Search

Το αντικείμενο της παρούσας διατριβής είναι η φωτοηλεκτροχημική παραγωγή ηλεκτρικού ρεύματος και υδρογόνου με χρήση φωτοστοιχείων καυσίμου. Τα φωτοστοιχεία καυσίμου είναι φωτοηλεκτροχημικά κελιά τα οποία αποτελούν έναν εναλλακτικό τρόπο μετατροπής της ηλιακής ενέργειας σε χρήσιμες μορφές ενέργειας, με παράλληλη κατανάλωση και αποδόμηση διαφόρων ειδών οργανικών ή ανόργανων ρύπων που χρησιμοποιούνται ως θυσιαζόμενες ενώσεις. Ένα φωτοστοιχείο καυσίμου αποτελείται από το ηλεκτρόδιο της ανόδου στο οποίο εναποτίθεται ένας νανοδομημένος ημιαγωγός που δρα ως φωτοκαταλύτης και το αντιηλεκτρόδιο στο οποίο εναποτίθεται ο ηλεκτροκαταλύτης. Τα δύο αυτά ηλεκτρόδια είναι βυθισμένα σε ένα διάλυμα ηλεκτρολύτη στο οποίο προστίθεται η θυσιαζόμενη ένωση και συνδέονται μεταξύ τους μέσω εξωτερικού ηλεκτρικού κυκλώματος. Φωτόνια με ενέργεια υψηλότερη από το ενεργειακό χάσμα του ημιαγωγού της φωτοανόδου, απορροφώνται από τον ημιαγωγό και μετατρέπονται σε ζεύγη ηλεκτρονίων-οπών. Στη συνέχεια, οι φωτοπαραγόμενες οπές στη ζώνη σθένους, κινούνται προς τη διεπιφάνεια ημιαγωγού/ηλεκτρολύτη όπου συμμετέχουν σε αντιδράσεις οξείδωσης των θυσιαζόμενων ενώσεων, ενώ τα ηλεκτρόνια μεταφέρονται μέσω του εξωτερικού κυκλώματος στο αντιηλεκτρόδιο όπου και συμμετέχουν σε αναγωγικές αντιδράσεις. Στα φωτοστοιχεία καυσίμου τα οποία περιείχαν οργανική θυσιαζόμενη ένωση χρησιμοποιήθηκε η αιθανόλη, ως αντιπροσωπευτικό δείγμα αλκοολών που μπορούν να βρεθούν σε διάφορα είδη αποβλήτων και παραπροϊόντων βιομάζας. Ως φωτοκαταλύτης χρησιμοποιήθηκε το TiO2¬ φωτοευαισθητοποιημένο ή μη με διαφορετικά είδη κβαντικών τελειών ενώ έγινε επίσης μελέτη διαφορετικών ηλεκτροκαταλυτών με στόχο την αντικατάσταση του λευκόχρυσου. Εκτός από τις διάφορες κατηγορίες οργανικών ενώσεων που μπορούν να δράσουν ως θυσιαζόμενες ενώσεις σε ένα φωτοστοιχείο καυσίμου, υπάρχουν και ανόργανες ενώσεις που μπορούν να λειτουργήσουν ως αποτελεσματικοί δέκτες οπών και να ευνοήσουν το διαχωρισμό των φορέων φορτίου. Σε αυτή την περίπτωση, χρησιμοποιήθηκαν δύο διαφορετικά μίγματα θείου (S/Na2S και Na2S/Na2SO3) τα οποία είναι παραπροϊόντα της επεξεργασίας ορυκτών καυσίμων. Στην πρώτη περίπτωση, η μελέτη βασίστηκε μόνο στην παραγωγή ηλεκτρικού ρεύματος καθώς η χρήση μίγματος S/Na2S οδηγεί στη δημιουργία ιόντων S22- η αναγωγή των οποίων ανταγωνίζεται την αναγωγή των υδρογονοκατιόντων με αποτέλεσμα να μην ευνοείται ο σχηματισμός αερίου υδρογόνου. Αντίθετα, στην περίπτωση της χρήσης ηλεκτρολύτη που περιείχε Na2S/Na2SO3 μελετήθηκε τόσο η παραγωγή ηλεκτρικού ρεύματος όσο και η παραγωγή υδρογόνου. Ως φωτοάνοδος χρησιμοποιήθηκε το TiO2 συνδυασμένο με διαφορετικά είδη κβαντικών τελειών ενώ οι ηλεκτροκαταλύτες που μελετήθηκαν ήταν κυρίως θειούχα μέταλλα (CuS, CoS, Cu2S) καθώς ο λευκόχρυσος δεν είναι σταθερός σε θειούχους ηλεκτρολύτες και αυξάνει την αντίσταση μεταφοράς φορτίου.Επίσης, στα πλαίσια μελέτης της φωτοηλεκτροχημικής παραγωγής υδρογόνου, εκτός από το TiO2 έγινε σύνθεση και μελέτη των WO¬3 και BiVO4. Τα δύο αυτά υλικά είναι ημιαγωγοί μεσαίου ενεργειακού χάσματος και πλεονεκτούν έναντι της τιτάνιας καθώς εμφανίζουν απορρόφηση στην ορατή περιοχή του φάσματος.

The development of porous carbon materials as highly efficient, durable, and economic electrocatalysts for oxygen reduction reaction (ORR) is of great importance for realizing practical applications of many significant energy conversion and storage devices. Herein, we demonstrate a general approach to porous carbons decorated with boron centers and atomically dispersed Fe–Nx species (denoted as FeBNC). The as-prepared FeBNC can serve as efficient electrocatalysts for ORR in an alkaline medium with a half-wave potential of 0.838 V vs RHE, comparable to that of the state-of-the-art porous carbon catalysts and the benchmark system Pt/C. Theoretical calculation reveals that incorporation of boron dopant into traditional Fe–Nx species-enriched porous carbons significantly lowers the energy barrier for oxygen reduction and therefore boosts the overall performance. This work not only provides an easy method to synthesize B-doped Fe–Nx centers-enriched porous carbons as highly efficient electrocatalysts for ORR a...

Bismuth vanadate is a promising visible-light driven photocatalyst due to its convenient band gap value and position. However, the efficiency of BiVO4 is very low due to high recombination rates. In this work, a comparative study between pristine and Nb-modified BiVO4 films was made. Modified BiVO4 has shown a hierarchical nanostructure, but its energy band gap remained equal to pristine BiVO4. The IPCE efficiency of Nb-BiVO4 increased importantly. The ideal nominal content of Nb(V) in BiVO4 was 10 at%. At such atomic rate, the quantity of photoelectrochemically produced hydrogen by water splitting significantly increased compared to pristine BiVO4. Photocatalytic degradation of organic compounds by Nb-modified bismuth vanadate films also exhibited higher oxidation rate.

Owing to their unique structure and intriguing properties, 2D transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), have attracted tremendous attention. Chemical surface functionalization of TMDs can be used to tune their properties and broaden their application perspective. Unfortunately, covalent functionalization of TMDs into sandwich-type hybrid materials remains challenging, owing to the chemically rather inert basal plane and the poor solution processability of TMDs. Herein, we report an efficient approach for the preparation of MoS2-templated, cobalt-containing conjugated microporous polymer sandwiches (MoS2−Co−MP), starting from basal-plane-functionalized MoS2. The resulting MoS2−Co−CMP hybrids can easily be converted into MoS2-cored, hierarchically porous carbon materials (MoS2−Co−C) of high specific surface area through direct pyrolysis. The MoS2−Co−C 2D materials exhibit excellent oxygen reduction reaction activity, approaching the performance of commercial Pt/C catalysts. Moreover, MoS2−Co−C shows a promising electrochemical energy storage capability, with a high capacitance value up to 288 F g−1 coupled with remarkable cycle stability. Such a promising performance can be attributed to synergistic effects between the MoS2 template and the graphitized, hierarchically porous carbon shells with a homogeneous distribution of nitrogen centres as well as probable involvement of catalytically active Co−N or/and Co−N−C structural motifs.

WO 3 photoanodes have been constructed by a simple soft chemistry procedure that produced efficient mesoporous nanocrystalline films. These photoanodes absorbed visible light and could be efficiently employed for photoelectrochemical hydrogen production under electric bias. The current increased and the rate of hydrogen production more than tripled in the presence of a small quantity of ethanol showing that such photoanodes may be successfully used in alternative photoelectrochemical installations for solar fuel production by consumption of organic wastes.

Nanocrystalline bismuth vanadate was deposited by simple wet chemistry procedure on FTO electrodes in order to construct visible light responsive photoanodes, which were employed for photoelectrochemical hydrogen production by water splitting. The specific surface area of the films was controlled by the presence of a surfactant template (Triton X-100). It was, however, found that when the quantity of surfactant was relatively high, the stoichiometry of the photocatalyst as well as its activity was lost. The obtained materials were characterized by SEM, XRD, AFM, BET and UV–Vis spectroscopy. Optimized photoanodes were employed in photoelectrochemical cells for water splitting and hydrogen production under electric and chemical bias. Maximum hydrogen production rate was 0.15 mmol/h under electric bias of 1.4 V vs Ag/AgCl plus 0.37 V chemical bias.

Cadmium selenide has been deposited on nanoparticulate titania films by one-step dynamic electrodeposition in aqueous environment using cadmium sulfate and selenium oxide mixtures as precursors. The molar concentration of cadmium sulfate was always 100 times higher than SeO 2 . Amorphous CdSe films have been obtained and they were characterized by several spectroscopic and microscopic techniques. Nanoparticulate titania films supported on transparent FTO electrodes, sensitized by the deposited CdSe, were employed as photoanode in photoelectrochemical cells using Cu 2 S/brass-foil as counter electrode. The applicability of the sensitized photoanode was tested for the production of hydrogen by water reduction under basic conditions, using a standard mixture of sodium sulfide and sodium sulfite as sacrificial agent. The present work shows that a functional cell can be easily obtained using non-sophisticated procedures that provide conditions for upscaling.

The main objective of this study was the comparison of activated sludge reactors with reactors containing biocarriers using a wastewater containing phenanthrene as a model compound simulating the presence of toxic substances. Five sequencing batch reactors were used. One contained a porous polyvinyl alcohol gel (PVA-gel) biocarrier and another had a high-density polyethylene biocarrier, while the other three reactors consisted of conventional activated sludge. The addition of phenanthrene at low concentration (15 μg/L in influent wastewater) did not adversely affect the removal efficiencies of chemical oxygen demand (COD) and ammonium (i.e. nitrification performance). However, with a higher addition of phenanthrene (150 μg/L in influent wastewater), a reduction in COD removal efficiency and an inhibitory effect on denitrification was observed. Generally, nutrient removal was poor, with the exception of denitrification in the reactor containing the PVA-gel. It seems that PVA-gel beads allow the for...

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.

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.

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.

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 MSxNy (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 MSxNy catalysts, leading to enhanced HER kinetics. Toward the activity origin of MS2N2, the experimental and theoretical results disclose that the metal atoms are preferentially protonated and then the production of H2 is favored on the MN active sites after a heterocoupling step involving a N-bound proton and a metal-bound hydride. Moreover, the tuning of the metal centers in MS2N2 leads to the HER performance in the order of FeS2N2 > CoS2N2 > NiS2N2. 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.

Photoelectrochemical cells have been constructed using quantum dot sensitized nanocrystalline titania photoanodes and were operated under photo-fuel-cell operation to produce electricity or water-splitting operation to produce hydrogen. In the first case, the cell functioned in the presence of an alkaline electrolyte using ethanol as fuel. The obtained data allowed to define the optimal thickness of the titania film, which was approximately 15 μm. Functional sensitizers were CdS or ZnSe combined with CdS. Small band gap quantum dot sensitizers like CdSe and PbS were not functional in photo-fuel-cells, owing to the limited oxidative power of their valence-band holes. For water-splitting operation, we mainly used a S 2− /SO 3 2− electrolyte. In that case, panchromatic sensitization is possible. Thus the photoanode, which gave the highest hydrogen production rate, was constructed by combining TiO 2 /FTO with ZnS/CdSe/CdS quantum dots. Hydrogen was produced by applying an external bias of 0.5 V. In the absence of bias the quantity of hydrogen was very small. Hydrogen production rate was also very small in a purely alkaline environment with ethanol as fuel.

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 Sola...

Quantum dot sensitized solar cells have been constructed using photoanodes bearing nanocrystalline titania and CdS, CdSe and ZnS quantum dots. The combined CdSe/CdS system provides enhanced photon absorption in a large range of the visible spectrum, while ZnS plays a passivation and stabilization role. This was verified by micro-Raman scattering studies as a function of light soaking of successively sensitized photoanodes. Annealing of photoanodes resulted in increasing quantum dot size as verified by both UV-vis diffuse reflectance and Raman spectroscopy. It was also found that a composite CdSxSe1−x species was formed at the interface of CdS and CdSe layer, which constitutes a stabilization factor and is further stabilized by annealing, that influences the electric characteristics of the sensitized photoanodes. Single CdS sensitizers, when annealed gave increased photocurrents but suffered a fast decay. Annealing of fully sensitized photoanodes gave a diverse behavior, markedly stabilizing photocurrents under N2 atmosphere. All cells worked well by using a Pt-free cathode made by chemically growing Cu2S on a brass foil. The employed innovative cell structure provided liberty in designing and optimizing the device geometry.

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.

Solid-state quantum dot sensitized solar cells based on nanocrystalline titania and poly(3-hexylthiophene) demonstrated an improved conversion efficiency by a combination of ZnSe and CdSe quantum dot sensitizers. Thus, a combination of 30% ZnSe–70%CdSe gave higher efficiency than CdSe or ZnSe alone. This result is justified by the combination of energy states, which facilitates charge transfer and leads to charge separation. Characterization of the photoanodes was made by UV–vis absorption spectrophotometry and micro-Raman spectroscopy, which showed a strong coupling between quantum dots facilitating charge transfer. Another important finding was that in the presence of a small quantity of Na2S a further increase of cell efficiency was observed, underlining the importance of ionic additives in modulating cell characteristics.

Photoanode electrodes have been made by layer deposition of CdS, CdSe and ZnS quantum dots on nanocrystalline titania and were studied by micro-Raman and photoluminescence techniques or were assembled with a Cu 2 S-on-brass cathode to make quantum dot sensitized solar cells. Spectroscopic characteristics of the photoanodes and electric characteristics of the cells were studied as a function of the illumination time and the photoanode material and structure. Combined semiconductor quantum dot sensitizers gave more stable photoanodes than simpler components. Stability increased by successive CdS, CdSe and ZnS deposition. ZnS was the most crucial component the presence of which on the top of the subsequent layers consolidated stability. Furthermore, analysis of the Raman spectra revealed the formation of CdS x Se 1- x composites at the interface of the CdS/CdSe layers, which may be an additional stabilization factor. Thick cells were used in this work where the distance between anode and cathode electrodes was 5 mm. Despite this large distance, the cells produced high currents, thanks to the high ionic conductivity of aqueous polysulfide electrolyte and the liberty in configuring the active area of the cathode electrode.

Quantum dot sensitized solar cells were constructed using nanoparticulate titania or titania nanotubes and a combination of CdS, CdSe and ZnS quantum dots as visible light sensitizers. The latter were deposited in three stages, first a combined ZnS25%–CdS75% directly on mesoporous titania, then CdSe and, finally, a ZnS layer on the top. Cathode electrodes carried metal sulfide electrocatalysts. Three cases were examined, CuS on FTO, CoS on FTO and Cu2S grown on brass. Their behavior was equivalent, however, CuS/FTO gave the highest efficiency of 3.5%. Contrary to the common practice, thick cells were used where the distance between anode and cathode electrode was 5 mm and the cell was constructed in such as manner as to provide versatility in geometry. Thus despite this large distance between the two electrodes, which is huge compared to the distance of a few tens of micrometers in the case of standard thin cells, the electric characteristics of the cell were satisfactory thanks to the liberty in designing the active area of each electrode. It was found that metal sulfides deposited on either the anode or the cathode electrode must be annealed in inert atmosphere before cell assembly. Otherwise they may give higher but decaying currents. Unusually high curr nts were attributed to u stable species, which are oxidized during cell operation.

Photo-fuel-cells constitute an alternative means of solar energy conversion. A photo-fuel-cell is a photoelectrochemical cell, which produces electricity by photocatalytically degrading organic substances. Since photocatalysts are not selective towards the photodegradable substance, any organic material or a mixture of materials, including water-soluble wastes can play the role of the fuel. The photo-fuel-cell may be used to produce hydrogen by photoelectrocatalytic reforming of a fuel or by water splitting, however, this operation can be carried out only under bias. Production of electricity is an unbiased processes generating substantial electric power. More power can be generated by employing two-compartment cells, where the compartments are separated by an ion-transfer membrane. Some commercial products have been tried as ion-transfer membranes, however, this matter remains a technological challenge. The data presented in this work are based on a nanocrystalline titania photoanode sensitized in the Visible by quantum dot sensitizers.

This chapter presents PhotoFuelCells as an alternative means of conversion of solar energy into electricity or hydrogen. PhotoFuelCells are photoelectrochemical cells, which can consume an inorganic or (mainly) organic substance to produce a useful form of energy. If the consumed substance is a waste, PhotoFuelCells offer the double environmental benefit of renewable energy production with environmental remediation. A PhotoFuelCell functions by employing a photocatalyst to carry out oxidation and an electrocatalyst to carry out reduction reactions. The chapter is a review of the authors' recent work in this domain but it also discusses some recent unpublished data, as examples of PhotoFuelCell application.

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.

Hydrogen can be produced by photoelectrochemical water splitting in the presence of S 2 − /SO 3 2− sacrificial agent by using a Cu 2 S electrocatalyst grown on brass by chemical treatment in HCl and polysulfide solution. Photoelectrochemical behavior of Cu 2 S/brass was equivalent to that of Pt nanoparticles mixed with carbon black and deposited on a carbon cloth electrode, i.e. the electrocatalyst typically used in fuel cells and electrolyzers. Thus photoelectrochemical water splitting can be achieved in a Pt-free device. The photoanode was made of nanocrystalline titania on FTO sensitized by quantum dots. Combined ZnS/CdSe/CdS quantum dots were by far more effective for hydrogen production than single-component CdS sensitizer.

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. Cu

Oxygen reduction reaction (ORR) lies at the heart of many renewable energy technologies. Development of facile approaches toward highly active carbon materials for ORR is urgently desirable. We accidentally discovered a method for preparing heteroatom-doped carbon catalyst through an in situ self-templating and self-sacrificing strategy during the carbonization of a single polymer precursor. The prepared N,S co-doped and hierarchical porous carbon possesses a large specific surface area up to 2446 m2 g−1, which is the state-of-the-art among the porous carbons prepared by direct pyrolysis of linear polymer precursors. The use of in situ nanoarchitecturing and active-site engineering ensures that the N,S co-doped porous carbon catalyst has sufficient active sites for charge/mass transport and catalysis. As expected, as-prepared carbonaceous electrocatalyst exhibited superior ORR activity in both alkaline and acidic electrolytes and outperformed the state-of-the-art benchmark Pt/C and the best carbonaceous electrocatalysts for ORR in aqueous media so far. Moreover, the N,S co-doped carbon catalyst was demonstrated to be a highly efficient air cathode for Zn–air batteries. Thus, this study provides a new and economical method of fabricating feasible carbon electrocatalysts with exceptional catalytic activities for energy conversion and storage.

Nanocrystalline titania photoanodes, sensitized with CdS, ZnS, CdSe, Sb