In this paper, the author describes several new electrochemical reactions in molten salts (MSs) and ionic liquids (ILs), which includes the development of new ILs, and their applications. First, the electrochemical reduction of solid SiO2 to Si in molten CaCl2 at 1123 K is mentioned. The author explains the mechanism how the insulating SiO2 is electrochemically reduced to silicon. A new production method of solar grade silicon (SOG-Si) is introduced as one of the promising applications of the electrochemical reduction of SiO2. The author also mentions another new production method of SOG-Si which utilizes a liquid Si–Zn cathode in molten CaCl2. Secondly, the author describes the electrodeposition of silicon and titanium from water-soluble KF–KCl MSs. Compact crystalline Si films are electrodeposited from molten KF–KCl–K2SiF6 at 923 K. When K2TiF6 and Ti are added in molten KF–KCl, Ti(III) ions are produced by the proportionation reaction between Ti(IV) and Ti(0), in which compact Ti films can be electrodeposited. Thirdly, the author describes the binary and ternary mixtures of M[TFSA] (M = Li, Na, K, Rb, and Cs; TFSA = bis(trifluoromethylsulfonyl)amide) ILs as a new class of electrolytes for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) operating in the intermediate temperature range. Fourthly, the development of binary and ternary mixtures of M[FSA] (M = Li, Na, K, Rb, and Cs; FSA = bis(fluorosulfonyl)amide) ILs and their applications for NIBs are mentioned; the binary and ternary mixtures of M[FSA] are unique in the points that they are inorganic compounds and have low melting temperatures. Finally, the author explains the development of several binary mixtures of M[FSA]–[Ocat][FSA] (M = Li, Na, and K; Ocat = organic cations) ILs which can be used for LIBs, NIBs, and KIBs (K-ion batteries). As a typical application of the M[FSA]–[Ocat][FSA] ILs, the development of practical NIB with a capacity of 27 Ah is described.
Single-atom catalysts (SACs), which are composed of singly isolated metal sites and heterogeneous supports, have recently attracted intensive attention as a novel category of electrocatalysts. SACs can not only ultimately reduce the loading amount of noble metals but also exhibit unique reaction activity and selectivity for many reactions. However, the design flexibility of conventional supports of SACs, including nanocarbons and metal oxides, is poor. Therefore, designing coordination environments of metal centers in SACs, which are one of the most significant parameters governing their electrocatalytic properties, has been challenging. This review outlines the synthesis of two kinds of SACs with defined coordination structures and their unique electrocatalytic activities. First, graphenes doped with metals (Fe, Cu, and Ni) and nitrogen atoms, which were prepared by a short-duration heat treatment, function as efficient electrocatalysts for oxygen reduction reactions and CO2 reduction reactions. Second, metal-doped covalent organic frameworks, which are a class of porous conjugated polymers, exhibit unique electrocatalytic selectivity compared with bulk metals.
In this comprehensive paper, three redox-neutral reactions, including [2 + 2] and [4 + 2] cycloadditions and vinylcyclopropane rearrangements, are outlined from the viewpoint of energy conversion. These reactions demonstrate the power of electrosynthesis in the field of synthetic organic chemistry not only from the viewpoint of energy conversion but also from that of redox economy because four-, five-, and six-membered-ring skeletons are constructed without a change in oxidation state of the growing molecules in synthetic routes. The key for all of the reactions is precise control of single-electron transfer (SET) in lithium perchlorate/nitromethane solution, where oxidative SET is facilitated and the thus-generated radical cations are highly stabilized. SET processes can be visualized by plotting the highest occupied molecular orbital and spin density distributions to obtain theoretical pictures for a mechanistic understanding of the reactions; the deduced mechanisms are in good accordance with the reactions’ formal expressions.
Since reversible lithium (de)intercalation of layered LiCoO2 and its high-voltage operation were discovered by Mizushima, Goodenough, and coworkers in 1980, layered lithium transition metal oxides have been mainly used as positive electrode materials for Li-ion batteries. Since 2010, layered sodium transition metal oxides have been further considered for Na-ion batteries, and new materials have also been developed. Recently, layered potassium transition metal oxides have attracted attention for their potential use in K-ion batteries. This article summarizes our studies on layered transition metal oxides as positive electrode materials for Li-, Na-, and K-ion batteries. The correlation between their crystal structures and electrochemical properties is also discussed. Comprehensive and systematic studies of intercalation materials based on the three alkali metals will provide insight into the unique chemistry and development of new layered oxide materials for next-generation batteries.
We prepared a high-quality magnetic BaCoTiFe10O19 ferrite-based composite sheet and BaCoTiFe10O19 thin films exhibiting in-plane magnetic anisotropy. In-plane magnetic anisotropy was realized by controlling particle orientation to achieve high-resonance frequency values. The dynamic permeability of the samples in the GHz-frequency region was evaluated by the short-circuited microstripline method up to 30 GHz. Controlling the propagation of electromagnetic waves with GHz frequencies is extremely difficult. Nevertheless, the ferromagnetic resonance phenomena were observed, and the obtained sheet showed a resonance frequency value of approximately 4 GHz. However, the intrinsic ferromagnetic resonance behaviors were not elucidated for the thin films. The obtained BaCoTiFe10O19 powder/polymer composite sheet and BaCoTiFe10O19 thin films may have the potential for use at GHz frequencies and as anisotropic-electromagnetic wave suppression materials.
A combination of sequential Injection Analysis (SIA) system and the ion-selective electrode (ISE) detector is considered to be one of a very effective analytical method. Reaction of lactate with lactate oxidase produces pyruvic acid and hydrogen peroxide. When the produced hydrogen peroxide is reacted with 4-aminoantipyrine (4-AAP) and 4-fluorophenol (4-FP) in the presence of horseradish peroxidase (HRP), F− ion is produced. The concentration of lactate can be indirectly determined by measuring the F− ion by F− ISE. Based on this principle of detection of lactate, a new measurement method of lactate using an SIA with ISE detector was developed.
The commercial pure aluminum (1A95) and its micro-alloyed with Mg and further particle composited with SiC were prepared using the vortex casting method, their electrochemical performances as the anodes in Aluminum-air batteries were investigated using open-circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and galvanostatic discharge. The results indicate that Al-Mg/SiC composite anode shows much better corrosion resistance and discharge performance than others. Impedance spectra and surface observation show that SiC addition obviously changed the anode dissolution morphology, which means that the SiC particles serve as attachment points for hydroxide products and impurities and reduce the parasitic corrosion reactions. As SiC particles fall off the substrate, SiC shedding disrupts the Al(OH)3 film, reducing the accumulation of hydrogen molecules on the pore surface, the more fresh surface is exposed, which promoted the corrosive effect of the anode. So, the Al-Mg/SiC composite anode has higher electrochemical activity and the open-circuit potential shifts more negatively. Even though the shedding of SiC particles might cause a relatively higher rate of dissolution during the spontaneous oxide growth/oxide dissolution process, the formation of MgAl2O4 cladding SiC particles reduces the Al/SiC interface energy, lowers corrosion rate and improves the composite anode efficiency and capacity density.
LiMn2O4 cathodes are prepared by a rapid microwave-induced solution combustion method. Their initial discharge capacity is significantly improved by introducing trace Ni. For example, the LiMn1.975Ni0.025O4 (LMNO-0.025) cathode releases an initial discharge capacity of 134.1 mAh g−1 at 1 C, even 144.5 mAh g−1 at 0.5 C, which is far higher than of the pristine LiMn2O4 cathode (119.7 mAh g−1 at 1 C) and close to its theoretical capacity of 148.2 mAh g−1. After 1000 cycles, the capacity retention of the LMNO-0.025 sample reaches 56.53 % at 1 C, and it can be presented the higher capacity retentions of 66.79 % at 5 C and 66.89 % at 20 C. The robust structure stability proved by XRD, SEM and HRTEM data, and the good kinetics testified by CV and EIS data of the Ni-doped material are responsible to improve the high-rate capability and long cycle properties.
Lithium Metal Battery (LMB) has attracted much attention to realize higher energy of rechargeable battery. However, LMB has not been commercialized due to its low safety and poor cycle performance. Recently, the highly concentrated (3–5 mol dm−3) electrolytes have been utilized, because of oxidation stability and thermal stability. So, in this study, lithium bis(fluorosulfonyl) imide (LiFSI)/ethylene carbonate (EC) electrolytes with different concentrations were investigated. With increasing concentration, amount of non-coordinating free EC solvent and FSI− ion decreased, leading to high stability of both the solvent and anion. In addition, the volume expansion of lithium metal was more suppressed with increasing the concentration of the electrolyte, leading to better contact between lithium metal and the separator without any free voids. XPS results showed that the SEI layer consisted of several inorganic components with high Li+ ion conductivity, such as Li-F, Li-N, Li2-S2 and Li2-S. Such SEI layer enabled more stable dissolution and deposition of lithium metal. In conclusion, the best molar ratio between LiFSI and EC was found to be 1 : 2.0 in this study.
We investigated the applicability of ionic-liquid electrolytes to FeSi2/Si composite electrode for lithium-ion batteries. In conventional organic-liquid electrolytes, a discharge capacity of the electrode rapidly faded. In contrast, the electrode exhibited a superior cycle life with a reversible capacity of 1000 mA h g(Si)−1 over 850 cycles in a certain ionic-liquid electrolyte. The difference in the cycle life was explained by surface film properties. In addition, the rate performance of the FeSi2/Si electrode improved in another ionic-liquid electrolyte. Remarkably, lithiation of only Si in FeSi2/Si composite electrode occurred whereas each FeSi2- and Si-alone electrode alloyed with Li in the ionic-liquid electrolyte. FeSi2 certainly covered the shortcomings of Si and the FeSi2/Si composite electrode exhibited improved cycle life and rate capability compared to Si-alone electrode.
It is widely recognized that heat-resistant separators improve the safety of lithium-ion batteries. However, it is not clear how the separator contributes to battery safety, or how much heat resistance is required. The state of separators when a short circuit occurs in batteries was simulated and the following tests were conducted. As a first test, the edges of the separator were restrained, and hot air was applied only to a limited part of the specimen. For the second test, a model battery system was constructed and electrical heat generation during the early stage of a short circuit was observed. In these tests, several types of separators were compared with respect to the extent of damage and electrical heat generation. The separators that gave good results during nail penetration tests showed limited damage and electrical heat generation during the previously discussed tests. The heat resistance of a separator should be discussed with regard to whether it can maintain the separator function during electrical heat generation via short circuit.
The oxygen chemical potential is an essential indicator for the rate-limiting step of the oxygen reduction reaction in high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs). However, standard electrochemical measurements cannot successfully analyze the oxygen potential profile. Herein, the relationship between oxygen deficiencies and the valence state was determined directly through operando X-ray absorption spectroscopy. To compare the rate-limiting reactions in SOFC cathodes, dense thin-film electrodes of La0.6Sr0.4CoO3−δ (LSC) on a Ce0.9Gd0.1O1.95 (GDC) electrolyte, La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) on a Y0.1Ce0.9O1.95 (YDC) electrolyte, and La0.9Sr0.1MnO3±δ (LSM) on a Zr0.92Y0.08O1.96 (YSZ) electrolyte were examined as model SOFC cathodes. Variations in the oxygen chemical potential of the electrodes with and without cathodic polarization were experimentally evaluated from the energy shift of the transition metal (Co, Fe, and Mn) K-edge X-ray absorption. It was found that the oxygen chemical potential of the LSC and LSCF electrodes was reduced by applying a cathodic potential and that this change in the oxygen chemical potential occurred mainly on the electrode surface. This result directly demonstrates that the electrochemical oxygen reduction at the cathode is rate-controlled by surface reactions. By contrast, the oxygen potential of LSM changes not at the electrode surface but inside the electrode, which demonstrates that oxide ion diffusion is the rate-determining step for the LSM/YSZ model electrode. This study directly reveals the different rate-determining steps of the electrode reaction for various SOFC cathodes.
Development of highly active bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is required for air electrodes of zinc-air secondary batteries (ZAB). In this study, we synthesize spinel-type MnCo2O4 (MCO) nanoparticles on highly graphitized platelet-type carbon nanofibers (pCNF) via a solvothermal method. The pCNF is selected as carbon support in this study because of the excellent stability against anodic degradation under the OER condition. The MCO nanoparticles of 2–5 nm in diameter are uniformly dispersed on pCNF and the catalyst exhibits high activities for ORR due to strong interaction pCNF and MCO, in addition to the improvement of OER activities. The MCO/carbon hybrids show comparable electrocatalytic performances to state-of-the-art bifunctional electrodes for OER and ORR.
China is short of high grade bauxite for the production of smelting grade alumina. With the massive exploitation of potassium containing bauxite in China, the produced Bayer alumina contains a non-negligible content of potassium oxide. When this kinde of alumina is used as the raw material for aluminum electrolysis, the potassium would cause multiple effects to the cell performance. In this paper, the potassium concentrations of fresh alumina, secondary alumina, aluminum fluoride, anode, electrolyte, anode cover, cathode block, carbon lining, silicon carbide block and product aluminum et al. in 160 kA and 200 kA prebaked aluminum reduction cells were tested and a potassium balance model was preliminarily given. Initial results showed that most of the potassium was brought into the cell by the potassium containing impurities in the fresh alumina. The only effective way for potassium removal from the cell was through the carbon residue though nearly 80 wt% of the potassium would stay and enrich in different positions of the cell.