Liquid Water Formation Research Articles

Three-dimensional transient two-phase study of the cathode side of a PEM fuel cell

A three-dimensional transient two-phase isothermal model has been developed for the cathode side of a proton exchange membrane fuel cell (PEMFC). This has been done in order to fully investigate the effects and the time variation of liquid water formation as well as the gas phase transport under the start up condition. It is considered that the generated water in the cathode catalyst layer (CL) is liquid water and that the gas diffusion layer (GDL) is hydrophobic. A non-equilibrium water condensation-evaporation is also assumed. The time variations of liquid water distribution in along-channel and through-plane directions are investigated. This is to determine the liquid water accumulation at the start up time (above the channel under the CL), then the movement of the liquid water in the domain and the final accumulation at the steady state condition (above the rib and near the CL). It has also been found that it takes less time for a high average current density to attain the steady state condition which is due to the capillary pressure gradient inside the porous media. Validation of the numerical results has been implemented via a polarization curve comparison with the experimental data. Both sets show good agreement.

Short Note: Liquid water formation around rocks and meteorites on Antarctic Polar Plateau ice

Short Note: Liquid water formation around rocks and meteorites on Antarctic Polar Plateau ice

Passivity based control of a distributed PEM fuel cell model

This work considers the controlled load change of proton exchange membrane (PEM) fuel cells. Due to the intrinsic nonlinearities of fuel cells, load changes are quite challenging. In the case of a low temperature PEM fuel cell, there is the possibility of undesired liquid water formation. Most available control concepts are heuristic linear controller structures based on a perfectly mixed fuel cell model. In this work a nonlinear controller for one-dimensional spatially distributed model of a PEM fuel cell is presented. The fuel cell model is derived from first principles. The concept of passivity is used to design the controller. A suitable control Lyapunov function is chosen and passivity of the fuel cell is shown. A state-feedback law is derived that can guarantee stability of the closed-loop system over a wide range of operation conditions. In order to make the feedback law applicable to fuel cells with limited measurement information an observer is designed. In a final step the state-feedback law and the observer are combined to an output-feedback controller.

Increasing the efficiency of a portable PEM fuel cell by altering the cathode channel geometry: A numerical and experimental study

Portable fuel cells are receiving great attention today mainly because their energy density is higher than any portable battery solution. Among other types, portable polymer electrolyte membrane (PEM) fuel cells are an established technology where research on increasing their efficiency is leading product development and manufacturing. The objective of this work was to study and evaluate the redesign of a commercial portable fuel cell, improving its efficiency. A three-dimensional model of the original PEM fuel cell with parallel plus a transversal flow channel design was developed using Comsol Multiphysics, including the effects of liquid water formation and electric current production. Using this model, the effects of different channel geometries and respective cathode flow rates on the cell’s performance, including the local transport characteristics, were studied. Laboratory tests with various fuel cell stacks using the new channels structure were effectuated for an evaluation of the fuel cell’s performance, showing improvements in its efficiency of up to 26.4%.

Water Vapor Absorption in Porous Media Polluted by Calcium Nitrate Studied by Time Domain Nuclear Magnetic Resonance

Nuclear magnetic resonance relaxation analysis of liquid water (1)H nuclei in real porous media, selected for their similar composition (carbonate rocks) and different pore space architecture, polluted with calcium nitrate, is presented to study the kinetics of water condensation and salt deliquescence inside the pore space. These phenomena are responsible for deterioration of porous materials when exposed to environmental injury by pollution in a humid atmosphere. The theory is well described for simple pore geometries, but it is not yet well understood in real porous media with wide distributions of pore sizes and connections. The experiment is performed by following in time the formation of liquid water inside the pore space by T(1) and T(2) relaxation time distributions. The distributions allow one to see the effects of both the salt concentration and the pore space structure on the amount of water vapor condensed and its kinetics. It is shown that, for a given lithotype, even with different amounts of pollutant, the rate-average relaxation time T(1ra) tends to increase monotonically with NMR signal, proportional to the amount of liquid water. T(1ra) is often inversely associated with surface-to-volume ratio. This suggests a trend toward the filling of larger pores as amounts of liquid water increase, but it does not indicate a strict sequential filling of pores in order of size and starting with the smallest; in fact, relaxation time distributions show clearly that this is not the case. Increased amounts of salt lead to both markedly increased rates and markedly increased amounts of water absorption. NMR measurements of amounts of water, together with relaxation time distributions, give the possibility of information on the effect of pollution in porous materials exposed to humid atmospheres but sheltered from liquid water, even before the absorption of large amounts of moisture and subsequent damage. These phenomena are of importance also in other fields, such as the exploitation of geothermal energy.

In situ Visualization of Liquid Water Formation and Transport Processes in PEM Fuel Cells

not Available.

Study of the effects of various parameters on the transient current density at polymer electrolyte membrane fuel cell start-up

An unsteady two phase nonisothermal model is used to study the start-up characteristics of a polymer electrolyte membrane fuel cell (PEMFC). The model is 3D in nature and a PEMFC with serpentine flow field and 9 channels is simulated. The base case is run with equilibrium or inflow initial conditions, i.e. the initial conditions inside the fuel cell are set to be equal to inflow conditions. The change in current density, at different potentials, with time is plotted. The effect of different parameters on the transient current density behavior is studied. Single phase and two phase models are compared to understand the effect of liquid water on the performance at start-up and is seen to affect the current undershoot. Isothermal and nonisothermal models are also compared to see the effect of temperature. Following this, the gas diffusion layer thickness and the porosity are varied and are seen to have a significant effect on the current density. While both have an immediate effect on the mass transport time scales, the current density is affected by both transport and liquid water formation. The time scales of the above two mentioned phenomenon determine how the current behaves at start-up.

Special regions in Mars exploration: Problems and potential

“Special regions” on Mars are areas designated in the COSPAR planetary protection policy as areas that may support Earth microbes inadvertently introduced to Mars, or that may have a high probability of supporting indigenous martian life. Since absolutely nothing is known about martian life, the operational definition of a special region is a place that may allow the formation and maintenance of liquid water, on or under the surface of Mars. This paper will review the special-regions concept, the implications of recent recommendations on avoiding them, and the work of the Mars science community in providing an operational definition of those areas on Mars that are “non-special.”

Evidence of liquid water formation during methane hydrates dissociation below the ice point

Dissociation of small methane hydrate samples formed from water droplets of size 0.25–2.5 mm has been investigated below the ice melting point in the temperature range of 240–273 K, where the self-preservation effect is observed for bulk hydrates. The experiments included optical microscopy observations combined with P– T measurements of the dissociation conditions for the methane hydrates. For the first time, the formation of supercooled liquid water during the hydrate dissociation was reliably detected in the temperature range of 253–273 K. The formation of the liquid phase was visually observed. The induction time of the ice nucleation for the metastable liquid water depended from the dissociation temperature and a size of water droplets formed during the hydrate dissociation. It was found that in the temperature range of 253–273 K values of the dissociation pressure for the small hydrate samples fall on the extension of the water–hydrate–gas equilibrium curve into the metastable region where supercooled water exist. The average molar enthalpy of 51.7 kJ/mol for the dissociation of the small methane hydrate samples in the temperature range of 253–273 K was calculated using Clausius–Clapeyron equation. This value agrees with the enthalpy of dissociation of bulk methane hydrates into water and gas at temperatures above 273 K.

Otimização dos tempos de mistura e decantação no processo de coagulação/floculação da água bruta por meio da <em>Moringa oleifera</em> Lam

Several natural coagulants/flocculants are being studied to remove color and turbidity from raw water; however, natural coagulants have demonstrated advantages over chemical ones. Moringa oleifera Lam is a natural polymer that has become an important alternative in water treatment, acting as a clarifying agent due to the presence of a cationic protein that destabilizes the particles contained in water in liquid form. This work proposes the optimization of the coagulation, flocculation and decantation times, using different concentrations of the solution extracted from the pulp of Moringa oleifera Lam seeds as coagulant, in order to verify its efficiency in removing color and turbidity from water. Jar tests were carried out, using different mixture and decantation times, with varying concentrations of the biopolymer. It was verified that the time required to achieve a fast and slow mixture, as well as the concentration of the coagulant, influenced the removal of parameters during the process of coagulation/flocculation/sedimentation.

Neutron imaging investigation of liquid water distribution in and the performance of a PEM fuel cell

In this study neutron radiography is applied to investigate the performance of a polymer electrolyte membrane (PEM) fuel cell based on the effect of liquid water accumulation in the cell. Dynamic performance tests have been carried out on a PEM fuel cell with a specially designed serpentine flow channel under various operating conditions and simultaneous measurements of accumulated liquid water with neutron imaging. Liquid water tends to accumulate in the gas diffusion layer (GDL) adjacent to the flow channel area while the liquid water formed in the GDL next to the channel land area seems to be effectively removed by the cross leakage flow through the porous GDL between the adjacent flow channels. The amount of liquid water accumulation in the cell is dependant on the cell operating temperature, the pressure drop in the flow channel and the current density under the present test conditions of fixed stoichiometry. It is shown that the cell performance is strongly affected by the presence and accumulation of liquid water, especially at high current densities. This phenomenon results in performance hysteresis for load variations. The rate of liquid water production is also mathematically modeled to analyze the effect of the cell operating temperature and pressure drop on the liquid water formation in a cell. The model result shows good agreement with experimental measurements. The history of liquid water accumulation is also analyzed.

Reaction Kinetics at the Triple-Phase Boundary in PEM Fuel Cells

A mathematical model is developed to describe the reaction dynamics in the vicinity of the triple-phase boundary (TPB), which is an important part of the pore scale structure of the catalyst layer in proton exchange membrane fuel cells. The model incorporates coupled diffusion, migration, and reaction phenomena of the chemical components in an undersaturated air pore and ionomer. One challenging feature of the work is the description of the TPB by a system of nonlinear partial differential equations (PDEs), coupling bulk, and surface-diffusion phenomena, which offers an approach to study the rarely investigated proton surface diffusion along the air pore surface. A numerical technique is implemented, taking into account the particular form of the domain, in order to solve the nonlinear PDE system efficiently. Several numerical results are discussed, including a sensitivity analysis with respect to the physical reference case and geometric parameters. The results indicate that surface diffusion might play a major role for the reaction kinetics, but only if the air pore is void of liquid water. In contrast, the formation of liquid water in the gas pores will turn surface diffusion into bulk diffusion, with the latter resembling the Grotthus mechanism.

Effects of flow channel geometry on cell performance for PEM fuel cells with parallel and interdigitated flow fields

A 3D numerical model was developed to explore the effects of the cathode flow channel configuration on the local transport phenomena and cell performance for parallel and interdigitated flow fields in proton exchange membrane (PEM) fuel cells. The effect of liquid water formation on the reactant transport is taken into account in the model. For operating voltages greater than 0.7 V, the electrochemical reaction rates are low with a small amount of oxygen consumption and liquid water production, and all cell designs provide sufficient oxygen for the electrochemical reactions. Thus, the flow channel aspect ratio and the flow channel cross-sectional area have little effect on the cell performance. For operating voltages lower than 0.7 V, as the operating voltage decreases the electrochemical reaction rates gradually increase with a large amount of oxygen consumption and liquid water production, so the cell performance is strongly dependent on the flow field design. For the parallel flow field design, lower flow channel aspect ratios and flow channel cross-sectional area areas improve liquid water removal, thus, decreasing both improves cell performance. However, the interdigitated design has an optimal aspect ratio of 1.00 and an optimal cross-sectional area of 1.000 mm × 1.000 mm.

Effects of serpentine flow field with outlet channel contraction on cell performance of proton exchange membrane fuel cells

A serpentine flow field with outlet channels having modified heights or lengths was designed to improve reactant utilization and liquid water removal in proton exchange membrane (PEM) fuel cells. A three-dimensional full-cell model was developed to analyze the effects of the contraction ratios of height and length on the cell performance. Liquid water formation, that influences the transport phenomena and cell performance, was included in the model. The predictions show that the reductions of the outlet channel flow areas increase the reactant velocities in these regions, which enhance reactant transport, reactant utilization and liquid water removal; therefore, the cell performance is improved compared with the conventional serpentine flow field. The predictions also show that the cell performance is improved by increments in the length of the reduced flow area, besides greater decrements in the outlet flow area. If the power losses due to pressure drops are not considered, the cell performance with the contracted outlet channel flow areas continues to improve as the outlet flow areas are reduced and the lengths of the reduced flow areas are increased. When the pressure losses are also taken into account, the optimal performance is obtained at a height contraction ratio of 0.4 and a length contraction ratio of 0.4 in the present design.

Effect of humidity of reactants on the cell performance of PEM fuel cells with parallel and interdigitated flow field designs

This study investigates the effects of the relative humidity (RH) of the reactants on the cell performance and local transport phenomena in proton exchange membrane fuel cells with parallel and interdigitated flow fields. A three-dimensional model was developed taking into account the effect of the liquid water formation on the reactant transport. The results indicate that the reactant RH and the flow field design all significantly affect cell performance. For the same operating conditions and reactant RH, the interdigitated design has better cell performance than the parallel design. With a constant anode RH = 100%, for lower operating voltages, a lower cathode RH reduces cathode flooding and improves cell performance, while for higher operating voltages, a higher cathode RH maintains the membrane hydration to give better cell performance. With a constant cathode RH = 100%, for lower operating voltages, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction, but also increases the difference in the water concentrations between the anode and cathode, which enhances back-diffusion of water from the cathode to the anode, thus reducing cathode flooding to give better performance. However, for higher operating voltages, the cell performance is not dependent on the anode RH.

Reactants flow behavior and water management for different current densities in PEMFC

Computational fluid dynamics analysis was carried out to investigate the reactants flow behavior and water management for proton exchange membrane fuel cell (PEMFC). A complete three-dimensional model was chosen for single straight channel geometry considering both anode and cathode humidification. Phase transformation was included in the model to predict the water vapor and liquid water distributions and the overall performance of the cell for different current densities. The simulated results showed that for fully humidified conditions hydrogen mole fraction increases along the anode channel with increasing current density, however, at higher current densities it decreases monotonically. Different anode and cathode humidified conditions results showed that the cell performance was sufficiently influenced by anode humidification. The reactants and water distribution and membrane conductivity in the cell depended on anode humidification and the related water management. The cathode channel–GDL (Gas Diffusion Layer) interface experiences higher temperature and reduces the liquid water formation at the cathode channel. Indeed, at higher current densities the water accumulated in the shoulder area and exposed higher local current density than the channel area. Higher anode with lower cathode humidified combination showed that the cell had best performance based on water and thermal management and caused higher velocity in the cathode channel. The model was validated through the available literature.

Effects of Flow Channel Area Ratio on Local Transport Characteristics and Cell Performance of 3D PEMFCs

The objective of this work is to examine the effects of the flow channel area ratio (40%, 50%, and 60%) on the reactant gas transport and the cell performance of proton exchange membrane fuel cells (PEMFCs) with parallel and interdigitated flow channel designs. The effects of liquid water formation on the reactant gas transport are taken into account in the present study. Higher flow area ratios are expected to provide more area for fuel diffusion into the gas diffusion layer to enhance the electrochemical reaction rate and improve cell performance. Numerical predictions show that the cell performance for parallel flow channel design can be enhanced by increasing the flow channel area ratio and the best overall cell performance occurs for a flow channel area ratio of 60%. For the interdigitated flow channel design, because the fuel transport is by forced convection rather than diffusion, the flow channel area ratio has less effect on cell performance. Taking into account liquid water removal and pressure losses, the optimal cell performance occurs for a flow channel area ratio of 50% for the interdigitated flow channel design. The results also show that the flow channel area ratio and liquid water distribution significantly influence cell performance at lower voltages.

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.

Liquid Water Formation and Transport in the PEFC Anode

Transparent polymer electrolyte fuel cells (PEFCs) with parallel- and serpentine-channel flowfields have been developed to study liquid water formation and transport on the anode side. In situ observations reveal that liquid water in the anode channels results from condensation of water on cooler and more hydrophilic channel walls and that water available for condensation in the anode comes either from the cathode through membrane transport or from hydrogen consumption. No water droplets can be found on the anode gas diffusion layer (GDL) surface, in sharp contrast with the cathode side. Moreover, GDL wettability has much influence on water distribution in the anode for the cell operated at low current density . Using hydrophobic GDL at low current density, water is prone to condense on the channel surfaces rather than inside hydrophobic carbon paper GDL. The condensed water then accumulates and results in channel clogging in the anode. In contrast, by using untreated carbon paper as the anode GDL, it is found that channel clogging by liquid water is avoided under similar operating conditions. This finding implies that water distributions in the anode with hydrophilic and hydrophobic anode GDLs differ much from each other at low current density. Finally, experiments suggest that water condensation on the channel surfaces is a primary mechanism for liquid water formation and anode flooding and that elevating the anode plate temperature modestly is effective to avoid surface condensation and mitigate anode flooding.

Numerical study of cell performance and local transport phenomena in PEM fuel cells with various flow channel area ratios

Three-dimensional models of proton exchange membrane fuel cells (PEMFCs) with parallel and interdigitated flow channel designs were developed including the effects of liquid water formation on the reactant gas transport. The models were used to investigate the effects of the flow channel area ratio and the cathode flow rate on the cell performance and local transport characteristics. The results reveal that at high operating voltages, the cell performance is independent of the flow channel designs and operating parameters, while at low operating voltages, both significantly affect cell performance. For the parallel flow channel design, as the flow channel area ratio increases the cell performance improves because fuel is transported into the diffusion layer and the catalyst layer mainly by diffusion. A larger flow channel area ratio increases the contact area between the fuel and the diffusion layer, which allows more fuel to directly diffuse into the porous layers to participate in the electrochemical reaction which enhances the reaction rates. For the interdigitated flow channel design, the baffle forces more fuel to enter the cell and participate in the electrochemical reaction, so the flow channel area ratio has less effect. Forced convection not only increases the fuel transport rates but also enhances the liquid water removal, thus interdigitated flow channel design has higher performance than the parallel flow channel design. The optimal performance for the interdigitated flow channel design occurs for a flow channel area ratio of 0.4. The cell performance also improves as the cathode flow rate increases. The effects of the flow channel area ratio and the cathode flow rate on cell performance are analyzed based on the local current densities, oxygen flow rates and liquid water concentrations inside the cell.