Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates

We present a study of proton exchange membrane (PEM) fuel cells with parallel channel flow fields for the cathode, dry inlet gases, and ambient pressure at the outlets. The study compares the performance of two designs: a standard, non-porous graphite cathode plate design and a porous hydrophilic carbon plate version. The experimental study of the non-porous plate is a control case and highlights the significant challenges of operation with dry gases and non-porous, parallel channel cathodes. These challenges include significant transients in power density and severe performance loss due to flooding and electrolyte dry-out. Our experimental study shows that the porous plate yields significant improvements in performance and robustness of operation. We hypothesize that the porous plate distributes water throughout the cell area by capillary action; including pumping water upstream to normally dry inlet regions. The porous plate reduces membrane resistance and air pressure drop. Further, IR-free polarization curves confirm operation free of flooding. With an air stoichiometric ratio of 1.3, we obtain a maximum power density of 0.40 W cm⁻², which is 3.5 times greater than that achieved with the non-porous plate at the same operating condition.     

An experimental investigation of electro-osmotic drag coefficients in a polymer electrolyte membrane fuel cell

Through the use of a water balance experiment, the electro-osmotic drag coefficients of Nafion 115 were obtained under several conditions (as a function of water content and thermodynamics conditions). For the cases when the anode was fully hydrated (corresponding to water content λ ≈ 14 in the adjacent membrane) and the cathode suffered from drying when dry air was supplied (λ ≈ 2), the electro-osmotic drag coefficients varied from 0.82 (±0.06) to 0.50 (±0.03) H₂O/H⁺ when the current density varied from 0.4 to 1.0 A cm⁻² (95% confidence level). When the current density increased, the electro-osmotic drag coefficient decreased. When the water content at the anode increased from λ ≈ 5 to λ ≈ 14, the cathode was supplied with dry air (λ ≈ 2), and the fuel cell discharged constant current density at 0.6 A cm⁻², the electro-osmotic drag coefficient increased from 0.44 (±0.06) to 0.68 (±0.06) H₂O/H⁺ (95% confidence level). Higher relative humidity gas leads to a higher electro-osmotic drag coefficient at constant current density.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

Ethanol/water mixture permeation through a Nafion based membrane electrode assembly

In the present work, the permeation behavior of ethanol/water mixtures through a Nafion^®-115 based membrane electrode assembly (MEA) has been investigated. The crossover measurements were carried out in a single fuel cell test apparatus. Ethanol aqueous solutions at different concentrations were supplied to the anode compartment while high-purity dry helium was fed to the cathode in order to sweep off the permeated water and ethanol. The quantitative analysis of water and ethanol from the cathode effluent has been carried out on-line by a GC under the following operation conditions: T_cell = 30–90 °C, ethanol aqueous solution concentration C_ethanol = 0–12.0 mol L⁻¹, helium flow rate at the cathode F_He in the range of 80–1500 mL min⁻¹ and liquid solution flow rate to the anode F_l = 0.2 mL min⁻¹.     

Preparation of Pt/zeolite–Nafion composite membranes for self-humidifying polymer electrolyte fuel cells

A novel Pt/zeolite–Nafion (PZN) polymer electrolyte composite membrane is fabricated for self-humidifying polymer electrolyte membrane fuel cells (PEMFCs). A uniform dispersion of Pt nanoparticles with an average size of 3 nm is achieved by ion-exchange of the zeolite HY. The Pt nanoparticles embedded in the membrane provide the catalytic sites for water generation, whereas the zeolite HY-supported Pt particles absorbs water and make it available for humidification during cell operation at elevated temperature. Compared with the performance of ordinary membranes, the performance of cells with PZN membranes is improved significantly under dry conditions. With dry H₂ and O₂ at 50 °C, the PZN membrane with 0.65 wt.% of Pt/zeolite (0.03 mg Pt cm⁻²) gives 75% of the performance obtained at 0.6 V with the humidified reactants at 75 °C. Impedance analysis reveales that an increase in charge-transfer resistance is mainly responsible for the cell performance loss operated with dry gases.     

Transient behavior of water generation in a proton exchange membrane fuel cell

The effect of water generation on the performance of proton exchange membrane fuel cell (PEMFC) was investigated by using a periodical linear sweep method. Three different kinds of I–V curves were obtained, which reflected different amount of water uptake in the fuel cell. The maximum water uptake that could avoid flooding in the fuel cell and the hysteresis of water diffusion were also discussed. Quantitative analysis of water uptake and water transport phenomena in this study were conducted both experimentally and theoretically. Results showed that the water uptake capacity for the fuel cell under no severe flooding was 27.837 mg cm⁻². The transient response of the internal resistance indicated that the high frequency resistance (HFR) lagged the current with a value of about 20 s. The effect of purging operation on the internal resistance of the fuel cell was also explored. Experimental data showed that the cell experienced a continuous 8-min purging process can maintain at a relatively steady and dry state.     

Tortuosity in anode-supported proton conductive solid oxide fuel cell found from current flow rates and dusty-gas model

Ba(Ce_0.8Y_0.2)O_3−δ anode-supported proton conductive solid oxide fuel cells were fabricated and tested. By changing the H₂ partial pressure at the anode side, the effect of anodic concentration polarization on open-circuit voltage of the cell was observed. Saturation current densities under concentration polarization were obtained from different anode thickness cells and were used for tortuosity calculation. The calculation is based on the dusty-gas model which includes Knudsen diffusion and Stefan-Maxwell equation terms. The tortuosity value for our supporting anode is 1.55 ± 0.1 which is in a physically reasonable range for modern porous anode materials. The tortuosity that we found is independent of the cell testing temperature and anode thickness, which is consistent with the fact that tortuosity is a geometric factor of the anode structure. The derived equation also can be used for predicting the effect of varying the anode thickness, porosity and pore size. Also, the concentration of the gases as a function of position across the anode is determined.     

High-resolution in-plane investigation of the water evolution and transport in PEM fuel cells

High-resolution synchrotron X-ray radiography is used to study the evolution of primary water clusters and the transport of liquid water from the catalyst layer through the gas diffusion layer (GDL) to the gas channels of a low temperature polymer electrolyte membrane (PEM) fuel cell. The liquid water content is quantified separately in the respective components; in the hydrophobic microporous layer (MPL) almost no liquid water can be observed. In the adjacent GDL, depending on the current density i₀ water clusters are formed which lead to a diffusion barrier for the reactant gases. Water transport dynamics are explained and a recently proposed eruptive mechanism describing the transport from the GDL to the gas channels is imaged in a pseudo three-dimensional representation [A. Bazylak, D. Sinton, Z.-S. Liu, N. Djilali, J. Power Sources 163 (2007) 784–792; S. Litster, D. Sinton, N. Djilali, J. Power Sources 154 (2006) 95–105; I. Manke, Ch. Hartnig, M. Grünerbel, W. Lehnert, N. Kardjilov, A. Haibel, A. Hilger, H. Riesemeier, J. Banhart, Appl. Phys. Lett. 90 (2007) 174105]. Based on a high temporal resolution the dynamics of the liquid water transport are observed; transient conditions resembling dynamic operation of the fuel cell are studied and an estimation of the time required to reach equilibrium conditions is given. The obtained spatial resolution of 3 μm is far below commonly used techniques such as neutron radiography or ¹H NMR. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multi-phase flow are addressed.     

Oxygen-selective immobilized liquid membranes for operation of lithium-air batteries in ambient air

In this work, nonaqueous electrolyte-based Li-air batteries with an O₂-selective membrane have been developed for operation in ambient air of 20-30% relative humidity (RH). The O₂ gas is continuously supplied through a membrane barrier layer at the interface of the cathode and ambient air. The membrane allows O₂ to permeate through while blocking moisture. Such membranes can be prepared by loading O₂-selective silicone oils into porous supports such as porous metal sheets and Teflon (PTFE) films. It was found that the silicone oil of high viscosity shows better performance. The immobilized silicone oil membrane in the porous PTFE film enabled the Li-air batteries with carbon black air electrodes to operate in ambient air (at 20% RH) for 16.3 days with a specific capacity of 789 mAh g⁻¹ carbon and a specific energy of 2182 Wh kg⁻¹ carbon. Its performance is much better than a reference battery assembled with a commercial, porous PTFE diffusion membranes as the moisture barrier layer on the cathode, which only had a discharge time of 5.5 days corresponding to a specific capacity of 267 mAh g⁻¹ carbon and a specific energy of 704 Wh kg⁻¹ carbon. The Li-air battery with the present selective membrane barrier layer even showed better performance in ambient air operation (20% RH) than the reference battery tested in the dry air box (<1% RH).     

A novel full-field experimental method to measure the local compressibility of gas diffusion media

The gas diffusion medium (GDM) in a proton exchange membrane (PEM) fuel cell needs to simultaneously satisfy the requirements of transporting reactant gases, removing product water, conducting electrons and heat, and providing mechanical support to the membrane electrode assembly (MEA). Concerning the localized over-compression which may force carbon fibers and other conductive debris into the membrane to cause fuel cell failure by electronically shorting through the membrane, we have developed a novel full-field experimental method to measure the local thickness and compressibility of GDM. Applying a uniform air pressure upon a thin polyimide film bonded on the top surface of the GDM with support from the bottom by a flat metal substrate and measuring the thickness change using the 3-D digital image correlation technique with an out-of-plane displacement resolution less than 0.5 μm, we have determined the local thickness and compressive stress/strain behavior in the GDM. Using the local thickness and compressibility data over an area of 11.2 mm × 11.2 mm, we numerically construct the nominal compressive response of a commercial Toray™ TGP-H-060 based GDM subjected to compression by flat platens. Good agreement in the nominal stress/strain curves from the numerical construction and direct experimental flat-platen measurement confirms the validity of the methodology proposed in this article. The result shows that a nominal pressure of 1.4 MPa compressed between two flat platens can introduce localized compressive stress concentration of more than 3 MPa in up to 1% of the total area at various locations from several hundred micrometers to 1 mm in diameter. We believe that this full-field experimental method can be useful in GDM material and process development to reduce the local hard spots and help to mitigate the membrane shorting failure in PEM fuel cells.     

Application of water barrier layers in a proton exchange membrane fuel cell for improved water management at low humidity conditions

This study discusses the use of an additional layer in the cathode side of a proton exchange membrane fuel cell (PEMFC) for improved water management at dry conditions. The performance of fuel cells deteriorates significantly when low to no gas humidification is used. This study demonstrates that adding a non-porous material with perforations, such as stainless steel, between the cathode flow field plate and the gas diffusion layer (GDL) improves the water saturation in the cathode GDL and catalyst layer, increases the water content in the anode, and keeps the membrane hydrated. The slight voltage drop in the performance as a result of transport limitations is justifiable since the overall durability of the cell at these extreme conditions is enhanced. The results show that the perforated layer(s) enhances the operational life of the PEMFC under completely dry conditions. These extreme conditions (dry gases without humidification, 90 kPa, 75 °C) were used to accelerate the failure modes in the fuel cells.     

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.     

Towards hydrogen production using a breathable electrode structure to directly separate gases in the water splitting reaction

A novel electrode design to directly separate the gases and improve the efficiency of the water splitting reaction is described. In this work, platinum was used as a model catalyst, deposited on porous membranes with different pore size and shape. The O₂ evolution rate was monitored at the gaseous side of these breathable electrodes. We show that the hydrophobic Goretex^® membrane electrodes provide a highly efficient removal of the gases, breathing out 92% of expected O₂ during water splitting, and thereby also largely avoiding the well known migration of oxygen to the cathode in the absence of a separator in the cell. The breathable structure is also shown to operate as a hydrogen electrode. The ability to separate the two gases, without the need for a separator, decreases gas cross-over and thereby enhances the coloumbic efficiency. Merging this approach with catalysts and photocatalysts of a variety of types e.g. non-precious metal and metal oxides will allow fabrication of cost efficient and straightforward water splitting devices.     

An onboard hydrogen generation method based on hydrides and water recovery for micro-fuel cells

Micro-proton exchange membrane fuel cells are considered to be the next generation power sources for micro-scale power applications, but onboard hydrogen storage and generation with high energy density at the small scale is still a technical barrier. This paper introduces a hydrogen generation method based on an onboard hydride fuel and a byproduct water recovery mechanism for micro-hydrogen PEM fuel cells. The water recovery is carried out by water diffusion from the more humid cathode side to the less humid anode side through the proton exchange membrane. The micro-fuel cells based on this water recovery method were constructed and tested. The results demonstrate that the relative humidity has a significant affect on the fuel cell performance as well as the opening area on the cover layer, the type of hydrides, and the thickness of the Nafion membrane also can affect the fuel cell performance. A 10 mm³ prototype water recovery micro-fuel cell has been built and tested, and the device has produced a maximum power density of 104 W L⁻¹ and a maximum energy density of 313 W h L⁻¹.     

Synthesis and hydrogen permeation of Ni-Ba(Zr0.1Ce0.7Y0.2)O3−δ metal-ceramic asymmetric membranes

Ni-Ba(Zr_0.1Ce_0.7Y_0.2)O_3−δ (BZCY) metal-ceramic asymmetric membranes consisted of Ni-BZCY top membrane and porous substrate were successfully prepared and developed as hydrogen permeation membrane for the first time via a method to combine co-pressing technique and two-step sintering process. The uniform fine NiO-BZCY composite powders as the precursor of top membranes were co-synthesized through the citrate-nitrate combustion route (co-synthesis method), which was the key to fabricating Ni-BZCY thin membrane. The homogeneity and phase structure of two phases in powders were characterized using element-map technique and X-ray diffraction analysis, respectively. The fluxes through a metal-ceramic membrane of about 30-μm-thickness were measured as a function of temperature under different feed gas hydrogen partial pressures. The results indicated the asymmetric membrane displayed high hydrogen permeation flux and using 80%H₂/N₂ (with 3% of H₂O) as feed gas and dry high purity argon as sweep gas, a maximum flux of 2.4 × 10⁻⁷ mol cm⁻² s⁻¹ was achieved at 900 °C, exhibiting the predominance of asymmetric structures.     

Numerical studies of liquid water behaviors in PEM fuel cell cathode considering transport across different porous layers

In this paper, a two-phase two-dimensional PEM fuel cell model, which is capable of handling liquid water transport across different porous materials, is employed for parametric studies of liquid water transport and distribution in the cathode of a PEM fuel cell. Attention is paid particularly to the coupled effects of two-phase flow and heat transfer phenomena. The effects of key operation parameters, including the outside cell boundary temperature, the cathode gas humidification condition, and the cell operation current, on the liquid water behaviors and cell performance have been examined in detail. Numerical results elucidate that increasing the fuel cell temperature would not only enhance liquid water evaporation and thus decrease the liquid saturation inside the PEM fuel cell cathode, but also change the location where liquid water is condensed or evaporated. At a cell boundary temperature of 80 °C, liquid water inside the catalyst layer and gas diffusion media under the current-collecting land would flow laterally towards the gas channel and become evaporated along an interface separating the land and channel. As the cell boundary temperature increases, the maximum current density inside the membrane would shift laterally towards the current-collecting land, a phenomenon dictated by membrane hydration. Increasing the gas humidification condition in the cathode gas channel and/or increasing the operating current of the fuel cell could offset the temperature effect on liquid water transport and distribution.     

Passive water management at the cathode of a planar air-breathing proton exchange membrane fuel cell

Water management is a significant challenge in portable polymer electrolyte membrane (PEM) fuel cells and particularly in proton exchange membrane (PEM) fuel cells with air-breathing cathodes. Liquid water condensation and accumulation at the cathode surface is unavoidable in a passive design operated over a wide range of ambient and load conditions. Excessive flooding or dry out of the open cathode can lead to a dramatic reduction of fuel cell power. We report a water management design based on a hydrophilic and electrically conductive wick. A prototype air-breathing fuel cell with the proposed water management design successfully operated under severe flooding conditions, ambient temperature 10 °C and relative humidity of 80%, for up to 6 h with no observable cathode flooding or loss of performance.     

Water and air management systems for a passive direct methanol fuel cell

In this paper water and air management systems were developed for a miniature, passive direct methanol fuel cell (DMFC). The membrane thickness, water management system, air management system and gas diffusion electrodes (GDE) were examined to find their effects on the water balance coefficient, fuel utilization efficiency, energy efficiency and power density. Two membranes were used, Nafion^® 112 and Nafion^® 117. Nafion^® 117 cells had greater water balance coefficients, higher fuel utilization efficiency and greater energy efficiency. A passive water management system which utilizes additional cathode gas diffusion layers (GDL) and a passive air management system which makes use of air filters was developed and tested. Water management was improved with the addition of two additional cathode GDLs. The water balance coefficients were increased from −1.930 to 1.021 for a cell using a 3.0 mol kg⁻¹ solution at a current density of 33 mA cm⁻². The addition of an air filter further increased the water balance coefficient to 1.131. Maximum power density was improved from 20 mW cm⁻² to 25 mW cm⁻² for 3.0 mol kg⁻¹ solutions by upgrading from second to third generation GDEs, obtained from E-TEK. There was no significant difference in water management found between second and third generation GDEs. A fuel utilization efficiency of 63% and energy efficiency of 16% was achieved for a 3.0 mol kg⁻¹ solution with a current density of 66 mA cm⁻² for third generation GDEs.     

Water management in a single cell proton exchange membrane fuel cells with a serpentine flow field

Gas and water management is the key to achieving good performance from a polymer electrolyte membrane fuel cell (PEMFC) stack. Imbalance between production and evaporation rates can result in either flooding of the electrodes or membrane dehydration, both of which severely limit fuel cell performance. In the present study, a mathematical model was developed to evaluate moisture profiles of hydrogen and air flows in the flow field channels of both the anode and the cathode. For model validation, a single fuel cell was designed with an active area of 200 cm². Six humidity sensors were installed in the flow fields of both the anode and the cathode at 457 mm, 1266 mm and 2532 mm from the inlets. The experiment was performed using an Arbin Fuel Cell Test Station. The temperature was varied (25 °C, 40 °C, 50 °C and 60 °C), while hydrogen and air velocities were fixed at 3 L min⁻¹ and 6 L min⁻¹, respectively, during the operation of the single cell. The feed relative humidity at the anode was fixed at 1.0, while the feed relative humidity at the cathode was fixed at 0.005 (dry air). All humidity sensor readings were taken at steady state after 2 h of operation. Model predictions were then compared with experimental results by using the least squares algorithm. The moisture content was found to decrease along the flow field at the anode, but to increase at the cathode. The moisture content profile at the anode was shown to depend on the moisture Peclet number, which decreased with temperature. On the other hand, the moisture profile at the cathode was shown to depend on both the Peclet number and the Damkohler number. The trend of the Peclet number in the cathode followed closely that of the anode. The Damkohler number decreased with temperature, indicating increasing moisture mass transfer with temperature. The moisture profile models were successfully validated by the published data of the estimated overall mass transfer coefficient and moisture effective diffusivity of the same order of magnitude. The strategy of saturating the hydrogen feed and using dry air, as in the present work, has been shown to successfully prevent water droplet formation in the cathode, and hence prevent flooding.     

Structural optimization of the direct methanol fuel cell passively fed with a high-concentration methanol solution

In a high-concentration direct methanol fuel cell (HC-DMFC), the methanol crossover is typically decreased to an acceptable level by two main mechanisms: high methanol transport resistance between the anode reservoir and the membrane electrode assembly (MEA), and high water back flow from the cathode to the anode. Based on the semi-passive HC-DMFC fabricated in this work, the effects of methanol barrier layer (MBL) thickness and electrolyte membrane thickness on cell performance, methanol and water crossover, and fuel efficiency have been studied. The results showed that a thicker MBL could significantly decrease the methanol and water crossover by increasing the mass transport resistance between the anode reservoir and the MEA, while a thinner Nafion^® membrane could also significantly decrease the methanol and water crossover by enhancing the water back flow from the cathode through the electrolyte membrane to the anode. Using Nafion^® 212 as the electrolyte membrane, and a 6.4 mm porous PTFE plate as the MBL, a semi-passive HC-DMFC operating at 70 °C produced the maximum power density of 115.8 mW cm⁻² when 20 M methanol solution was fed as the fuel.