Gas Diffusion Layer Flooding Research Articles

Geometric Catalyst Utilization in Zero-Gap CO2 Electrolyzers.

The electrochemical reduction of CO2 (CO2RR) on silver catalysts has been demonstrated under elevated current density, longer reaction times, and intermittent operation. Maintaining performance requires that CO2 can access the entire geometric catalyst area, thus maximizing catalyst utilization. Here we probe the time-dependent factors impacting geometric catalyst utilization for CO2RR in a zero-gap membrane electrode assembly. We use three flow fields (serpentine, parallel, and interdigitated) as tools to disambiguate cell behavior. Cathode pressure drop is found to play the most critical role in maintaining catalyst utilization at all time scales by encouraging in-plane CO2 transport throughout the gas-diffusion layer (GDL) and around salt and water blockages. The serpentine flow channel with the highest pressure drop is then the most failure-resistant, achieving a CO partial current density of 205 mA/cm2 at 2.76 V. These findings are confirmed through selectivity measurements over time, double-layer capacitance measurements to estimate GDL flooding, and transport modeling of the spatial CO2 concentration.

An Unscented Kalman Filter Based on-line Diagnostic Approach For PEM Fuel Cell Flooding

Poor water management usually leads to various degrees of flooding in the hydrogen type fuel cell, affecting both the instantaneous performance and the long-term durability of the system adversely. While a lot of fuel cell diagnostic tools exist that could be utilized for the flooding diagnostics, most of these approaches are intrusive, requiring special modification to the fuel cell that affects its integrity, or special equipment (e.g. AC spectrometer) that adds to the complexity and cost of the system, and therefore are not considered to be a viable solution for the on-board integration of the diagnostic scheme.This paper proposes a model based approach for the fuel cell flooding diagnostics problem, utilizing only the cell current and voltage, and the inlet pressures of the fuel cell as the input signals of the diagnostic scheme. A diagnostic-oriented fuel cell system dynamic model is developed to incorporate the effects of the fault, i.e. the flooding, on the system dynamics. For simplicity, only the cathode channel flooding, the cathode gas diffusion layer (GDL) flooding, and the anode channel flooding are considered while we neglect the mass transport loss through the anode GDL. The cathode channel flooding and the GDL flooding diagnostic problems are decoupled and formulated as standard joint state and parameter estimation problems, with the amounts of the liquid water treated as varying system parameters to be identified. The unscented Kalman Filter technique has been applied to solve these problems. Simulation results validate the applicability of the cascading unscented Kalman filter design for flooding diagnostics.

3D experimental visualization of water flooding in proton exchange membrane fuel cells

In Proton Exchange Membrane Fuel Cell (PEMFC) heat and water are generated as a side production. The accumulated water leads to a decrease in PEMFCs' efficiency. Hence, water has to be removed in order to prevent flooding inside the cell. In this study, by using experimental transparent Gas Diffusion Layer (GDL) water transport in PEMFC is visualized, and the capillary fingering mechanisms are studied. The effect of adding Micro Porous Layer (MPL) on water flooding inside the GDL is also examined. The images of high-speed camera confirmed the characteristics of capillary fingering in the GDL. It is observed that lateral flow on the surface of Catalyst Layer (CL) leads to an increase in water flooding in GDL while PEMFC efficiency is decreased. By using the MPL and increasing its thickness, the amount of lateral flow on the CL is decreased, and the generated fingers are entered into the GDL at specific discrete locations on the surface of the MPL. In this condition, the fingers choose straight paths to reach the gas channel (GC). Adding the MPL, with a maximum thickness of 2 cm, leads to a 51.5% reduction in the expansion length of the penetrating fluid inside the GDL in comparison with the GDL without MPL. Also, at the end of the test, the residual volume of the non-wetting fluid inside the GDL and MPL layers is measured. By employing the MPL and increasing its thickness, the volume of the non-wetting fluid inside the cell is reduced down to 38% which causes a decrease in saturation level of water in the GDL, and the cell performance is also improved.

Anode water removal and cathode gas diffusion layer flooding in a proton exchange membrane fuel cell

Abstract Anode water removal (AWR) is studied as a diagnostic tool to assess cathode gas diffusion layer (GDL) flooding in PEM fuel cells. This method uses a dry hydrogen stream to remove product water from the cathode, showing ideal fuel cell performance in the absence of GDL mass transfer limitations related to water. When cathode GDL flooding is limiting, the cell voltage increases as the hydrogen stoichiometry is increased. Several cathode GDLs were studied to determine the effect of microporous layer (MPL) and PTFE coating. The largest voltage gains occur with the use of cathode GDLs without an MPL since these GDLs are prone to higher liquid water saturation. Multiple GDLs are studied on the cathode side to exacerbate GDL flooding conditions to further confirm the mechanism of the AWR process. Increased temperature and lower cathode RH allow for greater overall water removal so the voltage improvement occurs faster, though this leads to quicker membrane dehydration.

The gas diffusion layer in polymer electrolyte membrane fuel cells: A process model of the two-phase flow

A two-phase flow process model for the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell, considering also the cathode catalyst layer (CL), is presented. For this purpose, a systematic analysis of the factors affecting flooding and drying, including the liquid accumulation in the gas channel (CH), was performed using a one-dimensional reference model for the GDL and a compact channel model. The treatment proposed for the CH–GDL interface was compared with other boundary conditions in the literature. It was concluded that the liquid accumulation in the channel is determinant for estimating the steady state and transient GDL flooding, but that predicting the saturation level in the CL can help for determining operation policies for precluding flooding in the GDL–CL composite, in the absence of an adequate channel model. Bifurcation behavior, associated with the water phase change, was identified by means of the compact model.

Numerical modeling and analysis of micro-porous layer effects in polymer electrolyte fuel cells

Abstract It is well known that a micro-porous layer (MPL) plays a crucial role in the water management of polymer electrolyte fuel cells (PEFCs), and thereby, significantly stabilizes and improves cell performance. To ascertain the exact roles of MPLs, a numerical MPL model is developed in this study and incorporated with comprehensive, multi-dimensional, multi-phase fuel-cell models that have been devised earlier. The effects of different porous properties and liquid-entry pressures between an MPL and a gas diffusion layer (GDL) are examined via fully three-dimensional numerical simulations. First, when the differences in pore properties and wettability between the MPL and GDL are taken into account but the difference in the entry pressures is ignored, the numerical MPL model captures a discontinuity in liquid saturation at the GDL|MPL interface. The simulation does not, however, capture the beneficial effects of an MPL on cell performance, predicting even lower performance than in the case of no MPL. On the other hand, when a high liquid-entry pressure in an MPL is additionally considered, the numerical MPL model predicts a liquid-free MPL and successfully demonstrates the phenomenon that the high liquid-entry pressure of the MPL prevents any liquid water from entering the MPL. Consequently, it is found from the simulation results that a liquid-free MPL significantly enhances the back-flow of water across the membrane into the anode, which, in turn, helps to avoid membrane dehydration and alleviate the level of GDL flooding. As a result, the model successfully reports the beneficial effects of MPLs on PEFC performance and predicts higher performance in the presence of MPLs (e.g., an increase of 67 mV at 1.5 A cm−2). This study provides a fundamental explanation of the function of MPLs and quantifies the influence of their porous properties and the liquid-entry pressure on water transport and cell performance.

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.

Estimation of flooding in PEMFC gas diffusion layer by differential pressure measurement

The flooding, especially in gas diffusion layer (GDL), is one of the critical issues to put PEMFC to practical use. However, the experimental data of the flooding in GDL is so insufficient that the optimization design related to the water management for GDL has not established. In this study we developed a method to estimate the water saturation, namely the ratio of liquid water to pore volume in GDL. We fabricated a simple interdigitated cell where the supply gas is enforced to flow under rib. This structure enables to estimate the liquid water ratio in GDL by the measurement of differential pressure through the cell. We operated the cell and measured the differential pressure, and succeeded in estimating the water saturation, which changed largely with changing cell operation condition. In addition to this deferential pressure measurement, we measured the ionic resistance in polymer electrolyte membrane by ac impedance method. We evaluated and discussed the influence of the water saturation on cell voltage.

Analysis of GDL flooding effects on PEMFC performance

One of the major concerns of the gas diffusion layer (GDL) inside a proton exchange membrane fuel cell (PEMFC) is water management. Treatment of the pores of the GDL can affect the PEMFC performance due to the degree of water flooding inside the GDL. In this work, GDL flooding was investigated using a simplified approach in an established numerical model. The predictions are compared with experimental data of GDLs with different treatments. Local distributions inside the cell are given and the effect of homogenous GDL flooding on these is discussed.

Estimation of Flooding in PEMFC Gas Diffusion Layer by Differential Pressure Measurement

The flooding, especially in gas diffusion layer (GDL), is one of the critical issues to put PEMFC to practical use. However, the experimental data of the flooding in GDL is so insufficient that the optimization design to solve the flooding problem in GDL has not established until now. In this study we show a method to estimate the water saturation, namely the liquid water ratio for unit volume in GDL. We fabricated a simple interdigitated cell where the supply gas is enforced to flow under rib. This structure made it possible to capture the water droplet in GDL with the measurement of differential pressure through the cell. We operated the cell and measured the differential pressure, and succeeded in estimating the water saturation, which largely changed with changing cell operation condition. In addition to deferential pressure measurement, we measured the ionic resistance in polymer electrolyte membrane by AC impedance method. We evaluated the effect of the water saturation on the decrease of cell voltage.

Mass transfer in porous systems, flooded in part with water, of gas diffusion and catalytic layers of the cathode of a fuel cell with a solid polymer electrolyte

Mass transfer in porous gas diffusion and catalytic layers of the cathode of a hydrogen-air fuel cell with a solid polymer electrolyte is considered. The transport processes are considered with allowance made for the partial flooding of porous systems of these layers with water, which forms during the fuel cell operation. The consideration also allows for the influence of the diluent gas present when air oxygen is used as the oxidant. The fraction of water-flooded pores is calculated within percolation theory as a function of structural parameters of the porous system. Conditions leading to the beginning of the gas diffusion layer flooding are presented.

The role of ambient conditions on the performance of a planar, air-breathing hydrogen PEM fuel cell

This paper presents experimental data on the effects of varying ambient temperature (10–40 °C) and relative humidity (20–80%) on the operation of a free-breathing fuel cell operated on dry-hydrogen in dead ended mode. We visualize the natural convection plume around the cathode using shadowgraphy, measure the gas diffusion layer (GDL) surface temperature and accumulation of water at the cathode, as well as obtain polarization curves and impedance spectra. The average free-convection air speed was 9.1 cm s −1 and 11.2 cm s −1 in horizontal and vertical cell orientations, respectively. We identified three regions of operation characterized by increasing current density: partial membrane hydration, full membrane hydration with GDL flooding, and membrane dry-out. The membrane transitions from the fully hydrated state to a dry out regime at a GDL temperature of approximately 60 °C, irrespective of the ambient temperature and humidity conditions. The cell exhibits strong hysteresis and the dry membrane regime cannot be captured by a sweeping polarization scan without complete removal of accumulated water after each measurement point. Maximum power density of 356 mW cm −2 was measured at an ambient temperature of 20 °C and relative humidity of 40%.

A Nonisothermal, Two-Phase Model for Polymer Electrolyte Fuel Cells

A model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes is developed to investigate liquid water distribution and flooding in polymer electrolyte fuel cells (PEFCs) under nonisothermal conditions. The thermal model accounts for irreversible heat and entropic heat generated due to electrochemical reactions, Joule heating arising from protonic/electronic resistance, and latent heat of water condensation and/or evaporation. A theoretical analysis is presented to show that in the two-phase zone, water transport via vapor-phase diffusion under the temperature gradient is not negligible, with a magnitude comparable to the water production rate in PEFCs. Detailed numerical results further reveal that the vapor-phase diffusion enhances water removal from the gas diffusion layer (GDL) under the channel and exacerbates GDL flooding under the land. Simultaneously, this vapor-phase diffusion provides a new mechanism for heat removal through a phase change process in which water evaporates at the hotter catalyst layer, diffuses through the interstitial spaces of the GDL, and condenses on the cooler land surface. This new heat removal mechanism resembles the heat pipe effect. Three-dimensional simulations for a full PEFC using this nonisothermal, two-phase model are presented for the first time. Separate velocity fields of gas and liquid phases are given, clearly illustrating that the vapor-phase diffusion and capillary-driven liquid water transport in a GDL aid each other in water removal along the through-plane direction under the channel area, but oppose each other along the in-plane direction between the channel area and land.