This headline article focuses on the oxygen reduction reaction (ORR), cathodic reaction of fuel cells, on well-defined high index planes of Pt and Pd for the elucidation of factors enhancing the activity for the ORR. The surfaces with (111) terrace edge have high activity for the ORR on Pt electrodes. Pt(331) = 3(111)-(111) gives the highest activity for the ORR in the stepped surfaces of Pt. Incorporation of one kink atom among eleven step atoms further enhances the activity of Pt(331). On single crystal electrodes of Pd, however, terrace edge deactivates the ORR and wide (100) terrace enhances the activity. Effects of Pt oxides (PtOH and PtO) on the ORR have been examined using vibrational spectroscopy. Infrared reflection absorption spectroscopy (IRAS) shows that PtOH prevents the ORR on Pt electrodes. Nanoparticle surface enhanced Raman spectroscopy (NPSERS) indicates that PtO also blocks the ORR on Pt(100) of which ORR activity is the lowest. The ORR activity of Pt is also enhanced by the modification of the surfaces by amines with long alkyl chains such as octylamine (OA) and amine with pyrene ring (PA). Modification by OA/PA increases the ORR activity on n(111)-(111) surfaces of Pt with terrace atomic rows more than 7. The activity of flat Pt(111) is enhanced most remarkably by OA/PA. However, the ORR of Pt(100), of which surface is also composed of flat terrace, is deactivated significantly.
Structural effects on the activity for the oxygen reduction reaction (ORR) have been studied on single crystal electrodes of Pt modified with six aromatic organic molecules (AOMs). The AOMs examined affect the ORR activity slightly. However, the activity of the sites uncovered by AOMs increases after the modification: the ORR activity of uncovered Pt(111) area after the modification of phthalocyanine is 2.5 times as high as that of bare Pt(111). t-BuTAP and iron (II) phthalocyanine also enhance the ORR on Pt(997). These facts show that adsorbed AOMs can enhance the ORR activity of the uncovered active sites on Pt electrodes.
The electrochemical behavior and specific adsorption of an ionic liquid, 1-butyl-3-methylimidazolium iodide, on a Au(111) electrode surface were investigated via voltammetric analyses, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). The electrochemical potential window and the reductive desorption of I adatoms were evaluated using voltammetric techniques. The XPS and STM results supported the specific adsorption of I adatoms on Au(111). Furthermore, high-resolution STM images revealed the formation of characteristic nanostructured rings consisting of imidazolium cations on I adatoms for the first time.
For conventional electrocatalysts for the nitrate reduction reaction, Pt substrates have been used and modified with Sn adatoms to form Sn/Pt interfaces, which work as highly efficient catalytic active sites. In this work, Pt nanoparticles were deposited on fluorine-doped tin oxide (FTO) substrates by cathodic arc plasma deposition (APD) to produce Pt/Sn interfaces. As expected, APD–Pt/FTO showed the electrocatalytic nitrate reduction activity in acidic media and the Pt/Sn interfaces worked as the catalytic active site. Our interfacial design concept and synthetic approach will offer new catalyst development strategies at nanoscale.
In order to clarify the nanoparticles formation process of the metal sputtering onto ionic liquid, which is the facile method to prepare metal nanoparticles, the vacuum-liquid interface was captured by a camcorder while conducting gold sputtering. Because of intense color of Au clusters, move of Au clusters by convection of ionic liquid was clearly observed. The convection is a dominant factor in a transport of the resulting Au nanoparticles from the surface to interior of the ionic liquid.
Tetrahydrofuran solutions containing alkylmagnesium halides (Grignard reagent) provide effective halo-magnesium species on magnesium surface, and deliver reversible magnesium deposition–dissolution without overpotential by a passivation layer on magnesium surface. For this study, the reaction between passivation layer and halogen-containing species has been reproduced by the immersion of magnesium plate in alkylbromides. The immersion in various alkylbromides, especially 1,6-dibromohexane, reduced the passivation overpotential significantly. The treated surface is unstable and replaced by soaking in tetrahydrofuran. XPS reveals that a thick oxide layer remains, while multiple bromine species distribute in the surface layer after immersion in 1,6-dibromohexane.
Electrodeposition of cadmium (Cd) was investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) using CdCl2 as the source of Cd species in the presence of excess chloride ion. Raman spectroscopy and potentiometric measurement suggested formation of a cadmium tetrachlocomplex, [CdCl4]2−, in BMPCl/BMPTFSA. Cyclic voltammetry showed the possibility of electrochemical reduction from [CdCl4]2− to Cd(0) with interesting unusual electrochemical behavior, probably derived from the potential-dependent electric double layer structure typical to the ionic liquid. Electrodeposition of Cd was performed by potentiostatic electrolysis and the deposits were characterized by X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy.
The characteristics for 3,4,9,10-perylenetetracarboxylic-bisbenzimidazole (PTCBI, an n-type semiconductor) and 29H,31H-phthalocyanine (H2Pc, a p-type semiconductor) as organic p/n bilayer and bulk heterojunction (BHJ) photoelectrodes were studied for the photooxidation of thiol. Based on the analysis in their absorption spectra, a new absorption band in the longer wavelength (λ > 800 nm) for both bilayer and co-deposited photoelectrode suggested a formation of charge transfer complex. A photoanodic current was observed at λ ∼ 880 nm for the both bilayer and co-deposited electrodes, while no absorption and photocurrent for single layers of PTCBI and H2Pc. By assuming the Langmuir adsorption equilibrium at the solid/water interface, the kinetic parameters for the photoanodic current of thiol was analyzed for the longer wavelength of irradiation (λ ∼ 900 nm), and it was indicated that the rate of oxidation in the co-deposited was higher than that of the bilayer due efficient charge separation in the charge transfer complex.
We investigated the oxygen reduction reaction (ORR) activity of several monolayer-thick Co-deposited Pt(111) model catalyst surfaces (n-monolayer(ML)-Co/Pt(111): n = 0.13–2.0) in 0.1 M KOH. Cobalt layer thicknesses of less than 0.25 ML enhance the ORR activity in contrast to clean Pt(111) surfaces. The maximum activity enhancement factor of 0.25 ML-Co/Pt(111) was ca. 1.7. The rotating ring–disk electrode measurements of n ML-Co/Pt(111) surfaces show an increase in the generation rates of HO2− at ∼0.8 V vs. reversible hydrogen electrode (RHE). This suggests that the deposited Co, in form of (hydro)oxide in 0.1 M KOH, functions as oxidation site and activates the Pt sites for the ORR. The results demonstrate that tuning the Pt and Co alloy compositions at the topmost surface is the key in developing highly active Pt-based alloy catalysts for the ORR, not only in acid but also in alkaline electrolytes.
An amorphous 80LiCoO2·20Li2SO4 (mol%; Li1.2Co0.8S0.2O2.4) thin film electrode is prepared by pulsed laser deposition (PLD). Although the Li1.2Co0.8S0.2O2.4 electrode material has been synthesized via mechanochemistry and includes a cubic LiCoO2 crystalline phase, amorphous Li1.2Co0.8S0.2O2.4 (without the cubic phase active material) is successfully obtained using the PLD method. The atomic ratio of Li/Co in the obtained film as determined by ICP-AES is identical to that of the milled powder. X-ray photoelectron spectroscopy reveals that the thin film mainly contains Co3+ and SO42−. The amorphous Li1.2Co0.8S0.2O2.4 thin film electrode is directly deposited on the 90Li3BO3·10Li2SO4 (mol%) oxide electrolyte to form closely-attached electrode-electrolyte interface. The fabricated all-solid-state cells shows a higher discharge capacity than the cell fabricated using the electrode derived from the mechanochemically prepared Li1.2Co0.8S0.2O2.4, which partially includes electrochemically-inactive cubic LiCoO2 nanoparticles. It is noted that complete amorphization of Li1.2Co0.8S0.2O2.4 is effective in increasing reversible capacities.
Deposition and dissolution of copper were investigated by electrochemical quartz crystal microbalance (EQCM) technique using a Pt-coated quartz crystal resonator in contact with a separator in order to simulate the configuration of practical batteries. The admittance analysis indicated the quartz crystal resonated even in contact with a glass separator impregnated with the electrolyte under pressure. The mass change estimated from the change in the resonance frequency during deposition and dissolution of Cu on a Pt-coated resonator in contact with the separator was linearly related to the passed electric charge and closed to that without the separator, suggesting the mass monitoring according to Saurbrey equation is possible in the presence of the separator on the electrode. The highly sensitive mass monitoring with an overtone resonance was also found to be possible in the presence of the separator using the third harmonic vibration mode.
Lithium bis(oxalato)borate (LiBOB) has attracted much attention as an alternative lithium salt in lithium-ion batteries. At positive electrodes, LiBOB is considered to form a protective surface-film. However, its protection-ability is not understood in detail. In this study, the surface-film formation behavior and the protection-ability on a LiMn2O4 thin-film electrode in LiBOB/propylene carbonate (PC) was investigated using spectroscopic methods and a redox reaction of ferrocene. The surface film formed in LiBOB/PC at 55°C is much thinner than that in LiClO4/PC. The redox reaction of ferrocene on cycled LiMn2O4 showed that the electronic passivation did not occur during cycling even at 55°C. On the basis of the results, a degradation behavior of LiMn2O4 was discussed.
Electrochemical behavior of Sn was investigated on a GC electrode in an aprotic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing BMPCl and SnCl2 at 298 K. SnCl2 was found to dissolve as [SnCl4]2− by potentiometry. The formal potential of [SnCl4]2−/Sn was −1.67 V vs. Ag|Ag(I) (−1.24 V vs. ferrocene|ferrocenium), which was more negative than that in BMPTFSA without chloride ions. Electrodeposition of Sn was possible by reduction of [SnCl4]2− in the ionic liquid. The dissociation of [SnCl4]2− was suggested from the diffusion coefficient of [SnCl4]2−. Tubular deposits with an inner diameter of 1–1.5 µm were obtained in addition to granular deposits by potentiostatic cathodic reduction at −2.0 V.
Acid-treated Ketjen Black (a-KB) carbon supports were prepared to investigate how oxidation of the carbon surface influences La0.6Sr0.4MnO3 (LSM) nanoparticle distribution, and conjugation to the carbon support. 30 wt.% LSM-loaded a-KB (LSM/a-KB) materials were prepared as air-electrode catalysts for rechargeable lithium-air batteries (LABs). a-KB exhibited a significant degree of O-containing (C-O, COO) surface functional groups, which resulted in the formation of smaller LSM nanoparticles and enhanced homogeneity over the carbon support when compared with the pristine KB support. Consequently, C-O-Mn bonds were formed, which increased the Mn oxidation state, and concomitantly enhanced conjugation resulting in improved catalytic activity. Additionally, the overpotential was reduced during charging (Li2O2 decomposition). Furthermore, LSM/a-KB enhanced the cyclability of the LAB test cell. Scanning electron microscopy observations revealed that LSM/a-KB efficiently decomposed the Li2O2 deposition layer, even after the 15th charge cycle when compared with LSM/KB. The LSM/a-KB air-electrode exhibited a more homogeneous and smaller-sized (and/or amorphous) Li2O2 deposition after discharging. Therefore, the oxidation of the carbon surface, resulting in enhanced LSM nanoparticle distribution on, and conjugation to, the a-KB surface, influences the homogeneity of the Li2O2 deposition onto the support during the discharge process leading to its facile decomposition during the following charge process.