Platinum catalysts carbon corrosion

Polymer electrolyte membrane (PEM) fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen front the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers. Such reformers could use different type of fuels, for instance methanol (Figure 7.9). 

Another reason for higher polarization of the oxygen electrode is corrosion (oxidation) of the carbon material serving as support of the platinum catalyst. This causes loss of catalyst s contact with the support (Cai et al 2006). That is a de facto exclusion of part of the catalyst. The loss of Pt-C bonding also favors recrystallization of the platinum catalyst. 

Pt/C and PtCo/C catalysts High temperature operation effect on carbon corrosion, platinum dissolution, and sintering Arico et ai, 2008... 

Platinum catalyst Surface area loss due to carbon corrosion and increasing platinum particle size Cai et al., 2006... 

Platinum catalyst Potential static holding conditions and potential step conditions effect on platinum dissolution and carbon corrosion Shao et al., 2008... 

Better stabilities were achieved investigating heat-treated catalysts (section Molecular Centers in Carbonized Materials ). Nevertheless, in most of the cases, even after the heat treatment, considerable decreases of performance were already observed after a few hours of operation time [69, 159, 168, 184, 188, 213-216]. For catalysts prepared by a heat treatment of either iron porphyrin or iron phthalocyanine, it was shown that they are tolerant toward CO [217, 218]. Therefore, we assume that in general, CO-poisraiing can be mled out as degradation mechanism for carbonized materials. Possible degradation mechanisms could be addressed to (1) a corrosion of carbon as known from platinum catalysts, (2) an inactivation by leaching of active sites, or (3) a deactivation of active species (e.g., by blocking of the centers by intermediates or the final products) [71, 98, 99, 102, 190, 191, 210, 211, 219, 220],... 

Figure 23.30. Comparison of the cumulative carbon corrosion following a 24-h 1.2 V potentiostatic hold at 80 °C in 1 M H2SO4 for two commercial carbons COl, C02 and one heat-treated carbon C03, and platinum and Pt/Co alloy catalysts on these carbons [95]. (Reprinted from Journal of Power Sources, 166(1), Prasanna M, Cho EA, Kim H-J, Oh I-H, Lim T-H, Hong S-A, Performance of proton-exchange membrane fuel cells using the catalyst-gradient electrode technique, 18-25, 2007, with permission from Elsevier.)...

Higher operational temperatures of HT-PEMFCs as compared to LT-PEMFCs impose more challenges with catalyst degradation. This issue coupled with the presence of free acid in the membrane obviously aggravates both carbon corrosion and platinum dissolution, which in turn triggers significant agglomeration of the... 

Carbon corrosion and platinum dissolution in the acidic electrolyte at elevated temperatures are well recognized from the early years of research on PAFCs and are definitely relevant to HT-PEMFCs based on the acid-doped FBI membranes. Both mechanisms are enhanced at higher temperatures and higher electrode potentials. This should be taken into account when platinum alloy catalysts are considered for the HT-PEMFC. More efforts are also needed to develop resistant support materials based on either structured carbons or non-carbon alternatives. 

Roen et al. (2004) examined the effect of platinum on carbon dioxide emissions three synthesized in-house MEAs (carbon, 10 wt%and 39 wt%platinum supported on carbon catalysts) were potential cycled between 0 and 1 V (vs RHE) at 65 °C (100% RH, hydrogen/air or oxygen as reactants) and their carbon dioxide emissions were measured by mass spectrometry. The presence of Pt enhanced carbon corrosion rate since Pt catalyzes CO2 formation at low potentials (-0.55-0.65 V vs RHE) (Willsau and Heitbaum, 1984) and increases CO2 emission rates at 1V (vs SHE) (Roen et al., 2004). It was also reported that the carbon corrosion rate is enhanced as the range of potential cycling is increasing (higher anodic and lower cathodic potentials) due to the formation of defects (Stevens et al., 2005) on carbon support by chemical oxidation in low potentials and the presence of a harsh electro-oxidation envirorunent at high potentials (Maass et al., 2008). 

The membrane electrode assembly (MEA) is the heart of a fuel cell stack and most likely to ultimately dictate stack life. Recent studies have shown that a considerable part of the cell performance loss is due to the degradation of the catalyst layer, in addition to membrane degradation. The catalyst layer in PEMFCs typically contains platinum/platinum alloy nanoparticles distributed on a catalyst support to enhance the reaction rate, to reach a maximum utilization ratio and to decrease the cost of fuel cells. The carbon-supported Pt nanoparticle (Pt/C) catalysts are the most popular for PEMFCs. Catalyst support corrosion and Pt dissolution/aggregation are considered as the major contributions to the degradation... 

Conductive polymers, such as polyacetylene, polythiophene, polypyrrole, polyisothianaphthene, polyethylene dioxythiophene, polyaniUne, and so on, have interesting properties that make them suitable for use in PEMFCs (Heeger, 2001 Shirakawa, 2001). Their electroconductivity and noncarbon functionalities allow some of them to perform effectively as alternative carbon catalysts or with carbon supports to enhance their catalytic effects. Huang et al. utilized polypyrrole as a conductive polymer support for a platinum catalyst active for the ORR (Huang et al., 2009). Their results show significant resistance to carbon corrosion and improved conductivity over traditional Pt/C catalysts. They report that the platinum on polypyrrole catalyst (Pt/Ppy) has well-dispersed platinum particles of about 3.6 nm in diameter. CV scans up to 1.8 V revealed that there was httle carbon support corrosion on the Pt/Ppy and a twofold increase in activity than Pt black at 0.9 V. 

Various metals/metallic oxides have been coated onto carbons with the aim to improve the platinum tolerance to poisons, increase the platinum utilization and to avoid carbon corrosion of the catalyst support. Sn, Ru, Ti, and Co metals as well as their oxides have been reported on in fair amounts as being transition metals capable of increasing the platinum utilization by removing hydroxide species that would be present on the platinum such that the platinum can further catalyze the ORR. Although these materials have been coated onto the carbon as a support, there are alloying characteristics that occur with the impregnated platinum which result and thus will not be touched up in this section. However, many of these oxides and metals have been used as stand-alone catalyst supports without the use of a carbon substrate, and are discussed further in Section 3.5.3. 

In fuel cells, where platinum catalysts deposited onto carbon black or other carbon supports are often used, corrosion of the carbon support is a very troublesome problem. This corrosion not only leads to a deterioration of the contact between catalyst and substrate that is needed for it to function, but may also cause the catalyst to drop away from the electrode. 

Fig. 19 Kinetics of carbon corrosion for approximately 50wt% platinum catalysts on various carbon-supports (A-D) and of the OER (E). The horizontal dashed line indicates the maximum local Hj starvation current, equivalent to the crossover current, for fully developed local starvation. MEA specifications and test conditions were as follows 0.4 mgp using an...

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