Passive Water Management Research Articles

(Invited) Addressing Durability in Long-Life, Low-Power Fuel Cells

Fuel Cells’ scalability and energy density make them excellent candidates for different applications, especially difficult to electrify applications. Their design flexibility can also be applied to low power, long-life applications. LANL has constructed a unique design to provide continuous, low power (< 100 µW) for very long duration (multiple decades). This LANL system operates passively, without active control of cell parameters, such as temperature and humidity. In addition, the system does not require interventions like refueling or maintenance during its operational lifetime. While the application and operational conditions of the systems mentioned here differ greatly from those of light and heavy-duty vehicles (e.g. ambient temperature operation, passive water management, low current densities), many of the modes of degradation are similar, and listed below: Membrane thinningDegraded performance due to loss of electrochemical surface area (ECSA)Loss of water management due to changes in hydrophobicityIncreased cell resistance due to corrosion of cell components To better understand these degradation processes LANL has performed accelerated stress tests (ASTs) to induce and quantify the membrane thinning, loss of ECSA, hydrophobicity, and bipolar plate corrosion. Those ASTs include long term OCV holds with reactants present, high potential cycling (1-1.5 V) in flooded conditions, potential cycling to induce catalyst Ostwald ripening, and exposure of cell components to corrosive environments.

Intensification of water management during climate change

The current management of the world’s fresh water resources is not optimal. Due to the uneven distribution of water, many parts of the world are entering a passive water management balance due to climate change and the growth of the human population. The Czech Republic is one of the countries with a passive balance. With ongoing climate change, it will be difficult in many regions to maintain sufficient raw water for technological and technical purposes without intensification in the water management process. Scientific progress and current technical possibilities offer ways to increase the way water is treated and the possibilities of re-using the realized water in the area of its use for various purposes, from the water source to the final cycle. One of these possibilities and the way of intensive use of water is dealt with in the following article.

Force Scaling Comparison of Transport Phenomena in Proton Exchange Membrane Fuel Cell Flow Channels

Abstract Liquid–gas two-phase flow in flow channels of proton exchange membrane (PEM) fuel cells has been investigated extensively in the literature; however, a comparison between the order of the magnitude of the forces occurring within the flow channels has not been documented. A comparison is relevant due to increased interest in practical active and passive water management strategies. The present study compares the magnitude of the forces experienced by liquid water residing in the flow channels. An analytical model of a 20-cm-long flow channel was analyzed, and key forces were compared in the stream-wise coordinate. Results clearly reinforce the dominance of the surface tension forces over other forces applied in the channel while also demonstrating how they change with key variables. For a cathode stoichiometric ratio of 1, the surface tension effects were calculated to be three orders of magnitude greater than the gravitational effects, the second largest force scale, for a droplet diameter of 0.1 mm. For larger droplets, this difference becomes smaller but the surface tension effects remain dominant. The results are useful for flow-field designers where water removal using complex geometry and hydrophobic coatings are being explored.

Imaging and Modeling of Passive Water Management in a Miniature Fuel Cell

Fuel cells have potential for small power applications that require long periods, as much as several decades, of uninterrupted power. Small microwatt fuel cell stacks of varying membrane thickness were designed to allow for passive water management. The passive water management system was incorporated into the stack design by use of exposed Nafion membrane for lateral water transport. 1 An experimental and theoretical study of water management across the operating temperature range was conduction, including operando neutron imaging and the development of a multiphysics two-dimensional model. Stack testing was done between -55 °C to 85 °C using a baseline output current of 10 µA and periodic pulses of 4.5 mA lasting 100 ms, every 100 s, or an average current of 16 µA. The stack spatial water concentration was monitored during various operating conditions through in situ neutron imaging. Figure 1 shows the main points of interest on the stack. Operating temperatures ranged from -55 °C to 85 °C to analyze water formation and subsequent water removal. Freezing and below-freezing temperatures were controlled by use of a liquid nitrogen cooled series of insulated boxes to prevent external water condensation during imaging. These measurements suggest successful water transport laterally, enabling operation from the sub-freezing environment to temperatures near the boiling point of water. Figure 2a depicts a constant operation for two hours at 16 µA and -20 ºC and the subsequent drying process in Figure 2b. Water production due to gas crossover in the stacks was quantified at below-freezing temperatures. At temperatures at and below -20 °C, the water production due to gas crossover was greater than the amount of water removal via the passive water management strategy with the current turned off, rather than a drying of the cell as expected. With the gases turned off as well, drying occurs. This indicates that the cell design must allow for water (ice) build-up during operation at these sub-freezing temperatures for successful operation. To avoid large amounts of hydrogen loss due to membrane crossover, we employ multiple layers of membrane to make a thick, mechanically reinforced and chemically stabilized membrane. A two-dimensional multiphysics model for evaluation of water management was also developed to further describe and explore the passive water management system under different conditions. The fundamental model in its present form shows a similar shape but indicates faster water build-up than experimental data. This discrepancy is currently being resolved to allow for a more accurate representation. Acknowledgments: This work was supported by the U.S. Department of Energy.

Imaging and Modeling of Passive Water Management in a Miniature Fuel Cell

A small microwatt fuel cell stack with 8 cells was designed, fabricated, and tested for passive operation with pure H2 and O2 to provide continuous power for multiple decades with uncontrolled fluctuating ambient conditions. The stack was designed to operate with dead ended gas flows with water removal via passive membrane diffusion to notches in the bipolar plates. Two stacks with active areas of 0.2 cm2, one with 200 µm of membrane and the other with 400 µm, were assembled and tested at varying temperatures and current conditions. Temperatures ranged –55 °C to 70 °C, with focus on the lower temperatures. Factors affecting water production and removal included temperature, gas crossover, and membrane thickness. Currents were applied from 16 µA to 10 mA after the cells were stabilized at temperature, followed by a period of drying to observe water transport to the external peripheries of the cell. A two-dimensional multiphysics model was developed to further explore the water management system in varying conditions.

Microwatt Fuel Cell for Long-Term and Wide Ambient Temperature Range Operation

A small microwatt fuel cell stack and system was designed, fabricated, and tested for passive operation with pure H2 and O2 to provide continuous power for multiple decades with uncontrolled fluctuating ambient conditions. The stack was designed to operate with dead ended gas flows with water removal via passive membrane diffusion to notches in the bipolar plates. Additional requirements for decades long operation is minimizing H2 and O2 consumption, specifically membrane cross-over; to minimize cross-over, stacks were built with layered membranes of 100 µm and 400 µm. The stack spatial water concentration level was monitored during operation by in situ neutron imaging to evaluate the passive water management design. Operation was verified by testing at temperatures ranging from -55 °C to 80 °C, including periodic pulses in power.

Air-breathing fuel cells fed with dimethoxymethane (DMM) vapor

The dimethoxymethane vapor-fed fuel cell (V-DMMFC) described in this paper is an innovative design for passive water management without the need for a complex micro-fluidic subsystem. DMM is a promising fuel for vapor-fed fuel cells because it has high energy density, an absence of C–C bonds, and lower toxicity as well as high vapor pressure. In a polarization measurement, the V-DMMFC showed an open circuit voltage of around 0.55 V and a high power density of 34.7 mW cm − 2 at 0.315 V, which is higher than that of a fuel cell with MeOH vapor. The V-DMMFC achieved a fast response in fuel feeding and a higher Faradic efficiency (76.1%) than that of fuel cells fed with MeOH vapor (53.7%); these improvements result from a higher vapor pressure and a lower fuel crossover of DMM. For constant voltage (CV) mode, the V-DMMFC provided evidence for the long-term stability of the device and for the sustainability of humidity within the self-humidified MEA without any additional provision of water.

Water Management Issues for Direct Borohydride/Peroxide Fuel Cells

This study evaluated water management strategies to lengthen the run time of a batch fueled direct sodium borohydride/peroxide (NaBH4/H2O2) proton exchange membrane fuel cell. The term “batch fueled” refers specifically to a fuel tank containing a fixed volume of fuels for use in the run. The length of a run using a fixed fuel tank is strongly influenced by water dynamics. The water that reacts at the anode is produced at the cathode, and is transported through the membrane via drag and diffusion. Resulting concentration changes in the fuel of the NaBH4/H2O2 fuel cell were modeled to evaluate the run lifetime. The run time is defined as the amount of time required for NaBH4 or for NaBO2 (the byproduct compound) to reach either solubility limit or until the fuel is depleted, whichever occurs first. As part of the evaluation, an “effective” H2O drag coefficient (net drag minus back diffusion) with Nafion® 112 was experimentally determined to be 1.14 and 4.36 at 25°C and 60°C, respectively. The concentrations of the NaBH4 and NaBO2 solutions were calculated as a function of initial concentration, and for the case where H2O was supplied to the anode compartment during operation. Several strategies to increase the run time by both passive and active water management were considered. It is found that the run time is increased from 10 W h to 57 W h, with a decrease in the initial NaBH4 concentration from 30 wt % (typically employed in these cells) to 10 wt %. Adding 0.125 ml/min H2O to the bulk anode solution increases the run time of a 10 wt % NaBH4 solution by a factor of 1.6. Adding 0.225 ml/min H2O to 30 wt % NaBH4 bulk solution increases the run time by a factor of 4.4. While attractive for increasing run time, the practicality of water addition depends on its availability or requires incorporation of an added unit, designed to separate and recirculate water from the cathode solution.

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.

In Situ Polymerized Wicks for Passive Water Management and Humidification of Dry Gases

We develop a passive water management system that employs in-situ polymerized wick structures that can be integrated with state-of-the-art flow field designs (e.g., stamped metal or injection molded). We develop a simple model to examine the feasibility of using a pre-humidifier to evaporate fuel cell product water into the cathode inlet dry-gas stream. We then demonstrate the effectiveness of this idea in a custom-fabricated integrated wick system. We also experimentally study the performance of our novel wick design for flood mitigation in a parallel flow field using fully humidified gases.

In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells

Air-delivery is typically the largest parasitic loss in PEM fuel cell systems. We develop a passive water management system that minimizes this loss by enabling stable, flood-free performance in parallel channel architectures, at very low air stoichiometries. Our system employs in situ-polymerized wicks which conform to and coat cathode flow field channel walls, thereby spatially defining regions for water and air transport. We first present the fabrication procedure, which incorporates a flow field plate geometry comparable to many state-of-the-art architectures (e.g., stamped metal or injection molded flow fields). We then experimentally compare water management flow field performance versus a control case with no wick integration. At the very low air stoichiometry of 1.15, our system delivers a peak power density of 0.68 W cm −2. This represents a 62% increase in peak power over the control case. The open channel and manifold geometries are identical for both cases, and we demonstrate near identical inlet-to-outlet cathode pressure drops at all fuel cell operating points. Our water management system therefore achieves significant performance enhancement without introducing additional parasitic losses.

In Situ Polymerized Wicks for Passive Water Management and Humidification of Dry Gases

not Available.

Passive water management for µfuel-cells using capillary microstructures

In this work we present a novel system for the passive water management in polymer electrolyte fuel cells (PEMFC) based on capillary effects in microstructures. The system removes abundant water that occurs at low temperatures at a fuel cell cathode and secures the humidity of the electrolyte membrane on higher temperatures. Liquid water is removed by hydrophilic gas supply channels with a tapered cross section as presented previously, and further transported by a system of capillary channels and a layer of nonwoven material. To prevent the membrane from running dry, a storage area in the nonwoven layer is introduced, controlled by a novel passive capillary overflow valve. The valve controls whether water is stored or finally disposed by gravity and evaporation. Experiments in a model system show that the nonwoven material is capable of removing all liquid water that can be produced by the fuel cell. A miniaturized fuel cell utilizing the novel water removal system was fabricated and experiments show that the system can stabilize the performance during changes of electrical load. Clearing the drowned miniaturized fuel cell flow field was proven and required 2 min. To make the capillary effects available for the originally hydrophobic graphite composite materials that were used to fabricate the flow fields, hydrophilic grafting based on photochemistry was applied to the material and contact angles of about 40° could be achieved and preserved for at least three months.

Waterbird Response to Wetlands Restored Through the Conservation Reserve Enhancement Program

Abstract: Conservation programs that facilitate restoration of natural areas on private land are one of the best strategies for recovery of valuable wetland acreage in critical ecoregions of the United States. Wetlands enrolled in the Conservation Reserve Enhancement Program (CREP) provide many ecological functions but may be particularly important as habitat for migrant and resident waterbirds; however, use of, and factors associated with use of, CREP wetlands as stopover and breeding sites have not been evaluated. We surveyed a random sample of CREP wetlands in the Illinois River watershed in 2004 and 2005 to quantify use of restored wetlands by spring migrating and breeding waterbirds. Waterbirds used 75% of wetlands during spring migration. Total use‐day abundance for the entire spring migration ranged from 0 to 49,633 per wetland and averaged 6,437 ± 1,887 (SE). Semipermanent wetlands supported the greatest total number of use‐days and the greatest number of use‐days relative to wetland area. Species richness ranged from 0 to 42 (x̄ = 10.0 ± 1.5 [SE]), and 5 of these species were classified as endangered in Illinois. Density of waterfowl breeding pairs ranged from 0.0 pairs/ha to 16.6 pairs/ha (x̄ = 1.9 ± 0.5 [SE] pairs/ha), and 16 species of wetland birds were identified as local breeders. Density of waterfowl broods ranged from 0.0 broods/ha to 3.6 broods/ha and averaged 0.5 ± 0.1 (SE) broods/ha. We also modeled spring stopover use, waterbird species richness, and waterfowl reproduction in relation to spatial, physical, and floristic characteristics of CREP wetlands. The best approximating models to explain variation in all 3 dependent variables included only the covariate accounting for level of hydrologic management (i.e., none, passive, or active). Active management was associated with 858% greater use‐days during spring than sites with only passive water management. Sites where hydrology was passively managed also averaged 402% greater species richness than sites where no hydrologic management was possible. Density of waterfowl broods was 120% greater on passively managed sites than on sites without water management but was 29% less on sites with active compared to passive hydrologic management. Densities of waterfowl broods also were greatest when ratios of open water to cover were 70:30. Models that accounted for vegetation quality and landscape variables ranked lower than models based solely on hydrologic management or vegetation cover in all candidate sets. Although placement and clustering of sites may be critical for maintaining populations of some wetland bird species, these factors appeared to be less important for attracting migrant waterbirds in our study area. In the context of restored CREP wetlands, we suggest the greatest gains in waterbird use and reproduction may be accomplished by emphasizing site‐specific restoration efforts related to hydrology and floristic structure. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):654–664; 2008)

Passive water removal in fuel cells by capillary droplet actuation

In this paper, a microstructured flow field for passive water management in proton exchange membrane fuel cells (PEMFC) is presented. It is based on the transport of liquid water droplets by capillary forces. A triangular microchannel forces droplets to detach from a fuel cell's gas diffusion layer (GDL) or membrane electrode assembly (MEA). Thus, it ensures proper oxygen supply of the reactive area. Water droplets are lifted into a secondary channel, and removed from the fuel cell by capillary action. Water formation in the flow field channels has been studied by analytic models and experiments. Three droplet configurations were identified, which show different properties in terms of capillary transport. Preferred shapes cover the GDL only slightly and can be found for contact angles typical for fuel cell materials. The actuation mechanism was studied by simulations for different designs, with varying contact angles and geometries. The new channel design was compared in a test fuel cell to standard rectangular channels, in the initial phase of the cell, at low working temperatures (22–30 °C). The new flow field design stabilized the cell at 95% of its initial performance compared to 60% when using the standard design.

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.

Vapor feed direct methanol fuel cells with passive thermal-fluids management system

The present paper describes a novel technology that can be used to manage methanol and water in miniature direct methanol fuel cells (DMFCs) without the need for a complex micro-fluidics subsystem. At the core of this new technology is a unique passive fuel delivery system that allows for fuel delivery at an adjustable rate from a reservoir to the anode. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The innovative thermal management mechanism is the key for effective operation of the fuel cell system. The vapor feed DMFC reached a power density of 16.5 mW cm −2 at current density of 60 mA cm −2. A series of fuel cell prototypes in the 0.5 W range have been successfully developed. The prototypes have demonstrated long-term stable operation, easy fuel delivery control and are scalable to larger power systems. A two-cell stack has successfully operated for 6 months with negligible degradation.