Physical and photo-electrochemical characterizations of α-Fe2O3. Application for hydrogen production

The optical, electrical and photo-electrochemical properties of dense hematite α-Fe₂O₃ have been studied for the photo-catalytic hydrogen production. The band gap was evaluated at 1.96 eV from the diffuse reflectance spectrum and the transition is directly allowed; further indirect transition occurs at 2.04 eV. The oxygen deficiency permits the altering of the transport properties and the oxide exhibits n type behavior with activation energy of 0.11 eV. α-Fe₂O₃ is found to be photo-electrochemically active. The flat band potential V_fb (−0.51 V_SCE) and the density N_D (19.12 × 10¹⁹ cm⁻³) were obtained respectively by extrapolating the linear part to C⁻² = 0 and the slope of the Mott–Schottky plot. The complex impedance pattern is circular in the high frequency region followed by a straight line in the low frequency one, a behavior attributed to the Warburg ionic diffusion. The conduction band edge (−0.62 V_SCE) lies below the H₂O/H₂ level (−0.50 V_SCE) and Fe₂O₃ offers the possibility to be used as hydrogen photocathode. The best activity was obtained in SO₃²⁻ (0.5 M, pH 13.8) solution with a rate evolution of 6 ml (g catalyst)⁻¹ min⁻¹.     

Physical and photoelectrochemical studies for hydrogen photo-evolution over the spinel ZnCr2O4

The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr₂O₄. The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V_SCE and an exchange current density of 50 μA cm⁻² have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be −0.39 V_SCE. ZnCr₂O₄/S₂O₃²⁻ is a self driven system where absorption of light promotes electrons into the conduction band with a potential (−1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S₂O₆²⁻ and S₂O₄²⁻ with water. A comparative study with CuCr₂O₄ is reported.     

Characterization of new heterosystem CuFeO2/SnO2 application to visible-light induced hydrogen evolution

Photo-assisted H₂ evolution has been realized over the new heterosystem CuFeO₂/SnO₂ without any noble metal and was studied in connection with some physical parameters. The delafossite CuFeO₂ has been prepared by thermal decomposition from various salts. The polarity of generated voltage is positive indicating that the materials exhibit p-type conductivity whereas the electroneutrality is achieved by oxygen insertion. The plot of the logarithm (conductivity) vs. T⁻¹ gives average activation energy of 0.12 eV. CuFeO₂ is a narrow band gap semiconductor with an optical gap of 1.32 eV. The oxide was characterized photoelectrochemically; its conduction band (−1.09 V_RHE) is located below that of SnO₂ (−0.86 V_RHE) at pH ∼13.5 itself more negative than the H₂O/H₂ level leading to a thermodynamically favorable H₂ evolution under visible irradiation. The sensitizer CuFeO₂, working as an electron pump, is stable towards photocorrosion by hole consumption reactions involving the reducing agents X²⁻ (=S₂O₃²⁻ and SO₃²⁻). The photoactivity was dependent on the precursor and the best performance (0.026 ml h⁻¹ mg⁻¹) was obtained in S₂O₃²⁻ (pH ∼13.5) over CuFeO₂ synthesized from nitrate with a mass ratio (CuFeO₂/SnO₂) equal to unity. A quantum yield of 0.5% was obtained under polychromatic light. H₂ liberation occurs concomitantly with the oxidation of S₂O₃²⁻ to dithionate and sulfate. The tendency towards saturation, in a closed system, is mainly ascribed to the competitive reduction of the end product S₂O₆²⁻.     

Photo-catalytic hydrogen production over Fe2O3 based catalysts

The hydrogen photo-evolution was successfully achieved in aqueous (Fe_1−xCr_x)₂O₃ suspensions (0 ≤ x ≤ 1). The solid solution has been prepared by incipient wetness impregnation and characterized by X-ray diffraction, BET, transport properties and photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-type conductivity with activation energy of ∼0.1 eV and the conduction occurs via adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is ∼0.2 V cathodic with respect to the flat band potential, implying a small existence of surface states within the gap region. The absorption of visible light promotes electrons into (Fe_1−xCr_x)₂O₃-CB with a potential (∼−0.5 V_SCE) sufficient to reduce water into hydrogen. As expected, the quantum yield increases with decreasing the electro affinity through the substitution of iron by the more electropositive chromium which increases the band bending at the interface and favours the charge separation. The generated photo-voltage was sufficient to promote simultaneously H₂O reduction and SO₃²⁻ oxidation in the energetically downhill reaction (H₂O + SO₃²⁻ → H₂ + SO₄²⁻, ΔG = −17.68 kJ mol⁻¹). The best activity occurs over Fe_1.2Cr_0.8O₃ in SO₃²⁻ (0.1 M) solution with H₂ liberation rate of 21.7 μmol g⁻¹ min⁻¹ and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S₂O₆²⁻.     

Relaxor ferroelectric and photocatalytic behaviour of Ba0.785Bi0.127Y0.017TiO3 composition

The composition of Ba_0.785Bi_0.127Y_0.017TiO₃ (BaBiYTiO) belongs to the BaTiO₃-Bi₂O₃-Y₂O₃ system. A dielectric study on ceramics performed at temperatures in the range (77 K-500 K) and frequency (10²-2 × 10⁵ Hz) revealed ferroelectric relaxor behaviour with a phase transition close to room temperature. The conductivity of BaBiYTiO obeys to an Arrhenius-type law with activation energy equals to 0.21 eV, in conformity with a small polaron hopping where most electrons are localized. The oxide is lightly doped leading to a wide space charge region (480 nm) in which photoeffect occurs. The presence of domains promotes the separation of the charge carriers through conversion of light into chemical energy. The electrode acquired n-type behaviour, evidenced from the negative thermopower and anodic photocurrent. The flat band potential V_fb (−0.46V_SCE) and the electron density N_D (2.82 × 10¹⁵ cm⁻³) were determined in KOH solutions (0.5 M). The Nyquist plot exhibits two well defined time constants characteristic of bulk and grains boundaries contributions and an equivalent electrical circuit has been proposed according to the Randles model. The energy band diagram shows the potentiality of the oxide for the solar-energy conversion. BaBiYTiO has been tested successfully for H₂ production upon visible light when combined to the delafossite CuFeO₂ as sensitizer. An evolution rate of 24 μmol mn⁻¹ and a quantum yield of 0.4% under polychromatic light were obtained.     

A novel one-step electrodeposition to prepare single-phase CuInS2 thin films for solar cells

Copper indium disulfide (CuInS₂) thin films have been successfully prepared on Ni substrates using a novel one-step potentiostatic electrodeposition combined with a potassium hydrogen phthalate (C₈H₅KO₄) complexing agent, accompanied by annealing at 350 °C. Electrodeposition in the solution of Cu and In salts and sodium thiosulfate (Na₂S₂O₃) containing an adequate concentration of C₈H₅KO₄ (e.g., [C₈H₅KO₄]=23 mM) provides thin films comprised of a CuInS₂ single phase as the bulk composition, without forming Cu_xS secondary phases. In addition to the effect on bulk-phase compositions, the adjustment of [C₈H₅KO₄] causes variation in morphology and atomic composition of the film surface. The surface states of the films change from the Cu-rich rough surface at low [C₈H₅KO₄] (15 mM) to the In-rich smooth surface at high [C₈H₅KO₄] (23 mM). The higher [C₈H₅KO₄] induces the grains constructing the film to interconnect and form a densely packed CuInS₂ film without voids and pinholes. The single-phase and void-free CuInS₂ film shows a band gap of 1.54 eV, satisfying the requirement of the absorber layers in solar cells. The electrical properties tests denote its n-type conductivity with a resistivity of 9.6×10⁻⁵ Ω cm, a carrier concentration of 2.9×10²¹ cm⁻³ and a carrier mobility of 22.2 cm² V⁻¹ s⁻¹.     

Correlation between electrical properties and microstructure of nanocrystallized V2O5-P2O5 glasses

It was shown that by thermal nanocrystallization of a 90V₂O₅·10P₂O₅ glass one can obtain a novel nanomaterial exhibiting enhanced electronic conductivity. Using a combination of methods: DTA, SEM, XRD and impedance spectroscopy (IS), it was possible to find correlation between microstructure and electrical properties of the obtained material and to optimize conditions of its synthesis. The room temperature electronic conductivity of the nanocrystallized samples is σ₂₅ = 2 × 10⁻³ S cm⁻¹ and is by a factor of 25 higher than the conductivity of the as-received glass. The nanocrystallized material is thermally stable up to ca 400 °C, which is about 150 °C above the glass transition temperature of the original glass. Maximum electronic conductivity of the thermally treated samples reaches 2 × 10⁻¹ S cm⁻¹ at ca 400 °C. The activation energy for these samples (0.28 eV) are substantially lower than that found for the starting glass (0.34 eV). The experimental results were discussed in terms of a model proposed in this paper and based on a “core-shell” concept. The results obtained here can be important for the progress in the search of novel nanocrystalline cathode materials for applications in Li-ion batteries.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Performance of LiNi0.5Mn1.5O4 prepared by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared through a solid-state reaction using various Ni precursors. The effect of precursors on the electrochemical performance of LiNi_0.5Mn_1.5O₄ was investigated. LiNi_0.5Mn_1.5O₄ made from Ni(NO₃)₂·6H₂O shows the best charge–discharge performance. The reversible capacity of LiNi_0.5Mn_1.5O₄ is about 145 mAh g⁻¹ and remained 143 mAh g⁻¹ after 10 cycles at 3.0–5.0 V. The XRD results showed that the precursors and the dispersion methods had significant effect on their phase purity. Pure spinel phase can be obtained with high energy ball-milling method and Ni(NO₃)₂·6H₂O as precursor. Trace amount of NiO and Li₂MnO₃ phase were detected in LiNi_0.5Mn_1.5O₄ with manual-mixture method and using Ni(CH₃COO)₂·6H₂O, NiO and Ni₂O₃ as precursors.     

Li3V2(PO4)3 cathode material synthesized by chemical reduction and lithiation method

The monoclinic-type Li₃V₂(PO₄)₃ cathode material was synthesized via calcining amorphous Li₃V₂(PO₄)₃ obtained by chemical reduction and lithiation of V₂O₅ using oxalic acid as reducer and lithium carbonate as lithium source in alcohol solution. The amorphous Li₃V₂(PO₄)₃ precursor was characterized by using TG–DSC and XPS. The results showed that the V⁵⁺ was reduced to V³⁺ by oxalic acid at ambient temperature and pressure. The prepared Li₃V₂(PO₄)₃ was characterized by XRD and SEM. The results indicated the Li₃V₂(PO₄)₃ powder had good crystallinity and mesoporous morphology with an average diameter of about 30 nm. The pure Li₃V₂(PO₄)₃ exhibits a stable discharge capacity of 130.08 mAh g⁻¹ at 0.1 C (14 mA g⁻¹).     

Improvement in electrochemical performance of V2O5 by Cu doping

Cu_0.04V₂O₅ was prepared by a precipitation method followed by heat treatment at 300 and 600 °C. The material prepared at 300 °C showed porous morphology, whereas that prepared at 600 °C was highly crystalline. X-ray diffraction, Raman scattering and Fourier transform infrared spectroscopy showed both materials exhibiting the same structure as that of V₂O₅, with a slight lattice expansion. X-ray absorption spectroscopy confirmed the presence of V⁴⁺ cations in Cu_0.04V₂O₅, which would increase the electronic conductivity of V₂O₅. Cu_0.04V₂O₅ showed better electrochemical performance than V₂O₅ because of its high electronic conductivity and good structural stability. The material prepared at 600 °C delivered a reversible discharge capacity over 160 mAh g⁻¹ after 60 cycles at a C rate of C/5.6. The material prepared at 300 °C showed good high-rate performance, which delivered a reversible capacity ∼100 mAh g⁻¹ when cycled at C/1.9. The discrepancy in the rate performance of Cu_0.04V₂O₅ was attributed to the morphology of materials.     

Photocatalytic water splitting with In-doped H2LaNb2O7 composite oxide semiconductors

The In-doped HLaNb₂O₇ oxide semiconductors synthesized by solid-state reaction followed by an ion-exchange reaction were found to be a novel composite photocatalyst system with enhanced activity for water splitting. Pt was incorporated in the interlayer of In-doped HLaNb₂O₇ by the stepwise intercalation reaction. The In-doped HLaNb₂O₇ powder samples were characterized with X-ray diffraction (XRD) and UV-vis diffuse reflectance spectrometry. The photocatalytic activities of Pt-loaded In-doped HLaNb₂O₇ and individual precursor materials were evaluated by H₂ evolution from aqueous CH₃OH solution under UV light irradiation. It was found that the composite In-doped HLaNb₂O₇ showed a higher H₂ evolution rate in comparison with individual materials. The hydrogen production activity of In-doped HLaNb₂O₇ was greatly enhanced by Pt co-incorporation. The In content in the In-doped HLaNb₂O₇ system was discussed in relation to the photophysical and photocatalytic properties. As In content equal 5 mol%, the HLaNb₂O₇:In/Pt showed a photocatalytic activity of 354 cm³ g⁻¹ hydrogen evolution in 10 vol% methanol solution under irradiation from a 100 W mercury lamp at 333 K for 3 h.     

Study of Pt electrode dissolution in H2O2-containing H2SO4 solution using an electrochemical quartz crystal microbalance

Pt electrode dissolution has been investigated using an electrochemical quartz crystal microbalance (EQCM) in H₂O₂-containing 0.5 mol dm⁻³ H₂SO₄. The Pt electrode weight-loss of ca. 0.4 μg cm⁻² is observed during nine potential sweeps between 0.01 and 1.36 V vs. RHE. In contrast, the Pt electrode weight-loss is negligible without H₂O₂ (<0.05 μg cm⁻²). To support the EQCM results, the weight-decrease amounts of a Pt disk electrode and amounts of Pt dissolved in the solutions were measured after similar successive potential cycles. As a result, these results agreed well with the EQCM results. Furthermore, the H₂O₂ concentration dependence of the Pt weight-decrease rate was assessed by successive potential steps. These EQCM data indicated that the increase in H₂O₂ accelerates the Pt dissolution. Based on these results, H₂O₂ is known to be a major factor contributing to the Pt dissolution.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

Study of the Li insertion into V2O5 films deposited by CVD onto various substrates

Vanadium oxide films were synthesised by chemical vapour deposition (CVD) from pure of triisopropoxyvanadium oxide (VO(OC₃H₇)₃) and oxygen as precursors. The influence of the substrate on the crystallinity of the vanadium oxide films was studied before and after annealing at 500 °C. On mica substrates, as-deposited film was composed of crystalline V₂O₅ as revealed by XRD. On Pt, Ti, stainless steel, glass and F-doped SnO₂ substrates, an annealing procedure was required to get V₂O₅. SEM investigations have clearly evidence V₂O₅ plates but the kinetics growth seems to be strongly dependent on the nature of the substrate. The insertion/extraction of Li⁺ into the host structure was investigated in 1 M LiClO₄-PC with annealed V₂O₅ films deposited on Ti, Pt and stainless steel substrates. The best electrochemical performances were obtained in the potential range 3.8–2.8 V versus Li/Li⁺ with V₂O₅ films deposited onto stainless steel substrate: the reversible capacity reaches after subsequent cycles was about 115 mAh g⁻¹ (rate C/23). In a wider potential range (between 3.8 and 2.2 V versus Li/Li⁺), V₂O₅ deposited onto Ti substrate exhibited the higher electrochemical performances (220 mAh g⁻¹ for a rate of C/23).     

Synthesis and characterization of Li3V(2 − 2x/3)Mgx(PO4)3/C cathode material for lithium-ion batteries

Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C (x = 0, 0.15, 0.30, 0.45) composites have been synthesized by the sol-gel assisted solid state method, using adipic acid C₆H₁₀O₄ (hexanedioic acid) as carbon source. The particle size of the composites is ∼1 μm. During the pyrolysis process, Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C network structure is formed. The effect of Mg²⁺ doped on the electrochemical properties of Li₃V₂(PO₄)₃/C positive materials has been studied. Li₃V_1.8Mg_0.30(PO₄)₃/C as the cathode materials of Li-ion batteries, the retention rate of discharge capacity is 91.4% (1 C) after 100 cycles. Compared with Li₃V₂(PO₄)₃/C, Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C composites have shown enhanced capacity and retention rate capability. The long-term cycles and ex situ XRD tests disclose that Li₃V_1.8Mg_0.30(PO₄)₃ exhibits higher structural stability than the undoped system.     

Basic molten salt process—A new route for synthesis of nanocrystalline Li4Ti5O12-TiO2 anode material for Li-ion batteries using eutectic mixture of LiNO3-LiOH-Li2O2

A nanocrystalline Li₄Ti₅O₁₂-TiO₂ duplex phase has been synthesized by a simple basic molten salt process (BMSP) using an eutectic mixture of LiNO₃-LiOH-Li₂O₂ at 400-500 °C. The microstructure and morphology of the Li₄Ti₅O₁₂-TiO₂ product are characterized by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The sample prepared by heat-treating at 300 °C for 3 h (S-1) reveals dense agglomerates of ultra-fine nanocrystalline Li₄Ti₅O₁₂; with heat treatment at 400 °C for 3 h (S-2), there is a duplex crystallite size (fine < 10 nm, and coarse > 20 nm) of Li₄Ti₅O₁₂-TiO₂; at 500 °C for 3 h (S-3), a much coarser and less-dense distribution of lithium titanate (crystallite size ∼15-30 nm) is observed. According to the results of electrochemical testing, the S-2 sample shows initial discharge capacities of 193 mAh g⁻¹ at 0.2 C, 168 mAh g⁻¹ at 0.5 C, 146 mAh g⁻¹ at 1 C, 135 mAh g⁻¹ at 2 C, and 117 mAh g⁻¹ at 5 C. After 100 cycles, the discharge capacity is 138 mAh g⁻¹ at 1 C with a capacity retention of 95%. The S-2 sample yields the best electrochemical performance in terms of charge-discharge capacity and rate capability compared with other samples. Its superior electrochemical performance can be mainly attributed to the duplex crystallite structure, composed of fine (<10 nm) and coarse (>20) nm nanoparticles, where lithium ions can be stored within the grain boundary interfaces between the spinel Li₄Ti₅O₁₂ and the anatase TiO₂.     

Electrochemical capacitance of NiO/Ru0.35V0.65O2 asymmetric electrochemical capacitor

A designed asymmetric hybrid electrochemical capacitor was presented where NiO and Ru_0.35V_0.65O₂ as the positive and negative electrode, respectively, both stored charge through reversible faradic pseudocapacitive reactions of the anions (OH⁻) with electroactive materials. And the two electrodes had been individually tested in 1 M KOH aqueous electrolyte to define the adequate balance of the active materials in the hybrid system as well as the working voltage of the capacitor based on them. The electrochemical tests demonstrated that the maximum specific capacitance and energy density of the asymmetric hybrid electrochemical capacitor were 102.6 F g⁻¹ and 41.2 Wh kg⁻¹, respectively, delivered at a current density of 7.5 A cm⁻². And the specific energy density decreased to 23.0 Wh kg⁻¹ when the specific power density increased up to 1416.7 W kg⁻¹. The hybrid electrochemical capacitor also exhibited a good electrochemical stability with 83.5% of the initial capacitance over consecutive 1500 cycle numbers.     

CoFe2O4 spinel protection coating thermally converted from the electroplated Co-Fe alloy for solid oxide fuel cell interconnect application

CoFe₂O₄ has been demonstrated as a potential spinel coating for protecting the Cr-containing ferritic interconnects. This spinel had an electrical conductivity of 0.85 S cm⁻¹ at 800 °C in air and an average coefficient of thermal expansion (CTE) of 11.80 × 10⁻⁶ K⁻¹ from room temperature to 800 °C. A series of Co-Fe alloys were co-deposited onto the Crofer 22 APU ferritic steel via electroplating with an acidic chloride solution. After thermal oxidation in air at 800 °C, a CoFe₂O₄ spinel layer was attained from the plated Co_0.40Fe_0.60 film. Furthermore, a channeled Crofer 22 APU interconnect electrodeposited with a 40-μm Co_0.40Fe_0.60 alloy film as a protective coating was evaluated in a single-cell configuration. The presence of the dense, Cr-free CoFe₂O₄ spinel layer was effective in blocking the Cr migration/transport and thus contributed to the improvement in cell performance stability.