**4. Energy storage through functional porous carbon obtained from frozen tofu (organic matter)**

Frozen tofu is a source of carbon and nitrogen [16]. By using one-step carbonization activation method, it can be converted into oxygen-doped carbon and nitrogen-doped carbon, respectively. By one-step carbonization, sponge-like carbon (co-doped) has a maximum surface area of 3134 m2 g−1. High volumes of

mesopores (1.11 cm3 g−1) and micropores (0.71 cm3 g−1) are present in this hierarchical (graded) porous carbon [16, 17]. Scientists have discovered that this acquired carbon is used to make supercapacitors which acts as electrodes, in 1 M aqueous electrolyte sulfuric acid; it has a remarkable capacitance of 243 Fg−1 (evaluated at 0.1 Ag−1); after 10,000 cycles, it has a capacitance retentiveness of 93% at 10 Ag−1. In BMIMBF4 (1-butyl-3-methylimidazolium tetra fluoroborate) liquid ion electrolyte, the said carbon shows a precise capacity of 170 Fg−1 (assessed in 1 Ag−1) along with a valuable effectiveness (135 F g−1 at 20 Ag−1) ensuring the power density of 72 W h kg−1 (at 889 W kg−1). A carbon supercapacitor (derived from frozen tofu) can comfortably drive 25 light-transmitting diodes in excess of 2 minutes [16]. The fabrication flow chart for porous carbon through one-step carbonization is as shown in **Figure 3** [16].

Further, systematic steps involved to get porous carbon through carbonization are freeze drying, KOH activation, temperature treatment, and carbonization, respectively, as are shown in flow chart diagram (**Figure 3**). Many energy storage devices are available worldwide but because of potential applications, electrochemical appliances such as lithium-ion batteries (LIBs) and supercapacitors have an appreciable fascination [16, 17]. On the one hand, supercapacitors have active charging/discharge performance and also a fine power density greater than 10 k W kg−1 ; on the other hand, LIBs have a great energy density (usually 100–200 W h kg−1), yet it has a longer charging time. Two mechanisms mainly used by supercapacitors in order to store energy are as follows [17, 18]:


The lithium ions in LIBs are moved between anode and cathode, which results in lithium ions storage or discharge through distinct means according to the materials of the electrode. Because of many properties such as chemical stability, pattern porosity, and high electrical conductivity, for both commercial LIBs (e.g., graphite) and supercapacitors (e.g., activated carbon), carbon materials were selected as effective materials [16, 19]. It has been determined that macropores act as ion-buffering storage in porous carbon materials in supercapacitors and mesopores contribute channels for the transport of ions to micropores, where they were ultimately deposited. In the LIB (graphite) anodes, the intercalated LiC compound is the result of the complete intercalation of lithium ions that limits the reversible Li-ion storage potential for graphite (approximately 372 mA hg−1) [16]. In addition,


**45**

*Advanced Carbon Functional Materials for Superior Energy Storage*

the transit kinetics and accordingly the energy density [16, 20].

employed due to its great specific surface area (greater than 2000 m2

will make effective manufacturing possible through treating of precursors of nanostructured carbon in order to get novel carbon compounds [21]. For instance, platelets of graphene may be entirely transformed to a three-dimensional porous

0.6–5 nm by microwave-exfoliated graphite oxide activation of KOH [16, 21]. By KOH activation of C micro tubes, scientists have documented a porous (spongelike) carbon made up of macropores (numerous microns in size) and micropores (0.47 nm in size). The conductivity and wettability of carbon compounds can be enhanced by adequate heteroatom doping. Additionally, nitrogen is a donor molecule of electrons, as well as in a graphite matrix, they have the ability to promote conductivity of the n-type. As the LIB anode, carbon N-doping has proved itself to support the Li injection because of the hybridization of the long pair of nitrogen electrons with carbon π electrons. In recent times, by activation of C with KOH in an ammonia environment, scientists have achieved porous N-doped carbon (7.5 wt %); the carbon was found to have a reversible power of 1900 mA h g−1 at 0.1 Ag−1

furthermore, after 800 cycles (at 2 Ag−1), the capacity is 600 mA hg−1, respectively. Porous carbon content obtained from renewables can be more environmentally

capacitor, showed 73.2 F g−1 capacitance (at 0.2 A g−1). The porous carbon (obtained from frozen tofu) is known to be suitable anode specimens for LIBs, because of their bigger surface areas, porous hierarchical structures, and heteroatom doping. In short, we can easily obtain doped N porous hierarchical carbon from frozen tofu, using single-step carbonization activation. Frozen tofu is environment-friendly,

sustainable relative to porous carbon products acquired from wood, polymer blends, tar, and other resources. Researchers have demonstrated that porous carbons can be obtained from various biomass sources such as rice husks, fungi, water bamboo, and rice straw for energy storage applications. For instance, researchers have informed that the willow catkin could be transformed into a cross-linked polymer carbon laminate co-doped with sulfur (S) and N by one-step pyrolysisactivation synthesis. In 1 M Na2SO4, the carbon demonstrated a remarkable electrochemical efficiency with a specific capacitance of 298 F g−1 at 0.5 A g−1 and magnificent cycling endurance at 5 A g−1 after 10,000 cycles with only 2% capacitance loss. Tofu consists of moisture, carbohydrates, proteins, and trace concentration of minerals; it is an available resource and is considered as a renewable fuel for nitrogen and carbon [16, 22]. Not long ago, scientists have stated the molten salt synthesis of strongly (N-doped) porous carbon which may be obtained from tofu, in which LiCl/KCl (45/55 in weight) is a eutectic mixture (which functioned as the activator) was used as the solvent to dilute LiNO3. The collected carbon (N-content:

permeable carbon was too employed in the form of an electromagnetic scaffold or including the electrode in the LIBs, since the spongy (porous) arrangement is known to increase the contact areas of the electrode-electrolyte and decrease the length of the path for ions/electrons transport, which leads to an improvement in

Power density of industrial supercapacitors is usually lower than 6 W h kg−1 and that is beyond from long-term electronic equipment requirements [20]. However, the efficiency and performance of the LIB anodes must also be further enhanced. With regard to the two applications, spongy carbon of great electrical drivability and a customized 3D design is required. In the previous couple of centuries, by using different strategies like self-assembly, activation, and templating, several porous carbon materials have been researched [16, 18, 20]. Among available

methods, an efficient method is activation, to increase the surface area of carbon by making several micropores. In supercapacitors, activated carbon is commercially

g−1). Activation

;

g−1 and distribution of pore size is

g−1) in a 1 M Na2SO4 symmetric super-

*DOI: http://dx.doi.org/10.5772/intechopen.93355*

(spongy) carbon with a SSA of up to 3100 m2

4.72 wt%, density: 0.84 g cm−3, SSA: 1202 m2

**Figure 3.** *The fabrication flow chart for porous carbon through one-step carbonization [16].*

#### *Advanced Carbon Functional Materials for Superior Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.93355*

*Advanced Functional Materials*

g−1) and micropores (0.71 cm3

chical (graded) porous carbon [16, 17]. Scientists have discovered that this acquired carbon is used to make supercapacitors which acts as electrodes, in 1 M aqueous electrolyte sulfuric acid; it has a remarkable capacitance of 243 Fg−1 (evaluated at 0.1 Ag−1); after 10,000 cycles, it has a capacitance retentiveness of 93% at 10 Ag−1. In BMIMBF4 (1-butyl-3-methylimidazolium tetra fluoroborate) liquid ion electrolyte, the said carbon shows a precise capacity of 170 Fg−1 (assessed in 1 Ag−1) along with a valuable effectiveness (135 F g−1 at 20 Ag−1) ensuring the power density of 72 W h kg−1 (at 889 W kg−1). A carbon supercapacitor (derived from frozen tofu) can comfortably drive 25 light-transmitting diodes in excess of 2 minutes [16]. The fabrication flow chart for porous carbon through one-step carbonization is as

Further, systematic steps involved to get porous carbon through carbonization are freeze drying, KOH activation, temperature treatment, and carbonization, respectively, as are shown in flow chart diagram (**Figure 3**). Many energy storage devices are available worldwide but because of potential applications, electrochemical appliances such as lithium-ion batteries (LIBs) and supercapacitors have an appreciable fascination [16, 17]. On the one hand, supercapacitors have active charging/discharge performance and also a fine power density greater than 10 k W kg−1

on the other hand, LIBs have a great energy density (usually 100–200 W h kg−1), yet it has a longer charging time. Two mechanisms mainly used by supercapacitors in

• Pseudocapacitive electrodes store ions established at the electrode-electrolyte

The lithium ions in LIBs are moved between anode and cathode, which results

• Dual layer electrodes with capacitive electricity preserve energy through the desorption and adsorption of ions on a large field of spongy (porous)

in lithium ions storage or discharge through distinct means according to the materials of the electrode. Because of many properties such as chemical stability, pattern porosity, and high electrical conductivity, for both commercial LIBs (e.g., graphite) and supercapacitors (e.g., activated carbon), carbon materials were selected as effective materials [16, 19]. It has been determined that macropores act as ion-buffering storage in porous carbon materials in supercapacitors and mesopores contribute channels for the transport of ions to micropores, where they were ultimately deposited. In the LIB (graphite) anodes, the intercalated LiC compound is the result of the complete intercalation of lithium ions that limits the reversible Li-ion storage potential for graphite (approximately 372 mA hg−1) [16]. In addition,

*The fabrication flow chart for porous carbon through one-step carbonization [16].*

g−1) are present in this hierar-

;

mesopores (1.11 cm3

shown in **Figure 3** [16].

materials.

order to store energy are as follows [17, 18]:

interface for immediate Faradaic reactions.

**44**

**Figure 3.**

permeable carbon was too employed in the form of an electromagnetic scaffold or including the electrode in the LIBs, since the spongy (porous) arrangement is known to increase the contact areas of the electrode-electrolyte and decrease the length of the path for ions/electrons transport, which leads to an improvement in the transit kinetics and accordingly the energy density [16, 20].

Power density of industrial supercapacitors is usually lower than 6 W h kg−1 and that is beyond from long-term electronic equipment requirements [20]. However, the efficiency and performance of the LIB anodes must also be further enhanced. With regard to the two applications, spongy carbon of great electrical drivability and a customized 3D design is required. In the previous couple of centuries, by using different strategies like self-assembly, activation, and templating, several porous carbon materials have been researched [16, 18, 20]. Among available methods, an efficient method is activation, to increase the surface area of carbon by making several micropores. In supercapacitors, activated carbon is commercially employed due to its great specific surface area (greater than 2000 m2 g−1). Activation will make effective manufacturing possible through treating of precursors of nanostructured carbon in order to get novel carbon compounds [21]. For instance, platelets of graphene may be entirely transformed to a three-dimensional porous (spongy) carbon with a SSA of up to 3100 m2 g−1 and distribution of pore size is 0.6–5 nm by microwave-exfoliated graphite oxide activation of KOH [16, 21]. By KOH activation of C micro tubes, scientists have documented a porous (spongelike) carbon made up of macropores (numerous microns in size) and micropores (0.47 nm in size). The conductivity and wettability of carbon compounds can be enhanced by adequate heteroatom doping. Additionally, nitrogen is a donor molecule of electrons, as well as in a graphite matrix, they have the ability to promote conductivity of the n-type. As the LIB anode, carbon N-doping has proved itself to support the Li injection because of the hybridization of the long pair of nitrogen electrons with carbon π electrons. In recent times, by activation of C with KOH in an ammonia environment, scientists have achieved porous N-doped carbon (7.5 wt %); the carbon was found to have a reversible power of 1900 mA h g−1 at 0.1 Ag−1 ; furthermore, after 800 cycles (at 2 Ag−1), the capacity is 600 mA hg−1, respectively.

Porous carbon content obtained from renewables can be more environmentally sustainable relative to porous carbon products acquired from wood, polymer blends, tar, and other resources. Researchers have demonstrated that porous carbons can be obtained from various biomass sources such as rice husks, fungi, water bamboo, and rice straw for energy storage applications. For instance, researchers have informed that the willow catkin could be transformed into a cross-linked polymer carbon laminate co-doped with sulfur (S) and N by one-step pyrolysisactivation synthesis. In 1 M Na2SO4, the carbon demonstrated a remarkable electrochemical efficiency with a specific capacitance of 298 F g−1 at 0.5 A g−1 and magnificent cycling endurance at 5 A g−1 after 10,000 cycles with only 2% capacitance loss. Tofu consists of moisture, carbohydrates, proteins, and trace concentration of minerals; it is an available resource and is considered as a renewable fuel for nitrogen and carbon [16, 22]. Not long ago, scientists have stated the molten salt synthesis of strongly (N-doped) porous carbon which may be obtained from tofu, in which LiCl/KCl (45/55 in weight) is a eutectic mixture (which functioned as the activator) was used as the solvent to dilute LiNO3. The collected carbon (N-content: 4.72 wt%, density: 0.84 g cm−3, SSA: 1202 m2 g−1) in a 1 M Na2SO4 symmetric supercapacitor, showed 73.2 F g−1 capacitance (at 0.2 A g−1). The porous carbon (obtained from frozen tofu) is known to be suitable anode specimens for LIBs, because of their bigger surface areas, porous hierarchical structures, and heteroatom doping. In short, we can easily obtain doped N porous hierarchical carbon from frozen tofu, using single-step carbonization activation. Frozen tofu is environment-friendly,

#### *Advanced Functional Materials*

cheap, and extendable biomaterial precursor. It has showed significant SSA of 3134 m2 g−1 and significant pore diameter of 1.82 m3 g−1 on the activation conditions, and this is better than traditional biomass-originated active carbon products. In 1 M H2SO4, supercapacitors based on porous carbon (from frozen tofu) revealed 243 F g−1 specific capacitance, and in BMIMBF4/AN, it showed an extraordinary power density of about 72 W h kg−1 at an ordinary 889 W kg−1 energy density. Such carbonization procedure offers a potentially helpful strategy from abundant supportable resources to design carbon electrode materials with supreme execution for supercapacitors and LIBs, respectively [6, 11, 16, 22].
