Bioelectrocatalytic mediatorless dioxygen reduction at carbon ceramic electrodes modified with bilirubin oxidase

Carbon ceramic electrodes were prepared by sol-gel processing of a hydrophobic precursor - methyltrimethoxysilane (MTMOS) - together with dispersed graphite microparticles according to a literature procedure. Bilirubin oxidase (BOx) was adsorbed on this electrode from buffer solution and this process was followed by atomic force microscopy (AFM). The electrodes exhibited efficient mediatorless electrocatalytic activity towards dioxygen reduction. The activity depends on the time of adsorption of the enzyme and the pH. The electrode remains active in neutral solution. The bioelectrocatalytic activity is further increased when a fraction of the carbon microparticles is replaced by sulfonated carbon nanoparticles (CNPs). This additive enhances the electrical communication between the enzyme and the electronic conductor. At pH 7 the carbon ceramic electrode modified with bilirubin oxidase retains ca. half of its highest activity. The role of the modified nanoparticles is confirmed by experiments in which a film embedded in a hydrophobic silicate matrix also exhibited efficient mediatorless biocatalytic dioxygen reduction. Scanning electrochemical microscopy (SECM) of the studied electrodes indicated a rather even distribution of the catalytic activity over the electrode surface.     

SECM imaging of electrocatalytic activity for oxygen reduction reaction on thin film materials

The oxygen reduction reaction (ORR) on sputtered Pt thin films in acidic solution was successfully studied by scanning electrochemical microscopy (SECM) in a modified tip generation-substrate collection (TG-SC) mode. SECM images of ORR activity in different sample areas were obtained and it is shown that this TG-SC SECM technique can be used to screen electrocatalytic activity of continuous thin film samples efficiently and quickly for the ORR in an acidic medium. It is observed that this technique is not very sensitive to the tip-substrate separation within a certain range. The SECM images obtained are strongly dependent on the substrate potential. The advantages of this technique for studying ORR electrocatalysts are discussed.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Semiconducting photocathodes for the reduction of dioxygen: Part I. Characterisation of crystalline and amorphous p-Si

As a first step toward the development of photocathodes for use in water-purification reactors, we have investigated the behaviour of p-type Si(1 0 0)-oriented single crystals and glow-discharge a-Si:H layers as photocathodes for the reduction of dissolved dioxygen. The electrode surfaces were prepared by HF rinsing and were characterised by cyclic voltammetry in the dark and under tungsten-halogen lamp illumination. This study has been performed in dilute-buffered fluoride medium, in dilute sulphuric acid, and lastly in three buffered phosphate media at pH 2, 7 and 12. A study using a rotating-disk electrode (RDE) arrangement permits to distinguish the contribution from dioxygen reduction (mass-transport dependent) and that from hydrogen evolution (kinetically limited). Crystalline Si and a-Si:H appear to exhibit rather similar behaviours. The Si surface acquires catalytic properties for dioxygen reduction upon negative pre-polarisation. However, these catalytic properties do not appear usable in practice for efficient operation, which calls for chemical modifications of the silicon surface.     

Electrochemistry in confined microsystems: Part 1. CV-SECM of reversible system at a confined disk ultramicroelectrode

This work is the first part of a series of papers concerning the study of electrochemical method in case of ultramicroelectrode (UME) confinement in integrated chemical microsystems. This paper is devoted to the discussion of numerical simulation obtained in the case of a transient electrochemical method - i.e. cyclic voltammetry (CV) - applied to a finite disk ultramicroelectrode confined in front of an insulating surface. This configuration corresponds to scanning electrochemical microscopy (SECM) in negative feedback configuration. We describe a new electrochemical method, with acronym CV-SECM, based on the coupling of cyclic voltammetry with scanning electrochemical microscopy. The main advantage of this method is the obtention of a time resolved measurement simultaneously with a high local resolution. The results of the simulation show that adjusting the scan speed of the potential sweep allows the exploration of a controlled range of distance between the UME and the insulating surface. The reverse peak current obtained in cyclic voltammetry exhibits unexpected variations with the approach distance of the UME: (i) a regime of depletion of the electroactive reactant during the forward scan at short distance, associated to a decrease of the peak current and (ii) a regime of accumulation of the electroactive product at larger distance, associated to an increase of the reverse peak current identified in comparison with the free diffusion regime.     

Electron transfer to sulfides:: Heterogeneous kinetics, CS bond cleavage, and self-protonation

The problem of characterizing the steps associated with the dissociative reduction of sulfides has been addressed. The electrochemical reduction of diphenylmethyl para-methoxyphenyl sulfide in N,N-dimethylformamide, on both glassy carbon and mercury electrodes, was chosen as a test system. The electrode process involves the slow heterogeneous outer-sphere electron transfer to the sulfide, the fast cleavage of the CS bond, the reduction of the ensuing carbon radical, and the self-protonation triggered by the generation of the strong base Ph₂CH⁻. The latter reaction is rather slow, in agreement with the large intrinsic barriers characterizing proton transfers between CH-acids and carbon bases. The dissociative reduction was studied in the presence of an exogenous acid. The results, obtained by convolution analysis, point to a stepwise DET mechanism in which the ET step is accompanied by rather large reorganization energy. Similar results were obtained on both electrode materials. Analysis of the heterogeneous electron transfer and associated CS bond cleavage indicate that the reduction of this and other sulfides lies between the stepwise dissociative electron transfers leading to the formation of stiff π^* radical anions and those going through the intermediacy of loose σ^* radical anions.     

Film formation from monodisperse acrylic lattices: Part 3: Drying and ageing of acrylic latex films

The influence of drying/ageing on the structure and properties of acrylic latex films was investigated using turbidity measurements in combination with gravimetry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and water vapour permeability. Ageing above the minimum film formation temperature (MFFT) leads to marked changes in a dried latex film, whereas, after ageing below the MFFT, changes are hardly found. Above the MFFT there is a continuous change in the film properties with time. This becomes obvious from the decreases in the regenerated interference minimum, water vapour permeability and corrugation height. The influence of ageing on the water absorption of the films is less straight forward. It was expected that films with a more compact structure would absorb less water. This is correct for short drying times only, from 0.5 to 3 h. Ageing of better-dried films, however, yields the opposite result: by increase of the ageing time from 3 to 150 h the water uptake increases. There are various reasons for this increase; they are discussed briefly.     

Electrocatalytic reduction of dioxygen at a modified glassy carbon electrode based on Nafion-dispersed single-walled carbon nanotubes and cobalt–porphyrin with palladium nanoparticles in acidic media

Cathodic dioxygen (O₂) reduction was performed at a modified glassy carbon electrode (GCE) by single-walled carbon nanotubes (SWCNT)/Nafion^® (NF) film with cobalt (II) tetra (2-amino-phenyl) porphyrin (CoTAPP) and palladium (Pd) nanoparticles incorporated and employed as doping agents. Both the electrochemical behavior of SWCNT with a P(CoTAPP)–Pd nanoparticle matrix and the electrocatalytic reduction of O₂ were investigated using transmission electron microscopy (TEM), cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) techniques in 0.1 mol l⁻¹ H₂SO₄ aqueous solutions. The electrocatalytic reduction of O₂ at the SWCNT/NF/P(CoTAPP)–Pd composite film established a pathway of four-electron transfer reductions into H₂O. Hydrodynamic voltammetry revealed that the modified electrode was catalyzed effectively by the four-electron transferred reduction of dioxygen into H₂O with minimal generation of H₂O₂. The SWCNT/NF/P(CoTAPP)–Pd composite film showed a highly efficient electrocatalytic performance. P(CoTAPP)–Pd was an effective mediator for the reduction of dioxygen and was responsible for the enhanced catalytic activity.     

High temperature behaviour of self-consolidating concrete: Microstructure and physicochemical properties

This paper presents an experimental study on the properties of self-compacting concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete mixture were tested. These concrete mixtures come from the French National Project B@P. The specimens of each concrete mixture were heated at a rate of 1 °C/min up to different temperatures (150, 300, 450 and 600 °C). In order to ensure a uniform temperature throughout the specimens, the temperature was held constant at the maximum temperature for 1 h before cooling. Mechanical properties at ambient temperature and residual mechanical properties after heating have already been determined. In this paper, the physicochemical properties and the microstuctural characteristics are presented. Thermogravimetric analysis, thermodifferential analysis, X-ray diffraction and SEM observations were used. The aim of these studies was in particular to explain the observed residual compressive strength increase between 150 and 300 °C.     

Graphite flake carbon composites with a ‘sinterable’ microbead matrix: I. Mechanical properties

In this paper we report a study of the effect of graphite flakes of different size and volume fraction on the mechanical properties of a fine-grained carbon produced by the ‘sinterable’ route. Mesophase microbeads have been used as a matrix and the volume percent and size of the graphite flakes have been varied. It is shown that the flakes significantly increase the work of fracture of the composite, the effect being dependent on both flake size and volume fraction. However, there is a corresponding decrease in the Young Modulus and flexural strength of the composites. The flakes are not bonded to the structure and effectively act as inherent ‘crack-like’ pores. Flakes aligned perpendicular to the surface form the flaws that control the fracture stress. However, they also contribute significantly to the bridging stresses in the wake of the crack, so enhancing the work of fracture. The results should be useful in understanding the role of graphite-flake inclusions in modifying the properties of carbon materials.     

Development of low-pH cementitious materials for HLRW repositories: Resistance against ground waters aggression

One of the most accepted engineering construction concepts of underground repositories for high radioactive waste considers the use of low-pH cementitious materials. This paper deals with the design of those based on Ordinary Portland Cements with high contents of silica fume and/or fly ashes that modify most of the concrete “standard” properties, the pore fluid composition and the microstructure of the hydrated products. Their resistance to long-term groundwater aggression is also evaluated. The results show that the use of OPC cement binders with high silica content produces low-pH pore waters and the microstructure of these cement pastes is different from the conventional OPC ones, generating C-S-H gels with lower CaO/SiO₂ ratios that possibly bind alkali ions. Leaching tests show a good resistance of low-pH concretes against groundwater aggression although an altered front can be observed.     

Measurement and modelling of the film casting process: 2. Temperature distribution along draw direction

In the case film process a polymer melt is extruded through a slit die, stretched in air and cooled on a chill roll. During the path in air the melt cools and a reduction of both thickness and width takes place; obviously, temperature distribution, thickness and width reductions are function of draw ratio and stretching distance.Temperature distribution along the draw direction was measured as function of flow rate during film casting experiments performed with an iPP resin. A non-contacting method of measurement, based on a narrow-band IR pyrometer, was adopted.A good qualitative agreement is shown between experimental temperature data and predictions of a model accounting of radiation emissivity dependence upon film thickness. Differences are consistent with discrepancies of film thickness evolution along draw direction, indeed the model slightly over predicts both film thickness reduction and, parallel, temperature decrease along the draw direction.     

New quality criteria in wastewater reuse : The case of Gran Canaria

Wastewater reuse plays a key role in this vital cycle of water because it is able to reduce the wastewater spilled. Simultaneously, the supply of water for specific uses is increased. A new regulation (Royal Decree 1620/2007) came into force in Spain in December 2007 and regulates the basic conditions for the wastewater reuse and establishes the required criteria of quality to waters according to use.In the present paper, different kinds of tertiary treatments in reuse of wastewater are described. We focused on the applied tertiary treatments in most wastewater treatment plants of Gran Canaria Island. Particularly, we analysed Hoya del Pozo wastewater treatment plant due to its wide number of technologies and processes.Finally, we reviewed the quality criteria used in the regenerated waters, in agreement with the new regulations.     

Test of vanadium pentoxide as anode for the electrooxidation of toluene: A theoretical approach of the electrode process

Vanadium pentoxide (V₂O₅) films were prepared by electrochemical and thermal decomposition of vanadyl sulphate on titanium dioxide covered titanium plates and glassy carbon discs. The prepared material by thermal decomposition showed high surface area and good physical stability; while the electrodeposited films, although being homogeneous, showed poor adhesion. The V₂O₅ electrodes were chemically and electrochemically stable in aqueous (1 M H₂SO₄ + 1 M NaOH, pH 3) and organic (0.1 M But₄NPF₆ + CH₃CN) solutions. In both cases, a well defined electrochemical response was observed. At the experimental conditions, the prepared materials were not active for the electrooxidation of toluene. The theoretical modeling suggests that the lack of activity is due to the weak interaction between toluene and the V₂O₅ surface.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

Toughening effect of carbon nanotubes and carbon nanofibres in epoxy adhesives for joining carbon fibre laminates

The effect of carbon nanotubes (CNTs) and carbon nanofibres (CNFs) on mode I adhesive fracture energy (G_IC) of double cantilever beam (DCB) joints of carbon fibre-reinforced laminates bonded with an epoxy adhesive has been studied. It was observed that the presence of carbon nanofillers in the epoxy adhesive results in a significant increase in the propagation value of mode I adhesive fracture energy with CNTs producing the largest increase. The toughening mechanisms, analysed using scanning electron microscopy (SEM), for the two nanofiller systems differed: pull-out with CNFs, and pull-out and crack bridging with CNTs. At the macroscopic level there was also a change in the failure mode, with an increased proportion of delamination occurring in the nanoreinforced joints in comparison with the unreinforced. Two different surface treatments were also applied to the laminates: grit blasting and atmospheric plasma. The highest fracture energy was obtained in the grit blasted joints.     

Investigation of flow boiling in circulating three-phase fluidised bed: Part II: Theoretical correlation

The following part of this paper reviews existing theoretical correlations to predict the behaviour of two-phase (liquid-solid) and three-phase (liquid-solid-vapour) fluidised beds as well as models describing heat transfer coefficients. Moreover, a theoretical correlation is developed to describe heat transfer during boiling in a three-phase circulating fluidised bed. The approach uses earlier work on two-phase (liquid-vapour) flow boiling, two-phase (liquid-solid) fluidised beds and three-phase (liquid-vapour-solid) circulating fluidised beds. The correlation developed is validated against experimental data obtained in Part I of the presented paper. The model's ability to predict the experimental data has been successfully demonstrated. The developed expression for heat transfer coefficients is written as follows:

α_T.F.B.-[(α_nb)^t_R+(α_cb)^t_R]^1/t_R     

Direct numerical simulation (DNS) modeling of PEFC electrodes: Part II. Random microstructure

The direct numerical simulation (DNS) method, developed for modeling the cathode catalyst layer (CL) of a polymer electrolyte fuel cell (PEFC) in Part I, is further extended wherein the catalyst layer is described as a random three-dimensional porous structure. A random CL microstructure is obtained using a computer-generated random number with specified porosity and pore size as the input structural parameters. Some statistical features of the CL and their dependence on the porosity are identified and demonstrated. The charge and species conservation equations are solved directly on this microscopically complex structure. The results from the DNS calculation are compared with the one-dimensional macrohomogeneous predictions and the Bruggeman factor for transport property correction is evaluated, which can be used as direct input into the macroscopic fuel cell models.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part II: Computational modeling and design

The cathode catalyst layer in direct methanol fuel cells (DMFCs) features a large thickness and mass transport loss due to higher Pt loading, and therefore must be carefully designed to increase the performance. In this work, the effects of Nafion loading, porosity distribution, and macro-pores on electrochemical characteristics of a DMFC cathode CL have been studied with a macro-homogeneous model, to theoretically interpret the related experimental results. Transport properties in the cathode catalyst layers are correlated to both the composition and microstructure. The optimized ionomer weight fraction (22%) is found to be much smaller than that in H₂ polymer electrolyte fuel cells, as a result of an optimum balance of proton transport and oxygen diffusion. Different porosity distributions in the cathode CLs are investigated and a stepwise distribution is found to give the best performance and oxygen concentration profile. Influence of pore defects in the CLs is discussed and the location of macro-pores is found to play a dual role in affecting both oxygen transport and proton conduction, hence the performance. The reaction zone is extended toward the membrane side and the proton conduction is facilitated when the macro-pores are near the gas diffusion layer.