Anodic Structures Research Articles

Development of a Novel Structured Mesh-Type Pd/γ-Al2O3/Al Catalyst on Nitrobenzene Liquid-Phase Catalytic Hydrogenation Reactions

Nitrobenzene liquid-phase catalytic hydrogenation is commonly regarded as one of the most effective technologies for aniline production. The traditional granular catalysts have the disadvantages that the reactor bed pressure drop is large and the mass transfer efficiency between gas and liquid phases is low. In this study, a novel structured mesh-type Pd/γ-Al2O3/Al catalyst was prepared by anodic oxidation and pore structures of γ-Al2O3/Al supports were constructed by acid pore-widening treatments. The results showed that acid pore-widening treatments can improve the pore size of γ-Al2O3/Al supports; the support with HNO3 pore-widening treatment exhibited the largest pore size, being enlarged from 3.7 nm to 4.6 nm. The Pd/γ-Al2O3/Al catalysts prepared with different acid pore-widening treatment supports contribute to the increased active metal Pd loading, more Pd0 content, and better dispersion of the Pd particles. The catalyst prepared with HNO3 pore-widening treatment support exhibited the largest active metal Pd loading, enlarging from 1.82% to 1.95%, the largest Pd0 content being enlarged from 52.1% to 58.5% and the smallest Pd particle size being reduced from 103 nm to 41 nm, resulting in the highest nitrobenzene conversion, increasing from 67.2% to 74.3%. Eventually, we calculated that the pressure drop of structured catalysts was 1/72 of that of granular catalysts, resulting in a better diffusion of the H2 through nitrobenzene solution to active sites on the catalyst surface and a significant increase in the catalytic activity.

Analyzing the degradation mechanism of solid oxide fuel cell during different time periods

Improving the lifetime and robustness of solid oxide fuel cells (SOFCs) is crucial for their commercial viability. This study aims to elucidate the degradation mechanism of SOFCs during galvanostatic tests and understand the internal and external factors influencing the cell degradation. Three nominally identical cells underwent galvanostatic testing for different durations to illustrate the evolution mechanism of SOFCs degradation at different stages. The electrochemical impedance spectroscopy (EIS) measurements under the open circuit voltage (OCV) and the direct current (DC) bias at different periods were carefully analyzed. By combining distribution of relaxation times (DRT) with equivalent circuit model (ECM) fitting, the contribution of each electrode process to cell performance degradation during galvanostatic testing was quantified. The polarization impedance, primarily associated with the charge transfer reactions in the anode, exhibited continuous increase under DC bias, contributing more than 80 % to the increase overall polarization impedance. Conversely, the cathode, related to O2 dissociation and diffusion process, contributed 7.2 %. The rapid degradation in the early stage was attributed to the degeneration of the anode microstructure, resulting from the formation of numerous isolated micron Ni particles and a subsequent decrease in three phase boundary (TPB) density. The anode microstructure after galvanostatic testing was characterized, revealing a novel Ni migration evolution mechanism. This mechanism involves the rapid agglomeration of Ni crystal particles at the beginning test, subsequent diffusion as micro-particles within 25 h, and eventual coarsening into dispersive agglomerates over time. The proposed mechanism of anodic microstructure evolution offers insights for optimizing anodic structures and lays the foundation for enhancing the long-term stability of SOFCs.

A New Approach of Electrolytic Metal Manganese with Lower Energy Consumption and Fewer Spherical Dendrites Based on a Hyperchaotic Circuit with Directly Offset Boosting

Electrolysis is an important way to produce manganese metal, but the low current efficiency and random growth of dendrites have always been challenging problems for enterprises. The lack of understanding of the dynamic system during the electrolysis process is the main reason for the accurate control of the electrolysis process. Based on this consideration, a new four-dimensional continuous hyperchaotic system with high Lyapunov exponents is designed. The amplitude control, frequency modulation, and offset boosting of the hyperchaotic system are obtained through the selection of feedback term. A circuit simulation and corresponding simplified circuit are established. In addition, the actual hyperchaotic circuit is applied to the manganese electrolysis process through the self-designed current amplification module (the amplification of [Formula: see text] signal is realized by the offset boosting control). The experimental results of the hyperchaotic electrolysis of metal manganese showed that the hyperchaotic current can delay the occurrence time of electrochemical oscillation, and reduce the generation of cathode metal manganese dendrites. Furthermore, the results show that the hyperchaotic current can enhance the current efficiency and reduce the energy consumption. Based on the new experiment, it is suggested that the formation of anodic porous structures, whose primary phase compositions were PbSO4, MnO2, and Mn2O3, is one factor for the occurrence of electrochemical oscillations, while the conversion between Mn[Formula: see text] and Mn[Formula: see text] is another main factor for the mutation of electrochemical signal (manganese autocatalysis).

BiVO4-modified anodic nanocoral WO3 structures for enhancement of photoelectrochemical performance

Nanostructuring of metal oxides is a straightforward strategy for enhancing photoelectrochemical (PEC) performance. The present work demonstrates the anodization of W metal to determine the optimum WO3 nanostructure for use in photoanodes. Anodization was performed in a fluoride-containing 1 M Na2SO4 electrolyte. Under optimum conditions, WO3 nanocoral structures were fabricated. The forming mechanism of the nanocoral structures was elucidated by the generation and development of nanobubbles at the metal/oxide interface. To enhance the PEC performance, BiVO4 layers were coated on the anodic WO3 nanocoral structures and annealed at 500 °C in air atmosphere. The prepared BiVO4/WO3 nanocoral layers were characterized by SEM, XRD, EIS and UV–vis spectroscopy. They were then used as photoanodes for PEC and IPCE measurements. Consequently, photocurrent density was improved to 0.42 mA/cm2 at 1.23 V vs. RHE and IPCE at 400 nm (21%) was enhanced by 21 times due to the producing band alignment between BiVO4 and WO3.

Anodic SnO2 Nanoporous Structure Decorated with Cu2O Nanoparticles for Sensitive Detection of Creatinine: Experimental and DFT Study.

Advanced anodic SnO2 nanoporous structures decorated with Cu2O nanoparticles (NPs) were employed for creatinine detection. Anodization of electropolished Sn sheets in 0.3 M aqueous oxalic acid electrolyte under continuous stirring produced complete open top, crack-free, and smooth SnO2 nanoporous structures. Structural analyses confirm the high purity of rutile SnO2 with successful functionalization of Cu2O NPs. Morphological studies revealed the formation of self-organized and highly-ordered SnO2 nanopores, homogeneously decorated with Cu2O NPs. The average diameter of nanopores is ∼35 nm, while the average Cu2O particle size is ∼23 nm. Density functional theory results showed that SnO2@Cu2O hybrid nanostructures are energetically favorable for creatinine detection. The hybrid nanostructure electrode exhibited an ultra-high sensitivity of around 24343 μA mM-1 cm-2 with an extremely lower detection limit of ∼0.0023 μM, a fast response time (less than 2 s), and wide linear detection ranges of 2.5-45 μM and 100 μM to 15 mM toward creatinine. This is ascribed to the creation of highly active surface sites as a result of Cu2O NP functionalization, SnO2 band gap diminution, and the formation of heterojunction and Cu(1)/Cu(ll)-creatinine complexes through secondary amines which occur in the creatinine structure. The real-time analysis of creatinine in blood serum by the fabricated electrode evinces the practicability and accuracy of the biosensor with reference to the commercially existing creatinine sensor. The proposed biosensor demonstrated excellent stability, reproducibility, and selectivity, which reflects that the SnO2@Cu2O nanostructure is a promising candidate for the non-enzymatic detection of creatinine.

Incorporation of Phase Change Materials into the Surface of Aluminum Structures for Thermal Management.

This article explores the concept of generating a porous anodic layer on the surface of a metallic component to host a phase change material (PCM) aiming to reduce the peak temperatures that the host structure will experience. The conditions to fabricate a porous anodic layer on top of an aluminum substrate were determined through varying anodization conditions: solution concentration, voltage employed, and anodization times. Pore sizes were characterized using scanning electron microscopy. The alkane n-eicosane was selected as PCM, introduced within the porous anodic annealed layer using vacuum impregnation and the thin film composite structure sealed. Epoxy resin and a metallic paste were tested as sealants. Thermal tests were performed to compare the behavior of aluminum alloy substrates anodized and sealed with and without PCM. The results showed pores with diameters in the 5–85 nm range, with average values that increased as the time of anodization was extended. The aluminum alloy impregnated with n-eicosane presents lowered surface peak temperatures during heating cycles than the samples that were only anodized or than the base alloy, demonstrating the potential of PCM incorporated in the superficial microstructure of anodic structures to manage, to a certain extent, peak transient thermal loads.

Flexible anodic SnO2 nanoporous structures uniformly coated with polyaniline as a binder-free anode for lithium ion batteries

• SnO 2 @PANI anodes were successfully synthesized through wet-chemical approach. • Anodic nanoporous SnO 2 leads to shortened lithium ion diffusion lengths. • Uniformly deposited PANI onto nanoporous SnO 2 serves as an effective buffer layer. • Optimized thickness of PANI layer is controlled by adjusting the scan rate. Herein we report a wet-chemical process to obtain binder-free anodes for lithium-ion batteries (LIBs) using a straightforward and easily scalable approach. Electrochemical deposition, anodization, and electropolymerization are used to firmly attach a nanoporous and flexible SnO 2 film, uniformly coated with polyaniline (PANI), onto a Cu substrate. The three-dimensional, flexible, and nanoporous SnO 2 @PANI electrodes exhibit good specific capacity, capacity retention, and charge transfer resistance. Its favorable electrochemical properties are attributed to the large surface area, high electrical conductivity, and effective reduction of volume expansion. These results demonstrate that the proposed approach to forming SnO 2 @PANI electrodes is a promising strategy for constructing high-performance binder-free anodes for emerging flexible LIBs.

Degradation and utilization of EPS from excessive activated sludge by interaction of electrogenesis and light stimulation

Sticky and gel-like extracellular polymeric substances (EPS) could provide the protective shielding and prevent the microbial cellrupture and lysis as well as increase the difficulty of excessive activated sludge (EAS) biodegradation. Microbial fuel cells (MFCs) are able to accelerate the degradation of EPS and gain electric energy recovery from wastewater simultaneously. In order to discover the role of light stimulation on MFC performance, four reactors were operated in dark (SD), in LED light (SL), in electrogenesis (ED) and in combination of electrogenesis and light (EL), respectively. EPS characteristic, electrochemical performance and anodic microbial community structures were investigated and compared. The results showed that single light stimulation distinctly improved the total EPS especially polysaccharides content in soluble EPS, and Pseudomonas (31.85% relative abundance) as well as Acinetobacter (10.2% relative abundance) became the dominant genera. Single electrogenesis mainly promoted degradation of negatively charged proteins, which was further proved by the enrichment of typical proteolytic and amino acids fermented bacterial genera like Proteiniphilum and Cloacibacillus. Furthermore, the electrochemical activated Petrimonas and syntrophic Syntrophomonas occupied the dominant position indicating the existence of interspecific electron transfer. Moreover, the predominance of Acinetobacter (44.2% relative abundance) and Escherichia/shigella (7.09% relative abundance) in EL reactor implied the synergetic improvement of electrogenesis and light stimulation. In the interaction effect, light stimulation optimized the electricity generation and proteins degradation of MFC.

Rumen Inoculum Enhances Cathode Performance in Single-Chamber Air-Cathode Microbial Fuel Cells.

During the last decade, bioprospecting for electrochemically active bacteria has included the search for new sources of inoculum for microbial fuel cells (MFCs). However, concerning power and current production, a Geobacter-dominated mixed microbial community derived from a wastewater inoculum remains the standard. On the other hand, cathode performance is still one of the main limitations for MFCs, and the enrichment of a beneficial cathodic biofilm emerges as an alternative to increase its performance. Glucose-fed air-cathode reactors inoculated with a rumen-fluid enrichment and wastewater showed higher power densities and soluble chemical oxygen demand (sCOD) removal (Pmax = 824.5 mWm−2; ΔsCOD = 96.1%) than reactors inoculated only with wastewater (Pmax = 634.1 mWm−2; ΔsCOD = 91.7%). Identical anode but different cathode potentials suggest that differences in performance were due to the cathode. Pyrosequencing analysis showed no significant differences between the anodic community structures derived from both inocula but increased relative abundances of Azoarcus and Victivallis species in the cathodic rumen enrichment. Results suggest that this rarely used inoculum for single-chamber MFCs contributed to cathodic biofilm improvements with no anodic biofilm effects.

Preparation and Preliminary Evaluation of Silver-Modified Anodic Alumina for Biomedical Applications

The present study reports a specific method for preparation of silver-modified anodic alumina substrates intended for biomaterial applications. Al2O3 coatings were obtained by anodization of technically pure aluminum alloy in sulfuric acid electrolyte. Silver deposition into the pores of the anodic structures was carried out employing in situ thermal reduction for different time periods. The obtained coatings were characterized using scanning electron microscopy (SEM), potentiodynamic scanning after 168 h in 3.5% NaCl solution and bioassays with human fibroblast and NIH/3T3 cell lines. The modified alumina substrates demonstrated better biocompatibility compared to the control anodic Al2O3 pads indicated by increased percent cell survival following in vitro culture with human and mouse fibroblasts. The Ag-deposition time did not affect considerably the biocompatibility of the investigated anodic layers. SEM analyses indicated that mouse NIH/3T3 cells and human fibroblasts adhere to the silver-coated alumina substrates retaining normal morphology and ability to form cell monolayer. Therefore, the present studies demonstrate that silver coating of anodic alumina substrates improves their biocompatibility and their eventual biomedical application.

Mixed anodic oxides for forming-free memristors revealed by combinatorial screening of hafnium-tantalum system

A combinatorial screening approach has been designed for identification of Hf-Ta anodic memristors with enhanced properties. Three distinct compositional zones with similar characteristics have been identified, two of which are relevant for memristive applications. In a Hf-poor zone, (< 50 at.% Hf), the memristive behavior is similar to that of anodic Ta2O5. A second compositional zone (Hf content between 50 and 70 at.%) produced anodic memristors which dramatically differ. Devices in this region resemble forming-free unipolar behavior and show resistive states ratios of more than 8 orders of magnitude while retaining and enduring more than one million switching cycles. Photoelectron spectroscopy with soft and hard X-rays has been applied for understanding the chemistry of these special anodic structures and enrichment of Hf species close to the surface of anodic oxides has been evidenced. Unlike bipolar memristors from the first zone, the newly identified unipolar memristors show an in-depth homogeneous local chemical state of Hf and lower electronic polarizabilities for both O and Hf. High-resolution transmission electron microscopy reveals HfO2 crystallites embedded into an amorphous oxide matrix. These entities formed during the anodization process are considered to be responsible for the coexistence of threshold and non-volatile memristive characteristics.

Anodic TiO2 Nanotubes on 3D-Printed Titanium Meshes for Photocatalytic Applications.

In this work, large 3D Ti meshes fabricated by direct ink writing were wirelessly anodized for the first time to prepare highly photocatalytically active TiO2 nanotube (TNT) layers. The use of bipolar electrochemistry enabled the fabrication of TNT layers within the 3D Ti meshes without the establishment of an electrical contact between Ti meshes and the potentiostat, confirming its unique ability and advantage for the synthesis of anodic structures on metallic substrates with a complex geometry. TNT layers with nanotube diameters of up to 110 nm and thicknesses of up to 3.3 μm were formed. The TNT-layer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO2-nanoparticle-decorated and nonanodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. The results presented here open new horizons for the employment of anodized 3D Ti meshes in various flow-through (photo)catalytic reactors.

(Invited) Recent Advances in Structural Engineering of Nanoporous Anodic Alumina and Applications

In the last years structurally engineered nanoporous materials have attracted an increasing interest due to their potential applications in different research fields such as nanophotonic, biotechnology, sensing, catalysis, drug delivery, tissue engineering, batteries, membrane separation, and so on. Among other, nanoporous anodic alumina (NAA) has become an interesting material as a result of their outstanding set of properties, cost-competitive fabrication processes and fully scalable process compatible with conventional micro- and nanofabrication technology.Nanoporous anodic alumina is obtained by the electrochemical etching of aluminum. Under specific anodization condition presents a self-ordered hexagonal pore distribution of parallel cylindrical nanopores with pore diameters between 5 and 300 nm. Its geometric characteristics such as pore diameter, pore length and separation distance can be tuned by the anodization conditions (voltage, current density, temperature and electrolyte concentration) [1-2]. Their chemical resistance, thermal stability, and intrinsic photoluminescence are some of the outstanding properties of NAARecently, different anodization and post-anodization treatments (etching and annealing) have been developed to create new structures and pore geometries such as modulated, funnel-like, serrated-like, tip-like, etc. The application of periodic variations of current or voltage during the anodization is transferred to the material as the periodic variation of the pore diameter and consequently, it is possible design, 2D and 3D structures. In particular, it is very interesting the design photonic structures with tunable stop bands within the UV-VIS-NIR range by applying cycling anodization processes [3-4]. Another interesting example of structural engineering is the fabrication of anodic alumina nanotubes by pulse anodization performed in galvanostatic conditions. The length of nanotubes and inner and outer diameters are tailor-engineered by varying pulse period and current density level [5].In addition, its effective surface area makes of NAA an interesting platform for sensing and loading and releasing of active agents. For example, the surface of NAA can be easily functionalized to be selective for specific molecules and covered with biodegradable, chemical, or pH responsive agents to trigger and regulate the release. Another approach is to decorate the surface with magnetic or metallic nanoparticles. The surface modification of nanoporous anodic alumina can provide new properties and functionalities that can be advantageously used besides the structural engineering for innovative applications.In this talk, we will present recent advances in the design and fabrication of nanostructures based on nanoporous anodic alumina. We introduce different electrochemical approaches to modify the pore geometry during or after the fabrication processes. We will show how different nanoporous anodic alumina structures can be designed with particular photonic properties and how engineered porous anodic alumina nanotubes can be chemically modified and decorated with magnetic nanoparticles. Finally, we present different applications of nanoporous anodic alumina for optical sensing, drug delivery and cell culture.

(Invited) Recent Advances in Structural Engineering of Nanoporous Anodic Alumina and Applications

The structural engineering of nanoporous anodic alumina with controllability, reproducibility, cost-effective, and well-organized processes presents excellent opportunities for the obtention of several structures with photonic and optical properties to enable their application in several fields such as chemical and optical biosensing, drug delivery, photocatalysis, optical encoding and others. For this reason, we present a comprehensive study of the recent advances in the engineering of nanoporous anodic alumina structures and their application in optical sensing.

Rough 1D photonic crystals: A transfer matrix approach

Abstract Interfacial roughness is prevalent in 1D photonic crystals and other layered structures, but is not generally accounted for in their design nor the analysis of their optical properties due to a lack of simple theoretical approaches. We present a transfer matrix formalism to incorporate the effects of interfacial roughness in the optical properties of stratified systems such as 1D photonic crystals and apply it to calculate the optical response of some nanoporous anodic alumina and porous silicon structures. We have validated our formalism by comparing our results to some experiments.

Generation scenarios of anodic structures and experimental realization of turbulence in unmagnetized plasma

In the present work, we report development of a DC glow discharge plasma (GDP) set-up to study controlled evolution of anodic structures having distinctive geometry, size and layers, generated in front of a positively biased electrode, submerged in unmagnetized plasma. For such an anodic structure, we have also investigated the condition under which the turbulence is triggered. Characteristic of these structures, generated in front of a positively biased electrode, depends on multiple parameters such as the ratio of anode to cathode size, electrode separation, gas pressure, biasing configuration such as anode bias, cathode bias and grounding schemes. We attempted to classify different anodic structures observed experimentally, as anode glow, fireball, anode spot, double layer and multiple double layers (MDLs) based on its physical characteristics. Among these structures the present investigation is focused on MDLs. The number of layers, observed in MDLs varied from as high as six to as low as zero, by controlling the operating discharge parameters, externally. Diagnostics were carried out using Langmuir probe. The analysis of floating potential fluctuations corresponds to a multiple anodic structure showed emergence of turbulence, at its critical stage, satisfying condition for self-organized criticality (SOC). This was identified with three slopes observed in the power spectrum, resembling the sand-pile model. Though, the GDP is completely different from that of the magnetically confined plasma, the nature of turbulence observed with SOC, is very similar to that observed in the scrape of layer of fusion devices. Therefore, the present investigation could provide new approach to study turbulence of similar nature, under an experimental condition that is free from the complexities of complicated field geometries used in confinement devices.

Anodic alumina oxide surfaces prepared by dual hard and mild anodization at subzero temperature: Surface microscopic characterization and influence on wettability

Multistage anodizations allow the preparation of new generations of ordered anodic alumina oxide porous structures with large interpore distances, yet the influence of second anodization and pore widening times on pore dimensions and wettability of the obtained structures has not yet been fully studied. The main objective of this study was to characterize the obtained pores (dimensions and morphology), for different anodizations and pore widening times, and to investigate their respective influence on surface wettability. Anodic alumina oxide ordered porous surfaces were prepared using a dual hard and mild anodization process with oxalic and phosphoric acids at subzero temperature, with or without pore widening. Statistical analyses were realized on pore measurements from samples produced with a large range of anodization durations (7 different tested times). The obtained surfaces possessed regular ordered pores of 99 nm and 307 nm for respectively median pore diameter and inter-pore distance, before pore widening, and did not conform to expected theoretical values for either usual lone mild or hard anodization conditions. The slow growth of pore depth (median of 11 nm/min) observed during second anodization could be explained by the subzero electrolyte temperature used. Pore widening effectively increased pore diameter, but had statistically a bigger impact on diameters for shorter second anodization time than for longer ones. Also, pore widening significantly decreased pore depth as well as the oxide barrier thickness. Additionally, porosity increased from 9.57% to 14.8% after 3 hours of pore widening, but did not significantly affect surface wettability, which was however increased when compared to flat control surfaces. The obtained results showed that a dual anodization with different electrolytes at a subzero temperature had a significant impact on pores dimensions. A discussion on the respective effects of surface chemistry and surface roughness in the wetting evolution was proposed.

Multilevel self-organization on anodized aluminium: discovering hierarchical honeycomb structures from nanometre to sub-millimetre scale.

Herein, the one-step spontaneous formation of multiscale hierarchical honeycomb-like textures on anodized aluminium is reported for the first time. A self-organized array of unprecedentedly large sub-millimetre hemispherical pores was observed on the upper hierarchy level, while superimposed well-ordered hexagonal cells on the lower hierarchy level maintained their classically known nanoscale self-ordering periodicity (50-60 nm). The larger macroscopic cells (0.45-0.90 mm) show great morphological similarity to the conventional nanosized porous anodic structures, but arise on a purely metallic surface without any analytically detectable amount of deposited alumina. The observed cell size expansion (approximately by a factor of 104) during Al anodizing is colossal in comparison with the size variation ranges reported in the literature so far. It reveals a dramatic nonlinearity effect in a dynamic self-organizing system upon a moderate change of experimental parameters (specifically, this work focuses on the effect of anodizing temperature). Such multilevel self-organization in a single aluminium anodizing process sheds new light on the origin of self-ordered anodic structures and allows rethinking the whole range of opinions regarding their formation mechanism that have been proposed to date.

Electrochemical Effects of Annealing Temperature on Anodic 304 Stainless Steels

Supercapacitors, as an energy storage device, have attracted growing interests [1]. Recently, iron oxides (a-Fe2O3 and Fe3O4) have been demonstrated to present comparable pseudocapacitive properties [2]. It was further observed that nanoporous oxide layers have been obtained on various SS substrates by electrochemical anodization process. They can provide more reaction sites due to high surface area, and acceptable electron conductivity due to multiple valence states of formed Fe3O4 for potential supercapacitor electrodes. Meanwhile, it has been envisaged that annealing at various temperatures may induce morphologgical modification and crystal-phase transition, which will strongly influence the supercapacitance behavior [3]. However, the effects of annealing temperature on supercapacitance behavior of anodic oxide films have not been well studied. In the experiments, SUS 304-type SS foils were used as the anode for prorous formation. The anodization of SS foils was carried out in ethylene glycol containing 0.1 M distilled water and 0.1 M NH4F using a two-electrode cell. The SS foils worked as anode and graphite plate as cathode. During the anodization process, the long-term anodization was kept by slightly decreasing electrolyte temperature to rescrict the temperature rise. The as-fabricated samples were soaked in 99.9% ethanol for 1 h followed by annealing at 200-600℃ in air for 3h, respectively. Fig.1 presents the SEM images of self-organized porous anodic films formed on the surface of SS foils by constant potential anodization of 50 V for 1 h, respectively annealed at 200-600℃ in air. The self-organized pores exhibited a quasi-honeycomb structure with pore diameters ranging from 45 to 60 nm as shown in Fig. 1a. As shown in Fig. 1b, the thickness of 8.12 μm anodic layer was formed at 50 V for 1h and some cracks were further clearly observed across the anodic films by the residual stress produced during film growing process and further amplified during subsequent annealing process. Remarkably, the decrease of the pore size from the top to the bottom of the porous anodic film was clearly observed by SEM pictures shown in Fig. 1c, the pore size varies from 53±10nm (top) to 33±7nm (bottom). An anodic etching model will be applied to explain the phenonomenon. XRD spectra further reveals the annealed anodic films own crystalline structures, which are predominantly composed of three phases of hematite (Fe2O3), eskolaite (Cr2O3) and spinel (Fe3O4 and FeCr2O4), which was clearly increased with the increasing annealing temperature. Fig. 2 (a) shows the C-V curves of the anodic layers formed at different annealing temperature. When tested as a binder-free electrode, all C-V curves exhibit a nearly symmetric CV plots, indicating good reversibility of faradaic redox reactions. Clearly, the anodic layer annealed at 400℃ exhibits the maximum specific area capacitance among the five samples, which can be explained by the increased amount of Fe2O3 and Fe3O4. Fig. 2 (b) shows the C-V curves of the anodic layers (400℃) recorded at the scan rates from 5 to 100 mV s-1. The enclosed area under the CV curve increases with the scan rate increases from 5 to 100 mV s-1. Similar results were observed in GCD plots as shown in Fig. 2 (c) and (d). The anodic layers exhibit the specific area capacitance ~ 54.5, 145.3, 80.6 and 18.3mF cm-2 at 1 mA cm-2, resepectively for the annealing temperature from 300-600℃. The optimum one exhibited the higher area capacitance ~ 145.3 mF cm-2 at 1 mA cm-2 and ~ 76 mF cm-2 at 4 mA cm-2, and the capacitance still retained 53.3% of its initial value after 1000 cycles. The Nyquist plots are presented in Fig. 2e to assess the resistance and capacitance of the formed anodic films. The lower resistance of anodic film annealed at 600℃ was attributed to the increased amount of high conductive Fe3O4. In summary, nanostructured anodic films have been fabricated on 304 stainless steel via constant-potential anodization instead of galvanostatic anodization. The supercapacitance behavior of anodic film is strongly dependent on the annealing temperature. When tested as a binder-free electrode, the optimum anodic layers (400℃) exhibit the highest area capacitance ~ 145.3 mF cm-2 at 1 mA cm-2, and the capacitance still retained 53.3% of its initial value after 1000 cycles.

Anodic Fabrication of Ti-Ni-Si-O Nanostructures on Ti10Ni5Si Alloy.

Ti-Ni-Si-O nanostructures were synthesized on Ti10Ni5Si alloy through an electrochemical anodization in electrolyte solutions containing ammonium fluoride (NH4F). The anodic oxide structures were affected by the electrochemical anodization parameters, including the electrolyte viscosity, water content, anodization potential and anodization time. Using an anodization potential of 40 V for 90 min in an ethylene glycol/glycerol electrolyte with 3 vol.% deionized water, highly ordered self-organized nanotube arrays were obtained in the α-Ti phase region of the alloy substrate, with an average inner diameter of 70 nm and a wall thickness of about 12 nm. Self-organized nanopore structures with an average pore diameter of 25 nm grew in the Ti5Si3 phase region. Only etching pits were found in the Ti2Ni phase region. The Ti-Ni-Si-O nanostructures were characterized using scanning electron microscopy and energy dispersive spectroscopy. In addition, a formation mechanism of different nanostructures was presented.