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Magnesium-sulfur batteries are one of the most promising next-generation battery systems due to their high energy density, low cost, and high level of safety. However, the reaction mechanisms are not well understood, and in particular, the discharge reaction products have not yet been identified. Here we show that zinc blende magnesium sulfide is observed as a reaction product after discharging in magnesium-sulfur batteries. When magnesium reacts electrochemically with sulfur in a sulfone-based magnesium electrolyte, sulfur becomes amorphous consisting of magnesium and sulfur in the cathode. In this study, it has been found that the amorphous material has an unusual local structure, which is not related to the most stable rock salt phase of magnesium sulfide but rather the metastable zinc blende phase. It was indicated that this material realizes the reversibility of magnesium-sulfur batteries.

We have developed the passive-type biofuel cell, in which two-electron oxidation of glucose and four-electron reduction of O2 occur at pH 7 in mediated bioelectrochemical processes, having the maximum power density of 5.0 mW/cm2 at 500 mV. This performance has been achieved by using a new electrolyte solution (Imidazole/HCl buffer, 2 M, pH 7), instead of the general solution (Sodium phosphate buffer, 1 M, pH 7); the maximum power density was a 3.0 mW/cm2 at the same conditions. By introducing the new electrolyte solution into our biofuel cell, we have succeeded in enhancing the catalytic current because the proton transfer into electrodes would increase compared with the phosphate buffer system.

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Mediated biosensors consisting of an oxidase and peroxidase (POx) have attracted increasing attention because of their wide applicability. However, since most of oxidases utilize artificial electron acceptors in place of dioxygen, the competition between O2 and the electron acceptor in the mediated sensors is anticipated. This has been evidenced with a glucose oxidase (GOx)- and POx-coentrapped and ferrocene-embedded carbon paste electrode, which exhibits peak-shaped current-time curves at increased concentrations of glucose and also gives a peak-shaped calibration curve. Digital simulation has been applied to clarify the cause of such unusual responses, by taking into account the ping-pong enzyme kinetics on two- and three-substrate models for POx and GOx, respectively. The simulation has well reproduced such unusual responses and has clearly revealed that the depletion of O2 in the enzyme layer is the most important factor responsible for such unusual responses. To overcome such a drawback of oxidase/POx bienzyme sensors, it is expected to be essential to decrease the rate of the oxidase reaction. In contrast, increase in the POx activity is useful to improve the sensitivity. According to the simulation-based expectation, the GOx and POx concentrations in the bienzyme sensor are adjusted to exhibit normal behavior with high sensitivity.