Agnieszka Pawlicka

Nuclear magnetic resonance and conductivity study of starch based polymer electrolytes

Abstract Nuclear magnetic resonance (NMR) spectroscopy and complex impedance spectroscopy have been used to study the polymer electrolytes formed by amylopectin rich starch, plasticized with glycerol and containing lithium perchlorate. The 7 Li and 1 H NMR linewidth narrowing occurs close to the glass transition temperature ( T g ) of the plasticized electrolytes. The heteronuclear decoupling NMR experiments suggest a weaker Li–polymer interaction in the plasticized electrolyte when compared with the unplasticized ones. The effects of the plasticizer on the ionic mobility in these starch-based electrolytes, as measured by NMR spin-lattice relaxation and conductivity, are discussed. The 7 Li NMR relaxation results indicate that the ionic mobility in these plasticized electrolytes seems to be controlled by the plasticizer molecules. The dynamical parameters obtained from the conductivity and NMR data demonstrate that the Li + mobility in our systems is comparable to those found in others plasticized polymer electrolytes.

Preparation of transparent CeO2–TiO2 coatings for electrochromic devices

Abstract Thin films of mixed oxides of CeO2–TiO2 with a molar ratio of Ce/Ti of 0.5 have been obtained by the sonocatalytic sol–gel method. The precursor sols consist of a mixture of Ce(NH4)2(NO3)6, titanium alkoxide (Ti(OiPr)4) and isopropanol. The films were deposited using the dip-coating technique and calcined at 450°C in an oxygen atmosphere. Xerogels were characterized by thermal analysis (DTA–TG) and X-ray diffraction. The films prepared from the precursor sols were subjected to electrochemical measurements (cyclic voltammetry and chronoamperometry), SEM and AFM. Films of CeO2–TiO2 approx. 100 nm thick present a good electrochemical response under Li+ ions insertion. The cathodic and anodic charges at a scan velocity of 50 mV/s were about 16 mC/cm2 and the process was fully reversible. The values measured up to 4500 cycles were constant indicating a good electrochemical stability of the films. The stability test and optical measurements confirm that the films can be used as ion storage (counter-electrode) in electrochromic devices.

Ionic Conductivity Thermogravimetry Measurements of Starch-Based Polymeric Electrolytes

Natural polymers are a very interesting matrix to obtain solid polymeric electrolytes (SPE). The principal advantage comes from their particularly interesting biodegradation properties due to the natural origin and also very low cost and good physical and chemical properties. These polymers contain heteroatoms in their structure and for this reason can interact with protons or lithium ions leading to the ionic conduction. Among different natural polymers, starch-based SPEs show good opto-electrochemical characteristics and can be applied to electrochemical devices. This work presents the results of starch and starch derivatives-based electrolytes, which were characterized by impedance spectroscopy and thermal analysis. The ionic conductivity results obtained for these SPEs varied from 10−6 S/cm to 10−4 S/cm at room temperature, depending on the sample, and increased following Arrhenius model. The samples showed good stability up to 200°C evidenced by thermal analysis (TGA). Good conductivity results combined with transparency and good adhesion to the electrodes have shown that starch-based SPEs are very promising materials to be used as solid electrolytes in electrochromic devices.

Two methods of obtaining sol–gel Nb2O5 thin films for electrochromic devices

Nb2O5 coatings prepared by the sol–gel route using the dip coating technique with sols prepared from alkoxide or chloroalkoxide (two different methods: sonocatalytic and conventional) precursors are a promising alternative for WO3 electrochromic coatings. The crystalline films (TT phase) sintered at 560 and 600°C are transparent and present a deep blue colour under Li+ ion insertion. Electrochemical stability is excellent as these systems are fully reversible and stable and no change in colour, amount of charge exchanged and corrosion effect could be observed after 2000 voltammetry cycles between 2 and −1.8 V versus Ag. The electrochemical and optical properties of these coatings have been determined as a function of sintering temperature. The superficial structure was visualized by atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the Li+ diffusion coefficient was determined for both kinds of coatings.

Nuclear magnetic resonance and conductivity study of HEC:polyether-based polymer electrolytes

Measurements of 1H and 7Li nuclear magnetic resonance (NMR) and ionic conductivity are reported for the polymer electrolytes formed by hydroxyethylcelluloses (HEC), poly(ethylene oxide) diisocyanate (DPEO) and LiClO4. The NMR experiments exhibit the qualitative features associated with Li+ and polymer chain mobilities in polymer electrolytes, namely the presence of well-defined 7Li and 1H spin–lattice relaxation rate maxima (T1−1). The single absorption band in the proton spectra and the observed 1H and 7Li exponential relaxation indicate that the HEC/DPEO/LiClO4 polymer electrolytes are predominantly amorphous. The conductivity values obtained by a.c. measurements rise above 10−4 S/cm at 370 K and are slightly higher for the samples with a greater oxygen content in the HEC molecule. The parameters estimated from the data indicate that the conductivity and the mobilities of Li+ and polymer chains are comparable to those found in the PEO8LiClO4 polymer electrolyte.

ELECTROCHROMIC DEVICES WITH SOLID ELECTROLYTES BASED ON NATURAL POLYMERS

Solid-state electrochromic devices(ECDs)are of considerable technological and commercial interest because of their controllable transmission, absorption, and/or reflectance. For instance, a major application of these devices is in smart windows that can regulate the solar gains of buildings and also in glare attenuation in automobile rear view mirrors. A typical electrochromic device has a five-layer structure: GS/TC/EC/IC/IS/TC/GS, where GS is a glass substrate, TC is a transparent conductor, usually ITO(indium tin oxide)or FTO(fluorine tin oxide), EC is an electrochromic coating(typically WO3, Nb2O5, Nb2O5:Li+or Nb2O5–TiO2), IC is an ion conductor(solid or liquid electrolyte), in our case polymeric films based on natural polymers like starch or cellulose, and IS is an ion storage coating(typically CeO2–TiO2, CeO2–ZrO2, or CeO2–TiO2–ZrO2).This paper describes properties of two electrochromic devices with a WO3/natural-polymer-based electrolyte/CeO2–TiO2configuration.

WO3 nanorods created by self-assembly of highly crystalline nanowires under hydrothermal conditions.

WO3 nanorods and wires were obtained via hydrothermal synthesis using sodium tungstate as a precursor and either oxalic acid, citric acid, or poly(methacrylic acid) as a stabilizing agent. Transmission electron microscopy images showed that the organic acids with different numbers of carboxylic groups per molecule influence the final sizes and stacking nanostructures of WO3 wires. Three-dimensional electron diffraction tomography of a single nanocrystal revealed a hexagonal WO3 structure with preferential growth along the c-axis, which was confirmed by high-resolution transmission electron microscopy. WO3 nanowires were also spin-coated onto an indium tin oxide/glass conducting substrate, resulting in the formation of a film that was characterized by scanning electron microscopy. Finally, cyclic voltammetry measurements performed on the WO3 thin film showed voltammograms typical for the WO3 redox process.

DSC and solid state NMR characterization of hydroxyethylcellulose/polyether films

Thin solid films of hydroxyethylcellulose/oligoether were obtained by a grafting reaction of HEC with diisocyanates, and were then characterized by DSC, FTIR, NMR and complex impedance techniques. The transparency of these films in the visible range of the electromagnetic spectrum, the low T g , the good chain mobility and ionic conductivity of 8.8 x 10 -4 Scm -1 at 333 K, show that they can be used as solid electrolytes in electrochromic devices.

Alternative Nb2O5-TiO2 thin films for electrochromic devices

Thin films of mixed oxides of Nb2O5-TiO2 with molar ratio Nb/Ti = 3, have been prepared by sol-gel sonocatalytic method. The precursor sols consist of a mixture of niobium salt NbCl5, titanium alkoxide Ti(OPri)4, isopropanol and acetic acid. After addition of all components the mixture was sonicated. The films were deposited by dip-coating technique on ITO-Asahi Glass and then submitted to a final heat treatment at 560°C during 3 h in atmosphere of oxygen. The electrochromic properties of the films were investigated using in situ spectroelectrochemical method. The films exhibit a blue-gray coloration under lithium ions insertion with a reversible variation of the transmittance in the visible and near infrared range between 90% and 20%. The xerogels of Nb2O5-TiO2 also were characterized by X-ray diffraction, thermal analysis (DTA/TGA) and IR measurements. The morphology of the films was observed by scanning electron microscopy (SEM) and the topography by atomic force microscopy (AFM). These films appear suitable for architectural applications, where a minimum performance in depth color is required.

Agar-Based Gel Electrolyte for Electrochromic Device Application

Gel Polymer Electrolytes (GPE) based on agar and containing LiClO4 have been prepared, characterized and applied to electrochromic devices. The ionic conductivity revealed the best result of 6.5 × 10−5 S/cm for the sample with 17 wt.% of LiClO4, which increased to 5.4 × 10−4 S/cm at 72°C. The GPE have been used in electrochromic devices (ECD) with K-glass/WO3/GPE/CeO2-TiO2/K-glass configuration. The ECD changed transmittance values up to 30% between the colored and transparent states. The charge density measurements revealed an increase of 5.5 to 7.5 mC/cm2 from the first to 500th cycles and then a decrease to 4.4 mC/cm2 during the next 4500 cycles. Coloration efficiency (η) of 25 cm2/C was obtained.