Metal-based fuel cell

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

In a low-temperature fuel cell, hydrogen gas is oxidized into protons, electrons, and other by-products when other fuels are used at the anode. At the cathode of the fuel cell, the oxygen is reduced, leading to formation of water. Both the anodic and cathodic reactions require electrocatalysts to reduce the overpotentials and increase reaction rates. In the state-of-the-art low-temperature fuel cells, Pt-based materials are used as the electrocatalysts for both the reactions however, the high cost and limited resources of this precious metal are hindering the commercialization of fuel cells. Recent efforts have focused on the discovery of electrocatalysts with little or no Pt for oxygen reduction reaction (ORR) [1-3]. 

The reaction increases the Hj content of syngas, CO + Hj mixtures, and can be used in conjunction with the Fischer-Tropsch process. There has been renewed interest in this reaction as a source of for fuel cells. Metal carbonyls of Fe, Ru and Rh, under either acidic or basic conditions, are effective homogeneous catalysts. Fachinetti and co-workers have investigated the speciation and reactivity of Rh CCOju " and RujCCOjjj under acidic conditions. These systems are complicated by reactions such as disproportionation and complexation of the catalyst by the conjugate base of the acid. 

Various materials have been studied as catalysts for anion-exchange membrane fuel cells, metal-air cells, etc. These electrocatalysts which were studied and developed for alkali-based fuel cells are shown in Table 1. 

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

Conversely, the oxygen reduction reaction (ORR) has also been widely studied, in particular for its requirement in proton exchange membrane (PEM) fuel cells. Platinum based catalysts are typically used to increase the sluggish kinetics, although other noble metal catalysts and carbon... 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Solid oxide fuel cells consist of solid electrolytes held between metallic or oxide elecU odes. The most successful fuel cell utilizing an oxide electrolyte to date employs Zr02 containing a few mole per cent of yttrium oxide, which operates in tire temperature range 1100-1300 K. Other electrolytes based... 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Binary systems of ruthenium sulfide or selenide nanoparticles (RujcSy, RujcSey) are considered as the state-of-the-art ORR electrocatalysts in the class of non-Chevrel amorphous transition metal chalcogenides. Notably, in contrast to pyrite-type MS2 varieties (typically RUS2) utilized in industrial catalysis as effective cathodes for the molecular oxygen reduction in acid medium, these Ru-based cluster materials exhibit a fairly robust activity even in high methanol content environments of fuel cells. 

Recent testing in phosphoric acid fuel cells has shown improved performance using promoted Ft on carbon catalysts in the air cathode. The promoters are oxides of the base transition metals, e.g., Ti (O,... 

High-temperature solid-oxide fuel cells (SOFCs). The working electrolyte is a solid electrolyte based on zirconium dioxide doped with oxides of yttrium and other metals the working temperatures are 800 to 1000°C. Experimental plants with a power of up to lOOkW have been built with such systems in the United States and Japan. 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

Lee K, Savadogo O, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. 2006. Methanol-tolerant oxygen reduction electrocatalysts based on Pd-3D transition metal alloys for direct methanol fuel cells. J Electrochem Soc 153 A20-A24. 

Campbell S. 2006. Ballard Power System. Development of transition metal/chalcogen based cathode catalysts for PEM fuel cells. DOE Hydrogen Program Review, May 16-19, Washington, DC. Available at http //www.hydrogen.energy.gov/annual review06 fuelcells.html (click on catalysts section). 

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