Synthesis and Electrocatalytic Properties of Structure Engineered First-Row Transition Metal Derivatives

Abstract

Hydrogen is a green energy carrier, producing only water when combusted, and a hydrogen economy has been considered the ideal green economy for human society. Water electrolysis can produce high-purity hydrogen on a large scale, and if the electricity used in water electrolysis is obtained from renewable energy, a sustainable energy chain can be achieved. Fuel cell technology offers a highly efficient way of converting chemical energy from a fuel into electricity through an electrochemical reaction. Fuel cells are expected to be one of the mainstream energy conversion devices for many applications such as the transportation and portable electronic systems. Hydrogen fuel cell technology is, of course, the ideal choice. However, the hydrogen storage is still a big challenge due to its gaseous nature, extremely low boiling point, and high inflammability. While advanced hydrogen storage technology is under development, fuel cells using liquid fuels (e.g. hydrazine) need to be developed. The key to both water electrolysis and fuel cells is the electrocatalyst. Currently, the noble metal based materials are still the state-of-the-art electrocatalysts for water electrolysis and in fuel cells in terms of catalytic activity and catalyst durability. However, their scarcity and high price hinder their widespread commercial use. Therefore, it is imperative to develop earth-abundant, low-cost electrocatalyst materials that have high catalytic activity comparable to or even better than the noble metal based electrocatalysts. Nowadays, the research emphasis of earth-abundant electrocatalysts is thus primarily placed on enhancing the catalytic activity or lowering the overpotential that is needed to drive the electrochemical reactions. The catalytic performance of an electrocatalyst is associated with its surface area, near-surface structure, electronic structure, conductivity, crystal size, etc. Rational structural modification of the electrocatalyst materials and/or architectural design of the catalyst electrodes can help enlarge the surface area, increase the active sites, tune the electronic structure and conductivity, and so on. In this dissertation, a series of strategies (e.g. hydrogenation, solvothermal reduction, and electrochemical tuning) have been developed to fabricate structure-tuned electrocatalyst materials for electrochemical water splitting and electro-oxidation of hydrazine. Well-defined Co/Co₃O₄ and Co/CoO core-shell heterostructures have been found to be highly active towards hydrogen evolution reaction (HER) and hydrazine oxidation, respectively. FeNi₃/NiFeOₓ nanohybrids have been thoroughly characterized for HER and oxygen evolution reaction (OER). Nano-on-micro Cu has been explored as a highly efficient catalyst towards electro-oxidation of hydrazine. Cobalt hydroxide carbonate with rich grain boundaries has been shown to be a highly efficient non-metallic electrocatalyst towards hydrazine oxidation.