Nitrogen and oxygen co-doped hierarchical porous carbon for high performance supercapacitor electrodes

Chemical Physics Letters 730 (2019) 32–38

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Nitrogen and oxygen co-doped hierarchical porous carbon for high performance supercapacitor electrodes ⁎

T

⁎

Pengzhen Wanga,1, Wanxia Luoa,1, Nannan Guoa, Luxiang Wanga, , Dianzeng Jiaa, , Zongbin Zhaob, Su Zhanga, Mengjiao Xua a

Key Laboratory of Material and Technology for Clean Energy, Ministry of Education, Key Laboratory of Advanced Functional Materials, Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang Autonomous Region, PR China b School of Chemical Engineering, Dalian University of Technology, Dalian, PR China

H I GH L IG H T S

pitch as carbon source to prepare hierarchical porous carbon with high N and O contents. • Using Zn(NO ) as the template, as well as the porogen and N sources. • Choosing • The sample shows a high specific capacity and excellent cyclability as supercapacitor electrode material. 3 2

A R T I C LE I N FO

A B S T R A C T

Keywords: Petroleum pitch Aqua-mesophase pitch Hierarchical porous carbon Supercapacitor electrodes

In this work, a facile template-assisted method has been applied to prepare hierarchical porous carbon (HPC), using the aqua-mesophase pitch (AMP) as the carbon precursor obtained through liquid phase oxidation, zinc nitrate hexahydrate (ZNH) as template, porogen and nitrogen source. The as-obtained HPC exhibits a high specific capacitance of 235.5 F g−1 at a current density of 1 A g−1, excellent rate capability with a capacitance retention of 58.6% at 50 A g−1. This research may deliver a scalable avenue for the design of novel carbon-based nanoarchitecture materials and effective utilization of abundant petroleum pitch resources for energy storage.

1. Introduction Electrical double layer capacitors (EDLCs) have shown great potential for new energy storage devices because of their high power density, fast charge-discharge processes, outstanding reversibility, long cycling life [1–4]. An ideal electrode material is equipped with optimized surface properties, pore structure and conductivity for efficient transport of ions and electrons [5]. Hierarchical porous carbon (HPC) with high specific surface area, excellent electronic conductivity, high chemical stability and simple preparation methods have attracted much attention. The micropores provide effective charge storage, the mesopores equipped with lower ions transport resistance benefit to electrolyte ions transport, while the macropores can reserve electrolyte thereby shortening the transport distances [6]. Therefore, the electrochemical performance depends on the morphologies of carbon materials and their pore structures, which could improve the electrolyte permeability, ion/electron transport, and ion-adsorbing surface area

[1]. By now, the attention of HPC materials is focused on (1) preparation of HPC with controllable pore size, (2) regulation of composition, (3) developing new activation methods, (4) exploring cheap carbon sources. The petroleum pitch is one of the considerably abundant mineral resources which has been extensively utilized for energy engineering, chemical industry and carbon production. In previous researches, the petroleum pitch has already been utilized to synthesize many carbon materials, including carbon fibers [7,8], hollow carbon spheres [9], and graphene nanosheets [10,11]. The general strategy for the synthesis of HPC is the template method with three processes: template replication, carbonization and removal of the template, respectively [12]. Nevertheless, the method is often impeded by complex synthesis processes, uneven pore size distribution, low nitrogen and oxygen content. Many studies have shown that oxygen and nitrogen functional groups in carbon materials can directly participate in redox reaction and lead to pseudocapacitive effect [13–16]. The oxygen and nitrogen functional

⁎

Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (D. Jia). 1 Authors with equal contributions to this work. https://doi.org/10.1016/j.cplett.2019.05.032 Received 1 April 2019; Received in revised form 17 May 2019; Accepted 18 May 2019 Available online 22 May 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.

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groups can also improve the wettability of carbon materials. However, low porosity and limited nitrogen and oxygen content still restricted the electrochemical performance of carbon materials [17]. In this paper, we prepared the aqua-mesophase pitch (AMP) with high oxygen content via liquid phase oxidation of petroleum pitch, and then used template-assisted method to prepare HPC. Through liquid phase oxidation, the surface of AMP is modified with rich oxygen functional groups, and its reactive activity, loading capacity, dispersion in polar solvents are significantly increased. The as-prepared HPC possess several features: enriched in-plane holes, high specific surface area (818 m2 g−1), high nitrogen and oxygen content of 7.13 at.% and 6.48 at.%, respectively. The specific capacitance of HPC1:4 is 235.5 F g−1 at 1 A g−1 with unapparent capacity fading even after 10,000 cycles at 10 A g−1. The excellent electrochemical performance of the HPC1:4 is not only attributed to the moderate specific surface area, but also the high nitrogen and oxygen content, rich active site, sufficient ion migration pathway and high electric conductivity.

Fig. 2. IR spectra of petroleum pitch and AMP.

2. Experimental

2.3. Characterization

2.1. Preparation of the aqua-mesophase pitch (AMP) First, 5 g petroleum pitch (from Kalamayi Oil Field) was put in a three-neck glass flask. Then, 25 mL HNO3 (67 wt%) and 75 mL concentrated H2SO4 (98 wt%) were slowly and carefully added into the flask under continuous stirring. The mixture was heated to 75 °C using water bath and refluxed under this temperature for 6 h. After cooled down to room temperature, the materials were separated by centrifugation and repeatedly washed with deionized water. After drying at 80 °C for 8 h, the aqua-mesophase pitch (AMP) was obtained.

The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM, S-4800, Hitachi, Japan), X-ray diffraction patterns (XRD, D8, Bruker, Germany) using Cu Ka radiation at 3 kW, Fourier transform infrared spectroscopy (FT-IR, VERTEX70, Bruker, Germany), Raman spectroscopy (SENTERRA, Bruker, Germany). The specific surface area and the pore size distribution were calculated according to N2 adsorption – desorption isotherms (ASAP 2020 Analyzer, Micromeritics, USA). The surface element compositions were investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA).

2.2. Preparation of the HPC

2.4. Electrochemical evaluation of materials

HPC was synthesized by one step carbonization and activation process of AMP using zinc nitrate hexahydrate (ZNH) as template and porogen, the schematic of the preparation process of the HPC is shown in Fig. 1. In a typical preparation process, 0.5 g AMP and a certain amount of the ZNH were uniformly dispersed into 50 mL of ethanol with the assistance of ultra-sonication and vigorous stirring. The mixture was dried in an oven at 80 °C to remove ethanol and then carbonized under nitrogen atmosphere at 700 °C for 1 h. Finally, the HPC was obtained by fully removing the template using diluted HCl and washing with deionized water to neutral. The samples were devoted as HPCratio, the “ratio” represents the ratio of AMP to ZNH. For example, HPC1:4 was prepared using 0.5 g AMP and 2 g ZNH. For comparison, 0.5 g petroleum pitch and 2 g ZNH were dispersed into 50 mL tetrahydrofuran, the obtained sample after carbonization was named as PC1:4. The impact of carbonization temperature on micromorphology and electrochemical performance of HPC were shown in Figs. S3 and S4.

All electrochemical measurements were carried out by an electrochemical workstation (CHI-760E, Chenhua, China) at room temperature. The electrochemical tests were constructed by platinum electrode as the counter electrode, standard Hg/HgO electrode as reference electrode, the synthesized samples as working electrode respectively, and 6 M KOH aqueous solution served as electrolyte. The working electrodes were constructed by mixing the as-prepared samples (80 wt %), acetylene black (10 wt%), and polytetrafluoroethylene (10 wt%) to form a uniform slurry, the weight of which is 2 mg. Then the slurry was pressed onto nickel foam (1 cm × 1 cm), and then dried at 110 °C for 10 h in a vacuum oven and compacted at 15 MPa. The voltage window was set from −1 V to 0 V. The electrochemical impedance spectroscopy (EIS) was performed in the range of frequencies between 0.01 Hz and 10 kHz by applying 5 mV voltage.

Fig. 1. Schematic diagram of the HPC preparation process. 33

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Fig. 3. (a) XRD, (b) Raman patterns of the as-prepared HPC.

Fig. 4. (a) XPS survey spectra of HPC1:0, HPC1:3, HPC1:4, HPC1:5 and PC1:4, (b–d) C1s, O1s and N1s spectrum of HPC1:4.

1246 cm−1 and 1726 cm−1, and peaks of symmetrical stretching vibration and asymmetric stretching vibration of aromatic nitro (eNO2) at 1545 cm−1 and 1349 cm−1. The quite broad absorption peak at 3450 cm−1 is stretching vibration absorption bands of the associated OeH after oxidation which demonstrates the existence of carboxyl groups [21]. These results suggest that large amounts of oxygen-containing groups appear in the AMP. The representative XRD patterns of the samples are shown in Fig. 3a. The characteristic peaks at 24° and 43° can be ascribed to the (0 0 2) and (1 0 0) diffraction of graphite structure, respectively [22,23]. The broad and dispersive character of XRD patterns demonstrate that all the HPC are amorphous carbon. In order to investigate the final form of ZNH in the mixture after carbonization, the XRD pattern of the mixture before removal of the template was exhibited in Fig. S1. Compared with the standard XRD pattern of ZnO (JCPDS No. 36-1451) and ZNH directly annealed at 700 °C under N2 atmosphere, it can be concluded that ZNH was turned into ZnO after carbonization. Fig. 3b is the Raman spectra of the as-prepared samples. Two peaks located at 1350 cm−1 and 1580 cm−1 corresponding to the D (disorder and defects) and G (graphitic) band of carbons. Generally, the ratio of ID/IG means its defect density [24–26]. The ratio of ID/IG increases along with

Table 1 Surface composition of the samples derived from XPS analyses. Samples

C (at.%)

N (at.%)

O (at.%)

PC1:4 HPC1:0 HPC1:3 HPC1:4 HPC1:5

88.10 88.80 86.75 86.39 87.89

5.00 5.04 7.00 7.13 6.45

6.91 6.16 6.24 6.48 5.66

3. Results and discussion Fig. 2 shows optical photographs and the FT-IR analysis images of the petroleum pitch and AMP. The characteristic absorption peak at ca. 721 cm−1, 1458 cm−1 and 2927 cm−1 in the spectrum of petroleum pitch can be ascribed to the rocking vibrations of eCH2e, whose intensity is corresponding to numbers of groups. For the AMP, those peaks almost disappear in the spectrum, because of the break of alkane chains between macromolecules and the reduction in the amount of methylene groups [18–20]. The spectrum of AMP exists stretching vibration peaks of sulfonic acid group and carboxyl groups (C]O) at 34

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Fig. 5. SEM images of (a) HPC1:0, (b) HPC1:3, (c1-c2) HPC1:4, (d) HPC1:5, (e) PC1:4.

Fig. 6. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of the HPC.

decrease with the increasing of ZNH. HPC1:4 possesses the highest nitrogen and the second highest oxygen content of 7.13 at.% and 6.48 at. %, respectively. In addition, C 1s, N 1s, O 1s spectra of each sample are presented in Fig. S2. The C 1 s spectrum of the HPC1:4 was proved in the form of CeC, CeN, CeO, C]O and C(O)O (Fig. 4b), and the O 1s region (Fig. 4c) was found in the form of C]O and CeO. Furthermore, the

the increase of the amount of ZNH. More ZNH introduces more nanopores and destroys the graphite crystallite, and thus the HPC1:5 exhibits a high defect density and a relative low degree of graphitization [27]. The XPS survey spectra show that all the samples have enriched oxygen and nitrogen elements on the surface (Fig. 4a). As seen in Table 1, the nitrogen and oxygen contents in HPC first increase then

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insures better conductivity during electrochemical procedure. However, the SEM image of PC1:4 presented in Fig. 5e displays compact block without obvious porous structure. The N2 adsorption and desorption isotherms and pore size distribution of the obtained samples are shown in Fig. 6. All isotherms are type IV isotherm with a typical hysteresis loop in the intermediate relative pressure range, suggesting that these samples possess a hierarchical pore distribution that consists of micropores and mesopores. The formation of micropores are created by the escape of small gas molecules from carbon precursor and ZNH during the carbonization [36], while mesopores are formed after removing ZNH from carbon matrix. The BET surface area and pore structure parameters are shown in Table 2. The specific surface area and microporous volume of PC1:4 are the smallest in all the samples. Both the total pore volume and specific surface area increase with the increasing of ZNH content, however, the microporous volume of the samples is almost unchanged and mesoporous volume increases gradually after adding ZNH, which leads to a decrease of the ratio of micropores to mesopores. The main reason for this is that high ZNH content produces a large number of mesopores, resulting in the collapse of micropores. That is why HPC1:5 has the smallest micropores/mesopores ratio, although it possesses the highest specific area (851.3 m2 g−1) and pore volume (1.059 cm3 g−1). The as-prepared samples have three-dimensional interconnected network architecture, leading to their larger specific surface area, multiple active sites, competent ion migration pathway. In addition, high nitrogen and oxygen contents make HPC more promising as a higher capacitance electrode material for supercapacitors [28]. HPC1:4 possesses the largest area of cyclic voltammetry (CV) curve in all samples, suggesting that it has the largest specific capacitance (Fig. 7a). It is noteworthy that the CV curves of the samples depart from rectangular shape at 10 mV s−1, and the galvanostatic charge-discharge (GCD) profiles (Fig. 7b) are slightly distorted from the shape of triangle. These phenomena suggest that the energy storage of HPC is controlled by electric double layer and pseudocapacitive behavior at the same time [37–40]. The electric double layer capacitance of HPC is derived from the conductive carbon scaffolds with large specific surface area, while the surface nitrogen and oxygen species can introduce extra pseudocapacitance. Nitrogen and oxygen functional groups are introduced into AMP by pitch oxidation using concentrated sulfuric acid

Table 2 Textual parameters of the HPC measured by N2 adsorption-desorption isotherms. Samples

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

Vmicroc (cm3 g−1)

Vmesod (cm3 g−1)

Vmicro/Vmeso

PC1:4 HPC1:0 HPC1:3 HPC1:4 HPC1:5

214.3 249.6 805.1 818.6 851.3

0.254 0.288 0.621 0.651 1.059

0.078 0.097 0.339 0.340 0.357

0.176 0.191 0.282 0.311 0.702

0.443 0.508 1.202 1.093 0.509

a b c d

BET specific surface area (SBET). Total pore volume. Volume of micropores. Volume of mesopores.

high-resolution spectrum of N 1s region (Fig. 4d) of the HPC1:4 can be divided into three different components at 398.6, 400.7 and 403.8 eV corresponding to the pyridinic N, pyrrolic N and pyridinic N+eO− [28–31], respectively. The pyridinic N and pyrrolic N species are more active than pyridinic N+eO− [32], the percentage of different types of N species in the samples are summarized in Table S1. The contents of pyridinic N and pyridinic N+eO− in HPC are higher than that of PC, and pyrrolic N content in the former is lower than that of the later. After adding ZNH, there is little difference in N species content among HPC1:3, HPC1:4 and HPC1:5. The nitrogen and oxygen surface groups not only enhance the hydrophilia of the HPC but also introduce extra pseudocapacitance through faradaic reactions to improve the specific capacitance of HPC-based supercapacitors [33,34]. The morphologies of the samples were characterized by scanning electron microscopes (SEM). As seen in Fig. 5, the enriched in-plane holes obviously appear with the increase of ZNH content. When the mass ratio of AMP/ZNH is higher than 1:3, the morphologies of obtained samples exhibit the nanosheet-type and interconnected with each other (Fig. 5c and d). The addition of ZNH can feasibly vary the cross linkage state of AMP in the curing process and facilitate the generation of the hierarchical porous structure during carbonization process [35]. In addition, there are a large number of macropores and mesopores ascribed to the loose porous structure in the samples, which could reinforce the ion transfer process, the interrelated architecture

Fig. 7. (a) CV curves under 10 mV s−1, (b) GCD profiles under 1 A g−1 of HPC, (c) CV curves of the HPC1:4 under different scan rates from 10 to 500 mV s−1, (d) Specific capacitance of the samples at different specific current density, (e) cycle performance of the HPC1:4 under 10 A g−1, (f) EIS spectra of the samples. 36

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P = E /Δt

and nitric acid solutions. The following redox reactions that give rise to pseudocapacitance could occur [16,41]:

(7)

where *C stands for the carbon network. Pyridinic and pyrrolic nitrogen functional groups were verified to have significant influence on the capacitance through the faradic reactions [42]:

where Cm is the specific capacitance (the mass is the sum of HPC1:4 in two working electrodes), ΔV is the voltage range, Δt is the discharge time. As shown in Fig. S5a, a yellow LED can be lit by HPC1:4-based devices in series. Based on GCD data (Fig. S5b), Ragone plot was plotted in Fig. S5c. The energy density is 10.4 Wh kg−1 at the power density of 250 W kg−1. Moreover, the energy density of 6.6 Wh kg−1 still remains at a high-power density of 5002 W kg−1. These results suggest that HPC1:4 is expected to be the most potential electrode material for the supercapacitor.

*CHeNH2 + 2OH− ⇔ *C]NH + 2H2O + 2e−

4. Conclusion

*CeOH ⇔ C]O + H + e +

−

e*COOH ⇔ e*COO + H + e +

−

*C]O + e ⇔ *CeO

−

(1) −

(2)

−

*CeNH2 + 2OH ⇔ *CeNHOH + H2O + 2e

(3)

−

(4) (5)

In conclusion, hierarchical porous carbon has been successfully prepared from petroleum pitch via a simple liquid phase oxidation method combined with a template-assisted method. The liquid phase oxidation and the addition of ZNH benefit to increasing the contents of N and O, constructing reasonable morphology and pore structure of the samples, which further improve the electrochemical performance of the samples. The total pore volume and specific surface area of the samples increase along with the increasing of ZNH content, while the nitrogen and oxygen contents of the samples first increase to maximum and then decrease. Under the optimum preparation condition, the specific surface area of the HPC1:4 reaches 818 m2 g−1, and the contents of N and O are 7.13 at.% and 6.48 at.%, respectively. In brief, nitrogen and oxygen co-doping can improve the specific capacitance because of pseudocapacitive effect. The specific capacitance is 235.5 F g−1 at 1 A g−1 for HPC1:4 and the capacity is 138 F g−1 at 50 A g−1. The sample has excellent cyclic stability with no obvious capacity degradation even after 10,000 cycles at 10 A g−1. The good electrochemical properties can be attributed to its large specific surface areas, hierarchical structure, and high nitrogen and oxygen contents. This work demonstrates that the AMP is a suitable precursor for the preparation of advanced carbon materials, which will develop a new pathway to convert non-renewable petroleum pitch into clean energy.

The CV profiles of the HPC1:4 at different scanning rates (Fig. 7c) reveal its rapid charge-discharge performance, which is detected even at 500 mV s−1. Based on the GCD test, the specific capacitance of PC1:4 and HPC1:0 are 95.5 and 20F g−1, 125 and 25F g−1 at 1 and 50 A g−1 (Fig. 7d), respectively. The low nitrogen content, small specific surface area and pore volume lead to poor electrochemical performance of PC1:4 and HPC1:0. However, the specific capacitance of the HPC1:4 is up to 235.5 and 138 F g−1 at the same current density. In addition, the HPC1:4 exhibits barely decay of capacity even after 10,000 cycles at 10 A g−1 (Fig. 7e), suggesting the excellent cycling stability. EIS is an effective test method to evaluate resistance and electrochemical kinetics of materials. As shown in Fig. 7f, the Nyquist plots show semicircles in high frequency region of each spectrum, which describe the charge transfer resistances (Rct) for these electrodes. The oblique lines in low frequency region, which are concerned with ion diffusion in the electrode materials [43,44]. Obviously, the radii of HPC samples are decreasing with the increase of ZHN amount, implying the reduced charge transfer resistances of HPC. The slope of oblique lines of HPC samples go up along with the increase of ZNH, reflecting the increase of mass transfer rate between the electrode and electrolyte. The resistance of HPC1:5 is smaller than that of HPC1:4, which can be ascribed to HPC1:5 has higher mesoporous volume, then efficiently facilitate the ions diffusion. However, the capacitance of HPC1:5 is lower than that of HPC1:4, because HPC1:5 has lower ratio of micropore to mesopore, and the smaller content of N, O elements, which can introduce extra pseudocapacitance. Overall, HPC1:4 shows high specific capacitance (235.5 F g−1 at 1 A g−1), excellent rate performance (138 F g−1 at 50 A g−1) and cycle stability (almost no attenuation after 10,000 cycles at 10 A g−1), which are better than that of previously reported carbon materials (Table S2). The excellent electrochemical performance of HPC1:4 should be attributed to the synergistic effect of high nitrogen and oxygen content, large specific surface area and hierarchical porous structure with reasonable pore-size distribution. Firstly, the presence of nitrogen and oxygen functional groups can improve the wettability of HPC1:4 getting the electrolyte easily to contact the electrode, which is conducive to making full use of the specific surface area [34,45]. Additionally, nitrogen and oxygen functional groups can introduce extra pseudocapacitance via redox reactions. Secondly, the large specific surface area provides plenty of active sites to achieve high double layer capacitance. Thirdly, the ratio of micropore to mesopore is close to 1:1 in HPC1:4, high microporous volume can store more charge by effectually adsorbing the electrolyte ions, while the interconnected mesopores reduce the diffusion and transmission path of electrolyte ions, which ensure the excellent rate capability even at high current density [6,46]. To further evaluate its potential for practical applications, a symmetric two-electrode system was assembled in 6 M KOH electrolyte using HPC1:4 as electrode material. The energy density (E) at a power density (P) is calculated according to Eq. (6) and (7) [32]:

E = Cm × (ΔV )2 /2

Acknowledgements This work is supported by the National Nature Science Foundation of China (21671166, U1703251 and 21701138), Program for Tianshan Innovative Research Team of Xinjiang Uygur Autonomous Region (2018D14002). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.05.032. References [1] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [2] S.G. Dai, B.T. Zhao, C. Qu, D.C. Chen, D. Dang, B. Song, B.M. deGlee, J.W. Fua, C.G. Hu, C.P. Wong, M.L. Liu, Controlled synthesis of three-phase NixSy/rGO nanoflake electrodes for hybrid supercapacitors with high energy and power density, Nano Energy 33 (2017) 522–531. [3] J.H. Li, Z.C. Liu, Q.B. Zhang, Y. Cheng, B.T. Zhao, S.G. Dai, H.H. Wu, K.L. Zhang, D. Ding, Y.P. Wu, M.L. Liu, M.S. Wang, Anion and cation substitution in transitionmetal oxides nanosheets for highperformance hybrid supercapacitors, Nano Energy 57 (2019) 22–33. [4] Q.B. Zhang, Z.C. Liu, B.T. Zhao, Y. Cheng, L. Zhang, H.H. Wu, M.S. Wang, S.G. Dai, K.L. Zhang, D. Ding, Y.P. Wu, M.L. Liu, Design and understanding of dendritic mixed-metal hydroxide nanosheets@N-doped carbon nanotube array electrode for highperformance asymmetric supercapacitors, Energy Storage Mater. 16 (2019) 632–645. [5] Y.P. Zhai, Y.Q. Dou, D.Y. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828–4850. [6] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. 120 (2008) 379–382.

(6)

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