Hydrogen Storage Properties of Complex Metal Hydride-Carbon Materials

Hydrogen storage properties of NaAlH₄ can be improved by mixing it with Co/carbon catalysts. The importance of cobalt nanoparticles was established by varying the amount of active cobalt in the reaction vessel. This was achieved either by keeping the weight of NaAlH₄ and carbon constant while varying the amount of Co/carbon catalyst used or by poisoning the cobalt active site with CO. Another approach to improve the hydrogen storage properties of complex metal hydride is through nano-confinement of the hydride in carbon pores. LiBH₄–carbon composite formed via liquid phase infusion of LiBH₄ into carbon was examined for reversibility in the hydriding/dehydriding cycles. The physical constraints imposed by the carbon nano-pores appeared to be effective in deterring the loss of cyclability due to sintering of the hydride. The loss in hydrogen storage appeared to be the result of LiBH₄ reaction with impurities/reactive functional groups on the carbon.     

Hydrogen storage properties of Pd nanoparticle/carbon template composites

Theoretical studies predict improved hydrogenation properties for hybrid carbon/metal composites. The hydrogen storage capacity of ordered porous carbon containing Pd clusters was measured. The C/Pd composite was obtained by chemical impregnation of an ordered porous carbon template (CT) with a H₂PdCl₄ solution followed by a reduction treatment. 10 wt.% of palladium clusters were introduced in the carbon porosity; the Pd clusters (2 nm in size) being homogeneously distributed. Thermodynamic hydrogenation properties of both Pd-free CT and the Pd-10 wt.% CT composite have been determined by hydrogen isotherm sorption measurements and thermal desorption spectroscopy (TDS) analysis. The introduction of the palladium into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 1.6 MPa, since here the hydrogen uptake is being attributed to physisorption on the carbon. However, at room temperature and moderate pressure (0.5 MPa), the filling of the CT with 10 wt.% nanocrystalline Pd results in an hydrogen uptake eight times larger than that of the Pd-free CT. After the second cycle, a good reversibility is observed. TDS measurements confirm that the sharp increase of the hydrogen uptake is due to the presence of the Pd clusters in the carbon porosity.     

Preparation and catalytic efficiency of mixed noble metal catalysts on electrochemically activated carbon fibre supports

Mixed noble metal catalytic systems were prepared on electrochemically activated PAN-based carbon fibre supports through a procedure of repetitive cation exchange between the acidic (-OOOH and-OH) groups of the electrooxidized carbon surface and noble metal salts and subsequent cathodic reduction of the exchanged noble metal ions to the metallic state.The whole investigation was carried out for the binary system Ag-Pd. The catalytic efficiency of palladium deposited on silver is more pronounced compared to a Pd-deposition obtained after only one Pd²⁺-exchange procedure and subsequent cathodic reduction, whereas an inhibition of the catalytic activity of palladium is noticed when it is covered by silver deposition. The catalytic efficiency of the obtained mixed noble metal catalysts was studied by means of hydrogen-adsorption and absorption profiles in H₂SO₄, the electroreduction of nitrobenzene in aqueous methanolic H₂SO₄ solutions and the hydrogenation reaction of nitrobenzene to aniline in methanolic solutions. The study was completed by impedance spectroscopic measurements at the potential of hydrogen evolution in H₂SO₄ solutions.Mixed Ag-Pd systems are even more stable than double Pd-Pd depositions on activated carbon fibre supports, as shown by the fact that they retain their mechanical stability and catalytic activity even after prolonged storage in aqueous or methanolic solutions, as well as after ultrasonic treatment.     

In situ synthesis and hydrogen storage properties of PdNi alloy nanoparticles in an ordered mesoporous carbon template

Organized mesoporous carbon has been used as a nanoreactor to prepare PdNi metallic particles using an incipient wetness method starting from Pd and Ni salts. The final composite material consists of nanosized metallic particles of an alloy with composition Pd_0.60Ni_0.40 highly dispersed within the carbon host structure. The thermodynamic hydrogenation properties of both the PdNi-free OMC and the Pd_0.60Ni_0.40-OMC composite have been determined by hydrogen isotherm sorption measurements. The introduction of the palladium–nickel alloy into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 2 MPa, since the hydrogen uptake is mainly attributed to physisorption on the carbon surface. However, at room temperature and moderate pressure (0.5 MPa), the filling of the OMC with nanocrystalline Pd_0.60Ni_0.40 results in larger hydrogen uptake than that of the PdNi-free OMC.     

基于活性碳纤维毡复合吸附剂的储热性能

研制了一种以活性碳纤维毡为基质、氯化锂（LiCl）为吸湿盐的复合吸附剂,并辅以硅溶胶进行固化成型。该复合吸附剂可用于以水为吸附质的热化学吸附储热系统,并对其微观结构、吸附性能和储热性能进行了表征研究。制备了不同含盐量的复合吸附剂样品,并根据样品的溶液泄漏现象,确定ACFLi30为最佳样品。通过试验测量,获得ACFLi30样品的热导率、孔比表面积、孔体积和孔径等参数。并对多种温湿度工况下的平衡和动态吸附性能进行测试,研究了不同温湿度条件下样品的吸附特性。结果表明样品的吸水量可达1.1 g/g（20℃、75% RH）。利用同步热分析仪测试了复合吸附剂的储热密度,ACFLi30的质量和体积储热密度分别达到1.08 kW·h/kg和588.2 kW·h/m³。与膨胀蛭石和活性氧化铝等基质相比,活性碳纤维毡基质在体积储热密度更具优势。     

碳纤维复合材料高压储氢容器研究与结构设计

氢能的储运技术是氢能利用的关键环节之一。对车用燃料氢气的储存技术进行了综述和比较,提出高压储氢是氢气储存的主要方向之一。阐述了碳纤维复合材料高压容器的技术原理和结构特点,通过改进压力容器的结构形式,达到防止氢气渗漏、延长容器使用寿命、提高系统储氢量等综合性能。     

Removal of sulfur compounds from utility pipelined synthetic natural gas using modified activated carbons

Synthetic natural gas (SNG), which is produced from petroleum and distributed via pipeline in Honolulu by The Gas Company, was analyzed using a gas chromatograph equipped with a sulfur chemiluminescence detector (GC/SCD). Hydrogen sulfide (H₂S), methyl mercaptan (MM), ethyl mercaptan (EM), dimethylsulfide (DMS), dimethyl disulfide (DMDS), tetrahydrothiophene (THT), ethyl disulfide (EDS), and one unidentified compound (UN1) were detected. Among these sulfur compounds, THT is added as an odorant and was present in the highest concentration.A commercial activated carbon (Calgon OLC plus 12X30) was modified by oxidation and impregnation methods and the resulting materials were evaluated for their ability to adsorb sulfur compounds present in SNG. The evaluation results indicate that all of the modification methods can improve the retention of individual sulfur compounds or the total sulfur capacity compared with the untreated virgin carbon. It is also found that activated carbons impregnated with metal impurities have different selectivity for sulfur compounds. Cu and Zn loaded carbons had the highest capacity for H₂S removal, Fe loaded carbon was more efficient for DMS removal (the most difficult S compound to remove), and carbon oxidized by HNO₃ was the best for THT removal.Based on these findings, a composite sorbent consisting of Cu loaded and Fe loaded carbons was designed and tested. The test results indicate that the composite sorbent had improved performance in the removal of individual sulfur compound. A linear programming model was used to design a composite sorbent optimized to minimize the required sorbent mass based on a 1-kW scale fuel cell system service target. Validation tests showed that the optimized sorbent required less of the individual modified carbon components than when they were individually used for the same sulfur removal target.     

Effect of morphology on the performance of metal-hydride electrodes

Electrochemical studies on AB₂ type Zr_0.5Ti_0.5V_0.6Cr_0.2Ni_1.2 metal hydride electrodes with varying particle size suggests that the electrodes with alloy particles of about 60 m yield the optimum performance. The values for diffusion coefficient of hydrogen in the alloy particles > 25 m are found to be nearly invariant. Both a.c. impedance and linear polarization data on electrodes with varying particle size suggest that the charge-transfer resistance depends on state-of-charge of the electrodes. A comparison of scanning electron micrographs of fresh electrodes and subsequent to their prolonged charge-discharge cycling suggests that the metal hydride particles develop stress-induced cracks owing to their inherent expansion and contraction during the hydriding/dehydriding processes.     

Purification of Polluted Source Water with Ozonoation and Biological Activated Carbon

In order to determine effective processes for purifying polluted source waters 1n Harbin City, various processes consisting of ozonation, sand filtration, and/or GAC filtration and adsorption, i.e., ozonation ( “O₃” Process), ozonatlon/sand filtration ( “O₃ + SF” Process), ozonation/biological activated carbon ( “O₃ + BAC” Process), ozonation/sand filtration/biological activated carbon ( “O₃ + SF + BAC” Process), and granular activated carbon (“GAC” Process) were tested In an 8 m³/d capacity pilot plant. In addition, a small plant of 500 L/d capacity was used to conduct comparative studies between the two processes “GAC” and “O₃ + BAC”, as well as two types of carbon.     

等离子体法制备掺杂Ni碳材料的储氢性能

引言氢能是可再生的理想洁净能源,人类出于对环境的保护和化石燃料趋于短缺的考虑,太阳能和氢能的利用已是本世纪能源领域发展的必然趋势。特别是在燃料电池迅速发展并获得突破的今天,更安全、灵活和有效的氢能储藏技术的研究受到全球的广泛关注[1]。在吸附储氢材料中,碳质材料由于其质量轻、比表面积高和合适的结构引起了广泛的重视[2-10]。Yang等[11-12]发现在碳上掺杂过渡金属(Rh、Pt、     

Alkali-activated waste ceramics: Importance of precursor particle size distribution

The waste ceramics belongs to wide range of aluminosilicate materials which can be alkaline-activated to geopolymer cement – possible “green” alternative to conventional Portland cement. The studied ceramic material is generated during the size adjustment of ceramic building blocks by means of grinding. It means that most of the material is very fine, but it contains also some larger shards. This ceramic powder was used as geopolymer precursor “as received” and after removal of particles retained on 1, 0.5 and 0.125 mm sieves. These four types of precursor were activated by sodium silicate (SiO₂/Na₂O = 1) solution. The prepared mortars were tested for strength, basic physical properties, transport parameters and characterized by help of XRD and thermal analysis. It was found that the best mechanical performance provided the precursor after removal of particles retained on 0.5 mm sieve thanks to the highest geopolymerization rate. The presence of coarser particles in precursor gave rise to porosity, what consequently influenced transport parameter of geopolymers towards the lower thermal conductivity and faster moisture transport.     

Low temperature electrochemical properties of LaNi4.6−xMn0.4Mx (M = Fe or Co) and effect of oxide layer on EIS responses in metal hydride electrodes

Iron is a key element in the development of Co-free AB₅-type hydrogen storage alloys. The aim of this work is to systematically investigate the effects of Fe and Co on the electrochemical properties of LaNi_4.6−xMn_0.4M_x (M = Fe or Co, x = 0, 0.25, 0.5 and 0.75) hydrogen storage alloys under relatively low temperatures (273, 253 and 233 K). The results showed that substitution of Fe for Ni reduced the low temperature electrochemical performance much more seriously than that of Co. Exchange current density (I₀), charge-transfer resistance (R_ct) and hydrogen diffusion coefficient (D) were determined based on the study of linear polarization, electrochemical impedance spectrum (EIS) and galvanostatic discharge, respectively. Both the hydrogen diffusion in the bulk of alloy particles and the electrochemical reaction at the alloy electrolyte interface were found to be greatly limited as the decrease of temperature. During the EIS analysis, interestingly, we found that the semicircle in the high frequency region increased dramatically with the decrease of temperature. The electrochemical process corresponding to this semicircle was proposed to be related to the oxide layer on the surface of alloy particles. Novel explanations of EIS response in metal hydride electrodes were proposed accordingly.     

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.     

Chloromethylated Activated Carbon: A Useful New Synthon for Making a Novel Class of Sorbents for Heavy Metal Separations

The chloromethylation of activated carbon is described. Chloromethylation was found to produce a carbon derivative with a surface area of 1310 m²/g and no significant change in the pore structure. The product was found to contain ～1.5 mmole of [sbnd]CH₂Cl groups per g of material, similar to the functional density reported in the original Merrifield resin synthesis. Displacement of the benzylic chloride was achieved by treating this material with an excess of sodium thiosulfate in refluxing aqueous methanol. The resulting Bunte salt was then hydrolyzed by treatment with warm 3 M HCl to afford the corresponding thiol (“AC-CH₂-SH”) cleanly and in high yield. AC-CH₂-SH was found to be an effective heavy metal sorbent, efficiently capturing Hg, Pb, Ag, and Cu. Sorption kinetics were rapid, with equilibrium achieved in less than 30 minutes.     

The effect of processing conditions on microstructure of Pd-containing activated carbon fibers

Palladium-doped activated carbon fibers are being evaluated as candidate materials for enhanced hydrogen storage at near ambient conditions. Pd-doped fibers were spun using a Pd salt mixed with an isotropic pitch precursor. Experimental techniques such as in situ X-ray analysis, thermogravimetric studies, scanning transmission electron microscopy and gas adsorption were employed to understand how processing conditions for the production of Pd-doped activated carbon fibers affect the microstructure, pore development, and dispersion of metal particles throughout the fibers. The results showed that PdO phase is present in the stabilized fibers and that this oxide phase is stable up to about 250 °C. The oxide phase transforms into Pd metal with increasing heat treatment temperature, going through the formation of an intermediate carbide phase. Sintering of Pd particles was observed with heat treatment at temperatures over 750 °C. It was also found that pore development during physical activation with CO₂ was not significantly affected by the presence of Pd particles within the fibers.     

Single Pd atoms in activated carbon fibers and their contribution to hydrogen storage

Palladium-modified activated carbon fibers (Pd-ACF) were synthesized by melt-spinning, carbonization and activation of an isotropic pitch carbon precursor premixed with an organometallic Pd compound. The hydrogen uptake at 25 °C and 20 bar on Pd-ACF exceeded the expected capacity based solely on Pd hydride formation and hydrogen physisorption on the microporous carbon support. Aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom spatial resolution provided unambiguous identification of isolated Pd atoms occurring in the carbon matrix that coexist with larger Pd particles. First principles calculations revealed that each single Pd atom can form Kubas-type complexes by binding up to three H₂ molecules in the pressure range of adsorption measurements. Based on Pd atom concentration determined from STEM images, the contribution of various mechanisms to the excess hydrogen uptake measured experimentally was evaluated. With consideration of Kubas binding as a viable mechanism (along with hydride formation and physisorption to carbon support) the role of hydrogen spillover in this system may be smaller than previously thought.     

Carbon nanotube growth from alkali metal salt nanoparticles

We demonstrate that alkali metal salts, including KCl, NaCl, K₂SO₄, Na₂SO₄, K₂CO₃, and Na₂CO₃, can act as catalysts for carbon nanotube (CNT) growth in chemical vapor deposition (CVD). The solution of alkali metal salt, water and ethanol was nebulized and was introduced into the CVD reactor, producing CNT with a multi-walled structure. Individual CNT are terminated with an onion-shaped carbon tip even when different alkali metal salt catalysts are used. Through observation and analysis of the catalyst particles and the resulting product, we elucidate the mechanism by which the alkali metal salt nanoparticles are served as “seeds” and provide nucleation sites for CNT growth. The ethanol decomposes to release carbon atoms into the catalyst particles, and the carbon nucleates and then begins to assemble on the surface of the catalyst particles, resulting in the CNT growth. By altering growth conditions, branched CNT and single-walled CNT also can be grown on alkali metal salt nanoparticles.     

活性氧化铝基质新型复合吸附剂的制备和储热性能

研制了一种以活性氧化铝为基质、CaCl₂为吸湿盐的新型复合吸附剂,可用于以水为吸附质的热化学吸附储热系统,并对其内部结构、吸附性能和储热性能进行了研究。利用恒温恒湿箱确定出现溶液泄漏现象的最大含盐量,并对30℃和多种相对湿度工况下的动态和平衡吸附特性进行测量,研究了含盐量和相对湿度对吸附剂的吸附特性的影响,结果表明含盐量和相对湿度越大,复合吸附剂的吸水能力越强。利用全自动比表面积与孔隙度分析仪测量材料的比表面积和孔体积,利用同步热分析仪测试了复合吸附剂的储热密度,其中含盐量最高的复合吸附剂的储热密度最高,质量和体积储热密度分别达到0.51 kW·h/kg和610.2 kW·h/m³,具有良好的储热性能。     

Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents

A series of templated carbons with various high surface areas (2033–3798 m²/g) have been prepared using various microporous zeolites as hard templates. Molecular hydrogen storage and spiltover hydrogen storage on these templated carbons were investigated and compared with superactivated carbon AX-21 and other reported porous carbon sorbents at 298 K and 100 atm. Two relationships between the surface areas of these carbons and their hydrogen capacities were obtained. The relationship between molecular hydrogen capacity and surface area showed a 0.23 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, indicating that merely increasing surface areas of the carbon sorbents cannot achieve a significant molecular hydrogen capacity at ambient temperature. Spiltover hydrogen storage was achieved by doping Pt nanoparticles (as dissociative hydrogen source) on these carbons (spiltover hydrogen receptor). Our first result on the relationship between the spiltover hydrogen capacity and surface area showed 0.4 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, which indicated that storage via spillover can lead to an average of 70% enhancement compared to molecular hydrogen storage.     

Metal-assisted hydrogen storage on Pt-decorated single-walled carbon nanohorns

The catalytic dissociation of hydrogen molecules by metal nanoparticles and spillover of atomic hydrogen onto various supports is a well-established phenomenon in catalysis. However, the mechanisms by which metal catalyst nanoparticles can assist in enhanced hydrogen storage on high-surface area supports are still under debate. Experimental measurements of metal-assisted hydrogen storage have been hampered by inaccurate estimation of atomically stored hydrogen deduced from comparative measurements between metal-decorated and undecorated samples. Here we report a temperature cycling technique combined with inelastic neutron scattering (INS) measurements of quantum rotational transitions of molecular H₂ to more accurately quantify adsorbed hydrogen aided by catalytic particles using single samples. Temperature cycling measurements on single-wall carbon nanohorns (SWCNHs) decorated with 2–3 nm Pt nanoparticles showed 0.17% mass fraction of metal-assisted hydrogen storage (at ≈0.5 MPa) at room temperature. Temperature cycling of Pt-decorated SWCNHs using a Sievert’s apparatus also indicated metal-assisted hydrogen adsorption of ≈0.08% mass fraction at 5 MPa at room temperature. No additional metal-assisted hydrogen storage was observed in SWCNH samples without Pt nanoparticles cycled to room temperature. The possible formation of C–H bonds due to spilled-over atomic hydrogen was also investigated using both INS and density functional theory calculations.