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Fabrication of Hierarchical Porous Carbon Nanoflakes for High-Performance Supercapacitors Yamin Yao, Yunqiang Zhang, Li Li, Shulan Wang, Shi Xue Dou, and Xuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10593 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017
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ACS Applied Materials & Interfaces
Fabrication of Hierarchical Porous Carbon Nanoflakes for High-Performance Supercapacitors Yamin Yao1, Yunqiang Zhang1, Li Li1, Shulan Wang1*, Shixue Dou2*, and Xuan Liu3* 1
Department of Chemistry, School of Science, Northeastern University, Shenyang,
110819, China. 2
Institute for Superconducting and Electronic Materials, University of Wollongong,
Wollongong, NSW 2522, Australia. 3
Department of Materials Science and Engineering, Carnegie Mellon University,
Pittsburgh, PA, 15213, USA
ABSTRACT: In the current work, the carbon nanoflakes (CNs-Fe/KOH) and porous carbon (PC-Ni/KOH)
have
been produced
by using Fe(NO3)3/KOH
and
Ni(NO3)2/KOH as the co-graphitization/activation catalysts to treat the natural plane tree fluff, respectively. The as-prepared carbon materials show different morphologies when treated with different metal ions. Compared with PC-Ni/KOH, the CNs-Fe/KOH have both high graphitization degree (IG/ID = 1.53) and large SBET (1416 m2/g). In a three-electrode setup, the CNs-Fe/KOH electrode shows a high specific capacitance of 253 F/g at 10 A/g, with a capacitance retention of 92.64% after 10000 cycles in 2 M H2SO4 aqueous solution, which is far better than the sample without Fe3+ addition. In 1 M LiPF6 in ethylene carbonate/diethyl carbonate organic solution, CNs-Fe/KOH based symmetric supercapacitor also presents an excellent 1
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specific capacitance of 32.2 F/g at 1 A/g. In addition, an energy density of 39.98 W h/kg can be achieved at the power density of 1.49 kW/kg. Influence of metal ions on the morphology, structure as well as electrochemical performance of the carbon materials are further analyzed in detail. The current work provides a novel path for design and fabrication of supercapacitor electrode materials with promising electrochemical performances.
KEYWORDS: biomass; porous carbon nanoflakes; co-graphitization/activation; heteroatom doping; supercapacitor
1. INTRODUCTION Supercapacitors (SCs) are attracting the increased attentions as the promising energy storage devices with high power density,1 wide operational temperature range,2 and long cycling stability,3 which are currently unattainable in other energy devices, such as lithium battery. The extensive applications of SCs cover the mobile electrical systems, high-power industrial equipment, consumer electronics, and hybrid electric vehicle, etc.4 The commercial SCs are principally based on carbon based materials (activated carbons, ordered porous carbons, graphitized carbons, et al.), which are limited by the unfavorable energy density (E, 3~5 W h/kg) and electrode kinetic problems associated with inner-pore ion transportation.1,5 In the past decades, two methods are commonly used to increase the E of SCs through improving the gravimetric specific capacitance (Csp) with innovatory active materials or choosing the organic/ionic liquid electrolytes with high charge-discharge voltage (∆V).6 Carbon 2
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materials combined with pseudo-capacitive materials (metal oxides/hydroxides or conductive polymers) can enhance their electrochemical performance based on reversible faradic reactions between the active materials and electrolyte.7,8 The inherent capacitance feature of the carbon materials is particularly critical since most of the pseudo-capacitive materials have some drawbacks including low electronic conductivity and poor cycling stability. Specific surface area is one of the crucial factors to determine the Csp of carbon-based SCs due to the electric double-layer capacitance
(EDLC)
mechanism
of
pure
electrostatic
adsorption
at
the
electrode/electrolyte interfaces.9 In addition, the structural factors, such as the heteroatom content, bonding states and graphitization degree also influence the Csp of the active materials apparently. Porosity and pore size distribution of carbon-based electrode materials affect the electrolyte ion diffusion and electron transportation, which can cause the low rate capability and large IR drop.10 Some of the modifications for graphene,11 carbon nanotubes,12 and fullerene13 can combine the structural advantages with pores at different length scales, high ion-accessible specific surface area and excellent conductivity into the single component, but the fabrication has high cost that limits the commercial use of EDLC. Therefore, an economic and innovatory method to treat the carbon material with excellent capacitor performances is in great necessity. Improving the graphitization degree is an effective method to enhance the electron transfer rate and structural stability, which is beneficial to increase the electrochemical performance of carbon materials. High pyrolysis temperatures can increase the 3
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graphitization degree of carbon materials due to the formation of conjugated carbon atoms in the sp2 state, but decrease the Brunauer–Emmett–Teller (BET) specific surface area (SBET) dramatically simultaneously with the collapse of pore structure.14 The “bottom-up” catalytic route which can prepare the graphitized carbon with remarkable electronic conductivity and structural stability from molecular carbon precursors, has suffered from the limited Csp because of the low SBET (
