585 divacancy-defective germanene as a hydrogen separation membrane: A DFT study

As sustainable and clean energy, hydrogen is the most attractive and promising energy source in the future. Membrane separation is attractive due to its high hydrogen separation performance and low energy consumption. Van-der-Waals-corrected density functional theory (DFT) calculations are performed to investigate the hydrogen separation performance of 585 divacancy-defective germanene (585 germanene). It is found that the 585 germanene presents a surmountable energy barrier (0.34 eV) for hydrogen molecule passing through the membrane, and that membrane exhibits extremely high selectivity for H₂ molecules over CO, CO₂, N₂, CH₄ and H₂S molecules in a wide range of temperatures. Meanwhile, the hydrogen permeance of 585 germanene can reach 1.94 × 10^?7 mol s^?1 m^?2 Pa^?1 at the low limit temperature of methane reforming (at 450 K), which is higher than the industrially acceptable gas permeance. With high selectivity and permeance, the 585 germanene is a promising candidate for hydrogen separation.     

First-principles study of LiBH4 nanoclusters interaction with models of porous carbon and silica scaffolds

Density functional theory calculations of an interaction of LiBH₄ represented by n = 2−6 and 12 formula units nanoclusters with models of activated carbon and porous silica show that on both non-defective substrates only physisorption is observed for all cluster sizes. The binding energies are low, reaching up to −43 kJ/mol for smallest clusters. The charge transfer between LiBH₄ and the support is not observed. On defective graphene (LiBH₄)₂ may adsorbed dissociatively. Hydrogens detached from BH₄ groups saturates under-coordinated C atoms while the binding between BH₃ moiety and underlying C atoms restores sp³-hybridization in the BH₄ group. The dissociative adsorption of LiBH₄ clusters leads to the retrieval of the three-fold coordination of the C atoms, the subsequent (LiBH₄)₂ physisorps with the differential heat of adsorption not exceeding −46 kJ/mol. The present calculations indicate that chemical interaction between matrix and lithium borohydride, leading to a destabilization of LiBH₄, takes place until substrate's defects remain unsaturated.     

Promotion of H2 adsorption performance on InN monolayer by embedding Cu atom: A first-principles study

Hydrogen energy as a clean energy has great application potential, and finding efficient hydrogen storage materials has become the current research hotspot. This work studied the structure, electronic properties, thermodynamic properties and H₂ adsorption performance of InN, N-defect (V_N–InN), In-defect (V_In–InN), Cu atom substitutes N atom embedded InN (Cu/V_N–InN) and Cu atom substitutes In atom embedded InN (Cu/V_In–InN) by density functional theory (DFT). The results show that all of the five InN materials have good thermal stability at room temperature (300 K), and the structural stability of the defective InN increases after embedding of Cu atom. Meanwhile, the hydrogen interaction on the five InN materials was investigated. Cu/V_In–InN has the best performance for H₂ adsorption among the five InN materials. The adsorption energy for Cu/V_In–InN can reach ?0.769 eV, which is 4.5 times better than original InN nanosheet. After adsorbing 5H₂ molecules, the average adsorption energy is ?0.399 eV that indicates Cu/V_In–InN structure still has possibility of adsorbing more hydrogen molecules and it has the potential to become a new hydrogen storage material.     

Hydrogen storage performance of platinum supported carbon nanohorns: A DFT study of reaction mechanisms,thermodynamics, and kinetics

Platinum (Pt) is one of a robust hydrogen dissociative catalyst. However, the migration of dissociated hydrogens from Pt nanoparticles to carbon supports such as graphene and carbon nanotube are energetically unfavorable reactions. To enhance the hydrogen storage via migration mechanism, carbon nanohorn is applied as a support for Pt nanoparticles (Pt and Pt₄). The H₂ storage performance of Pt and Pt₄ supported on the mono-vacancy carbon nanohorn (vNH) has been investigated by using density functional theory calculations. The Pt and Pt₄ firmly deposit at the vacancy site through the three strong Pt–C bonds with binding energies about ?7.0 eV, which can prevent the metal desorption and migration. The mechanism of H₂ storage starts with H₂ adsorption followed by H₂ spillover reaction. The calculation results reveal that the supported Pt nanoparticles are the active sites for H₂ dissociative adsorption while the high curvature surface of carbon nanohorn is the active area for accommodating the migrated H atoms from the spillover reaction. Remarkably, the hydrogen spillover reactions over Pt– and Pt₄-supported on vNHs in this study are spontaneous at room temperature with highly exothermic reaction energy. The fundamental understanding obtained from this study is beneficial for further design and synthesis of high-performance materials for H₂ storage applications.     

Facile construction of phosphate incorporated graphitic carbon nitride with mesoporous structure and superior performance for H2 production

Novel mesoporous phosphate incorporated g-C₃N₄ (CNM-Px) polymeric material was synthesized via a facile hydrothermal-calcination method, using melamine as precursor and phosphoric acid as dopant. The successful incorporation of phosphate into the framework of g-C₃N₄ nanosheets was verified by XRD, FT-IR and XPS characterizations and the possible formation mechanism was put forward. The as-fabricated CNM-Px samples were applied to photocatalytic hydrogen evolution reaction and exhibited remarkably improved photocatalytic performance both under simulated sunlight and visible light irradiation. The concentration of phosphoric acid was also well tuned and the optimal concentration was 2.5 mol L^?1. The hydrogen evolution rate of the optimized sample CNM-P2.5 (the concentration of treating phosphoric acid was 2.5 mol L^?1) reached 8163 μmol g^?1 h^?1 under simulated sunlight irradiation, which is 3.7 times higher than that of pristine g-C₃N₄ (CNM). It also showed dramatically improved hydrogen evolution rate under visible light irradiation, which was 2105 μmol g^?1 h^?1, about 6.7 times higher than that of CNM. The excellent photocatalytic activity of CNM-Px samples is due to the synergic advantages of larger surface area and reduced recombination of photo-generated electrons and holes. This study paves the way for tailoring design and synthesis of highly active metal-free carbon nitride materials for photocatalytic hydrogen evolution.     

Preparation,characterization and gas permeation properties of PDMS/PEI composite asymmetric membrane for effective separation of hydrogen from H2/CH4 mixed gas

In this work, PDMS/PEI membranes were synthesized and sorption and permeation of H₂/CH₄ mixture were studied. The influence of pressure, temperature and feed composition were investigated. It was shown that permeances increased and selectivity decreased with an increment in the feed temperature. Increasing feed pressure caused a decline in gas permeance and increased selectivity. Higher concentrations of hydrogen in the feed declined the selectivity. The effect of different non-solvents was explained by their effect on precipitation time and it was concluded that water made the membrane denser while isopropanol forms a sponge-like structure. Coagulation bath temperature made the membrane denser. Film casting and dip-coating techniques were used to prepare selective membranes. Obtained selectivity results introduced dip-coating as a better method than film casting. Sequential coating improved selectivity of the prepared membrane. Finally, sequential coating with different concentrations was applied and enhanced selectivity significantly from about 22 to more than 70.     

DFT study of H2 adsorption on Pd-decorated single walled carbon nanotubes with C-vacancies

Carbon nanotubes are considered important materials for hydrogen storage. Although the C–H interaction is very weak at room temperature, the incorporation of highly dispersed Pd nanoparticles increases the H₂ adsorption on carbon surfaces. In this work we performed density functional theory studies of H₂ adsorption on single walled carbon nanotubes (SWCNTs) with C-vacancies and a Pd decoration. We used the VASP and SIESTA codes. Our calculations show that Pd adsorption is favored on the C-vacancies of the (5,5) SWCNT, while H₂ adsorption also occurs preferentially on C-defective sites.     

Theoretical study and performance evaluation of hydrogen production by 200 W solid oxide electrolyzer stack

High temperature steam electrolyzers, taking advantage of high temperature heat, can produce more hydrogen by using less electrical energy than low temperature electrolyzers. This paper presents an experimental study on hydrogen production by using a 200 W solid oxide stack working in reverse mode. A thermodynamic study of the process was performed by measuring the heat and mass balance of stack at different operating conditions. Different definitions of efficiency were used to highlight the limit and potential of the process. The I–V curve, the flow rate measurements and the GC analysis on outlet flows were used to calculate the hydrogen and oxygen productions. In addition, the influence of steam dilution, water utilization and operating temperature on conversion efficiency and stack's thermal balance was evaluated. With this aim, the tests were performed at three operating temperature (700 °C, 750 °C and 800 °C) over a range of steam inlet concentration from 50% to 90% and water utilization up to 70%. The hydrogen and oxygen flows produced by electrolysis, at different loads, were directly measured after water condensation: net flows up to 2.4 ml/(min cm²) of hydrogen and 1.2 ml/(min cm²) of oxygen were measured and compared to the theoretical ones, showing a good agreement.     

Fabrication and characterization of poly(phenylene oxide)/SBA-15/carbon molecule sieve multilayer mixed matrix membrane for gas separation

A novel multilayer mixed matrix membrane (MMM), consisting of poly(phenylene oxide) (PPO), large-pore mesoporous silica molecular sieve zeolite SBA-15, and a carbon molecular sieve (CMS)/Al₂O₃ substrate, was successfully fabricated using the procedure outlined in this paper. The membranes were cast by spin coating and exposed to different gases for the purpose of determining and comparing the permeability and selectivity of PPO/SBA-15 membranes to H₂, CO₂, N₂, and CH₄. PPO/SBA-15/CMS/Al₂O₃ MMMs with different loading weights of zeolite SBA-15 were also studied. This new class of PPO/SBA-15/CMS/Al₂O₃ multilayer MMMs showed higher levels of gas permeability compared to PPO/SBA-15 membranes. The permselectivity of H₂/N₂ and H₂/CH₄ combinations increased remarkably, with values at 38.9 and 50.9, respectively, at 10 wt% zeolite loading. Field emission scanning electron microscopy results showed that the interface between the polymer and the zeolite in MMMs was better at a 10 wt% loading than other loading levels. The increments of the glass transition temperature of MMMs with zeolite confirm that zeolite causes polymer chains to become rigid.     

Enhancing energy storage efficiency of lithiated carbon nitride (C7N6) monolayers under co-adsorption of H2 and CH4

There is a great interest in the design of innovative concepts and strategies of nitrogen rich carboneous materials for exploring their hydrogen (H₂) storage properties. Methane (CH₄) storage can be an alternative to H₂ because the combustion energy of the former is around three times higher than the latter. However, strong inter-molecular repulsion between the CH₄ molecules is a major bottleneck to achieve a high gravimetric density. In this study, we use first principles density functional calculations to investigate the coadoption of H₂ and CH₄ on Li decorated carbon nitride (Li–C₇N₆) monolayer. The repulsion between CH₄ molecules has been avoided by keeping them in asymmetric configuration whereas the repulsion between CH₄–H₂ is in moderation due to the exploitation of open Li doped sites on C₇N₆ surface. Though Li–C₇N₆ has a lower H₂ or CH₄ storage capacity due to weak van der Waals interactions, the capacity could be doubled with a novel strategy of co-mixing of H₂ with CH₄ which results into a significantly high gravimetric density of 8.1 wt%. This clearly shows that the CH₄–H₂ co-mixing strategies have the potential to further propel the prospects of C₇N₆ monolayers for reversible clean energy storage applications.     

A DFT study on enhanced adsorption of H2 on Be-decorated porous graphene nanosheet and the effects of applied electrical fields

The adsorption of the hydrogen molecule on the pure porous graphene nanosheet (P-G) or the one decorated with Be atom (Be-G) was investigated by the first-principle DFT calculations. The Be atom was adsorbed on the P-G with a binding energy of ?1.287 eV to successfully establish the reasonable Be-G. The P-G was a poor substrate to interact weakly with the H₂, whereas the Be-G showed a high affinity to the adsorbed H₂ with an enhanced adsorption energy and transferred electrons of ?0.741 eV and 0.11 e, respectively. A molecular dynamics simulation showed that the H₂ could also be adsorbed on the Be-G at room temperature with a reasonable adsorption energy of ?0.707 eV. The interaction between the adsorbed H₂ and the Be-G was further enhanced with the external electrical fields. The applied electrical field of ?0.4 V/Å was found to be the most effective to enhance the adsorption of H₂ on the Be-G with the modified adsorption energy and the improved transferred electrons being ?0.708 eV and 0.17 e, respectively. Our study shows that the Be-G is a promising substrate to interact strongly with the H₂ and could be applied as a high-performance hydrogen gas sensor, especially under the external electrical field.     

Theoretical study of hydrogen production by ethanol steam reforming: Technical evaluation and performance analysis of catalytic membrane reactor

Ethanol steam reforming over a Co/Al₂O₃ catalyst was studied theoretically in a catalytic PdAg membrane reactor (CMR). A mathematical model has been developed to elucidate the behavior of CMR by taking into account the chemical reactions, heat and mass transfer phenomena. The effect of operating parameters on the performance of CMR has been evaluated in terms of ethanol conversion, hydrogen recovery and hydrogen yield. The results revealed the high performance of this configuration is related to the continuous removal of hydrogen from the retentate side, shifting the reaction equilibrium towards hydrogen formation. Sensitivity analysis of operating parameters indicate that ethanol conversion is favored at higher temperatures, pressures, sweep ratios and feed molar ratios. Moreover, increasing the feed molar ratio enhances the ethanol conversion, and decreases the hydrogen recovery due to reduction of partial pressure of hydrogen and consequently decreasing the driving force for the hydrogen permeation through the membrane.     

A DFT study on the hydrogen storage performance of MoS2 monolayers doped with group 8B transition metals

The adsorption of hydrogen (H₂) molecules on MoS₂ monolayers doped with Fe, Co, Ni, Ru, Rh, Pd, Os, Ir or Pt was calculated via first-principle density functional theory (DFT). The H₂ was found to interact most strongly with the MoS₂ doped with Os with a higher adsorption energy of ?1.103 eV. Investigations of the adsorptions of two to five H₂ molecules on Os-doped MoS₂ monolayers indicate that there are at most four H₂ interacting stably with the substrate with a promising average adsorption energy of ?0.792 eV. Molecular dynamics simulations also confirmed that the four H₂ molecules can still be reasonably adsorbed and stored on the Os-doped MoS₂ monolayer with a comparable average adsorption energy of ?0.713 eV at 300 K. This study indicates that MoS₂ monolayer doped with Os is a promising substrate to interact strongly with H₂ and can be applied to effectively store H₂ at room temperature.     

Synthesis,characterization and corrosion performance evaluation of DDR membrane for H2 separation from HI decomposition reaction

An all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure has been developed for separation of H₂ from HI/I₂ mixture of HI decomposition reaction. In this work, the DDR zeolite membrane was synthesized on the seeded clay-alumina substrate within 5 days. The seeds were synthesized by sonication mediated hydrothermal process within short crystallization time which enhanced the nucleation for the membrane growth. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDAX). The selectivity of hydrogen with respect to CO₂ and Ar was evaluated by single gas permeation studies at room temperature. The tests for corrosion resistance were carried out upto 120 h with both support and DDR membrane at 130 °C which confirmed the stability of membrane under the harsh HI/I₂ environment.     

Theoretical study on the performance of an integrated ground-source heat pump system in a whole year

Being environmental friendly and with the potential of energy-efficiency, ground-source heat pump (GSHP) systems are widely used. However, in southern China, there exists large difference between cooling load in summer and heating load in winter. Thus the increase of soil temperature gradually year-by-year will decrease the COP of the GSHP system. In this paper, the configuration of a vertical dual-function geothermal heat exchanger (GHE) used in an integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, which charged cold energy to the soil at night and produced chilled water at daytime in summer, and supplied hot water for heating in winter, is presented. This is then followed by reporting the development of the mathematical model for the GHE considering the impact of the coupled heat conduction and groundwater advection on the heat transfer between the GHE and its surrounding soil. The GHE model developed was then integrated with a water-source heat pump and a building energy simulation program together for a whole ISCS&GSHP system. Then the operation performance of the ISCS&GSHP system used for a demonstration building is studied. These simulation results indicated the system transferred 71.505% of the original power consumption at daytime to that at nighttime for the demonstration building. And the net energy exchange in the soil after one-year operation was only 2.28% of the total cold energy charged. Thus we can see the feasibility of the ISCS&GSHP system technically.     

Biogas use in high temperature fuel cells: Enhancement of KOH-KI activated carbon performance toward H2S removal

Activated carbons (ACs) treated with KOH-KI are very effective sorbents for deep H₂S removal, as required by biogas use in high temperature fuel cell systems. For this application, the performance of a commercial KOH-KI treated AC was investigated through a systematic study based on dynamic adsorption tests. With reference to the composition of a real biogas produced in a wastewater treatment plant located in Barcelona, the present work presents a sensitivity performance analysis on singular and synergetic effects of gas matrix, humidity and oxygen on AC KOH-KI performance.The results revealed a positive role of water (up to 90% of relative humidity (R.H.)) for different gas matrices, enhanced by the simultaneous presence of small percentages of oxygen (2%_v). A relevant influence of gas matrix composition was found (except for the case of oxygen addition to dry inlet streams), specifically in terms of a marked negative effect of CO₂ and a significant sorption capacity increase for high percentage of methane. Sulfur dioxide was not detected in the outlet gas-phase for the investigated operating parameters (O₂ 2%_v, R.H. 0–90%, H₂S 100 ppm_v, temperature 45 °C). Therefore, even in the case of further oxidation of adsorbed elemental sulfur to SO₂, this product could be completely removed by AC KOH-KI.     

Effect of coatings on long term behaviour of a commercial stainless steel for solid oxide electrolyser cell interconnect application in H2/H2O atmosphere

K41X (AISI 441) stainless steel evidenced a high electrical conductivity after 3000 h ageing in H₂/H₂O side when used as interconnect for solid oxide electrolyser cells (SOEC) working at 800 °C. Perovskite (La_1 − xSr_xMnO_3 − δ) and spinel (Co₃O₄) oxides coatings were applied on the surface of the ferritic steel for ageing at 800 °C for 3000 h. Both coatings improved the behaviour of the steel and give interesting opportunities to use the K41X steel as interconnect for hydrogen production via high temperature steam electrolysis. Co₃O₄ reduced into Co leading to a very good Area Specific Resistance (ASR) parameter, 0.038 Ω cm². Despite a good ASR (0.06 Ω cm²), La_1 − xSr_xMnO_3 − δ was less promising because it partially decomposed into MnO and La₂O₃ during ageing in H₂/H₂O atmosphere.     

CO2-selective membranes for hydrogen purification and the effect of carbon monoxide (CO) on its gas separation performance

Industrial hydrogen production may prefer CO₂-selective membranes because high-pressure H₂ can therefore be produced without additional recompression. In this study, high performance CO₂-selective membranes are fabricated by modifying a polymer–silica hybrid matrix (PSHM) with a low molecular weight poly(ethylene glycol) dimethyl ether (PEGDME). The liquid state of PEGDME and its unique end groups eliminate the crystallization tendency of poly(ethylene glycol) (PEG). The methyl end groups in PEGDME hinder hydrogen bonding between the polymer chains and significantly enhance the gas diffusivity. In pure gas tests, the membrane containing 50 wt% additive shows CO₂ gas permeability and CO₂/H₂ selectivity of 1637 Barrers and 13 at 35 °C, respectively. In order to explore the effect of real industrial conditions, the gas separation performance of the newly developed membranes has been studied extensively using binary (CO₂/H₂) and ternary gas mixtures (CO₂/H₂/carbon monoxide (CO)). Compared to pure gas performance, the second component (H₂) in the binary mixed gas test reduces the CO₂ permeability. The presence of CO in the feed gas stream decreases both CO₂ and H₂ permeability as well as CO₂/H₂ selectivity as it reduces the concentration of CO₂ molecules in the polymer matrix. The mixed gas results affirm the promising applications of the newly developed membranes for H₂ purification.     

Preparation and characterization of multi-walled carbon nanotube/PBNPI nanocomposite membrane for H2/CH4 separation

By combining organic polymers normally used to make membrane filters with inorganic substances, multi-walled carbon nanotube (MWCNTs), an extraordinary ability to separate H₂ from CH₄ was developed in this study. A series of MWCNTs/PBNPI nanocomposite membrane with a nominal MWCNTs content between 1 and 15 wt% were prepared by solution casting method, in which the very fine MWCNTs were embedded into glassy polymer membrane. Detailed characterizations, such as morphology, thermal stability and crystalline structure have been conducted to understand the structures, composition and properties of nanocomposite membranes. The results found that this new class of membrane had increased permeability and enhanced selectivity, and a useful ability to filter gases and organic vapours at the molecular level.     

Parametric study of anodic microstructures to cell performance of planar solid oxide fuel cell using measured porous transport properties

This study reports effects of porosity (?), permeability (k) and tortuosity (τ) of anodic microstructures to peak power density (PPD) of a single-unit planar anode-supported SOFC based on 3D electrochemical flow models using measured porous transport properties. Applying particle image velocimetry, a transparent porous rib-channel with different ? is applied to measure an effective viscosity (μ_e) in the Brinkman equation commonly used to predict flow properties in porous electrodes. It is found that, contrary to the popular scenario, μ_e is not equal to the fluid viscosity (μ_f), but it is several orders in magnitude smaller than μ_f resulting in more than 10% difference on values of PPD. Numerical analyses show: (1) while keeping k and τ fixed with ? varying from 0.2 to 0.6, the highest PPD occurs at ? = 0.3 where the corresponding triple-phase-boundary length is a maximum; (2) PPD increases slightly with k when k ≤ 10⁻¹¹ m² due to the diffusion limitation in anode; and (3) PPD decreases with τ when τ > 1.5 due to the accumulation of non-depleted products. Hence, a combination of ? = 0.3, k = 10⁻¹¹ m², and τ = 1.5 is suggested for achieving higher cell performance of planar SOFC.