Multifunctional oxidation electrocatalysts for direct alkaline fuel cells
The need for the development of new technologies to reduce our dependence on fossil fuels requires the combination of different energy sources such as wind, solar, nuclear as well as new energy storage and powering devices. Amongst these new technologies fuel cells are a promising technology capable of transforming chemical energy stored in fuels into electric power at higher efficiencies than combustion processes. However, the commercialization of fuel cells has been limited due to the high costs associated with electrocatalysts needed for the corrosive environments in which proton-exchange membrane fuel cells operate. Electrocatalysts for such fuel cells are based on expensive noble metals such as platinum. Nevertheless, the resurging interest on the development of alkaline fuel cells presents a number of advantages addressing the limitations of proton-exchange membrane fuel cells. Alkaline fuel cells operate at high pHs which allow the use of a wider variety of inexpensive and abundant materials such as transition and rare-earth metals. Moreover, faster kinetics have been reported in alkaline environments for both oxidation and reduction processes occurring at each of the electrodes. The discussion related to the first part of this dissertation focuses on the development of novel electrocatalysts for the oxidation of hydrazine for application in direct hydrazine alkaline fuel cells. Hydrazine is a carbon-free nontraditional fuel with high energy density (5kWh/kg), which is often considered a green fuel since its oxidation only produces nitrogen and water and does not contribute to the production of greenhouse gases emissions. It has been reported that transition metal catalysts such as Ni and Co demonstrate better performances than the commonly used Pt catalyst. Based on these preliminary findings, we have developed novel electrocatalysts with enhanced performance due to the addition of a second metal. α-NiZn electrocatalysts have shown improved performance due to an intrinsic effect cause by the alloying of an electron-dense atom such as Zn with Ni. Moreover, enhanced performance was also observed by the addition of a second phase, La(OH)3. La(OH)3 promotes the catalytic oxidation of hydrazine by providing oxygen species to the surface of the electrode for the dehydrogenation of hydrazine. Extensive ex situ characterization of materials using a number of different electron microscopy and X-ray spectroscopy techniques in combination with in situ electrochemical infrared studies provided insightful knowledge about the role of the components in the mechanism of the reaction. The knowledge gained from the studies performed for the development catalyst for hydrazine oxidation was applied to a more complex reaction, the electrooxidation of ethanol in alkaline media. Complex kinetics have been reported for the oxidation reaction of ethanol resulting in only a partial oxidation producing acetate. Highly active Pd/SnO2 catalysts were developed with three times the performance of Pd. Moreover, thorough understanding of the mechanisms of the ethanol reaction at different electrolyte concentrations was carried out using in situ infrared studies. Results show that better performances were obtained at 1 M KOH, but ethanol only partially oxidized to acetate. On the other hand, when the concentration of the electrolyte was reduced to 0.1 M KOH complete oxidation of ethanol to CO2 was observed. However, this resulted in higher overpotentials and lower rate constants. Mechanistic studies of reactions in both electrolytes concluded that higher concentrations of electrolyte allow for the oxidation of ethanol to occur at lower overpotentials due to the availability of hydroxide ions at the surface of the electrode, which participate in the oxidation of the adsorbed ethanol species. On the other hand, by the decreasing the concentration of electrolyte, diffusion of the hydroxide ions to the surface of the electrode is limited allowing the oxidation of ethanol to proceed to completion without desorbing the intermediate acetate product.
Electrocatalysts, Alkaline Fuel Cells; Proton exchange membrane fuel cells., Hydrazine--Oxidation., Ethanol--Oxidation., Metal catalysts., Electrocatalysis.
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