Modulating the electronic structure and pseudocapacitance of δ-MnO2 through transitional metal M (M = Fe, Co and Ni) doping

Sulfur-bridged bonds enabled structure modulation and space confinement of MnS for superior sodium-ion capacitors

Manganese sulfide (MnS) is a promising converion-type anode for sodium storage, owing to the virtues of high theoretical capacity, coupled with it crustal abundance and cost-effectiveness. Nevertheless, MnS suffers from inadequate electronic conductivity, sluggish Na+ reaction kinetics and considerable volume variation during discharge/charge process, thereby impeding its rate capability and capacity retention. Herein, a novel lamellar heterostructured composite of Fe-doped MnS nanoparticles/positively charged reduced graphene oxide (Fe-MnS/PG) was synthesized to overcome these issues. The Fe-doping can accelerate the ion/electron transfer, endowing fast electrochemical kinetics of MnS. Meanwhile, the graphene space confinement with strong MnSC bond interactions can facilite the interfacial electron transfer, hamper volume expansion and aggregation of MnS nanoparticles, stabilizing the structural integrity, thus improving the Na+ storage reversibility and cyclic stability. Combining the synergistic effect of Fe-doping and space confinement with strong MnSC bond interactions, the as-produced Fe-MnS/PG anode presents a remarkable capacity of 567 mAh/g at 0.1 A/g and outstanding rate performance (192 mAh/g at 10 A/g). Meanwhile, the as-assembled sodium-ion capacitor (SIC) can yield a high energy density of 119 Wh kg-1 and a maximum power density of 17500 W kg-1, with capacity retention of 77 % at 1 A/g after 5000 cycles. This work offers a promising strategy to develop MnS-based practical SICs with high energy and long lifespan, and paves the way for fabricating advanced anode materials.

3D printing of layered vanadium disulfide for water-in-salt electrolyte zinc-ion batteries

Miniaturized aqueous zinc ion batteries are attractive energy storage devices for wearable electronics, owing to their safety and low cost. Layered vanadium disulfide (VS2) has demonstrated competitive charge storage capability for aqueous zinc ion batteries, as a result of its multivalent states and large interlayer spacing. However, VS2 electrodes are affected by quick oxide conversion, and they present predefined geometries and aspect ratios, which hinders their integration in wearables devices. Here, we demonstrate the formulation of a suitable ink for extrusion-based 3D printing (direct ink writing) based on micro flowers of layered VS2 obtained using a scalable hydrothermal process. 3D printed architectures of arbitrary design present electrochemically active, porous and micron-sized struts with tuneable mass loading. These were used as cathodes for aqueous zinc-ion battery electrodes. The 3D printed VS2 cathodes were assembled with carbon/zinc foil anodes to form full cells of zinc-ion, demonstrating a capacity of ∼1.98 mA h cm-2 with an operating voltage of 1.5 V. Upon cycling a capacity retention of around 65% was achieved after ∼100 cycles. The choice of the electrolyte (a water-in-salt electrolyte) and the design of the pre-processing of the 3D printed cathode ensured improved stability against dissolution and swift oxidation, notorious challenges for VS2 in an aqueous environment. This works paves the way towards programmable manufacturing of miniaturized aqueous batteries and the materials processing approach can be applied to different materials and battery systems to improve stability.

MOF-derived NiAl2O4/NiCo2O4 porous materials as supercapacitors with high electrochemical performance

Metal-organic framework compounds are extensively utilized in various fields, such as electrode materials, owing to their distinctive porous structure and significant specific surface area. In this study, NiCoAl-MOF metal-organic framework precursors were synthesized by a solvothermal method, and NiAl2O4/NiCo2O4 electrode materials were prepared by the subsequent calcination of the precursor. These materials were characterized by XRD, XPS, BET tests, and SEM, and the electrochemical properties of the electrode materials were tested by CV and GCD methods. BET tests showed that NiAl2O4/NiCo2O4 has an abundant porous structure and a large specific surface area of up to 105 m2 g-1. The specific capacitance of NiAl2O4/NiCo2O4 measured by the GCD method reaches up to 2870.83 F g-1 at a current density of 1 A g-1. The asymmetric supercapacitor NiAl2O4/NiCo2O4//AC assembled with activated carbon electrodes has a maximum energy density of 166.98 W h kg-1 and a power density of 750.00 W kg-1 within a voltage window of 1.5 V. In addition, NiAl2O4/NiCo2O4 materials have good cycling stability. These advantages make it a good candidate for the application of high-performance supercapacitors.

Designing Yolk-Shell Nanostructures for Reversible Water-Vapor-Responsive Dual-Mode Switching of Fluorescence and Structural Color

Metal halide perovskites offer ample opportunities to develop advanced optoelectronic devices. This work showcases that the integration of metal halide perovskites into metal oxide nanoshells with controllable interior cavities can enable water-vapor-responsive dual-mode switching of fluorescence and structural color. Through a ship-in-a-bottle method to introduce a controlled amount of CsPbBr3 into MnO2 nanoshells, we have designed CsPbBr3@MnO2 yolk-shell nanostructures, which can uptake a defined amount of water to exhibit rapid (less than 1 s) and reversible (≥100 cycles) responses in both fluorescence on-off and color change when exposed to dynamic water vapor. These responses originate from the water-triggered phase transformation of CsPbBr3 to CsPb2Br5 and the structural color change of the MnO2 shell. The altered electronic and bonding structure at the oxide-halide interface, rapid water accumulation in the yolk-shell cavity, and protective effect of the oxide shell facilitate the reversible transformations. The response characteristics of the yolk-shell nanostructures have been further demonstrated in fabricating patterned films capable of multiple fluorescence/structural color responses, highlighting their potential for applications in advanced anticounterfeiting and encryption.

Engineering the crystal facets of α-MnO2 nanorods for electrochemical energy storage: experiments and theory

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO2 nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn3+ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO2 nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g-1 and 133.0 F g-1 than their α-MnO2-200 counterparts (67.5 F g-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO2. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.

Porous prussian blue analogs derived nickel-iron bimetallic phosphide nanocubes on conductive hollow mesoporous carbon nanospheres for stable and flexible high-performance supercapacitor electrode

Nickel-iron bimetallic phosphide (Ni-Fe-P) is the ideal battery-type materials for supercapacitor in virtue of high theoretical specific capacitance. Nevertheless, its actual adhibition is astricted on account of inferior rate capability and cyclic stability. Herein, we constructed hierarchical core-shell nanocomposites with hollow mesoporous carbon nanospheres (HMCS) packaged via prussian blue analogs derived Ni-Fe-P nanocubes (Ni-Fe-P@HMCS), as a positive electrode for hybrid supercapacitor (HSC). Profiting from the cooperative effects of Ni-Fe-P nanocubes with small size and good dispersibility, and HMCS with continuously conductive network, the Ni-Fe-P@HMCS composite electrode with abundantly porous architectures presents an ultrahigh gravimetric specific capacity for 739.8 C g-1 under 1 A g-1. Specially, the Ni-Fe-P@HMCS electrode presents outstanding rate capability of 78.4% (1 A g-1 to 20 A g-1) and cyclic constancy for 105% after 5000 cycles. Density functional theory implies that the composite electrode possesses higher electrical conductivity than bare Ni-Fe-P electrode by reason of the incremental charge density, and the electrons transferring from NiFe3P4 to HMCS layers. Additionally, the assembled Ni-Fe-P@HMCS//HMCS HSC facility delivers the high energy density for 64.1 Wh kg-1, remarkable flexibility and mechanical stability. Thus, this work proffers a viable and efficacious measure to construct ultra-stability electrode for high-performance portable electronic facilities.

Green Synthesis of NiFe2O4 Nano-Spinel Oxide-Decorated Carbon Nanotubes for Efficient Capacitive Performance-Effect of Electrolyte Concentration

Energy storage applications received great attention due to environmental aspects. A green method was used to prepare a composite of nickel-iron-based spinel oxide nanoparticle@CNT. The prepared materials were characterized by different analytical methods like X-ray diffraction, X-ray photon spectroscopy (XPS), scanning electron microscopy (SEM), and transmitted electron microscopy (TEM). The synergistic effect between nickel-iron oxide and carbon nanotubes was characterized using different electrochemical methods like cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS). The capacitances of the pristine NiFe2O4 and NiFe2O4@CNT were studied in different electrolyte concentrations. The effect of OH- concentrations was studied for modified and non-modified surfaces. Furthermore, the specific capacitance was estimated for pristine and modified NiFe2O4 at a wide current range (5 to 17 A g-1). Thus, the durability of different surfaces after 2000 cycles was studied, and the capacitance retention was estimated as 78.8 and 90.1% for pristine and modified NiFe2O4. On the other hand, the capacitance rate capability was observed as 65.1% (5 to 17 A g-1) and 62.4% (5 to 17 A g-1) for NiFe2O4 and NiFe2O4@CNT electrodes.

Effect of bismuth doping on zircon-type gadolinium vanadate: Effective electrocatalyst for determination of hazardous herbicide mesotrione

Pesticides are used to promote the growth of plants and crops by killing weeds and other pests. On the other hand, overused and unused pesticides can leach into groundwater and agricultural lands, easily contaminating water, air, and soil resources. Doping with metal ions is an effective method to improve the catalytic activity of potential electrode materials. In the present study, an electrochemical sensor based on Bi³⁺-doped gadolinium vanadate nanoparticles (GVB NPs) was fabricated for sensitive and selective detection of harmful pesticide mesotrione (MST). The crystalline nature, functional groups, and elemental composition of the prepared electrocatalysts were confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Field-emission scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) showed that the undoped gadolinium vanadate had a rice-like nanostructure and was designated as GV NRs, while GVB had the morphology of nanoparticles. The fabricated electrode exhibited a well-resolved MST reduction peak in cyclic voltammetry and linear sweep voltammetry (LSV). Bismuth doping effectively enhanced the MST reduction and produced a stronger cathodic current response than bare and GV NRs-modified GCE. Moreover, GVB NPs/GCE show a nanomolar detection limit of 45 nM with a sensitivity of 0.43 μA μM^-1 cm^-2. The proposed sensor showed good repeatability, reproducibility, and stability in LSV analysis. The fabricated MST sensor was successfully applied to the analysis of real samples (river water and corn) with good recovery results.

Suppressed Layered-to-Spinel Phase Transition in δ-MnO2 via van der Waals Interaction for Highly Stable Zn/MnO2 Batteries

Although birnessite-type manganese dioxide (δ-MnO₂ ) with a large interlayer spacing (≈7 Å) is a promising cathode candidate for aqueous Zn/MnO₂ batteries, the poor structural stability associated with Zn²⁺ intercalation/deintercalation limits its further practical application. Herein, δ-MnO₂ ultrathin nanosheets are coupled with reduced graphene oxide (rGO) via van der Waals (vdW) self-assembly in a vacuum freeze-drying process. It is interesting to find that the presence of vdW interaction between δ-MnO₂ and rGO can effectively suppress the layered-to-spinel phase transition in δ-MnO₂ during cycling. As a result, the coupled δ-MnO₂ /rGO hybrid cathode with a sandwich-like heterostructure exhibits remarkable cycle performance with 80.1% capacity retained after 3000 cycles at 2.0 A g^-1 . The first principle calculations demonstrate that the strong interfacial interaction between δ-MnO₂ and rGO results in improved electron transfer and strengthened layered structure for δ-MnO₂ . This work establishes a viable strategy to mitigate the adverse layered-to-spinel phase transition in layered manganese oxide in aqueous energy storage systems.

Engineering a Novel AgMn2O4@Na0.55Mn2O4 Nanosheet toward High-Performance Electrochemical Capacitors

Manganese oxides, as a type of two-dimensional (2D) material with high specific area and low cost, are considered promising energy storage materials. Here, we report novel AgMn2O4/Na0.55Mn2O4 nanosheets created by a popular liquid precipitation method with different AgNO3 contents, and their corresponding physical and electrochemical characterizations are performed. The results show that the ultra-thin Na0.55Mn2O4 nanosheets were combined with the AgMn2O4 nanoparticles and an enhancement in their specific capacity was observed compared to the pristine sheets. This electrode material displays a peak specific capacitance of 335.94 F g−1 at 1 A g−1. Using an asymmetric supercapacitor (ASC) assembled using a positive electrode made of AgMn2O4/Na0.55Mn2O4 nanosheets and a reduced graphene oxide (rGO) negative electrode, a high energy density of 65.5 Wh kg−1 was achieved for a power density of 775 W kg−1. The ASC showed good cycling stability with a capacitance value maintained at 90.2% after 10000 charge/discharge cycles. The excellent electrochemical performance of the device was ascribed to the heterostructures and the open space formed by the interconnected manganese oxide nanosheets, which resulted in a rapid and reversible faraday reaction in the interface and further enhanced its electrochemical kinetics.

Mechanochemically synthesized Fe-Mn binary oxides for efficient As(III) removal: Insight into the origin of synergy action from mutual Fe and Mn doping

Iron manganese oxide resources are widely derived from the geological structure, and their combinations play an important role in the migration and transformation of arsenic. Iron oxide and manganese oxide exist generally in a mixed state in Fe-Mn oxides synthesized via the well studied co-precipitation methods using potassium permanganate and manganese/iron sulfates. Herein, a newly designed Fe-Mn-O compositing oxide with Fe-MnO₂, Mn-Fe₂O₃, (Fe_0.67Mn_0.33)OOH solid solution and FeOOH as the main components, simply through solvent-free mechanical ball milling pyrolusite (MnO₂) and ferrihydrite (FeOOH) together has been reported. Atomic-scale integrations by doping Fe and Mn with each other were detected and an adsorption-oxidation bifunctionality was achieved, where Fe-doped MnO₂ served as oxidizer for As(III) and amorphous/ground FeOOH acted as adsorbent first for As(III) and then As(V) from the oxidization. The maximal adsorption for As(III) could reach 44.99 mg/g and over 82.5% of As(III) was converted to As(V). More importantly, high removal ability of arsenic worked in a wide pH range of 2-10.5%, and 87.2% of its initial adsorption-oxidation capacity could be kept even after 5-cycles reuse for treating 20 mg/L As(III) with a dosage at 1 g/L. Together with the enhanced adsorption capacity by the milled FeOOH, surface electron transfer efficiency of the developed Fe-MnO₂ surrounded with Mn-Fe₂O₃ has been studied for the first time to understand the oxidization effect to As(V). Besides the environment-friendliness of ball milling method, the prepared sample is quite stable without noticeable metal release into solution. Mechanism studies of arsenic removal by the as-prepared Fe-Mn-O oxide provide a new direction for improving the oxidation efficiency of MnO₂ to As(III) based on the widely available cheap Mn and Fe oxides, contributing to the development of advanced oxidization process in the treatment of waste water.

Enhanced electrochemical performance in an aluminium doped δ-MnO2 supercapacitor cathode: experimental and theoretical investigations

One of the biggest challenges faced by the layered manganese oxide MnO₂ used as a supercapacitor cathode is the capacity fading caused by chemo-mechanical degradation and/or structural transformation occurring in the discharging/charging process. Here, based on systematical experimental measurements and theoretical calculations, we show that both the stability and rate performance of the δ-MnO₂ supercapacitor cathode can be significantly enhanced by Al doping. Compared with pure δ-MnO₂, Al doping (δ-Al_0.06MnO₂) clearly improves the specific capacitance (7% enhancement at 0.1 mA cm^-2) and cycling stability (12% enhancement after 5000 cycles) simultaneously. These improvements can be attributed to the enhanced electronic transport and formation of more active sites, which are introduced by Al doping. Additionally, our calculations demonstrate that the doped systems (Al atoms located at Mn or O sites) show smaller surface energies than that of pure δ-MnO₂, which hinders side reactions or structure transformations and leads to a better cycling lifetime. Our work gives a comprehensive understanding of the impacts on the performance of δ-MnO₂ introduced by Al doping, and provides a feasible scheme to study the electrochemical mechanism of metal-doped δ-MnO₂.

Dual modulation of the morphology and electric conductivity of NiCoP on nickel foam by Fe doping as a superior stability electrode for high energy supercapacitors

Nickel-cobalt bimetallic phosphide (NiCoP) is a potential electrode material for supercapacitors on account of its high theoretical specific capacitance. However, its practical application is restricted because of its relatively poor cycling stability and rate performance. Herein, we constructed self-standing NiCoP nanowires and Fe doped NiCoP nanoarrays with different iron ion concentrations on nickel foam (Fe-NiCoP/NF-x%, x = 4, 6.25, 12.5, 25) as a positive electrode for asymmetric supercapacitors (ASCs). The morphological result reveals that the nanostructure of the material evolves from nanowires to nanosheets with the iron doping concentration, and the Fe-NiCoP/NF-12.5% nanosheets possess a more stable structure than NiCoP/NF nanowires. The density functional theory analysis implies that the conductivity of the material enhances after Fe doping because of the increased charge density and electron states. The combination of multicomponents and structural advantages endows the optimal Fe-NiCoP/NF-12.5% electrode with an ultrahigh areal capacitance of 9.93 F cm^-2 (2758.34 F cm^-3) under 1 mA cm^-2, excellent rate capability (82.58% from 1 mA cm^-2 to 50 mA cm^-2) and superior cycling stability (95.72% retention over 5000 cycles under 20 mA cm^-2), and the areal capacitance of Fe-NiCoP/NF-12.5% is 2.27 times higher than that of the pristine NiCoP/NF electrode at 1 mA cm^-2. Moreover, the assembled Fe-NiCoP/NF-12.5%//activated carbon ASC device delivers a high energy density of 0.327 mW h cm^-2 (60.43 mW h cm^-3) at 1.10 mW cm^-2 (202.54 mW cm^-3). Therefore, this strategy may provide a novel route for the application of NiCoP with its intrinsic advantages in the energy storage field.

Highly Effective Self-Propagating Synthesis of Lamellar ZnO-Decorated MnO2 Nanocrystals with Improved Supercapacitive Performance

A series of MO_x (M = Co, Ni, Zn, Ce)-modified lamellar MnO₂ electrode materials were controllably synthesized with a superfast self-propagating technology and their electrochemical practicability was evaluated using a three-electrode system. The results demonstrated that the specific capacitance varied with the heteroatom type as well as the doping level. The low ZnO doping level was more beneficial for improving electrical conductivity and structural stability, and Mn10Zn hybrid nanocrystals exhibited a high specific capacitance of 175.3 F·g⁻¹ and capacitance retention of 96.9% after 2000 cycles at constant current of 0.2 A·g⁻¹. Moreover, XRD, SEM, and XPS characterizations confirmed that a small part of the heteroatoms entered the framework to cause lattice distortion of MnO₂, while the rest dispersed uniformly on the surface of the carrier to form an interfacial collaborative effect. All of them induced enhanced electrical conductivity and electrochemical properties. Thus, the current work provides an ultrafast route for development of high-performance pseudocapacitive energy storage nanomaterials.

Immobilized Co2+ and Cu2+ induced structural change of layered double hydroxide for efficient heterogeneous degradation of antibiotic

In this study, MgMn-layered double hydroxide (MgMnLDH) exhibited excellent remediation functionality for heavy metals-antibiotics combined pollution. On the one hand, Co²⁺ and Cu²⁺ was efficiently immobilized on MgMnLDH with maximum quantity of 4.30 and 10.65 mmol g^-1, respectively. A series of characterizations reflected the changes in structure and physicochemical properties of MgMnLDH after the immobilization. Density functional theory calculations (DFT) confirmed that the binding modes were lattice substitution for Co²⁺ and surface precipitation for Cu²⁺. On the other hand, the immobilized heavy metals enhanced the heterogeneous degradation for sulfamethoxazole (SMX) by peroxymonosulfate (PMS) activation. Complete degradation was achieved within 10 min in MgMnLDH-Co-4/PMS system and 60 min in MgMnLDH-Cu/PMS system, while only 20% in MgMnLDH/PMS system. The pH adaptability, reusability, stability and activation mechanism of two systems were systematically compared. The superior degradation performance of MgMnLDH-Co-4 benefited from the intense Co/Mn synergism and abundant oxygen vacancies, which could accelerate electron transfer during PMS activation process. The applicability of two catalysis system was confirmed in purifying other antibiotics and actual wastewater. The results highlight the importance of structural control in heterogeneous catalysis and provide new idea for environmental remediation.

Enhanced capacitive performance of a Ag-functionalized low crystalline Co3O4/graphene composite

The addition of Ag increased the capacitance of Co3O4 nanowires by about 5.8 times.

Two-step hydrothermal synthesis of (NH4)xWO3 hollow spherical and hierarchical structures

Two novel (NH₄)_xWO₃ structures were prepared by a two-step hydrothermal route. Also, the possible formation mechanism of these two (NH₄)_xWO₃ structures was proposed and discussed.

High areal capacitance of manganese oxide electrodes with cerium as rare earth modification

Manganese oxides have attracted wide attention as promising electrode materials for high-energy density supercapacitors. However, the electrochemical performance of the manganese oxide materials deteriorates considerably with the increase in mass loading due to their moderate electronic and ionic conductivities. This phenomenon leads to low areal capacitance, which limits the practical application of these materials. Herein, we perform a potentiostatic electrodeposition of manganese oxides with Ce as rare earth (RE) modification on a nickel (Ni) foam substrate to achieve high areal capacitance. Under optimum conditions, manganese oxide nanosheets are axially grown on Ni foam to form a hierarchically porous network nanostructure, which ensures facile ionic and electric transport. The Ce-modified manganese oxide with the Mn:Ce molar ratio of 1:0.1 yields an outstanding areal capacitance of 3.67 F cm^-2 at 2 mA cm^-2 and a good rate capability compared with the capacitance of 2.59 F cm^-2 at 2 mA cm^-2 of pure manganese oxide without the addition of Ce. This result verifies the importance of Ce modification to manganese oxides. Our results suggest the important role played by the RE element Ce in enhancing the electrochemical performance of high areal capacitance manganese oxide electrodes, which is essential to bringing them one step toward further practical applications.

High-rate asymmetrical supercapacitors based on cobalt-doped birnessite nanotubes and Mn-FeOOH nanotubes

Co-Doped MnO2 nanotubes (Co-MnO2-5) were prepared as the positive electrode of supercapacitors via a simple one-step hydrothermal method. Co doping and one-dimensional tunneling of nanotubes result in low internal resistance and good ionic contact, enhancing the conductivity and electrochemical performance of the electrodes.

Construction of Defect-Rich Ni-Fe-Doped K0.23 MnO2 Cubic Nanoflowers via Etching Prussian Blue Analogue for Efficient Overall Water Splitting

Designing elaborate nanostructures and engineering defects have been promising approaches to fabricate cost-efficient electrocatalysts toward overall water splitting. In this work, a controllable Prussian-blue-analogue-sacrificed strategy followed by an annealing process to harvest defect-rich Ni-Fe-doped K_0.23 MnO₂ cubic nanoflowers (Ni-Fe-K_0.23 MnO₂ CNFs-300) as highly active bifunctional catalysts for oxygen and hydrogen evolution reactions (OER and HER) is reported. Benefiting from many merits, including unique morphology, abundant defects, and doping effect, Ni-Fe-K_0.23 MnO₂ CNFs-300 shows the best electrocatalytic performances among currently reported Mn oxide-based electrocatalysts. This catalyst affords low overpotentials of 270 (320) mV at 10 (100) mA cm^-2 for OER with a small Tafel slope of 42.3 mV dec^-1 , while requiring overpotentials of 116 and 243 mV to attain 10 and 100 mA cm^-2 for HER respectively. Moreover, Ni-Fe-K_0.23 MnO₂ CNFs-300 applied to overall water splitting exhibits a low cell voltage of 1.62 V at 10 mA cm^-2 and excellent durability, even superior to the Pt/C||IrO₂ cell at large current density. Density functional theory calculations further confirm that doping Ni and Fe into the crystal lattice of δ-MnO₂ can not only reinforce the conductivity but also reduces the adsorption free-energy barriers on the active sites during OER and HER.

Ti-Based Oxide Anode Materials for Advanced Electrochemical Energy Storage: Lithium/Sodium Ion Batteries and Hybrid Pseudocapacitors

Titanium-based oxides including TiO₂ and M-Ti-O compounds (M = Li, Nb, Na, etc.) family, exhibit advantageous structural dynamics (2D ion diffusion path, open and stable structure for ion accommodations) for practical applications in energy storage systems, such as lithium-ion batteries, sodium-ion batteries, and hybrid pseudocapacitors. Further, Ti-based oxides show high operating voltage relative to the deposition of alkali metal, ensuring full safety by avoiding the formation of lithium and sodium dendrites. On the other hand, high working potential prevents the decomposition of electrolyte, delivering excellent rate capability through the unique pseudocapacitive kinetics. Nevertheless, the intrinsic poor electrical conductivity and reaction dynamics limit further applications in energy storage devices. Recently, various work and in-depth understanding on the morphologies control, surface engineering, bulk-phase doping of Ti-based oxides, have been promoted to overcome these issues. Inspired by that, in this review, the authors summarize the fundamental issues, challenges and advances of Ti-based oxides in the applications of advanced electrochemical energy storage. Particularly, the authors focus on the progresses on the working mechanism and device applications from lithium-ion batteries to sodium-ion batteries, and then the hybrid pseudocapacitors. In addition, future perspectives for fundamental research and practical applications are discussed.

Experimental and Theoretical Investigation of the Effect of Oxygen Vacancies on the Electronic Structure and Pseudocapacitance of MnO2

Defect engineering is an effective way to modulate the intrinsic physicochemical properties of materials. In this work, δ-MnO₂ with oxygen vacancies is fabricated by a simple oxidation or reduction process, and the relationship between the electronic structure and pseudocapacitance is systematically studied through experimental analysis and theoretical calculations. The peaks in the Raman spectra of the as-prepared samples are shifted compared with those of pure MnO₂ and the Mn³⁺ /Mn⁴⁺ ratio and O species content also change after the introduction of oxygen vacancies. The optimized samples exhibit a better specific capacitance of 207 F g^-1 after the oxidation process and 181.4 F g^-1 after the reduction treatment compared with only 143.9 F g^-1 for the pure MnO₂ . The samples obtained through the oxidation or reduction process also retain 93.3 or 86.4 % of the initial capacity after 5000 cycles. The excellent properties are attributed to the enhanced conductivity and increased surface reactivity or electrochemically active sites. Theoretical calculations demonstrate that the presence of oxygen vacancies leads to an increase in the density of states, which improves the redox reaction of MnO₂ . This study will provide a reference for exploring and designing highperformance pseudocapacitive materials.

Fe-Doped α-MnO2 nanorods for the catalytic removal of NOx and chlorobenzene: the relationship between lattice distortion and catalytic redox properties

Controllably tuning redox performance is one of the key targets in catalysis.