Advisor(s)

Sanjeev Mukerjee (1960-)

Contributor(s)

Eugene S. Smotkin, David (David Edward) Budil, Geoffrey Davies, Kuzhikalail M. Abraham

Date of Award

2011

Date Accepted

4-2011

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Science. Department of Chemistry and Chemical Biology.

Keywords

physical chemistry, chemistry, alkaline Medium, alkaline membrane fuel cell, electrocatalysis, oxygen reduction, renewable energy, specific adsorption

Subject Categories

Oxidation-reduction reaction, Electrocatalysis

Disciplines

Catalysis and Reaction Engineering | Chemistry

Abstract

Oxygen Reduction Reaction (ORR) is an overarching technological and fundamental challenge in the context of electrochemical energy conversion. Recent developments in alkaline ionomer membranes that transport hydroxide anions have spurred research interests in the Alkaline Membrane Fuel Cell (AMFC) technology. Among several challenges such as improving hydroxide anion conductivity and anodic alcohol oxidation, a key source of overpotential loss is the cathodic ORR process. In this scenario, the fundamental understanding and design of novel electrocatalysts for ORR assume a crucial role. While Pt-based materials have been the mainstay in acidic medium, the dispensability of the so called "acid-stability" criterion permits the use of several non-noble metal based electrocatalysts for ORR in alkaline medium. In this dissertation a combination of catalyst synthesis, electrochemical studies and X-ray absorption spectroscopy (XAS) investigations have been carried to understand ORR in alkaline medium. Such a study is expected to provide a detailed understanding of the oxygen reduction mechanisms, pathways and ultimately methods to design novel electrocatalysts.

Chapter 1 begins with a broad introduction to the global energy scenario, the niche position that electrochemical energy conversion enjoys and the fundamentals of electrocatalysis and XAS technique. Pt-based materials, the subject of Chapter 2, act as model systems to understand the mechanisms/pathways under more or less ideal conditions. Typically electrocatalytic reactions are treated as inner-sphere electron transfer processes whereas the possibilities of a 'surface-independent outer-sphere electron transfer component' step in the overall inner-sphere electrocatalytic process have not come to fore of the discussion. Such a scenario is observed during ORR in alkaline medium, where the specifically adsorbed hydroxide anions are found to mediate/promote outer-sphere electron transfer to solvated molecular oxygen.

Chapter 2 discusses ORR on chalcogen modified ruthenium nanoparticles (Ru/C, Se/Ru/C, Se/RuMo/C, S/Ru/C, S/RuMo/C) synthesized in-house via aqueous routes. This class of materials was chosen since the ability of chalcogen to suppress oxide formation on transition metals is likely to promote direct molecular adsorption of O2. Chapter 3 delves into the biomimetic materials such as iron porphyrin. Here, the origin of electrocatalytic activity of heat treated transition metal macrocycles has been studied using a combination of square wave voltammetry, X-ray absorption near edge spectroscopy (XANES) and Delta-Mu techniques. It is found that 1) an anodic shift in the metal center redox potential, 2) the stabilization of ferrous-hydroperoxyl adduct due to double-layer electrostatic interactions, and 3) the influence of atomic vacancy defects on graphitic carbon surfaces play key role in improving ORR activity upon heat treatment.

In Chapter 5, the durability of perfluorinated sulfonic acid proton exchange ionomer membranes are investigated under fuel cell operating conditions using a novel array-electrode assembly setup. Correlation of membrane degradation with the peroxide yield is obtained. Fenton type mechanism of peroxide radical generation from H2O2 formed due to two electron pathway of ORR is found to be the dominant membrane degrading factor.

Finally the concluding chapter presents the evaluation of electrocatalysts in Alkaline Membrane Fuel Cells (AMFC). The initial results are very promising and warrants further intense research. Importance of certain challenges such as electrocatalyst design, specific adsorption of quaternary ammonium cations, and study of alkaline anode-membrane double layer structure are pointed out for future directions.

Document Type

Dissertation

Rights Information

Copyright 2011

Rights Holder

Nagappan Ramaswamy


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