Superoxide (Electro)Chemistry on Well-Defined Surfaces in Organic

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Superoxide (Electro)Chemistry on Welldefined Surfaces in Organic Environments. Bostjan Genorio, Jakub Staszak-Jirkovsky, Rajeev S. Assary, Justin G. Connell, Dusan Strmcnik, Charles E. Diesendruck, Pietro Papa Lopes, Vojislav R. Stamenkovic, Jeffrey S. Moore, Larry A Curtiss, and Nenad M. Markovic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12230 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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The Journal of Physical Chemistry

Superoxide (Electro)Chemistry on Well-Defined Surfaces in Organic Environments. Bostjan Genorio†,‡, Jakub Staszak-Jirkovský†, Rajeev S. Assary†, Justin G. Connell†, Dusan Strmcnik†, Charles E. Diesendruck§, Pietro P. Lopes†, Vojislav R. Stamenkovic,† Jeffrey S. Moore§, Larry A. Curtiss†, and Nenad M. Markovic†* †

Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

‡ University of Ljubljana, Faculty of Chemistry and Chemical Technology, Ljubljana, SI-1000, Slovenia § Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

KEYWORDS superoxide anion, well-characterized interfaces, solid electrolyte interface, outer-sphere reaction, solvent decomposition, nucleophilic attack, radical hydrogen abstraction

ABSTRACT: Efficient chemical transformations in energy conversion and storage systems depend on understanding superoxide anion (O2-) electrochemistry at atomic and molecular levels. Here, a combination of experimental and theoretical techniques are used for rationalizing, and ultimately understanding, the complexity of superoxide anion (electro)chemistry in organic environments. By exploring the O2 + e- ↔ O2- reaction on well-characterized metal single crystals (Au, Pt, Ir), Pt single crystal modified with a single layer of graphene (Graphene@Pt(111)) and glassy carbon (GC) in 1,2 dimethoxyethane (DME) electrolytes we demonstrate that: (i) the reaction is an outer-sphere process; (ii) the reaction product O2can “attack” any part of the DME molecule, i.e., the C-O bond via nucleophilic reaction and the C-H bond via radical hydrogen abstraction; (iii) the adsorption of carbon-based decomposition products and

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the extent of formation of a “solid electrolyte interface” (“SEI”) increases in the same order as the reactivity of the substrate, i.e., Pt(hkl)/Ir(hkl) >>> Au(hkl)/GC > Gaphene@Pt(111); and (iv) the formation of the “SEI” layer leads to irreversible superoxide electrochemistry on Pt(hkl) and Ir(hkl) surfaces. We believe this fundamental insight provides a pathway for the rational design of stable organic solvents that are urgently needed for the development of a new generation of reliable and affordable battery systems.

Introduction The superoxide anion (O2-), a one-electron adduct of dioxygen (O2 + e- ↔ O2-), is a free radical that plays a significant role in a number of processes relevant to physiology, biochemistry, biology,1,2 inorganic and organic chemistry,3,4 and electrochemistry.5–10 In their early work, Sawyer and co-workers explored how the thermodynamics and kinetics of the superoxide anion are highly dependent on the nature of solvents and on the electrode materials used.5 In aqueous solutions, for example, the most dominant characteristic of O2- is a strong interaction with hydrogen, leading to the hydration of O2- ions by water (heat of hydration –∆H = 418 kJ) and a consequent shift in the O2/O2- redox potential. Due to its strong hydration, the O2- radical anion exhibits low reactivity in aqueous media, which led Sawyer and coworkers to conclude that, in fact, the prefix “super” in superoxide does not imply an exceptional reactivity of O2- species.4 Another key electrochemical property in aqueous solutions is the deprotonation of water molecules by O2- (acting as a strong Brønsted base) that is followed either by disproportionation to form peroxide-like species and dioxygen

8

or further electrochemical reduction to water (acid solu-

tions) and hydroxyl ions (alkaline media).11 In the aprotic environments commonly used in battery systems, O2- is quite stable because disproportionation to give the peroxide dianion (O22-) is highly unfavorable.12 It is also well established that in organic environments O2- is a strong nucleophile that is able to attack different parts of organic molecules. This, in turn, leads to the decomposition of organic solvents.13 It was also shown previously that there is a weak interaction between polycrystalline electrode


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materials and O2- ions in organic solvents, signaling that superoxide electrochemistry might be an inner sphere reaction involving direct bond formation and orbital overlap with the surface.5,14 Our recent report, however, revealed that the O2 + e- ↔ O2- reaction on gold single crystal surfaces in pure organic environments is not a structure-sensitive process15, proceeding via an outer-sphere mechanism where neither O2 nor O2- interact strongly with gold surface atoms.16,17 Although this observation, together with observations from other groups, helped to elucidate the role of oxygen reaction intermediates in H2O-mediated H+-O2 (aqueous)15 and Li+-O216 or Na+-O218 (organic) electrochemistry, it has also raised many intriguing questions. For example: does the O2 + e- ↔ O2- reaction become a structuresensitive process on the more catalytically-active platinum single crystal surfaces; what part(s) of organic molecules are “attacked” by O2-; and perhaps most intriguingly, do these decomposition products interact with the electrode surface after nucleophillic attack (presumably forming a carbon-based adlayer which for our purposes here we termed as the “solid electrolyte interphase”, “SEI”); or do they diffuse away into the bulk of the electrolyte? Resolving these fundamentally important issues requires superoxide electrochemistry to be explored at well-defined interfaces in impurity-free organic electrolytes, which surprisingly are still missing. It is reasonable to anticipate that answering these questions will make it possible to establish functional links between the (electro)chemistry of superoxide anions and their role in the scission of different parts of organic molecules. Establishing such functional links will ultimately lead to deeper understanding of the still poorly-understood interfacial processes during charging-discharging processes in organic electrolytes. In this paper, we present an approach that can be used to rationalize, and ultimately understand, the complexity of superoxide anion (electro)chemistry in organic environments. This approach entails building electrochemically-relevant interfaces on well-defined gold, platinum and iridium single crystal surfaces, platinum single crystals modified with a single graphene layer, and glassy carbon electrodes in a water-free 1,2-dimethoxyethane (DME) solvent with tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. A combination of experimental and theoretical techniques are



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used to demonstrate, at atomic- and molecular-levels, that: the O2 + e- ↔ O2- reaction is indeed an outersphere reaction; every bond in DME is accessible to nucleophilic or abstraction attack by O2-; and an “SEI” is predominately formed on catalytically active platinum and iridium surfaces. The insights from the fundamental understanding provided in this study indicates a pathway towards the rational design of stable organic solvents, which are urgently needed for the development of new generation of reliable and affordable battery systems.

Experimental Chemicals and General Purification Procedures. All experiments were performed in dried glassware under Ar atmosphere unless stated otherwise. 1,2-dimethoxyethane (DME) CHROMASOLV®, for HPLC grade from Sigma-Aldrich was dried over activated basic Al2O3, filtered and distilled over liquid Na/K alloy. Caution: All steps involving Na/K alloy should be carried out with extreme caution under strict exclusion of air or moisture, under inert gas and appropriate personal protection (hood, blast shields, face shield, protective and fire resistant clothing) should be used and worn at all times. Tetrabutylammonium hexafluorophosphate (TBAPF6) for electrochemical analysis from Sigma-Aldrich was dried in vacuum (~10-2 mbar) at 100oC for 24 h. NaH, Tetraethylene glycol dimethyl ether (TEGDME), ethylene glycol, and CD3I were used as received. 0.1M HClO4 was prepared from 70% perchloric acid OmniTrace Ultra™ from EDM (7g) and MilliQ water (500 mL) and purified electrochemically. 0.15M TBAPF6 in DME was prepared in a glovebox and used without further purification Synthesis of 1,2-dimethoxyethane-d6 (d-DME). To an oven-dried, 250 mL 2-necked round-bottom flask, dry NaH (9.0 g, 0.375 mol) was added under dry N2. The vessel was sealed with septa and TEGDME (~100 mL) was added using a syringe. The mixture was cooled with an ice bath. Ethylene glycol (10 g, 0.161 mol) dissolved in TEGDME (50 mL) was added to the reaction mixture dropwise. After all the glycol was added, the mixture was left stirring at room temperature for 3 h. The mixture was again cooled with an ice bath and CD3I (50 g, 0.345 mol) was slowly added drop-wise. The reaction ACS Paragon Plus Environment

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mixture was solidified and allowed to stand for at least 48 h. The crude product was then purified by distillation. The first fraction – pure compound was collected at 82°C as a colorless liquid (9.2 g, 59% yield). NMR in accordance to literature.19 Electrochemistry. Au(100), Au(111), Pt(100), Pt(111), Ir(111) single crystals and glassy carbon of 6mm in diameter were used in the experiments. Au crystals were annealed before each experiment using RF induction heating at 800oC under a 3% H2 in Ar stream. Pt and Ir crystals were annealed before each experiment using RF induction heating at 1100oC under a 3% H2 in Ar stream. For the ORR, a 10% O2/Ar mixure was used as a reagent. Water content in electrolytes was measured using Karl Fischer titration (Metler-Toledo) placed inside an Ar filled glove-box with H2O level below 0.5 ppm. Water concentration of supporting electrolytes was measured before each experiment and was always < 0.2 ppm (method detection limit). All experiments were carried out inside of an Ar filled glove-box and in a home-designed glass cell with ~40 ml of electrolyte. A three-electrode system was employed with Pt wire as a counter electrode and Ag/Ag+ nonaqueous quasi-reference electrode (QRE) reference electrode separated by a Luggin capillary. The counter electrode compartment was separated from the main compartment by a frit. The Ag/Ag+ QRE reference was sealed in a separate compartment divided from the rest of the cell by Vycor tip. The working electrode was tightly held in Kel-F collet and sealed by a PTFE u-cap to ensure that only the defined surface was in contact with the electrolyte. The reference potential was calibrated by measurement of the ferrocene redox couple.20 For the experiments in organic solvents, all potentials were recalculated vs. the standard Li/Li+ couple. The aqueous experiments were performed in a glass cell in a three-electrode setup. An Ag/AgCl reference electrode was employed and the potential was recalculated with respect to the reversible hydrogen electrode (RHE) scale. The Pine Instruments rotators used in the RDE measurements were set to 400RPM unless stated otherwise. Autolab potentiostats were used in the electrochemical measurements, and IR drop correction was used in all the experiments.



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X-ray Photoelectron Spectroscopy (XPS) Measurements. A Specs PHOIBOS 150 hemispherical analyzer was used to measure the XPS spectra. All samples were transferred to the UHV system within 2 min after the end of electrochemical experiments to avoid excessive oxidation in air. A monochromatic Al Kα X-ray source (1486.6 eV) was used for all measurements with an analytical spot size of 3.5 mm × 1.0 mm and a 45° takeoff angle. All measurements were performed at room temperature under ≤ 6 x 10– 10

mbar vacuum with a pass energy of 20 eV. Au samples were referenced against the Au 4f7/2 line at

84.0 eV, Pt samples were referenced against the Pt 4f7/2 line at 71.2 eV, and Ir samples were referenced against the Ir 4f7/2 line at 60.9 eV. Subtractively Normalized Interfacial Fourier Transform Infrared Reflectance Spectroscopy (SNIFTIRS). A Thermo Nicolet 8700 FTIR spectrometer was used for the SNIFTIRS experiments. The experiments were carried out in a home-designed sealed cell to prevent contamination with ambient moisture. The electrochemical setup was similar in principle to the electrochemical experiments described above. A reflectance geometry was employed with a CaF2 prism and p-polarized light filter. A Pike optical box was used to control the incidence beam angle close to ~63 degrees. Computational details. All calculations presented herein are computed using the Gaussian 09 software.21 The B3LYP/6-31+G(d) level of theory is used to compute electronic energies, enthalpies and spectra, unless mentioned otherwise. This level of theory provides adequate accuracy for computing reaction energetics.22–24

Results and Discussion We start by summarizing electrocatalytic trends for the O2/O2- redox couple on Au(111), Au(100), glassy carbon, Pt(100), Pt(111), Ir(111) and Pt(111) modified by a single graphene layer in impurity-free DME/TBAPF6 electrolytes saturated with a 10% O2/Ar mixture (Figure 1a, for details see the methods section). Corresponding results for Au(100) and Pt(100) surfaces are summarized in Figure S1 and S2, respectively. The cyclic voltammograms (CVs) and corresponding polarization curves, summarized in ACS Paragon Plus Environment

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Figure 1a, reveal that the same reversible O2/O2- redox couple, centered at ~2.2 V with a peak separation of ~75 mV, is observed on Au(111) (Figure 1a, i), Au(100) (Figure S1) and glassy carbon (Figure 1a, ii). The polarization curves observed under hydrodynamic conditions on these three surfaces are almost identical, confirming that the O2 + e- ↔ O2- reaction is indeed an outer sphere electron transfer process, at least on materials that are catalytically inactive for the adsorption of O2 and its intermediates in aqueous environments.25 The results obtained on Pt(111) (Figure 1a, iii), Pt(100) (Figure S2) and Ir(111) (Figure 1a, iv) under the same experimental conditions reveal both differences and similarities with the corresponding gold single crystals. Similar to Au(hkl), the O2 + e- ↔ O2- reaction is not a structure sensitive process, e.g., the same activity is observed for Pt(111) and Pt(100) (Figure S2). In contrast to Au(hkl) (and carbon), however, the O2/O2- redox couple on Pt(hkl) and Ir(111) is a rather irreversible process, with the peak separation increasing from 0.37 V to 0.45 V with progressive electrode potential cycling (Figure S3). A similar increase in peak separation is also observed for Ir(111) (Figure S4). Furthermore, comparison of polarization curves reveals that gold and carbon are more active than platinum electrodes; namely the onset potential for the reaction on Pt(111) and Pt(100) is shifted to more negative potentials. At first this observation is surprising considering that platinum is one of the best catalysts for the oxygen reduction reaction (ORR),11 in impurity-free aqueous environments.15 In the presence of even trace levels of organic impurities in aqueous systems the ORR is strongly inhibited due to the adsorption of organic species and concomitant poisoning of Pt active sites.26,27 It is reasonable, therefore to anticipate the the higher superoxide anion irreversibility on Pt(hkl) Ir(hkl) in Figure 1 is the result of the irreversible adsorption of various organic species (see also Figure S5). An ex situ experimental method comprised of four distinct steps was developed in order to probe the chemical nature of any adsorbed organic molecules on Pt which, for simplicity, will be termed as a “solid electrolyte interface” (e.g., “Pt-SEI” layer, Figure 1b). Certainly, there is a clear distinction between the tens of nm thick SEI layer that is typically discussed by the Li-ion battery community,28 and the “SEI” layer formed in our experiments. In particular, after two potential cycles in an electrochemical



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cell containing organic electrolyte (Step 1), the electrode was transferred to ultrahigh vacuum (UHV) to perform X-ray photoelectron spectroscopy (XPS) and characterize the chemical nature of organic adsorbates (Step 2). Deconvolution of the C 1s XPS spectra on Pt(111), summarized in Figure 1c, reveals five distinct peaks corresponding to the five different chemical environments of carbon atoms on the surface. The lowest binding energy peak (284.3 eV) corresponds to emission from the C=C (sp2) core hybridized carbon orbital, followed by the C-C (sp3) core orbital (285.2 eV), the ether C-O bond (286.3 eV), carbonyl C=O bond (287.9 eV) and carboxylate O-C=O bond (289.0 eV). The relative percentages of each species are summarized in Table S6a. We note in passing that almost identical XPS results are observed on Ir(111) (Figure S7 and Table S6c).

Figure 1 a) Cyclic voltammetry (sweep rate; 100mV/s) and polarization curves by RDE (rotation rate; 400 RPM) of O2 electrochemistry in dry 0.15M TBAPF6/DME saturated with 10% O2 in Ar on i) Au(111), ii) Glassy carbon, iii) Pt(111), iv) Ir(111), and v) Graphene@Pt(111). b) Shematics of solid electrolyte interface (SEI) formation with corresponding electrode


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activity increase. c) X-ray photoelectron spectroscopy (XPS) C1s spectra with deconvolution of SEI formed during O2 electrochemistry in dry 0.15M TBAPF6/DME saturated with 10% O2 on Pt(111) working electrode.

The presence of carbonyl and carboxylate species on the Pt(111) surface suggests that there is indeed deposition of organic species on the electrode surface during cycling, and that these organic species are likely decomposition products related to the interaction of DME with O2- species formed during the ORR. Examination the C 1s XPS spectra of Au(111) (Figure S8 and Table S6b) indicates that there is a significantly lower percentage of carbonyl (C=O) and carboxylate (O-C=O) species present after electrochemical cycling, highlighting the weak interaction of O2--formed decomposition products with gold surface atoms. Following XPS characterization, the electrodes were transferred to an aqueous 0.1 M HClO4 electrolyte hoping that the adlayer will remained stable so that it will be possible to assess the surface coverage of adsorbed carbon species (Step 3). Indeed, comparison of the CVs for a clean Pt(111) electrode and a Pt(111) surface modified by an “SEI” layer (Figure S5) reveals that the charge under the peaks corresponding to adsorbed hydrogen (0.05-0.45V) and hydroxyl species (0.65-0.85V) is considerably reduced on the “Pt(111)-SEI” electrode. Based on differences in pseudocapacitance for the hydrogen adsorption on bare Pt(111) (160 µC/cm2) and Pt(111) covered by an “SEI” (54 µC/cm2), the surface coverage by carbon species (ΘC) is estimated to be ~0.7 ML. Furthermore, we found that the “SEI” layer was extremely stable in the potential region between 0.05 and 0.85 V, suggesting that organic molecules are irreversibly adsorbed (chemisorbed) and that the kinetics of superoxide electrochemistry on Pt(hkl) surfaces in organic environments are predominantly governed by the nature/coverage of the “SEI” layer. The fact that similar behavior was observed for the Ir(111) electrode (Figure S9) indicates that “SEI” formation is universal on Pt-based electrode materials. These results highlight that special care should be taken when investigating organic electrochemistry (in particular Li-O2) on catalytically active Ptbased materials.



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Figure 2 a) Cyclic voltammetry of Au(111) surface in 0.15M TBAPF6/DME saturated with O2. In situ infrared reflectionabsorption difference spectra in 0.15M TBAPF6/DME saturated with O2 at Au(111) during b) oxidation (red), i.e. sweep to more positive potentials and c) subsequent reduction (blue). Sweep rate =10mV s-1.

Given the remarkable stability of the “SEI”-modified Pt(hkl) surfaces in aqueous environments, electrodes were returned to organic electrolytes (Step 4) to investigate whether the superoxide electrochemistry was at all modified due the surface treatment in the prior three steps. Interestingly, the fact that the polarization curves and CVs recorded in this final step (Figure S10) are identical to those in Figure 1a, iii for the “pristine” Pt(111) electrode strongly suggests that a stable “SEI” is formed in the presence of O2- anions. Thus, the kinetics of the O2 + e- ↔ O2- reaction on Pt and Ir is primarily controlled by the nature of “SEI” and not by the energy of adsorption of O2 and O2-on free metal sites. As a first approximation, it is reasonable to suggest that the observed irreversibility on Pt and Ir is a consequence of lower electronic conductivity of the “SEI” layer then naked metal sites. In turn, affecting (attenuate) the rate of charge transfer from/to an electrode surface. To support the hypothesis that the “SEI” conductivity con-


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trols the kinetics of the reaction, we synthesized an “artificial SEI layer” on Pt(111) consisting of an electronically conductive, single layer of graphene (a full paper for this graphene system is in preparation). As summarized in Figure 1a, v, a highly reversible redox couple is observed on Pt(111) with a single graphene layer which is “resistant” to the formation/effect of “SEI” layer formed on Pt and Ir, supporting our supposition that the conductivity of the “SEI” indeed play an important role in the O2 to O2redox process. The small difference in the reversibility observed on gold and graphene is likely due to the formation of a small amount of “SEI” layer on the Au surface, which is confirmed by XPS and electrochemical measurements (Table S6b and Figure S8). Finally, the results for graphene modified Pt(111) show unambiguously that the superoxide electrochemistry is an outer-sphere reaction. Having demonstrated that the irreversible adsorption of decomposition products play a central role in governing the kinetics of the O2 + e- ↔ O2- reaction in organic electrolytes, it is important to understand what part of the DME molecule is prone to nucleophillic attack by O2- in order to better understand how to eliminate the formation of such detrimental “SEI” layers. To resolve this issue we make use of a combination of experimental and theoretical techniques capable of providing information at atomic and molecular levels. We begin by further investigating the chemical nature of decomposition products via in situ, subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS) on Au(111) in oxygen saturated TBAPF6/DME electrolytes (Figure 2). We chose to study this system because Au(111) is free of the “SEI” layer, so the fast production of O2- allows superoxide anions to be formed (and trapped) within a thin electrolyte layer (~ 10 µm) between the Au(111) electrode and spectroscopic prism window upon one-electron reduction of O2. A central feature of the FTIR difference spectra obtained during the potential sweep from 3.3 to 1.0 V is a sharp positive band at 1607 cm-1 (Figure 2c), which is tentatively assigned to C=O stretch of TBA+-formate (R-COO- TBA+).29 Cycling to positive potentials from 3.0 V to 4.2 V shows a negative peak at the same band frequency in the difference spectra (Figure 2b), signaling the consumption (oxidation) of formate anions and the concomitant production of CO2 (appearance of a sharp positive band at 2345 cm-1). Although the FTIR data re-



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veal that the DME molecules are not stable against nucleophilic attack by superoxide ions, the mechanism by which such stable ethers are attacked is still unclear. Developing such understanding is critical to learn how to design a stable organic solvent for the ORR. To provide an atomistic understanding of the superoxide-mediated decomposition of DME and identify likely decomposition pathways, density functional theory (DFT) calculations were performed at the B3LYP/6-31+G(d) level of theory. First, selected reactions of DME and likely fragments with oxygen and superoxide are considered and their computed energetics (17 reactions) in the gas phase are summarized in Table S11, and suggest that the oxidation reactions of the DME molecule are highly exothermic in nature (Figure 3a). For example, the oxidation of DME to form four formate molecules is exothermic by ~10 eV (Table S11 reaction 1). Also, partial oxidation of DME (Table S11 reaction 6) to form one formate molecule is found to be exothermic by ~ 4 eV. Thus, the exothermic nature of the reaction increases with respect to the degree of oxidation. In general, upon removing hydrogen atoms or breaking C-O bonds, the DME molecule is increasingly vulnerable to irreversible (exothermic) oxidation reactions (Table S11 reactions 5 to 17). Therefore, computation suggests that the rate-determining step in DME decomposition is the initiation reaction (Figure 3a). Given that the reactivity of the superoxide anion radical towards DME is due to its combined anionic and radical nature, it is possible that superoxide could initiate C-O bond cleavage via nucleophilic reaction and C-H bond cleavage via radical hydrogen abstraction reactions. Since the superoxide formed at the interface is likely to be more reactive due to a lack of solvation, kinetics computed in the gas–phase are adequate to determine whether or not such pathways are favorable. The computed enthalpies of activation of the C-O bond breaking (i: primary ‘CH3’ center and ii: secondary ‘CH2’ center) and the C-H bond breaking (iii: primary ‘CH3’ center and iv: secondary ‘CH2’ center) are shown in Figure 3a. Indeed, computation reveals that the C-O bond cleavage proceeds via nucleophilic attack of the carbon by the superoxide anion (primary or secondary). A detailed pathway for the ‘CH3-O’ bond-scission reaction profile is presented in the SI (Figure S12a). Furthermore, computation shows that C-H bond cleavage


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proceeds via hydrogen abstraction by the superoxide anion radical, as proton abstraction is both kinetically and thermodynamically unlikely due to the highly endothermic nature of the reaction. A detailed reaction pathway for DME decomposition initiated via hydrogen abstraction (‘CH2-H’) is described in the SI (Figure S12b). In general, the computed gas-phase enthalpies of activation are all less than 0.8 eV, suggesting the kinetic feasibility of these reactions regardless of the nature of the type of the bonds (C-H or C-O) or their positions (primary or secondary). These reactions result in the oxidation of carbon atoms via a process in which an initiation reaction takes place and results in the formation of formate ions due to subsequent exothermic oxidation reactions (Figure S12). Additionally, we note that that the activation enthalpies for all initiation reactions (Figure 1a reactions i- iv) are increased to ~1.5 eV in the presence of a solvent dielectric (acetone, ε=20.6), indicating that the reactions of the superoxide anion in the bulk solvent medium are kinetically much slower compared with the gas phase reactions (i.e. reactions taking place in the double layer) summarized in Table S13. In order to determine if even the most stable part of the DME molecule is attacked by O2-, experiments were undertaken with DME that had been selectively labeled by deuterium atoms at the methyl groups D3C-O-CH3-CH2-O-CD3 (d-DME) (Figure 3b). Figure 3c shows that the d-DME yielded significant, 17 ± 5 cm-1, redshifts of the 1607 cm-1 band to ~1590 cm-1, thereby determining that CH3-R (the α-carbon) group are also a target of the reactive superoxide. The observed redshift is also supported by the DFT calculations, which predict redshifts for carboxylate (RCOO-) species with different length of R and therefore different placements of deuterium (Table S14). These calculations further show that D must be present at the primary carbon of the COO- group in order to induce a significant shift of the C=O stretch such as that observed in this study. Nevertheless, the fact that DFT calculations predict that the α-carbon is the most stable part of the DME molecule suggests that superoxide anions will likely attack the βcarbon as well as the C-O bond, implying that the entire DME molecule is subject to nucleophilic attack and decomposition during the ORR. As discussed above, these decomposition products either stay in the electrolyte, as is the case for glassy carbon, Au(hkl) and graphene-modified Pt(111), or are chemisorbed



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to form a stable “SEI” layer that can significantly alter the physicochemical properties of the interface, as is the case for Pt and Ir single crystal electrodes.

Figure 3 a) Schematic of the 0.15M TBAPF6/DME electrolyte decomposition in saturated 10% O2 in Ar . The rate limiting step is the C-O (i and ii; blue arrows) and C-H (iii and iv; red arrows) bond cleavage by superoxide to form intermediates (radicals, anionic species), while subsequent oxidation reactions that results in the formation of carbonyls, carboxylates and carbonates are largely exothermic in gas phase. b) Synthesis of the 1,2-dimethoxyethane-d6 (d-DME). c) A snapshot detail from in situ infrared reflection-absorption spectroscopy measurement in 0.15M TBAPF6 saturated with O2 in DME (black line) and d-DME (red line) at Au(111); this detail of the near 1600 cm-1 region and comparison between d-DME and regular DME was obtained in similar experiment as in Figure 2 and corresponds to a difference spectra at 1.0 V vs. Li/Li+ with respect to a reference spectra at 3.0 V.

Conclusions In conclusion, we used a combination of in situ (CV, FTIR) and ex situ (XPS) experimental and theoretical (DFT) techniques to explore the complexity of superoxide anion (electro)chemistry in in ACS Paragon Plus Environment

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TBAPF6/DME electrolyte. First, by focusing on the O2 + e- ↔ O2- reaction on well-characterized Au(hkl), Pt(hkl), Ir(hkl), Graphene@Pt(111) and glassy carbon surfaces we show unambiguously that the reaction is an outer-sphere process. Based on this finding, it is reasonable to anticipate that the structure sensitivity and high activity of Pt in aqueous media is not solely controlled by the energy of adsorption of O2 and reaction intermediates. We also find that although O2- produced during the ORR attacks every part of the DME molecule (either the C-O bonds via nucleophilic reaction or the C-H bonds via radical hydrogen abstraction), the probability of bond scission by O2- decreases in the order C-O > CH2 > CH3. Both FTIR and computational results indicate that the resulting decomposition product is the formate ion. We also found that although electrolyte decomposition products interact strongly with Pt(hkl) and Ir(hkl) surface atoms, consequently leading to the irreversible adsorption of C=O, and O-C=O species, the interaction of these decomposition products with Au(hkl), GC and, in particular, graphene@Pt(111) is rather weak. The degree of reversibility of the O2/O2- redox couple, decreasing in the order Graphene@Pt(111) > Glassy carbon ~ Au(hkl) >>> Pt(hkl) ~ Ir(hkl), is inversely proportional to the surface coverage of “SEI”. The fundamental insight provided by this study offers a foundation for a surface science approach that is urgently needed for the development of new generation of reliable and affordable battery systems.

ACKNOWLEDGMENT This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract no. DE-AC02-06CH11357. Work related to electrochemistry in aqueous solutions was supported by the US Department of Energy, Basic Energy Science, Materials Science and Engineering Division. We gratefully acknowledge the computing resources provided on "Fusion," a 320-node ACS Paragon Plus Environment


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computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory.

ASSOCIATED CONTENT Supporting Information. Additional CVs, polarization curves, XPS, and computational results are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected]

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Figure 1 a) Cyclic voltammetry (sweep rate; 100mV/s) and polarization curves by RDE (rotation rate; 400 RPM) of O2 elec-trochemistry in dry 0.15M TBAPF6/DME saturated with 10% O2 in Ar on i) Au(111), ii) Glassy carbon, iii) Pt(111), iv) Ir(111), and v) Graphene@Pt(111). b) Shematics of solid electrolyte interface (SEI) formation with corresponding electrode activity increase. c) X-ray photoelectron spectroscopy (XPS) C1s spectra with deconvolution of SEI formed during O2 elec-trochemistry in dry 0.15M TBAPF6/DME saturated with 10% O2 on Pt(111) working electrode. 84x58mm (300 x 300 DPI)



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Figure 2 a) Cyclic voltammetry of Au(111) surface in 0.15M TBAPF6/DME saturated with O2. In situ infrared reflection-absorption difference spectra in 0.15M TBAPF6/DME saturated with O2 at Au(111) during b) oxidation (red), i.e. sweep to more positive potentials and c) subsequent reduction (blue). Sweep rate =10mV s-1. 84x45mm (300 x 300 DPI)


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Figure 3 a) Schematic of the 0.15M TBAPF6/DME electrolyte decomposition in saturated 10% O2 in Ar . The rate limiting step is the C-O (i and ii; blue arrows) and C-H (iii and iv; red arrows) bond cleavage by superoxide to form intermediates (radicals, anionic species), while subsequent oxidation reactions that results in the formation of carbonyls, carboxylates and carbonates are largely exothermic in gas phase. b) Synthesis of the 1,2-dimethoxyethane-d6 (d-DME). c) A snapshot detail from in situ infrared reflectionabsorption spectroscopy measurement in 0.15M TBAPF6 saturated with O2 in DME (black line) and d-DME (red line) at Au(111); this detail of the near 1600 cm-1 region and comparison between d-DME and regular DME was obtained in similar experiment as in Figure 2 and corresponds to a difference spectra at 1.0 V vs. Li/Li+ with re-spect to a reference spectra at 3.0 V. 71x49mm (300 x 300 DPI)


Superoxide (Electro)Chemistry on Well-Defined Surfaces in Organic

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