Nafion influence on oxygen reduction reaction catalyzed by ptfree materials.

Fuel cells for low temperature applications are the subject of intensive research because they may offer a reduction of primary fossil fuel consumption and greenhouse gas emission. However, one most problematic aspect arises from the kinetic slowness of the oxygen reaction. For this reason, electrode materials are, at present, based on platinum nano-dispersed onto carbon. Nevertheless, the natural scarcity and cost of platinum justify the search for alternate, non-precious materials, even though less catalytically performing. Their utilization may introduce unexpected or unpredictable electrocatalytic behaviour, because of the unknown interactions between these new materials and Nafion. In fact, in fuel cells Nafion is used as both the electrode separator and as an ion-conducting binder of the catalyst layer affecting platinum activity and fuel cell performance [1]. Catalytic sites of platinum and platinum-free catalysts obviously have different nature, thus implying possibly different interaction mechanisms with the Nafion binder, to affect catalytic activity. In this work, we present some results about the oxygen reaction reduction (ORR) on a Pt/C commercial catalyst and on lab-synthesized platinum-free catalysts in acid, that were obtained by varying the electrode preparation method and the added Nafion quantity. In a first method a Nafion suspension was deposited onto a catalytic layer previously dried onto the Rotating Disk Electrode (RDE) graphitic tip, in a second one Nafion was directly added to catalyst suspensions, before the RDE tip deposition. Voltammetric results show that the catalytic layer thickness and the relative Nafion quantity affect the experimental I/E curve morphology and the presence of a well defined limiting current regime. Optimized experimental conditions (relative quantity of Nafion to carbon quantity) to obtain good results were determined differing for Pt/C and Pt-free catalysts. [1] M. Watanabe, H. Igarashi, and K. Yosioka Electrochimica Acta, 40, 1995, 329.