Numerical study on hydrogen and thermal storage performance of a sandwich reaction bed filled with metal hydride and thermochemical material

Hydrogen storage and release process of metal hydride (MH) accompany with large amount of reaction heat. The thermal management is very important to improve the comprehensive performance of hydrogen storage unit. In present paper, thermochemical material (TCM) is used to storage and release the reaction heat, and a new sandwich configuration reaction bed of MH-TCM system was proposed and its superior hydrogen and thermal storage performance were numerically validated. Firstly, the optimum TCM distribution with a volume ratio (TCM in inner layer to total) of 0.4 was derived for the sandwich bed. Then, comparisons between the sandwich reaction bed and the traditional reaction bed were performed. The results show that the sandwich MH-TCM system has faster heat transfer and reaction rate due to its larger heat transfer area and smaller thermal resistance, which results in the hydrogen storage time is shortened by 61.1%. The heat transfer in the reaction beds have significant effects on performance of MH-TCM systems. Increasing the thermal conductivity of the reaction beds can further reduce the hydrogen storage time. Moreover, improving the hydrogen inflation pressure can result in higher equilibrium temperature, which is beneficial for the enhancing heat transfer and hydrogen absorption rates.     

Comparative structural and electrochemical study of high density spherical and non-spherical Ni(OH)2 as cathode materials for Ni-metal hydride batteries

In this paper we compare the behavior of non-spherical and spherical β-Ni(OH)₂ as cathode materials for Ni-MH batteries in an attempt to explore the effect of microstructure and surface properties of β-Ni(OH)₂ on their electrochemical performances. Non-spherical β-Ni(OH)₂ powders with a high-density are synthesized using a simple polyacrylamide (PAM) assisted two-step drying method. X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), thermogravimetric/differential thermal analysis (TG-DTA), Brunauer-Emmett-Teller (BET) testing, laser particle size analysis, and tap-density testing are used to characterize the physical properties of the synthesized products. Electrochemical characterization, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test, is also performed. The results show that the non-spherical β-Ni(OH)₂ materials exhibit an irregular tabular shape and a dense solid structure, which contains many overlapped sheet nano crystalline grains, and have a high density of structural disorder and a large specific surface area. Compared with the spherical β-Ni(OH)₂, the non-spherical β-Ni(OH)₂ materials have an enhanced discharge capacity, higher discharge potential plateau and superior cycle stability. This performance improvement can be attributable to a higher proton diffusion coefficient (4.26 × 10⁻⁹ cm² s⁻¹), better reaction reversibility, and lower electrochemical impedance of the synthesized material.     

Synthesis and crystal structure analysis of complex hydride Mg(BH4)(NH2)

Complex hydride Mg(BH₄)(NH₂), which consists of double anion BH₄⁻ and NH₂⁻, was synthesized and the crystal structure was analyzed by synchrotron X-ray diffraction. The mixture sample of Mg(BH₄)₂ + Mg(NH₂)₂ prepared by ball milling was reacted and crystallized to Mg(BH₄)(NH₂) by heating at about 453 K. This crystal phase transforms into amorphous phase above 473 K and subsequently the dehydrogenation begins. The crystal structure of Mg(BH₄)(NH₂) was determined from measurement data at 453 K (chemical formula: Mg_0.94(BH₄)₁(NH₂)_0.88, crystal system: tetragonal, space group: I4₁ (No.80), Z = 8, lattice constants: a = 5.814(1), c = 20.450(4) Å at 453 K). Mg(BH₄)(NH₂) is ionic crystal which the cation (Mg²⁺) and the anions (BH₄⁻ and NH₂⁻) are stacking alternately along the c-axis direction. Two BH₄⁻ and two NH₂⁻ tetrahedrally coordinate around Mg²⁺ ion.     

Obtaining particles with the structure Mg@C and (Mg@C)@Pd,their properties and stability in the hydrogenation/dehydrogenation processes

In this work, we studied the change in the properties of powders with a core (magnesium) – shell structure (carbon and carbon/palladium) in the process of hydrogenation/dehydrogenation with hydrogen (99.995 wt%). Magnesium powders were obtained by plasma chemical synthesis in an atmosphere of argon containing a small amount of hydrogen (2–3 at.%) and nitrogen (8–9 at.%), when performing a low-frequency arc discharge between a tungsten electrode and a magnesium melt. The shell (carbon and carbon/palladium) was deposited in a plasma generator with vortex and magnetic stabilization. For all samples, a decrease in the sorption capacity of hydrogen was observed as a result of successive cycles of sorption and desorption reactions. It was found that the reason for this fall is associated with the formation of the MgO and Mg(OH)₂ phase, which prevents the diffusion of hydrogen. The carbon shell provides a more complete hydrogenation of the magnesium particles, and an additional palladium shell increases the resistance to cyclic hydrogenation/dehydrogenation and reduces the temperature of these processes. According to the data obtained, powders with particles (Mg@C)@Pd can absorb the largest amount of hydrogen (6.9 wt%) for the duration of 5 cycles, after which the protective shell of the particles begins to collapse and a loss of sorption capacity is observed.     

Decreasing the thermal dehydrogenation temperature of methylamine borane (MeAB) by mixing with poly(methyl acrylate) (PMA)

This paper reports on a hydrogen storage material of poly(methyl acrylate) and methylamine borane (PMA/MeAB) composite, which is synthesized by a simple solution-blending process at room temperature. The thermal decomposition process of the as-prepared composite is investigated by temperature programmed desorption/mass spectrometer (TPD/MS), thermogravimetry (TG) measurements and water displacement method. It is found that PMA/MeAB100 (100 mg PMA with 100 mg MeAB) starts to release H₂ at the temperature of 90.5 °C with the dehydrogenation peak centered at 120.7 °C. This is about 20 °C lower than that of neat MeAB. Meanwhile, the evaporation of MeAB and the volatile byproducts from the dehydrogenation stage of PMA/MeAB100 are also suppressed. The present result shows that the dehydrogenation property of MeAB is enhanced by using PMA/MeAB composite.     

The influence of different additives on the solid-state reaction of magnesium hydride (MgH2) with Si

The possibility of increasing the solid-state reaction rate of MgH₂ with Si by modifying the mixture preparation method and adding chemical compounds or elements, such as niobium pentafluoride (NbF₅), nano-titanium (IV) oxide (TiO₂), nano-chromium (III) oxide Cr₂O₃, yttrium, and nano- and microsized nickel, was investigated. The results show that changing the milling parameters of the MgH₂ and Si mixture in a planetary ball mill greatly affects the rate of direct reaction between them and allows the reaction to take place at temperatures as low as 200 °C with an equilibrium pressure over 1 bar. Moreover, all additives significantly enhanced the reaction speed. The reaction pathway was found to be different for decomposition in a hydrogen vacuum and pressures of several bars. Reactions of the powders investigated did not occur at temperatures below 150 °C.     

Hydrolysis characteristics of calcium hydride (CaH2) powder in the presence of ethylene glycol,methanol, and ethanol for controllable hydrogen production

Calcium hydride (CaH₂) reacts vigorously with water, liberating hydrogen gas. For the development of an effective hydrogen storage system, it is an absolute necessity to control the rate of hydrogen production. In the present study, the effects of different solvent, ethylene glycol, methanol, and ethanol, on the hydrolysis of CaH₂ for controllable hydrogen production were investigated. Reactions were performed at different temperatures (20, 40, and 60°C) in order to calculate the kinetic parameters. The Arrhenius equation was used to calculate the activation energies. The activation of energy of the hydrolysis reaction of CaH₂ in an ethanol solution (E_a = 20.03 kJ/mol) was found to be less than the other reactions.     

Enhanced dehydrogenation performance of ammonia borane con-catalyzed by novel TiO2(B) nanoparticles and bio-derived carbon with well-organized pores

A novel TiO₂(B) confined in porous bio-derived carbon has been prepared for dehydrogenated catalyzation of NH₃BH₃. The microstructural characterizations of as-prepared samples show that the nanoconfinement in well-organized micro/mesopores of carbon can avoid the aggregations of TiO₂(B) nanoparticles and NH₃BH₃. The dehydrogenation measurement demonstrates the dehydrogenated thermodynamic and dynamic properties of NH₃BH₃ could be improved under the con-catalyzation of TiO₂(B) and porous carbon. The results suggest that both TiO₂(B) and porous bio-derived C are promising catalysts. Additionally, it also provides a high-value solution for the disposal of agricultural wastes.     

Evaluation of Bunsen reaction at elevated temperature and high pressure in continuous co-current reactor in iodine-sulfur thermochemical process

Bunsen reaction is one of the three reaction steps of iodine-sulfur process. In present study, Bunsen reaction is carried out in co-current reactor to identify effect of different operating conditions on concentrations of Bunsen reaction product mixture. Bunsen reaction studies have been done in tubular reactor, which is made of tantalum tube and stainless steel jacket, in 50–80 °C temperature range, 2–6 bar (g) pressure range. Feed flow rates are varied for HIx (mixture of hydroiodic acid, water and iodine) 1.2 l/h - 3 l/h, SO₂ 0.02 g/s – 0.24 g/s and O₂ 0.008 g/s ?0.016 g/s. It has been observed that, increasing SO₂ feed flow rate and pressure results in increased mole fraction of HI in HIx phase and H₂SO₄ in sulfuric acid phase. Increase in temperature increased the mole fraction of HI in HIx phase but decreased the mole fraction of H₂SO₄ in sulfuric acid phase. Increase in feed I₂/H₂O ratio and HIx feed flow rate, decreased the mole fraction of HI in HIx phase. Higher pressure improved the conversion of Bunsen reactants to products.     

Development of platinum assisted ternary catalyst with high activity and selectivity at working temperature of proton-exchange membrane fuel cells for preferential oxidation of CO

The development of superior catalysts for preferential oxidation of CO has always maintained challenging in heterogeneous catalysis. Herein it is found that the Pt_0.05/CuO–CeO₂ catalyst exhibited high activity and selectivity for preferential oxidation of CO at working temperature of proton-exchange membrane fuel cells (~80 °C). The studies indicate that the presence of platinum and cerium promote the generation of oxygen vacancy and the dissociation of O₂ molecules. Meanwhile, the introduction of platinum facilitates the reduction of copper and cerium species as well as enhances the quantity of lattice oxygen. The timely update of lattice oxygen promotes CO oxidation with the help of oxygen vacancy. The finding may provide new ideas for developing the excellent ternary catalyst for preferential oxidation of CO at the proton-exchange membrane fuel cells working temperature.     

Numerical investigation of H2 absorption in an adiabatic high-temperature metal hydride reactor based on thermochemical heat storage: MgH2 and Mg(OH)2 as reference materials

A two-dimensional mathematical model to predict the thermal performance of an adiabatic hydrogen storage system based on the combination of magnesium hydride and magnesium hydroxide materials has been developed. A simple geometry consisting of two coaxial cylinders filled with the hydrogen and thermochemical heat storage materials was considered. The main objective was to gain a better knowledge on the thermal interaction between the two storage media, and to determine the dependence of the hydrogen absorption time on the geometric characteristics of the reactor as well as the operation conditions and the thermophysical properties of the selected materials. The dimensions of the two compartments where the two materials are filled were chosen based on the results of a preliminary analytical study in order to compare the absorption times obtained analytically and numerically. The numerical results have shown that the hydrogen absorption process can be completed in a shorter interval of time than analytically as a result of the larger temperature gradient between the magnesium hydride and magnesium hydroxide beds. This was mainly due to variation of temperature in the thermochemical heat storage material during the more realistic dehydration reaction in the numerical solution. Larger temperature gradients, thus a faster hydrogen absorption process can also be achieved by increasing the hydrogen absorption pressure. Moreover, it was found that the increase of the thermal conductivity of the magnesium hydroxide material is crucial for a further improvement of the performance of the MgH₂–Mg(OH)₂ combination reactor.     

Preparation of graphite carbon/Prussian blue analogue/palladium (GC/PBA/pd) synergistic-effect electrocatalyst with high activity for ethanol oxidation reaction

In this paper, newly graphite carbon/Prussian blue analogue/palladium (GC/PBA/Pd) synergistic-effect electrocatalyst for ethanol oxidation reaction were developed, with Co-based PBA (Co₃[Co(CN)₆]₂) as a co-catalyst. Structural analysis shows that the Co₃(Co(CN)₆)₂ nanoparticles were highly dispersed and inlaid on surface of GC nanosheets with outstanding structural stability. The GC/Co₃(Co(CN)₆)₂/Pd electrocatalyst exhibits significantly enhanced electrocatalytic activity towards ethanol oxidation with a maximum mass activity of 2644 A g^?1 Pd GC/Pd, which is more than double that of GC/Pd electrocatalyst (1249 A g^?1). Excellent electrochemical stability is also demonstrated for this GC/Co₃(Co(CN)₆)₂/Pd electrocatalyst. The enhanced electrocatalytic activity can be attributed to the synergistic effects of GC support and Co₃(Co(CN)₆)₂ promoter on the Pd electrocatalysts, in which Co₃(Co(CN)₆)₂ acts as a co-catalyst and GC acts as a conductive support.     

Investigation on PM formation from combustion of lignite with high contents of AAEMs (alkali and alkaline earth metals) and Fe: Effect of the reaction temperature

Coal-fired power plants require higher flexibility and a broader range of the operating temperature than before to accommodate the load regulation of the power grid. The relationship between the reaction temperature and the characteristics of particulate matter (PM) need to be better understood. In this study, Zhundong coal combustion was conducted in a drop tube furnace at different reaction temperatures in air. The PM characteristics and elemental contributions are investigated in detail. The experimental results show that the mass yields of PM_0.4 and PM_0.4-10 are non-monotonic with the reaction temperature. The competition between the generation of inorganic fumes and the removal of inorganic fumes by Si–Al-bearing minerals governs the mass yield of PM_0.4. At higher reaction temperature, generation of Ca, Mg, Fe-containing fumes increases, contributing most to the increment of PM_0.4; while the sulfation of chlorides is inhibited, resulting in more Cl in PM_0.4. The S content in PM_0.4 is mainly affected by the sulfation of AAEMs (alkali and alkaline earth metals) oxides. The mass yield of PM_0.4-10 is controlled by the competition between the fragmentation of char or mineral particles and the coalescence of mineral particles. For Zhundong coal combustion, the reaction temperature is recommended to be 1273K–1373K to control PM emission.     

Texture/phase evolution during microwave fabrication of nanocrystalline multicomponent (Cu/Zn/Al)O metal oxides with varying diethylene glycol content applied in hydrogen production

A series of Cu/Zn/Al mixed oxides, as steam methanol reforming catalysts, were synthesized via the microwave assisted combustion synthesis method using diethylene glycol as the organic fuel. The nanocatalysts were analyzed by XRD, FESEM, EDX, BET, H₂-TPR and FTIR techniques to ensure authenticity of the synthesis steps and pursuing the effect of the fuel/nitrate ratio on their physicochemical properties. The results proved the necessity of defining an optimum fuel/nitrate ratio for the combustion synthesis method. Fuel/nitrate ratio affects significantly on crystal growth and crystalline facets size. Proper crystallography of CuO/ZnO/Al₂O₃ (DEG/Nitrate = 3) nanocatalyst along with higher specific surface area and distributed particle size, made it predictable that it could result in higher methanol conversion in the steam methanol reforming process. The catalytic performance studies justified assumptions, since the CZA (DEG/N = 3) presented higher methanol conversion and selectivity toward desired products as well as its high stability.     

Insights into the reaction mechanisms of methanol decomposition,methanol oxidation and steam reforming of methanol on Cu(111): A density functional theory study

Cu-based catalysts have been widely used for hydrogen production from methanol decomposition, methanol oxidation and steam reforming of methanol (MSR). In this study, we have systematically identified possible reaction paths for the thermodynamics and dynamics involved in the three reactions on a Cu(111) surface at the molecular level. We find that the reaction paths of the three reactions are the same at the beginning, where methanol scission is favourable involving O–H bond scission followed by sequential dehydrogenation to formaldehyde. Formaldehyde is an important intermediate in the three reactions, where direct dehydrogenation of formaldehyde to CO is favourable for methanol decomposition; for methanol oxidation, formaldehyde tends to react with oxygen to form dioxymethylene through C–H bond breaking and finally the end products are mainly CO₂ and hydrogen; for MSR, formaldehyde tends to react with hydroxyl to form hydroxymethoxy through formic acid and formate formation, followed by dissociation to CO₂. CH₂O formation from methoxy dehydrogenation is considered to be the rate-limiting step for the three reactions. In general, the thermodynamic and kinetic preference of the three reactions shows the order methanol oxidation > MSR > methanol decomposition. Methanol oxidation and MSR are faster than methanol decomposition by about 500 and 85 times at typical catalytic conditions (e.g., 523 K), respectively. The result may be useful for computational design and optimization of Cu-based catalysts.     

Activity,stability and deactivation behavior of Au/CeO2 catalysts in the water gas shift reaction at increased reaction temperature (300 °C)

The effect of increasing the reaction temperature to 300 °C on the activity, stability and deactivation behavior of a 4.5 wt.% Au/CeO₂ catalyst in the water gas shift (WGS) reaction in idealized reformate was studied by kinetic and spectroscopic measurements at 300 °C and comparison with previously reported data for reaction at 180 °C under similar reaction conditions [A. Karpenko, Y. Denkwitz, V. Plzak, J. Cai, R. Leppelt, B. Schumacher, R.J. Behm, Catal. Lett. 116 (2007) 105]. Different procedures for catalyst pretreatment were used, including annealing at 400 °C in oxidative, reductive or inert atmospheres as well as redox processing. The formation/removal of stable adsorbed reaction intermediates and side products (surface carbonates, formates, OH_ad, CO_ad) was followed by in situ IR spectroscopy (DRIFTS), the presence of differently oxidized surface species (Au⁰, Au^0′, Au³⁺, Ce³⁺) was evaluated by XPS. The reaction characteristics at 300 °C generally resemble those at 180 °C, including (i) significantly higher reaction rates, (ii) comparable apparent activation energies (44 ± 1/50 ± 1 kJ mol⁻¹ vs. 40 ± 1 kJ mol⁻¹ at 180 °C), (iii) a correlation between deactivation of the catalyst and the build-up of stable surface carbonates, and (iv) a decrease of the initially significant differences in activity after different pretreatment procedures with reaction time. Different than expected, the tendency for deactivation did not decrease with higher temperature, due to enhanced carbonate decomposition, but increases.     

A vibrational spectroscopic and modeling study of poly(2,5-benzimidazole) (ABPBI) – Phosphoric acid interactions in high temperature PEFC membranes

This paper reports a FT-ATR-IR spectroscopic study on proton conducting poly(2,5-benzimidazole) (ABPBI) membranes doped with orthophosphoric acid. The analysis of the vibrational profiles is a good diagnostic tool to help understand the interactions occurring between the phosphoric acid and the polymer membranes. The experimental data show evidence that an acid-base proton exchange reaction has occurred between the imidazole moieties in the polymer chain and phosphoric acid to produce dihydrogen phosphate ions and protonated imidazolium cations in ABPBIⁿ⁺. Vibrational modes associated with the dihydrogen phosphate ions are evident in the FT-IR spectra at lower doping levels and then become partially masked by the large amount of free phosphoric acid at high acid concentrations. Several bands in the FT-IR and FT-Raman spectra attributed to mixed modes containing varying contributions from NH bending motions exhibit high frequency shifts upon protonation the imidazole moieties. The correlatively assigned experimental vibrational bands were compared with calculated normal modes for small molecule models. The optimized geometry of the benzimidazolium dimer suggests that protonation of ABPBI results in a perturbation of the extended conjugated π system and allows rotation of the benzimidazole monomer units along the polymer chain. The results described here provide insight into the roles of phosphoric acid and ABPBI in the conduction mechanism of polybenzimidazole systems.     

Synthesis of mesoporous silica (SBA-16) nanoparticles using silica extracted from stem cane ash and its application in electrocatalytic oxidation of methanol

In this work, a new catalyst based on modified mesoporous silica SBA-16 is proposed and used for electrochemical oxidation of methanol. Mesoporous silica SBA-16 nanoparticles are synthesized hydrothermally under the acidic medium using SiO₂/F127/BuOH/HCl/H₂O gel. Pure SiO₂ powder is prepared from inexpensive and environmentally friendly silica source of stem cane ash (SCA). The synthesized SBA-16 is characterized using X-ray diffraction, scanning electronic microscopy, transmission electron microscopy, Brumauer–Emmett–Teller (BET) and FT-IR techniques. The synthesized SBA-16 is modified with Ni(II) by dispersion in a 0.1 M nickel chloride solution. A modified carbon paste electrode (CPE) is prepared by mixing of NiSBA-16 to carbon paste (NiSBA-16CPE). The electrocatalytic oxidation of methanol was studied on modified electrode by cyclic voltammetry and chronoamperometry. From cyclic voltammetry, it is observed that the oxidation current is extremely increased by using NiSBA-16CPE compared to the nonmodified CPE. The incorporation of Ni²⁺ into SBA-16 channels provides the active sites for catalysis of methanol oxidation. Also, the rate constant for the catalytic reaction (k) of methanol is obtained.     

Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.