Cathode Flooding Research Articles

Stabilization of polymer electrolyte fuel cell voltage with reduced-order Lyapunov exponent feedback and corrective pressure perturbations

Polymer electrolyte fuel cell (PEFC) system efficiency can be decreased by instabilities resulting from the accumulation of water in the cathode as well as by excessive air delivery parasitic loads used to prevent liquid water accumulation. In this work, we present a new instability detection diagnostic tailored for the nonlinear and chaotic dynamics of PEFC operation with multi-phase flow in the gas channels. The instability statistic, the Lyapunov exponent of the reduced-order voltage return map, λ, is a measure of the exponential rate of divergence in the dynamic voltage signal measured from the fuel cell. A key advantage of this statistic for embedded control is that it is a self-referencing measure of the system stability for feedback and is not based on an a priori performance threshold. Our experiments demonstrate that the Lyapunov exponent statistic provides a warning typically 100 s in advance of significant power loss. Using this statistic as a control diagnostic, a new control scheme that detects PEFC instability in real time and mitigates it with pressure perturbations was applied experimentally to several fuel cell systems, including one that simulates stack operation. Our control scheme resulted in increased PEFC power, decreased cathode flooding leading to a lower parasitic load for air delivery, and stable PEFC performance.

On the effect of gas diffusion layers hydrophobicity on direct methanol fuel cell performance and degradation

Degradation and mass transport phenomena management are two of the main issues hindering direct methanol fuel cell commercialization. Water and methanol crossover through the membrane, regulated by both anode and cathode gas diffusion layers hydrophobic properties, is widely studied in the literature, while the effect of mass transport phenomena evolution on the direct methanol fuel cell degradation has not been investigated yet. This work aims to present a combined experimental and modeling analysis on the effect of the gas diffusion layers hydrophobicity on DMFC degradation, through the comparison of performance characterization and degradation tests of two different fuel cells. In one of them, the lower diffusion layer hydrophobicity and the absence of anode microporous layer determines the onset of cathode flooding, negatively affecting performance and degradation. However, the cathode surface area loss is similar between the two fuel cells, meaning that flooding does not involve modifications in cathode permanent degradation mechanisms, but it mainly determines the amplification of the cathode surface area loss effects.

An experimental investigation of temperature distribution and flooding phenomena of cathode flow fields in a proton exchange membrane (PEM) fuel cell

Water management is considered to be one of the main issues to be addressed for the performance improvement of proton exchange membrane (PEM) fuel cells. In this paper, to investigate cathode flooding and its relationship with temperature distribution, an experimental study was carried out on cathode sides of an operating single PEM fuel cell. For the direct visualization of temperature fields and water transport in cathode flow channels, a transparent cell was designed and manufactured using quartz window. Liquid water transport and distribution in the flow channels were investigated experimentally. Also, the visualization of temperature distributions in the cathode channels was made by using an IR (infra-red) camera. Results indicate that the temperature rise near the exit of cathode flow channels was found. It is expected that this study can effectively contribute to get the detailed data on water transport linked with thermal management during the operation of a PEM fuel cell.

The Effect of Cathode Pressure Perturbations on the Voltage Stability of a Simulated Fuel Cell Stack

Current strategies for mitigating power loss due to cathode flooding in polymer electrolyte fuel cells are aimed at avoiding the unstable two-phase slug flow regime in the air delivery microchannels by delivering parasitically expensive excessive air flow rates. In this work, we investigate ways to stabilize the slug flow regime and recover voltage by applying low cost cathode pressure perturbations. We experimentally investigate the voltage response to the magnitude and frequency of pressure perturbations applied to a simulated fuel cell stack. The results are applicable to active fuel cell water management.

Simultaneous Electricity Generation and Microbially-Assisted Electrosynthesis in MFCs

Microbial Fuel Cells (MFCs) are bio-electrochemical transducers that generate electricity as a direct result of microbial metabolism, when breaking down organic matter for continuous growth and maintenance. On the other hand, Microbial Electrolysis Cells (MECs) consume electricity to drive chemical reactions and recover hydrogen or other high value chemicals, at the cathode half-cell [1]. In MFCs, electric current is generated when for every electron donated to the electrode surface, a proton is transferred from the anode to the cathode. Other cations such as Na+ or K+ may be present in significantly higher concentrations and are more likely to be transferred through a cation exchange membrane [2]. This normally results in electro-osmotically dragged water from the anode to the cathode, and often results in a phenomenon known as cathode flooding, which is a problem for MFCs and other chemical fuel cells. To this day, bio-electrosynthesis has not been reported for energy-generating MFCs, since it is associated with energy-consuming MECs. The main aim of this work was therefore to investigate the effects on MFC performance of low-cost catalyst-free electrode materials, in conjunction with cation and water transport to the cathode half-cell, in the context of beneficial water accumulation and recovery of valuable resources. Materials and Methods Twelve dual–chamber MFCs were tested in triplicate groups assembled with carbon veil anode electrodes. Half-cell chambers were 25mL each and activated sewage sludge (Wessex Water) was used as the inoculum. The cathode chambers contained carbon based electrodes mechanically pressed against the CEM separator. The tested cathode electrodes included: Microporous Layer on carbon cloth (MPL), carbon fibre veil (CV), MPL on carbon fibre veil (CV MPL) and activated carbon (AC). Results and Discussion Results showed that the range of Pt-free cathodes including plain carbon fibre veil, activated carbon, and microporous layer (MPL) in dual-chamber MFCs generated electric current with simultaneous catholyte generation in the cathode chamber.During MFC operation, the production of catholyte on the surface of the cathode electrode was a direct result of electricity generation, and power output has been correlated with catholyte volume. Moreover, the pH of the formed catholyte (>13) and conductivity, showed gradual increase with current generation. The MFC system fed with real wastewater supplemented with sodium acetate showed sodium recovery on the cathode in the form of sodium carbonate salts. Similarly, when the anode feedstock was supplemented with potassium acetate, KOH was formed on the cathode half-cell with additional crystallisation of potassium salts.This paper demonstrates an innovative and energy-efficient system that exploits microbially assisted electrosynthesis for the recovery of valuable elements from wastewater, in the form of chemicals (NaOH, KOH) and electricity. Conclusions This approach leads to carbon capture through wet caustic scrubbing on the cathode, which locks the carbon dioxide into carbonate salts. Acknowledgements This work has been supported by the Bill & Melinda Gates Foundation, grant no. OPP1094890, and the UK EPSRC, grant numbers EP/I004653/1 and EP/L002132/1.

Charles W.Tobias Young Investigator Award: Understanding Transport Phenomena in Polymer-Electrolyte Fuel Cells

One of the keys to optimizing polymer-electrolyte-fuel-cell performance is the understanding of transport phenomena. This is especially critical when liquid water exists in the cell as it can result in flooding and blockage of the electrocatalytic sites. In this award talk, transport phenomena in general will be discussed with a focus on efforts conducted at LBNL in terms of both modeling and advanced diagnostics. Thus, this talk will discuss the current macroscopic modeling approaches being undergone in terms of multiphase flow and fuel-cell behavior at low and subzero temperatures. In addition, diagnostic work on effective properties will be discussed. Topics to be explored in depth include transport in GDLs and catalyst-layer ionomer.For GDLs, there has been a study on the effective properties including diffusivity, thermal conductivity, and adhesion force for GDLs that demonstrate different performance at low temperatures. Both modeling and experiments show that the key is water removal out of the anode to decrease the cathode flooding. Such removal is enhanced by decreasing adhesion force at the GDL/channel surface owing to structural hetergeities in fiber density.For catalyst-layer ionomer, it will be shown how this could be an important issue for the observed increased mass-transport limitations for low Pt loadings. In this structure, confinement and substrate interactions dominate and result in increased transport resistance for the ionomer thin-film surrounding the catalyst sites. Ex-situ thin-film studies show how the morphology changes to more of a membrane interfacial-type behavior as the thickness decreases, thereby decreasing the water content in agreement with water-content measurements in catalyst layers. Mitigation of this issue is paramount to the future commercial viability of polymer-electrolyte fuel cells. AcknowledgementsThis work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.

Equivalent Circuit Modeling of PEM Fuel Cell Degradation Combined With a LFRC

This approach investigates with an equivalent circuit modeling a polymer electrolyte membrane (PEM) fuel cell degradation combined with a low-frequency ripple current (LFRC) that may shorten the fuel cell life and worsen the fuel efficiency. Through the immediate measurement of the impedance curves on single cells obtained after cycling for hours and operating in cathode flooding and membrane dehydration modes at variable frequencies, it has been shown that the impedance magnitude of a fuel cell injecting a LFRC increased when compared with that of a high-frequency ripple current. This study develops these investigations one step further by designing equivalent circuit elements like resistance and capacitance by electrochemical impedance spectroscopy. Impedance-based model is able to describe the dynamic behavior of the PEM fuel cell. For validation of the proposed fuel cell model, an experimental study has been carried out for a 1.2-kW Nexa module.

Significant performance improvement in terms of reduced cathode flooding in polymer electrolyte fuel cell using a stainless-steel microcoil gas flow field

Flooding at the cathode is the greatest barrier to increasing the power density of polymer electrolyte fuel cells (PEFCs) and using them at high current densities. Previous studies have shown that flooding is caused by water accumulation in the gas diffusion layer, but only a few researchers have succeeded in overcoming this issue. In the present study, microcoils are used as the gas flow channel as well as the gas diffuser directly on the microporous layer (MPL), without using a conventional carbon-fiber gas diffusion layer (GDL), to enable flood-free performance. The current–voltage curves show flooding-free performance even under low air stoichiometry. However, the high-frequency resistance (HFR) in this case is slightly higher than that in grooved flow channels and GDLs. This is due to the differences in the electron conduction path, and the in-plane electron conductivity in the MPL is the key to enhancing the microcoil fuel cell performance.

Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell

Water management is vital for the successful development of PEM fuel cells. Water should be carefully balanced within a PEM fuel cell to meet the conflicting requirements of membrane hydration and cathode anti-flooding. In order to understand the key factors that can improve water management and fuel cell performance, the cathodes with different structures and properties are prepared and tested in this study. The experimental results show that even though no micro-porous layer (MPL) is placed between the cathode catalyst layer (CCL) and macro-porous substrate (MaPS), a hydrophobic CCL is effective to prevent cathode flooding and keep membrane hydrated. The impedance study and the analysis of the polarization curves indicate that the optimized hydrophobic micro-porous structure in the MPL or the hydrophobic CCL could be mainly responsible for the improved water management in PEM fuel cells, which functions as a watershed to provide wicking of liquid water to the MaPS and increase the membrane hydration by enhancing the back-diffusion of water from the cathode side to the anode side through the membrane.

Role of the Cathode Microporous Layer in a PEMFC

The role of the cathode microporous layer (MPL) of the gas diffusion layer in a polymer electrolyte exchange membrane fuel cell was investigated by comparing the performance of several cells containing different MPLs. MPLs were prepared with different hydrophobic agent contents to impart different water capillary pressures and hydrophobic properties. Experimental data were evaluated according to the water vapor activity of the cathode, which was modified with the capillary pressure of the MPL. As a result, the modified activity was determined to be a key factor to explain the occurrence of cathode flooding. Liquid water saturation in the diffusion media and the cathode catalyst layer (CCL) are determined by the balance of capillary pressures of the pores of each layer. Increase of the diffusion resistance of oxygen in the CCL is significant, while that in the diffusion media is moderate, which indicates that the CCL is susceptible to flooding.

The effects of the gravity on transient responses and cathode flooding in a proton exchange membrane fuel cell

The work presented in this paper includes contributions that provide insight into liquid water transport in the proton exchange membrane fuel cell (PEMFC).The purpose of the present study is to investigate the influence of gravity on the cathode flooding by accumulated liquid water at the interface between catalyst layer and gas diffusion layer (CL–GDL interface) in a PEMFC. We introduce a one-dimensional and transient model based on numerical resolution of mass transport of liquid water and oxygen in the porous GDL domain. Obtained results indicate that gravity has significant effects on the transient time and on the cell voltage at the steady regime. In this work, both opposing and aiding gravity cases, as reference to capillary actions, are considered.

Large–scale DMFC stack model: The effect of a condensation front on stack performance

The cathode flooding has a significant impact on DMFC performance. Liquid water reduces diffusion of oxygen through the cathode GDL. Oxygen transport problems on the cathode side redistribute the rate of methanol consumption and eventually lead to transport limitations on the anode side. Our model shows that there is a critical level of flooding of the cathode backing layer below which the cell performance suffers greatly, eventually running the risk of cell reversal. This critical point is reached when the oxygen diffusivity in the flooded domain drops by a factor of five with respect to the normal non-flooded level.

Experimental study on mitigating the cathode flooding at low temperature by adding hydrogen to the cathode reactant gas in PEM fuel cell

Severe flooding can be critical in a fuel cell vehicle operating at a high current density, and in a fuel cell vehicle at the initial stage of start up. It is often difficult to remove the condensed water from the cathode gas diffusion layer (GDL) of the fuel cell because of the surface tension between the water and the GDL. In this research, in order to remove the condensed water from the cathode GDL, a small amount of hydrogen was injected into the cathode reactant gases. The results showed that the hydrogen addition method successfully removed the liquid water from the cathode GDL. Water removal was verified for various hydrogen flow rates and hydrogen addition durations. Furthermore, the dew point temperature of the outlet gas at the cathode was observed to determine the amount of water removed from the cathode GDL. In addition, the water droplet in the cathode gas flow channel was visualized by using a transparent cell. Furthermore, degradation tests are also performed. Considering the degradation test, the hydrogen addition method is expected to be effective in mitigating cathode flooding.

Impedance-based diagnosis of polymer electrolyte membrane fuel cell failures associated with a low frequency ripple current

This work deals with a diagnosis of cathode flooding and membrane drying associated with a low frequency ripple current of a polymer electrolyte membrane fuel cell (PEMFC) based on impedance measurement on 12 single cells using electrochemical impedance spectroscopy (EIS). Average values of the identified model parameters obtained from direct measurement of the impedance curves of 12 single cells obtained after cycling for hours at variable frequencies, it has been found that impedance magnitude of a fuel cell injecting a low frequency ripple current (100 Hz) increased when compared with those injecting high frequency ripple currents (1 kHz and 10 kHz). Based on these investigations, additional impedance measurements are directly conducted to gain insight into cathode flooding and membrane drying concerning a low frequency ripple current. Regardless of operating frequency of ripple current, two PEMFC failures lead to an increase in the impedance magnitude in comparison with that of a fresh cell. Specifically, it is shown that a low frequency ripple current more accelerates the PEMFC degradation associated with two PEMFC failures.

A model and simulation of cathode flooding and drying on unsteady proton exchange membrane fuel cell

A water balance has a significant impact on the overall system performance in proton exchange membrane fuel cell. An actual fuel cell application has a dynamic electrical load which means also dynamic electrical current. Therefore, since this electrical current is known, the water production from the fuel cell reaction is also able to be predicted. As long as the fuel cell water transportation model is provided, the present liquid water inside the porous medium is also able to be modeled. A model of the liquid water saturation level in a fuel cell in unsteady load condition was proposed. This model is a series of the water transportation model of water saturation level for the final output of proton exchange membrane (PEM) fuel cell to predict the flooding or drying of PEM fuel cell. The simulation of vehicle fuel cell in different dynamic load profiles and different inlet air conditions was done using this model. The simulation result shows that PEM fuel cell with different dynamic load profiles has different liquid water saturation level profiles. This means that a dynamic load fuel cell requires also a dynamic input air humidification.

Water transport and flooding in DMFC: Experimental and modeling analyses

Water transport control is the major issue of direct methanol fuel cell. High water flow rates through the fuel cell imply extra water feeding at the anode and flooding at the cathode. In the present work, water transport and flooding in the direct methanol fuel cell are investigated through both experimental and modeling analyses and an interpretation of such phenomena is proposed. The model is validated on the experimental data of two different fuel cells in an extensive range of operating conditions. The analysis elucidates that water transport through the cathode diffusion layer is determined by vapor diffusion, slightly affected by current density, and by liquid water permeation proportional to current density, that occurs when liquid pressure in the electrode exceeds a threshold value. To simulate the effects of cathode diffusion layer flooding two mechanisms must be considered simultaneously: superficial pore obstruction, proportional to liquid water concentration in cathode channel, and bulk pore obstruction, proportional to liquid water permeation. The modeling analysis proposes the correlations to reproduce the effects of cathode flooding and permits to discuss the onset and magnitude of such phenomenon and the influence of micro-porous layer.

Numerical study on the cathode flooding of direct humidified proton exchange membrane fuel cell

In proton exchange membrane fuel cell (PEMFC) stack, the water management in the cell is required in order to ensure high cell performance. The water dry-out interferes with the proton migration in the electrode and electrolyte, and the water flooding interferes with the reactant gas transfer in the gas diffusion layer, consequently these phenomena affect the cell performance. One of the approaches to prevent the dry-out, a direct humidified cell with the channel to supply liquid phase water has been considered. In this cell configuration, sufficient water will be supplied to the anode, however the migrated water to the cathode from the anode shall be removed immediately in order to prevent the flooding. In this paper, numerical analysis that includes the behavior of the electrode reaction, heat transfer, gas flow, and evaporation and condensation of water was conducted regarding a direct humidified model cell. The co-flow and counter-flow of fuel and oxidant gases were considered for the pressure conditions of 1 atm and 3 atm. The water distribution in the vapor phase and liquid phase at the oxidant gas and cathode were calculated, and as a result of those, the flooding characteristics for each condition were evaluated.

Modeling of spontaneous hydrogen evolution in a direct methanol fuel cell

Abstract Hydrogen evolution takes place in the negative electrode of a direct methanol fuel cell with insufficient oxygen supply. In this work, we developed a two-dimensional computational model to investigate the formation and evolution of the galvanic region (GR) and the coupled electrolytic region (ER) where this spontaneous hydrogen evolution occurs. Special attention is paid to the species, potential and current distributions at different air flow rates (AFRs), and the effect of water flooding is highlighted. The open-circuit potential (OCP) of the cell is found to be quite sensitive to the AFR, especially just after the emergence of the abnormal ER. It is also demonstrated that the extent of cathode flooding has dramatic impacts on the cell behavior, which may be the major reason for the observed uncertainties of experimentally measured OCPs and current distributions.

Effect of inlet Stoichiometry in to Anode and Cathode Channels on Performance of Polymer Electrolyte Membrane Fuel Cell by using Three-Dimesional Mathematical Modeling

One of the most important performance in polymer electrolyte membrane fuel cell, is the problem in the water balance. At the same time being so that wet membrane for proton transfer,it can prevent from the cathode flooding and anode drying. Inlet stoichiometry to anode and cathode channels is one of the most important parameters in maintaining this balance. In this study, the effect of inlet stoichiometry on distribution of local current density, oxygen concentration and activation overpotential investigated by three-dimentional mathematical modeling which is according to conservation law, electrochemical reactions, ohmic losses and mass diffution with assuming the spiral arrangement of the flow channel, ideal gas mixture and being fully hydrated membrane. Modeling results, shows that increasing the inlet stoichiometry to cathode channel due to more air entering channel, resulting in more water leaving channel and lead to reduce water flooding and thus increase in electrochemical reaction and result to increase current density, activation overpotential and decrease Oxygen concentration. Increasing stoichiometry of anode also has a similar effect with the cathode stoichiometry. Because of this increase resulting to increased the hydrogen concentration along the anode channel, and thus, the electrochemical reaction speed up and therefor results to current density and activation overpotential increment

Visualization of flooding in a single cell and stacks by using a newly-designed transparent PEMFC

The flow phenomenon in the fuel-cell channels is difficult to understand and predict because of the two-phase flow. Proton exchange membrane fuel cells (PEMFCs) with transparent windows are widely used for visualizing the two-phase flow in the channels. In this paper, the visualization of the two-phase flow in the channels was accomplished under various current-density conditions using a transparent cell. The visualization of the single serpentine flow field clearly reveals that anode flooding is more severe than cathode flooding. The main cause for anode flooding is a low gas-flow rate in the channel because of the absence of the carrier gas. In addition, flooding is more significant under a low current-density condition than under a high current-density condition; under the latter condition, there is significantly more reaction heat that prevents flooding. The flow phenomena in the PEMFC stack were also visualized by electrically connecting three transparent cells in series and supplying fuel to each cell from a manifold. Sudden voltage drops and overshoots were detected, and the voltage fluctuations were found to be strongly related to flooding.