Defect-Driven Oxide Transformations and the Electrochemical Interphase

Abstract

Conspectus The redox reaction pathway is crucial to the sustainable production of the fuels and chemicals required for a carbon-neutral society. Our society is becoming increasingly dependent on devices using batteries and electrolyzers, all of which rely on a series of redox reactions. The overall properties of oxide materials make them very well suited for such electrochemical and catalytic applications due to their associated cationic redox properties and the static site–adsorbate interactions. As these technologies have matured, it has become apparent that defect-driven redox reactions, defect-coupled diffusion, and structural transformations that are both time- and rate-dependent are also critical materials processes. This change in focus, considering not only redox properties but also more complex, dynamic behaviors, represents a new research frontier in the molecular sciences as they are strongly linked to device operation and degradation and lie at the heart of various phenomena that take place at electrochemical interfaces. Fundamental studies of the structural, electronic, and chemical transformation mechanisms are key to the advancement of materials and technological innovations that could be implemented in various electrochemical systems. In this Account, we focus on recent studies and advances in characterizing and understanding the dynamic redox evolution and structural transformations that take place in model perovskites and layered oxides under reactive conditions and correlate them with degradation mechanisms and operations in electrolyzers and batteries. We show that the dynamic evolution of oxygen vacancies and cationic migration in the surface or bulk occurs at the solid–liquid interface, using a combination of different synchrotron-based X-ray spectroscopies and scattering probes. Detailed redox–structure–reactivity correlation studies show how defects and diffusion processes can be tailored to drive various physical and chemical transformations in electrolyzers and batteries. We also highlight a strong correlation between oxygen redox reactivity and structural reorganization in both model thin films and particles, helping to bridge the gap between fundamental studies of the reaction mechanism and device applications. On the basis of these findings, we discuss strategies to probe and tune the redox reactivity and structural stability of the redox-active oxide interphase toward devising efficient pathways for energy and chemical harvesting.

Intercalation of Hydrogen in Perovskite Oxide for Pseudocapacitive Energy Storage

“…Such a storage mechanism is essentially triggered by a water dissociation reaction, which has been reported in oxides at high temperature . Thus, the charge storage and transport in oxides are inevitably dictated by the defect-relevant reactions, such as dissociative water incorporation and hydrogen incorporation reaction. , In aqueous electrolytes, hydrogen storage in perovskites can be realized by a surface reaction followed by a hydrogen intercalation. Mn-based perovskite oxides and as-derived surface hydroxides have been reported to exhibit superior reactivity for water splitting reaction. , As described by the Volmer reaction − H 2 O + e − ⇌ H ( ad ) + OH − water molecules from the electrolyte can form adsorbed hydrogens H (ad) on the oxide surface and release OH – , accompanied by the reduction of adjacent metal ions (Figure a).…”

“…41 Thus, the charge storage and transport in oxides are inevitably dictated by the defect-relevant reactions, such as dissociative water incorporation and hydrogen incorporation reaction. 33,42 In aqueous electrolytes, hydrogen storage in perovskites can be realized by a surface reaction followed by a hydrogen intercalation. Mnbased perovskite oxides and as-derived surface hydroxides have been reported to exhibit superior reactivity for water splitting reaction.…”

“…Figure a–c shows the ex-situ NEXAFS spectra of LaMnO 3 under different conditions, collected in total electron yield (TEY) mode, which gives a probing depth of only a few nm. − NEXAFS spectra at La M 4 -edge suggests La being trivalent on the surface of pristine LaMnO 3 , which remains unchanged when the material is soaked in KOH for 50 h (Figure a). As expected, their spectra are identical after 20 and 100 cycles, indicating La is not involving the charge storage reaction.…”

Unraveling the Role of Defects in Electrocatalysts for Water Splitting: Recent Advances and Perspectives

“…XANES is highly sensitive to valence states and symmetry, while EXAFS enables the investigation of short-range order, which mainly contains the coordination number and local atomic distance. 169,170 This information could be essential to support the analysis of defects in the HER and OER catalysts.…”

“…XAS has been extensively applied to obtain insight into electronic structures and local geometries. , The XAS spectrum includes two regions, that is, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES is highly sensitive to valence states and symmetry, while EXAFS enables the investigation of short-range order, which mainly contains the coordination number and local atomic distance. , This information could be essential to support the analysis of defects in the HER and OER catalysts.…”