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Organic 3D interconnected graphene aerogel as cathode materials for high-performance aqueous zinc ion battery
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Organic 3D interconnected graphene aerogel as cathode materials for high-performance aqueous zinc ion battery Ruibai Cang, Ke Ye∗, Kai Zhu, Jun Yan, Jinling Yin, Kui Cheng, Guiling Wang, Dianxue Cao∗
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Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang, China
a r t i c l e
i n f o
Article history: Received 3 August 2019 Revised 26 September 2019 Accepted 27 September 2019 Available online xxx Keywords: Aqueous battery Zinc ion battery Poly 3,4,9,10-perylentetracarboxylic dianhydride Graphene aerogel Cathode materials
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a b s t r a c t Aqueous rechargeable zinc ion batteries are very attractive in large-scale storage applications, because they have high safety, low cost and good durability. Nonetheless, their advancements are hindered by a dearth of positive host materials (cathode) due to sluggish diffusion of Zn2+ in the solid inorganic frameworks. Here, we report a novel organic electrode material of poly 3,4,9,10-perylentetracarboxylic dianhydride (PPTCDA)/graphene aerogel (GA). The 3D interconnected porous architecture synthesized through a simple solvothermal reaction, where the PPTCDA is homogenously embedded in the GA nanosheets. The self-assembly of PPTCDA/GA coin-type cell will not only significantly improve the durability and extend lifetime of the devices, but also reduce the electronic waste and economic cost. The self-assembled structure does not require the auxiliary electrode and conductive agent to prepare the electrode material, which is a simple method for preparing the coin-type cell and a foundation for the next large-scale production. The PPTCDA/GA delivers a high capacity of ≥200 mAh g–1 with the voltage of 0.0∼1.5 V. After 300 cycles, the capacity retention rate still close to 100%. The discussion on the mechanism of Zn2+ intercalation/deintercalation in the PPTCDA/GA electrode is explored by Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations. The morphology and structure of PPTCDA/GA are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction With the development of science and technology, human living standards have improved. Rapid development has caused our environmental pollution problems to become more and more serious, so developing green energy storage devices is one of the significant works. The aqueous rechargeable battery owns the advantages of environmentally friendly nature, low cost and high safety. The first aqueous rechargeable battery was reported in 1994 [1], named as the aqueous lithium rechargeable battery. The LiMn2 O4 was used as cathode and V2 O5 as anode in 5 and 0.001 mol dm–3 LiNO3 aqueous electrolyte, but the Li ion battery did not have a long cycling performance [2]. Until the emergence of phosphates family appeared in 2007, LiV3 O8 , Lix TiO2 and Li(Na)Ti2 (PO4 )3 used as anode in lithium (sodium) rechargeable batteries solved the problem that the rechargeable aqueous lithium (sodium) batteries does not have a suitable anode materials [3–6]. However, in a multivalent
∗
Corresponding authors. E-mail addresses: [email protected] (K. Ye), [email protected] (D. Cao).
ionized (e.g. magnesium; zinc) aqueous battery, the anodic materials were still few. In addition, the magnesium metal as anode was also easy to be oxidized in the air. Thus, at present, the development of aqueous magnesium ion rechargeable battery was limited. Luckily, zinc metal is stable in the air and can be used as anode in aqueous battery, so the V2 O5 , MnO2 and Prussian blue analogs as cathode have been studied for rechargeable aqueous zinc batteries [7]. In the 1980s, conductive polymers were extolled as promising materials for the next generation of environmentally benign and efficient batteries [8–11]. Organic compounds have many typical features. For example, the main feature is the diversity and flexibility of the structure and molecular-level designability. This is the reason that there are many organic compounds materials used in lithium (sodium) battery [12,13] and the used electrode materials have excellent specific capacity, cycling stability and high rate capability [14]. Conjugated organic materials are the most typical electrode materials [15], such as Na2 TP, Na2 DBQ and 3Q of aqueous rechargeable sodium batteries [16–18]. At present, most rechargeable aqueous zinc ion battery used zinc metal as anode, confining p-chloranil inside nanochannels of mesoporous nanochannels of CMK-3 as cathode in aqueous zinc ion electrolyte, which
https://doi.org/10.1016/j.jechem.2019.09.026 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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delivers a high capacity of 200 mAh g–1 [12]. The acidity and alkalinity of electrolyte have great influence on the capacity of hydrogen and oxygen evolution. PTO as cathode in mild aqueous electrolyte can deliver a high capacity of 336 mAh g–1 and the problem of hydrogen and oxygen evolution can be solved [19]. However, most organic materials included poly (3,4,9,10perylentetracarboxylic dianhydride) (PPTCDA) of typical conjugated carbonyl polymer have the low conductivity. In order to improve the conductivity of the electrode material, we used the one-step synthesis method to prepare the PPTCDA/graphene aerogel (GA) (PPTCDA/GA). As known, GA is a material with large open pores, which can promote the transfer speed of electrons to achieve a stable effect of long cycle and make the electrode material fully contact and store sufficient electrolyte to improve the electrochemical performance for aqueous rechargeable batteries. The GA has a large surface area and ultra-light density, in which 3D porous structure can improve their electrical conductivity [20–22]. The conductivity of the electrode material is largely improved by combining the PPTCDA with GA, and the electrode material is obtained with better performance. Herein, we reported an aqueous cointype cell with the PPTCDA/GA as cathode and zinc metal foil as anode in the aqueous zinc solution. The PPTCDA/GA materials can effectively solve the problem of poor conductivity of traditional organic materials, which can be used as cathode in the coin-type full cell. Due to the self-assembly of PPTCDA/GA, the coin-type full cell does not require a binder and a conductive agent in a mild 2.0 mol L–1 ZnSO4 aqueous electrolyte (pH≈6). All components in this battery are low toxic (or not toxic) and beneficial to the development of environmental protection. The GA provides a high ionic conductivity in the neutral environment. Overall, the PPTCDA/GA||Zn coin-type full cell has a high specific capacity of 281 mAh g–1 at the current density of 100 mA g–1 and a high coulombic efficiency close to 100%. Most importantly, a cell stack with the specific capacity of 281 mAh g–1 can be continuously operated for 300 cycles at the current density of 100 mA g–1 . Compared with the traditional aqueous batteries, the coin-type cell was more portable with good reliability and applicability for this newgeneration PPTCDA/GA||Zn.
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2. Experimental
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2.1. Characterizations
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The reversible Zn-ion coordination mechanisms on PPTCDA/GA cathode were examined by ex-situ Fourier transform infrared spectroscopy (FT-IR). The elemental distributions of PPTCDA/GA electrodes were characterized by scanning transmission electron microscopy (STEM). The elemental existence and valent state of PPTCDA/GA electrodes before and after discharge/charge was examined by X-ray photoelectron spectroscopy (XPS).
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2.2. Electrode preparation and battery fabrication
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The PPTCDA/GA was set in a mild 2.0 mol L–1 ZnSO4 aqueous solution after 24 h. For the electrochemical test in a typical cointype cell, the PPTCDA/GA diameter is 14 mm and the mass loading of PPTCDA/GA is 3.0 mg. A piece of Zn metal foil (≥99.999%, SigmaAldrich) was used as anode and also as counter electrode for cointype cell. The glass microfiber filters were used as separator.
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3. Results and discussion
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The preparation process of PPCTDA was referenced in the previous work [23]. The SEM and TEM images of PPTCDA/GA electrode material were shown in Figs. 1 and 2. It was clear from the SEM images (Fig. 1) that the PPTCDA with an irregular rod
Fig. 1. SEM images of the PPTCDA/GA with 3D interconnected porous structure.
Fig. 2. TEM images of the PPTCDA/GA with 3D interconnected porous structure.
structure formed on the silk GA. The high-magnification SEM image (Fig. 1d) confirmed that the PPTCDA was wrapped in the silky graphene and kept its original appearance. To further investigate the detailed microstructure of the synthesized PPTCDA/GA materials, TEM test was carried out. From typical TEM images in Fig. 2, the monodispersed PPTCDA nanorod with irregularity size are embedded in the GA nanosheets. To check the change of structure and phase for the obtained PPTCDA and PPTCDA/GA, the X-ray diffraction (XRD) measurement was employed (Fig. 3a). In the XRD pattern of PPTCDA/GA, the broad diffraction peak located at 25.5°, corresponding to GA. However, there was no peak at 25.5° in the PPTCDA pattern, suggesting the PPTCDA/GA successfully synthesized. In order to further determine the successful combination PPTCDA with GA, FT-IR measurement was utilized. Fig. 3(b) shows the FT-IR spectra of the PPTCDA (red line) and PPTCDA/GA (blue line). From the FT-IR spectra of PPTCDA/GA, a peak appeared at ∼2926 cm–1 , which was corresponded to the -CH2 stretching vibration group originated from GA. In addition, the carbonyl bonds (C=O) peak at ∼1650 cm–1 and C=N peak at ∼1708 cm–1 were due to the imprint groups from PPTCDA. It is clearly seen that the PPTCDA and graphene oxide (GO) precursor exhibits a lower crystalline before calcination, which is in accordance with the reported work [11,24]. The synthesis process of PPTCDA/GA is illustrated in Scheme 1. The GO hydrogel was changed to the graphene hydrogel (GH) via a hydrothermal method after heating at 150 °C for 16 h in the 20 mL Teflon-lined stainless steel. Then the as-prepared PPTCDA/GH was freeze-dried for 24 h to obtain the PPTCDA/GA. The characteristic peaks of GO are significantly higher than that of the original PPTCDA. After 24 h freeze drying of PPTCDA/GA, the PPTCDA/GA was soaked in the 2.0 mol L–1 ZnSO4 aqueous solution for 24 h. Afterwards, the PPTCDA/GA was used as cathode and zinc metal as anode to assemble as the coin-type cell (Figs. S1 and S2). In addition, the stability of PPTCDA/GA demonstrated as a video show in the additional supporting information.
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 3. XRD patterns (a) and FT-IR spectra (b) of PPTCDA and PPTCDA/GA.
Scheme 1. Schematic illustration of the synthesis process of PPTCDA/GA.
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The electrochemical performance of PPTCDA/GA has been studied versus Zn by employing cyclic voltammetry (CV) at different scan rates from 0.1 to 1.0 mV s–1 in the coin-type battery test. Fig. 4(a) shows the electrochemical behavior of PPTCDA/GA in the 2.0 mol L–1 ZnSO4 aqueous solution. The three oxidation peaks and two reduction peaks were clearly observed in the CV curves, which are the same as those in the galvanostatic charge-discharge test. The oxidation peaks at 0.66 V and 0.73 V are very close, which is likely due to the intercalation process of Zn2+ in the PPTCDA/GA with the C=O conversion to C–O. The voltage gap between the redox peaks (called as electrode polarization) reflects the electrochemical kinetics speed for a specific electrode reaction. The calculated b-values of the five peaks are 0.650, 0.670, 0.690, 0.680 and 0.673 in the 2.0 mol L–1 ZnSO4 aqueous electrolyte (Fig. 4b), which means that the charge storage is affected by the diffusion process. This is the main reason that the PPTCDA/GA materials as cathode have a high capacity. From cyclic voltammetry (CV) test, there are multiple redox peaks, indicating that the process of zinc ions embedded in the materials is a slow process. Meanwhile, the contribution of PPTCDA/GA||Zn coin-type cell to the capacitance of the battery was determined by combining the relationship between capacitive and diffusion control behavior. Fig. 4(c) shows a typical voltage profile of capacitive current (violet region) in comparison with the total current of the zinc coin-type cells. Hence, the capacitance ratio in the battery can be determined by Zn2+ semi-infinite linear. The b value can be obtained through the algorithm of Dunn’s method [25], implying that excessive scan rate has a great influence on capacitive behavior. Thus, the scan rate was selected to be 0.1 mV s–1 and the surface-control contribution is 41.7% for the Zn2+ storage. As the PPTCDA/GA used as cathode in the coin-type cells with different scan rates of 0.2, 0.3, 0.5, 0.8 and
1 mV s–1 , the surface control contribution is 41.7%, 45.6%, 45.8%, 48.0%, 47.8% and 48.1% (Fig. 4d), respectively, which is responsible for a high-rate capability of the coin-type cell. Galvanostatic charge-discharge cycling of the coin-type cell with the PPCTDA/GA as cathode and zinc foil as anode in 2.0 mol L–1 ZnSO4 aqueous electrolyte are shown in Fig. 5(a). The galvanostatic charge-discharge measurements are tested in the electrochemical window of 0.0 V to 1.5 V, which is the safe voltage range for the aqueous battery. According to the capacity of the anode and cathode, the specific anode/cathode mass ratio was balanced at 1:1. The coin-type cell shows a reversible specific capacity of 281 mAh g–1 at a current density of 100 mA g–1 (The galvanostatic charge/discharge of PPTCDA as cathode was also shown in Fig. S3). At the same time, the pH value of electrolyte were accordingly adjusted (pH≈6, 4.5 and 3.1), the electrode material has the best electrochemical performance in the electrolyte concentration with pH≈6.0 (Fig. S4). The coin-type cell had good cycle stability and the cell charge-discharge curve was not significantly changed at 1st, 2nd, 3rd, 5th, 10th, 50th and 100th. The PPTCDA/GA as cathode in the aqueous rechargeable zinc ion battery can be utilized as promising large-scale energy storage technologies. In addition, the coin-type cell shows the divalent Zn-ion can be reversibly intercalated/deintercalated from the PPTCDA/GA. The rate capability of the coin-type cell in aqueous 2.0 mol L–1 ZnSO4 electrolyte was investigated and showed the excellent electrochemical performance with a specific capacity of 281 mAh g–1 at the current density of 100 mA g–1 . When the current density increased to 10 0 0 mA g–1 , the electrode was not polarized and maintained good cycle stability. After 300 cycles, the specific capacity of 102 mAh g–1 was maintained (Fig. 5c). Furthermore, PPTCDA/GA showed excellent cyclic capacity retention without any capacity decrease after 300 cycles and also showed excellent cycling stability with close to 100% (Fig. 5b). In the meantime, we also need to consider the problem of metallic dendrites of the anodic zinc sheet. The cycle stability of the coin-type cell was tested for 300 cycles. There was no obvious change in the cycle stability process, proving that the electrode material has good stability (Fig. S5). Previous report demonstrated that the ion-coordination reaction may lead to the dissolution of organic electrode materials, which should be urgently solved [17]. Fortunately, in this work, after polymerization the PTCDA,
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 4. (a) Cyclic voltammetry curves at different scan rates. (b) The b-value of different redox peaks, determined from the log(i) versus log(v) plots. (c) The k1 analysis of PPTCDA/GA electrode at 0.1 mV s–1 . (d) The contribution radio of different scan rates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the PPTCDA does not cause the dissolution in mild aqueous solution. The 3,4,9,10-perylentetracarboxylic dianhydride and ethylene diamine (EDA) were used by reflux to obtain the PPTCDA product. The redox graphite was coated on PPTCDA with PPTCDA/GA. The coin-type cell not only has good stability in aqueous solution but also greatly enhances the electrical conductivity of the electrode material resulted from Nyquist plots of zinc metal foil and PPTCDA/GA electrode. The electrochemical impedance spectroscopic (EIS) measurement of PPTCDA and PPTCDA/GA was showed in Fig. S6. A typical Nyquist plot shows the PPTCDA/GA has better conductivity. In order to further prove the good electrical conductivity of PPTCDA/GA, the battery was tested at different current densities of 50, 10 0, 20 0, 30 0, 50 0 and 10 0 0 mA g–1 , and the specific capacities maintain at 305, 281, 255, 200, 173 and 103 mAh g–1 in the aqueous 2.0 mol L–1 ZnSO4 electrolyte (Fig. 5c and d), which is higher than that of other aqueous rechargeable batteries, such as 140 mAh g–1 of spinel Mn3 O4 //Zn [26], 81.5 Wh kg–1 of PVA/Zn(CF3 SO3 )2 ) [27], 97 mAh g–1 of Zn||ZVO [28], 59.2 mAh g−1 of Zn3 [Fe(CN)6 ]2 ||Zn [29]. Meanwhile, the comparison of electrochemical performance on the different zinc ion battery systems is listed in Table 1 and exhibited in Fig. S7 [18–21,26,27,30–32]. For further verifying the mechanism of Zn2+ intercalation and deintercalation in the PPTCDA/GA, the PPTCDA/GA was used as
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Table 1. A comparison of the conditions used for different zinc battery systems. Cathode/anode/Zn
Electrolyte
Voltage
Spinel Mn3 O4 [26] PVA/Zn(CF3 SO3 )2 ) [27]
2.0 M ZnSO4 Self-healing electrolyte 1.0 M ZnSO4 0.5 M Zn(CH3 COO)2 3 M ZnSO4 0.2 M ZnSO4 1.0 M ZnSO4 1.0 M Zn ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 2.0 M ZnSO4
1.2 1.35
201 Wh kg−1 81.5 Wh kg−1
1.5 1.1 1.73 1.73 0.65 1.8 0.7 1.5 1.5 1.5 1.5
150 Wh kg−1 97 mAh g−1 59.2 mAh g−1 52.5 mAh g−1 62.7 Wh kg−1 185 mAh g−1 80.5 Wh kg−1 168 Wh kg−1 118 mAh g−1 102 Wh kg−1 225.75 Wh kg−1
p-chloranil/CMK [21] Na3 V2 (PO4 )3 [30] Zn3 [Fe(CN)6 ]2 //Zn [29] CuFeCN6 [15] VS2 [12] Zn||ZVO [28] Zn0.25 V2 O5 nH2 O [26] ZMD/MnO2 [31] V0.05 MnO2 [32] Fe(CN)6 [33] OUR WORK
Capacity/Energy density
cathode and zinc as anode in the coin-type cell. Seven points of the galvanostatic discharge/charge at 100 mA g–1 were selected to measure the ex FT-IR (Fourier-transform infrared). Fig. 6(a) shows the galvanostatic charge/discharge potential profiles for the discharge (from initial state (1) to 1.5 V (7)). The point (1) is the fresh electrode, corresponding to the -CH2 stretching vibration group at
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 5. The electrochemical performance of the PPTCDA/GA as cathode and zinc foil as anode for the full cell in 2.0 mol L–1 ZnSO4 aqueous solution. (a) Galvanostatic charge/discharge potential profiles of the 1st, 2nd, 3rd, 5th, 10th, 50th, and 100th cycles at the current density of 100 mA g–1 in a potential range of 0.0∼1.5 V. (b) Cycle performance at a current density of 100 mA g–1 . (c) Galvanostatic charge/discharge at 50, 10 0, 20 0, 30 0, 50 0 and 10 0 0 mA g–1 . (d) Cycle stability at a current density of 100 mA g–1 .
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∼2926 cm–1 originated from GA [21], carbonyl bonds (C=O) at ∼1650 cm–1 and C=N at ∼1708 cm–1 from PPTCDA (Fig. 3b). To further prove the group structure of the electrode material, the four points from (1) to (4) for the discharge to 0.0 V were selected and measured by FT-IR test. The four points were the discharge of the coin-type cell. During the discharge, the carbonyl groups shrinkage vibration peak gradually weakens. At the same time, the enolate groups (∼1375 and ∼1105 cm–1 ) shrinkage vibration peak stretch remains progressive enhancement, indicating that the process of zinc ion embedding is the carbonyl groups stretch and peter out to enolate groups. Furthermore, there appears a second carbonyl bonds (C=O) at ∼1650 cm–1 , which is the skeleton vibration of benzene ring. Thus, the process of zinc ion intercalation is the process of breaking the carbonyl groups double bond into an ethanol groups. As the double bond breaks, the zinc-ion inserts into the enolate group. At the (5)–(7) points, corresponded to the PPTCDA/GA cathode charge to 1.5 V, the enolate groups shrinkage vibration peak gradually weakens. At the same time, the carbonyl groups shrinkage vibration peak stretch remains progressive enhancement, until returning to the same stretching vibration peak as the fresh electrode. In all the discharge/charge process, the -CH2 group shows no change. So the zinc ion can freely insert/deinsert in the PPTCDA/GA and no damage to the GA group in the Zn2+ intercalation/(de)intercalation process was observed. This phenomenon is quite different from that of monovalent ion such as Li+ , Na+ , or K+ (the existed problems of va-
lence and ion radius). A carbonyl group space can not accommodate one zinc ion due to the limitation of enolate group space. The enolate groups can not possibly come from a single PPTCDA/GA molecule, and so it must be no less than two adjacent enolate group molecules in the PPTCDA/GA that coordinate to one Zn2+ . The cathode is a PPTCDA/GA with the condensation of PPTCDA and GA (Scheme S1). The monomer is denoted as R, each monomer contains two imide groups for Zn2+ , and oxygen from adjacent monomers can ligate a single Zn2+ , leading to the stoichiometry of Zn-R. In the real situations, the metal cations can be thermally distributed in the polymer chain, and their definite positions relative to the monomers are not fixed. For the qualitative purposes and consideration of computational cost, we truncated the polymer chain into monomers. The ratio of Zn2+ and R anion is determined by charge balance, which is an assumption that charge balance can be achieved in the real case. Fig. 6(c) shows the XPS spectra of discharge to 0.0 V (red line) and the fresh electrode (black line) of PPTCDA/GA-cathode. For the fresh electrode, the C 1 s (285.0 eV) and O 1 s peaks from GA and PPTCDA can be clearly seen. When discharge to 0.0 V (red line), the characteristic peak of Zn 2P1/2 and Zn 2P2/3 was appeared, indicating that zinc ions are embedded in the electrode material during the discharge process. Fig. 6(d) shows the Zn 2P1/2 and Zn 2P2/3 peaks. When the electrode was fresh, the electrode has no characteristic peak of zinc. When the electrode discharge to 0.0 V, the Zn 2P1/2 and Zn 2P2/3 peaks are located at 1045/1020 eV, which
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 6. (a) Charge/discharge potential profiles of the coin-type cell, where different SOC is marked, points (1)-(4) for the discharge to 0.0 V and points (5)-(7) for the charge to 1.5 V. (b) FT-IR spectra of the PPTCDA/GA corresponding to the selected SOC points in (a). XPS patterns of the PPTCDA/GA fresh and reduced electrodes (c); Zn 2P3/2 and Zn 2P1/2 core level spectra of fresh and discharge to 0.0 V (d); C core level spectra of fresh and discharge to 0.0 V (e and f).
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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is significantly different from the fresh electrode. For the C of XPS explorations of fresh electrode, there were different carbonbased functional groups. The carbonyl groups (287.8 eV), C–C, C–N, C–OH (285.4 eV) and C–C (284.1 eV) were appeared. The C is from conductive agent (acetylene black), and the PPTCDA also consists of C–C, C=O [29]. After discharge to 0.0 V, the XPS measurement are enolate groups (C–O) (285.4 eV), C–C (284.1 eV) and C 1 s (285.0 eV), and the C=O was obviously disappeared. At the same time, there appears a distinct characteristic peak of C–C–O (285.4 eV), which means the Zn ion insertion and leads to the change between C=O and C–O, consistent with zinc ion intercalation/deintercalation in the PPTCDA/GA electrode. Thus, zinc ions can be freely intercalated/deintercalated in the electrode material during the charging and discharging process. However, due to the limitations of C–O group space, a single C–O group in the PPTCDA molecule is unable to hold a zinc ion, and so it must be no less than two adjacent PPTCDA/GA molecules that coordinate to one Zn2+ . Above all, Zn2+ ions insert into the lattice of PPTCDA/GA cathode after the double bonds cleavage. At the discharge to 0.0 V, the C–C, C–N, C–OH (285.4 eV) and C–C (284.1 eV) are the same as the fresh electrode, suggesting zinc ion intercalation/deintercalation to PPTCDA/GA with no structural change of the electrode material. Finally, the results obtained by XPS are consistent with the FT-IR results.
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The preparation of PPTCDA/GA not only improved the conductivity but also enhanced the electrochemical performances. In order to avoid the dendrite problem of zinc metal foils after longcycling galvanostatic charge and discharge process, we combined the PPTCDA with grapheme and adjusted the pH value of electrolyte to prolong the cycle performance to 300 cycles. Although the working voltage of zinc electrode in aqueous battery is lower than that of organic electrolyte zinc-ion battery, the specific capacity of as-prepared coin-type cell is higher than other zinc aqueous battery. At the same time, the cell has a stable cycling life for up to 300 charge-discharge cycles with 99% capacity retention. The PPTCDA/GA electrode delivers a reversible capacity of 281 mAh g–1 at 100 mAh g–1 with the coulombic efficiency close to 100%. Even though current density is high at 10 0 0 mAh g–1 , all electrochemical platforms remain unchanged. At the different scan rate, b-value can be obtained from CV that b = 0.650, 0.670, 0.690, 0.680 and 0.673, and the surface-control contribution is 41.7% for the Zn2+ storage. So the coin-type cell is indeed battery performance. Therefore, the safe and non-toxic nature of aqueous rechargeable zinc-ion battery plays a significant role in our green energy construction. We believe that it would become a new green energy storage device, which can replace the traditional battery.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (51672056), Excellent Youth Project
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of Natural Science Foundation of Heilongjiang Province of China (YQ2019B002), China Postdoctoral Science Foundation (2018M630307 and 2019T120220) and Fundamental Research Funds for the Central Universities (HEUCFD201732).
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.026.
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References
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Organic 3D interconnected graphene aerogel as cathode materials for high-performance aqueous zinc ion battery Ruibai Cang, Ke Ye∗, Kai Zhu, Jun Yan, Jinling Yin, Kui Cheng, Guiling Wang, Dianxue Cao∗
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Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang, China
a r t i c l e
i n f o
Article history: Received 3 August 2019 Revised 26 September 2019 Accepted 27 September 2019 Available online xxx Keywords: Aqueous battery Zinc ion battery Poly 3,4,9,10-perylentetracarboxylic dianhydride Graphene aerogel Cathode materials
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a b s t r a c t Aqueous rechargeable zinc ion batteries are very attractive in large-scale storage applications, because they have high safety, low cost and good durability. Nonetheless, their advancements are hindered by a dearth of positive host materials (cathode) due to sluggish diffusion of Zn2+ in the solid inorganic frameworks. Here, we report a novel organic electrode material of poly 3,4,9,10-perylentetracarboxylic dianhydride (PPTCDA)/graphene aerogel (GA). The 3D interconnected porous architecture synthesized through a simple solvothermal reaction, where the PPTCDA is homogenously embedded in the GA nanosheets. The self-assembly of PPTCDA/GA coin-type cell will not only significantly improve the durability and extend lifetime of the devices, but also reduce the electronic waste and economic cost. The self-assembled structure does not require the auxiliary electrode and conductive agent to prepare the electrode material, which is a simple method for preparing the coin-type cell and a foundation for the next large-scale production. The PPTCDA/GA delivers a high capacity of ≥200 mAh g–1 with the voltage of 0.0∼1.5 V. After 300 cycles, the capacity retention rate still close to 100%. The discussion on the mechanism of Zn2+ intercalation/deintercalation in the PPTCDA/GA electrode is explored by Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations. The morphology and structure of PPTCDA/GA are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction With the development of science and technology, human living standards have improved. Rapid development has caused our environmental pollution problems to become more and more serious, so developing green energy storage devices is one of the significant works. The aqueous rechargeable battery owns the advantages of environmentally friendly nature, low cost and high safety. The first aqueous rechargeable battery was reported in 1994 [1], named as the aqueous lithium rechargeable battery. The LiMn2 O4 was used as cathode and V2 O5 as anode in 5 and 0.001 mol dm–3 LiNO3 aqueous electrolyte, but the Li ion battery did not have a long cycling performance [2]. Until the emergence of phosphates family appeared in 2007, LiV3 O8 , Lix TiO2 and Li(Na)Ti2 (PO4 )3 used as anode in lithium (sodium) rechargeable batteries solved the problem that the rechargeable aqueous lithium (sodium) batteries does not have a suitable anode materials [3–6]. However, in a multivalent
∗
Corresponding authors. E-mail addresses: [email protected] (K. Ye), [email protected] (D. Cao).
ionized (e.g. magnesium; zinc) aqueous battery, the anodic materials were still few. In addition, the magnesium metal as anode was also easy to be oxidized in the air. Thus, at present, the development of aqueous magnesium ion rechargeable battery was limited. Luckily, zinc metal is stable in the air and can be used as anode in aqueous battery, so the V2 O5 , MnO2 and Prussian blue analogs as cathode have been studied for rechargeable aqueous zinc batteries [7]. In the 1980s, conductive polymers were extolled as promising materials for the next generation of environmentally benign and efficient batteries [8–11]. Organic compounds have many typical features. For example, the main feature is the diversity and flexibility of the structure and molecular-level designability. This is the reason that there are many organic compounds materials used in lithium (sodium) battery [12,13] and the used electrode materials have excellent specific capacity, cycling stability and high rate capability [14]. Conjugated organic materials are the most typical electrode materials [15], such as Na2 TP, Na2 DBQ and 3Q of aqueous rechargeable sodium batteries [16–18]. At present, most rechargeable aqueous zinc ion battery used zinc metal as anode, confining p-chloranil inside nanochannels of mesoporous nanochannels of CMK-3 as cathode in aqueous zinc ion electrolyte, which
https://doi.org/10.1016/j.jechem.2019.09.026 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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delivers a high capacity of 200 mAh g–1 [12]. The acidity and alkalinity of electrolyte have great influence on the capacity of hydrogen and oxygen evolution. PTO as cathode in mild aqueous electrolyte can deliver a high capacity of 336 mAh g–1 and the problem of hydrogen and oxygen evolution can be solved [19]. However, most organic materials included poly (3,4,9,10perylentetracarboxylic dianhydride) (PPTCDA) of typical conjugated carbonyl polymer have the low conductivity. In order to improve the conductivity of the electrode material, we used the one-step synthesis method to prepare the PPTCDA/graphene aerogel (GA) (PPTCDA/GA). As known, GA is a material with large open pores, which can promote the transfer speed of electrons to achieve a stable effect of long cycle and make the electrode material fully contact and store sufficient electrolyte to improve the electrochemical performance for aqueous rechargeable batteries. The GA has a large surface area and ultra-light density, in which 3D porous structure can improve their electrical conductivity [20–22]. The conductivity of the electrode material is largely improved by combining the PPTCDA with GA, and the electrode material is obtained with better performance. Herein, we reported an aqueous cointype cell with the PPTCDA/GA as cathode and zinc metal foil as anode in the aqueous zinc solution. The PPTCDA/GA materials can effectively solve the problem of poor conductivity of traditional organic materials, which can be used as cathode in the coin-type full cell. Due to the self-assembly of PPTCDA/GA, the coin-type full cell does not require a binder and a conductive agent in a mild 2.0 mol L–1 ZnSO4 aqueous electrolyte (pH≈6). All components in this battery are low toxic (or not toxic) and beneficial to the development of environmental protection. The GA provides a high ionic conductivity in the neutral environment. Overall, the PPTCDA/GA||Zn coin-type full cell has a high specific capacity of 281 mAh g–1 at the current density of 100 mA g–1 and a high coulombic efficiency close to 100%. Most importantly, a cell stack with the specific capacity of 281 mAh g–1 can be continuously operated for 300 cycles at the current density of 100 mA g–1 . Compared with the traditional aqueous batteries, the coin-type cell was more portable with good reliability and applicability for this newgeneration PPTCDA/GA||Zn.
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2.1. Characterizations
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The reversible Zn-ion coordination mechanisms on PPTCDA/GA cathode were examined by ex-situ Fourier transform infrared spectroscopy (FT-IR). The elemental distributions of PPTCDA/GA electrodes were characterized by scanning transmission electron microscopy (STEM). The elemental existence and valent state of PPTCDA/GA electrodes before and after discharge/charge was examined by X-ray photoelectron spectroscopy (XPS).
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2.2. Electrode preparation and battery fabrication
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The PPTCDA/GA was set in a mild 2.0 mol L–1 ZnSO4 aqueous solution after 24 h. For the electrochemical test in a typical cointype cell, the PPTCDA/GA diameter is 14 mm and the mass loading of PPTCDA/GA is 3.0 mg. A piece of Zn metal foil (≥99.999%, SigmaAldrich) was used as anode and also as counter electrode for cointype cell. The glass microfiber filters were used as separator.
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3. Results and discussion
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The preparation process of PPCTDA was referenced in the previous work [23]. The SEM and TEM images of PPTCDA/GA electrode material were shown in Figs. 1 and 2. It was clear from the SEM images (Fig. 1) that the PPTCDA with an irregular rod
Fig. 1. SEM images of the PPTCDA/GA with 3D interconnected porous structure.
Fig. 2. TEM images of the PPTCDA/GA with 3D interconnected porous structure.
structure formed on the silk GA. The high-magnification SEM image (Fig. 1d) confirmed that the PPTCDA was wrapped in the silky graphene and kept its original appearance. To further investigate the detailed microstructure of the synthesized PPTCDA/GA materials, TEM test was carried out. From typical TEM images in Fig. 2, the monodispersed PPTCDA nanorod with irregularity size are embedded in the GA nanosheets. To check the change of structure and phase for the obtained PPTCDA and PPTCDA/GA, the X-ray diffraction (XRD) measurement was employed (Fig. 3a). In the XRD pattern of PPTCDA/GA, the broad diffraction peak located at 25.5°, corresponding to GA. However, there was no peak at 25.5° in the PPTCDA pattern, suggesting the PPTCDA/GA successfully synthesized. In order to further determine the successful combination PPTCDA with GA, FT-IR measurement was utilized. Fig. 3(b) shows the FT-IR spectra of the PPTCDA (red line) and PPTCDA/GA (blue line). From the FT-IR spectra of PPTCDA/GA, a peak appeared at ∼2926 cm–1 , which was corresponded to the -CH2 stretching vibration group originated from GA. In addition, the carbonyl bonds (C=O) peak at ∼1650 cm–1 and C=N peak at ∼1708 cm–1 were due to the imprint groups from PPTCDA. It is clearly seen that the PPTCDA and graphene oxide (GO) precursor exhibits a lower crystalline before calcination, which is in accordance with the reported work [11,24]. The synthesis process of PPTCDA/GA is illustrated in Scheme 1. The GO hydrogel was changed to the graphene hydrogel (GH) via a hydrothermal method after heating at 150 °C for 16 h in the 20 mL Teflon-lined stainless steel. Then the as-prepared PPTCDA/GH was freeze-dried for 24 h to obtain the PPTCDA/GA. The characteristic peaks of GO are significantly higher than that of the original PPTCDA. After 24 h freeze drying of PPTCDA/GA, the PPTCDA/GA was soaked in the 2.0 mol L–1 ZnSO4 aqueous solution for 24 h. Afterwards, the PPTCDA/GA was used as cathode and zinc metal as anode to assemble as the coin-type cell (Figs. S1 and S2). In addition, the stability of PPTCDA/GA demonstrated as a video show in the additional supporting information.
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 3. XRD patterns (a) and FT-IR spectra (b) of PPTCDA and PPTCDA/GA.
Scheme 1. Schematic illustration of the synthesis process of PPTCDA/GA.
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The electrochemical performance of PPTCDA/GA has been studied versus Zn by employing cyclic voltammetry (CV) at different scan rates from 0.1 to 1.0 mV s–1 in the coin-type battery test. Fig. 4(a) shows the electrochemical behavior of PPTCDA/GA in the 2.0 mol L–1 ZnSO4 aqueous solution. The three oxidation peaks and two reduction peaks were clearly observed in the CV curves, which are the same as those in the galvanostatic charge-discharge test. The oxidation peaks at 0.66 V and 0.73 V are very close, which is likely due to the intercalation process of Zn2+ in the PPTCDA/GA with the C=O conversion to C–O. The voltage gap between the redox peaks (called as electrode polarization) reflects the electrochemical kinetics speed for a specific electrode reaction. The calculated b-values of the five peaks are 0.650, 0.670, 0.690, 0.680 and 0.673 in the 2.0 mol L–1 ZnSO4 aqueous electrolyte (Fig. 4b), which means that the charge storage is affected by the diffusion process. This is the main reason that the PPTCDA/GA materials as cathode have a high capacity. From cyclic voltammetry (CV) test, there are multiple redox peaks, indicating that the process of zinc ions embedded in the materials is a slow process. Meanwhile, the contribution of PPTCDA/GA||Zn coin-type cell to the capacitance of the battery was determined by combining the relationship between capacitive and diffusion control behavior. Fig. 4(c) shows a typical voltage profile of capacitive current (violet region) in comparison with the total current of the zinc coin-type cells. Hence, the capacitance ratio in the battery can be determined by Zn2+ semi-infinite linear. The b value can be obtained through the algorithm of Dunn’s method [25], implying that excessive scan rate has a great influence on capacitive behavior. Thus, the scan rate was selected to be 0.1 mV s–1 and the surface-control contribution is 41.7% for the Zn2+ storage. As the PPTCDA/GA used as cathode in the coin-type cells with different scan rates of 0.2, 0.3, 0.5, 0.8 and
1 mV s–1 , the surface control contribution is 41.7%, 45.6%, 45.8%, 48.0%, 47.8% and 48.1% (Fig. 4d), respectively, which is responsible for a high-rate capability of the coin-type cell. Galvanostatic charge-discharge cycling of the coin-type cell with the PPCTDA/GA as cathode and zinc foil as anode in 2.0 mol L–1 ZnSO4 aqueous electrolyte are shown in Fig. 5(a). The galvanostatic charge-discharge measurements are tested in the electrochemical window of 0.0 V to 1.5 V, which is the safe voltage range for the aqueous battery. According to the capacity of the anode and cathode, the specific anode/cathode mass ratio was balanced at 1:1. The coin-type cell shows a reversible specific capacity of 281 mAh g–1 at a current density of 100 mA g–1 (The galvanostatic charge/discharge of PPTCDA as cathode was also shown in Fig. S3). At the same time, the pH value of electrolyte were accordingly adjusted (pH≈6, 4.5 and 3.1), the electrode material has the best electrochemical performance in the electrolyte concentration with pH≈6.0 (Fig. S4). The coin-type cell had good cycle stability and the cell charge-discharge curve was not significantly changed at 1st, 2nd, 3rd, 5th, 10th, 50th and 100th. The PPTCDA/GA as cathode in the aqueous rechargeable zinc ion battery can be utilized as promising large-scale energy storage technologies. In addition, the coin-type cell shows the divalent Zn-ion can be reversibly intercalated/deintercalated from the PPTCDA/GA. The rate capability of the coin-type cell in aqueous 2.0 mol L–1 ZnSO4 electrolyte was investigated and showed the excellent electrochemical performance with a specific capacity of 281 mAh g–1 at the current density of 100 mA g–1 . When the current density increased to 10 0 0 mA g–1 , the electrode was not polarized and maintained good cycle stability. After 300 cycles, the specific capacity of 102 mAh g–1 was maintained (Fig. 5c). Furthermore, PPTCDA/GA showed excellent cyclic capacity retention without any capacity decrease after 300 cycles and also showed excellent cycling stability with close to 100% (Fig. 5b). In the meantime, we also need to consider the problem of metallic dendrites of the anodic zinc sheet. The cycle stability of the coin-type cell was tested for 300 cycles. There was no obvious change in the cycle stability process, proving that the electrode material has good stability (Fig. S5). Previous report demonstrated that the ion-coordination reaction may lead to the dissolution of organic electrode materials, which should be urgently solved [17]. Fortunately, in this work, after polymerization the PTCDA,
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 4. (a) Cyclic voltammetry curves at different scan rates. (b) The b-value of different redox peaks, determined from the log(i) versus log(v) plots. (c) The k1 analysis of PPTCDA/GA electrode at 0.1 mV s–1 . (d) The contribution radio of different scan rates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the PPTCDA does not cause the dissolution in mild aqueous solution. The 3,4,9,10-perylentetracarboxylic dianhydride and ethylene diamine (EDA) were used by reflux to obtain the PPTCDA product. The redox graphite was coated on PPTCDA with PPTCDA/GA. The coin-type cell not only has good stability in aqueous solution but also greatly enhances the electrical conductivity of the electrode material resulted from Nyquist plots of zinc metal foil and PPTCDA/GA electrode. The electrochemical impedance spectroscopic (EIS) measurement of PPTCDA and PPTCDA/GA was showed in Fig. S6. A typical Nyquist plot shows the PPTCDA/GA has better conductivity. In order to further prove the good electrical conductivity of PPTCDA/GA, the battery was tested at different current densities of 50, 10 0, 20 0, 30 0, 50 0 and 10 0 0 mA g–1 , and the specific capacities maintain at 305, 281, 255, 200, 173 and 103 mAh g–1 in the aqueous 2.0 mol L–1 ZnSO4 electrolyte (Fig. 5c and d), which is higher than that of other aqueous rechargeable batteries, such as 140 mAh g–1 of spinel Mn3 O4 //Zn [26], 81.5 Wh kg–1 of PVA/Zn(CF3 SO3 )2 ) [27], 97 mAh g–1 of Zn||ZVO [28], 59.2 mAh g−1 of Zn3 [Fe(CN)6 ]2 ||Zn [29]. Meanwhile, the comparison of electrochemical performance on the different zinc ion battery systems is listed in Table 1 and exhibited in Fig. S7 [18–21,26,27,30–32]. For further verifying the mechanism of Zn2+ intercalation and deintercalation in the PPTCDA/GA, the PPTCDA/GA was used as
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Table 1. A comparison of the conditions used for different zinc battery systems. Cathode/anode/Zn
Electrolyte
Voltage
Spinel Mn3 O4 [26] PVA/Zn(CF3 SO3 )2 ) [27]
2.0 M ZnSO4 Self-healing electrolyte 1.0 M ZnSO4 0.5 M Zn(CH3 COO)2 3 M ZnSO4 0.2 M ZnSO4 1.0 M ZnSO4 1.0 M Zn ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 1.0 M ZnSO4 2.0 M ZnSO4
1.2 1.35
201 Wh kg−1 81.5 Wh kg−1
1.5 1.1 1.73 1.73 0.65 1.8 0.7 1.5 1.5 1.5 1.5
150 Wh kg−1 97 mAh g−1 59.2 mAh g−1 52.5 mAh g−1 62.7 Wh kg−1 185 mAh g−1 80.5 Wh kg−1 168 Wh kg−1 118 mAh g−1 102 Wh kg−1 225.75 Wh kg−1
p-chloranil/CMK [21] Na3 V2 (PO4 )3 [30] Zn3 [Fe(CN)6 ]2 //Zn [29] CuFeCN6 [15] VS2 [12] Zn||ZVO [28] Zn0.25 V2 O5 nH2 O [26] ZMD/MnO2 [31] V0.05 MnO2 [32] Fe(CN)6 [33] OUR WORK
Capacity/Energy density
cathode and zinc as anode in the coin-type cell. Seven points of the galvanostatic discharge/charge at 100 mA g–1 were selected to measure the ex FT-IR (Fourier-transform infrared). Fig. 6(a) shows the galvanostatic charge/discharge potential profiles for the discharge (from initial state (1) to 1.5 V (7)). The point (1) is the fresh electrode, corresponding to the -CH2 stretching vibration group at
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 5. The electrochemical performance of the PPTCDA/GA as cathode and zinc foil as anode for the full cell in 2.0 mol L–1 ZnSO4 aqueous solution. (a) Galvanostatic charge/discharge potential profiles of the 1st, 2nd, 3rd, 5th, 10th, 50th, and 100th cycles at the current density of 100 mA g–1 in a potential range of 0.0∼1.5 V. (b) Cycle performance at a current density of 100 mA g–1 . (c) Galvanostatic charge/discharge at 50, 10 0, 20 0, 30 0, 50 0 and 10 0 0 mA g–1 . (d) Cycle stability at a current density of 100 mA g–1 .
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∼2926 cm–1 originated from GA [21], carbonyl bonds (C=O) at ∼1650 cm–1 and C=N at ∼1708 cm–1 from PPTCDA (Fig. 3b). To further prove the group structure of the electrode material, the four points from (1) to (4) for the discharge to 0.0 V were selected and measured by FT-IR test. The four points were the discharge of the coin-type cell. During the discharge, the carbonyl groups shrinkage vibration peak gradually weakens. At the same time, the enolate groups (∼1375 and ∼1105 cm–1 ) shrinkage vibration peak stretch remains progressive enhancement, indicating that the process of zinc ion embedding is the carbonyl groups stretch and peter out to enolate groups. Furthermore, there appears a second carbonyl bonds (C=O) at ∼1650 cm–1 , which is the skeleton vibration of benzene ring. Thus, the process of zinc ion intercalation is the process of breaking the carbonyl groups double bond into an ethanol groups. As the double bond breaks, the zinc-ion inserts into the enolate group. At the (5)–(7) points, corresponded to the PPTCDA/GA cathode charge to 1.5 V, the enolate groups shrinkage vibration peak gradually weakens. At the same time, the carbonyl groups shrinkage vibration peak stretch remains progressive enhancement, until returning to the same stretching vibration peak as the fresh electrode. In all the discharge/charge process, the -CH2 group shows no change. So the zinc ion can freely insert/deinsert in the PPTCDA/GA and no damage to the GA group in the Zn2+ intercalation/(de)intercalation process was observed. This phenomenon is quite different from that of monovalent ion such as Li+ , Na+ , or K+ (the existed problems of va-
lence and ion radius). A carbonyl group space can not accommodate one zinc ion due to the limitation of enolate group space. The enolate groups can not possibly come from a single PPTCDA/GA molecule, and so it must be no less than two adjacent enolate group molecules in the PPTCDA/GA that coordinate to one Zn2+ . The cathode is a PPTCDA/GA with the condensation of PPTCDA and GA (Scheme S1). The monomer is denoted as R, each monomer contains two imide groups for Zn2+ , and oxygen from adjacent monomers can ligate a single Zn2+ , leading to the stoichiometry of Zn-R. In the real situations, the metal cations can be thermally distributed in the polymer chain, and their definite positions relative to the monomers are not fixed. For the qualitative purposes and consideration of computational cost, we truncated the polymer chain into monomers. The ratio of Zn2+ and R anion is determined by charge balance, which is an assumption that charge balance can be achieved in the real case. Fig. 6(c) shows the XPS spectra of discharge to 0.0 V (red line) and the fresh electrode (black line) of PPTCDA/GA-cathode. For the fresh electrode, the C 1 s (285.0 eV) and O 1 s peaks from GA and PPTCDA can be clearly seen. When discharge to 0.0 V (red line), the characteristic peak of Zn 2P1/2 and Zn 2P2/3 was appeared, indicating that zinc ions are embedded in the electrode material during the discharge process. Fig. 6(d) shows the Zn 2P1/2 and Zn 2P2/3 peaks. When the electrode was fresh, the electrode has no characteristic peak of zinc. When the electrode discharge to 0.0 V, the Zn 2P1/2 and Zn 2P2/3 peaks are located at 1045/1020 eV, which
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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Fig. 6. (a) Charge/discharge potential profiles of the coin-type cell, where different SOC is marked, points (1)-(4) for the discharge to 0.0 V and points (5)-(7) for the charge to 1.5 V. (b) FT-IR spectra of the PPTCDA/GA corresponding to the selected SOC points in (a). XPS patterns of the PPTCDA/GA fresh and reduced electrodes (c); Zn 2P3/2 and Zn 2P1/2 core level spectra of fresh and discharge to 0.0 V (d); C core level spectra of fresh and discharge to 0.0 V (e and f).
Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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is significantly different from the fresh electrode. For the C of XPS explorations of fresh electrode, there were different carbonbased functional groups. The carbonyl groups (287.8 eV), C–C, C–N, C–OH (285.4 eV) and C–C (284.1 eV) were appeared. The C is from conductive agent (acetylene black), and the PPTCDA also consists of C–C, C=O [29]. After discharge to 0.0 V, the XPS measurement are enolate groups (C–O) (285.4 eV), C–C (284.1 eV) and C 1 s (285.0 eV), and the C=O was obviously disappeared. At the same time, there appears a distinct characteristic peak of C–C–O (285.4 eV), which means the Zn ion insertion and leads to the change between C=O and C–O, consistent with zinc ion intercalation/deintercalation in the PPTCDA/GA electrode. Thus, zinc ions can be freely intercalated/deintercalated in the electrode material during the charging and discharging process. However, due to the limitations of C–O group space, a single C–O group in the PPTCDA molecule is unable to hold a zinc ion, and so it must be no less than two adjacent PPTCDA/GA molecules that coordinate to one Zn2+ . Above all, Zn2+ ions insert into the lattice of PPTCDA/GA cathode after the double bonds cleavage. At the discharge to 0.0 V, the C–C, C–N, C–OH (285.4 eV) and C–C (284.1 eV) are the same as the fresh electrode, suggesting zinc ion intercalation/deintercalation to PPTCDA/GA with no structural change of the electrode material. Finally, the results obtained by XPS are consistent with the FT-IR results.
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The preparation of PPTCDA/GA not only improved the conductivity but also enhanced the electrochemical performances. In order to avoid the dendrite problem of zinc metal foils after longcycling galvanostatic charge and discharge process, we combined the PPTCDA with grapheme and adjusted the pH value of electrolyte to prolong the cycle performance to 300 cycles. Although the working voltage of zinc electrode in aqueous battery is lower than that of organic electrolyte zinc-ion battery, the specific capacity of as-prepared coin-type cell is higher than other zinc aqueous battery. At the same time, the cell has a stable cycling life for up to 300 charge-discharge cycles with 99% capacity retention. The PPTCDA/GA electrode delivers a reversible capacity of 281 mAh g–1 at 100 mAh g–1 with the coulombic efficiency close to 100%. Even though current density is high at 10 0 0 mAh g–1 , all electrochemical platforms remain unchanged. At the different scan rate, b-value can be obtained from CV that b = 0.650, 0.670, 0.690, 0.680 and 0.673, and the surface-control contribution is 41.7% for the Zn2+ storage. So the coin-type cell is indeed battery performance. Therefore, the safe and non-toxic nature of aqueous rechargeable zinc-ion battery plays a significant role in our green energy construction. We believe that it would become a new green energy storage device, which can replace the traditional battery.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (51672056), Excellent Youth Project
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of Natural Science Foundation of Heilongjiang Province of China (YQ2019B002), China Postdoctoral Science Foundation (2018M630307 and 2019T120220) and Fundamental Research Funds for the Central Universities (HEUCFD201732).
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.026.
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References
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Please cite this article as: R. Cang, K. Ye and K. Zhu et al., Organic 3D interconnected graphene aerogel as cathode materials for highperformance aqueous zinc ion battery, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.026
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