Biomass-derived carbon electrode materials for supercapacitors

Abstract

This review provides a summary of recent research progress towards biomass-derived carbon electrode materials, including specific cellulose-, lignin- and hemicellulose-derived carbon electrode materials, for supercapacitors. Various lab-scale methods for preparing biomass-derived carbons, including carbonisation and/or activation conditions are discussed. Control over the pore structure, electrical conductivity, and surface functional groups of biomass-derived carbons for enhancing electrocapacitive performance is analysed. Emphasis is made on discussing cellulose-, lignin- and hemicellulose-derived carbon electrode materials for supercapacitor applications. Future research trends in this field are projected.

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1. Introduction

1.1 Supercapacitors

The development of sustainable and clean energy, such as solar and wind energy, relies significantly upon advanced energy storage systems, such as rechargeable batteries and supercapacitors (SCs).^1–3 On the other hand, the rapidly growing market for portable electronic devices requires more and more energy storage systems with a high performance.⁴ SCs with distinctive advantages such as high power density, long cycle life (>100 000 cycles), and safety⁵ will find more and more applications in the future as an alternative energy storage resource for rechargeable batteries.^6,7 A couple of excellent review articles have discussed the device configuration and energy storage mechanism of SCs.^1,5,8,9

SCs have already found wide applications as an energy storage system and/or a power source for hybrid and electric vehicles, smart grids, uninterruptible power supply (UPS) systems and many consumer electronics.¹⁰ Their excellent reliability under severe climate conditions, combined with the growing popularity of the automatic start–stop technology, enables SCs to become more and more popularly used in buses, trucks, trains, trams and metros.¹⁰ In these applications, SCs capture power from the regenerative braking system and release energy to assist in acceleration.¹¹ Besides, SCs can offer rapid storage and efficient delivery of energy in heavy-duty applications even under harsh conditions. For instance, burst power and good low temperature operation performance make SCs widely used for hybrid forklift and cranes.⁶

Recent research interest has also been focused on integrating photovoltaics (PVs) with SCs, in which the SC serves as both an energy reservoir and a power buffer.¹² As a clean and renewable energy supply, PV solar cells are expected to play a leading role in future global sustainable energy development. However, the energy converted from a solar cell is intermittent. SCs can alleviate the intermittence, making sustainable energy supply available at night and on cloudy days.¹³ In the past decade, the integration system packed as a parallel combination of PVs and SCs or a photo-supercapacitor has witnessed substantial progress at the laboratory scale.^14,15 Another interesting application of SCs is for the intelligent wireless sensor (IWS). Currently, battery-powered or wired sensors that can provide different kinds of data from our surroundings are widely used. In the near future, wireless sensors needed to be installed in remote areas or incorporated in some special structures. The energy supply of the IWS can be vibration, radiofrequency, thermal and electromagnetic energy harvesting.^16,17 To store the harvested energy for the IWS, the energy storage system needs to fulfill several properties, i.e., a sufficient number of charge and discharge cycles without deterioration during the lifetime of the device, high energy density and a low self-discharge.¹⁶

One of the key issues of SCs is their low energy density. Currently commercially available SCs can only provide an energy density of less than 10 W h kg⁻¹. This is much inferior than that from lithium-ion batteries (LIBs), which can provide an energy density of more than 180 W h kg⁻¹.^18,19 Therefore, both the scientific community and companies are eager to enhance the energy density of SCs by developing new electrode materials, novel electrolytes with a wide operation voltage window, or an ingenious device design.^18,19 In this review, we will mainly focus on the development of electrode materials from biomass.

1.2 Electrode materials for supercapacitors

SCs store energy via two mechanisms; one is the electric double-layer (EDL) mechanism and the other is the redox reaction mechanism. Thus there are two types of SCs, i.e., EDL capacitors (EDLCs) and pseudocapacitors. The electrode materials for the former are mainly activated carbons (ACs) while for the latter they can be metal oxides and conducting polymers. The electrode material plays a significant role in the electrocapacitive performance of SCs.

The criteria for evaluating SC electrode materials include specific capacitance, stability against the electrolyte and cycling, and cost. In terms of specific capacitance, the specific surface area (SSA), pore size and geometry, and electronic and ionic conductivity are important parameters to consider.^19,20

Generally, pseudocapacitive materials exhibit a higher specific capacitance than EDL electrodes; thus pseudocapacitors always have a higher energy density than EDLCs. However, carbon-based materials tend to have a better cycle stability and rate capability, and thus EDLCs can be generally operated at high charge and discharge rates with a lifetime of over a million cycles.²⁰ Therefore, carbon materials have been the focus on searching for advanced electrode materials for SCs.

1.3 Biomass and biomass-derived carbon materials

Biomass is an energy source most often refers to plants or plant-derived, animal-derived and marine organism-derived materials.^21,22 Because biomass is earth abundant, renewable, and low cost, biomass and biomass-derived materials have been exploited for various applications, such as CO₂ capture,^23,24 hydrogen storage,^25–27 water treatment,^28,29 catalysis and energy storage.^30–35 This review mainly focuses on plants or plant-derived biomass as a source for making SC electrode materials.

Plants or plant-derived biomass is often referred to as lignocellulose, which is mainly composed of carbohydrate polymers (cellulose and hemicellulose) and aromatic polymers (lignin and tannin).³⁶ Carbon materials can be prepared directly from lignocellulose or its derivatives, such as cellulose, lignin and hemicellulose. Recently, Zhang et al.²¹ reviewed the applications of cellulose and alginate for both LIBs and SCs. Jabbour et al.³⁷ summarised the progress of cellulose and cellulose derivative based LIBs. Ma et al.³⁸ reviewed carbon-based materials derived from waste for water remediation and energy storage. Dutta et al.³⁹ presented biomass-biopolymer derived hierarchically porous carbons (HPCs) for applications ranging from CO₂ capture and carbon photonic crystal sensors to Li–S batteries and SCs. In this article, we provide an overview of biomass-derived carbon materials as electrodes for SCs.

2. Biomass-derived carbon for supercapacitors

2.1 Methods

To convert biomass into carbon, different carbonisation methods (e.g., pyrolysis and hydrothermal carbonisation) and activation methods (e.g., physical and chemical activations) can be used. As illustrated in Scheme 1, both physical and chemical means, or a combination of both, have been employed to transfer biomass into value-added carbon materials. By controlling parameters such as temperature, time, and reagents involved in the preparation, carbon materials with different porosities, surface properties, morphologies, and costs can be obtained.

Scheme 1 Common methods for converting biomass to carbon materials.

Pyrolysis and hydrothermal carbonisation (HTC) are the two common methods used to carbonise biomass. Pyrolysis is carried out in an inert or limited oxygen atmosphere at elevated temperatures while HTC refers to a thermo-chemical process used to convert biomass to carbonaceous materials.^40,41

The main pyrolysis products obtained from biomass depend on the temperature, temperature ramping rate, particle size and catalyst used.⁴² HTC is performed in a pressurized aqueous environment at a relatively low temperature, typically in the range of 120–250 °C,⁴³ with or without the aid of a catalyst.⁴⁴ It mimics the natural coalification of biomass, although the reaction rate is higher and the reaction time is shorter compared to the hundreds of years of slow nature coalification of biomass.⁴⁰ In recent years, several review articles regrading hydrothermal conversion of biomass have been published.^30,41,43,45 As a thermo-chemical conversion technique, the HTC can be influenced by several parameters, such as temperature, residence time, precursor concentration and catalyst. It uses subcritical water for the conversion of a biomass to carbonaceous products, resulting in efficient hydrolysis and dehydration of precursors and bestowing hydrochars with a high and tunable content of oxygen-containing functional groups (OFG).⁴¹ Other functionalities, e.g., nitrogen-containing groups, can also be introduced into hydrochars by using dopant-containing precursors or additives.⁴⁵ The use of HTC for the conversion of biomass into carbon materials has received considerable attention for various applications such as catalysis,^46,47 CO₂ capture,^48,49 and energy storage.^43,50–55

Activation is a process of converting carbonaceous materials into AC. Both physical and chemical activation means can be used.⁵ Physical activation is usually conducted immediately after the pyrolysis step at high temperatures (up to 1200 °C) in an atmosphere of steam or CO₂.²⁵ Chemical activation is implemented with a chemical agent at a temperature typically ranging from 450 to 900 °C. The commonly used chemical activation agents include KOH, NaOH, ZnCl₂, FeCl₃, H₃PO₄, and K₂CO₃.

2.2 Carbonisation

2.2.1 Pyrolysis. Porous carbon materials from biomass are usually obtained by pyrolysis with subsequent activation to increase the SSA and pore volume.^1,8,56 Research has also shown that high-performance carbon electrode materials can be obtained by direct pyrolysis of biomass without the activation step.^51,57–61 Béguin and co-workers^57,58 reported a high surface area SC carbon electrode material prepared from waste seaweeds without activation. Biswal et al.⁵⁹ demonstrated the preparation of a high SSA microporous carbon without using the activation step. The plant-leave-derived carbon exhibited a high specific capacitance of 400 F g⁻¹ at a current density of 0.5 A g⁻¹ in aqueous 1 M H₂SO₄ electrolyte. These biomass precursors all have a relatively high content of impurities such as Na, K, Ca, and Mg, which are known to be good porogens.⁴⁴ These impurities can act as an activation reagent during pyrolysis.

2.2.2 Hydrothermal carbonisation (HTC). The HTC process yields a partially carbonised product, called hydrochar, which has a high density of oxygen-containing groups and a low degree of condensation.⁴¹ Such HTC hydrochars have been directly used as electrodes for SCs.^51,62–64 However, the HTC-derived hydrochars possess poorly developed porosity with a low SSA. Thus, subsequent carbonisation/activation is needed for improving the material's physical and chemical properties.^{44,50,52–55,65–70}

Zhu et al.⁶⁵ proposed and demonstrated a hydrothermally assisted pyrolysis procedure to produce fungi-derived electrode materials for SCs. By HTC at 120 °C for 6 h, the product was characterised by small particle sizes (50–200 nm), high oxygen content (13.4 wt%) and a low surface area (14 m² g⁻¹). To increase the porosity and improve the electronic conductivity, the hydrochar was further pyrolysed at 700 °C for 3 h. The ultimately obtained carbon material had a SSA of 80 m² g⁻¹ and an oxygen content of 5 wt%. In spite of the still low SSA after the pyrolysis, the material exhibited a specific capacitance of 196 F g⁻¹ at a scan rate of 1 mV s⁻¹, a value comparable to that of commercial ACs such as Maxsorb.

Actually, highly porous carbon materials based on hydrochar normally have a relatively high amount of heteroatoms.^71–73 Thus, the hydrochar from the HTC process is an excellent precursor for the production of carbon with tuneable surface functionality and porosity.⁷⁴ Torres et al.⁷⁵ prepared AC materials doped with N and O heteroatoms by activation of hydrochars with KOH. The presence of nitrogen and oxygen groups was found to improve both the capacitance and charge transfer especially at high current densities. Wei et al.⁷⁰ demonstrated that hydrochars with networks of uniformly distributed oxygen can be efficiently transformed into microporous carbons with a high SSA and large pore volume of interconnected pores. In their experiment, the transformation was accomplished via HTC at 230–250 °C for 2 h and subsequent chemical activation at 700–800 °C for 1 h. The carbon electrode produced from wood saw dust exhibited both a high capacitance of 236 F g⁻¹ and a rapid charge/discharge capability in a symmetric two-electrode SC using an organic electrolyte.

2.3 Activation

Chemical and physical activation are the two mainly employed methods to obtain biomass-derived ACs. Compared with chemical activation, the latter one is simpler and more environmentally friendly but usually conducted at a higher temperature. Currently, chemical activation is more commonly used because of lower activation temperature, less activation time, higher yield, higher SSA and well-developed pores. Anyhow, the chemical activation method does have several disadvantages, such as the need for water for the necessary post-activation washing to thoroughly wash away any impurities generated during the activation process and the handling of the contaminated water.^20,22,76,77

2.3.1 Physical activation. Physical activation is generally conducted in the presence of a gas, mainly including air, steam, or CO₂. Air activation is usually conducted under relatively low temperature, <500 °C. Recently, Mitlin and co-workers described the preparation of high-performance carbon electrode materials using chicken eggshell membranes as the precursor, which has a high content of nitrogen and a continuous network formed by interwoven and coalescing carbon fibers.^44,78 They combined the pyrolysis/air activation methods to maintain the interconnected porous structure of the eggshell membranes. The carbonisation was conducted at 800 °C for 2 h and the activation was done in air at 300 °C for 2 h. The air activation increased the SSA to 221 m² g⁻¹. The macroscopic porous structure was preserved. The resulting electrode material with 8 wt% nitrogen displayed a specific capacitance of 297 F g⁻¹ in 1 M KOH electrolyte and a cycling efficiency of 97% after 10 000 cycles. The air activation step has also been shown to significantly influence the porous properties of biomass-derived carbon.^79–81

Steam is a readily available activating agent used for biomass materials due to its low cost and the lack of post-activation process to remove by-products.⁸ The steam activation process is always combined with pyrolysis as a single step. It can generate rich surface oxygen-containing groups (defects), which lead to a poor electrical conductivity of the resulting carbon. Some researchers have systematically studied steam activation of biomass.^82–87 Li's group studied the influence of the steam activation time and water flow rate on the texture and electrochemical performance of ACs derived from coconut shell⁸⁶ and corncob residue.⁸⁵ They found that the mesoporosity increased considerably with the increase of activation time and water flow rate, which enabled the sample to have high rate capability and cycle stability. Jin et al.⁸⁷ investigated in detail the effect of steam activation time on the porosity and surface area of the activated carbon fibres (ACFs). Steam activation of liquefied wood with various activation times (20, 60, 100, 140, 180 and 220 min) was conducted. By controlling the activation time, the mesopore/micropore ratio could be effectively tuned and the micropore and mesopore surface area increased with the rising activation time before burn-off. The sample with the highest micropore surface area and a relatively high proportion of mesopores in the range of 3–4 nm presented a specific capacitance of 280 F g⁻¹ at 0.5 A g⁻¹ in 0.5 M H₂SO₄ and excellent rate performance as well as good cyclic stability.

CO₂ activation, which is based on the controlled gasification of a char with CO₂ gas at a high temperature, is the most commonly used physical activation process. Studies have shown that smaller sized microporous carbons exhibit a larger capacitance.⁷ However, CO₂ activation-derived carbon tends to have a high fraction of mesorpores and an increased average pore size, as the large dimension of CO₂ molecules tends to restrict the accessibility of CO₂ into micropores.^{8,74,84,88,89} Besides, researchers found that, compared with KOH activation, as seen in Fig. 1, CO₂ activation could result in a little higher degree of graphitisation with apparently oriented multilayer domains and graphene sheets stacked in parallel in the structure of the porous carbons.⁹⁰

Fig. 1 XRD patterns of the activated carbon materials with CO₂ and KOH activation. The inset is a sketch map for the calculation of the R values.⁹⁰

Systematic research has been conducted focusing on CO₂ activation of polymer-derived carbons⁸⁴ and hard/soft-template carbons^89,91 as well as their applications for SCs^92,93 and CO₂ adsorption.^94,95 However, research focusing on CO₂ activation of biomass-derived carbons for SCs is still relatively less.^96,97

Compared with steam and CO₂ activation, air activation needs a lower temperature. But the former two are more commonly used than the latter as the ACs produced with steam or CO₂ tend to have a wider pore size distribution.^98,99 Research on the former has further shown that steam activation produces a larger development of mesopores and macropores than CO₂ activation.^98,100 However, research for physical activation towards biomass is still relatively insufficient as chemical activation is more commonly used. For instance, Osswald⁸⁸ and Yan⁹¹et al. have systematically investigated the porosity, SSA and stability of carbide-derived carbon materials physically activated using CO₂, air or steam. By contrast, such detailed research on physical activation for biomass is little, especially combined with their further application for SC electrodes.

2.3.2 Chemical activation.
2.3.2.1 KOH activation. KOH is a most often used chemical for activating biomass-derived carbons. A rough carbon surface could be induced by the activation of KOH, which brings about a high SSA and a porous structure that are advantageous in charge storage.¹⁰¹ The detailed mechanism of KOH activation has not been fully understood because of the complex variables including the experimental parameters as well as the reactivity of different carbon precursors.^90,102–105 In a general view, the development of a large SSA and high porosity in KOH-activated carbons is the result of the synergistic, comprehensive actions, including chemical activation and carbon lattice expansion by metallic K intercalation.⁷⁶

For a given carbon precursor, experimental variables of KOH activation include the mass ratio of KOH/biomass, heating rate, activation temperature and time. The normally adopted variables are the following: (1) the KOH/biomass mass ratio ranges from 2 to 5; (2) a heating rate of 3–10 °C min⁻¹; (3) the activation temperature and time are 550–900 °C and 1–4 h, respectively. Karthikeyan et al.¹⁰⁶ studied the chemical activation of pine cone petal powders with KOH/biomass mass ratios of 1, 3 and 5, respectively. The mixture was then pyrolysed at 750 °C under an Ar flow at a heating rate of 5 °C min⁻¹ for 1.5 h. The highest SSA was obtained from the sample prepared with a KOH/biomass ratio of 5. A symmetric SC fabricated with this carbon showed an energy density of ∼61 W h kg⁻¹ at a power density of ∼0.39 kW kg⁻¹ with an excellent capacitance retention of ∼90% after 20 000 cycles in an organic electrolyte.

KOH-activated, in a single or two-step process, biomass-derived carbons of good performance have been reported. Li et al.¹⁰⁷ described a one-step process for the preparation of nitrogen-doped activated carbon with corncob, KOH, and NH₃ as the carbon source, activating agent and nitrogen source, respectively. The corncob powders were mixed with KOH in a 1 : 3 mass ratio. Then the mixture was heated to the desired temperature under a N₂ or NH₃ flow for a certain time. The obtained sample had a narrow micro- to meso-pore distribution ratio and showed a high SSA of 2900 m² g⁻¹ with a moderate N content of 4 wt%, and delivered a specific capacitance of up to 185 F g⁻¹ in an organic electrolyte at a current density of 0.4 A g⁻¹.

In comparison, the two-step KOH activation method is more often used to prepare biomass-derived carbon materials for SCs. Biomass is always pre-treated by HTC,^{52–55,67,70,73,75,108} or pre-carbonisation^68,109–112 or pyrolysis^78,113–120 before KOH activation. Qian et al.¹⁰⁹ prepared heteroatom doped porous carbon flakes from human hair fibres using the two-step KOH method, combining pre-carbonisation with KOH activation. In their experiment, hair fibres were firstly pre-carbonised at 300 °C for 1.5 h and then mixed with KOH (W_KOH/W_carbon = 2 : 1) and further activated at 700, 800 or 900 °C, respectively. In the pre-carbonisation step, some surface functional groups or unstable component of human hairs, which act as active sites in the chemical activation with KOH, are likely to decompose, with a moderate amount of N, O and S retained.^68,112 The material activated at 800 °C has a SSA of 1306 m² g⁻¹ with a doping of N, O and S, 4.38, 5.39, 1.51%, respectively. The temperature of pre-carbonisation noticeably affects the chemical composition, surface area and porosity development.^111,112 Further research is needed to study the influence of the pre-carbonisation temperature on the performance of biomass-derived carbons as SC electrodes.

Hou et al.¹¹⁶ prepared carbons with a micro/mesopore interconnected structure through pyrolysis and KOH activation in a two-step process. Rice brans were carbonised under a N₂ atmosphere at a rate of 3 °C min⁻¹ from room temperature to 700 °C for 1 h and was then KOH-activated at 850 °C for 1 h. Porous carbons with a SSA of 2475 m² g⁻¹ and a pore volume of 1.21 cm³ g⁻¹ (40% for mesopores) were obtained. It exhibited high specific capacitance especially at large current densities in 6 M KOH electrolyte, i.e., 265 and 182 F g⁻¹ at 10 and 100 A g⁻¹ respectively. Moreover, an energy density of 70 W h kg⁻¹ and a power density of 1223 W kg⁻¹ were obtained in an ionic liquid (IL). Wang et al.¹¹⁴ used waste celtuce leaves to prepare porous carbons. Pyrolysed at 600 °C, followed by KOH activation at 800 °C for 1 h, the as-prepared carbon has a high SSA of 3404 m² g⁻¹ and a large pore volume of 1.88 cm³ g⁻¹. As an electrode, it exhibited specific capacitances of 421 and 273 F g⁻¹ at a current density of 0.5 A g⁻¹ in three and two-electrode systems, respectively.

Conventionally, precursors and solid KOH, with different mass ratios, are thoroughly blended or ground in an agate mortar and then carbonised to a certain temperature at a certain rate and maintained for several hours.^{52–54,68,70,73,75,78,106–109,112,113,116–118,120,121} Some researchers have tried to modify the conventional method by impregnation of the precursor with KOH aqueous solution and some good results were obtained.^{67,110,111,114,115,119,122–125} Precursors were first pyrolysed or pre-carbonised and then dispersed and stirred in aqueous KOH with different mass ratios of KOH/C, followed by an evaporation step until a stable slurry or colloidal solution was obtained. Subsequently, the mixture was annealed under the same conditions of the conventional method. Ruoff's¹²⁵ and Huang's¹²² groups both used such a modified KOH activation method in their experiments, using graphene oxide and polypyrrole micro-sheets as the precursor respectively. It is believed that the phase separation between hydrophobic carbon and water during the activation process leads to both mesopores and macropores. Therefore 3D HPCs of high SSA with excellent electrocapacitive performance were obtained. However, detailed research on modified KOH activation using biomass as the precursor is still less.

2.3.2.2 ZnCl₂ activation. ZnCl₂ is another commonly used chemical activation agent for converting biomass-derived carbons into porous ACs.^126–136 It acts as a dehydrating agent during the activation process and it also has a deoxygenation effect at high temperatures by removing oxygen in the form of water as well as by carbothermal reduction.^137,138 ZnCl₂-activated biomass-derived carbons of high performance, through a simple one-step process, have been reported.

Rufford et al.¹³³ employed the ZnCl₂ activation method to prepare porous carbon with a SSA as high as 1000 m² g⁻¹. The carbon prepared at 750 °C with a ZnCl₂ to sugar cane bagasse weight ratio of 1 delivered the highest specific capacitance at low current densities. At current densities greater than 1 A g⁻¹, however, the carbon with mesopores that was prepared at 900 °C with a ZnCl₂ to bagasse ratio of 3.5 showed the most stable electrochemical performance. These results demonstrate the benefit of mesopores to energy storage at fast charge–discharge rates, i.e., acting as reservoirs for electrolyte ions and facilitating ion transport through the carbon pore network.¹³⁹

Hou and co-workers¹²⁹ prepared hierarchically porous nitrogen-doped carbon via simultaneous ZnCl₂ activation and graphitisation, in a one-step process. Natural silk was mixed with ZnCl₂ and FeCl₃ solution in a certain ratio, followed by annealing at 900 °C for 1 h. The as-obtained carbon consisted of 2D nanosheet architecture with a hierarchical porosity, high SSA (2494 m² g⁻¹), rich N-doping (4.7%), and defects. The synergistic effect of these characteristics enables the as-obtained carbon to display high energy storage performance. Tested in an IL electrolyte two-electrode system, it exhibited a capacitance of 242 F g⁻¹, an energy density of 90 W h kg⁻¹ at a power density of 875 W kg⁻¹ and high cycling life stability (9% loss after 10 000 cycles).

2.3.2.3 Other activation reagents. Besides KOH and ZnCl₂, several other reagents are also used for biomass chemical activation for SCs, such as H₃PO₄ (ref. 140–142) and KHCO₃.¹⁴³ Some researchers have also employed microwave-induced activation^144–148 or physicochemical activation^149–157 consisting of a chemical activation step followed by physical activation to further control the porosity development and tune the PSD of activated carbons.^22,25 However, there is still relatively less research on their application for biomass-derived SC electrodes.

KOH, ZnCl₂ and H₃PO₄ are the three mainly used chemical activation agents. In comparison, H₃PO₄-activated carbons usually have a relatively low SSA of below 1000 m² g⁻¹ while both KOH and ZnCl₂ activation can easily produce a higher SSA. To enhance the SSA, KOH is a kind of oxidant, while ZnCl₂ is a dehydrating and deoxygenation agent. Currently, KOH-activation is preferred to be used for biomass-derived ACs for SC electrodes as it can easily produce hierarchical porous carbons with an even high SSA of beyond 3000 m² g⁻¹ (Table 1).

Table 1 Summary of biomass-derived carbon electrodes for SCs through pyrolysis and/or activation

Materials	Pre-activation treatment	Activation agent	Activation temperature and time (°C)/(h)	SSA (m^2 g^−1)	C (F g^−1) (symmetric SCs)	Measurements at	Electrolyte	Ref.
Seaweed	Pyrolysis	NA	NA	746	264	2 mV s^−1	1 M H2SO4	58
Eggplant	Freeze drying/pyrolysis	NA	NA	950	121	5 mV s^−1	6 M KOH	60
Dead Neem leaves	Pyrolysis	NA	NA	1230	400	0.5 A g^−1	1 M H2SO4	59
Eggshell membranes	Pyrolysis	Air	300/2	221	205	2 A g^−1	1 M H2SO4	78
Corncob	NA	Steam	850/0.75	1210	120	1 A g^−1	6 M KOH	85
Wood	NA	Steam	850/3	3223	247	0.5 A g^−1	1 M H2SO4	87
Coconut shell	NA	Steam	800/1	1532	192	1 A g^−1	6 M KOH	86
Coffee endocarp	Pyrolysis	CO2	800/2	709	176	—	1 M H2SO4	97
Fungi	HTC/pyrolysis	NA	NA	80	196	1 mV s^−1	6 M KOH	65
Wood saw dust	HTC	KOH	800/1	2967	236	1 mV s^−1	TEABF4/AN	70
Pollen	HTC	KOH	900/1	3037	185	1 A g^−1	TEABF4/AN	54
Tobacco rods	HTC	KOH	800/1	−2000	263	0.5 A g^−1	6 M KOH	52
Microalgae	HTC	KOH	700/—	2130	200	0.1 A g^−1	6 M LiCl	73
Human hair	Pre-carbonisation	KOH	800/2	1306	340	1 A g^−1	6 M KOH	109
Cornstalk core	Pre-carbonisation	KOH	800/3	2139	317	1 mV s^−1	6 M KOH	110
Bean dregs	Pre-carbonisation	KOH	700/1	2876	280	0.1 A g^−1	1 M H2SO4	68
Rice bran	Pyrolysis/600	KOH	850/1	2475	323	0.1 A g^−1	6 M KOH	116
Ginkgo shells	Pyrolysis/600	KOH	700/1	1775	237	2 mV s^−1	6 M KOH	113
Celtuce	Pyrolysis/600	KOH	88/1	3404	273	0.5 A g^−1	6 M KOH	114
Broad beans	Pyrolysis/800	KOH	650/1	655	202	0.5 A g^−1	6 M KOH	115
Pine cone petal	NA	KOH	750/1.5	3850	198	0.25 A g^−1	1 M LiPF6	106
Corncob	NA	KOH/NH3	400/—	2900	185	0.4 A g^−1	1.2 M LiPF6	107
Silk fibroin	NA	KOH	800/3	2557	264	0.1 A g^−1	1 M H2SO4	101
Coffee beans	NA	ZnCl2	900/1	1021	134	0.05 A g^−1	1 M TEABF4/AN	131
Coffee beans	NA	ZnCl2	900/1	1019	368	0.05 A g^−1	1 M H2SO4	132
Sugar cane bagasse	NA	ZnCl2	900/1	1788	300	0.25 A g^−1	1 M H2SO4	133
Banana fiber	NA	ZnCl2	800/1	1097	296	0.5 A g^−1	1 M Na2SO4	126
Chestnut shell	NA	ZnCl2	800/1.5	1987	92	10 A g^−1	6 M KOH	127
Silk	NA	ZnCl2	900/1	2494	242	0.1 A g^−1	EMIMBF4	129
Coconut shell	NA	ZnCl2	900/1	1874	276	1 A g^−1	6 M KOH	130
Coffee bean	NA	H3PO4	800/0.5	742	160	1 A g^−1	1 M H2SO4	140
Cotton stalk	NA	H3PO4	800/2	1481	114	0.5 A g^−1	1 M TEABF4/AN	141
Bamboo	NA	KHCO3	800/1	1425	187	0.5 A g^−1	6 M KOH	143
Rice husk	NA	Microwave-assisted ZnCl2	600 W/1/3	1552	94	0.05 A g^−1	1 M Et4NBF4/PC	146
Bagasse pith	NA	Microwave-assisted ZnCl2	700 W/0.25	—	138	0.2 A g^−1	1 M EMIMBF4	158
Cassava peel	NA	KOH/CO2	800/3	1186	264	—	0.5 M H2SO4	153
			800/1
Oil palm	Pre-carbonisation	KOH/CO2	800/—	1704	150	—	1 M H2SO4	157
			800/3

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2.4 Structure control

High SSA biomass-derived activated carbons contain predominantly micropores with a high pore tortuosity, which poses a large resistance to ion transport, leading to poor power density and rate capability of SC devices.^25,159 In addition, pore geometry and dimension as well as electrical conductivity are also important parameters determining SC performance.^143,160,161 Therefore, pore structure control and graphitisation degree are important for the electrocapacitive performance of biomass-derived activated carbons.¹⁶²

The pore structure of carbon materials can be made to have hierarchical pores in two dimensions (2D) or three dimensions (3D) to facilitate ion transport and provide a robust interface for charge storage.¹⁶² The transport behaviour of electrolyte ions in pores is significantly determined by pore length, pore size and tortuosity. The ion transport time (τ) is given by equation τ = L²/d,¹⁶³ where L refers to the ion transport distance and d is the ion transport coefficient. A porous carbon with macropores, mesopores and micropores well-distributed in 2D or 3D with low-resistant ion-transport paths is ideal for EDLCs. This enables active ions in micropores to have nanometre transport distances from adjacent mesopores and macropores, thus shortening the transport time.^139,162,164 Through careful control over the carbonisation and activation conditions, biomass-derived carbon electrodes with 2D^{53,129,164–167} or 3D^{78,116,118,143,168–170} hierarchical pore structures of high SSA have been reported, as shown in Table 2, which exhibited both high specific capacitance and excellent rate capability.

Table 2 Rate capability of biomass-derived carbon electrodes for SCs versus pore structure

Precursor	SSA (m^2 g^−1)	Pore structure	S meso+macro/St	V meso+macro/Vt	C 1 (F g^−1) (symmetric SCs)	C 2 (F g^−1) (symmetric SCs)	Rate capability	Electrolyte	Ref.
Silk proteins	2557	Microporous	0.34	—	264 (0.1 A g^−1)	162 (6.2 A g^−1)	61%	1 M H2SO4	101
Broad beans	655	Rich in micropores	—	—	202 (0.25 A g^−1)	129 (10 A g^−1)	63%	6 M KOH	115
Sucrose	2094	Microporous	—	—	224 (0.2 A g^−1)	91 (20 A g^−1)	41%	6 M KOH	212
Nutshell	1069	2D microporous	—	0.17	261 (0.2 A g^−1)	97 (8 A g^−1)	37%	6 M KOH	165
Acacia gum	1832	Microporous	0.11	0.19	272 (1 A g^−1)	160 (10 A g^−1)	59%	6 M KOH	213
Glucose	2600	Oriented and interlinked 2D hierarchical porous	0.31	0.69	257 (0.5 A g^−1)	184 (100 A g^−1)	72%	6 M KOH	166
Corn gluten meal waste	3353	Interconnected meso/microporous	—	—	298 (0.5 A g^−1)	215 (10 A g^−1)	72%	6 M KOH	214
Bagasse	2296	Hierarchical porous	0.26	0.33	180 (0.2 A g^−1)	128 (15 A g^−1)	71%	6 M KOH	215
Bamboo-based industrial by-products	1472	Beehive-like hierarchical nanoporous	—	0.21	301 (0.1 A g^−1) (three electrode)	192 (100 A g^−1) (three electrode)	64%	6 M KOH	216
Corn husk	867	3D hierarchical porous	0.14	0.27	260 (1 A g^−1)	228 (10 A g^−1)	88%	6 M KOH	176
Artemia Cyst shells	1758	3D hierarchical porous	0.21	0.30	369 (0.5 A g^−1) (three electrode)	334 (10 A g^−1) (three electrode)	91%	1 M H2SO4	217
Rice bran	2475	3D porous	0.15	0.39	323 (0.1 A g^−1)	265 (10 A g^−1)	82%	6 M KOH	116
Waste wood shavings	3223	Porous carbon fibre	0.29	0.49	247 (0.5 A g^−1)	227 (10 A g^−1)	92%	1 M H2SO4	87
Gelatin	3012	Hierarchical porous	—	0.33	385 (0.05 A g^−1)	281 (5 A g^−1)	73%	6 M KOH	218
Lignin-derived by-products	2218	Interconnected hierarchical porous	—	—	312 (1 A g^−1)	254 (80 A g^−1)	81%	6 M KOH	207
Lignin	907	3D hierarchical porous	0.15	0.21	165 (0.05 A g^−1)	124 (10 A g^−1)	75%	1 M H2SO4	168

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Using glucose as a precursor, Zheng et al.¹⁶⁶ prepared 2D porous carbon nanosheets with an excellent capacitive performance. During the activation process, potassium species acted as not only an activator but also a melt template that led to the oriented nanosheet structure. The obtained 2D porous carbon exhibited a specific capacitance of 257 F g⁻¹ at a current density of 0.5 A g⁻¹ and it was maintained at 184 F g⁻¹ at 100 A g⁻¹. The large amount of micropores and small mesopores led to a high SSA of around 2600 m² g⁻¹ which gave rise to the high specific capacitance. The interlinked 2D hierarchical porous structure facilitated ion transport, thus enabling the ultrahigh rate capability. Hao et al.¹⁶⁹ prepared 3D hierarchical porous carbon electrode materials using bagasse as the raw material. They assembled solid state symmetric SCs and due to the advantages of the aforementioned hierarchically porous structure, a comparable high specific capacitance of 142 F g⁻¹ at a current density of 0.5 A g⁻¹ was obtained. The solid state SCs displayed a good rate capability and an excellent capacitance retention of 93.9% over 5000 cycles.

Improving the graphitisation of AC simultaneously enhances the electric conductivity of the electrode and its surface wettability towards aqueous electrolytes, which can facilitate ion diffusion and electron transfer, thus improving the electrochemical performance.¹³⁰ High-temperature treatment can enhance graphitisation but it is of high energy consumption. Besides, it will also decrease the SSA and pore volume of the AC.

Catalytic graphitisation by means of a transition metal is an effective way to obtain ACs with a certain graphitisation degree.¹¹³ Coupling chemical activation with catalytic graphitisation enables the preparation of porous carbons with a high SSA and an excellent electrocapacitive performance.^{53,113,128–130,167} Sun et al.¹³⁰ synthesised porous graphene-like nanosheets (PGNSs) with a large SSA via a simultaneous activation-graphitisation route using coconut shell as the precursor. FeCl₃ and ZnCl₂, functioning as a graphitic catalyst and activating agent respectively, were simultaneously introduced into the skeleton of the coconut shell through coordination of the metal precursors with the functional groups in the coconut shell, thus simultaneously carrying out activation and graphitisation. The obtained PGNSs possessed good electrical conductivity due to a high graphitisation degree, a SSA of 1874 m² g⁻¹ and a pore volume of 1.21 cm³ g⁻¹. With no addition of conductive additives, it exhibited a specific capacitance of 268 F g⁻¹ at 1 A g⁻¹ in KOH electrolyte. Besides, it also displayed a capacitance of 196 F g⁻¹ at 1 A g⁻¹ in an organic electrolyte. An energy density of 54.7 W h kg⁻¹ was obtained at a power density of 10 kW kg⁻¹ (Fig. 2).

Fig. 2 HR-TEM (a), Raman spectra (b), rate performance (c) and Ragone plot (d) of PGNS.¹³⁰

2.5 Heteroatom-doped biomass-derived carbon

It has been a widely accepted strategy to enhance the specific capacitance of a carbon electrode without sacrificing the high rate capability and long cycle life through doping with heteroatoms, such as N, O, P, S, and B. Some researchers obtained heteroatom-doped biomass-derived carbon by mixing a biomass precursor with other heteroatom-containing precursors^171–173 or through post-treatment.¹⁷⁴

However, as a naturally available resource, the most interesting and convenient spot for biomass is that it can be transformed into heteroatom-contained carbon by a simple one-step calcination without the addition of any other precursors.^{52,67,114,115,129,175–177}

Hou et al.¹²⁹ prepared nitrogen-doped hierarchically porous carbon nanosheets via simultaneous activation and graphitisation, using natural silk. The carbon material exhibited good performance for both SC and lithium-ion batteries. Xu et al.¹¹⁵ prepared sulfur and nitrogen dual-doped porous carbon materials using broad bean shells which are abundant in amino acids and vitamins as the precursor. Broad beans were thermally treated at 800 °C for 2 h and then activated with a KOH ethanol solution at 650 °C for 1 h under nitrogen with a heating rate of 3 °C min⁻¹. The doped sulfur increased the space utilisation by a specific electrosorption of electrolyte ions. The incorporation of nitrogen increased the electrical conductivity as well as the wettability of the electrode. In addition, both sulphur- and nitrogen-containing groups contributed to pseudocapacitance. Therefore, the prepared sample exhibited a specific capacitance of 202 F g⁻¹ at a current density of 0.5 A g⁻¹ in 6 M KOH aqueous electrolyte and maintained 129 F g⁻¹ at 10 A g⁻¹ in spite of the moderate SSA (655 m² g⁻¹) of the carbon.

3. Lignocellulose-derived carbon

During the past decade, biomass especially lignocellulose has attracted researchers' huge interest and research worldwide.^21,58 However, the dependence of electrochemical performance upon the composition of lignocellulose is still unclear. Therefore, it is necessary and a trend to separately study cellulose, lignin and hemicellulose, three main components of lignocellulose, for the transformation and application of biomass into SC electrodes.¹⁷⁸ Some researchers have conducted such research to some extent and in the following part, we will give them a brief review separately.

3.1 Cellulose

3.1.1 Properties of cellulose. As the main skeleton component of lignocellulose, cellulose is one of the most important natural polymers. It is almost inexhaustible and can be mass produced on an industrial scale.¹⁷⁹ Cellulose exists in various forms from micrometric cellulose fibres to nanocellulose and water/solvent soluble cellulose derivatives. Nanocellulose, cellulose with one dimension in the nanometre range, has attracted an ever-increasing attention. Formed by the repeated connection of D-glucose, nanocellulose has a number of important properties such as hydrophilicity, high strength and stiffness, broad chemical-modification capacity, biodegradability and renewability, as well as some specific features of nanomaterials.^21,179–181 Based on dimensions, functions and preparation methods, nanocellulose can be classified in three main subcategories, i.e., microfibrillated cellulose, nanocrystalline cellulose and bacterial nanocellulose.¹⁷⁹

3.1.2 Cellulose-derived carbon for supercapacitors. Currently, cellulose-derived carbons for SC electrodes have been researched mainly through two directions. One is the direct utilisation of commercial cellulose as the carbon precursor,^{62,70,71,75,182–185} the other is first isolating cellulose from lignocellulose by different extraction processes and then researching their electrochemical performance as electrodes after transforming them into ACs.^{169,176,186,187}

Sevilla and coworkers^62,70,71,75 produced activated carbon materials with cellulose as the carbon precursor. By HTC at 230–250 °C for 2 h and subsequent KOH activation at 700–800 °C for 1 h, the carbon electrode produced displayed a capacitance of 140 F g⁻¹ at 10 mV s⁻¹ and an excellent rate capability. Tam et al.¹⁸³ conducted a one-step pyrolysis on melamine-formaldehyde cellulose nanocrystals. The sample pyrolysed at 900 °C displayed a capacitance value of 352 F g⁻¹ at a current density of 5 A g⁻¹ in a three-electrode system with 1 M H₂SO₄ electrolyte. Ji's group also presented a simple one-step fabrication methodology for nitrogen-doped nanoporous carbon membranes via annealing cellulose filter paper under NH₃.¹⁸⁸ They discovered that the doped nitrogen played an important role in the activation of carbon under NH₃, leading to a large SSA. Nitrogen doping (up to 10.3 at%) occurred during cellulose pyrolysis under NH₃ at as low as 550 °C. At 700 °C or above, N-doped carbon further reacted with NH₃, resulting in a large SSA. Compared with conventional AC (1533 m² g⁻¹), the N-doped nanoporous carbon (1326 m² g⁻¹) exhibited more than double the unit area capacitance (90 vs. 41 mF m⁻²).

An interesting route recently has been attracting researchers' increasing interest, i.e., dissolving cellulose in other solutions, especially in NaOH/urea solution.^184,185,189 Zhao et al.¹⁸⁴ prepared meso–microporous activated carbons via the template method in combination with pyrolysis and ZnCl₂ chemical activation using cellulose and biowaste lignosulphonate as the precursors. Cellulose was first regenerated to couple with the silica template. Lignosulphonate was then cast into the composites to fill voids and produce mesoporous carbon. A post-ZnCl₂ activation was employed to further optimize the pore structure. The as obtained sample exhibited specific capacitances of 286 F g⁻¹ and 141 F g⁻¹ at current densities of 0.25 A g⁻¹ and 10 A g⁻¹, respectively in 6 M KOH electrolyte. However, the whole procedure of this route is kind of complex when considering the practical application for cellulose.

Some researchers tried to isolate cellulose from lignocellulose using different isolation processes and then transform them into ACs as SC electrodes, as shown in Fig. 3.^{169,176,186,187} In the approach reported by Wan's group¹⁷⁶ as seen in Fig. 3a, corn husks were first added into KOH solution and subsequently refluxed at 80 °C for 4 h. The obtained colloidal liquid was filtered by using a stainless steel mesh. The obtained solid residue, i.e., cellulose, was carbonised at 800 °C for 1 h. The final obtained AC possessed a 3D architecture, a SSA of 928 m² g⁻¹, uniform pore size and rich O-doping (17.1%). It exhibited a specific capacitance of 260 F g⁻¹ at 1 A g⁻¹ and maintained up to 228 F g⁻¹ at 10 A g⁻¹. Besides, it also displayed an energy density of 21 W h kg⁻¹ at a power density of 875 W kg⁻¹ in Na₂SO₄ aqueous electrolyte with a cell voltage of 1.8 V.

Fig. 3 A scheme showing isolating cellulose from (a) corn husk¹⁷⁶ and (b) bagasse¹⁶⁹ using two different isolation processes.

As shown in Fig. 3b, Hao et al.¹⁶⁹ purified cellulose from bagasse by alkaline hydrolysis and then bleaching using a sodium chlorite/glacial acetic acid mixture. 7 wt% of the obtained cellulose was dispersed into the solvent mixture of NaOH/urea/H₂O (7.5 : 11.5 : 81) precooled to −12 °C under vigorous stirring for 4 h at −6 °C. The cellulose sol was then freeze dried at −80 °C for 12 h. The obtained aerogel was pyrolysed and KOH-activated at different temperatures. At last, 3D hierarchical porous ACs were obtained and they exhibited good performance in solid state symmetric SCs.

3.2 Lignin

3.2.1 Properties of lignin. Lignin, constituting 15–35% of the typical dry lignocellulose by weight and 40% by energy, is the secondly most abundant biopolymer from lignocellulose and the main one based on aromatic units.^36,190,191 It is estimated that over 70 million tons of lignin are produced annually as the low-value by-product in pulp or paper industry.^192,193 Currently, around 95% of them are directly burned which has a series of disadvantages. The remaining 5% is used for commercial applications including dispersants, additives, resin and binder compositions, oil well drilling, carbon black, water treatment, battery expanders and so on.^36,194 Especially, in recent years, when the new concept of biorefinery emerged, developing the use of lignin as a high value-added product has aroused researchers' huge interest for lignin.^190,194

Generally, lignin can be classified into two main categories: sulfur lignin and sulfur-free lignin. Sulfur lignin includes Kraft lignin (alkali lignin) and lignosulfonates, while the latter includes soda lignin and organosolv lignin.

Lignin has a carbon to oxygen ratio of above 2 : 1, and thus is more energy dense than cellulose and hemicellulose. Compared with cellulose, there is a less detailed elucidation of lignin's structure by the experimental method. However, the currently commercially available lignin, which is the by-product of the cellulose industry, is more easily dissolved in aqueous solvent than cellulose.¹⁹⁵ Therefore, it seems more easily for lignin acting as the carbon precursor for SC electrodes.

3.2.2 Lignin-derived carbon for supercapacitors. The lignin-based carbon materials may be a favourable choice for the electrode materials of SCs, since some functional groups on the surface may enhance the pseudocapacitance.¹⁹⁴ Some electrochemically inert functional groups on the carbon surface can improve the wettability of the carbon electrode to boost the specific capacitance by increasing surface utilisation and pore access.^25,196 In recent years, a certain amount of research focusing on lignin-derived carbons for SCs has been conducted mainly through two routes.

One of the interesting routes is the synthesis of lignin-based carbon fibre through electrospinning.^197–200 Hu et al.¹⁹⁷ prepared AC fibres through both NaOH (Na-ACFs) and KOH (K-ACFs) activation using low sulfonated alkali lignin as the precursor. The hydrophilic and high SSA ACFs exhibited large-size nanographites and good electrical conductivity to demonstrate a good electrochemical performance. K-ACFs showed a specific capacitance of 344 F g⁻¹ at 10 mV s⁻¹, much higher than that of Na-ACF. The superior electrochemical properties of SC constructed with K-ACF over Na-ACF were attributed mainly to the higher microporosity and more narrowly distributed pore size.

Another interesting route is to prepare lignin-derived porous carbons through a template^201–205 or template-free^{168,206–209} method. As shown in Fig. 4a, Saha et al.²⁰¹ synthesised mesoporous carbon using the surfactant Pluronic F127, a triblock copolymer, as the template. Subsequent CO₂ physical activation and KOH chemical activation enhanced the SSA of the pristine mesoporous carbon to 624 and 1148 m² g⁻¹ respectively, with the present of a certain percent of micropores. The CO₂-activated and KOH-activated mesoporous carbon exhibited a capacitance of 102.3 and 91.7 F g⁻¹ respectively. Zhang et al.¹⁶⁸ prepared lignin-derived HPCs (LHPCs) through a template-free method, as seen in Fig. 4b. Lignin was first dissolved in KOH solution and then solidified. In the composite, KOH crystallised into small particles with different sizes. Then the composites were pyrolysed at 700 °C for 2 h when the crystallised KOH particles act as both the template and activation agent. The finally obtained LHPCs consist of a 3D hierarchical porous network as well as a large amount of oxygen-containing groups, which contributes to the pseudocapacitance. It exhibited a capacitance of 165 F g⁻¹ at 0.05 A g⁻¹ and maintained 123 F g⁻¹ at 10 A g⁻¹. Besides, the hierarchical porous structure enables it to maintain over 97% of the initial value at 1 A g⁻¹ after 5000 cycles.

Fig. 4 Scheme of preparing a lignin-derived carbon electrode through a (a) template²⁰¹ or (b) template-free method.¹⁶⁸

3.3 Hemicellulose

Hemicellulose, constituting approximately 20–35% of lignocellulose, is the second most common polysaccharide in nature. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. Therefore, it can be easily hydrolysed in dilute acid or base.²¹⁰ However, research on hemicellulose is less as most researchers focus on the other two main components of lignocellulose, i.e., cellulose and lignin. Therefore review on the application of hemicellulose for SC electrodes here will be relatively brief.

Wang et al.²¹¹ and Falco et al.⁶⁶ extracted hemicellulose from biomass through base and acid hydrolysis, respectively. In Wang's experiment, a certain amount of hemp stems powder was soaked in 6 wt% NaOH aqueous solution with stirring for 12 h at 40 °C. The hemicellulose was then isolated by precipitation of filtrate with two volumes of ethanol. The extracted hemicellulose was added into 5 wt% H₂SO₄ solution and was then HTC treated at 160 °C for 12 h. By contrast, Falco and coworkers impregnated corncob and spruce in diluted acid solution and the hydrolysed hemicellulose products was HTC treated at 200 °C for 24 h. Lastly, both samples were KOH-activated to increase the SSA and porosity. The finally obtained samples exhibited a specific capacitance of 240 F g⁻¹ at 0.1 A g⁻¹ in 6 M KOH and 315 F g⁻¹ at 0.25 A g⁻¹ in 0.5 M H₂SO₄, respectively. Anyhow, more research focusing on hemicellulose-derived carbon electrode materials for SCs is needed (Table 3).

Table 3 Summary of lignocellulose-derived carbon electrodes for SCs

Main materials	Activation	Modification	SSA (m^2 g^−1)	C (F g^−1) (symmetric SCs)	Measurements at	Electrolyte	Ref.
Lignosulphonate cellulose	ZnCl2	NA	856	286	0.25 A g^−1	6 M KOH	184
Cellulose filter paper	NH3	N-Doped	1326	120	1 A g^−1	2 M KOH	188
Bagasse-derived cellulose	KOH	NA	1892	142	0.5 A g^−1	KOH/PVA gel	169
Corn husk-derived cellulose	NA	NA	867	260	1 A g^−1	6 M KOH	176
Cellulose acetate	Steam	MWNT	1120	145	10 A g^−1	6 M KOH	178
Paper cellulose	NA	SWNT	—	200	—	1 M H2SO4	219
Textiles	NA	SWNT	—	140	20 μA cm^−2	Organic electrolyte	220
Bacterial nanocellulose paper	NA	CNT	—	50.5	1 A g^−1	Ion gel	221
Cellulose nanofiber	NA	RGO	—	207	5 mV s^−1	H2SO4/PVA	222
Cellulose nanocrystals	NA	PPy	—	336	—	0.1 M KCl	187
Cellulose sponge	NA	MnO2/CNT	—	1000	1 mV s^−1	1 M Na2SO4	223
Cellulose fibres	NA	CNT/MnO2		327	10 mV s^−1	1 M Na2SO4	224
Cellulose nanofibers	NA	Ni(OH)2	—	172 (asymmetric)	1 mV s^−1	6 M KOH	225
Low sulfonated alkali lignin	KOH	NA	1400	344	10 mV s^−1	6 M KOH	197
Alcell lignin	NA	NA	930	116	1 A g^−1	1 M H2SO4	204
Hardwood kraft lignin	CO2	NA	624	102	1 mV s^−1	6 M KOH	201
Lignin	NA	NA	803	208	0.1 A g^−1	6 M KOH	203
Lignin-derived by-products	KOH	NA	2218	141	1 A g^−1	EMI–BF4	207
Alkali lignin	KOH	NA	3775	286	0.2 A g^−1	6 M KOH	208
Kraft lignin	KOH	NA	1406	87	0.1 A g^−1	1.5 M NEt4BF4/ACN	206
Solvent lignin	KOH	N-Doped	3130	306	0.1 A g^−1	KOH/PVA	196
Softwood lignin	KOH	S/O-doped	1800	231	1 A g^−1	EMI–BF4	226
Lignin	NA	BC	199	124	0.5 A g^−1	6 M KOH	227
Solvent lignin	KOH	Aniline	2265	336	1 A g^−1	6 M KOH	228
Natural lignin	NA	rGO/PEDOT	—	144	0.1 A g^−1	0.1 M HClO4	229
Sodium lignosulphonate	NA	NiO	802	880 (three-electrode)	1 A g^−1	6 M KOH	230
Hemp-derived hemicellulose	KOH	NA	3062	240	0.1 A g^−1	6 M KOH	211
Corncob-derived hemicellulose	KOH	NA	2300	315	0.25 A g^−1	0.5 M H2SO4	66

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3.4 Modification of lignocellulose-derived carbon

Researchers could conduct similar physical or chemical processes on lignocellulose to get a high SSA or hierarchically porous structure as depicted in Section 2.4. In this section, we will preferably focus on other modifications of lignocellulose-derived carbons.

3.4.1 Modification to improve EDLC. Effective SSA and charge separation distance are the two main factors that determine EDLC.²²⁷ Increasing the SSA, creating a hierarchical porous structure, and enhancing the electrical conductivity and wettability are several corresponding methods to improve EDLC.²⁰ Besides the general applied physical or chemical activation methods, researchers have also tried to enhance EDLC through compositing cellulose-derived or lignin-derived carbons with other electrochemically functional materials such as CNTs and RGO.^{178,219,220,222,227,231}

Cui's group made conductive and stretchable SWNT-paper^219,231 and SWNT-textile²²⁰ electrodes for SCs by using simple solution processes, as shown in Fig. 5. In their study, paper or ordinary textiles were explored as a platform for SCs by integration with SWNTs via rod coating or dipping and drying methods. The coated cellulose films, functioning as both electrodes and current collectors, show high conductivity, porosity, and robust chemical and mechanical stability, which lead to high-performance SCs. Deng¹⁷⁸ and Kang et al.²²¹ also researched modification on cellulose-derived carbons with CNTs.

Fig. 5 Cellulose/SWNT composite electrode through a simple rod coating²¹⁹ (a) or dipping and drying²²⁰ (c) method, and their electrochemical performance respectively (b) and (d).

Gao et al.²²² reported a cellulose nanofiber-reduced graphene oxide (CNF–RGO) hybrid aerogel as the electrode material for all-solid-state flexible SCs. In their experiment, CNF–RGO hybrid hydrogels were prepared by acidizing homogeneous solution of CNFs and GO nanosheets with hydrochloric acid vapor. Then the hybrid aerogel was prepared by supercritical CO₂ drying. The finally obtained flexible SCs exhibited a capacitance of 207 F g⁻¹ at 5 mV s⁻¹ in H₂SO₄/PVA gel electrolyte. It showed a good electrochemical stability under bent state. The capacitance remained at 207 F g⁻¹ in bent state (180°) and did not change obviously after 100 bending cycles.

Biomass-based carbon aerogels represent an important novel research direction in aerogel development. However, lignin-derived aerogel is brittle and fragile. Xu et al.²²⁷ toughened lignin-resorcinol-formaldehyde (LRF) aerogel using bacterial cellulose (BC) through a catalyst-free process. The toughened and graphitised lignin-derived aerogel exhibited a core–shell nanostructure and it can undergo at least 20% reversible compressive deformation. The large mesopore ratio and core–shell nanostructure of the sample, with BC-derived carbon nanofiber as the backbone and LRF-converted carbon as the coating respectively, enable it to show a high areal capacitance of 62.2 μF cm⁻² at 0.5 A g⁻¹ with a relatively low SSA, 119.4 m² g⁻¹.

3.4.2 Modification to improve pseudocapacitance. Combining pseudocapacitance with EDLC in one electrode can significantly improve the capacitance value thus enhancing the energy density. Pseudocapacitance can be integrated with lignocellulose-derived carbon through modification with conducting polymers, transition metal oxides, or heteroatom doping. In general, limited research has currently been done in this branch.

Both conducting polymer and metal oxides can provide higher capacitance than carbon materials. However, their electrochemical stability and conductivity are relatively poor compared with porous carbon. Therefore, researchers coated a conductive polymer^{187,228,229,232–235} or metal oxides^{199,223–225,230} on cellulose-derived or lignin-derived porous carbons, which function as the backbone.

Alshareef's group²²³ fabricated a “sponge supercapacitor” using a MnO₂–CNT–sponge hybrid electrode. The macroporous nature of the cellulose-sponge as well as the porous nature of the electrodeposited MnO₂ nanoparticles provided a double porous electrode structure, giving rise to good conductivity and full accessibility of the electrolyte to MnO₂. The capacitance was dramatically increased to 1230 F g⁻¹ (based on the mass of MnO₂) at a scan rate of 1 mV s⁻¹ in a three-electrode system. Besides, it also showed excellent rate capability and cycle stability.

Chen et al.²³⁰ incorporated NiO nanoparticles into lignin-derived mesoporous carbon (MPC) using a LC phase-templating preparation method. The obtained core–shell structured NiO@MPC composite not only increased the utilization of NiO, but also improved its electrical conductivity and mechanical strength. It exhibited a specific capacitance of 880.2 F g⁻¹ at a current density of 1 A g⁻¹, with enhanced rate capability and cycle stability—90.9% and 93.7% of the capacitance value was maintained at a current density of 10 A g⁻¹ and after 1000 GCD cycles, respectively.

Another direction to increase the capacitance of lignocellulose-derived carbon materials is by heteroatom doping. Wide research has been focused on heteroatom doping, including N, O, S, and P, on polymer-derived or biomass-derived carbon electrodes. However, there is still little research on doping heteroatoms in specific cellulose,¹⁸⁸ lignin^196,205,226 or hemicellulose-derived carbons and more research is required.

4. Conclusions and prospects

Biomass is a good resource for making functional carbon materials for electrocapacitive energy storage applications such as electrodes, separators, or binders. Recent research has shown that affordable biomass-derived carbon materials with electrocapacitive properties comparable to commercial activated carbon can be prepared using simple carbonisation and/or activation methods. Advanced hierarchical porous biomass-derived carbons of excellent electrocapacitive performance can be obtained by using the chemical activation methods using, for example, potassium hydroxide. To enable biomass-derived carbon materials to find practical applications in energy storage, challenges and issues must be addressed.

First, considering the diversity of biomass resources, fundamental research focusing on the effect of biomass composition on the electrocapacitive properties of resulting carbon is needed. Perhaps, it could be a good practice to do this kind of research with cellulose-, lignin- and hemicellulose-derived carbon materials.

Second, the majority of the reported biomass-derived electrode materials of excellent electrocapacitive performance were prepared at the lab scale. There is a need to do scale-up research to develop a process protocol for further development of the technology.

Third, while hierarchical porous biomass-derived carbons of high specific surface area can be prepared using the KOH-activation method, it has little control over the pore geometry, pore size and pore connection. Besides, the KOH-activation method is unfavourable for creating graphitic carbons, which determines the electrical conductivity and surface wettability towards an electrolyte. KOH activation in combination with thermal treatment may enable one to prepare biomass-derived carbons of a high specific surface area, appropriate pore structure, good electrical conductivity and good wettability towards electrolytes.

Forth, there has been an increasing demand for flexible supercapacitor devices with advantages of portability and flexibility for portable electronics applications. Biomass offers opportunities for making flexible electrodes as fibres or foams. This will open up a new research area.

Fifth, the energy density of supercapacitors can be enhanced by increasing electrode capacitance or widening the operation voltage. Adding pseudocapacitive materials to biomass-derived carbon such as heteroatoms, metal oxides, or conductive polymers, is a future research direction to increase the capacitance. Widening of the voltage window by using ionic liquids as electrolytes or configuring asymmetric cells or fabricating hybrid capacitors will be another effective way to improve both energy density and power density.

Acknowledgements

This work was supported by the Australian Research Council Discovery Project (DP 130101870) and the University of Queensland Research and Teaching Fellowship Program (2015000144). HL wishes to acknowledge the China Scholarship Council (CSC) and the University of Queensland for providing scholarship.

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