Progress in supercapacitors: roles of two dimensional nanotubular materials

Abstract

Overcoming the global energy crisis due to vast economic expansion with the advent of human reliance on energy-consuming labor-saving devices necessitates the demand for next-generation technologies in the form of cleaner energy storage devices. The technology accelerates with the pace of developing energy storage devices to meet the requirements wherever an unanticipated burst of power is indeed needed in a very short time. Supercapacitors are predicted to be future power vehicles because they promise faster charging times and do not rely on rare elements such as lithium. At the same time, they are key nanoscale device elements for high-frequency noise filtering with the capability of storing and releasing energy by electrostatic interactions between the ions in the electrolyte and the charge accumulated at the active electrode during the charge/discharge process. There have been several developments to increase the functionality of electrodes or finding a new electrolyte for higher energy density, but this field is still open to witness the developments in reliable materials-based energy technologies. Nanoscale materials have emerged as promising candidates for the electrode choice, especially in 2D sheet and folded tubular network forms. Due to their unique hierarchical architecture, excellent electrical and mechanical properties, and high specific surface area, nanotubular networks have been widely investigated as efficient electrode materials in supercapacitors, while maintaining their inherent characteristics of high power and long cycling life. In this review, we briefly present the evolution, classification, functionality, and application of supercapacitors from the viewpoint of nanostructured materials to apprehend the mechanism and construction of advanced supercapacitors for next-generation storage devices.

Fig. 1. Roadmap of the evolution, progress, and developments in supercapacitors.

Fig. 2. An overview of the taxonomical classification of supercapacitors. The lightning bolt symbol represents energy and the cylindrical stack represents the storage.

Fig. 3. (a) Typical Ragone plot showing power against energy density. (b) Specific energy/power performance for different energy storage devices. (c) Ragone plot showing the recent status of supercapacitors, asymmetric supercapacitors, and hybrid supercapacitors. (d) Illustration of various components used in the design of hybrid materials for energy storage. (a) has been adapted/reproduced from ref. 473 with permission from Nature Materials, Springer Nature. (c–e) have been adapted/reproduced from ref. 30 with permission from Chemical Society Reviews, Royal Society of Chemistry.

Fig. 4. (a) Schematic representation of basic construction of a supercapacitor. (b) Cross-sectional interface of the electrochemical double layer structure between a porous carbon negative electrode and an aqueous electrolyte. E_N, E_P, and E_s are respectively the potentials of the negative and positive electrodes, and the electrolyte. (c) Potential distribution in the electrolyte solution between the negative and positive electrodes. (d) Energy storage mechanisms in supercapacitors e.g. electrostatic storage mechanism in double-layer capacitors & electrochemical storage mechanism in pseudo-capacitors. (e) The charge is stored electrostatically or non-faradaically using a double layer (Helmholtz layer) in which the charge accumulates on the electrode surface by following the natural attraction of unlike charges diffused across the separator into the pores of the electrodes in the electrolyte medium. The arrows represent the polarized solvent molecular layer in the inner Helmholtz plane and the arrows with positive ions represent solvated cations. (f) The charge is stored faradaically or electrochemically through the transfer of charge across the interface. This phenomenon is achieved through electrosorption, reduction–oxidation reactions, and an intercalation process. The arrows with negative ions represent redox ions and arrows with positive ions depict solvated cations. (g) Depiction of symbols in (a and b). (b, c and g) have been adapted/reproduced from ref. 41 with permission from Taylor & Francis.

Fig. 5. (a). Comparison of the charge and discharge cycles of batteries vs. supercapacitors. Pink color signifies the discharge time cycle and light green represents recharge timing (b). (c) The cyclic voltammograms of positive and negative electrodes in a three-electrode cell and galvanostatic charge and discharge plots for a two-electrode cell for rechargeable batteries and supercapacitors respectively. U_max and U_min are the maximum and minimum cell voltages; U_dis is the average discharge voltage; τ and t: end times of the first charge and discharge cycle, τ ≥ (t − τ). 2τ and 2t: end times of the second charge and discharge cycle. (d) Charge–discharge voltage curves (referred from Battery University™ sponsored by Cadex Electronics Inc.). (c and d) have been adapted/reproduced from ref. 41 and 474 with permission from Taylor & Francis and Battery University™ Copyright ©2003–2018 Cadex Electronics Inc. (Cadex) respectively.

Fig. 6. (a) Ordered structures with slit pores, nanotubes and asymmetric nanoporous carbons. (b) Symmetric supercapacitor based on metal oxide framework derived nanoporous carbon. (c) Stacked geometry model for a carbon-based electrode material consisting of GO electrodes with pure EMI + BF₄−. (d) Porous electrodes made of carbide derived carbon immersed in a BMI-PF₆ ionic liquid electrolyte. (a) has been adapted/reproduced from ref. 42 with permission from Springer Nature. (b) has been adapted/reproduced from ref. 475 with permission from the Royal Society of Chemistry. (c) has been adapted/reproduced from ref. 476 with permission from the Royal Society of Chemistry. (d) has been adapted/reproduced from ref. 477 with permission from American Chemical Society.

Fig. 7. (a) Solvated ions residing in pores with varying adjacent pore walls. A decrease in the pore size leads to high or improved capacitance with the efficient approach of ion mobility towards the pore walls. The detrimental effect on capacitance can be observed with the increase in pore size as per Chmiola et al. (b) The plot depicts effect of synthesis temperature on the SSA, and the average pore size of CDC. Redline illustrates the SSA values and Blue line depicts the pore size. (b) has been adapted/reproduced from ref. 62 with permission from the American Association for the Advancement of Science.

Fig. 8. (a) Schematic cross section of a negatively polarized electrode pore showing accommodation of one line of ions in the order of d < 2a < 2d. One dimensional lattice representation of distribution of ions. Reprinted with permission from Royal Society Publishing. (b) The specific voltage-dependent differential capacitance of a single pore per unit surface area. The effect of the diameter of the pore, shown for an ion diameter of 0.7 nm (upper panel) and 1 nm (lower panel); the effective dielectric constant of the pore interior, ε = 2. Due to saturation of charge density, the capacitance vanishes very quickly with voltage. (a and b) have been adapted/reproduced from ref. 73 with permission from the Royal Society of Chemistry.

Fig. 9. Classification of electrolytes with electrochemical supercapacitor effects based on electrolytes.

Fig. 10. (a) Schematic view of a charged capacitor (left) and (b) supercapacitor (right): solid black—electrodes; dashed—separator; yellow—electrolyte; nanotubes schematically shown in shades of gray; numbers indicate the hierarchical system of pores 1—small pores and 2 big ones; ” +”—positive charge; ” ⊖”—negative ion; ” −”—negative charge (electrons); ” ⊕”—positive ion. (c) Illustration of a part of a supercapacitor consisting of monolayers of nanotubes.

Fig. 11. A schematic diagram showing the discovery, morphologies, properties, and synthesis of carbon nanotubes.

Fig. 12. (a). Type of nanotube. (b) Pathways of obtaining nanotubes, e.g., rolling of the sheet, (c) the surface of a core-type template, and (d) the internal surface of a sheath-type template. Reprinted with permission from Springer. (a–d) have been adapted/reproduced from ref. 478 with permission from Springer Nature.

Fig. 13. CNT network for supercapacitor applications and its effects.

Fig. 14. Self-charging biological supercapacitor; capacitive side was built using graphite foil modified with a polyaniline/CNT composite and the charging side was prepared using an enzymatic fuel cell designed from 3D gold NP (yellow color) based nanobiostructures.

Fig. 15. Relational graph depicting specific capacitance vs. energy density in various nanotube network-based supercapacitors.

Fig. 16. (a and b) Highly porous ZnO tetrapodal network template transformed into a nano-microtubular carbon based aerographite tetrapod network through the CVD process. Hydrogen gas molecules selectively etch ZnO and toluene gas simultaneously deposits carbon at high temperatures inside the CVD chamber. (a) The SEM image demonstrates the tetrapodal geometry of ZnO structures. (b) Converted carbon based network tetrapodal morphology of the aerographite network. The conversion process takes place via selective removal of ZnO. SEM images of aerographite conversion from ZnO: (c) partially converted, (d) almost converted, and (e) completely converted (crumbled aerographite network). (f) Schematic of sacrificial growth of aerographite tubular carbon arms over ZnO arms through a belt-like growth process. Aerographite structures retain the initial ZnO template architecture which is mainly tetrapodal. The aerographite tetrapods are hollow from inside with a nano-microtubular arm morphology in contrast to the initial ZnO template which is solid. (g) TEM image of typical aerographite particles in the ‘normal’ hollow, closed graphitic shell modification on a lacey carbon grid. (h) HR-TEM image with the parallel view of an aerographite closed shell with a low curvature (diameter of the tubular ligament ≈ 2000 nm). (i) EFTEM was used to investigate the thickness of the intact hollow closed shell aerographite ligament (red line indicates the locations where the thickness measurement was performed). (a–i) have been adapted/reproduced from ref. 385 with permission from Elsevier.

Fig. 17. Typical SEM and TEM images of NSMs, e.g. 0D: (a) quantum dots, (b) nanoparticle arrays, (c) core–shell nanoparticles, (d) hollow cubes, and (e) nanospheres. 1D: (a) nanowires, (b) nanorods, (c) nanotubes, (d) nanobelts, (e) nanoribbons, and (f) hierarchical nanostructures. 2D: (a) junctions (continuous islands), (b) branched structures, (c) nanoplates, (d) nanosheets, (e) nanowalls and (f) nanodisks. 3D: (a) nanoballs (dendritic structures), (b) nanocoils, (c) nanocones, (d) nanopillars and (e) nanoflowers. This figure has been adapted/reproduced from ref. 243 with permission from Elsevier.

Fig. 18. Schematic illustration of different heterogeneous materials based on structural complexity. This figure has been adapted/reproduced from ref. 259 with permission from the Royal Society of Chemistry.

Fig. 19. (a) Carbon materials used in the work of Nomura et al. where they have demonstrated a superstable mesoporous carbon sheet made of edge-free graphene walls. The figure depicts (a) SWCNTs (single-walled carbon nanotubes), (b) rGO (reduced graphene oxide), (c) AC (activated carbon), (d) GMS (graphene mesosponge) powder preparation, and (e) sheet-moulded Al₂O₃ nanoparticles and a GMS sheet with cross-sectional SEM and TEM images. (f) Cyclic voltammetric scans of (a) SWCNTs, (g) rGO (reduced graphene oxide) and (h) GMS (graphene mesosponge) performed at 10 mV s⁻¹ in 1.5 M TEMA-BF₄/PC. (i) Ragone plot measured at upper limit voltage. (j) Capacitance vs. cycle numbers. (a–j) have been adapted/reproduced from ref. 320 with permission from the Royal Society Of Chemistry.

Fig. 20. (a) Application of graphene-based supercapacitors in various sectors. (b) Graphene-based supercapacitors or miniaturized bioelectronics. (c) Properties affecting the synthesis of graphene-based supercapacitors varying from 2D graphene to 3D curved graphene. (d) Improved pacemakers and implantable medical devices. (e) Graphene-based supercapacitors for tissue engineering. (f) Graphene-based supercapacitors for drug-delivery. (g) Antibacterial activity of graphene with altered biological properties.

Fig. 21. Metal oxide (MO_x) based composite materials for advanced supercapacitors, including composites doped with metal and nonmetallic materials.

Pritam Kumar Panda

Anton Grigoriev

Yogendra Kumar Mishra

Rajeev Ahuja