Cathode electrocatalyst

Grinberg VA, Pasynskii AA, Kulova TL, Maiorova NA, Skundin AM, Khazova OA, Law CG (2008) Tolerant-to-methanol cathodic electrocatalysts based on organometallic clusters. Russ J Electrochem 44 187-197... 

Fang, B., et al., High Pt loading on functionalized multiwall carbon nanotubes as a highly efficient cathode electrocatalyst for proton exchange membrane fuel cells. Journal of Materials Chemistry, 2011. 21(22) p. 8066-8073. 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

This type of N-doped soot catalyst is of particular interest for the development of advanced fuel cells. As this type of catalyst is not poisoned by carbon monoxide, it is a promising candidate for O2 cathodes in methanolconsuming fuel cells (132). In methanol-combusting cells, diffusive transport of methanol from the anode to the cathode cannot be avoided, with the consequence that the activity of Pt-activated cathodes becomes severely impaired by CO poisoning of the Pt catalyst therefore, a CO-insensitive cathodic electrocatalyst seems to be indispensible. Yet the longevity of this type of catalyst is still in dispute (133). 

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. 

A cathodic electrocatalyst, which uses the protons and electrons produced in the anodic section for the electrocatalytic reduction of C02. 

The next advance in development of PAFC binary-alloy cathode electrocatalysts was the use of Pt-Cr alloys.27 In this patent, it was disclosed that with the platinum-vanadium alloy in 99 % phosphoric acid at 194 °C and at an electrode potential 0.9 volts, over 67 % by weight of the vanadium had dissolved in the first 36 hours. In the case of Pt-Cr, only 37 % had dissolved under the same conditions. It is not clear from the descriptions in these patents whether or not there is any unreacted vanadium or chromium present in the catalyst because it is not identified that all of the vanadium or all of the chromium was initially alloyed with the platinum. It is conceivable that significant amounts of the non-noble metal components are not fully reacted. 

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-... 

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. 

However, the performance of a Galvanic or electrolytic cell is not dictated only by the quality of its anodic and cathodic electrocatalysts but also by the resistance of the electrolyte between the two electrodes, which must be minimized, and by the rates of mass transport of reactants and products to and from the two electrodes. 

The electrochemical processes in the fuel cell take place at the interface between the dispersed anode and cathode electrocatalysts and hydrated ionomer electrolyte. In the hydrogen/air or reformate/air fuel cell, the processes at the anode and cathode, respectively, are as follows ... 

Figure 5b. Performance of MEAs with 60 wt.% Pt/ Carbon Cathode Electrocatalyst at 0.8 V...

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

Platinum was the initial choice for both the anodic and cathodic electrocatalysts for the following reasons (1) platinum shows minimum degradation or corrosion in acid or when used as an anodic or cathodic electrocatalyst (2) recent technical advances have been made in forming efficient high surface area porous platinum electrocatalyst structures at the surface of PEM membranes and (3) preliminary work... 

Tsivadze AY, Tarasevich MR, Kuzov AV, Romanova lA, Pripadchev DA (2008) New nanosized cathode electrocatalysts tolerant to ethanol. Doklady Phys Chem 421 166-169... 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

The relationship between metal prices and relative abundance of the chemical elements in the Earth s upper continental crust is shown in Fig. 1. One can observe that Pt, being one of the rarest metals is the most expensive one. Its finite deposits are concentrated in just a few countries. Alone the high and volatile price of this metal could be a potential showstopper for fuel cells in any tmly mass market application such as transportation. Hence, in order to develop and commercialize fuel cells, it is necessary and challenging to partially reduce or completely replace Pt as the cathode electrocatalyst using non-precious metals such as cobalt, iron, molybdenum, tungsten, etc. 

Fang, B., Kim, J.H., Yu, J.S. Colloid-imprinted carbon with superb nanostructure as an efBcient cathode electrocatalyst support in proton exchange membrane fuel cell. Electrochem. Commun. 10(4), 659-662 (2008)... 

Palladium-alloy materials have been recently introduced as a promising ORR cathode electrocatalyst for replacing Pt [91-98], Pd alloys are considerably less expensive than Pt. Besides, experimental studies indicate that they have high methanol tolerance for direct methanol fuel cells (DMFCs) in which the methanol crossover to the cathode significantly decreases the cell s efficiency [99, 100]. 

Yuan XX, Zeng X, Zhang HI, Ma ZF, Wang CY (2010) Improved performance of proton exchange membrane fuel cells with p-toluenesulfonic acid-doped Co-PPy/C as cathode electrocatalyst. J Am Chem Soc 132(6) 1754—1755... 

Ohnishi R, Takahashi Y, Takagaki A, Kubota J, Domen K (2008) Niobium oxides as cathode electrocatalysts for platinum-free polymer electrolyte fuel cells. Chem Lett 37 838-839... 

Non-Pt Cathode Electrocatalysts for Anion-Exchange-Membrane Fuel Cells... 

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