1 Introduction

The development of economy of any society depends on many factors and among them one of the most important is reducing fossil fuel consumption and greenhouse gases. Nowadays, researchers are working on renewable energy sources to meet the objective of production of electricity and further improvement of energy efficiency. To store the harvest energy, a storage system is immensely required, such as primary, secondary batteries (rechargeable) and electrochemical supercapacitors. The electrochemical capacitors/ultracapacitors are known for their long cycle life, moderate energy densities and fast charge/discharge rate, which make them useful in the field of energy storage systems for portable electronic devices, and hybrid electric vehicles with rechargeable batteries. Supercapacitors complement batteries, because the device applications require a large rapid pulse of energy or high power with quick repetitive recharging. Supercapacitors are being used in hybrid electric vehicles, because of its tremendous high power requirement. The high power densities, extended cycle lifetimes, and a broad operating potential range are some of the attractive parameters of electrochemical capacitors, but it needs improvement in operating temperature and specific energy [1]. Low energy density of supercapacitors prohibits the replacement of battery with it, and tremendous efforts have been taken to increase the energy density, which can be achieved through the design of new electrode materials, new electrolytes, and new electrochemical concepts. Although, the electrolyte is equally important in the supercapacitors, as it influences the capacitance values and rate capability (related to radius of the ions, ionic hydration sphere in aqueous electrolytes, solvation/de-solvation energy of the ions, conductivity and mobility of the electrolyte ions) of the device. Nevertheless, in this particular review, we will be focused on the electrode compartment of the device. The important factor is to choose materials to derive the electroactive electrode, which should be from sustainable, natural, abundant, low to moderate cost resources. According to the type of electrode materials and charge storage mechanism, the supercapacitors are classified into two major categories: (1) electric double-layer capacitors (EDLCs) based on various carbons materials, that store energy via electrostatically, (2) pseudocapacitors based on metal oxides and conducting polymers, that store energy via fast faradaic (redox) reactions. RuO2, MnO2 and electrically conducting polymers store charges based on faradaic redox reactions, and exhibit pseudocapacitances 100 times greater than electrical double-layer (EDL) capacitance. However, redox capacitors suffer from poor stabilities (< 1000 charge/ discharge cycles), self-discharge, ageing or fast capacitance decay. In the case of conductive polymers, such as polyaniline, polypyrrole, and polythiophene-based supercapacitors, they provide improved capacitive performance, but their poor mechanical properties need to be improved. Pseudo-capacitance in metal oxides and nitrides is a result of redox reactions comprising diverse oxidation states within an operational potential window, somewhat similar to electrode materials of batteries. Consequently, the poisoning of the electrode material and irreversible oxidation is responsible for the cyclability deterioration.

The structure of carbon material differs such as activated carbons, carbon nanofibers, carbon nanotubes, and graphene. The researchers have explored utility of these different carbon structures as electrode materials for supercapacitors [2, 3]. On the other hand, activated carbons have become the most important part of the EDLC device. The most vital property of carbons is their diversity of textures/structures and surface chemistries. This property makes carbon material the most used electrode material, as one can alter its electrochemical performance according to used electrolytic media and system configurations. The EDL capacitance arises from voltage-driven electrosorption of ions takes place due to the electrostatic charge accumulation from charge carrier electrolytes into the different layers of ions on the electrode/electrolyte interfaces [4]. The classical Helmholtz–Gouy–Stern double-layer approach [4,5,6] partially explain charging process and behaviour of the electrode–electrolyte interface. The computational and experimental findings are trying to explain charge storage phenomena microscopically with the help of all relevant parameters, although recent explorations have extended the understanding of the related phenomenon [7, 8]. But, there overall capacitance does not exceed 100 F/g and energy density is low to utilize it in real practical applications.

To enhance the specific capacitance and energy density of supercapacitors, certain sincere efforts have been taken to integrate both types of charge storage mechanism in one electrode, to complement the shortcoming of low capacitive carbons with unstable conducting polymer and low conducting metal oxides. Thus, the enhanced specific capacitance of carbon materials has been achieved due to extensive and intensive research by introducing metal oxides, [9] conducting polymers [10] and/or functional groups (such as nitrogen and phosphorus groups) [11, 12] into carbon materials. Transitional metal oxides and conducting polymers exhibit a much greater pseudocapacitance than functional groups. However, these materials involve a fast capacitance degradation during the charge–discharge processes, and stability is a serious issue with them that limits their practical applications. The carbon as nanotubes or graphene is also an excellent support for pseudocapacitive materials such as transition metal oxides and electrically conducting polymers. The cost of high purity carbon nanotubes and graphene is a practical constraint.

2 Materials and methods

To achieve high capacitance, high specific energy-keeping specific power with a long cycle life, as discussed in detail in the previous section, there are several specifically designed carbon precursors that have been explored.

2.1 Advanced class of carbon precursors

  1. (i)

    Metal organic frameworks (MOFs) or porous coordination polymers are potential templates to build microporous carbons owing to their large carbon contents. A primary carbon precursor is impregnated and then polymerized inside the micropores of MOFs. Upon thermal carbonization, the development of porous carbon networks and decomposition of MOFs occur simultaneously. MOFs play a role as a sacrificial template as well as a secondary carbon precursor. The limited stability of the framework can collapse in the decomposition process. These carbons possess broad pore size distribution and carbonization conditions are not optimized fully.

  2. (ii)

    Zeolites and mesoporous silicas using nano-casting with hard templates (solid templates) to prepare ordered microporous and mesoporous carbons, although this method is somewhat complex and unfavourable for large-scale production.

  3. (iii)

    Polyacrylonitrile (PAN) is a relatively high yield carbon precursor with high nitrogen content. It is used to fabricate heteroatoms, e.g., nitrogen-doped porous/non-porous carbons for electrode materials of supercapacitors. The 1-D electro-spun carbon nanofibers (CNFs) derived from PAN precursors possess good conductivity and various porosity features.

2.2 Advantages of porous carbon materials

The porous carbon materials have high surface area, low density, semiconducting, good polarization behaviour, easy availability of wide range and low-price precursors. We can prepare several micro- and nano-structures of these carbon materials. These several advantages of porous carbons make them attractive for variety of applications. These applications are specific advantages of porous carbon materials only. These carbons are being used in many applications ranging from adsorbents and catalytic supports, water desalination, capacitive deionization, electrode materials for batteries, fuel cells, supercapacitors, and drug delivery carriers. They have ability to store hydrogen also. Their nano-structures offer ballistic transport properties. Apart from all above advantages, there are few very sever disadvantages, as discussed in next section.

2.3 Drawbacks of porous materials

  1. (i)

    The electrical conductivity has been decreased by increasing surface porosity, owing to the non-compatibility of the conductive pathways and increase of bulk resistance due to the presence of micropores (certain empty space) that limits accessibility to the electrode surface.

  2. (ii)

    Porous carbon has a high surface activity related to the presence of various oxygen functionalities that is pH-sensitive. These high surface area carbon electrode materials are unfavourable for the power performance of supercapacitors, susceptible to poisoning by organic compounds due to these hetero-functional sites and pH. The decomposition of electrolytes occurs on these functional sites, and harms the life cycle performance of electrochemical capacitor cells.

  3. (iii)

    The specific volumetric capacitance is more a reliable parameter to calculate the efficiency of a material and high porosity limits the volumetric capacitance due to low packing density of porous materials. Highly porous nanostructures having large surface area are able to improve gravimetric capacitance but not to volumetric capacitance.

  4. (iv)

    High cost of templated carbons, carbon nanotubes and graphene materials, etc.

  5. (v)

    Limited specific capacitance of cheaper activated carbons

These above-mentioned shortcomings restrict their application in technology. To overcome the above-mentioned drawbacks, more conducting, high-density and oxygen-free carbons are required. In the series of synthesis advancements, few non-porous materials have been reported.

To obtain a detailed understanding of the science and technology of non-porous carbon/carbon supercapacitors, this review describes the design and complete understanding of non-porous carbons in terms of interaction involved between doped heteroatoms and its surface. We conclude certain key aspects of various non-porous carbon materials have been synthesized to improve the energy density of supercapacitors. The material–electrolyte interaction will be discussed later in detail.

2.4 Non-porous carbon structures and their performance

There is a need for non-porous carbon containing the functional groups/heteroatoms in the skeleton of carbon structure of these materials, as it will provide purely pseudocapacitive redox reactions, so that charge storage mechanism is based on Faradic reactions. However, few reports describe, the influence of surface composition (chemical species) and structure (structural defects) on the interaction between electrolyte with electrodes under neutral, constant potentials, and fluctuating voltages. The ion confinement in functionalized surface and correlation study of experimental results with computational models to deliver details into supercapacitor charging/discharging processes is reported.

Heteroatom-enriched carbon materials belong to the class of pseudocapacitive materials along with conductive polymers, metal oxides and nitrides. These non-porous carbons feature reactive edge sites, semi-conductive electronic band structures, and robust surface chemistries [13, 14].

The surplus pseudocapacitance originates in non-porous carbons due to heteroatoms in its structural skeleton and functional surface groups given the Faradic redox reactions. The non-porous carbons with functional groups enhance capacitance keeping the good cycling feature of carbon-based supercapacitors [15]. The interaction between functional defects and electrolytes occurs at the electrode–electrolyte interface. Functional groups containing N, P, B, S etc alter ideal graphene’s π–π bonding symmetry and increase theoretical quantum capacitance by 15–50 µF cm−2 [16,17,18]. Further, these heteroatom-based species interact with ions and affect the ionic orientation on the pore wall with functional group, that hinder the ionic mobility and decrease specific capacitance by 2–3 µF cm−2 [7, 19]. While the surface functional groups (–HCOO, –O–C = O, etc.,) due to their acidic and basic nature interact with ions and restrict their movement, sometimes surface groups electrosorb ions at the narrow pore openings and block rapid electrolyte transport [20, 21]. Furthermore, the intrinsic metallic-like conductivity of carbon electrodes decreases due to the presence of functional groups. Further, the ion confinement in micropores, surface chemistry, and disorder at the electrode–electrolyte interface affect charge storage, because confined ions exhibit lower mobility, [22] lower electrolyte densities (ρ ≈ 0.8ρbulk), maximize ion transport rates [23]. Furthermore, short range and lower pore loading of localised electrolytes and heterogeneous pore filling with varying local density of ions are the consequences of functional groups [14, 24,25,26].

As discussed in the above section about the various aspects of non-porous carbon, the porosity of the carbon with the functional groups is not much of a useful choice, and further it restricts its utility as a stable supercapacitive electrode.

The pyrolysis of organic precursors or carbonization of polymeric materials can be used to achieve highly porous carbon materials, this similar method can be used to prepare non-porous carbons removing activation step. Table 1 describes the different materials and methods used to obtain different types of carbon structures.

Table 1 Materials and methods used to obtain different types of carbon structures
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2.5 Modified graphene: non-porous

In the series of the development of non-porous carbons, various efforts have been made by scientific community worldwide. But, few methods are specifically designed and dedicated to completely new carbon structures. There are reports to increase the packing density of nanostructured carbons using the irreversible method of chemical pressure, for example, in graphene hydrogel films. Therefore, Yang et al. [27] tried to increase the packing density of graphene from 0.13 to 1.3 g cm−3 via synthesizing graphene hydrogel films by capillary pressure. Highly compact graphene films ~ 1.33 g cm−3 has been obtained. These could achieve high volumetric capacitance ~ 255.5 F cm−3 using these graphene films as electrode in aqueous electrolyte at 0.1 Ag−1. This specific capacitance value is comparable to the existing porous carbon materials.

The conventional nanocasting technique can be used to prepare polymer nanofibers. The fabrication process for carbon nanofibers often comprises additional treatments such as high temperature carbonization and chemical activation. Carbon materials processed using such techniques may achieve high capacitance due to these porous carbons, but their rate capabilities still cannot meet the need for high volumetric power delivery. Therefore, a simple synthesis route without any additional activation, which produces non-porous/low surface area carbon materials with high capacitance and high rate capability is desirable. High volumetric carbons can be achieved by simple methods such as doping of heteroatoms in these non-porous high-density carbons.

The groups containing O, N, F or P can increase the capacitance of carbon electrodes as a consequence of the Faradaic redox effect involved in the charge transfer between electrodes and electrolytes [28,29,30,31].

2.6 N-doped non-porous carbon

Nitrogen-containing carbons induce pseudocapacitance by enhancing the charge mobility of negative charges on carbon surfaces, and this could improve the electronic conductivity. Nitrogen doping gives fast Faradic redox reactions between the nitrogen functional groups and ions in the electrolytes and thus greatly improving the capacitance. Therefore, nitrogen groups have been proven to be electrochemically active groups. Polyacrylonitrile (PAN) has been used as a carbon precursor to fabricate nitrogen-enriched carbons owing to its high carbon yield and high nitrogen content. This has been used as electrode materials for supercapacitors. Other precursors also have been used to incorporate nitrogen into the carbon matrix to achieve porous as well as non-porous carbons. The nitrogen (N2) doping in carbon materials is done using ammonia (NH3) gas/air. On the other hand, N-containing carbon precursors, such as melamine, polyacrylonitrile, polyvinylpyridine, and quinoline-polymerized pitch have been used. There are few reports, where, N-doped carbon has enough porous as a result of the soft/hard template or activation process. Thus, the pseudocapacitance is a surplus contribution to the double-layer capacitance. Few researchers [32,33,34] prepared N-containing carbon–nanotube (CNT) composites from biopolymers as well as N-enriched carbon with a large amount of nitrogen residues from melamine resins. They found less surface area for N-enriched carbon rather than N-containing carbon–nanotubes (CNT) composites. However, low surface area N-enriched carbon was able to give more capacitance in acidic and alkali electrolytes than N-containing carbon nanotube composites. Their study promotes that porous carbon materials are not mandatory to achieve a high value of specific capacitance. Later, Gao Qing Lu et al. reported [35] a new type of non-porous N-enriched carbon material synthesized from low surface area N-enriched carbon using an ammonia treatment. They synthesized many samples derived from melamine. Initially, they synthesized two sets of samples:

  1. (i)

    Melamine–mica composite which was carbonized at 1000 °C.

  2. (ii)

    Later these composites were stabilized in air for 4 h at 250 °C prior to carbonization.

Further, they performed two different treatments on these samples to increase the nitrogen content in the samples.

  1. (i)

    The above samples were subjected to further nitrogen enrichment in pure ammonia gas at 400 °C.

  2. (ii)

    As ammonia interacts with oxygen groups, initial carbon samples were peroxidised with 30% nitric acid before ammonia treatment.

They found that the surface area of the starting carbon materials reduced considerably after above treatments. High density of surface functionalities possibly blocks the pores which may result in carbon shrinkage. Therefore, materials which were stabilized before carbonization and other treatments were found to have a lesser surface area than the samples prepared without a stabilization process. They calculated the micropore volumes for all the prepared samples and found it to be nearly 0.01 cm3 g−1 using N2 and Co2 adsorption. These non-porous carbon materials will not contribute to double-layer capacitance, as ion transport must be restricted due to the absence of larger pores. Gao Qing Lu et al. [35] performed detailed analysis of XPS measurements of all the prepared samples and found that peroxidised carbon samples were significantly different from their non-preoxidised counterparts with respect to having different amounts of quaternary N-3 peak. It indicates that peroxidised and non-preoxidised samples were having different surface chemistry, which can be understood using XPS spectra. Various nitrogen-containing functional groups can be associated to carbon matrix as shown in Fig. 1.

Fig. 1
figure 1

Schematic diagram of the carbon matrix, representing the locations of the various nitrogen (N)-containing functional groups [35]

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During the treatment of carbon samples with ammonia gas, N-containing groups can be introduced in carbon matrix. This is attained via reactions of NH3 with oxygen surface functionalities. These reactions can produce four different types of N-functionalities. The reactions of NH3 with side carboxylic acid sites and the ring system, produce the lactams and imids, while the reactions with the side groups only results in the amides and amines. Their XPS results clearly show that more effective nitrogen enrichment is possible in carbon materials subjected to peroxidation, because the reaction of ammonia with oxygen is responsible for the absorption of more nitrogen into the material. The lactams and imids group formation need the presence of three oxygen atoms in the material, however, amid and amine group formation need only one oxygen group in the material. Their results are well in agreement with this theory. A high presence of lactams and imids in peroxided carbon samples and a high presence of amide and amine groups in non-preoxidised carbon samples were confirmed by XPS spectra.

The electrochemical performance of the carbon material will differ for same carbon having different functional groups. Therefore, to check the role of nitrogen functionalities in the capacitive performance of supercapacitors, they performed the detailed electrochemical testing of all carbon samples. Their cyclic voltammetry results show that preoxidised samples showed higher electrical conductivities than their precursors. Many research works have proven the electrochemical response of nitrogen groups in carbon matrix successfully. Very interesting outcomes have been discussed in detail. Generally, the surface area of these carbons is very low. Often, the co-existence of small amount of micropores with heteroatom groups makes it complex to study the true role of the heteroatom groups, but the overall dominant behaviour is of non-porous type.

2.7 P-doped non-porous carbon

In this order, phosphorous-doped carbon nanofibres have been reported recently. The phosphorus doping has been proven as a promising strategy to fabricate high-capacitance carbon-based electrode materials for supercapacitors. The introduction of phosphorus groups in non-porous nanocarbons also improves the capacitive performance of carbon. Xiaoping Yang et al. [36] reported the effect of phosphorous groups on the supercapacitive performance of carbon nanofibers (P-CNFs). They used a simple method to fabricate P-CNTs. Without any additional activation agent, P-CNFs were fabricated through electrospinning of the precursor solution containing polyacrylonitrile and phosphoric acid. P-CNFs show excellent electrochemical performance. They showed better electrical double-layer characteristic for P-CNFs got enhanced electrical double-layer characteristic without pseudocapacitance characteristic confirmed by CV measurements rather than pure CNFs. The rectangular shape of CV was attributed to the oxygen-rich phosphorus functionalities which could improve the wettability between the electrode materials and the electrolyte as well as enhance the adsorption of ions in the electrolyte. The electrochemical impedance spectroscopy shows that phosphorus doping in carbon nanofibers has better electrochemical performance than without doped samples. Their results clearly show that supercapacitor formed using P-CNFs have a fast ion transfer from the electrolyte to the electrode materials than undoped CNFs.

2.8 Nitrogen–phosphorus co-doped non-porous carbon nanofibers

Recently, heteroatoms co-doping has attracted considerable attention owing to its unique effect on the electrochemical properties of carbon materials. Nevertheless, the mechanism of heteroatoms co-doping is not so obvious, because mostly heteroatoms co-doped carbon materials are porous, which makes it tough to exactly estimate the role of the heteroatom functionalities in the overall capacitance of the carbon electrodes [37]. Nitrogen doping induces pseudocapacitance by improving the charge mobility of negative charges on carbon surfaces, and therefore, prominently improves the capacitance. The low surface-area nonporous N/P co-doped CNFs (where the EDL capacitance derived from the large SSA and micropores could be ignored) offer good opportunity to distinguish the functions of nitrogen and phosphorus groups. Therefore, X. Yan et al. [37] had produced nitrogen/phosphorus dual-doped non-porous carbon nanofibers (N/P-NPCNFs). They used polyacrylonitrile (PAN) as a carbon precursor. Without adding any additional activation, N/P-NPCNFs were produced by the electrospinning of the PAN solutions by adjusting the amount of phosphoric acid (H3PO4) added to the precursor solutions, and subsequent heat treatments.

They prepared many samples using the similar procedure via changing the mass ratio of PAN to H3PO4 in the electrospinning solution. Fibrous morphology was observed in SEM images [37]. The doping of N/P increases the average diameter of the CNFs as shown in the SEM images in Fig. 2.

Fig. 2
figure 2

The SEM images of a typical non-porous carbon nanofibers [37]

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The presence of phosphorus hinders the carbonization of polyacrylonitrile (PAN), given the amorphous nature of CNFs. CNFs have a very small surface area. N-doping facilitates the generation of pyrrole-like nitrogen. The phosphorus functionalities suppress the unstable surface oxygen groups, which favours stability. The capacitance value increases significantly from 120 to 230 F g−1 with the increase of phosphorous content from 0% to 9% and increase in the pyrrol-type nitrogen from 40 to 65% which also contribute to this enhancement (Fig. 3), [37]. Even at a high current density of 30 A g−1, its capacitance is as high as 155.5 F g−1 with long-term stability ~ 8000 cycles. They clearly showed (Fig. 3) that capacitance depends on the phosphorus content as well as pyrrol-like nitrogen content. Pseudocapacitive Faradic reactions occur between the pyrrol-like nitrogen (N-X) groups and ions in the electrolyte which results in high specific capacitance.

Fig. 3
figure 3

Dependence of specific capacitance of non-porous carbon nanofibers as a function of phosphorus content with N-X (pyrrole-like nitrogen) content [37]

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It should be noted that there may be a synergetic effect between the electrochemically active nitrogen groups and the phosphorus groups.

Maximum N/P-doped sample exhibits a widened electrochemical potential window up to 1.4 V in 1 M H2SO4 electrolyte. This widening of the electrochemical window was attributed to the blockage of phosphorus groups to the electrochemical active oxidation sites, and or to the decrease of electrochemical active oxidation sites which are replaced by phosphorus groups.

In another report [37, 39], phosphorus-doped carbons are further explored for a future optimal design and fabrication of high-performance nitrogen–phosphorus co-doped carbon materials. The capacitive behaviour of CNFs in different electrolytes (H2SO4, Li2SO4 and K2SO4) has been investigated. They compared the capacitive performance of nitrogen–phosphorus co-doped CNFs electrode with pure nitrogen-doped non-porous CNFs electrodes. The average diameter of N/P non-porous carbon nanofibers was found to be much larger than that of nitrogen-doped non-porous carbon nanofibers, but both types of CNFs had a smooth surface. Cyclic voltammogram (CV) curves of pure nitrogen-doped CNFs with different electrolytes (H2SO4, Li2SO4, Na2SO4 and K2SO4) showed tremendously distorted rectangular shapes and an extremely small capacitance. Generally, nitrogen-doped carbons store energy by a simple electrostatic interaction between electrolyte ions and charge at the electrode surface in neutral electrolytes [40]. Thus, CV curves of nitrogen–phosphors co-doped samples with different electrolytes (H2SO4, Li2SO4, Na2SO4 and K2SO4) show a rectangular shape [39]. However, pure nitrogen-doped carbons showed almost a triangular shape indicating capacitance due to pseudocapacitive interactions between the H+ ions and the heteroatom nitrogen and oxygen groups. High areal capacitance per unit area ~ 8.5 F m−2 in acidic electrolyte may be due to the adsorbed electrolyte ions by the phosphors groups that might partially de-solvate, shortening the distance between the carbon surface and the adsorbed ions. With K2SO4 electrolyte, the contribution of capacitance due to nitrogen group is found to be more at low current densities and this is because of the advantage of the strong adsorption of K+ to the pyrrole-like nitrogen configurations. EIS measurements indicate that N/P-NPCNF electrode greatly enhances surface wettability induced by the phosphorus groups and offers much better capacitance. This is attributed to the introduction of the phosphorus groups that really facilitate the adsorption of the H+ ions and 84.3% capacitance retention in 1 M H2SO4. This excellent rate capability may be explained by the following reasons: high electrolyte conductivity, high ionic mobility, and the hydrogen bonding between the H+ ions and the oxygen-rich phosphorus groups and thus this can lead to a great number of the H+ ions to be located at the electrode/electrolyte interface and thus shorten the ion diffusion distance from the electrolyte to the carbon surface.

2.9 Fluorine and nitrogen co-doped carbon microspheres

As discussed earlier, nitrogen doping greatly helps to improve capacitance as it contributes pseudocapacitance by improving the charge mobility of negative charges on carbon surfaces. Fluorination is also an effective approach to improve electrochemical capacitive performance of carbon electrodes as a result of increased electrical conductivity because of the semi-ionic bonding features in non-aqueous electrolytes, enhanced polarization from the highly electronegative fluorine functional groups and the refinement of pore structures/surfaces [41, 42]. However, Li Z. et al. [43] reported that volumetric performance for heteroatom-enriched carbon materials is still low because of their relatively low densities and large surface areas. Later, Faming Gao et al. [44] reported another approach to grow high-performance electrodes with ultrahigh volumetric capacitance and exceptional cyclic stability for electrochemical energy storage. They used low temperature solvothermal route which is one of the synthesis methods used to synthesize non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen. Electrode materials prepared with this method have high volumetric capacitance with high mass loading and high charge rates for long-living electrochemical energy storage systems. The fluorine doping gives an acidic flavour to carbon as electron acceptors, while, nitrogen doping provides basic characteristics, inducing electron-donor properties. These active centres may contribute to pseudo-capacitance through additional Faradic interactions [31].

The carbon microspheres were found to be monodispersed and they have a uniform diameter of ~ 4 µm (Fig. 4). These microspheres have a smooth surface as well as nearly perfect geometry of CM-NF.

Fig. 4
figure 4

SEM images of nitrogen–fluorine co-doped carbon microspheres’ (CM-NFs) electrodes. Scale bar, 15 mm and 1 mm (inset) [44]

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Figure 5c–f shows a uniform distribution of C, F and N species within CM-NF spheres. The CM-NF and CM-N indicate dominant features of amorphous carbon, however, a greater degree of graphitization was found for CM-NF than CM-N. The CM-NF, CM-N and commercially purchased pure CM carbon materials have very low surface areas of 1.4, 1.9 and 1.0 m2 g−1, respectively.

Fig. 5
figure 5

SEM images and the corresponding EDS elemental mappings/elemental distribution of nitrogen–fluorine co-doped carbon microspheres’ (CM-NFs) electrodes: carbon (red); nitrogen (yellow); and fluorine (purple). Scale bar, 2 mm [44]

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The presence of ultra-micropores in the size range ~ 0.55 nm and a total pore volume of ~ 0.015 cm3 g−1 were confirmed from the CO2 sorption data. Small electrolyte ions may easily move into these ultra-micropores and can diffuse rapidly also. Thus, these micropores enable Faradaic reactions both at the surface and in the bulk. They found a rectangular-like shape for CM-NF than CM-F and CM-F than CM in CV curves, indicating greatly enhanced capacitance due to the F doping. They proposed that N and F doping in the CM could not only increase the surface wettability between electrolyte and electrode materials, but could also participate in the pseudocapacitance reaction, which contributes to the pseudocapacitance, given the volumetric capacity ~ 521 F cm−3 for CM-NF. This is possibly due to synergistic effects, affecting charge transfer in donor/acceptor characteristics, upon fluorination and in both acid and base electrolytes. The pseudocapacitive reactions attributed to pyridine-N, pyridine-N-oxide and pyrrol-type nitrogen groups. The high nitrogen doping concentration ~ 8–9% resulted in high capacitance. A high volumetric energy density of 18.1 Whl−1 is attained by CM-NF electrode in basic solution. The volumetric energy density of the whole supercapacitor could be much higher than that of commercial devices owing to the high volume fraction of the electrodes in the device stack and high mass loading (16.8 mg cm−2). The excellent capacitance retention is reported for high rate applications. The capacitance retention of the CM-NF is 64% in KOH and 63% in H2SO4, as current density increases from 0.1 to 5 Ag−1, which is much higher than that of 50% in KOH and 43% in H2SO4 for the CM-N. The excellent performance without loss of specific capacitance after 20,000 cycles in KOH and 10,000 cycles in H2SO4 is achieved. No obvious phase transformation was found upon prolonged cycling and the broadening of the (002) peak after 5000 and 10,000 cycles suggest the continuous loss of layer ordering upon ion intercalation. The fluorine atoms can induce redistribution of charges of N atoms and decrease the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels for CM-NF, and thus, further responsible for the reduction of the charge transfer resistance of electrodes, which is calculated by DFT and verified by EIS. As observed from previous reports [45,46,47], these nitrogen groups have a significant role in inducing pseudocapacitance by improving the charge mobility of the carbon and by originating the negative charges on the carbon surface, resulting in ion doping/de-doping as observed in a similar case of conducting polymers. The higher amount of graphitization of the CM-NF may also efficiently facilitate the transport of electrolyte ions and high electrical conductivity during the charge/discharge process [48]. Table 2 describes the charge storage mechanism in different types of carbons reviewed in this article.

Table 2 Dominant charge storage mechanism in different types of carbon
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An interesting report on poly(o-phenylenediamine)-derived extremely low surface area carbon materials has been published for high-performance supercapacitors [49], and surprisingly this material performed almost in a equal capacity as large surface area carbons. A quantitative analysis of reactive carbon edge sites has been described, and this can be useful to understand the possible reactions of ion and catalytic species with the carbon surface. The effect and estimation of graphene sheet size in the condition of high temperature treatment have been well-documented, and the various aspects of non-porous carbons have been reported in detail in ref [50].

2.10 Boron and nitrogen-doped non-porous carbon

To study the fundamental mechanism of capacitance due to these heteroatoms, well-designed boron and nitrogen-doped non-porous carbon have been studied [51]. The following factors are found to be the key to the performance of these heteroatom-doped non-porous carbons: (1) better wettability, (2) the decrease of ESR, (3) the contribution of space-charge-layer capacitance, and (4) the occurrence of pseudocapacitance. It is expected that N-doping improved the wettability in the aqueous electrolytes and B-doping was effective for the reduction in intrinsic resistance of carbon. Although, the influence of doping on the space-charge-layer capacitance was not observed in non-porous carbon-coated AAOs, the effect of redox reactions is the only possible explanation for capacitance enhancement. Finally, one of the good reports [52] was that a specific capacitance of 20% nitrogen-doped carbon nanospheres is 191.9 F g−1, which is 14 times higher than that of the undoped carbon nanospheres with good retention up to 10,000 cycles. The performance of supercapacitors based of different non-porous heteroatom-doped carbons is presented in Table 3.

Table 3 The performance of supercapacitors based of different non-porous heteroatom-doped carbons
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2.11 Nitrogen and phosphorous co-doped non-porous carbon

N and P co-doped carbon is reported to be derived from polybenzoxazine–melamine polyphosphate precursor [53]. The molar ratio between the carbon atoms of benzoxazines and the phosphorus atoms of melamine polyphosphate was kept to 20:1, and the resulted carbon denoted as C/P-20-1 shows the best optimized performance. The C/P-20-1 showed enhanced specific capacitance, rate capability and cycling stability which exhibited high specific capacitances of 173.2 F g−1(see Fig. 6). The capacitance retention of the supercapacitor was reported to be 79.9%. The power density of 25 kW kg−1 and energy density of 5.55 Wh kg−1 are reported.

Fig. 6
figure 6

a CV curves at different scan rates and b GCD curves at different current densities of C/P-20-1 electrode in a three-electrode system. All measurements were reported in the voltage range of − 0.1–0.9 V versus SCE [53]

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As the scan rates increases, CV curves were shown to retain a capacitive charge storage nature. GCD curves at different current densities (marked on Fig. 6a, b) exhibit a symmetric triangular shape. Both types of electrochemical studies confirm high rate capability and cycling reversibility due to improved electronic conductivity by the N-doping. The low voltage drops in discharge profile, indicating its small internal resistance due to the nitrogen doping, is further confirmed by EIS spectrum, which shows a smaller value of contact resistance, equivalent series internal resistance and charge transfer resistance (Fig. 7).

Fig. 7
figure 7

Nyquist plots of the N and P co-doped (C/P-20-1) carbon electrode-based supercapacitor in a two-electrode system [53]

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2.12 Nitrogen, phosphorous and silicon-doped non-porous carbon nanofibers

Silicon (Si)-doped carbons also have attracted attention in strategy for high-performance supercapacitor electrodes. The tetraethyl orthosilicate (TEOS) and phosphorus (H3PO4) activation of polyacrylonitrile (PAN) through electrospinning and subsequent thermal treatment was used for preparation of N-, P- and Si-doped non-porous carbon nanofibers [54]. High gravimetric capacitance of 243.7 F g−1 at 0.5 A g−1, and capacitance retention rate > 100% after 8000 cycles at a high current density of 30 A g−1 with volumetric capacitance of 209 F cm−3 at 30 A g−1 (due to a high packing density) are reported [54]. The silicon doping contributes to better conductivity and P doping improves the wettability of the CNFs, resulting in a smaller charge diffusion resistance which obtained a better electrochemical performance (Fig. 8). The CV curves (Fig. 8) of different N/P/Si-doped carbon nanofibers at a scan rate of 20 mV s− 1 in 1 M H2SO4 aqueous solution shows capacitive feature for all the samples prominent pseudo peaks attributed to the faradaic redox reactions of N, P and Si dopants. Possible faradaic redox reactions are proposed as shown in the Fig. 9.

Fig. 8
figure 8

The electrochemical performance of the N-, P- and Si-decorated CNF-based cells using a three-electrode system a CV curves at a scan rate of 20 mV s−1 b GCD plots at the current density of 0.5 A g−1. c Specific capacitance versus current density (0.5–30 A g−1). d Impedance curves in the frequency range of 0.01–105 Hz [54]

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Fig. 9
figure 9

The possible Faradaic redox reactions of the functional groups involving N, O, P, Si doped on the surface of CNFs [54]

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These heteroatoms (N, P, and Si) work as active sites, they interact with electrolyte ions, and promote reversible redox reactions, which provide surplus pseudocapacitance. In addition, the surface wettability of the electrodes could also be enhanced due to a higher oxygen content. The pyridinic and pyridonic forms of N and high N-6 (42.7%) and N-5 (55.3%) content increase the pseudocapacitance through the as-mentioned redox reactions. The pseudocapacitance can be tailored by altering the doping percentage of N, P, and Si. The doping percentage of N, P, and Si could be changed by altering the percentage of phosphoric acid and TEOS doping amount.

The above discussion shows that the enhancement in the capacitance value is significant in different heteroatom-doped carbons. The cycling life, power density and energy density are not optimized in many cases. There is much scope to optimize such electrode systems for high-performance supercapacitors. The volumetric characterization is incomplete in many reports. The high volumetric capacitance achieved by tailoring the density, doping, texture and shape of carbon materials results in their gravimetric capacitance decreasing. A good volumetric performance is required to make these carbon materials useful for practical applications.

2.13 Non-porous carbon in other electrochemical applications

Similar type of carbons has also been tested for other applications like batteries. Inexpensive, and facile methods that have used to prepare carbons with high sulphur content has been used to synthesize Li–S batteries [55]. Kan Mi et al. [55] reported N and O in situ dual-doped non-porous carbon material (NONPCM). Importantly, NONPCM can be synthesized on a large scale via the green hydrothermal carbonization method. The excellent cycle stability and rate performance of NONPCM with a sulphur confinement of 80–90 wt% in active sites, exclusive of physical entrapment have been achieved using this material. In addition to that, turbostratic-structured carbon nanospheres (CNSs) of size ~ 100 nm with N-functional groups have been reported [56]. A higher specific capacity is observed for these CNSs as anode in lithium ion battery as compared to the carbon spheres derived from sucrose as well as a greater rate capability than commercial mesophase carbon microbeads. In another report on nitrogen-doped carbon nanoparticles, which is produced by flame synthesis, it also performed well as anode material for rechargeable lithium ion batteries [57]. There was a report on sodium ion batteries, fluorine-doped carbon particles derived from lotus petioles that are also employed in high-performance anode materials. These F-doped carbons performed much better than high surface area carbons [58]. On the other hand, heteroatom-doped graphitic carbon acts as a catalyst for efficient electrocatalysis of oxygen reduction reaction along with other porous carbon nanostructures, which showed great performance and provided an insight into cheaper carbon-based, metal-free substitute to precious Pt-based compounds [59].

Recently, there was a report on lithium ion batteries with non-porous carbon anode [60]. The spent coffee ground (SCG)-derived carbon (C-SCG) was employed as anode in LIB which delivered a specific capacity of 360 mAh g−1, reversible capacity of 285 mAh g−1 at current density of 0.1 A g−1 and coulombic efficiency nearly to 100%, for which a landmark performance was reported for biomass-derived carbons. The electrochemical performance of the LIB cells is depicted in Fig. 10.

Fig. 10
figure 10

Electrochemical performance of the non-porous carbon material (C-SCG) in a half-cell of LIB. a CV curves at 0.1 mV s−1, b charge–discharge curves, c specific capacity versus cycle number, both at a current rate of 0.1 A g−1 and d rate performance [60]

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3 Conclusions and future perspectives

The immense importance of volumetric parameters (capacitance, energy and power) is established due to the use of electrochemical capacitor in the development of electric vehicles and smart devices. Various high surface area carbon-based electrodes have been successfully utilized in the fast development of supercapacitors, while being limited by their low volumetric and high gravimetric performance owing to their porous structure and low density. The most recent progress in non-porous heteroatom-doped carbon with a high volumetric performance stressed upon the aim to clarify the significance of the volumetric performance and reviewed the approaches for improving the volumetric performance, mainly with heteroatom-doped carbon-based electrode materials. The reports on heteroatom functionalization of carbon electrode materials show potential to stop deterioration in the cyclability, creating a applied approach to develop high-capacitance device for commercial use. The transformation of supercapacitor electrodes from normal low-density to high-density carbons is emphasized as a capable remedy for below the mark performance issues, and their applications in next-generation supercapacitors are also discussed in detail. These carbons have also found applications in lithium–sulphur, lithium ion, and sodium ion batteries and can be used as a catalyst for effective electrocatalysis of oxygen reduction reaction.