In situ voltammetric de-alloying of fuel cell catalyst electrode layer: A combined scanning electron microscope/electron probe micro-analysis study

In situ voltammetric de-alloying, i.e. partial selective dissolution of less noble alloy components, is a recently proposed, effective strategy to prepare active electrocatalysts for the oxygen reduction reaction (ORR) [S. Koh, P. Strasser, J. Am. Chem. Soc. 129 (2007) 12624–12625; R. Srivastava, P. Mani, N. Hahn, P. Strasser, Angew. Chem. Int. Ed. 46 (2007) 8988–8991]. However, in situ de-alloying of bimetallics inside electrode layers of membrane-electrode-assemblies (MEAs) seems to defy the requirement of keeping the membrane free of cationic contaminants; yet, when followed by ion exchange, de-alloyed cathodes result in previously unachieved single cell activities of polymer electrolyte membrane fuel cell cathode layers of up to 0.4 A mg_Pt⁻¹ at 900 mV cell voltage. The effects of voltammetric Cu de-alloying on the MEA have never been studied before. In the present study, we therefore address this issue and report detailed scanning electron microscope (SEM) imaging of the morphology and electron probe micro-analysis (EPMA) mapping of a MEA at various stages of the de-alloying and ion-exchange process. We investigate the significant loss of Cu from the cathode particle catalyst after de-alloying, demonstrate how the membrane can be cleaned from Cu-ion contamination using ion exchange with protons from liquid inorganic acids, and show that Cu ion exchange does ultimately not affect the activated catalyst particles inside the cathode layer. We correlate the microscopic study of the MEA with its cyclic voltammetric response curves as well as the single cell polarization data.     

Study of methanesulfonic acid effect on electrosynthesis of lead dioxide to provide more environmentally electrolyte selection

Lead dioxide has absorbed much attention for its good resistance to corrosion, high electronic conductivity, good stability, long lifetime, low cost as electrode material. However, there still exist problems about the selection of electrolyte, which may bring trouble in energy conservation and environmental protection. The present study was to provide more environmentally electrolyte of methanesulfonic acid (MSA) and evaluate MSA effect on electrosynthesis of lead dioxide by using rotating disk electrode (RDE) and scanning electron microscope (SEM) as well as X-ray diffraction (XRD) so as to make sure suitable concentration of MSA and to gain more environmental and energy benefit. The results show that methanesulfonic acid has great influence on the electrodeposition of lead dioxide and adequately characterise the PbO₂ synthesized on Pt electrode surface. By controlling the composition of the electrodeposition bath at MSA concentration 0.1 M and Pb(II) 0.2 M can we get better reversible process, longer electrode lifetime and more satisfactory environmental protection.     

The corrosion of nickel–aluminium bronze in seawater

Nickel–aluminium bronze (NAB) alloys show good corrosion resistance under marine conditions. The corrosion behaviour of cast and wrought NAB alloys is illustrated in this work through a range of electrochemical techniques including open-circuit potentiometry with time, oxygen reduction voltammetry, NAB dissolution voltammetry, potential step (or flow step) current transients and linear polarisation resistance. The galvanic coupling of NAB to stainless steel or copper is examined by zero resistance ammetery. The importance of using controlled flow working electrodes is illustrated by the use of a rotating disc electrode, a rotating cylinder electrode and a bimetallic (NAB/copper–nickel) rotating cylinder electrode. In addition to controlling the hydrodynamics, such electrodes allow charge transfer data to separate from those of mass transport control under mixed kinetic control. Longer term seawater immersion trials on planar coupons coupled to titanium or cupronickel are also reported. The relative contributions of erosion and corrosion attack are considered using a wall-jet electrode and the corrosion characteristics of NAB are compared to those of copper and copper–nickel in chloride media.     

Effect of chemisorbed carbon monoxide on Pt/C electrode on the mechanism of the hydrogen oxidation reaction

The influence of poisoning of Pt catalyst by CO on the kinetics and mechanism of H₂ oxidation reaction (HOR) at Pt/C electrode in 0.5 mol dm⁻³ HClO₄, saturated with H₂ containing 100 ppm CO, was examined with rotating disc electrode (RDE) at 22 °C. Commercial carbon black, Vulcan XC-72 was used as support, while Pt/C catalyst was prepared by modified polyol synthesis method in an ethylene glycol (EG) solution. The kinetically controlled current (I_k) for the HOR at Pt/C decreases significantly at CO coverage (Θ_CO) > 0.6. For Θ_CO < 0.6 the HOR takes place through Tafel-Volmer mechanism with Tafel reaction as rate-determining step at the low CO coverage, while Volmer step controls the overall reaction rate at the medium CO coverage. When CO coverage is higher then 0.6, Heyrovsky-Volmer mechanism is operative for the HOR with Heyrovsky as the rate-determining step (rds).     

Effects of base and filler chemistry and weld techniques on equiaxed zone formation in Al–Zn–Mg alloy welds

Abstract

Equiaxed zone (EQZ) formation in Al–Zn–Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al–Zn–Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.     

轻型车扩展地区实际道路NOx排放研究

基于轻型国Ⅵ柴油车安装的后处理前、后氮氧传感器进行实际道路排放RDE(real driving emission)测试和估算.测试地区分别为环境温度40℃以上的吐鲁番高温地区、海拔2 800 m的格尔木高原地区和环境温度-25℃的海拉尔高寒地区.测试结果表明:在不同环境温度和压力下,由于采用整车热管理策略,确保了选择性催化还原(selective catalytic reduction,SCR)系统内部温度处于NOx转化高效温度范围,在城市、城郊和高速工况下,NOx 的转化效率均在85％以上;相比高温地区,高原地区车辆的NOx 原排约提高3.23倍,高寒地区约提高1.81倍.现有轻型国Ⅵ车辆要满足2023年RDE法规的NOx排放限值,优化车辆原始NOx排放至关重要.     

Formation and behaviour study of an environment-friendly corrosion inhibitor by electrochemical methods

This paper deals with the characterization of an environment-friendly corrosion inhibitor formulated for industrial cutting fluids. Polarization measurements and electrochemical impedance spectroscopy (EIS) tests reveal the formation of a 3D film at the interface metal/elecrolyte. This film blocks iron ions oxidation by forming a barrier on the metallic surface. Studies concerning, respectively, inhibitor concentration, immersion time and temperature show that effects of these three parameters are strongly linked and influence considerably the protective effect of the inhibitor. XPS analyses confirm formation of an inhomogeneous layer submitted to a water-uptake with time when inhibitor is not added in necessary concentration.     

Electrochemical studies of acid-promoted disproportionation of nitroxyl radical

The acid-promoted disproportionation of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) was studied in order to elucidate the recycling process of the catalytic oxidation of alcohol by nitroxides. Potentiometry, cyclic and hydrodynamic voltammetries with a rotating disk electrode (RDE) have been used to investigate the formation of a nitrosonium cation and a hydroxylamine by disproportionation of nitroxides in acetonitrile. The addition of p-toluenesulfonic acid caused the rest potential of the TEMPO/nitrosonium cation couple to shift in a more positive direction and decreased the peak current in the cyclic voltammograms. The subsequent addition of 2,6-lutidine as a Lewis base made the rest potential and the peak current recover. These phenomena corresponded to the recycling of the catalytic process by TEMPO in a basic aprotic solution.     

Estimation of the effectiveness factor of an outer-sphere redox couple (Fe3+/Fe2+) using rotating disk Ti/IrO2 electrodes of different loading

The effectiveness factor; E _f , defined as the fraction of the surface that participates effectively in a given reaction, is an important parameter when operating three-dimensional (3D) electrodes. The rotating disk electrode (RDE) technique with the Fe³⁺/Fe²⁺ redox couple as a probe reaction has been used for the evaluation of the effectiveness factor of 3D Ti/IrO₂ electrodes with different IrO₂ loading. For this purpose, steady-state polarization measurements using Ti/IrO₂ rotating disk electrodes in 0.5 M Fe³⁺/Fe²⁺ in 1 M HCl were carried out under well-defined hydrodynamic conditions. The low-field approximation relation has been used for the estimation of the exchange current densities j ₀, of the Fe³⁺/Fe²⁺ redox couple. It was found for this redox couple that the effectiveness factor is very low (<2%) and essentially the 2D electrode surface area works effectively in the steady-state polarization measurements.     

Effect of chemisorbed CO on MoOx–Pt/C electrode on the kinetics of hydrogen oxidation reaction

The influence of poisoning of MoO_x–Pt catalyst by CO on the kinetics of H₂ oxidation reaction (HOR) at MoO_x–Pt electrode in 0.5 mol dm⁻³ HClO₄ saturated with H₂ containing 100 ppm CO, was examined on rotating disc electrode (RDE) at 25 °C. MoO_x–Pt nano-catalyst prepared by the polyole method combined with MoO_x post-deposition was supported on commercial carbon black, Vulcan XC-72. The MoO_x–Pt/C catalyst was characterized by TEM technique. The catalyst composition is very similar to the nominal one and post-deposited MoO_x species block only a small fraction of the active Pt particle surface area. MoO_x deposition on the carbon support can be ruled out from the EDAX results and from the low mobility of these oxides under used conditions. Based on Tafel–Heyrovsky–Volmer mechanism the corresponding kinetic equations from a dual-pathway model were derived to describe oxidation current–potential behavior on RDE over entire potential range, at various CO coverages. The polarization RDE curves were fitted with derived polarization equations according to the proposed model. The fitting showed that the HOR proceeded most likely via the Tafel–Volmer (TV) pathway. A very high electrocatalytic activity observed at MoO_x–Pt catalyst for the hydrogen oxidation reaction in the presence of 100 ppm CO is achieved through chemical surface reaction of adsorbed CO with Mo surface oxides.