1086
Improved Cathode Catalyst Layers for Proton Exchange Membrane Fuel Cells
Extensive research activities have been carried out on fuel cells worldwide during recent decades, with particular interest and focus on proton-exchange membrane fuel cell (PEMFC) systems.
PEMFC is highly relevant for environmental friendly (clean, zero CO2-emission) transport1. This fuel cell type is the most promising candidate for transportation applications due to its relative low operating temperature and fast start-up2, 3. However, the main drawback with PEM fuel cell is a scarcity of platinum (catalyst) and issues related to the durability of the components. Kjelstrup et al.4 introduced a systematic membrane electrode assembly (MEA) design procedure by combining it with an entropy production minimization procedure to optimize the energy efficiency and the catalyst utilization in PEMFC. It was proposed that the concentration polarization that dominates the PEMFC performance at high current density operation can be minimized by cleverly designing the gas and water distribution in the catalyst layer.
In this work, two different techniques (template-assisted indentation and pore formers) were employed to fabricate the catalyst layers with uniform pore sizes and homogeneous distribution.
Experimental
In the former method, the templates with cylindrical pillars depending on the requirement of pore sizes were prepared through photolithography on Si substrates. The Si substrate that holds the template was then hot pressed (at 130 °C, 80 kg cm-2 for 3 minutes) against the cathode side of the catalyst–coated membranes (CCM) (Gore PRIMEA) to obtain uniform channels in the layer.
In the second method, monodispersed (synthesized in-house) polystyrene particles with the particle size of 1 µm was used as a pore forming agent. Cathodes were spray coated on Nafion 212 with the catalyst ink containing 70 wt% polystyrene particles, 60 wt% Pt/C (0.12 mg cm-2 Pt) and 17 wt% Nafion. The polystyrenes from the dry catalyst layer were then selectively dissolved in ethyl acetate to form uniform pores.
Fuel cell testings were carried out with the catalyst layers formed through these two methods. MEAs with a geometrical area of 5 cm2 were fabricated by sandwiching the CCM between two gas diffusion layers (GDL). The GDLs of type H2315 I2 C6 used in this work were from Freudenberg FCCT.
The fuel cells were operated at 60 oC with pure hudrogen and synthetic air. All gases were pre-humidified to 100% RH on the cathode side and 80% RH on the anode side. The flow rates of 1.5/2.0 stoic on the fuel side and 2 stoic on the oxidant side were used. The MEAs were conditioned by pulsing between 0.6 and 0.25 A cm-2 for 24 hours before recording the polarization curve.
Results
Fig. 1 shows the morphology of Gore MEA after indentation with 4 µm template. It is observed that the formation of the channels is uniform through out the catalyst layer with some debris from the broken template. The polarization curve for the indented MEA is shown in Fig. 2. As expected a noticeable, although minor, difference in the fuel cell performance at high current densities is obtained.
Similar differences in the polarization curves are also seen in Fig. 3 where a comparison between a normal and porous MEA is made. Furthermore, the differences in the fuel cell performance are more pronounced here. This could be due to differences in the size of the pores generated in these two catalyst layers (4 µm vs 1 µm); smaller the pore size better the performance. But, these are only preliminary conclusions as the CCMs were made through different methods. Nevertheless, it is identified that the presence of macro pores in the catalyst layers have a positive effect on the fuel cell performance. Further studies such as porosity, pore size distributions, influence of catalyst layer thickness and fuel cell testings are being carried out for better understanding.
This work is supported by The Research Council of Norway.
References
1. F. Barbie, T. Góme, J. Hyd. Energy, 22 (1997) 1027-1037
2. S. Gottensfeld, T. Zawodzinski, Adv. Electrochem. Sci. Eng., 5 (1997) 195-301
3. D. Pant, A. Singh, G.V. Bogaert, Y.A. Gallego, L. Diels, K. Vanbroekhoven, Renew. Sust. Energy Rev., 15 (2011) 1305-1313
4. S. Kjelstrup, M.-O. Coppens, J. G. Pharoah, P. Pfeifer, Energy&Fuels, 24 (2010) 5097-5108