A new route for the formation of self-organized anodic porous alumina in neutral electrolytes

The present work reports the formation of regular porous aluminum oxide layers in neutral fluoride containing (NH₄)₂SO₄ electrolytes. For a fluoride free (NH₄)₂SO₄ electrolyte only irregular and thin porous alumina layers can be grown under these conditions. Upon addition of small amounts of fluorides highly regular, smooth high aspect ratio pore arrays can be produced. Pores with a typical diameter of approximately 50 nm and a length of several micrometers are formed. This finding significantly widens the spectrum of synthesis routes for ordered porous alumina structures and templates.     

Formation of hexagonally ordered nanoporous anodic zirconia

The present work demonstrates that highly ordered porous anodic zirconia (PAZ) arrays with cell diameters ranging from 70 to 120 nm can be grown in fluoride containing glycerol electrolytes. We show that this morphology (in contrast to the typically observed nanotubular layers) can simply be obtained by controlling the water content in the electrolyte during the anodization process. It is proposed that the morphology transition from pores to tubes is based on the rate of preferential etching at the hexagonal cell triple points in the oxide. This finding allows producing void free highly defined nanoporous layers for various applications.     

Fast fabrication of Ta2O5 nanotube arrays and their conversion to Ta3N5 for efficient solar driven water splitting

This work shows that highly ordered and mechanically stable micrometer-long Ta₂O₅ nanotube arrays can be fabricated by galvanostatic anodization in a few seconds. Typically, ~ 7.7 μm long nanotubes can be grown at 1.2 A cm^− 2 in only 2 s. Such nanotubes can be converted to Ta₃N₅ nanotube arrays by nitridation. Photoelectrochemical (PEC) water splitting using AM 1.5G illumination yields for the Ta₃N₅ nanotube photoanode modified with cobalt phosphate (Co–Pi) remarkable photocurrents of 5.9 mA cm ⁻² at 1.23 V_RHE and 12.9 mA cm ⁻² at 1.59 V_RHE and after Ba-doping a value of 7.5 mA cm ⁻² at 1.23 V_RHE is obtained.     

Self-organized porous and tubular oxide layers on TiAl alloys

The present paper describes the electrochemical fabrication of nanostructured oxide films on a TiAl intermetallic compound. The alloy is investigated under conditions where the individual alloying elements show the growth of ordered oxide structures, i.e. anodization is carried out in fluoride containing and fluoride free H₂SO₄ electrolytes. In 1 M H₂SO₄ the alloy shows randomly ordered nanoporous oxide structures while in HF-containing electrolytes highly ordered films can be formed. The key factor that affects the morphology is the anodizing potential. At low potentials (∼10 V) self-organized nanopores are formed whereas at higher potentials (∼40 V) separation of the pore walls and therefore formation of nanotubes can be observed. The results clearly indicate that on TiAl a wide range of nanoscale morphologies can be achieved ranging from random porous to organized pores to organized tubes.     

Formation of porous TiOx biomaterials in H3PO4 electrolytes

In this work, formation of porous TiO_x layers and theirs corrosion behavior were studied. Application of H₃PO₄ electrolytes results in porous TiO_x formation. The process is enhanced by small amount of HF content in the electrolyte. The HF results in higher current density, enhancing dissolution. Small 0.5% HF concentration results in nanopores formation, with pore diameter of about 45 nm. Increase of HF concentration up to 10% results in pores with average diameter of about 5.2 μm. An increase of etching time results in larger pore diameter, but between large 2–5 μm diameter pores smallest ones were observed with diameter below 200 nm. In the initial etching process a remnants of the flat surface are presents with initial cracks in the surface, indicating places for growth of the pores.The TiO_x layers can be used as a biomaterial. The corrosion behavior of the layer investigated in Ringer’s solution, revealed an excellent corrosion resistance, with respect to pure Ti.     

Formation of mesoporous germanium double layers by electrochemical etching for layer transfer processes

We produce uniform mesoporous single- and multilayers on 4 in. p-type Ge wafers by means of electrochemical etching in highly concentrated HF-based electrolytes. Pore formation by anodic etching in germanium leads to a constant dissolution of the already formed porous layer plus substrate. Alternating the etching bias from anodic to cathodic bias enhances the passivation of the pore walls and substrate. The formation of porous multilayers is possible, since the starting layer is not dissolved during the formation of the separation layer. We report on the production of mesoporous double layers in Ge with different porosities. The change in the porosity of the porous layers is achieved by varying the anodic etching current and the HF concentration of the electrolyte. Porosities in the range of 25–65% are obtained for etching current densities of 1–15 mA cm^?2 with the specific resistivity of the Ge substrates lying in the (0.020–0.032) Ω cm range and electrolyte HF concentrations in the range of 35–50 wt.%.     

Titanium oxide nanotubes prepared in phosphate electrolytes

In the present communication, we report the electrochemical formation of self-organized titanium oxide nanotubes (π-TiO₂) prepared in fluoride ion containing phosphate electrolytes. The morphology of the π-TiO₂ layers (particularly the pore diameter and length) is affected by the electrochemical conditions used (applied potential, electrolyte composition, pH, and anodizing time). Under specific sets of conditions highly self-organized titanium oxide nanotubes are formed with diameters varying from approx. 40 nm to 100 nm and length from approx. 100 nm to 4 μm. XPS investigations show that the nanotubes formed in phosphate solutions contain a significant amount of phosphorous species.     

Tungsten doping of Ta3N5-nanotubes for band gap narrowing and enhanced photoelectrochemical water splitting efficiency

Ordered W-doped Ta₂O₅ nanotube arrays were grown by self-organizing electrochemical anodization of TaW alloys with different tungsten concentrations and by a suitable high temperature ammonia treatment, fully converted to W:Ta₃N₅ tubular structures. A main effect found is that W doping can decrease the band gap from 2 eV (bare Ta₃N₅) down to 1.75 eV. Ta₃N₅ nanotubes grown on 0.5 at.% W alloy and modified with Co(OH)_x as co-catalyst show ~ 33% higher photocurrents in photoelectrochemical (PEC) water splitting than pure Ta₃N₅.     

Preparation of porous polymer electrolyte by a microwave assisted effervescent disintegrable reaction

Porous polymer membranes with sub-micrometer pores were successfully prepared by a novel microwave assisted effervescent disintegrable reaction. The fine connected porous structure was obtained by promoting effervescent disintegrable reaction between citric acid and sodium bicarbonate due to the assistance of microwave. The ionic conductivity of the prepared gelled polymer electrolyte is up to 1.17 × 10^?3 S cm^?1 and electrochemical window 4.5 V. This method provides a convenient route to prepare porous polymer electrolyte, which will greatly promote the practical application of porous polymer electrolytes.     

Electrochemical characterisation of MBaCo3ZnO7+δ (M = Y,Er, Tb) as SOFC cathode material with low thermal expansion coefficient

The electrochemical oxygen activation at high temperature was studied on a new class of oxygen-store material based on the system YBaCo₄O_7+δ. Three different porous layers made of YBaCo₃ZnO_7+δ, ErBaCo₃ZnO_7+δ and TbBaCo₃ZnO_7+δ were electrochemically tested as oxygen activation coatings and showed a very promising activity. The envisaged applications for these materials are principally as SOFC cathodes and as catalytic layer on oxygen membranes. The electrochemical performance followed the order Tb ? Y > Er at any tested temperature. Area specific resistance for the best performing material (TbBaCo₃ZnO_7+δ) ranges from 30 mΩ cm² at 850 °C to 0.46 Ω cm² at 650 °C. High temperature XRD showed that the thermal expansion coefficient (25–900 °C) in air of TbBaCo₃ZnO₇ is 9.45 × 10^?6 K^?1, which evidences the good thermochemical compatibility of this cobalt-rich electrocatalyst with YSZ/GDC electrolytes.     

Electrochemical formation of self-organized zirconium titanate nanotube multilayers

The present work reports the formation of multilayers of self-organized zirconium titanate nanotubes by anodizing a Ti–35Zr alloy in 1 M (NH₄)₂SO₄ + 0.5 wt% NH₄F electrolytes. It was found that multilayers consisting of different diameter nanotubes can be produced by repeated anodization steps under different conditions. Formation of new nanotubes starts in the gaps between the existing tubes. The process allows the formation of multilayer stacks consisting of layers of several 100 nm in length and adjustable nanotube diameters in a range from 50 to 180 nm.     

Fabrication of irregular-layer-free and diameter-tunable Ni–Ti–O nanopores by anodization of NiTi alloy

Ni–Ti–O nanopores (NPs) free of irregular surface layers and with tunable diameters are prepared by anodizing NiTi alloy in glycerol containing H₂O and NaCl. In an electrolyte composed of glycerol, 10 vol% H₂O and 0.6 M NaCl, NPs with diameters between 23 and 39 nm can be produced at anodization voltages between 20 and 80 V. In this electrolyte system, the irregular oxide layer on the surface can be completely removed chemically and/or mechanically during anodization. The resulting Ni–Ti–O NPs with tunable diameters should prove useful, for example, in energy, environmental and biomedical applications.     

A simple cathodic process for carboxylating noble metals and generating new versatile electrode interfaces

The electrochemical reactivity of polarized metals such as platinum, palladium, and rhodium toward carbon dioxide in aprotic dimethylformamide (DMF) solutions of tetramethylammonium tetrafluoroborate (TMABF₄) is presented. The capacity of metals such as Pd and Pt to cathodically insert the electrolytes under superdry conditions (via the generation of organometallic intermediates analogous to Zintl metals) is combined with the concomitant carboxylation of those metals within a potential range from − 1 V to − 2.5 V vs. Ag/AgCl/KCl(sat). Under these conditions, dense surface carboxylation of these precious metals occurs, totally suppressing their catalytic activity. Thick layers of the carboxylated metals (platinum-CO₂^– and palladium-CO₂⁻) are chemically stable and may then be further functionalized for specific applications.     

Characterization of thin InP anodic oxide layers: Correlation of morphological investigations with chemical and electrical properties

Depending on the applied electrochemical parameters, various oxide films can be grown onto InP in aqueous media. In this work, two oxide layers have been grown in borate buffer solution at pH = 9 by applying a low (0.2 mA cm⁻²) or a high (30 mA cm⁻²) current density, but a similar coulometric charge. Capacitance–voltage measurements performed before and after the anodic processes have been made to investigate the electrical properties of new interfaces, while X-ray photoelectron spectroscopy (XPS) analysis and atomic force microscopy (AFM) observations were used to access to the chemical and topographic aspects of the two oxidized surfaces. It is demonstrated that AFM observations coupled with electrochemical and XPS measurements is a good probe for the study of thin oxide on InP. A correlation between the anodization parameters and the resulting electrical and morphological aspects of the anodic layers is clearly evidenced.     

Achieving high electrode specific capacitance with materials of low mass specific capacitance: Potentiostatically grown thick micro-nanoporous PEDOT films

Electrode materials for supercapacitors are at present commonly evaluated and selected by their mass specific capacitance (C_M, F g⁻¹). However, using only this parameter may be a misleading practice because the electrode capacitance also depends on kinetics, and may not increase simply by increasing material mass. It is therefore important to complement C_M by the practically accessible electrode specific capacitance (C_E, F cm⁻²) in material selection. Poly[3,4-ethylene-dioxythiophene] (PEDOT) has a mass specific capacitance lower than other common conducting polymers, e.g. polyaniline. However, as demonstrated in this communication, this polymer can be potentiostatically grown to very thick films (up to 0.5 mm) that were porous at both micro- and nanometer scales. Measured by both cyclic voltammetry and electrochemical impedance spectrometry, these thick PEDOT films exhibited electrode specific capacitance (C_E, F cm⁻²) increasing linearly with the film deposition charge, approaching 5 F cm⁻², which is currently the highest amongst all reported materials.     

Sulfone-based electrolytes for high-voltage Li-ion batteries

Sulfone-based electrolytes have been investigated as electrolytes for lithium-ion cells using high-voltage positive electrodes, such as LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ spinels, and Li₄Ti₅O₁₂ spinel as negative electrode. In the presence of imide salt (LiTFSI) and ethyl methyl sulfone or tetramethyl sulfone (TMS) electrolytes, the Li₄Ti₅O₁₂/LiMn₂O₄ cell exhibited a specific capacity of 80 mAh g^?1 with an excellent capacity retention after 100 cycles. In a cell with high-voltage LiNi_0.5Mn_1.5O₄ positive electrode and 1 M LiPF₆ in TMS as electrolyte, the capacity reached 110 mAh g^?1 at the C/12 rate. When TMS was blended with ethyl methyl carbonate, the Li₄Ti₅O₁₂/LiNi_0.5Mn_1.5O₄ cell delivered an initial capacity of 80 mAh g^?1 and cycled fairly well for 1000 cycles under 2C rate. The exceptional electrochemical stability of the sulfone electrolytes and their compatibility with the Li₄Ti₅O₁₂ safer and stable anode were the main reason behind the outstanding electrochemical performance observed with high-potential spinel cathode materials. These electrolytes could be promising alternative electrolytes for high-energy density battery applications such as plug-in hybrid and electric vehicles that require a long cycle life.     

Titania nanotube formation in chloride and bromide containing electrolytes

Titania nanotubes and nanofibers were grown by anodization of titanium in fluoride-free electrolytes composed of NaCl and KBr dissolved in either water, ethylene glycol (EG), glycerol, or 50/50 mixtures of water and EG, and water and glycerol. The tubes and fibers grew out of pits in the titanium foil. The 15 nm diameter of the nanotubes was significantly smaller, and the growth rates were much faster than those of tubes developed in fluoride solutions. Nanotubes were nucleated in all electrolytes investigated, but the tubes’ lengths were limited to a few nms in EG and glycerol. Nanofibers produced in the aqueous solutions and nanotubes formed in the 50/50 aqueous mixtures grew to many tens of microns in less than 60 s.     

Electrochemical reactions of reversible intercalation in Chevrel compounds for cationic transfer – Principle and application on Co2+ ion

The process is an application of the reversible electrochemical reactions of insertion and deinsertion in the mineral host matrix M_xMo₆S₈ Chevrel phases to carry out the transfer of cations between two electrolytes. The device and the associated process are built around an electrochemical transfer junction (ETJ) placed between two tanks. The whole allows a transfer of cobalt cations (∼5 × 10⁻⁶ mol/h/cm² for a junction thickness of 4 mm) by application of a current density controlled between electrodes placed in both tanks.     

Study on electrochemical performance of activated carbon in aqueous Li2SO4, Na2SO4 and K2SO4 electrolytes

The electrochemical performances of activated carbon (AC) in 0.5 mol/l Li₂SO₄, Na₂SO₄ and K₂SO₄ aqueous electrolytes were investigated. The cyclic voltammetric results at different scan rates show that the rate behaviors of AC in the three electrolytes improve in the order of Li₂SO₄ < Na₂SO₄ < K₂SO₄. This improvement can be mainly ascribed to the following two reasons: (1) the decreasing equivalent series resistance in the order of Li₂SO₄ > Na₂SO₄ > K₂SO₄, which is the main factor influencing the maximum output power, and (2) the increasing migration speed of hydrated ions in the bulk electrolyte and in the inner pores of AC electrode in the order of Li⁺ < Na⁺ < K⁺. Their cycling behaviors do not show any differences in capacitive fading. The above results provide valuable information to explore new hybrid supercapacitors.     

Assessing Si-based anodes for Ca-ion batteries: Electrochemical decalciation of CaSi2

Density functional theory (DFT) calculations are used to investigate the basic electrochemical characteristics of Si-based anodes in calcium ion batteries (CIBs). The calculated average voltage of Ca alloying with fcc-Si to form the intermetallic Ca_xSi phases (0.5 < x ≤ 2) is of 0.4 V, with a volume variation of 306%. Decalciation of the lower Ca content phase, CaSi₂, is predicted at an average voltage between 0.57 V (formation of Si-fcc, 65% volume variation) and 1.2 V (formation of metastable deinserted-Si phase, 29% volume variation). Experiments carried out in conventional alkyl carbonate electrolytes show evidence that electrochemical “decalciation” of CaSi₂ is possible at moderate temperatures. The decalciation of CaSi₂ is confirmed by different characterization techniques.