Diffusion Of Liquid Water Research Articles

Drying model and drying characteristic analysis of multiphase porous medium for flake materials

The main purpose of this paper is to optimize the drying process and gain a fundamental understanding of the movement of different phases during the drying process of cigarette flakes. Therefore, the drying model of a multiphase porous medium for cigarette flakes has been established in this paper. Binary diffusion of water vapor and gas pressure transport, as well as capillary diffusion of liquid water and gas pressure transport, are considered in this model. The non-equilibrium equations are used to calculate the evaporation rate, and the fluxes of water vapor and liquid water are also obtained. The results indicated that the stochastic stacking model is suitable for studying cigarette flake drying. Because water evaporation absorbs more heat than hot air, the temperature of the cigarette flakes decreases during the drying process. The water activity, vapor density, and vapor mole fraction of the medium layer of cigarette flakes were significantly higher than those of the upper and lower layers. The highest water vapor and liquid water fluxes occur in the medium of the drying process. Additionally, it was observed that the saturated vapor pressure was higher than the equilibrium vapor pressure and steam pressure, non-equilibrium evaporation occurs in the medium of drying. The correlation between the evaporation rate and the drying temperature and humidity of the cigarette flakes was found to be positive.

Enhancing sludge hot-pressing drying by sawdust: Sensitivity analysis of heat and mass transfer physical properties of sludge, process performance, and kinetics

Hot-pressing drying shows promise in sludge treatment. Optimization often focuses on temperature and external load. However, the impact is restricted by sludge inherent properties, which can be changed by sludge conditioning. Due to coupling effects, altering individual sludge properties is difficult. Key sludge properties that significantly affect heat and mass transfer remain unclear, hampering identification of specific conditioning techniques. To address this gap, the effects of sludge specific heat, thermal conductivity, water activity, moisture diffusivity, and moisture permeability on drying performance were explored using sensitivity analysis. Sawdust conditioning and kinetics were employed to improve drying performance, validate sensitivity analysis findings, and elucidate drying mechanisms. Sensitivity analysis revealed that liquid water permeability significantly positively influenced drying performance by enhancing liquid water convection. Sawdust conditioning and kinetics demonstrated that adding 20 % sawdust increased drying flux from 31 to 62 kgwater·m⁻²·h⁻¹. The drying process included warm-up and constant rate, the first falling rate, and the second falling rate periods. Enhanced moisture transfer was dominated by breakthrough of interfacial gas film, transformation from liquid water diffusion to convection, and facilitated water vapor diffusion, respectively. Hence, this study provided novel insights, strategies, and direction of guidance for revealing sludge drying mechanisms and improving drying performance.

Ruggedness test made with mortars by free water intake test

Capillary movement in porous building materials is modelled by unsaturated flow. Material property needed for the calculations is liquid water diffusivity which is usually estimated by water absorption coefficient and capillary saturation moisture content. While determining of water absorption coefficient can be done according to EN-ISO 15148:2002 there is still lack of knowledge about sources of uncertainty in the experiment. In this paper author presents results of ruggedness tests done with mortar. The sources of errors investigated here was the shape and water inflow area of the specimen. Some specimens were also deprived of side sealing and sealing of upper surface as it was known to have impact on results. Results shows there is no considerable impact on measured capillary properties if the specimen sides are sealed or not. Nor does affect the immersed surface area of the specimen. The shape of the immersed surface instead has significant impact on values of both water absorption coefficient and capillary saturation of the specimen.

Experimental study on hygrothermal properties of recycled aggregate concrete

Recycled aggregate concrete has great significance from the environmental protection perspective. However, the current research findings on its hygrothermal properties are sparse. In this study, the apparent density, thermal conductivity, mass moisture content, water vapor permeability coefficient, water absorption coefficient, and liquid water diffusion coefficient of recycled aggregate concrete were determined by experimental methods. The effect of temperature and humidity on thermal conductivity was also studied. Furthermore, the fitting relations of the thermal conductivity of recycled aggregate concrete with the two variables of temperature and relative humidity were presented. The isothermal moisture absorption curve and the fitting relationship of water vapor permeability coefficient with relative humidity were also developed. The results indicate that the thermal conductivity of the eight recycled concrete specimens in the dry state at 25°C ranges from 0.994 to 1.242 W/(m·K). The thermal conductivity increases with the increase in temperature. When the temperature rises from 25°C to 35°C, the thermal conductivity of recycled aggregate concrete increases by 3.8%–13.5%. With the increase of relative humidity, the thermal conductivity of recycled aggregate concrete shows an increasing trend, followed by a steady state, and finally the increasing trend, showing a cubic function relationship between them. When the relative humidity increases from 0% to 95%, the thermal conductivity increases by about 20%. The mass moisture content and the water vapor permeability coefficient increase with relative humidity. The results of this study can provide basic information for the study on the heat and moisture transfer of recycled aggregate concrete, and enhance the database of hygrothermal properties of building materials.

Preparation of PLA/PEG@SDS microfibers‐based nonwovens via melt‐blown process parameters: Wound dressings with enhanced water wetting performance

AbstractPolylactic acid (PLA) melt‐blown nonwovens are promising materials for medical and healthcare applications due to their properties such as softness, breathability, biodegradability, and biocompatibility. However, their widespread commercial application has been limited by their poor mechanical properties and insufficient hydrophilicity limit. To address these limitations, we prepared polylactic acid/polyethylene glycol@sodium dodecyl sulfate (PLA/PEG@SDS) microfibers‐based nonwovens using a melt‐blown process. The average fiber diameter of PLA/PEG@SDS microfibers nonwovens could be tuned from 1 to 13.9 μm by regulating the die temperature or hot air pressure. In addition, increasing the die temperature from 210°C to 225°C resulted in a 140.3% and 124.8% increase in the breaking resistance along the mechanical direction (MD) and transverse direction (CD), respectively. Meanwhile, the increase in the breaking resistance was observed between the hot air pressure and the mechanical strength of the PLA/PEG@SDS microfibers‐based nonwovens. Notably, the prepared samples exhibited a short wetting time of 3.438 s, a fast water absorption rate of 68.401%·s−1, and an excellent liquid water diffusion of 5.86 mm s−1. These results suggest that the PLA/PEG@SDS microfibers nonwovens prepared using melt‐blown process demonstrate efficient wetting performance, which paves the way for the industrial production of medical‐grade materials such as wound dressings.

Combined Description of the Equation of State and Diffusion Coefficient of Liquid Water Using a Two-State Sanchez-Lacombe Approach.

Water is one of the most important compounds on Earth, yet its material properties are still poorly understood. Here, we use a recently developed two-state, two-(time)scale (TS2) dynamic mean-field model combined with the two-state Sanchez-Lacombe (SL) thermodynamic theory in order to describe the equation of state (density as a function of temperature and pressure) and diffusivity of liquid water. In particular, it is shown that in a relatively wide temperature and pressure range (160 K < T < 360 K; 0 < P < 100 MPa), density and self-diffusion obey a special type of dynamic scaling, similar to the "τTV" scaling of Casalini and Roland, but with the negative exponent γ. The model predictions are consistent with experimental data. The new equation of state can be used for various process models and generalized to include multicomponent mixtures.

Simulation of moisture transport in fired-clay brick masonry structures accounting for interfacial phenomena

This paper presents a numerical study on moisture transport in brick masonry walls with a special focus on the simulation of their hygric performance as well as the hydraulic phenomena at the brick-mortar interface. A diffusivity model based on Fick's law is used to describe the moisture transport accounting for both liquid and water vapor movement. The necessary hygric parameters are obtained directly from experimental tests or determined by curve fitting. The proposed model is validated with respect to water absorption and drying tests. The good-fitness of the results is qualitatively assessed and an overall good agreement is found between the simulated and measured curves. It is demonstrated that the chosen liquid water diffusivity expression needs to be adjusted to represent drying processes; the necessary adjustment is made through a diffusivity factor implemented in the original analytical expression. The interface impact on water absorption is introduced as a hydraulic resistance. Moreover, it is hypothesized that the presence of successive interfaces entails an additive in-series effect. Conversely, the interfacial impact on drying is negligible. Finally, the proposed model is extended to different modeling approaches commonly used for mechanical studies of masonry. The necessary input data, modeling methodology, advantages and disadvantages associated with each modeling strategy are also discussed. The present study points out the need of studying water absorption in multi-layered structures made up of constituents with relatively similar hygric behavior. In such cases, the impact of imperfect contact at the interface between materials is not negligible.

Effect of ion chelator on water sorption and transport behavior of unsaturated mortars with high volume fly ash and blast furnace slag

This paper investigated the effect of ion chelator on water dependency of water transport and fixation in mortars with high volume of fly ash (FA) and blast furnace slag (BFS). Ion chelator (CA) as a new type of crystalline admixture could improve self-healing performance of cementitious materials by crack-healing and pore filling. The porosity of mortar was measured by vacuum saturation and pore size distribution of mortar was tested by low-field nuclear magnetic resonance (LF NMR). The water sorption of mortar was evaluated by water vapor adsorption desorption balance. The water transport of mortar with and without crack was tested by capillary water absorption. Based on the experimental results, the water distribution of capillary water absorption in mortar was calculated. Results showed that the pore structure of mortar was improved evidently by CA, and the pores (>1μm) and the total porosity in mortar containing 50 %BFS were significantly reduced. CA had no obvious effect on the water vapor diffusion coefficient of mortar, but has a significant impact on the liquid water diffusion coefficient, especially in pure cement and containing 50 % BFS mortar. The total capillary water absorption and capillary absorption coefficient of mortars were reduced by CA, especially in pure cement and containing 50 % BFS mortar. The addition of CA decreased the water penetration in mortar, the reduction rates of water penetration depth in experimental groups (with CA) at 360 min were 12.4 % (100 %OPC), 6.0 % (30 %FA), 21.3 % (50 %BFS) and 10.8 % (16 %FA 24 %BFS) compared to control group (no CA). After curing for 42d, the normalized water absorption coefficients S” of cracked M100CA and M50BFS50CA were 55.1 % and 72.0 %, respectively. Ion chelator significantly improved internal pore structure and crack healing ability of the mortar containing 50 %BFS, and then delayed the water diffusion.

Dynamic performance for a kW-grade air-cooled proton exchange membrane fuel cell stack

In this study, a kW-grade air-cooled proton exchange membrane fuel cell (PEMFC) stack with a dead-end anode (DEA) operation is designed and manufactured. The gravity-assisted drainage principle is applied for the stack to design the wettability of gas diffusion layers (GDLs) and the anode channel geometry, which can help the liquid water that diffuses to the anode to drain out of the anode porous electrode and move down the anode channel outlets. As a result, the stack can stably operate in a long purge interval of 268 s and in a short purge time of 2 s. In addition, using this design, only four small-power fans are employed to pump air to the cathode to provide oxygen for the electrochemical reaction and cool the stack. With a constant load current of 30, 45, or 60 A, the stack output voltage is experimentally tested at various cathode air flow rates (CAFRs). The local temperatures (60 measurement points) inside the stack and the pressure differences across anode channels are also monitored to understand heat dissipation and the back diffusion of liquid water. In a wide range of operating conditions, the designed stack possesses superior and stable voltage output characteristics with relatively uniform temperature distributions. The measured maximum output power is 3.83 kW, and the parasitic powers of fans are only 80∼112 W.

Heat and mass transfer at interfaces in decomposition of methane hydrate under combustion

The thermodynamic and kinetic mechanism of decomposition of gas hydrate with burning is still in its infancy. The heat and mass transfer are analyzed based on accurate positioning of interfaces in the decomposition of methane hydrate under combustion by ReaxFF molecular dynamics simulation. There is a transition region between the solid–liquid interface of decomposition and the burning boundary. Results show that the water layer in transition region largely hinders the diffusion of methane produced by decomposed hydrate so that hydrate cannot burn steadily. Heat flux at burning boundary is larger than at solid–liquid interface in the early stage of decomposition, so thermal transport in burning boundary drives decomposition of hydrate. The heat flux at solid–liquid interface is larger than at burning boundary in the later stage, and the diffusion and extrusion of liquid water with heat transport through the transition region become the new driving force. The interfacial thermal resistance of solid–liquid interface decrease rapidly to the order of 10−10 m2·K/W at different temperatures. 23.9 % of the heat from burning process at 1600 K can maintain entire decomposition of hydrate within 3000 fs. Although the formation of transition region doesn’t significantly reduce the decomposition rate of hydrate, it is unfavorable for burning. The discharge of water layer in transition region should be focused on in the future.

The Influence of Pre-Irradiation and Simultaneous Grafting Methods on the Physicochemical Properties of Polyethylene-Based Anion-Exchange Membranes and Ionomers

Controlling the synthesis parameters during radiation-induced grafting (RIG) is a key feature to design anion-exchange membranes (AEMs) with enhanced properties that lead to high performances and stability. The present study provides a systematic investigation of the effect of RIG methods on physicochemical properties of LDPE-based AEMs with similar degree of grafting (DoG) and ion exchange capacity (IEC). The LDPE films were grafted with 4-vinylbenzyl chloride using two methods: i) pre-irradiation (PIM), using electron-beam (EB), and ii) simultaneous (SM), using gamma-ray (γ), both in air atmosphere and with different absorbed doses. The AEMs grafted by SM were irradiated with 20, 25, and 30 kGy, while the ones grafted by PIM were irradiated with doses of 30, 70, and 100 kGy. Samples grafted by the two different methods have the same molecular structure, but distinct physicochemical properties due to markedly differences in the degree of crosslinking. The studies have shown that grafting step directly influences the internal structure and morphology of AEMs. Multi-technique characterization demonstrated that RIG method determines the mechanical properties, water transport, and the distribution of ionic groups in AEMs. These characteristics have direct impact on fuel cell performance, anion-conductivity, and AEM stability.Figure 1 shows the polarization curves of anion-exchange membrane fuel cells (AEMFCs) operating at 80 °C with AEMs grafted by PIM and SM. Comparing AEMFCs performances containing AEMs with similar IECs from the two different grafting methods, it is possible to notice that AEMs synthesized by PIM, in general, promote better fuel cell performances than the corresponding ones prepared by SM. The main differences in the polarization curves are evidenced in the ohmic and the mass transport limiting regions. This result suggests that water management is enhanced in AEMs synthesized by PIM compared to the SM-AEMs. A membrane that allows rapid back diffusion of liquid water from the anode to the cathode, should reduce the current losses due to mass transport limitations.The gel content results confirmed the presence of more crosslinking in membranes synthesized by PIM than in the ones synthesized by the SM. As a consequence of that, enhanced mechanical properties were found for PIM-AEMs, indicating polymer backbone reinforcement due to high degree of crosslinking. Small-angle X-ray scattering measurements have confirmed that the microstructure of PIM and SM AEMs are different and this feature is directly related to ion conduction and water transportation.Figure 1. AEMFCs performances at 80 °C with LDPE-based AEMs grafted by the pre-irradiation method and by the simultaneous method. H2 anode gas flow=0.8 L min-1, O2 cathode gas flow=0.5 L min-1, both supplied unpressurized with optimal dewpoint temperatures ranging from 77 to 80 °C. AEMs are referred according to the grafting method and absorbed dose. 30 and 70-EB-PIM refer to the samples irradiated with 30 and 70 kGy, respectively, in the EB accelerator and grafted by the pre-irradiation method. 20 and 25-γ-SM refer to AEMs irradiated with 20 and 25 kGy, respectively, by γ-rays and grafted by the simultaneous method. Figure 1

An energy-conservative many-body dissipative particle dynamics model for thermocapillary drop motion

The thermocapillary motion of a drop on a solid substrate is a common phenomenon in daily life and many industrial fields. The motion can be significantly affected by the temperature gradient of the substrate and the properties of the liquid, such as surface tension, viscosity, thermal coefficient, density, and diffusivity. In this study, a numerical model based on modified many-body dissipative particle dynamics was developed to capture correctly the temperature dependence of a fluid. The momentum, thermal diffusivity, viscosity, and surface tension of liquid water at various temperatures ranging from 273 to 373 K were used as examples to verify the proposed model. The results calculated with this model for heat conduction in a liquid–solid system are in good agreement with those calculated with Fourier's law. The approach successfully modeled the thermocapillary motion of a liquid water droplet on a hydrophobic substrate with a temperature gradient. The migration of the droplet on a flat solid substrate was induced by the difference in surface tension due to the temperature gradient. The migration velocity increased with the temperature difference, which is in agreement with the present theoretical analysis and dynamic van der Waals theory. The modified numerical model proposed in this work could be used to study heat and mass transfer across a free interface, such as Marangoni convection in multiphase fluid flow.

Investigation of phosphoric acid and water transport in the high temperature proton exchange membrane fuel cells using a multiphase model

AbstractA three‐dimensional, nonisothermal, and multiphase model of high temperature proton exchange membrane fuel cells is built to investigate water and phosphoric acid transportation, in which a spherical agglomerate model considering catalyst layer structure and liquid saturation is applied to determine the electrochemical kinetics in the cathode catalyst layer. Experimental polarization curve, water proportion in the anode outlet gas, and phosphoric acid distribution are selected for validation. It is found that the simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of current density and stoichiometry on the variable distributions are analyzed. The results show that water in anode is mainly from cathode by concentration diffusion of liquid water, and the proportion of anode outlet water to the total produced water decreases slightly with the increase of current density. A higher current density leads to a greater electromigration of phosphoric acid from cathode to anode and a higher liquid phase fraction in anode, while a lower phosphoric acid concentration in the fuel cells.

Dependency of conservation conditions of Takase stone Buddha, Oita, Japan, on anisotropy of liquid water diffusivity

Takase Stone Buddhas is one of the important old stone buddha sculptures curved into the inner wall of a cave in Oita, Japan. It is located in the cave curved into the cliff of a hill of volcanic tuff. In general, because the cave is currently protected from rain and direct solar radiation by the roof shelter and waterproof treatment, the Buddhas is well conserved and no currently ongoing weathering can be clearly observed. However, because of a high ground water level, there is a concern in the influence of water evaporation at and near the surface of the stone buddhas and the wall of the cave on their deterioration. In the past, we conducted a long-term field survey of conservation environment to obtain yearly data set of the conservation environment that can be used as input of numerical simulations of heat and water transport in the material. In this paper, we report measurement data of the ground water level as well as the liquid water diffusivity of the tuff stone that significantly affects the conservation condition. We also performed numerical analyses on heat and moisture transport in the tuff stone layer and stone buddhas. The simulation results show that the conservation condition of Takase stone Buddha strongly depends on the anisotropy of liquid water diffusivity of the tuff stone.

Reinforced contact between sludge and hot wall for enhancing conductive drying by applying external load: Heat and mass transfer analysis

Conductive drying is widely used in sludge management for emission reduction and energy recovery. However, interfacial vapor film between sludge and hot wall inhibits drying efficiency. This study employed external load to reinforce the contact of sludge with hot wall for facilitating interfacial heat transfer. Subsequently, a drying model was developed for heat and mass transfer mechanism analysis. The influence of external loads was explored on conductive drying behavior at 180 °C and 0, 25, 100 kPa. The drying rates were increased with external load. The drying process consisted of the warm-up, constant rate, and two falling rate periods. Without external load, moisture migration was dominated by liquid water diffusion induced by capillary force in the first falling rate period. However, at 100 kPa, the convective flow of water and vapor dominated the moisture migration in the first falling rate period due to increased interfacial heat transfer, evaporation rate, and gas pressure. Then vapor diffusion was predominant owing to enhanced gas diffusivity. Besides, this study provided essential information on design optimization of industrial applications, such as moisture content prediction for searching for equilibrium points between energy consumption and resource recovery and the critical factors for enhancing moisture migration.

Modified Carbon Cryogel As Tunable Electrocatalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Pt-free catalysts for Oxygen Reduction Reaction (ORR) are broadly studied for Proton Exchange Membrane Fuel Cells (PEMFC) in order to widely develop hydrogen as a fuel for individual transportation. To reduce the performance gap between noble and non-noble catalysts, kinetics and thermodynamics are to be considered. The kinetics of the reaction is influenced by both the turn over frequency of the catalytic site and the density of said sites in the materials. The diffusion of gaseous reactants and liquid water produced is equally important as it modifies the thermodynamic of the reaction. For a long time now, Fe-C-N materials have been identified as the most promising Pt-free electrocatalysts for ORR despite the limited knowledge on the actual structure of the involved sites. Several approaches are currently studied to produce materials with high active site density and high porosity. Different carbon structures are modified to include active sites. The most reported and promising routes are Metal Organic Frameworks, polymers on carbon nanoparticles and silica templated organic structures[1], [2]. With the recent works on the structure and activity of Fe-N4 active sites[3], the carbon plane surrounding active sites seem to have a great impact on the sites’ Turn-Over Frequency (TOF). Cheaper routes are explored to produce tunable carbon structure adapted to the requirement of ORR catalysis. The sol-gel polymerization of organic polymer is a newly investigated route[4], [5]. In this approach, carbon cryogels are chosen for the tenability of their porous structure and the versatility of precursors that can be tested. Porous Fe-C-N catalysts are synthesized in one-step by a modified resorcinol-formaldehyde aerogel method. Gelation, drying and pyrolysis steps are carefully studied and optimized to obtain a high density of accessible catalytic sites. As iron is both a precursor of the active site and a catalyst for the graphitization of carbon during the pyrolysis, its impact on the final structure is studied. Different types of precursors are used in several loadings. The family of materials therefor obtained presents various degrees of graphitization and TOF of the active sites. Structure-Performance relationships are investigated for this family of materials. Electrochemical performances of the materials are assessed in RDE-setup in acidic electrolyte as well as in differential cell of PEMFC. The porosity of the formed materials is measured with nitrogen adsorption. The electrochemical performances in acidic electrolyte are comparable with state of the art materials.[1] A. Serov, K. Artyushkova, et P. Atanassov, « Fe-N-C Oxygen Reduction Fuel Cell Catalyst Derived from Carbendazim: Synthesis, Structure, and Reactivity », Adv. Energy Mater., vol. 4, no 10, p. 1301735, 2014, doi: 10.1002/aenm.201301735.[2] E. Proietti et al., « Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells », Nat. Commun., vol. 2, p. 416, 2011, doi: 10.1038/ncomms1427.[3] T. Asset et P. Atanassov, « Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells », Joule, vol. 4, no 1, p. 33–44, janv. 2020, doi: 10.1016/j.joule.2019.12.002.[4] A. Sarapuu, L. Samolberg, K. Kreek, M. Koel, L. Matisen, et K. Tammeveski, « Cobalt- and iron-containing nitrogen-doped carbon aerogels as non-precious metal catalysts for electrochemical reduction of oxygen », J. Electroanal. Chem., vol. 746, p. 9–17, 2015, doi: 10.1016/j.jelechem.2015.03.021.[5] Y. Wang et S. Berthon-Fabry, « One-Pot Synthesis of Fe-N-Containing Carbon Aerogel for Oxygen Reduction Reaction », Electrocatalysis, nov. 2020, doi: 10.1007/s12678-020-00633-8.

Experimental study of hygro-thermal characteristics of novel cement-cork mortars

Since cork has low density, low thermal conductivity, and water resistance, mixing cork with mortar can effectively improve the thermal and moisture insulation of mortar materials. In this study, cement-cork mortars with different cork volume admixtures was prepared. The fresh state consistency and compressive strength of cement-cork mortars were studied. The porosity and pore distribution characteristics of the prepared samples were obtained using microparticle mercury porosimeter. The thermo-physical parameters (thermal conductivity, specific heat, heat storage coefficient, and thermal diffusivity) and hygric properties (isothermal adsorption curve, liquid water diffusion coefficient, and water vapor permeability) of cement-cork mortars were studied experimentally. Meanwhile, The variation of thermo-physical parameters of the prepared samples with temperature (within the range of 20–70 °C) and moisture content (with the relative humidity range of 0–100%) were obtained. The results show that the hygro-thermal parameters of cement-cork mortars decreased with the increase of cork content (the largest decrease in thermal conductivity was 52.6%, whereas the drop in liquid water diffusion coefficient was 87.9%). Furthermore, temperature and moisture content had great influence on the thermo-physical parameters of the samples (the maximum growth rates of thermal conductivity and specific heat within the studied temperature range were 7.5% and 27.3%, respectively, whereas the corresponding values within the studied range of moisture content were 60.7% and 26.7%, respectively).

One-Pot Synthesis of Pt-Free 3D-Catalyst for PEMFC

Pt-free catalysts for Oxygen Reduction Reaction (ORR) are broadly studied for Proton Exchange Membrane Fuel Cells (PEMFC) in order to widely develop hydrogen as a fuel for individual transportation. To reduce the performance gap between noble and non-noble catalysts, kinetics and thermodynamics are to be considered. The kinetics of the reaction is influenced by both the turn over frequency of the catalytic site and the density of said sites in the materials. The diffusion of gaseous reactants and liquid water produced is equally important as it modifies the thermodynamic of the reaction. For a long time now, Fe-C-N materials have been identified as the most promising Pt-free electrocatalysts for ORR despite the limited knowledge on the actual structure of the involved sites. Several approaches are currently studied to produce materials with high active site density and high porosity. Different carbon structures are modified to include active sites. The most reported and promising routes are Metal Organic Frameworks, polymers on carbon nanoparticles and silica templated organic structures.In this new approach, carbon cryogels are chosen for their tunability and high specific surface area. Porous Fe-C-N catalysts are synthesized in one step by a modified resorcinol-formaldehyde aerogel method. Gelation, drying and pyrolysis steps are carefully studied and optimized to obtain a high density of accessible catalytic sites. The porosity of the formed materials is measured with nitrogen adsorption. Values of the Brunauer Emmett Teller (BET) surface area up to 800 m2/g are obtained. The impact of different synthesis parameters on catalytic activity is studied. To assess the catalyst’s performances toward ORR, rotating ring Disk Electrode measurements as well as Fuel Cell polarization curves will be reported.

Coalbed methane diffusion and water blocking effects investigated by mesoscale all-atom molecular dynamic simulations.

In coalbed methane extraction processes, the water blocking effect (WBE) is a formation damage that limits the extraction efficiency. To investigate WBE mechanisms at the molecular level, realistic coal models must be developed to simulate the interplay between methane and liquid phase water in a coal matrix's mesopores and macropores. This study built a massive and highly scalable coal tube model with accurate all-atom force fields. Based on this model, we investigated the adsorption and diffusion of methane and liquid water in the mesopores of coal. We found that methane forms multiple layers of adsorption on the coal surface, and the diffusivity of methane strongly depends on pore sizes and the presence of water. When both methane and liquid water were loaded in the coal tube, the liquid phase formed a nearly impenetrable barrier that prevented methane diffusion. This work provides insights into the mechanism of the WBE and can facilitate further studies on WBE alleviating strategies.

Translational and Rotational Diffusion in Liquid Water at Very High Pressure: A Simulation Study.

Translational and rotational diffusion coefficients of liquid water have been computed from molecular dynamics simulation with a recent polarizable potential at 298, 400, and 550 K at very high pressure. At 298 K, the model reproduces the initial increase and the occurrence of a maximum for the translational and rotational diffusion coefficients when the pressure increases. At 400 and 550 K, translational and rotational diffusion coefficients are found to monotonically decrease when pressure increases in the gigapascal range, with the translational coefficient decreasing faster than the rotational one. At 400 K, such an evolution of the rotational diffusion coefficient contrasts with quasielastic neutron scattering results predicting a near independence of the rotational diffusion with a pressure increase above ≃0.5 GPa. An interpretation is proposed to explain this discrepancy. The pressure dependence of the translation-rotation coupling is analyzed. The anisotropy of rotational diffusion is investigated by computing the rotational diffusion tensor in a molecular system of axes and the reorientational correlation times of rank 1 and rank 2 of the inertia axes and of the OH bond vector. Deviation of the simulation data with respect to the predictions of the isotropic Debye model of rotational diffusion are quantified and can be used to estimate experimental rotational diffusion coefficients from experimental reorientational correlation times.