The effect of Nafion content in a graphitized carbon nanofiber-based anode for the direct methanol fuel cell

The performance and stability of a direct methanol fuel cell (DMFC) with membrane electrode assemblies (MEA) using different Nafion® contents (30, 50 and 70 wt% or MEA30, MEA50 and MEA70, respectively) and graphitized carbon nanofiber (GNF) supported PtRu catalyst at the anode was investigated by a constant current measurement of 9 days (230 h) in a DMFC and characterization with various techniques before and after this measurement. Of the pristine MEAs, MEA50 reached the highest power and current densities. During the 9-day measurement at a constant current, the performance of MEA30 decreased the most (−124 μV h−1), while the MEA50 was almost stable (−11 μV h−1) and performance of MEA70 improved (+115 μV h−1). After the measurement, the MEA50 remained the best MEA in terms of performance. The optimum anode Nafion content for commercial Vulcan carbon black supported PtRu catalysts is between 20 and 40 wt%, so the GNF-supported catalyst requires more Nafion to reach its peak power. This difference is explained by the tubular geometry of the catalyst support, which requires more Nafion to form a penetrating proton conductive network than the spherical Vulcan. Mass transfer limitations are mitigated by the porous 3D structure of the GNF catalyst layer and possible changes in the compact Nafion filled catalyst layers during constant current production.

► DMFC MEAs were fabricated with GNF-supported PtRu as the anode catalyst. ► The effect of anode Nafion content on performance was studied at 30, 50 and 70 wt%. ► Anode with 50 wt% of Nafion produced the highest power density. ► Anode with 70 wt% of Nafion was the most stable. ► GNF based DMFC anode requires more Nafion for optimal performance than Vulcan based.