The effect of microstructure on the hydrogenation of Mg/Fe thin film multilayers

Nanoconfined magnesium hydride can be simultaneously protected and thermodynamically destabilized when interfaced with materials such as Ti and Fe. We study the hydrogenation of thin layers of Mg (<14 nm) nanoconfined in one dimension within thin film Fe/Mg/Fe/Pd multilayers by the optical technique Hydrogenography. The hydrogenation of nanosized magnesium layers in Fe/Mg/Fe multilayers surprisingly shows the presence of multiple plateau pressures, whose nature is thickness dependent. In contrast, hydrogen desorption occurs via a single plateau which does not depend on the Mg layer thickness. From structural and morphological analyses with X-ray diffraction/reflectometry and cross-section TEM, we find that the Mg layer roughness is large when deposited on Fe and furthermore contains high-angle grain boundaries (GB's). When grown on Ti, the Mg layer roughness is low and no high-angle GB's are detected. From a Ti/Mg/Fe multilayer, in which the Mg layer is flat and has little or no GB's, we conclude that MgH₂ is indeed destabilized by the interface with Fe. In this case, both the ab- and desorption plateau pressures are increased by a factor two compared to the hydrogenation of Mg within Ti/Mg/Ti multilayers. We hypothesize that the GB's in the Fe/Mg/Fe multilayer act as diffusion pathways for Pd, which is known to greatly alter the hydrogenation behavior of Mg when the two materials share an interface.     

Hydrogen sensing by plasmon decoupling effect in nanostructured Pd/Au films

Plasmon coupling effect occurs in plasmonic nanostructures when interparticle distances are in the order of particle size leading to spectral shifts in the plasmonic band. This effect has been recently highlighted for measurement of fluctuations in the interparticle distance at nanoscale level. In this study, nanostructured thin Au films were deposited on quartz substrates by pulsed laser deposition (PLD) for sensing of hydrogen gas. A blue shift from 730 to 560 nm in LSPR of Au films was observed when substrate temperatures rises from 25 to 600 °C due to variation in morphology of films from a continuous surface composed of tiny agglomerates to granular surface composed of bigger particles with increased interparticle spacing. For plasmon coupling sensing of hydrogen, a thin Pd film was deposited on top of nanostructured Au films. Upon hydrogen exposure, up to12 nm blue shift within few seconds was observed depending on hydrogen concentration. Based on field emission scanning electron microscope (FESEM) images and finite-difference time-domain (FDTD) simulations, this plasmon sensing is explained by hydrogen-induced decoupling due to the formation of surface stresses in Pd, which can affect the LSPR via an increase in interparticle spacing of Au nanoislands.     

Effects of surface modification by MgO on interfacial reactions of lithium cobalt oxide thin film electrode

Magnesium oxide (MgO)-modified lithium cobalt oxide (LiCoO₂) thin film electrodes were prepared by pulsed laser deposition (PLD) and effects of surface modification by MgO on interfacial reactions of LiCoO₂ were studied. The modification by MgO was carried out by PLD on LiCoO₂ thin film electrode successively after the deposition of LiCoO₂ thin film by PLD. Auger electron spectroscopy suggested that Mg dispersed uniformly in nano-scale on the film electrode. Cyclic voltammetry measurements clearly showed that MgO modification suppresses the increase of resistances caused by repetition of lithium-ion insertion–extraction reaction charged up to 4.2 V versus Li/Li⁺. Moreover, MgO modification decreased the activation energy of lithium-ion transfer reaction at LiCoO₂ thin film electrode–electrolyte interface, indicating that the modification by MgO affects the kinetics of lithium-ion transfer reaction at LiCoO₂–electrolyte interface.     

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.     

Hydrogen storage properties of magnesium nanotrees investigated by a quartz crystal microbalance system

We report enhanced low temperature hydrogen storage properties of magnesium “nanotrees” fabricated by glancing angle deposition (GLAD) method. The arrays of nanotrees and conventional thin films of elemental Mg have been deposited directly onto gold coated unpolished quartz crystal substrates. Mg nanotrees were about 15 μm in height, 10 μm by 1 μm in lateral size, and were composed of “nanoleaves” of about 20 nm in thickness, 2 μm length, and 1 μm width. Hydrogen absorption and desorption properties of Mg nanotrees and thin films were investigated using a quartz crystal microbalance (QCM) testing system that is capable of measuring weight changes with a nanogram sensitivity. QCM absorption tests were performed at temperatures 100, 200, and 300 °C under 30 bars of H₂ pressure. Measurements revealed that Mg nanotrees can absorb hydrogen at significantly higher weight percentage (wt%) and faster rates compared to conventional Mg films under similar conditions. Hydrogen storage of Mg thin film was observed to be at 0.02, 0.30 and 3.91 wt% (weight percentage), while it reached to 1.26, 3.75, and 5.86 wt% for nanotrees at temperatures 100, 200, and 300 °C, respectively, after 150 min. In addition, the results of desorption experiments show that Mg nanotrees can start to release hydrogen at temperatures as low as 100 °C at a rate of 0.11 wt% (vs. 0.01 wt% for thin film at the same temperature) with desorption rates reaching to 1.05 wt% at 200 °C (0.26 wt% for thin film) and 2.57 wt% at 300 °C (1.45 wt% for thin film), which are considerably lower desorption temperatures compared to previously reported values for bulk Mg (>300 °C). The enhancement in hydrogen absorption and desorption properties of Mg nanotrees is believed to originate from their thin and isolated nanoleaves that also have an improved oxidation resistance property.     

Electrochemical deposited Mg-PPy multilayered film to store hydrogen

The stability of magnesium and its hydride (Mg/MgH₂) against moisture and oxygen can be improved by forming a multilayered structure with a protective polymeric layer. Herein, we report for the first time the fabrication of such a multilayered structure via an electrochemical deposition method consisting of the successive depositions of Mg on electropolymerised polypyrrole (PPy) films. The thickness of the PPy and the Mg layers was ～1 μm, which is larger than the Mg/Pd multilayers prepared via physical deposition methods, owing to the higher surface roughness of electropolymerised films. However, such films displayed remarkable hydrogen storage properties. Hydrogenation of the PPy/Mg film was achieved at 100 °C and hydrogen release started from 125 °C with a peak at ～215 °C. When covered by a second PPy layer, the hydrogen desorption temperature increased slightly to 230 °C. These hydrogen absorption and desorption temperatures are significantly lower than that of pristine Mg/MgH₂ micron sized powders and this was achieved without any additional catalyst. Furthermore, such hydrogenated PPy/Mg/PPy film structures were found to be stable in air even after one week.     

An experimental feasibility study of vanadium oxide films on metallic bipolar plates for the cold start enhancement of fuel cell vehicles

An experimental study of vanadium oxide polycrystalline films deposited onto 316L stainless steel bipolar plates as an efficient Joule heating source for fuel cell vehicles was conducted by carefully modulating the negative temperature coefficients of the electrical resistance of the films at subzero temperatures. To fabricate the thin films, a well-mixed precursor solution of vanadium alkoxide and organic cosolvent was prepared by the hydrolytic sol-gel route and then coated on the pre-cleaned flat surface of 316L stainless steel plates with natural passive oxide layers by a dip-coating method. Subsequently, the variation of the nonlinear electrical resistance of the thin film was measured simultaneously over a wide temperature range of −20 to 80 °C, allowing direct detection of the surface temperature of the thin films. In addition, the adhesion, microstructures, compositions, and morphologies of the vanadium oxide thin films were investigated using the ASTM D3359 method, XRD, FE-SEM, and XPS analyses. A remarkable result from this study was that a temperature increase of 41.65 °C was induced by significant Joule heating of the vanadium sesquioxide films on metallic bipolar plates, i.e. approximately 1.8-folded more than the minimum requirement of Joule heating, at a current density of 0.1 A·cm⁻² at −20 °C. Thus, it was concluded that thermal dissipation from the resistive vanadium oxide films with a negative temperature coefficient can be effectively used as a self-heating source to melt frozen water at subzero ambient temperatures, particularly for fuel cell vehicles.     

A study on properties of yttrium-stabilized zirconia thin films fabricated by different deposition techniques

This paper investigates the micro-structural, chemical and crystalline properties of yttrium-stabilized zirconia (YSZ) thin films by using pulsed laser deposition (PLD), atomic layer deposition (ALD) and sputter. Atomic ratio of Y:Zr of YSZ thin films fabricated by three different deposition methods was adjustable. ALD YSZ with smaller grains has high density compared to PLD YSZ and sputter YSZ. On the other hand, the low crystallinity of ALD YSZ can be supplemented by annealing process. From these experimental results, ALD YSZ thin film has the characteristics that satisfy requirements for using an electrolyte of thin film solid oxide fuel cells.     

Improved electrical conductivity in Pr2Ni(Cu,Ga)O4 film with nano thickness

Pr_1.91Ni_0.71Cu_0.24Ga_0.05O₄ (PNCG) thin film with few 100 nm thickness was prepared on polycrystalline MgO substrate with pulsed laser deposition (PLD) method. The prepared film was dense and uniform, and formation of Pr₂NiO₄ phase was observed after a post annealing treatment. Electrical conductivity was significantly changed in the film and increase in conductivity was observed when the film thickness was 320 nm. However, the conductivity decreased with decreasing the film thickness less than 300 nm and Hall coefficient measurements suggested that the electronic hole concentration increased, however its mobility decreased in PNCG film because of the expanded lattice. Increased conductivity in the PNCG film with 320 nm could be explained by the increased amount of electronic hole and its high mobility. XPS measurement also showed that Pr and Ni were an oxidized state comparing with that in bulk and so excess oxygen may be introduced in the PNCG thin film by charge compensation.     

Microchip power compensated calorimetry applied to metal hydride characterization

In this work, we show the suitability of the thin film membrane-based calorimetric technique to measure kinetically limited phase transitions such as the dehydrogenation of metallic hydrides. Different compounds such as Mg, Mg/Al and Mg₈₀Ti₂₀ have been deposited over the active area of the microchip by electron beam evaporation. After several hydrogenation treatments at different temperatures to induce the hydride formation, calorimetric measurements on the dehydrogenation process of those thin films, either in vacuum or in air, are performed at a heating rate of 10 °C/min. We observe a significant reduction in the onset of dehydrogenation for Mg₈₀Ti₂₀ compared with pure Mg or Mg/Al layers, which confirms the beneficial effect of Ti on dehydrogenation. We also show the suitability of the membrane-based nanocalorimeters to be used in parallel with optical methods. Quantification of the energy released during hydrogen desorption remains elusive due to the semi-insulating to metallic transition of the film which affects the calorimetric trace.     

Pulsed laser deposition of yttria stabilized zirconia for solid oxide fuel cell applications

Yttria stabilized zirconia (YSZ) films were grown by pulsed laser deposition (PLD) from a 8YSZ target using a KrF excimer laser source (248 nm). The films have been deposited under oxygen atmospheres on porous NiO/YSZ substrates heated from room temperature to 600 °C. YSZ films were obtained in the range of 1–2 μm thickness. The films have been investigated with respect to surface morphology, microstructure and film–substrate interface interaction. The film morphology varied from columnar to an irregular crystalline structure depending on the oxygen pressure and the substrate temperature. In all cases the films consisted of YSZ with the cubic fluorite structure. The formation of oxide layers under low oxygen pressures on the NiO/YSZ substrates is due to a film–substrate redox interaction. The NiO grains close to the coating interface are partially reduced and serve as an oxygen source for the oxidation of the film. The measured He leakage rates to analyze the gas tightness of the YSZ films have so far shown no improvements as compared with uncoated substrates.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.     

Pd/Mg/Pd thin films prepared by pulsed laser deposition under different helium pressures: Structure and electrochemical hydriding properties

Three-layered Pd/Mg/Pd thin films were prepared by pulsed laser deposition in the presence of helium gas. For Pd layer deposition, the He pressure was fixed at 200 mTorr whereas different pressures of He were used for Mg layer deposition (50, 200 and 600 mTorr). The degree of crystallinity and of (001) texture in the Mg layer increase with increasing He pressure. In addition, the increase in He pressure upon Mg deposition greatly accentuates the roughness of the Mg layer, which induces an extension of the outer Pd/Mg interface region. In contrast, the inner Pd/Mg interface is sharp for all the Pd/Mg/Pd films. The electrochemical hydrogen sorption properties of the Pd/Mg/Pd films are improved by increasing the He pressure for Mg layer deposition. However, the maximum H-solubility in the Mg layer remains low (H/Mg ∼0.26) and is not significantly increased by the presence of the inner Pd layer, indicating that Mg hydride phase is confined in the outer Pd/Mg interface region.     

Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte

Solid oxide fuel cells (SOFC) using a pulsed laser deposited bi-layer electrolyte have been successfully fabricated and have shown very good performance at low operating temperatures. The cell reaches power densities of 0.5 W cm⁻² at 550 °C and 0.9 W cm⁻² at 600 °C, with open circuit voltage (OCV) values larger than 1.04 V. The bi-layer electrolyte contains a 6–7 μm thick samarium-doped ceria (SDC) layer deposited over a ∼1 μm thick scandium-stabilized zirconia (ScSZ) layer. The electrical leaking between the anode and cathode through the SDC electrolyte, which due to the reduction of Ce⁴⁺ to Ce³⁺ in reducing environment when using a single layer SDC electrolyte, has been eliminated by adopting the bi-layer electrolyte concept. Both ScSZ and SDC layers in the bi-layer electrolyte prepared by the pulsed laser deposition (PLD) technique are the highly conductive cubic phases. Poor conductive (Zr, Ce)O₂-based solid solutions or β-phase ScSZ were not found in the bi-layer electrolyte prepared by the PLD due to low processing temperatures of the technique. Excellent reliability and flexibility of the PLD technique makes it a very promising technique for the fabrication of thin electrolyte layer for SOFCs operating at reduced temperatures.     

Hydrogen absorption and optical properties of Pd/Mg thin films prepared by DC magnetron sputtering

Hydrogen storage in metallic thin films in the form of metal hydride have a great potential to solve the hydrogen storage challenges and also thin films offer an opportunity to grow new samples fast with novel structures. In the present work the ex situ study on structural, optical and hydrogen storage properties of Pd-capped Mg thin films have been investigated. The nano structured Pd-capped Mg thin films have been prepared by DC magnetron sputtering on glass substrate. The as deposited and hydrogenated samples have been characterized by XRD and FESEM. The content of hydrogen in thin films has measured by using Elastic Recoil Detection Analysis (ERDA) technique with 120 MeV₁₀₇ Ag⁺⁹ ions. The temperature dependent hydrogen contents in thin film samples have been estimated and the saturation of hydrogen absorption has been observed at 250 °C among all studied samples. In the optical reflectance spectra, Mg hydride samples have been observed partially transparent in comparison to as deposited Mg film.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.     

Hydrogen generation from photocatalytic water splitting over TiO2 thin film prepared by electron beam-induced deposition

Photocatalytic TiO₂ thin films were prepared via an electron beam-induced deposition (EBID) method. The effects of post-calcination treatment on the properties of the prepared TiO₂ thin films were studied. X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometry (SEM-EDS), and UV–V is absorption spectrometry were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorbance of the prepared TiO₂ thin films. The photoelectrochemical characteristics of the TiO₂ thin films were investigated with a potentiostat. Under UV irradiation, a photocurrent of ˜2.1 mA was observed for the TiO₂ thin film with post-calcination at 500 °C. A water-splitting reaction was conducted over the TiO₂ thin film with the best photoelectrochemical performance. The yields of hydrogen and oxygen were 59.8 and 30.6 μmole, respectively, after 8 h of reaction under UV irradiation.     

Enhanced oxidation resistance of magnesium nanorods grown by glancing angle deposition

Oxidation behavior of magnesium thin films and nanorods were investigated in the temperature range of 25-550 °C by using thermal gravimetric analysis. Arrays of vertical magnesium nanorods were deposited by the DC magnetron sputtering glancing angle deposition technique, while the magnesium thin films were deposited using the same system but at normal incidence. The morphologies and corresponding crystal structure of the samples were analyzed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction methods, respectively. We report that the Mg thin films showed oxidation induced weight gain starting from room temperature. On the other hand, Mg nanorods did not show any indication of significant oxidation at temperatures below 350 °C. Enhanced oxidation resistance of Mg nanorods was also confirmed by quartz crystal microbalance measurements. At temperatures higher than 350 °C, Mg nanorods started to get oxidized and their weight increased at a similar rate to that of Mg thin films. We argue that reduced oxidation of Mg nanorods is mainly attributed to their single crystal nature. Magnesium nanorods’ reduced oxidation can potentially play a key role in hydrogen storage and gas sensing applications.     

Intermediate temperature solid oxide fuel cells using LaGaO3 based oxide film deposited by PLD method

Dense La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM, 5 μm in thickness)/Ce_0.8Sm_0.2O_2-δ (SDC, 400 nm in thickness) bilayer films were deposited on a dense NiO (Fe₃O₄)-SDC anode substrate by a pulsed laser deposition (PLD) method. After in-situ reduction, the substrate turned to be porous and it can be used as a porous anode substrate. The power density was strongly affected by the oxide ion conductor combined with LSGM and it was found that SDC is the most useful for achieving the high power density. Preparation of Sm_0.5Sr_0.5CoO₃ cathode film by PLD method is also studied for decreasing the contact resistance of cathode. Preparation of SSC film by PLD is effective for decreasing cathodic overpotential and 400 nm thick film is the most effective for achieving the high power density. At 773 K, the maximum power density of the cell becomes higher than 500 mW/cm².