Electrocatalysts chemical stability

In a fuel cell, the electrocatalysts generate electrical power by reducing the oxygen at the cathode and oxidizing the fuel at the anode [1], Pt and Pt alloys are the most commonly used electrocatalysts in PEFCs due to their high catalytic activity and chemical stability [99-103]. 

Advances in electrochemical systems rest in large measure with the evolution of new materials that exhibit chemical stability in severe environments, high electrocatalytic activity, rapid ion conductivity, etc. Examples include RuOx-TiOy-Ti electrocatalysts, the polymer Nafion, yttrium-stabilized zirconate and beta-alumina electrolytes, and metastable alloys produced by rapid solidification processing. 

A chemically stable ORR electrocatalyst should not be oxidized or corroded by O2 or proton. However, some catalyst components such as the alloy metals such as Fe, Co, Ni, Mn, Cu, etc., in Pt alloy catalysts could be chemically oxidized by either proton or O2 to leach out. Some metal oxide supports and non-noble metal catalysts could also dissolve in acidic environment. Therefore, developing ORR catalysts and support materials with chemical stability is necessary. 

Oxide-based cathode catalysts are entirely new non-precious metal cathode catalysts for low-temperature fuel cells such as jxtlymer electrolyte fuel cells (PEFCs). These catalysts were developed from a viewpoint that high chemical stability was essentially required for the cathode for PEFCs. The cathode catalysts for PEFCs are exposed to an acidic and oxidative atmosphere, that is, a strong corrosive environment, therefore, even platinum nanoparticles dissolved during a long-time operation. This instability of electrocatalysts is one of the factors which hindered the wide commercialization of PEFCs. 

Of practical importance is the contribution that is made by carbonaceous materials as an additive to enhance the electronic conductivity of the positive and negative electrodes. In other electrode applications, carbon serves as the electrocatalyst for electrochemical reactions and/or the substrate on which an electrocatalyst is located. In addition, carbonaceous materials are fabricated into solid structures which serve as the bipolar separator or current collector. Clearly, carbon is an important material for aqueous-electrolyte batteries. It would be very difficult to identify a practical alternative to carbon-based materials in many of their battery applications. The attractive features of carbon in electrochemical applications are its high electrical conductivity, acceptable chemical stability, and low cost. These characteristics are important for the widespread acceptance of carbon in aqueous electrolyte batteries. 

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... 

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

The field of stability still lacks definite concepts for guiding the search for stabilizers. Nevertheless, efforts in this direction are many and positive improvements have been documented. This is the field of additives and dopants , and therefore of electronic interactions and chemical shifts. Most of the remedies thus far proposed are not known with certainty because applied to proprietary electrocatalysts. A careful investigation of their actions would be necessary. In fact, fundamental research is still mostly aimed at investigating electrocatalytic and mechanistic aspects. 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

Poly(amidoamine) dendrimers of the fourth generation with -OH terminal groups were used as templates to produce stable metal nanoparticles [170-172], The dendrimers (in aqueous solution) are first loaded with a predetermined amount of Cu + or Pt + metal ions following chemical reduction, metal nanoparticles are formed inside the dendrimer structure, where they are protected from agglomeration. This procedure permits both particle stability and control over particle size. Dendrimers containing Pt metal clusters were also attached to gold electrodes, and were found to be active as electrocatalysts for O2 reduction [172]. This demonstrates that the nanoparticles inside the dendrimer can mediate electron transfer processes between the electrode surface and reactants in solution. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

Fuel cells, due to their higher efficiency in the conversion of chemical into electrical energy vhth respect to thermo-mechanical cycles, are another major area of R D that has emerged in the last decade. Their effective use, ho vever, still requires an intense effort to develop ne v materials and catalysts. Many relevant contributions from catalysis (increase in efficiency of the chemical to electrical energy conversion and the stability of operations, reduce costs of electrocatalysts) are necessary to make a step for vard in the application of fuel cells out of niche areas. This objective also requires the development of efficient fuel cells fuelled directly vith non-toxic liquid chemicals (ethanol, in particular, but also other chemicals such as ethylene glycol are possible). Together vith improvement in other fuel cell components (membranes, in particular), ethanol direct fuel cells require the development of ne v more active and stable electrocatalysts. 

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... 

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