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The (2 × 2) tunnels structured manganese dioxide nanorods with α phase for lithium air batteries
Superlattices and Microstructures 90 (2016) 184e190
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The (2 2) tunnels structured manganese dioxide nanorods with a phase for lithium air batteries Zafar Khan Ghouri a, b, Awan Zahoor c, Nasser A.M. Barakat b, d, *, Mohammad S. Alsoufi e, Tahani M. Bawazeer f, Ahmed F. Mohamed e, g, Hak Yong Kim a, b, ** a
Advanced Materials Institute for BIN Convergence, Department of BIN Convergence Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea c Department of Chemical Engineering, NED University of Engineering & Technology, University Road, Karachi 75270, Pakistan d Department of Chemical Engineering, Faculty of Engineering, El-Minia University, El-Minia, Egypt e Department of Mechanical Engineering, Collage of Engineering and Islamic Architecture, Umm Al-Qura University, Makkah, Saudi Arabia f Department of Chemistry, Faculty of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia g Department of Mechanical Engineering, Collage of Engineering, Sohag University, Sohag, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2015 Accepted 10 December 2015 Available online 13 December 2015
The (2 2) tunnels structured manganese dioxide nanorods with a phase (a-MnO2) are synthesized via simplistic hydrothermal method at low temperature. The obtained tunnels structured aeMnO2 nanorods are characterized by, Transmission electron microscopy, Scanning electron microscopy, and X-ray diffraction techniques. The oxygen reduction reaction (ORR) activity was studied by cyclic voltammetry and rotating ring-disc electrode voltammetry techniques in alkaline media. Moreover; the highly electrocatalytic tunnels structured aeMnO2 nanorods were then also applied as cathode in rechargeable LieO2 cells. The LieO2 cells exhibited initial discharge capacity as high as ~4000 mAh/g with the tunnels structured aeMnO2 nanorods which was double the original capacity of the cells without any catalyst. Also we obtained 100% round trip efficiency upon cycling with limited capacity for more than 50 cycles. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Tunnels structure a-MnO2 Nanorods Cyclic voltammetry Lithium air batteries Air cathode
1. Introduction In order to satisfy the increasing demand of clean and high-efficient energy, alternative resource of energy independent of fossil fuels, must be developed [1e8], among which lithium air batteries are attractive because they are compact, lightweight and more importantly they employ a lighter air cathode operating on environmentally abundant oxygen. The inherent energy density of lithium is more than 11,000Wh kg1 (close to gasoline with 13,000Wh kg1), which makes LieO2 batteries even more attractive for electric vehicle and energy storage.
* Corresponding author. Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea. ** Corresponding author. Advanced Materials Institute for BIN Convergence, Department of BIN Convergence Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea. E-mail addresses: [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). http://dx.doi.org/10.1016/j.spmi.2015.12.012 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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Amongst all the potential benefits of lithium air batteries, their large scale production and commercialization is still hindered by various limitations. One of the important factors that limit the LieO2 batteries is the slow kinetics of oxygen reduction reaction (ORR) on cathode side of the battery. Recently ORR has attracted particular attention since it is an important process to control the overall cell performance. Electro-catalysts play a key role in ORR mechanism and until now platinum is known to be the best electro-catalyst for the ORR in alkaline media [9]. Also, the mostly investigated ORR catalysts are based on noble metals such as platinum, palladium, gold, etc., because of their best overall catalytic performance [9,10]. However, due to high cost, and limited availability of these noble metals needs to be replaced by new electro-catalysts which are inexpensive, abundant and comparable electrocatalytic performance to that of noble metal catalysts. Numerous non-precious materials such as carbons, polymers, transitional metals and their oxides have already been suggested and investigated as an alternative materials for the ORR [11e13]. Among them manganese oxide seems to be the most promising and widely used catalyst in alkaline electrolyte because of its abundance, low cost, high catalytic activity and non-toxicity. MnOx particles incorporated transition metal based materials also shows good ORR activity compared to benchmark commercial Pt/C based catalyst [14]. MnOx can be applied as promising catalysts in air electrode for both alkaline fuel cells and metal-air batteries like LieO2 batteries. The role of catalysts in LieO2 batteries is mainly to reduce the voltage gap between the discharges and recharge processes and thereby to increase the efficiency of the cells [15]. Thus the capacity and cyclic performance of the cell are directly related to the nature of electro-catalyst used at the cathode. So far, there have been various MnO2 catalysts explored for ORR and LieO2 cells including MnO2 with different structures [16e18], MnO2 combined with metals and metal oxides [19,20] and MnO2 supported on carbons [21e23], etc. Even though nanostructured manganese oxides are promising electrocatalysts for the ORR and promising catalyst in air cathode, their wide spread implementation hinges upon their development with high activity and stability. In addition, structure-activity relationships for nanostructured MnO2 for the ORR are still needed to be investigated further to elucidate the factors that determine catalytic performance. Herein, we report a simple, facile and templet free approach for preparing the (2 2) tunnels structured manganese dioxide nanorods with a phase. The novel shaped MnO2 nanostructures obtained via simplistic hydrothermal route at low temperature. The synthesized (2 2) tunnels structured manganese dioxide nanorods were investigated for ORR activity and finally used for LieO2 batteries application. Moreover, their structure-activity relations are also explained in this article. 2. Experimental 2.1. Synthesis of (2 2) tunnels structured manganese dioxide nanorods Potassium per manganite (Showa Chemicals Co. Ltd, Japan, 99.3%) and Nitric Acid (Samchun pure Chemicals Co. Ltd, Korea, 60%) were used as starting chemicals without further purification. In a typical synthesis, 50.0 mmol of KMnO4 and 5.1 mL HNO3 were dissolved in 58 mL of twice-distilled water (18.3 MU) with vigorous stirring to form the precursor solution. Subsequently, after stirring for about 30 min, the solution was treated in a stainless steel Teflon-lined autoclave equipped with a microwave heating, at 120 C for 12 h, and then was cooled down to room temperature and the product was filtered, washed with distilled water and dried at 80 C for 12 h in air. The final product, tunnel structured a-MnO2 nanorods were characterized by various techniques. 3. Characterization techniques Uniformity, chemical composition and morphology of the samples were checked with field emission scanning electron microscopy (FESEM, JSM-6700F) and high resolution transmission electron microscopy (HRTEM JEM-2010, JEOL) equipped with EDS system. The phase structures of the as-prepared samples were determined by powder X-ray diffraction spectrum (XRD, Shimadzu XRD-6000, Cu KR, l 1.5406 Å). Electrocatalytic measurements for ORR were carried out on a computerized potentiostat instrument (model CHI700C) at room temperature in a three electrode system using 0.1 M KOH as electrolyte. Linear sweep voltammetry (LSV) were recorded at a scan rate of 5 mV1, with a disk rotation rate of 1600 rpm, and the cyclic voltammetry (CV) was recorded at a scan rate of 50 mV1. 4. Preparation of electrode For ORR studies, the synthesized (2 2) tunnels structured manganese dioxide nanorods were mixed with carbon powder (Cabot Vulcan XC-72) in the weight ratio of 3:7 to ensure sufficient electronic conductivity. 5 mg of as prepared catalyst was dispersed ultrasonically in 75 mL of diluted nafion alcohol solution (5 wt.%), and about 20 mL of the suspension was pipetted onto a glassy carbon substrate. Pt wire and Hg/HgO were used as the counter and the reference electrode respectively. Prior to measurement O2 was bubbled directly into the cell for at least 1 h. For Lithium air battery studies, the air cathodes were prepared by mixing as prepared (2 2) tunnels structured manganese dioxide nanorods. a-MnO2 nanorods catalyst and Ketjen black (EC 600JD) conductive carbon in the ratio of 1:2 with teflonised acetylene black (TAB) binder (60%) in isopropyl alcohol.
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The mixture was prepared into a fine pellet of about 1 cm diameter and the pellet was pressed on a Ni mesh current collector with a diameter of 1.2 cm. Thus prepared electrode is then dried in vacuum over night at 100 C and used as cathode in Li air battery. 5. Lithium air battery studies Lithium air battery applications were studied by Swagelok™ type cell with Li metal anode, LiTFSI (TEGDME) (1:1) electrolyte and KB cathode with and without a-MnO2 nanorod as catalyst. Each cell were assembled in argon filled glove box under room temperature and purged with oxygen before cycle performance. The charge discharge profiles of the cells were tested in BTS 2004 (Japan) system at 1 atm O2 atmosphere. 6. Results and discussion Fig. 1 shows X-ray diffraction patterns of the as synthesize (2 2) tunnels structured manganese dioxide nanorods with a phase. It can be observed that various sharp diffraction peaks located at 2q of 12.7, 18.1, 25.7, 28.8 , 36.6 , 37.5 , 39.0 ,41.2 , 41.9 , 49.8 , 56.3 , 60.2 , 65.2 , 69.7 and 72.7 correspond well to the (110), (200), (220), (310), (400), (211), (330), (420), (301), (411), (600), (521), (002), (541) and (312) planes are characteristic ones for the a-MnO2 phase and characterizing this support as crystalline oxide with tetragonal structure, [space group: 14/m] according to the JCPDS 44-0141 with lattice constants a ¼ b ¼ 9.78 Å, c ¼ 2.86 Å; a ¼ b ¼ g ¼ 90 . The crystal construction of a-MnO2 is assembled from binary chains of octahedral [MnO6] which form (2 2) tunnels with an opening as large as 0.47 nm. This tunnel like construction is ideal as electrode material for batteries [1]. Scanning electron microscopy was employed to observe the nano-morphology of as synthesize (2 2) tunnels structured a-MnO2. FESEM images of the nanostructured a-MnO2 are shown in Fig. 2(a and b). The FESEM images show that all the nanostructured a-MnO2 had rod-like shapes that are closely and haphazardly aligned. The average diameters of the nanorods were 85 nm and length in few hundreds of nanometers. Comprehensive analysis of the particle morphology was made using TEM & HRTEM measurements and the results were presented in Fig. 3(a and b). The TEM image (Fig. 3(a)) illustrates clearly the synthesized (2 2) tunnels structured a-MnO2, having a rod-like shape and are associated together. The HRTEM image (Fig. 3(b)) shows uniform lattice fringes with average inter planar spacing of 0.48 nm corresponding to (200) plane of tetragonal structure (marked by arrows). 7. Electrocatalytic properties In order to examine the effect of the a-MnO2 nanorods on the electro-catalytic ORR performance, linear sweep voltammograms (LSVs) were recorded in O2 saturated 0.1 M KOH at a scan rate of 5 mV/s over the electrode rotation rate of 1600 rpm and are shown in Fig. 4(a). The extracted data is given in Table 1. For comparison ORR activity of simple prepared MnO2 nanomaterials (bulk), glassy carbon and commercial 20 wt.% Pt/C under the same experimental condition are also given in Table 1. Starting at 0.3 V and scanning the cathode potential, a fast increase of currents in the mixed kinetic diffusion control
Fig. 1. XRD spectrum of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
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Fig. 2. (a and b) FESEM images of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
region (0.04 to 0.3 V) is followed by the appearance of diffusion-limiting current. The a-MnO2 nanorods shows a faster current drop exhibiting positively shifted onset potential with a high steady state current. It exhibits a limiting current value close to that of Pt/C. The onset potential of a-MnO2 nanorods is 0.02 V whereas for Pt/C is 0.08 V. The a-MnO2 nanorods exhibits better performance than bulk MnO2 in terms of limiting current, half wave potential and onset potential. a-MnO2 nanorods also show far better electrocatalytic activity than glassy carbon electrode. Fig. 4(b) shows the CV of the a-MnO2 nanorods electrode in 0.1 M KOH solution saturated with O2 at a scan rate of 50 mV/s. For comparison CV of glassy carbon and bulk MnO2 electrodes are also included. Table 1 summarizes the corresponding electrochemical properties. The catalytic activity of a catalyst to the ORR could be qualitatively estimated from the peak current and the reduction peak potentials. There is significant difference between the peak potentials of glassy carbon, bulk MnO2 and a-MnO2 nanorods, and the peak potentials of a-MnO2 nanorods are positively shifted to 0.20 V. The CV curves of a-MnO2 nanorods showed (not included in here) a well-defined cathodic peak even after 10 cycles, showing the stability of the samples. 8. Lithium air battery performance The Swagelok™ cells were assembled to investigate the catalytic effect of a-MnO2 nanorods. The KB air cathode with and without a-MnO2 nanorod catalyst were tested versus (Li/Liþ) lithium metal anode for cycling performance and the discharge charge profiles were observed in the potential range of 2.0e4.3 V. The open circuit voltage of the Li air battery was 3.3 V with a discharge plateau of 2.75 V. The cells with a-MnO2 nanorod catalyst exhibit a very high initial discharge capacity of 3997(~4000) mAh/g at 0.1 mA/cm2 current density with a charge capacity of 3772 mAh/g. This was double the capacity obtained without any catalyst with only KB cathode. The charge discharge profiles of the LieO2 cells are given in Fig. 5(a). On second discharge a good reversible capacity with 93% capacity retention was obtained. The obtained initial discharge capacity is much better than the earlier reports [24e26]. This superior performance of a-MnO2 nanorods catalyst might be due to the unique crystal structure and the porosity of aMnO2. The crystal structure of the a-polymorph consists of 2 2 tunnels formed by edge- and corner-sharing MnO6
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Fig. 3. (a) TEM image (b) HRTEM image of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
Fig. 4. (a) Linear sweep voltammetry of samples obtained at a rotation rate of 1600 rpm and a scan rate of 5 mV/s in 0.1 M KOH and (b) Cyclic voltammetry of samples obtained in 0.1 M KOH solution saturated with O2 at a scan rate of 50 mV/s.
octahedral. Johnson [27] and Rossouw [28] have explained that Li2O2 can be incorporated within the MnO2 tunnels with the O2 ions located at the tunnel centers and the Liþ ions coordinated between these central O2 ions, the latter forming the walls of the tunnels.
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Table 1 Onset potentials, limiting current, half wave potentials, and peak potential and peak current of different samples (values derived from Fig. 4(a and b). Samples
Limiting current (mA)
Onset potential (V)
Halfwave potential (V)
Peak current (mA)
Peak potential (V)
Glassy carbon MnO2 Bulk a-MnO2 nanorod Commercial Pt/C
0.24 0.66 0.76 0.78
0.25 0.037 0.026 0.085
0.33 0.13 0.16 0.03
0.05 0.15 0.27
0.35 0.25 0.20
Fig. 5. (a) Charge-discharge profile of air cathode with and without aeMnO2 nanorods catalyst and (b) Charge and discharge cycling profile of air cathode with aeMnO2 nanorods catalyst.
The ability of a-MnO2 to incorporate Li2O2 suggests the possibility of incorporating the Liþ and O2 2 ions on the surface of the material promoting the reversibility of the Li2O2 formation. Recently, Zhang et al. [29] also reported a significant increase in discharge capacity and cycling of LieO2 cells with a-MnO2 catalysts due to the reaction between the discharge product Li2O2 and a-MnO2 on charging. It has been suggested that in the presence of the catalyst, the Li2O2 deposits may be less dense and more porous than the ones formed in the absence of the catalyst hence preserving the ability of the reagents to diffuse into the carbon voids through the orifices occupied with such porous deposits. This may in turn increase the discharge capacity [30]. The difference between the charge and discharge voltages DV was 1.2 V, demonstrating that the battery had become more reversible. Whereas, the voltage gap of KB cathode without any catalyst is 0.6 V higher than with a-MnO2 nanorods catalyst as seen from Fig. 5(a). In other words addition of a-MnO2 nanorods catalysts have decreased the over potential of the cell. The addition of a-MnO2 really increases the overall potential enhancement in the cell cycling process which in turn enhances the specific power density of the cell as well. It is obvious that the addition of nanorods catalyst is highly effective for increasing the discharge capacity and reducing the over potential problem in Li air battery. Considerable cycling performance can be improved by limiting the depth of discharge and charging [31]. Fig. 5(b) presents the cycle efficiency with respect to cycle number for a cell cycled at a rate of 0.1 mA/cm2 and at a fixed capacity of 0.5 mAh. A uniform discharge/charge profile is observed up to 50 cycles. There is a small increase in over potential in every cycle and the increase of over potential with the limited capacity may be attributed to strong bonding between LiO2 and initial bare surface of a-MnO2 nanorods or its deep embedment into open structured surface frame with (2 2) tunnels [10,22]. Hence the round-trip efficiency of rechargeable Lieair batteries is limited by the increasing over potential upon charging. This can also be attributed to slow oxidation kinetics of Li2O2 formed upon discharge. But even after 50th cycle, the cycling profile maintains steady round trip efficiency without any capacity fade. 9. Conclusions A catalyst is the key component in electrode material. A systematic study has been carried out on the catalytic properties of MnO2 based nanostructures as electro-catalyst for oxygen reduction reaction in alkaline media. Thus prepared (2 2) tunnels structured a-MnO2 nanorods show superior electrocatalytic performance with positive peak potential. The catalysts were applied in LieO2 battery applications and excellent cycling is obtained. The specific capacity is almost doubled and the charge potential is lowered with a-MnO2 nanorods catalyst. Also the reversibility of LieO2 battery could be retained up to 50 cycles.
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Acknowledgments This Research was financially supported by National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No.2014R1A4A1008140) and this work was also supported by the IT R&D Program of MKE/KEIT ( Design and Development of fiber-]based flexible display) No.10041957. References [1] Z.K. Ghouri, M. Shaheer Akhtar, A. Zahoor, N.A.M. Barakat, W. Han, M. Park, B. Pant, P.S. Saud, C.H. Lee, H.Y. Kim, High-efficiency super capacitors based on hetero-structured a-MnO2 nanorods, J. Alloys Compd. 642 (2015) 210e215. [2] Z.K. Ghouri, N.A.M. Barakat, M. Park, B.-S. Kim, H.Y. Kim, Synthesis and characterization of Co/SrCO3 nanorods-decorated carbon nanofibers as novel electrocatalyst for methanol oxidation in alkaline medium, Ceram. Int. 41 (2015) 6575e6582. [3] Z.K. Ghouri, N.A.M. Barakat, M. Obaid, J.H. Lee, H.Y. Kim, Co/CeO2-decorated carbon nanofibers as effective non-precious electro-catalyst for fuel cells application in alkaline medium, Ceram. Int. 41 (2015) 2271e2278. [4] Z.K. Ghouri, N.A.M. Barakat, A.-M. Alam, M.S. Alsoufi, T.M. Bawazeer, A.F. Mohamed, H.Y. Kim, Synthesis and characterization of nitrogen-doped & CaCO3-decorated reduced graphene oxide nanocomposite for electrochemical supercapacitors, Electrochim. Acta 184 (2015) 193e202. [5] Z.K. Ghouri, N.A. Barakat, A.-M. Alam, M. Park, T.H. Han, H.Y. Kim, Facile synthesis of Fe/CeO2-doped CNFs and their capacitance behavior, Int. J. Electrochem. Sci. 10 (2015) 2064e2071. [6] J. Kim, Z.K. Ghouri, R.Z. Khan, T. An, M. Park, H.-Y. Kim, Experimental study on synthesis of Co/CeO2-doped carbon nanofibers and its performance in supercapacitors. [7] Z.K. Ghouri, N.A. Barakat, H.Y. Kim, Synthesis and electrochemical properties of MnO2 and Co-decorated graphene as novel nanocomposite for electrochemical super capacitors application, Energy Environ. Focus 4 (2015) 34e39. [8] Z.K. Ghouri, N.A.M. Barakat, H.Y. Kim, Influence of copper content on the electrocatalytic activity toward methanol oxidation of CocCuy alloy nanoparticles-decorated CNFs, Sci. Rep. 5 (2015) 16695. [9] J. Zhang, K. Sasaki, E. Sutter, R. Adzic, Stabilization of platinum oxygen-reduction electrocatalysts using gold clusters, Science 315 (2007) 220e222. [10] M. Shao, Palladium-based electrocatalysts for hydrogen oxidation and oxygen reduction reactions, J. Power Sources 196 (2011) 2433e2444. vre, E. Proietti, F. Jaouen, J.P. Dodelet, Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells, Science 324 [11] M. Lefe (2009) 71e74. [12] X. Wang, J.S. Lee, Q. Zhu, J. Liu, Y. Wang, S. Dai, Ammonia-treated ordered mesoporous carbons as catalytic materials for oxygen reduction reaction, Chem. Mater. 22 (2010) 2178e2180. [13] Z. Shi, H. Liu, K. Lee, E. Dy, J. Chlistunoff, M. Blair, P. Zelenay, J. Zhang, Z.S. Liu, Theoretical study of possible active site structures in cobalt-polypyrrole catalysts for oxygen reduction reaction, J. Phys. Chem. C 115 (2011) 16672e16680. bart, A.J. Paterson, J. Bao, P.G. Bruce, a-MnO2 nanowires: a catalyst for the O2 electrode in rechargeable lithium batteries, Angew. Chem. 120 [14] A. De (2008) 4597e4600. [15] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Oxygen electrocatalysts in metaleair batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (2014) 7746e7786. [16] W. Xiao, D. Wang, X.W. Lou, Shape-controlled synthesis of MnO2 nanostructures with enhanced electrocatalytic activity for oxygen reduction, J. Phys. Chem. C 114 (2009) 1694e1700. [17] F. Cheng, Y. Su, J. Liang, Z. Tao, J. Chen, MnO2-based nanostructures as catalysts for electrochemical oxygen reduction in alkaline media, Chem. Mater. 22 (2009) 898e905. [18] F.H. Lima, M.L. Calegaro, E.A. Ticianelli, Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction, Electrochim. Acta 52 (2007) 3732e3738. [19] H. Cheng, K. Scott, Selection of oxygen reduction catalysts for rechargeable lithiumeair batteriesdmetal or oxide? Appl. Catal. B Environ. 108 (2011) 140e151. [20] T.N. Lambert, D.J. Davis, W. Lu, S.J. Limmer, P.G. Kotula, A. Thuli, M. Hungate, G. Ruan, Z. Jin, J.M. Tour, GrapheneeNiea-MnO2 and eCuea-MnO2 nanowire blends as highly active non-precious metal catalysts for the oxygen reduction reaction, Chem. Commun. 48 (2012) 7931e7933. k, Carbon-supported manganese oxide nanoparticles as electrocatalysts for the oxygen reduction reaction [21] I. Roche, E. Chaînet, M. Chatenet, J. Vondra (ORR) in alkaline medium: physical characterizations and ORR mechanism, J. Phys. Chem. C 111 (2007) 1434e1443. [22] E. Benbow, S. Kelly, L. Zhao, J. Reutenauer, S. Suib, Oxygen reduction properties of bifunctional a-manganese oxide electrocatalysts in aqueous and organic electrolytes, J. Phys. Chem. C 115 (2011) 22009e22017. [23] Y.-G. Wang, L. Cheng, F. Li, H.-M. Xiong, Y.-Y. Xia, High electrocatalytic performance of Mn3O4/mesoporous carbon composite for oxygen reduction in alkaline solutions, Chem. Mater. 19 (2007) 2095e2101. [24] L. Zhang, Z. Wang, D. Xu, J. Xu, X. Zhang, L. Wang, a-MnO2 hollow clews for rechargeable Li-air batteries with improved cyclability, Chin. Sci. Bull. 57 (2012) 4210e4214. [25] J. Zeng, J.R. Nair, C. Francia, S. Bodoardo, N. Penazzi, Li-O2 cells based on hierarchically structured porous alpha-MnO2 catalyst and an imidazolium based ionic liquid electrolyte, Int. J. Electrochem Sci. 8 (2013) 3912e3927. [26] Y. Qin, J. Lu, P. Du, Z. Chen, Y. Ren, T. Wu, J.T. Miller, J. Wen, D.J. Miller, Z. Zhang, In situ fabrication of porous-carbon-supported a-MnO2 nanorods at room temperature: application for rechargeable LieO2 batteries, Energy & Environ. Sci. 6 (2013) 519e531. [27] C. Johnson, M. Mansuetto, M. Thackeray, Y. Shao-Horn, S. Hackney, Stabilized alpha-MnO2 electrodes for rechargeable 3 V lithium batteries, J. Electrochem. Soc. 144 (1997) 2279e2283. [28] M. Rossouw, D. Liles, M. Thackeray, W. David, S. Hull, Alpha manganese dioxide for lithium batteries: a structural and electrochemical study, Mater. Res. Bull. 27 (1992) 221e230. [29] G. Zhang, J. Zheng, R. Liang, C. Zhang, B. Wang, M. Au, M. Hendrickson, E. Plichta, a-MnO2/carbon nanotube/carbon nanofiber composite catalytic air electrodes for rechargeable lithium-air batteries, J. Electrochem. Soc. 158 (2011) A822eA827. [30] A. Kraytsberg, Y. Ein-Eli, Review on Lieair batteriesdopportunities, limitations and perspective, J. Power Sources 196 (2011) 886e893. [31] D. Xu, Z.-L. Wang, J.-J. . Xu, L.-L. Zhang, X.b. 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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The (2 2) tunnels structured manganese dioxide nanorods with a phase for lithium air batteries Zafar Khan Ghouri a, b, Awan Zahoor c, Nasser A.M. Barakat b, d, *, Mohammad S. Alsoufi e, Tahani M. Bawazeer f, Ahmed F. Mohamed e, g, Hak Yong Kim a, b, ** a
Advanced Materials Institute for BIN Convergence, Department of BIN Convergence Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea c Department of Chemical Engineering, NED University of Engineering & Technology, University Road, Karachi 75270, Pakistan d Department of Chemical Engineering, Faculty of Engineering, El-Minia University, El-Minia, Egypt e Department of Mechanical Engineering, Collage of Engineering and Islamic Architecture, Umm Al-Qura University, Makkah, Saudi Arabia f Department of Chemistry, Faculty of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia g Department of Mechanical Engineering, Collage of Engineering, Sohag University, Sohag, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2015 Accepted 10 December 2015 Available online 13 December 2015
The (2 2) tunnels structured manganese dioxide nanorods with a phase (a-MnO2) are synthesized via simplistic hydrothermal method at low temperature. The obtained tunnels structured aeMnO2 nanorods are characterized by, Transmission electron microscopy, Scanning electron microscopy, and X-ray diffraction techniques. The oxygen reduction reaction (ORR) activity was studied by cyclic voltammetry and rotating ring-disc electrode voltammetry techniques in alkaline media. Moreover; the highly electrocatalytic tunnels structured aeMnO2 nanorods were then also applied as cathode in rechargeable LieO2 cells. The LieO2 cells exhibited initial discharge capacity as high as ~4000 mAh/g with the tunnels structured aeMnO2 nanorods which was double the original capacity of the cells without any catalyst. Also we obtained 100% round trip efficiency upon cycling with limited capacity for more than 50 cycles. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Tunnels structure a-MnO2 Nanorods Cyclic voltammetry Lithium air batteries Air cathode
1. Introduction In order to satisfy the increasing demand of clean and high-efficient energy, alternative resource of energy independent of fossil fuels, must be developed [1e8], among which lithium air batteries are attractive because they are compact, lightweight and more importantly they employ a lighter air cathode operating on environmentally abundant oxygen. The inherent energy density of lithium is more than 11,000Wh kg1 (close to gasoline with 13,000Wh kg1), which makes LieO2 batteries even more attractive for electric vehicle and energy storage.
* Corresponding author. Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea. ** Corresponding author. Advanced Materials Institute for BIN Convergence, Department of BIN Convergence Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea. E-mail addresses: [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). http://dx.doi.org/10.1016/j.spmi.2015.12.012 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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Amongst all the potential benefits of lithium air batteries, their large scale production and commercialization is still hindered by various limitations. One of the important factors that limit the LieO2 batteries is the slow kinetics of oxygen reduction reaction (ORR) on cathode side of the battery. Recently ORR has attracted particular attention since it is an important process to control the overall cell performance. Electro-catalysts play a key role in ORR mechanism and until now platinum is known to be the best electro-catalyst for the ORR in alkaline media [9]. Also, the mostly investigated ORR catalysts are based on noble metals such as platinum, palladium, gold, etc., because of their best overall catalytic performance [9,10]. However, due to high cost, and limited availability of these noble metals needs to be replaced by new electro-catalysts which are inexpensive, abundant and comparable electrocatalytic performance to that of noble metal catalysts. Numerous non-precious materials such as carbons, polymers, transitional metals and their oxides have already been suggested and investigated as an alternative materials for the ORR [11e13]. Among them manganese oxide seems to be the most promising and widely used catalyst in alkaline electrolyte because of its abundance, low cost, high catalytic activity and non-toxicity. MnOx particles incorporated transition metal based materials also shows good ORR activity compared to benchmark commercial Pt/C based catalyst [14]. MnOx can be applied as promising catalysts in air electrode for both alkaline fuel cells and metal-air batteries like LieO2 batteries. The role of catalysts in LieO2 batteries is mainly to reduce the voltage gap between the discharges and recharge processes and thereby to increase the efficiency of the cells [15]. Thus the capacity and cyclic performance of the cell are directly related to the nature of electro-catalyst used at the cathode. So far, there have been various MnO2 catalysts explored for ORR and LieO2 cells including MnO2 with different structures [16e18], MnO2 combined with metals and metal oxides [19,20] and MnO2 supported on carbons [21e23], etc. Even though nanostructured manganese oxides are promising electrocatalysts for the ORR and promising catalyst in air cathode, their wide spread implementation hinges upon their development with high activity and stability. In addition, structure-activity relationships for nanostructured MnO2 for the ORR are still needed to be investigated further to elucidate the factors that determine catalytic performance. Herein, we report a simple, facile and templet free approach for preparing the (2 2) tunnels structured manganese dioxide nanorods with a phase. The novel shaped MnO2 nanostructures obtained via simplistic hydrothermal route at low temperature. The synthesized (2 2) tunnels structured manganese dioxide nanorods were investigated for ORR activity and finally used for LieO2 batteries application. Moreover, their structure-activity relations are also explained in this article. 2. Experimental 2.1. Synthesis of (2 2) tunnels structured manganese dioxide nanorods Potassium per manganite (Showa Chemicals Co. Ltd, Japan, 99.3%) and Nitric Acid (Samchun pure Chemicals Co. Ltd, Korea, 60%) were used as starting chemicals without further purification. In a typical synthesis, 50.0 mmol of KMnO4 and 5.1 mL HNO3 were dissolved in 58 mL of twice-distilled water (18.3 MU) with vigorous stirring to form the precursor solution. Subsequently, after stirring for about 30 min, the solution was treated in a stainless steel Teflon-lined autoclave equipped with a microwave heating, at 120 C for 12 h, and then was cooled down to room temperature and the product was filtered, washed with distilled water and dried at 80 C for 12 h in air. The final product, tunnel structured a-MnO2 nanorods were characterized by various techniques. 3. Characterization techniques Uniformity, chemical composition and morphology of the samples were checked with field emission scanning electron microscopy (FESEM, JSM-6700F) and high resolution transmission electron microscopy (HRTEM JEM-2010, JEOL) equipped with EDS system. The phase structures of the as-prepared samples were determined by powder X-ray diffraction spectrum (XRD, Shimadzu XRD-6000, Cu KR, l 1.5406 Å). Electrocatalytic measurements for ORR were carried out on a computerized potentiostat instrument (model CHI700C) at room temperature in a three electrode system using 0.1 M KOH as electrolyte. Linear sweep voltammetry (LSV) were recorded at a scan rate of 5 mV1, with a disk rotation rate of 1600 rpm, and the cyclic voltammetry (CV) was recorded at a scan rate of 50 mV1. 4. Preparation of electrode For ORR studies, the synthesized (2 2) tunnels structured manganese dioxide nanorods were mixed with carbon powder (Cabot Vulcan XC-72) in the weight ratio of 3:7 to ensure sufficient electronic conductivity. 5 mg of as prepared catalyst was dispersed ultrasonically in 75 mL of diluted nafion alcohol solution (5 wt.%), and about 20 mL of the suspension was pipetted onto a glassy carbon substrate. Pt wire and Hg/HgO were used as the counter and the reference electrode respectively. Prior to measurement O2 was bubbled directly into the cell for at least 1 h. For Lithium air battery studies, the air cathodes were prepared by mixing as prepared (2 2) tunnels structured manganese dioxide nanorods. a-MnO2 nanorods catalyst and Ketjen black (EC 600JD) conductive carbon in the ratio of 1:2 with teflonised acetylene black (TAB) binder (60%) in isopropyl alcohol.
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The mixture was prepared into a fine pellet of about 1 cm diameter and the pellet was pressed on a Ni mesh current collector with a diameter of 1.2 cm. Thus prepared electrode is then dried in vacuum over night at 100 C and used as cathode in Li air battery. 5. Lithium air battery studies Lithium air battery applications were studied by Swagelok™ type cell with Li metal anode, LiTFSI (TEGDME) (1:1) electrolyte and KB cathode with and without a-MnO2 nanorod as catalyst. Each cell were assembled in argon filled glove box under room temperature and purged with oxygen before cycle performance. The charge discharge profiles of the cells were tested in BTS 2004 (Japan) system at 1 atm O2 atmosphere. 6. Results and discussion Fig. 1 shows X-ray diffraction patterns of the as synthesize (2 2) tunnels structured manganese dioxide nanorods with a phase. It can be observed that various sharp diffraction peaks located at 2q of 12.7, 18.1, 25.7, 28.8 , 36.6 , 37.5 , 39.0 ,41.2 , 41.9 , 49.8 , 56.3 , 60.2 , 65.2 , 69.7 and 72.7 correspond well to the (110), (200), (220), (310), (400), (211), (330), (420), (301), (411), (600), (521), (002), (541) and (312) planes are characteristic ones for the a-MnO2 phase and characterizing this support as crystalline oxide with tetragonal structure, [space group: 14/m] according to the JCPDS 44-0141 with lattice constants a ¼ b ¼ 9.78 Å, c ¼ 2.86 Å; a ¼ b ¼ g ¼ 90 . The crystal construction of a-MnO2 is assembled from binary chains of octahedral [MnO6] which form (2 2) tunnels with an opening as large as 0.47 nm. This tunnel like construction is ideal as electrode material for batteries [1]. Scanning electron microscopy was employed to observe the nano-morphology of as synthesize (2 2) tunnels structured a-MnO2. FESEM images of the nanostructured a-MnO2 are shown in Fig. 2(a and b). The FESEM images show that all the nanostructured a-MnO2 had rod-like shapes that are closely and haphazardly aligned. The average diameters of the nanorods were 85 nm and length in few hundreds of nanometers. Comprehensive analysis of the particle morphology was made using TEM & HRTEM measurements and the results were presented in Fig. 3(a and b). The TEM image (Fig. 3(a)) illustrates clearly the synthesized (2 2) tunnels structured a-MnO2, having a rod-like shape and are associated together. The HRTEM image (Fig. 3(b)) shows uniform lattice fringes with average inter planar spacing of 0.48 nm corresponding to (200) plane of tetragonal structure (marked by arrows). 7. Electrocatalytic properties In order to examine the effect of the a-MnO2 nanorods on the electro-catalytic ORR performance, linear sweep voltammograms (LSVs) were recorded in O2 saturated 0.1 M KOH at a scan rate of 5 mV/s over the electrode rotation rate of 1600 rpm and are shown in Fig. 4(a). The extracted data is given in Table 1. For comparison ORR activity of simple prepared MnO2 nanomaterials (bulk), glassy carbon and commercial 20 wt.% Pt/C under the same experimental condition are also given in Table 1. Starting at 0.3 V and scanning the cathode potential, a fast increase of currents in the mixed kinetic diffusion control
Fig. 1. XRD spectrum of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
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Fig. 2. (a and b) FESEM images of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
region (0.04 to 0.3 V) is followed by the appearance of diffusion-limiting current. The a-MnO2 nanorods shows a faster current drop exhibiting positively shifted onset potential with a high steady state current. It exhibits a limiting current value close to that of Pt/C. The onset potential of a-MnO2 nanorods is 0.02 V whereas for Pt/C is 0.08 V. The a-MnO2 nanorods exhibits better performance than bulk MnO2 in terms of limiting current, half wave potential and onset potential. a-MnO2 nanorods also show far better electrocatalytic activity than glassy carbon electrode. Fig. 4(b) shows the CV of the a-MnO2 nanorods electrode in 0.1 M KOH solution saturated with O2 at a scan rate of 50 mV/s. For comparison CV of glassy carbon and bulk MnO2 electrodes are also included. Table 1 summarizes the corresponding electrochemical properties. The catalytic activity of a catalyst to the ORR could be qualitatively estimated from the peak current and the reduction peak potentials. There is significant difference between the peak potentials of glassy carbon, bulk MnO2 and a-MnO2 nanorods, and the peak potentials of a-MnO2 nanorods are positively shifted to 0.20 V. The CV curves of a-MnO2 nanorods showed (not included in here) a well-defined cathodic peak even after 10 cycles, showing the stability of the samples. 8. Lithium air battery performance The Swagelok™ cells were assembled to investigate the catalytic effect of a-MnO2 nanorods. The KB air cathode with and without a-MnO2 nanorod catalyst were tested versus (Li/Liþ) lithium metal anode for cycling performance and the discharge charge profiles were observed in the potential range of 2.0e4.3 V. The open circuit voltage of the Li air battery was 3.3 V with a discharge plateau of 2.75 V. The cells with a-MnO2 nanorod catalyst exhibit a very high initial discharge capacity of 3997(~4000) mAh/g at 0.1 mA/cm2 current density with a charge capacity of 3772 mAh/g. This was double the capacity obtained without any catalyst with only KB cathode. The charge discharge profiles of the LieO2 cells are given in Fig. 5(a). On second discharge a good reversible capacity with 93% capacity retention was obtained. The obtained initial discharge capacity is much better than the earlier reports [24e26]. This superior performance of a-MnO2 nanorods catalyst might be due to the unique crystal structure and the porosity of aMnO2. The crystal structure of the a-polymorph consists of 2 2 tunnels formed by edge- and corner-sharing MnO6
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Fig. 3. (a) TEM image (b) HRTEM image of the as-synthesized (2 2) tunnels structured a-MnO2 nanorods.
Fig. 4. (a) Linear sweep voltammetry of samples obtained at a rotation rate of 1600 rpm and a scan rate of 5 mV/s in 0.1 M KOH and (b) Cyclic voltammetry of samples obtained in 0.1 M KOH solution saturated with O2 at a scan rate of 50 mV/s.
octahedral. Johnson [27] and Rossouw [28] have explained that Li2O2 can be incorporated within the MnO2 tunnels with the O2 ions located at the tunnel centers and the Liþ ions coordinated between these central O2 ions, the latter forming the walls of the tunnels.
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Table 1 Onset potentials, limiting current, half wave potentials, and peak potential and peak current of different samples (values derived from Fig. 4(a and b). Samples
Limiting current (mA)
Onset potential (V)
Halfwave potential (V)
Peak current (mA)
Peak potential (V)
Glassy carbon MnO2 Bulk a-MnO2 nanorod Commercial Pt/C
0.24 0.66 0.76 0.78
0.25 0.037 0.026 0.085
0.33 0.13 0.16 0.03
0.05 0.15 0.27
0.35 0.25 0.20
Fig. 5. (a) Charge-discharge profile of air cathode with and without aeMnO2 nanorods catalyst and (b) Charge and discharge cycling profile of air cathode with aeMnO2 nanorods catalyst.
The ability of a-MnO2 to incorporate Li2O2 suggests the possibility of incorporating the Liþ and O2 2 ions on the surface of the material promoting the reversibility of the Li2O2 formation. Recently, Zhang et al. [29] also reported a significant increase in discharge capacity and cycling of LieO2 cells with a-MnO2 catalysts due to the reaction between the discharge product Li2O2 and a-MnO2 on charging. It has been suggested that in the presence of the catalyst, the Li2O2 deposits may be less dense and more porous than the ones formed in the absence of the catalyst hence preserving the ability of the reagents to diffuse into the carbon voids through the orifices occupied with such porous deposits. This may in turn increase the discharge capacity [30]. The difference between the charge and discharge voltages DV was 1.2 V, demonstrating that the battery had become more reversible. Whereas, the voltage gap of KB cathode without any catalyst is 0.6 V higher than with a-MnO2 nanorods catalyst as seen from Fig. 5(a). In other words addition of a-MnO2 nanorods catalysts have decreased the over potential of the cell. The addition of a-MnO2 really increases the overall potential enhancement in the cell cycling process which in turn enhances the specific power density of the cell as well. It is obvious that the addition of nanorods catalyst is highly effective for increasing the discharge capacity and reducing the over potential problem in Li air battery. Considerable cycling performance can be improved by limiting the depth of discharge and charging [31]. Fig. 5(b) presents the cycle efficiency with respect to cycle number for a cell cycled at a rate of 0.1 mA/cm2 and at a fixed capacity of 0.5 mAh. A uniform discharge/charge profile is observed up to 50 cycles. There is a small increase in over potential in every cycle and the increase of over potential with the limited capacity may be attributed to strong bonding between LiO2 and initial bare surface of a-MnO2 nanorods or its deep embedment into open structured surface frame with (2 2) tunnels [10,22]. Hence the round-trip efficiency of rechargeable Lieair batteries is limited by the increasing over potential upon charging. This can also be attributed to slow oxidation kinetics of Li2O2 formed upon discharge. But even after 50th cycle, the cycling profile maintains steady round trip efficiency without any capacity fade. 9. Conclusions A catalyst is the key component in electrode material. A systematic study has been carried out on the catalytic properties of MnO2 based nanostructures as electro-catalyst for oxygen reduction reaction in alkaline media. Thus prepared (2 2) tunnels structured a-MnO2 nanorods show superior electrocatalytic performance with positive peak potential. The catalysts were applied in LieO2 battery applications and excellent cycling is obtained. The specific capacity is almost doubled and the charge potential is lowered with a-MnO2 nanorods catalyst. Also the reversibility of LieO2 battery could be retained up to 50 cycles.
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Acknowledgments This Research was financially supported by National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No.2014R1A4A1008140) and this work was also supported by the IT R&D Program of MKE/KEIT ( Design and Development of fiber-]based flexible display) No.10041957. References [1] Z.K. Ghouri, M. Shaheer Akhtar, A. Zahoor, N.A.M. Barakat, W. Han, M. Park, B. Pant, P.S. Saud, C.H. Lee, H.Y. Kim, High-efficiency super capacitors based on hetero-structured a-MnO2 nanorods, J. Alloys Compd. 642 (2015) 210e215. [2] Z.K. Ghouri, N.A.M. Barakat, M. Park, B.-S. Kim, H.Y. Kim, Synthesis and characterization of Co/SrCO3 nanorods-decorated carbon nanofibers as novel electrocatalyst for methanol oxidation in alkaline medium, Ceram. Int. 41 (2015) 6575e6582. [3] Z.K. Ghouri, N.A.M. Barakat, M. Obaid, J.H. Lee, H.Y. Kim, Co/CeO2-decorated carbon nanofibers as effective non-precious electro-catalyst for fuel cells application in alkaline medium, Ceram. 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