Cesium and rubidium salts of Keggin-type heteropolyacids as stable meso-microporous matrix for anode catalyst for H2/O2 Proton Exchange Membrane Fuel Cell, Direct Methanol Fuel Cell and Direct Ethanol Fuel Cell

In various applications, fuel cells are widely recognized as very attractive devices to obtain directly electric energy from the electrochemical combustion of chemical products. Low temperature fuel cells, which generally utilize proton electrolyte membranes, seem to be also of utility for a large of power applications. However, the final choice of the fuel is still difficult and depends greatly on the field of application. Utilization of pure hydrogen or hydrogen-rich gases, rather than alcohols, as fuels in Polymer Electrolyte Membrane Fuel Cell (PEMFC) leads to higher electric efficiencies. Due to the problems related to the hydrogen storage, the hydrogen-oxygen PEMFC is the best choice for stationary applications. Direct alcohols (methanol and ethanol) fuel cells based on solid polymer electrolytes are widely proposed for portable and mobile applications due to relatively low prices of the methanol and ethanol fuels and easiness of storage. The major impediment in the development of fuel cells operating on hydrogen, methanol, and ethanol is deactivation of the anode electrocatalyst by trace level of CO. Thus, the most active Pt electrocatalyst for the oxidation of hydrogen and alcohols (e.g. methanol, and ethanol) is deactivated by strong adsorption of carbon monoxide, and leads to decreased performance of fuel cell. Therefore, a new inexpensive, stable electrocatalyst must be developed, which is tolerant to high levels of CO (particularly, for direct alcohol fuel cell) or that could preferably utilize CO. There are currently two state-of-the-art methods which increase CO tolerance of the fuel cell anode. One method is to use a Pt-alloy catalyst (e. g. PtRu, PtSn, PtMo,PtW). The other method is to use the zeolite as matrix, and limit the preferential formation of CO clusters on platinum by the steric constraints imposed by the zeolites framework, followed by facile oxidation to CO2 by interaction with the surface or bridged hydroxyls of the zeolites. Furthermore, the porous nature of the zeolite support material provides relatively improved gas permeability and minimizes the disadvantage associated with restricted gas diffusion in the electrode. The ideal support would also an enhanced electrochemically active surface area by dispersion of the metal catalyst. Keggin-type heteropolyacids show appreciable acid catalytic properties that are of practical importance as illustrated in numerous recent reviews. The fact that heteropolyanions undergo spontaneous adsorption (from aqueous solution) on various substrates provides a simple tool for modification of electrode surfaces. Among other important issues related to electrocatalysis are their ability to undergo fast reversible multi-electron transfers and super-acid properties resulting in the increased availability and mobility of protons at the electrocatalytic interfaces. Consequently, polyacids were considered for fuel cell research. With respect to the oxygen reduction, the adsorbed heteropolyacid (particularly H3PW12O40) nanostructures do not seem to block access of reactant molecules to catalytic Pt. Further, their presence at the interface shifts formation of the inhibiting Pt-oxo (PtOH or PtO) species towards more positive potentials thus increasing the potential range where catalytic metallic (Pt0) sites exist. It is reasonable to expect that, by analogy to the activating role of WO3 and partially reduced hydrogen tungsten oxide bronzes during elect oxidation of methanol, the related heteropolyblue tungstates should also enhance reactivity of Pt during oxidation of organic fuels. Stanis and co-workers reported that the addition of adsorbed HPAs can improve the performance of Pt anodes in a fuel cell under CO-poisoned conditions. Also Farell et al. postulated that HPAs can act as cocatalysts with platinum for methanol electrooxidation. Among limitations in practical applications of heteropolyacids is their very good solubility in aqueous solutions including acids as well as their ability to undergo desorption during long-term operation. Thus it is necessary to stabilize heteropolyacid layers at electrocatalytic interfaces without loosing their activating properties. An interesting alternative arises from the possibility of formation of acidic salts of heteropolytungstates or molybdates by partial exchange of protons with large cations (such as Cs+, Rb+, NH4+ or K+) in the parent heteropolyacid. Consequently, a watersoluble polyacid of low surface area ( minus 5 m2g-1) is transferred into a water-insoluble acid salt precipitate characterized by the surface area exceeding 100 m2g-1. Contrary to zeolites exchanged with alkali metals, heteropolyacid salts remain strong acidity. The resulting materials have occurred to be efficient solid shape-selective acid catalysts for a variety of organic liquid-phase reactions that include hydrogenations and oxidations. The pore size and acidity of heteropolyacid salt can be controlled by the cation content. For example, when the Cs content, x, is initially increased in CsxH3- xPW12O40 from 0 to 2, the number of surface protons decreases but, later, it significantly increases when x changes from 2 to 3 to show the highest surface acidity at x = 2.5. In the present work, we consider the salts of Keggin-type heteropolyacids containing 2.5 moles of Cs+ and Rb+ cations in 1 mole of the heteropoly salt. The system with such Cs or Rb-content is micro-mesoporous, and it is characterized by very good stability and, while being insoluble in water, it exhibits high acidity (proton availability and mobility). The aim of this work is to study the applicability of the Cs and Rb salts of Keggin type heteropolyacids as a stable meso-microporous matrix for anode catalyst for H2/O2 Proton Exchange Membrane Fuel Cell, Direct Methanol Fuel Cell and Direct Ethanol Fuel Cell. The experimental part is divided into four chapters (from 6 to 9). In the Chapter 6, we present characterization of cesium, rubidium and ammonium salts of Keggin types heteropolyacids by infrared spectroscopy (IR), scanning electron microscopy (SEM), and cyclic voltammetry (CV) measurements. In Chapter 7, we illustrate incorporation and activation of Pt centers in the conductive high surface-area zeolite-type robust, Cs2.5H0.5PW12O40 matrices through application of mixing and electrochemical methods. To evaluate electrocatalytic activity towards the hydrogen oxidation of the investigated electrode material, we have performed the diagnostic rotating disc electrode voltammetric measurements. In a case of the catalytic layer prepared by corrosion of Pt counter electrode (electrochemical method), CO-stripping and HRTEM measurements have been performed to comment the electrochemically active area of catalyst, dimensions of Pt particles and platinum loading. In Chapter 8, we report on the performance of electrocatalysts (prepared by mixing method) towards methanol oxidation in acidic media. The Cs and Rb salts of H3PW12O40, H3PMo12O40, H4SiW12O40, H4SiMo12O40 were used as zeolite matrix for Pt40%/Vulcan XC-72 carbon nanoparticles (Pt/C). The techniques of cyclic voltammetry, staircase voltammetry, chronoamperometry, electrochemical impedance spectroscopy, CO stripping voltammetry were applied to compare the Pt-base electrocatalyst activity and stability to methanol oxidation. In Chapter 9, the system composed of Pt40%/Vulcan XC-72 carbon modified with Cs2.5-HPAs matrix (prepared by mixing method) has been examined with respect to ethanol electrooxidation by several different electrochemical techniques. Comparison has been made to commercial Pt/C.