Carbon nanotubes (CNTs) are known as nano-architectured allotropes of carbon, having graphene sheets that are wrapped forming a cylindrical shape. Rolling of graphene sheets in different ways makes CNTs either metals or narrow-band semiconductors. Over the years, researchers have devoted much attention to understanding the intriguing properties CNTs. They exhibit some unusual properties like a high degree of stiffness, a large length-to-diameter ratio, and exceptional resilience, and for this reason, they are used in a variety of applications. These properties can be manipulated by controlling the diameter, chirality, wall nature, and length of CNTs which are in turn, synthesis procedure-dependent. In this review article, various synthesis methods for the production of CNTs are thoroughly elaborated. Several characterization methods are also described in the paper. The applications of CNTs in various technologically important fields are discussed in detail. Finally, future prospects of CNTs are outlined in view of their commercial applications.

Conducting polymers and carbon-based materials such as graphene oxide (GO) and activated carbon (AC) are the most promising capacitive materials, though both offer charge storage through different mechanisms. However, their combination can lead to some unusual results, offering improvement in certain properties in comparison with the individual materials. Cycling stability of supercapacitors devices is often a matter of concern, and extensive research is underway to improve this phenomena of supercapacitive devices. Herein, a high-performance asymmetric supercapacitor device was fabricated using graphene oxide–polyaniline (GO@PANI) nanocomposite as positive electrode and activated carbon (AC) as negative electrode. The device showed 142 F g⁻¹ specific capacitance at 1 A g⁻¹ current density with capacitance retention of 73.94% at higher current density (10 A g⁻¹). Most importantly, the device exhibited very high electrochemical cycling stability. It retained 118.6% specific capacitance of the starting value after 10,000 cycles at 3 Ag⁻¹ and with coulombic efficiency of 98.06 %, indicating great potential for practical applications. Very small solution resistance (Rs, 0.640 Ω) and charge transfer resistance (Rct, 0.200 Ω) were observed hinting efficient charge transfer and fast ion diffusion. Due to asymmetric combination, potential window was extended to 1.2 V in aqueous electrolyte, as a result higher energy density (28.5 Wh kg⁻¹) and power density of 2503 W kg⁻¹ were achieved at the current density 1 Ag⁻¹. It also showed an aerial capacitance of 57 mF cm⁻² at current 3.2 mA cm⁻². At this current density, its energy density was maximum (0.92 mWh cm⁻²) with power density (10.47 W cm⁻²).

Owing to its exciting physicochemical properties and doping–dedoping chemistry, polyaniline (PANI) has emerged as a potential adsorbent for removal of dyes and heavy metals from aqueous solution. Herein, we report on the synthesis of PANI composites with magnetic oxide (Fe₃O₄) for efficient removal of Basic Blue 3 (BB3) dye from aqueous solution. PANI, Fe₃O₄, and their composites were characterized with several techniques and subsequently applied for adsorption of BB3. Effect of contact time, initial concentration of dye, pH, and ionic strength on adsorption behavior were systematically investigated. The data obtained were fitted into Langmuir, Frundlich, Dubbanin-Rudiskavich (D-R), and Tempkin adsorption isotherm models for evaluation of adsorption parameters. Langmuir isotherm fits closely to the adsorption data with R² values of 0.9788, 0.9849, and 0.9985 for Fe₃O₄, PANI, and PANI/Fe₃O₄ composites, respectively. The maximum amount of dye adsorbed was 7.474, 47.977, and 78.13 mg/g for Fe₃O₄, PANI, and PANI/Fe₃O₄ composites, respectively. The enhanced adsorption capability of the composites is attributed to increase in surface area and pore volume of the hybrid materials. The adsorption followed pseudo second order kinetics with R² values of 0.873, 0.979, and 0.999 for Fe₃O₄, PANI, and PANI/Fe₃O₄ composites, respectively. The activation energy, enthalpy, Gibbs free energy changes, and entropy changes were found to be 11.14, −32.84, −04.05, and −0.095 kJ/mol for Fe₃O₄, 11.97, −62.93, −07.78, and −0.18 kJ/mol for PANI and 09.94, −74.26, −10.63, and −0.210 kJ/mol for PANI/Fe₃O₄ respectively, which indicate the spontaneous and exothermic nature of the adsorption process.

The synthesis of promising nanocomposite materials can always be tricky and depends a lot on the method of synthesis itself. Developing such synthesis routes, which are not only simple but also can effectively catch up the synergy of the compositing material, is definitely a worthy contribution towards nanomaterial science. Carbon-based materials, such as graphene oxide, and conjugative polymers, such as conductive polyaniline, are considered materials of the 21st century. This study involves a simple one pot synthesis route for obtaining a nanocomposite of polyaniline and graphene oxide with synergistic effects. The study was carried out in a systematic way by gradually changing the composition of the ingredients in the reaction bath until the formation of nanocomposite took place at some particular reaction parameters. These nanocomposites were then utilized for the fabrication of electrodes for aqueous symmetric supercapacitor devices utilizing gold or copper as current collectors. The device manifested a good capacitance value of 264 F/g at 1 A/g, magnificent rate performance, and capacitance retention of 84.09% at a high current density (10 A/g) when gold sheet electrodes were used as the current collectors. It also showed a capacitance retention of 79.83% and columbic efficiency of 99.83% after 2000 cycles.

Water pollution caused by industrial wastes containing heavy metals and dyes is a major environmental problem. This study reports on the synthesis, characterization, and utilizations of Polyaniline (PANI) and its composites with Fe₃O₄ for the removal of hexavalent chromium Cr(VI) and divalent nickel Ni(II) ions from water. The adsorption data were fitted in Freudlich, Langmuir, Tempkin, Dubbanin–Ruddishkawich (D–R), and Elovich adsorption isotherms. The Freundlich isotherm fits more closely to the adsorption data with R² values of 0.9472, 0.9890, and 0.9684 for adsorption of Cr(VI) on Fe₃O₄, PANI, and PANI/Fe₃O₄ composites, respectively, while for adsorption of Ni(II) these values were 0.9366, 0.9232, and 0.9307 respectively. The effects of solution pH, initial concentration, contact time, ionic strength, and adsorbent dosage on adsorption behavior were investigated. The adsorption ability of composites was compared with pristine PANI and Fe₃O₄ particles. Activation energy and other thermodynamic properties such as changes in enthalpy, entropy, and Gibbs free energy indicated spontaneous and exothermic adsorption.

Conducting polymers (CPs), especially polyaniline (PANI) based hybrid materials have emerged as very interesting materials for the adsorption of heavy metals and dyes from an aqueous environment due to their electrical transport properties, fascinating doping/de-doping chemistry and porous surface texture. Acid Blue 40 (AB40) is one of the common dyes present in the industrial effluents. We have performed a comparative study on the removal of AB40 from water through the application of PANI, magnetic oxide (Fe₃O₄) and their composites. Prior to this study, PANI and its composites with magnetic oxide were synthesized through our previously reported chemical oxidative synthesis route. The adsorption of AB40 on the synthesized materials was investigated with UV-Vis spectroscopy and resulting data were analyzed by fitting into Tempkin, Freundlich, Dubinin–Radushkevich (D–R) and Langmuir isotherm models. The Freundlich isotherm model fits more closely to the adsorptions data with R² values of 0.933, 0.971 and 0.941 for Fe₃O₄, PANI and composites, respectively. The maximum adsorption capacity of Fe₃O₄, PANI and composites was, respectively, 130.5, 264.9 and 216.9 mg g⁻¹. Comparatively good adsorption capability of PANI in the present case is attributed to electrostatic interactions and a greater number of H-bonding. Effect of pH of solution, temperature, initial concentration of AB40, contact time, ionic strength and dose of adsorbent were also investigated. Adsorption followed pseudo-second-order kinetics. The activation energy of adsorption of AB40 on Fe₃O₄, PANI and composites were 30.12, 22.09 and 26.13 kJmol⁻¹ respectively. Enthalpy change, entropy change and Gibbs free energy changes are −6.077, −0.026 and −11.93 kJ mol⁻¹ for adsorption of AB40 on Fe₃O₄. These values are −8.993, −0.032 and −19.87 kJ mol⁻¹ for PANI and −10.62, −0.054 and −19.75 kJ mol⁻¹ for adsorption of AB40 on PANI/Fe₃O₄ composites. The negative sign of entropy, enthalpy and Gibbs free energy changes indicate spontaneous and exothermic nature of adsorption.