Electrocatalysts performance testing

Subsequent deployment of the new catalyst in the cathode layer of small-area MEAs first, then large-area MEAs, and finally fuel cell stacks represents the typical series of performance tests to check the practical viability of novel ORR electrocatalyst materials. Figure 3.3.15A shows the experimental cell voltage current density characteristics (compare to Figure 3.3.7) of three dealloyed Pt-M (M = Cu, Co, Ni) nanoparticle ORR cathode electrocatalysts compared to a state-of-the-art pure-Pt catalyst. At current densities above 0.25 A/cm2, the Co- and Ni-containing cathode catalysts perform comparably to the pure-Pt standard catalyst, even though the amount of noble metal inside the catalysts is lower than that of the pure-Pt catalyst by a factor of two to three. The dealloyed Pt-Cu catalyst is even superior to Pt at reduced metal loading. 

Several of the other tested electrocatalysts give weaker performances than Pt-Ru, regardless of the anode potential, but some of the ternary catalysts such as Pt-Ru-Co, Pt-Ru-Ni, or Pt-Ru-Fe, exhibited good performances but showed, however, poor stability under working conditions, the corrosion of the three metals, Co, Ni, and Fe, occurring in strong acidic media. 

Adzic and coworkers proposed a radically new approach in electrocatalysis and catalysis that can alleviate both problems. It is based on a catalyst consisting of only a submonolayer Pt deposited on carbon-supported Ru nanoparticles. The Pt submonolayer on Ru (PtRu2o) electrocatalyst demonstrated higher CO tolerance than commercial catalysts in rotating disk experiments. Tests of the long-term stabihty of the fuel cells detected no loss in perform-... 

Fuel-cell tests offer the ultimate verification of the usefulness of an electrocatalyst by determining its long-term stability under real operating conditions. They were performed on single cells using electrodes of 50 cm and an anode catalyst loading of 0.2 mg... 

Figure 21. Long-term test of the performance stability of the PtRu2o electrocatalyst in an operating fuel cell. The fuel cell voltage at constant current of 0.4 A cm is given as a function of time for the electrode of 50 cm with an anode containing to 0.18 mg Ru cm and 0.018 mg Pt cm / (approximately 1/10 of the standard Pt loading) and a standard air cathode with a Pt/C electrocatalyst. The fuel was clean H2 or H2 with 50 ppm of CO and 3% air temperature 80 C.

In methane good performances were reported at 1000°C when Pt or Ni metals were present. Originally, it was believed that reduced ceria was a good electrocatalyst for direct methane oxidation.This was based on the observation that in blind tests with the Pt current collector directly on the YSZ electrolyte, a very high polarisation resistance was obtained, and thus it was concluded that Pt was almost inert for the reactions in question as also reported by Steele et al. The... 

Synthesize binary and ternary Pt-alloy electrocatalysts by spray pyrolysis and test their performance in MEAs. 

Continue to improve the structure and performance of the cathode layer and test the performance of newly discovered electrocatalyst compositions in MEA configuration that are identified by the rapid screening technique to have enhanced kinetic performance. 

Recently, Lamy and co-workers [4,5] described that PtSn/C electrocatalysts were more active than PtRu/C electrocatalysts for ethanol oxidation. For electrocatalysts prepared by co-impregnation-H2 reduction and Bonneman methods, they found that the optimum tin composition was in the range of 10-20 at.%. In these conditions, the electrode activity was enhanced and the CO-intermediates coming from ethanol dissociative chemisorption were reduced. Xin and co-workers [6-9] prepared PtRu/C and PtSn/C electrocatalysts by a polyol method and tested for ethanol oxidation. It was observed that the addition of some elements, like W, could improve the PtRu/C electrocatalyst activity. However, the activities of the PtRu/C electrocatalysts were inferior to those of PtSn/C electrocatalysts. It was also found that PtSn/C electrocatalysts with Pt Sn atomic ratios of 60 40 and 50 50 were more active than electrocatalysts with 75 25 and 80 20 atomic ratios. Thus, it seems that the performance of PtSn/C electrocatalysts depends greatly on their preparation procedure. 

Case Study 3 Surface-doped Pt/Ru/Co Carbon Based on the above-mentioned DFT calculations performed by Norskov [168] we have prepared trimetallic electrocatalysts having PtRu/C surface-doped with Co(0) in order to produce highly active but at the same time CO tolerant electrocatalysts. For example, Pt/Ru/Fe/C, Pt/Ru/Ni/C, and Pt/Ru/Co/C systems were manufactured with the metal ratios being 45 45 10 a/o and a total metal loading of 20 wt.% on Vulcan XC 72. The resulting catalysts were compared with the industrial PtsoRuso standard catalyst under identical conditions. Full characterization was done via a combination of TEM, XRD, XPS, andXAS measurements, further BET, and electrochemical tests [171]. 

The performance of the Pt-Au/C (2 1) and Pt/C catalysts toward ORR in the presence and absence of methanol was evaluated by RDE experiments and the results are shown in Fig. 5.6a. As can be observed, Pt-Au/C (2 1) and Pl/C possesses similar ORR half-wave potentials, but Pt-Au/C showed higher ORR activity in the presence of methanol. The electrocatalysts have also been tested in a DMFC [27]. The cell polarization data for Pt-Au/C catalysts with different Pt to Au atomic ratios in a methanol/02 DMFC are presented in Fig 5.6b. The results obtained for Pt/C were also included for comparison. These results show that the DMFCs with Pt-Au/C cathodes perform better than Pt/C. It was noted that the DMFC with Pt-Au/ C (2 1) in the cathode shows an enhanced peak power density of 120 mW cm , compared to 80 mW cm observed for the DMFC with a Pt/C cathode. As argued by the authors [27], since the anodes in both the DMFCs are identical, the enhanced performance for the DMFC with Pt-Au/C catalyst is clearly due to the synergistic promotion of ORR on the catalyst. 

For application of ORR electrocatalyst in PEM fuel cells, the catalyst must be used to prepare the CL, which is then bonded into the MEA for fuel cell testing. The composition, structure, and preparation procedure all have strong effects on the fuel cell performance. For example, even an ORR catalyst has high ORR activity, if the CL and its MEA preparation are not optimized, it is not necessary that fuel cell would have a high performance. Therefore, the optimum preparation of CL and its MEA are very important in fiilly realizing the catalyst activity. 

Ir- and Rh-based catalysts have been investigated for ethanol oxidation reaction in acidic media, and there have been some interesting and reasonably promising results [116-118]. Lamy et al. [116] carried out in situ FTIR study on poly crystalline Ir and Rh electrodes and showed ethanol oxidation on Ir leading selectively to either acetic acid or acetaldehyde, while Rh is abetter catalyst in the total oxidation of ethanol to CO2. Cao et al. [117] studied carbon-supported IrsSn nanoparticle electrocatalyst, and the fuel cell test results showed that the overall performance of Ir3Sn/C was comparable to that of the PtsSn/C catalyst. 

To improve the electrocatalytic activity of platinum and palladium, the ethanol oxidation on different metal adatom-modified, alloyed, and oxide-promoted Pt- and Pd-based electrocatalysts has been investigated in alkaline media. Firstly, El-Shafei et al. [76] studied the electrocatalytic effect of some metal adatoms (Pb, Tl, Cd) on ethanol oxidation at a Pt electrode in alkaline medium. All three metal adatoms, particularly Pb and Tl, improved the EOR activity of ft. More recently, Pt-Ni nanoparticles, deposited on carbon nanofiber (CNE) network by an electrochemical deposition method at various cycle numbers such as 40, 60, and 80, have been tested as catalysts for ethanol oxidadmi in alkaline medium [77]. The Pt-Ni alloying nature and Ni to ft atomic ratio increased with increasing of cycle number. The performance of PtNi80/CNF for the ethanol electrooxidation was better than that of the pure Pt40/CNF, PtNi40/CNF, and PtNi60/CNF. 

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