Choice of the Ionomer for Phosphoric Acid-Doped High Temperature Proton Exchange Membrane Fuel Cells (PEMFC)

Recent progress has been made on PEMFC operating from 100^oC to 200^oC under ambient pressure. This strategy is recognized as a promising solution to the current limitation of low temperature (below 100^oC) technique. The advantages of operation at such elevated temperature are: 1) improved kinetics of cathode and anode reaction; 2) improved tolerance of anode fuel contaminants; 3) no humidification of fuel needed and better life time in sub-zero temperature; 4) improved gas diffusion, solubility and permeability; 5) simplified cooling system and increase of power density [1-3].

However, many challenges for materials come with the advantages. For example, carbon support corrosion is accelerated, especially for phosphoric acid-doped membrane fuel cells. Catalyst sintering and dissolution also happen along with the carbon corrosion [3]. For >150^oC temperature operation, Nafion membrane suffer from dehydration and loss of proton conductivity. PBI-based membranes, together with other sulfonated polyaromatic electrolytes, are typically used because of the high mechanical strength at high temperature and retain of phosphoric acid in the polymer chain network for proton transport [3]. Long-time durability has also been proved form PBI-based membranes [3].

Although we may have the right membrane to work with, the role of ionomer played in the catalyst layer (CL) and its compatibility with the membrane cannot be neglected [3]. In Nafion-based membrane fuel cells, the ionomer serves as a proton conductor forming a triple-phase boundary [5], whereas in phosphoric acid-doped membrane fuel cells, the ionomer is a binder that helps maintaining the phosphoric acid in the CL [6]. In addition, the interfacial resistance between the CL and proton exchange membrane (PEM) is found to be dependent on the choice of ionomer [3]. For phosphoric acid-doped PBI-based PEMFC, different types of ionomer have been reported, such as Nafion, polyvinylidene difluoride (PVDF), PBI and a combination of PBI and PVDF [6,7]. Studies on the PBI content in the CL have shown that the ionomer is able to penetrate to the primary pores of carbon and the mean pore size increase slightly with the increase of ionomer content. However, an optimal content of around 3 wt% [6] and 10-20 wt% [7] is independently concluded from the two studies.

In this study, we attempt to investigate the effect of ionomer type and content on the pore structure, oxygen and air diffusivity and overall cell performance. We employed a one-step dry CL deposition method, reactive spray deposition technology (RSDT), to fabricate catalyst coated membrane (CCM), in which a thin-film CL was directly deposited onto the PEM through a flame-initiated mechanism. RSDT not only has more precise control over the electrode microstructure and component-level dispersion, but also eliminates several traditional manufacturing steps than ink-based deposition process [8]. An Air quench was incorporated into the RSDT process that allows for the introduction of carbon and ionomer at lower temperatures than the primary flame zone, which prevents thermal damage during deposition.

The membranes under study included a Celtec-P PBI-based membrane and a pyridine and polyaromatic ether membrane. Nafion, PVDF and PBI were selected as the binder. Catalyst layers with platinum loading of 0.3 mg/cm² and electrode thickness of ~15 µm were produced; see Figure 1. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used to investigate the catalyst particles and the ionomer distribution. Pore size distribution was obtained through nitrogen adsorption. Single-cell test was performed on a 25 cm²CCM at 160^oC at ambient pressure. Gas transport property during the fuel cell test was investigated by electrochemical impendence spectroscopy. 

Figure 1. a) CCM cross section structure prepared by RSDT with PVDF as binder; b) Pore size distributions for CCM with varying Nafion/Carbon weight ratio. 

References:

[1] Chandan, A. et al. J. Power Sources, 2013, 231, 264-278.

[2] Zhang, J. et al. J. Power Sources, 2006, 160, 872-891.

[3] Shao, Y. et al. J. Power Sources, 2007, 160, 872-891.

[4] Li, Q. et al. Prog. Polym. Sci., 2009, 34, 449-477.

[5] Song, Y. et al. J. Power Sources, 2006, 154, 138-144

[6] Lobato, J. et al. Int. J. Hydrogen Energy, 2010, 35, 1347-1355.

[7] Kim, J. et al. J. Power Sources, 2007, 170, 275-280.

[8] Roller, J. et al J. Electrochem.Soc., 2013, 160, F716-F730 (2013).