In this critical Review we focus on the evolution of the hybrid ion capacitor (HIC) from its early embodiments to its modern form, focusing on the key outstanding scientific and technological questions that necessitate further in-depth study. It may be argued that HICs began as aqueous systems, based on a Faradaic oxide positive electrode (e.g., Co3O4, RuOx) and an activated carbon ion-adsorption negative electrode. In these early embodiments HICs were meant to compete directly with electrical double layer capacitors (EDLCs), rather than with the much higher energy secondary batteries. The HIC design then evolved to be based on a wide voltage (∼4.2 V) carbonate-based battery electrolyte, using an insertion titanium oxide compound anode (Li4Ti5O12, LixTi5O12) versus a Li ion adsorption porous carbon cathode. The modern Na and Li architectures contain a diverse range of nanostructured materials in both electrodes, including TiO2, Li7Ti5O12, Li4Ti5O12, Na6LiTi5O12, Na2Ti3O7, graphene, hard carbon, soft carbo...

Review of Hybrid Ion Capacitors: From Aqueous to Lithium to Sodium.

Conducting polymers (CPs) have received much attention in both fundamental and practical studies because they have electrical and electrochemical properties similar to those of both traditional semiconductors and metals. CPs possess excellent characteristics such as mild synthesis and processing conditions, chemical and structural diversity, tunable conductivity, and structural flexibility. Advances in nanotechnology have allowed the fabrication of versatile CP nanomaterials with improved performance for various applications including electronics, optoelectronics, sensors, and energy devices. The aim of this review is to explore the conductivity mechanisms and electrical and electrochemical properties of CPs and to discuss the factors that significantly affect these properties. The size and morphology of the materials are also discussed as key parameters that affect their major properties. Finally, the latest trends in research on electrochemical capacitors and sensors are introduced through an in-depth discussion of the most remarkable studies reported since 2003.

Electrical and Electrochemical Properties of Conducting Polymers.

We have synthesized the hybrid supercapacitor electrode of Co3O4 nanoparticles on vertically aligned graphene nanosheets (VAGNs) supported by carbon fabric. The VAGN served as an excellent backbone together with the carbon fabric, enhancing composites to a high specific capacitance of 3480 F/g, approaching the theoretical value (3560 F/g). A highly flexible all-solid-state symmetric supercapacitor device was fabricated by two pieces of our Co3O4/VAGN/carbon fabric hybrid electrode. The device is suitable for different bending angles and delivers a high capacitance (580 F/g), good cycling ability (86.2% capacitance retention after 20 000 cycles), high energy density (80 Wh/kg), and high power density (20 kW/kg at 27 Wh/kg). These excellent electrochemical performances, as a result of the particular structure of VAGN and the flexibility of the carbon fabric, suggest that these composites have an enormous potential in energy application.

All-Solid-State Symmetric Supercapacitor Based on Co3O4 Nanoparticles on Vertically Aligned Graphene

Binary metal oxides with three-dimensional (3D) superstructure have been regarded as desirable electrode materials for the supercapacitor due to the combination of the improved electrical conductivity and effective porous structure. 3D hierarchical flower-shaped nickel cobaltite (NiCo2O4) microspheres have been fabricated by a rapid and template-free microwave-assisted heating (MAH) reflux approach followed by pyrolysis of the as-prepared precursors. The flower-shaped NiCo2O4 microspheres, composed of ultrathin nanopetals with thickness of about 15 nm, are endowed with large specific surface area (148.5 m(2) g(-1)) and a narrow pore size distribution (5-10 nm). The as-fabricated porous flower-shaped NiCo2O4 microspheres as electrode materials for supercapacitor exhibited high specific capacitance of 1006 F g(-1) at 1 A g(-1), enhanced rate capability, and excellent electrochemical stability with 93.2% retention after 1000 continuous charge-discharge (CD) cycles even at a high current density of 8 A g(-1). The desirable integrated performance enables it to be a promising electrode material for the electrochemical supercapacitor (EC).

Rapid microwave-assisted green synthesis of 3D hierarchical flower-shaped NiCo₂O₄ microsphere for high-performance supercapacitor.

We attempt to meet the general design requirements for high-performance supercapacitor electrodes by combining the strategies of lightweight substrate, porous nanostructure design, and conductivity modification. We fabricate a new type of 3D porous and thin graphite foams (GF) and use as the light and conductive substrates for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes. The nanowire core is Co3O4, and the shell is a composite of conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) and metal oxide (MnO2). To show the advantage of this integrated electrode design (viz., GF + Co3O4/PEDOT–MnO2 core/shell nanowire arrays), three other different less-integrated electrodes are also prepared for comparison. Full supercapacitor devices based on the GF + Co3O4/PEDOT–MnO2 as positive electrodes exhibit the best performance compared to other three counterparts due to an optimal design of structure and a synergistic effect.

https://dr.ntu.edu.sg/bitstream/10356/103926/1/Accepted%20MS.pdf

A new type of porous graphite foams and their integrated composites with oxide/polymer core/shell nanowires for supercapacitors : structural design, fabrication, and full supercapacitor demonstrations

NiO nanomaterial was synthesized at different calcination temperatures using cetyltrimethyl ammonium bromide (CTAB) as surfactant via microwave method. Thermogravimetric studies revealed the decomposition details of Ni(OH)2 precursor. The structure and morphology of the NiO was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). NiO calcined at 300 °C shows a nanoflake-like structure. A possible formation mechanism has been discussed with time evolution study. Electrochemical studies indicate that the sample calcined at 300 °C exhibits better charge storage. The NiO nanoflakes exhibit maximum specific capacitance of 401 F g–1 at a current density of 0.5 mA cm–2. The energy generated and hence the charges collected from wind and solar panels are slow but in many applications the power delivery has to be at a faster rate. Considering this aspect, slow-charge and fast-discharge tests have bee...

Supercapacitor studies on NiO nanoflakes synthesized through a microwave route.

Microwave assisted synthesis of Co3O4 nanoparticles for high-performance supercapacitors

Via the green chemistry route, Mn3O4/amorphous carbon nanoparticles have been synthesized. Dextrose was used as the reducing agent, and starch was used as the capping agent. The X-ray diffraction patterns reveal the Hausmannite tetragonal structure of the synthesized Mn3O4 particles. EDAX analysis confirms the presence of carbon and stoichiometry of Mn3O4. Morphological studies reveal the nanospherical nature of the synthesized particles. The FTIR spectra confirm the presence of Mn–O bonds. Mn3O4/AC 500 exhibits highest specific capacitance of 522 F g–1 at a specific current of 1 A g–1, when measured from the charge–discharge process. This value is superior to previous reports on Mn3O4 nanoparticles as an electrode for supercapacitors. Higher energy density of 58.72 W h kg–1 could be observed for Mn3O4/AC 500, which is higher than lead acid batteries and comparable to those for the nickel hydride batteries. These results indicate that Mn3O4/AC 500 is a promising electrode for supercapacitor applications.

Synthesis of Mn3O4/Amorphous Carbon Nanoparticles as Electrode Material for High Performance Supercapacitor Applications

Hybrid supercapacitor devices based on MnCo 2 O 4 as the positive electrode and FeMn 2 O 4 as the negative electrode

NiO/C nanocomposites were synthesized at different calcination temperatures using starch as the stabilizing agent. The NiO/C nanocomposites were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The samples calcined at 500 °C exhibit a highly porous nature. Electrochemical investigations of NiO/C were carried out using cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. NiO/C-5 shows excellent electrochemical performance compared to NiO/C-4 and NiO/C-6. The NiO/C-5 electrode exhibits a specific capacitance value of 644 F g–1 at a scan rate of 2 mV s–1. The charge transfer resistance is 0.59 Ω for the NiO/C-5 electrode. This NiO/C-5 appears to be a promising electrode material for supercapacitor applications.

Sadayappan Nagamuthu

Papers

Supercapacitor studies on NiO nanoflakes synthesized through a microwave route.

Microwave assisted synthesis of Co3O4 nanoparticles for high-performance supercapacitors

Synthesis of Mn3O4/Amorphous Carbon Nanoparticles as Electrode Material for High Performance Supercapacitor Applications

Hybrid supercapacitor devices based on MnCo 2 O 4 as the positive electrode and FeMn 2 O 4 as the negative electrode

Porous NiO/C Nanocomposites as Electrode Material for Electrochemical Supercapacitors