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Bimetallic-organic coordination polymers to prepare N-doped hierarchical porous carbon for high performance supercapacitors
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Bimetallic-organic coordination polymers to prepare N-doped hierarchical porous carbon for high performance supercapacitors Danchen Fua, Zeyu Chenb, Chuying Yub, Xiaolan Songa,∗∗, Wenbin Zhongb,∗ a b
School Minerals Processing and Bioengineering, Central South University, Changsha, 410083, China College of Materials Science and Engineering, Hunan University, Changsha, 410082, China
ARTICLE INFO
ABSTRACT
Keywords: Bimetallic-organic coordination polymers N-doped porous carbon High specific surface area Supercapacitors
Single metal-organic coordination polymers have limited functions as precursors for porous carbon electrode materials. The construction of bimetallic organic coordination polymers can effectively utilize the advantages of each single metal-organic coordination polymer to improve the performance of the derived carbon materials. Herein, High performance nitrogen-doped porous carbon (BCFe–Ni) have been produced by directly carbonizing bimetallic organic coordination polymers formed by 4,4′-bipyridine (BPD) reaction with FeCl3 and NiCl2. The BCFe–Ni exhibits high nitrogen content (12.66 at%), large specific surface area (1049.51 m2 g−1) and hierarchical porous structure, which contributes to an excellent gravimetric specific gravity of 320.5 F g−1 and 108% of specific capacitance retention after 10000 cycles. The BCFe–Ni assembled symmetrical supercapacitor shows an energy density of 18.3 W h kg−1 at a power density of 350 W kg−1. It is expected that the as-prepared N-doped porous carbon derived from bimetallic-organic coordination polymer is a promising electrode material for high performance energy storage devices.
1. Introduction Supercapacitors have the characteristics of high power density, good cycle stability and high rate capacity, which show great prospect in the field of clean energy storage [1,2] Porous carbons are the most widely used as electrode materials for electric double-layer supercapacitors because of their excellent properties such as high specific surface area, excellent conductivity and good chemical stability [3–5]. However, carbon materials exhibit a general capacitance of 100–200 F g−1, which greatly limits their practical applications. One effective way to improve the electrochemical properties of porous carbon is to increase the specific surface area and optimize pore size distribution [1,2,6–8]. The construction of hierarchical porous structure is helpful as the micropores can provide a large number of ion absorption sites to enhance the charge storage capacity, and mesoporous can shorten the distance of ion transport and promote the transport of ions, while macropores can serve as ion buffer pools and provide a stable supply of electrolyte ions [6,9]. The introduction of heteroatoms such as nitrogen atoms into the porous carbon framework can provide pseudo-capacitance and significantly improve the wettability of the materials [10,11]. Therefore, the heteroatom doping is considered as another feasible approach to effectively improve the
electrochemical performance of the carbon materials [5,12,13]. It is well known that the development of carbon materials with high nitrogen content and large specific surface area is of great importance. Metal-organic coordination polymers (MOCPs) or metal-organic frameworks (MOFs) that composed of metal ions and organic ligands through coordination bonds have been used in gas storage and separation, catalysis, sensors, energy storage and conversion, and so on [14–18]. It is reported that MOCPs and MOFs show excellent electrochemical performance and are promising electrode materials. For example, Huawei Song and co-works prepared one-dimensional MOF nanowires and used in lithium ion batteries, which exhibit high energy density of 875 W h kg−1 at a power density of 6422 W kg−1 [19]. The MOF bulks incorporated with various dimensions inorganic nano-domains show largely enhanced Li-ion charge storage capability, which is twice more than that of the metal-based MOF bulks [20]. On the other hand, when nitrogen-containing organic molecules are selected as ligands, the nitrogen atoms existed in the ligands can be well maintained in the carbon skeleton after pyrolysis, resulting in the formation of nitrogen-doped porous carbon [21–25]. For example, MOF-5 [26], CYCU-3 [27], ZIF-8 [24,25,28,29], ZIF-67 [23] and ZIF-7 [30] were used as precursors to prepare nitrogen-doped porous carbons. In our previous work, N-doped porous carbon/multi-walled carbon nanotubes
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Song), [email protected] (W. Zhong).
∗∗ ∗
https://doi.org/10.1016/j.pnsc.2019.08.014 Received 19 June 2019; Received in revised form 29 August 2019; Accepted 30 August 2019 1002-0071/ © 2019 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Danchen Fu, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.08.014
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with high N content has also been produced by the carbonization of multi-walled nanotubes and MOCP composite, in which MOCP was formed by coordination reaction between 4,4′-bipyridine (BPD) and FeCl3. [31] Moreover, high electrochemical performance N-doped porous carbon/graphene has also been prepared from MOCP/GO composite. The MOCP is formed by the coordination reaction between BPD and various metal ions (Cu2+, Fe3+ and Zn2+) [32]. However, it is difficult for single metal-organic coordination polymer derived porous carbon to achieve simultaneous high nitrogen doping content and high specific surface area. Many efforts have been made to produce carbon materials with high electrochemical performance. For example, biligand metal-organic coordination polymers were synthesized by performing double-coordination reaction between nitrogenous heterocyclic ligand BPD and aromatic amine ligand (p-phenylenediamine (PPD), 4,4′-diaminobiphenyl (AMP) and 3,3′-diaminobenzidine (DAB)) with metal ions. Facile control over the N-doped content, N species, microstructure and graphitization level was fulfilled in the biligand metal-organic coordination polymers derived N-doped porous carbon, which further leads to prominent electrochemical properties [33]. Moreover, it is reported that the ZIF-67 derived porous carbon shows highly graphitic walls and high conductivity by the catalytic graphitization of Co species [23] while the ZIF-8 exhibits large specific surface area and high nitrogen-doping level due to the activation effect of Zn species at high temperature [24,25]. Combining their advantage, bimetallic organic frameworks such as (Cox·Zn1−x(MeIm)2), Zn–Fe-ZIF were exploited as precursors to produce porous carbon materials with tailored functionalities [26,34,35]. In addition, carbon materials derived from Fe and Ni-based MOFs also show many advantages. The nitrogen-doped porous carbon material derived from Fe-MOF possesses a high specific surface area and numerous micropores but low graphitization level [11,16,32], resulting in a high specific capacitance but poor rate capability. On the other hand, the Ni-MOF derived carbons show good rate performance and cycle stability, attributed to the catalytic graphitization effect of Ni and the formation of mesoporous structure. Whereas, the specific capacitance is relatively low due to the low specific surface area [36–39]. Thus, it is promising to design bimetallic MOFs based on synergetic properties for the development of carbon electrode materials [36,40]. In the present work, bimetallic MOCPs with combined advantages of Fe-MOCP and Ni-MOCP have been designed utilizing Ni2+ and Fe3+ as metal ions and 4,4 ′-bipyridine as ligand. High performance porous carbon material (BCFe–Ni) has been successfully derived from the bimetallic MOCP by adjusting the content ratio of Ni2+ and Fe3+ in the polymer. BCFe–Ni possesses a high nitrogen content of 12.66 at%, large specific surface area of 1049.51 m2 g−1, and excellent high gravimetric specific capacitance of 320.5 F g−1. The assembled symmetrical supercapacitor has a superior energy density of 18.3 W h kg−1 at a power density of 350 W kg−1.
into the above mixture. The molar ratio of FeCl3:NiCl2 was 7:3 and the molar ratio of (FeCl3 + NiCl2): BPD was 1:1. The reaction was kept for 12 h. The resulting product was washed with ethanol and then centrifuged under 5000 rpm for 4 times with each for 10 min. The obtained composite was dried in the oven for 12 h at a temperature of 90 °C, and then annealed under nitrogen atmosphere at 650 °C for 2 h at a heating rate of 5 °C min−1. The resulted BCFe–Ni was washed with 2 M HCl (500 mL) and deionized water (2000 mL), and dried in oven at 90 °C for 12 h. The preparation of BCFe and BCNi is identical to that of BCFe–Ni but in absence of NiCl2 and FeCl3, respectively. The molar ratio of metal ions to BPD was in 1:1. 2.3. Characterizations The structures and morphologies of the as-prepared samples were characterized by scanning electron microscope (SEM, Hitachi, S-4800) and transmission electron microscope (TEM, FEI, Titan G2 60–300). The X-ray diffraction (XRD) patterns of the samples were obtained by Brucker D8-Advance diffraction using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) were carried out on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) with a 1486.6 eV Al Kα X-ray source. Raman spectra were recorded at 633 nm laser light on a Raman spectrometer (LR-3, Varian, USA) in the range of 300–3000 cm−1. Pore structure of the samples was analyzed by physical adsorption of N2 at 77 K (Micromeritics ASAP 2020) after vacuum drying at 200 °C overnight. The specific surface area was calculated by Brunauer-EmmettTeller (BET) method and micropore surface area was calculated using tplot method. The pore size distribution (PSD) was obtained via the nonlocal density functional theory (DFT) method. 2.4. Electrochemical measurements
2. Experimental
Preparation of working electrode: The active material, carbon black, polytetrafluoroethylene (PTFE) were mixed in deionized water with a mass ratio of 8:1:1, and ultrasonic for 20 min to achieve a homogeneous slurry. Then slurry was evenly coated on a stainless steel (1 cm2), pressed under 20 MPa for 5 min, and dried in oven at 90 °C for 12 h. The weight of active substances on each electrode is about 2.0 mg. Electrochemical measurements include cyclic voltammetry (CV) electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD) were carried out based on three-electrode system and two-electrode system in an electrochemical workstation (CHI 760E Shanghai, Chenhua, China), using 1 M H2SO4 aqueous solution as electrolyte. In the three-electrode system, platinum (Pt) and saturated calomel were used as counter electrode and reference electrode, respectively. CV and GCD curves were measured in the range of −0.2−0.8 V. EIS was measured in a frequency range of 0.01–105 Hz with the amplitude of 5 mV alternating (AC). In three-electrode system, the gravimetric specific capacitance of the electrode was calculated from GCD curves based on equation (1):
2.1. Materials
Cm =
4,4′-bipyridine (BPD, 98%) was purchased from Meryer Chemical Technology Co, Ltd. (Shanghai, China). Ferric chloride hexahydrate (FeCl3·6H2O), nickel chloride hexahydrate. (NiCl2·6H2O) and anhydrous ethanol were purchased from Sinopharm Chem. Reagent Co, Ltd. (Beijing, China). All chemicals were of analytical reagent grade and used directly without any further purification.
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), m is the mass of the active material (g), t is the discharge time (s), I is the current (A), ΔV is the potential range (V). In two-electrode system, the gravimetric specific capacitance of the supercapacitor electrode was calculated from GCD curves based on equation (2):
Cm =
2.2. Preparation of the BCFe–Ni
I×t . m× V
4I × t m× V
(1)
(2)
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), m is the mass of the active material (g), t is the discharge time (s), I is the current (A), ΔV is the potential range (V). The energy density (E) and the power density (P) were calculated
BCFe–Ni was prepared based on the following process: 1.21 g FeCl3·6H2O and 0.46 g NiCl2·6H2O were dissolved in 50 mL anhydrous ethanol under stirring. 1 g BPD (dissolved in 50 mL ethanol) was added 2
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Fig. 1. SEM images of (a, b) BCNi, (c, d) BCFe and (e, f) BCFe–Ni.
based on equations (3) and (4):
3. Result and discussion
E=
Cm × ( V ) 2 8 × 3.6
(3)
P=
3600 × E t
(4)
The microstructure and morphology of the as-prepared samples were characterized by SEM (Fig. 1). BCNi shows a porous structure composed of wrinkled sheets with thin pore walls, due to the catalytic effect of Ni species (Fig. 1a and b). BCFe presents a sheet-like porous structure with much thicker pore walls, (Fig. 1c and d). BCFe–Ni shows a coral-like crosslinked hierarchical porous structure, which is quite different from that of the BCFe and BCNi, attributed to the synergetic effect of Fe3+ and Ni2+ ions. The morphology of related MOCP precursors were also characterized by SEM, which shows granular accumulation
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), ΔV is the potential range (V), t is the discharge time (s). 3
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Fig. 2. TEM images of (a, b) BCNi, (c, d) BCFe and (e, f) BCFe–Ni at different magnifications (Insets are high magnification results).
structure (Fig. S1). The microstructure of the MOCP precursors are different from the derivative carbon, mainly due to the melting of precursors and effect of metal ions during the pyrolysis process [41]. Fig. 2 shows the TEM images of the as-prepared samples. The TEM images of BCNi, further confirms the sheet-like structure with the appearance of mesopores and macropores and some micropores. Lattice fringes can be observed in the high-resolution transmission electron microscopy (HRTEM) image, indicating a relatively high degree of graphitization. BCFe possesses much thicker sheets, in consistent with the SEM results. The structure is more amorphous with the presence of
abundant micropores. (Fig. 2c and d). The TEM images of BCFe–Ni reveals the coral-like structure, in which hierarchical porous structure containing micropores, mesopores and macropores can be observed. The ordered lattice fringes can be detected in the HRTEM image, indicating a high degree of graphitization and crystallinity. The XRD patterns of the as-prepared samples are shown in Fig. 3a. All samples show a broad peak at about 25.3° and a weak peak at 43.0°, corresponding to the (002) and (110) crystal planes of graphic carbon [42], respectively [28,43]. The (002) peak of BCNi is slightly sharp comparing to that of the BCFe and BCFe–Ni, indicating a relatively higher 4
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Fig. 3. (a) XRD patterns and (b) Raman spectra of BCFe–Ni, BCNi and BCFe.
graphitization level associated to the catalytic effect of Ni species. The Raman spectra of the as-prepared samples are shown in Fig. 3b. Two distinct peaks at ca. 1350 and 1570 cm−1 can be observed in all the samples, which are related to the D and G peaks, respectively. The D peak refers to the defects and disorders in the graphite structure, and the G peak represents the vibration peak of the sp2 hybridized carbon. The integrated intensity ratio of D peak to G peak (ID/IG) reflects the graphitization level of the samples [44,45]. The ID/IG values of BCFe–Ni, BCNi and BCFe are 2.83, 2.81 and 2.98, respectively. The BCNi exhibits the lowest ID/IG value, indicating the highest degree of graphitization. The ID/IG value of BCFe–Ni is mediate compared to that of BCNi and BCFe, due to a synergistic effect of Ni2+ and Fe3+ ions. It is suggested that the graphitization level of the carbons can be controlled by adjusting the molar ratio of Ni2+ to Fe3+. The Raman results are in accordance with the XRD and TEM results. Fig. 4 displays the N2 adsorption/desorption isotherms of the asprepared samples. The BCNi exhibits type IV isotherm with a H3 type hysteresis loop. There is no obvious increase of adsorption at low relative pressures (P/P0 < 0.1), indicating the lack of micropores. The appearance of hysteresis loop at medium relative pressure (0.1 < P/ P0 < 0.85) and uptake at high relative pressure section (P/P0 > 0.85) suggest the existence of mesopores and macropores, respectively [29,46]. The BCFe shows type I isotherm, with a tense adsorption under low relative pressure (P/P0 < 0.1) and sharp increase of adsorption volume at high relative pressure (P/P0 > 0.85) associated to the abundant micropores and macropores, respectively [47]. The BCFe–Ni possesses a type IV isotherm with a sharp increase under low relative
pressure and a steady adsorption under moderate and high relative pressure. It is indicated that the BCFe–Ni has a hierarchical porous structure with dominant micropores and certain amount of mesopores and macropores [47]. These are consistent with the SEM and TEM observance. The specific surface areas of BCFe–Ni, BCNi and BCFe are 1049.51, 488.47 and 1244.71 m2 g−1, respectively (Table 1). The BCFe has the largest specific surface area due to a great deal of Fe species embedded in carbon before acid treatment [32]. The BCNi shows the lowest specific surface area mainly attributed to the lack of micropores. The BCFe–Ni exhibits a mediate specific surface area, correlated to the synergetic effect of Ni2+ and Fe3+. The pore size distributions of all samples obtained based on DFT method are shown in Fig. 4b [48]. The BCFe–Ni and BCFe show a dominant microporous structure with peaks centered at ca. 0.8 and 0.9 nm, respectively. The BCNi has much less micropores with pore size of 1 nm. The pore size distributions of all samples obtained based on BJH methods are shown in the inset of Fig. 4b. The mesopore sizes of the BCNi are much larger than those of the BCFe–Ni and BCFe, result in a much larger average pore size. The BCFe–Ni, BCNi and BCFe have average pore diameters of 3.51, 38.22 and 1.73 nm, respectively. The well-constructed hierarchical porous structure of BCFe–Ni is beneficial for the fast transportation of ions and charges, which can contribute to the improvement of electrochemical performance. The chemical compositions of the samples were analyzed by XPS. The XPS survey spectrum of BCFe–Ni, BCNi and BCFe are shown in Fig. 5a. Three characteristic peaks appeared at 286.4 eV, 400.0 eV and 532.1 eV correspond to C 1s, N 1s and O 1s, respectively. The relative
Fig. 4. (a) Nitrogen adsorption−desorption isotherms and (b) pore size distribution of BCFe–Ni, BCNi and BCFe determined by the DFT method (the inset of the image show the pore size distribution obtained using the BJH method). 5
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components are listed in Table S3. The presence of oxygen containing functional groups can promote the surface wettability. Moreover, the OII and O-III can provide pseudocapacitance in acidic electrolyte based on the following reactions [1,21,51].
Table 1 Characteristic surface areas and pore structures of BCFe–Ni, BCNi and BCFe. Samples
BCNi BCFe BCFe–Ni
BET surface area (m2 g−1)
Total pore volume
Micropore volume
Average pore size
Stotal
Smicro
(cm3 g−1)
(cm3 g−1)
(nm)
488.47 1244.71 1049.51
53.79 1158.70 924.99
4.67 0.54 0.77
0.02 0.45 0.36
38.22 1.73 3.51
> C= O+ H+ + e
>C
COO + H+ + e > C= O+ e
>C
OH
COOH O
(5) (6) (7)
The electrochemical properties of the as-prepared samples in threeelectrode system are shown in Fig. 6. Fig. 6a shows the CV curves of BCNi, BCFe and BCFe–Ni at a scan rate of 5 mV s−1. The samples exhibit rectangular shape with a set of peaks at ca. 0.5 V, implying the coexistence of electrical double layer capacitance (EDLC) and pseudocapacitance. Fig. 6b shows the GCD curves of the samples at 0.5 A g−1. The gravimetric specific capacitances of BCNi, BCFe and BCFe–Ni at current density of 0.5 A g−1 are 228.8, 260.0 and 320.5 F g−1, respectively. The obtained gravimetric specific capacitance is higher or comparable to the report on N-doped porous carbon derived from MOCPs or MOFs [21–25,28,52]. Fig. 6c shows the rate capability of the samples. The BCNi exhibits a relatively low specific capacitance but high rate capability while BCFe possesses an improved specific capacitance but poorer rate capability. It is suggested that the catalytic effect of Ni species can promote the graphitization of carbon and improve the rate performance, but the specific surface areas is relatively low. Whereas, BCFe derived from MOCP owns high the specific surface area and delivered high capacitance, but the formation of amorphous carbon would deteriorate the rate properties. The BCFe–Ni possesses the highest gravimetric specific capacitance which can be attributed to the wellconstructed hierarchical porous structure and large specific surface area by Fe3+ species removed [32]. Meanwhile, the high N-5 and N-6
contents of C, N and O are listed in Table S1. The BCNi, BCFe and BCFe–Ni possess high nitrogen content of 13.99, 13.48 and 12.66 at%, respectively, and oxygen content of 3.64, 7.9 and 5.31 at%, respectively. The nitrogen-doping content in carbon matrix can effectively enhance conductivity and wettability as well as provide pseudocapacitance through redox reactions [43]. The high resolution N 1s spectra of BCNi (Fig. 5b), BCFe (Fig. 5c) and BCFe–Ni (Fig. 5d) can be deconvoluted into four peaks, which can be assigned to pyridinic-N (N-6, ~398.3 eV), pyrrolic-N (N-5, ~400.0 eV), quaternary (N-Q, ~401.1 eV) and pyridine-N-oxide (N-X, 403.4–405.6 eV) [43,49,50]. The relative contents of nitrogen species of all the samples are shown in Table S2. The total contents of N-5 and N-6 in BCNi, BCFe and BCFe–Ni are 57.06% 64.81% and 60.92%, respectively, and the N-Q content is 34.24%, 29.86% and 34.04%, respectively. It is reported that N-Q can improve the conductivity and facilitate the ion transportation while the N-5 and N-6 can enhance the specific capacitance through Faradaic redox reactions [21,34,49,50]. The high resolution O 1s spectra of the samples can be divided into three peaks, namely carbonyl (C]O, O–I, ~531.0 eV); hydroxyl or ether (C–OH/COC, O-II, ~532.4 eV); and carboxyl group (COOH, O-III, ~533.6eV) (Fig. S2). The relative contents of oxygen
Fig. 5. (a) XPS survey of BCFe–Ni, BCNi and BCFe and XPS spectrum of N 1s in (b) BCFe–Ni, (c) BCNi and (d) BCFe. 6
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Fig. 6. (a) CV curves at a scan rates of 5 mV s−1, (b) GCD curves at a current density of 0.5 A g−1, (c) the specific capacitance at different current densities for BCFe–Ni, BCNi and BCFe, (d) Nyquist plots with an amplitude of 5 mV s−1 over a frequency range from 0.01 Hz to 100 kHz (Inset shows the magnified high-frequency regions).
0.6:0.4, 0.7:0.3, and 0.8:0.2, respectively. With the increase of the Fe3+ content, the specific capacitance of the sample first increases and then decreases. It is implied that the presence of the Fe3+ can enlarge the specific surface area, resulted in the improvement of specific capacitance. However, the excess Fe3+ would lead to the formation of abundant micropores, which would not be complete accessed by ion. Additionally, the production of amorphous carbon can also decrease the conductivity. Therefore, the optimism Fe3+/Ni2+ molar ratio is determined to be 0.7:0.3. The electrochemical properties of the samples with a Fe3+/Ni2+ molar ratio of 0.7:0.3 at different pyrolysis temperatures are shown in Figs. S3d–h. The gravimetric specific capacitances of samples prepared at the pyrolysis temperatures of 600 °C, 650 °C and 700 °C are 241.4, 320.5 and 271.4 F g−1, respectively, at a current density of 0.5 A g−1. The sample at a pyrolysis temperature of 650 °C exhibited the highest specific capacitance which can be attributed to a balance between nitrogen content and graphitization level [24,32]. It is known that the nitrogen content would decrease while the degree of graphitization would increase with the increase of the pyrolysis temperature [57,58]. Thus, a pyrolysis temperature of 650 °C is selected. The as-prepared samples were further assembled into symmetrical supercapacitors using 1 M H2SO4 as the electrolyte to evaluate the electrochemical properties and their potential applications in the field of energy storage. The selection of potential window of BCNi, BCFe–Ni and BCFe is shown in Fig. S5. The CV measurements can be stably operated in a potential window of 0–1.4 V without obvious deformation, which is selected as the operation potential window. The CV curves of the BCNi, BCFe–Ni and BCFe present a rectangular-like shape (Fig. 7a). The linearly symmetric triangular shaped GCD curves of BCNi, BCFe–Ni and BCFe at a current density of 0.5 A g−1 are shown in Fig. 7b, which
content as well as high O-II and O-III content can provide pseudocapacitance to further improve the capacitance properties. The excellent rate capability of BCFe–Ni can be attributed to the catalytic effect of Ni2+ that improve the graphitization level and the hierarchical porous structure with relative larger average pore size (3.51 nm) that provide fast ion/charge transportation and more electroactive sites. It is indicated that the incorporation of two different metal ions shows synergetic effect to achieve excellent specific capacitance and rate capability. Fig. 6d shows the Nyquist plots of samples, which were fitted to the equivalent circuit model reported by Hung et al. [53] and Wang et al. [54] The Rct is the charge transfer resistance, W is the Warburg resistance, C is the capacitance and the Rs is the bulk solution resistance. The BCNi, BCFe and BCFe–Ni exhibit Rct of 1.95, 2.34 and 2.07 Ω, respectively, and the Rs of BCNi, BCFe and BCFe–Ni is 1.17 Ω. Near vertical lines were observed in the low frequency region of the Nyquist plots for all the samples. Fig. S3 shows the cycle stability of the as-prepared samples at a current of 10 A g−1. The BCNi, BCFe and BCFe–Ni can retain 100%, 95% and 108% of the initial capacitance, respectively, after 10,000 cycles. Interestingly, the specific capacitance of the BCFe–Ni increases after the cycling test, possibly due to the activation of extra active sites [32,55].This excellent chemical stability can be attributed to relative high graphitization degree, stable reversible redox reaction and excellent nitrogen-oxygen doping function [46,47,56]. The effect of various experimental conditions on the electrochemical properties of the BCFe–Ni has also been investigated (Fig. S4). The electrochemical properties of samples with different Fe3+/Ni2+ molar ratios are shown in Figs. S3a–c. The gravimetric specific capacitance of the samples are 231.4, 290, 320.5 and 293.8 F g−1 at the current density of 0.5 A g−1, in a Fe3+/Ni2+ molar ratio of 0.5:0.5, 7
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Fig. 7. Electrochemical performance of BCFe–Ni, BCNi and BCFe assembled two-electrode symmetric supercapacitor measured in the voltage window of 0–1.4 V: (a) CV curve with a scan rate of 10 mV s−1, (b) Galvanostatic charge/discharge curves at a current density of 0.5 A g−1, (c) The gravimetric specific capacitances at different current density ranging from 0.5 to 20 A g−1, (d) Ragone plots of BCFe–Ni, BCNi and BCFe.
reveal good EDLC capacitive behavior. The gravimetric specific capacitances of BCNi, BCFe–Ni and BCFe are 181.2, 269.2 and 199.7 F g−1, at a current density of 0.5 A g−1, respectively with a capacitance retention of 70.6, 77.1 and 21.7 F g−1, at a current density of 20 A g−1, respectively (Fig. 7c). Fig. 7d shows the Ragone plot of BCFe–Ni, BCNi and BCFe and the comparison with other previously reported works. The BCNi, BCFe–Ni and BCFe possess an energy density of 12.33, 18.32 and 13.59 W h kg−1 at a power density of 350.0 W kg−1, respectively and can maintain an energy density of 4.8, 5.2 and 1.5 W h kg−1 at a power density of 14000 W kg−1, respectively. The energy density of BCFe–Ni is comparable or higher than many reported porous carbon materials [59–64].
high performance carbon materials for energy storage applications.
4. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.08.014.
Conflicts of interest There are no conflicts to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NO. 51803053 and 51873057). Appendix A. Supplementary data
Novel bimetallic organic coordination polymer has been designed by coordination reaction between 4,4′-bipyridine and two metal ions Fe3+ and Ni2+. High performance nitrogen-doped porous carbon BCFe–Ni has been prepared using bimetallic organic coordination polymer as precursor and nitrogen source. The BCFe–Ni possesses hierarchical porous structure, large specific surface area (1049.51 m2 g−1) and high nitrogen content (12.66 at%), due to the synergetic manipulation of two types of metal ions. As a result, BCFe–Ni exhibits a high gravimetric specific capacitance of 320.5 F g−1 at a current density of 0.5 A g−1 and excellent cycle stability of 108% after 10000 cycles. The BCFe–Ni assembled symmetric supercapacitor shows high energy density of 18.3 W h kg−1 at a power density of 350 W kg−1. The strategy to construct bimetallic organic coordination polymer is feasible to prepare
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Original research
Bimetallic-organic coordination polymers to prepare N-doped hierarchical porous carbon for high performance supercapacitors Danchen Fua, Zeyu Chenb, Chuying Yub, Xiaolan Songa,∗∗, Wenbin Zhongb,∗ a b
School Minerals Processing and Bioengineering, Central South University, Changsha, 410083, China College of Materials Science and Engineering, Hunan University, Changsha, 410082, China
ARTICLE INFO
ABSTRACT
Keywords: Bimetallic-organic coordination polymers N-doped porous carbon High specific surface area Supercapacitors
Single metal-organic coordination polymers have limited functions as precursors for porous carbon electrode materials. The construction of bimetallic organic coordination polymers can effectively utilize the advantages of each single metal-organic coordination polymer to improve the performance of the derived carbon materials. Herein, High performance nitrogen-doped porous carbon (BCFe–Ni) have been produced by directly carbonizing bimetallic organic coordination polymers formed by 4,4′-bipyridine (BPD) reaction with FeCl3 and NiCl2. The BCFe–Ni exhibits high nitrogen content (12.66 at%), large specific surface area (1049.51 m2 g−1) and hierarchical porous structure, which contributes to an excellent gravimetric specific gravity of 320.5 F g−1 and 108% of specific capacitance retention after 10000 cycles. The BCFe–Ni assembled symmetrical supercapacitor shows an energy density of 18.3 W h kg−1 at a power density of 350 W kg−1. It is expected that the as-prepared N-doped porous carbon derived from bimetallic-organic coordination polymer is a promising electrode material for high performance energy storage devices.
1. Introduction Supercapacitors have the characteristics of high power density, good cycle stability and high rate capacity, which show great prospect in the field of clean energy storage [1,2] Porous carbons are the most widely used as electrode materials for electric double-layer supercapacitors because of their excellent properties such as high specific surface area, excellent conductivity and good chemical stability [3–5]. However, carbon materials exhibit a general capacitance of 100–200 F g−1, which greatly limits their practical applications. One effective way to improve the electrochemical properties of porous carbon is to increase the specific surface area and optimize pore size distribution [1,2,6–8]. The construction of hierarchical porous structure is helpful as the micropores can provide a large number of ion absorption sites to enhance the charge storage capacity, and mesoporous can shorten the distance of ion transport and promote the transport of ions, while macropores can serve as ion buffer pools and provide a stable supply of electrolyte ions [6,9]. The introduction of heteroatoms such as nitrogen atoms into the porous carbon framework can provide pseudo-capacitance and significantly improve the wettability of the materials [10,11]. Therefore, the heteroatom doping is considered as another feasible approach to effectively improve the
electrochemical performance of the carbon materials [5,12,13]. It is well known that the development of carbon materials with high nitrogen content and large specific surface area is of great importance. Metal-organic coordination polymers (MOCPs) or metal-organic frameworks (MOFs) that composed of metal ions and organic ligands through coordination bonds have been used in gas storage and separation, catalysis, sensors, energy storage and conversion, and so on [14–18]. It is reported that MOCPs and MOFs show excellent electrochemical performance and are promising electrode materials. For example, Huawei Song and co-works prepared one-dimensional MOF nanowires and used in lithium ion batteries, which exhibit high energy density of 875 W h kg−1 at a power density of 6422 W kg−1 [19]. The MOF bulks incorporated with various dimensions inorganic nano-domains show largely enhanced Li-ion charge storage capability, which is twice more than that of the metal-based MOF bulks [20]. On the other hand, when nitrogen-containing organic molecules are selected as ligands, the nitrogen atoms existed in the ligands can be well maintained in the carbon skeleton after pyrolysis, resulting in the formation of nitrogen-doped porous carbon [21–25]. For example, MOF-5 [26], CYCU-3 [27], ZIF-8 [24,25,28,29], ZIF-67 [23] and ZIF-7 [30] were used as precursors to prepare nitrogen-doped porous carbons. In our previous work, N-doped porous carbon/multi-walled carbon nanotubes
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Song), [email protected] (W. Zhong).
∗∗ ∗
https://doi.org/10.1016/j.pnsc.2019.08.014 Received 19 June 2019; Received in revised form 29 August 2019; Accepted 30 August 2019 1002-0071/ © 2019 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Danchen Fu, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.08.014
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with high N content has also been produced by the carbonization of multi-walled nanotubes and MOCP composite, in which MOCP was formed by coordination reaction between 4,4′-bipyridine (BPD) and FeCl3. [31] Moreover, high electrochemical performance N-doped porous carbon/graphene has also been prepared from MOCP/GO composite. The MOCP is formed by the coordination reaction between BPD and various metal ions (Cu2+, Fe3+ and Zn2+) [32]. However, it is difficult for single metal-organic coordination polymer derived porous carbon to achieve simultaneous high nitrogen doping content and high specific surface area. Many efforts have been made to produce carbon materials with high electrochemical performance. For example, biligand metal-organic coordination polymers were synthesized by performing double-coordination reaction between nitrogenous heterocyclic ligand BPD and aromatic amine ligand (p-phenylenediamine (PPD), 4,4′-diaminobiphenyl (AMP) and 3,3′-diaminobenzidine (DAB)) with metal ions. Facile control over the N-doped content, N species, microstructure and graphitization level was fulfilled in the biligand metal-organic coordination polymers derived N-doped porous carbon, which further leads to prominent electrochemical properties [33]. Moreover, it is reported that the ZIF-67 derived porous carbon shows highly graphitic walls and high conductivity by the catalytic graphitization of Co species [23] while the ZIF-8 exhibits large specific surface area and high nitrogen-doping level due to the activation effect of Zn species at high temperature [24,25]. Combining their advantage, bimetallic organic frameworks such as (Cox·Zn1−x(MeIm)2), Zn–Fe-ZIF were exploited as precursors to produce porous carbon materials with tailored functionalities [26,34,35]. In addition, carbon materials derived from Fe and Ni-based MOFs also show many advantages. The nitrogen-doped porous carbon material derived from Fe-MOF possesses a high specific surface area and numerous micropores but low graphitization level [11,16,32], resulting in a high specific capacitance but poor rate capability. On the other hand, the Ni-MOF derived carbons show good rate performance and cycle stability, attributed to the catalytic graphitization effect of Ni and the formation of mesoporous structure. Whereas, the specific capacitance is relatively low due to the low specific surface area [36–39]. Thus, it is promising to design bimetallic MOFs based on synergetic properties for the development of carbon electrode materials [36,40]. In the present work, bimetallic MOCPs with combined advantages of Fe-MOCP and Ni-MOCP have been designed utilizing Ni2+ and Fe3+ as metal ions and 4,4 ′-bipyridine as ligand. High performance porous carbon material (BCFe–Ni) has been successfully derived from the bimetallic MOCP by adjusting the content ratio of Ni2+ and Fe3+ in the polymer. BCFe–Ni possesses a high nitrogen content of 12.66 at%, large specific surface area of 1049.51 m2 g−1, and excellent high gravimetric specific capacitance of 320.5 F g−1. The assembled symmetrical supercapacitor has a superior energy density of 18.3 W h kg−1 at a power density of 350 W kg−1.
into the above mixture. The molar ratio of FeCl3:NiCl2 was 7:3 and the molar ratio of (FeCl3 + NiCl2): BPD was 1:1. The reaction was kept for 12 h. The resulting product was washed with ethanol and then centrifuged under 5000 rpm for 4 times with each for 10 min. The obtained composite was dried in the oven for 12 h at a temperature of 90 °C, and then annealed under nitrogen atmosphere at 650 °C for 2 h at a heating rate of 5 °C min−1. The resulted BCFe–Ni was washed with 2 M HCl (500 mL) and deionized water (2000 mL), and dried in oven at 90 °C for 12 h. The preparation of BCFe and BCNi is identical to that of BCFe–Ni but in absence of NiCl2 and FeCl3, respectively. The molar ratio of metal ions to BPD was in 1:1. 2.3. Characterizations The structures and morphologies of the as-prepared samples were characterized by scanning electron microscope (SEM, Hitachi, S-4800) and transmission electron microscope (TEM, FEI, Titan G2 60–300). The X-ray diffraction (XRD) patterns of the samples were obtained by Brucker D8-Advance diffraction using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) were carried out on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) with a 1486.6 eV Al Kα X-ray source. Raman spectra were recorded at 633 nm laser light on a Raman spectrometer (LR-3, Varian, USA) in the range of 300–3000 cm−1. Pore structure of the samples was analyzed by physical adsorption of N2 at 77 K (Micromeritics ASAP 2020) after vacuum drying at 200 °C overnight. The specific surface area was calculated by Brunauer-EmmettTeller (BET) method and micropore surface area was calculated using tplot method. The pore size distribution (PSD) was obtained via the nonlocal density functional theory (DFT) method. 2.4. Electrochemical measurements
2. Experimental
Preparation of working electrode: The active material, carbon black, polytetrafluoroethylene (PTFE) were mixed in deionized water with a mass ratio of 8:1:1, and ultrasonic for 20 min to achieve a homogeneous slurry. Then slurry was evenly coated on a stainless steel (1 cm2), pressed under 20 MPa for 5 min, and dried in oven at 90 °C for 12 h. The weight of active substances on each electrode is about 2.0 mg. Electrochemical measurements include cyclic voltammetry (CV) electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD) were carried out based on three-electrode system and two-electrode system in an electrochemical workstation (CHI 760E Shanghai, Chenhua, China), using 1 M H2SO4 aqueous solution as electrolyte. In the three-electrode system, platinum (Pt) and saturated calomel were used as counter electrode and reference electrode, respectively. CV and GCD curves were measured in the range of −0.2−0.8 V. EIS was measured in a frequency range of 0.01–105 Hz with the amplitude of 5 mV alternating (AC). In three-electrode system, the gravimetric specific capacitance of the electrode was calculated from GCD curves based on equation (1):
2.1. Materials
Cm =
4,4′-bipyridine (BPD, 98%) was purchased from Meryer Chemical Technology Co, Ltd. (Shanghai, China). Ferric chloride hexahydrate (FeCl3·6H2O), nickel chloride hexahydrate. (NiCl2·6H2O) and anhydrous ethanol were purchased from Sinopharm Chem. Reagent Co, Ltd. (Beijing, China). All chemicals were of analytical reagent grade and used directly without any further purification.
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), m is the mass of the active material (g), t is the discharge time (s), I is the current (A), ΔV is the potential range (V). In two-electrode system, the gravimetric specific capacitance of the supercapacitor electrode was calculated from GCD curves based on equation (2):
Cm =
2.2. Preparation of the BCFe–Ni
I×t . m× V
4I × t m× V
(1)
(2)
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), m is the mass of the active material (g), t is the discharge time (s), I is the current (A), ΔV is the potential range (V). The energy density (E) and the power density (P) were calculated
BCFe–Ni was prepared based on the following process: 1.21 g FeCl3·6H2O and 0.46 g NiCl2·6H2O were dissolved in 50 mL anhydrous ethanol under stirring. 1 g BPD (dissolved in 50 mL ethanol) was added 2
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Fig. 1. SEM images of (a, b) BCNi, (c, d) BCFe and (e, f) BCFe–Ni.
based on equations (3) and (4):
3. Result and discussion
E=
Cm × ( V ) 2 8 × 3.6
(3)
P=
3600 × E t
(4)
The microstructure and morphology of the as-prepared samples were characterized by SEM (Fig. 1). BCNi shows a porous structure composed of wrinkled sheets with thin pore walls, due to the catalytic effect of Ni species (Fig. 1a and b). BCFe presents a sheet-like porous structure with much thicker pore walls, (Fig. 1c and d). BCFe–Ni shows a coral-like crosslinked hierarchical porous structure, which is quite different from that of the BCFe and BCNi, attributed to the synergetic effect of Fe3+ and Ni2+ ions. The morphology of related MOCP precursors were also characterized by SEM, which shows granular accumulation
Where Cm is the gravimetric specific capacitance of the electrode material (F g−1), ΔV is the potential range (V), t is the discharge time (s). 3
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Fig. 2. TEM images of (a, b) BCNi, (c, d) BCFe and (e, f) BCFe–Ni at different magnifications (Insets are high magnification results).
structure (Fig. S1). The microstructure of the MOCP precursors are different from the derivative carbon, mainly due to the melting of precursors and effect of metal ions during the pyrolysis process [41]. Fig. 2 shows the TEM images of the as-prepared samples. The TEM images of BCNi, further confirms the sheet-like structure with the appearance of mesopores and macropores and some micropores. Lattice fringes can be observed in the high-resolution transmission electron microscopy (HRTEM) image, indicating a relatively high degree of graphitization. BCFe possesses much thicker sheets, in consistent with the SEM results. The structure is more amorphous with the presence of
abundant micropores. (Fig. 2c and d). The TEM images of BCFe–Ni reveals the coral-like structure, in which hierarchical porous structure containing micropores, mesopores and macropores can be observed. The ordered lattice fringes can be detected in the HRTEM image, indicating a high degree of graphitization and crystallinity. The XRD patterns of the as-prepared samples are shown in Fig. 3a. All samples show a broad peak at about 25.3° and a weak peak at 43.0°, corresponding to the (002) and (110) crystal planes of graphic carbon [42], respectively [28,43]. The (002) peak of BCNi is slightly sharp comparing to that of the BCFe and BCFe–Ni, indicating a relatively higher 4
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Fig. 3. (a) XRD patterns and (b) Raman spectra of BCFe–Ni, BCNi and BCFe.
graphitization level associated to the catalytic effect of Ni species. The Raman spectra of the as-prepared samples are shown in Fig. 3b. Two distinct peaks at ca. 1350 and 1570 cm−1 can be observed in all the samples, which are related to the D and G peaks, respectively. The D peak refers to the defects and disorders in the graphite structure, and the G peak represents the vibration peak of the sp2 hybridized carbon. The integrated intensity ratio of D peak to G peak (ID/IG) reflects the graphitization level of the samples [44,45]. The ID/IG values of BCFe–Ni, BCNi and BCFe are 2.83, 2.81 and 2.98, respectively. The BCNi exhibits the lowest ID/IG value, indicating the highest degree of graphitization. The ID/IG value of BCFe–Ni is mediate compared to that of BCNi and BCFe, due to a synergistic effect of Ni2+ and Fe3+ ions. It is suggested that the graphitization level of the carbons can be controlled by adjusting the molar ratio of Ni2+ to Fe3+. The Raman results are in accordance with the XRD and TEM results. Fig. 4 displays the N2 adsorption/desorption isotherms of the asprepared samples. The BCNi exhibits type IV isotherm with a H3 type hysteresis loop. There is no obvious increase of adsorption at low relative pressures (P/P0 < 0.1), indicating the lack of micropores. The appearance of hysteresis loop at medium relative pressure (0.1 < P/ P0 < 0.85) and uptake at high relative pressure section (P/P0 > 0.85) suggest the existence of mesopores and macropores, respectively [29,46]. The BCFe shows type I isotherm, with a tense adsorption under low relative pressure (P/P0 < 0.1) and sharp increase of adsorption volume at high relative pressure (P/P0 > 0.85) associated to the abundant micropores and macropores, respectively [47]. The BCFe–Ni possesses a type IV isotherm with a sharp increase under low relative
pressure and a steady adsorption under moderate and high relative pressure. It is indicated that the BCFe–Ni has a hierarchical porous structure with dominant micropores and certain amount of mesopores and macropores [47]. These are consistent with the SEM and TEM observance. The specific surface areas of BCFe–Ni, BCNi and BCFe are 1049.51, 488.47 and 1244.71 m2 g−1, respectively (Table 1). The BCFe has the largest specific surface area due to a great deal of Fe species embedded in carbon before acid treatment [32]. The BCNi shows the lowest specific surface area mainly attributed to the lack of micropores. The BCFe–Ni exhibits a mediate specific surface area, correlated to the synergetic effect of Ni2+ and Fe3+. The pore size distributions of all samples obtained based on DFT method are shown in Fig. 4b [48]. The BCFe–Ni and BCFe show a dominant microporous structure with peaks centered at ca. 0.8 and 0.9 nm, respectively. The BCNi has much less micropores with pore size of 1 nm. The pore size distributions of all samples obtained based on BJH methods are shown in the inset of Fig. 4b. The mesopore sizes of the BCNi are much larger than those of the BCFe–Ni and BCFe, result in a much larger average pore size. The BCFe–Ni, BCNi and BCFe have average pore diameters of 3.51, 38.22 and 1.73 nm, respectively. The well-constructed hierarchical porous structure of BCFe–Ni is beneficial for the fast transportation of ions and charges, which can contribute to the improvement of electrochemical performance. The chemical compositions of the samples were analyzed by XPS. The XPS survey spectrum of BCFe–Ni, BCNi and BCFe are shown in Fig. 5a. Three characteristic peaks appeared at 286.4 eV, 400.0 eV and 532.1 eV correspond to C 1s, N 1s and O 1s, respectively. The relative
Fig. 4. (a) Nitrogen adsorption−desorption isotherms and (b) pore size distribution of BCFe–Ni, BCNi and BCFe determined by the DFT method (the inset of the image show the pore size distribution obtained using the BJH method). 5
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components are listed in Table S3. The presence of oxygen containing functional groups can promote the surface wettability. Moreover, the OII and O-III can provide pseudocapacitance in acidic electrolyte based on the following reactions [1,21,51].
Table 1 Characteristic surface areas and pore structures of BCFe–Ni, BCNi and BCFe. Samples
BCNi BCFe BCFe–Ni
BET surface area (m2 g−1)
Total pore volume
Micropore volume
Average pore size
Stotal
Smicro
(cm3 g−1)
(cm3 g−1)
(nm)
488.47 1244.71 1049.51
53.79 1158.70 924.99
4.67 0.54 0.77
0.02 0.45 0.36
38.22 1.73 3.51
> C= O+ H+ + e
>C
COO + H+ + e > C= O+ e
>C
OH
COOH O
(5) (6) (7)
The electrochemical properties of the as-prepared samples in threeelectrode system are shown in Fig. 6. Fig. 6a shows the CV curves of BCNi, BCFe and BCFe–Ni at a scan rate of 5 mV s−1. The samples exhibit rectangular shape with a set of peaks at ca. 0.5 V, implying the coexistence of electrical double layer capacitance (EDLC) and pseudocapacitance. Fig. 6b shows the GCD curves of the samples at 0.5 A g−1. The gravimetric specific capacitances of BCNi, BCFe and BCFe–Ni at current density of 0.5 A g−1 are 228.8, 260.0 and 320.5 F g−1, respectively. The obtained gravimetric specific capacitance is higher or comparable to the report on N-doped porous carbon derived from MOCPs or MOFs [21–25,28,52]. Fig. 6c shows the rate capability of the samples. The BCNi exhibits a relatively low specific capacitance but high rate capability while BCFe possesses an improved specific capacitance but poorer rate capability. It is suggested that the catalytic effect of Ni species can promote the graphitization of carbon and improve the rate performance, but the specific surface areas is relatively low. Whereas, BCFe derived from MOCP owns high the specific surface area and delivered high capacitance, but the formation of amorphous carbon would deteriorate the rate properties. The BCFe–Ni possesses the highest gravimetric specific capacitance which can be attributed to the wellconstructed hierarchical porous structure and large specific surface area by Fe3+ species removed [32]. Meanwhile, the high N-5 and N-6
contents of C, N and O are listed in Table S1. The BCNi, BCFe and BCFe–Ni possess high nitrogen content of 13.99, 13.48 and 12.66 at%, respectively, and oxygen content of 3.64, 7.9 and 5.31 at%, respectively. The nitrogen-doping content in carbon matrix can effectively enhance conductivity and wettability as well as provide pseudocapacitance through redox reactions [43]. The high resolution N 1s spectra of BCNi (Fig. 5b), BCFe (Fig. 5c) and BCFe–Ni (Fig. 5d) can be deconvoluted into four peaks, which can be assigned to pyridinic-N (N-6, ~398.3 eV), pyrrolic-N (N-5, ~400.0 eV), quaternary (N-Q, ~401.1 eV) and pyridine-N-oxide (N-X, 403.4–405.6 eV) [43,49,50]. The relative contents of nitrogen species of all the samples are shown in Table S2. The total contents of N-5 and N-6 in BCNi, BCFe and BCFe–Ni are 57.06% 64.81% and 60.92%, respectively, and the N-Q content is 34.24%, 29.86% and 34.04%, respectively. It is reported that N-Q can improve the conductivity and facilitate the ion transportation while the N-5 and N-6 can enhance the specific capacitance through Faradaic redox reactions [21,34,49,50]. The high resolution O 1s spectra of the samples can be divided into three peaks, namely carbonyl (C]O, O–I, ~531.0 eV); hydroxyl or ether (C–OH/COC, O-II, ~532.4 eV); and carboxyl group (COOH, O-III, ~533.6eV) (Fig. S2). The relative contents of oxygen
Fig. 5. (a) XPS survey of BCFe–Ni, BCNi and BCFe and XPS spectrum of N 1s in (b) BCFe–Ni, (c) BCNi and (d) BCFe. 6
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Fig. 6. (a) CV curves at a scan rates of 5 mV s−1, (b) GCD curves at a current density of 0.5 A g−1, (c) the specific capacitance at different current densities for BCFe–Ni, BCNi and BCFe, (d) Nyquist plots with an amplitude of 5 mV s−1 over a frequency range from 0.01 Hz to 100 kHz (Inset shows the magnified high-frequency regions).
0.6:0.4, 0.7:0.3, and 0.8:0.2, respectively. With the increase of the Fe3+ content, the specific capacitance of the sample first increases and then decreases. It is implied that the presence of the Fe3+ can enlarge the specific surface area, resulted in the improvement of specific capacitance. However, the excess Fe3+ would lead to the formation of abundant micropores, which would not be complete accessed by ion. Additionally, the production of amorphous carbon can also decrease the conductivity. Therefore, the optimism Fe3+/Ni2+ molar ratio is determined to be 0.7:0.3. The electrochemical properties of the samples with a Fe3+/Ni2+ molar ratio of 0.7:0.3 at different pyrolysis temperatures are shown in Figs. S3d–h. The gravimetric specific capacitances of samples prepared at the pyrolysis temperatures of 600 °C, 650 °C and 700 °C are 241.4, 320.5 and 271.4 F g−1, respectively, at a current density of 0.5 A g−1. The sample at a pyrolysis temperature of 650 °C exhibited the highest specific capacitance which can be attributed to a balance between nitrogen content and graphitization level [24,32]. It is known that the nitrogen content would decrease while the degree of graphitization would increase with the increase of the pyrolysis temperature [57,58]. Thus, a pyrolysis temperature of 650 °C is selected. The as-prepared samples were further assembled into symmetrical supercapacitors using 1 M H2SO4 as the electrolyte to evaluate the electrochemical properties and their potential applications in the field of energy storage. The selection of potential window of BCNi, BCFe–Ni and BCFe is shown in Fig. S5. The CV measurements can be stably operated in a potential window of 0–1.4 V without obvious deformation, which is selected as the operation potential window. The CV curves of the BCNi, BCFe–Ni and BCFe present a rectangular-like shape (Fig. 7a). The linearly symmetric triangular shaped GCD curves of BCNi, BCFe–Ni and BCFe at a current density of 0.5 A g−1 are shown in Fig. 7b, which
content as well as high O-II and O-III content can provide pseudocapacitance to further improve the capacitance properties. The excellent rate capability of BCFe–Ni can be attributed to the catalytic effect of Ni2+ that improve the graphitization level and the hierarchical porous structure with relative larger average pore size (3.51 nm) that provide fast ion/charge transportation and more electroactive sites. It is indicated that the incorporation of two different metal ions shows synergetic effect to achieve excellent specific capacitance and rate capability. Fig. 6d shows the Nyquist plots of samples, which were fitted to the equivalent circuit model reported by Hung et al. [53] and Wang et al. [54] The Rct is the charge transfer resistance, W is the Warburg resistance, C is the capacitance and the Rs is the bulk solution resistance. The BCNi, BCFe and BCFe–Ni exhibit Rct of 1.95, 2.34 and 2.07 Ω, respectively, and the Rs of BCNi, BCFe and BCFe–Ni is 1.17 Ω. Near vertical lines were observed in the low frequency region of the Nyquist plots for all the samples. Fig. S3 shows the cycle stability of the as-prepared samples at a current of 10 A g−1. The BCNi, BCFe and BCFe–Ni can retain 100%, 95% and 108% of the initial capacitance, respectively, after 10,000 cycles. Interestingly, the specific capacitance of the BCFe–Ni increases after the cycling test, possibly due to the activation of extra active sites [32,55].This excellent chemical stability can be attributed to relative high graphitization degree, stable reversible redox reaction and excellent nitrogen-oxygen doping function [46,47,56]. The effect of various experimental conditions on the electrochemical properties of the BCFe–Ni has also been investigated (Fig. S4). The electrochemical properties of samples with different Fe3+/Ni2+ molar ratios are shown in Figs. S3a–c. The gravimetric specific capacitance of the samples are 231.4, 290, 320.5 and 293.8 F g−1 at the current density of 0.5 A g−1, in a Fe3+/Ni2+ molar ratio of 0.5:0.5, 7
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Fig. 7. Electrochemical performance of BCFe–Ni, BCNi and BCFe assembled two-electrode symmetric supercapacitor measured in the voltage window of 0–1.4 V: (a) CV curve with a scan rate of 10 mV s−1, (b) Galvanostatic charge/discharge curves at a current density of 0.5 A g−1, (c) The gravimetric specific capacitances at different current density ranging from 0.5 to 20 A g−1, (d) Ragone plots of BCFe–Ni, BCNi and BCFe.
reveal good EDLC capacitive behavior. The gravimetric specific capacitances of BCNi, BCFe–Ni and BCFe are 181.2, 269.2 and 199.7 F g−1, at a current density of 0.5 A g−1, respectively with a capacitance retention of 70.6, 77.1 and 21.7 F g−1, at a current density of 20 A g−1, respectively (Fig. 7c). Fig. 7d shows the Ragone plot of BCFe–Ni, BCNi and BCFe and the comparison with other previously reported works. The BCNi, BCFe–Ni and BCFe possess an energy density of 12.33, 18.32 and 13.59 W h kg−1 at a power density of 350.0 W kg−1, respectively and can maintain an energy density of 4.8, 5.2 and 1.5 W h kg−1 at a power density of 14000 W kg−1, respectively. The energy density of BCFe–Ni is comparable or higher than many reported porous carbon materials [59–64].
high performance carbon materials for energy storage applications.
4. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.08.014.
Conflicts of interest There are no conflicts to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NO. 51803053 and 51873057). Appendix A. Supplementary data
Novel bimetallic organic coordination polymer has been designed by coordination reaction between 4,4′-bipyridine and two metal ions Fe3+ and Ni2+. High performance nitrogen-doped porous carbon BCFe–Ni has been prepared using bimetallic organic coordination polymer as precursor and nitrogen source. The BCFe–Ni possesses hierarchical porous structure, large specific surface area (1049.51 m2 g−1) and high nitrogen content (12.66 at%), due to the synergetic manipulation of two types of metal ions. As a result, BCFe–Ni exhibits a high gravimetric specific capacitance of 320.5 F g−1 at a current density of 0.5 A g−1 and excellent cycle stability of 108% after 10000 cycles. The BCFe–Ni assembled symmetric supercapacitor shows high energy density of 18.3 W h kg−1 at a power density of 350 W kg−1. The strategy to construct bimetallic organic coordination polymer is feasible to prepare
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