Warburg Coefficient Research Articles

Effect of Oxygen Functionalities inside Carbon Micropore on Lithium-Sulfur Battery Performance

Introduction Lithium-sulfur batteries (LSB) using a sulfur cathode with a high specific capacity (1672 mAh g-1) have been regarded as one of the most promising candidates for large-scale energy storage systems, while the sulfur cathode's drawback hinders the commercial application of LSB. Soluble Li2Sx (x = 4 – 8) generated during a charge-discharge process dissolves into electrolytes, which induces LSB's self-discharge and capacity decay. The main efforts to overcome this issue have been on physical confinement of sulfur in the pores of porous carbons [1,2]; however, carbon materials, being non-polar in nature, interact weakly with the polar Li2Sx, which still leads to low cycle stability of LSB. Hence, the surface modification to porous carbon, which provides polar-polar interaction between the carbon surface and Li2Sx, has been performed, suppressing the Li2Sx dissolution and improving LSB's performance [1,3].On the other hand, we reported that polar-oxygen functionalities on microporous carbon (MC), which are generated with oxidants, significantly improve the discharge capacity of LSB [4]. The formation of Li2Sx (x = 4 – 8) is inhibited in micropores (pore diameter < 2 nm) [5]; therefore, the oxygen functional groups probably improve the discharge capacity of LSB assembled with a MC-sulfur composite cathode due to factors other than the suppression effect of Li2Sx dissolution. Our previous study revealed that oxygen functionalities on the MC surface contribute to the enhanced electrochemical performance of the LSB by preventing electrolyte decomposition and thus decreasing the resistance of the cathode electrolyte interface (Rcei) [6]. However, the ratio of Rcei to the total resistance of the LSB is small; the oxygen functional groups would improve the performance of LSB by affecting other resistance. Herein, we investigate the effect of surface oxygen functionalities on charge transfer resistance (Rct) and Li+ diffusion of LSB. Our report would provide deep insight into the surface modification of carbon materials as LSB’s cathode. Method 2.1. Preparation of Oxidized MC-Sulfur composite MC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 h (Ox MC). Washed and dried Ox MC was mixed with sulfur at a weight ratio of 48: 52. The mixture was heated at 155ºC for 5 h (Ox MC-S). 2.2. Assembling of Cells To obtain the Ox MC-S cathode, the slurry prepared by mixing the Ox MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a weight ratio of 89: 5: 3: 3 was coated on an Al foil. The cells with the Ox MC-S cathode and Li metal anode were assembled in a glove box. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI): tetraglyme (G4): 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte. 2.3. Activation Energy Measurement with Electrochemical Impedance Spectroscopy (EIS) To elucidate the effect of oxidation treatment on the Rct and Warburg impedance (Rw) of Li-S batteries, activation energy measurement was carried out at 2.5 V in the 50th charging process. The temperature range was 0 – 25°C, and the alternating current voltage amplitude was 10 mV. Rw was investigated with the calculation of the Warburg coefficient (σ). Major results and conclusion The activation energy of Rct (Ect) and σ were determined to investigate the effects of oxygen functional groups formed at MC surfaces on charge transfer processes and Li+ diffusion. While Ect increased from 0.65 to 0.72 eV by oxidation treatment, the frequency factor increased from 17.7 to 20.9; this indicates that the oxygen functional groups improve the number of reaction sites where three substances (carbon, active material, and electrolyte) come into contact with each other. The σ of MC-S and Ox MC-S were 281.2 and 178.9 Ω s-1/2, respectively. Therefore, a decrease in Rw due to oxygen functional groups was suggested.The charge transfer process and Li+ diffusion occur inside the microporous carbon particles where the confined sulfur is present. We processed the Ox MC-S particle from the cathode after the 50th charging using focused ion beam (FIB) milling to obtain a cross-section and observed it with STEM-EELS. This result will also be reported in our presentation.This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST.[1] X. Ji et al., Nat. Mater., 8 (2009) 500.[2] T. Takahashi et al., Prog. Nat. Sci., 25 (2015) 612.[3] L. Yoshida et al., Electrochim. Acta, 429 (2022) 141000.[4] S. Okabe et al., Electrochemistry, 85 (2017) 671.[5] Y. Xiao et al., RSC Adv., 10 (2020) 39875.[6] L. Yoshida et al., 242nd ECS Meeting s, Z01-2299 (2022).

Co-electrodeposited poly (3, 4-ethylenedioxythiophene) (PEDOT)-multiwall carbon nanotubes (MWCNT) hybrid electrodes based solid-state supercapacitors using ionic liquid gel electrolyte for energy storage with pulsed power capabilities

Electrochemical and energy storage properties of poly(styrene sulfonate) doped poly (3, 4-ethylenedioxythiophene) (PEDOT:PSS) conducting polymer hybridized with the multiwall carbon nanotubes (MWCNT) are studied as solid-state supercapacitor with anion-doped ionic liquid (IL) gel electrolyte. The PEDOT: PSS-MWCNT composite electrodes were formed by co-electrodeposition using sequential 4 mA cm−2 current density with 10 ms current pulses in an aqueous medium with highly dispersed MWCNTs using Na-dodecyl sulfate (SDS) surfactant, PSS dopant (10–20 %) and EDOT monomer. Raman spectral study confirms with ≤10%PSS, the MWCNTs are moored into PEDOT chains via covalent bonding with carbons at α and β-position as PEDOT grows over flexible graphite substrate. Specific capacitance of 138 F g−1 discerned from cyclic voltammetry in IL-gel electrolyte is higher than reported in liquid electrolytes and chemically polymerized electrodes. Electrical double layer (EDL) capacitance analysis shows high 71.2 mF cm−2 contribution originated from MWCNTs with 10 % PSS, and slightly lower 48.8 mF cm−2 with 20 % PSS. The performance optimized PEDOT: PSS (10 %)- MWCNT (1 mg) based supercapacitors show large (43 %) access to the electroactive nanoporous surface regions contributing to capacitive charge and 57 % electrode has access to charge storage via diffusion limited Faradaic process. Nyquist plots analyzed for Warburg coefficient show minimal charge transfer resistance unimpacted by ionic diffusion. From the frequency response of the real and imaginary impedances pulsed power is assessed as 6.33 kW kg−1 at energy density 3.43 Wh kg−1. Charge-discharge behavior of optimized supercapacitor over 0.05–2.0 A g−1 current densities is triangular, highly symmetrical. Ragone plot shows high energy density of 7.7 Wh kg−1 at specific power 5.3 kW kg−1 plateauing out to 4.4 Wh kg−1 at higher specific power of 11.5 kW kg−1. Charge-discharge cycle analysis shows merely 20 % loss of specific capacitance by ∼500 cycles largely stabilizing for over 2000 cycles while the Coulomb efficiency remaining invariant at 95 %.

Mechanism of Improved Lithium-Sulfur Battery Performance by Oxidation Treatment to Microporous Carbon as Sulfur Matrix

1. IntroductionLithium-sulfur (Li-S) batteries are rechargeable devices assembled with a sulfur cathode and a lithium metal anode. Li-S batteries have twice the volumetric energy density and 5 times the gravimetric energy density of lithium-ion batteries (LIB). Hence, Li-S batteries are expected to be applied to stationary power sources and EV vehicles [1]. However, Li-S batteries have the following issues:・Sulfur and the final discharge product (Li2S) are insulators.・In the discharge process, sulfur expands up to 1.8 times, so the structure of batteries is unstable.・Intermediate products (Li2Sx (x = 4 – 8)) dissolve in an electrolyte; Li2Sx (x = 4 – 8) diffused to an anode to provide an insulating layer at the anode surface.・In the charge process, Li2Sx (x = 4 – 8) causes redox shuttling.As a result, Li-S batteries cannot charge and discharge stably. In order to deal with this problem, Nazar et al. reported the method that sulfur is confined in porous carbon [2]. This approach provides a cathode realizing good electronic conductivity, restriction of sulfur expansion, and suppression of Li2Sx (x = 4 – 8) dissolution. Although these improved characteristics allow Li-S batteries to operate, The discharge capacity of Li-S batteries is still not high enough and this needs to be addressed.In our previous study, we reported that oxidation treatment to microporous carbon (MC) with HNO3 improves Li-S batteries' discharge capacity [3]. Moreover, we clarified that the discharge capacity of Li-S batteries has an approximate proportional relation with the amount of oxygen-containing functional groups on the MC surface [4].This work attempts to elucidate the mechanism of improved Li-S battery performance by oxidation treatment to MC. Our report would lead to the proposal of a novel strategy to improve the performance of Li-S batteries.2. Method2.1 Preparation of Oxidized MC-Sulfur composite (Ox MC-S)MC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 h. By vacuum filtration and washing with deionized water, Ox MC was obtained. Ox MC dried in vacuum at 80ºC overnight was mixed with sulfur at a weight ratio of Ox MC: S = 48: 52. The mixture was thermally annealed at 155ºC for 5 h (Ox MC-S). Untreated MC was also composited with sulfur by the same method (MC-S).2.2 Assembling of CellsEach MC-S cathode was prepared by mixing the MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89: 5: 3: 3 and coating the resulting aqueous slurry on an Al foil current collector. The cells with the MC-S electrode and Li metal foil as an anode were assembled in a glove box filled with Ar. Lithium bis(trifluorosulfonyl)imide (LiTFSI): tetraglyme (G4): hydrofluoroether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte.2.3 Electrochemical Impedance Spectroscopy (EIS)To elucidate the effect of oxidation treatment on the internal resistance of Li-S batteries, EIS was carried out at various potentials (Discharge 2.0 – 1.0 V and Charge 1.0 – 3.0 V). The obtained Nyquist plots were used for the evaluation of solid electrolyte interphase (SEI) resistance (Rsei), charge-transfer resistance (Rct), and Warburg impedance (Rw). Rw was investigated with the calculation of Warburg coefficient (σ).3. Major results and conclusionSince oxidation treatment to MC significantly increased the discharge capacity of Li-S batteries [3][4], it was expected that oxidation treatment would lower the internal resistance of Li-S batteries. EIS of MC-S and Ox MC-S at various potentials showed that oxidation treatment reduced Rsei by an average of 12.2 Ω. This indicates that the SEI thickness was reduced, or the SEI was composed of highly ion-conductive components by the oxidation treatment. Rct decreased only at lower potentials, and the Warburg coefficient decreased except at the end of charge and discharge potential. These results suggest that the oxidation treatment decreases overall resistance, but especially SEI resistance and Warburg impedance, which may improve the discharge capacity of Li-S batteries.We will also report the activation energy of Rsei and Rct and mechanism analysis of decreasing Rsei by oxidation to MC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST.[1] Y. Guo et al., Angew. Chem. Int. Ed., 52 (2013) 13186.[2] X. Ji et al., Nat. Mater., 8 (2009) 500.[3] S. Okabe et al., Electrochemistry, 85 (2017) 671.[4] L. Yoshida et al., ECS 238th PRiME Meeting s (2020).

Correlating between the height of three-dimensional core-shell electrodes and ion transport for their electrochemical performance

The height of electrodes for in-situ grown 3D core-shell electrodes on a collector, which can directly affect the transport of electrolyte ions, is an important issue that needs to be concerned. Herein, nanocone Co2(OH)2CO3 (CCH) with different heights were synthesized as representative demos to explore the possible factors, including the height h, ion diffusion coefficient D, and Warburg coefficient σ for addressing the ion transport issue. A compromise balance between the height and ion diffusion coefficient was obtained after a series of dynamic analyses. The results showed that the optimal height of CCH can improve ion transport and shorten the charge transfer distance while avoiding stacking and curling. The best electrochemical performance renders the minimum Warburg and maximum ion diffusion coefficients. Furthermore, in-situ EIS and Raman showed that the active sites of CCH were the exposed Co in octahedral units, which can form linkage of octahedrally-electrolyte upon suitable cycling height can reveal more cations, thereby producing more extraordinary electrochemical performance. This work emphasizes the importance of ion diffusion behavior for electrochemical performance within electrodes. It gives a guide for constructing core-shell electrodes with a high electrochemical performance by engineering the microscopic height of materials.

Encapsulating Sn-Cu alloy particles into carbon nanofibers as improved performance anodes for lithium-ion batteries

The SnCu-SiO2 @CNFs composite nanofibers are successfully fabricated by electrospinning and subsequent heat treatments, in which the Sn-Cu alloy particles are uniformly confined in carbon nanofibers (CNFs) in the presence of SiO2. The characterization results show that incorporating SiO2 into the CNFs matrix can mitigate the problem of the phase separation between Sn-Cu alloy particles and CNFs and increase the fibrous structural integrity of the composite electrodes. Importantly, the SnCu-SiO2 @CNFs-1.0 composite electrode displays the improved electrochemical performance when the amount of Cu(Ac)2 is controlled at 1.0 mmol, in which the discharge specific capacity of the electrode not only delivers 501.8 mAh/g after 100 cycles at 100 mA/g, but also retains 467.4 mAh/g after rate cycles when the current density is abruptly recovered to 100 mA/g. Moreover, the SnCu-SiO2 @CNFs-1.0 electrode can possess an enhanced Li+ diffusion coefficient of approximately 1.58 × 10−16 cm2 s−1 and a high Warburg coefficient of 568.9 Ω cm2 s−0.5 at room temperature. The enhanced electrochemical performance can be partly ascribed to the complete encapsulation of Sn-Cu alloy particles by the CNFs matrix, and partly to the promotion of the over-all electronic conductivity. Furthermore, the flexible and conductive CNFs matrix and the stable Cu6Sn5 phase can conjoinedly accommodate the volume change of the electrodes during charge/discharge processes. Therefore, the facile strategy for encapsulating Sn-Cu alloy particles into CNFs may offer some a prospective instruction for the alloy anodes in the field of energy storage.

Scalable synthesis of porous silicon nanoparticles from rice husk with the addition of KBr as a scavenger agent during reduction by the magnesiothermic method as anode lithium-ion batteries with sodium alginate as the binder

Porous silicon nanoparticle has been synthesized via a highly scalable heat scavenger-assisted magnesiothermic reduction of rice husk. Addition of KBr as scavenger agent for the highly exothermic magnesium reduction process. Porous silicon nanoparticles are a promising anode material for lithium-ion batteries. Effective binders can keep porous silicon nanoparticle anode materials from breaking down and losing their anode capacity because of the massive volume changes during alloy dealloying. The porous silicon nanoparticles are characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), nitrogen adsorption, and scanning electron microscope (SEM). The application of silicon nanoparticles on lithium-ion batteries with sodium alginate as the binders resulted in a good performance. The cyclic voltammetry (CV) curve of PSiNPs-1 shows a reduction peak at 0.17 V with oxidation peaks at 0.77 V. PSiNPs-1.5 shows a reduction peak at 0.18 V with oxidation peaks at 0.83 V. PSiNPs-2 shows a reduction peak at 0.20 V with oxidation peaks at 0.83 V. The PSiNPs-1 after the first cycle shows the charge-transfer resistance (Rct) value of 488 Ω, lower than PSiNPs-1.5 and PSiNPs-2. It indicates an improved charge transferability, confirming the role of porous silicon nanoparticles in enhancing electrical conductivity. Warburg coefficient of PSiNPS-1 shows lower impedance, suggesting the greatly enhanced lithium ions transport inside the active material particles. In comparison, the PSiNPs-1 electrode delivers a high initial specific capacity of 2777 mAh g−1 and maintains a specific capacity of 2579 mAh g−1 after 100 cycles.

Effect of MgO on electrochemical properties of silicon-based anode composite material

A silicon (Si)/magnesia (MgO)/graphite composite (denoted as SMG@C) was synthesized by high-energy ball milling (HEBM) via the reduction of silica (SiO) by magnesium (Mg) and subsequent addition of graphite, followed by pitch pyrolysis for carbon coating. The particles of MgO in the SMG@C were removed by hydrogen chloride (HCl) leaching to form the Si/graphite composite (denoted as SG@C). The microstructural characteristics of the two composite materials were analyzed by X-ray diffraction and transmission electron microscopy and their electrochemical properties were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, and capacity cycling tests. Results indicated that the a homogeneous mixture of nanoscale Si and MgO was fabricated by HEBM, and some of the Si and MgO formed a specific crystallographically coherent interface which was determined as Si(220)//MgO(200). The values of Warburg coefficient for SMG@C and SG@C were determined to be 1.84 Ω/S1/2 and 203.42 Ω/S1/2, respectively. When the SMG@C anode composite material was cycled for 440 cycles at a current density of 200 mA/g, its reversible capacity retention rate was 87.8%, showing excellent long-cycle performance compared to the SG@C. The presence of MgO in the anode composite material greatly improved the electrochemical properties.

Measurement of Impedance of AGM Solar Battery for RAPS Applications

The paper deals with the measurement of the cell impedance parameters during discharging and charging of the AGM 200Ah 6V Sun Power lead acid battery. Real and imaginary part of impedance of the battery were measured by PEIS method. Results of the impedance changes during discharging and charging were plot to Nyquist diagrams. Important values - ohmic resistance RS, charge transfer resistance RCT, double layer capacity CDL and Warburg coefficient σ were found during discharging and charging of the solar battery.

NH3 modified β-Na0.33V2O5 with high capacity for aqueous zinc ion battery cathode

Vanadium-based materials with high capacity and stability are needed to promote aqueous zinc ion batteries in large-scale energy storage. NH3 modified β-Na0.33V2O5 is prepared by a facile hydrothermal method and subsequent calcination. The NH3 modification mechanism of β-Na0.33V2O5 is proposed, which includes forming vacancies, structural defects and V-O-N bonds after escaping most ammonia molecules from β-Na0.33V2O5’s framework, and retaining a small part of ammonia molecules in its structure. In the Zn/β-Na0.33V2O5 battery system, the β-Na0.33V2O5 delivers a high capacity of 430 mAh g−1 at 200 mA g−1 and good structural stability. After 50th cycles at 200 mA g−1, the β-Na0.33V2O5 modified by NH3 under N2 atmosphere possesses a lower Warburg coefficient of 19.90 Ωcm2s−0.5 than 29.3 Ωcm2s−0.5 of β-Na0.33V2O5 without modification. The rigid β-Na0.33V2O5 is opened by small NH3 molecules, which provides new horizons for optimizing tunneled vanadium-based materials for aqueous zinc ion batteries.

Chiral carbon nanotubes decorated MoS2 nanosheets as stable anode materials for sodium-ion batteries

Constructing carbon-based metal sulfides hybrids are efficient strategy for improving the sodium storage performance. And novel carbon materials are attractive in the development of electrodes. Herein, chiral amphiphiles are utilized to design chiral (C-CNTs) and straight (S-CNTs) nitrogen-doped CNTs through polycondensation and supramolecular assemble process, followed by pyrolysis. Besides, the CNTs can serve as skeleton, and MoS2 nanosheets are then anchored on the mesoporous CNTs matrix by hydrothermal reaction. Well-aligned architectures are obtained for MoS2/S-CNTs, while MoS2/C-CNTs are porous with short tubular morphology. As anode materials, MoS2/C-CNTs electrode outperforms the MoS2/S-CNTs counterparts and exhibits superior cycle stability and rate performances. It delivers a capacity of 368.8 mA h g−1 after 300 cycles at a high current density of 2 A g−1. Based on kinetics analysis, the percentage of capacitive charge storage is around 93.6% at the scan rate of 2 mV s−1. The phenomenon can be elucidated by the fast charge transfer resistance and small Warburg coefficient of MoS2/C-CNTs, which are superior to MoS2/S-CNTs. Therefore, the unique chiral structure combines the merits of expanded layer-spacing, nitrogen doping with high conductivity, and mesoporous channels, ensuring fast ion diffusion and pseudocapacitive behavior. This work provides an opportunity for fabricating other metal sulfides/C-CNTs SIBs anode materials with excellent cycling stability.

Reduction of Cd(II) Ions in the Presence of Tetraethylammonium Cations. Adsorption Effect on the Electrode Process

The effect of the adsorption of tetraethylammonium (TEA) cations, which present both ionic and organic characteristics, on the reduction of Cd(II) ions have been studied from dc and ac measurements at the dropping mercury electrode. The resistance to the charge transfer (Rct) and Warburg coefficient (σ) parameters have been determined through impedance measurements. Thus, the global velocity constant has been obtained. The reduction process of Cd(II) in perchloric media is reversible and is affected by the adsorption of TEA cations, especially at high TEA concentrations. Values of E1/2, half wave potential, and DO, diffusion coefficient, obtained from both dc and ac measurements agree. The velocity constants show a decrease as TEA concentration increases, with values ranging from 0.6 to 0.01 cm·s−1. The inhibitory effect of TEA adsorption on the electrode process and the relationship between electrode coverage, θ, and velocity constants, K, using several isotherm equations, have been discussed. The best fit was obtained with the equation K = 0K(1 − θ)a with an a value close to three, indicating a blocking effect and electrostatic repulsion due to TEA.

Synergetic Strategy for the Fabrication of Self-Standing Distorted Carbon Nanofibers with Heteroatom Doping for Sodium-Ion Batteries.

Currently, the limited availability of lithium sources is escalating the cost of lithium-ion batteries (LIBs). Considering the fluctuating economics of LIBs, sodium-ion batteries (SIBs) have now drawn attention because sodium is an earth-abundant, low-cost element that exhibits similar chemistry to that of LIBs. Despite developments in different anode materials, there still remain several challenges in SIBs, including lighter cell design for SIBs. The presented work designs a facile strategy to prepare nitrogen-doped free-standing pseudo-graphitic nanofibers via electrospinning. A structural and morphological study implies highly disordered graphitic structured nanofibers having diameters of ∼120–170 nm, with a smooth surface. X-ray photoelectron spectroscopy analysis showed that nitrogen was successfully doped in carbon nanofibers (CNFs). When served as an anode material for SIBs, the resultant material exhibits excellent sodium-ion storage properties in terms of long-term cycling stability and high rate capability. Notably, a binder-free self-standing CNF without a current collector was used as an anode for SIBs that delivered capacities of 210 and 87 mA h g–1 at 20 and 1600 mA g–1, respectively, retaining a capacity of 177 mA h g–1 when retained at 20 mA g–1. The as-synthesized CNFs demonstrate a long cycle life with a relatively high Columbic efficiency of 98.6% for the 900th cycle, with a stable and excellent rate capacity. The sodium storage mechanisms of the CNFs were examined with various nitrogen concentrations and carbonization temperatures. Furthermore, the diffusion coefficients of the sodium ions based on the electrochemical impedance spectra measurement have been calculated in the range of 10–15–10–12 cm2 s–1, revealing excellent diffusion mobility for Na atoms in the CNFs. This study demonstrates that optimum nitrogen doping and carbonization temperature demonstrated a lower Warburg coefficient and a higher Na-ion diffusion coefficient leads to enhanced stable electrochemical performance. Thus, our study shows that the nitrogen-doped CNFs will have potential for SIBs.

Optimization of Cathode Material Components by Means of Experimental Design for Li-Ion Batteries

The positive electrode of a Li-ion battery is made from active material, a binder and conductive material. In this work, the direct and interaction effects of these components on electrochemical performance of a Li-ion cell is studied using design of experiments. For this purpose, a two-level full factorial design was used and interactions were analyzed using analysis of variance. The content of carbon black and graphite as conductive materials and polyvinylidene difluoride as binder are considered as input parameters. Theactive material is NCM622 in the current work. Initial discharge capacity and Warburg coefficient obtained from impedance spectroscopy are output responses. Eleven experiments along with the central points were conducted to determine the relationship between process parameters and the output responses. This information is needed to optimize the output. It was observed that all three parameters are significant. In low carbon black and graphite content, the binder has a negative effect on both parameters, but in high levels of carbon black content it has a positive effect due to the better connectivity of carbon particles with each other. The results show that interaction of carbon black-binder and carbon black-graphite are also significant. Mathematical models were presented in order to optimize the parameters.

Synthesis of MoO₃/V₂O5/C Composite as Novel Anode for Li-Ion Battery Application.

The MoO₃/V₂O5/C, MoO₃/C and V₂O5/C are synthesized by electrospinning combined with heat treatment. These samples are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and thermogravimetric analysis (TG) techniques. The results show that sample MoO₃/V₂O5/C is a composite composed from MoO₃, V₂O5 and carbon. It takes on morphology of the nanofibers with the diameter of 200~500 nm. The TG analysis result showed that the carbon content in the composite is about 40.63%. Electrochemical properties for these samples are studied. When current density is 0.2 A g-1, the MoO₃/V₂O5/C could retain the specific capacity of 737.6 mAh g-1 after 200 cycles and its coulomb efficiency is 92.99%, which proves that MoO₃/V₂O5/C has better electrochemical performance than that of MoO₃/C and V₂O5/C. The EIS and linear Warburg coefficient analysis results show that the MoO₃/V₂O5/C has larger Li+ diffusion coefficient and superior conductivity than those of MoO₃/C or V₂O5/C. So MoO₃/V₂O5/C is a promising anode material for lithium ion battery application.

A Novel Approach to the Synthesis of Alloy: PEG400 Containing ZnNi and ZnNiCr Alloy Coatings and Their Anticorrosion Performance

ZnNi and ZnNiCr alloy coatings were electrochemically deposited onto carbon steel (CS) in the presence of PEG400 for the first time. Scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) analyses showed that polyethylene glycol (PEG) was incorporated into ZnNi and ZnNiCr alloy coatings. Although it contained active zinc, the corrosion resistance of all coatings increased as the exposure time to the corrosive environment increased, and all coatings exhibited Warburg impedance. Furthermore, Warburg coefficients were incredibly higher for PEG incorporated alloy coatings than for those without PEG, which was attributed to the filling of the pores by PEG in addition to the oxide layers formed on alloy surfaces.

The impact of air humidity on the operation of proton exchange membrane fuel cells determined using dynamic electrochemical impedance spectroscopy

This paper describes the changes in the fuel cell impedance across a range of current loads and humidity changes. The changes of each equivalent circuit parameters representing specific processes occurring in fuel cells, as a function of humidity and current changes, were determined. Within the combined scope of current and humidity, the optimal working conditions of the cells were determined, corresponding to the lowest impedance values. The influence of humidity on the charge transfer resistance behavior and the Warburg coefficient for varying current load values were analyzed in detail.

Novel hierarchical structural SnS2 composite supported by biochar carbonized from chewed sugarcane as enhanced anodes for lithium ion batteries

A novel SnS2/B composite with hierarchical structure consisting of SnS2 nanosheet arrays and biochars carbonized from the chewed sugarcane is successfully prepared using a simple one-step hydrothermal method, which is confirmed by FE-SEM, XRD, XPS, and TG analyses. These results indicate that the SnS2/B-9 composite anode with optimal weight ratio of biochar to SnS2 powders approximating 9% can exhibit the excellent stable structure and crystallinity. The assembled cell with the SnS2/B-9 delivers a high initial discharge specific capacity of 1107.4 mAh g−1 with a coulombic efficiency of 54.8% and retains 369.1 mAh g−1 after 50 cycles at 100 mA g−1. Moreover, it can possess an enhanced lithium ion diffusion coefficient of approximately 5.517E−14 cm2 s−1 and a high Warburg coefficient of 31.48 Ω cm2 s−0.5 at room temperature. These outstanding electrochemical performance as well as environmental friendliness of the as-prepared SnS2/B-9 composite make it possible to be a potential anode material for the lithium ion battery.

Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes

The chemical and surface modification of activated carbon (AC) through the incorporation of nanoparticles (NPs) is a simple and efficient strategy to enhance the charge transfer as well as the capacitance behavior of AC. Herein, ZnO-NPs were successfully attached at the surface of AC via one-step hydrothermal process to form ZnO-NPs@AC (ZAC). The introduced ZAC-material was studied by surface characterization techniques including transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) to investigate the morphological and chemical characteristics of ZAC. After that, ZAC was utilized as a working electrode (WE) material for supercapacitor in alkaline and acidic electrolytes. The specific capacitance (Cs) values of ZAC showed an enhanced performance as compared with that of the pristine AC. Moreover, the low series resistance and high Warburg co-efficient of ZAC as compared to those of the AC indicated the faster −OH diffusion inside the ZAC working electrode. Additionally, the phase angle of ZAC is closer to the ideal capacitor than that of AC. Overall, this study proved that ZnO-NPs can enhance the AC capacitance behavior through the easy chemical modification of AC.

Decoupling and correlating the ion transport by engineering 2D carbon nanosheets for enhanced charge storage

The high aspect ratio of two-dimensional (2D) carbon materials feature unique superiorities and great potential in supercapacitors. The transport of electrolyte ions inside the 2D carbon electrode that can directly affect the overall electrochemical performance, is a key issue that need to be concerned. Nevertheless, the research regarding this process always lags far behind the material concerns and the intrinsic factors to affect ion transport remain obscure. Here we for the first time focus on and correlate the potential and possible factors for addressing this issue using 2D porous carbon nanosheets with tunable thickness as representative demos. A compromise balance between the thickness and ion transport behavior is presented and investigated. The optimal thickness of nanosheets can effectively avoid curling and stacking of ultrathin nanosheets, shorten the distance of ion transport and accelerate the ion diffusion, which render the material with the low resistance corresponding to minimum Warburg coefficient and maximum ion diffusion coefficient for fast transport of electrolyte. A superior capacitance value of 280 F g−1 can be obtained at 0.5 A g−1, with a retention rate of 81% even at a large current density of 100 A g−1. Besides, the symmetric supercapacitor with 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte (EMIMBF4) delivers an energy density of 94 Wh kg−1 at 1.8 kW kg−1. This work illustrates the importance for transferring the focus from carbon materials to electrolyte ion transport behavior within electrode and provides a guidance for regulating the ion transport by engineering the microscopic thickness of advanced materials in energy-related fields.

Electrochemical corrosion behavior of micrometer‐sized porous Ti3SiC2 compounds in NaCl solution

The electrochemical corrosion behavior of reactively synthesized porous Ti3SiC 2 in 1 M NaCl solution was investigated compared with that of porous TiAl. Porous Ti 3SiC 2 and TiAl have high‐purity single phase characteristics with open porosities of 48% and 41% and maximum apertures of 6.9 and 16.3 µm, respectively, which show positive direction shift behavior of open circuit potential at the early immersion stage with stable potential ranges of 0.25~0.27 and −0.36~−0.38 V (vs. saturated calomel), respectively. The electrochemical corrosion procedures of the two porous compounds exhibit three‐stage characteristics of activation, passivation, and breakdown. Porous Ti 3SiC 2 has a much larger ohmic resistance and Warburg coefficient than TiAl. The passivation films of the two porous compounds consist of an inner layer with a larger resistance and a porous outer layer. Porous Ti 3SiC 2 has a significantly larger resistance and relatively smaller capacitance for the inner passivation film, indicating better corrosion resistance stability.