Stuart M. Holmes

Supercapacitance from Cellulose and Carbon Nanotube Nanocomposite Fibers

Multiwalled carbon nanotube (MWNT)/cellulose composite nanofibers have been prepared by electrospinning a MWNT/cellulose acetate blend solution followed by deacetylation. These composite nanofibers were then used as precursors for carbon nanofibers (CNFs). The effect of nanotubes on the stabilization of the precursor and microstructure of the resultant CNFs were investigated using thermogravimetric analysis, transmission electron microscopy and Raman spectroscopy. It is demonstrated that the incorporated MWNTs reduce the activation energy of the oxidative stabilization of cellulose nanofibers from ∼230 to ∼180 kJ mol–1. They also increase the crystallite size, structural order, and electrical conductivity of the activated CNFs (ACNFs). The surface area of the ACNFs increased upon addition of nanotubes which protrude from the fiber leading to a rougher surface. The ACNFs were used as the electrodes of a supercapacitor. The electrochemical capacitance of the ACNF derived from pure cellulose nanofibers is demonstrated to be 105 F g–1 at a current density of 10 A g–1, which increases to 145 F g–1 upon the addition of 6% of MWNTs.

Optimization of the Mechanical Performance of Bacterial Cellulose/Poly(L-lactic) Acid Composites

Understanding the nature of the interface between nanofibers and polymer resins in composite materials is challenging because of the complexity of interactions that may occur between fibers and between the matrix and the fibers. The ability to select the most efficient amount of reinforcement for stress transfer, making a saving on both cost and weight, is also a key part of composite design. The use of Raman spectroscopy to investigate micromechanical properties of laminated bacterial cellulose (BC)/poly(l-lactic) acid (PLLA) resin composites is reported for the first time as a means for understanding the fundamental stress-transfer processes in these composites, but also as a tool to select appropriate processing and volume fraction of the reinforcing fibers. Two forms of BC networks are investigated, namely, one cultured for 3 days and another for 6 days. The mechanical properties of the latter were found to be higher than the former in terms of Youngs modulus, stress at failure, and work of fracture. However, their specific Youngs moduli (divided by density) were found to be similar. Youngs modulus and stress at failure of transparent predominantly amorphous PLLA films were found to increase by 100 and 315%, respectively, for an 18% volume fraction of BC fibers. BC networks cultured for 3 days were shown to exhibit enhanced interaction with PLLA because of their higher total surface area compared, as measured by nitrogen adsorption, to the material cultured for 6 days. This enhanced interaction is confirmed by using the Raman spectroscopic approach, whereby larger band shift rates, of a peak initially located at 1095 cm(-1), with respect to both strain and stress, are observed, which is a quantitative measure of enhanced stress transfer. Thermal analysis (differential scanning calorimetry) and electron microscopy imaging (scanning electron microscopy) of the samples also confirms the enhanced coupling between the resin and the BC networks cultured for 3 days, compared to those cultured for 6 days. These results are shown to have implications for the use of BC networks for composite reinforcement, whereby less material can be used for the same specific mechanical properties. The technique also gives opportunities to study the interfaces in these composite materials in detail.

In situ FTIR study of the formation of MCM-41

The formation of both siliceous and aluminium-containing MCM-41 has been followed insitu using attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectroscopy together with a range of complementary techniques. Key stages in the reaction are reflected by changes in the intensity of IR bands at 1030 and 1105 cm−1, which are assigned to the internal Si–O vibrations of Si(OSi)3 and Si(OSi)4 groupings (Q3 and Q4, respectively). Assignments were confirmed using solid state NMR. Under the conditions of these experiments, the reaction rate is limited by the relatively slow dissolution of the silica source. The resulting silicate oligomers then react more rapidly with surfactant cations in a co-operative assembly process to give the embryonic MCM-41 structure. Slower condensation reactions subsequently increase the product Q4/Q3 ratio as further crosslinking develops. From an analysis of both the CH3 head group and CH2 in-chain IR vibrations, the surfactant appears to be complexed or micellised at all times during the synthesis.

Synthesis of MCM-41 materials: an in situ FTIR study

Abstract A new approach has been developed to monitor the synthesis of MCM-41 mesoporous materials based on in situ ATR FTIR measurements. The transformation between Q4 and Q3 species, which is an essential step in the synthesis and structural development of MCM-41 materials, can be followed using this technique.

A solvent extraction method to study the location and concentration of coke formed on zeolite catalysts

Abstract The location and composition of the coke formed over H-ZSM-5 zeolites during the cracking of hexadecane at 350°C and at 20 atmospheres was studied. A solvent extraction technique using two different solvents located the coke either on the external surface or within the pores of the zeolite and GC-MS analysis characterised the soluble coke. Coke was also examined using 13C MAS NMR and high resolution TEM. Substituted benzenes were identified as the major precursors to coke formation, and the external coke had a graphitic structure. The contact time was shown to affect only the rate of formation but not the composition of the coke.

Acid sites in mesoporous materials: A DRIFTS study

Abstract A range of MCM-41 materials have been prepared and characterised employing diffuse reflectance FTIR spectroscopy (DRIFTS) to investigate fundamental and combination frequencies of the OH groups in amorphous and crystalline aluminosilicates. Comparative characterisation of mesoporous materials shows that SiOH groups in Al-MCM-41 are acidic, unlike those in Si-MCM-41. At the same time, no indication of the presence of bridging hydroxyls in any of the studied MCM-41 samples has been found. It appears that the ‘mild acidity’ of Al-MCM-41 materials is not due to the bridging Al(OH)Si groups, which are commonly found in zeolites, but it is associated with SiOH groups similar to those in amorphous aluminosilicates. Acidic properties of these OH groups are probably modified by the inductive effect of the Al present in the structure, which however does not affect their stretching frequency. A significant concentration of Bronsted acid sites is observed in the Al grafted sample obtained via a post-synthesis modification of Si-MCM-41 indicating that this may be a suitable way to generate acid sites compared with the aluminium introduction during the synthesis.

Evaluation of porous carbon substrates as catalyst supports for the cathode of direct methanol fuel cells.

This paper presents an investigation into the effect of the cathode catalyst substrate morphology on the performance of supported catalysts for direct methanol fuel cells (DMFC). Two porous carbons were synthesized by a sacrificial templating method, one using disordered, macroporous silica diatomaceous earth, the other using ordered, mesoporous SBA-15. The Pt particles deposited onto the synthesized mesoporous substrate and commercial Vulcan XC-72 were shown to have fairly uniform dispersion, with particle size in the range 2–3 nm, and high catalyst surface areas. In situ single cell DMFC testing showed that the Pt catalyst supported on carbon based on diatomaceous earth (Pt/C–Celatom) exhibited the lowest cell impedance, which may be attributed to the large pores, which facilitate oxygen transport, and to the low porosity, which enhances protonic and electronic conductivity of the catalyst layer. In DMFC polarization tests, the Pt/C–Celatom also exhibited the highest power output of the tested catalysts, with a peak power density 28% better than the current commercial standard, Vulcan XC-72.

The direct synthesis of pure zeolite-A using ‘virgin’ Kaolin

Here we demonstrate the conversion of a Nigerian Kaolin, straight from the source without any form of pre-treatment, directly into pure zeolite-A. The method involves a rapid and low temperature meta-kaolinization step followed by chemical conversion directly into pure zeolite-A. The synthesis step acts as the purification step removing the 80% quartz impurity simultaneously with zeolite growth. The whole process is designed to be both economical and straightforward to facilitate commercialisation.

Potentiometry in aqueous solutions using zeolite films

The potentiometric behaviour of films of zeolite toward aqueous phase cations is described. The stability of the potentiometric responses toward various analytes is used to diagnose the applicability of these materials for electroanalysis. Size selective behaviour with regard to cationic species is observed: cations with a crystallographic diameter exceeding that of the zeolite pores gave no measurable potential response. The zeolite films were formed using three distinct preparation routes: pressing discs of zeolite powder (for zeolite Y, zeolite A and mordenite), growing a free-standing membrane of zeolite (for sodalite) and using a secondary growth step to heal defects in pressed zeolite A discs. The approach presented is the first report of the use of coherent polycrystalline films, consisting solely of zeolite, in a potentiometric study.

The removal of caesium ions using supported clinoptilolite

In this paper, the sorptive kinetic and diffusional characteristics of caesium ion removal from aqueous solution by carbon-supported clinoptilolite composites are presented. Natural clinoptilolite was supported on carbonaceous scaffolds prepared from date stones. Thermal treatment was applied to produce voids in the carbon which was conditioned using polydiallyldimethylammonium chloride to facilitate the clinoptilolite attachment. This method allowed the formation of a consistent zeolite layer on the carbon surface. The composite was applied in the removal of non-radioactive caesium ions showing an enhanced uptake from 55 mg g(-1) to 120.9 mg g(-1) when compared to clinoptilolite. Kinetic studies using Pseudo First Order model revealed an enhanced rate constant for carbon-clinoptilolite (0.0252 min(-1)) in comparison with clinoptilolite (0.0189 min(-1)). The Pseudo-First Order model described the process for carbon-clinoptilolite, meanwhile Pseudo Second Order model adjusted better for pure clinoptilolite. Diffusivity results suggested that mass transfer resistances involved in the Cs(+) sorption are film and intraparticle diffusion for natural clinoptilolite and intraparticle diffusion as the mechanism that controls the process for carbon-clinoptilolite composite. The most significant aspect being that the vitrified volume waste can be reduced by over 60% for encapsulation of the same quantity of caesium due to the enhanced uptake of zeolite.