Size Of Electrolyte Cation Research Articles

Role of the Supporting Electrolyte Ions and Additives on the Electron Transport Properties of Electrografted Films Bearing Ferrocenyl Moieties

AbstractThe electrochemical oxidation of 6‐ferrocenylheptanoate was used to modify the surface of glassy carbon electrodes with thin films bearing ferrocene molecules. These films grow layer by layer and behave as surface redox catalyst during the electrografting. The electrografted films have a structure with pinholes or channels, whose existence was deduced from the redox behavior of the film in acetonitrile solution containing different supporting electrolytes and organic additives. The electron transport through this film requires the presence of the electrolyte anions in the film channels. However, the voltametric feature of the grafted films is sensitive to the electrolyte cation size. The supporting electrolyte anions inside the film channels stabilize the ferrocenium species during the film oxidation. However, the presence of the cations is required to keep the inner charge balance. The inclusion of both anions and cations inside the grafted film determines the surface coverage obtained from the voltammetric experiments.

Influence of supporting electrolyte on the pseudocapacitive properties of MnO2/carbon nanotubes

The aim of this study was to prepare MnO2 from a MnCl2 solution by electrode polarisation at a constant potential. MnO2 was deposited at platinum and platinum modified by carbon nanotubes (CNTs) with a varying degree of oxidation. The mass of the electrodeposited MnO2 was calculated from the current transient. An electrochemical quartz crystal microbalance (EQCM) enabled the determination of the correction factor which considers the presence of the equivalent amount of water within the MnO2 layer. The obtained electrodes (MnO2, MnO2/CNT, MnO2/CNT-EO1 and MnO2/CNT-EO2) were tested in NaCl, LiCl and MnCl2 electrolytes by cyclic voltammetry, demonstrating the best pseudocapacitive properties in the MnCl2 solution. It was proven that the electrolyte cation size, its valence and the hydration shell significantly influence the MnO2 pseudocapacitive redox reaction. The pseudocapacitive properties of MnO2 were improved by MnO2 deposition onto CNTs and oxidised CNT electrodes. The oxidation of CNTs was accomplished by a simple electrochemical procedure in a Na2SO4 solution (CNT-EO1 and CNT-EO2 electrodes). Electrochemical impedance spectroscopy has shown that charge transfer resistance has increased by oxidation process. However, it did not influence the overall pseudocapacitive properties significantly.

Promoter Effects of Alkali Metal Cations on the Electrochemical Reduction of Carbon Dioxide.

The electrochemical reduction of CO2 is known to be influenced by the identity of the alkali metal cation in the electrolyte; however, a satisfactory explanation for this phenomenon has not been developed. Here we present the results of experimental and theoretical studies aimed at elucidating the effects of electrolyte cation size on the intrinsic activity and selectivity of metal catalysts for the reduction of CO2. Experiments were conducted under conditions where the influence of electrolyte polarization is minimal in order to show that cation size affects the intrinsic rates of formation of certain reaction products, most notably for HCOO-, C2H4, and C2H5OH over Cu(100)- and Cu(111)-oriented thin films, and for CO and HCOO- over polycrystalline Ag and Sn. Interpretation of the findings for CO2 reduction was informed by studies of the reduction of glyoxal and CO, key intermediates along the reaction pathway to final products. Density functional theory calculations show that the alkali metal cations influence the distribution of products formed as a consequence of electrostatic interactions between solvated cations present at the outer Helmholtz plane and adsorbed species having large dipole moments. The observed trends in activity with cation size are attributed to an increase in the concentration of cations at the outer Helmholtz plane with increasing cation size.

Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO2 over Ag and Cu.

Electrolyte cation size is known to influence the electrochemical reduction of CO2 over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pKa for cation hydrolysis decreases and is sufficiently low for hydrated K+, Rb+, and Cs+ to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO2. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H2 and CH4, and an increase in Faradaic efficiencies for CO, C2H4, and C2H5OH, in full agreement with experimental observations for CO2 reduction over Ag and Cu.

Solid-State Electrochemistry of a Semiconducting MMX-Type Diplatinum Iodide Chain Complex

Electron-transfer-facilitated dissolution, ion insertion, and desorption associated with an MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complex (MMX chain) have been investigated for the first time. K2(NC3N)[Pt2(pop)4I]·4H2O (1) (NC3N(2+) = (H3NC3H6NH3)(2+); pop = P2H2O5(2-)) is a semiconductor with a three-dimensional coordination-bond and hydrogen-bond network included in the chain. The cyclic voltammetry of 1 was studied by using 1-modified electrodes in contact with acetonitrile solutions containing electrolyte. The chemical reversibility for oxidation of 1 depended on the electrolyte cation size, with large cations such as tetrabutylammonium (Bu4N(+)) being too large to penetrate the pores formed by the loss of K(+) and NC3N(2+) upon oxidation. The potential for reduction of 1 decreased as the cation size increased. The presence of the acid induced additional well-defined processes but with gradual solid dissolution, attributed to the breaking of the coordination-bond networks.

Electrochemical and Theoretical Investigation of Corannulene Reduction Processes

The voltammetric generation of corannulene anions was investigated over a large range of experimental conditions comprising either "traditional" electrochemical solvents, such as dimethylformamide, acetonitrile, and tetrahydrofuran, or "unconventional" solvents, such as liquid ammonia, liquid methylamine, or liquid dimethylamine, and several different supporting electrolytes. Strong ion pairing effects were found to dominate the electrochemical generation of corannulene higher anions, and through the suitable choice of the solvent/electrolyte system, we observed, for the first time, the reversible electrochemical generation of up to the triply reduced corannulene. The standard potentials obtained experimentally compared rather well with the theoretical values calculated by ab initio and density functional methods, in which solvation and ion pairing effect were explicitly taken into account. In particular, the calculations considered the effect of the electrolyte cation size on ion pairing in order to rationalize the occurrence of the third reduction within the experimental potential window.

Revisiting the heterogeneous electron-transfer kinetics of nitro compounds

Electrochemical impedance spectroscopy has been used to determine the heterogeneous electron-transfer kinetic parameters for the reduction of the following nitrobenzene derivatives in acetonitrile containing 0.10 M tetrabutylammonium hexafluorophosphate at a mercury working electrode: 2-methylnitrobenzene( 1), 2,6-dimethylnitrobenzene( 2), 2,4,6-trimethylnitrobenzene( 3), 2,3,5,6-tetramethylnitrobenzene ( 4), pentamethylnitrobenzene ( 5), 2,4,6-triisopropylnitrobenzene ( 6) and 2,4,6-tri- tert-butylnitrobenzene ( 7). The results are discussed together with previously published results for nitromethane ( 8), nitroethane ( 9), 2-nitropropane ( 10) and 2-methyl-2-nitropropane ( 11). In general, a decrease in the standard electron-transfer rate constant, k s, is seen on going from 1 to 7 and this trend continues for 8–11. Double layer effects are shown to be of minor importance. A previous explanation, based on increasing outer reorganization energies on going from 1 to 11 is called into question by the fact that the Gibbs energies of solvation of the radical anions of 1, 3 and 8 have been found to be almost identical. Arguments are presented supporting the idea that the outer reorganization energy should scale linearly with the Gibbs energy of solvation. A previously presented model, used to explain the effect of electrolyte cation size on the values of k s, has been extended to include the effect of reactant size. It is inferred that the decrease in k s on going from 8 (CH 3NO 2) to 11 ((CH 3) 3CNO 2) results at least partly from the increased size of the alkyl group, which causes the average distance of closest approach to increase from 8 to 11 resulting in an increased tunneling distance and smaller rate constants. This suggests that the “true” values of k s for 8–11, corrected for this distance effect, are roughly constant. Finally, semi-empirical molecular orbital methods (AM1; 6 and 7) and density functional theory (B3LYP; 1–5; 8–11) were used to estimate the contributions of the inner reorganization energy for 1–11. These contributions are small for 1 but steadily increase on going to 5. The inner reorganization contribution for 8–11 is quite substantial, ca. 4–5 kcal/mol (1 kcal/mol = 4.184 kJ/mol). This suggests that the inner reorganization term is a very significant factor underlying the sharp decrease in k s on going from 1 to 11. The principal structural change causing the calculated larger values of inner reorganization energy for 1–5 is the significant turning of the nitro group out of the plane of the ring in the neutral, a feature that is resisted in the radical anion. For 8–11, pyramidalization at nitrogen in the radical anion is the most significant structural change.

Mercury(II) Immobilized on Carbon Nanotubes: Synthesis, Characterization, and Redox Properties

Mercury(II)-modified carbon nanotubes can be readily prepared by reacting purified/oxidized carbon nanotubes (CNTs) with a Hg(NO3)2 aqueous solution. Two types of surface-confined Hg(II) species are formed and have been identified as (CNT-COO)2HgII and (CNT-O)2HgII. These two complexes have a surface concentration ratio of about 30%:70%, on the basis of data obtained from high-resolution XPS spectra, Raman spectroscopy, and electrochemical measurements. The electrochemical behavior of Hg(II)-modified CNTs adhered to electrode surfaces in contact with CH3CN (electrolyte) strongly depends on the nature of the working electrode used and the size of electrolyte cation. Significant voltammetric changes also are observed after the addition of water to the initially water-free acetonitrile electrolyte solution. At a glassy carbon electrode and using NaClO4 as the electrolyte, a proposed mechanism is operative. However, at a gold-coated quartz-crystal electrode, Hg formed after reduction reacts with the Au to form Hg−Au alloy which has a very positive stripping peak potential value compared to that for the Hg film formed on glassy carbon surfaces. The influence of the electrolyte cation size on the reduction of Hg(II)-modified CNTs is attributed to the intercalation of electrolyte cations.

Solvent and support electrolyte effects on the catalytic activity of Ti/RuO 2 and Ti/IrO 2 electrodes: oxidation of isosafrole as a probe model

The electrocatalytic behavior of Ti/RuO 2 and Ti/IrO 2 electrodes was investigated as function of the supporting electrolyte and solvent. A decrease was observed in the electrochemically active area of the electrode as the cation size increases. The oxidation potential of a series of organic solvents tested showed a straight relationship with Gutmann's donor number: the higher the GDN the more easily is the solvent oxidized. Oxidation of isosafrole, ISF, was used as a probe reaction to examine the influence of the supporting electrolyte cation size and the composition of the organic solvents on the electrocatalytic activity of the electrodes. The global catalytic activity for ISF oxidation is independent of the size of the supporting electrolyte cation and of the electrode material. For both electrode materials investigated it was verified that the following sequence holds for the influence of the solvent on the overall catalytic efficiency of ISF oxidation: AN>PC>DMSO≈DMF.

Influence of the cation size on the charge compensation process in poly(1-naphthol) coated electrodes: Multiple internal reflection FTIR spectroscopy (MIRFTIRS) and probe beam deflection (PBD) study

The electrochemical oxidation-reduction process of poly(1-naphthol) (poly(NAP-1) film was studied by in situ FTIR spectroscopy and probe beam deflection techniques. On oxidation, the polymer film is positively charged and on reduction, it is in its neutral form. It is demonstrated that the charge compensation process depends on the electrolyte cation size. With bulky cations the role of the charge compensation is taken by the anion. With small cations a mixed contribution of anions and cations to the process is observed. Anion exchange is predominant at short periods (seconds) but cation exchange is also involved if one waits until equilibrium is established after several minutes.

Electrochemical behavior of methyl viologen in zeolite particle films

Zeolites NaA and NaY were coated onto platinum electrodes. The effects of channel diameter in the zeolites, size of electrolyte cation, solvent, and solubility of methyl viologen on electron transfer and ion-exchange behavior were studied. When exposed to electrolyte solutions containing methyl viologen ( N, N′-dimethyl-4,4′-bipyridyl dichloride), ion-exchange of the viologen occured into zeolite Y (large pores) but not into zeolite A (small pores). In water, electrochemistry of methyl viologen in NaY zeolite on the electrode surface resembled that of methyl viologen at clean platinum. When the zeolite-coated electrode was used in acetonitrile, however, cyclic voltammetric peak shapes changed dramatically. Generation of the neutral viologen species by subsequent reduction of the methyl viologen cation radical appeared to lead to a comproportionation reaction of the neutral species with the dication remaining in the zeolite, to regenerate the cation radical. The electrochemical behavior of methyl viologen exchanged into NaY zeolite in acetonitrile depended on the size of electrolyte cations. Electron transfer and loss of methyl viologen vy leaching were fast in the presence of lithium perchlorate solution and slow in tetraethylammonium and tetrabutylyammonium perchlorate solutions. A mixed electrolyte (tetraheptylammonium bromide + lithium perchlorate) solution prevented solubility of methyl viologen while (facilitating charge transfer.