Interface physicochemical processes controlling sulphate anion incorporation in porous anodic alumina and their dependence on the thermodynamic and transport properties of cations

Aluminium was anodised in H₂SO₄, LiHSO₄, NaHSO₄, KHSO₄, Mg(HSO₄)₂ and Al(HSO₄)₃ electrolytes. The kinetics of growth of porous anodic alumina films and of the pore wall oxide dissolution during anodisation was studied. Based on the derived kinetic parameters, suitable physicochemical processes in the barrier layer electrolyte interface controlling the anion incorporation in the barrier layer were suggested and relevant models were formulated. According to these processes Al³⁺ and H⁺ ions are rejected from the pore base surface in the attached double layer, where Al³⁺ ions are solvated, and are transferred to the pore filling solution. The strongly different mobilities of Al³⁺ and H⁺ and the necessary space negative charge density distribution in the double layer result in similar concentration distributions of Al³⁺ and anions inside it, which differ strongly from that of H⁺. These Al³⁺ and anion concentrations increase with decreasing mobility of the main cations in the solution which depends on their hydration enthalpy and transport mechanism. The concentration of incorporated anions inside both a thin surface layer of the barrier layer and the double layer vary similarly. For identical surface density and base diameter of pores the decrease of the above mobility reinforces anion incorporation.     

Solid surface and field catalysed interface formation of colloidal Al<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>3</Subscript> during Al anodizing affecting the kinetics and mechanism of development and structure of porous oxides

Overall kinetic and potentiometric studies of the growth of porous anodic alumina films in saturated H₂SO₄+Al₂(SO₄)₃ electrolyte showed non-saturation conditions inside the pores and supersaturation conditions at the pore surface/electrolyte interface where the field and the solid surface catalyse the formation of colloidal Al₂(SO₄)₃ micelles. Suitable high-strength field thermodynamically sustained electrochemical and chemical kinetic equations were formulated. It was shown that the diameter and surface fraction of charge exchange at the pore bases, the real pore wall surface fraction where oxide dissolution occurs, and its rate are strongly affected by the conditions. The mechanism of growth and structure of the films are quite different from those in H₂SO₄. A mechanism of regular film growth is imposed and the critical current density, above which pitting appears, strongly increases. The formulated theory may predict improved or new Al anodizing technologies. Electronic Publication     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

XPS spectroscopic study of potentiostatic and galvanostatic oxidation of Pt electrodes in H2SO4 and HClO4

The surface oxides produced from potentiostatic and galvanostatic oxidation of Pt electrodes in HClO₄ and H₂SO₄ are examined using X-ray photoelectron spectroscopy. The oxide I species produced as the initial oxidation product by successively more anodic potentiostatic oxidation in 0.2 M HClO₄ is found to have a Pt²⁺ oxidation state, a binding energy characteristic of neither PtO, Pt(OH)₂ or PtO₂, and a limiting thickness of 8 Å. Galvanostatic oxidation in HClO₄ and H₂SO₄ is found to produce PtO₂·H₂O as an unlimiting growth oxide or a limiting growth oxide layer depending on the concentration of the acid electrolyte. The incorporation of the acid electrolyte anion in the surface layer is shown to have an effect on which type of oxide layer is produced. X-ray decomposition and chemical modification by Ar⁺ stripping are shown to produce chemical artifacts complicating any interpretation of a Pt oxide surface layer.     

Thermodynamic representation of the NaCl + Na2SO4 + H2O system with electrolyte NRTL model

A comprehensive thermodynamic model based on the electrolyte NRTL (eNRTL) activity coefficient equation is developed for the NaCl + H₂O binary, the Na₂SO₄ + H₂O binary and the NaCl + Na₂SO₄ + H₂O ternary. The NRTL binary parameters for pairs H₂O-(Na⁺, Cl⁻) and H₂O-(Na⁺, SO₄²⁻), and the aqueous phase infinite dilution heat capacity parameters for ions Cl⁻ and SO₄²⁻ are regressed from fitting experimental data on mean ionic activity coefficient, heat capacity, liquid enthalpy and dissolution enthalpy for the NaCl + H₂O binary and the Na₂SO₄ + H₂O binary with electrolyte concentrations up to saturation and temperature up to 473.15 K. The Gibbs energy of formation, enthalpy of formation and heat capacity parameters for solids NaCl_(s), NaCl·2H₂O_(s), Na₂SO_4(s) and Na₂SO₄·10H₂O_(s) are obtained by fitting experimental data on solubilities of NaCl and Na₂SO₄ in water. The NRTL binary parameters for the (Na⁺, Cl⁻)-(Na⁺, SO₄²⁻) pair are regressed from fitting experimental data on dissolution enthalpies and solubilities for the NaCl + Na₂SO₄ + H₂O ternary.     

The elementary physicochemical processes during the first stage of formation of the negative lead-acid battery plate

During the first formation stage of the negative plate in H₂SO₄ solution lead monoxide and tribasic sulphate transform into Pb and PbSO₄ giving rise to a Pb?PbSO₄ zone. It was established that these processes take place within a thin layer (the δ-layer) situated between the Pb?PbSO₄ zone and the paste. The rate determining step is the transport (by diffusion and migration) of H⁺ and SO₄^2? ions from the bulk of the electrolyte towards the δ-layer. An equation is derived for the dependence of the concentration of the H₂SO₄ in the Pb?PbSO₄ zone on the thickness of this zone. It was found that during formation until a limiting thickness of the Pb?PbSO₄ zone, PbSO₄ originates mainly at the expense of the H₂SO₄ present in the bulk of the electrolyte. When the formation rate of PbSO₄ becomes very small the reaction of PbSO₄ reduction to Pb takes place. This reaction proceeds in a second reaction layer (the α-layer) which is located in the Pb?PbSO₄ zone at the plate surface. Subsequently, the formation of PbSO₄ in the δ-layer takes place at the expense of H₂SO₄ generated in the α-layer. The model is confirmed by the experimentally determined distribution of PbSO₄ across the cross-section of the plate during formation.     

Aluminium anodising in ultra-dense sulfate baths: discovery by overall kinetic and potentiometric studies of the critical role of interface colloidal Al2(SO4)3 nanoparticles in the mechanism of growth and nanostructure of porous oxide coatings

The solubility of Al₂(SO₄)₃ in H₂SO₄ at different concentrations was determined and showed a minimum at 95% w/v. Overall kinetic and potentiometric studies of Al anodising were performed in large ranges of concentrations of saturated H₂SO₄ solutions and current densities. During anodising quasi-steady-state supersaturation and unsaturation conditions for concentrations below and above 95% w/v dominate in the pore-filling solution affecting those in the oxide–electrolyte interface. Interface colloidal Al₂(SO₄)₃ nanoparticles form occupying surface fractions increasing with salt concentration, supersaturation, field strength in the pore base surface and current density increase and temperature decrease. These control the mechanism and kinetics of growth and structural parameters of films and impose the growth of non-pitted uniform films up to current densities higher than in unsaturated baths, more effectively under supersaturation conditions. Well-defined peaks of structural parameters appear depending on thickness and current. Thus optimal regularly grown films of desired nanostructure and the introduction of new anodising technologies can be achieved.     

Aluminium anodising in low acidity sulphate baths: growth mechanism and nanostructure of porous anodic films

Overall kinetic and chronopotentiometric studies were performed during Al anodising in H₂SO₄, 0–5% w/v, bath solutions pure and saturated by Al₂(SO₄)₃. Peculiarities in film growth mechanism and nanostructure in these cases appeared, like significant differences of porosity and its dependence on film thickness, different critical current density above which pitting appears, salt deposition on pitted surface regions in saturated bath, etc. The different conditions inside pores are responsible for this behaviour like almost depletion of H⁺ during a long initial transient stage in the first case, supersaturation and formation of Al₂(SO₄)₃ nanoparticle micelles on pore surface in the second case, etc. Differences in film growth mechanism also appeared between these and alike baths at higher acidity. Anodising in low acidity saturated baths shows superiority for growing low porosity films at specific conditions. New technologies may be suggested to produce optimal films of desired structure.     

Numerical Solution of the Problem of the Limiting Current for the Copper Electrodeposition from a Solution of Cupric Sulfate and Sulfuric Acid in Conditions of Natural Convection

Equations for a boundary layer, written in self-similar variables, are integrated numerically to obtain distributions of concentration of ions Cu²⁺, H⁺, and SO₄ ^2–; a hydrodynamic velocity; partial currents of electrolyte components; and the limiting current of copper deposition and determine the Rayleigh number for the system under consideration. The results are compared with an approximate analytical solution obtained earlier.     

SIFDT study of the SO+2/H2 H-atom abstraction reaction

The exothermic H-atom abstraction reaction of SO⁺₂ with H₂ has been studied in a selected ion flow drift tube (SIFDT) over a range of center-of-mass energies from thermal (300 K) to about 0.12 eV. The measured rate coefficient at 300 K is 4.2 × 10⁻¹² cm³ s⁻¹ which is very much less than the Langevin capture rate. The increase in rate coefficient with ion kinetic energy gives a linear Arrhenius-type plot with a slope that indicates a barrier of ∼5 kJ mol⁻¹ exists on the potential surface. The H₂SO⁺₂ potential surface is also explored in an ab initio investigation using the G2 procedure. An (SO⁺₂.H₂)¹ transition state between reactants and products is identified, corresponding to the barrier found from experiments.     

Proton affinity of SO3

Collision-induced dissociation (CID) of the radical cation H₂SO₄⁺ gives the product pairs H₂O⁺+SO₃ and HO+HSO₃⁺ with a 1:3 ratio that is essentially independent of collision energy. Statistical analysis of the two channels indicates that the proton affinity of HO is 3±4 kJ/mol lower than that of SO₃. This can be used to derive PA(SO₃)=591±4 kJ/mol at 0 K and 596±4 kJ/mol at 298 K. Previously, Munson and Smith bracketed the proton affinity as PA(HBr)=584 kJ/mol<PA(SO₃)<PA(CO)=594 kJ/mol. The threshold of 152±16 kJ/mol for formation of H₂O⁺+SO₃ indicates that the barrier to CID is small or nonexistent, in contrast to the substantial barriers to decomposition for H₃SO₄⁺ and H₂SO₄.     

Mitigating Swelling of the Solid Electrolyte Interphase using an Inorganic Anion Switch for Low-temperature Lithium-ion Batteries

In overcoming the Li⁺ desolvation barrier for low-temperature battery operation, a weakly-solvated electrolyte based on carboxylate solvent has shown promises. In case of an organic-anion-enriched primary solvation sheath (PSS), we found that the electrolyte tends to form a highly swollen, unstable solid electrolyte interphase (SEI) that shows a high permeability to the electrolyte components, accounting for quickly declined electrochemical performance of graphite-based anode. Here we proposed a facile strategy to tune the swelling property of SEI by introducing an inorganic anion switch into the PSS, via LiDFP co-solute method. By forming a low-swelling, Li₃PO₄-rich SEI, the electrolyte-consuming parasitic reactions and solvent co-intercalation at graphite-electrolyte interface are suppressed, which contributes to efficient Li⁺ transport, reversible Li⁺ (de)intercalation and stable structural evolution of graphite anode in high-energy Li-ion batteries at a low temperature of −20 °C.     

Fabrication of AAO films with controllable nanopore size by changing electrolytes and electrolytic parameters

In this paper, we fabricate two kinds of anodic aluminum oxide (AAO) films with controllable nanopore size by changing electrolytes and electrolytic parameters. The first AAO film with a four-layer structure was fabricated by sequential anodization of aluminum in aqueous solution of H₂SO₄, H₂C₂O₄, malonic acid, and tartaric acid at different anodic oxidation voltages. The average pore diameter of the as-prepared AAO film is 25 nm in the first layer, 54 nm in the second layer, 68 nm in the third layer, and 88 nm in the fourth layer, respectively. The pore densities of each layer decrease downwards to Al substrate, which are 300?×?10⁸, 100?×?10⁸, 21?×?10⁸, and 6.9?×?10⁸ cm^?2, respectively. Furthermore, another AAO film with periodically changed pore diameter was fabricated by alternating anodization of aluminum in aqueous solution of H₃PO₄ and tartaric acid under galvanostatic mode. The anodization processes present approximately identical best ordering voltage (195 V) in H₃PO₄ and tartaric acid under galvanostatic mode. The pore diameter with periodic change can be enlarged through a pore-widening treatment. Both AAO films with special nanopore structures can be used not only as templates for preparing nano-array materials whose pore diameter presents periodic change or gradual increase, but also as nanofilters to separate materials in some special media.     

Formulation of a holistic model for the kinetics of steady state growth of porous anodic alumina films

A holistic model for the kinetics of steady state growth of porous anodic alumina films in oxalic acid, H₂C₂O₄, solution was developed not necessarily requiring the adoption of any ‘a priori’ mechanism of porous film growth. By this model the effect of anodising conditions on the transport numbers of Al³⁺ cations and O²⁻ anions across the barrier layer was revealed. The cation (anion) transport number decreased (increased) with current density, increased (decreased) with temperature and was unaffected by the concentration of electrolyte or pH. A complementary atomistic-ionic kinetic model was developed that fully justified these results and showed that the activation distances of Al³⁺ and O²⁻ transport are comparable, but the activation energy of Al³⁺ transport is lower mainly due to the much smaller size of Al³⁺. The validity of the model was tested on the basis of SEM observations, while structural features and the rate of pore wall dissolution were determined.     

Electrocatalytic Determination of Sulfite at Immobilized Microdroplet Liquid|Liquid Interfaces: The EIC′ Mechanism

Liquid|liquid interfaces provide a natural boundary and a reactive interface where an organic phase is in contact with an aqueous analyte. The selectivity of ion transfer processes at liquid|liquid interfaces can help to provide sensitivity, introduce reactive reagents, or allow analyte accumulation at the electrode surface. In this study, microdroplet deposits of the organic liquid 4‐(3‐phenylpropyl)‐pyridine (PPP) with the ferrocenylmethyl‐dodecyldimethylammonium⁺ (FDA⁺) redox system are deposited onto a basal plane pyrolytic graphite electrode and employed to transfer anions from the aqueous into the organic phase. A clear trend of more hydrophobic anions transferring more readily (at more negative potentials) is observed and an ESI‐mass spectrometry method is developed to confirm the transfer. Subsequently, the electrocatalytic oxidation of sulfite, SO₃^2?, within the organic phase and in the presence of different electrolyte anions is investigated. Competition between sulfite transfer and inert anion transfer occurs. The electrocatalytic sulfite oxidation is suppressed in the presence of PF₆^? and occurs most readily in the presence of the hydrophilic nitrate anion. The resulting process can be classified as an electrocatalytic EIC′‐process (E: electron transfer; I: ion transfer; C: chemical reaction step). The effectiveness of the electrocatalytic process is limited by i) competition during anion transfer and ii) the liquid|liquid interface acting as a diffusion barrier. The analytical sensitivity of the method is limited to ca. 100 μM SO₃^2? (or ca. 8 ppm) and potential approaches for improvement of this limit are discussed.     

Substituent effects on the kinetics of the cerium(IV) oxidation of mandelic acids in sulfate media. Anomalous kinetic behavior of the methoxy derivative

The kinetics of the cerium(IV) oxidation of p-nitro and p-methoxymandelic acids have been investigated in H₂SO₄-MHSO₄ (M⁺ = Li⁺, Na⁺, K⁺) and H₂SO₄-MClO₄ (M⁺ = H⁺, Na⁺) mixtures at a constant total electrolyte concentration of 2.00 mol/dm³. The oxidation of p+nitromandelic acid proceeds through two [H⁺]-independent paths, as was also observed for some substituted mandelic acids studied previously. The kinetic behavior of the p-methoxy derivative differs from that of the other mandelic acids in that (1) the oxidation occurs via two [H⁺]-dependent paths, (2) the reaction rate is anomalously high, (3) the activation enthalpy and entropy of the overall process are markedly lower. It provides strong support to the suggestion that a different mechanism is operative. The substituent effects and the reaction mechanism are discussed.     

Effect of water and fluoride content on morphology and barrier layer properties of TiO2 nanotubes grown in ethylene glycol-based electrolytes

This paper studies the effect of H₂O and NH₄F content on morphology and barrier layer properties of TiO₂ nanotubes grown by potentiostatic anodization in ethylene glycol-based electrolytes. The increase in these two variables leads to an increase in the chemical attack of the formed oxide. However, each of these variables plays a different role in the formation of TiO₂ nanotubes. On the one hand, a higher percentage of H₂O in the electrolyte leads to a transition from a nanoporous to a nanotubular structure, as well as to a greater diameter of the tubes and a decrease in their length and barrier layer thickness. In contrast, a higher NH₄F concentration decreases nanotube diameter and increases their length modifying barrier layer properties due to insertion of F^? ions into the lattice. This diminishes the barrier layer resistance, but increases both the adsorption and the diffusion coefficient of F^? ions. The different roles of H₂O and NH₄F in film formation are also associated with the presence of sub-oxides detected by XPS.     

Mass transport in a boundary layer and in an ion exchange membrane: Mechanism of concentration polarization and water dissociation

In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the membrane surface through a boundary layer. In a boundary layer formed on an anion exchange membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation exchange membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion exchange membrane and by migration on the cation exchange membrane. Ion transport in a boundary layer on the cation exchange membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion exchange membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the membrane. The water dissociation reaction is generated in an ion exchange membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation exchange membrane comparing to the phenomenon in the strong base anion exchange membrane due to lower forward reaction rate constant in the cation exchange membrane. In the strong acid cation exchange membrane, the intensity of electric potential is larger than the values in the strong base anion exchange membrane. Accordingly, the stronger repulsive force is developed between ion exchange groups (SO ₃ ^? groups) and co-ions (OH^? ions) in the cation exchange membrane, and the water dissociation reaction is suppressed. In the strong base anion exchange membrane, the repulsive force between ion exchange groups (N⁺(CH₃)₃ groups) and co-ions (H⁺ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation exchange membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion exchange membrane.     

Peculiarities of copper underpotential deposition and nucleation on polycrystalline platinum in the presence of acetonitrile: Rotating ring-disk electrode

The initial stages of copper electrocrystallization on polycrystalline platinum in 0.5 M H₂SO₄ + 10 mM CuSO₄ + 0–200 mM acetonitrile (AcN) solutions are studied by the methods of cyclic voltammetry and potentiostatic current transients on a ring-disk electrode. Adsorbed AcN molecules accelerate both the underpotential deposition and the bulk deposition of copper due to the local electrostatic effects on the charged interface. With the increase in the additive concentration in solution, the contribution of the production of copper ions Cu⁺ is observed to increase due to the formation of Cu(AcN) _x ⁺ comlexes, particularly, for [AcN] ≥ 4 mM when the concentrations of acetonitrile and copper sulfate become comparable. In the presence of AcN, as well as in the copper sulfate supporting electrolyte, the adatomic layer is formed via the mechanism of the two-dimensional growth of Cu(1 × 1) phase islets.     

甲醇浓度对Pt/C催化剂氧还原活性的影响

采用半池考察了Pt/C催化剂在含不同浓度甲醇的0.5mol/L硫酸中的氧还原活性(ORR).研究发现,当甲醇浓度为0.1mol/L时,Pt/C催化剂的ORR活性最高,在催化层上热压商品NafionNRE-212膜后也出现同样趋势.线性扫描伏安曲线显示,压膜前后的Pt/C催化剂的ORR活性在含0.1mol/L甲醇的0.5mol/L硫酸中几乎没有变化.电化学阻抗谱结果表明,在该溶液中,Nafion膜的电阻比在其它电解液中低,这可能是导致Pt/C催化剂ORR活性提高的主要原因.有必要关注Nafion膜的这一异常性质并通过特殊设计后用于电池堆,以提高燃料电池性能.