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Dragomir M.,University of Nova Gorica | Arcon I.,University of Nova Gorica | Arcon I.,Jozef Stefan Institute | Arcon I.,CO NOT | And 3 more authors.
Acta Materialia | Year: 2013

Studies performed on the NdVO4-BiVO4 system showed the existence of the BixNd1-xVO4 homogeneity range for x ≤ 0.49(1). Extended X-ray absorption fine structure and X-ray diffraction analyses confirmed that Bi3+ is incorporated onto the Nd site in the NdVO4 crystal structure with some distortion of the local structure. Surprisingly, the unit cell volume decreases with the increase in the content of the larger Bi3+ ion. On the other side of the NdVO4-BiVO4 system, Nd3+ does not enter the BiVO4 structure but forms the NdVO4-based secondary phase. Ultraviolet-visible spectroscopy showed that the band gap of NdVO 4 can be reduced to below 3.1 eV by the Bi doping. New emissions that do not exist for NdVO4 have been found in the 650-675 nm range of BixNd1-xVO4 photoluminescence spectra. The observed chemical and optoelectronic properties were explained on the basis of the hybridization of Bi 6s2 and O 2p orbitals. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Pregelj B.,Jozef Stefan Institute | Pregelj B.,CO NOT | Vrecko D.,Jozef Stefan Institute | Vrecko D.,CO NOT | And 2 more authors.
Journal of Power Sources | Year: 2011

Polymer electrolyte membrane fuel cells are proving to be a clean and efficient source of energy. Nowadays, extensive research efforts are being focused on bringing this technology to everyday use. An important aspect when integrating fuel cells in practical applications is their ability to respond to load demand. With respect to this, due to their complex internal dynamics, fuel cells belong to the group of more slowly responding sources. In order to make them more generally applicative they are often connected with a battery or a super-capacitor via a power converter to form a hybrid power source. A control algorithm, designed for such a system, represents an interesting challenge: it has to adapt to varying working conditions and operate optimally in terms of efficiency and reliability, while minimizing any impacts on the degradation of the components. Here, we present an approach using supervisory control automaton that switches between the system's operational modes and sets the references for the lower-level control loops. The evaluation of the efficiency and degradation is carried out in a simulation using a model of the widely used 1.2-kW Ballard Nexa power module. © 2011 Elsevier B.V.

Mihelcic M.,Slovenian National Institute of Chemistry | Francetic V.,University of Ljubljana | Pori P.,Chemcolor Sevnica D.o.o. | Gradisar H.,Slovenian National Institute of Chemistry | And 3 more authors.
Applied Surface Science | Year: 2014

Polyhedral oligomeric silsesqioxanes (POSS) compounds consisting of [RSiO3/2]n groups organized in the form of various polyhedra (Tn, n = 3, 6, 8, 10, 12, ....) have not often been used as pigment surface modifiers. Their interactions with pigments are not known in detail and coatings deposited from pigments modified by POSS dispersants are rare. Identification of interactions between a dispersant and the surface of pigments is important from the point of view of obtaining stable pigment dispersions enabling the deposition of optical coatings with high pigment loading, low haze and mechanical integrity. Thin TiO2 (anatase) pigment coatings (70-260 nm) were deposited from pigment dispersions prepared by milling metatitanic acid (mTiA) powder agglomerates with trisilanol heptaisobutyl silsesquioxane dispersant (trisilanol POSS) in butanol and hexane. The results of TEM, EDAX and TG measurements confirmed the influence of trisilanol POSS dispersant on the formation of a dispersion with a uniform distribution of mTiA and rutile (mTiR) nanoparticles with a size of about 30 ± 5.0 nm and 90 ± 5.0 nm, respectively, as determined from dynamic light scattering (DLS) measurements. The mTiA/trisilanol POSS dispersions with added titanium tetraisopropoxide were deposited on fluorine-doped tin oxide (FTO) coated glass (spin-coating) and indium tin oxide coated polymeric substrate (ITO PET) (coil-coating) and thermally treated at 150 °C. UV-vis spectra, AFM and SEM results showed that the pigment coatings exhibited low haze (up to 6%), low surface roughness (up to 20 nm) and uniform morphology. mTiA/trisilanol POSS interactions were assessed from the frequency shifts of the SiOSi stretching modes of trisilanol POSS, while the adsorption of the dispersant was followed from the intensity changes of the Corresponding CH 3 and CH2 stretching modes, confirming the gradual occupation of the mTiA crystalline sites by trisilanol POSS dispersant. Examination of IR vibrational spectra showed that trisilanol POSS interacted with the mTiA surface by establishing hydrogen bonding. The advantage of using trisilanol POSS dispersant was demonstrated by the enhanced electrochromic effect of the mTiA pigment coatings. © 2014 Elsevier B.V. All rights reserved.

Pivko M.,Slovenian National Institute of Chemistry | Pivko M.,CO NOT | Bele M.,Slovenian National Institute of Chemistry | Bele M.,CO NOT | And 8 more authors.
Chemistry of Materials | Year: 2012

A two-step procedure for preparation of LiMnPO 4 with small particle size (15-20 nm) and embedded in a carbon matrix is presented. The crucial point that prevents excessive particle growth is the avoidance of lithium in the first firing step, so that small Mn 2P 2O 7 particles embedded in carbon are obtained. Because of the carbon matrix, the Mn 2P 2O 7 particles also cannot grow in the second step, which involves lithiation and heating to 700 °C in argon. The prepared LiMnPO 4 shows a high theoretical capacity (up to 95% of the theoretical value) and a stable cycling (>130 mAh/g, even after 100 cycles at 55 °C and a rate of C/20). At room temperature and using the CC-CV mode, the performance is comparable to the best result shown in the literature so far. Finally, the performance of LiMnPO 4 is briefly compared with that of LiFePO 4. © 2012 American Chemical Society.

Mazaj M.,Slovenian National Institute of Chemistry | Mali G.,Slovenian National Institute of Chemistry | Rangus M.,Slovenian National Institute of Chemistry | Zunkovic E.,Slovenian National Institute of Chemistry | And 4 more authors.
Journal of Physical Chemistry C | Year: 2013

Structural dynamics of Ca(BDC)(DMF)(H2O) with rhombic-shaped channels and 44 net topology upon heating and hydration were elucidated by using complementary methods of diffraction (XRD) and spectroscopy (FT-IR, MAS NMR, EXAFS, XANES). During heating the Ca(BDC)(DMF)(H2O) framework underwent structural changes in two steps. The first change at 150 °C includes breaking of Ca-O bonds with H2O and DMF molecules. In this step, DMF is removed from the surface or near the surface of the crystals. The affected parts of the crystals are transformed to a new nonporous Ca-BDC(400) phase that prevents the diffusion of DMF from the cores of the crystals. Second transition at 400 C led to the complete transformation to Ca-BDC(400). This phase is reversibly transformed to a pseudo-3-D framework Ca(BDC)(H2O)3 upon exposure to humid environment. We proposed mechanisms of Ca-BDC(RT) → Ca-BDC(400) and Ca-BDC(400) → Ca(BDC)(H2O)3 transformations, which include breaking of the bonds between Ca2+ and carboxylate groups, rotating of BDC ligand, and recoordination of COO- groups to Ca2+ centers. The crystal-to-crystal transformations are driven by the tendencies to change the bonding modes between COO- and Ca2+ with the change of Ca2+ coordination number. Thus the decrease in Ca2+ coordination number, which is usually a consequence of activation, does not lead to the expansion or contraction of the pores, but it leads to pronounced structural rearrangement. Such behavior can explain the lack of porosity in Ca-MOF systems. © 2013 American Chemical Society.

Pivko M.,Slovenian National Institute of Chemistry | Pivko M.,CO NOT | Arcon I.,CO NOT | Arcon I.,University of Nova Gorica | And 8 more authors.
Journal of Power Sources | Year: 2012

Two stable modifications of A 3V 2(PO 4) 3 (A = Na or Li) were synthesized by citric acid assisted modified sol-gel synthesis. The obtained samples were phase pure Li 3V 2(PO 4) 3 and Na 3V 2(PO 4) 3 materials embedded in a carbon matrix. The samples were tested as half cells against lithium or sodium metal. Both samples delivered about 90 mAh g -1 at a C/10 cycling rate. The change of vanadium oxidation state and changes in the local environment of redox center for both materials were probed by in-situ X-ray absorption spectroscopy. Oxidation state of vanadium was determined by energy shift of the absorption edge. The reversible change of valence from trivalent to tetravalent oxidation state was determined in the potential window used in our experiments. Small reversible changes in the interatomic distances due to the relaxation of the structure in the process of alkali metal extraction and insertion were observed. Local environment (vanadium-oxygen bond distances) after 1st cycle were found to be the same as in the starting material. Both structures have been found very rigid without significant changes during alkali metal extraction. © 2012 Elsevier B.V.

Gasperin M.,Jozef Stefan Institute | Boskoski P.,Jozef Stefan Institute | Debenjak A.,Jozef Stefan Institute | Petrovcic J.,Jozef Stefan Institute | Petrovcic J.,CO NOT
Fuel Cells | Year: 2014

This paper presents an alternative computational method for on-line estimation and tracking of the impedance of PEM fuel cell systems. The method is developed in order to provide the information to diagnostics and health management system. Proper water management remains the main issue influencing the reliability and durability of PEM fuel cell technology. While literature reviews reveal the thorough understanding of the underlying processes and extensive experimental work, the existing implementations rely on expensive hardware or time consuming computational methods. In this scope, we will show how the characteristic values of the fuel cell impedance, required by the diagnostic system, can be computed by robust and computationally efficient algorithms, which are suitable for implementation in embedded systems. The methods under consideration include continuous-time wavelet transform (CWT) and extended Kalman filter (EKF). The CWT is a time-frequency technique, which is suitable for tracking transient signal components. The EKF is a stochastic signal processing method, which provides confidence measures for the estimates. The paper shows, that both methods provide accurate estimates for diagnostics of FCS and can perform on-line tracking of these features. The performance of the algorithms is validated on experimental data from a commercial fuel cell stack. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Conradi M.,Slovenian Institute of Metals And Technology | Kocijan A.,Slovenian Institute of Metals And Technology | Zorko M.,Slovenian National Institute of Chemistry | Jerman I.,Slovenian National Institute of Chemistry | Jerman I.,CO NOT
Progress in Organic Coatings | Year: 2012

In this paper we focus on the preparation of thin polymer coatings synthetized from 30-nm and 600-nm silica particles dispersed in polyvinyl chloride (PVC) and deposited on two different steel substrates: duplex DSS 2205 and austenitic AISI 316L steel. We show that a silica surface modification with silane IO 7T 7(OH) 3 (trisilanol isooctyl polyhedral oligomeric silsesquioxane, POSS) significantly improves its dispersion properties when mixed with PVC. For comparison, the surface morphology and surface roughness of PVC coatings filled with both silanated and as-received (non-silanated) silica fillers were analyzed with scanning electron microscopy (SEM) and atomic force microscopy (AFM) when sprayed on the steel surface. The effect of the silica silanization is later on reflected in a decreased average surface roughness in the silanated, compared to non-silanated, silica/PVC-coatings. The wetting properties of the silanated and non-silanated silica/PVC-coatings on DSS 2205 and AISI 316L were investigated using contact-angle and surface-energy measurements, indicating an increased surface hydrophilicity in terms of a decreased static water contact angle and an increased total surface energy compared to the uncoated specimens. Finally, the beneficial corrosion resistance of the silica/PVC coatings was confirmed with potentiodynamic polarization spectroscopy in a 3.5% NaCl solution. © 2012 Published by Elsevier B.V. All rights reserved.

Conradi M.,Slovenian Institute of Metals And Technology | Zorko M.,Slovenian National Institute of Chemistry | Jerman I.,Slovenian National Institute of Chemistry | Jerman I.,CO NOT | And 3 more authors.
Polymer Engineering and Science | Year: 2013

This article focuses on the preparation and mechanical properties of silica/poly(vinyl chloride) (PVC) composites enriched with 60% mass ratio of 130 nm and 30 nm silica sphere fillers. Silica particles were pre-treated with silane, IO7 T7(OH)3 (trisilanol isooctyl polyhedral-oligomeric silsesquioxane) to prevent agglomeration. The dispersion and interfacial compatibility of silica particles in a PVC matrix were investigated by scanning electron microscopy. The composite mechanical properties were characterized by tensile test, revealing improved Young modulus and tensile strength. Compared to pure PVC, the stiffness of 30 nm and 130 nm silica/PVC composites is on average increased by 30-40%, respectively. Similar trend was observed for the composite tensile strength on the change of the silica size. In contrast, elongation at break decreased for both composites compared to pure PVC, for 15% in 30 nm and for 30% in 130 nm silica/PVC composite. Copyright © 2012 Society of Plastics Engineers.

Colovic M.,Slovenian National Institute of Chemistry | Jerman I.,Slovenian National Institute of Chemistry | Jerman I.,CO NOT | Gaberscek M.,Slovenian National Institute of Chemistry | And 3 more authors.
Solar Energy Materials and Solar Cells | Year: 2011

The main objective of this study was to broaden the assortment of I -/I3 - redox ionic liquids using polyhedral oligomeric silsesquioxanes (POSS) acting as nanobuilding blocks for the construction of functionalized 1,3-alkylimidazolium iodide solid (melting temperature 150200 °C) and room temperature (RT) ionic liquids. The structural characteristics of the synthesised final ionic liquids and the corresponding intermediates were determined using 1H, 29Si NMR and infrared spectroscopic measurements. Raman spectra were next reported, in order to demonstrate the presence of polyiodides formed after the addition of iodine and the formation of redox electrolytes. Ionic conductivity values obtained from the impedance (EIS) spectra were determined in the temperature interval from room temperature up to 100 °C. Finally, a hybrid electrochromic cell was constructed from room temperature MePrImI x - IO7 T8 POSS (x=1, 1.2, 3 and 5) ionic liquids encapsulated between a lithiated WO3 working and Pt counter-electrode, and colouringbleaching changes assessed for cells cycled up to 1000 repetitive cycles. © 2011 Elsevier B.V. All rights reserved.

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