Influence of ionomer content in catalyst layers on direct methanol fuel cell performance

The ionomer content in catalyst layers has a marked influence on direct met hanol fuel cell (DMFC) performance. In an anode which contains unsupported PtRu as the catalyst, the recast ionomer may not always be necessary becaus e the protonic conductivity of hydrous RuOx, the presence of which is infer red from the X-ray diffraction pattern, may be sufficient to allow effectiv e utilization of catalyst sites. To examine interpenetration of catalyst an d membrane material as a possible explanation for the lack of an apparent n eed of added ionomer, ultramicrotomed thin sections of the membrane-electro de assembly (MEA) were examined by scanning electron microscopy. Microscopi c examination of such MEA cross sections revealed significant porosity in l ayers made by mixing unsupported catalysts with recast ionomer Images of su ch sections did not reveal significant interpenetration, supporting the int erpretation that hydrous RuOx may by itself provide sufficient protonic con ductivity in PtRu catalyst layers prepared with no added ionomer. In contra st we: show that the presence of recast ionomer in DMFC cathodes based on u nsupported Pt as the catalyst is essential for optimum DMFC performance, be cause the recast ionomer is the primary source of protonic conductivity in the latter case. Having shown its potential function as proton conductor, w e stress that Ru oxide is apparently not the key for maximizing DMFC anodic activity. (C) 1999 The Electrochemical Society. S0013-4651(99)05-024-7. Al l rights reserved.