CdS1−yTey thin films: Production and bandgap investigation

In CdS/CdTe solar cells it is necessary to determine the efficiency limitations related with the intermixing at the interface between the CdS window layer and CdTe absorber layer. So understanding the properties of the solid solution (CdS_xTe_1?x on the CdTe side which is CdTe rich and CdS_1?yTe_y on the CdS side which is CdS rich) that is always formed in this region is essential. We produced thin films of CdS_1?yTe_y solid solution-which is CdS rich – by first producing CdS:In thin films on glass substrates by the spray pyrolysis technique and then annealing the films in nitrogen atmosphere at 400 °C in the presence of Te vapor. We are the first who produce this solid solution by this simple and low cost method. The composition and morphology of the films were determined by energy dispersive X-ray detection (EDAX) measurements and scanning electron microscopy (SEM) observations respectively. Eight values of y in the range 0 ? y ? 0.2845 were obtained. The transmittance was measured and used to investigate the optical bandgap energy by using the second derivative of the absorbance. It is found that the films show a single hexagonal phase for y ? 0.0852 and then a mixed (hexagonal and cubic) phase for 0.0997 ? y ? 0.2845. Bandgap energies in the range 2.259 ? E_g ? 2.528 eV were obtained. Urbach tailing in the bandgap was also investigated.     

Chemical and structural instabilities of lithium ion battery cathodes

The chemical and structural stabilities of various layered Li_1−xNi_1−y−zMn_yCo_zO₂ cathodes are compared by characterizing the samples obtained by chemically extracting lithium from the parent Li_1−xNi_1−y−zMn_yCo_zO₂ with NO₂BF₄ in an acetonitrile medium. The nickel- and manganese-rich compositions such as Li_1−xNi_1/3Mn_1/3Co_1/3O₂ and Li_1−xNi_0.5Mn_0.5O₂ exhibit better chemical stability than the LiCoO₂ cathode. While the chemically delithiated Li_1−xCoO₂ tends to form a P3 type phase for (1 − x) < 0.5, Li_1−xNi_0.5Mn_0.5O₂ maintains the original O3 type phase for the entire 0 ≤ (1 − x) ≤ 1 and Li_1−xNi_1/3Mn_1/3Co_1/3O₂ forms an O1 type phase for (1 − x) < 0.23. The variations in the type of phases formed are explained on the basis of the differences in the chemical lithium extraction rate caused by the differences in the degree of cation disorder and electrostatic repulsions. Additionally, the observed rate capability of the Li_1−xNi_1−y−zMn_yCo_zO₂ cathodes bears a clear relationship to cation disorder and lithium extraction rate.     

Electrocatalysis of oxygen reduction on carbon supported Ru-based catalysts in a polymer electrolyte fuel cell

Powder of nanosized particles of Ru-based (Ru_x, Ru_xSe_y and Ru_xFe_ySe_z) clusters were prepared as catalysts for oxygen reduction in 0.5 M H₂SO₄ and for fuel cells prepared by pyrolysis in organic solvent. These electrocatalysts show a high uniformity of agglomerated nanometric particles. The reaction kinetics were studied using rotating disk electrodes and an enhanced catalytic activity for the powders containing selenium and iron was observed. The Ru-based electrocatalysts were used as the cathode in a single prototype PEM fuel cell, which was prepared by spray deposition of the catalyst on the surface of Nafion^® 117 membranes. The electrochemical performance of each single fuel cell was compared to that of a platinum/platinum conventional membrane electrode assembly (MEA), using hydrogen and oxygen feed streams. A maximum power density of 140 mW cm⁻², at 80 °C with 460 mA cm⁻² was obtained for the Ru_xFe_ySe_z catalysts; approximately 55% lower power density than that obtained with platinum.     

Evaluation of discharge and cycling properties of skutterudite-type Co1−2yFeyNiySb3 compounds in lithium cells

The preparation, structural characterization and study of the electrochemical behavior in lithium cells of Co_1−2yFe_yNi_ySb₃ (0.125<y≤0.5) compounds is described. The refinement of X-ray diffraction (XRD) patterns evidenced the skutterudite-type structure for all compositions, with an increasing partial filling of 2a sites. The first discharge of lithium anode cells shows two plateaus. The plateau at higher potential, which occurs at ca. 0.8 V and extends from ca. 150 to 250 Ah/kg depending on “y”, is mainly assigned to side reactions of lithium with the electrolyte, as confirmed by ⁷Li NMR. The second plateau occurs at 0.5–0.6 V and is assigned to Li–Sb alloys formation. The charge of the cell shows that only the second step is reversible. Further discharge/charge cycles present a plateau at 0.8 V in the discharge and at 1.05 V in the charge, which agree well with the Li–Sb alloying/de-alloying process. Extended cycling results on a loss of capacity, to stabilize over 100 Ah/kg after 20 cycles.     

Synthesis,structural and electrochemical properties of pulsed laser deposited Li(Ni,Co)O2 films

We report the epitaxial growth of the LiNi_1−yM_yO₂ films (M = Co, Co–Al) on heated nickel foil using pulsed laser deposition in oxygen environment from lithium-rich targets. The structure and morphology was characterized by X-ray diffractometry, electron scanning microscopy and Raman spectroscopy. Data reveal that the formation of oriented films is dependent on two important parameters: the substrate temperature and the gas pressure during ablation. The charge–discharge process conducted in Li-microcells demonstrates that effective high specific capacities can be obtained with films 1.35 μm thick. Stable capacities of 83 and 92 μAh cm⁻² μm are available in the potential range 4.2–2.5 V for LiNi_0.8Co_0.2O₂ and LiNi_0.8Co_0.15Al_0.05O₂ films, respectively. The self-diffusion coefficient of Li ions determined from galvanostatic intermittent titration experiments is found to be 4 × 10⁻¹² cm² s⁻¹.     

Electrochemical insertion of lithium and sodium into (MoO2)2P2O7

Insertion of lithium and sodium into phosphate (MoO₂)₂P₂O₇ was investigated electrochemically to determine the usefulness as a possible cathode for ion-transfer secondary batteries. Specific charges of up to 250 mA h g⁻¹ were obtained for A/(MoO₂)₂P₂O₇ (A: Li, Na) cells with liquid organic electrolytes in the first reduction half-cycle at room temperature. Intercalation processes under constant current densities of 0.2 mA cm⁻² were reversible within the range of composition 0.85<x<4.0 for lithium and 0.5<x<3.1 for sodium in A_x(MoO₂)₂P₂O₇ (A: Li, Na), respectively. Structural changes induced by lithium or sodium intercalation were followed by ex situ X-ray diffraction measurements, and the phase change from the crystal to the amorphous was observed in both cases.     

Preparation and characterization of composite electrodes of coconut-shell-based activated carbon and hydrous ruthenium oxide for supercapacitors

The relationship between the structure-specific capacitance (F g⁻¹) of a composite electrode consisting of activated coconut-shell carbon and hydrous ruthenium oxide (RuO_x(OH)_y) has been evaluated by impregnating various amounts of RuO_x(OH)_y into activated carbon that is specially prepared with optimum pore-size distribution. The composite electrode shows an enhanced specific capacitance of 250 F g⁻¹ in 1 M H₂SO₄ with 9 wt.% ruthenium incorporated. Chemical and structural characterization of the composites reveals a homogeneous distribution of amorphous RuO_x(OH)_y throughout the porous network of the activated carbon. Electrochemical characterization indicates an almost linear dependence of capacitance on the amount of ruthenium owing to its pseudocapacitive nature.     

Thermalgravimetry–mass spectrometry studies on the thermal stability of graphite anodes with electrolyte in lithium-ion battery

Thermal reactions of a lithiated graphite anode in 1 M LiPF₆-ethylene carbonate (EC)/dimethyl carbonate (DMC) (50:50 vol.%) in the temperature range 40–320 °C were investigated by TG–MS analysis. Studies by TG–MS during thermal reactions detected a small exothermic peak around 140 °C due to CO₂ (m/z = 44) evolution, which suggests partial destruction of the SEI formed on the graphite and/or decomposition of the electrolyte through the SEI. In addition, the main exothermic reaction above 280 °C, which is associated with simultaneous evolution of C₂H₄O (m/z = 44), is caused by direct reaction of the lithiated graphite with solvent.     

Electrochemical lithium insertion into amorphous MoO3·2H2O

Following the route of synthesis of β-MoO₃ through soft chemistry methods a new amorphous material with composition MoO₃·2H₂O has been detected. The hydrated molybdenum oxide showed the capacity for electrochemical lithium insertion. The maximum amount of lithium incorporated in this material (∼3.3 Li/Mo) leads to a specific capacity of 490 Ah kg⁻¹. The charge–discharge curve showed a good reversibility in the potential range from 3.2 to 1.1 V versus Li⁺/Li⁰ where the cell voltage decreased monotonously as a function of the degree of lithium inserted. The electrochemical features of amorphous MoO₃·2H₂O suggest that it can be considered as a possible cathode candidate in rechargeable lithium batteries.     

Crystal chemistry and electrochemical characterization of layered LiNi0.5−yCo0.5−yMn2yO2 and LiCo0.5−yMn0.5−yNi2yO2 (0 ≤ 2y ≤ 1) cathodes

The crystal chemistry and electrochemical performance of the layered LiNi_0.5−yCo_0.5−yMn_2yO₂ and LiCo_0.5−yMn_0.5−yNi_2yO₂ oxide cathodes for 0 ≤ 2y ≤ 1 have been investigated. Li₂MnO₃ impurity phase is observed for Mn-rich compositions with 2y > 0.6 in LiNi_0.5−yCo_0.5−yMn_2yO₂ and 2y < 0.2 in LiCo_0.5−yMn_0.5−yNi_2yO₂. Additionally, the Ni-rich compositions encounter a volatilization of lithium at the high synthesis temperature of 900 °C. Compositions around 2y = 0.33 are found to be optimum with respect to maximizing the capacity values and retention. The rate capabilities are found to bear a strong relationship to the cation disorder in the layered lattice. Moreover, the evolution of the X-ray diffraction patterns on chemically extracting lithium has revealed the presence of Li₂MnO₃ phase in addition to the layered phase for the composition LiNi_0.25Co_0.25Mn_0.5O₂ with an oxidation state of manganese close to 4+, which results in a large anodic peak at around 4.5 V due to the extraction of both lithium and oxygen.     

Metal oxyfluorides TiOF2 and NbO2F as anodes for Li-ion batteries

Lithium insertion and extraction in to/from the oxyfluorides TiOF₂ and NbO₂F is investigated by galvanostatic cycling, cyclic voltammetry and impedance spectroscopy in cells using Li-metal as a counter electrode at ambient temperature. The host compounds are prepared by low-temperature reaction and characterized by powder X-ray diffraction (XRD), Rietveld refinement and Brunauer, Emmett and Teller (BET) surface area. Crystal structure destruction occurs during the first-discharge reaction with Li at voltages below 0.8–0.9 V for Li_xTiOF₂ as shown by ex situ XRD and at ≤1.4 V for Li_xNbO₂F to form amorphous composites, ‘Li_xTi/NbO_y–LiF’. Galvanostatic discharge–charge cycling of ‘Li_xTiO_y’ in the range 0.005–3.0 V at a current density of 65 mA g⁻¹ gives a capacity of 400 (±5) mAh g⁻¹ during 5–100 cycles with no noticeable capacity fading. This value corresponds to 1.52 mol of recycleable Li/Ti. The coulombic efficiency (η) is >98%. Results on ‘Li_xNbO_y’ show good reversibility of the electrode and a η >98% is achieved only after 10 cycles (range 0.005–3.0 V and at 30 mA g⁻¹) and a capacity of 180 (±5) mAh g⁻¹ (0.97 mol of Li/Nb) was stable up to 40 cycles. In both ‘Li_xTiO_y’ and ‘Li_xNbO_y’, the average discharge and charge voltages are 1.2–1.4 and 1.7–1.8 V, respectively. The impedance spectral data measured during the first cycle and after selected numbers of cycles are fitted to an equivalent circuit and the roles played by the relevant parameters as a function of cycle number are discussed.     

Effect of (Al,Mg) substitution in LiNiO2 electrode for lithium batteries

Stabilized lithium nickelate is receiving increased attention as a low-cost alternative to the LiCoO₂ cathode now used in rechargeable lithium batteries. Layered LiNi_1−x−yM_xM_yO₂ samples (M_x = Al³⁺ and M_y = Mg²⁺, where x = 0.05, 0.10 and y = 0.02, 0.05) are prepared by the refluxing method using acetic acid at 750 °C under an oxygen stream, and are subsequently subjected to powder X-ray diffraction analysis and coin-cell tests. The co-doped LiNi_1−x−yAl_xMg_yO₂ samples show good structural stability and electrochemical performance. The LiNiAl_0.05Mg_0.05O₂, cathode material exhibits a reversible capacity of 180 mA h g⁻¹ after extended cycling. These results suggest that the threshold concentration for aluminum and magnesium substitution is of the order of 5%. The co-substitution of magnesium and aluminium into lithium nickelate is considered to yield a promising cathode material.     

Effect of ultra-thin polymer membrane electrolytes on dye-sensitized solar cells

In-situ ultra-thin porous poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF–HFP) membranes were prepared by a phase inversion method on TiO₂ electrodes coated with Ru N-719 dye. These membranes were then soaked in the organic liquid electrolyte to form the in-situ ultra-thin porous P(VDF–HFP) membrane electrolytes. Dye-sensitized solar cell (DSC) using the membrane electrolyte exhibited an open-circuit voltage (V_oc) of 0.751 V, a short-circuit current (J_sc) of 16.260 mA cm^?2 and a fill factor (FF) of 0.684 under an incident light intensity of 1000 W m^?2 yielding an energy conversion efficiency (η) of 8.35%. The V_oc, FF and η of the solar cell using the membrane electrolyte increased by about 5.8%, 2.2% and 5.7%, respectively, when compared with the corresponding values of a cell using liquid electrolyte. However, the J_sc decreased by about 2.1%.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

Synthesis and characterization of Nd0.6Sr0.4Co1−yMnyO3−δ (0 ≤ y ≤ 1.0) cathodes for intermediate temperature solid oxide fuel cells

Nd_0.6Sr_0.4Co_1−yMn_yO_3−δ (0 ≤ y ≤ 1.0) oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). The samples form a single-phase solid solution with an orthorhombic perovskite structure and the lattice parameters and volume increase with increasing Mn content y. The degree of oxygen loss at high temperatures and the thermal expansion coefficient (TEC) decrease with increasing y due to a stronger MnO bond. The electrical conductivity decreases with y and the system exhibits a metal to semiconductor transition at around y = 0.2. The electrocatalytic activity and power density measured with single cell SOFC decrease with increasing y due to a decrease in oxygen exchange and mobility as well as charge transfer kinetics, arising from a decrease in the oxide ion vacancy concentration and electrical conductivity.     

Behavior of lanthanum-doped ceria and Sr-, Mg-doped LaGaO3 electrolytes in an anode-supported solid oxide fuel cell with a La0.6Sr0.4CoO3 cathode

Anode-supported solid oxide fuel cells (SOFCs) with lanthanum-doped ceria (LDC)/Sr-, Mg-doped LaGaO₃ (LSGM) bilayered or LDC/LSGM/LDC trilayered electrolyte films were fabricated with a pure La_0.6Sr_0.4CoO₃ (LSC) cathode. The behaviors of the two electrolytes in cells were investigated by using scanning electron microscopy, impedance spectroscopy and cell performance measurements. The reactions between LSGM and anode material can be suppressed by applying a ca. 15 μm LDC film. Due to the Co diffusion from the LSC cathode to the LSGM electrolyte during high temperature sintering, the electronic conductivity of the LDC electrolyte cannot be completely blocked with an LSGM layer below 50 μm, which leads to open-circuit potentials of these cells of ca. 0.988 V at 800 °C. The electrical conductivities of LDC and LSGM electrolytes in the cells under operation conditions are obtained from the dependence of the cell ohmic resistance on the electrolyte thickness. The electrical conductivity of LDC electrolyte is ca. 0.117 S cm⁻¹ at 800 °C on the bilayered electrolyte cells with a 50 μm LSGM layer. The bilayer electrolyte cells with a 25 μm LDC layer at 800 °C, had a cell ohmic resistance two-stage linear dependence on the LSGM layer thickness, which showed the electrical conductivity of ca. 1.9 S cm⁻¹ for the LSGM layer below 50 μm and 0.22 S cm⁻¹ for the LSGM layer above 100 μm. With a LDC/LSGM/LDC trilayered electrolyte film for the anode-supported cell, an open-circuit potential of 1.043 V was achieved.     

Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing

Anaerobic treatment of solid wastes from potato processing was studied in completely stirred tank reactors (CSTR) at 55 °C. Special attention was paid to the effect of increased organic loading rate (OLR) on the biogas yield in long-term experiments. Both biogas yield and CH₄ in the biogas decreased with the increase in OLR. For OLR in the range of 0.8 gl⁻¹ d⁻¹–3.4 gl⁻¹ d⁻¹, biogas yield and CH₄ obtained were 0.85 l g⁻¹–0.65 l g⁻¹ and 58%–50%, respectively. Biogas yield y as a function of maximum biogas yield y_m, reaction rate constant k and HRT are described on the basis of a mass balance in a CSTR and a first order kinetic. The value of y_m can be obtained from curve fitting or a simple batch test and k results from plotting y/(y_m−y) against 1/OLR from long-term experiments. In the present study values for y_m and k were obtained as 0.88 l g⁻¹ and 0.089 d⁻¹, respectively. The simple model equations can apply for dimensioning completely stirred tank reactors (CSTR) digesting organic wastes from food processing industries, animal waste slurries or biogas crops.     

Performance of a solid oxide fuel cell utilizing hydrogen sulfide as fuel

The electrochemical performance of a hydrogen sulfide solid oxide fuel cell having the configuration H₂S, Pt/(ZrO₂)_0.92(Y₂O₃)_0.08/Pt, air has been examined at atmospheric pressure and 750–800°C, using both pure and 5% H₂S anode feed streams. The performance of the cell is higher when using diluted H₂S feed compared with pure H₂S feed: current densities up to 100 mA cm⁻² and power densities up to 15.4 mW cm⁻² have been achieved using diluted H₂S gas (5%) at 800°C. However, the platinum anode degrades over time in H₂S stream due to the formation of PtS. Electrochemical oxidation of H₂S on the Pt anode significantly accelerated its degradation. Polarization and impedance spectroscopy measurements show that at low current density (i) electrochemical reaction is the major cause of polarization in the fuel cell. Ohmic loss due to the resistance of the electrolyte material and the electrical connecting wire is a major part of cell polarization at high i.     

Ni1−xCux alloy-based anodes for low-temperature solid oxide fuel cells with biomass-produced gas as fuel

Ni–Cu alloy-based anodes, Ni_1−xCu_x (x = 0, 0.05, 0.2, 0.3)–Ce_0.8Sm_0.2O_1.9 (SDC), were developed for direct utilization of biomass-produced gas in low-temperature solid oxide fuel cells (LT-SOFCs) with thin film Ce_0.9Gd_0.1O_1.95 electrolytes. The alloys were formed by in situ reduction of Ni_1−xCu_xO_y composites synthesized using a glycine-nitrate technique. The electrolyte films were fabricated with a co-pressing and co-firing technique. Electrochemical performance of the Ni_1−xCu_x–SDC anode supported cells was investigated at 600 °C when humidified (3% H₂O) biomass-produced gas (BPG) was used as the fuel and stationary air as the oxidant. With Ni–Cu alloys as anodes, carbon deposition was substantially suppressed and electrochemical performance of the cells was sustained for much longer periods of time. For example, the power export of a Ni–SDC supported cell was only 50% of the initial value (200 mW cm⁻² at 0.5 V) after 20 min, while Ni_0.8Cu_0.2–SDC supported cells could maintain 90% of the initial power density (250 mW cm⁻² at 0.5 V) over a period of 10 h. The improved performance of the Ni–Cu alloy-based anodes is worth considering in developing SOFCs fueled directly with dilute hydrocarbons such as gases derived from biomass.     

Thermodynamic evaluation of methanol steam reforming for hydrogen production

Thermodynamic equilibrium of methanol steam reforming (MeOH SR) was studied by Gibbs free minimization for hydrogen production as a function of steam-to-carbon ratio (S/C = 0–10), reforming temperature (25–1000 °C), pressure (0.5–3 atm), and product species. The chemical species considered were methanol, water, hydrogen, carbon dioxide, carbon monoxide, carbon (graphite), methane, ethane, propane, i-butane, n-butane, ethanol, propanol, i-butanol, n-butanol, and dimethyl ether (DME). Coke-formed and coke-free regions were also determined as a function of S/C ratio.Based upon a compound basis set MeOH, CO₂, CO, H₂ and H₂O, complete conversion of MeOH was attained at S/C = 1 when the temperature was higher than 200 °C at atmospheric pressure. The concentration and yield of hydrogen could be achieved at almost 75% on a dry basis and 100%, respectively. From the reforming efficiency, the operating condition was optimized for the temperature range of 100–225 °C, S/C range of 1.5–3, and pressure at 1 atm. The calculation indicated that the reforming condition required from sufficient CO concentration (<10 ppm) for polymer electrolyte fuel cell application is too severe for the existing catalysts (T_r = 50 °C and S/C = 4–5). Only methane and coke thermodynamically coexist with H₂O, H₂, CO, and CO₂, while C₂H₆, C₃H₈, i-C₄H₁₀, n-C₄H₁₀, CH₃OH, C₂H₅OH, C₃H₇OH, i-C₄H₉OH, n-C₄H₉OH, and C₂H₆O were suppressed at essentially zero. The temperatures for coke-free region decreased with increase in S/C ratios. The impact of pressure was negligible upon the complete conversion of MeOH.