Kok Siong Siow

Determination of ascorbic acid in a mixture of ascorbic acid and uric acid at a chemically modified electrode

Abstract The electrochemical behavior of ascorbic acid (AA) at a 3,4-dihydroxybenzaldehyde (DHB) modified glassy carbon electrode has been examined by voltammetry and amperometry. The results indicated that the modified electrode reduces the overpotential of AA oxidation, while the oxidation process of uric acid (UA) is little affected. The anodic peak due to the oxidation of AA was pH dependent and occurred at 0.15 V (vs. SCE) at pH 7.4, which is at least 250 mV more negative than that at a bare glassy carbon electrode. Rotating disk electrode experiments revealed that the oxidation current is solely controlled by mass-transport process in solution. AA was detected amperometrically in a flow injection system at the modified electrode with excellent sensitivity. Calibration curves were linear over the concentration range of 0.7 gmM to 1.0 mM with a pH 7.4 phosphate buffer solution as the carrier. The detection limit was 0.30 μM. Selectivity was illustrated by the analysis of ascorbic acid in the presence of uric acid. The modified electrode also shows good anti-fouling properties towards surface active materials.

Low‐Temperature Synthesized LiV3 O 8 as a Cathode Material for Rechargeable Lithum Batteries

LiV{sub 3}O{sub 8} was synthesized at 300--350 C by reaction of Li{sub 2}CO{sub 3} and NH{sub 4}VO{sub 3} which were blended in an aqueous solution. The product was termed LT LiV{sub 3}O{sub 8} and its preparation reaction was studied by thermographic analyses-depolarization thermocurrent. LT LiV{sub 3}O{sub 8} showed a high discharge capacity of 300 mAh/g active material, and maintained a capacity of 275 mAh/g after 15 cycles. Its electrochemical behavior as a cathode material for rechargeable lithium batteries was studied by galvanostatic charge-discharge and cyclic voltammetry. The structure and morphology of LT LiV{sub 3}O{sub 8} were characterized by infrared, X-ray diffraction, scanning electron microscopy, and their relationships with good electrochemical performance were studied.

Synthesis and characterization of the hollandite-type MnO2 as a cathode material in lithium batteries

Hollandite-type MnO2 (HMDO) with hydronium as tunnel counter ion was synthesized by oxidation of MnSO4 with ozone in a concentrated sulfuric acid solution.Its composition and structure were analyzed and characterized by inductive coupled plasma atomic emission spectrometry (ICP-AES), titration, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM).Hydronium can be exchanged with hydrated lithium ion and the exchanged HMDO can be dehydrated at 300°C. Cyclic voltammetry and galvanostatic charge–discharge study showed that HMDO with lithium ion exchanged and dehydrated had potential to be used as a cathode material in lithium secondary batteries.

Catalytic-adsorptive stripping voltammetric determination of molybdenum in plant foodstuffs.

In acetate buffer solution (pH 3.5) containing oxine and chlorate, ultratrace amounts of molybdenum can be determined after adsorptive accumulation of the Mo(VI)-oxine complex on a hanging mercury drop electrode, coupled with the catalytic effect on the reduction of chlorate. Under optimized conditions, the catalytic-adsorptive stripping voltammetric procedure gives excellent selectivity and an extremely low detection limit of 1.7 pM molybdenum (60 s accumulation). The stripping peak current increases linearly with molybdenum concentration between 10 pM and 5.0 nM. The procedure is applied to determine traces of molybdenum in plant foodstuffs.

Self-assembled conducting polymer monolayers of poly(3-octylthiophene) on gold electrodes

Conducting polymer monolayers of poly(3-octylthiophene) (POT) on gold electrodes were prepared by a self-assembly technique. Infrared spectroscopic, ellipsometric and contact angle goniometric measurements indicate a highly organized and densely packed supermolecular structure on the gold surface. Upon chemical doping in an aqueous solution, the monolayer shows a dramatic change in voltammetric responses towards the redox probe in the solution. The self-assembled conducting polymer films exhibit better thermal stability in wet environments, suggesting that multi-anchoring effects of the polymer improve the integrated adhesion and the binding stability of the self-assembled POT monolayers. The monolayers were also characterized by electrochemical techniques.

Mechanical properties and ionic conductivities of plasticized polymer electrolytes based on ABS/PMMA blends

Abstract A new plasticized polymer electrolyte composed of a blend of poly(acrylonitrile–butadiene–styrene) (ABS) and poly(methyl methacrylate) (PMMA) as a host polymer, mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a plasticizer, and LiClO 4 as a salt was studied. Owing to the different miscibility of ABS and PMMA with the plasticizer, phase separation takes place in the electrolyte system as revealed from SEM studies. The ionic conductivity of the electrolytes decreased with the increasing ABS/PMMA ratio and increased with the increasing plasticizer content at the LiClO 4 content of 15%. Such conductivity behaviour could be explained in terms of the morphology and the thermal characteristics of the electrolytes. The mechanical property of the electrolytes was much improved compared with that of the electrolyte system containing pure PMMA, plasticizer and lithium salt.

Tin-based oxide anode for lithium-ion batteries with low irreversible capacity

Abstract Several kinds of tin-oxide composites have been synthesised that can replace the carbon-based lithium intercalation materials as the anode of lithium-ion batteries. The electrochemical behaviour of these materials is investigated in ethylene carbonate and dimethyl carbonate (EC–DMC) based solutions containing lithium salts, LiClO 4 . The lithium intercalation capacities in the tin-oxide materials prepared from the same raw materials and by the same method depend on the heat-treatment temperature. A higher charge capacity and lower irreversible capacity material based on Sn 2 P 2 P 7 is obtained by preparation at high temperature (700°C). The element manganese is added to these materials to obtain a material with lower irreversible capacity. This material (SnMn 0.5 PO 4 ), prepared by the same method, exhibits a lower irreversible capacity. X-ray diffraction shows that this material has an amorphous structure.

Ionic conductivity and electrochemical characterization of novel interpenetrating polymer network electrolytes

Abstract We report here our investigation into the ionic conductivity and other electrochemical properties of this micro-phase separation type solid-state electrolyte (SPE). The novel polymer electrolyte has been obtained by swelling an interpenetrating polymer network (IPN) with liquid electrolyte solutions of inorganic lithium salts dissolved in a plasticiser or mixture of plasticizers such as ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL) and dimethyl carbonate (DMC). The interpenetrating networks are prepared by sequential interpenetration of cross-linked methoxyoligo(oxyethylene)methacrylate (Cr-MOE n M, where n represents number of unit –CH 2 CH 2 O–) and cross-linked poly(methylmethacrylate) (PMMA). The IPN electrolytes exhibit conductivities in the range of 4.5×10 −4 to 1.4×10 −3 S cm −1 at ambient temperature. Cyclic voltammetry of the IPN electrolytes on stainless steel electrode shows electrochemical stability windows of 5.0, 4.2 and 4.0 V vs. Li + /Li for IPN electrolytes with 1 M LiClO 4 /EC-DMC (1:1 by volume), 1 M LiBF 4 /γ-BL and 1 M LiSO 3 CF 3 /EC-PC (1:1 by volume), respectively. The impedance of the Li/electrolyte interface for the IPN electrolyte with 1 M LiClO 4 /EC-DMC under open circuit conditions is found to increase rapidly over the first 30 h and then level off, in contrast to the case for the Cr-MOE n M network electrolyte (i.e., a network without PMMA) where the impedance increases continuously with time.

A.C. impedance study on the interface of lithium and polymer electrolyte based on lithium-N(4-sulfophenyl) maleimide

Abstract The properties of the interface between lithium electrode and polymer electrolyte, poly[lithium- N (4-sulfophenyl) maleimide-co-methoxy oligo(ethyleneoxy) methacrylate] with single lithium ionic conduction, have been investigated by A.C. impedance technique. The impedance spectrum for stainless steel electrode shows a semicircle and a straight line, while that for lithium electrode has two separate imperfect semicircles and a spur. This difference suggests that a passivation layer has formed between the interface of the Li/polymer electrolyte. The dependence of the impedance spectra on storage time and temperature has been interpreted using an assumed equivalent circuit. It is revealed that the passivation layer is composed of a combination of solid inorganic and polymeric salts resulting from the reactions of lithium metal with the residual moisture and the polymer electrolyte, and has the characteristics of ionic conductive behavior. It grows rapidly on the surface of the lithium electrode during the initial period after the assembly of cells. The growth of the passivation layer will affect the nature of the polymer electrolyte and lead to an increase in the bulk resistance of the polymer electrolyte. Using the impedance spectra of cells with the stainless steel electrode, the dielectric property of the polymer electrolyte has also been studied.

Determination of trace amounts of iron by catalytic-adsorptive stripping voltammetry.

A highly sensitive and selective voltammetric procedure is described for the determination of trace amounts of iron. The procedure is based on the adsorptive collection of an iron-thiocyanate-nitric oxide complex on a hanging mercury drop electrode. The adsorbed complex catalyzes the reduction of nitrite in solution, which gives a detection limit of 40 ppt iron (30 s accumulation). The stripping current increases linearly with iron concentration up to 80 ppb. The relative standard deviations are 4.2% and 1.6% at 0.5 ppb and 40 ppb respectively. Most of the common ions, except cobalt, do not interfere with the determination of iron. The procedure is applied to determine iron in biological samples, natural waters and analytical-grade chemicals.