R. J. Latham

Polymer electrolytes based on modified natural rubber

Abstract Modified natural rubber polymer hosts having low transition glass temperatures have been investigated for use in polymer electrolytes. Two types of modified natural rubber, namely 25% epoxidised natural rubber (ENR-25) and 50% epoxidised natural rubber (ENR-50) were employed in conjunction with poly(ethylene oxide), PEO. Results are reported for ionic conductivity and thermal properties for both unplasticized and plasticized polymer electrolyte systems with lithium triflate. The samples were in the form of free standing films with the thickness 0.2–0.5 mm and mixtures of ethylene carbonate (EC) and propylene carbonate (PC) were used as plasticizers. Unplasticized modified natural rubber-based systems exhibit ionic conductivities in the range 10−6 to 10−5 S cm−1 at ambient temperatures. Incorporating 100% of EC/PC by weight fraction of polymer (ENR/PEO) to the systems yielded mechanically stable films and ionic conductivities in the range of 10−4 S cm−1 at ambient temperature.

Conducting polymer‐based electrochemical redox supercapacitors using proton and lithium ion conducting polymer electrolytes

Polypyrrole-based solid-state redox supercapacitors have been constructed using proton and lithium ion conducting polymer electrolytes: poly(vinyl alcohol) (PVA)–H3PO4, and poly(ethylene oxide) (PEO)–LiCF3SO3 plasticized with poly(ethylene glycol) (PEG). The capacitors have been characterized using a.c. impedance, cyclic linear sweep voltammetry and galvanostatic charge–discharge methods. Redox capacitors based on polypyrrole show large values of capacitance (about 1·5–5·0 mFcm-2) (equivalent to a single electrode capacitance of 40–84 Fg-1 of polypyrrole) for both the electrolytes. The values of capacitance have been found to be stable up to 1000 charge–discharge cycles between 0 and 1·0 V.

Studies on all solid state electric double layer capacitors using proton and lithium ion conducting polymer electrolytes

Proton and lithium ion conducting polymer electrolytes, based on poly(vinyl alcohol)–H3PO4 and poly(ethylene oxide)–LiCF3SO3 plasticised with poly(ethylene glycol), have been used in the construction of electric double layer capacitors with both high density graphite sheet and activated carbon fabric electrodes. The polymer electrolytes have room temperature ionic conductivities (ca. 10−4–10−3 S cm−1) that make them suitable for use in thin-film form in devices. The performance characteristics of the capacitors have been studied using impedance analysis, linear-sweep voltammetry and charge–discharge methods. The supercapacitors based on the activated carbon fabric have characteristically large values of overall capacitance of 360–470 mF cm−2 [equivalent to single electrode capacitance 70–90 F (g carbon)−1] with the proton conducting electrolyte. Systems based on lithium ion conducting polymer electrolytes, however, have a much lower capacitance of ca. 20 mF cm−2 (equivalent to a single electrode capacitance of ca. 4 F g−1).

Supercapacitor devices using porous silicon electrodes

Electrical double layer (EDL) supercapacitors have been constructed using gold coated porous silicon (PSi) electrodes in a 0.25 M TEABF4/PC solution. As a comparison with the PSi, graphite paper, carbon cloth and ITO on glass electrodes have also been tested using the same electrolyte. The capacitors have been characterised using a.c. impedance spectroscopy and cyclic voltammetry (normal staircase mode). Devices using PSi electrodes showed a capacitance of approximately 0.2 mF cm−2 (equivalent to 5 mF g−1). In comparison, devices based on ITO on glass electrodes had a capacitance of 0.76 mF cm−2. Those based on graphite-paper electrodes gave 10 mF cm−2 (equivalent to 131 mF g−1) and those using carbon cloth gave 600 mF cm−2 (equivalent to 35 F g−1).

Supercapacitors using polymer electrolytes based on poly(urethane)

Abstract Electrical double layer supercapacitors have been constructed from a poly(urethane) electrolyte, [Poly(urethane):ethylene carbonate:propylene carbonate:lithium perchlorate in the ratio of 1:2:2:0.1 based on the mass of the polymer as the electrolyte], using both the high surface area carbon cloth and carbon composite electrodes. The cells have been characterised using the AC impedance spectroscopy, galvanostatic charge–discharge experiments, and cyclic voltammetry. Capacitance values of up to 35 and 5.5 F g −1 for the composite and carbon cloth electrodes were respectively attained. The supercapacitor cells showed a good stability up to 1000 charge–discharge cycles, retaining 80% of their original capacity at 1000 cycles.

Polymer electrolyte based solid state redox supercapacitors with poly (3-methyl thiophene) and polypyrrole conducting polymer electrodes

Types I and II solid state redox supercapacitors have been constructed using polypyrrole (pPy) and poly (3-methyl thiophene) (pMeT) conducting polymer electrodes with lithium ion conducting polymer electrolyte poly(ethylene oxide) (PEO)-LiCF3SO3 plasticised with poly (ethylene glycol) (PEG). The performance of the capacitors has been characterised by a.c. impedance, linear sweep voltammetry, galvanostatic charge-discharge methods and long term cycling tests. The asymmetric type II capacitors with p-doped pPy and pMeT electrodes give a capacitance value ∼ 2 mF cm−2 (equivalent to 18 Fg−1 of the total mass of the electrodes) and can be charged up to the voltage of 1.7 V. The symmetric type 1 capacitors of the configuration pPy | polymer electrolyte | pPy and pMeT | polymer electrolyte | pMeT show comparable values of capacitance but they are limited to the working voltage of <1.0 V.

Stiffness as a function of moisture content in natural materials:Characterisation of hoof horn samples

Hoof horn, which forms the capsule at the lower part of the legs of many grazing animals including equids (horses, donkeys and mules), is a composite natural material based on α-keratin. Its function is influenced by the tubular and intertubular material and is modulated by the moisture content. There is a requirement to adopt a standard approach to drying regimes and sampling protocols in order to make progress in understanding how the biomechanical properties of hoof horn are related to its structure. In this work the stiffness of donkey hoof has been examined using a three point bending technique and the effect of hydration has been investigated. Also the tubule density properties of this hoof horn material are reported.

Cation-oxygen geometry in polymer electrolytes: interpretation of EXAFS results

Two issues are of current interest in the field of ionically conducting polymers (polymer electrolytes): these are ion pairing and possible interference of the polymer-cation interaction by water. EXAFS was chosen as a suitable technique to probe local structure surrounding the cations. The systems studied were PEOn:ZnX2, where n= 6–15 and X = Cl, Br or I. They were chosen in order to ascertain the reliability of information pertaining to oxygen neighbours when the system under investigation contains heavy counterions. The results reveal, as expected, that the information about numbers of oxygen nearest neighbours is qualitative rather than quantitative, and firmer conclusions can be drawn for the lighter counterions. Cations and anions were found to be in close proximity, thus confirming ion pairing in PEO-zinc polymer electrolytes; this is in accord with recent observations of zinc diffusion.

Zinc polymer electrolytes in battery systems

Abstract We have previously reported results of our studies of structure-conductivity relationships for polymerer electrolytes of the form PEO n ·ZnX 2 . In this paper we report the results of investigations of battery systems based on these electrolytes. Results will be presented for OCV and discharge curves for loaded cells of the type: Zn/polymer electrolyte/MnO 2 . We are particularly interested in the speciation between oxidation states of manganese as a function of the degree of cell discharge, and have carried out determinations by chemical methods based on polarography. Preliminary studies indicate the presence of Mn II in cells discharged at various rates. The discharge times for a series of optimised cells show an exponential decrease with increasing load. This is consistent with a low electrolyte conductivity and less than ideal cathode conductivitt, which leads to an increased “front face” reaction with increasing load.

Plasticiser-induced local structure in polymer electrolytes

Results are reported for EXAFS local structural studies of polymer electrolytes incorporating nickel bromide. The bench-mark system PEO8 : NiBr2 was studied at 25 and 170 °C [PEO = poly(ethylene oxide)]. Nickel was shown to be surrounded by both oxygen and bromine nearest neighbours, but at high temperature the number of oxygen neighbours was considerably reduced. For the more dilute PEO100 : NiBr2 there are no bromine nearest neighbours, indicating an absence of ion association. The local structure in solutions of nickel bromide in poly(ethylene glycol) is shown to be substantially different from the bench-mark system. For electrolyte films made from nickel bromide, PEO and a plasticiser it is shown that when the plasticiser is methyl end-capped poly(ethylene glycol) the local structure surrounding nickel is virtually unchanged from that of the bench-mark polymer electrolyte. When hydroxy end-capped poly(ethylene glycol) is used, the structure closely resembles that in the solution of the low molecular weight oligomer only. It is concluded that the choice of platiciser is important.