Electrochemical faceting of metal electrodes

Electrochemical faceting is a term recently coined to denote the preferred crystallographic orientation of grains in polycrystalline metals developed when they are subjected to periodic potential perturbations of determined characteristics. Electrochemical faceting can also be applied to small single crystal beads resulting from melting polycrystalline metal wires. By properly adjusting the conditions defining the periodic perturbation, the resulting electrode surface acquires different preferred orientations which, depending on the nature of the electrode metal, can be followed either electrochemically through conventional voltammetry in the H and O electroadsorption/electrodesorption potential range, and in upd of different metals, or by scanning electron microscopy. The procedure has already been successfully applied to platinum, gold, rhodium and palladium. Electrochemical faceting involves at least two stages, namely, the initiation stage related to an electroadsorption process and a propagation stage associated with the electrodissolution and electrodeposition of the base metal in the acid electrolyte. Stabilization procedures for the freshly oriented surfaces and roughness development are also considered.     

The influence of electrolyte purity on electrochemical measurements using platinum electrodes

The influence of electrolyte purity on electrochemical processes at low-area platinum electrodes has been studied by a fast potential-step method and by cyclic voltammetry. Pre-purification of the electrolyte had no significant influence on the metal surface area available for H adsorption determined immediately after an electrochemical cleaning step. Maintaining the electrode potential within the ‘double-layer’ region of platinum (at 0.5 V vs rhe) for relatively long periods of time (1–30 min) results in the progressive suppression of H adsorption, owing to the adsorption of impurities blocking active Pt sites. All samples of electrolyte, irrespective of the degree of pre-purification, exhibited this effect. In the determination of current-time transients during the first few hundred milliseconds of the methanol electro-oxidation reaction, impurities in the untreated electrolyte contribute to the measured anodic currents at Pt electrodes working in the potential range of 0.4–0.6 V. The anomalous effects observed within this potential range can be largely eliminated by pre-electrolysis of the H₂SO₄ electrolyte for several days using Pt wire gauze electrodes at 2.1 V.     

Electrochemical surface reshaping of polycrystalline platinum: Morphology and crystallography

A novel process of morphological evolution was observed on a polycrystalline platinum electrode surface during intensive potential perturbation in sulfuric acid electrolyte. The process was started from a smooth polycrystalline platinum surface, and ended up with a roughened overlayer which is composed of numerous morphological units. According to its appearance, the unit is named “feather-like morphology (FLM)” which contains finer structures at nanometer scale. The active surface area of platinum at different stages during the morphological evolution was estimated by cyclic voltammetry. Electron microscopic measurements provided both morphological details and crystallographic information.     

The external-pulse introduced pattern in a system with a closer reference electrode and a quasi-one-dimensional ribbon working electrode

In the present work the electrochemical pattern formation was investigated during electro-oxidation of formic acid on platinum electrode. The working electrode is quasi-one-dimensional ribbon polycrystalline platinum. The stationary domains (sDs) have been successfully captured after an external pulse was triggered. At a relatively high conductivity a new kind of electrochemical pattern was found and it is named as cyclopean pattern. In a nonlinear electrochemical system with an N-type negative differential resistance, the theoretical simulation of the spatial temporal structure is able to predict the existence of stationary domains. Increasing the electrolyte conductivity promotes the formation of stationary domains and cyclopean pattern. The difference of the potential amplitudes in two phases (domains) becomes smaller in an electrolyte with a higher conductivity.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. II. Identification of potential fluctuation sources for unstressed electrodes

In this second part of the study of electrochemical noise (EN) generated by hydrogen evolution on a vertical cylindrical AISI 4140 steel electrode under galvanostatic control in 0.5 M sulphuric acid, the potential fluctuations induced by the growth and detachment of hydrogen bubbles at the electrode surface were analysed. They could be related to fluctuations of various quantities: electrode active surface due to bubble screening effects, concentration of dissolved hydrogen in the electrolyte close to the electrode surface, and metal-hydrogen interactions (MHI) on or beneath the electrode surface. The existence of MHI and their influence on faradaic potential fluctuations could be revealed by comparing the noise features on steel and platinum. The influence on EN of the charging cathodic current density, the presence of dissolved oxygen in the solution, and the electrode rotation speed was investigated in the absence of stress applied to the electrode. In the third paper of this series, the effect of hydrogen embrittlement on potential fluctuations of stressed electrodes will be examined.     

Enhanced cyclability and surface characteristics of lithium batteries by Li-Mg co-deposition and addition of HF acid in electrolyte

The effect of Mg(ClO₄)₂ and HF acid in 1 M LiPF₆/EC + DEC + DME electrolyte on the cycle behaviors and the surface characteristics of the lithium electrode were investigated. The morphology of the electrode is greatly improved using Li-Mg co-deposition and HF acid, and results in the enhancement of the cyclability observed by electrochemical measurement. The scanning electron microscopy images of the electrode surface show densely deposited lithium-magnesium alloy particles with a hemispherical shape on the entire surface of the electrode. The improved coulombic efficiency was attributed to the synergistic effect due to the formation of Li-Mg alloy and LiF caused by Li-Mg co-deposition and HF to the electrolyte, respectively.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

Oxytetracycline nanosensor based on poly-ortho-aminophenol/multi-walled carbon nanotubes composite film

Poly-ortho-aminophenol (PoAP) and multi-walled carbon nanotubes (MWCNTs) were deposited on the platinum electrode using cyclic voltammetry technique to form the Pt/PoAP/MWCNTs nanosensor for the electrochemical determination of oxytetracycline as analyte. This electrochemical nanosensor with good uniformity and high surface area was prepared in the presence of an ionic surfactant (sodium dodecyl sulfate) as electrolyte to suspend carbon nanotubes within the PoAP and improve the stability and electroactivity of the composite film. The surface morphology of the prepared nanosensor was characterized by scanning electron microscopy and showed a three-dimensional network structure. The influence of several parameters such as number of potential cycles, scan rate and pH of the solution on the electrochemical response of the resultant electrode was investigated. The prepared electrode functioned as a selective recognition element for oxytetracycline determination. It showed excellent electrochemical response to oxytetracycline at low oxidative potential in buffer solution of pH 2.0, with good stability and sensitivity. Under the optimal experimental conditions, the electrochemical response of the sensor was linear with respect to the concentration of oxytetracycline in a dynamic range of 0.2 μM–0.25 mM. The detection limit of the fabricated nanosensor was calculated as 0.10 μM (signal/noise = 3). This sensor was used successfully for the oxytetracycline determination in real samples with recoveries of 96.9–103.5 %.     

Electrochemical study of benzene on Pt of various surface structures in alkaline and acidic solutions

The electrochemical behaviour of benzene on platinum electrodes (polycrystalline and single-crystal electrodes) has been studied in acidic and alkaline solutions. In acid solutions the reduction of benzene to cyclohexane takes place in all the platinum surface structure employed, however it does not occur in alkaline media (0.1 M NaOH). In this case, the hydrogen adsorption-desorption processes displace the adsorbed benzene from the electrode surface. The oxidation of benzene is also affected by the pH of the electrolyte and also by the surface structure of the platinum electrode used. In acid solutions, this oxidation at higher potentials (1.4 V vs. RHE) yields CO₂, benzoquinone and α,β-unsaturated esters or lactones, however in alkaline media carbonate anions coming from CO₂ and salts of carboxylic acids have been detected by in situ FTIR spectroscopy using a platinum polycrystalline electrode. Bulk electrolysis of benzene solutions using a platinum electrode in acid and alkaline media was performed in order to confirm the results obtained by spectroscopic measurements.     

电化学刻蚀制备宏孔硅研究

半导体硅在含HF电解液阳极氧化体系中可以形成不同的腐蚀形态。当采用背面照明,且反应电流密度小于某个临界值时,可以在N型硅抛光片沿(100)晶向刻蚀产生宏孔结构,控制电化学条件能够调整宏孔的孔径、间距等形貌参数。本文采用相同电阻率N型抛光(100)硅片,考查了恒流条件下不同工作电流、照明强度对刻蚀反应以及刻蚀表面形貌的影响。     

硝基苯在离子液体BPyBF4/H2O中的电化学行为

在离子液体1-丁基吡啶四氟硼酸盐(BPyBF₄)的水溶液中,采用循环伏安法(CV)、计时电流法(CA)、恒电位电解及原位红外光谱法(in-situ FTIRS)等研究了硝基苯在铂电极上的电化学还原行为。研究结果表明:硝基苯在离子液体BPyBF₄/H₂O体系中的还原过程是受扩散控制的,其还原产物主要是偶氮苯,经估算得到硝基苯在该体系中的扩散系数为3.1×10^-7 cm²·s^-1,提高温度和加入一定浓度的水均有利于硝基苯在该体系中的电化学还原。     

电解质对卵磷脂双分子层电化学行为的影响

实验利用自组装技术在铂电极上制备支撑的卵磷脂双层膜（s-BLM）作为生物膜的模型,利用循环伏安法（cV）和电化学阻抗谱（EIS）对由电解质KCI引起的s-BLM通透性的变化进行了研究。结果表明,KCI可与s-BLM发生较强的相互作用,导致s—BLM表面磷脂分子的有序排列受到影响,产生一些离子通道,增加了对探针分子电流响应,同时降低了s—BLM的电阻。     

The electrocatalytic oxidation of ascorbic acid on polyaniline film synthesized in the presence of β-naphthalenesulfonic acid

Polyaniline-β-naphthalenesulfonic acid composite film on platinum electrode surface has been synthesized via the electrochemical polymerization of aniline in the presence of β-naphthalenesulfonic acid (NSA). FT-IR, UV-vis and electrochemical characterization indicate the formation of the doped polyaniline. Further investigations found that the polyaniline (PAN) doped with NSA extended the electroactivity of PAN in neutral and even in alkaline media. The PAN-NSA composite film coated platinum electrodes are shown to be good electrocatalytic surfaces for the oxidation of ascorbic acid (AA) in phosphate buffer solution (PBS) of pH 7.0. The anode peak potential of AA shifts from 0.62 V at bare platinum electrode to 0.34 V at the PAN-NSA composite modified platinum electrode with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5-60 mM using cyclic voltammetry. The kinetics of the catalytic reaction is investigated using rotating disk electrode (RDE) voltammetry, cyclic voltammetry and chronoamperometry. The results are explained using the theory of electrocatalytic reactions at chemically modified electrodes. The PAN-NSA composite film on the electrode surface shows good reproducibility and stability.     

基于原位红外光谱的水相苯酚电氧化机理研究

采用电化学原位红外光谱技术,研究了苯酚在Pt电极表面的电化学氧化机理。在0.1 mol/L Na₂SO₄溶液中,Pt电极上电化学氧化苯酚的反应电位为+0.9~1.0 V（vs SCE）、析氧电位为+1.3 V;电化学原位红外光谱结果表明,当电位<0.9 V时,苯酚氧化产物主要为苯二酚、醌及少量醇类物质;电位0.9~1.1 V时,苯环结构被破坏,氧化产物主要为酮、酸、醇和CO₂;根据官能团吸收峰的变化,苯酚在Pt电极表面氧化经历如下途径：苯酚→苯二酚→苯醌→酮、醇、酸→CO₂。同时研究了NH₄⁺对苯酚在Pt电极表面的电氧化的影响,结果表明在低电位区（<0.9 V）对苯酚氧化构成竞争。     

Metal electrodes bonded on solid polymer electrolyte membranes (SPE)—The behaviour of platinum bonded on SPE for hydrogen and oxygen electrode processes

The electrochemical characteristics of a platinum electrode directly bonded to one side of a solid polymer electrolyte membrane (Pt-SPE) were studied. Current-potential relations of hydrogen and oxygen adsorbed on Pt-SPE were observed to be similar to those on a platinum metal electrode independently of the presence/absence of electrolytic solution at the platinum side of Pt-SPE. Hydrogen evolution, hydrogen ionisation, and oxygen reduction were examined at the condition that the side at the platinum of Pt-SPE was free from liquid but filled with the gases concerned     

Electrocatalytic oxidation of methanol and C1 molecules on highly dispersed electrodes Part II: Platinum-ruthenium in polyaniline

The oxidation of methanol and C1 molecules at electrodes modified with polyaniline and particles of platinum and ruthenium has been studied in aqueous HClO₄ electrolyte. The platinum and ruthenium particles were incorporated into the polyaniline film by electrochemical reduction. The activity for the oxidation of C1 molecules is higher for bimetallic electrodes than for polyaniline-coated electrodes modified with platinum alone. Indeed, a negative shift of more than 100 mV is observed as compared to the potential obtained with a polyaniline film modified by pure platinum. Moreover, the oxidation of methanol is faster and more complete on the Pt-Ru modified polyaniline electrode, since carbon dioxide is the main reaction product.     

Nitric oxide reduction at noble metal electrodes: a voltammetric study in acid solution

Features of the electrochemical reduction of nitric oxide on platinum, palladium, rhodium and ruthenium in aqueous perchloric acid solutions (0.33–1.0 M) are compared. The results from voltammetric studies (ie linear potential sweep and rotating disc electrode) using the bulk metal electrodes are described and compared with residual current voltage plots in acid electrolyte alone. In general, three nitric oxide reduction peaks are observed on the metals. The most anodic peak, at ca E = 0.15 V vs sce is attributed to the one-electron reduction of nitric oxide to an adsorbed NOH intermediate on a bare metal surface (ie one free of oxides or adsorbed hydrogen). The other two peaks occur in potential regions where adsorbed hydrogen is present on the metal surface (ca E = 0.0 and −0.20 V, respectively). The co-adsorbed hydrogen complicates the analysis and precludes an unambiguous interpretation of these two peaks. However, they apparently reflect nitric oxide reduction to nitrogen, hydroxylamine and/or ammonia. In a cathodic scan on the rhodium electrode, a current plateau is seen instead of the first (most anodic) peak, a probable consequence of oxide film formation with subsequent chemical complications. On the ruthenium electrode the first two (most anodic) peaks are not observed probably due to a relatively stable oxide layer. Reaction selectivities at metal black gas diffusion cathodes operating in an electrogenerative (ie galvanic) mode with perchloric acid electrolyte are compared with the voltammetric results at the corresponding bulk electrodes. Dinitrogen formation is observed on the platinum and rhodium black electrodes as suggested from voltammetric results. A series-parallel reaction sequence is proposed to explain the results. Limitations of using simple voltammetric techniques for predicting behavior of large scale preparative electrochemical reactors are discussed.     

Electrochemical synthesis and optimization of poly(4-methoxyphenol) film as a sensor material

This article describes that electrochemical polymerization of 4-methoxyphenol in the presence of enzyme glucose oxidase produces adherent polymeric films containing the active enzyme onto the surface of platinum electrodes. Polymeric electrodes prepared in this one-step procedure can be operated for the glucose determination. The effects of the electrochemical polymerization parameters (for example, concentrations of monomer, electrolyte, and enzyme; film thickness; and polymerization potential) on the electrode preparation and the effects of amperometric measurement parameters (for example, pH, temperature) on the amperometric responses to the glucose of the prepared electrodes were systematically investigated, and optimal values were determined. Furthermore, glucose specificity and storage stability of the enzyme electrode were investigated. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1941–1947, 1998     

Electrocatalyzed synthesis of polypeptides on platinum surface in concentrated glycine electrolytes and ab initio calculations coupled to spectroscopic analysis

The anodic oxidation of glycine in aqueous electrolyte on smooth platinum electrode was carried out by electrochemical quartz crystal microbalance coupled to cyclic voltammetry technique. Here, we performed electrochemical experiments in concentrated glycine-based electrolyte. Contrary to studies made in diluted medium, the reaction we describe leads to a strongly grafted polymer on the surface at pH values superior to 6, and up to 13, in an irreversible way. The electrodeposited mass is even very significant at pH 13. Several methods such as AFM topography and spectroscopic techniques were performed to characterize the resulting coating. The polymer growth in alkaline conditions during the anodic oxidation of glycine in water probably involves an electrocatalytic step. We showed the presence of amide bonds and then polypeptide formation on the smooth platinum surface. Periodic ab initio calculations on polyglycine II were performed and compared to XPS and vibrational spectra.     

Theoretical Study of the Acetonitrile Flip-Flop with the Electric Field Orientation: Adsorption on a Pt(111) Electrode Surface

The interaction of acetonitrile on Pt(111) electrode has been studied using density functional theory methods. Periodic calculations for a coverage of 0.25 show that the most stable adsorption mode of acetonitrile in UHV conditions on Pt(111) is the side-on η²(C,N) state. In this state, the short-bridge surface site is energetically preferred to the long-bridge site. The nitrogen end-on on top of a surface platinum atom is also likely to occur. An external uniform electric field introduced in the cluster models perpendicular to the surface serves to mimic the electrode potential. It makes the adsorbed molecule move and orient perpendicular to the surface. Depending on the direction of the electric field, the acetonitrile is oriented with the nitrogen atom or the methyl group toward the surface. Our results are in agreement with recent SFG experiments performed in electrochemical environments.