Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 41339-41346
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Selenium Encapsulated into Metal−Organic Frameworks Derived N‑Doped Porous Carbon Polyhedrons as Cathode for Na−Se Batteries Qiuju Xu,†,‡ Ting Liu,†,‡ Yi Li,†,‡ Linyu Hu,†,‡ Chunlong Dai,†,‡ Youquan Zhang,†,‡ Yan Li,§ Dingyu Liu,†,‡ and Maowen Xu*,†,‡
ACS Appl. Mater. Interfaces 2017.9:41339-41346. Downloaded from pubs.acs.org by UNIV OF WOLLONGONG on 08/04/18. For personal use only.
†
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P.R. China ‡ Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, P.R. China § College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, P.R. China S Supporting Information *
ABSTRACT: The substitution of Se for S as cathode for rechargeable batteries, which confine selenium in porous carbon, attracts much attention as a potential area of research for energy storage systems. To date, there are no reports about metal− organic frameworks (MOFs) to use for Na−Se batteries. Herein, MOFs-derived nitrogen-doped porous carbon polyhedrons (NPCPs) have been obtained via facile synthesis and annealing treatment. Se is encapsulated into the mesopores of carbon polyhedrons homogeneously by melt-diffusion process to form Se/NPCPs composite, using as cathode for advanced Na−Se batteries. Se/NPCPs cathode exhibits excellent rate capabilities of 351.6 and 307.8 at 0.5C and 2C, respectively, along with good cycling performance with high Coulombic efficiency of 99.7% and slow decay rate of 0.05% per cycle after 1000 cycles at 2C, which result from the NPCPs having a unique porous structure to accommodate volumetric expansion of Se during discharge− charge processes. Nitrogen doping could enhance the electrical conductivity of carbon matrix and facilitate rapid charge transfer. KEYWORDS: metal organic frameworks, nitrogen-doped porous carbon polyhedrons, Se-based material, volumetric expansion, Na−Se batteries
1. INTRODUCTION Recently, Na-ion batteries have been highlighted for renewable energy grid storage and electric vehicles due to the abundance and low cost of sodium.1,2 The traditional high-temperature Na−S batteries have been developed for large-scale renewable energy storage because of the outstanding advantages including high theoretical specific energy density (760 W h kg−1), highenergy efficiency (approaching 100%), low material cost (sufficiency of S and Na in the earth’s crust), and long cycle life.3 Although the high-temperature Na−S batteries have already been manufactured commercially, their high operation temperature (>300 °C) is the major defect. In addition, the use of β-alumina solid electrolyte and the high operating temperature increase the cost of battery manufacturing, maintenance, and safety concerns.4 Therefore, it would be promising to © 2017 American Chemical Society
explore novel cathode materials for Na-ion batteries, which can be operated at room temperature. Selenium has intrinsic electrical conductivity (1 × 10−3 S −1 m ), which is much higher than that of sulfur (5 × 10−28 S m−1) due to the semiconductor properties of Se, so it may offer higher capacity utilization and better rate capability.5 And its theoretical volumetric capacity density (3253 mA h cm−3) is as high as that of sulfur (3467 mA h cm−3).6,7 Se possesses higher reaction activity with Na at room temperature, making it a prospective choice for ambient applications. As a result, Se has been regarded as a promising cathode for Na-ion batteries. Received: September 21, 2017 Accepted: November 7, 2017 Published: November 7, 2017 41339
DOI: 10.1021/acsami.7b14380 ACS Appl. Mater. Interfaces 2017, 9, 41339−41346
Research Article
ACS Applied Materials & Interfaces
Figure 1. XRD patterns (a); Raman spectra (b) of pristine Se, NPCPs, and Se/NPCPs composite.
compared to other mesoporous carbons.11 Not only could the hollow structure offer continuous electron transportation path, but also it can buffer volume changes.22,23 When used as cathode for Na−Se batteries first, Se/NPCPs composite shows superior electrochemical performance.
Similar to S, Se-based cathode also sustains large volume expansion, resulting in low Coulombic efficiency and fast capacity fading.5 Confinement of selenium in porous carbon and the structure of nanofibrous and nanoporous selenium have been used to overcome such difficulty.8 Combined Se with carbon material can prominently improve the cycle performance, Coulombic efficiency, and selenium utilization because of the increasing electronic and ionic conductivities.9,10 Luo et al.11 prepared selenium-impregnated carbon composites by infusing Se into mesoporous carbon at high temperature under vacuum as cathode for Li−Se batteries and Na−Se batteries. Yu’s group12 developed selenium/porous carbon nanofibers composite electrode to buffer the shuttle reaction of intermediate polyselenides during cycling processes. Wang et al.13 reported that organic selenide fiber was prepared by electrospinning technique and delivered high capacities. Goodenough’s group14 successfully loaded Se molecules into the microporous slits of carbons to give a long cycling life. Ding et al.15 created a cathode based on capsuling selenium into cellulose-derived carbon nanosheets. Among all the above strategies, diverse carbon-matrixes-derived porous carbon were developed to encapsulate Se as cathode for Na−Se batteries. Metal organic frameworks (MOFs) have attracted extensive attention because of their uniquely porous structure, large specific surface area, and microstructural controllability, showing various applications in C2H2 storage, hydrogen evolution, CO2 adsorption, electrocatalytic water splitting, etc. Many porous carbon materials derived from MOFs with abundant pores were used for confining S or Se and then as cathode for Li−S batteries,16,17 Na−S batteries,18,19 and Li−Se batteries.20,21 However, until now, there are no reports about MOFs-derived porous carbon to infuse Se as cathode for Na− Se batteries. Herein, we prepared nitrogen-doped porous carbon polyhedrons (NPCPs) derived from ZIF-67 (a subclass of MOFs) for the loading of Se. Such NPCPs hold abundant mesopores with diameter >2 nm and the unique porous structure makes it possible for Se to be firmly confined in the mesopores by melting-diffusion progress. Compared with other porous carbons, the NPCPs do not need KOH activation to create more pores12 and possess larger surface area and pore volume
2. EXPERIMENTAL SECTION Preparation of Se/NPCPs Composite. In a typical preparation, 2.5 mmol of Co(NO3)2·6H2O was dissolved in 50 mL of methanol to form a clear red solution under stirring, which was subsequently poured into the other solution made from 25 mmol of 2methylimidazole (C4H6N2) dissolved in 50 mL of methanol under stirring for 5 min, and then kept still for 24 h at ambient temperature. Then obtained precipitates were collected by filtration, washed with ethanol several times, and dried at 60 °C for 12 h. Finally, purple ZIF67 was obtained. Then ZIF-67 was heated at 700 °C for 3 h under Ar atmosphere with a heating rate of 5 °C min−1. To remove Co species, the black product after carbonization was immersed into HCl solution. After filtration and being washed with deionized water, ZIF-67 was successfully turned into NPCPs. Then pristine Se powder and NPCPs were grinded and mixed in agate mortar with weight ratio of 2:1. Subsequently, the mixture was transferred to a tube furnace and heated at 260 °C for 20 h under argon atmosphere to obtain Se/ NPCPs composite. Materials Characterization. Crystallographic structure of samples were investigated with an X-ray diffractometer (Maxima-X XRD-7000) and Cu Kα radiation. Raman patterns were acquired by using Invia Refl (Renishaw, UK). The morphologies and microstructures of products were inspected by using field-emission scanning electron microscopy (FESEM, JSM-7800F), transition electron microscopy (TEM, JEM-2100), and STEM (Tecnai-G2-F30). The BET specific surface areas and pore size distributions were tested by Quadrasorb evo 2QDS-MP-30 (Quantachrome Instruments, USA). The weight percent of Se was determined with a thermogravimetric analyzer (TGA, Q50). Moreover, X-ray photoelectron spectroscopy (XPS) characterization was analyzed with an ESCALAB 250Xi electron spectrometer. Electrochemical Measurements. The cathode electrode was prepared by mixing active material (Se/NPCPs composite), acetylene black (AB), and sodium alginate ((C6H7O6Na)n, SA) water-soluble binder with weight ratio of 80:10:10 in agate mortar. Then the slurry was covered on aluminum foil and dried overnight at 60 °C under vacuum. There was ∼2.0 mg cm−2 active material (∼0.8 mg cm−2 Se) loaded on the electrode and the specific capacity was calculated based 41340
DOI: 10.1021/acsami.7b14380 ACS Appl. Mater. Interfaces 2017, 9, 41339−41346
Research Article
ACS Applied Materials & Interfaces
Figure 2. FESEM images (a−c) and STEM elemental mapping images of C (d), N (e), and Se (f); XPS spectrum of C 1s (g), N 1s (h), and Se 3d (i) of Se/NPCPs composite. on the mass of Se (1C equals 675 mA g−1). The Se/AB cathode was made by the same method. CR2032 coin cells were used and assembled in an Ar-filled glovebox. In coin cells, we used Se/NPCPs composite and Se/AB as cathode, Celgard 2400 as the separator, and a disk of sodium foil as reference electrode. The electrolyte was 1 M NaClO4 dissolved in mixed solvent of EC-DEC with 3 wt % FEC. The amount of the electrolyte in the cell is 45 μL. Cyclic voltammetry (CV) test was conducted with an Arbin Instruments Testing System. And the galvanostatic discharge−charge test was achieved on a LAND instrument testing system. Electrochemical impedance spectroscopy (EIS) test was carried on a CHI 600c electrochemical workstation.
is attributed to chain-structured Se.11 However, those three peaks can not be observed while a peak of ∼260 cm−1 exists in Se/NPCPs composite; the only peak explains Se8 ring.15 It can be concluded the selenium exists in Se/NPCPs composite, and the Se8 ring has appropriate sizes and dimensions to fit into the porous carbon easily. Figure S2 shows the morphologies of ZIF-67 and NPCPs. The particles of ZIF-67 hold polyhedral shapes as well as uniform distribution and size of ∼250 nm with smooth surfaces. After carbonization, porous carbon material inherits original polyhedral uniform morphologies and dimensions with shrinkage while the smooth surfaces become distinctly rough. Compared with NPCPs, Se/NPCPs composite maintains homogeneous dispersion in Figure 2a and holds few bulk Se on the surface in Figure 2b,c, accounting for the successful encapsulation of Se inside the NPCPs. The Se content in Se/ NPCPs composite was 48.5 wt %, which was measured by TGA in Figure S3. Meanwhile, HAADF-STEM image and corresponding element mappings of C, N, and Se are shown in Figure 2d−f, which not only clearly shows uniform distribution of Se but also verifies homogeneous presence of effective N in the carbon matrix. And FESEM element mapping images also agree with that (Figure S4). The XPS analysis was tested to research chemical valence state and component of Se/NPCPs composite. Figure S5 reveals the survey spectra, which testify to the existence of C, N, and Se. The peaks of the C 1s spectrum at 284.5, 285.6, 286.7, and 288.1 eV correspond to C−C, CN, C−N, and CO, respectively, as shown in Figure 2g.6,26,27 The existence of N element can be verified by the N 1s spectra in Figure 2h. The spectra show the different oxidation states of nitrogen atoms, including pyrrolic N at 399.3 eV, graphitic N at 400.9 eV, and oxidized N at 402.6 eV.28 The doped nitrogen can improve the
3. RESULTS AND DISCUSSION Figure 1a reveals XRD patterns of pristine Se and products. For pristine Se, diffraction peaks correspond with trigonal crystalline (JCPDS 06-0362). Broad diffraction peak of NPCPs around 26° exhibits characteristics of graphitized carbon,21 and the low-intensity peaks located at 45° and 76.5° are residue Co, which are in accord with the peaks 44.2° (111) and 75.9° (220) of Co (JCPDS 15-0806). After heat treatment of Se and NPCPs, both diffraction peaks of carbon and Se are observed distinctly, indicating that Se exists in carbon matrix. In addition, the XRD pattern of ZIF-67 is displayed in Figure S1 of the Supporting Information and all diffraction peaks correspond well with previous research.24 The structure characteristics of pristine Se, NPCPs, and Se/ NPCPs composite are further researched by Raman spectroscopy in Figure 1b. Both Raman spectra of NPCPs and Se/ NPCPs composite show carbon peaks at 1320 cm −1 (amorphous structure, D band) and 1580 cm−1 (graphitized structure, G band). In particular, the G band with lower intensity demonstrates the partially graphitized nature of NPCPs.25 For pristine Se, the peaks at 144 and 460 cm−1 represent Se12 with a ring structure while the peak at 235 cm−1 41341
DOI: 10.1021/acsami.7b14380 ACS Appl. Mater. Interfaces 2017, 9, 41339−41346
Research Article
ACS Applied Materials & Interfaces
Figure 3. TEM and HRTEM images of NPCPs (a, c, e) and Se/NPCPs composite (b, d, f); adsorption−desorption isotherms (g); pore size distributions (h) of NPCPs and Se/NPCPs composite; schematic diagram (i) of the preparation process of Se/NPCPs composite.
Figure 4. (a) CV curves of Se/NPCPs composite; (b) discharge−charge profiles of Se/NPCPs composite at 0.5C; (c) rate capability of Se/NPCPs composite; (d) discharge−charge profiles of Se/NPCPs composite at different current densities.
keeps the original morphology after Se encapsulation. In terms of Figure 3a, polyhedrons with particle size ∼250 nm can be observed clearly under low magnification, and those small black spots are residual Co particles and Figure S6 shows the amount of Co is 3.49 wt %. Hollow carbon matrix is filled with Se after heat treatment at 260 °C, as shown in Figure 3b. Figure 3c,d further proves distinctions of porous structure before and after Se encapsulation; it can be observed easily that the carbon matrix NPCPs hold mesopores while Se exists in porous carbon
electronic conductivity of porous carbon. In particular, pyrrolic N is more favorable because it can create numerous extrinsic defects and active sites.28 Figure 2i displays the spectra of Se 3d; two peaks at 55.9 and 55.1 eV correspond to Se 3d3/2 and Se 3d5/2, which are attributed to spin−orbit interaction.21,29 Furthermore, the peak at 58.8 eV fits with Se−O bonding.30 TEM images further reveal the morphology and unique porous structure of NPCPs (Figure 3a,c). TEM images of Se/ NPCPs composite are shown in Figure 3b,d; the carbon matrix 41342
DOI: 10.1021/acsami.7b14380 ACS Appl. Mater. Interfaces 2017, 9, 41339−41346
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Cycling performance and Coulombic efficiency of Se/NPCPs composite electrode at 2C. (b) Nyquist plots of the battery with the fresh Se/NPCPs composite cathode and after cycling. (c) FESEM images of Se/NPCPs composite after cycling.
Figure 4a. There are two reduction peaks and one oxidation peak in the first cycle, indicating two-step phase change reaction to form Na2Se and single-phase transformation reaction from Na2Se to Se. The reduction peaks at ∼1.8 and ∼1.25 V correspond to reactions of Se and Na: step one, Se turns into the intermediate phase Na2Sen (n ⩾ 4); step two, Na2Sen turns into final Na2Se phase. The single oxidation peak at ∼2.0 V indicates direct phase change from Na2Se to Se.5,6 Notably, the reduction peak emerged at ∼1.8 V almost disappears, and the reduction peak emerged at ∼1.25 V shifts to ∼1.5 V in subsequent cycles, which could be attributed to the electrochemical activation process in the first discharge process.33 In terms of the oxidation peak, it shifts to ∼1.95 V subsequently. The coincident peaks after second cycle demonstrate the good electrochemical stability of the electrode. The galvanostatic discharge−charge test was conducted at 0.5C within the voltage window of 2.5−0.5 V in Figure 4b and the voltage platforms consist of the peak positions in CV curves. The Na−Se batteries can deliver initial discharge capacity of 976.1 mA h g−1, with 437.3 mA h g−1 being reversible. The reason for excellent capacity is high conductivity of carbon and electrochemical reaction with homogeneous Se, and irreversible capacity in the first cycle may be attributed to SEI (solid electrolyte interface) formation and some irreversible capturing of Na within carbon matrix at high potentials.15 Besides, the capacity fading of second cycle may be attributed to the unavoidable polyselenides dissolution within electrolyte.The profiles of the 2nd, 5th, 10th, 20th, and 50th cycles almost overlap, suggesting stable electrochemical performance. The Se/NPCPs composite electrode shows excellent rate capability, as exhibited in Figure 4c. It delivers discharge capacity of 392.2, 351.6, 327.7, 307.8, and 282.3 mA h g−1 at 0.2C, 0.5C, 1C, 2C, and 3C, respectively. The capacity regains 377.8 mA h g−1 when returning to 0.2C after operating at various rates, exhibiting an excellent rate performance of Se/ NPCPs composite. The decreasing discharge capacity of Se/ NPCPs composite at 0.2C may be caused by incomplete utilization of Se which attached onto polyhedrons surface. The Coulombic efficiency increased gradually, which could be ascribed to the activation process of Se. With further enhancement of the current density to 0.5C, 1C, 2C, and 3C,
after its encapsulation. Figure 3e,f displays the HRTEM of NPCPs and Se/NPCPs composite. In Figure 3e, the disordered fringes denote the graphite carbon in NPCPs, and lattice fringes in Figure 3f can be assigned to (101) lattice interplanar spacing of Se, demonstrating existence of Se inside Se/NPCPs composite. To further research the porous features, BET specific surface areas and pore size distributions are both exhibited in Figure 3g,h. Figure 3g illustrates the BET specific surface areas before and after Se encapsulation. As shown in Figure S7, the BET surface area of NPCPs reach up to 568.207 m2 g−1 which can provide sufficient internal space for Se loading. For comparison, Se/NPCPs composite only holds specific surface area of 19.648 m2 g−1. The obvious decline demonstrates that Se was successfully encapsulated into the internal space of porous carbon matrix, which is consistent with TEM results. Furthermore, the decrease of pore volume (Figure S7) can also prove this. Figure 3h displays the pore size distributions of NPCPs and Se/NPCPs composite. It is observed that NPCPs consist of abundant mesopores with pore diameter >2 nm, and the hysteresis loop at N2 pressures (P/P0 = 0.4−0.95) in Figure 3g indicates the presence of mesopores.31,32 After encapsulation of Se, the amount of mesopores performs sharp reduction, demonstrating that Se was successfully encapsulated into the mesopores. The pore volume of the NPCPs is 0.403 cm3 g−1 (Figure S7), which corresponds to a theoretical Se loading of ∼63 wt % (calculated based on the density of Se, 4.26 g cm−3).14 On the basis of TEM and BET results, the schematic diagram that exhibits a transformation course from ZIF-67 to eventual Se/NPCPs composite vividly is shown in Figure 3i. Obtained purple precursor was transformed to nitrogen-doped porous carbon polyhedrons by annealing at 700 °C for 3 h, while the carbon matrix holds a mass of mesopores. Subsequently, Se particles were successfully encapsulated inside the hollow area of NPCPs via melting−diffusion process at 260 °C for 20 h. Therefore, the unique carbon matrix provides a conductive framework for uniform Se dispersion. To explore the redox reactions during discharge−charge processes, CV curves of Se/NPCPs composite were obtained at a scan rate of 0.1 mV s−1 between 2.5 and 0.5 V, as shown in 41343
DOI: 10.1021/acsami.7b14380 ACS Appl. Mater. Interfaces 2017, 9, 41339−41346
Research Article
ACS Applied Materials & Interfaces discharge capacity of 345, 324.6, 302.2, and 276.6 mA h g−1 remained after 20, 30, 40, and 50 cycles, respectively. This excellent rate capability of Se/NPCPs is significant and meaningful for the practical application of Na−Se batteries. Figure 4d exhibits discharge−charge voltage profiles of Se/ NPCPs cathode at different current rates (0.2C, 0.5C, 1C, 2C, and 3C) within the voltage window of 2.5−0.5 V. Discharge and charge curves overlap at current rates below 1C, indicating tiny polarization and excellent rate capability during electrochemical reaction. It can be seen that both discharge capacity and discharge plateau decline at higher current rates above 1C due to higher ohmic and kinetic overpotentials on this occasion.16 However, inclined platforms can still be observed. To tune the amount of Se, we used different Se/NPCPs ratios (that is, 13:6 and 7:3) during the heat treatment; the Se content calculated from the TGA curves for the Se/NPCPs composite was ∼55 and 60 wt %, respectively (Figure S8a,b). The capacity of the composite with ∼55 wt % Se is lower and decreases rapidly compared to that with ∼50 wt %; however, the performance of ∼60 wt % Se is rather worse (Figure S8e). It may be caused by the samples holding numerous bulk Se on the surface clearly in Figure S8c,d, which result in poor conductivity and capacity fading. Besides excellent rate capability, Se/NPCPs composite shows good cycling performance as well, which is illustrated in Figure 5a. The enhancement of specific capacity before the 34th cycle could be attributed to the activation process of Se.34 The decreasing capacity may be ascribed to incomplete utilization of Se. When it reaches the 1000th cycle, a discharge capacity of 161.4 mA h g−1 is still retained with Coulombic efficiency staying around 99.7%. Additionally, the capacity decline per cycle is merely 0.05%. Compared with other cathode materials,11−14,23,29 our work manifests lower capacity decay rate (Figure S9). The good cycling performance could be ascribed to the unique hollow structure of NPCPs material. Nitrogen doping could enhance the electrical conductivity and facilitate rapid charge transfer.35 To prove the stability when the Se reacts with Na+ more completely, the cycling performance of lower current density is demonstrated in Figure S10. It exhibits a reversible capacity of 366.3 mA h g−1 after 100 cycles at 0.2C. To research the kinetics of the electrode process, Nyquist plots before cycling and after 1000 cycles at 2C are tested and displayed in Figure 5b. The Rct (212 Ω) before cycling suggests higher impedance due to the lower electrochemical contact.35 And the Rct decreases by around 88% after 1000 cycles, indicating that the NPCPs matrix acts could enhance chargetransfer ability of Na−Se batteries. For comparison, electrochemical performance of Se/AB was investigated and displayed in Figure S11, in which the capacity decreases rapidly to 50 mA h g−1 until the 70th cycle, which is inferior to Se/NPCPs composite. To further investigate the structural stability of the Se/NPCPs composite, FESEM analysis was tested on the cycled electrode after 1000 cycles at 2C in Figure 5c. The morphology of Se/NPCPs composite maintained well after overlong cycling, indicating that the unique structure of NPCPs can effectively buffer the volume change. The pore volume of the NPCPs and Se/NPCPs is 0.403 and 0.030 cm3 g−1, respectively (Figure S7). The pore volume of Na2Se is
