Oxygen Reduction Products Research Articles

A hydrogen peroxide microelectrode to use in bioelectrochemical systems

Polarized electrodes are used in bioelectrochemical systems (BES) in which the electrodes are used to grow biofilms or remove biofilms. In either case H2O2 is generated as an intermittent product of oxygen reduction. Traditionally, a H2O2 microelectrode (HPM) consists of a sealed platinum wire with a tip a few μm in diameter. This microelectrode is connected to an external reference electrode (which is also used as the counter electrode) and polarized to +0.8VAg/AgCl to measure H2O2 concentration. This traditional H2O2 microelectrode (tHPM) works well for measuring H2O2 concentrations in biofilms growing on non-polarized surfaces. It is a common misconception that this tHPM will work correctly in an electric field. We observed experimentally that the tHPM produces artifacts in measurements when it is used in BES. This has not been noticed before. These undesired effects needed to be resolved before H2O2 concentration could be measured in BES. The goals of this work were to (1) describe the artifacts related to the use of tHPM in BES, (2) develop a H2O2 microelectrode which does not suffer from these artifacts, (3) verify the operation of the new microelectrode in BES, and (4) improve the sensitivity of the microelectrode. In this work, we built an all-in-one HPM (aHPM) in which the reference electrode and the working electrode were placed a few 100μm away from each other in a sealed configuration. We verified that our aHPM, unlike the tHPM, is not affected by an interfering electric field and generates reproducible, correct measurements in BES. We compared H2O2 flux measurements from the electrode in a BES with depth profiles measured using aHPM and tHPM to identify artifacts and reliability. Finally, after proving the reliability of the aHPM in BES, we improved its sensitivity. The aHPM demonstrated a linear calibration in the range of 0.87μM to 166.8μM H2O2 concentration with a 1.69μM limit of detection (S/N=2). Although our goal was to develop an HPM which can operate in an electric field, the developed microelectrode can be used in many other applications requiring μM resolution.

Surface studies of lithium–oxygen redox reactions over HOPG

The O2/Li2O2 electrode reaction has been studied on low surface area HOPG electrodes in 0.1M LiPF6 in dimethyl sulfoxide (DMSO) electrolyte. Studies were performed using electrochemical cells coupled to a XPS spectrometer and to an AFM microscope. AFM images after electrochemical treatment at cathodic potentials exhibited 20 to 100nm in height features, whereas anodic treatment showed a thin film of about 1nm thickness deposited over the HOPG electrode. XPS spectra after electrochemical treatment showed surface species due to DMSO and LiPF6 decomposition. These findings indicate the high reactivity of oxygen reduction products towards the electrolyte and the solvent. The unwanted deposits formed under electrochemical operation cannot be completely eliminated from the surface even after applying high anodic potentials. This highlights the known loss of capacity of Li-air batteries, issue that must be overcome for successful applications.

Stability of Glyme-Solvate Ionic Liquid Electrolyte in the Presence of Oxygen Reduction Product

The lithium-oxygen (Li-O2) battery continues to attract much attention as one of the more promising future energy storage technologies, due in part to its high theoretical gravimetric energy density.[1] In earlier work, we reported the use of a solvate ionic liquid (SIL) as electrolyte for the Li-O2 system, and demonstrated reversible electrochemical reaction could be achieved via cyclic voltammetry.[2] In the current work, we report on our further study of the SIL system, applied to a functioning Li-O2 battery and here present an investigation probing one of the key challenges for the Li-O2battery, namely the reactivity of the electrolyte. SILs are exemplified by the equimolar binary mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4), shown in Fig. 1(a), which forms a room temperature liquid comprised of a solvate cation and counter-anion, [Li(G4)1][TFSA]. The intriguing thermal and physicochemical properties of such SILs, including negligible vapor pressure, high ionicity and a low proportion of free (un-coordinated) solvent, have been previously reported.[3] SILs also possess attractive characteristics as electrolytes for electrochemical devices including high molar Li ion concentration and high transference numbers.[4] We have previously demonstrated the use of SILs for battery systems, utilizing both conventional Li intercalation electrodes, and metallic lithium, notably the lithium-sulfur battery. Fig 1(b) shows a typical discharge-recharge curve for the application of [Li(G4)1][TFSA] in a Li-O2battery. The high charge over-potential observed is a widely reported characteristic of this cell chemistry, and not only reduces the overall energy efficiency of the cell, but also provides an oxidative potential sufficient to decompose typical dilute glyme-based electrolytes, contributing to low cycle-ability. As we have previously demonstrated, the formation of a SIL results in an improved oxidative stability, due to the lowering of the HOMO level of the glyme upon coordination with Li,[4,5] and thus the use of a SIL electrolyte provides a means to ensure electrolyte stability at the high oxidative potential observed during recharge. However, the reactivity of the oxygen reduction intermediate (O2 - radical) and product (Li2O2) with electrolyte solvent is another widely reported challenge, and has not yet been clarified. Here we report on our study of the stability of the SIL compared to the dilute solutions, and comment on these findings in light of recent reports of (i) a decreased reaction barrier for simulated C-O bond breaking of G4 coordinating with Li+,[6] and (ii) the switch in the principal contributor to reductive stability (LUMO) from solvent to anion for highly concentrated acetonitrile/TFSA solutions.[7] References [1] (a) J. Christensen, P. Albertus, R. S. Sanchez-Carrera, T. Lohmann, B. Kozinsky, R. Liedtke, J. Ahmed, A. Kojic, J. Electrochem. Soc., 2012, 159, R1. (b) A. C. Luntz, B. D. McCloskey, Chem. Rev., 2014, 114, 11721. [2] (a) R. Tatara, N. Tachikawa, H.-M. Kwon, K. Ueno, K. Dokko, M. Watanabe, Chem. Lett., 2013, 42, 1053. (b) R. Tatara, H.-M. Kwon, K. Ueno, N. Tachikawa, K. Dokko, M. Watanabe, ECS Meeting s, 2014, MA2014-04, 532.[3] (a) T. Mandai, K. Yoshida, K. Ueno, K. Dokko, and M. Watanabe, Phys. Chem. Chem. Phys., 2014, 16, 8761. (b) K. Ueno, R. Tatara, S. Tsuzuki, S. Saito, H. Doi, K. Yoshida, T. Mandai, M. Matsugami, Y. Umebayashi, K. Dokko, M. Watanabe, Phys. Chem. Chem. Phys., 2015, 17, 8248.[4] K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K. Dokko, M. Watanabe, J. Am. Chem. Soc., 2011, 133, 13121.[5] S. Tsuzuki, W. Shinoda, S. Seki, Y. Umebayashi, K. Yoshida, K. Dokko, M. Watanabe, ChemPhysChem, 2013, 14, 1993.[6] Y. Okamoto, Y. Kubo, J. Phys. Chem. C, 2013, 117, 15940.[7] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc., 2014, 136, 5039. Acknowledgements This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) and by the Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Figure 1

Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non-Aqueous Electrolyte Containing Phenol as a Proton Source.

The non-aqueous Li-air (O2 ) battery has attracted intensive interest because it can potentially store far more energy than today's batteries. Presently Li-O2 batteries suffer from parasitic reactions owing to impurities, found in almost all non-aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O2 (-) ), a reactive, one-electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non-aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O2 (-) , not HO2 , is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O2 (-) is explained using density functional theory (DFT) calculations.

Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non‐Aqueous Electrolyte Containing Phenol as a Proton Source

AbstractThe non‐aqueous Li–air (O2) battery has attracted intensive interest because it can potentially store far more energy than today′s batteries. Presently Li–O2 batteries suffer from parasitic reactions owing to impurities, found in almost all non‐aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O2−), a reactive, one‐electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non‐aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O2−, not HO2, is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O2− is explained using density functional theory (DFT) calculations.

Oxygen Reduction and Evolution Examined Using Quartz Crystal Microbalance and Electrochemical Methods in Non-Aqueous Li-O2 Batteries

The Lithium-Oxygen battery is one of the most promising battery technologies being researched, it is also one of the more challenging ones due to the various problems associated with the battery. One of these problems is the poor understanding of the reaction occurring during oxygen reduction and evolution. It is known that the intermediates of the reaction are highly reactive and take part in unwanted side reactions. It is also known that the oxygen reduction products are poorly soluble in the electrolyte. This study sets out to further examine these effects. Qualitatively these aspects are somewhat known but quantitatively the answer is less straight forward. By combining a Quartz Crystal Microbalance with electrochemical techniques the mass, current and voltage can be measured simultaneously to help shed further light on the species solubility and occurrence of side reactions. Obtaining this information about the chemical nature of the species formed will serve as a well needed compliment to the more frequently used surface characterization methods used when trying to determine the reactions occurring at the oxygen electrode.

Oxygen electroreduction in propylene carbonate solutions

The reaction of oxygen electroreduction is studied on various catalytically active materials: compact and disperse platinum, gold, and carbon in a nonaqueous solvent (propylene carbonate). In aqueous media at medium and low pH, the platinum activity is known to far exceed the activities of gold and carbon due to the high adsorption properties of platinum. It is shown that in the nonaqueous solvent medium, in the high current region, platinum demonstrates the activity lower as compared with Au and C. This is associated with the combined effect of the surface passivation by the products of solvent reaction with oxygen reduction products and the surface blockage with a film formed by oxygen-containing compounds of alkali metals.

The role of surface oxygenated-species and adsorbed hydrogen in the oxygen reduction reaction (ORR) mechanism and product selectivity on Pd-based catalysts in acid media.

Oxygen reduction reaction (ORR) is investigated on bulk PdO-based catalysts (oxides of Pd and Pd3Co) in oxygen-saturated 0.1 M HClO4 to establish the role of surface oxides and adsorbed hydrogen in the activity and product selectivity (H2O/H2O2). The initial voltammetric features suggest that the oxides are inactive toward ORR. The evolution of the ORR voltammograms and potential-dependent H2O2 generation features on the PdO catalyst suggest gradual and parallel in situ reduction of the bulk PdO phase below ∼0.4 V in the hydrogen underpotential deposition (Hupd) region; the reduction of the bulk PdO catalyst is confirmed from the X-ray photoelectron spectra (XPS) and X-ray diffraction (XRD) patterns. The potential-dependent H2O2 generation features originate due to the presence of surface oxides and adsorbed hydrogen; this is further confirmed using halide ions (Cl(-) and Br(-)) and peroxide as the external impurities.

Oxygen Reduction and Evolution Examined Using Quartz Crystal Microbalance and Electrochemical Methods in Non-Aqueous Li-Ion Electrolytes

The Lithium-Oxygen battery is one of the most promising battery technologies being researched, it is also one of the more challenging ones due to the various problems associated with the battery. One of these problems is the poor understanding of the reaction occurring during oxygen reduction and evolution. It is known that the intermediates of the reaction are highly reactive and take part in unwanted side reactions. It is also known that the oxygen reduction products are poorly soluble in the electrolyte.This study sets out to further examine these effects. Qualitatively these aspects are somewhat known but quantitatively the answer is less straight forward. By combining a Quartz Crystal Microbalance with electrochemical techniques the mass, current and voltage can be measured simultaneously to help shed further light on the species solubility and occurrence of side reactions. Obtaining this information about the chemical nature of the species formed will serve as a well needed compliment to the more frequently used surface characterization methods used when trying to determine the reactions occurring at the oxygen electrode.

Redox Shuttles for Sustained Oxygen Reduction in Non-Aqueous Lithium Electrolyte

Direct electrochemical oxygen reduction in non-aqueous electrolytes results in the deposition of an insoluble product on the electrode surface, thereby blocking access of oxygen to the surface for continued reaction (Fig. 1). Once the surface is completely covered, the reaction stops and the discharge is ended. Alternatively, in the presence of a soluble redox shuttle, oxygen reduction can occur homogeneously in solution near the air interface (Fig. 2). This has the effect of displacing oxygen reduction away from the electrode surface, which remains active to regenerate the shuttle to the reduced form. A further possibility is mediation of the oxygen reduction reaction in order to favour the formation of lithium peroxide rather than radical products that may degrade the electrolyte (Fig. 3). Given a suitable shuttle reagent, this scheme may have several potential advantages, for example,Avoidance of electrode passivation by insoluble lithium oxidesAn increased, chemical driving force for oxygen adsorption into the electrolyteIncreased mass transfer to the electrode surfaceEnhanced kinetics of lithium peroxide formationMinimisation of side reactions that may degrade the electrolyte We will present results on Ethyl Viologen acting as a shuttle, including its reduction at the electrode, its reactions with oxygen, and its stability to the products of oxygen reduction under various conditions. Acknowledgement This work was supported by the European Union’s Seventh Framework Programme under EC-GA No. 265971 ‘LABOHR’ and the University of Southampton.

An in-situ Raman study of the oxygen reduction reaction in ionic liquids

In-situ Raman spectroscopy is applied, for the first time, to elucidate the reaction products of oxygen reduction in two types of ionic liquids: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C2mimTFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI). The degradation of C2mimTFSI by superoxide attack is evidenced by the appearance of bands characteristic of amide and carbonate compounds. On the contrary, Pyr14TFSI is found to be resistant towards degradation. It is observed that superoxide is the first product of oxygen reduction in Pyr14TFSI, and the formation of Li2O2 is observed at longer times.

Composites of Metal Nanoparticles and Reduced Graphene Oxide for Rechargeable Li-O2 Battery

A metal-air battery functions involving the principles of both batteries and fuel cells. The anode of a metal–air cell is stored inside the cell and O2 for the air-electrode is supplied either from atmosphere or a tank. Zinc-air cells are commercially successful. In recent years, research activities on rechargeable Li-air cells have become intense due to their anticipated high energy density, which is close to specific energy of gasoline. Development of rechargeable Li-air cells involves several issues related to the negative Li electrode, the positive O2 electrode as well as the electrolyte. Among these problems, studies on rechargeable O2 electrode kinetics in non-aqueous electrolytes are important. Development of an appropriate catalyst, which allows both O2 reduction and its evolution with fast kinetics and high efficiency, is a challenging problem. Reduced graphene oxide (RGO) and its composites have emerged as important materials for energy storage systems because of high electronic conductivity and high surface area of RGO. In the present study, graphite is converted to graphite oxide by Hummer’s method and converted to graphene oxide by ultra-sonication. Graphene oxide is chemically reduced to RGO by NaBH4. Au-RGO is also prepared by a similar route. RGO and Au-RGO are characterized by physicochemical studies. Au nanoparticles of average size of 5.2 nm are distributed uniformly over RGO sheets (Figure: 1). Further characterization studies of RGO and Au-RGO are carried out by powder XRD, UV-Visible, Raman spectroscopy and XPS. Li-O2(RGO) and Li-O2(Au-RGO) cells are assembled in Ar atmosphere using Li foil as the negative electrode, the catalyst coated carbon paper as the positive electrode and 1.0 M LiPF6 dissolved in TEGDME as the electrolyte. A glass absorbing mat is used as the separator. Typical charge-discharge curves of Li-O2(RGO) and Li-O2(Au-RGO) are shown in Fig. 2. The discharge capacities obtained for the first discharge are 956 and 1738 mAh g-1, respectively, for Li-O2(RGO) and Li-O2(Au-RGO) cells at a discharge current of 0.2 mA cm-2. Li-O2(Au-RGO) cells are cycled at 0.6 mA cm-2 over about 120 charge-discharge cycles. Although about 1500 mAh g-1 discharge capacity is obtained for the cycle, there is a rapid decrease of capacity in the initial stages of cycling, and then there is a gradual decrease. Discharge capacity of about 500 mAh g-1 is obtained for the 120th cycle. The moderate stability of the Li-O2(Au-RGO) cells is attributed to nanoparticles of Au, which are uniformly distributed on RGO sheets. Powder XRD pattern of Au-RGO catalyst used for cycling of Li-O2(Au-RGO) cells indicate the presence of Li2O and Li2O2, which are the products of oxygen reduction in the non-aqueous electrolyte. Results of these investigations will be presented.Reference:1. K. M. Abraham and Z. Jiang, J. Electrochem. Soc., 1996, 143, 1-5.2.Yi-Chun Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham and Y. Shao-Horn, Energy Environ. Sci., 2013, 6, 750-768.3. S. Kumar, C. Selvaraj, N. Munichandraiah and L. G. Scanlon RSC Adv.,2013, 3 , 21706-21714.

AFM study of oxygen reduction products on HOPG in the LiPF6–DMSO electrolyte

Ex situ atomic force microscopy (AFM) has been used to study the morphology of oxygen reduction products in the LiPF6-dimethyl sulfoxide (DMSO) electrolyte, i.e. Li2O2 on a highly oriented pyrolytic graphite (HOPG) surface. Both cyclic voltammetry and chronoamperometry have shown that at low cathodic polarization the initial deposits decorate the edge steps of HOPG. At higher overpotentials a massive deposit covers the terraces. Upon charging the battery cathode Li2O2 oxidation and dissolution do not take place until high overpotentials are reached at which solvent decomposition has been demonstrated by in situ FTIR studies.

Studies of Li-Air Cells Utilizing Dimethyl Sulfoxide-Based Electrolyte

Dimethyl Sulfoxide (DMSO) was evaluated as a practical solvent for the rechargeable lithium air battery. Redox characteristics of the dissolved oxygen and its reduction products in the presence of lithium hexafluorophosphate (LiPF6) supporting electrolyte were studied via cyclic, rotating disk (RDE) and ring-disk (RRDE) electrode voltammetry. The DMSO medium facilitates reversible reduction and oxidation processes in contrast to other solvent-based electrolytes studied. Galvanostatic discharge-charge cycling of the Li-O2 cells has shown characteristics of rechargeability expected from voltammetric studies. Multiple high-efficiency discharge-charge cycles are possible if the depth of discharge of the carbon cathode is limited to avoid excessive passivation by the discharge products. The discharge voltage of this Li-O2 cell is higher than cells assembled with other non-aqueous organic electrolytes, an attribute ascribed to the stability of superoxide (O2−), the one-electron reduction product of oxygen.

A redox shuttle to facilitate oxygen reduction in the lithium air battery

A novel design of the non-aqueous lithium air cell is presented with a demonstration of a new reaction concept, involving a soluble redox shuttle to catalyse oxygen reduction. In principle, this can relieve the requirement for fast diffusion of molecular oxygen from the air interface to the positive electrode. To demonstrate this concept, ethyl viologen ditriflate was dissolved in BMPTFSI, reduced at a carbon electrode and regenerated by aspiration with oxygen. Useful shuttle behaviour, confirmed by several reduction–oxidation cycles, was observed in the case where the electrolyte contained at least 0.3M lithium salt. The beneficial effect of the salt was attributed to its critical role in converting superoxide, which would otherwise destroy the shuttle, into the more desirable product of oxygen reduction, lithium peroxide.

Synthesis, Characterization, and Structural Modeling of High‐Capacity, Dual Functioning MnO2 Electrode/Electrocatalysts for Li‐O2 Cells

AbstractIt has become clear that cycling lithium‐oxygen cells in carbonate electrolytes is impractical, as electrolyte decomposition, triggered by oxygen reduction products, dominates the cell chemistry. This research shows that employing an α‐MnO2/ramsdellite‐MnO2 electrode/electrocatalyst results in the formation of lithium‐oxide‐like discharge products in propylene carbonate, which has been reported to be extremely susceptible to decomposition. X‐ray photoelectron data have shown that what are likely lithium oxides (Li2O2 and Li2O) appear to form and decompose on the air electrode surface, particularly at the MnO2 surface, while Li2CO3 is also formed. By contrast, cells without α‐MnO2/ramsdellite‐MnO2 fail rapidly in electrochemical cycling, likely due to the differences in the discharge product. Relatively high electrode capacities, up to 5000 mAh/g (carbon + electrode/electrocatalyst), have been achieved with non‐optimized air electrodes. Insights into reversible insertion reactions of lithium, lithium peroxide (Li2O2) and lithium oxide (Li2O) in the tunnels of α‐MnO2, and the reaction of lithium with ramsdellite‐MnO2, as determined by first principles density functional theory calculations, are used to provide a possible explanation for some of the observed results. It is speculated that a Li2O‐stabilized and partially‐lithiated electrode component, 0.15Li2O·α‐LixMnO2, that has Mn4+/3+ character may facilitate the Li2O2/Li2O discharge/charge chemistries providing dual electrode/electrocatalyst functionality.

Stability Comparison towards Oxygen Reduction Products between Oligoether Silane and Propylene Carbonate for Lithium-Air Batteries

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Microelectrochemical Detection of Reaction Products of Oxygen Reduction and Oxygen Evolution Reactions

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Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys

Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au–Pd surface composition and distribution of Pd within a Au0.7Pd0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H2O2 as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au–Pd surface composition and in remarkable structural memory effects.

Increased Stability Toward Oxygen Reduction Products for Lithium-Air Batteries with Oligoether-Functionalized Silane Electrolytes

The successful development of Li-air batteries would significantly increase the possibility of extending the range of electric vehicles. There is much evidence that typical organic carbonate based electrolytes used in lithium ion batteries form lithium carbonates from reaction with oxygen reduction products during discharge in lithium-air cells so more stable electrolytes need to be found. This combined experimental and computational study of an electrolyte based on a tri(ethylene glycol)-substituted trimethylsilane (1NM3) provides evidence that the ethers are more stable toward oxygen reduction discharge species. X-ray photoelectron spectroscopy (XPS) and FTIR experiments show that only lithium oxides and no carbonates are formed when 1NM3 electrolyte is used. In contrast XPS shows that propylene carbonate (PC) in the same cell configuration decomposes to form lithium carbonates during discharge. Density functional calculations of probable decomposition reaction pathways involving solvated oxygen reduction species confirm that oligoether substituted silanes, as well as other ethers, are more stable to the oxygen reduction products than propylene carbonate. These results indicate that the choice of electrolyte plays a key role in the performance of Li-air batteries.