Keywords

1 Introduction

Porous carbon materials (PCMs) are important members in the big family of porous materials. PCMs could be prepared from various precursors including synthetic polymers, nature polymers, and carbon-rich small molecules (Borchardt et al. 2017; Gonzalez-Garcia 2018; Wang et al. 2020b). PCMs are widely applied in energy storage and conversion devices, electrocatalysis, water purification, electromagnetic shielding, microwave absorption, and other fields (Zhai et al. 2011; He et al. 2019c; Tian et al. 2020). PCMs show advantages in high yield, low cost, tunable microstructures, high specific surface area (SSA), stability, etc. Tons of PCMs are produced globally every year mainly from low cost biomass. The price of most PCMs could be below 1 $/g. The pore size of PCMs could be adjusted in the range of millimeter to micrometer to nanometer, and the corresponding specific surface area could be varied from a few square meters per gram to thousands square meters per gram. PCMs with pore size in the range of millimeter to sub-micrometer could be obtained from lyophilization/freeze drying, chemical blowing/(gas) foaming, template methods, etc. Usually, pores with the diameter below 100 nm are named nanopores. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), nanopores can be divided into macropores (>50 nm), mesopores (2–50 nm), and micropores (0.2–2 nm). Micropores can be divided into super-micropores (>0.7 nm) and ultra-micropores (0.2–0.7 nm) (Liu et al. 2017). As to PCMs, ultra-micropores are always formed from the release of volatile gas and the condensation of carbon framework. The super-micropores of PCMs could be generated by the physical/chemical activation process (Yin et al. 2020). The formation of mesopores and macropores of PCMs always requires soft/hard templates within nano-size (Nishihara and Kyotani 2012; Zhu et al. 2019; Yan et al. 2021; He et al. 2019b).

Electrochemical double-layer capacitors (EDLCs) are relatively new energy storage devices which store the charge at the electrode/electrolyte interface, via physical ion adsorption/desorption process (He and Chen 2015; Simon et al. 2014). EDLCs possess the merits of high power density, good stability, easy maintenance, wide operating temperature, and low cost (Wang et al. 2016). PCMs are preferred electrode materials for EDLCs attributing to their good conductivity/stability, large specific surface area, tunable pore structure, and low cost (Merlet et al. 2012). The gravimetric specific capacitance of PCMs spans from a few Faraday per gram to hundred Faraday per gram mainly depending on the specific surface area and pore structure. In this chapter, the capacitance performance of PCMs synthesized via the physical/chemical activation process is summarized. The pore structure optimization process and the influence of pore structure on the capacitance performance of PCMs are also discussed. It is expected that this chapter could offer some enlightenments to the researchers’ focus on improving the capacitance performance of PCMs.

2 Capacitance Performance of PCMs Synthesized with Activation Agents

PCMs are widely used as electrochemical energy storage devices because of their high SSA, multi-dimensional and tunable pore structure, good electrical conductivity, and low cost. Pore-size distribution and SSA are the main issues that dominate the performance of carbon-based supercapacitors. PCMs with large SSA and controllable pore size are good electrode materials for EDLCs. It is generally believed that microspores can enhance the electric double-layer capacitance of materials, mesopores provide low resistance channel for electrolyte ion transport, and macropores can be used as buffer reservoirs to store electrolyte ions (Wang et al. 2019). Activation is considered as an effective method to increase the SSA of carbon materials and introduce stratified porosity (Ding et al. 2021; He et al. 2013; Wang et al. 2020a).

Porous activated carbon is generally prepared from carbon precursors by the carbonization–activation approach. Activated carbon materials used in supercapacitors must meet the following three requirements: (1) a high SSA, (2) a low internal resistance, and (3) a microstructure conducive to the electrolyte entering the inner surface. High SSA is conducive to high specific capacitance. Low internal resistance determines high power density. Special pore structure is conducive to the formation of electrolyte wetting interface (Li et al. 2020c; Zhu and Xu 2020; Wang et al. 2020c).

The activation methods for preparing porous activated carbon can be divided into three categories: physical activation, chemical activation, and self-activation (Li et al. 2021b; Cao et al. 2021). In addition, it can be subdivided. Usually, the preparation process of porous activated carbon can be divided into one-step or two-step methods. One-step method means carbonization–activation occurs simultaneously. For the two-step method, the first step is carbonization, and the second step is activation that is using activator to further make pores. The different types of activation methods and its mechanisms, and some commonly used activators are described in detail below.

2.1 Physical Activation

Physical activation is usually carried out at high temperature (600–1200 °C) in the presence of oxidizing gases (Li et al. 2021b; Yin et al. 2020). Usually, air (Cai et al. 2020; Leng et al. 2017), carbon dioxide (Ding et al. 2021; Vinayagam et al. 2020; Elmouwahidi et al. 2020; Jin et al. 2016), and steam (Gao et al. 2020a; Qin 2019; Pallarés et al. 2018; Lee et al. 2016) can act as activation agents (Table 1). Most studies consider that activation temperature and activation time are the main factors affecting physical activation. Some studies have also investigated the effect of flow rate on pore structure (Qin 2019). Common physical activation methods are discussed in detail below.

Table 1 Summary of porous carbons produced by physical activation
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2.1.1 Carbon Dioxide Activation

CO2 is widely distributed on the earth and is rich in content. In those oxidizing gases, CO2 can be used as an environment-friendly activator of materials, which is not only green but also easy to deal with, and has a good application prospect (Li et al. 2020c). The activation mechanism of CO2 is shown in Eq. (1) (Navarro et al. 2007). In general, the CO2 activation degree is mainly affected by the activation temperature, time, and the concentration of CO2 (Zhu and Xu 2020).

$$ {\text{C}} + {\text{CO}}_{2} \to 2{\text{CO}} $$
(1)

Veerakumar et al. studied the capacitance performance of porous carbon nanosheets prepared by the carbonization of paper flower via chemical and physical activation (Veerakumar et al. 2020). The effect of CO2 activation temperature on the SSA and pore-size distribution of the carbon materials was discussed. The influence on the capacitance performance of paper flower-derived carbon materials was also analyzed. They first activated the carbon material with ZnCl2, followed by CO2 activation at 700 °C and 800 °C, respectively (Fig. 1). The results show that the sample PFC-800 has the maximum SSA and pore volume, which are 1800 m2 g−1 and 1.16 cm3 g−1, respectively. In a three-electrode system, the specific capacitances of PFC-800 are 91.2 F g−1 at 12 A g−1 which is more than twice the value of PFC-700 (44.9 F g−1). The cycle stability of PFC800 is better than that of the activated sample at 700 °C, with a loss of only 3% after 10,000 charge–discharge cycles. They also studied the adsorption capacity of the carbon material to sunset yellow, as shown in the lower right corner of Fig. 1a, and found that PFC-800 had a good adsorption capacity to this pigment (273.6 mg g−1). Elmouwahidi et al. prepared a kind of carbon microsphere and then activated it with CO2 to investigate the effect of activation time (30, 60, 90, 120 min at 900 °C) on the material's pore structure (Elmouwahidi et al. 2020). Their results indicate that in the process of CO2 activation, micropores are mainly produced, and the increase of micropores is especially obvious after 30 min, but the rise decreases when the time exceeds 60 min and reaches the peak at 90 min. After activation for 30 min, the formation of ultra-micropores is detected. With the time extension, the pores became wider. When the activation time increases to 90 min, the SSA (1523 m2 g−1), average pore size (0.95 nm), and pore volume all reach the maximum. Although CO2 can play an active role in opening and expanding the pores, different activation conditions can produce different pore structures and SSA and influence the capacitance performance of porous carbon.

Fig. 1
figure 1

a Preparation schematic of PFC, b the pore diameter of PFC samples at different activation temperatures, c cyclic performance of PFC-800 (Veerakumar et al. 2020)

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2.1.2 Steam Activation

As one of the commonly used physical activation methods, steam activation is not only cheap and easy to operate, but also an environmentally friendly activating agent. Steam activation is beneficial to the expansion of micropores, and thus, the activated carbon prepared by this activation method has a lower micropore volume (Zhu and Xu 2020; Pallarés et al. 2018). The steam activation mechanism is shown in Eq. (2). First, water vapor is adsorbed on the surface of the carbon, releasing hydrogen and oxygen (Navarro et al. 2007). Then, the presence of hydrogen prevents the continuous reaction of the active site, and the released oxygen further reacts with the carbon monoxide separated from the surface of the carbon to form carbon dioxide and at the same time produces a large number of micro-/mesopores (Sevilla et al. 2021; Li et al. 2020c; Yin et al. 2020).

$$ {\text{C}} + {\text{H}}_{2} {\text{O}} \to {\text{CO}} + {\text{H}}_{2} $$
(2)

For the stream activation process, the main factors affecting the pore structure of ACs are the activation temperature, activation time, and steam flow rate (Qin 2019). Lee et al. studied the capacitance performance of pitch-based activated carbon fibers (ACFs) produced by steam activation for different times (Lee et al. 2016). At the fixed activation temperature of 900 ℃, the activation time was varied from 10 to 40 min. SSA of the as-prepared porous carbon spans from 1520 to 3230 m2 g−1, and the pore volume increases from 0.61 cm3 g−1 up to 1.87 cm3 g−1. The specific capacitance increases from 1.1 to 22.5 F g−1. Figure 2 shows that increasing activation time can produce larger pores, which also indicates that steam activation is conducive to mesopore development.

Fig. 2
figure 2

a–e Scanning electron microscopy (SEM) images of activated carbon fiber with different steam activation times: a as-received CF, b ACF-10, c ACF-20, d ACF-30, e ACF-40, f the pore-size distribution curves of different samples (Lee et al. 2016)

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2.1.3 Air Activation

Compared with CO2 and steam activation, air activation is less used and the activation temperature is lower. However, the redox reaction of oxygen and carbon is a severe exothermic reaction, and the direct use of air to activate carbon may cause excessive combustion of the raw materials and fail to effectively form pores (Li et al. 2020c). Leng et al. successfully synthesized activated carbon by carbonizing dead pine needles (PN) in the air at different temperatures (Leng et al. 2017). The products show pore volume of 0.25–0.49 cm3 g−1 and SSA of 461–783 m2 g−1. In 1 M H2SO4 electrolyte, the PN-1000 exhibits a specific capacitance of 223 F g−1 at 0.5 A g−1 and maintains 150 F g−1 even at 100 A g−1.

2.2 Chemical Activation

Chemical activation is one of the most used methods for the preparation of PCM (Table 2). Unlike physical activation, chemical activation takes place at slightly lower temperatures (i.e., 400–900 °C) and involves the dehydration by certain agents, such as phosphoric acid, zinc chloride, and alkaline hydroxide (Li et al. 2021b; Zeng et al. 2014). Compared with physical activation, chemically activated porous carbon shows higher porosity development with porous network combining micropores and mesopores. Moreover, its selectivity is more extensive. At present, there are many kinds of chemical activators that have been used to prepare PCMs. The most used ones are KOH and ZnCl2. Chemical activators can be divided into three main groups: acids, alkalis, and salts. The commonly used chemical activation methods are discussed below.

Table 2 Summary of porous carbons produced by chemical activation
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2.2.1 Acid Activation

Common acid activating agents include H3PO4 (Cheng et al. 2015, 2021; Budinova et al. 2006), H2SO4 (Kigozi et al. 2020), HNO3 (Li et al. 2016; Pourhosseini et al. 2017; Zhu et al. 2017; Bo et al. 2018), etc. Phosphoric acid is used more often in those acidic activating agents. H3PO4 activation is related to its ability to promote bond cleavage and cross-linking formation. It can form phosphate bonds with the carbon precursors, resulting in a cross-linked structure. The activation process is roughly divided into six aspects as shown in Eqs. (3)–(8) (Myglovets et al. 2014; Olivares-Marín et al. 2006; Shi et al. 2019; Zhang et al. 2020):

$$ n{\text{H}}_{3} {\text{PO}}_{4} \mathop{\longrightarrow}\limits^{{{100}\;^{ \circ } {\text{C}} \sim {400}\;^{ \circ } {\text{C}}}}{\text{H}}_{n + 2} {\text{P}}_{n} {\text{O}}_{3n + 1} + (n - 1){\text{H}}_{2} {\text{O}} $$
(3)
$$ 2{\text{H}}_{3} {\text{PO}}_{4} \to {\text{P}}_{2} {\text{O}}_{5} + 3{\text{H}}_{2} {\text{O}} $$
(4)
$$ 2{\text{H}}_{3} {\text{PO}}_{4} \mathop{\longrightarrow}\limits^{{{ > 500}\;^{ \circ } {\text{C}}}}{\text{H}}_{2} {\text{P}}_{2} {\text{O}}_{7}^{2 - } + {\text{H}}_{2} {\text{O}} + 2{\text{H}}^{ + } $$
(5)
$$ 2{\text{H}}_{2} {\text{P}}_{2} {\text{O}}_{7}^{2 - } \to {\text{P}}_{4} + 6{\text{O}}_{2} + 2{\text{H}}_{2} {\text{O}} $$
(6)
$$ 2{\text{P}}_{2} {\text{O}}_{5} + 5{\text{C}} \to {\text{P}}_{4} + 5{\text{CO}}_{2} $$
(7)
$$ {\text{P}}_{2} {\text{O}}_{5} ({\text{liquid}}) \to {\text{P}}_{2} {\text{O}}_{5} ({\text{gas}}) $$
(8)

Cheng et al. first activated shiitake mushrooms with H3PO4 at 500 °C and then activated them with KOH for a second time (Cheng et al. 2015). The surface area of the activated sample (PAC-500) in the first step is 1341 m2 g−1, and the pore volume is 2.02 cm3 g−1. The size of pores formed is mainly between 2 and 5 nm, and considering that phosphoric acid reacts with carbon above 500 °C to form phosphate bond (Myglovets et al. 2014), so those nanopores are mainly due to the dehydration effect. After the activation of H3PO4, the rich channels are more conducive for the entry of KOH electrolyte into the interior, and the contact surface is larger to achieve a better effect. Finally, after two steps of activation and carbonization, the shiitake mushroom-derived carbon showed an interconnected hierarchical porous network. The obtained hierarchical pores exhibited a high specific capacitance of 306 F g−1 at 1 A g−1 in KOH electrolyte; even when the current density rises to 30 A g−1, the capacitance retention is 78.4%.

Unlike H3PO4, H2SO4 and HNO3 activations mainly rely on their strong oxidizing property, not only can react with carbon atoms to produce some gas, which can achieve pore-forming, but also can enhance the surface wettability of the active porous carbon (Li et al. 2016; Zhu et al. 2017; Kigozi et al. 2020). Thus, enhance the capacitance performance of the carbon material.

2.2.2 Alkali Activation

Alkali activation is the most widely used activation method in chemical activation. The carbon materials are often activated by KOH to make pores to enhance the capacitive performance (Wang and Kaskel 2012). It was found that the KOH activation could inhibit the generation of tar, reduce the activation temperature, and accelerate the reaction rate. KOH activation also shows the advantages of high yield, low reaction temperature, high SSA, and micropore development (Kumar et al. 2020; Sun et al. 2020; Li et al. 2020c). Chemical activation mechanism of KOH is suggested as Eqs. (9)–(13) (Li et al. 2021b; Lozano-Castelló et al. 2007; Yin et al. 2020). KOH dehydration to form K2O (Eq. 9) and then at about 400 °C is converted into K2CO3 (Eq. 10). At 600 °C, KOH is completely consumed. When the temperature rises to above 700 °C, K2CO3 and K2O are reduced to metallic potassium (Eqs. 12–13). In the process of activation, CO2, H2O, CO, H2, and other gaseous substances are released, which play the role of physical activation (Gao et al. 2020b; Sun et al. 2020; Wang et al. 2019; Li et al. 2020c; Yin et al. 2020).

$$ 2{\text{KOH}} \to {\text{K}}_{2} {\text{O}} + {\text{H}}_{2} {\text{O}} $$
(9)
$$ {\text{K}}_{2} {\text{O}} + {\text{CO}}_{2} \to {\text{K}}_{2} {\text{CO}}_{3} $$
(10)
$$ {\text{K}}_{2} {\text{CO}}_{3} \to {\text{K}}_{2} {\text{O}} + {\text{CO}}_{2} $$
(11)
$$ {\text{K}}_{2} {\text{CO}}_{3} + 2{\text{C}} \to 2{\text{K}} + 3{\text{CO}} $$
(12)
$$ {\text{K}}_{2} {\text{O}} + {\text{C}} \to 2{\text{K}} + {\text{CO}} $$
(13)

The activation temperature (Guo et al. 2016; Kim et al. 2021; Sun et al. 2020; Li et al. 2020b), immersion time (Yakaboylu et al. 2021), concentration of KOH (Chen et al. 2016), and the mass ratio of carbon precursors to KOH (Guo et al. 2016; Lv et al. 2019; Mo et al. 2020; Yu et al. 2019) will affect the pore-size distribution, porosity, and SSA of the carbon materials. Guo et al. studied the influence of KOH/carbon source mass ratio and activation temperature on capacitive performance of soybean root-derived hierarchical porous carbon (SRPC) (Fig. 3) (Guo et al. 2016). When the mass ratio of KOH to soybean root is 4:1 and the activation temperature is 800 °C, the porous carbon obtained has the largest SSA (2143 m2 g−1) and the largest proportion of micropore SSA (1772 m2 g−1). The mesopore volume of SRPC-4K is 0.13 cm3 g−1, and the micropore volume is 0.81 cm3 g−1. It has the richest hierarchical micro-/mesoporous structure. The high microporosity is conducive to the accumulation of charge, and the mesopores can transport abundant electrolyte into the micropores, thus enhancing the capacitive performance of porous carbon. When used as electrodes of SC, SRPC-4K displays a high specific capacitance of 276 F g−1 at 0.5 A g−1 and 98% capacitance retention after 10,000 cycles at 5 A g−1 (Fig. 3d) in a two-electrode configuration in 6 M KOH electrolyte. In conclusion, although KOH activation is used for pore-forming, appropriate activation conditions are needed to achieve the best capacitance performance for different carbon sources.

Fig. 3
figure 3

a SEM images of SRPC-4K, b N2 physisorption isotherms, c charge/discharge curves at 1 A g−1, d cyclic stability at 5 A g−1 for 10,000 cycles of SRPC-4K (inset: charge/discharge curves of the 1st and 10,000th cycles) (Guo et al. 2016)

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2.2.3 Salt Activation

In addition to acid and alkali activation, there is a class of salt-activating agents. Those reagents include some metal chlorides, decomposable salts, and oxidizing salts.

2.2.3.1 Metal Chlorides

In the preparation of porous carbon, metal chlorides, such as ZnCl2 (Ahmed et al. 2019; Li et al. 2021a; Ma et al. 2015), FeCl3 (Bedia et al. 2018; Hou et al. 2018; Li et al. 2020a), NaCl (Wang et al. 2018; Zong et al. 2020; Thileep Kumar et al. 2018), KCl (Wang et al. 2018; Thileep Kumar et al. 2018), and CuCl2 (Liu et al. 2018; Chen et al. 2019, 2021; Tian et al. 2019), are often used as activators.

ZnCl2 is mainly acted as a dehydrating agent and can be deoxidized in the form of water (Zhu and Xu 2020). Beyond that, ZnCl2 can also converted to ZnO (Eq. 14), which further etches carbon and be reduced to Zn (Eq. 15) (Kim et al. 2007; Wang et al. 2019). Better porous structure and higher carbon yield can be obtained. The activation temperature of ZnCl2 is usually between 600 and 1000 °C. When the temperature is above 700 °C, ZnCl2 volatilizes without post-treatment. The factors affecting the pore formation of ZnCl2 are close to those of KOH, but the activation temperature and impregnation ratio are more important. Ahmed et al. used pollen as a carbon source, and they investigated the effect of impregnation ratio with zinc chloride on the pore structure of porous carbon (Ahmed et al. 2019). The optimum mesopore ratio can be obtained by the appropriate impregnation ratio. If the impregnation ratio is too large, ZnCl2 will destroy the walls between adjacent micropores. It is confirmed that when the mass ratio of carbon source ZnCl2 is 1:2, the SSA of the resulting porous carbon is 1422.66 m2 g−1, the total pore volume is 0.77 cm3 g−1, the micropore volume is 0.33 cm3 g−1, and the mesopore volume is 0.44 cm3 g−1. Using ionic liquid electrolyte, the activated carbons achieved large magnitudes of energy density (~42 Wh kg−1) and power density (~19 kW kg−1).

$$ {\text{ZnCl}}_{2} + 1/2{\text{O}}_{2} \to {\text{ZnOCl}} + 1/2{\text{Cl}}_{2} $$
(14)
$$ 2{\text{ZnO}} + {\text{C}} \to 2{\text{Zn}} + {\text{CO}}_{2} $$
(15)

In recent years, some metal chlorides have also acted as molten salts, reacting with carbon at high temperatures to produce porous structures, which is so-called molten salt etching method. Wang et al. adopted non-toxic NaCl and KCl mixed salt as reaction media to regulate the activation process of corn straw (Wang et al. 2018). Hierarchical porous carbon sheets (HPCSs) with good capacitive properties were successfully prepared. They found that when the temperature is higher than 800 °C, Cl− etched the carbon skeleton to form micro- and mesopores. The hierarchical porous carbon obtained by this strategy has a SSA of 1588 m2 g−1 and a high specific capacitance of 407 F g−1 at 1 A g−1 in the three-electrode system.

CuCl2 is another molten salt that can be used as an activation agent. The mechanisms of CuCl2 porogen are described as Eqs. (16–17) (Liu et al. 2018)

$$ 3{\text{CuCl}}_{2} + {\text{C}} + 2{\text{H}}_{2} {\text{O}} \to {\text{Cu}} + 2{\text{CuCl}} + {\text{CO}}_{2} + 4{\text{HCl}} $$
(16)
$$ 4{\text{CuCl}} + {\text{C}} + 2{\text{H}}_{2} {\text{O}} \to 4{\text{Cu}} + {\text{CO}}_{2} + 4{\text{HCl}} $$
(17)

The porous carbon can maintain the spherical shape (Fig. 4a) of the original carbon precursor, with the pores distributed below 2 nm and pore-size distribution centered at 1.2 nm. As is shown in Fig. 4b, under the same conditions, compared with the activation of ZnCl2 (Z-RPC) and KOH (K-RPC), the pore-size distribution of the former is wide (1–60 nm), while that of the latter is mainly below 3 nm. Moreover, the porous carbon obtained by the activation of CuCl2 shows a high SSA of 2488 m2 g−1 and presents an outstanding gravimetric capacitance of 390 F g−1 at 0.5 A g−1 in 6 M KOH electrolyte (Fig. 4c). There is no doubt that molten salt etching is milder than acid/base activation, but it is more expensive and unsuitable for practical production.

Fig. 4
figure 4

a Schematic illustration of the preparation of RPC, b pore-size distribution (PSD) curves of RPC, Z-RPC, and K-RPC, c specific capacitances of RPC, Z-RPC, K-RPC, and the blank sample at different current densities (Liu et al. 2018)

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2.2.3.2 Decomposable Salts

There are various salts that can be decomposed to produce gases, which can etch carbon to produce pores, such as carbonates (Mao et al. 2020; Ma et al. 2019; Tang et al. 2018), acetates (Li et al. 2018b, 2019), bicarbonates (Liang et al. 2021; Sevilla and Fuertes 2016), nitrates (Li et al. 2018a; Huo et al. 2020; Wang et al. 2017), and so on. Li et al. produced a macro- and mesoporous carbon material by using the decomposable and water-removable NaNO3 as the porogen (Li et al. 2018a). Above 600 °C, NaNO3 begins to decompose, generating N2, O2, and NO, which is conducive to the formation of mesopores (Eqs. 18 and 19) (Li et al. 2018a). The pore size of the material is affected by the dosage of NaNO3. The modified carbon material showed small mesopores of 2–4 nm and larger mesopores of 5–50 nm, with a SSA of 2872.2 m2 g−1. Due to the synergistic effect of suitable macro-mesoporous ion-diffusion channels and continuous conductive network, the porous carbon material exhibits a high power density. Huo et al. used NaNO3 and KOH to activate soluble starch to produce interconnected layered porous carbon networks (Fig. 5a) (Huo et al. 2020). As can be seen from Fig. 5b, the PCMs prepared by NaNO3 and KOH activation (HPCNS) possess micropores, mesopores, and macropores at the same time. However, the materials prepared only with KOH (MICPCS) have no obvious mesoporous and macroporous characteristics. The results showed that the HPCN prepared at 800 °C had a maximum specific surface area of 1864 m2 g−1 and a total pore volume of 2.22 cm3 g−1. In a three-electrode system with 6 M KOH electrolyte, it shows good capacitance performance. The specific capacitance reaches 385 F g−1 at 1 A g−1. Even when the current density is up to 100 A g−1, HPCNs can achieve a capacitance retention of 75.6% (Fig. 5c).

Fig. 5
figure 5

a Schematic illustration of the synthesis of HPCNS, b N2-adsorption/desorption isotherms, c specific capacitance as a function of the current densities (Huo et al. 2020)

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$$ 12{\text{NaNO}}_{3} \to 6{\text{Na}}_{2} {\text{O}} + {\text{N}}_{2} + 10{\text{O}}_{2} + 10{\text{NO}} $$
(18)
$$ 2{\text{Na}}_{2} {\text{O}} + {\text{C}} \to 4{\text{Na}} + {\text{CO}}_{2} $$
(19)

Some oxidizing salts are also used in the preparation of porous activated carbon. For example, KMnO4 gradually decomposes into oxygen, water-soluble potassium salt, and non-water-soluble manganese salt in the process of continuous pyrolysis, which are beneficial to the formation of hierarchical pores (Qiu et al. 2019a, b; Yi et al. 2020). In addition, KNO3 (Leng et al. 2020; Wang et al. 2017) can also be used as an oxidizing salt for the activation of porous carbon.

2.3 Self-activation

Although the above-mentioned activation methods and activation reagents have been widely used in the preparation of PCMs, they inevitably have some disadvantages, such as corrosive reagents, complex activation operation, post-treatment, and secondary pollution. Self-activation can simplify the activating process without the need for additional activation reagents and reducing the cost. Its pore generation mechanisms include three aspects: (1) chemical activation (it contains some inorganic salts, organic salts, and some special components, which can be used for activation), (2) physical activation (utilization of CO2 and H2O produced by pyrolysis), and (3) removal of inorganic nanoparticles in carbon source (Sevilla et al. 2021). The main influencing factors of self-activation are carbonization temperature and time (Wang et al. 2021; Gao et al. 2021; He et al. 2019a; Bhat et al. 2021). Niu et al. fabricated mesopore-dominant porous carbon as electrode materials by direct pyrolysis (600, 850, 900, 1000, 1100, 1200 °C) of cattle bones in an Ar atmosphere (Niu et al. 2017). With the increase of temperature, the surface of carbon changes from nonporous to porous. The porous carbon obtained at 1100 °C presented a hierarchical porous structure with a large SSA of 2096 m2 g−1. The mesopore volume is 1.83 cm3 g−1, and the conductivity reached 5141 S m−1. PC-1100 displayed a high specific capacitance of 258 F g−1 at a current density of 5 A g−1 in a symmetrical supercapacitor. Even at 100 A g−1, a high capacitance of 176 F g−1 was maintained.

3 Summary and Perspectives

PCMs are promising electrode materials for supercapacitors attributing to their rich precursors, low cost, high specific surface area, tunable pore structure, and good stability. PCMs could be prepared from various precursors including biomass, natural polymers, synthetic macromolecules, and carbon-rich small molecules by pyrolysis at high temperature. However, the carbonaceous products obtained from directly carbonization of precursors often show porous-less or even nonporous feature with limited SSA. It is necessary to improve the SSA and adjust the pore structure of carbon materials to enhance their capacitance performance. Physical/chemical activation methods are proved to be effective approaches.

Nowadays, PCMs have produced tons of kilograms annually through carbonization–activation way. However, there are some issues still unsolved which block the production and application of PCMs. As to the physical activation, steam and CO2 are the commonly used activation agents which are low corrosive to the production facilities and more favorable for practical application. But physical activation always results in PCMs with narrowly distributed micropores, and the particle and microdomain sizes are reduced. Physical activation also shows the shortage in product yield, tap density, and SSA. As to chemical activation, corrosive reagents like ZnCl2, KOH, NaOH, H2SO4, HNO3, and K2CO3 are the traditional activation agents. Compared with physical activation, chemical activation shows advantages in relatively low activation temperature, high yield, large SSA, and high mesopore ratios. However, chemical activation often requires high mass ratio of corrosive activation agents to carbon sources and generates great amounts of contaminants. Hence, tradition chemical activation method is only limit to laboratory-scale research.

In recently years, great efforts have been investigated on the exploration of much less corrosive chemical activation agents and molten salts (KCl, NaCl, CuCl2, FeCl3, NiCl2, etc.), decomposable salts (nitrates, carbonates, bicarbonates, acetates, oxalates, etc.), and oxidative salts (KMnO4, KClO4, (NH4)2S2O8, H2O2, etc.) are proved to be better choices. Among these new chemical activation agents, potassium salts and sodium salts are more preferred owing to their water solvable nature which are benefit for post-treatment. Moreover, potassium salts and sodium salts could be served as templates to generate hierarchical pores. Ammonium salts are also good choices attributing to their fully decomposable nature which could act as chemical blowing/foaming agents generating lots of macropores. Besides, ammonium salts could be used as nitrogen sources to introduce nitrogen species to contribute pseudocapacitance and modify the surface feature of carbon materials.

The pore structure optimization process is complicated and sometimes uncontrollable. It is very hard to obtain PCMs with idea pore structure from only one method within one-step experiments. As to application in supercapacitors, the ratio among macropores, mesopores, and micropores of PCMs should be well adjusted to keep a balance on specific capacitance, rate performance, stability, and tap density. From a production point of view, it is more favorable to synthesize PCMs from one-step carbonization and activation process with multifunctional agents (e.g., KCl, NaCl, NH4NO3). It is necessary to combine activation with other methods including template, chemical vapor deposition, electrospinning, hydrothermal/solvothermal method to design PCMs with finely tuned pore structures, and good capacitive performance.