Application of Danckwerts-type boundary conditions to the modeling of the thermal behavior of metal hydride reactors

The paper presents a model-based investigation of a metal hydride reactor applied as a solid state hydrogen storage device. The elements of a metal hydride reactor are hydrogen supply duct, internal hydrogen distribution, hydride bed, reactor shell and the flow domain of the heat transfer fluid. Internal hydrogen distribution and hydride bed are porous media. Therefore, hydrogen flows through non-porous and porous regions during its reversible exothermic absorption and endothermic desorption, respectively. The interface between porous and non-porous regions is a discontinuity with respect to energy transport mechanisms. Hence, Danckwerts-type boundary conditions for the energy balance equation are introduced. Application of the first and second law of thermodynamics to the interface reveals that temperature jumps may occur at the hydrogen inlet but are not allowed at the hydrogen outlet. Exemplarily the loading behavior of a metal hydride storage tank based on sodium alanate is analyzed. It is demonstrated and experimentally validated that only Danckwerts-type boundary conditions predict the important cooling effect of the inlet hydrogen on the exothermic absorption process correctly.     

The electrochemical impedance of metal hydride electrodes

The electrochemical impedance responses for different laboratory type metal hydride electrodes were successfully modeled and fitted to experimental data for AB₅ type hydrogen storage alloys as well as one MgNi type electrode. The models fitted the experimental data remarkably well. Several AC equivalent circuits have been proposed in the literature. The experimental data, however, could not always be satisfactorily approximated. The approximation model presented here exhibits smooth fit to the experimental results for all frequencies in the whole range from 10 kHz to 0.1 mHz. Equivalent circuits, explaining the experimental impedances in a wide frequency range for electrodes of hydride forming materials mixed with copper powder, were obtained. Both charge transfer and spherical diffusion of hydrogen in the particles are important sub processes that govern the total rate of the electrochemical hydrogen absorption/desorption reaction. To approximate the experimental data, equations describing the current distribution in porous electrodes were needed. Indications of one or more parallel reduction/oxidation processes competing with the electrochemical hydrogen absorption/desorption reaction were observed. The impedance analysis was found to be an efficient method for characterizing metal hydride electrodes in situ.     

Activated carbon fibres and composites on its base for high performance hydrogen storage system

To obtain high hydrogen sorption capacity and reduce the time of the cycle (adsorption/desorption) in the gas storage system a new composite material (metal hydride particles on the activated carbon fibre matrix) was suggested. Different adsorbent materials such as activated carbon fibre “Busofit”, granular activated carbon and new composite sorbent (metal hydride La_0.5Ni₅Ce_0.5 particles on the activated carbon fibre “Busofit”) were tested. Effect of the carbon sorbent nature and metal hydride content is important to choose the optimal sorbent bed.In this paper, a thermally regulated storage system for hydrogen was numerically analyzed and experimentally validated. A two-dimensional transient model was used to analyze the influence of the thermal control on the operating characteristics of the flat sectional vessel. The evolution of the temperature, pressure and volumetric density of hydrogen inside the vessel during the charging/discharging is discussed. It was shown that heat pipe based thermal control of the process increase the efficiency of the hydrogen storage. Such vessels are interesting to be applied in fuel cells used for vehicle or dual-fuel engine car (hydrogen/gasoline, hydrogen/methane).     

Metal hydride characteristics by Zr and Ti

Hydrogen absorption experiments were carried out using zirconium (Zr) and titanium (Ti) in the form of a metal sponge, strip and rod to investigate the metal hydride characteristics. The Zr and Ti sponges showed a high hydrogen absorption capacity despite a low reaction temperature. The H/M, which indicates the capacity of hydrogen absorption, was measured at 2.0 for the Zr/Ti sponge at 25 ‡C. In the case of the Zr/Ti strip and rod, however, the hydrogen absorption capacity was very low at 25 ‡C. The capacity of hydrogen absorption increased with an increase in the reaction temperature. When the Ti strip was not activated, the H/M ratio was measured at 0.58. When the Ti strip was once and twice activated at 800 ‡C for 1 hour, the H/M ratio increased to 1.6 and 1.83, respectively. The hydrogen absorption capacity decreased with the increment of concentration of helium in hydrogen due to a blanketing effect of metal surface by the helium. A pulverizing phenomenon during the metal hydriding was observed in both the Zr/Ti strip and the Zr/Ti rod. However, this pulverizing phenomenon was not observed in the Zr/Ti sponges because of their high surface area.     

Hydrogen in aluminum during alkaline corrosion

The thermodynamic state of hydrogen in aluminum during alkaline corrosion was investigated, using a two-compartment hydrogen permeation cell with an Al/Pd bilayer membrane. The open-circuit potential of the Pd layer in a pH 7.0 buffer solution was monitored to sense the hydrogen chemical potential, μ_H. At pH 12.5-13.5, the measurements established a minimum μ_H of 0.55 eV relative to the ideal gas reference, equivalent to a H₂ gas pressure of 5.7 GPa. Statistical mechanics calculations show that vacancy-hydrogen defects are stable in Al at this condition. A dissolution mechanism was proposed in which H at very high μ_H is produced by oxidation of interfacial aluminum hydride. The mechanism explains the observed rapid accumulation of H in the metal by extensive formation of vacancy-hydrogen defects.     

Long-term electrolytic hydrogen permeation in nickel and the effect of vanadium species addition

Nickel cathodes have been found to become deactivated under long-term polarization in the H₂ evolution region during alkaline water electrolysis. The cause of deactivation was examined using steady state polarization and measurement of hydrogen permeation through nickel foil in 8 mol/l KOH at 70 °C and 100 mA/cm². The long-term (over 50 h) permeation behaviour was explained by formation and growth of a nickel hydride phase. The rise in hydrogen overpotential was ascribed to an increase of the hydrogen surface coverage on the newly formed hydride. The effect of an electrolyte additive (a vanadium salt) on the hydrogen overpotential and permeation rate was also investigated. Upon addition of dissolved V₂O₅, the permeation rate was found to increase quickly and then slowly decrease to a steady value close to that measured for hydride-free nickel. Meanwhile, the hydrogen overpotential was observed to recover back to nearly its initial value for fresh nickel. The exhibited behaviour was attributed to decomposition of the hydride phase, after deposition of a vanadium-bearing compound. The prolonged contact between Ni and V was proposed as the main reason for hydride decomposition. The addition of more vanadium had no further result on the hydrogen overpotential.     

耦合相变储热的金属氢化物反应器吸氢过程模拟

基于金属氢化物储氢反应,建立了相变材料蓄热的固体储氢反应器模型,模拟研究了吸氢压力等操作参数及相变材料的相变温度、固(液)态导热系数、相变潜热等物性参数对固体储氢反应器工作过程的影响. 结果表明,相变材料的固态导热系数和相变潜热对固体储氢反应器性能的影响较小,相变温度和液态导热系数对反应器性能影响较大. 相变温度越低,液态导热系数越大,储氢反应器性能越好. 在使用最优的相变材料储能时,提高充入氢气的压力可加快反应速率,强化相变材料的传热,有助于进一步优化反应器的储氢性能.     

Hydrogen adsorption and storage on porous materials

The development of safe and efficient methods of hydrogen storage is a prerequisite for the use of hydrogen with fuel cells for transport applications. In this paper, results available for adsorption of hydrogen on porous materials, ranging from activated carbons to metal organic framework materials, are discussed. The results indicate that up to 5 and 7.5 wt% of hydrogen can be stored on porous carbon and metal organic framework materials, respectively, at 77 K. The amounts of hydrogen adsorbed on porous materials at ambient temperatures and high pressures are much lower (0.5 wt%). The strong temperature dependence of hydrogen physisorption on porous materials is a limitation in the application of this method for hydrogen storage in addition to storage capacity requirements.     

Comparison of dual fluidized bed steam gasification of biomass with and without selective transport of CO2

A comparison of dual fluidized bed gasification of biomass with and without selective transport of CO₂ from the gasification to the combustion reactor is presented. The dual fluidized bed technology provides the necessary heat for steam gasification by circulating hot bed material that is heated in a separate fluidized bed reactor by combustion of residual biomass char. The hydrogen content in producer gas of gasifiers based on this concept is about 40 vol% (dry basis). Addition of carbonates to the bed material and adequate adjustment of operation temperatures in the reactors allow selective transport of CO₂ (absorption enhanced reforming—AER concept). Thus, hydrogen contents of up to 75 vol% (dry basis) can be achieved. Experimental data from a 120 kW_{Fuel input} pilot plant as well as thermodynamic data are used to determine the mass- and energy-balances. Carbon, hydrogen, oxygen, and energy balances for both concepts are presented and discussed.     

Some aspects of the EIS study of the rhodium electrode in the hydrogen evolution region

General expression for the frequency dependence of the faradaic electrode admittance is derived for the case when the rate of the hydrogen evolution reaction (HER) depends on the potential, potential dependent surface fraction of adsorbed hydrogen, and potential dependent surface or subsurface concentration of the molecular or absorbed hydrogen, respectively. The simplified version of the general expression was adjusted to take into account two different specific reaction mechanisms of HER, and applied in the analysis of the experimentally measured impedance spectra of the rhodium/H₂SO₄ electrode in the region of low cathodic overpotentials. The results of the fitting procedure suggested proceedings of HER in parallel with diffusion associated with some absorption of hydrogen within the rhodium metal layer.     

Controlled synthesis of nanoplate,nanoprism and nanopyramid-shaped CdSe decorated on porous TiO2 photocatalysts for visible-light-driven hydrogen evolution

Herein, we report a successful synthesis of porous TiO₂ monoliths decorated with unique nanoplate, nanoprism, and nanopyramid-shaped CdSe particles through a mild selenylation of CdO embedded inside porous TiO₂ monoliths via a hydrothermal method in a very controlled manner. Compared with pure TiO₂, as-synthesized CdSe/TiO₂ photocatalyst not only enhances light absorption but also leads to a highly efficient charge-carrier separation. Particularly, the nanoplate-shaped 7% CdSe/TiO₂ photocatalyst (molar percentages of CdSe to TiO₂ is 7:100) exhibits an exceptional hydrogen evolution rate up to 3650?μmol?h^?1 g^?1 without resorting to any noble-metal co-catalysts under visible-light irradiation owing to synergistic effects envisaged by a rational material design. Our results may provide a useful strategy to develop a highly-efficient visible-light-driven hydrogen production system via water splitting.     

Hydrogen electrosorption into Pd-Pt-Au ternary alloys

Thin layers of Pd-Pt-Au alloys were prepared by metal codeposition at constant potential from chloride solutions. The process of hydrogen electrosorption into Pd-Pt-Au alloys was investigated in acidic solution (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry, also coupled with the electrochemical quartz crystal microbalance. It was found that Pd alloying with both Pt and Au decreases the maximum hydrogen solubility, but improves the kinetics of absorbed hydrogen oxidation, which is mirrored in a negative shift of the potential of hydrogen desorption peak and shorter hydrogen desorption time. In Pd-Pt-Au alloys the effect of absorption/desorption hysteresis and the stresses connected with hydrogen absorption are reduced in comparison with pure Pd. After prior hydrogen absorption in Pd-Pt-Au alloys, surface oxides are formed and reduced at potentials even by 200 mV lower than before hydrogen treatment.     

Quasi-1D and 3D TPOX porous media diffuser reformer model

This paper focuses on the numerical simulations of methane thermal partial oxidation reforming process within inert porous media and their comparison with experiments. In order to produce hydrogen rich mixtures and for the sake of reaction stability, the reformer consists on a diffuser filled with porous media. The validity of using a quasi-1D approach to model this system is explored based on 3D simulations of the isothermal fluid flow through the porous solid structure. Several fluid flow cases were taken into account as well as two different porous materials, Al₂O₃ fiber lamellae and SiSiC foam. The detailed fluid flow information obtained from the 3D study was used to provide the realistic cross-sectional area variation of the quasi-1D model. The quasi-1D 12-steps reduced chemistry model predictions are in very satisfactory agreement with the temperature and concentration fields measured within the diffuser porous reformer.     

Facile fabrication of 3D interconnected porous boron doped polymeric g-C3N4 with enhanced visible light photocatalytic hydrogen evolution and dye contaminant elimination

Researchers have attempted to developing high-efficiency catalysts for photocatalytic hydrogen evolution and organic pollution elimination simultaneously to alleviate the issues of energy shortage and water pollution. In this work, we fabricated 3D interconnected porous boron doped polymeric g-C₃N₄ catalysts with efficient photocatalytic activity for hydrogen evolution and dye contaminant elimination under visible-light irradiation. The as-fabricated catalysts exhibited significantly enhanced hydrogen evolution (4.37 mmol g ^?1 h^?1) and RhB contaminant elimination (96.37%) activity. Based on characterization and photocatalytic tests, an enhanced mechanism of the superior photocatalytic performance was proposed: 3D interconnected porous structure and B-doping have a synergistic effect on the greatly improved photocatalytic activity. The 3D interconnected structures endowed g-C₃N₄ with a higher specific surface area and abundant active sites and improved the capacity of rapid absorption to facilitate the photocatalytic process. B doping provided enhanced visible-light absorption capacity and a narrowed bandgap and served as a “highway” for electron-hole pairs to facilitate migration and separation and suppress the combination of photogenerated carriers. Besides, the possible mechanism of enhanced photocatalytic performance was elucidated according to the results of characterization measurements and active species analysis.     

Effects of hydrogen on the corrosion of pure magnesium

Electrochemical measurements and capacitance measurements were performed for better understanding of the effects of hydrogen on the corrosion of pure magnesium. Anodic polarization curves, activation energy (E_a), pitting initiation time and electrochemical noise (EN) were carried out, which showed that hydrogen had a strong influence on the corrosion of magnesium. There existed a highest corrosion resistance of magnesium, when a series of cathodic charging current density were applied to the specimens due to the optimum hydrogen concentration in the hydride coating (MgH₂) on the surface. Mott-Schottky results confirmed that there was a hydride coating on the charged magnesium. Hydrogen ionized as H⁻ and depleted donor/electron, which induced the inversion of semi-conductivity from N-type to P-type.     

Combustion wave propagation in magnesium/water mixtures: Experiments and model

Metal (Al, Mg)/water mixtures are of interest for hydrogen generation and propulsion. Due to the highly exothermic metal-water reaction, such mixtures, upon ignition, exhibit self-sustained propagation of combustion wave with simultaneous release of hydrogen from water. In this work, experiments with stoichiometric Mg/H₂O mixtures were conducted. Uniform propagation of the combustion wave was observed, and the effect of metal particle size, in the range , on the combustion front velocity was determined. Based on the obtained results, a mathematical model for combustion wave propagation in Mg/H₂O mixtures was developed. In this model, the combustion wave structure includes a thin water-boiling front, a preheat zone with water vapor flowing through the porous medium and a wide zone of reaction between the formed water vapor and the metal. This diffusion-limited model, solved analytically, predicts the front velocity and thermal profile of the combustion wave for different metal particle sizes. A satisfactory agreement between the experimental and modeling results is demonstrated, and features to improve the model are identified. With some modifications, the model can also be applied to combustion wave propagation in nanoaluminum/water mixtures.     

Electrolytic loading of hydrogen in Zr65(Pd80Rh20)35 and Zr65Pd35 alloys prepared by mechanical grinding and in their oxidised derivatives

Zr₆₅(Pd₈₀Rh₂₀)₃₅ and Zr₆₅Pd₃₅ alloys were prepared by mechanical grinding of stoichiometric amounts of either ZrH₂ and Pd₈₀Rh₂₀ or ZrH₂ and Pd. Following a suggestion in the literature, these alloys were converted by thermal oxidation in air into the corresponding ZrO₂(Pd₈₀Rh₂₀) and ZrO₂Pd compounds, which are probably composed of Pd₈₀Rh₂₀ and Pd nanostructures embedded in ZrO₂.The aim of our work thereafter was to investigate hydrogen storage in these nanostructures. Both the alloys and their oxidation derivatives were thus loaded with hydrogen by cathodic reduction at 25 °C in 6 M KOH, and loaded hydrogen was then determined by anodic extraction. On comparing the hydriding extent of the alloys with that of the corresponding oxidation derivatives, the metal clusters which formed after thermal oxidation are probably much larger in PdRh than in Pd. The former exhibits the high decomposition pressure typical of massive Pd₈₀Rh₂₀ hydride, and the maximum [H]/[Pd_0.8Rh_0.2] atom ratio is ≈0.82. Conversely, the amount of hydrogen extracted from reduced ZrO₂Pd samples prepared in optimal conditions fits [H]/[Pd] atom ratios between 1 and 2. The shape of the electrochemical hydrogen desorption isotherms indicates that the hydrogen in excess of the β-Pd hydride phase is probably stored in a new, very stable form.     

Hydrogen absorption into alpha titanium in acidic solutions

The absorption of hydrogen into commercially pure alpha titanium (Grade-2) has been studied on oxide-free (pH ≤ 3) surfaces at 25 °C under both potentiostatic and galvanostatic conditions. The rate of hydrogen absorption was found to be extremely rapid on oxide-free surfaces and the formation of surface hydrides catalyzed the proton reduction process. Complete hydride coverage of the surface very significantly suppressed the rate of hydrogen absorption, which is consistent with the known decrease in diffusivity of hydrogen in the hydride phase. SIMS imaging showed that absorption occurred preferentially at iron-containing intermetallic particles located along grain boundaries, leading to the coexistence of cathodically active hydrided sites and anodically active alpha titanium sites.     

Synthesis of three-dimensional hierarchically porous reduced graphene oxide–TiO2 nanocomposite for enhanced hydrogen storage

Three-dimensional hierarchically porous graphene-TiO₂ (3D-HPGT) nanocomposites were synthesized through electrostatic assembly method. The obtained 3D-HPGT nanocomposites exhibited hierarchically porous structure with multi-level pores (macro-, meso- and micropores), high specific surface area (705?m²/g), large pore volume (0.41?cm³/g) and higher hydrogen storage capacity. At the pressure of 5?bar, 3D-HPGT nanocomposite showed a maximum hydrogen capacity of 4.11 and 1.48?wt% at 77 and 298?K, respectively, which were much higher than those of previously reported graphene-based materials. The enhanced hydrogen storage capacities were attributed to the three-dimensional hierarchically porous structure, evenly distributed TiO₂ nanoparticles on the graphene nanosheets, strong attachment of TiO₂ nanoparticles to the underlying graphene nanosheets, and hydrogen spillover effect originated from TiO₂ nanoparticles.     

Investigation of hydrogen adsorption-absorption on iron by EIS

This work concerns the interaction between hydrogen and iron in the cathodic potential region. It was motivated by the need for a better understanding of the hydrogen insertion mechanism in metals. Electrochemical deposits of iron with various thicknesses were carried out on a gold substrate and they were characterized by impedance measurements under hydrogen evolution conditions.The processes occurring at the surface and within the iron films deposited on a gold electrode were studied using various electrochemical techniques. The impedance and voltammetric behaviours were strongly dependent on the film thickness. The main result of this study is that the charge transfer resistance increases with the film thickness. A model of the adsorption-absorption of hydrogen into iron films was proposed. It considered two types of absorbed hydrogen in a sublayer richer in hydrogen than the bulk metal. There, the hydrogen transported in the film coexists with another one coming from a direct absorption mechanism and “trapped” in some sites. The two absorbed hydrogen are reversibly exchanged. The interaction of hydrogen with palladium and iron was compared. It was concluded that for iron, the insertion of hydrogen results from a competition between a one-step direct hydrogen absorption mechanism and the classical two-step indirect penetration in the metal via the adsorbed hydrogen. At the end, a possible explanation of the deep penetration of the direct hydrogen absorption in the metal, detected by the impedance analysis, is proposed. It is based on the imperfections of numerous first layers of the deposited metals on polycrystalline gold.