Investigation of water transport through membrane in a PEM fuel cell by water balance experiments

Water balance in a polymer electrolyte membrane fuel cell (PEMFC) was investigated by measurements of the net drag coefficient under various conditions. The effects of water balance in the PEMFC on the cell performance were also investigated at different operating conditions. Experimental results reveal that the net drag coefficient of water through the membrane depended on current density and humidification of feed gases. It was found that the net drag coefficient (net number of water molecules transported per proton) ranged from −0.02 to 0.93, and was dependent on the operating conditions, the current load and the level of humidification. It was also found that the humidity of both anode and cathode inlet gases had a significant effect on the fuel cell performance. The resistance of the working fuel cell showed that the membrane resistance increased as the feed gas relative humidity (RH) decreased. The diffusion of water across Nafion membranes was also investigated by experimental water flux measurements. The electro-osmotic drag coefficient was evaluated from the experimental results of water balance and diffusion water flux measurements. The value of electro-osmotic drag coefficient, ranging from 1.5 to 2.6 under various operating conditions, was in agreement with literature values. The electro-osmotic drag coefficient, the net flux of water through the membrane and the effective drag as a function of operating conditions will also provide validation data for the fuel cell modeling and simulation efforts.     

Performance and water transport behaviour in Polymer Electrolyte Membrane fuel cells

Water management in fuel cells is important for avoiding the phenomenon of flooding or dehydration in the stack and for maintaining good fuel cell performance and durability. This study focuses on the evaluation of the dynamic performance and behaviour (purge cycle) of the commercial Polymer Electrolyte Membrane (PEM) fuel cell stack towards water transport (water balance) at different operating conditions. The stack was operated at different current loads (0–10 A) and operating temperature (ambient to 50 °C). The results indicated that the measured water accumulation in the stack increased with the increase in current load. The optimal current load was 4 A, with calculated efficiency of 62.8%. The optimal operating temperature was 40 °C, resulting in calculated efficiency of 52.3%. At higher temperature, the fuel cell performance decreased, and the measured water balance was not properly distributed, which could be due to the dehydration and low conductivity of the electrolyte membrane. It can be concluded that the behaviour and performance of the stack, as well as the water balance in the stack, were influenced by the operating conditions. Moreover, this study improves the understanding of fuel cell performance and behaviour based on evaluation of the water balance.     

Water management studies in PEM fuel cells,Part I: Fuel cell design and in situ water distributions

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.     

Visualisation of water accumulation in the flow channels of PEMFC under various operating conditions

The accumulation of water in the cathode/anode serpentine flow channels of a transparent PEMFC has been investigated by direct visualisation where water droplets and slugs formed in these channels were quantified over a range of operating conditions. Four operating parameters concerning air stoichiometry, hydrogen stoichiometry, cell temperature, and electric load were examined to evaluate their effects on the formation and extraction of water from the flow channels. The results showed that hydrogen and air stoichiometry contribute almost equally to the water formation process in the cathode channels. However, their effects on the water extraction from the channels were quite different. Air stoichiometry proved capable of extracting all the water from the cathode channels, without causing membrane dehydration, contrary to hydrogen. Increasing the operating temperature of the cell was found to be very effective for the water extraction process; a temperature of 60 °C was sufficient to evaporate all the water in the channels as well as enhancing the fuel cell current. The electric load was strongly associated to the water formation in the channels but had no influence on water extraction. Finally, no water was present in the anode flow channels under all examined operating conditions.     

Stability and performance improvement of a polymer electrolyte membrane fuel cell stack by laser perforation of gas diffusion layers

The performance and stability of a hydrogen-driven polymer electrolyte membrane fuel cell stack (6-cell PEFC stack) are investigated with regard to pore flooding within the gas diffusion layers (GDLs). Two short stacks with various GDLs (Toray TGP-H-060 untreated and laser-perforated) were characterized at different operating conditions by several characterization techniques such as constant current load, polarization curve, chronoamperometry and chronovoltammetry. The experimental results reveal that the perforation of the cathode GDLs improves the water transport in the porous media and thus the performance as well as the stability of the operating stack in medium and high current density range. A reduced pore flooding is verified when using the customized laser-perforated GDLs. The GDL perforation has a huge potential to balance the inhomogeneous in-plane saturation conditions between the inlet and outlet area of the cell and to compensate to a certain degree the effects of temperature distribution within a stack regarding the water management.     

Experimental study on water transport coefficient in Proton Exchange Membrane Fuel Cell

Water transport within Proton Exchange Membrane Fuel Cell (PEMFC) is investigated by systematic measurements of the water transport coefficient, defined as the net water flux across the membrane divided by the water production. It is recorded for various operating conditions (current density, gas stoichiometry, air inlet relative humidity, temperature, pressure) in a fuel cell stack fed by dry hydrogen. The measurement of the water transport coefficient shows that a significant fraction of water is collected at the anode while water is produced or injected at the cathode. Moreover, in usual operating conditions, liquid water is present at the cell outlet not only in the cathode but also in the anode. Contrary to the electrical performances, ageing has no influence on the water transport coefficient, which allows the comparison between data collected at different periods of the fuel cell lifetime. From this comparison, it was found that the hydrogen flow rate, the amount of vapor injected at cathode inlet, and the temperature are the main parameters influencing the water transport coefficient. It is shown that air and hydrogen stoichiometry present significant effects on water transport but only through these parameters.     

Simultaneous direct visualisation of liquid water in the cathode and anode serpentine flow channels of proton exchange membrane (PEM) fuel cells

Water flooding is detrimental to the performance of the proton exchange membrane fuel cell (PEMFC) and therefore it has to be addressed. To better understand how liquid water affects the fuel cell performance, direct visualisation of liquid water in the flow channels of a transparent PEMFC is performed under different operating conditions. Two high-resolution digital cameras were simultaneously used for recording and capturing the images at the anode and cathode flow channels. A new parameter extracted from the captured images, namely the wetted bend ratio, has been introduced as an indicator of the amount of liquid water present at the flow channel. This parameter, along with another previously used parameter (wetted area ratio), has been used to explain the variation in the fuel cell performance as the operating conditions of flow rates, operating pressure and relative humidity change. The results have shown that, except for hydrogen flow rate, the wetted bend ratio strongly linked to the operating condition of the fuel cell; namely: the wetted bend ratio was found to increase with decreasing air flow rate, increasing operating pressure and increasing relative humidity. Also, the status of liquid water at the anode was found to be similar to that at the cathode for most of the cases and therefore the water dynamics at the anode side can also be used to explain the relationships between the fuel cell performance and the investigated operating conditions.     

Water balance in a polymer electrolyte fuel cell system

Polymer electrolyte fuel cell (PEFC) systems operating on carbonaceous fuels require water for fuel processing. Such systems can find wider applications if they do not require a supply of water in addition to the supply of fuel, that is, if they can be self-sustaining based on the water produced at the fuel cell stack. This paper considers a generic PEFC system and identifies the parameters that affect, and the extent of their contribution to, the net water balance in the system. These parameters include the steam-to-carbon and the oxygen-to-carbon ratios in the fuel processor, the electrochemical fuel and oxygen utilizations in the fuel cell stack, the ambient pressure and temperature, and the composition of the fuel used. The analysis shows that the amount of water lost from the system as water vapor in the exhaust is very sensitive to the system pressure and ambient temperature, while the amount of water produced in the system is a function of the composition of the fuel. Fuels with a high H/C (hydrogen to carbon atomic ratio) allow the system to be operated as a net water producer under a wider range of operating conditions.     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.     

Numerical study of water management in the air flow channel of a PEM fuel cell cathode

The water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. By enabling the volume of fraction (VOF) model, the air–water two-phase flow can be simulated under different operating conditions. The effects of channel surface hydrophilicity, channel geometry, and air inlet velocity on water behavior, water content inside the channel, and two-phase pressure drop are discussed in detail. The results of the quasi-steady-state simulations show that: (1) the hydrophilicity of reactant flow channel surface is critical for water management in order to facilitate water transport along channel surfaces or edges; (2) hydrophilic surfaces also increase pressure drop due to liquid water spreading; (3) a sharp corner channel design could benefit water management because it facilitates water accumulation and provides paths for water transport along channel surface opposite to gas diffusion layer; (4) the two-phase pressure drop inside the air flow channel increases almost linearly with increasing air inlet velocity.     

Simulation of an air conditioning absorption refrigeration system in a co-generation process combining a proton exchange membrane fuel cell

In this work, a computer simulation program was developed to determine the optimum operating conditions of an air conditioning system during the co-generation process. A 1 kW PEMFC was considered in this study with a chemical/electrical theoretical efficiency of 40% and a thermal efficiency of 30% applying an electrical load of 100%. A refrigeration-absorption cycle (RAC) operating with monomethylamine–water solutions (MMA–WS), with low vapor generation temperatures (up to 80 °C) is proposed in this work. The computer simulation was based on the refrigeration production capacity at the maximum power capacity of the PEMFC. Heat losses between the fuel cell and the absorption air conditioning system at standard operating conditions were considered to be negligible. The results showed the feasibility of using PEMFC for cooling, increasing the total efficiency of the fuel cell system.     

Water equilibria and management using a two-volume model of a polymer electrolyte fuel cell

In this paper, we introduce a modified interpretation of the water activity presented in Springer et al. [T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, Polymer electrolyte fuel cell model, J. Electrochem. Soc. 138 (8) (1991) 2334–2342]. The modification directly affects the membrane water transport between the anode and the cathode (two electrodes) of the polymer electrolyte membrane (PEM) fuel cell in the presence of liquid water inside the stack. The modification permits calibration of a zero-dimensional isothermal model to predict the flooding and drying conditions in the two electrodes observed at various current levels [D. Spernjak, S. Advani, A.K. Prasad, Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell, in: Proceedings of the Fourth International Conference on Fuel Cell Science, Engineering and Technology (FUELCELL2006-97271), June 2006]. Using this model the equilibria of the lumped water mass in the two electrodes are analyzed at various flow conditions of the stack to determine stable and unstable (liquid water growth) operating conditions. Two case studies of water management through modification of cathode inlet humidification and anode water removal are then evaluated using this model. The desired anode water removal and the desired cathode inlet humidification are specified based upon (i) the water balance requirements, (ii) the desired conditions in the electrodes, and (iii) the maximum membrane transport at those conditions.     

Analysis on the heating performance of a gas engine driven air to water heat pump based on a steady-state model

In this study, the heating performance of a gas engine driven air to water heat pump was analyzed using a steady state model. The thermodynamic model of a natural gas engine is identified by the experimental data and the compressor model is created by several empirical equations. The heat exchanger models are developed by the theory of heat balance. The system model is validated by comparing the experimental and simulation data, which shows good agreement. To understand the heating characteristic in detail, the performance of the system is analyzed in a wide range of operating conditions, and especially the effect of engine waste heat on the heating performance is discussed. The results show that engine waste heat can provide about 1/3 of the total heating capacity in this gas engine driven air to water heat pump. The performance of the engine, heat pump and integral system are analyzed under variations of engine speed and ambient temperature. It shows that engine speed has remarkable effects on both the engine and heat pump, but ambient temperature has little influence on the engine’s performance. The system and component performances in variable speed operating conditions is also discussed at the end of the paper.     

Experimental investigation of the role of a microporous layer on the water transport and performance of a PEM fuel cell

A highly reliable experimental system that consistently closed the overall water balance to within 5% was developed to study the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the water transport and performance of a standard 100 cm² active area PEM fuel cell. Various combinations of cells were built and tested with PTLs at the electrodes using either carbon fibre paper with a MPL (SGL 10BB) or carbon fibre paper without a MPL (SGL 10BA). The net water drag coefficient at three current densities (0.3, 0.5 and 0.7 A cm⁻²) for two combinations of anode/cathode relative humidity (60/100% and 100/60%) and stoichiometric ratios of H₂/air (1.4/3 and 1.4/2) was determined from water balance measurements. The addition of a MPL to the carbon fibre paper PTL at the cathode did not cause a statistically significant change to the overall drag coefficient although there was a significant improvement to the fuel cell performance and durability when a MPL was used at the cathode. The presence of a MPL on either electrode or on both electrodes also exhibited similar performance compared to when the MPL was placed at the cathode. These results indicate that the presence of MPL indeed improves the cell performance although it does not affect the net water drag coefficient. The correlation between cell performance and global water transport cannot be ascertained and warrants further experimental investigation.     

凝汽器抽气管道加装冷却器的研究

针对胜利发电厂射水抽气设备工作水温度偏高的现状,提出在凝汽器与抽气器之间的抽气管路上安装一个冷却器,使从凝汽器抽出的气、汽混合物中的水蒸气凝结疏出的方法,降低抽气设备工作水温度,提高其抽气能力,从而提高凝汽器的真空。     

Water balance for the design of a PEM fuel cell system

The design of a proton exchange membrane (PEM) fuel cell system is important for the optimization of the function of supporting parameters in the fuel cell. The water balance in a PEM fuel cell is investigated based on the water transport phenomena. In this investigation, the diffusion of water from the cathode side to the anode side of the cell is observed to not occur at 20% relative humidity at the cathode (RHC) and 58% relative humidity at the anode (RHA). The minimum concentration of condensed water at the cathode side is observed at a cathode gas inlet relative humidity of 40% RHC–92% RHC and at temperatures between 343 K and 363 K. RHC operating conditions that are greater than 90% and at a temperature of 363 K increased the concentration of condensed water and occurred quickly, which result in a water balance that became difficult to control. On the anode side, the condensation of water is observed at operating temperatures of 353 K and 363 K.     

Water balance simulations of a polymer-electrolyte membrane fuel cell using a two-fluid model

A previously published computational multi-phase model of a polymer-electrolyte membrane fuel cell cathode has been extended in order to account for the anode side and the electrolyte membrane. The model has been applied to study the water balance of a fuel cell during operation under various humidification conditions. It was found that the specific surface area of the electrolyte in the catalyst layers close to the membrane is of critical importance for the overall water balance. Applying a high specific electrolyte surface area close to the membrane (a water-uptake layer) can prevent drying out of the anode and flooding at the cathode while the average membrane water content is only weakly affected. The results also indicate that in contrast to common presumption membrane dehydration may occur at either anode or cathode side, entirely depending on the direction of the net water transport because the predominant transport mechanism is diffusion. Consequently, operating conditions with a high net water transport from anode to cathode should be avoided as it is important to keep the cathode catalyst layer well humidified in order to prevent high protonic losses. Addition of the micro-porous layer did not affect the overall water balance or membrane water content in our study.     

Modeling efficiency and water balance in PEM fuel cell systems with liquid fuel processing and hydrogen membranes

Integrating PEM fuel cells effectively with liquid hydrocarbon reforming requires careful system analysis to assess trade-offs associated with H₂ production, purification, and overall water balance. To this end, a model of a PEM fuel cell system integrated with an autothermal reformer for liquid hydrocarbon fuels (modeled as C₁₂H₂₃) and with H₂ purification in a water–gas-shift/membrane reactor is developed to do iterative calculations for mass, species, and energy balances at a component and system level. The model evaluates system efficiency with parasitic loads (from compressors, pumps, and cooling fans), system water balance, and component operating temperatures/pressures. Model results for a 5-kW fuel cell generator show that with state-of-the-art PEM fuel cell polarization curves, thermal efficiencies >30% can be achieved when power densities are low enough for operating voltages >0.72 V per cell. Efficiency can be increased by operating the reformer at steam-to-carbon ratios as high as constraints related to stable reactor temperatures allow. Decreasing ambient temperature improves system water balance and increases efficiency through parasitic load reduction. The baseline configuration studied herein sustained water balance for ambient temperatures ≤35 °C at full power and ≤44 °C at half power with efficiencies approaching ∼27 and ∼30%, respectively.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Effects of operating parameters on the transient response of proton exchange membrane fuel cells subject to load changes

Water management is critical to the steady state and transient performance of proton exchange membrane (PEM) fuel cell systems. The dynamic behavior of PEM fuel cells is profoundly related to the dynamics of water transport in the fuel cell system, which include electro-osmotic drag and back-diffusion of water through membrane and rate at which water is generated, supplied and removed from the electrodes. In particular, for low humidity operations, water transport dynamics plays a dominant role in determining the time taken to reach steady state. Toward an understanding of the fuel cell dynamics for dry operations, numerical simulations are carried out for a single channel polymer electrolyte (PEM) fuel cell undergoing step change in cell voltage over wide range of operating conditions. A detailed model-based parametric study is carried out to analyze the effects of operating conditions on membrane water content and the time taken by the single cell system to reach steady state. Based on the studies, design windows are presented that limit the time taken to reach steady state to a desired value. Optimum parameter combinations that minimize the transient time are also identified from the studies.