Water Management Layer Research Articles

Experimental measurements of fuel and water crossover in an active DMFC

This study measured polarization curves as well as the high-frequency resistance of active direct methanol fuel cell (DMFC) operates at around 80 °C with active controls of temperature, methanol concentration, airflow rate, and relative humidity. The relative humidity of the air did not have noticeable impacts on the fuel cell unless the operating temperature was near the evaporation temperature of water (100 °C). The hydrophobic water management layer (WML) between the membrane electrode assembly (MEA) and cathode air channel increases the mass transfer resistance and improves the water retention in MEA. Adding a WML increased the peak power density, decreased the ohmic resistance, and improved the fuel efficiency of the fuel cell, especially when it operated near 100 °C. This study also quantitatively measured methanol and water crossover as well as the fuel efficiency at different operating currents. The fuel efficiency increased significantly with the increase of the current density. Using a hydrophobic fuel management layer (FML) between the anode fuel channel and MEA reduced the fuel and water crossover rates and increased the ohmic resistance due to the decrease of the water content of the Nafion membrane. The FML improved fuel efficiency by reducing the methanol crossover. The combination of the FML and WML enabled the steady operation of DMFC using highly concentrated methanol solutions (up to 75 wt%).

Numerical simulation on mass transport in a passive vapor-fed direct methanol fuel cell operating with neat methanol

A direct methanol fuel cell (DMFC) operating with neat methanol is a promising power candidate for electronic and portable devices because of its high specific energy. But complex mass transport continues to hinder the reliability of such a DMFC. In this work, a two-dimensional isothermal model for a passive vapor-fed DMFC operating with neat methanol is developed to explore the triple-phase (gas, liquid and dissolved phases) mass transport and phase change. The simulation results indicate that the addition of water management layer at the cathode effectively facilitates the water recovery to the anode, especially under lower relative humidity condition. The increase of the methanol vapor supply rate alleviates the concentration polarization at high current densities, but the aggravated methanol crossover by an excessive fuel supply decreases the cathode performance. The elevated operating temperature enhances the electrochemical kinetics and mass transport, while excessively high temperature leads to the degradation of the cell performance due to significant water loss. The present work provides an effective theoretical guidance for the design and operation of a passive vapor-fed DMFC operating with neat methanol.

Computational analysis of the impact of a micro porous layer (MPL) on the characteristics of a high temperature PEMFC

High-temperature PEM fuel cells (HT-PEMFCs) are of current interest due to their favorable characteristics in relation to PEM fuel cells operating at a low temperature (LT-PEMFCs). The benefits concern poisoning of the CO catalyst, water management, thermal management and humidity issues. The major challenge facing these fuel cells is to have the membrane performing well at high temperature. This manuscript reveals the impact of an added micro porous layer (MPL) at both the cathode and anode electrodes by using a three-dimensional (3-D) numerical single-phase model. The bipolar plates have serpentine flow channels for the reactants. The most important result is that the MPL, known as the water management layer (WML) in LT-PEMFCs, improves the performance of an HT-PEMFC in terms of enhanced mass and heat transfer. The difference in the polarization curve and the three overpotentials (activation, ohmic, concentration) between two cases, i.e., with and without MPL, prove that the HT-PEMFC performance improvement is most significant for the ohmic overpotential (about 130% decrease at high current densities). In addition, a more uniform temperature distribution is obtained within the fuel cell components, especially in the membrane electrode assembly (MEA). This improved the performance of the membrane and accordingly a slight improvement of the fuel cell performance. The result shows a difference (at the point of maximum power) between the curves of with and without MPL of about 35%.

Constructing a Cathode Catalyst Layer of a Passive Direct Methanol Fuel Cell with Highly Hydrophilic Carbon Aerogel for Improved Water Management.

Nitrogen-doped porous carbon materials show excellent water adsorption ability by forming strong hydrogen bonding between water molecules and the doped atoms. When these porous carbon materials are used to construct a water management layer (WML) of a passive direct methanol fuel cell (DMFC), high water concentration and hydraulic pressure formed inside the cathode catalyst layer would facilitate the water recovery from cathode to anode. In this paper, a highly hydrophilic nitrogen-doped carbon aerogel was synthesized by the carbonization of hydrogel precursors composed of resorcinol, formaldehyde, and graphene oxide under ammonia, and it was used for the first time to construct the WML for liquid-feed and vapor-feed passive DMFCs. The results show that the WML significantly improves the output performance of the liquid-feed DMFC by enhancing the water recovery, which is characterized and proved by the smaller cathode polarization, the slightly increased anode polarization, and a released cathode water flooding situation. A new method was also proposed to study the in situ methanol crossover of DMFCs, which confirmed that the methanol crossover during the discharge was reduced by the WML. As for the vapor-feed DMFCs, the WML reduces both the cathode and anode polarizations significantly, which increases the output performance greatly. This study opens a new window for the design and optimization of the membrane assembly electrode of DMFCs.

Fuel and Water Management in Direct Methanol Fuel Cells

This study has designed and setup experiments to measure the performance of MEAs made from Johnson Matthey’s gas diffusion electrodes (anode: 3 mg/cm2 Pt and 1.5 mg/cm2 Ru; cathode: 1.25 mg/cm2 Pt). The polarization curves of MEAs with fuel and water management layers, which are porous materials with different wettability and mass transfer resistance, have been experimental measured with dilute methanol solutions (3M or less). In addition, the water crossover, and methanol crossover, as well fuel efficiency, energy efficiency have been estimated from changes of weight and concentration of methanol solutions before and after constant current discharge experiments. Results show that adding a fuel management layer between the methanol channel and the MEA can significantly reduce the water and methanol crossover rate and increase the fuel and energy efficiency. The net water flux across the electrolyte membrane could be less than the water flux caused by electro-osmotic drag flux. In other words, the water flux by diffusion and convection could be from cathode to anode when a fuel management layer is applied. However, the fuel management layer also reduces the water flux from the channel to the MEA and increases the ohmic resistance of the electrolyte membrane. A water management layer between the MEA and cathode channel is recommended to reduce the water loss through the cathode, reduce the water and methanol and water crossover rate, and increase the fuel efficiency. This water management layer does not deteriorate the water flooding since the operating current of methanol fuel cells (<0.5 A/cm2) are typically much lower than that of proton exchange membrane. The limiting current with or without the water management layer keeps almost the same. In addition, if a fuel management layer is applied to reduce the methanol crossover, a thinner electrolyte membrane is recommended because it requires less water to be hydrated and it has relatively low ohmic resistance.

Micro DMFC for Portable Applications

Objective and results Methanol fuel cell systems have the potential of being part of a portable energy system with both high energy density and fast refueling. A hearing aid is an example of a device where fast and easy refueling/recharging and high energy density is an important requirement. The direct methanol fuel cell (DMFC) system presented here is considered novel as it presents new solutions to miniaturization and, to our knowledge, has the smallest volume reported in literature for a self-contained system. Furthermore, it has high energy density, and has been thoroughly tested with many cells actively running for more than 2 years. The fuel cell developed is a vapor feed DMFC. It is omnidirectional and fully self-contained, weighing less than 1 g with fuel and packaging. The design has been optimized for maximum energy and power density for the given footprint. At room temperature (25 °C) a gravimetric energy density of 350 Wh/kg has been obtained. Scaling the reservoir can lead to a cell with an energy density of more than 700 Wh/kg. In the specific case of powering hearing aids, the current design of the micro fuel cell is capable of powering a hearing aid for up to 2 days, with a refueling time of 20 seconds. At the DTI testing facilities, up to 500 cells have been evaluated simultaneously and the oldest cells have been running for 7 years. More than 1500 cells have been tested and currently 60 cells have been operating for more than 20.000 hours. The accumulated runtime of the tested cells is above 6 million hours thus providing significant amount of data for use in development of the cells. Excellent degradation levels have been shown. One example is a cell exhibiting a degradation of 1 µV/h during its 25.000 hours lifetime, but with no measurable degradation over the last 20.000 hours. Fuel cell design The presented work will focus on the fuel cell system design and how it enables the described figures of merit. The cell consists of three primary parts. One part termed the powerpack, which contains the functional parts of the fuel cell. These are the pervaporation membrane, current collectors and membrane electrode assembly (MEA). The two other primary parts are a fuel reservoir, containing the methanol solution, and a valve which enables refueling of the system. Figure 1.A shows a drawing of the powerpack and the different elements inside the powerpack, which will be the key topic of the presented work. The powerpack is build by placing layers of the different parts inside a metal cup (the anode cup), which acts as the anode current collector. The design has been developed to minimize packaging, maximize active MEA area and enable easy assembly. Traditional gaskets take up a large part of the potential active MEA area in small scale systems. In the design shown in Figure 1.A the gasket on the anode side has been removed. In the latest design traditional gaskets have been removed and the polymer electrolyte membrane (PEM) acts as both ion conductor, gasket and insulator. This is achieved by forming the PEM into a cup, which continues up the sides of the anode cup, thus separating the anode side from the cathode side. This simplifies assembly and results in a significant increase in voltage output. The CO2 produced at the anode side is vented through a small hole in the sidewall of the anode cup. A pervaporation membrane, placed in the bottom of the anode cup, prevents direct liquid contact to the fuel reservoir, thus enabling vapor feed operation. This design makes the fuel cell omnidirectional. There is no liquid contact to the CO2 hole, regardless of the direction the fuel cell has. Figure 1: A) An illustration of a powerpack and the layers inside it: 1: Isolator, 2: cathode current collector, 3: water management layer (WML), 4: cathode gas diffusion electrode, 5: gasket, 6: polymer electrolyte membrane, 7: anode gas diffusion electrode, 8: (WML), 9: pervaporation limiter (PL), 10: pervaporation membrane, 11: anode cup (and current collector). B) A picture of an actual fuel cell. The size is approximately 9x8x5.5 mm3. Figure 1

Effects of an MPL on water and thermal management in a PEMFC

In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC.

Water Management in Low Temperature Fuel Cells through in-Operando X-Ray Computed Tomography

For polymer-electrolyte fuel cells (PEFCs) and anion exchange membrane fuel cells (AEMFCs) optimal water management is essential. During start-up and at low operating temperatures, where water is present in a liquid form, it can block reactant delivery to electrocatalyst sites, resulting in significant mass-transport losses, flooding and low fuel cell power densities. Using synchrotron X-ray computed tomography (CT) we have previously shown how water transport properties depend on porous materials morphology and chemical composition, as well as cell compression.1-3 These ex- and in-situ studies isolate specific porous membrane / electrode assembly (MEA) component’s effect on water management (such as micro-porous (MPL) or gas-diffusion layer (GDL)). To understand water transport under operating conditions we use in-operandoX-ray CT studies, where water distribution is observed under various steady-state conditions. In-operando single serpentine channel (1x1 mm) fuel cell testing hardware for synchrotron X-ray CT is used with active area of 1 cm2. Steady-state water generation and removal is observed in PEFCs and AEMFCs for temperature of 30oC with chronoamperometric holds at current densities ranges of 20 – 100 mA/cm2. Furthermore, comparison of water distribution in PGM and PGM-free catalysts for both type of fuel-cells will be presented. In all of these systems water pooling is observed at the component interfaces, indicating the importance of interfaces design. Acknowledgment: The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Moisturized anode and water management in a passive vapor-feed direct methanol fuel cell operated with neat methanol

This paper investigates the moisturized anode and water management of a vapor-feed direct methanol fuel cell operated with neat methanol. Three methods of water management are experimentally compared, including water storage in a fuel reservoir, active water vapor supply and water recovery from the cathode to the anode. A water management layer for water recovery is introduced to the cathode, which is made of a quasi-superhydrophobic sintered porous metal plate (SPMP) to enhance water back diffusion (WBD). Results prove that each of these methods can improve the cell performance. WBD enhancement based on the use of a SPMP is proven to be the most effective way. It is also found that combination of different methods may more promote the cell performance. Using a WBD enhancement layer under the condition of active water vapor supply can completely eliminate performance decline in the early stage of constant-load discharging. For fully-passive operation, a higher catalyst loading at the cathode helps retain stable performance when a WBD enhancement layer is used. Based on this design, the passive vapor-feed DMFC fed with neat methanol can achieve a maximum power density of 21 mW cm−2.

Passive vapor-feed direct methanol fuel cell using sintered porous metals to realize high-concentration operation

The use of methanol vapor is helpful to reduce the effect of methanol crossover in a direct methanol fuel cell. This study develops a passive vapor-feed direct methanol fuel cell (VF-DMFC) based on a pervaporation membrane and sintered porous metals which act as functional layers for effective control of the mass transfer process. The feasibility of using this method is experimentally validated. For the cathode, a sintered porous metal-fiber plate (SPMFP) with great hydrophobicity is used as a water management layer to enhance water back diffusion from the cathode to the anode. Results indicate that the use of a SPMFP promotes a higher cell performance especially when a higher methanol concentration is used. The highest peak power density of 19.3mW/cm2 is achieved at an ambient temperature when 12M methanol is supplied. Neat-methanol operation is also viable under this condition. The cathode current collector (CC) with a window-like pattern yields a higher cell performance than the parallel-channel. For the anode, the use of a hydrophilic sintered porous metal-powder plate (SPMPP) embedded in the anode CC reduces the cell performance due to limitation of methanol delivery. This work also reports the influences of methanol concentration and vapor chamber temperature.

A numerical investigation of the effects of membrane swelling in polymer electrolyte fuel cells

A two-dimensional computational fluid dynamics model of PEM fuel cell is developed by taking into account the electrochemical, mass and heat transfer process occurring in the cathode compartment. Additionally, this model includes the effect of water content in the membrane swelling phenomenon. Several parameters such as gases temperature, inlet velocity and membrane characteristics are too investigated to establish their effect on the PEM fuel cell performance. The membrane water content and the air fraction variation in the gas channel are examined for diverse values of Reynolds number. In particular, the desirable inlet flow for enhancing the performance of the PEM fuel cell is determined by examining membrane water content patterns. The methodology in this study is useful to the control of water management and gas diffusion layer design.

Characteristics of heat and mass transport in a passive direct methanol fuel cell operated with concentrated methanol

In this work, the effect of cell structure on the overall cell performance as well as the heat and mass transport in a passive direct methanol fuel cell (DMFC) operated with concentrated methanol solution is experimentally investigated. The influences of the anode methanol vapor transport layer (VTL) and cathode water management layer (WML) are examined. The variations of cell temperature under different working conditions are also recorded. The results show that both the anode VTL thickness and cathode WML thickness greatly influence the cell performance and the species transport in the cell, which should be carefully optimized. Also, it is shown that the difference between the stable cell temperature under the open circuit condition and that under constant voltage discharging condition is quite small for a passive vapor-feed DMFC operated with concentrated fuel. Theoretical analysis and experimental results show that the transport resistance of methanol in the membrane electrode assembly (MEA) is much smaller than that in the other anode components of the cell structure.

Improving the water management and cell performance for the passive vapor-feed DMFC fed with neat methanol

A passive vapor-feed direct methanol fuel cell (DMFC) was experimentally investigated to improve its water management and cell performance when neat methanol was directly used. The effects of different water management approaches, including the addition of a water management layer (WML) and a hydrophobic air filter layer (AFL), and the use of thinner membrane on the cell performance, internal resistance, and fuel efficiency were investigated. The transient discharging behavior and long-term stability of the passive vapor-feed DMFC with the optimized water management were also studied. The results showed that by adding a WML and an AFL, or thinning the membrane thickness, the water management capability can be highly improved, not only enhancing the water recovery from the cathode to the anode, leading to a lower internal resistance and better cell performance, but also curbing the methanol crossover, increasing the fuel efficiency. It is also seen from the long-term constant-voltage test that the discharged current varied with the methanol concentration in the tank and the ambient temperature, while no evident permanent performance degradation was encountered after the 150 h test.

Development of a High Performance Passive Vapor-Feed DMFC Fed with Neat Methanol

A passive vapor-feed direct methanol fuel cell (DMFC), which has a membrane vaporizer and a hydrophobic vapor transport layer (VTL), was investigated experimentally to improve its performance when highly concentrated or neat methanol was directly used. DMFCs with different structure parameters, such as the VTL thickness, open area ratio of the vaporizer, and thickness of the water management layer, were tested using different methanol concentrations varying from 6 M to neat methanol. The results showed that the performance of the passive vapor-feed DMFC mainly depended on the trade-off between the methanol vapor supply to the anode and methanol crossover through the membrane. Thickening the VTL, or decreasing the open area ratio of the vaporizer, effectively decreased the transport rate of methanol vapor to the anode, leading to an increase in the concentration of the optimum feeding methanol. The hydrophobic water management layer in the cathode was also useful in lowering the methanol crossover and increasing the membrane hydration level. Through optimizing the structure parameters, a passive vapor-feed DMFC fed with neat methanol, which showed a peak power density of and a high fuel efficiency of 62% at room temperature, was developed.

Methanol and water crossover in a passive liquid-feed direct methanol fuel cell

Methanol crossover, water crossover, and fuel efficiency for a passive liquid-feed direct methanol fuel cell (DMFC) were all experimentally determined based on the mass balance of the cell discharged under different current loads. The effects of different operating conditions such as current density and methanol concentration, as well as the addition of a hydrophobic water management layer, on the methanol and water crossover were investigated. Different from the active DMFC, the cell temperature of the passive DMFC increased with the current density, and the changes of methanol and water crossover with current density were inherently coupled with the temperature rise. When feeding with 2–4 M methanol solution, with an increase in current density, both the methanol crossover and the water crossover increased, while the fuel efficiency first increased but then decreased slightly. The results also showed that a reduction of water crossover from the anode to the cathode was always accompanied with a reduction of methanol crossover. Not only did the water management layer result in lower water crossover or achieve neutral or reverse water transport, but it also lowered the methanol crossover and increased the fuel efficiency.

Preparation of water management layer and effects of its composition on performance of PEMFCs

A membrane electrode assembly (MEA) with a novel water management layer (WML) used in proton exchange membrane fuel cell (PEMFC) was prepared. The so called WML, which was located between the carbon paper and the catalyst layer, was a sublayer composed of carbon and hydrophobic PTFE. Various parameters of the WML, including carbon loading, PTFE content and species, sintering time and temperature and pore formers, were investigated in this study. As demonstrated in our experimental results, the performance of the membrane electrode assembly (MEA) PEMFC could be significantly improved by WML in the condition of operation with dry reactive gases. The MEA with the WML exhibited more stable performance than the situation of MEA without WML during a long time running period.

Influence of water transport properties through gas diffusion layer on flooding in PEFC

How the property of liquid water transport through a gas diffusion layer (GDL) influences the cell performance on electrode flooding was investigated by examining the cell performance and measuring the pressure to dehydrate liquid water from a GDL on a polymer electrolyte fuel cell (PEFC). A PEFC is required to operate under high humidity condition and high gas utilization aimed at high performance and high system efficiency. However, conversely, sudden decrease of the cell performance is caused by the water flooding in some types of GDL which disturbs the gas diffusion to reaction area under above conditions. The GDLs fabricated by four manufacturers which have different characters of structure, affinities for water, with and without through-holes, with and without a water management layer were examined by the single cell generation examination. Focusing attention on the experimental fact of the recent researches, the GDL is impregnated with water when electrode flooding is caused. As the examination outside of the cell that imitates the actual generation, the property of liquid water transport was evaluated by measuring dehydration pressure of through plane direction from water impregnated GDL. By the comparison limiting current determined by the IV curve with dehydration pressure, it was found that strong negative correlation existed between flooding phenomena in the GDL and pressure to dehydrate from the GDL.

Study on Water Management of Polymer Electrolyte Fuel Cell (3rd Report, Influence of Gas Channel Height on Water Transportation in PEFC Cell)

We have experimentally investigated influences of cathode channel heights on vapor permeation flux through gas diffusion layer (GDL) of polymer electrolyte fuel cell (PEFC). Vapor permeation flux through GDL was remarkably influenced by channel velocity. In the analysis of these experiments with a vapor transport model, it was shown that the resistance associated with the vapor-concentration boundary layer was dominant in the low channel velocity less than 0.5m/s, and the resistance for the diffusivity of GDL became dominant as channel velocity increases. Influences of the channel height on the vapor permeation flux were verified by the power generation tests using a single cell. In the cell tests, anode channel height was fixed at 0.5 mm and cathode channel heights were changed at 0.2 mm, 0.5 mm, 1 mm and 3 mm. Under the high humidity condition, the cell performance was improved by using a separator with the shallow channel height. Such a cell performance was well correlated with the vapor permeation flux through GDL. In addition, influences of oxidant utilization and water management layer (WML) on the cell performance were evaluated.

A novel method for the preparation of a PEMFC water management layer

A novel rolling method for the preparation of the water management layer (WML) of a PEMFC was developed. In this method, the WML was prepared without dependence on the carbon paper substrate. The obtained WML was in a filmed state. The thickness, PTFE content and sintering condition of the rolled WML film were investigated. The MEA with the WML film exhibited a more stable performance than the MEA without the WML during a long running period. The water management ability of the WML film was confirmed.

Design of low-humidification PEMFC by using cell simulator and its power generation verification test

As long as the perfluorinated proton exchange membrane (PEM) is used for the electrolyte, both the cell performance and life are highly dependent upon the water content in the electrolyte. On the other hand, pre-humidification of fuel and oxidant gases complicates the PEMFC system and prevents it from possible cost reduction measures. In this study, in order to maintain a membrane electrode assembly (MEA) with a satisfactory water content by only the water produced in catalyst layer through the electrode reaction without prior humidification of both the fuel and oxidant gases, a novel gas diffusion layer (GDL) was fabricated. This was achieved by coating a water management layer (WML) onto a traditional GDL in order to place the WML between the traditional GDL and the catalyst layer of the PEMFC. This study describes the significant balance of water with WML in the fuel cell using both simulation and experimental analysis.