Valve Metal Oxide Research Articles

On the Occurrence of Domain Boundaries and Defects in Self-Organized Ordered Porous Anodic Alumina Systems.

The formation of domains and defects is a part of every self-organized structure without prepatterning which evolves from the electrochemical oxidation of valve metals like Ti, Al or Ta. Defects and domains, especially in highly ordered porous alumina films (PAOX), are one of the most studied phenomena in the current literature. The focus is on understanding their formation in order to find parameters to minimize their amount for applications that require the most highly organized structures. In this letter we give a solution to this problem by showing that the occurrence of defects and domains is an unavoidable consequence of the underlying self-organized process due to the simultaneous impact of the minimum (MINEP) and maximum (MAXEP) entropy production principle. As a consequence, experimental procedures to manufacture defect-free PAOX-films from a self-organization process face a fundamental limitation.

Application features of microarc oxidation technology

The technology of microarc oxidation of valve metals is one of the promising methods for engineering the working surface of friction units of modern technology. As a result of the versatility of the technology, it is possible to obtain composite ceramic coatings and materials of various types. Their properties are set by the electrical modes of material formation, the chemical composition of the electrolyte, and the possible further modification of the ceramic matrix with micro- and nanosized tribofillers. Based on the practical results of their research in the field of creating coatings of various types by microarc oxidation, an analysis is given of the main areas of application of microplasma electrolytic oxidation technology, their advantages over other methods of surface modification, structure and properties of the materials obtained. It is possible to distinguish the modification of the friction surface of a part by the method of microplasma electrolytic oxidation in order to increase its hardness and wear resistance. Good results were obtained in increasing the wear resistance of the hardened working surface of the spinning machine parts, the number of equipment repairs was reduced by more than 20 times. The use of microarc oxidation is promising for the preparation of composite coatings, which are a ceramic matrix in which solid lubricating dispersed particles are embedded. The technology of forming a matrix on aluminum, modified with dispersed magnetite, graphite and molybdenum disulfide, has been developed. According to the results of comparative tribotechnical tests, it was found that the intensity of linear wear of the material filled with MoS2 is 3 times, and Fe3O4 - 1.6 times lower than that of the coatings without filler. Based on the technology of microplasma electrolytic oxidation, an original technology has been developed for obtaining mineral-ceramic material, which is a matrix of aluminum oxide and dispersed diamond inclusions. Abrasive wheels made from this material have a consistently high volumetric cutting ability, 1.5-3.5 times higher than the traditional analogues and are characterized by high diamond retention. The possibilities of microplasma oxidation have not yet been fully explored, the most promising direction being the creation of nanostructured coatings for a specific technological task.

Analysis of electrical response, gas evolution and coating morphology during transition to soft sparking PEO of Al

The soft sparking regime of Plasma Electrolytic Oxidation (PEO) of valve metals attracts increasing attention as it enables a better process control with improved energy efficiency and coatings with enhanced performance to be obtained. Here, the discharges softening phenomenon was characterised by evaluation of changes in electrical signal, composition of gaseous products and morphology of the metal-oxide interface in PEO coatings on Al. The coatings were formed using two electrical regimes retarding or promoting soft sparking conditions. Harmonic analysis of PEO process has been performed using independent evaluation of the positive and negative half-waves of polarisation signal. The composition of gas evolved during the PEO process was evaluated by a quantitative gas analyser. Coatings were characterised by scanning and transmission electron microscopy with energy dispersive spectroscopy and small area electron diffraction. It was found that discharge softening is accompanied with a decrease in oxygen evolution rate, an increase in equivalent capacitance of anodic process as well as structural and compositional changes near the metal-oxide interface. The experimental findings provide further support to the explanation of discharge softening phenomenon by narrowing the active zone at the metal-oxide interface due to intercalation of hydrogen species into anodic alumina under cathodic polarisation. Such mechanistic understanding allows rational regime selection, in-situ process control and optimisation of the coating properties to be performed that paves the way to intellectual manufacturing approach based on careful process monitoring in operando.

Black Tungsten Oxide Nanofiber as a Robust Support for Metal Catalysts: High Catalyst Loading for Electrochemical Oxygen Reduction.

Black valve metal oxides with low oxygen vacancies are identified to be promising for various industrial applications, such as in gas sensing, photocatalysis, and rechargeable batteries, owing to their high reducibility and stability, as well as considerable fractions of low-valent metal species and oxygen vacancies in their lattices. Herein, the nanofiber (NF) of black oxygen-deficient tungsten trioxide (WO3- x ) is presented as a versatile and robust support for the direct growth of a platinum catalyst for oxygen reduction reaction (ORR). The nonstoichiometric, poorly crystallized black WO3- x NFs are prepared by electrospinning the W precursor into NFs followed by their low-temperature (650 °C) reductive calcination. The black WO3- x NFs have adequate electrical conductivity owing to their decreased bandgap and amorphous structure. Remarkably, the oxygen-deficient surface (surface O/W = 2.44) facilitates the growth of small Pt nanoparticles, which resist aggregation, as confirmed by structural characterization and computational analysis. The Pt-loaded black WO3- x NFs outperform the Pt-loaded crystalline white WO3- x NFs in both the electrochemical ORR activity and the accelerated durability test. This study can inspire the use of oxygen-deficient metal oxides as supports for other electrocatalysts, and can further increase the versatility of oxygen-deficient metal oxides.

Stabilization of an iridium oxygen evolution catalyst by titanium oxides

The anodic oxygen evolution reaction (OER) has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up-to-date Iridium is known as the best catalyst material for the OER in acidic media due to its relatively high activity and long-term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One promising way would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the OER of an oxidized Ir–Ti thin film material library covering the composition range from 20–70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and x-ray photoelectron spectroscopy. Overall, our data shows that Ir–Ti mixtures can be promising OER catalysts with both high activity and high stability.

(Invited) Multiscale Modelling of Titanium Oxide Growth and Dissolution in Fluoride-Containing Electrolytes

Nanostructured valve metal oxides have been extensively investigated due to the variety of applications related to their valuable chemical and physical properties, including mechanical and thermal resistance, catalytic and photocatalytic activity, and biocompatibility. The technology to form well defined porous anodic oxides on titanium will strongly benefit from an optimization of the process parameters based on deterministic modelling. The relatively large number of adjustable parameters often precludes the unambiguous interpretation of steady-state and transient electrochemical data in terms of a unique kinetic model.An approach to overcome this ambiguity by parameterization of the models of barrier film growth [1, 2] on Ti and its alloys (Figure 1) using a combination of in-situ EIS in a large frequency range, dynamic high-frequency impedance and photocurrent measurements, combined with ex-situ characterization of the oxides by X-ray photoelectron spectroscopy (XPS) is presented. Additional estimates of kinetic parameters are derived from density functional theory (DFT) modelling of adsorption of water and fluoride on Ti and TiO2 [3, 4]. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring are described and the significance of the obtained parameters for the kinetics of barrier film growth and dissolution is outlined. Stancheva, M. Bojinov, Electrochim. Acta 78, 65 (2012).Bojinov, I. Betova, V. Karastoyanov, Electrochim. Acta 344, 136137 (2020).Stancheva, B. Diawara, F. Lebreau, M. Bojinov, J. Electrochem. Soc. 161, E3188 (2014).Betova, M. Bojinov, V. Karastoyanov, M. Stancheva, Computational Mater. Sci.171, 109260 (2020). Acknowledgments: The financial support of the National Science Fund of Bulgaria under contract ДН19/3-2017 „Modeling, optimization and modification of nanoporous anodic oxide films as catalysts for water photoelectrolysis“is gratefully acknowledged. Figure 1

ЭЛЕКТРООСАЖДЕНИЕ СПЛАВА Pb-Sn(TiOx) ИЗ ЩЕЛОЧНОГО ЭЛЕКТРОЛИТА

In this work, the production of thermal and electrolytic triple alloy Pb-Sn (TiOx) is investigated and some properties of the obtained material are considered. Conclusions are made on the advantage of the electrolytic method in comparison with the thermal one. An alkaline tin and lead electrolyte containing potassium metatitanate has been developed. According to the criteria of the maximum content of titanium, dissipative power and surface quality, its optimal composition is determined: КОН – 100–150 g/l, SnCl2 – 16 g/l, (СН3СОО)2Pb – 75 g/l, К2ТіО3 – 0.2 g/l, glycerol – 50–60 g/l. The optimal conditions for electrolysis are a temperature of 20–30 ˚С, current density ік = 20 mA/cm2; the ratio of the cathode and anode surfaces is not less than 1 : 2. The passivated coatings photocatalytic activity research results together with their corrosion tests data and surface morphology analysis confirmed their high quality. The obtained alloy can be used as a corrosion-resistant coating, as a substrate for photo- and electroactive electrodes based on valve metal oxides, etc.

Stacked High Voltage Al Electrolytic Capacitors Using Zr-Al-O Composite Oxide

A stacked high-voltage (900 V) Al electrolytic capacitor made with ZrO2 coated anode foils, which has not been studied so far, is realized and the effects of Zr-Al-O composite layer on the electric properties are discussed. Etched Al foils coated with ZrO2 sol are anodized in 2-methyl-1,3-propanediol (MPD)-boric acid electrolyte. The anodized Al foils are assembled with stacked structure to prepare the capacitor. The capacitance and dissipation factor of the capacitor with ZrO2 coated anode foils increase by 41 % and decrease by 50 %, respectively, in comparison with those of Al anode foils. Zr-Al-O composite dielectric layer is formed between separate crystalline ZrO2 with high dielectric constant and amorphous Al2O3 with high ionic resistivity. This work suggests that the formation of a composite layer by coating valve metal oxide on etched Al foil surface and anodizing it in MPD-boric acid electrolyte is a promising approach for high voltage and volume efficiency of capacitors.

Counterbalanced Valve Metal Oxide as a Reliable Dielectric Layer for Electrowetting-on-dielectric Devices

Author(s): Chen, S; Kim, CJCJ | Abstract: Valve metal oxide (anodized oxide) is used in a mirrored configuration to serve as self-limiting dielectric layers of electrowetting-on-dielectric (EWOD) devices. Resembling nonpolar electrolytic capacitors, the two opposing metal-dielectric layers always counterbalance each other so that voltages of any polarity can be applied without current flow. By using tantalum pentoxide as the valve metal oxide for experimental evaluation, the mirrored configuration is compared with the usual configuration and further compared with silicon oxide and silicon nitride, which are two dielectric materials commonly used for EWOD. Experiments with a range of applied biases confirm that in a mirrored configuration, one of the two metal-dielectric-electrolyte combinations is always reverse-biased to prevent the current leakage, which is the most common mode of failure for the EWOD devices. The utility is demonstrated by manipulating droplets of KC1 solution on both the parallel-plate and one-plate EWOD devices.

Noble Metal Decorated Anodic TiO2 Nanotubes: Excellent (Electro)Catalyst

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The most established member of this group of materials is nanoporous Al2O3, which has been prepared two decades ago by anodic oxidation of Al into perfectly ordered, honeycomb-like porous structures employing suitable electrochemical conditions [1]. Owing to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, its porous membranes have been since than widely used as template material of choice for a range of materials [2-4]. Self-organized TiO2 nanotube layers have received the highest attention after Al2O3 motivated by its range of applications, including photocatalysis, water splitting, solar cells and biomedical uses. Very significant research efforts have led to reproducible synthesis of self-organized TiO2 nanotube layers by means of anodic oxidation, during which the starting Ti substrate is converted into a highly-ordered nanotubular layer by anodization in suitable electrolytes [5-8]. Many new applications can be derived upon modification of the nanotubes by decoration, filling or coating by secondary materials [9]. Atomic layer deposition (ALD) has been shown to be an effective technique to coat TiO2 nanotube layers homogenously with secondary materials or to decorate them with noble metals [9]. The resulting structures offer many advantages and opportunities in various applications. TiO2 nanotubes in particular appeal as highly ordered electron-conductive supports for noble metal particles. This presentation will focus in detail on the decoration of the nanotube layers of different aspect ratios by noble metals using ALD. We will show and discuss our recent catalytic results on water splitting [10] and electrocatalytic results on the oxidation of methanol [11].

Highly Conductive Off-Stoichiometric Zirconium Oxide Nanofibers with Controllable Crystalline Structures and Bandgaps and Improved Electrochemical Activities

The structural and morphological control of durable valve metal oxides with bandgaps over 5 eV (e.g., ZrO2) paves the way for the development of bifunctional electrochemical energy devices with both good stabilities and electronic conductivities. Herein, a tailored synthesis of highly conductive off-stoichiometric ZrO2–x nanofiber materials under a controlled reducing atmosphere is reported. The bandgap and corresponding charge conductivity of ZrO2–x are simultaneously tuned (in the range of visible colors (white, brown, and black)) by generating reduced Zr3+ and oxygen vacancies. The morphological and structural evolution of the ZrO2–x nanofibers obtained under different reducing atmospheres are investigated in detail. Electrochemical kinetics in aqueous and nonaqueous media are promoted by employing a darker ZrO2–x nanofiber electrode. The functionalizing valve metal oxides with a facile charge transfer inspire an advanced design for future electrochemical and electronic devices.

The Composition and Physicochemical Properties of PbO2–TiO2 Composite Materials Deposited from Colloid Electrolytes

The effect of the deposition conditions from colloid electrolytes on the chemical and phase composition, texture and electrocatalytic activity of PbO2–TiO2 composite materials was studied. The composition of the composites depends on the electrolysis regimes, the charge of the dispersed phase particles and the electrode, as well as the precipitation rate of lead dioxide, and the concentration of the components in the solution. By varying the electrolysis regimes and the composition of the electrolyte, composites with a TiO2 content of up to 27 wt % can be obtained. The phase composition and texture of the resulting composites are determined by the electrolysis regimes and the composition of the electrolytes used. In addition, the presence of TiO2 particles in the electrolyte leads, as a rule, to a decrease in crystal size and an increase in the content of α-phase of lead dioxide in the precipitate. Introduction of valve metal oxide particles into the lead dioxide materials generally results in an increase in OER overvoltage and in the rate of p-nitroaniline oxidative degradation due increasing the number of oxygen-bound particles firmly bound to the electrode surface and the parallel flow of photocatalytic processes at TiO2 centers, providing additional quantity of oxygen-containing oxidants of radical and peroxide nature. The use of composite materials as an active and transition layer of low-wearing anodes makes it possible to obtain electrodes with a long service lifetime.

Anodic Oxidation of Bismuth Towards Various Nanostructures

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The two most established members of this group of materials are nanoporous Al2O3 and TiO2 nanotubes [1,2]. While Al2O3 membranes have been since than widely used as template material of choice for a range of materials [3-5] due to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, TiO2 nanotubes received high attention because of their applications in e.g. photocatalysis, water splitting, solar cells and biomedical uses [6]. Bismuth oxide is another interesting material and its nanostructures show an enhanced photoelectrochemical performance [7,8]. However, the formation of bismuth nanostructures, such as bismuth oxide nanopores [7, 8] or bismuth phosphate nanorods [9], by anodization is not well explored by now. Similar to the formation of nanoporous Al2O3, nanostructured Bi2O3 can be produced in acidic, F--free electrolytes [10]. In this presentation, we will demonstrate the anodization of bismuth in various electrolytes towards morphologically very interesting Bi2O3 nanostructures with many potential applications. References H. Masuda, K. Fukuda, Science, 268 (1995) 1466. J.M. Macak et al., Curr. Opin. Solid State Mater. Sci. 1-2 (2007) 3.K. Nielsch, F. Müller, A.-P. Li, U. Gösele, Adv. Mater. 12 (2000) 582. H. Asoh et al., J. Electrochem. Soc. 148 (2001) B152.J. Kolar, J. M. Macak, K. Terabe, T. Wagner, J. Mater. Chem. C, 2 (2014) 349.K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 114 (2014) 9385.X. Lv, J. Zhao, X. Wang, X. Xu, L. Bai, B. Wang, J. Solid. Stat Electrochem. 17 (2013) 1215.K.C. Chitrada, K. S. Raja, R. Gakhar, D. Chidambaram, J. Electrochem. Soc. 162 (2015) H380.M. Yang, N. K. Shrestha, R. Hahn, P. Schmuki, Electrochem. Solid-State Lett. 13 (2010) C5. H. Sopha, V. Podzemna, L. Hromadko, J. M. Macak, Electrochem. Commun. 84 (2017) 6-9.

Nanometric Fe-substituted ZrO2 on Carbon Black: a Novel PGM-Free ORR Catalyst for PEMFCs

The independence from Platinum-Group-Metal (PGM) is one of the major goals towards the commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs), due to the limited availability of PGM and their considerable loading on the Oxygen-Reduction-Reaction (ORR), increasing their cost contribution to the overall system. On the other hand, the challenges for PGM-free ORR catalysts are an activity approaching that of Pt and a long-term operational stability in the strong acidic environment of a PEMFC. Low-cost FeNC catalysts fulfill the high activity requirement, but they still face fast degradation in acidic environment [1, 2]. On the contrary, pure valve-metal oxides as non-PGM ORR catalysts need a significant improvement in ORR activity and H2O yield but they are very promising in terms of stability during PEMFC operation [3, 4]. The aim of this contribution is to combine the intrinsic high ORR activity of Fe with the stability of valve-metal oxides, via partial substitution of Zr4+ with Fe3+ in the ZrO2 structure [5]. The formation of a solid solution of oxides of the two aliovalent-metals should also provide oxygen vacancies or uncoordinated metal sites at the oxide surface, hypothesized to be correlated to an increased ORR activity of valve-metal oxides [6, 7]. In this study, to obtain Fe-substituted ZrO2 we used a homogeneous mixture of the two metals at the molecular level, by employing two soluble organometallic precursors, i. e., zirconium (IV) tetra-tert-butyl dichloro phthalocyanine (ZrCl2Pc(tBu)4) and iron(II) tetra-tert-butyl phthalocyanine (FePc(tBu)4). Graphitized Ketjen-Black carbon is first impregnated with the precursors and then heat-treated as described in [8, 9]. XRD and TEM characterization show that our catalysts consist of ZrO2 particles of about 3 nm (Fe-substituted) and 7 nm (pure ZrO2). Mössbauer spectroscopy reveals that the Fe moieties are isolated and in Fe3+ electronic configuration at high spin, typical of an oxidic environment. This is confirmed by soft XAS data at the Fe L-edge and XPS data, pointing to the desired FexZr1-xO2-δ phase. We already published an evaluation of the ORR mass activity of FexZr1-xO2-δ in comparison to Fe-only and ZrO2-only catalysts, using a thin-film rotating (ring) disk electrode (RRDE) setup [9]. Using the preferred catalyst Fe0.07Zr0.93O2-δ, the variation of its mass activity and selectivity as a function of the loading is further discussed, with a H2O2 yield even lower in comparison to an FeNC catalyst [10]. The catalyst mass activity and its Arrhenius analysis in a single PEMFC at ≈0.4 mgcat/cm² is compared to the RDE results (Figure 1) [11]. The Arrhenius analysis resulted in an ORR activation energy of 16 kJ/mol (RDE) and 18 kJ/mol (PEMFC) at 0.4 VRHE, significantly lower than our best pure-ZrO2 catalyst reported (21 kJ/mol from RDE and 29 kJ/mol from PEMFC data) [4]. Stability tests and loading optimization of nanometric FexZr1-xO2-δ will be the future outlook. Acknowledgements: This work was supported by the Bayerische Forschungsstiftung (Project ForOxiE², AZ 1143-14).

Anodic Oxidation of Bismuth Towards Various Nanostructures

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The two most established members of this group of materials are nanoporous Al2O3 and TiO2 nanotubes [1,2]. While Al2O3 membranes have been since than widely used as template material of choice for a range of materials [3-5] due to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, TiO2 nanotubes received high attention because of their applications in e.g. photocatalysis, water splitting, solar cells and biomedical uses [6]. Bismuth oxide is another interesting material and its nanostructures show an enhanced photoelectrochemical performance [7,8]. However, the formation of bismuth nanostructures, such as bismuth oxide nanopores [7, 8] or bismuth phosphate nanorods [9], by anodization is not well explored by now. Similar to the formation of nanoporous Al2O3, nanostructured Bi2O3 can be produced in acidic, F--free electrolytes [10]. In this presentation, we will demonstrate the anodization of bismuth in various electrolytes towards morphologically very interesting Bi2O3 nanostructures with many potential applications. References H. Masuda, K. Fukuda, Science, 268 (1995) 1466.J.M. Macak et al., Curr. Opin. Solid State Mater. Sci. 1-2 (2007) 3.K. Nielsch, F. Müller, A.-P. Li, U. Gösele, Adv. Mater. 12 (2000) 582.H. Asoh et al., J. Electrochem. Soc. 148 (2001) B152.J. Kolar, J. M. Macak, K. Terabe, T. Wagner, J. Mater. Chem. C, 2 (2014) 349.K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 114 (2014) 9385.X. Lv, J. Zhao, X. Wang, X. Xu, L. Bai, B. Wang, J. Solid. Stat Electrochem. 17 (2013) 1215.K.C. Chitrada, K. S. Raja, R. Gakhar, D. Chidambaram, J. Electrochem. Soc. 162 (2015) H380.M. Yang, N. K. Shrestha, R. Hahn, P. Schmuki, Electrochem. Solid-State Lett. 13 (2010) C5.H. Sopha, V. Podzemna, L. Hromadko, J. M. Macak, Electrochem. Commun. 84 (2017) 6-9.

Application of oxide-metallic catalysts on valve metals for ecological catalysis

It is shown that a promising technique for obtaining oxide-metallic catalysts is the plasma-electrolytic oxidation (PEO) of valve metals, particularly aluminum and titanium alloys. Such a mode of synthesis makes it possible to form catalytically active materials with a developed surface, high content of dopants, and a broad scope of application over a single-stage technological process. The d-metals, in particular, manganese and cobalt are most promising as the dopants for oxide-metallic catalysts. Employing results of the experimental studies, we demonstrated pathways to control the composition and degree of surface development of the mangan- and cobalt-containing oxide-metallic systems by using complex electrolytes. It is established that the obtained oxide coatings are characterized by high catalytic activity in the model conversion reactions of carbon (II) oxide. In terms of such critical process parameters as the degree of conversion and ignition temperature, they are not inferior to, and in some cases, outperform industrial platinum catalysts. The use of manganese-containing oxide-metallic coating of the piston in an internal combustion engine leads to lower fuel consumption and a reduction in the toxicity of gas emissions

Synergistic Effect on the Activity of Fe+ZrO2 Non-PGM ORR Catalysts for PEMFC

High cost and limited availability of platinum-based catalysts for the Oxygen Reduction Reaction (ORR) is a major hurdle towards the commercialization of Proton Exchange Membrane Fuel Cell (PEMFC) systems. Until today, replacing Pt by Pt-free and finally by noble-metal-free ORR catalysts is still restricted to the laboratory scale. High activity and durability are the main requirements for new catalysts. The currently most favored materials are based on the MNxCy system (M = Fe, Co) due to their low cost and reasonable ORR activity, but they still cannot be commercialized due to their poor stability in corrosive environment [1, 2]. On the other hand, valve-metal oxides as non-PGM ORR catalysts are promising based on their stability in acid, but their ORR activity still needs to be improved [3, 4]. Recently, our group published an article in which the synthesis of carbon-supported zirconia (ZrO2) nanoparticles has been optimized towards ORR activity by varying the heat treatment atmosphere and temperature [5]. Based on our findings, we concluded that the intrinsic ORR activity of the pure valve-metal oxides is far too low to be practically employed in fuel cells. Since it is hypothesized that the active sites correlate with the presence of oxygen vacancies or uncoordinated metal sites at the oxide surface [6, 7], a way to enhance their ORR activity would be to substitute part of the valve metals in the oxide with aliovalent metals. In this study, we have chosen iron as a substituent, since it is ORR-active and since it is known that it can be incorporated into the ZrO2 structure [8]. In this contribution, a series of ORR catalysts consisting of Fe containing ZrO2 nanoparticles and supported on graphitized Ketjen-Black carbon (ZrO2/KBGraph) are studied. In order to get a homogeneous mixture of the two metals at the molecular level, the synthesis route employed two soluble organometallic precursors, viz., zirconium (IV) tetra-tert-butyl dichloro phthalocyanine (ZrCl2Pc(tBu)4) and iron(II) tetra-tert-butyl phthalocyanine (FePc(tBu)4). The carbon support was first impregnated with the mixed precursor solution and then heat-treated as described in [9]. From XRD characterization it is evident that we obtain ZrO2 particles with a particle size much smaller than 10 nm. Electrochemical measurements of the ORR activity are performed at 20 °C using a thin-film rotating disk electrode (RDE) setup in 0.1 M HClO4 at 20 °C, 1600 rpm, reporting the capacitively-corrected anodic scans [10]. As shown in Figure 1, the carbon-supported nanoparticles prepared by adding Fe to ZrO2 show a significantly increased activity when compared to pure Fe or ZrO2 samples heat treated under the same conditions. It is thus evident that a clear synergistic effect between the two components results in a much more ORR-active catalyst. A detailed structural characterization of the active catalyst and their further optimization will be the focus of this presentation.

Self-Organized TiO2 Nanotubes: Influence of the Ti Microstructure on the Nanotube Growth

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The most established member of this group of materials is nanoporous Al2O3, which has been prepared two decades ago by anodic oxidation of Al into perfectly ordered, honeycomb-like porous structures employing suitable electrochemical conditions [1]. Owing to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, its porous membranes have been since than widely used as template material of choice for a range of materials [2-4]. Recently, TiO2 has received the highest attention after Al2O3 motivated by its range of applications, including photocatalysis, water splitting, solar cells and biomedical uses. Very significant research efforts have led to reproducible synthesis of self-organized TiO2 nanotube layers by means of anodic oxidation, during which the starting Ti substrate is converted into a highly-ordered nanotubular layer by anodization in suitable electrolytes [5-7]. Although advancements in the anodic synthesis of self-organized TiO2 nanotube layers have been presented over the past years [8], the degree of ordering has not reached so far the level known from porous alumina [1]. Numerous factors influence the ordering and the homogeneity of the TiO2 nanotube layers. In the presentation, we demonstrate significant advancements in the understanding of the influence of Ti substrates on the TiO2 nanotube growth compared to the recent state-of-art. Using SEM, AFM, GDOES and optical measurements we will show how to obtain a high degree of ordering, uniformity and homogeneity in the nanotube structure of TiO2 by tailoring the anodization protocol [9, 10]. Furthermore, based on SEM, EBSD and TEM measurements, we will demonstrate how the Ti grain structure influences the lateral uniformity of the nanotube layers [11].

Anodic Oxidation of Bismuth in Halide Containing Electrolytes

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The two most established members of this group of materials are nanoporous Al2O3 and TiO2 nanotubes [1,2]. While Al2O3 membranes have been since than widely used as template material of choice for a range of materials [2-4] due to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, TiO2 nanotubes received high attention because of their applications in e.g. photocatalysis, water splitting, solar cells and biomedical uses [5]. Bismuth oxide is another interesting material and its nanostructures show an enhanced photoelectrochemical performance [6,7]. However, the formation of bismuth nanostructures, such as bismuth oxide nanopores [6,7] or bismuth phosphate nanorods [8], by anodization is not well explored by now. Similar to the formation of nanoporous Al2O3 nanoporous Bi2O3 was produced in acidic, F--free electrolytes. In this presentation, we demonstrate the anodization of bismuth in halide containing electrolytes similar to the electrolytes classically used for the formation of TiO2 nanotubes.

Resistive switching in TiO2 nanocolumn arrays electrochemically grown

Resistive switching in metal oxides, especially in TiO2, has been intensively investigated for potential application in non-volatile memory microdevices. As one of the working mechanisms, a conducting filament consisting of a substoichiometric oxide phase is created within the oxide layer. With the aim of investigating the filament formation in spatially confined elements, we fabricate arrays of self-ordered TiO2 nanocolumns by porous-anodic-alumina (PAA)-assisted anodizing, incorporate them into solid-state microdevices, study their electron transport properties, and reveal that this anodizing approach is suitable for growing TiO2 nanostructures exhibiting resistive switching. The electrical properties and resistive switching behavior are both dependent on the electrolytic formation conditions, influencing the concentration and distribution of oxygen vacancies in the nanocolumn material during the film growth. Therefore, the PAA-assisted TiO2 nanocolumn arrays can be considered as a platform for investigating various phenomena related to resistive switching in valve metal oxides at the nanoscale.