Fluorinated ionic liquids are cutting-edge materials investigated for electrolytic media for energy-related applications. Although their industrial usages are being spread, waste treatment techniques for these materials are not well established, because they are thermally and chemically stable, owing to the presence of strong carbon-fluorine bonds, and incineration produces hydrogen fluoride gas, which seriously damages incinerators. We describe herein our recent efforts to decompose fluorinated ionic liquids to F− ions (i.e., mineralization) by use of superheated water, with the aim for closing the loops on fluorine element. A methodology that enables complete mineralization of the ionic liquids bearing [(CF3SO2)2N]− anion moiety is demonstrated.
Mixing of a Pt-sputtered protic ionic liquid (PIL) containing 3,4-ethylenedioxythiophene and a carbon support at room temperature for 3 min produces carbon-supported Pt nanoparticles (Pt-NPs), in which the PIL acts as an adhesive. Its electrochemical oxidation generates a conducting poly(3,4-ethylenedioxythiophene) (PEDOT) network in the thin PIL layer that is present between Pt-NPs and the carbon support. The prepared material possesses high electrocatalytic activities toward oxygen reduction reaction and it exhibits high durability against carbon corrosion-inducing accelerated deterioration test.
This paper reports that the fully-discharged graphite-fluoride Li primary battery (GF/Li battery) can be regenerated as a hybrid capacitor with a higher energy density than the electric double layer capacitor (EDLC) using an activated carbon electrode. The graphite-fluoride (GF) positive electrode of the GF/Li battery is electrochemically defluorinated during the fully-discharged process to be converted to a nanocomposite consisting of carbon and LiF particles. The nanocomposite as the discharge product behaves as a capacitor-like electrode, so the fully-discharged GF/Li battery can be stably charged/discharged as a hybrid capacitor with the capacitor-type electrode (defluorinated GF electrode) and the battery-type negative electrode (Li metal). This hybrid capacitor, i.e., “graphite-fluoride Li capacitor (GF/Li capacitor)”, exhibited the maximum volumetric energy density of 52 Wh L−1 (at the power density of 71 W L−1), which is higher than that of the EDLC and comparable to that of the Li-ion capacitor. In this paper, the improvement of the cyclability by using the graphite/Li bilayer negative electrode and the charge-discharge mechanism are also discussed for the GF/Li capacitor.
Lithium-excess transition metal (M) oxyfluorides, LixMO1+x−yFy, have received considerable attention as positive electrode materials for lithium-ion batteries. Little is known about the relationship between the crystallinity and electrochemical reactivities although this offers further understanding and improvement of LixMO1+x−yFy, because conventionally ball-milled LixMO1+x−yFy exhibits limited crystallinity, i.e., amorphous-like nanoparticles. We herein adapted a high-pressure/high-temperature method at 12 GPa and 1000 °C to synthesize high-crystallinity LixMO1+x−yFy (M = Fe, Mn, V, Nb, Mo, and W) and investigated the electrochemical properties of this series. Rietveld analyses based on X-ray diffraction (XRD) and cross-sectional elemental mapping clarified that Li3VO3F and Li4WO4F crystalized as an almost-single-phase rock-salt structure with homogenous cation/anion distributions and formed well-faceted particles with sizes of 1–20 µm. Their rechargeable capacities over 1.8–5.0 V vs. Li+/Li were ∼40 mAh g−1 and ∼10 mAh g−1, respectively. According to ex situ XRD measurements of the cycled Li3VO3F electrodes, these partial rechargeable capacities were caused by a decomposition reaction during the initial charge, which differed from the topotactic reaction proposed for the low-crystallinity phases. This information is helpful for designing the microstructure of LixMO1+x−yFy to improve its performance now that both crystal and amorphous LixMO1+x−yFy phases are attainable.
A carbon paste electrode (CPE) modified with polytetrafluoroethylene (PTFE) powder and perfluoropolyether (PFPE) oil was studied. These two fluorinated materials were simply mixed with graphite powder. The voltammetric behavior was compared with those of non-fluorinated materials, i.e., polyethylene (PE) powder and liquid paraffin (LP) oil. The CPE with PTFE and PFPE gave a wider polarizable potential window and a lower charging current than the non-fluorinated CPEs. When the ionic strength of the test solution was sufficiently low, the electrostatic interaction between the reactant and the fluorinated electrode surface was observed. The CPE with PTFE and PFPE was stable in a tetrahydrofuran (THF) solution, whereas other CPEs were not, due to the dissolution of the non-fluorinated materials.
Anodic fluorination of N-(diphenylmethyleneamino)-2,2,2-trifluoroethane and N-[bis(methylthio)methyleneamino]-2,2,2-triphenylethane in acetonitrile containing poly(HF) salt ionic liquids afforded monofluorinated products in moderate to good yields. On the other hand, anodic fluorination of N-[bis(methylthio)methylene]glycine methyl ester provided mono- and difluoroproducts depending on the amount of electricity passed. This is the first successful electrochemical fluorination of open-chain α-amino acid derivatives. Cathodic Michael addition of N-(diphenylmethyleneamino)-2,2,2-trifluoroethane to activated olefins such as acrylate and acrylonitrile was also successfully carried out using a titanium cathode.
Ni–Al layered double hydroxide (LDH) was prepared on a steel plate and fumed alumina by the liquid phase deposition (LPD) method with NO2− inserted between the layers for rust prevention, and its corrosion-inhibition effect was investigated. Anion exchange of the synthesized LDH was carried out by immersing it in aqueous KOH and NaNO2 solutions, separately. The composition of the obtained Ni–Al LDH is typically [Ni(II)0.69Al(III)0.31(OH)2]OH0.31 in OH−-type LDH. Electrochemical measurements revealed the expansion of the passive area and a decrease in the corrosion current in the sample mixed with LDH, compared to the case with the pure polyacrylic gel sample. In particular, the corrosion current was reduced to less than half of the original in the samples mixed with NO2−–LDH. For the polyacrylic gel containing LDH, the anodic current was suppressed at all concentrations of the aqueous NaCl solution, and the corrosion current was approximately the same as the result of the polarization measurement. The corrosion-inhibiting effects due to Cl− absorption and NO2− release were confirmed. A good rust preventive effect was also observed in the hydrous LDH gel sheet intended to be exposed to the atmosphere where an arbitrary amount of Cl− ions is dissolved.
Electric double layer capacitors are energy storage devices with advantages of fast charge-discharge and long life span. Surface modification of activated carbon electrodes is an effective way to improve their performance. For this purpose, deoxofluorination of activated carbon with sulfur tetrafluoride was attempted in this study. Successful introduction of fluorine atom on the surface of activated carbon resulted in the increased capacitance and improved coulombic efficiencies in electrochemical tests for electric double layer capacitors.
A voltammetric method has been developed for the determination of the acid dissociation constant (pKa) of (RfSO2)2CHR (Rf = perfluoroalkyl), which is a strongly acidic molecule that serves as an acid catalyst. In acetonitrile solution containing small quantity of the acid, the reduction peak potential of quinone caused by the acid shifted to a more positive side accompanied by an increase in the acidity of the acid. This relationship was in good agreement with an equation derived from the Nernst equation. The present finding has been successfully applied to the determination of the pKa of the acids in acetonitrile with values ranging from 7 to 17.
Metal-assisted etching has attracted increasing attention as a method to produce porous silicon (Si). We previously found that gold (Au)-particle-assisted etching and platinum (Pt)-particle-assisted etching cause general corrosion of the Si substrate, but not in the case of silver (Ag)-particle-assisted etching [A. Matsumoto, et al., RSC Adv., 10, 253 (2020)]. In this work, we discussed the mechanism of the general corrosion with electrochemical approaches. We demonstrated that potentials of the Au- and Pt-deposited Si during the metal-particle-assisted etching are higher than that of the bare Si in the etchant, but not in the case of the Ag-deposited Si. We also performed electrochemical etching of the bare Si by applying the potential during the Pt-particle-assisted etching, resulting in the formation of a mesoporous layer which was dissolved in the etchant. We concluded that the general corrosion occurs during the metal-particle-assisted etching due to the dissolution of the mesoporous layer formed by anodic polarization of the Si substrate.
To explore the effect of the presence of a self-assembled monolayer (SAM) of an alkyl thiol derivative as an underlayer of a Nafion film, redox reaction of methyl viologen (MV) at the Au/SAM/Nafion (1 µm-thick) interface was characterized using the results of the voltammetric and electroreflectance measurements. The perfluorinated SAM slowed the kinetics of the interfacial electron transfer process compared to a Au/Nafion electrode, whereas 1-dodecanethiol SAM accelerated it. However, both the SAMs decreased the voltammetric current by half. Importance of the approach to underlay a SAM was discussed.
Field effect transistor (FET) biosensors are capable of detecting various biomolecules, although challenges remain in the detection of uncharged molecules. In this study, the detection of uncharged cortisol was demonstrated by interfacial design using a technique to immobilize target-bound aptamers. The target-bound aptamers, which formed a higher-order structure than target-unbound aptamers, expanded the distance between adjacent aptamers and reduced the steric hindrance to the conformational change. The density-controlled aptamers efficiently induced their conformational changes with the cortisol binding, which resulted in the improvement of the sensitivity of FET biosensors.
Water electrolysis cell in which the product gases was separated from liquid water on the surface of the electrode was developed. In order to realize the separation between gas and water, interdigitated diffusion layer (GDL) was designed, and the surface of the GDL was covered by catalyst to form electrode. When the pressurized water was supplied, the water directly made a contact to the proton conductive membrane. Due to the hydrophobic surface condition of the GDLs, gas/water separation along the surface of the electrode was completed.
Control of light transmittance is important for optoelectronic devices such as smart windows and switchable photodetectors. In particular, that for infrared light can be used for tuning thermal flows through windows. Plasmonic compound nanomaterials are suitable for those devices because of their electrochemically tunable localized surface plasmon resonance (LSPR). We combined CuS nanoplates with MoO3 nanosheets for developing complementary near infrared electrochromic cell. CuS loses its plasmonic absorption (peaked at ∼1100 nm) when it is reduced, as opposed to MoO3, whose absorption at >600 nm is suppressed when it is oxidized. An assembled smart window with rocking-chair type lithium ion transport exhibits wide transmittance changes in the near infrared region.
The oxygen reduction reaction (ORR) has been studied on low-index planes of Pd in 0.05 M XOH (X = Li, Na, K, and Cs) solutions to determine the effects of cations on ORR activity. The order of ORR activity is Pd(110) < Pd(100) < Pd(111) in all solutions. This order differs from that in HClO4: Pd(110) < Pd(111) ≪ Pd(100). Cationic species do not affect the ORR activity on Pd(110) and Pd(100). In contrast, the ORR activity of Pd(111) increases in the order of Li+ < Na+ < K+ < Cs+. The lower the cation hydration energy, the higher the ORR activity on Pd(111), as is also the case on Pt(111). The ORR activity of Pd(111) in CsOH is 1.8 times higher than that of Pd(100) in HClO4.
We studied the average crystal structural change during charging and discharging of a 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 battery cathode in combination with a Li-metal anode, graphite anodes, and three types of separators. Prelithiation of the graphite anode enabled stable charge and discharge of the cathode. The average structure of the cathode after the 5th discharge did not change significantly due to the difference in the anodes and separators. However, after the 55th discharge, the difference in the electron density distribution and the distortion of the M-O6 octahedra for the different separators increased. The findings suggest that the changes in the crystal structure of a cathode over a long-term cycle should be studied with the anodes and separators used in actual batteries rather than Li-metal anodes and conventional separators.
To establish the kinetics of the lithium insertion materials used as the electrodes in lithium-ion batteries, it is important to clarify the diffusion phenomena of lithium ions in the solid phase. Since there are two variations of the rate-determining step of a lithium insertion electrode (Li-ion diffusion in solid particles and that in the electrolyte solution), these have to be differentiated in any investigation of the diffusion phenomena of Li ions in a solid. In this study, we calculated the rate capability of diluted electrodes using a charge–discharge simulation program and proved that the transport of Li ions in the solid phase is the rate-determining step for a diluted electrode with an extremely low active material content. In addition, the solid-state Li-ion diffusion coefficient can be calculated by analyzing the results of the rate capability tests. These results reveal that the diluted electrode method is very useful for elucidating the solid-state Li ion diffusion phenomena of lithium insertion materials. As such, the rate capability tests provide a new means of analyzing the solid-state diffusion coefficient.
In recent years, carbon films have attracted interest as electrode materials for electrochemical sensors due to their wide potential window and acceptable electrochemical activity. The films can be fabricated into electrodes of any shape and size. The electrochemical performance of the films depends on their structural features such as the ratio of the sp2 and sp3 bonds, and the surface functional groups. Here, we report the control of both structure and electrochemical properties based on carbon films fabricated by easily available direct current (DC) magnetron sputtering equipment. The sp3 concentration (sp3/sp3 + sp2) can be controlled from 0.22 to 0.29 by changing the substrate bias power, which can be characterized with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The potential window became wider as the sp3 concentration increased, similar to the results we previously obtained with films formed by unbalanced magnetron (UBM) sputtering. The electrochemical properties of DC magnetron sputtered carbon films were examined with four species, namely Ru(NH3)62+/3+, Fe(CN)63−/4−, Fe2+/3+, and dopamine. The peak separation of these species depended on the content of the edge plane of the film for Fe(CN)63−/4− and surface functional groups containing oxygen for Fe2+/3+, and both edge plane and surface oxygen-containing groups for dopamine. These results indicate that our carbon films perform sufficiently well for use in electroanalysis and are not inferior to previously reported carbon film electrodes.
In order to develop high-energy batteries, it is important to understand the charge/discharge characteristics of the Li-metal negative electrode when operating with high Li utilization; these characteristics determine the practical capacity of the negative electrode. In this study, electrochemical properties and deposition/dissolution behavior of Li metal negative electrodes in a VS4/Li battery with high Li utilization and current density were investigated. The potentials of the positive and negative electrodes were measured separately using a three-electrode cell. During discharge (Li dissolution) at the negative electrode, a semi-quantitative correlation was observed between the Coulombic efficiency and the capacity at which the slope of the potential curve increased sharply. The Coulombic efficiency of the negative electrode improved when vinylene carbonate (VC) or fluoroethylene carbonate (FEC) was added to the electrolyte. Granular particles were found to be deposited on the entire surface of the charged negative electrodes. The average particle size followed the order FEC addition > VC addition > no addition. A mixture of fine fibrous and cord-shaped residues was observed in the discharged negative electrode when the electrolyte was used without additives. In contrast, almost exclusively fibrous residues were observed when the FEC-added electrolyte was used. The cell capacity decreased mainly because of the Li depletion of the negative electrode without additives, while the capacity reduction was mainly attributed to the degradation of the positive electrode with additives.
Electrochemical reactions in positive and negative electrodes during recovery from capacity fades in lithium ion battery cells were evaluated for the purpose of revealing the recovery mechanisms. We fabricated laminated type cells with recovery electrodes, which sandwich the assemblies of negative electrodes, separators, and positive electrodes. The positive electrodes were replenished with Li+ by applying current between the recovery and the positive electrodes. A discharge curve analysis revealed that Li+ replenishment enabled the cells to recover from the capacity fade originating from capacity slippage between the positive and the negative electrodes. However, an issue is low recovery efficiency, which is defined as the ratio of recovery capacity of capacity slippage to the electric charge between the recovery and the positive electrodes. The cause of low recovery efficiency was elucidated by evaluating the positive and the negative electrodes after replenishment. It was found that the following mechanisms are involved in the replenishment of the positive electrodes: (a) Li+ are intercalated into the positive electrodes as they are released from the recovery electrodes, which significantly contributes to recovery from capacity slippage; however, (b) some amount of Li+ is released from the planes of the negative electrodes facing the positive electrodes as they are intercalated into the planes of the negative electrodes facing the recovery electrodes, which does not significantly contribute to recovery. Consequently, the recovery efficiencies were less than 50 %. We conclude that, to increase recovery efficiency, process (b) should be suppressed.
For electrocatalysts of oxygen evolution reaction (OER), a new accelerated durability test (ADT) protocol is presented. The protocol is designed to closely mimic the fluctuations of renewable energies. The unit cycle of the current ADT protocol represents the “ON/OFF” operation mode. In the “ON” step, the electrolyzer operates under a DC current of 0.6 A cm−2. In the “OFF” step, the electrocatalyst is subjected to a constant potential that is clearly more cathodic than its OER onset potential (namely, 0.3, 0.5, and 0.7 V vs. RHE) for 10 or 60 s. The transition from the “ON” state to the “OFF” state occurs through a cathodic linear sweep voltammetry of a fast sweep rate to mimic the sudden changes in the renewable power. A NiCoOx/Ni-mesh electrode was used as a case study. The electrode showed remarkable durability under continuous operation (i = 0.6 A cm−2) for about 900 hours. However, it did suffer severe degradation after a certain number of ADT cycles, and the rate of degradation mainly depends on the potential value and the duration of the “OFF” step. Interestingly, the inclusion of the 10-sec open-circuit potential step after the “ON” step clearly mitigates the impact of energy fluctuations on the durability of OER electrocatalysts.
In this work, a novel electroless deposition process on the anion exchange membrane (AEM) is proposed. AEM surface has a positively charged functional group, which in general does not allow the catalyst particle, such as Pd, to be formed on the surface. Hence, a different strategy from the conventional catalyzation process was required. We found that the sensitization process using Sn-containing solution, which is widely applied in the electroless plating on nonconductive substrates, hindered the Pd particle modification, which hence inhibited the following deposition reaction. Our several experiments and density functional theory analyses suggest that for Pd particle modification, anion in the bath turned out to play a key role. In particular, Cl− provides the sufficiently strong connection between the precursor Pd2+ and positive functional group of the substrate. This leads to favorable deposition of Pd catalyst particles and metal layer formation on the AEM. Therefore, we conclude that just a single pre-treatment to immerse the AEM films into PdCl2/HCl solution is capable to perform electroless plating on it. We applied the novel process to the electrode formation, such as Pt and Ni–P, on the AEM for hydrogen evolution reaction (HER) as a case study. Both Pt and Ni–P was successfully formed on the AEM. The electrochemical measurements show that those electrodes are able to serve as the catalytic electrode for HER. The electroless process proposed here opens possibility of the direct metal fabrication on ion exchange membrane surface.
All solid state Lithium-Sulfur batteries can effectively solve the problem of the conventional Li-S batteries with a liquid electrolyte. However, they still do not achieve sufficient cycle stability and rate capability because of high interfacial resistance between the electrode and the solid electrolyte. Hybrid-electrolyte structure using a liquid electrolyte and a solid electrolyte can efficiency solve the interfacial problem. Here, we demonstrate the effect of Li salt concentration in the liquid electrolyte between the sulfur cathode and the solid electrolyte Li7La3Zr2O12 (LLZ) on the electrochemical properties in Lithium-Sulfur batteries employing hybrid-electrolyte structure. Furthermore, the interfacial reactivity between the liquid electrolyte and LLZ is investigated. With increasing Li salt concentration, the electrochemical performances including the utilization of sulfur, the cycle stability and coulombic efficiency are improved because the dissolution of lithium polysulfides during cycle into the liquid electrolyte at cathode side is inhibited. The interfacial layer is formed on LLZ surface during discharge-charge cycle by a contact of the liquid electrolyte with LLZ, leading to increasing the interfacial resistance. We believe that this study helps to improve the electrochemical properties of Lithium-Sulfur batteries with hybrid electrolyte concepts.
Full cells employing Li3VO4 (LVO) and Li3V2(PO4)3 (LVP) as anode and cathode, respectively, are energy storage devices offering high power and cyclability. Such full cells, termed as LVO//LVP, were constructed in this study, and they exhibited low capacity retention (72 %) over 1000 cycles at a high temperature of 50 °C. We clarified the capacity degradation mechanism using charge-discharge cycling simulations based on a difference in coulombic efficiency (CE) between two electrodes with/without a capacity decay at electrode materials. Simulation results indicate that the low CE of LVO accompanied with a cyclic capacity decay of LVO was responsible for the full cell capacity degradation. The LVO capacity decay was further elucidated by experimental evidences, showing that the cycled LVO was covered by resistive polymeric films derived from the electrolyte reductive decomposition. Indeed, the capacity retention of full cell cycling was improved to 86–96 % by mitigating the effect of such side reaction, demonstrating the credibility and effectivity of our simple cycling simulation. Our finding may help to elucidate the degradation mode of the full cell cycling with less experimental efforts and work out own strategy to mitigate the degradation.
A divalent Mn-Ni oxide solid solution electrode was prepared by a hydrothermal method, and its electrochemical characteristics were investigated. As a result of optimizing preparation and heat treatment conditions and including nanocarbon as a conducting material, the potential window of electrode was successfully enlarged compared to that previously reported, reaching a specific capacitance of 363 F g−1. An aqueous electrolyte asymmetric supercapacitor consisting of electrodes of the Mn-Ni oxide including carbon (Mn-Ni-O/C) and activated carbon operated to an upper limit voltage of 2.5 V, leading to an energy density of 21.0 Wh kg−1 and an average power density of 21.8 kW kg−1. Moreover, the relationship between the oxidation number and the potential of the electrode was studied by X-ray photoelectron spectroscopy. For the first time, the oxidation number of Mn in the electrode was observed to change approximately in the range from divalent to tetravalent during the charge-discharge. This can be a very important guideline for increasing the capacitance and energy density of the Mn-based oxide electrodes.
Recently, a smaller-ion shell (SiS) model was developed to describe the electrostatic interaction between ions in solutions (D. Fraenkel, Mol. Phys., 108, 1435 (2010)). The analytical formula based on the SiS model reproduces experimental data very well with only one fitting parameter (namely, ion size parameter, a). However, a numerical simulation of the SiS model does not agree with the analytical formula. This indicates that the analytical formula does not represent the SiS model correctly. Therefore, the ion size parameter (a) obtained by fitting Fraenkel’s analytical formula to the experimental data is physically meaningless.
New programs for fitting using an equivalent circuit model and data visualization of electrochemical impedance spectroscopy were developed using Python and its open-source libraries. Executable programs have a graphical user interface and can run on a Microsoft Windows operating system without utilizing other platform applications. Many practical functions were implemented, e.g., supporting various file formats, graphical support functions to obtain a good initial guess, display interactive three-dimensional plots of the spectrum, a modified logarithmic Hilbert integral transform test, display multiple spectra data, etc. The developed executable programs were released by free of charge for use under the MIT license.