Triiodide Diffusion Coefficient Research Articles

Balance between the physical diffusion and the exchange reaction on binary ionic liquid electrolyte for dye-sensitized solar cells

A comprehensive characterizations of viscosities, conductivities, triiodide diffusion coefficients, charge-transfer resistances and photovoltaic performance of a potential dye-sensitized solar cell (DSC) electrolyte systems based on binary ionic liquid (IL) mixtures, namely, 1-ethyl-3-methylimidazolium dicyanamide (EMIDCA)/1-methyl-3-propylimidazolium iodide (PMII) with a fixed iodine concentration at varying EMIDCA volume fraction are investigated in the present study. Viscosity and conductivity values are accurately correlated with regard to temperature and EMIDCA volume fraction. The triiodide diffusion coefficients, the predominant electrolyte parameter for limitation of DSC efficiency, are determined by symmetrical cell methods. The physical diffusion and exchange reactions between the iodide and triiodide dominate the apparent triiodide diffusion coefficients at different range of EMIDCA volume fraction. A balance between the viscosity-dependent physical diffusion and the exchange reactions can get at an optimal volume percents of EMIDCA. Impedance spectroscopy and photovoltaic results both support the existence of an optimized binary IL electrolyte composition. Hence, for optimizing an IL-based electrolyte in regards to triiodide transport, a low viscosity is not the exclusive crucial factor since exchange reactions transport effects also play an important role to resolve the diffusion limitation of DSC efficiency.

Promoting effect of layered titanium phosphate on the electrochemical and photovoltaic performance of dye-sensitized solar cells.

We reported a composite electrolyte prepared by incorporating layered α-titanium phosphate (α-TiP) into an iodide-based electrolyte using 1-ethyl-3-methylimidazolium tetrafluoroborate(EmimBF4) ionic liquid as solvent. The obtained composite electrolyte exhibited excellent electrochemical and photovoltaic properties compared to pure ionic liquid electrolyte. Both the diffusion coefficient of triiodide (I3−) in the electrolyte and the charge-transfer reaction at the electrode/electrolyte interface were improved markedly. The mechanism for the enhanced electrochemical properties of the composite electrolyte was discussed. The highest conversion efficiency of dye-sensitized solar cell (DSSC) was obtained for the composite electrolyte containing 1wt% α-TiP, with an improvement of 58% in the conversion efficiency than the blank one, which offered a broad prospect for the fabrication of stable DSSCs with a high conversion efficiency.

Electrochemical measurement of triiodide diffusion coefficients in blends of ionic liquids: Results for improving a critical parameter of dye-sensitized solar cells

Triiodide diffusion coefficients were determined in two ionic liquid based electrolyte systems for dye-sensitized solar cells. The electrolytes comprised iodine, 1-methyl-3-propylimidazolium iodide ([MPIM][I]), and either 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Determination of triiodide diffusion coefficients was performed by two independent methods, steady-state cyclic voltammetry at ultramicroelectrodes and polarization measurements at thin layer cells, both temperature-dependent and over a broad mixing range with varying ionic liquid molar ratios. The triiodide diffusion coefficients strongly increase with increasing temperature and therefore decreasing electrolyte viscosity. However, the triiodide diffusion coefficients stay almost constant with decreasing [MPIM][I] mole fraction or they even have a maximum at high [MPIM][I] mole fractions and therefore high viscosities. The Einstein–Stokes ratios for both systems strongly increase with increasing [MPIM][I] mole fraction. So as the Einstein–Stokes equation is not obeyed, a strong non-Stokesian charge transport can be assumed to occur in both electrolyte systems.

Electrochemical characterization and photovoltaic performance of the binary ionic liquid electrolyte of 1-methyl-3-propylimidazolium iodide and 1-ethyl-3-methylimidazolium tetrafluoroborate for dye-sensitized solar cells

Determination of triiodide diffusion coefficient, charge-transfer resistance and photovoltaic performance of the binary ionic liquid electrolyte of 1-methyl-3-propylimidazolium iodide (MPII) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4) for dye-sensitized solar cells (DSSCs) were performed at varying volume ratios of MPII/EMIMBF 4. Compared to pure MPII ionic liquid, the addition of EMIMBF 4 not only increased triiodide diffusion coefficient, but also decreased charge-transfer resistance at Pt/electrolyte interface, leading to the notably improved photovoltaic performance of DSSC. The highest photoelectric conversion efficiency of 4.99% was obtained under light intensity of 100 mW/cm 2 for the DSSC with the binary MPII/EMIMBF 4 ionic liquid electrolyte when the volume ratio of MPII/EMIMBF 4 was 50/50, which was much higher than that of pure MPII ionic liquid electrolyte (2.6%).

Preparation of 1-methyl-3-propylimidazolium acetate and its application in dye sensitized solar cells

In this paper, we reported the preparation of 1-methyl-3-propylimidazolium acetate (MPIAc), which proceeded via the metathesis of 1-methyl-3-propylimidazolium iodide (MPII) and lead acetate or potassium acetate. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, MPIAc and MPII with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide increased and those of iodide slightly increased with the weight ratio increase of MPIAc and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I 2, 0.10 M LiI, 0.50 M 4- tert-butylpyrdine in the binary ionic liquid electrolyte of MPIAc (employing potassium acetate) and MPII (weight ratio 0.2), gave short circuit photocurrent density of 9.40 mA cm −2, open circuit voltage of 0.62 V, and fill factor of 0.57, corresponding to the photoelectric conversion efficiency of 3.34% at the illumination (air mass 1.5, 100 mW cm −2).

In situ quaterizable oligo-organophosphazene electrolyte with modified nanocomposite SiO 2 for all-solid-state dye-sensitized solar cell

Two latent chemical cross-linked precursors: (1) oligo-organophosphazene with propionitrile and iodo-(ethylene oxide- co-propylene oxide) and (2) oligo-organosiloxane grafting oligo-ethylene oxide and propylene oxide dimethylamine are synthesized to form all-solid-state polymer electrolyte for dye-sensitized solar cells employing in situ quaterization reaction. This novel electrolyte shows about 6.79 × 10 −7 cm 2 s −1 of the apparent diffusion coefficient of triiodide. The disintegrating temperature of 181 °C demonstrates good thermal stability of the solid-state electrolyte. Proper amount of 7 nm SiO 2 modified with 3-triethoxysilylpropyl oligo(oxyethylene- co-oxypropylene) monomethyl urethane is added to the electrolyte and the photocurrent conversion efficiency reaches 2.66% compared with that of 1.81% with no additive.

Synthesis of perylene monoimide derivative and its use for quasi-solid-state dye-sensitized solar cells based on bare and modified nano-crystalline ZnO photoelectrodes

A novel perylene monoimide derivative, PCA, with a strongly electron-donating cyclohexylimide segment, bulky alkylphenoxy substituents at the 1,7-bay positions of the perylene core and an acid anhydride as the strong coupling group was synthesized and used as sensitizer for dye-sensitized solar cells (DSSCs). PCA was readily soluble in common organic solvents, decomposed above 390 °C and had glass transition temperature of 64 °C. The absorption curves had maximum at 505–512 nm, with thin film absorption onset at 643 nm corresponding to an optical band gap of 1.93 eV. We have fabricated the quasi-solid-state DSSCs with ZnO nano-crystalline films deposited from different values of pH. It is found that the power conversion efficiency (PCE) is high when the pH value of sol gel is about 9. The PCE of DSSC fabricated with the gold-coated ZnO photoanode is higher than that for bare ZnO photoanode. This enhancement is attributed to the formation of Schottky barrier at the Au/ZnO interface that blocks the electron transfer from ZnO to dye and electrolyte, increasing the electron density in the conduction band of ZnO. The incorporation of TiO2 nanoparticles in polymer gel electrolyte further increases the PCE of DSSC up to 3.0%. The enhancement is primarily explained by studying the dark reaction, diffusion coefficient of tri-iodide and exchange current density in the interface of electrolyte/Pt counter electrolyte, and lifetime of electrons in the anode film.

Temperature Dependence of the Non-Stokesian Charge Transport in Binary Blends of Ionic Liquids

A comprehensive characterization of an ionic liquid based electrolyte for dye-sensitized solar cells (DSSC) was performed by determination of triiodide diffusion coefficients, viscosities, specific conductivities, and densities. The observed non-Stokesian transport behavior was ensured by determination of triiodide diffusion coefficients with two independent methods, steady-state cyclic voltammetry at ultramicroelectrodes, and polarization measurements at thin layer cells. The electrolyte, consisting of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-methyl-3-propylimidazolium iodide ([MPIM][I]), and iodine, was examined at fixed iodine concentration over a broad mixing range with varying ionic liquid molar ratio and over a broad temperature range as well. The triiodide diffusion coefficients and the specific conductivity increase with decreasing [MPIM][I] mole fraction or increasing temperature, caused by decreasing electrolyte viscosity. The Einstein−Stokes ratios strongly increase with i...

An improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate and its application in dye sensitized solar cells

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA), which proceeded via efficient reaction of 1-methylimidazole and ethyl trifluoroacetate under solvent-free conditions using Teflon-lined, stainless steel autoclaves. It was shown that the procedure was simple and eco-friendly. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, EMITA and 1-methyl-3-propylimidazolium iodide (MPII) with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide slightly increased and those of iodide decreased with the weight ratio increase of EMITA and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I 2, 0.10 M LiI, 0.50 M 4- tert-butylpyrdine in the binary ionic liquid electrolyte of EMITA and MPII (weight ratio 1:2), gave short circuit photocurrent density of 7.88 mA cm −2, open circuit voltage of 0.61 V, and fill factor of 0.67, corresponding to the photoelectric conversion efficiency of 3.22% at the illumination (Air Mass 1.5, 100 mW cm −2).

An improved preparation of 1-methyl-3-propylimidazolium iodide and its application in dye-sensitized solar cells

In this paper, we reported an improved preparation of 1-methyl-3-propylimidazolium iodide (MPII), which was the alkylation reaction of n-propyl iodide and 1-methylimidazole under solvent-free conditions by Teflon-lined, stainless autoclaves. It was shown that the resulting MPII was high pure, the conversion rate of 1-methylimidazole was close to 100% and the procedure was simple and eco-friendly. Moreover, the apparent diffusion coefficients of triiodide and iodide in the mixture with different ratios of MPII and 3-methoxypropionitrile were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. The dye-sensitized solar cells with the electrolyte, which was composed of 0.13 M I 2, 0.10 M LiI, 0.50 M 4- tert-butylpyrdine in the mixture of 3-methoxypropionitrile and MPII (weight ratio 0.65:1), gave short circuit photocurrent density of 14.82 mA/cm 2, open-circuit voltage of 0.69 V, and fill factor of 0.66, corresponding to the photoelectric conversion efficiency of 6.73% at the illumination (AM 1.5, 100 mW/cm 2).

Electrochemical Characterization of 1-Ethyl-3-methylimidazolium Thiocyanate and Measurement of Triiodide Diffusion Coefficients in Blends of two Ionic Liquids

Electrochemical Characterization of 1-Ethyl-3-methylimidazolium Thiocyanate and Measurement of Triiodide Diffusion Coefficients in Blends of two Ionic Liquids

Characterisation of DSSC-electrolytes based on 1-ethyl-3-methylimidazolium dicyanamide: Measurement of triiodide diffusion coefficient, viscosity, and photovoltaic performance

A comprehensive characterisation of an ionic liquid based electrolyte for dye-sensitised solar cells (DSSC) was performed by determination of triiodide diffusion coefficients, viscosities and photovoltaic performances. The electrolyte, consisting of 1-ethyl-3-methylimidazolium dicyanamide, 1-methyl-3-propylimidazolium iodide (MPII), and iodine, was examined at varying ionic liquid molar ratio and fixed iodine concentration, as well as at fixed ionic liquid molar ratio and varying iodine concentration. All measurements were conducted at controlled temperature over a broad temperature range to analyse the electrolyte properties in view of thermal stress of the DSSC during later practical application. The triiodide diffusion coefficient increases with decreasing MPII concentration or increasing temperature, caused by decreasing electrolyte viscosity. Additionally, strong non-Stokesian behaviour over the whole ionic liquid mixing range was found, decreasing at higher temperatures. Photovoltaic measurements showed that the DSSC efficiency is limited by triiodide diffusion at low temperatures and low iodine concentrations whereas at high temperatures it is limited by recombination processes, leading to a maximum for DSSC efficiency at a certain temperature. The exchange between diffusion limitation of DSSC efficiency and limitation by recombination and therefore the maximum for DSSC efficiency is shifted to lower temperatures by increasing iodine concentration.

A microelectrode study of triiodide diffusion coefficients in mixtures of room temperature ionic liquids, useful for dye-sensitised solar cells

The diffusion coefficients of triiodide in binary mixtures of ionic liquids at 25 ± 0.05 °C were determined via steady-state cyclic voltammetry at platinum disk microelectrodes in five different electrolyte systems, all representing potential ionic liquid based electrolytes for dye-sensitised solar cells. These electrolytes were composed of iodine, 1-methyl-3-propylimidazolium iodide (acting as iodide source) and a second lower viscous ionic liquid, namely 1-ethyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium tetrafluoroborate or 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and were investigated either at fixed ionic liquid composition and varying iodine concentration or at fixed iodine concentration and varying 1-methyl-3-propylimidazolium iodide content. To check and optimise our measurement set-up, four Pt disk microelectrodes with four different electrode radii (0.3–5 µm) were tested at a well known system (ferrocene/tetraethylammonium tetrafluoroborate/acetonitrile solutions). The radius of each electrode was controlled by scanning electron microscopy at the beginning and during the work.

A new ionic liquid based quasi-solid state electrolyte for dye-sensitized solar cells

Ionic liquid based quasi-solid state electrolytes were prepared by solidifying the ionic liquid electrolytes containing 1-hexyl-3-methylidazolium iodide (HMImI) or a binary mixture of HMImI and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4) with an ionic liquid polymer poly (1-oligo (ethylene glycol) methacrylate-3-methylimidazolium chloride) (P(MOEMImCl)), respectively. The improvements of the ionic conductivity and the apparent diffusion coefficient of triiodide are observed in the binary ionic liquid mixture gel electrolyte. The thermal stability of the quasi-solid state electrolytes is determined by thermogravimetric analysis. The ionic liquid based quasi-solid state electrolytes were successfully used to fabricate the dye-sensitized solar cells achieving the conversion efficiency of 6.1% under the light intensity of 100 mW cm −2.

The influence of the metal cation and the filler on the performance of dye-sensitized solar cells using polymer-gel redox electrolytes

New composite PEO-based polymer-gel electrolytes were developed, using propylene carbonate (PC) plasticizer and iodide/triiodide (I −/I 3 −) redox couple. The iodide anion was introduced in the form of the corresponding metal salts: CsI, RbI and LiI and the influence of the metal cation was examined. The non-volatile and high polar character of the PC plasticizer results to excellent ionic conductivity values of the electrolyte samples (up to 1.6 mS cm −1), as well as remarkable stability over time. Linear sweep voltametry measurements revealed noticeable triiodide diffusion coefficient values (in the order of 2 × 10 −6 cm s −1). The thermograms confirm the amorphicity of the electrolytes (crystallinity values below 20%). All the experimental results showed that the introduction of inorganic titania filler (Degussa P25) into the polymer matrix reduces even more the crystallinity of the samples and enhances the mobility of the redox couple. Nanocrystalline photoelectrochemical solar cells were fabricated with the newly developed composite electrolytes and were tested with I– V measurements. All cells presented high efficiency values for this type of composite electrolytes, with the best performance attained in the case of the PEO-TiO 2-PC(CsI/I 3 −) electrolyte, with overall energy conversion efficiency ( η) of 2.87% under standard AM1.5 simulated solar irradiation. Our results clearly support the redox couple mobility dependence on the cation nature.

Synthesis of 3-hexyl-1-methylimidazolium Iodide and Its Application in Dye-Sensitized Solar Cells

利用高压釜使1-甲基咪唑和碘代正己烷直接反应生成离子液体1-甲基-3-己基咪唑碘,并考察了温度对反应的影响.结果表明,升高反应温度,可使1-甲基咪唑和碘代正己烷按化学计量比进行反应,产率近100%,该方法操作简单,所得产品纯度高,反应过程不需要使用有机溶剂,且对环境友好.同时还测量了I3^-在1-甲基-3-己基咪唑碘和1-甲基咪唑混合液中的表观扩散系数及其所组成的染料敏化太阳电池的光伏性能.

Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids

Results of diffusion coefficient measurements of triiodide in a mixture of two ionic liquids (1-methyl-3-propylimidazolium iodide and 1-butyl-3-methylimidazolium tetrafluoroborate) at 25°C are described in this paper. Four electrochemical methods for measuring diffusion coefficients of triiodide were evaluated for their reliability and performance, including impedance spectroscopy and polarization measurements at thin layer cells as well as cyclic voltammetry and chronoamperometry at microelectrodes of different radii. Viscosities of the blends were measured to investigate the transport behaviour of triiodide ions used in Grätzel-type dye-sensitized solar cells.

Enhanced exchange current density and diffusion coefficient of iodide-based liquid electrolyte by layered α-zirconium phosphate

Here, we firstly reported a composite electrolyte prepared by adding α-zirconium phosphate into the iodide/triiodide liquid electrolyte including 4- tert-butylpyridine (TBP). The obtained composite electrolyte exhibited very good electrochemical property. The exchange current density ( I 0) and the diffusion coefficient of triiodide ( D I 3 - ) were both enhanced markedly. It was proved that the intercalation behavior of TBP into layered α-ZrP could be responsible for their enhancement. The mechanism for the enhancement of I 0 and D I 3 - were investigated in detail, supported by X-ray diffraction (XRD) electrochemical impedance spectra (EIS) and limiting current measurements.

Study of the iodide—tri-iodide redox electrode in dimethylsulphoxide

The electrochemistry of solutions of sodium iodide and sodium iodide with added iodine dissolved in dimethyl sulphoxide has been investigated in the temperature range from 25 to 45°C. The current voltage curves were determined using a platinum rotating disk electrode working at 300 up to 6000 rpm. Both anodic and cathodic processes were considered, in particular when solutions with added iodine were employed. The anodic current/voltage curves are characterized by two well defined steps, the total limiting current being proportional to the iodide-ion concentration, while the cathodic current/voltage curve involves only one step, the limiting current density being linearly related to the tri-iodide-ion concentration. From the diffusion-controlled processes, the diffusion coefficients of the diffusing species in both the cathodic and anodic processes were evaluated. The ratio between the diffusion coefficients of tri-iodide and those of the iodide ions is in agreement with the predictable values based on the results of the corresponding aqueous systems. The current/voltage curves in the region before the limiting current densities are appreciably irreversible. Those curves were obtained by the method of Frumkin and Tedoradse for intermediate kinetics, and the kinetic parameters of the reactions, including the reaction orders, have been calculated. The kinetics of the reactions are discussed in terms of three possible reaction schemes which comprise the initial participation of iodide or tri-iodide ions. The experimental transfer coefficients for the cathodic and anodic processes add up to one.