Extraordinary Capacitance and Stability of Carbon Electrode for Electrochemical Capacitors

Abstract

To further improve the energy density and reliability of carbon based electrochemical capacitors, such as electric double-layer capacitors and lithium ion capacitors, it is necessary to increase both the electric double layer capacitance and stability of the porous carbon electrodes used in the capacitors. Not only the specific surface area and pore size, but also the pore shape and pore size distribution, and the analytical methods to properly evaluate the pore structure should be considered when improving the capacitance. In the case of the high stability to high voltage charging, it is important to design and control the three-dimensional structure of the electrode as well as the crystal structure and crystallinity of the carbon. In this review, some advanced examples of the recent research on these topics will be discussed.

1. Introduction

Nanoporous carbons, such as activated carbon, are key materials for use in electrochemical capacitors, especially in electric double layer capacitors (EDLCs) and lithium ion capacitors (LICs).^1,2 The energy (E) stored in these carbon-based electrochemical capacitors is proportional to the capacitance (C) of the capacitor and the square of the maximum charging voltage (V) as follows.^1–3

  

\begin{equation} E = CV^{2}/2 \end{equation} (1)

The electrochemical capacitors have the disadvantage of a lower energy density compared to other similar energy storage devices that are rechargeable batteries, such as lithium-ion batteries, thus much research has been done to improve the energy density.^4–7

The capacitance of the carbon electrochemical capacitors is determined by the electric double layer capacitance of the porous carbon electrode. In particular, the double layer capacitance of the porous carbon electrode has been extensively studied worldwide in terms of its correlation with the pore structure of the electrode.^4–11 For example, the relationship between the specific surface area and the gravimetric capacitance (specific capacitance normalized by the weight of the electrode) has been significantly discussed. However, the volumetric capacitance (specific capacitance normalized by the volume of the electrode), which is related to the volumetric energy density, is more important for the practical use of the capacitors for energy storage than the gravimetric capacitance.^12,13 The gravimetric capacitance (C_g) is roughly linear to the specific surface area (S) with the proportionality constant (the areal capacitance, C_a) while the volumetric capacitance (C_v) is also related to the electrode bulk density (ρ) in addition to the specific surface area and the areal capacitance as follows.⁷

  

\begin{equation} C_{\text{g}} = C_{\text{a}}S \end{equation} (2)

  

\begin{equation} C_{\text{v}} = C_{\text{g}}\rho = C_{\text{a}}S\rho \end{equation} (3)

To improve the volumetric capacitance, it is very important to increase the areal capacitance which is the specific capacitance by the electrode surface area because of the trade-off of the surface area and the electrode bulk density. The areal capacitance is sensitive to the pore size (pore diameter or width) and morphology, the size of the electrolyte ion (solvated ion in some cases), the surface chemical state, such as surface functional groups, or the crystal structure of the pore wall. Understanding the controlling factor of the areal capacitance is very significant, but undoubtedly difficult.

Equation 1 indicates that the other way to improve the energy density of the carbon-based capacitors is to increase the maximum charging voltage, which is basically determined by the electrochemical window of the electrolyte. Therefore, organic electrolytes using organic solvents with a wide electrochemical window are widely used in the electrochemical capacitors for the energy storage. Using an electrolyte with a wider electrochemical window should result in a capacitor with a higher maximum charging voltage, but the actual maximum voltage is not as high as expected. The electrochemical window also depends on the electrode material. Most of the electrochemical window data have been obtained using noble metals (e.g., platinum and gold) or glass-like carbon electrodes as test electrodes, not the practical porous carbon electrodes in the electrochemical capacitors. The electrochemical window of the porous carbon, such as activated carbon, tends to be narrower than that of glass-like carbon electrodes. A number of factors, including pore structure, surface functionality, crystal structure (e.g., sp³/sp²), and crystallinity, influence the electrochemical window of the carbon electrodes. Therefore, the material design of the porous carbon electrode should also be aimed at improving the maximum voltage charging voltages for the capacitors.^1,8,9 Charging with a voltage higher than the maximum charging voltage causes the Coulombic efficiency to decrease and electrochemical decomposition (Faradaic process) to occur, resulting in the destruction of the electrodes and the degradation of the capacitor. The degradation mechanism of the capacitor by high-voltage charging is complicated and difficult to understand, so it has not yet been well clarified despite of a lot of efforts.^14–19 Recently, some characteristic carbon electrodes (e.g., particle-boundary free, edge-site free, diamond-based types) have been developed with an excellent durability against high-voltage charging compared to conventional activated carbon electrodes. The stable high voltage charge/discharge operation is closely related not only to the increase in the energy density but also to the further improvement of the reliability for the carbon-based capacitors. Therefore, the realization of the durable carbon electrode creates a new area in the studies of the carbon-based electrochemical capacitors which until now have only focused on the improvement of the capacitance.

In this review, based on the above background data, some novel research examples of the porous carbon electrode for use in the capacitors are introduced from the viewpoint of extraordinary capacitance and stability.

2. Porous Carbon with Worm-like Shape and Attention for Pore Structure Analysis

The pore structure of porous carbon electrodes is an important factor governing the EDLC performance. The pore spaces act as sites for the electric double-layer formation. The capacitance (C) is proportional to the electrode area (S) and inversely proportional to the thickness (δ) of the electric double-layer as follows.

  

\begin{equation} C = \int \frac{\varepsilon_{0}\varepsilon_{r}}{\delta}ds = \frac{\varepsilon_{0}\varepsilon_{r}}{\delta}S \end{equation} (4)

(ε₀: permittivity in vacuum, ε_r: relative permittivity of the electric double-layer)

Porous carbon materials with high specific surface areas have been used as EDLC electrodes.²⁰ In aqueous electrolytes, oxygen-containing functional groups (OGCFs) at the pore entrance affect the access of electrolyte ions to the pores. Here, the group of the author (K.U.) describes the relationship between pore structure and capacitance, focusing on non-aqueous electrolyte systems, where the influence of the OGCFs is negligible. Microporous carbons have a large specific surface area. However, electrolyte ions exist as solvated ions with size of 1–2 nm in bulk electrolytes: mesoporous carbons with pore size of 2–50 nm have long been believed to be ideal electrodes for the EDLC. Previous studies have reported that mesoporous carbon electrodes exhibit performance of approximately 100–140 F g⁻¹ as gravimetric capacitance.^21,22 The expression of the areal capacitance (expressed in µF cm⁻²) is an indicator to consider the efficiency with which the electrode surface is used as sites for the formation of the electric double layer.

Gogotsi and Simon’s groups reported that carbide-derived carbon (CDC) with an average pore size smaller than the solvated electrolyte ion size exhibited a higher normalized capacitance (µF cm⁻²) than previously reported mesoporous carbons.²³ They compared the average pore size of CDC to the bare electrolyte ion size and attributed the unique enhancement of capacitance in microporous carbon to the desolvation of electrolyte ions. This unique phenomenon in micropores, which differs from that in the bulk phase, was reported in the field of adsorption science in the 1990s.^24–26 One representative phenomenon is called as “quasi-pressure effect.” The molecules introduced into the micropores behave as if they are in a high-pressure field.^27–29 The strong interaction from pore walls should facilitate the desolvation of electrolyte ions.

The capacitance of porous carbon electrodes is strongly related to their pore structures (pore shape and pore size distribution). The pore shapes of the porous carbon electrodes were clarified by transmission electron microscopy (TEM). Figure 1a shows TEM images of representative porous carbons. Depending on the raw material and activation method, the pore shapes of carbon materials are broadly categorized into slit and worm-like (WL) shapes with curvature. As shown in Fig. 1b, porous carbons with WL-shaped pores (WcX: X = 1–6, sample number) exhibit a higher gravimetric capacitance than those with slit-shaped pores (ScX: X = 1–7, sample number). This result indicates that the pore shape of the electrodes strongly contributes to the desolvation process and enhances gravimetric capacitance. Recently, pore size distribution (PSD) analysis using computer simulations has been widely used. Although the PSD is obtained by using the model structure, the surface area of the pores with each pore size can be determined from the integration of the PSD. Figure 1b shows each surface area of the pores classified by the pore size (w) that the solvents, solvated ions, and desolvated ions can access, as well as the total specific surface area. Focusing on the total specific surface area and gravimetric capacitance of each electrode, the relationship between the total specific surface area and gravimetric capacitance is not proportional. For example, comparing the Wc4 and Wc6 samples classified in the WL shape pore group, Wc6 has a smaller total specific surface area than Wc4, but it shows a higher gravimetric capacitance. Considering the gravimetric capacitance from the ratio of the surface area where solvated ions can form electric double layer (S₁) to the surface area where desolvated ions can form electric double layer (S₂), it was found that the electrode with twice as large S₂ as S₁ exhibited the highest gravimetric capacitance.³⁰ Each specific surface area of S₁ and S₂ was determined from the integration of PSDs based on the specific surface area (dS/dw). The integration curve proposes the surface area of pores with the target pore size estimated from the solvated and desolvated ion sizes. These data of these porous carbons with worm-like (WL) shapes indicate that the areal capacitance can be enhanced by the pore shape and the PSD. In other words, it is important to consider not only the total specific surface area and average pore size, but also the pore shape and PSD to design optimum porous carbon electrodes against electrolytes and the areal capacitance.

Figure 1.

(a) TEM images of porous carbons, (b) Specific surface areas (blue: w < 0.5 nm, red: w = 0.5–1.5 nm, green: w = 1.5–3.0 nm, gray: w > 3.0 nm, orange: total) and gravimetric capacitance of various porous carbons (dotted line: the gravimetric capacitance of a ordered mesoporous carbon). [Left: porous carbon with slit-shaped pores, Right: porous carbon with worm-like shaped pores]

The areal capacitance, normalized by the surface area of the porous carbon electrode is obtained by dividing the gravimetric capacitance by the total specific surface area. Gas adsorption isotherms have been widely used to determine the specific surface areas of porous carbons. Since the pores accessible to adsorbents depend on the type of adsorbent, temperature, and adsorption equilibrium time, these conditions are quite important for comparing the specific surface areas of porous carbons. For example, the specific surface areas determined from the N₂ adsorption isotherm at 77 K and the Ar adsorption isotherm at 87 K are different. The specific surface area also depends on the analytical methods. Figure 2 shows the total specific surface area of microporous carbon determined from various analytical methods applied to N₂ adsorption isotherms at 77 K. Representative classical methods for determining the specific surface area are the Brunauer–Emmett–Teller (BET) method³¹ and the subtracting pore effect (SPE) method³² using high-resolution α_s-plot.³³ Additionally, the PSD method provides the surface area for each pore size as well as the total specific surface area. The PSD is determined by fitting the experimental adsorption isotherm with the theoretical adsorption isotherms for each pore size obtained from the density functional theory (DFT) method^34–36 and the grand canonical Monte Carlo (GCMC) method^37,38 applied to pore structure models. The DFT and GCMC methods yield different specific surface areas due to different calculation processes for obtaining theoretical adsorption isotherms. In addition, equipment for measuring adsorption isotherms produced by different manufacturers uses different theoretical adsorption isotherms and algorithms to calculate PSD, which may result in different specific surface areas even on the same method. Because the strength of the interaction between the adsorbate and pore wall depends on the pore shape, the PSD depends on the model pore structure. Although simple pore structures (e.g., slit and cylinder structures) have been widely used as pore models, such structural models are idealized and neglect important features of the real porous structures. Recently, GCMC methods using realistic three-dimensional porous carbon structures (pore size <5 nm) as model pore structures have been proposed.³⁹ Such realistic model structures are effective for evaluating the pore structures of carbon electrodes without an ordered structure. However, as shown in Fig. 2, the specific surface area strongly depends on the measurement conditions and the analytical methods. It is important to consistently evaluate the specific surface area to design porous carbon electrodes.

Figure 2.

Total specific surface area determined from the GCMC method (slit and realistic models), DFT method (slit and quenched solid models from different manufactures), BET method and alpha-s method, and normalized capacitance.

3. Seamless Activated Carbon Electrode

The practical porous carbon electrodes for the EDLCs usually have a composite structure consisting of porous carbon (activated carbon) particles, a conductive additive (carbon black), and a binder, as shown in Fig. 3a. The composite structure is also typical for the electrodes of electrochemical capacitors or rechargeable batteries, where the conductive additive effectively connects the active material particles electrically, thereby reducing the internal resistance of the electrode. However, the degradation of the binder or the deposition of decomposition products between the active material particles under aggressive operating conditions (e.g., overcharging) destroys the electric network in the electrode and increases the internal resistance, resulting in an actual loss of the capacitor performance.^1,40 Such an electric network degradation, as well as the problem of micropore blockage caused by decomposition products, hinders the improvement of the withstand voltage of the EDLCs and makes it difficult to improve the energy density and the further reliability of the capacitor.

Figure 3.

Schematic illustration of the electrode for the EDLC. (a) conventional activated carbon composite electrode, (b) seamless activated carbon electrode.¹

Based on this problem, the group of the author (S.S.), in collaboration with AION Co. Ltd., have developed an activated carbon monolith without particle boundaries, i.e., a “seamless activated carbon electrode”.^1,2,40,41 A schematic illustration of the seamless activated carbon electrode is shown in Fig. 3b. Figure 4 shows the scanning electron microscope (SEM) images of the conventional activated carbon composite electrode for the EDLC and the seamless activated carbon electrode. The seamless activated carbon is prepared by carbonizing and activating a consecutively macroporous phenolic resin plate in a manner similar to the conventional activated carbon powder. The carbon matrix of the seamless activated carbon has well-developed micropores. The consecutive macropores not only provide diffusion paths for the activation gases, such as CO₂, but also for electrolyte penetration. The seamless structure of the electrode, in which no particle boundaries exist, achieves excellent durability against high-voltage charging.

Figure 4.

SEM images of (a) the conventional activated carbon composite electrode for the EDLC and (b) seamless activated carbon electrode.

Figure 5 shows the Ragone plots of the EDLC cell with the seamless activated carbon electrodes before and after the durability test (floating charge test with high voltage). For comparison, the data using the typical activated carbon (YP50F, specific surface area: about 1600 m² g⁻¹) composite electrode (electrode bulk density: 0.65 g cm⁻³), which is widely used for the EDLC, are also plotted. The seamless activated carbon electrode is a high-density type (specific surface area: about 1700 m² g⁻¹, electrode bulk density: 0.53 g cm⁻³). The floating charge voltage in this durability test (3.5 V) was much higher than the maximum voltage of the typical commercialized EDLC with an organic electrolyte (about 2.8 V). The Ragone plot in Fig. 5 clearly shows that the seamless activated carbon electrode maintained its electrode performance while the composite electrode lost most of its discharge capability after the durability test. The graphene mesosponge (GMS) sheet electrode discussed in the next section, which is also a seamless porous carbon electrode, realizes EDLCs with a very excellent voltage durability.⁴² Therefore, there is no doubt that the seamless structure for the electrode is very effective for improving the durability against high voltage charging of the EDLCs.

Figure 5.

Ragone plots of the EDLC (two-electrode) cell using a densified seamless activated carbon electrode or conventional activated carbon (YP50F) composite electrode before and after the floating charge durability test (3.5 V, 100 h, 70 °C). Reprinted with permission from Ref. 2. Copyright 2022 Elsevier.

Surface modification, such as nitrogen doping, of the porous carbon electrode also successfully improves the durability of the EDLC. The nitrogen surface functionalities have the effect of improving the stability to high-voltage charging and charge-discharge cycling^{1,9,10,40,41,43,44} which is considered to be due to the suppression of undesirable electrochemical decomposition at the electrode surface or at the interface between the active layer and the current collector. The nitrogen doping reduces the micropore blockage and any increase in the internal resistance, which further improves the high voltage charging stability of the seamless activated carbon electrode.⁴¹

4. Graphene Mesosponge

As discussed in the preceding section, expanding the voltage range is an important direction for the development of electrochemical capacitors with high energy density. Regarding the voltage stability of carbon electrodes, there are two key factors. One of them is the improvement of the stability at the electrolyte/carbon interface, while the other involves fabricating a seamless carbon structure to exclude binder polymers and eliminate inter-particle resistance between carbon particles.⁴⁰ In this chapter, the author (H.N.)’s group introduce a novel material known as graphene mesosponge⁴⁵ as a state-of-the-art carbon electrode solution to tackle these two problems.

Since electric double-layer capacitance primarily relies on the surface area of the electrode, carbon materials employed in electrodes are required to possess a high specific surface area. On the other hand, carbon materials intrinsically exhibit anisotropic nanostructures comprising basal planes and edge sites. While the basal planes are chemically inert, the edge sites cause electrolyte decomposition, leading to electrochemical capacitor degradation, especially under high voltage.^15,46 Consequently, the optimal structure for a carbon electrode combines a high specific surface area with edge-free properties to simultaneously guarantee high electric double-layer capacitance and voltage stability. Three-dimensional graphene architectures known as Mackay crystals⁴⁷ or carbon schwarzites⁴⁸ fulfill the aforementioned criteria due to their continuous graphene-based frameworks without edge sites. However, these are theoretical materials, and the synthesis of realistic materials that meet the requirements is demanded. In 2016, a groundbreaking mesoporous carbon material called graphene mesosponge (GMS) was developed using the synthesis scheme shown in Fig. 6.⁴⁵ GMS can be synthesized via specific chemical vapor deposition,^49,50 allowing the growth of a minimally stacked graphene layer onto the entire surface of metal oxide nanoparticles such as Al₂O₃, MgO, and surface-modified SiO₂.^48,51 Following the template removal under conditions that prevent additional stacking,⁵² edge sites are removed via zipping reactions occurring at temperatures exceeding 1600 °C.⁵³ GMS possesses developed mesoporosity, a high specific surface area (∼2000 m² g⁻¹), excellent conductivity, and high oxidation resistance,^45,54 all of which align with the characteristics anticipated in structures like carbon schwarzites composed of edge-free and minimally stacked graphene framework. Moreover, GMS exhibits a distinctive elasticity that makes it further apart from other nanoporous carbon materials, opening up new possibilities for innovative applications utilizing mechanical deformation.⁵⁵ Thus, GMS is expected for various applications, including electrochemical capacitors,⁴⁵ fuel cells,⁵⁶ Li-O₂ batteries,^57,58 and catalyst supports.⁵⁹

Figure 6.

A schematic of the synthesis of GMS with transmission electron microscope images at each step. (a) Alumina nanoparticles (Al₂O₃-NP) as a nanosized substrate for CVD. (b) Carbon-coated Al₂O₃-NP (denoted as Al₂O₃-NP/C). (c) GMS prepared by high-temperature treatment (1800 °C) of carbon mesosponge (CMS) obtained by removing Al₂O₃-NP from Al₂O₃-NP/C by chemical etching. (d) Atomic-resolution TEM image of GMS. Some graphene domains are highlighted by circles. The nanographene domains coalesce, and many wrinkles form in GMS. The scale bars are 10 nm in (a–c) and 2 nm in (d). Reprinted with permission from Ref. 45. Copyright 2016 Wiley.

Figure 7a illustrates the cyclic voltammogram of GMS in an Et₄NBF₄/propylene carbonate electrolyte compared to a conventional electrode carbon (Kuraray YP-50F). Similar to single-walled carbon nanotubes,⁶⁰ capacitance of GMS increases as the potential rises, owing to its specific density of state.^61,62 While YP-50F shows an anodic peak corresponding to electrolyte decomposition above 0.8 V vs. Ag/AgClO₄, GMS is stable. Thus, GMS exhibits increasing capacitance up to 1.8 V (Fig. 7b). Due to its high-voltage operation, a supercapacitor incorporating GMS demonstrates over twice the energy density compared to YP-50F (Fig. 7c).

Figure 7.

Electrochemical behavior of GMS. (a) Cyclic voltammograms (1st cycle) of GMS and YP-50F measured by using a three-electrode cell in 1 M Et₄NBF₄/PC at 25 °C with a scan rate of 1 mV s⁻¹. (b) Specific capacitance measured by galvanostatic charge/discharge cycling versus the upper-limit potential. The lowest potential is fixed at −0.5 V, and the upper-limit potential is expanded stepwise by 0.1 V from 0.5 to ≈1.9–2.0 V. (c) Ragone plots obtained using two-electrode cells for GMS and YP-50F. Reprinted with permission from Ref. 45. Copyright 2016 Wiley.

It is also possible to fabricate a seamless GMS sheet using a self-supported Al₂O₃ template, as depicted in Fig. 8. Similar to the case of activated carbons,⁴⁵ voltage stability can be further improved by fabricating a seamless electrode. A symmetric electrochemical capacitor utilizing the GMS sheet demonstrates outstanding voltage stability, reaching up to 4.4 V at room temperature, even when using conventional organic electrolytes.⁴² The capacitance and voltage stability of GMS both exceed those of single-walled carbon nanotubes, which were previously regarded as the most high-performance electrode material for symmetric supercapacitors primarily relying on double-layer capacitance. Furthermore, the nanostructures of GMS, particularly pore size, can be finely tuned over a broad range.⁶³ Thus, GMS is expected to be a next-generation electrode material for high-voltage electrochemical capacitors.

Figure 8.

Photographs of sheet-molded Al₂O₃ nanoparticles (template) and a resulting seamless GMS sheet. Cross-sectional scanning electron microscope and transmission electron microscope images of the GMS sheet are also shown. Reprinted with permission from Ref. 42. Copyright 2019 The Royal Society of Chemistry.

5. Boron-doped Diamond

Boron-doped diamond (BDD) electrodes are known for their wide potential window in aqueous and nonaqueous electrolyte solutions as well as their excellent physical and chemical stability. They are considered promising for various electrochemical applications, including efficient electrolysis and highly sensitive electrochemical sensors.⁶⁴ It is also recognized that the BDD electrode surface shows poor electrocatalytic activity on the surface. Nevertheless, BDD electrodes could considerably enhance cell voltage and electrolyte stability in the electrochemical capacitors owing to their ability to suppress reactions with solvents and electrolytes. Particularly in aqueous electrolytes, the wide potential window of over 3 V offered by BDD electrodes may enable the development of the electrochemical capacitors with high energy and power densities. However, their limited electric double-layer capacitance that is suitable for electroanalytical applications, renders them less ideal for the EDLCs.

Typically produced as polycrystalline thin films on conductive substrates via chemical vapor deposition (CVD), BDD electrodes have a relatively small specific surface area. To leverage BDD electrodes for the electrochemical capacitors, it is essential to develop methods for fabricating BDD with larger specific surface areas. Porous BDD can be created by the techniques in the top-down approach that includes the creation of nanohoneycomb diamond electrodes through oxygen plasma etching with nanoporous alumina membranes,⁶⁵ vertically-aligned diamond whiskers via reactive ion etching,⁶⁶ and porous BDDs through steam activation,⁶⁷ catalytic etching,⁶⁸ and two-step heat treatment methods,⁶⁹ have been explored. Alternatively, the bottom-up approach has been utilized to produce porous BDD electrodes using three-dimensional template substrates such as polypyrrole,⁷⁰ silicon nanowires,⁷¹ porous Ti,⁷² and quartz glass filters.⁷³ Although these methods may introduce more crystal defects and higher levels of sp² carbon impurities, the resulting BDD electrodes maintain a wide potential window, making them viable for the electrochemical capacitors.

BDD powder (BDDP) and boron-doped nanodiamond (BDND) are also being investigated for their high specific surface areas.⁷⁴ The particle size of BDDP ranges from 150 nm to 60 µm and can be adjusted through modifying the substrate diamond particle size.^75,76 Cyclic voltammetry (CV) studies of 150 nm BDDP in 1 M H₂SO₄, which were conducted in a symmetric two-electrode system at a sweep rate of 10 mV s⁻¹, demonstrated a considerably large cell voltage of 1.5 V without electrolyte electrolysis.⁷⁵ This performance markedly exceeded that of activated carbon (AC), which showed a cell voltage of only 0.8 V,⁷⁵ underscoring the potential of BDD materials as electrode materials for aqueous EDLCs.

BDND is a conductive diamond powder obtained by depositing BDD on detonation ND substrates with a primary particle size of approximately 4–5 nm using microwave plasma-assisted CVD.⁷⁷ Although the primary ND particles are about 4–5 nm in size, BDD and sp² carbon by-products are deposited on the surfaces of ND agglomerates, resulting in BDND particles of several hundred nanometers in size. Despite these huge sizes, BDND possesses a large specific surface area of approximately 650 m² g⁻¹, attributed to the interparticle voids and the high surface area of its sp² carbon components. The conductivity of BDND is in the order of 0.1 S cm⁻¹.

CV measurements of BDND in 1 M H₂SO₄ at a scan rate of 10 mV s⁻¹ in a symmetric two-electrode system revealed a cell voltage of up to 1.8 V (Fig. 9).⁷⁷ The capacitance of BDND, influenced by its substantial specific surface area, is comparable to that of AC. The cell voltage increased to 2.8 V when using saturated NaClO₄ as the electrolyte, attributed to the “water-in-salt” electrolyte nature of highly concentrated NaClO₄. This electrolyte exhibits a large overpotential in water molecule electrolysis due to strong coordination of most water molecules with Na⁺ ions.^78,79 This synergy between an electrode material with a wide potential window and an aqueous electrolyte with a similarly wide window is crucial for achieving high energy and power densities in aqueous EDLCs.

Figure 9.

CVs in 1 M H₂SO₄ and saturated NaClO₄ with a symmetric AC and BDND two-electrode system. Scan rate was 10 mV s⁻¹.⁷⁷

Ragone plots illustrate the relationship between energy and power densities derived from capacitance values obtained from CV at various scan rates (10–1,000 mV s⁻¹) and cell voltages of 0.8 V for the AC/1 M H₂SO₄ system, 1.8 V for the BDND/1 M H₂SO₄ system, and 2.8 V for both AC and BDND in saturated NaClO₄ (Fig. 10). They demonstrate the superior performance of the BDND/saturated NaClO₄ system, which exhibited energy densities greater than 10 Wh kg⁻¹ and power densities exceeding 10⁴ W kg⁻¹.⁷⁷ The higher bulk density of BDND (0.52 g cm⁻³) compared with AC (0.23 g cm⁻³) underscored the potential of BDND for enhancing the volumetric performance of the active material layer in compact and safe aqueous EDLCs with high energy and power densities.

Figure 10.

Ragone plots showing power density vs. energy density (a) per unit weight of active material and (b) per unit volume of electrode layer of AC and BDND electrode cells.⁷⁷

Given the current limitations in mass-producing BDND, its initial application in small devices is advisable until scalable production methods are developed. It is expected that BDND-based aqueous EDLCs with high energy density can be developed through the preparation of pseudocapacitors by complexation of BDND with redox-active materials and the application of BDND to hybrid capacitors. Furthermore, BDND’s potential to enhance capacitor durability, especially when combined with organic electrolytes due to its electrocatalytic inactivity, is also noteworthy.

6. Conclusion

The recent advanced research studies (e.g., porous carbon with worm-like shapes, seamless activated carbon, graphene mesosponge, and boron-doped diamond) were reviewed for the carbon based electrochemical capacitors. They demonstrated the importance of the pore shape, three-dimensional structure of the electrode, carbon crystallinity, and consistent evaluation of the pore structure to enhance the capacitance and stabilize the high-voltage charging. The author believes that the reviewed topics suggest a significant potential to successfully realize technological innovation of the carbon based electrochemical capacitors with an excellent capacitance and durability.

Acknowledgment

This work was financially supported by JSPS KAKENHI Grant. No. 17H03123, No. 23H02048, No. 23H00227, JST A-STEP Grant No. JPMJTR22T6, Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), the 3rd period of SIP “Creating a materials innovation ecosystem for industrialization” (Funding agency: NIMS).

CRediT Authorship Contribution Statement

Soshi Shiraishi: Writing – original draft (Equal)

Koki Urita: Writing – original draft (Equal)

Hirotomo Nishihara: Writing – original draft (Equal)

Takeshi Kondo: Writing – original draft (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 17H03123

Japan Society for the Promotion of Science: 23H02048

Japan Society for the Promotion of Science: 23H00227

Adaptable and Seamless Technology Transfer Program through Target-Driven R and D: JPMJTR22T6

Council for Science, Technology and Innovation

Footnotes

S. Shiraishi, K. Urita, and T. Kondo: ECSJ Active Members

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Biographies

Soshi Shiraishi (Professor, Graduate School of Science and Technology, Gunma University)

Soshi Shiraishi was born in 1970. He graduated from Graduate School of Engineering, Kyoto University in March 1995, and earned Doctor of Energy Science in 1999. He worked in Gunma University as Research Associate in 1997–2006 and promoted to Associate Professor in 2006 and Professor in 2013. He was awarded Young Researcher Award (Sano Award), Scientific Achievement Award from The Electrochemical Society of Japan in 2006, 2021, respectively.

His research interests are carbon-based electrochemical devices such as electrochemical capacitors and lithium batteries. Hobby: experiments, reading comics.

Koki Urita (Associate Professor, Graduate School of Integrated Science and Technology, Nagasaki University)

Koki Urita was born in 1980. He graduated from Graduate School of Science, Chiba University and earned Doctor of Science in March 2008. He worked in Nagasaki University as Assistant Professor in 2009–2018 and promoted to Associate Professor in 2019. He has spent in Prof. Simon’s group at Paul Sabatier University (France) as a visiting researcher in 2013–2014. He was awarded Encouragement Award from The Japan Society on Adsorption in 2015.

His research interests are phenomena in narrow pores of porous carbon electrodes under charge-discharge process. Hobby: camping, fishing and finding new foods.

Hirotomo Nishihara (Professor, Advanced Institute for Materials Research, Tohoku University)

Hirotomo Nishihara received his PhD from Kyoto University (Japan) in 2005. Shortly after earning his doctoral degree, he accepted a position as an Assistant Professor at Tohoku University. He was promoted to Associate Professor in 2011 and Full Professor in 2020. In 2022, he took his innovations into the business sphere by establishing a startup company, 3DC. As the Chief Scientific Officer, he is dedicated to the industrialization of a novel porous carbon material he developed.

Takeshi Kondo (Professor, Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science)

Takeshi Kondo was born in 1976. He graduated from Graduate School of Engineering, The University of Tokyo in March 2004, and earned Ph.D in Engineering. He moved to Tokyo University of Science as Research Associate and promoted to Associate Professor in 2019 and Professor in 2024. He was awarded Young Researcher Award of The Electrochemical Society of Japan (Sano Award) in 2010.

His research interests are application of diamond to electrochemical devices. Hobby: reading, walking.