Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10474-10482
From Dead Pine Needles to O, N Codoped Activated Carbons by a One-Step Carbonization for High Rate Performance Supercapacitors Changyu Leng,† Kang Sun,*,† Jihui Li,† and Jianchun Jiang† †
Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab for Biomass Chemical Utilization, Key and Open Lab on Forest Chemical Engineering, SFA, Nanjing 210042, China
ABSTRACT: Dead pine needle derived O, N codoped activated carbons were prepared by an easy one-step carbonization without adding any chemical reagents. The as-obtained PN-X samples had a high rate performance as electrodes for supercapacitors. It was due to the unique structure of cells in dead pine needles, abundant contents of metal elements (K, Na, Ca, Mg) and a large number of heteroatom contents (O, N). The role of different metal atoms and air for the formation of pores, and the mechanism of one-step carbonization were proposed. Micro/Meso/Macropores are beneficial to ion adsorption and transportation, and the doping O and N can improve physical and chemical properties of the carbon surface and increase electrochemical active sites to generate additional pseudocapacitance. All chemical and physical properties of PN-X samples were provided in detail. Especially, the PN-1000 showed a high specific capacitance of 223 F g−1 (178 F cm−3) at 0.5 A g−1 while it still had 150 F g−1 (120 F cm−3) even at 100 A g−1. The results suggested the unique PN-1000 to be a promising electrode material for supercapacitors in many practical applications. KEYWORDS: Dead pine needle, One-step carbonization, O, N codoped, Activated carbon, High rate performance, Supercapacitor
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INTRODUCTION With the increasing demands and challenges of energy storage, there is an urgent need for developing high-performance energy storage devices and advanced functional materials. Compared with Li-ions batteries, supercapacitors have a high power density, rapid charge−discharge rate and long cycle life, which have been widely applied in many practical areas, such as electric vehicles, electronic devices, subway systems and so on. In the past decades, activated carbons were engineered as electrode materials for supercapacitors.1−5 Especially, biomassderived activated carbons (ACs) have attracted lots of attention because of (a) low cost of raw materials and processes, (b) high chemical stability and thermal stability, which affect the cycling life, (c) high specific surface area, which can generate a high specific capacitance, (d) excellent electrical conductivity, which is crucial to the power density and the rate capability, (e) controlled porosity and abundant pores whose size can be adapted to electrolyte ions.6−10 In recent years, it is confirmed that ACs should have a large volume of micropores for energy storage, mesopores as ion transport channels for fast delivery ions, and macropores for ion storage.11−15 Although biomassderived ACs as electrode materials for supercapacitors exhibit a © 2017 American Chemical Society
high specific capacitance, few of them show excellent rate performance, which is one of the most important characteristics in practical application.16−20 The traditional methods of preparing ACs are mainly chemical activation, physical activation and chemical−physical activation.21−23 Some ACs have a high specific surface area, large pore volume and great electrochemical performance. However, they have lots of disadvantages, for example, the chemical activations usually need a large number of chemical reagents while alkali and acid can eat away the inside of the container at a high temperature. The excess agents can not be recycled easily and pollute the environment. Some dead leaves were used to prepare ACs by traditional activation with abundant alkali or acid, which were nothing novel.24−28 If the unique cell nanostructure of biomass materials can be used to prepare ACs without any agents, only by an easy one-step carbonization, it will have a promising application and the above problems can also be solved. Received: July 21, 2017 Revised: September 11, 2017 Published: October 12, 2017 10474
DOI: 10.1021/acssuschemeng.7b02481 ACS Sustainable Chem. Eng. 2017, 5, 10474−10482
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Nanostructures of Dead Pine Needles and PN-1000a
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(a) Dead pine needles; (b) longitudinal section of the dead pine needles; (c) cross section of the dead pine needles; (d) PN-1000; (e) internal profile of PN-1000; (f) cross section of PN-1000; (g) external surface of PN-1000; panels b′, c′, e′, f′ and g′ are magnified images of the red part of panels b, c, e, f and g.
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In other words, biomass waste has been a problem in many countries recently. It is a rich source of carbon, heteroatoms or metal elements. However, most of biomass waste is just burnt or buried to produce ash, greenhouse gases or heavy metal circulation. It is meaningful that they can be used to prepare carbons as value-added products, such as materials for supercapacitor electrodes.29−32 In this paper, dead pine needles are used as a precursor to prepare activated carbons by an easy carbonization without any chemical reagents adding during the whole process and the as-obtained ACs also have a high specific surface area and large pore volume, especially, high rate performance as electrode materials for supercapacitors. It is due to the unique cell nanostructure of dead pine needles, abundant content of metal elements (K, Na, Ca, Mg) and a large number of heteroatom content (O, N). The role of different metal atoms and air for the formation of pores, and the mechanism of one-step carbonization are proposed in detail. This paper aims to propose an idea that biomass waste derived porous carbon materials should be designed from the nanostructure of cells in raw materials. The metal elements and heteroatoms of the biomass should also be considered and used, which can replace chemical reagents and avoid environment pollution to realize the green chemistry.
EXPERIMENTAL SECTION
Materials. The dead pine needles (Cedrus) were collected in northern China in autumn. All other chemicals were purchased from Aladdin and used without further treatment. Preparation of Dead Pine Needle-Derived Activated Carbons. The dead and dry pine needles were collected in great quantity, then cleaned, washed and dried at 110 °C in an oven for 12 h. The dry pine needles were cut into pieces and kept in a completely dry atmosphere. During the one-step carbonization process, 10 g of dead pine needle pieces was heated in a crucible from room temperature to 600, 800, 1000 and 1200 °C with a heating rate of 10 °C min−1 and kept at the final temperature for 1 h in air. The asobtained samples were denoted as PN-X (X = 600, 800, 1000, 1200). Scheme 1 shows the nanostructures of dead pine needles and PN1000. As shown in Scheme 1b, the longitudinal section of dead pine needles showed the porous rod-like structure, which was derived from empty plant cell cavities. The cross section of dead pine needles in Scheme 1c exhibited honeycomb-type pores. It can be clearly seen in Scheme 1e,f,g that the carbon framework of PN-1000 reserved the structures of cell cavities in dead pine needles to a great extent. The internal profile of PN-1000 in Scheme 1e showed the wavy carbon framework with porous nanostructures of dead pine needle cells after carbonization. The cross section of PN-1000 in Scheme 1f also showed abundant pores, which confirmed the wavy nanostructures in the internal profile of PN-1000 from another angle. The external surface of PN-1000 in Scheme 1g exhibited carbon layers with interconnected pores. So, the empty cell cavities of dead pine needles were considered and used to prepare carbon materials with unique porous carbon frameworks. 10475
DOI: 10.1021/acssuschemeng.7b02481 ACS Sustainable Chem. Eng. 2017, 5, 10474−10482
Research Article
ACS Sustainable Chemistry & Engineering Structure Characterization. Thermogravimetric analysis (TGA) was performed to find the pyrolysis temperature and demonstrated the possible reaction of metal ions during the carbonization process. The elemental contents of green and dead pine needles were obtained from an elemental analyzer (vario EL cube CHNOS, Germany). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the variation tendency of surface morphology and porous nanostructures. The activated carbon structure was investigated by Raman spectra and X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) measurements were also carried out to analyze the surface atomic content. N2 adsorption (77K) was used to analyze the specific surface area and pore size distribution of the porous carbon samples. The specific surface area (SSA) was calculated from the N2 isotherm with p/p0 in the range 0.1−0.25 by applying the BET method. The pore size distribution (PSD) were also calculated by the DFT equation using N2 adsorption data. The total pore volume was confirmed by the amount of nitrogen adsorbed (p/p0 = 0.9). The average pore sizes of micropores and mesopores were estimated by the DFT method with the p/p0 values between 10−7−0.1 and 0.1−0.99, respectively. Cyclic voltammetry (CV), galvanostatic charge−discharge curve (GCD) and electrochemical impedance spectroscopy (EIS) were carried out on the electrochemical workstation (Bio-Logic, France). Electrochemical Measurements. The typical three-electrode system and two-electrode system were both used to text the electrochemical performance of all samples. All electrochemical measurements were carried out on an electrochemical station (VMP3, Bio-Logic). The working electrode was prepared by coating 5 wt % polytetrafluoroethylene, 10 wt % acetylene black and 85 wt % PN-X activated carbons fully and grounding in a mortar. Then the slurry was pressed on a stainless steel to form the working electrode in which active material is contained. The mixture was pressed on a stainless steel plate under 10 M Pa for 1 min. The thickness of electrode material was 100 μm, the electrode area was 1 cm2. The coin-type supercapacitor (two-electrode system) was assembled by using two similar electrodes separated by a piece of polypropylene membrane in 1 M H2SO4 aqueous electrolyte. A platinum foil and Ag/ AgCl were used as the counter and reference electrodes in threeelectrode system, respectively. The voltage window for aqueous electrolyte was set to be 0−0.9 V. And the galvanostatic charge− discharge curves were performed under a current density range from 0.1 to 10 A g−1. The scan rate was in a range 5−100 mV s−1. Electrochemical impedance spectroscopy measurements were collected with frequencies ranging from 100 kHz to 10 mHz.
and can hold up some space in the carbon frameworks while Na, K play the role of forming pores and getting through channels during the carbonization process. The role of different metal atoms during the formation of pores were discussed and combined with the thermogravimetric analysis (TGA) and N2 adsorption. As shown in Figure 1, there were several stages of thermogravimetric analysis process of dead pine needles, the
Figure 1. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) analysis (left and right y axes, respectively) under an air atmosphere, heating rate: 10 °C min−1.
first stage (up to 200 °C) was associated with the desorption of absorbed water. The second stage (250 °C-375 °C) had the most obvious weight loss, which corresponded to CO2 and H2O evolution. It was due to the decarboxylation and dehydration of the materials, respectively. The next step, the range of 400−550 °C, corresponded to the degradation of dead pine needles. And the rapid elimination of C−H, C−O, C−C bonds released gaseous volatiles, such as CO, CO2 and CH4. Dead pine needles also had some oweight losses of 600−900 °C, which demonstrated the decomposition of carbon frameworks. The sodium/potassium (Na/K) compounds generated during heat treatment had a crucial role in the formation of the micro/mesopores network and the development of channels. As shown in Scheme 2, first, sodium/potassium carbonates were formed (
