Evolution of oxygen reduction current and biofilm on stainless steels cathodically polarised in natural aerated seawater

The aim of a series of works recently performed at ISMAR was to provide new useful information for a better understanding of the mechanisms by which bacteria settlement causes corrosion on Stainless Steels (SS) and similar active-passive alloys exposed to seawater. In this work, the evolutions of cathodic current, bacteria population, and electronic structure of the passive layer were investigated on SS samples polarised at fixed potentials during their exposure to natural seawater. It was found that, during the first phase of biofilm growth, cathodic current increase is proportional to the number of settled bacteria at each fixed potential. However, the proportionality factor between settled bacteria and cathodic current depends on imposed potential. In particular, the proportionality factor strongly decreases when the potential is increased above a critical value close to −150 mV Ag/AgCl. This effect seems to be correlated with the electronic structure of the passive layer. Indeed, the outer part of the passive layer on tested SS was found to behave like a conductor at potentials more active than −150 mV Ag/AgCl, and like an n-type semiconductor at more noble potentials.     

Electrodeposited amorphous iron(III) oxides as anodes for photoelectrolysis of water

Deposition of amorphous iron(III)-oxide films on a conducting glass substrate was achieved via a cathodic bias in a 0.1 M hydrated ammonium iron(II) sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O) solution at −1.6 V versus Ag/AgCl. Analysis by X-ray absorption near edge structure confirmed the iron(III) feature of the amorphous films. The deposited films exhibited n-type semiconducting characteristics by showing photoresponses under an anodic bias. The Mott–Schottky method and cyclic voltammetry were employed to characterize the semiconducting properties of the deposited films, which included the band gap (2.2 eV), the potentials of the conduction and valence band edges and flat band (−0.6, +1.6 and −0.58 V versus Ag/AgCl at pH 7, respectively), and the donor density (1 × 10²²/cm³). The deposited iron(III)-oxide films were suitable to serve as an anode for water splitting under illumination.     

Improving the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes by a surface-coating layer of GdBaCo2O5+δ

A GdBaCo₂O_5+δ layer was coated on the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes to enhance their oxygen permeability by employing the fast oxygen adsorption/desorption surface-exchange properties of the GdBaCo₂O_5+δ material. The oxygen flux of the coated and uncoated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes was measured in the temperature range of 600–850 °C. The results reveal that the oxygen-permeation flux of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes coated by a GdBaCo₂O_5+δ layer shows significant enhancement. The GdBaCo₂O_5+δ layer coated on the oxygen desorption side (He side) has much effect than that coated on the oxygen adsorption side (air side). At 850 °C, the oxygen flux with a single coating layer on the air side can rise 16%, while a single coating on the helium side will result into a rise of 23%.     

Coating electroaccretion on galvanized iron and aluminum in seawater

Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawater was investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential <−1.2 V versus Ag/AgCl, coating is composed of aragonite and brucite as calcareous deposits on pure iron, aragonite forming before the growth of brucite. Even when coupled to a magnesium anode, the zinc layer can corrode and large aggregates of brucite and aragonite form on the bare steel. Coatings are composed of zinc hydroxychloride Zn₅(OH)₈Cl₂·H₂O and aragonite without brucite if electroaccretion is performed in natural seawater at potential >−1.2 V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg₄Al₂(OH)₁₄·2H₂O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg₆Al₂(OH)₁₈·4H₂O islands.     

In situ infrared spectroscopic characterization of a bismuth–ethanol interface

The structure and composition of substances adsorbed to a Bi(0 0 1) electrode in ethanolic LiClO₄ solution were studied by cyclic voltammetry, electrochemical impedance and infrared reflectance spectroscopic methods. An analysis of the results demonstrates that at negative surface charge densities, there are no chemisorbed particles at the bismuth–ethanol solution interface. In solutions containing dissolved oxygen, an insoluble surface compound was detected at positive surface charge densities (E > −0.38 V vs. Ag|AgCl). In a behavior very different from that of the platinum–electrolyte interface, no ethanol oxidation products were detected on the Bi electrode. Absorption peaks measured in the infrared spectra are mainly caused by the variation of solvated perchlorate anion adsorption resulting from changes in surface charge density with the variation of the bismuth electrode's potential.     

Properties of Nb-doped SrCo0.8Fe0.2O3−d perovskites in oxidizing and reducing environments

The phase stability of SrCo_0.8Fe_0.2O_3−d perovskite doped with niobium was studied by in situ high-temperature X-ray diffraction in the temperature range of 30–1000 °C and oxygen partial pressure 0.2–10⁻⁵ atm. The stability of the cubic perovskite structure in a wide range of oxygen partial pressures is the main advantage of SrCo_0.8−xFe_0.2Nb_xO_3−d (x = 0.1–0.3) system in comparison with SrCo_0.8Fe_0.2O_3−d. It is suggested that equilibrium of the thermal expansion with changes of the oxygen non-stoichiometry leading to the same lattice parameters in the oxidizing and reducing environments at the catalytic temperatures is a necessary requirement for stable operation of perovskite as an oxygen-conducting membrane. In the case of SrCo_0.8−xFe_0.2Nb_xO_3−d perovskite this condition is met at x = 0.2. This makes the SrCo_0.6Fe_0.2Nb_0.2O_3−d composition promising for application as oxygen-conducting membrane.     

An in situ spectroelectrochemical Raman investigation of Au electrodeposition and electrodissolution in KAu(CN)2 solution

Spectroelectrochemical behaviour of CN^– on a Au electrode in a KAu(CN)₂ bath at pH 6.3 was studied by in situ confocal Raman spectroscopy. Internal (CN stretch) and external (Au–CN) CN^–-related frequencies were investigated under potentiostatic control in a potential interval spanning cathodic and anodic ranges (–1800 to +1200 mV vs Ag/AgCl). Electrochemical behaviour was assessed by cyclic voltammetry. Stark-shifted Au–NC^– species are the dominating ones under cathodic polarization. Above the hydrogen evolution potential a Au–H stretch band can also be observed. At open circuit Au–CN^– species tend to prevail, while anodic conditions relate to the enhanced formation of Au(CN)₂ ^– and OCN^–.     

An electrochemical study of cathodic protection of steel used for marine structures

Impressed current cathodic protection can result in hydrogen embrittlement, which can cause trouble with high-strength steels, particularly at welds. Therefore, the limiting potential for hydrogen embrittlement should be examined in detail as a function of the cathodic protection potential. This study investigated the effects of post-weld heat treatment (PWHT) on marine structural steels from an electrochemical viewpoint. In addition, the slow strain rate test (SSRT) was used to investigate both the electrochemical and mechanical effects of PWHT on impressed current cathodic protection. According to the SSRT, the optimum cathodic protection potential was −770 mV [with a saturated calomel electrode (SCE)]. SEM fractography analysis showed that the fracture morphology at an applied cathodic protection potential of −770–850 mV (SCE) was a dimpled pattern with ductile fractures, while a transgranular pattern was seen at potentials below −875 mV (SCE). Therefore, the cathodic protection potential range should be −770–850 mV (SCE).     

Preparation and electrochemical properties of functionalized ionic liquid self-assembled modified graphite electrode

A modified graphite electrode with functionalized ionic liquid (IL) pyridinium derivative of β-cyclodextrin ([CDbPy]BF₄) was prepared by layer-by-layer self-assembly technique. With ferrocene as probe, the characterization of the (CDIL/PDDA)_n/GE SAMs in the solution of phosphate (PBS, pH 7.0) was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronocoulometry. The electrochemical behavior of p-chloronitrobenzene (p-CNB) at the modified electrode was studied. It was found that the modified electrode could catalyze the reduction of p-CNB and made the cathode peak move about 100 mV in positive direction in the solution of 0.1 mol/L PBS (pH 7.0). Differential pulse voltammetry (DPV) was applied to the determination of p-CNB in waste water with satisfactory results. The detection limit and the linear range of the concentration of p-CNB to the reduction peak current were 8.0 × 10⁻⁸ mol/L and 3.0 × 10⁻⁷–1.0 × 10⁻⁵ mol/L, respectively.     

The reduction of dispersed indigo by cathodically formed 1,2,4-trihydroxynaphthalene

The indirect cathodic reduction of the vat dye indigo (C.I. Vat Blue 1) by cathodically reduced Lawsone (2-hydroxy-1,4-naphthoquinone; C.I. Natural Orange 6) was studied in aq. solution at different pH values. Cyclic voltammetry and spectroelectrochemistry were used to investigate the electrochemical behavior of 2-hydroxy-1,4-naphthoquinone at a hanging mercury drop electrode. The cathodic peak potential (E_p)_d measured at 0.1 mM lawsone solution at a scan rate of 50 mV s^?1 changed from ?425 mV at pH 7, to ?730 mV at pH 11.5 and ?750 mV at pH 13 (vs. Ag/AgCl, 3 M KCl). Particularly at pH values of 8–9 and 11.5–13 voltammograms indicated successful, indirect cathodic reduction of the dye in which the cathodically reduced 2-hydroxy-1,4-naphthoquinone acted as soluble mediator. The linear relationship obtained for (I_p)_d vs. v^1/2 is indicative of a diffusion-controlled electrode reaction mechanism. In the presence of dispersed indigo, the overall cathode reaction is similar to the E_cat process with continuous regeneration of the electroactive species. Spectrochemical experiments were used to prove the indirect cathodic reaction of dispersed vat dyes by 2-hydroxy-1,4-naphthoquinone.     

In situ STM studies of Ga electrodeposition from GaCl3 in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide

In the present paper the electrodeposition of Ga on Au(1 1 1) from 0.5 mol L⁻¹ GaCl₃ in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, [Py_1,4]TFSA, has been investigated by in situ scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and cyclic voltammetry (CV). The CV shows two redox processes: the one at −0.3 V vs. Pt is attributed to a Ga deposition on a Ga layer formed during an electroless deposition process at OCP and/or to the formation of a Au–Ga alloy; the other one at −0.9 V is due to the bulk deposition of Ga. The XPS measurement reveals that there is an oxide layer on the top of the gallium electrodeposit due to exposure to air. In situ STM measurements show that the first layer of the Ga deposit consists of islands of 10–30 nm in width and several nanometers in height surprisingly the result of an electroless deposition at the OCP. If the electrode potential is further reduced the bulk deposition of Ga sets in.     

Electrochemical characteristics and dyeing properties of selected 9,10-anthraquinones as mediators for the indirect cathodic reduction of dyes

Substituted 9,10-anthraquinones were characterised using cyclic voltammetry for their potential to serve as mediators for the indirect cathodic reduction of dispersed sulphur dyes, vat dyes and indigo. Besides characterisation of the electrochemical behaviour, the compounds were assessed in model dyeing experiments for chemical stability and staining of cotton fabric. 1,2-, 1,4- and 1,8-dihydroxy-9,10-anthraquinones exhibited negative mid-point potential ((E_p)_d + (E_p)_a)/2 of −713 to −820 mV (vs. Ag/AgCl, 3 M KCl). Under model dyebath conditions staining of cotton fabric occurred. 9,10-anthraquinone-1,5-disulfonate and 9,10-anthraquinone-2,6-disulfonate both imparted low staining to cotton; their mid-point potentials (−698 mV and −475 mV, respectively) limit their applicability to the reduction of sulphur dyes. Dihydroxy substituted 9,10-anthraquinone-sulfonates exhibited low staining owing to their good solubility. The mid-point potentials of 1,4-dihydroxy-9,10-anthraquinone-2-sulfonate and 5,8-dihydroxy-9,10-anthraquinone-2-sulfonate were only −618 mV and −675 mV respectively. Of the 9,10-anthraquinoids studied, 1,2-dihydroxy-9,10-anthraquinone-3-sulfonate (C.I. Mordant Red 3) could be identified as the most promising reversible redox couple, displaying a mid-point potential of −848 mV, low staining of cotton fabric and sufficient chemical stability under dyebath conditions.     

Kinetic and mechanistic evaluation of tetrahydroborate ion electro-oxidation at polycrystalline gold

The anodic oxidation of tetrahydroborate ion is studied in NaOH at stationary and rotating polycrystalline Au disk electrodes. Linear sweep and cyclic voltammetry are applied varying the scan and rotation rate from 0.005 to 51.200 V s⁻¹ and from 52.3 to 314.1 rad s⁻¹, correspondingly. The effects of variation of BH₄⁻ and NaOH concentrations as well as of the potential limits of the ranges studied have been initially followed. Most of the experiments have been carried out with 10.9 mM NaBH₄ in 1.04 M NaOH at 293 K in the potential range from −1.300 to 0.900 V (vs. Ag/AgCl). It is found that 6 electrons are exchanged in the overall oxidation transformation. The kinetic analysis of the processes determining the two anodic peaks recorded under static conditions at scan rates lower than 0.500 V s⁻¹ shows that 1.4 electrons are exchanged in the potential range of the first one (at ca −0.5 V), while the rate of the second one (at ca +0.3 V) is determined by a quasi-reversible 1-electron transfer reaction. A kinetic evidence for the participation of surface bound intermediates in the electro-oxidation process is provided. Two additional well outlined anodic peaks are recorded in the aforementioned potential range under specific experimental conditions. A quasi-8 electron mechanism involving four oxidation and hydrolysis steps is advanced to explain the experimental results. It accounts for the involvement of borohydride oxidation species and the Au⁺/Au³⁺ mediator couple.     

pH Dependent redox behaviour of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) - Cyclic voltammetry in presence of dispersed vat dye

The redox chemistry of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) was studied as function of pH by photometry, potentiometric titration and cyclic voltammetry. Dependent upon solution pH three species of the oxidised form of Alizarin Red S are present. Electrochemical reduction of the anthraquinone group leads to 1,2,9,10-tetrahydroxy-anthracene-3-sulfonate. The four phenolic hydroxyl groups dissociate with increasing pH value, above pH 12 a tetra anion is the main species in solution. Cyclic voltammetry experiments in unbuffered solution indicate that the overall cathodic reduction contains electron transfer and pH-dependent protonation reactions. Above pH 12 dispersed Vat Yellow 1 could be reduced by indirect cathodic electron transfer using the fully deprotonated system as reversible redox couple. In cyclic voltammetry a reduction potential of (E_p)_d = −900 mV (vs. Ag/AgCl/3 M KCl) was observed, which is sufficiently negative to reduce Vat Yellow 1. No indication for an indirect cathodic reduction of Vat Yellow 46 could be registered.     

Electrochemical properties of A-site deficient SOFC cathodes under Cr poisoning conditions

Under solid oxide fuel cells (SOFCs) operating atmosphere, volatile Cr species are generated over the chromia scale of chromia-forming alloy interconnects, poisoning the cathodes and causing the rapid degradation of the fuel cell performance. In this study, the influence of the cathode composition and A-site deficiency of Perovskite ABO₃ cathodes on their tolerance to Cr poisoning was investigated in detail through the electrochemical polarization and electrochemical impedance spectroscopy (EIS) analysis. Results showed that 5 mol% La-site deficient cathodes, La_0.75Sr_0.2BO_3−δ (B = Mn, Fe), presented a good performance in resistance to Cr poisoning and electrocatalytic activity to oxygen reduction reaction in the presence of alloy interconnects under a constant current density of 0.2 A cm⁻² in oxygen at 1073 K. B-site atom of Perovskite ABO₃ cathodes has a strong influence on the mechanism of Cr deposition and cathodes performance, while proper A-site deficiency can greatly improve the cathodes electrochemical performance and slow down Cr deposition.     

Layered NdBaCo2−xNixO5+δ perovskite oxides as cathodes for intermediate temperature solid oxide fuel cells

The effect of Ni substitution on the crystal chemistry, thermal and electrochemical properties, and catalytic activity for oxygen reduction reaction of the layered NdBaCo_2−xNi_xO_5+δ perovskite oxides has been investigated for 0 ≤ x ≤ 0.6. The oxygen content (5 + δ) and oxidation state of the (Co, Ni) ions in the air-synthesized NdBaCo_2−xNi_xO_5+δ samples decrease with increasing Ni content, accompanied by a structural transition from tetragonal (0 ≤ x ≤ 0.4) to orthorhombic (x = 0.6). Similarly, the thermal expansion coefficient (TEC) and electrical conductivity also decrease with increasing Ni content. The x = 0.2 and 0.4 samples exhibit slightly improved performance as cathodes in single cell solid oxide fuel cell (SOFC) compared to the x = 0 sample, which is in accordance with the ac-impedance data. Among the samples studied, the x = 0.4 sample exhibits a combination of low thermal expansion and high catalytic activity for the oxygen reduction reaction in SOFC.     

pH Dependent redox behaviour of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) – Cyclic voltammetry in presence of dispersed vat dye

The redox chemistry of Alizarin Red S (1,2-dihydroxy-9,10-anthraquinone-3-sulfonate) was studied as function of pH by photometry, potentiometric titration and cyclic voltammetry. Dependent upon solution pH three species of the oxidised form of Alizarin Red S are present. Electrochemical reduction of the anthraquinone group leads to 1,2,9,10-tetrahydroxy-anthracene-3-sulfonate. The four phenolic hydroxyl groups dissociate with increasing pH value, above pH 12 a tetra anion is the main species in solution. Cyclic voltammetry experiments in unbuffered solution indicate that the overall cathodic reduction contains electron transfer and pH-dependent protonation reactions. Above pH 12 dispersed Vat Yellow 1 could be reduced by indirect cathodic electron transfer using the fully deprotonated system as reversible redox couple. In cyclic voltammetry a reduction potential of (E_p)_d = −900 mV (vs. Ag/AgCl/3 M KCl) was observed, which is sufficiently negative to reduce Vat Yellow 1. No indication for an indirect cathodic reduction of Vat Yellow 46 could be registered.     

Effect of electrolyte addition on activity of (Ga1−xZnx)(N1−xOx) photocatalyst for overall water splitting under visible light

Effects of electrolyte addition on photocatalytic activity of (Ga_1−xZn_x)(N_1−xO_x) modified with either Rh_2−yCr_yO₃ or RuO₂ nanoparticles as cocatalysts for overall water splitting under visible light (λ > 400 nm) are investigated. The cocatalyst Rh_2−yCr_yO₃ is confirmed to selectively promote the photoreduction of H⁺, while RuO₂ functions as both H₂ evolution sites and as efficient O₂ evolution sites. The activity of Rh_2−yCr_yO₃-loaded (Ga_1−xZn_x)(N_1−xO_x) is found to be suppressed in the presence of Cl⁻, which undergoes oxidation by photogenerated holes in the valence band of (Ga_1−xZn_x)(N_1−xO_x). Alkaline- and alkaline earth-metal cations in the reactant solution compensate the negative effect of Cl⁻ to a certain extent depending on the metal cation employed. Among the electrolytes examined, the addition of an appropriate amount of NaCl or A₂SO₄ (A = Li, Na, or K) to the reactant solution without pH control is found to increase activity by up to 75% compared to the case without additives. Direct splitting of seawater to produce H₂ and O₂ is also demonstrated using Rh_2−yCr_yO₃-loaded (Ga_1−xZn_x)(N_1−xO_x) catalyst under visible light.     

Characterization of polymeric film of a new manganese phthalocyanine complex octa-substituted with 2-diethylaminoethanethiol, and its use for the electrochemical detection of bentazon

Manganese acetate octakis-(2-diethyaminoethanethiol) phthalocyanine (AcMnODEAETPc) was newly synthesized and characterized by spectroscopic and electrochemical methods. Solution electrochemistry of the complex showed three redox processes assigned to Mn^IIIPc⁻¹/Mn^IIIPc⁻², Mn^IIIPc⁻²/Mn^IIPc⁻² and Mn^IIPc⁻²/Mn^IIPc⁻³ species. The new molecule was polymerized onto a glassy carbon electrode (GCE) to form thin films of different thickness, giving poly-10-AcMnODEAETPc-GCE, poly-20-AcMnODEAETPc-GCE and poly-30-AcMnODEAETPc-GCE, where 10, 20 and 30 represent the number of voltammetry scans during polymerization. Three distinct redox processes were observed on the modified electrode in 0.1 M phosphate buffer solution, pH 5, which confirmed the formation of the polymer. The current signal due to the herbicide, bentazon, was dependent on film thickness; the best signal was obtained on poly-20-AcMnODEAETPc-GCE while poly-10-AcMnODEAETPc-GCE gave the least signal. However, the signals due to the herbicide were better on the different films compared to the bare electrode. Electrochemical impedance spectroscopy (EIS) technique revealed that differences in film thickness offered different charge transfer resistances, R_ct, hence difference in current signals for bentazon oxidation were observed on these films. A Tafel slope of 77 mV/decade, obtained for the herbicide on poly-20-AcMnODEAETPc-GCE, denotes a fast one electron transfer followed by a slow chemical step in the electro-oxidation of bentazon. The voltammetry signals of the herbicide on the films indicated the likely involvement of ring-based redox processes in the detection of the herbicide. A plot of background corrected current response, on this film, versus the concentration of bentazon was linear within the range 50–750 μM with a detection limit of 2.48 × 10⁻⁷ M.     

Electrochemical diamond sensors for TNT detection in water

An electrochemically stabilized boron doped diamond electrode prepared by chemical vapour deposition (CVD) is used for electrochemical TNT sensing in aqueous solutions. Square wave voltammograms (SWVs) exhibit three highly resolved peaks at −0.47, −0.62 and −0.76 V vs. Ag–AgCl reference electrode, respectively. The current vs. TNT concentration plot shows a linear relationship with a same slope for the two first TNT peaks at μg L⁻¹ and mg L⁻¹ concentration ranges. Detection and quantification limits of 10 and 25 μg L⁻¹, respectively, were obtained without any preconcentration step. Relative standard deviation (RSD) of less than 1% measured over 10 runs has been found for the −0.47 V peak current showing the very high stability of the electrode without any significant fouling effect. An interference study with nitro aromatic compounds of the same family (nitro toluene and dinitrotoluene) has shown that the −0.47 V reduction peak enables TNT discrimination. Measurement of TNT in a natural medium (sea water without any purification step except filtering) has been also investigated.