Exsolution Modeling and Control to Improve the Catalytic Activity of Nanostructured Electrodes

Precise Regulation of In Situ Exsolution Components of Nanoparticles for Constructing Active Interfaces toward Carbon Dioxide Reduction

Metal nanocatalysts supported on oxide scaffolds have been widely used in energy storage and conversion reactions. So far, the main research is still focused on the growth, density, size, and activity enhancement of exsolved nanoparticles (NPs). However, the lack of precise regulation of the type and composition of NPs elements under reduction conditions has restricted the architectural development of in situ exsolution systems. Herein, we propose a strategy to attain a regulated distribution of exsolved transition metals (Cu, Ni, and Fe) on Sr2Fe1.2Ni0.2Cu0.2Mo0.4O6-δ medium-entropy perovskite oxides by varying the oxygen partial pressure (pO2) gradient in the mixture. At 800 °C, the unitary Cu, binary Cu-Ni, and ternary Cu-Ni-Fe NPs are exsolved as pO2 decreases from high to low. Combining experimental and theoretical simulations, we further corroborate that solid oxide electrolysis cells with ternary alloy clusters at the CNF@SFO interface exhibit superior CO2 electrolytic performance. Our results provide tailored strategies for nanostructures and nanointerfaces for studying metal oxide exsolution systems, including fuel electrode materials.

Quantitative Unraveling of Exsolved Heteroboundaries for High-Temperature Electrocatalysis

The application of perovskite oxide for high-temperature electrocatalysis is hindered by its limited activity. Exsolution is a smart strategy that allows the enrichment of the perovskite's surface with highly reactive phases, yielding heteroboundaries. However, the identification of the exact catalytic role of this complex architecture is still elusive. Here we presented a quantitative analysis of the CO2 electroreduction reactivity of a series of perovskite thin film platforms (La0.4Ca0.4Ti0.94Ni0.06O3, LCTN) boosted by exsolved heteropical nanoparticles (particularly for Ni and NiO). The cross-scale electrochemical characterizations, together with density functional theory (DFT) modelings, have shown clear evidence that the boundary length of the NP/perovskite interface is strictly correlated with the CO2RR activity. The intrinsic reaction rate per active site at the NiO/LCTN boundary demonstrates a highest turnover frequency (TOF) of 7.05 ± 0.75 × 104 s-1 at 800 °C, which is 2.5 times and 4 orders of magnitude better than that of Ni/LCTN and LCTN, respectively. The ab initio molecular dynamics (AIMD) proves that the CO2 absorption at the NiO/LCTN boundary leverages a bidentate carbonate modality with a reduced dissociation energy barrier. Moreover, a multifold enhancement in oxygen exchange rate was confirmed, which correlated to the facilitated oxygen ion hopping between adjacent TiO6 octahedrons. Further near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) during CO2 electrolysis on model electrolyzers reveals the crucial role of the NiO/LCTN boundary in stabilizing oxidized carbon intermediates, raising the onset potential threshold of adventitious carbon as well as preventing its build-up.

Optimizing Reversible Exsolution and Phase Transformation in Double Perovskite Sr2Fe1.5-xCoxMo0.5O6-δ Electrodes for High-Performance Symmetric Solid Oxide Cells

Double perovskite (DP) oxides are promising electrode materials for symmetric solid oxide cells (SSOCs) due to their excellent electrochemical activity and stability. B-site cation doping in DP oxides affects the reversibility of phase transformation and exsolution, which plays a crucial role in the catalyst recovery. Yet, few studies have been conducted on this topic. In this study, the Sr2Fe1.5-xCoxMo0.5O6-δ (CSFM, x = 0, 0.1, 0.3, 0.5) DP system demonstrates modulated exsolution and phase transformation reversibility by manipulating the oxygen vacancy concentration. The correlation between Co-doping level and oxygen vacancy concentration is investigated to optimize the exsolution and phase transformation properties. Sr2Fe1.2Co0.3Mo0.5O6-δ (3CSFM) exhibits reversible transformation between DP and Ruddlesden-Popper phases with a high density of exsolved CoFe nanoparticles under redox atmospheres. The quasi-symmetric cell with 3CSFM shows a peak power density of 1.27 W cm-2 at 850 °C in H2 fuel cell mode and a current density of 2.33 A cm-2 at 1.6 V and 800 °C in H2O electrolysis mode. The 3CSFM electrode exhibits robust stability during continuous operation for ≈700 h. These results demonstrate the significant role of B-site doping in designing DP materials capable of dynamic phase transformation in diverse environments.

Hierarchical exsolution in vertically aligned heterostructures

Metal nanoparticle exsolution from metal oxide hosts has recently garnered great attention to improve the performance of energy conversion and storage devices. In this study, the nickel exsolution mechanisms in a vertically aligned nanostructure (VAN) thin film of heteroepitaxial (Sr0.9Pr0.1)0.9Ti0.9Ni0.1O3-δ-Ce0.9Gd0.1O1.95 with a columnar architecture was investigated for the first time. Experimental results and Density Functional Theory (DFT) calculations reveal that the multiple vertical interphases in a VAN with a hierarchical arrangement provide faster and more selective Ni diffusion pathways to the surface than traditional bulk diffusion in epitaxial films. Kinetic studies conducted at different temperatures and times indicate that the nucleation process of the exsolved metal nanoparticles primarily takes place at the surface through the phase boundaries of the columns. The vertical strain is crucial in preserving the film's microstructure, yielding a robust heteroepitaxial architecture after reduction. This innovative heteromaterial opens up new possibilities for designing efficient devices through advanced structural engineering to achieve controlled nanoparticle formation.

In Situ Self-Assembled Active and Stable Ir@MnOx/La0.7Sr0.3Cr0.9Ir0.1O3-δ Interfaces for CO2 Electrolysis

Solid oxide electrolysis cells are prospective approaches for CO2 utilization but face significant challenges due to the sluggish reaction kinetics and poor stability of the fuel electrodes. Herein, we strategically addressed the long-standing trade-off phenomenon between enhanced exsolution and improved structural stability via topotactic ion exchange. The surface dynamic reconstruction of the MnOx/La0.7Sr0.3Cr0.9Ir0.1O3-δ (LSCIr) catalyst was visualized at the atomic scale. Compared with the Ir@LSCIr interface, the in situ self-assembled Ir@MnOx/LSCIr interface exhibited greater CO2 activation and easily removable carbonate intermediates, thus reached a 42 % improvement in CO2 electrolysis performance at 1.6 V. Furthermore, an improved CO2 electrolysis stability was achieved due to the uniformly wrapped MnOx shell of the Ir@MnOx/LSCIr cathode. Our approach enables a detailed understanding of the dynamic microstructure evolution at active interfaces and provides a roadmap for the rational design and evaluation of efficient metal/oxide catalysts for CO2 electrolysis.

Local Structures of Ex-Solved Nanoparticles Identified by Machine-Learned Potentials

In this study, we identify the local structures of ex-solved nanoparticles using machine-learned potentials (MLPs). We develop a method for training machine-learned potentials by sampling local structures of heterointerface configurations as a training set with its efficacy tested on the Ni/MgO system, illustrating that the error in interface energy is only 0.004 eV/Å2. Using the developed scheme, we train an MLP for the Ni/La0.5Ca0.5TiO3 ex-solution system and identify the local structures for both exo- and endo-type particles. The established model aligns well with the experimental observations, accurately predicting a nucleation size of 0.45 nm. Lastly, the density functional theory calculations on the established atomistic model verify that the kinetic barrier for the dry reforming of methane are substantially reduced by 0.49 eV on the ex-solved catalysts compared to that on the impregnated catalysts. Our findings offer insights into the local structures, growth mechanisms, and underlying origin of the catalytic properties of ex-solved nanoparticles.

Nanoparticle Exsolution on Perovskite Oxides: Insights into Mechanism, Characteristics and Novel Strategies

Supported nanoparticles have attracted considerable attention as a promising catalyst for achieving unique properties in numerous applications, including fuel cells, chemical conversion, and batteries. Nanocatalysts demonstrate high activity by expanding the number of active sites, but they also intensify deactivation issues, such as agglomeration and poisoning, simultaneously. Exsolution for bottom-up synthesis of supported nanoparticles has emerged as a breakthrough technique to overcome limitations associated with conventional nanomaterials. Nanoparticles are uniformly exsolved from perovskite oxide supports and socketed into the oxide support by a one-step reduction process. Their uniformity and stability, resulting from the socketed structure, play a crucial role in the development of novel nanocatalysts. Recently, tremendous research efforts have been dedicated to further controlling exsolution particles. To effectively address exsolution at a more precise level, understanding the underlying mechanism is essential. This review presents a comprehensive overview of the exsolution mechanism, with a focus on its driving force, processes, properties, and synergetic strategies, as well as new pathways for optimizing nanocatalysts in diverse applications.