Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Synthesis and hydrogenation properties of Mg–La–Ni–H system by reactive mechanical alloying

We have synthesized Mg–30 mass%LaNi_2.28 composite material and investigated its hydrogenation behaviour. The reactive mechanical alloying process of the mixture of Mg and LaNi_2.28 was studied. It is found that a composite of _MgH2${\mathrm{MgH}}_2$, _La4H12.9${\mathrm{La}}_4{\mathrm{H}}_{12.9}$ and _Mg2NiH4${\mathrm{Mg}}_2{\mathrm{NiH}}_4$ formed after 80 h ball-milling under 3.0 MPa hydrogen. Scanning electron microscopic analysis indicated that these new phases are distributed homogeneously. This composite shows excellent hydriding properties even at moderate temperature. Under 3.0 MPa hydrogen pressure it absorbed more than 80% of its full capacity in the temperature range of 473–553 K within less than 1 min. The maximum hydrogen absorption capacity at 553 K is 5.4 mass%. The enhanced hydriding properties could be attributed to the fine and uniform particles and a synergeticly catalytic effect generated by mechanical milling.     

Hydrogen production via hydrolysis reaction from ball-milled Mg-based materials

Hydrogen generation by hydrolysis of Mg and MgH₂${}_2$ has been investigated in pure water and 1 M KCl. It has been found that hydrolysis reaction of Mg and Mg–Ni composite, both obtained by high-energy ball milling, is faster and extensive when they are immersed in 1 M KCl. In contrast, milled Mg and Mg–Ni composite in pure water, MgH₂${}_2$ and MgH₂${}_2$–Ni composites in pure water and in 1 M KCl show low yield and reactivity. Hydrolysis kinetics and yield are maximum with Mg–10 at% Ni composite milled for 30 min, so reaction is fully completed within an hour in the presence of chloride ions. It is related to the creation of micro-galvanic cells between Mg and dispersed Ni elements, accentuating greatly Mg corrosion in highly conductive aqueous media. A significant increase of the H₂${}_2$ production is also observed with 30 min milled Mg sample, likely because of the accentuation in the pitting corrosion resulting from the creation of numerous defects and fresh surfaces through the milling process. On the other hand, intensive ball milling of pure magnesium has no effect on the Mg reactivity in pure water. Ball milling effect is likely masked by the significant Mg passivation in pure water. A correlation is established between the conversion yield of ball-milled MgH₂${}_2$ powder in pure water and its effective surface area, which is increased by the milling process. Ni addition has no effect on the hydrolysis reaction in nonconductive media (i.e. pure water) and with nonconductive material (i.e. MgH₂${}_2$).     

Composite structure and hydrogen storage properties in Mg-base alloys

In order to improve the hydrogen absorption/desorption kinetic properties of Mg and Mg–Ni alloys, composite hydrogen storage alloys in the form of powder and film have been synthesized and investigated. For fabricating the composite powder, Mg or Mg–Ni powder was mechanically alloyed with _MmNi3.5(CoAlMn_)1.5${\mathrm{MmNi}}_{3.5}(\mathrm{CoAlMn}{)}_{1.5}$ alloy. For the preparation of the film with a composite structure, evaporation deposition and magnetron sputtering methods have been used to fabricate Mg–Ni film with multi-phase structure and Mg/Mm–Ni and Mg–Ni/Mm–Ni multi-layer film. By controlling the fabrication process, the microstructure feature, such as phase constituent, grain size, interlayer distances, interlayer boundary structure, of the composite can be modified. To reveal the influence of the composite structure on the hydrogen absorption/desorption kinetic properties of Mg and Mg–Ni alloys, hydrogen storage properties of the composite were measured with their microstructure features varied systematically. The present work shows that the hydrogen sorption properties of Mg and Mg–Ni-based alloys can be substantially improved by forming composites having proper microstructure features.     

The effects of Si and expanded PTFE substrates on formation and hydrogenation of Mg and Mg–Ti films

In this work, we investigated how DC and pulsed DC Ar gas plasma treatment changes surface topography and chemical composition of silicon and expanded Polytetrafluoroethylene (ePTFE) substrates and how different surface pretreatment techniques can affect the formation and hydrogenation of Mg and Mg–Ti films. It is observed that pre-treating Si and ePTFE substrates with different plasma modes results in significant changes of microstructures of as-deposited Mg and Mg–Ti films. After the hydrogenation of Mg films at 20 bar H₂ pressure and 180 °C temperature the formation of crystalline MgH₂ phase is observed only for the films deposited on plasma non-treated Si substrates which had the films with smallest dimensions of its columnar structure. It is known that, usually, Ti additives has positive effect on hydrogenation properties of Mg, but in this study independently of the used substrate pre-treatment technique after the hydrogenation of Mg–Ti films their XRD analysis showed no peaks of the crystalline hydride phase. However, depending on the surface properties of the substrate after hydrogenation Mg–Ti films also have several disparities which are discussed and attributed to the potentially related substrate features.     

Function of Al on the cycling behavior of the La–Mg–Ni–Co-type alloy electrodes

The cycling behavior of the _{La0.7Mg0.3Ni2.65-xCo0.75Mn0.1Alx}${\mathrm{La}}_{0.7}{\mathrm{Mg}}_{0.3}{\mathrm{Ni}}_{2.65-x}{\mathrm{Co}}_{0.75}{\mathrm{Mn}}_{0.1}{\mathrm{Al}}_x$(x=0,0.3)$(x=0,0.3)$ alloy electrodes was systematically investigated by XRD, SEM, EIS, XPS and AES measurements, and the function of Al in the La–Mg–Ni-based alloys and the reasons for the improvement of the cycling stability of the alloy electrode with Al were discussed. Results show that the cycling behavior of the _{La0.7Mg0.3Ni2.35Co0.75Mn0.1Al0.3}${\mathrm{La}}_{0.7}{\mathrm{Mg}}_{0.3}{\mathrm{Ni}}_{2.35}{\mathrm{Co}}_{0.75}{\mathrm{Mn}}_{0.1}{\mathrm{Al}}_{0.3}$ alloy electrode can be divided into three stages, i.e., the pulverization and Mg oxidation stage, the Mg oxidation and La and/or Al oxidation stage, and the La and Al oxidation and Al oxide film protection stage. The improvement of the cycling stability of the alloy electrode with Al can be ascribed to two factors. One is the decrease in the pulverization of the alloy particles during charge/discharge cycling due to the alloy with Al undergoes a smaller cell volume expansion and contraction. The other is the increase in the anti-oxidation/corrosion due to the formation of a dense Al oxide film during cycling, which is believed to be the most important reason for the improvement of the cycling stability of the La–Mg–Ni–Co–Mn–Al-type alloy electrodes.     

Microstructures and electrochemical characteristics of the La0.75Mg0.25Ni2.5Mx (M=Ni, Co; x=0–1.0) hydrogen storage alloys

In order to investigate the influences of the stoichiometric ratios of B/A (A: gross A-site elements, B: gross B-site elements) and the substitution of Co for Ni on the structure and the electrochemical performances of the _AB2.5–3.5${\mathrm{AB}}_{2.5–3.5}$-type electrode alloys, the La–Mg–Ni–Co system _{La0.75Mg0.25Ni2.5Mx}${\mathrm{La}}_{0.75}{\mathrm{Mg}}_{0.25}{\mathrm{Ni}}_{2.5}{\mathrm{M}}_x$ (M=Ni$\mathrm{M}=\mathrm{Ni}$, Co; x=0$x=0$, 0.2, 0.4, 0.6, 0.8, 1.0) alloys were prepared by induction melting in a helium atmosphere. The structures and electrochemical performances of the alloys were systemically measured. The obtained results show that the structures and electrochemical performances of the alloys are closely relevant to the M content. All the alloys exhibit a multiphase structure, including _LaNi2${\mathrm{LaNi}}_2$, (La,Mg)_Ni3$(\mathrm{La},\mathrm{Mg}){\mathrm{Ni}}_3$ and _LaNi5${\mathrm{LaNi}}_5$ phases, and the major phase in the alloys changes from _LaNi2${\mathrm{LaNi}}_2$ to (La,Mg)_Ni3+_LaNi5$(\mathrm{La},\mathrm{Mg}){\mathrm{Ni}}_3+{\mathrm{LaNi}}_5$ with the variety of M content. The electrochemical performances of the alloys, involving the discharge capacity, the high rate discharge (HRD) ability, the activation capability and the discharge potential characteristics, significantly improve with increasing M content. When M content x$x$ increases from 0 to 1.0, the discharge capacity rises from 177.7 to 343.62  mAh/g for the alloy (M=Ni)$(\mathrm{M}=\mathrm{Ni})$, and from 177.7 to 388.7 mAh/g for the alloy (M=Co)$(\mathrm{M}=\mathrm{Co})$. The cycle stability of the alloy first mounts up then declines with growing M content. The substitution of Co for Ni significantly ameliorates the electrochemical performances. For a fixed M content (x=1.0)$(x=1.0)$, the substitution of Co for Ni enhances the discharge capacity from 343.62 to 388.7 mAh/g, and the capacity retention ratio (_S100)$(S_{100})$ after 100 charging–discharging cycles from 51.45% to 61.1%.     

Effect of boron additive on electrochemical cycling life of La–Mg–Ni alloys prepared by casting and rapid quenching

In order to improve the electrochemical performances of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, a trace of B was added in La₂Mg(Ni_0.85Co_0.15)₉ alloy. La₂Mg(Ni_0.85Co_0.15)₉B_x${}_x$ (x=0,0.05,0.1,0.15,0.2$x=0,0.05,0.1,0.15,0.2$) hydrogen storage alloys were prepared by casting and rapid quenching. The electrochemical charging-discharging cycling lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of B additive on the microstructures and cycling lives of as-cast and quenched alloys were investigated in detail. The results show that the as-cast and quenched alloys are composed of the (La, Mg)Ni₃ phase (PuNi₃ structure), the LaNi₅ phase and the LaNi₂ phase. A trace of the Ni₂B phase exists in the as-cast alloys containing B. The Ni₂B phase in the alloys containing B nearly disappears after rapid quenching and the relative ratio of each phase in the alloys changes with the variety of the quenching rate. The addition of B obviously enhances the charging-discharging cycling stabilities of the as-cast and quenched alloys. When B content x$x$ increases from 0 to 0.2, the cycling lives of the as-cast and quenched at 20 m/s alloys were increased from 72 to 94 cycles and from 86 to 104 cycles, respectively.     

Changes of hydrogen storage properties of MgH2 induced by heavy ion irradiation

In order to understand the influence of defect zones on desorption behavior of _MgH2${\mathrm{MgH}}_2$, Xe 120 keV ion irradiation of this material has been performed. DSC, SEM measurements, and SRIM calculations have been used to characterize induced modifications and its influence on the hydrogen desorption behavior of _MgH2${\mathrm{MgH}}_2$. We have demonstrated that the near-surface area of _MgH2${\mathrm{MgH}}_2$ plays the crucial role in hydrogen desorption kinetics. DSC analysis provides clear picture of vacancies influence on H diffusion and desorption in _MgH2${\mathrm{MgH}}_2$, and points out that there is possibility to control the thermodynamic parameters by controlled ion bombardment.     

Electrocatalytic activities of nanocomposite Ni81P16C3 electrode for hydrogen evolution reaction in alkaline solution by electrochemical impedance spectroscopy

Kinetics of hydrogen evolution reaction (HER) was studied in 1 M NaOH at 298 K on nickel–phosphorous–carbon (_Ni81P16C3)$({\mathrm{Ni}}_{81}{\mathrm{P}}_{16}{\mathrm{C}}_3)$ composite electrode. Evaluation of the electrode activities was carried out by steady-state polarization Tafel curves, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV).     

Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal-supported catalysts

Most of the hydrogen production processes are designed for large-scale industrial uses and are not suitable for a compact hydrogen device to be used in systems like solid polymer fuel cells. Integrating the reaction step, the gas purification and the heat supply can lead to small-scale hydrogen production systems. The aim of this research is to study the influence of several reaction parameters on hydrogen production using liquid phase reforming of sugar solution over Pt, Pd, and Ni supported on nanostructured supports. It was found that the desired catalytic pathway for _H2${\mathrm{H}}_2$ production involves cleavage of C–C, C–H and O–H bonds that adsorb on the catalyst surface. Thus a good catalyst for production of _H2${\mathrm{H}}_2$ by liquid-phase reforming must facilitate C–C bond cleavage and promote removal of adsorbed CO species by the water–gas shift reaction, but the catalyst must not facilitate C–O bond cleavage and hydrogenation of CO or _CO2${\mathrm{CO}}_2$. Apart from studying various catalysts, a commercial Pt/γ$\gamma$-alumina catalyst was used to study the effect of temperature at three different temperatures of 458, 473 and 493 K. Some of the spent catalysts were characterised using TGA, SEM and XRD to study coke deposition. The amorphous and organised form of coke was found on the surface of the catalyst.     

Interactions of hydrogen flames with walls: Influence of wall temperature,pressure, equivalence ratio,and diluents

The thermal and chemical effects of one-dimensional, premixed hydrogen flames quenching against a single surface are studied numerically using a detailed chemical mechanism. The results for stoichiometric _H2–_O2${\mathrm{H}}_2–{\mathrm{O}}_2$ flames impinging on a 750 K inert wall agree qualitatively with prior published results. Other wall boundary conditions studied include an adiabatic wall and isothermal walls with temperatures ranging from 298 to 1200 K. Chemical pathway analysis of the detailed hydrogen mechanism reveals the growing importance of radical recombination reactions near the inert walls for increasing wall temperature. Implementation of a H, O, and OH radical sink at the surface of a 750 K isothermal wall results in significantly lower heat generation near the surface. Investigations of various gas properties include changes to equivalence ratio (0.7–2.0), chamber pressure (1 and 2 bar), and inert gas (_N2)$({\mathrm{N}}_2)$ addition.     

Effects of B addition on the hydrogen absorption–desorption property of Ti0.32Cr0.43V0.25 alloy

The effects of boron addition on the hydrogen absorption–desorption properties of the _{Ti0.32Cr0.43V0.25}${\mathrm{Ti}}_{0.32}{\mathrm{Cr}}_{0.43}{\mathrm{V}}_{0.25}$ alloy were studied. Boron was added either directly or indirectly through a mother alloy _Ti0.75B0.25${\mathrm{Ti}}_{0.75}{\mathrm{B}}_{0.25}$. Direct boron addition caused the decrease in the titanium content of the BCC matrix through formation of Ti–B phases, resulting in the decrease in the lattice constant. Conversely, mother alloy addition increased the titanium content and the lattice constant of the matrix, for it contained enough titanium to contribute to the matrix even after forming the second phase TiB. Such lattice constant changes caused by boron addition resulted in drastic changes in hydrogen plateau pressure and great decrease in effective hydrogen storage capacity.     

Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications

The present study compares the dehydrogenation kinetics of (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ systems and their vulnerabilities to the NH₃ emission problem. The (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixture. On the basis of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system are discussed in light of the newly proposed reaction mechanism.     

Effects of metal oxides addition on the performance of La1.3CaMg0.7Ni9 hydrogen storage alloy

The _{La1.3CaMg0.7Ni9}${\mathrm{La}}_{1.3}{\mathrm{CaMg}}_{0.7}{\mathrm{Ni}}_9$ alloy was modified with various metal oxides (_Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$, _TiO2${\mathrm{TiO}}_2$, _Cr2O3${\mathrm{Cr}}_2{\mathrm{O}}_3$, ZnO), and the effects of metal oxides addition on the electrochemical properties of the _{La1.3CaMg0.7Ni9}${\mathrm{La}}_{1.3}{\mathrm{CaMg}}_{0.7}{\mathrm{Ni}}_9$ hydrogen storage alloy were investigated. The catalytic effects of metal oxides are found. Not only the discharge capacity but also the high-rate dischargeability (HRD) is improved by addition of 5 wt% _TiO2${\mathrm{TiO}}_2$, _Cr2O3${\mathrm{Cr}}_2{\mathrm{O}}_3$, and ZnO, while the cyclic stability does not change except for addition of _Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$. The low-temperature property is enhanced more obviously by addition of _TiO2${\mathrm{TiO}}_2$, _Cr2O3${\mathrm{Cr}}_2{\mathrm{O}}_3$, and ZnO. The electrochemical kinetics is also measured by the linear polarization and electrochemical impedance spectroscopy (EIS). In addition, the hydrogen absorption kinetic behavior is measured by gas–solid reaction.