Imaging the hydrogenation of Mg thin films

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH₂ is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO₂). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.     

Metal buffer layer inserted switchable mirrors

Thin buffer layers of hydrogen diffusive metals such as Ti, Nb, and V were inserted between a Mg₄Ni thin film and a Pd top layer, which were prepared by DC magnetron sputtering. Their optical, electrochemical properties, and switching durability were investigated using both gasochromic and electrochromic switching methods. It has been proved that Ti, Nb, and V buffer layers can protect the migration of Mg to surface of thin film during the switching processes, and also service as a protection layer against the oxidization of Mg. These metal buffer layers do not affect the hydrogenation of Mg–Ni alloy mirrors system when switching with hydrogen gas or electrochromic, rather, its dehydrogenation speed is accelerated greatly. Switching cyclic number of those metal buffer layer inserted mirrors achieved 400–500 cycles which was enhanced ca. 3 times than non-inserted one.     

Concentration-dependent hydrogen diffusion in hydrogenation and dehydrogenation of vanadium-coated magnesium nanoblades

We carry out a computational investigation to show how the exponential concentration dependence of hydrogen diffusion, which was recently verified in a combined experimental and analytical study, could affect the characteristics of hydrogenation and dehydrogenation of a vanadium-coated magnesium nanoblade. A reaction model is built that separates hydrogen surface sorption and interior diffusion during the hydrogenation/dehydrogenation process. For the hydrogenation process, the hydrogen surface adsorption is much faster than the hydrogen diffusion, resulting in high hydrogen concentration buildup at the surface at a relatively low temperature. With increasing temperature, the hydrogen diffusion time decreases more rapidly than the hydrogen surface adsorption time. This leads to a relatively low-gradient diffusion field in the nanoblade during most time of the hydrogenation process, and no shell-core structure with a finite hydride layer is observed. However, for the dehydrogenation process, when hydrogen molecules are released at the surface, a hydride core is formed inside the nanoblade and the interface recedes gradually. The receding rate of the hydride core is determined by the hydrogen molecule release rate. In a two-dimensional simulation with decorated vanadium catalyst islands on the surface, isolated interior hydride islands are sometimes observed before the hydride core entirely fades away. The hydride core boundary is sharper at lower temperature when the surface reaction rate is high relative to the interior diffusion rate.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

In-situ investigation on hydrogenation-dehydrogenation of Pd–Ag alloy films

The present work reports on synthesis of Pd–Ag nano-composite films by magnetron co-sputtering and the structural changes in the alloy film during hydrogenation and dehydrogenation. Synchrotron X-ray diffraction is employed in-situ to reveal subtle structural changes occurring during hydrogenation and dehydrogenation processes, an aspect not investigated so far. It is revealed that the nanocomposite film having 88 at% Pd shows the formation of α-phase as an intermediate phase, however, completion of the hydrogenation process yields only β-phase. No β-phase formation is observed in nanocomposite thin film containing 54 at% of Pd, suggesting the suppression of formation of β-phase with increase in Ag concentration. On dehydrogenation, the peak returns to its original position i.e. the value before hydrogenation. The data also demonstrated that the addition of Ag in Pd results in complete removal of dissolved hydrogen thereby eliminates the problem of hysteresis. The study shows that the lower concentrations of Ag in Pd are better in terms of extent of peak-shift on hydrogenation/dehydrogenation and faster response/recovery kinetics.     

Analysis and improvement of cyclic stability of H2 sensing properties of Pd/Mg-Ni films

The nominal electrical resistivity of palladium coated magnesium-nickel (Pd/Mg-Ni) films was measured by exposing the films iteratively to hydrogen (H₂) at a concentration of CH₂ for hydrogenation and air for dehydrogenation. When a low CH₂ was used, the film remained in an amorphous α-phase. H atoms interacted “interstitially” with the atomic network, and the H₂ detection sensitivity S was relatively stable in the cyclic test. If higher CH₂ values were used, the film was partially or completely transformed to an amorphous β-phase. Significant volumetric breathing occurred in the course, leading to severe roughening of the film and oxidation of the Mg-Ni layer. S became unstable. These suggestions are supported by the results of film thickness measurements, atomic force microscopy and X-ray photoelectron spectroscopy analyses. Stability of S of a Pd/Mg-Ni film can be greatly improved by either (i) operating the film in a low CH₂ environment to prevent substantial volume breathing, or (ii) choosing an appropriate thickness of the Pd layer to optimize both oxidation resistance and sensing response of the film sensor.     

Nano-Pd/CeO2 catalysts for hydrogen storage by reversible benzene hydrogenation/dehydrogenation reactions

Nano-CeO₂ supports, which have different structure from different preparation methods, were used to prepare nano-Pd/CeO₂ catalysts. The hydrogen storage capacity of prepared nano-Pd/CeO₂ catalysts were studied via vapor phase benzene hydrogenation and cyclohexane dehydrogenation reactions. Results show that the prepared Pd/CeO₂ catalysts exhibit excellent benzene hydrogenation and cyclohexane dehydrogenation performances. The catalytic performance of the Pd/CeO₂ catalysts is related to the dispersion of metallic Pd, hydrogen adsorption-desorption ability and structure of Pd/CeO₂ catalysts and so on. And those properties are also directly affected by the morphology and mesoporous structure of the prepared nano-Pd/CeO₂ catalysts that can be regulated by CeO₂ support preparation methods. The synergistic effect between metal Pd, CeO₂ support and their structures can effectively promote benzene hydrogenation and cyclohexane dehydrogenation, thus promoting hydrogen storage capacity. The prepared Pd/CeO₂-HT catalyst, which has high specific surface area, developed pore structure and highly dispersed metal Pd species, exhibits superior catalytic performances. And, the Pd/CeO₂-HT catalyst exhibits superior catalytic hydrogen storage performances. The benzene conversion over it at 200 °C reaches 99.5%. Whereas the cyclohexane conversion at 450 °C is 65.3%, and the H₂ production capacity is 73.77 g/h.     

Thermodynamics, stress release and hysteresis behavior in highly adhesive Pd-H films

We investigate the role of clamping on the thermodynamics of highly adhesive metal hydride thin films. Using Pd as a model system, we add Ti as an intermediate adhesion layer to increase the interaction with the substrate. We show that Pd/Ti films remain clamped during (de-)hydrogenation while the stress release occurs by means of rearrangement and pile-up of the material. The compressive stress build-up reaches a value of about 1.5 GPa during hydrogen absorption. The enthalpy of hydride formation and decomposition, measured using Hydrogenography is found to decrease and increase by about 2.7 and 1.3 kJ/mol H₂ respectively, as compared to buckled Pd films. A simple model confirms that the change in the thermodynamics and the asymmetric expansion of the hysteresis correlate with the mechanical work needed to accommodate the stress induced plastic deformations in clamped Pd/Ti films during the (de-)hydrogenation cycle.     

Pd nanoparticles immobilized on aniline-functionalized MXene as an effective catalyst for hydrogen production from formic acid

A simple wet chemical method was used to prepare two-dimensional transition metal carbides (MXene); PDA-MXene was prepared by alkalization of p-phenylenediamine (PDA) on MXene. And further, Pd metal nanoparticles (NPs) were conveniently loaded on the surface to catalyze the dehydrogenation of formic acid. The as-prepared Pd/PDA-MXene catalyst for the formic acid dehydrogenation was characterized by XRD, IR, TEM, and XPS. Pd-NPs with a size of about 4 nm were formed upon the PDA-MXene support surface and were well dispersed. The Pd/PDA-MXene exhibited good catalytic activity in the formic acid decomposition process without any additives, and the turnover frequency value at 50 °C was 924.4 h⁻¹, which is comparable to most of the reported noble metal catalysts under similar conditions. It is worth mentioning that the prepared catalyst maintained good catalytic activity in five consecutive catalytic cycles of the formic acid dehydrogenation experiment.     

Poly-Si thin films by metal-induced growth for photovoltaic applications

Poly-Si films were produced using a metal-induced growth technique by sputtering from an n-type Si target onto a 50 nm thick Co seed-layer at 625°C. Silicon grew heteroepitaxially on the CoSi₂ layer formed due to the reaction between the sputtered Si atoms and Co at the beginning stage of deposition. A 5 μm thick Si film with grain features up to 1 μm was produced on the thin and flexible tungsten substrate by using a two-step sputtering method. The films also have a natural texture structure on the surface that is strongly recommended in thin-film solar cells in order to obtain high current density by increasing incident light trapping. After post-sputtering annealing at 700°C, the measured minority carrier lifetime for poly-Si film was 1.33 μs which shows the film to be suitable for photovoltaic applications. To explore the photovoltaic applications by using MIG poly-Si films, Au/n-Si Schottky photodiodes were fabricated due to the process simplicity. The effects of different parameters, which include film doping density, active-layer thickness, Si film surface conditions and hydrogenation, were studied. It was found that with the increasing of doping density, the open-circuit voltage (V_oc) increased while short-circuit current density (J_sc) decreased. Increasing the poly-Si active-layer thickness tended to improve the light absorption with an increased J_sc, but the V_oc was decreased due to a higher value of reverse saturation current. Because the metal/semiconductor interface condition facilitates the carrier transport in Schottky devices, the earlier study of modifying the Si surface by polishing showed an improved V_oc. The overall photo response was further improved by plasma hydrogenation.     

Atomistic insight into dehydrogenation and oxidation of aluminum hydride nanoparticles (AHNPs) in reaction with gaseous oxides at high temperature

Metal hydride additives, aluminum hydride in particular, have extensively been applied in solid rocket motor propellants. This work employed the reactive force field molecular dynamics to elaborate the underlying mechanism for the oxidation of highly active aluminum hydride nanoparticles (AHNPs) by gaseous oxides (CO, CO₂, NO, and NO₂). The results showed that AHNPs first went through four stages: dehydrogenation (<84 ps), Al nucleation and growth (>25 ps), micro-explosion (~31 ps), and oxidation (>28 ps). The dehydrogenation of AHNPs surface overlaps with the Al nucleation in the preheating stage and prevents the oxidation of Al by gaseous oxides. Only a small part of Al on the surface is oxidized to form a thin and uneven oxide film (0.18–0.54 nm). In the core, the formed H₂ is hindered by the shell and gradually gathers into H₂ bubbles. H₂ bubbles have great kinetic energy and become a micro-explosion promoter, eventually causing nanoparticles to burst at high temperatures. The micro-explosion accelerates the dissociation of gaseous oxides. This study provides an in-depth understanding of the mechanism of dehydrogenation and oxidation of metal hydride nanoparticles.     

Magnetron sputtering as a method for introducing catalytic elements to magnesium hydride

Magnesium hydride is a very interesting material suggested to be used for storing hydrogen because of its high capacity (～7.5 wt%). However its application is limited by high decomposition temperature and poor kinetics. This paper presents results of the preliminary study on the new approach to introduction of surface dopants by magnetron sputtering on powdery substrates. Thin films of nickel were successfully deposited on the magnesium hydride powder. SEM observations and EDS elements mapping show uniform and continuous layers, with the thickness up to 320 nm, formed on hydride grains. It was proven by measurements with Sievert's method that such a surface modification increases the H₂ dissociation/recombination speed and effectively enhances hydrogenation/dehydrogenation reaction rate. The DSC measurements show the reduction of the activation energy of hydrogen desorption by ～150 kJ/mol and a decrease of the decomposition temperature by 50 K. The study shows that uniform nickel coating of hydride powder by magnetron sputtering can be considered as an effective way for introduction of the catalytic elements and improvement of the hydrogen storage properties of the magnesium hydride.     

Hydrogenation properties of pure magnesium and magnesium–aluminium thin films

We have studied the hydrogenation/dehydrogenation behaviour of multilayered stacks of Pd/Mg/Pd and Pd–Fe(Ti)–Mg–Al–Mg–Fe(Ti)–Pd grown by electron beam physical vapour deposition. The palladium coating was deposited at both sides of the structure to ensure a fast dissociation rate and good transport properties for hydrogen as well as to avoid oxidation of magnesium either from atmosphere as from the substrate surface. Fe and Ti layers were included in the stack composition in order to assess their possible catalyst effect as well as to prevent the formation of Mg_xPd_y intermetallics during the thermal treatments. We have studied the structure evolution after thermal treatments as well as after the hydrogenation and dehydrogenation processes using XRD. We have also followed the reactions kinetics by resistometry and differential scanning calorimetry. The nanostructured Mg films have been hydrogenated at temperature as low as 50 °C in few minutes. Adding aluminium to magnesium has improved its hydrogenation capacity. We have also observed that the formation of an Mg_xAl_y intermetallic before hydrogenation improves the storage capacity. We have confirmed that titanium is a better catalyst for the hydrogenation/dehydrogenation of the Mg films.     

Synergetic catalyst effect of Ni/Pd dual metal coating accelerating hydrogen storage properties of ZrCo alloy

Aimed at enhancing the hydrogen absorption/desorption performances of ZrCo system, Ni/Pd dual metal coating is employed on ZrCo alloy combined with the electroless plating and displacement plating. The effects of Ni/Pd dual metal coating on the microstructure, hydrogen storage performance of ZrCo alloys were investigated systematically. The results show that Ni/Pd dual metal coating deposits on the surface of ZrCo sample successfully with the thickness of 500 nm. The hydrogen absorption kinetic property is substantially enhanced for ZrCo alloy after Ni/Pd dual metal coating, which is owing to the catalytic effect of Ni/Pd coating. Further, the activation energies (E_a) for hydrogen absorption and desorption are calculated using the Arrhenius Equation and Kissinger method, respectively. Compared with the bare ZrCo, the activation energies of the Ni/Pd coated samples for hydriding/dehydriding process decrease which facilitate the hydrogenation/dehydrogenation reaction. This work introduces a rational approach by building new catalytic coating on the hydrogen storage materials to improve the hydriding/dehydriding kinetic performance.     

Selective hydrogen absorption from gaseous mixtures by BCC Ti-V alloys

Hydrogen absorption behaviour of the BCC Ti-V alloys has been studied as related to their potential application for the selective hydrogen absorption from mixtures with active gases containing CO and steam at high temperatures. Alloys and their hydrides were characterised using Scanning Electron Microscopy (SEM), Thermal Desorption Spectroscopy (TDS), Temperature Programmed Reaction (TPR) and in situ synchrotron X-ray powder diffraction (SR-XRD). BCC Ti_0.8V_0.2 and Ti_0.9V_0.1 were able to absorb 3.95 wt.% H by forming a FCC type hydride γ−(Ti,V)H₂. Nanoparticles of Pd and Pd/Pt were electroless deposited on the surface of the hydrides to catalyse hydrogen absorption. TPR tests showed that such alloys were capable of absorbing hydrogen even when substantial amounts of CO and H₂O were present in the gas stream. Nanoparticles of Pd/Pt provide a better performance as compared to Pd alone. In situ SR-XRD has been used to probe the mechanisms of hydrogenation and dehydrogenation.     

Reversible hydrogen storage in vapour deposited Mg-5 at.% Pd powder composites

Mg-5 at.% Pd powder composites derived from multilayered films of Mg and Pd deposited in Pd/Mg/Pd/Mg/Pd layer configuration by thermal evaporation reversibly store about 3.5 wt.% hydrogen up to 15 cycles under mild conditions of pressure and temperature. Hydrogenation takes place at 0.15 MPa hydrogen pressure while dehydrogenation occurs in a dynamic rotary vacuum. Each process is completed in about three hours. The temperature of a dehydrogenation or hydrogenation step is about 5–10 K higher than the preceding hydrogenation or dehydrogenation step. The hydrogenation temperature of the first cycle is 343 K whereas the dehydrogenation temperature of the 15th cycle is 423 K. The hydrogen storage capacity of composite is the manifestation of fine-grained microstructure of Mg and the catalytic properties of Pd. It declines beyond 423 K due to the exhaustion of metallic Pd as a result of the formation of Mg–Pd intermetallic compounds. This approach presents a simple and rapid method of preparing Mg–Pd composites for hydrogen storage applications.     

Hydrogen storage properties of nano-structured 0.65MgH2/0.35ScH2

The intermetallic compound Mg_0.65Sc_0.35 was found to form a nano-structured metal hydride composite system after a (de)hydrogenation cycle at temperatures up to 350 °C. Upon dehydrogenation phase separation occurred forming Mg-rich and Sc-rich hydride phases that were clearly observed by SEM and TEM with the Sc-rich hydride phase distributed within Mg/MgH₂-rich phase as nano-clusters ranging in size from 40 to 100 nm. The intermetallic compound Mg_0.65Sc_0.35 showed good hydrogen uptake, ca. 6.4 wt.%, in the first charging cycle at 150 °C and in the following (de)hydrogenation cycles, a reversible hydrogen capacity (up to 4.3 wt.%) was achieved. Compared to the as-received MgH₂, the composite had faster cycling kinetics with a significant reduction in activation energy E_a from 159 ± 1 kJ mol⁻¹ to 82 ± 1 kJ mol⁻¹ (as determined from a Kissinger plot). Two-dehydrogenation events were observed by DSC and pressure–composition-isotherm (PCI) measurements, with the main dehydrogenation event being attributed to the Mg-rich hydride phase. Furthermore, after the initial two cycles the hydrogen storage capacity remained unchanged over the next 55 (de)hydrogenation cycles.     

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN_x film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1–10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN_x-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based on our results, in which defect passivation is governed by the release of hydrogen from the SiN_x film due to annealing, the generation of vacancies during Al–Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3–1.5% absolute when compared to slow, belt furnace contact firing. This enhancement was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%.     

Rapid activation of MmNi5−xMx based MH alloy through Pd nanoparticle impregnation

Faster activation of a multi-component AB₅ based alloy metal hydride electrode through Pd nanoparticle (NP) impregnation is demonstrated. Pd nanoparticle impregnated MmNi_5−xM_x based alloy was prepared and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and elemental mapping techniques. Electro-catalytic activity of laminar metal hydride electrodes containing Pd nanoparticles and micrometer size Ni particles was studied. Hydrogen absorption efficiency of the nanocomposite electrodes was compared with the metal hydride electrodes without Pd nanoparticles. The incorporation of nanostructured materials in the metal hydride alloy increased its hydrogen absorption capacity at the initial stage and activated much faster, indicating its good prospect for energy storage applications.     

The mechanism of the water dissociation and dehydrogenation of glycerol on Au (111) and PdAu alloy catalyst surfaces

Hydrogen is an energy carrier found from renewable sources such as biomass, geothermal, solar, or wind. Water splitting and dehydrogenation of glycerol is a sustainable process of H₂ production from renewables because water is abundant, and the glycerol is formed from the biomass-derived compounds. However, finding a suitable and best catalyst for these processes is challenging. Thus, this paper proposed a theoretical study to find the mechanism of the dissociation of water and dehydrogenation of glycerol using Au metal and PdAu alloy catalysts using the density functional theory (DFT) method. Four PdAu alloys have been constructed with different atomic compositions ranging from 1 to 3 of Pd metal to Au metal. The result showed strong adsorption on the Pd₁Au₃ catalyst surface, and the water splitting is best on the Pd₃Au₁ catalyst surface. Simultaneously, the glycerol adsorption on catalyst surfaces is tested before proceeding for the complete dehydrogenation mechanism of glycerol. Strong adsorption was found at the Pd₁Au₃ catalyst compared to other catalyst surfaces on the glycerol adsorption. The dehydrogenation mechanism was found toward a downhill reaction and removed eight hydrogens from the glycerol compared to Au metal, referring to easy dehydrogenation of glycerol using the alloy PdAu. The final species that adsorbed on the Pd₁Au₃ surface is the carbon monoxide will be turned later into carbon dioxide.