Application of the Scanning Kelvin Probe for the study of the corrosion resistance of interfacial thin organosilane films at adhesive/metal interfaces

The Scanning Kelvin Probe is introduced as a real time non-destructive in situ technique for the detection of de-adhesion at adhesive/metal oxide interfaces. Iron substrates and an epoxy adhesive served as model systems. Iron surfaces were coated with ultra-thin organosilane plasma polymer films from a microwave discharge and 3-(trimethoxysilyl)-propylamine films from dilute water based solutions. Surface and film characterisation was done by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS). The effect of these interfacial films on the stability of the adhesive/metal joint was studied in corrosive environments. The Scanning Kelvin Probe allows the measurement of electrode potentials at buried polymer/metal interfaces with a spatial resolution of about 100 m. The electrode potential characterises the reactivity of the interface. Moreover, by the variation of the oxygen partial pressure in the measurement chamber, local anodes and cathodes underneath the polymer can be distinguished. The kinetics of electrochemical de-adhesion can be effectively slowed down by thin 3-(trimethoxysilyl)-propylamine films at the interface. The effect of the adhesion promoter can be further improved when a thin SiO_x layer, which inhibits electron transfer reactions, and is deposited on the iron surface prior to coating with the adhesion promoter.This paper is dedicated to Mike Owen on occasion of his winning the DeBruyn medal, the first silicon chemist to do so.     

A glucose electrode for fermentation control

An oxygen-stabilized enzyme electrode was applied to monitor the glucose concentration in a fermentor during a batch culture ofCandida utilis. The electrode contains an electrolysis circuit for generation of oxygen within the enzyme layer that keeps the oxygen activity in that layer at the same level as that of the surrounding broth. The electrolysis current is used as a measure of the glucose concentration in the broth. The glucose analysis continued without major disturbances when the dissolved oxygen pressure gradually decreased during the fermentation and also when the broth was subjected to a sudden increase in dissolved oxygen tension. The electrode could also be used in an anaerobic broth. Then the reference electrode was replaced by a constant reference potential that simulated a reference oxygen activity.     

Electrocatalytic and Biosensing Properties of Aerobic Carbon Monoxide Dehydrogenase from Hydrogenophaga Pseudoflava Immobilized on Au Electrode towards Carbon Monoxide Oxidation

Herein, an enzyme‐electrode based on the oxygen‐insensitive carbon monoxide dehydrogenase (CODH) containing molybdenum (Mo) and copper (Cu), flavin adenine dinucleotide (FAD) and two different [2Fe‐2 S] clusters as cofactors from the aerobic bacterium Hydrogenophaga pseudoflava, is proposed as a platform for dissolved CO concentration monitoring. The immobilized CODHs on Au electrode retain their catalytic activity and demonstrates changes to cyclic voltammetry and amperometry signals upon interactions with various dissolved CO concentrations. Cyclic voltammetry shows that CODHs are capable of direct electron transfer without any mediator use as oxidative current which starts around ?0.268 V (vs Ag/AgCl) is observed in the presence of CO. When CO‐saturated standard solution was spiked sequentially into the gas‐tight reactor, amperometry analysis shows current increased accordingly with response time within 5 s. Our study demonstrates that this enzyme‐electrode is promising to serve as platform for developing an on‐line dissolved CO concentration monitoring tool which is essential to fill in the gap for conventional technologies which are limited to off‐line measurement.     

气体扩散层中聚四氟乙烯的分布对质子交换膜燃料电池水淹的影响

通过在不同真空度下进行碳纸的聚四氟乙烯（PTFE）浸渍处理,考察了PTFE在碳纸中的分布对燃料电池水淹情况的影响. 碳纸PTFE浸渍过程中,抽真空作用可以将碳纸微孔中存留的空气移除,使PTFE更均匀地扩散到内部微孔中. 碳纸的断面电镜照片显示真空浸渍可以改善PTFE的分布. 在总浸渍量相同时,由于真空浸渍使更多的PTFE进入到碳纸内部微孔,故其表面的PTFE比例减少. 实验进一步考察了碳纸中亲水孔和憎水孔的分布,结果表明真空浸渍处理的碳纸具有更高比例的憎水孔. 将不同处理方法的碳纸制备成膜电极,通过全尺寸电池考察其性能,结果表明PTFE的均匀分布可以改善电池性能,并且电化学阻抗分析表明其有利于改善水淹问题.     

Kinetics of the oxygen transfer reaction at the (U0.5Sc0.5)O2±x/YSZ interface by impedance spectroscopy

The complex impedance dispersion analysis technique was used to study the electrode kinetics of (U_0.5Sc_0.5)O_2±x, a fluorite type solid solution material potentially suitable as electrode for low temperature oxygen sensors. Variables included the temperature and oxygen partial pressure. The effect of heat treatment on the interfacial contact resistance and the electrode morphology was also investigated. A single are for the electrode reaction was observed over most of the experimental ranges of temperature and oxygen partial pressure. The angle of depression of the electrode are was small (8–18°) compared with platinum electrodes (20–45°). The activation energy for the overall electrode reaction was between 170 and 180 kJ mol^?1. The average value for the pressure exponent, determined from the oxygen partial pressure dependence of the electrode resistance, was 0.16. A mechanism for the oxygen transfer reaction is proposed. Materials of this type show promise for future use in low temperature oxygen sensors.     

Optimizing Weights of Platinum in the Active Layer of the Cathode of a Hydrogen–Oxygen Fuel Cell with a Solid Polymer Electrolyte

The active layer of the cathode of a hydrogen–oxygen fuel cell with a solid polymer electrolyte is computer simulated. The active mass of the electrode consists of substrate grains (agglomerates of carbon particles with Pt particles embedded into them) and grains of a solid polymer electrolyte (Nafion). The substrate grains presumably contain hydrophobic pores, which facilitate the oxygen penetration into the active mass. A calculation of characteristics of such an electrode focuses on the optimization of platinum weights. The principal parameters of the system are concentration and size of grains of substrate and Nafion, Pt concentration in substrate grains, average diameter of hydrophobic pores in substrate grains, and the electrode polarization. The optimum, at a given electrode polarization, electrochemical activity of the active layer, its thickness, and the platinum weight are calculated. A link between these quantities and principal parameters of the active layer is revealed.     

Formation of porous silicon by metal particle enhanced chemical etching in HF solution and its application for efficient solar cells

Porous Si was formed on n-Si wafers, modified with fine Pt particles, by simply immersing the wafers in a HF solution without a bias or an oxidizing agent. The Pt particles were deposited onto n-Si wafers by electrodeposition or electroless displacement deposition. SEM images show that many pores, ranging between 0.1 and 0.8 μm in diameter and covered with a luminescent nanoporous layer, were formed only on the Pt-modified area of the n-Si surface by immersion in 7.3 M HF solution for 24 h. The weight loss of Pt-electrodeposited n-Si wafer was 0.46 mg cm⁻², corresponding to ca. 2 μm in thickness. The weight loss and the structure of porous Si changed with the etching conditions, such as concentration of dissolved oxygen in the HF solution, distribution density of metal particles, and different kinds of metal particles. A photoelectrochemical solar cell equipped with a Pt-particle-modified porous n-Si electrode gave 13.3 mW cm⁻² of maximum output power, which corresponds to a 13% conversion efficiency and is higher than that for the Pt-particle-modified flat n-Si electrode.     

Micro-tubular solid oxide fuel cells fabricated by phase-inversion method

A novel structured micro-tubular solid oxide fuel cell (MT-SOFC) has been fabricated by combining a phase-inversion, dip-coating and high temperature co-sintering process with impregnation of the electrode catalyst into a porous electrode matrix. The asymmetric porous anode made by phase-inversion is divided into two different layers, a thick fuel delivery layer with large finger-like pores and a thin function layer with small finger-like pores. The MT-SOFC demonstrates maximum power densities of 0.44, 0.54, 0.65 and 0.78 W/cm² at 650, 700, 750 and 800 °C, respectively with H₂–15%H₂O as fuel and ambient air as oxidant. Combining the power output with the quick start-up behavior, novel structured MT-SOFC offers a potential solution for rapid start-up high performance power devices.     

Studying the process of oxygen ionization on a porous metal hydride electrode

Problems that are connected with utilization of oxygen evolving during overcharge of the nickel oxide electrode in sealed nickel metal hydride batteries are considered. It is established experimentally that the rate of the process of oxygen reduction in conditions of forced gas supply into pores of a metal hydride electrode increases by two orders of magnitude as compared with the intensity of this process during natural convection. Up to 80% of evolved oxygen undergo ionization on a metal hydride electrode in these conditions even in a regime of forced (hour-long) charge of a model sealed nickel-metal hydride battery. The dependence of the current density of oxygen reduction at a metal hydride electrode on the filling of the electrode’s porous space by oxygen is estimated with the aid of manometric and potentiostatic methods. It is shown that practically all the oxygen ionization current is generated at the walls of gas-filled electrode pores, under thin electrolyte films, with a local current intensity of 1–3 mA cm^?2.     

Polypyridylpropyne‐Pd and ‐Pt porphyrin coating for visualization of oxygen pressure

Poly[1‐trimethylsilyl‐1‐propyne‐co‐1‐(3‐pyridyl)propyne] 1 was prepared both as a polymer‐ligand of a palladium porphyrin (PdOEP) and of a platinum porphyrin (PtTFP) and as a highly gas‐permeable polymer matrix of the porphyrin. The porphyrin acted as a phosphorescence probe which could be quenched with oxygen and sense the oxygen partial pressure. 1 gave a smooth and tough coating with a thickness of ca. 2 µm which homogeneously involved the porphyrin. The porphyrin‐ 1 coatings displayed strong red‐colored phosphorescences (the emission maximum at 670 and 650 nm for PdOEP and PtTFP, respectively), and their intensity significantly decreased with an increase in the oxygen partial pressure on the coating. The high oxygen‐quenching efficiency or the high oxygen pressure sensitivity of the porphyrin's phosphorescence was observed even at cryogenic temperature. Aggregation of the porphyrin was suppressed in the coating by ligation of the porphyrin with the nitrogenous residue of 1 to significantly reduce spatial noise in the phosphorescence measurement or the oxygen‐pressure sensing. PtTFP‐ 1 was coated on the surface of a delta wing model. The oxygen‐pressure distribution on the coated model was successfully visualized in a cryogenic wind tunnel test. Copyright © 2008 John Wiley & Sons, Ltd.     

基于四苯乙烯自聚集膜增强电化学发光灵敏检测溶解氧

该文发展了一种基于四苯乙烯(TPE)在电极表面自聚集增强鲁米诺电化学发光信号的新技术,并实现了溶解氧的定量测定。将TPE分子修饰至金电极表面,在室温下形成多孔隙、无定形的聚集态自组装薄膜,有效增加了电极比表面积与孔隙的协同作用,高效捕获氧气分子O_2与超氧阴离子自由基O■,使鲁米诺的电化学发光信号得到显著增强。TPE修饰电极具有较高的稳定性与较好的重现性。利用氮气排除氧气,可改变鲁米诺电化学发光强度,进而测定溶液中溶解氧的含量。研究显示,通氮时间(1～6 min)与溶解氧浓度在1.63～0.190 mg/L范围内呈良好线性关系,相关系数(r~2)为0.990 9,溶解氧初始浓度的最低检出限为1.89 mg/L。方法操作简单、检测快速、信号变化灵敏,为溶解氧的灵敏检测提供了新方法。     

Calculation of the Liquid and Gas Permeability of Hydrophobic Low-Porosity Membranes of an Arbitrary Thickness

A method for calculating the liquid and gas permeability of hydrophobic low-porosity membranes of an arbitrary thickness is described. The calculation is based on the solution of a problem on percolation—the procedure of finding the distribution of liquid and gas over the membrane thickness. The dependence of the permeability for liquid on the share of pores that are potentially accessible to being filled with liquid is obtained for both thin and thick membranes. This dependence is of a universal nature and can easily be recalculated into a dependence of permeability on the pressure drop for membranes with any distribution of pores by size. Numerical estimates of principal characteristics for a membrane that possesses pores of three types are performed. The characteristics in question include permeabilities for liquid and gas; fluxes of the liquid; critical pressures, at which the permeability for liquid turns other than zero; and the working range of pressures, in which the membrane is capable of working normally. All these data permit the optimization of the operation of similar membranes, in particular, gas-delivering membranes that are used in hydrogen–oxygen fuel cells with a solid polymer electrolyte.     

Low loaded Pt-Co catalyst surfaces optimized by magnetron sputtering sequential deposition technique for PEM fuel cell applications: physical and electrochemical analysis on carbon paper support

A series of thin Pt-Co films with different metal ratios were deposited by using the sequential cosputtering directly on a commercial hydrophobic carbon paper substrate at room temperature and in ultra-high vacuum (UHV) conditions. Their electrocatalytic properties toward the oxygen reduction reaction were investigated in 0.5 M H₂SO₄ solution by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on rotating disc electrode (RDE). The results showed that Pt particles, deposited by dc-magnetron gun, surround the large Co-clusters deposited by rf-magnetron gun. In addition, the increase of Co content led to an increase in the electrochemical active surface area (EASA) from 23.75 m²/gPt to 47.54 m²/gPt for pure Pt and Pt:Co (1:3), respectively, which corresponded the improvement of the utilization of Pt by a factor of 1.91. This improvement indicated that the sequential magnetron cosputtering was one of the essential technique to deposit homogeneous metal clusters with desirable size on the gas diffusion layer by adjustment plasma parameters.     

Interpretation of water isotherm hysteresis for an activated charcoal using stochastic pore networks

Water vapor adsorption equilibria on activated carbons typically exhibit hysteresis. The size and shape of the hysteresis loop which separates the adsorption and desorption branches is a strong function of the pore size and interconnectivity of the pores. Neither conventional pore filling models nor statistical thermodynamics approaches provide a means for predicting the extent of hysteresis from only adsorption measurements. This work uses the Kelvin Equation in conjunction with the structural concept of a stochastic pore network to describe measured water isotherms on BPL carbon. Using a pore segment distribution function determined from the adsorption branch, it is shown that totally random assemblies underestimate the extent of hysteresis. It is possible, however, to closely fit the measured BPL-water hysteresis loop using a patchy heterogeneity in which a proportion of the larger pores are preferentially located on the exterior, mid-range pores are concentrated in a sub-surface layer and some large pores form shielded voids behind much smaller pores.Nomenclature p vapor phase partial pressure of sorbate - p _sat saturation vapor pressure of sorbate - R gas constant - r pore radius - T absolute temperature - t adsorbed layer thickness - V _L molar volume of adsorbed phase - surface tension - contact angle     

Hydrophobically modified polyelectrolyte for improved oxygen barrier in nanobrick wall multilayer thin films

The influence of attaching hydrophobic side groups to a polyelectrolyte, used for deposition of a multilayer oxygen gas barrier thin film, was investigated. Polyethyleneimine (PEI) was labeled with pyrene and deposited in “quadlayers” of PEI, poly(acrylic acid), PEI, and sodium montmorillonite clay using layer‐by‐layer assembly. Thin films made of three repeating quadlayers using unmodified PEI had much lower density (1.24 g/cm³) than pyrene‐labeled PEI‐based films (1.45 g/cm³), which is believed to be the result of greater chain coiling from the increased hydrophobicity of pendant pyrene groups. This increased density in pyrene‐labeled PEI layers allowed three quadlayers to match the oxygen transmission rate of a four quadlayer film made with unmodified PEI. This discovery provides an additional tool for tailoring the barrier behavior of clay‐based multilayer thin films that could prove useful for a variety of packaging applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1153–1156     

Studying the fine microstructure of the passive film on nanocrystalline 304 stainless steel by EIS,XPS, and AFM

The fine microstructure of the passive films on nanocrystalline (NC) and coarse crystalline (CC) 304 stainless steels (SSs) in 0.5 M H₂SO₄ were investigated by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The results indicate that the passive film on both CC and NC SSs exhibits a two-layer microstructure consisting of a compact inner layer and a porous outer layer. Some hydrated compounds (HC) were present in the porous outer layer of NC SS but not CC SS in 0.5 M H₂SO₄ solution. The pores in the outer layer of the NC SS were observed to be in the nanoscale by AFM. HC filling of the pores in the passive film on NC SS may occur due to capillary forces endowed by the nanosize pores. XPS analysis of the passive films on both CC and NC SSs, however, does not show such a composition difference which is attributed to dehydration occurring in the XPS vacuum chamber. Both the inner and outer layers of the NC SS were determined by EIS analysis to be more compact and protective than the corresponding films on CC SS as evidenced by the lower Q value, higher n, and much higher R value in the corresponding layer.     

Voltammetric measurement of pH using a mixed self-assembly of macrocyclic nickel(II) complex and diimine on gold electrode

Redox active mixed self-assembled monolayers (SAMs) of a macrocyclic nickel complex, dinickel (II)(2,2^′-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide (MNC) and a electrochemically derived diimine, 4^′-mercapto-N-phenylquinone diimine (NPQD) on gold electrode is used for the voltammetric measurement of solution pH (pH 6–9.4). The formal potential of MNC is insensitive to the solution pH and it serves as a reference molecule, whereas the redox reaction of NPQD is pH dependent and it serves as an indicator molecule. The formal potential of NPQD shifts −56 mV while changing the solution pH by one unit. The formal potential of the bifunctional self-assembly is insensitive to oxygen in solution and the dissolved oxygen does not interfere with the voltammetric measurement.     

Bioelectrocatalytic Oxygen Reduction by Laccase Immobilized on Various Carbon Carriers

Laccase is an enzyme that is used for fabricating cathodes of biofuel cells. Many studies have been aimed at searching the ways for enhancing specific electrochemical characteristics of cathode with the laccase- based catalyst. The electroreduction of oxygen on the electrode with immobilized laccase proceeds under the conditions of direct electron transfer between the electrode and active enzyme center. In this work, the effect of oxygen partial pressure on the electrocatalytic activity of laccase is studied. It is shown that, at the concentrations of oxygen dissolved in the electrolyte higher than 0.28 mM, the process is controlled by the kinetics of the formation of laccase–oxygen complex, whereas at lower concentrations and a polarization higher than 0.3 V, the process is limited by the oxygen diffusion. A wide range of carbon materials are studied as the carriers for laccase immobilization: carbon black and nanotubes with various BET specific surface areas. The conditions, which provide the highest surface coverage of carbon material with enzyme in the course of spontaneous adsorptive immobilization and the highest specific characteristics when using a “floating” electrode simulating a gas-diffusion electrode, are determined: 0.2 M phosphate-acetate buffer solution; oxygen atmosphere; the carrier material (nanotubes with a BET surface area of 210 m²/g and a mesopore volume of 3.8 cm³/g); and the composition of active mass on the electrode (50 wt % of carbon material + 50 wt % of hydrophobized carbon black).     

Measurement of Dissolved Oxygen in Biological Fluids by Using a Modified Carbon Paste Electrode

A modified carbon paste electrode was constructed for the determination of dissolved oxygen using diamino‐o‐benzoquinone (DABQ) as the modifier. The electrochemical behavior of the electrode in citrate buffer (pH 2.0) was studied. In the presence of dissolved oxygen (DO) both cathodic and anodic peak currents decreased, indicating a chemical reaction between modifier and O₂. The decrease in peak current was linearly proportional to the amount of dissolved oxygen in the concentration range of 252–1260 μM of DO. The electrode was utilized in the determination of DO in urine samples. The relative error and RSD of the method were 1.6% and 4.1%, respectively. The electrode was applied more than two months for the determination of DO without any significant divergence in its voltammetric response.     

Interaction between a stationary mercury electrode and a hydrogen bubble in an aqueous sodium dodecyl sulphate solution

The stability and rupture of thin liquid films formed from an aqueous solution of Na₂SO₄ (0.05 mol dm⁻³) in the presence of 5 × 10⁻⁵ mol dm⁻³ sodium dodecyl sulphate between a stationary mercury electrode and a hydrogen bubble has been investigated as a function of electrode potential. The electrostatic component of disjoining pressure has been calculated using the results of capacity measurement for the mercury-solution interface. Special attention has been paid to films formed on positively charged mercury surfaces. In this case, despite the positive electrode polarization, the outer Helmholtz plane potential is found to be negative due to the high surface activity of the dodecyl sulphate anion. The van der Waals component of disjoining pressure has been calculated on the basis of a double sheath model of the two interacting surfaces, taking into consideration the orientation of the adsorbed surfactant layers at the interfaces. Calculations of the total disjoining pressure can explain film stability at negative mercury potentials, but do not explain film rupture when the polarization of the mercury is positive. The existence of a hydrophobic attractive interaction is postulated in the latter case.