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.     

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 management in alkaline anion exchange membrane fuel cell anode

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.     

Experimental study of water transport in a polybenzimidazole-based high temperature PEMFC

The present work reports a systematic experimental analysis on water transport in a phosphoric acid doped polybenzimidazole-based high temperature PEM fuel cell. Two sets of polarization curves are run with dry and alternatively humidified reactants, covering a wide range of fuel cell operating temperatures and stoichiometries. With dry feed streams, up to 18% of water produced by electrochemical reaction is found on anode side proving the presence of water transport from cathode electrode. Under the investigated conditions, water transport across the membrane is independent of fuel cell temperature but strongly dependent on reactants stoichiometry and humidification. Such parameters can even determine a change in water transport direction. Humidification causes a limited drop in membrane proton resistivity (around 6 mΩ cm²); conversely a slight decrease in fuel cell performances (−5 to −20 mV) is measured.     

Sensitivity analysis of a polybenzimidazole‐based polymer fuel cell and insight into the effect of humidification

The present work describes a systematic investigation of the effect of operating temperature, cathode stoichiometry, anode stoichiometry and reactants humidification rate on the behavior of a polybenzimidazole‐based high temperature polymer fuel cell. The effect of reactants humidification was also considered; actually, in real applications, the syngas holds great amounts of water. Furthermore, water diffuses through the membrane and reaches the cathode side where it adds to the water produced by the electrochemical reaction. The investigation is based on the analysis of polarization curves measured under different operating conditions. Anode stoichiometry has no impact on the fuel cell voltage, while cathode stoichiometry and fuel cell temperature are relevant. When the anode stream is humidified, negligible effects take place; conversely, when the cathode stream is humidified, a consistent drop in the fuel cell voltage is observed, with a consequent drop in the power output. When air is saturated at 70 °C, a power loss of 8% and 27% takes place at 0.55 A cm^?2 and 0.9 A cm^?2, respectively. Such a finding might represent an issue when high power densities are pursued. The effect of cathode humidification was further investigated by means of electrochemical impedance spectroscopy and cyclic voltammetry. Thanks to dedicated tests, the effect of water in the cathode feed stream was clarified. Cathode humidification increases the electrode catalyst active area due to the dilution of the phosphoric acid retained in the electrode. Conversely, the presence of water hinders the oxygen mass transport to the catalyst active sites. Copyright © 2013 John Wiley & Sons, Ltd.     

Water transport in the proton-exchange-membrane fuel cell: measurements of the effective drag coefficient

The water transport in proton-exchange-membrane fuel cells has been experimentally investigated by measurements of the effective or net drag coefficient. Results are presented for a wide range of operating conditions (current density, temperature, pressure, stoichiometry and humidity of the inlet gases), as well as for different types of membrane-electrode-assemblies. It was found that the humidity and the stoichiometry of the inlet gases had a large effect on the drag. Of the material properties investigated, the membrane thickness was found to be the most significant parameter. Inspection of the cell performances showed that drying of the cathode was much more detrimental for the cell performance than drying of the anode. This was ascribed to increased activation losses, which turned out to be extremely sensitive to the type of cathode used.     

Water transport characteristics of a PEM fuel cell at various operating pressures and temperatures

In this investigation, water in a single-cell proton exchange membrane (PEM) fuel cell was managed using saturated hydrogen and dry air. The experiment was conducted at temperatures of 40, 50 and 60 °C and pressures of 1 and 1.5 bar at both the anode and cathode gas inlets. The feed velocities of hydrogen and air were fixed at 3 and 6 L min⁻¹, respectively. After reaching steady-state conditions, the relative humidity along the single serpentine gas channel was measured. From the experimental data, water transport properties were characterized based on a membrane hydration model. The electro-osmotic drag coefficient, water diffusion coefficient, membrane ionic conductivity and water back-diffusion flux were significantly influenced by the water content in the membrane of the PEM fuel cell. The water content depended on the relative humidity profile along the gas channel. In this investigation, a negative value for the water back-diffusion flux was measured; thus, the transport of water from the cathode to the anode did not occur. This phenomenon was due to the large water concentration gradient between the anode and cathode. Therefore, this strategy successfully prevented flooding in the PEM fuel cell.     

Water distribution measurement for a PEMFC through neutron radiography

Neutron radiography has been used for in situ and non-destructive visualization and measurement technique for liquid water in a working proton exchange membrane fuel cell (PEMFC). In an attempt to differentiate water distribution in the anode side from that in the cathode side, a specially designed cell was machined and used for the experiment. The major difference between our design and traditional flow field design is the fact the anode channels and cathode channels were shifted by a channel width, so that the anode and cathode channels do not overlap in the majority of the active areas.

The neutron radiography experiments were performed at selected relative humidities, and stoichiometry values of cathode inlet. At each operating condition, the water distribution in anode/cathode gas diffusion layers (GDLs) was obtained. Image processing with four different spatial masks was applied to those images to differentiate liquid water in four different types of areas. Results indicate that the reactant gas relative humidity and stoichiometry significantly influence current density distribution and water distribution.     

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.     

Three-dimensional modeling of water transport in PEMFC

A three dimensional two phase flow model is proposed to study transport phenomena in a PEMFC. In order to capture the effects of liquid water on the performance of the fuel cell, all regions are modeled from the anode to the cathode as having finite thickness. The geometry of the bipolar plate is modeled in detail to capture the effect of liquid water accumulation under the channel rib. This model takes into account the effect of temperature and inlet RH of both the anode and cathode. The three-dimensional model uses the finite volume method to solve the equations of mass conservation, momentum, energy, species transfer and protonic potential. These equations include the effect of liquid water on the transport properties as well as the electrochemical source. The effects of water on ohmic losses are presented for different humidity conditions of the anode and cathode at various fuel cell temperatures.     

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.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Improving dynamic performance of proton-exchange membrane fuel cell system using time delay control

Transient behaviour is a key parameter for the vehicular application of proton-exchange membrane (PEM) fuel cell. The goal of this presentation is to construct better control technology to increase the dynamic performance of a PEM fuel cell. The PEM fuel cell model comprises a compressor, an injection pump, a humidifier, a cooler, inlet and outlet manifolds, and a membrane-electrode assembly. The model includes the dynamic states of current, voltage, relative humidity, stoichiometry of air and hydrogen, cathode and anode pressures, cathode and anode mass flow rates, and power. Anode recirculation is also included with the injection pump, as well as anode purging, for preventing anode flooding. A steady-state, isothermal analytical fuel cell model is constructed to analyze the mass transfer and water transportation in the membrane. In order to prevent the starvation of air and flooding in a PEM fuel cell, time delay control is suggested to regulate the optimum stoichiometry of oxygen and hydrogen, even when there are dynamical fluctuations of the required PEM fuel cell power. To prove the dynamical performance improvement of the present method, feed-forward control and Linear Quadratic Gaussian (LQG) control with a state estimator are compared. Matlab/Simulink simulation is performed to validate the proposed methodology to increase the dynamic performance of a PEM fuel cell system.     

Steady state and dynamic performance of proton exchange membrane fuel cells (PEMFCs) under various operating conditions and load changes

The steady-state performance and transient response for H₂/air polymer electrolyte membrane (PEM) fuel cells are investigated in both single fuel cell and stack configurations under a variety of loading cycles and operating conditions. Detailed experimental parameters are controlled and measured under widely varying operating conditions. In addition to polarization curves, feed gas flow rates, temperatures, pressure drop, and relative humidity are measured. Performance of fuel cells was studied using steady-state polarization curves, transient I–V response and electrochemical impedance spectroscopy (EIS) techniques. Different feed gas humidity, operating temperature, feed gas stoichiometry, air pressure, fuel cell size and gas flow patterns were found to affect both the steady state and dynamic response of the fuel cells. It was found that the humidity of cathode inlet gas had a significant effect on fuel cell performance. The experimental results showed that a decrease in the cathode humidity has a detrimental effect on fuel cell steady state and dynamic performance. Temperature was also found to have a significant effect on the fuel cell performance through its effect on membrane conductivity and water transport in the gas diffusion layer (GDL) and catalyst layer. The polarization curves of the fuel cell at different operating temperatures showed that fuel cell performance was improved with increasing temperature from 65 to 75 °C. The air stoichiometric flow rate also influenced the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane and water flooding in cathode side. The fuel cell steady state and dynamic performance also improved as the operating pressure was increased from 1 to 4 atm. Based on the experimental results, both the steady state and dynamic response of the fuel cells (stack) were analyzed. These experimental data will provide a baseline for validation of fuel cell models.     

Treatment of two-phase flow in cathode gas channel for an improved one-dimensional proton exchange membrane fuel cell model

It has been reported recently that water flooding in the cathode gas channel has significant effects on the characteristics of a proton exchange membrane fuel cell. A better understanding of this phenomenon with the aid of an accurate model is necessary for improving the water management and performance of fuel cell. However, this phenomenon is often not considered in the previous one-dimensional models where zero or a constant liquid water saturation level is assumed at the interface between gas diffusion layer and gas channel. In view of this, a one-dimensional fuel cell model that includes the effects of two-phase flow in the gas channel is proposed. The liquid water saturation along the cathode gas channel is estimated by adopting Darcy’s law to describe the convective flow of liquid water under various inlet conditions, i.e. air pressure, relative humidity and air stoichiometry. The averaged capillary pressure of gas channel calculated from the liquid water saturation is used as the boundary value at the interface to couple the cathode gas channel model to the membrane electrode assembly model. Through the coupling of the two modeling domains, the water distribution inside the membrane electrode assembly is associated with the inlet conditions. The simulation results, which are verified against experimental data and simulation results from a published computational fluid dynamics model, indicate that the effects of relative humidity and stoichiometry of inlet air are crucial to the overall fuel cell performance. The proposed model gives a more accurate treatment of the water transport in the cathode region, which enables an improved water management through an understanding of the effects of inlet conditions on the fuel cell performance.     

Performance analysis of a solid oxide fuel cell with reformed natural gas fuel

In the present study a two‐dimensional model of a tubular solid oxide fuel cell operating in a stack is presented. The model analyzes electrochemistry, momentum, heat and mass transfers inside the cell. Internal steam reforming of the reformed natural gas is considered for hydrogen production and Gibbs energy minimization method is used to calculate the fuel equilibrium species concentrations. The conservation equations for energy, mass, momentum and voltage are solved simultaneously using appropriate numerical techniques. The heat radiation between the preheater and cathode surface is incorporated into the model and local heat transfer coefficients are determined throughout the anode and cathode channels. The developed model has been compared with the experimental and numerical data available in literature. The model is used to study the effect of various operating parameters such as excess air, operating pressure and air inlet temperature and the results are discussed in detail. The results show that a more uniform temperature distribution can be achieved along the cell at higher air‐flow rates and operating pressures and the cell output voltage is enhanced. It is expected that the proposed model can be used as a design tool for SOFC stack in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell— part II: Parameter sensitivity analysis

The operating and designing parameters have significant influences on the performance of an air-cooled proton exchange membrane fuel cell. Figuring out the parameter sensitivity helps select the appropriate operating point and the geometry size for a fuel cell. In this paper, parameter sensitivity analysis is conducted for the performance and the internal transport phenomena of an air-cooled proton exchange membrane fuel cell based on different air stoichiometries, air relative humidities and air flow field designs. The numerical results show that large air stoichiometry helps lower the single cell temperature, keeps the membrane better hydrated, and improves cell performance. Especially, the fluctuation of water content always exists periodically for the case of different air stoichiometry, where the minimum value of water content appears underneath the cathode channel in contrast to the maximum value appearing underneath the cathode rib. Furthermore, the maximum periodic fluctuation amplitude of water content is even more than 8 for the case of air stoichiometry of 150. The water flooding phenomenon becomes severe with the increase of air stoichiometry. Air with larger relative humidity also increases the single cell performance by improving the hydration of the membrane. However, water flooding becomes worse with the increment of air relative humidity. The narrower channel design for the cathode flow field not only leads to a more uniform current density distribution but also keeps the membrane better hydrated and thus enhances the cell performance.     

Water transport characteristics in a passive liquid-feed DMFC

A two-dimensional, two-phase, non-isothermal model was developed to investigate the water transport characteristics in a passive liquid-feed direct methanol fuel cell (DMFC). The liquid–gas two-phase mass transport in the porous anode and cathode was formulated based on multi-fluid model in porous media, and water and methanol crossover through the membrane were considered with the effect of diffusion, electro-osmotic drag, and convection. The model enabled numerical investigation of the effects of various operating parameters, such as current density, methanol concentration, and air humidity, as well as the effect of the cathode hydrophobic air filter layer, on the water transport and cell performance. The results showed that for the free-breathing cathode, gas species concentration and temperature showed evident differences between the cell and the ambient air. The use of a hydrophobic air filter layer at the cathode helped to achieve water recovery from the cathode to the anode, although the oxygen transport resistance was increased to some extent. It was further revealed that the water transport can be influenced by the ambient relative humidity.     

Study on the effect of humidity and stoichiometry on the water saturation of PEM fuel cells

This paper investigates the effects of relative humidity (RH) and stoichiometry of reactants on the water saturation and local transport process in proton exchange membrane fuel cells. A two‐dimensional model was developed, taking into account the effect of the formation of liquid water on the reactant transport. The results indicate that the reactant RH and stoichiometry significantly affect cell performance. At a constant anode RH = 100%, a lower cathode RH maintains membrane hydration to give better cell performance. At a constant cathode RH = 100%, 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. This enhances the back‐diffusion of water from the cathode to the anode, reducing possible flooding for better cell performance. Higher anodic stoichiometry results in the reduction of cathodic water saturation by increasing water back‐diffusion, thereby enhancing fuel cell performance. Higher cathodic stoichiometry also reduces water saturation by drying more liquid water to increase cathode local current density. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-isothermal dynamic modelling and optimization of a direct methanol fuel cell

A non-isothermal dynamic optimization model of direct methanol fuel cells (DMFCs) is developed to predict their performance with an effective optimum-operating strategy. After investigating the sensitivities of the transient behaviour (the outlet temperature, crossovers of methanol and water, and cell voltage) to operating conditions (the inlet flow rates into anode and cathode compartments, and feed concentration) through dynamic simulations, we find that anode feed concentration has a significantly larger impact on methanol crossover, temperature, and cell voltage than the anode and cathode flow rates. Also, optimum transient conditions to satisfy the desired fuel efficiency are obtained by dynamic optimization. In the developed model, the significant influence of temperature on DMFC behaviour is described in detail with successful estimation of its model parameters.