Recent Studies on Supercapacitors with Next-Generation Structures

Abstract

Supercapacitors have shown great potential as a possible solution to the increasing global demand for next-generation energy storage systems. Charge repositioning is based on physical or chemical mechanisms. There are three types of supercapacitors-the electrochemical double layer, the pseudocapacitor, and a hybrid of both. Each type is further subdivided according to the material used. Herein, a detailed overview of the working mechanism as well as a new method for capacitance enhancement are presented.

Keywords: EDLC; Helmholtz model; electrical double layer capacitor; energy storage system; micropores; nanopores; negative capacitance; pseudocapacitor; supercapacitor; ultracapacitor.

Figure 1

Classification of supercapacitors [24].

Figure 2

Models of the electrochemical double layer. (a) Helmholtz model; (b) Gouy–Chapman or diffuse model; (c) Stern model; (d) Grahame model; (e) Bockris, Devanathan, and Muller model [25,26,27].

Figure 3

(a) Brief structure of a supercapacitor with a view of cylindrical theory (endohedral capacitors/electric double-cylinder capacitor), (b) shape of the electrolyte ion placed and the cylindrical electrode according to the pore size. The first picture shows that the ions are not very involved in charge storage because they have a very small pore structure. The second figure shows one ion interacting with an electrode to store charge. The third figure shows more ions forming the same layer and acting on the electrode. (c) The shape of pores observed in a three-dimensional shape; “a” is the distance from the center to a layer of several ions, the radius of the pores is expressed as “b”, and the cylinder length is expressed as “L” [21,45,46].

Figure 4

(a) Brief structure of activated carbon particles with various pore sizes, not based on a circle, but based on the electrode theory of walls and walls. (b) Nanopores do not have the minimum space required to form an electrical layer with solvated ions, (c) nanopores have minimal space to form an electrical layer with solvated ions, (d) nanopores have extra space to form an electrical layer with solvated ions, (e) nanopores have sufficient space for each electrode such that solvated ions can form an electrical layer with each electrode, (f) electric layer for carbon grain at the surface, (g) schematic diagram of normalized capacitance according to pore size. When the pore size is less than or equal to a, which is indicated by a red-colored line, (b,c) is applicable, and when the pore size is more than a and less than or equal to b, which is indicated by a yellow-colored line, (c,d) is applicable. If the pore size is larger than b, which is indicated by a green-colored line with a conventional view, (e,f) is applied [25,39,40,42].

Figure 5

An analysis model for the theory of nanopores, showing a model that combines the theory of a structure made of cylinders and micropores [25,39,40,42,45,48].

Figure 6

(A–C) Macro/micropole-dominated carbon on SEM (scanning electron microscope) images with different scale (10 μm and 1 μm) and (D) element mapping of highly porous activated carbon [47]. Reprinted with permission from Ref. [47]. Copyright 2017 American Chemical Society.

Figure 7

Schematic diagram of a capacitor formed by a solvated cation with a negative polarity located in the middle of two electrodes separated by a single layer of charge [39,44,50].

Figure 8

Illustrations and plots to express the integration of capacitance caused by both the dielectric layer and the ferroelectric layer to enhance capacity. (a) Schematic of the dielectric capacitor including D_d-E_d curve, C_d⁻¹D_d c-urve, and F_d-D_d curve without external bias. (b) Schematic of ferroelectric capacitor including P_f-E_f curve, C_f⁻¹-P_f curve shown as “s” and F_f-Pd_f curve with polarization on 2Q. (c–e) Schematic of capacitor compounded with both ferroelectric and dielectric layer shows significant charge storage which is expressed as the area above the Q-V curve. (c) shows a state at zero bias, (d) demonstrates at V_a, and (e) expresses at V_max. Reprinted with permission from Ref. [16]. Copyright 2019. The authors are under Creative Commons License 4.0 (https://creativecommons.org/licenses/by/4.0/).

Figure 9

Plots of capacitor compounded with both a ferroelectric (HZO) and dielectric layer (Al₂O₃) with various layer thicknesses. (a) Efficiency (%) versus discharged energy density 〈W_d〉(J/cm⁻³) for 7.7 nm HZO compounded with 1.5 nm to 4 nm Al₂O₃ layer thicknesses. (b) Efficiency (%) versus discharged energy density 〈W_d〉(J/cm⁻³) for 4 nm Al₂O₃ compounded with 7.7 nm and 11.3 nm HZO thicknesses. (c) Charging (Q_c), Discharging (Q_d), and Charge loss (Q_loss), which means Q_c-Q_d, according to measured charges versus applied voltage at capacitor fabricated with the 4 nm Al₂O₃/7.7 nm HZO. (d) Charges versus applied voltage with an experimental and theoretical capacity increase (W/W₀). Reprinted with permission from Ref. [16]. Copyright 2019. The authors are under Creative Commons License 4.0 (https://creativecommons.org/licenses/by/4.0/).

Figure 10

Plots of major factors with the stability of the capacitor composed with ferroelectric/dielectric stack. (a) Both capacitance density C (left) and loss factor tan (δ) (right) versus frequency without DC bias for frequency stability. (b) Discharged energy density 〈W_d〉 (left) and efficiency (right) versus cycling times under 10.6 V for cycling stability. (c) 〈W_d〉 versus frequency by pulse under 12.7 V for frequency stability. (d) 〈W_d〉 (left) and efficiency (right) versus temperature at 14.4 V for temperature stability. Reprinted with permission from Ref. [16]. Copyright 2019. The authors are under Creative Commons License 4.0 (https://creativecommons.org/licenses/by/4.0/).

Figure 11

Nyquist plot based on the real and imaginary parts at the complex impedance (Zr + jZi) of MFM capacitor stacked with Pt–ZrO2–Pt. Resistor-capacitor (RC) dummy cell provided by Metrohm AUTOLAB, KM Utrecht, Netherlands is exhibited in an insert with RC component 100 Ω and an RC element (1 μF) capacitance with 1 MΩ resistance integrated in parallel. Reprinted with permission from Ref. [52]. Copyright 2019. The authors are under Creative Commons License 4.0 (https://creativecommons.org/licenses/by/4.0/).

Figure 12

Schematic illustration of a supercapacitor classified by capacitive type. (a) Schematic illustration of EDLC, which has cations and anions that are physically layered from electrodes. The schematic cyclic voltmeter has the same shape as a capacitor. (b) Schematic illustration of redox pseudocapacitor, which stores and releases energy through chemical adsorption reactions at the surface of the electrode. The schematic cyclic voltmeter is not the same as a capacitor but has a similar shape. (c) Schematic illustration of intercalation pseudocapacitor, which stores and releases energy through chemically transform reactions based on the faradaic charge storage mechanism. The schematic cyclic voltmeter shows the characteristics between the capacitor and the battery. (d) Schematic illustration of hybrid capacitor integrated both electrode types, which is EDLC and redox pseudocapacitive material. (e) Schematic illustration of hybrid capacitor integrated with both electrode types, which are EDLC and intercalation pseudocapacitive material [5,6,10,21,55].