Jelena M. Jaksic
University of Belgrade
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Featured researches published by Jelena M. Jaksic.
Electrochimica Acta | 2000
Jelena M. Jaksic; M.V. Vojnovic; N.V. Krstajić
The hydrogen evolution reaction (HER) on Ni, Mo and MoNi intermetallic compound was investigated with ac impedance and dc polarization measurements in 1.0 mol dm−3 NaOH solution at 25°C. The rate constants of the forward and backward reactions of Volmer, Heyrovsky and Tafel steps were estimated by a nonlinear fitting method. MoNi alloy electrode provides a much lower overpotential for the HER than Ni and Mo electrodes, sometimes which exhibit similar catalytic activity. The main pathway for the HER at above mentioned electrode materials is Volmer–Heyrovsky with Heyrovsky as the rate determining step.
International Journal of Hydrogen Energy | 1998
Jelena M. Jaksic; Nikola M. Ristic; Nedeljko V. Krstajić; Milan M. Jakšić
Abstract The Brewer valence-bond theory for bonding in metals and intermetallic phases has been employed, together with Fermi dynamics, to correlate with the electrocatalytic properties of both individual and composite transition metal catalysts for the hydrogen electrode reactions (HELR). It has been inferred that the electrocatalytic activity of both individual transition metals and their intermetallic phases and alloys for both hydrogen evolution (HER) and its oxidation (HOR), primarily correlates with the electronic density of states and obeys typical laws of catalysis reflected in the first place in the existence of volcano plots along the Periodic .Table. Since the bonding effectiveness of both individual and intermetallic hypo–hyper- d -electronic transition metal composite electrocatalysts correlates in a straightforward manner with their electrocatalytic activity, such evidence strongly suggests Fermi energy, as a typical elementary binding energy, which otherwise stays in the linear relation with cohesive energy, this forms the basis in investigation and correlation of electrocatalytic activity. Due to the fact that the Fermi wave-vector represents the individual and collective (alloys and intermetallic phases) bulk property of the available electronic number density (or its concentration, n , i.e., k F = (3 π fn2 n) 1 fn3 ), and in a straightforward manner correlates with the electronic density of states at the Fermi level, and thereby defines all metallic properties of a metal (and intermetallics) as a solid with a Fermi surface, including electrocatalytic features, it has been taken as the main parameter to correlate with the exchange current density in the hydrogen electrode reactions.
International Journal of Hydrogen Energy | 1998
Jelena M. Jaksic; Nedeljko V. Krstajić; Bane N. Grgur; Milan M. Jakšić
Abstract Hydridic and electrocatalytic properties of hypo-hyper-d-electronic combinations of transition metals in their intermetallic phases and alloys for the hydrogen storage, hydridic batteries and its electrode reactions (HELR) have been considered in the light of Fermi dynamics (or the electronic density of states), work function and the Brewer or Miedema intermetallic bonding theory (structural factors). It has been pointed out that such an intermetallic hypo-hyper-d-electronic interaction of transition metals (or the doped effect of a hyper-d- upon the bulk or surface of a hypo-d-electronic metal, or vice versa), which leads to the defined optimal mutual (bulk or surface) electronic density of states for both hydridic storage and/or electrocatalytic reaction (cathodic evolution (HER) and/or anodic oxidation (HOR) of hydrogen), imposes the same catalytic effect as the Non-Faradaic promotion by induced polarization, or the so-called NEMCA effect (Non-Faradaic Electrochemical Modification of Catalytic Activity). The main impact has been imposed on the most promising hydridic battery system (Ti-Ni, crystalline and sintered), as well as on typical electrocatalytic issues (Mo-Co, Mo-Ni, Zr-Ni). It has also been inferred that each phase diagram of a hypo-hyper-d-electronic combination of transition metals behaves in both the hydridic and electrocatalytic sense as the part of Periodic Table: intermetallic phases and alloys between two pure constituents obey typical volcano plots for both hydridic and catalytic activity in accordance with the changes in their electronic density of states and well-defined crystal structure, thereby resulting in intermetallic bonding effectiveness and a mutual work function.
Electrochimica Acta | 1994
Milan M. Jakšić; Jelena M. Jaksic
There has been inferred that the electrocatalytic activity of both individual transition metals and their intermetallic phases and alloys for hydrogen evolution primarily correlates with the electronic density of states and obeys typical laws of catalysis reflected in the first place in the volcano plots along the periodic table. Due to the fact that the intermetallic bonding effectiveness of hypo-hyper-d-electronic transition metal composite electrocatalysts correlates in a straightforward manner with their electrocatalytic activity, such state of evidence strongly suggests the Fermi energy, as a typical atomic binding energy, for the basis in investigation and correlation of electrocatalytic activity. Since the Fermi wave-vector represents the individual and collective (alloys and intermetallic phases) bulk property of the available electronic number density [or its concentration, n, ie, kF = (3π2n)13], and in a straightforward manner correlates with the electronic density of states at the Fermi level, and thereby defines all metallic properties of a metal (and intermetallics) as “a solid with a Fermi surface”, including electrocatalytic features, it has been taken as the main parameter to correlate with the exchange current density in the hydrogen evolution reaction (her). It has been inferred that the Fermi wave-vector, as the main electronic feature of metal and intermetallic phases, has already been implicitly comprised in kinetic relations of the exchange current density, otherwise decisive for electrocatalytic activity. The Fermi wave-vector therefore is considered as the main governing parameter to estimate and predict electrocatalytic activity of intermetallic electrocatalysts of transition metals. The latter is implied within the Thomas-Fermi approximation based upon the assumption that a local internal chemical potential of electrons (read the electrochemical potential or consequently the redox potential of an electrode) can be defined as a function of the electron concentration at that point. Electrode potential and kinetic relations imply the latter as the macroscopic law.
Advances in Physical Chemistry | 2011
Georgios D. Papakonstantinou; Jelena M. Jaksic; Diamantoula Labou; Angeliki Siokou; Milan M. Jakšić
The core subject of the present paper represents the interrelated spillover (effusion) phenomena both of the primary oxide and the H-adatoms, their theory and practice, causes, appearances and consequences, and evidences of existence, their specific properties, and their alterpolar equilibria and kinetic behavior, structural, and resulting catalytic, and double layer charging features. The aim is to introduce electron conductive and d-d interactive individual and composite (mixed valence) hypo-d-oxide compounds, of increased altervalent capacity, or their suboxides (Magneli phases), as the interactive catalytic supports and therefrom provide (i) the strong metal-support interaction (SMSI) catalytic effect and (ii) dynamic spillover interactive transfer of primary oxides (M-OH) and free effusional H-adatoms for further electrode reactions and thereby advance the overall electrocatalytic activity. Since hypo-d-oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. In fact, altervalent hypo-d-oxides impose spontaneous dissociative adsorption of water molecules and then spontaneously pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/H xWO 3) under cathodic and/or its hydrated state (Pt/W(OH) 6), responsible for Pt-OH effusion, under anodic polarization, this way establishing instantaneous reversibly revertible alterpolar bronze features (Pt/H 0.35WO 3 ⇔ Pt/W(OH) 6) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured-type electrocatalysts, even of mixed-valence hypo-d-oxide structures (Pt/H 0.35WO 3/TiO 2/C, Pt/H xNbO 3/TiO 2/C), have for the first time been synthesized by the sol-gel methods and shown rather high stability, electron conductivity, and nonexchanged initial pure monobronze spillover and catalytic properties. Such a unique electrocatalytic system, as the striking target issue of the present paper, has been shown to be the superior for substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The main target of the present thorough review study has been to throw some specific insight light on the overall spillover phenomena and their effects in electrocatalysis of oxygen and hydrogen electrode reactions from diverse angles of view and broad contemporary experimental methods and approaches (XPS, FTIR, DRIFT, XRD, potentiodynamic spectra, UHRTEM).
Chemical Industry & Chemical Engineering Quarterly | 2008
Jelena M. Jaksic; Caslav Lacnjevac; Nedeljko V. Krstajić; Milan M. Jakšić
The aim of the present paper has been to introduce the electron conductive and d-d-interactive individual and composite hypo-d-oxides of the increased altervalent capacity, or their suboxides (Magneli phases), as catalytic supports and therefrom provide: (i) The Strong Metal-Support Interaction (SMSI) effect, and (ii) the Dynamic spillover interactive transfer of primary oxides (M-OH) for further electrode reactions, and thereby advance the overall electrocatalytic activity. The d-band has been pointed out as the bonding, adsorptive and catalytic orbital. In the same context, the phenomenon and significance of the d-d-correlations both in heterogeneous catalysis and electrocatalysis are displayed and inferred. Since hypo-d-oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. Potentiodynamic experiments have shown that the reversible peak of the primary oxide growth on Pt, Ru and Au supported upon hypo-d-oxides and suboxides becomes distinctly increased in the charge capacity and shifts to remarkably more negative potential values, so that it starts even within the range of H-adatoms desorption, while its reduction extends until and merge with the UPD of hydrogen atoms. With wet tungstenia doped titania supported Pt catalyst in membrane cells these peaks dramatically increase in their charge capacity and reversibly become shrunk with a decreased moisture content in the feeding inert gas mixture, and vice versa. Such distinct potentiodynamic scans, in conjunction with some broaden complementary kinetic electrocatalytic improvements rising from the same hypo-d-oxide and/or suboxide interactive support effects, have been proved to be the best and comparable experimental evidence for the spillover effect of primary oxides.
International Journal of Hydrogen Energy | 2004
Lj.M. Vračar; N.V. Krstajić; S.G. Neophytides; Jelena M. Jaksic
Abstract Potentiodynamic studies of the underpotential deposition of hydrogen (Hupd) on MoPt4 electrode in 0.5 mol dm −3 HClO4 aqueous solution in the range of temperature from 275 to 312 K are made and thermodynamic state functions for the hydrogen adsorption are determined. Theoretical treatment of the experimental results is derived from Hupd electrochemical adsorption isotherm. It is determined that Δ G ads θ (H upd ) , as a function of temperature, varies with the surface coverage from ∼−20 kJ mol −1 (θ=0) . The increase of Δ G ads θ (H upd ) with the surface coverage indicates the repulsive interactions between Hupd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and entropy of adsorption are calculated. The values of these functions are determined to be Δ H ads (θ=0) θ =4.5 kJ mol −1 and Δ S ads (θ=0) θ =82 J mol −1 K −1 . The value of ΔHadsθ allows determinations of the bond energy between electrode surface and Hupd, which is found to be E M–H =213 kJ mol −1 for θ=0. The lateral repulsion interactions are the reasons why M–Hupd bond energy decreases significantly with the increase of coverage so the saturation coverage is much less than 1 in the UPD potential region.
Materials Science Forum | 2004
Jelena M. Jaksic; N.V. Krstajić; Lj.M. Vračar
Potentiodynamic studies of the underpotential deposition of hydrogen (Hupd) on MoPt3 electrode in 0.5 mol dm -3 HClO4 aqueous solution in the temperature range from 275 K to 317 K are made and thermodynamic state functions for the hydrogen adsorption are determined. Theoretical treatment of the experimental results is derived from Hupd electrochemical adsorption isotherm. It is determined that G ads (Hupd), as a function of temperature varies with the surface coverage from -16 ± 3 kJ mol -1 ( = 0). The increase in G ads(Hupd) with the surface coverage increase indicates repulsive interactions between Hupd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and entropy of adsorption are calculated. The values of these functions are detemined to be H ads ( =0) = -60.9 kJ mol and S ads( =0) = 151 J mol -1 K -1 . The value of H ads allows determination of the bond energy between electrode surface and Hupd, that is found to be EM-H = 279 kJ mol -1 for = 0. The lateral repulsion interactions are the reason why M-Hupd bond energy decreases significantly with the increase of coverage so, the saturation coverage is much less than 1 in the UPD potential region.
Archive | 2016
Milan M. Jakšić; Angeliki Siokou; Georgios D. Papakonstantinou; Jelena M. Jaksic
Ever since Sir William Grove invented gas fed fuel cells (FC), the main electrocatalytic challenge has been to establish the reversible oxygen electrode (ROE) to take advantage of the entire thermodynamically available current/voltage range between hydrogen and oxygen evolving limits. This challenge is the main subject of the present study.
Surface Science | 2007
Nedeljko V. Krstajić; Ljiljana M. Vračar; Velimir Radmilovic; S.G. Neophytides; Miranda Labou; Jelena M. Jaksic; Reidar Tunold; Polycarpos Falaras; Milan M. Jakšić