Filtering carbon dioxide through carbon nanotubes

Fall into the pseudo-decoupling trap: Type identification, trend characterization and solution path of carbon decoupling trap in urban agglomerations of China

This study investigates the stability and sustainability of carbon decoupling in urban agglomerations across China, where the strong coupling between economic growth and carbon emissions poses significant challenges. Despite efforts in energy conservation and emission reduction, urban agglomerations have seen unsatisfactory results. By analyzing the real-pseudo decoupling states in 19 urban agglomerations from 2007 to 2020, the objective of this study is to identify the type and trend characteristics of carbon decoupling traps and to propose solution paths for maintaining decoupling stability. Major Findings: (1) The decoupling state exhibits volatility and instability in urban agglomerations, making them susceptible to decoupling traps. (2) Most urban agglomerations remain un-decoupled, with a few cities achieving real decoupling and gradually shifted from northeast to southeast, while pseudo-decoupling and un-decoupled cities consistently cluster in the southwest and northwest regions. (3) Real-pseudo decoupling is driven by a combination of endogenous and exogenous factors, with energy structure, population density, and environmental regulation intensity emerging as pivotal influencers. (4) Geographical factors exhibit both commonalities and variations in their impact on real-pseudo decoupling. By identifying real-pseudo decoupling states and their driving factors, this study proposes strategic solution paths to overcome carbon constraints and achieve stable decoupling in urban agglomerations, contributing to the broader goals of sustainable economic and environmental development.

Stochastic model for the transfer of gaseous particles in polymer-carbon-nanotube nanocomposites with interfacial regions

In this work, a stochastic model of gaseous transfer in polymer-carbon-nanotube (CNT) nanocomposites is presented. The model takes into account interfacial areas, i.e., polymer depletion regions. The local regime of transport is controlled by the density of the polymer. In a dense polymer, this regime corresponds to the ordinary diffusion, while in free volume regions, it corresponds to the ballistic transport. The introduction of a free volume and/or a depleted polymer layer near to a CNT wall leads to the emergence of anomalous diffusion. We have demonstrated how the anomalous diffusion regime changes in the presence of nanotubes for different distributions of polymer density. The presented approach allows us to describe the threshold effect in the diffusion coefficient as a function of CNTs density in polymer-CNT nanocomposites.

Understanding water slippage through carbon nanotubes

It is a formidable challenge to understand water slippage through carbon nanotubes (CNTs), despite its great significance in fundamental research and technology. Herein, we propose an effective scheme to describe water slippage properties by extending two friction models - the phononic friction model and Einstein's diffusion model, both relying on the potential corrugation of water slippage. Our scheme effectively captures the tube-size effect on the viscosity and slippage of water molecules through CNTs. It also identifies the experimentally reported size-dependent transition from continuum to sub-continuum flow and further reveals that this transition is likely to be determined by the hydrogen bond instead of the structural transition or entropic change. Besides, the size-dependence of slip lengths is found to be controllable by temperature. Our methods are thus expected to be a useful basis for further studies on substance transport under confinement.

Desalination behavior analysis of interior-modified carbon nanotubes doped membrane by dielectric spectrum and molecular simulation

Carbon nanotube (CNT)-doped polyamide (PA) membranes have attracted much attention in reverse osmosis (RO) membranes due to their significant advantages of water flux and desalination. In this study, we synthesized multi-walled carbon nanotube (MWNT)/PA RO membrane by 12-oxodidodecanoic acid methyl ester group interior-modified MWNTs (MWNT-C₁₄H₂₅O₄). Then, their mechanism of desalination behavior was successfully analyzed by combining dielectric relaxation spectrum (DRS) and molecular dynamics (MD) simulation. DRS analysis mainly focuses on two aspects: (1) the water volume fraction, average pore size and dielectric parameters of MWNT-C₁₄H₂₅O₄/PA and PA membranes were obtained by model analysis of DRS data. These data of MWNT-C₁₄H₂₅O₄/PA membrane are higher than PA membrane, which indicates that the water flux of the MWNT-C₁₄H₂₅O₄/PA membrane was higher than that of the PA membrane. (2) Further analysis shows that the MWNT-C₁₄H₂₅O₄/PA membranes have high average charge density, ion solvation barrier and reflection coefficient, which indicates that the added interior-modified MWNT can improve the salt rejection of PA membranes. In the microscopic aspect, the desalination behavior of the MWNT-C₁₄H₂₅O₄/PA and PA membrane was analyzed from the aspects of free volume distribution, the dynamic diffusion process of water and ions. The results show that the microscopic data of dynamic simulation well support the conclusion of the DRS method. This study provides a convenient methodology to characterize the properties of the membrane from the aspect of membrane structure.

Gas Hydrate Combustion in Five Method of Combustion Organization

Experiments on the dissociation of a mixed gas hydrate in various combustion methods are performed. The simultaneous influence of two determining parameters (the powder layer thickness and the external air velocity) on the efficiency of dissociation is studied. It has been shown that for the mixed hydrate, the dissociation rate under induction heating is 10–15 times higher than during the burning of a thick layer of powder, when the combustion is realized above the layer surface. The minimum temperature required for the initiation of combustion for different combustion methods was studied. As the height of the sample layer increases, the rate of dissociation decreases. The emissions of NO_x and CO for the composite hydrate are higher than for methane hydrate at the same temperature in a muffle furnace. A comparison of harmful emissions during the combustion of gas hydrates with various types of coal fuels is presented. NO_x concentration as a result of the combustion of gas hydrates is tens of times lower than when burning coal fuels. Increasing the temperature in the muffle furnace reduces the concentration of combustion products of gas hydrates.

Multiscale Computational Fluid Dynamics

Computational Fluid Dynamics (CFD) has numerous applications in the field of energy research, in modelling the basic physics of combustion, multiphase flow and heat transfer; and in the simulation of mechanical devices such as turbines, wind wave and tidal devices, and other devices for energy generation. With the constant increase in available computing power, the fidelity and accuracy of CFD simulations have constantly improved, and the technique is now an integral part of research and development. In the past few years, the development of multiscale methods has emerged as a topic of intensive research. The variable scales may be associated with scales of turbulence, or other physical processes which operate across a range of different scales, and often lead to spatial and temporal scales crossing the boundaries of continuum and molecular mechanics. In this paper, we present a short review of multiscale CFD frameworks with potential applications to energy problems.

Enhanced water transport through a carbon nanotube controlled by the lateral pressure

The transport of water through carbon nanotubes (CNTs) is now of great importance in bionanotechnology and of considerable interest for potential nanofluidic applications. In this paper, we show by molecular dynamics simulations that the permeation of single-file water molecules through a CNT can be significantly improved by means of tuning the direction of pressure difference, i.e. introducing an additional lateral pressure to the longitudinal one. The water flow exhibits an interesting maximum behavior with the change of lateral pressure, deciphered by the breakdown of single-file water chain inside the CNT. The translocation time decreases monotonously with the increase of lateral pressure and exhibits a clear bifurcation due to the longitudinal pressure, corresponding to the flow enhancement. Therefore, the lateral pressure will increase the difficulty for water entering, while promotes the water conduction inside the CNT, whose competition ultimately leads to the flow maximum behaviors. Along with the water reducing inside the CNT, the CNT switches between the filling and empty states with the unique distributions of water dipole orientation, density and H-bond number. Our results indicate that tuning the direction of pressure difference should be a significant new strategy for enhancing the water permeability, where the key lies in the breakdown of single-file water chain and are thus insightful for future studies.

Solvent-free nanofluid with three structure models based on the composition of a MWCNT/SiO2 core and its adsorption capacity of CO2

A series of core/shell nanoparticle organic/inorganic hybrid materials (NOHMs) with different weight ratios of two components, consisting of multi-walled carbon nanotubes (MWCNTs) and silicon dioxide (SiO₂) as the core were synthesized. The NOHMs display a liquid-like state in the absence of solvent at room temperature. Five NOHMs were categorized into three kinds of structure states based on different weight ratio of two components in the core, named the power strip model, the critical model and the collapse model. The capture capacities of these NOHMs for CO₂ were investigated at 298 K and CO₂ pressures ranging from 0 to 5 MPa. Compared with NOHMs having a neat MWCNT core, it was revealed that NOHMs with the power strip model show better adsorption capacity toward CO₂ due to its lower viscosity and more reactive groups that can react with CO₂. In addition, the capture capacities of NOHMs with the critical model were relatively worse than the neat MWCNT-based NOHM. The result is attributed to the aggregation of SiO₂ in these samples, which may cause the consumption and hindrance of reactive groups. However, the capture capacity of NOHMs with the collapse model was the worst of all the NOHMs, owing to its lowest content of reactive groups and hollow structure in MWCNTs. In addition, they presented non-interference of MWCNTs and SiO₂ without aggregation state.

The adsorption and fast transport of Xe in single walled carbon nanotubes

Combined GCMC and MD simulations have been used to investigate the adsorption and diffusion of Xe gases in carbon nanotubes (CNTs) at different conditions.

Synthesis, characterization, and CO2 capture study of micro-nano carbonaceous composites

The micro-nano carbonaceous composite activated carbon fiber/carbon nanotube (ACF/CNTs) was obtained by chemical vapor deposition technology with CNTs growth on the substrate ACF, and the composite was further modified by branched polyethyleneimine (PEI). The morphological features of the as-grown ACF/CNTs and PEI-modified samples were analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and thermal gravimetric analysis respectively. Physical properties of the samples were recorded by conducting N2 adsorption/desorption at 77K. CO2 capture tests indicated that both the presence of CNTs and PEI increased the CO2 adsorption capacity, due to the unique hollow tubular structure of CNTs and poly amino groups of PEI. The CO2 adsorption capacities of ACF/CNTs and ACF/CNTs-PEI were tested to be 66.2 mg/g and 98.8 mg/g, respectively at 30°C, which were much higher than that of unmodified ACF (42.4 mg/g). Increasing adsorption temperature up to 60°C further promoted the CO2 adsorption capacity of ACF/CNTs-PEI (121.2 mg/g) due to the maximum equilibrium adsorption between the chemical and physical adsorption at this temperature. Cyclic CO2 adsorption tests via thermal desorption proved that ACF/CNTs-PEI had a good regenerability of 96.2%, suggesting this material is a promising CO2 adsorbent for practical applications.

Enhanced carbon dioxide adsorption through carbon nanoscrolls

Over the last few years, significant efforts have been devoted to exploring the capabilities of carbon based structures for gas separation and filtration. In the present study the layering behavior of carbon dioxide transported through carbon nanoscrolls is examined through molecular dynamics simulations. The layering arrangements are investigated for carbon nanoscrolls with intralayer distances spanning from 4.2 to 8.3 Å at temperature of 300 K and pressures ranging from 5 to 20 bars. Characteristic layering structures are developed around the internal and external surfaces of the nanoscroll for all the examined cases. It is shown that the number of layers, their relative strength, and the starting point of bifurcation phenomena vary as a function of the nanoscrolls' intralayer distance, scroll's core radius, CO2 density, and gas structure interactions. It is also shown that the number of carbon dioxide molecules adsorbed per scroll's carbon particles is a function of the scroll's surface-to-volume ratio and is maximized under certain structural configurations.