Biljana Šljukić

An Overview of the Electrochemical Reduction of Oxygen at Carbon-Based Modified Electrodes

We present an overview of the electrochemical reduction of oxygen in water, focussing on carbon-based and modified carbon electrodes. This process is of importance for gas sensing, in fuel cells and in the electrosynthesis of hydrogen peroxide.

Carbon-supported Mo2C electrocatalysts for hydrogen evolution reaction

Molybdenum carbide (Mo2C) nanoparticles supported on two different carbon materials, carbon nanotubes and carbon xerogel, were prepared and characterised using X-ray diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, nitrogen sorption and X-ray photoelectron spectroscopy. The analyses showed similar composition (ca. 27 wt% of Mo2C) and crystallite size (22–28 nm) for the two samples, but significantly different morphologies and specific surface areas. These were subsequently tested as electrocatalysts for hydrogen evolution reaction (HER) in acid media. Using linear scan voltammetry and electrochemical impedance measurements the main reaction parameters were determined, including Tafel slope, charge transfer coefficient and exchange current density. Capacitance properties were examined and correlated with the electrocatalysts activity for HER. The stability of the two materials was also investigated and proved to be very good.

Physics of Electrolytic Gas Evolution

A brief analysis of the physics and effects of electrolytic gas evolution is presented. Aspects considered include bubble nucleation, growth and detachment; enhancement of mass and heat transfer; and decrease of apparent electrical conductivity of bubble containing electrolytes. This analysis is mainly oriented to hydrogen-/oxygen-evolving electrodes.

Electrochemical detection of arsenic on a gold nanoparticle array

The detection of As(III) was investigated on a gold nanoparticle array. At the first stage, gold nanoparticles were synthesized on glassy carbon microspheres. The resulting hybrid material was characterized by SEM and the sizes of the nanoparticles were found to be in the range 20–200 nm. At the second stage, glassy carbon microspheres decorated with Au nanoparticles were abrasively attached to the surface of a basal-plane pyrolytic electrode. The resulting gold nanoarray was characterized by the reduction of surface gold oxides. Furthermore, it was found to have good characteristics for the sensing of arsenic via anodic stripping voltammetry with a limit of detection of 0.8 μM and a sensitivity of 0.91 C M−1.

Molybdenum Carbide Nanoparticles on Carbon Nanotubes and Carbon Xerogel: Low-Cost Cathodes for Hydrogen Production by Alkaline Water Electrolysis.

Low-cost molybdenum carbide (Mo2 C) nanoparticles supported on carbon nanotubes (CNTs) and on carbon xerogel (CXG) were prepared and their activity for the hydrogen evolution reaction (HER) was evaluated in 8 m KOH aqueous electrolyte at 25-85 °C. Measurements of the HER by linear scan voltammetry allowed us to determine Tafel slopes of 71 and 74 mV dec(-1) at 25 °C for Mo2 C/CNT and Mo2 C/CXG, respectively. Stability tests were also performed, which showed the steady performance of the two electrocatalysts. Moreover, the HER kinetics at Mo2 C/CNT was enhanced significantly after the long-term stability tests. The specific activity of both materials was high, and a higher stability was obtained for the activated Mo2 C/CNT (40 A g(-1) at -0.40 V vs. the reversible hydrogen electrode).

The thermodynamics of sequestration of toxic copper(II) metal ion pollutants from aqueous media by L-cysteine methyl ester modified glassy carbon spheres

L-Cysteine methyl ester modified glassy carbon powder (CysOMe-GC) has been shown to have potential for the removal of toxic heavy metal ions, such as copper(II), cadmium(II) and arsenic(III), from “real” water samples. To develop this material for environmental applications we must develop an understanding of the thermodynamic parameters controlling the uptake of metal ions by the modified carbon powder. Here we characterise the material using X-ray photoelectron spectroscopy (XPS), before investigating the effect of varying the solution pH, the concentration of copper(II) ions and the mass of CysOMe-GC powder added to the solution using square wave voltammetry (SWV). This data allows us to understand the thermodynamics controlling the copper(II) ion uptake and elucidate that the adsorption of copper(II) onto the CysOMe modified surface is controlled by a Freundlich isotherm.

Disposable manganese oxide screen printed electrodes for electroanalytical sensing

Manganese dioxide screen printed graphite electrodes for electro-analytical sensing purposes have been fabricated. The prepared sensors exhibit attractive performances as electro-catalysts for the sensing of nitrite ions and ascorbic acid with detection limits comparable or lower than those obtainable with other electrochemical sensors. The manganese dioxide platforms are also explored towards the electrochemical reduction of oxygen which has importance in electrochemical energy storage. While the platforms are found to operate by a four-electron reduction producing water (rather than hydrogen peroxide which is undesirable), a large overpotential, compared to other possible electrode compositions reported in the literature, is required. Nevertheless, the promising results towards the electro-analytical sensing of nitrite ions and ascorbic acid and also given the disposable nature and scale of economies make these electrodes useful as sensor platforms.

The search for stable and efficient sonoelectrocatalysts for oxygen reduction and hydrogen peroxide formation: azobenzene and derivatives

We report the electrochemical reduction of oxygen using glassy carbon electrodes modified with azobenzene, hydroazobenzene, or fast black K salt (2,5-dimethoxy-4-[(4-nitrophenyl)azo] benzenediazonium tetrachlorozincate). The performance of the electrodes under ultrasound was explored. At the highest intensity (87 W cm−2) with one hour continuous insonation, voltammetric signals for azobenzene decreased by 7%, hydroazobenzene by 9% and fast black K by 18%. The catalytic rate constants of the immobilised species towards oxygen reduction were assessed via cyclic, rotating disc and sono-voltammetry. The rate constants were found to be 6.1 × 103 M−1 s−1 for azobenzene, 7.4 × 103 M−1 s−1 for hydroazobenzene and 10.4 × 103 M−1 s−1 for fast black K. These values suggest the use of these modified electrodes as practical hydrogen peroxide generators.

Anion- or Cation-Exchange Membranes for NaBH4/H2O2 Fuel Cells?

Direct borohydride fuel cells (DBFC), which operate on sodium borohydride (NaBH4) as the fuel, and hydrogen peroxide (H2O2) as the oxidant, are receiving increasing attention. This is due to their promising use as power sources for space and underwater applications, where air is not available and gas storage poses obvious problems. One key factor to improve the performance of DBFCs concerns the type of separator used. Both anion- and cation-exchange membranes may be considered as potential separators for DBFC. In the present paper, the effect of the membrane type on the performance of laboratory NaBH4/H2O2 fuel cells using Pt electrodes is studied at room temperature. Two commercial ion-exchange membranes from Membranes International Inc., an anion-exchange membrane (AMI-7001S) and a cation-exchange membrane (CMI-7000S), are tested as ionic separators for the DBFC. The membranes are compared directly by the observation and analysis of the corresponding DBFC’s performance. Cell polarization, power density, stability, and durability tests are used in the membranes’ evaluation. Energy densities and specific capacities are estimated. Most tests conducted, clearly indicate a superior performance of the cation-exchange membranes over the anion-exchange membrane. The two membranes are also compared with several other previously tested commercial membranes. For long term cell operation, these membranes seem to outperform the stability of the benchmark Nafion membranes but further studies are still required to improve their instantaneous power load.

Hydrogen peroxide sensing at MnO2/carbonized nanostructured polyaniline electrode

Manganese dioxide modified carbonized nanostructured polyaniline (MnO2/Carb-nanoPANI) was prepared via a novel hydrothermal procedure. The synthesized material was characterized using XRD, SEM and TG-DTA analysis. Furthermore, MnO2/Carb-nanoPANI was examined as electrode material for potential application in the field of electroanalysis. It showed a high electrocatalytic activity for the sensing of hydrogen peroxide in an aqueous media.