Ian C. Graham

(Advisor: Prof. Lauren Garten)

Will defend a doctoral thesis entitled,

Oxygen Stoichiometry and Phase Stability of Barium Nickelate Oxides for Energy and Catalytic Applications

on

Friday, September 11 at 12 p.m. (EDT)

Kendeda Building Room 210

and 

Virtually via Zoom

Committee

• Prof. Lauren Garten - School of Materials Science and Engineering (advisor)
• Prof. Angus Wilkinson - School of Materials Science and Engineering and School of Chemistry and Biochemistry
• Prof. Mark Losego - School of Materials Science and Engineering
• Prof. Faisal Alamgir - School of Materials Science and Engineering
• Prof. Marta Hatzell - School of Mechanical Engineering and School of Chemical and Biomolecular Engineering

Abstract

Rising global energy demands and the urgent need to reduce greenhouse gas emissions underscore the need for multifunctional materials capable of enabling heterogeneous catalysis, carbon capture, and piezoelectric applications. Barium nickelate (BaNiO3-x, BNO) offers a unique platform for exploring these functionalities owing to the redox flexibility of nickel (Ni2+ to Ni4+). Changes in nickel oxidation state are accompanied by a series of phase transitions spanning stoichiometries from BaNiO2 in the Cmcm space group to BaNiO3 in the P63/mmc space group. In addition to the centrosymmetric end members, BNO also forms in the non-centrosymmetric P63mc and R32 space groups, enabling the potential for piezoelectric functionality. Despite differing stoichiometries and crystal symmetries, BNO polymorphs retain common structural motifs with minimal changes in lattice parameters. The combination of Ni redox flexibility and the high degree of structural similarity among phases enables enhanced oxygen exchange, critical for oxygen-mediated applications such as thermochemical fuel production and oxygen pumping. Collectively, the compositional, structural, and redox flexibility of BNO establishes it as a compelling platform for studying multifunctional behavior in complex oxides. Therefore, this dissertation investigates the synthesis, phase stability, and functional behavior of BNO, with emphasis on oxygen storage, carbon capture, and the stabilization of non-centrosymmetric phases for future piezoelectric applications.

This work first advances the phase-stoichiometry relationship in BNO through a systematic exploration of the effects of calcination temperature and atmosphere on phase formation, stoichiometry, and morphology of sol-gel derived powders. Precise control of processing conditions enabled the selective synthesis of BNO powders exhibiting predominantly the P63mc, P63/mmc, or rare and unindexed h-BaNiO2.36 phase. Temperature was the dominant factor in determining oxygen stoichiometry, with compositions varying from BaNiO2.76 at 800 °C to BaNiO2.25 at 1000 °C. Powder morphology was governed by both temperature and oxygen flow rate, as increasing temperature transformed porous sponge morphologies to dendritic structures while increasing oxygen flow promoted agglomeration. Ultimately, we established a framework for selectively controlling the phase formation, stoichiometry, and morphology of BNO to tailor the material for a wide application space.   

Building upon the phase-stoichiometry relationship, pulsed laser deposition and epitaxial stabilization were used to synthesize metastable and non-centrosymmetric R32 BNO thin films. Through the modulation of substrate setpoint temperature and deposition oxygen partial pressure, we established that R32 BNO accommodates oxygen stoichiometries ranging from BaNiO2.26 to BaNiO2.14 through the stabilization of a mixture of Ni2+, Ni3+, and Ni4+ oxidation states. Variations in oxygen stoichiometry enabled the tuning of the electrical conductivity of R32 BNO from 0.137 to 0.161 S/m. Taken collectively, these findings establish a foundation for future studies of piezoelectricity in BNO. 

The redox flexibility of Ni and the structural similarity among BNO polymorphs motivated an investigation of its redox mechanisms and oxygen storage capacity. This work reveals that oxygen exchange in BNO proceeds through reversible exsolution and reincorporation of NiO, rather than solely through the Ni4+/Ni2+ redox couple. Here, we establish the oxygen storage capacity of BNO at 5,027 µmol O/gabsorbent, exceeding the theoretical capacity associated with the BaNiO2 to BaNiO3 transition by 14%, with the excess capacity arising from reversible NiO exsolution and reincorporation. These findings establish BNO as a new benchmark oxygen storage material and identify reversible metal oxide exsolution as a promising strategy for enhancing the oxygen storage performance of next-generation oxygen storage materials. 

Finally, reversible NiO exsolution was leveraged for high-temperature carbon capture. Through the exsolution of NiO and formation of the Ba-rich h-BaNiO2.36 phase, BNO exhibited substantial CO2 uptake via the formation of BaCO3. The thermal stability of BaCO3 enables carbon capture up to 1100 °C, enabling carbon capture at temperatures relevant to industrial exhaust streams while minimizing the need for extensive flue gas cooling. Collectively, this dissertation advances the fundamental understanding of structure, processing, and functionality within BNO and establishes a framework for its application across a range of energy and environmental technologies.