中村 将志

© 2020 Elsevier Ltd The oxidation of dopamine (DA) on a boron-doped diamond (BDD) electrode was investigated using in situ attenuated total reflection infrared spectroscopy (ATR-IR) and infrared reflection absorption spectroscopy (IRAS). Voltammogram showed multiple anodic/cathodic peaks for the oxidation/reduction of DA and its oxidized derivatives on the BDD electrode. The oxidation/reduction species on the surface and in solution were assigned using ATR-IR and IRAS, respectively. The anodic oxidation of DA promoted polymerization, and the polymerized DA (PDA) was continuously deposited on BDD without being reduced by potential cycles, which resulted in the irreversible behavior of the voltammogram. A decrease in the oxidation current of DA by potential cycles was due to the deposition of PDA. Some intermediate quinone species, such as dopaminequinone and dopaminechrome, were reversibly reduced to hydroquinone.

© 2018 Elsevier B.V. Many time-resolved measurements of electrochemical interface have been developed in conformity with the time scale of various transition. X-ray diffraction using synchrotron radiation is a powerful tool for structural determination of electrical double layer in real time. This short review describes structural dynamics of interfacial ions during the faraday and non-faraday processes in the time scale from microsecond to second.

The interfacial structure between aqueous electrolytes and the epitaxial graphene on a SiC(0001) electrode has been determined using X-ray diffraction. The electrolyte and electrode potential dependences are investigated, and it is found that the water bilayer is stabilized on the graphene surface in a similar fashion to icelike structure. There are no specific adsorbed ions and no layer formation of electrolyte ions at the Helmholtz plane, which differs from the double-layer structure found on metal electrodes remarkably. The layer spacing of the water bilayer depends on the electrode potential, indicating that water reorientation occurs. The applied electrode potential is strongly related to the potential drop across the interface induced by the electric dipole field of the bilayer water. A small double-layer current results from non-faradaic charge by the reorientation of the bilayer water.

The interfacial structure and activity for the oxygen reduction reaction (ORR) were investigated on a PtNi surface alloy on a Pt(111) electrode (PtNi/Pt(111)). The PtNi surface alloy was prepared by thermal annealing of Ni2+ modified on Pt(111) at 573-803 K. After optimizing the alloying temperature and the amount of added Ni, the ORR current density of PtNi/Pt(111) at 0.9 V (reversible hydrogen electrode) is enhanced 9.5 times compared with that of Pt(111), and the activity is decreased by 24% after 1000 potential cycles. The atomic composition and subsurface structure of PtNi/Pt(111) were determined by in situ infrared reflection-absorption spectroscopy and X-ray diffraction. The surface contains a (111)-oriented Pt-skin and the subsurface of the 2nd-5th layers of the PtNi alloy contains less than 11% Ni atoms. The layer spacings of the surface alloy layers are slightly expanded compared with those of bare Pt(111). Homogeneous alloying with a small amount of Ni in the subsurface layers achieves the high ORR activity and durability.

The effects of alkali meal cation on the surface oxidation and alcohol oxidation reactions on Au(111) have been investigated using surface X-ray diffraction and infrared spectroscopy. It is known that alkali metal cations strongly affect the alcohol oxidation reactions on Pt(111); however, the oxidation reactions on Au(111) do not depend on alkali metal cations. Infrared spectroscopy reveals the formation of adsorbed OH in LiOH and CsOH solutions below the second anodic peak at 1.3 V. This result indicates that surface oxidation processes do not depend on alkali metal cations below 1.3 V. The interfacial structure, including the outer layer, of an Au(111) electrode has been determined using X-ray diffraction in LiOH and CsOH. During the surface oxidation, the Au(111) surface in CsOH gets roughened more remarkably than that in LiOH above 1.3 V. Li+ has a protective effect against surface roughening. Thus, the cationic effect is weaker in the potential region lower than the second anodic peak, which does not affect the lifting of the surface reconstruction and the alcohol oxidation reactions.

The adsorption of hydroxide (OHad) on Pt has been studied on the low index planes of Pt using infrared reflection absorption spectroscopy (IRAS) in electrochemical environments. We discuss the correlation between the integrated band intensity of the bending mode of OH of Pt-OH (delta(PtOH)) and the charge density of the oxide formation of Pt. The band of dPtOH is observed around 1100 cm(-1), and the onset potential depends on the surface structure. The onset potential of dPtOH on Pt(110) and Pt(100) overlaps with the hydrogen adsorption/desorption potential region. The order of the integrated band intensity of delta(PtOH) at 0.9 V vs RHE is opposite to the order of the oxygen reduction reaction (ORR) activity. This finding supports that the OHad is one of the species deactivating the ORR.

Transitional structures of Cs+ at the outer Helmholtz plane (OHP) have been determined using time-resolved X-ray diffraction during the double-layer charging/discharging on the Ag(100) electrode in CsBr solution. At the double-layer potential region at which c(2 X 2)-Br is formed on Ag(100), the transient current comprises two exponential terms with different time scales: a rapid and a slow one are due to the dielectric polarization of water molecules and the transfer of Cs+, respectively. The slow term is composed of different dynamic processes of Cs+ during charging and discharging. When the potential is stepped in the positive direction, the coverage of Cs+ at the OHP decreases. In this step, the transient X-ray intensity at the (0 0 1) reflection, which is sensitive to the OHP structure, shows that Cs+ is released from the OHP according to exponential function of time. The decay of transient intensity of X-ray has a time scale similar to that of the current transient measurement. On the other hand, the accumulation process of Cs+ from the diffuse double layer to the OHP comprises two different kinetic processes after a potential step in the negative direction: a rapid one is the accumulation of Cs+ near the outer layer, and a slow one is the structural stabilization of the Cs+ layer.