Efficiency improvement of dye-sensitized solar cells based on electrodeposited TiO2 films by low temperature post-treatment

Porous crystalline TiO₂ films can be prepared at low temperatures (80 °C) by surfactant-assisted electrodeposition from TiCl₃ solution. Nevertheless, up to now calcination at high temperatures (typically 450 °C) was still necessary to establish a good performance of these films in dye-sensitized solar cells (DSSC). With this study we report that water vapour treatment at much lower temperatures (150 °C) for 1 week improves the performance of the films in DSSC to the same degree as calcination although the overall crystallinity remains lower. Reason for the good efficiency is that the porous structure stays intact and thus the dye molecules can be better adsorbed. Avoiding high temperatures during the preparation process of TiO₂ films for the application in DSSC enables the use of polymer substrates for the fabrication of flexible solar cells.     

Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells

Nanostructured porous zinc oxide electrodes for use in dye-sensitized solar cells (DSSCs) were coated with thin niobium oxide layers by using sol–gel transformation of niobium pentaethoxide in air. Coating solutions were prepared by mixing niobium pentaethoxide and ethanol. A dip-coating technique was adopted at a low withdrawal speed of 100 μm s⁻¹. The coated electrodes were then heat-treated at temperatures between 400 and 600 °C. The presence of niobium in the coated electrodes was confirmed by X-ray photoelectron spectroscopy. As expected, the niobium oxide layers worked as an energy barrier between the ZnO electrode and electrolyte. Open-circuit voltage (V_OC) of the cells using the coated electrodes was then enhanced up to 0.768 V, which was attributable to the suppression of the recombination of photogenerated electrons with oxidized species in electrolytes. An additional benefit of the coating was that grain growth of ZnO particles in the electrodes was hindered and short-circuit photocurrent density (J_SC) was kept relatively high due to large amounts of adsorbed dye. An overall light-to-electricity conversion efficiency was increased to a maximum of 5.19%, indicating that the proper coating technique was the key for improving the performance of ZnO-based DSSCs.     

Synthesis and characterization of poly(1-vinyl-3-alkylimidazolium) iodide polymers for quasi-solid electrolytes in dye sensitized solar cells

A series of new poly(1-vinyl-3-alkylimidazolium) iodide polymers with different alkyl derivatives such as methyl, propyl and perflurodecyl have been synthesized. The alkyl substituent influenced some properties such as solubility, thermal stability, glass transition and crystallinity of the polymers. For instance, polymer having the propyl substituent was soluble in solvents of intermediate polarity such as acetonitrile, chloroform and THF, the one with the methyl substituent was only soluble in very polar solvents such as water and methanol and the fluorinated polymer was only soluble in DMF. The alkyl substituent also influenced the thermal stability in the order methyl > propyl > perflurodecyl and all the polymers thermally decomposed between 250 and 400 °C in nitrogen. The poly(1-vinyl-3-alkyl-imidazolium) iodide polymers having propyl and methyl substituents were amorphous polymers showing a glass transition temperature of 43 and 21 °C, respectively; and perflurodecyl polymers were semi-crystalline with a Tm at 153 °C and a Tg at 20 °C, as indicated by differential scanning calorimetry.Polymer electrolytes were formulated as mixtures of the ionic liquid 1-methyl-3-propylimidazolium iodide and the poly(1-vinyl-3-alkylimidazolium) iodide polymers. These polymer electrolytes showed ionic conductivities in the range of 10⁻³ to 10⁻⁷ S/cm at room temperature which strongly depended on the ionic liquid content. Finally, poly(1-vinyl-3-propyl-imidazolium) iodide was used to obtain gel electrolytes by adding it to a typical acetonitrile electrolyte used in dye sensitized solar cells (DSSCs). Solar cells with 1 cm² area prepared using the polymer gel electrolyte yielded a maximum light-to-electricity conversion efficiency of 3.73%.     

An approach to laminated flexible Dye sensitized solar cells

We have built TiO₂ Dye sensitized solar cells (DSSCs) that combined flexible TiO₂ photoanodes coated on ITO/PET substrates with a gel electrolyte based on PVDF-HFP-SiO₂ films. Titanium isopropoxide (TiP₄) was used as additive to TiO₂ nanoparticles for increasing power conversion efficiency in Dye sensitized solar cell electrodes prepared at low-temperature (130 °C). An efficiency η_AM1.5G = 3.55% on ITO/PET substrates is obtained at 48 mW/cm² illumination with a standard liquid electrolyte based on methoxypropionitrile. Among several solvents forming gels with PVDF-HFP-SiO₂, N-methyl (pyrrolidone) (NMP) was found to enable the most stable devices. A power conversion efficiency η_AM1.5G = 2% was obtained under 10 mW/cm² with flexible TiO₂-ITO-PET photoanodes and the PVDF-HFP-SiO₂ + NMP gel electrolyte.     

Gel electrolyte-based flexible electrochromic devices showing subtractive primary colors

We studied flexible electrochromic devices containing phthalate derivative which showed reversible color change between water transparent and subtractive primary color by electrochemical reaction. Poly(vinyl butyral) (PVB)-based gel electrolyte was prepared to apply the flexible electrochromic device. The ionic conductivity of the gel electrolyte depended on the polymer content, and was higher than 10⁻⁴ S/cm at 25 °C at the PVB content of 33 wt%. Redox reaction of phthalate derivatives was successfully achieved in the gel electrolyte. It is revealed that PVB-based gel electrolyte works well as the material for flexible electrochromic devices showing subtractive primary colors.     

Decomposition of polyurethane foams by alkanolamines

Methylene diphenyl isocyanate (MDI)-based polyurethane flexible foams were decomposed by alkanolamines without a catalyst at 150°C. The decomposition products were completely separated into two layers. The upper liquid layer was a polyether polyol, and the lower liquid layer was methylene diphenyl amine (MDA) and alkanolamine derivatives. These layers were analyzed using gel permeation chromatography (GPC) and ¹³C-NMR. The upper layer was obtained as a relatively pure polyether. The lower layer was observed to be MDA and 2-hydroxyethyl carbamic acid ester prepared from alcoholysis products during the decomposition reaction. It was thought that the decomposition process of polyurethane foam by alkanolamines was not aminolysis but alcoholysis. © 1994 John Wiley & Sons, Inc.     

Photocatalytic degradation of Rhodamine B dye under UV/solar light using ZnO nanopowder synthesized by solution combustion route

Nano-sized ZnO powder with crystallite size in the range 12 to 50 nm were prepared by solution combustion route. The product was characterized by powder X-ray diffraction (PXRD), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). Photocatalytic degradation of rhodamine B (RB) dye was carried out with ZnO nanopowder. The effect of parameters such as the crystallite size, amount of catalyst, concentration of the dye, pH and irradiation on photocatalytic degradation of RB is studied. The results reveal that the maximum decolorization (more than 95%) of dye occurred with ZnO catalyst in 8 min of stirring at basic pH under solar light irradiation. It was also found that chemical oxygen demand (COD) reduction takes place at a faster rate under solar light as compared to that of UV light. The results suggest that, the ZnO solar photocatalytic irradiation is better than the calcined ZnO/solar and UV light irradiation.     

Carbon formation promoted by hydrogen peroxide in supercritical water

A limited use of hydrogen peroxide in supercritical water has produced graphitic carbons from hydrocarbons at the low temperature of 400 °C. The choice of precursor hydrocarbons leads to morphological and microstructural variations. The use of n-hexane has provided chain-like interlinked nanoparticles comprised of graphitic layers, while benzene has been converted to a fine spherical shape of colloidal carbons with sub-micron size. The microstructure of the colloidal carbons is comprised of smaller aromatic clusters than the hexane-derived graphitic layers. Furthermore, the graphitization of the colloidal carbons at 2300 °C has induced not closed shell-like graphitic layers with concentric arrangement but ribbon-like graphitic layers growing in no particular direction. The arbitrary direction of the graphitization demonstrates the smallness of the aromatic clusters allowing for their flexible rearrangement.     

Physico-Chemical Features and Catalytic Activity of Sulfated Zirconia Prepared by Sol–Gel Method. The Role of the Solvent Evaporation Step

ZrO₂–SO₄ powders have been prepared by following a single-step sol–gel preparative route using zirconium propoxide as the starting compound. Sulfuric acid was employed both as the sulfating agent and as the catalyst of the polycondensation reaction in the gel formation. Two different series of dried precursors were obtained by either evaporating the solvent in an oven at 100°C (xerogels) or in supercritical conditions (aerogels). All the samples were calcined at three temperatures (470, 550, and 630°C) for the same time length (5 h). The powders were characterized for phase composition–crystallinity, surface area–porosity, sulfur content and surface state (XPS). The catalytic activity of the calcined samples was tested in the isomerization of n-butane in a continuous system at 150°C in absence of H₂ and 250°C in presence of H₂. The role played by the conditions of the solvent elimination, at the end of the sol–gel reaction, in affecting the physico-chemical and catalytic properties of the powders is discussed.     

Titanium dioxide sols synthesized by hydrothermal methods using tetrabutyl titanate as starting material and the application in dye sensitized solar cells

TiO₂ sols were synthesized via hydrothermal reaction using a less hydrolysable alkoxide or tetrabutyl titanate as the starting material. Using characterizations of X-ray diffraction, dynamic laser scattering and transmission electron microscopy, particle coarsening dynamics along with the particle dispersion behavior in the sols were studied via changing hydrothermal temperature from 160 °C to 250 °C. It was observed that, pure anatase phase was obtained in the whole examined temperature range. With temperature increasing, the particle size distribution in the sols first narrowed, and then broadened. Mono-dispersed nanocrystallites were obtained at the temperature of 220 °C. Aggregations were formed between the nanocrystallites at other temperatures. The causes of the aggregating behavior were analyzed and found to be different when the temperature was less than or beyond 220 °C. The as-synthesized TiO₂ sols were used in fabrication of dye sensitized nanocrystallite solar cell. The cells assembled from temperature of 220 °C came out with the best photo-to-electric energy conversion efficiency, which is ascribed to the improved dispersion of particles in TiO₂ sols.     

The application of P(MMA-co-MAA)/PEG polyblend gel electrolyte in quasi-solid state dye-sensitized solar cell at higher temperature

A poly(methyl methacrylate-co-methacrylate acid)/poly(ethylene glycol) [P(MMA-co-MAA)/PEG] polyblend with viscoelasticity was synthesized by a copolymerizing reaction between methyl methacrylate (MMA) and methacrylate acid (MAA) using azobisisobutyronitrile (AIBN) as initiator in poly(ethylene glycol) (PEG) methanol solution. Then, a polyblend gel electrolyte was prepared by adding KI and I₂ to P(MMA-co-MAA)/PEG system. The influence of compositions of the polyblend gel electrolyte on the ionic conductivity and the effect of temperature on photoelectronic performance of quasi-solid state dye-sensitized solar cell (QS-DSSC) were discussed. It was found that the polyblend gel electrolyte was a good candidate as high-temperature electrolyte for QS-DSSCs. Under an optimized condition, the highest conductivity of the polyblend gel electrolyte was 2.70 mS/cm² at 30 °C. Based on the polyblend gel electrolyte, a light-to-electricity conversion efficiency of 4.85% for QS-DSSC was achieved under AM 1.5 simulated solar light illumination at 60 °C.     

Fabrication of high performance Pt counter electrodes on conductive plastic substrate for flexible dye-sensitized solar cells

Pt counter electrodes (CEs) with different platinum loading have been prepared using chemical reduced method on flexible indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for dye-sensitized solar cells (DSSCs). H₂PtCl₆·6H₂O terpineol solutions were screen printed on the transparent ITO-PEN substrates. After drying, H₂PtCl₆ was reduced by treating it in NaBH₄ solution followed by the hydrothermal treatment at 100 °C. The obtained Pt CEs with different Pt-loading (2.4-7.7 μg/cm²) were characterized by SEM, XPS, electrochemical impedance and transmission spectrum measurement. The Pt CEs show high catalytic activity, low charge transfer resistance (0.26-1.38 Ω cm²) and good light transmittance (about 70% at 400-800 nm). The light-to-electricity conversion efficiency of the flexible DSSC fabricated with the prepared Pt CE and the TiO₂ photoanode prepared on Ti substrate by screen printing technique attains 5.41% under the simulated AM 1.5 sunlight, which is almost same as that based on the thermal decomposited Pt CE on FTO-glass. Compared with other methods to prepare Pt CEs, chemical reduced method is simple and suitable for flexible polymer substrates and the large scale preparation of DSSCs.     

Modeling and high-resolution-imaging studies of water-content profiles in a polymer-electrolyte-fuel-cell membrane-electrode assembly

Water-content profiles across the membrane electrode assembly of a polymer-electrolytefuel cell were measured using high-resolution neutron imaging and compared to mathematical-modeling predictions. It was found that the membrane held considerably more water than the other membrane-electrode constituents (catalyst layers, microporous layers, and macroporous gas-diffusion layers) at low temperatures, 40 and 60 °C. The water content in the membrane and the assembly decreased drastically at 80 °C where vapor transport and a heat-pipe effect began to dominate the water removal from the membrane-electrode assembly. In the regimes where vapor transport was significant, the through-plane water-content profile skewed towards the cathode. Similar trends were observed as the relative humidity of the inlet gases was lowered. This combined experimental and modeling approach has been beneficial in rationalizing the results of each and has given insight into future directions for new experimental work and refinements to currently available models.     

Characteristics of nanosized Bi-based glass powders prepared by flame spray pyrolysis as transparent dielectric layer material

Bi-based glass powders with particle size of 34 nm were prepared by high-temperature flame spray pyrolysis. The glass transition temperature (T_g) of the powders was 442 °C. Dielectric layers fired at temperatures of 480 and 500 °C contained voids, while those fired at temperatures above 540 °C had clean surfaces and no voids. The dielectric layers sintered at temperatures of 560 and 580 °C had transmittances of 70% in the visible range. Further, it was observed that the dielectric layers formed from the nanosized glass powders obtained from spray solutions containing excess boron had higher transmittances (80% in the visible range at a sintering temperature of 580 °C) than the layers formed from spray solutions containing stoichiometric amounts of boron.     

Temperature/pH-induced morphological regulations of shell cross-linked graft copolymer assemblies

Shell cross-linked (SCL) assemblies are prepared from the thermally induced three-layered, onion-like micelles of graft copolymers comprising acrylic acid (AAc) and 2-methacryloylethyl acrylate (MEA) as the backbone units and poly(N-isopropylacrylamide) (PNIPAAm) and monomethoxy poly(ethylene glycol) (mPEG) as the grafts via radical polymerization of the MEA residues within the AAc-rich interfacial layers in the aqueous phase of pH 5.0 at 60 °C. The resulting nanosized SCL assemblies exhibit versatile structural regulations in a fully reversible manner in response to changes in pH and temperature. At 20 °C, SCL assemblies retain the morphology of vesicle-like hollow microspheres with pH-controlled water influx and particle size. At pH 7.0, SCL assemblies remain invariant in both vesicular structure and size irrespective of the temperature increase beyond the coil-to-globule phase transition of PNIPAAm grafts occurring primarily in a highly individual manner. When the temperature increases from 20 to 60 °C at pH 5.0, the hollow particle size is greatly reduced, accompanied by the development of hydrophobic, impermeable PNIPAAm lumens attached to the inside surfaces of the interfacial gel layers. In addition, SCL assemblies undergo a dramatic thermally induced transformation from the vesicle-like to micelle-like morphology by virtue of yielding hydrophobic PNIPAAm inner cores at pH 3.0. The thermally evolved morphology of SCL assemblies is governed by the vesicle structure in response to the effect of pH on the AAc ionization within the interfacial gel layers at ambient temperature.     

Preparation of titanium nitride ultrafine powders by sol–gel and microwave carbothermal reduction nitridation methods

Titanium nitride ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction methods. The influences of reaction temperature, molar ratio of Ti to C, addition of crystal seeds and amount of NH₄F on the synthesis of titanium nitride were studied. The results show that excess amount of carbon, addition of crystal seeds and NH₄F plays a positive effect on the preparation of TiN at low temperature. The inceptive formation temperature of TiN ultrafine powders is about 800 °C, and pure TiN can be prepared at 1000 °C. Field emission-scanning electron microscopy (FE-SEM) was used to get the micrograph of the TiN powder, it shows that the size of the powders synthesized at 1000 °C is about 0.1–0.5 μm.     

Highly porous TiO2 films from anodically deposited titanate hybrids and their photoelectrochemical and photocatalytic activity

Hybrid films of TiO₂ and benzoquinone, its derivative 2-methyl-benzoquinone or the dye 2,9,16,23-tetrasulfophthalocyaninatonickel(II) were prepared by anodic electrodeposition from titanium alkoxide solutions. Calcination of the films at 450 °C led to removal of the organics and the formation of crystalline and highly porous TiO₂ films as seen in XRD and Kr adsorption measurements, respectively. In dye-sensitized solar cells the films achieved an overall light-to-electricity conversion efficiency of 0.8% despite a low film thickness of 0.55 μm. In the photocatalytic decomposition of methylene blue the films showed photonic efficiencies of up to 0.09% for film thicknesses around 0.5 μm, which is much higher than those of comparable TiO₂ films prepared by sol–gel method.     

Novel carbon-supported Fe-N electrocatalysts synthesized through heat treatment of iron tripyridyl triazine complexes for the PEM fuel cell oxygen reduction reaction

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) was used as a ligand to prepare iron-TPTZ (Fe-TPTZ) complexes for the development of a new oxygen reduction reaction (ORR) catalyst. The prepared Fe-TPTZ complexes were then heat-treated at temperatures ranging from 400 °C to 1100 °C to obtain carbon-supported Fe-N catalysts (Fe-N/C). These catalysts were characterized in terms of catalyst composition, structure, and morphology by several instrumental methods such as energy dispersive X-ray, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. With respect to the ORR activity, the Fe-N/C catalysts were also evaluated by cyclic voltammetry, as well as rotating disk and ring-disk electrodes. The results showed that among the heat-treated catalysts, that obtained at a heat-treatment temperature of 800 °C is the most active ORR catalyst. The overall electron transfer number for the catalyzed ORR was determined to be between 3.5 and 3.8, with 10-30% H₂O₂ production. The ORR catalytic activity of this catalyst was also tested in a hydrogen-air proton exchange membrane (PEM) fuel cell. At a cell voltage of 0.30 V, this fuel cell can give a current density of 0.23 A cm⁻² with a maximum MEA power density of 0.070 W cm⁻² indicating that this catalyst has potential to be used as a non-noble catalyst in PEM fuel cells.     

Surface characterization and catalytic activity of sulfated-hafnia promoted zirconia catalysts for n-butane isomerization

Sulfated-hafnia promoted zirconia containing various compositions of hafnia (1-10 wt.%) were prepared by precipitation method in an attempt to ultimately carry n-butane isomerization reaction. The catalyst samples were calcined under dry air flow at 600 °C. The structural and crystal changes were monitored by FTIR and XRD whereas the textural changes were estimated by low temperature N₂ adsorption. FTIR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted and Lewis acid sites from pyridine adsorption. The catalytic activity, stability and selectivity of the catalyst samples were tested for n-butane isomerization at 250 °C. The results reveal that the existence of a small content of hafnia increases the surface sulfate density, stabilizes the tetragonal phase of zirconia, increases the amount and strength of Brönsted acid sites and enhances the initial catalytic activity of the samples. Increasing hafnia content to 5 and 10 wt.% is accompanied with the formation of hafnia monoclinic phase which causes a drastic decrease in the amount of Brönsted acid sites and the initial activity of the catalyst. The isomerization of n-butane occurred through bimolecular pathway with the formation of iso-butane, propane and pentanes as primary products. In spite of the high activity of the samples, the catalyst deactivates rapidly during the first hour of the reaction due to coke formation, retreating the catalyst at 450 °C in the presence of dry air before the catalytic reaction lead to regeneration of the initial catalytic activity of the catalyst which implies that the coke formation is the main source for catalyst deactivation.     

Solid oxide derived from waste shells of Turbonilla striatula as a renewable catalyst for biodiesel production

Biodiesel production via transesterification of mustard oil with methanol using solid oxide catalyst derived from waste shell of Turbonilla striatula was investigated. The shells were calcined at different temperatures for 4 h and catalyst characterizations were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) and Brunauer-Emmett-Teller (BET) surface area measurements . Formation of solid oxide i.e. CaO was confirmed at calcination temperature of 800 °C. The effect of the molar ratio of methanol to oil, the reaction temperature, catalyst calcination temperature and catalyst amount used for transesterification were studied to optimize the reaction conditions. Biodiesel yield of 93.3% was achieved when transesterification was carried out at 65 ± 5 °C by employing 3.0 wt.% catalyst and 9:1 methanol to oil molar ratio. BET surface area indicated that the shells calcined in the temperature range of 700 °C-900 °C exhibited enhanced surface area and higher pore volume than the shells calcined at 600 °C. Reusability of the catalysts prepared in different temperatures was also investigated.