Liquid Saturation Research Articles

Microscale structure optimization of catalyst layer for comprehensive performance enhancement in proton exchange membrane fuel cell

The catalyst layer plays a significant role in promoting the redox reaction of proton exchange membrane fuel cells (PEMFC). Especially at high current densities, as the oxygen reduction rate increases and more water is produced, the pores of the catalyst layer are easily clogged, thus preventing the oxygen from reaching the reactive sites. In this work, a cathode catalyst layer with micro-grooved structure is proposed to optimize the water-gas transport and output performance of PEMFC. Three-dimensional two-phase numerical simulation results show that the catalyst layer with grooves significantly improves the water-gas transport performance of PEMFC, with a maximum increase of 4.76 % in output performance. Combining the response surface methodology and non-dominated sorting genetic algorithm-II for multi-objective optimization (MOO), the optimal solution corresponding to anode/cathode relative humidity and ionomer volume fraction is confirmed to be 86.73 %, 80.10 %, and 0.2592, respectively. The optimization objectives (oxygen concentration, liquid saturation, and net power density) are improved by 3.88 %, 1.95 %, and 1.79 %, respectively. The differences between the three optimization objectives obtained by numerical simulation verification are only 0.61 %, 0.32 %, and 0.55 %, indicating that the MOO results are reliable and accurate.

Coupled optimization of auxiliary channels and porosity gradient of GDL for PEMFC

In this study, we proposed a novel under-rib auxiliary channel and developed a systematic methodology combining computational fluid dynamics (CFD), an artificial neural network (ANN), and a particle swarm optimization (PSO) algorithm for the coupled optimization of the auxiliary-channel structure and gas diffusion layer (GDL) porosity gradient in a proton exchange membrane fuel cell (PEMFC). First, we developed a three-dimensional multiphase numerical model of a non-isothermal parallel flow-field PEMFC. ANN was then applied to obtain a mapping relationship between auxiliary channels structure, GDL porosity gradient and output performance of PEMFC with finite CFD data. Finally, PSO was employed to perform an optimization search to determine the optimal parameter pairing, resulting in a 16.7 % increase in maximum power density and a 20.6 % increase in the current operating range. In addition, the optimized PEMFC exhibited superior mass transfer and water removal capabilities, where the average oxygen concentration in the intermediate section of cathode GDL was increased by 33.3 % and the maximum liquid water saturation was reduced by 23.5 %. Moreover, the coupled optimization of the auxiliary channels and the GDL porosity gradient can increase power density by at least 8.7 % and current density by 9.1 % compared to single component optimization.

Operando Scanning Small-/Wide-Angle X-ray Scattering for Polymer Electrolyte Fuel Cells: Investigation of Catalyst Layer Saturation and Membrane Hydration- Capabilities and Challenges.

Polymer electrolyte fuel cells are an essential technology for future local emission-free mobility. One of the critical challenges for thriving commercialization is water management in the cells. We propose small- and wide-angle X-ray scattering as a suitable diagnostic tool to quantify the liquid saturation in the catalyst layer and determine the hydration of the ion-conducting membrane in real operating conditions. The challenges that may occur in operando data collection are described in detail─separation of the anode and cathode, cell alignment to the beam, X-ray radiation damage, and the possibility of membrane swelling. A synergistic development of experimental setup, data acquisition, and data interpretation circumvents the major challenges and leads to practical and reliable insights.

Numerical investigation and experimental verification of liquid water dynamic transfer characteristics in the flow field of PEMFC with dead-ended anode during gas purging

Water accumulation in the anode during the operation of a proton exchange membrane fuel cell with the dead-ended anode (DEA) has a significant impact on its performance. In this study, numerical simulation and experimental verification were combined to investigate the water accumulation and water dynamic transfer characteristics in the anode flow field during dead-ended operation and gas purging, respectively. The effects of placement orientation, surface wettability, and purge time on the water dynamic transfer characteristics are investigated. The results show that the placement orientation has significant effects on the water distribution characteristics and drainage for PEMFC. When placed vertically and with a purging time of 50 ms, the liquid water saturation in the channel is 5.52 %, which drains 14.91 % more water than that of horizontal placement. Besides, Water discharge velocity and water removal ratio should be considered comprehensively in the development of the purge strategy, the optimal purging time is 70 ms with the water discharge velocity and water removal rate are 0.39 μl/ms and 97.34 %, respectively. Finally, the transfer process of liquid water in the flow field with DEA mode is observed by visual experiment, and the results of the numerical simulation are verified.

Multifunctional Eco-Friendly Adsorbent Cryogels Based on Xylan Derived from Coffee Residues.

Agricultural and animal farming practices contribute significantly to greenhouse gas (GHG) emissions such as NH3, CH4, CO2, and NOx, causing local environmental concerns involving health risks and water/air pollution. A growing need to capture these pollutants is leading to the development of new strategies, including the use of solid adsorbents. However, commonly used adsorbent materials often pose toxicity and negative long-term environmental effects. This study aimed to develop responsive eco-friendly cryogels using xylan extracted from coffee parchment, a typical residue from coffee production. The crosslinking in cryogels was accomplished by "freeze-thawing" and subsequent freeze-drying. Cryogels were characterized in terms of morphology by using scanning electron microscopy, porosity, and density by the liquid saturation method and also moisture adsorption and ammonia adsorption capacity. The analysis showed that the porosity in the cryogels remained around 0.62-0.42, while the apparent densities varied from 0.14 g/cm3 to 0.25 g/cm3. The moisture adsorption capacity was the highest at the highest relative humidity level (80%), reaching 0.25-0.43 g of water per gram of sample; the amount of water adsorbed increased when the xylan content in the cryogel increased up to 10% w/v, which was consistent with the hygroscopic nature of xylan. The ammonia adsorption process was modeled accurately by a pseudo-second-order equation, where the maximum adsorption capacity in equilibrium reached 0.047 mg NH3/g when xylan reached 10% w/v in cryogels, indicating a chemisorption process. The cryogels under investigation hold promise for ammonia adsorption applications and GHG separation, offering a sustainable alternative for gas-capturing processes.

A comprehensive study of parameters distribution in a short PEM water electrolyzer stack utilizing a full-scale multi-physics model

Proton exchange membrane (PEM) electrolyzer is regarded as one of the most promising technologies for hydrogen production. To meet the requirements of large-scale hydrogen production, PEM electrolyzer stack that composed by series units are necessary. However, the composition brings extra problems such as maldistribution from the manifold to each unit, which results in parameters differences among the units and further gives rise to stack degradation. In this study, a full-scale three-dimensional multi-physics model is developed to simulate a short PEM electrolyzer stack with four single electrolyzer units. All the electrolyzer components and connections (e.g. manifold) are included in the model, and the temperature distribution, liquid saturation, membrane water content and electrolyzer performance are investigated in detail. The results indicate that performance of each single electrolyzer decreases from the first to the last unit along the water flow direction. It is recommended that the inlet temperature of electrolyzer stack should not exceed 60°C. To ease the thermal management, the water supply mode for cathode side can obviously lower the stack temperature to a preferred range, and improve the temperature uniformity. This study provides a comprehensive understanding of the distribution behaviors inside the electrolyzer stack and guides the operating parameters optimization.

Induced polarization of clay-rich materials — Part 3: Partially saturated mixtures of clay and pyrite

Source rocks for oil and gas are often associated with shales that are rich in pyrite and kerogen. Induced polarization is a suitable tool to characterize these formations. We develop a new experimental database of 43 laboratory experiments using mixtures of clay and pyrite under fully or partially water-saturated conditions. The liquid water saturation is in the range of 20%–100%, whereas the pyrite content is in the range of 0%–17%. Spectral-induced polarization measurements are performed in the frequency range of 0.1 Hz to 45 kHz at room temperature (approximately 25°C ± 1°C). The complex conductivity spectra are fitted with a Cole-Cole model, and the Cole-Cole parameters are determined using a stochastic procedure based on a Markov chain Monte Carlo sampler. The Cole-Cole parameters associated with low-frequency dispersion are then plotted as a function of the (water) saturation and pyrite content (volume fractions). Four predictions of the model are tested against the experimental data. We find that the model is able to explain the results, including (1) the chargeability depends on the pyrite content in a predictable way, (2) the (instantaneous) conductivity depends on the saturation according to Archie’s law, (3) the Cole-Cole exponent does not depend on the saturation and pyrite content (except at very small pyrite content of <1 vol%), and (4) the relaxation time is inversely proportional to the instantaneous conductivity. We also develop a more advanced petrophysical model using a double Cole-Cole distribution, in which one distribution is associated with the clay minerals and the other is associated with the pyrite.

Optimal design and performance analysis of anode flow channels in proton exchange membrane water electrolyzers

In proton exchange membrane water electrolysis, the water/oxygen distribution characteristics in the anode channel significantly affect the electrolyzer performance. In this paper, a three-dimensional two-phase non-isothermal numerical model of the electrolyzer is established, in which the reaction kinetics equation is modified theoretically and is validated experimentally. Two novel anode channel structures (Straight-through turbulent flow channel and Mesh-like structured flow channel) are designed and their multiphysical field distribution characteristics are compared with the traditional straight-through flow channel. Results show that, for Straight-through flow channel structure, the liquid water saturation and temperature gradient between the under-channel and under-rib areas are large, resulting in bubble accumulation under the ribs, deterioration of heat and mass transfer, and harming the electrolyzer reaction rate. The two novel structures both have velocity component perpendicular to the channel flow direction, promoting the expulsion of oxygen accumulated in the anode porous electrode, contributing to more uniform distribution characteristics of gas-liquid two-phase flow and current density. At 2.5 V, the current density of Straight-through flow channel, Straight-through turbulent flow channel, and Mesh-like structured flow channel are 4.12, 4.14, and 4.53 A/cm2, respectively. The excellent heat and mass transfer characteristics enable the Mesh-like structured flow channel to have the best electrochemical performance.

An Analytical Relative Permeability Model Considering Flow Path Structural Characteristics for Gas-Liquid Two-Phase Flow in Shale Fracture

Summary Relative permeability models are essential in describing the multiphase fluid flow in reservoir rocks. Literature work has shown that the existing theoretical models of relative permeability cannot perfectly describe the two-phase flow experimental data in fractures because those models are mostly developed for porous media (such as sandstone) or proposed without fully taking the specific characteristics of two-phase flow into consideration. In this paper, we propose a theoretical two-phase flow relative permeability model based on the tortuous flow channels, considering the structural characteristics of two-phase flow in the fractures. This model considers that the gas and liquid flow through different channels of different shapes and sizes at the same time. The formula for two-phase relative permeability was derived from cubic law in fracture and Darcy’s law, with the influence of the slip effect of the gas phase also considered. The results from different models were compared using several series of experimental data. The model proposed in this paper has a better fit than the others for the raw experimental data. This study demonstrates that it is crucial to take the flow paths and distribution of the two phases into consideration to model the two-phase flow in fracture accurately. This work also found that the tortuosity of the gas channel at the irreducible liquid saturation has a negative effect on gas relative permeability but positive to liquid relative permeability. Moreover, the model demonstrates that the decrease in aperture leads to an increase in the gas relative permeability due to gas slippage, while the impact of gas slippage reduces under high pressure.

Phase Saturation Control on Vorticity Enhances Mixing in Porous Media

AbstractMixing controls the fate of any solute entering porous media. Hence, an understanding of the involved processes is essential for assessing subsurface contamination and planning for its protection. However, the three‐dimensional mechanisms dominating solute mixing in the presence of several fluid phases in the pore space, and their dependency on phase saturation degree (fraction of the pore volume occupied by a phase) are unknown. Here, we analyze solute mixing in unsaturated porous media at the pore scale using X‐ray micro‐tomography performed with synchrotron radiation at unprecedented temporal and spatial resolutions for such an investigation. Transport experiments through a synthetic, sand‐like porous medium, followed in 4D using a contrast solution, are performed at different liquid phase saturation degrees. The results reveal larger solute's front deformation at lower saturation, which translates into an enhanced mixing with time. Using different topological indexes, defined based on a description of the liquid phase geometry and of the resulting hydrodynamics, we show an increase in the spatial convergence of flow streamlines at lower saturation, which, in turn, leads to a strengthened helical flow inside the liquid phase. Consequently, this increases the number of shear‐ and vorticity‐dominated deformation regions, as characterized by larger negative and positive Q‐criterion values, respectively. These findings represent a major step toward understanding the control of both saturation and the system's heterogeneity on solute mixing, essential, among others, to assess reactivity in porous media.

Numerical simulation of gradient catalyst layer design in proton exchange membrane water electrolyzer with enhanced performance

The catalyst loading, ionomer content and agglomerate radius in the catalyst layer (CL) of proton exchange membrane water electrolyzer (PEMWE) plays critical role on its electrochemical performance. Conventionally, the CL in PEMWE is modeled with uniform composition and homogeneous domain. In this study, a two-dimensional PEMWE model considering linear gradient distribution of catalyst, ionomer and agglomerate radius in CL was developed and their effect on the electrolyzer performance were systematically investigated. The results show that, when the total catalyst, ionomer loading and the average agglomerate radius in CL are kept constant, the optimum PEMWE performance was achieved with low catalyst loading, high ionomer content, and small agglomerates radius distribution at the CL-PEM interface. Compared with the uniform CL, this optimized gradient CL design results into a 7.01% improvement in the PEMWE current density. Meanwhile, under the gradient CL design, increased maximum temperature and oxygen concentration with lower liquid water saturation is found inside PEMWE, which might affect the temperature distribution uniformity within the PEMWE. The results of this work provide insightful guidance for the CL optimization design of PEMWE.

Investigating the Effects of Mixing Dynamics on Twin-Screw Granule Quality Attributes via the Development of a Physics-Based Process Map.

Twin-screw granulation (TSG) is an emerging continuous wet granulation technique that has not been widely applied in the industry due to a poor mechanistic understanding of the process. This study focuses on improving this mechanistic understanding by analyzing the effects of the mixing dynamics on the granule quality attributes (PSD, content uniformity, and microstructure). Mixing is an important dynamic process that simultaneously occurs along with the granulation rate mechanisms during the wet granulation process. An improved mechanistic understanding was achieved by identifying and quantifying the physically relevant intermediate parameters that affect the mixing dynamics in TSG, and then their effects on the granule attributes were analyzed by investigating their effects on the granulation rate mechanisms. The fill level, granule liquid saturation, extent of nucleation, and powder wettability were found to be the key physically relevant intermediate parameters that affect the mixing inside the twin-screw granulator. An improved geometrical model for the fill level was developed and validated against existing experimental data. Finally, a process map was developed to depict the effects of mixing on the temporal and spatial evolution of the materials inside the twin-screw granulator. This process map illustrates the mechanism of nucleation and the growth of the granules based on the fundamental material properties of the primary powders (solubility and wettability), liquid binders (viscosity), and mixing dynamics present in the system. Furthermore, it was shown that the process map can be used to predict the granule product quality based on the granule growth mechanism.

Preliminary study on double-sided direct shear test of microbial mortar

A new method of strengthening masonry structures of ancient architecture has been proposed, which involves the use of microbial grouting to generate mineralized mortar. To study the shear mechanical properties of microbial mortar, a comparative experiment was designed with 9 groups of specimens, including acid washing method to test the amount of calcium carbonate, penetration method to test compressive strength of mortar, double-sided direct shear method to test shear strength of mortar and get shear stress-strain curve. According to the experimental results, when the brick joint is thick, more bubbles remain at the contact between the brick and the mortar, reducing the saturation of the grouting liquid and affecting the amount of calcium carbonate generated, thereby affecting the shear strength of the mortar. When the brick is placed vertically, the contact area between the brick and the mortar is on the side, and it is not easy for bubbles to remain, so the amount of calcium carbonate generated is high, and the corresponding shear strength of the mortar is high. Reducing the concentration of calcium solution and implementing bidirectional perfusion can slow down the deposition of calcium carbonate and blockage of perfusion pathways, thereby improving the homogeneity of the carbonate level. A well-graded filling of sand particles is an effective measure to improve the binding efficiency of calcium carbonate.These measures not only increase the production of calcium carbonate, but also result in higher compressive and shear strength.The shear stress-displacement curve of microbial mortar specimens with damagedmortar layer exhibits strain-softening behavior, while the specimens with inter facial damage show strain-hardening behavior.The shear strength of specimens with interface damage is lower than that of specimens with mortar layer damage, so measures should be taken to avoid interface damage.

Numerical investigation into the thermo-hydro-mechanical behavior in unsaturated soils considering hypergravity effect

Unsaturated soils are commonly found in coastal regions and marine sediments, where understanding the thermo-hydro-mechanical (THM) behavior, especially regarding phase transition, is crucial for related practical engineering applications. Centrifugal model offers an effective means to reflect the prototype with smaller scale and shorter duration under hypergravity conditions. Present studies on similarity based on governing equation analysis of the THM coupled responses are mainly focused on saturated rather than unsaturated soils, nor involved with different boundary conditions. This paper theoretically derives the similarity of THM behavior of unsaturated soils between the centrifugal model and prototype, and numerically assesses the similarity considering Dirichlet / Neumann condition based on the open-source FEM code OpenGeoSys. In addition, numerical results shows that phase transition between liquid water and water vapor will occur under temperature variation, and pore pressure difference rather than initial pore pressure causes the difference of THM coupled responses. Significant pore pressure variation will be amplified with increase of gas diffusion coefficient, and large air entry pressure corresponds to high liquid saturation and obvious phase transition.

Measuring and modeling of liquid water transport in a carbon paper with the aid of centrifuge experiment

In previous macroscopic average models of proton exchange membrane fuel cells (PEMFCs), liquid water was assumed to be continuous throughout the carbon paper. The transport of disconnected water drops in carbon paper has long been neglected in these models, which tend to underestimate water saturation in carbon paper. This study proposes a two-phase flow model that considers the transport of disconnected water drops to simulate the air-water transport in carbon paper. The proposed model was established based on the relationship between capillary force and liquid water saturation, and this relationship was obtained by conducting centrifuge experiments. In the centrifuge experiment, initially, the water saturation inside the carbon paper sample was one, and the centrifugal acceleration was zero. As the centrifugal acceleration increased, the water saturation decreased owing to the centrifugal force. After twenty seconds, the water saturation no longer decreases because the centrifugal force on the liquid water is balanced by the capillary force. Centrifugal acceleration was used to evaluate the capillary force. When the centrifugal acceleration was 185 ms−2, the saturation of the water drops in the balanced state was 32.0%. As the centrifugal acceleration increases to 739 ms−2, the saturation decreases to 6.5%. According to the experimental results, a function correlating the saturation and centrifugal acceleration, that is, the capillary force, was established. Finally, the results show that the centrifugal acceleration is more than 18.5 times that of the gravitational acceleration, indicating that the effect of gravity on the motion of water drops is negligible.

Unveiling the impact of pore structure of cathode catalyst layer on proton exchange membrane cell performance

This work quantitatively explores the impact of pore structure of cathode catalyst layer (CCL) on the overall performance of low-Pt loading electrodes in Proton Exchange Membrane Fuel Cell (PEMFCs), providing guidelines for designing high-performance CCL. The model predictions align closely with experimental measurements. The results reveal that the best cell performance occurs when the ratio of secondary to primary pores is 2, which is attributed to the balance between liquid water saturation and the diffusivity of gaseous species within the catalyst layer. In addition, higher total porosity or higher Pt loading require more primary pores to achieve a remarkable improvement in performance. Furthermore, the influence of total porosity on the peak power density of fuel cell depends on pore size distribution. The optima total porosities are 0.5 and 0.7 when the ratios of secondary to primary pores are 2 and 1, respectively.

Randomness effect of platinum loading distribution in a proton exchange membrane fuel cell: A numerical simulation

The platinum loading is randomly distributed within catalyst layers because of the fabrication way, whereas the platinum loading is considered to follow the homogeneous distribution in the traditional catalyst layer model. The cell performance and heat and species transport laws of the fuel cell with platinum loading random distribution within catalyst layers are not yet clear. Therefore, in this work, a two-dimensional, non-isothermal proton exchange membrane fuel cell model considering the two-phase flow and agglomerate structure of catalyst layers is established to simulate the fuel cell having the catalyst layer where platinum loading is random distribution in a way that is closer to the actual structure. The effect of random distribution of platinum loading on the electrical property, species transfer and temperature distribution is analyzed. Results suggest that the randomness influence needs to be considered when the local distributions of parameters are studied and the research accuracy of parameters is relatively high. In contrast with the homogeneous distribution of platinum loading, the cell performance for the normal random distribution and uniform random distribution cases is reduced. The effects of catalyst loading random patterns on cell performance, species and temperature distributions are different, and the decrease of cell performance for the uniform random distribution case is more obvious. Furthermore, the influence degree of randomness on each parameter is different. The distributions of electrochemical reaction rate, oxygen concentration and temperature display random characteristics, while the liquid water saturation distribution is less influenced by randomness and does not show an obvious random change. These findings can provide a reference for the randomness verification in random simulations and deepen the understanding of species and heat transfer laws in the catalyst layer which is closer to the reality to improve the accuracy of fuel cell simulations.

Modelling of CO2 Sequestration in Deep Saline Aquifer Reservoir using Surface Evolver Software

A strategy to mitigate CO2 concentration in the atmosphere is injection of supercritical CO2 in porous sedimentary medium filled with saline water where CO2 gets trapped in the pores. Sequestration is resultant of two processes, drainage and imbibition in accordance with capillary law. Simulation of the pore-scale trapping has been performed on reservoir scale. A simple model reservoir consisting of 1000 tetrahedral subunits was constructed. Using Surface Evolver software, pore volueach subunit. The CO2 pressure and liquid saturation curves of the reservoir displayed hysteretic character, indicating that the evolution of the system is pathway-dependent. The simulated curves illustrate the pathway dependency of the residual gas saturation, an important parameter controlling the amount of CO2 stored in the reservoir, hence the efficiency of the CO2 storage. Our study also shows the existence of critical thresholds in terms of in/out gassing during the drainage or imbibition processes

Optimal design of locally improved structure for enhancing mass transfer in PEMFC cathode flow field

At high current density, the mass transfer in proton exchange membrane fuel cells (PEMFC) is significantly influenced by the flow field structure. In this paper, a bilateral track flow field (BTFF) is proposed to improve the transverse and longitudinal mass transfer as well as the water removal capacity of PEMFC. The geometry of the BTFF is optimized by orthogonal tests to obtain the optimal combination of geometric parameters and levels. Then using numerical modeling, the impact of the optimized BTFF (BT6) on the efficiency of the PEMFC was examined. The results show that BT6 significantly improves the utilization of the under-rib region by shortening the reactive gas diffusion path length to obtain a more uniform oxygen distribution, and the average oxygen concentration in the midline of the under-rib of the BT6 is increased by 102.60%, furthermore, the current density is increased by 16.32% compared with that of the parallel flow field (case0). In addition, by reducing the cross-sectional area of the gas flow channel, the BT6 has a higher gas flow velocity, which accelerates the removal of cathode liquid water, and the average liquid water saturation in the midline of the under-rib of the BT6 is reduced by 15.26% compared with case0. Finally, by analyzing the net power density of PEMFC, it can be found that the net power density of BT6 is increased by 15.68% compared with case0, which has good practical application value.

Effects of cathode gas diffusion layer porosity and contact angle distributions along through-plane direction on the performance of polymer electrolyte membrane fuel cell

In the present work, a numerical model for polymer electrolyte membrane fuel cell (PEMFC) has been employed to investigate the effects of porosity and contact angle distributions of the porous layer on the performance of PEMFC. The three-dimensional, two-phase, and non-isothermal flow solver considers heat transfer, electrochemical reaction, and liquid water saturation. Various cases with different cathode gas diffusion layer (GDL) porosities are performed. The results showed that a better performance is achieved when the GDL porosity is increased because of the reduced mass transport resistance of oxygen. The effects of the porosity distributions on the PEMFC performance were examined by comparing the predictions from the cases with non-uniform porosity and uniform porosity in the cathode GDL. It was shown that the current density of the cases with non-uniform porosity is higher than that of the uniform porosity case when the operation voltage is low. This is due to the fact that non-uniform porosity distributions in the cathode GDL enhance the reactant transport. When the operation voltage is high, the current densities of the cases with non-uniform porosity and uniform porosity are almost the same. Moreover, the influence of contact angle distributions in cathode GDL was explored. Various cases with gradient-decreasing, gradient-increasing, and uniform constant contact angles from GDL/channel interface to GDL/catalyst layer interface were studied. It was found that the case with gradient-increasing contact angle has a better performance due to the larger capillary pressure difference to remove more water. The poor performance of the case with gradient-decreasing contact angle is related to the phenomenon of flooding in the catalyst layer.