Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge

Abstract: Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation's oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoOx) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO3-{\delta} or the brownmillerite SrCoO2.5. Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200~300 {\deg}C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low temperature redox activity in SrCoO3-{\delta} is attributed to a small Gibbs free energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low temperature cathode materials.

• The paper introduces epitaxial stabilization via pulsed laser epitaxy, achieving reversible redox transitions at low temperatures (200–300 °C).
• The paper reports a four orders of magnitude resistivity change and a shift from antiferromagnetic insulating to ferromagnetic metallic behavior.
• The paper validates phase-specific oxygen stoichiometry using XRD, XAS, and XMCD, highlighting improved catalytic and energy storage potential.

Insights into Reversible Redox Reactions in Epitaxially Stabilized SrCoO_x

This paper investigates the reversible redox reactions in epitaxially stabilized strontium cobaltites (SrCoO_x), achieved through the pioneering growth of single crystalline thin films of brownmillerite SrCoO<sub\>2.5</sub> and perovskite SrCoO<sub\>3-δ</sub>. The study emphasizes their fast, reversible phase transformations at low temperatures, a characteristic previously unattainable due to the high thermodynamic barrier and cation oxidation state fixity in transition-metal oxides (TMOs).

Reversible redox activity in materials like SrCoO_x is vital given its implications for energy storage, conversion systems, and catalysis. The epitaxial stabilization, carried out via pulsed laser epitaxy (PLE), effectively induces reversible phase transitions in these materials within a temperature range of 200–300 °C. This temperature is significantly lower than traditional methods, which often require temperatures exceeding 700 °C and prolonged annealing durations.

A comprehensive analysis using x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) underscores the contrasting electronic and magnetic properties that emerge due to this reversible transformation. In particular, the SrCoO<sub\>3-δ</sub> phase adopts a ferromagnetic metallic behavior with a saturation magnetization of approximately 2.3 μ_B/C at 10 K, in contrast to the antiferromagnetic insulating state of SrCoO<sub\>2.5</sub>. Noteworthy is the observed four orders of magnitude difference in resistivity between these two phases at room temperature.

The XAS conducted at the Co L-edge and the O K-edge further confirms the associated oxygen stoichiometry, highlighting a significant electron conductivity increase when transitioning to the perovskite phase. The epitaxial films demonstrate clear oxygen stoichiometry with a remarkably stable high-oxygenated state, δ ≤ 0.1, without requiring traditional post-growth oxidation, thereby improving film quality and uniformity.

Thermodynamic calculations illustrate that the small Gibbs free energy difference between the brownmillerite and perovskite phases is fundamental to their low-temperature phase transition. The paper compares these findings against SrMnO_x systems and highlights a distinctive Gibbs energy reduction for SrCoO<sub\>3-δ</sub>, critical for topotactic phase transformations.

The paper also ventures into the catalytic potential of SrCoO<sub\>2.5</sub> using a custom micro-reactor, focusing on CO oxidation, demonstrating its active catalytic behavior above 320°C—indicating potential applicability in redox reactions beyond the constraints of high operations temperatures.

The implications of this research are multifaceted. Practically, the ability to switch phases quickly and at lower temperatures without structural degradation opens new pathways for developing efficient, durable electrochemical sensors and cathode materials. Theoretically, it sheds light on topotactic transformations in TMOs and the role of ordered oxygen vacancy channels, promoting further exploration in fast ionic conductiveness and material design.

Future developments might expand this framework, exploring the controlled engineering of other multivalent oxides, thereby enhancing the application scope from solid-state fuel cells to advanced catalytic reactors, emphasizing sustainability and practicality in transition-metal oxide utilization. This study's methodology and findings could inform the synthesis and stability analysis of a wide range of epitaxial thin film structures for innovative technological applications.