(457c) Experimental Study of CO and Temperature Impact On High Temperature Proton Exchange Membrane Fuel Cell (HT-PEM FC) Performance and Current Distribution

Experimental Study of CO and temperature impact on High Temperature Proton Exchange Membrane Fuel Cell (HT-PEM FC) performance and current distribution

This experimental study investigates the impact of the fuel contaminant carbon monoxide (CO) and different operating temperature levels on the performance, durability and current distribution of a segmented 10 cm² High Temperature Proton Exchange Membrane Fuel Cell (HTPEMFC).

Segmented current measurements, i-V and i-P characterization, electrochemical impedance spectroscopy (EIS) and accompanied on-line gas analysis are the utilized methods of characterization.

The current distribution experiments are realized on a prototype HT-PEM fuel cell with an active area of 10 cm² and an 8-fold segmented, two-channel meander cathode flow-field-design with an 8-fold segmented current collector design.

The current collector rods are contacted with the channel segments by the application of pressure via the end plates.

Current distribution measurement data is obtained through the segmented current collector measurement module. The different voltage drops along the precisely defined resistors are in correlation with the prevailing current flow via Ohm's Law and thus can be attributed to the eight different segments.

The 10 cm² HT-PEMFC is operated with PBI membranes. Celtec^®-P 1000 MEAs are produced at BASF Fuel Cell GmbH using Celtec^®-P membranes and electrodes produced at BASF Fuel Cell Inc. The cathode contains a Vulcan XC 72 supported Platinum-alloy with 0.75 mg_Pt cm^-2. The anode contains a Vulcan XC 72 supported Platinum catalyst with 1 mg_Pt·cm^-2. The thickness of the membrane in the MEA is approximately 50?75 μm.

These MEAs are operating best at temperatures between 120°C and 180 °C. Therefore they are especially suitable in reformed-hydrogen-based PEM fuel cell applications. Due to these high operating temperatures, CO tolerances up to 3-5% can be achieved.

After a sufficiently long period of equilibration time on a new operational point, the characterization experiments and measurements are conducted galvanostatically.

The current distribution along the segmented cathode flow-field remains unchanged during the variation of temperature. With raising temperature, an increase in CO₂ generation is observed both at anode and cathode for a given operational point. A decrease in CO₂ generation with increasing cell voltage is observed on the anode side, whereas the opposite effect of increased CO₂ generation with higher voltages is noticed at the cathode side. According to the results of the EIS measurements, increasing the operating temperature enhances mass transport, decreases the resistance to ionic conduction in the membrane and increases the reaction rates.

Given the fact for strongly linear voltage-temperature behaviour of the cell ranging from 160°C - 145°C, no apparently significant cur-rent shift is visible in any of the eight segments. Starting at 145°C the cell voltage is dropping nonlinearly with the following implications: The segments at the hydrogen inlet are the locations for a negative current shift of up to -20% relative to the initial amperage. The segments at the hydrogen outlet are the locations of an observed positive current shift of 15% - 20%.

As a competitive adsorption mechanism of CO and H₂ on the Pt surface is going to occur, the operation at higher temperatures will increase the ability of the fuel cell anode to perform in the presence of CO by decreasing the coverage of CO on the catalyst surface, thereby increasing hydrogen coverage. Unlike in low temperature PEM fuel cells, the CO poisoning is immediately effecting the HT-PEM and thus, the fuel cell performance can be immediately restored after poisoning.

The CO contents of 0.5 - 15% result in slightly linear performance losses in the current-density range between 0.1 and 0.2 A?cm^-2. At middle current densities of 0.4 - 0.6 A?cm^-2 the performance loss due to increasing CO content is more evident.  At high CO contents up to 15%, a Z-form of the polarization curves is observed at the high current density regions.

The results of the EIS measurements indicate a strong dependency of R_Ω on the CO concentration. In addition, the kinetic resistance R_fA+R_fC is clearly increasing with higher CO exposition levels.

In the current distribution measurements, the expected voltage drop with increasing CO concentration in the anode gas feed is observed and the measured overall current is slightly dropping as well, due to increasing membrane and kinetic resistances. As hydrogen is consumed alongside the flowfield-segment setup, a smaller fraction of hydrogen is supplied to the last segments. As a consequence the hydrogen/carbon monoxide equilibrium is shifted towards higher CO-coverage onto the Pt-catalyst sites. These segments are therefore impinged with higher resistances resulting in a current shift towards the segments with lower resistances. This CO concentration ? current shift behaviour is found out to be of linear nature.