In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water
solution of... more In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water solution of sodium tungstate (Na2WO42H2O) with the addition of the pure and Ce-loaded zeolites clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized with respect to their morphologies and chemical and phase compositions using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray diffraction. The prepared coatings contained -alumina, WO3, and metallic tungsten. The surface morphologies of the obtained coatings strongly depended on the PEO processing time; the roughness of all coatings increased with PEO time, while porosity decreased with PEO processing time as a result of microdischarge coalescence and growth. All coatings contained elements originating from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for 10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for various applications.
Two types of Cu-modified Pd catalysts supported on high area carbon were prepared: Pd nanoparticl... more Two types of Cu-modified Pd catalysts supported on high area carbon were prepared: Pd nanoparticles modified with a sub-monolayer of underpotentially deposited Cu (Cu@Pd/C) and Pd-Cu alloy nanoparticles (Pd-Cu/C), and examined for the ethanol oxidation reaction (EOR) in alkaline solution. The catalysts were characterized by energy-dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, as well as cyclic voltammetry. As reference catalysts, Pd/C and Pt/C were used. The electrochemically active surface area of all samples was determined from CO ads and Cu upd desorption and Pd oxide reduction, and used to assess their intrinsic activity for EOR. Intimate contact of Pd with Cu atoms enhanced its activity, regardless of the type of bimetal catalyst. The atomic Pd:Cu ratio between 2:1 and 4:1 appears to be optimal for high activity. The most active catalyst under the potentiodynamic conditions was Cu@Pd/C with θ(Cu) = 0.21,although Pd-Cu/C was superior during the potentiostatic test. All bimetallic catalysts surpassed Pd/C in mass activity. The EOR activity of Pt/C was higher compared to Pd-based catalysts at low potentials, both in terms of specific and mass activity, but with a significant decline over a 30-min potentiostatic stability test.
Efficient cathodes for the hydrogen evolution reaction (HER) in acidic water electrolysis rely on... more Efficient cathodes for the hydrogen evolution reaction (HER) in acidic water electrolysis rely on the use of expensive platinum group metals (PGMs). However, to achieve economically viable operation, both the content of PGMs must be reduced and their intrinsically strong H adsorption mitigated. Herein, we show that the surface effects of hydrogenated TiO2 nanotube (TNT) arrays can make osmium, a so far less-explored PGM, a highly active HER electrocatalyst. These defect-rich TiO2 nanostructures provide an interactive scaffold for the galvanic deposition of Os particles with modulated adsorption properties. Through systematic investigations, we identify the synthesis conditions (OsCl3 concentration/temperature/reaction time) that yield a progressive improvement in Os deposition rate and mass loading, thereby decreasing the HER overpotential. At the same time, the Os particles deposited by this procedure remain mainly sub-nanometric and entirely cover the inner tube walls. An optimally balanced Os@TNT composite prepared at 3 mM/55°C/30 min exhibits a record low overpotential (η) of 61 mV at a current density of 100 mA cm −2 , a high mass activity of 20.8 A mg Os −1 at 80 mV, and a stable performance in an acidic medium. Density functional theory calculations indicate the existence of strong interactions between the hydrogenated TiO2 surface and small Os clusters, which may weaken the Os−H* binding strength and thus boost the intrinsic HER activity of Os centers. The results presented in this study offer new directions for the fabrication of cost-effective PGM-based catalysts and a better understanding of the synergistic electronic interactions at the PGM|TiO2 interface.
The growth of thin oxide coatings on the aluminum substrate in water-based sodium tungstate elect... more The growth of thin oxide coatings on the aluminum substrate in water-based sodium tungstate electrolyte by plasma electrolytic oxidation (PEO) is discussed and experimentally illustrated. The growth is carried out using a distinctive ultra-low duty cycle pulsed direct current (DC) power supply. During the PEO processing elements present in micro-discharges are identified using standard optical emission spectroscopy (OES) technique. The spectral line shape analysis of the first two hydrogen Balmer lines shows the presence of two types of micro-discharges. Obtained coatings are also characterized with respect to their morphology and chemical and phase composition. It is shown that coatings are composed of Al, O, and W, featuring low roughness and porosity. Partial crystallization of the coatings resulted in identification of WO 3 , W 3 O 8 , and γ-Al 2 O 3 crystalline phases.
Hydrated iridium oxyhydroxide (IrO x) films were electrochemically deposited from an alkaline oxa... more Hydrated iridium oxyhydroxide (IrO x) films were electrochemically deposited from an alkaline oxalic solution at constant anodic potentials and by applying a potential cycling protocol, in both cases with variation of the electrodeposi-tion time. From UV-vis spetroscopy of the solution for the deposition and their characterization it was concluded that a mixture of Ir(III)/Ir(IV) monomers participates in the deposition of IrO x film. X-ray photoelectron spectroscopy (XPS) of IrO x films indicated that both types of films contained hydrated Ir(IV) hydroxide as the dominant species, but in the film deposited by potential cycling the presence of the additional Ir(III) species was evident. The scanning electon mi-croscopy (SEM) analysis of the surface morphology revealed that films deposited by potential cycling were more uniform than the films deposited at a constant potential. The amount of electrochemically active Ir-species on the surface of deposited IrO x films was estimated from the voltammetric charge of the Ir(III)/Ir(IV) transition. Depending on the film electrodeposition parameters, the values between 15 and 1080 nmol cm −2 were obtained. The electrochemically active surface area (ECSA) of IrO x films was calculated from cyclic voltammetry and electrochemical impedance spec-troscopy (EIS) measurements and ranged from 3 to 131 cm 2 per 1 cm 2 of geometric surface area for various films. The activity and stability of IrO x films toward oxygen evolution reaction (OER) was investigated in 0.5 M H 2 SO 4 solution under potentiostatic conditions. The intrinsic activity, stated as turnover frequency and specific current density normalized per ECSA, showed that the OER activity of IrO x films deposited by potential cycling are up to two and a half times higher than the activity of films deposited at a constant anodic potential. Potentiostatic stability test showed a decrease in OER current over time for both type of the films. Determination of ECSA, the amount of electroactive Ir species, XPS spectrum and SEM imaging after the test indicated that the decrease in OER activity was caused by partial dissolution and delamination of the film as well as by oxidation of highly active hydroxide Ir(III) species.
International Journal of Electrochemical Science, 2020
Pt, PtSn and PtSnO2 catalysts supported on high surface area carbon synthesized by microwave assi... more Pt, PtSn and PtSnO2 catalysts supported on high surface area carbon synthesized by microwave assisted polyol procedure were tested for methanol oxidation. Based on TGA, EDX and XRD analysis, PtSn/C is composed of Pt and Pt3Sn phase while the rest of Sn is present in a form of very small tin oxide particles. This paper focuses on structure-activity relationships for CO tolerance and methanol oxidation reactions after addition of Sn to Pt catalysts. Alloying of Sn with Pt improves the rate of CO oxidation despite the fact that the pure Sn does not react with CO and therefore activity for methanol oxidation increases ~ 2 times in comparison to Pt/C catalyst. PtSn/C catalyst shows small advantage in comparison with PtSnO2/C catalyst due to the alloyed Sn and its electronic effect. Long term stability tests also confirmed that PtSn/C catalyst is somewhat better in comparison to PtSnO2/C.
Two types of iridium oxide films formed at the Ti 2 AlC substrate were investigated: (1) anodical... more Two types of iridium oxide films formed at the Ti 2 AlC substrate were investigated: (1) anodically electrodeposited iridium oxide film from the solution based on IrCl 3 xH 2 O; (2) iridium oxide film prepared by cycling thin layer of electrodeposited Ir in the 0.5 M H 2 SO 4 from − 0.25 to 1.20 V. It was shown that during anodic electrodeposition of iridium oxide film (1) coulombic efficiency decreases with increasing anodic potential, being only 3% at E = 0.7 V vs. SCE and 26% at E = 0.62 V vs. SCE. A pair of peaks corresponding to the transition Ir(III)-oxide/Ir(IV)-oxide was present on the CVs recorded in 0.5 M H 2 SO 4. While cycling pure Ir thin layer in the solution of 0.5 M H 2 SO 4 from − 0.25 to 1.20 V (2) up to 100 cycles, typical CV response was characterized with the prepeak and a pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. With the increase of cycles number to 150, additional peak at potential of 1.0 V appeared. This peak was formed on the account of pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. The oxygen evolution reaction (OER) was investigated at both iridium oxide films. It was shown that the Tafel slope for the OER was~40 mV dec −1 for the first polarization curve, confirming that the rds was a reaction S-OH → SO ads + H + + e −. As the number of recorded polarization curves increased, the activity of both types of iridium oxide films decreased, due to dissolution of iridium oxide films at the potentials of the OER. It is shown that anodically electrodeposited iridium oxide film is more active for the OER than that obtained by cycling electrodeposited iridium layer. However, both iridium oxide films exhibited insufficient stability.
Developing ultraefficient electrocatalytic materials for the hydrogen evolution reaction (HER) wi... more Developing ultraefficient electrocatalytic materials for the hydrogen evolution reaction (HER) with low content of expensive platinum group metals (PGMs) via low-energy-input procedures is the key to the successful commercialization of green water electrolysis technologies for sustainable production of high-purity hydrogen. In this study, we report a facile room-temperature synthesis of ultrafine metallic Ir nanoparticles on conductive, proton-intercalated TiO 2 nanotube (H-TNT) arrays via galvanic displacement. A series of experiments demonstrate that a controlled transformation of the H-TNT surface microstructure from neat open-top tubes to disordered nanostripe bundles ("nanograss") is highly beneficial for providing an abundance of exposed Ir active sites. Consequently, for nanograss-engineered composites, outstanding HER activity metrics are achieved even at very low Ir(III) precursor concentrations. An optimum Ir@TNT cathode loaded with 5.7 mg Ir cm À2 exhibits an overpotential of À63 mV at À100 mA cm À2 and a mass activity of 34 A mg Ir À1 at À80 mV under acidic conditions, along with excellent catalytic durability and structural integrity. Density functional theory (DFT) simulations reveal that the hydrogen-rich TiO 2 surface not only stabilizes the deposited Ir and weakens its H binding strength to a moderate intensity, but also actively takes part in the HER mechanism by refreshing the Ir catalytic sites near the Ir|H-TiO 2 interface, thus substantially promoting H 2 generation. The comprehensive characterization combined with theory provides an in-depth understanding of the electrocatalytic behavior of H-TNT supported PGM nanoparticles and demonstrates their high potential as competitive electrocatalyst systems for the HER.
We report a mesoporous composite system consisting of carbon fiber cores surrounded with Co 1.5 M... more We report a mesoporous composite system consisting of carbon fiber cores surrounded with Co 1.5 Mn 1.5 O 4 spinel nanocrystal shells, synthesized by a simple two-step process involving single-nozzle co-electrospinning and subsequent calcination. Benefiting from the obtained core-shell structure, this composite has exhibited high specific capacitance in the two-electrode configuration, up to 384 F g −1 at 0.28 A g −1 , with no capacitance loss after 2000 cycles at 50 mV s −1. The incorporation of spinel nanocrystals improved the capacitive performances of composite fibers due to a synergistic effect of redox-active shells and the conductive cores, making this novel material promising for symmetrical supercapacitors.
Formation of the honeycomb-like electrodes of copper by the regime of reversing current (RC) in t... more Formation of the honeycomb-like electrodes of copper by the regime of reversing current (RC) in the second range has been investigated. Morphological and structural characteristics of this electrode type obtained by various parameters of RC regimes were examined by the techniques of scanning electron and optical microscopies, while the amount of hydrogen produced during electrodeposition process was quantified by determination of the average current efficiency for hydrogen evolution reaction. To optimize the process of formation of the honeycomb-like electrodes, the following parameters of square wave RC regimes were analyzed: the cathodic current density, the same anodic to cathodic time ratios but various durations of the cathodic and the anodic pulses, and the various values of the anodic to cathodic time ratios. The minimal amount of hydrogen spent for formation of the honeycomb-like electrodes with maximal number of holes formed from detached hydrogen bubbles is obtained with the anodic to cathodic time ratio of 0.50 and duration of the cathodic and anodic pulses of 2 and 1 s, respectively. To explain formation of the honeycomb-like electrodes of optimal morphological and structural characteristics, the upgraded mathematical model defining the RC regime in the second range was proposed and discussed.
Rising demands for renewable energy sources have led to the development of dye sensitized solar c... more Rising demands for renewable energy sources have led to the development of dye sensitized solar cells. It is a challenge to find a good and low cost sensitizer, which has a low environmental impact. In this work, we conducted spectroscopic and electrochemical experiments, as well as quantum-chemical calculations of the natural pigment hypericin, in order to provide insight into its sensitizing efficiency. To this end, three identical cells were made and characterized. Although this pigment exhibited good adsorption onto a semiconductor surface, a high molar absorption coefficient (43 700 L mol −1 cm −1) and favorable alignment of energy levels and provided a long lifetime of electrons (17.8 ms) in the TiO 2 photoanode, it was found that the efficiency of hypericin-sensitized solar cells was very low, only 0.0245%. We suggest that this inefficiency originated from a low injection of electrons into the conduction band of TiO 2. This conclusion is supported by the density functional theory calculations which revealed a low electron density in the anchoring groups of electronically excited hypericin. The results of this work could be valuable not only in the photovoltaic aspect, but also for application of hypericin in medicine in photodynamic therapy.
Electrodeposition of copper in the hydrogen co-deposition range by the regime of reversing curren... more Electrodeposition of copper in the hydrogen co-deposition range by the regime of reversing current (RC) in the second range has been investigated by determination of the average current efficiency for hydrogen evolution reaction and by scanning electron (SEM) and optical (OM) microscopic analysis of the obtained deposits. Keeping the cathodic current density, the cathodic and the anodic pulses constant in all experiments, the anodic current density (j a) values were varied: 40, 80, 160, 240 and 320 mA cm −2. The Cu deposits produced by the RC regimes with different anodic current density values were compared with that obtained in a constant galva-nostatic regime (DC) at the current density equal to the cathodic current density in the RC regimes. The honeycomb like structures were formed in the DC regime and by the RC regimes with j a of 40 and 80 mA cm −2. The hole size in them was in the 60-70 μm range. Due to the decrease of quantity of evolved hydrogen with increasing anodic current density, the larger dish-like holes with dendrites at their bottom and shoulder were formed with j a values of 160, 240 and 320 mA cm −2. The maximum number of holes, and hence, the largest specific surface area of the honeycomb-like electrodes was obtained with j a = 80 mA cm −2 , that can be ascribed to a suppression of coalescence of neighboring hydrogen bubbles. Application of the RC regime also led to the increase of uniformity of structures, what is concluded by cross section analysis of the formed honeycomb-like electrodes. For the first time, mechanism of Cu electrodeposition in the hydrogen co-deposition range by the RC regime in the second range was proposed and discussed.
Ag-In, Pd-In and CoIn alloy coatings were electrodeposited under galvanostatic conditions from di... more Ag-In, Pd-In and CoIn alloy coatings were electrodeposited under galvanostatic conditions from different solutions. The appearance and the composition of coatings were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The appearance of spatio-temporal structures on the surface od electrodeposited alloy layers is characteristic for all three indium containing alloys. Well-distinguished spirals were detected only at the surfaces of Ag-In and CoIn alloys, while for Pd-In alloy spatio-temporal structures are characterized with the valleys of different shapes surrounded by hills. In the case of Ag-In and Pd-In alloy coatings the cross-section does not show the presence of alloy layers with different composition, due to the fact that spatio-temporal structures are of large dimensions and cannot be seen on the high magnification images of the cross-sections. EDS line scan of the cross-section of CoIn alloy layers indicates the presence of layers with different compositions at high magnification, confirming that spatio-temporal structures are placed not only on the surface, but also in the bulk of the alloy coating.
Electrodeposited NiSn alloy coatings onto Ni 40 mesh substrate were tested for application as cat... more Electrodeposited NiSn alloy coatings onto Ni 40 mesh substrate were tested for application as cathodes and anodes in the cell for alkaline water electrolysis in 30 wt% KOH at 80 °C. The "accelerated service life test" (ASLT) was performed for the hydrogen evolution reaction (HER), as well as for the oxygen evolution reaction (OER), and compared to that recorded for the Ni coating (Ni-dep) and Ni-mesh for both reactions. The morphology and chemical compositions of the NiSn and Ni coatings were investigated by scanning electron mi-croscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), while their surface composition was investigated by X-ray photoelectron spectroscopy (XPS) before and after the ASLT for both reactions, respectively. By measuring the potential at j = 0.3 A cm − 2 it was shown that during the ASLT the NiSn alloy coating catalytic activity for the HER decreases (about 24 mV after 25 cycles), while the catalytic activity for the OER increases (about 50 mV after 25 cycles), so that the cell voltage decreases for about 26 mV. The Ni-dep and Ni-mesh electrodes catalytic activity was found to increase for the HER (for about 103 mV), as well as for the OER (for about 52 mV) during the ASLT. Hence, the cell voltage for the Ni-dep and Ni-mesh electrodes decreased from 2.402 V to 2.245 V during the ASLT, while that for the NiSn electrode decreased from 1.967 V to 1.941 V. The cell voltage saving with the NiSn electrodes amounts to about 435 mV before the ASLT and about 304 mV after the ASLT. SEM results showed that no changes in the morphology of as prepared samples could be detected after the ASLTs for both reactions. EDS analysis confirmed that some changes occurred during the ASLT, particularly for the oxygen content in the surface layer. Similar conclusions were made from the XPS analysis.
Noble metal-based materials are inevitable components of cathodes for the hydrogen evolution reac... more Noble metal-based materials are inevitable components of cathodes for the hydrogen evolution reaction (HER) in future water electrolysis systems for clean hydrogen fuel production. However, designing active and durable nanostructured catalysts with low amount of costly noble metals is still a great challenge. Herein, we show that Pd nanoparticles (NPs) can be synthesized on the highly developed surface of anodically grown TiO2 nanotube (TNT) arrays by applying a simple galvanic displacement strategy. In a two-step procedure, air-annealed TNT arrays are first cathodically protonated and then partially reoxidized by Pd(II) ions from a PdCl2 solution while providing a scaffold for the metallic Pd deposit. Structural and electrochemical characterizations reveal that the Pd content and the width of the Pd-populated zone of the tube walls are in correlation with the tube length. The Pd@TNT composites display remarkable HER activity in 1 M HClO4 delivering a current density of –10 mA cm–2 at an overpotential of –38 mV and a Tafel slope of only –13 mV/dec. More impressively, the mass and apparent activity of the Pd@TNTs is superior to even commercial Pt/C at higher current densities. The composites also show stable chronopotentiometric response over 25 h and a negligible HER overpotential increase after potential cycling tests. The exceptional performance of the Pd@TNT cathodes is assigned to the unique semiconducting properties of the three-dimensional, interactive TNT supporting structures that, on the one hand, provide abundance of Pd active sites with optimized atomic hydrogen binding energy for the cathodic HER, but on the other hand, prevent anodic degradation of the Pd catalyst.
In our previous paper, we described in detail studies of Sn influence on electrocatalytic activit... more In our previous paper, we described in detail studies of Sn influence on electrocatalytic activity of PtSn catalyst for CO and formic acid oxidation (Stevanović et al., J. Phys. Chem. C, 118 (2014) 278–289). The catalyst was composed of a Pt phase, Pt 3 Sn alloy and very small SnO 2 particles. Different electrochemical treatment enabled studies of PtSn/ C having Sn both in surface and subsurface layers and skeleton structure of this catalyst with Sn only in subsurface layers. The results obtained revealed the promotional effect of surface Sn whether alloyed or as oxide above all in preventing accumulation of CO and blocking the surface Pt atoms. As a consequence , in formic acid oxidation, the currents are not entering the plateau but increasing constantly until reaching a maximum. It was concluded that at lower potentials the effect of Sn on formic acid oxidation was predominantly electronic but with increasing the potential bi-functional mechanism prevailed due to the leading role of SnO 2. This role of SnO 2 is restated in the present study. Therefore, CO and formic acid oxidation were examined at PtSnO 2 /C catalyst. The catalyst was synthesised by the same microwave-assisted polyol procedure. According to XRD analysis, the catalyst is composed of a Pt phase and SnO 2 phase. The reactions were examined on PtSnO 2 /C catalyst treated on the same way as PtSn/C. Comparing the results obtained, the role of SnO 2 is confirmed and at the same time the significance of alloyed Sn and its electronic effect is revealed.
Tungsten based support was prepared by polycondensation of resorcinol and formaldehyde from ammon... more Tungsten based support was prepared by polycondensation of resorcinol and formaldehyde from ammonium metatungstate, in the presence cetyltrimethylammonium bromide (CTABr) surfactant. Pd nanocatalyst on this support was synthesized by borohydride reduction method. The obtained materials were characterized by High Resolution Transmission Electron Microscopy (HRTEM), Electron Energy Loss Spectroscopy (EELS), X-ray Photoelectron Spectroscopy (XPS) and electrochemical measurements. TEM analysis revealed Pd nanoparticles size in the range of a few nanometers, even the clusters of single Pd atoms. X-Ray Photoelectron Spectroscopy was applied to determine surface composition of the substrates. It was found that tungsten based support consisted of W, WC and WO 3 species. The presence of metallic palladium – Pd(0) in the Pd/W@WCWO 3 catalyst was revealed, as well. The catalytic activity and stability for the oxygen reduction were investigated in acid and alkaline solutions, by cyclic voltammetry and linear sweep voltammetry at the rotating disc electrode. The catalysts' activities were compared to the carbon supported Pd nanoparticles (Vulcan XC 72). WC supported Pd nanoparticles have shown high activity and superior stability, comparable even to Pt based catalysts, especially in alkaline electrolytes.
The oxygen evolution reaction (OER) was studied at pure Ni and Ni-(Ebonex/Ir) composite coatings ... more The oxygen evolution reaction (OER) was studied at pure Ni and Ni-(Ebonex/Ir) composite coatings in 1 M NaOH solution at 25 oC. Ni-(Ebonex-supported Ir) coatings were electro-deposited from a nickel Watts bath containing different concentrations of suspended Ebonex/Ir particles (0-2 g dm-3) onto a Ni 40 mesh substrate. The surface morphology of the coatings was examined by scanning electron microscopy (SEM), the surface composition by energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRPD) and X-ray photoelectron spectroscopy (XPS), whereas the electrochemical properties were studied by electrochemical impedance spectroscopy (EIS), polarization measurements and cyclic voltammetry (CV). It was shown that the roughness factor of Ni-(Ebonex/Ir) composite coatings calculated relative to the surface area of the pure Ni sample increased with the increasing content of Ebonex/Ir particles in the bath to a maximum value of 40.6. All samples displayed a Tafel slope of about 60 mV dec-1 in the potential range corresponding to lower current densities for the OER. The increase of the apparent activity for the OER at Ni-(Ebonex/Ir) coatings compared with the pure Ni coating was attributed only to the increase of the electrochemically active surface area. Although the pure Ni coating initially exhibited higher intrinsic catalytic activity for the OER than the composite coatings, it also showed a drastic loss of activity after subjecting to continuous oxygen evolution at j = 50 mA cm-2 for 24 h (deltaE = 395 mV). At the same time, the OER overpotential at Ni-(Ebonex/Ir) coatings only negligibly increased after the stability test (deltaE = 22 mV). The improved retention of catalytic activity observed with Ni-(Ebonex/Ir) coatings was ascribed to the presence of IrO2 , which inhibited the formation of the inactive gamma-NiOOH phase.
In this paper, we propose a novel method for preparation of high surface area titania nanoparticl... more In this paper, we propose a novel method for preparation of high surface area titania nanoparticles, and investigate their photovoltaic performance in dye sensitized solar cells (DSSCs). The precursor compound titanium(IV)-isopropoxide is hydrolyzed in the presence of nonionic surfactant (Triton X100) and ethylenediaminetetraacetic acid sodium salt (EDTA-Na 2) and thermally transformed into gel. The gel is autoclaved, resulting in submicronic micelles which are further processed into mesoporous films. The films are deposited from paste, composed of ultrasonically broken micelles and the organic ingredients. The crack-free film structures, composed of sub–25 nm pure anatase particles are observed using the appropriate instrumental techniques: scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), UV-VIS-NIR spectroscopy. The large surface area of 158 m 2 g -1 and mesoporosity are confirmed using the data obtained from the nitrogen adsorption-desorption isotherm. The photovoltaic performance of the operating N719-sensitized solar devices are tested using electrochemical impedance spectroscopy (EIS), open-circuit voltage decay (OCVD) and by recording current density-voltage (J-V) curves. The cell exhibits promising photocurrent density up to 11.7 mA cm -2 , and the photo-to-electric power efficiency of 5.22%, for cell area of 0.24 cm 2 , under 100 mW cm -2 halogen light source.
Pd-Ni/C catalyst was synthesized employing a borohydride reduction method. The high area Ni was f... more Pd-Ni/C catalyst was synthesized employing a borohydride reduction method. The high area Ni was first dispersed on the carbon support and then modified by Pd nanoparticles. Transmission electron microscopy confirmed relatively even distribution of Ni across the carbon support with discrete pal-ladium particles of about 3.3 nm mean diameter on it. Cyclic voltammetry confirmed the presence of Ni on the catalyst surface. The activity of the Pd-Ni/C catalysts for ethanol oxidation reaction (EOR) in alkaline solution was tested under the potentiodynamic and potentiostatic conditions and the results were compared to those obtained on the Pd/C catalyst. It was found that Pd-Ni/C is more active for the EOR compared to Pd/C by a factor up to 3, depending on the type of experiments and whether specific activity or mass activity are considered. During the potentiodynamic stability test an interesting phenomenon of activation of Pd-Ni/C catalyst was observed. It was found that maximum activity is attained after fifty cycles with the positive potential limit of 1.2 V, regardless of whether they were performed in the electrolyte with or without ethanol. It was postulated that potential cycling of the Pd-Ni surface causes reorganization of the catalyst surface bringing Pd and Ni sites to a more suitable arrangement for the efficient ethanol oxidation.
In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water
solution of... more In this paper, we employed plasma electrolytic oxidation (PEO) of aluminum in a water solution of sodium tungstate (Na2WO42H2O) with the addition of the pure and Ce-loaded zeolites clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized with respect to their morphologies and chemical and phase compositions using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray diffraction. The prepared coatings contained -alumina, WO3, and metallic tungsten. The surface morphologies of the obtained coatings strongly depended on the PEO processing time; the roughness of all coatings increased with PEO time, while porosity decreased with PEO processing time as a result of microdischarge coalescence and growth. All coatings contained elements originating from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for 10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for various applications.
Two types of Cu-modified Pd catalysts supported on high area carbon were prepared: Pd nanoparticl... more Two types of Cu-modified Pd catalysts supported on high area carbon were prepared: Pd nanoparticles modified with a sub-monolayer of underpotentially deposited Cu (Cu@Pd/C) and Pd-Cu alloy nanoparticles (Pd-Cu/C), and examined for the ethanol oxidation reaction (EOR) in alkaline solution. The catalysts were characterized by energy-dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, as well as cyclic voltammetry. As reference catalysts, Pd/C and Pt/C were used. The electrochemically active surface area of all samples was determined from CO ads and Cu upd desorption and Pd oxide reduction, and used to assess their intrinsic activity for EOR. Intimate contact of Pd with Cu atoms enhanced its activity, regardless of the type of bimetal catalyst. The atomic Pd:Cu ratio between 2:1 and 4:1 appears to be optimal for high activity. The most active catalyst under the potentiodynamic conditions was Cu@Pd/C with θ(Cu) = 0.21,although Pd-Cu/C was superior during the potentiostatic test. All bimetallic catalysts surpassed Pd/C in mass activity. The EOR activity of Pt/C was higher compared to Pd-based catalysts at low potentials, both in terms of specific and mass activity, but with a significant decline over a 30-min potentiostatic stability test.
Efficient cathodes for the hydrogen evolution reaction (HER) in acidic water electrolysis rely on... more Efficient cathodes for the hydrogen evolution reaction (HER) in acidic water electrolysis rely on the use of expensive platinum group metals (PGMs). However, to achieve economically viable operation, both the content of PGMs must be reduced and their intrinsically strong H adsorption mitigated. Herein, we show that the surface effects of hydrogenated TiO2 nanotube (TNT) arrays can make osmium, a so far less-explored PGM, a highly active HER electrocatalyst. These defect-rich TiO2 nanostructures provide an interactive scaffold for the galvanic deposition of Os particles with modulated adsorption properties. Through systematic investigations, we identify the synthesis conditions (OsCl3 concentration/temperature/reaction time) that yield a progressive improvement in Os deposition rate and mass loading, thereby decreasing the HER overpotential. At the same time, the Os particles deposited by this procedure remain mainly sub-nanometric and entirely cover the inner tube walls. An optimally balanced Os@TNT composite prepared at 3 mM/55°C/30 min exhibits a record low overpotential (η) of 61 mV at a current density of 100 mA cm −2 , a high mass activity of 20.8 A mg Os −1 at 80 mV, and a stable performance in an acidic medium. Density functional theory calculations indicate the existence of strong interactions between the hydrogenated TiO2 surface and small Os clusters, which may weaken the Os−H* binding strength and thus boost the intrinsic HER activity of Os centers. The results presented in this study offer new directions for the fabrication of cost-effective PGM-based catalysts and a better understanding of the synergistic electronic interactions at the PGM|TiO2 interface.
The growth of thin oxide coatings on the aluminum substrate in water-based sodium tungstate elect... more The growth of thin oxide coatings on the aluminum substrate in water-based sodium tungstate electrolyte by plasma electrolytic oxidation (PEO) is discussed and experimentally illustrated. The growth is carried out using a distinctive ultra-low duty cycle pulsed direct current (DC) power supply. During the PEO processing elements present in micro-discharges are identified using standard optical emission spectroscopy (OES) technique. The spectral line shape analysis of the first two hydrogen Balmer lines shows the presence of two types of micro-discharges. Obtained coatings are also characterized with respect to their morphology and chemical and phase composition. It is shown that coatings are composed of Al, O, and W, featuring low roughness and porosity. Partial crystallization of the coatings resulted in identification of WO 3 , W 3 O 8 , and γ-Al 2 O 3 crystalline phases.
Hydrated iridium oxyhydroxide (IrO x) films were electrochemically deposited from an alkaline oxa... more Hydrated iridium oxyhydroxide (IrO x) films were electrochemically deposited from an alkaline oxalic solution at constant anodic potentials and by applying a potential cycling protocol, in both cases with variation of the electrodeposi-tion time. From UV-vis spetroscopy of the solution for the deposition and their characterization it was concluded that a mixture of Ir(III)/Ir(IV) monomers participates in the deposition of IrO x film. X-ray photoelectron spectroscopy (XPS) of IrO x films indicated that both types of films contained hydrated Ir(IV) hydroxide as the dominant species, but in the film deposited by potential cycling the presence of the additional Ir(III) species was evident. The scanning electon mi-croscopy (SEM) analysis of the surface morphology revealed that films deposited by potential cycling were more uniform than the films deposited at a constant potential. The amount of electrochemically active Ir-species on the surface of deposited IrO x films was estimated from the voltammetric charge of the Ir(III)/Ir(IV) transition. Depending on the film electrodeposition parameters, the values between 15 and 1080 nmol cm −2 were obtained. The electrochemically active surface area (ECSA) of IrO x films was calculated from cyclic voltammetry and electrochemical impedance spec-troscopy (EIS) measurements and ranged from 3 to 131 cm 2 per 1 cm 2 of geometric surface area for various films. The activity and stability of IrO x films toward oxygen evolution reaction (OER) was investigated in 0.5 M H 2 SO 4 solution under potentiostatic conditions. The intrinsic activity, stated as turnover frequency and specific current density normalized per ECSA, showed that the OER activity of IrO x films deposited by potential cycling are up to two and a half times higher than the activity of films deposited at a constant anodic potential. Potentiostatic stability test showed a decrease in OER current over time for both type of the films. Determination of ECSA, the amount of electroactive Ir species, XPS spectrum and SEM imaging after the test indicated that the decrease in OER activity was caused by partial dissolution and delamination of the film as well as by oxidation of highly active hydroxide Ir(III) species.
International Journal of Electrochemical Science, 2020
Pt, PtSn and PtSnO2 catalysts supported on high surface area carbon synthesized by microwave assi... more Pt, PtSn and PtSnO2 catalysts supported on high surface area carbon synthesized by microwave assisted polyol procedure were tested for methanol oxidation. Based on TGA, EDX and XRD analysis, PtSn/C is composed of Pt and Pt3Sn phase while the rest of Sn is present in a form of very small tin oxide particles. This paper focuses on structure-activity relationships for CO tolerance and methanol oxidation reactions after addition of Sn to Pt catalysts. Alloying of Sn with Pt improves the rate of CO oxidation despite the fact that the pure Sn does not react with CO and therefore activity for methanol oxidation increases ~ 2 times in comparison to Pt/C catalyst. PtSn/C catalyst shows small advantage in comparison with PtSnO2/C catalyst due to the alloyed Sn and its electronic effect. Long term stability tests also confirmed that PtSn/C catalyst is somewhat better in comparison to PtSnO2/C.
Two types of iridium oxide films formed at the Ti 2 AlC substrate were investigated: (1) anodical... more Two types of iridium oxide films formed at the Ti 2 AlC substrate were investigated: (1) anodically electrodeposited iridium oxide film from the solution based on IrCl 3 xH 2 O; (2) iridium oxide film prepared by cycling thin layer of electrodeposited Ir in the 0.5 M H 2 SO 4 from − 0.25 to 1.20 V. It was shown that during anodic electrodeposition of iridium oxide film (1) coulombic efficiency decreases with increasing anodic potential, being only 3% at E = 0.7 V vs. SCE and 26% at E = 0.62 V vs. SCE. A pair of peaks corresponding to the transition Ir(III)-oxide/Ir(IV)-oxide was present on the CVs recorded in 0.5 M H 2 SO 4. While cycling pure Ir thin layer in the solution of 0.5 M H 2 SO 4 from − 0.25 to 1.20 V (2) up to 100 cycles, typical CV response was characterized with the prepeak and a pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. With the increase of cycles number to 150, additional peak at potential of 1.0 V appeared. This peak was formed on the account of pair of peaks corresponding to the Ir(III)/Ir(IV)-oxide transition. The oxygen evolution reaction (OER) was investigated at both iridium oxide films. It was shown that the Tafel slope for the OER was~40 mV dec −1 for the first polarization curve, confirming that the rds was a reaction S-OH → SO ads + H + + e −. As the number of recorded polarization curves increased, the activity of both types of iridium oxide films decreased, due to dissolution of iridium oxide films at the potentials of the OER. It is shown that anodically electrodeposited iridium oxide film is more active for the OER than that obtained by cycling electrodeposited iridium layer. However, both iridium oxide films exhibited insufficient stability.
Developing ultraefficient electrocatalytic materials for the hydrogen evolution reaction (HER) wi... more Developing ultraefficient electrocatalytic materials for the hydrogen evolution reaction (HER) with low content of expensive platinum group metals (PGMs) via low-energy-input procedures is the key to the successful commercialization of green water electrolysis technologies for sustainable production of high-purity hydrogen. In this study, we report a facile room-temperature synthesis of ultrafine metallic Ir nanoparticles on conductive, proton-intercalated TiO 2 nanotube (H-TNT) arrays via galvanic displacement. A series of experiments demonstrate that a controlled transformation of the H-TNT surface microstructure from neat open-top tubes to disordered nanostripe bundles ("nanograss") is highly beneficial for providing an abundance of exposed Ir active sites. Consequently, for nanograss-engineered composites, outstanding HER activity metrics are achieved even at very low Ir(III) precursor concentrations. An optimum Ir@TNT cathode loaded with 5.7 mg Ir cm À2 exhibits an overpotential of À63 mV at À100 mA cm À2 and a mass activity of 34 A mg Ir À1 at À80 mV under acidic conditions, along with excellent catalytic durability and structural integrity. Density functional theory (DFT) simulations reveal that the hydrogen-rich TiO 2 surface not only stabilizes the deposited Ir and weakens its H binding strength to a moderate intensity, but also actively takes part in the HER mechanism by refreshing the Ir catalytic sites near the Ir|H-TiO 2 interface, thus substantially promoting H 2 generation. The comprehensive characterization combined with theory provides an in-depth understanding of the electrocatalytic behavior of H-TNT supported PGM nanoparticles and demonstrates their high potential as competitive electrocatalyst systems for the HER.
We report a mesoporous composite system consisting of carbon fiber cores surrounded with Co 1.5 M... more We report a mesoporous composite system consisting of carbon fiber cores surrounded with Co 1.5 Mn 1.5 O 4 spinel nanocrystal shells, synthesized by a simple two-step process involving single-nozzle co-electrospinning and subsequent calcination. Benefiting from the obtained core-shell structure, this composite has exhibited high specific capacitance in the two-electrode configuration, up to 384 F g −1 at 0.28 A g −1 , with no capacitance loss after 2000 cycles at 50 mV s −1. The incorporation of spinel nanocrystals improved the capacitive performances of composite fibers due to a synergistic effect of redox-active shells and the conductive cores, making this novel material promising for symmetrical supercapacitors.
Formation of the honeycomb-like electrodes of copper by the regime of reversing current (RC) in t... more Formation of the honeycomb-like electrodes of copper by the regime of reversing current (RC) in the second range has been investigated. Morphological and structural characteristics of this electrode type obtained by various parameters of RC regimes were examined by the techniques of scanning electron and optical microscopies, while the amount of hydrogen produced during electrodeposition process was quantified by determination of the average current efficiency for hydrogen evolution reaction. To optimize the process of formation of the honeycomb-like electrodes, the following parameters of square wave RC regimes were analyzed: the cathodic current density, the same anodic to cathodic time ratios but various durations of the cathodic and the anodic pulses, and the various values of the anodic to cathodic time ratios. The minimal amount of hydrogen spent for formation of the honeycomb-like electrodes with maximal number of holes formed from detached hydrogen bubbles is obtained with the anodic to cathodic time ratio of 0.50 and duration of the cathodic and anodic pulses of 2 and 1 s, respectively. To explain formation of the honeycomb-like electrodes of optimal morphological and structural characteristics, the upgraded mathematical model defining the RC regime in the second range was proposed and discussed.
Rising demands for renewable energy sources have led to the development of dye sensitized solar c... more Rising demands for renewable energy sources have led to the development of dye sensitized solar cells. It is a challenge to find a good and low cost sensitizer, which has a low environmental impact. In this work, we conducted spectroscopic and electrochemical experiments, as well as quantum-chemical calculations of the natural pigment hypericin, in order to provide insight into its sensitizing efficiency. To this end, three identical cells were made and characterized. Although this pigment exhibited good adsorption onto a semiconductor surface, a high molar absorption coefficient (43 700 L mol −1 cm −1) and favorable alignment of energy levels and provided a long lifetime of electrons (17.8 ms) in the TiO 2 photoanode, it was found that the efficiency of hypericin-sensitized solar cells was very low, only 0.0245%. We suggest that this inefficiency originated from a low injection of electrons into the conduction band of TiO 2. This conclusion is supported by the density functional theory calculations which revealed a low electron density in the anchoring groups of electronically excited hypericin. The results of this work could be valuable not only in the photovoltaic aspect, but also for application of hypericin in medicine in photodynamic therapy.
Electrodeposition of copper in the hydrogen co-deposition range by the regime of reversing curren... more Electrodeposition of copper in the hydrogen co-deposition range by the regime of reversing current (RC) in the second range has been investigated by determination of the average current efficiency for hydrogen evolution reaction and by scanning electron (SEM) and optical (OM) microscopic analysis of the obtained deposits. Keeping the cathodic current density, the cathodic and the anodic pulses constant in all experiments, the anodic current density (j a) values were varied: 40, 80, 160, 240 and 320 mA cm −2. The Cu deposits produced by the RC regimes with different anodic current density values were compared with that obtained in a constant galva-nostatic regime (DC) at the current density equal to the cathodic current density in the RC regimes. The honeycomb like structures were formed in the DC regime and by the RC regimes with j a of 40 and 80 mA cm −2. The hole size in them was in the 60-70 μm range. Due to the decrease of quantity of evolved hydrogen with increasing anodic current density, the larger dish-like holes with dendrites at their bottom and shoulder were formed with j a values of 160, 240 and 320 mA cm −2. The maximum number of holes, and hence, the largest specific surface area of the honeycomb-like electrodes was obtained with j a = 80 mA cm −2 , that can be ascribed to a suppression of coalescence of neighboring hydrogen bubbles. Application of the RC regime also led to the increase of uniformity of structures, what is concluded by cross section analysis of the formed honeycomb-like electrodes. For the first time, mechanism of Cu electrodeposition in the hydrogen co-deposition range by the RC regime in the second range was proposed and discussed.
Ag-In, Pd-In and CoIn alloy coatings were electrodeposited under galvanostatic conditions from di... more Ag-In, Pd-In and CoIn alloy coatings were electrodeposited under galvanostatic conditions from different solutions. The appearance and the composition of coatings were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The appearance of spatio-temporal structures on the surface od electrodeposited alloy layers is characteristic for all three indium containing alloys. Well-distinguished spirals were detected only at the surfaces of Ag-In and CoIn alloys, while for Pd-In alloy spatio-temporal structures are characterized with the valleys of different shapes surrounded by hills. In the case of Ag-In and Pd-In alloy coatings the cross-section does not show the presence of alloy layers with different composition, due to the fact that spatio-temporal structures are of large dimensions and cannot be seen on the high magnification images of the cross-sections. EDS line scan of the cross-section of CoIn alloy layers indicates the presence of layers with different compositions at high magnification, confirming that spatio-temporal structures are placed not only on the surface, but also in the bulk of the alloy coating.
Electrodeposited NiSn alloy coatings onto Ni 40 mesh substrate were tested for application as cat... more Electrodeposited NiSn alloy coatings onto Ni 40 mesh substrate were tested for application as cathodes and anodes in the cell for alkaline water electrolysis in 30 wt% KOH at 80 °C. The "accelerated service life test" (ASLT) was performed for the hydrogen evolution reaction (HER), as well as for the oxygen evolution reaction (OER), and compared to that recorded for the Ni coating (Ni-dep) and Ni-mesh for both reactions. The morphology and chemical compositions of the NiSn and Ni coatings were investigated by scanning electron mi-croscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), while their surface composition was investigated by X-ray photoelectron spectroscopy (XPS) before and after the ASLT for both reactions, respectively. By measuring the potential at j = 0.3 A cm − 2 it was shown that during the ASLT the NiSn alloy coating catalytic activity for the HER decreases (about 24 mV after 25 cycles), while the catalytic activity for the OER increases (about 50 mV after 25 cycles), so that the cell voltage decreases for about 26 mV. The Ni-dep and Ni-mesh electrodes catalytic activity was found to increase for the HER (for about 103 mV), as well as for the OER (for about 52 mV) during the ASLT. Hence, the cell voltage for the Ni-dep and Ni-mesh electrodes decreased from 2.402 V to 2.245 V during the ASLT, while that for the NiSn electrode decreased from 1.967 V to 1.941 V. The cell voltage saving with the NiSn electrodes amounts to about 435 mV before the ASLT and about 304 mV after the ASLT. SEM results showed that no changes in the morphology of as prepared samples could be detected after the ASLTs for both reactions. EDS analysis confirmed that some changes occurred during the ASLT, particularly for the oxygen content in the surface layer. Similar conclusions were made from the XPS analysis.
Noble metal-based materials are inevitable components of cathodes for the hydrogen evolution reac... more Noble metal-based materials are inevitable components of cathodes for the hydrogen evolution reaction (HER) in future water electrolysis systems for clean hydrogen fuel production. However, designing active and durable nanostructured catalysts with low amount of costly noble metals is still a great challenge. Herein, we show that Pd nanoparticles (NPs) can be synthesized on the highly developed surface of anodically grown TiO2 nanotube (TNT) arrays by applying a simple galvanic displacement strategy. In a two-step procedure, air-annealed TNT arrays are first cathodically protonated and then partially reoxidized by Pd(II) ions from a PdCl2 solution while providing a scaffold for the metallic Pd deposit. Structural and electrochemical characterizations reveal that the Pd content and the width of the Pd-populated zone of the tube walls are in correlation with the tube length. The Pd@TNT composites display remarkable HER activity in 1 M HClO4 delivering a current density of –10 mA cm–2 at an overpotential of –38 mV and a Tafel slope of only –13 mV/dec. More impressively, the mass and apparent activity of the Pd@TNTs is superior to even commercial Pt/C at higher current densities. The composites also show stable chronopotentiometric response over 25 h and a negligible HER overpotential increase after potential cycling tests. The exceptional performance of the Pd@TNT cathodes is assigned to the unique semiconducting properties of the three-dimensional, interactive TNT supporting structures that, on the one hand, provide abundance of Pd active sites with optimized atomic hydrogen binding energy for the cathodic HER, but on the other hand, prevent anodic degradation of the Pd catalyst.
In our previous paper, we described in detail studies of Sn influence on electrocatalytic activit... more In our previous paper, we described in detail studies of Sn influence on electrocatalytic activity of PtSn catalyst for CO and formic acid oxidation (Stevanović et al., J. Phys. Chem. C, 118 (2014) 278–289). The catalyst was composed of a Pt phase, Pt 3 Sn alloy and very small SnO 2 particles. Different electrochemical treatment enabled studies of PtSn/ C having Sn both in surface and subsurface layers and skeleton structure of this catalyst with Sn only in subsurface layers. The results obtained revealed the promotional effect of surface Sn whether alloyed or as oxide above all in preventing accumulation of CO and blocking the surface Pt atoms. As a consequence , in formic acid oxidation, the currents are not entering the plateau but increasing constantly until reaching a maximum. It was concluded that at lower potentials the effect of Sn on formic acid oxidation was predominantly electronic but with increasing the potential bi-functional mechanism prevailed due to the leading role of SnO 2. This role of SnO 2 is restated in the present study. Therefore, CO and formic acid oxidation were examined at PtSnO 2 /C catalyst. The catalyst was synthesised by the same microwave-assisted polyol procedure. According to XRD analysis, the catalyst is composed of a Pt phase and SnO 2 phase. The reactions were examined on PtSnO 2 /C catalyst treated on the same way as PtSn/C. Comparing the results obtained, the role of SnO 2 is confirmed and at the same time the significance of alloyed Sn and its electronic effect is revealed.
Tungsten based support was prepared by polycondensation of resorcinol and formaldehyde from ammon... more Tungsten based support was prepared by polycondensation of resorcinol and formaldehyde from ammonium metatungstate, in the presence cetyltrimethylammonium bromide (CTABr) surfactant. Pd nanocatalyst on this support was synthesized by borohydride reduction method. The obtained materials were characterized by High Resolution Transmission Electron Microscopy (HRTEM), Electron Energy Loss Spectroscopy (EELS), X-ray Photoelectron Spectroscopy (XPS) and electrochemical measurements. TEM analysis revealed Pd nanoparticles size in the range of a few nanometers, even the clusters of single Pd atoms. X-Ray Photoelectron Spectroscopy was applied to determine surface composition of the substrates. It was found that tungsten based support consisted of W, WC and WO 3 species. The presence of metallic palladium – Pd(0) in the Pd/W@WCWO 3 catalyst was revealed, as well. The catalytic activity and stability for the oxygen reduction were investigated in acid and alkaline solutions, by cyclic voltammetry and linear sweep voltammetry at the rotating disc electrode. The catalysts' activities were compared to the carbon supported Pd nanoparticles (Vulcan XC 72). WC supported Pd nanoparticles have shown high activity and superior stability, comparable even to Pt based catalysts, especially in alkaline electrolytes.
The oxygen evolution reaction (OER) was studied at pure Ni and Ni-(Ebonex/Ir) composite coatings ... more The oxygen evolution reaction (OER) was studied at pure Ni and Ni-(Ebonex/Ir) composite coatings in 1 M NaOH solution at 25 oC. Ni-(Ebonex-supported Ir) coatings were electro-deposited from a nickel Watts bath containing different concentrations of suspended Ebonex/Ir particles (0-2 g dm-3) onto a Ni 40 mesh substrate. The surface morphology of the coatings was examined by scanning electron microscopy (SEM), the surface composition by energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRPD) and X-ray photoelectron spectroscopy (XPS), whereas the electrochemical properties were studied by electrochemical impedance spectroscopy (EIS), polarization measurements and cyclic voltammetry (CV). It was shown that the roughness factor of Ni-(Ebonex/Ir) composite coatings calculated relative to the surface area of the pure Ni sample increased with the increasing content of Ebonex/Ir particles in the bath to a maximum value of 40.6. All samples displayed a Tafel slope of about 60 mV dec-1 in the potential range corresponding to lower current densities for the OER. The increase of the apparent activity for the OER at Ni-(Ebonex/Ir) coatings compared with the pure Ni coating was attributed only to the increase of the electrochemically active surface area. Although the pure Ni coating initially exhibited higher intrinsic catalytic activity for the OER than the composite coatings, it also showed a drastic loss of activity after subjecting to continuous oxygen evolution at j = 50 mA cm-2 for 24 h (deltaE = 395 mV). At the same time, the OER overpotential at Ni-(Ebonex/Ir) coatings only negligibly increased after the stability test (deltaE = 22 mV). The improved retention of catalytic activity observed with Ni-(Ebonex/Ir) coatings was ascribed to the presence of IrO2 , which inhibited the formation of the inactive gamma-NiOOH phase.
In this paper, we propose a novel method for preparation of high surface area titania nanoparticl... more In this paper, we propose a novel method for preparation of high surface area titania nanoparticles, and investigate their photovoltaic performance in dye sensitized solar cells (DSSCs). The precursor compound titanium(IV)-isopropoxide is hydrolyzed in the presence of nonionic surfactant (Triton X100) and ethylenediaminetetraacetic acid sodium salt (EDTA-Na 2) and thermally transformed into gel. The gel is autoclaved, resulting in submicronic micelles which are further processed into mesoporous films. The films are deposited from paste, composed of ultrasonically broken micelles and the organic ingredients. The crack-free film structures, composed of sub–25 nm pure anatase particles are observed using the appropriate instrumental techniques: scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), UV-VIS-NIR spectroscopy. The large surface area of 158 m 2 g -1 and mesoporosity are confirmed using the data obtained from the nitrogen adsorption-desorption isotherm. The photovoltaic performance of the operating N719-sensitized solar devices are tested using electrochemical impedance spectroscopy (EIS), open-circuit voltage decay (OCVD) and by recording current density-voltage (J-V) curves. The cell exhibits promising photocurrent density up to 11.7 mA cm -2 , and the photo-to-electric power efficiency of 5.22%, for cell area of 0.24 cm 2 , under 100 mW cm -2 halogen light source.
Pd-Ni/C catalyst was synthesized employing a borohydride reduction method. The high area Ni was f... more Pd-Ni/C catalyst was synthesized employing a borohydride reduction method. The high area Ni was first dispersed on the carbon support and then modified by Pd nanoparticles. Transmission electron microscopy confirmed relatively even distribution of Ni across the carbon support with discrete pal-ladium particles of about 3.3 nm mean diameter on it. Cyclic voltammetry confirmed the presence of Ni on the catalyst surface. The activity of the Pd-Ni/C catalysts for ethanol oxidation reaction (EOR) in alkaline solution was tested under the potentiodynamic and potentiostatic conditions and the results were compared to those obtained on the Pd/C catalyst. It was found that Pd-Ni/C is more active for the EOR compared to Pd/C by a factor up to 3, depending on the type of experiments and whether specific activity or mass activity are considered. During the potentiodynamic stability test an interesting phenomenon of activation of Pd-Ni/C catalyst was observed. It was found that maximum activity is attained after fifty cycles with the positive potential limit of 1.2 V, regardless of whether they were performed in the electrolyte with or without ethanol. It was postulated that potential cycling of the Pd-Ni surface causes reorganization of the catalyst surface bringing Pd and Ni sites to a more suitable arrangement for the efficient ethanol oxidation.
Electrodeposition and Surface Finishing – Fundamentals and Applications, Modern Aspects of Electrochemistry, No. 57, S.S. Djokić (Ed.), Chapter 1, 1–84, Mar 2014
v Izvod da je u manjoj meri prisutna i MoNi 4 faza. TEM analiza istaloženih prahova je pokazala i... more v Izvod da je u manjoj meri prisutna i MoNi 4 faza. TEM analiza istaloženih prahova je pokazala istovremeno prisustvo amorfnih i kristalnih čestica. Kristalne čestice svih prahova sadrže pretežno MoO 3 i MoNi 4 fazu, dok je kod praha istaloženog pri odnosu Ni/Mo = 1/0,3 uočeno i prisustvo NiO faze. Otkriveno je da je formiranje NiMoO 4 faze posledica reakcije u čvrstom stanju između NiO i MoO 3 na povišenim temperaturama. Na osnovu rezultata TEM analize pretpostavljen je mehanizam elektrohemijskog taloženja prahova legura Mo-Ni-O sistema po prvi put u svetskoj literaturi.
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Papers by Uroš Lačnjevac
solution of sodium tungstate (Na2WO42H2O) with the addition of the pure and Ce-loaded zeolites
clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized
with respect to their morphologies and chemical and phase compositions using scanning electron
microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray
diffraction. The prepared coatings contained
-alumina, WO3, and metallic tungsten. The surface
morphologies of the obtained coatings strongly depended on the PEO processing time; the roughness
of all coatings increased with PEO time, while porosity decreased with PEO processing time as
a result of microdischarge coalescence and growth. All coatings contained elements originating
from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher
photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels
were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for
10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO
coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for
various applications.
solution of sodium tungstate (Na2WO42H2O) with the addition of the pure and Ce-loaded zeolites
clinoptilolite and 13 X for the preparation of oxide coatings. The obtained coatings were characterized
with respect to their morphologies and chemical and phase compositions using scanning electron
microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray
diffraction. The prepared coatings contained
-alumina, WO3, and metallic tungsten. The surface
morphologies of the obtained coatings strongly depended on the PEO processing time; the roughness
of all coatings increased with PEO time, while porosity decreased with PEO processing time as
a result of microdischarge coalescence and growth. All coatings contained elements originating
from the substrate and from the electrolytes. Coatings containing zeolites with Ce showed higher
photoactivity than those with immobilized pure zeolites. The highest photocatalytic activity levels
were observed for coatings containing immobilized Ce-exchanged clinoptilolite processed for
10 min. It was observed that both clinoptilolite and 13X zeolites improved the features of the PEO
coatings in a similar manner, making natural and abundant clinoptilolite an excellent candidate for
various applications.