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.     

Enhanced photocatalytic hydrogen evolution under visible light over Cd1−xZnxS solid solution with cubic zinc blend phase

A series of Cd_1−xZn_xS solid solutions were synthesized at 80 °C with the assistance of sodium dodecylsulfate. The structures, optical properties and morphologies of the solid solutions have been studied by X-ray diffraction, UV–vis diffuse reflectance spectroscopy, and transmission electron microscopy. The photocatalytic H₂ evolution over the solid solutions under visible-light irradiation was investigated and the highest rate reached 2640 μmol h⁻¹ g⁻¹ even without any co-catalysts. The solid solution with optimum performance exhibited cubic structure rather than previously-reported hexagonal one and the possible reasons were discussed. Moreover, the effects of sacrificial reagents on the photocatalytic H₂ evolution were explored by using Na₂S solution with different concentration.     

Enhancement of hydrogen production by the filamentous non-heterocystous cyanobacterium Arthrospira sp. PCC 8005

The efficiency of hydrogen production by different cyanobacterial species depends on several external factors. We report here the factors enhancing hydrogen production by filamentous non-heterocystous cyanobacterium Arthrospira sp. PCC 8005. Cells adapted to dark-anaerobic conditions produced hydrogen consistent with increased hydrogenase activity when supplemented with Fe²⁺. Stimulation of hydrogen production could be achieved by addition of reductants, either dithiothreitol or β-mercaptoethanol with higher production observed with the latter. Additionally, Fe²⁺ and β-mercaptoethanol added to nitrogen- and sulphur-deprived cells significantly stimulated H₂ production with maximal value of 5.91 ± 0.14 μmol H₂ mg Chla⁻¹ h⁻¹. Glucose and a small increase of osmolality imposed by either NaCl or sorbitol enhanced hydrogen production. High rates of hydrogen production were obtained in cells adapted in nitrogen-deprived medium with neutral and alkaline external pH, significant decrease of hydrogen production occurred under acidic external pH.     

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.     

Hydrogen production and end-product synthesis patterns by Clostridium termitidis strain CT1112 in batch fermentation cultures with cellobiose or α-cellulose

Hydrogen (H₂) production and end-product synthesis were characterized in a novel, mesophilic, cellulolytic, anaerobic bacterium, Clostridium termitidis strain CT1112, isolated from the gut of the termite, Nasutitermes lujae. Growth curves, pH patterns, protein content, organic acid synthesis, and H₂ production were determined. When grown on 2 g l⁻¹ cellobiose and 2 g l⁻¹ α-cellulose, C. termitidis displayed a cell generation time of 6.5 h and 18.9 h, respectively. The major end-products synthesized on cellobiose included acetate, hydrogen, CO₂, lactate, formate and ethanol, where as on cellulose, the major end-products included hydrogen, acetate, CO₂ and ethanol. The concentrations of acetate were greater than ethanol, formate and lactate on both cellobiose and α-cellulose throughout the entire growth phase. Maximum yields of acetate, ethanol, hydrogen and formate on cellobiose were 5.9, 3.7, 4.6 and 4.2 mmol l⁻¹ culture, respectively, where as on cellulose, the yields were 7.2, 3.1, 7.7 and 2.9 mmol l⁻¹ culture, respectively. Hydrogen and ethanol production rates were slightly higher in C. termitidis cultured on cellobiose when compared to α-cellulose. Although, the generation time on α-cellulose was longer than on cellobiose, H₂ production was favored corresponding to acetate synthesis, thereby restricting the carbon flowing to ethanol. During log phase, H₂, CO₂ and ethanol were produced at specific rates of 4.28, 5.32, and 2.99 mmol h⁻¹ g dry weight⁻¹ of cells on cellobiose and 2.79, 2.59, and 1.1 mmol h⁻¹ g dry weight⁻¹ of cells on α-cellulose, respectively.     

Prontonic ceramic membrane fuel cells with layered GdBaCo2O5+x cathode prepared by gel-casting and suspension spray

In order to develop a simple and cost-effective route to fabricate protonic ceramic membrane fuel cells (PCMFCs) with layered GdBaCo₂O_5+x (GBCO) cathode, a dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte was fabricated on a porous anode by gel-casting and suspension spray. The porous NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (NiO–BZCY7) anode was directly prepared from metal oxide (NiO, BaCO₃, ZrO₂, CeO₂ and Y₂O₃) by a simple gel-casting process. A suspension of BaZr_0.1Ce_0.7Y_0.2O_3−δ powders synthesized by gel-casting was then employed to deposit BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) thin layer by pressurized spray process on NiO–BZCY7 anode. The bi-layer with 10 μm dense BZCY7 electrolyte was obtained by co-sintering at 1400 °C for 5 h. With layered GBCO cathode synthesized by gel-casting on the bi-layer, single cells were assembled and tested with H₂ as fuel and the static air as oxidant. An open-circuit potential of 0.98 V, a maximum power density of 266 mW cm⁻², and a low polarization resistance of the electrodes of 0.16 Ω cm² was achieved at 700 °C.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C. Part 1: In the absence of humidity

The presence of CO in the H₂-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H₂ activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H₂-D₂ exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy (E_a) of H₂-D₂ exchange reaction (representing H₂ activation) from approximately 4.5-5.3 kcal mole⁻¹ (Bernasek and Somorjai (1975) [24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3-19.7 kcal mole⁻¹ (in the presence of 10-70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 °C in H₂ with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 °C in H₂ after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H₂-D₂ exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated.     

p-Cu2O/n-ZnO heterojunction applied to visible light Orange II degradation

The p-Cu₂O/n-ZnO system is studied for its potential use as a photoactive heterojunction able to highly perform under visible light. The main application deals with the degradation of organic dyes such as Orange II and the effects of Cu₂O amount, Orange II concentration and light intensity are investigated. Results show that the kinetics of degradation follows a pseudo-first order and the optimum sensitization effect is obtained using a 70% concentration of Cu₂O. The degradation rate reaches its maximum (R_initial = 22.45 × 10⁻² mg l⁻¹ min⁻¹) at 15 mg l⁻¹ of Orange II. The effect of the irradiation intensity is also investigated taking the electrical energy consumption per order of magnitude (EE/O) as a figure of merit. The highest efficiency is obtained at an irradiation intensity of ∼122 × 10⁻⁵ kW with k_OBS = 14.97 × 10⁻³ min⁻¹ and EE/O, which corresponds to 12.54 kW h m⁻³ of energy consumption. This heterojunction allows a ∼25% saving of electrical energy in comparison to the p-Cu₂O/n-TiO₂ system, demonstrating the important role of the collector.     

A pilot-scale study of biohydrogen production from distillery effluent using defined bacterial co-culture

We evaluated the feasibility of improving the scale of hydrogen (H₂) production from sugar cane distillery effluent using co-cultures of Citrobacter freundii 01, Enterobacter aerogenes E10 and Rhodopseudomonas palustris P2 at 100 m³ scale. The culture conditions at 100 ml and 2 L scales were optimized in minimal medium and we observed that the co-culture of the above three strains enhanced H₂ productivity significantly. Results at the 100 m³ scale revealed a maximum of 21.38 kg of H₂, corresponding to 10692.6 mol, which was obtained through batch method at 40 h from reducing sugar (3862.3 mol) as glucose. The average yield of H₂ was 2.76 mol mol⁻¹ glucose, and the rate of H₂ production was estimated as 0.53 kg/100 m³/h. Our results demonstrate the utility of distillery effluent as a source of clean alternative energy and provide insights into treatment for industrial exploitation.     

The effect of irradiance growing on hydrogen photoevolution and on the kinetic growth in Rhodopseudomonas palustris, strain 42OL

The relationship between hydrogen generation and the age of culture was investigated under fed-batch growth conditions. The specific growth rate (μ_e) was determined during the log phase of the growth curve and the μ_eMax was 0.02643 h⁻¹. Boltzmann's sigmoidal regression model was used to determine the specific rate of hydrogen evolution (μH): the maximum was 0.04440 h⁻¹. At low irradiance (36–75 W m⁻²), an inverse relationship was found between μH and I; after increasing the irradiance further, μH reached a plateau (0.00916 h⁻¹). The maximum reactor yield of cumulative hydrogen (4.5 l) was obtained at an irradiance of 320 W m⁻², but the highest hydrogen evolution rate (17.217 ml h⁻¹) was achieved at 500 W m⁻². The light conversion efficiency reached its maximum (6.91%) at the lowest irradiance investigated (36 W m⁻²); when the irradiance increased further, it decreased progressively down to 0.36%.     

Water–gas shift reaction in a Pd membrane reactor over Pt/Ce0.6Zr0.4O2 catalyst

A water–gas shift (WGS) Pt/Ce_0.6Zr_0.4O₂ catalyst has been prepared, which exhibits much faster kinetics than conventional high-temperature ferrochrome catalysts in the temperature range most suitable for operation of WGS Pd membrane reactors, i.e. above 623 K. The performance of the Pt catalyst was tested in a reactor furnished with a supported, 1.4 μm thick high-flux Pd membrane using feeds obtained by autothermal reforming of natural gas. CO conversion remained above thermodynamic equilibrium up to feed space velocities of 9100 l kg⁻¹ h⁻¹ at 623 K, P_total = 1.2 MPa and steam-to-carbon ratio S/C = 3, but H₂ recovery decreased from 84.8% at GHSV = 4050 l kg⁻¹ h⁻¹ to 48.7% at the highest space velocity. This rapid decline of separation performance is attributed to slow H₂ diffusion through the catalyst bed, suggesting that external mass flow resistance has a significant impact on the H₂ permeation rate in such membrane reactors. This could be minimized by the development of WGS catalysts with even faster kinetics which would allow further reduction of the catalyst bed height.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

The influence of the CO inhibition effect on the estimation of the H2 purification unit surface

The CO inhibition effect on H₂ permeance through commercial Pd-based membranes was analysed by means of permeation measurements at different CO compositions (0–30% molar) and temperatures (593–723 K) with the aim to determine the increase of the membrane area in order to compensate the H₂ flux reduction owing to the CO inhibition effect. The permeance of H₂ fed with carbon monoxide was observed to decrease with respect to the case of pure hydrogen. At 647 K the H₂ permeance of a pure feed of 316 μmol m⁻² s⁻¹ Pa^−0.5 reduces progressively until 275 μmol m⁻² s⁻¹ Pa^−0.5 when 15% or more of CO is present in the system, until it reaches a plateau at 20%. The inhibition effect occurring when CO is present in the feed stream reduces with the progressive temperature increase; the reduction of the permeance decreases exponentially by 23% at 593 K and by 3% at 723 K with 10% of CO. The inhibition effect is seen to be reversible. An H₂ flux profile in a Sieverts' plot shows the effect produced by the increase of the CO composition along the Pd-based membrane length. The H₂ flux profile allows the area of a Pd-based membrane to be evaluated in order to have the same permeate flow rate of H₂ when it is fed with CO or as a pure stream. Moreover, a qualitative comparison between the H₂ flux profiles and a previously proposed model has been carried out.     

Synthesis and characterization of new ternary transition metal sulfide anodes for H2S-powered solid oxide fuel cell

A number of ternary transition metal sulfides with general composition AB₂S₄ (where A and B are different transition metal atoms) have been prepared and investigated as potential anode catalysts for use in H₂S-powered solid oxide fuel cells (SOFCs). For the initial screening, polarization resistance of the materials was measured in a two electrode symmetrical cell at 700–850 °C. Vanadium-based materials showed the lowest polarization resistance, and so were chosen for subsequent full cell tests using the configuration [H₂S, AV₂S₄/YSZ/Pt, air] (where A = Ni, Cr, Mo). MoV₂S₄ anode had superior activity and performance in the full cell setup, consistent with results from symmetrical cell tests. Polarization curves showed MoV₂S₄ had the lowest potential drop, with up to a 200 mA cm⁻² current density at 800 °C. The highest power density of ca. 275 mW cm⁻² at 800 °C was obtained with a pure H₂S stream. Polarization resistance of materials was a strong function of current density, and showed a sharp change of slope attributable to a change in the rate-limiting step of the anode reaction mechanism. MoV₂S₄ was chemically stable during prolonged (10 days) exposure to H₂S at 850 °C, and fuel cell performance was stable during continuous 3-day operation at 370 mA cm⁻² current density.     

Fabrication and characterization of a co-fired La0.6Sr0.4Co0.2Fe0.8O3−δ cathode-supported Ce0.9Gd0.1O1.95 thin-film for IT-SOFCs

A dense membrane of Ce_0.9Gd_0.1O_1.95 on a porous cathode based on a mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was fabricated via a slurry coating/co-firing process. With the purpose of matching of shrinkage between the support cathode and the supported membrane, nano-Ce_0.9Gd_0.1O_1.95 powder with specific surface area of 30 m² g⁻¹ was synthesized by a newly devised coprecipitation to make the low-temperature sinterable electrolyte, whereas 39 m² g⁻¹ nano-Ce_0.9Gd_0.1O_1.95 prepared from citrate method was added to the cathode to favor the shrinkage for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ. Bi-layers consisting of <20 μm dense ceria film on 2 mm thick porous cathode were successfully fabricated at 1200 °C. This was followed by co-firing with NiO–Ce_0.9Gd_0.1O_1.95 at 1100 °C to form a thin, porous, and well-adherent anode. The laboratory-sized cathode-supported cell was shown to operate below 600 °C, and the maximum power density obtained was 35 mW cm⁻² at 550 °C, 60 mW cm⁻² at 600 °C.     

Co-doped Sr2FeNbO6 as cathode materials for intermediate-temperature solid oxide fuel cells

Sr₂Fe_1−xCo_xNbO₆ (0.1 ≤ x ≤ 0.9) (SFCN) oxides with perovskite structure have been developed as the cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). These materials are synthesized via solid-state reaction and characterized by XRD, SEM, electrical conductivity, AC impedance spectroscopy and DC polarization measurements. The reactivity tests show that the Sr₂Fe_1−xCo_xNbO₆ electrodes are chemically compatible with the Zr_0.85Y_0.15O_1.925 (YSZ) and Ce_1.9Gd_0.1O_1.95 (CGO) electrolytes at 1200 °C, and the electrode forms a good contact with the electrolyte after sintering at 1200 °C for 12 h. The total electrical conductivity that has a considerable effect on the electrode properties is determined in a temperature range from 200 °C to 800 °C. The highest conductivity of 5.7 S cm⁻¹ is found for Sr₂Fe_0.1Co_0.9NbO₆ at 800 °C in air. The electrochemical performances of these cathode materials are studied using impedance spectroscopy at various temperatures and oxygen partial pressures. Two different kinds of reaction rate-limiting steps exist on the Sr₂Fe_0.1Co_0.9NbO₆ electrode, depending on the temperature. The Sr₂Fe_0.1Co_0.9NbO₆ electrode on CGO electrolyte exhibits a polarization resistance of 0.74 Ω cm² at 750 °C in air, which indicates that the Sr₂Fe_0.1Co_0.9NbO₆ electrode is a promising cathode material for IT-SOFCs.     

Synthesis and electrochemical properties of Li1−xFe0.8Ni0.2O2–LixMnO2 (Mn/(Fe + Ni + Mn) = 0.8) material

A new type of Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ (Mn/(Fe + Ni + Mn) = 0.8) material was synthesized at 350 °C in air atmosphere using a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−xFeO₂–Li_xMnO₂ (Mn/(Fe + Mn) = 0.8) with much reduced impurity peaks. The Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell showed a high initial discharge capacity above 192 mAh g⁻¹, which was higher than that of the parent Li/Li_1−xFeO₂–Li_xMnO₂ (186 mAh g⁻¹). We expected that the increase of initial discharge capacity and the change of shape of discharge curve for the Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell is the result from the redox reaction from Ni²⁺ to Ni³⁺ during charge/discharge process. This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles.     

La substituted Sr2MnO4 as a possible cathode material in SOFC

Sr_2−xLa_xMnO_4+δ (x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr_2−xLa_xMnO_4+δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm⁻¹ was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr_1.4La_0.6MnO_4+δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm² at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr_1.4La_0.6MnO_4+δ electrode.     

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.