Net Water Transport Coefficient Research Articles

Low Crossover of Methanol and Water Through Thin Membranes in Direct Methanol Fuel Cells

Low crossover of both methanol and water through a polymer membrane in a direct methanol fuel cell (DMFC) is essential for using high concentration methanol in portable power applications. A novel design of the membrane-electrolyte assembly (MEA) has been developed in this work to attain low methanol crossover, low water crossover, and high cell performance simultaneously. The anode catalyst layer, in the form of a catalyzed diffusion medium (CDM), serves as a methanol diffusion barrier to reduce methanol crossover. In addition, a highly hydrophobic cathode microporous layer (MPL) is employed to build up the hydraulic pressure at the cathode and hence drive the product water from the cathode into the anode to offset the water dragged by electro-osmosis. The new MEA, consisting of a CDM anode, a thin Nafion membrane, and a carbon cloth precoated with an MPL on the cathode, is shown to attain: (i) a net water transport coefficient through the membrane smaller than 0.8 at and 0.4 at ; (ii) fuel efficiency of ; and (iii) a steady-state power density of at ca. and with low stoichiometric flow rates of ambient dry air and methanol solution.

Simultaneous Measurements of Species and Current Distributions in a PEFC under Low-Humidity Operation

An experimental technique has been developed to conduct simultaneous measurements of species concentration and current density distributions using a segmented cell of . Twelve segmented subcells in conjunction with a multichannel potentiostat were employed to measure the current distribution, and two sets of multiposition microactuators and micro gas chromatographs were used to measure gas concentration distributions on the anode and cathode sides simultaneously. Distribution data are shown systematically for a polymer electrolyte fuel cell (PEFC) operated under low-humidity cathode conditions and at and . The concentration profile for the PEFC is generally linear and can be well predicted by a simple mass balance analysis, the anode water concentration profile is closely tied to the net water transport rate through the membrane, and the net water transport coefficient ranges between 0.05 and 0.30 even under very dry cathode conditions, suggesting the presence of strong back-diffusion of water from the cathode to the anode despite rather dry cathode gas.

Water Transport Through Nafion 112 Membrane in DMFCs

Water management has emerged as a significant challenge in portable direct methanol fuel cells, DMFCs. Excessive water crossover through the membrane causes water loss in the anode and flooding in the cathode. We report anovel DMFC design based on Nafion 112 and a cathode gas-diffusion layer coated with a microporous layer. This DMFC operated with ambient air yielded a stable power density of 56 mW/cm 2 at 60°C, while the net water transport coefficient was only 0.64 compared to a typical value of 3 for Nafion 117 membranes. In addition, the fuel efficiency was about 74-92%.

Water balance in a free-breathing polymer electrolyte membrane fuel cell

Water balance in a free-breathing polymer electrolyte membrane fuel cell was studied, focusing on the effect of anode conditions. The methods used were current distribution measurement, water collection from the anode outlet, and the measurement of cell polarization and resistance. Current density levels were 100 and 200 mA cm−2, temperature levels were 40 and 60 °C, and hydrogen stoichiometry range was from 1.5 to 2.5. The direction of hydrogen flow was varied. The fraction of product water exiting through the anode outlet varied from 0 to 58%, and it was found to increase with increasing temperature and hydrogen flow rate. When the general direction of hydrogen flow was against the direction of air flow, the percentage of water removal through the anode was smaller and the current distributions were more even than in the cases where the direction was the same as that of the air flow. This probably resulted from a more favorable distribution of water over the active area. The results also indicate that the net water transport coefficient varies across the active area. In further measurements, operation with the anode side in dead-end mode was investigated. It was also found that water distribution was more favorable when the general direction of hydrogen flow was against the air flow.