Design of 3D Carbon Nanotube Monoliths for Potential-Controlled Adsorption

The design of 3D monoliths provides a promising opportunity to scale the unique properties of singular carbon nanotubes to a macroscopic level. However, the synthesis of carbon nanotube monoliths is often characterized by complex procedures and additives impairing the later macroscopic properties. Here, we present a simple and efficient synthesis protocol leading to the formation of free-standing, stable, and highly conductive 3D carbon nanotube monoliths for later application in potential-controlled adsorption in aqueous systems. We synthesized monoliths displaying high tensile strength, excellent conductivity (up to 140 S m−1), and a large specific surface area (up to 177 m2 g−1). The resulting monoliths were studied as novel electrode materials for the reversible electrosorption of maleic acid. The process principle was investigated using chronoamperometry and cyclic voltammetry in a two-electrode setup. A stable electrochemical behavior was observed, and the synthesized monoliths displayed capacitive and faradaic current responses. At moderate applied overpotentials (± 500 mV vs. open circuit potential), the monolithic electrodes showed a high loading capacity (~20 µmol g−1) and reversible potential-triggered release of the analyte. Our results demonstrate that carbon nanotube monoliths can be used as novel electrode material to control the adsorption of small organic molecules onto charged surfaces.

On the relationship between limiting current and mass-transport impedance at zero frequency

The zero-frequency reversible mass transport impedance (measured at the reversible potential) is shown to be simply related to the limiting current density (far from the reversible potential). For mass transport of a single species exchanging n electrons with the electrode, their product is RT/nF . For the two-species case, O + ne -> R, the same product is found, provided the harmonic mean of the anodic and cathodiclimiting currents is used. This applies for most of the mass-transport situations found in electrochemistry, including with forced convection or for electrodes without uniform accessibility.

Limitations in Electrochemical Determination of Mass-Transport Parameters: Implications for Quantification of Electrode Kinetics Using Data Optimisation Methods

Voltammetric quantification of the electrode kinetics for the quasi-reversible reaction requires detailed experiment–theory comparisons. Ideally, predicted data derived from the theoretical model are fitted to the experimental data by adjusting the reversible potential (E0), heterogeneous electron transfer rate constant at E0 (k0), and charge transfer coefficient α, with mass-transport and other parameters exactly known. However, parameters relevant to mass transport that include electrode area (A), diffusion coefficient (D), and concentration (c), are usually subject to some uncertainty. Herein, we examine the consequences of having different combinations of errors present in A, D, and c in the estimation of E0, k0, and α on the basis of the a.c. (alternating current) voltammetric experiment–theory comparisons facilitated by the use of a computer-assisted parameter optimisation algorithm. In most cases, experimentally reasonable errors (<10 %) in the mass-transport parameters do not introduce significant errors in recovered E0, k0, and α values. However, a pernicious situation may emerge when a slight overestimation of A, D or c is included in the model and results in erroneous identification of a reversible redox process as a quasi-reversible one with a report of apparently quantifiable kinetic parameters k0 and α.

Preparation and Utilization of Alumina Oxide Membranes for Sensor Devices

This work describes preparation process of free-standing alumina membranes used in sensor devices for separation or purification (increased selectivity, and sensitivity) purposes. Nanoporous alumina membranes were prepared using anodic oxidation of aluminium foil in two types of acidic electrolytes and characterized using scanning electron microscopy. Membranes with pore diameters of 90 nm and 30 nm and thicknesses of 115 µm and 163 µm respectively were obtained. Fabrication of membranes with different post-treatment was also done. In this post-treatment process, etching of non-anodized aluminium and opening of barrier layer were replaced with application of reversible potential with equal magnitude as anodization voltage.