Recent advancements in zero- to three-dimensional carbon networks with a two-dimensional electrode material for high-performance supercapacitors

Supercapacitors have gained significant attention owing to their exceptional performance in terms of energy density and power density, making them suitable for various applications, such as mobile devices, electric vehicles, and renewable energy storage systems. This review focuses on recent advancements in the utilization of 0-dimensional to 3-dimensional carbon network materials as electrode materials for high-performance supercapacitor devices. This study aims to provide a comprehensive evaluation of the potential of carbon-based materials in enhancing the electrochemical performance of supercapacitors. The combination of these materials with other cutting-edge materials, such as Transition Metal Dichalcogenides (TMDs), MXenes, Layered Double Hydroxides (LDHs), graphitic carbon nitride (g-C₃N₄), Metal-Organic Frameworks (MOFs), Black Phosphorus (BP), and perovskite nanoarchitectures, has been extensively studied to achieve a wide operating potential window. The combination of these materials synchronizes their different charge-storage mechanisms to attain practical and realistic applications. The findings of this review indicate that hybrid composite electrodes with 3D structures exhibit the best potential in terms of overall electrochemical performance. However, this field faces several challenges and promising research directions. This study aimed to highlight these challenges and provide insights into the potential of carbon-based materials in supercapacitor applications.

Compressive-Strain-Facilitated Fast Oxygen Migration with Reversible Topotactic Transformation in La0.5Sr0.5CoOx via All-Solid-State Electrolyte Gating

Modifying the crystal structure and corresponding functional properties of complex oxides by regulating their oxygen content has promising applications in energy conversion and chemical looping, where controlling oxygen migration plays an important role. Therefore, finding an efficacious and feasible method to facilitate oxygen migration has become a critical requirement for practical applications. Here, we report a compressive-strain-facilitated oxygen migration with reversible topotactic phase transformation (RTPT) in La_0.5Sr_0.5CoO_x films based on all-solid-state electrolyte gating modulation. With the lattice strain changing from tensile to compressive strain, significant reductions in modulation duration (∼72%) and threshold voltage (∼70%) for the RTPT were observed, indicating great promotion of RTPT by compressive strain. Density functional theory calculations verify that such compressive-strain-facilitated efficient RTPT comes from significant reduction of the oxygen migration barrier in compressive-strained films. Further, ac-STEM, EELS, and sXAS investigations reveal that varying strain from tensile to compressive enhances the Co 3d band filling, thereby suppressing the Co-O hybrid bond in oxygen vacancy channels, elucidating the micro-origin of such compressive-strain-facilitated oxygen migration. Our work suggests that controlling electronic orbital occupation of Co ions in oxygen vacancy channels may help facilitate oxygen migration, providing valuable insights and practical guidance for achieving highly efficient oxygen-migration-related chemical looping and energy conversion with complex oxides.

Activating Surface Lattice Oxygen of a Cu/Zn1-xCuxO Catalyst through Interface Interactions for CO Oxidation

Surface lattice oxygen in metal oxides is a common participant in many chemical reactions. Given this, the structural design of catalysts to activate lattice oxygen and moreover investigations into the effect of lattice oxygen on reaction pathways are hot topics. With this in mind, herein we prepare CuO-Zn_1-xCu_xO (ZCO) nanofibers akin to the Trojan horse legend and via an in situ reduction obtain activated Cu/Zn_1-xCu_xO (Cu/ZCO) nanofibers. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy reveal that surface lattice oxygen of Cu/ZCO is effectively activated from inert O^2- to reactive O^2-x. This activation stems from the enhanced covalence of metal-oxygen bonds and the electron transfer between Cu and the support. Online mass spectrometry reveals that Cu/ZCO with activated lattice oxygen exhibits a higher Mars-van Krevelen reaction efficiency during the CO oxidation process. This study offers a new avenue to engineer interface interactions, given, as highlighted here, the importance of surface lattice oxygen in oxide supports during the catalytic process.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO₃ exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO₃ in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu³⁺ ions stabilize at both Ba²⁺ and Sn⁴⁺ sites consistent with the highly negative formation energy of around -6.26 eV. Eu³⁺ doping leads to an intense ⁵D₀→⁷F₁ orange emission and a feeble ⁵D₀→⁷F₂ red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of Eu_Ba and Eu_Sn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn²⁺ in the PL of BaSnO₃ or Eu²⁺ in the PL of BaSnO3:Eu³⁺. Interestingly, when co-doped, Eu³⁺ stabilizes at Sn⁴⁺ sites whereas La³⁺ stabilizes at Ba²⁺ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La³⁺ ions lead to lowering of symmetry around Eu³⁺ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO₃ in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO₃ and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO₃ is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

Amorphous Boron Dispersed in LaCoO3 with Large Oxygen Vacancies for Efficient Catalytic Propane Oxidation

Unsatisfactory oxygen mobility is a considerable barrier to the development of perovskites for low-temperature volatile organic compounds (VOCs) oxidation. This work introduced small amounts of dispersed non-metal boron into the LaCoO₃ crystal through an easy sol-gel method to create more oxygen defects, which are conducive to the catalytic performance of propane (C₃ H₈ ) oxidation. It reveals that moderate addition of boron successfully induces a high distortion of the LaCoO₃ crystal, decreases the perovskite particle size, and produces a large proportion of bulk Co²⁺ species corresponding to abundant oxygen vacancies. Additionally, surface Co³⁺ species, as the acid sites, which are active for cleaving the C-H bonds of C₃ H₈ molecules, are enriched. As a result, the LCB-7 (molar ratio of Co/B=0.93:0.07) displays the best C₃ H₈ oxidation activity. Simultaneously, the above catalyst exhibits superior thermal stability against CO₂ and H₂ O, lasting 200 h. This work provides a new strategy for modifying the catalytic VOCs oxidation performance of perovskites by the regulation of amorphous boron dispersion.

Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized eg Electronic Structures by Manganese Dopant

Perovskite oxides as bifunctional electrocatalysts toward oxygen reduction (ORR) and oxygen evolution reactions (OER) have been investigated for decades because of the flexible and adjustable electronic structures. For example, by optimizing the strength of the Co-O bond, the ORR and OER activity of a typical perovskite oxide, LaCoO₃, can be improved, but they are still unsatisfying. The insufficient insights into the effects of secondary metal dopants at the B-site on the electronic structure and activity, especially for ORR, significantly limit the R&D of bifunctional perovskite oxide catalysts. In this work, a series of LaMn_xCo_1-xO₃ (x = 0, 0.25, 0.3, 0.35, 0.5, 1) catalysts are prepared by a polyol-assisted solvothermal method to investigate the structure-property relationships between the B-site metal substitution and the electrochemical performance of perovskite oxides catalysts. The optimized LaMn_0.3Co_0.7O₃ catalyst demonstrates an enhanced half-wave potential of 0.72 V for ORR, 52 mV higher than that of the pristine LaCoO₃ (0.668 V). Meanwhile, the OER overpotential of LaMn_0.3Co_0.7O₃ catalyst is 416 mV, which is reduced by 64 mV compared to LaCoO₃ (480 mV). It is revealed that the appropriate Mn dopant efficiently optimizes the covalency of Co-O bonds and significantly reduces the e_g orbit-filling electron from 1.23 of pristine LaCoO₃ to 1.02 in LaMn_0.3Co_0.7O₃ (very close to theoretical value 1). This work paves a new way to design and synthesize bifunctional perovskite oxide electrocatalyst for ORR and OER.

Controllable P Doping of the LaCoO3 Catalyst for Efficient Propane Oxidation: Optimized Surface Co Distribution and Enhanced Oxygen Vacancies

The properties of LaCoO₃ are modified by a controllable P doping strategy via a simple sol-gel route. It is demonstrated that appropriate P doping is beneficial for forming a relatively pure perovskite phase and hinders the growth of perovskite nanoparticles. The combined results of density functional theory (DFT), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), temperature-programmed reduction of hydrogen (H₂-TPR), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia (NH₃-TPD) reveal that appropriate P doping gives rise to more oxygen vacancies, optimized distribution of Co ions, and improved surface acidity, which are beneficial for the adsorption of active oxygen species and the activation of propane molecules, resulting in an excellent catalytic oxidation performance. Especially, LaCo_0.97P_0.03O₃ exhibits more surface-active oxygen species, higher bulk Co³⁺ proportion, increased surface Co²⁺ species, and increased acidity, resulting in its superior propane oxidation performance, which is dominated by the Langmuir-Hinshelwood mechanism. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that the presence of P will accelerate oxygen mobility, which in turn promotes the oxidation rate. Moreover, the obtained LaCo_0.97P_0.03O₃ catalyst displays excellent thermal stability during the 60 h durability test at 400 °C and strong resistance against 5 vol % H₂O and/or 5 vol % CO₂ for prolonged 150 h.

Co3O4-CuCoO2 Nanomesh: An Interface-Enhanced Substrate that Simultaneously Promotes CO Adsorption and O2 Activation in H2 Purification

Nanomaterials are widely used as redox-type reaction catalysts, while reactant adsorption and O₂ activation are hardly to be promoted simultaneously, restricting their applications in many important catalytic fields such as preferential CO oxidation (CO-PROX) in H₂-rich stream. In this work, an interface-enhanced Co₃O₄-CuCoO₂ nanomesh was initially synthesized by a hydrothermal process using aluminum powder as a sacrificial agent. This nanomesh is systematically characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, Raman spectroscopy, X-ray absorption near-edge spectroscopy, hydrogen temperature-programmed reduction, and oxygen temperature-programmed desorption. It is demonstrated that the nanomesh possesses high-density nanopores, enabling a large number of CO adsorption sites exposed to the surface. Meanwhile, electron transfer from O^2- to Co³⁺/Co²⁺ and the weakened bonding strength of Co-O bond at surfaces promoted the oxygen activation and redox ability of Co₃O₄. When tested as a catalyst for CO-PROX, this nanomesh with an optimized pore structure and a surface electronic structure, exhibits a strikingly high catalytic oxidation activity at low temperatures as well as a broader operation temperature window (i.e., CO conversion >99.0%, 100-200 °C) in the CO selective oxidation reaction. The present finding should be highly useful in promoting the quest for better CO-PROX catalysts, a hot topic for proton exchange membrane fuel cells and automotive vehicles.

Switchable Intrinsic Defect Chemistry of Titania for Catalytic Applications

The energy crisis is one of the most serious issue that we confront today. Among different strategies to gain access to reliable fuel, the production of hydrogen fuel through the water-splitting reaction has emerged as the most viable alternative. Specifically, the studies on defect-rich TiO2 materials have been proved that it can perform as an efficient catalyst for electrocatalytic and photocatalytic water-splitting reactions. In this invited review, we have included a general and critical discussion on the background of titanium sub-oxides structure, defect chemistries and the consequent disorder arising in defect-rich Titania and their applications towards water-splitting reactions. We have particularly emphasized the origin of the catalytic activity in Titania-based material and its effects on the structural, optical and electronic behavior. This review article also summarizes studies on challenging issues on defect-rich Titania and new possible directions for the development of an efficient catalyst with improved catalytic performance.

Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La_0.4Sr_0.6CoO_3-δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co⁴⁺ and O^2-. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O^2-) to form more reactive oxygen species (O^{2- x}), which reduces the activation energy barriers of molecular O₂ and leads to a very high CO catalytic activity. The revealing of the activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity.

Oxygen defect engineering by the current effect assisted with temperature cycling in a perovskite-type La0.7Sr0.3CoO3 film

Introducing and modulating the oxygen deficiency concentration have been received as an effective way to obtain high catalytic activity in perovskite oxides. However, it is difficult to control the oxygen vacancy in conventional oxygen defect engineering due to harsh reaction conditions at elevated temperatures and the reducing atmosphere, which make it impractical for many technological applications. Herein, we report a new approach to oxygen defect engineering based on the combination of the current effect and temperature cycling at low temperature. Our investigations revealed that the electrical conductivity of the (011)-La_0.7Sr_0.3CoO₃/PMN-PT film changes continuously from metallicity to insulativity under repeated transport measurements below room temperature, which indicates the transformation of the Co⁴⁺ state to Co³⁺ in the film. Further experiments and analysis revealed that oxygen vacancies can be well regulated by the combined current effect and temperature cycling in repeated measurements, which results in a decrease of Co⁴⁺/Co³⁺ and thus the remarkable variation of conductive properties of the film. Our work provides a simple and highly efficient method to engineer oxygen vacancies in perovskite-type oxides and brings new opportunities in designing high-efficiency oxidation catalysts.

Enhanced CO catalytic oxidation by Sr reconstruction on the surface of LaxSr1-xCoO3-δ

Surface electronic structure of solid materials plays a critical role in heterogeneous catalysis. However, surface chemical composition of the perovskite oxides is usually dominated by segregated A-site cations and the amount of oxygen vacancies is relatively low, which seriously restricts their catalytic oxidation property. Here, we prepare perovskite La_xSr_1-xCoO_3-δ (x=0.3, 0.5, 0.7) with different Sr doping amount and experiment results show that perovskite LSCO with higher content of surface Sr possesses more oxygen vacancies and better catalytic activity. On this basis, we develop a new experimental strategy to create more surface oxygen vacancies to promote their CO catalytic activity. In this method, we use high active hydrogen atoms (BH^4-) as reductant to realize surface in-situ chemical composite modification of La_xSr_1-xCoO_3-δ (x=0.3, 0.5, 0.7), which causes their surface reconstruction (surface Sr enrichment). The regulation mainly focuses on the atomic layer level without damaging their bulk phase structure. Different from traditional high temperature annealing under reducing atmosphere, this method is high-efficiency, green and controllable. Furthermore, we study the surface reconstruction process and demonstrated that it is atomic layer engineering on the surface of La_xSr_1-xCoO_3-δ (x=0.3, 0.5, 0.7) by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS). Our experiment results also show that these samples treated by this method exhibit superior activity for CO oxidation compared with original samples.