Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO3 Thin Films

Interfacial charge transfer and its impact on transport properties of LaNiO3/LaFeO3 superlattices

Charge transfer or redistribution at oxide heterointerfaces is a critical phenomenon, often leading to remarkable properties such as two-dimensional electron gas and interfacial ferromagnetism. Despite studies on LaNiO3/LaFeO3 superlattices and heterostructures, the direction and magnitude of the charge transfer remain debated, with some suggesting no charge transfer due to the high stability of Fe3+ (3d5). Here, we synthesized a series of epitaxial LaNiO3/LaFeO3 superlattices and demonstrated partial (up to ~0.5 e-/interface unit cell) charge transfer from Fe to Ni near the interface, supported by density functional theory simulations and spectroscopic evidence of changes in Ni and Fe oxidation states. The electron transfer from LaFeO3 to LaNiO3 and the subsequent rearrangement of the Fe 3d band create an unexpected metallic ground state within the LaFeO3 layer, strongly influencing the in-plane transport properties across the superlattice. Moreover, we establish a direct correlation between interfacial charge transfer and in-plane electrical transport properties, providing insights for designing functional oxide heterostructures with emerging properties.

Uncovering an Interfacial Band Resulting from Orbital Hybridization in Nickelate Heterostructures

The interaction of atomic orbitals at the interface of perovskite oxide heterostructures has been investigated for its profound impact on the band structures and electronic properties, giving rise to unique electronic states and a variety of tunable functionalities. In this study, we conducted an extensive investigation of the optical and electronic properties of epitaxial NdNiO3 synthesized on a series of single-crystal substrates. Unlike nanofilms synthesized on other substrates, NdNiO3 on SrTiO3 (NNO/STO) gives rise to a unique band structure featuring an additional unoccupied band situated above the Fermi level. Our comprehensive investigation, which incorporated a wide array of experimental techniques and density functional theory calculations, revealed that the emergence of the interfacial band structure is primarily driven by orbital hybridization between the Ti 3d orbitals of the STO substrate and the O 2p orbitals of the NNO thin film. Furthermore, exciton peaks have been detected in the optical spectra of the NNO/STO film, attributable to the pronounced electron-electron (e-e) and electron-hole (e-h) interactions propagating from the STO substrate into the NNO film. These findings underscore the substantial influence of interfacial orbital hybridization on the electronic structure of oxide thin films, thereby offering key insights into tuning their interfacial properties.

Comparing the Impacts of Strain Types on Oxygen-Vacancy Formation in a Perovskite Oxide via Nanometer-Scale Strain Fields

The utilization of an in-plane lattice misfit in an oxide epitaxially grown on another oxide with a different lattice parameter is a well-known approach to induce strains in oxide materials. However, achieving a sufficiently large misfit strain in this heteroepitaxial configuration is usually challenging, unless the thickness of the grown oxide is kept well below a critical value to prevent the formation of misfit dislocations at the interface for relaxation. Instead of adhering to this conventional approach, here, we employ nanometer-scale large strain fields built around misfit dislocations to examine the effects of two distinct types of strains─tension and compression─on the generation of oxygen vacancies in heteroepitaxial LaCoO3 films. Our atomic-level observations, coupled with local electron-beam irradiation, clarify that the in-plane compression notably suppresses the creation of oxygen vacancies, whereas the formation of vacancies is facilitated under tensile strain. Demonstrating that the defect generation can considerably vary with the type of strain, our study highlights that the experimental approach adopted in this work is applicable to other oxide systems when investigating the strain effects on vacancy formation.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

Bifunctional Square-Planar NiO4 Coordination of Topotactic LaNiO2.0 Films for Efficient Oxygen Evolution Reaction

The high-efficient and low-cost oxygen evolution reaction (OER) is decisive for applications of oxide catalysts in metal-air batteries, electrolytic cells, and energy-storage technologies. Delicate regulations of active surface and catalytic reaction pathway of oxide materials principally determine thermodynamic energy barrier and kinetic rate during catalytic reactions, and thus have crucial impacts on OER performance. Herein, a synergistic modulation of catalytically active surface and reaction pathway through facile topotactic transformations switching from perovskite (PV) LaNiO3.0 film to infinite-layer (IL) LaNiO2.0 film is demonstrated, which absolutely contributes to improving OER performance. The square-planar NiO4 coordination of IL-LaNiO2.0 brings about more electrochemically active metal (Ni+ ) sites on the film surface. Meanwhile, the oxygen-deficient driven PV- IL topotactic transformations lead to a reaction pathway converted from absorbate evolution mechanism to lattice-oxygen-mediated mechanism (LOM). The non-concerted proton-electron transfer of LOM pathway, evidenced by the pH-dependent OER kinetics, further boosts the OER activity of IL-LaNiO2.0 films. These findings will advance the in-depth understanding of catalytic mechanisms and open new possibilities for developing highly active perovskite-derived oxide catalysts.

Strain Engineering in Perovskites: Mutual Insight on Oxides and Halides

Perovskite materials have garnered significant attention over the past decades due to their applications, not only in electronic materials, such as dielectrics, piezoelectrics, ferroelectrics, and superconductors but also in optoelectronic devices like solar cells and light emitting diodes. This interest arises from their versatile combinations and physiochemical tunability. While strain engineering is a recognized powerful tool for tailoring material properties, its collaborative impact on both oxides and halides remains understudied. Herein, strain engineering in perovskites for energy conversion devices, providing mutual insight into both oxides and halides is discussed. The various experimental methods are presented for applying strain by using thermal mismatch, lattice mismatch, defects, doping, light illumination, and flexible substrates. In addition, the main factors that are influenced by strain, categorized as structure (e.g., symmetry breaking, octahedral distortion), bandgap, chemical reactivity, and defect formation energy are described. After that, recent progress in strain engineering for perovskite oxides and halides for energy conversion devices is introduced. Promising methods for enhancing the performance of energy conversion devices using perovskites through strain engineering are suggested.

Direct observation of the dynamic reconstructed active phase of perovskite LaNiO3 for the oxygen-evolution reaction

Ni-based transition metal oxides are promising oxygen-evolution reaction (OER) catalysts due to their abundance and high activity. Identification and manipulation of the chemical properties of the real active phase on the catalyst surface is crucial to improve the reaction kinetics and efficiency of the OER. Herein, we used electrochemical-scanning tunnelling microscopy (EC-STM) to directly observe structural dynamics during the OER on LaNiO3 (LNO) epitaxial thin films. Based on comparison of dynamic topographical changes in different compositions of LNO surface termination, we propose that reconstruction of surface morphology originated from transition of Ni species on LNO surface termination during the OER. Furthermore, we showed that the change in surface topography of LNO was induced by Ni(OH)2/NiOOH redox transformation by quantifying STM images. Our findings demonstrate that in situ characterization for visualization and quantification of thin films is very important for revealing the dynamic nature of the interface of catalysts under electrochemical conditions. This strategy is crucial for in-depth understanding of the intrinsic catalytic mechanism of the OER and rational design of high-efficiency electrocatalysts.

Deeper mechanistic insights into epitaxial nickelate electrocatalysts for the oxygen evolution reaction

Mass production of green hydrogen via water electrolysis requires advancements in the performance of electrocatalysts, especially for the oxygen evolution reaction. In this feature article, we highlight how epitaxial nickelates act as model systems to identify atomic-level composition-structure-property-activity relationships, capture dynamic changes under operating conditions, and reveal reaction and failure mechanisms. These insights guide advanced electrocatalyst design with tailored functionality and superior performance. We conclude with an outlook for future developments via operando characterization and multilayer electrocatalyst design.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Promoting the Oxygen Evolution Activity of Perovskite Nickelates through Phase Engineering

Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO₃ by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO₃ film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO₃ single crystalline film. The obtained current density of 1038 μΑ cm^-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.

Development of Perovskite Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Perovskite oxides are studied as electrocatalysts for oxygen evolution reactions (OER) because of their low cost, tunable structure, high stability, and good catalytic activity. However, there are two main challenges for most perovskite oxides to be efficient in OER, namely less active sites and low electrical conductivity, leading to limited catalytic performance. To overcome these intrinsic obstacles, various strategies are developed to enhance their catalytic activities in OER. In this review, the recent developments of these strategies is comprehensively summarized and systematically discussed, including composition engineering, crystal facet control, morphology modulation, defect engineering, and hybridization. Finally, perspectives on the design of perovskite oxide-based electrocatalysts for practical applications in OER are given.

Local-electrostatics-induced oxygen octahedral distortion in perovskite oxides and insight into the structure of Ruddlesden-Popper phases

As the physical properties of ABX₃ perovskite-based oxides strongly depend on the geometry of oxygen octahedra containing transition-metal cations, precise identification of the distortion, tilt, and rotation of the octahedra is an essential step toward understanding the structure-property correlation. Here we discover an important electrostatic origin responsible for remarkable Jahn-Teller-type tetragonal distortion of oxygen octahedra during atomic-level direct observation of two-dimensional [AX] interleaved shear faults in five different perovskite-type materials, SrTiO₃, BaCeO₃, LaCoO₃, LaNiO₃, and CsPbBr₃. When the [AX] sublayer has a net charge, for example [LaO]⁺ in LaCoO₃ and LaNiO₃, substantial tetragonal elongation of oxygen octahedra at the fault plane is observed and this screens the strong repulsion between the consecutive [LaO]⁺ layers. Moreover, our findings on the distortion induced by local charge are identified to be a general structural feature in lanthanide-based A_n + 1B_nX_3n + 1-type Ruddlesden-Popper (RP) oxides with charged [LnO]⁺ (Ln = La, Pr, Nd, Eu, and Gd) sublayers, among more than 80 RP oxides and halides with high symmetry. The present study thus demonstrates that the local uneven electrostatics is a crucial factor significantly affecting the crystal structure of complex oxides.

Unusual Role of Point Defects in Perovskite Nickelate Electrocatalysts

Low-cost transition-metal oxide is regarded as a promising electrocatalyst family for an oxygen evolution reaction (OER). The classic design principle for an oxide electrocatalyst believes that point defect engineering, such as oxygen vacancies (V_O^..) or heteroatom doping, offers the opportunities to manipulate the electronic structure of material toward optimal OER activity. Oppositely, in this work, we discover a counterintuitive phenomenon that both V_O^.. and an aliovalent dopant (i.e., proton (H⁺)) in perovskite nickelate (i.e., NdNiO₃ (NNO)) have a considerably detrimental effect on intrinsic OER performance. Detailed characterizations unveil that the introduction of these point defects leads to a decrease in the oxidative state of Ni and weakens Ni-O orbital hybridization, which triggers the local electron-electron correlation and a more insulating state. Evidenced by first-principles calculation using the density functional theory (DFT) method, the OER on nickelate electrocatalysts follows the lattice oxygen mechanism (LOM). The incorporation of point defect increases the energy barrier of transformation from OO*(V_O) to OH*(V_O) intermediates, which is regarded as the rate-determining step (RDS). This work offers a new and significant perspective of the role that lattice defects play in the OER process.

Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis

Structure-activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO₃ epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO₂ surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.

Electronic Structure Tuning of 2D Metal (Hydr)oxides Nanosheets for Electrocatalysis

2D metal (hydr)oxide nanosheets have captured increasing interest in electrocatalytic applications aroused by their high specific surface areas, enriched chemically active sites, tunable physiochemical properties, etc. In particular, the electrocatalytic reactivities of materials greatly rely on their surface electronic structures. Generally speaking, the electronic structures of catalysts can be well adjusted via controlling their morphologies, defects, and heterostructures. In this Review, the latest advances in 2D metal (hydr)oxide nanosheets are first reviewed, including the applications in electrocatalysis for the hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction. Then, the electronic structure-property relationships of 2D metal (hydr)oxide nanosheets are discussed to draw a picture of enhancing the electrocatalysis performances through a series of electronic structure tuning strategies. Finally, perspectives on the current challenges and the trends for the future design of 2D metal (hydr)oxide electrocatalysts with prominent catalytic activity are outlined. It is expected that this Review can shed some light on the design of next generation electrocatalysts.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Effect of Lattice Strain on the Formation of Ruddlesden-Popper Faults in Heteroepitaxial LaNiO3 for Oxygen Evolution Electrocatalysis

A great deal of research has recently been focused on Ruddlesden-Popper (RP) two-dimensional planar faults consisting of intervened [AO] monolayers in an ABO₃ perovskite framework due to the structurally peculiar shear configuration. In this work, we scrutinize the effect of elastic strain on the generation behavior of RP faults, which are electrocatalytically very active sites for the oxygen evolution reaction (OER), in (001) epitaxial LaNiO₃ thin films through by using two distinct single-crystal substrates with different cubic lattice parameters. Atomic-scale direct observations reveal that RP faults can be more favorably created when tensile misfit strain is exerted. Furthermore, we demonstrate that the controlled growth of thin films show notably enhanced OER activity by the RP faults. The findings in this study highlight the impact of symmetry-breaking defect formation for better oxygen electrocatalysis in perovskite oxides.

In Vivo Glutamate Sensing inside the Mouse Brain with Perovskite Nickelate-Nafion Heterostructures

Glutamate, one of the main neurotransmitters in the brain, plays a critical role in communication between neurons, neuronal development, and various neurological disorders. Extracellular measurement of neurotransmitters such as glutamate in the brain is important for understanding these processes and developing a new generation of brain-machine interfaces. Here, we demonstrate the use of a perovskite nickelate-Nafion heterostructure as a promising glutamate sensor with a low detection limit of 16 nM and a response time of 1.2 s via amperometric sensing. We have designed and successfully tested novel perovskite nickelate-Nafion electrodes for recording of glutamate release ex vivo in electrically stimulated brain slices and in vivo from the primary visual cortex (V1) of awake mice exposed to visual stimuli. These results demonstrate the potential of perovskite nickelates as sensing media for brain-machine interfaces.

Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface-Mounted Metal-Organic Frameworks

Metal-organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a "strain modulation" approach has been applied through the use of surface-mounted NiFe-MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA cm-2 at an overpotential of only ≈210 mV. It demonstrates operational long-term stability even at a high current density of 500 mA cm-2 and exhibits the so far narrowest "overpotential window" ΔEORR-OER of 0.69 V in 0.1 m KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts.

Rare-earth-containing perovskite nanomaterials: design, synthesis, properties and applications

As star material, perovskites have been widely used in the fields of optics, photovoltaics, electronics, magnetics, catalysis, sensing, etc. However, some inherent shortcomings, such as low efficiency (power conversion efficiency, external quantum efficiency, etc.) and poor stability (against water, oxygen, ultraviolet light, etc.), limit their practical applications. Downsizing the materials into nanostructures and incorporating rare earth (RE) ions are effective means to improve their properties and broaden their applications. This review will systematically summarize the key points in the design, synthesis, property improvements and application expansion of RE-containing (including both RE-based and RE-doped) halide and oxide perovskite nanomaterials (PNMs). The critical factors of incorporating RE elements into different perovskite structures and the rational design of functional materials will be discussed in detail. The advantages and disadvantages of different synthesis methods for PNMs will be reviewed. This paper will also summarize some practical experiences in selecting suitable RE elements and designing multi-functional materials according to the mechanisms and principles of REs promoting the properties of perovskites. At the end of this review, we will provide an outlook on the opportunities and challenges of RE-containing PNMs in various fields.

Tuning the Electronic Structure of LaNiO3 through Alloying with Strontium to Enhance Oxygen Evolution Activity

The perovskite oxide LaNiO3 is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO3 films (La1- x Sr x NiO3) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at x = 0.5 comparable to the state-of-the-art catalyst Ba0.5Sr0.5Co0.8Fe0.2O3- δ . By combining X-ray photoemission and X-ray absorption spectroscopies with density functional theory, it is shown that an upward energy shift of the O 2p band relative to the Fermi level occurs with increasing x in La1- x Sr x NiO3. This alloying step strengthens Ni 3d-O 2p hybridization and decreases the charge transfer energy, which in turn accounts for the enhanced OER activity.

Recent Advances in Oxygen Electrocatalysts Based on Perovskite Oxides

Electrochemical oxygen reduction and oxygen evolution are two key processes that limit the efficiency of important energy conversion devices such as metal–air battery and electrolysis. Perovskite oxides are receiving discernable attention as potential bifunctional oxygen electrocatalysts to replace precious metals because of their low cost, good activity, and versatility. In this review, we provide a brief summary on the fundamentals of perovskite oxygen electrocatalysts and a detailed discussion on emerging high-performance oxygen electrocatalysts based on perovskite, which include perovskite with a controlled composition, perovskite with high surface area, and perovskite composites. Challenges and outlooks in the further development of perovskite oxygen electrocatalysts are also presented.