Nobumitsu Hirai

Combined in situ EC-AFM and CV measurement study on lead electrode for lead–acid batteries

Abstract An electrochemical atomic force microscope (EC-AFM) was used to study the reaction of a lead electrode in sulfuric acid electrolyte, when the reaction corresponding to what occurs at the negative electrode of a lead–acid battery took place. At first, the AFM was applied to observation of the lead electrode during cyclic-voltammetry (CV) measurement, and was found to be useful to obtaining continuous in situ images of the surface morphology. These AFM images dynamically showed the surface morphology change during the oxidation/reduction cycle. From these observation results, it was visually confirmed that the quick deposition of lead sulfate crystals occurs after super-saturation phenomena at the oxidation peak on CV, and that the slow dissolving of the lead sulfate crystals occurs after the reduction peak. AFM images of the lead sulfate morphology after oxidation were then compared with those in a different potential sweeping rate and electrolyte concentration at CV. It was clearly found that the crystal size becomes smaller when the potential sweeping rate is fast or the electrolyte concentration is high. We also compared the difference in AFM images and SEM images that were observed on the same electrode sample.

In situ analysis of electrochemical reactions at a lead surface in sulfuric acid solution

Abstract An Atomic Force Microscope (AFM) was used to analyze the surface of a lead plate when the reaction corresponding to what occurs at the negative electrode of a lead acid battery, Pb+SO 4 2− ⇄PbSO 4 +2e, took place. At the beginning, when the lead plate, on the surface of which lead oxide already existed, was in contact with sulfuric acid, lead sulfate crystals were formed and gradually grew with time without applying any potential. In situ AFM observation of the formation and the growth of lead sulfate at the potential corresponding to the discharge reaction was attempted. And it was possible along with the reduction of the lead sulfate, which formed in discharging process. The results also showed that the chemically formed lead sulfate crystals were clearly different in appearance from the electrochemically formed ones. These results were also applied to a study of the mechanism of the sulfation process that often occurs in the active material of the negative electrode of a lead acid battery.

Study of charge acceptance for the lead-acid battery through in situ EC-AFM observation — influence of the open-circuit standing time on the negative electrode

An influence of the open-circuit standing time after oxidation of the lead electrode was investigated for understanding charge acceptance of the negative electrode of a lead-acid battery. It was confirmed by a potentiostatic transient experiment that charge acceptance of the lead electrode heavily depended on the standing time before charging, and charge acceptance decreased if the standing time was longer. This tendency was conspicuous at the initial period of the reduction process in particular. When the behavior of the lead electrode surface during the open-circuit standing after oxidation was observed by in situ electrochemical atomic force microscope (EC-AFM), it was found that some lead sulfate crystals gradually grow with time to bigger crystals having a smooth surface, retaining basically its morphology of previous one. We have concluded that the standing time dependence of the charge acceptance is caused by this behavior of the crystals. Also, it was confirmed that charge acceptance of a valve regulated lead-acid (VRLA) battery with different standing times can be explained by result of the potentiostatic transient test and the EC-AFM observation.

Enhanced diffusion of surface atoms at metal/electrolyte interface under potential control

We have investigated the surface self-diffusion coefficient (DS) on Ag(100) in aqueous 50 mM H2SO4 solution using electrochemical atomic force microscopy (ECAFM). The results were compared with those observed on Au(100). The DS values of Ag(100) increased exponentially with potential between −50 and 350 mV (vs. normal hydrogen electrode, NHE). From the DS–E relationship, we propose that the activation energy of surface diffusion decreases because of the surface excess charge. We also found that the DS values on Ag(100) were a hundred times larger than those on Au(100) within the potential range from 150 to 350 mV (vs. NHE).

Comparative kinetic study of Cd diffusion into Au(100) and Ag(100) during electrodeposition

The kinetics of thin alloy film formation and growth during Cd electrodeposition on Au(100) and Ag(100) has been studied at room temperature in 50 mM H2SO4 + 1 mM CdSO4 solution within the Cd underpotential range from − 0.3 to − 0.45 V. Electrochemical experiments and atomic force microscopy studies have shown that the overall alloying process at the Cd2+/Au(100) and Cd2+/Ag(100) interfaces consisted of two processes: one very fast, which occurred within a few atomic layers and is characterized by a diffusion coefficient, D≈10−16 cm2 s−1, and another much slower, which is characterized by D≈10−19 cm2 s−1, suggesting a solid state diffusion process. Based on a new kinetic approach to the alloying process at the metal/electrolyte interface in the underpotential region, the differences in kinetic parameters are discussed.

Wetting of Au and Ag Particles on Monocrystalline Graphite Substrates

The wetting behavior of Au and Ag particles on a monocrystalline graphite substrate was investigated using the microscopic sessile drop method under a purified Ar atmosphere at 1300 K. The measured contact angles of the liquid Au and Ag on monocrystalline graphite substrates of (0001) face were 129° and 124°, respectively. It is believed that the interaction at the interface is dominated by the physical bonding (van der Waals interaction).

Intrinsic contact angle and contact interaction between liquid silver and solid graphite

The intrinsic contact angle between liquid metal and a solid substrate is important for interpreting the contact interaction between them. However, the apparent contact angle revealed in a wetting test does not always coincide with the intrinsic angle. In this study, the intrinsic contact angle of liquid silver on monocrystalline and polycrytalline graphite substrates was investigated by means of a sessile drop method under a 10%H2−Ar atmosphere at 1273 K. The intrinsic contact angles were estimated to be 124° for the monocrystalline (C surface) and 127° for the polycrystalline.

In situ atomic force microscopy observation on the decay of small islands on Au single crystal in acid solution

The decay process of small islands atop Au single crystals was observed by in situ atomic force microscopy in 50 mM H2SO4 aqueous solution at 1050 mV (vs normal hydrogen electrode, where no reaction current was detected). It was found that the area of islands on the terrace decreases linearly with time and the decay rate of the islands in solution is more than 30 times faster than that reported in air. We also found that the island in the same sulfuric solution at 1050 mV decayed more rapidly on Au(100) than on Au(111). We discuss the role of the excess charge at the metal/electrolyte interface, namely the electric double layer, which may affect the surface diffusion process of atoms atop the electrode in aqueous solution.

Decay of nano-islands on the surface of a Au(111) electrode in contact with sulfuric acid solution

Abstract The decay of nano-islands on the surface of a Au(111) electrode, in contact with 50 mM sulfuric acid aqueous solution under an applied potential in the range of 0.15–1.2 V, has been investigated using electrochemical atomic force microscopy (EC-AFM). The results are compared with those previously obtained for Au(100). With either orientation, it is found that the area of the top layer of multi-layered islands decreases linearly with time at any applied potential. It was also found that the decay rate, defined as the rate of decrease of the number of atoms in the top-layer of the islands per second, increases with the magnitude of the applied potential. Further, the decay rate of the top layer of the islands on Au(111) is almost the same as that on Au(100).

In-situ Electrochemical Atomic Force Microscopy on Au(100) in Room Temperature Ionic Liquid: 1-Ethyl-3-methyl-imidazolium Bis(trifluoromethanesulfonyl) Imide

A Au(100) single crystal surface in 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl) imide (EMImTFSI) was investigated by means of in-situ electrochemical atomic force microscopy (EC-AFM). High-resolution EC-AFM images with fourfold symmetry and an interatomic distance of 0.3 nm, corresponding to a bare and unreconstructed Au(100)-(1×1) structure, were observed in the potential range from -0.9 to 0.3 V vs Ag/Ag+ in 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMImBF4). The top layer of a multilayered island on Au(100) in EMImTFSI collapsed at a constant decay rate and the decay rate had a positive potential dependence when the Au(100) electrode was kept at any potential between -0.6 and 0.9 V vs Ag/Ag+ in EMImBF4. It was also found that the decay rate in EMImTFSI was higher than that in EMImBF4 in the potential region from 0.1 to 0.9 V vs Fc/Fc+ electrode.