N. Sharma

FTIR investigations on ion–ion interactions in liquid and gel polymeric electrolytes: LiCF3SO3-PC-PMMA

FTIR spectroscopy and ionic conductivity measurements have been employed to study the solvation and conduction mechanism of lithium ions in aprotic liquid electrolytes of LiCF3SO3-PC (lithium trifluromethanesulfonate-propylene carbonate) and gel electrolytes containing poly(methylmethacrylate) (PMMA) additionally. Cation solvation occurs via interactions between Li+ ions with the ring oxygens as well as the carbonyl oxygen of PC molecules over the entire salt concentration range under investigation (0.025–2 M) for both liquid and gel electrolytes. The ionic conductivity decline for concentrations ≥0.8 M LiCF3SO3-PC is attributed to the formation of ion pairs and/or triplets. Presence of (a) free CF3SO3− ions in 0.025 and 0.05 molar LiCF3SO3-PC solutions, (b) the LiCF3SO3 structure with a monodentate coordinated lithium ion for concentrations ≥0.5 molar LiCF3SO3-PC systems and (c) ion triplets comprising two cations and an anion with a bidentate bridging structure in 2 M LiCF3SO3-PC electrolytes, has been established. Ionic conductivity performance concurs with our infrared results. Gel electrolytes containing upto 15 wt% of PMMA have been found to exhibit liquid like behavior but 2 M LiCF3SO3-PC systems that incorporate 25 wt% of polymer show a distinct Li+–O=C (of PMMA) interaction which is unambiguously determined from the remarkable changes observed for the νsC=O) and νs(SO3)—the symmetric stretching vibrational modes.

Conductivity and viscosity of liquid and gel electrolytes based on LiClO4, LiN(CF3SO2)2 and PMMA

Abstract Liquid electrolytes comprising lithium salts of the type LiX (X=N(CF 3 SO 2 ) 2 , ClO 4 ) dissolved in an aprotic solvent PC and a binary mixture of PC and EC show σ 25(max) of the order of 10 −2 S cm −1 . Gel electrolytes synthesized by the incorporation of PMMA up to 25 wt.% in these liquid electrolytes show insignificant changes in the σ 25 value, retaining the liquid-like behaviour with σ 25(max) ∼10 −3 S cm −1 . The small σ 25 decline in contrast to the dramatic η 25 increase upon PMMA incorporation in liquid electrolytes exemplifies that PMMA plays a nonsolvating, passive role in these systems. The conduction mechanism that persists in liquid electrolytes is apparently preserved in gel electrolytes based on PMMA. Gel electrolytes can be visualized as highly viscous liquid electrolytes encaged in a polymeric matrix as thermal dependence of conductivity and viscosity for liquid and gel electrolytes follow nearly the same trend. PMMA imparts mechanical stability to gels thereby promoting ease of device fabrication. Nominal variance of conductivity over a wide temperature range makes these gel electrolytes ideal candidates for electrochromic windows (ECWs).

FTIR investigations of solid precursor materials for sol-gel deposition of WO3 based electrochromic films

Peroxypolytungstic acid derivative (APTA) and peroxypolytungstic acid derivative coupled with a dicarboxylic acid additive i.e., oxalic acid dihydrate (APTA + OAD) in xerogel form, synthesized by a wet chemistry route were the solid sol-gel precursors for casting WO3 films. These bulk materials have a complex structure owing to the presence of several groups such as acetate, peroxy anions and water molecules in their basic matrix. Additionally, oxalate ions constitute an integral part of the structure in the APTA + OAD xerogel. Detailed FTIR analysis of these coordination compounds has thrown light on the modes of association of the various anions or groups with tungsten metal ion. The mode of coordination of the acetate ion as a bidentate as well as a monodentate ligand enabling chelate formation and the several other chemical linkages prevalent in APTA have been established. Upon the incorporation of OAD in APTA, the net ramifications are drastic changes in the structure inclusive of changes in the nature and strength of metal - ligand bonding, which is exemplified by FTIR studies of (APTA + OAD) xerogel.

FTIR and absorption edge studies on tungsten oxide based precursor materials synthesized by sol-gel technique

Abstract The sol–gel technique has been employed for synthesizing three precursor materials for the deposition of tungsten oxide based electrochromic films. The ion-exchange route yields tungstic acid precursor (ITA) which has a strong tendency to polymerize rapidly. Addition of hydrogen peroxide and/or acetic acid suppresses the gelation process and an ethanol soluble precursor material (AIPTA) possessing a low degree of polymerization and high chemical stability is obtained. Another precursor material APTA endowed with similar desirable characteristics is produced from direct reaction between tungsten metal powder and a mixture of hydrogen peroxide and acetic acid. These materials were characterized by FTIR and absorption edge studies and compared in terms of their degree of polymerization and modes of association of various functionalities with the tungsten metal ion.

FTIR investigations of tungsten oxide electrochromic films derived from organically modified peroxotungstic acid precursors

Alcoholic solutions of acetylated peroxotungstic acid (APTA) and oxylated APTA (ox-APTA) precursors are used for casting films by a spin coating technique. Fourier transform infrared (diffuse reflectance mode) spectroscopy is used to investigate the linkages prevalent in the constituents of the films in the as-deposited as well as annealed states. The modes of association of peroxo and acetate/oxalate in the form of bidentate, chelating or bridging ligands is highlighted. The spectra clearly reveal the gradual decomposition of peroxo and organic ligands and also the hydrated state of film at various stages of the annealing treatment. Films based on ox-APTA precursor exhibit better electrochromic performance in contrast to films based on APTA alone. This feature of ox-APTA films which is of enormous practical importance, has been established by spectral studies, as heat-treated ox-APTA films have a microstructure which is entirely different from that of heat-treated APTA films.

An insight into the interactions between LiN(CF3SO2)2 - γBL/DMF — PMMA by FTIR spectroscopy

FTIR spectroscopic analysis has been carried out for liquid electrolytes containing lithium —(trifluormethanesulfonimide or imide) salt as the ion source, a binary solvent composed of γBL and DMF and gel electrolytes containing PMMA. These studies illustrate that for all electrolytes, the cation (Li+) — solvent interaction is predominant and occurs through the carbonyl oxygen and the electron rich nitrogen atom of the solvating medium i.e., the binary solvent. Ionic conductivity trends upon varying lithium imide concentration, exhibit a single maximum in both liquid and gel polymeric electrolytes. The conductivity at 25 °C (σ25) decline at high salt concentrations attributable to ion aggregation or cation-anion association, has been explained on the basis of detailed spectral analysis. Addition of PMMA as a gelatinizing agent to liquid electrolytes does not affect the conduction mechanism drastically, which is evident from conductivity measurements and is supplemented by spectral studies.

Effect of mixed salts on the properties of gel polymeric electrolytes

Nonaqueous unimolar solutions of lithium perchlorate (LiClO 4 ) and lithium trifluoromethanesulfonate or triflate (LiCF 3 SO 3 ) in γ-butyrolactone (γBL) mixed in (a) 1:1 and (b) 3:2 volume ratios yield mixed salt electrolytes with good conductivity performance with σ 25 approaching 10 -2 S cm -1 . Addition of 10 wt.% of poly(methylmethacrylate) (PMMA) to these liquid electrolytes manifests in producing highly viscous gel electrolytes with η 25 in the range of 10 4 -10 5 cSt and σ 25 greater than 10 -3 S cm -1 . Conductivity and viscosity studies reveal that PMMA plays an insignificant role in controlling the conduction mechanism in gel electrolytes, as they tend to retain the liquid-like behaviour. Also, the dimensional strength bestowed to gels by the polymer stimulates their use in electrochromic windows (ECWs). Inconsequential variation of a over the operational temperature range of an ECW renders them to be ideal candidates for the same.

Impedance studies of Li inserted sol–gel-derived WO3 films

Abstract Sol–gel-derived films of WO3 deposited on transparent conducting electrodes (TCEs) have been investigated by impedance spectroscopy in a gel electrolyte as a function of frequency from 200 kHz to 1 mHz at various applied voltages. These studies have revealed that electron injection at the TCE/WO3 interface occurring at high frequencies and charge transfer and diffusion controlled Li insertion into a WO3 film at low frequencies govern the colouration mechanism of WO3 films. The surface of WO3 films has been shown to be nearly homogeneous which has been reaffirmed by AFM studies.

Ion Exchange Derived Precursor Materials for Deposition of WO3 Electrochromic Films: Spectroscopic Investigations

The ion exchange route was first successfully used by Livage et al. for depositing WO3 films exhibiting reversible electrochromic effect. We report in this paper the preparation of a solid precursor material by chemically modifying this route. The short lived ion exchange derived sol on addition of H2O2 and acetic acid on further processing, yields a solid precursor material with (1) unusually enhanced stability and (2) easy solvation in alcohol. UV and infrared spectroscopic investigations give strong evidence for these two properties as manifestations of the lower degree of polymerization and coordination of the acetate ion as a chelating ligand with the tungsten atom.

Transmission enhancement in tin oxide conductive coatings for ECWs

Abstract Transmission in commercially available fluorine doped tin oxide conductive coatings on glass (FTOs) exhibits dramatic enhancement when coated with either an electrochromic tungsten oxide coating or an electrochemically active molybdenum doped tin oxide coating via a “wet chemical deposition” technique. The former acts as the primary electrochromic electrode while the latter behaves as the electrochemically inert (passive) counter electrode in electrochromic window (ECW) configuration. This paper furnishes an insight into how cloudy or poorly transmitting FTOs can be modified so as to be suitable candidates for the electrochromic window application.