Galvanostatic Cathodic Reduction Research Articles

In-situ Analysis of the Solid-Electrolyte Interphase Formed in Li[N(CF3SO2)2] and Li[N(FSO2)2] Tetraglyme Solvate Ionic Liquids

Li metal has attracted attention as the negative electrode for secondary batteries due to its low density and negative redox potential. However, its negative operating potential also leads to the decomposition of electrolytes, resulting in the formation of some interfacial phase on the anode, i.e., solid-electrolyte interphase (SEI). The decomposition of the electrolyte causes the low coulombic efficiency of the Li anode. Therefore, the characterization of SEI is necessary in order to utilize the Li anode.Solvate ionic liquids (SILs) composed of 1:1 equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) or bis(fluorosulfonyl)amide (LiFSA) and glyme have been investigated as the alternative electrolytes for lithium secondary batteries because of their high Li+ concentration [1, 2]. The deposition and dissolution of Li were reported to be possible in the SILs, although the cathodic decomposition of the electrolyte also proceeded at the potential more positive than that of Li(I)/Li. We have already reported the improvement of the morphology of Li deposits and the cyclability of the Li anode in LiTFSA-triglyme (G3) by the formation of SEI on a Cu electrode at 0 V vs. Li|Li(I) prior to the deposition of Li [3]. The cyclability of the Li anode in LiFSA-tetraglyme (G4) was found to be superior to that in LiTFSA-G4. However, the difference in the stability of Li deposits in these SILs has not been elucidated. In the present study, the formation of SEI on a Cu electrode and the interfacial reaction of the Li deposits were investigated in LiTFSA-G4 and LiFSA-G4 using electrochemical impedance spectroscopy (EIS) combined with electrochemical quartz crystal microbalance (EQCM).The solvate ionic liquids were prepared by mixing the equimolar quantities of G4 and LiTFSA or LiFSA. EIS and EQCM measurement were conducted on the same Cu-coated EQCM electrode (fundamental frequency: 9 MHz) using an impedance-type EQCM system (SEIKO EG&G, QCM922A) and a computerized electrochemical system (Hokuto Denko, HZ-7000). Li foil was used as a reference and counter electrode, respectively. SEI was formed at 0 V vs. Li|Li(I) on the Cu-coated EQCM electrode prior to the galvanostatic deposition of Li. EIS was performed at the open circuit potential (OCP) in the frequency range from 20 kHz to 1.0 Hz with an amplitude of 5 mV rms during SEI formation and after Li deposition.The resonance frequency (f) of a Cu-coated EQCM electrode gradually decreased with keeping the potential of the electrode at 0 V. A semicircle corresponding to the parallel circuit of the resistance and capacitance of SEI was observed in the Nyquist plots of the Cu electrode after keeping the potential at 0 V. The resistance of SEI, R SEI, increased with the lapse of time over 24 hours, indicating the formation and growth of SEI on the Cu electrode at 0 V in LiTFSA-G4 and LiFSA-G4. The changes in f and R SEI in LiFSA-G4 were smaller than those in LiTFSA-G4, suggesting the SEI formed in LiFSA-G4 was lighter and more conductive.The larger change in f of the Cu-EQCM electrode than that calculated from the passed electric charge implied that SEI was also formed on Li deposits during the galvanostatic cathodic reduction at 0.01 mA cm–2, as observed during cyclic voltammetry in LiTFSA-G3 [4]. Δf in LiFSA-G4 was smaller while the changes in the resonance resistance mainly reflecting the surface roughness of the electrode were close in the SILs, suggesting the reactivity against Li was low in LiFSA-G4. The electrode potential of the Cu-EQCM electrode changed over from that of Li(I)/Li after few tens of hours, suggesting the SEI formed in SILs did not suppress completely the reaction between Li and the electrolyte at the OCP. The transition time of the potential in LiFSA-G4 was longer than that in LiTFSA-G4, indicating Li deposits were more stable in LiFSA-G4.References H. Moon, et al., J. Phys. Chem. C, 118, 20246 (2014).S. Terada, et al., Aust. J. Chem., 72, 70 (2019).N. Serizawa, et al., J. Electrochem. Soc., 167, 110560 (2020).N. Serizawa, et al., J. Electrochem. Soc., 160, A1529 (2013). AcknowledgementThis study was supported in part by the JSPS KAKENHI (Grant No. 19K15682) and Advanced Low Carbon Technology Research and Development Program (ALCA, JPMJAL1301).

Electrodeposition of Palladium Nanoparticles on Carbon Nanotubes Dispersed in an Ionic Liquid

Electrodeposition of palladium nanoparticles (PdNPs) was investigated on single walled carbon nanotubes (SWCNTs) or few walled carbon nanotubes (FWCNTs) dispersed in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing [PdBr4]2 -. Ionic liquids have been expected to be alternative electrolytes for electrodeposition of various metals and alloys because of such unique characteristics as wider electrochemical potential window, negligible vapor pressure, less flammability, and wider usable temperature range, compared with conventional organic electrolytes. In addition, the ionic liquids have been known to have ability to disperse nanoparticles without any additives[1]. Although several studies have reported the preparation of metal nanoparticles by electrochemical cathodic reduction in ionic liquids, there are some problems in the utilization of the nanoparticles dispersed in ionic liquids. Platinum nanoparticles (PtNPs) prepared in ionic liquids have been reported to be collected on such carbon materials as carbon nanotubes (CNTs) and carbon black with elevating temperature[2,3]. It has been reported that CNTs are dispersed in ionic liquids and act as an electrode[4]. We have already reported the electrodeposition of PtNPs is possible on CNTs dispersed in BMPTFSA containing [PtBr4]2–[5]. The average particle size of the PtNPs was 1.7 nm, which was independent on the concentration of [PtBr4]2– and the kind of CNTs. In the present study, the electrodeposition of PdNPs was investigated on SWCNTs and FWCNTs dispersed in BMPTFSA containing [PdBr4]2– in order to examine the effect of the deposition potential on the average particle size of PdNPs because the deposition potential of Pd is more positive than that of Pt[3,6]. In addition, the effect of the catalyst remaining in CNTs on electrodeposition of PdNPs was examined using high purity (> 99.6% purity) FWCNT synthesized by the fluidized bed method[7].Galvanostatic cathodic reduction was conducted in BMPTFSA containing CNTs and [PdBr4]2– at room temperature, resulted in deposition of NPs, which were identified as metallic Pd by energy dispersive X-ray analysis (EDX) and transmission electron mocroscopy (TEM). The average potential of [PdBr4]2– during the galvanostatic cathodic reduction was –1.2 V, which was close to that reported on a planar glassy carbon electrode[6]. Potentiostatic cathodic reduction at different potentials indicated that the average particle size of PdNPs decreased with lowering the potential. Pd nuclei are considered to occur and grow on the CNTs dispersed in the ionic liquid. As the potential becomes more negative, the cations of the ionic liquid are expected to be accumulated on the surface of the nuclei and CNTs in order to compensate the excess negative charge. The cations strongly attracted on the surface are considered to inhibit the growth of the PdNPs and affect their average particle size. On the other hand, the remaining catalyst in the CNTs was found to have little effect on the average particle size of PdNPs.

Electrochemical Behavior of a Ni Chlorocomplex in a Lewis Basic Ionic Liquid Containing Chloride Ion

The electrochemical behavior of a chlorocomplex of nickel (Ni) was investigated in a Lewis basic and hydrophobic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Cl–. NiCl2 dissolved in BMPTFSA with addition of 0.5 M BMPCl. The dissolved species of Ni(II) in BMPTFSA was identified as [NiCl4]2– by UV-visible spectrometry and magnetic susceptibility balance. The cathodic and anodic currents assignable to deposition and dissolution of Ni, respectively, were observed in cyclic voltammetry. The granular deposits of Ni were obtained on a glassy carbon electrode at 25 and 100 °C. Ni nanoparticles were dispersed in the electrolyte by galvanostatic cathodic reduction. The initial stage of Ni deposition was found to be regarded as instantaneous. The diffusion coefficient of [NiCl4]2– was calculated to be 6.6 × 10–8 cm2 s–1.

Electrode Reactions of Platinum Bromide Complexes in an Amide-Type Ionic Liquid

The electrode reactions of platinum bromide complexes were investigated in an amide-type room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA). Platinum bromides were dissolved in BMPTFSA with addition of BMPBr. Ultraviolet and visible absorption spectra of the electrolytes revealed that Pt(II) and Pt(IV) species were dissolved as [PtIIBr4]2− and [PtIVBr6]2−, respectively. The cyclic voltammogram in BMPTFSA containing 10 mM [PtIVBr6]2− showed two reduction peaks at −1.2 and −2.6 V, which can be assigned to reduction of [PtIVBr6]2− and [PtIIBr4]2−, respectively. The diffusion coefficients of the [PtIIBr4]2− and [PtIVBr6]2− at 25°C were determined to be 1.5 × 10−7 and 0.49 × 10−7 cm2 s−1 in BMPTFSA, respectively, by chronoamperometry. Electrodeposition of metallic platinum was possible by galvanostatic cathodic reduction on a glassy carbon electrode in BMPTFSA containing 10 mM [PtIIBr4]2−. In addition, platinum nano-particles (2–3 nm in diameter) formed and dispersed in the ionic liquid after potentiostatic cathodic reduction at −2.6 V. Platinum nano-particles could be collected by heating the ionic liquid containing the nano-particles with carbon black at 200°C.

Passive-Active Transition Behavior of 304 Stainless Steel

The passive-active transition behavior of a 304 stainless steel was investigated by observing the self-activation behavior and nano-scale galvanostatic cathodic reduction experiment. The self-activation time, τ0 was dependent remarkably on concentration of sulfuric acid. It was appeared that applied nano-scale cathodic current density dissolved the passive film on a 304 stainless steel surface and shortened the activation time in galvanostatic cathodic reduction experiments. The applied cathodic current density was proportional to the reciprocal of activation time. From this linear relationship, the rate of the self-activation process, i0 was obtained. The i0 increased with increasing H2SO4 concentration. And i0 also increased with increasing passivation potential and passivation time. The stability of passive film increased in accordance with increasing τ0 and decreasing i0. Therefore, it was concluded that the stability of passive film on austenitic stainless steel is evaluated by the kinetic parameters of the self-activation rate i0 and the self-activation time, τ0.

Reconstruction in Air of an Iron Passive Film Formed at −0.4 V in a Borate Buffer Solution

The reconstruction in air of passive films on iron formed at -0.4 V in a borate buffer solution was investigated by STM and galvanostatic cathodic reduction. With exposure to air (<30% RH), the apparent thickness rapidly increased in initial stages until a critical exposure duration of about 130 min, and after a transition between 130 and 360 min, slowly increased in the following stages, and the changes of surface structures from the amorphous state to short-range order and then to long-range order and further to crystal structures with a wide terrace were observed. These processes are suggested to be attributed to rapid dehydration of Fe 3 + and Fe 2 + hydroxides and partial rapid oxidation upon removal from electrolytic solution after passivation, steady oxidation in air until the attainment of a passive state to be more protective one at t = t c , and reconstruction in air, respectively. The reconstruction process remarkably improved the resistance of the passive films against open-circuit breakdown. The behavior of passive films formed at -0.4 V observed during air exposure is actually self-protective.

Electrochemical synthesis of calcium carbonate coatings on stainless steel substrates

Calcite CaCO 3 has been electrocrystallized on stainless steel substrates by the galvanostatic cathodic reduction of aqueous calcium bicarbonate solutions. The deposition is controlled by pH changes occurring close to the cathode due to electrogeneration of base. The deposit morphology varies from facetted rhombs observed at low (1–20 mA cm −2) current densities to corner-rounded particles observed at high (40 mA cm −2) current densities.

Effect of cathodic reduction on catalytic activity of amorphous alloy electrodes for electrooxidation of sulfite

Active electrode materials for a new zinc electrowinning process, in which the thermodynamic cell voltage is about half that of the conventional process by replacing oxygen evolution by anodic oxidation of SO2 produced in the zinc smelting process have been studied. Immersion in HF solution and subsequent cyclic voltammetry (CV) in sulfuric acid are known to be effective surface activation treatments of the amorphous alloy electrodes. The galvanostatic cathodic reduction (CR) treatment was applied to obtain further activation for sulfite oxidation for HF- and CV-treated electrodes prepared from amorphous nickel-valve metal-platinum group metal alloys. This treatment has been found to be effective in enhancing the activity. Among the amorphous Ni-40Nb alloys containing platinum group elements, the platinum-containing electrode showed the highest catalytic activity, which was higher than that of platinized platinum. Furthermore, the electrocatalytic activities of CR-treated electrodes prepared from amorphous alloys containing platinum and rhodium, and platinum and ruthenium were higher than that of the electrode containing only platinum. According to XPS analysis of the amorphous Ni-40Nb-1Pt-1Ru alloy specimen the enrichment of platinum and ruthenium occurred by CV treatment, and a small amount of oxidized platinum and ruthenium species remained on the electrode surfaces, but most of them were cathodically reduced to the metallic state by CR treatment. High catalytic activities for sulfite oxidation can be attributed to the metallic state of platinum and ruthenium contained in the alloy electrodes, even though the activity of these electrocatalysts is higher than that of pure Pt or Ru.

Mechanism of anodic dissolution and passivation of iron—II. Comparison of the behavior in neutral benzoate and acetate buffer solutions

The kinetics of the dissolution and passivation of Fe has been studied in neutral benzoic acid/sodium benzoate buffer solution as a function of the total benzoate buffer concentration and pH. The results have been obtained from potential sweep experiments at 5 mV s−1, which corresponds to quasi steady-state conditions. Two anodic peaks were found in the active dissolution region of the anodic voltammogram. A reaction order of about −1 with respect to benzoate concentration and of about 2 with respect to pH has been found for the Tafel region of the first peak. The corresponding reaction orders were −1 and 0.7 for the second peak. Galvanostatic cathodic reduction experiments were used to characterize the surface film formed during the anodic sweep. Anodic sweep experiments were also performed using mixtures of benzoate and acetate buffers. The behavior of Fe in benzoate and acetate buffer solutions has been compared and contrasted. Benzoate buffer is found to be a much better inhibitor than acetate buffer, and the results can be understood in terms of a strong interaction between the Fe surface and benzoate-containing adsorbates.

Mechanism of anodic dissolution and passivation of iron—I. Behavior in neutral acetate buffer solutions

The kinetics of the anodic dissolution and passivation of Fe have been studied in neutral and slightly acidic acetic acid/sodium acetate buffer solution as a function of the total acetate buffer concentration and pH. The results have been obtained from potential sweep experiments at 4 mV s−1, which corresponds to quasi-steady state conditions. Two anodic peaks were found in the active dissolution region of the voltammogram. A reaction order of −0.7 ± 0.2 with respect to acetate concentration and of 1.4 ± 0.2 with respect to pH has been found for the Tafel region of the first peak; more complicated behavior was found for the remainder of the voltammogram. Galvanostatic cathodic reduction experiments were used to characterize the surface film formed during the anodic sweep. A mechanism is proposed which accounts for the observed reaction orders, and simulates the observed anodic voltammograms in a semi-quantitative fashion. The mechanism provided a simple, physical expalantion for the observed negative reaction order with respect to acetate concentration.

Influence of the Anodic Oxide Film on Pitting of Iron

The fundamental processes leading to the initiation of pitting on pure Fe in acetate and borate solutions containing halide ions are discussed. The results of potentiostatic step experiments show that pit initiation is associated with a critical stage in the development of the passive oxide film, likely corresponding to a certain critical oxide thickness. The critical oxide thickness depends on the identity (chloride or bromide) and the concentration of the halide ion in solution, and on the supporting electrolyte, but not on the anodic potential. Acetate is shown to inhibit pitting. The critical oxide film characteristics have been investigated using galvanostatic cathodic reduction techniques. Scanning electron microscopy with computerized image analysis has been used to study the size distribution of the pits formed in these experiments. The results can be used to support the proposed model for pitting.

Passivation of iron and its cathodic reduction studied with a rotating ring-disk electrode

Passivation of iron in various slightly acidic to neutral buffer solutions was studied with a rotating ring-disk electrode (RRDE). Ferrous ions was always detected at the ring electrode at the initial stage of potentiostatic passivation. Galvanostatic cathodic reduction in borate buffer solution of pH 8.39 revealed that two stages of reduction of the passive film occurred and ferrous ion yields during reduction were different from each other. The passivation potential and time dependence of the chemical composition of films were estimated and illustrated from the ferrous ion yields detected on the ring electrode.

The use of 18O/SIMS and electrochemical techniques to study the formation of the passive film on Fe-6Cr alloy

The passivation of Fe-6Cr alloy in pH 8.4 borate buffer has been investigated using a combination of electrochemical and surface analytical techniques, and the results have been compared to those previously obtained for pure Fe in the same solution. The electropolish film on Fe-6Cr could not be removed by galvanostatic cathodic reduction. Electrochemical results suggested that it was removed after a sufficiently vigorous potentiostatic cathodic treatment; they were confirmed using the 18O/SIMS technique. A potentiostatic treatment at −1.7 V was used to provide a reproducible, bare, starting surface. 18O/SIMS results indicated that the thickness of the passive film on Fe-6Cr was less than that on Fe. In addition, the Fe-6Cr exhibited a smaller potential range over which the anodic film was stable to air exposure than did pure Fe. A comparison of plots of thickness versus anodic charge for Fe and Fe-6Cr suggested that the composition of the passive film on Fe-6Cr is a function of potential, the film being more Cr-rich at lower anodic potentials.

Use of 18O/SIMS and Electrochemical Techniques to Study the Reduction and Breakdown of Passive Oxide Films on Iron

Secondary ion mass spectrometry (SIMS) has been used to study the cathodic removal and open‐circuit breakdown of passive oxide films formed on Fe in 18O‐enriched borate buffer solution. For film removal by galvanostatic cathodic reduction, the initial stage of reduction resulted in a layer by layer thinning of the film. The rates of film removal with cathodic charge passed were determined to be 1.56 and 0.30 nm/mC cm−2 during the two distinct stages of reduction. These results are analyzed in terms of the various models for the passive film on Fe. Immersion of the passivated Fe electrodes in acidic sulfate solutions on open circuit resulted in a distinct potential arrest followed by a very rapid potential decrease. The length of this arrest is strongly dependent on the oxide film thickness and on the solution pH. 18O SIMS and electrochemical results are discussed in terms of mechanisms of reductive and chemical dissolution on open circuit.

ChemInform Abstract: Isothermal Magnetic Phase Transitions Controlled by Reversible Electron/Ion Transfer Reactions.

AbstractIn the course of isothermal galvanostatic cathodic reduction of the spinel CuCr2Se4 in copper‐containing electrolytes at 300 K cubic, nonstoichiometric phases (I) are formed.

Quasi‐One‐Dimensional Behaviour of Hydrogen in H0.35MoO3 and H0.33WO3 as Revealed by Proton NMR

AbstractThe 1H NMR has been studied in the hydrogen bronzes H0.35MoO3 and H0.33WO3 prepared by galvanostatic cathodic reduction. Both systems reveal an extreme anisotropy of the hydrogen mobility due to quasi‐one‐dimensional behaviour. – In H0.35MoO3 the temperature dependent 1H wideline data give strong evidence that hydrogen is clustered in quasi‐one‐dimensional channels. Analysis of the temperature dependence of the second moment M2, the transverse relaxation time T2, and the spin‐lattice relaxation time T1 yields essentially the same valueEA = 30 kJ/mol for the activation energy of the translational motion of hydrogen within the channels. In H0.33WO3 evidence for the quasi‐one‐dimensional behaviour is provided by the frequency dependence of T1 and the marked non‐exponential recovery of the nuclear magnetization in the vicinity of the T1 minimum. The activation energy for the translational motion is EA = 3.6 kJ/mol only, indicating high mobility of hydrogen in this bronze.

A resistometric study of the passive film on an evaporated iron electrode

Cathodic reduction mechanisms of the passive film formed on an evaporated iron electrode in a neutral solution were studied by chronopotentiometry, colorimetric analysis and measurement of the electrical conductance of the film electrode (resistometry). By galvanostatic cathodic reduction, the outer γ-Fe 2O 3 layer was reduced to ferrous ion and dissolved with nearly 100% current efficiency without showing any change in the conductance of the film. The inner magnetite layer was partly reduced to metallic iron and the residual part was dissolved as ferrous ion. The latter reaction was predominant in low pH. From the increase in conductance in the initial part of cathodic reduction, it was concluded that adsorbed oxygen fortified the passivity by lowering the conductance of the passive film.

A resistometric study of the passive film on an evaporated iron electrode

Cathodic reduction mechanisms of the passive film formed on an evaporated iron electrode in a neutral solution were studied by chronopotentiometry, colorimetric analysis and measurement of the electrical conductance of the film electrode (resistometry). By galvanostatic cathodic reduction, the outer γ-Fe2O3 layer was reduced to ferrous ion and dissolved with nearly 100% current efficiency without showing any change in the conductance of the film. The inner magnetite layer was partly reduced to metallic iron and the residual part was dissolved as ferrous ion. The latter reaction was predominant in low pH. From the increase in conductance in the initial part of cathodic reduction, it was concluded that adsorbed oxygen fortified the passivity by lowering the conductance of the passive film.

Changes in the conductance of passivated iron thin films during galvanostatic cathodic reduction

The change in the conductance and the potential of passivated films on iron were measured in a neutral borate—boric acid buffer solution (pH 8·39) during galvanostatic cathodic reduction. The conductance increased abruptly at the onset of cathodic current, then remained constant and then increased gradually. The latter two stages corresponded well to the potential plateaus due to the reduction of γ-Fe 2O 3 layer and Fe 3O 4 layer in chronopotentiogram respectively. The initial jump in conductance was attributed to the reduction of the outermost excess oxygen. The resistivities of the γ-Fe 2O 3 layer and Fe 3O 4 layer were estimated from the slope of the conductance—electric charge curves. From the results, it was suggested that the existence of the outermost excess oxygen played an important role in passivity phenomena.

Ellipsometry of the passivation film on iron in neutral solution

Ellipsometric and electrochemical measurements have been made of the passivation film on iron during its potentiostatic anodic formation and galvanostatic cathodic reduction in boric-acid-sodium-borate buffer solution. The complex refractive index estimated ellipsometrically of the cathodically reduced iron surface is identical with that of iron, indicating absence of any oxide films on it. The index of the passivation film comes close to that of spinel-type iron oxide, Fe 3O 4 or γ-Fe 2O 3. The passivation occurs at a minimum film thickness of about 9 Å and the film thickens up to 50 Å at the onset potential of oxygen evolution. The film thickness is linearly related to the oxidation potential in a limited potential region, which extrapolates to a film thicker than a monolayer even at zero anode overvoltage. The distribution of the potential in the film is discussed by a non-uniform film model consisting of an inner layer of intrinsic, anhydrous ferric oxide and an outer layer of hydrous, semiconducting ferric oxide. Ellipsometric measurements also reveal that a surface film is formed on actively dissolving iron, and electrochemical measurements disclose transpassivation to occur above a critical potential in the oxygen-evolution region.