Magnetism and lithium diffusion in LixCoO2 by a muon-spin rotation and relaxation (μSR) technique

Microscopic magnetism of the electrochemically Li-deintercaleted Li_xCoO₂ powders has been investigated by muon-spin rotation and relaxation (μ⁺SR) spectroscopy in the temperature (T) range between 10 and 300 K. Weak transverse-field μ⁺SR measurements indicate that localized moments appear in LiCoO₂ below 60 K, while both Li_0.53CoO₂ and Li_0.04CoO₂ are paramagnetic even at 10 K. Zero-field μ⁺SR measurements for the samples with x = 0.53 and 0.04 show that the field distribution width (Δ) due to randomly oriented nuclear magnetic moments of ⁷Li and ⁵⁹Co decreases monotonically with increasing T up to 250 K, and then it decreases steeper (increasing slope (dΔ/dT)) above 250 K. Because the muon hopping rate (ν) is almost T independent for Li_0.53CoO₂ below 300 K, the decrease in Δ suggests that the time scale of Li⁺ diffusion in Li_xCoO₂ is within a microsecond scale.     

Electrochemical study on Mn-substitution in LiFePO4 olivine compound

Olivine compounds LiFe_1−xMn_xPO₄ (0.0 < x < 0.4) were prepared by the solid-state reaction, and the electrochemical properties were studied in order to examine the effects of Mn²⁺-substitution. The substitution led to the modification of the electrochemical performance, such as initial capacity, capacity fading and polarization. From the cyclic voltammetry, it was found that the effective Li⁺ ionic diffusion coefficient was always larger in the charging process than in the discharging process and that it became larger with an appropriate amount substitution. The structural analysis on the chemical-delithiated compounds exhibited anomalous expansion of the unit cell along c-axis with the substitution, while the lattice parameters of the pristine compounds obeyed Vegard's law. The relationship between the Li⁺ ion diffusion and the bottle-neck area of (0 1 0) zigzag path was discussed. From the results, it was considered that Mn²⁺ had no direct contribution on the electrochemical reaction but influenced both electronic and ionic conductivities, which led to some modifications in the electrochemical performance.     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

Microscopic magnetism in lithium insertion materials of LiNi1−xCoxO2 (x = 0, 1/4, 1/2, 3/4, and 1)

The magnetic nature of lithium insertion materials of LiNi_1−xCo_xO₂ (x = 0, 1/4, 1/2, 3/4, and 1) were investigated by means of positive muon-spin rotation/relaxation (μ⁺SR) spectroscopy combined with X-ray diffraction (XRD) analyses and susceptibility measurements. Zero field μ⁺SR spectra for all the samples below 300 K were well fitted by a dynamic Kubo–Toyabe function, indicating the existence of randomly oriented magnetic moments even at 2 K, i.e., disordered state. The field distribution width Δ due to magnetic Ni³⁺ ions decreases exponentially with increasing x, suggesting that the Co substitution is likely to simply dilute Ni moments. This also supports that cobalt and nickel ions are homogeneously distributed in a solid matrix even in a muon-scale (microscopically), which is consistent with the results of macroscopic measurements.     

Thermal expansion and electrochemical properties of Ni-doped GdBaCo2O5+δ double-perovskite type oxides

Layered GdBaCo_2 −x Ni_xO_5 + δ (0 ≤ x ≤ 0.3) complex oxides were synthesized and investigated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFCs). All compositions formed an orthorhombic double-perovskite structure after calcination at 1000 °C for 5 h. The thermal expansion coefficient (TEC) was effectively decreased due to the partial substitution of Ni for Co, but the cathodic polarization resistance slightly increased with the increasing Ni content. Among the tested oxides, the GdBaCo_1.7Ni_0.3O_5 + δ composition showed a fairly reduced TEC (15.5 × 10⁻⁶ K⁻¹) and reasonably low polarization resistances (e.g., 0.54 Ωcm² at 600 °C), which was considered as a promising candidate for IT-SOFCs.     

A porous spherical aggregation of Li4Mn5O12 nanorods and its electrochemical performance

A porous spherical aggregation of Li₄Mn₅O₁₂ nanorods with the particle size of 3 μm is prepared by oxidizing LiMn₂O₄ powder with (NH₄)₂S₂O₈ under hydrothermal conditions. The result displays that concentration of (NH₄)₂S₂O₈ plays a key role in forming the porous spherical aggregation and the optimal concentration of oxidant is found to be 1.5 mol L⁻¹. The mechanism for the formation of the porous spherical aggregation is proposed. The electrochemical capacitance performance is tested by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge. The porous spherical aggregation exhibits a good electrochemical performance. It could deliver 375 F g⁻¹ within potential range 0-1.4 V at a scan rate of 5 mV s⁻¹ in 1 mol L⁻¹ Li₂SO₄ and the value is cut down to less than 0.024 F g⁻¹ per cycling period in 1000 cycles.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.     

A cheap asymmetric supercapacitor with high energy at high power: Activated carbon//K0.27MnO2·0.6H2O

Studies of the electrochemical behavior of K_0.27MnO₂·0.6H₂O in K₂SO₄ show the reversible intercalation/deintercalation of K⁺-ions in the lattice. An asymmetric supercapacitor activated carbon (AC)/0.5 mol l⁻¹ K₂SO₄/K_0.27MnO₂·0.6H₂O was assembled and tested successfully. It shows an energy density of 25.3 Wh kg⁻¹ at a power density of 140 W kg⁻¹; at the same time it keeps a very good rate behavior with an energy density of 17.6 Wh kg⁻¹ at a power density of 2 kW kg⁻¹ based on the total mass of the active electrode materials, which is higher than that of AC/0.5 mol l⁻¹ Li₂SO₄/LiMn₂O₄. In addition, this asymmetric supercapacitor shows excellent cycling behavior without the need to remove oxygen from the electrolyte solution. This can be ascribed in part to the stability of the lamellar structure of K_0.27MnO₂·0.6H₂O. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).     

Microstructural and electrical properties of different-sized aluminum-alloyed contacts and their layer system on silicon surfaces

The firing of screen-printed aluminum pastes is well established for the formation of a back surface field (BSF) and back contacts since many years in silicon solar cell fabrication. In this paper we investigate the electrical and microstructural properties of Al-alloyed contacts and their layer system, consisting of (i) the Al-doped p⁺-layer (2-14 μm), the eutectic layer (1-15 μm) and the layer of paste residuals (20-100 μm). We show the influence of process parameters like the amount of printed paste, the alloying time and the peak temperature. Special emphasis is devoted to the properties of small alloyed screen-printed aluminum structures for the formation of local aluminum back contacts. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electrochemical capacitance voltage (ECV) and conductivity measurements have been applied to characterize the samples. A simple model is qualitatively augmented to describe all effects occurring in a technical alloying process. For example, for increasing aluminum amounts, a saturation of the p⁺-layer thickness was found in the range of 10 mg/cm². For small screen-printed structures, the p⁺-layer is formed very homogenously and with a greater thickness compared to samples with a full-area Al metallization, which have been processed with similar alloying conditions. For the eutectic layer a high electrical conductivity of about 16×10⁶ S/m, only 2-3 times below that of pure aluminum has been determined. This is advantageous for the lateral conductivity especially for high paste amounts and small structures, which feature a relatively thick eutectic layer.     

The discharge properties of Na/Ni3S2 cell at ambient temperature

The discharge properties of a Na/Ni₃S₂ cell using 1 M NaCF₃SO₃ in tetra(ethylene glycol)dimethyl ether liquid electrolyte were investigated at room temperature. The products were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Electrochemical properties of Na/Ni₃S₂ cells were also presented by cyclic voltammetry and the galvanostatic current method. Na/Ni₃S₂ cells have an initial discharge capacity of 420 mAh g⁻¹ with a plateau potential at 0.94 V versus Na/Na⁺. After the first discharge, Ni₃S₂ and Na react at room temperature and then form sodium sulfide (Na₂S) and nickel. Sodium ion can be partially deintercalated from Na₂S charge reaction. The discharge process can be explained as follows: Ni₃S₂ + 4Na ↔ 3Ni + 2Na₂S.     

A study of vapor CdCl2 treatment by CSS in CdS/CdTe solar cells

We report the effect of CdCl₂ vapor treatment on the photovoltaic parameters of CdS/CdTe solar cells. Vapor treatment allows combining CdCl₂ exposure time and annealing in one step. In this alternative treatment, the CdS/CdTe substrates were treated with CdCl₂ vapor in a close spaced sublimation (CSS) configuration. The substrate temperature and CdCl₂ powder source temperature were 400 °C. The treatment was done by varying the treatment time (t) from 15 to 90 min. Such solar cells are examined by measuring their current density versus voltage (J-V) characteristics. The open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) of our best cell, fabricated and normalized to the area of 1 cm², were V_oc = 663 mV, J_sc = 18.5 mA/cm² and FF = 40%, respectively, corresponding to a total area conversion efficiency of η = 5%. In cells of minor area (0.1 cm²) efficiencies of 8% have been obtained.     

Gallium (III) sulfide as an active material in lithium secondary batteries

In an attempt to identify an active material for use in lithium secondary batteries with high energy density, we investigated the electrochemical properties of gallium (III) sulfide (Ga₂S₃) at 30 °C. Ga₂S₃ shows two sloping plateaus in the potential range between 0.01 V and 2.0 V vs. (Li/Li⁺). The specific capacity of the Ga₂S₃ electrode in the first delithiation is ca. 920 mAh g⁻¹, which corresponds to 81% of the theoretical capacity (assuming a 10-electron reaction). The capacity in the 10th cycle is 63% of the initial capacity. Ex situ X-ray diffraction and X-ray absorption fine structure analyses revealed that the reaction of the Ga₂S₃ electrode proceeds in two steps: Ga₂S₃ + 6Li⁺ + 6e⁻ ? 2Ga + 3Li₂S and Ga + xLi⁺ + xe⁻ ? Li_xGa.     

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.     

Ba0.5Sr0.5Co0.8Fe0.2O3 − δ + LaCoO3 composite cathode for Sm0.2Ce0.8O1.9-electrolyte based intermediate-temperature solid-oxide fuel cells

A novel Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ + LaCoO₃ (BSCF + LC) composite oxide was investigated for the potential application as a cathode for intermediate-temperature solid-oxide fuel cells based on a Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. The LC oxide was added to BSCF cathode in order to improve its electrical conductivity. X-ray diffraction examination demonstrated that the solid-state reaction between LC and BSCF phases occurred at temperatures above 950 °C and formed the final product with the composition: La_0.316Ba_0.342Sr_0.342Co_0.863Fe_0.137O_3 − δ at 1100 °C. The inter-diffusion between BSCF and LC was identified by the environmental scanning electron microscopy and energy dispersive X-ray examination. The electrical conductivity of the BSCF + LC composite oxide increased with increasing calcination temperature, and reached a maximum value of ∼300 S cm⁻¹ at a calcination temperature of 1050 °C, while the electrical conductivity of the pure BSCF was only ∼40 S cm⁻¹. The improved conductivity resulted in attractive cathode performance. An area-specific resistance as low as 0.21 Ω cm² was achieved at 600 °C for the BSCF (70 vol.%) + LC (30 vol.%) composite cathode calcined at 950 °C for 5 h. Peak power densities as high as ∼700 mW cm⁻² at 650 °C and ∼525 mW cm⁻² at 600 °C were reached for the thin-film fuel cells with the optimized cathode composition and calcination temperatures.     

Influence of substrates on photoelectrochemical performance of sprayed n-CdIn2S4 electrodes

Cadmium indium sulphide (CdIn₂S₄) electrodes have been prepared onto the preheated fluorine doped tin oxide (FTO) coated glass and stainless steel (SS) substrates at optimized deposition conditions by using spray pyrolysis. Influence of substrates on the photoelectrochemical (PEC) performance has been carried out using cell configuration n-CdIn₂S₄/1 M (NaOH + Na₂S + S)/C for studying the current-voltage (I-V), photovoltaic output, photovoltaic rise and decay, photo and spectral responses and capacitance-voltage (C-V) characteristics. The junction ideality factor in dark (n_D) and light (n_L), series and shunt resistances (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The measured fill factor (FF) and cell efficiency (η) of the cells are found to be 0.47%, 0.38%, and 1.06%, 0.38% for FTO and SS substrates respectively. The Energy band diagram of band bending has been constructed using the physical parameters estimated from Mott-Schottky plots. Mott-Schottky plots shows the flat-band potential (V_fb) of CdIn₂S₄ films to be −1.15 V/SCE and −0.90 V/SCE on FTO and SS substrates respectively.     

Nearly carbon-free printable CIGS thin films for solar cell applications

In order to fabricate low cost and printable CuIn_xGa_1−xSe_yS_2−y (CIGS) thin film solar cells, a precursor solution based method was developed. Particularly, in this method, nearly carbon-free CIGS film was obtained by applying a three-step heat treatment process: the first for the elimination of carbon residue by air annealing, the second for the formation of CIGS alloy by sulfurization, and the third for grain growth and densification in the CIGS film by selenization. The film also revealed very large grains with a low degree of porosity, similar to those produced by the vacuum based method. A solar cell device with this film showed current-voltage characteristics of J_sc=21.02 mA/cm², V_oc=451 mV, FF=47.3%, and η=4.48% at standard conditions.     

Properties of the lithium and graphite-lithium anodes in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide

The ternary [Li⁺]_0.09[MePrPyr⁺]_0.41[NTf₂⁻]_0.50 room temperature ionic liquid was obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([MePrPyr⁺][NTf₂⁻]), and studied as an electrolyte for lithium-ion batteries. The graphite-lithium (C₆Li) anode, working together with vinylene carbonate as an additive showed ca. 90% of its initial discharge capacity after 50 cycles. The addition of vinylene carbonate to the neat ionic liquid results in the formation of the protective coating (SEI) on both the lithium and graphite anodes. The SEI formation increases the rate of the charge transfer reaction as well as protects the anode from chemical passivation (corrosion). The graphite-lithium (C₆Li) anode shows good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate as an additive to the ionic liquid.     

In situ quantification of the in-plane water content in the Nafion membrane of an operating polymer-electrolyte membrane fuel cell using H micro-magnetic resonance imaging experiments

Spatial, quantitative, and temporal information regarding the water content distribution in the transverse-plane between the catalyst layers of an operating polymer-electrolyte membrane fuel cell (PEMFC) is essential to develop a fundamental understanding of water dynamics in these systems. We report ¹H micro-magnetic resonance imaging (MRI) experiments that measure the number of water molecules per SO₃H group, λ, within a Nafion^®-117 membrane between the catalyst stamps of a membrane-electrode assembly, MEA. The measurements were made both ex situ, and inside a PEMFC operating on hydrogen and oxygen. The observed ¹H MRI T₂ relaxation time of water in the PEM was measured for several known values of λ. The signal intensity of the images was then corrected for T₂ weighting to yield proton density-weighted images, thereby establishing a calibration curve that correlates the ¹H MRI density-weighted signal with λ. Subsequently, the calibration curve was used with proton density weighted (i.e., T₂-corrected) signal intensities of transverse-plane ¹H MRI images of water in the PEM between the catalyst stamps of an operating PEMFC to determine λ under various operational conditions. For example, the steady state, transverse-plane λ was 9 ± 1 for a PEMFC operating at ∼26.4 mW cm⁻² (∼20.0 mA, ∼0.661 V, 20 °C, flow rates of the dry H₂(g) and O₂(g) were 5.0 and 2.5 mL min⁻¹, respectively).     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.