Development of novel anodic and cathodic materials applied in proton exchange membrane, direct methanol, alkaline and phosphoric acid fuel cells

• Online Access: http://hdl.handle.net/2047/d20000290

This dissertation summarizes the author's research effort to identify and synthesize novel electrocatalysts for application in proton exchange membrane fuel cells (PEMFCs), direct alcohol (acid and alkaline) fuel cells (DAFCs) and phosphoric acid fuel cells (PAFCs). Electrocatalysis enables modification of rates of electrochemical reactions to achieve maximum selectivity, yield and efficiency. It is an important chemical process during the operation of fuel cells. "Electrocatalysts" is a term in the field of Electrochemistry. In fuel cells, they are various metal-containing catalysts used to enhance the rates of the half reactions that comprise the fuel cell. The behavior of state-of-the-art Pt-based electrocatalysts highly depends on the composition (nominal composition, surface composition), structure, morphology, particle size, degree of alloying, and oxide content, among other properties. The principle of "rule-of-thumb" has been utilized for a few decades to synthesize electrocatalysts. In this work, a microemulsion method was fully studied and taken advantage to control particle size, catalyst morphology, and crystalline shape as well as to form catalyst layers. This method accelerates the conversion of new materials synthesis from "art" to "science". As a typical example, synthesis of carbon supported PtCo using microemulsions, including simultaneous and sequential reduction procedures in both acid and alkaline media was reported. As-prepared PtCo/C catalysts showed better performance towards oxygen reduction reaction in PEMFC than commercial Pt/C catalyst. In addition, a carbon-supported PtAu alloy core with a Ru shell (PtAu@Ru/C) catalyst was synthesized using a water-in-oil microemulsion method and heated at 220°C. It was found that gold cluster in the PtAu@Ru/C catalyst improve the stability of Pt and Ru significantly by interacting with Pt and Ru to raise their oxidation potential, which is a promising step towards resolving the problem of Ru dissolution for the practical application of PtRu/C catalyst in direct methanol fuel cells. Pt-based binary or ternary alloy (nano) materials are still dominant and irreplaceable electrocatalysts in the field of acid fuel cells. However, more choices are available for choosing materials as electrocatalysts for alkaline fuel cell. As a series of non-Pt materials, Pd based alloy nanoparticles were prepared by a chemical reduction method. Voltammetric and chronoamperometric measurements showed higher current density and longer-term stability for ethanol oxidation in high pH environments with palladium alloy nanocatalysts than with a commercial Pt/C catalyst. Overall, the Pd-based alloy catalysts represent promising candidates for the electrocatalytic oxidation of ethanol, and Pd4Au/C displays the best catalytic activity among them for the ethanol oxidation in alkaline media. Phosphoric acid fuel cells (PAFCs) have been commercialized successfully and used for stationary applications with a combined heat and power efficiency of about 80%. However, there is still a lot of room for improvements in this technology through further research and development. One major factor limiting the performance of PAFCs is the sluggish ORR kinetics in H3PO4, which is attributed chiefly to the impeding effect of phosphate ion adsorption on ORR activity. In this study, a Pt based Ni alloy catalyst was synthesized in-house using components that may possess novel functions. Detailed electrochemical and X-ray absorption spectroscopy (XAS) investigations have been carried out on our electrocatalysts under in-situ conditions. Using the Δμ-XANES analysis, it was found that despite being smaller than the Pt/C (E-TEK) catalysts, the Pt-Ni/C catalysts are less susceptible to PO43- anion adsorption/poisoning. To conclude, owing to lower susceptibility to poisoning by PO43- ions, the Pt-Ni catalysts can be expected to perform better than Pt/C (E-TEK) catalysts in a PAFC.