Nanostructured carbon electrodes

In its conducting form, carbon has proven to be a versatile, robust and high performing electrode material in areas such as energy conversion, energy storage and even medical bionics. In our laboratories we have been interested in the fabrication and utilization of nanostructured electrodes based on more recently discovered forms of carbon. These include carbon nanotubes and graphene.

Nano-carbon electrodes for thermal energy harvesting

Thermogalvanic cells are capable of converting waste heat (generated as a by-product of almost all human activity) to electricity. These devices may alleviate the problems associated with the use of fossil fuels to meet the world's current demand for energy. This review discusses the developments in thermogalvanic systems attained through the use of nano-carbons as the electrode materials. Advances in cell design and electrode configuration that improve performance of these thermo converters and make them applicable in a variety of environments are also summarized. It is the aim of this review to act as a channel for further developments in thermogalvanic cell design and electrode engineering.

Demonstration of the advantages of using bamboo-like nanotubes for electrochemical biosensor applications compared with single walled carbon nanotubes

The modification of glassy carbon electrodes with random dispersions of nanotubes is currently the most popular approach to the preparation of carbon nanotube modified electrodes. The performance of glassy carbon electrodes modified with a random dispersion of bamboo type carbon nanotubes was compared with single walled carbon nanotubes modified glassy carbon electrodes and bare glassy carbon electrodes. The electrochemical performance of all three types for electrode were compared by investigating the electrochemistry with solution species and the oxidation of guanine and adenine bases of surface adsorbed DNA. The presence of edge planes of graphene at regular intervals along the walls of the bamboo nanotubes resulted in superior electrochemical performance relative to SWNT modified electrodes from two aspects. Firstly, with solution species the peak separation of the oxidation and reduction waves were smaller indicating more rapid rates of electron transfer. Secondly, a greater number of electroactive sites along the walls of the bamboo-carbon nanotubes (BCNTs) resulted in larger current signals and a broader dynamic range for the oxidation of DNA bases.

The voltammetric reduction, deprotonation and surface activity of ruthenium photovoltaic sensitizers in acetone

The reductive voltammetry of the photovoltaic sensitizer [(H2-dcbpy)2Ru(NCS)2] (H2-dcbpy=2,2′-bipyridine-4,4′-dicarboxylic acid) and [(H3-tctpy)Ru(NCS)3]− (H3-tctpy=2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylic acid) has been investigated in acetone. Significant surface interactions at both platinum and glassy carbon electrodes occur at 0.6 V prior to the reversible potential expected for ligand-based reduction process of the fully protonated acids. The origin of the surface interactions are attributed to the acid–base behaviour of the compounds, combined with overall deprotonation and reduction to hydrogen, since repetitive cycling of the potential reveals well-defined reversible reduction processes in the negative potential range, resulting from formation of doubly deprotonated [(H-dcbpy−)2Ru(NCS)2]2− and singly deprotonated [(H2-tctpy−)Ru(NCS)3]2−, respectively. The extent of the surface interactions has been estimated by electrochemical quartz crystal microbalance and chronocoulometric measurements. Under certain conditions, a thick conducting polymer consisting of several hundred monolayers is formed.

Cerium acetylacetonates—new aspects, including the lamellar clathrate [Ce(acac)4]·10H2O

Reactions of CeCl₃·7H₂O and Ce(NO₃)₃·6H₂O with Naacac or NH₄acac in aqueous solution at 21 and 45 °C yielded the trihydrate [Ce(acac)₃(H₂O)₂]·H₂O and the dihydrate [Ce(acac)₃(H₂O)₂], respectively, whereas similar treatment of (NH₄)₂[Ce(NO₃)₆] gave the trihydrate at both temperatures. Desiccation of the hydrates over silica gel left the dihydrate unchanged, whereas the trihydrate underwent decomposition rather than dehydration. Aerial oxidation of [Ce(acac)₃(H₂O)₂] in CH₂Cl₂ and toluene yielded α-[Ce(acac)₄] and β-[Ce(acac)₄], respectively, the structure of the former being re-determined with improved precision. Careful treatment of aqueous (NH₄)₄[Ce(SO₄)₄] and Hacac (initially pH 1–2) with aqueous ammonia to pH 5 precipitated hydrated [Ce(acac)₄], from which [Ce(acac)₄]·10H₂O was isolated as unstable, light-sensitive single crystals, and the structure was determined. The complex is a laminar clathrate containing layers of Ce(acac)₄ molecules sandwiched between extensive hydrogen-bonded layers of water molecules which do not interact with the metal. Electrochemical experiments confirmed the unstable nature of hydrated Ce^III(acac)₃, while the reduction of [Ce(acac)₄] yielded well-defined cyclic voltammograms in acetonitrile and acetone, corresponding to a quasi-reversible process. For the [Ce^IV(acac)₄]/[Ce^III(acac)₄]⁻redox couple, a calculated reversible potential of 0.22±0.02 V versus SHE was obtained in acetone or acetonitrile (0.1 M Bu₄NPF₆) at both gold and glassy carbon electrodes. This potential is consistent with the ease of both oxidation and reduction of cerium acetylacetonate complexes as found in the synthetic studies.

A novel non-enzymatic glucose sensor based on nickel (II) oxide electrospun nanofibers

A new enzymeless glucose sensor has been fabricated via electrospinning technology and subsequent calcination. The morphology and structure of the as-prepared nanofibers have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The electrocatalytic oxidation of glucose in alkaline medium at nickel oxide modified glassy carbon electrodes has been investigated. The modified electrodes offer excellent electrocatalytic activity toward the glucose oxidation at low positive potential (0.3 V). Glucose has been determined chronoamperometrically at the surface of NiO nanofibers modified electrode in 0.5 mM NaOH. Under the optimized condition, the calibration curve is linear in the concentration range of 2 × 10−3 mM∼1 mM, and 1 mM∼9.5 mM. The detection limit (signal-to-noise 3) and response time are 3.394 × 10−6 M and 2 s, respectively. The NiO electrospun nanofibers is easy to prepare and feasible in economy. The modified electrode is steady and can be used repeatedly, so it is reasonable to expect its broad use in non-enzymatic glucose sensor.

Evidence for the direct interaction between methylene blue and guanine bases using DNA-modified carbon paste electrodes

It is explored that methylene blue interacts with the guanine bases specifically, rather than the bases of ss-DNA in general. This interaction can be used as a method of quantifying the amount of oligonucleotide that is immobilized onto an electrode surface.

Preparation of composite electrodes with carbon nanotubes for lithium-ion batteries by low-energy ball milling

Some of the prospective electrode materials for lithium-ion batteries are known to have electronic transport limitations preventing them from being used in the electrodes directly. In many cases, however, these materials may become practical if they are applied in the form of nanocomposites with a carbon component, e.g. via incorporating nanoparticles of the phase of interest into a conducting network of carbon nanotubes. A simple way to prepare oxide-carbon nanotube composites suitable for the electrodes of lithium-ion batteries is presented in this paper. The method is based on low-energy ball milling. An electrochemically active but insulating phase of LiFeTiO4 is used as a test material. It is demonstrated that the LiFeTiO4-carbon nanotube composite is not only capable of having significantly higher capacity (∼105-120 mA h g-1vs. the capacity of ∼65-70 mA h g -1 for the LiFeTiO4 nanoparticles) at a slow current rate but may also operate at reasonably high current rates. © the Partner Organisations 2014.

Electrochemistry of tris(2,2′-bipyridyl) cobalt(II) in ionic liquids and aprotic molecular solvents on glassy carbon and platinum electrodes

The tris(2,2′-bipyridyl) complexes of cobalt(II) and (III) ([Co(bpy)3]^2+/3+) produce a redox couple of great interest in thermoelectrochemical cells and dye sensitized solar cells including both types of devices based on ionic liquid electrolytes. We present a systematic study of the electrochemistry of [Co(bpy)3]²⁺ [NTf2]^-2 in two ionic liquids (ILs) based on the 1-ethyl-3-methylimidazolium (C2mim) cation and two ILs based on the 1-butyl-1-methylpyrrolidinium cation (C4mpyr), as well as three aprotic molecular solvents. Platinum (Pt) and glassy carbon (GC) working electrodes were compared. In all solvents better electrochemical responses were observed on GC, which yielded higher currents in the cyclic voltammograms and lower rate constants for the redox reaction. The [Co(bpy)3]^1+/2+ couple is also readily observed, but this redox reaction is chemically irreversible, possibly because the [Co(bpy)3]¹⁺ complex dissociates. However, the [Co(bpy)3]^1+/2+ reaction is chemically reversible in all of the solvents studied, except 3-methoxypropionitrile, if excess of 2,2′-bipyridyl is added to the solution.

Lithium-ion capacitors with 2D Nb2CTx (MXene) - carbon nanotube electrodes

There is a growing interest to hybrid energy storage devices, such as lithium-ion capacitors, in which battery-type electrodes are combined with capacitor-type ones. It is anticipated that the energy density (either gravimetric or volumetric) of lithium-ion capacitors is improved if pseudocapacitive or fast insertion materials are used instead of conventional activated carbon (AC) in the capacitor-type electrode. MXenes, a new family of two-dimensional transition metal carbides, demonstrate metallic conductivity and fast charge-discharge behavior that make them suitable for this application. In this study, we move beyond single electrodes, half-cell studies and demonstrate three types of hybrid cells using Nb2CTx-carbon nanotube (CNT) films. It is shown that lithiated graphite/Nb2CTx-CNT, Nb2CTx-CNT/LiFePO4 and lithiated Nb2CTx-CNT/Nb2CTx-CNT cells are all able to operate within 3 V voltage windows and deliver capacities of 43, 24 and 36 mAh/g (per total weight of two electrodes), respectively. Moreover, the polarity of the electrodes can be reversed in the symmetric Nb2CTx-CNT cells from providing a positive potential between 0 and 3 V to a negative one from -3 to 0 V. It is shown that the volumetric energy density (50-70 Wh/L) of our first-generation devices with MXene electrodes exceeds that of a lithium titanate/AC capacitor.

Hydrangea-like multi-scale carbon hollow submicron spheres with hierarchical pores for high performance supercapacitor electrodes

Uniform hydrangea-like multi-scale carbon hollow submicron spheres (HCSSg) are fabricated by a simple hydrothermal method using glucose as carbon source and fibrous silicon dioxides spheres as shape guide. Structure characterization suggests that petal-like partially graphitized carbon nanosheets with the thickness of about 10 nm arranged in three dimensions (3D) to form the hydrangea-like hollow spheres (size ranging from 250 to 500 nm) with mesoporous channels, which can be conducive to be a high specific surface area (934 m2 g-1) and bulk density (0.87 cm g-3), hierarchical pores structure with good conductivity. As a result, the HCSSg has been demonstrated to be a supercapacitor electrode material with high gravimetric (386 F g-1 at 0.2 A g-1) and outstanding volumetric (335 F cm-3) capacitance, good rate capability and cycling stability with 94% capacitance retention after 5000 cycles in aqueous electrolytes, thus suggesting its application potential.

Protein electrochemistry using aligned carbon nanotube arrays

The remarkable electrocatalytic properties and small size of carbon nanotubes make them ideal for achieving direct electron transfer to proteins, important in understanding their redox properties and in the development of biosensors. Here, we report shortened SWNTs can be aligned normal to an electrode by self-assembly and act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWNTs, in this case, microperoxidase MP-11. The efficiency of the electron transfer through the SWNTs is demonstrated by electrodes modified with tubes cut to different lengths having the same electron-transfer rate constant.

MoO3 nanoparticles dispersed uniformly in carbon matrix : a high capacity composite anode for Li-ion batteries

A MoO₃-carbon nanocomposite was synthesized from a mixture of MoO₃ and graphite by a controlled ball milling procedure. The as-prepared product consists of nanosized MoO₃ particles (2-180 nm) homogeneously distributed in carbon matrix. The nanocomposite acts as a high capacity anode material for lithium-ion batteries and exhibits good cyclic behavior. Its initial capacity exceeds the theoretical capacity of 745 mA h g^-1 in a mixture of MoO₃ and graphite (1:1 by weight), and the stable capacity of 700 mA h g^-1 (94% of the theoretical capacity) is still retained after 120 cycles. The electrode performance is linked with the unique nanoarchitecture of the composite and is compared with the performance of MoO₃-based anode materials reported in the literature previously (nanoparticles, ball milled powders, and carbon-coated nanobelts). The high value of capacity and good cyclic stability of MoO₃-carbon nanocomposite are attractive in respect to those of the reported MoO₃ electrodes.