1,354,494 research outputs found
Physically-based modelling of solid oxide fuel cells: overcoming the three-phase boundary paradigm
No full textWithin composite electrodes for solid oxide fuel cells (SOFCs), electrochemical reactions take place at the so-called three-phase boundary (TPB), which is the contact perimeter where the electron-conducting phase, the ion-conducting phase and the porous phase meet. Such a TPB is conventionally regarded as a mono-dimensional line and efforts have been made to increase its length per unit of electrode volume in order to reduce the activation losses.
In this study, by using physically-based modelling, 3D tomography and impedance spectroscopy, we show that the electrochemical reactions take place within an extended region around the geometrical TPB line. Such an extended region is in the order of 4 nm in Ni-ScSZ anodes [1] while approaches hundreds of nanometres in LSM-YSZ cathodes [2]. These findings have significant implications for preventing the degradation of nanostructured anodes, which is due to the coarsening of the fractal roughness of Ni nanoparticles [1], as well as for the optimisation of composite cathodes, indicating that the adsorption and surface diffusion of oxygen limit the rate of the oxygen reduction reaction (ORR) [2]. In both anodes and cathodes, the results point out that the surface properties of the materials are key in determining the performance and lifetime of SOFCs, demonstrating the benefits of adopting a model-based approach in the study of fuel cell electrodes.
References
[1] A. Bertei, E. Ruiz-Trejo, K. Kareh, V. Yufit, X. Wang, F. Tariq, N.P. Brandon, Nano Energy 38 (2017) 526.
[2] A. Bertei, M.P. Carpanese, D. Clematis, A. Barbucci, M.Z. Bazant, C. Nicolella, Solid State Ionics 303 (2017) 181
Morphology and electrochemical activity of SOFC composite cathodes: II. Mathematical modelling
No full textThis paper presents a mathematical model of mass and charge transport and electrochemical reaction in porous composite cathodes for application in solid oxide fuel cells. The model describes a porous composite cathode as a continuum, and characterises charge and mass transfer and electrochemical kinetics using effective parameters (i.e. conductivity, diffusivity, exchange current) related to morphology and material properties by percolation theory. The model accounts for the distribution of morphological properties (i.e. porosity, tortuosity, density of contacts among particles) along cathode thickness, as experimentally observed on scanning electron microscope images of LSM/YSZ cathodes of varying thickness. This feature allows the model to reproduce the dependence of polarisation resistance on thickness, as determined by impedance spectroscopy on LSM/YSZ cathodes of varying thickness. Polarisation resistance in these cathodes is almost constant for thin cathodes (up to 10 A mu m thickness), it sharply decreases for intermediate thickness, to reach a minimum value for about 50 A mu m thickness, then it slightly increases in thicker cathodes
The thermodynamics of protonation and copper(II) complex formation of a-aminomalonic acid
No full textThe thermodynamic functions (ΔG⊖, ΔH⊖, and ΔS⊖) for the protonation of α-aminomalonic acid were determined in aqueous solution (25°C, l = 0.1 mol dm-3 NaCl). The basicity constants and enthalpy changes were evaluated from potentiometric and calorimetric data for both the primary amino and the carboxylate groups. On the basis of the thermodynamic changes a closed structure is hypothesized with the protonated amino nitrogen hydrogen-bonded to both the carboxylate groups to form two five-membered rings. The protonation of the CO2- groups provokes opening of the rings. A relatively high stability constant for complexation with Cu2+ was determined, however in a narrow range of low pH. The co-ordination to the copper(II) occurs both with nitrogen and the two oxygen donor atoms, to form two five-membered chelate rings
Amidic alginate hydrogel for nucleus pulposus replacement
No full textDegeneration of intervertebral discs is the most common cause of back pain. The first phase of this degenerative process involves the nucleus pulposus (NP). A rapid recovery of this structure can prevent further degradation of the annulus fibrosus. A new amidic derivative of alginate (AAA) was developed to obtain a polysaccharide possessing some of the physical–chemical properties of Hyal (i.e. viscosity) without losing the rigidity of the native alginate structure. The modified polysaccharide was crosslinked using 1.3 diaminopropane as crosslinking agent.
The hydrogel obtained was characterized in terms of water uptake and rheological behavior. In particular, the viscoelastic behavior of the hydrogel was determined in shear stress under dynamic conditions and compared with the behavior of nondegenerated human lumbar NP. We then assessed the effect of the AAA hydrogel on NHC (Normal Human Chondrocyte) cell viability and on the production of important extracellular matrix factors, such as glycosaminoglycans and Type II collagen. In conclusion, the results achieved in this study demonstrated that the amidic alginate based scaffold is a promising material to be utilized in the replacement of NP
Protonation thermodynamics of membranes grafted with polyelectrolytes controlling solute permeability
No full textThe protonation of two polyelectrolytes with carboxyl groups, grafted on porous cellulose and polyurethane supports, was evaluated by potentiometry, titration calorimetry and Fourier transform IR (FT-IR) spectroscopy in aqueous media. The large potentiometric hysteresis loops observed were indicative of interactions between functional groups of the graft chains and the substrate. This was confirmed by FT-IR spectroscopic and calorimetric analysis. The thermodynamic properties are reviewed in relation to applications concerning solute permeability. The pH-sensitive porous membrane, having a ''chemical valve'' function, self-regulated insulin release in response to the pH of the surrounding solution. The porosity of the membrane was controlled by the charge state conformations of grafted polyacids
The role of plasma proteins and stress in the assesment of hemocompatibility
No full textThe physiological and psychological conditions of subjects supplying blood for hemocompatibility tests significantly affect the behavior of platelets in terms of both adhesion and activation. The responses of platelets to a standard biomaterial, polyethylene (PE), were examined with blood collected from male rabbits both in basal conditions and after stress, Different media were utilized. First, platelet-rich plasma (PRP) was used to obtain a PE response to contact with platelets. Then platelets drawn from PRP were isolated and washed with Krebs-Ringer solution. One aliquot was suspended in serum (Pw-S) where fibrinogen was absent, another aliquot in Krebs-Ringer solution (Pw-KR) tin order to avoid the influence of the plasma proteins on platelets), and a third aliquot in the original plasma from which the platelets were drawn (Pw-PPP) tin order to restore the initial condition of the plasma but with washed platelets). The analysis of platelet adhesion and morphology was performed by Scanning Electron Microscopy (SEM). Differences in platelet adhesion and morphology were observed with four different media in nonstressed animals, with Pw-PPP showing a higher number and Pw-S and PW-KR lower numbers. Platelet morphology indicated low levels of activation. The platelets drawn from stressed subjects could not be counted in either PRP or PPP medium because they were fully aggregated and adhered; in contrast, in Pw-KR and Pw-S, no significant differences were found with respect to nonstressed conditions, and there was little difference in platelet morphology. All of these factors underline the role of plasma proteins, in particular fibrinogen, in the stress-induced activation of platelet
Novel carboxymethylcellulose-based microporous hydrogels suitable for drug delivery
No full textSeveral materials capable of acting as structures for controlled release were analysed for the fabrication of matrices. Among those used, hydrophilic polysaccharides appeared to be the most suitable materials. Carboxymethylcellulose (a semi-synthetic polysaccharide) was chemically cross-linked with a 60% and 90% cross-linking degree in order to obtain hydrogels and utilised as matrix for the realisation of controlled drug release systems. The morphology of the gels was changed in order to obtain a microporous structure with different porosity (14, 30 and 40 μm). The obtained porous matrices were characterised in terms of pore density, dimension and swelling behaviour. The influence of both the pore dimension and technique of loading on the release kinetics was analysed. By increasing the pore dimension the release of ibuprofen-lysin was slower. Inducing the microporous structure after the loading of the hydrogel with the drug resulted in a slower release. © VSP 2004
ESF Exploratory Workshop on Proton Conducting Materials for Next-generation Solid Oxide Fuel Cells, Genova, Italy, 2007 Foreword
No full textThe Convenors are grateful to the Standing Committee for the Physical and Engineering Sciences (PESC) of the European Science Foundation (ESF) for the sponsorship of this Exploratory Workshop on Proton Conducting Materi- als for Next-Generation Solid Oxide Fuel Cells.
Following ESF indications, with up to 30 participants from several different countries, our aim was to group prominent researchers in the fields of PC-SOFC and of traditional SOFC, in order to achieve synergy among sci- entists aware of the opportunities and issues relating to these two technologies. The choice was difficult and, in addition to leading researchers in the two fields, a few scientists involved in the FP7-project IDEAL-Cell were invited. This was because IDEAL-Cell, through the development of an original fuel cell coupling a protonic, with a traditional, SOFC, really matched the spirit the Workshop
The immobilization of titania nanoparticles on hyaluronan films and their photocatalytic properties
No full textWe have developed a method to bind titania nanoparticles onto hyaluronic films (HA) photoimmobilized on silanized glass. Titania nanoparticles were deposited on the HA films from commercially available dispersions by casting and dip-coating methods at various pHvalues. XPS was used to monitor the deposition of titania and to estimate the surface coverage of the nanoparticles. The topography of the titania-modified HA films was investigated by means of AFM. XPS results indicate that the titania surface coverage depends on the preparation method and the pH of the dispersion. We found that the maximum titania nanoparticle surface coverage was obtained by the casting method with the formation of aggregates and multilayers of particles. The titania surface coverage for the surfaces prepared
by the dip-coating method is pH-dependent. The surfaces prepared at pH 2 show a surface coverage of 65% and a rather uniform distribution of particles. We found that titania nanoparticles are anchored in a stable way to the HA substrate in a phosphate buffer solution (PBS) and that the interaction between the HA and the titania is through the carbonyl group of
carboxylates and amidic groups of the polymer. AFM images clearly show that titania nanoparticles are uniformly distributed over the HA films. By measuring the average diameter
and the average height of the nanoparticles deposited on HA films it appears that the particles are partially embedded in the polysaccharide films. The results of the study on the photobleaching of methylene blue indicate that the characteristic photocatalytic activity of titania is maintained when the nanoparticles are anchored to the HA substrate
The role of Fibrinogen conformation on platelets activation
No full textAbstract
Platelet adhesion and activation induced by fibrinogen (Fbg) coating on polysaccharide layers of hyaluronic acid (Hyal) and its sulfated derivative (HyalS) were analyzed. Hyal or HyalS was coated and grafted on the glass substrate using a photolithographic method. The Fbg coating was achieved by two different routes: the immobilization of Fbg by means of covalent bond to the polysaccharide layers and the mere adsorption of Fbg to Hyal and HyalS surfaces. Platelet adhesion and activation to the surfaces were evaluated using, respectively, scanning electron microscopy (SEM) and quantifying the release of Platelet Factor 4 by ELISA. The method used for the coating of the surfaces with the Fbg influenced the platelet response. In fact, platelet adhesion and activation took place on surfaces covered by bound Fbg but not on those containing adsorbed Fbg. To explain this difference, the molecular mechanism involved in the Fbg--platelet interaction was investigated blocking platelet membrane receptors by monoclonal antibodies. Because the interaction between Fbg and the GPIIb/IIIa platelet membrane receptor was the only molecular pathway involved, Fbg conformation after the interaction (adsorption or binding) with the Hyal and the HyalS chains and the role of serum proteins adsorbed on the Fbg containing surfaces were accurately analyzed. Both adsorbed and bound Fbg prevented the adsorption of further serum proteins; consequently, a direct interaction between Fbg and platelets was supposed and the different platelet behavior was ascribed to the different conformational changes that occurred after the adsorption and the chemical binding of the Fbg to the Hyal and HyalS surfaces
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