Biomass-derived porous carbons as supercapacitor electrodes -A review

Majid Shaker,Ali Asghar Sadeghi Ghazvini ,CAO Wei-qi ,Reza Riahifar ,GE Qi

（1.Chongqing 2D Materials Institute, Liangjiang New Area, Chongqing 400714, China;2.Department of Materials Science and Engineering, Tarbiat Modares University, Tehran 3177983634, Iran;3.Key Laboratory of Optoelectronic Technology & System Ministry of Education, Chongqing University, Chongqing 400044, China;4.Department of Nanotechnology and Advanced Materials, Materials and Energy Research Center of Iran, Tehran 14115-111 Iran）

Abstract：Electrochemical capacitors,also called supercapacitors (SCs),have been gaining a more significant position as electrochemical energy storage devices in recent years.They are energy storage devices with a considerable power density,a satisfactory energy density and a long-life cycle,suitable for a large number of applications.The further development of these devices relies on providing suitable,low-cost,environmentally friendly,and abundant materials for use as the active materials in the electrodes.Among the current materials used,activated carbons have a superior performance.Their excellent electrochemical performance,high specific surface area,high adsorption,tunable surface chemistry,fast ion/electron transport,abundant functional moieties,low cost,and abundance have made them promising candidates as SC electrodes.These advantages can be enhanced if the activated carbons are prepared from biomass precursors.Recently,scientists have focused on biomass because it is abundant and renewable,low cost,simply processed,and environmentally friendly.The fundamentals of SCs as an electrochemical energy storage device are discussed and biomass from various sources is categorized and introduced.Finally,the activation techniques for these biomass precursors and their use as electrode materials for SCs are discussed.

Key words:Supercapacitor；Electrode；Porous carbon；Biomass；Activation

1 Introduction

The global issue of clean energy storage has been gaining rising significance since last century.Despite the fact that fossil fuels have been considered as the primary energy supply,their limited resources and high demands have made them a costly and unreliable source of energy.Besides,the use of fossil fuels is becoming constrained by governments because of their environmental pollutions and their undeniable effect on global warming.Therefore,it is a wise choice to replace fossil fuels with fresh and environmentally friendly sources of energy such as solar,hydroelectric,nuclear and wind.To consume the energy produced from these clean sources,it is necessary to store and release energy whenever and wherever are needed.At present,supercapacitors (SCs),also called electrochemical capacitors,are one of the foremost energy storage devices owing to the high power density and satisfying energy density that they provide[1,2].SCs store and release electrical energy only in a few seconds,but in comparison with secondary ion batteries,their energy density is one order of magnitude smaller[3].Currently,fabrication of materials possessing high power density of SCs and large energy density of secondary ion batteries is a hot research topic[2,4].Through this approach,one can see a brilliant outlook by the extensive replacement of electrochemical energy storage devices,which makes green energy competitive with fossil fuels.

SCs are favored because of their high power density while having a prominent energy density[5].SCs are indeed a newborn generation of traditional capacitors with exceptional energy density.They store energy as electric charge at the electrolyte-porous carbon or ceramic electrode interface[6].They are ideal for fast energy storage and release,but there is still a large gap left to be filled to achieve the ideal energy density for such devices.Compared with conventional capacitors,they have 10 000 times more energy density owing to their porous structure and energy storage mechanism.The stored energy in the typical capacitors can be as high as μF or mF level,whereas the capacitance of SCs based on their electrode and electrolyte easily reaches tens,hundreds and thousands of F[7].They also have high reliability due to exceptional capacity retention even after a large number of charge/discharge cycles,which makes them highly desirable for electric vehicles.Although there are several types of SCs that have high power densities and long cycle life,their limited energy densities have constrained their applications in comparison with secondary ion batteries.

For better understanding performance of energy storage devices,it is significant to observe Ragone plot in Fig.1.According to this figure,fuel cells possess the highest energy density while they suffer from a low power density.Energy density of batteries is relatively a little bit lower.However,even the batteries with improved power density,still do not provide high enough power density.Although SCs have large power density but do not have high energy density as compared with batteries and fuel cells,they are at present being employed in a wide variety of fields as electric buses,cars,cranes,trains,and elevators owing to their superb performance.This phenomenon is caused by the non-similar charge storage mechanisms of SCs and secondary ion batteries.SCs store electric charge in the electrode-electrolyte interface via electrostatic or faradaic reactions,whereas secondary ionbatteries store energy via ion-intercalation inside the electrode active materials[8,9].

Fig.1 Specific power and energy capabilities of electrochemical energy storage devices.

Nowadays,SCs play a key role in various fields for storing and releasing energy.A wide variety of advanced materials are researched and reviewed as SC electrode materials in multiple articles[10].Amongst them,biomass-derived activated carbons are of extremely high significance for the further development and future electrochemical capacitors owing to their remarkable advantages for the production of cost-effective commercial SC electrode materials[11].To date,a large number of research groups have focused on this topic.Therefore,at present,we need to gather the results of recent experiments in a systematic way as a guideline for the fabrication of biomass-derived activated carbons and their applications as SC electrode materials.Thus,it is necessary to review SCs,the sources and compositions of biomass,the techniques of converting biomass to porous carbons,and their performance as SC electrode active materials.

Biomass such as wood,dried leaves,stem,flowers,seeds,fruits,and agricultural products can be employed to produce carbon materials.Biomass are suitable precursors for synthesizing carbon materials because they are environmentally friendly,recyclable,sustainable,abundant and nontoxic.Therefore,it is significant to study biomass as the precursors of carbon materials for a wide variety of applications including supercapacitor electrode materials.

Currently,in the literature,there is a considerable number of review papers on biomass-derived porous carbons for multiple applications.For instance,some papers focus on activated carbons derived from specific precursors such as lignocellulosic precursors[12],oil palm[13],wood[14],constituents of plants[15]and agricultural waste[16],while other researchers have authored their papers regarding the applications of the biomass-derived activated carbons in heavy metals removal[17],sequestration of dyes[18],electrode materials of hybrid electrochemical capacitors[19]and adsorption[20].Meanwhile,some papers have only probed the synthesis techniques for the preparation of activated carbons from biomass such as hydrothermal[21],microwave[22],chemical[23]and physical[24]routes.The difference of the current study with previous publications is that in this work,we have summarized the use of biomass-derived activated carbons as one of the most promising electrode materials for SCs through a comprehensive and unique approach by classifying biomass precursors into 5 main groups regarding their origin with focusing on their chemical composition,electrochemical performance in SCs and preparation methods.In this work,firstly various types of SCs and their charge storage mechanisms are reviewed.Then,the composition,resources,microstructures,properties and applications of main classes of biomass-derived porous carbons as SC electrodes are discussed in details.

2 Energy storage devices

2.1 EDLCs

Double-layer concept has been proposed by scientists since the 19thcentury when research on colloidal suspensions was conducted,Helmholtz model was proposed and the double-layer concept was developed[25].Although this phenomenon was discovered early,the pragmatic application of doublelayer capacitors was discovered in 1957 by General Electric,when this company patented an electric double-layer capacitor capable of storing electrical energy[26].Nowadays,a wide variety of commercial EDLCs is produced and distributed by various manufacturers for satisfying the global rising demand of high-power energy storage devices.

The energy and power densities of electrochemical energy storage devices are displayed in Fig.1 as the Ragone plot.The special position of SCs in this plot means they can provide energy and power close to that of batteries and capacitors.Therefore,SCs are favored because they are able to bridge the existing gap between aluminum electric capacitors and batteries.The energy density of SCs is several orders of magnitude larger than that of traditional capacitors.Nonetheless,it is constrained to around 10 kW kg?1[27].The energy-storage mechanism of batteries has limited their power density to lower than 1 kW kg?1,whereas SCs output much higher power density[28].

A SC cell is made of two electrodes with a separator between them.The electrodes are made of the same material with the same weight in symmetric cells while the electrodes are made of different materials in asymmetric cells.The role of the separator is to keep the electrodes from an electron short circuit while it is soaked in the electrolyte.The separator material needs to be ion-permeable to provide ion transfer.It also should have ionic conductance,low ionic resistance and low thickness simultaneously to achieve the best performance.In the literature,glass or ceramic fiber separators are usually reported in aqueous electrolyte systems,whereas polymer or paper ones are used in organic electrolyte systems[29].The electrolyte breakdown potential,which occurs at one of the working electrodes,limits the achievable cell voltage.The equivalent series resistance (ESR) of a cell is strongly affected by electrolyte conductivity of a separator.The breakdown voltage of aqueous electrolytes is around 1 V,which is much lower than the attainable voltage by utilizing organic liquids or ionic liquids as electrolytes.Even so,the higher conductivity of aqueous electrolytes makes them a better option for high power applications in comparison with organic ones.In addition,handling aqueous electrolytes and assembling cells using aqueous electrolytes is more facile and cheaper.

EDLCs,pseudocapacitors and hybrid capacitors are distinguishable regarding their cell configurations and storage mechanisms.As the charge storage mechanism in EDLCs is electrostatic (physical) and it requires more surface area for more stored electric charge,EDLC electrode materials should have high specific surface area (＞1 000 m2g?1).Moreover,the resultant capacitance of these materials is greater than those of traditional capacitors.The most suitable option for this purpose is porous carbons owing to their low cost,industrial scale production and availability.Pseudocapacitors,which often use metal oxides,polymers and sometimes functionalized porous carbons as electrode materials,and combine two electric charge storage mechanisms of electrostatic and pseudocapacitive electric charge storage mechanisms,provide higher specific capacitance than EDLCs.The pseudocapacitive charge storage mechanism relies on fast redox reactions happening on the electrode surface,not in the bulk like batteries.Despite that,their similarity to batteries is swelling and shrinkage of electrode materials caused by their mechanical changes during fast redox reactions,which deteriorates their stability.Therefore,the cycle life of pseudocapacitive materials is lower in comparison with EDLC electrode materials.In order to improve the shortcomings of EDLCs,pseudocapacitors and batteries by producing electrode materials with the merits of each type,scientists have tried to combine these electrode materials together and assemble hybrid capacitors[30].The description of such devices and their differences is fully developed in the coming sections.

In order to know the current status of SCs in the world,it is useful to observe the following Table 1.In the Table 1,some manufacturers of SCs for high power applications and some of their products are observable and some specifications of these SCs are tabulated.

Table 1 Commercial SCs and their specifications produced by multiple manufacturers.

2.2 Pseudocapacitors

The so-called pseudocapacitors are another type of supercapacitors that store energy differently from the EDLCs.They store energy not by the electrostatic mechanism.Energy storage occurs both on the electrode-electrolyte interface and in the bulk close to the electrode surface by the quick oxidation/reduction re-actions called Faradaic reactions. Compared with EDLCs,pseudo-capacitors exhibit larger capacitance values due to additional charging transfer during operation[38].However,they suffer from the electrode material degradation caused by faradic reactions,which shortens their life cycle.Polymers and metal oxides are the most common-used materials in pseudocapacitors due to their rapid Faradic reactions,low prices,simple fabrication and relatively long life cycles[39].New materials such as sulfides,carbides,phosphides and nitrides are employed by the researchers[40–42].Pseudocapacitors have a very fast charging time of several seconds to minutes compared with the batteries.

By analyzing energy density vs time in the charging process of a high-power Li-ion batteries and EDLCs,it is found that the specific capacity for the battery is constant only for charging periods longer than 10 min[43].On the contrary,for an EDLC,the specific capacitance is always constant.Specific energy reduction of the battery for short time periods are due to resistive losses from the difficulty in electron motion and ion transport.In batteries,this shortcoming can also initiate fire disasters.There is a gap to be filled with pseudocapacitors between the feat time of lithium ion batteries and EDLCs (10 s to 10 min).

2.3 Asymmetric SCs

Compared with EDLCs,pseudocapacitors provide higher energy density but suffer from a lower power density[38].EDLCs and pseudocapacitors are employed independently in various applications according to their properties.But bringing them together and creating a new form of SCs is interesting.Therefore,researchers have been developing a new form of energy storage system called "asymmetric supercapacitors (ASCs)",made up of two dissimilar electrodes[44].In these devices,the positive electrode(cathode) consists of a pseudocapacitive material(Faradaic) while the negative electrode (anode) is made of a capacitive material.Ordinary SCs typically have a hydrolysis voltage window limited by 1 V in aqueous systems,whereas organic electrolytes are able to increase energy density effectively through widening voltage window of up to 5 V.However,their lower electrical conductivity,high viscosity,and non-environmentally friendliness have restricted their applications[45]. Fortunately,combining dissimilar electrodes used in ASCs can increase the energy density through enhancing the operating voltage[46].The working mechanism of an ASC is depicted in Fig.2.The positive and negative electrodes of this device are made of two distinct materials.The unique performance of ASCs has made them a promising candidate for the next generation of energy storage systems.

Fig.2 Schematic illustration of asymmetric SCs.

The limited voltage window (＜1 V) of aqueous EDLCs leads to their moderate energy density.In spite the fact that almost all commercial SCs employ organic electrolytes to boost their power density up to 10 Wh kg?1through voltage window widening up to 2.5-2.85 V,an ASC with an optimum design can reach a voltage window above 2 V in aqueous systems[47].Hence,aqueous ASCs are capable of delivering an energy density of with organic-electrolytebased EDLCs.However,reaching the technology of aqueous ASCs is not the end of research on ASCs.Researchers are also extremely fascinated by the use of organic electrolytes to further enhance the voltage window of ASCs and fabricate high-power density ASCs.This concept has been affirmed successfully,with the aid of expanding working voltage range[48],to increase the energy density to 128 Wh kg?1.

2.4 Hybrid SCs

It is clear that because of their above-mentioned shortages,neither batteries nor SCs are ideal and flawless devices for storing energy.But when we approach this problem optimistically,each device has its unique superiorities.The energy density of batteries is high and the power density of SCs is excellent.The combination of SCs and batteries produces strengthened electrochemical energy storage devices.When they are combined,the overall efficiency is improved by various materials storing energy via dissimilar mechanisms.The aim of research in this field is to fabricate complementary hybrid materials and hybrid devices made of dissimilar electrodes,enjoying the benefits of each component and overcoming their shortcomings.

In order to enhance the properties and efficiency of energy storage systems,hybrid materials with their individual components result in a synergistic effect.The combination of electrically conductive and electroactive materials is the most enticing combination among existing hybrid materials.For instance,since several years ago,hybrid composites of electrodes consisting of conductive carbons and electroactive oxides have been recruited in rechargeable Li-ion batteries[49].The concept behind this composition is to increase the electrical conductivity of an improperly conductive electroactive material by inserting carbon between its grains.Naturally,not all combinations work because an optimization is essential.To establish optimal conditions across percolation paths,size and ratio of the components should be controlled.It is worth mentioning that in such composites,each component retains its own crystal structure,electrochemical,and physical properties.

Hybrid materials are one step ahead in incorporation compared with composite materials.In reality,the chemical mix of organic and inorganic materials contributes to new phases of basically fresh features originated from combination in molecular level.The manufacture of hybrid materials strongly relies on their chemistry,not on the interactions between consisting components like weak van der Waals interaction,ionic,hydrogen-bonding and strong covalent bonding[50].

Fig.3 indicates that the term hybrid is employed for two purposes.The first is a hybrid device,consisting of a negative SC-type electrode (anode) and a battery-type positive electrode (cathode).The second concept concerns hybrid materials in hybrid devices.In such cases,a cathode consists of a hybrid material made up of a battery-type and a SC-type electrode.At present,many hybrid devices and materials are being researched and produced in laboratories and factories around the world to manufacture electrochemical energy storage devices lying on the diagonal of the Ragone plot.Being located on the diagonal of the Ragone plot means that new systems with high power and high energy density are under development.Scientists have employed different materials such as oxides,carbonaceous materials,and conductive polymers to achieve this objective[51,52].

Fig.3 Schematic illustrations of different hybridization approaches of supercapacitor and battery electrodes and materials.

2.5 Differences between pseudocapacitors,asymmetric supercapacitors and hybrid supercapacitors

In order to distinguish clearly between pseudocapacitors,asymmetric supercapacitors and hybrid supercapacitors,the differences of these devices are elaborated.For this purpose,Brousse et al.[53]published a paper on defining pseudocapacitors and hybrid supercapacitors (HSCs).The word "pseudo-capacity" is used to describe the distinctive unique capacitive behavior of certain materials,for instance MnO2and RuO2in the field of electrochemical energy storage devices[54–56].The linear dependence of the ammount of stored charge on the varying potential in the cyclic voltammetry (CV) curve is present in these products.Although their capacitance origin is different from EDLCs,they exhibit capacitive behavior.

Energy storage in pseudocapacitors takes place by transfer of charge through quick redox reactions,rather than the accumulation of electrostatic charge in dual EDLC layers.Although there are obvious distinctions between EDLCs,pseudocapacitors,and batteries,sometimes battery-type materials such as Ni(OH)2are reported as pseudocapacitor-type materials[57,58].The same problem also happens for irreversible electrical materials[59,60].For materials with a faradic demeanor that have electrochemical electrode signature of batteries,the use of word pseudocapacitance must be avoided.Moreover,there have been several compounds that concurrently exhibit multiple behaviors,such as birnessite-type MnO2,which demonstrates both pseudocapacitive surface activity and faradaic contribution due to bulk intercalation reactions[61].

The electrochemical behavior of any reporting material therefore seems important to be explained in detail to prevent readers from being confused.Brousse et al.[54]indicated that the phrase“ASC”can be only ascribed to the devices composed of one pseudocapacitor-type electrode and one EDLC-type electrode,whereas the word "hybrid" should be employed for those devices,which use one battery-type electrode.We are in accordance with this reasonable practice,so as to avoid further misunderstanding for readers.

3 Electrode materials

The most significant components of an electrical energy storage device (EESD) are its electrodes[62,63].Hence,the electrodes have a certain crucial position in SCs and the batteries[2].The main factor that determines the energy density and power density of an EESD is its electrode materials.At present,plenty of various kinds of electrode materials are being utilized in SCs,such as carbon-based materials,metal nitrides,metal oxides,conductive polymers,MXenes,metal organic frameworks,black phosphorus,covalent organic frameworks,sulfides and composite materials[64–68].The wide variety of available materials allows scientists to access materials with alterable characteristics.It also lets producers to fabricate SCs with controllable performance according to the requirements of the endusers.Although many efforts have been made to develop and improve performance of SCs,the discovery of the electrochemical properties of new materials does not put an end in this field.The prelude to materials with better electrochemical properties has always opened the door to the unlimited development of SCs.

The high cost,complex synthesis routes,and some shortcomings of polymers,oxides,MXenes,etc.are pushing the research directions towards utilizing low-cost and abundant materials for fabricating practical electrode materials.Carbonaceous materials such as porous carbon derived from metal-organic-frameworks (MOFs)[69],graphene[70,71],graphene oxide[72,73],graphite[74],carbon nanotubes[75],fullerene[76],and other porous carbons[77]have revealed excellent performance as SC electrodes solely or in combination with other materials.Among the above-mentioned carbon materials,porous carbons have played important roles in research and industry for further development of SCs.Porous carbons provide a large surface area exposed to the electrolyte,which is favored for the fast interfacial charge-storage reactions.More importantly,their inexpensiveness,abundance of precursors,and a wide variety of resources have made them reliable and promising candidates for further energy storage research and investment.

3.1 Biomass-derived carbons as SC electrodes

Using biomass as raw material,one can obtain different types of carbonaceous materials based on the synthesis and processing routes.Withal,it is possible to take advantage of biomass and synthesize specific carbonaceous materials such as graphene[78],graphene quantum dots[79],carbon quantum dots[80],porous carbons[81].Among several types of carbonaceous materials used in SCs,activated carbons are the most reported ones in the literature[82].Activated carbons are often intrinsically amorphous and have high porosity because of their production process and treatment[83].The properties of porous or activated carbons including surface area,pore size distribution,pore structure,chemical polarity,and surface functional groups depend greatly on the raw materials,activation agents and activation conditions[84].Unfortunately,presently,the main precursors of commercial activated carbons are fossil fuel-based precursors such as petroleum and coal,which require high cost and are not environmentally friendly.Thus,there is increasing interest in the biomass derived activated carbons due to their low cost,environmental friendliness,availability,renewability and high porosity.

Since the beginning of research on biomass-derived activated carbons,there have been numerous publications about using a large variety of biomass precursors like peanut shells[16],seaweed[86],bamboo[87],rice husk[88],waste coffee beans[89],silk[90],cassava peel waste[91],coffee endocarp[92],apricot shell[93],camellia oleifera shell[94],oil palm empty fruit bunch[95],poplar wood[96],etc.as electrode materials for EESDs.As an example,Fig.4 represents a work that the authors used soybean to prepare porous carbons and the microstructures of their synthesized activated carbons by TEM and SEM images[85].The SEM and TEM images display the abundant macro and mesopores existing in the microstructure of the synthesized activated carbons,providing suitable paths for the mobility of electrolytes and ions.

Fig.4 (a) Schematic illustration of scalable synthesis of porous carbon materials,(b) and (c) SEM images of SBC-600,(d) and (e) TEM images of SBC-600[85].Reproduced with permission.

3.2 Biomass-derived carbons:Sources,compositions and SC applications

Biomass can be used as a source of energy.Actually,it is a renewable organic material derived from live organisms (plants and animals).The saliency of biomass is its multifarious existing kinds on the Earth.Agriculture and related industries,municipal wastes,food industry wastes,purposely grown crops such as corn,sugarcane and wheat are the major sources of biomass that have the potential to be employed for electrochemical energy storage applications.The increasing human population in the world requires more food,and this food needs to be prepared via farming.This process leaves over a huge quantity of biomass as bio-waste,which if we look at it wisely,it can be converted to a cheap and good precursor for biomassderived carbonaceous materials used in different fields.Some of these biomass are bio-wastes,which are burned as fuel to generate energy (feedstock for boilers),fertilizers (after burning),mulching for oil palm plantation,while the others are left in open space,which have harmful environmental effects.On the other hand,many plants are neither eatable nor used in industries,thereby making them a suitable option as biomass precursors.

3.2.1 Chemical composition of biomass

The chemical composition,microstructure,and preparation techniques of biomass-derived carbons have a direct impact on their final composition,microstructure and efficiency.Thus,it is important to know certain variables to master the processing of biomassderived carbons for the applications as SC electrodes.

As it is observable in the Table 2,carbon,oxygen,and hydrogen are the main constituents of biomass,whereas nitrogen and sulfur are minor constituents.Besides these elements,ash and small quantities of other elements including alkali metals,alkaline earth metals,and heavy metals are other constituents of biomass.Nevertheless,in comparison with other elements,carbon has the highest ratio in biomass composition and after that oxygen and hydrogen.According to the essence of biomass,not only the building molecules are different but also the ratio of elements is variable.For example,wood residue and saw dust,which are classified as woody biomass in Table 2,have 51.4 wt.% and 32.1 wt.% carbon,respectively.Besides,the oxygen content in furniture waste and municipal solid waste varies about 30%.Even in some cases,other elements such as potassium,sodium,magnesium,and chlorine can exist in biomass.The proportion of constituent elements is a function of different variables such as environment,biomass species,geographical,and geological situation of the region and growing conditions.

It is observable in Table 2 that containing high quantities of carbon and oxygen is the characteristic of agricultural and woody biomass.These two elements contribute to the formation of more char and the high calorific value of the products[118].Algae biomass has the highest amount of hydrogen,which makes it desirable for hydrogen production.Nonetheless,its carbon quantity is not different from other biomass.Human,animal,and industrial waste biomass contain less oxygen than other types of biomass but the hydrogen content does not show any significant difference.

3.2.2 Biomass from forest plants and residues

Kim et al.prepared activated carbons from bamboo by carbonization and further activation by steam[119].Altering activation conditions led to a wide range of specific capacitance and surface area of 5 to 60 F g?1and 445 to 1 025 m2g?1respectively (Table 3).Kim’s group took benefit of KOH as an activation agent and obtained samples with specific capacitances ranging from 15 to 65 F g?1[120](more details of their work is depicted in Table 3).They found that the specific capacitance has a linear relationship with the biomass to activating agent ratio.

Table 3 reveals that in studies by Wu and colleagues,an activated carbon was prepared from firewood by steam activation at 900 °C[121].This activated carbon could achieve 96,120 and 89 F g?1specific capacitances in H2SO4,HNO3and NaNO3electrolytes,respectively.This group continued their research on the same material using other activating agents such as KOH and KOH+CO2and concluded that the BET surface area can be controlled by changing activation temperature.

In another research,the pinecone hull was activated under a CO2atmosphere at 800 °C to obtain a new microporous carbon[122].This carbon used as a lithium secondary battery anode material had a capacity of 357 mAh g?1and a coulombic efficiency of 98.9% current density of 10 mA g?1.Table 3 depicts some publication results about activated carbons derived from forest crops and its waste.It can be perceived that the synthesis route,has a significant effect on the BET specific area.Besides,the same material can output different capacitances in different electrolytes.

3.2.3 Biomass from agricultural products and agricultural wastes

Agriculture is a product of human activity,which is being done for thousands of years and cannot bestopped due to the significance of food for human beings.Besides,its waste is a global issue because of its huge worldwide quantity and its effects on the environment.Consequently,agriculture crops and agriculture wastes are cheap,available,and sustainable options as biomass sources for carbon materials used in energy storage devices.This kind of biomass is usually made of varying proportions of hemicellulose,lignin,and cellulose,which are lignocellulosic materials.Small quantities of sugar,protein,lipids,and starches can also be found in these materials[129].The molecules building lignocelluloses are strongly intermeshed and chemically bonded by both covalent linkage and non-covalent forces[130].Table 4 shows that their compositions are not the same from one plant species to another.For instance,as demonstrated in Table 4,the weight ratios of hemicellulose,cellulose and lignin in agricultural biomass vary from approximately 14%,13% and 7.5% to 40%,50% and 35%.As an example,banana waste contains a relatively low quantity of lignocellulosic constituent,while these ratios in sugarcane bagasse are around two times higher.

Table 2 Carbon,oxygen,hydrogen,nitrogen and sulfur contents in biomass obtained from different sources.

Table 3 Biomass precursors from forest crops and residues chosen for the preparation of electrode materials in capacitors.

Table 4 The proportions of hemicellulose,cellulose and lignin (Lignocellulosic constituents) of some agricultural wastes and residuals.

Fig.5 exhibits some kinds of biomass sources as precursors and their conversion to biomass-derived carbons and biofuel.In addition to biofuel production from agricultural biomass,there have been numerous publications reporting the use of agricultural waste biomass like cassava peel waste[91],sugarcane bagasse[142],apricot shell[93],sunflower seed shell[143],coffee endocarp[144],camellia oleifera shell[94],oil palm empty fruit bunch[95],argan seed shell,poplar wood[96],and peanut shell[145]as raw materials to produce activated carbons for applications as EDLC electrodes.

Fig.5 Photographic and schematic illustration of some biomass sources and their conversion to biomass carbons and biofuel.

As an example,Peng et al.[146]employed waste tea-leaves as the precursor of a porous carbon (Fig.6a).They activated the carbonized biomass with KOH and obtained an ultra-high porous activated carbon with a BET specific surface area of 2 841 m2g?1.Their samples displayed a maximum specific capacitance of 330 F g?1at a current density of 1 A g?1as well as an ideal capacitive behavior in an aqueous KOH electrolyte.Besides,this biomass-derived activated carbon exhibited an excellent cycle stability of～92% capacitance retention after 2 000 charge/discharge cycles.

All the XRD patterns of the activated carbons in Fig.6b depict a wide diffraction peak at 43°,ascribedto the (100) diffraction of graphitic carbon with the amorphous character.The high intensity of the 5 samples in the low-angle region is indicative of abundant micropores in the structure[147].Fig.6c demonstrates that the C 1s spectrum can be deconvoluted into four peaks ascribed to carbon atoms bonded to carbon and oxygen atoms.These observable four peaks,which are found in almost all activated carbons are related to O―C＝O (289.7 eV),C＝O(287.7 eV),C―O (286.4 eV),and C＝C―C bonds(284.7 eV)[148].In addition,a qualitative identification of functional groups in Fig.6d was achieved by FT-IR spectroscopy in the range of 4 000–400 cm?1.The wide band of O―H stretching vibration is observable at around 3 443 cm?1,whereas the bands at 1 050,1 438,and 1 657 cm?1are ascribed to C―O stretching of ester,C＝C vibrations in aromatic rings,and COO?anion stretching,respectively[149].The oxygencontaining moieties including pyrone-like functionalities (C―O and C＝O) on the surface of porous carbons have a significant contribution to electrochemical capacitive performance.

Fig.6 (a) Schematic of activated carbon production process from tea-waste,(b) XRD patterns of the derived activated carbons,(c) the C 1s XPS spectra of an activated carbon,(d) GCD curves of the pyrolyzed and KOH activated carbon at a constant current density of 1 A g?1,in a 2 mol L?1 KOH aqueous electrolyte[146].Reproduced with permission.

The effectiveness of activation is presented by Fig.6e,where it shows a large difference in the GCD profiles of pyrolyzed and activated carbons.Indeed,activation creates more functional moieties and a higher specific surface area exposable to electrolyte and ions,thereby positively influences the electrochemical performance of carbonaceous SC electrodes[150].The GCD curves also indicate a capacitive behavior with a small internal resistance.

Table 5 displays a number of activated carbons synthesized from agriculture biomass and compares their specific surface area,specific capacitance,and activation methods.In terms of specific surface area,activated carbons prepared from corn grains and apricot shell through KOH and NaOH activation reached ultrahigh BET specific surface area values of 3 199 and 2 335 m2g?1,respectively.Moreover,the activated carbon derived from peanut meal exhibited an extremely high specific capacitance of 525 F g?1in 1 mol L?1H2SO4electrolyte.Besides,jujube-derived activated carbon also revealed an excellent electrochemical performance as the electrode of SCs in the same electrolyte with a specific capacitance of 499 F g?1.

Table 5 Specific capacitances of agriculture-derived porous carbons reported in literature.

3.2.4 Biomass from industrial wastes

The food industry is a great industry,which produces a wide variety of by-products and residues that are capable of being used as biomass sources.This type of waste materials are generated from different steps of food processing from meat production to confectionery production and some other industries suchas cartons and newspapers.Although these industries are not directly related to the food industry,they make a huge quantity of waste materials available as biomass.

This biomass can be categorized into two types of solid and liquid wastes.The solid wastes mainly include peelings and scraps from vegetables and fruit,foods that do not pass quality control,fiber and pulp from sugar such as sugar cane bagasse and starch extraction,coffee grounds,and filter sludge.Unfortunately,these wastes are often disposed of in the environment,which is harmful to the environment.On the other hand,liquid waste comes from washing fruits,vegetables,and meat,blanching vegetables and fruits,pre-cooking poultry,fish and other kinds of meats,cleaning,and processing steps of foods and drinks such as beer making and winemaking.These waste waters contain different types of materials such as proteins,starches,sugars,and other dissolved and undissolved organic materials coming from biomass.Paper and pulp industries are the other highly polluting industries that generate large amounts of wastewater.The waste waters coming from these industries contain diverse materials like wood and other raw materials,chemicals used for processing,and other byproducts produced while processing.These industrial wastes are capable of being fermented to produce ethanol or be anaerobically digested to generate biogas.In addition,they can be employed for producing carbonaceous materials for energy storage and several other techniques,which can take the most benefit of industrial wastes.

Fig.7 shows an example of porous carbons derived from recycled waste paper[162].The authors activated waste newspaper with KOH.The amorphous nature of the synthesized activated carbons was investigated by XRD (Fig.7a).The wide peaks at 22°and 41° are related to the (100) and (111) planes,respectively.The BET specific surface area and pore size of the optimum sample were 416 m2g?1and 5.9 nm,respectively.The electrochemical performance of this activated carbon was evaluated in a 6.0 mol L?1KOH aqueous electrolyte by CV and charge-discharge tests.The Nyquist plots of the assembled SCs in a 3-electrode system in Fig.7b depict a small internal resistance (Rs) and a very low chargetransfer resistance (Rct).These data indicate a low electrical resistance and proper conductivity of the electrode materials and electrolytes.The CV curves revealed a maximum specific capacitance of 180 F g?1at a scan rate of 2 mV s?1as shown in Fig.7c.The specific capacitance vs the cycle number (Fig.7d)of the two activated carbons for 2 300 cycles shows a negligible capacity fading after cycling.

Fig.7 (a) XRD patterns of activated carbons prepared from newspaper recycled waste (1) RF gel and (2) WP carbon,(b) Nyquist plot of activated carbons,(c) CV profiles of activated carbons obtained at 10 mV s?1 and (d) long cycle-life of the activated carbons[162].Reproduced with permission.

Table 6 lists the published data about the performance and synthesis routes of industrial waste-derived porous carbons as SC electrodes.It is observable that researchers have used different industrial wastes as biomass sources and used multiple activation and pyrolysis techniques for the production of activated carbons for the use as supercapacitor electrodes.

3.2.5 Biomass from domestic wastes

Yearly,millions of tonnes of household wastes are collected with the vast majority disposed of in open fields,which can be used for energy production and energy storage applications.If the domestic wastes are lucky,they can be converted into energy by natural anaerobic digestion in the engineered landfills,otherwise,they will face direct combustion.The useable parts for the production of carbons for energy storage from biomass are plant and animal products such as cardboard,paper,grass clippings,food waste,wood,leaves,and leather products.This class of biomass is important due to its almost free cost and its contribution to a clean environment if managed properly.Managing the waste materials can also help those countries,which are suffering from a lack of space for their domestic waste disposing of.

Due to the serious challenges caused by this type of biomass and the interest of municipalities and environmental organizations managers to overcome the corresponding issues,there have been abundant publications in this area.Some of these publications are tabulated in Table 7.It is tried to gather the existing information about the preparation of electrochemical capacitors from domestic wastes.For instance,Gomaa A.et al.[172]impregnated palm kernel shell(ACPKS) with CaO derived from eggshell,as an activation agent.The heat-treated samples exhibited a highly porous honeycomb structure by taking benefit of the homogeneous distribution of CaO nanoparticles.CaO nanoparticles with the size distribution in the range of 30 to 50 nm were homogeneously distributed on the surface of the activated carbon and created porous CaO/ACPKS.The prepared porous carbons demonstrated a maximum surface area of 776.4 m2g?1.Moreover,both ACPKS and CaO/ACPKS samples showed a comparable level of mesoporosity with an average pore size equal to 3.9 and 3.5 nm,respectively.The prepared materials were evaluated as SC electrodes and showed a specific capacitance of 222 F g?1at 0.025 A g?1for CaO/ACPKS,which is three times more than that of ACPKS.

Table 6 Different biomass from industrial wastes used as precursors of carbons in SCs and their maximum specific capacitances.

Table 7 Different biomass from domestic wastes used as precursors of electrode materials in capacitors.

3.2.6 Biomass from marine sources

Marine biomass is a rather new segment of thebiomass precursors for the synthesis of carbonaceous materials.The term“marine biomass”is mostly used to refer to all types of biomass extracted or produced from oceans,seas,lakes,rivers,and such sources.China’s coastline is prolonged approximately 14 500 km extended from the Bohai Gulf in the north to the Gulf of Tonkin in the south of China.As there are many countries having long coastlines,marine bioenergy and biomass could have an interesting potential for the production and saving energy.The main difference between marine biomass with other biomass is their compositions.Marine biomass (plants)contains high contents of proteins and carbohydrates,which makes its resultant biomass-derived carbon different from other biomass growing on the land.Although there have been numerous reports about using chitosan[177],seaweed[178],marine wastes[179],fish scale[180],etc.,there is still a long way remaining for optimization and industrialization of carbons derived from marine biomass for energy storage[181].

The most well-known source of marine biomass,algae,refers to a great diversity of organisms including various species from microscopic cyanobacteria,to giant kelp.Among their attractive characteristics for biomass-derived carbon production are that they can be grown with minimal impact on freshwater resources.They do not need land for growing up.Ordinary people without any special education can grow it up,even children.Moreover,there is no need to clean water and they can be produced using saline and wastewater.These types of plants have a high flash point,are biodegradable,and relatively harmless to the environment if leftover.A wide variety of products are derived from algae such as cosmetics,health products,biofuels,and carbonaceous materials depending on the desired applications.A cheaper source of marine biomass is its waste.This waste sometimes comes from huge quantities of seaweed in oceans or seas invading beaches or of wastes and residues by the fish farm industry.

To convert marine biomass to porous carbons applicable in SCs,researchers have employed various methods such as KOH[183],ZnCl2[184],H3PO4activation[185],direct carbonization[186],and hydrothermal carbonization[187].For instance,Divya and Rajalakshmi[182]used a type of dried waste seaweed and simply pyrolyzed this raw material at high temperatures in a furnace under argon atmosphere for 3 h as shown in Fig.8a.Raman spectroscopy,which is a suitable technique to study the nature and degree of graphitization in carbonaceous materials was used in this work (Fig.8b).Generally,the ratio of the intensity disordered structure (ID) to the intensity graphitic structure (IG) reveals the degree of graphitization and the defects in the carbon network[178].The prepared carbon samples showed typicalDandGbands at 1 353 and 1 591 cm?1,respectively with aIDtoIGratio close to 1.This ratio is indicative of a highly disordered structure.The CV curves tested from 10 to 50 mV s?1scan rates depict rectangular shapes,indicative of capacitive behavior.Finally,the assembled SCs in 1 mol L?1H2SO4had a high energy density of 52 Wh kg?1at a power density of 104 W kg?1and maintained a 27.08 Wh kg?1energy density at a high power density of 520.8 W kg?1.

Fig.8 (a) Schematic illustration of the synthesis process of functional carbons from seaweed,(b) Raman spectrum of TC-900,(c) CV profiles of TC-700 at 10–50 mV s?1 and (d) Ragone plots of the assembled SCs in 1 mol L?1 H2SO4 aqueous electrolyte[182].Reproduced with permission.

4 Preparation methods of porous carbons from biomass

It is important to prepare high-quality activated carbons from biomass for electrochemical energy storage.Typically,biomass is first converted to pyrolyzed carbon in the absence of oxygen in inert gasses like (N2and Ar) or transformed into biochar through hydrothermal routes.The biomass is often pyrolyzed at a temperature of 400 to 850 °C.Then,the pyrolyzed carbon is activated through different methods such as heating in a furnace or microwave in the presence of activation agents or an activating environment.The activation can be catergoried into physical,chemical,physiochemical,and microwave activation .

4.1 Physical activation

Physical activation involves two steps.Firstly,the biomass (raw material) is converted to a carbonaceous material through pyrolysis.Secondly,additional activation with oxidizing gasses such as CO2,air,steam,and their mixtures is carried out at 600-900 °C to increase the specific surface area of the electrode active materials.

What makes CO2an interesting activating agent is its cleanness (no polluting waste) and easy handling[188].The use of a controllable activating agent such as carbon dioxide gas makes the control of product properties easier,especially around 800 °C,where the reaction rate is relatively slow[189].On the other hand,CO2cannot be offered as an activating agent for all carbonaceous materials,because different materials have dissimilar reaction rates with different activating agents as reported by Aworn et al.[190].The key role of the activation is to generate pores to increase surface area available for electrochemical reactions at the interface of electrodeelectrolyte.Physical activation happens when a part of carbon is burned off.The final characteristics are influenced by different variables.For instance,carbon burn-offs depend heavily on the temperature,time,gas type,flow rate and heating method.

Fig.9 shows the nitrogen adsorption test results of porous carbons prepared by Yuhe Cao’s group[191].It is found that the porosity of samples exhibits a maximum with the activation time by CO2gas.The porosity was deteriorated beyond 90 min.

Fig.9 (a) N2 isothermal adsorption/desorption isotherms on the ACs,(b) DFT pore size distribution curves of the ACs and (c) cumulative pore volumes of the ACs[191].Reproduced with permission.

4.2 Chemical activation

Chemical activation takes the benefit of kineticcontrolled chemical reactions in special environments.Precursors and activating agent/agents such as KOH[192],NaOH[193],K2CO3[194],H3PO4[195],ZnCl2[196],and FeCl3[197]are mixed together via wet or dry techniques in chemical activation.Later,this mixture is heated usually under an inert atmosphere at 300–950 °C and the carbonaceous materials are decomposed partially to enhance porosity.The two most used activating agents for chemical activation are potassium hydroxide (KOH)[198]and zinc chloride(ZnCl2)[199].The chemical activation method is of high efficiency for producing porous carbons from various sources including biomass[200].Nonetheless,the washing procedure with diluted acids after activation to remove residues and unreacted reactants makes this method costly,and time and energy-consuming.

Zhi Liu et al.[201]tried activating waste soybean pods using two activating agents of KOH and ZnCl2separately and found that for this precursor,KOH resulted in richer textural properties as compared with ZnCl2(Fig.10).The activated carbon obtained by KOH activation had plenty of micro-,mesopores,and interconnected macropores,with a high surface area of 2 245 m2g?1and a large pore volume of 1.35 cm3g?1.This activated carbon used as a SC electrode in 6 mol L?1KOH in a three-electrode system,showed a specific capacitance of 321.1 F g?1at a current density of 1 A g?1.This KOH activated carbon used as the electrode material in a symmetric SC device with 1 mol L?1Na2SO4neutral aqueous solution as the electrolyte,exhibited an energy density of 22.28 Wh kg?1at a power density of 450 W kg?1.Fig.8a displays the CV curves of their samples.These activated carbons showed a rectangular CV curve in a 1 V voltage window,indicative of an ideal capacitive behavior.This fact was also confirmed by the triangular GCD curves(Fig.8b).Moreover,their specific capacitance decays at high current densities were small as shown in Fig.8c.Besides,these porous carbons had small resistances(Fig.8d).

Fig.10 Electrochemical behavior of the SPAC electrodes in a three-electrode system:(a) CV tests were done at a scan rate of 50 mV s?1,(b) GCD profiles at a current density of 1 A g?1,(c) specific capacitances retention at varied current densities,(d) Nyquist plots[201].Reproduced with permission.

4.3 Microwave induced/assisted activation

In microwave heating,the carbon material is heated directly by conversion of microwave energy via dipole rotation and ionic conduction inside the particles[202].Microwave induced/assisted activation is desired for overcoming the defects of the conventional heating process in physical and chemical activation.The temperature gradient existing in conventional heating results in the inhomogeneous microstructure of the activated carbons[203].Thus,microwave heating,a way of uniform and thorough heating,is used to overcome this challenge.

In microwave heating,the thermal gradient is unlike conventional heating (Fig.11).It means that inside the particles is hot and the outer surface is cold.This method of heating is believed to be time and energy saving due to its short time required for completing the activation reactions[204].Although there have been successful reports about using microwave heating for preparing activated carbons,there is a long way to go from lab-scale to industrial production of activated carbons using microwave irradiation technique[205].Moreover,most biomass materials suffer from relatively low microwave absorption capacity when subjected to microwave radiation.As a result,these materials sometimes cannot reach the target temperature by microwave heating[206].

Fig.11 Schematic illustration of temperature gradient and direction of heat transfer (Left) microwave heating and (Right) conventional heating(red-high temperature,blue-low temperature).

4.4 Self-activation

This method does not employ any external activating agent for activation.For this purpose,the gases emitted when biomass is carbonized can be used for activation,which means converting carbonaceous biomass into activated carbons in one step.This method can also be considered as a physical self-activation technique[207].The other self-activation methods take the benefit of already existing inorganic materials in biomass structure for its in-situ activation (chemical self-activation)[208].But the frustrating point about self-activation is that controlling the quantity and composition of emitted gasses or already existing elements in the biomass structure is not as easy as physical and chemical activation techniques.

4.5 Template technique

In order to control the morphology of particles and the porosity of activated carbons for special applications,template methods are introduced and developed.Hard templating/nanocasting usually requires inorganic materials as a template such as silica for producing porous carbons from a biomass precursor[209].After activation,the hard template can be easily removed by alkaline washing.On the other hand,the soft templating methods take the benefit from polymeric reactions between molecule chains to endow particles with special shapes like spheres[210].This technique does not need any template removal step after activation.Overall,though employing a template method can lead to controlled porosity and morphology with high accuracy,the cost of applying this method is usually high.

5 Conclusions and outlook

Nowadays,SCs are considered an important type of energy storage devices with outstanding properties.They are highly desired because of their excellent performance as electrochemical energy storage devices with a long life-cycle,considerable power density,satisfying energy density and reliability.Such requirements are essential in multiple fields such as electric vehicles,fast trains,solar systems and wind power conversion systems.

More improvement and commercialization of SCs after a few decades of research and development needs cost-effective,renewable,and environmentally friendly electrode materials.These electrode materials need to possess high adsorption,high specific surface area,inherently doped elements,fast ion/electron transport,and tunable surface chemistry.Fortunately,activated carbons have all the above-mentioned features.

To prepare cost-effective porous carbons,researchers need to utilize materials that enjoy unlimited sources,low cost,facile processing,and environmentally friendliness.One of the best options to achieve this aim is to employ biomass as the precursors of activated carbons.Thus,in this paper,SCs are described basically followed by an introduction and classification of biomass originating from different sources.Then,the applications of biomass-derived porous carbons as electrodes of SCs and the corresponding activation methods are reviewed.

To synthesize activated carbons from biomass,to date,a considerable number of synthesis routes are developed.Physical activation employing air,CO2,and steam,chemical activation with some activation agents including KOH,NaOH,NaCl,ZnCl2,and H3PO4,self-activation,template method,and microwave induced method all result in different properties of the final products.Besides,each synthesis approach possess its particular pros and cons,which greatly influence the physicochemical and electrochemical properties of the obtained activated carbons.As a result,scientists and manufacturers of activated carbons from biomass ought to select the right method for synthesizing activated carbons regarding the desired electrochemical properties as SC electrodes.

The activated carbons made of biomass have been reported to possess excellent physicochemical and electrochemical properties.Some of these carbonaceous materials have exhibited BET surface areas above 3 000 m2g?1.Furthermore,their chemical composition can be precisely altered through the biomass precursor selection and synthetic route.For instance,the quantity of oxygen atoms and functional moieties in the structure as well as some doped elements such as nitrogen and sulfur are modifiable regarding the requirements.In addition,SCs made of biomass-derived activated carbons have revealed hundreds of F g?1in both aqueous and organic electrolytes.

The further development of SCs relies on the low-cost and abundant precursors for carbonaceous electrode materials.Porous carbons derived from biomass raw materials have shown to be one of the best candidates as the electrodes of high-performance SCs.As a result,scientists have been making tremendous endeavors to synthesize advanced carbon materials employing biomass from a wide variety of sources.Nonetheless,these obtained lab-scale technologies need to be scaled up to let everybody enjoy the numerous benefits of biomass-derived porous carbons.Overall,it is necessary to discover more novel,practical,and green methods for the fabrication of porous carbons from nature,satisfying the requirements of markets and industries.