Matthew M. Mench

In situ water distribution measurements in a polymer electrolyte fuel cell

The overall water vapor balance and concentration distribution in the flow channels is a critical phenomenon affecting polymer electrolyte fuel cell (PEFC) performance. This paper presents, for the first time, results of a technique to measure in situ water vapor, nitrogen and oxygen distribution within the gas channels of an operating PEFC. The use of a gas chromatograph (GC) to measure high levels of water saturation directly, without dehumidification of the flow stream, is a unique aspect of this work. Following careful calibration and instrumentation, a gas chromatograph (GC) was interfaced directly to the fuel cell at various locations along the serpentine anode and cathode flow paths of a specially designed fuel cell. The 50 cm 2 active area fuel cell also permits simultaneous current distribution measurements via the segmented collector plate approach. The on-line GC method allows discrete measurements of the water vapor content up to a fully saturated condition about every 2 minutes. Water vapor and other species distribution data are shown for several inlet relative humidities on the anode and cathode for different cell voltages. For the thin electrolyte membranes used (51 m), there is little functional dependence of the anode gas channel water distribution on current output. For thin membranes, this indicates that there is little gradient in the water activity between anode and cathode, indicating diffusion can offset electro-osmotic drag under these circumstances ( i< 0.5 A/cm 2 ). This technique can be used for detailed studies on water distribution and transport in the PEFC.

In Situ Current Distribution Measurements in Polymer Electrolyte Fuel Cells

bCD-adapco Japan, Limited, Yokohama, Japan There has been much recent interest and development of methods to accurately measure the current distribution in an operating polymer electrolyte fuel cell ~PEFC!. This paper presents results from a novel technique that uses a segmented flow field with standard, nonaltered membrane electrode assemblies and gas diffusion layers. Multiple current measurements are taken simultaneously with a multichannel potentiostat, providing high-resolution temporal and spatial distribution data. Current distribution data are shown that display the distributed effects of cathode stoichiometry variation and transient flooding on local current density. It is shown that the time scale for liquid accumulation in gas diffusion layer pores is much greater than that of any electrochemical or gas-phase species transport process. In order to facilitate state-of-the-art PEFC model validation, an idealized single-pass serpentine flow field was used, and the exact geometry is presented.

Impact of Initial Biofilm Growth on the Anode Impedance of Microbial Fuel Cells

Electrochemical impedance spectroscopy (EIS) was used to study the behavior of a microbial fuel cell (MFC) during initial biofilm growth in an acetate‐fed, two‐chamber MFC system with ferricyanide in the cathode. EIS experiments were performed both on the full cell (between cathode and anode) as well as on individual electrodes. The Nyquist plots of the EIS data were fitted with an equivalent electrical circuit to estimate the contributions of various intrinsic resistances to the overall internal MFC impedance. During initial development of the anode biofilm, the anode polarization resistance was found to decrease by over 70% at open circuit and by over 45% at 27 µA/cm2, and a simultaneous increase in power density by about 120% was observed. The exchange current density for the bio‐electrochemical reaction on the anode was estimated to be in the range of 40–60 nA/cm2 for an immature biofilm after 5 days of closed circuit operation, which increased to around 182 nA/cm2 after more than 3 weeks of operation and stable performance in an identical parallel system. The polarization resistance of the anode was 30–40 times higher than that of the ferricyanide cathode for the conditions tested, even with an established biofilm. For a two‐chamber MFC system with a Nafion® 117 membrane and an inter‐electrode spacing of 15 cm, the membrane and electrolyte solution dominate the ohmic resistance and contribute to over 95% of the MFC internal impedance. Detailed EIS analyses provide new insights into the dominant kinetic resistance of the anode bio‐electrochemical reaction and its influence on the overall power output of the MFC system, even in the high internal resistance system used in this study. These results suggest that new strategies to address this kinetic constraint of the anode bio‐electrochemical reactions are needed to complement the reduction of ohmic resistance in modern designs. Biotechnol. Biotechnol. Bioeng. 2008;101: 101–108.

An In Situ Method for Determination of Current Distribution in PEM Fuel Cells Applied to a Direct Methanol Fuel Cell

This paper describes and demonstrates a new method for determination of current density distribution in an operating polymer electrolyte membrane ~PEM! fuel cell. The technique is a modification of the current mapping technique that relies on an array of shunt resistors embedded within a current collecting plate. Standard, nonaltered membrane electrode assemblies are utilized with gas diffusion layers in direct contact with an electrically segmented current collector/flow field. Multiple current measurements are taken simultaneously, allowing transient distribution detection with a multichannel potentiostat. Both steady state and transient data are presented for an operating liquid fed direct methanol fuel cell. Cathode flooding is predicted, and shown to occur at relatively high cathode flow rates. This technique can contribute to knowledge and understanding of key phenomena including water management and species distribution in PEM fuel cells.

Investigation of Temperature-Driven Water Transport in Polymer Electrolyte Fuel Cell: Phase-Change-Induced Flow

The objective of this work is to investigate phase-change-induced water transport of polymer electrolyte fuel cell materials subjected to a temperature gradient. Contrary to thermo-osmotic flow in fuel cell membranes, a net flux of water was found to flow from the hot to the cold side of the full membrane electrode assembly. The key to this is the existence of some gas phase in the catalyst layer or other porous media. This mode of transport is a result of phase-change-induced flow. The measured water transport through the membrane electrode assembly is the net effect of mass diffusion as well as thermo-osmosis in the membrane, which moves counter to the direction of the phase-change-induced flow. Arrhenius functions that are dependent on material set, temperature gradient, and average temperature across the materials were developed that describe the net flux. In addition to direct quantification, phase-change-induced flow was visualized and confirmed using high-resolution neutron radiography.

Characterization of microbial fuel cells at microbially and electrochemically meaningful time scales.

The variable biocatalyst density in a microbial fuel cell (MFC) anode biofilm is a unique feature of MFCs relative to other electrochemical systems, yet performance characterizations of MFCs typically involve analyses at electrochemically relevant time scales that are insufficient to account for these variable biocatalyst effects. This study investigated the electrochemical performance and the development of anode biofilm architecture under different external loadings, with duplicate acetate-fed single-chamber MFCs stabilized at each resistance for microbially relevant time scales. Power density curves from these steady-state reactors generally showed comparable profiles despite the fact that anode biofilm architectures and communities varied considerably, showing that steady-state biofilm differences had little influence on electrochemical performance until the steady-state external loading was much larger than the reactor internal resistance. Filamentous bacteria were dominant on the anodes under high external resistances (1000 and 5000 Ω), while more diverse rod-shaped cells formed dense biofilms under lower resistances (10, 50, and 265 Ω). Anode charge transfer resistance decreased with decreasing fixed external resistances, but was consistently 2 orders of magnitude higher than the resistance at the cathode. Cell counting showed an inverse exponential correlation between cell numbers and external resistances. This direct link of MFC anode biofilm evolution with external resistance and electricity production offers several operational strategies for system optimization.

Liquid Water Storage, Distribution, and Removal from Diffusion Media in PEFCS

Liquid water storage in the diffusion media (DM) of polymer electrolyte fuel cells (PEFCs) is a function of design geometry, surface geometry, and operating conditions, and the DM, water storage, and can affect transient response, degradation via ionic contaminants, pressure loss, and freeze-thaw behavior. Neutron imaging was used to quantify the liquid water distribution in a PEFC under a variety of flow rates, humidities, and currents with paper or cloth DM. For a wide range of conditions, the paper DM held roughly 60% of the total water stored under the landings and the remaining 40% in, or under, the channels. The cloth DM had a nearly 50:50 channel to land liquid water distribution. From 0.2 to 1.0 A/cm 2 current conditions, the paper DM held 174% more water per volume of DM under the landings than cloth, resulting in a very high liquid saturation and eventual flooding. Increasing flow rate decreased the total liquid water content, mostly from removal of droplets. The residual liquid water under the lands was removed with increased flow rate more readily using the cloth DM, thus it was a more effective material for low power purge. Transient testing showed the time scale of significant liquid water accumulation is on the order of minutes.

Validated Leverett Approach for Multiphase Flow in PEFC Diffusion Media III. Temperature Effect and Unified Approach

The final paper in this series is devoted to delineating the effects of temperature on the multiphase transport characteristics of thin-film polymer electrolyte fuel cell (PEFC) diffusion media (DM). Direct measurements of capillary pressure-saturation of various commercial DM coated with a wide range of poly(tetrafluoroethylene) (PTFE) loadings (from 5 to 20 wt % PTFE) were performed at different operating temperatures (20, 50, and 80°C). The benchmark data gathered from these experiments (available upon request) were compiled into the existing database generated from the first and second phase of this study, which examined the hydrophobicity and compression effects. The expanded database was then utilized to deduce a unified form of an empirical correlation appropriate for the tested DM. This semiempirical approach can predict capillary pressure of the tested DM as a function of liquid saturation, hydrophobic additive content, uncompressed porosity, compression pressure, and operating temperature within an uncertainty of ±14% of the measured capillary pressure over the entire saturation domain, showing considerable improvement over the traditional Leverett approach.

Characterization of Interfacial Structure in PEFCs: Water Storage and Contact Resistance Model

In this work, an analytical model of the microporous layer MPL and the catalyst layer CL interface under compression is developed to investigate the effects of the MPL|CL interfacial morphology on the ohmic and mass transport losses at the MPL|CL interface in a polymer electrolyte fuel cell PEFC. The model utilizes experimentally measured surface profile data as input. Results indicate that the uncompressed surface morphology of mating materials, the elasticity of PEFC components, and the local compression pressure are the key parameters that influence the characteristics of the MPL and CL contact. The model predicts that a 50% drop in the MPL and CL surface roughness may result in nearly a 40% drop in the MPL|CL contact resistance. The model also shows that the void space along the MPL|CL interface can potentially store a significant amount of liquid water 0.9‐3.1 mg/cm 2 , which could result in performance loss and reduced durability. A 50% drop in the MPL and CL surface roughness is expected to yield nearly a 50% drop in the water storage capacity of the MPL|CL interface. The results of this work provide key insights that will enhance our understanding regarding the complex relation between MPL|CL interfacial structure and cell performance.

Direct Dimethyl Ether Polymer Electrolyte Fuel Cells for Portable Applications

Dimethyl ether ~DME! is a potential fuel for direct oxidation fuel cells that combines the main advantages of hydrogen ~pumpless fuel delivery! and methanol ~high energy density storage!. DME also has low toxicity compared to methanol, making it a potential fuel for portable applications. This paper describes performance aspects and limitations of the DME fuel cell. At the anode, there is a critical balance between water and DME availability for reaction that suggests a thin electrolyte to promote back diffusion of water to the anode is desirable for high performance. However, excessive DME or DME intermediate crossover reaction losses with a Pt/Ru anode and Pt cathode catalyst preclude use of the thinnest electrolytes available.