Oxygen Reduction Reaction in Ionic Liquids: Fundamentals and

Perspective pubs.acs.org/journal/ascecg

Oxygen Reduction Reaction in Ionic Liquids: Fundamentals and Applications in Energy and Sensors Asim Khan, Christian A. Gunawan, and Chuan Zhao* School of Chemistry, The University of New South Wales, Kensington, New South Wales 2052, Australia ABSTRACT: The oxygen reduction reaction plays a vital role in several processes and applications including energy and gas sensors that have been widely studied in aqueous and organic solvents. Although ionic liquids (ILs) have been known for a century, they gained substantial attention of researchers only a few decades ago as solvents for many applications like sensors, synthesis, catalysis, electrodeposition and energy applications. Use of ILs as an electrolyte for fuel cells, Li−O2 batteries and electrochemical gas sensors can resolve the issues related to instability/decompsotion, flamability and evaporation of electrolytes. Understanding the fundamentals of the ORR in ILs is essential for the development of these devices. Both the electrode materials and structure of ILs have significant effects on ORR. In addition, solubility and diffusion of O2 play an important part. This review focuses on recent advancements of ORR in ILs for energy (Li−O2 batteries and fuel cells) and electrochemical gas sensor applications. A brief introduction of ILs, followed by ORR mechanism in both aprotic and protic ILs are presented. In addition, the influence of electrode materials and ILs structure on ORR and solubility and mass transport of O2 in vaious IL-based electrolytes are also presented. Finally, some future directions with special emphasis on Li−O2 batteries, proton exchange membrane fuel cells (PEMFCs) and gas sensors are suggested. KEYWORDS: Oxygen reduction reaction, Ionic liquids, Electrochemical energy conversion and storage, Electrochemical gas sensors

■

INTRODUCTION

The ORR process in aqueous and nonaqueous media is well established.2,5,18 In aqueous solution, ORR generally adopts two pathways: (i) direct four electron pathway to form H2O, or (ii) peroxide pathway to form either H2O or H2O2 depending on the electrocatalysts used:5 In aqueous solvent: (a) Direct 4e− pathway Alkaline solution:

The increased demand for renewable energy worldwide has stimulated the development of high energy density electrochemical storage and conversion devices. The oxygen reduction reaction (ORR) is of great importance for energy conversion and storage devices such as fuel cells and metal−air batteries. It is also the key reaction in oxygen sensors that have been widely used in the manufacturing and automotive industries. The ORR is a complex reaction and generally influenced by both the electrode materials and electrolytes. There are several recent reviews available on the electrocatalysts for ORR, and its electrochemistry in aqueous and organic solvents is well established.1−9 Ionic liquids (ILs), which are a class of “green” and designable electrolytes with a unique set of properties, have attracted enormous interest in recent years as promising electrolyte for fuel cells, metal−air batteries and gas sensors.10−15 However, few reviews on ORR in ILs are available.16,17 This paper aims to fill the gap and reviews the fundamental of ORR in ILs with emphasis on its practical advances in Li−O2 batteries and PEMFCs and gas sensors applications. The energy application of ORR in ILs is limited to Li−O2 batteries and PEMFCs and only the progress made from 2000 and beyond are included. In the case of O2 gas sensors, only the developments made from 2011 and beyond are included. © 2017 American Chemical Society

O2 + 2H 2O + 4e− → 4OH−

E o = 0.401 V vs NHE (1)

Acidic solution: O2 + 4H+ + 4e− → 2H 2O

E o = 1.23 V vs NHE

(2)

(b) Peroxide pathway Alkaline solution: O2 + H 2O + 2e− → OH− + HO2− E o = −0.065 V vs NHE

(3)

followed by either HO2− + H 2O + 2e− → 3OH−

E o = 0.867 V vs NHE (4)

Received: February 8, 2017 Revised: March 7, 2017 Published: March 10, 2017 3698

DOI: 10.1021/acssuschemeng.7b00388 ACS Sustainable Chem. Eng. 2017, 5, 3698−3715

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ACS Sustainable Chemistry & Engineering

specific cations or anions, the reason why ILs are sometimes called “designer solvent”. A wide variety of cations and anions can be combined to form a staggering number of ILs. However, it is important to study the structure and intramolecular interactions of ILs while selecting them for a specific application because the properties of ILs vary significantly with the nature and size of cations and anions. For example, for ILs with same cations, the weaker coordinating anions like bis(trifluoromethylsulfonyl)imide ([NTf2]) result in lower viscosity and water immiscibility whereas ILs with strongly coordinated anions such as NO3− and Cl− have high viscosity and are water miscible.21,24 Similarly, increasing the alkyl or fluorinated alkyl chains of cations can result in higher viscosity and water immiscibility.21,24,25 ILs can be generally categorized into two broad subclasses: (i) aprotic ionic liquids (AILs) and (ii) protic ionic liquids (PILs). AILs consist of ions which do not have active protons in their structures, e.g., 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2]. The aprotic nature, wide electrochemical window and good stability against the attack of superoxide are the advantages of AILs as an electrolyte for Li−air batteries.10,26 PILs are formed by the transfer of proton from Brønsted acid to Brønsted base. Although the charge on the cations of AILs is generally stable, the proton transfer from acid to base is reversible and the strength of the acid and base governs the degree of proton transfer (eq 11). The proton transfer process can be improved by combining strong acid and strong base so the transferred proton will remain located on the base until the decomposition temperature is reached.26,27

or −

−

2HO2 → 2OH + O2

(5)

Acidic solution: O2 + 2H+ + 2e− → 2H 2O2

E o = 0.67 V vs NHE

(6)

followed by either H 2O2 + 2H+ + 2e− → 2H 2O

E o = 1.77 V vs NHE (7)

or 2H 2O2 → 2H 2O + O2

(8)

In nonaqueous organic solvent: O2 + e− → O2•−

O2•− + e− → O2 2 −

(9) (10)

Ionic Liquids (ILs). In general, ILs are referred to salts composed entirely of ions with a melting point below 100 °C. The ILs that are liquids at and below room temperature are known as room temperature ionic liquids (RTILs). The low melting point of ILs is due to the delocalized charge distribution on larger asymmetric cations and weak interactions between the ions.19 It is generally postulated that the first IL discovered by Walden in 1914 was ethylammonium nitrate (EAN), having a melting point 12−14 °C, and has since been extensively studied because of its water like properties.20 Although ILs have been known for a century, they gained substantial attention of researchers few decades ago as solvent for many applications like sensors, synthesis, catalysis, electrodeposition and energy applications.21−23 Commonly reported ILs consist of large asymmetric organic cations and organic or inorganic anions, Scheme 1. Classes of ILs. The growing interest in ILs as solvent/ electrolyte for energy applications is attributed to their unique set of properties like negligible vapor pressure, good thermal stability, high ionic conductivity, nonflammability and wide electrochemical window (up to 6 V).24 In addition, these properties can be altered to suit a particular task by selecting

HA + B ⇌ [A]− + [BH]+

(11)

One of the most important properties of PILs is the presence of active protons, which make them an attractive electrolyte for fuel cells.11,20,28−31 In addition, good thermal stability of PILs has made them suitable candidates as an electrolyte for nonhumidified proton exchange membrane fuel cell. ORR in Aprotic Ionic Liquids (AILs). Similar to nonaqueous aprotic solvents, the O2 undergoes one electron reduction in AILs to form superoxide (eq 9).32,33 The O2 reduction in mixtures of 1-ethyl-3-methylimidazolium chloride [EMIM]Cl and aluminum chloride (AlCl3) was first investigated by Carter et al.34 O2 is reduced to O2•−, which was unstable in the presence of protic impurities. Various studies have reported one electron quasi-reversible reduction process of O2 in AILs under anhydrous conditions.16,17,32,33,35,36 Under aprotic conditions O2•− can further undergo one electron reduction to form peroxide dianion (O22−) electrochemically (eq 10).17,37 However, O22− is extremely reactive, and can react with O2 to form O2•−.37,38 Because generated superoxide is a strong nucleophile, it can react with protonic impurities like H2O to form HO2− through irreversible disproportionation reactions (eqs 12 and 13). Upon the addition of H2O into AILs, O2 undergoes two electron reduction (eq 14, overall reaction of eqs 12 and 13), resulting in the disappearance of the anodic peak, an increase in the O2 reduction current and a positive shift of the cathodic peak potential16,32,37,39−44 (Figure 1).

Scheme 1. Molecular Structures of Commonly Used Cations and Anions of RTILs

3699

O2•− + H 2O → HO2• + HO−

(12)

O2− + HO2• → O2 + HO2−

(13)

O2 + H 2O + 2e− → HO2− + HO−

(14)


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to different levels of impurities present in AILs. Further work is required to understand the precise role of the ILs structure on the reduction of O2. Effect of Electrode Material on ORR. The electrode material also has considerable effect on the catalytic reduction of O2. The thermodynamics and kinetics of O2/O2•− couple, and the effect of electrode material on O2 reduction have been extensively studies by both Compton46,49,54,57−59 and Ohsaka.38,39,41,52,60 The influence of electrode materials on overpotential, heterogeneous rate constant and reaction mechanism of the ORR in various AILs have been investigated. The heterogeneous rate constant (khet) for O2 to O2•− in AILs was mostly found in the order GC > Au > Pt.39,54 In contrast to solid electrode (Au), the O2 undergoes two electron quasireversible redox reaction and a lower overpotential for ORR in [EMIM][BF4]-based ionic liquid at hanging mercury drop electrode (HDME). Catalytic activity of HDME is correlated to the adsorption of [EMIM] cations on hanging drop mercury electrodes (HDME) surface.41 Compared to GC, a sluggish kinetics at boron doped diamond (BDD) have been reported57 whereas multiwalled carbon nanotubes modified edge-plane pyrolytic graphite (EPPG) exhibited faster kinetics of the ORR in various AILs.60 The khet values for O2/O2•− redox reaction as a function of both electrode material and IL structures are shown in Table 1. It is evident from the data that changes in

Figure 1. Cyclic voltammograms obtained at an Au electrode (d = 51.6 mm) in O2-saturated [EMIM][BF4] containing less than ca. 2.1 mM H2O (dotted line) and 2.64 M H2O (solid line). Potential scan rate: 100 mV s−1. (Reprinted from ref 39 with permission from The Electrochemical Society. Copyright 2004 The Electrochemical Society.)

The influence of weak acids, like phenols, on O2 reduction in [NTf2]−-based AILs has been investigated.45,46 In the presence of phenol, the superoxide is protonated by the phenol to produce HO2• and phenolate ions. Compared to O2•−, HO2• is easily reduced by another O2•− leading to overall two electron reduction of O2 to form H2O2 in AILs.45 The protonation of O2•− by phenols in AILs was found to be similar to that in DMSO and DMF.3,47 A different mechanism of ORR in [EMIM][NTf2] in the presence of 4-t-butyl phenol has been reported, where phenolate ion is largely formed by the deprotonation of phenol by O22− instead of O2•−.46 Effect of AIL Cations on ORR. The structure of cations also plays an important role in determining the stability of O2•−. The O2•− was found to be unstable in pyridinium-based AILs.48 The O2•− can also react with many IL cations such as imidazolium and phosphonium to undergo proton abstraction reactions and is more stable in the presence of aliphatic and alicyclic ammonium cations such as quaternary ammonium and pyrrolidinium.16,37,49,50 Ion-pairing of O2•− with imidazolium cations has been extensively studied using voltammetry,37,40,49 digital simulation,49 ab initio molecular orbital calculation, electrochemical quartz crystal microbalance51 and UV−visible spectroscopy.40,52 The association constant and free-energy change (ΔG) of the ion-pairing of O2•− with [EMIM] have been determined to be 13.5 M−1 and −6.4 kJ mol−1, respectively. The first-order rate constant for association of O2•− with [BMIM] was found to be 1.4−2.4 × 10−3 s−1.49,52 The strong ion-pairing of imidazolium cation with O2•− may be attributed to relatively localized positive charge on imidazolium cation (carbon atoms at position 2, 4 and 5).37 However, there are some discrepancies in the literature regarding the stability of O2•− in phosphonium-based AILs. Compton and co-workers have reported that O2•− is reactive and abstract proton from trihexyl(tetradecyphosphonium) cations, and O2 reduction in these AILs occurred via a two electron reaction. It was demonstrated that the reactive O2•− can be trapped by the presence of CO2.42,53,54 On the other hand, MacFarlane and co-workers have reported the chemically reversible reduction of O2 in trihexyl(tetradecyphosphonium)based AILs and O2•− was found to be stable even in the presence of water.16,55 The stability of O2•− in these AILs was attributed to strong ion-pairing between O2•− and phosphonium cations (confirmed by computational quantum chemistry) and long-term stability of AILs was observed in the presence of O2•− and O22−.55,56 The contradictions may be due

Table 1. Heterogeneous Rate Constant (khet) Obtained by Voltammetric Investigations of the Oxygen−Superoxide Couple in a Range of RTILs khet at electrode material (103 cm s−1) RTIL system

Pt

Au

N-hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222][NTf2]) N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][NTf2]) tris(n-hexyl)tetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P14666][NTf2]) tris(n-hexyl)tetradecylphosphonium trifluorotris(pentafluoroethyl)phosphate ([P14666][FAP]) 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) 1-propyl-3-methylimidazolium tetrafluoroborate ([PMIM][BF4]) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) [BMP][NTf2]/DMSO o-xylene-α,α′-diylbis(1,2-dimethyl-1H-imidazol-3ium) di(bis(trifluoromethylsulfonyl)imide ([DiIM][NTf2])/DMSO

3.0

5.0

54

0.8

3.5

54

0.14

1.77

54

0.05

0.11

54

0.94

2.2

6.4

39

1.5

3.3

7.3

39

0.83

1.2

7.5

39

3.6

61 62

8.6

GC

ref

both the electrode materials and IL structures can have a significant influence of on the rate constant of O2/O2•− redox reaction. Table 1 covers a small range of materials and IL structures. Still, a large collection of electrode material and IL combinations remains to be investigated for better understanding of their role on the kinetics of ORR. O2 Solubility and Mass Transport in AILs. The mass transport and solubility of O2 are crucial in determining the rate capabilities and discharge capacities of Li−O2 batteries.63,64 A large number of data regarding the solubility (c) and diffusion coefficient of O2 (DO2) as well as diffusion coefficient of O2•− (DO2•−) in various AILs have been reported.33,48,54,58,59,65−67 Discrepancies have been found in the values reported for both 3700


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ACS Sustainable Chemistry & Engineering Table 2. Diffusion Coefficient and Solubility of O2 in Various RTILs c (mM)

IL [EMIM][BF4] [PMIM][BF4] [BMIM][BF4] [BMP][NTf2]

N-hexyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([HMP][NTf2]) [EMIM][NTf2] [BMIM][NTf2] 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BMMIM][NTf2]) [BMIM][PF6] [N6222][NTf2] [P14666][NTf2] [P14666][FAP] N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide ([PP13][NTf2]) N-methyl-N-methoxyethylpiperidinium bis(trifluoromethylsulfonyl)imide ([PP11O2][NTf2]) N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][NTf2]) N-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([PP14][NTf2]) 1-butyl-1-methylazepanium bis(trifluoromethylsulfonyl)imide ([Aze14][NTf2])

DO2 and c, which are possibly due to techniques used, level of purity and temperature difference. Table 2 summarizes DO2 and c values in some selected RTILs. The DO2 values reported in AILs are 1 order of magnitude slower than in nonaqueous organic solvent such as DMSO, DMF and acetonitrile.3,68 The c of O2 in AIL is in the range from 1.0 mM to ∼14.5 mM. Increase in temperature results in an increase in DO2 and a corresponding decrease in the c of O2.33,48,58,59,67 The Stokes−Einstein relationship for O2 molecule in RTILs has also been investigated.33,39,58,65 It has been reported that the smaller size of O2 molecule, compared to the solvent molecules, results in the movement of O2 molecule through the interstices of the IL ions. Therefore, diffusion does not fully obey the Stokes−Einstein relationship.33,39,58,65,69 The understanding of influence of IL structures on the diffusion and solubility of O2 is still limited and requires more detailed studies. Steady-state voltammetry of O2 in RTILs at an ultramicroelectrode reveals the asymmetry between the forward (plateau) and backward scan (peak),33,70,71 Figure 2. This asymmetry in CV is due to the significant difference between DO2 and DO2•− that has been observed in AILs, which is more pronounced in more viscous AILs.33,70,71 As mentioned earlier, the O2•− interacts strongly with the surrounding IL, and therefore IL-effects on the electrocatalytic reduction of O2 are apparent. ORR in AILs for Li−O2 Batteries. Among metal−air batteries, Li−air batteries, hereafter referred to as Li−O2 batteries, have the highest specific energy density.72 Like fuel cells, Li−O2 batteries, are energy conversion devices which are composed of Li anode (acts as fuel), ion conducting electrolyte and have oxygen as cathode material which does not need to be stored inside the battery. The air electrode generally consists of

DO2 (10−6 cm2 s−1)

1.1 ± 0.2 0.97 ± 0.05 1.1 ± 0.1 4.1 13.6 ± 0.8 3.6 2.9 6.1 ± 0.5 7.1 3.9 14.5 3.9 3.1 4.3 3.6

17 ± 2 13 ± 2 12 ± 1 1.79 1.8 ± 0.2 5.49 12 5.2 ± 0.4 1.1 6.3 2.5 7.3 8.76 1.8 5.1

3.0 3.9 3.9 6.0 ± 0.5 7.8 ± 1.5 4.6 4.4 4.4

2.5 4.55 4.6 7.5 ± 0.6 6.1 ± 1.1 3.0 5.0 5.1

8.48 ± 0.09 7.98 ± 0.20

1.82 ± 0.02 1.50 ± 0.04

η (mPa·S)

T °C

ref

37 103 180

25 25 25 25 25 25 30 35 23 25 25 20 25 23

39 39 39 58 33 58 67 54 51 66 48 70 58 51 58,59

259 119 119

25 25 25 35 35 25 25 25

58 58 59 54 54 66 66 66

178 323

25 25

69 69

89 60 89 172 101

52 25

Figure 2. Cyclic voltammetry of O2 reduction in [Et3BuN][NTf2] (30 min saturation) at a 5-μm-radius disk gold ultramicroelectrode. Scan rate 0.1 V s−1. (Reprinted from ref 71 with permission from the American Chemical Society. Copyright 2007 American Chemical Society.)

a gas diffusion layer and a catalytic (reaction) layer. The Li−O2 battery is generally categorized into two types according to the electrolyte used: (i) nonaqueous systems and (ii) aqueous systems,73,74 though there are other architectures like solid-state and mixed aqueous/nonaqueous systems.72,75 The following reactions take place at the electrodes:72,74,75 Anode: Li → Li+ + e−

(15)

Cathode: in nonaqueous electrolyte Li+ + O2 + e− → LiO2 3701

E o = 3.0 V vs Li/Li+

(16)


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ACS Sustainable Chemistry & Engineering 2LiO2 → Li 2O2 + O2 LiO2 + Li+ + e− → Li 2O2

(17)


Li 2O2 + 2Li+ + 2e− → 2Li 2O

(18)

E o = 2.91 V vs Li/Li+ (19)

in aqueous electrolyte (alkaline conditions) 2Li+ +

1 O2 + 2e− + 2H 2O → 2LiOH 2


(20)

A nonaqueous system has drawn more attention and has been widely investigated because of its advantages (viz. higher theoretical and specific energy density) over an aqueous system.73,76 The Li−O2 batteries are still in the research and development phase as numerous challenges have to be addressed before their practical application. Some of the major ORR-related challenges are discussed here. First, the currently reported nonaqueous electrolytes have shown chemical instability against the attack of nucleophiles such as O2•−, forming side products like lithium carbonate, lithium alkyl carbonates and LiOH. These side products are not desirable for rechargeable Li−O2 as these products oxidize at much higher charging potential than Li2O2 decreasing the cell efficiency.72,77,78 The problems related to the use of nonaqueous organic solvents in aprotic Li−O2 batteries, like evaporation of solvent and decomposition of electrolyte by discharge products can be addressed by the use of AILs. Negligible vapor pressure, hydrophobic nature, large potential window, high thermal stability and good electrochemical stability of AILs offer several advantages over nonaqueous organic solvents and make them appealing for Li−O2 batteries. Compared to propylene carbonate (PC), a commonly used organic solvent for Li batteries, [BMP][NTf2] exhibits excellent stability to reactive products such as O2•− and LiO2.79,80 The stability of [BMP][NTf2] is attributed to its delocalized positive charge80 and much slower diffusion of O2•− in [BMP][NTf2].79 The degree of association of O2•·− with cations of ILs is far less compared to the reaction of O2•− with metal cations e.g., lithium, sodium, and potassium ions.12,38,50 Allen et al.,12 have explained the influence of alkali metal ions (Li+, Na+ and K+) on ORR mechanism in AILs on the basis of hard soft acid base (HSAB) theory. It was assumed that IL cations such as [EMIM], a soft Lewis acid, stabilized the O2•− by ion-pairing (O2•−...[EMIM]), which oxidized back to form [EMIM] and O2, Figure 3. On the other hand, the addition of hard Lewis acids like Li+ reacted with O2•− to form LiO2 which disproportionate to form more stable Li2O2. However, similar to the majority of nonaqueous solvents, the Li2O2 is sparingly soluble in ILs and only reoxidise at much higher potentials, which results in clogging the pores of gas diffusive cathode.76,81,82 Kuboki et al.83 were among the first to report that hydrophobic AILs can be used as an electrolyte for Li−O2. Among different hydrophobic AILs reported, the primary Li− O2 cell based on [EMIM][NTf2] electrolyte exhibited the highest capacities (5630 mAh g−1 at a current density 0.01 mA cm−2). No significant changes in discharge behavior were observed for 56 days when cell was operated in air. This stability was attributed to hydrophobic nature and negligible vapor pressure of [EMIM][NTf2]. This study has stimulated significant interest in the use of AILs for Li−O2 applications

Figure 3. Cyclic voltammogram of neat [EMIM][NTf2] along with various salts at 0.025 M concentration on a GC electrode at 100 mV s−1. (Reprinted from ref 12 with permission from the American Chemical Society. Copyright 2012 American Chemical Society.)

and since then several investigations using AILs as electrolytes have been reported.30,66,67,84−89 More recently, Elia et al.90 reported a range of techniques utilized to evaluate the performance of the [BMP][NTf2]-based Li−O2 cell. It was demonstrated that if appropriately optimized, the IL-based Li− O2 battery can exhibit excellent cycling capability and improved energy efficiency (ca. 82%). Compared to organic solvents, lower charge overpotential has been observed in ILs-based Li− O2 cell.80,90,91 The selection of a suitable IL structure is crucial for stability of electrolyte in a Li−O2 system. Stability of various electrolytes has been investigated using differential electrochemical mass spectrometry, voltammetry, rotating ring-disk electrode (RRDE), 13C nuclear magnetic resonance (13C NMR), Fourier transform-infrared (FT-IR) and in situ Raman spectroscopy.79,91−94 Among these, pyrrolidinium-based ILs are found to be more stable than imidazolium and quaternary ammoniumbased ILs for Li−O2 systems. However, long-term stability is still an issue that requires further research. In addition, other issues such as high viscosity, poor mass transport of ions, sluggish kinetics and low solubility of Li salts result in lower discharge capacities.67,72,80,95,96 Attempts have been made to address these issues by using solvents with higher donor number (DN) like dimethyl sulfoxide (DMSO)97,98 and 1-methylimidazole ([MIM])98 and incorporating additives such as tris(pentafluorophenyl) borane (TPFPB)82 in the electrolyte to stabilize/solvate the reduction products by decreasing the Lewis acidity of Li+. However, the spectroelectrochemical analysis showed decomposition of DMSO99−101 whereas additives also decrease the conductivity and enhance the viscosity of electrolyte resulting in reduced discharge capacity.102 Fewer studies have been carried out by combining the ILs with organic solvents to overcome the above drawbacks and to achieve better Li−O2 battery performance by integrating useful properties of both components. Mixing [PMP][NTf2] with propylene carbonate PC (1:1 v/v) enhances the stability of electrolyte against O2•−, increases conductivity and improves discharge capacity of Li−O2 cell. However, no influence on the charging potential was observed.103 Similar results were obtained when [BMP][NTf2] was added into tetra ethylene glycol dimethyl ether (TEGDME). In this case, the addition of [BMP][NTf2] considerably lowers the charge potential of Li−O2 cell.104 Recently, Zhao et al. 3702


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membrane by the electrochemical attack of the HO•− and HO2• radicals greatly decreases the durability of PEMFCs.31 Although, recent advances have been made to reduce the cost and increases the durability of PEMFCs,121 there is still the need for fundamental research to develop membranes which have little or no dependence on humidification and can operate at higher temperature. As discussed above, good thermal stability and presence of active protons of PILs have made them suitable candidates as an electrolyte for nonhumidified PEMFCs. Recently, the ORR has been studied in a range of room temperature PILs.122 The solubility of O2 was found to be higher than the aqueous electrolyte while diffusion coefficient of O2 in these PILs being 1 order of magnitude lower than that obtained in the aqueous electrolyte. Cyclic voltammetric studies and the digital simulation suggest that reduction of oxygen in these PILs proceeds via two one-electron processes (ECEC mechanism) to H2O2. The influence of temperature on kinetics and thermodynamic properties like activation energy of ORR, activation energy of diffusion and enthalpy of dissolution of oxygen was also investigated. Among the PILs studied (bis(methoxyethyl)ammonium benzoate, bis(methoxyethyl)ammonium sulfamate, triethylammonium methanesulfonate, bis(methoxyethyl)ammonium acetate, pyrrolidinium acetate, ethylenediamine acetate and EAN), EAN has higher O2 solubility, lower activation energies and much higher enthalpy of dissolution than phosphoric acid. However, the ORR in these PILs even at Pt electrode remains restricted to twoelectron reduction to form H2O2. The first evidence showing that O2 can be reduced to H2O via four electron reduction process in RTILs was reported by Switzer et al.123 They extensively investigated the ORR in [BMMIM][TfO] over a wide range of pKa (∼30 pKa unit) in the presence of protic additives. The proton activity of additives (pKa value) in [BMMIM][TfO] exhibits significant influence on the onset potential of ORR, Figure 5. The rotating ring-disk electrode (RRDE) experiments revealed that the addition of protic species in IL results in a four-electron reduction pathway for O2 on Pt, indicating the high catalytic activity of Pt. On the other hand, ORR is limited to a two-electron process to form H2O2 on GC and was attributed to the absence of Hads and Oads on GC and four electron reduction of O2 can only be achieved on GC by applying significantly higher potentials.123 PozoGonzalo et al.124 also reported that four electron reduction of O2 to H2O can be achieved by the addition of proton donor species like ethylene glycol (EG) in trihexyl(tetradecyl)phosphonium chloride [P14666]Cl. In this case, the increased ORR activity was attributed to the self-stabilization of ethylene glycol after deprotonation via an internal hydrogen bond. PILs as an Electrolyte for PEMFCs. In contrast to AILs, PILs have active protons and can be used as proton conductor in H2/O2 fuel cells.28,125,126 Many PILs show good thermal stability (up to 150 °C) and high ionic conductivity and can be used in mesothermal nonhumidified fuel cells.11,28,127−129 Watanabe and co-workers showed that ORR and hydrogen oxidation reaction (HOR) can take place in PILs prepared by combining organic amines with bis(trifluoromethylsulfonyl) amide (HNTf2) under anhydrous conditions.28,125,126,130 The PILs showed high conductivity and excellent thermal stability (up to 400 °C). Compared to 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]), an AIL, where no definite increase in current was observed under H2 atmosphere at a Pt electrode, a significant increase in the

investigated the effect of ILs structures on the ORR/OER in mixed electrolyte in the presence and absence of Li salts.61 In all of ILs/DMSO mixed electrolytes, the reduction of O2•− to O22− is observed at a more positive potential than in pure DMSO electrolyte (Figure 4). This positive shift in the

Figure 4. Cyclic voltammograms for the ORR in O2 saturated (a) 3.5 M [BMPy][NTf2]/DMSO, (b) 3.5 M [BMIM][BF4]/DMSO and (c) 0.1 M TBAPF6/DMSO at GC disk electrode at scan rate of 100 mV s−1. (Reprinted from ref 61 with permission from the American Chemical Society. Copyright 2016 American Chemical Society.)

reduction potential is attributed to the strong attraction between the cations of ILs and O2•−. Higher current density and better reversibility for ORR-OER processes were observed in mixed electrolytes compared to individual DMSO or IL electrolytes, which has important implications to high energy density of Li−O2 batteries.61,105 Addition of AILs into organic solvents like DMSO, TEGDME and 1,2-dimethoxyethane (DME) results in enhanced solubility of alkaline peroxides, increased discharge capacity and improved kinetics and reversibility of ORR-OER processes.62,95,104 However, it should be noted that the ratio of IL/organic solvent has a significant effect on the mechanism of ORR-OER processes and plays an important role in achieving these properties. A comparison of different nonaqueous Li−O2 battery electrolytes is summarized in Table 3. ORR in Protic Ionic Liquids (PILs). Fuel cell is an energy conversion device which converts chemical energy stored in fuels into electrical energy. The core component of the PEMFCs is membrane electrode assembly (MEA), which comprises a proton exchange membrane (PEM), and catalyst layers and gas diffusive layers (GDL). The PEM acts as a gas separator and isolates anode from cathode which contains carbon supported catalyst layers applied between PEM and GDL. Platinum (Pt) is the benchmark catalyst for ORR. However, it is known that the poisoning of Pt by CO adsorption,118 Pt dissolution in acidic media and carbon corrosion especially at higher cell potential can decrease the durability of PEMFCs.119 Operating at higher temperature reduces the CO poisoning;120 however, the membrane has to be humidified for efficient proton conduction and this limits the operating temperature.119 In addition, the degradation of 3703


200 mA gc−1

α-MnO2 (42 wt %)/C (Super P) α-MnO2 (42 wt %)/C (Super P) α-MnO2 (42 wt %)/C (Super P) Ru/CNTs mesoporous β-MnO2 (70 wt %)/C (Super S) binder free graphene foams carbon free nanoneedles-Co3O4 10%MnO2/C vertically aligned CNTs (VACNT) MnCo2O4-graphene (∼0.5 mg cm−2) 3D LaFeO3 carbon (Super P) carbon (Ketjan Black) carbon (Ketjan Black) carbon (Ketjan Black) carbon (Ketjan Black) carbon (Ketjan Black) catalyst-free meso/macroporous carbon (ZL) nanoporous gold (NPG)

0.3 M LiNTf2/[PMP][NTf2]

0.3 M LiNTf2/PC

1 M LiTfO-TEGDME

1 M LiPF6/ethylene carbonate/dimethyl carbonate (1:1 v/v)

3704

0.4 M LiClO4/[PP13][NTf2]

1 M LiNO3/N,N-dimethylacetamide (DMA)

0.5 M LiNTf2/[BMIM][NTf2]

0.5 M LiNTf2/[BMP][NTf2]

0.5 M LiNTf2/[BMIM][NTf2]/[BMP][NTf2] (9:1)



0.1 M LiNTf2/[BMP][NTf2]

0.1 M LiClO4/DMSO

1 M LiNTf2/TEGDME

1 M LiClO4/PC

1 M LiNTf2/[PP13][NTf2]

1 M LiNTf2/TEGDME

1 M LiTfO/TEGDME

200 mA gc−1

α-MnO2 (42 wt %)/C (Super P)

0.3 M LiNTf2/N-propyl-N-methyl pyrrolidinium bis(trifluoromethansulfony)imide) [PMP][NTf2]/PC (50/50 v/ v) 0.3 M LiNTf2/[PMP][NTf2]/PC (70/30 v/v)

300 mA g−1 100 mA gCo3O4−1 0.02 mA cm−2 0.2 mA cm−2 400 mA g−1 0.15 mA cm−2 0.1 mA cm−2 0.1 mA cm−2 0.1 mA cm−2 0.1 mA cm−2 0.1 mA cm−2 0.1 mA cm−2 0.08 mA cm−2 500 mA gAu−1

0.10 mA cm−2 300 mA g−1

200 mA gc−1

Co3O4 (30 wt %)/C

0.5 M LiClO4-TEGDME (20% by wt)-DMSO

current density 0.1 mA cm−2 0.1 mA cm−2 200 mA gc−1

Co3O4 (30 wt %)/C

cathode material

0.5 M LiClO4/[EMIM][NTf2] (20% by wt)-DMSO

electrolyte

discharge/charge capacity cut-off

40

1000 mA h g−1

25 25 50 50 50 15 100

400 mAh g−1 400 mAh g−1 400 mAh g−1 400 mAh g−1 200 mAh g−1 300 mAh gAu−1

80

1000 mA h gcarbon−1 ∼220 mAh gcarbon−1 400 mAh g−1

124

20

30

50

20

2.0−4.2

2.0−4.2

2.0−4.2

2.0−4.2

2.0−4.2

2.5−4.2

2.2−4.4

3.75 −4.3

2.0−4.0

2.30−4.20

2.00−4.50

50 50

2.25−4.35

2.25−4.35

2.25−4.35

2.25−4.35

2.25−4.30

2.25−4.30

voltage range (V)

70

70

70

70

50

65

cycles

800 mAh gVACNT−1

500 mAh g−1

1000 mAh g−1

154 mAh g

−1

835 mA h gelectrode−1 835 mA h gelectrode−1 750 mAh g−1 (maximum capacity) 683 mAh g−1 (maximum capacity) 533 mAh g−1 (maximum capacity) 880 mAhg−1 (maximum capacity)

Table 3. Comparison of Different Nonaqueous Li−O2 Battery Electrolytes performance

charging potential is below 3.80 V; cell with ZL/GC cathode shows ∼80% recharge efficiency after 15 cycles retaining 95% of its capacity after 100 cycles. More than 99% Li2O2 formation

68% Coulombic efficiency after 28 cycles (charge capacity retention 73%)


55% Coulombic efficiency after 30 cycles


low overpotential, high specific capacity, good rate capability and cycle stability up to 124 cycles cell shows consistent charging profile and good capacity retention; O2 is the primary gaseous product formed during charging 60% Coulombic after 20 cycles (charge capacity retention 73%)

cells show high capacity, low overpotential and good cycling stability

showed a lower charge potential∼ 3.83 V; no significant changes in the discharge/ charge profiles were observed during 50 cycles 60% retention of discharge capacity after 30 cycles; compared to PC electrolyte the voltage gasp is reduced from 1.30 to 0.75 V; charging voltage ∼3.20 V retention of discharge capacity can reach up to 56% after 20 cycles

round-trip efficiency of up to 78%; stable charge voltage ∼3.85 V for 20 cycles

81% capacity retention after 50 cycles

retains 64% of its initial capacity

retains only 25% of the initial capacity

retains only 7.69% of the initial capacity

retains only 42.3% of the initial capacity

retains 94.6% of the initial capacity

performance is limited to 50 cycles

recharge potential is

Oxygen Reduction Reaction in Ionic Liquids: Fundamentals and

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