Shujie Hu

Type

Text

Type

Dissertation

Advisor

Khalifah, Peter G | Koch, Stephen | Wong, Stanislaus.

Date

2015-05-01

Keywords

Electrochemistry, ORR, structure | Chemistry

Department

Department of Chemistry.

Language

en_US

Source

This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree.

Identifier

http://hdl.handle.net/11401/77072

Publisher

The Graduate School, Stony Brook University: Stony Brook, NY.

Format

application/pdf

Abstract

Hydrogen fuel cells are a promising energy conversion technology because they are capable of efficiently producing electricity from fuels while avoiding the production of environmental pollution when clean fuels such as hydrogen gas are used. However, Pt and other noble metals that are traditionally used as electrocatalysts for hydrogen fuel cells are scarce and expensive, and is a major factor limiting the commercialization of fuel cells. Therefore, we have searched for new non-noble metal cathode catalysts for the oxygen reduction reaction (ORR) of fuel cells. Three classes of materials have been investigated: the Nb-substituted titanates La2(Ti1 xNbx)2O7 with a 110-perovskite related structure, the Ti-vacancy phases La5Ti4O15 and La9Ti7O27 with 111-perovskite related structures, and a solid solution of magnesium molybdenum oxynitrides, MgxMo1 xOyNz,with rock salt related structures. It was determined that the solubility limit for Nb in La2(Ti1 xNbx)2O7 occurred for x ~ 0.1. However, neither this phase nor the Ti-vacancy phases La5Ti4O15 and La9Ti7O27 showed measureable activity for the ORR, though it is noted that these compounds may be suitable supports for other ORR electrocatalysts due to the conductivity derived from their partially-reduced Ti cations. In contrast, the magnesium molybdenum oxynitrides showed modest ORR activity in acidic conditions, and good activity in basic conditions when prepared on a carbon support (MgxMo1 xOyNz/C). The mechanisms of these catalysts were investigated through electrochemical techniques, while a detailed structural investigation of MgxMo1 xOyNz compounds was carried out through the co-refinement of X-ray and neutron diffraction data studies of these nanoscale materials. The optimum synthetic temperature for maximum activity was determined for a variety of substitution levels. The best overall activity in both acidic and basic electrolytes was observed for Mg0.25Mo0.75OyNz/C synthesized at 700 °C. | Hydrogen fuel cells are a promising energy conversion technology because they are capable of efficiently producing electricity from fuels while avoiding the production of environmental pollution when clean fuels such as hydrogen gas are used. However, Pt and other noble metals that are traditionally used as electrocatalysts for hydrogen fuel cells are scarce and expensive, and is a major factor limiting the commercialization of fuel cells. Therefore, we have searched for new non-noble metal cathode catalysts for the oxygen reduction reaction (ORR) of fuel cells. Three classes of materials have been investigated: the Nb-substituted titanates La2(Ti1 xNbx)2O7 with a 110-perovskite related structure, the Ti-vacancy phases La5Ti4O15 and La9Ti7O27 with 111-perovskite related structures, and a solid solution of magnesium molybdenum oxynitrides, MgxMo1 xOyNz,with rock salt related structures. It was determined that the solubility limit for Nb in La2(Ti1 xNbx)2O7 occurred for x ~ 0.1. However, neither this phase nor the Ti-vacancy phases La5Ti4O15 and La9Ti7O27 showed measureable activity for the ORR, though it is noted that these compounds may be suitable supports for other ORR electrocatalysts due to the conductivity derived from their partially-reduced Ti cations. In contrast, the magnesium molybdenum oxynitrides showed modest ORR activity in acidic conditions, and good activity in basic conditions when prepared on a carbon support (MgxMo1 xOyNz/C). The mechanisms of these catalysts were investigated through electrochemical techniques, while a detailed structural investigation of MgxMo1 xOyNz compounds was carried out through the co-refinement of X-ray and neutron diffraction data studies of these nanoscale materials. The optimum synthetic temperature for maximum activity was determined for a variety of substitution levels. The best overall activity in both acidic and basic electrolytes was observed for Mg0.25Mo0.75OyNz/C synthesized at 700 °C. | 162 pages

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