Scheme of thermal compression of hydrogen (TCH) using MmNi4.25Al0.75 recovered with ethyl alcohol and handled under non protective atmospheres

A MmNi_4.25Al_0.75 intermetallic was obtained by low energy mechanical alloying and low temperature heating at 600 °C for 24 h under Ar. The intermetallic was recovered from milling chamber using ethyl alcohol, dried, stored and handled under air at room conditions. Structure was characterized by XRD. A maximum stability temperature of 160 °C was obtained from non-isothermal DSC measurement under air. The kinetics of oxidation at 200 °C was analyzed. A maximum reaction degree (α = 0.1) was obtained after 2500 s of treatment. The hydrogen sorption properties of samples were studied by volumetric measurements. Hydrogen maximum mass percent capacity (mass %) was reached in less than 300 s. The thermodynamic sorption properties were measured. Values of ΔH_f = −29 ± 2 kJ mol⁻¹ and ΔS_f = 197 ± 10 J mol⁻¹ K⁻¹ were obtained for absorption process and ΔH_d = 28 ± 2 kJ mol⁻¹ and ΔS_d = 189 + 10 J mol⁻¹ K⁻¹ were obtained for desorption process. From these results, a one-stage of thermal compression of hydrogen is proposed with a standard compression ratio (R_c) of 5.71 in the 25–80 °C range.     

Mechanical milling of Mg,Ni and Y powder mixture and investigating the effects of produced nanostructured MgNi4Y on hydrogen desorption properties of MgH2

It has been reported that intermetallic compounds could be used to improve hydrogen storage properties of Mg-based alloys. In this study, an attempt was made to synthesize the MgNi₄Y compound from pure Ni, Mg and Y elemental powders via the combination of mechanical milling and heat treatment methods. In this regard, powders were ball milled in different conditions and then heat treated at 400 and 600 °C for 4 h to investigate the effect of temperature on the formation of MgNi₄Y intermetallic compound. The characteristics of mixtures were evaluated via (XRD) and (SEM) methods. It was found from the results of XRD analysis that ternary intermetallic compound was not formed completely via ball milling alone. Nanostructured intermetallic compound was formed after heat treatment of milled powder at 600 °C for 25 h. Furthermore, addition of 5 and 10 wt% of the produced intermetallic compound to MgH₂ decreased hydrogen desorption temperature and increased released hydrogen content.     

A two-stage hydrogen compressor based on (La,Ce,Nd,Pr)Ni5 intermetallics obtained by low energy mechanical alloying – Low temperature annealing treatment

La_0.67Ce_0.19Nd_0.08Pr_0.06Ni₅ was synthesized by low energy mechanical alloying. The AB₅ was milled up to completion stage to reach the final composition and appropriate particle size distribution and microstructure characteristics. Crystallite size, strain and sorption properties of as-milled samples were evaluated. After milling, La_0.67Ce_0.19Nd_0.08Pr_0.06Ni₅ and previously obtained LaNi₅ were annealed at 600 °C for 24 h. An improvement in both microstructural and hydrogen sorption properties was found. Equilibrium hydrogen sorption properties were obtained and quantified in the 25–90 °C range. From these results, a two-stage hydrogen compressor was proposed. In the first stage, hydrogen is absorbed by LaNi₅ at 575 kPa and 25 °C and desorbed at 1365 kPa and 90 °C. In the second stage, this fluid is absorbed by La_0.67Ce_0.19Nd_0.08Pr_0.06Ni₅ at 745 kPa and 25 °C and desorbed at 2100 kPa and 90 °C. As a result, a global compression ratio of 3.65 is reached using this scheme.     

Characterization of LaNi4.70Al0.30 handled in air and application to a scheme of thermal compression of hydrogen

LaNi_4.70Al_0.30 was characterized by SEM, EDS and XRD. The structure was refined by the Rietveld method. The intermetallic stability temperature range in air was analyzed by DSC. The intermetallic is destabilized at T > 160° C. The intermetallic was annealed at this temperature for 24 h in air. After that, the pressure-composition-isotherms were measured. The thermodynamic properties were calculated from the Van’t Hoff diagrams. Values obtained were ΔH_f = 30 ± 2 kJ/mol and ΔS_f = 0.13 ± 0.01 kJ/mol for absorption process and ΔH_d = 31 ± 2 kJ/mol and ΔS_d = 0.14 ± 0.01 ± kJ/mol for desorption process. From these results, a scheme of thermal compression of hydrogen (TCH) was proposed. The scheme has a practical compression ratio (R_c) of 7.2 in the 25–100 °C temperature range and 1–1000 kPa pressure range.     

Hydrogen compression characteristics of a dual stage thermal compressor system utilizing LaNi5 and Ca0.6Mm0.4Ni5 as the working metal hydrides

The research performed herein consisted of the design, construction, and testing of a dual stage metal hydride hydrogen compression system intended to be used with lower grade geothermal or waste energy sources. The metal hydrides used in this study were LaNi₅ and Ca_0.6Mm_0.4Ni₅. A Finite Time Thermodynamics (FTT) model was also developed and the model proved useful in determining how the compression results and energy requirements for the system change with variations in the system parameters. Dual stage system results showed a final compression ratio of approximately 12 when using cooling and heating temperatures of 10 °C and 90 °C, respectively. The final output pressures and compression ratios were found to follow an upward trend when increasing the heating bath temperatures. It can be concluded from the experimental results, that though the dual stage hydrogen compression system has room for improvement, it is an effective way of compressing the hydrogen from low initial pressures while using low grade energy sources.     

Catalytic effect of melt-spun Ni3FeMn alloy on hydrogen desorption properties of nanocrystalline MgH2 synthesized by mechanical alloying

To improve hydrogen desorption properties of magnesium hydride, a composite material with composition of MgH₂-5 at% Ni₃FeMn has been prepared by co-milling MgH₂ powder with Ni₃FeMn alloy either in the form of as-cast (sample A) or melt-spun ribbon (sample B). The study has shown that the addition of Ni₃FeMn alloy to magnesium hydride can yield a finer particle size after mechanical alloying (MA). As a consequence, the desorption temperature of mechanically activated MgH₂ for 30 h has decreased from 319 °C to 307 °C for sample A and to 290 °C for sample B. Furthermore, some favorable effects of Ni₃FeMn alloy on hydrogen desorption kinetics have been observed. Further improvement in the hydrogen desorption of melt-spun containing composite can be related to higher hardness value of the melt-spun powder compared to the as-cast alloy, and probably a more homogeneous distribution of the alloyed elements.     

Hydrogen storage properties of Ti0.72Zr0.28Mn1.6V0.4 alloy prepared by mechanical alloying and copper boat induction melting

In the present study, two process techniques, mechanical alloying and innovative vacuum copper boat induction melting, were used to produce Ti_0.72Zr_0.28Mn_1.6V_0.4 alloy for hydrogen storage applications. The hydrogen absorption and desorption properties of the alloy were studied. The material structure and phases were characterized by XRD and SEM. The hydrogen absorption and desorption properties of the alloy were measured by an automatically controlled Sieverts apparatus. The results showed that the samples consisted of two main phases, C14 Lave phase and V-base solid solution phase. The maximum capacity of abs/desorption was achieved at mediate temperature (150 °C). The hydrogen capacity of the induction melted samples in various temperatures was higher than that for the samples produced by mechanical alloying method. The maximum absorption capacity of the induction melted and mechanically alloyed samples were 2 and 1.2 wt%, respectively. The maximum desorption capacity of the induction melted and mechanically alloyed samples were 0.45 and 0.1 wt%, respectively.     

Hydrogen sorption behavior of mechanically synthesized Mg–Ag alloys

A systematic study of the interactions between hydrogen and Mg–Ag alloys prepared by mechanical alloying is presented in this paper. The alloys were chosen to cover both the two-phase limited solubility and hypoeutectic regions (3, 5, 10, 20 wt. % Ag in Mg) as well as the intermetallic phases (γ′-AgMg₄, ε-AgMg₃ and AgMg). The hydrogen-absorption abilities of the samples were investigated for the two states: directly after mechanical alloying and after annealing. It was found that all of the chosen alloys could be very effectively synthesized with mechanical alloying, a process that also was found to extend the solubility limits compared to those on known phase diagrams. The synthesis is greatly enhanced by further annealing which allows us to obtain well crystallized samples with phase compositions in accordance with these phase diagrams. In most cases, the obtained alloys were able to absorb significant amounts of hydrogen while magnesium hydride and AgMg intermetallics were formed as a residual product and phase, respectively, that did not react with hydrogen under the chosen conditions. The magnesium hydride formed in such reactions seemed to have a very similar decomposition temperature and was not significantly influenced by the silver additive, which was not found to greatly catalyze the MgH₂ decomposition reaction in such cases. Samples characterized directly after ball milling showed higher reaction kinetics with hydrogen.     

Improved hydrogen desorption in lithium alanate by addition of SWCNT-metallic catalyst composite

A LiAlH₄/single walled carbon nanotube (SWCNT) composite system was prepared by mechanical milling and its hydrogen storage properties investigated. The SWCNT - metallic particle addition resulted in both a decreased decomposition temperature and enhanced desorption kinetics compared to pure LiAlH₄. The decomposition temperature of the 5 wt.% SWCNT-added LiAlH₄ sample was reduced to 80 °C and 130 °C for the first and second stage, respectively, compared with 150 °C and 180 °C for as-received LiAlH₄. In terms of the desorption kinetics, the 5 wt.% SWCNT-added LiAlH₄ sample released about 4.0 wt.% hydrogen at 90 °C after 40 min dehydrogenation, while the as-milled LiAlH₄ sample released less than 0.3 wt.% hydrogen for the same temperature and time. Differential scanning calorimetry measurements indicate that enthalpies of decomposition in LiAlH₄ decrease with added SWCNTs. The apparent activation energy for hydrogen desorption was decreased from 116 kJ/mol for as-received LiAlH₄ to 61 kJ/mol by the addition of 5 wt.% SWCNTs. It is believed that the significant improvement in dehydrogenation behaviour of SWCNT-added LiAlH₄ is due to the combined influence of the SWCNT structure itself and the catalytic role of the metallic particles contained in the SWCNTs. In addition, the different effects of the SWCNTs and the metallic catalysts contained in the SWCNTs were also investigated, and the possible mechanism is discussed.     

Hydrogen sensing properties of ZnO nanorods: Effects of annealing,temperature and electrode structure

In this study, the hydrogen (H₂) sensing properties of vertically aligned zinc oxide (ZnO) nanorods were investigated depending on annealing, Pd coating, temperature and electrode structure. ZnO nanorods were fabricated by using hydrothermal method on a glass substrate and an indium tin oxide (ITO) coated glass substrate. In order to determine the effects of annealing on the H₂ sensor performance, the nanorods were heated at 500 °C in dry air. H₂ sensing measurements were done in the temperature range of 25–200 °C. It was found that, the sensor response of Pd coated ZnO nanorods were much higher than the un-coated nanorods due to the catalytic effect of Pd thin film. Moreover, the un-annealed samples showed better sensor response than the annealed samples due to the number of oxygen deficiency. In addition, the lateral electrode structure showed higher sensor response than the sandwich electrode structure.     

Hydrogen storage properties of Mg-10 wt% Ni alloy co-catalysed with niobium and multi-walled carbon nanotubes

Mg-10 wt% Ni alloys containing up to 1 wt% Nb were fabricated by a casting technique, followed by ball-milling with 5 wt% multi-walled carbon nanotubes. Further mechanical alloying with 1.5, 3, and 5 at % Nb was applied to a cast Mg-10 wt% Ni-370 ppm Nb alloy to investigate the catalytic role of Nb in hydrogen dissociation. The microstructure and distribution of Nb and Mg₂Ni in the alloys were characterised by SEM. The absorption and desorption kinetics of the samples were measured by Sieverts’ apparatus at various temperatures. The results show that addition of Nb during casting accelerates the hydrogen diffusion compared to the cast binary Mg-10 wt% Ni alloy. Moreover, ball-milling of the alloy with metallic niobium leads to the formation of BCC phase of Mg-Nb solid solution, which significantly improves the hydrogenation properties of the alloy. DSC results show that mechanical alloying of Mg-10 wt%Ni-370 ppm Nb with Nb in excess of 1.5 wt% decreases the desorption temperature by approximately 100 °C compared to the ball-milled cast alloy.     

Dehydriding properties of γ-AlH3

AlH₃ is a promising hydrogen storage material due to its high hydrogen capacity (10 wt%) and relatively low dehydriding temperature. In this work, γ-AlH₃ was prepared by organometallic synthesis method and the effects of ball milling on dehydriding properties of γ-AlH₃ were investigated systematically. Experimental results shows that as-prepared γ-AlH₃ releases about 8.3 wt% of hydrogen in the temperature range of 130–160 °C at a heating rate of 2 °C/min. Ball milling significantly improves the dehydriding behavior of γ-AlH₃. DSC-MS analysis reveals that the dehydriding temperature of γ-AlH₃ ball-milled for 10 h decreases by around 30 °C. In addition, the dehydriding activation energy of γ-AlH₃ ball-milled for 2 h decreased from 87 to 68 kJ/mol. Isothermal dehydriding measurements demonstrate that duration needed to release 90% hydrogen for as-prepared γ-AlH₃ is 280 min, but it takes only 82 min after ball milled for 10 h to release the same amount of hydrogen. Moreover, the dehydriding path of γ-AlH₃ is changed by ball milling. As-prepared γ-AlH₃ transforms to α-AlH₃ before dehydriding, while ball-milled γ-AlH₃ prefers to dehydride directly without firstly transforming to α-AlH₃.     

Electrochemical performance of Ni-RE (RE = rare earth) as electrode material for hydrogen evolution reaction in alkaline medium

In this work, Ni-RE (RE = rare earth, La, Ce) materials were obtained by the solid state reaction using as Ni source: a) metal acetylacetonates and b) metal powders while rare earth element was added from acetylacetonates. These materials were synthesized for 3 h at different temperatures (800 °C or 900 °C, 1000 °C and 1200 °C) in order to evaluate their electrochemical performance on the Hydrogen Evolution Reaction (HER). The effects of the sintering temperature and the Ni source on the morphology, structure and particle size were evaluated and correlated with the displayed catalytic activity. The results showed that depending of the added rare earth element and Ni source the formed compounds varied from a mixture of oxides (NiO, CeO₂) and intermetallic compounds (LaNiO₃) at low annealing temperatures up to the formation of the NiO-CeO₂, NiO-LaNiO₃ and NiO-La₄Ni₃O₁₀ compounds at 1200 °C. From Scanning Electron Microscopy (SEM) results, it was observed that the agglomerates of Ni-RE electrode materials presented a more uniform shape (semispherical) and lower crystal sizes (0.2-2.0 μm) using acetylacetonate precursors than that obtained with Ni powders (5-50 μm). It was found that the individual organization of the nickel particles and their electrocatalytic activity is affected by diverse factors: a) the type of precursor used in the synthesis, b) the reaction temperature and c) the synergetic effect caused by the addition of the rare earth metal, which seems to be better for lanthanum than for cerium. The Tafel parameters of the stabilized Ni-RE electrodes revealed that the formation of Ni-La intermetallic compounds at low temperature favors the current densities on the HER. Thus a clear dependence of the electrocatalytic activity on the source of these Ni-RE materials was observed.     

Effects of CNTs on the hydrogen storage properties of MgH2 and MgH2-BCC composite

MgH₂ with 10 wt.% Ti_0.4Mn_0.22Cr_0.1V_0.28 alloy (termed the BCC alloy for its body centred cubic structure) and 5 wt.% carbon nanotubes (CNTs) were prepared by planetary ball milling, and its hydrogen storage properties were compared with those of the pure MgH₂ and the binary mixture of MgH₂ and the BCC alloy. The sample with CNTs showed considerable improvement in hydrogen sorption properties. Its temperature of desorption was 125 °C lower than for the pure sample and 59 °C lower than for the binary mixture. In addition, the gravimetric capacity of the ternary sample was 6 wt.% at 300 °C and 5.6 wt.% at 250 °C, and it absorbed 90% of this amount at 150 s and 516 s at 300 °C and 250 °C, respectively. It can be hypothesised from the results that the BCC alloy assists the dissociation of hydrogen molecules into hydrogen atoms and also promotes hydrogen pumping into the Mg/BCC interfaces, while the CNTs facilitate access of H-atoms into the interior of Mg grains.     

Synthesis of Co2FeAl alloys as highly efficient electrocatalysts for alkaline hydrogen evolution reaction

The development of cost-effective non-precious metal electrocatalysts is a major challenge for water splitting applications, but it is important for the realization of renewable energy systems. Alloying has proved an effective way to design metal-based electrocatalysts, and by controlling the annealing temperature, the surface morphology and crystallinity of the alloy can be tuned to control the hydrogen evolution reaction (HER) performance. In this work, with a simple coprecipitation method, we have prepared Co₂FeAl alloys at different annealing temperatures (550 °C–670 °C), which exhibit excellent crystallinity and electrocatalytic performance for HER in alkaline solution. Among all conditions, the Co₂FeAl alloys prepared at 620 °C shows the better crystallinity and the higher purity, and it could achieve a low overpotential of 149 mV at 10 mA cm^?2 in alkaline solution. The overpotential demonstrates persistent stability with only 3 mV change after over 1000 cycles. Both density functional theory (DFT) calculations and experimental results revealed that alloying optimizes the electronic structure near the Fermi surface of the system, improving the electron transport efficiency and enhancing the catalytic activity. These Co₂FeAl alloys are appealing candidates for high-performance alkaline HER electrocatalytic electrodes in water electrolysis due to their outstanding electrocatalytic properties.     

Hydrogen production by aqueous phase catalytic reforming of glycerine

Hydrogen is believed to be the one of the main energy carriers in the near future. In this research glycerine, which is produced in large quantities as a by-product of biodiesel process, was converted to hydrogen aiming to contribute to clean energy initiative. Conversion of glycerol to hydrogen was achieved via aqueous-phase reforming (APR) with Pt/Al₂O₃ catalyst. The experiments were carried out in an autoclave reactor and a continuous fixed-bed reactor. The effects of reaction temperature (160-280 °C), feed flow rate (0.05-0.5 mL/dak) and feed concentration (5-85 wt-% glycerine) on product distribution were investigated. Optimum temperature for hydrogen production with APR was determined as 230 °C. Maximum gas production rate was found at the feed flow rates around 0.1 mL/min. It was also found that hydrogen concentration in the gas product increased with decreasing glycerol concentration in the feed.     

Cycle life and hydrogen storage properties of mechanical alloyed Ca1−xZrxNi5−yCry; (x = 0, 0.05 and y = 0, 0.1)

CaNi₅–based alloys have been synthesized by mechanical alloying followed by isothermal annealing. The formation of the CaNi₅ structure occurred when the milled powders were heated at 800 °C under vacuum for 3 h. The abundance of CaNi₅ phase in the alloys ranges from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi₅ whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption plateau regions are narrowed compared to those of pure CaNi₅. Substitution of Zr into the Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where a flat plateau is maintained. The reaction enthalpy ΔH for both absorption and desorption are directly proportional to the unit cell volume of the alloys. The hydrogen storage capacity of all alloys rapidly decays for the first 50 cycles at 85 °C followed by a more gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200 cycles is in the range of 65–75% of the initial capacity.     

Significantly improved dehydrogenation of LiAlH4 destabilized by K2TiF6

The effects of K₂TiF₆ on the dehydrogenation properties of LiAlH₄ were investigated by solid-state ball milling. The onset decomposition temperature of 0.8 mol% K₂TiF₆ doped LiAlH₄ is as low as 65 °C that 85 °C lower than that of pristine LiAlH₄. Isothermal dehydrogenation properties of the doped LiAlH₄ were studied by PCT (pressure–composition–temperature). The results show that, for the 0.8 mol% K₂TiF₆ doped LiAlH₄ that dehydrogenated at 90 °C, 4.4 wt% and 6.0 wt% of hydrogen can be released in 60 min and 300 min, respectively. When temperature was increased to 120 °C, the doped LiAlH₄ can finish its first two dehydrogenation steps in 170 min. DSC results show that the apparent activation energy (E_a) for the first two dehydrogenation steps of LiAlH₄ are both reduced, and XRD results suggest that TiH₂, Al₃Ti, LiF and KH are in situ formed, which are responsible for the improved dehydrogenation properties of LiAlH₄.     

A comparative study for synthesis methods of nano-structured (9Ni–2Mg–Y) alloy catalysts and effect of the produced alloy on hydrogen desorption properties of MgH2

9Ni–2Mg–Y alloy powders were prepared by arc melting, induction melting, mechanical alloying, solid state reaction and subsequent ball milling processes. The results showed that melting processes are not suitable for preparation of 9Ni–2Mg–Y alloy due to high losses of Mg and Y. Therefore, 9Ni–2Mg–Y alloy powder was prepared by three methods including: 1) mechanical alloying, 2) mechanical alloying + solid state reaction + ball milling, and 3) mixing + solid state reaction + ball milling. The prepared 9Ni–2Mg–Y alloy powders were compared for their catalytic effects on hydrogen desorption of MgH₂. It is found that 9Ni–2Mg–Y alloy powder prepared by mechanical alloying + solid state reaction + ball milling method has a smaller particle size (1–5 μm) and higher surface area (1.7 m² g⁻¹) than that of other methods. H₂ desorption tests revealed that addition of 9Ni–2Mg–Y alloy prepared by mechanical alloying + solid state reaction + ball milling to MgH₂ decreases the hydrogen desorption temperature of MgH₂ from 425 to 210 °C and improves the hydrogen desorption capacity from 0 to 3.5 wt.% at 350 °C during 8 min.     

Zero drift suppression for PdNi nano-film hydrogen sensor by vacuum annealing

Zero drift is one of the important factors affecting the accuracy of hydrogen sensor. To restrain the zero drift, the PdNi nano-film hydrogen sensors were vacuum annealed and effects of different vacuum annealing temperatures on the micro-structure, morphology and hydrogen sensing performances were explored in detail. The results show that with the annealing temperature increasing, the grains grow and the lattice constantly decreases. And the morphology of the surface grains changes from cone to sphere when the annealing temperature is more than 250 °C. Moreover, we provide a concept of EZD (effect of zero drift) to quantitative analysis of the effect of zero drift on hydrogen sensing performance. Repeatability tests reveal that annealing can inhibit EZD when the annealing temperature is more than 150 °C, which is attributed to defects of PdNi nano-film reducing and stability improving. However, increasing annealing temperature leads to the deterioration of sensing performances such as response (Rs), response time (t_rs) and recover time (t_rc), especially when the annealing temperature is more than 250 °C. Only annealing at 250 °C can decrease EZD availably and make the PdNi nano-film hydrogen sensor maintain a high Rs and short t_rs and t_rc. In addition, the sensor annealed at 250 °C is appropriate for detecting hydrogen with low concentration and the detection limit is low to 2 ppm.