Rational Surface Tailoring Oxygen Functional Groups on Carbon

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Rational surface tailoring oxygen functional groups on carbon spheres for capacitive mechanistic study Dongdong Zhang, Jianlong Wang, Chong He, Yuzi Wang, Taotao Guan, Jianghong Zhao, Jinli Qiao, and Kaixi Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22370 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Materials & Interfaces

Rational Surface Tailoring Oxygen Functional Groups

on

Carbon

Spheres

for

Capacitive

Mechanistic Study Dongdong Zhang

a, b

, Jianlong Wang a, b, Chong He b, Yuzi Wang a, b, Taotao Guan a, b, Jianghong

Zhao c, Jinli Qiao d*, Kaixi Li a, b* a

Institute of Coal Chemistry, Chinese Academy of Sciences, 27 Taoyuan South Road, Taiyuan

030001, China b

Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy

of Sciences, Beijing 010049, China c

Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University,

92 Wucheng Road, Taiyuan 030006, China d

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Shanghai

Institute of Pollution Control and Ecological Security, College of Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai, 201620, China KEYWORDS: carbon spheres, micro-meso-macropore, HNO3 hydrothermal oxidation, oxygen functional groups, capacitive mechanism

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ABSTRACT: Porous carbons represent a typical class of electrode materials for electric double layer capacitors. However, less attention focus on the study of capacitive mechanism of electrochemically active surface oxygen groups rooted in porous carbons. Herein, the degree and variety of oxygen surface groups of HNO3 modified samples (N-CS) are finely tailored by a mild hydrothermal oxidation (0.0 ~ 3.0 mol L-1), while the micro-meso-macroporous structures are efficiently preserved from the original sample. Thus, N-CS is a suitable carrier for separately discussing the contribution of oxygen functional groups on electrochemical property. The optimized N-CS shows a high capacitance of 279.4 F g-1 at 1 A g-1, exceeding 52.8% of pristine CS (182.8 F g-1 at 1 A g-1) in KOH electrolyte. Further deconvoluted the redox peaks of CV curves, we find that the pseudocapacitance not only associates with the surface-controlled faradaic reaction at high scan rate, but also dramatically stems from the diffusion-controlled capacitance through potassium and hydroxyl ions insertion/deinsertion into the underutilized micropores at low scan rate. The assembled supercapacitor based on N-CS presents stable energy density of 5 Wh kg-1 over a wide range of power density of 250 ~ 5000 W kg-1, which is higher than 0.0N-CS in KOH electrolyte. In TEABF4 electrolyte, the energy density of N-CS supercapacitor is 26.9 Wh kg-1 at the power density of 1350 W kg-1, and exhibits an excellent cycling stability with capacitance retention of 93.2 % at 2 A g-1 after 10,000 cycles. These results demonstrate that surface oxygen groups alter the capacitive mechanism and contribution of porous carbons.


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INTRODUCTION Supercapacitors have attracted considerable urgent exploitation over recent years because of their fast charge-discharge rate, high power density, and long cycle-life, and those advantages endow them with unique applications in regenerative brake, electrocar, portable electronics, and many others.1-3 Generally, the charge storage mechanism in supercapacitors can be split into two ways: electric double layer capacitance (EDLC) and pseudocapacitance. The EDLC is stored via electrostatic

accumulation

of

pseudocapacitance arises from

ions

at

the

electrode/electrolyte

the reversible redox reactions,

interface;

but

the

electro-sorption, and

under-potential deposition at/near the electrode surface based on metal oxides, conductive macromolecules, or hybrid carbon materials. In principle, EDLCs often exhibit higher energy efficiency and power density than pseudocapacitors, yet lower capacity.4-7 In the midst of various available electrodes for supercapacitors, porous carbons are a promising candidate because of its unique physicochemical characters, for example, good conductivity, high specific surface area, and tunable porous structure or surface chemistry.4,8-10 Various porous carbons derived from polymer, pitch, biomass, and metal organic frameworks have been extensively studied for electric double layer capacitive materials, 11-14 but the as-prepared carbon materials inevitably present a certain amount of oxygen associated with the precursor or synthetic process. Recent studies have confirmed that porous carbons with residual oxygen not only improve the electrodes wettability and reversible pseudocapacitive reactions but also affect the charge or mass transfer in KOH electrolyte.9-10,15-16 From the perspective of practical application, there is ever increasing interest in efficiently integrating EDLC and pseudocapacitance in one electrode for achieving high energy density without sacrificing power density and cycle life. However, less attention



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focus on exploring the capacitive mechanism and half-quantifying the capacitance contribution of oxygen functional groups anchored in micro-meso-macropore of porous carbons. In the recent years, HNO3 liquid phase oxidation has been deliberately applied to tailor the surface oxygen groups of carbons to optimize the electrochemical performance, including activated carbons,17,18 carbon aerogels,19 ordered mesoporous carbons (OMCs),20-21 carbon nanotubes (CNTs),22-24 carbide-derived carbons (CDCs),25-26 and graphene. 27 Different oxygen functional groups (for example, carboxylic acids, anhydrides, lactones, phenols and quinones) can be easily incorporated into carbon materials, but the structures and conductive network skeletons of those carbons are partially disruption under the HNO3 oxidation processing. To compensate the loss of electrical conductivity, controllable annealing or reduction process at a particular temperature is the most common strategies.20,28 However, for all of these surface-modified carbons, the variety of oxygen surface groups is different for each of temperature due to different thermal stability and reducibility. After the removal of lively oxygen atoms, it is hard to control the pore distribution and accessibility. Beyond that, the decrease of surface oxygen groups makes precise observation of rapid redox reactions difficult. On this level, it is hard to eliminate the influence of variable pore structures on capacitance and half-quantitatively calculate the individual capacitive contribution of oxygen groups. As we have seen, rational surface tailoring oxygen functionalities at high concentration and similar species without significant textural collapse of carbon materials has remained a challenge. Moreover, it is essential to understand the relationship between oxygen content and its capacitive contribution. In our previous work, carbon microspheres (CS) constructed with nanoparticles and contained little

micropores

but

abundant

meso-macropores

were

fabricated

through


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inverse-microemulsion-polymerization combining with carbonization. When the CS used for electrode materials in KOH electrolytes, the macropores play the role of ion buffer, mesopores promote the ions diffusion rate; and micropores provide charge adsorption sites. 29 The abundant hierarchical meso-macropores are also favorable for sufficient rapid transport of HNO3 and oxygen grafting on the wall of pores. This rigid sphere morphology assembled by carbon nanoparticles with star-studded micropores also provides an enough buffer space to reduce the destruction of the carbon skeleton. Thus, less destruction of porous structures can be elaborately designed by controlling the HNO3 oxidation condition. Herein, inspired by the hierarchical porous structure of CS, we design a mild strategy for skillfully constructing functionalized CS via HNO3 hydrothermal oxidation. The evolution and quantity of surface oxygenic functional groups in the CS can be finely controlled through adjusting the HNO3 concentrations between 0.0~3.0 mol L-1. The modified 0.5N-CS exhibits an improved specific capacitance of 279.4 F g-1 at 1 A g-1 in KOH electrolyte, which increases by 52.8% (182.8 F g-1 at 1 A g-1) in contrast with the pristine carbon sphere and is similar to its KOH activated counterparts (275 F g-1 at 1 A g-1).29 Exceptionally, because of the similar micro-meso-macroporous structures and oxygen functional groups distribution but totally different oxygen amount, the capacitance contribution of surface oxygen functional groups is systematically investigated. These findings also demonstrate the principal capacitance realized from oxygen groups can be maximized by optimizing the oxidation condition for porous carbons.

EXPERIMENTAL SECTION Fabrication of N-CS. CS was synthesized via a simple polymerization and carbonization method that was firstly developed by our group.29 During the experiment, thermoplastic phenolic



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resin (10 g), ethylene glycol (40 g), and methenamine (1 g) were blending and drastically stirring until forming transparent liquid.29 Then the solution was dropwise poured into conduction oils (80 °C) along with 300 rpm, and take 0.5 °C/min at a heating rate up to 120 °C. Afterward the reaction temperature cooled down to room temperature. Vacuum filtration was adopted and constantly leached with alcohol until no residual conduction oils, and desiccated at 100 °C about 12 h. Finally, the resin spheres were annealed at 800 °C for 1 h under N2 with 2.5 °C /min and named CS. The CS (1g) was soaked with 40 ml HNO3 with concentration of 0.2, 0.5, 1.0, and 3.0 mol·L-1, and stirred 0.5 h at 25 °C. The resulting turbid liquid was treated at 120 °C for 4 h in a Teflon-lined autoclave under continuous stirring at 100 rpm. After reaction, the reactants were filtrated and washed with H2O until constant pH. And the collected carbons were labeled as x-N-CS, where x was the HNO3 concentrations. A control experiment was conducted by using H2O instead of the HNO3 and named 0.0N-CS. Characterization. The morphology was observed on JEOL JSM-700 Scanning Electron Microscope (SEM) at an acceleration voltage of 5.0 kV. Transmission electron microscope (TEM) was taken on JEM-2100 with an accelerating voltage of 200 kV. Thermogravimetric (TG) and Derivative Thermogravimetric (DTG) were conducted using Ta Q500 from 25 to 800 °C at 5 °C/min under Ar atmosphere. Thermogravimetric Analysis/Mass Spectrometry (TGA-MS) was characterized on SETSYS EVOLUTION TGA from 25 to 800 °C at 5 °C/min under Ar atmosphere. Fourier transform infrared (FT-IR) spectroscope was analyzed with Bruker Vertex70 spectroscope with wavenumber of 4000 cm-1~500 cm-1. N2 adsorption-desorption isothermal was measured on Micromeritics ASAP 2020 at 77K. Specific surface area of N-CS was obtained by the Brunauer-Emmet-Teller (BET) method; pore size distribution was analyzed


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by density functional theory (DFT) method. X-ray diffraction (XRD) measurements was taken on a Bruker D8 at room temperature (Cu Ka radiation, λ=0.15406). Raman spectra were measured on a Nanofinder 3.0 Raman spectrometer using laser excitation at 488 nm. The date of X-ray photoelectron spectroscopy (XPS) was recorded on an AXIS Ultra DLD spectrometer with monochromatic Mg Ka (1486.6 eV). Elemental analysis was conducted with a Vario ELcube elemental analyzer. Electrochemical measurements. Active material, acetylene black, and polytetrauoroethylene binder with weight proportion of 8:1:1 were blended into a smooth paste by adding moderate ethanol, then evenly coated on foam Ni (1 cm  1 cm, ~5 mg), then dried at 100 °C for next measuring. The electrochemical performance was carried out using three-electrode test in 6 M KOH. Pt foil was used as counter electrode; Hg/HgO was used as reference electrode. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and Electrochemical impedance spectroscope (EIS) were measured on CHI 660E workstation at 25 °C. CV plots were recorded between -1~0 V at scan rate 10 ~ 100 mv s-1. EIS plots were obtained in 0.01Hz~100 KHz by applying 5 mV amplitude. For the symmetric supercapacitor (CR 2032-type coin cell), two pieces carbon electrodes (circular Ni foam, Φ=10mm) were assembled with polypropylene as separator. 6 M KOH and 1 M tetraethylammomium tetrafluoroborate/acetonitrile (TEABF4/AN) were used as the electrolyte, respectively. The symmetric supercapacitor based on TEABF4/AN electrolyte was assembled in glove box. The CV and GCD curves were measured using CHI 660E workstation at 25 °C with voltage window ranged from 0 ~ 1 V in 6 M KOH and 0 ~ 2.7 V in 1 M TEABF4/AN. The cycle performance, leakage current, and self-discharging curves were measured by Land CT2001A.



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RESULTS AND DISCUSSION As depicted in Figure 1a, the N-CS samples with tunable surface chemical were fabricated from CS by a mild HNO3 hydrothermal process, and the detailed operating procedure was discussed in the Experimental section. The SEM images shown in Figure 1b-c demonstrate the smooth spherical appearance morphologies 0.0N-CS and 0.5N-CS before and after HNO3 hydrothermal process. The average diameter of carbon spheres is about 40 µm. In additions, by further observing fine microstructures of 0.5N-CS, we discover that the interconnected carbon framework assembled through abundant nanoparticles with size about 40 nm is well preserved.29 The developed meso-macropores interspersed among the carbon nanoparticles are also clearly seen in TEM images (Figure 1d-e). The small particles size is highly beneficial to reduce mass transport and charge transfer resistances in the charge-discharge process.30 This structure also can significantly contribute to the rapid diffusion of nitric acid and confine the oxidation reaction of acid and lively carbon atoms in nanoscale. It should be noted that there has not been any substantial change of the morphology features of CS after a facile low concentration nitric acid hydrothermal treatment. Consequently, HNO3 hydrothermal approach is a moderate method to modify the surface chemical state with different oxygen functional groups.

(a)


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(d)

100 nm

(e)

50 nm

Figure 1. (a) Schematic diagram of the fabrication of N-CS from CS; SEM images of (b) 0.0N-CS, and (c) 0.5N-CS; (d-e) TEM images of 0.5N-CS. To further assess the pore structural differences of 0.0N-CS, 0.2N-CS, 0.5N-CS, 1.0N-CS, and 3.0N-CS, N2 sorption isotherms were measured at 77 K, and the detailed textural parameters were summarized in Table S1. As described in Figure 2a, N-CS share combined characteristic of type І/IV.31,32 It is clear that all the samples possess the analogous adsorption quantities of 200 cm3 g-1 at low relative pressures, which implies the presence of comparable amounts of micropores before and after HNO3 oxidation. Moreover, the parallel hysteresis loops are also observed in N-CS at the high relative pressure range (P/P0 > 0.70), which indicate that HNO3 hydrothermal oxidation is a mild approach to alleviate the meso-macroporous structural collapse. Figure 2b shows the pore size distributions (PSD) profiles of N-CS centered on bimodal



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distribution with micropores (

Rational Surface Tailoring Oxygen Functional Groups on Carbon

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