Influence of Ionomer Content in Catalyst Layers on Direct Methanol Fuel Cell Performance

Abstract

The ionomer content in catalyst layers has a marked influence on direct methanol fuel cell (DMFC) performance. In an anode which contains unsupported PtRu as the catalyst, the recast ionomer may not always be necessary because the protonic conductivity of hydrous RuOx, the presence of which is inferred from the X-ray diffraction pattern, may be sufficient to allow effective utilization of catalyst sites. To examine interpenetration of catalyst and membrane material as a possible explanation for the lack of an apparent need of added ionomer, ultramicrotomed thin sections of the membrane-electrode assembly (MEA) were examined by scanning electron microscopy. Microscopic examination of such MEA cross sections revealed significant porosity in layers made by mixing unsupported catalysts with recast ionomer. Images of such sections did not reveal significant interpenetration, supporting the interpretation that hydrous RuOx may by itself provide sufficient protonic conductivity in PtRu catalyst layers prepared with no added ionomer. In contrast we show that the presence of recast ionomer in DMFC cathodes based on unsupported Pt as the catalyst is essential for optimum DMFC performance, because the recast ionomer is the primary source of protonic conductivity in the latter case. Having shown its potential function as proton conductor, we stress that Ru oxide is apparently not the key for maximizing DMFC anodic activity.

The Influence of Pt Loading, Support and Nafion Content on the Performance of Direct Methanol Fuel Cells: Examined on the Example of the Cathode

“…It is well known that the N/C ratio is a crucial factor for the performance and stability of a FC [11,12]. The DMFC performances, i.e.…”

“…0, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]However, the optimum N/C ratios are found to be lower, namely 0.7 and 0.5 for the VC and HG supports, respectively. The optimum N/C value using VC as catalyst support is essentially the same as reported by Liu and Song et al [19,20] i.e.…”

“…It is well known that properties of the catalyst layer such as catalyst loading, Nafion content, and the thickness of the catalyst layer affect the performance of a fuel cell (FC) [11][12][13][14]. Improvements in the FC performance can be achieved by increasing the electrochemically active surface area (EASA) of the catalyst layer.…”

“…Solutions of Nafion are impregnated into the electrode catalyst layer to increase the contact area between the Pt catalyst and the Nafion ionomer. In this way, the Pt utilization can be increased, resulting in higher FC performances if an optimum amount of ionomer is used [9][10][11][12][13][14].…”

“…It has been recognised for some time that the optimal structure for high catalyst utilization is a catalyst layer that is thin, with an appropriate mixture of catalyst and ionomer [12,21].…”