Stability of electrocatalysts

The activity of catalysts for electrochemical reactions (like that of catalysts for chemical reactions in general) as a rnle faffs olf with time. The degree and rate of this decline depend on a large nnmber of factors the catalyst type, its method of preparation, its working conditions (composition of the electrolyte solution, temperature. 

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

The adsorption and accumulation of various impurities from the electrolyte or surrounding atmosphere on the catalyst surface. The rate of accumulation of impurities on the catalyst surface depends on its activity for adsorption, which often is parallel to its catalytic activity. 

In principle, such N4 compounds in their heat-treated version, as weU as chalcogenids, could be used in the cathodes of aU types of low- or medium-temperature fuel cells (PEMFCs, DLFCs, PAFCs, and AFCs). However, when such catalysts were used not in liquid electrolyte solutions bnt in contact with ion-exchange membranes, certain difficulties were seen. Up to now, most manufacturers of commercialized fuel cells of these types prefer to use the more reliable platinum catalysts. 

In a review by Wang (2005), detailed data can be found for different types of ORR catalysts without platinum, both for the variety with other noble metals and that without noble metals. 

In alkaline solutions, bifunctional properties are exhibited by catalysts having the pyrochlore structure A2B2O7, where A = Pb, Bi and B = Ru, Ir (Horowitz et al., 1983), and by oxide catalysts having the perovskite structure (e.g., Lao,6Cao,4Co03) (Wu et al., 2003). The properties of bifunctional oxygen electrodes are discussed in greater detail in a paper by Jorissen (2006). 

As a rule, the activity of catalysts for electrochemical reactions (like that of catalysts for chemical reactions in general) falls off with time. The degree and 

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The stability of electrocatalysts for PEMFCs is increasingly a key topic as commercial applications become nearer. The DoE has set challenging near-term durability targets for fuel cell technology (automotive 5,000 h by 2010 stationary 40,000 h by 2011) and has detailed the contribution of the (cathode) catalyst to these. In particular, for automotive systems as well as steady-state stability, activity after simulated drive cycles and start-stop transients has been considered. In practice, both these treatments have been found to lead to severe degradation of the standard state-of-the-art Pt/C catalyst, as detailed next. 

There is still no consistent definition about the catalyst s stability in literature. The stability of electrocatalyst may be defined as the percentage loss of electrocatalytic activity (or electro-catalytic current density) after a certain period of electrocatalytic reaction ... 

In general, both the catalyst s catalytic activity and stability are strongly dependent on the catalyst material s chemical and physical properties, composition, morphology, and structure. For ORR catalysis, in order to improve both the activity and stability of electrocatalysts, various Wnds of materials including noble metals, non-noble metals, metal oxides, chalcogenides, metal... 

Two main phenomena have been identified to infiuence the long-term stability of electrocatalysts catalyst dissolution and Ostwald ripening. 

Dassenoy F, Vogel W, Alonso-Vante N (2002) Structural Studies and Stability of Cluster-fike R%Sey Electrocatalysts. JPhys Chem B 106 12152-12157... 

Zhang J, Sasaki K, Sutter E, Adzic RR. 2007b. Stabilization of platinum oxygen reduction electrocatalysts using gold clusters. Science 315 220-222. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

Shi C, Mak KW, Chan K-S, Anson EC. 1995. Enhancement by surfactants of the activity and stability of iridium octaethyl porphyrin as an electrocatalyst for the four-electron reduction of dioxygen. J Electioanal Chem 397 321. 

Only a limited number of true metal complex electrocatalysts have been proposed for proton reduction due to the difficulty inherent in the bielectronic nature of this reaction. It is obvious that the design of such electrocatalysts must take into account the lowering of the overpotential for proton reduction, the stability of the catalytic system, and the regeneration of the starting complex. 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

Phosphoric Acid Fuel Cell (PAFC) Phosphoric acid concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt. 

Investigations of enzyme-catalyzed direct electron transfer introduce the basis for a future generation of electrocatalysts based on enzyme mimics. This avenue could offer new methods of synthesis for nonprecious metal electrocatalysts, based on nano-structured (for example, sol—gel-derived) molecular imprints from a biological catalyst (enzyme) with pronounced and, in some cases, unique electrocatalytic properties. Computational approaches to the study of transition state stabilization by biocatalysts has led to the concept of theozymes . " ... 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

Zhang, J., K. Sasaki, E. Sutter, and R. R. Adzic. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. Sci-... 

Structural and compositional characterization of individual elements of a combinatorial library can be important for the initial validation of a particular combinatorial synthesis method. Many earlier reports on combinatorial synthesis and screening of electrocatalysts fall short of reporting the complete structural and compositional characterization of individual library elements of interest. The workflow described here includes catalyst characterization before and after screening, thereby establishing an activity-composition-structure-stability relationship for electrocatalysts. This can be relevant in light of the extreme conditions present in a conventional fuel cell environment. 

It became obvious that long-term stability of high surface area electrocatalysts was as important, or even more important than short-term activity. Luczak36 and Landsman pioneered the heat treatment of ternary alloy electrocatalysts in order to provide an ordered crystallite structure. This work was followed in Japan by Itoh and Katoh, and subsequently by... 

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