Hydriding Kinetics Of Mg Research Articles

Tuning Mg hydriding kinetics with nanocarbons

Mg-nanocarbon composites demonstrate fast reversible hydriding kinetics. The kinetics is strongly correlated with the structure of the added nanocarbon allotrope. We conducted a systematic exploration of the reversible hydriding kinetics of Mg upon addition of nanocarbons with different dimensionalities, namely, 1D carbon nanotubes, 2D graphene nanoplatelets and 3D activated carbon. Our findings showed that the Mg (de)hydriding rate could be tuned by the dimensionality and defect density of the nanocarbon. The resulting kinetic enhancement was explained in terms of the hydrogen spillover mechanism. The use of nanocarbons offers an alternative to the utilization of high concentrations of expensive transition-metal catalysts for hydrogen storage.

Enhanced hydriding kinetics of Mg-10 at% Al composite by forming Al 12 Mg 17 during hydriding combustion synthesis

Abstract Mg-based alloys are widely considered as promising materials for hydrogen storage. However, owing to the high chemical activity of Mg, the surface of Mg powder is always covered with a layer of close packed MgO, which can retard the hydrogenation of Mg. In this paper, 10 at% Al was added to accelerate the hydriding kinetics of Mg through tabletting and grinding treatments. After heating to 420 °C and subsequent holding at 340 °C for 120 min under 1.5 MPa hydrogen pressure, the Mg-10 at% Al composite could absorb 6.34 wt% H2 in the process of hydriding combustion synthesis (HCS) while the same treated pure Mg powder was barely hydrided. Upon heating from room temperature to 420 °C, Al alloyed with Mg to form an Al12Mg17 phase on the surface of Mg particles and the newly formed alloy was oxide-free, which would react with H2 when the temperature decreased to 340 °C. Then hydrogen was able to get into the inner part of the Mg particle from the former Al12Mg17 sites where the MgO shell had been broken in the alloying period to further react with unhydrided Mg.

Hydrogen storage characteristics of melt spun Mg–23.5Ni–5Cu alloys mixed with LaNi 5 and/or Nb 2O 5

Mg-based materials have a high potential for commercialization due to the large hydrogen storage capacity, low cost and abundance in the Earth's crust. Despite these advantages, reaction kinetics of Mg with hydrogen is slow. Various investigations to increase the hydriding kinetics of Mg were performed. Although LaNi 5 has a low hydrogen storage capacity, it can store hydrogen at a low temperature. In this study, a Mg–23.5Ni–5Cu alloy was prepared by gravity casting method in a large quantity (approximately 7.5 kg), and then melt-spun and heat-treated. LaNi 5, Nb 2O 5 or LaNi 5 + Nb 2O 5 were added to the Mg–23.5Ni–5Cu alloy by reactive mechanical grinding in a planetary mill. The hydrogen storage capacity and hydriding behavior of the Mg–23.5Ni–5Cu alloys with LaNi 5, Nb 2O 5 or LaNi 5 + Nb 2O 5 added were then investigated.

Modeling and analyzing the hydriding kinetics of Mg–LaNi 5 composites by Chou model

Chou model was used to analyze the influences of LaNi 5 content, preparation method, temperature and initial hydrogen pressure on the hydriding kinetics of Mg–LaNi 5 composites. Higher LaNi 5 content could improve hydriding kinetics of Mg but not change hydrogen diffusion as the rate-controlling step, which was validated by characteristic reaction time t c. The rate-controlling step was hydrogen diffusion in the hydriding reaction of Mg–30 wt.% LaNi 5 prepared by microwave sintering (MS) and hydriding combustion synthesis (HCS), and surface penetration was the rate-controlling step of sample prepared by mechanical milling (MM). Rising temperature and initial hydrogen pressure could accelerate the absorption rate. The rate-controlling step of Mg–30 wt.% LaNi 5 remained hydrogen diffusion at temperatures ranging from 302 to 573 K, while that of Mg–50 wt.% LaNi 5 changed from surface penetration to hydrogen diffusion with increasing initial hydrogen pressure ranging from 0.2 to 1.5 MPa. Apparent activation energies of absorption for Mg–30 wt.% LaNi 5 prepared by MS and MM were respectively 25.2 and 28.0 kJ/mol H 2 calculated by Chou model. Kinetic curves fitted and predicted by Chou model using temperature and hydrogen pressure were well exhibited.

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 Mg 20Ni 8Cu 2 and Mg 20Ni 8Co 2 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.

Improving MgH 2 formation kinetics and its effect on NaBH 4 synthesis

Mg and Mg-based alloys are not only promising hydrogen storage materials due to their high storage capacities, but also potential reducing agents in the synthesis of sodium borohydride NaBH 4, an attractive hydrogen source with 10.6 wt% hydrogen content. In this study, the MgH 2 formation kinetics in pure Mg and Mg–23.5 wt% Ni was studied and correlated with NaBH 4 synthesis kinetics. It was found that the hydriding kinetics of Mg–23.5 wt% Ni was significantly improved owing to the fine laminar Mg/Mg 2Ni structure. Ballmilling and surface treatment made the eutectic alloy absorb 2.0 wt% hydrogen at 353 K. When these materials were applied for the NaBH 4 synthesis, it was found that the eutectic Mg–Ni alloy enhanced the NaBH 4 formation kinetics at high temperatures. However, NaBH 4 was not produced at 353 K although MgH 2 could be formed.

Kinetic analysis of Mg–8 mol%LaNi 0.5 composite

The aim of this paper is to apply a new kinetic model in order to investigate kinetic mechanism of the hydriding kinetics at 553 K and hydrogen pressure range from 0.256 to 0.992 MPa in the two-phase (α–β) region of the Mg–8 mol%LaNi 0.5 composition prepared by different methods. Much attention was put on the influence of the fabrication technique and the initial hydrogen pressure on hydriding kinetics of Mg–8 mol%LaNi 0.5 composite. This model has been validated by comparison of our calculation results with experimental data. The hydriding kinetic mechanism of Mg–8 mol%LaNi 0.5 was determined and the rate-limited step in the two-phase (α–β) region is hydrogen diffusion in the β phase.

Hydriding kinetics and process parameters in reactive milling

We focus on the hydriding kinetics of Mg 2Ni/Ni nanocrystalline and Zr 55Cu 30Al 10Ni 5 amorphous powders under reactive milling conditions. A methodological approach, based on mechanochemical parameters, is presented to quantify the different behaviour shown by the two systems. A decelerating course characterized the H 2 absorption rate in the Mg based powders. Although they were able to absorb H 2 under static conditions, a speeding up of the kinetics was observed under milling. The mechanochemical gain, g m , that is the rate increase at the milling outset, was found to depend on the specific milling intensity I M . By contrast an incubation period preceded the absorption in the case of the Zr based amorphous powders. Furthermore, S-shaped trends described the reactive behaviour in this case even if the kinetics was still dependent on I M . Irrespective of the milling intensity it was found that the same mechanical work, that is the energy dose, was required to reach the maximum hydriding rate.

A study of the hydriding kinetics of Mg-(10–20 w/o) LaNi 5

The addition of LaNi 5 allows a considerable increase in the hydrogenation rate of magnesium according to Tanguy et al. [ Mater. Res. Bull. 11, 1441 (1976)]. The rate-controlling step was determined by comparing their empirical results with the rate equations of each step of the kinetic hydriding model established earlier. The initial rate of the reaction is controlled by the step involving hydrogen adsorption on the metal surface after which the rate-controlling step changes to the diffusion of hydrogen through the hydride phase.