2023 Volume 72 Issue 1 Pages 11-32
Nanoarchitectonics integrates nanotechnology with numerous scientific disciplines to create innovative and novel functional materials from nano-units (atoms, molecules, and nanomaterials). The objective of nanoarchitectonics concept is to develop functional materials and systems with rationally architected functional units. This paper explores the progress and potential of this field using biomass nanoarchitectonics for supercapacitor applications as examples of energetic materials and devices. Strategic design of nanoporous carbons that exhibit ultra-high surface area and hierarchically pore architectures comprising micro- and mesopore structure and controlled pore size distributions are of great significance in energy-related applications, including in high-performance supercapacitors, lithium-ion batteries, and fuel cells. Agricultural wastes or natural biomass are lignocellulosic materials and are excellent carbon sources for the preparation of hierarchically porous carbons with an ultra-high surface area that are attractive materials in high-performance supercapacitor applications due to high electrical and ion conduction, extreme porosity, and exceptional chemical and thermal stability. In this review, we will focus on the latest advancements in the fabrication of hierarchical porous carbon materials from different biomass by chemical activation method. Particularly, the importance of biomass-derived ultra-high surface area porous carbons, hierarchical architectures with interconnected pores in high-energy storage, and high-performance supercapacitors applications will be discussed. Finally, the current challenges and outlook for the further improvement of carbon materials derived from biomass or agricultural wastes in the advancements of supercapacitor devices will be discussed.
This paper, entitled biomass nanoarchitectonics for supercapacitor applications, describes the significance and recent progress in this field. In particular, since nanoarchitectonics is a new concept, the introduction is divided into (i) general background on nanoarchitectonics and (ii) specific background on biomass-derived carbons in supercapacitor applications.
The development of human society has been driven by new materials that have been created one after another 1) . In particular, various scientific fields have been systematized since the 20th century, and these have accelerated the advancement of new functional materials. Recent reviews and research results show that organic chemistry 2) , 3) , 4) , inorganic chemistry 5) , 6) , 7) , polymer chemistry 8) , 9) , 10) , coordination chemistry 11) , 12) , 13) , supramolecular chemistry 14) , 15) , 16) , bio-related chemistry 17) , 18) , 19) , and other materials sciences 20) , 21) , 22) have contributed significantly to the development of new functional materials and their concepts. Furthermore, not only the creation of materials themselves but also the control of their micro- and nanostructures has produced a variety of functions. The development of devices through microfabrication has brought about the super-integration of functions through the miniaturization of functional units 23) , 24) , 25) . In addition, controlling the size of a material and its internal structure at the nanometer level can change the material’s properties. For example, phenomena such as quantum effects 26) , 27) , 28) and the development of nanostructure-specific properties 29) , 30) , 31) have been discovered. Such developments have progressed remarkably since the later period of the 20th century under the name of nanotechnology. It has become possible to monitor, estimate, and control structures at the atomic and molecular level 32) , 33) , 34) .
At the same time, the research field of creating functional material systems through the successful design and assembly of components has also developed. For example, molecular recognition 35) , 36) , 37) and self-assembly 38) , 39) , 40) in supramolecular chemistry, Langmuir-Blodgett (LB) method 41) , 42) , 43) and layer-by-layer (LbL) assembly 44) , 45) , 46) as thin film technology, metal-organic frameworks (MOFs) 47) , 48) , 49) and various coordination structures 50) , 51) , 52) in coordination chemistry, nanoporous materials 53) , 54) , 55) and covalent organic frameworks (COFs) 56) , 57) , 58) in materials science, and the other materials science 59) , 60) , 61) are contributing to the development of new functional materials. The basic fields of interface science 62) , 63) , 64) and oil science 65) , 66) have also made significant contributions. Together with nanotechnology, the development of these fields has come at a time when they should be integrated into a new concept. As Richard Feynman proposed nanotechnology 67) , 68) , so did Masakazu Aono for nanoarchitectonics at the beginning of the 21st century 69) . As a post-nanotechnology concept, nanoarchitectonics integrates nanotechnology with various scientific disciplines to create novel materials from nano building blocks such as atoms, molecules, and nanomaterials with innovative functionalities (Fig. 1) 70) , 71) . Nanoarchitectonics connects nanotechnology and materials science and has recently been assumed to correlate with materials informatics 72) , 73) .
Outline of nanoarchitectonics concept, history, and methodology.
In nanoarchitectonics, functional material systems are built from nano-units by selecting and combining various processes 74) , 75) , which include the manipulation of atom/molecule, physical/chemical material transformation, self-assembly/self-organization, orientation control and organization by external fields, nano/micro fabrication, and biochemical treatments. Some operations are based on equilibrium processes, while others are non-equilibrium. Material fabrication by ordinary self-assembly is based on equilibrium processes, resulting in relatively simple structures. In nanoarchitectonics, on the other hand, several of them are often combined, which is advantageous in obtaining asymmetric or hierarchical structures 76) . For example, obtaining a basic structure by molecular self-assembly and then creating a material through template synthesis, carbonization processes, and forming it into thin films, etc., may be considered a typical process of formation of hierarchical structures in nanoarchitectonics. In addition, nanoscale phenomena are subject to multiple ambiguities, including thermal fluctuations, statistical distributions, and quantum effects. Therefore, rather than simply summing the effects, they are harmonized together 77) . It could be a functional material system organization similar to a biological system.
The above principles of nanoarchitectonics are universally applicable to many materials and applications. The use of nanoarchitectonics is widely considered from basic to applied fields. Fundamental applications include synthesis of various materials 78) , 79) , 80) , organization of structures 81) , 82) , 83) , understanding of physical phenomena 84) , 85) , 86) , and elucidation of biological phenomena 87) , 88) , 89) . Applications include catalysts 90) , 91) , 92) , sensors 93) , 94) , 95) , devices 96) , 97) , 98) , energy applications 99) , 100) , 101) , environmental applications 102) , 103) , 104) , and biomedical applications 105) , 106) , 107) . Since materials are made of atoms and molecules, nanoarchitectonics, which is the creation of functional materials from nanounits, may be said to be a concept that architects all materials. In analogy to the Theory of Everything in physics 108) , nanoarchitectonics may be called the Method for Everything in materials science 109) .
The goal of nanoarchitectonics is to develop functional material systems in which various functional units are rationally architectured as highly advanced systems like biological systems 110) . However, this paper will show that it can also be applied to more practical fields. Energy and resource issues are of great interest as practical problems these days. In particular, there is an urgent need to construct highly functional energy materials and devices using abundant, untapped resources such as biomass. For this purpose, it is necessary to convert naturally occurring materials into advanced nanostructured materials 111) , 112) , 113) . This is where nanoarchitectonics can make a significant contribution. This paper explores the progress and potential of this field using biomass nanoarchitectonics and supercapacitors as examples of energetic materials and devices (Scheme 1). In the next section, a more detailed background is given.
Schematic representation of biomass nanoarchitectonics for supercapacitor application.
The continuous increase in energy demand has diminished fossil fuels and enhanced the necessity of cost-effective and high-efficiency alternative energy storage systems. Current state-of-art electrochemical energy storage systems such as lithium-ion batteries, supercapacitors (SCs), fuel cells, and solar cells are widely used in electronics and electric vehicles 114) , 115) , 116) , 117) , 118) , 119) , 120) . Among these energy storage systems, SCs are of great interest because of their ultra-fast charging/discharging dynamics, high power density (>10 kW/kg), outstanding long cycle life (>10,000), low internal resistance, high-rate performance, and easy and safe operation 115) , 116) , 117) , 118) , 119) , 120) , 121) , 122) , 123) , 124) . SCs have been extensively explored in transportable electronic/microelectronic devices, memory backup systems, recent hybrid electric vehicles, and industrial energy and power management 120) , 121) , 122) , 123) , 124) , 125) . Generally, electrochemical capacitors are classified into two distinct types depending on the energy storage mechanism: SCs also called electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) 126) , 127) , 128) , 129) , 130) . Energy storage involves the accumulation of electrical charges in the form of electrical double layer at the electrode surface in the SCs. It does not undergo faradic reactions; therefore, electrostatic charge diffusion is crucial in SCs 126) , 127) , 128) , 129) , 130) . On the other hand, redox reactions take place in PCs due to the charge transfer from electrolyte solution to the electrode surface. Therefore, electrochemical energy storage (specific energy) in PCs is higher compared to EDLCs but the cycle life and rate performance of PCs are poor 131) , 132) , 133) . Recently, researchers have proposed new material systems for a better energy storage system with high capacitances and longer cycle lifetime by the unification of SCs and PCs, and hence, known as hybrid supercapacitors 134) , 135) , 136) , whose essential components are carbon composites (binary or ternary) with metal oxides, and conductive polymers. Varieties of pseudocapacitive materials, oxides of different novel metals such as manganese cobalt, nickel, cerium, vanadium, and iron have been investigated 137) , 138) , 139) , 140) , 141) , 142) , 143) , 144) , 145) .
Despite the several advantages, SCs deliver small specific energy (1-10 Wh/kg), which is quite lower compared to batteries. Lithium-ion batteries can deliver energy density more than 160 Wh/kg. Therefore, SCs are yet far from the commercial success and to fulfill the upcoming energy requirements, advanced supercapacitors need to be designed which can provide high energy densities sustaining high-power density and prolonged cycle life and high-rate performance. High specific capacitance (C s) electrode material could deliver high energy density SCs upon operating at a wide potential (V) window as the energy storage is proportional to C s V 2 146) . The operating voltage can be expanded in non-aqueous electrolyte or ionic liquids electrolyte. However, due to low ionic conductivities, overall performance is limited. Furthermore, the system is highly sensitive to the moisture and hence needs costly facilities to assemble the devices. In addition, two different electrode materials can be assembled into asymmetric supercapacitor to widen window. In general sense, one of the best ways of enhancing the overall performance of the SCs is to design new materials with superior specific capacitances.
Several intrinsic properties of the electrodes including, surface textures (specific surface area, and pore structure), conductivity, and chemical stability mainly influence the electrochemical performance of the SCs 147) , 148) , 149) , 150) . Therefore, it is a key challenge to nanoarchitect high-energy performance SCs electrode materials sustaining other intrinsic properties such as fast charging-discharging dynamics, high-power density, and long cycle life. Of several supercapacitor electrode materials explored, carbon-based materials such as graphene, carbon nanotubes, fullerenes, carbon nanofibers, carbon nanohorns, and nanoporous carbons are the most extensively used in energy storage supercapacitors due to their excellent porosity, enhanced surface area, well defined pore size distributions, outstanding electrical conductivity, and high thermal and chemical stability 151) , 152) , 153) , 154) , 155) , 156) , 157) , 158) , 159) , 160) . Among them, activated carbons from biomass are still the most popular electrode material for SCs applications due to the abundant sources, low cost, and simple nanoarchitectonics method 161) , 162) , 163) , 164) , 165) .
In this review, we will discuss the latest developments and advancements in the nanoarchitectonics of hierarchically porous carbons materials derived from various biomass or agricultural wastes and their use as the electrode materials in supercapacitors applications. We will overview the importance of the nanoarchitectonics concept to tune the surface textural properties (surface area, and pore size), pore engineering, systematic tuning of the hierarchical porous architectures that will directly influence the performance of supercapacitors.
Nanoporous activated carbons imply the carbonaceous materials and can be produced by thermochemical methods. Common nanoarchitectonics methods for producing porous activated carbons include pyrolysis of synthetic carbon sources and MOFs, template method including soft and hard templates, hydrothermal carbonization, microwave-assisted hydrothermal carbonization, and activation of the biomasses that essentially contain cellulose, hemicellulose, and lignin using different activating agents 166) , 167) , 168) , 169) , 170) . The porosity properties of the prepared carbon materials mainly depend on starting material itself and the synthetic conditions. Thus, by optimization of the synthetic conditions or carbon sources the surface area, and porosity properties can be manipulated. In this regard, concept of nanoarchitectonics would be very useful for designing the high-performance supercapacitor application enabling the free control of the surface textural properties, engineering the pore structures, and fine control of hierarchical porous structures. Hierarchically porous carbon materials with micro-meso-macro pore structures and having 3D network structures have been extensively explored in wider areas from energy storage SCs, lithium-ion batteries, and hydrogen storage, energy conversion fuel cells, and solar cells, and sensing or separation 146) , 147) , 171) , 172) , 173) , 174) , 175) . Owing to the exceptional physicochemical properties of the nanoporous carbon materials including, high surface areas, large pore volumes, defined fine pore structure, good conductivity, and excellent thermal and chemical stabilities nanoporous carbons are the most promising materials and have enormous potential in the targeted energy storage SCs applications. However, a key challenge yet exits in the nanoarchitectonics of dimensionally controlled porous carbons with tunable porosity and surface area, hierarchical pore structures with interconnected networks 146) . Moreover, considering the sustainability, cost, and environmental issues, the production of such materials should use natural, abundant, and renewable resources and include easy and energy-saving method. The following section outlines the latest progresses in the natural and renewable biomass-derived hierarchically porous carbons and their applications in energy storage SCs.
The growing energy consumption in the current developed society requires prominent energy storage devices and efficient energy conversion system to fulfill the everyday energy demands, which mainly depends on the development of novel and high-efficiency electrode materials. In this regard, carbon materials especially with high porosity and large surface area with tunable pore structures have drawn much attention. Biomass nanoarchitectonics to produce high-performance carbon materials with high porosity and exceptional physicochemical properties would be promising materials for the various energy and environment related applications and features the friendly green chemistry concept 176) .
Biomasses are plants or plant-based materials in nature and are the major sources to produce carbon worldwide. A value to the waste biomass can be added by burning the forest byproducts and agricultural resides. However, this process causes air pollution and is not efficient energy utilization. Biomass is rich carbon source and has been used as the starting materials to produce porous carbons by direct pyrolysis or activation method. Using different biomasses with a proper activator various surface area activated carbons have been reported 177) , 178) , 179) , 180) . Due to the presence of only micropores such activated carbons have limited use in supercapacitors as the electrolyte ion diffusion is restricted and is not favorable in micropores. Therefore, several nanoarchitectonics strategies were employed to optimize the hierarchy in the pore structures which lead to the formation of both micro- and mesopores in the carbon framework to enhance the materials’ electrochemical applications including energy storage and conversions. Nevertheless, biomass-derived carbon materials exhibit unevenness due to the complex structure and composition of the biomass. In addition, the biomass carbons have irregular pore structure, and due to the limited pore channels the ion diffusion distance and resistance in the electrolyte increase, which significantly reduce the rate capability of the assembled supercapacitor devices. Therefore, it is yet challenging to develop suitable method for the nanoarchitectonics of the potential carbon materials with optimum porosity properties using biomass carbon sources 181) , 182) . Hereafter, we will discuss the synthesis and nanopore engineering of porous carbon materials obtained from various biomass and their applications in supercapacitors.
Carbon content in the biomass is found to be in the range of 45 to 50%. Therefore, they are a good source of carbon. Using the simple thermochemical conversion methods carbonaceous materials can be prepared. In this process, most of the carbons are kept and other unnecessary elements are eliminated. The structural and surface textural properties of the obtained carbon materials rely on the nanoarchitectonics method and source of carbon. Several biomasses have been explored. Carbon materials that we used in our everyday life are derived from agricultural wastes such as coconut shell, and woods by treating them in an inert gas at higher temperatures called direct carbonization. Biomass is essentially composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are long linear and branched chain crystalline polysaccharide, respectively, while lignin is a three-dimensional complex macromolecule structure, which functions as a binder and wraps the cellulose as a framework and hemicellulose as filler forming a stable structure. Therefore, the stable structure of the biomass framework become fragile and unstable particularly when portion or part of the lignin and hemicellulose are eliminated by heat-treatment, and a stacked dense carbon structure is formed. By activation process more active sites can be created forming porous structures in the carbon skeleton. Investigations have shown that porous carbon materials with a micro- and mesopore structures with higher specific surface area can be formed from the activation of cellulose compared to hemicellulose and lignin-derived carbons.
Generally, direct carbonization of the biomass is conducted by the heat-treatment from mild- to high temperature ranges. Heat-treatment causes the disintegration of the biomass and volatile organic components and other the non-carbon species such as H, N, O, and S are eliminated forming carbonaceous char with porous structure. The major biomass components cellulose, hemicellulose, and lignin upon calcination under inert gas atmosphere may go through various chemical routes. Generally, the first stage (<100°C) includes the volatilization of moisture or evaporation of water. Hemicellulose starts to degrade at 220°C and the process continues up to about 315°C. From 315 to 400°C the pyrolysis of cellulose proceeds with the degradation of carbohydrates and lipids, which released volatile materials or gases including, CO2, CH4, CO, and some organics causing obvious mass loss in this range. Pyrolytic decomposition of the lignin covers a wide range of temperatures. Thus, the residual solid is mainly carbon after the pyrolysis, whose performance in electrochemical applications largely depend on the surface textural and structural properties such as porosity, morphology, and physicochemical properties. For example, formation of sp2-rich carbon during carbonization is advantageous for the electrochemical applications due to the enhanced conductivity. Therefore, proper choice of carbon precursors and the carbonization conditions including heating ramp, hold time, and temperature are crucial to the textural properties, functionality, and performance of the final product. In the recent time, a mild activation (chemical) process is commonly used to optimize the porous architectures of the directly carbonized carbon materials by mixing carbon or carbon precursors with different activating agents such as phosphoric acid (H3PO4), sodium hydroxide (NaOH), potassium hydroxide (KOH), sulfuric acid (H2SO4), potassium chloride (KCl), calcium chloride (CaCl2), zinc chloride (ZnCl2), potassium carbonate (K2CO3) etc. at the elevated temperatures 183) , 184) , 185) .
Directly carbonized biomass carbons do not have well-developed porous structures. Therefore, activation is essential to enhance porosity, and hence, the performance and functions of the resulting carbon materials. The activation can be accomplished either by physical or chemical method. The scale up production of carbon materials is based on physical activation. This involves a two-step process. First, carbonization of precursor an inert atmosphere at a proper temperature usually <800°C, and second, activation using oxidants such as CO2, air or steam or both at high temperatures between 800 to 1100°C. The carbon materials produced by physical activation have low specific surface area (500 - 1000 m2/g), which is yet far from the requirement of high energy storage electrochemical supercapacitor systems. Therefore, chemical activation method is employed to further enhance surface area. Chemical activation involves direct mixing of precursor (or pre-carbonized biochar) with activator (such as acids, alkali, or salts) followed by calcination or carbonization at high temperatures (up to 1000°C) in a continuous flow of the inert gas or under the inert gas condition. The activators used in chemical activation inhibit the tar formation and also reduce volatile products thus producing the high yield carbon materials with well-developed porosity and high surface area. Therefore, chemical activation is preferred as it leads to the improved porosity at the cost of less energy. Specific surface areas and pore volumes of chemically activated carbon materials are much higher compared to the carbon materials obtained by the physical activation method. Especially the porosity, hierarchical pore structures and pore size distribution are adjustable by tuning temperature, hold time, and mixing ratio of the activators 183) , 184) , 185) .
Recently, several interesting novel carbon materials with excellent surface textural properties and hierarchically porous architectures have been realized from natural, abundant, renewable, inexpensive, and environmentally friendly high-carbon-content lignocellulosic precursors. For example, Shrestha, Ariga, and coworkers 186) have prepared porous activated carbon materials by the H3PO4 activation of bamboo, an abundant natural biomaterial and investigated their electrochemical supercapacitances performances. The activation was based on the low energy method and hence the carbonization was performed at 400°C and the effect of mixing ratio of H3PO4 on the porosity properties and hence supercapacitance performances was thoroughly examined. H3PO4 was impregnated with bamboo powder (BP) (H3PO4:BP=0.1:1, 0.4:1, 0.7:1, 1:1, 1.5:1, and 2:1) and the mixture was calcined at 400°C in N2 atmosphere. Due to the low-temperature carbonization the obtained carbon materials contain various oxygenated surface functional groups with amorphous structure and containing hierarchical micro- and mesopore structures. Fine carbon granules with irregular shape and size were confirmed by scanning and transmission electron microscopy images (SEM, TEM, and HR-TEM) (Fig. 2). The carbon particles contain large number of macropores with channel like morphology (Fig. 2a). High resolution SEM image revealed plenty of mesopores with pore size ranges from 10 to 50 nm (Fig. 2b). SEM analyses shows an increase in mesoporosity with increases in H3PO4 mixing ratio from 0.1 to 1 and then remains almost unchanged. Highly porous with interconnected mesopores could be observed in TEM image (Fig. 2c), which can promote the ion diffusion and contribute to improve the supercapacitance performance. Bamboo-derived carbons exhibited amorphous and graphitic structure as confirmed by HR-TEM image (Fig. 2d). Nitrogen sorption isotherm analyses revealed that specific surface areas in the range 218 - 1431 m2/g and pore volumes in the range of 0.26 - 1.26 cm3/g.
Electron microscopic observations of phosphoric acid-activated bamboo-derived nanoporous carbons: (a, b）SEM images, (c) TEM image, and (d) HR-TEM image. Inset of panel (d) represents the selected area electron diffraction pattern.
Bamboo carbons show excellent energy storage properties. As shown in Fig. 3a galvanostatic charge/discharge curves of the optimal sample prepared at a mixing ratio of 1:1 (NCM_1) display EDLC type of energy storage mechanism. Figure 3b shows that the discharge times of electrodes prepared at different H3PO4 concentration increases with H3PO4 impregnation ratio demonstrating higher energy storage capacity, which is supported by the porosity properties. The specific capacitance (C s) estimated from the discharge curve at different current densities is presented in Fig. 3c. The electrode prepared at impregnation ratio of 1:1 gave a maximum specific capacitance of 206 F/g at a current density of 1 A/g followed by a good cycle life sustaining 93% capacity after 1000 successive charging/discharging cycles inferring that the bamboo-derived nanoporous carbon materials have a considerable potential in energy storage supercapacitors applications.
The electrochemical energy storage performance of the nanoporous carbon materials (NCM) prepared from bamboo: (a) Galvanostatic charge/discharge curves at different current densities from 1-10 A/g for NCM_1 sample, (b) discharge profiles at 1 A/g of the carbon samples prepared at different mixing ratio of phosphoric acid, (c) the specific capacitance vs. current density, and (d) cycle performance of the electrode prepared with the optimal sample, NCM_1.
Next, Shrestha, and coworkers 187) have studied the effect of activators on the surface area and porosity properties of Areca catechu nut (ACN) carbon. ACN powder was chemically activated at low temperature (400°C) suing different activators such as NaOH, ZnCl2 and H3PO4 and investigated the effect of these activators on the surface area, porosity, structural properties, surface functional groups, and energy storage supercapacitance performance. ACN-derived NCMs are amorphous carbon and exhibit hierarchical micro- and mesopore architectures depending on the activator. Total specific surface area and total pore volume were found in the range 25 - 1985 m2/g and 0.12 - 3.42 cm3/g, respectively having obtained the best textural properties by the H3PO4 activation. All the carbon materials contain commonly carboxyl, carboxylate, carbonyl, and phenolic oxygenated surface functional groups. As expected, the directly carbonized sample without any activators ended up with the large size macropore on the surfaces of the irregular shaped carbon particles. NaOH activation also could not develop high porosity. While the porosity (microporosity) was enhanced with ZnCl2 activation although mesopore structure was not fully developed. The surface textural properties turned out to be significantly different with the H3PO4 activation. The obtained carbon materials exhibited heterogeneous distribution of mesopores structures because of aggressive attack of the H3PO4 on the carbon skeleton during the activation process. These results demonstrated that H3PO4 as an effective activator to create hierarchically micro-mesoporous structures in the carbon framework at the cost of low energy, i.e., at low temperature carbonizations (400°C). Transmission electron microscopy images (Figs. 4a, 4b: ZnCl2 activation) and (Figs. 4c, 4d: H3PO4 activation) clearly show the highly porous network surface structure. Mesopores are not clearly seen in TEM image of ZnCl2 system (Fig. 4a) as most of the pore lie in the micropore region. However, it is obvious in the H3PO4 activation system (Fig. 4c). Amorphous with partial graphitic carbon microstructure commonly observed in activated nanoporous carbon materials can be seen in both the ZnCl2 and H3PO4 activated systems (Figs. 4b, 4d).
TEM observations of the Areca catechu nut-derived nanoporous carbon materials: (a) TEM, and (b) HR-TEM image of ZnCl2 activated sample, and (c) TEM and (d) HR-TEM images of H3PO4 activated sample. Inset of panels (a) and (c) represent the selected area electron diffraction patterns. Reprinted with permission from ref. 187, copyright 2017 Springer Science+Business Media, LLC, part of Springer Nature.
Porosity dependent to the activator could be seen in the nitrogen adsorption-desorption isotherms (Fig. 5a). The isotherms of the directly carbonized and NaOH activated samples indicate nonporous carbon. While the adsorption isotherm of ZnCl2 activated sample exhibited Type-I behavior suggesting microporous structure. Combined type-I/type-IV sorption isotherm of H3PO4 activated carbon indicates the hierarchically micro- and mesopore structures. Large nitrogen adsorption at a lower relative pressure can be assigned to the micropores filling and the hysteresis loop at higher relative pressure is caused by the capillary condensation occurred in the mesopores channels. Histogram of the pore size distributions obtained by Barrett-Joyner-Halenda (BJH) method (Fig. 5b), and density functional theory (DFT) (Fig. 5c) further confirmed the hierarchical pore structures in the ACN-derived carbon materials. Brunauer-Emmett-Teller (BET) surface areas of the directly carbonized sample was poor (5 m2/g). It is slightly improved with NaOH activation (25 m2/g) and significantly improved with ZnCl2 (1550 m2/g) and H3PO4 activation (1985 m2/g) due to the well-developed porosity and hierarchically porous structures. The total pore volume of the optimal carbon material was found to 3.426 cm3/g, which is much larger than the commercial carbon materials obtained by the direct carbonization or physical activation.
Surface textural properties of Areca catechu nut-derived nanoporous carbon materials (NCMs) : (a) nitrogen adsorption/desorption isotherms, (b) mesopore size distribution profiles from BJH analyses, and (c) the corresponding micropore size distribution profiles from the DFT analyses. NCM_H3PO4, NCM_ZnCl2, and NCM_NaOH correspond to the carbon materials obtained by the H3PO4, ZnCl2, and NaOH activation, respectively. NCM_DC represents the directly carbonized sample without activating agents. Reprinted with permission from ref. 187, copyright 2017 Springer Science+Business Media, LLC, part of Springer Nature.
TEM observations and nitrogen adsorption isotherms revealed that the activator ZnCl2 involves micropore formation. Whereas H3PO4 activation results in micro- and mesopores in carbon framework. NaOH could not create pores at this carbonization temperature, which could be due to the different reaction dynamics and requirement of higher energy for the efficient activation with NaOH activator. These results clearly showed that the pore formation mechanism largely depend on the type of activators. Note that ZnCl2 which is essentially a dehydrating agent fosters the extraction of water molecules from lignocellulosic structure and carbonization proceeds at the loss of small amount of carbon. Moreover, ZnCl2 inhibits the tar formation and thus contributes to create micropore structures working as a template of pore formation. Removal of the ZnCl2 that has been intercalated in carbon matrix by washing forms void space 188) . The pore formation mechanism is different with H3PO4 activation. H3PO4 promotes bond cleavage through acid catalytic action and combines with cellulose/hemicellulose forming the phosphate ester, which upon thermal breakdown creates pores in the carbon skeleton 189) . While, NaOH involves in the destruction of cellulose structure producing heterogeneous structure which eventually causes the pore coalescence resulting macropore structures.
Due to the well-developed porosity with hierarchical micro- and mesopore structures which promotes ions transport to the electrode surface, H3PO4 activated ACN-derived carbon materials presented outstanding supercapacitance property with the longest discharge time in chronopotentiometry measurements. A high specific capacitance of 342 F/g was recorded at a scan rate of 5 mV/s and 265 F/g at current density of 1 A/g. The electrode also sustained 46% capacitance at 10 A/g followed by outstanding cycle performance with 97% capacity retention after 5,000 successive charging/discharging cycles suggesting good rate performance of the electrode. ZnCl2 activated electrode achieved 185 F/g at 5 mV/s with a cycle stability of 91% after 5000 cycles. These results demonstrated the vital role of the activator to control and structural and surface textural properties and H3PO4 and ZnCl2 activated Areca catechu nut-derived nanoporous carbons could work as efficient electrode materials in supercapacitors applications.
Shrestha, and coworkers have recently shown that ZnCl2 activation of Choerospondias axillaris (Lapsi) seed stone result in micro- and mesoporous carbons 190) . Lapsi seed powder was blended with ZnCl2 (at weight ratio of 1:0, 1:0.5, 1:1, 1:2, 1:4) and carbonized at 700°C. The obtained carbon materials possess graphitic microstructure with interconnected mesopore structures (Figs. 6a, 6b: SEM, TEM & HR-TEM). Depending on the impregnation amount of ZnCl2, specific surface areas and pore volumes are found in the ranges from 931 to 2272 m2/g and 0.998 to 2.845 cm3/g, respectively. Owing to the ultra-high surface areas, well-defined pore sizes followed by the interconnected mesopores Lapsi seed-derived porous carbons displayed brilliant energy storage capacity of 284 F/g at 1 A/g followed by the high-rate performance sustaining 67.7% capacity at 20 A/g with exceptional long cycle life of 99% following 10,000 successive cycles. Their results highlighted the important role of carbonization temperature to optimize textural properties of the biomass-derived carbon material and ZnCl2 activation would be efficient at higher temperature. Furthermore, an agro-waste biomass, Lapsi could be a sustainable carbon sources to produce higher performance carbon materials with significant potential in supercapacitors.
SEM and TEM observation of the ZnCl2 activated Lapsi seed carbon: (a) SEM image revealing mesopore structure, (b) TEM image showing micro/mesopore structures, and (c) HR-TEM image displaying amorphous carbon with graphitic microstructure.
We have overviewed so far, the structural, porosity, and energy storage properties of the biomass carbons obtained by the chemical activation method using H3PO4, NaOH, and ZnCl2 activators. Note that of the several activators such as H3PO4, NaOH, H2SO4, KCl, CaCl2, ZnCl2, KOH, K2CO3, etc., commonly employed to generate porosity in the carbon materials, KOH have shown to be the most promising activator. Exceptionally high specific surface area (up to 3000 m2/g) carbons have been obtained by the KOH activation and these materials are highly advantageous in high energy storage applications. Three main activation processes are involved during KOH activation: First, KOH and other potassium compounds, such as K2CO3 and K2O (generated in calcination process), react with carbon and generates the porous structure; second, the intermediate products (H2O and CO2) formed during the process promotes the additional growth of the porosity through the gasification of carbon; Finally, the K (as prepared during the process) gets intercalated into the carbon lattice expanding the lattice. The removal or washing out of these K and other K compounds result in the formation of porous structure. With the KOH activation process, micro/mesoporosity can be controlled by adjusting the amount of KOH, carbonization temperature and other carbonization condition of ramp, hold time and atmosphere 191) .
Karnan, Sathish, and co-workers 192) demonstrated that KOH activated corncob carbon material functions as an excellent supercapacitor electrode material. The BET surface area was estimated 800 m2/g and the material exhibit both the micro and mesoporous structures. In an aqueous electrolyte (1 M H2SO4) the electrode showed an exceptional specific capacitance of 390 F/g at 0.5 A/g. Furthermore, they have also tested the supercapacitance performance in three different ionic liquids (namely 1-ethyl-3-methyl imidazoliumbistrifluro methyl sulfonyl amide [EMIM] [TFSA], 1-n-butyl-3-methyl imidazolium hexafluoro phosphate [BMIM] [PF6] and 1-ethyl-3-methyl imidazolium tetrafluro borate [EMIM] [BF4]). Among them the ionic liquid EMIMBF4 showed excellent capacitive behavior resulting in a high energy density of 25 Wh/kg and power density of 174 W/kg. The assembled supercapacitor device in the ionic liquid, which after charging only for 10 s could power a red LED for more than 4 min demonstrating that the corncob derived nanoporous carbons have potential as the electrode materials for applications in supercapacitors. Karnan, Sathish, and co-workers have also prepared activated nanoporous carbons by the KOH activation of cauliflower waste and investigated the energy storage properties 193) . KOH activated the carbon exhibit micro- mesopore architectures. The surface area and pore volume were found to be 1500 m2/g and 0.275 cm3/g, respectively. As the result, the material shows excellent electrochemical energy storage supercapacitance performance. Fabricated symmetric supercapacitor device delivered energy densities of 29 Wh/kg in ionic liquid electrolyte. The energy storage capacity the device could be improved and a maximum energy density of 58.5 Wh/kg could be achieved by adding an additive (0.01 M KI in Na2SO4 electrolyte).
KOH activation method has been extended to produce carbon materials from different biomass and obtained two-dimensional porous carbon materials. Selvaraj, and coworkers 194) have demonstrated ultra-high surface area carbon nanosheets from Prosopis Juliflora wood. The obtained carbon materials achieved ultra-high specific surface area (2943 m2/g), and large pore volume (1.83 cm3/g) with micro-meso- and macropores. They have found that lower temperature carbonization (700 and 800°C) result in microporous carbon and higher temperature carbonization (900°C) resulted in hierarchical porous structures (micro- and mesoporous carbon). Owing to the increased porosity, ultrahigh surface area and hierarchical porosity supercapacitor electrode achieved 588 F/g capacitance at 0.5 A/g followed by an excellent cycle life of 92.5% after 6000 charging/discharging cycles. The assembled symmetric supercapacitor device in a neutral aqueous (1 M Na2SO4) electrolyte gave a high energy density of 56.73 Wh/kg. These results demonstrated that the energy storage supercapacitance performance could be increased by the textural and physicochemical properties of the carbon materials that can be produced from the natural biomass or agro-wastes.
Using KOH as an activator our group has fabricated series of ultra-high surface area hierarchically porous carbons comprising of micro-mesopore architectures from different biomass and investigated their electrochemical supercapacitance performance. Our results demonstrated that agro-waste natural carbon source precursors have potential to the mass production of hierarchically porous carbons with exceptional surface area and pore volume that are essentially required in the next generation advanced supercapacitors. For example, KOH activation of Sapindus mukorossi (Washnut) seed resulted in the porous carbons having hierarchical micro- and mesoporous structures 195) . Due to the well-developed porosity, Washnut seed carbon materials exhibited ultra-high surface areas of 2185 m2/g and a large pore volume of 2.002 cm3/g. As a result, the optimal sample (carbonized at 900°C: WNC_K900) showed outstanding supercapacitance with 288.7 F/g capacitance at 1 A/g. Moreover, the rate performance of the electrode was very good as it could sustain 67.2% capacitance at a 50 A/g followed by long cycle life (Fig. 7).
The electrochemical supercapacitance performance of KOH activated Washnut seed carbons: (a) Galvanostatic charge/discharge (GCD) curves at a current density of 1 A/g, the GCD curves vs. current density for (b) the sample carbonized at 800°C (WNC_K800), (c) sample carbonized at 900°C (WNC_K900), and (d) sample carbonized at 1000°C, (e) comparison of the specific capacitance, and (f) cycle performance of the selected samples (WNC_K800 and WNC_K900). Reprinted with permission from ref. 195, copyright 2021 Chemical Society of Japan.
Recently, Maji and coworkers 196) have reported the ultra-high surface area (2104.3 m2/g) and large pore volumes (1.386 cm3/g) hierarchically micro- and mesopore structured carbon materials by the KOH activation of Artocarpus heterophyllus (Jackfruit) seed. The obtained carbon was amorphous in nature with a partial graphitic microstructure and performed excellently as the electrode materials in electrical-double layer capacitors with an aqueous electrolyte (1 M H2SO4). Electrode prepared from the sample with excellent textural parameters displayed 323.8 F/g, a high specific capacitance, at a current density of 1 A/g and retained 53.7% capacity at 50 A/g, a high current density, which is a clear indication of the electrode’s good rate performance due to quick ion diffusion to the electrode surface, which was also verified using molecular simulations. The simulation study showed that the capacitance increases as the electrolyte ions enters into the pores, but at the same time reduces the charge rate compared to non-porous electrode prepared from the directly carbonized sample having low porosity. Furthermore, Jackfruit seed-derived carbon electrode showed exceptional long cycle life sustaining 97% of the capacitance after 10,000 consecutive charging/discharging cycles revealing the substantial possibility of Jackfruit-seed-derived hierarchically porous carbon materials in the high-performance supercapacitor applications.
In a recent report, we have found that KOH activation of Nelumbo nucifera (Lotus) seed results in self-nitrogen-doped porous carbon materials with an ultrahigh surface area (2489.6 m2/g) 197) . The preparation method included the mixing of Lotus seed biochar with KOH pellet (1:1 wt. ratio) and carbonizations at high temperatures (600 to 1000°C) in an inert gas atmosphere. The obtained carbon samples displayed both the type-I and type-IV nitrogen sorption isotherms (Fig. 8a) with a clear H4 hysteresis loop demonstrating the hierarchically porous structures, which is obvious in the histograms of the pore size distributions determined from the analyses of nitrogen sorption isotherms by DFT (Fig. 8b) and BJH method (Fig. 8c). It is interesting to note that the hysteresis loop grows with increase in the carbonization temperature, implying the formation of more mesopores in the materials carbonized at higher temperature. The electrochemical supercapacitance performance studied in an aqueous 1 M H2SO4 electrolyte solution confirmed the EDLC type energy storage mechanism, see quasi-rectangular shaped CV curves (Fig. 9a). Total integral current collection in the CV curves, which reflects the energy storage capacity of the electrode agrees with the surface area and suggested that the higher the surface porosity the higher the energy storage capacity. The total integral current within the CV curve increases with the potential sweep and the intrinsic EDLC’s type energy storage mechanism sustained underlining the key role of the mesoporous channels in the electrode material for the fast ion transport (Fig. 9b). The electrode prepared from the best sample achieved 434.5 F/g capacitance at 5 mV/s, which can be attributed to the outstanding surface area properties with self-doped-nitrogen. The specific capacitance of all the samples is compared (Fig. 9c). The capacitance retention performances of the electrodes are exceptionally good (Fig. 9d). For example, LTSC_K1000 electrode sustained 83.4% capacitance at 500 mV/s suggesting a faster ion diffusion. The electrodes also showed outstanding cycling performance with 99% cycle life after a long continuous cycle (10,000 charging/discharging). These results clearly demonstrated that an agro-waste, Nelumbo nucifera seed, represents a low-cost natural carbon source for producing high-performance self-nitrogen doped porous carbon materials having substantial possibilities in supercapacitors applications.
Porosity properties of the KOH activated Nelumbo nucifera (Lotus) seed-derived hierarchically porous carbons (LTSC_K600, LTSC_K700, LTSC_K800, LTSC_K900, and LTSC_K1000) by nitrogen adsorption/desorption: (a) Nitrogen sorption isotherms, (b) pore size distribution of micropores, and (c) pore size distribution of mesopores. LTS_800 represent the directly carbonized sample.
The electrochemical energy storage supercapacitance performance of Lotus seed-derived carbon materials: (a) Cyclic voltammograms (CV) of LTS_800, LTSC_K600, LTSC_K700, LTSC_K800, LTSC_K900, and LTSC_K1000 recorded at a fixed scan rate of 50 mV/s, (b) CV profiles of LTSC_K900 (best sample in terms of surface textural properties) vs. scan rates (5 to 500 mV/s), (c) specific capacitance calculated from the CV curves for all the samples vs. scan rate, and (d) the corresponding specific capacitance retention profiles.
Similarly, Shrestha, and coworkers 198) have recently reported hierarchically porous carbons by the KOH activation of Phoenix dactylifera (date) seed. The obtained carbon materials have excellent surface porosity properties: specific surface area reached to 2383.2 m2/g followed by quite high pore volume (1.76 cm3/g) due to hierarchically pore architectures and interconnected micro- and mesoporous structures. As expected, the carbon materials displayed very good charge storage property as electrode materials of supercapacitor. A high specific capacitance of 386 F/g was recorded at 1 A/g for the electrode prepared from the best sample followed by the significant capacitance holding of 63% at 50 A/g and a long cycle life of 98% after 10,000 successive charging/discharging cycles demonstrating the key role of the hierarchical porous structures to improve the performance of electrode materials in commercial, and advanced supercapacitors.
The examples discussed in this review highlights the importance of biomass nanoarchitectonics in energy storage supercapacitor applications. It should be noted that the biomass or agricultural waste are cost effective and sustainable carbon sources for the scale-up production of nanoporous activated carbons. Nanoarchitectonics of high-performance porous carbon materials requires the fundamental understanding of activation process, selection of activators, impregnation ratio, and carbonization time and temperature for tuning and optimization of surface areas and optimal pore size distributions of the carbon materials. We believe that this review explains the importance of biomass nanoarchitectonics to produce ultra-high surface area, high porosity with well-defined and tunable pore size, and hierarchical pore structures that are essentially required to develop the high energy storage advanced supercapacitors 199) , 200) , 201) .
This short review article explores the progress and potential of this field using biomass nanoarchitectonics for supercapacitor applications as examples of energetic materials and devices. Biomass-derived hierarchically porous carbon materials comprising micro- and mesopore architectures with ultra-high surface area, well-developed porosity, and well-defined pore size distribution that are highly essential in the high energy storage supercapacitors are overviewed. The outstanding potential of the ultra-high surface area biomass-derived porous carbon materials obtained by activation method as the high energy storage electrode materials in advanced supercapacitors is highlighted. Several recent examples of biomass as the sustainable carbon sources and the derived porous carbon materials’ energy storage supercapacitance performance have been discussed. Since the porosity properties of porous carbon materials depend on the synthetic conditions, synthetic method, and carbon sources themselves, optimization of the production process using the nanoarchitectonics concept would be beneficial to improve the electrode material’s energy storage capacity enabling the free structure and surface textural properties control by engineering and fine control of hierarchically porous structures. Owing to the unique physicochemical properties of the hierarchically porous carbon materials with micro-meso-macro pore structures and having 3D network structures with interconnected mesopores, they are advantageous in energy storage supercapacitors applications. Plant-based biomass is rich carbon source and has been employed as the starting materials to produce porous carbons by pyrolysis or activation method. Directly carbonized biomass carbons are poor in textural properties. Due to the lack of well-developed porous structures, they exhibit low specific surface area. As a result, the electrochemical energy storage capacity or other energy related performance are poor. Therefore, activation is required to improve the properties, performance, and functions of the resulting carbon materials. The activation can be achieved either by physical or chemical method. Using the appropriate activator several new ultra-high surface area porous carbon materials have been reported from various biomass. Because of the nanoporous structures, extraordinary surface areas, enormous pore volumes, tunable pore size, good conductivity, and excellent thermal and chemical stabilities these carbons are the most promising electrode materials and have enormous potential in the energy storage applications. However, carbon materials obtained from the activation of biomass-derived display irregularities in the pore structures due to the complex structure and composition of the biomass. Hence, the ion diffusion distance and resistance in the electrolyte increase because of the limited pore channels, resulting in the significant reduction of the rate capability of the supercapacitor devices. Therefore, it is yet challenging to develop and optimize relevant nanoarchitectonics method to produce high performance carbon materials with optimum porosity properties from the sustainable biomass carbon sources. A key challenge is the scale up production of dimension-controlled porous carbons with controllable surface area, tunable pore size with interconnected mesopore networks. These features are extremely important for the high energy storage sustainable supercapacitors. Considering the sustainability, environmental and cost issues, nanoarchitectonics of high-performance porous carbon materials should be easy, time and energy-saving, and cost-effective.
This research was partially funded by JSPS KAKENHI Grant Number JP20H00392, JP20H00316, and JP20K05590.
The authors declare no conflict of interest.
