A new route to control texture of materials: Nanostructured ZnFe2O4 photoelectrodes

Studies were conducted to investigate the influence of deposition solution composition (methanol ≤ the deposition solvent ≤ ethanol) on their physical and chemical properties that matters in the aerosol formation and subsequent decomposition during the aerosol assisted chemical vapour deposition (AACVD) of ZnFe₂O₄ electrodes. The FEGSEM studies found that the change of composition of deposition solution produced a dramatic change in the ZnFe₂O₄ electrode texture. The ZnFe₂O₄ electrodes deposited from methanol as well as predominately methanolic solvents had a relatively compact morphology. In contrast, the electrodes deposited from ethanol as well as predominately ethanolic solvents showed highly textured rod-like structure at nanoscale. The change in electrode texture is explained in terms of changes occurred in precursor decomposition pathways from heterogeneous and homogeneous when the composition of deposition solution is systematically varied. The photoelectrochemical (PEC) properties of all ZnFe₂O₄ electrodes were studied by recording J–V characteristics under AM1.5 illumination and the photocurrent spectra. The textured electrodes exhibited a significantly higher photocurrent compared to their compact counterparts. This is attributed to the improved photogenerated minority carrier collection at the ZnFe₂O₄/electrolyte interface as the average feature size gradually decreased. The photocurrent density (at 0.25 V vs. Ag/AgCl/3M KCl) increases rapidly when the electrode is deposited from the solvent containing 60% ethanol and above, which is in close agreement with the textural changes taken place in ZnFe₂O₄ electrodes.     

Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications

The photoelectrochemical (PEC) properties of heterostructured CdS/BiVO₄ and BiVO₄/CdS film electrodes on conducting glass for hydrogen production under visible light were investigated. These two types heterostructured film electrodes were prepared using spin coating method and ultrasonic spray pyrolysis method. The structural analyses of the prepared films were determined by using XRD, SEM, EDX and UV–vis. Photoelectrochemical measurements were carried out in a convenient three electrodes cell with 0.5 M Na₂SO₃ aqueous solution. In order to investigate band gap influence of electrode PEC property, a series ITO/Cd_1−xZn_xS/BiVO₄ and ITO/BiVO₄/Cd_1−xZn_xS (x = 0 ∼ 1) film electrodes were also synthesized. After PEC test, a maximum photocurrent density from ITO/CdS/BiVO₄ film electrode was confirmed. The maximum photocurrent density, 3 times and 113 times as that of single CdS film electrode and single BiVO₄ film electrode, respectively. Incident photon to current conversion (IPCE) of as prepared film electrodes were measured and the value were 65% (ITO/CdS/BiVO₄), 22% (single CdS film) and 10% (ITO/BiVO₄/CdS) at 480 nm with 0.3 V external bias. Comparison with ITO/BiVO₄/CdS electrode and single Cd_1−xZn_xS electrodes, the heterostructured ITO/CdS/BiVO₄ electrode can effectively suppress photogenerated electron-hole recombination and enhance light harvesting. Therefore, the ITO/CdS/BiVO₄ electrode gave the maximum photocurrent density and IPCE value.     

Relationships between processing,morphology and discharge capacity of the composite electrode

The cycled capacity of Li_1.1V₃O₈ based positive electrodes varies between 100 and 250 mAh g⁻¹ (C/5 rate, 3.3–2 V) depending on the processing parameters. The initial volatile solvent concentration has a strong impact on the distribution of the electrode constituents. For a concentration below the optimal one, the mechanical energy available for mixing is insufficient to overcome viscosity forces and to reach a good dispersion of the constituents in the bulk of the electrode. Above the optimal concentration, settling of the Li_1.1V₃O₈ and carbon black particles in the low viscosity suspensions creates a concentration gradient. In these two cases the electrochemical performance are degraded. The viscosity of the electrode slurry must be systematically adjusted since the grain size and density depend on the active material.     

Synthesis and evaluation of (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF)–Y0.08Zr0.92O1.96 (YSZ)–Gd0.1Ce0.9O2−δ (GDC) dual composite SOFC cathodes for high performance and durability

This paper investigates a (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O₃ (LSCF)–Y_0.16Zr_0.92O_1.96 (YSZ)–Gd_0.1Ce_0.9O_2−δ (GDC) dual composite cathode to achieve better cathodic performance compared to an LSM/GDC–YSZ dual composite cathode developed in previous research. To synthesize the structures of the LSCF/GDC–YSZ and LSCF/YSZ–GDC dual composite cathodes, nano-porous composite cathodes containing LSCF, YSZ, and GDC were prepared by a two-step polymerizable complex (PC) method which prevents the formation of YSZ–GDC solid solution. At 800 °C, the electrode polarization resistance of the LSCF/YSZ–GDC dual composite cathode showed to be significantly lower (0.075 Ω cm²) compared to that of a commercial LSCF–GDC cathode (0.195 Ω cm²), a synthesized LSCF/GDC–YSZ dual composite cathode (0.138 Ω cm²), and an LSM/GDC–YSZ dual composite cathode (0.266 Ω cm²) respectively. Moreover, the Ni–YSZ anode-supported single cell containing the LSCF/YSZ–GDC dual composite cathode achieved a maximum power density of 1.24 W/cm² and showed excellent durability without degradation under a load of 1.0 A/cm² over 570 h of operation at 800 °C.     

Regulating the hole transfer from CuWO4 photoanode to NiWO4 electrocatalyst for enhanced water oxidation performance

The electrocatalyst coupling with CuWO₄ has resulted in a comparable or worse performance when compared to the bare CuWO₄. This work attempts to address this challenge by coupling CuWO₄ with NiWO₄ electrocatalyst that can form a Type-II heterojunction with a suitable energy-level alignment allowing for effective hole transfer from CuWO₄ to NiWO₄ electrocatalyst. We applied thermal annealing to the WO₃ nanoplates by adding Cu(NO₃)₂ and Ni(NO₃)₂ precursors to obtain the CuWO₄/NiWO₄ composite with common anions. A high surface-to-volume ratio, perfect interface lattice match, suitable energy level alignment, and high electrocatalytic activity were exhibited in the composite. These characteristics led to a 100 mV negative shift on the onset potential compared to the pure CuWO₄ photoanode. Moreover, it featured a 0.7-fold higher photocurrent density than that of the pure CuWO₄ photoanode. Only 9% of photocurrent density decreased after 4 h of photo-irradiation, demonstrating excellent photostability. Our mechanism study demonstrated that NiWO₄ could act as a semiconductor to form a Type-II heterojunction with CuWO₄, promoting hole transfer from the CuWO₄ valence band to the NiWO₄. Meanwhile, the NiWO₄ effectively injects the separated holes into the water solution as a promising electrocatalyst, thus enhancing the overall water splitting performance. This work provides an important design consideration by focusing on the corrected level alignment and lattice match for developing the CuWO₄/electrocatalyst system to work effectively.     

Enhancing photoelectrochemical activity of nanocrystalline WO3 electrodes by surface tuning with Fe(Ⅲ)

Tungsten oxide (WO₃) photoelectrodes with the surface tuned by Fe(Ⅲ) for photoelectrochemical water splitting were successfully synthesized. Nanostructured WO₃ films were prepared using doctor blade method, then a facile and economical deposition-annealing process was employed to fabricate Fe(Ⅲ) modified WO₃ films. The resulting composite's structural and optical properties were analyzed by SEM, EDX, XRD, UV–Vis spectrometry and XPS. The photoelectrochemical properties were evaluated by photocurrent density under 500 W Xe lamp with an intensity of 100 mW/cm². The Fe(Ⅲ) modified WO₃ electrode exhibited a larger photocurrent than the pure WO₃ electrode. Significantly, the optimized Fe(Ⅲ) modified WO₃ film achieved the maximum photocurrent density of 1.18 mA/cm² at 0.8 V vs. Ag/AgCl in the 0.2 M Na₂SO₄. The enhanced photocurrent was attributed to the extension of the light response and the electron hole separation at the interface Fe(Ⅲ)/WO₃ which was confirmed by Mott–Schottky and electrochemical impedance spectroscopy.     

Preparation and electrochemical properties of strontium doped Pr2NiO4 cathode materials for intermediate-temperature solid oxide fuel cells

Pr_2−xSr_xNiO₄ (PSNO, x = 0.3, 0.5 and 0.8) cathode materials for intermediate-temperature solid oxide fuel cell (IT-SOFC) were synthesized by a glycine-nitrate process using Pr₆O₁₁, Ni(NO₃)₂·6H₂O and SrCO₃ powders as raw materials. Phase structure of the synthesized powders was characterized by X-ray diffraction analysis (XRD). Microstructure of the sintered PSNO samples was observed and thermal expansion coefficient (TEC) and electrical conductivity were investigated. Electrochemical impedance spectroscopy (EIS) measurement of the PSNO materials on Sm_0.2Ce_0.8O_1.9 (SCO) electrolyte was carried out, and single cells based on the PSNO cathodes were also assembled and their performances were tested. The results show that the synthesized PSNO powders have pure K₂NiF₄-type structure and the PSNO materials are chemically stable with Sm_0.2Ce_0.8O_1.9 (SCO) electrolyte. The sintered PSNO samples have porous and fine microstructure with pore size smaller than 1 μm. Average thermal expansion coefficient of the PSNO materials is about 12–13 × 10⁻⁶ K⁻¹ at 200–800 °C and the electrical conductivity is in the range of 70–120 Scm⁻¹ at 800 °C. Area specific resistance (ASR) of the Pr_2−xSr_xNiO₄ materials on SCO electrolyte is 0.407 Ωcm², 0.126 Ωcm² and 0.112 Ωcm² for x = 0.3, 0.5 and 0.8 at 800 °C, respectively. Maximum open circuit voltage (OCV) and power density of the single NiO-SCO/SCO/PSNO cells are 0.75 V and 298 mWcm⁻² at 700 °C, respectively, which indicates that Pr_2−xSr_xNiO₄ may be a potential cathode material for IT-SOFC.     

Hydrothermal synthesis of hydrated vanadium oxide nanobelts using poly (ethylene oxide) as a template

With the aim of obtaining nanodevices as batteries, sensors and fuel cells, we prepared V₂O₅ and V₃O₇·H₂O nanobelts by a simple hydrothermal process using poly (ethylene oxide) (PEO) as a template. The yielding percentage of the nanomaterial is less in polymer-free V₂O₅ nanobelts and material size is also big. It is apparent that PEO used V₃O₇·H₂O form a continuous and relatively homogeneous matrix with a clearly 1–5 μm long and 50–150 nm diameter nanobelts morphology. The SEM micrographs suggest that there is no bulk deposition of polymer on the surface of the nano-crystallites. Strong interaction between the vanadyl group and the polymer during the formation process has been identified by the shifts of the vanadyl vibration peaks. The CV curve of the electrode made of the V₃O₇·H₂O nanobelts have higher current densities than the CV curve of the electrode made of V₂O₅ nanobelts.     

Contribution of Nafion loading to the activity of catalysts and the performance of PEMFC

Different amounts of Nafion loadings (in the range of 0–2.0 mg cm⁻²) were added to a catalyst containing 0.5 mg cm⁻² of Pt; these were prepared by spraying a Nafion solution on an electrode surface. The effect of Nafion loading on the activity of the catalyst and the performance of a proton exchange membrane fuel cell (PEMFC) was investigated by using electrochemical methods such as direct current polarization (using an I–V curve), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear scan voltammetry (LSV). The results of the I–V and EIS were compared in order to resolve the ohmic resistance (R_Ω, calculated from the I–V curve) into interfacial and internal resistances (R_if and R_s, simulated from the EIS). The analysis of the electrochemical data revealed that the interfacial resistance (R_if) is closely related to the reactive region of three-phase zones (interfaces among the reactants, electrolyte and catalyst), and it provides a major parameter for diagnosing the activity of the catalysts and the performance of the PEMFC.     

The effect of preparation method on the catalytic performance over superior MgO-promoted Ni–Ce0.8Zr0.2O2 catalyst for CO2 reforming of CH4

The effect of preparation method on MgO-promoted Ni–Ce_0.8Zr_0.2O₂ catalysts was investigated in CO₂ reforming of CH₄. Co-precipitated Ni–MgO–Ce_0.8Zr_0.2O₂ exhibited very high activity as well as stability (X_CH4 > 95% at 800 °C for 200 h) due to high surface area, high dispersion of Ni, small Ni crystallite size, and easier reducibility. Four elements (Ni, Mg, Ce, and Zr) are located at the same position for the co-precipitated catalyst, resulting in easier reducibility.     

Electrochemical performance of La2Cu1−xCoxO4 cathode materials for intermediate-temperature SOFCs

K₂NiF₄-type structure oxides La₂Cu_1−xCo_xO₄ (x = 0.1, 0.2, 0.3) are synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. The results show that no reaction occurs between La₂Cu_1−xCo_xO₄ electrode and Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte at 1000 °C. The electrode forms good contact with the electrolyte after sintering at 800 °C for 4 h in air. The electrode properties of La₂Cu_1−xCo_xO₄ are studied under various temperatures and oxygen partial pressures. The optimum composition of La₂Cu_0.8Co_0.2O₄ results in 0.51 Ω cm² polarization resistance (R_p) at 700 °C in air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer process. La₂Cu_0.8Co_0.2O₄ cathode exhibits the lowest overpotential of about 50 mV at a current density of 48 mA cm⁻² at 700 °C in air.     

Characteristics of nano La0.6Sr0.4Co0.2Fe0.8O3−δ-infiltrated La0.8Sr0.2Ga0.8Mg0.2O3−δ scaffold cathode for enhanced oxygen reduction

The chemical compatibility and electrochemical properties of nanoLa_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-infiltrated La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) scaffold were manufactured and assessed for the application as a solid oxide fuel cell cathode with an LSGM electrolyte. When the LSCF and LSGM powder mixture was fired above 950 °C, the characteristic peaks of the two materials merged and an insulation peak (derived from LaSrGaO₄) was observed. To prevent reactions between LSCF and LSGM, an infiltration technique was utilized with the LSGM as a scaffold. Using this infiltration technique, nano LSCF particles (approximately 100 nm) can be uniformly coated on the LSGM scaffold surface. Good nano particle adhesion was observed at the LSGM/LSCF interface, even at relatively low firing temperatures (850 °C). The cathode polarization resistance (R_p) of the nano LSCF infiltrated LSGM scaffold cathode was lower than that of a conventional LSCF cathode. The improvement in performance of the nano LSCF-infiltrated cathode was attributed to an increase in the number of triple phase boundaries (TPB) as a result of the nano LSCF coating. In addition, the oxygen reduction reaction (ORR) paths were extended from the TPBs to the LSCF surface because LSCF particles are considerably smaller than the LSCF oxygen ion penetration depth (3–4 μm) over the temperature range of 700 °C–800 °C.     

Electrochemical characterization of Pr2CuO4 cathode for IT-SOFC

The electrochemical properties of Pr₂CuO₄ (PCO) electrode screen-printed on Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte were investigated. PCO was synthesized by a solid-state route from the stoichiometric mixture of oxides at 1273 K, 20 h. Thermogravimetric analysis (TGA) of PCO both in air and Ar demonstrated its stability up to 1173 K. X-ray powder diffraction study of the PCO–CGO mixture annealed in air at 1173 K for 100 h did not reveal chemical interaction between materials. The oxygen reduction on porous PCO electrodes applied on CGO electrolyte was studied in a symmetrical cell configuration by AC impedance spectroscopy at OCV conditions at 773–1173 K and pO₂$p_{{\text{O}}_2}$ = 10⁻⁴–1 atm. Analysis of the data revealed that depending on temperature and oxygen partial pressure different rate-determining steps of the overall oxygen reduction reaction take place. Calculated value of area specific resistance (ASR) of PCO electrode is 1.7 ± 0.2 Ω cm² at 973 K in air and it is constant after 6 subsequent thermocycles. We have found that oxygen reduction on PCO applied on CGO takes mainly place at the triple-phase boundary (TPB) since Adler–Lane–Steele (ALS) model is not valid. Therefore electrochemical characteristics of PCO electrode can be improved by further optimization of both microstructure of the electrode and electrode/electrolyte interface and PCO can be considered as a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC).     

The advanced photocatalytic performance of V doped CuWO4 for water splitting to produce hydrogen

In the study, we successfully conducted vanadium doping to improve photocatalytic performance of the CuWO₄ for water splitting to produce hydrogen. The doping mechanism, optimal doping ratio and material stability were investigated by various characterization methods and water splitting experiments. We found that the V substituted several W elements of the CuWO₄ crystal. In the V–CuWO₄, V dopant existed in form of the V⁵⁺, which created new energy level between the conduction band (CB) and the valence band (VB) of the CuWO₄ to improve charge transfer as well as to prevent the e⁻ and h⁺ recombination of the material. The substitution of W by V dopant also led the formation of Cu⁺ and W⁵⁺ in the CuWO₄ crystal. The formation of Cu⁺ and W⁵⁺ in the CuWO₄ crystal not only narrowed the energy band gap but also increased the CB potential of the material. Therefore, the V–CuWO₄ generated significant amount of e⁻ under visible light and the generated e⁻ was strong enough to react with H⁺ to produce H₂. The optimal V/W ratio for maximum improving photocatalytic performance of the CuWO₄ was 6 wt%. Finally, we investigated that our prepared V–CuWO₄ showed high stability during long-term water splitting process.     

Photoelectrochemical water oxidation of LaTaON2 under visible-light irradiation

Lanthanum tantalum oxynitride (LaTaON₂) powders were prepared by one-step flux method. LaTaON₂ photoanodes, which are fabricated by using LaTaON₂ powders, are found to exhibit photoelectrochemical activity for overall water splitting. The photocurrent for LaTaON₂ photoelectrodes was ca. 120 μA cm⁻² at 1.5 V vs. reversible hydrogen electrode (RHE) in 1 M NaOH aqueous solutions (pH = 13.6) under AM 1.5 G simulated sunlight irradiation (100 mW cm⁻²). The photocurrent of LaTaON₂ photoelectrode from back-side illumination is much larger than that from front-side illumination, suggesting that the photoelectrochemical property is mainly limited by poor continuous electron transport in the bulk. Further efforts to ameliorate the electron transport in the bulk of LaTaON₂ photoelectrodes are expected to significantly improve their photoelectrochemical performance.     

Visible light driven nanocrystal anatase TiO2 doped by Ce from sol–gel method and its photoelectrochemical water splitting properties

Visible light driven nanocrystal anatase TiO₂ was prepared by doping rare earth element Ce through sol–gel method. UV–Vis diffusion reflectance spectrum indicated its absorption edge extended to about 550 nm, red shifting about 170 nm compared with that without doping. Ce doping TiO₂ showed obvious anodic photocurrent effect for water splitting under visible light irradiation (λ > 420 nm) in photoelectrochemical measurement with three electrodes configuration. Ce doping TiO₂ showed higher photocurrent density than that of without doping TiO₂ under full arc irradiation. Furthermore, the electronic structures for CeO₂ and TiO₂ were analyzed theoretically based on the first principle calculation. As a result, the electronic structure for Ce doping TiO₂ is proposed as the overlap and some degree of hybridization among splitting occupied Ce 4f and unoccupied Ce 4f with O 2p and Ti 3d respectively. The visible light responsive property is mainly due to the transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d.     

Characterization of LaNi4.70Al0.30 handled in air and application to a scheme of thermal compression of hydrogen

LaNi_4.70Al_0.30 was characterized by SEM, EDS and XRD. The structure was refined by the Rietveld method. The intermetallic stability temperature range in air was analyzed by DSC. The intermetallic is destabilized at T > 160° C. The intermetallic was annealed at this temperature for 24 h in air. After that, the pressure-composition-isotherms were measured. The thermodynamic properties were calculated from the Van’t Hoff diagrams. Values obtained were ΔH_f = 30 ± 2 kJ/mol and ΔS_f = 0.13 ± 0.01 kJ/mol for absorption process and ΔH_d = 31 ± 2 kJ/mol and ΔS_d = 0.14 ± 0.01 ± kJ/mol for desorption process. From these results, a scheme of thermal compression of hydrogen (TCH) was proposed. The scheme has a practical compression ratio (R_c) of 7.2 in the 25–100 °C temperature range and 1–1000 kPa pressure range.     

Study of the electrocatalytic properties of Ir4(CO)12 for the oxygen reduction and hydrogen oxidation reactions,in the absence and presence of fuel cell contaminants

Carbonyl cluster compounds are promising potential substitutes for platinum as catalysts in fuel cells. In this work, we have used Ir₄(CO)₁₂ as a new electrocatalyst to study its activity for the Oxygen Reduction Reaction (ORR) in the absence and presence of methanol in different concentrations (1.0 and 2.0 mol L⁻¹), and the Hydrogen Oxidation Reaction (HOR) with pure hydrogen and hydrogen/carbon monoxide mixtures ([CO] = 100 ppm and 0.5%). This iridium cluster can perform both the ORR and HOR, and it exhibits the important property of being tolerant to the aforementioned contaminants to an important extent, in contrast to platinum, which is easily deactivated even by traces of CO or methanol. The compound was electrochemically studied by rotating disk electrode (RDE) measurements, using the cyclic and linear sweep voltammetry (CV and LSV) techniques. The ORR temperature dependence (293 K–333 K) was studied and kinetic parameters such as the Tafel slope, charge transfer coefficient, exchange current density and activation energy were calculated from the LSV polarization curves. This carbonyl complex is one of the few transition metal clusters whose catalytic activity for these reactions has been reported.     

Significantly improved dehydrogenation of LiAlH4 destabilized by K2TiF6

The effects of K₂TiF₆ on the dehydrogenation properties of LiAlH₄ were investigated by solid-state ball milling. The onset decomposition temperature of 0.8 mol% K₂TiF₆ doped LiAlH₄ is as low as 65 °C that 85 °C lower than that of pristine LiAlH₄. Isothermal dehydrogenation properties of the doped LiAlH₄ were studied by PCT (pressure–composition–temperature). The results show that, for the 0.8 mol% K₂TiF₆ doped LiAlH₄ that dehydrogenated at 90 °C, 4.4 wt% and 6.0 wt% of hydrogen can be released in 60 min and 300 min, respectively. When temperature was increased to 120 °C, the doped LiAlH₄ can finish its first two dehydrogenation steps in 170 min. DSC results show that the apparent activation energy (E_a) for the first two dehydrogenation steps of LiAlH₄ are both reduced, and XRD results suggest that TiH₂, Al₃Ti, LiF and KH are in situ formed, which are responsible for the improved dehydrogenation properties of LiAlH₄.     

Characterization and optimization of La0.8Sr0.2Sc0.1Mn0.9O3−δ-based composite electrodes for intermediate-temperature solid-oxide fuel cells

Composite electrodes composed of a perovskite-type La_0.8Sr_0.2Sc_0.1Mn_0.9O_3−δ (LSSM) and a fluorite-type scandium-stabilized zirconia (ScSZ) were prepared and evaluated as potential cathodes for intermediate-temperature solid-oxide fuel cells. Characterization was made by phase reaction, electrochemical impedance spectroscopy, step current polarization and I–V tests. The phase reaction between LSSM and ScSZ occurred at 1150 °C or higher; however, it had a minor effect on the electrode performance. The formation of a composite electrode led to an obvious improvement in both charge transfer and surface-related processes. With the increase of ScSZ content, the rate-limiting step of oxygen reduction reaction steadily changed from mainly a surface diffusion process to an electron transfer process. The optimal ScSZ content and sintering temperature of the electrode layer were found to be 20 wt.% and 1100–1150 °C, respectively. Under optimal conditions, an anode-supported single cell with LSSM + ScSZ composite cathode showed high power densities of ∼1211 and 386 mW cm⁻² at 800 and 650 °C, respectively.