Palladium Membrane with High Density of Large-Angle Grain Boundaries to Promote Hydrogen Diffusivity

A higher density of large-angle grain boundaries in palladium membranes promotes hydrogen diffusion whereas small-angle grain boundaries suppress it. In this paper, the microstructure formation in 10 µm thick palladium membranes is tuned to achieve a submicronic grain size above 100 nm with a high density of large-angle grain boundaries. Moreover, changes in the grain boundaries’ structure is investigated after exposure to hydrogen at 300 and 500 °C. To attain large-angle grain boundaries in Pd, the coating was performed on yttria-stabilized zirconia/porous Crofer 22 APU substrates (intended for use later in an ultracompact membrane reactor). Two techniques of plasma sprayings were used: suspension plasma spraying using liquid nano-sized powder suspension and vacuum plasma spraying using microsized powder as feedstock. By controlling the process parameters in these two techniques, membranes with a comparable density of large-angle grain boundaries could be developed despite the differences in the fabrication methods and feedstocks. Analyses showed that a randomly oriented submicronic structure could be attained with a very similar grain sizes between 100 and 500 nm which could enhance hydrogen permeation. Exposure to hydrogen for 72 h at high temperatures revealed that the samples maintained their large-angle grain boundaries despite the increase in average grain size to around 536 and 720 nm for vacuum plasma spraying and suspension plasma spraying, respectively

Palladium Membrane with High Density of Large-Angle Grain Boundaries to Promote Hydrogen Diffusivity

A higher density of large-angle grain boundaries in palladium membranes promotes hydrogen diffusion whereas small-angle grain boundaries suppress it. In this paper, the microstructure formation in 10 µm thick palladium membranes is tuned to achieve a submicronic grain size above 100 nm with a high density of large-angle grain boundaries. Moreover, changes in the grain boundaries’ structure is investigated after exposure to hydrogen at 300 and 500 °C. To attain large-angle grain boundaries in Pd, the coating was performed on yttria-stabilized zirconia/porous Crofer 22 APU substrates (intended for use later in an ultracompact membrane reactor). Two techniques of plasma sprayings were used: suspension plasma spraying using liquid nano-sized powder suspension and vacuum plasma spraying using microsized powder as feedstock. By controlling the process parameters in these two techniques, membranes with a comparable density of large-angle grain boundaries could be developed despite the differences in the fabrication methods and feedstocks. Analyses showed that a randomly oriented submicronic structure could be attained with a very similar grain sizes between 100 and 500 nm which could enhance hydrogen permeation. Exposure to hydrogen for 72 h at high temperatures revealed that the samples maintained their large-angle grain boundaries despite the increase in average grain size to around 536 and 720 nm for vacuum plasma spraying and suspension plasma spraying, respectively

Wasserstofftransport in dünnen Filmen : Mg-MgH2 und Ti-TiH2

Hydrogen storage has become progressively important due to increasing energy demand. Magne-sium (Mg/MgH2) is one of the most promising elements of hydrogen uptake, however, the slow kinetics and need for high temperatures during dehydrogenation make this material challenging for mobile applications. Meanwhile, Titanium (Ti/TiH2/TiO2) draws attention due to its catalytic effect in hydrogenation of other metals with higher capacities. A comprehensive way to quantitatively char-acterize the kinetics of hydride formation in both systems (Mg and Ti) is shown here. A technique allowing a large range of pressures and temperatures (room temperature to 300 °C and from 0.05 bar up to 100 bar) is developed successfully. Thin films (50-1000 nm), deposited by ion beam sput-tering (PVD), are used because of their smooth surface and defined structure. In order to study hydrogen transport precisely, X-ray diffraction (XRD), electron microscopy (SEM/FIB/TEM) and electric resistance measurements are used. In the case of Mg, while a Pd coating is used as catalyst, the hydride is formed from the surface towards the substrate and transformation in the morpholo-gy is observed. Parabolic law is followed and the diffusion coefficient of hydrogen in MgH2 is ob-tained at room temperature (2.67 · 10-17 cm2/s). Additionally, a model is created to fit the experi-mental change in resistance during hydrogen loading and shows the changes in the behavior of thicker layers. The interface between Pd/Mg is discussed, since Mg5Pd2 and Mg6Pd are formed at high temperatures and are most dominant over dehydrogenation. However, at room temperature, this interface appears to be more stable. The activation energy of hydrogenation is calculated ex-perimentally from an Arrhenius plot to be equal to Ea = 22.6 ± 2.0 kJ/mol and the pre-factor D0 = 3904 cm2/s. Additional attention is given to magnesium hydride as an anode electrode in Li-ion bat-teries. TEM investigations of thin film electrodes demonstrate the complete lithiation of the mate-rial however, with drastic volume changes, leading to bad reversibility. In Ti the thin oxide layer naturally formed on the surface, appears to play a dominant role in the kinetics of hydrogen transport leading to a linear kinetics. A pressure dependency is observed, while an experimental evaluation of the permeation coefficient in the oxide is also discussed. Important information on the hydrogen transport is obtained in both systems, giving an input for further improvements of such hydrides.Die Wasserstoffspeicherung ist aufgrund des steigenden Energiebedarfs zunehmend wichtig gewor-den. Magnesium (Mg / MgH2) ist eines der vielversprechendsten Elemente der Wasserstoffaufnahme, die langsame Kinetik und der Bedarf an hohen Temperaturen während der Dehydrierung machen die-ses Material jedoch zu einer Herausforderung für mobile Anwendungen. Auch macht Titan (Ti / TiH2 / TiO2) aufgrund seiner katalytischen Wirkung bei der Hydrierung anderer Metalle mit höheren Kapazi-täten auf sich aufmerksam. Ein umfassender Weg zur quantitativen Charakterisierung der Kinetik der Hydridbildung in beiden Systemen (Mg und Ti) wird hier gezeigt. Eine Technik, die einen großen Bereich von Drücken und Temperaturen (R.T. bis 300 ° C und von 0,05 bar bis 100 bar) ermöglicht, wurde er-folgreich entwickelt. Dünne Schichten (50-1000 nm), abgeschieden durch Ionenstrahl-Sputtern (PVD), werden wegen ihrer glatten Oberfläche und definierten Struktur verwendet. Um den Wasser-stofftransport genau zu untersuchen, werden Röntgenbeugung (XRD), Elektronenmikroskopie (SEM / FIB / TEM) und elektrische Widerstandsmessungen verwendet. Im Fall von Mg wird, wenn Pd als Kata-lysator an der Oberfläche verwendet wird, das Hydrid von der Oberfläche in Richtung auf das Substrat gebildet. Gleichzeitig wird eine Veränderung der Morphologie beobachtet. Ein parabolisches Zeitge-setz wird befolgt und der Diffusionskoeffizient in MgH2 wird experimentell bei R.T. (2.67 · 10-17 cm2/s) bestimmt. Darüber hinaus wurde erfolgreich ein Modell erstellt, um die experimentelle Widerstands-änderung während der Wasserstoffbeladung zu berücksichtigen und die Veränderungen im Verhalten dickerer Schichten zu zeigen. Die Grenzfläche zwischen Pd / Mg wird diskutiert, da Mg5Pd2 und Mg6Pd bei hohen Temperaturen gebildet werden und die Dehydrierung kontrollieren. Bei R.T. scheint diese Grenzfläche jedoch stabil zu sein. Die Aktivierungsenergie der Hydrierung wird experimentell aus ei-nem Arrhenius-Diagramm berechnet, zu Ea = 22 ± 2 kJ/mol bei einem Vorfaktor von D0 = 3904 cm2/s. Zusätzliches Augenmerk wird auf Magnesiumhydrid als Anodenelektrode in Li-Ionen-Batterien gelegt. TEM-Untersuchungen von Dünnschichtelektroden zeigen die vollständige Lithiierung des Materials, je-doch mit drastischen Volumenänderungen, die im Laufe der Zeit zu einer schlechten Reversibilität füh-ren. In Ti scheint die natürlich gebildete dünne Oberflächenoxidschicht eine dominante Rolle in der Kinetik des Wasserstofftransports zu spielen, was zu einer linearer Kinetik der Hydridbildung führt. Die Druckabhängigkeit wird beobachtet und der experimentelle Bewertung des Permeationskoeffizient im Oxid ebenfalls quantifiziert. Wichtige Information über den Wasserstofftransport in beiden Systemen wird so erhalten, was wesentlichen Input für weitere Verbesserungen solcher Hydride liefert

Hydrogen sorption kinetics in MgH2 and TiH2 thin films

The diffusion mechanism of H in metals and metal hydrides is studied particularly at high H2 pressures. Thin films of Mg and Ti offer a convenient tool to quantify the atomic transport. We show how different parameters of hydrogenation affect the kinetics. At 200°C, an interface controlled reaction is predominant and a linear regime of hydrogenation is observed, whereas at 300°C a parabolic regime is detected. In Mg, the hydride forms from the surface to the substrate whereas in Ti, growth of TiH2 starts from the substrate. Linear kinetics is seen during hydrogenation of Ti films, which is due to the naturally formed oxide layer on top, measured to be about 10nm thick. In the studied high pressure regime, the hydrogenation is not pressure dependent any more. Quantitative calculation of the growth rate and the diffusion coefficient of H in the hydrides is presented.</p

The role of surface oxides on hydrogen sorption kinetics in titanium thin films

Titanium is presently discussed as a catalyst to accelerate the hydrogenation kinetics of hydrogen storage materials. It is however known that H absorption in Ti decisively depends on the surface conditions (presence or absence of the natural surface oxide). In this work, we use Ti thin films of controlled thickness (50–800 nm) as a convenient tool for quantifying the atomic transport. XRD and TEM investigations allow us to follow the hydrogenation progress inside the film. Hydrogenation of TiO2 /Ti bi-layers is studied at 300 °C, for different durations (10 s to 600 min) and at varying pressures of pure H2 atmosphere. Under these conditions, the hydrogenation is found to be linear in time. By comparing films with and without TiO2 , as well as by studying the pressure dependence of hydrogenation, it is demonstrated that hydrogen transport across the oxide represents the decisive kinetic barrier rather than the splitting of H2 molecules at the surface. Hydrogenation appears by a layer-like reaction initiated by heterogeneous nucleation at the backside interface to the substrate. The linear growth constant and the H diffusion coefficient inside the oxide are quantified, as well as a reliable lower bound to the hydrogen diffusion coefficient in Ti is derived. The pressure dependence of hydrogen absorption is quantitatively modelled. </p

Ionic conductivity of melt-frozen LiBH4 films

The fast Li conductivity of LiBH4 envisages its use in all-solid-state batteries. Powders are commonly applied. But here, we study the formation of dense micrometer films by melting, spinning and subsequent solidifying. Characterized by electron microscopy, and spectroscopy (EDX/XPS/impedance), a reversible phase transformation is confirmed as well as a maximum conductivity of 103 S cm−1

improving plasma sprayed Raney type nickel–molybdenum electrodes towards high performance hydrogen evolution in alkaline medium

Rationally designed free standing and binder free Raney type nickel–molybdenum (ni–Mo) electrodes produced via atmospheric plasma spraying (APS) are developed by correlating APS process parameters with the microstructure of electrodes and their electrochemical performance in alkaline media. the results revealed that the electrode morphology and elemental composition are highly affected by the plasma parameters during the electrode fabrication. It is found that increasing plasma gas flow rate and input plasma power resulted in higher in-flight particle velocities and shorter dwell time, which in result delivered electrodes with much finer structure exhibiting homogeneous distribution of phases, larger quantity of micro pores and suitable content of Ni and Mo. Tafel slope of electrodes decreased with increasing the in-flight particles velocities from 71 to 33 mV dec-1 in 30 wt.% KOH. However, beyond a critical threshold in-flight velocity and temperature of particles, electrodes started to exhibit larger globular pores and consequently reduced catalytic performance and higher Tafel slop of 36 mV dec-1 in 30 wt.% KOH. Despite slightly lower electrochemical performance, the electrodes produced with highest plasma gas flow and energy showed most inter-particle bonded structure as well as highest stability with no measurable degradation over 47 days in operation as HER electrode in 30 wt.% KOH. The Raney-type Ni–Mo electrode fabricated at highest plasma gas flow rate and input plasma power has been tested as HER electrode in alkaline water electrolyzer, which delivered high current densities of 0.72 and 2 A cm-2 at 1.8 and 2.2 V, respectively, representing a novel prime example of HER electrode, which can synergistically catalyze the HER in alkaline electrolyzer. This study shows that sluggish alkaline HER can be circumvented by rational electrode composition and interface engineering

A Review of the MSCA ITN ECOSTORE Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Sklodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discusse