Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol–gel method

A mesoporous TiO₂/WO₃ nanohoneycomb at a molar ratio of 3:1 was prepared by sol–gel method for photoelectrochemical splitting of water. In order to create a highly porous structure, the composite TiO₂/WO₃ with a block copolymer internal template was deposited on the substrate covered with polystyrene (PS) nanospheres. A mesoporous TiO₂/WO₃ composite nanohoneycomb was obtained after removing the PS spheres and copolymer by thermal treatment. It exhibited a lower band gap energy than TiO₂ so that the optical absorption edge was shifted toward the visible light region. It also showed a better photoelectrochemical efficiency of water splitting and higher production of hydrogen due to lower energy gap, higher reactive surface area, and better charge separation efficiency.     

On the processes of photo-induced hydrogen generation from methanol solution by Ta3N5 and WO3

The hydrogen production rates from deionized water and 20% methanol solution, with or without the presence of Ta₃N₅, WO₃, and the indirect Z-scheme Ta₃N₅/WO₃, were investigated. Under irradiation of a 300 W Xe lamp, all of these three catalysts assisted hydrogen generation in deionized water. In the methanol solution, Ta₃N₅, and WO₃ reduced the hydrogen generation, but Ta₃N₅/WO₃ significantly enhanced the hydrogen production rate by seven times. Under visible light irradiation, the effects of the three catalysts are different from those under full spectrum irradiation. The mechanisms based on the competition of methanol decomposition and water reduction in the presence of catalyst under different irradiation conditions are proposed to explain the different hydrogen generation behaviors.     

Fabrication,photoelectrochemical and electrocatalytic activity of 1D linear Co(Ⅱ) and Fe(Ⅲ) TPP-based coordination compounds

Application of transition metal elements in catalysis has become a research hotspot in recent years. Here, two kinds of transition metal-centered HER electrocatalyst of Co(Ⅱ)TPP-based coordination compounds and Fe(Ⅲ)TPP-based coordination compounds are reported. Both of coordination compounds show high photocurrent response and excellent hydrogen evolution activity. The most attraction is that FeTPP-OA/PVP58, FeTPP-PTA/PVP58, FeTPP-OA/PVP1300 and FeTPP-PTA/PVP1300 possess a special surface with a big spine-like cross which is different to the regular pyramid morphology of the other coordination compounds, and these coordination compounds display superior HER performance compare to the other samples. Especially, FeTPP-OA/PVP58 exhibits a low overpotential of 83 mV at the current density of 10 mA cm⁻² and an ultralow Tafel slope of 39 mV dec⁻¹ which is close to the Pt/C (29 mV dec⁻¹). The low charge transfer resistance of 14.4 Ω and high photocurrent of 3 μA under visible light illumination also reveal the outstanding photoelectrochemical property of FeTPP-OA/PVP58. This work provides a novel insight into the design of transition metal-centered HER electrocatalyst with high-efficiency electrocatalytic activity and low cost.     

Enhancing surface activity of La0.6Sr0.4CoO3-δ cathode by a simple infiltration process

Nanoscaled La_0.6Sr_0.4CoO_3?δ (LSC) prepared by infiltration has been investigated as an effective catalyst to promote oxygen reduction reaction (ORR) performance for solid-oxide fuel cell. The area specific resistance of original cathode is appreciably reduced by the infiltration of LSC nanoparticles into the porous backbones of LSC cathode. The Arrhenius plots reveal that the reduction is originated from the pre-exponential factor, not the activation energy, suggesting that the rate of ORR has been enhanced by the infiltrated LSC nanoparticles. The infiltrated cell shows a high power density of 800 mW cm^?2 at 700 °C, which is more than two times higher than those measured in the same conditions for a similar cell with pristine LSC cathode. Our results demonstrate that simple infiltration process without further heating at high temperature is a potential way to enhance electrochemical performance and to reduce the operating temperature.     

Enhanced electrochemical performance and stability of (La,Sr)MnO3–(Gd,Ce)O2 oxygen electrodes of solid oxide electrolysis cells by palladium infiltration

Palladium-impregnated or infiltrated La_0.8Sr_0.2MnO₃–Gd_0.2Ce_0.8O_1.9 (LSM-GDC) composites are studied as the oxygen electrodes (anodes) for the hydrogen production in solid oxide electrolysis cells (SOECs). The incorporation of small amount of Pd nanoparticles leads to a substantial increase in the electrocatalytic activity and stability of the LSM-GDC oxygen electrodes. The electrode polarization resistance (R_E) at 800 °C on a 0.2 mg cm⁻² Pd-infiltrated LSM-GDC electrode is 0.13 Ω cm², significantly smaller than 0.42 Ω cm² for the reaction on the pure LSM-GDC electrodes. The overpotential loss is also substantially reduced after the Pd infiltration; at an anodic overpotential 50 mV and 800 °C, the current increases from 0.15 A cm⁻² for the pure LSM-GDC anode to 0.47 A cm⁻² on a 0.3 mg cm⁻² Pd-infiltrated LSM-GDC. The infiltrated Pd nanoparticles enhance the stability of the LSM-GDC oxygen electrodes and are most effective in the promotion of the diffusion, exchange and combination processes of oxygen species on the surface of LSM-GDC particles, leading to the increase in the oxygen evolution reaction rate.     

Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe₂O₃) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm². The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm² at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.     

Enhanced sintering behavior mechanism of nanocrystalline BaCe0.8Sm0.2O3−δ by Cu doping

Nano-sized BaCe_0.8Sm_0.2O_3−δ and Cu-doped BaCe_0.8Sm_0.2O_3−δ proton conducting electrolyte powders are synthesized by citric-nitrate method, and then the powder properties are investigated. The synthesized BaCe_0.8Sm_0.2O_3−δ powder acquires pure perovskite structure after heat treatment above 1100 °C, while impurity phases such as BaCO₃ and Ce_0.8Sm_0.2O_2−δ are formed below 1100 °C. The BaCe_0.8Sm_0.2O_3−δ and Cu-doped BaCe_0.8Sm_0.2O_3−δ showed similar powder characteristics, except the shrinkage rate. The sintering temperature for densification of the synthesized BCS are significantly reduced as much as ∼300 °C by small amount of Cu. Compared to drastic reduction in sintering temperature, the total conductivity and the activation energy of Cu doped BCS turn out to deviate negligibly from those of pure BCS.     

Synthesis and hydrogen storage property tuning of Zr(BH4)4·8NH3 via physical vapour deposition and composite formation

Zr(BH₄)₄·8NH₃ is considered to be a promising solid state hydrogen-storage material, due to its high hydrogen capacity and low dehydrogenation temperature. However, the possible applications of Zr(BH₄)₄·8NH₃ have been greatly hampered by the complicated and less applicable synthesis process, which must be operated at relatively low temperature (<20 °C). Herein, we reported a simple and facile “heating-(ball milling) BM vial” method via physical vapour deposition to tackle this issue. By this technique, Zr(BH₄)₄·8NH₃ was successfully synthesized. Furthermore, composite formation by adding 10 wt% NaBH₄ to the as-prepared Zr(BH₄)₄·8NH₃ was found to be able to lower down the dehydrogenation peak of Zr(BH₄)₄·8NH₃ from 130 to 75 °C and more excitingly, the possible emission of B₂H₆ and NH₃ from dehydrogenation of only Zr(BH₄)₄·8NH₃ was completely suppressed after addition of NaBH₄. This research presents a new hydrogen-storage system based on Zr(BH₄)₄·8NH₃+NaBH4 composite and it also implies a new development methodology of future hydrogen storage materials.     

Enhancing sinterability and electrochemical properties of Ba(Zr0.1Ce0.7Y0.2)O3-δ proton conducting electrolyte for solid oxide fuel cells by addition of NiO

The influence of NiO on the sintering behavior and electrical properties of proton conducting Ba(Zr_0.1Ce_0.7Y_0.2)O_3-δ (BZCY7) as an electrolyte supporter for solid oxide fuel cells is systematically investigated. SEM images and shrinkage curve demonstrate that the sinterability of the electrolyte pellets is dramatically improved by doping NiO as a sintering aid. The sintering aid amount and sintering temperature are optimized by analyzing the linear shrinkage, grain size and morphology for a series of sintered BZCY7 electrolyte pellets. Almost full dense electrolyte pellets are successfully formed by using 0.5–1.0 wt% NiO loading after sintering at 1400 °C for 6 h. The linear shrinkage of 0.5 wt% NiO modified BZCY7 sample is about 14.25% higher than that without NiO addition (4.81%). Energy dispersive X-ray spectroscopy analysis indicate that partial NiO might dissolve into the perovskite lattice structure and the other NiO react with BZCY7 to form BaY₂NiO₅ secondary phase as a sintering aid. Excessive NiO is especially detrimental to the electrical properties of BZCY7 and thus lower the open circuit voltage. The electrochemical performance for a series of single cells with different concentration NiO modified BZCY7 electrolyte are measured and analyzed. The optimized composition of 0.5 wt% NiO modified BZCY7 as an electrolyte support for solid oxide fuel cell demonstrates a high electrochemical performance.     

Performance and stability of (La,Sr)MnO3–Y2O3–ZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions

The electrochemical performance and stability of (La,Sr)MnO₃–Y₂O₃–ZrO₂ (LSM-YSZ) composite oxygen electrodes is studied in detail under solid oxide electrolysis cells (SOECs) operation conditions. The introduction of YSZ electrolyte phase to form an LSM-YSZ composite oxygen electrode substantially enhances the electrocatalytic activity for oxygen oxidation reaction. However, the composite electrode degrades significantly under SOEC mode tested at 500 mA cm⁻² and 800 °C. The electrode degradation is characterized by deteriorated surface diffusion and oxygen ion exchange and migration processes. The degradation in electrode performance and stability is most likely associated with the breakup of LSM grains and formation of LSM nanoparticles at the electrode/electrolyte interface, and the formation of nano-patterns on YSZ electrolyte surface under the electrolysis polarization conditions. The results indicate that it is important to minimize the direct contact of LSM particles and YSZ electrolyte at the interface in order to prevent the detrimental effect of the LSM nanoparticle formation on the performance and stability of LSM-based composite oxygen electrodes.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

Hydrogen gas sensing properties of WO3 sputter-deposited thin films enhanced by on-top deposited CuO nanoclusters

Magnetron-based gas aggregation cluster source (GAS) was used to prepare high-purity CuO (cupric oxide) nanoclusters on top of sputter-deposited thin film of tungsten trioxide (WO₃). The material was assembled as a conductometric hydrogen gas sensor and its response was tested and evaluated. It is demonstrated that addition of CuO clusters noticeably enhances the sensitivity of the pure WO₃ thin film. With an increasing amount of CuO clusters the sensitivity of CuO/WO₃ system rises further. When CuO clusters form a sufficiently thick and compact layer, the resistance response is reversed. Based on the sensorial behavior, conventional and near-ambient pressure X-Ray photoemission spectroscopies, and resistivity measurements, we propose that the sensing mechanism is based on the formation of nano-sized p-n junctions in between p-type CuO and n-type WO₃. The advantages of the GAS technique for preparing sensorial and/or catalytically active materials are emphasized.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Pd doped WO3 films prepared by sol–gel process for hydrogen sensing

The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride salt was dissolved in the sol with different Pd:W molar ratios and coated on Al₂O₃ substrates by spin coating method. XRD and XPS techniques were used to analyze the crystal structure and chemical composition of the films before and after heat treatment at 500 °C. We observed that Pd can modify the growth kinetic of tungsten trioxide nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 2.5 × 10⁴ at room temperature in hydrogen concentration of 0.1% in air. Considering all sensing parameters, an optimum working temperature of 100 °C was obtained.     

Performance and durability of an anode-supported solid oxide fuel cell with a PdO/ZrO2 engineered (La0.8Sr0.2)0.95MnO3-δ-(Y2O3)0.08(ZrO2)0.92 composite cathode

PdO/ZrO₂ co-infiltrated (La_0.8Sr_0.2)_0.95MnO_3-δ-(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM-YSZ) composite cathode (PdO/ZrO₂+LSM-YSZ), which adsorbs more oxygen than equal amount of PdO/ZrO₂ and LSM-YSZ, is developed and used in Ni-YSZ anode-supported cells with YSZ electrolyte. The cells are investigated firstly at temperatures between 650 and 750 °C with H₂ as the fuel and air as the oxidant and then polarized at 750 °C under 400 mA cm^?2 for up to 235 h. The initial peak power density of the cell is in the range of 438–1207 mW cm^?2 at temperatures from 650 to 750 °C, corresponding to polarization resistance from 1.04 to 0.35 Ω cm². This result demonstrates a significant performance improvement over the cells with other kinds of LSM based cathode. The cell voltage at 750 °C under 400 mA cm^?2 decreases from initial 0.951 to 0.89 V after 170 h of current polarization and remains essentially stable to the end of current polarization. It is identified that the self-limited growth of PdO particles is responsible for the cell voltage decrease by reducing the length of triple phase boundary affecting the high frequency steps involved in oxygen reduction reaction in the cathode.     

High performance solid oxide electrolysis cells using Pr0.8Sr1.2(Co,Fe)0.8Nb0.2O4+δ–Co–Fe alloy hydrogen electrodes

A novel ceramic hydrogen electrode material consisting of K₂NiF₄-type structured Pr_0.8Sr_1.2(Co,Fe)_0.8Nb_0.2O_4+δ (K-PSCFN) matrix with homogenously dispersed nano-sized Co–Fe alloy (CFA) has been demonstrated by annealing perovskite Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (P-PSCFN) in H₂ at 900 °C. Impedance spectra and voltage–current density curves of the La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) electrolyte supported solid oxide electrolysis cell (SOEC) with a configuration of K-PSCFN–CFA/LSGM/Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ (BCFN) have been evaluated as a function of the operating temperature and feeding gas absolute humidity (AH) to characterize the cell performance. Cell polarization resistances (R_p) were as low as 0.77 and 0.31 Ω cm² under open circuit voltage (OCV) and 60% absolute humidity (AH) at 800 and 900 °C, respectively. The cell has demonstrated good stability for high temperature stream electrolysis, and a hydrogen production rate of 707 ml/cm² h calculated from the Faraday's law has been achieved under an electrolysis voltage of 1.3 V and 60 vol.% AH at 900 °C. The cell performance results indicate that K-PSCFN–CFA is a promising hydrogen electrode for high temperature solid oxide electrolysis cells.     

Synthesis and characterization of composite visible light active photocatalysts MoS2–g-C3N4 with enhanced hydrogen evolution activity

Molybdenum disulfide (MoS₂) and graphitic carbon nitride (g-C₃N₄) composite photocatalysts were prepared via a facile impregnation method. The physical and photophysical properties of the MoS₂–g-C₃N₄ composite photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microcopy (HRTEM), ultraviolet–visible diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The photoelectrochemical (PEC) measurements were tested via several on–off cycles under visible light irradiation. The photocatalytic hydrogen evolution experiments indicate that the MoS₂ co-catalysts can efficiently promote the separation of photogenerated charge carriers in g-C₃N₄, and consequently enhance the H₂ evolution activity. The 0.5wt% MoS₂–g-C₃N₄ sample shows the highest catalytic activity, and the corresponding H₂ evolution rate is 23.10 μmol h⁻¹, which is enhanced by 11.3 times compared to the unmodified g-C₃N₄. A possible photocatalytic mechanism of MoS₂ co-catalysts on the improvement of visible light photocatalytic performance of g-C₃N₄ is proposed and supported by PL and PEC results.     

Photoelectrochemical water splitting at nanostructured α-Fe2O3 electrodes

In this study, nanostructured α-Fe₂O₃ thin films were deposited by simple electrodeposition for photoelectrochemical water splitting. Post-annealing temperature was found to have drastic effect on photoactivity of these films. SEM analysis illustrated that size of nanoparticles increases with annealing temperature. The current–potential characteristics showed that the water-splitting photocurrent strongly depends on post-annealing temperature. A maximum photocurrent density of 0.67 mA/cm² was observed at 1.23 V versus reversible hydrogen electrode (RHE) under standard illumination conditions (AM 1.5 G 100 mW/cm²), and the water-splitting current was over 1.0 mA/cm² before the dark current flow starts (at 1.55 V versus RHE). The electrode shows an onset potential as low as 0.8 V (versus RHE) for water photooxidation, which is one of the best results reported for hematite photoanodes. This high photoactivity of electrodes is attributed to the preferential growth of hematite nanostructures along the most conductive plane (001) and incorporation of Sn in film from the substrate at high annealing temperature. The best-performing electrode shows an incident photon conversion efficiency (IPCE) of 12% at 400 nm (in 1 M NaOH at 1.23 V versus RHE), which indicate the improved light-harvesting properties of these nanostructures.     

Enhancing preferential oxidation of CO in H2 on Au/α-Fe2O3 catalyst via combination with APTES/SBA-15 CO2-sorbent

Au/α-Fe₂O₃ was combined with a CO₂-sorbent (3-aminopropyltriethoxysilane (APTES) grafted on SBA-15 and hereafter denoted as APTES/SBA-15) to enhance preferential oxidation (PROX) of CO in H₂. The CO₂ molecules could be rapidly adsorbed on APTES/SBA-15 at low temperatures below 50 °C with a capacity of 0.68 mmol CO₂/g-sample, and desorbed at a temperature range of 50 °C–80 °C. Three different configurations of the Au/α-Fe₂O₃ catalyst and the CO₂-sorbent were tested in the PROX reaction, namely (i) the sorbent-free (catalyst//SBA-15//catalyst) configuration, (ii) the packed three-layer configuration (catalyst//CO₂-sorbent//catalyst), and (iii) the mechanically mixed catalyst and CO₂-sorbent configuration. Compared to configuration (i), configuration (ii) achieved an average 10% higher CO conversion at 50 °C and a GHSV of 65000 h⁻¹. However, the CO concentration could not be lowered to below 70 ppm from 2000 ppm using configuration (ii) at a GHSV of 10000 h⁻¹. Thus, a 5-layer configuration (catalyst//CO₂-sorbent//catalyst//CO₂-sorbent//catalyst) was used, and the CO concentration was lowered to ca. 25 ppm. The mechanism for enhancement of the PROX reaction by the continuous removal of CO₂ by the CO₂-sorbent is discussed and attributed to reduction of the surface carbonate on the Au/α-Fe₂O₃ catalyst formed during the PROX process.     

Releasing 9.6 wt% of H2 from Mg(NH2)2–3LiH–NH3BH3 through mechanochemical reaction

Ball milling the mixture of Mg(NH₂)₂, LiH and NH₃BH₃ in a molar ratio of 1:3:1 results in the direct liberation of 9.6 wt% H₂ (11 equiv. H), which is superior to binary systems such as LiH–AB (6 equiv. H), AB–Mg(NH₂)₂ (No H₂ release) and LiH–Mg(NH₂)₂ (4 equiv. H), respectively. The overall dehydrogenation is a three-step process in which LiH firstly reacts with AB to yield LiNH₂BH₃ and LiNH₂BH₃ further reacts with Mg(NH₂)₂ to form LiMgBN₃H₃. LiMgBN₃H₃ subsequently interacts with additional 2 equivalents of LiH to form Li₃BN₂ and MgNH as well as hydrogen.