Abstract
In this study, a set of experimental techniques have been adopted to characterize the fundamental properties of machine prepared catalyst layers (CLs) formed by different Pt:C ratios, highly graphitized carbon and 3M ionomer (825EW). Single cell performance with these catalyst layers was evaluated in order to analyze the relationship with the structural properties. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) nitrogen adsorption were conducted in a series of CLs with Pt:C ratio variations at a fixed ionomer:carbon (IC) ratio to investigate the microstructure including materials dispersion and porosity. This analysis was performed on both the CL in a fresh (as-prepared) condition and after subjected to a standard PEMFC protocol. TEM images along with Pt size distributions of the 50:50 Pt:C fresh and post-operation samples are shown in Fig. 1. A significant widening of the Pt size distribution and a possible tendency towards a multimodal distribution is observed after FC operation. This behavior can be explained by a combination of Pt particle coalescence and dissolution/re-precipitation processes within the solidionomer, as has been suggested in previous studies.1 In order to assess the ionomer behavior in the machine prepared CLs, proton conductivity measurements and estimation of the proton conduction tortuosity was performed as a function of relative humidity (RH) in the single cell. Water uptake isotherms of the as-prepared CLs have also been acquired under well controlled RH levels. In contrast to typical Nafion®-based fuel cells, the 3M fuel cell reached its optimum performance at 60%-70% RH and suffered dramatic mass transfer losses at saturated humidity levels. Therefore, the 3M fuel cells are able to function fully under drier conditions than the Nafion units. However, the high sensitivity to the cells’ water content requires efficient water management during operation, especially at higher current densities. References More, K.L., Borup, R., and Reeves K.S. ECS Transactions, 3 (1) 717-733 (2006) Figure 1
Published Version
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