Performance Characteristics of a Free-Breathing Polymer Electrolyte Fuel Cell with Various Channel Shapes
Authors: Nakashima Kohei , Yoshida Masao , Kondo Yuki 
Published: December 2024

Free-breathing fuel cells of channel-type cathodes have straight vertical channels with open ends to supply oxygen to their cathode by natural convection. The channel structure should efficiently feed air to the cathode and ensure uniform contact between the electrode and the electrolyte. Several studies have reported the performance characteristics of free-breathing fuel cells with channel-type cathodes, including the effects of the width and the depth of the channels, the thickness of the gas diffusion layers, and the air blowing into the channels. However, we have seen no full reports investigating the performance characteristics of free-breathing fuel cells as a function of their channel shapes. This study utilized a small free-breathing fuel cell, with an active area of 4 cm2 (2 cm × 2 cm), and channel cathodes with various channel shapes, to experimentally investigate the effect of channel structure on cell performance, to numerically analyze the natural convection air flow in channels, and to compare with the results of performance characteristics.

Our free-breathing fuel cell used a solid polymer electrolyte membrane of Nafion 112, with a thickness of 50 μm. The electrodes with gas diffusion layers were carbon paper (TGP-H-120) with a thickness of 0.37 mm, loaded with 1.0 mg/cm2 platinum. A separator on the anode had a meander channel with a width of 2 mm and a depth of 1 mm. The various channel-type separators on the cathode were based on a separator with four straight channels with a width of 3 mm, a depth of 3 mm, and a rib width of 2 mm, then some of the ribs were eliminated or their shapes were modified to promote natural convection through the channels.

Experiments were carried out with the cell center plane perpendicular to the ground, and the channels of the cathode separator oriented vertically. The cell voltage and resistance (at a frequency of 10 kHz) were measured, by increasing current density in increments of 5 mA/cm2, until the cell voltage dropped to 0.3 V, at a non-humidified hydrogen supply rate of 7.0 cm3/min (normal). Experiments were conducted at ambient temperatures of 16 - 22 ºC and relative humidities of 35 - 40%.

The experimental results indicated that, compared with a conventional cathode separator with four straight channels, cell performance was improved by eliminating some of the channel ribs and increasing the opening ratio, up to a point. A separator with 2 mm square ribs rotated by 45° further improved cell performance. However, when the opening ratio became too large, cell performance leveled off.

CFD ULTIMATE (Autodesk Inc.) software was used to analyze the natural convection air flow. The analytical model consisted of the cathode separator, the electrode, the gasket, and their separating spaces, as well as spaces above and below the cathode separator (3 mm thick x 31 mm wide x 100 mm long). Natural convection of air was analyzed in these spaces. The temperature was set at 25 ºC on the electrode surface and 20 ºC on the other surfaces. The gauge pressure at the inlet of the space below the separator and at the outlet of the space above was set to 0 Pa. No air flow was allowed in the vertical direction on the sides of the spaces above and below the separator. The turbulence model was a low Reynolds number k-ε model and the analysis method was incompressible steady flow.

The analysis results showed that, compared with a conventional cathode separator with four straight channels, eliminating some of the ribs and increasing the opening ratio resulted in higher flow velocities in the channels. In addition, a separator with 2 mm square ribs rotated by 45° showed even higher velocities next to the ribs. These analysis results revealed that separators with modified channels improved cell performance by promoting natural convection.