Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interface

Synthesis of 14-oxa-1,4,8,11-tetraazabicyclo[9,5,3]nonadecane (L1) and a Spectroscopic and Structural Study of [Ni(L1)(ClO4)](ClO4) and of the Macrobicyclic Precursor Diamide Complex, [Ni(HL2)](ClO4); Chloride Substitution Kinetics of the Corresponding [Ni(III)(L1)]3+ Species

The pentadentate ligand 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane (L1) has been synthesized by the high dilution cyclization of 1-oxa-4,8-diazacyclododecane ([10]aneN2O) (1) with 1,3-bis(α-chloroacetamido)propane (2) and subsequent reduction of the diamide intermediate. The structure [Ni(L1)(ClO4)](ClO4) (P21/c (no. 14), a = 8.608(3), b = 16.618(3), c = 14.924(4) Å, β = 91.53(3)° converged at R = 0.050 (Rw = 0.046) for 307 parameters using 2702 reflections with I > 2σ(I). For the nickel(II) complex of the (monodeprotonated) precursor diamide ligand 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane-3,9-dione (H2L2), [Ni(HL2)](ClO4) (Pbca (no. 61), a = 15.1590(3), b = 13.235(2), c = 18.0195(6) Å), the structure converged at R = 0.045 (Rw = 0.038) for 265 parameters using 1703 reflections with I > 3σ(I). In the reduced system, the cyclam-based ligand adopts a trans-III configuration. The [Ni(L1)(ClO4)]2+ ion is pseudooctahedral with the Ni−O(ether) 2.094(3) Å distance shorter than the Ni−O(perchlorate) 2.252(4) Å. The nickel(II) and nickel(III) complexes are six-coordinate in solution. Oxidation of [Ni(L1)(OH2)]2+ with K2S2O8 in aqueous media yielded an axial d7 Ni(III) species (g⊥ = 2.159 and g⊥ = 2.024 at 77 K). The [Ni(L1)(solv)]2+ ion in CH3CN showed two redox waves, NiII/I (an irreversible cathodic peak, Ep,c = −1.53 V) and NiIII/II (E1/2 = 0.85 V (reversible)) vs Ag/Ag+. The complex [Ni(HL2)](ClO4) displays square-planar geometry with monodeprotonation of the ligand. The ether oxygen is not coordinated. Ni−O(3) = 2.651(6) Å and Ni−O(3a) = 2.451(12) Å, respectively. The NiIII/II oxidation at E1/2 = 0.24 V (quasi-reversible) vs Ag/Ag+ is considerably lower than the saturated system. The kinetics of Cl- substitution at [Ni(L1)(solv)]3+ are pH dependent. Detachment of the ether oxygen atom is proposed, with insertion of a protonated water molecule which deprotonates at a pKa more acidic than in the corresponding cyclam complex. Mechanistic implications are discussed

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interfaces. © 2009 The American Physical Society

Aluminium oxidation at high anodic potentials in an AlCl <inf>3</inf>-containing air- and water-stable ionic liquid solution

The observation of a multi-step stripping process at high positive potentials is reported for aluminium electrodeposition from a solution of AlCl3 in the air- and water-stable ionic liquid 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Cyclic voltammograms recorded on an aluminium-coated boron-doped diamond electrode revealed complex oxidative behaviour, suggesting that a passive layer forms on the freshly deposited aluminium that hinders electro-oxidation. High positive potentials are required to induce chloride attack of the passive film, composed of Al-NTf 2 compounds, and facilitate the rapid oxidation of the underlying aluminium metal. These results have important implications in bipolar-pulse electroplating and electrorefining applications where electrodissolution of aluminium is required. © 2013 Published by Elsevier B.V. All rights reserved

Favourable surface properties of boron-doped diamond electrodes for aluminium electrodeposition from ionic liquids

Aluminium electrodeposition on a boron-doped diamond (BDD) electrode was studied in the ionic liquid: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C 4mpyr][NTf 2]. Cyclic voltammograms recorded on BDD electrodes in 1 m solutions of AlCl 3 in [C 4mpyr][NTf 2] were compared with those recorded on Au, Al and glassy carbon (GC) electrodes. The BDD electrode exhibits similar voltammetric responses for Al electrodeposition as on metallic electrodes but without interferences from alloy formation or reaction of the electrode material. Conversely, nucleation and growth of Al is significantly more difficult on freshly abraded GC. The electrochemical stability and activity of BDD over a wide potential range make it a highly favourable surface for studying metal deposition at very negative potentials in ionic liquids. © 2012 Elsevier B.V

The influence of thermal degradation on the electrodeposition of aluminium from an air- and water-stable ionic liquid

Aluminium electrodeposition is demonstrated from a thermally degraded ionic liquid solution. NMR and voltammetric analyses established that Al3+ reduction was remarkably similar to that in non-degraded IL solutions suggesting that the electroactive metal-containing species was unaffected by heat treatment. Electron microscopy revealed a significant grain refinement of the deposited metal. © the Owner Societies 2013

Electrical double-layer capacitance in room temperature ionic liquids: Ion size and specific adsorption effects

The electrical double-layer structure and capacitance in room temperature ionic liquids at electrified interfaces were systematically studied with use of the self-consistent mean-field theory. The capacitance curve departs from symmetry with respect to the point of zero charge when unequal ion-size is implemented or when specific adsorption of ions is introduced. For the case of unequal ion-size, the shape of the capacitance curve is strongly determined by the size of the counterion and only weakly influenced by the co-ion size. When present, specifically adsorbed ions would change the capacitance within a limited range of applied potential from the point of zero charge, which itself varies with the strength of specific adsorption

N-donor functionalised ionic liquids: their thermo-physical, electrochemical and transport properties

Two functionalised room temperature ionic liquids 1-ethyl-1,4-dimethyl piperazinium bis{(trifluoromethyl)sulfonyl}amide (1) and 1-(2-dimethylaminoethyl)dimethylethylammonium bis{(trifluoromethyl)sulfonyl}amide (2) were synthesised. Properties measured were the electrochemical window, melting point, glass and decomposition temperatures, density, thermal expansivity, viscosity, conductivity and ion self-diffusion coefficients. These were compared with those of 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}amide (3), as a benchmark salt. A marked difference in thermal stability was observed between the two tertiary amine functionalised salts and the non-functionalised benchmark system, the former being up to 170 K less stable than the latter. The compounds of this study also have narrower electrochemical windows than structural counterparts lacking a tertiary amine function, by about 3 V. The ion self-diffusion coefficients and conductivities are higher for the open chain (2) than for the cyclic (1) at each temperature, and the viscosity less. The transport properties are examined using fractional Stokes – Einstein and Nernst – Einstein relations and the velocity correlation coefficients (VCC) calculated. The fractional Stokes – Einstein exponents range from (0.95 to 0.98). The Nernst – Einstein equation deviation parameters (&#916;) are such that the mean of the like-ion VCCs is greater than that for the unlike ions: &#916; = (0.36 and 0.42) respectively, values typical for ionic liquids. The VCCs are analysed in terms of the anti-correlated motion of ions, a feature of one-component molten salts, that contrasts with the correlated motions of ions in electrolyte solutions. According to the VCC analysis, the distinct diffusion coefficients follow the order D--