Transition Metal Complexes with Macrocyclic Ligands Serve as

All the M-TPP complexes exhibited a redox pair of peaks at around 3.0–4.5 V, which were assigned to the M(III)/M(II) redox process.(18, 19) It was con...
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Transition Metal Complexes with Macrocyclic Ligands Serve as Efficient Electrocatalysts for Aprotic Oxygen Evolution on Li2O2 Shoichi Matsuda,† Shigeki Mori,‡ Kazuhito Hashimoto,*,†,§ and Shuji Nakanishi*,† †

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Molecular Science, Integrated Center for Sciences, Ehime University, Matsuyama, Ehime 790-8577, Japan § Research Centre of Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan S Supporting Information *

ABSTRACT: Since the oxygen evolution reaction (OER) in aprotic Li ion electrolytes is a crucial reaction in the charging process of nonaqueous aprotic Li− air batteries, there is a strong demand for decreasing the overpotential by developing more efficient OER catalysts. Herein, we investigated the effect of addition of transition metal complexes with macrocyclic ligands, such as porphyrins and phthalocyanines, for OER in aprotic Li ion electrolytes. Electrochemical experiments using a three-electrode system revealed that such complexes functioned as efficient OER catalysts, in which the center metal in the complex played an essential role in the catalytic process. Among the metal complexes studied, cobalt tert-butylphthalocyanine was found to be the best catalyst: the charging potential was lowered from 4.1 V to about 3.4 V at 1 μA/cm2 by addition of 1 mM catalyst

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INTRODUCTION Lithium (Li)−air batteries have the potential to achieve a more than 3-fold greater energy density than Li ion batteries.1−5 A typical nonaqueous rechargeable Li−air battery is composed of Li metal as the negative electrode, a Li ion conducting nonaqueous electrolyte, and porous carbon as the positive electrode. During the discharge process, atmospheric oxygen is reduced in pores of the positive electrode and then combines with the Li ions to form solid lithium peroxide (Li2O2). The reverse reaction occurs during the charging process. Thus, in an ideal Li−air battery, the following reaction should proceed efficiently at the surface of the positive electrode: 2Li 2O2 ⇄ O2 + 2Li+ + 2e−

overcome by developing diffusive catalysts that can be repeatedly adsorbed on the growing Li2O2 front. Recent studies have demonstrated that redox-active small molecules, such as tetrathiafulvalene, iodine ion, and iron phthalocyanine, could serve as the diffusive catalyst for positive electrode reactions.10−12 Among the redox-active molecules that potentially catalyze aprotic OER, transition metal complexes with macrocyclic ligands are especially attractive in that (1) such complexes have higher solubility in many kinds of organic electrolytes and (2) the redox properties can be flexibly tuned by modifying the molecular structure and/or replacing the center metal ions. Herein, we investigated the catalytic properties of a series of metal macrocyclic complexes in order to further decrease the overpotential of aprotic OER.

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One of the major challenges for realization of rechargeable nonaqueous Li−air batteries is to increase the round-trip energy efficiency by decreasing large overpotentials for both the forward and backward reactions at the positive electrode.1−5 Since the overpotential in oxygen evolution reaction (OER) is much larger (>1 V) than that in oxygen reduction reaction (ORR, ∼0.3 V), there is a strong demand for developing efficient OER catalysts to improve the energy efficiency. To date, a number of OER catalysts have been reported for aprotic Li−air batteries, such as noble metals, metal oxides, and metal nitrides.6−9 They were used in the form of heterogeneous catalysts grafted on carbon substrates. However, such supported catalysts are gradually buried in the formed Li2O2, resulting in the disappearance of catalytic activity. In fact, the loss of catalytic effect at deep discharge conditions has been reported in heterogeneous catalyst systems.6,7 This problem could be © XXXX American Chemical Society

EXPERIMENTAL METHODS Preparation of Catalysts. Tetra-tert-butylphthalocyanine cobalt (Co-tert-butyl-Pc) and tetrabutoxyphthalocyanine cobalt (Co-butoxy-Pc) were synthesized by a modified literature method.13 A typical synthetic procedure is as follows. A mixture of phthalonitrile (4-tert-butylphthalonitrile or 4-butoxyphthalonitrile) (10 mmol), cobalt salt (2.5 mmol) and 1,8diazabicyclo[5.4.0]undec-7-ene (10 mmol) was heated to reflux in 1-hexanol. The obtained precipitate was washed with 3% HCl and water, chromatographed on silica with chloroform, Received: September 2, 2014 Revised: November 15, 2014

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and purified by recrystallization with toluene. The other transition metal complexes were obtained from Aldrich. Electrochemical Measurements. A single-chamber (6 mL volume), three-electrode system was used to investigate the OER properties of transition metal complexes. Li metal (Honjo Metal Co., Ltd.) was used as the counter and reference electrodes. We used an electrochemical-grade glassy carbon plate (GC, Tokai Carbon Co., Ltd.; 10 mm × 10 mm × 0.5 mm) as the working electrode. The use of a GC plate is advantageous in the present study, aiming to clarify the mechanism of the catalytic effect of transition metal complexes, because (1) the electrode surface area can be precisely defined and (2) linear diffusion of transition metal complexes and Li ion to the electrode can be expected.14 The GC electrode was polished with alumina paste (1.0 μm, 0.05 μm, MicroPolish, Buehler), sonicated in acetone, and dried at 90 °C under vacuum prior to use. Bis(trifluoromethane)sulfonimide lithium salt (Li TFSA, Sigma−Aldrich, 0.1 M ) dissolved in diethylene glycol dimethyl ether (DME, Wako) was used as the electrolyte. Li TFSA was dried for 12 h at 120 °C under vacuum, and 3 Å molecular sieves (Wako) were immersed in the DME to remove residual water. The electrolyte was purged with O2 or Ar for 10 min before the measurements. The electrochemical cell was assembled and disassembled in an argon glovebox. A potentiostat (VMP3, Bio-Logic Science Instruments) was used for all electrochemical measurements in this study. X-ray Photoelectron Spectroscopic and Scanning Electron Mixroscopic Analyses. X-ray photoelectron spectroscopy (XPS; Axis Ultra, Kratos Analytical Co.) with monochromal Al Kα X-rays at hν = 1486.6 eV was used for characterization of the deposited products. A field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi) was used to observe the morphology of the deposited products. GC electrodes for the analysis were rinsed with DME more than 2 times and dried under vacuum to remove the residual solvent. The samples were transferred into each chamber with minimal exposure to the ambient atmosphere.

Figure 1. Catalytic effect of transition metal complexes with macrocyclic ligands on aprotic OER. (a) Anodic chronopotentiograms obtained in the presence and absence of 1 mM M-TPP by use of the Li2O2-formed GC electrode in Ar atmosphere. (b) Cyclic voltammograms obtained in the presence of 1 mM M-TPP by use of pristine GC electrode in Ar atmosphere. Scan rate was 10 mV/s.

the OER catalyzed by M-TPPs. The film of electrochemically preformed Li2O2 had 1 μm-sized toroidal-like structures about 1 μm in diameter, as shown in Figure 2a. This is the typical structure of the electrochemically formed Li2O2.16 Note that Li2O2 was deposited even at positions where the toroidal-like structures were not formed (for example, point A in Figure 2a). In contrast, such toroidal aggregates vanished completely from the electrode surface after anodic chronopotentiometry in both the presence and absence of Co-TPP (Figure 2b,c). The elemental compositions of the electrode surface were also analyzed by XPS. Electrochemically formed Li2O2 exhibited peaks at 54 eV (in the Li 1s region) and 531 eV (in the O 1s region) that were essentially identical to the reported peaks17 (Figure 3). We confirmed that these XPS peaks were greatly diminished after anodic chronopotentiometry in both the presence and absence of Co-TPP. These results indicated that decomposition of Li2O2 proceeded even during the OER catalyzed by the Co-TPP. It was also confirmed that Li2O2 was formed after cathodic chronopotentiometry in the presence of M-TPP, indicating that the presence of M-TPPs does not interfere with Li2O2 formation associating with oxygen reduction reaction (Figures S2 and S3, Supporting Information). Next, we investigated the relationship between OER activity and redox potential of the center metals in M-TPP. Cyclic voltammograms (CV) obtained in Ar-purged Li ion electrolytes containing 1 mM M-TPPs against a pristine GC electrode are shown in Figure 1b. All the M-TPP complexes exhibited a

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RESULTS AND DISCUSSION First, to determine the relationship between the central metal species of the complex and its OER activity, we investigated a series of metal tetraphenylporphyrins (M-TPP, M = Co, Zn, Mn, Cu or Fe). In our experiments, Li2O2 films were galvanostatically preformed on the working electrode in O2saturated Li ion electrolyte without M-TPP at a current density of 1 μA/cm2 for 3 h (Figure S1, Supporting Information). Next, the electrolyte was replaced with Ar-purged Li ion electrolyte containing 1 mM M-TPP, and the system was subjected to anodic chronopotentiometry. Representative results of anodic chronopotentiometry in the presence of various M-TPP complexes are shown in Figure 1a. In the absence of M-TPP, an electrode potential reached at about 4.1 V when 3 μA·h (corresponding to 10 mC) of electrical charge flowed. This potential profile was consistent with previous studies.15,16 In contrast, in the presence of 1 mM M-TPPs, the overpotential of OER decreased and an electrode potential of ca. 3.6−4.0 V was exhibited. Among the M-TPPs we investigated, Co-TPP showed the best catalytic effect. Considering that Li2O2 is electrochemically decomposed during the charging process of Li−air batteries, we analyzed the surface of the positive electrode after anodic chronopotentiometry to determine whether Li2O2 was decomposed during B

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Figure 2. (a) SEM image of GC electrode surface on which Li2O2 film is predeposited. (b, c) Images of GC electrode surfaces taken after anodic chronopotentiometry with Li2O2 predeposited electrode in the presence (b) and absence (c) of 1 mM Co-TPP. Scale bar 1 μm.

Figure 3. XPS analyses of electrodeposits: XPS spectra of GC electrode obtained after anodic chronopotentiometry (CP) with (red line) and without (black line) 1 mM Co-TPP in (a) Li 1s region and (b) O 1s region.

redox pair of peaks at around 3.0−4.5 V, which were assigned to the M(III)/M(II) redox process.18,19 It was confirmed that the essentially same CV was obtained in O2 atmosphere and also for the GC electrode covered with deposited Li2O2 (Figure S4, Supporting Information). It can be seen that the redox potential of the M(III)/M(II) increased in the following order: Co < Zn < Mn < Cu < Fe. Notably, this order is consistent with the order of thepositive electrode potential at 1 μA/cm2 (i.e., OER activity; Figure 1a). Thus, the results suggested that the center metal ion in the M-TPP complexes plays an essential role in the catalytic process of OER. As the stability of the catalyst is an important issue to be addressed, we evaluated it by conducting 100 cycles of CV. The reversible redox peaks were observed stably even after 100 cycles of CV, indicating the high stability of these metal complexes in the potential region (Figure S5, Supporting Information). On the basis of the above results, we predicted that metal macrocyclic complexes with electron-donating groups would exhibit better catalytic activity, since introduction of an electron-donating group into a metal macrocyclic complex generally increases the electron density of the metal center, resulting in a negative shift of the redox potential of the center metal ion. Because Co-TPP showed the best OER activity among the M-TPPs (Figure 1a), hereafter we focused on Cobased complexes. We investigated the effect of tetramethoxyporphyrin cobalt (Co-MeTPP), tetra-tert-butylphthalocyanine cobalt (Co-tert-butyl-Pc), and tetrabutoxyphthalocyanine cobalt (Co-butoxy-Pc), all of which possess electron-donating groups in the ligands. Co-tert-butyl-Pc and Co-butoxy-Pc were synthesized by a literature protocol.13 The redox potential of Co(III)/Co(II) shifted in the negative direction upon introduction of electron-donating groups (Figure 4b), which was in good agreement with the reported shift of redox potentials in aqueous electrolytes.18,19 The OER activities of these cobalt complexes were evaluated by anodic chronopo-

Figure 4. Catalytic effect of cobalt complexes with macrocyclic ligands for aprotic OER. (a) Anodic chronopotentiograms obtained in the presence and absence of 1 mM cobalt complexes by use of the Li2O2formed GC electrode in Ar atmosphere. (b) Cyclic voltammograms obtained in the presence of 1 mM cobalt complexes by use of pristine GC electrode in Ar atmosphere. Scan rate was 10 mV/s.

tentiometry with a GC electrode on which Li2O2 film was electrochemically preformed. The results revealed that all the C

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* E-mail [email protected].

cobalt macrocyclic complexes exhibited higher OER activity than Co-TPP (Figure 4a). Note that the decomposition of Li2O2 during OER was also confirmed in the presence of these cobalt complexes (Figure S6, Supporting Information). Among the cobalt macrocyclic complexes, cobalt tert-butylphthalocyanine exhibited the best catalytic performance with charging at around 3.4 V. In Figure 5, onset potentials of OER were plotted against redox potential of M(III)/M(II) in the series of metal

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Research Fellow of Japan Society for the Promotion of Science.

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(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (2) Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Horn, Y. S. Lithium−Oxygen Batteries: Bridging Mechanistic Understanding and Battery Performance. Energy Environ. Sci. 2013, 6, 750−768. (3) Li, F.; Zhang, T.; Zhou, H. Challenges of Non-Aqueous Li−O2 Batteries: Electrolytes, Catalysts, and Anodes. Energy Environ. Sci. 2013, 6, 1125−1141. (4) Abraham, K. M.; Jiang, Z. A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery. J. Electrochem. Soc. 1996, 143, 1−5. (5) Jung, H. G.; Hassoun, J.; Park, J. B.; Sun, Y. K.; Scrosati, B. An Improved High-Performance Lithium-Air Battery. Nat. Chem. 2012, 4, 579−585. (6) Shui, J. L.; Karan, N. K.; Balasubramanian, M.; Li, S. Y.; Liu, D. J. Fe/N/C Composite in Li-O2 Battery: Studies of Catalytic Structure and Activity toward Oxygen Evolution Reaction. J. Am. Chem. Soc. 2012, 134, 16654−16661. (7) Lu, J.; Lei, Y.; Lau, K. C.; Luo, X.; Du, P.; Wen, J.; Assary, R. S.; Das, U.; Miller, D. J.; Elam, J. W.; et al. A Nanostructured Cathode Architecture for Low Charge Overpotential in Lithium-Oxygen Batteries. Nat. Commun. 2013, 4, 2383−2391. (8) Kim, B. G.; Kim, H. J.; Back, S.; Nam, K. W.; Jung, Y.; Han, Y. K.; Choi, J. W. Improved Reversibility in Lithium-Oxygen Battery: Understanding Elementary Reactions and Surface Charge Engineering of Metal Alloy Catalyst. Sci. Rep. 2014, 4, 4225−4233. (9) Sun, B.; Munroe, P.; Wang, G. Ruthenium Nanocrystals as Cathode Catalysts for Lithium-Oxygen Batteries with a Superior Performance. Sci. Rep. 2013, 3, 2247−2253. (10) Chen, Y.; Freunberger, S. A.; Peng, Z.; Fontaine, O.; Bruce, P. G. Charging a Li-O2 Battery Using a Redox Mediator. Nat. Chem. 2013, 5, 489−494. (11) Lim, H. D.; Song, H.; Kim, J.; Gwon, H.; Bae, Y.; Park, K. Y.; Hong, J.; Kim, H.; Kim, T.; Kim, Y. H.; et al. Superior Rechargeability and Efficiency of Lithium-Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst. Angew. Chem., Int. Ed. 2014, 53, 3926−3931. (12) Sun, D.; Shen, Y.; Zhang, W.; Yu, L.; Yi, Z.; Yin, W.; Wang, D.; Huang, Y.; Wang, J.; Wang, D.; et al. A Solution-Phase Bifunctional Catalyst for Lithium-Oxygen Batteries. J. Am. Chem. Soc. 2014, 136, 8941−8946. (13) Tomoda, H.; Saito, S.; Shiraishi, S. Synthesis of Metallophthalocyanines from Phthalonitrile with Strong Organic Bases. Chem. Lett. 1983, 3, 313−316. (14) Matsuda, S.; Hashimoto, K.; Nakanishi, S. Efficient Li2O2 Formation via Aprotic Oxygen Reduction Reaction Mediated by Quinone Derivatives. J. Phys. Chem. C 2014, 118, 18397−18400. (15) McCloskey, B. D.; Scheffler, R.; Speidel, A.; Bethune, D. S.; Shelby, R. M.; Luntz, A. C. On the Efficacy of Electrocatalysis in Nonaqueous Li-O2 Batteries. J. Am. Chem. Soc. 2011, 133, 18038− 18041. (16) Lu, Y.-C.; Shao-Horn, Y. Probing the Reaction Kinetics of the Charge Reactions of Nonaqueous Li−O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 93−99. (17) Nasubulin, E.; Xu, W.; Engelhard, M. H.; Nie, Z.; Burton, S. D.; Cosimbescu, L.; Gross, M. E.; Zhang, J. G. Effects of Electrolyte Salts

Figure 5. Onset potentials of OER plotted against redox potential of M(III)/M(II) in transition metal complexes with macrocyclic ligands.

macrocyclic complexes investigated in this study. A linear correlation was observed between these two factors with a correlation coefficient of R2 = 0.95, which clearly indicated the direct relationship between OER activity and redox potential of the center metal ion.

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CONCLUSIONS We demonstrated that OER in aprotic Li ion electrolyte associated with Li2O2 decomposition could proceed efficiently by adding transition metal complexes with macrocyclic ligands to the system. It was shown that the redox potential of the metal complexes determined the onset potential of OER, indicating that the complex served as the electron mediator. Namely, the complex is electrochemically oxidized at the surface of GC and/or Li2O2, and then the electrochemically oxidized complex reacts with the Li2O2, resulting in oxygen evolution associated with Li2O2 decomposition and reduction of the complex. On the basis of these results, it can be expected that further decreases in the onset potential of the OER can be achieved by sophisticated choice of metal macrocycles. Considering that decreasing the large hysteresis in charge− discharge cycles originating from the large overpotential for the OER is one of the great challenges in battery development, we anticipate that our results will contribute to the development of an aprotic Li−air battery with high round-trip efficiency.

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Six figures showing cathodic chronopotentiometry, XPS spectra of electrodeposits, and electrochemical analysis of transition metal complex. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

* E-mail [email protected]. D

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