Measurement of the current distribution in a direct methanol fuel cell - Confirmation of parallel galvanic and electrolytic operation within one cell

Current production in fuel cells is typically unequally distributed along the cell surface due to inhomogeneous concentration of reactants and temperature. The inhomogeneities in fuel cells can result in reduced output power and accelerated ageing. To quantify the inhomogeneities a measurement system has been developed which allows measuring the local distribution of current and temperature in hydrogen and direct methanol fuel cells. With this system we are able to directly observe the coexistence of galvanic and electrolytic domains in a single channel direct methanol fuel cell (the electrolytic domain is the domain where electrolysis occurs in contrast to the galvanic domain where the fuel cell process takes place). The measurement device also allows for the measurement locally resolved impedance spectra. (c) 2007 Elsevier B.V. All rights reserved

The influence of GDL wettability on DMFC performance: A combined local current distribution and high resolution neutron radiography study

The influence of the anode and cathode GDL wettability on the current and media distribution was studied using combined in situ high resolution neutron radiography and locally resolved current distribution measurements. MEAs were prepared by vertically splitting either the anode or cathode carbon cloth into a less hydrophobic part (untreated carbon cloth 'as received') and a more hydrophobic part (carbon cloth impregnated by PTFE dispersion). Both parts were placed side by side to obtain a complete electrode and hot-pressed with a Nation membrane. MEAs with partitioned anode carbon cloth revealed no difference between the untreated and the hydrophobised part of the cell concerning the fluid and current distribution. The power generation of both parts was almost equal and the cell performance was similar to that of an undivided MEA (110 mW cm(-2), 300 mA cm(-2), 70 degrees C). In contrast, MEAs with partitioned cathode carbon cloth showed a better performance for the hydrophobised part, which contributed to about 60% of the overall power generation. This is explained by facilitated oxygen transport especially in the hydrophobised part of the cathode gas diffusion layer. At an average current density of 300 mA cm-2, a pronounced flooding of the cathode flow field channels adjacent to the untreated part of GDL led to a further loss of performance in this part of the cell. The low power density of the untreated part caused a significant loss of cell performance, which amounted to less than 40 mW cm(-2) (at 300 mA cm(-2)). (C) 2010 Elsevier B.V. All rights reserved

Neutron radiography and current distribution measurements for studying the influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells

The influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells DMFCs was studied. All measurements were performed with DMFC stack cells A 314.75 cm2 . The local and temporal water distributions in the flow field channels during DMFC operation were visualized by means of through plane neutron radiography. Current and temperature distributions were measured simultaneously by the segmented cell technology. Additionally, the time dependent current distribution, cell performance and pressure drop were measured. Cathode flow field designs with channel and grid structures were compared. The cathode flow field channels were impregnated by either hydrophobizing or hydrophilizing agents or used as received. It turned out that hydrophobized and partially also untreated flow fields cause large water droplets in the cathode channels. The water droplets cause a blocking of the air flow and consequently a lower and more unstable fluctuating performance, less steady current and temperature distributions, and higher pressure drops between cathode inlet and outlet. Because of their two dimensional design, grid flow fields are less prone to water accumulations. The best results are achieved with a hydrophilized grid flow field that has a channel depth and width of 1.5mm each C GR1

In-plane Neutron Radiography for Studying the Influence of Surface Treatment and Design of Cathode Flow Fields in Direct Methanol Fuel Cells

The influence of surface treatment and design of cathode flow fields in direct methanol fuel cells was investigated by in plane neutron radiography and measurements of cell performance and pressure drop along the cathode channels. A specially designed test cell and neutron radiography set up allows for studying the water distribution in an in plane viewing direction. A temporal resolution of down to 10 s was used while an image resolution of approximately 80 mm could be obtained. The cathode flow fields were either impregnated by a hydrophobizing or hydrophilizing agent or left untreated. It turned out that hydrophobic channel walls lead to the formation of large water droplets, which partially block the air flow in the cathode channels. Their periodical growth and discontinuous removal leads to an unstable and fluctuating operation. Hydrophilized cathode flow fields, on the other hand, ensure a stable operation due to removal of excess water by a continuous water film. Two different cell designs including untreated cathode flow fields with either dual channel or grid design were compared. The grid flow field was superior with regard to the stability of cell performance and less prone to the formation and removal of water droplet

Neutron radiography and current distribution measurements for studying cathode flow field properties of direct methanol fuel cells

The influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells (DMFCs) was studied. All measurements were performed with DMFC stack cells (A = 314.75 cm2). The local and temporal water distributions in the flow field channels during DMFC operation were visualized by means of through-plane neutron radiography. Current and temperature distributions were measured simultaneously by the segmented cell technology. Additionally, the time-dependent current distribution, cell performance and pressure drop were measured. Cathode flow field designs with channel and grid structures were compared. The cathode flow field channels were impregnated by either hydrophobizing or hydrophilizing agents or used as received. It turned out that hydrophobized and partially also untreated flow fields cause large water droplets in the cathode channels. The water droplets cause a blocking of the air flow and consequently a lower and more unstable (fluctuating) performance, less steady current and temperature distributions, and higher pressure drops between cathode inlet and outlet. Because of their two-dimensional design, grid flow fields are less prone to water accumulations. The best results are achieved with a hydrophilized grid flow field that has a channel depth and width of 1.5 mm each (‘C-GR15’

Water Evolution in Direct Methanol Fuel Cell Cathodes studied by Synchrotron X-Ray Radiography

Water evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X ray radiography. Radiographs with a spatial resolution of around 5 lm were taken every 5 s. Special cell designs allowing for through plane and in plane viewing were developed, featuring two mirror symmetrical flow field structures consisting of one channel with the through plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar