Shinji Nohara

The possible use of polymer gel electrolytes in nickel/metal hydride battery

Abstract Alkaline polymer gel electrolyte was prepared from potassium salt of cross-linked poly(acrylic acid) and KOH aqueous solution in order to investigate the applicability of the polymer gel electrolyte to alkaline secondary batteries such as nickel/metal hydride (Ni/MH) battery. The polymer gel electrolyte was found to have high ionic conductivity and wide potential window almost comparable to the KOH aqueous solution over the wide ranges of KOH concentration and temperature. An experimental Ni/MH cell was assembled using polymer gel electrolyte and its charge–discharge and capacity retention characteristics were tested. The cell using the polymer gel electrolyte exhibited somewhat better charge–discharge and capacity retention characteristics at 25 °C compared with a similar type of cell assembled using KOH aqueous solution.

Electrochemical characteristics of a homogeneous amorphous alloy prepared by ball-milling Mg2Ni with Ni

Ball-milling Mg2Ni with metallic Ni (70 wt.% vs. Mg2Ni) lead to the formation of a homogeneous amorphous alloy, which exhibited a maximum discharge capacity of ca. 870 mAh g (Mg2Ni)−1 at 30°C. Its electrochemical and microstructural characteristics indicated that the homogeneous amorphous structure of the alloy was an important factor for such an improvement in charge–discharge characteristics.

Hydriding and electrochemical characteristics of a homogeneous amorphous Mg2Ni-Ni composite

Abstract Hydriding and electrochemical characteristics of a homogeneous amorphous Mg 2 Ni-Ni composite prepared by ball-milling of Mg 2 Ni alloy with Ni (70 wt.% vs. Mg 2 Ni) were investigated. It was found that the Mg 2 Ni-Ni composite absorbed hydrogen to the amount of 4.0 wt.% vs. Mg 2 Ni [2.4 wt.% vs. (Mg 2 Ni+70 wt.% Ni)] at a high rate under a hydrogen pressure of 3 MPa at 30°C. Furthermore, this alloy exhibited an extremely high discharge capacity of ∼1082 mAh g(Mg 2 Ni) −1 [636 mAh g(Mg 2 Ni+70 wt.% Ni) −1 ] at 30°C, which exceeded the theoretically calculated value of 999 mAh g(Mg 2 Ni) −1 on the basis of Mg 2 NiH 4 for crystalline Mg 2 Ni.

Electrochemical characterization of new electric double layer capacitor with polymer hydrogel electrolyte

Abstract A new electric double layer capacitor (EDLC) was constructed by using polymer hydrogel electrolyte prepared from crosslinked potassium poly(acrylate) and KOH aqueous solution, and its electrochemical characteristics were investigated by cyclic voltammetry and charge–discharge cycle tests, compared with a case of the cell using only a KOH aqueous solution as an electrolyte. As a result, the cell with the polymer hydrogel electrolyte was found to exhibit higher capacitance than that with the KOH aqueous solution and excellent high-rate dischargeability. The impedance spectroscopic measurements suggested that the higher capacitance could be ascribed to the pseudocapacitance. These results indicate the potential applicability of the polymer hydrogel electrolyte to EDLCs as an electrolyte with good performance.

Electrochemical characteristics of an amorphous Mg0.9V0.1Ni alloy prepared by mechanical alloying

Abstract Electrochemical characteristics of an amorphous MgNi alloy with Mg partially substituted by V were investigated. A Mg 0.9 V 0.1 Ni alloy prepared by mechanical alloying (MA) exhibited much better cycle life than MgNi alloy. It was found that the partial substitution of Mg in MgNi with V could suppress the formation of Mg(OH) 2 on the alloy surface during the charge–discharge cycling in alkaline solution. This may have unveiled an important factor to improve cycle life of the Mg-based alloy for use in nickel–hydrogen batteries.

All solid-state nickel/metal hydride battery with a proton-conductive phosphoric acid-doped silica gel electrolyte

Abstract All solid-state nickel/metal hydride (Ni/MH) battery was fabricated by using a proton-conductive phosphoric acid-doped silica gel as an electrolyte. The H3PO4-doped silica gel was dried in vacuum at various temperatures for 1 h before fabricating the battery in order to reduce the possibility of corrosion by water on the surface and in the micropores of the gel. The influence of drying temperature on the crystallinity, water content and conductivity of the gel was investigated. The conductivity depended on the drying temperature of the gel. The fabricated all solid-state Ni/MH battery was able to operate several tens of charge–discharge cycles at relatively high current density although the utilization of the battery was low.

Effect of ball-milling on electrochemical and physicochemical characteristics of crystalline Mg2Ni alloy

Two-step surface modifications for crystalline Mg2Ni alloy were tried to improve the charge–discharge characteristics at 30°C. The surface modification of the alloy by ball-milling led to the improvement of the discharge capacity, probably due to the increase in surface area and the decrease in diffusion length for the desorption of absorbed hydrogen as the result of the production of nanocrystallites. The subsequent surface modification with graphite led to a further increase in discharge capacity due to the improvement of electrocatalytic activity.

Compositional and structural characteristics of MgNi alloy prepared by mechanical alloying for use as negative electrodes in nickel-metal hydride batteries

From the elemental analysis by EPMA, it was found that the bulk of MgNi alloy particles prepared by mechanical alloying (MA) were uniform composition with MgNi = 1 and a nickel-rich surface, while in the bulk of the alloy particles prepared by induction melting (IM) the MgNi ratio was not uniform. Moreover, it was confirmed by transmission electron microscopy (TEM) and electron diffractometry that the MgNi alloys prepared by MA and IM had amorphous and crystalline structures, respectively. This was believed to be responsible for the difference of the discharge capacities between two kinds of MgNi alloys.

A new electrode material for nickel-metal hydride batteries : MgNi-graphite composites prepared by ball-milling

Abstract MgNi–graphite composites prepared by ball-milling were found to show greatly enhanced charge–discharge characteristics with respect to the original MgNi alloy. There was an optimal ball-milling time for the preparation of the MgNi graphite composite with enhanced electrode performance, when the modification with graphite was limited to the surface layer of MgNi alloy. Raman and XPS investigations on the composites indicated a decline in the π-electron character of graphite and changes in the chemical states of the constituents on alloy surface, suggesting the possibility of charge transfer between graphite and MgNi alloy during ball-milling, which resulted in an increase in the surface Ni/Mg ratio.

Effects of Ti–V substitution on electrochemical and structural characteristics of MgNi alloy prepared by mechanical alloying

Abstract Electrochemical and structural characteristics of amorphous Mg 0.9 Ti 0.1 Ni, Mg 0.9 V 0.1 Ni and Mg 0.9 Ti 0.06 V 0.04 Ni alloys prepared by mechanical alloying (MA) were investigated. In charge–discharge cycle tests, the decay of discharge capacity with increasing cycle number was suppressed by partial substitution with either Ti or V, and the alloy partially substituted with both Ti and V, Mg 0.9 Ti 0.06 V 0.04 Ni, exhibited further improved cycle performance. Results of X-ray diffractometry (XRD) indicated that the oxidation of Mg on the alloy surface during the charge–discharge cycles could be fairly suppressed by the partial substitution with both Ti and V. Moreover, it was found from Auger electron spectroscopy (AES) that thickness of oxidized surface layer of the MgNi alloy particle was decreased by the introduction of both Ti and V. It was suggested that composite oxide layer in which both Ti and V species existed might be very effective in suppressing the oxidation of the alloy surface, leading to the synergistic effect on charge–discharge cycle performance.