Habib Rostaghi Chalaki

Sluggish oxygen reduction reaction (ORR) remains a critical barrier to advancing intermediate-temperature electrochemical energy devices. Here, we demonstrate that strain engineering in two platforms, epitaxial thin films and freestanding membranes, systematically tunes ORR kinetics in Ruddlesden-Popper LaSrCoO₄. In epitaxial films, film thickness is varied to control in-plane tensile strain, whereas in freestanding membranes strain relaxation during the release step using water-soluble sacrificial layers produces flat or wrinkled architectures. Electrochemical impedance spectroscopy analysis reveals more than an order of magnitude increase in the oxygen surface exchange coefficient for tensile-strained films relative to relaxed films, together with a larger oxygen vacancy concentration. Wrinkled freestanding membranes provide a further increase in oxygen surface exchange kinetics and a lower activation energy, which are attributed to increased active surface area and local strain variation. These results identify epitaxial tensile strain and controlled wrinkling as practical design parameters for optimizing ORR activity in Ruddlesden-Popper oxides.

ABO₃ perovskite oxides are a versatile class of materials whose surfaces and interfaces play essential roles in sustainable energy technologies, including catalysis, solid oxide fuel and electrolysis cells, thermoelectrics, and energy-relevant oxide electronics. The interplay between point defects and surface reconstructions strongly affects interfacial stability, charge transport, and catalytic activity under operating conditions. This review summarizes recent progress in understanding how oxygen vacancies, cation nonstoichiometry, and electronic defects couple to atomic-scale surface rearrangements in representative perovskite systems. We first revisit Tasker’s classification of ionic surfaces and clarify how defect chemistry provides compensation mechanisms that stabilize otherwise polar or metastable terminations. We then discuss experimental and theoretical insights into defect-mediated reconstructions on perovskite surfaces and how they influence the performance of energy conversion devices. Finally, we conclude with a perspective on design strategies that leverage defect engineering and surface control to enhance functionality in energy applications, aiming to connect fundamental surface science with practical materials solutions for the transition to sustainable energy.

A deterministic platform for engineering epitaxial strain in CaMnO_3-δ (CMO) thermoelectric thin films is demonstrated using pulsed laser deposition, enabling precise control of the interplay between strain state and oxygen vacancy formation. High-quality epitaxial CMO films are grown on four different single crystalline substrates, which impose fully relaxed, partially relaxed, low tensile, and high tensile strain states, respectively. Increasing tensile strain induces a monotonic expansion of the unit cell volume and a systematic rise in oxygen vacancy concentration. Oxygen vacancies increase carrier concentration but decrease mobility due to enhanced scattering. Reducing tensile strain suppresses scattering of electrons by oxygen vacancies and increases both electrical conductivity (σ) and the Seebeck coefficient (S), mitigating the conventional inverse relationship between S and σ. Fully relaxed films exhibit σ approximately four orders of magnitude higher at room temperature than highly tensile strained films. These relaxed films also show the highest power factor (PF = S²·σ), exceeding strained films by up to six orders of magnitude. Strain-controlled oxygen vacancies thus provide a direct route to optimize charge transport and maximize the thermoelectric performance of CMO thin films.

Sluggish oxygen reduction reaction (ORR) remains a critical barrier to advancing intermediate-temperature electrochemical energy devices. Here, we demonstrate that strain engineering in two platforms, epitaxial thin films and freestanding membranes, systematically tunes ORR kinetics in Ruddlesden-Popper LaSrCoO4. In epitaxial films, the thickness is varied to control in-plane tensile strain, whereas in freestanding membranes strain relaxation during the release step of fabrication with water soluble sacrificial layers produces flat or wrinkled architectures. Electrochemical impedance spectroscopy analysis reveals more than an order of magnitude increase in the oxygen surface exchange coefficient for tensile-strained films relative to relaxed films, together with a larger oxygen vacancy concentration. Wrinkled freestanding membranes provide a further increase in oxygen surface exchange kinetics and a lower activation energy, which are attributed to increased active surface area and local strain variation. These results identify epitaxial tensile strain and controlled wrinkling as practical design parameters for optimizing ORR activity in Ruddlesden-Popper oxides.

ABO3 perovskite oxides are a versatile class of materials whose surfaces and interfaces play essential roles in sustainable energy technologies, including catalysis, solid oxide fuel and electrolysis cells, thermoelectrics, and energy-relevant oxide electronics. The interplay between point defects and surface reconstructions strongly affects interfacial stability, charge transport, and catalytic activity under operating conditions. This review summarizes recent progress in understanding how oxygen vacancies, cation nonstoichiometry, and electronic defects couple to atomic-scale surface rearrangements in representative perovskite systems. We first revisit Tasker’s classification of ionic surfaces and clarify how defect chemistry provides compensation mechanisms that stabilize otherwise polar or metastable terminations. We then discuss experimental and theoretical insights into defect-mediated reconstructions on perovskite surfaces and how they influence the performance of energy conversion devices. Finally, we conclude with a perspective on design strategies that leverage defect engineering and surface control to enhance functionality in energy applications, aiming to connect fundamental surface science with practical materials solutions for the transition to sustainable energy.

A deterministic platform for engineering epitaxial strain in CaMnO3 (CMO) thermoelectric thin films is demonstrated using pulsed laser deposition, enabling precise control of the interplay between strain state and oxygen-vacancy formation. High-quality epitaxial CMO films are grown on four different single-crystalline substrates, which impose fully relaxed, partially relaxed, low-tensile, and high-tensile strain states, respectively. Increasing tensile strain induces a monotonic expansion of the unit-cell volume and a systematic rise in oxygen vacancy concentration. Oxygen vacancies increase carrier concentration but decrease mobility due to enhanced scattering. Reducing tensile strain suppresses vacancy scattering and increases both electrical conductivity (σ) and the Seebeck coefficient (S), mitigating the conventional inverse relationship between S and σ. Fully relaxed films exhibit σ approximately four orders of magnitude higher at room temperature than highly tensile-strained films. These relaxed films also show the highest power factor (PF = S2・σ), exceeding strained films by up to six orders of magnitude. Strain-controlled oxygen vacancies thus provide a direct route to optimize charge transport and maximize the thermoelectric performance of CMO thin films.