**1. Introduction**

Energy is necessary in day today life. In future, energy generation with sufficient and sustainable methods will be a major concern. Currently, over 75% of the energy used by society is produced by fossil fuels, including coal, natural gas, and oil, neither of which are renewable. As a result, energy must always be replaced by renewable energy before fossil fuels run out. Excessive use of fossil fuels leads to global warming. It releases harmful gases directly into the environment [1,2]. To address the challenges related to environmental pollution and energy production, fuels with petroleum must be transformed to renewable and sustainable energy sources. Unutilized waste materials and biomass can be used effectively in synthesizing of carbon materials for energy storage and conversion devices. The carbonization process carried out by heating biomasses under inert gas condition and high temperature. In contrast, the heteroatoms (oxygen, sluphur and nitrogen etc.,) in the networks of the biological macromolecules escapes there by leaving the carbon skeletons with the porous shape. So when residual carbon skeletons are activated, they may form linked 3D structures with considerably high conductivity, porosity and surface area, making them attractive candidates in energy storage applications. There are various biomass-derived carbon materials using highly effective green energy storage devices. Finding alternative energy resources that use green energy (biomass) and clean energy is essential [3,4]. So far, several storage technologies have been developed such as capacitors, batteries, fuel cells, and supercapacitors [1,2,5]. Supercapacitors are in the field of electrochemical devices with their remarkable fast charging speed, high power density, light weight, safe operation, and long life cycle [3,4,6]. Supercapacitors are divided into symmetric and asymmetric supercapacitors [5,7]. Electrochemical double-layer

**Citation:** Bejjanki, D.; Banothu, P.; Kumar, V.B.; Kumar, P.S. Biomass-Derived N-Doped Activated Carbon from Eucalyptus Leaves as an Efficient Supercapacitor Electrode Material. *C* **2023**, *9*, 24. https://doi.org/10.3390/c9010024

Academic Editor: Enrico Andreoli

Received: 20 December 2022 Revised: 19 January 2023 Accepted: 23 January 2023 Published: 17 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

capacitors (EDLC) and pseudo capacitors are classified as symmetric supercapacitors, while the hybrid capacitor is an asymmetric supercapacitor. The charge storage mechanism classifies the behavior of supercapacitor. The EDLC stores energy electrostatically while pseudo capacitors store energy via electrochemical redox or faradaic reactions. The Supercapacitor consists of 4 major components they are electrolyte, electrode, current collector, and separator. Among all the components, the performance of supercapacitor is mainly based on the electrode material [8]. Activated carbon, carbon nanotubes, and carbon nanofibers are utilized as electrode materials in EDLC, whereas metal-based oxides (RuO2, Co3O4, MnO2, NiO) and conducting polymers (polyaniline, poly-(3,4-ethylenedioxythiophene), and polypyrrole) are employed as pseudo capacitor materials, whereas the pseudocapacitors are chemical stability, high conductivity, corrosion resistance, controlled pore structure, temperature stability, environmental friendliness, and low cost [9].

In recent years, biomass-derived carbon material as electrode material gained a wide variety of attention, because of its unique properties such as high specific surface area (SSA), excellent electrical conductivity, and low production cost. Carbon materials exhibited in different morphologies, such as graphene, carbon nanotubes (CNT), carbon sphere, and carbon nanoparticle, thus have been looked into supercapacitor applications. Although, these electrode materials show short-time durability, low energy density, and high power density with less porosity [4,10,11]. The biomass-derived activated carbon has been widely used as electrode material for supercapacitor application. Considering the importance of activated carbon from biomass in an energy storage device, the literature has been carried out to know the chemical composition of various biomass such as Noeli et al. have studied the physical characteristics of dried bananas leaving and it has a carbon content of 43.5 wt.%. [12], D. Pujol et al. reported the elemental analysis of coffee waste extracted from soluble coffee industry with a carbon wt.% has 57 [13], Salwan et al. studied the chemical composition of black tea waste and algae both biomass waste has a carbon wt. percent of 30 & 28 respectively [14], and Yang Liu et al. studied biomass waste of willow leave, and which has 45 wt.% of carbon [15], Nannan et al. and Saad A et al. studied waste biomass, i.e., tremella also known as white fungus, and Egyptin mango leaves both biomass have the carbon as wt.% 40.25 and 40.7, respectively [16,17], and Grima et al., studied the carbonaceous residue of eucalyptus leaves biomass, and it has 74.5 wt.% carbon [18]. Compared to all other biomasses, the eucalyptus has the highest carbon content (approximately 75%) [18]. The chemical activation technique used to synthesize activated carbon will help to lower its issues and can increase its active sites. The chemical activating agent are ZnCl2, KOH, H3PO4, NaOH, K2CO3, H2SO4, NaCl, and CaCl<sup>2</sup> [19–21]. The ZnCl<sup>2</sup> has been used as activating agent to prepare activated carbon mostly for lignocellulosic biomass due which acts as dehydrating and dampening agent during the chemical activation. The ZnCl<sup>2</sup> activation causes swelling in the cellulose structure due to electrolytic action which also leads to increasing the surface area of activated carbon. The ZnCl<sup>2</sup> activated carbon (AC) has been widely used as electrode material in energy storage application. Furthermore, AC having low graphitization degrees usually have poor electric conductivity, which significantly limits the quick charge and discharge, especially at higher current densities. It's been proven that modifying AC with heteroatom species such as (oxygen, sulphur, and nitrogen functional groups), not only alter the conductivity of carbon network, but also helps in the permeability ions into electrode and electrolytes.

In this study, the chemical activation technique has been employed to synthesize activated carbon, using the carbonation method at various temperatures from 400 ◦C to 800 ◦C using ZnCl<sup>2</sup> as an activating agent. As prepared activated carbon was studied to determine the effect of temperature on its morphology and specific capacitance. Among all the AC, AC-700 ◦C has shown the most promising results in terms of phase, morphology, structure, composition, and electrochemical measurements. In order to synthesize N-doped activated carbon, urea was used as a nitrogen precursor and a chemical activation process was applied.
