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The use of membranes for gas upgrading has increasingly become of interest as it has shown great potential for efficient and affective gas purification and a pathway to green energy. The emission of greenhouse gases to the atmosphere has detrimental effects on the economy in terms of global warming which has led to many natural disasters, heat waves, food shortage, loss of life and property. To combat this, studies of capturing and utilizing greenhouse gases are ongoing. In this paper, the study of biogas components (methane and carbon dioxide) diffusion through membranes are studied to employ its use as a solution for the challenge. The study involved the use of membranes of different pore sizes (15, 200 and 6000nm) to ascertain the flow characteristics and regime of the gases under different operating conditions. Single gas permeation tests were conducted, and the results show the flow of gases is dependent on factors including molecular weight, kinematic diameter and viscosity of the gas components. It was observed that pressure has a greater influence on the gas flow through membranes compared to temperature with the effect of pore size having the greatest impact. The flux of methane through the membrane is greater than that of carbon dioxide in regular pore geometry and depicts a greater potential for upgrading of biogas.

Rull cave is a karstic cave located in Vall d’Ebo (Alicante, Southeast of Spain) developed in massive Miocene conglomerates and Cretaceous limestones. Processes in soil above Rull cave and outdoor atmosphere directly influence the underground environment of the cave. Continuous and discrete monitoring of cave atmosphere and soil (from 2012 to 2022) allows to characterise the spatial distribution and temporal variations of the gaseous concentration (222Rn and CO2) and understand the relationship between the processes which occurred in the 3-component system (soil-cave-atmosphere). Besides the presence of visitors, Rull cave maintains stable values of mean temperature (16.2 °C) and relative humidity (97.6%). In an annual cycle the cave presents two different gaseous stages (stagnation and ventilation). Maximum average values of CO2 and 222Rn concentration are reached within the stagnation stage, in the warmest period of the year. On the contrary, in the ventilation stage (in the coldest months) the cave reaches the lowest concentrations in its inner atmosphere. For the study period, daily average CO2 and 222Rn concentrations are 2008 ppm and 1745 Bq/m3, respectively. Results show that the dynamics of 222Rn and CO2 in the cave air follow different patterns defined by the complex relationships between external and internal factors. Findings from this study provide substantial information about the environmental situation of the cave atmosphere in terms of air quality for visitors and workers., This work was supported by the Spanish Ministry of Science, Innovation and Universities (project RTI2018-099052-BI00), and Regional Government of Comunidad Valenciana (Spain) (project AICO/2020/175), and with collaboration of the project PID2019-110603RB-I00. A pre-doctoral research fellowship (PRE2019-088294) was awarded to S.G.O. (linked to the project RTI2018-099052-BI00). A pre-doctoral research fellowship (GRI-SOLIAP/2020/124) was awarded to M.C.R. by the Department of Innovation, Universities, Science and Digital Society of the Generalitat Valenciana, and a pre-doctoral research fellowship (FPU20/05157) was awarded to N.G.M. by the Spanish Ministry of Science, Innovation and Universities.

The performance and operation stability of proton exchange membrane fuel cells (PEMFCs) are closely related to the transportation of reactants and water management in the membrane electrode assembly (MEA) and flow field. In this paper, a new three-dimensional wave parallel flow field (WPFF) in cathode was designed and analyzed throughout simulation studies and an experimental method. The experimental results show that the performance of PEMFC with WPFF outperforms that of PEMFC with straight parallel flow field (SPFF). Specifically, the peak power density increased by 13.45% for the PEMFC with WPFF as opposed to PEMFC with SPFF. In addition, the flow field with area of 11.56 cm2 was formed by the assembly of transparent end plate used for cathode and the traditional graphite plate used for anode. To understand the mechanism of the novel flow field improving the performance of PEMFC, a model of PEMFC was proposed based on the geometry, operating conditions and MEA parameters. The thickness of gas diffusion layers (GDL), catalytic layers (CL) and proton exchange membrane were measured by scanning electron microscope. The simulation result shows that compared with SPFF, the WPFF based PEMFC promote the oxygen transfer from flow channel to the surface of CL through GDL, and it was beneficial to remove the liquid water in the flow channel and the MEA.

The effect of triple doping with chromium, lanthanum and barium on the mechanical, degassing and gas diffusion properties of fine-crystalline corundum synthesized in a supercritical water fluid was studied. The influence of chromium monodoping on the mechanical and optical properties of fine-crystalline corundum was also investigated. It was found that even small amounts of dopants of lanthanum and chromium during triple and monodoping of corundum significantly improve its mechanical properties (increase the abrasive ability) and reduce it several times the content of volatile gas-liquid impurities, which contributes to increase transparency of the resulting ceramics. The role of the interaction of a chromium impurity with oxygen vacancies in corundum on the increase in the static strength of crystals was suggested. The diffusion coefficients of water in doped corundum were determined. Comparison of the degassing and diffusion properties of corundum doped with La, Cr, Ba with undoped corundum and corundum doped with lanthanum was carried out. The results obtained contribute to the creation of new materials based on doped corundum with high abrasive, luminescent, and gas diffusion characteristics.

Two-dimensional (2D) ZnSnO3 is a promising candidate for future gas sensors due to its high chemical response and excellent electronic properties. However, the preparation of 2D ZnSnO3 nanosheets by utilizing soluble inorganic salts and nonorganic solvents remains a challenge. In this work, 2D ZnSnO3 was synthesized via a facile graphene oxide (GO)-assisted co-precipitation method, in which inorganic salts in the aqueous phase replaced metal organic salts in a non-aqueous system. Meanwhile, a “dissolution and recrystallization” mechanism was proposed to explain the transformation from 3D nanocubes to 2D nanosheets. In comparison, the 2D ZnSnO3 nanosheets showed a higher response to formaldehyde (HCHO) at low operating temperature (100 °C). The response (Ra/Rg) of the 2D ZnSnO3 sensor to 10 ppm HCHO was as high as 57, which was approximately 5 times the response of the ZnSnO3 nanocubes sensor. However, the ZnSnO3 nanocubes sensor showed better gas sensing performance to ethanol at high temperature (200 °C). Different gas-sensitive properties were attributed to the different gas diffusion and adsorption processes caused by the morphology and nanostructure. Moreover, both sensors could detect either 0.1 ppm HCHO or ethanol at their optimum operating temperature. This work presents a relatively economical method to prepare 2D compound metal oxides, provides a novel “dissolution and recrystallization” mechanism for 2D multi-metal oxide preparation, and sheds light on the great potential of high-efficiency HCHO and/or ethanol gas sensors.

C/C-BN composites and Cf/BN/PyC composites exhibiting different structures for pyrolytic carbon (PyC) and boron nitride (BN) were studied comparatively to determine their oxidation behavior. This study used five types of samples. Porous C/C composites were modified with silane coupling agents (APS) and then fully impregnated in water-based slurry of hexagonal boron nitride (h-BN); the resulting C/C-BN preforms were densified by depositing PyC by chemical vapor infiltration (CVI), resulting in three types of C/C-BN composites. The other two Cf/BN/PyC composites were obtained by depositing a BN interphase and PyC in carbon fiber preforms by CVI; one was treated with heat, and the other was not. This study was focused on determining how the PyC deposition mechanism, morphology and pore structure were affected by the method of BN introduction. In the 600–900 °C temperature range, the Cf/BN/PyC composites and C/C composites underwent oxidation via a mixed diffusion/reaction mode. The C/C-BN composites had a different pore structure due to the formation of nodules comprising h-BN particles; both interfacial debonding and cracking were reduced, resulting in higher resistance to gas diffusion, lower oxidation rate and larger activation energy (Ea) in the temperature range 600–800 °C. In addition, the mechanism for oxidation of C/C-BN composites gradually exhibited diffusion control at 800–900 °C because the formation of h-BN oxidation products healed the defects. The oxidation mechanism was more dependent on pore structure than on BN structure or content.

Clinoptilolite is a low-cost and abundant natural zeolite with a high CO2 selectivity, which is abundant in many regions around the world, but with limited industrial use. By combining experimental and theoretical (molecular simulation) approaches, the present work aims at contributing with fundamental data to assist the design of adsorption processes based on this zeolite. A sample of naturally occurring (raw) clinoptilolite was characterized by TGA, XRD, XRF, N2 adsorption isotherms at -196 °C and CO2 at 0 °C. Single-component (CO2, N2, CH4 and O2) and binary (CO2/CH4 and CO2/N2) equilibrium isotherms were measured gravimetrically. Column dynamics experimental data were obtained in order to validate a fixed-bed model, which was then used to evaluate the performance of separation processes. Molecular simulation (MS) was used to build a predictive zeolite model validated with experimental data for CO2, N2, CH4 and O2 able to investigate adsorption of mixtures and gas diffusion. TGA results demonstrated structural stability up to 600 °C with the X-ray diffraction pattern revealing clinoptilolite as the main phase, together with cristobalite and other silicon phases as contaminants. The textural properties obtained from N2 adsorption isotherms at -196 °C of the studied sample were comparable to those reported in the literature, though inaccurate for the actual clinoptilolite structure, which poses significant diffusional resistance for N2 at such temperature. IAST combined with Sips model was used to predict the behavior of mixtures. The proposed clinoptilolite MS model was used to verify the suitability of IAST in predicting mixture equilibrium on clinoptilolite and to predict the selectivity of CO2 over O2. The MS model was able to assess the equilibrium effects in the adsorption of the CH4-N2 mixture and to confirm the relationship between the diffusivity of gases and the positioning and nature of cations in the structure. The fixed-bed model highlighted the different performances of the raw and exchanged clinoptilolite samples, including the theoretical Na-clinoptilolite crystal predicted by the MS model. Together with experimental data, the MS model can be used to aid the design of separation processes of such gas stream with multiple components using clinoptilolite.

CO2-enhanced coalbed methane (CO2-ECBM) technology can be applied not only to extract coal seam gas, but also to geologically store a large amount of CO2 produced in various industries, thus reducing the CO2 concentration in the atmosphere and protecting the earth’s environment from the adverse effects of climate change. For CO2 injection and methane recovery from coal seams, the differences in the gas adsorption and diffusion characteristics due to the properties of the coal seam are crucial. In this study, first, intact and tectonic coals were collected to conduct a proximate analysis and high-pressure methane adsorption/desorption tests. Subsequently, the adsorption characteristics of the intact and tectonic coals with different particle sizes were analyzed. A time-dependent dynamic diffusion coefficient was introduced to investigate the diffusion characteristics of the two coals. The results showed that the intact and tectonic coals have higher adsorption capacity for CO2 than methane under the same particle size, demonstrating the potential for storing a large amount of CO2 in coal seams. The time-dependent dynamic diffusion coefficient decreased significantly in the initial stages and then tended to stabilize as the desorption progressed. To better explain the gas diffusion characteristics of the coal, a gas diffusion mechanism model was established to analyze the variation in the diffusion coefficient over time. In addition, a new method for reducing the pressure difference was proposed to maximize CO2 storage and methane recovery. Finally, the environmental implications and safety of CO2 storage in coal seams during the implementation of a CO2-ECBM project was analyzed.

The electrochemical carbon dioxide (CO2) reduction provides a means to upgrade CO2 into value‐added chemicals. When powered by renewable electricity, CO2 electroreduction holds the promise of chemical manufacturing with carbon neutrality. A commercially relevant CO2 electroreduction process should be highly selective and productive toward desired products, energetically efficient for power conversion, and stable for long‐term operation. To achieve these goals, designing gas‐diffusion catalytic electrodes and prototyping reactors built upon in‐depth understandings of the reaction mechanisms are of paramount importance. In this review, the fundamentals of gas‐diffusion electrodes are briefly presented. Then, the most recent advances in developing high‐performance CO2 reduction using gas‐diffusion electrodes are overviewed. Reactor engineering aiming at enhancing productivity, energy efficiency, CO2 single‐pass utilization, and operating lifetime is further discussed. Challenges in developing CO2 electroreduction systems are included. The prospects for advancing CO2 electroreduction toward practical applications are also narrated.

Gas diffusion in a porous medium is a very complex process. Aiming at accurately describing the whole process of gas diffusion in a coal particle, a dynamic diffusion model of capillary pores in a coal particle (hereafter referred to as CPCPDD model) which is dependent on both the pore size and the pore pressure and based on the fractal characteristics of these capillary pores was established according to the characteristics of gas diffusion in different-sized pores. Next, parameters of the CPCPDD model were obtained through gas desorption, low-temperature nitrogen adsorption and mercury injection experiments on a coal particle. Furthermore, a comparative analysis was made between the solution results of the CPCPDD model and the data obtained from the gas desorption experiment. The results suggest that the CPCPDD model proposed in this study can accurately describe the whole process of gas diffusion in a coal particle. The diffusion coefficient obtained by the CPCPDD model is a variable related to the pore diameter and the pore pressure, and the classical uni-pore diffusion model, bi-disperse diffusion model and the multi-pore diffusion model are specific manifestations of the CPCPDD model in different pore size ranges. The research results boast crucial theoretical significance for in-depth understanding and interpretation of gas migration in coal reservoirs.

Two-dimensional (2D) π-conjugated conductive metal–organic frameworks (cMOFs, 2DπcMOF) with modulated channel sizes and a broad conductivity range have been reported in the last decade. In contrast, the corresponding comparative studies on their effects on chemiresistive sensing performances, which measure the resistive response toward external chemical stimuli, have not yet been reported. In this work, we sought to explore the structure–performance relationships of honeycomb-like 2D π-conjugated cMOF chemiresistive gas sensors with channel sizes less than 2 nm (the mass transport issue) and broad conductivity in the range from ∼10−8 S cm−1 to 1 S cm−1 (the charge transport issue). As a result, we found that the cMOF with a lower conductivity facilitates the much more sensitive response toward the charge transfer of the adsorbed gases (relative increases in resistance: R = 63.5% toward 100 ppm of NH3 for the as prepared Cu–THQ sensor with the conductivity of ∼10−8 S cm−1). Interestingly, the cMOF with a medium channel size (Cu–THHP–THQ) exhibited the fastest response speed in sensing, although it contains H2en2+ as neutralizing counterions in the channels. From the evaluation of the pore size distribution, it is found that the overall porosity (meso- & micro-pores) of cMOFs, rather than the pore size of the honeycomb structure, would determine their sensing speed. When comparing the performance of two different morphologies of nanorods (NRs) and nanosheets (NSs), NRs showed a slower response and extended recovery time, which can be ascribed to the slower gas diffusion in the more extended 1D channel. Altogether, our results demonstrate the first systematic studies on the effect of various structural parameters on the chemiresistive sensor performance of cMOFs.

Atmospheric gases trapped in polar ice have been used to reconstruct polar and global climate changes, providing better time resolution when less diffused. Experiments have shown that gas diffusion...

The adsorption behaviors of CO2, CH4 and N2 on MER zeolites were investigated by Grand Canonical Monte Carlo (GCMC) simulation method. The calculated pure gas uptake agreed well with the experimental data, which proved that the current simulation model and COMPASS force field are reliable. On this basis, molecular dynamics (MD) simulations were carried out for diffusion and separation of CO2, CH4 and N2 on K-MER zeolites, with the silicon MER zeolite as the reference. The results show that mean squared displacement (MSD) versus simulation time is sublinear. In MER zeolites, the configuration diffusion regime is dominant resulted from the tight fit of the gas molecules and the zeolite pore size. The diffusion of CO2, CH4 and N2 in MER zeolites with three-dimensional cage structures is anisotropic. The gas molecules diffuse preferentially along with the direction of x axis in K-MER zeolites. Extra-framework cations in zeolite plays important influence on the gas diffusion. In K-MER zeolites, the self-diffusion coefficients of CO2 and N2 are negative correlated with loading, whereas for the self-diffusion coefficient of CH4, it firstly increases and then decreases with the increase of loading. All of the self-diffusion coefficients of CO2, CH4 and N2 increase with the elevation of temperature. The order of diffusion activation energy is N2 (16.51 kJ/mol)>CH4 (8.39 kJ/mol)>CO2 (4.38 kJ/mol). K-MER zeolite membrane has good separation selectivity for gas mixture system of CO2/CH4, CO2/N2 and N2/CH4.The permeance of CO2 and N2 through K-MER zeolite membrane is as high as 104 GPU (1 GPU=3.35×10−10 mol/(s·m2·Pa)).

Porous carbon materials, which are widely used as gas diffusion layer (GDL) in PEFCs, have several issues including high cost and mechanical strength. In order to reduce the cost and thickness of GDLs, we focus on the possible use of metallic Ti and Sn as alternative GDL materials. In this study, Sn/Ti and Sn/Sn GDLs are prepared, which are based on porous Ti or Sn plates coated with Sn layer. Microstructural observations using scanning electron microscope (SEM) confirmed that porous Sn dendrite layer could be formed on the metallic GDLs. Current-voltage measurements of the cells using these materials revealed that the porous structure of the GDLs, on which Sn dendrites were formed, is advantageous to improve electrical contact.

The dynamics of water droplets on fiber layers are investigated by fiber-scale simulation using volume of fluid (VOF) method. The effects of fiber spaces, contact angles at fibers and numbers of layers on droplet dynamics are investigated. Force analysis for the pinned and moving droplets on fiber layers shows that the resistant forces from the wetted fibers at the edge positions rather than middle positions are the dominant force to control the droplet mobility. Force analysis for droplets on the dual-layers fiber layer are also conducted. The results show that the second layer contributes little to hinder the droplet directly due to the insignificant resistant force they provided. However, the second fiber layer enhances the total resistant force from the first layer. Furthermore a simplified model is used to compare with the fiber model. The results indicate that the simplified model can also be used to predict the droplet mobility once the contact angle is given.

In this paper, a hydrothermal method was applied to synthesize the nanosheet-like pure ZnO and 0.5%, 1 and 3% Co-doped ZnO (Co-ZnO). The pristine and Co-doped ZnO flower-like particles were assembled by porous nanosheets, with the uniform diameter about 18 μm. The N2-BET test found that Co doping significantly increased the specific surface area of the material which was conducive to gas diffusion and adsorption. HRTEM presented that 1% Co-ZnO nanosheets were composed of coral-like nanoparticles. The lattice distances 0.259 nm and 0.276 nm correspond to (002) and (100) crystal plane of ZnO. The gas sensing properties reveal that the 1% Co-doped ZnO present an outstanding enhanced sensitive performance comparing with pure ZnO to ethanol. To 100 ppm target gas, the response increased from 103 to 279.8 and the optimal operating temperature decreased from 369 to 348 °C, and the recovery time decreased from 40 to 18 s. The increased surface carrier concentration which promoted oxygen adsorption by Co was considered to be the key factor to improve the performance.

Developing highly efficient and selective electrocatalysts for the CO2 reduction reaction to produce value-added chemicals has been intensively pursued. We report a series of Cux Oy Cz nanostructured electrocatalysts derived from a Cu-based MOF as porous self-sacrificial template. Blending catalysts with polytetrafluoroethylene (PTFE) on gas diffusion electrodes (GDEs) suppressed the competitive hydrogen evolution reaction. 25 to 50 wt % teflonized GDEs exhibited a Faradaic efficiency of ≈54 % for C2+ products at -80 mA cm-2 . The local OH- ions activity of PTFE-modified GDEs was assessed by means of closely positioning a Pt-nanoelectrode. A substantial increase in the OH- /H2 O activity ratio due to the locally generated OH- ions at increasing current densities was determined irrespective of the PTFE amount.

The CO2 reduction reaction (CO2RR) is a potential means of using renewable electricity to synthesize commodity chemicals and fuels. The CO2RR can be performed at industrially relevant product formation rates in an electrolyser, which must simultaneously manage the transport of electrons, water, CO2 and protons at a cathode. Gas diffusion electrodes (GDEs) and polymer electrolyte membranes are used to mediate these critical processes. Consequently, the design and development of GDEs and membranes tailored for the CO2RR is critical. In this Review, we discuss how the properties of GDEs and polymer electrolyte membranes affect CO2RR electrolysis. The CO2 reduction reaction can be used to produce carbon-neutral chemicals using renewable electricity. This Review presents material design strategies for controlling the transport of products and reactants in electrochemical reactors that convert CO2 emissions into chemicals and fuels.

The local pH near the surface of a CO₂ reduction electrocatalyst strongly impacts catalytic selectivity and activity. Here, confocal fluorescence microscopy was used to map the electrolyte pH near a copper gas diffusion electrode during CO₂ reduction with micron spatial resolution in three dimensions. We observed that the local pH increased from pH 6.8 to greater than pH 10 as the current density was increased from 0 to 28 mA/cm² in a 100 mM KHCO₃ electrolyte. Variations in the pH across the surface indicate areas of locally increased activity. Within deep trenches of the active layer, the local pH increases as trench width decreases. Computational models confirm these experimental results and also showed that the catalyst found within narrow trenches is more active than that found at the surface of the electrode. This study suggests that the overpotential required to perform selective CO₂ reduction can be reduced by increasing the density of narrow trench regions in the microporous layer.

Thomas Graham discovered the law that bears his name while studying gas diffusion into air and other gases. He also found that the same relationship held with gas effusion, the movement of gases through a pinhole into a vacuum. Modern understanding of diffusion and effusion is based on kinetic-molecular theory, and it is generally accepted that Graham’s law as originally stated holds for effusion. Diffusion is more complicated, involving intermolecular collisions and composition gradients. This article describes a simple laboratory activity in which diffusion of gases into air though an open syringe is studied over time. The activity could be adapted and modified into an inquiry-based independent study activity or undergraduate research project.