Electrocatalysts for the ORR

Finally, it should be remarked that a catalyst for the ORR should provide a balance between being sufficiently active to activate O2 and releasing it as HgO, and noble enough not to be oxidized itself by water oxidation which may occur at the typical potentials for cathode operation. 

It is well established that the reaction rate of the ORR on Pt (hkl) surfaces is structure-sensitive. This is because of the different adsorption strengths of the electrolytes the stronger the adsorption, the more pronounced the structural effect. Remarkably, the same activation energy in both acid (ca. 42 IJ/mol) and alkaline solution (ca. 40 kj/mol), at the reversible potential, and at 0.8 V, has been reported. This feature indicates that the structure sensitivity arises from geometrical factors in the pre-exponential term of Eq. (9.21). 

ORR as a function of particle size in different electrolytes open circles 98% H5PO4 180°C full circles 0.5 M H2SO4 25°C and open squares 97% H3PO4 at 

While the studies above demonstrate that certain surfaces show superior activity for the ORR, it is difficult (to say the least) to synthesize Pt catalysts exposing only a desired individual surface. Therefore, other approaches are required in order to increase the activity for the ORR. 

We have hinted above that by alloying, the surface structure and electronic density of a given surface can be modified. Accordingly, the interaction with adsorbates, and hence the catalytic performance, can be engineered. A great deal of effort has been devoted to the preparation, characterization, and study of alloys of the composition PtgX (X = Fe, Co, Ni, Cr, Mn). In a seminal work, Jalan and Taylor identified carbon supported PtCr alloy as the most active alloy for the ORR in phosphoric acid fuel cells. They also proposed PtNi and PtCo as the next best alloys. This line of research was further explored by other groups Mukeijee et 

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The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

The dependence of the Gibbs free energy pathway on electrode potential (Figure 3.3.10A) manifests itself directly in the experimental current potential characteristic illustrated in Figure 3.3.10B. At 1.23 V, no ORR current is measureable, while with decreasing electrode potentials the ORR current increases exponentially until at +0.81 V, processes other than surface kinetics (e.g. mass transport) begin to limit the overall reaction rate. Figure 3.3.10B represents a typical performance characteristic of a Pt or Pt-alloy electrocatalyst for the ORR. 

The oxygen reduction reaction (ORR), the importance of which has to be underlined both firom a fundamental point of view and for its implication in electrochemical power sources and in the corrosion of metals [135], has been thoroughly investigated at modified electron conducting polymers. Both metallic particles and transition metal complexes were considered as suitable electrocatalysts for the ORR, and were dispersed in different electron conducting polymers (PPy, PAni, etc). 

The results obtained with bulk alloys can be also translated to more practical carbon supported PtM catalysts. In general, a certain increase in the mass activity (A/gpt) for the ORR reaction was found in PtM (M = Cr, V, Mn, Ni, Co) as compared to Pt/C. Again, it is difficult to extract accurate conclusions to explain the role of the foreign metal on the activity of carbon supported bimetallic (or polymetallic) electrocatalysts for the ORR since features such as particle size and size distribution, or metal loading cannot always be eliminated by normalization procedures. Paulus et studied... 

Details on the scale-up synthesis of Pt monolayer electrocatalysts for the ORR can be found in reference. The activity of the Pt monolayer can be further increased by depositing it onto a core of inter-metallic structures such as PtPb or PtFe. The Pt/PtPb catalyst showed a current density of 2.7 mA/cm ggo ,gjric at E = 900 mV. Furthermore, the Pt/PtPb catalyst is stable after 6,000 cycles between 0.6 and 1.13V and shows better stability than carbon supported Pt/PtPb. 

Recently it has been reported that Ru addition to PtCo/C results in catalysts with improved tolerance to methanol under typical ORR reaction conditions i.e. potentials more positive than 0.7 V NHE and O2 saturated acid electrolyte. This is because under these reaction conditions, upper oxide Ru species are stable and hinder methanol adsorption. Another interesting alternative is the Ru-based chalcogenides. In particular Ru Sey-based electrocatalysts have received a great deal of attention because of their high tolerance to methanol, even if their performance as electrocatalysts for the ORR is inferior to Pt/C by =40%. ... 

Thus, electrocatalysts for the ORR have to present electronic structures that result in adsorption forces that strike these two competing steps while strong adsorption leads to facilitated 0-0 bond breaking, weak adsorption tend to facilitate the O-H bond formation (hydrogen addition). This produces the so-called volcano plot of the activity as a function of the adsorption strength on the catalyst surface [7-9]. 

Many ORR experiments have been made on electrocatalysts composed by Pt and Pd with the addition of non-noble metals, such as Co, Fe, and Ni. However, under electrochemical conditions these non-noble metals might leach out from the electrocatalyst, as demonstrated in previous investigations [25]. In order to avoid this problem, Yang and co-authors [26] have investigated PdPt-based electrocatalysts for the ORR in absence and in the presence of methanol, because the long-term stability of Pd in acidic solution is comparable to that of Pt (but this depends on the potential - additionally, Pt may stabilize Pd atoms in the alloy). It was report a novel strategy for surface and structure-controlled synthesis of carbon-supported Pd3Pti nanoparticles for the ORR as well as for methanol-tolerant ORR electrocatalyst. The influence of the surface composition and structure of the PdsPti/C on the ORR activity in the absence and presence of methanol was also reported. 

Koslowski UI, Abs-Wurmbach I, Fiechter S, Bogdanoff P (2008) Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR a correlation of kinetic current density with the site density of Fe-N-4 Centers. J Phys Chem C 112(39) 15356-15366... 

In previous years, platinum has been widely used as electrocatalyst for the ORR because it is the most efficient pure catalyst material. However, its scarcity and associated high cost motivated the development of alloy materials to reduce the load of Pt. Moreover, the ORR kinetics is slow on pure Pt catalysts, requiring a high overpotential due to the presence of oxygen and hydroxyl strongly bonded to the surface as it was determined by electronic structure calculations [18]. Therefore, there is a need for catalysts with low cost and enhanced ORR activity to replace pure Pt. 

Characterization of electrocatalysts for the ORR therefore routinely involves determination of their selectivity, which essentially requires the determination of the number of electrons transferred, the percentage of H2O2 generated, and the rate of decomposition of H2O2. Rotating disk electrode (RDE) voltammetry and... 

The onset potential difference for the orr at Ag and Pt as a function of reaction temperature is shown in Fig. 12. Their potential difference is obviously decreased with increasing temperature. At room temperature, the difference is approximately 0.10 V. It falls in a range of 0.02 to 0.03 V when the temperature is higher than 130 °C. This indicates that Ag is a very promising electrocatalyst for the orr in the intermediate temperature range. Although the stability of Ag in alkaline media is questioned, a few strategies have been proven efficient to this issue. 

Another example of a MOF-based electrocatalyst for the ORR is given by Cu3(btc)2 (Figure 5d) and by Cu bipy btc. The bipy auxiliary ligand was necessary to stabilize the structime in aqueous mediiun, as the Cu btc MOF [Cu3(btc)2l was not stable enough. A large increase in the reduction current in 02-saturated medium attests... 

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