Date on Master's Thesis/Doctoral Dissertation

5-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Ramezanipour, Farshid

Committee Co-Chair (if applicable)

Buchanan, Robert

Committee Member

Buchanan, Robert

Committee Member

Liu, Jinjun

Committee Member

Sumanasekera, Gamini

Author's Keywords

Electrocatalyst; materials; conductivity; energy; magnetism; water splitting

Abstract

Ruddlesden Popper (RP) oxides are perovskite-derived functional materials with the general formula An-1Aʹ2BnO3n+1, where A/Aʹ is often a lanthanide or alkaline earth metal, and B is usually a transition metal. These materials contain perovskite-like connectivity, where BO6 units share apexes to form layers that are stacked above each other. The number of layers in each stack is represented by n in the above formula. The spaces between stacks are often occupied by lanthanide or alkaline-earth metals (Aʹ-site metals), which also reside in intra-stack spaces (A-site) between the octahedra. Oxide materials derived from perovskites have been studied for a wide range of applications, such as solar cells, batteries, catalysts, and capacitors. We have studied the electrocatalytic properties of RP materials for water splitting. This is an important application given the need for efficient and economical electrocatalysts for the two half-reactions of water-splitting, namely hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER). The benchmark catalysts to overcome the sluggish kinetics of OER and HER are Ru, Ir, and Pt-based catalysts. Such catalysts are expensive

since they use precious metals, and some of them have stability issues. Owing to their compositional diversity and the thermodynamic stability, multi-element transition metal oxides are promising candidates for electrocatalytic water splitting. The variation of A and B-site metals in RP oxides can lead to changes in physical and chemical properties, such as crystal structure, magnetism, conductivity, and electrochemical catalysis. In this study, several bilayered (n = 2) RP oxides are synthesized followed by systematic study of magnetic, charge transport, pseudocapacitive, and electrocatalytic properties. The effect of structural symmetry on electrocatalytic properties in two isoelectronic materials, Sr2LaMn2O7 and Ca2LaMn2O7, which were synthesized by varying the A-site cation, was investigated. The structural changes identified are associated with systematic variation in the magnetic, electrical charge transport and electrocatalytic properties toward both components of water splitting, namely OER and HER. The effect of the B-site cation on electrocatalytic performance of two RP materials, Sr2LaFeMnO7 and Sr2LaCoMnO7 was also studied. Here, the variation in the B-site cation can result in significant changes in magnetic properties, charge-transport properties, and the activity toward the HER and OER processes. Together with the A/Aʹ and B-site changes of RP oxides, oxygen vacancies in these oxide materials strongly affect the properties including conductivity and electrocatalytic activity. Systematic trends in the series of materials, Sr3Ti2-xMxO7-δ (M = Mn, Fe, Co; x = 0, 1) were observed with the creation of oxygen vacancies which correlate with electrocatalytic and charge-transport properties.

The bilayered RP materials have been also explored to understand the oxide intercalation based pseudocapacitance. The change in pseudocapacitive properties as a function of structural symmetry in two materials Ca2LaMn2O7 and Sr2LaMn2O7 was studied in the alkaline medium. Overall, in the bilayered RP oxides, the electrical, electrochemical and electrocatalytic characteristics can be modified by changing the elemental composition, symmetry, and oxygen vacancies.

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