Low platinum loading catalyst

Most recently, an alternative low-platinum-loading catalyst layer structure has been developed by Wilson at LANL. In this structure, recast ionomer is used instead of PTFE to bind the catalyst layer structure together, and the low-loading catalyst layer is applied to the membrane, rather than to the gas-diffusion structure (mode A3)... 

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... 

Inspite of the extremely low platinum loading level of this catalyst and the low engine exhaust temperatures in the test, it gave relatively good conversion of gas phase HC and CO (37% and 26%, respectively) in the fresh state. Gas phase activity was decreased however, after extended engine aging. 

Taking these findings into account, even ultra low platinum loadings could sustain significant current densities for hydrogen oxidation at low overpotentials. However, for practical fuel cells, one has to take into account that hydrogen fuel never is completely pure while platinum catalysts are very susceptible to poisoning by polar substances such as CO, H2S, NH3 etc. [35, 36]. 

Low platinum loading, high activity, and more durable catalysts still remain as critical challenges for PEMFCs for automotive applications. Further... 

Optimization of electrode and MEA performance with new electrode materials (catalysts, catalyst supports, and ionomers [2]), particularly for high-current density operation with low platinum loadings. 

Typically, electrodes can be cast as thin films and transferred to the membrane or applied directly to the membrane. Alternatively, the catalyst-electrode layer may be deposited onto the gas diffusion layer (GDL), then bonded to the membrane. Low platinum loading electrodes (<... 

O Hayre R, Lee SJ, Cha SW, Ptinz FB. A sharp peak in the performance of sputtered platinum fuel cells at ultra-low platinum loading. J Power Sourees 2002 109 483-93. Passos RR, Paganin VA, TicianeUi EA. Studies of the performanee of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes. Electrochim Acta 2006 51 5239-45. 

TicianeUi and co-workers [126] fabricated the low-platinum loading PEM fuel cells featuring PI PE-bound catalyst layers. Cheng and co-workers [128] developed conventional PI PE-bound catalyst layer electrodes for direct comparison with the current thin-film method. The typical process employed for forming the PTFE-bound catalyst layer MEA in their study is detailed in the following [125],... 

O Hayre et al. (2002) have reported on their development of a catalyst layer with ultra-low platinum loading. Their paper suggests that they are developing these electrodes for use in micro-fuel cells since it was stated that the sputtering process is compatible with many other integrated circuit fabrication techniques. 

Using a carbon-supported Pt catalyst to replace Pt black can reduce the platinum loading by a factor of 10—from 4 to 0.4 mg/cm [74]. However, the platinum utilization in this PTFE-bound catalyst layer still remains low in the vicinity of 20% [75,76]. 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

Adjacent the ionomeric membrane on both sides are the catalyst layers (Fig. 1). As described above, these are platinum black/PTFE composites with high platinum loadings (typically 4 mg Pt/cm on each electrode) or composites of carbon-supported platinum and recast ionomer, with or without added PTFE, of much lower platinum loading (as low as 0.1 mg Pt/cm on each electrode). The electrochemical processes in the fuel cell take place at these electrocatalysts. In the hydrogen (or methanol reformate)/air fuel cell, the processes at the anode and cathode, respectively, are ... 

This mode of anode catalyst cleansing was recently reexamined by Wilson and others in PEFCs utilizing thin-film catalysts of ultra-low catalyst loading (0.10-0.15 mg/cm) [21]. As shown in Fig. 14, a thin-film anode catalyst of ultra-low platinum... 

Fig. 14. Cleansing by oxygen bleeding of a platinum anode catalyst in the presence of 5-20 ppm CO in the hydrogen fuel, demonstrated for a platinum anode catalyst of ultra-low loading (0.14 mg Pt/cm ), consisting of a Pt/C//ionomer thin film composite bonded to the membrane [21]. (Reprinted by permission of the American Chemical Society).

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