Sonia Dsoke - Academia.edu (original) (raw)

Papers by Sonia Dsoke

The Journal of Physical Chemistry C, 2012

A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1... more A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2:3679 | 2.0 alpha 39) 2012/05/08 15:22:00 | PROD-JCA1 | rq_334126 | 5/23/2012 14:45:21 | 12 56 A major source of catalyst degradation is the loss of 57 electrochemically active surface area (ECSA). In several studies 58 the loss of active platinum by dissolution of Pt was 59 observed. 6−10 Dissolved Pt species can migrate and leave the 60 catalytically active zone (e.g., Pt particles can be detected in the 61 ionomer phase, outside the conductive carbon support 7,8 ) or 62 redeposited on other (larger) particles due to their higher 63 equilibrium potential for dissolution (Ostwald ripening). 6,10 64 There is also evidence for ECSA loss due to other mechanisms, 65 such as agglomeration (due to carbon corrosion or the 66 migration of nanoparticles on the carbon support) and 67 coalescence, which was demonstrated. e.g.. in refs 7−14. 68 Attempts to distinguish the relative contribution of each 69 mechanism have also been made, 6,8 but the matter is still under 70 debate. Many works have shown that alloying Pt with transition 71 metals can also improve the stability of the catalyst, particularly 72 those containing cobalt (see, e.g,. refs 3, 12, and 13 and 73 references therein). 74 Several hypotheses have been put forward in the literature to 75 justify the reasons for Pt-based alloys activity and durability 76 increase with respect to pure Pt catalyst. Many experimental 77 and theoretical works show that this enhancement could be 78 attributed mainly to changes in the surface structure and 79 chemical composition of the near-surface region (segregation to 80 the surface and dissolution of atoms and atomic species) and 81 also (as a consequence) in the local geometric structure (i.e., 82 Pt−Pt bond distance, number of Pt nearest neighbors), 83 electronic structure (electron density of states in the Pt 5d 84 band, strength of interaction between the Pt and the 3d-85 transition metal atoms), and nature and coverage of surface 86 oxide layers. 3,4,15−20 However, many of these works considered 87 extended alloy surfaces, and obviously, the mechanisms 88 established for bulk surfaces need not be applicable to 89 nanoparticles. Moreover, often the catalyst was degraded 90 (dealloyed, sintered, annealed, acid treated) in conditions 91 which only mimic the real fuel cell environment. 92 a R [Å] is the mean bond length, σ 2 [10 −3 Å 2 ] is the Debye−Waller-like parameter, and N c is the coordination number.

Journal of Solid State Electrochemistry, 2014

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) ... more A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of highsurface-area zeoli te-t ype aci dic salt of cesium phosphododecatungstate (Cs 2.5 H 0.5 PW 12 O 40 ) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2-5 μg cm −2 ) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs 2.5 H 0.5 PW 12 O 40free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1-2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.

Journal of Power Sources, 2015

ABSTRACT This study shows how the simple modulation of the cathode/anode mass ratio, in a Li-ion ... more ABSTRACT This study shows how the simple modulation of the cathode/anode mass ratio, in a Li-ion capacitor based on activated carbon (AC) and Li4Ti5O12 (LTO), results in a drastic increase in performance. Starting with a device balanced in the classical way (with an AC/LTO mass ratio of 4.17), the cathode/anode mass ratio has been reduced to 1.54 and then to 0.72. At a high power density, the device with a cathode/anode mass ratio of 0.72 shows the highest energy density. In fact, at 2.3 kW L-1, it delivers an energy density of 31 Wh L-1, which is almost 10 times greater than the energy obtained with a capacitor balanced with an AC/LTO ratio of 4.17 (3.68 Wh L-1). Moreover, the reduction in the cathode/anode mass ratio from 4.17 to 0.72 improves the cycling stability with a factor of 4.8 after 1000 cycles at 10C. Electrochemical impedance spectroscopy reveals that the better power performance is due to reduced diffusion and charge transfer resistances. In addition, the anode polarization is less pronounced for the system with a lower AC/LTO mass ratio, leading to a minimization in electrolyte decomposition on the anode surface and therefore limiting the increase in the electrode resistance during cycles.

The present invention relates to the field of fuel cell, in particular to electrode compositions ... more The present invention relates to the field of fuel cell, in particular to electrode compositions for fuel cells (PEMFC - Polymer Electrolyte Membrane Fuel Cell) with low platinum content and discloses the use of a compound of formula M1xHy-xM2M312O40, wherein M1 is selected from the group consisting of Cs+, Rb+, K+, NH4+, x represents the number salified protons from M1 in the corresponding acid, M2 is P or Si and M3 is W or Mo; y = 3, when M2 is P and y = 4 when M2 is Si. Said compound is used in a composition with a noble metal or an alloy thereof, preferably Platinum, for the preparation of an electrode composition for fuel cells.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015

A structural study of Pt nano-catalysts is presented in the paper. The innovation in the case of ... more A structural study of Pt nano-catalysts is presented in the paper. The innovation in the case of the considered catalytic materials resides in the use a meso-microporous inorganic matrix of heteropolyacid salt of composition X 2.5 H 0.5 YMo 12 O 40 where X = Rb, Cs and Y = P, Si as a catalyst support. Metallic nanoparticles were created by platinum ions reduction in a hot H 2 /Argon stream and were mainly located into matrix pores. As the porous structure can be controlled by the type and content of the cation used, the desired/optimal Pt nanoparticle size can be precisely defined by using a proper heteropolyacid salt composition. Multiple-scattering extended X-ray absorption fine structure (MS EXAFS) analysis was applied to obtain the relation between the matrix composition and the size of the catalyst nanoparticles. The results showed that it was only the secondary structure form of the heteropolymolybdate salt (i.e. Rb or Cs cations) that influenced the metallic nanoparticle size. The smaller the X atom ionic radii, the larger the mean diameter of the embedded Pt nanoparticles.

J. Mater. Chem. A, 2015

ABSTRACT Nanostructured Li3V2−xNix(PO4)3 (x = 0, 0.05, and 0.1) cathode materials, with a mean pa... more ABSTRACT Nanostructured Li3V2−xNix(PO4)3 (x = 0, 0.05, and 0.1) cathode materials, with a mean particle dimension ranging from 200 to 63 nm, are successfully synthesized with poly(acrylic acid) and D-(+)-glucose as carbon sources. All three samples show a monoclinic crystalline structure as confirmed by X-ray diffraction and Rietveld analysis. Ni-doping improves the specific capacity of Li3V2(PO4)3/C. Between 3.0 and 4.3 V vs. Li+/Li, all cathodes exhibit good rate capability, even at high C-rates. For these reasons, they are good candidates for high power and energy applications, in particular for the development of high energy density supercapacitors. Li3V1.95Ni0.05(PO4)3/C, because of its highest specific discharge capacity (93 mA h g−1 at 100 C) and capacity retention of 97% after 1000 cycles, is selected for building an asymmetric supercapacitor with activated carbon as the anode. At a power density of 2.8 kW L−1, the asymmetric system delivers 18.7 W h L−1, a value five orders of magnitude higher than that of the symmetric capacitor at the same power level.

Journal of Solid State Electrochemistry, 2014

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) ... more A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of highsurface-area zeoli te-t ype aci dic salt of cesium phosphododecatungstate (Cs 2.5 H 0.5 PW 12 O 40 ) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2-5 μg cm −2 ) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs 2.5 H 0.5 PW 12 O 40free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1-2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.

The Journal of Physical Chemistry C, 2012

A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1... more A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2:3679 | 2.0 alpha 39) 2012/05/08 15:22:00 | PROD-JCA1 | rq_334126 | 5/23/2012 14:45:21 | 12 56 A major source of catalyst degradation is the loss of 57 electrochemically active surface area (ECSA). In several studies 58 the loss of active platinum by dissolution of Pt was 59 observed. 6−10 Dissolved Pt species can migrate and leave the 60 catalytically active zone (e.g., Pt particles can be detected in the 61 ionomer phase, outside the conductive carbon support 7,8 ) or 62 redeposited on other (larger) particles due to their higher 63 equilibrium potential for dissolution (Ostwald ripening). 6,10 64 There is also evidence for ECSA loss due to other mechanisms, 65 such as agglomeration (due to carbon corrosion or the 66 migration of nanoparticles on the carbon support) and 67 coalescence, which was demonstrated. e.g.. in refs 7−14. 68 Attempts to distinguish the relative contribution of each 69 mechanism have also been made, 6,8 but the matter is still under 70 debate. Many works have shown that alloying Pt with transition 71 metals can also improve the stability of the catalyst, particularly 72 those containing cobalt (see, e.g,. refs 3, 12, and 13 and 73 references therein). 74 Several hypotheses have been put forward in the literature to 75 justify the reasons for Pt-based alloys activity and durability 76 increase with respect to pure Pt catalyst. Many experimental 77 and theoretical works show that this enhancement could be 78 attributed mainly to changes in the surface structure and 79 chemical composition of the near-surface region (segregation to 80 the surface and dissolution of atoms and atomic species) and 81 also (as a consequence) in the local geometric structure (i.e., 82 Pt−Pt bond distance, number of Pt nearest neighbors), 83 electronic structure (electron density of states in the Pt 5d 84 band, strength of interaction between the Pt and the 3d-85 transition metal atoms), and nature and coverage of surface 86 oxide layers. 3,4,15−20 However, many of these works considered 87 extended alloy surfaces, and obviously, the mechanisms 88 established for bulk surfaces need not be applicable to 89 nanoparticles. Moreover, often the catalyst was degraded 90 (dealloyed, sintered, annealed, acid treated) in conditions 91 which only mimic the real fuel cell environment. 92 a R [Å] is the mean bond length, σ 2 [10 −3 Å 2 ] is the Debye−Waller-like parameter, and N c is the coordination number.

The Journal of Physical Chemistry B, 2006

In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalati... more In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalation of lithium from LiCoO2 using a specially designed electrochemical cell. The AC-dispersions have been correlated with the cell parameters obtained from the X-ray spectra. The correlation confirms previous hypothesis on the interpretation of the AC-dispersions in terms of an equivalent circuit comprising an element that relates the change of the intrinsic electronic conductivity, occurring at the early stages of deintercalation, to the semiconductor to metal transition caused by the change of the cell parameters.

Physical Chemistry Chemical Physics, 2009

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt lo... more We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

Journal of Power Sources, 2008

The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan ... more The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan XC-72 catalyst using X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XAS results have been obtained ex situ and in situ using a specially optimized for XAS measurement fuel cell (down to 6 keV). The results are compared with those obtained for pure Pt catalyst on the same carbon support under the same working conditions.

Journal of Power Sources, 2009

The electrochemical behavior of composite anodes prepared either by mixing partially oxidized gra... more The electrochemical behavior of composite anodes prepared either by mixing partially oxidized graphite and Cu powders or by coating the pristine partially oxidized graphite electrodes with few-nanometerthick Cu layers has been studied by slow-scan-rate cyclic voltammetry (SSCV) and galvanostatic charge/discharge cycles over the temperature range of −30 • C to 20 • C. The interfacial intercalation/ deintercalation kinetics has also been investigated using electrochemical impedance spectroscopy (EIS).

Journal of Power Sources, 2010

The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced ... more The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced by preparing graphite-tin composite electrodes. The anodes are prepared by mixing partially oxidized graphite with nanometric Sn powder or by coating the oxidized graphite electrode with a thin Sn layer. Long-term cycling stability and intercalation/deintercalation performances of the composite anodes in the temperature range 20 • C to −30 • C are evaluated. Kinetics is investigated by cyclic voltammetry and electrochemical impedance spectroscopy, in the attempt to explain the role of Sn in reducing the overall electrode polarization at low temperature. Two possible mechanisms of action for bulk metal powder and surface metal layer are proposed.

Journal of Power Sources, 2013

ABSTRACT Electrochemical double layer capacitors (EDLCs) are conceived as high power devices. Gen... more ABSTRACT Electrochemical double layer capacitors (EDLCs) are conceived as high power devices. Generally, power improvement is mainly related to lowering the device impedance (Pmax = V2/4R). However, the supercapacitor impedance is the sum of several contributions (electrolyte, active material, contact between the active material and the current collector, electrode porosity). In this work we demonstrate that the resistance at the interface between the electrode and the current collector and as well the diffusion resistance can be properly controlled during the electrode preparation. In our preparation method the electrode are compacted by using a hot-press device. During the electrode fabrication parameters such as temperature and pressure are systematically modulated in order to find the best condition for increasing the contact between the electrode and the current collector while maintaining good electrode porosity.

Journal of Non-Crystalline Solids, 2008

The paper presents an X-ray absorption fine structure (XAFS) investigation of Pt supported on Vul... more The paper presents an X-ray absorption fine structure (XAFS) investigation of Pt supported on Vulcan XC-72 operating as a cathode catalyst in a polymer electrolyte membrane fuel cell (PEM FC). XAFS spectroscopy was performed in situ using a cell developed by the authors and optimized for absorption measurements. Low-noise spectra were obtained using the transmission mode over the Pt L 3 XAFS range, enabling multiple-scattering XAFS data-analysis for the Pt nanocrystalline system under operating conditions. Accurate measurements of the local structure were performed at various potentials and working temperatures. Changes in the near-edge structures reflecting variations in the Pt electronic structure were observed for various potential values in the cathode activation region. The Pt average local geometric structure was found to be practically potential-independent. The increase in structural disorder caused by the higher working FC temperature was not found to affect the catalyst's performance in the kinetically controlled region.

Journal of Non-Crystalline Solids, 2014

In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD)... more In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations of the changes in the local geometric and electronic structure of Pt nanoparticles used as a cathode catalyst in proton exchange membrane fuel cell (PEMFC), working under controlled potential cycling conditions. The body of the results obtained suggests that in the first stage of PEMFC operation, small particle dissolution was a dominant process. Subsequent 100 h of work led to the progressive agglomeration of nanoparticles followed by a pronounced growth of the mean nanoparticle size. At the same time, high-quality XAFS spectra analysis demonstrated that negligible changes in structural local ordering and a slight increase in Pt 5d-electron density occurred during the whole FC operation period under consideration.

Journal of Electroanalytical Chemistry, 2010

The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of metha... more The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of methanol and ethanol is increased by admixing them with zeolite-type cesium salts of heteropolymolybdic and heteropolytungstic acids: Cs 2.5 H 0.5 PMo 12 O 40 , Cs 2.5 H 1.5 SiMo 12 O 40 , Cs 2.5 H 0.5 PW 12 O 40 , and Cs 2.5 H 1.5 -SiW 12 O 40 . It is apparent from IR measurements that these salts remain the Keggin-type structure. They are electroactive and undergo reversible redox transitions, as well as they are hydrated and contain mobile protons. As evidenced from cyclic voltammetric, stair-case voltammetric and chronoamperometric diagnostic experiments, the electrocatalytic enhancement effect has been most pronounced upon application of phosphododecamolybdate and phosphododecatungstate salts. The overall activation effect may also reflect the micro and mesoporous (zeolite-type) high surface area morphology of polyoxometallate cesium salts. The presence of well-defined polyoxometallate clusters in the vicinity of Pt is expected increase population of reactive oxo groups at the electrocatalytic interface.

International Journal of Hydrogen Energy, 2011

The oxygen reduction reaction (ORR) has been studied using different types of carbon supported Pt... more The oxygen reduction reaction (ORR) has been studied using different types of carbon supported Pt-Co alloys and (Pt-Co)-Cs 2.5 H 0.5 PW 12 O 40 composite electrodes. The composite electrodes, prepared using different methods, have been characterized by RDE voltammetry in O 2 saturated acidic media. The composite electrodes show better performances toward ORR than the commercial pristine catalyst in terms of mass activities and half wave potentials. The composite prepared using the in house Pt-Co alloy shows the higher catalytic activity. The analysis of XRD spectra of the home-made Pt-Co alloy reveals the presence of different phases (fct and fcc) that probably justify the better catalytic properties. (S. Dsoke).

Electrochimica Acta, 2005

The electrochemical behavior of composite electrodes obtained by mixing graphite (Timrex KS-15 by... more The electrochemical behavior of composite electrodes obtained by mixing graphite (Timrex KS-15 by Timcall), partially oxidized by thermal treatment, with nanometric metal particles (Au, Ag, Ni, Cu, Al, Sn) at about 1% (w/o) is presented. The charge-discharge properties of the composite electrodes have been studied in the temperature range 20 to −30 • C in 1 M LiPF 6 EC-DEC-DMC (1:1:1). The main effect is a general improvement of the cycling behavior at any temperature. In particular, at −30 • C about 30% of the theoretical intercalation capacity is retained by electrodes containing Cu, Al and Sn. At the same temperature, the composites containing the above metals show evidences of lithium staging. This may indicate that certain metals affect the kinetics of phase transformation that, together with other effects including charge transfer resistance, lithium diffusion coefficient and polarization due to SEI and solvent conductivities, seems to be the main cause of the poor intercalation capacity of graphite anodes at low temperature.

The Journal of Physical Chemistry C, 2012

A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1... more A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2:3679 | 2.0 alpha 39) 2012/05/08 15:22:00 | PROD-JCA1 | rq_334126 | 5/23/2012 14:45:21 | 12 56 A major source of catalyst degradation is the loss of 57 electrochemically active surface area (ECSA). In several studies 58 the loss of active platinum by dissolution of Pt was 59 observed. 6−10 Dissolved Pt species can migrate and leave the 60 catalytically active zone (e.g., Pt particles can be detected in the 61 ionomer phase, outside the conductive carbon support 7,8 ) or 62 redeposited on other (larger) particles due to their higher 63 equilibrium potential for dissolution (Ostwald ripening). 6,10 64 There is also evidence for ECSA loss due to other mechanisms, 65 such as agglomeration (due to carbon corrosion or the 66 migration of nanoparticles on the carbon support) and 67 coalescence, which was demonstrated. e.g.. in refs 7−14. 68 Attempts to distinguish the relative contribution of each 69 mechanism have also been made, 6,8 but the matter is still under 70 debate. Many works have shown that alloying Pt with transition 71 metals can also improve the stability of the catalyst, particularly 72 those containing cobalt (see, e.g,. refs 3, 12, and 13 and 73 references therein). 74 Several hypotheses have been put forward in the literature to 75 justify the reasons for Pt-based alloys activity and durability 76 increase with respect to pure Pt catalyst. Many experimental 77 and theoretical works show that this enhancement could be 78 attributed mainly to changes in the surface structure and 79 chemical composition of the near-surface region (segregation to 80 the surface and dissolution of atoms and atomic species) and 81 also (as a consequence) in the local geometric structure (i.e., 82 Pt−Pt bond distance, number of Pt nearest neighbors), 83 electronic structure (electron density of states in the Pt 5d 84 band, strength of interaction between the Pt and the 3d-85 transition metal atoms), and nature and coverage of surface 86 oxide layers. 3,4,15−20 However, many of these works considered 87 extended alloy surfaces, and obviously, the mechanisms 88 established for bulk surfaces need not be applicable to 89 nanoparticles. Moreover, often the catalyst was degraded 90 (dealloyed, sintered, annealed, acid treated) in conditions 91 which only mimic the real fuel cell environment. 92 a R [Å] is the mean bond length, σ 2 [10 −3 Å 2 ] is the Debye−Waller-like parameter, and N c is the coordination number.

Journal of Solid State Electrochemistry, 2014

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) ... more A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of highsurface-area zeoli te-t ype aci dic salt of cesium phosphododecatungstate (Cs 2.5 H 0.5 PW 12 O 40 ) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2-5 μg cm −2 ) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs 2.5 H 0.5 PW 12 O 40free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1-2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.

Journal of Power Sources, 2015

ABSTRACT This study shows how the simple modulation of the cathode/anode mass ratio, in a Li-ion ... more ABSTRACT This study shows how the simple modulation of the cathode/anode mass ratio, in a Li-ion capacitor based on activated carbon (AC) and Li4Ti5O12 (LTO), results in a drastic increase in performance. Starting with a device balanced in the classical way (with an AC/LTO mass ratio of 4.17), the cathode/anode mass ratio has been reduced to 1.54 and then to 0.72. At a high power density, the device with a cathode/anode mass ratio of 0.72 shows the highest energy density. In fact, at 2.3 kW L-1, it delivers an energy density of 31 Wh L-1, which is almost 10 times greater than the energy obtained with a capacitor balanced with an AC/LTO ratio of 4.17 (3.68 Wh L-1). Moreover, the reduction in the cathode/anode mass ratio from 4.17 to 0.72 improves the cycling stability with a factor of 4.8 after 1000 cycles at 10C. Electrochemical impedance spectroscopy reveals that the better power performance is due to reduced diffusion and charge transfer resistances. In addition, the anode polarization is less pronounced for the system with a lower AC/LTO mass ratio, leading to a minimization in electrolyte decomposition on the anode surface and therefore limiting the increase in the electrode resistance during cycles.

The present invention relates to the field of fuel cell, in particular to electrode compositions ... more The present invention relates to the field of fuel cell, in particular to electrode compositions for fuel cells (PEMFC - Polymer Electrolyte Membrane Fuel Cell) with low platinum content and discloses the use of a compound of formula M1xHy-xM2M312O40, wherein M1 is selected from the group consisting of Cs+, Rb+, K+, NH4+, x represents the number salified protons from M1 in the corresponding acid, M2 is P or Si and M3 is W or Mo; y = 3, when M2 is P and y = 4 when M2 is Si. Said compound is used in a composition with a noble metal or an alloy thereof, preferably Platinum, for the preparation of an electrode composition for fuel cells.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015

A structural study of Pt nano-catalysts is presented in the paper. The innovation in the case of ... more A structural study of Pt nano-catalysts is presented in the paper. The innovation in the case of the considered catalytic materials resides in the use a meso-microporous inorganic matrix of heteropolyacid salt of composition X 2.5 H 0.5 YMo 12 O 40 where X = Rb, Cs and Y = P, Si as a catalyst support. Metallic nanoparticles were created by platinum ions reduction in a hot H 2 /Argon stream and were mainly located into matrix pores. As the porous structure can be controlled by the type and content of the cation used, the desired/optimal Pt nanoparticle size can be precisely defined by using a proper heteropolyacid salt composition. Multiple-scattering extended X-ray absorption fine structure (MS EXAFS) analysis was applied to obtain the relation between the matrix composition and the size of the catalyst nanoparticles. The results showed that it was only the secondary structure form of the heteropolymolybdate salt (i.e. Rb or Cs cations) that influenced the metallic nanoparticle size. The smaller the X atom ionic radii, the larger the mean diameter of the embedded Pt nanoparticles.

J. Mater. Chem. A, 2015

ABSTRACT Nanostructured Li3V2−xNix(PO4)3 (x = 0, 0.05, and 0.1) cathode materials, with a mean pa... more ABSTRACT Nanostructured Li3V2−xNix(PO4)3 (x = 0, 0.05, and 0.1) cathode materials, with a mean particle dimension ranging from 200 to 63 nm, are successfully synthesized with poly(acrylic acid) and D-(+)-glucose as carbon sources. All three samples show a monoclinic crystalline structure as confirmed by X-ray diffraction and Rietveld analysis. Ni-doping improves the specific capacity of Li3V2(PO4)3/C. Between 3.0 and 4.3 V vs. Li+/Li, all cathodes exhibit good rate capability, even at high C-rates. For these reasons, they are good candidates for high power and energy applications, in particular for the development of high energy density supercapacitors. Li3V1.95Ni0.05(PO4)3/C, because of its highest specific discharge capacity (93 mA h g−1 at 100 C) and capacity retention of 97% after 1000 cycles, is selected for building an asymmetric supercapacitor with activated carbon as the anode. At a power density of 2.8 kW L−1, the asymmetric system delivers 18.7 W h L−1, a value five orders of magnitude higher than that of the symmetric capacitor at the same power level.

Journal of Solid State Electrochemistry, 2014

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) ... more A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of highsurface-area zeoli te-t ype aci dic salt of cesium phosphododecatungstate (Cs 2.5 H 0.5 PW 12 O 40 ) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2-5 μg cm −2 ) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs 2.5 H 0.5 PW 12 O 40free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1-2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.

The Journal of Physical Chemistry C, 2012

A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1... more A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2:3679 | 2.0 alpha 39) 2012/05/08 15:22:00 | PROD-JCA1 | rq_334126 | 5/23/2012 14:45:21 | 12 56 A major source of catalyst degradation is the loss of 57 electrochemically active surface area (ECSA). In several studies 58 the loss of active platinum by dissolution of Pt was 59 observed. 6−10 Dissolved Pt species can migrate and leave the 60 catalytically active zone (e.g., Pt particles can be detected in the 61 ionomer phase, outside the conductive carbon support 7,8 ) or 62 redeposited on other (larger) particles due to their higher 63 equilibrium potential for dissolution (Ostwald ripening). 6,10 64 There is also evidence for ECSA loss due to other mechanisms, 65 such as agglomeration (due to carbon corrosion or the 66 migration of nanoparticles on the carbon support) and 67 coalescence, which was demonstrated. e.g.. in refs 7−14. 68 Attempts to distinguish the relative contribution of each 69 mechanism have also been made, 6,8 but the matter is still under 70 debate. Many works have shown that alloying Pt with transition 71 metals can also improve the stability of the catalyst, particularly 72 those containing cobalt (see, e.g,. refs 3, 12, and 13 and 73 references therein). 74 Several hypotheses have been put forward in the literature to 75 justify the reasons for Pt-based alloys activity and durability 76 increase with respect to pure Pt catalyst. Many experimental 77 and theoretical works show that this enhancement could be 78 attributed mainly to changes in the surface structure and 79 chemical composition of the near-surface region (segregation to 80 the surface and dissolution of atoms and atomic species) and 81 also (as a consequence) in the local geometric structure (i.e., 82 Pt−Pt bond distance, number of Pt nearest neighbors), 83 electronic structure (electron density of states in the Pt 5d 84 band, strength of interaction between the Pt and the 3d-85 transition metal atoms), and nature and coverage of surface 86 oxide layers. 3,4,15−20 However, many of these works considered 87 extended alloy surfaces, and obviously, the mechanisms 88 established for bulk surfaces need not be applicable to 89 nanoparticles. Moreover, often the catalyst was degraded 90 (dealloyed, sintered, annealed, acid treated) in conditions 91 which only mimic the real fuel cell environment. 92 a R [Å] is the mean bond length, σ 2 [10 −3 Å 2 ] is the Debye−Waller-like parameter, and N c is the coordination number.

The Journal of Physical Chemistry B, 2006

In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalati... more In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalation of lithium from LiCoO2 using a specially designed electrochemical cell. The AC-dispersions have been correlated with the cell parameters obtained from the X-ray spectra. The correlation confirms previous hypothesis on the interpretation of the AC-dispersions in terms of an equivalent circuit comprising an element that relates the change of the intrinsic electronic conductivity, occurring at the early stages of deintercalation, to the semiconductor to metal transition caused by the change of the cell parameters.

Physical Chemistry Chemical Physics, 2009

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt lo... more We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

Journal of Power Sources, 2008

The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan ... more The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan XC-72 catalyst using X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XAS results have been obtained ex situ and in situ using a specially optimized for XAS measurement fuel cell (down to 6 keV). The results are compared with those obtained for pure Pt catalyst on the same carbon support under the same working conditions.

Journal of Power Sources, 2009

The electrochemical behavior of composite anodes prepared either by mixing partially oxidized gra... more The electrochemical behavior of composite anodes prepared either by mixing partially oxidized graphite and Cu powders or by coating the pristine partially oxidized graphite electrodes with few-nanometerthick Cu layers has been studied by slow-scan-rate cyclic voltammetry (SSCV) and galvanostatic charge/discharge cycles over the temperature range of −30 • C to 20 • C. The interfacial intercalation/ deintercalation kinetics has also been investigated using electrochemical impedance spectroscopy (EIS).

Journal of Power Sources, 2010

The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced ... more The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced by preparing graphite-tin composite electrodes. The anodes are prepared by mixing partially oxidized graphite with nanometric Sn powder or by coating the oxidized graphite electrode with a thin Sn layer. Long-term cycling stability and intercalation/deintercalation performances of the composite anodes in the temperature range 20 • C to −30 • C are evaluated. Kinetics is investigated by cyclic voltammetry and electrochemical impedance spectroscopy, in the attempt to explain the role of Sn in reducing the overall electrode polarization at low temperature. Two possible mechanisms of action for bulk metal powder and surface metal layer are proposed.

Journal of Power Sources, 2013

ABSTRACT Electrochemical double layer capacitors (EDLCs) are conceived as high power devices. Gen... more ABSTRACT Electrochemical double layer capacitors (EDLCs) are conceived as high power devices. Generally, power improvement is mainly related to lowering the device impedance (Pmax = V2/4R). However, the supercapacitor impedance is the sum of several contributions (electrolyte, active material, contact between the active material and the current collector, electrode porosity). In this work we demonstrate that the resistance at the interface between the electrode and the current collector and as well the diffusion resistance can be properly controlled during the electrode preparation. In our preparation method the electrode are compacted by using a hot-press device. During the electrode fabrication parameters such as temperature and pressure are systematically modulated in order to find the best condition for increasing the contact between the electrode and the current collector while maintaining good electrode porosity.

Journal of Non-Crystalline Solids, 2008

The paper presents an X-ray absorption fine structure (XAFS) investigation of Pt supported on Vul... more The paper presents an X-ray absorption fine structure (XAFS) investigation of Pt supported on Vulcan XC-72 operating as a cathode catalyst in a polymer electrolyte membrane fuel cell (PEM FC). XAFS spectroscopy was performed in situ using a cell developed by the authors and optimized for absorption measurements. Low-noise spectra were obtained using the transmission mode over the Pt L 3 XAFS range, enabling multiple-scattering XAFS data-analysis for the Pt nanocrystalline system under operating conditions. Accurate measurements of the local structure were performed at various potentials and working temperatures. Changes in the near-edge structures reflecting variations in the Pt electronic structure were observed for various potential values in the cathode activation region. The Pt average local geometric structure was found to be practically potential-independent. The increase in structural disorder caused by the higher working FC temperature was not found to affect the catalyst's performance in the kinetically controlled region.

Journal of Non-Crystalline Solids, 2014

In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD)... more In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations of the changes in the local geometric and electronic structure of Pt nanoparticles used as a cathode catalyst in proton exchange membrane fuel cell (PEMFC), working under controlled potential cycling conditions. The body of the results obtained suggests that in the first stage of PEMFC operation, small particle dissolution was a dominant process. Subsequent 100 h of work led to the progressive agglomeration of nanoparticles followed by a pronounced growth of the mean nanoparticle size. At the same time, high-quality XAFS spectra analysis demonstrated that negligible changes in structural local ordering and a slight increase in Pt 5d-electron density occurred during the whole FC operation period under consideration.

Journal of Electroanalytical Chemistry, 2010

The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of metha... more The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of methanol and ethanol is increased by admixing them with zeolite-type cesium salts of heteropolymolybdic and heteropolytungstic acids: Cs 2.5 H 0.5 PMo 12 O 40 , Cs 2.5 H 1.5 SiMo 12 O 40 , Cs 2.5 H 0.5 PW 12 O 40 , and Cs 2.5 H 1.5 -SiW 12 O 40 . It is apparent from IR measurements that these salts remain the Keggin-type structure. They are electroactive and undergo reversible redox transitions, as well as they are hydrated and contain mobile protons. As evidenced from cyclic voltammetric, stair-case voltammetric and chronoamperometric diagnostic experiments, the electrocatalytic enhancement effect has been most pronounced upon application of phosphododecamolybdate and phosphododecatungstate salts. The overall activation effect may also reflect the micro and mesoporous (zeolite-type) high surface area morphology of polyoxometallate cesium salts. The presence of well-defined polyoxometallate clusters in the vicinity of Pt is expected increase population of reactive oxo groups at the electrocatalytic interface.

International Journal of Hydrogen Energy, 2011

The oxygen reduction reaction (ORR) has been studied using different types of carbon supported Pt... more The oxygen reduction reaction (ORR) has been studied using different types of carbon supported Pt-Co alloys and (Pt-Co)-Cs 2.5 H 0.5 PW 12 O 40 composite electrodes. The composite electrodes, prepared using different methods, have been characterized by RDE voltammetry in O 2 saturated acidic media. The composite electrodes show better performances toward ORR than the commercial pristine catalyst in terms of mass activities and half wave potentials. The composite prepared using the in house Pt-Co alloy shows the higher catalytic activity. The analysis of XRD spectra of the home-made Pt-Co alloy reveals the presence of different phases (fct and fcc) that probably justify the better catalytic properties. (S. Dsoke).

Electrochimica Acta, 2005

The electrochemical behavior of composite electrodes obtained by mixing graphite (Timrex KS-15 by... more The electrochemical behavior of composite electrodes obtained by mixing graphite (Timrex KS-15 by Timcall), partially oxidized by thermal treatment, with nanometric metal particles (Au, Ag, Ni, Cu, Al, Sn) at about 1% (w/o) is presented. The charge-discharge properties of the composite electrodes have been studied in the temperature range 20 to −30 • C in 1 M LiPF 6 EC-DEC-DMC (1:1:1). The main effect is a general improvement of the cycling behavior at any temperature. In particular, at −30 • C about 30% of the theoretical intercalation capacity is retained by electrodes containing Cu, Al and Sn. At the same temperature, the composites containing the above metals show evidences of lithium staging. This may indicate that certain metals affect the kinetics of phase transformation that, together with other effects including charge transfer resistance, lithium diffusion coefficient and polarization due to SEI and solvent conductivities, seems to be the main cause of the poor intercalation capacity of graphite anodes at low temperature.