CuInS2/SiNWs/Si composite material for application as potential photoelectrode for photoelectrochemical hydrogen generation

This work aims at developing a new composite material based on nanosized semiconducting CuInS₂ (CIS) particles combined with silicon nanowires grown on a silicon substrate (SiNWs/Si) for photoelectrochemical (PEC)-splitting of water. The CIS particles were prepared via a colloidal method using N-methylimidazole (NMI) as the solvent and an annealing treatment. The SiNWs were obtained by chemical etching of silicon (100) substrates assisted by a metal. The CIS/SiNWs/Si composite material was obtained by deposition of an aliquot of a suspension of CIS particles onto the SiNWs/Si substrate, using spin coating followed by a drying step. The XRD pattern demonstrated that CuInS₂ grows in the tetragonal/chalcopyrite phase, while SiNWs/Si presents a cubic structure. The SEM images show semi-spherical particles (～10 nm) distributed on the surface of silicon nanowires (～10 μm). The EIS measurements reveal n-type conductivity for CIS, SiNWs/Si and CIS/SiNWs/Si materials, which could favour the oxidation reaction of water molecules.     

Characteristics of Li3V2(PO4)3/C powders prepared by ultrasonic spray pyrolysis

Li₃V₂(PO₄)₃ and Li₃V₂(PO₄)₃/C powders are prepared by ultrasonic spray pyrolysis from spray solutions with and without sucrose. The precursor powders have a spherical shape and the crystal structure of V₂O₃ irrespective of the concentration of sucrose in the spray solution. The powders post-treated at 700 °C have the pure crystal structure of the Li₃V₂(PO₄)₃ phase irrespective of the concentration of sucrose in the spray solution. The Li₃V₂(PO₄)₃ powders prepared from the spray solution without sucrose have a non-spherical shape and hard aggregation. However, the Li₃V₂(PO₄)₃/C powders prepared from the spray solution with sucrose have a spherical shape and non-aggregation characteristics. The Li₃V₂(PO₄)₃ powders prepared from the spray solution without sucrose have a low initial discharge capacity of 122 mAh g⁻¹. However, the Li₃V₂(PO₄)₃/C powders prepared from the spray solutions with 0.1, 0.3, and 0.5 M sucrose have initial discharge capacities of 141, 130, and 138 mAh g⁻¹, respectively. After 25 cycles, the discharge capacities of the powders formed from the spray solutions with and without 0.1 M sucrose are 70% and 71% of the initial discharge capacities, respectively.     

Field effect surface passivation of SiO2/Si interfaces by heat treatment with high-pressure H2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H₂O vapor heating. Heat treatment with 2.1×10⁶ Pa H₂O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO_x film/Si. Additional deposition of 140 nm-SiO_x films (x<2) with a high density of fixed positive charges on the SiO₂/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO₂. Additional deposition of 210 nm SiO_x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO_x film and the heat treatment with high-pressure H₂O vapor is effective for low-temperature passivation of the silicon surface.     

An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.     

Novel synthesis of LiFePO4-Li3V2(PO4)3 composite cathode material by aqueous precipitation and lithiation

LiFePO₄-Li₃V₂(PO₄)₃ composite cathode material is synthesized by aqueous precipitation of FeVO₄·xH₂O from Fe(NO₃)₃ and NH₄VO₃, following chemical reduction and lithiation with oxalic acid as the reducer and carbon source. Samples are characterized by XRD, SEM and TEM. XRD pattern of the compound synthesized at 700 °C indicates olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ are co-existed. TEM image exhibits that LiFePO₄-Li₃V₂(PO₄)₃ particles are encapsulated with a carbon shell 5-10 nm in thickness. The LiFePO₄-Li₃V₂(PO₄)₃ compound cathode shows good electrochemical performance, and its discharge capacity is about 139.1 at 0.1 C, 135.5 at 1 C and 116 mA h g⁻¹ at 3 C after 30 cycles.     

Design and simulation of antireflection coating for application to silicon solar cells

Phosphorous silicate glass (PSG) was formed at the temperature range 800–900°C during diffusion of phosphorous (P) into Si wafers from liquid POCl₃ source in ambient atmosphere of N₂ and O₂. The thickness and refractive indices were measured by an ellipsometer. The refractive index increased with the temperature of formation upto 875°C and then became constant at which point PSG is saturated with P. From the growth rate data at different temperatures, the linear and parabolic activation energies were determined as 0.79 and 1.43 eV for parabolic and linear rate constants, respectively. Therefore, growth rate of PSG is higher than thermal SiO₂. The PSG films were found to have refractive indices 1.85, 1.78, 1.74 and 1.71 for forming temperature 800°C, 825°C, 850°C and 875°C, respectively. Reflectivity varied from 2.5% to 7.5% in the wavelength range 450–700 nm. SIMS depth profiling suggests that there has been a pile up of P on the Si side at the Si/SiO₂ (PSG) interface.     

Characteristics and structural change of layered polysilane (Si6H6) anode for lithium ion batteries

Layered polysilane (Si₆H₆) has a graphite-like structure with higher capacity than crystalline silicon. The rate of increase of the thickness of a layered polysilane electrode after 10 charge-discharge cycles was smaller than that for a Si powder electrode, although the layered polysilane electrode has higher capacity. The structural changes of electrochemically lithiated and delithiated layered polysilane at room temperature were studied using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Layered polysilane became amorphous by insertion of lithium to 0 V, whereas insertion of lithium into crystalline silicon produces Li₁₅Si_4. Layered polysilane maintained an amorphous state during lithium insertion and deinsertion, whereas silicon changed between Li₁₅Si₄ and amorphous Li_xSi, which explains the smaller volume change of a layered polysilane electrode compared with a Si powder electrode.     

Cycleable graphite/FeSi6 alloy composite as a high capacity anode material for Li-ion batteries

FeSi₆/graphite composite was prepared by mechanical ball milling. The FeSi₆ alloy particles consist of an electrochemically active silicon phase and inactive phases FeSi₂, distributed uniformly in the graphite matrix. The composite anode offers a large reversible capacity (about 800 mAh g⁻¹) and good cycleability, due to the buffering effect of the inactive FeSi₂ phase and graphite layers on the volumetric changes of Si phase during lithium–Si alloying reaction. Since FeSi₆ alloy is a low-cost industrial material, this alloy compound provides a possible alternative for development of high capacity lithium-ion batteries.     

Membrane electrode assemblies doped with H3PO4 for high temperature proton exchange membrane fuel cells

H₃PO₄ content plays a critical role in high temperature proton exchange membrane fuel cells (HT-PEMFC), as it is responsible for the majority of the conductivity of the key components under high temperature operation. The conductivities of commercial AB-PBI membranes doped by immersing in 85 wt.% H₃PO₄ for different times and temperatures are investigated. The effect of H₃PO₄ loading in electrodes, including the AB-PBI polymer and a Pt/C catalyst, is also studied. The as-prepared electrodes and membranes are combined to fabricate a membrane electrode assembly for HT-PEMFCs. The results reveal that AB-PBI membranes doped with 85 wt.% H₃PO₄ at 90 °C for 9 h display a maximum conductivity of 33 mS cm⁻¹. This membrane was selected and combined with electrodes including 15 wt.% AB-PBI and 0.75 mg cm⁻² Pt with different H₃PO₄ loadings. A maximum current density of 260 mA cm⁻² was achieved in the as-prepared MEA (with 5 mg cm⁻² H₃PO₄ in electrodes) operating at 0.6 V and 160 °C, using oxygen and hydrogen.     

Effect of Co3(PO4)2 coating on Li[Co0.1Ni0.15Li0.2Mn0.55]O2 cathode material for lithium rechargeable batteries

To prepare a high-capacity cathode material with improved electrochemical performance for lithium rechargeable batteries, Co₃(PO₄)₂ nanoparticles are coated on the surface of powdered Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂, which is synthesized by a simple combustion method. The coated powder prepared under proper conditions for Co₃(PO₄)₂ content and annealing temperature shows an optimum coating layer that consists of a Li_xCoPO₄ phase formed by reaction with lithium impurities during heat treatment. A sample coated with 3 wt.% Co₃(PO₄)₂ and annealed at 800 °C proves to be the best in terms of specific capacity, cycle performance and rate capability. Thermal stability is also enhanced by the coating, as demonstrated a decrease in the onset temperature and/or the heat generated during thermal runaway.     

Thin nanostructured LiMn2O4 films by flame spray deposition and in situ annealing method

A new approach has been developed to rapidly synthesize nanostructured LiMn₂O₄ thin films by flame spray deposition (FSD) and in situ annealing. A precursor solution of lithium acetylacetonate and manganese acetylacetonate in an organic solution was supplied through a flame spray pyrolysis (FSP) reactor. The liquid solution spray was ignited and stabilized by a premixed methane/oxygen flame ring surrounding the FSP nozzle. Thus, LiMn₂O₄ nanoparticles were formed by combustion and deposited onto a current collector followed by in situ annealing. Two different types of current collectors, i.e. stainless steel and aluminum coated with carbon-based primer were tested. The prepared thin films were characterized by X-ray diffraction and field-emission scanning electron microscopy. The electrochemical properties of the thin films were evaluated by cyclic voltammetry and galvanostatic cycling. The LiMn₂O₄ films exhibited good cyclability. Films that underwent sintering and crystal growth during in situ annealing developed more robust film structures on the current collector surface and exhibited better electrochemical performance than poorly adhered films.     

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).     

Electrochemical properties of LiFe0.9Mn0.1PO4/Fe2P cathode material by mechanical alloying

LiFePO₄, olivine-type LiFe_0.9Mn_0.1PO₄/Fe₂P composite was synthesized by mechanical alloying of carbon (acetylene back), M₂O₃ (M = Fe, Mn) and LiOH·H₂O for 2 h followed by a short-time firing at 900 °C for only 30 min. By varying the carbon excess different amounts of Fe₂P second phase was achieved. The short firing time prevented grain growth, improving the high-rate charge/discharge capacity. The electrochemical performance was tested at various C/x-rate. The discharge capacity at 1C rate was increased up to 120 mAh g⁻¹ for the LiFe_0.9Mn_0.1PO₄/Fe₂P composite, while that of the unsubstituted LiFePO₄/Fe₂P and LiFePO₄ showed only 110 and 60 mAh g⁻¹, respectively. Electronic conductivity and ionic diffusion constant were measured. The LiFe_0.9Mn_0.1PO₄/Fe₂P composite showed higher conductivity and the highest diffusion coefficient (3.90 × 10⁻¹⁴ cm² s⁻¹). Thus the improvement of the electrochemical performance can be attributed to (1) higher electronic conductivity by the formation of conductive Fe₂P together with (2) an increase of Li⁺ ion mobility obtained by the substitution of Mn²⁺ for Fe²⁺.     

A novel H3PO4/Nafion–PBI composite membrane for enhanced durability of high temperature PEM fuel cells

A novel phosphoric acid doped Nafion–polybenzimidazole (H₃PO₄/Nafion–PBI) composite membrane was prepared and the H₂/O₂ single cell durability was tested at 150 °C without humidification. The durability was improved 55% compared with that of phosphoric acid doped polybenzimidazole (H₃PO₄/PBI). During the durability test, the hydrogen permeability of the membrane and the internal resistance of the single cell were detected using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), respectively. Before and after the durability test, the mechanical strength of the membranes was measured by stress–strain tests. The results of characterization indicated that the enhanced durability of the membrane attributed to the improved mechanical strength, which benefited from the presence of Nafion in the Nafion and PBI matrix. The preliminary results suggested that the novel H₃PO₄/Nafion–PBI composite membrane is a good candidate in high temperature PEMFC for achieving longer cell lifetime.     

Catalytic decomposition of methane over enteromorpha prolifera-based hierarchical porous biochar

Biochar is a potential catalyst for methane decomposition (CMD) owing to its environmental-friendly and application prospects. In this work, the hierarchical porous biochar was prepared by carbonization and H₃PO₄ activation using Enteromorpha prolifera (EP) as precursor, respectively. The results show that when the ratio of H₃PO₄/EP is 1.5, the maximum CH₄ conversion is 46%, along with hydrogen output of 396 mmol/g_cat, which is 5.8 times as that of the unactivated biochar. The characterization results by XPS, Raman, SEM and HRTEM indicate that P element is inserted in carbon layer in the form of C–O–P, resulting in lattice distortion of carbon layer and larger defect density, and C–O–P plays a dominant role in initial CH₄ conversion. The mesopores formed by H₃PO₄ activation alleviate the influence of the deposited carbon on the catalyst and decrease the deactivation rate, thereby exhibiting better performance in CMD.     

An experimental investigation into micro-fabricated solid oxide fuel cells with ultra-thin La0.6Sr0.4Co0.8Fe0.2O3 cathodes and yttria-doped zirconia electrolyte films

Thin-film solid oxide fuel cells (SOFCs) were fabricated with both Pt and mixed conducting oxide cathodes using sputtering, lithography, and etching. Each device consists of a 75–150 nm thick yttria-stabilized zirconia (YSZ) electrolyte, a 40–80 nm porous Pt anode, and a cathode of either 15–150 nm dense La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) or 130 nm porous Pt. Maximum powers produced by the cells are found to increase with temperature with activation energies of 0.94–1.09 eV. At 500 °C, power densities of 90 and 60 mW cm⁻² are observed with Pt and LSCF cathodes, respectively, although in some conditions LSCF outperforms Pt. Several device types were fabricated to systematically investigate electrical properties of components of these fuel cells. Micro-fabricated YSZ structures contacted on opposite edges by Pt electrodes were used to study temperature-dependent in-plane conductivity of YSZ as a function of lateral size and top and bottom interfaces. Si/Si₃N₄/Pt and Si/Si₃N₄/Au capacitor structures are fabricated and found to explain certain features observed in impedance spectra of in-plane and fuel cell devices containing silicon nitride layers. The results are of relevance to micro-scale energy conversion devices for portable applications.     

Synthesis of plate-like Li3V2(PO4)3/C as a cathode material for Li-ion batteries

Plate-like Li₃V₂(PO₄)₃/C composite is synthesized via a solution route followed by solid-state reaction. The Li₃V₂(PO₄)₃/C plates are 40-100 nm in thicknesses and 2-10 μm in lengths. TEM images show that a uniform carbon layer with a thickness of 5.3 nm presents on the surfaces of Li₃V₂(PO₄)₃ plates. The apparent Li-ion diffusion coefficient of the plate-like Li₃V₂(PO₄)₃/C is calculated to be 2.7 × 10⁻⁸ cm² s⁻¹. At a charge-discharge rate of 3 C, the plate-like Li₃V₂(PO₄)₃/C exhibits an initial discharge capacity of 125.2 and 133.1 mAh g⁻¹ in the voltage ranges of 3.0-4.3 and 3.0-4.8 V, respectively. After 500 cycles, the electrodes still can deliver a discharge capacity of 111.8 and 97.8 mAh g⁻¹ correspondingly, showing a good cycling stability.     

Rate-controlling species for the sintering of LiTi2(PO4)3

A series of Li deficient LiTi₂(PO₄)₃ samples were prepared and sintered and the density was measured to determine the rate-controlling species for sintering of LiTi₂(PO₄)₃. It was observed that as the Li content decreased the density decreased. This result suggests that oxygen does not control sintering. A comparison of the LiTi₂(PO₄)₃ sintering data to sintering and diffusion data in olivine, which exhibits a similar framework structure to LiTi₂(PO₄)₃, suggests that P is the species which controls sintering. This suggestion was confirmed by the density results of a Li excess LiTi₂(PO₄)₃ sample.     

Impact of substrate material and annealing conditions on the microstructure and chemistry of yttria-stabilized-zirconia thin films

Si-diffusion from Si-based substrates into yttria-stabilized-zirconia (YSZ) thin films and its impact on their microstructure and chemistry is investigated. YSZ thin films used in electrochemical applications based on micro-electrochemical systems (MEMS) are deposited via spray pyrolysis onto silicon-based and silicon-free substrates, i.e. Si_xN_y-coated Si wafer, SiO₂ single crystals and Al₂O₃, sapphire. The samples are annealed at 600 °C and 1000 °C for 20 h in air. Transmission electron microscopy (TEM) showed that the Si_xN_y-coated Si wafer is oxidized to SiO_z at the interface to the YSZ thin film at temperatures as low as 600 °C. On all YSZ thin films, silica is detected by X-ray photoelectron spectroscopy (XPS). A particular large Si concentration of up to 11 at% is detected at the surface of the YSZ thin films when deposited on silicon-based substrates after annealing at 1000 °C. Their grain boundary mobility is reduced 2.5 times due to the incorporation of SiO₂. YSZ films on Si-based substrates annealed at 600 °C show a grain size gradient from the interface to the surface of 3 nm to 10 nm. For these films, the silicon content is about 1.5 at% at the thin film's surface.     

Improvement of characteristics of new-type solar cells, having a “transparent conductor/thin Si02 layer with ultrafine metal particles as conductive channels/n-Si” junction

New-type solar cells, having a structure “transparent conductor/thin Si0₂ layer with ultrafine metal islands as conductive channels/n-Si” have been prepared by forming a very thin (< 1.0 nm) silicon oxide (Si0₂) layer as well as platinum (Pt) islands (5–50 nm in size) embedded in it on a single crystal n-type silicon (n-Si) wafer, followed by the deposition of an indium tin oxide (ITO) film (200 nm thick) by the electron-beam evaporation method. The open-circuit photovoltages (V_oc) of the solar cells of the above structure were relatively low, 0.25–0.47 V, but they increased very much to 0.50–0.59 V if a thin (3–10 nm) layer of an organic compound such as copper phthalocyanine (CuPc) was pre-deposited on the Pt-island modified n-Si wafer before the ITO deposition. The reason for the beneficial effect of the pre-deposition of the thin CuPc layer was investigated in detail, and it has been found that certain crystal defects are formed in n-Si near the n-Si/Si0₂ interface during the ITO deposition in the absence of the CuPc layer. The formation of such defects is prevented in the presence of the CuPc layer, which leads to a decrease in surface carrier recombination and hence to the increase in V_oc.