OER Mechanism Research Articles

Investigation of Ether-Based Ionic Liquid Electrolytes for Lithium-O2 Batteries

The growing global energy demand of modern society is urging to find large-scale sources, which are more sustainable and environmentally friendly than the oil-based ones. At present, the capacity of the state-of-the-art lithium ion battery is limited and therefore, radically different approaches are required in order to fulfil the very demanding requests of energy storage. Rechargeable Li-O2/air batteries can potentially represent an interesting solution because they theoretically can store more energy than Li-ion batteries. At present numerous aspects of Li-O2 batteries deserve to be carefully investigated before becoming a commercial reality and among them the electrolyte is one of the main challenges.1 Ether based electrolytes have attracted a lot of interest because they combine several beneficial attributes; they are safe, low cost, stable up to 4.5V and they are of low volatility. Tetra(ethylene) glycol dimethyl ether (TEGDME) has been recently employed in Li-O2 batteries and with very good electrochemical performances.2 Hydrophobic ionic liquids (ILs) have been largely employed in case of the conventional Li-ion cells, however a few details on their behaviour in Li-O2batteries are still available.In this work, we investigated the potential use as electrolytes of mixtures of TEGDME and ether functionalized ionic liquids (ILs). We show the results of the characterization of mixtures having different ratio TEGDME-ILs and containing LiCF3SO3 or LiTFSI salt by means of thermal analysis, conductivity and electrochemical stability measurements. In addition, their application in Li-O2 batteries has been assessed, investigating Li/TEGDME-IL/O2 cells by means of cycling voltammetry (CV) and galvanostatic charge-discharge tests. Oxygen reduction and evolution reactions (ORR and OER) have been studied in TEGDME-ILs mixtures to address the mechanism and reversibility of ORR and OER. Our results demonstrate that oxygen reduction and evolution take place in a reversible way and that ether-based electrolytes containing ionic liquids worthwhile of being further investigated for rechargeable Li-O2cells.[1] D. Capsoni, M. Bini, S. Ferrari, E. Quartarone, P. Mustarelli J. Power Sources(2012) 220, 253.[2] H.-G. Jung, J. Hassoun, J.-B. Park, Y.-K. Sun, B. Scrosati Nat. Chem. (2012) 4, 579

In Situ Probing of the Catalyst-DMSO Interface in Li-O2 Cells

The Li-O2 cell, with a theoretical energy density of ~11686 Wh/kg, is an attractive technology for improving upon the current generation of rechargeable batteries [1]. However, poor cyclablility and low energy efficiencies make the application of these cells impractical in their current state. Selection of suitable electrolytes and catalysts to increase round trip efficiencies, improve rate capability, reduce the generation of undesirable side products, and promote practical cycling are necessary.Much literature has focused on selecting non-aqueous electrolyte solvents and electrode materials that yield lithium peroxide as the main discharge product. Ether-based electrolytes (i.e. dimethoxyethane, tetraglyme) have been employed for their improved stability over carbonates, but have still been shown to decompose upon cycling [2,3]. Our recent study shows that dimethyl sulfoxide (DMSO) -based electrolytes are both more stable than ethers and provide a kinetic advantage to the discharge and charge reactions [4]. The oxygen electrode material also has a profound effect on the cyclability of Li-O2 cells. Carbon has been shown to degrade upon repeated cycling [5] and exhibits high overpotentials for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. Noble metal catalysts (i.e. Pt, Pd, Au) supported on carbon can promote ORR at lower overpotentials than carbon alone and demonstrate better cycle stability [6].Here we examine the catalyst-electrolyte interface using in-situ techniques coupled with ex-situ morphological characterization. The electrolyte solvent, DMSO, is considered a probe for studying the environment at the interface during cycling by surface-enhanced Raman spectroscopy (SERS). Pt, Pd, and Au containing all-metal oxygen electrodes prepared by galvanic displacement and electrodeposition are used, thus eliminating carbon. We find that the proximity of polar solvent molecules to the electrode surface increases during both the discharge and charge (somewhat counterintuitively). We find that the selection of noble metal catalysts has a significant effect on this solvent-surface behavior. DMSO orientation at the interface is also influenced by the applied electric field. Our results indicate that the formation of discharge products does not limit electrolyte access to the electrode surface over a single cycle but may do so upon repeated cycling, leading to selective deactivation. While lithium peroxide was detected in-situ, ex-situ microscopy confirms that the platelet morphology of the discharge products is thin and significantly porous. The morphology of the discharge products, influenced by the choice of catalyst, is also seen to affect the interfacial behavior during charge.Electrolyte proximity (the formation of a double layer) and ion exchange during reactions were also studied by in-situ electrochemical impedance spectroscopy. The impedance of the oxygen electrodes exhibit minima which correspond to bulk ORR and intermediate OER potentials. These findings, in addition to cyclic voltammetry and long term galvanostatic cycling, complement the in-situ SERS data. This work represents the first of its kind to study the catalyst-electrolyte interface in-situ at a molecular level with a variety of catalysts and observe electrolyte migration in Li-O2 cells. Our findings have great implications in understanding the catalytic mechanism of ORR and OER with noble metals in a range of electrolytes.[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, and W. Wilcke, J. Phys. Chem. Lett., 2010, 1, 2193–2203.[2] B. D. McCloskey, R. Scheffler, A. Speidel, G. Girishkumar, and A. C. Luntz, J. Phys. Chem. C, 2012, 116, 23897–23905.[3] B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov, and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997–1001.[4] F. S. Gittleson, R. C. Sekol, G. Doubek, M. Linardi, and A. D. Taylor, Phys. Chem. Chem. Phys., 2014.[5] M. M. O. Thotiyl, S. A. Freunberger, Z. Peng, and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494–500.[6] Y.-C. Lu, H. A. Gasteiger, and Y. Shao-Horn, J. Am. Chem. Soc., 2011, 133, 19048–19051.

Dimensionally Stable Anodes (DSA) for Electrochemical Water Splitting: OER Kinetics and Mechanism at RuO2/ NiOx Modified Electrodes in Aqueous Base

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Electrocatalytic activity of nickel oxide nanoparticles-modified electrodes: Optimization of the loading level and operating pH towards the oxygen evolution reaction

The current study addresses the superior electrocatalytic activity of nickel oxide nanoparticles (nano-NiO x ) modified GC, Au and Pt electrodes towards the OER. The electrodeposition of nickel oxide nanoparticles (with an average particle size of 80 nm) are believed to enhance the OER reaction. NiOOH phase, as shown from XRD data, participates in the OER mechanism in such a way to facilitate the charge transfer during various steps in the reaction mechanism through a reversible transformation of NiOOH to NiO 2. Optimizing the loading level and the operating pH of the proposed catalyst has been carried out.

Electrochemical characterisation and oxygen evolution at a heavily boron doped diamond electrode

Characterisation of a commercial heavily doped BDD electrode demonstrated it contains a small sp 2 content, which on anodic potential scanning, is oxidised to CO/CO 2. This surface modification alters the electrode activity, increasing the overpotential for the hydrogen and oxygen evolution reactions (HER and OER). Ex situ and in situ investigations indicate film morphology is mainly composed of “chain of hills”, presenting relatively high differential capacitance values and morphology factor, which is attributed to the effect of surface states and high surface roughness of the BDD film. The voltammetric behaviour depends on the applied potential; the heavily doped BDD electrode behaving as a metallic electrode at more anodic potentials. Polarisation curves (potentiostatic (1 mV s −1) or galvanostatic (point-by-point)), recorded at different temperatures and H 2SO 4 concentrations, lead to the same conclusions. The high Tafel coefficients and low apparent electronic transfer coefficient ( α A) are independent of overpotential and temperature but show a dependence on H 2SO 4 concentration. The linear relationship observed between the apparent electrochemical enthalpy of activation (Δ H # η) and overpotential supports α A is constant. An OER mechanism was proposed taking into account the absence of adsorption sites at the BDD surface. The OER is inhibited, explaining the high overpotentials and elevated Δ H # η values.