Ian Graham

(Advisor: Prof. Lauren Garten)

Will propose a doctoral thesis entitled,

Developing Piezoelectric BaNiO3-x Thin Films to Promote the Catalytic Activity in the Oxygen Evolution Reaction

on

Friday, November 22 at 2:00 p.m. (EST)

Parker H. Petit Institute of Bioengineering and Bioscience (IBB) Suddath Seminar Room 1128

and

Virtually via Teams

Committee

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

Abstract

Barium nickelate (BaNiO3-x, BNO) has been shown to have an order of magnitude higher catalytic activity than one of the current state-of-the-art oxygen evolution reaction (OER) catalysts, iridium oxide (IrO2). However, the overpotential needed for OER must be reduced to increase the energy efficiency of the production of fuel from the electrolysis of water. The goal of this proposed thesis is to investigate the role of oxygen vacancies, crystal structure, and piezoelectricity in BNO on the reduction of the overpotential needed for OER catalysis.

The first principal objective of this thesis is to determine how to control the phase formation pathway of sol-gel synthesized BNO. Previous literature shows that OER catalytic activity is increased when the eg orbital filling of the transition metal ion is near unity which would occur in BNO if the average Ni oxidation was reduced to Ni3+. BNO can accommodate a wide range Ni oxidation states by varying the concentration of oxygen vacancies, but the associated change in crystal structure with oxygen vacancy concentrations is not yet established. I hypothesize that the addition of a reductive species during the calcination step will allow for the selection of different phases of BNO, because of the variable oxidation state of Ni within each phase of BNO. We will investigate the impact of oxygen vacancy concentration on the stoichiometry and crystal structure of BNO by adding reductive precursors, increasing the temperature, and decreasing oxygen partial pressure during calcination.

The second principal objective of this thesis is to stabilize oriented R32 BNO thin films via pulsed laser deposition (PLD). The piezoelectric R32 phase of BNO is particularly interesting because the presence of piezoelectricity in other materials has shown increases in the catalytic activity by enhancing adsorption and desorption and by increasing the amount of charge carriers at the surface that can readily participate in redox reactions. The growth of oriented thin films of the R32 phase of BNO is essential in determining the magnitude of the piezoelectric coefficients and piezocatalytic coupling. I hypothesize that decreasing oxygen activity during deposition will stabilize the metastable piezoelectric R32 phase of BNO because oxygen vacancies are accommodated in the R32 structure by the presence of three unique Ni sites. We will determine the processing window for the R32 phase of BNO by varying temperature, oxygen partial pressure, and laser fluence and determine the effect these processing parameters have on the crystal structure, orientation, and stoichiometry during thin film growth. Once we have grown oriented thin films, we will then measure the piezoelectric coefficient of the R32 phase of BNO by using an e31,f wafer flexure method.

       Finally, the last principal objective of this thesis is to determine the effect that crystal structure, stoichiometry, and Ni oxidation state have on the catalytic activity of BNO for OER catalysis. I hypothesize that by controlling the concentration of oxygen vacancies to target an eg orbital filling near unity in a non-centrosymmetric crystal structure, then the catalytic activity of BNO will increase because the O p-band center will shift toward the Fermi energy. We will determine the effect of piezoelectricity and eg orbital filling on the catalytic activity in BNO by measuring the overpotential required to maintain a current density of 10 mA/cm2 under different mechanical stresses (i.e. various magnitudes of piezopotential). In summary, this work will give insight into the BNO phase diagram by determining the crystal structure and associated stoichiometries that are allowed in various temperature and pO2 ranges. This work will also provide insight into ways to increase the OER activity of ABO3 type catalysts.