discharge cycles at 1 A g^-1, owing to the high mechanical strength of the MCMs. Therefore, this synthesis strategy could provide an efficient and scalable solution for the development of supercapacitor electrode materials.

Hydrogen peroxide-induced growth of hierarchical Ni3S2 nanorod/sheet arrays for high performance supercapacitors

The rational design and simple synthesis of high performance electrodes are important aspects of energy storage fields. However, it is difficult to determine a facile preparation method to obtain hierarchical Ni₃S₂ nanorod/sheet arrays. Herein, hydrogen peroxide-induced growth by the hydrothermal method is used to fabricate the hierarchical Ni₃S₂ nanorod/sheet arrays on nickel foam substrates. Hydrogen peroxide can accelerate the hydrolysis of Na₂S₂O₃ to release sulfur ions and then induce formation of the hierarchical nanorod/sheet. The one-dimensional nanorod skeleton acting as high-speed electron transfer channels can support the nanosheets. The two-dimensional nanosheets can provide abundant active edge sites and protect the backbone from electrochemical corrosion. The hierarchical structure integrates the advantages of sufficient active sites, effective protection and high-speed electron transfer. The electrode based on the Ni₃S₂ nanorod/sheet delivers a high specific capacitance of 6.87 F cm^-2 at 2 mV s^-1 (6.24 F cm^-2 at 5 mA cm^-2) and long cycling stability (85.7% capacitance retention at 15 mA cm^-2 after 3000 cycles). The asymmetric supercapacitor gains a high energy density of 1.16 mWh cm^-3 at 15.00 W cm^-3. The hierarchical Ni₃S₂ nanorod/sheet arrays are expected to be candidates for the electrodes of practical supercapacitors.

Bimetal-organic framework derived Cu(NiCo)2S4/Ni3S4 electrode material with hierarchical hollow heterostructure for high performance energy storage

Rational design of electrical active materials with high performance for energy storage and conversion is of great significance. Herein, Cu(NiCo)₂S₄/Ni₃S₄, a three-dimensional (3D) hierarchical hollow heterostructured electrode material, is designed by etching the well-defined bimetal organic framework (MOF) via sequential in-situ ion-exchange processes. This trimetallic sulfides with unique structure provide large surface area, hierarchical pore distribution and enhanced electrical conductivity, can enrich the active sites for redox reactions, facilitate electrolyte penetration and rapid charge transfer kinetics. As a result, the Cu(NiCo)₂S₄/Ni₃S₄ electrode exhibits a high specific capacitance of 1320 F/g at 1 A/g and excellent rate performance (only 15% of capacitance is attenuated when the current density is increased by 20 times). Furthermore, a fabricated hybrid supercapacitor of Cu(NiCo)₂S₄/Ni₃S₄/AC can deliver a maximum energy density of 40.8 Wh/kg, remarkable power density of 7859.2 W/kg and superior cycling stability (85% retention of capacitance after 5000 cycles), demonstrating great potential for practical applications in energy storage and conversion devices.

Metal–organic frameworks with different spatial dimensions for supercapacitors

Recent progress in MOF materials for SCs with different spatial dimensions, such as 2D MOFs, including conductive MOFs and nanosheets, and 3D MOFs, categorized as single metallic and multiple metallic MOFs, are reviewed.

Hierarchical NiCo2S4@NiCoP core-shell nanocolumn arrays on nickel foam as a binder-free supercapacitor electrode with enhanced electrochemical performance

A novel hierarchical core-shell nanocolumn array, with NiCo₂S₄ hollow nanowire (NiCo₂S₄ H-NW) as the core and NiCoP nanosheet (NiCoP NS) as the shell, has been directly synthesized on nickel foam (NF) as self-supported, binder-free electrode for high-performance supercapacitors. The morphological characterizations reveal that the diameter of NiCo₂S₄ H-NW core is ∼100 nm and the diameter of single NiCo₂S₄@NiCoP core-shell nanocolumn is ∼250 nm. Through a series of electrochemical tests and the analysis of charge storage kinetics, hierarchical NiCo₂S₄@NiCoP/NF electrode presents high areal specific capacitance of 5.98 F/cm² at 1 mA/cm², outstanding rate capability (70.29% capacitance retention with the current density increased from 1 to 50 mA/cm²) and superior cycling stability (92.94% of original capacity is retained after 5000 cycles at 10 mA/cm²). The prominent performance of NiCo₂S₄@NiCoP/NF electrode could be resulted from their unique hierarchical core-shell nanocolumn structure, which could offer abundant active sites near the interface for fast electrochemical reaction, and validly avoid the collapse of internal structure for the stability of whole structure in the repeated electrochemical measurement. The novel NiCo₂S₄@NiCoP/NF electrode offers a new method for future electrochemical energy storage devices with high-stability.