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Raptis D.,University of Patras | Dracopoulos V.,FORTH ICE HT | Lianos P.,University of Patras
Journal of Hazardous Materials | Year: 2017

The present work has studied renewable hydrogen production by photoelectrocatalytic degradation of model organic substances representing biomass derived organic wastes. Its purpose was to show that renewable energy can be produced by consuming wastes. The study has been carried out by employing nanoparticulate WO3 photoanodes in the presence of ethanol, glycerol or sorbitol, i.e. three substances which are among typical biomass products. In these substances, the molecular weight and the number of hydroxyl groups increases from ethanol to sorbitol. The photocurrent produced by the cell was the highest in the presence of ethanol, smaller in the case of glycerol and further decreased in the presence of sorbitol. The photocurrent was roughly the double of that produced in the absence of an organic additive thus demonstrating current doubling phenomena. Hydrogen was produced only under illumination and was monitored at two forward bias, 0.8 and 1.6 V vs Ag/AgCl. Hydrogen production rates followed the same order as the photocurrent thus indicating that hydrogen production by reduction of protons mainly depends on the current flowing through the external circuit connecting photoanode with cathode. The maximum solar-to-hydrogen efficiency reached by the present system was 2.35%. © 2017 Elsevier B.V.

Avgouropoulos G.,FORTH ICE HT | Avgouropoulos G.,University of Patras | Neophytides S.G.,FORTH ICE HT
Journal of Applied Electrochemistry | Year: 2012

Copper-based reforming catalyst was placed adjacent to ADVENT Technologies high-Temperature polymer electrolyte membrane/electrode assembly in a novel internal reforming methanol fuel cell (IRMFC) and tested for their electrochemical properties and chemical stability under various methanol/water anode feedstreams. Polarization measurements and AC impedance spectroscopy combined with measurements of reactor outlet composition were carried out. Methanol is being reformed inside the anode compartment of the fuel cell at 200 °C producing H 2, which is readily oxidized at the anode to produce electricity. The reformer provides enough hydrogen supply for efficient fuel cell operation at 600 mV with 0.2 A cm -2 and a hydrogen stoichiometric ratio of 1.2 (λ H2 = H 2 fed/H 2 reacted = 1.2). However, unreacted MeOH (∼5 %) in combination with low H 2 content poisons the anode electrode and the cell performance rapidly decreases. Gradual recovery of the initial performance under pure H 2 is observed after switching to pure H 2. A slight improvement of the cell's design by the introduction of a pre-reforming step significantly improves the electrocatalytic behavior. © Springer Science+Business Media B.V. 2012.

Pop L.-C.,University of Patras | Dracopoulos V.,FORTH ICE HT | Lianos P.,University of Patras
Applied Surface Science | Year: 2015

Photoelectrocatalytic hydrogen production was realized my means of a double electrode carrying photocatalyst and electrocatalyst, deposited side by side on an FTO electrode, acting as a "Photoelectrocatalytic Leaf". As photocatalyst we used plain commercial nanoparticulate titania and as electrocatalyst a conductive carbon film made by a commercial carbon paste enriched with a small quantity of Pt nanoparticles (0.0134 mg/cm2). This quantity of Pt is much smaller than used in other applications and it may be further optimized. Hydrogen was produced in an alkaline environment in the presence of ethanol acting as sacrificial agent. A few variants of electrode geometry were studied in order to set the basic terms for efficient hydrogen production. It was found that optimal electrode geometry necessitates a much larger area for photocatalyst coverage than electrocatalyst and that it is preferable to divide photocatalyst and electrocatalyst areas in alternating zones. © 2015 Elsevier B.V. All rights reserved.

Sfaelou S.,University of Patras | Sygellou L.,FORTH ICE HT | Dracopoulos V.,FORTH ICE HT | Travlos A.,Greek National Center For Scientific Research | Lianos P.,University of Patras
Journal of Physical Chemistry C | Year: 2014

Nanocrystalline titania films deposited on FTO transparent electrodes were sensitized by CdS nanoparticles synthesized in situ by the SILAR method. Three precursor solutions were employed for adsorption of Cd2+ ions: nitrate, sulfate, and acetate. The CdS load and the size of CdS nanoparticles varied a lot from one precursor to the other. The highest load and the largest nanoparticles were obtained in the case of cadmium acetate, the smallest in the case of nitrate, while sulfate gave intermediate values. Ultraviolet photoelectron spectroscopy was employed to measure the ionization potential (and valence band potential) for pure titania and for sensitized titania in the three cases. The obtained values were 7.4 eV for pure titania and 6.7, 6.6, and 5.7 eV for CdS-sensitized titania made by using cadmium nitrate, sulfate, and acetate precursors, respectively. The corresponding band gap values ranged between 2.5 and 2.3 eV. The thus characterized photoanodes were employed to make Quantum Dot Sensitized Solar Cells and PhotoFuelCells. Acetate-derived photoanodes were more effective in the case of Quantum Dot Sensitized Solar Cells, while nitrate-derived precursors were more effective in the case of PhotoFuelCells. This reversed behavior is due to the fact that, in the first type of cells, what matters is mainly the extent of sensitization capacity, while in the case of PhoFuelCells, what matters most is the capacity of the combined semiconductor to oxidize the fuel. © 2014 American Chemical Society.

Nikolakopoulou A.,University of Patras | Tasis D.,FORTH ICE HT | Sygellou L.,FORTH ICE HT | Dracopoulos V.,FORTH ICE HT | And 3 more authors.
Electrochimica Acta | Year: 2013

Graphene oxide has been synthesized by oxidation of graphite flakes and has been deposited on trans-parent conductive electrodes by casting of its aqueous suspension. Then it was annealed at different temperatures to produce thermally reduced graphene oxide and thus to be employed as counter electrode in quasi-solid-state dye-sensitized solar cells. A variety of techniques has been employed, namely, XPS, TGA, XRD, Raman, FE-SEM and electrochemical analysis to characterize the different phases of graphene oxide reduction, to determine the temperature of completion of thermal reduction and to optimize graphene oxide film thickness. Employment of thermally reduced graphene oxide as electro-catalyst on counter electrodes gave satisfactory current and voltage but very poor fill factor. Then the conductive polymer PEDOT has been deposited on the graphene oxide film by simple and rapid one-step potentiostatic electro deposition. The mixed PEDOT/reduced-graphene-oxide film demonstrated abehavior comparable with Pt nanoparticulate electrocatalyst. © 2013 Elsevier Ltd. All rights reserved.

Balis N.,University of Patras | Dracopoulos V.,FORTH ICE HT | Bourikas K.,Hellenic Open University | Lianos P.,University of Patras
Electrochimica Acta | Year: 2013

Quantum dot sensitized solar cells have been constructed using photoanodes made of nanocrystalline titania and an optimized combination of ZnS, CdS and CdSe nanoparticles. Pt, CoS and CuS have been used as electrocatalysts on counter electrodes. Attachment of quantum dot sensitizers on mesoporous titania was made by successive ionic layer adsorption and reaction and by chemical bath deposition obeying a certain order, where the first layer was crucial in defining the quality and the quantity of the subsequent layers as well as of the ensuing solar conversion efficiency. Thus the first quantum dot layer consisted of 75% CdS and 25% ZnS and it was followed by a CdSe layer and by an additional ZnS layer on the top. The quantity of material deposition seems to be affected not only by the employed deposition method but also and mainly by the nature of the underlying layer. Optimized anode electrodes led to solar cells producing high current densities but did not much affect open-circuit voltage. The maximum solar conversion efficiency reached in this work was 2.7% and was obtained by using CuS electrocatalyst. Both CoS and CuS gave high currents and this was in line with the low charge transfer resistances recorded in their case. © 2013 Elsevier Ltd. All rights reserved.

Pop L.-C.,University of Patras | Sfaelou S.,FORTH ICE HT | Lianos P.,University of Patras
Electrochimica Acta | Year: 2015

Current-voltage curves recorded during the study of photoelectrochemical cells functioning in the presence of aqueous electrolytes contain a characteristic anodic current feature, which may interfere in the interpretation of the results. This anodic current is observed at negative potentials and it derives from the adsorption of cations into the mesoporous structure of nanoparticulate titania photoanodes. The shape and the height of the peak depends on the size of the cation, its concentration, the thickness of the titania film and the nature of the counter ion and the corresponding pH. This anodic current is similar to the one observed in other works during ion storage and electrochromism studies with mesoporous titania. In the presence of an organic sacrificial agent, like ethanol, adsorption of cationic species is still strong but it decreases at higher organic content. © 2015 Elsevier Ltd. All rights reserved.

Syrrokostas G.,University of Patras | Siokou A.,FORTH ICE HT | Leftheriotis G.,University of Patras | Yianoulis P.,University of Patras
Solar Energy Materials and Solar Cells | Year: 2012

This work investigates the stability of platinum (Pt) electrodes prepared by electrodeposition and thermal decomposition of hexachloroplatinic acid (H 2PtCl 6) solutions. To this aim, the electrodes were stored in an electrolyte solution (0.5 M KI, 0.05 M I 2 in 90%/10% PC/EG) or in air, within a closed vessel, for up to 70 days. A drop of up to 40% in the current density for triiodide reduction was caused by storage in the electrolyte solution and of about 15%-20% when the electrodes were stored in air. Electrolyte storage was found to cause a 40% increase in the activation energy of both kinds of electrodes. XPS measurements have shown that the main degradation mechanism in the case of electrolyte storage is the dissolution of Pt from the substrate. Regeneration of the electrodes by heat or acid treatment was not possible. The observed electrode degradation is expected to appear in dye sensitized solar cells using this kind of counter electrodes and thus affect their long term stability. © 2012 Elsevier B.V. All rights reserved.

Trunov M.L.,Uzhgorod National University | Lytvyn P.M.,Institute of Semiconductor Physics of Ukraine | Yannopoulos S.N.,FORTH ICE HT | Szabo I.A.,Debrecen University | Kokenyesi S.,Debrecen University
Applied Physics Letters | Year: 2011

Surface relief gratings formation in amorphous selenium thin films in two recording configurations with light intensity modulation were studied in situ by real-time atomic force microscopy and diffraction efficiency measurements. We report observation of mass transport effect in films induced by band-gap irradiation when the light polarization of the recording beams has a component along the light intensity gradient (p-p scheme of recording) that allows obtaining giant stable gratings in this versatile chalcogenide material. On the contrary, only a pure scalar weak grating caused by photoinduced volume shrinkage is obtained in the s-s recording configuration, even for long-term irradiation. © 2011 American Institute of Physics.

Sfyris D.,FORTH ICE HT | Sfyris G.I.,LMS | Galiotis C.,FORTH ICE HT
International Journal of Solids and Structures | Year: 2015

Abstract We lay down a nonlinear elastic constitutive framework for the modeling of some 2D crystals of current interest. The 2D crystals we treat are graphene, hexagonal boron nitride and some metal dichalcogenides: molybdenium disulfide (MoS2), tungsten selenium (WSe2), and niobium diselenide (NbSe2). We first find their arithmetic symmetries by using the theory of monoatomic and diatomic 2-nets. Then, by confinement to weak transformation neighborhoods and by applying the Cauchy-Born rule we are able to use the symmetries continuum mechanics utilizes: geometric symmetries. We give the complete and irreducible representation for energies depending on an in-plane measure, the curvature tensor and the shift vector. This is done for the symmetry hierarchies that describe how symmetry changes at the continuum level: C→C→C1 for monoatomic 2-nets, and C→C→C1 for diatomic two nets. We stress that we do not take into account how energy behaves at the transition regime. Having these energies at hand we are able to evaluate stresses and couple stresses for each symmetry regime. These quantities participate to the field equations: the momentum equation, the moment of momentum equation and the equation ruling the shift vector. By making specific assumption for the loading histories that correspond to in-plane biaxial tension/compression as well as in-plane simple shear, we obtain necessary expressions for the shift vector components in order all field equations to be satisfied. When the algebraic equations ruling the shift vector render infinite solutions, we use the Cauchy-Kowalevski theorem-when it applies-for the momentum equation viewed as a quasilinear system in order to single out the unique solution. © 2015 Elsevier Ltd.

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