Spectroscopic Investigation for Oxygen Reduction and Evolution

Subscriber access provided by UNIV OF ARIZONA

Article

Spectroscopic Investigation for Oxygen Reduction and Evolution Reactions with TTF as a Redox Mediator in Li-O Battery 2

Yu Qiao, and Shen Ye J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11692 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Spectroscopic Investigation for Oxygen Reduction and Evolution Reactions with TTF as a Redox Mediator in Li-O2 Battery *

Yu Qiao and Shen Ye

Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan Abstract To develop a lithium-oxygen (Li-O2) battery with a high specific energy, the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been systematically investigated on gold and porous carbon

electrodes in a DMSO-based electrolyte solution containing

trathiafulvalene (TTF) as a redox mediator by using in situ UV-Vis absorption spectroscopy and surface-enhanced Raman vibrational spectroscopy (SERS) in combination with ex situ infrared and Raman spectroscopies. Our results demonstrate that TTF definitely reduces the overpotential for the OER process and restrains the decomposition of the carbon electrode and solvent during the OER while the functionality of the redox mediator can change with the electrode materials and morphologies. No free TTF+ was observed in solution during a round-trip galvanostatic ORR/OER cycle on the gold electrode surface. The electrochemically generated TTF+ was mainly consumed by the oxidative decomposition of lithium superoxide (LiO2) in solution but not lithium peroxide (Li2O2) on the electrode surface formed during the ORR, different from that previously proposed. On the other hand, Li2O2 and Li2CO3 were observed during the ORR on the porous carbon electrode surface both in the TTF-free and TTF-containing solutions. The TTF+ can mediate the oxidation of the Li2O2 during the OER. Furthermore, the accumulation of free TTF+ was observed in solution during a round-trip galvanostatic ORR/OER cycle on the porous carbon electrode. The amount of the excess TTF+ in solution well corresponds to that of the by-product of Li2CO3 formed on the carbon electrode surface. TTF is unlikely to work as an ideal redox mediator 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

at the porous carbon electrode as expected. A better understanding for the mechanism and surface reactions for the ORR/OER cycle on these electrodes has been achieved by the present study.

Introduction The lithium-oxygen (Li-O2) battery is attracting much research interest due to its higher specific energy compared with the conventional Li-ion batteries and is regarded as one of the most promising power sources for electric vehicles (EVs).1-5 However, a number of barriers, such as poor cycle life, low reversible capacity and high charge overpotential, significantly hinder its practical application.6-9 For example, the charging voltage for the battery may increase as high as 4.5 V, which induces a low efficiency and oxidative decomposition of organic solvents and carbon-based electrode.10-14 Many by-products including lithium carbonate (Li2CO3) and lithium carboxylates are generated by the oxygen reduction process (ORR, i.e., discharge process) / oxygen reduction process (OER, i.e., charge process) cycle and deposited on the electrode surface that block the reactive sites, leading to a gradual capacity fade.11, 14-15 As a result, a higher overpotential for the OER is necessary to decompose these surface species. A significant number of cathode catalysts has been developed to reduce the OER overpotential,16 such as carbon nanotubes/nanofibers,17-18 metal oxides,19-20 noble metal nanoparticles,21-22 perovskites,23 and pyrochlore.24 The apparent overpotential for the OER can be reduced to as low as 4.0 V but still too high for practical applications. The accumulation of by-products still occurs on the electrode surface.10, 25 Due to the insulating property of lithium peroxide (Li2O2), which can make a high barrier for the charge transfer with the electrode surface,7, 26-30 a solid-phase catalyst cathode seems to be difficult to fully solve the problem. One expects to solve the charge transfer problem by using homogenous catalysts as a mobile charge carrier. Recently, redox mediators soluble in the 2


Page 2 of 58

Page 3 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrolyte solution have been applied for this purpose.31-36 By utilizing the redox

process

of

tetrathiafulvalene

(TTF),

Bruce

and

co-workers

demonstrated that the OER potential on a nano porous gold (NPG) electrode dropped from 4.1 V to 3.4 V at a low polarization current. As a result, the round-trip efficiency was significantly improved and the possible cycle number has been extended to as long as 100. They proposed that 1e–-oxidized TTF, TTF+, oxidizes the Li2O2 in the solid-state at a lower OER potential and TTF is regenerated as a redox mediator.31 Inspired by Bruce’s studies, a number of redox mediators, such as iron phthalocyanine (FePc),32 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO),33 and iodide (I–),34-36 have been investigated for the OER in the Li-O2 batteries. These redox mediators seem to be able to facilitate the oxidation of Li2O2 and therefore reduce the OER overpotential during the first several cycles. However, the stability and cycling reversibility are still not good enough. For example, although the activity of the mediator showed a good stability on the gold electrode surface,31 the capacity on the porous carbon electrode suffered from a serious loss and the OER potential significantly increased only after several cycles on the porous carbon catalysts.33 On the other hand, the reaction mechanisms for the redox mediators during the ORR/OER processes have not been quantitatively analyzed which also hinders the development of a novel redox mediator. Recently, we demonstrated that not only Li2O2 was formed on a gold cathode surface during the ORR, a large amount of lithium superoxide (LiO2) was also produced in a DMSO-based solution based on in situ UV-Vis absorption spectroscopy and surface enhanced Raman spectroscopy (SERS) measurements.37 We found that the reaction yields for Li2O2 and LiO2 can be significantly affected by the surface morphologies of the electrode. In the present study, we further systematically investigated the roles and reaction mechanisms of TTF in the ORR/OER on different kinds of electrode materials (gold and porous carbon). First of all, we evaluated the basic features of TTF 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

on a gold electrode surface in a DMSO-based electrolyte solution and found that the concentration of TTF+ in solution can be quantitatively determined by an in situ UV-Vis absorption measurement. We further investigated the ORR/OER on a gold electrode and a porous carbon electrode. To independently determine the products during the OER, a porous carbon catalyst mixed with the desired amount of Li2O2 was prepared and evaluated during the direct OER process. A number of new findings has been obtained based on the experimental observations and analyses on the gold and porous carbon electrodes in a DMSO-based electrolyte solution with TTF: (1) TTF does not show any apparent influence on the ORR, but largely reduces the overpotential for the OER with quite different features on the two electrodes. (2) Free TTF+ was not observed in the solution during a round-trip galvanostatic ORR/OER cycle on the gold electrode surface where the electrochemically-generated TTF+ was mainly consumed by the oxidative decomposition of LiO2 but does not play a similar role for Li2O2. (3) Not only Li2O2 but also Li2CO3 were formed during the ORR on the porous carbon electrode surface either in the TTF-free or TTF-containing solutions. TTF+ was accumulated in solution during a round-trip galvanostatic ORR/OER cycle on the porous carbon electrode. The amount of the excess TTF+ in solution well corresponds to that of by-product of Li2CO3 formed on the carbon electrode surface. Although TTF restrains the decomposition of the carbon electrode and solvent by reducing the OER overpotential, the by-products (such as Li2CO3) are still formed during the ORR which can significantly affect the reversible capacity of the Li-O2 battery.

Experimental Section Materials: TTF was purchased from Tokyo Chemical Industry Co., Inc. The typical concentration of TTF in the electrolyte used in the study was 10 mM. All other chemicals were purchased from Kanto Chemical Co., Inc. (Tokyo, 4


Page 4 of 58

Page 5 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Japan) unless otherwise mentioned. DMSO of electrochemical grade was dried in freshly activated molecular sieves (types 3 Å and 4 Å) for several days

before

use.

Battery

grade

tetra-n-butylammonium

perchlorate

(TBAClO4) and lithium perchlorate (LiClO4) were used for preparing the electrolytes. The typical concentration of the electrolyte was 0.5 M. Ketjenblack EC600JD (BET 1400 m2/g, Lion Co., Inc.) and lithium iron phosphate (LiFePO4, Clariant Co., Inc., Canada) were dried at 120 °C for 12h under vacuum before the electrode preparation procedures. The excess water in commercially available Li2O2 (Sigma-Aldrich Co., Inc.) and Li2CO3 powder (Tokyo Chemical Industry Co., Inc.) were remove by further drying at 100 °C for 12 h under vacuum. Electrodes Preparation: The flat gold cathode was prepared by a sputtering method onto a stainless steel substrate (6.0 mm in diameter) or a slide glass substrate (10 mm×10 mm) using an auto fine coater (JFC-1300, JEOL) in an Ar-plasma environment (10 Pa, 40 mA current) for 300 s. The thickness of the thin gold film was approximately 300 nm. The surface of the sputtered thin gold film was rough with many small islands of approximately 50 nm dimensions. The special surface morphology on a sputtered gold electrode can provide a strong surface enhancement effect for the Raman measurement, which is very helpful for obtaining surface information as reported in our previous paper.37 The gold electrode was used in subsequent experiments without further treatment. The porous carbon cathodes were fabricated onto a carbon paper using a slurry of Ketjenblack. The slurry of Ketjenblack was prepared using a polyvinylidene fluoride (PVDF) binder and N-2-methyl pyrrolidone (NMP) dispersing agent in the ratio of 20∶1∶308 (weight) with stirring for 3 min at 1000 rpm and then uniformly coated onto a carbon paper current collector. The porous carbon electrode with preloaded Li2O2 was prepared by mixing Li2O2 powder into the former Ketjenblack-based slurry in which the amount of Li2O2 was adjusted to have a capacity of 400 µAh/cm2. The electrode was 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dried in a vacuum at 110 °C for 15 h and then uniformly cut into a square (typically 10 mm × 10 mm). It was then transferred to an argon-filled glove box (Labmaster MB10-C, MBraun) after vacuum dried to avoid exposure to the lab air. The LiFePO4-based electrode was prepared by mixing LiFePO4 with Ketjenblack and PVDF in the ratio of 8:1:1 (weight), then coated on a nickel mesh. When LiFePO4 was used as the reference electrode, it was first pre-oxidized (ca. 1.0 mAh) in a 0.5 M LiClO4-DMSO solution. To keep the equilibrium potential for the LiFePO4/LixFePO4 (3.45 V vs. Li/Li+) stable as a reference electrode, the electrode was further covered by a porous glass microfiber paper (No. 1820-021, Whatman).38 To avoid the reaction of TTF with the Li metal, LiFePO4 and the pre-oxidized LiFePO4 electrode were used as the counter and reference electrode, respectively. All the potentials in this paper are referred to Li/Li+. All of the electrodes were transferred into a glove box after vacuum drying to avoid exposure to the open air. To characterize the electrode surface by ex situ analyses after the cycle, the cycled electrodes were first washed 3 times with boiling dimethyl carbonate (DMC) in the glove box and then dried in a vacuum drying oven for 4h to totally volatilize the DMC at 55 °C. Electrochemical Measurements: All the electrochemical and spectroscopic experiments were carried out under the control of a potentiostat (Potentiostat /Galvanostat 2020, TOHO Technical Research) and a function generator (Function Generator 2230, TOHO Technical Research) at room temperature. The current and potential outputs from the potentiostat were recorded by a multifunction data acquisition module (USB-6211, National Instruments) controlled by LabVIEW. Typically, cyclic voltammograms (CVs) were recorded at the scan rate of 10 mV/s in the range between 2.0 V and 4.2 V from the open circuit potential (OCP) otherwise noted. Galvanostatic measurements were recorded at the current density of 100 µA/cm2. The OCP was typically 3.2 V in this study. 6


Page 6 of 58

Page 7 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In situ SERS and ex situ Raman Measurements: The Raman spectra were recorded using a Renishaw inVia microscope. As described in our previous paper, a homemade cell has been optimized for the in situ Raman observations.37 Since SERS does not work on the porous carbon electrode, only ex situ Raman measurements were carried out. To obtain a better S/N ratio, the excitation light (He-Cd laser at 785 nm) was focused on the electrode surface through a 50× objective and the acquisition time was extended to 12 s. The spectral resolution of the Raman spectra in the study was ca. 1.0 cm–1. Infrared Measurements: A combined mid-IR/far-IR spectrometer, i.e., Perkin-Elmer Frontier, was also employed for product characterization on the electrode surface, especially in the mid-IR frequency region (500-4000 cm–1). The FTIR measurement was carried out using a GladiATR accessary with a diamond crystal (Pike Technology) purged with dry air. During testing, the ATR accessary was kept in a relatively independent atmosphere separated by open air and continuously purged with dry air. Typically, 64 interferograms were accumulated for one spectrum with a resolution of 1.0 cm–1. In situ UV-Vis Measurements: The in situ UV-vis bulk electrolysis cell (Figure S1b) was modified from a commercial UV-vis cell (10 mm optical path length) with a three-electrode configuration which does not block the light path, similar to that used in our previous studies.37 Typically, 3.0 mL of the electrolyte solution was added to the UV-Vis cell, and the geometric area of the electrode was 1.0 cm2. The electrolyte solution was purged in purified Ar or O2 (Hokkaido Air & Water) for 30 min before each experiment. The detail has been described in our previous paper used for the flat gold electrode.37 Due to the high specific area (nearly 1.4 m2/mg) of the porous carbon electrode used in the study, O2 dissolved in the DMSO-based electrolyte solution (merely 2.0 mM /cm3) is not sufficient to provide its electrode reaction (see below). To solve the problem, a gas diffusion electrode was designed by covering a porous carbon electrode surface with a PTFE film (pore size 100 nm, Toyo Roshi Kaisha, Ltd.) and O2 gas was continuously 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 58

blown towards the electrode surface at the flow rate of nearly 10 mL/min (Figure S1). The UV/Vis absorption spectra were recorded by a UV-Vis spectrometer (Lambda

650,

Perkin-Elmer)

with

double

monochromators

and

a

photomultiplier detector. The UV/Vis measurements were carried out to detect the superoxide species (based on its 1πu → 1πg transition around 250 nm)39-41 and TTF cation radical (based on its π → π* transition around 437 nm and 583 nm).42-43 See the text below for details. In order to dissolve the all the TTF+ in solution including those trapped in the porous carbon electrode, the cycled porous carbon cathode was directly under the ultrasonic irradiation (15 s) in the same cell after the counter and reference electrodes were removed from the UV-Vis cell. After the solution was centrifuged (8000 rpm, 3 min), a certain amount of the transparent solution on the upper layer in the cell was used for further UV-Vis characterizations.

Results and Discussion 1. General Features of TTF The general electrochemical behaviors and spectroscopic features of TTF were first characterized in a DMSO-based electrolyte solution. Figure 1a shows a CV (10 mV/s, 2.0 - 4.2 V) of a sputtered gold electrode in an

8


Page 9 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ar-saturated 0.5 M LiClO4-DMSO solution containing 10 mM TTF. Two pairs of redox peaks were observed at 3.81/3.68 V and 4.01 V/3.89 V, attributed to the redox reaction between TTF+/TTF and TTF2+/TTF+, respectively.44 Both the anodic and cathodic peak currents are proportional to the square root of the scan rate, indicating that both of the redox processes in the O2-free DMSO solution are diffusion-controlled processes (Fig. S2).31 The anodic charge passed in the positive-going sweep is 78 mC/cm2, while the cathodic charge passed is only –45 mC/cm2. The cathodic charge is only 58% of the anodic one, indicating a lower Coulombic efficiency. The TTF+ and TTF2+ species generated in the positive-going sweep diffuse into the bulk solution, while those in the bulk solution are unable to be fully reduced in the subsequent negative-going sweep. This should be attributed to the low concentration gradient of the TTF+ and TTF2+ in the negative-going sweep. As the scan rate increases (decreases), a lower (more) anodic/anodic charge passes, resulting in a lower (higher) Coulombic efficiency (see Table S1 in Supporting Information). Figure 1b shows a potential profile during a galvanostatic oxidation process to a capacity of 10 µAh/cm2 (i.e., 36 mC/cm2) at the current density of 100 µA/cm2. A potential plateau appears at 3.67 V, corresponding to the first anodic peak of TTF observed in the CV (Fig. 1a). Furthermore, an in situ UV-Vis absorption measurement was used to determine the concentration profiles for TTF and TTF+ during the redox

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

process. As shown in Fig. 2, TTF itself shows an intense absorption peak at 451 nm (black trace), which can be assigned to the π→π* transition in TTF.42-43 The absorption coefficient for TTF at λ= 451 nm was estimated to be 288 L/mol (see Fig. S3). When an anodic galvanostatic polarization at 100 µA/cm2 was applied to the gold electrode in the in situ UV-Vis cell, two broad absorption bands appeared at 435 and 579 nm (blue trace, Fig. 2). These peaks can be assigned to the π → π* transitions of the TTF+ species in the solution.42-43 A good linear relationship was found between the peak intensity at 579 nm and the anodic charges passed. The absorption coefficient for TTF+ at λ= 579 nm was estimated to be 235 L/mol (see Fig. S3). By using this value, we are able to estimate the concentration of TTF+ in the solution during the reaction process.

2. ORR/OER Processes on a Flat Gold Electrode. 2.1 Electrochemical Behaviors. Figure 3a shows the CVs (10 mV/s) of a gold electrode in O2-saturated 0.5 M LiClO4-DMSO solution with 0 (blue trace) and 10 mM TTF (red trace). The cathodic and anodic currents in the CV observed in LiClO4-DMSO saturated by O2 should be attributed to the ORR (LiO2 and Li2O2 formation) and OER (Li2O2 and LiO2 oxidation), respectively.37

10


Page 10 of 58

Page 11 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The addition of TTF only affects the current profiles in the OER (Fig. 3a). By adding TTF to the solution (red trace), the current profile of the OER in the potential region of the first anodic peak (3.39 V) was almost identical to that of the TTF-free solution (blue trace) but largely increases as the potential became more positive. The CV (red trace) is quite similar to that observed in the Ar-saturated TTF-contained solution (black dash trace) except for a small anodic shoulder (3.45 ~ 3.70 V, marked by the green shadow) and slightly higher anodic peak currents were observed. As reported in our previous paper, in addition to Li2O2 deposited on the gold electrode surface, a large amount of LiO2 is simultaneously produced in the DMSO solution during the ORR. For example, the partial yields for LiO2 (Li2O2) during the potential sweep at 10 mV/s and 2 mV/s, were found as high as 73 % (27 %) and 86% (14%), respectively.37 In the positive-going sweep during the OER, the Li2O2 started to be oxidized during the first anodic peak. By considering the equilibrium potential for these reactions, (1) O2 + e– ⇄ O2–

E0 = 2.65V

Li+ + O2– ⇄ LiO2 (2) O2 + 2Li+ + 2e– ⇄ Li2O2

E0 = 2.96 V

(3) TTF+ + e– ⇄ TTF

E0 = 3.66 V

(4) TTF2+ + e– ⇄ TTF+

E0 = 4.05 V

One expects that TTF+ or TTF2+ can thermodynamically oxidize LiO2 (or O2–) and Li2O2 while these Li oxides cannot oxidize TTF.44 However, as shown in Fig. 3a, no difference was observed in the potential region of the first anodic peak with TTF addition, indicating that TTF may not be strongly involved in the oxidation of Li2O2 which mainly occurs during the first anodic peak.37 This should be attributed to the fact that the potential is too low to generate TTF+ However, as the potential becomes higher, TTF+ cations start to be electrochemically generated, either electrochemical oxidation or TTF+-induced chemical oxidations of Li2O2 deposited on the electrode surface and LiO2 dissolved in the solution may occur. This should contribute to the increase of 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the anodic current in the current shoulder (3.45 ~ 3.70 V). As the potential further increases, the generation rate of TTF+ or TTF2+ is higher, and the chemical oxidation rate of LiO2 by TTF+ or TTF2+ in solution will be even faster. However, it is hard to make a conclusive discussion only by the potential sweep as Fig. 3a. Figure 3b shows galvanostatic ORR/OER cycles on a flat gold electrode in an O2-saturated 0.5 M LiClO4-DMSO electrolyte solution (with and w/o TTF) at a polarization current of 100 µA/cm2. By using galvanostatic control, one is able to control the reaction rate (i.e., current) and reaction amount (i.e., charge) during the ORR/OER processes under the desired potential regulation. The galvanostatic control will be mainly used in the results reported later in the study. During this measurement, the ORR and OER capacities in the cycle were exactly controlled to 10 µAh/cm2 on the gold electrode (i.e., -100 µA/cm2 and +100 µA/cm2, respectively, for a period of 360 s) with an upper potential limit of 4.0V. As shown by the black trace in Fig. 3b, during the ORR stage, the electrode potential quickly decreases from the OCP (3.2 V) to 2.6 V and remains almost constant until the target capacity (10 µAh/cm2, point A) is reached. No difference is observed in the ORR with the addition of TTF, similar to that of the CV measurements (Fig. 3a). As the polarization is switched to the OER, more complicated potential profiles are observed which significantly depend on the TTF in the solution. In a TTF-free solution (blue trace, Fig. 3b), the potential jumps to 2.7 V and slowly increases to 2.8 V with a narrow plateau with a capacity of ca. 1 µAh/cm2 (point B) and then monotonically increases to 4.0 V (point C, upper potential limit). The total OER capacity from point A to C is only 3.2 µAh/cm2, much lower than the ORR capacity (10 µAh/cm2), indicating an irreversible ORR/OER cycle. The OER capacity contains oxidation of the Li2O2 formed on the electrode surface as well as the LiO2 in the solution. Due to the diffusion rate of the LiO2 in the bulk solution to the electrode surface being lower than the surface reaction rate (i.e., 100 µA/cm2), 12


Page 12 of 58

Page 13 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the over-potential rapidly increases and the OER soon terminates at the upper potential limit (4.0 V).37 As TTF is added to the solution, the potential profile (red trace, Fig. 3b) in the first stage (point A to B2) is almost identical in comparison to the TTF-free solution (blue trace, Fig. 3b), but significantly changes in the second stage (from point B2 to C2). The potential increase becomes very slow from ca. 3.45 V (point B2) and finally reaches a plateau at 3.67 V (point C2). This is consistent with the current shoulder around 3.45 ~ 3.70 V observed in the potential sweep (shaded area in Fig. 3a). By adding a 10 mM TTF solution, the same ORR capacity as that of the ORR with a final potential of 3.67 V has been apparently reached, indicating that a reversible capacity is realized in which TTF+ plays important roles to mediate the oxidation of LiO2 species dissolved in solution. To further distinguish the potential profiles and the role of TTF during the OER, an additional experiment was carried out as described. After the ORR capacity (10 µAh/cm2) in the O2-saturated LiClO4-DMSO with 10 mM TTF is reached, the potential was switched to the OCP and the electrolyte solution in the cell was replaced by the fresh solution and then polarized in the OER under the same condition (green trace, Fig. 3b). By the solution replacement, it is expected that the soluble products and intermediates generated in the DMSO during the ORR (especially LiO2) can be removed from the cell, while the species deposited on the electrode surface (Li2O2) should remain the same.37 As shown by the green trace in Fig. 3b, after a narrow plateau around 3.2 V (with a capacity of ca. 1 µAh/cm2, point B3), the potential quickly increases to 3.67 V as a new plateau. As discussed above, the narrow plateau around 3.2 V can be attributed to the oxidation of Li2O2 on the electrode surface, which has a lower equilibrium potential compared to that of TTF+. In the second plateau, only the electrochemical oxidation of TTF occurs, but the TTF+ generated here has no partner (i.e., LiO2 or Li2O2) to oxidize and remain in solution. 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

One may ask why the initial oxidation potential plateau changes before (ca. 2.7 V) and after the solution replacement (ca. 3.2 V) in Fig. 3b. In fact, the potential plateau around 2.7 V should be attributed to the oxidation of the LiO2 accumulated on the electrode surface by the galvanostatic ORR process at a reaction rate of –100 µA/cm2. As the LiO2 near to the electrode surface is used up, the electrode potential quickly increases since the diffusion rate of the LiO2 from bulk solution is too low to meet with the galvanostatic OER rate (+100 µA/cm2). When the electrolyte solution after the ORR is replaced by a fresh one, such potential plateau disappears and a potential plateau due to oxidation of Li2O2 deposited on the Au electrode surface appears around 3.2 V. This can be further confirmed by the analysis towards the in situ SERS results shown in the following part. In short summary, a full reversible capacity for the ORR/OER cycle on a gold electrode can be realized by adding TTF to the DMSO-based electrolyte solution. On the other hand, only a 31.5% capacity can be realized in a TTF-free DMSO-based solution (upper potential limit of 4.0 V). We initially consider that The TTF+ helps the chemical oxidation of LiO2 in solution, but not Li2O2 on the gold electrode surface based on the present electrochemical measurement. However, several questions have not yet been critically answered. (1) It is still hard to exactly answer whether TTF+ is involved in the oxidation of Li2O2 on the gold electrode surface before one can directly measure Li2O2 on the electrode surface with/without TTF in solution; (2) There is no direct information about the concentration for TTF+ during the reaction, thus its role in the ORR/OER cycle is not clear. (3) It has not been confirmed that the electrochemically generated TTF+ species fully reacts with the LiO2 and/or Li2O2 as a redox mediator? (4) What is the exact action objective of TTF/TTF+ as redox mediator during the OER process, Li2O2 or LiO2 or both of them? (5) Does the role of TTF change with the electrode materials? The in situ spectroscopic investigation will help us to quantitatively answer these questions in the following sections. 14


Page 14 of 58

Page 15 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2 In situ UV-Vis Characterization. In the present study, in situ UV-Vis absorption spectroscopy was employed to probe the reaction products and intermediates in solution during the ORR/OER cycle. Figure 4a shows the in situ UV-Vis absorption spectra during galvanostatic polarization in an O2-saturated 0.5 M LiClO4-DMSO solution on a gold electrode, similar to that in Fig. 3b. After an ORR with a capacity of 10 µAh/cm2, an intense absorption peak is observed at 252 nm (black solid trace), while nothing can be observed at the OCP (black dot trace). As previously reported, this peak should be assigned to the 1πu → 1πg transition of the superoxide (LiO2 or O2–) in the solution.39-41 Based on its absorption coefficient,39-41 the amount of superoxide was estimated to be 0.336 µmol/cm2 (n2, Table 1), which is ca. 90% of the ORR charge assuming a 1-e– reduction of O2 (n1, Table 1, 0.37 µmol/cm2). In a Li-free solution, the amount of superoxide determined by a UV-Vis measurement (n2, Table S2) was in excellent agreement with that estimated from the ORR charge (n1, Table S2). Part of the ORR charges in the Li-contained DMSO solution (i.e., n1 – n2, 0.038 µmol/cm2) was used to form Li2O2. The charge for the Li2O2 formation only accounts for approximately 10% of the total ORR charge here. The width of the potential plateau at 3.2 V of the galvanostatic OER process after the solution replacement (point A to B3, Fig. 3b) gives the similar amount of the Li2O2, 1 µAh/cm2 (i.e., 10% of the total ORR capacity). As shown in Fig. 3b, the potential quickly increases to 4.0 V during the OER although only 31.5% of the ORR capacity was used, implying that most of the ORR products have not been re-oxidized in the OER to 4.0 V. On the other hand, as shown by the blue trace in Fig. 4a, an intense peak at 252 nm can still be observed at 4.0 V. The peak height keeps ca. 68.5% of the original height soon after the ORR. In other words, only 31.5% of LiO2 in solution has been re-oxidized at 4.0 V based on the in situ UV-Vis observation, which is in excellent agreement with that obtained from the OER capacity (Fig. 3b). This 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

indicates that the quick potential increase during the OER should be related to the increase in the overpotential for oxidation of LiO2 in the DMSO solution, which is hard to diffuse back the electrode surface to meet the reaction rate (+100 µA/cm2). In order to understand the influence of TTF, in situ UV-Vis measurements (Figs. 4b and 4c) were carried out during the ORR/OER cycle on a gold electrode under similar conditions. As shown in Fig. 4b, no UV-Vis peak corresponding to the TTF+ species is observed after either the ORR (black trace) or the subsequent OER (red trace) during the ORR/OER cycle with is exactly the same amount of capacity. However, if we provide an additional overcharge in the OER (Fig. 4c) after a full ORR/OER cycle of 10 µAh/cm2, two peaks clearly appear in the UV-Vis spectra at 435 and 579 nm (blue trace, Fig. 4c), corresponding to those of TTF+ (Fig. 2), even for only a 10% overcharge in the OER (i.e., 1 µAh/cm2). These peaks increase with the continuous OER overcharge process (Fig. 4c). The amounts of TTF+ estimated by the UV-Vis measurement exactly equaled the overcharged capacities. Once the OER overcharge flows, free TTF+ is present in solution since no LiO2 species can be chemically oxidized after a full ORR/OER cycle. This confirms that TTF+ exactly reacts with all the LiO2 during the OER as a redox mediator, and neither the superoxide nor TTF+ is present in the solution at the end of a full ORR/OER cycle. Unfortunately, we were unable to directly observe LiO2 during the ORR in

16


Page 16 of 58

Page 17 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the DMSO-based electrolyte solution containing TTF like that in the TTF-free solution (Fig. 4a). This is mainly associated with the extremely strong absorption of TTF in the wavelength region for LiO2 (248 - 300 nm). We have to make our discussions on the reaction mechanisms based on the UV-Vis observations for TTF+. As a further examination, when we directly charged the electrode in the OER direction (i.e., direct OER without ORR), two intense peaks immediately appeared at 439 ad 579 nm in the UV-Vis spectrum (purple dash trace, Fig. 4b). Free TTF+ is present in solution since the reaction products and intermediates of the ORR are not available in the direct OER process and no TTF+ can be consumed. Furthermore, one can see from the Fig. 4b that the amount of TTF+ species after the direct OER process (purple dash trace) is slightly higher than that OER process in the replaced electrolyte solution (green trace). The small difference between the two UV-Vis spectra is not due to the experimental error but contains important messages. From the operation procedures for each spectrum, the difference should be attributed to the TTF+ species associated with the oxidation of Li2O2. Based on the absorbance difference at 579 nm, we are able to determine the amount of the TTF+ associated (n1–n2, 0.037 µmol/cm2), thus Li2O2 (0.0185 µmol/cm2) is estimated to be formed on the gold electrode surface. The amount of Li2O2 estimated here is in agreement with that observed in a TTF-free solution (0.019 µmol/cm2, the upper right, Table 1). However, we are unable to decide whether the Li2O2 is

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrochemically oxidized or chemically oxidized by the TTF+ species only from the UV-Vis observations since both processes can induce the same change in the UV-Vis spectra. This problem is expected to be solved by the following SERS measurement by which the Li2O2 can be directly detected.

3. In situ SERS Characterization. Figure 5 shows in situ SERS spectra (450 ~ 1100 cm–1) recorded on a sputtered gold electrode surface during the galvanostatic ORR/OER cycles similar to those in Fig. 3b. The SERS spectrum observed at the OCP in the LiClO4-DMSO electrolyte solution with 10 mM TTF (bottom trace, Fig. 5b) shows a number of strong peaks attributed to the vibrational modes of the solvent and electrolyte. For example, a pair of peaks at 666 and 698 cm–1 can be assigned to the CSC symmetric and asymmetric stretching mode of DMSO, respectively, while that at 1044 cm–1 is assigned to the S-O stretching mode of DMSO.37, 45-46 However, we are unable to see any SERS contributions from the TTF species at the OCP. As the ORR starts, a new SERS peak appears at 788 cm–1. This peak is undoubtedly assigned to the O-O stretching mode of Li2O2 deposited on the gold electrode surface.12 The identical SERS spectrum was also observed during the ORR and OER in either the TTF-free (Fig. 5a) or TTF-containing 18


Page 18 of 58

Page 19 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Fig. 5b) solutions. During the OER in the TTF-free solution, the SERS peak intensity for Li2O2 gradually decreases and completely disappears around the charge capacity of 2.5 µAh/cm2 (3.63 V, Figs. 5a and 3b). During the OER in the TTF-containing solution, Li2O2 suffers from a similar decrease and disappears as early as 2.5 µAh/cm2 (3.47 V, Figs. 5b and 3b). No SERS peak for TTF or TTF+ can be observed till the end of the OER process (3.67 V, Figs. 5b and 3b, point C2). As the electrolyte solution after the ORR was replaced by a fresh solution (green traces, Figs. 5c and 3b), the Li2O2 disappeared after the potential plateau of OER at 3.2 V (with a capacity of 1.0 µAh/cm2). Figure 6 compares the SERS intensities for Li2O2 (O-O stretch at 788 cm-1) and potential profiles for the different stages in the ORR/OER cycle shown in Fig. 5 as a function of the ORR/OER capacity. From these plots, one can quantitatively know the coverage of Li2O2 at a different capacity or potential. It is interesting to note that although the potential profiles during the galvanostatic OER are quite different in the TTF-free and TTF-containing solutions (Fig. 3b), the capacity dependent SERS profiles for Li2O2 during the OER (and ORR) observed in the two solutions almost overlap each other (Figs. 6a and 6b). No TTF dependence was observed for the formation and oxidation of Li2O2 on the gold electrode surface (see Fig. S9). Figure 6c shows 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the same process as in Fig. 6b except for the solution was replaced by the same fresh solution after the ORR (corresponding to Fig. 5c). The same dropping rate with the capacity was observed in a TTF-free solution under the same experimental conditions (Fig. S9). No dependence of TTF on the SERS peak for Li2O2 was also observed after the solution replacement. The decrease in the SERS peak intensity for Li2O2 with capacity (Fig. 6c) is faster that without replacement (Figs. 6b and 6c). This should be attributed to the fact that the oxidation reaction of LiO2 simultaneously with oxidation reaction of Li2O2 (see our detailed deconvolution results already published).37 As a summary, the present SERS analyses quantitatively confirms that the TTF+ is not involved in the oxidation of Li2O2 similar to that also expected from the previous electrochemical measurements (Fig. 3). When we further charge over the capacity (i.e., higher than 10µAh /cm2 ) in the OER process, a SERS peak at 745 cm–1 and a group of SERS peaks around 486 cm–1 immediately appear (Figs. 5b and 5c). These peaks are attributed to the adsorption of TTF+ on the gold electrode surface.31, 47 The adsorption of TTF+ on the gold electrode surface is observed as soon as the LiO2 in solution is fully consumed by TTF+. This timing is consistent with the in situ UV-Vis observations that free TTF+ species appears in solution after a full ORR/OER cycle on the gold electrode (Fig. 4c). The present results provides strong evidence that TTF+ only mediates the oxidation of LiO2 but is not involved in the chemical oxidation of Li2O2 on the gold electrode. This is different from that proposed by Bruce and co-workers who proposed that TTF works as a mediator for Li2O2 on the NPG electrode, but they did not mention anything about the reaction of LiO2 in the solution mainly based on the electrochemical characterizations.31 Furthermore, Li2CO3, which should give a band at 1087 cm–1, was not observed on the gold electrode surface during the ORR/OER in either the TTF-free or TTF-containing solutions. 20


Page 20 of 58

Page 21 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7 shows (a) potential profiles and (b) SERS intensity for Li2O2 during five ORR/OER cycles in the 0.5 M LiClO4/DMSO solution with 10 mM TTF (each capacity was controlled at 10 µAh/cm2). As shown in Fig. 7a, the OER potential is stable around 3.67 V during the cycles. When TTF is not included in the solution, the OER potential quickly increases to a potential higher than 4.0 V (black dash trace). At the same time, SERS measurements confirm the reversible formation of Li2O2 on the gold electrode surface after each ORR (Fig. 7b) under the same conditions. The reversible electrochemical and SERS behaviors can be repeated many cycles if the oxygen concentration in the solution can be maintained constant. These results demonstrate that TTF significantly improves the reversible capacity at a much lower OER potential on the gold electrode and plays important roles as a redox mediator.

3. ORR/OER Processes on Porous Carbon Electrode. 3.1 Electrochemical Behaviors After investigating the role of TTF as a redox mediator on the gold electrode, we focused our attention on the porous carbon electrode, which is widely used as a practical electrode material. Figure 8 shows the galvanostatic ORR/OER cycles on a porous carbon electrode in a 0.5 M LiClO4-DMSO solution with (red trace) and without the TTF additive (blue trace) in the order of 1st ORR → 1st OER → 2nd ORR. Both

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the ORR and OER capacities were controlled at 200 µAh/cm2, much higher than that of the gold electrode due to carbon’s large surface area with the three-phase interface features. It should be mentioned that the total ORR capacity of the porous carbon electrode before the potential decay (i.e., depth of discharge) was 2250 µAh/cm2. Only 10% of the total capacity was used in the present experiments (i.e., many reaction sites are still available after the ORR). TTF does not show any influence on the potential profiles in the 1st ORR but significantly affects that in the 1st OER, similar to those observed on the gold electrode. In the TTF-free solution, the OER potential quickly increases to 4.0 V and gradually reaches a plateau around 4.2 V (blue trace). In contrast, when the TTF is included in the solution (red trace), the OER potential increases to 3.3 V and gradually climbs to 3.7 V, approximately 0.5 V lower than that in the TTF-free solution (blue trace). As discussed above, the OER potential reduction should be attributed to the redox reaction of TTF+/TTF (Eq. 3), where the electrochemically-generated TTF+ can chemically oxidize Li2O2 and/or LiO2. The potential profiles in the 2nd ORR are different in the two solutions (Fig. 8). After the 1st OER, the potential sharply drops to a plateau at 2.7 V in the TTF-free electrolyte, while in the TTF-containing solution, the potential slowly decreases to the same plateau in the 2nd ORR. Therefore, TTF seems to be involved in the ORR/OER cycles on the carbon

22


Page 22 of 58

Page 23 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode. However, the chemical species both in solution and on the electrode

surface

are

still

unknown

only

by

the

electrochemical

characterization, thus the reaction mechanism mediated by TTF is also still not clear. We will further discuss these issues based on the Raman, infrared and UV-Vis measurements described below. As mentioned in the experimental section, due to the porous properties of the carbon materials, only ex situ observations were carried out.

3.2 Ex situ Raman Characterization. Figure 9a shows ex situ Raman spectra (600 - 1700 cm–1) recorded on the porous carbon electrode surfaces during the galvanostatic ORR/OER cycles at OCP (point A), end of the 1st ORR (point B), middle of the 1st OER (point C), end of the 1st OER (point D) as well as the beginning (with a capacity of 15 µAh/cm2, point E) and end (point F) of the 2nd ORR in the two solutions. The colors and notation of the Raman spectra (Fig. 9a) correspond to those of the electrochemical characterization (Fig. 8).As a comparison, standard Raman spectra for several commercially available materials, such as Li2O2, Li2CO3 and Ketjenblack, are also shown as references. The Raman spectrum at the OCP (point A) shows a pair of broad peaks at 1298 and 1591 cm–1. In comparison with the standard Raman spectra, the pair of broad peaks should be assigned to the D-band and G-band of the carbon from the Ketjenblack, respectively.48 The Raman peak intensities for the Dand G-bands are almost constant during the ORR/OER cycles and independent of the TTF addition. After the 1st ORR with a capacity of 200 µAh/cm2, two peaks are clearly observed at 784 cm–1 and 1087 cm–1 on the electrode surface in both the TTF-free (point B1) and TTF-containing solutions (point B2). No significant difference in the Raman spectra was observed between the two solutions soon after the ORR. In comparison with the Raman spectra for the standard materials, these two new peaks should be assigned to the O-O stretching 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mode of Li2O2 and CO3 symmetric stretching mode of Li2CO3,49 respectively, indicating that not only Li2O2, but Li2CO3 was also produced on the porous carbon electrode surface after the ORR in the two solutions. As mentioned before, Li2CO3 was not observed on the gold electrode surface during the ORR/OER cycle (Fig. 5). As the electrode is forced to the OER, differences in the Raman spectra were observed between the two cases. In the TTF-containing solution, the Raman peak at 784 cm–1 gradually decreased and finally disappeared at the end of the OER while that at 1087 cm–1 remains almost identical (point C2 and D2, Fig. 9). This indicates that Li2O2 is fully decomposed, while Li2CO3 on the porous carbon electrode surface does not increase during the OER in the TTF-containing solution. However, if TTF is not included in the solution, nearly 20% of the Li2O2 still remains on the porous carbon electrode surface at the end of the OER (point C1 and D1, Fig. 9). The amount of Li2CO3 doubled after the OER in comparison to that after the ORR. As the electrode is forced to the 2nd ORR process, at the very beginning stage (point E, with an ORR capacity of 15 µAh/cm2), the Raman peak intensity for the Li2O2 in the TTF-free solution slightly increases while no Li2O2 is observed in the TTF-containing solution (point E1 and E2). As the 2nd ORR further progresses, the Raman intensities for Li2O2 and Li2CO3 increase.

24


Page 24 of 58

Page 25 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


When the ORR reached the target capacity (point F), the Raman peak intensity for Li2O2 in the TTF-containing solution is nearly 10% lower than that observed after the 1st ORR (point B). In the TTF-free solution, the same amounts of Li2O2 and Li2CO3 increased in the 2nd ORR as those formed in the 1st ORR. It is interesting to note that the Li2O2 formed on the porous carbon electrode by the 1st ORR cannot be fully decomposed during the 1st OER in the TTF-free solution. However, similar amount of Li2O2 as that in the 1st ORR is formed in the 2nd ORR in the TTF-free solution. As mentioned above, only 10% of the full capacity for the porous carbon electrode (200 µAh/cm2) is used in the ORR/OER cycle, the surface states induced by the previous cycle can be ignored when the cycle number is not too high. The reason for the lower amount of Li2O2 formed in the 2nd ORR in the TTF-containing solution will be discussed later in combination with the UV-Vis observations. On the other hand, more Li2CO3 is accumulated on the porous carbon surface in the TTF-free solution which is formed during both the ORR and OER. In the TTF-containing solution, Li2CO3 is only generated in the ORR, but not in the OER. Therefore, the porous carbon surface will be irreversibly covered by more Li2CO3 in the TTF-free solution. As shown in Fig. 9b, the Li2CO3 cannot be decomposed in the present OER even if TTF is present in the solution and finally blocks all the surface sites. The Li2CO3 was not observed on the gold electrode (Fig. 5) either during the ORR and OER processes even if TTF is not included in the solution. In order to fully understand the influence of Li2CO3 on the formation of the Li2O2, several questions about the formation of Li2CO3 on the carbon electrode surface have to be exactly answered: (1) Which contributes to the Li2CO3 formation during the ORR, oxidation of the organic solvent or carbon electrode or both; (2) What induces the reaction for the Li2CO3 formation, Li2O2 itself and LiO2? (3) How much Li2O2 and Li2CO3 was formed on the electrode surface during the ORR/OER cycle. These are critical to 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

understanding the reaction mechanism of TTF during the ORR/OER process and we will answer these questions one-by-one. Question 1. Carbon source for Li2CO3 formation. From a comparison with the results obtained on the gold and carbon electrode surfaces, it is easier to conclude that the Li2CO3 produced during the ORR may mainly come from the carbon electrode since such phenomena were not observed on the gold electrode surface. We also carried out preliminary experiments for the ORR on a glassy carbon and found that the amount of Li2CO3 was very low (ca. 2% vs. Li2O2) at the end of the ORR. The porous carbon electrode with a large surface area and three-phase interfacial structure may play roles in the formation of Li2CO3. As we extend the ORR to a higher capacity, for example, 2000 µAh/cm2, decomposition of the organic solvent becomes more obvious. While in the present ORR/OER cycles with a lower capacity, the oxidation of the carbon electrode will be the main reason for the Li2CO3 formation. Question 2. Species oxidize carbon electrode to form Li2CO3 during the ORR. It is hard to answer which active species, Li2O2 and/or LiO2, oxidize the carbon electrode surface only from the comparison between the ORR behaviors on the gold and porous carbon electrodes. To exactly answer the question, we designed an elegant experiment using a porous carbon electrode with preloaded Li2O2 (see experimental section). Under such a condition, no LiO2 is present in the system which is only available during the ORR. By checking the direct OER process on this electrode, we are able to distinguish the influence of LiO2 from Li2O2. Figure 10a shows the galvanostatic ORR/OER cycle of the Li2O2-preloaded porous carbon electrode in a 0.5 M LiClO4-DMSO solution with (red trace) and without the TTF additive (blue trace). The amount of Li2O2 mixed into the carbon electrode was accurately controlled at a capacity of 400 µAh/cm2. In the TTF-free electrolyte (blue trace, Fig. 10a), the OER potential rapidly increased to ca. 4.0 V and then slightly dropped to 3.8 V at a capacity of 170 µAh/cm2. After that, the OER potential gradually increased to 4.4 V at the 26


Page 26 of 58

Page 27 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


target capacity (400 µAh/cm2). As the 10 mM TTF is added to the solution, the OER potential profiles become much simpler (red trace, Fig. 10a). The potential quickly increased to 3.67 V and remained constant until the target capacity was reached. The OER overpotential is basically decreased in comparison to that in the TTF-free solution. As discussed above, the potential plateau at 3.67 V should be attributed to the redox reaction of TTF+/TTF (Eq. 3).

Figure 10b shows the ex situ Raman spectra (600 - 1700 cm–1) recorded on the Li2O2-preloaded porous carbon electrode surfaces at the OCP (point A), middle of the OER (point B), and end of the OER (point C). The Raman spectrum at the OCP (point A) shows a sharp peak at 784 cm–1 and a pair of broad peaks at 1298 and 1591 cm–1. As described above, the pair of broad peaks should be assigned to the D-band and G-band of the carbon from the Ketjenblack, respectively, while the sharp peak at 784 cm–1 is attributed to the O-O stretch of Li2O2 preloaded into the carbon. It is important to note that the peak at 1087 cm–1, attributed to the symmetric CO3 stretching of Li2CO3, was not observed at the OCP. Thus, the chemical reaction between Li2O2 and the carbon electrode to form Li2CO3 seems to be not very important being under the detection limit of the Raman measurement. The Raman peak intensities for the D- and G-bands are almost constant during the OER and are independent of the TTF addition. However, the 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Raman peak for Li2O2 shows totally different behaviors in the two solutions. In the TTF-free solution (blue traces, Figs. 10b and 10c), the peak intensity gradually decreased to 30% at the final target capacity (point C1). Nearly 30% of the Li2O2 still remained on the carbon surface after the OER. At the same time, a small peak at 1087 cm–1 (Li2CO3) gradually appeared during the OER (blue traces, Figs. 10b and 10c), indicating that Li2CO3 was formed during the OER in the TTF-free solution. On the other hand, in the TTF-containing solution (red traces, Figs. 10b and 10c), the sharp peak for Li2O2 largely decreases at half of the target capacity (point B2) and completely disappears at the end of the OER (point C2). No peak for Li2CO3 was observed in the TTF-containing solution. Therefore, the Li2CO3 formed during the OER in the TTF-free solution is expected to be produced by the electrochemical oxidation of the carbon electrode and solvent, but not the chemical reaction with Li2O2 preloaded into the carbon electrode (such a peak was not observed at the OCP). Similar results have also been confirmed by the ex situ IR measurement (Fig. S11), in which the chemically produced Li2CO3 only accounts for 1.6% compared with the amount of Li2O2. This very little amount of Li2CO3 may be below the detection limitation of the present Raman measurement. With the addition of TTF as a redox mediator, the OER overpotential is decreased to 3.67 V. This can significantly restrain the decomposition of solvent (DMSO) as well and the carbon electrode, resulting in a higher reversible efficiency. Figure 10c summarizes the Raman band intensity for the Li2O2 as a function of the OER capacity. It is clear that the peak decreases quicker and totally disappears in the TTF-containing solution at the final target OER capacity. Question 3. Absolute amount of Li2CO3 and Li2O2 formed on the electrode surface. Figure 11 shows an IR spectrum (500 ~ 1650 cm–1) obtained after the ORR for a capacity of 200 µAh/cm2 (point B, Fig. 8) in the TTF-free solution. Three main IR peaks are observed. When compared with the standard IR spectrum of Li2O2 and Li2CO3, the peaks at 593 and 860 cm–1 can be assigned to that of 28


Page 28 of 58

Page 29 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the Li-O stretching mode of Li2O2 and C-O bending mode of Li2CO3, respectively, on the carbon electrode surface. The IR measurement confirms again that the main products at the end of the ORR are the Li2O2 and Li2CO3 species. This is generally consistent with the observations by Raman measurement (Fig. 9). To quantitatively determine the absolute amounts of Li2CO3 and Li2O2 on the carbon surface, a number of ex situ IR measurements were carried out on

the different mixing ratios of Li2CO3 and Li2O2 (Fig. S11). It was found that the IR intensity ratio of Li2CO3 and Li2O2 is linearly proportional to their molar ratio in the mixture (Fig. 11). Based on this relationship, the molar ratio of Li2CO3 and Li2O2 can be estimated from the IR intensity ratio. For example, the ratio of the IR band intensity of Li2CO3 (860 cm–1) and Li2O2 (593 cm–1) was found to be 0.098 from the figure. The molar ratio of Li2CO3 and Li2O2 was estimated to be 0.106. The ratios for Li2O2 and Li2CO3 are estimated as 90.4 % and 9.6%, respectively. In the present study, Li2O2 and Li2CO3 are assumed to be the main products during the ORR on the porous carbon surface under the experimental conditions. This is reasonable since the amounts for the other products are very low. For example, the reaction yield of LiO2, which is a main product on the gold electrode, only accounts for 3% for an extended ORR capacity to 2250 µAh/cm2 on the porous carbon electrode. Thus, the amount of LiO2 formed can be ignored under the present low capacity (10%, 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200 µAh/cm2). Based on this assumption, the amount of Li2CO3 can be directly calculated from the ORR capacity (200 µAh/cm2) as 0.36 µmol/cm2. As mentioned above, this is a rough calculation since other possible products, such as LiO2, HCOOLi and CH3COOLi as well as Li2SO4,14 are not considered here.

3.3 UV-Vis Characterization. At the same time, we also made a quantitative analysis for all the free TTF+ including that trapped in the porous carbon electrode. As mentioned in

the experimental section, the electrode in the same cell was directly put into an ultrasound cleaner, forcing the TTF+ species trapped in the porous carbon electrode to dissolve into the solvent. After the centrifuging procedure, the solution was analyzed by UV-Vis measurement. As shown in the inset of Figure 12, an absorption peak is clearly observed at 579 nm at the end of the 1st OER process (point D) and the very beginning of the 2nd ORR process (point E). On the other hand, no peak was observed at points B and C, and disappeared at point F. Figure 12 summarizes the amounts of TTF+ (green blocks) during the 1st OER and 2nd ORR. The amount of free TTF+ increased from the end stage of the 1st OER and soon decreased to zero in the early stage of the following 2nd

30


Page 30 of 58

Page 31 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ORR. The total amount of free TTF+ after the 1st OER (point D) was quantitatively estimated as 0.77 µmol/cm2 (i.e., 20 µAh/cm2 in capacity). As already mentioned, if TTF+ is used as an ideal redox mediator, free TTF+ should not exist after a full cycle. Although Li2O2 can be chemically oxidized by TTF+, the same chemical oxidation does not occur on the Li2CO3. Since the TTF largely reduces the OER overpotential (ca. 0.5 V), the potential is too low to electrochemically decompose the Li2CO3 on the porous carbon electrode surface (normally up to 4.0 V).14 Therefore, after the chemical oxidation of Li2O2 on the electrode surface, the electrochemically generated TTF+ species have no partner to oxidize and accumulate in the solution as an “overcharged” product. This assumption is confirmed as the total amount of free TTF+ in the solution that was found to equal that of Li2CO3 produced during the ORR. Furthermore, the excess TTF+ species are present in the subsequent ORR and will be electrochemically reduced to TTF in the early stage of the 2nd ORR, producing a considerable amount of capacity (19.6 µAh/cm2). However, this capacity is not a meaningful ORR capacity since it is not due to the oxygen reduction.

This is the reason that an additional potential profile appears in

the early stage during the 2nd ORR process (Fig. 12). Since part of the ORR capacity has been used to regenerate TTF from TTF+, there is less Li2O2 formed on the porous electrode surface under the identical ORR capacity of 200 µAh/cm2 (see Fig. 9b).

3.3 Quantitative Discussions on the ORR/OER Cycles on the Porous Carbon Electrode Surface Table 2 summarizes the distribution of the ORR/OER capacities (cycle capacity is 200 µAh/cm2) and the amount of products generated on the each stage based on the in situ UV, ex situ Raman/IR and electrochemistry measurements in the TTF-free (left column) and TTF-containing solutions (right column). 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 58

During the 1st ORR, capacities of 181 and 19 µAh/cm2 were used to produce Li2O2 and Li2CO3 on the porous carbon electrode, respectively, in both solutions. Approximately 10% of ORR capacity was consumed in the formation of by-product Li2CO3. At the end of the following OER process in the TTF-free solution, 33 µAh/cm2 of Li2O2 still remained on the carbon electrode surface while the amount of Li2CO3 increased to 38 µAh/cm2 (i.e., net increase from the previous ORR was 38 – 19 = 19 µAh/cm2). Two sources were considered to contribute for the Li2CO3 capacity: (1) oxidative decomposition of carbon electrode and/or organic solvent (m1); and (2) electrochemical decomposition of Li2CO3 in the positive potential region (m2). Based on the experimental conditions given above, we have the following simultaneous equations, m1 – m2 = 19

(A-1)

(181 – 33) + m1 + m2 = 200

(A-2)

Then, we have m1 = 35.5 µAh/cm2, m2 = 16.5 µAh/cm2. This demonstrates that in addition to the oxidative decomposition of carbon electrode and/or solvent to form Li2CO3 (35.5 µAh/cm2), Li2CO3 itself electrochemically decomposed on the positive potential region (16.5 µAh/cm2). When the TTF is included in the solution, all of Li2O2 was oxidized and no increase of Li2CO3 was observed at the end of the OER. On the same time, 19 µAh/cm2 of TTF+ was observed in the solution by UV-Vis observation (right column, Table 2). The similar simultaneous equations can be given as, m1’ – m2’ = 0

(B-1)

181 + m1’ + m2’ + 19 = 200

(B-2)

From (B-1) and (B-2), we have m1’ = m2’ = 0. This indicates that neither oxidation of carbon/solvent nor Li2CO3 decompositions occurs in the OER in the TTF-containing solution. This should be associated to the reduction of OER overpotential by the TTF. It is noteworthy that although no Li2CO3 was formed during the OER in the TTF-containing solution, nearly the same 32


Page 33 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amount of TTF+ to that of Li2CO3 formed in the ORR (20 µAh/cm2) remained in solution at the end of OER (Table 2). During the 2nd ORR in the TTF-free solution, 212 µAh/cm2 of Li2O2 and 60 µAh/cm2 of Li2CO3 were observed, thus 179 µAh/cm2 of Li2O2 and 22 µAh/cm2 of Li2CO3 were newly formed on the carbon electrode surface during the ORR (left column, Table 2). As many active surface sites are still available on the porous carbon electrode (the real capacity is approximately 2250 µAh/cm2), the similar amount of Li2O2 was formed although more Li2CO3 was formed in the ORR/OER cycle. When the TTF is added in the solution, the 2nd ORR gave 161 µAh/cm2 of Li2O2 with an increase of 19 µAh/cm2 of Li2CO3 to 39 µAh/cm2, while the TTF+ fully disappeared (right column, Table 2). The amount of Li2O2 formation seems to decrease from the 1st ORR (181 µAh/cm2). This is just due to an additional capacity is necessary to oxidize the TTF+ formed in the previous OER process.

4. Different ORR/OER Depth on Porous Carbon Electrode

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 13a shows the galvanostatic ORR/OER cycles on a porous carbon electrode with different capacities in the TTF-containing solution. Up to a capacity of 400 µAh/cm2 (green trace), the ORR potential always held at 2.7 V. The potential profile is very similar to that of the 200 µAh/cm2 (Fig. 8). In the reversed OER, the potential quickly increased to a narrow plateau around 3.4 V, then gradually increased to 3.8V. All the cycles have a reversible capacity due to the addition of TTF. Figure 13b shows a series of ex situ Raman spectra observed at the OCP and the end of several ORRs with capacities from 100 to 400 µAh/cm2. With an increase in the ORR capacity, the Raman peaks at 784 and 1087 cm–1 synchronously increase, indicating that the amounts of Li2O2 and Li2CO3 increase with the ORR (Fig. 9b). The growth rates for both Li2O2 and Li2CO3 seem to be constant during the ORR. Furthermore, as described in the previous sections, the amounts of Li2O2 and Li2CO3 have also been investigated by ex situ IR measurements at various ORR capacities (Fig. 14a). The IR peak intensities at 593 and 860 cm–1 increase with the ORR capacity, which is consistent with the ex situ Raman observation. The ratio of the peak intensity (860 cm–1@Li2CO3: 593 cm–1@ Li2O2) at different ORR capacities is very close to the value of 0.097 (highlighted by orange

34


Page 34 of 58

Page 35 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


circle), which indicates that the molar ratio of Li2CO3 and Li2O2 is around 0.10. This indicates that the component ratio of the ORR products does not change with the ORR capacity (upper limit: 400 µAh/cm2). The growth rates for the two products remain almost constant during the ORR process. There is also a constant ratio of capacity (9.6%) devoted to the produce of the Li2CO3 species during the ORR, showing good agreement with our previous results. Finally, the UV-Vis absorption spectroscopy was used to quantitatively determine the amount of TTF+ after the ORR/OER cycles with different capacities on a porous carbon electrode (Fig. 14b). The intensity of the peak at 579 nm at the end of each OER process uniformly increases with the increase in the ORR/OER capacity. This indicates that as an indicator for the overcharged product on the porous carbon electrodes, the amount of TTF+ increases during the ORR/OER capacity with cycling. The increase in the TTF+ species at the end of the OER is mainly attributed to the increase in the absolute amount of Li2CO3 produced during the ORR. The amount of TTF+ is directly proportional to the ORR/OER capacity (Table 3). This result is also consistent with the constant moles ratio of the Li2O2 and Li2CO3 species for different ORR/OER capacities obtained in the IR measurements. The quantitative analysis of the TTF+ after the OER is an effective way to determine the amount of by-products formed during the ORR. This provides a new insight into the evaluation of the stability of the Li-O2 system towards both cathode materials and electrolyte solvents.

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 58

As discussed above, although TTF can reduce the OER overpotential for both gold and porous carbon electrodes, it is unable to stop the side reactions during the ORR, especially on the carbon electrode surface. TTF does not work as an ideal redox mediator at the porous carbon electrode as we expected. The development of a redox mediator which can control the oxidative reactions with a superoxide is highly desired.

Conclusion In summary, we have presented a systematic study of the ORR/OER cycles on a gold electrode and porous carbon electrode in DMSO-based electrolyte solution by in situ SERS and UV-Vis observations in combination with ex situ Raman,

IR,

UV-Vis

observations

and

electrochemistry.

Our

results

demonstrate that TTF definitely reduces the overpotential for the OER process and restrains the decomposition of the carbon electrode and solvent during the OER, while the functionality of the redox mediator can change with the electrode materials and morphologies. Free TTF+ was not observed in solution during a round-trip galvanostatic ORR/OER cycle on the gold electrode surface. It was found that the electrochemically generated TTF+ was mainly consumed by the oxidative decomposition of LiO2 in solution, but not Li2O2 on the gold electrode surface formed during the ORR. On the other hand, Li2O2 and Li2CO3 were observed during the ORR on the porous carbon electrode surface both in the TTF-free and TTF-containing solution. The TTF can mediate the oxidation of Li2O2 formed on the porous carbon electrode. The formation of Li2CO3 with a simultaneous decomposition reaction only occurs during the OER in the TTF-free solution. Furthermore, free TTF+ was found to accumulate in solution during a round-trip galvanostatic ORR/OER cycle on the porous carbon electrode. The amount of the excess TTF+ in solution well corresponds to that of the Li2CO3 by-product formed during the 36


Page 37 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ORR on the carbon electrode surface. TTF is unlikely to work as an ideal redox mediator at the porous carbon electrode as expected.

Associated Content Supporting Information: Schematics of the cells employed in this study; in situ SERS measurements during the ORR/OER in the Ar-saturated electrolyte solution;

Electrochemical

behaviors,

in

situ

UV-Vis

and

SERS

characterizations on gold electrode in TBAClO4-DMSO electrolyte solutions; Overcharge analysis on gold electrode; Baseline subtraction process for ex situ Raman spectra; Mid-IR calibration curves employed for component analysis on Li2O2-preloaded porous carbon electrodes. The Supporting Information is available free of charge on the ACS Publications website.

Author Information Corresponding Author: *Tel: +81-117069126. Email: [email protected]. Notes: The authors declare no competing financial interest.

Acknowledgments This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA), specially promoted research for innovative next generation batteries (SPRING) from the Japan Science and Technology Agency (JST). The authors want to express their special thanks to Prof. Kubo and Mr. Kumura from NIMS for guidance on the preparation of the porous carbon electrode employed in this study. Q.Y. acknowledges a scholarship from the China Scholarship Council (CSC). The authors thank Prof. Kohei Uosaki for his stimulating discussions and comments. The authors thank Dr. Can Liu and Miss Yingying Zhou for their assistance with some of the experiments.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC)

38


Page 38 of 58

Page 39 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Abraham, K. M.; Jiang, Z., A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen

Battery. J Electrochem Soc 1996, 143, 1-5. 2. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li-O-2 and Li-S Batteries with High Energy Storage. Nature Materials 2012, 11, 19-29. 3. Lu, Y. C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y., Lithium-Oxygen Batteries: Bridging Mechanistic Understanding and Battery Performance. Energ Environ Sci 2013, 6, 750-768. 4. Luntz, A. C.; McCloskey, B. D., Nonaqueous Li-Air Batteries: A Status Report. Chem Rev 2014, 114, 11721-11750. 5. Adams, B. D.; Black, R.; Williams, Z.; Fernandes, R.; Cuisinier, M.; Berg, E. J.; Novak, P.; Murphy, G. K.; Nazar, L. F., Towards a Stable Organic Electrolyte for the Lithium Oxygen Battery. Adv Energy Mater 2015, 5. 6. Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W., Lithium - Air Battery: Promise and Challenges. J Phys Chem Lett 2010, 1, 2193-2203. 7. Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.; Kozinsky, B.; Liedtke, R.; Ahmed, J.; Kojic, A., A Critical Review of Li/Air Batteries. J Electrochem Soc 2012, 159, R1-R30. 8. Grande, L.; Paillard, E.; Hassoun, J.; Park, J. B.; Lee, Y. J.; Sun, Y. K.; Passerini, S.; Scrosati, B., The Lithium/Air Battery: Still an Emerging System or a Practical Reality? Adv Mater 2015, 27, 784-800. 9. Lu, J.; Li, L.; Park, J. B.; Sun, Y. K.; Wu, F.; Amine, K., Aprotic and Aqueous Li-O-2 Batteries. Chem Rev 2014, 114, 5611-5640. 10.

McCloskey, B. D.; Scheffler, R.; Speidel, A.; Bethune, D. S.; Shelby, R. M.; Luntz, A. C., On the

Efficacy of Electrocatalysis in Nonaqueous Li-O-2 Batteries. Journal of the American Chemical Society 2011, 133, 18038-18041. 11.

McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshoj, J. S.;

Norskov, J. K.; Luntz, A. C., Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li-O-2 Batteries. J Phys Chem Lett 2012, 3, 997-1001. 12.

Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G., A Reversible and Higher-Rate Li-O-2

Battery. Science 2012, 337, 563-566. 13.

Laoire, C. O.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M., Rechargeable

Lithium/Tegdme-Lipf6/O-2 Battery. J Electrochem Soc 2011, 158, A302-A308. 14.

Thotiyl, M. M. O.; Freunberger, S. A.; Peng, Z. Q.; Bruce, P. G., The Carbon Electrode in

Nonaqueous Li-O-2 Cells. Journal of the American Chemical Society 2013, 135, 494-500. 15.

McCloskey, B. D.; Scheffler, R.; Speidel, A.; Girishkumar, G.; Luntz, A. C., On the Mechanism

of Nonaqueous Li-O-2 Electrochemistry on C and Its Kinetic Overpotentials: Some Implications for Li-Air Batteries. J. Phys. Chem. C 2012, 116, 23897-23905. 16.

Shao, Y. Y.; Park, S.; Xiao, J.; Zhang, J. G.; Wang, Y.; Liu, J., Electrocatalysts for Nonaqueous

Lithium-Air Batteries: Status, Challenges, and Perspective. Acs Catal 2012, 2, 844-857. 17.

Mitchell, R. R.; Gallant, B. M.; Thompson, C. V.; Shao-Horn, Y., All-Carbon-Nanofiber

Electrodes for High-Energy Rechargeable Li-O-2 Batteries. Energ Environ Sci 2011, 4, 2952-2958. 18.

Zhang, T.; Zhou, H. S., A Reversible Long-Life Lithium-Air Battery in Ambient Air. Nat

Commun 2013, 4. 39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

Jian, Z. L.; Liu, P.; Li, F. J.; He, P.; Guo, X. W.; Chen, M. W.; Zhou, H. S., Core-Shell-Structured

Cnt@Ruo2 Composite as a High-Performance Cathode Catalyst for Rechargeable Li-O-2 Batteries. Angew. Chem.-Int. Edit. 2014, 53, 442-446. 20.

Cui, Y. M.; Wen, Z. Y.; Liu, Y., A Free-Standing-Type Design for Cathodes of Rechargeable

Li-O-2 Batteries. Energ Environ Sci 2011, 4, 4727-4734. 21.

Li, F. J.; Tang, D. M.; Chen, Y.; Golberg, D.; Kitaura, H.; Zhang, T.; Yamada, A.; Zhou, H. S.,

Ru/Ito: A Carbon-Free Cathode for Nonaqueous Li-O-2 Battery. Nano Lett 2013, 13, 4702-4707. 22.

Lu, Y. C.; Gasteiger, H. A.; Shao-Horn, Y., Catalytic Activity Trends of Oxygen Reduction

Reaction for Nonaqueous Li-Air Batteries. Journal of the American Chemical Society 2011, 133, 19048-19051. 23.

Xu, J. J.; Xu, D.; Wang, Z. L.; Wang, H. G.; Zhang, L. L.; Zhang, X. B., Synthesis of

Perovskite-Based Porous La0.75sr0.25mno3 Nanotubes as a Highly Efficient Electrocatalyst for Rechargeable Lithiumoxygen Batteries. Angew. Chem.-Int. Edit. 2013, 52, 3887-3890. 24.

Oh, S. H.; Black, R.; Pomerantseva, E.; Lee, J. H.; Nazar, L. F., Synthesis of a Metallic

Mesoporous Pyrochlore as a Catalyst for Lithium-O-2 Batteries. Nat. Chem. 2012, 4, 1004-1010. 25.

Shao, Y. Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J. G.; Wang, Y.; Liu, J., Making

Li-Air Batteries Rechargeable: Material Challenges. Adv Funct Mater 2013, 23, 987-1004. 26.

Lu, Y. C.; Kwabi, D. G.; Yao, K. P. C.; Harding, J. R.; Zhou, J. G.; Zuin, L.; Shao-Horn, Y., The

Discharge Rate Capability of Rechargeable Li-O-2 Batteries. Energ Environ Sci 2011, 4, 2999-3007. 27.

Ganapathy, S.; Adams, B. D.; Stenou, G.; Anastasaki, M. S.; Goubitz, K.; Miao, X. F.; Nazar, L.

F.; Wagemaker, M., Nature of Li2o2 Oxidation in a Li-O-2 Battery Revealed by Operando X-Ray Diffraction. Journal of the American Chemical Society 2014, 136, 16335-16344. 28.

Mo, Y. F.; Ong, S. P.; Ceder, G., First-Principles Study of the Oxygen Evolution Reaction of

Lithium Peroxide in the Lithium-Air Battery. Phys Rev B 2011, 84. 29.

Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M., Studies of

Li-Air Cells Utilizing Dimethyl Sulfoxide-Based Electrolyte. J Electrochem Soc 2013, 160, A259-A267. 30.

Viswanathan, V.; Thygesen, K. S.; Hummelshoj, J. S.; Norskov, J. K.; Girishkumar, G.;

McCloskey, B. D.; Luntz, A. C., Electrical Conductivity in Li2o2 and Its Role in Determining Capacity Limitations in Non-Aqueous Li-O-2 Batteries. J Chem Phys 2011, 135. 31.

Chen, Y. H.; Freunberger, S. A.; Peng, Z. Q.; Fontaine, O.; Bruce, P. G., Charging a Li-O-2

Battery Using a Redox Mediator. Nat. Chem. 2013, 5, 489-494. 32.

Sun, D., et al., A Solution-Phase Bifunctional Catalyst for Lithium-Oxygen Batteries. Journal

of the American Chemical Society 2014, 136, 8941-8946. 33.

Bergner, B. J.; Schurmann, A.; Peppler, K.; Garsuch, A.; Janek, J., Tempo: A Mobile Catalyst

for Rechargeable Li-O-2 Batteries. Journal of the American Chemical Society 2014, 136, 15054-15064. 34.

Lim, H. D., et al., Superior Rechargeability and Efficiency of Lithium-Oxygen Batteries:

Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst. Angew. Chem.-Int. Edit. 2014, 53, 3926-3931. 35.

Yu, M. Z.; Ren, X. D.; Ma, L.; Wu, Y. Y., Integrating a Redox-Coupled Dye-Sensitized

Photoelectrode into a Lithium-Oxygen Battery for Photoassisted Charging. Nat Commun 2014, 5. 36.

Zhu, Y. G.; Jia, C. K.; Yang, J.; Pan, F.; Huang, Q. Z.; Wang, Q., Dual Redox Catalysts for Oxygen

Reduction and Evolution Reactions: Towards a Redox Flow Li-O-2 Battery. Chem Commun 2015, 51, 9451-9454.

40


Page 40 of 58

Page 41 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37.

Qiao, Y.; Ye, S., In Situ Study of Oxygen Reduction in Dimethyl Sulfoxide (Dmso) Solution: A

Fundamental Study for Development of the Lithium-Oxygen Battery. J. Phys. Chem. C 2015, 119, 12236-12250. 38.

McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; Scheffler, R.; Speidel, A.; Sherwood,

M.; Luntz, A. C., Limitations in Rechargeability of Li-O-2 Batteries and Possible Origins. J Phys Chem Lett 2012, 3, 3043-3047. 39.

Kim, S.; Dicosimo, R.; Filippo, J. S., Spectrometric and Chemical Characterization of

Superoxide. Anal Chem 1979, 51, 679-681. 40.

Gampp, H.; Lippard, S. J., Reinvestigation of 18-Crown-6 Ether Potassium Superoxide

Solutions in Me2so. Inorg Chem 1983, 22, 357-358. 41.

Hunter-Saphir, S. A.; Creighton, J. A., Resonance Raman Scattering from the Superoxide Ion.

J Raman Spectrosc 1998, 29, 417-419. 42.

Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P. E., Optical-Properties of the

Radical Cation Tetrathiafulvalenium (Ttf+) in Its Mixed-Valence and Mono-Valence Halide Salts. Phys Rev B 1979, 19, 730-741. 43.

Tanaka, K.; Kunita, T.; Ishiguro, F.; Naka, K.; Chujo, Y., Modulation of Morphology and

Conductivity of Mixed-Valence Tetrathiafulvalene Nanofibers by Coexisting Organic Acid Anions. Langmuir 2009, 25, 6929-6933. 44.

Chahma, M.; Wang, X. S.; van der Est, A.; Pilkington, M., Synthesis and Characterization of a

New Family of Spin Bearing Ttf Ligands. J Org Chem 2006, 71, 2750-2755. 45.

Martens, W. N.; Frost, R. L.; Kristof, J.; Kloprogge, J. T., Raman Spectroscopy of Dimethyl

Sulphoxide and Deuterated Dimethyl Sulphoxide at 298 and 77 K. J Raman Spectrosc 2002, 33, 84-91. 46.

Shen, A. J.; Pemberton, J. E., Interfacial Structure of Dimethylsulfoxide at Ag Electrodes from

Surface Enhanced Raman Scattering and Differential Capacitance. J Electroanal Chem 1999, 479, 21-31. 47.

Berlinsky, A. J.; Hoyano, Y.; Weiler, L., Raman-Spectra of Tetrathiofulvalene (Ttf). Chem Phys

Lett 1977, 45, 419-421. 48.

Ferrari, A. C.; Robertson, J., Interpretation of Raman Spectra of Disordered and Amorphous

Carbon. Phys Rev B 2000, 61, 14095-14107. 49.

Bates, J. B.; Boyd, G. E.; Brooker, M. H.; Quist, A. S., Raman Spectra of Molten Alkali-Metal

Carbonates. J Phys Chem-Us 1972, 76, 1565-&.

41



(a)

(b)

3.8

4.01 V 3.81 V

3.67 V voltage plateau

3.7

2

+

1.0

10mM TTF 0.5M LiClO4-DMSO (Ar) CV 2.0~4.2 V 10 mV/s

Potential (V vs. Li/Li )

1.5

Current (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 42 of 58

0.5 0.0

3.89 V

-0.5

3.6 3.5 3.4

10mM TTF in 0.5 M LiClO4-DMSO (Ar)

3.3

2

100 uA/cm on gold electrode -1.0

3.2

3.68 V 2.0

2.5

3.0

3.5

4.0

4.5

0

2

4

6

8

10

2

+

Capacity (uAh/cm )


Figure 1. (a) Cyclic voltammogram (CV) of a gold electrode (2.0-4.2 V) at a scan rate of 10 mV/s in the Ar-saturated 0.5 M LiClO4-DMSO with 10mM TTF. (b) Galvanostatic oxidation polarization at 100 μA/cm2 in the same electrolyte solution.


Page 43 of 58

4.0 3.5

0.9

435 nm 451 nm

UV-vis Absorbance

3.0

10mM TTF in DMSO (DMSO as ref.) + 2 TTF in DMSO (charged 100Ah/cm ) (10mM TTF-DMSO as ref.)

2.5

0.8 0.7 0.6 0.5

2.0 0.4

579 nm

1.5

0.3

1.0

0.2

0.5

0.1

0.0

0.0

400

500

600

700

UV-vis Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


800

Wavelength (nm)

Figure 2. UV-Vis absorption spectra of 0.5 M LiClO4-DMSO with 10 mM TTF before (black trace) and after galvanostatic oxidation polarization with a capacity of 100 μAh/cm2 (blue trace).



(a)

1.5

(b)

0.5 M LiClO4-DMSO CV 2.0~4.2 V 10 mV/s

1.0


2

Current Density (mA/cm )

TTF-contained (Ar) TTF-contained (O2) TTF-free (O2)

+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

0.5

0.0

-0.5

Page 44 of 58

4.2 4.0

10mM TTF in Li-DMSO 2 100uA/cm on gold electrode

3.8

C3

3.6

C1

C2

3.4 ORR process B2 OER added TTF B3 OER without TTF replace electrolyte after ORR

3.2 3.0

B1

2.8

A

2.6

-1.0 2.0

2.5

3.0

3.5

4.0

0

2

4

6

8

10

2

+

Capacity (uAh/cm )


Figure 3. (a) CVs (10mV/s) of a gold electrode in O2-saturated 0.5 M LiClO4-DMSO solution with 0 mM (blue trace) and 10mM TTF (red trace). A CV in Ar-saturated 0.5 M LiClO4-DMSO with 10 mM TTF is also shown for comparison (black dash trace). (b) Galvanostatic ORR/OER curves of a gold electrode at 100 μA/cm2 in O2-saturated 0.5 M LiClO4-DMSO with and without TTF. For comparison, after the ORR in solution with 10 mM TTF, the solution in the cell was replaced by fresh solution at OCP (ca. 3.0 V), then a galvanostatic OER is recorded (green trace). Point A and C refer to the end of ORR and OER process, respectively, while point B is at an OER capacity of 1 μAh/cm2. The subscript indicates OER condition (1: TTF-free; 2: TTF-contained; 3: TTF-contained solution after electrolyte replacement). See the text for details. ACS Paragon Plus Environment

(b)

0.35

TTF-free Li-DMSO 0.30

0.06

end of OER after electrolyte replacement (point C3)

0.05

0.25

UV-vis Abs

0.04

0.20 OCP end of ORR (point A) end of OER (point C1)

0.15 0.10

0.04 2

0.03 0.02 end of ORR (point A) end of OER (point C2)

250

260

270

280

Wavelength (nm)

290

300

400

3.9

3.3

overcharge

Charge

Discharge

3.6

OOO

2

C2

100 uA/cm

3.0 2.7

O 2

1 uAh/cm 2 2 uAh/cm 2 5 uAh/cm

OA 0

5

0.03

10/0 5 2 Capacity (uAh/cm )

10

15

end of ORR (point A) end of OER (point C2) overcharge process 1 uAh/cm2 2 uAh/cm2 5 uAh/cm2

0.02 0.01

0.00

0.00

0.06 0.05

direct charge for 10 uAh/cm

0.01

0.05

(c)

10mM TTF Li-DMSO

Potential (V)

(a)

UV-vis Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


UV-vis Abs

Page 45 of 58

0.00 500

600

700

Wavelength (nm)

800

400

500

600

700

800

Wavelength (nm)

Figure 4. (a) In situ UV-Vis absorption spectra (248-300 nm) observed in 0.5 M LiClO4-DMSO at OCP (dotted trace), after ORR at point A (black trace) and after OER at point C1. (b) In situ UV-Vis absorption spectra (400-800 nm) in 0.5 M LiClO4-DMSO with 10 mM TTF at end of ORR (point A) and OER (point C2). The dotted trace is a UV-Vis spectrum after a direct OER in the same solution while the green trace is obtained in an OER after the ORR solution in the cell is replaced by a fresh solution. (c) In situ UV-Vis absorption spectra (400-800 nm) in 0.5 M LiClO4-DMSO with 10 mM TTF with different OER overcharge degrees. The inset shows the potential profiles during the ORR/OER cycle. See the text for details. ACS Paragon Plus Environment


(a)

(b)

TTF-free

(c) Electrolyte Replacement

TTF-contained

2.0

Overcharge

adsorbed TTF

Li2O2 10.0

1.0

6.0 2.5

10.0 6.0 3.0 1.5

10.0 6.0 3.0 1.5

10.0 6.0 3.0 1.5

OCP

OCP

OCP

700

800

900 1000 1100 -1

Raman Shift (cm )

1.0 0

500

600

700

800

900 1000 1100 -1

Raman Shift (cm )

ORR

600

OER

OER

Li2O2

2.5 1.0 0

500

+

1.0

10.0 6.0 2.5 1.0 0

Li2O2

ORR

2.0

adsorbed TTF

+

Overcharge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 46 of 58

500

600

700

800

900 1000 1100 -1

Raman Shift (cm )

Figure 5. Capacity dependent in situ Raman spectra observed on a gold electrode surface in the O2-saturated 0.5 M LiClO4-DMSO recorded during galvanostatic ORR/OER cycle and OER overcharge processes in (a) TTF-free solution, (b) and (c) 10 mM TTF-contained solution. The details of the electrochemical conditions are the same as that in Figure 3b. Spectra at different ORR/OER/overcharge states ACS Paragon Plus Environment are separated by different colors (as shown in Figure 3b). All the Raman spectra are offset for clarity. See text for details.

Page 47 of 58


OER

3.6 3.4

TTF-free

3.2 3.0

over charge

2.8 2.6 4.2

(b)

Raman Intensity (a.u.)

+

+


4.0

ORR

(a)

3.8

2.5 uAh/cm2

3.8 3.6 3.4

TTF-contained

3.2 3.0 2.8

+

(c)



2.6 4.2

4.0

1.0 uAh/cm2

3.8 3.6

Raman Intensity at 788 cm-1 (O-O stretch in Li2O2)

3.4 3.2 3.0

Electrolyte Replacement

2.8 2.6

0

2.5

5

7.5

10/0

2.5

5

7.5

10/0


4.0


4.2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

2.5

2

Capacity (Ah/cm )

Figure 6. Capacity dependence of the Raman peak intensity at 788 cm-1 (blocks and triangles) and potential (curves) observed on a gold electrode surface in corresponding electrolyte solutions from Figure 5. The galvanostatic ORR/OER/overcharge curves are also shown. The electrochemical conditions are the same as those in Figure 3b. Spectra at different ORR/OER/overcharge states are separated by different colors (as shown in Figure 3b). ACS Paragon Plus Environment


1st

+

3.8 3.6

2nd

TTF free

3rd

4th

5th

3.4 3.2 3.0 2.8

3.0k

2.5k

2.0k

1.0k 20

40

60

80

End of each ORR Peak intensity of Li2O2 End of each OER Baseline intensity

1.5k

2.6 0

5th

4th

3rd

2nd

1st

amount of Li2O2


4.0

Gold electrode 10mM TTF 0.5 M Li-DMSO 2 1-5th galvanostatic cycles at 100 A/cm

(b) 3.5k -1

4.2

Raman Intensity @ 788 cm

(a)

during ORR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 48 of 58

100

2


OCV 0

20

40

60

80

100

2


Figure 7. (a) Galvanostatic ORR/OER cycles of a gold electrode at 100 μA/cm2 in O2-saturated 0.5 M LiClO4-DMSO with 10mM TTF during 1-5th cycles. The ORR/OER capacity are both limited to 10 μAh/cm2. As a comparison, the results observed in the TTF-free solution during the 1st cycle and also shown (black dash). (b) Raman peak intensity for Li2O2 (hollow marks) on a gold electrode surface at different ORR/OER conditions correspond to (a). The peak intensities are directly read from the in situ Raman spectra without baseline subtraction. The peak intensities at different cycles are separated by different colors. ACS Paragon Plus Environment

Page 49 of 58

1st ORR

4.2

+


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


2nd ORR

1st OER

D

3.9 without TTF

3.6

added TTF

A

3.3

E (15 Ah/cm2)

C

3.0

F

B 2.7

0

100

200/0

100

200/0

100

200

2

Capacity (Ah/cm ) Figure 8. Galvanostatic ORR/OER curves of the porous carbon electrode at 100 μA/cm2 in 0.5 M LiClO4-DMSO solution saturated by O2. The electrodes in both the TTF-free (blue trace) and TTF-containing (red trace) solutions are observed during 1st ORR- 1st OER and 2nd ORR processes. Different ORR/OER states are denoted by different characters: A-OCP; B-end of 1st ORR; C-middle of 1st OER (100 μAh/cm2); Dend of 1st OER; E- 2nd ORR at 15 μAh/cm2); F-end of 2nd ORR. See the text for details. ACS Paragon Plus Environment


point B1 point B2 point A

600

800

Li2CO3

1000

D

1200

KB-CP G

1400 -1

Raman Shift (cm )

1600

Standard Materials

Intensity of Li2O2 TTF-free TTF-contained

F

D

3.6

E

A

4.0

600

3.2

300

B

2.8

-1

0

1500

4.4

Absolute Raman Intensity of Li2CO3 TTF-contained TTF-free

4.0

1000 3.6 500

3.2

0

2.8

0

100

200/0

100

200/0

100

+

Li2O2

4.4

C

1200 900

2nd ORR


point C1 point C2

Raman Intensity @784 cm

point D1 point D2

1500

-1

point E1 point E2

1st OER

1800 Absolute Raman

+


point F1 point F2

1st ORR

Raman Intensity @1087 cm

(b)

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 50 of 58

200

2

Capacity (uAh/cm )

Figure 9. (a) Capacity dependent ex situ Raman spectra are shown at different ORR/OER depths. The Raman spectrum for original electrode and standard samples (Li2O2 and Li2CO3) are shown for comparison. The electrochemical conditions and meanings for each characters are given in caption in Fig. 8. The subscript indicates the electrolyte solution: 1. TTF-free (blue curves); 2. TTF-contained (red curves). All the Raman spectra are offset for clarity. (b) Capacity dependence of Raman peak for Li2O2 and Li2CO3 are also summarized at different depths of OER in both the TTF–free (blue trace) and TTF-containing (red trace) conditions. The galvanostatic ORR/OER curves ACS Paragon Plus Environment are also shown. The fitting and baseline subtraction processes are shown in Fig. S10. See the text for details.

Page 51 of 58

+

C1

Ketjenblack electrode TTF-contained TTF-free

4.2

3.9

B1

3.6

C2

B2

2

100 Ah/cm Galvanostatic Charging 0.5 M LiClO4-DMSO

3.3

point A

point C1 point C2


4.5

Li2O2-preloaded

(c)

4.0k

Raman Intensity -1 @ 784 cm

(b)

2.0k

Absolute Raman Intensity of Li2O2 TTF contained TTF free

0.0

point B1 point B2 point A (OCP) KB-CP

Li2CO3

Li2O2

D

G

Standard Materials

Raman Intensity -1 @ 1087cm

(a) Potential (V vs. Li/Li )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


2k

Absolute Raman Intensity of Li2CO3 TTF contained TTF free

1k

0

3.0 0

100

200

300 2

Charge Capacity (Ah/cm )

400

600

800

1000

1200

1400

1600

-1

Raman Shift (cm )

0

100

200

300

400 2

Charge Capacity (Ah/cm )

Figure 10. (a) Galvanostatic charge curves of porous carbon electrode mixed with commercial Li2O2 (equal to a capacity of 400 μAh/cm2) at 100 μA/cm2 in 0.5 M LiClO4-DMSO saturated by O2. The electrodes in both the TTF-free (blue trace) and TTF-containing (red trace) electrolyte are both observed. (b) Capacity dependent ex situ Raman spectra are shown at different OER depths. The Raman spectrum for original electrode and standard samples (Li2O2 and Li2CO3) are shown for comparison. All the Raman spectra are offset for clarity. Different OER states were denoted by different characters in (a) and (b): A-OCP; B-middle of OER (200 μAh/cm2); C-end of 1st OER (400 μAh/cm2). (c) Capacity dependence of Raman peak at Li2O2 and Li2CO3 are also summarized at different OER depths in both the TTF–free (blue trace) and TTF-containing (red trace) conditions. See the text for details. ACS Paragon Plus Environment


0.3 -1

Measurements after 1st ORR process

0.2

linear fit

Ratio in peak intensity

-1

Li2CO3@860cm / Li2O2@600cm

0.02

Mid-IR Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

0.01

Page 52 of 58

-1

860cm / 593cm

-1

= 0.098

0.1

Ratio in moles Li2CO3 / Li2O2 0.0 0.0

0.1 0.2 Li2CO3 / Li2O2 in moles

= 0.106

0.3

0.00 Li2O2 Li2CO3 -0.01 500

750

1000

1250

1500

-1

Wavenumber (cm )

Figure 11. Ex situ mid-IR spectra observed on the porous carbon electrode in 0.5 M LiClO4-DMSO electrolyte recorded at the end of the 1st ORR process. The spectra for standard Li2O2 and Li2CO3 samples are shown for comparison. Inset shows the IR calibration curve obtained by mixing known amounts of Li2O2 and Li2CO3 at different moles ratios. Note that the isolated Li2O2 peak at 600 cm-1 and Li2CO3 peak at 880 cm-1 are chosen for the quantitative analysis. The blue circular ring highlights the value obtained from the electrode after the 1st ORR process and the corresponding values are also listed up in the figure. See the text for details. ACS Paragon Plus Environment

Page 53 of 58

2nd ORR

1st OER 4.2

TTF-contained TTF-free UV-vis Abs

0.06

D

+


3.9

579 nm TTF

3.6

0.08

+

D point Abs=0.06

0.06

0.03

E point Abs=0.01

B,C,F 0.00 400

500

C

3.3

600

700

800

Wavelength (nm)

0.04

E 0.02

3.0

F

B

2.7

100

200/0

100

+

0

0.00

UV-vis Absorance (@579 nm, TTF )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


200

2

Capacity (uAh/cm )

Figure 12. Capacity dependence of the UV-vis adsorption peak intensity at 579 nm (green blocks) and galvanostatic ORR/OER curve (red trace) observed in TTF-containing Li-DMSO electrolyte during the 1st OER and 2nd ORR processes. A part of the 1st ORR process is not shown in this figure. Inset shows the UV-Vis spectra observed at different states (marked by letters from B to F) during cycling. The ORR/OER curves in the TTF-free solution are also shown for comparison (blue dash line). The electrochemical conditions and meanings for these characters are given in Fig. 8. See the text for details. ACS Paragon Plus Environment


(c)


3.6 3.4 3.2 3.0

-1

after discharge to different capacity 2 (uAh/cm )

+


3.8

(b)

10mM TTF in 0.5 M Li-DMSO 100 uA/cm2

400 300 200 100 OCP

2.8

KB-CP

Li2O2

Li2CO3

D

G

3k

3k

Li2O2 Li2CO3

2k

2k

1k

1k

0

0

-1

2.6

Absolute Raman Intensity @ 784 cm

(a)

Absolute Raman Intensity @ 1087 cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 54 of 58

0

100

200

300 2

Capacity (uAh/cm )

400

600

800

1000

1200

1400 -1

Raman Shift (cm )

1600

0

100

200

300

400 2

Discharge Capacity (Ah/cm )

Figure 13. (a) Galvanostatic ORR/OER curves of the porous carbon electrode at 100 μA/cm2 in 0.5 M LiClO4-DMSO with 10 mM TTF saturated by O2. Curves at different ORR/OER depths are separated by different colors. (b) Capacity dependent ex situ Raman spectra are shown at different ORR depth. The Raman spectrum for original electrode and standard samples (Li2O2 and Li2CO3) are shown for comparison. All the Raman spectra are offset for clarity. (c) Capacity dependence of Raman peak at Li2O2 (blue blocks) and Li2CO3 (red triangles) are also summarized at different ORR depths in both TTF-containing solutions. See the text for details. ACS Paragon Plus Environment

Page 55 of 58

0.15

Discharge to 100 uAh/cm2 200 300 400

-1

0.04

Li2CO3@860cm / Li2O2@600cm

-1

0.20

0.02

(b) 0.20

linear fit

Discharge Ratio in moles capacity Li2CO3 / Li2O2

0.10

100 200 300 400

0.05

0.04

0.08

0.12

0.16

0.103 0.106 0.104 0.101

0.20

Li2CO3 / Li2O2 in moles

0.15

UV-vis Abs

(a) Mid-IR Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


After full ORR/OER process at different discharge-charge depth +

579 nm TTF

0.10

400 uAh/cm

2

300 uAh/cm

2

200 uAh/cm

2

0.05 0.00

100 uAh/cm

Li2O2 Li2CO3

500

750

1000

2

0.00 1250 -1

Wavenumber (cm )

1500

400

500

600

700

800

Wavelength (nm)

Figure 14. (a) Ex situ IR spectra observed on the porous carbon electrode surface in 0.5 M LiClO4-DMSO recorded at different stage of ORR. The spectra for standard Li2O2 and Li2CO3 samples are shown for comparison. Inset shows the mid-IR calibration curve obtained by mixing known amounts of Li2O2 and Li2CO3 with different moles ratios (expansion from Figure 11 in the low ratio part). The isolated Li2O2 peak at 600 cm-1 and Li2CO3 peak at 880 cm-1 are chosen for the quantitative analysis. The orange circle highlights the values obtained from the electrodes on different ORR depths and the corresponding values are also listed up in the figure. (b) UV-Vis absorption spectra observed after a complete ORR/OER process at different ORR/OER depths (marked by different colors in Figure 13a). The peak intensity at Paragon Environment 579 nm corresponds to the amount of the TTF+ remainingACS inside thePlus porous carbon electrode after the OER process. See the text for details.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 a 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 56 of 58

Table 1. Moles amount of charge during ORR/OER and UV-Vis absorbance during a galvanostatic ORR/OER cycle in 0.5 M LiClO4-DMSO solution. The mole amounts of Li2O2 obtained by two different methods are also given. See the text for details. Galvanostatic

End of ORR (point A, without TTF) End of OER (point C3, replace electrolyte)

Capacity (μAh/cm2)

n1 -7 (x10 mol)

10

3.74

10

3.74

UV-Vis Absorbance a 0.304 (@252 nm, O2-) 0.0264 (@579 nm, TTF+)

Li2O2 n2 -7 (x10 mol)

(n1-n2)/2 (x10-8 mol)

3.36

1.9

3.37

1.85

Amount of superoxide in TTF-free electrolyte at the end of ORR was determined form UV absorbance at 250 nm. Amount of TTF+ at the end of OER process in TTF-contained electrolyte after electrolyte replacement was determined from UV absorbance at 579 nm. The amount of TTF+ at end of OER equals to the amount of superoxide produced during ORR process, which removed by the electrolyte replacement step at the end of ORR.


Page 57 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Table 2. The distribution of the ORR/OER capacities and the amount of products formed in each stage in the TTFfree and TTF-containing solutions. The target capacity for each cycle is 200 μAh/cm2. The m1 (m1‘) and m2 (m2‘) denote oxidative decomposition of carbon/solvent and electrochemically decomposed Li2CO3, respectively, in the OER process. See the text for details. TTF-free solution

TTF-containing solution

Capacity (μAh/cm2)

Li2O2

Li2CO3

Li2O2

Li2CO3

TTF+

1st ORR

181

19

181

19

0

1st OER

33

37

0

19

20

m1=35.5 m2=16.5 2nd ORR

212

60

m1’=0 m2‘=0 161


39

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 58 of 58

Table 3. UV-Vis absorbance at 579 nm at end of each OER process, corresponding mole number of TTF+ and capacity for Li2CO3 during different ORR/OER depths in 10mM TTF-containing LiClO4-DMSO solution. The efficiency is estimated by ratio of C2 (capacity for Li2CO3) and C1 (cycled capacity). See the text for details. ORR/OER Capacity Absorbance @579 nm TTF+ (x10-7 mol) (C1, μAh/cm2) (at end of OER) (at end of OER)

a Capacity

Capacity for Li2CO3 (C2, μAh/cm2) a

Efficiency (C2/C1)

100

0.03

3.75

10.03

0.100

200

0.06

7.7

20.6

0.103

300

0.089

11.39

30.47

0.102

400

0.117

14.92

39.93

0.100

for Li2CO3, C2, was determined from the amount of TTF+ after the OER cycle.


Spectroscopic Investigation for Oxygen Reduction and Evolution

Recommend Documents