Investigation of hydrogen isotope effect on storage properties of Zr–Co–Ni alloys

The deuterium desorption pressure-composition isotherms (PCIs) of ZrCo_1−xNi_x-D₂ (x = 0, 0.1, 0.2 and 0.3) systems were generated in this study in the temperature range of 524–603 K using Sievert's type volumetric apparatus. Thermodynamic parameters like enthalpy and entropy change for deuterium desorption reactions involved in the ZrCo_1−xNi_x-D₂ systems were derived using the equilibrium pressure data of PCIs. In order to interpret the hydrogen isotope effect on the storage behaviour of ZrCo_1−xNi_x alloys, the results obtained in the present study were compared with the earlier reported data on the ZrCo_1−xNi_x-H₂ (x = 0, 0.1, 0.2 and 0.3) systems. This comparison revealed that these alloys show normal hydrogen isotope effect where the equilibrium pressure of D₂ is higher than that of H₂ at all experimental temperatures. Based on these observation, it is expected that at the ITER SDS operating conditions the equilibrium pressure of tritium, deuterium and hydrogen will follow the order: p(T₂) > p(D₂) > p(H₂) for these alloys.     

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr_0.8Ti_0.2-xNb_xCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr_0.8Ti_0.2-xNb_xCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr_0.8Ti_0.2-xNb_xCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.     

Hydrogen isotope effect of nanoporous palladium

Materials used in hydrogen isotopes separation are crucial in modern hydrogen energy field. In this article, a promising material nanoporous palladium was prepared and its relative properties were studied. Nanoporous palladium with pore scale of about 5 nm was fabricated by free dealloying corrosion. Its kinetic and PCT curves of hydrogen/deuterium adsorption at room temperature were tested using a Sievert-type volumetric apparatus. Microstructures were observed with Scanning Electron Microscopy and Atomic Force Microscopy. Crystalline structure was characterized with X-Ray Diffractometer before and after deuterium adsorption. Comparative experiments with spongy palladium that is commonly used in relative industry were also carried out. According to the results, nanoporous palladium shows faster hydrogen/deuterium adsorption rate than spongy palladium, which is due to its nanoporous structure that supplies a large amount of specific surface area. PCT curves of hydrogen/deuterium adsorption in nanoporous palladium among 298–338 K were tested and plateau pressures at different temperatures were obtained. Deuterium/hydrogen isotopes separation factors were calculated using plateau pressures above and were plotted with temperature. It's found that nanoporous palladium shows larger hydrogen isotope separation factors compared with spongy palladium at temperatures no higher than 323 K. These findings illustrate that nanoporous palladium would be an ideal material for hydrogen isotopes separation applications.     

Hydrogen storage in binary and ternary Mg-based alloys: A comprehensive experimental study

This study focused on hydrogen sorption properties of 1.5 μm thick Mg-based films with Al, Fe and Ti as alloying elements. The binary alloys are used to establish as baseline case for the ternary Mg–Al–Ti, Mg–Fe–Ti and Mg–Al–Fe compositions. We show that the ternary alloys in particular display remarkable sorption behavior: at 200 °C the films are capable of absorbing 4–6 wt% hydrogen in seconds, and desorbing in minutes. Furthermore, this sorption behavior is stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys. Even after 100 absorption/desorption cycles, no degradation in capacity or kinetics is observed. For Mg–Al–Fe, the properties are clearly worse compared to the other ternary combinations. These differences are explained by considering the properties of all the different phases present during cycling in terms of their hydrogen affinity and catalytic activity. Based on these considerations, some general design principles for Mg-based hydrogen storage alloys are suggested.     

Time and frequency domain analysis of hydrogen permeation across PdCu metallic membranes for hydrogen purification

Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd_0.47Cu_0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.     

A comparative study of catalytic dehydrogenation of perhydro-N-ethylcarbazole over noble metal catalysts

N-ethylcarbazole is one of the most promising liquid organic hydrogen carriers (LOHCs) as it can be catalytically hydrogenated and dehydrogenated at relatively moderate temperatures. In the present work, we report a systematic study on dehydrogenation of perhydro-N-ethylcarbazole over several important supported noble metal catalysts to identify the optimal catalyst for temperature-controlled dehydrogenation. The reaction takes three consecutive stages with two intermediates of octahydro-N-ethylcarbazole and tetrahydro-N-ethylcarbazole. The initial catalytic activity of the selected noble metal catalysts for the dehydrogenation process was found to follow the order of Pd > Pt > Ru > Rh. 100% selectivity toward the final product of N-ethylcarbazole and fully dehydrogenation was achieved over the supported Pt and Pd catalysts. The kinetics of the three stage dehydrogenation processes over the catalysts was studied and the rate constants were derived. The results indicate that the dehydrogenation reaction rate decreases significantly with the reaction stage for all the selected noble catalysts and the conversion from tetrahydro-N-ethylcarbazole to N-ethylcarbazole was found to be the rate-limiting step of the entire reaction process.     

Effect of Mo–Ni treatment on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloys

In order to improve kinetic properties of La–Mg–Ni-based hydrogen storage alloys, Mo–Ni treatment was applied to La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. FESEM results showed that after Mo–Ni treatment some network-shaped substance with nano-size formed on the surface of the alloy particles. The EDS results revealed increase in Ni content and emerge of Mo element. EIS and Linear polarization showed that charge-transfer resistance decreased and exchange current density increased for the treated alloy electrode, and the high rate dischargeability (HRD) was consequently improved. HRD at 1500 mA/g increased from 22.5% to 39.5%. Mo- and Ni-single treatments were performed compared with the Mo–Ni treatment, and the results showed that the single treatment improved HRD slightly, far less than the Mo–Ni treatment.     

Repair of Pd-based composite membrane by polishing treatment

In this study, the effect of module configuration on the performance of Pd-based membrane devices was examined using a Pd-Au composite membrane deposited on a porous nickel support. Hydrogen permeation flux, recovery, and CO₂ enrichment were experimentally examined using two different modules. The module configuration that had a narrower space between the surface of the membrane and cover plate provided a large linear flow velocity of the gas mixture, which allowed for a reduction in the concentration polarization. The CO₂ enrichment capacity of the membrane module with a space distance of 0.4 mm was 4 times higher than the module with a space distance of 2.5 mm. In regards to the process design, the membrane with an effective area of 16.6 cm² could enrich 40% of the CO₂ at a flow rate of 2000 ml min⁻¹ up to 87% with a hydrogen recovery ratio of >90% at 673 K and a total feed pressure of 1600 kPa.     

Substitutional V–Pd alloys for the membranes permeable to hydrogen: Hydrogen solubility at 150–400 °C

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V-xPd alloys (x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–10⁶) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10⁻⁴ to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys.     

Tailoring thermal resistance of porous materials with void filling for improved hydrogen adsorption

Porous materials such as 5A molecular sieve (5A) display huge thermal resistance due to high porosity and lots of voids between grains that is negative to hydrogen isotope separation engineering. Generally, introducing thermal conductive fillers contributes to reducing thermal resistance while results in decreasing volume ratio of porous materials and then certainly causes declined hydrogen adsorption capacity. Here, a liquid polydimethylsiloxane (PDMS) is pressed into the voids between 5A grains, which is further developed into a silicon, oxygen and carbon (SiOC) structure suffering from 350 °C sintering. 5A/SiOC composite at 10 wt% polydimethylsiloxane (5A/SiOC10) displays 0.74 W/mK of thermal conductivity, which is about 300% higher than that of neat 5A. More importantly, enhanced rather than reduced hydrogen adsorption capacities at a fixed volume of the composite are determined. 5A/SiOC10 shows adsorption capacities of H₂ (175.4 mL/cm³) and D₂ (188.4 mL/cm³) while neat 5A shows that of H₂ (171.6 mL/cm³) and D₂ (165.8 mmol/cm³) at 77 K with 1 bar. Besides, enhanced thermal conductivity of porous materials shortens the cycle time of hydrogen isotope separation that contributes to reducing energy consumption. This work proposes a novel strategy on void filling to tailor thermal resistance of porous materials, which open a window to improve hydrogen isotope separation with thermal management materials.     

Hydrophobic platinum–polytetrafluoroethylene catalyst for hydrogen isotope separation

A composite catalyst, platinum supported on polytetrafluoroethylene (Pt/PTFE), has been successfully prepared by compression moulding forming and used for hydrogen isotope separation by hydrogen–water isotope exchange. The as-prepared Pt/PTFE was characterized by nitrogen adsorption. The results of the catalytic activity for hydrogen–water isotope exchange show that Pt/PTFE has high catalytic activity. The effects of different factors, such as flow rate, temperature, molar flow ratio of hydrogen gas to feed water and time have also been investigated. The present study shows a promising choice of Pt/PTFE as a composite catalyst for hydrogen isotope separation.     

An analogy of interstitial site occupancy and hydrogen induced disproportionation of Zr1−xTixCo ternary alloys

The hydrogen induced disproportionation behavior of Ti-substituted ZrCo alloys was investigated to explore their suitability for International Thermonuclear Experimental Reactor (ITER) Storage and Delivery System (SDS). The isothermal disproportionation studies on Ti-substituted alloys were carried out in conditions simulating ITER SDS i.e. 750 K temperature and 100 kPa hydrogen pressure. It was observed that the rate of disproportionation of Ti-substituted ZrCo alloys was found to vary as ZrCo > Zr_0.9Ti_0.1Co > Zr_0.7Ti_0.3Co > Zr_0.8Ti_0.2Co. X-ray diffraction measurements revealed the formation of TiCoH phase along with Ti-substituted ZrCo₂ and ZrH₂ phases as a result of disproportionation reaction of alloys. Neutron diffraction measurements on deuterides indicated that deuterium occupancy in 8e site and the corresponding Zr-D distance provide the primary driving force for disproportionation of alloys to take place. A plausible potential energy profile based on thermodynamic and kinetic considerations was proposed to explain the disproportionation mechanism of alloys.     

The hydriding kinetics of Mg–Ni based hydrogen storage alloys: A comparative study on Chou model and Jander model

Two kinds of kinetic models, which are Jander model and Chou model, were applied to investigate the hydriding kinetic behavior of Mg–Ni based alloys. By comparing the calculated values with experimental data, it can be seen that both models were successfully used in the diffusion-controlled hydrogen absorption process of Mg–Ni system. However, Chou model was not only convenient for use but also gave a set of physical meaningful explicit analytic expressions. Chou model should be preferentially recommended to deal with the calculation at multi-temperatures and multi-pressures without multistep calculation. The application of Chou model to Mg₂₀Ni₈Cu₂ and Mg₂₀Ni₈Co₂ alloys shows that the calculated results agreed well with the experimental data and it is reasonable to expect that this model will also suitable for other Mg–Ni based alloys if the mechanism is similar.     

Enhanced hydrogen generation behaviors and hydrolysis thermodynamics of as-cast Mg–Ni–Ce magnesium-rich alloys in simulate seawater

In this study, we developed as-cast (Mg10Ni)_1-xCe_x (x = 0, 5, 10, 15 wt%) ternary alloys by using a flux protection melting method and investigated their hydrolysis hydrogen generation behaviour in simulate seawater. The phase compositions and microstructures of as-cast (Mg10Ni)_1-xCe_x ternary alloys are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with electron energy dispersion spectrum (EDS) and transition electron microscope (TEM). Their kinetics, thermodynamics, rate-limiting steps and apparent activation energies are investigated by fitting the hydrogen generation curves at different temperatures. With increasing Ce content, the (Mg10Ni)_1-xCe_x ternary alloys show increased electrochemical activities and decreased eutectic. When 10 wt% and 15 wt% Ce added, the active intermediate phase of Mg₁₂Ce has been observed. The hydrogen generation capacity of (Mg10Ni)₉₅Ce₅ is as high as 887 mLg⁻¹ with a hydrolysis conversion yield of 92%, which is higher than that of Mg₁₀Ni alloys (678 mLg⁻¹) with a yield only 75% at 291 K. The initial hydrolysis reaction kinetics of Mg–Ni–Ce alloys is mainly controlled by the electrochemical activity and the mass transfer channels formed in the alloys. Such a structure-property relationship will provide a possible strategy to prepare Mg-based alloys with high hydrogen conversion yield and controlled hydrolysis kinetics/thermodynamics.     

Methane catalytic decomposition integrated with on-line Pd membrane hydrogen separation for fuel cell application

In this study, 70 wt.% Ni/Al₂O₃ was prepared via a glycine–nitrate combustion method and applied as the catalyst for decomposing methane into hydrogen and carbon nanotubes that can be applied in polymer-electrolyte-membrane fuel cell (PEMFC). The methane conversion and the hydrogen content in the effluent gas reached 71 and 83%, respectively, at an operating temperature of 700 °C under ambient pressure. I–V tests demonstrated that the methane is inert to the electro-catalyst and acts mainly as a diluting gas. A porous Al₂O₃-supported thin-film Pd membrane was integrated with the catalytic methane decomposition process. Due to the high initial hydrogen content, even an imperfect Pd membrane, effectively increased the hydrogen content to >98%, which resulted in only a slight performance loss of ∼10% compared to the application of pure hydrogen as the fuel. The advantages, such as continuous hydrogen separation, simple process, high reliability and value-added by-product, all make this process highly attractive for future PEMFC application.     

Diffusion barrier coating using a newly developed blowing coating method for a thermally stable Pd membrane deposited on porous stainless-steel support

In this study, we present an intermetallic diffusion barrier coating using a newly developed blowing coating method for a thermally stable Pd-based composite membrane on porous stainless steel (PSS). A tubular PSS sample with 1/2 inch (12.7 mm) in diameter and 450 mm in length was used for the support. The support was welded with a stainless-steel cap and a 450-mm-long stainless-steel tube for each end. Before the diffusion barrier coating, the large-sized entrance pores were gradually blocked with sub-micron (～500 nm) and nano (50–80 nm) yttria stabilized zirconia (YSZ). 8YSZ, i.e., 8wt.% YSZ, was used for the interdiffusion barrier material and dispersed on the surface of the pre-treated PSS using the blowing coating method. The blowing coating method has 4 steps: (i) spraying the 17 wt. % 8YSZ paste on the surface of the PSS tube, (ii) blowing the paste using compressed air, (iii) drying at room temperature, and (iv) heat treatment at 923 K for 2 h in air. Steps (i) and (ii) were repeated 3 times to have a ～240 nm thick 8YSZ layer. A thin palladium layer (～3 μm) was deposited on the pre-treated PSS using electroless plating, and the membrane stability was tested at 673–773 K for ～ 200 h. A hydrogen permeation flux of 9.86 × 10^?2 mol m^?2 s^?1 and an H₂/N₂ selectivity of 595 were obtained at 773 K and a transmembrane pressure difference of 20 kPa. The surface and cross-sectional SEM/EDX analysis confirmed that the 8YSZ layer sufficiently prevented the interdiffusion between Pd and PSS elements, such as Fe, Cr and Ni.     

Surface roughness improvement of Hastelloy X tubular filters for H2 selective supported Pd–Ag alloy membranes preparation

Thin Pd–Ag layers have been successfully deposited on ceramic supports with controlled surface characteristics. The need for less fragile membranes and ease of sealing and connection leads to the study of metallic supports for thin Pd-based membrane development. Metallic supported membranes are prone to intermetallic diffusion issues so an interdiffusion barrier must be introduced. However, metallic supports with sufficient surface quality for direct membrane deposition are expensive and not readily available in the market. It is thus important to study how to improve surface roughness of commercially available rough metallic filters, in order to allow deposition of a smooth, delamination-free Pd–Ag layer.This work reports a first attempt towards a standardized preparation procedure for Pd-based membranes on cheap, rough, and unrefined Hastelloy X tubular filters. The focus is on surface roughness reduction, in order to allow the deposition of a smooth, uniform Pd–Ag selective layer. The surface roughness of the tubes is tuned via 1) polishing and 2) addition of a smoothening interdiffusion barrier layer based on a boehmite dip-coated dispersion. The polishing time was assessed by studying the average support's roughness variation, permeation behavior and ability to retain ceramic coating. It was found that the best trade-off between polishing extent and gas permeance of the support amounts to 6 h. Moreover, it was assessed that increasing the boehmite load in the interdiffusion barrier precursor solution leads to thicker layers and larger surface roughness reduction, but greater solution instability. 1,2%wt of boehmite load proved the best trade-off between layer reproducibility and support coverage. Different dipping-sintering routes were evaluated in order to improve surface's suitability for electroless plating: a single interdiffusion layer deposition route proved the most suitable for Pd–Ag deposition. The electroless plating performed onto the treated supports results in a continuous Pd–Ag layer, proving Pd–Ag deposition possible on the selected filters.