Electrochemical activity of platinum, gold and glassy carbon electrodes in water-in-salt electrolyte

Journal Pre-proof Electrochemical activity of platinum, gold and glassy carbon electrodes in water-insalt electrolyte Laura Coustan, Daniel Bélanger PII:

S1572-6657(19)30806-9

DOI:

https://doi.org/10.1016/j.jelechem.2019.113538

Reference:

JEAC 113538

To appear in:

Journal of Electroanalytical Chemistry

Received Date: 10 April 2019 Revised Date:

19 September 2019

Accepted Date: 30 September 2019

Please cite this article as: L. Coustan, D. Bélanger, Electrochemical activity of platinum, gold and glassy carbon electrodes in water-in-salt electrolyte, Journal of Electroanalytical Chemistry (2019), doi: https:// doi.org/10.1016/j.jelechem.2019.113538. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

ELECTROCHEMICAL ACTIVITY OF PLATINUM, GOLD AND GLASSY CARBON ELECTRODES IN WATER-IN-SALT ELECTROLYTE

Laura Coustan and Daniel Bélanger

Département de Chimie, Université du Québec à Montréal, Case Postale 8888, succursale Centre-Ville, Montréal, Québec, H3C 3P8, Canada

*Corresponding author: [email protected]

Revised: September 19, 2019

ABSTRACT The effect of potential cycling conditions on redox processes observed in the potential region of electrochemical stability window, between hydrogen and oxygen evolution reactions, of platinum, gold and glassy carbon electrodes has been investigated in aqueous lithium bis(trifluoromethylsulfonyl)imide (LITFSI) solutions with concentration ranging from 1 to 21 molal (moles of salt/kg of water). Following potential cycling to positive potential upon which oxidation of Pt is barely observable in contrast to Au, which is characterized by a well-defined oxidation wave, both electrodes showed on the return scan a cathodic wave corresponding to the electrochemical reduction of corresponding metal oxides. In the potential region more negative than about 0 V, voltammograms recorded by avoiding scanning to a positive potential limit reaching the onset of the oxygen evolution reaction, show no well-defined cathodic waves prior to hydrogen evolution reaction. This provides strong evidence that TFSI anions are not electrochemically reduced in this potential region at Pt, Au and glassy carbon electrodes. Furthermore, X-ray photoelectron spectroscopy measurements of electrode surface, following potential cycling to a value negative enough to reach the onset of hydrogen evolution, showed formation of surface layer with a high fluorine/sulfur ratio (higher than 3 expected for TFSI) that could be explained by chemical degradation of TFSI by species generated during hydrogen evolution with preferential dissolution of sulfur-based compounds.

Keywords. Water-in-salt electrolyte, lithium bis(trifluoromethylsulfonyl)imide, platinum, gold, glassy carbon, solid-electrolyte interphase.

2

1. INTRODUCTION Electrochemical technologies depend on redox processes taking place at electrode surfaces. These redox processes are strongly influenced by the nature of the electrode, the electrolyte and the chemical composition of the electrode/electrolyte interface.[1–5] Electrolyte and solvent

decomposition

determine

the

electrochemical

stability

window

of

an

electrode/electrolyte system. In aqueous electrolyte, the thermodynamic electrochemical stability window set at 1.23 V is established by electrochemical processes involving electrolysis of water with formation of hydrogen (reduction process) and oxygen (oxidation process).[4,5] Electrochemical technologies often rely on the rate of these two competitive reactions compared to the rate of electrochemical processes of interest (eg. electrodeposition, oxidation of waste). The useful and practical range of most electrodes can be higher in some specific conditions. For instance, this can occur due to larger overpotential, and slower electron transfer kinetics, for hydrogen and oxygen evolution reactions taking place at the electrode surface. Consequently, the onset of hydrogen and oxygen evolution reactions are shifted to more negative and positive potentials, respectively.[4–6] A typical example of industrial relevance is the slow hydrogen and oxygen evolution reactions in a lead-acid battery that enables a cell voltage of 2 V, which is much larger than the thermodynamic electrochemical stability of water.[7] Recently a larger electrochemical stability window has been demonstrated in highly concentrated solutions also called water-in-salt, water-in-bisalt [8–18] and hydrate melt electrolytes.[19] These electrolytes contain a very high concentration of salt with a solubility as high as 37 m (37 moles of salt per kg of water) at room temperature.[16] Such solutions have

3

been used for applications in aqueous rechargeable batteries [15,17,18] as well as electrochemical capacitors [20,21] and have been also the subject of modelling studies.[20,22,23] For these electrolytes, the wider electrochemical stability window is believed to be due to the lower amount of free water present in the electrolyte and formation of a solid electrolyte interphase at the electrode surface.[8,18,24] The solid electrolyte interface would be formed by reductive decomposition of bis(trifluoromethylsulfonyl)imide (TFSI) anions that led to the formation of a LiF film and the electrochemical reduction of traces of O2 and CO2 present in the electrolyte that generated an insulating Li2CO3 deposit.[14] On the other hand, we have demonstrated that the electrochemical reduction of TFSI anions is barely occurring within the electrochemical

stability

window

of

a

stainless

steel

electrode

in

lithium

bis(trifluoromethylsulfonyl)imide (LiTFSI) solution.[25] Furthermore, combination of chemical, electrochemical and operando methods enabled to demonstrate that formation of a fluorinated solid-electrolyte-interphase occurs by reaction of TFSI anions with hydroxyls species generated during electrochemical reduction of water to form hydrogen. [26] Herein, we extend our recent study [27] by investigating the effect of potential cycling conditions on electrochemical processes occurring at platinum, gold and glassy carbon electrodes in LiTFSI electrolyte for concentration ranging between 1 and 21 m. This study is motivated by the possibility of developing electrochemical technologies with these electrodes in presence of superconcentrated electrolytes. Thus, one specific goal of this work is to determine if the electrochemical behaviour of TFSI anions depends on the nature of the electrode. The electrode surface was also characterized by X-ray photoelectron spectroscopy following specific potential cycling experiments to probe the surface of the electrodes.

4

2. EXPERIMENTAL SECTION Chemicals. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was purchased from Solvionic SA.

Preparation of solutions. LiTFSI salt was kept under vacuum at 60°C before weighing in air. MilliQ water was used to prepare the solutions. To the mass of LiTFSI required for concentration of 1, 5 and 21 m (moles of salt per kg of water), the appropriate amount of water was added. The solutions were degassed with nitrogen for two hours prior to electrochemical experiments. The approximate pH values of the solutions, measured with a pH paper, are found to between 6 and 7. Due to the high concentration of salt and possible presence of HF, a standard glass electrode was not used [28].

Electrodes and electrochemical cell. Platinum (0.02 cm²), gold (0.02 cm²) and glassy carbon (0.07 cm²) disk electrodes were used. Platinum, gold and glassy carbon electrodes were cleaned by polishing with 1 μm alumina, rinsed with Nanopure water, sonicated in fresh Nanopure water for 10 min and finally put in boiling water for few seconds.[20,22,23] Platinum gauze and Ag wire were used as counter and reference electrode, respectively. Platinum electrode was ultrasonically cleaned with water and ethanol, annealed in a flame, cooled and washed again in Nanopure water. Electrochemical measurements were performed at 21+1°C in a two-compartment Pyrex cell with working and counter electrodes placed in two different compartments connected by a glass frit. A Solartron multipotentiostat (model 187) controlled by a computer with the Corrware software was used.

5

Cyclic voltammetry was carried out at scan rate of 10 mV.s-1. Cyclic voltammograms shown below are for the first cycle. In one set of experiments, the electrodes were scanned from 0 V to a potential giving an arbitrarily chosen current density of about 1 mA/cm2 in the positive potential direction and subsequently in negative potential direction to a potential until a similar current density was recorded. In a second series of experiments, the electrode potential was only swept from 0 V to the same negative potential limit.

X-ray photoelectron spectroscopy. X-ray photoelectron spectra were recorded using a VG Escalab 3 MKII spectrometer employing a focused, achromatic Mg Kα source (hν=1253.3 eV), with the X-ray gun set to 300 W and X-ray incident angle of 90°. A 2×3 mm surface was analyzed. A pass energy of 100 eV was used for survey scans and 20 eV for high-resolution scans. After cycling in a selected potential range, the electrode was ultrasonically cleaned with Nanopure water, rinsed two times with water and dried under vacuum at 60°C overnight. The samples were pre-pumped in a preparative chamber to a pressure lower than 6.7×10−9 mbar before transfer into the analytical spectrometer chamber for measurement. The high-resolution spectra were corrected to the C1s peak at 284.5 eV. The atomic concentrations (at. %) of each element were determined from the relative peak areas of the survey spectra and the corresponding sensitivity factors (C1s = 0.25; O1s = 0.66; Cr2p = 2.30; F1s = 1.00; Fe2p = 3.00) and the equation: at. % = ܽ. % = (

஺೔ /௦೔

)

ಲ ஊ( ೔ ) ೞ೔

where Ai is the area of the peak for element 1 and si is the sensitivity factor of the element.

6

3. RESULTS AND DISCUSSION We have recently reported on the electrochemical behaviour of Pt, Au and glassy carbon electrode in presence of aqueous LiTFSI solutions in the concentration range of 1 to 21 m.[25] The focus of this earlier study was to investigate the effect of LiTFSI concentration on the electrochemical stability window, which became larger upon increasing the concentration of the salt. Here, we extend this earlier work and report a detailed investigation of the effect of potential cycling conditions on redox processes observed in the potential region of electrochemical stability window, between hydrogen and oxygen evolution reactions.

Electrochemical characterization. Platinum. Figure 1 shows voltammetric curves (partial cyclic voltammograms and linear sweep voltammograms) for a platinum electrode in aqueous LiTFSI solution of various concentrations ranging from 1 to 21 m at a scan rate of 10 mV/s. Two sets of voltammograms were recorded. For a first one depicted in Figure 1A, the potential was scanned in positive direction to a potential giving an arbitrarily chosen current density of about 1 mA/cm2 and subsequently in negative potential direction to a potential until a similar current density was recorded. The voltammogram for a Pt electrode shows an onset of oxidation presumably corresponding to oxygen evolution and platinum oxidation at about 1.0 V, which is shifted approximately to 1.5 V when the concentration of LiTFSI salt is increased from 1 to 21 m. Platinum oxide formation is not clearly observed but is probably occurring concomitantly with the oxygen evolution reaction since on the reverse scan, the first cathodic wave centered between 0 and 0.2 V is attributed to reduction of platinum oxide. The smaller voltammetric charge of the cathodic wave for the

7

smaller LiTFSI concentration reflects lower extent of platinum oxidation due to the lowest potential limit used in linear sweep voltammetry. In addition, a significant shift to less positive potential is seen when LiTFSI concentration is decreased. Such a shift is typically observed when metal oxide thickness is increasing due to an increase of potential drop across the oxide layer.[29,30] However, the observed behaviour cannot be explained by the presence of a thicker oxide layer for lower LiTFSI concentration. Thus, this observation is unclear at the moment and will require further investigation. The second wave recorded at more negative potential (centered between -0.4 and -0.7 V) is followed by a significant increase of current due to hydrogen evolution reaction. The latter is not significantly influenced by LiTFSI concentration. The electrochemical processes giving rise to the second cathodic wave, with a current density ranging between 43 and 70 µA/cm2 (Table 1) as seen in inset of Figure 1, has been previously attributed to electrochemical reduction of TFSI anions, on the basis of electrochemical, spectroscopic and modelling studies.[31,32] The shift of this cathodic wave seems to parallel that of the first cathodic wave (observed between 0 and 0.25 V). In the second set of experiments, the initial scan was initiated in negative direction and potential excursion in positive potential region avoided. In this case, Figure 1B shows that almost only double-layer capacitive current was observed between 0 and -1.0 V before the onset of hydrogen evolution reaction. A very low current intensity reduction wave (Table 1) can be detected but its intensity is significantly lower than that seen on Figure 1A when the potential sweep was initially performed in positive potential direction. This unambiguously suggests that, in contrast to previous reports [8,27,29,33] , the second reduction wave centered between -0.4 and -0.7 V is unlikely related to electrochemical reduction of TFSI anions.

8

Gold. A similar general trend was observed for a gold electrode characterized in presence of the same LiTFSI solutions (Figure 2 and Table 2). Similarly to Pt, only low current intensity redox waves are observed between 0 and -1.2 V when the initial potential scan direction is towards negative potential (Figure 2B). Thus, there is no electrochemical evidence for reduction of TFSI anions on gold electrode in the potential range investigated in our work. Nonetheless, in comparison to Pt, some differences are observed on the voltammetric curves. Firstly, the onset of hydrogen and oxygen evolution reactions is shifted to more negative and positive potential, respectively. This is in line with our previous report [27] and the fact that the two above processes display slower kinetics on Au than on Pt [25]. Noteworthy, is the clearer observation of the oxidation of gold electrode illustrated by the anodic wave located between 0.3 and 1.2 V, depending on LiTFSI concentration. The evolution of the potential of gold oxide reduction peak and associated voltammetric charge is similar to those observed on platinum (vide supra).

Glassy carbon. A similar set of experiments was carried out with a glassy carbon electrode and corresponding voltammetric curves are presented in Figure 3. The voltammetric curves initiated in positive potential direction show a well-defined anodic wave, at about 1.5 V, only for the more concentrated solution. This wave can be tentatively attributed to oxidation of glassy carbon. It overlaps with oxygen evolution reaction for the lower concentration solution. As previously reported [25], a significant shift of onset of the anodic wave corresponding to oxygen evolution reaction is observed upon LiTFSI concentration increase and can be explained by the decrease of concentration of free water molecules [8,15,25,34]. The reverse sweep in positive

9

direction shows distinct features compared to Pt and Au electrodes. Only one reduction wave is seen at about – 0.75 V that is not strongly affected by salt concentration. Similarly to stainless steel [27], Pt (Figure 1) and Au (Figure 2), a well-defined reduction wave is not observed between about 0 and – 1 V when potential cycling was initiated in negative potential direction (Figure 3B). Again, this would rule out the possibility that this wave could be attributed to electrochemical reduction of TFSI anions. In fact, upon cycling to negative potential, hydrolysis of TFSI- anions occurs by reaction with hydroxyls generated during hydrogen evolution when the electrode potential is sufficient to electrochemically reduce water.[26] This will be discussed further below. X-ray photoelectron spectroscopy. Figure 4 shows a set of XPS survey spectra recorded for a bare Pt electrode and also following potential cycling in different conditions and different LiTFSI concentrations. The spectra for Pt electrodes cycled from 0 to 1.8 and -1.3 V (curves PN), 0 to 1.8 V (curves P) and 0 to - 1.3 V (curves N) are presented in Figures 4a, 4b and 4c, respectively. A comparison of the survey spectrum of bare Pt with those of electrodes cycled in LiTFSI electrolytes show an attenuation of the intensity of Pt4f peaks owing to the presence of a surface layer on the latter. The surface atomic concentrations of these Pt electrodes for relevant atoms are reported in Table 4. In all cases, the surface concentration of fluorine and sulfur (determined from F1s and S2p peaks, respectively) increase with when LiTFSI concentration is changed from 1 to 5 m and do not significantly varied upon increase of concentration to 21 m. Interestingly, potential cycling direction and conditions have no significant effect on F and S concentration. Note that in these experiments, the potential limits were set to values sufficient to observe an arbitrarily chosen

10

current of about 1 mA/cm2 that would correspond to onset of hydrogen and oxygen evolution reactions. In contrast to stainless steel electrode [27], electrodes only cycled to negative potential values (Figure 4b, 4c, curve c) show F and S surface concentration similar to those of electrodes only cycled to positive potential. A similar trend was observed for Au (Figure SM1 and Table SM1) and glassy carbon electrodes cycled in various LiTFSI aqueous solutions (Figure SM2 and Table SM2). For example, an increase of F/Au ratio is observed upon increase of LiTFSI concentration (Table SM1), similar to observation with Pt electrodes (Table 4) albeit F/Au ratios are slightly higher (for the more concentrated solutions). This is also demonstrated by the more important attenuation of Au4f peak than Pt4f. A close look at data of Tables 4, SM1 and SM2 revealed significant differences between F/S ratio of Pt, Au and glassy carbon electrodes cycled in various experimental conditions. Pt and glassy carbon electrodes cycled to positive potential values in the higher LiTFSI concentration solution show F/S ratios close to 3, as expected for pristine TFSI anions. On the other hand, F/S ratio is always much higher (> 10) for Pt, Au and glassy carbon electrodes cycled to only negative potential values sufficient to reach the onset of hydrogen evolution reaction. The presence of pristine TFSI anions, adsorbed at electrode surface, will give rise to F/S ratio of 3. That would require that the strength of interaction of anions with the electrode surface is strong enough to resist thorough rinsing, prior to XPS measurements. Nonetheless, it is not possible to distinguish with another mechanism that would involve adsorption of TFSI degradation products. On the other hand, F/S ratio higher than 10 of the surface layer clearly suggests that degradation of TFSI anions occurred (see below).

11

Discussion. Electrochemical processes are strongly influenced by chemical composition of the electrode/electrolyte interface. For example, hydrogen evolution reaction, which involves electrochemical formation of adsorbed hydrogen as initial step, is strongly influenced by the presence of adsorbed species or a surface layer. These two are related to the chemical composition of the electrolyte. It has been recently reported that a solid electrolyte interphase consisting in a mixture of LiF and Li2CO3 is formed at the surface of battery electrode materials by electrochemical reduction of TFSI anions of LITFSI electrolyte and O2/CO2 dissolved in the electrolyte, respectively [8,14]. Furthermore, density functional theory calculations predicted that electrochemical reduction of TFSI, more specifically Li2(TFSI)(H2O)x, should occur at 2.9 V vs. Li (about - 0.3 V vs. Ag) [8,35]. Our voltammetry data, more specifically when the potential sweep is initiated in negative direction, do not support this hypothesis. DFT calculations also revealed that free TFSI anions should be reduced at much lower potential (- 1.8 V vs. Ag or 1.4 V vs. Li).[35,36] The electrochemical behaviour of electrolyte containing TFSI anions has been investigated for both non-aqueous [36,38] and ionic liquid [31,39] as solvent and its role in the formation of a solid electrolyte interphase. A solid electrolyte interphase can be formed by electrochemical decomposition of TFSI anions to form species such as LiF, Li2S2O4, LiSO3CF3 and Li2NSO2CF3.[37,40–43] Interestingly, in ionic liquid containing LiTFSI, the cathodic peak observed at 1.4 V vs Li was attributed to electrochemical reduction of TFSI catalyzed by adsorption of Li+ [30]. Furthermore, electrochemical reduction of TFSI-based ionic liquid at various metal electrodes including Pt and Au was observed at - 2 V vs Fc/Fc+ (-1.53 V vs. Ag) that is more negative that the negative potential limit reached in our work with Pt and Au electrodes [35].

12

These observations, although not in aqueous electrolytes, seem to suggest that the cathodic wave found between - 0.3 and - 0.75 V in our study could be due to electrochemical reduction of TFSI anions is unlikely. On the other hand, potential cycling to negative potential values corresponding to onset of hydrogen evolution reaction allowed formation of a solid-electrolyteinterphase containing fluorinated species that could be formed by reaction of TFSI anions with hydroxyls species generated during electrochemical reduction of water to form hydrogen[26]. Moreover, XPS data of Table 4 revealed that the sulfur content of this solid-electrolyteinterphase is smaller than the stoichiometric ratio of S and F of TFSI anions. It has been proposed that electrochemical reduction/degradation of TFSI anions could occur by cleavage of .

the S-N bond and lead to two species, a radical ( NSO2CF3-) and a new anion (SO2CF3-) that still have a F/S ratio of 3.[35] These initial degradation fragments can undergo further chemical reduction by cleavage of their S-C bond.[35] Here, we propose that species generated during hydrogen evolution can react with the aforementioned degradation fragments to yield final degradation products. The low sulfur content (with respect to fluorine) of the surface layer can be possibly explained by the higher solubility of sulfur-based species (eg. Li2S2O4) in water than fluorinated compounds. Voltammograms recorded for Pt, Au and glassy carbon electrodes were analyzed further by comparison of limiting currents observed after the second reduction wave prior to onset of hydrogen evolution. Tables 1, 2 and 3 report these values for LiTFSI solutions with Pt, Au and glassy carbon electrodes, respectively. The limiting current observed at about - 0.9 V, for voltammograms recorded by starting potential cycling in positive potential direction, increases when LiTFSI concentration is raised from 1 to 21 m. The peak current intensity of the second

13

cathodic peak observed between - 0.3 and - 0.75 V depends also on LiTFSI concentration (see Figure 1) and follows the same trend. Gold and glassy carbon electrodes showed similar electrochemical behaviour (Figures 2, 3, Tables 2, 3). These observations are in sharp contrast with results obtained for a stainless steel electrode for which a lower current was recorded in presence of the more concentrated (21 m) solution.[8,27] In the case of stainless steel electrode, the second cathodic wave was related to reduction of corrosion products formed during positive potential excursion.[27] In contrast to stainless steel that can corrode upon polarization at high positive potential, platinum and gold undergo surface oxidation to form corresponding surface metal oxides that can be electrochemically reduced between 0 and 0.5 V (first reduction wave) on the reverse scan (Figures 1 and 2) (vide supra). Thus, we hypothesized that the second reduction wave observed between - 0.3 and - 0.75 V could be due to pseudocapacitance related to electrochemically induced adsorption of TFSI anions or TFSI anions degradation products that is occurring at high positive potential or to reduction of species formed during this positive potential excursion [25,31]. This is currently investigated in our laboratory and results will be reported in due course.

4. CONCLUSION The electrochemical characterization of Pt, Au and glassy carbon electrodes in aqueous LiTFSI solutions with concentration ranging from 1 to 21 m and involving potential cycling up to onset of oxygen evolution reaction revealed, following sweep reversal, a cathodic wave for potential more negative than about 0 V that could not be related to direct electrochemical reduction of TFSI anions. This is suggested by the absence of a well-defined redox wave between 0 and onset

14

of hydrogen evolution when potential cycling was initiated in negative potential direction. This behaviour was found to be similar to that of stainless steel electrode.[27] XPS measurements revealed the formation of surface layer (eg. solid-electrolyte-interphase) upon cycling to potential limits reaching either hydrogen or oxygen evolution reactions. When the electrode potential was cycled to positive values corresponding to onset of oxygen evolution, the composition of the surface layer on Pt and glassy carbon electrode suggest the presence of TFSIderived species. In contrast, potential cycling by using an initial potential of 0 V and only in negative potential direction resulted in surface layer with a high fluorine/sulfur ratio (higher than 3) that could be explained by chemical degradation of TFSI by species generated during hydrogen evolution. Presumably, chemical degradation of TFSI yields sulfur compounds that are more soluble in water than fluorine-based species. Finally, it is worth noting that some caution must be exercised when analyzing XPS data owing to uncertainty of the effect of washing on the species present at electrode surfaces. More specifically, species forming the solid-electrolyteinterphase could be dissolved during washing. Dissolution of solid-electrolyte-interphase components can also occur for electrode left in contact with the electrolyte.[44] As final remark, it is important to discuss the relevance of results obtained with Pt, Au and glassy carbon for application of water-in-salt electrolytes in electrochemical energy storage systems. It has been stated in the literature that formation of a solid-electrolyte-interphase before the onset of hydrogen evolution enabled a shift of the negative potential limit of an anode to more negative values.[8,9,18,44] Our results [27], those of this work and of Grimaud and coworkers[26] do not seem to support this claim because a solid-electrolyte-interphase is formed only upon hydrogen evolution. On the other hand, in the case of battery anode, it may

15

be possible that reduced species of the anode chemically degrade TFSI anions to form a solidelectrolyte-interphase. It will be worth investigating if this can occur and demonstrate that much work is needed to better understand the complex behaviour of electrode materials in superconcentrated electrolytes.

5. ACKNOWLEDGMENTS The financial contribution of the Natural Sciences and Engineering Research Council of Canada (Discovery Grant to DB) is acknowledged. DB was also partially supported by the “Chaire de recherche stratégique sur les nouveaux matériaux pour les technologies de l’énergie” from UQAM (2016-2018).

16

FIGURE CAPTIONS Figure 1. (A) Voltammograms of Pt electrode from 0 V to a positive potential limit and to a negative potential limit and (B) Linear sweep voltammogram from 0 V to a negative potential limit, in 1 m LiTFSI (red), 5 m LiTFSI (blue) and 21 m LiTFSI (black) and their respective magnifications at a scan rate of 10 mV/s.

Figure 2. (A) Voltammograms of Au electrode from 0 V to a positive potential limit and to a negative potential limit and (B) Linear sweep voltammogram from 0 V to a negative potential limit, in 1 m LiTFSI (red), 5 m LiTFSI (blue) and 21 m LiTFSI (black) and their respective magnifications at a scan rate of 10 mV/s.

Figure 3. (A) Voltammograms of glassy carbon electrode from 0 V to a positive potential limit and to a negative potential limit and (B) Linear sweep voltammogram from 0 V to a negative potential limit, in 1 m LiTFSI (red), 5 m LiTFSI (blue) and 21 m LiTFSI (black) and their respective magnifications at a scan rate of 10 mV/s.

Figure 4. XPS survey spectra of: (A) bare platinum electrode and in: (B) 1 m, (C) 5 m and (D) 21 m aqueous LiTFSI solutions following one sweep from an initial potential of 0 to 1.8 V and reverse sweep to - 1.8 V (spectra PN), one potential sweep from 0 to 1.8 V (spectra P) and one potential sweep between 0 and - 1.8 V (spectra N).

17

Figure 1

18

Figure 2

19

Figure 3

20

Figure 4

21

Table 1. Current density (I) and potential (E) values of cathodic peaks from voltammogram of platinum electrodes at 10 mV.s-1. Pt

1 m LiTFSI

5 m LiTFSI

21 m LiTFSI

E

I

Q

E

I

Q

E

I

Q

V vs. Ag

µA.cm-²

mC.cm-²

V vs. Ag

µA.cm-²

mC.cm-²

V vs. Ag

µA.cm

Cathodic peak 1

0

85

2.0

0.05

179

3.1

0.16

186

2.7

Cathodic peak 2

-0.72

93

-

-0.59

118

-

-0.46

110

-

-2

mC.cm-²

Table 2. Current density (I) and potential (E) values of cathodic peaks from voltammogram of gold electrode at 10 mV.s-1. Au

1 m LiTFSI

Cathodic peak 1 Cathodic peak 2

5 m LiTFSI

21 m LiTFSI

E V vs. Ag 0.5

I µA.cm-2

Q mC.cm-²

E V vs. Ag

I µA.cm-2

Q mC.cm-²

I µA.cm-2

Q mC.cm-²

1.5

E V vs. Ag 0.69

23

0.66

0.55

95

95

1.5

-0.08

170

-

-0.34

70

-

-0.38

47

-

Table 3. Current density (I) and potential (E) values of the cathodic peaks from voltammogram of glassy carbon electrodes at 10 mV.s-1. GC

Cathodic peak 1 Cathodic peak 2

1 m LiTFSI

5 m LiTFSI

E

I

V vs. Ag

µA.cm

- 0.82

42

-2

21 m LiTFSI

E

I

V vs. Ag

µA.cm

- 0.64

54

- 1.01

49

-2

E

I

V vs. Ag

µA.cm

- 0.72

65

-2

22

Table 4. Surface atomic concentration (at. %) of platinum, sulfur, carbon, nitrogen, oxygen and fluorine together with F/Pt and F/s ratios determined from XPS survey spectra of Pt electrodes.

Element Binding Energy, eV Pt 4f 72.6 S 2p 168.9 C 1s 284.8 N 1s 399 O 1s 531.8 F 1s 688.2 F/Pt ratio F/S ratio

Bare Pt

1 m LiTFSI P PN

N

21.1

15.1

27.7

5 m LiTFSI P PN

N 15

4.8

N

21 m LiTFSI P PN

20.2

14.8

17.4

15

20.7

12.3

1.9

3.7

4.6

1.2

3.9

5.3

54.7

59.7

14.9

50.7

22

32.9

25.1

41

20.3

32.2

2.7

3.4

5.3

3.9

2.8

2.1

3.3

2.7

3.4

3.3

19.6

16.2

32.2

19.5

38.8

32.3

34.3

26

34.2

31.6

3.7 0.24

13.5 0.49 2.8 -

7.8 0.52

13.3 0.66 7

11.2 0.76 3

14.9 0.86 3.2

12 0.8 10

17 0.82 4.4

13.7 1.11 2.6

-

23

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