Electrodes catalyst aging

Regarding catalyst aging. Darling and Meyers have proposed a mechanistic model, based on empirical parameters, of the Pt oxidation/dissolution in a PEMFC cathode, largely cited by experimentalists in subsequent papers. By using classical Butler-Volmer equations written in terms of the CL electrode potential and empirical parameters (e.g. symmetry factors, zero-exchange current and... 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

Hie samples were prepared fixim tetraethyiorthosilicate, phosphoric ackl (85%) and hydrated aluminum chloride or nitrate. TEOS is first hydrolyzed by water in ethanol solution, with HCl as a catalyst. An aqueous solution of the aluminum salt and phosphoric acid is then carefiilly added. That clear acidic solution is injected at the base of a reactor containing an ammonia solution kept at 0 C and pH 8 by a pump coupled to a pH electrode. The gel is left for ageing in the mother solution at pH = 8 under slight stirring, washed with water and isopropanol and finally dried by water exchange in isopropanol. 

For the simulation of the degradation, the model accounts for the numerical feedback between the sub-models describing the non-aging mechanisms (e.g., water transport across the porous electrode) and the sub-models describing the aging mechanisms (e.g., carbon corrosion or catalyst dissolution in the case of PEMFCs) (Fig. 14). At each time step of the simulation, the performance part of the model calculates the... 

A fuel cell decays with time, and the rate of decay determines its durability. The decay is related to the aging of the fuel cell components, especially the membrane electrolyte, the catalysts, and the catalyst support. The decay of the membrane will cause its thinning and mechanical property deterioration. The loss of its mechanical properties often causes a fuel cell to fail prematurely and catastrophically. The decay of the catalyst is normally due to the particle size increase and the particle dissolution and redistribution. Catalyst decay rarely causes a sudden failure of a cell. The decay of the catalyst-support is often related to its corrosion. Corrosion makes the electrode more prone to flooding and accelerates the growth and redistribution of the catalyst particles. 

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