Constant Current Polarization Research Articles

Overlimiting Currents and Sand’s Time Behaviors in Solid Polymer Electrolytes

Dendrite growth in solid polymer electrolytes has been frequently analyzed since it was the critical issue that limited its applications. For the liquid electrolyte systems, the dilute solution theory and the classic Nernst-Planck equation have been proven to be useful tools for analysis of ion transport dynamics and especially the dendrite initiation at Sand’s time. However, characterization of the Sand’s time in solid polymer electrolyte systems is challenging and also seldomly performed. From the experimental perspective, operando observations have been done, but the true local current density was different from the geometrical average current density since there was always heterogeneous current distribution for millimeter-scale electrolytes. From the theoretical perspective, the dendrite initiation time followed the trend predicted by Sand’s equation, but discrepancies were found such as dendrites observed at under-limiting currents or the order-of-magnitude extended Sand’s time compared to theoretical predictions. Here, we use transparent microcapillary cells for solid polymer electrolyte systems to overcome these challenges. These specialized cells allow direct operando optical observations, while the micron-scale cross-sectional area minimizes the discrepancy between true local and averaged geometrical current densities. Comparison between operando image and voltage response during constant current polarization shows that, unlike liquid electrolyte cases, the dendrite initiation for solid polymer electrolytes doesn’t always trigger a voltage spike. We also derived transport parameters from the measured Sand’s time, limiting current density, and conductivity and the cross-validated transport parameters were used to calculate theoretical Sand’s time that can be compared with the experimental values. Using the analytical solution from the Nernst-Planck equation and numerical calculations using Newman’s concentrated solution model and COMSOL Multiphysics, the predicted Sand’s time for the dilute and concentrated solution theory both matched closely with our experimental values. This work demonstrates that while the polarization process and the onset of lithium dendritic growths in solid polymer electrolytes can be still accurately predicted by the dilute solution theory, it may not always result in voltage responses similar to that of the liquid electrolyte cases. It’s also suggested that ensuring the homogenous distribution of lithium flux and avoiding the localized overlimiting current density is the key to realizing the dendrite-free polymer electrolytes.

Current-driven solvent segregation in lithium-ion electrolytes

Liquid lithium-battery electrolytes universally incorporate at least two solvents to balance conductivity and viscosity. Almost all continuum models treat cosolvent systems such as ethylene carbonate:ethyl-methyl carbonate (EC:EMC) as single entities whose constituents travel with identical velocities. We test this “single-solvent approximation” by subjecting LiPF 6 :EC:EMC blends to constant-current polarization in Hittorf experiments. A Gaussian process regression model trained on physicochemical properties quantifies changes in composition across the Hittorf cell. EC and EMC are found to migrate at noticeably different rates under applied current, demonstrating conclusively that the single-solvent approximation is violated and that polarization of salt concentration is anticorrelated with that of EC. Simulations show extreme solvent segregation near electrode/liquid interfaces: a 5% change in EC:EMC ratio, post-Hittorf polarization, implies more than a 50% change adjacent to the interface during the current pulse. Understanding how lithium-ion flux induces local cosolvent or additive imbalances suggests new approaches to electrolyte design. • Applied current rearranges neutral cosolvents in lithium-ion electrolytes • A solution-property machine-learning model pinpoints electrolyte composition changes • This polarization effect is expected to be most severe near electrode interfaces While it is understood that electrolyte solvents are consumed throughout the lifetime of battery operation, less is known about intra-cycle variation in cosolvent composition. Wang et al. pair polarization experiments with machine learning to measure solvent segregation occurring under applied current. This effect is especially prominent near electrode surfaces and may illuminate guiding principles for electrolyte design.

The critical role of aluminum sulfate as electrolyte additive on the electrochemical performance of lead-acid battery

Aluminum sulfate is inexpensive, non-toxic and non-hazardous and has the potential to become an ideal electrolyte additive for lead-acid batteries. This paper investigates in depth on the effect of electrolyte additives in lead-acid batteries under high rate charging and discharging conditions. This research work proves that aluminum sulfate in the electrolyte can affect the rapid accumulation of lead sulfate formation and growth in high rate charging and discharging process and then significantly inhibit sulfation, which reduces the internal resistance of the electrode plate and substantially improves the electrochemical performance of the battery. The high-rate partial charge state (1C charging and discharging rate) cycle (HRPSoC) life (8003 times) of the battery with aluminum sulfate additive is 13 times that of the blank battery. The battery test results show that the battery has excellent performance in charge acceptance test and constant voltage and constant current polarization test. We also found aluminum sulfate could repair the spent batteries effectively.

Suppression of Sr surface segregation in La0.8Sr0.2Co0.2Fe0.8O3-δ via Nb doping in B-site

Sr surface segregation has been one of the main reasons for the cathode performance degradation during the long-term operation of solid oxide fuel cells (SOFC). Investigation on Sr segregation mechanism and proposing strategies on suppressing the Sr surface segregation are significant for SOFC development. In this paper, La 0.8 Sr 0.2 Co 0.2 Fe 0.8-x Nb x O 3-δ (LSCFNb, x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) are prepared via sol-gel method. The electrochemical performance and long-term stability are tested through electrochemical impedance spectroscopy (EIS) and constant current polarization. The results show that the long-term stability of LSCFNb cathodes are strongly affected by Nb content. Combining the results of ICP and XPS, it's revealed that the Sr surface segregation can be effectively suppressed with the increase of Nb content. The LSCFNb cathodes gain the optimal electrochemical performance when x = 0.04, with minimum polarization resistance of 0.27 Ω cm 2 at 750 °C and oxygen reduction reaction activation energy of 1.54 eV. After cathodic polarization for 144 h, the polarization resistance and activation energy of LSCFNb4 cathode increase slightly, revealing it a promising cathode material for SOFC research.

Effects of Gd0.8Ce0.2O1.9−δ coating with different thickness on electrochemical performance and long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ cathode in SOFCs

The commercialization of Solid oxide fuel cells (SOFCs) has always been limited by the poor catalytic activity and the severe degradation of cathode in the intermediate and low operating temperature. Here we report a Gd0.8Ce0.2O1.9−δ (GDC) coated La0.8Sr0.2Co0.2Fe0.8O3-δ (LSCF) composite cathode material, which can significantly improve the electrochemical performance and durability of LSCF cathode. The effects of different GDC coating thickness on the electrochemical performance and long-term working stability of LSCF cathode are investigated, and the optimal coating thickness is established. The polarization impedance of GDC coated LSCF (LSCF@GDC) cathode with 9 nm of GDC coating is 0.08 Ω cm2 at 800 °C, which is only one quarter of that of the raw LSCF cathode, and the degradation rate of constant current polarization with 100 mA cm−2 is only 0.42%/100 h at 700 °C, which is far less than that of the raw LSCF cathode. The X-ray photoelectron spectroscopy (XPS) results show that the degree of Sr segregation decreases with the increase of the thickness of the coated GDC layer. The potential LSCF@GDC composite material is expected to increase the operability of SOFCs and accelerate its commercialization.

Electrochemical properties of Li6+yLa3−yBayNbZrO12 lithium garnet oxide solid-state electrolytes with co-doping barium and zirconium

In this work, we co-doped barium (Ba) and zirconium (Zr) in a lithium garnet oxide, to improve its electrochemical properties. In the doping range of y = 0–1.0, the Li6+yLa3−yBayNbZrO12 (LLBNZO) oxides maintain a cubic crystal structure. A secondary phase of BaZrO3 emerges when y ≥ 0.75. The ionic conductivity of the developed garnet oxides first increases with Ba-doping in the low doping range and then decreases in the high doping range. It reaches the maximum value in Li6.5La2.5Ba0.5NbZrO12 when y = 0.5. Cyclic voltammetry shows that the garnet oxides have a wide electrochemical voltage window up to 6 V. The constant-current polarization results on the symmetric cells assembled from Li6.5La2.5Ba0.5NbZrO12 reveal that this garnet oxide is chemically stable against metallic lithium and has stable cycling performance.

Effect of optimal GDC loading on long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 composite cathodes

Three types of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 (LSCF/GDC) composite cathodes with different optimal GDC loading are fabricated through electrospinning, screen printing and solution infiltration method. Constant current polarization with current density of 100 mA cm−2 at 750°C is applied to test the stability of LSCF/GDC composite cathodes. After constant current polarization for 144 h, the polarization resistance (Rp) of 280 nm-nanofiber skeletal LSCF/GDC composite cathode after pore-forming exhibits the minimum increase, from 0.062 Ω cm2 to 0.098 Ω cm2. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) results show that the microstructure and surface chemical composition of the cathode maintain stable during the constant current polarization. Combined with the X-ray diffraction (XRD) result, a relationship among GDC loading, stress, Sr surface segregation and long-term stability is established.

Boron Trifluoride Anionic Side Groups in Polyphosphazene Based Polymer Electrolyte with Enhanced Interfacial Stability in Lithium Batteries.

A modified polyphosphazene was synthesized using a mixed substitution at phosphorus consisting of 2-(2-methoxyethoxy)ethoxy side groups and anionic trifluoroborate groups. The primary goal was to increase the low lithium ion conductivities of the conventional lithium salt containing poly[2-(2-methoxyethoxy)ethoxy-phosphazene] (MEEP) by the immobilized anionic groups. As in previous studies, the mechanical stability was stabilized by UV induced radiation cross linking. By variation of the molar ratio between different side groups, mechanical and electrochemical properties are controllable. The polymer demonstrated large electrochemical stability windows ranging between 0 and 4.5 V versus the Li/Li+ reference. Total and lithium conductivities of 3.6 × 10−4 S·cm−1 and 1.8 × 10−5 S·cm−1 at 60 °C were revealed for the modified MEEP. When observed in special visualization cells, dendrite formation onset time and short-circuit time were determined as 21 h and 90 h, respectively, under constant current polarization (16 h and 65 h for MEEP, both with 15 wt % LiBOB), which hints to a more stable Li/polymer interface compared to normal MEEP. The enhanced dendrite suppression ability can be explained by the formation of a more conductive solid electrolyte interphase (SEI) and the existence of F-contained SEI components (such as LiF). With the addition of ethylene carbonate–dimethyl carbonate (EC/DMC) to form MEE-co-OBF3P gel polymer, both total and lithium conductivity were enhanced remarkably, and the lithium transference numbers reached reasonable values (σtotal = 1.05 mS·cm−1, σLi+ = 0.22 mS·cm−1, = 0.18 at 60 °C).

An Exceptionally Facile Synthesis of Highly Efficient Oxygen Evolution Electrodes for Zinc‐Oxygen Batteries

AbstractZinc‐oxygen batteries are promising candidates for electrical vehicles and electric grid energy storage due to their low cost, high safety levels, and low environmental impact. The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) represent the most significant processes in zinc‐oxygen batteries. The development of nonprecious metal catalysts for OER with satisfactory performances and low cost (especially prepared by a straightforward synthesis route), still poses a significant challenge. Herein, we report an exceptionally facile and easily scalable method to produce NiFe layered double hydroxide (LDH) coated nickel foam by immersing nickel foams into aqueous solutions containing NiCl2 and FeCl2 and keeping them standing for a certain period of time. Without any other complicated technique or organic reagent, microflower‐shaped NiFe LDHs can be obtained. The resulting electrode shows excellent OER performance with a low overpotential of 210 mV at a 10 mA cm−2 constant current polarization and stable operation for more than 100 charge‐discharge cycles in zinc‐oxygen batteries.

Electrochemical behavior of Pb–Ag–Nd alloy during pulse current polarization in H2SO4 solution

The anodic layer and oxygen evolution behavior of Pb–Ag–Nd alloy during pulse current polarization and constant current polarization in 160 g/L H2SO4 solution was comparatively investigated by chronopotentiometry, SEM, XRD, EIS and Tafel techniques. The results show that the anodic layer on Pb–Ag–Nd alloy formed through pulse current polarization is more intact and presents fewer micro-holes than that formed through constant current polarization. This could be attributed to the low current density period, which works as a ‘recovery period’. During this period, the oxygen evolution reaction is less intense, which benefits the recovery of porous anodic layer. Pb–Ag–Nd anode also shows a lower anodic potential during pulse current polarization, which is in accordance with its smaller charge transfer resistance and smaller Tafel slope coefficient at high over-potential region. The lower anodic potential could be ascribed to the higher concentration of PbO2 in the anodic layer, which promotes the formation of more reactive sites for the oxygen evolution reaction.

Study on Critical Corrosive Ratio of Reinforced Concrete Based on Monte-Carlo Method

Based on the relationship between the displacement and the corresponding strain in protective layer concrete during their corrosive expanding, the strain in protective layer arrives the critical strain have been think as the cracking condition, then a certainty critical corrosive ratio model has been constructed in this paper. Considering the probability distribution and the statistical characteristics of concrete protective layer thickness c, reinforced bar diameter dst and the effectively volume expansion coefficient neff, the Monte-Carlo method have been used to simulate the critical corrosive ratio ρcr, its probability distribution has been shown in good agreement with Weibull distribution and lognormal distribution in results. Through the constant current polarization accelerated corrosion test, the critical corrosive model has been validated. With comparison of critical corrosive ratio which be obtained with the test and the model simulation results respectively, where a good agreement have been shown.

Properties of passive film formed on 316L / 2205 stainless steel by Mott-Schottky theory and constant current polarization method

Semiconductor properties of the passive films formed on 316L and 2205 stainless steel were studied by Electrochemical Impedance Spectroscopy (EIS) in the high-temperature acetic acid. The results showed that the corrosion resistance of 2205 was higher than that of 316L, and the passive films formed on 316L and 2205 stainless steel showed p-type and n-type semiconductor behavior, respectively. Destruction and self-repairing of passive films were studied by using the constant current polarization method. The results showed that for 316L, the self-repairing process would occur when the destruction was lower than the critical extent or it would not do; for 2205, the self-repairing process only happened in a short time when the destruction was in the same extent as 316L.

PEM Fuel Cell Contamination: Effects of Operating Conditions on Toluene-Induced Cathode Degradation

Toluene, one of many volatile organic compounds existing in the atmosphere, was studied as an impurity or contaminant in the airstream of proton exchange membrane (PEM) fuel cells. The effects of toluene contamination on fuel cell performance were investigated with constant-current polarization discharge under various operating conditions, including current density, back pressure, and air stoichiometry. The severity of the contamination increased with increasing current density, toluene concentration, and air stoichiometry, but decreased with increasing back pressure. The effect of toluene contamination on current dynamic operation was also studied by cycling the current density from and then to in the presence of toluene. Cell performance degradation during current dynamic operation was accelerated by the presence of toluene. Electrochemical impedance measurements and analysis revealed that both kinetic resistance and mass transfer resistance were increased as a result of toluene contamination, but the increase in kinetic resistance was dominant.

Polymer electrolyte membrane fuel cell contamination: Testing and diagnosis of toluene-induced cathode degradation

The effects of toluene contamination on the performance of polymer electrolyte membrane (PEM) fuel cells were investigated, using various levels of toluene concentration in the air streams, under different operational conditions and with different catalyst loadings. Constant-current polarization and electrochemical impedance spectroscopy (EIS) were conducted to analyze the poisoning behaviour of toluene. The severity of the contamination effect increased with an increase in both the current density and the toluene concentration, but decreased with an increase in both the relative humidity (RH) and the cathode-side Pt loading. The toluene-poisoned fuel cell could not be fully recovered by replacing toluene-contaminated air with pure air. EIS measurements revealed that both kinetic resistance and mass transfer resistance increased as a result of toluene contamination, while membrane resistance remained unchanged. However, the increase in kinetic resistance was a major contributor to cell performance degradation.

Studies of the pulse charge of lead-acid batteries for PV applications: Part I. Factors influencing the mechanism of the pulse charge of the positive plate

The mechanism of the positive plate charge in pulse regime was studied in model lead-acid cells with one positive and two negative plates (8 Ah each) and Ag/Ag 2SO 4 reference electrodes. The results showed that the evolution of the electrode potential is much slower on the positive plate than on the negative plate. Regardless of this fact, the calculated capacitive current of charge and self-discharge of the electrochemical double layer (EDL) during the “ON” and “OFF” half-periods of the pulse current square waves is comparable with the charge current amplitude. The result is due to the high values of the EDL on the surface of the lead dioxide active material. The influence of different factors like state of charge, state of health, pulse frequency, current amplitude and open circuit stay before the polarization was discussed. The previously determined optimal frequency of 1 Hz was associated with a maximum in the average double layer current on frequency dependence. The average double layer current is also maximal at SOC between 75 and 100%. The exchange of the constant current polarization with pulse polarization does not change substantially the mechanism and the overvoltage of the oxygen evolution reaction on the positive plate. The mechanism of the self-discharge of the EDL was also estimated analyzing long-time PPP transients (up to 2 h). It was found that when the PPP is lower than 1.2 V the preferred mechanism of EDL self-discharge is by coupling with the lead sulphate oxidation reaction. At higher values of PPP the EDL self-discharge happens via oxygen evolution. The high faradic efficiency of the pulse charge is due to the chemical oxidation of the Pb(II) ions by the O atoms and OH radicals formed at the oxygen evolution both during the “ON” and “OFF” periods.

Preparation of nanocrystalline palladium on p-Si by electroless deposition and its photo-electrochemical hydrogen evolution property

The Pd-modified p-Si electrode was prepared by means of electroless deposition. The average diameter of Pd nanoparticles was about 80 nm from SEM observations. XRD results showed that the structure of Pd coatings was nanocrystalline. The catalyzing properties of the Pd-modified p-Si electrode for hydrogen evolution under illumination and in dark were studied with the constant current polarization method. The polarization potential for hydrogen evolution reaction (HER) of the Pd/p-Si electrode under illumination was 250 mV lower than that in dark, and was 450 mV lower than that of p-Si in dark at the current density of 2.5 mA·cm −2. The results of electrochemistry impedance spectra (EIS) showed that electrochemistry HER resistance of the modified electrode under illumination decreased from 593.12 to 442.20 Ω·cm 2, and the HER rate increased evidently.

Ionic conductance of gel electrolyte using a polyurethane matrix for rechargeable lithium batteries

Polymeric gel electrolyte films consisting of poly(ethylene oxide)-based polyurethane containing lithium salt (LiClO 4 or LiPF 6) dissolved in propylene carbonate (PC) have been prepared. The ionic conductance and electrochemical properties of the gel films were examined. The obtained gel film was self-standing, transparent and flexible with high mechanical strength. Thermal analysis of the gel film showed that it is homogeneous and amorphous over a wide temperature range. The highest conductivity, 1.51 mS cm −1 at 60 °C, was obtained for the polymeric gel containing 50 wt.% of 1.5 mol dm −3 LiClO 4/PC. The applicability of the present gel system to a rechargeable lithium battery was briefly examined by cyclic voltammetry and constant-current polarization using metallic lithium electrodes.

Studies of a Model Mesh Electrode O2,Pt/O2–under Galvanostatic Polarization

The impedance of model platinum mesh electrode in contact with a doped zirconium-dioxide-based single-crystal electrolyte is studied by impedance spectroscopy under constant-current polarization. The electrode is distinguished for its readily determinable geometrical characteristics of the metal/electrolyte interface, in particular, the length of the triple-phase boundary and the electrode contact area. Three different orientations of the electrolyte face are studied. The impedance dispersion is discussed in terms of the electrode prehistory, orientation, and potential.

Polymeric gel electrolytes using a network matrix with carbonyl groups for rechargeable lithium batteries

A novel polymer matrix containing carbonyl groups in its network structure has been examined as the base component of polymeric gel electrolytes for rechargeable lithium batteries. The gel electrolyte was prepared by solidification based on photo-induced radical polymerization of a mixture of ethylmethacrylmethyl carbonate (EMMC) and diethyleneglycol dimethacrylate (DEGDM) that contains LiPF 6 as the electrolytic salt with a liquid component of ethylene carbonate (EC) and dimethyl carbonate (DMC). The obtained gel film was flexible and self-standing with proper mechanical strength. The ionic conductivity of the polymeric film was about 1 mS cm −1 at room temperature. The introduction of carbonyl groups to the polymer matrix was effective to hold the liquid component as homogeneous gel phase. The applicability of the present gel system to rechargeable battery was examined by cyclic voltammetry and constant-current polarization of metallic lithium and lithium titanate electrodes.

Electrode System Ni/NayAlxV12O30/Al3+: An Impedance Spectroscopy Study

The impedance of electrochemical cells Ni/VOB/Al3+, Ni/VOB/Na+, and Ni/VOB/Ni (where VOB stands for vanadium oxide bronzes of the general formula Na y Al x V12O30, with 0 < x ≤ 1.7) is studied both at equilibrium and a constant-current polarization. Variations in the VOB composition and aqueous electrolytic solutions in contact with the bronze as a result of the passing of the current are monitored. The results of these measurements are employed to substantiate a model that describes the occurrence of processes of current production in the Ni/VOB/Al3+ electrode system. The overvoltage is defined to a substantial degree by the aluminum diffusion. The diffusion coefficients for aluminum, its partial conductivity, and transport number are estimated.