Dielectric and conduction mechanism studies of PVA-orthophosphoric acid polymer electrolyte

Polyvinyl alcohol (PVA)-based proton conducting polymer electrolytes have been prepared by the solution cast technique. The conductivity is observed to increase from 10⁻⁹ to 10⁻⁴ S cm⁻¹ as a result of orthophosphoric acid (H₃PO₄) addition. The plot of conductivity vs temperature shows that a phase transition occurred at 343 K in the sample PVA-33 wt% H₃PO₄. The β-relaxation peak is observed at 313 K. The glass transition temperature of PVA-33 wt% H₃PO₄ is 343 K. Orthophosphoric acid seems to play a dual role, i.e., as a proton source and as a plasticizer. The ac conductivity σ _ac = Aω ^s was also calculated in the temperature range from 303 to 353 K. The conduction mechanism was inferred by plotting the graph of s vs T from which the conduction mechanism for sample PVA-17 wt% H₃PO₄ was inferred to occur by way of the overlapping large polaron tunneling (OLPT) model and the conduction mechanism for the sample PVA-33 wt% H₃PO₄ by way of the correlated barrier height (CBH) model.     

Impedance spectroscopy studies of poly (methyl methacrylate)-lithium salts polymer electrolyte systems

In the present work, five systems of samples have been prepared by the solution casting technique. These are the plasticized poly(methyl methacrylate) (PMMA-EC) system, the LiCF₃SO₃ salted-poly(methyl methacrylate) (PMMA-LiCF₃SO₃) system, the LiBF₄ salted-poly(methyl methacrylate) (PMMA-LiBF₄) system, the LiCF₃SO₃ salted-poly(methyl methacrylate) containing a fixed amount of plasticizer ([PMMA-EC]-LiCF₃SO₃) system, and the LiBF₄ salted-poly(methyl methacrylate) containing a fixed amount of plasticizer ([PMMA-EC]-LiBF₄) system. The conductivities of the films from each system are characterized by impedance spectroscopy. The room temperature conductivity in the pure PMMA sample and (PMMA-EC) system is 8.57 × 10⁻¹³ and 2.71 × 10⁻¹¹ S cm⁻¹, respectively. The room conductivity for the highest conducting sample in the (PMMA-LiCF₃SO₃), (PMMA-LiBF₄), ([PMMA-EC]-LiCF₃SO₃), and ([PMMA-EC]-LiBF₄) systems is 3.97 × 10⁻⁶, 3.66 × 10⁻⁷, 3.40 × 10⁻⁵, and 4.07 × 10⁻⁷ S cm⁻¹, respectively. The increase in conductivity is due to the increase in number of mobile ions, and decrease in conductivity is attributed to ion association. The increase and decrease in the number of ions can be implied from the dielectric constant, ɛ_r-frequency plots. The conductivity–temperature studies are carried out in the temperature range between 303 and 373 K. The results show that the conductivity is increased when the temperature is increased and obeys Arrhenius rule. The plots of loss tangent against temperature at a fixed frequency have showed a peak at 333 K for the ([PMMA-EC]-LiBF₄) system and a peak at 363 K for the ([PMM-EC]-LiCF₃SO₃) system. This peak could be attributed to β-relaxation, as the measurements were not carried out up to glass transition temperature, T _g. It may be inferred that the plasticizer EC has dissociated more LiCF₃SO₃ than LiBF₄ and shifted the loss tangent peak to a higher temperature. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006     

Studies on the structure and transport properties of hexanoyl chitosan-based polymer electrolytes

Polymer electrolytes composed of hexanoyl chitosan as the host polymer, lithium trifluoromethanesulfonate (LiCF₃SO₃) as the salt, diethyl carbonate (DEC)/ethylene carbonate (EC) as the plasticizers were prepared and characterized by X-ray diffraction and impedance spectroscopy. The X-ray diffraction results reveal the variation in conductivity from structural aspect. This is reflected in terms of amorphous content. Sample with higher amorphous content exhibits higher conductivity. In order to further understand the source of the conductivity variation with varying plasticizers compositions as well as temperatures, the ionic charge carrier concentration and their mobility in polymer electrolyte were determined. The Rice and Roth model was proposed to be used to estimate the ionic charge carrier concentration, n. Knowing n and combining the result with dc conductivity, the mobility of the ionic charge carrier can be calculated. It is found that the conductivity change with DEC/EC composition is due mainly to the change in ionic charge carrier concentration while the conductivity change with temperature is due primarily to the change in mobility.     

Effects of MnO<Subscript>2</Subscript> nano-particles on the conductivity of PMMA-PEO-LiClO<Subscript>4</Subscript>-EC polymer electrolytes

Li-ion rechargeable batteries based on polymer electrolytes are of great interest for solid state electrochemical devices nowadays. Many studies have been carried out to improve the ionic conductivity of polymer electrolytes, which include polymer blending, incorporating plasticizers and filler additives in the electrolyte systems. This paper describes the effects of incorporating nano-sized MnO₂ filler on the ionic conductivity enhancement of a plasticized polymer blend PMMA–PEO–LiClO₄–EC electrolyte system. The maximum conductivity achieved is within the range of 10⁻³ S cm⁻¹ by optimizing the composition of the polymers, salts, plasticizer, and filler. The temperature dependence of the polymer conductivity obeys the VTF relationship. DSC and XRD studies are carried out to clarify the complex formation between the polymers, salts, and plasticizer.     

Structural and electrical characterisation of strontium zirconate proton conductor co-sputter deposited coatings

SrZr_1−x Y _x O₃ coatings were co-sputtered from metallic Zr–Y (84–16 at.%) and Sr targets in the presence of a reactive argon–oxygen gas mixture. The structural and chemical features of the film have been assessed by X-ray diffraction and scanning electron microscopy. The electrical properties have been investigated for different substrates by Complex Impedance Spectroscopy as a function of crystalline state, temperature and atmosphere. The as-deposited coatings are amorphous and crystallise after annealing at 673 K for 2 h under air. The stabilisation of the perovskite structure is a function of the nominal composition. The films are dense and present a good adhesion on different substrates. Crystallisation and mechanical stresses are detected by alternating current (AC) impedance spectroscopy. Significant ionic conductivity in the 473–823 K temperature range is evidenced in air. Two different conduction regimes in the presence of steam are attributed to a modification of the charge carrier nature. In spite of low conductivity values (σ ~10⁻⁶ S.cm⁻¹ at 881 K), the activation energies are in agreement with that of Y-doped strontium zirconate ceramics (~0.7 eV in air).     

Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique

The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO₂) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω ^s . The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.     

Solid polymeric electrolyte of PVC–ENR–LiClO<Subscript>4</Subscript>

The ionic conductivity of PVC–ENR–LiClO₄ (PVC, polyvinyl chloride; ENR, epoxidized natural rubber) as a function of LiClO₄ concentration, ENR concentration, temperature, and radiation dose of electron beam cross-linking has been studied. The electrolyte samples were prepared by solution casting technique. Their ionic conductivities were measured using the impedance spectroscopy technique. It was observed that the relationship between the concentration of salt, as well as temperature, and conductivity were linear. The electrolyte conductivity increases with ENR concentration. This relationship was discussed using the number of charge carrier theory. The conductivity–temperature behaviour of the electrolyte is Arrhenian. The conductivity also varies with the radiation dose of the electron beam cross-linking. The highest room temperature conductivity of the electrolyte of 8.5 × 10⁻⁷ S/cm was obtained at 30% by weight of LiClO₄. The activation energy, E _a and pre-exponential factor, σ _o, are 1.4 × 10⁻² eV and 1.5 × 10⁻¹¹ S/cm, respectively.     

Ionic conductance behaviour of plasticized polymer electrolytes containing different plasticizers

The effect of different plasticizers on the properties of PEO-NH₄F polymer electrolytes has been studied. Aprotic organic solvents like propylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone (γ-BL), dimethylacetamide (DMA), dimethylformamide (DMF), diethylcarbonate (DEC) and dimethylcarbonate (DMC) having different values of donor number, dielectric constant, viscosity etc. have been used as plasticizers in the present study. The addition of plasticizer has been found to modify the conductivity of polymer electrolytes by increasing the amorphous content as well as by dissociating the ion aggregates present in polymer electrolytes at higher salt concentrations. The conductivity enhancement with different plasticizers has been found to be closely related to the donor number of the plasticizer used rather than its dielectric constant. The increase in conductivity with the addition of plasticizer has further been found to be dependent upon the level of ion association present in the electrolytes. The variation of conductivity as a function of plasticizer concentration and temperature has also been studied and maximum conductivity of ∼ 10⁻³ S /cm at room temperature has been obtained. X-ray diffraction studies show an increase of amorphous content in polymer electrolytes with the addition of plasticizers.     

Evidence of ion pair breaking by dispersed polymer in polymer gel electrolytes

Non-aqueous polymer gel electrolytes containing trifluoromethanesulfonic acid (HCF₃SO₃) and polyethylene oxide (PEO) as the gelling polymer has been studied. The increase in conductivity observed with the addition of PEO to liquid electrolytes has been explained to be due to the breaking of ion aggregates present in electrolytes at higher acid concentrations. The increase in free H⁺ ion concentration upon breaking of ion aggregates has also been observed in pH measurements and viscosity of gel electrolytes has been found to increase with PEO addition. Polymer gel electrolytes containing dimethylacetamide (DMA) have σ ∼ 10⁻² S/cm at room temperature and are stable over −50 to 125 °C range of temperature. Gels based on propylene carbonate (PC) and ethylene carbonate (EC) are stable in the −50 to 40 °C temperature range and loose their gelling nature above 40 °C.     

Evidence of ion pair breaking by dispersed polymer in polymer gel electrolytes

Non-aqueous polymer gel electrolytes containing trifluoromethanesulfonic acid (HCF₃SO₃) and polyethylene oxide (PEO) as the gelling polymer has been studied. The increase in conductivity observed with the addition of PEO to liquid electrolytes has been explained to be due to the breaking of ion aggregates present in electrolytes at higher acid concentrations. The increase in free H⁺ ion concentration upon breaking of ion aggregates has also been observed in pH measurements and viscosity of gel electrolytes has been found to increase with PEO addition. Polymer gel electrolytes containing dimethylacetamide (DMA) have σ ∼ 10⁻² S/cm at room temperature and are stable over −50 to 125 °C range of temperature. Gels based on propylene carbonate (PC) and ethylene carbonate (EC) are stable in the −50 to 40 °C temperature range and loose their gelling nature above 40 °C.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Electrical properties of nickel-doped arsenic trisulphide

Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As₂S₃ are reported. The a.c. conductivity of pure As₂S₃ obeys a well-known relationship: σ_ac ∝ω ^s. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As₂S₃. But a different behaviour of a.c. conductivity has been observed for the nickel doped As₂S₃. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σ_ac ∝ω ^s) for nickeldoped As₂S₃ is similar to pure As₂S₃ at lower temperatures. But at higher temperatures, ln σ_ac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D ^0′) in the band gap. Single polaron hopping is expected to occur between theseD ^0‘ andD ⁺ states. Furthermore, allD ⁺,D ^0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As₂S₃. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.     

Hyperfine Interactions of 181Ta in Zr2Ni Observed Using PAC Spectroscopy

We have measured nuclear electric–quadrupole interactions (EQI) at ¹⁸¹Ta impurities substituted as Hf atoms into the Zr site in Zr₂Ni. Using perturbed-angular-correlation (PAC) spectroscopy, we measured the EQI over temperatures ranging from 10 to 1200 K. Over the entire range of temperature, the Zr₂Ni crystal has a bct Al₂Cu structure that includes a single Zr site. The crystal field symmetry surrounding this site is rather low, giving rise to a highly asymmetric electric-field gradient tensor. At 10 K, the EQI is characterized by an angular frequency ω₀=601(3) Mrad s⁻¹, and an asymmetry parameter η=0.835(2). At 1200 K, ω₀ decreases to 516(3) Mrad s⁻¹, and η also decreases to 0.790(4). Although weak, the temperature dependence of ω₀ is consistent with a (1−BT ^3/2) power law, in which B=6×10⁻⁶ K^−3/2. The EQI also manifests a very narrow linewidth. We observed no evidence either for magnetic ordering or for structural phase transitions in the temperature range covered by this experiment. Moreover, the sharpness of the EQI indicates that the samples as prepared are remarkably free of strain and defects. These results indicate that the Zr₂Ni structure does not promote the formation of defects and that the power-law dependence of ω₀ on T is insensitive to the asymmetric nature of the crystal. This revised version was published online in September 2006 with corrections to the Cover Date.     

Structural,thermal and ion transport studies on nanocomposite polymer electrolyte-{(PEO + SiO<Subscript>2</Subscript>):NH<Subscript>4</Subscript>SCN} system

Development and characterisation of polyethylene oxide (PEO)-based nanocomposite polymer electrolytes comprising of (PEO-SiO₂): NH₄SCN is reported. For synthesis of the said electrolyte, polyethylene oxide has been taken as polymer host and NH₄SCN as an ionic charge supplier. Sol–gel-derived silica powder of nano dimension has been used as ceramic filler for development of nanocomposite electrolytes. The maximum conductivity of electrolyte ∼2.0 × 10⁻⁶ S/cm is observed for samples containing 30 wt.% silica. The temperature dependence of conductivity seems to follow an Arrhenius-type, thermally activated process over a limited temperature range.     

Role of polyvinyl alcohol in the conductivity behaviour of polyethylene glycol-based composite gel electrolytes

An attempt has been made in the present work to combine gel and composite polymer electrolyte routes together to form a composite polymeric gel electrolyte that is expected to possess high ionic conductivity with good mechanical integrity. Polyethylene glycol (PEG) based composite gel electrolytes using polyvinyl alcohol (PVA) as guest polymer have been synthesized with 1 molar solution of ammonium thiocyanate (NH₄SCN) in dimethyl sulphoxide (DMSO) and electrically characterized. The ionic conductivity measurements indicate that PEG:PVA:NH₄SCN-based composite gel electrolytes are superior (σ _max = 5.7 × 10⁻² S cm⁻¹) to pristine electrolytes (PEG:NH₄SCN system) and conductivity variation with filler concentration remains within an order of magnitude. The observed conductivity maxima have been correlated to PEG:PVA:NH₄SCN-and PVA:NH₄SCN-type complexes. Temperature dependence of conductivity profiles exhibits Arrhenius behaviour in low temperature regime followed by VTF character at higher temperature.      

Conductivity studies of plasticized anhydrous PEO-KOH alkaline solid polymer electrolyte

Polyethylene oxide (PEO)–potassium hydroxide (KOH)-based alkaline solid polymer electrolyte films have been prepared by using methanol as solvent. The highest room temperature ionic conductivity of (2.1 ± 0.5) × 10⁻⁸ S cm⁻¹ was achieved for the composition of 70 wt% PEO:30 wt% KOH. The addition of plasticizer, ethylene carbonate, propylene carbonate, or polyethylene glycol to the highest conductivity of PEO–KOH system helps to increase the ambient ionic conductivity to the order of 10⁻⁶–10⁻⁴ S cm⁻¹. The log σ vs 1/T plot of PEO–KOH showed a small conductivity decrease at 50–60 °C range. The small decrease and the hysteresis that occur during the heating–cooling cycle was overcome by the presence of the plasticizer. X-ray diffraction observation supports the conductivity results.     

PEO electrolytes containing dioctyl phthalate (DOP) for dye-sensitized nanocrystalline TiO2 solar cells

In this paper, we aim to prepare polymer electrolytes consisting of NaI and I₂ dissolved in poly(ethylene oxide) (PEO) and dioctyl phthalate (DOP) as an additive and apply the electrolytes to dye-sensitized solar cells (DSSC). Upon the incorporation of salt, the phthalic-stretching C=O bands of DOP in Fourier transform infrared spectra shifted to a lower wave number (Δf = 93 cm⁻¹), confirming the unusual strong complex formation between sodium ions and phthalic oxygen. Coordinative interactions and structural changes of PEO/NaI/I₂/DOP electrolytes have also been characterized by wide angle X-ray scattering, presenting an almost amorphous structure of the polymer electrolytes. The ionic conductivity of the polymer electrolytes reached ∼10^–4 S/cm at room temperature at the mole ratio of [EO]:[Na]:[DOP] = 10:1:0.5, as determined by the four-probe method. DSSC using the polymer electrolytes and conductive indium tin oxide glasses exhibited 2.9% of overall energy conversion efficiency (=P _max/P _in × 100) at one sun condition (100 mW/cm²). The good interfacial contact between the electrolytes and the dye-attached nanocrystalline TiO₂ layers were verified by field-emission scanning electron microscopy.     

Effect of plasticizer on structural and electrical properties of nanocomposite solid polymer electrolytes

The solid polymer electrolyte films based on polyethylene oxide, NaClO₄ with dodecyl amine modified montmorillonite as filler, and polyethylene glycol as plasticizer were prepared by a tape casting method. The effect of plasticization on structural, microstructural, and electrical properties of the materials has been investigated. A systematic change in the structural and microstructural properties of plasticized polymer nanocomposite electrolytes (PPNCEs) on addition of plasticizer was observed in our X-ray diffraction pattern and scanning electron microscopy micrographs. Complex impedance analysis technique was used to calculate the electrical properties of the nanocomposites. Addition of plasticizer has resulted in the lowering of the glass transition temperature, effective dissociation of the salt, and enhancement in the electrical conductivity. The maximum value of conductivity obtained was ∼4.4 × 10⁻⁶ S cm⁻¹ (on addition of ∼20% plasticizer), which is an order of magnitude higher than that of pure polymer nanocomposite electrolyte films (2.82 × 10⁻⁷ S cm⁻¹). The enhancement in conductivity on plasticization was well correlated with the change in other physical properties.     

Performance evaluation of PVdF gel polymer electrolytes

A series of gel polymer electrolytes containing PVdF as homo polymer, a mixture of 1:1 Ethylene Carbonate (EC) : Propylene Carbonate (PC) as plasticizer and lithium-bistrifluoromethane sulphone imide [imide — LiN (CF₃SO₂)₂] has been developed. Amounts of polymer (PVdF), plasticizer and the imide lithium salt have been varied as a function of their weight ratio composition in this regard. Dimensionally stable films possessing appreciable room temperature conductivity values have been obtained with respect to certain weight ratio compositions. However, conductivity data have been recorded at different possible temperatures, i.e., from 20 °C to 65 °C. XRD and DSC studies were carried out to characterize the polymer films for better amorphicity and reduced glass transition temperature, respectively. The electrochemical interface stability of the PVdF based gel polymer electrolytes over a range of storage period (24 h – 10 days) have been investigated using A.C. impedance studies. Test cells containing Li/gel polymer electrolyte (GPE)/Li have been subjected to undergo 50 charge-discharge cycles in order to understand the electrochemical performance behaviour of the dimensionally stable films of superior conductivity. The observed capacity fade of less than 20% even after 50 cycles is in favour of the electrochemical stability of the gel polymer electrolyte containing 27.5% PVdF −67.5 % EC+PC −5% imide salt. Cyclic voltammetry studies establish the possibility of a reversible intercalation — deintercalation process involving Li⁺ ions through the gel polymer electrolyte.     

Ac electrical parameters of Al-ZnPc-Al organic semiconducting films

The ac electrical parameters of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180–390 K and frequency between 0.1 and 20 kHz. Aluminum electrode contacts were utilized to sandwich the organic ZnPc semiconducting films. Capacitance and loss tangent decreased rapidly with frequency at high temperatures, but at lower temperatures a weak variation is observed. An equivalent circuit model assuming ohmic contacts could qualitatively and successfully explains capacitance and loss tangent behavior. The ac conductivity showed strong dependence on both temperature and frequency depending on the relevant temperature and frequency range under consideration. Ac conductivity σ (ω) is found to vary with ω, as ω ^s with the index s ≤ 1.35 suggesting a dominant hopping conduction process at low temperatures (< 250 K) and high frequency. The conductivity of some samples did not increase monotonically with temperature. This behavior was attributed to oxygen exhaustion of the sample as its temperature is increased. The ac conductivity behavior at low temperatures of ZnPc films could be described well by Elliott model assuming hopping of charge carriers between localized sites.