Oxygen-Vacancy Abundant Ultrafine Co3O4/Graphene Composites for High-Rate Supercapacitor Electrodes.

The primary purpose of this research is to discover new supercapacitor electrode materials to anticipate future requirements for achieving higher-performing materials for energy storage applications. Therefore, iron cobalt oxide was investigated as a more practical and affordable technique to generate multicationic oxide materials for use as supercapacitor electrodes. In this context, one-dimensional nanostructured binary metal oxides have garnered significant attention in the field of supercapacitor (SC) applications due to their exceptional capability for fast-charge transportation. In particular, high-performance pseudocapacitor electrodes could now be made using highly aligned nanospherical arrays directly grown on conducting substrates. The iron cobalt oxide (FeCo2O4 (FCO)) electrodes on carbon fiber cloth (CFC) have porous structures constructed from several small building blocks of primary nanospherical, contributing to the nanospherical-like morphology. With a surface area of 130.04 m2 g-1, the FCO-CFC nanocomposite electrode considerably increased the pseudocapacitors’ electrochemical activity. Moreover, the FCO-CFC nanocomposite electrode demonstrated exceptional cyclic stability, i.e., 66% retention of capacitance at a current density of 10 mA g-1 after a process of 1000 cycles and a current density of 10 mA g-1 at a surprisingly high specific capacitance of 225 F g-1 for a nanocomposite electrode. In addition, the unique porous nanospherical texture, the good conductivity, and the high effectiveness can be credited to the asymmetric supercapacitor employing FCO-CFC electrodes that achieved acceptable electrochemical efficiency due to the synergistic interaction between the FCO and the CFC.

A novel core–shell heterostructure with multi-walled carbon nanotubes as the core and graphene oxide nanoribbons as the shell (MWCNT@GONR), fabricated by the facile unzipping of MWCNTs with the help of microwave energy, was used as a supercapacitor (SC) electrode material. Graphene nanopowder (GNP) and multi-walled carbon nanotubes (MWCNTs) have also been applied as SC materials for comparison. A smooth surface and a tube-like structure are found for the GNP and MWCNTs, respectively, while for the MWCNT@GONR material, graphene oxide sheet structures are observed on both sides of central nanotube cores that retain their tube-like structure. The specific capacitance is much better for the SC electrode with the MWCNT@GONR (252.4 F g−1) compared to the SC electrodes with commercial MWCNTs (39.7 F g−1) and GNP (19.8 F g−1), as determined using cyclic voltammetry (CV) at a scan rate of 50 mV s−1, which is due to the defective edges of the nanostructures in the former. The SC electrode with the MWCNT@GONR also exhibits good stability and capacitance retention even after 1000 cycles of galvanostatic charge–discharge testing, indicating its potential as a SC material. CV, galvanostatic charge–discharge (GC/D) and electrochemical impedance spectroscopy (EIS) were applied to analyze the SC performance.

Copper tin sulfide, Cu4SnS4 (CTS), a ternary transition-metal chalcogenide with unique properties, including superior electrical conductivity, distinct crystal structure, and high theoretical capacity, is a potential candidate for supercapacitor (SC) electrode materials. However, there are few studies reporting the application of Cu4SnS4 or its composites as electrode materials for SCs. The reported performance of the Cu4SnS4 electrode is insufficient regarding cycle stability, rate capability, and specific capacity; probably resulting from poor electrical conductivity, restacking, and agglomeration of the active material during continued charge-discharge cycles. Such limitations can be overcome by incorporating graphene as a support material and employing a binder-free, facile, electrodeposition technique. This work reports the fabrication of a copper tin sulfide-reduced graphene oxide/nickel foam composite electrode (CTS-rGO/NF) through stepwise, facile electrodeposition of rGO and CTS on a NF substrate. Electrochemical evaluations confirmed the enhanced supercapacitive performance of the CTS-rGO/NF electrode compared to that of CTS/NF. A remarkably improved specific capacitance of 820.83 F g-1 was achieved for the CTS-rGO/NF composite electrode at a current density of 5 mA cm-2, which is higher than that of CTS/NF (516.67 F g-1). The CTS-rGO/NF composite electrode also exhibited a high-rate capability of 73.1% for galvanostatic charge-discharge (GCD) current densities, ranging from 5 to 12 mA cm-2, and improved cycling stability with over a 92% capacitance retention after 1000 continuous GCD cycles; demonstrating its excellent performance as an electrode material for energy storage applications, encompassing SCs. The enhanced performance of the CTS-rGO/NF electrode could be attributed to the synergetic effect of the enhanced conductivity and surface area introduced by the inclusion of rGO in the composite.

Major concerns about the effects of increasing fossil fuel consumption on the environment and energy security have prompted the development of sustainable and environmentally-friendly energy conversion and storage technologies based on electrochemical processes (e.g. water electrolysers, batteries and supercapacitors). Electrode materials are a key component of these technologies, and high-performance electrode systems are essential for the realization of a clean-energy-based economy. Numerous efforts have been made to develop advanced electrode materials for energy conversion and storage applications. However, current electrode synthesis methods are usually energy-intensive, not environmentally friendly, difficult to scale, or costly to produce. This thesis aims to utilize electrode structure engineering to develop highperformance electrodes based on earth-abundant materials via low-cost, energy-efficient and green synthesis strategies. Further, the applications of these electrodes in various energy conversion and storage applications are explored. Nickel-iron oxides or hydroxides are considered promising electrocatalysts for the oxygen evolution reaction, featuring a high activity and long cycling life in alkaline solution. A room temperature, electroless method has been developed here to grow nickel-iron hydroxides on a nickel foam current collector. The activity of nickel foam for the oxygen evolution reaction can be remarkably enhanced by simply immersing the nickel foam in a ferric nitrate solution at room temperature. During this process, the oxidation of the nickel foam surface by ferric nitrate ions increases the near-surface concentration of hydroxide ions, which results in the in situ deposition of a highly active, amorphous nickel-iron hydroxide layer. This phenomenon is described in Chapter 2 of this thesis. Carbon cloth is a widely-adopted current collector for the fabrication of electrodes. A facile, two-step method has been investigated here to turn commercial carbon cloth into a high-performance electrode for zinc-air batteries. Mild acid oxidation followed by air calcination directly activate carbon cloth to generate uniform, nanoporous and superhydrophilic surface structures with optimized, oxygen-rich functional groups and dramatically increased surface area. This two-step-activated carbon cloth exhibits superior bifunctional oxygen electrocatalytic activity and durability. A rechargeable, flexible zinc-air battery using the activated carbon cloth as a binder-free, flexible air electrode yields a remarkably high peak power density, high flexibility, and good cycling performance, with a small charge-discharge voltage gap. This work is elaborated in Chapter 3. Cost-effective synthesis of large-scale, uniform electrode materials with high activity and cycling stability is challenging. In Chapter 4, a reaction environment confinement strategy for scalable and reproducible production of nanostructured materials is proposed. Nickel foam is simply immersed in metal nitrate aqueous solution, with the volume of solution per unit area of nickel foam kept very low. A precisely designed reactor with a spiral tunnel ensures the same width of solution on each side of the nickel foam. The reaction environment is confined to ensure reproducible and uniform synthesis of nanostructured materials across the Ni foam. This approach has the largest REAVC (ratio of electrode area to precursor volume consumption) value reported so far, 2.0 cm2 mL-1. The synthesized nickel-iron hydroxides/nickel foam electrodes with uniformity in both microstructure and electrochemical properties exhibit remarkable activity for both the oxygen evolution reaction and hydrogen evolution reaction. Manganese oxides are a class of promising electrode materials for high performance supercapacitors. However, not all types of manganese oxides with different phases are electrochemically active, and their crystal structures have a considerable effect on their capacitance. In Chapter 5, a facile strategy is developed for the transformation of manganese oxide from the orthorhombic to birnessite crystal structure. The product exhibits significantly enhanced electrochemical performance as a supercapacitor electrode. This work opens up new possibilities for changing the crystal structure of manganese oxides towards optimized properties in electrochemical applications. This thesis makes significant contributions to our understanding of electrode structure engineering, materials science and electrochemical energy conversion and storage through: (i) designing novel nanostructured nickel-foam-based electrode systems with high electrocatalytic activity towards water oxidation via a simple immersion strategy at ambient temperature; (ii) developing facile activation procedures to endow commercially available, inactive carbon cloth with oxygen-rich functional groups and high oxygen electrocatalytic activity; (iii) controlling ion diffusion in a confined zone for uniform deposition of active materials over large-size electrodes, electrodes useful for various electrochemical applications; (iv) probing the phase transformation of manganese oxides from orthorhombic to birnessite, a material with enhanced electrochemical performance; (v) investigating the growth mechanisms of these advanced electrode materials to understand the origin of their exceptional activity.

Recent advances on supercapacitor electrode materials from biowastes- a review

Spindle-shape ferric oxyhydroxides with nano-sized grains for efficient oxygen evolution reaction and supercapacitors

Strong interaction between polyaniline and carbon fibers for flexible supercapacitor electrode materials

As one of the promising energy storage and conversion systems, supercapacitors (SCs) are highly favored owing to their high power density and good service life. Among all the key components of supercapacitor devices, the design and investigation of electrode materials play an essential role in determining the whole electrochemical charge storage performance. Recently, nanocarbon‐based materials (e.g., graphene, carbon dots, graphene quantum dots, etc.) have been widely used as SC electrode materials because of their good physical structure and chemical properties, providing a new route to further improve the energy density and life span of SCs. Here, we review the latest progress of nanocarbon‐based materials (including nanocarbon and nanocarbon‐based composite materials) as electrode materials in SCs application. The recent progress of carbon dots, graphene, carbon nanotubes, and other nanocarbon materials electrodes is summarized, while the capacitance and energy density of the above nanocarbon electrodes still need to be improved. Then, the preparation and performance of nanocarbon‐based composite electrodes comprising transition metal oxides, conductive polymer, and metal–organic framework derived porous carbon are reviewed. Finally, we outline major challenges and propose some ideas on building better nanocarbon‐based SC electrodes.

Novel three-dimensional architectured ZnMgAl ternary layered double hydroxide@reduced graphene oxide nanocomposites as electrode material for high-performance supercapacitor

Supercapacitors (SCs) have attracted increasing attention owing to their unique properties, such as fast charge and discharge rates, long cycle life and high power density. However, the key drawback of SCs is the limited energy density. An effective strategy to address this issue is to enhance electrode capacitance, which can be achieved by a number of approaches, including increasing specific surface area, incorporating metal oxides or conducting polymers to introduce pseudocapacitance, and tailoring the electrode pore structure and surface chemistry to match well with the electrolyte. This project aims to develop graphite-derived composite electrode materials for improving SCs performance, in particular with regard to energy density. Graphene is considered a promising electrode for SCs1, 2 because of its high specific surface area and excellent electron conductivity.3, 4 Graphene can be prepared from natural graphite via either a physical approach5 or a chemical venue.6 The latter is the most commonly used method in preparing graphene materials (strictly speaking, reduced graphene oxide materials), which have a fairly low specific surface area due to agglomeration upon reduction of graphite oxide for recovering the conjugated p carbon bonds. To maintain the high specific surface area of reduced graphene oxide, researchers have reported various strategies, such as pillaring between graphene sheets,7-9 fabrication of graphene superstructures,10-12 and self-assembly of graphene sheets.13-15 In this PhD thesis work, graphite or expanded graphite was used directly to prepare composite electrode materials. Instead of exfoliation of graphite oxide to graphene oxide followed by pillaring, electroactive materials were directly added to graphite or graphite oxide. Described in Chapter 4 is regarding manganese oxide decorated graphite oxide composite electrode material, which were prepared by using a one-pot method. Manganese oxide particles were directly dispersed on graphite oxide surface during the synthesis of graphite oxide using the modified Hummersr method with potassium manganese permanganate. The morphology and crystalline phase of the manganese oxide can be controlled by adjusting the mass ratio of KMnO4 to graphite. The sample prepared at the mass ratio of 2 exhibited the highest specific capacitance among all GrO-MnOx composites, revealing the importance of morphology and ii phase of manganese oxides on the electrocapacitive properties of GrO-MnOx composite electrodes. Chapter 5 reports a vacuum-thermal synthesis method for preparing composite electrode materials consisting of reduced graphene oxide (RGO) and nickel oxide (NiO) nanoparticles. Graphene oxide was thermally expanded (EGO) in vacuum and simultaneously nickel (II) acetylacetonate was decomposed to form NiO nanoparticles between graphene layers. This method allowed the simultaneous reduction of graphene oxide and homogeneous dispersion of NiO nanoparticles into graphene layers with well-maintained structural and electrochemical advantages. The EGO-NiO composite exhibited a specific capacitance of 880 F g-1 and a 93.1% retention of its initial capacitance after 5000 cycles at a high current density of 5 A g-1 . Chapter 6 presents composite electrode materials consisting of expanded graphite (EG), carbon CNT and NiO were prepared by a chemical vapour deposition process followed by KOH activation. The EG-CNT-NiO ternary composite inherited the advantages of the excellent electric conductivity of the EG-CNT scaffold and the pseudocapacitance of NiO, thereby, showed an improved electrochemical performance compared to EG when applied as electrode material for SCs. The EG-CNT-NiOrs capacitance reached 294 F g-1 and 93.4% retention of its initial capacitance after 3000 cycles. In Chapter 7, a method for preparing RGO/cellulose paper (CP) materials is discussed. CP provides a continuous network for impregnation of GO. Both the GO content and the KOH activation temperature were studied and optimized. The RGO/CP composite with a GO content of 0.2 mg cm-2 and KOH activated at 800 dC delivered a specific capacitance of 220 F g-1 in 6M KOH and 97.6% capacitance remained after 10000 cycles. An all-solid state supercapacitor device employing the optimized composite as both electrodes and PVA-H2SO4 gel as electrolyte showed a capacitance of 49 mF cm-2 at 2 mV s-1 . In summary, four strategies for preparing graphite-derived composite electrode materials with high electrocapacitive performance are demonstrated in this PhD thesis. Based on the work covered in this thesis, it can be concluded that the synergetic effects of graphite-derived materials and metal oxides reveal the structure-property relationships. At first, the electric double layer capacitance from graphite-derived materials and pseudocapacitance from metal oxides both contribute to electrocapacitive performance. Secondly, the conductive substrate iii (graphite-derived materials) provide the support for metal oxides deposition. Lastly, maximizing the electrode performance relies on tailoring composite structure such as species, phase, and morphology of metal oxides. It is noted that in situ strategy is a more effective way to hybridise electrocapacitive materials with graphite-derived materials. Moreover, the p bonds in the carbon plane may provide new possibilities to modify or functionalize graphite-derived materials. Further research should focus on the interfacial interactions between substrate and hybridising blocks. The design of hybrid electrode materials give insight to develop SCs with high performance.

Critical review on recent developments in conducting polymer nanocomposites for supercapacitors

MnO2@PCs (porous carbons) exhibiting high energy and power density are utilized as supercapacitor electrodes and prepared by impregnating porous carbons (PCs) derived from coal tar pitch (CTP) with KMnO4 as the manganese source. This study systematically investigates the impact of MnO2 loading on the microstructure and electrochemical performance in sample. It is found that the specific surface areas (SSA) of all MnO2@PCs significantly reduced compared to that of the PCs 2789 m2 g−1. The suggested mechanism might be a combination of the energy storage mechanism of dual layer capacitors with pseudo‐capacitance due to redox reactions of MnO2. Notably, MnO2@PCs‐0.0075 exhibits a maximum SSA of 1454.62 m2 g−1. Its specific capacitance reached 561 F g−1 at 0.5 A g−1, while the capacitance of the PCs increased by 81.5% to 309 F g−1. Remarkably, the Coulombic efficiency remained at 100%. The power density and energy density are determined in a two‐electrode test system to be 0.5 kW kg−1 and 58.01 Wh kg−1, respectively, at 0.5 A g−1. Concluding from these results and related literature, the MnO2 content significantly influences the electrochemical performance, suggesting that MnO2@PCs‐0.0075 could be a promising supercapacitor (SC) electrode material, provided its capacitance retention is enhanced.

Recent advances in metal organic framework (MOF) as electrode material for super capacitor: A mini review

Effect of temperature on redox active sites in sulfide based nanorods composites electrode material in a hybrid supercapacitor for energy storage and biomedical applications

Three-dimensional interconnected nitrogen-doped mesoporous carbons as active electrode materials for application in electrocatalytic oxygen reduction and supercapacitors