High performance of catalysts supported by directly grown PTFE-free micro-porous CNT layer in a proton exchange membrane fuel cell

Abstract

A proton exchange membrane fuel cell (PEMFC) uses a solid polymer electrolyte, viz. Nafion®, sandwiched between the two electrodes. Nafion® not only plays the role as an electronic insulator and gas barrier but also allows rapid proton transport and supports high current densities. In order to maintain the high proton conductivity of Nafion®, humidified H₂ and O₂ are passed through the two electrodes. However, water gets easily condensed in the electrodes. This process, called water-flooding, degrades the performance of PEMFC. Hence, a hydrophobic agent, viz. polytetrafluoroethylene (PTFE), is normally incorporated into the electrodes to prevent this phenomenon. Since it is electrically insulating, the incorporation of PTFE increases the internal resistance of the fuel cell. In this study, we successfully demonstrate a PEMFC with catalyst layer comprising of low loading of platinum nanoparticles (0.05 mg cm⁻²) supported by a directly grown micro-porous carbon nanotube (CNT) layer without incorporation of PTFE, (Pt/MPL-CNT). This cell performs well without exhibiting water-flooding. A commercial electrode, the catalyst layer of which was supported by a conventional micro-porous layer of carbon black mixed with 30 w.t.% PTFE, was used as a reference (Pt/PTFE-MPL-CB). In the single cell tests, PEMFCs with 0.05 mg cm⁻²Pt/MPL-CNT and 0.25 mg cm⁻²Pt/PTFE-MPL-CB were used at the cathodes. These cells yielded maximum power densities of 902 mW cm⁻² and 824 mW cm⁻², respectively, at 70 °C when operated with H₂/O₂. Notably, the Pt-loading of Pt/MPL-CNT cell is one-fifth of that of Pt/PTFE-MPL-CB, but the former still outperforms the latter. It is shown that the directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which are the properties responsible for the high performance obtained here.