Michael D. Horne

Random assemblies of microelectrodes (RAM™ electrodes) for electrochemical studies

Abstract The application of random assemblies of microdisks (RAM™ electrodes) to electrochemical studies is described. These devices have working surfaces intersected by hundreds or thousands of disk-shaped microelectrodes that are capacitively, resistively, and diffusively independent. They therefore produce current–time responses of the same form as single microelectrodes, but many times larger. This property allows experimental data to be obtained on the benchtop without shielding and without significant mains interference — something that is impossible with single microelectrodes. The design criteria of random assemblies are summarized, and examples of their utility in a wide range of electrochemical experiments are given.

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.

Structure of [C4mpyr][NTf2] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C(4)mpyr][NTf(2)]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C(4)mpyr](+) at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C(4)mpyr](+) onto the gold electrode.

Aluminium speciation in 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide/AlCl3 mixtures

Electrodeposition of aluminium is possible from solutions of AlCl(3) dissolved in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C(4)mpyrNTf(2)) ionic liquid. However, electrodeposition is dependant on the AlCl(3) concentration as it only occurs at concentrations >1.6 mol L(-1). At these relatively high AlCl(3) concentrations the C(4)mpyrNTf(2)/AlCl(3) mixtures exhibit biphasic behaviour. Notably, at 1.6 mol L(-1) AlCl(3), aluminium can only be electrodeposited from the upper phase. Conversely, we found that at 3.3 mol L(-1) aluminium electrodeposition can only occur from the lower phase. The complex chemistry of the C(4)mpyrNTf(2)/AlCl(3) system is described and implications of aluminium speciation in several C(4)mpyrNTf(2)/AlCl(3) mixtures, as deduced from Raman and (27)Al NMR spectroscopic data, are discussed. The (27)Al NMR spectra of the C(4)mpyrNTf(2)/AlCl(3) mixtures revealed the presence of both tetrahedrally and octahedrally coordinated aluminium species. Raman spectroscopy revealed that the level of uncoordinated NTf(2)(-) anions decreased with increasing AlCl(3) concentration. Quantum chemical calculations using density functional and ab initio theory were employed to identify plausible aluminium-containing species and to calculate their vibrational frequencies, which in turn assisted the assignment of the observed Raman bands. The data indicate that the electroactive species involved are likely to be either [AlCl(3)(NTf(2))](-) or [AlCl(2)(NTf(2))(2)](-).

Speciation of Aluminium in Mixtures of the Ionic Liquids [C3mpip][NTf2] and [C4mpyr][NTf2] with AlCl3: An Electrochemical and NMR Spectroscopy Study†

This paper reports on the electrodeposition of aluminium on several substrates from the air- and water-stable ionic liquids 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)amide ([C(3)mpip][NTf(2)]) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C(4)mpyr][NTf(2)]), which contain anhydrous AlCl(3). At an AlCl(3) concentration of 0.75 molal, no evidence for aluminium electrodeposition was observed in either system at room temperature. However, aluminium electrodeposition becomes feasible upon heating the samples to 80 degrees C. Aluminium electrodeposition from bis(trifluoromethylsulfonyl)amide-based ionic liquids that contain AlCl(3) has previously been shown to be very dependent upon the AlCl(3) concentration and has not been demonstrated at AlCl(3) concentrations below 1.13 molal. The dissolution of AlCl(3) in [C(3)mpip][NTf(2)] and [C(4)mpyr][NTf(2)] was studied by variable-temperature (27)Al NMR spectroscopy to gain insights on the electroactive species responsible for aluminium electrodeposition. A similar change in the aluminium speciation with temperature was observed in both ionic liquids, thereby indicating that the chemistry was similar in both. The electrodeposition of aluminium was shown to coincide with the formation of an asymmetric four-coordinate aluminium-containing species with an (27)Al chemical shift of delta=94 and 92 ppm in the [C(3)mpip][NTf(2)]-AlCl(3) and [C(4)mpyr][NTf(2)]-AlCl(3) systems, respectively. It was concluded that the aluminium-containing species that give rise to these resonances corresponds to the electroactive species and was assigned to [AlCl(3)(NTf(2))](-).

Transport, electrochemical and thermophysical properties of two N-donor-functionalised ionic liquids

Two N-donor-functionalised ionic liquids (ILs), 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)amide (1) and 1-(2-dimethylaminoethyl)-dimethylethylammonium bis(trifluoromethylsulfonyl)amide (2), were synthesised and their electrochemical and transport properties measured. The data were compared with the benchmark system, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (3). Marked differences in thermal and electrochemical stability were observed between the two tertiary-amine-functionalised salts and the non-functionalised benchmark. The former are up to 170 K and 2 V less stable than the structural counterpart lacking a tertiary amine function. The ion self-diffusion coefficients (Di ) and molar conductivities (Λ) are higher for the IL with an open-chain cation (2) than that with a cyclic cation (1), but less than that with a non-functionalised, heterocyclic cation (3). The viscosities (η) show the opposite behaviour. The Walden [Λ[proportionality](1/η)(t) ] and Stokes-Einstein [Di /T)[proportionality](1/η)(t) ] exponents, t, are very similar for the three salts, 0.93-0.98 (±0.05); that is, the self-diffusion coefficients and conductivity are set by η. The Di for 1 and 2 are the same, within experimental error, at the same viscosity, whereas Λ for 1 is approximately 13% higher than that of 2. The diffusion and molar conductivity data are consistent, with a slope of 0.98±0.05 for a plot of ln(ΛT) against ln(D+ +D- ). The Nernst-Einstein deviation parameters (Δ) are such that the mean of the two like-ion VCCs is greater than that of the unlike ions. The values of Δ are 0.31, 0.36 and 0.42 for 3, 1 and 2, respectively, as is typical for ILs, but there is some subtlety in the ion interactions given 2 has the largest value. The distinct diffusion coefficients (DDC) follow the order D(d)__ < D(d)++ < D(d)+_, as is common for [Tf2N](-) salts. The ion motions are not correlated as in an electrolyte solution: instead, there is greater anti-correlation between the velocities of a given anion and the overall ensemble of anions in comparison to those for the cationic analogue, the anti-correlation for the velocities of which is in turn greater than that for a given ion and the ensemble of oppositely charged ions, an observation that is due to the requirement for the conservation of momentum in the system. The DDC also show fractional SE behaviour with t~0.95.

Comparison of Diffusivity Data Derived from Electrochemical and NMR Investigations of the SeCN¯/(SeCN)2/(SeCN)3¯ System in Ionic Liquids

Electrochemical studies in room temperature ionic liquids are often hampered by their relatively high viscosity. However, in some circumstances, fast exchange between participating electroactive species has provided beneficial enhancement of charge transport. The iodide (I¯)/iodine (I(2))/triiodide (I(3)¯) redox system that introduces exchange via the I¯ + I(2) ⇌ I(3)¯ process is a well documented example because it is used as a redox mediator in dye-sensitized solar cells. To provide enhanced understanding of ion movement in RTIL media, a combined electrochemical and NMR study of diffusion in the {SeCN¯-(SeCN)(2)-(SeCN)(3)¯} system has been undertaken in a selection of commonly used RTILs. In this system, each of the Se, C and N nuclei is NMR active. The electrochemical behavior of the pure ionic liquid, [C(4)mim][SeCN], which is synthesized and characterized here for the first time, also has been investigated. Voltammetric studies, which yield readily interpreted diffusion-limited responses under steady-state conditions by means of a Random Assembly of Microdisks (RAM) microelectrode array, have been used to measure electrochemically based diffusion coefficients, while self-diffusion coefficients were measured by pulsed field gradient NMR methods. The diffusivity data, derived from concentration and field gradients respectively, are in good agreement. The NMR data reveal that exchange processes occur between selenocyanate species, but the voltammetric data show the rates of exchange are too slow to enhance charge transfer. Thus, a comparison of the iodide and selenocyanate systems is somewhat paradoxical in that while the latter give RTILs of low viscosity, sluggish exchange kinetics prevent any significant enhancement of charge transfer through direct electron exchange. In contrast, faster exchange between iodide and its oxidation products leads to substantial electron exchange but this effect does not compensate sufficiently for mass transport limitations imposed by the higher viscosity of iodide RTILs.

Some photoelectrochemical insights into galena flotation

Abstract A photoelectrochemical study has been made of the interaction between ethyl xanthate anions and freshly-abraded galena surfaces at pH 9.2. It is found that galena behaves as a semiconductor having mixed ionic and electronic conduction. The implications for research on galena flotation are discussed.

A scanning tunneling microscopy study of the surface microstructure of alpha‐ and beta‐lead dioxide

The surface microstructures of specimens of pure alpha‐ and beta‐lead dioxide (PbO2 ) have been resolved with scanning tunnel microscopy (STM). Observed crystallite sizes are compared with values deduced from x‐ray diffraction (XRD), and the same data are compared with results from other characterization methods including scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurements, mercury porosimetry and electrochemical reduction. The large difference in surface area between alpha‐ and beta‐PbO2 is explained by microcracks permeating the beta structure. The STM images show these microcracks clearly.

A reference half-cell capillary that improves the high frequency performance of the potentiostat/whole-cell combination

In a recent paper in this journal, Wiles et al. [l] described an experimental artifact associated with impedance measurements in resistive media: namely, a semi-circle in complex plane plots at high frequencies. The cause of this artifact was traced back to the restricted volume (and hence poor conductance) of that part of the electrolyte solution that was inside the Luggin capillary. Because the latter was connected via stray capacitance to ground, a phase shift was introduced into the