Improved hydrogen adsorption of ZnO doped multi-walled carbon nanotubes

Hydrogen storage is still one of the most important problems to improve hydrogen energy usage widespread. New materials capable of storing hydrogen with high efficiency must be introduced to overcome this problem. In recent years, addition of metals or inorganic compounds to multiwalled carbon nanotubes (MWCNTs) has been generally used for hydrogen uptake studies to enhance adsorption property of the nanotubes. In this study, Zinc oxide (ZnO) nanoparticles doped MWCNTs (ZnO-MWCNTs) have been produced as new reversible hydrogen storage materials, and we have investigated characterization of ZnO-MWCNTs by XRD, SEM, TGA, TEM and BET analyses. The functionalized MWCNTs and ZnO doped MWCNTs were subjected to hydrogenation step by dynamic gas sorption analyser under pressure of 5–50 bar. The hydrogen uptake capacities of the materials under different pressures were measured gravimetrically. It was indicated that by controlling the pressures for hydrogenation of ZnO-MWCNTs induces the spillover of ZnO nanoparticles in the layer of MWCNTs which in return with high hydrogen adsorption capacity. Consequently, the hydrogen adsorption of the functionalized MWCNTs (f-MWCNTs) and the ZnO-MWCNTs were achieved to be 1.05 wt% and 2.7091 wt% under pressure of 50 bar as maximum.     

Synergistic effects of multiwalled carbon nanotubes and Al on the electrochemical hydrogen storage properties of Mg2Ni-type alloy prepared by mechanical alloying

Mg_2−xAl_xNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by XRD, SEM and TEM. XRD analysis results indicate that Al substitution results in the formation of AlNi-type solid solution that can interstitially dissolve hydrogen atoms. In contrast, the addition of MWCNTs hardly affects the XRD patterns. SEM observations show that after co-milling with 5 wt. % MWCNTs, the particle sizes of both Mg₂Ni and Mg_1.75Al_0.25Ni milled alloys are decreased explicitly. The TEM images reveal that ball milling is a good method to cut long MWCNTs into short ones. These MWCNTs aggregate along the boundaries and surfaces of milled alloy particles and play a role of lubricant to weaken the adhesion of alloy particles. The majority of MWCNTs retain their tubular structure after ball milling except a few MWCNTs whose tubular structure is destroyed. Electrochemical measurements indicate that all milled alloys have excellent activation properties. The Mg_1.75Al_0.25Ni-MWCNTs composite shows the highest discharge capacity due to the synergistic effects of MWCNTs and Al on the electrochemical hydrogen storage properties of Mg₂Ni-type alloy. However, the improvement on the electrode cycle stability by adding MWCNTs is unsatisfactory.     

Enhancement of hydrogen storage capacity of multi-walled carbon nanotubes with palladium doping prepared through supercritical CO2 deposition method

Pd doped Multi-Walled Carbon Nanotubes were prepared via supercritical carbon dioxide deposition method in order to enhance the hydrogen uptake capacity of carbon nanotubes at ambient conditions. A new bipyridyl precursor that enables reduction at moderate conditions was used during preparation of the sample. Both XRD analyses and TEM images confirmed that average Pd nanoparticle size distribution was around 10 nm. Hydrogen adsorption and desorption experiments at room temperature with very low pressures (0–0.133 bar) were conducted together with temperature programmed desorption (TPD) and reduction (TPR) experiments on undoped and doped materials to understand the complete hydrogen uptake profile of the materials. TPD experiments showed that Pd nanoparticles increased the hydrogen desorption activity at moderate temperatures around at 38 °C while for undoped materials it was determined around at 600 °C. Moreover, a drastic enhancement of hydrogen storage was recorded from 44 μmol/g sample for undoped material to 737 μmol/g sample for doped material through adsorption/desorption isotherms at room temperature. This enhancement, also verified by TPR, was attributed to spillover effect.     

Beryllium-doped single-walled carbon nanotubes with Stone-Wales defects: A promising material to store hydrogen at room temperature

Hydrogen storage in single-walled carbon nanotubes containing the Stone-Wales defects and doped with metal atoms (titanium and beryllium) has been studied using molecular dynamics simulations and density functional theory calculations. Although, Be is known to be toxic at high temperatures, Be-doped SWCNT shows a promising potential to exceed the DOE target at moderate temperatures and pressures. One of the major advantages of doping Be is its lower atomic weight, which increases the gravimetric storage capacity compared to SWCNTs doped with heavy-wight Ti atoms. In addition, the binding energy of Be is higher than that of Ti, which enhances the capture of hydrogen molecules. The gravimetric and volumetric storage capacities depend not only on the dopant atom but also on the location of doping. SWCNTs in which Be is doped on the octagonal ring of the Stone-Wales defects exhibits higher storage capacity than Be doped on defect-free SWCNTs. At room temperature (298 K), the storage capacity of Be-doped SWCNT containing the Stone-Wales defect exceeds the DOE target of 5.5 wt% (gravimetric) and 40 g H₂/L (volumetric) at a pressure of 267 atm, which is significantly lower than that used in high pressure vessels.     

The effect of various acids treatment on the purification and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of HCl, HNO₃, H₂SO₄ and HF acids on the purification and the electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) were studied. The MWCNTs were synthesized on Fe–Ni catalyst by thermal chemical vapor deposition method. The X-ray diffraction and thermal gravimetric analysis results indicated that the MWCNTs purified by HF acid had the highest impurities as compared with the other acids. The N₂ adsorption results at 77 K indicated that all the samples were mainly mesoporous and the purified MWCNTs by HF acid had the highest surface area as compared with the other acids. The hydrogen storage capacities of the purified MWCNTs by the following acids were in ascending order as: H₂SO₄, HCl, HNO₃ and HF. It was found that the 1–2 nm micropores in the MWCNTs are very important for hydrogen storage. Further, the presences of catalyst and defective sites in MWCNTs influence the hydrogen storage capacity.     

Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites

Hydrogen storage capacity of a pristine multi-walled carbon nanotubes is increased 10-fold at 298 K and an equilibrium hydrogen pressure of ～23 atm, upon addition of a hydrogen spillover catalyst cobalt- and copper oxide, from 0.09 to 0.9 wt.%. In situ reduction method is utilized to synthesize Co-oxide/MWCNTs and Cu-oxide/MWCNTs composite. Blocking of channels and pores of MWCNTs by oxide nanoparticles during preparation method is responsible for low BET specific surface area of composites compared to pristine sample. X-ray diffraction, scanning, and transmission electron microscopy demonstrates nanostructural characterization of MWCNTs and composites. Thermogravimetric analysis of two oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage of both composites is attributed to the spillover mechanism due to decoration of Co and Cu-oxide nanoparticles on the outer surface of MWCNTs.     

Hydrogen uptake of Ti-decorated multiwalled carbon nanotube composites

In this study, the hydrogen storage capacity of purified multiwalled carbon nanotubes (MWCNTs) was enhanced from 13- to 15-fold at a temperature of 298 K and pressure of 2.0 MPa, upon incorporation of 1.57–1.88 wt% of ultrafine Ti nanoparticles. The effect of a hydrogen spillover Ti catalyst on MWCNTs prepared using the sputtering method was investigated. A comparison between the hydrogen uptake by MWCNTs sputtered with Ti for 3000 s and that for 6000 s was also performed using the Sievert's volumetric apparatus. The significant enhancement in hydrogen uptake was attributed to the interfacial diffusion of hydrogen from Ti to the MWCNTs. The re-adsorption of hydrogen on the pristine MWCNTs and Ti-decorated MWCNTs dehydrogenated at 200 °C indicated that the samples did not compromise their reversible hydrogen uptake during the hydrogenation–dehydrogenation cycles. It was also found that longer exposure of Ti to the MWCNTs during sputtering improved the hydrogen storage capacity. This improvement could be attributed to the presence of a higher amount of Ti deposited on the MWCNTs, as indicated by thermogravimetric analysis study.     

Enhanced hydrogen storage performance in Pd3Co decorated nitrogen/boron doped graphene composites

In order to harness the potential of hydrogen as an alternative energy carrier, overcoming the barrier related to its storage is of utmost importance. In this direction, it has been shown that dissociation of hydrogen molecules into atoms followed by their adsorption onto high surface area nanomaterials like reduced graphene oxide is a promising pathway. In the present study, we have exploited this pathway, commonly known as the “spillover mechanism” and achieved a hydrogen storage capacity of ～4.6 wt% at 30 bar and 25 °C in Pd₃Co decorated boron doped graphene composite. We demonstrate that optimum loading of transition metal alloy nanoparticles coupled with heteroatom (nitrogen or boron) doped graphene support is an efficient, easy and cost effective avenue towards meeting the US department of energy (DOE) targets for gravimetric hydrogen storage capacity at room temperature and moderate pressures.     

A study on the effects of Fex/Niy/MgO(1−x−y) catalysts on the volumetric and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of various ratios of Fe/Ni/MgO and growth temperatures on yield, diameter and quality of multi-walled carbon nanotubes (MWCNTs) were studied. Thermal gravimetric analysis (TGA) confirmed that the MWCNT yield depends on Fe/Ni ratio with the following order; Fe_0.5 Ni_0.5 > Fe > Fe_0.75 Ni_0.25 > Fe_0.25 Ni_0.75 > Ni. The results indicated that there is an optimum temperature (940 °C) for the MWCNT growth both from quality and quantity (yield) aspects as compared to other temperatures. Moreover, the changes on Fe/Ni to MgO ratio for the MWCNT growth revealed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest quality and surface area as compared to the other ratios. The hydrogen storage capacities of MWCNTs grown on Fe/Ni/MgO with various ratios obtained by using volumetric technique were in ascending order as 17.5/17.5/65, 15/15/70, 12.5/12.5/75, 10/10/80, 20/20/60, 22.5/22.5/55 and 25/25/50. In addition, the defective sites and mean diameter of the MWCNTs influenced the desorption temperature of stored hydrogen. Hydrogen storage by using electrochemical technique showed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest hydrogen storage capacity compared with the other ratios. Based on electrochemical analysis, there are two regimes for hydrogen adsorption on the MWCNTs, one at about 0.8 V and the other at 0.15 V. The study on two kinds of adsorption region showed that the ratio of hydrogen storage capacity at 0.8 V to hydrogen storage capacity at 0.15 V increased with the increasing of the mean diameter of MWCNTs. The ratio reached to maximum value for the MWCNTs grown on Fe/Ni/MgO with the ratio of 20/20/60 as compared to the other ratios.     

The effects of bromine treatment on the hydrogen storage properties of multi-walled carbon nanotubes

The effects of bromine treatment on the properties of multi-walled carbon nanotubes (MWCNTs) such as surface porosity, sp² hybridization, functional groups and hydrogen storage capacity were studied and compared with treated MWCNTs by HCl, HNO₃ and H₂SO₄ acids. The treatments affect the graphitization properties (sp² hybridization) and porous structures of MWCNTs by enlarging the specific surface area and the micro-pore volume. In addition, the hydrogen storage capacity of the treated MWCNTs was also investigated by volumetric technique. It is found that the destroying of sp² hybridization of bromine treated MWCNTs increases hydrogen adsorption sites and decreases hydrogen desorption temperature.     

Thermal Modeling of Mg2Ni-Based Solid-State Hydrogen Storage Reactor

In this paper, a numerical study of coupled heat and hydrogen transfer characteristics in an annular cylindrical hydrogen storage reactor filled with Mg₂Ni is presented. An unsteady, two-dimensional (2-D) mathematical model of a metal hydride reaction bed of cylindrical configuration is developed for predicting the hydrogen storage capacity. The effect of volumetric radiation is accounted in the thermal model. Effects of hydride bed thickness, initial absorption temperature, hydride bed thermal conductivity, and hydrogen supply pressure on the hydrogen storage capacity are studied. A thinner hydride bed is found to enhance the hydriding rate, accomplishing a rapid reaction. At an operating condition of 20 bar supply pressure and 573 K initial absorption temperature, Mg₂Ni stores about 36.7 g hydrogen per kg alloy. For a given bed thickness and an overall heat transfer coefficient, there exists an optimum value of hydride bed thermal conductivity. The present numerical results are compared with the experimental data reported in the literature, and good agreement was observed.     

Synthesis and electrochemical properties of binary MgTi and ternary MgTiX (X = Ni,Si) hydrogen storage alloys

Mg-based hydrogen storage alloys are promising candidates for many hydrogen storage applications because of the high gravimetric hydrogen storage capacity and favourable (de)hydrogenation kinetics. In the present study we have investigated the synthesis and electrochemical hydrogen storage properties of metastable binary Mg_yTi_1?y (y = 0.80–0.60) and ternary Mg_0.63Ti_0.27X_0.10 (X = Ni and Si) alloys. The preparation of crystalline, single-phase, materials has been accomplished by means of mechanical alloying under controlled atmospheric conditions. Electrodes made of ball-milled Mg_0.80Ti_0.20 powders show a reduced hydrogen storage capacity in comparison to thin films with the same composition. Interestingly, for a Ti content lower than 30 at.% the reversible storage capacity increases with increasing Ti content to reach a maximum at Mg_0.70Ti_0.30. The charge transfer coefficients (α) and the rate constants (K₁ and K₂) of the electrochemical (de)hydrogenation reaction have been obtained, using a theoretical model relating the equilibrium hydrogen pressure, electrochemically determined by Galvanostatic Intermittent Titration Technique (GITT), and the exchange current. The simulation results reveal improved values for Mg_0.65Ti_0.35 compared to those of Mg_0.80Ti_0.20. The addition of Ni even more positively affects the hydrogenation kinetics as is evident from the increase in exchange current and, consequently, the significant overpotential decrease.     

The mechanism and sorption kinetic analysis of hydrogen storage at room temperature using acid functionalized carbon nanotubes

The present work investigates the effect of acid functionalization of multiwalled carbon nanotubes (MWCNTs) on the physisorption based mechanism of hydrogen storage at room temperature. For this purpose, a suite of functionalized CNT samples is synthesized and subjected to a comprehensive range of material characterization techniques and hydrogen storage measurements. Nitric acid (HNO₃) and the mixture of sulphuric acid and nitric acid (H₂SO₄:HNO₃) are used for the synthesis at oxidation temperatures of 80 °C and 100 °C. Electron microscopy and X-ray photoelectron spectroscopy results reveal that acid functionalization causes major alternation in the physicochemical properties of the CNTs due to the varied concentration of oxygen functional groups. Particularly, the H₂SO₄:HNO₃ functionalized sample at 100 °C is found to have the highest interlayer spacing, oxygen to carbon ratio (26.09 at. %), defect content, and specific surface area (215.3 m²/g). These features collectively contribute to substantially improved hydrogen storage properties, including a ～150% increase in the hydrogen storage capacity at 298 K and 50 bar. Furthermore, kinetic analysis shows that the desorption follows a multiple diffusion process which is sensitive to the oxygen functional groups and structural defects, hence reducing the rate of desorption; whereas the adsorption is controlled by a more rapid, three-dimensional diffusion process.     

Hydrogenation behavior of high-energy ball milled amorphous Mg2Ni catalyzed by multi-walled carbon nanotubes

Amorphous Mg₂Ni alloy was successfully synthesized by means of mechanical alloying. Then, the multi-walled carbon nanotubes (MWCNTs) were added by high-energy ball milling to catalyze the amorphous alloy. The X-ray diffraction (XRD) spectroscopy reveal that the as-cast Mg₂Ni alloy has presented a completely amorphous state under specific conditions of high-energy ball milling process. Different process parameters of ball-to-powder ratio (10:1, 20:1, 40:1) and milling time have been attempted for the preparation of amorphous Mg₂Ni alloy. The results show that the milling time and ball-to-powder weight ratio have significantly influence on the amorphization process of crystalline Mg₂Ni alloy. Before and after the milling, phase compositions and microstructures of the prepared materials were characterized by XRD, scanning electron microscope (SEM), electron energy dispersion spectrum (EDS) and transition electron microscope (TEM) approaches. The morphology of composite Mg₂Ni/MWCNTs was investigated, the TEM images show that the MWCNTs imbed on the surface of the particles after milling for 1 h, and the MWCNTs with and without tubular structure have been observed. The hydrogen storage properties of amorphous Mg₂Ni alloys were improved by the catalytic effect of MWCNTs. The catalytic effect and mechanism of MWCNTs on the hydrogen storage properties of amorphous Mg₂Ni alloy are discussed and investigated.     

Enhanced hydrogen generation by hydrolysis of LiBH4 doped with multiwalled carbon nanotubes for micro proton exchange membrane fuel cell application

LiBH₄ has a high hydrogen storage capacity and could potentially serve as a superior hydrogen storage material. However, during the hydrolysis process for hydrogen generation, the agglomeration of the hydrolysis product of LiBH₄ limits its full utilization. In order to completely release the stoichiometric amount of H₂ from LiBH₄ hydrolysis, multiwalled carbon nanotubes (MWCNTs) were doped with LiBH₄ by mechanical milling. The results show that MWCNT carried LiBH₄ can slowly react with water vapor at room temperature which is 25 °C lower than the reaction temperature of neat LiBH₄. Agglomeration can be avoided when the addition of MWCNTs exceeds 7 wt.%, which results in a complete hydrolysis process. The total hydrogen capacity is 7.5 wt.%. The enhanced hydrolysis of LiBH₄ can be attributed to the MWCNTs which increased the contact areas between LiBH₄ and water and created gas channels for hydrogen diffusion. The performance of a micro proton exchange membrane fuel cell connected to MWCNT-doped LiBH₄ powder packed-bed reactor was examined. The result demonstrates that doping with MWCNTs enhanced the hydrogen generation of LiBH₄ hydrolysis. MWCNT-doped LiBH₄ can be applied as hydrogen source of fuel cells.     

Hydrogen storage in platinum loaded single-walled carbon nanotubes

To study the hydrogen storage capacity, platinum (Pt) nanoparticles were deposited on single-walled carbon nanotubes (SWNT) using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as a precursor. To verify Pt deposition on the surface of the SWNT, a Transmission Electron Microscope (TEM) was used to obtain surface morphology. The TEM images show that Pt nanoparticles were homogeneously distributed on the surface of SWNT. Commercial SWNT were also used to compare the results. Thermal Gravimetric Analysis at heating rate of 5 °C/min is measured for pure SWNT and Pt loaded SWNT. Before hydrogen storage measurements these samples were reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 hour. Hydrogen storage capacity of these SWNT was investigated under 25 bar pressure and room temperature as well as liquid nitrogen temperature.     

Impact of surface modifications on hydrogen uptake by Fe@f-MWCNTs and Cu@f-MWCNTs at non-cryogenic temperatures

A comprehensive study has been conducted to evaluate the hydrogen uptake capacity of carboxylate functionalized multi-walled carbon nanotubes (f-MWCNTs) after strategic incorporation of Fe and Cu nanoparticles on the surface. Metal decorated multi-walled carbon nanotubes (Fe@f-MWCNTs and Cu@f-MWCNTs) were prepared by refluxing various concentrations of metal precursor and f-MWCNTs in different reaction medium such as water, amine and DMF. The prepared materials were characterized by FT-IR, powder XRD, SEM, TEM and BET analyzer. The adsorption isotherms revealed that the hydrogen storage capacity of Fe@f-MWCNTs and Cu@f-MWCNTs was 0.55 and 0.68 wt%, respectively, at 253 K and 70 bar. Similarly, both compounds showed 0.39 and 0.5 wt% adsorption at 298 K and 70 bar, respectively. The uptake of hydrogen by metal decorated multi-walled carbon nanotubes was remarkably enhanced by a factor of 2 and 5 times that of Pristine MWCNT at 253 K and 298 K, respectively.     

Enhanced hydrogen storage capacity of Pt-loaded CNT@MOF-5 hybrid composites

We report on an easy synthesis method for the preparation of a hybrid composite of Pt-loaded MWCNTs@MOF-5 [Zn₄O(benzene-1,4-dicarboxylate)₃] that greatly enhanced hydrogen storage capacity at room temperature. To prepare the composite, we first prepared Pt-loaded MWCNTs, which were then incorporated in-situ into the MOF-5 crystals. The obtained composite was characterized by various techniques such as powder X-ray diffractometry, optical microscopy, porosimetry by nitrogen adsorption, and hydrogen adsorption. The analyses confirmed that the product has a highly crystalline structure with a Langmuir specific surface area of over 2000 m²/g. The hybrid composite was shown to have a hydrogen storage capacity of 1.25 wt% at room temperature and 100 bar, and 1.89 wt% at cryogenic temperature and 1 bar. These H₂ storage capacities represent significant increases over those of virgin MOF-5s and Pt-loaded MWCNTs.     

Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures

In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi₅ utilizing the hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the effect of hydrogen pressure on the ignition temperature and the initial activation of hydrogenation. In the experiments, La₂O₃, Ni, and Ca were dry-mixed, and then heated at 0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from bulk to powder with hydrogen pressure. This was probably caused by volume expansion due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The results of this research can be applied as an innovative production route for LaNi₅ without the conventional melting of La and Ni.     

Hydrogen storage properties of nanocrystalline and amorphous Pr–Mg–Ni-based alloys synthesized by mechanical milling

To improve the hydrogen storage properties of PrMg₁₂ alloy, one Mg atom was replaced by one Ni atom in the alloy. And then the PrMg₁₁Ni alloy was milled with two or three times Ni powers. The as-milled PrMg₁₁Ni + x wt.% Ni (x = 0, 100, 200) (PrMg₁₁Ni-xNi (x = 0, 100, 200)) alloys possess a nanocrystalline and amorphous structure. The gaseous hydrogen storage properties were tested by Sievert apparatus and Differential scanning calorimetry (DSC) connected with a H₂ detector. The results indicate that increasing Ni content have obvious improvement effects on the gaseous hydrogen storage kinetics and thermodynamics for both the hydrogenation and dehydrogenation reaction. Furthermore, the variation of milling time also has an obvious influence on gaseous hydrogen storage properties of the alloys. The hydrogen storage capacity firstly increases and decreases again with prolonging milling time, and the peak values are 5.859 wt.% for the x = 100 alloy and 6.164 wt.% for the x = 200 alloy. The hydrogen absorption rates have similar trends, but the hydrogen desorption rates always increase. The hydrogen desorption activation energy markedly lowers with Ni content and milling time increasing, which is the real reason for the improvement of hydrogen desorption kinetics.