Preparation of 1-methyl-3-propylimidazolium acetate and its application in dye sensitized solar cells

In this paper, we reported the preparation of 1-methyl-3-propylimidazolium acetate (MPIAc), which proceeded via the metathesis of 1-methyl-3-propylimidazolium iodide (MPII) and lead acetate or potassium acetate. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, MPIAc and MPII with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide increased and those of iodide slightly increased with the weight ratio increase of MPIAc and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I₂, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of MPIAc (employing potassium acetate) and MPII (weight ratio 0.2), gave short circuit photocurrent density of 9.40 mA cm⁻², open circuit voltage of 0.62 V, and fill factor of 0.57, corresponding to the photoelectric conversion efficiency of 3.34% at the illumination (air mass 1.5, 100 mW cm⁻²).     

An improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate and its application in dye sensitized solar cells

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA), which proceeded via efficient reaction of 1-methylimidazole and ethyl trifluoroacetate under solvent-free conditions using Teflon-lined, stainless steel autoclaves. It was shown that the procedure was simple and eco-friendly. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, EMITA and 1-methyl-3-propylimidazolium iodide (MPII) with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide slightly increased and those of iodide decreased with the weight ratio increase of EMITA and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I₂, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of EMITA and MPII (weight ratio 1:2), gave short circuit photocurrent density of 7.88 mA cm⁻², open circuit voltage of 0.61 V, and fill factor of 0.67, corresponding to the photoelectric conversion efficiency of 3.22% at the illumination (Air Mass 1.5, 100 mW cm⁻²).     

Quasi solid state polymer electrolyte with binary iodide salts for photo-electrochemical solar cells

Quasi-solid-state polymer electrolytes can be used in dye sensitized solar cells (DSSCs) in order to overcome various problems associated with liquid electrolytes. Prior to fabricating commercially viable solar cells, the efficiency of quasi solid state DSSCs needs to be improved. Using electrolytes with a binary iodide mixture is a novel technique used to obtain such efficiency enhancement. In this work we report both conductivity and solar cell performance enhancements due to incorporation of a mixture containing LiI and tetrahexylammonium iodide in a quasi-solid-state electrolyte. The conductivity of the electrolyte increases with added amounts of LiI and thus the highest conductivity, 3.15 × 10⁻³ S cm⁻¹ at 25 °C, is obtained for the electrolyte 100 wt% LiI. The predominantly ionic behavior of the electrolytes was established from dc polarization measurements. The iodide ion conductivity, measured using iodine pellet electrodes decreased somewhat with increasing amount of LiI even though the overall conductivity increased. However, the highest efficiency was obtained for the DSSC containing a polymer electrolyte with Hex₄N⁺I^¯:LiI = 1:2 mass ratio. This cell had the largest short circuit current density of about 13 mA cm⁻² and more than 4% overall energy conversion efficiency. The results thus show that electrolytes with Hex₄N⁺I^¯/LiI mixed iodide system show better DSSC performance than single iodide systems.     

Improved dye-sensitized solar cells by composite ionic liquid electrolyte incorporating layered titanium phosphate

We reported a composite electrolyte prepared by incorporating layered α-titanium phosphate (α-TiP) into a binary ionic liquid of 1-propyl-3-methylimidazolium iodide (PMII) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF₄) (volume ratio, 13:7) electrolyte. The addition of α-TiP markedly improved the photovoltaic properties of dye-sensitized solar cells (DSSCs) compared to that without α-TiP. The enhancement was explained by improved diffusion of tri-iodide ions, suppressed electron recombination with in the electrolyte and increased lifetime of electrons in mesoscopic TiO₂ film.     

Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells

A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I-V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs.     

The potential of polyurethane bio-based solid polymer electrolyte for photoelectrochemical cell application

A photoelectrochemical cell was developed from bio-based polyurethane (PU), solid polymer electrolyte with lithium iodide as conducting material. At the initial stage, PU prepolymer was prepared via prepolymerization technique by reacting palm kernel oil-based monoester-OH (PKO-p) and 2,4′-methylene diphenyl diisocyanate (2,4′-MDI). The polyurethane electrolyte film was then prepared by inclusion of varying amount of lithium iodide (LiI) via solution casting technique. The formation of urethane linkages (NHCO backbone) and the chemical interaction between segmented polyurethane and lithium ion from LiI salts were confirmed by ATR-FTIR technique. Thermal studies carried out by TGA have proven the occurrence of polymer-salt complexation. Structural analysis by XRD has revealed that polyurethane electrolytes with 25 wt.% LiI reduced the semi-crystalline characteristics of plasticized polyurethane. The SEM morphological observation on the fractured film indicated the absence of phase separation. The ionic conductivity increased with the addition of 25 wt.% LiI resulted in the highest conductivity of 7.6 × 10⁻⁴ S cm⁻¹. The temperature dependence conductivity of the electrolytes obeyed the Arrhenius law with the pre-exponential factor, σ_o of 2.4 × 10⁻³ S cm⁻¹ and activation energy, E_a of 0.11 eV. A dye-sensitized solar cell of FTO/TiO₂-dye/PU-LiI-I₂/Pt give a response under light intensity of 100 mW cm⁻² indicated the photovoltaic effect with the J_sc of 0.06 mA cm⁻² and V_oc of 0.14 V respectively. These properties exhibited promising potentials for photoelectrochemical cell giving the focus on bio-based polymer electrolyte.     

Low molecular mass organogelator based gel electrolyte gelated by a quaternary ammonium halide salt for quasi-solid-state dye-sensitized solar cells

Quasi-solid-state dye-sensitized solar cells (DSC) are fabricated using tetradodecylammonium bromide as a low molecular mass organogelator (LMOG) to form gel electrolyte with a high solution-to-gel transition temperature (T_SG) of 75 °C to hinder flow and volatilization of the liquid. The steady-state voltammograms reveal that the diffusion of the I₃⁻ and I⁻ in the gel electrolyte is hindered by the self-assembled network of the gel. An increased interfacial exchange current density (j₀) of 4.95 × 10⁻⁸ A cm⁻² and a decreased electron recombination lifetime (τ) of 117 ms reveal an increased electron recombination at the dyed TiO₂ photoelectrode/electrolyte interface in the DSC after gelation. The results of the accelerated aging tests show that the gel electrolyte based dye-sensitized solar cell can retain over 93% of its initial photoelectric conversion efficiency value after successive heating at 60 °C for 1000 h, and device degradation is negligible after one sun light soaking with UV cutoff filter for 1000 h.     

Dye-sensitized, nano-porous TiO2 solar cell with poly(acrylonitrile): MgI2 plasticized electrolyte

Dye-sensitized solar cells are promising candidates as supplementary power sources; the dominance in the photovoltaic field of inorganic solid-state junction devices is in fact now being challenged by the third generation of solar cells based on dye-sensitized, nano-porous photo-electrodes and polymer electrolytes. Polymer electrolytes are actually very favorable for photo-electrochemical solar cells and in this study poly(acrylonitrile)-MgI₂ based complexes are used. As ambient temperature conductivity of poly(acrylonitrile)-salt complexes are in general low, a conductivity enhancement is attained by blending with the plasticizers ethylene carbonate and propylene carbonate. At 20 °C the optimum ionic conductivity of 1.9 × 10⁻³ S cm⁻¹ is obtained for the (PAN)₁₀(MgI₂)_n(I₂)_n/10(EC)₂₀(PC)₂₀ electrolyte where n = 1.5. The predominantly ionic nature of the electrolyte is seen from the DC polarization data. Differential scanning calorimetric thermograms of electrolyte samples with different MgI₂ concentrations were studied and glass transition temperatures were determined. Further, in this study, a dye-sensitized solar cell structure was fabricated with the configuration Glass/FTO/TiO₂/Dye/Electrolyte/Pt/FTO/Glass and an overall energy conversion efficiency of 2.5% was achieved under solar irradiation of 600 W m⁻². The I-V characteristics curves revealed that the short-circuit current, open-circuit voltage and fill factor of the cell are 3.87 mA, 659 mV and 59.0%, respectively.     

Quasi-solid-state dye-sensitized solar cells with a novel efficient absorbent for liquid electrolyte based on PAA–PEG hybrid

A novel efficient absorbent for liquid electrolyte based on poly(acrylic acid)–poly(ethylene glycol) (PAA–PEG) hybrid is prepared. The highest value of liquid electrolyte absorbency about 3.65 is achieved. The polymer gel electrolyte shows a moderate value of ionic conductivity about 3.24 mS cm⁻¹ and high chemical stability. Based on the polymer gel electrolyte, a quasi-solid-state dye-sensitized solar cell was fabricated and its overall energy conversion efficiency of 3.19% was obtained under irradiation of 100 mW cm⁻².     

Physical and photoelectrochemical characterizations of hematite α-Fe2O3: Application to photocatalytic oxygen evolution

The physical properties and photoelectrochemical characterization of α-Fe₂O₃, synthesized by co-precipitation, have been investigated in regard to solar energy conversion. The optical gap is found to be 1.94 eV and the transition is indirectly allowed. The chemical analysis reveals an oxygen deficiency and the oxide exhibits n-type conductivity, confirmed by a negative thermopower. The plot log σ vs 1/T shows linearity in the range (400-670 K) with the donor levels at 0.14 eV below the conduction band and a break at ∼590 K, attributed to the ionization of the donors. The conduction occurs by small polaron hopping through mixed valences Fe^2+/3+ with an electron mobility μ_400 K of 10⁻³ V cm² s⁻¹. α-Fe₂O₃ exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as bias-free O₂-photoanode. The flat band potential V_fb (−0.45V_SCE) and the electron density N_D (1.63 × 10¹⁸ cm⁻³) were determined, respectively, by extrapolating the linear part to C⁻² = 0 and the slope of the Mott Schottky plot. At pH 6.5, the valence band (+1.35V_SCE) is suitably positioned with respect to the O₂/H₂O level (+0.62 V) and α-Fe₂O₃ has been evaluated for the chemical energy storage through the photocatalytic reaction: (, ΔG = 213.36 kJ mol⁻¹). The best photoactivity occurs in solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm³ (g catalyst)⁻¹ h⁻¹.     

Enhanced performance of a quasi-solid-state dye-sensitized solar cell with aluminum nitride in its gel polymer electrolyte

The effects of incorporation of aluminum nitride (AlN) in the gel polymer electrolyte (GPE) of a quasi-solid-state dye-sensitized solar cell (DSSC) were studied in terms of performance of the cell. The electrolyte, consisting of lithium iodide (LiI), iodine (I₂), and 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN), was solidified with poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The 0.05, 0.1, 0.3, and 0.5 wt% of AlN were added to the electrolyte for this study. XRD analysis showed a reduction of crystallinity in the polymer PVDF-HFP for all the additions of AlN. The DSSC fabricated with a GPE containing 0.1 wt% AlN showed a short-circuit current density (J_SC) and power-conversion efficiency (η) of 12.92±0.54 mA/cm² and 5.27±0.23%, respectively, at 100 mW/cm² illumination, in contrast to the corresponding values of 11.52±0.21 mA/cm² and 4.75±0.08% for a cell without AlN. The increases both in J_SC and in η of the promoted DSSC are attributed to the higher apparent diffusion coefficient of I⁻ in its electrolyte (3.52×10⁻⁶ cm²/s), compared to that in the electrolyte without AlN of a DSSC (2.97×10⁻⁶ cm²/s). At-rest stability of the quasi-solid-state DSSC with 0.1 wt% of AlN was found to decrease hardly by 5% and 7% at room temperature and at 40 °C, respectively, after 1000 h duration. The DSSC with a liquid electrolyte showed a decrease of about 40% at room temperature, while it virtually lost its performance in about 150 h at 40 °C. Explanations are further substantiated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and by porosity measurements.     

Structure, morphology, and cathode performance of Li1−x[Ni0.5Mn1.5]O4 prepared by coprecipitation with oxalic acid

The cathode materials Li_1−x[Ni_0.5Mn_1.5]O₄ prepared by coprecipitation from acetate solution by oxalic acid and annealing at 900 °C in air had the preferred disordered Ni and Mn on the 16d octahedral sites of a spinel structure. The coprecipitation method provides better crystallinity than the phase previously obtained by quenching from the melt. Polycrystalline octahedral-shaped particles with smooth surfaces contained trace amounts of a Li_yNi_1−yO impurity that introduced some Mn(III) into the spinel phase. Half-cells cycled at 0.2 C rate between 3.5 and 4.8 V versus Li exhibited a flat voltage V ≈ 4.7 V with a small step at x ≈ 0.5 and a capacity at room temperature of 130 mAh g⁻¹ that showed no fade after 50 cycles. A small capacity fade was initiated with a cut-off voltage ≥4.9 V; a significant capacity loss between 2 and 5 C cycling rates was reversible to 134 mAh g⁻¹ on returning to 0.1 C after 50 cycles at 10 C between 3.5 and 5.0 V.     

Landfill leachate treatment by solar-driven AOPs

Sanitary landfill leachate resulting from the rainwater percolation through the landfill layers and waste material decomposition is a complex mixture of high-strength organic and inorganic compounds which constitutes serious environmental problems. In this study, different heterogeneous (TiO₂/UV, TiO₂/H₂O₂/UV) and homogenous (H₂O₂/UV, Fe²⁺/H₂O₂/UV) photocatalytic processes were investigated as an alternative for the treatment of a mature landfill leachate. The addition of H₂O₂ to TiO₂/UV system increased the reduction of the aromatic compounds from 15% to 61%, although mineralization was almost the same. The DOC and aromatic content abatement is similar for the H₂O₂/UV and TiO₂/H₂O₂/UV processes, although the H₂O₂ consumption is three times higher in the H₂O₂/UV system. The low efficiency of TiO₂/H₂O₂/UV system is presumably due to the alkaline leachate solution, for which the H₂O₂ becomes highly unstable and self-decomposition of H₂O₂ occurs. The efficiency of the TiO₂/H₂O₂/UV system increased 10 times after a preliminary pH correction to 4. The photo-Fenton process is much more efficient than heterogeneous (TiO₂, TiO₂/H₂O₂/UV) or homogeneous (H₂O₂/UV) photocatalysis, showing an initial reaction rate more than 20 times higher, and leading to almost complete mineralization of the wastewater. However, when compared with TiO₂/H₂O₂/UV with acidification, the photo-Fenton reaction is only two times faster.The optimal initial iron dose for the photo-Fenton treatment of the leachate is 60 mg Fe²⁺ L⁻¹, which is in agreement with path length of 5 cm in the photoreactor. The kinetic behaviour of the process (60 mg Fe²⁺ L⁻¹) comprises a slow initial reaction, followed by a first-order kinetics (k = 0.020 , r₀ = 12.5 mg ), with H₂O₂ consumption rate of k_H2O2 = 3.0 mmol H₂O₂, and finally, the third reaction period, characterized by a lower DOC degradation and H₂O₂ consumption until the end of the experiment, presumably due to the formation of low-molecular-weight carboxylic groups. A total of 306 mM of H₂O₂ was consumed for achieving 86% mineralization (DOC_final = 134 mg L⁻¹) and 94% aromatic content reduction after 110 kJ_UV L⁻¹, using an initial iron concentration of 60 mg Fe²⁺ L⁻¹.     

Cycling and rate performance of Li-LiFePO4 cells in mixed FSI-TFSI room temperature ionic liquids

A study is conducted of the performance of lithium iron(II) phosphate, LiFePO₄, as a cathode material in a lithium secondary battery that features an ionic liquid electrolyte solution and a metallic lithium anode. The electrolyte solution comprises an ionic liquid of a N-methyl-N-alkyl-pyrrolidinium (alkyl = n-propyl or n-butyl) cation and either the bis(fluorosulfonyl)imide [(FSO₂)₂N⁻] or bis(trifluoromethanesulfonyl)imide [(F₃CSO₂)₂N⁻] anion, together with 0.5 mol kg⁻¹ of lithium bis(trifluoromethanesulfonyl)imide salt. For N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide, coin cells discharging at rates of C/10 and 4C yield specific capacities of 153 and 110 mAh g⁻¹, respectively, at an average coulombic efficiency of 99.8%. This performance is maintained for over 400 cycles at 50 °C and therefore indicates that these electrolyte solutions support long-term cycling of both LiFePO₄ and metallic lithium while, due to the negligible volatility of ionic liquids, surrounding the lithium in an inherently safe, non-flammable medium.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.