Pressure-induced evolution in the durability of nickel-metal hydride batteries under high-current charge.

We found that an AAA-type battery (min. 750 mAh) pressurized with Ar or N2 at pressures of up to 5 MPa exhibited a significant durability enhancement even under high-current conditions. As an example of a charge-discharge cycle test at 3 amperes, the residual ratio of capacity at atmospheric pressure decreased to approximately 90% of the standard capacity before 50 cycles. However, at a pressure of 3 MPa of N2, the capacity remained at more than 90% until 180 cycles. With an increase in the pressure, the residual ratio of capacity was further improved. It has been considered that, at the positive electrode of the Ni-MH battery, the chemical reaction from nickel(II) hydroxide (Ni(OH)2) crystals to nickel oxide hydroxide (NiOOH) crystals occurs during the charging process. However, X-ray diffraction (XRD) results in the present study do not support this solid-solid reaction between these two types of crystal. To address this contradiction, we propose a different reaction mechanism by introducing the concept of non-crystalline fine particles of compounds, which are undetected by XRD. This mechanism clearly explains how the pressure affects the durability improvement. Pressurized batteries, which are capable of fast charge-discharge operation under high-current conditions, can provide a new route for application fields of unconventional energy storage.

Sensitive Quantitative Analysis of Strontium in Microdroplet by Surface-enhanced Laser-induced Breakdown Spectroscopy Using Porous Silicon.

Surface-enhanced laser-induced breakdown spectroscopy (SELIBS) is a promising method for microanalysis of liquid samples. We previously demonstrated that the SELIBS signal was significantly enhanced by using porous silicon (Si) instead of flat Si. In this work, we dried aqueous droplets containing 1 - 200 ppb strontium (Sr) on porous Si substrates and evaluated the quantitative performance by analyzing the dry residues. A linear calibration curve for the Sr quantification (R2 = 0.998) was obtained and an LOD was 0.67 ppb.

Kinetic Modelling of Electroless Nickel-Phosphorus Plating under High Pressure.

In electroless nickel-phosphorus plating (ENPP), growth of the plated layer under high pressure was found to be faster than under ambient pressure. To quantitatively elucidate the effect of high pressure on the mechanism of the ENPP reaction, we propose a kinetic model that takes into account both mass transfer and reaction of the chemical species present in the plating solution. We solved the mass balance equations between the chemical species to calculate the transient changes in the thickness of the plated layer as well as the concentrations of the chemical species in the plating solution. By fitting the calculated results to the experimentally acquired results based on the nonlinear least square method, we determined such parameters as the film mass transfer coefficient, the adsorption constants, and the reaction rate constants of the chemical species in the model. As a result, we found that the film mass transfer coefficient under high pressure was greater than that under ambient pressure and revealed the dependence of the coefficient on pressure. The transient changes in the concentrations of the chemical species in the plating solution that we calculated based on the kinetic model employing our estimated parameters closely modeled the experimental results with the determination coefficients being mostly over 99%.

General corrosion during metal-assisted etching of n-type silicon using different metal catalysts of silver, gold, and platinum.

Metal-assisted etching is a promising technique for microfabrication of semiconductors. In this method, porous silicon (Si) can be produced with a very simple procedure, and various nanostructures can be designed by changing the catalyst patterns. The kind of metal catalysts is one of the key factors to control the porous structure. In this work, we performed the etching of n-type Si (100) in a hydrofluoric acid solution containing hydrogen peroxide in the dark using silver, gold, and platinum particles electrolessly deposited at a constant coverage, and demonstrated the difference in the porous structures obtained for the different kind of metal catalysts. By comparing the mass loss of substrates with the depth of pores formed under the metal particles, we found that general corrosion occurred on the top-surface of the Si substrate around the metal particles even under the dark condition. The general corrosion depended on the metal species and it was explained by the formation and dissolution of a mesoporous layer. The kind of metal catalysts influences the dissolution of the Si surface not only under the metal catalysts but also between them.

Catalytic activity of noble metals for metal-assisted chemical etching of silicon.

Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.