Hydrolytic dehydrogenation of ammonia borane catalyzed by carbon supported Co core-Pt shell nanoparticles

Co core-Pt shell nanoparticles (denoted as Co_1−x@Pt_x where x = 0.33, 0.43, 0.60, 0.68, 0.82) and carbon supported Co core-Pt shell nanoparticles (denoted as Co_1−x@Pt_x/C where x = 0.60, 0.68, 0.82) (Co_1−x@Pt_x/C = 43%), which are synthesized through a polyol reduction process with oleic acid as a surfactant, have been investigated as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃) at 25 ± 0.5 °C. The as-prepared Co core-Pt shell nanoparticles are uniformly dispersed on carbon surface with diameters of about 3 nm. It is found that the catalysts show favorable performance toward the hydrolysis of NH₃BH₃ and the catalytic activity is associated with the ratio of Pt to Co. Among the catalysts studied, Co_0.32@Pt_0.68/C (Co_0.32@Pt_0.68/C = 43%) displays the highest catalytic performance, delivering a high hydrogen-release rate of 4874 mL min⁻¹ g⁻¹ (per catalyst).     

Catalytic hydrolysis of ammonia borane via magnetically recyclable copper iron nanoparticles for chemical hydrogen storage

In this work, a series of new Cu_1−xFe_x alloy nanoparticles (NPs) have been successfully in situ synthesized by a very simple method and used as catalysts for hydrogen generation from the aqueous solution of ammonia borane (AB) under ambient atmosphere at room temperature. The prepared nanoalloys exhibit excellent catalytic activity, especially for Cu_0.33Fe_0.67 sample outperform the activity of monometallic counterparts, and even of Cu@Fe core–shell NPs. By using an external magnet, these catalysts can be readily separated from the solution for recycle purpose, and can keep the high activity even after 8 times of recycle under ambient atmosphere. The hydrolysis activation energy for the Cu_0.33Fe_0.67 alloy NPs was measured to be approximately 43.2 kJ/mol, which is lower than most of the reported activation energy values for the same reaction using many different catalysts except for some noble-metal containing catalysts, indicating the superior catalytic performance of Cu_0.33Fe_0.67 nanocatalysts.     

Carbon-supported Ni1−x@Ptx (x = 0.32, 0.43, 0.60, 0.67, and 0.80) core-shell nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on the carbon supported Ni core-Pt shell Ni_1−x@Pt_x/C (x = 0.32, 0.43, 0.60, 0.67, and 0.80) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The catalysts are prepared through a polyol synthesis process with oleic acid as the surfactant. The structure, morphology, and chemical composition of the obtained samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), inductively coupled plasma emission spectroscopy (ICP), and nuclear magnetic resonance (NMR). The results show that the Ni core-Pt shell nanoparticles are uniformly dispersed on the carbon surface with the diameters of 2-4 nm, and furthermore, the catalysts show favorable performance toward the hydrolysis of NH₃BH₃. Among the nanoparticles, Ni_0.33@Pt_0.67/C displays the highest catalytic activity, delivering a high hydrogen release rate of 5469 mL min⁻¹ g⁻¹ and a low activation energy of 33.0 kJ mol⁻¹.     

Synthesis and electrochemical properties of Li1−xFe0.8Ni0.2O2–LixMnO2 (Mn/(Fe + Ni + Mn) = 0.8) material

A new type of Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ (Mn/(Fe + Ni + Mn) = 0.8) material was synthesized at 350 °C in air atmosphere using a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−xFeO₂–Li_xMnO₂ (Mn/(Fe + Mn) = 0.8) with much reduced impurity peaks. The Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell showed a high initial discharge capacity above 192 mAh g⁻¹, which was higher than that of the parent Li/Li_1−xFeO₂–Li_xMnO₂ (186 mAh g⁻¹). We expected that the increase of initial discharge capacity and the change of shape of discharge curve for the Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell is the result from the redox reaction from Ni²⁺ to Ni³⁺ during charge/discharge process. This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles.     

Hydrogen generation from hydrolysis of NH3BH3 by an electroplated Co–P catalyst

This paper investigates the effect of an electroplated Co–P catalyst on hydrogen generation kinetics from hydrolysis of NH₃BH₃. The Co–P catalyst is composed of an amorphous Co–P phase and Co nanoparticles. An increase in NH₃BH₃ concentration caused the hydrogen generation rate to increase dramatically. The Co–P catalyst shows a large hydrogen generation rate for 2 wt% NH₃BH₃ solution at 30 °C. This is 1.8 times higher than that of the Pt/C catalysts and 6 times higher than that of Ru catalysts. The activation energy for hydrolysis of NH₃BH₃ by the Co–P catalyst is calculated to be 22 kJ/mol, which is close to that of noble metal-based catalysts.     

PtxNi1−x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on Pt_xNi_1−x (x = 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The Pt_xNi_1−x catalysts were prepared through a redox replacement reaction with a reverse microemulsion technique. The structure, morphology, and chemical composition of the obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), and inductively coupled plasma emission spectroscopy (ICP). The results show that the diameters of the Pt_xNi_1−x nanoparticles are about 2–4 nm, and the Pt atomic contents in the catalysts were 35%, 44%, 65%, 75%, and 93%, respectively. It is found that the catalytic activity toward the hydrolysis of NH₃BH₃ is correlated with the composition of the Pt_xNi_1−x catalysts. The annealing of Pt_0.65Ni_0.35 at 300 °C for 1 h increases the crystallinity of the nanoparticles, but shows almost the same activity as that without annealing. Among the as-prepared Pt_xNi_1−x nanoparticles, Pt_0.65Ni_0.35 displays the highest catalytic performance, delivering a high hydrogen-release rate of 4784.7 mL min⁻¹ g⁻¹ and a low activation energy of 39.0 kJ mol⁻¹.     

(Ni0.75Fe0.25-xMgO)/YSZ anode for direct methane solid-oxide fuel cells

(Ni_0.75Fe_0.25-xMgO)/YSZ samples—with a varying weight percentage x (0, 5%, 10%) of MgO with respect to Ni_0.75Fe_0.25—were prepared and studied as anodes for intermediate temperature solid oxide fuel cells (SOFCs) operated on humidified methane (3% H₂O). Among the cells with different anode compositions, it was found that the cell with the (Ni_0.75Fe_0.25-5%MgO)/YSZ anode showed the highest power density, giving 648 mW cm⁻² at 800 °C. The cells with MgO-doped anodes were able to operate stably for 20 h under a current density of 0.53 A cm⁻² at 700 °C without observed degradation, while the cells without MgO degraded rapidly. The mechanisms responsible for the superior performance and duration of the (Ni_0.75Fe_0.25-5%MgO)/YSZ anode were analyzed.     

Hydrogen production from ethanol steam reforming over core–shell structured NixOy–, FexOy–, and CoxOy–Pd catalysts

The present work focused on the investigation of the hydrogen generation through the ethanol steam reforming over the core–shell structured Ni_xO_y–, Fe_xO_y–, and Co_xO_y–Pd loaded Zeolite Y catalysts. The transmission electron microscopy (TEM) image of Ni_xO_y–Pd represented a very clear core–shell structure, but the other two catalysts, Co_xO_y– and Fe_xO_y–Pd, were irregular and non-uniform. The catalytic performances differed according to the added core metal and the support. The core–shell structured Co_xO_y–Pd/Zeolite Y provided a significantly higher reforming reactivity compared to the other catalysts. The H₂ production was maximized to 98% over Co_xO_y–Pd(50.0 wt%)/Zeolite Y at the conditions of reaction temperature 600 °C, CH₃CH₂OH:H₂O = 1:3, and GHSV (gas hourly space velocity) 8400 h⁻¹. In the mechanism that was suggested in this work, the cobalt component played an important role in the partial oxidation and the CO activation for acetaldehyde and CO₂ respectively, and eventually, cobalt increased the hydrogen yield and suppressed the CO generation.     

Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0-0.20) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of (La_0.7Mg_0.3)_1−xCe_xNi_2.8Co_0.5 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated. The results show that all alloys consist of (La, Mg)Ni₃ and LaNi₅ phases. The cyclic stability (S₁₀₀) of the alloy electrodes increases from 58.7% (x = 0) to 69.8% (x = 0.20) after 100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8% (x = 0) to 69.6% (x = 0.10), then decreases to 65.1% (x = 0.20) at the discharge current density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy electrodes are also improved by increasing Ce content.     

Structural characteristics and hydrogen storage properties of Ca3.0−xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys

The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca_3.0−xMg_xNi₉ (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi₃-type (Ca, Mg)Ni₃ main phase decreased with increasing Mg content. The 6c site of PuNi₃-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic properties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5–2.0, the Ca_3.0−xMg_xNi₉ alloys had favorable enthalpy (ΔH) and entropy (ΔS) of hydride formation.     

Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS + MM)

The effect of Cu-doping on the hydrogen storage properties of Mg₉₅Ni₅Cu_x (x = 0, 0.5, 1, 2) prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. For dehydriding properties, the dehydriding temperature onset decreases from 450 K for Mg₉₅Ni₅ to 420 K for Mg₉₅Ni₅Cu₂. Additionally, the activation energy for dehydriding decreases from 116 kJ/mol for Mg₉₅Ni₅ to 98 kJ/mol for Mg₉₅Ni₅Cu₂, indicating that the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen absorption capacity of Mg₉₅Ni₅Cu₂ at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM pretreatment before HCS. The factors for the improvement of the hydrogen storage properties are discussed in this paper.     

Synthesis and electrochemical performance of La0.6Ca0.4Fe1−xNixO3 (x = 0.1, 0.2, 0.3) material for solid oxide fuel cell cathode

Polycrystalline samples of La_0.6Ca_0.4Fe_1−xNi_xO₃ (x = 0.1, 0.2, 0.3) (LCFN) are prepared by liquid mix method. The structure of the polycrystalline powders is analyzed with X-ray powder diffraction data. The XRD patterns are indexed as the orthoferrite similar to that of LaFeO₃ having a single phase with orthorhombic perovskite structure (Pnma). The morphological characterization is performed by scanning electron microscopy (SEM) obtaining a mean particle size less than 300 nm.Polarization resistance is studied using two different electrolytes: Y-stabilized zirconia (YSZ) and Sm-doped ceria (SDC). Electrochemical impedance spectroscopy (EIS) measurements of LCFN/YSZ/LCFN and LCFN/SDC/LCFN test cells are carried out. These electrochemical experiments are performed at equilibrium from 850 °C to room temperature, under both zero dc current intensity and air. The best value of area specific resistance (ASR) obtained is 0.88 Ω cm², corresponding to the La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ material using SDC as electrolyte. The dc four-probe measurement indicates that La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ exhibits fairly high electrical conductivity, over 300 S cm⁻¹ at T > 500 °C.     

Effect of oxygen non-stoichiometry and temperature on cation ordering in LiMn2−xNixO4 (0.50 ≥ x ≥ 0.36) spinels

The effect of oxygen stoichiometry on the transition metal ordering and electrochemical activity of LiMn_2−xNi_xO₄ solid solutions was investigated. Temperature–oxygen-partial-pressure–composition (pO₂–T–x) diagrams of ordered and disordered phases of LiMn_2−xNi_xO₄ (0.50 ≥ x ≥ 0.36) in the vicinity of order–disorder transition temperature (T_c) was constructed by means of infrared spectroscopy, thermogravimetric analysis and galvanostatic measurements. Despite their simplicity and limitations over traditional diffraction techniques, all three techniques offered near excellent capability to distinguish ordered and disordered phases. The effect of oxygen-partial-pressure (pO₂) in the annealing atmosphere and nickel content of the spinel on T_c was studied. The transition temperature increased with pO₂ and nickel content, except in oxygen-rich (pO₂ = 1) atmosphere for the maximum nickel content spinel of LiMn_1.5Ni_0.5O₄. An explanation for the dependence of the transition temperature on the two variables and changes induced by the post-fabrication heat treatments is provided.     

Effect of Fe substitution on the structure and properties of LnBaCo2−xFexO5+δ (Ln = Nd and Gd) cathodes

The effect of Fe substitution for Co on the crystal chemistry, thermal and electrical properties, and catalytic activity for oxygen reduction reaction of the layered LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) perovskite has been investigated. The air-synthesized LnBaCo_2−xFe_xO_5+δ samples exhibit structural change with increasing Fe content from tetragonal (0 ≤ x ≤ 1) to cubic (1.5 ≤ x ≤ 2) for the Ln = Nd system and from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) for the Ln = Gd system. The thermal expansion coefficient (TEC) and electrical conductivity decrease with increasing Fe content in LnBaCo_2−xFe_xO_5+δ. While the substitution of a small amount of Fe (x = 0.5) for Co leads to slightly improved performance in solid oxide fuel cells (SOFC), larger Fe contents (x ≥ 1.0) deteriorate the fuel cell performance. In the Ln = Gd system, the better performance of the x = 0.5 sample is partly due to the improved chemical stability with the LSGM electrolyte at high temperatures. With an acceptable electrical conductivity of >100 S cm⁻¹ at 800 °C, the x = 0.5 sample in the LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) system offers promising mixed oxide-ion and electronic conducting (MIEC) properties.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Effect of substituting Al for Co on the hydrogen-storage performance of La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys

La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys were prepared by induction melting, and the effects of partially substituting Al for Co on the structure and hydrogen-storage properties of the alloys were investigated systematically. It is found that La(Ni, Co, Al)₅ phase with hexagonal CaCu₅-type structure, LaNi₃ phase with PuNi₃ structure and MgNi₂ phase exist as the main phases in La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys, and the cell volume of the La(Ni, Co, Al)₅ phase increases with the amount of Al added. The results show that the substitution of Al for Co can reduce the plateau pressure and the hysteresis between hydrogen absorption and desorption, and improve the hydrogen-absorption capacity and thermal stability of the hydride. Moreover, the addition of Al can delay the oxidation of the surface layer of the alloy electrodes in electrolyte, slow down the capacity degradation and prolong the cycling lifetime, and enhance the electrocatalytic activity of the hydrogen-storage electrodes for hydrogen oxidation.     

Room temperature hydrogen generation from aqueous ammonia-borane using noble metal nano-clusters as highly active catalysts

Nano-clusters of noble metals Ru, Rh, Pd, Pt and Au have been supported on γ-Al₂O₃, C and SiO₂, of which the catalytic activities have been investigated for hydrolysis of NH₃BH₃. Among these catalysts, the Ru, Rh and Pt catalysts exhibit high activities to generate stoichiometric amount of hydrogen with fast kinetics, whereas the Pd and Au catalysts are less active. Support effect has been studied by testing the hydrogen generation reaction in the presence of Pt supported on γ-Al₂O₃, VULCAN^® carbon and SiO₂, and it is found that Pt on γ-Al₂O₃, which has the smallest particle size, is the most active. Concentration dependence of the hydrogen generation from aqueous NH₃BH₃ solutions has been investigated in the presence of Pt/γ-Al₂O₃ by keeping the amount of Pt/γ-Al₂O₃ catalyst unchanged, which exhibits that the hydrogen release versus time (ml H₂ min⁻¹) does not significantly change with increasing the NH₃BH₃ concentration, indicating that the hydrogen release rate is not dependent on the NH₃BH₃ concentration and the high activity of the Pt catalyst can be kept at high NH₃BH₃ concentrations. Activation energies have been measured to be 23, 21 and 21 kJ mol⁻¹ for Ru/γ-Al₂O₃, Rh/γ-Al₂O₃ and Pt/γ-Al₂O₃ catalysts, respectively, which may correspond to the step of B–N bond breaking on the metal surfaces. The particle sizes, surface morphology and surface areas of the catalysts have been obtained by TEM and BET experiments.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Nanoporous Ni-based catalysts for hydrogen generation from hydrolysis of ammonia borane

We report nanoporous Ni, Ni–Fe, and Ni–Pt as catalysts for hydrogen generation from hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB). The Ni and Ni–Fe nanoparticles with diameters of 20–25 nm were synthesized by a colloidal method in starch-containing aqueous solution. They exhibited considerable in situ catalytic performance but severely lost activity after separating from the reaction solution. Nanoporous Ni_1−xPt_x (x = 0.01, 0.08 and 0.19) with particle size below 5 nm was prepared from the isolated Ni nanoparticles through a replacement reaction. After centrifugation, drying, washing, and annealing, the obtained nanoporous Ni–Pt could attain remarkable activity, high hydrogen generation rate and efficiency, and low activation energy.     

Thermal stability of LiPF6-based electrolyte and effect of contact with various delithiated cathodes of Li-ion batteries

Thermal stability of LiPF₆-based electrolyte (1 M LiPF₆/EC + DMC) was studied by in-situ FTIR spectroscopy and C80 calorimetry, which indicated that the electrolyte underwent furious polymerization and decomposition reactions and sharp heat flow was generated below 225 °C. The thermal stability of the electrolyte in contact with various delithiated cathodes (Li_xCoO₂, Li_xNi_0.8Co_0.15Al_0.05O₂, Li_xNi_1/3Co_1/3Mn_1/3O₂, Li_xMn₂O₄, Li_xNi_0.5Mn_0.5O₂, Li_xNi_0.5Mn_1.5O₄ and Li_xFePO₄) was also investigated by C80 calorimetry. The results show that the cathode materials except for Li_xFePO₄ usually have an enhancement effect on the decomposition of the electrolyte, but Li_xFePO₄ exhibits a suppression effect on the reactions of the electrolyte. Li_xFePO₄ is found to be with excellent thermal stability. Among the other cathodes, Li_xCoO₂, Li_xNi_0.8Co_0.15Al_0.05O₂, Li_xNi_0.5Mn_0.5O₂ and Li_xNi_0.5Mn_1.5O₄ promote the decomposition of electrolyte by releasing oxygen and thus considered not favorable for safety, but Li_xNi_1/3Co_1/3Mn_1/3O₂ with a lesser reaction heat and Li_xMn₂O₄ with even less heat flow in the low temperature range (50-225 °C) are believed as promising cathodes for better safety. By comparing X-ray diffraction (XRD) patterns of these cathode materials at room temperature and those heated to 300 °C in the presence of the electrolyte, we have found that Li_xFePO₄ only has experienced tiny structure change, which is greatly different from the other cathode materials.