IrxCo1−x (x = 0.3–1.0) alloy electrocatalysts, catalytic activities, and methanol tolerance in oxygen reduction reaction

Novel methanol-tolerant catalysts for oxygen reduction reaction (ORR), Ir_xCo_1−x/C (x = 0.3–1.0), were synthesized by a conventional impregnation method. These carbon-supported catalysts showed particle sizes of 2.7–5.0 nm. The catalyst activity and the catalyzed ORR kinetics were characterized by cyclic voltammetry and rotating disk electrode methods. Among these Ir_xCo_1−x/C catalysts, the alloy with a formula of Ir_xCo_1−x/C with x value in the range of 0.7–0.8 exhibited the highest mass and specific activities. Compared to a Pt/C catalyst, these alloy catalysts have much stronger methanol tolerance in terms of ORR onset potential (or open-circuit potential). Based on the rotating disk electrode measurements, it was confirmed that these Ir_xCo_1−x/C alloy catalysts could catalyze a complete four-electron transfer reaction of oxygen to water. These results strongly suggest that the novel Ir–Co metal alloy catalysts synthesized in this work could be promising for DMFC cathodes.     

Electrochemistry of poly(vinylferrocene) modified electrodes in aqueous acidic media

A cyclic voltammetric study of the electrochemistry and chemical stability of the poly(vinylferrocene) (PVFc) redox couple, coated on a gold substrate, in aqueous solutions of H₂SO₄, HClO₄ and HCl was carried out. It was found that the anodic peak potential (E_pa) did not depend on the acid concentration in the range (1.0 × 10⁻² to 1.0 × 10⁻⁷ mol L⁻¹). However, the E_pa values shifted linearly to less positive potentials when investigated in more concentrated acid solutions in the range 1–5 mol L⁻¹. The slope of the E_pa versus acid concentration graph was found to be in the order H₂SO₄ > HCl > HClO₄. In this regard PVFc behaved very similar to 1,1′-bis(11-mercaptoundecyl)ferrocene (Fc(C₁₁SH)₂) except for its chemical stability. In H₂SO₄ media the PVFc was found to be much less stable than 1,1′-Fc(C₁₁SH)₂. The dependence of E_pa on acid concentration could be used to monitor state of charge of lead-acid batteries. However, for this application Fc(C₁₁SH)₂ would be a better choice because of its superior chemical stability.     

A new electrode for a poly(pyrrole)-based rechargeable battery

The use of dodecylbenzenesulfonate-doped poly(pyrrole) films, PPYDBS, as a secondary battery electrode was studied. The redox and morphologic properties of these films are suitable for battery application. Films were synthesized by electrolysis of pyrrole and sodium dodecylbenzenesulfonate aqueous solutions with a current density of 1.0 mA cm⁻² and were switched in LiC1O₄ 1.0 M propylene carbonate solutions (PC) by cyclic voltammetry. In these experiments an apparent diffusion coefficient of 3.7x10⁻⁹ cm²s⁻¹ has been found. Charge/discharge tests at ±50, ±100, ±150 and ±200 μA cm⁻² were done for a PPYDBS/ LiC1O₄, PC/Li battery. The open-circuit voltage was 3.2 V after 30 h, the specific capacity 53 A h kg⁻¹ and the energy density 154 W h kg⁻¹. These values indicate that this polymer can be used as an electrode in a rechargeable battery.     

Synthesis of polyaniline/graphite composite as a cathode of Zn-polyaniline rechargeable battery

The characteristics of polyaniline/graphite composites (PANi/G) have been studied in aqueous electrolyte. PANi/G films with different graphite particle sizes were deposited on a platinum electrode by means of cyclic voltammetry. The film was employed as a positive electrode (cathode) for a Zn-PANi/G secondary battery containing 1.0 M ZnCl₂ and 0.5 M NH₄Cl electrolyte at pH 4.0. The cells were charged and discharged under a constant current of 0.6 mA cm⁻². The assembled battery showed an open-circuit voltage (OCV) of 1.55 V. All the batteries were discharge to a cut off voltage of 0.7 V. Maximum discharge capacity of the Zn-PANi/G battery was 142.4 Ah kg⁻¹ with a columbic efficiency of 97–100% over at least 200 cycles. The mid-point voltage (MPV) and specific energy were 1.14 V and 162.3 Wh kg⁻¹, respectively. The constructed battery showed a good recycleability. The structure of these polymer films was characterized by FTIR and UV–vis spectroscopies. Electrochemical impedance spectroscopy (EIS) was used as a powerful tool for investigation of charge transfer resistance in cathode material. The scanning electron microscopy (SEM) was employed as a morphology indicator of the cathodes.     

Synthesis and electrochemical studies of the 4 V cathode, Li(Ni2/3Mn1/3)O2

Layered Li(Ni_2/3Mn_1/3)O₂ compounds are prepared by freeze-drying, mixed carbonate and molten salt methods at high temperature. The phases are characterized by X-ray diffraction, Rietveld refinement, and other methods. Electrochemical properties are studied versus Li-metal by charge–discharge cycling and cyclic voltammetry (CV). The compound prepared by the carbonate route shows a stable capacity of 145 (±3) mAh g⁻¹ up to 100 cycles in the range 2.5–4.3 V at 22 mA g⁻¹. In the range 2.5–4.4 V at 22 mA g⁻¹, the compound prepared by molten salt method has a stable capacity of 135 (±3) mAh g⁻¹ up to 50 cycles and retains 96% of this value after 100 cycles. Capacity-fading is observed in all the compounds when cycled in the range 2.5–4.5 V. All the compounds display a clear redox process at 3.65–4.0 V that corresponds to the Ni^2+/3+–Ni^3+/4+ couple.     

Electrochemical characteristics and cycle performance of LiMn2O4/a-Si microbattery

A LiMn₂O₄ thin film and an amorphous Si (a-Si) thin film were prepared by radio-frequency (rf) magnetron sputtering. Each thin film was electrochemically evaluated by cyclic voltammetry (CV) and galvanostatic cycling. The rate of capacity fade on cycling was monitored as a function of the voltage window and current density. This was compared with the cycle performance of cathode and anode using two kinds of electrolyte, 1 M LiPF₆ in EC/DMC and PC, for 100 cycles. It was found that the discharge capacity of optimized LiMn₂O₄/a-Si full-cell reached 24 μAh/(cm²-μm) in the first cycle, and a reversible capacity of about 16 μAh/(cm² μm) was still maintained after 100 cycles. In a voltage window of 3.0–4.2 V, LiMn₂O₄/a-Si full-cell exhibits relatively stable cycle performance compared to a voltage window of 2.75–4.2 V.     

Capacity fading with oxygen loss for manganese spinels upon cycling at elevated temperatures

The nominal LiMn₂O₄ and Li-doped spinels with different oxygen stoichiometry were prepared and investigated for capacity fading upon cycling at elevated temperatures. The discharge plateau at 3.2 V originating from oxygen defects in manganese spinels is observed to grow very quickly to nearly a maximum scale in initial 15 cycles at 60 °C. Meanwhile, the majority of capacity fading is lost. Therefore, the quick capacity fading in the initial stage is associated with the increase of oxygen deficiencies or oxygen loss upon cycling. It is proposed that the oxygen loss is originated from the decomposition of instable spinel phases that containing little Li cations on the 8a sites ([□₁]_8a[Mn_2−x]_16d[O_4−δ□_δ]_32e, etc.), which are formed upon charging to the upper voltage limit. This phenomenon is much severe for nominal LiMn₂O₄ spinels with oxygen deficiencies. After partial substitution of Mn with Li, part of the Li cations on the 8a sites will be retained upon charging to the upper voltage limit. Thereafter, the cycling performance can be improved for the stabilized spinel phases formed upon charging.     

On the reversibility of hydrogen storage in Ti- and Nb-catalyzed Ca(BH4)2

The hydrogen sorption properties of calcium borohydride (Ca(BH₄)₂) catalyzed with a small amount of TiF₃, TiCl₃, NbF₅ or NbCl₅ are investigated using thermal analyses and X-ray diffraction. NbF₅ exhibits the best performance among all the catalysts; it causes a decrease in the hydrogen desorption temperature which leads to hydrogen absorption at practical temperature and pressure conditions. The hydrogen content of Ca(BH₄)₂ with NbF₅ reaches about 5.0 wt.% after hydrogen absorption at 693 K for 24 h under 90 bar of hydrogen. The main dehydrogenation product of Ca(BH₄)₂ with NbF₅ is a CaH_2−xF_x solid solution with a CaF₂ (C1) structure, while pure Ca(BH₄)₂ produces CaH₂ after hydrogen desorption.     

Characterization of Cu(InxGa1−x)2Se3.5 thin films prepared by rf sputtering

Cu(In_xGa_1−x)₂Se_3.5 thin films were fabricated by rf sputtering from CuIn_xGa_1−xSe₂ and Na mixture target by controlling the mixture ratio. X-ray diffraction analyses show that the structure of Cu(In_xGa_1−x)₂Se_3.5, thin films is different from chalcopyrite structure: especially, CuIn₂Se_3.5 thin films have a defect chalcopyrite structure. The lattice parameters for Cu(In_xGa_1−x)₂Se_3.5 thin film are slightly smaller than those for CuIn_xGa_1−xSe₂ thin film and linearly decreased with increasing Ga content. The optical absorption coefficients for Cu(In_xGa_1−x)₂Se_3.5, thin films exceed 2 × 10⁴ cm⁻¹ in energy region above the fundamental band edge. The band gap for Cu(In_xGa_1−x)₂Se_3.5 thin films is larger than that for CuIn.Ga_1−x2Se₂ with the same Ga content and increased with increasing Ga content.     

Synthesis and characterization of osmium carbonyl cluster compounds with molecular oxygen electroreduction capacity

A transition metal cluster electrocatalyst based on Os_x(CO)_n was synthesized by pyrolysis of Os₃(CO)₁₂ in 1,2-Dichlorobenzene (b.p.≈180°C) under inert atmosphere (N₂). The electrocatalytic parameters of the oxygen reduction reaction (ORR) for an Os_x(CO)_n catalyst were studied with a rotating disk electrode in 0.5 MH₂SO₄ electrolyte. The diffusion coefficient and solubility of O₂ in 0.5 MH₂SO₄ were calculated. Koutecky–Levich analysis of the linear voltamperometry data showed that the reaction follows first-order kinetics and the value of the Koutecky–Levich slope indicates a multielectron charge transfer during the ORR. The value of the Tafel slope obtained from the mass transfer corrected Tafel plots is 131 mV/decade. The performance of the catalyst in a H₂/O₂ PEM fuel cell cathode was evaluated and found to be nearly as good as that of Pt.     

Application of hydrogen-storage alloy electrode in electrochemical reduction of glucose

Electrolysis of glucose was performed at constant potential using hydrogen-storage alloy as catalytic reduction electrode. A superficial treatment and activation of the hydrogen-storage alloy powder had an obvious effect on improving the current efficiency for the generation of sorbitol. The most favorable results were obtained when a current density of 8 mA cm⁻², a voltage of 4.0–5.0 V and a pH of 8 at a temperature of 30 °C are used. Prior to use the electrode was treated for 2 h at 80 °C with a solution containing KBH₄ (0.5 mol/l), KF (0.5 mol/l) and KOH (6 mol/l). Under optimized conditions the current efficiency for sorbitol by electrolyzing glucose exceeded 90%. The yield of sorbitol was around 80%. The lifetime of the electrode was tested and analyzed, and methods of regeneration are discussed.     

Effect of substituting Mm with La on the electrochemical performances of Co-free LaxMm1−x(NiMnSiAlFe)4.9 (x = 0–1) electrode alloys prepared by casting and rapid quenching

In order to enhance the electrochemical capacity of the Co-free AB₅-type electrode alloy, Mm in the alloys was substituted with La and Co-free La_xMm_1−x(NiMnSiAlFe)_4.9 (x = 0, 0.45, 0.75, 1.0) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of the substituting Mm with La on the electrochemical performances of the as-cast and quenched alloys were investigated in detail. The obtained results show that substituting Mm with La can enhance markedly the capacities of the as-cast and quenched alloys. When the amount of substituting Mm with La, x increased from 0 to 1.0, the maximum capacity of the as-cast alloys at 0.2C rate increased from 273.45 to 304.47 mAh g⁻¹, and the capacity retaining rate (R_h) increased from 59.16 to 59.86%. The capacity of the as-quenched alloys with a quenching rate of 10 m s⁻¹ increased from 236.83 to 300.31 mAh g⁻¹, and the capacity retaining rate (R_h) decreased from 78.69 to 62.29%. The substituting Mm with La had an insignificant effect on the activation capabilities of the as-cast and quenched alloys.     

Photooxidation of aqueous ammonia with titania-based heterogeneous catalysts

The photoassisted oxidation of dilute aqueous ammonia (0–100 μM) to NO₂⁻ and NO₃⁻ was investigated over heterogeneous catalysts of TiO₂ doped with iron oxide in quartz vessels and with radiation from mercury lamps. Effects on the reaction rate of varying concentration, radiation intensity, amount of catalyst, and pH were studied. The rate of ammonia consumption increased with radiation intensity and was independent of the amount of catalyst over the range of 0.1–1.0 g suspended in 50 ml of solution. In acidic solutions (pH 1–6), the simultaneous action of catalyst and radiation was needed, while in strongly basic solutions (pH 12) only illumination was required. Moreover, the nature and yields of products were dependent on pH: in acidic solutions NO₃⁻ was the main product and in basic solutions NO₂⁻ was the primary one. Control experiments showed that the reaction rate increased with pH in the absence of catalyst, and the catalyst was active only when irradiated. The reaction was first-order in NH₃. The chemical reaction and the mechanism of the photooxidation of NH₃ are discussed.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Preparation and properties of transparent conducting zinc oxide and aluminium-doped zinc oxide films prepared by evaporating method

Undoped and aluminium-doped zinc oxide films have been prepared by thermal evaporation of zinc acetate [Zn(CH₃COO)₂ 2H₂O] and aluminium chloride [AlCl₃] onto a heated glass substrate. The structural and optoelectrical properties of the films have been studied. The effects of heat treatment for the as-deposited films in air and vaccum are investigated. Highly transparent films with conductivity as low as 2×10⁻³ Ω cm can be produced by controlling the deposition parameters. The electron carrier densities are in the range 0.2–7×10¹⁹ cm⁻³ with mobilities of 22–58 cm² V⁻¹ s⁻¹.     

Large area CIGS2 thin film solar cells on foils: nucleus of a pilot plant

In earlier research, conversion efficiency of 10.4% (AM1.5) and 9.9% (AM0) has been achieved on small area CuIn_xGa_1−xS₂ (CIGS2) solar cell on 127 μm thick stainless steel substrate. The area of research is mainly focused on studying CIGS2 thin films as solar cell absorber material and growing high efficiency cells on ultralightweight and flexible metallic foils such as 127 μm thick stainless steel and SiO₂ coated 25 μm thick Ti foils. This paper presents the scaling up process of CIGS2 thin film substrate from 2.5 × 2.5 cm² to 10 × 10 cm². Initial scaling up efforts focused on achieving uniform thickness and stress-free films. Process of scaling up consisted of refurbishment of selenization/sulfurization furnace, design and fabrication of scrubber and enlargement of new CdS deposition setup. The scaling up from 2.5 × 2.5 cm² to 10 × 10 cm² substrate size has laid the foundation for PV Materials Lab of Florida Solar Energy Center becoming the nucleus of a pilot plant.     

Performance of a direct methanol fuel cell (DMFC) at low temperature: Cathode optimization

The effects of Pt loading, Nafion content in the cathode and membrane–electrode assembly (MEA) preparation techniques (CCS_cathode/CCS_anode and CCM_cathode/CCS_anode) on the performance of MEAs for direct methanol fuel cells (DMFC) were studied. The MEA performance was analyzed with polarization curves, electrochemical impedance spectroscopy and scanning electron microscopy data. It was shown, that the cathode prepared by the catalyst coated membrane (CCM) method forms a mainly microporous and mesoporous structure, whereas the catalyst coated substrate (CCS) method generates macroporosity together with micropores and mesopores. The power density of the CCM_cathode/CCS_anode typed MEAs strongly depends on the CCM-cathode composition: Pt loading and Nafion content in the cathode. Nafion (10.7 wt.%) was found to be an optimum for DMFC performance, and at this composition, the power density gradually increased with the Pt loading up to 6.0 mg cm⁻². At higher Nafion contents, a significant mass transfer limitation at high Pt loadings occurs. Comparing the CCM and CCS methods of the cathode fabrication, the latter revealed a higher power density, which reached 104 mW cm⁻² at 0.4 V and 70 °C owing to the lack of significant mass transfer limitations. This behavior indicates that in addition to Pt loading and Nafion content, the cathode pore structure is critical to DMFC MEA performance.     

Oxygen reduction on platinum electrode coated with Nafion

The kinetics of the reduction of oxygen on platinum covered by a Nafion^® film in sulfuric acid (0.5 M) has been studied in order to determine to what extent the solid polymer electrolyte modifies this reaction. As electrode we used a rotating electrode which is particularly well adapted to the measurement of the permeability D_fC_f (product of the diffusion coefficient and of the oxygen concentration in the film) of oxygen in the Nafion^® film. This product is of the order of 6×10⁻¹² M cm⁻¹ s⁻¹ whatever the state of division of the platinum, and is of the same order of magnitude as the permeability D_sC_s of oxygen in the adjacent sulfuric acid solution. It is shown, moreover, that the oxygen concentration in the film is very high, about five times that in the solution.     

Preparation and properties of LiCoyMnxNi1−x−yO2 as a cathode for lithium ion batteries

The preparation of LiCo_yMn_xNi_1−x−yO₂ from LiOH·H₂O, Ni(OH)₂ and γ-MnOOH in air was studied in detail. Single-phase LiCo_yMn_xNi_1−x−yO₂ (0y0.3 and x=0.2) is obtained by heating at 830–900°C. The optimum heating temperatures are 850°C for y=0–0.1 and 900°C for y=0.2–0.3. Excess lithium (1z1.11 for y=0.2) and the Co doping level (0.05y0.2) do not significantly affect the discharge capacity of Li_zCo_yMn_0.2Ni_0.8−yO₂. The doping of Co into LiMn_0.2Ni_0.8O₂ accelerates the oxidation of the transition metal ion, and suppresses partial cation mixing. Since the valence of the manganese ion in LiMn_0.2Ni_0.8O₂ is determined to be 4, the formation of a solid solution between LiCo_yNi_1−yO₂ and Li₂MnO₃ is confirmed.     

Effects of catalysts doping on the thermal decomposition behavior of Zn(BH4)2

Transition metal complex borohydrides, Zn(BH₄)₂ have been synthesized by solid-state mechanochemical process. Various catalysts such as TiCl₃, TiF₃, nanoNi, nanoFe, Ti, nanoTi, and Zn were used to dope the borohydride in order to lower the decomposition temperature in the range of , without a significant reduction in the hydrogen content per total weight of the sample. In addition, the structural, bonding and thermal characteristics of undoped and catalysts doped Zn(BH₄)₂ were compared and analyzed to find out the optimum catalyst and dopant concentration. Among the different catalysts, 1.5 mol% of nanoNi obtained from Quantum Sphere Inc. was found to possess the optimum behavior in terms of fast kinetics and lowering the melting and decomposition temperature of Zn(BH₄)₂.