Perovskite oxide derivatives for enhanced electrochemical water splitting and pseudocapacitor applications.

Author

Md. Sofiul Alom, University of LouisvilleFollow

Date on Master's Thesis/Doctoral Dissertation

12-2023

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

Zamborini, Francis

Committee Member

Sumanasekera, Gamini

Author's Keywords

Water splitting; pseudocapacitance; perovskite oxide; material synthesis; hydrogen evolution reaction (HER); oxygen evolution reaction (OER)

Abstract

The global energy landscape is at a crossroads, marked by surging demand, finite fossil fuel reserves, and escalating environmental concerns stemming from carbon emissions. To address these challenges and transition towards a sustainable energy future, this dissertation embarks on a multidisciplinary exploration of perovskite oxides and their derivatives as catalysts and materials for advanced electrochemical water splitting and pseudocapacitive energy storage. In the quest for efficient water splitting catalysts, a series of quasi-2D oxides, SrLaAl_1/2M_1/2O₄ (M = Mn, Fe, Co), was synthesized and systematically studied where the B-site comprised of both transition metals and main group metals. These materials, characterized by 2-dimensional layers of octahedrally coordinated transition metals separated by alkaline-earth or rare-earth metals, exhibited promising ability to catalyze both OER and HER. Among them, SrLaAl_1/2Co_1/2O₄ emerged as a standout performer, exhibiting significantly reduced overpotentials for OER and HER, along with enhanced reaction kinetics. This superiority was attributed to a combination of factors, including stronger bond covalency driven by the higher electronegativity of Co, as well as the advantageous electronic configuration of trivalent cobalt. Remarkably, the correlation between electrocatalytic activity and electrical conductivity unveiled in these materials underscores their potential in sustainable energy applications. Moving forward, the exploration of layered oxides was refined to focus exclusively on materials comprising transition metals at the B-site, leading to improved performance. These materials revealed remarkable electrocatalytic activity for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Electrical charge-transport studies on SrLaFe_1-xCo_xO_4-δ, spanning a wide temperature range, unveiled their semiconducting behavior, with increasing electrical conductivity at higher temperatures. The apex of this series, SrLaCoO_4-δ, showcased superior electrical charge transport and electrocatalytic prowess for OER and HER. This bifunctional catalyst also featured significant polyhedral distortion and oxygen vacancies, which are responsible for the enhanced performance. Further efforts to advance electrocatalysts for green hydrogen generation led to the incorporation of oxygen vacancies in perovskite oxide framework. Detailed characterizations, including iodometric titration, X-ray diffraction, and X-ray photoelectron spectroscopy, verified the formation of oxygen vacancies. The oxygen-deficient material La₂MnCoO_6-δ (LaMn_0.5Co_0.5O_3-δ), isomorphic to its parent stoichiometric counterpart, exhibited a remarkable enhancement in electrocatalytic properties for both hydrogen-evolution and oxygen-evolution reactions, owing to the strategically introduced oxygen vacancies. Another instance of the creation of oxygen defects in perovskite oxides, particularly through a controlled reduction, demonstrated a dramatic improvement in electrocatalytic activity for both HER and OER. The resulting material, LaFe_0.5Ni_0.5O₃-R, sharing the same crystal structure as the parent compound, exhibited significantly reduced overpotentials, notably achieving an overpotential for OER comparable to precious metal catalysts like IrO₂. Furthermore, the mass activity and reaction kinetics witnessed substantial enhancement upon the introduction of oxygen vacancies, accompanied by a remarkable eight-fold increase in turnover frequency (TOF). These observations highlight the profound influence of oxygen-vacancy-induced changes in coordination number, aiding reaction intermediate adsorption, and charge-transport properties, enabling facile electron transfer for both HER and OER. In addition to electrochemical water splitting, the research extends to pseudocapacitive energy storage, where three materials from a family of layered oxides are explored. These materials, featuring 2-dimensional layers of transition metals separated by alkaline-earth or rare-earth metals, demonstrate pseudocapacitive charge-storage properties. Systematic trends reveal enhanced charge-storage properties in relation to Co-concentration, structural distortion, and oxygen-deficiency in SrLaFe_1-xCo_xO_4-δ. Symmetric pseudocapacitor cells based on these materials exhibit substantial specific capacitance, energy density, and power density, coupled with remarkable stability even after 1000 charge-discharge cycles, emphasizing the potential of these layered oxides for energy storage applications. In summary, this comprehensive dissertation underscores the pivotal role of perovskite oxides and their derivatives in advancing sustainable energy solutions. The multifaceted investigations into electrochemical water splitting and pseudocapacitive energy storage offer valuable insights and open new avenues for the development of efficient and environmentally friendly energy technologies.

Recommended Citation

Alom, Md. Sofiul, "Perovskite oxide derivatives for enhanced electrochemical water splitting and pseudocapacitor applications." (2023). Electronic Theses and Dissertations. Paper 4210.
https://doi.org/10.18297/etd/4210