PEFC Cathode

Giaracteristic Physical and Operational Parameters for the PEFC Cathode and DMFC Anode 

Calculations show that solution to the optimization problem strongly depends on the dimensionless working current density jo. This parameter varies in a wide range between different types of cells. A discussion of solutions relevant to the CCL in a PEFC and to the ACL in a DMFC is given below. The respective typical Jo s differ by an order of magnitude (0.6667 for PEFC and 6.67 for DMFC, Table 4.5). This leads to dramatic differences in the optimization results. The results for optimal and uniform shapes are compared at the same value of the total potential loss fjo, that is, optimization raises the cell current density Jo- 

FIGURE 4.34 (a) The uniform (solid line) and optimal (dashed line) relative ionomer content in the cathode catalyst layer (CCL) of a PEFC. (b) The respective distributions of overpotential (two upper curves) and proton current density (lower curves) through the CCL. 

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Specific aspects examined here include insights and conclusions derived from the most recently performed density functional theory (DFT) calculations, which have been based on a comprehensive model of the electrochemical interface, and the strong disagreements (which seem to defy all recent theoretical efforts) that remain regarding proper interpretation of experimental ORR results and proper identihcation of the ORR mechanism in a PEFC cathode employing Pt catalysts. 

What is behind the apparent disagreements between Tafel slopes and reaction orders reported from recent investigations of the ORR at PEFC cathode catalysts and the slopes and reaction orders measured earlier for model systems of low Pt surface area Is the ORR process at a dispersed Pt catalyst possibly different in nature from the ORR process at low-surface-area Pt ... 

Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2) as the first and rate-limiting step be defended in light of the recently reported apparent Tafel slope and reaction order for ORR in the PEFC cathode ... 

Is there a way to verify experimentally not only that the measured low Tafel slope of 60mV/decade in the fuel-cell-relevant potential range is fully explainable by an intrinsic slope of 120 mV/decade for ORR at Pt metal, but also that this intrinsic Tafel slope can be experimentally verified within the fuel-cell-relevant potential range In fact, an elegant response to this challenge has recently been provided by impedance spectra measured for the ORR process at Pt/C in a PEFC cathode... 

Gode P, Jaouen F, Lindbergh G, Lundblad A, Sundholm G (2003) Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode. Electrochim Acta 48(15) 4175-87... 

Liu, Y. et al.. Zirconium oxide for PEFC cathodes, Electrochem. Solid-State Lett., 8, A400, 2005. 

ORR activity as the recast ionomer film gradually dried again while in contact with saturated water vapor at 80 °C. These observations are important in the context of ORR catalytic activity in PEFC cathodes. The sensitivity of the rate of ORR to interfacial water content (Fig. 9) and the tendency of the ionomer to dry up at elevated temperatures when in contact with saturated water vapor, clearly suggest that the level of humidification at the PEFC cathode catalyst should be kept high by maintaining some water in liquid state in contact with the cathode catalyst layer. 

The additional problem identified recently, of in-situ generation of CO by reduction of CO2 within the PEFC cathode, can be minimized by the use of a PtRu anode catalyst. Low PtRu loading (0.1 mg/cm ) seems to be effective in maintaining the anodic hydrogen oxidation activity while minimizing the rate of CO2 reduction. 

Wilson and co-workers have measured platinum catalyst ripening in PEFC cathodes of ultra-low platinum loading which operated continuously for 25(X)h at a cell voltage of 0.5 V, on pressurized hydrogen and air [46]. Results obtained for the cathode catalyst show that slow catalyst ripening takes place in these PEFC cathodes. The typical degree of ripening for Pt/C catalysts can be summarized as a decrease of platinum surface area from an initial value of 100 m /g to 70 m /g after 1000 h and to 40 m /g after 2500 h. The results of particle size distribution analyses for as-supplied... 

Fig. 26. Mass distributions of Pt/C (a) as cast catalyst, (b) PEFC cathode catalyst after 1320 of operation, and (c) PEFC cathode catalyst after 2200 h of operation [46]. (Reprinted by permission of the Electrochemical Society).

The first numerical models of PEFC were developed about 15 years ago. Springer et al. published a one-dimensional (ID) steady-state model of PEFC [7]. At the same time Bernardi and Verbrugge developed a model of the PEFC cathode [5] (and later extended it to the whole cell [6]), which in many respects is close to the model of Springer et al. 

Fig. 28 Mass distributions of carbon-supported Pt catalyst particles (a) in the as-cast catalyst layer, (b) after 1300 hours of operation, and (c) after 2200 hours of operation as PEFC cathode catalyst [51].

Uniqueness of the Pt Catalyst The high overpotential at the PEFC cathode is the single most important source of loss in the PEFC, as it is in other low-temperature fuel cells. The sluggish nature of the ORR process is immediately reflected by the measured open-circuit voltage of around 1.0 V, to be compared with the thermodynamically expected value of... 

A cathode potential of 0.7 V is required to reach an ORR current density of 1 A per cross-sectional square centimeter of a PEFC air cathode at 80 °C, corresponding to a cell voltage loss of 0.5 V. This is, by far, the largest loss in a hydrogen/air PEFC. Furthermore, such cathode performance requires Pt or Pt alloy catalyst of significant cost. The demonstrated 2006 PEFC cathode technology of 0.2 g Pt/kWpeak, translates, at the level of Pt... 

In a subsequent attempt to mimic more closely the composition and structure within the fuel-cell cathode ([87]), a recast Nation film was applied to a Pt microelectrode in order to study the rate of the ORR as function of temperature and humidification conditions and the cell employed was as described in Fig. 37. This configuration comes closest to PEFC cathode conditions, in that the Pt surface is in contact with the recast ionomer, rather... 

PEFC cathodes. The sensitivity of the rate of ORR to interfacial water content and the tendency of the ionomer to dry up at elevated temperatures when in contact... 

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