Structures and hydrogen storage performances of rare earth-Mg-Ni-Mn-based AB<Subscript>2</Subscript>-type alloys applied to Ni-MH battery

The La_1-x Ce _x MgNi_3.5Mn_0.5 (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were fabricated by melt spinning technology. The effects of Ce content and spinning rate on the structures and electrochemical performances of the alloys were investigated systematically. The identification of X-ray diffraction (XRD) and SEM revealed that the experimental alloys consisted of a major phase LaMgNi₄ and a secondary phase LaNi₅. The variations of Ce content and spinning rate obviously changed the phase abundance of the alloys, namely LaMgNi₄ phase increasing and LaNi₅ phase decreasing with the increase of Ce content and spinning rate. The electrochemical test showed that the as-cast and as-spun alloys achieved the maximum discharge capacities just at the first cycling without any activation. With the increase of Ce content and spinning rate, the discharge capacity of the alloys first increased and then decreased, and the electrochemical kinetics of the alloys, involving the high rate discharge ability (HRD), hydrogen diffusion coefficient (D), limiting current density (I _L), and charge transfer rate, had a similar variation trend, whereas their cycle stabilities have shown increasing trend.     

Structure and electrochemical hydrogen storage behaviors of alloy Co2B

The alloys Co₂B were prepared by two ways of high temperature solid phase process and arc melting, the structure of the alloys was characterized by XRD and SEM. It showed that it was structure of tetragonal Co₂B.The electrochemical experimental results demonstrated that the Co₂B prepared by two means both showed excellent cycling stability. The initial discharge capacity of Co₂B prepared by the high temperature solid phase process was 480.3 mA h g⁻¹, there was no distinct declination after 70 charge–discharge cycles and the capacity kept about 195 mA h g⁻¹. Co₂B prepared by the high temperature solid phase process showed very good electrochemical reversibility in CV curves. The hydrogen storage mechanism was also discussed, it confirmed that the high initial capacity of Co₂B prepared by the high temperature solid phase process was due to the oxidation of Co and B₂O₃, and it was irreversible.     

贮氢合金表面分析和金属氢化物电极电催化活性

贮氢合金表面状态和组成是影响金属氢化物(MH)电极电催化活性的重要因素, MH电极的表面改性处理是改善电极性能的有效方法。用XPS, ICP, BET方法分析了处理后贮氢合金表面组成和状态的变化。讨论了化学还原处理对MH电极电催化活性的影响, 结果表明: 化学还原处理大大提高了MH电极反应的交换电流密度和减低了电极反应活化能。     

Electrochemical behavior of LiCoO2 as aqueous lithium-ion battery electrodes

Despite the large number of studies on the behavior of LiCoO₂ in organic electrolytes and its recent application as a positive electrode in rechargeable water battery prototypes, a little information is available about the lithium intercalation reaction in this layered compound in aqueous electrolytes. This work shows that LiCoO₂ electrodes can be reversibly cycled in LiNO₃ aqueous electrolytes for tens of cycles at remarkably high rates with impressive values specific capacity higher than 100 mAh/g, and with a coulomb efficiency greater than 99.7%. Stable and reproducible cycling measurements have been made using a simple cell design that can be easily applied to the study of other intercalation materials, assuming that they are stable in water and that their intercalation potential range matches the electrochemical stability window of the aqueous electrolyte. The experimental arrangement uses a three-electrode flooded cell in which another insertion compound acts as a reversible source and sink of lithium ions, i.e., as the counter electrode. A commercial reference electrode is also present. Both the working and the counter electrodes have been prepared as thin layers on a metallic substrate using the procedures typical for the study of electrodes for lithium-ion batteries in organic solvent electrolytes.     

Photoassisted charge behavior of hydrogen storage alloy-TiO2/Pt electrodes

The photoassisted charge behavior of hydrogen storage alloy modified with TiO2/Pt nanocomposites (HSA-TiO2/Pt electrode) was investigated. The HSA-TiO2/Pt electrode can be photocharged under current. The mechanism of photoassisted behavior of the HSA-TiO2/Pt electrode was explained through the results of cyclic voltammogram and impedance measurements of the HSA-TiO2/Pt electrode. Upon illumination, the photogenerated electrons can charge the electrode, but the photogenerated holes may oxidize the hydrogen storage alloy to form a layer of metal oxide. Because the current could keep the electrode active, the H atoms produced by photogenerated electrons diffused to the hydrogen storage alloy and a metal hydride formed. The electrode delivered a higher discharge capacity due to the assistance of photocharge.     

Electrochemical and surface science investigations of PtCr alloy electrodes

Electrodes of supported Pt, modified with Cr, have shown an increase in electrochemical activity for oxygen reduction in phosphoric acid fuel cells over supported Pt only electrodes. To clarify the role of chromium and its chemical nature at the electrode surface, we have characterized a series of Pt_xCr(_1-x) bulk alloys (x = 0.9, 0.65, 0.5, 0.2) by electrochemical and ex-situ surface science methods. In this paper we report the surface characterization of native and post-electrochemical electrodes by XPS, cyclic voltammetry in 0.05 M H₂SO₄ and 85% H₃PO₄, and analysis of 0.05 M H₂SO₄ electrolyte following electrochemical treatment. The surface Cr(1 to 2 nm) was oxidized to Cr³⁺ oxide for surfaces at open circuit and those exposed to potentials < + 1.3 V vs DHE in 0.05 M H₂SO₄ and < + 1.55 V vs. DHE in 85% H₂PO₄. In 0.05 M H₂SO₄ the Cr component was electrooxidized to solube Cr⁶⁺ species at potentials > +1.3 V with the extent of Cr dissolution dependent on initial alloy stoichiometry. Alloys with Cr content 0.5 are capable of producing (dependent on time spent at potentials above +1.3 V in 0.05 M H₂SO₄) very porous Pt-rich surfaces. Loss of Cr was also observed in 85% H₃PO₄ for the alloys with Cr content 0.5, although at the more positive potential limit of +1.55 V. For the Pt_0.2Cr_0.8, treatment in 85% H₃PO₄ at +1.4 V and above led to the appearance of Pt⁴⁺ and Cr⁶⁺ species, apparently stabilized in a porous phosphate overlayer up to 5 nm thick (dependent on time spent at potentials above this limit). The enhancement reported for supported Pt+Cr oxygen cathodes is discussed in the light of these results.     

Efficient hydrogen storage with the combination of lightweight Mg/MgH2 and nanostructures

Efficient hydrogen storage plays a key role in realizing the incoming hydrogen economy. However, it still remains a great challenge to develop hydrogen storage media with high capacity, favourable thermodynamics, fast kinetics, controllable reversibility, long cycle life, low cost and high safety. To achieve this goal, the combination of lightweight materials and nanostructures should offer great opportunities. In this article, we review recent advances in the field of chemical hydrogen storage that couples lightweight materials and nanostructures, focusing on Mg/MgH(2)-based systems. Selective theoretical and experimental studies on Mg/MgH(2) nanostructures are overviewed, with the emphasis on illustrating the influences of nanostructures on the hydrogenation/dehydrogenation mechanisms and hydrogen storage properties such as capacity, thermodynamics and kinetics. In particular, theoretical studies have shown that the thermodynamics of Mg/MgH(2) clusters below 2 nm change more prominently as particle size decreases.     

Correlation between composition and hydrogen storage behaviors of the Li2NH-MgNH combination system

Hydrogen storage performances of a Li(2)NH-xMgNH combination system (x = 0, 0.5, 1 and 2) are investigated for the first time. It is found that the hydrogenated samples with MgNH exhibit a significant reduction in the dehydrogenation temperatures. Mechanistic investigations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio between MgNH and Li(2)NH. As a consequence, tuning of thermodynamics is achieved for hydrogen storage in the Li(2)NH-xMgNH system by changing the reaction routes, which is ascertained to be the primary reason for the reduction in the operating temperature for hydrogen desorption. Specifically, it is found that under 105 atm hydrogen (140-280 °C) 5.6 wt% hydrogen is reversibly stored in the Li(2)NH-0.5MgNH combination system, which is greater than in the well-investigated Mg(NH(2))(2)-2LiH system.     

Electrochemical properties of LaY2Ni9 hydrogen storage alloy,used as an anode in nickel-metal hydride batteries

The electrochemical properties of LaY₂Ni₉ alloy used as an anode in nickel-metal hydride batteries were investigated at ambient and at different temperatures. Several techniques, such as the galvanostatic charging and discharging, the constant potential discharge, and the potentiodynamic polarization, were applied to characterize these electrochemical properties. The discharge capacity of the LaY₂Ni₉ alloy increases to reach 258 mAh g^?1 after 5 cycles and decreases to 140 mAh g^?1 after 100 cycles then stabilizes around this same value indicating good cycling held. The hydrogen diffusion coefficient D _H in the bulky alloy is estimated to be (1.02?±?0.11)?×?10^?11 cm² s^?1 correlated with the good stability of electrochemical capacity after 100 cycles. The evolution of the ratio \( \frac{D_{\mathrm{H}}}{a^2} \) and the corrosion current density and potential are correlated with the evolution of the electrochemical capacity during the activation and for a long cycling. The enthalpy, the entropy, and the apparent activation energy of the LaY₂Ni₉ hydride formation are evaluated. The calculated results show that the enthalpy change, the entropy change, and the activation energy are (?42.64?±?1.08), (56.85?±?2.11), and (14.84?±?0.35)?kJ mol^?1, respectively.     

Remarkable hydrogen storage properties for nanocrystalline MgH2 synthesised by the hydrogenolysis of Grignard reagents

The possibility of generating MgH(2) nanoparticles from Grignard reagents was investigated. To this aim, five Grignard compounds, i.e. di-n-butylmagnesium, tert-butylmagnesium chloride, allylmagnesium bromide, m-tolylmagnesium chloride, and methylmagnesium bromide were selected for the potential inductive effect of their hydrocarbon group in leading to various magnesium nanostructures at low temperatures. The thermolysis of these Grignard reagents was characterised in order to determine the optimal conditions for the formation of MgH(2). In particular, the use of di-n-butylmagnesium was found to lead to self-assembled and stabilized nanocrystalline MgH(2) structures with an impressive hydrogen storage capacity, i.e. 6.8 mass%, and remarkable hydrogen kinetics far superior to that of milled or nanoconfined magnesium. Hence, it was possible to achieve hydrogen desorption without any catalyst at 250 °C in less than 2 h, while at 300 °C, hydrogen desorption took only 15 min. These superior performances are believed to result from the unique physical properties of the MgH(2) nanocrystalline architecture obtained after hydrogenolysis of di-n-butylmagnesium.     

Preparation and hydrogen storage properties of nanostructured Mg2Cu alloy

We successfully synthesized Mg₂Cu alloys from the metal nanoparticles, which are produced from hydrogen plasma-metal reaction method, in two ways. One is under 0.1 MPa argon at 673 K and the other is under 4.0 MPa hydrogen at 673 K. The structure, morphology and reaction mechanism were studied. The hydrogen absorption and the pressure-composition isotherm properties of the obtained Mg₂Cu alloy under hydrogen were studied. The van’t Hoff equation and the formation enthalpy and entropy of the resulting hydride (MgH₂+MgCu₂) were obtained from the equilibrium plateau pressures of the desorption isotherms. Nanostructured Mg₂Cu shows excellent hydrogen storage properties because nanostructured materials have more surface area and more defects, which means more nucleation sites with hydrogen, and smaller particles, which means shorter diffusion distance for hydrogen in the alloys particles.     

Bonding and stability of the hydrogen storage material Mg(2)NiH(4)

Structural stability and bonding properties of the hydrogen storage material Mg(2)NiH(4) (monoclinic, C2/c, Z = 8) were investigated and compared to those of Ba(2)PdH(4) (orthorhombic, Pnma, Z = 8) using ab initio density functional calculations. Both compounds belong to the family of complex transition metal hydrides. Their crystal structures contain discrete tetrahedral 18 electron complexes T(0)H(4)(4-) (T = Ni, Pd). However, the bonding situation in the two systems was found to be quite different. For Ba(2)PdH(4), the electronic density of states mirrors perfectly the molecular states of the complex PdH(4)(4-), whereas for Mg(2)NiH(4) a clear relation between molecular states of TH(4)(4-) and the density of states of the solid-state compound is missing. Differences in bonding of Ba(2)PdH(4) and Mg(2)NiH(4) originate in the different strength of the T-H interactions (Pd[bond]H interactions are considerably stronger than Ni[bond]H ones) and in the different strength of the interaction between the alkaline-earth metal component and H (Ba[bond]H interactions are substantially weaker than Mg[bond]H ones). To lower the hydrogen desorption temperature of Mg(2)NiH(4), it is suggested to destabilize this compound by introducing defects in the counterion matrix surrounding the tetrahedral Ni(0)H(4)(4-) complexes. This might be achieved by substituting Mg for Al.     

Electrochemical capacities and corrosion of TiNix and its zirconium-substituted alloy hydride electrodes

The hydrogen-absorbing capacities, the discharging capacities and the corrosion resistance of TiNi_x (x = 0.5–1.0) and zirconium-substituted Ti_1−yZr_yNi_x (y = 0.05–0.7) electrodes were examined. The zirconium-substituted alloy electrode generally had a larger capacity and a better corrosion resistance than the TiNi_x electrode. It is believed that the stable passive oxide film formed on the surface protects the underlying metal from oxidation. Zirconium substitution helps in the formation of this film, but sometimes inhibits hydrogen discharge.     

Electrochemical investigation of the anodic hydrogen evolution on MgZn2, Mg2Si,and Al4Cu2Mg8Si7 intermetallic phases

Journal of Solid State Electrochemistry - The anodic hydrogen evolution (AHE) on Mg, MgZn2 (η-phase), Al4Cu2Mg8Si7 (Q-phase), and Mg2Si (β-phase) intermetallic compounds has been...     

Electrochemical properties of Mg2Ni and Mg2Ni2 prepared by mechanical alloying

Crystallized Mg₂Ni and Mg₂Ni₂ amorphous alloys synthesized by mechanical alloying at room temperature were found to present first discharge capacities of 270 mAh/g and 500 mAh/g, respectively. These capacities decrease upon subsequent cycling to reach 30 mAh/g and 70 mAh/g after 60 charge/discharge cycles. The largest initial capacity, measured for the Mg₂Ni₂ composition, is ascribed to its amorphous nature, while its poor capacity retention upon cyding appears to originate from a fine Ni dispersion within the Mg/Ni matrix. This dispersion enables a better protection of the Mg against oxidation during cycling. We show, however, that this protection of Mg by Ni is not sufficient to avoid a strong corrosion of Mg in the KOH electrolyte during cycling, leading to the formation of Mg(OH)₂. Received: 8 October 1997 / Accepted: 20 January 1998     

Ca2+- and Mg2+-doped covalent organic frameworks exhibiting high hydrogen and acetylene storage

A multiscale theoretical investigation has been performed to study the hydrogen and acetylene storage in Ca²⁺- and Mg²⁺-doped COFs (COF-105 and COF-108). The first-principles calculations show that the Ca²⁺ and Mg²⁺ can be immobilized at the COFs surfaces, and the doped Ca and Mg cations can adsorb five H₂ molecules and three C₂H₂ molecules with ideal binding energies. The Grand Canonical Monte Carlo (GCMC) simulations were carried out to obtain the hydrogen and acetylene uptakes of Ca²⁺- and Mg²⁺-doped COFs at room temperature in the different pressure ranges. Our results demonstrate that, at T = 298 K and p = 100 bar, the total gravimetric uptakes of H₂ in Ca²⁺-doped COF-105 and COF-108 reach 6.78 and 6.54 wt%, respectively, and a higher uptakes of 7.14 and 7.27 wt% have been reached for Mg²⁺-doped COF-105 and COF-108, respectively. At T = 298 K and p = 1 bar, the acetylene uptakes of Ca²⁺-doped COF-105, Ca²⁺-doped COF-108, Mg²⁺-doped COF-105, and Mg²⁺-doped COF-108 are 406.42, 366.24, 308.07, and 319.88 cm³/g (corresponding to the excess uptakes of 358.37, 316.38, 236.7109, and 245.42 cm³/g), respectively. The Ca²⁺-doped COF-105 displays a highest acetylene storage capacity among all materials reported. The Ca²⁺- and Mg²⁺-doped COFs can be very practical hydrogen or acetylene storage medium in the future.     

Triplet state photosensitization of nanocrystalline metal oxide electrodes by zinc-substituted cytochrome c: application to hydrogen evolution

The interfacing of nanostructured semiconductor photoelectrodes with redox proteins is an innovative approach to the development of artificial photosynthetic systems. In this paper, we have investigated the photoinduced electron-transfer reactions of zinc-substituted cytochrome c, ZnCyt-c, immobilized on mesoporous, nanocrystalline metal oxide electrodes. Efficient electron injection from the triplet state of ZnCyt-c is observed into TiO(2) electrodes (t(50%) approximately 100 micros) resulting in a long-lived charge-separated state (lifetime of up to 0.4 s). Further studies were undertaken as a function of electrolyte pH and metal oxide employed. Optimum yield of a long-lived charge-separated state was observed employing TiO(2) electrodes at pH 5, consistent with our previous studies of analogous dye-sensitized metal oxide electrodes. The addition of EDTA as a sacrificial electron donor to the electrolyte resulted in efficient photogeneration of molecular hydrogen, with a quantum yield per one absorbed photon of 10 +/- 5%.     

Superior hydrogen storage properties of LiBH4 catalyzed by Mg(AlH4)2

Mg(AlH(4))(2) is found to provide a synergistic effect on improving the de-/rehydrogenation properties of LiBH(4). The Mg(AlH(4))(2)-catalyzed LiBH(4) exhibits lower dehydrogenation temperature and faster de-/rehydrogenation kinetics than the individually MgH(2)- or Al-catalyzed LiBH(4).     

Preparation and characterization of ordered porous carbons for increasing hydrogen storage behaviors

We prepared ordered porous carbons (PCs) by using a replication method that had well-organized mesoporous silica as a template with various carbonization temperatures in order to investigate the possibility of energy storage materials. The microstructure and morphologies of the samples are characterized by XRD, TEM, and FT-Raman spectroscopy. N₂ adsorption isotherms are analyzed by the t-plot method, as well as the BET and the H–K method in order to characterize the specific surface area, pore volume, and pore size distribution of the samples, respectively. The capacity of the hydrogen adsorption of the samples is evaluated by BEL-HP at 77 K and 1 bar. From the results, we are able to confirm that the synthesis of the samples can be accurately governed by the carbonization temperature, which is one of the effective parameters for developing the textural properties of the carbon materials, which affects the behaviors of the hydrogen storage.     

Preparation of platinum-decorated porous graphite nanofibers, and their hydrogen storage behaviors

In this work, the hydrogen storage behaviors of porous graphite nanofibers (GNFs) decorated by Pt nanoparticles were investigated. The Pt nanoparticles were introduced onto the GNF surfaces using a well-known chemical reduction method. We investigated the hydrogen storage capacity of the Pt-doped GNFs for the platinum content range of 1.3-7.5 mass%. The microstructure of the Pt/porous GNFs was characterized by X-ray diffraction and transmission electron microscopy. The hydrogen storage behaviors of the Pt/GNFs were studied using a PCT apparatus at 298 K and 10 MPa. It was found that amount of hydrogen stored increased with increasing Pt content to 3.4 mass%, and then decreased. This result indicates that the hydrogen storage capacity of porous carbons is based on both their metal content and dispersion rate.