Investigation on hydrogen production using multicomponent aluminum alloys at mild conditions and its mechanism

A series of Al alloys with low melting point metals Ga, In, Sn as alloy elements were fabricated using mechanical alloying method. The phase compositions and morphologies of different Al alloys were characterized by XRD and SEM techniques. The reaction of the Al alloys with water for hydrogen evolving at mild conditions (at room temperature in neutral water) was studied. The results showed that there were no hydrogen yields for binary Al–Ga, Al–In, Al–Sn and the ternary Al–Ga–Sn alloys. The hydrogen yields were observed for Al–Ga–In and Al–In–Sn ternary alloys. The Al–In–Sn alloys showed an even faster hydrogen generation rate and higher yields than Al–Ga–In alloys. Based on the ternary Al–Ga–In and Al–In–Sn system, the hydrogen production property of quaternary Al–Ga–In–Sn was greatly improved. The hydrogen conversion efficiency of the optimized Al–3%Ga–3%In–5%Sn alloy was nearly 100% in tap water. The highest hydrogen generation rate reached 1560 mL/g min in distilled water or deionized water. It was suggested that both the embrittlement of Al by liquid Ga–In–Sn eutectic and the active points formed by intermetallic compounds In₃Sn and InSn₄ may be attributed to the high activity of Al–Ga–In–Sn alloys at room temperature.     

Effect of Ti on the microstructure and Al–water reactivity of Al-rich alloy

Ti-bearing Al alloys (0.1–1 wt.%) were prepared using arc melting techniques. Their microstructures were investigated using XRD and SEM/EDX, and found to depend strongly on Ti contents. Al grains are columnar as Ti contents are lower, but they are refined and turn into equiaxed ones when Ti contents are higher. The particle sizes of Ga–In–Sn phase decrease with Al grain refinement. Al–water reactivities were also investigated under different water temperatures. Kinetic measurements found that Ti prohibits Al–water reaction and reduces hydrogen yields when alloys contain little Ti. However, Al reacts with water fast and hydrogen yields rise with the increase of Ti contents of alloys. Reasons concerning the variations of microstructures and Al–water reactivities with Ti additions are discussed.     

Insight into the reactivity of Al–Ga–In–Sn alloy with water

An Al alloy ribbon with finer Al grains was prepared using a rapid spinning technique, and then was annealed at different temperatures to modify its microstructures, such as: Al grain size, size and number of Ga–In–Sn phase. The microstructures and phase compositions of the as-prepared and those annealed ribbons were investigated by means of XRD and SEM/EDX. The reaction of Al and the grain boundary phase was measured using DSC. Based on DSC analysis and other experiments, the formation of Al–Ga–In–Sn eutectic was suggested the origin of the alloy being capable of splitting water. Kinetic measurements found that the H₂ generation rate depends strongly on the microstructures of ribbons. An analytical expression was established to calculate the H₂ generation rates of ribbons with the measured microstructure parameters, and the calculated results agreed well with measurements.     

Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water

In this paper, a series of Al-based materials were prepared by ball milling and/or melting. The XRD, SEM and TG-DTA techniques were used for sample analyses. Effects of different metals such as Zn, Ca, Ga, Bi, Mg, In and Sn on the hydrogen generation through hydrolysis of the Al alloy were evaluated in pure water. The results showed that mechanical milling was more favorable than the melting method to synthesize the Al alloys containing some metals with lower melting point and easier vaporization in the melting process. Addition of Bi and Sn could more significantly enhance Al reactivity with water in Al alloy than other metals such as Zn, Ca and Ga. Especially Al–Bi alloy had a faster hydrolysis rate than Al–Sn alloy at room temperature. For Al–Bi alloy, the addition of Zn and Ga accelerated the alloy hydrolysis while the effect of addition of other metals (Sn, In, Mg) on the hydrolysis of the alloy was reverse. Furthermore, the effect of some compounds (NaCl, _MgCl2${\mathrm{MgCl}}_2$, _CaH2${\mathrm{CaH}}_2$) on the hydrolysis of the Al–Bi alloy was explored. It showed that the milling Al–Bi alloy together with the compounds could accelerate the formation of its mico-galvanic cell between the anode (Al) and cathode (Bi). The alloy composition was therefore optimized to be Bi, Zn, Ga, _CaH2${\mathrm{CaH}}_2$ and Al. The optimized Al alloy demonstrated a high hydrogen generation rate and theoretic hydrogen yields.     

Liquid phase-enabled reaction of Al-Ga and Al-Ga-In-Sn alloys with water

The water-reactivity of Al-Ga and Al-Ga-In-Sn alloys is investigated as a means to utilize the chemical potential energy of Al to split water for the production of H₂. Al in bulk quantities of these alloys participates in a heterogeneous reaction with water to produce H₂ and α-Al(OH)₃ (bayerite). Low melting point phases in these alloys are believed to enable the observed reaction upon liquefaction by providing a means of transport for Al in the alloys to reach a reaction site. In the Al-Ga binary system, this reaction-enabling phase is shown to form at a temperature corresponding to the system’s eutectic melting point. In the Al-Ga-In-Sn quaternary system this reaction-enabling phase liquefies at 9.38 °C, as shown using differential scanning calorimetry (DSC). Alloys with the composition 50 wt% Al-34 wt% Ga-11 wt% In-5 wt% Sn are reacted with distilled water in a series of controlled experiments, and H₂ yield from these reactions is measured as a function of time and temperature. Applying kinetic analysis to the yield data shows the apparent activation energy for the reaction process to be 43.8 kJ/mol. A physicochemical model for the alloy-water reaction is presented in the context of the observed experimental results and relevant scientific literature.     

Hydrogen generation through rolling using Al–Sn alloy

Mechanically treated aluminum-tin (Al–Sn) alloy, a novel hydrogen-generating material, was fabricated and found to react directly and immediately with water at room temperature. The maximum yield of hydrogen per unit volume of alloy was 2259 mL/cm³ (0.202 g/cm³). The mass ratio of the generated hydrogen and the Al–Sn alloy material was 4.86%. This percentage is much higher than that of traditional hydrogen storage alloys and can compete with metal hydrides. The combination of Al–Sn alloy powder and carbon nanotubes (CNTs) produced a new kind of Al–Sn/CNT composite that also reacts with water at room temperature. Al–Sn/CNT composites were synthesized using a high temperature and high-pressure method. When CNT content was held constant, composites with single-walled CNTs had higher reaction rates than those with multi-walled CNTs. The effects of mechanical treatment and CNT addition on enhancing the reaction between Al–Sn alloys or Al–Sn/CNTs and water were also analysed.     

Investigation on microstructure and hydrogen generation performance of Al-rich alloys

In order to study the effect of cooling rate on the microstructures and hydrogen generation performance of Al alloys, two ingots (20 g and 45 g) with a composition of 94 Al, 3.8 Ga, 1.5 In and 0.7 Sn (in mass%) were prepared by arc melting under high purity argon atmosphere, and a rod (10 g) with the same composition was cast in a vacuum chamber. The microstructures and phase compositions of the three samples were investigated by means of X-ray diffraction and scanning electron microscope with energy dispersed X-ray. The melting point of the grain boundary phase was measured using differential scanning calorimeter. Based on the structural analysis, samples with different but uniform grain sizes were cut from these alloys for H₂ generation. The reactions of Al alloys with water were measured at different temperatures. The measured H₂ generation rates were found to increase rapidly once the grain size was reduced below 50 μm. An isothermal kinetic model was employed to analyze the measured kinetic data so as to obtain kinetic parameters of reactions. The reaction order (n) for these alloys was found to be about 0.7. The activation energy (E_a) decreases with grain size d, i.e., 30% reduction of E_a as d was reduced from 258 to 23 μm. A mechanism of Al alloy corrosion in water was proposed.     

Feasibility study of hydrogen production for micro fuel cell from activated Al–In mixture in water

A safe and environmental-friendly method of hydrogen production from milled Al–In–Zn–salt mixture in water was proposed in this paper. The 10 h—milled Al–In–Zn–salt mixture had high reactivity and produced hydrogen in water at room temperature. Its improved reactivity came from that the additive Zn and salts facilitate to the negative shift of Al–In alloy and benefited the combination of Al, In and Zn in the milling process. Optimized the composition content, 1 g of 10 h—milled Al—5 wt%In—3 wt%Zn—2 wt%NaCl mixture had highest hydrogen yield of 1035 mL hydrogen/1 g Al in 4 min of hydrolysis reaction in water, corresponding to 9.21 wt% hydrogen (excluding water mass). Hydrogen supplying from milled Al–In–Zn–salt mixture was performed for micro fuel cell and 0.96 W was produced with the stable hydrogen supply rate. Therefore, the milled Al–In–Zn–salt mixture could be a feasible alternative for providing a source of CO₂ free hydrogen production to supply micro fuel cell.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Effect of heat treatment and chemical composition on the corrosion behavior of Ni–Al intermetallics in molten (Li + K) carbonate

The corrosion performance of several Ni–Al alloys in 62 mol% Li₂CO₃–38 mol% K₂CO₃ at 650 °C has been studied using the weight loss technique. Alloys included 50Ni–50Al at.% (NiAl) and 75Ni–25Al at.% (Ni₃Al) alloys with additions of 1, 3 and 5 at.% Li each one, with or without a heat treatment at 400 °C during 144 h. For comparison, AISI-316L type stainless steel was also studied. The tests were complemented by X-ray diffraction, scanning electronic microscopy and micro-analyses. Results showed that NiAl-base alloy without heat treatment presented the lowest corrosion rate even lower than Ni₃Al alloy but still higher than conventional 316L-type stainless steel. In general terms, by either by heat treating these base alloys or by adding Li, the mass loss was increased. This effect was produced because by adding Li the adhesion of the external protective layer was decreased by inducing a higher number of discontinuities inside the grain boundaries. When the alloys were thermally annealed, these irregularities in the grain boundaries disappeared, decreasing the number of paths for the outwards diffusion of Al from the alloy to form the external, protective Al₂O₃ layer.     

Hydrogen production for micro-fuel-cell from activated Al–Sn–Zn–X (X: hydride or halide) mixture in water

A systematic investigation of hydrogen production from milled Al–Sn–Zn–X (X: hydride or halide) mixtures in pure water was performed at room temperature. The hydrolysis mechanism of the mixtures was based on the work of micro-galvanic cell between aluminum and tin in water where aluminum reacted with water to generate AlOOH (Boehmite) and hydrogen. It was found that many effects such as milling time, temperature, additives and mass ratio had a significant role in the hydrogen production rate, especially that of the additives (hydride or halide) led to reduction of crystallite size and accumulation of uniform mixing. They also produced a lot of heat and the conductive ions which simulated the work of micro-galvanic cell. The milled Al–Sn–Zn–X (X: hydride or halide) mixtures had high reactivity and Al–Sn–Zn–MgH₂ mixture produced 790 mL g^?1 hydrogen in 5 min of the hydrolysis reaction with the activation energy of 17.570 kJ mol^?1, corresponding to 7.04 wt.% hydrogen excluding water mass. Therefore, a new method of CO₂ free and safe hydrogen production for micro-fuel-cell was obtained from the activated aluminum alloys in water.     

Fabrication of AlLi and Al2Li3/Al4Li9 intermetallic compounds by molten salt electrolysis and their application for hydrogen generation from water

The concept of “hydrogen on demand” has been widely publicized. More importantly, the materials used to produce hydrogen on demand should be in themselves safe to handle. In present work, Al–Li intermetallic compounds (IMC) were fabricated in air by electrolysis from LiCl–KCl molten salt at 480 ± 25 °C. Bulk AlLi IMC and the bulk compound with mixture of Al₂Li₃ and Al₄Li₉ (Al₂Li₃/Al₄Li₉ IMC) were not pyrophoric and can be safely handled in air. When both compounds in contact with water, vigorous reaction occurred and H₂ was directly produced. The by-products after H₂ generation from AlLi IMC were a mixture of Li-containing α-Al and Li–Al hydrotalcite (hereafter referred to as Li–Al LDH). The by-product after H₂ generation from Al₂Li₃/Al₄Li₉ compound was a mixture of LiOH·H₂O and Li–Al LDH. Those by-products can be easily separated from water and may be reused as a resource. Approximately 500–860 ml of H₂ per weight (g) of the IMC compounds was generated in deionized water at room temperature. Experimentally, AlLi IMC powder and Al₂Li₃/Al₄Li₉ compound exhibit gravimetric hydrogen capacity of 7.0 wt.% and 5.4 wt.%, respectively. Although the energy consumed for fabricating Al–Li IMC compounds is a little larger than the energy provided by the generated H₂, the Al–Li IMC compounds are promising materials for producing hydrogen on demand without the necessity of hydrogen storage.     

Electrochemical investigation of the anodic corrosion of Pb–Ca–Sn–Li grid alloy in H2SO4 solution

The steady-state and anodic corrosion of Pb–0.17 wt.% Ca–0.88 wt.% Sn, and Pb–0.17 wt.% Ca–0.88 wt.% Sn–0.06 wt.% Li alloys in 4.5 M H₂SO₄ at 25 °C were studied using cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. The experimental results show that the lithium added to Pb–Ca–Sn alloy increases corrosion resistance in equilibrium potential and inhibits the growth of the anodic corrosion layer.     

Experiment assessment of hydrogen production from activated aluminum alloys in portable generator for fuel cell applications

An experiment assessment of hydrogen production from activated aluminum alloy in portable hydrogen generator for fuel cell applications was investigated. The optimum hydrogen capacity of the high–reactive Al–Bi–NaCl alloys (the abbreviation of milled material of aluminum, bismuth and NaCl particles) is about 9–9.4 wt.%, meeting the targets (9 wt.%) of the US Department of Energy in 2015. Hydrogen production rate can be controlled via controlling the water flow rate in the generator, being 1.369–6.198 L hydrogen/min while the water flow rate ranges in 5–20 mL/min. The larger water flow rate often leads to higher temperature and results in unsafety in the generator as the hydrolysis reaction of aluminum alloy and water releases 15 kJ/g heat. However, the heat problem can be successfully eliminated by using effective cooling stytles, which enable the maximum temperature of Al–H₂O mixture (the abbreviation of hydrolysis products of aluminum alloy in water) controlled less than 474 K even though the water flow rate is 20 mL/min. Therefore, the experiment results show that the portable hydrogen generator from aluminum alloy could supply the CO₂–free, high hydrogen capacity and safe hydrogen for fuel cell applications.     

Development of Fe–Ni/YSZ–GDC electrocatalysts for application as SOFC anodes: XRD and TPR characterization and evaluation in the ethanol steam reforming reaction

Electrocatalysts based on Fe–Ni alloys were prepared by means of modified Pechini and physical mixture methods and using on a composite of Yttria Stabilized Zirconia (YSZ) and Gadolinia-Doped Ceria (GDC) as support. The former method was based on the formation a polymeric precursor that was subsequently calcined; the later method was based on the mixture of NiO and the support. The resulting composites had 35 wt.% metal load and 65 wt.% support (70 wt.% YSZ and 30 wt.% GDC mixture) (cermets). The samples were then characterized by Temperature-Programmed Reduction (TPR) and X-Ray Diffraction (XRD) and evaluated in the ethanol steam reforming at 650 °C for 6 h in the temperature range of 300–900 °C. The XRD results showed that the bimetallic sample calcined at 800 °C formed a mixed oxide (NiFe₂O₄) with a spinel structure, which, after reduction in hydrogen, formed Ni–Fe alloys. The presence of Ni was observed to decrease the final reduction temperature of the NiFe₂O₄ species. The addition of iron to the nickel anchored to YSZ–GDC increased the hydrogen production and inhibited carbon deposition. The resulting bimetallic 30Fe5Ni sample reached an ethanol conversion of about 95% and a hydrogen yield up to 48% at 750 °C. In general, ethanol conversion and hydrogen production were independent of the metal content in the electrocatalyst. However, the substitution of nickel for iron significantly reduced carbon deposition on the electrocatalyst: 74, 31, and 9 wt.% in the 35Ni, 20Fe15Ni, and 30Fe5Ni samples, respectively.     

Effect of addition of rhenium to Pt-based anode catalysts in electro-oxidation of ethanol in direct ethanol PEM fuel cell

Breaking of C–C bond at low temperature to completely oxidize ethanol in direct ethanol fuel cell (DEFC) is the limiting factor for the development of DEFC as alternative source of power in portable electronic equipment. Binary and ternary Pt based catalysts with addition of Re, Pt–Re/C (20:20), Pt–Sn/C (20:20), Pt–Re–Sn/C (20:10:10) and Pt–Re–Sn/C (20:5:15) catalysts were prepared from their precursors by co-impregnation reduction method to study electro-oxidation of ethanol in DEFC. The electrocatalysts characterized by transmission electron microscope, scanning electron microscope, energy dispersive X-ray, and X-ray diffraction shows the formation of above mentioned bi- and tri-metallic catalyst with size ranges from 6 to 16 nm. Electrochemical analyses by cyclic voltammetry, linear sweep voltammetry and chronoamperometry show that Pt–Re–Sn/C (20:5:15) gives higher current density compared to that of Pt–Re/C (20:20) and Pt–Sn/C (20:20). The addition of Re to Pt–Sn/C is conducive to electro-oxidation of ethanol in DEFC. The power density obtained using Pt–Re–Sn/C(20% Pt, 5% Re, 15% Sn by wt) (30.5 mW/cm²) as anode catalyst in DEFC is higher than that for Pt–Re–Sn/C(20% Pt, 10% Re, 10% Sn by wt) (19.8 mW/cm²), Pt–Sn/C (20% Pt, 20% Sn by wt) (22.4 mW/cm²) and Pt–Re/C (20% Pt, 20% Re by wt) (9.8 mW/cm²) at 100 °C, 1 bar, with catalyst loading of 2 mg/cm² and 5 M ethanol as anode feed.     

Role of the composition and preparation method in the activity of hydrotalcite-derived Ru catalysts in the catalytic partial oxidation of methane

Hydrotalcite-derived Ru catalysts were tested in the catalytic partial oxidation of CH₄ to produce syngas. The effect of Ru content, oxidic matrix composition, and preparation procedure on chemical–physical properties and performances of catalysts was studied. Bulk catalysts (0.25 and 0.50 wt.% Ru) were obtained via Ru/Mg/Al hydrotalcite-type (HT) precursors with carbonates or silicates as interlayer anions. A supported catalyst was prepared by impregnation on a calcined Mg/Al–CO₃ HT. Ru/γ-Al₂O₃ was evaluated for comparison. Both the Ru dispersion and the interaction with the support decreased as the Ru loading increased and when silicates were present due to RuO₂ segregation. Regardless of the Ru loading, carbonate-derived catalysts performed better than those containing silicates. The increased Ru loading improved the initial activity, but deactivation occurred after high temperature tests. Stability tests for shorter contact times over a 0.25 wt.% bulk sample obtained from Ru/Mg/Al HT with carbonates showed a tendency to deactivate at 750 °C.     

Effects of Bi composition on microstructure and Al-water reactivity of Al-rich alloys with low-In

Bi-bearing Al-Ga-In-Sn quinary alloys were prepared by a high-temperature melting technique. The alloys primarily consist of Al(Ga) matrix and Ga, In, Sn, Bi (GISB) grain boundary phase, mainly in the form of Ga-InSn₄-InBi. The microstructure of GISB particles was obviously equiaxed with the increasing Bi dosage. Al-water reaction was tested at 40 °C. Owing to the Bi-doping, the hydrogen generation yields of alloys with InSn₄ intermetallic compound are obviously improved and hydrogen release rates gradually tend to be stable, which show great potential in applications. At the dosage of 2.53 wt% Bi, the hydrogen generation performance of alloys was more prominent in Al-water reaction, including a theoretical hydrogen generation yield and hydrogen released extremum rate to ～0.076 L/min·g Al alloy. Furthermore, the Al-water reaction mechanism of Bi-bearing Al-rich low-In quinary alloys has been put forward.     

Two-step sintering ceria-based electrolytes

Yttrium and gadolinium-doped ceria-based electrolytes (20 at% dopant cation) with and without small Ga₂O₃-additions (0.5 mol%) were fired at peak temperatures of 1250 and 1300 °C, or following a two-step sintering profile including one peak temperature and subsequent dwell at 1150 °C, 10 h. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. Average grain sizes in the range 150–250 nm and densifications up to about 94% were found dependent on the sintering profile and presence of Ga. The grain boundary arcs in the impedance spectra increased significantly with Ga-doping, cancelling the apparently positive role of Ga on bulk transport, evidenced mostly in the case of yttrium-doped materials.     

Electrochemical hydriding of Mg–Ni–Mm (Mm = mischmetal) alloys as an effective method for hydrogen storage

In this work, the electrochemical hydriding method was used for storing hydrogen in four binary Mg–Ni (Ni content from 15 to 34 wt.%) alloys and one ternary Mg–26Ni–12Mm alloy. Both the as-cast and powdered alloys were hydrided in a 6 M KOH solution at 80 °C for 120–480 min. The structures and phase compositions of the alloys, both before and after hydriding, were studied using optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Differential scanning calorimetry and mass spectrometry were used to study the dehydriding process. In the case of as-cast alloys, the best combination of hydriding parameters (maximum hydrogen concentration on surface; depth of hydrogen penetration) was achieved in the Mg–26Ni alloy. In the case of powdered alloys, the Mg–34Ni alloy absorbed the highest amount of hydrogen, nearly 4.5 wt.%. The only hydride formed during hydriding was the MgH₂ hydride. The results of the mass spectrometry analysis reveal a significant thermodynamic destabilization of magnesium hydride due to Ni and Mm. The decomposition temperature of MgH₂ was reduced by more than 200 °C. The results are discussed in relation to the electronic structure and atomic size of the alloying elements and the structural variations in the alloys.