Water transport in polymer membranes for PEMFC

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Abstract

To determine the net electro-osmotic drag coefficient of Nafion 115 and Hanwha membrane, we measured the fluxes of water discharged from anode and cathode at different current densities. Also, we investigated the contribution of water supply for membrane from anode and cathode. When the cathode was humidified, water supply for membrane at low current densities was achieved via the cathode, but the contribution of the anode became more important as current density gradually increased. The net electro-osmotic drag coefficient decreased sharply with current density, but it had a nearly constant value over 200 mA cm−2. When the cathode was not humidified, at low current densities, most of water generated at cathode was supplied for membrane, but water supply from cathode at high current densities decreased proportionately, and the net electro-osmotic drag coefficient showed larger value.

Introduction

Polymer electrolyte membrane fuel cell (PEMFC) has many advantages compared with other types of fuel cell as power generators for transport [1]. Using a solid polymer electrolyte such as Nafion® solves a corrosion problem of cells and makes it possible to operate at low temperature. In the early years, the PEMFC was developed for space and military, but it has been developed for electric vehicles as a hybrid power system with battery since the late 1980's [2].

In PEMFC, polymer membrane requires supplying with water in order to retain its proton conductivity, which depends strongly on the hydration of the polymer, and water management, which is essential for the enhancement of cell performance 3, 4, 5, 6, 7. Up to now, water has been supplied for the membrane indirectly by humidifying reactants, and then water content of membrane changes dynamically with the operating conditions.

Two main reasons cause water molecules to move within the membrane. One is an electro-osmotic drag by potential gradient and the other is a back diffusion by concentration gradient of water 3, 8. When protons move into the cathode by potential difference, they carry several water molecules in the form of the hydrated proton. At the same time, water generation by electrochemical reaction provides additional water at cathode. So a concentration profile of water is formed between anode and cathode [9].

Electro-osmotic drag coefficient is defined as the number of water molecules transported from anode to cathode per proton. Earlier methods to determine it measured the flux of water across the membrane at constant current [3]. However, the changes in the pretreatment of the membrane permitted a variation in the water content of membranes that were equilibrated with liquid water, and thus it was not easy to measure an electro-osmotic drag coefficient experimentally. This method still does not allow an exact examination of electro-osmotic drag coefficient at low water content.

We have measured the fluxes of water discharged from anode and cathode at different current densities and determined the net electro-osmotic drag coefficient with current density. Also, we investigated the contribution of water supply for membrane from anode and cathode.

Section snippets

Membrane and electrode assembly (MEA) fabrication

Nafion® 115 (127 μm) and Hanwha membrane (70 μm) were used as an electrolyte, and E-Tek electrode (0.4 mg Pt cm−2) was used for fabricating a MEA. The electrode was impregnated with ionomer (5 wt.% Nafion® solution, Aldrich) after cutting the area of 50 cm2, and then it was dried in the oven at 80°C for 2 h. The electrolyte membrane was pretreated by boiling in 3 wt.% H2O2 and H2SO4, respectively, to remove impurities and to transform into the H+ form. The MEA was fabricated by hot-pressing

Results and discussion

All experiments were performed at 70°C (1 atm) and an electrolyte membrane was humidified by reactant gases passing through a humidifier. In PEMFC, continuous water supply for membrane is required to retain the membrane's conductivity. Excessive water supply, however, can cause a drop of performance due to flooding in the electrode, especially at cathode. In addition, water generation at cathode is intended to provide additional water for the membrane. Therefore, water management of the cathode

Conclusions

Water management is essential for the performance enhancement of PEMFC, and an analysis of water transport in the membrane and water supply to the membrane are necessary to evaluate the effect of humidification on cell performance.

In this study, water transport within the membrane and the water supply for the membrane (Nafion® 115 and Hanwha membrane) were investigated by measuring the fluxes of water discharged from anode and cathode at different current densities. When the cathode was

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