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Oxygen Reduction Reactions in Ionic Liquids and the Formulation of a General ORR Mechanism for Li−Air Batteries † † † † ‡ Chris J

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Oxygen Reduction Reactions in Ionic Liquids and the Formulation of a General ORR Mechanism for Li−Air Batteries † † † † ‡ Chris J Article pubs.acs.org/JPCC Oxygen Reduction Reactions in Ionic Liquids and the Formulation of a General ORR Mechanism for Li−Air Batteries † † † † ‡ Chris J. Allen, Jaehee Hwang, Roger Kautz, Sanjeev Mukerjee, Edward J. Plichta, ‡ † Mary A. Hendrickson, and K. M. Abraham*, † Department of Chemistry and Chemical Biology, Northeastern University Center for Renewable Energy Technology (NUCRET), Northeastern University, Boston, Massachusetts 02115, United States ‡ Power Division, U.S. Army RDECOM CERDEC CP&I, RDER-CCP, 5100 Magazine Road, Aberdeen Proving Ground, Maryland 21005, United States ABSTRACT: Oxygen reduction and evolution reactions (ORRs and OERs) have been studied in ionic liquids containing singly charged cations having a range of ionic radii, or charge densities. Specifically, ORR and OER mechanisms were studied using cyclic and rotating disk electrode voltammetry in the neat ionic liquids (ILs), 1-ethyl- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and 1-methyl-1-butyl-pyrrolidinium bis- fl (tri ouromethanesulfonyl)imide (PYR14TFSI), and in their solutions containing LiTFSI, NaPF6, KPF6, and tetrabutylam- fl monium hexa uorophosphate (TBAPF6). A strong correlation was found between the ORR products and the ionic charge density, including those of the ionic liquids. The observed trend is explained in terms of the Lewis acidity of the cation present in the electrolyte using an acidity scale created from 13C − 13 NMR chemical shifts and spin lattice relaxation (T1) times of C O in solutions of these charged ions in propylene carbonate (PC). The ionic liquids lie in a continuum of a cascading Lewis acidity scale with respect to the charge density of alkali metal, IL, and TBA cations with the result that the ORR products in ionic liquids and in organic electrolytes containing any conducting cations can be predicted on the basis of a general theory based on the hard soft acid base (HSAB) concept. ■ INTRODUCTION electrolytes5,6 judicially selected on the basis of their Lewis − fi Lithium-ion (Li-ion) batteries have an undeniable influence acid base properties as de ned by Guttmann donor and over our daily lives. Vested in this battery technology are acceptor numbers. The Guttmann donor number (DN) mobile electronics, load-leveling infrastructures, and electric measures the electron-donating capacity of the solvent to propulsion vehicles. Despite their ubiquity, limits on energy form complexes with Lewis acids such as Li+. density and the high cost of commercialized Li-ion batteries Ionic liquids as a class of electrolytes for Li batteries offer have accelerated efforts for alternative rechargeable battery several potential advantages over traditional nonaqueous − fi systems, such as the nonaqueous Li O2 battery, rst realized organic solvents. Besides a negligible vapor pressure, high 15 years ago.1 The 5280 Wh/kg theoretical energy density of ionic conductivity, and nonflammability, they can be designed − − ’ Li O2 is 7 8 times that of today s best Li-ion battery, and it to offer enhanced hydrophobicity and large electrochemical offers a long-term solution to energy independence. stability windows that are highly desirable for their use in the ffi A primary concern facing this power source is the ine cient Li−air battery. In this work, we have studied oxygen reduction rechargeability of insoluble LixOy discharge products that 2 and evolution reactions (ORRs and OERs, respectively) in two accumulate on the O2 electrode. This leads to poor cell ionic liquids (ILs), namely, 1-ethyl-3-methylimidazolium bis- performance, stemming from large cathode impedances and the (trifluoromethylsulfonyl)imide (EMITFSI), and 1-methyl-1- associated voltage gaps between oxygen reduction reactions 3,4 butyl-pyrrolidinium bis(triflouromethanesulfonyl)imide (ORRs) and oxygen evolution reactions (OERs). Our recent work on the mechanism of ORRs in nonaqueous electrolytes (PYR14TFSI). The PYR14 cation is abbreviated as PYR in this has revealed that the properties of the organic solvent play a report. The cations and common anion of these ionic liquids significant role in the nature of the final reduction product are shown in Scheme 1. formed, and the stability of the intermediates through which − the conversion of O2 to Li2O occurs in the discharge of a Li Received: July 6, 2012 O2 cell. We have gained this insight from a detailed study of the Revised: September 2, 2012 ORR intermediates and products in a series of organic Published: September 5, 2012 © 2012 American Chemical Society 20755 dx.doi.org/10.1021/jp306718v | J. Phys. Chem. C 2012, 116, 20755−20764 The Journal of Physical Chemistry C Article Scheme 1. Ionic Liquid Cations and Anion Used in This was first measured versus Li/Li+ using a Li foil placed next to Study the reference electrode in the ionic liquid. With this value of the Ag/Ag+ electrode potential versus Li/Li+, the measured electrode potentials from the CV and RDE cells were converted to the Li/Li+ voltage scale reported throughout this article. Propylene carbonate (PC) (anhydrous, 99.7%, Sigma- Aldrich) was used as the solvent for all NMR measurements along with various salts, including lithium bis- (trifluoromethanesulfonyl)imide (LiTFSI) (Purolyte), lithium fl fl hexa uorophosphate (LiPF6) (Purolyte), sodium hexa uoro- phosphate (NaPF6) (98%, Sigma-Aldrich), potassium hexa- fl uorophosphate (KPF6) (98%, Sigma-Aldrich), tetrabutylam- fl monium hexa uorophosphate (TBAPF6) (Fluka), and the two ionic liquids, EMITFSI and PYRTFSI. 13C NMR samples run on a Varian 400 MHz NMR utilized The ORR and OER mechanisms were studied using cyclic external referencing to preserve salt−solvent interactions. The and rotating disk electrode voltammetry (CV and RDE, tube configuration consisted of three components: (1) a respectively) in the neat ionic liquids and in their solutions Wilmad NMR tube (100 mHz, Economy) with 1 M salt containing singly charged cation salts, which included LiTFSI, solutions in propylene carbonate, (2) an inner PYREX capillary NaPF , KPF , and tetrabutylammonium hexafluorophosphate melting point reference tube with acetone d-6 and tetrame- 6 6 fl (TBAPF6). A correlation between the ORR products and the thylsilane (TMS) (1% v/v), and (3) Master ex peroxide-cured Lewis acidity of the cation present in the electrolyte was silicone to hold the capillary melting point tube in place. developed with the help of an acidity scale of the various Electrolyte preparation and tube assembly was done in a cations, including those of the ionic liquids. The acidity scale controlled-atmosphere argon glovebox. Before all NMR was created from 13C NMR chemical shifts and spin−lattice experiments, the instrument was locked with acetone d-6 fi 13 13 relaxation (T1) times, speci cally of the C O moiety, of the placed in the capillary tube. The C chemical shifts of PC and solutions of these salts in propylene carbonate (PC). its solutions were referenced to the 13C peaks of TMS. − 13 Remarkably, we have found that the ionic liquids lie in a Inversion recovery C T1 measurements utilized the same continuum of a falling Lewis acidity scale with respect to the tube configuration and were made on a Varian 500 MHz charge density of alkali metal, ILs, and TBA cations with the instrument. The 90° pulse width was calibrated on each sample result that the ORR products in ionic liquids and in organic before measuring T1, and eight acquisition arrays were used. electrolytes containing any conducting cations can be predicted Two scans were taken for each sample. on the basis of a general theory based on the hard soft acid base (HSAB) concept. These results are presented in detail in this ■ RESULTS AND DISCUSSION paper. The present results complement our recent work that An initial look at the role cations play on ORRs can be gleaned provided initial evidence for the correlation between the charge from the cyclic voltammograms (CVs) obtained with and 5,6 + density of ions and the ORR products. This work provides without 25 mM TBA in O2-saturated EMITFSI and PYRTFSI. experimental evidence for the ability of the HSAB theory to These CVs measured on Au and glassy carbon (GC) working explain the ORR mechanism and reaction products in any electrodes are presented in Figure 1. There is no significant nonaqueous electrolytes and, perhaps, even in aqueous change in the ORR following addition of TBA+. Increasing the electrolytes. TBA+ concentration 20× to 0.5 M made no change to the peak positions. An estimate of formal potentials (Eo′) in both neat ■ EXPERIMENTAL SECTION The ionic liquids 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)imide (EMITFSI) and 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYRTFSI) were synthesized through an aqueous ion-exchange reaction. The details of the synthesis and purification are described elsewhere.7,8 Structures were confirmed with 1H fi NMR, and a H2O content below 25 ppm was veri ed with Karl Fisher coulometry. Cyclic voltammetry (CV) measurements were performed on an Autolab potentiostat (Eco Chemie B.V.) equipped with a frequency response analyzer for iR correction. Electrochemical O2 half-cell studies were carried out in a controlled-atmosphere argon glovebox outfitted with a high- purity O2 gas source (99.995%). The O2 half-cell consisted of a planar glassy carbon (⌀ = 6 mm) or Au (⌀ = 5 mm) disk working electrode, a Ag/Ag+ reference electrode, and a Pt mesh counter electrode. The reference electrode was composed of 0.1 M silver fl + tri ouromethanesulfonate (AgCF3SO3) (99% , Sigma Aldrich) Figure 1. Cyclic voltammograms on Au and GC electrodes in O2- EMITFSI solution contained in a glass tube with a Ag wire and saturated EMITFSI and PYRTFSI both with (red) and without fi tted with a Vycor frit. In an argon atmosphere, its potential (black) 0.025 M TBAPF6. Scan rate = 100 mV/s. 20756 dx.doi.org/10.1021/jp306718v | J. Phys. Chem. C 2012, 116, 20755−20764 The Journal of Physical Chemistry C Article 11,12 ILs can be calculated based on peak potentials (Epa + Epc/2).
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