Capacitive Contribution Research Articles

Unmodified α-LiFe5O8 as potential anode material of lithium ion battery and the revelation of conversion mechanism

Spinel structured α-LiFe5O8 were prepared via ball milling combined with a straightforward solid-phase method, using Fe2O3 and Li2CO3 as the raw materials. The material maintained a stable capacity of 440 mAh g−1 after 200 cycles at 900 mA g−1 with Coulomb efficiency more than 95 % and also showed facile lithium ion diffusion dynamics. The calculated diffusion coefficient of α-LiFe5O8 ranged around 10−10-10−12 cm2 s−1. Diffusion controlled contribution was dominant over capacitive contribution in the sweeping range 0.1–1.5 mV s−1. A new intercalation-decomposition-conversion mechanism was proposed on the basis that there existed a small pit at the initial stage of the discharge plateau and a significantly low initial discharge plateau voltage was observed compared with the followed discharge cycles. Based on the DFT calculations and the phase diagram of the Li–Fe–O system, we supposed that for the initial discharge of the material, after 1 mol of lithium ion intercalation per unit formula, α-LiFe5O8 decomposed to LiFeO2 and Fe3O4. The followed discharge was the conversion of both LiFeO2 and Fe3O4 to metal Fe and Li2O. Thereafter, the system cycled between Fe2O3 at the fully charged state and metal Fe and Li2O at the fully discharged state.

Constant potential energetics of metallic and semiconducting electrodes: A benchmark study on 2D materials.

Grand-canonical (GC) constant-potential methods within an implicit solvent environment provide a general approach to compute the potential-dependent energetics at electrified solid-liquid interfaces with first-principles density-functional theory. Here, we use a mindfully chosen set of 27 isostructural 2D metal halides MX2 to analyze the variation of this energetics when the electronic structure changes from metallic to semiconducting and insulating state. Apart from expectable changes due to the opening up of the electronic bandgap, the calculations also show an increasing sensitivity to the numerical Brillouin zone integration and electronic smearing, which imposes computational burdens in practice. We rationalize these findings within the picture of the total interfacial capacitance arising from a series connection of the electrochemical double-layer capacitance and the so-called quantum capacitance resulting from the filling of electronic states inside the electrode. For metals, the electrochemical double-layer capacitance dominates at all potentials, and the entire potential drop takes place in the electrolyte. For semiconductors, the potential drop occurs instead fully or partially inside the electrode at potentials within or just outside the bandgap. For 2D semiconductors, the increased sensitivity to numerical parameters then results from the concomitantly increased contribution of the quantum capacitance that is harder to converge. Fortunately, this understanding motivates a simple extension of the CHE + DL approximation for metals, which provides the approximate GC energetics of 2D semiconductors using only quantities that can be obtained from computationally undemanding calculations at the point of zero charge and a generic double-layer capacitance.

Electrical and Ferroelectric Properties of Strontium Doped Lead-Free BCZT Ceramics

Herein, we synthesized Ba0.9-xCa0.1SrxTi0.8Zr0.2O3 (x = 0.05, 0.1, 0.15, 0.2) ceramic samples by employing a solid-state reaction technique. The variation of dielectric constant is studied concerning temperature in the range 1 kHz to 1 MHz frequency. Variation of dielectric constant ( εr ) with temperature reveals that charge carrier dispersion pushes the transition temperature towards higher frequency. Impedance analysis reveals that the maximum value of the imaginary part of impedance i.e., Z″ maxima, increases in all samples except for x = 0.05, which shows the opposite behaviour. This behaviour is due to the calcium (Ca2+) substitution at the titanium (Ti4+)-site at low doping concentration of Sr2+ ions. Nyquist plots show the presence of a single semi-circular arc, indicating the contribution of resistance and capacitance through grain boundary. PE hysteresis loops reveal that the values of saturation polarization ( Ps ) decrease with increasing Sr-doping, from 10.989 μC cm−2 to 6.586 μC cm−2, whereas the remnant polarization ( Pr ) shows variable values with increasing Sr-doping.

Synergistic effect of co-sputtered tungsten-titanium nitride as electrode material for efficient hybrid supercapacitors

The dawn of bimetallic transition metal nitrides has attracted considerable interest as battery grade electrode material for potential energy storage applications. In addition, it is essential to investigate binder-free processes to improve the performance of the fabricated electrodes. In this study, binder-free tungsten‑titanium nitrides (W-TiN) are deposited through RF/DC magnetron co-sputtering onto the conducting nickel foam (NF). SEM, EDX and X-ray diffraction are exploited to investigate surface morphology, elemental composition, and structural properties of sputtered materials. The W-TiN electrodes are characterized through electrochemical investigation in half-cell configuration. The tested W-TiN electrode is further utilized with activated carbon (AC) electrode to develop hybrid supercapacitor device W-TiN//AC. The hybrid device revealed a maximum energy density (Es) of 88.8 Wh/kg and power density (Ps) 1700 W /kg. To further understand the mechanism of hybrid devices, the capacitive and diffusive contributions are computed using linear and quadradic models. This study provides a new direction to integrate co-sputtered binder-free electrode materials and devices for large scale production of advanced hybrid energy storage devices.

Effect of temperature on the chemical activation of carbon nanospheres and their efficiency in supercapacitors

We report the synthesis of carbon spheres (CSs) by pyrolysis of soybean oil and their chemical activation employing acid baths at different temperatures. This allows to study the effect of these experimental parameters on the efficiency of these materials for their use as supercapacitor electrodes. The CSs were characterized by SEM, Raman, XRD, FT-IR, BET, and TEM to reveal the relationship between the electrochemical properties and the structure. Using nitric acid as the activating agent causes the formation of micropores and a high density of oxygenated functional groups in the CSs. In contrast, the mixture of sulfuric and nitric acids destroys the CSs, producing coalescence. The capacitance of the CSs was ~103 F g−1 at 5 mV s−1, and the contribution of electrical double-layer capacitance and pseudocapacitance was determined using cyclic voltammograms. Moreover, it is observed that after 1000 galvanostatic charge-discharge cycles at 2 A g−1, the CSs with the larger electrochemical capacitance increase their capacitive performance up to 38 %, maintaining the electrical double-layer capacitance contribution constant. Therefore, we show that controlling the density of oxygenated functional groups anchored through an increase in temperature during acid treatment and generating high surface areas are key parameters to obtain high capacitance.

Measurement and Characterization of the Electrical Properties of Actin Filaments.

Actin filaments, as key components of the cytoskeleton, have aroused great interest due to their numerous functional roles in eukaryotic cells, including intracellular electrical signaling. The aim of this research is to characterize the alternating current (AC) conduction characteristics of both globular and polymerized actin and quantitatively compare their values to those theoretically predicted earlier. Actin filaments have been demonstrated to act as conducting bionanowires, forming a signaling network capable of transmitting ionic waves in cells. We performed conductivity measurements for different concentrations of actin, considering both unpolymerized and polymerized actin to identify potential differences in their electrical properties. These measurements revealed two relevant characteristics: first, the polymerized actin, arranged in filaments, has a lower impedance than its globular counterpart; second, an increase in the actin concentration leads to higher conductivities. Furthermore, from the data collected, we developed a quantitative model to represent the electrical properties of actin in a buffer solution. We hypothesize that actin filaments can be modeled as electrical resistor-inductor-capacitor (RLC) circuits, where the resistive contribution is due to the viscous ion flows along the filaments; the inductive contribution is due to the solenoidal flows along and around the helix-shaped filament and the capacitive contribution is due to the counterion layer formed around each negatively charged filament.

Home-made chemical vapor deposition-based synthesis of binder-free nanostructured magnesium-molybdenum-sulfide electrode materials for supercapacitor application

Molybdenum disulfides (MoS2) is considered as promising electrode materials (E-M) for high electrochemical performance (ECP) of lithium-ion batteries and supercapacitors due to their layered nanostructures. Herein, MoS2, MgS and hetrostructure MgS/MoS2 E-Ms comprising of nano-particles/sheets/needles like structures are synthesized on Nickel foam (Ni–F) by a simple home-made chemical vapor deposition (CVD) technique. The XRD analysis confirmed the formation of MoS2, MgS and MgS/MoS2 polycrystalline E-Ms growing along different directions. The FESEM analysis is confirmed that the addition of MgS layer over the surface of MoS2 change the surafce morphology from nano-particles to nano-needles. The change in structural and microstructural features is indicated the appearance of defects, micro-strain and dislocations density which are fruitful for excellent ECP. The cyclic voltammetry analysis is indicated the pseudocapacitive nature of the synthesized E-Ms. The maximum specific capacitance values for MoS2/Ni–F, MgS/Ni–F and MgS/MoS2/Ni–F E-Ms are 1273, 1688 and 3934 F/g at 1 A/g respectively. Moreover, the addition of MgS layer over MoS2/Ni–F is revealed the excellent cyclic stability of 98.76 % at current density of 10 A/g after 4000 cycles, energy density values are ranged from 79 to 136 W h/kg with homologous power density of 250–2500 W/kg and very small charge transfer resistance. The simulations of Power's Law and Dunn's model is confirmed that the excellent ECP is due to both capacitive and diffusive controlled contributions of heterostructured MgS/MoS2/Ni–F E-Ms. Additionally, MgS/MoS2/Ni–F E-Ms have both battery and pseudocapacitive nature due to unique nano-needles like morphology. The excellent electrochemical properties of MgS/MoS2/Ni–F E-Ms substantiate their potential applications in portable energy storage devices.

Multilayer boron doped Si@SiOx/C from Si-Ca alloy for lithium-ion battery anodes

SiOx has been considered as a promising anode for next-generation lithium-ion batteries. However, the poor rate performance caused by the low electrical conductivity is a bottleneck of its application. Approaches involving carbon composites, boron doping and two-dimensional structure have been pursued to improve the electrical conductivity. In this work, through the combination of these strategies, a multilayer boron doped Si@SiOx/C composites are fabricated by a chemical exfoliation of CaSi2 with B2O3 and CaCO3. The production exhibits favorable properties for lithium-ion battery anodes, such as proper specific surface area, complete multilayer structure, boron doped and carbon composite structure. The as-obtained B-Si@SiOx/C-700 delivers a capacity of 659.3 mAh g−1 after 500 cycles at 1 A g−1. Especially, at 4 A g−1, the capacity retention of the B-Si@SiOx/C-700 anode is still 40.9 % of its average discharge capacity at 0.2 A g−1. The capacitive contribution to the total Li+ storage capacity for the B-Si@SiOx/C-700 electrode is 83.6 % at 1.0 mV s−1. This work presents a simple and efficient strategy to construct a multilayer doped structure to improve electrochemical properties.

Investigating the influence of copper benzene-1,2-dicarboxylate (Cu-BDC) and benzene-1,3,5-tricarboxylate ligands (Cu-BTC) on the electrochemical capacity of hybrid supercapacitors

The elevated energy demand and crises have rooted the urge to develop advanced electrode materials that can overcome the energy dilemma present all over the globe. Metal-organic frameworks (MOFs) have emerged as promising electrode materials in recent times due to their better electrochemical properties. Herein the metal ligand synergy produced in MOFs is observed for the same metal center (Cu) with different ligands i.e., benzene-1,3,5-tricarboxylate (1,3,5-BTC) and benzene-1,2-dicarboxylate (1,2-BDC). The hybrid device of the best performing MOF (Cu-1,3,5-BTC//AC) reveals the energy and power density of 88.32 Wh kg−1 and 680 W kg−1, respectively. Even at the highest current density of 15 A/g, the device retained the Es of 21.25 Wh kg−1 and Ps of 12,750 W kg−1. Furthermore, the semi-empirical approach was utilized for the evaluation of capacitive and diffusive contributions.

Novel paddy-like Bi-Sb-Cu alloy fabricated via pulse reverse potential electrodeposition enabling high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) have gained remarkable interest as cost-effective and scalable energy storage devices. Nevertheless, their further application has been significantly hindered due to the limited selection of desirable anode materials that can provide both high capacity and excellent cycling stability. Herein, we developed a hierarchical paddy-like Bi42.3Sb35.2Cu22.5 alloy anode on Cu foil via template-free pulse reverse potential electrodeposition. The Bi42.3Sb35.2Cu22.5 anode obtains a considerable specific capacity of 529.5 mAh·g−1 after 120 cycles with a remarkable capacity retention of 95.5 %, signifying a capacity loss of only 0.0375 % per cycle. However, Bi44.9Sb55.1 anodes obtained by the pulse potential electrodeposition strategy reveal a conventional irregular cluster structure, delivering only 64.5 mAh·g−1 after 100 cycles and also demonstrating inferior rate performance compared to Bi42.3Sb35.2Cu22.5 anodes. The enhanced cycling performance of the paddy-like Bi42.3Sb35.2Cu22.5 anode is primarily attributed to the introduced Cu-doped network facilitating the rapid diffusion of Na+ and also ensuring the stability of the electrode structure. Moreover, the unique paddy-like structure of Bi42.3Sb35.2Cu22.5 mitigates volume expansion-related stress and facilitates efficient charge transport through open channels. This is further corroborated by conducting CV measurements, indicating a greater capacitive contribution in the case of the Bi42.3Sb35.2Cu22.5 electrode compared to the Bi44.9Sb55.1 electrode. Additionally, SEM images show a stabilized surface morphology for the Bi42.3Sb35.2Cu22.5 electrode after cycling, demonstrating improved electrochemical performance. This work offers new insights into the development of high-performance anode materials for SIBs.

Improved Na+ adsorption performance and storage mechanism of cobalt hexacyanoferrate/polyaniline composite during the hybrid capacitive deionization process

In order to realize high desalting capacity of capacitive deionization (CDI) system, cobalt hexacyanoferrate/polyaniline (CoHCF/PANI) composite with hollow hierarchical tubular structure was finely prepared by employing conductive hollow PANI tube as the interconnection skeleton for CoHCF cubic particles grown in situ, thereby endowing this composite with good conductivity, abundant redox active sites and stable structure. The synergistic effect between the CoHCF layer and PANI layer makes the CoHCF/PANI electrode possess superior specific capacitance (263.8F·g−1) and high contribution of the capacitance behavior (87.8 %) at 5 mV·s−1. Compared with either the single PANI or CoHCF electrode, the CoHCF/PANI electrode exhibits outstanding salt adsorption capacity (30.48 mg·g−1), rapid salt adsorption rate (3.66 mg·g−1·min−1) and excellent cycle stability (retention rate 98.1 %). Different from the CoHCF/PANI(+)‖AC(−) cell, there is no inversion peak in the AC(+)‖CoHCF/PANI(−) cell, which testifies that the CoHCF/PANI electrode is easier to capture Na+ ions. Based on the XPS spectra before and after desalination tests, the Na+ adsorption mechanism of the CoHCF/PANI electrode as cathode for the hybrid capacitive deionization (HCDI) system was further explored. It is found that the change of element valence in CoHCF/PANI composites (reduction of Fe3+ to Fe2+ and Co3+ to Co2+ in CoHCF, partial reduction of −C = N+ to −N–H- in PANI) is the main factor leading to Na+ intercalation. This study emphasizes the significance for fine design of the electrode material and strategic configuration of the HCDI cell.

Electrode Spacing Effects on the Capacitance of Glassy Carbon Electrodes in Aqueous Electrolytes

Matched glassy carbon electrodes in aqueous K2SO4 electrolytes were used to examine the effects of opposing electrode spacing on capacitive performance. Planar non-porous glassy carbon electrodes were used to avoid complications with porosity and roughness. Electrode spacing effects were examined in terms of device and individual electrode performance, using cyclic voltammetry, coupled with its deconvolution into residual, diffusional, and capacitive processes. Decreasing the spacing between electrodes led to a decrease in capacitive contributions, and a relative increase in diffusional and residual contributions, implying that individual electrodes were influencing the behaviour of each other. This is also consistent with the use of more dilute electrolytes. Electrode behaviour was modelled using the Poisson-Boltzmann equation, together with its integrated outputs of electric field and potential difference. For electrodes with the same amount of charge and a similar diffuse layer thickness, the electric field and potential drop was diminished because of their charge interaction. Conversely, it is shown that for a similar potential drop across the electrodes, the variable controlled in a cyclic voltammetry experiment, more charge accumulation is needed at the electrode-electrolyte interface to compensate for the counter charge generate from the opposing electrode.

Effect of electrolytes on supercapacitive behavior of cubic shaped pristine cuprous oxide synthesized via sol-gel approach

The present study uses the sol-gel approach to employ cuprous oxide (Cu2O) as an electrode material for the electrochemical energy storage application. Scanning electron microscopy (SEM) examines the structural and surface morphological characteristics of the synthesized material, and pure phase development is verified by the X-ray Diffraction (XRD) patterns. The synthesized material has a high value of specific capacitance, notable porosity, and a well-defined cubic shape. In the three-electrode set-up, the electrochemical analysis uses different aqueous electrolytes, i.e., KOH and NaOH. The electrochemical outcomes indicate that the synthesized material has a maximum specific capacitance (Cs) of 362.77 Fg-1 with excellent stability (104 %) in 6 M NaOH than KOH electrolyte (225 Fg-1). EIS data for Cu2O-based electrodes have been modeled using various equivalent circuits. The ESR value of Cu2O in NaOH electrolyte is much smaller (0.503 Ω) than in KOH electrolyte, indicating the creation of more active sites and enhanced accessibility of ions. Capacitive and diffusion current contributions are shown by using the Trassati and Dunn techniques. Thus, all our findings suggest that sol-gel synthesized pristine cuprous oxide is a potential option for energy storage gadgets.

Superior electrochemical performance of metal ligand framework-neodymium oxide composite for energy storage devices

Battery supercapacitor hybrids are gaining increasing importance in the realm of energy storage, driven by their exceptional electrochemical attributes, where the choice of electrode material emerges as a paramount factor for their efficacy. Here the enhancement in the electrochemical activity from the synergistic effect produced by combining copper benzene tetra carboxylate with neodymium oxide (Cu-1,3,5-BTC/Nd2O3) along with the individual samples Cu-1,3,5-BTC, and Nd2O3, is studied via various electrochemical measurements which includes CV, GCD and EIS in half cell assembly and for real-world applications in full cell assembly. The hybrid device achieved a specific capacity (Qs) of 390 C/g with an energy density (Es) of 91 Wh/kg and power density (Ps) of 5950 W/kg respectively. Semi-empirical approaches such as linear and quadratic models were further employed and compared to scrutinize the CV curves and to deconvolute the capacitive and diffusive contribution.

Heterostructure of reduced graphene oxide supported tin (IV) sulfide nanopetals as an anode material for Sodium/Potassium-ion batteries: Evidence for the formation of C-S bond

Anodes that engage in conversion as well as alloying reactions are highly attractive candidates for sodium/potassium-ion batteries (SIBs and PIBs) because of their high theoretical specific capacities. Herein, Tin sulphide@reduced graphene oxide (SnS2 nanopetal@rGO) composite material is investigated as an advanced anode material for SIBs and PIBs. In this work, a simple hydrothermal synthesis of ultrathin SnS2 nanopetals covalently decorated on the surface of rGO is demonstrated as an anode material for SIBs and PIBs. The as prepared SnS2@rGO displays an initial charge capacity of 749 (at 0.2 A g−1) and 852 mAh g−1 (at 0.1 A g−1) for SIBs and PIBs respectively. The SnS2@rGO hybrid exhibited excellent cycle life which is attributed to the introduction of rGO in the composite as well as the in-built formed C-S bond. Moreover, the rGO matrix, firmly anchored with C–S bonds, envelops the outer SnS2, effectively inhibiting direct contact between SnS2 and the electrolyte. These combined effects contribute to impede the irreversible conversion of sulfur to sulfite, thus ensuring excellent structural stability throughout electrochemical cycling. The well-engineered nanoarchitecture not only guarantees the fast electrode kinetics, but also confirms excellent pseudo capacitance contribution during repetitive cycles which is confirmed by kinetic studies. Thus, SnS2@rGO is found to be a most promising electrode for sodium/potassium-ion batteries.

Pore structure and oxygen content design of amorphous carbon toward a durable anode for potassium/sodium‐ion batteries

AbstractBoth sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs) are considered as promising candidates in grid‐level energy storage devices. Unfortunately, the larger ionic radii of K+ and Na+ induce poor diffusion kinetics and cycling stability of carbon anode materials. Pore structure regulation is an ideal strategy to promote the diffusion kinetics and cyclic stability of carbon materials by facilitating electrolyte infiltration, increasing the transport channels, and alleviating the volume change. However, traditional pore‐forming agent‐assisted methods considerably increase the difficulty of synthesis and limit practical applications of porous carbon materials. Herein, porous carbon materials (Ca‐PC/Na‐PC/K‐PC) with different pore structures have been prepared with gluconates as the precursors, and the amorphous structure, abundant micropores, and oxygen‐doping active sites endow the Ca‐PC anode with excellent potassium and sodium storage performance. For PIBs, the capacitive contribution ratio of Ca‐PC is 82% at 5.0 mV s−1 due to the introduction of micropores and high oxygen‐doping content, while a high reversible capacity of 121.4 mAh g−1 can be reached at 5 A g−1 after 2000 cycles. For SIBs, stable sodium storage capacity of 101.4 mAh g−1 can be achieved at 2 A g−1 after 8000 cycles with a very low decay rate of 0.65% for per cycle. This work may provide an avenue for the application of porous carbon materials in the energy storage field.

Exploring the synergistic potential of mixed metal sulfides in hybrid supercapacitors

In energy reservoirs, hybrid supercapacitors (HSCs) have gained incredible recognition owing to their exceptional specific energy (Es), Specific power (Ps), and high cyclic stability. However, electroactive electrode materials with flashing electrochemical features and high storage ability are still required. In this regard, nickel-cobalt binary sulfide composites need further exploration as an electrode material owing to their high electrochemical properties, better conductivity, excellent redox reversibility, and high-rate capability. Here, we account the electrochemical activities and synthesis of composites including (nickel and cobalt sulfides). The composites were analyzed for structural and morphological analysis using XRD and SEM techniques, respectively. Amidst all other compositions, Ni0.75Co0.25S exhibited the optimum electrochemical performance measured in half-cell assembly with specific capacity (Qs) of 1147.6 Cg−1 and 900 Cg−1 at lowest scan rate and current density respectively. This electroactive electrode material together with activated carbon (AC) was further utilized in two cell assembly. This Hybrid device revealed ES of 50.21 Whkg−1 and Ps of 9350 Wkg−1 with capacity retention of 80.30% after consecutive charge discharge. The performance analysis of the actual device was further examined using a simulation approach, focusing on both capacitive and diffusive contributions by comparing the results of both linear and quadratic models. These attractive results revealed by Ni0.75Co0.25S signify it as favorable electrode material for hybrid supercapacitor applications.

Regulating nitrogen/sulfur terminals on 3D porous Ti3C2 MXene with enhanced reaction kinetics toward high-performance alkali metal ion storage

In this paper, we have developed a simple and efficient sulfur-amine chemistry strategy to prepare a three-dimensional (3D) porous Ti3C2Tx composite with large amounts of N and S terminal groups. The well-designed 3D macroporous architecture presents enlarged interlayer spacing, large specific surface area, and unique porous structure, which successfully solves the re-stacking issue of MXene during storage and electrode fabrication. It is the amount of concentrated hydrochloric acid added to the S-EDA (ethylenediamine)/MXene colloidal suspension that is critical to the formation of 3D morphology. In addition, N and S terminals on MXene could improve the adsorption ability of K+. Owing to the synergistic effect of the structure design and terminal modification, the N, S codoped three-dimensional porous Ti3C2Tx (3D-NSPM) material shows a high surface capacitive contribution and rapid diffusion kinetics for K+ and Na+. As a result, the as-prepared 3D-NSPM delivers high reversible capacity (237 and 273 mAh g−1 at 0.1 A g−1 for PIBs and SIBs, respectively), superb cycling stability (84.9% capacity retention after 10,000 cycles at 1 A g−1 in PIBs and 74.0% capacity retention after 2200 cycles at 1 A g−1 in SIBs), and excellent rate capability (111 and 196 mAh g−1 at 5 A g−1 for PIBs and SIBs, respectively), which are superior to other MXene-based anodes for PIBs and SIBs. Moreover, the described strategy provides a new insight for constructing the 3D porous structure from 2D building blocks beyond MXene.

Confined grotthuss proton-conduction along polyoxometalate chains inside carbon nanotubes for high-rate charge storage

Redox-active materials with excellent rate capability and cycling performance are in highly demand for electrochemical energy storage and conversion applications. Here, we unveil the confined proton-conduction behavior of one-dimensional polyoxometalate chains inside single-walled carbon nanotubes (SWNTs). The polyoxometalate molecules including phosphomolybdic acid {HPMo} and phosphotungstic acid {HPW} are encapsulated within SWNTs via host–guest recognition, driven by the electron transfer from nanotubes to polyoxometalates. Impressively, the redox hybrids of polyoxometalate@SWNTs deliver abnormal rate performance in acidic solution with capacity retentions over 73 % at high sweep rate of 1000 mV s−1, relative to the capacity at 10 mV s−1. This value is comparable to that of the pure SWNTs, typically known as double-layer capacitive materials. Electrochemical analyses reveal the Grotthuss proton-conduction mechanism of {HPMo} in SWNTs with a low activation energy of 0.11 eV. Therefore, the {HPMo}@SWNT hybrid demonstrates quasi diffusion-free characteristic with dominant capacitive contribution over 60 % at different sweep rates. Molecular dynamic simulations and density functional theory calculations evidence the strong burst-like transport of protons in the {HPMo}@SWNT hybrid. This process is dynamically favored in the continuous and long-range hydrogen-bond network along polyoxometalate chains with bound/free water molecules inside SWNTs. The superiority of nanoconfined Grotthuss mechanism may engender an exciting avenue for developing high-performance energy storage materials.

Revealing the role of calcium ion intercalation of hydrated vanadium oxides for aqueous zinc-ion batteries

Exploring suitable high-capacity V2O5-based cathode materials is essential for the rapid advancement of aqueous zinc ion batteries (ZIBs). However, the typical problem of slow Zn2+ diffusion kinetics has severely limited the feasibility of such materials. In this work, unique hydrated vanadates (CaVO, BaVO) were obtained by intercalation of Ca2+ or Ba2+ into hydrated vanadium pentoxide. In the CaVO//Zn and BaVO//Zn batteries systems, the former delivered up to a 489.8 mAh g−1 discharge specific capacity at 0.1 A g−1. Moreover, the remarkable energy density of 370.07 Wh kg−1 and favorable cycling stability yard outperform BaVO, pure V2O5, and many reported cathodes of similar ionic intercalation compounds. In addition, pseudocapacitance analysis, galvanostatic intermittent titration (GITT) tests, and Trasatti analysis revealed the high capacitance contribution and Zn2+ diffusion coefficient of CaVO, while an in-depth investigation based on EIS elucidated the reasons for the better electrochemical performance of CaVO. Notably, ex-situ XRD, XPS, and TEM tests further demonstrated the Zn2+ insertion/extraction and Zn-storage mechanism that occurred during the cycle in the CaVO//Zn battery system. This work provides new insights into the intercalation of similar divalent cations in vanadium oxides and offers new solutions for designing cathodes for high-capacity aqueous ZIBs.