Experimental and first principle studies on hydrogen desorption behavior of graphene nanofibre catalyzed MgH2

With the combination of experiment and first-principles theory, we have evaluated and explored the catalytic effects of graphitic nanofibres for hydrogen desorption behaviour in magnesium hydride. Helical form of graphene nanofibres (HGNF) have larger surface area, curved configuration and high density of graphene layers resulting in large quantity of exposed carbon sheet edges. Therefore they are found to considerably improve hydrogen desorption from MgH₂ at lower temperatures compared to graphene (onset desorption temperature of MgH₂ catalyzed by HGNF is 45 °C lower as compared to MgH₂ catalyzed by graphene). Using density functional theory, we find that graphene sheet edges, both the zigzag and armchair type, can weaken MgH bonds in magnesium hydride. When the MgH₂ is catalyzed with higher electronegative and reactive graphene edge of graphene, the electron transfer occurs from Mg to carbon, due to which MgH₂ is dissociated into hydrogen and MgH component. The Mg gets bonded with the graphene edge carbon atoms in the form of CMgH and CH bonds. In the as formed CMgH, the graphene edges “grab” more electronic charge as compared to the normal charge donation of Mg to H. This leads to the weakening of the MgH bond, causing hydrogen to desorbs at lower temperatures.     

Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni₃C, Ni₃N, NiO and Ni₂P) are synthesized by chemical methods. The MgH₂-X (X = Ni₃C, Ni₃N, NiO and Ni₂P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P. The onset dehydrogenation temperatures of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H₂ within 20 min at 300 °C. The activation energies of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 97.8, 100.0, 119.7 and 132.5 kJ mol^?1, respectively. It' found that the MgH₂Ni₃C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.     

The catalytic effect of an inert additive (SrTiO3) on the hydrogen storage properties of 4MgH2Na3AlH6

Investigations on the catalytic effects of a non-reactive and stable additive, SrTiO₃, on the hydrogen storage properties of the 4MgH₂Na₃AlH₆ destabilized system were carried out for the first time. The Na₃AlH₆ compound and the destabilized systems used in the investigations are prepared using ball milling method. The doped system, 4MgH₂Na₃AlH₆SrTiO₃, had an initial dehydrogenation temperature of 145 °C, which 25 °C lower as compared to the un-doped system. The isothermal absorption and desorption capacity at 320 °C has increased by 1.2 wt% and 1.6 wt% with the addition of SrTiO₃ as compared to the 4MgH₂Na₃AlH₆ destabilized system. The decomposition activation energy of the doped system is estimated to be 117.1 kJ/mol. As for the XRD analyses at different decomposition stages, SrTiO₃ is found to be stable and inert. In addition to SrTiO₃, similar phases are found in the doped and the un-doped system during the decomposition and dehydrogenation processes. Therefore, the catalytic effect of the SrTiO₃ is speculated owing to its ability to modify the physical structure of the 4MgH₂Na₃AlH₆ particles through pulverization effect.     

In situ preparation of N doped orthorhombic Nb2O5 nanoplates /rGO composites for photocatalytic hydrogen generation under sunlight

The synthesis of nitrogen doped orthorhombic niobium oxide nanoplates/reduced graphene oxide composites (NNb₂O₅/rGO) and their photocatalytic activity towards hydrogen generation from water and H₂S under natural sunlight has been demonstrated, uniquely. Nanostructured NNb₂O₅/rGO is synthesized by in situ wet chemical method using urea as a source of nitrogen and optimized by varying percentage of graphene oxide (GO). X?ray diffraction (XRD) study reveals that NNb₂O₅ have orthorhombic crystal structure with crystalline size, 35 nm. Further, X?ray photoelectron spectroscopy (XPS) confirm the presence of nitrogen and rGO in NNb₂O₅/rGO nanocomposite. Morphological features of (NNb₂O₅/rGO) were examined by FE?SEM and FE?TEM showed Nb₂O₅ nanoplates of diameter 25–40 nm anchored on 2D rGO. Diffuse reflectance spectra depicts the extended absorbance in the visible region with band gap of 2.2 eV. Considering the band gap in the visible region, the photocatalytic hydrogen generation from water and H₂S has been performed. The 1 wt % rGO hybridized NNb₂O₅ (S2) exhibited superior photocatalytic hydrogen generation (537 μmol/h) from water and (1385 μmol/h) from H₂S under sunlight. The improved photocatalytic activity is attributed due to an extended absorbance in the visible region, modified electronic structure upon doping and formation of well defined NNb₂O₅/rGO interface, provides large surface area, accelerates the supression of electron and hole pairs recombination rate. In our opinion, this works may provides facile route for energy efficient and economic approach for fabrication of NNb₂O₅/rGO nanocomposites as a visible light active photocatalyst.     

Performance of CoNiO spinel oxide coating on AISI 430 stainless steel as interconnect for intermediate temperature solid oxide fuel cell

In order to decrease oxide growth kinetics, maintain suitable conductivity and prevent Cr-volatilization of AISI 430 stainless steels (430 SS) as the interconnect for intermediate temperature solid oxide fuel cells (SOFCs), a CoNiO spinel oxide protective coating has been successfully fabricated on the 430 SS specimen using a simple and cheap process with two steps: 1) electroplation of CoNi alloy layer and 2) pre-oxidation treatment to convert the CoNi alloy into spinel oxide. The CoNiO spinel layer on the 430 SS (CoNiO 430 SS) is dense and uniform with 8–10 μm thickness. And the CoNiO spinel oxide protective coating consists of a main face-centered-cubic (fcc) NiCo₂O₄ spinel phase and a minor fcc NiO phase. Compared with bare 430 SS, the oxidation resistance and the conductivity of the CoNiO 430 SS have been improved remarkably under simulated typical SOFC operating cathode conditions (at 800 °C in air). After an isothermal oxidation test at 800 °C, the area specific resistance (ASR) of CoNiO 430 SS is much lower and stable (0.1 Ω cm² for 100 h and 0.9 Ω cm² for 600 h) than that of bare 430 SS (1.2 Ω cm² for 100 h and 2.4 Ω cm² for 600 h). These performances of CoNiO 430 SS imply that it can be a promising candidate interconnect for solid oxide fuel cell.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

Numerical analysis of H2 formation during partial oxidation of H2SH2O upon activation of oxidizer by an electric discharge

The numerical analysis of H₂ production during partial oxidation of H₂SH₂O in a plug-flow reactor at atmospheric pressure and a rather low temperature (T₀ = 500 K) was conducted, when the oxidizer (oxygen or air) was preliminarily activated by an electrical discharge with different values of reduced electric field and input energy. It was shown that a significant hydrogen yield in flow reactor can be obtained only after ignition of the mixture. The ignition delay length depends on the reduced electric field E/N and input energy E_s in the discharge and is minimal at E/N～8–10 Td for the discharge in oxygen and at E/N～4–10 and 120–150 Td in air discharge, when O₂(a¹Δ_g) mole fraction in the discharge products is maximal. If the H₂SH₂OO₂(air) mixture ignites inside the flow reactor, the mole fraction of hydrogen and its relative yield do not depend on E/N. The relative hydrogen yield increases monotonically with an addition of water to H₂S. It was found, that the approach based on the partial oxidation of the H₂SH₂O mixture upon activation of oxygen by an electric discharge can ensure very low energy cost for H₂ production. The minimum specific energy requirement, obtained for the H₂SO₂ mixture, was found to be 0.83 eV/(molecule H₂) and 0.18 eV/(molecule H₂S) at atmospheric pressure and can be further decreased if the energy released during partial oxidation of H₂S is spent on heating the reagents. The use of air as an oxidizer requires higher energy costs and seems to be less promising.     

Synergic effect of vanadium trichloride on the reversible hydrogen storage performance of the MgMgH2 system

Vanadium trichloride (VCl₃) is one of the best catalysts for the hydrogenation-dehydrogenation MgMgH₂ system. X-ray photoelectron spectroscopy (XPS) has shown that VCl₃ reduced to metallic vanadium during ball milling along with MgH₂. The in-situ-formed metallic vanadium doped over the MgH₂ surface which has shown an excellent catalytic effect on hydrogenation-dehydrogenation of the MgMgH₂ system. The catalyzed surface reduced the activation energies of hydrogenation-dehydrogenation reactions and correspondingly on-set hydrogenation-dehydrogenation temperatures. The microstructural analysis has also shown an excellent grain refinement property of VCl₃ which reduced the crystallite size of MgH₂. The decreased crystallite size decreases the diffusion path length of hydrogen and increases the active surface area which eventually enhances the hydrogenation-dehydrogenation kinetics of MgMgH₂.     

Hydrogen storage properties of nanostructured 2MgH2Co powders: The effect of high-pressure compression

In this study, MgH₂ and Co powders were mechanically milled in the molar ratio 2:1 and compressed to hard-packed cylindrical pellets. The microstructure, phase changes, and hydrogen storage properties of the mechanically milled 2MgH₂Co powder and the 2MgH₂Co compressed pellet were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and synchronous thermal (DSC/TG) analyses. Dehydrogenation of the 2MgH₂Co compressed pellet is mainly due to the decomposition of Mg₂CoH₅ while it is the dehydriding of MgH₂ for the milled 2MgH₂Co powder. Pressure composition absorption isotherms of the 2MgH₂Co powder and 2MgH₂Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg₆Co₂H₁₁ and Mg₂CoH₅ hydride phases. For the compressed 2MgH₂Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H₂/55.70±3.34 kJ/mol H₂ for Mg₂CoH₅ and -81.89±10.39 kJ/mol H₂/74.47±5.27 kJ/mol H₂ for Mg₆Co₂H₁₁. In contrast, hydrogenation/dehydrogenation enthalpies of Mg₂CoH₅ and Mg₆Co₂H₁₁ mechanically milled 2MgH₂Co powder were −73.98±10.1 kJ/mol H₂/71.67±1.38 kJ/mol H₂ and -96.86±8.73 kJ/mol H₂/89.95±10.81 kJ/mol H₂, respectively. Fast hydrogenation was observed in the dehydrided 2MgH₂Co compressed pellet with about 2.75 wt% absorbed in less than 1 min at 300 °C and a maximum hydrogen storage capacity of 4.43 wt% (2.32 wt% for the 2MgH₂Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH₂Co compressed pellet (64.34 kJ-mol⁻¹ H₂) is lower than the mechanically milled 2MgH₂Co powder (73.74 kJ-mol⁻¹ H₂). The results show that mechanical milling followed by high-pressure compression is an efficient method for the synthesis of Mg-based complex hydrides with superior hydrogen sorption properties.     

Study on steam reforming of coal tar over NiCo/ceramic foam catalyst for hydrogen production: Effect of Ni/Co ratio

The effect of Ni/Co ratio on the catalytic performance of NiCo/ceramic foam catalyst for hydrogen production by steam reforming of real coal tar was studied. The NiCo/ceramic foam catalyst was synthesized by deposition-precipitation (DP) method and characterized with different methods. The experiments were conducted in a two-stage fixed-bed reactor. The results showed that the reducibility of the metallic oxides in bimetallic NiCo/ceramic foam catalysts was influenced obviously by the Ni/Co ratio.Both gas and hydrogen yield increased first and then decreased with the decline of Ni/Co ratio, and the highest hydrogen yield of 31.46 mmol g^?1 was obtained when the Ni/Co ratio was 5/5. The lowest coke deposition of 0.34 wt% was generated at the same Ni/Co ratio. The lifetime test showed the catalyst maintained catalytic activity after 14 cycles (28 h), indicating the coal tar steam reforming on NiCo/ceramic foam catalyst is a promising method for hydrogen production.     

Tailoring the hydrogen absorption desorption's dynamics of MgMgH2 system by titanium suboxide doping

Titanium suboxide (TiO) is one of the best catalysts which improved the hydrogen absorption-desorption property of MgH₂ Mg system. The TiO catalyzed Mg MgH₂ have shown a remarkably reduced apparent activation energy and enhanced the hydrogen absorption-desorption kinetics. The X-ray photoelectron spectroscopy (XPS) analysis has indicated that the oxidation state of Ti in TiO remains unchanged during ball milling and hydrogen absorption-desorption of TiO-doped-MgH₂. The X-ray diffraction (XRD) analysis further confirms the XPS result. The TiO has shown the excellent catalytic effect on the MgMgH₂ system which remarkably reduced the hydrogen absorption-desorption temperatures.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

Hydrogen evolution in the dehydrogenation of methylcyclohexane over Pt/CeMgAlO catalysts derived from their layered double hydroxides

Pt/CeMgAl layered double hydroxides with different Ce contents were prepared by one-step co-precipitation method, which underwent calcination and reduction with hydrogen and were finally converted into Pt/CeMgAlO catalysts. These catalysts were tested in the dehydrogenation of methylcyclohexane (MCH) into toluene to produce hydrogen. The addition of CeO₂ promoted the dispersion of Pt and decreased the Pt particle size. During the dehydrogenation reaction, toluene was the only liquid product and its selectivity was higher than 99.9%. MCH conversion increased with the reaction temperature rising. The conversion and hydrogen evolution rate on Pt/Ce₁₄MgAlO350 reached up to 98.5% and 1358.6 mmol/g_Pt/min at 350 °C. Moreover, Pt/CeMgAlO catalysts exhibited no acidity and presented a high anti-coking ability and good stability. These results suggest that Pt/CeMgAlO catalysts have potential industrial application for hydrogen energy utilization.     

Hydrogen-iodide decomposition over PdCeO2 nanocatalyst for hydrogen production in sulfur-iodine thermochemical cycle

A highly active and stable catalyst for hydrogen-iodide decomposition reaction in sulfur-iodine (SI) cycle has been prepared in the form of PdCeO₂ nanocatalyst by sol-gel method with different calcination temperatures (300 °C, 500 °C, and 700 °C). XRD and TEM confirmed a size around 6–8 nm for PdCeO₂ particles calcined at 300 °C. Raman study revealed large number oxygen vacancies in PdCeO₂-300 when compared to PdCeO₂-500 and PdCeO₂-700. With increase in calcination temperature, the average particle size increased whereas the specific surface area and number of oxygen vacancies decreased. Hydrogen-iodide catalytic-decomposition was carried out in the temperature range of 400°C–550 °C in a quartz-tube, vertical, fixed-bed reactor with 55 wt % aqueous hydrogen-iodide feed over PdCeO₂ catalyst using nitrogen as a carrier gas. PdCeO₂-300 showed hydrogen-iodide conversion of 23.3%, which is close to the theoretical equilibrium conversion of 24%, at 550 °C. It also showed a reasonable stability with a time-on-stream of 5 h.     

Fatigue limit of carbon and CrMo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.     

Dehydrogenation steps and factors controlling desorption kinetics of a MgCe hydrogen storage alloy

Experimentally systematical comparisons are carried out in this work to clarify dehydrogenation steps of Mg-based hydrogen storage alloys during the overall desorption process. Different forms of MgH₂CeH_2.73 composite powders are prepared by high energy ball milling, partial dehydrogenation and annealing. For partially dehydrogenated samples, the desorption temperature and desorption activation energy decrease significantly considering the fact that primary-precipitated metal Mg phase on the surface of MgH₂ can act as nucleate precursors. No significant difference in isothermal desorption kinetics is observed for MgH₂CeH_2.73 powders with different grain sizes. However, particle size reduction facilitates desorption at temperatures below 300 °C. As minor Ni is distributed on the surface, both onset and peak temperatures in thermal desorption decrease for MgH₂CeH_2.73 composite. The reduced activation energy by Ni addition is comparable to the value caused by partial dehydrogenation. Recombination of hydrogen atoms plays an important role during dehydrogenation. The obtains in this work can be expected to provide guidelines to improve desorption kinetics of Mg-based alloys.     

Hydrogen storage in MgH2LaNi5 composites prepared by cold rolling under inert atmosphere

Mg-AB₅ composites are promising systems for hydrogen storage applications, due to their possibility of hydrogen cycling at relatively low temperatures. Traditionally, these composites are mainly processed by high-energy ball milling (HEBM) techniques employing longer processing times. In this study, cold rolling was applied to prepare MgH₂LaNi₅ composites and the hydrogen storage properties were investigated. The materials were processed using a vertical rolling mill under argon atmosphere, leading to a good homogeneity and no contamination at shorter processing times. The mixture of MgH₂-1.50 mol.% LaNi₅ showed the best hydrogen storage properties at 200 °C and 100 °C and the lowest desorption temperature even when compared to cold rolled MgH₂. The results indicate that the composite MgH₂LaNi₅ is transformed into a mixture of three phases MgH₂, Mg₂NiH₄ and LaH₃ upon hydrogen absorption/desorption cycles. The synergetic effect among these phases when in appropriate proportion in the sample seems to play a crucial role in the acceleration of hydrogen absorption/desorption kinetics at lower temperatures in comparison to MgH₂.     

Enhanced thermal diffusivity and dehydrogenation of 2LiNH2MgH2 by doping with super activated carbon

Doping with the additives in metal-N–H system has been regarded as one of the most effective approaches to improve its hydrogen storage properties. Herein, we prepared super activated carbon (SuperC) through the activation of commercial activated carbon by KOH and evaluated its effect on dehydrogenation properties of 2LiNH₂MgH₂. Our studies show that doping with SuperC could effectively lower its dehydrogenation temperatures. For instance, 2LiNH₂MgH₂–10 wt% SuperC can release 4.86 wt% of hydrogen upon heating up to 300 °C with the onset and peak dehydrogenation temperatures of 71 °C and 168 °C, respectively. Moreover, the release of byproduct NH₃ was successfully suppressed. Measurement of thermal diffusivity suggests that the enhanced dehydrogenation properties may be ascribed to the improved heat transfer for solid-solid reaction resulting from doping with SuperC.     

Promoted activity of porous silica coated Ni/CeO2ZrO2 catalyst for steam reforming of acetic acid

Porous silica coated Ni/CeO₂ZrO₂ catalysts were synthesized for steam reforming of acetic acid. The silica coated Ni/CeO₂ZrO₂ catalyst showed a significantly enhanced activity (95% acetic acid conversion) than the Ni/CeO₂ZrO₂ catalysts (62% acetic acid conversion) at a low temperature (550 °C). Interaction between Ni/CeO₂ZrO₂ and silica layer was proved to be a crucial role on enhancing of catalytic activities. Further characterization (XPS, H₂-TPR) indicates this interaction facilitates the steam reforming reaction and raises the selectivity of CO by modifying the surface Ni electronic structure. In addition, the coated catalyst also exhibited a good stability and no obvious deactivation was detected at 550 °C and 600 °C within 30 h.     

Mesocrystalline Ta2O5 nanosheets supported PdPt nanoparticles for efficient photocatalytic hydrogen production

We successfully synthesized mesocrystalline Ta₂O₅ nanosheets supported bimetallic PdPt nanoparticles by the photo-reduction method. The as-prepared mesocrystalline Ta₂O₅ nanosheets in this work showed amazing visible-light absorption, mainly because of the formation of oxygen vacancy defects. And the as-prepared bimetallic PdPt/mesocrystalline Ta₂O₅ nanaosheets also showed highly enhanced UV–Vis light absorption and highly improved photocatalytic activity for hydrogen production in comparison to that of commercial Ta₂O₅, mesocrystalline Ta₂O₅ nanosheets, Pd/mesocrystalline Ta₂O₅ nanosheets and Pt/mesocrystalline Ta₂O₅ nanosheets. The highest photocatalytic hydrogen production rate of PdPt/mesocrystalline Ta₂O₅ nanaosheets was 21529.52 g^?1 h^?1, which was about 21.2 times of commercial Ta₂O₅, and the apparent quantum efficiency of PdPt/mesocrystalline Ta₂O₅ nanaosheets for hydrogen production was about 16.5% at 254 nm. The highly enhanced photocatalytic activity was mainly because of the significant roles of PdPt nanoparticles for accelerating the charge separation and transport upon illumination. The as-prepared PdPt/mesocrystalline Ta₂O₅ nanaosheets in this work could serve as an efficient photocatalyst for green energy production.