Redox capacitance of poly-3,4-ethylenedioxythiophene studied by cyclic voltammetry and faradaic impedance spectroscopy

The paper presents results of studying the electrochemical properties of poly-3,4-ethylenedioxythiophene films using the methods of cyclic voltammetry and faradaic impedance spectroscopy in acetonitrile and propylene carbonate solutions of different electrolytes: LiClO₄, Bu₄NBF₄, Bu₄NPF₆. The effect of the film synthesis conditions, the nature of anion and solvent, and the supporting electrolyte concentration on the film redox capacitance is discussed. Main attention is paid to the comparison of values characterizing the redox capacitance of a poly-3,4-ethylenedioxythiophene film C _lf (determined using the faradaic impedance method) and C _cv (determined using the cyclic voltammetry method) and the studies of their dependence on the varied experimental factors. The experimental C _lf vs. E curves are analyzed using the relationships of a model of a uniform film.     

Electrochemical formation and properties of two-component films of transition metal complexes and C60 or C70

The electrochemically active polymers have been formed during electro-reduction carried out in solution containing fullerenes, C₆₀ or C₇₀, and transition metal complexes of Pd(II), Pt(II), Rh(III), and Ir(I). In these films, fullerene moieties are covalently bounded to transition metal atoms (Pd and Pt) or their complexes (Rh and Ir) to form a polymeric network. All films exhibit electrochemical activity at negative potentials due to the fullerene cages reduction process. For all studied metal complexes, yields of formation of films containing C₇₀ are higher than yields of electrodeposition of their C₆₀ analogs. C₇₀ /M films also exhibit higher porosity in comparison to C₆₀/M layers. The differences in film morphology and efficiency of polymer formation are responsible for differences in electrochemical responses of these films in acetonitrile containing supporting electrolyte only. C₇₀/M films shows more reversible voltammeric behavior in negative potential range. They also show higher potential range of electrochemical stability. Processes of film formation and electrochemical properties of polymers depend on the transition metal ions or atoms bonding fullerene cages into polymeric network. The highest efficiency of polymerization was observed for fullerene/Pd and fullerene/Rh films. In the case of fullerene/Pd films, the charge transfer processes related to the fullerene moieties reduction in negative potential range exhibit the best reversibility among all of the studied systems. Capacitance performances of C₆₀/Pd and C₇₀/Pd films deposited on the porous Au/quartz electrode were also compared. Capacitance properties of both films are significantly affected by the conditions of electropolymerization. Only a fraction of the film having a direct contact with solution contributes to pseudocapacitance. Capacitance properties of these films also depend on the size of cations of supporting electrolyte. The C₇₀/Pd film exhibits much better capacitance performance comparison to C₆₀/Pd polymer.     

Capacitance spectroscopy of alumina sol–gel capacitors with Al top contacts

We measured the complex capacitance C(f) of ITO/sol–gel alumina/Al capacitors deposited on glass (and some on stainless steel foil) in the frequency range 15 Hz–10 MHz. The sol–gel films were deposited by dip-coating and following a two-step process. The capacitance C(f) found was much higher than that of a pure Al₂O₃-film due to the remaining porosity of the film and the uptake of H₂O from the environment. The C(f) curves are useful as a sensitive probe for the porosity of the sol–gel film. In particular the evolution of the capacitance curve with time after drying has been measured. The curves can qualitatively be understood by modelling the capacitor as a (nearly) percolating random insulator/conductor network, using the effective medium approximation. However a detailed understanding of the relation between the C(f,t) curves and the structure of the films requires a more elaborate model. Films sintered at 500–550 °C for 1 h initially behave exactly as not sintered films but in contrast with the latter they improve slowly over time. Fast sintering yields better films.     

Gas transport properties of annealed polyimide films

Sorption and permeation of CO₂ in various annealed polyimide (PI) films were investigated. Dual-mode sorption and partial immobilization models were used to analyze the data. Sorption of CO₂ in PI film quenched from above the glass transition temperature (T_g) is greater than in film as received. In fact, sorption is decreased over the entire pressure range by cooling the film slowly. These changes in sorption of CO₂ can be attributed to a change in the Langmuir sorption capacity C′_H by annealing, since the other dual-mode sorption parameters, k_D and b, are almost independent of annealing. The value of C′_H is increased by quenching, and decreased by slow cooling from above T_g. The two diffusion coefficients D_D and D_H according to the Henry and Langmuir modes, respectively, for CO₂ also depend markedly on annealing. Diffusion coefficients of quenched PI films are increased and those of film cooled slowly are decreased compared with values for PI film as received. The change in D_H is larger than that in D_D. The permeability coefficient of quenched PI films at 100 cmH_g is about 1.7 times that of PI film as received. The film structure formed by quenching can enhance permselectivity.     

Comparison of electrochemical properties of two-component C60-Pd polymers formed under electrochemical conditions and by chemical synthesis

The electrochemical behavior of C₆₀-Pd polymer formed under electrochemical conditions and by the chemical synthesis was examined. In these polymers, fullerene moieties are covalently bonded to palladium atoms to form a polymeric network. Both materials deposited at the electrode surface show electrochemical activity at negative potentials due to the reduction of fullerene cage. Electrochemically formed thin polymeric films exhibit much more reversible voltammetric response in comparison to chemically synthesized polymers. The morphology and electrochemical behavior of chemically synthesized C₆₀-Pd polymer depend on the composition of grown solution. Chemical polymerization results in formation of large, ca. 50 μm, crystallic superficial structures that are composed of regular spherical particles with a diameter of 150 nm. The capacitance properties of C₆₀-Pd films were investigated by cyclic voltammetry and faradaic impedance spectroscopy. Specific capacitance of chemically formed films depends on the conditions of film formation. The best capacitance properties was obtained for films containing 1:3 fullerene to Pd molar ratio. For these films, specific capacitance of 35 Fg^?1 was obtained in acetonitrile containing (n-C₄H₉)₄NClO₄. This value is much lower in comparison to the specific capacitance of electrochemically formed C₆₀-Pd film.     

Capacitance and photocurrent study of electronic properties of anodic oxide films on Nb and Ta. Evaluation of the ionized donor concentration profile in Nb2O5 film

Electronic properties of electrochemically formed oxide films on Nb were studied by photocurrent and differential capacitance measurements in 0.025 M KH₂PO₄+0.025 M Na₂HPO₄ electrolyte, pH 6.9. Oxide films of n-type conductivity were formed galvanostatically for final potentials ranging from 4 to 230 V. Measurements were performed in two potential regions, which correspond to formation of a depleted layer of variable thickness at relatively low potentials, and to complete depletion of oxide films of electronic charge carriers at higher potentials. In the first potential region the behavior of both capacitance and photocurrent, was governed by a build up of a depleted layer of potential dependent thickness. In the second, high potential, region, which extends up to the oxide film formation potential, the photocurrent and capacitance of oxide films in most features followed the trends typical of dielectric films containing defects and traps. The photocurrent and capacitance measurements on presumably dielectric oxide films formed on Ta were staged for comparison. The capacitance–potential measurements performed in the first potential region enabled us to construct the ionized donor concentration profile across the Nb₂O₅ film width. The limitations on the use of the C–E profiling method for electrochemically formed oxide layers are considered.     

Electrochemical and Electrochromic Properties of Polymer Complex Films Composed of Polytetramethyleneviologen and Poly-[p-Styrenesulfonic Acid] Containing a Conductive Powder

The electrochemical and electrochromic properties of polymer films containing a conductive powder (SnO₂/TiO₂) have been investigated. The films are complexes of polytetramethyleneviologen and poly(p-styrenesulfonic acid). It was found that the coloration (purple) and bleaching rates of the composite films increase markedly with increasing conductive powder content(x). The coloring and bleaching of the composite film with x = 95 wt% were about 7 and 44 times faster, respectively, than those for an equivalent film without conductive powder. This increase in the rate of color change by introduction of a conductive powder was found to be correlated with the apparent diffusion coefficient (D _app) for the diffusionlike charge-transport process within the composite films which increases with increasing x. The D _app for the reduction process of the film with x = 95 wt% was larger by about 3 orders of magnitude than that for the unfilled film.     

Diffusion of nonionic azo dyes in nylon-6

Diffusion of 2,4-dinitroaniline and three nonionic azo dyes in Nylon-6 film was studied by analysis of the concentration-distance curves (profiles) of penetrants in the polymer. Actual diffusivities D_(c) of penetrants in polymer, diffusion coefficients as a function of the concentration C_f of penetrant in polymer, were calculated from the profile. It was found that D_(c) is almost constant or decreases gradually with decreasing C_f in the range of high-medium C_f but decreases appreciably with decreasing C_f at low C_f. The change in D_(c) with C_f was explained in terms of the dual-mode sorption-diffusion model. The penetrants diffuse in the polymer as two distinct species, i.e., a dissolved species and an adsorbed species. The former is the penetrant taken up by the polymer by a partition mechanism (dissolved species) and the latter is that taken up by Langmuir sorption (adsorbed species). The actual diffusivity D_P(c) of the dissolved species decreases with decreasing C_f. While the actual diffusivity D_L(c) of the adsorbed species normally increases gradually with decreasing C_f. D_P(c) is usually larger than D_L(c). © 1993 John Wiley & Sons, Inc.     

Conformational Study on Thin Films of Symmetric AnB2nAn Triblock Copolymer

Summary: The behavior of symmetric A_nB_2nA_n triblock copolymer films confined between two hard neutral walls was explored by Monte Carlo simulation. The thicknesses of the films were between ≈1R_g0 and ≈7R_g0, where R_g0 is the unperturbed radius of gyration in the bulk. The confinement leads to a lamellar structure normal to the wall and the order‐disorder transition (ODT) temperature was found to be a function of film thickness. When the film thickness (D) was less than a critical value, D_C, which is between 3R_g0 and 4R_g0, the ODT temperature (T*_ODT) reduced by chain length N (T*_ODT/N) decreased with decreasing film thickness. However, T*_ODT/N was nearly independent of the film thickness when it was greater than D_C. In the case of strong confinement (D < D_C), the B block shrinks along the direction perpendicular to the wall and stretches along the direction parallel to the wall with decreasing film thickness, and the volume occupied by the B block shrinks. Under weak confinement conditions (D > D_C), the volume of the B block is nearly independent of film thickness. The conformations of the B block in the disordered state are quite different from those in the lamellae. If the film is thick enough, the volume of the B block approaches its value in the unperturbed state, regardless of the morphology. When temperature decreases, the B block stretches in the direction perpendicular to the A/B interface and shrinks in the other two directions. In addition, decreasing the temperature leads to the chains adopting two main extreme conformations, coiling or stretching as much as they can. The scaling behavior of the fraction of bridge chains vs. the temperature obtained in the weak segregation limit was different from that predicted in the strong segregation limit.

Schematic diagram of the X, Y and Z axis definition.     

Redox Active Two‐Component Films of Palladium and Covalently Linked Zinc Porphyrin–Fullerene Dyad

Redox active films have been generated electrochemically by the reduction of dyads consisting of fullerene C₆₀ covalently linked to zinc meso‐tetraphenyloporphyrin, ZnP? C₆₀, and palladium acetate. The films are believed to consist of a polymeric network formed via covalent bonds between the palladium atoms and the fullerene moieties. In these films, the zinc porphyrin moiety is covalently linked to the polymeric chains through the pyrrolidine ring of the fullerene. The ZnP? C₆₀/Pt films are electrochemically active in both positive and negative potential excursions. At positive potentials, two oxidation steps for the zinc porphyrin are observed. In the negative potential range, electron transfer processes involving the zinc porphyrin and the fullerene entities are observed. Film formation is also accompanied by palladium deposition on the electrode surface. The presence of a metallic phase in the film influences its morphology, structure and electrochemical properties.     

Channeling Exciton Migration into Electron Transfer in Formamidinium Lead Bromide Perovskite Nanocrystal/Fullerene Composites

Hydrophobically capped nanocrystals of formamidinium lead bromide (FAPbBr₃) perovskite (PNC) show bright and stable fluorescence in solution and thin‐film states. When compared with isolated PNCs in a solution, close‐packed PNCs in a thin film show extended fluorescence lifetime (ca. 4.2 μs), which is due to hopping or migration of photogenerated excitons among PNCs. Both fluorescence quantum efficiency and lifetime decrease in a PNC thin film doped with fullerene (C₆₀), which is attributed to channeling of exciton migration into electron transfer to C₆₀. On the other hand, quenching of fluorescence intensity of a PNC solution is not accompanied by any change in fluorescence lifetime, indicating static electron transfer to C₆₀ adsorbed onto the hydrophobic surface of individual PNCs. Exciton migration among close‐packed PNCs and electron transfer to C₆₀ places C₆₀‐doped PNC thin films among cost‐effective antenna systems for solar cells.     

Electrochemically Polymerized N,N-Dimethylaniline Films Containing Tris-(Bathophenanthroline Disulfonato)Iron(II/III) Complexes

The electropolymerization of N, N-dimethylaniline (DMA) was carried out in an aqueous CF₃COONa solution (pH 1.0) containing DMA in the presence of tris(bathophenanthroline disulfonato)iron(II), Fe(bphen)₃ ^4-. Poly(N, N-dimethylanilinium trifluoroacetate) (PDMA) film was formed on electrode surfaces and, at the same time, Fe(bphen)₃ ^4- ions were stably confined in the formed PDMA film by electrostatic interaction between them and the positively charged quaternary ammonium sites of the PDMA film. The PDMA-Fe(bphen)₃ ^4-/3- film thus prepared displayed well-defined reversible electroactivity and electrochromic properties ascribable to those of the Fe(bphen)₃ ^4-/3- couple confined in the film. The PDMA-Fe(bphen)₃ ^4- film is red, and the PDMA-Fe(bphen)₃ ^3- film is colorless. The response rate of the color change to a potential pulse was found to be correlated with the kinetic parameters characterizing the rate of the overall charge-transfer reaction at the PDMA-Fe(bphen)₃ ^4-/3- film-coated electrode, that is, the apparent diffusion coefficient (D _app) for the homogeneous charge-transport process within the film and the standard rate constant (k) of the heterogeneous electron-transfer reaction at the electrode/film interface. For the PDMA-Fe(bphen)₃ ^4-/3- film with larger k° and D _app values, the response rate of the color change was larger, Further, k°, D _app, and response rate depended on the concentration (C°) of the Fe(bphen)₃ ^4- (or Fe(bphen)₃ ^3-) confined in the PDMA film; and at a given film thickness, the lower C°, the higher were k°, D _app, and response rate. At a given C°, the thinner the film thickness, the greater was the response rate.     

Investigation into Charge Transport Dynamics of [Os(bpy)2(picolinate)]Cl Nafion Films

This work examines the charge transport properties of redox films of the immobilized enzyme mediator [Os(bpy)₂(picolinate)]Cl ([Os(bpy)₂Pic]⁺). Chronoamperometry was used to calculate D_CT values (typically of the order of 1.5×10^?8 cm² s^?1) and the effect of %loading of redox material was determined for three electrolytes over a range of concentrations. The data obtained implies that charge transfer within the film occurs via a redox site diffusion mechanism. The concentration of redox sites was determined using an approximate film thickness of 7 μm, as determined from profilometry studies. Experiments were also performed using immobilized carbon nanotubes within the redox film in order to examine the impact of the increased surface area and conductivity. The presence of the carbon nanotubes had the effect of doubling the surface coverage values and enhancing D_CT^1/2 C_M values from 1.9×10^?9 to 16.1×10^?9 mol cm^?1 s^?1.     

Stability studies of an electrochemically formed [C60]fullerene-palladium two-component film

The stability of C₆₀ and palladium two-component films, C₆₀/Pd, has been investigated. The effect of different polymerization conditions on the electrochemical stability of the film upon prolonged potential cycling has been studied. Stable voltammetric behavior was observed for polymers formed at potentials less negative than the potential of third C₆₀ reduction step. The incorporation of palladium particles into the structure of C₆₀/Pd polymers increases the polymer stability. The C₆₀/Pd films are doped with supporting electrolyte cations during reduction. The size of these cations is a crucial factor in determining the stability of the film. A strong solvent effect on the potential stability of the film was also observed. The wildest range of stable voltammetric properties was found for acetonitrile and N,N-dimethylformamide. No effect of the temperature on the film stability was observed. The results reported in this work allow for the determination of the optimal conditions for the formation of stable C₆₀/Pd films.     

A new type of piezoelectric detector in liquid: Part 2. Computation of the equivalent circuit parameters of a piezoelectric crystal with a separated electrode and of series piezoelectric sensors in a non-electrolyte solution

An improved equivalent circuit model is proposed for a piezoelectric crystal with a separated electrode. Equations are derived for the equivalent circuit parameters in a non-electrolyte solution and are verified experimentally. The resonance frequency f_s is given by f_s = f₀[1 + C₁/2(C′₀ + C_s)], where f₀, C₁ and C′₀ are the resonance frequency, motional capacitance and the shunt capacitance of the crystal respectively and C_s is the solution capacitance. The mechanical quality factor is the same as that of the crystal. The motional resistance, motional inductance, motional capacitance and shunt capacitance are respectively K², K², K⁻² and K⁻¹ times those of the crystal, where K = 1 + C′₀/C_s. The influence of the permittivity, density and viscosity of the solution and the configuration of the sensors on the equivalent parameters are investigated. The equivalent circuit parameters of a series piezoelectric crystal are also calculated and measured.     

Triplet excited states of nitronaphthalenes. II. b-Nitronaphthalene

Nanosecond flash photolysis of b-nitronaphthalene (b-NO₂C₁₀H₇) in nonpolar and polar solvents shows a transient species with maximum absorption and lifetime dependent on solvent polarity. In deaerated n-hexane the absorption maximum and lifetime (1/k) are 425 nm and 530 nsec, while in deaerated ethanol the corresponding values are 470 nm and 1.7 ·sec. This transient absorption is attributed to the triplet excited state of b-NO₂C₁₀H₇, and the observed red shift as well as its longer lifetime in polar solvents are indicative of the intramolecular charge transfer character of this state. The change of dipole moment accompanying the transition T₁ → T_n, as well as rate constants for electron and proton transfer reactions involving the T₁ state of b-NO₂C₁₀H₇, were determined. The spectroscopic and kinetic data obtained in this work indicate that the triplet state of b-NO₂C₁₀H₇ behaves like a n-π* state in nonpolar media, while in polar solvents the n-π* character of the state is reduced with a simultaneous increase in the charge transfer character.     

Achieving high electrode specific capacitance with materials of low mass specific capacitance: Potentiostatically grown thick micro-nanoporous PEDOT films

Electrode materials for supercapacitors are at present commonly evaluated and selected by their mass specific capacitance (C_M, F g⁻¹). However, using only this parameter may be a misleading practice because the electrode capacitance also depends on kinetics, and may not increase simply by increasing material mass. It is therefore important to complement C_M by the practically accessible electrode specific capacitance (C_E, F cm⁻²) in material selection. Poly[3,4-ethylene-dioxythiophene] (PEDOT) has a mass specific capacitance lower than other common conducting polymers, e.g. polyaniline. However, as demonstrated in this communication, this polymer can be potentiostatically grown to very thick films (up to 0.5 mm) that were porous at both micro- and nanometer scales. Measured by both cyclic voltammetry and electrochemical impedance spectrometry, these thick PEDOT films exhibited electrode specific capacitance (C_E, F cm⁻²) increasing linearly with the film deposition charge, approaching 5 F cm⁻², which is currently the highest amongst all reported materials.     

Sorption and diffusion of gases in a polyimide

Sorption and diffusion of gases (CO₂, N₂, and He) in a polyimide (PI2080) film were measured by using an apparatus which gives the sorption rate curves from the initial state to the equilibrium state. Nonlinear isotherms observed for CO₂ sorption were interpreted successfully in terms of the dual-mode model for sorption in glassy polymers. Linear isotherms observed for N₂ and He seemed to obey Henry's law. Two diffusion coefficients (D_I and D_E) were obtained using the short-time method and the long-time method for a Fickian diffusion model, together with the equilibrium solubility (C_e) from each experiment. The initial sorption rate curves agreed with the calculated curves using D_I, however near sorption equilibrium the curves are in accord with the calculated curves using D_E. These observations suggest that some relaxation process is superimposed on the diffusion process. The non-Fickian transport data were correlated successfully with a model that combines time-dependent diffusion and the Fickian model.     

Lupane type triterpenes as structuring elements in the monolayers and films of lecithin and fullerene derivatives

Using compression and wetting isotherm analysis, it was shown that lupane triterpenes (betulinol, betulinol diacetate, betulinic acid) change crucially the state of monolayers and films of C₆₀ fullerene, nitroxide malonate C₆₀ methanofullerene, and lecithin and also the films of their mixtures at the water—air interface. The structuring action of triterpenes in the presence of lecithin and C₆₀ fullerene at high triterpene contents in the film gives rise to uniaxially oriented films similar to thin films of triterpenes (atomic force microscopy data). The formation of mixed bis-nitroxide malonate methanofullerene—triterpene films with excess of the latter affords structures shaped like crater-like bowls. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1369–1378, July, 2008.     

Electron transfer reaction of electrochemically polymerized tris-(amino-phenanthroline)-iron(II/III) complex films

New films of iron complex with 4,7-bis(2-aminophenyl)-methylaminosulfonylphenyl)-1,10-phenanthroline (APP) and 5-amino-1,10-phenanthroline (AP) are prepared on the electrode surface of In–Sn oxide conducting glass (ITO) by electrochemical oxidation. The thickness (Φ) of the films prepared on the ITO can be controlled by the number of cycles of the potential scan. The resulting film-coated electrodes show well-defined reversible vol-tammograms corresponding to the redox reaction of the Fe(II/III) complexes in 0.1 M NaClO₄ acetonitrile (AN), a mixture of butylene carbonate (BC) and propylene carbonate (PC) and poly2-hydroxyethylmethacrylate gel containing BC and PC. The electron transfer processes within the films can be treated apparently as diffusional processes characterized by the rate constants of the apparent diffusion coefficient (D_app). The value of D_app increase from 1.0 × 10^?9 to 1.6 × 10^?8 cm² sec^?1 as the Fe complex concentration (C_Fe) increases from 0.06 to 1.04 M for the [Fe(AP)₃] complex film (Φ=0.80 μm) in 0.2 M NaClO₄/AN solution. The D_app value for the [Fe(APP)₃) complex film (C_Fe = 0.19 M , Φ= 0.78 μm) is 3.5 × 10^?9 cm² sec^?1 in 0.2 M NaClO₄/AN solution. The D_app values of the [Fe(AP)₃] complex film in the PC + BC mixture and gel containing 1.0 M NaClO₄ were smaller than those obtained in AN solution by an order of magnitude. The dependence of the apparent formal potential of the Fe(II/III) redox reaction for the [Fe(AP)₃] complex film on the activity of NaClO₄ supporting electrolyte in AN shows that Na⁺ moves preferentially across the polymer/solution interfaces during the redox reaction. The Fe(II/III) redox reaction of the Fe complex films shows reversible electrochromic response between red and colorless.