Stavroula Sfaelou

Photocatalysis for Renewable Energy Production Using PhotoFuelCells

The present work is a short review of our recent studies on PhotoFuelCells, that is, photoelectrochemical cells which consume a fuel to produce electricity or hydrogen, and presents some unpublished data concerning both electricity and hydrogen production. PhotoFuelCells have been constructed using nanoparticulate titania photoanodes and various cathode electrodes bearing a few different types of electrocatalyst. In the case where the cell functioned with an aerated cathode, the cathode electrode was made of carbon cloth carrying a carbon paste made of carbon black and dispersed Pt nanoparticles. When the cell was operated in the absence of oxygen, the electrocatalyst was deposited on an FTO slide using a special commercial carbon paste, which was again enriched with Pt nanoparticles. Mixing of Pt with carbon paste decreased the quantity of Pt necessary to act as electrocatalyst. PhotoFuelCells can produce electricity without bias and with relatively high open-circuit voltage when they function in the presence of fuel and with an aerated cathode. In that case, titania can be sensitized in the visible region by CdS quantum dots. In the present work, CdS was deposited by the SILAR method. Other metal chalcogenides are not functional as sensitizers because the combined photoanode in their presence does not have enough oxidative power to oxidize the fuel. Concerning hydrogen production, it was found that it is difficult to produce hydrogen in an alkaline environment even under bias, however, this is still possible if losses are minimized. One way to limit losses is to short-circuit anode and cathode electrode and put them close together. This is achieved in the “photoelectrocatalytic leaf”, which was presently demonstrated capable of producing hydrogen even in a strongly alkaline environment.

Sulfur-doped porous carbon nanosheets as high performance electrocatalysts for PhotoFuelCells

Sulfur-doped porous carbon nanosheets have been prepared by pyrolysis of graphene-coupled conjugated microporous polymers under an inert atmosphere. The obtained carbon nanosheets exhibited large specific surface areas up to 642 m2 g−1 and high sulfur weight content up to 7.11%. These highly porous carbon nanosheets have been studied as metal-free oxygen reduction electrocatalysts in alkaline environments and they were found to undergo oxygen reduction via a major 4-electron transfer pathway. They were then examined as substitutes for Pt–carbon electrocatalysts in PhotoFuelCells functioning in the presence of ethanol as a model fuel. It has been shown that sulfur doped porous carbon nanosheets yield functional cells with approximately the same characteristics as those employing Pt–carbon electrocatalysts, therefore, they mark a new class of metal-free catalysts.

BiOI solar cells

An inorganic solar cell was constructed using a thin compact supporting layer of titania with BiOI nanoflakes as a functional material, a Pt/FTO cathode and a I3−/I− redox electrolyte. The efficiency of the cell was 1.03% but this leaves a lot of ground for improvement, which is mainly expected to come from the optimization of the BiOI nanostructure.

Quantum Dot Sensitized Titania as Visible-light Photocatalyst for Solar Operation of Photofuel Cells

Abstract Nanocrystalline titania photoanodes, sensitized with CdS, ZnS, CdSe, Sb2S3 and PbS quantum dots have been employed in photofuel cells functioning with alkaline electrolyte and ethanol as fuel. Ethanol was photocatalytically oxidized producing electric current. It was found that medium band gap semiconductors, obtained by mixing ZnS and CdS, were the best sensitizers since they combine sensitization capacity in the Visible with sufficient oxidative power for oxidizing ethanol. However, in the presence of small band gap semiconductors, like CdSe, Sb2S3 and PbS, the oxidative power is diminished and the system demonstrates a poor behavior. Nanocrystalline titania sensitized by ZnS-CdS makes photoanodes with remarkable stability in the presence of ethanol, which acts as sacrificial agent.

Toward Activity Origin of Electrocatalytic Hydrogen Evolution Reaction on Carbon‐Rich Crystalline Coordination Polymers

The fundamental understanding of electrocatalytic active sites for hydrogen evolution reaction (HER) is significantly important for the development of metal complex involved carbon electrocatalysts with low kinetic barrier. Here, the MSx Ny (M = Fe, Co, and Ni, x/y are 2/2, 0/4, and 4/0, respectively) active centers are immobilized into ladder-type, highly crystalline coordination polymers as model carbon-rich electrocatalysts for H2 generation in acid solution. The electrocatalytic HER tests reveal that the coordination of metal, sulfur, and nitrogen synergistically facilitates the hydrogen ad-/desorption on MSx Ny catalysts, leading to enhanced HER kinetics. Toward the activity origin of MS2 N2 , the experimental and theoretical results disclose that the metal atoms are preferentially protonated and then the production of H2 is favored on the MN active sites after a heterocoupling step involving a N-bound proton and a metal-bound hydride. Moreover, the tuning of the metal centers in MS2 N2 leads to the HER performance in the order of FeS2 N2 > CoS2 N2 > NiS2 N2 . Thus, the understanding of the catalytic active sites provides strategies for the enhancement of the electrocatalytic activity by tailoring the ligands and metal centers to the desired function.

Studying the Formation of Biofilms on Supports with Different Polarity and Their Efficiency to Treat Wastewater

The main objective of this study was the evaluation of biofilm formation onto different supports and of biofilm efficiency to treat wastewater. Two different reactors were used, one with porous polyvinyl alcohol gel (PVA) biocarrier and another with a high-density polyethylene (PE) biocarrier. The reactor performance was evaluated and the biofilm formed was analyzed with potentiometric mass titrations. The biofilm formation was monitored with diffuse reflectance spectroscopy. The presence of the support did not alter the nature of the biofilm. However, the quantity of the biofilm formed was higher when polar surface groups were present on the support.

Synergistic Effect of CdS and PbS Sensitizers in Quantum Dot Sensitized Solar Cells

Quantum dot sensitized solar cells have been constructed by using photoanodes based on nanoparticulate titania sensitized with PbS or CdS quantum dot semiconductors and by their combination. PbS alone gave a pure behavior excluding it as sensitizer. When CdS was added on the top of the PbS layer, an impressive synergy was demonstrated inducing a ten-fold current increase. In addition, the presence of CdS provided protection to the combined sensitizer thus offering a satisfactory solar cell performance.

Quantum-dot-sensitized Solar Cells with Metal Electrodes

Abstract Quantum dot sensitized solar cells have been made by using nanocrystalline titania as photocatalyst, sensitized in the Visible by a combination of quantum dot sensitizers: first a layer of CdS, followed by deposition of CdSe and finally a passivation layer of ZnS on the top. An inox grid was used as anode electrode and its functionality was compared with that of transparent fluorine-doped tin oxide (FTO) electrodes. Cu2S, chemically grown on a brass foil, was used as cathode electrode. The cell operated with an aqueous polysulfide electrolyte. The electrochemical potential of the Cu2S/brass electrode in combination with the polysulfide electrolyte was measured and found +0.33 V vs RHE. The highest solar conversion efficiency of 4.1% was obtained with a photoanode of 1 cm2 active geometrical area grown on an FTO electrode. The efficiency of the cell dramatically dropped when the active area of the photoanode increased. This was the result of the fact that transparent FTO electrodes are low conductivity materials with limited ability of photogenerated charge collection, leading to a very low value of fill factor. The substitution of FTO by an inox grid leads to an impressive increase of the fill factor. However, the overall efficiency of the cells based on inox photoanodes remained at low level necessitating an improvement of catalyst deposition conditions in that case.