Fotis Paloukis | Foundation for Research and Technology - Hellas (original) (raw)

Papers by Fotis Paloukis

Macromolecules, May 18, 2011

Novel aromatic polyethers bearing polar pyridine units along the main chain and side cross-linkab... more Novel aromatic polyethers bearing polar pyridine units along the main chain and side cross-linkable propenyl groups have been successfully synthesized. Their properties relating to their ability to be used as polymer electrolyte membranes for high temperature fuel cell applications, were thoroughly investigated. Cross-linked membranes were obtained by thermal curing of the cross-linkable polymers with the use of a bisazide as the cross-linking agent. The glass transition temperatures of the cross-linked membranes were determined by dynamic mechanical analysis and found to be higher compared to the neat polymers proving the successful cross-linked network. The doping ability in phosphoric acid and the proton conductivity of the cross-linked membranes were higher compared to the noncross-linked analogues. Finally, membrane electrode assemblies (MEAs) were constructed and tested in a single cell at temperatures between 180 and 220°C. The superior performance of the cross-linked membranes in combination with the operating stability at 200°C for 48 h demonstrate the potential use of these materials as electrolytes for high temperature PEM fuel cells.

ECS transactions, Dec 17, 2019

Aromatic polysulfones containing 2,5-bis(4-hydroxyphenyl) pyridine, or tetramethyl-biphenyl-diol ... more Aromatic polysulfones containing 2,5-bis(4-hydroxyphenyl) pyridine, or tetramethyl-biphenyl-diol units or 2,5-bis(4-methyl phenyl)-hydroquinone have been synthesised employing various comonomers' ratios. The resulting soluble high molecular weight copolymers were thoroughly investigated as potential electrolytes for high temperature PEM fuel cells. Excellent film forming properties, mechanical integrity, as well as high thermal and oxidative stability were confirmed for all the polymeric materials. The above characteristics, in combination with the high phosphoric acid doping level which results in conductivities in the range of 10-2 S cm-1, enabled the use of these poylelctrolytes as the proton conducting membranes in fuel cells operating at temperatures up to 200°C. A discussion on the effect of the chemical structure on the membranes' properties and fuel cell performance is herein presented.

Chemical Engineering Science, Aug 1, 2007

Numerical simulation has been used to show the feasibility of the autothermal cogeneration of syn... more Numerical simulation has been used to show the feasibility of the autothermal cogeneration of synthesis gas and electricity in a solid oxide fuel cell (SOFC) by the electrochemical partial oxidation of CH 4. Owing to the large positive entropy change of the CH 4 partial oxidation reaction and its low heating value, severe cooling effect is being induced in the SOFC due to heat absorbance by the reaction products. For this reason the autothermal operation of the SOFC reactor cannot be secured. As it is shown this can be overcome by combining the dynamic operation of the SOFC under forced periodic reversal of the flow and the bleeding of a small amount of CH 4 (< 2.5%) in the oxidant stream (cathode). In this respect the catalytic combustion of CH 4 , on the perovskite cathodic electrode, provides the necessary energy demand so that in combination with flow reversal operation the SOFC is maintained ignited even at inlet temperature as low as 300 K. It is shown that the overall thermodynamic efficiency of the process can by far exceed unity (> 2), thus revealing the unique property of the SOFCs to produce high-quality energy and useful chemicals.

Journal of Physical Chemistry B, Dec 30, 2003

Cyclic voltammetry (CV) and X-ray and UV photoelectron spectroscopy (XPS, UPS) were employed to s... more Cyclic voltammetry (CV) and X-ray and UV photoelectron spectroscopy (XPS, UPS) were employed to study the electrochemical deposition and dissolution of nickel on a polycrystalline gold electrode, in a 0.1 M NiSO4 electrolyte. The modification of the Ni/Au interface upon thermal treatment in ultrahigh vacuum was also investigated. Depending on the applied potential, two distinct Ni chemical states were observed. At −0.5 V versus SCE an oxidized Ni phase, mainly Ni(OH)2, was formed, whereas at potentials lower than −0.8 V nucleation of reduced Ni species surrounded by hydroxidized nickel occurred. The systematic investigation of the adsorbate species binding energies as a function of the applied potential revealed that hydroxidized nickel precipitates on the gold electrode, while reduced nickel species directly deposit on the electrode surface via the reduction of Ni2+ ions. Heating the Ni/Au interface to 650 K decomposes the adsorbed nickel film forming a new compound containing a Au−Ni intermetallic compo...

ECS transactions, Sep 25, 2009

A new approach towards the development of electrocatalytic layers for use in high temperature PEM... more A new approach towards the development of electrocatalytic layers for use in high temperature PEM fuel cells is reported. Modified carbon nanotubes (CNTs) were used as the support. The aim was to achieve a uniform distribution of polar groups, which can interact with phosphoric acid, on the surface of the modified carbon support. MWNTs were selected due to their unique properties and were surface modified introducing pyridine based groups on the side walls. The different supports were thoroughly characterized by means of relevant techniques. Platinum was deposited on the new carbon supports resulting in the newly synthesized catalysts. Initial measurements of the catalytic activity towards oxygen reduction were performed in order to evaluate the potential use of these materials as catalytic layers in HT PEMFCs.

Surface Science, Mar 1, 2004

The thermal stability of Ni overlayers electrochemically prepared on a polycrystalline vanadium (... more The thermal stability of Ni overlayers electrochemically prepared on a polycrystalline vanadium (V) surface, was studied by means of X-ray and UV photoelectron spectroscopies (XPS, UPS). The results showed that upon cathodic polarization, a mixed Ni-Ni(OH) 2 adlayer is formed, while V surface becomes partially oxidized into vanadium oxide (V 2 O x). The applied potential during nickel electrodeposition is crucial for both Ni-Ni(OH) 2 composition and V 2 O x film thickness. Annealing the Ni-Ni(OH) 2 /V 2 O x interface under UHV conditions leads to the gradual reduction of the films and the formation of a new Ni-V intermetallic phase. A solid state reaction accompanies the decomposition of nickel and vanadium oxides resulting in oxygen migration from the Ni adlayer to the V support. The electronic structure of Ni-V intermetallic phase formed after oxide decomposition is compared to previously reported results for Ni alloys and intermetallic compounds.

Nano-Structures & Nano-Objects, 2020

The performance and stability of electrocatalysts strongly depend on the physicochemical characte... more The performance and stability of electrocatalysts strongly depend on the physicochemical characteristics, such as the surface area, the crystalline structure, size and shape of the particles and the interactions with the support. Platinum (Pt) is one of the most versatile elements in catalysis, efficiently mediating a multitude of chemical reactions. Reducing the demand for expensive Pt is a major driving force in catalysis research. The objective of the present study is the development of electrocatalysts with innovative features and focuses on the synthesis and characterization of highly dispersed Pt supported catalysts using functionalized Multi-Wall Carbon Nanotubes (f-MWCNT) as the substrate. The choice of the substrate was based on the unique properties of carbon nanotubes that generate strong interest for their use in many nano-material devices in catalysis, batteries, optics, gas storage, electronics, sensors and others. Various chemical pathways were used to modify the sidewalls of MWCNT and introduce several covalently attached groups like pyridine, sulfonic acid, carboxyl, benzene or zwitterion type moieties. Using these substrates, deposition of Pt nanoparticles was realized by means of the polyol synthetic procedure. The parameters of the synthetic procedure differentiate the reduction/deposition mechanism. The surface chemistry and functionalities of the support and the diverse reaction conditions resulted in differentiations of the properties, not only in terms of quantitative deposition, but also with respect to platinum oxidation state, nanoparticle size and dispersion and overall catalyst morphology. The combined characterization techniques used shed light into the fundamental understanding of the support effect on the final properties, while metal-support interactions are intensely discussed.

Applied Surface Science, 2021

Abstract Using chemically modified supports is a strategy to differentiate metal deposition, cata... more Abstract Using chemically modified supports is a strategy to differentiate metal deposition, catalyst morphology and properties and, ultimately, the electrochemical activity towards the multiparametric and structure-sensitive Oxygen Reduction Reaction (ORR). Functionalized carbon nanotubes with pyridine groups, oxpyMWCNT, is a support that allows fine spatial dispersion of nanoparticles. This work reports the development of Pt supported on oxpyMWCNT with different metal loadings using the polyol method in alkaline reaction media. The effect of the pH value of the reaction solution and the reduction time on the deposition, the morphological and surface characteristics of the formed electrocatalysts was thoroughly investigated. The electrochemical performance of the aforementioned electrocatalysts towards ORR was evaluated. By using the aforementioned support, small particle size and fine and homogeneous dispersion were achieved that allowed the correlation between the properties and the electrocatalytic activity. Thus, there is a discussion about how mass and specific activity towards ORR are simultaneously affected by metal-support interactions, interparticle distance (particle proximity) and nanoparticle size.

ACS Applied Energy Materials, 2019

Due to their fuel flexibility and high efficiency, solid oxide cells are a promising technology f... more Due to their fuel flexibility and high efficiency, solid oxide cells are a promising technology for sustainable energy production and storage. Nickel in combination with yttria-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC), forming Ni-YSZ or Ni-GDC cermets respectively, are the most widely adopted electrodes in solid oxide fuel cell fabrication. Currently, there is an increasing interest in cermet electrodes for hydrogen generation through high temperature steam electrolysis using solid oxide electrolysis cells (SOECs). However, durability remains a major issue for reliable operation of SOEC systems. A variety of processes accountable for permanent performance degradation of SOECs has been identified based on post-mortem cell analysis. Besides, transient/reversible degradation processes are typically examined by indirect methods, like impedance spectroscopy. The reason is that the application of material characterization techniques during SOEC operational conditions is challenging. In this work we provide a direct correlation between Ni-YSZ and Ni-GDC electrode surface oxidation states and their performance during steam electrolysis using operando experimental evidence provided by near ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopy. We show that nickel surface oxidation may induce significant performance degradation in Ni-YSZ cathodes while, on the contrary, having minor effects in Ni-GDC. Remarkably, we found that in the case of Ni-GDC electrodes, small modification of the GDC oxidation state can have an important impact on the electrolysis performance. The results highlight the crucial role of the cathode electrode surface oxidation state on the SOEC functionality and have potential implications for the design and operation strategies of more efficient and durable SOEC devices.

ACS Applied Materials & Interfaces, 2017

Understanding the surface chemistry of electrode materials under gas environments is important in... more Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La0.75Sr0.25Cr0.9Fe0.1O3, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La0.75Sr0.25Cr0.9Fe0.1O3, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Journal of Catalysis, 2017

The solid oxide electrolysis cell (SOEC) technology has a huge potential for future mass producti... more The solid oxide electrolysis cell (SOEC) technology has a huge potential for future mass production of hydrogen, mainly due to its high electrical-to-chemical energy conversion efficiency. However, the durability and the performance of SOEC devices are inferior to that of other competitive electrolysis technologies inhibiting the commercialization of SOECs. Despite the fact that Ni-based cermets are currently the most widely used cathode materials for SOEC, change of the nickel oxidation state has been accused as a major issue limiting the performance of these devices. In this work we provide operando experimental evidence of the active surface oxidation state and composition of nickel/doped-ceria cermets under water electrolysis conditions using ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, combined with quantitative spectra simulation. Remarkably under specific operational conditions, nickel is maintained in a partially oxidized state which, counterintuitive to the expected behavior, can be beneficial to the cell performance. This finding may initiate new improvement strategies for SOEC electrodes based on thorough optimization of the operational conditions, in order to engineer in situ the most propitious electrode configuration.

ChemPhysChem, 2016

Nickel/doped‐ceria composites are promising electrocatalysts for solid‐oxide fuel and electrolysi... more Nickel/doped‐ceria composites are promising electrocatalysts for solid‐oxide fuel and electrolysis cells. Very often steam is present in the feedstock of the cells, frequently mixed with other gases, such as hydrogen or CO2. An increase in the steam concentration in the feed mixture is considered accountable for the electrode oxidation and the deactivation of the device. However, direct experimental evidence of the steam interaction with nickel/doped‐ceria composites, with adequate surface specificity, are lacking. Herein we explore in situ the surface state of nickel/gadolinium‐doped ceria (NiGDC) under O2, H2, and H2O environments by using near‐ambient‐pressure X‐ray photoelectron and absorption spectroscopies. Changes in the surface oxidation state and composition of NiGDC in response to the ambient gas are observed. It is revealed that, in the mbar pressure regime and at intermediate temperature conditions (500–700 °C), steam acts as an oxidant for nickel but has a dual oxidant/...

Journal of Catalysis, 2020

Perovskite materials are typically used as oxygen electrodes of solid oxide fuel and electrolysis... more Perovskite materials are typically used as oxygen electrodes of solid oxide fuel and electrolysis cells (SOC). The high stability of the perovskite structure in oxidative environments makes it a good candidate as a cathode electrode for steam electrolysis SOC as well. In this work, we investigate SOC with La 0.75 Sr 0.25 Cr 0.9 Fe 0.1 O 3 perovskite cathodes employing near ambient pressure X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies combined with online electrical measurements. Based on operando experimental evidences the surface state of the perovskite electrode is directly associated with the electrocatalytic performance of the cell. The results indicate that under steam electrolysis operating conditions the well-known Sr surface enrichment is accompanied by Cr segregation and formation of SrCrO 4-like oxide. In contrast to the common perception of the role of surface chromites, we show here that its presence does not induce cell deactivation, but on the contrary, is beneficial for cell performance.

Energy Conversion and Management

Journal of Power Sources, Nov 1, 2016

Abstract In this paper, a study of the lithiation mechanism of micro-grain structured silicon ano... more Abstract In this paper, a study of the lithiation mechanism of micro-grain structured silicon anode is presented. Micro-grain amorphous silicon was deposited on special copper foil and it is shown that after several decades of galvanostatic cycles, it preserves its granular nature with minor degradation. In order to shed light on the lithiation mechanisms of the micro-grain silicon, Electrochemical Impedance Spectroscopy (EIS) was conducted on silicon half-cells at various State-of-Charge (SoC) and various discharging current values and the Solid-Electrolyte Interphase (SEI) R SEI and polarization resistance R pol were determined. Results reveal that R pol highly increases for cell voltages lower than 0.2 V and it strongly depends on the discharging C-rate. From X-ray Photoelectron Spectroscopy (XPS) measurements combined with surface sputtering, the existence of a Li x Si y O z interlayer between SEI and silicon is confirmed, which is believed to play an important role to the lithium kinetics. Finally, combining our results, a lithiation mechanism of the micro-grain silicon anode is proposed.

International journal of molecular sciences, 2015

The utilization of fermentation media derived from waste and by-product streams from biodiesel an... more The utilization of fermentation media derived from waste and by-product streams from biodiesel and confectionery industries could lead to highly efficient production of bacterial cellulose. Batch fermentations with the bacterial strain Komagataeibacter sucrofermentans DSM (Deutsche Sammlung von Mikroorganismen) 15973 were initially carried out in synthetic media using commercial sugars and crude glycerol. The highest bacterial cellulose concentration was achieved when crude glycerol (3.2 g/L) and commercial sucrose (4.9 g/L) were used. The combination of crude glycerol and sunflower meal hydrolysates as the sole fermentation media resulted in bacterial cellulose production of 13.3 g/L. Similar results (13 g/L) were obtained when flour-rich hydrolysates produced from confectionery industry waste streams were used. The properties of bacterial celluloses developed when different fermentation media were used showed water holding capacities of 102-138 g·water/g·dry bacterial cellulose, v...

Electrochimica Acta, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Abstract: The utilization of fermentation media derived from waste and by-product streams from bi... more Abstract: The utilization of fermentation media derived from waste and by-product streams from biodiesel and confectionery industries could lead to highly efficient production of bacterial cellulose. Batch fermentations with the bacterial strain Komagataeibacter sucrofermentans DSM (Deutsche Sammlung von Mikroorganismen) 15973 were initially carried out in synthetic media using commercial sugars and crude glycerol. The highest bacterial cellulose concentration was achieved when crude glycerol (3.2 g/L) and commercial sucrose (4.9 g/L) were used. The combination of crude glycerol and sunflower meal hydrolysates as the sole fermentation media resulted in bacterial cellulose production of 13.3 g/L. Similar results (13 g/L) were obtained when flour-rich hydrolysates produced from confectionery industry

Fuel cells using methanol as a fuel are promising future energy technology. Methanol is electro-o... more Fuel cells using methanol as a fuel are promising future energy technology. Methanol is electro-oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The excellent catalytic activity of Pt for methanol oxidation makes this metal electrocatalysts ideal for use as an anode in Direct Methanol fuel cells (DMFCs). However, Pt metal surface is easily poisoned at low temperatures by trace amounts of CO, which exists as a byproduct of methanol electrooxidation. Theoretical and experimental studies have shown that ternary PtRuCo catalyst significantly promotes the methanol oxidation reaction [1]. In this study we investigate the PtRuCo surface for the CH3OH and CO oxidation in the presence of H2O.

ECS Meeting Abstracts

It is well known that silicon presents one of the most important anode materials for the improvem... more It is well known that silicon presents one of the most important anode materials for the improvement of lithium-ion cells in terms of energy density. Indeed, silicon’s high theoretical specific capacity to lithium (more than 3800 mAh/g at room temperature), environmental friendliness, low potential compared to lithium and material abundance turn silicon to a strong candidate for the replacement of carbon-based anodes [1,2]. However, one of the main drawbacks of silicon’s application to the lithium-ion technology is its poor electrochemical cycling stability over several galvanostatic cycles, mainly due to silicon’s huge volume change (around 300%) during lithiation and delithiation that leads to high internal mechanical stress and therefore to film fracturing and delamination [3]. Many alternatives have been proposed that alleviate this mechanical expansion issue either through the use of nanostructured silicon sometimes combined with carbon-based materials or with special coatings ...