Charge Transport Diffusion Coefficient Research Articles

Revisiting the Mechanisms of Charge Transport in Solutions of Redox-Active Molecules Using Computer Simulations: When and Why Do Analytical Theories Fail?

Understanding charge transport is essential for the development of energy-storage applications. This work introduces a new theoretical methodology to model diffusive charge transport in solutions of redox-active molecules by combining Langevin dynamics for the spatial degrees of freedom and a master-equation formalism to describe the electron-hopping events between redox molecules. The model is used to analyze the effects of the concentration of the redox molecules and the strength of the intermolecular interactions on the charge-transport mechanism. In the past, the rate of charge transport has been modeled with the analytical Dahms-Ruff equation; however, this is a mean-field equation, whose range of validity has not been tested with less approximate theories. We show that the Dahms-Ruff equation fails to quantitatively predict the diffusion coefficient for charge transport for large concentrations of the redox species and high bimolecular electron-transfer rates, i.e., the most relevant conditions for energy-storage applications. Under these conditions, the diffusion coefficient for charge transport obtained from simulations is larger than that predicted from the Dahm-Ruff equation because of the formation of transient clusters of redox molecules. Also, intermolecular interactions, which are not taken into account by the Dahms-Ruff equation, play a central role in the charge transport of redox species. We show that the apparent diffusion coefficient experiences a maximum with respect to the strength of the intermolecular attractions. This maximum is traced back to the formation of clusters and their two opposite effects on the diffusion coefficient: electron hopping is fast within a cluster but inefficient between neighboring clusters.

Highly Electroconductive Metal-Organic Framework: Tunable by Metal Ion Sorption Quantity.

This study has developed a simple and innovative approach to design a metal-organic framework (MOF) with tunable conductivity. The in situ amine functionalized TMU-60 [Zn(OBA)(L*)·DMF] has the potential of converting to a conductor due to the selective and fast sorption of cadmium ion. In order to precisely monitor the cadmium penetration to the framework pores, in addition to full kinetic and thermodynamic studies of sorption, the time-dependent effects of Cd(II) penetration to MOF pores was taken by fluorescence and electrochemical methods. The results obviously prove that the conductivity of the structure can be adjusted by changing the Cd exposure time and the quantity of ion sorption. The presence of cadmium ions can enhance the ability of the framework to pass electrons through its constituents. Even though the porosity of the structure remains to a certain extent, the achieved tunable and highly stable electrical conductivity is provided by sorption of metal ions. More importantly, the conductivity has been improved from 53 × 10-6 to 1.8 × 10-2 S·cm-1 for TMU-60 and TMU-60-Cd compressed pellet (room temperature), respectively. The mechanism of charge transport through the pellet is proposed to be a hopping mechanism between metal nodes and localized Cd(II) after sorption, which is supported by chronoamperometry measurement. A fit of the time-dependent data to the Cottrell equation gave a charge-transport diffusion coefficient of 1.6 × 10-10 cm2/s throughout the structure. This novel approach not only enhances the conductivity of the MOF but also allows adjusting the conductivity according to the desired applications.

Scanning electrochemical microscopy imaging of poly (3,4-ethylendioxythiophene)/thionine electrodes for lactate detection via NADH electrocatalysis

Herein we report the use of scanning electrochemical microscopy (SECM) together with electrochemical and spectroscopic techniques to develop and characterise a stable and uniformly reactive chemically modified platinum electrode for NADH electrocatalysis. In order to achieve this, a range of different approaches for thionine entrapment within an electropolymerised poly (3,4-ethylendioxythiophene) (PEDOT) film were evaluated using SECM imaging in the presence of NADH, demonstrating the uniformity of the reactive layer towards NADH oxidation. The effect of electrolyte type and time scale employed during PEDOT electropolymerisation was examined with respect to thionine loading and the resulting charge transport diffusion coefficient (DCT) estimated via chronoamperometry. These studies indicated a decrease in DCT as thionine loading increased within the PEDOT film, suggesting that charge transport was diffusion limited within the film. Additionally, thionine functionalised nanotubes were formed, providing a stable support for lactate dehydrogenase entrapment while lowering the rate of thionine leaching, determined via SECM imaging. This enabled lactate determination at Eapp = 0.0 V vs Ag/AgCl over the range 0.25–5 mM in the presence of 1 mM NAD+.

Photocurrent induced by conducting channels of hole transporting layer to adjacent photoactive perovskite sensitized TiO2 thin film: solar cell paradigm.

A high performance perovskite solar cell was fabricated using the distinguished morphology of polyaniline nanoparticles (PANI-NPs) as an efficient hole transporting layer (HTL) with methylammonium lead iodide perovskite (CH3NH3PbI3) as sensitizer. PANI-NPs were simply synthesized by the oxidative chemical polymerization of aniline monomer at 0-5 °C. A reasonable solar-to-electricity conversion efficiency of ∼6.29% with a high short circuit current (JSC) of ∼17.97 mA/cm(2) and open circuit voltage (VOC) of ∼0.877 V were accomplished by Ag/PANI-NPs/CH3NH3PbI3/mp-anatase-TiO2/bl-TiO2/FTO perovskite solar cell. The transient photocurrent and photovoltage studies revealed that the fabricated solar cell showed better charge transport time, diffusion coefficient, diffusion length, and charge collection efficiency. Herein, the use of PANI-NPs as the HTL improved the charge carrier generation and the charge collection efficiency of the fabricated solar cell.

High efficiency electrochemiluminescence from polyaniline:ruthenium metal complex films

Electrochemiluminescent films have been formed by electrodepositing polyaniline, PANI, in the presence of [Ru(bpy)2PIC]2+; bpy is 2,2′-bipyridyl and PIC is (2,2′-bipyridyl)-2(4-carboxylphenyl) imidazo [4,5][1,10] phenanthroline. The homogeneous charge transport diffusion coefficient, DCT, for the Ru2+/3+ couple within the PANI film is 2.6±0.9×10−10cm2s−1. The large DCT facilitates a fast regeneration of Ru3+ and, coupled to a relatively rigid microenvironment, results in a high ECL intensity in the presence of tripropylamine as a co-reactant compared to [Ru(bpy)3]2+. Significantly, despite the conducting nature of the polymer backbone, the [Ru(bpy)2PIC]2+ loaded PANI has the highest efficiency, 1.00%, yet reported for a surface confined ruthenium complex.

Ti thin film towards the growth of crystalline-TiO2 nanostructures: stepped light-induced transient measurements of photocurrent and photovoltage in dye sensitized solar cell

Highly dense and well-defined TiO2 nanoflowers (NFs) were grown by the hydrothermal process on a titanium (Ti) coated FTO substrate. The Ti layer with a thickness of ~500–600 nm was deposited on FTO at room temperature with a pressure of ~5 mTorr using a Ti-source through RF magnetic sputtering. The unique TiO2 NF thin film substrate was applied as a photoanode for the fabrication of a dye sensitized solar cell (DSSCs). Each NF was made of uniform clover leaf-like petals of an average diameter of ~80–100 nm. The synthesized TiO2 NFs possessed a pure anatase phase with good crystal quality. The fabricated DSSC with TiO2 NF thin film photoanode accomplished a reasonably good overall solar-to-electricity conversion efficiency (η) of ~3.64% with a high short circuit photocurrent density (JSC) of ~9.6 mA cm−2. The improved performance and photocurrent density were explained by the charge transport time, diffusion coefficient, diffusion length and charge collection efficiency of the fabricated DSSC.

Charge Transport through Geobacter sulfurreducens Biofilms Grown on Graphite Rods

Biofilms of the electroactive bacterium Geobacter sulfurreducens were induced to grow on graphite-rod electrodes under a potential of 0 V (vs Ag/AgCl) in the presence of acetate as an electron donor. Increased anodic currents for bioelectrocatalytic oxidation of acetate were obtained when the electrodes were incubated for longer periods with periodic electron-donor feeding. The maximum current density for acetate oxidation increased 2.8-fold, and the biofilm thickness increased by 4.25-fold, over a time period of 83-147 h. Cyclic voltammetry in the presence of acetate supports a model of heterogeneous electron transfer, one electron at time, from biofilm to electrode through a dominant redox species centered at -0.41 V vs Ag/AgCl. Voltammetry performed under nonturnover conditions provided an estimate of the surface coverage of the redox species of 25 nmol/cm(2). This value was used to estimate a redox species concentration of 7.3 mM within the 34-μm-thick biofilm and a charge-transport diffusion coefficient of 3.6 × 10(-7) cm(2)/s. This value of diffusion coefficient is greater than that observed in traditional thin-film voltammetric studies with redox polymer films containing much higher surface concentrations of redox species and might be associated with proton transport to ensure electroneutrality within the biofilm upon electrolysis.

Host−Guest Directed Assembly of Gold Nanoparticle Arrays

The formation of a three-dimensional assembly of gold nanoparticles driven by host-guest interactions is described. Assembly is achieved via host-guest interactions between cyclodextrin-modified gold nanoparticles which associate with the adamantane unit of a redox active metal complex [Os(CAIPA)(3)](ClO(4))(2), where CAIPA is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]-phenanthroline-1-adamantylamine. The electrochemical properties of thin films formed on glassy carbon electrodes have been probed using cyclic voltammetry. In aqueous LiClO(4), the homogeneous charge transport diffusion coefficient, D(CT), is (8.4 +/- 0.4) x 10(-9) cm(2) s(-1) for both oxidation and reduction of the osmium complexes. Significantly, this charge transport rate is significantly larger than that obtained for a solid deposit of [Os(CAIPA)(3)](ClO(4))(2) alone where D(CT) is 2.3 x 10(-10) cm(2) s(-1). The higher D(CT) value observed for the nanoparticle arrays suggests that the incorporated nanoparticles facilitate electron transfer between the bound osmium centers.

Enhanced Electrochemiluminescence and Charge Transport Through Films of Metallopolymer-Gold Nanoparticle Composites

Water-soluble 4-(dimethylamino) pyridine (DMAP) stabilized gold nanoparticles (DMAP-AuNP) were synthesized by ligand exchange and phase transfer (toluene/water). The DMAP-AuNPs are positively charged with the core diameter of 4 +/- 1 nm. Metallopolymer-gold nanocomposites were prepared by mixing gold nanoparticles and [Ru(bpy)(2)PVP(10)](ClO(4))(2), in water at different mole ratios; bpy is 2,2'-bipyridyl and PVP is poly (4-vinylpyridine). The photoluminescence emission intensity of the metallopolymer decreases with increasing AuNP loading and approximately 57% of the emission intensity is quenched when the Au NP:Ru mole ratio is 14.8 x 10(-2). The rate of homogeneous charge transfer through thin layers of the nanocomposite deposited on glassy carbon electrodes increases with increasing nanoparticle loading. The homogeneous charge transport diffusion coefficient, D(CT), for the composite (AuNP:Ru mole ratio 13.2 x 10(-2)) is (2.8 +/- 0.8) x 10(-11) cm(2) s(-1) and is approximately 3-fold higher than that found for the pure metallopolymer. Significantly, despite the ability of the metal nanoparticles to quench the ruthenium-based emission, the electrochemiluminescence of the nanocomposite with a AuNP:Ru mole ratio of 4.95 x 10(-2) is approximately three times more intense than the parent metallopolymer. This enhancement arises from the increased rate of charge transport that leads to a greater number of excited states per unit time while minimizing the quenching effects. The implications of these findings for the design of electrochemiluminescent sensors are discussed.

Redox switching and oxygen evolution electrocatalysis in polymeric iron oxyhydroxide films

Outstanding issues regarding the redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been examined. Charge percolation through the hydrous layer has been quantified, using cyclic voltammetry, in terms of a charge transport diffusion coefficient D(CT) which admits a value of ca. 3 x 10(-10) cm2 s(-1). Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Redox induced switching dynamics of a three colour electrochromic metallopolymer film

Thin films of a novel Ru-phenolate based metallopolymer, [Ru(terpy)(box)PVP 20]PF 6, in which one in every twenty of the 4-vinyl pyridine monomer units is labelled with the ruthenium complex have been formed on glassy carbon electrodes, terpy is 2,2′:6′,2″-terpyridine, box is 2-(2-hydroxyphenyl)benzoxazole, and PVP is poly(4-vinylpyridine). Cyclic voltammetry and Raman spectroscopy reveal that the Ru 2+/3+ couple is electrochemically reversible but that the phenolate ligand based oxidation is irreversible. These redox processes are associated with reversible colour changes from wine red (reduced) to red orange (mixed composition) then to light green (oxidized) in the visible region and an irreversible change in the near-IR region, respectively. Scanning electron microscopy reveals that repeated switching in LiClO 4 aqueous solution does not induce any significant structural change within the deposit films. Cyclic voltammetry has been used to determine the electrochromic switching rate under semi-infinite linear diffusion conditions. In aqueous LiClO 4, the homogeneous charge transport diffusion coefficient, D CT, decreases from 3.6 ± 0.3 × 10 −13 to 2.7 ± 0.2 × 10 −13 cm 2 s −1 as the LiClO 4 concentration increases from 0.1 to 1.0 M. This weak dependence of D CT on electrolyte concentration suggests that counterion availability is not rate-determining and that the overall rate of charge transport through the metallopolymer film is limited by the rate of segmental polymer chain motion necessary to bring adjacent centres sufficiently close to allow electron transfer to occur. Also the impact of changing the identity of the charge compensating anion of the redox electrochromic switching rate has been investigated. Finally, the electronic conductivity has been determined using interdigitated array electrodes (IDAs).

Three colour electrochromic metallopolymer based on a ruthenium phenolate complex bound to poly(4-vinyl)pyridine

The photonic and electrochemical properties of a novel Ru–phenolate based metallopolymer are reported. The complex undergoes a ruthenium based reversible oxidation at approximately +0.400 V and irreversible box ligand oxidation at +0.800 V vs. Ag/AgCl. Oxidation of thin films in aqueous electrolyte at +0.500 V reversibly switches the colour from wine red to light green and a red orange colour is observed for mixed redox composition. In contrast, oxidation at potentials more positive than +1.500 V shows no visible colour change but produces a change in the near infra-red region. To determine the electrochromic switching rate and to identify the rate determining step of the, scan rate dependent cyclic voltammetry was performed under semi-infinite linear diffusion conditions in aqueous lithium perchlorate. These data reveal that the homogeneous charge transport diffusion coefficient, D CT, is 3.6 ± 0.2 × 10 −13 cm 2 s −1, i.e., under these conditions it takes approximately 90 s to fully oxidise a 100 nm thick film.

Impact of ion solvation on charge transport through [Os(bpy)2 (H2tzt) Cl]+ in the solid state

Microcavities have been formed at the tip of platinum microelectrodes and packed with [Os(bpy)2 (H2tzt) Cl]PF6, where bpy is 2,2′-bipyridyl and H2tzt is 3,6-bis(4-pyridyl)-dihydro-1,2,4,5-tetrazine. These solid deposits exhibit well defined electrochemical responses associated with the Os2+/3+ redox couple where the identity of the electrolyte anion is sodium perchlorate, bromide, chloride, iodide, fluoride or nitrate. Scanning electron microscopy of deposits on planar electrodes reveals that voltammetric cycling triggers morphological changes that depend on the identity of the electrolyte anion. The formal potential of the Os2+/3+ redox process ranges from 0.225 ± 0.015 V (perchlorate) to 0.390 ± 0.010 V (fluoride) and depends approximately linearly on the hydration energy of the anion. This result suggests that weakly hydrated anions are thermodynamically easier to incorporate within the solid deposit in response to redox switching, i.e., the anion desolvation energy influences the energetics of redox switching. Cyclic voltammetry has been used to determine the apparent charge transport diffusion coefficient, DCT, describing homogeneous charge transport through the deposit. The rates of charge transport depend significantly on the identity and concentration of the supporting electrolyte ranging from a minimum of 3.1 ± 0.5 × 10−12 cm2 s−1 in 0.1 M NaF to a maximum of 1.1 ± 0.1 × 10−10 cm2 s−1 in 1.0 M NaClO4. In NaClO4 supporting electrolyte, DCT is independent of the electrolyte concentration from 0.1 to 1.0 M suggesting that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid. In contrast, in NaBr solutions DCT is sensitive to the electrolyte concentration increasing from 0.2 ± 0.01 to 7.9 ± 0.1 × 10−11 cm2 s−1 on going from 0.1 to 1.0 M suggesting that the availability of charge compensating counterions within the solid limits the rate of charge transport. The free energy of at least partially desolvating charge compensating counterions prior to their incorporation within the oxidised deposit appears to control both the energetics and dynamics of charge transport.

Modulating the Redox Properties of an Osmium-Containing Metallopolymer through the Supporting Electrolyte and Cross-Linking

Thin films of the perchlorate salt of an [Os(N,N'-alkylated-2,2'-biimidazole3)2+/3+-containing polymer have been formed on planar platinum microelectrodes. The electrochemical response associated with the Os2+/3+ couple occurs at -0.19 V. In aqueous perchlorate media at near-neutral pH the voltammetric response is close to that expected for an electrochemically reversible reaction involving a surface-confined reactant. Chronoamperometry conducted on a microsecond time scale indicates that the film and solution resistances are comparable for low concentrations of supporting electrolyte. However, for LiClO4 concentrations greater than 0.4 M, RFilm contributes less than 25% of the overall cell resistance. These results suggest that when the film is dehydrated and the density of redox centers is increased, electron or hole hopping dominates the rate of homogeneous charge transport through the film. The rate of homogeneous charge transport, characterized by D(CT)1/2Ceff, where DCT is the homogeneous charge transport diffusion coefficient and Ceff is the effective concentration of osmium centers within the film, depends weakly on the concentration of LiClO4 as supporting electrolyte decreasing from (8.1 +/- 0.16) x 10(-9) to (4.7 +/- 0.4) x 10(-9) mol cm(-2) s(-1/2) as the perchlorate concentration increases from 0.1 to 1.0 M. These values are about 2 orders of magnitude lower than those of the chemically cross-linked chloride salt of the polymer. The rate of heterogeneous electron transfer is unusually rapid in this system and increases from (5.2 +/- 0.4) x 10(-3) to (7.8 +/- 0.4) x 10(-3) cm s(-1) on going from 0.1 to 0.4 M LiClO4 before becoming independent of the supporting electrolyte concentration at (9.2 +/- 0.6) x 10(-3) cm s(-1) for [LiClO4] > or = 0.6 M.

Electropolymerisation dynamics of a highly conducting metallopolymer: poly-[Os(4 ′-(5-(2,2 ′-bithienyl))-2,2 ′:6 ′,2 ″-terpyridine) 2] 2+

The potential, time, and concentration dependence of the potentiostatic electropolymerisation of the thienyl-substituted transition metal complex [Os(bttpy) 2 2+] onto platinum disk microelectrodes is reported, bttpy is 4 ′-(5-(2,2 ′-bithienyl))-2,2 ′:6 ′,2 ″-terpyridine. Oxidative electropolymerisation of the thienyl bridges is highly efficient with an electropolymerisation efficiency of 36 ± 3% being observed across a wide range of potentials and monomer concentrations. The osmium centres are oxidised when polymerisation proceeds and the deposited polymer is highly conducting allowing high surface coverage films, up to 6 × 10 −7 mol cm −2, to be deposited within 60 s. SEM imaging reveals that smooth films can be produced using moderate overpotentials for electrodeposition. Significantly, diffusional mass transport controls the rate of film deposition allowing the radial diffusion field found at microelectrodes to be exploited to favour film growth co-planar with the electrode surface. The electropolymerisation rate increases approximately linearly with increasing monomer concentration from approximately 8 to 400 μM and this first-order dependence likely arises from mass transport limitations. The rate of homogeneous charge transport through potentiostatically deposited films, characterised by D 1/2 C, where D is the apparent charge transport diffusion coefficient and C is the concentration of osmium centres, is 3.5 ± 0.5 × 10 −7 mol cm −2 s −1/2 and is largely insensitive to the deposition potential, and is very similar to that previously observed for potentiodynamically deposited films. This charge transport parameter is approximately two orders of magnitude larger than those found for non-conjugated bridges and is interpreted in terms of resonant superexchange across the quaterthienyl bridge.

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate–surfactant film modified electrode

Electrochemistry and electrocatalysis of binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) were investigated successfully in liquid crystal films of didodecyldimethylammonium bromide (DDAB) on pyrolytic graphite (PG) electrodes in aqueous solution. This new bi-CoPc–surfactant film modified electrode could reduce aggregation of bi-CoPc in aqueous solution, which was confirmed by the electronic absorption spectra in the visible region and images of transmission electron microscopy (TEM). The charge transport diffusion coefficient ( D ct) and apparent heterogeneous electrode reaction rate constant ( k s) for this modified electrode were estimated. The reduction of bi-CoPc ligand showed excellent electrocatalytic ability for the reduction of trichloroacetic acid (TCA) and catalytic current had a linear relationship with the concentration of TCA in the range of 3×10 −5–7.5×10 −3 M. The reduction of bi-Co(III)Pc/bi-Co(II)Pc couple exhibited new pattern of catalytic reactivity in the reduction of oxygen. Peak current for the cathodic reduction of oxygen is proportion to the square root of the scan rate in the range of 5–1000 mV s −1 in oxygen-saturated solution.

Nanoparticle–metallopolymer assemblies: charge percolation and redox properties

Stable gold nanoparticles have been prepared by the chemical reduction of tetrachloroaurate in the presence of PVP and the metallopolymer [Os(bpy)2(PVP)10Cl]+ films, where bpy is 2,2′-dipyridyl and PVP is poly(4-vinylpyridine). The maximum dry state conductivity observed is 3.5±0.3×103 and 8.7±0.1×102 S m−1 where the polymer matrix is PVP and the metallopolymer, respectively. The percolation threshold is significantly lower for the nanoparticle loaded metallopolymer, 0.07±0.01, compared with PVP, 0.31±0.04. This result suggests that the osmium centers provide an enhanced conductivity pathway in the dry state. The redox properties of thin films of the composites deposited on microdisk platinum electrodes in contact with aqueous electrolyte have been investigated. The charge transport diffusion coefficient, DCT, is approximately an order of magnitude larger for the composite containing the highest nanoparticle loading, 5.7±0.4×10−9 cm2 s−1, than that found for the pure metallopolymer, 5.7±0.2×10−10 cm2 s−1. A nanoparticle loading above the percolation threshold also promotes faster heterogeneous electron transfer increasing from 8.2±1.7×10−5 to 4.0±0.2×10−3 cm s−1.

Charge Transport Dynamics and Redox Induced Structural Changes within Solid Deposits of a Ruthenium Dimer

Solid particles of a ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+, have been abrasively attached to macroelectrodes and microelectrodes, where bpy is 2,2‘-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)-1,2,4-triazole. The voltammetry of these solids deposits is unusually ideal in NH4PF6-containing aqueous solutions, and the response is characterized by semi-infinite linear diffusion for scan rates between approximately 50 and 2000 mV s-1. SEM imaging reveals that sparse films of solid particles (1−10 μm in diameter) are efficiently transformed into microcrystals by voltammetric cycling. The charge transport diffusion coefficient, DCT, has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, DCT increases from 2.4 to 3.6 × 10-10 cm2 s-1 as the NH4PF6 concentration is increased from 0.1 to 2.0 M, while, for oxidation of the deposit, DCT increases from 1.1 to 3.9 × 10-10 cm2 s-1 over the same concentration range. The maximum DCT observed would correspond to an electron self-exchange rate constant of 1.1 × 105 M-1 s-1. Despite the smaller electron-transfer distance expected within the solid, this apparent self-exchange rate constant is more than 2 orders of magnitude smaller than that typically found for ruthenium bis-bpy complexes in solution. This observation suggests that ion rather than electron transfer may limit homogeneous charge transport through these solid deposits.

Redox switching in solid deposits:: triazole bridged osmium dimers

Solid deposits of the dimeric complex [Os(bpy) 2Cl 4-bpt Os(bpy) 2Cl]PF 6, where bpy is 2,2′-bipyridyl and bpt is 3,5-bis(pyridin-4-yl)-1,2,4,-triazole have been deposited onto platinum microelectrodes. These layers exhibit unusually ideal electrochemical responses over a wide range of electrolyte compositions and pH values. Scanning electron microscopy reveals that repeated switching of the redox composition of these layers does not induce any significant structural change within the deposits. Cyclic voltammetry (CV) has been used to determine the apparent charge transport diffusion coefficient, D CT, describing homogeneous charge transport through the deposit. D CT is independent of the electrolyte concentration suggesting that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid. In 1.0 M LiClO 4 and 1.0 M HClO 4, D CT values of 2.0±0.1×10 −10 and 1.7±0.4×10 −10 cm 2 s −1 are observed, corresponding to second order electron transfer rate constants of 1.8×10 7 and 3.0×10 7 M −1 s −1, respectively. The rate of heterogeneous electron transfer across the electrode ∣ deposit interface has been determined using fast scan CV. The standard heterogeneous electron transfer rate constant, k°, is 1.08±0.05 cm s −1 irrespective of the electrolyte pH. Significantly, this value is less than one order of magnitude smaller than that determined for a monomeric complex containing the same bridging ligand and redox active metal centre.

Electrochemical Study of the Binding Interaction Between Dihydroxybenzenesulfonates and Viologen-Linked Polypyrrole

This article discusses the enhanced oxidation of dihydroxybenzenesulfonates at poly[N,N′-bis(3-pyrrol-1-ylpropyl)-4,4′-bipyridinium]chloride modified electrodes in aqueous electrolytes. Voltammetric measurements of the anion concentration in the film demonstrated that the electroactive anions were highly concentrated in the polycationic film from their aqueous solutions. 4,5-Dihydroxy-1,3-benzenedisulfonate gave a distribution constant of about 2.8×104. The distribution constant of electroactive organic species decreased in the order of dianion > monoanion > noncharged molecule. Moreover, the reduced positive charge of the polymer weakened its binding to the organic anions. Furthermore, the formal potential of the polymer shifted to more negative values with increased loading. Thus, the electroactive species are incorporated into the cationic polymer primarily by electrostatic attraction, although hydrophobic interactions contribute somewhat to the binding, particularly that of less highly charged species. When the concentration of 2,5-dihydroxybenzenesulfonate in the film increased from 0.3 to 1.2 M, the apparent diffusion coefficient for charge transport decreased to 1/3–1/4. This dependence indicates a primary role of the physical diffusion of the electroactive anion in charge transport through the film. A decrease in the anion mobility with increased loading arises probably from the ionic association between the incorporated redox anion and polymer bipyridinium group, which is due to a reduction in film permittivity induced by cation exchange.