Kenichi Honda

Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders

WE report here the photoelectrocatalytic reduction of carbon dioxide to form organic compounds such as formic acid, formaldeyde, methyl alcohol and methane, in the presence of photosensitive semiconductor powders suspended in water as catalysts. Photocatalytic reaction kinetics were elucidated by reference to the theory of charge transfer at photoexcited semiconductors.

Recent topics in photoelectrochemistry: achievements and future prospects

The tremendous amount of research that has been carried out in the two closely related fields of semiconductor photoelectrochemistry and photocatalysis during the past three decades continues to provide fundamental insights and practical applications. The present review paper will attempt to describe some of the progress and resulting achievements in these two areas and to briefly discuss the future prospects. In order to provide a focal point, we will highlight work carried out in Japan over the last 5 years. However, we will try as much as possible to put this work into a global and historical context by tracing some of the key developments that have occurred outside this relatively narrow scope. We should note at the outset that we have made no attempt to cover the underlying theory or physics of photoelectrochemistry. Several excellent reviews have appeared during this same time period that cover fundamental and general aspects of photoelectrochemistry and photocatalysis.

Charge-controllable polypyrrole/polyelectrolyte composite membranes: Part II. Effect of incorporated anion size on the electrochemical oxidation-reduction process

Abstract Polypyrrole (PPy) films with various electrolyte anions were prepared by electropolymerization of pyrrole (py)in water. While the anodic doping process in the electropolymerization of py was scarcely affected by the size of the electrolyte anion, the oxidation-reduction (doping-undoping) process in the resulting PPy was influenced largely by the size of the incorporated anion (the dopant anion). Especially, an anionic polyelectrolyte (PE, an immobilized polymer dopant) incorporated in PPy was not released from the PPy matrix even when PPy was reduced electrochemically. The electroneutrality of PPy/PE (PE-incorporated PPy) was conserved by penetration of the electrolyte cation into the PPy matrix (pseudo-cathodic doping). The fixed charge polarity in PPy/PE was inverted reversibly between the positive and negative sign through the oxidation and the reduction of PPy. The PPy/PE composite acted as a “charge-controllable membrane”.

Suppression of Surface Dissolution of CdS Photoanode by Reducing Agents

The photoelectrochemical processes at CdS single crystal electrodes was studied, focusing mainly on the suppression of CdS dissolution by adding a variety of reducing agents differing widely in their redox potentials. The rotating ring-disk electrode technique served as a useful tool for the in situ determination of the dissolution rate of the electrode surface. Procedure and results are discussed. (WHK)

Electrochemical properties of Pt-modified nano-honeycomb diamond electrodes

The electrocatalytic behavior of boron-doped nanoporous honeycomb diamond films modified with Pt nanoparticles (10–150 nm) was examined with cyclic voltammetry (CV) and electrochemical impedance spectroscopy in acid solution. For two such films as well as an as-deposited film, the average number of exposed surface Pt atoms was estimated from CV to be 1.07(±0.05)×1015 cm−2 (real area, as estimated by SEM). These electrodes exhibited high electroactivity for hydrogen adsorption and oxidation of several alcohols. The current density (geometric basis) in the CV for methanol oxidation at a Pt-modified porous film with a pore diameter of 400 nm and a pore depth of 3 μm, was greatly enhanced, by a factor of 16, in comparison to the values obtained with a bulk Pt electrode. This enhancement is attributed to both the high surface area of the nano-honeycomb structure and the high electrocatalytic activity of Pt nanoparticles dispersed inside the pores. The electrocatalytic activities of the Pt-modified nano-honeycomb films were found to be dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols. Interestingly, ac impedance measurements have indicated a decrease in the penetration depths for a Pt-modified porous film with a pore diameter of 60 nm with increasing alcohol size, and an increase in the reaction resistances for ethanol oxidation with decreasing pore diameter.

Highly efficient quantum conversion at chlorophyll a–lecithin mixed monolayer coated electrodes

THE design of solar conversion systems based on the photosynthetic primary reactions has attracted much attention recently. For instance, photoelectrolysis at a chlorophyll (Chl) a-water aggregate coated platinum electrode as a photocathode has been studied by Fong et al.1–3, and simulation of the photoinduced electron transfer across the thylakoid membrane as well as of the biological ordered structure in a chloroplast has been attempted by Tien et al.4,5 using a Chl a-containing bilayer lipid membrane (BLM). We have already attempted to combine these different approaches6 by using a Chl a monolayer coated SnO2 transparent electrode as a photoanode, and a maximum photocurrent quantum efficiency of 12–16% was attained with Chl a-stearic acid mixed monolayer systems. In that case, however, a decrease of the quantum efficiency was observed at Chl a-stearic acid molar ratios <1.0; this might be due to the formation of pheophytin a and/or to a possible heterogeneity of the mixed monolayers. We describe here how we have overcome such problems by using a phospholipid instead of the fatty acid as a diluent for a Chl a monolayer. The hydrophobic interactions between Chl a and phospholipids have been well investigated both in vivo and in vitro, and the physical stability and photochemical activity of Chl a-synthetic lipid mixed layers were also confirmed in the BLM study7.

Charge-controllable poly pyrrole/poly electrolyte composite membranes: Part III. Electrochemical deionization system constructed by anion-exchangeable and cation-exchangeable polypyrrole electrodes

Abstract Chloride anion incorporated polypyrrole (PPy/Cl−) and poly(vinylsulfate) incorporated PPy (PPy/PVS−) electrodes were prepared by electrochemical polymerization of pyrrole in water. The PPy/Cl− and the PPy/PVS− had anion-exchange and cation-exchange abilities, respectively. Combination of these two PPy electrodes enabled electrochemical deionization from KCl aqueous solution and regeneration of the native PPy electrodes electrochemically. An efficient deionization was achieved without either decomposition of water on the PPy electrodes or self-discharge of the PPy. Concentrations of 10−3 mol 1−1 and 10−4 mol 1−1 KCl aqueous solutions were decreased to less than 10−4 mol 1−1 and 10−5 mol 1−1, respectively, by galvanostatic electrolysis with the PPy electrodes.

A novel type of polymer battery using a polypyrrole–polyanion composite anode

Polypyrrole incorporating an anionic polymer dopant was used as the anode of a polymer battery utilizing its pseudo-cathodic doping process.

Evidence for surface Ag+ complexes as the SERS-active sites on Ag electrodes

Abstract Evidence is given that SERS-active sites at Ag electrodes are associated with Ag + ions, forming sparingly soluble surface complexes with ligands such as pyridine molecules and halide ions. Such surface Ag + complexes contribute a factor of >800 to the overall (10 7 -fold) enhancement, possibly via a resonance Raman effect.

Impedance Characteristics of the Nanoporous Honeycomb Diamond Electrodes for Electrical Double-Layer Capacitor Applications

Nanoporous boron-doped diamond films with highly ordered pore structures and various pore dimensions, fabricated by oxygen plasma etching of polished polycrystalline diamond films through porous alumina masks, were characterized using electrochemical impedance measurements. The porous structures exhibited wide electrochemical potential windows (2.46-2.70 V), although somewhat less than that for the as-deposited films (3.04 V) because of surface damage due to the oxygen plasma treatment. A film of pore diam 400 nm and pore depth 3 μm exhibited a 400-fold increase in the capacitance in comparison to an as-deposited surface film. A film with 30 nm diam pores showed little enhancement in the capacitance at useful frequencies due to the high pore impedance. Impedance measurements carried out at −0.5 V vs. Ag/AgCl indicated a faradaic reaction, most likely hydrogen evolution. Although the dc current was small (ca. 3.0 μA cm−2 real area for the 60 nm honeycomb), it represents a pathway for charge leakage, limiting the negative potential limit for charge storage. However, similar limits were also observed for a representative acetylene black and activated carbon. The as-deposited diamond and nm honeycomb exhibited significantly higher effective potential limits for anodic charging due to the lack of surface oxidation current. This allows more charge to be stored per unit capacitance for diamond materials than for graphitic carbons. Due to the surface damage, the honeycomb sample did not store as much charge per unit capacitance as the as-deposited sample.