Performance analysis of three advanced controllers for polymerization batch reactor: an experimental investigation

The performances of three advanced non-linear controllers are analyzed for the optimal set point tracking of styrene free radical polymerization (FRP) in batch reactors. The three controllers are the artificial neural network-based MPC (NN-MPC), the artificial fuzzy logic controller (FLC) as well as the generic model controller (GMC). A recently developed hybrid model (Hosen et al., 2011a. Asia-Pac. J. Chem. Eng. 6(2), 274) is utilized in the control study to design and tune the proposed controllers. The optimal minimum temperature profiles are determined using the Hamiltonian maximum principle. Different types of disturbances are introduced and applied to examine the stability of controller performance. The experimental studies revealed that the performance of the NN-MPC is superior to that of FLC and GMC. © 2013 The Institution of Chemical Engineers.

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.

Performance enhancement of single-walled nanotube-microwave exfoliated graphene oxide composite electrodes using a stacked electrode configuration

We report the development of a stacked electrode supercapacitor cell using stainless steel meshes as the current collectors and optimised single walled nanotubes (SWNT)-microwave exfoliated graphene oxide (mw rGO) composites as the electrode material. The introduction of mw rGO into a SWNT matrix creates an intertwined porous structure that enhances the electroactive surface area and capacitive performance due to the 3-D hierarchical structure that is formed. The composite structure was optimised by varying the weight ratio of the SWNTs and mw rGO. The best performing ratio was the 90% SWNT-10% mw rGO electrode which achieved a specific capacitance of 306 F g-1 (3 electrode measurement calculated at 20 mV s-1). The 90% SWNT-10% mw rGO was then fabricated into a stacked electrode configuration (SEC) which significantly enhanced the electrode performance per volume (1.43 mW h cm-3, & 6.25 W cm-3). Device testing showed excellent switching capability up to 10 A g-1, and very good stability over 10000 cycles at 1.0 A g-1 with 93% capacity retention. © the Partner Organisations 2014.

The influence of graphene on the electrical communication through organic layers on graphite and gold electrodes

The influence of graphene on the electrical communication through organic layers fabricated on graphite and gold electrodes is investigated. These layers were prepared by in situ reductive adsorption of 4-aminobenzoic acid in the presence of NaNO2 and HCl to have surface bound carboxylic acid functionalities, followed by covalent attachment of 1-aminopyrene via an amide coupling reaction to have surface bound pyrene groups for graphene immobilization via noncovalent π-π stacking interaction. The coverage of the layers created via reductive adsorption on graphite electrodes was found to be much higher than that on gold electrodes. It was revealed that graphene significantly enhances the electrical communication through the layers on graphite electrodes but on gold electrodes the enhancement effect through the layers was only minor. However, when gold electrodes were modified with a self-assembled monolayer (SAM) of propanethiol the subsequent immobilization of graphene resulted in a significant enhancement of the electrical communication. It is also found that immobilization of graphene could affect the electron transfer between the redox probe, pyrene and the underlying electrodes. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the graphene sheets. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) were also used to characterize the stepwise modified electrodes. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Amine enrichment of thin-film composite membranes via low pressure plasma polymerization for antimicrobial adhesion

Thin-film composite membranes, primarily based on poly(amide) (PA) semipermeable materials, are nowadays the dominant technology used in pressure driven water desalination systems. Despite offering superior water permeation and salt selectivity, their surface properties, such as their charge and roughness, cannot be extensively tuned due to the intrinsic fabrication process of the membranes by interfacial polymerization. The alteration of these properties would lead to a better control of the materials surface zeta potential, which is critical to finely tune selectivity and enhance the membrane materials stability when exposed to complex industrial waste streams. Low pressure plasma was employed to introduce amine functionalities onto the PA surface of commercially available thin-film composite (TFC) membranes. Morphological changes after plasma polymerization were analyzed by SEM and AFM, and average surface roughness decreased by 29%. Amine enrichment provided isoelectric point changes from pH 3.7 to 5.2 for 5 to 15 min of plasma polymerization time. Synchrotron FTIR mappings of the amine-modified surface indicated the addition of a discrete 60 nm film to the PA layer. Furthermore, metal affinity was confirmed by the enhanced binding of silver to the modified surface, supported by an increased antimicrobial functionality with demonstrable elimination of E. coli growth. Essential salt rejection was shown minimally compromised for faster polymerization processes. Plasma polymerization is therefore a viable route to producing functional amine enriched thin-film composite PA membrane surfaces.

High-performance supercapacitor electrode materials from cellulose-derived carbon nanofibers

Nitrogen-functionalized carbon nanofibers (N-CNFs) were prepared by carbonizing polypyrrole (PPy)-coated cellulose NFs, which were obtained by electrospinning, deacetylation of electrospun cellulose acetate NFs, and PPy polymerization. Supercapacitor electrodes prepared from N-CNFs and a mixture of N-CNFs and Ni(OH)2 showed specific capacitances of ∼236 and ∼1045 F g(-1), respectively. An asymmetric supercapacitor was further fabricated using N-CNFs/Ni(OH)2 and N-CNFs as positive and negative electrodes. The supercapacitor device had a working voltage of 1.6 V in aqueous KOH solution (6.0 M) with an energy density as high as ∼51 (W h) kg(-1) and a maximum power density of ∼117 kW kg(-1). The device had excellent cycle lifetime, which retained ∼84% specific capacitance after 5000 cycles of cyclic voltammetry scans. N-CNFs derived from electrospun cellulose may be useful as an electrode material for development of high-performance supercapacitors and other energy storage devices.

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.

Understanding structure-function relationship in hybrid Co3O4-Fe2O3/C lithium-ion battery electrodes

A range of high-capacity Li-ion anode materials (conversion reactions with lithium) suffer from poor cycling stability and limited high-rate performance. These issues can be addressed through hybridization of multiple nanostructured components in an electrode. Using a Co3O4-Fe2O3/C system as an example, we demonstrate that the cycling stability and rate performance are improved in a hybrid electrode. The hybrid Co3O4-Fe2O3/C electrode exhibits long-term cycling stability (300 cycles) at a moderate current rate with a retained capacity of approximately 700 mAh g(-1). The reversible capacity of the Co3O4-Fe2O3/C electrode is still about 400 mAh g(-1) (above the theoretical capacity of graphite) at a high current rate of ca. 3 A g(-1), whereas Co3O4-Fe2O3, Fe2O3/C, and Co3O4/C electrodes (used as controls) are unable to operate as effectively under identical testing conditions. To understand the structure-function relationship in the hybrid electrode and the reasons for the enhanced cycling stability, we employed a combination of ex situ and in situ techniques. Our results indicate that the improvements in the hybrid electrode originate from the combination of sequential electrochemical activity of the transition metal oxides with an enhanced electronic conductivity provided by percolating carbon chains.