SOCl2 battery

Accepted Manuscript Binuclear metal phthalocyanines bonding with carbon nanotubes as catalyst for the Li/SOCl2 battery

Yan Gao, Siwen Li, Xiao Wang, Ronglan Zhang, Gai Zhang, Ying Zheng, Jianshe Zhao PII: DOI: Reference:

S1572-6657(17)30162-5 doi: 10.1016/j.jelechem.2017.03.013 JEAC 3178

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

3 December 2016 25 February 2017 6 March 2017

Please cite this article as: Yan Gao, Siwen Li, Xiao Wang, Ronglan Zhang, Gai Zhang, Ying Zheng, Jianshe Zhao , Binuclear metal phthalocyanines bonding with carbon nanotubes as catalyst for the Li/SOCl2 battery. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi: 10.1016/j.jelechem.2017.03.013

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Binuclear Metal Phthalocyanines Bonding with Carbon Nanotubes as Catalyst for the Li/SOCl2 Battery Yan GAOa,, Siwen LIa, Xiao WANGa, Ronglan ZHANGa, Gai ZHANGb, Ying ZHENGc, Jianshe ZHAOa,* a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xi’ an, Shaanxi 710069, China b School of Materials and Chemical Engineering, Xi’an Technological University, 710021， Xi’an, China c

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Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Abstract：

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Carbon nanotubes (CNTs) were chemically modified by a series of binuclear hexamino-metallo phthalocyanines (M2Pc2) which were produced by reducing the corresponding binuclear hexanitro-metallo phthalocyanines (M2HnPc2) (M=Mn(Ⅱ),Fe(Ⅱ),Co(Ⅱ),Ni(Ⅱ),Cu(Ⅱ),Zn(Ⅱ)). The compounds were

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characterized with different techniques: IR, UV-vis, XRD, SEM, XPS. The electrocatalytic performances of the CNTs bonding with M2Pc2 (M2Pc2-CNTs) to Li/SOCl2 battery was tested by adding these compounds into the electrolyte. The capacities of the battery were improved, with increases by 59.1-128.5%. In addition, sequence of the electrochemical catalytic performance of M2Pc2-CNTs is as follow: Fe>Co>Cu>Zn>Ni>Mn. The CNTs bonding with M2Pc2 can greatly promote the electronic transmission, which significantly improve the performance and the initial voltage of Li/SOCl2 battery. Cyclic voltammetry measurements are applied to verify the M2Pc2-CNTs possessing catalytic performances and a reasonable

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mechanism is proposed.

Keywords：Binuclear phthalocyanines; Carbon nanotubes; Li/SOCl2 battery; Catalyst 1. Introduction Carbon nanotubes (CNTs) are first found by lijima in 1911, which possess large specific surfaces, excellent electrical conductivities and unique chemical properties[1]. The structure of carbon nanotube is similar to the rolled graphite, which makes it excellently electrical and mechanical properties[2,3]. Carbon nanotube can conduct large current and process superb mechanical strength due to its structural cavity. Carbon nanotube is widely used in electrochemical materials, because of its unique structure and excellent performance[4]. Prior research has shown that binuclear 1

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phthalocyanine has good catalytic performance[5]. Binuclear metal phthalocyanine is widely used in solar cell, sensor, light-emitting device and nonlinear optical material[6,7]. Thus, metal phthalocyanine modification of carbon nanotubes will have potentially electrocatalytic properties to the Li/SOCl2 battery. The lithium-thionyl chloride (Li/SOCl2) battery is made up of anode, cathode and electrolyte: the lithium as the anode, carbon as the cathode, and the mixed solution (thionyl chloride/tetrachloride aluminum lithium) as the electrolyte. The Li/SOCl2 battery has some unique characteristics, such as high specific energy, high specific power, high voltage and low temperature performance, and those

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performances are widely applied in various aspects[8-10]. The theoretical voltage of Li/SOCl2 battery is 3.65V, but the actual voltage value is far less than the theory value. According to the cell reactions (Eq1-3), it can be found that LiCl and S are formed in the reaction process[11,12]. The precipitation of LiCl and S hindered the electronic transmission, resulting in the practical capacity of Li/SOCl2 much lower than its theoretical capacity. In order to overcome the problem of Li/SOCl2 battery, considerable catalyst has been adding to the battery. Anode: 2Li-2e → 2Li+ (1)

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Cathode: SOCl2+2e- → 1/2S↓+1/2SO2↑+2Cl(2) Cell: SOCl2+2Li → 1/2S↓+1/2SO2↑+2LiCl↓ (3) Recently, our group has studied the effect of metallophthalocyanines to Li/SOCl2 battery. It is found that when the metallophthalocyanines are added to the electrolyte, the discharge capability of the battery is improved obviously[13,14]. In this paper, two phthalocyanine rings of the metallophthalocyanines are connected by perylene group, which makes it have a bigger conjugate structure than the compounds of the previous paper. In addition, the carbon nanotubes are widely studied in modify electrodes in recent years. On the basis of previous research, CNTs modified by binuclear metal

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phthalocyanines were synthesized in this paper. The structures of the compounds were characterized by IR, XRD, SEM, XPS. The results show that the binuclear metal phthalocyanines (M2Pc2) and the CNTs are linked covalently and the complexes have prominent electrocatalytic effect on Li/SOCl2 battery. And the modified CNTs can lengthen the discharge time and improve the initial voltage of the battery effectively. In addition, the catalysis mechanism of the modified CNTs is proposed reasonably according to the cyclic voltammetry results. 2. Experimental 2.1 Materials and characterizations Perylene-3,4,9,10-tetracarboxylic dianhydride was purchased from J&K China Chemical Ltd.4-Nitrophthalimide was purchased from TCI (shanghai) Development 2

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Co.,Ltd. Carboxylic multi-walled carbon nanotubes was obtained from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. All the reagents were analytical grades and were used without further purification. LiAlCl4/SOCl2 electrolyte (battery grade), lithium foil (battery grade) and C film (battery grade) were obtained from Xi’an Institute of Electrical and Mechanical Services Information. IR spectra were measured on a Germany BRUKER VECT022 analyzer by using KBr pellets in the infrared region of 400-4000 cm-1. SEM spectra were tested on a Holland ESEM-FEG, Quanta 400FEG. XRD patterns were obtained by using a Bruker

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D8 Advance diffractometer. The XPS spectra were measured on an American PE company HI-540 X-ray photoelectron spectrometer. The Cyclic Voltammetry was performed on a Chinese Zhengzhou Shiruisi Technology Co. Ltd. RST5000 electrochemical workstation.

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2.2 Catalyst preparation The chemically modified CNTs by binuclear hexamino-metallo phthalocyanines (M2Pc2) were prepared following the method outlined in scheme 1.

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2.2.1 Synthesis of binuclear hexanitro-metalophthalocyanines Binuclear hexanitro-metal phthalocyanines M2HnPc2 (M = Mn(Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ)) were synthesized by solvent method. The general synthesis procedure was as follows: 4-nitrophthalimide (1.20 g, 6.25 mmol), perylene-3,4,9,10-tetracarboxylic dianhydride (0.40 g, 1 mmol), urea (3.4 g, 0.04 mol), M(OAc)2·4H2O (2 mmol), NH4Cl (1.70 g, 0.03 mol), (NH4)2Mo2O7 (0.15 g, 0.77 mmol) were mixed together and grinded evenly with agate mortar. The mixture was poured into a 250 ml three-necked bottle and immersed in 30 ml Jet fuel. The mixture

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was heated to 185 ℃ with magnetic stirrer and refluxed for 4 h. After cooling to room temperature, Jet fuel was removed and got bluish violet powder. The basic information and the IR spectra providing the evidence for their structure were presented as follows: Mn2HnPc2－bluish violet powder, yield 0.59 g (37.34%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 735, 1023, 1134, 1605, 1399, 1340, 1517. Fe2HnPc2－bluish violet powder, yield 0.42 g (26.59%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 721, 1010, 1112, 1598, 1440, 1347, 1534. Co2HnPc2－bluish violet powder, yield 0.56 g (35.44%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 728, 1097, 1149, 1594, 1407, 1325, 1517. Ni2HnPc2－bluish violet powder, yield 0.66 g (41.74%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 726, 1023, 1143, 1599, 1410, 1333, 1517. 3

ACCEPTED MANUSCRIPT Cu2HnPc2－bluish violet powder, yield 0.38 g (24.05%), and mp>300 ℃, IR (KBr pellet, cm-1): 736, 1023, 1134, 1591, 1407, 1340, 1524. Zn2HnPc2－bluish violet powder, yield 0.49 g (31.01%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 723, 1033, 1141, 1589, 1406, 1347, 1526.

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2.2.2 Synthesis of binuclear hexamino-metallo phthalocyanines (M2Pc2) M2HnPc2 (0.30 g) was added into a 100ml three neck bottle, and 5 ml DMF was added under incessant stirring. After temperature rose to 60 ℃, sodium sulfide nonahydrate (3.00 g) was added to the solution. The reaction mixture was refluxed for

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24 h. After mixture cooling to room temperature, the liquid was poured into 50ml distilled water. The resulting precipitate was filtered off by using G4 funnel under vacuum. The precipitate was repeatedly washed with HCl solution (2%) and NaOH solution (2%) until filtrate become neutral. Then the solid was refluxed in distilled water and anhydrous ethanol for 4 h, respectively. After filtration and drying, the powder M2Pc2 was obtained. 2.2.3 Purification

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The raw reacted mixture was refluxed in HCl solution (2%) for 8 h. Then the product was filtered off by using G4 funnel. The powder was refluxed in petroleum ether, acetone and dichloromethane for 4 h, respectively. After decompressional filtration, the solid was recrystallized with concentrated sulfuric acid. Decompressional again, the solid was washed with NaOH solution (2%), and deionized water until filtrate become neutral. The residue was refluxed in deionized and anhydrous ethanol for 4 h. After decompressional filtration, the product was dried for 12 h at 100 ℃. Then the product was dried for 24 h in the vacuum desiccator. The binuclear hexamino-metallo phthalocyanine (M2Pc2) was obtained. The basic

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informations were presented as follows: Mn2Pc2－black green powder, yield 0.12 g (42.85%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 733, 1057, 1147, 1617, 1399, 3309, 3480. Fe2Pc2－aquamarine powder, yield 0.12 g (46.15%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 725, 1054, 1141, 1598, 1445, 3308, 3469. Co2Pc2－black green powder, yield 0.16 g (61.53%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 727, 1077, 1149, 1600, 1427, 3356, 3458. Ni2Pc2－black green powder, yield 0.20 g (76.92%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 723, 1053, 1140, 1609, 1427, 3350, 3441. Cu2Pc2－black green powder, yield 0.21 g (80.21%), and m.p. >300 ℃, IR (KBr pellet, cm-1): 715, 1043, 1139, 1597, 1427, 3308, 3456. Zn2Pc2－black green powder, yield 0.18 g (69.23%), and m.p. >300 ℃, IR (KBr 4

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2.2.4 Synthesis of binuclear metalophthalocyanine-amide-carbon nanotubes Carbon nanotube (0.15 g) was added to the three necked bottle with anhydrous DMF (7 ml) under the N2 atmosphere. The mixture was heated to 76 ℃ with magnetic stirrer. Then SOCl2 solution was dropped to the mixture within 1 h by dropping funnel. The mixture was maintained 78 ℃ and reacted 48 h. Then the unreacted SOCl2 solution was steamed. After the temperature dropped to 60 ℃，M2Pc2（0.12 g）was added to the mixture, then triethylamine was dropped to the mixture within half hour

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by dropping funnel. After 48 h, the product was filtered off by using G4 funnel. Then the precipitate was refluxed in acetone for 4 h. After decompressional filtration, the product was dried for 12 h at 100℃, and then dried for 24 h in vacuum. The basic information and the IR spectra provided the evidence for their structure were presented as follows: Mn2Pc2-CNTs－dark blue powder, yield 0.20 g (74.07%), IR (KBr pellet, cm-1): 728, 1082, 1163, 1612, 1411, 3317, 1708. Fe2Pc2-CNTs－dark blue powder, yield 0.12 g (44.45%), IR (KBr pellet, cm-1):

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728, 1082, 1163, 1613, 1450, 3448, 1723. Co2Pc2-CNTs－dark blue powder, yield 0.18 g (66.67%), IR (KBr pellet, cm-1):

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721, 1097, 1149, 1605, 1444, 3419, 1719. Ni2Pc2-CNTs－dark blue powder, yield 0.16 g (60.53%), IR (KBr pellet, cm-1):

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728, 1079, 1141, 1607, 1436, 3441, 1721. Cu2Pc2-CNTs－dark blue powder, yield 0.22 g (82.61%), IR (KBr pellet, cm-1): 713, 1081, 1170, 1615, 1412, 3423, 1701. Zn2Pc2-CNTs－dark blue powder, yield 0.21 g (78.95%), IR (KBr pellet, cm-1):

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728, 1082, 1163, 1612, 1411, 3317, 1708.

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2.3 Electrochemistry testing The structural diagram of Li/SOCl2 battery is showed in Scheme 2. Cathode preparations: The cathode film was obtained by dispersing acetylene black and conductive agent in a certain proportion (9:1 wt %) in diluted Teflon emulsion, with constantly stirred until to form a paste. The paste was repeatedly rolled until to the provisions of the film was formed. The film was dried at 150 ℃ for 48 h. Battery assemblies: The all processes were performed in dry glovebox, in which the relative humidity was maintained below 2%, and the temperature is kept at ambient temperature. All of the experimental apparatus were kept dry. The prepared M2Pc2-CNTs (linked) (2 mg) were added to the electrolyte (2 mL), and the mixture was ultrasonicated for even dispersion. The performance of battery was evaluated 5

ACCEPTED MANUSCRIPT with a constant resistance of 40 Ω and an average current density of 70 mA·cm-2 until the assembled battery continuously discharged to 2 V. In this progress, the output voltage (U) and the discharge time (t) of the battery is measured.

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2.4 Cyclic voltammetry The CV measurement was carried out in dry glovebox, in which the relative humidity was maintained below 2% at ambient temperature. The CV measurement performed by three-electrode system: working electrode, reference electrode and

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auxiliary electrode. Glassy carbon electrode with a useful area of 0.25 cm2 was employed as the working electrode, two lithium slices were adopted as both the reference electrode and auxiliary electrode, copper was applied as a conductor wire. The measurement was carried out with scanning rate of 40 mV/s, 60 mV/s, 80 mV/s, 100 mV/s, respectively. The current voltage curves were recorded on electrochemical workstation.

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3.Results and discussions 3.1 IR Spectra

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Infrared spectra of CNTs (a), Ni2Pc2-CNTs (linked) (b), Ni2Pc2/CNTs (mixed) (c) and Ni2Pc2 (d) are showed in Fig. 1. The absorption peak of CNTs is not obvious, which is consistent with previous literature[15]. Obviously, the IR spectra of the Ni2Pc2-CNTs (linked), Ni2Pc2/CNTs (mixed) and Ni2Pc2 are showed the characteristic bands of phthalocyanine: the absorption peaks corresponding to C=N are appeared at about 1400 cm-1 region, strong peaks to C=C are at about 1600 cm-1 region, and characteristic peaks of phthalocyanine ring are observed at about 1530 cm-1,1120 cm-1, 1050 cm-1 and 750 cm-1. For Ni2Pc2, the double absorption peaks at range of 3350-3441 cm-1 are assigned to the group of amino, which confirms that the Ni2Pc2 is

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synthesized. Compared with Ni2Pc2, the peaks position of Ni2Pc2/CNTs (mixed) is appeared in same region. However, adding CNTs results in reducing the peak intensity of Ni2Pc2/CNTs (mixed). In addition, for Ni2Pc2-CNTs (linked), the strong peaks of amide bond are at 1701 cm-1 and 3326 cm-1, which verifies the bonding between CNTs and Ni2Pc2. 3.2 XRD analysis XRD patterns of CNTs, Ni2Pc2-CNTs (linked), Ni2Pc2/CNTs (mixed) and Ni2Pc2 are showed in Fig. 2. The diffraction pattern of pristine CNTs shows two typical peak at 25.2° and 44.6°, corresponding to the (002) reflection and (101) reflection, respectively. The pattern of CNTs shows that it has highly ordered structure. The XRD patterns of Ni2Pc2 show nine strong diffraction peaks in the range of 16° to 6

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32.7°, which is well consistent with previous reported[16]. Comparison with Ni2Pc2, the XRD pattern of Ni2Pc2-CNTs (linked) presents both diffraction peaks of Ni2Pc2 and the diffraction of CNTs, which confirms that Ni2Pc2 bond to CNTs forming Ni2Pc2-CNTs (linked), Ni2Pc2-CNTs (linked) also contain the highly order structures of CNTs. As for the pattern of Ni2Pc2/CNTs (mixed), two diffraction peaks of CNTs can be observed obviously. However, the diffraction peaks of Ni2Pc2 in Ni2Pc2/CNTs (mixed) are not obvious. The XRD results show that the desired material Ni2Pc2-CNTs (linked) is synthesized.

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3.3 SEM analysis Fig. 3 shows the SEM images of CNTs, Ni2Pc2-CNTs (linked), Ni2Pc2/CNTs (mixed) and Ni2Pc2. It is found that the morphology of CNTs is cylindrical tube (Fig. 3a). Many Ni2Pc2 particles connect with CNTs forming Ni2Pc2-CNTs (linked) (Fig. 3b). However, when the Ni2Pc2 is mixed with CNTs, large Ni2Pc2 particles suspended in the CNTs surface, not directly connected with the carbon nanotubes (Fig. 3c). Microstructure analysis by SEM indicates that the Ni2Pc2 particles are irregular bulk (Fig. 3d). The result is well consistent with the XRD analysis that Ni2Pc2 combine with CNTs produce Ni2Pc2-CNTs (linked).

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3.4 TEM analysis TEM images of CNTs and Ni2Pc2-CNTs (linked) are showed in Fig. 4. It can be seen that the pure CNTs is rigid rods, with a smooth surface. Compared with CNTs, the morphology of Ni2Pc2-CNTs (linked) is still cylindrical tube, but the surface became rough. This is due to that Ni2Pc2 is formed outside CNTs by chemical bond. In addition, the diameter of the Ni2Pc2-CNTs (linked) is bigger than that of CNTs. The TEM analysis reveal that the modified CNTs are synthesized by bonding with

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binuclear metal phthalocyanines. 3.5 X-ray photoelectron spectroscopy analysis In order to further confirm the structure of Ni2Pc2-CNTs (linked), XPS analysis of Ni2Pc2-CNTs(linked) has been carried out (Fig. 5). It can be seen that Ni2Pc2 exhibited three strong peaks at 860, 400 and 290 eV, which corresponds to the electronic states of Ni2p, N1s, C1s, respectively (Fig. 5a). The expended Ni2p region of Ni2Pc2 has two sharp peaks with binding energies at 872.56 eV and 855.40 eV, which confirms the existence of Ni(Ⅱ) in this compound (Fig. 5b). The analysis of C1s region shows three peaks. The strong peak at 285.36 eV is asssigned to the sp2 hybrid carbons of CNTs. Another C1s peak located at 289.2 eV is attributed to the C-N bonds in phthalocyanine, and at 287.2 eV is the carbon atoms of C=O bonds in 7

ACCEPTED MANUSCRIPT catalyst(Fig. 5c). The expanded N1s region shows four peaks (Fig. 5d). N1s peak display the presence of C=O at 401 eV, suggesting that a number of amide bond is created. Two peaks located at 399.3 eV and 400.2 eV are attributed to N=C and N-C in phthalocyanine ring, and at 393.6 eV is assigned to the nitrogen atoms of H2N-C bond. All of the data completely confirm that the CNTs are chemical modified by the binuclear nickel phthalocyanines through the covalent interaction.

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3.6 Electrocatalytic performance The typical discharge curves of Li/SOCl2 battery electrocatalyzed by

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M2Pc2-CNTs (linked) (M= Mn(Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ)) are showed in Fig. 6. The blank represents the battery in the absence of catalyst. As seen from curve 1, the initial voltage and the discharge time of blank experiment are 3.13 V and 607 s, respectively. The discharge curves of M2Pc2-CNTs (linked) with different central metals are shown in curve 2 to 7. In series of M2Pc2-CNTs (linked), the initial voltage increases by 0.07-0.13V. Simultaneously, the discharge time is lengthened by 288-681s. Among all catalysts, the Fe2Pc2-CNTs (linked) have best catalytic activity.

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In order to analyze the catalytic performance more directly, the capacity and the rate capability of the battery are calculated. The cell capacity of Li/SOCl2 battery is: C = ∫Idt = ∫U/Redt = ∑U/Redt = 1/Re∑UΔt (4) The cell capacity increasing rate of Li/SOCl2 battery is: X = (C﹣C0) / C0 ·100% (5)

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In the above formulas, C stands for the capacity of the Li/SOCl 2 battery, I represents the discharge current of the battery, U stands for the output voltage of the battery, Re stands for the electrical resistance, Δt represents the time of discharge, and

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C0 stands for the capacity of blank test. The capacity and capacity increasing rate of the Li/SOCl2 battery are showed in Fig. 7 and Fig. 8, respectively. The capacity of the battery electrocatalyzed by Fe2Pc2-CNTs (linked) is 96.09 mAh, which is higher than that of the blank (42.07 mAh) by the rate of 128.5%. As seen from Table 1, the initial voltage order of the linked M2Pc2-CNTs is marked by the central metal ion: Fe2+ > Co2+> Ni2+ > Mn2+ > Cu2+> Zn2+. The capacity order of the Li/SOCl2 battery is sorted: Fe2+ > Co2+ > Cu2+> Zn2+> Ni2+ > Mn2+. Above all, the order of catalytic activity is: Fe2+ > Co2+ > Cu2+> Zn2+> Ni2+ > Mn2+. Notably, the linked Fe2Pc2-CNTs have excellent catalytic activity.

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Blank Co(Ⅱ) Cu(Ⅱ) Fe(Ⅱ) Mn(Ⅱ) Ni(Ⅱ) Zn(Ⅱ)

3.09 3.20 3.22 3.18 3.20 3.19 3.19

810 1342 919 1028 1049 1196 817

50.68 95.84 73.98 73.09 74.23 86.46 58.54

88.49 34.43 42.39 46.45 69.02 15.44

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M2Pc2-CNTs

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The results show that M2Pc2-CNTs (linked) can improve the performance of Li/SOCl2 battery. The reason is that the carbon nanotube itself is electronic excellent conductor, when it is bonding with M2Pc2, the electronic transmission is greatly promoted.

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3.7 Cyclic voltammetric analysis In order to further investigate the electrocatalytic behavior of complex to Li/SOCl2 battery, cyclic voltammetric is tested on RT5000 electrochemical

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workstation (Fig. 9). Two current peaks are found from blank and all M2Pc2-CNTs (linked) samples, which is corresponding to SOCl2 and SCl2, respectively. Due to two reduction peaks and no oxidation peak, the reaction of battery is irreversible. That is, two electron transfer processes occur in battery system (shown in Eq.6 and 7). The high current peak at 2.6 V, which is mean that Eq.6 is the rate-controlling step. The low reduction at 3.6 V, which is mean that Eq.7 is the fast reaction step for the reductive reaction of SOCl2. SOCl2 + e → 1/2 SCl2 + 1/2 SO2 + Cl(6) 1/2 SCl2 + e → 1/2 S + Cl-

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The catalytic properties can be seen from the cyclic voltammetric analysis. The better catalytic activity has bigger reduction potential and higher reduction peak. The height of reduction peaks at 2.6 V is sorted as follows: Fe2+ > Co2+ > Cu2+ > Zn2+ > Ni2+>Mn2+. Simultaneously, the result further confirms the capacity order of the Li/SOCl2 battery: Fe2+ > Co2+ > Cu2+ > Zn2+ > Ni2+>Mn2+. And, the results show that the best catalyst among them is Fe2Pc2-CNTs (linked), and the worst is Zn2Pc2-CNTs (linked). This conclusion is in accordance with results of electrocatalytic performance. The cyclic voltammetric curves of battery catalyzed by Fe2Pc2, CNTs, Fe2Pc2-CNTs (linked) and Fe2Pc2/CNTs (mixed) are investigated at 100 mV/s (Fig. 10). The catalytic activity order is showed as follows: Fe2Pc2-CNTs(linked) > CNTs > Fe2Pc2/CNTs(mixed) > Fe2Pc2 > blank. The results show that the modified 9

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Fe2Pc2-CNTs (linked), CNTs, Fe2Pc2/CNTs(mixed) and the Fe2Pc2 can boost the catalytic activity to Li/SOCl2 battery, and the modified Fe2Pc2-CNTs is the best catalyst among them. The prominent electrocatalytic performance of the Fe2Pc2-CNTs (linked) may be attributed to the synergistic effect between the phthalocyanine and the CNTs, the transportation of the electrons in the battery catalyzed by Fe2Pc2-CNTs (linked) is sped up, which contributes to the major improvement of the capacity of the battery. The effect of the scanning rates on the electricatalytic activity of modified

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binuclear metal phthalocyanine is also investigated. Fig. 11 records at different scanning rates (40 ~ 100 mV/s) in electrolyte containing Cu2Pc2-CNTs (linked). With increasing the scanning rate, the reduction peaks are enhanced. Fig. 12 shows the plot of log ipc (ipc of SOCl2 in the rate-controlling step) vs Log v (v: scan rate) from CV results. The obtained linear equation is y = 13.116x + 410.52, with the correlation coefficient of 0.928. The results demonstrate that the main function of electrode surface is diffusion. At the same time, the catalyst played an important role in enhancing the performance of the battery.

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3.8 Catalytic mechanism The catalytic mechanism of the battery catalyzed by binuclear metalophthalocyanine has been investigated in some papers[17]. According to the CV test, the electrocatalytic process may experience three steps: (1) the SOCl2 bonds with catalyst M2Pc2-CNTs (linked) to form an adduct. (2) the adduct gains one electron to generate the intermediate M2Pc2--CNTs·1/2SCl2, SO2 and Cl-. (3) the intermediate M2Pc2 -CNTs·1/2SCl2 obtains another electron to produce S, Cl- and M2Pc2-CNTs is returned. The catalyst has beneficial effects on Li/SOCl2 battery in two aspects. One is

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that the SOCl2 can form an adduct by bonding with the center metal of catalyst, which reduces the energy barrier of the reducing reaction. Another aspect is that catalytst can effectively reduce the resistance of electron transfer. In the electrocatalytic process of the modified M2Pc2-CNTs, the CNTs and the phthalocyanine may show favorable synergistic effect, which is the main reason why the modified M2Pc2-CNTs have good catalytic activity. The catalytic cycle is described below: M2Pc2-CNTs + SOCl2 →M2Pc2-CNTs·SOCl2 M2Pc2-CNTs·SOCl2+e-→M2Pc2-CNTs·1/2SCl2+1/2SO2+Cl4. Conclusion

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In this paper, a series of binuclear metal phthalocyanines was designed, synthesized and characterized. Then, modified CNTs were synthesized by bonding with binuclear metal phthalocyanines. The results of catalytic activity show that the all of the catalysts can improve the capacity of Li/SOCl2 battery, with the capacity increased by 59.1-128.5%, and Fe2Pc2-CNTs (linked) is the best catalyst among them. Simultaneously, the CV analysis illustrates that the reaction of battery is irreversible and indirectly verifies the reaction mechanism. All of the results show that Fe2Pc2-CNTs (linked) can greatly improve the performance of Li/SOCl2 battery for the synergistic effect between the CNTs and the binuclear phthalocyanine.

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Acknowledgements The authors thank the National Natural Science Foundation of China (Nos.21371143, 21671157 and 21501139) for the financial support of this work. Reference

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[1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] Yingying Sun, HaiyingWang, Changqing Sun, Amperometric glucose biosensor based on layer-by-layer covalent attachment of AMWNTs and IO4−-oxidized GOx, Biosensors and Bioelectronics 24 (2008) 22–28. [3] X. H. Chen and H. H. Song, Multi walled carbon nanotube filled SBR rubber com-posites, New Carbon Mater 19 (2004) 214-218 [4] Baibarac M, Lira‐Cantú M, Oró‐Solé J, et al. Electrochemically functionalized

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[5] Li T, Peng Y, Li K, et al. Enhanced activity and stability of binuclear iron (III) phthalocyanine on graphene nanosheets for electrocatalytic oxygen reduction in acid, Journal of power sources 293 (2015) 511-518 [6] Zhu, B., Zhang, X., Han, M., Deng, P., & Li, Q, Novel planar binuclear zinc phthalocyanine sensitizer for dye-sensitized solar cells: Synthesis and spectral, electrochemical, and photovoltaic properties, Journal of Molecular Structure 1079 (2015) 61-66. [7] Liu, L., Guo, L. P., Bo, X. J., Bai, J., & Cui, X. J, Electrochemical sensors based on binuclear cobalt phthalocyanine/surfactant/ordered mesoporous carbon composite electrode, Analytica chimica acta, 673(1) (2010) 88-94. [8] K. M. Abraham, R. M. Mank, Some chemistry in the Li/SOCl2 Cell, Journal of the Electrochemical Society, 127(10) (1980) 2091-2096. 11

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[1] R. Kotz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochim. Acta 45 (2000) 2483. [9] Seung-Bok Lee, Su-Il Pyun, Eung-Jo Lee, Effect of the compactness of the lithium chloride layer formed on the carbon cathode on the electrochemical reduction of SOCl2 electrolyte in Li-SOCl2 batteries, Electrochimi. Acta 47 (2001) 855-864. [10] P. A. Bernstein, A. B. P. Lever, Two-electron oxidation of cobalt phthalocyanines by thionyl chloride. Implications for lithium/thionyl chloride batteries, Inorganic Chemistry 29 (1990) 608-616. [11] L. Cui, G. Lv, Z. Dou, and X. He, Fabrication of iron phthalocyanine/graphene

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Analytica chimica acta 673(1) (2010) 88-94. [17] W.S. Kima, Y.K. Choi, Electrocatalytic effects of thionyl chloride reduction by polymeric Schiff base transition metal (II) complexes, Appl. Catal A 252 (2003) 163.

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Fig. 1 IR spectra of a：CNTs，b：Ni2Pc2-CNTs (linked)，c： Ni2Pc2/CNTs (mixed)，d： Ni2Pc2 Fig. 2 X-ray diffraction spectra of a: CNTs, b: Ni2Pc2-CNTs (linked), c: Ni2Pc2/CNTs (mixed), d: Ni2Pc2 Fig. 3 SEM image of (a: CNTs, b: Ni2Pc2-CNTs (linked), c: Ni2Pc2/CNTs (mixed), d: Ni2Pc2 ) Fig.4 TEM image of a: CNTs, b: Ni2Pc2-CNTs (linked) Fig. 5 a: XPS whole scanning spectra in Ni2Pc2-CNTs(linked), b: core level spectra of Ni 2p, c: core level spectra of C1s, d: core level spectra of N1s Fig. 6 The U-t curve of Li/SOCl2 Fig. 7 The cell capacity of Li/SOCl2 battery CNTs-CO-NH-BinuclearMPc (M= Mn ,Fe, Co, Ni, Cu, Zn) Fig. 8 The cell capacity increasing rate of Li/SOCl2 battery CNTs-CO-NH-BinuclearMPc (M= Mn, Fe, Co, Ni, Cu, Zn) Fig. 9 Cyclic voltammogram of the Li/SOCl2 battery. Scan rate was 100 mV s-1. Fig. 10 Comparative cyclic voltammetries of a: blank, b: Fe2Pc2, c: Fe2Pc2-CNTs (linked),d: CNTs e: Fe2Pc2/CNTs (mixed) Fig. 11 Representative cyclic voltammograms for Fe2Pc2-CNTs (linked) at different scan rates. Fig. 12 Plot of scan rate versus root of the sweep rate for Fe2Pc2–CNTs (linked).

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Scheme 1 The synthetic reaction of the M2HnPc2, M2Pc2 and M2Pc2-CNTs

Scheme 2 The structural diagram of Li/SOCl2 battery

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Highlights: 1、The M2Pc2-CNTs catalysts were synthesized byCNTs bonding with M2Pc2. 2、The capacity of the battery electrocatalyzed by Fe2Pc2-CNTs (linked) is 96.09 Aˑh, which is higher than that of the blank (42.07 Aˑh) by the rate of 128.5%. 3、The synergistic effect between the CNTs and the binuclear phthalocyanine improve the

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4、The catalytic mechanism of the battery catalyzed by binuclear metalophthalocyanine has been

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