PacSurf2022 Session EH-MoM2: Surfaces and Interfaces for Efficient Power Conversion and Batteries

Pervoskite-type proton-conductive ceramic materials have high potentials for the application of sustainable energy fields, such as protonic ceramic fuel cell (PCFC), carbon dioxide conversion, ammonia fuel synthesis, etc. In this study, perovskite-type proton-conductive coatings, BaCeZrYO_3-δ (BCZY), were deposited on NiO/BaZrYO_3-δ (BZY) substrates by a solution precursor plasma spray (SPPS) method. The precursors were consisted the BCZY species of the nitrate salts and dissolved into de-ionic water. The NiO/BZY substrates were prepared by a die-pressing method. Several properties of the coatings were analyzed, including crystal phases by X-ray diffraction (XRD), microstructures by scanning electron microscope (SEM), electrical properties by electrochemical impedance spectroscopy (EIS), etc. The results of XRD show the coatings after post-coating annealing over 600^oC are mainly consisted of perovskite phases. The microstructure analysis of the BCZY coatings characterized by SEM show the coatings are mainly consisted of overlapped splats of BCZY materials and a few of surface pores can be found from the coating layers. The average thickness of the coating layers are about several microns. The formation mechanism of the BCZY coatings can be purposed as the BCZY precursor droplet breakup, liquid-phase evaporation, thermal pyrolysis of precursors, solid particles melting, and then the melting particles splatted and overlapped on the substrate to form the BCZY coatings. The details electrical property analysis of the BCZY coating samples are as mentioned in the following report.

Metal exsolution attracts much attention for the synthesis of nanostructured catalysts. Thermal reduction of doped perovskite parent oxides induces the migration of reducible cations from the oxide bulk to the surface and results in the self-assembly of metallic nanoparticles that serve as catalytic centers. Ideal exsolution-type catalysts therefore dynamically adapt with respect to the oxygen chemical potential. The parameters that influence the kinetics of nanoparticle exsolution, however remain an open question in the scientific community. Furthermore, little is known about the origin of stability limitations of exsolved nanoparticles that are typically anchored at the oxide support.

We employ epitaxial thin films model systems with atomically defined surfaces to investigate the exsolution response of Ni in SrTi_0.9Nb_0.05Ni_0.05O_3-ẟ and SrTi_0.95Ni_0.05O_3-ẟ of respective n–type and p-type defect chemistry. Here, we study the mass transfer of dopants towards the surface as well as the morphological evolution of the nanostructured catalysts. Our thin film approach enables to study differences in the exsolution behavior on the basis of well-defined (001) surfaces, eliminating uncertainties regarding the complex microstructure and anisotropy of ceramic oxides and the presence of secondary surface phases.

In order to assess the dynamic changes at the catalyst surface during metal exsolution, we conduct advanced in-situ spectroscopy and scattering analyses. In particular, we employ a recently developed technique that allows for the simultaneous analysis of the chemical and structural evolution of the solid-gas interface based on coupled in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in-situambient pressure grazing incidence X-ray scattering (AP-GIXS). Our investigations show that the defect chemistry of the near-surface region determines the dynamics of the exsolution response, where the formation of space charge regions at the perovskite surface plays a major role for the process.

We conclude that the mass transfer during metal exsolution and therefore the nanoparticle self-assembly at the perovskite surface is surface-controlled by interaction with the energy landscape of the complex oxide surface. Derived from our investigations, we propose a mechanistic model that describes how metal exsolution is mediated by the surface defect chemistry and we demonstrate surface engineering of the topmost atomic layers can be utilized as a novel strategy for the active control of the exsolution behavior. Moreover, our observations indicate that the different nature of the surface potential may influence the stability of exsolved nanoparticles.

Polymer electrolyte membrane water electrolyzers (PEMWEs) are of significant importance towards enabling the large-scale utilization of renewable hydrogen. Fabrication and integration of components, advancement of scalable manufacturing techniques, elucidation of degradation mechanisms, and development of degradation mitigation strategies all require extensive characterization of materials, components, and devices. The complexity of the systems and problems combined with the multitude of relevant scales require multi-technique characterization and motivate further developments of characterization methods and novel approaches. This talk will discuss some of our recent studies on both catalyst layers (CLs) and porous transport layers (PTLs) for application in PEMWEs. Specifically, this presentation will highlight the complementary nature of electron and X-ray microscopy and spectroscopy methods for the investigation of surfaces, interfaces, and interactions within the CLs comparing results for Ir and IrO₂-based CLs, and fresh and aged or tested CLs. Similarly, the characterization of PTLs requires advanced characterization to properly assess the elemental distribution and chemical speciation of PTLs and protective coatings.

Interfacial reactions driven by charge transfer occurring at the electrode – electrolyte interface contribute to the formation of complex interphasial layers that govern the performance of energy storage devices. These interfacial reactions are influenced by various factors such as surface chemistry of electrode, electrochemical properties of the electrolyte solution, and the working potential of the device. Precise capturing of critical interfacial reactions and subsequent evolution of solid-electrolyte interphase components is extremely challenging due to the complexity and heterogeneity of the interphasial reactive components, the reaction dynamics under variable potentials, and the smaller dimensions of interphase region. Probing complex interfacial processes requires the integration of advanced spectroscopies within an electrochemical device to monitor interfacial reactions and structural changes during operation. In this study, we investigated the interphasial evolution of MgV₂O₄ cathodes for rechargeable magnesium batteries using both in situ and ex situ X-ray photoelectron spectroscopy while also probing structural transformation of interphasial chemical species using operando Raman spectroscopy. Our study revealed that the solid electrolyte interphase layer on MgV₂O₄ was formed mainly by the decomposition of electrolyte anions that occurred at a very early stage of the initial charging process. The interphase layer also thickened with increased cell potential as well as repeated cycling. Post surface analysis of the MgV₂O₄ cathode after 20 cycles also confirmed surface conditioning processes that is well supported by its electrochemical performance in early cycles. Our study revealed insights into the surface evolution of MgV₂O₄, which provides crucial information for development of an oxide cathode for rechargeable magnesium batteries as well as other battery chemistries that involve sodium and lithium ions.