Electrocatalyst testing

The evaluation of catalysts for FEMFCs in recent years has been well defined experimentally and numerically. In particular, studies have correlated that observed in liquid electrolytes to that observed in MEAs (e.g., for cathode activity, see Gasteiger et al. ). 

The evaluation of catalysts typically uses two techniques. The first is evaluation as a thin layer on a bulk electrode (e.g., glassy carbon) in dilute liquid electrolyte (e.g., H2 4) either as a static electrode or an RDE. In the study of oxygen reduction, there has been much discussion as to the most appropriate electrolyte to use. In general, dilute perchloric acid (HCIOJ is preferred because of its noncoordinating nature, it is thus closest to the environment foxmd within a FEM catalyst layer with perfluorosulfonic acid ionomer. A possible alternative is trifluoromethylsulfonic acid (CF3SO3H), which mimics perfluorosulfonic acids closely, but there are relatively few studies with this acid. Rotating 

The second approach is to test catalysts as layers in full MEA sfrucfures. This has the advantage of testing catalysts under realistic conditions and in realistic environments. However, this approach depends on creating a near-optimal catalyst layer structure that shows high utilization of fhe cafalysf, fogefher wifh a structure that allows adequate hydration and reactant/product transport. 

With both approaches, it is key to establish the current regions where samples are under kinetic control to allow the correct comparison. Many reported comparisons of catalysfs in MEA sfrucfures point to differences in performance, which are attributed to intrinsic catalyst differences when it is clear that differences are due to mass fransport effects because of catalyst layer structure. To help overcome these difficulties, it is recommended that, for catalyst evaluation, pure reactants be used (e.g., O2 instead of air) and at relatively high stoichiometries. Use of current-voltage curves should be corrected for elecfrolyte or membrane resistances and Tafel analysis used to identify fhe kinefically confrolled current regions. 

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Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. The alcohol is fed directly into the fuel cell without any previous chemical modification and is oxidized at the anode while oxygen is reduced at the cathode. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. Among different electrocatalysts tested in the methanol oxidation, PtRu-based electrocatalysts were the most active [1-3]. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electrooxidation [3],... 

Out of more than forty different porphyrinic and non-porphyrinic organic and inorganic electrocatalysts tested for these properties, only three show the desired characteristics tetrakis (3-methoxy-4-hydroxyphenyl)porphyrin, me50-tri(V-methyl-4-pyridinium)-p-phenylene-5 -0-2, 3 -0-isopropylideneuridine (PUP), and NA -bis[5-p-phenylene-10,15,20-tris(3-methoxy-4-hydroxyphenyl)-porphyrin]-l,10-phenanthroline-4,7-diamide( 1,10-phen)(TMHPP)2." Detection limit for NO depends on film quality and varies between 10 and 10 M. The best sensitivity is usually obtained using a PUP and (l,10-phen)(TMHPP)2 films with Ni(II) as the central metal. If a reproducibility of preparation procedure is considered (l,10-phen)(TMHPP)2 Ni(II) is much superior (failure rate 1 10) in comparison to two other porphyrins. 

Molecules that undergo fast random polymerization on the electrode surface, like tetraphenylporphyrins with amino or pyrrole substituents on the phenyl ring, will form films with a well-developed surface but poor catalytic properties for NO oxidation and low electrical conductivity. Out of more than forty different porphyrinic and nonpor-phyrinic and organic and inorganic electrocatalysts tested for these properties, only three show the desired characteristics tetrakis (3-methoxy-4-hydroxyphenyl)... 

FIGURE 2.1 Particle size distribution of fresh and used Pt/C electrocatalysts, tested in constant-power mode, 0.12 W cm" at 333 K over 529 h. (Reproduced from Guilminot, E. et al. 2007b. /. Electrochem. Soc. 154 B96-B105. With permission from The Electro Chemical Society.)... 

Studies on the electrocatalytic activity of metal porphyrins are limited in comparison with those on other classes of macrocyclic metal complex. Among the few porphyrin complexes tested, cobalt porphyrins have been demonstrated to be efficient electrocatalysts for the reduction of C02 to CO... 

Polyaniline (PANI) was investigated as electrocatalyst for the oxygen reduction reaction in the acidic and neutral solutions. Galvanostatic discharge tests and cyclic voltammetry of catalytic electrodes based on polyaniline in oxygen-saturated electrolytes indicate that polyaniline catalyzes two-electron reduction of molecular oxygen to H2O2 and HO2". 

The electrocatalysts for oxygen reduction were prepared as follows. These complex compounds were inoculated onto the carbon (AG-3, BET area near 800 m2/g) by means of adsorption from dimethylformamide solutions. The portion of complex compound weighed so as to achieve 3% of Co content was mixed with the carbon, then 5 ml of dimethylformamide per 1 g of the carbon were added and the mixture was cured at room temperature for 24 hours. Series of samples obtained were thermally treated (pyrolyzed), and the resulting grafted carbons were tested as electrode materials in the reaction of molecular oxygen reduction. 

Myriads of electrocatalysts of the most varied composition have been tested thus far [36-38, 67]. It tvould be impractical to mention all of them and almost impossible to attempt to establish a rank of activity. Most of them exhibit electrocatalytic properties sufficiently close to each other. Under similar circumstances, it is only possible to summarize the situation briefly. 

The Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT) works on design, construction and testing of PEM components, electrochemical reactions/reactors, electrocatalysts for PEMs, hydrogen production by catalytic processes and water splitting. 

Alloys of noble metals have been tested for O2 reduction [31] and some should even be more efficient than Pt [63]. These metals are expensive so that other materials have been tested, like bronze, some being even slightly better electrocatalysts than platinum [45] however, they do not seem to be stable enough. The factors governing oxygen reduction on oxides used as cathodes in fuel cells have been reviewed recently [92]. 

Nickel boride (Ni2B) has been quoted sometimes as an excellent electrocatalyst in alkaline solution [3, 10, 400]. However, there exists only one paper describing a practical stability test [107]. A patent filed in 1974 describes the electroless formation... 

Fig. 11.9 Photograph of a 16-channel parallel rotating disk electrode (PRDE) test station for high-throughput screening of high surface-area carbon-supported electrocatalysts. The PRDE station is part of the secondary screening workflow for electrocatalysts.

Fig. 11.10 Parallel chronoamperometric screening of a 64-element, thin film electrocatalyst library for the oxidation of methanol. The library contained a diverse set of binary, ternary and quaternary electrocatalyst compositions consisting of Pt in combination with W, Ni, Co and Ru. The graph plots current vs. time and channel number. Conditions 1 M methanol, 0.5 M H2S04, room temperature, = + 450 mV/RHE, test time = 5 min. For clarity, channel numbers 2-4,10,12,19, 20, 23, 26-29, 42,45 and 57 are omitted. (Reproduced from [18]).

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. 

The experimental work done to date in the reaction kinetics of SO2 depolarized electrolyzers, electrocatalyst evaluation, and tests on small experimental cells has led to projections of cell voltage for a mature technology. Table III presents these projections, as a function of electrolyte concentration and operating temperature. These projections will be refined, as the development work progresses, to expand the matrix of variables, increase the confidence level in the figures, and define an optimum set of operating parameters. 

The thermally prepared oxides of the so-called rarer platinum metals are among the best electrocatalysts known for the oxygen gas evolution reaction from aqueous systems. Of these oxides, Ru02 exhibits the highest catalytic activity (at least in relatively short term tests) and has been investigated in most detail. Much of the published work on Ru02 has been stimulated by the success of Ru02-based anodes in chlor-alkali cells. 

These results not only represent the first time that water activation has been observed on a chalcogenide electrocatalyst, but also offer the first elucidation of the pertinent stracture/property relationships of this class of materials. As we are able to obtain and test single phases of higher purity, we anticipate being able to... 

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