Ex-situ XPS Research Articles

Gallium phosphide conformal film growth on in-situ tri-TBP dry-cleaned InGaP/GaAs using atomic hydrogen ALD

Nearly stoichiometric (high P/Ga ratio and low oxygen content) ∼ 20 nm GaP epi-layers were conformally deposited on the side-walls of InGaP layers using an atomic H ALD process with triethyl gallium (TEGa) and tri-tertiary butyl phosphine (tri-TBP). The key for uniform nucleation is an in-situ tri-TBP dry-clean in 40 mTorr Ar and 160 mTorr H2 without acidic wet-clean to generate a particle-free and damage-free InGaP surface which induces local epitaxy growth of homogeneous GaP thin films. The in-situ tri-TBP dry-clean removed the native oxide of InGaP, surface particles, and the residual impurities simultaneously. It is hypothesized that the key to the process is in-situ formation of PHx(C4H9)y surface adsorbates from atomic H and tri-TBP surface reactions. In-situ AES, cross-sectional STEM image, ex-situ XPS, AFM, and EDS confirmed the formation of high-quality GaP thin films on InGaP/GaAs with a high-quality interface and smooth surface morphology.

Promoted OH– adsorption and electron-transfer kinetics by electrospinning mono-disperse NiCo2S4 nanocrystals within porous CNFs for solid asymmetric supercapacitors

Bimetallic sulfide NiCo2S4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo2S4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH– adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo2S4 can reduce its OH– adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo2S4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo2S4/CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo2S4/CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo2S4/CNF gives the specific capacity of 757.97C g−1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo2S4/CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg−1 for liquid-state device and 1067.4 W kg−1 for solid-state device), high energy density (52.25 Wh kg−1 for liquid-state device and 48.54 Wh kg−1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo2S4/CNF composite.

Fe-N-C decorated fibrous network-wrapped biomass SiOx/C with gradient conductive structure for high performance Li-ion battery anodes

Fabrication of high performance silicon-based materials derived from natural biowastes plays a significant role in the green recycling of biomass resources. However, low utilization of organic component and poor lifespan hinders the its large scale applications. Herein, biomass derived SiOx/C-3 composite was encapsulated into Fe-N-C decorated carbon nanofibers network (Fe-N-C/SiOx/C-3) by a facile magnesiothermic reduction combined with electrostatic spinning strategy. The ingenious designed indirect contact biomass carbon/SiO2 interface effectively utilizes the organic components of rice husk and prevents the formation of SiC during reduction process. HAADF-STEM and XPS characterization confirmed the presence of Fe single atoms and the formation of Fe-N coordination bonds. Benefiting from the unique carbonaceous network and catalytic effect of Fe-N-C, the Fe-N-C/SiOx/C-3 exhibit excellent lithium storage properties (832.6 mAh g−1 after 250 cycles at 0.1 A g−1). Even at high current density (1 A g−1), the electrode can still remain a capacity of 602 mAh g−1 after 1000 cycles with capacity retention of 79.4%. The ex-situ SEM and XPS characterizations demonstrated that the gradient structure consisting of inner biomass-derived carbon and flexible carbonaceous networks enhanced the overall conductivity and structural integrity of the composite. Furthermore, the catalytic effect of Fe-N-C facilitates the rapidly formation of stable LiF-rich solid electrolyte interphase (SEI) films during charge/discharge process. The assembled LiFePO4‖Fe-N-C/SiOx/C-3 full cell shows excellent electrochemical performance (106.5 mAh g−1 after 100 cycles at 0.1A g−1), which provides insights into the fabrication of high performance biomass derived silicon based anodes.

Unlocking a new storage mechanism of high performance VN@N/S–C cathode: Role of electrochemical reduction

Vanadium-based compounds have earned incremental attention as cathode for aqueous zinc ion batteries (AZIBs) due to their ideal electrical conductivity, rich valence states and flexible crystal structures. Unfortunately, their storage mechanisms are confusing and not well-established, especially for nitride. Herein, we have constructed a cubic vanadium nitride and nitrogen/sulfur doped carbon composite (VN@N/S–C) as a cathode for AZIBs. An in-depth examination of ex-situ XRD and XPS reveal that the electrochemical reduction during the first discharge process promotes a structural transformation of cubic VN to layered Zn3+x(OH)2V2O7·2H2O, thus permitting subsequent insertion of Zn2+ in such layered structure. Ex-situ SEM studies of electrodes further identify a morphology change of nanoparticles to nanosheets, corresponding to VN and Zn3+x(OH)2V2O7·2H2O, respectively. New storage mechanism-the electrochemical reduction combined with the contributions from Zn2+ (de)intercalation, furnishes the VN@N/S–C cathode a high discharge capacity of 232 mA h g−1 after 400 cycles at 4 A g−1 and good rate performance (252 mA h g−1 at 6 A g−1). Even at a low temperature of 0 °C, the cathode still delivers an excellent capacity of 361 mA h g−1 after 300 cycles at 1 A g−1. This study offers a hint to improve the understanding on cubic structure for AZIBs.

Self-assembly tourmaline@BiFeO3 composites with enhanced polarization for dual-selective C3H6O and H2S detection

Ferroelectric oxides with great polarization have attracted much attention due to the polarization modulated carrier behaviors causing high gas sensitivity in detecting hazardous toxic gases. In this work, tourmaline@BiFeO3 (T@BFO) composites were fabricated by the sol-gel method based on the electrostatic self-assembly strategy for the C3H6O and H2S detection. It is found that BFO nanoparticles are uniformly distributed on tourmaline due to the strong electrostatic field of tourmaline. In comparison with pure BFO, the T@BFO composites showed an improved gas sensitivity. More importantly, the sensor based on 3.0-T@BFO can effectively detect C3H6O at 150 °C and H2S at 210 °C in limits of detection as low as 50 and 7 ppb, respectively. The results exhibit that 3.0-T@BFO composites have lower activation energies (Eact) of C3H6O at 150 °C and H2S at 210 °C, which endow 3.0-T@BFO with dual-selectivity for C3H6O and H2S. The Ex-situ XPS and density functional theory calculations further reveal that tourmaline activates the sensing site of BFO to achieve selective adsorption of C3H6O at Bi, Fe, and oxygen vacancies sites and H2S at surface lattice oxygens, Fe and oxygen vacancies sites. These results provide an efficient strategy to design practice multifunctional and highly sensitive gas sensors for detecting hazardous toxic gases.

FeIII Chelated with Humic Acid with Easy Synthesis Conditions and Good Performance as Anode Materials for Lithium-Ion Batteries.

In this work, we prepared a green, cheap material by chelating humic acid with ferric ions (HA-Fe) and used it as an anode material in LIBs for the first time. From the SEM, TEM, XPS, XRD, and nitrogen adsorption-desorption experimental results, it was found that the ferric ion can chelate with humic acid successfully under mild conditions and can increase the surface area of materials. Taking advantage of the chelation between the ferric ions and HA, the capacity of HA-Fe is 586 mAh·g-1 at 0.1 A·g-1 after 1000 cycles. Moreover, benefitting from the chelation effect, the activation degree of HA-Fe (about 8 times) is seriously improved compared with pure HA material (about 2 times) during the change-discharge process. The capacity retention ratio of HA-Fe is 55.63% when the current density increased from 0.05 A·g-1 to 1 A·g-1, which is higher than that of HA (32.55%) and Fe (24.85%). In the end, the storage mechanism of HA-Fe was investigated with ex-situ XPS measurements, and it was found that the C=O and C=C bonds are the activation sites for storage Li ions but have different redox voltages.

In-situ interfacial passivation and self-adaptability synergistically stabilizing all-solid-state lithium metal batteries

The function of solid electrolytes and the composition of solid electrolyte interphase (SEI) are highly significant for inhibiting the growth of Li dendrites. Herein, we report an in-situ interfacial passivation combined with self-adaptability strategy to reinforce Li0.33La0.557TiO3 (LLTO)-based solid-state batteries. Specifically, a functional SEI enriched with LiF/Li3PO4 is formed by in-situ electrochemical conversion, which is greatly beneficial to improving interface compatibility and enhancing ion transport. While the polarized dielectric BaTiO3-polyamic acid (BTO-PAA, BP) film greatly improves the Li-ion transport kinetics and homogenizes the Li deposition. As expected, the resulting electrolyte offers considerable ionic conductivity at room temperature (4.3 × 10−4 S cm−1) and appreciable electrochemical decomposition voltage (5.23 V) after electrochemical passivation. For Li-LiFePO4 batteries, it shows a high specific capacity of 153 mA h g−1 at 0.2 C after 100 cycles and a long-term durability of 115 mA h g−1 at 1.0 C after 800 cycles. Additionally, a stable Li plating/stripping can be achieved for more than 900 h at 0.5 mA cm−2. The stabilization mechanisms are elucidated by ex-situ XRD, ex-situ XPS, and ex-situ FTIR techniques, and the corresponding results reveal that the interfacial passivation combined with polarization effect is an effective strategy for improving the electrochemical performance. The present study provides a deeper insight into the dynamic adjustment of electrode-electrolyte interfacial for solid-state lithium batteries.

The synergistic effect of Lewis acidic etching V4C3(MXene)@CuSe2/CoSe2 as an advanced cathode material for aluminum batteries

Herein, we focused on the development of the V4C3 MXene composite bimetallic selenide heterostructure (V4C3@CuSe2/CoSe2) as a cathode material for aluminum batteries. This heterostructure was prepared through a Lewis melt salt etching and selenization process. By capitalizing on the synergistic effect between the bimetallic selenide and V4C3 MXene, V4C3@CuSe2/CoSe2 exhibited rapid charge transfer and demonstrated superior discharge specific capacity compared to V4C3 composite monometallic selenide. Furthermore, the incorporation of V4C3 improved the material's stability during charging/discharging. The initial discharge specific capacity of V4C3@CuSe2/CoSe2 reached an impressive 809 mAh g–1 at 1 A g–1. Even after nearly 3000 cycles, it retained a substantial capacity of 169.1 mAh g–1. Ex-situ XPS analysis confirmed the reversible valence transitions of Cu, Co, and Se elements as the main energy storage reactions taking place in the cathode material. Density functional theory analysis provided further insights, revealing that the strong metallic behavior of the heterostructure stemmed from the charge rearrangement facilitated by the bimetallic selenide structure and the optimization of the energy level structure. Additionally, the presence of the bimetallic selenide structure significantly improved the adsorption efficiency of [AlCl4]–. Overall, this research contributes to the advancement of rechargeable aluminum ion batteries and presents a promising avenue for future developments in composite metal selenide structures and MXene-based materials.

Construction of ion–electron conduction network on FeS2 as high-performance cathodes enables all-solid-state lithium batteries

The all-solid-state lithium batteries (ASSLBs) with high energy density are considered to be one of the most promising candidates for next-generation lithium battery systems. Nevertheless, the low ionic and electronic conduction inside the cathode and the poor interfacial contact of the cathode/electrolyte seriously impede the large-scale application of ASSLBs. In this work, a novel multiple ion–electron conductive network is constructed on the FeS2 cathode to realize a high-energy all-solid-state battery. The internal disordered carbon matrix acts as electronic network to accelerate the electronic transmission. Meanwhile, reduced graphene oxide (rGO) tightly wrapping FeS2/C microspheres’ surface serves as external electronic pathway. Moreover, the in-situ formed Li7P3S11 electrolyte infiltrates into the nanoparticles to improve lithium-ion transport kinetics. Therefore, the dual-carbon framework and Li7P3S11 coating layer strategies significantly enhance ion–electron transport kinetics and improve interfacial contact during cycling. As expected, the FeS2@C/rGO@Li7P3S11 cathode exhibits excellent rate capability and cycling stability, showing a reversible discharge capacity of 350.3 mAh/g at 0.5C after 200 cycles. More importantly, ex-situ XPS and dQ/dV results reveal that the synergistic effect of dual-carbon frameworks and Li7P3S11 coating layer not only provides fast electron–ion transfer channels, but also wraps the reaction products with poor electrochemical activity such as Fe0, FeSy, and S to accelerate the reaction kinetics and strengthens the reaction reversibility. This work provides valuable insights for improving the electrochemical performance and understanding the reaction mechanism of the conversion-type metal sulfide cathodes in ASSLBs.

Chromium induced nickel oxides leads to extraordinary enhancement in the performance of aqueous hybrid supercapacitors

Aqueous hybrid supercapacitors (AHSCs) have great potential in portable devices and other modern electronic products due to their high-power density, long life span, and environmentally friendly. However, compared with traditional Li-ion batteries, they are still not mature enough to meet market requirements due to their low energy density. In this study, we proposed to use nanostructured bi-metallic nickel-chromium oxide as an AHSC cathode. By using the synergetic effect arising from the two metal cations, Ni and Cr-ions, the energy storage performance of AHSCs was expected to be significantly improved. We synthesized the nanostructured bi-metallic nickel-chromium oxide (NiCrO3) with a facile hydrothermal method. The achieved NiCrO3 (NCO) has a sheet-like structure and a hexagonal shape in the nanoscale. The results show that NCO loading on carbon cloth (CC) as the working electrode (NCO@CC) has a high specific capacitance of 2,862 F/g at 1 A/g (corresponding capacity of 1,431 C/g) and a favorable cycling stability of 98.3% after 10,000 cycles at 10 A/g. The specific capacitance is about 2.3 and 5.9 times of pristine Cr2O3 and NiO in the same situation. The storage mechanism of charge was investigated by ex-situ XRD, XPS, and Raman spectroscopy, which reveals that the total stored charge is based on the combination of faradic and pseudocapacitive effects. Furthermore, AHSC was fabricated using the NCO@CC as cathode, activated carbon as anode, which showed promising performance (102.7 F/g at 2 A/g) with excellent rate-capability (58.5% at 20 A/g), long life span (48% after 50,000 cycles). Density function theory calculation reveals that NCO possesses higher conductivity than NiO and Cr2O3, and decreases the electron transport energy, which contributes to the improves the reactions kinetics.

The Role of Imidazolium 2-Cyanopyrrolide Ionic Liquid in Reduction of CO2 to CO, Formate, and Ethylene on Copper Electrode Revealed By SERS

Electrochemical conversion of CO2 over copper catalyst is known to produce higher value products such as methane, ethylene, formate and alcohols in aqueous electrolytes. Since CO2 is a stable molecule, the electrochemical reduction reaction requires significant overpotential (-1.9 V vs SHE) for the first electron transfer to form CO2 -. Ionic liquids (ILs) have high CO2 solubility and can be further functionalized to bind with CO2 such that CO2 can be brought to the electrode surface in a more reactive state. In this study, we studied electroreduction of CO2 over copper in non-aqueous electrolyte with the reactive IL 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]) in the concentration range of 0.1 to 1 M. The linear sweep voltammetry experiments show the absence of a noticeable Faradic current within the potential window of -1.8 to -2.2 V vs Ag/Ag+ under N2 demonstrating the stability of the imidazolium cation against electrochemical conversion, which is also confirmed by NMR analysis. In the presence of CO2, the onset potential of reduction is at -1.9 V vs Ag/Ag+ (-1.4 V vs. SHE) and does not change with IL concentration. By employing surface enhanced Raman spectroscopy (SERS), potential dependent changes in the interfacial microenvironment during CO2 reduction was revealed. Specifically, imidazolium enrichment and surface adsorbed CO formation were captured. These results confirmed the catalytic role of the IL by facilitating the transport of the reactive CO2 to the electrode surface via imidazolium in particular. On the other hand, surface coverage by imidazolium cations on the electrode seems to prohibit higher level of C-C couplings, thus limiting reaction products to C1 and C2 products. During 3 hrs of bulk electrolysis at -2.1 V (vs. Ag/Ag+) in an H-cell with non-aqueous negolyte containing the IL and an aqueous posolyte (proton source), gaseous products of CO and H2, liquid product of HCOO- and dissolved C2H4were determined by GC and NMR analysis, respectively. Oxidation of Cu electrode was separately captured by ex-situ XPS measurements after 3 hrs of bulk electrolysis. Even though high CO2 concentrations are achieved in ILs and they are stable at CO2 reduction conditions even after 3 hours on Cu surface, IL structure should be optimized to allow C-C coupling to obtain C2+ products.

Double carbon modified Na3V2(PO4)3 with dual charge carriers and high performance synthesized in-situ in polypyrrole and chitosan quaternary ammonium hydrogel

Sodium-ion batteries (SIBs) are regarded as the most promising battery to replace lithium-ion batteries (LIBs) in the future. Na3V2(PO4)3 (NVP), as an ideal positive electrode material for SIBs, is limited by poor electronic conductivity and low reversible capacity. In this paper, the double carbon resources of chitosan quaternary ammonium hydrogel (CHACC) and Polypyrrole (PPy) were introduced into NVP. Through the construction of double charge carriers, the high conductivity and the high reversible capacity of NVP which broke the theoretical limit were achieved. The introduction of CHACC brings the second charge carrier, ClO4−, in addition to Na+ in NVP system. Furthermore, the conductive network and defect-rich carbon coatings formed by CHACC and PPy significantly improve the electronic conductivity and kinetics of the electrode. Comprehensively, the detailed functional mechanism of dual carriers in NVP is deeply investigated by ex-situ FTIR and XPS, revealing that ClO4− utilizes the N+ in the carbonized substrate of CHACC and PPy as the binding sites to reversibly inserted/extracted from the carbon layer to provide additional capacities. Notably, the optimized CHACC-PPy-NVP submits superior sodium storage performance. It delivers a capacity of 148.3 and 139.3 mAh g−1 at 0.1 and 1C, significantly exceeding the theoretical capacity (117.6 mAh g−1). Even at 140C, it still maintains a capacity value of 88.9 mAh g−1 with 75.7 mAh g−1 remaining after 2000 cycles, corresponding to a low decay rate of 0.007 % per cycle.

Carbon dots confined nanosheets assembled NiCo2S4@CDs cross-stacked architecture for enhanced sodium ion storage

Na+ batteries (SIBs) have been emerging as the most promising candidate for the next generation of secondary batteries. However, the development of high-performance and cost-effective anode materials is urgently needed for the large-scale applications of SIBs. In this study, carbon dots confined bimetallic sulfide (NiCo2S4) architecture (NiCo2S4@CDs) was proposed and synthesized from assembling nanosheets into cross-stacked superstructure and the subsequent confinement of carbon dots. This novel decussated structure assembly from nanosheets is greatly beneficial to the structure stability of electrode material during the successive charge/discharge processes. Besides, the CDs based carbon conductive network can enhance the electrical conductivity for facilitating the easy transport of electron/Na+. Benefitting from these advantages, NiCo2S4@CDs exhibits high-rate performance and an ultralong cycling life in SIBs. Specifically, the specific capacity of NiCo2S4@CDs can reach the discharge specific capacity as high as 568.9 mAh/g at 0.5 A/g, which can also maintain 302.7 mAh/g after 750 cycles at 5.0 A/g. Additionally, ex-situ characterization techniques such as ex-situ XRD and ex-situ XPS were employed to further explore the sodium storage mechanism of the NiCo2S4@CDs anode.

Ultrafine manganese hexacyanoferrate with low defects regulated by potassium polyacrylate for high-performance aqueous Zn-ion batteries

Manganese hexacyanoferrate (MnHCF) is an ideal candidate for aqueous Zn-ion batteries (AZIBs) because of their high capacity and high operating voltage. Adding surfactant during the preparation process is an effective way to reduce the [Fe(CN)6] vacancies and water, leading to a stable structure and excellent electrochemical performance. However, the size of MnHCF becomes large (> 100 nm) after adding surfactant, resulting in the sluggish kinetics of Zn-ion transport and low utilization rate of active materials. Herein, ultrafine MnHCF (15 nm) (MnHCF-15) with few vacancies and low water content has been fabricated by introducing a typical surfactant of potassium polyacrylate (PAAK). Moreover, PAA-K can also precisely regulate the size of MnHCF, which exhibit a series of nano-sizes of 15, 50, 80, and 120 nm. The MnHCF-15 cathode exhibited a high capacity of 139.2 mA h g−1 at 0.05 A g−1 with a high operating voltage of 1.75 V. The excellent cycling stability of 92.5 %, 91.9 %, and 84.5 % capacity retention at 0.1, 0.2, and 0.5 A g−1 for 1000 cycles were also achieved because of the ultrafine size and the low defect of MnHCF-15. Ex-situ XRD and XPS were carried out to probe Zn-ion storage mechanism, indicating a reversible electrochemical reaction of the MnHCF-15 cathode. Impressively, the soft-packed AZIBs with high electrochemical performance highly reveal the practical application of the MnHCF-15. The results of this work offer a promising cathode material and lead to a new understanding on the nano-size effect of PBAs in AZIBs.

Vacancy and surface modulation engineering of CuxCo3-xO4 nanowires as an advanced cathode for zinc-ion hybrid supercapacitors

Zn-ion hybrid supercapacitors (ZHSCs) with high power and energy density have great potential in energy storage applications such as hybrid vehicles and renewable energy storage. However, the large radius of hydrated Zn2+-ions hampers their efficient storage in micropores with limited pore sizes, resulting in limited weight ratio capacitance and poor rate capability of ZHSCs. In this study, we developed the novel N-doped and oxygen vacancy-rich CCO nanowires (N-Ov-CCO@CC) architecture with the help of Chemical Vapor Deposition (CVD). Due to the guiding synergy of N-doping, defects, and surface engineering, N-Ov-CCO@CC exhibits significantly enhanced electrochemical performance. The N-Ov-CCO@CC single electrode exhibits excellent charge storage properties, including a high capacitance of 1480.7 F/g at 1 A/g, excellent rate-capability (88.4 % at 20 A/g), and excellent cycle stability of up to 90.1 % for 5000 cycles. The charge storage mechanism was analyzed by ex-situ XRD and XPS, and it reveals that the pseudocapacitive charge storage characteristics are dominant. Operating in the potential range of 1.2–2.0 V, the N-Ov-CCO@CC//Zn-ZHSC provides a high capacitance of 308.2 F/g at 1 A/g, excellent rate capability (86.9 % at 10 A/g), long lifetime (97 % after 10,000 cycles), and high specific energy/power (134.32 Wh/kg at 9507.6 W/kg). Density function theory (DFT) validations show that the N-Ov-CCO system possesses higher conductivities than Ov-CCO and pristine CCO. This work provides an effective strategy for constructing multifunctional electrochemical energy materials for ZHSCs.

Interface ion-exchange strategy of MXene@FeIn2S4 hetero-structure for super sodium ion half/full batteries

Herein, a well-designed hierarchical architecture of bimetallic transition sulfide FeIn2S4 nanoparticles anchoring on the Ti3C2 MXene flakes has been prepared by cation exchange and subsequent high-temperature sulfidation processes. The introduction of MXene substrate with excellent conductivity not only accelerates the migration rate of Na+ to achieve fast reaction dynamics but provides abundant deposition sites for the FeIn2S4 nanoparticles. In addition, this hierarchical structure of MXene@FeIn2S4 can effectively restrain the accumulation of MXene to guarantee the maximized exposure of redox active sites into the electrolyte, and simultaneously relieve the volume expansion in the repeated discharging/charging processes. The MXene@FeIn2S4 displays outstanding rate capability (448.2 mAh g−1 at 5 A g−1) and stable long cycling performance (428.1 mAh g−1 at 2 A g−1 after 200 cycles). Moreover, the Nay-In6S7 phase detected by ex-situ XRD and XPS characterization may be regarded as a “buffer” to maintain the stability of the Fe-based components and enhance the reversibility of the electrochemical reaction. This work confirms the practicability of constructing the hierarchical structure bimetallic sulfides with the promising electrochemical performance.

Hetero-structural and hetero-interfacial engineering of MXene@Bi2S3/Mo7S8 hybrid for advanced sodium/potassium-ion batteries

Herein, heterogeneous bimetallic sulfides Bi2S3/Mo7S8 nanoparticles anchored on MXene (Ti3C2Tx) nanosheets (MXene@Bi2S3/Mo7S8) were prepared through a solvothermal process and subsequent chemical vapor deposition process. Benefiting from the heterogeneous structure between Bi2S3 and Mo7S8 and the high conductivity of the Ti3C2Tx nanosheets, the Na+ diffusion barrier and charge transfer resistance of this electrode are effectively decreased. Simultaneously, the hierarchical architectures of Bi2S3/Mo7S8 and Ti3C2Tx not only effectively inhibit the re-stacking of MXene and the agglomeration of bimetallic sulfides nanoparticles, but also dramatically relieve the volume expansion during the periodic charge/discharge processes. As a result, the MXene@Bi2S3/Mo7S8 heterostructure demonstrated remarkable rate capability (474.9 mAh/g at 5.0 A/g) and outstanding cycling stability (427.3 mAh/g after 1400 cycles at 1.0 A/g) for sodium ion battery. The Na+ storage mechanism and the multiple-step phase transition in the heterostructures are further clarified by the ex-situ XRD and XPS characterizations. This study paves a new way to design and exploit conversion/alloying type anodes of sodium ion batteries with hierarchical heterogeneous architecture and high-performance electrochemical properties.

Mo-doped MnO2@CC electrode for high-performance 2.4 V aqueous asymmetric supercapacitors

Despite many advantages of supercapacitors based on aqueous electrolytes, the inherently low operating voltage severely limits their energy density and practical applicability to many applications. Thus, it is a key scientific challenge to broaden the potential window of electrode materials for supercapacitors. Here we report our findings in tailoring the Fermi level of MnO2-based electrode by doping with a high-valence Mo6+ ion, thus modulating its overpotential in aqueous electrolytes and potential window. For example, the redox potential of Mo-doped MnO2 is effectively increased, resulting in a wider potential window. When grown on a carbon support, the specific capacitance of Mo-doped MnO2@CC with a mass loading of 15.1 mg cm−2 reaches 3.22 F cm−2 in a wide potential window of 1.2 V. The enhanced performance is attributed to the additional redox behavior induced by the Mo dopant, as confirmed by in situ Raman and ex-situ XPS analyses. When coupled with a C-Ti3C2Tx/WO3@CC negative electrode to construct an ASC with a total mass loading of 35.8 mg cm−2, the ASC exhibits a wide voltage window of 2.4 V, demonstrating a remarkable energy density of 0.89 mWh cm−2 at a power density of 2.56 mW cm−2. In addition, the device is operational in a wide operating temperature range of −10 to 50 °C, and a voltage of ∼ 58% is retained after self-discharge for 48 h. This work provides some new insights into the development of aqueous ASC devices of commercial-grade mass loadings with high output voltage, high energy density, and slow self-discharge rates.

Tailoring covalent triazine frameworks anode for superior Lithium-ion storage via thioether engineering

The development of high-performance lithium-ion capacitors (LICs) requires the use of electrode materials with higher energy/power density, faster charging, and longer cycle-life. Herein, by taking the merit of the strong designability of covalent triazine frameworks (CTFs), two tailor-made thioether-functionalized CTFs (S-CTF-Et and S-CTF-Me) with tunable properties are designed and synthesized as anodes for LICs. Due to the rich redox active sites and highly accessible specific surface area of the S-CTF-Et anode, which delivers a very large reversible capacity up to 1334 mAh g−1 at 0.1 A g−1, good rate capability (520 mAh g−1 at 2 A g−1) and cycling stability. After being comprehensively studied via ex-situ FT-IR, ex-situ XPS analyses, and theoretical calculations, it is found that the lithium-storage mechanism involves multi-electron redox reactions of benzene and triazine rings and coordination of thioether-groups by the accommodation of Li+. As proof of the new concept, an all CTF-related LIC device is assembled with S-CTF-Et anode and CTF-derived porous carbon (CTF-800) cathode for the first time, which delivers competitive energy and power densities (179.3 Wh kg−1 and 10.7 kW kg−1) and a capacity retention of 81 % after 10 000 cycles at a large current density of 2 A g−1.

Identifying the in situ protection role of MnO2 nanosheets on Co oxide for superior water oxidation

As a limiting step of electrochemical water splitting, the oxygen evolution reaction calls for efficient catalysts to surmount the high energy barrier. In this work, the synthesis of O-deficient Co3O4 nanoparticles with abundant surface area was realized by applying two-dimensional MnO2 nanosheets as a self-protection agent. The strong interaction between Mn and Co was confirmed by our experiments and DFT calculations that the valence state is increased for Co while it is reduced for Mn during the annealing in reducing atmosphere. Mn3+δ with stronger Lewis acidity can protect Co2+ from being reduced when subjected to the H2 treatment. The H2– reduced CoMn oxide under 300 °C (CMO-H300) shows the best OER activity with an overpotential of 280 mV in 1 M KOH and 460 mV in 1 M phosphate buffer (PBS) at 10 mA cm-2 geo, which can be attributed to the optimal chemical state, appropriate oxygen vacancies and highly exposed surface area. Operando Raman spectroscopy combined with ex-situ XPS revealed the manner of active species evolution in CMO-FD (Freezing-drying CoMn oxide) and CMO-H300 under the OER process, which accounted for the dynamic change of chronoamperometry. Our findings provide a different viewpoint for novel electrocatalyst fabrication by introducing a self-sacrifice agent that in situ promotes electronic structure modulation and high surface area preservation.