Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

Nanostructured cadmium oxide thin films for hydrogen sensor

Cadmium oxide (CdO) thin films have been deposited by reactive dc magnetron sputtering on to glass substrates at room temperature and subsequently annealed at 275 °C for 3 h in vacuum. The as-deposited and annealed films were characterized for their structural and optical properties. The films were also tested for hydrogen sensing at different operating temperatures. The annealed samples showed better response down to 50 ppm at a relatively low operating temperature of 100 °C. The role of non-equilibrium surface states as a possible sensing mechanism is explained.     

Vapor-deposited iron sulfide films as a novel hydrogen permeation barrier for steel: Deposition condition,defect effect,and hydrogen diffusion mechanism

Hydrogen embrittlement is a serious problem in the oil/gas industry. In this work, various iron sulfide (FeS) films, including iron monosulfide (FeS), pyrite (FeS₂), and pyrrhotite (Fe₇S₈), were synthesized in X80 steel by chemical vapor deposition at 200 °C, 300 °C, 400 °C, and 500 °C. The corrosion resistance and hydrogen permeation properties of the FeS films were investigated through electrochemical methods. Results indicated that FeS films significantly improved the hydrogen barrier properties of the X80 steel, which was closely related to the crystal structure type and defects of FeS films. Defects like microcracks and pinholes during deposition can increase the porosity of the film, resulting in the film properties decreased. Moreover, FeS film (at 300 °C), which had the smallest apparent hydrogen diffusivity (D ? 2.64 × 10^?7 cm²/s) and apparent subsurface concentration (C_app ? 1.12 μmol/cm³), had the best hydrogen barrier properties. The corrosion resistance of FeS film (300 °C) was excellent.     

Production of hydrogen from hydrogen sulfide assisted by dielectric barrier discharge

Hydrogen production by non-thermal plasma (NTP) assisted direct decomposition of hydrogen sulfide (H₂S) was carried out in a dielectric barrier discharge (DBD) reactor with stainless steel inner electrode and copper wire as the outer electrode. The specific advantage of the present process is the direct decomposition of H₂S in to H₂ and S and the novelty of the present study is the in-situ removal of sulfur that was achieved by operating DBD plasma reactor at ∼430 K. Optimization of various parameters like the gas residence time in the discharge, frequency, initial concentration of H₂S and temperature was done to achieve hydrogen production in an economically feasible manner. The typical results indicated that NTP is effective in dissociating H₂S into hydrogen and sulfur and it has been observed that by optimizing various parameters, it is possible to achieve H₂ production at 300 kJ/mol H₂ that corresponds to ∼3.1 eV/H₂, which is less than the energy demand during the steam methane reforming (354 kJ/mol H₂ or ∼3.7 eV/H₂).     

Layer-by-layer self-assembly of manganese oxide nanosheets/polyethylenimine multilayer films as electrodes for supercapacitors

Multilayer thin films of manganese oxide nanosheets (MNSs) and polyethylenimine (PEI) polyelectrolyte have been fabricated onto various substrates via layer-by-layer self-assembly technique. UV–vis absorption spectra showed that the absorbance values at the characteristic wavelength of the multilayer films increased almost linearly with the number of PEI/MNS bilayers. Field emission scanning electron microscope (FESEM) images indicated that the surface of the multilayer film was rather smooth and dense. The electrochemical performances of (PEI/MNS)_n films on indium–tin oxide (ITO)-coated glass substrates were investigated by cyclic voltammetry and constant current charge–discharge test from 0 to 0.9 V in a 2 M KCl aqueous solution. The multilayer films showed excellent electrochemical activity, high reversibility and high power density. A specific capacitance value of 288 F g⁻¹ was obtained at a current density of 1.25 A g⁻¹ for (PEI/MNS)₁₀ film in 2 M KCl aqueous solution. The specific capacitance decreased 9.5% of initial capacity over 1000 cycles at a high current density of 2.5 A g⁻¹. These good electrochemical properties could be attributed to the special microstructure of the electrode.     

Correlative high-resolution imaging of hydrogen in Mg2Ni hydrogen storage thin films

Nanometer scale imaging of hydrogen in solid materials remains an important challenge for the characterization of advanced materials, such as semiconductors, high-strength metallic alloys, and hydrogen storage materials. Within this work, we demonstrate high-resolution imaging of hydrogen and deuterium within Mg₂Ni/Mg₂NiH₄ hydrogen storage thin films using an in-house developed secondary ion mass spectrometer (SIMS) system attached to a commercially available dual-beam focused ion beam - scanning electron microscope (FIB-SEM) instrument. We further demonstrate a novel approach to measure the size, shape, and distribution of the hydride phase in partially transformed films using laser scanning confocal microscopy (LSCM) to measure surface topography changes from the hydride phase volume expansion. Combining these techniques provides new insights on hydride nucleation and growth within the Mg₂NiH_x system. Finally, we demonstrate the efficacy of tracking deuterium as a hydrogen analog to reduce the background for SIMS imaging of hydrogen in high-vacuum chambers (∼10⁻⁶ mbar).     

Nano-scale bi-layer Pd/Ta, Pd/Nb, Pd/Ti and Pd/Fe catalysts for hydrogen sorption in magnesium thin films

We analyzed the elevated temperature volumetric hydrogen sorption behavior of magnesium thin films catalyzed by nano-scale bi-layers of Pd/Ta, Pd/Nb, Pd/Ti and Pd/Fe. Sorption of magnesium catalyzed by pure Pd was determined as a baseline. Sorption cycling demonstrated that when utilizing pure Pd and the Pd/Fe bi-layer catalysts the sorption kinetics of the Mg films rapidly degraded. However with the Pd/Nb, Pd/Ti and Pd/Ta bi-layer catalysts the composite remained cycleable. After multiple sorption cycles the Pd/Nb and Pd/Ti catalyst combinations possessed the fastest kinetics. X-ray diffraction analysis showed that NbH_0.5 and TiH₂ are formed during testing. Basic thermodynamic analysis indicates that NbH_0.5 and TiH₂ should be stable both during absorption and during desorption. We believe that this is why Nb and Ti are the most effective intermediate layers: The elements form stable hydrides at the Mg surfaces preventing complete Pd-Mg interdiffusion and/or acting as hydrogen catalysts and pumps.     

Interaction mechanism of hydrogen storage materials with layer-by-layer applied protective polyelectrolyte coatings

Metal hydrides are promising compounds for the storage of hydrogen especially in the transport sector. Their high reactivity with moisture and difficult handling in ambient atmosphere prevents their easy application in tanks and thus the market penetration of this very promising and clean technology. In the present work we investigated the way to protect sodium borohydride with organic polyelectrolytes like polyethyleneimine (PEI) and dicarboxy terminated poly(acrylonitrile-co-butadiene-co-acrylic acid) (PABA) with emphasis on the interaction of those polyelectrolytes with the SBH. The polyelectrolytes protect SBH either by electrostatic adsorption or by reacting with it, depending on the reactive groups present in their structure. PABA undergoes only an electrostatic interaction with the SBH substrate. PEI on the other hand interacts chemically with the SBH substrate and forms complexes with groups on the SBH surface. Both polyelectrolytes can be applied in a layer-by-layer approach on the SBH in order to protect it. Regarding the best order of the layer application XPS results yield that the formation of the boron-amino-complexes between the PEI and the SBH is independent of the order in which the PABA and PEI layers are applied. However, the rearrangement of the SBH surface groups due to the application of the PEI films has been found to be considerably influenced by the order of application. The effect of a PEI film has been mostly compensated by the subsequent application of a PABA film, while the application of PEI had a much larger influence on the surface of a PABA coated SBH sample. Thus, the best order of the layer application with respect to the application according to XPS is to deposit the PEI layer at first and the PABA layer afterwards.     

Photocatalytic semiconductor thin films for hydrogen production and environmental applications

The semiconducting thin film materials are the main key elements in the fields of photonic, electronic and magnetic devices. The preparation of materials in the form of thin films allows easy incorporation into various through changing the properties of materials and reducing the size. The core benefits of thin film systems are cost reduction and efficiency of devices. They are widely used for the protection of surfaces, chemical resistance and to modify electrical and optical properties. The study on semiconducting thin films is rapidly growing due to their constructive applications in wireless communication, solar cells, semiconductor devices, integrated circuits, magneto-optic memories, light emitting diodes, multifunctional protective coatings, liquid crystal displays, and so on. This review explains about recent achievements in the photocatalytic nanostructured semiconducting thin films, their deposition, fabrication methodologies, controlling of film micro/nanostructures, physicochemical properties, photocatalytic mechanism and its photocatalytic applications in the fields of energy such as hydrogen generation, CO₂ reduction to useful chemicals, and environmental remediation such as photodegradation of harmful bacteria and toxic chemicals.     

Graphene coating as a protective barrier against hydrogen embrittlement

The applicability of a graphene coating as a protective barrier against hydrogen embrittlement was studied. To simulate the hydrogen embrittlement, complex environment of tensile stress with simultaneous hydrogen charging was applied. The strain at fracture, ductility and ultimate tensile strength of graphene-coated copper under the charged condition were preserved above 95% comparing uncharged bare copper. After hydrogen charging for 12 h, the hydrogen content in graphene-coated copper was lower than that in bare copper. Using attenuated total reflectance infrared spectroscopy and Raman spectroscopy, it was verified that graphene can interrupt the hydrogen penetration by the formation of C–H sp³ bonds. Unfortunately, it induced a distortion of graphene structure, which increased the defects in the graphene. Nevertheless, the graphene coating is expected to decrease the hydrogen embrittlement susceptibility of metal substrate.     

Superior hydrogen absorption and desorption behavior of Mg thin films

Pd-capped Mg films prepared by magnetron sputtering achieved complete dehydrogenation in air at room temperature and behaved as favorable gasochromic switchable mirrors. Their cyclic hydrogen absorption and desorption kinetics in air were investigated by using the Bruggeman effective medium approximation. The overall activation energy was 80 kJ mol⁻¹, while the reaction orders controlling desorption were deduced to be n = 2 at 328 K and n = 1 at lower temperatures by analyzing the transmittance data. The hydrogen diffusion coefficient and the corresponding activation energy were calculated by electrochemical measurements. Mg thin films exhibited the smaller activation energy and remarkable diffusion kinetics at room temperature which implied potential applications in smart windows.     

Nanostructured hematite thin films produced by spin-coating deposition solution: Application in water splitting

Doped and undoped hematite films for photoelectrochemical hydrogen production were prepared by spin-coating deposition solution (SCDS). To understand the influence of the Si-doping and identify the critical parameters of the proposed SCDS method an extensive characterization was conducted. The Si-doped hematite exhibited higher photocurrent response when compared with undoped films. We have shown that the crystallographic orientation degree of the films appears to be a dominant factor affecting the photocurrent. The performance of our hematite electrodes is well below the maximum theoretical efficiency and the conceivable explanation could be given by the high value of recombination phenomena (electron/hole pair).     

Structural evolution of Pd-capped Mg thin films under H2 absorption and desorption cycles

In this paper we present a study on the relation between the evolution, upon successive H₂ cycling, of the crystalline order and the H₂ sorption properties of Pd-capped textured Mg thin films grown on Si and glass substrates.     

Highly conductive, methanol resistant fuel cell membranes fabricated by layer-by-layer self-assembly of inorganic heteropolyacid

Layer-by-layer (LBL) self-assembly is a simple and elegant method of constructing organic-inorganic composite thin films from environmentally benign aqueous solutions. In this paper, we utilize this method to develop proton-exchange membranes for fuel cells. The multilayer film is constructed onto the surface of sulfonated poly(arylene ether ketone) (SPAEK-COOH) membrane by LBL self-assembly of polycation chitosan (CTS) and negatively charged inorganic particle phosphotungstic acid (PTA). The highly conductive inorganic nanoparticles ensure SPAEK-COOH-(CTS/PTA)_n membranes to maintain high proton conductivity values up to 0.086 S cm⁻¹ at 25 °C and 0.24 S cm⁻¹ at 80 °C, which are superior than previous LBL assembled electrolyte systems. These multilayer systems also exhibit extremely low water swelling ratio and methanol permeability. The selectivity of SPAEK-COOH-(CTS/PTA)₈ is 2 orders of more than Nafion^® 117, which is attractive in direct methanol fuel cells (DMFCs).     

ZnSe thin films by chemical bath deposition method

The ZnSe thin films have been deposited onto glass substrates by the simple chemical bath deposition method using selenourea as a selenide ion source from an aqueous alkaline medium. The effect of Zn ion concentration, bath temperature and deposition time period on the quality and thickness of ZnSe films has been studied. The ZnSe films have been characterized by XRD, TEM, EDAX, TRMC (time-resolved microwave conductivity), optical absorbance and RBS techniques for their structural, compositional, electronic and optical properties. The as-deposited ZnSe films are found to be amorphous, Zn rich with optical band gap, Eg, equal to 2.9 eV.     

A comparative study on the hydrogen absorption of thin films at room temperature deposited on non-porous glass substrate and nano-porous anodic aluminum oxide (AAO) template

The influence of different morphology of the thin films deposited on the non-porous glass and nano-porous anodic aluminum oxide (AAO) substrates on the hydrogen absorption at room temperature was studied. A well-known sandwich-like Pd/Mg/Pd film was investigated. It is observed that the film deposited on the porous AAO template demonstrates a better hydrogen absorption in the H₂ pressure range 10-600 mbar with respect to the same film supported by the non-porous glass substrate. Moreover, the layer grown on the AAO, owing to its specific morphology inherited from the nano-porous substrate, has revealed its resistance toward stress accumulation caused by the lattice expansion, showing no buckle-to-crack network formation upon the hydrogen uptake. This interesting feature is expected to improve the cycling properties and structural stability of the system, and may help to investigate better the interaction of the H₂ with metal.     

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.     

Dielectric function of palladium capped zirconium thin films as a function of absorbed hydrogen

We report measurements of the optical transmission, between 240 and 1040 nm, and electrical resistivity of polycrystalline zirconium thin films as they absorb hydrogen. Both are measured as H₂ pressure is increased up to 880 mbar, at room temperature. Films, 20–22 nm thick, are deposited on fused quartz substrates by e-beam evaporation at 5.3 × 10^?7 mbar base pressure and covered with a 8.0 nm Pd over-coat. The morphology of the films is studied by means of AFM images. The complex refractive indices of Zr and Pd are extracted numerically from the transmission spectra by using a spectral projected gradient method for different hydrogen pressures. The corresponding dielectric functions for various Zr hydrogen concentrations are described with the parametric Drude-Lorentz and Brendel-Bormann (DL & BB) models. The Acceptance-Probability-Controlled Simulated Annealing approach is applied to calculate the parameters of the DL & BB model. This allows us to describe the effect of increasing hydrogen absorption on these parameters and in derived quantities, like the relaxation time and the effective mass of conduction electrons, the electrical resistance, the Fermi energy, and the electronic density of states at the Fermi level.     

Study on the hydrogen barrier performance of the SiOC coating

SiOC coatings were prepared on X70 pipeline steel substrate by a simple dipping method at low temperatures, and their performance of hindering hydrogen penetration was studied through electrochemical hydrogen permeation experiment. The sample thermal-treated at 120 °C achieved a low diffusion coefficient of hydrogen of 8.20 × 10^?9 cm² s^?1, which was nearly three orders of magnitude lower than 3.58 × 10^?6 cm² s^?1 for the X70 steel. This was due to that the amorphous coating did not provide a stable hydrogen diffusion channel, thus limiting hydrogen diffusion. Density functional theory (DFT) calculation further proved that hydrogen moleculars were difficult to be adsorbed at different sites on the surface of the coating.