Anode catalyst Introduction

This review will give a short introduction to the EXAFS and Ap XANES techniques and then focus on various investigations in PEM fuel cells. The advantages and limitations of XAS in fuel cell research will be compared to other characterization techniques, and then investigations of specific Pt-Ru anode catalysts presented. In the light of these recent results, the suitability of the XAS approach to reveal fundamental steps in the reaction mechanisms will be discussed. 

Here a mediator serves as an oxidant that enables the selective introduction of a functional group. The mediator is thereby reduced and can be regenerated at the anode, so that it acts as a redox catalyst (Eq. 17). The method is very powerful, as very selective oxidants, whose use is limited to smaller amounts due to their price, can be applied in catalytic quantities amounts for larger scale conversions. 

Tonkovich et al. [123] claimed a 90% size reduction due to the introduction of micro channel systems into their device, which made use of the hydrogen off-gas of the fuel cell anode burnt in monoliths at palladium catalyst to deliver the energy for the fuel evaporation. A metallic nickel foam 0.63 cm high was etched and impregnated with palladium to act as a reactor for the anode effluent It was attached to a micro structured device consisting of liquid feed supply channels and outlet channels for the vapor, the latter flowing counter-flow to the anode effluent... 

Spectroscopic investigations also showed that the introduction of the solid alkali did not lead to an anomalous population of rotational levels in NH but raised the vibrational temperature from = 3(XK) K to c= 4000 K. It was sugg ted that the recombination of ions, presumably N, on the catalyst surface leads to an increase in the number of nitrogen molecules at high levels of vibrational excitation. However, the location of NaOH catalyst at the cathode or anode does not influence significantly either the decomposition of ammonia or the synthesis of hydrazine (Fig. 25)... 

An important topic of research is the introduction of the catalyst in the microreactor. In brief solid catalysts can be incorporated on the interior of micromachined reaction channels, prior to or after closure of the channel, by a variety of strategies anodic oxidation, plasma-chemical oxidation, flame combustion synthesis, sol-gel techniques, impregnation, wash coating, (electro-)plating, aerosols, brushing, chemical vapor deposition, physical vapor deposition and nanoparticle deposition or self-assembly. Some of these methods can be applied in combination with photolithography or shadow masking. 

The DiSalvo group at Cornell Uifiversity has intensely studied intermetallics for formic acid electrooxidation and observed significant enhancements in turnover efficiencies [16, 46, 72-79]. Table 4.3 compares the activity of several extended intermetallic surfaces in comparison to a Pt baseline [16]. The onset potential relevant to enhanced reactivity through the direct dehydrogenation pathway was most impacted by the addition of Sb. The introduction of both Sn and Sb into the Pt unit cell negatively impacted the anodic peak current. While Bi increased the peak current, it had an adverse impact on the onset potential. It increased the onset potential by 0.06 V and nearly doubled the peak current. The key challenges related to intermetallics for DFAFCs are surfactant-free synthesis methods and reduced nanoparticle sizes (>10 nm) to improve mass activity of the catalyst [74, 75, 80]. Mastumoto et al. compared the mass activity of PtPb 10-20 nm intermetallic particles to a commercial nanocatalyst [79], During a 9 h hold at 0.197 V vs. RHE, the PtPb intermetallic catalyst demonstrated over a twofold sustained mass activity over that of Pd. 

As pointed out in the introduction, the activity for OER is a necessary property of the added/modified existing anode or cathode FC catalyst. However, the high OER activity, such as exhibited by the added Ru on Pt, must be able to withstand numerous high positive potential excursions during the lifetime of a FC stack in... 

As stated in the introduction, Ta coating may be used as substrate in the preparation of DSA oxygen electrodes it consists of a thin and porous layer of Iridium oxide, which acts as catalyst, obtained by thermal oxidation of an iridium compound on a valve metal. The lifetime of the anode in water electrolysis in extreme conditions of polarization (anodic current = 50 A/m ), acid concentration (30% m/m) and temperature (T = 80°C) is sensitive to the corrosion resistance of the valve metal This is shown on table I [24], which standardized life time (lifetime reported for the mass surface density of the catalyst Ir02) for some varieties of titanium base alloys and a tantalum coating as substrate ... 

The introduction of NMonNM deposition method has initiated applications as possible route to produce catalysts for fuel cells with improved peifoimance [37, 38, 42]. Different experimental methods were used to characterize electrosorptiOTi characteristics and activity of these modified bimetallic noble metal surfaces for different reactions [38,43,44]. These efforts have led to the design and characterization of one of the most efficient catalysts known today for polymer electrolyse membrane fuel cell anodes [4,45 7]. 

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