Efficient Li2O2 Formation via Aprotic Oxygen Reduction Reaction

Article pubs.acs.org/JPCC

Efficient Li2O2 Formation via Aprotic Oxygen Reduction Reaction Mediated by Quinone Derivatives Shoichi Matsuda,† 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 ‡ 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 reduction reaction (ORR) in aprotic Li ion electrolytes accompanied by Li2O2 formation is a crucial reaction for the discharge of nonaqueous aprotic Li−air batteries, there is a strong demand for a reduction in the overpotential of the reaction in order to improve the discharge performance. In the present work, we investigated the effect of the addition of quinone derivatives for ORR on carbon materials in aprotic Li+− electrolytes. Detailed electrochemical analysis revealed that the semiquinone species catalyze the aprotic ORR, resulting in the efficient Li2O2 formation. Among the quinone derivatives, benzoquinone exhibited the best catalytic performance, with an overpotential for the Li2O2 formation of less than 100 mV.

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INTRODUCTION A nonaqueous aprotic Li−air battery is one of the most promising devices, because it can potentially exhibit much higher energy density than today’s Li ion batteries.1−6 The Li− air battery is, in its most common configuration, composed of a Li metal anode, a Li ion conducting organic electrolyte (Li+− electrolyte), and a carbon-supported air cathode. For the discharge process, the oxidative dissolution of Li metal occurs at the anode. Li = Li+ + e−

is large. Therefore, development of electrocatalysts that accelerate the above reactions is strongly required for improved energy efficiency in Li−air batteries. However, catalysts supported on electrode substrates can function only in the initial stage, as the electrode surface is gradually covered by Li2O2 eq 4 and the supported catalysts are buried in the formed Li2O2 films.10,11 Therefore, diffusive catalysts that can be repeatedly adsorbed on the growing front of Li2O2 need to be developed. Quinone derivatives are potential candidates as diffusive catalysts for the following reasons: the quinone molecules (i) are known to efficiently mediate the reduction of molecular oxygen to superoxide anion radical in aprotic media12,13 and (ii) possess high solubility in organic electrolytes. Considering these advantageous properties, in the present work, we investigated the catalytic effect of quinone derivatives on the cathodic ORR in aprotic Li+−electrolyte associated with Li2O2 formation using a three-electrode system.

(1)

The electrons are transferred to the cathode through an external circuit, and then used for the reduction reaction of atmospheric oxygen, producing superoxide anions (O2). The Li ions (Li+) generated at the anode are transported to the cathode through the electrolyte, then react with O2 to form insoluble Li2O2 (Figure 1a). The total cathode reaction can be written as follows:7−9 O2 + e− = O−2 *

(2)

Li+ + O−2 * = LiO2

(3)

2LiO2 + Li+ + e− = 2Li 2O2

(4-1)

2LiO2 = Li 2O2 + O2

(4-2)

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EXPERIMENTAL METHODS Electrochemical Measurements. A single-chamber (6 mL in volume), three-electrode system was used to investigate the ORR properties of quinone molecules. 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.) with the size of a 10 × 10 × 0.5 mm3 as the working electrode. The use of GC plate is advantageous in the

or where * refers to a surface adsorbed species. For the charge process, the reverse reactions occur. Although porous carbon materials are generally used as the cathode material, the activation energy for the ORR on carbon © 2014 American Chemical Society

Received: May 18, 2014 Revised: July 19, 2014 Published: August 4, 2014 18397

dx.doi.org/10.1021/jp504894e | J. Phys. Chem. C 2014, 118, 18397−18400

The Journal of Physical Chemistry C

Article

Figure 1. Schematic of Li2O2 formation via aprotic ORR. The reaction process in the (a) presence of and (b) absence of quinone derivatives.

present study aiming to clarify the mechanism of the catalytic effect of quinone derivatives, as (1) the electrode surface area can be precisely defined and (2) liner diffusion of quinone molecules to the electrode can be expected. 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. All the quinone derivatives were purchased from Sigma-Aldrich and used as received. One M bis(trifluoromethane)sulfonimide lithium salt (Li TFSA) (Sigma-Aldrich) dissolved in diethylene glycol dimethyl ether (DME) (Wako), 1 M trifluoromethanesulfonate lithium salt (Li Trifl) (Sigma-Aldrich) dissolved in tetraethylene glycol dimethyl ether (TEGDME) (Wako) and 0.1 M lithium perchlorate (LiClO4) (Sigma-Aldrich) dissolved in DME were used as the electrolyte. Li TFSA, Li Trifl, and LiClO4 were dried for 12 h at 120 °C under vacuum and 3A molecular sieves (Wako) were immersed into DME or TEGDME to remove residual water. The electrolyte was purged using 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 the electrochemical measurements in this study. XPS and SEM Analyses. X-ray photoelectron spectroscopy (XPS; Axis Ultra, Kratos Analytical Co.) with monochromated Al Kα X-rays at hν = 1486.6 eV was used for characterization of the deposited products. Field-emission SEM (FE-SEM, S-4800, Hitachi) was used to observe the morphology of the deposited products. The 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 minimum exposure to the ambient atmosphere.

Figure 2. Catalytic effect of naphthoquinone for aprotic ORR. (a) Cyclic voltammogram (CV) obtained in with the presence of 1 mM naphthoquinone under O2-bubbled (red curve) or Ar-bubbled (black curve) condition, respectively. (b) Cathodic chronopotentiograms obtained in the presence (red curve) and absence (black curve) of 1 mM naphthoquinone.

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(Figure 2a, red curve). The onset potential of the cathode current is similar to the redox potential for the quinone/ semiquinone one electron transfer redox reaction (Qox/Qsem), indicating that the Qsem (and Qred) species catalyze the ORR. Considering that Li2O2 is formed on the cathode during the discharge process of Li−air batteries (eqs 4-1 or 4-2), we analyzed the cathodic deposits to reveal whether essentially the same reaction scheme is maintained even in the presence of quinones. In this experiment, the system was controlled under galvanostatic conditions (current density, 1 μA/cm2). In the presence of NQ, an electrode potential of ca. 2.77 V was exhibited (Figure 2b, red curve). For the control experiment without NQ, the electrode potential of 2.66−2.70 V was exhibited (Figure 2b, black curve) and almost no current could be observed at 2.77 V (SI Figure S1). Scanning electron microscopy (SEM) inspections after the galvanostatic experiments revealed that the cathode surfaces were fully covered by deposits, regardless of the presence of NQ (data not shown). Next, the elemental compositions of the deposits were analyzed by X-ray photoelectron spectra (XPS). The spectra for the

RESULTS AND DISCUSSION A cyclic voltammogram (CV) obtained in Ar-purged Li+− electrolyte containing 1 mM 1,4-naphthoquinone (NQ) is shown in Figure 2a (black curve), in which two redox peaks located at around 2.6 and 2.3 V (vs Li/Li+) are observed. The two redox peaks can be assigned to the lithium-coupled electron-transfer reactions of the quinone:14,15 Q ox + Li+ + e− = Q sem Li

(5)

Q sem Li + Li+ + e− = Q red Li 2

(6)

where Qox, Qsem, and Qred denote the oxidized form, semiquinone form, and reduced form of NQ, respectively. The first reduction reaction of quinone (eq 5) yields lithiated semiquinone (QsemLi), an intermediate species. The subsequent reduction reaction (eq 6) generates a fully reduced form of lithiated quinone (QredLi2). When the electrolyte was saturated with a dissolved O2, a cathodic current started at around 2.8 V 18398

dx.doi.org/10.1021/jp504894e | J. Phys. Chem. C 2014, 118, 18397−18400

The Journal of Physical Chemistry C

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sample prepared in the presence of NQ possessed clear peaks at around 54 eV in the Li 1s regions (red curve in Figure 3a) and

Figure 3. XPS analyses of the electrodeposits. XPS spectrum of the GC electrode obtained after cathodic chronopotentiometry with (red line) and without (black line) 1 mM naphthoquinone in (a) the Li 1s region and (b) the O 1s region.

at around 531 eV in the O 1s regions (red curve in Figure 3b). The combination of these two peaks, i.e., Li 1s@54 eV and O 1s@531 eV, accords with those for Li2O2 obtained in the absence of NQ (black curves in Figure 3a,b) and also with the reported data.16 These results clearly indicate that the addition of NQ leads to a decrease in the overpotential of the cathode reactions without changing the reaction scheme. It was confirmed that the deposited Li2O2 decomposed accompanied by oxygen evolution reaction in the subsequent anodic galvanostatic experiment, indicating NQ does not interfere the charging reactions. It should be noted here that the essentially same catalytic effect of NQ was observed even when the other electrolyte, such as 1 M Li Trifl/TEGDME and 0.1 M LiClO4/DME, was used. It is known that the redox potential of quinone molecules changes depending on the molecular structure. As the onset potential of the aprotic ORR was correlated with the redox potential of NQ (Figure 2a), it is worth investigating the effect of quinone derivatives with various redox potentials, such as anthraquinone (AQ), 2-methyl-1,4-naphthoquinone (MethylNQ), 2-methoxy-1,4-naphthoquinone (Metoxy-NQ), benzoquinone (BQ), tetramethyl-1,4-benzoquinone (Methyl-BQ), and methoxy-benzoquinone (Metoxy BQ), on the aprotic ORR. The results revealed that these quinone derivatives exhibited a catalytic effect on the ORR (Figure 4a) accompanied by Li2O2 formation (SI Figure S2). The onset potentials of the ORR were plotted against the midpoint potential of (Qox/Qsem) of the quinone derivatives investigated (Figure 4b). A linear correlation can be seen between these two factors with a correlation coefficient of R2 = 0.95. This result clearly indicates that the redox potential of Qox/Qsem of the quinone derivatives directly determines the onset potential of the ORR. For the Li2O2 formation, the redox potential of the Qox/Qsem should be more negative than the theoretical thermodynamic potential of the Li2O2 formation (U0 = 2.96 V).17 In other words, a quinone whose Qox/Qsem redox potential is more positive than the U0 cannot lead to Li2O2 formation even if the ORR proceeds. In good agreement with this argument, Li2O2 films were not formed when tetrachlorobenzoquinone with a redox potential of 3.1 V was added to the system (SI Figure S3) despite that the catalytic effect on the ORR was observed (SI Figure S4). Among the quinone derivatives, benzoquinone exhibited the best catalytic performance in the 1 M Li TFSA/DME electrolyte, with an

Figure 4. Catalytic effect of quinone derivatives for aprotic ORR. (a) Cathodic chronopotentiograms obtained in the presence of 1 mM quinone derivatives. (b) Onset potentials of the ORR plotted against the midpoint potential of (Qox/Qsem) of the quinone derivatives.

overpotential for the Li2O2 formation of less than 100 mV. Although the actual potential for the Li2O2 formation depends on the electrolyte used, the working principle of the quinonebased catalysts on efficient Li2O2 formations is considered to be true even for the other electrolytes. Thus, we demonstrated that ORR in an aprotic Li+− electrolyte associated with the Li2O2 formation could proceed efficiently by adding quinone derivatives to the system. This phenomenon can be explained by considering that Qsem species generally catalyze reduction of molecular oxygen to superoxide anion radicals in oxygenated aprotic electrolytes (Figure 1b). Q sem + O2 = Q ox + O−2

(7)

+

In the presence of Li , the QsemLi is formed by a lithiumcoupled one electron reduction reaction on the electrode (eq 5), and then molecular oxygen (O2) is converted to superoxide anion radicals (O2−) mediated by the QsemLi (eq 8). Q sem Li + O2 = Q ox + O−2 * + Li+

(8)

The superoxide anion radicals on the cathode surface (O2* ) formed by eq 8 further react with Li+, resulting in Li2O2 deposition (Figure 1b).

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CONCLUSIONS The semiquinone species plays a crucial role in the efficient Li2O2 formation via the aprotic ORR. Decreasing the large hysteresis in charge−discharge cycles, originating from the large overpotential for the ORR and OER is one of the great challenges in improving the Li−air battery performance. We anticipate that our results will contribute to the realization of nonaqueous aprotic Li−air batteries with a high round trip efficiency. 18399

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ASSOCIATED CONTENT

S Supporting Information *

Cathodic chronoamperogram obtained in the absence of quinone derivatives (Figure S1); XPS spectrum for the electrodeposits (Figure S2); XPS spectrum for the cathode surfaces after the potentiometric experiments (Figure S3); and catalytic effect of tetrachlorobenzoquinone for aprotic ORR (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*Phone: (+81) 3-5841-8389; e-mail: [email protected]. *Phone: (+81) 3-5841-8389; e-mail: [email protected]. ac.jp. 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. The authors thank Prof. Kiyoshi Kanamura of Tokyo Metropolitan University and Prof. Kohei Uosaki of National Institute for Materials Science for the valuable discussion.

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dx.doi.org/10.1021/jp504894e | J. Phys. Chem. C 2014, 118, 18397−18400