Suitable Thermodynamic Properties Research Articles

Achieving room temperature hydrogen storage reversibility in Nb-rich alloys of the Nb-Cr-Mn system

Recent developments in the design of body-centered cubic (BCC) multicomponent alloys via computational tools have demonstrated the possibility of obtaining alloys with excellent hydrogen storage behavior. In this work, we employ the CALPHAD (Calculation of Phase Diagrams) method to design Nb-rich alloys of the Nb-Cr-Mn system that present hydrogen storage reversibility at room temperature under moderate pressure conditions. We employ the valence electron concentration (VEC) factor as a compositional guide to select compositions with suitable thermodynamic properties. Using electric arc melting, we synthesize two alloys, namely Nb85Cr10Mn5 and Nb70Cr20Mn10, both forming predominant BCC solid solutions with VEC ∼ 5.2 and minor amounts of a eutectic microconstituent composed of BCC and Laves C14 phases. Both alloys are easily hydrogenated at room temperature without the need for an activation treatment. The Nb85Cr10Mn5 alloy reaches a storage capacity of 2.1 wt% of H (298 K; Peq∼ 20 bar) whereas the Nb70Cr20Mn10 alloy reaches a capacity of 1.4 wt% (298 K; Peq∼ 21 bar). Benefits in the storage kinetic performance are correlated with the BCC + C14 microstructure. Pressure-Composition-Temperature (PCT) diagrams show moderate values of equilibrium pressure for hydrogen storage reversibility at room temperature. Room temperature absorption/desorption cycling measurements demonstrated a reversible capacity of 1.2 wt% of H (Peq∼ 29 bar) for the Nb85Cr10Mn5 alloy and 0.8 wt% of H (Peq∼ 31 bar) for the Nb70Cr20Mn10 alloy after twenty cycles.

Working Fluid Selection for High-Temperature Heat Pumps: A Comprehensive Evaluation

Heat pumps have the potential for several applications across various industrial sectors, showcasing significant promise, especially in sectors such as pulp and paper, food and beverage, chemical, non-metallic minerals, and machinery. Envisioning the near future, there is confidence that heat pumps can achieve temperatures above 200 °C, offering substantial potential for utilization in these sectors. Nevertheless, a crucial aspect for the advancement of high-temperature heat pumps is the selection of the fluid. Fluid selection involves considerations of both thermodynamic efficiency and environmental impact, requiring fluids with zero ODP, negligible GWP, and no PFAS. Moreover, it is essential to consider the risks to human health associated with a specific fluid. Despite extensive research, particularly in the realm of vapour compression heat pumps, choosing the most suitable working fluid for these applications is a complex undertaking. Therefore, this paper conducts a theoretical analysis to evaluate potential fluids with suitable thermodynamic properties for high-temperature heat pumps (HTHPs). The comparative results gleaned from this study provide valuable insights for the comprehensive analysis of fluids, showing promise within temperature ranges dictated by specific applications. The metrics employed in the comparison emphasise the merits of fluids in terms of the overall performance, dimensions, and operating ranges of applicable compressor, heat exchange capacity, transport properties, and safety. One noteworthy finding from the analysis is that maintaining a constant HTHP lift (at 40 K) results in having the highest COP across all fluids when the condensing temperature ranges between 85% and 90% of their respective critical temperatures. According to the results of the analysis, natural fluids, including water and alcohols like ethanol or methanol, emerge as particularly compelling candidates.

Investigation on the Formation of Rare-Earth Metal Phenoxides via Metathesis

A number of alkali organometallic complexes with suitable thermodynamic properties and high capacity for hydrogen storage have been synthesized; however, few transition metal–organic complexes have been reported for hydrogen storage. Moreover, the synthetic processes of these transition metal–organic complexes via metathesis were not well characterized previously, leading to a lack of understanding of the metathesis reaction. In the present study, yttrium phenoxide and lanthanum phenoxide were synthesized via metathesis of sodium phenoxide with YCl3 and LaCl3, respectively. Quasi in situ NMR, UV-vis, and theoretical calculations were employed to characterize the synthetic processes and the final products. It is revealed that the electron densities of phenoxides in rare-earth phenoxides are lower than in sodium phenoxide due to the stronger Lewis acidity of Y3+ and La3+. The synthetic process may follow a pathway of stepwise formation of dichloride, monochloride, and chloride-free species. Significant decreases in K-band and R-band absorption were observed in UV-vis, which may be due to the weakened conjugation effect between O and the aromatic ring after rare-earth metal substitution. Two molecular structures, i.e., planar and nonplanar, are identified by theoretical calculations for each rare-earth phenoxide. Since these two structures have very close single-point energies, they may coexist in the materials.

Benzothioxanthene Dicarboximide as a Tuneable Electrogenerated Chemiluminescence Dye

AbstractOverlooked for years, the promising benzothioxanthene dicarboximide (BTXI) structure is nonetheless known to be an excellent fluorophore. As a further step towards the exploration of this rylene structure, the generation of electrochemiluminescence (ECL) in acetonitrile solutions is investigated herein as well as its direct comparison with a diphenylamine (DPA) derivative. Electrochemical characterization shows that the bare BTXI can be reversibly reduced or oxidized, characteristics which suggest the possibility that the electrogenerated reactive species can be exploited in the context of ECL. As a result, the BTXI moiety was found to be ECL‐active in both annihilation and co‐reactant mechanisms. Tri‐n‐propylamine (TPA) and benzoyl peroxide (BPO) were selected as sacrificial co‐reactants for promoting ECL. An ECL efficiency of 13 % (BTXI) and 6 % (BTXI‐DPA) compared to that of Ru(bpy)32+ was found when using TPA in oxidative ECL. Moreover, the ECL efficiency increased to 75 % and 55 % of Ru(bpy)32+ with BPO (i. e. reductive‐oxidation ECL) for BTXI and BTXI‐DPA, respectively. These results can be understood by calculating the free enthalpy of the redox reactions associated with each ECL pathway. Although the BTXI‐DPA red emitter exhibits more suitable thermodynamic properties, the BTXI green emitter is nonetheless more effective in combination with a co‐reactant due to a much larger photoluminescence quantum yield. The ECL spectra of the two molecules are compared to their photoluminescence spectra, revealing the involvement of the same excited states in the two different photoemission processes. In turn, the chemical engineering of BTXI is shown to be an effective platform for achieving tuneable ECL generation.

<i>In situ</i> resistance analysis of MgB<sub>2</sub> formation process from Mg(BH<sub>4</sub>)<sub>2</sub>

Mg(BH<sub>4</sub>)<sub>2</sub> was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH<sub>4</sub>)<sub>2</sub> began to decompose at about 300 ℃, and formed MgB<sub>2</sub> at the end of hydrogen desorption process with the weight content of 14.9% of hydrogen lost. Aside from the prominent hydrogen storage property, the decomposition process from Mg(BH<sub>4</sub>)<sub>2</sub> to MgB<sub>2</sub> can be a potential method for fabricating superconducting MgB<sub>2</sub> at a low sintering temperature. In this paper, MgB<sub>2</sub> bulk was prepared by an <i>in-situ</i> reaction, using the Mg(BH<sub>4</sub>)<sub>2</sub> pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH<sub>4</sub>)<sub>2</sub> decomposition process and the resistance-temperature (<i>R</i>-<i>T</i>) curve of this process was recorded. Phase of MgH<sub>2</sub>, Mg and B were formed as the block slowly release its hydrogen before MgB<sub>2</sub> occurred. According to the <i>R</i>-<i>T</i> curve, the phase formation of MgB<sub>2</sub> started in a relatively low temperature of 410 ℃. Because MgB<sub>2</sub> was critically formed by Mg and B derived from Mg(BH<sub>4</sub>)<sub>2</sub>, we can compare our formation temperature with previous study on MgB<sub>2</sub> prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH<sub>4</sub>)<sub>2</sub> decomposition was only 3.4 nm on average. The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity, which was the main reason for low sintering temperature. X-ray diffraction results showed that the purity of MgB<sub>2</sub> was 95.2%, and the size of MgB<sub>2</sub> grains was 10–18 nm. SEM images showed that the MgB<sub>2</sub> bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB<sub>2</sub> nanofibers can also be observed inside the bulk. In the superconductivity test, the superconducting transition temperature of the bulk was 35 K. After all, such <i>in situ</i> method to fabricate MgB<sub>2</sub> showed a great advantage in some aspects, as its low-cost precursors, low sintering temperature, small grain-size and high superconducting transition temperature in the formed MgB<sub>2</sub>, which have the potential in industrial scale fabrication of MgB<sub>2</sub> bulks and wires.

Analysis of Suitable Thermodynamic Properties and Conditions for Dew Point Cooling system

This study introduced dew point cooling system based on Maisotsenko Cycle Air-Conditioning (MAC/M-Cycle) for hot and humid climate. The purpose of this study to analyze the air thermodynamic properties and conditions supplied to the conditioned space. The room model has been developed to investigate the dry bulb temperature, specific humidity, and velocity distribution in the room. The simulation results showed a significance difference cooling capacity between these two systems. Consequently, this integrated system can realize significant energy saving and indoor air quality.

Retrofit of lower GWP alternative R449A into an existing R404A indirect supermarket refrigeration system

R404A is going to be phased out from most of the commercial refrigeration systems due to its high GWP value of 3943. R449A (GWP of 1282) has been proposed to replace R404A with only minor system modifications in supermarkets. This paper presents the measurements of a light retrofit replacement of R404A using R449A in a medium temperature indirect refrigeration system (secondary fluid temperature at the evaporator outlet between −9 and −4 °C). It has been demonstrated that with a slight expansion device adjustment and 4% increase of refrigerant charge, R449A can be used in this refrigeration system designed for R404A because of its suitable thermodynamic properties and acceptable maximum discharge temperature. At a secondary fluid temperature at condenser inlet of 30 °C, the COP of R449A nearly matches that of R404A (both were between 1.9 and 2.2), despite having approximately 13% lower cooling capacity. As a conclusion, attending to the GWP reduction and similar energy performance, it was demonstrated using the TEWI methodology that the use of the recently developed refrigerant R449A in these applications can reduce the total CO2 equivalent emissions of an indirect supermarket refrigeration system designed for R404A refrigerant.

Research on the flammable characteristics of fluoroethane (R161) and its binary blends

As one of the potential new generation refrigerants, R161 possesses the suitable thermodynamic properties and small climate change effects, but it is flammable. This paper contributed to its fundamental flammable characteristics theoretically and experimentally. Firstly, its flames were researched under various testing conditions. Then influences of temperature and vessel dimensions on the flammability limits of R161 were tested and analyzed. The results indicated that the lower flammability limits of R161 increased with the decrease of temperature while its upper flammability limits reduced oppositely, when initial temperature varied from 55 °C to −5 °C. Thirdly, a method was proposed to estimate the refrigerants' flammability limits, which concerned three impact parameters, the gas concentration, bond dissociation energies and enthalpy variations. Finally, the flame suppression effects of 1,1,1,2,3,3,3-heptafluoropropane (R227ea), trifluoroiodomethane (R13I1), and trans-1,3,3,3-tetrafluoropropene (R1234ze) to R161 were tested, and the critical flammability ratios (CFR) of some inhibitors to R161 were obtained.

Improved kinetics of the Mg(NH2)2–2LiH system by addition of lithium halides

The Mg(NH2)2–2LiH composite is a promising on-board hydrogen storage material due to its high reversible hydrogen capacity and suitable thermodynamic properties. However, the severe kinetic barrier inhibits its low temperature operation. In the present work, the additive effects of lithium halides on the Mg(NH2)2–2LiH system were studied systematically. Experimental results showed that, among all those lithium halides, the LiBr doped Mg(NH2)2–2LiH composite exhibited the best dehydrogenation performance. The hydrogen sorption and desorption rates of the Mg(NH2)2–2LiH–0.2LiBr sample are ∼3 and 2 times, respectively, faster than that of the pristine sample at 140 °C. At the same time, enhanced kinetics for hydrogen desorption was observed from an activation energy (Ea) of ca. 92 ± 9 kJ mol−1 which was significantly decreased by 35 kJ mol−1 compared with the pristine sample. Subsequently, a plausible mechanism for the modified dehydrogenation/re-hydrogenation process was proposed.

Effects of Al-based additives on the hydrogen storage performance of the Mg(NH2)2–2LiH system

The Mg(NH2)2-2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (~5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li3AlH6 was introduced to the Mg(NH2)2-2LiH system. Experimental results show that a 3.2% mol Li3AlH6-modified Mg(NH2)2-2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li3AlH6-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (Ea) of ca. 96.3 ± 9 kJ mol(-1) which was significantly decreased by 31 kJ mol(-1) from that of the Li3AlH6-free sample. The interaction of Li3AlH6 and Mg(NH2)2 during ball milling results in the formation of LiAl(NH)2, LiNH2 and Mg3N2. LiAl(NH)2 was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.

Thermodynamics study of hydrogen storage materials

The growing use of conventional energy such as fossil fuels results in problems degrading our environment. Hydrogen is frequently discussed as a clean energy in the future without pollution. However, efficient and safe storage of hydrogen constitute a key challenge and unresolved problem. One of the main options is solid-state storage technology. A successful solid-state reversible storage material should meet the requirements of high storage capacity, suitable thermodynamic properties, reversibility and fast adsorption and desorption kinetics. This feature article focuses mainly on the development of thermodynamic improvement of hydrogen storage materials in the past few years including the complex hydride, ammonia borane, and metal-organic frameworks.

Progress in improving thermodynamics and kinetics of new hydrogen storage materials

Hydrogen storage material has been much developed recently because of its potential for proton exchange membrane (PEM) fuel cell applications. A successful solid-state reversible storage material should meet the requirements of high storage capacity, suitable thermodynamic properties, and fast adsorption and desorption kinetics. Complex hydrides, including boron hydride and alanate, ammonia borane, metal organic frameworks (MOFs), covalent organic frameworks (COFs) and zeolitic imidazolate frameworks (ZIFs), are remarkable hydrogen storage materials because of their advantages of high energy density and safety. This feature article focuses mainly on the thermodynamics and kinetics of these hydrogen storage materials in the past few years.

Hydrogen release and structural transformations in LiNH 2–MgH 2 systems

Reactive hydride composites are good candidates for solid hydrogen storage due to their high gravimetric capacity, cyclability, and suitable thermodynamic properties. The LiNH 2–MgH 2 system is promising as changes in stoichiometry and milling conditions may result in tailoring of these properties. In this work, LiNH 2–MgH 2 with different ratios (Li2:Mg, Li:Mg) and ball milling conditions (100, 600 rpm) were investigated. Thermal desorption profiles shows hydrogen release starting at 125 °C for Li2:Mg 600 sample and at 225 °C for Li:Mg 600 sample, while for Li:Mg 100 sample simultaneous hydrogen and ammonia release at 175 °C is observed. In-situ synchrotron X-ray diffraction shows the related structural transformations, such as formation of Mg(NH 2) 2 and allotropic transformation of α into β-Li 2Mg(NH) 2 for Li2:Mg 600 sample at 350 °C or direct formation of β-Li 2Mg(NH) 2 for Li:Mg 100 sample at 370 °C. Different polymorphs of the LiMgN phase were also observed during cooling for these two samples. For the Li:Mg 600 sample, transformation occurs in a unique reaction from an unknown phase into β-Li 2Mg(NH) 2 at 290 °C. The unknown phase is indexed as a Fm3 m cubic similar to the high temperature γ-Li 2Mg(NH) 2.

Effect of additives on the synthesis and reversibility of Ca(BH4)2

Metal borohydrides are potential materials for solid-state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH4)2 is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high-pressure reactive ball milling. Starting from CaH2 and CaB6 with a TiCl3 or TiF3 as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H2. The presence of Ca(BH4)2 is confirmed by the presence of the stretching mode of the [BH4]- group in the infrared spectra of the as-milled samples. Using in situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH4)2 phase present after milling. The reversible decomposition/formation of Ca(BH4)2 is obtained with higher yield (57%) using higher temperature and TiF3 as additive but not with TiCl3 despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed. © 2009 Elsevier B.V. All rights reserved.

Solid-state hydrogen storage: Storage capacity, thermodynamics, and kinetics

Solid-state reversible hydrogen storage systems hold great promise for onboard applications. The key criteria for a successful solid-state reversible storage material are high storage capacity, suitable thermodynamic properties, and fast hydriding and dehydriding kinetics. The LiNH2 + LiH system has been utilized as an example system to illustrate these critical issues that are common among other solid-state reversible storage materials. The progress made in thermodynamic destabilization and kinetic enhancements via various approaches are emphasized. The implications of these advancements in the development of future solid-state reversible hydrogen storage materials are discussed.

The identification of a hitherto unknown intermediate phase CaB2Hx from decomposition of Ca(BH4)2

Ca(BH4)2 is a promising material for hydrogen storage due to its high gravimetric capacity and expected suitable thermodynamic properties. Experimental reports indicate a more complex reaction pathway than the previously proposed route, including formation of unknown intermediate phases. In this work, the reaction scheme of a mixture of γ- and β-Ca(BH4)2 was investigated using temperature programmed desorption and high-resolution synchrotron radiation powder X-ray diffraction. An unknown intermediate Ca–B–H-containing phase has been identified as a CaB2Hx compound where x most probably equals 2. A structure model is proposed.

Chemical-looping with oxygen uncoupling for combustion of solid fuels

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. The carbon dioxide from the combustion is inherently separated from the rest of the flue gases. CLOU is based on chemical-looping combustion (CLC) and involves three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen in the gas-phase (step 2) and where this gas-phase oxygen reacts with a fuel (step 3). In other proposed schemes for using chemical-looping combustion of solid fuels there is a need for an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds which then react with the oxygen carrier particles. The gasification of char with H 2O and CO 2 is inherently slow, resulting in slow overall rates of reaction. This slow gasification is avoided in the proposed process, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen. The process demands an oxygen carrier which has the ability to react with the oxygen in the combustion air in the air reactor but which decomposes to a reduced metal oxide and gas-phase oxygen in the fuel reactor. Three metal oxide systems with suitable thermodynamic properties have been identified, and a thermal analysis has shown that Mn 2O 3/Mn 3O 4 and CuO/Cu 2O have suitable thermodynamic properties, although Co 3O 4/CoO may also be a possibility. However, the latter system has the disadvantage of an overall endothermic reaction in the fuel reactor. Results from batch laboratory fluidized bed tests with CuO and a gaseous and solid fuel are presented. The reaction rate of petroleum coke is approximately a factor 50 higher using CLOU in comparison to the reaction rate of the same fuel with an iron-based oxygen carrier in normal CLC.

Dehydriding properties of Ti or/and Zr-doped sodium aluminum hydride prepared by ball-milling

The NaAlH4 complex is attracting great attention for its potential applications in hydrogen-powered fuel-cell vehicles due to its high hydrogen storage capacity and suitable thermodynamic properties. However, its practicable hydrogen storage capacity presently obtained is less than the theoretical capacity (5.6 wt.%). To improve the hydrogen capacity, we chose metallic Ti or/and Zr powder as catalyst dopants, and prepared the sodium aluminum hydride by hydrogenation of ball-milled NaH/Al mixture containing 10 mol% dopants with different proportions of Ti and Zr, and then investigated the effects on their hydrogen storage (dehydriding) properties. The results showed that different catalyst dopants affected the dehydriding properties greatly. The catalysis of metal Ti as a catalyst dopant alone on dehydriding kinetics for the entire dehydrogenation process of ball-milled (NaH/Al) composite was higher than that of adopting Zr alone. The synergistic catalytic effect of Ti and Zr together as co-dopants on the dehydrogenation process of (NaH/Al) composite was higher than that using only Ti or Zr as dopant individually. The composite doped with proper proportion of Ti and Zr together (8 mol% Ti+ 2 mol% Zr) as co-dopants exhibited the highest dehydriding kinetic property and desorption capacity.

Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2

Mg(BH4)2 is one of the few complex hydrides which have the potential to meet the requirements for hydrogen storage materials, because it contains 14.9 mass% H and has suitable thermodynamic properties. It has not been investigated for hydrogen storage applications yet. In this study, several ways to synthesize solvated and desolvated magnesium tetrahydroborate by wet chemical and mechanochemical methods were tested and compared. A direct synthesis by a reaction of MgH2 with aminoboranes yields magnesium tetrahydroborate quantitatively and in pure form. The method is also applicable to the synthesis of other tetrahydroborates. The products were characterized by elemental analysis, in situ X-ray diffraction (XRD), infrared spectroscopy (FTIR), and thermal analysis methods, such as thermogravimetric analysis (TGA-DSC) and high-pressure calorimetry under a hydrogen atmosphere (HP-DSC).

Carbon materials obtained from self-binding sugar cane bagasse and deciduous wood residues plastics

Abstract It is demonstrated that dispersed biomass residues (bagasse, sawdust) can be processed into hard carbonaceous blocks, panels or boards with good strength and thermodynamic properties. There are two possible approaches: to mould dispersed biomass charcoal with a phenol–formaldehyde binder or to produce this material by carbonising the biomass fiberboard prepared by making use of steam explosion autohydrolysis pulp or steam explosion lignin as a binder. In the first step, steam explosion lignin, as a modifier and a binder is introduced to the lignocellulosic biomass by impregnation or during the hot pressing process to form a hard fiberboard. By subsequent carbonisation of the fiberboard panels or blocks, carbonised panels or blocks with high bending and crushing strength and suitable thermodynamic properties are obtained due to the formation of an internal lignin reinforcement in cell lumina and impregnation of cell walls with lignin solution or molten lignin. The carbonised panels demonstrate a good dimensional stability after a standard treatment with water. The bending strength of the carbonised panels after 24 h soaking in water is 93% of that in dry state. The thermodynamic properties and porosity of the carbonised panels demonstrate their suitability for use as a building material. Lignin, a natural binder of fiberboards, has proven to be suitable for preparation of cabonaceous panels and boards. In this respect new carbon building blocks and panels from moulded biomass and carbonised steam exploded biomass act as a concentrated form of long term carbon storage and will be a factor stabilizing the growing CO2 concentration in the atmosphere. [Proceedings of the First Workshop of QITS, Materials Life-Cycle and Environmentally Sustainable Development, March 2–4, Campinas, UNU/IAS San Paulo, Brazil, 1998, pp. 95–101; Proceedings of the Workshop in “Targeting Zero Emissions for the Utilization of Renewable Resources”, ANESC, Tokyo, 1999, pp. 2–11.]