Liquid Electrolyte Solution Research Articles

Determination of the mass-transport properties of vanadium ions through the porous electrodes of vanadium redox flow batteries

This work is concerned with the determination of two critical constitutive properties for mass transport of ions through porous electrodes saturated with a liquid electrolyte solution. One is the effective diffusivity that is required to model the mass transport at the representative element volume (REV) level of porous electrodes in the framework of Darcy's law, while the other is the pore-level mass-transfer coefficient for modeling the mass transport from the REV level to the solid surfaces of pores induced by redox reactions. Based on the theoretical framework of mass transport through the electrodes of vanadium redox flow batteries (VRFBs), unique experimental setups for electrochemically determining the two transport properties by measuring limiting current densities are devised. The effective diffusivity and the pore-level mass-transfer coefficient through the porous electrode made of graphite felt, a typical material for VRFB electrodes, are measured at different electrolyte flow rates. The correlation equations, respectively, for the effective diffusivity and the pore-level mass-transfer coefficient are finally proposed based on the experimental data.

Imidazolium Based Ionic Liquid Electrolytes for Li-Ion Secondary Batteries Based on Graphite and LiFePO4

This manuscript reports the development of a safe and green lithium-ion battery using lithium iron phosphate (LiFePO4) as a positive electrode, graphite (Cgr) as a negative electrode, and an Ionic Liquid (IL) electrolyte solution based on 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide (C1C6ImNTf2). Vinylene carbonate (VC) was tested as an electrolyte additive. The electrochemical stability of (C1C6ImNTf2) and their binary mixture referred as (Li)(C1C6ImNTf2) were investigated. In addition the electrochemical behavior of the graphite electrode was studied through cyclic voltammetry experiments in the presence and absence of VC co- additive. Complementary cycling tests demonstrated the benefic effect of VC on the charge-discharge performances for the full Lithium ion configuration based on Cgr/LiFePO4.

Study on performances of ZSM-5 doped P(VDF-HFP) based composite polymer electrolyte prepared by steam bath technique

Several novel kinds of poly (vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) based composite polymer electrolyte (CPE) membranes doped with ZSM-5 (Zeolite sieve of molecular porosity) were fabricated by steam bath technique. The desirable CPE membranes were further prepared by immersing them into the liquid electrolyte solution of 1.0 M LiPF6-ethylene carbonate /dimethyl carbonate/ethylmethyl carbonate (v/v/v:1/1/1) for 1 h to be activated at room temperature. Physical and chemical properties of the as-prepared CPEs are studied by scanning electron microscope, Fourier transform infrared, thermogravimetry and differential scanning calorimetry and electrochemical methods. The results showed that the thermal and electrochemical stabilities of the CPEs can be reached to 350 °C and 5 V, respectively. Reciprocal temperature dependence of corresponding ionic conductivity follows Vogel–Tamman–Fulcher relation, and ionic conductivity at ambient temperature can be up to 5.1 mS cm−1. The fabricated Li/CPEs/LiCoO2 coin cells with the CPE membranes prepared by steam bath technique can achieve a high discharge capacity about 144.97 mAh g−1 at 0.1 C. At 1.0 C, the capacity can be kept to 89.84 % of that at 0.1 C for the cell. After 26 cycles, the cell prepared with these CPE membranes can keep 97.5 % of its initial discharge capacity. These excellent physicochemical and battery properties indicate that these novel CPEs can be potentially used as electrolyte in lithium ion polymer battery.

Ionic Current Rectification in Soft‐Matter Diodes with Liquid‐Metal Electrodes

AbstractA soft‐matter‐based diode composed of hydrogel and liquid metal (eutectic gallium indium, EGaIn) is presented. The ability to control the thickness, and thus resistivity, of an oxide skin on the metal enables rectification. First, a simple model system with liquid‐metal/electrolyte‐solution/Pt interfaces is characterized. The electrically insulating oxide skin on the EGaIn electrode is reduced or oxidized further depending on the direction of the bias, thereby allowing unidirectional ionic current. The forward current of the diode increases as the conductivity of the electrolyte increases, whereas backward current depends on the pH of the medium in contact with the insulating oxide layer on the EGaIn electrode. As a result, the diode shows a higher rectification ratio (defined as the ratio of forward to backward current measured at the same absolute bias) with more conductive electrolyte at neutral pH. Replacement of the liquid electrolyte solution with a hydrogel improves the structural stability of the soft diode. The rectification performance also improves due to the increased ionic conductivity by the gel. Finally, a diode composed entirely of soft materials by replacing the platinum electrode with a second liquid‐metal electrode is presented. Contacting each liquid metal with a polyelectrolyte gel featuring different pH values provided asymmetry in the device, which is necessary for rectification. A hydrogel layer infused with a strong basic polyelectrolyte removes the insulating oxide layer, allowing one interface with the EGaIn electrode to be conductive regardless of the direction of bias. Thus, the oxide layer at the other interface rectifies the current.

Properties of poly(vinylidene fluoride- co-hexafluoropropylene) gel electrolytes containing modified inorganic Al 2O 3 and TiO 2 filler, complexed with different lithium salts

The aim of the studies was to determine the effect of inorganic fillers surface modified with acidic groups on the properties of polymer gel electrolytes: matrix morphology, temperature dependence of ionic conductivity and interfacial stability. The effect of the concentration of the lithium salt on ionic conductivity was also studied. The poly(vinylidene fluoride- co-hexafluoropropylene) (PVdF/HFP) gels were prepared according to the so-called Bellcore process. Dry membranes were immersed in a liquid electrolyte solution. Two different lithium salts (LiCF 3SO 3 and LiN(CF 3SO 2) 2) in propylene carbonate (PC) were used as liquid electrolytes. The salt concentration in the solution varied from 0.05 M to 1 M. Changes in the ionic conductivity as a function of the type of the filler added were analyzed on the basis of impedance spectroscopy experiments. The structure and morphology were analyzed by means of DSC and scanning electron microscopy techniques. The stability of the gel electrolytes–lithium electrode interface was evaluated by impedance spectroscopy technique in Swagelok geometry against two lithium metal electrolytes over 25 days. Finally, the lithium transference number was calculated for some of the studied gels using the polarization method.

Metallisation of gel surfaces under ambient conditions

A room temperature method to coat a non-conducting gel phase with a metal is described, which uses galvanic displacement. Electrolytes are dissolved in the gel phase to allow metal deposition from an immiscible liquid electrolyte solution. Conformal deposition was achieved by imprinting the gel, followed by galvanic displacement of gold.

Facile Synthesis of TiO2 Inverse Opal Electrodes for Dye-Sensitized Solar Cells

Engineering of TiO(2) electrode layers is critical to guaranteeing the photoconversion efficiency of dye-sensitized solar cells (DSSCs). Recently, a novel approach has been introduced for producing TiO(2) electrodes using the inverted structures of colloidal crystals. This paper describes a facile route to producing ordered macroporous electrodes from colloidal crystal templates for DSSCs. Using concentrated colloids dispersed in a volatile medium, the colloidal crystal templates were obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Here, the effects of the structural properties of the inverse opal TiO(2) electrodes on the photovoltaic parameters of DSSCs were investigated. The photovoltaic parameters were measured as a function of pore ordering and electrode film thickness. Moreover, DSSC applications that used either liquid or viscous polymer electrolyte solutions were investigated to reveal the effects of pore size on performance of an inverse opal TiO(2) electrode.

Electrochemical Hydrogen Adsorption and Absorption. Part 1: Under-potential Deposition of Hydrogen

The review article presents an atomic-level discussion of the fundamental steps involved in hydrogen electro-adsorption on transition-metal electrodes. It also discusses the basic events involved in hydrogen physisorption and chemisorption under low-pressure gas-phase conditions in order to identify characteristics that are unique to the electrochemical environment. The electrochemical environment that comprises the solid electrode, the liquid electrolyte solution, and the electrostatic field creates favorable conditions for the electro-adsorption of two different hydrogen species, the under-potential and over-potential adsorbed hydrogen (HUPD and HOPD). The electro-adsorption of HUPD and HOPD occurs in two different potential ranges. The electro-adsorption of HUPD is characteristic of Pt group metals only, while the electro-adsorption of HOPD takes place at all electrode materials at which H2(g) can be electrolytically generated. The existence of HUPD and HOPD is supported by spectroscopic and thermodynamic results. A thermodynamic analysis of the electro-adsorption of HUPD results in the determination of thermodynamic state functions that govern the process, namely the standard Gibbs energy, enthalpy and entropy of electro-adsorption, $$ {\Delta_{{\rm{ec - ads}}}}G^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and $$ {\Delta_{{\rm{ec - ads}}}}S^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , the Gibbs energy of later interactions, $$ \omega \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and the M–HUPD surface bond energy, $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ . Thermodynamic data for the electro-adsorption of HUPD on polycrystalline electrodes (Pt(poly) and Rh(poly)) and on Pt(111) in several aqueous electrolyte (H2SO4, HClO4, and NaOH) solutions are reported. Their analysis and the analysis of $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ and $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ allow us to compare on a thermodynamic basis HUPD to Hchem, and to identify surface adsorption sites possibly occupied by HUPD and HOPD adatoms.

Effect, mechanism and recovery of nitrogen oxides poisoning on oxygen reduction reaction at Pt/C catalysts

The poisoning of nitrogen oxides (NO x ) on the oxygen reduction reaction (ORR) at the Pt/C catalyst has been studied for proton exchange membrane fuel cells by a three-electrode method in liquid electrolyte solution. The cyclic voltammetry (CV) results reveal that the absorption of NO x on metallic Pt is more significant than on Pt oxides, and this absorption is probably a chemical rather than an electrochemical process. Linear sweeping voltammetry (LSV) curves for the ORR show that it is the absorption of NO x on the Pt surface that results in significant performance degradation of Pt/C catalysts. This degradation is mainly due to the reduction of electrochemically active surface area, since the ORR mechanism remains almost the same after the NO x poisoning as revealed by similar Tafel slopes. Because lower potentials facilitate the reduction of NO x to water soluble N H 4 + , reducing the working potential can mitigate the poisoning of NO x . However, to completely recover the performance loss due to NO x poisoning through the potential sweeping, it is found that the oxidation removal is more efficient than the reduction removal.

High ionic conductivity of Mg–Al layered double hydroxides at intermediate temperature (100–200 °C) under saturated humidity condition (100% RH)

Ionic conduction properties of layered double hydroxides (LDHs), [Mg (1 − x) Al ( x) (OH) 2](CO 3) x/2 · nH 2O (═CO 3-type) and [Mg (1 − x) Al ( x) (OH) 2](OH) x · nH 2O (═OH-type), have been investigated in the temperature range of 25–200 °C under saturated water vapor pressure (100% RH). Conductivity was measured by the complex impedance method using pressed pellet samples. The conductivity of an OH-type sample ( x = 0.25) is as high as 4 × 10 − 3 S cm − 1 already at room temperature. On heating, it increases as slowly (the activation energy E a ∼ 55 meV) as those of liquid electrolyte solutions, suggesting that ions are transported via exchange of protons between OH − and H 2O species in the interlayer space of the LDH (a kind of the Grotthus mechanism). The conductivity at 200 °C reaches 3 × 10 − 2 S cm − 1 . Although the conductivity of the CO 3-type counterpart is considerably low (∼ 10 − 6 S cm − 1 ) at room temperature, it increases with increasing temperature with showing a jump by about four orders of magnitude around 80 °C, after which the compound is turned into an almost the same liquid-like conductor as the OH-type sample; the conductivity jump is probably due to the interlayer reaction CO 3 2− + H 2O = 2OH − + CO 2 (g). Thus those Mg–Al LDH materials show the potential application as solid electrolyte materials for intermediate temperature fuel cells, not only because they have high conductivity but also they should not suffer from the degradation by CO 2 which is a serious problem for conventional anion exchange membranes.

Novel Electroplating Method Using Foam of an Electrolyte Solution

A novel electroplating method in which the foam of an electrolyte solution was used instead of liquid electroplating solutions was developed. The foam of the electrolyte solution was generated by bubbling nitrogen gas at a rate of 50–100 mL/min through 20 mL of modified Watt bath in which 2–6 × 10−3 wt% of sulfuric acid monododecyl ester sodium salt (SDS) was added as a surfactant. The nitrogen gas was introduced through a glass filter. The foam of the electrolyte solution drew electric current sufficient to cause electrochemical deposition. Nickel films deposited on brass substrates by this method had a considerably smaller number of pits and pinholes when compared to films deposited using a liquid electrolyte solution. It is suggested that the reduction in the number of pits and pinholes was a result of the prompt removal of hydrogen bubbles from cathode surface as a result of interaction of the hydrogen bubbles with the bubbles in the foam of the electrolyte solution. Uniform films were obtained by using a foam with an average bubble diameter of the submillimeter scale. The use of the foam of the electrolyte solution also required the use of a lesser amount of electroplating solution.

Nonlinear Conductivity and Permittivity Spectra of Ion Conducting Glasses

Abstract We have measured third-order conductivity and permittivity spectra of a Li2O · Al2O3 · 4 SiO2 glass and of a 0.15 Na2O · 0.85 GeO2 glass by applying ac electric fields up to 100 kV cm−1. For the application of these high fields, the glass samples are interfaced with highly conductive liquid electrolyte solutions. In this field range, the electrical response due to ion dynamics in the bulk of a glass sample is weakly nonlinear, while the electrical reponse due to ion transport across the interface between glass sample and liquid electrolyte is strongly nonlinear. For both glasses, the real part of the third-order conductivity is characterised by a change in sign in a frequency range where the diffusive bulk ion dynamics passes over into the subdiffusive bulk ion dynamics. On the other hand, the real part of the third-order permittivity spectra is characterised by a change in sign at lower frequencies where ion transport across the glass / liquid electrolyte interface passes over into the bulk ion transport. We show that these changes in sign can be rationalised by considering an equivalent circuit for a disordered solid ion conductor interfaced with a highly conductive liquid. In this circuit, the bulk and interfacial resistances are assumed to decrease with increasing voltage.

Cycling performance of lithium-ion batteries assembled with a hybrid composite membrane prepared by an electrospinning method

A hybrid composite membrane is prepared by electrospinning poly(vinylidene fluoride- co-chlorotrifluoroethylene) copolymer and Al 2O 3 powder with a microfibrous form on both sides of a polyethylene membrane. The composite membrane shows better thermal stability and wettability for liquid electrolyte solution than polyethylene membrane. The lithium-ion cell assembled with the composite membrane exhibits good capacity retention and high rate performance due to the effective encapsulation of the electrolyte solution in the cell and the adhesive properties of the swollen polymer towards electrodes. The hybrid composite membrane prepared by the electrospinning method is expected to be a separator with enhanced thermal stability and good cycling performance for application in lithium-ion batteries.

Cycling and rate performance of Li–LiFePO 4 cells in mixed FSI–TFSI room temperature ionic liquids

A study is conducted of the performance of lithium iron(II) phosphate, LiFePO 4, as a cathode material in a lithium secondary battery that features an ionic liquid electrolyte solution and a metallic lithium anode. The electrolyte solution comprises an ionic liquid of a N-methyl-N-alkyl-pyrrolidinium (alkyl = n-propyl or n-butyl) cation and either the bis(fluorosulfonyl)imide [(FSO 2) 2N −] or bis(trifluoromethanesulfonyl)imide [(F 3CSO 2) 2N −] anion, together with 0.5 mol kg −1 of lithium bis(trifluoromethanesulfonyl)imide salt. For N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide, coin cells discharging at rates of C/10 and 4C yield specific capacities of 153 and 110 mAh g −1, respectively, at an average coulombic efficiency of 99.8%. This performance is maintained for over 400 cycles at 50 °C and therefore indicates that these electrolyte solutions support long-term cycling of both LiFePO 4 and metallic lithium while, due to the negligible volatility of ionic liquids, surrounding the lithium in an inherently safe, non-flammable medium.

Effect of amphiphilic hyperbranched-star polymer on the structure and properties of PVDF based porous polymer electrolytes

An amphiphilic hyperbranched-star polymer (HPE-g-MPEG) was synthesized by grafting methoxy poly(ethylene glycol) to the end of the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent. The synthesized amphiphilic hyperbranched-star polymer was blended with poly(vinylidene fluoride) (PVDF) to fabricate porous membranes via typical phase inversion process, and then the membranes were filled and swollen by a liquid electrolyte solution to form polymer electrolytes. The influences of HPE-g-MPEG on the morphology, crystallinity, liquid electrolyte uptake, mechanical properties of the porous membranes and the electrochemical properties of the activated membranes were investigated. It was found that the addition of HPE-g-MPEG resulted in a significant increase in porosity and a considerable reduction in crystallinity of the blend membranes, which favored the liquid electrolyte uptake and, consequently, led to a remarkable increase in ion conductivity at ambient temperature. The maximum ion conductivity observed in this study was 1.76 × 10 − 3 S/cm at 20 °C for the blend membrane with a HPE-g-MPEG/PVDF ratio of 3/10 (w/w).

Origin of Nanobubble - Formation of Stable Vacancy in Electrolyte Solution

As an elementary structure of nanobubbles, vacancy, i.e., a kind of vacuum void stably exists in a liquid electrolyte solution, which is composed of an inner wall negatively charged by anion and a positively charged ionic atmosphere (R. Aogaki, Electrochemistry 76, 458 (2008)). As a result, a strong repulsing pressure induced by the attraction of the ionic atmosphere is balanced with the action of surface tension. Coalescence of vacancies guarantees the same structure as vacancy, so that the internal pressure of nano-bubbles is kept low, almost equal to the atmospheric pressure. At the same time, the variation in the chemical potential of the homogeneous formation of nano-bubbles is calculated, and it is shown that after passing a critical state corresponding to a radius of 1 nm, nanobubbles are stably created.

In situ Raman spectroelectrochemistry of SWCNT bundles: Development of the tangential mode during electrochemical charging in different electrolyte solutions

The Raman spectra of metallic tubes in SWCNTs (Single wall carbon nanotubes) bundles during electrochemical doping have been investigated using three different electrolyte solutions: LiClO 4/CH 3CN, LiClO 4/propylenecarbonate/poly(methyl methacrylate) and LiClO 4/polyethyleneimine. Precise control of the electrochemical charging enabled us to follow the detailed development of the tangential displacement (TG) mode of SWCNT bundles in dependence on the electrode potential. The response of the TG mode to electrode potential was dependent on electrolyte used as a consequence of different doping efficiency. We demonstrate that the liquid electrolyte solution (LiClO 4/CH 3CN) has superior doping efficiency to those of both the tested polymer electrolytes.

Electro chemical properties of porous PVdF-HFP membranes prepared with different nonsolvents

Porous poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) membranes were prepared by solvent–nonsolvent evaporation technique. Morphology and porosity of the membranes were varied with different nonsolvents and had an effect on electrochemical properties. The porous membranes were functionalized with different liquid electrolyte solutions such as p-toluene sulfonic acid/phosphoric acid/sulfuric acid. Maximum electrolyte uptake and minimal electrolyte leakage were tailored by the optimized porosity of the membranes. Thermal behavior obtained in this study ensures the complete evaporation of nonsolvents and ensures its thermal stability. The pTSA-activated PVdF-HFP/THF membrane exhibited high ionic conductivity of about 27.27 mS/cm and a lower methanol permeability in the range of 9.7 × 10 −8 cm 2/s. High compatibility between pTSA solution and porous PVdF-HFP polymer electrolyte membrane enhances its electro chemical behavior than that of conventional liquid electrolytes.

Concentration Dependence of Ionic Transport in Dilute Organic Electrolyte Solutions

Ion transport is studied in dilute organic liquid electrolyte solutions in which close cation-anion interactions are minimized either through steric hindrance imposed by the bulky tetrabutylammonium cation or by strong solvation of alkali metal cations by DMSO or 1-propanol. In these solutions, the molar conductivity does not appear to depend on either the solvent viscosity or the size of the solvated charge carrier in a manner consistent with Walden's rule. The molar conductivities plotted as a function of the solvent dielectric constant from epsilon = 5.48 to 63.5 appear to lie on a smooth curve for a set of 0.0055 M solutions of tetrabutylammonium trifluoromethanesulfonate in a variety of aprotic solvents. The molar conductivity smoothly increases with increasing dielectric constant to a maximum at roughly epsilon = 33 and then decreases with further increase of the dielectric constant. The conductivity appears to depend only on the dielectric constant and not the specific functional group in this broad family of solvents. A similar plot for a series of linear alcohols as solvents also led to a smooth curve, although the values of the molar conductivity were lower than values in the aprotic solvents by almost an order of magnitude at corresponding values of the solvent dielectric constant.

Solid-State Electrochromic Devices via Ionic Self-Assembled Multilayers (ISAM) of a Polyviologen

A PV was synthesized by polymerization of N,N'-bis(δ-aminopropyl)-4,4'-bipyridinium bromide hydrobromide (APD) and isophthaloyl chloride (ISP). The PV was completely soluble in water as well as in organic solvents. The spectroelectrochemical and EC properties of the resulting ionic self-assembled multilayers of PV/poly-(2-acrylamido-2-methylpropanesulfonic acid) (PAMPs) were examined by cyclic voltammetry, FT-IR spectroscopy, UV-vis spectroscopy, optical switching and current density measurements. Solid state devices made of PV films sandwiched with PANI as a counter electrode have switching times of 100-250 ms. 40 Bilayer films of PV/PAMPs show high contrast (ΔT = 61%) in 0.1 M NaClO 4 liquid electrolyte solution and CE as high as 57 cm 2 . C -1 , one of the highest reported so far for any bipyridinium salt system.