The effect of vapor-grown carbon fiber as an additive to the catalyst layer on the performance of a direct methanol fuel cell

Abstract

Vapor-grown carbon fibers (VGCFs) were added to the anode catalyst layer of a direct methanol fuel cell to improve the cell performance through structural modification of the catalyst layer. The amount of VGCF varied up to 6 wt.% with respect to the weight of the PtRu black catalyst that was used. A catalyst layer with 2 wt.% VGCF loading showed the best cell performance. The electrodes that included the catalyst, VGCF, and gas diffusion layer, were directly examined by electron microscopic analyses. Electrochemical methods, such as cyclic voltammometry and impedence analysis, were applied to investigate the actual role of VGCF in the electrode. The porosity of the catalyst layer was increased by the addition of the fibers. This was clearly observed in pore diameters less than 1 μm. Sub-micron pore diameters are significant as they relate to micro-diffusive transport, compared to the macro-diffusion experienced by the large pores in the GDL. However, improved mass transport was only observed for 2 wt.% VGCF loading, probably due to insufficient optimization of the cell design. Microstructural and electrochemical analyses indicated that the improved performance was mainly ascribed to an increased electrochemically active surface area of the catalyst.