Nicola H. Perry

Hydration and hydrogenation upon steam exposure alter the local crystal structure, charge distribution, optical transmissivity, and electronic conductivity of BZFYO _{3− δ} thin films, revealing significant O 2p character of the p-type carriers.

Li _{3− x} OH _x Cl thin films were grown by PLD integrated with a glovebox and in situ temperature-dependent impedance spectroscopy. Room-temperature, vacuum deposition produced phase-pure, Li-rich, conductive films.

Electro-chemo-mechanical coupling involving the interplay between electrical, chemical, and mechanical states of materials, must be understood to improve materials performance (e.g., durability, efficiency) in solid-state electrochemical devices. Especially, Mixed ionic – electronic conducting oxides (MIECs) can undergo changes in stoichiometry with variations in the surrounding atmosphere, temperature, and voltages during process and operation. Such stoichiometry changes are also typically accompanied by the lattice distortions/strains, which is also referred to as chemical expansion, ε_c. Large chemical strains are typically associated with significant mechanical stresses, resulting in electrode cracks. Using Pr_0.2Ce_0.8O_2−δ(PCO20) as a model system, our previous work found such deleterious chemical expansion can be successfully suppressed by reducing the particle size. In this presentation, we will present a detailed analysis of the coefficient of chemical expansion (CCEs) vs. particle size, with quantitative measurements of the oxygen sub-stoichiometry by thermogravimetric analysis (TGA). The CCEs, defined as chemical strain over change in stoichiometry, are an important property that can reveal if the reduced chemical expansion is solely due to limited changes in defect concentrations. Here, we found the smaller particles generally exhibit lower CCEs compared to the larger particles, possibly due to the greater likelihood of defect interactions/clustering at higher oxygen substoichiometries. Additionally, in contrast to the typical linear strain vs. stoichiometry change behavior (constant CCE) observed for PCO bulk, the nanopowders in this study exhibit a highly non-linear relationship (variable CCE), which could be ascribed to the different chemical/coordination environment of the formed oxygen vacancies when the oxygen sub-stoichiometry changes. The potential impacts of oxygen vacancy positions and surface vs. bulk differences in chemical composition/valence states on CCEs will be further discussed.

Perovskite oxides have seen prevalent adoption in high-temperature electrochemical and sensing applications. The materials can be exposed to a wide range of temperatures and oxygen partial pressures during operation, which results in a varying concentration of oxygen vacancies. When an oxygen vacancy forms from an oxide ion exiting the lattice, two charge-compensating electrons may localize on the cations, increasing the radii of the cation and causing an expansion in the lattice. This type of gradual expansion induced by a change in composition is defined as stoichiometric chemical expansion, and a large coefficient of chemical expansion (CCE) could impair the performance and durability of the device.

For next-generation energy storage devices, solid state electrolytes have been widely studied due to their intrinsic safety, compatibility with high-voltage cathodes, and potential for high energy density. Among oxide solid electrolytes, NASICON-type lithium aluminum titanium phosphate (Li_1.3Al_0.3Ti_1.7(PO₄)₃, LATP) offers a compelling combination of relatively high Li⁺ conductivity and excellent oxidative stability, making it attractive for integration with cathodes. While LATP is not stable against lithium metal, it is well suited for cathode-side electrolytes in bilayer or multilayer architectures that decouple anode and cathode requirements – an approach increasingly relevant for manufacturable thin-film and microbattery platforms.

Metal exsolution becomes an emerging strategy to prepare nano-electrocatalysts for solid oxide fuel/electrolysis cell electrodes with enhanced surface electrochemical reaction kinetics. Compared to conventional deposition methods, in-situ exsolution can produce nanoparticles with anchored structure, resulting in enhanced thermal stability, resistance to agglomeration, and strong metal-support interactions. To date, perovskite oxides have been the most widely explored support oxides, owing to their flexible cation chemistry and B-sites that readily accommodate reducible metal dopants. However, the high-temperature reduction process and prolonged processing times required by most of them motivates the exploration of alternative oxide systems. Ceria-based systems could be a potential alternative host oxides for exsolution. The size mismatch between the 3d transition metal cations and the original fluorite cations may provide an additional elastic driving force for surface segregation, potentially resulting in a facile exsolution process. While the factors governing metal exsolution behavior (e.g., particle density) are well studied in perovskites, factors influencing exsolution in fluorite systems remain incompletely studied, warranting systematic investigation. In this work, oriented (Pr,Ce,Ni)O_2-δ thin films were used as the model system and prepared by pulsed laser deposition (PLD). We developed a high-throughput electrochemical oxygen-pumping cell with a lateral gradient in Nernst potential along the film to investigate the impact of electrochemical potential on exsolution density, using in-situ near-ambient pressure synchrotron X-ray photoelectron spectroscopy (NAP-XPS) and ex-situ atomic force microscopy (AFM). We found the exsolved Ni⁰ nanoparticle density is controlled by the bias and thus the effective oxygen partial pressure (pO₂). The correlation between applied bias and the exsolution behavior was further interpreted through a defect chemical model. Evaluating the electrochemical performance under reducing atmospheres, the significantly lowered polarization resistance of the Ni⁰ nanoparticle-bearing sample suggests the enhanced surface exchange kinetics after exsolution.

Ruthenium-based pyrochlores (A₂Ru₂O_7−δ) have attracted attention as non-Ir electrocatalysts that exhibit high oxygen evolution reaction (OER) activity and stability in acidic environments. Despite their promising performance, the role of defect chemistry and oxygen non-stoichiometry in governing their catalytic behavior is not fully understood. In particular, the possible involvement of lattice oxygen in the OER mechanism motivates a detailed investigation of oxygen vacancy formation, redox behavior, and charge transport in these materials.

Li_3-xOH_xCl compositions are intriguing battery solid electrolyte candidates, given their stability vs. Li metal anodes, mechanical compliance, and low melting points. These properties may enable high energy density, low stack pressure, and low-temperature manufacturing, respectively. Given modest Li-ion conductivities (~10^-3 mS/cm at 25 °C), their most compelling use may be in the form of thin functional layers, particularly at the anode side or in microdevices. Therefore, it is important to understand how the conductivity of thin films compares to that of bulk antiperovskites and how the film processing conditions affect the structure and transport behavior. Here we apply air-free pulsed laser deposition (PLD) with both in-situ and ex-situ impedance spectroscopy, XRD, SEM-EDS, and nuclear reaction analysis to investigate how target composition, laser fluence, and substrate temperature correlate to film composition, structure, and in-plane conductivity. Room-temperature-grown films exhibit the highest conductivities, antiperovskite phase, high Li/Cl ratios, and chemical homogeneity. Increasing deposition temperature or post-annealing decreases the Li/Cl ratio, increases LiCl phase fraction and compositional heterogeneity, and lowers conductivity. In-situ impedance measurements during film growth provide insight into how substrate temperature affects film formation. Ultimately, the ability to prepare pure antiperovskite films at room temperature with comparable conductivity to bulk cubic pellets, using carefully engineered PLD targets, encourages further investigation of their low-temperature manufacturing with scalable processes.

As protonic ceramic electrolysis cells emerge for efficient H₂ production, there is a need to develop air electrode materials enabling fast, durable steam splitting and proton incorporation. Single-phase triple conductors may fail to satisfy the myriad performance/stability requirements, and their critical charge-carriers (holes, oxygen vacancies, and protons) are in competition, limiting their concentrations. Instead, we propose task-sharing, vertically aligned nanocomposites (VANs), comprising a proton conductor (BaZr_0.9Y_0.1O_3-δ) and a redox-active mixed ionic electronic conductor (Ce_0.9Pr_0.1O_2-δ), that may enable rapid proton surface exchange at the solid–gas interface and transport along the solid–solid heterointerfaces. We grew VANs by pulsed laser deposition and investigated the interplay between their processing conditions, structure, and proton and oxygen surface exchange kinetics. We varied the substrate temperature, laser repetition rate, laser fluence, and processing oxygen pressure. The crystallinity and phases were characterized by grazing-incidence X-ray diffraction, and the strain and structural order as a function of depth were evaluated by angle-dependent synchrotron X-ray pair distribution function analysis. To evaluate the potential for interdiffusion, the formation energies of substitutional defects were simulated with density functional theory. Corresponding structural analysis and elemental mapping were performed by scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy, indicating distinct nanoscale compositional regions with a hierarchical structure embedded in individual VANs columns and minimal interdiffusion across a bilayer film. Proton and oxygen surface exchange coefficients (k_H, k_O) and polarization resistances were evaluated by electrical and optical relaxations and impedance spectroscopy of VAN-incorporated protonic ceramic electrochemical cells, respectively, at 400–500 °C, demonstrating values comparable to some of the best-known triple and mixed conductors.

In situ exsolution of metal nanoparticles is a promising strategy to prepare electrocatalysts with enhanced activity and resistance to agglomeration for efficient chemical transformations and energy conversion. Achieving a high nucleation density of nanoparticles under mild conditions and understanding how to tailor the process is important for performance of these electrodes in electrochemical cells. Here, we demonstrate facile exsolution of Ni nanoparticles using fluorite-structured (Pr,Ce)O_2-δ as the support oxide, driven by electrochemical potential and aided by the metastability of Ni in the solid solution (elastic driving force). We prepare single-phase oriented thin films of (Pr,Ce,Ni)O_2-δ (NPCO) on (Zr,Y)O_2-δ (YSZ) substrates by pulsed laser deposition. With the aid of a high-throughput electrochemical cell that provides a lateral gradient in Nernst voltage, we apply in situ near-ambient pressure synchrotron X-ray photoelectron spectroscopy and ex situ atomic force microscopy to investigate the impact of electrochemical potential on Ni nucleation density. We find that metallic Ni can be successfully exsolved at 550 °C upon cathodic biasing in 20 mTorr O₂, and its nucleation density increases with increasing electrochemical driving force/decreasing oxygen chemical potential. We further evaluate the electrochemical performance under highly reducing (fuel electrode) conditions by electrochemical impedance spectroscopy. With the exsolved Ni nanoparticles, the surface exchange coefficient of the NPCO is found to be ∼4× higher than for PCO without exsolution. This work confirms mixed conducting fluorites as beneficial host lattices for facile transition-metal exsolution and suggests the possibility for constructing an all ceria-based electrochemical cell with PCO serving as both the cathode and the anode.