Experimental

The working electrodes were prepared by a combined filtration/brushing procedure using platinum-on-carbon (Pt/C) catalysts (E-Tek), carbon powder (Vulcan XC-72, Cabot), a carbon cloth substrate (PWB-3, Stackpole), a Polytetrafluoroethylene (PIFE) suspension (Teflon T-3Ó, Du Pont) and a Nation® solution (Aldrich, 5 wt.% in 10% water/low aliphatic alcohols). Except where mentioned, the electrodes employed in the modules were made with 20 wt.% Pt/C, 1.1 mg Nation® cm"2, 0.4 mg Pt cm"2 in the catalyst layer and with 30 % PIFE and 50pm thickness in the diffusion layer. All the details regarding the procedures for the membrane & electrode assembly preparation were presented previously (6).

The main characteristics of the fuel cell stacks are summarized in the scheme presented in Figure 1. The bipolar and end plates were fabricated using a non porous furfural impregnated graphite material in which a series/parallel flow field was machined out. The gaskets were cut out from a silicon rubber fiberglass cloth. The cells with an active electrode area (geometric) of 20 cm2 were clamped between aluminum plates. The distribution of gases to the cells was done using an external parallel manifolding system made with stainless steel and containing individual needle valves for the control of the gas flow rates for each cell.

The cooling system was tested employing water cooling plates distributed every three cells throughout the stack. The temperature control is conducted using thermocouples inserted in predetermined places in the 6 and 21 cells stacks. The principle of operation is based in a constant coolant flow on/off concept, set to establish a maximum range of ± 3 °C in the highest point of the fuel cell operating temperature.

Humidification of the reactants was carried out by bubbling the gases through water contained in stainless steel bottles thermostated to the desired temperature. Measurements of the cell potential unipolar plates unipolar plates

membrane &

electrodes

Figure 1 - Scheme of the configuration of the multi-cell stacks.

membrane &

electrodes

Figure 1 - Scheme of the configuration of the multi-cell stacks.

as a function of the current density were made galvanostatically using an electronic load (HP-6050A, Hewlett Packard) and an IBM-PC compatible microcomputer equipped with a digital I/O and multiplexer plate (DAS-1600, Keithley). Testing of the stacks was conducted in a specially designed test station (14) employing non-pressurized H2/O2 reactants.

0 0

Post a comment