Noriyuki Sonoyama

Comparative Kinetic Study of Olivine Li x MPO 4 ( M = Fe , Mn)

A huge kinetic difference in olivine Li x MPO 4 (M = Fe,Mn) is demonstrated in a quantitative manner. Galvanostatic discharge profiles and the current relaxation to the stepwise anodic overvoltage (chronoamperometry) are comparatively measured for the Li x FePO 4 and Li x MnPO 4 under identical extrinsic conditions, which are carefully controlled and confirmed using Rietveld refinement for the X-ray diffraction profiles, direct texture observation by scanning electron microscope, Brunauer-Emmett-Teller surface area measurements, and tap density measurements. The current durability for Li x MnPO 4 is orders-of-magnitude inferior to that of Li x FePO 4 , the origin of which is clearly attributed to their intrinsic crystallographic and transport property differences. Heavy polaronic holes localized on the Mn 3+ sites are suggested as an important rate-limiting factor. In spite of the higher open-circuit voltage of Mn 3+ /Mn 2+ (4.05 V) compared to that of Fe 3+ /Fe 2+ (3.45 V) in the olivine framework, the abnormally large polarization may eliminate pure LiMnP0 4 as a practical lithium battery cathode due to much lower effective energy density than LiFePO 4 .

Fast Charging LiFePO4

An excellent fast charging ability for olivine LiFePO 4 is demonstrated with respect to its advantageous open-circuit voltage of 3.4 V vs. Li/Li + with an appropriately large overvoltage margin to the electrolyte oxidation, structural integrity under the nonequilibrium state induced by the large overpotential, and moderate electrochemical activity easily enhanced to the practical level by efficient nanocomposite formation. The current relaxation to the applied stepwise voltage of 4.2 V vs. Li/Li + for the Li x FePO 4 was fast enough to consume most of the theoretical capacity within 10-20 min and the speed is competitive with that of the conventional electrode composite of Li x Mn 2 O 4 , while the capacity consumption of Li x MnPO 4 under the identical experimental conditions was almost null.

Electrochemical reduction of CO2 at metal-porphyrin supported gas diffusion electrodes under high pressure CO2

Electrochemical reduction of CO2 at metal-meso-tetraphenylporphyrin (TPP) supported gas diffusion electrodes (GDEs) under CO2 at atmospheric pressure and 20 atm was carried out. At Co- and Fe-TPP supported GDEs that are comparatively active in the electrochemical reduction of CO2 under atmospheric CO2, the current efficiencies for the reduction of CO2 increased up to 97.4 and 84.6%, respectively, by an increase in CO2 pressure. At Cu- and Zn-TPP supported GDEs that showed low activity under atmospheric CO2, the current efficiencies for CO2 reduction increased up to 50.5 and 65.8%, respectively, under 20 atm CO2. At these active metal-TPP supported GDEs, the potential of CO2 reduction shifted positively by an increase in CO2 pressure. These results indicate that the increase in concentration of CO2 in the electrolyte solution caused by high pressure enhanced the electrocatalytic activity of metal-TPPs for CO2 reduction.

Continuous electrochemical decomposition of dichloromethane in aqueous solution using various column electrodes

Abstract Dichloromethane is one of the chlorohydrocarbons that contaminate the soil and groundwater. They are conventionally removed from water by the aeration method and aeration/activated carbon adsorption. These methods only remove the compounds from water into other phases, and therefore require a secondary treatment for decomposition of the pollutants. The electrochemical method is an appropriate method for the decomposition of halocarbons in water. However, dichloromethane is one of the most persistent halocarbons for electrochemical treatment. In this paper, we optimized the conditions for electrochemical decomposition of dichloromethane in water and attempted to continuously treat the dichloromethane solution using a flow cell. During batch electrolysis, a Cu wire electrode showed high electrocatalytic activity for the decomposition of dichloromethane and the optimum potential for the decomposition was −1.3 V vs. Ag/AgCl. During flow electrolysis, the conversion of dichloromethane was largely dependent on the packing material of the column electrodes. A Cu metal-powder column electrode showed higher activity than a Cu particle-impregnated carbon fiber electrode and activated carbon electrode. The Cu metal-powder column electrode decomposed 20 ppm dichloromethane in an aqueous solution with 100% conversion at a low flow rate. This result suggests that the structure of the Cu surface affects the adsorption of dichloromethane on the Cu electrodes.

Lithiation mechanism of new electrode material for lithium ion cells—the α-Fe2O3–SnO2 binary system

Abstract New electrode material for lithium ion cells using surface and intercalation reactions was found for the α-Fe 2 O 3 –SnO 2 binary system. The solid solution was synthesized by a wet preparation method using a precipitation from alkaline solutions containing Fe 3+ , Sn 4+ and SO 4 2− ions. The lithium cells with α-(Fe 2 O 3 ) 0.7 –(SnO 2 ) 0.3 cathode showed the discharge–charge capacity of 300 mA h g −1 . The reaction mechanism was examined by X-ray diffraction (XRD) and the 57 Fe and 119 Sn Mossbauer spectroscopy. Both the intercalation and the surface reactions participate in the charge–discharge process.

Electrochemical luminescence of ZnGa2O4 semiconductor electrodes activated with Cr and Co

Electrochemical luminescence of ZnGa2O4:Cr and ZnGa2O4:Co semiconductor electrodes was studied. Under cathodic polarization in Na2S2O8 electrolyte, these electrodes generated emission of ZnGa2O4 with a broad band from 350 to 650 nm together with red emission attributed to Cr3+ and Co2+. These emissions are explained by recombination between electrons and holes. Under anodic bias in Na2SO4 electrolyte, only the red emission was observed, accompanied with a sharp rise of anodic current at ca. +12.5 V. The emissions were explained by collision excitation of Cr3+ and Co2+ emission centers owing to avalanche breakdown.

Electrochemical luminescence of ZnGa2O4 and ZnGa2O4:Mn electrodes

Semiconductor electrodes of ZnGa2O4 and ZnGa2O4:Mn were prepared by reducing ZnGa2O4 and ZnGa2O4:Mn sintered disks in a hydrogen stream. Under cathodic polarization in S2O82− electrolyte solution, electrochemical luminescence (ECL) was observed with a peak at 485 and 500 nm for the ZnGa2O4 and ZnGa2O4:Mn electrodes, respectively. With increasing cathodic polarization, the 500 nm emission for the ZnGa2O4:Mn electrode shifted to 490 nm. The ECL is ascribed to the recombination between the electrons in the conduction band and the holes in the valence band for ZnGa2O4 or their recombination that takes place at the Mn2+ site for ZnGa2O4:Mn.

Electrocatalytic reduction of NO on metal electrodes and gas diffusion electrodes in an aqueous electrolyte

The electrochemical reduction of nitric oxide (NO) in an aqueous electrolyte was studied using different metal electrodes and gas diffusion electrodes (GDEs). The reduction product selectivity for NO reduction depends strongly on the type of electrode metal. NH3 was produced as the predominant reduction product on Ti, Ni, Cu, and Ag electrodes and N2 was produced predominantly on a glassy-carbon electrode. The current–potential curve in the NO atmosphere depended on the nature of the metal electrode. Product selectivity did not depend on the metal in electrolysis using GDEs. The predominant product was N2O and N2 on GDEs containing Ni, Pt, and Cu electrocatalysts.

A comparative study of semiconductor sensitization by micro-crystals of indium sulfide on various porous wide band gap semiconductor substrates

Abstract Semiconductor sensitization on various n-type wide band gap semiconductors was studied by micro-crystals of n-type indium sulfide. The generation of an anodic photocurrent on the electrode is explained from the viewpoint of semiconductor sensitization. A very high incident photon-to-current conversion efficiency (IPCE) of more than 80% was achieved on In 2 S 3 /In 2 O 3 electrodes in a polysulfide electrolyte. The observed values for the IPCE for In 2 S 3 /TiO 2 and In 2 S 3 /ZnO electrodes were rather low compared to that of In 2 S 3 /In 2 O 3 electrodes, in the same electrolyte. The semiconductor sensitization process was not observed on In 2 S 3 /ZnS electrodes. Different kinetics at the semiconductor/semiconductor interface are the reasons for the observed difference of the IPCE, for the electrodes.

Synthesis, structure and electrochemical properties of layered material, Li2/3[Mn1/3Fe2/3]O2, with mixed stacking states

Abstract The layered material, Li 2/3 [Mn 1/3 Fe 2/3 ]O 2 , was prepared by Li ion exchange reactions in alcohols from the host, Na 2/3 [Mn 1/3 Fe 2/3 ]O 2 . The structure of the stacking sequence was determined via the simulation of X-ray diffraction patterns and its electrochemical properties were characterized. The stacking sequence of the products depended on the reaction temperature and time. The structure of Li 2/3 [Mn 1/3 Fe 2/3 ]O 2 obtained at 85 °C has stacking faults including the O3, O2 and P2 type with the ratio of 4:4:2. The electrochemical properties of Li 2/3 [Mn 1/3 Fe 2/3 ]O 2 were studied using lithium cells.