Activated carbons prepared by indirect and direct CO₂ activation of lignocellulosic biomass for supercapacitor electrodes

Highlights

• Energy crop hybrid willow was converted to activated carbon by direct and indirect CO₂ activation.

• Optimized activation condition is at temperature of 800 °C and residence time of 60 min.

• Activated carbons of indirect route have higher capacitances and better cycling performance.

• Surface functional groups and porosity both contribute to the capacitance of activated carbons.

Abstract

Lignocellulosic biomass was converted into hierarchical porous carbon by using a physical activation technique under a carbon dioxide environment. Both direct and indirect CO₂ activation routes were utilized to investigate the effect of processing parameters and the kinetics of the activation. The porosity, surface chemistry, and morphology of the activated carbons were characterized in addition to their proximate and ultimate analyses. This was followed by the preparation of the activated carbon electrodes and the fabrication and electrochemical testing of these electrodes within a symmetrical supercapacitor cell. The results showed a dominant microporous structure along with the limited content of larger pores for the activated carbons prepared via both direct and indirect activation. Along with the preserved natural pore structure of the biomass, an engineered pore structure was achieved which is highly beneficial for the supercapacitors with respect to the transport and storage of ions. The morphological analysis also revealed their tortuous porous structure. The maximum specific capacitances of 80.9 and 92.7 F/g at the current density of 100 mA/g were achieved after direct and indirect activation routes, respectively. The surface functional groups were also found to play a significant role in the resultant electrochemical performance of the supercapacitors.

Introduction

Lignocellulosic biomass is a plant-based material which is mainly composed of cellulose, hemicellulose and lignin. It can be used as a feedstock to produce heat, such as firewood and pellets, or be converted into various biofuels [1,2]. In addition, it can be utilized to prepare renewable materials, such as cellulose and bio-adhesive [3,4]. The conversion of lignocellulosic biomass into biofuels and byproducts can be classified into three major pathways as thermal, chemical and biochemical. The lignocellulosic biomass typically includes wood and grass sources of which woody biomass is a major component. Typical woody biomass includes logging and mill residues, wood shavings and sawdust. Besides utilizing already existing residues, growing short-rotation woody crops is another way to develop woody biomass sources. For example, hybrid willow is a shrub woody crop which is an alternative energy source as promoted in recent years in the Northeast United States. Hybrid willow is a fast-growing high yield, and short rotation energy crop with less maintenance for stand establishment. It could be harvested as short as 3 years after planting. More importantly, it has been extensively cultivated and grown on disturbed mine and marginal agricultural lands in the northeastern United States. It has been popularly considered as a renewable biomass feedstock for value-added bioenergy products. Therefore, there is increasing interest in the growth and utilization of hybrid willow [5,6].

Activated carbon can be made from various precursors, such as coconut shell, peat, wood, coal, bamboo, and organic chemicals [[7], [8], [9]]. It is a carbonaceous and highly porous adsorptive material, which mainly contains carbon, with minor contents of oxygen and hydrogen, and limited amount of nitrogen and sulfur [10]. The activated carbon is an amorphous material, which typically has a well-organized pore network with a dominant microporous or mesoporous structure. Lignocellulosic biomass is a highly preferred precursor for the production of porous activated carbon materials due to its natural interconnected pore network [11]. The vertical and horizontal pores within the lignocellulosic biomass are usually connected with pits and perforation plates. These pore networks play a critical role in nutritional transportation for plants. In other words, they provide the capability of transporting nutritional elements including inorganic ions. Therefore, the conversion of the lignocellulosic biomass feedstocks into engineered, hierarchically porous carbon structures for supercapacitor applications has become an increasing trend in recent years [12]. Since these carbon products have well-developed networks of channels and pores that are beneficial to ion storage and diffusion, they are highly suitable for the electrochemical double layer capacitors (EDLCs) or commonly termed as supercapacitors. The accumulation of pure electrostatic charge on the interface between electrode and electrolyte generates the capacitance of the supercapacitor, which is mainly determined by the specific surface area of the carbon electrode that is accessible by the electrolyte ions [13]. Along with the pore structure and surface area, the surface chemistry and functional groups of the carbon materials are also critical to the adsorption behavior since they serve as electron donors or acceptors for the targeted ions [14].

To prepare activated carbon from lignocellulosic biomass, thermochemical and hydrothermal methods are typically employed. The thermochemical conversion process typically involves a one-step or two-step carbonization. In a one-step process, a biomass source is converted into porous carbon material by heating to a certain temperature range (600°-900 °C). This can be carried out either in an inert atmosphere along with the presence of a chemical activating agent (chemical activation) or in an activating gas environment, such as steam or carbon dioxide (physical activation) [15]. Two-stage carbonization usually refers to an initial pyrolysis step to convert the biomass into biochar, which is followed by a chemical or physical activation process [16]. The physical activation has three major reagents, which are air, steam and carbon dioxide (CO₂). The steam and CO₂ typically have an activation temperature ranging from 700° to 1000 °C, whereas the air has a lower temperature of activation at the range of 350°–550 °C [17,18]. The physical activation method is widely considered to be a cost-effective and efficient way to produce porous carbon materials [19]. More importantly, it is less corrosive when compared to the chemical activation method. The widely used chemical activation reagents, such as KOH and ZnCl₂, are highly corrosive, and they can react with the furnace components in such processes [20]. In addition, the physical activation route could be highly time efficient, since it typically does not require an extensive washing afterwards. The inorganic release, occurring along with the chemical activation route, may also lead to environmental pollution, which could be avoided by using the physical activation [15]. The CO₂ activation can promote the formation of meso/microporous structure, and also preserve the natural interconnected pore network of the biomass, particularly compared to the chemical activation that may destroy the pore structure. The main objective of this study was to investigate the effect of direct and indirect CO₂ activation on the physical and chemical properties of the activated carbons and the electrochemical performance of their supercapacitor electrodes. The temperature and residence time of the direct activation, and pyrolysis temperature were examined to understand their influence on the properties of activated carbons, and later, to optimize the process parameters. Finally, the microstructure and surface chemistry was investigated to understand the effect of processing parameters on the performance of the supercapacitor electrode. Understanding the key process parameters could be used as reference for the controlled preparation of activated carbon with well-developed characteristics in related applications of energy storage.