Design and Synthesis of Hierarchical SiO2@C/TiO2 Hollow Spheres for High-Performance Supercapacitors

Facile and cost-effective NiO/MgO-SiO2 composites for efficient oxygen evolution reaction and asymmetric supercapacitor systems

Biomass waste from grapefruit peel extract was used for the preparation of MgO-SiO2 composites in situ in order to develop effective electrocatalytic composites based on NiO/MgO-SiO2. The MgO-SiO2 composites were subsequently deposited with NiO using a modified hydrothermal method. The synthesized materials were analyzed to investigate their morphology, crystal structure, chemical composition, functional group, and optical band gap. The structural analysis allowed us to determine the orientation of the nanoparticles, the cubic phase of NiO and MgO, the significant loss of optical band gap, and the enriched functional groups on the surface of NiO/MgO-SiO2 composites. The electrochemical properties were investigated in the presence of an alkaline solution of KOH. To study the oxygen evolution reaction (OER) in 1 M KOH aqueous solution, different NiO/MgO-SiO2 composites were investigated. It was found that the NiO/MgO-SiO2 composite that contained the highest amount of MgO-SiO2 (sample 3) had a lower overpotential than the NiO/MgO-SiO2 composite with the lowest amount of MgO-SiO2. Sample 3 exhibited an overpotential of 230 mV at 10 mA cm-2 over a period of 40 hours with excellent stability. The superior electrochemical activity of the NiO/MgO-SiO2 composite (sample 3) was demonstrated in an energy storage device using 3 M KOH aqueous solution, and asymmetric supercapacitor devices were fabricated in 3 M KOH solution. According to the ASC's specifications, a specific capacitance of 344.12 F g-1 and an energy density of 7.31 W h kg-1 were found for the device at a fixed current density of 1.5 A g-1. After over 40 000 galvanic charge-discharge repeatable cycles at 1.5 A g-1, sample 3 of the NiO/MgO-SiO2 composite exhibited excellent cycling stability with 88.9% percent capacitance retention. During the performance evaluation of the NiO/MgO-SiO2 composites, grapefruit peel extract was confirmed as a potential biomass waste for the fabrication of high-performance energy conversion and storage devices.

The support effect on the performance of a MOF-derived Co-based nano-catalyst in Fischer Tropsch synthesis

The catalyst plays a central role in the Fischer-Tropsch synthesis (FTS) process, and the choice of catalyst support significantly impacts FTS catalyst performance by enhancing its attributes. In this study, the effects of utilizing various metal oxides-CeO2, ZrO2, and TiO2-on a cobalt-based FTS nanocatalyst are investigated by evaluating the catalyst's reducibility, stability, syngas chemisorption, intermediate species spillover, charge transfer, and metal-support interaction (MSI). This evaluation is conducted both theoretically and experimentally through diverse characterization tests and molecular dynamics (MD) simulations. Characterization tests reveal that the ceria-supported catalyst (Ceria Nano Catalyst, CNC) demonstrates the highest reducibility, stability, CO chemisorption, and spillover, while the zirconia-supported catalyst (Zirconia Nano Catalyst, ZNC) exhibits the highest hydrogen chemisorption and spillover. The MD simulation results align well with these findings; for instance, ZNC has the lowest hydrogen adsorption enthalpy (ΔHAds.), whereas CNC has the lowest ΔHAds. for CO. Additionally, MD simulations indicate that the titania-supported catalyst (Titania Nano Catalyst, TNC) possesses the highest MSI value, closely resembling that of ZNC, albeit with a minor difference. The TNC catalyst's performance in other tests is also similar to that of ZNC. Finally, FTS performance tests illustrate that the ZNC catalyst achieves the highest CO conversion at 88.1%, while the CNC catalyst presents the lowest CO conversion at 82.2%. Notably, the CNC catalyst showcases the highest durability, with only a 4.4% loss in CO conversion and an 8.55% loss in C5+ yield after 192 h of operation.

Modified preparation of Si@C@TiO2 porous microspheres as anodes for high-performance lithium-ion batteries

Microscale porous silicon materials have shown great application potential as anodes for next-generation lithium-ion batteries (LIBs); however, they face significant challenges, including mechanical structure instability, low intrinsic conductivity, and uncontrollable processing. In this study, a modified etching strategy combined with a facile sol-gel method is demonstrated to prepare microscale porous Si microspheres encapsulated by an inner amorphous carbon shell (≈10 nm) and an outer rigid anatase titanium oxide (TiO₂) shell (≈20 nm) (PSi@C@TiO₂), with the intact porous framework and core-shell-shell spherical structure. The interconnected pores can sufficiently accommodate the expansion of the Si core during lithiation. Moreover, the double shells can not only enhance the kinetic behavior of the PSi@C@TiO₂ microspheres, but can act as a compact fence to force the Si core to expand toward the internal pores during lithiation, ensuring the integrity of the porous spherical structure. As a result, the PSi@C@TiO₂ anodes show greatly superior high specific capacity, excellent rate capability, stable solid-electrolyte interphase (SEI) films and steady mechanical structure. It delivers a high reversible capacity of 1004 mA h g^-1 after 250 cycles at 0.5 A g^-1. This study provides a modified method to prepare microscale porous Si anodes with a stable mechanical structure and long cycle life for LIBs.

Synthesis, modification and application of titanium dioxide nanoparticles: a review

Titanium dioxide (TiO₂) has been heavily investigated owing to its low cost, benign nature and strong photocatalytic ability. Thus, TiO₂ has broad applications including photocatalysts, Li-ion batteries, solar cells, medical research and so on. However, the performance of TiO₂ is not satisfactory due to many factors such as the broad band gap (3.01 to 3.2 eV) and fast recombination of electron-hole pairs (10^-12 to 10^-11 s). Plenty of work has been undertaken to improve the properties, such as structural and dopant modifications, which broaden the applications of TiO₂. This review mainly discusses the aspects of TiO₂-modified nanoparticles including synthetic methods, modifications and applications.

Tunable Synthesis of Hierarchical Yolk/Double-Shelled SiOx @TiO2 @C Nanospheres for High-Performance Lithium-Ion Batteries

This work reports the preparation of unique hierarchical yolk/double-shelled SiO_x @TiO₂ @C nanospheres with different voids by a facile sol-gel method combined with carbon coating. In the preparation process, SiO_x nanosphere is used as a hard template. Etch time of SiO_x yolk affects the morphology and electrochemical performance of SiO_x @TiO₂ @C. With the increase in etch time, the yolk/double-shelled SiO_x @TiO₂ @C with 15 and 30 nm voids and the TiO₂ @C hollow nanospheres are obtained. The yolk/double-shelled SiO_x @TiO₂ @C nanospheres exhibit remarkable lithium-ion battery performance as anodes, including high lithium storage capacity, outstanding rate capability, good reversibility, and stable long-term cycle life. The unique structure can accommodate the large volume change of the SiO_x yolk, provide a unique buffering space for the discharge/charge processes, improve the structural stability of the electrode material during repeated Li⁺ intercalation/deintercalation processes, and enhance the cycling stability. The SiO_x @TiO₂ @C with 30 nm void space exhibits a high discharge specific capacity of ≈1195.4 mA h g^-1 at the current density of 0.1 A g^-1 after 300 cycles and ≈701.1 mA h g^-1 at 1 A g^-1 for over 800 cycles. These results suggest that the proposed particle architecture is promising and may have potential applications in improving various high performance anode materials.

Recent Advances in Synthesis and Applications of Carbon-Doped TiO2 Nanomaterials

TiO2 has been widely used as a photocatalyst and an electrode material toward the photodegradation of organic pollutants and electrochemical applications, respectively. However, the properties of TiO2 are not enough up to meet practical needs because of its intrinsic disadvantages such as a wide bandgap and low conductivity. Incorporation of carbon into the TiO2 lattice is a promising tool to overcome these limitations because carbon has metal-like conductivity, high separation efficiency of photogenerated electron/hole pairs, and strong visible-light absorption. This review would describe and discuss a variety of strategies to develop carbon-doped TiO2 with enhanced photoelectrochemical performances in environmental, energy, and catalytic fields. Emphasis is given to highlight current techniques and recent progress in C-doped TiO2-based materials. Meanwhile, how to tackle the challenges we are currently facing is also discussed. This understanding will allow the process to continue to evolve and provide facile and feasible techniques for the design and development of carbon-doped TiO2 materials.

Metal Nanoparticles Confined within an Inorganic-Organic Framework Enable Superior Substrate-Selective Catalysis

The search for catalysts with a perfect substrate selectivity toward the hydrogenation of nitroarenes is a goal of high importance, which still remains a significant challenge. Here, we designed a new type of catalyst with superior substrate selectivity by combining a space-confined effect and a hydrogen-bonding network, in which metal nanoparticles (MNPs) were confined in hierarchical hollow silica (HHS) with a poly(N-isopropylacrylamide) (PNIPA) coating. Given the strong induced properties of hydrogen-bond donors and acceptors in the HHS support and PNIPA coating, the as-synthesized catalyst would achieve perfect substrate selectivity for the hydrogenation of various nitroarenes and their mixture by thoroughly impeding the reduction of nitroarenes with any hydroxyl or carboxyl groups, which is typically very difficult to be realized over almost all of the reported supported-metal catalysts. Notably, the hydrogenation of nitroarenes can produce almost quantitative yields of anilines over the as-synthesized catalyst. Furthermore, density functional theory and experimental evidence are also provided for the hierarchical structure of HHS and PNIPA coating associated with substrates to demonstrate how a substrate could have access or be blocked into the confined active centers (MNPs). Therefore, this work would open a new window to design efficient catalysts for a wide variety of substrate-selective catalyses.

Improving the Supercapacitor Performance by Dispersing SiO2 Microspheres in Electrodes

This paper describes a simple, reproducible, and scalable procedure for the preparation of a SiO₂-containing supercapacitor with high cycle stability. A carbon mesoporous material (CMM) with a high specific surface area, CMK-3, was adopted as an electric double-layer capacitor (EDLC) active material for the preparation of electrodes for the supercapacitor. The optimized SiO₂ content decreased as the microsphere diameter decreased, and the optimal specific capacitance was obtained with 6 wt % SiO₂ microspheres (100 nm size). The capacitance improved from 133 to 298 F/g. The corresponding capacitance retention rate after 1000 cycles increased from 68.04 to 91.53%. In addition, the energy density increased from 21.05 to 26.25 Wh/kg with a current density of 1 A/g. Finally, similar results based on active carbon, CeO₂/CMK-3, and graphene/CNT/MnO_v composite electrodes demonstrated that the proposed method exhibits wide compatibility with diverse electrode materials.

Design and synthesis of 'single-crystal-like' C-doped TiO2 nanorods for high-performance supercapacitors

Although TiO₂ is widely used as a promising electrode material for supercapacitors, its potential application suffers from a critical limitation due to its poor electrical conductivity and low rate capability. Here, we report a cost-effective hydrothermal strategy to design and construct a novel 'single-crystal-like' C-doped TiO₂ electrode material. The as-synthesized electrode material combines the advantages of TiO₂, 'single-crystal-like' features and carbon doping, considerably improving the electrical conductivity of TiO₂. The electrochemical measurements demonstrate that the C-doped TiO₂ material presents an excellent specific capacitance (449.8 F g^-1 at 1 A g^-1), which approaches six times more than the value (77.3 F g^-1 at 1 A g^-1) of P25 electrodes, and far beyond the value of many previously reported TiO₂ electrodes. Therefore, this work explores a new method to design high performance electrochemical TiO₂ electrode materials by incorporating other dopants into the TiO₂ lattice.

An electroless-plating-like solution approach for the preparation of PS@TiO2@Ag core-shell spheres

PS@TiO₂@Ag spheres with triple-level core–shell nanostructures were prepared via a versatile coating procedure based on an electroless-plating-like solution deposition (EPLSD) method. A peroxo-titanium-complex (PTC) aqueous solution was used as the precursor to react with an aniline monomer in the EPLSD preparation. Aniline plays an important role in the TiO₂ layer anchoring process through the swollen effects of the PS cores. As extended, peroxo-metal-complex (PMC) with the d⁰ configuration can be introduced onto PS spheres to form varieties of PS@metal oxide core–shell structures by this method under mild conditions. Ag layers were then modified onto the PS@TiO₂ spheres via the photocatalytic method. By the extraction of the PS cores, hollow TiO₂ and TiO₂@Ag spheres could be obtained. The photochemical degradation of methylene blue (MB) under UV light irradiation was performed on the composite nanostructures.

PS@TiO₂@Ag spheres with a core–shell nanostructure were prepared by electroless-plating-like solution deposition (EPLSD) method, which can be alternatively extended to prepare PS@metal(1) oxide@metal(2) composite spheres and their relative hollow spheres.

Capacitive deionization for wastewater treatment: Opportunities and challenges

Capacitive deionization (CDI) is an emerging method for removal of charged ionic species from aqueous solutions, based on electrostatic interactions between (mostly) inorganic ions and porous carbon electrodes. Inspection of recent publications related to CDI processes, revealed that the majority of the publications are related to the removal of salt (NaCl) from the water (desalination) or electrosorption processes. However, such a water desalination is only one process in the improvement of the quality water, it is interesting to review the literature in the context of CDI processes for other water treatment processes. Herein wastewater treatments are discussed. In this paper, we critically review the last publications that relate to capacitive deionization with wastewater treatments. Since wastewater treatments may involve broad aspects, we address in this review four specific water treatment processes that are thought to be connected with CDI processes: organic fouling of CDI cells, removal of heavy metals by CDI processes, removal of organic micropollutants with CDI processes and disinfection with CDI processes. We also evaluate herein the status of several research efforts in this area and suggest future directions.

Bio-derived yellow porous TiO2: the lithiation induced activation of an oxygen-vacancy dominated TiO2 lattice evoking a large boost in lithium storage performance

Oxygen deficient TiO₂ has attracted extensive attention owning to its narrow bandgap and high electrical conductivity. In this work, novel yellow TiO₂ with hierarchically porous architecture is fabricated by a facile pyrolysis method in air via a biomass template. The obtained yellow TiO₂ exhibits interesting lithiation induced activation during cycling, which gives rise to a phase change from poorly crystallized TiO₂ to an amorphous phase, accompanied by a colour change from yellow to black. In contrast to the intercalation mechanism reported in most of the literature on the TiO₂ anode of LIBs, notably, the reversible redox reaction between Ti³⁺ and metal Ti can be verified in this case, demonstrating the novel conversion reaction mechanism of the TiO₂ electrode. Based on this, the yellow porous TiO₂ delivers enhanced electrochemical performance as an anode for LIBs with a superior capacity of 480 mA h g^-1 at 5 A g^-1 and a high capacity of 206 mA h g^-1 at 10 A g^-1 after 8000 cycles.

Hierarchically-Structured TiO2/MnO2 Hollow Spheres Exhibiting the Complete Mineralization of Phenol

Although TiO2 or MnO2-based materials have been widely used for the degradation of phenolic compounds, complete mineralization is still a challenge, especially for TiO2-based materials. Here, we devise a hierarchically-structured TiO2/MnO2 (HTM) hollow sphere, in which hollow TiO2 acts as a skeleton for the deposition of MnO2 in order to prevent the aggregation of MnO2 nanoparticles and to maintain its hollow structure. During the oxidation reaction, the as-synthesized HTM can fully exert their respective advantages of the TiO2 and MnO2 species to realize the first stage of the rapid oxidation degradation of phenol and the second stage of the complete photo-mineralization of residual phenol and its intermediates, which efficiently overcomes the incomplete mineralization of phenolic compounds. The degradation mechanism and pathway of phenol are also proposed according to the analysis of Mass Spectrometry (MS). Therefore, this work provides a new insight for exploring hierarchically-structured materials with two or more species.

CO2-Assisted synthesis of hierarchically porous carbon as a supercapacitor electrode and dye adsorbent

A facile and sustainable strategy was developed for the fabrication of hierarchically porous carbons with tunable pore size distributions and architectures.

Pushing the cycling stability limit of hierarchical metal oxide core/shell nanoarrays pseudocapacitor electrodes by nanoscale interface optimization

Three-dimensional hierarchical metal oxide core/shell nanowire arrays (HMONAs) have become promising pseudocapacitive materials due to their integrated smart architectures. However, these core/shell nanostructures have unsatisfactory structural stability and frequently suffer destruction during their fabrication process and their charge-discharge cycles, thus limiting their application lifespan. Herein, a general strategy based on the minimization of the lattice mismatch between the shell and the backbone at the nanoscale interface has been proposed to improve the cycling stability of the HMONAs. This strategy is achieved by a facile hydrothermal pretreatment under mild acidic condition, where a selective dissolution process occurs for interface optimization. To prove the concept, three typical HMONAs, α-MnO₂ nanotube@δ-MnO₂ nanosheet core/shell arrays, α-MnO₂ nanotube@NiO nanosheet core/shell arrays and Co₃O₄@MnO₂ core/shell nanoarrays, were synthesized for interface optimization. It was found that these thermodynamically unstable nanostructures in the shells of HMONAs can be selectively dissolved under a hydrothermal process, leading to enhanced stability of HMONAs. The comparison study indicates that all treated HMONAs exhibit excellent capacitance retention of 93.2% (MnO₂@MnO₂), 94.3% (MnO₂@NiO) and 95.3% (Co₃O₄@MnO₂) after 5000 cycles, which are 22.9%, 9.3% and 20.1% higher, respectively, than those of the untreated HMONAs. Furthermore, the symmetrical supercapacitors based on treated MnO₂@MnO₂ nanoarrays electrodes also demonstrate 92% capacitance retention after 5000 cycles, showing better comprehensive performance than their untreated counterpart (78% capacitance retention). The general strategy of nanoscale interface optimization provides new opportunities in pushing the cycling stability limit of HMONAs.

Hierarchically-structured SiO2-Ag@TiO2 hollow spheres with excellent photocatalytic activity and recyclability

A new protocol for constructing sandwich-like SiO₂-Ag@TiO₂ hollow spheres (SAT) is introduced, in which SiO₂ acts as an efficient support for the Ag nanoparticles (Ag NPs) immobilization, while TiO₂ maintains its hierarchical structure and prevents the aggregation of Ag NPs during the photocatalytic reaction. As a photocatalytic agent, the inner and outer surfaces of TiO₂ can be fully occupied by pollutants molecules because of its unique structure, which faster boosts the photo-generated electrons to transfer the substrates, leading to an enhanced photocatalytic performance. Compared with Ag NPs deposited on the surface of SiO₂@TiO₂ (STA), the as-synthesized SAT exhibits a markedly enhanced visible-light and UV light activity than STA for degrading tetracycline and traditional dyes. The excellent photocatalytic performances are ascribed to the enhanced transport paths of photo-generated electrons, reduced recombination probability of e^-/h⁺ pairs, and decreased threat of oxidation and corrosion. Especially, the SAT still maintains its photocatalytic efficiency after five consecutive runs even though the sample is recovered under visible-light irradiation, far beyond the reusability of STA under the same conditions. Therefore, the outstanding photocatalytic activity and excellent recyclability make SAT more potential to purify aquatic contaminants and to meet the demands of future environmental issues.

NiMoO4nanorod deposited carbon sponges with ant-nest-like interior channels for high-performance pseudocapacitors

Monolithic carbon sponges with uniformly deposited NiMoO₄nanorods and ant-nest-like interior channels are reported as elastic electrodes for asymmetric supercapacitors.

Hierarchical VOOH hollow spheres for symmetrical and asymmetrical supercapacitor devices

Hierarchical VOOH hollow spheres with low crystallinity composed of nanoparticles were prepared by a facile and template-free method, which involved a precipitation of precursor microspheres in aqueous solution at room temperature and subsequent hydrothermal reaction. Quasi-solid-state symmetric and asymmetric supercapacitor (SSC and ASC) devices were fabricated using hierarchical VOOH hollow spheres as the electrodes, and the electrochemical properties of the VOOH//VOOH SSC device and the VOOH//AC ASC device were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Results demonstrated that the electrochemical performance of the VOOH//AC ASC device was better than that of the VOOH//VOOH SSC device. After 3000 cycles, the specific capacitance of the VOOH//AC ASC device retains 83% of the initial capacitance, while the VOOH//VOOH SSC device retains only 7.7%. Findings in this work proved that hierarchical VOOH hollow spheres could be a promising candidate as an ideal electrode material for supercapacitor devices.