SECM imaging of electrocatalytic activity for oxygen reduction reaction on thin film materials

The oxygen reduction reaction (ORR) on sputtered Pt thin films in acidic solution was successfully studied by scanning electrochemical microscopy (SECM) in a modified tip generation-substrate collection (TG-SC) mode. SECM images of ORR activity in different sample areas were obtained and it is shown that this TG-SC SECM technique can be used to screen electrocatalytic activity of continuous thin film samples efficiently and quickly for the ORR in an acidic medium. It is observed that this technique is not very sensitive to the tip-substrate separation within a certain range. The SECM images obtained are strongly dependent on the substrate potential. The advantages of this technique for studying ORR electrocatalysts are discussed.     

Quantitative study of pH change during LaNi5−xAlx (x = 0, 0.3) discharge process by SECM

Oxygen reduction reaction (ORR) on Pt microelectrode was used for developing a micro pH sensor for scanning electrochemical microscopy (SECM) study in this work. When the potential of Pt microelectrode was held constant in ORR region, the ORR current (cathodic current) increased with decreasing solution pH and vice versa. The response time of the ORR current to pH changes was measured to be ca. 30 ms which implies that the pH response is fast enough for monitoring the temporal pH changes. Furthermore, a fine linear relationship was found to exist between the half wave potential of ORR (E_1/2) and the solution pH value, and the slope is −46 mV/pH. The Pt micro pH sensor was located 1 μm above the LaNi_5−xAl_x (x = 0, 0.3) substrate electrode surface in pH = 9 KOH solution to perform the tip-substrate voltammetry of SECM. In tip voltammogram, the ORR tip current qualitatively reflects the transit solution pH changes during LaNi_5−xAl_x discharge reaction. Also, the minimum values of the solution pH near LaNi₅ and LaNi_4.7Al_0.3 surface during the discharge reaction were quantitatively detected; they were 7.17 and 7.57, respectively. The result indicates that Al partial substitution for Ni degrades the maximum discharge ability of the alloy and decreases the hydrogen diffusion coefficient in alloy bulk.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Scanning electrochemical microscopy for the investigation of corrosion processes: Measurement of Zn2+ spatial distribution with ion selective microelectrodes

Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM) for chemical imaging of corrosion processes. They present higher chemical selectivity than conventional amperometric microdisks, and may be the only effective option to visualize the dissolution of metals with negative redox potentials in aqueous environments when the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions. A robust micro-sized ion selective electrode has been developed which allows the spatial distribution of Zn²⁺ during galvanic corrosion of a model Fe/Zn couple to be investigated using SECM. Owing to the low internal contact potential achieved with the novel design, the resistance of the micropipette electrodes is only fractions of the resistance of conventional micropipette electrodes of the same size. As a result, no special shielding of the microelectrodes is required and higher scanning rate can be used for scanning in the potentiometric modes using these micropipette tips. Concentration profiles over corroding surfaces measured with this technique will be presented.     

Ion transport and degradation studies of a polyaniline-modified electrode using SECM

The morphology and ion transport characteristics of polyaniline (PANI) films on a platinum electrode were investigated using scanning electrochemical microscopy method (SECM). An ultramicroelectrode (UME) tip was positioned close to the surface of a PANI-modified substrate electrode, and the current signal at the tip and substrate electrodes, during a substrate potential step or linear sweep were monitored simultaneously. Proton transport occurred on the PANI film at a pH of 6 during the first and second redox cycles of PANI was monitored at this pH. The chemical reaction pathway of PANI as a function of substrate potential was analyzed. Different species of degradation products were detected electrochemically at the SECM tip. Benzoquinone (BQ) was found to be the major product of anodic degradation of PANI. Chemical imaging of the anodic degradation of PANI was performed by SECM during applying different potential steps to the PANI. The anodic threshold of 0.75 V was suggested as the potential at which degradation of PANI begins.     

Reactivity at the film/solution interface of ex situ prepared bismuth film electrodes: A scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) investigation

Bismuth film electrodes (BiFEs) prepared ex situ with and without complexing bromide ions in the modification solution were investigated using scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). A feedback mode of the SECM was employed to examine the conductivity and reactivity of a series of thin bismuth films deposited onto disk glassy carbon substrate electrodes (GCEs) of 3 mm in diameter. A platinum micro-electrode (? = 25 μm) was used as the SECM tip, and current against tip/substrate distance was recorded in solutions containing either Ru(NH₃)₆³⁺ or Fe(CN)₆⁴⁻ species as redox mediators. With both redox mediators positive feedback approach curves were recorded, which indicated that the bismuth film deposition protocol associated with the addition of bromide ions in the modification solution did not compromise the conductivity of the bismuth film in comparison with that prepared without bromide. However, at the former Bi film a slight kinetic hindering was observed in recycling Ru(NH₃)₆³⁺, suggesting a different surface potential. On the other hand, the approach curves recorded by using Fe(CN)₆⁴⁻ showed that both types of the aforementioned bismuth films exhibited local reactivity with the oxidised form of the redox mediator, and that bismuth film obtained with bromide ions exhibited slightly lower reactivity. The use of SECM in the scanning operation mode allowed us to ascertain that the bismuth deposits were uniformly distributed across the whole surface of the glassy carbon substrate electrode. Comparative AFM measurements corroborated the above findings and additionally revealed a denser growth of smaller bismuth crystals over the surface of the substrate electrode in the presence of bromide ions, while the crystals were bigger but sparser in the absence of bromide ions in the modification solution.     

An SECM observation of dissolution distribution of ferrous or ferric ion from a polycrystalline iron electrode

The dissolution of iron as ferrous or ferric ion from a polycrystalline iron electrode during anodic polarization in pH 2.3 sulfate solution was evaluated by using scanning electrochemical microscopy (SECM). A graphite reinforcement carbon (GRC) microelectrode was employed as a probe electrode of SECM to detect ferrous or ferric ions dissolved from the iron electrode in the active-dissolution, passive or trans-passive region. The probe current above the iron electrode surface subjected to active-dissolution showed the dissolution distribution of ferrous ion, depending on the substrate grains. It was found that the active-dissolution rate of iron as ferrous ions from the grain on which the thicker film was formed in the passive region, was lower than that from the grain on which the thinner film was formed in the passive region.     

Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy

In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion.     

A mechanistic investigation of di-tert-butyl nitroxide using scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used to evaluate the electron-transfer mechanism of di-tert-butyl nitroxide (DTBN) in acetonitrile. The oxidation of DTBN is coupled to a rapid, irreversible chemical follow-up step that is difficult to characterize quantitatively with CV due to distortion of the voltammograms by solution resistance and mixed radial–linear diffusion within the scan rate region of interest. Collection efficiencies from the tip generation substrate collection (TGSC) mode of SECM were used to determine a rate constant of 21 s⁻¹ for the follow-up reaction. Collection efficiency versus distance plots obtained at 5 and 50 mM DTBN concentration are identical, confirming the first-order nature of the chemical reaction. Numerical simulation of linear scan voltammograms obtained at different tip/substrate distances provides a heterogeneous electron-transfer rate constant of 0.85 cm s⁻¹.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

Fabrication and characterization of an innovative integrated solid-state microelectrode

Currently, most commercial microelectrodes cannot be used for in vivo or in situ measurements due to size constraints of the reference and counter electrodes. Thus, integrated miniature microelectrodes are highly desirable. In this work, an integrated needle-type microelectrode consisting of a solid Ag/AgCl reference electrode, a gold counter electrode, and a gold working microelectrode was prepared using a microfabrication technique and an electrochemical modification method. The electrochemical performance of each electrode was characterized using cyclic voltammetry. The effects of chloride ion concentration on the potential of the Ag/AgCl reference electrode were also evaluated. The results showed that the integrated microelectrode displayed high stability, reproducibility, repeatability and reliability, and that it can be used to determine constituents, profiles and functions in situ.     

Sensing electrochemical activity in polymer coated metals during the early stages of coating degradation–Effect of the polarization of the substrate

Eventual limitations in the applicability of the SECM in the SG-TC mode to in situ monitor electrochemical activity in the proximity of a sub-centimeter defect (holiday) through the coating applied to protect a metal against the corrosive environments has been explored. The possible occurrence of chemical interferences between the chemical species involved in the corrosion reactions, as well as the formation of reactive secondary products, have been investigated because they would influence the faradaic signals registered at the SECM tip. In order to separate the contributions of the half-cell processes, polarization of the substrate was performed in this work, both negative and positive with respect to the corrosion potential of the sample in the test environment, and the signals detected at the tip were analyzed. By selecting different values for the potential applied to the tip, the local concentrations of Fe(II) ions, hydrogen peroxide and oxygen could be monitored. It has been observed that the oxygen reduction reaction occurring at the cathodic sites on the corroding steel surface can be accompanied by the formation of hydrogen peroxide as intermediate, and this species will interfere with the detection of dissolved metal at the tip.     

Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM)

Transition metal hexacyanoferrates are versatile inorganic compounds widely employed for the assembling of sensors and biosensors in a variety of different electroanaytical applications. A modified version of the recently introduced redox competition mode of scanning electrochemical microscopy (RC-SECM) was exploited to visualize the local electrocatalytic activity of microstructured Prussian blue (PB) films towards the reduction of H₂O₂ with improved lateral resolution. The PB films were electrochemically deposited in a spot on glassy carbon surfaces using a droplet cell. The influence of the potential applied to the PB modified surface on the current at the SECM tip was evaluated when both the sample and the SECM tip were competing for H₂O₂ in solution. Thus, high local electrocatalytic activity is indicated by low currents at the SECM tip. The same strategy was successfully employed for the characterization of the performances of a biosensor employing the enzyme glucose oxidase (GOx) immobilized within a polymer hydrogel matrix on the top of PB-modified glassy carbon electrodes.     

Power consumption and flow field characteristics of a coaxial mixer with a double inner impeller

A coaxial mixer meeting the actual demand of a system with high and variable viscosity is investigated. It has an outer wal-scraping frame and a double inner impeller consisting of a four-pitched-blade turbine and Rushton turbine. The power consumption and flow field characteristics of the coaxial mixer in laminar and transitional flow are simulated numerically, and then the distribution of velocity field, shear rate and mass flow rate are analyzed. The simulation results indicate that the outer frame has little effect on the power consumption of the double inner impeller whether in laminar or transitional flow, whereas the inner combined impeller has a great effect on the power consumption of the outer frame. Compared with the single rotation mode, the power consumption of the outer frame will decrease in co-rotation mode and increase in counter-rotation mode. The velocity, shear rate and mass flow rate are relatively high near the inner impeller in all operating modes, and only under double-shaft agitation wil the mixing performance near the free surface be improved. In addition, these distributions in the co-rotation and counter-rotation modes show little difference, but the co-rotation mode is recommended for the advantage of low power consumption.     

Scanning electrochemical microscopy characterization of bimetallic Pt–M (M = Pd, Ru, Ir) catalysts for hydrogen oxidation

A combinatorial screening method, combined with scanning electrochemical microscopy (SECM) in a tip-generation–substrate-collection (TG–SC) mode, was applied to systematically and rapidly identify potential bimetallic catalysts (Pt–M, M = Pd, Ru, Ir) for the hydrogen oxidation reaction (HOR). The catalytic oxidation of hydrogen on the candidate catalysts was further examined during cyclic voltammetric scans of the substrate with a tip close to the substrate. The quantitative rate of hydrogen oxidation on the candidate substrates was determined for different substrate potentials from SECM approach curves by fitting to a theoretical model. SECM screening results revealed that Pt₄Pd₆, Pt₉Ru₁ and Pt₃Ir₇ were the optimum composition of the catalysts from the Pt–Pd, Pt–Ru and Pt–Ir bimetallic systems for hydrogen sensors. The catalytic activity of the candidate catalysts in HOR was highly dependent on the substrate potential. The kinetic parameters for HOR were obtained on Pt₄Pd₆ (Tafel slope = 124 mV, k° = 0.19 cm/s, α = 0.52), Pt₉Ru₁ (Tafel slope = 140 mV, k° = 0.08 cm/s, α = 0.58) and Pt₃Ir₇ (Tafel slope = 114 mV, k° = 0.11 cm/s, α = 0.48) and compared with Pt (Tafel slope = 118 mV, k° = 0.17 cm/s, α = 0.5). Among the bimetallic catalysts studied, Pt₄Pd₆ exhibited the highest activity toward HOR with a high standard rate constant value in a wide range of applied potentials.     

电弧炉电极调节PDF自适应控制系统

将鲁棒性强、超调小、响应速度快、易实现的PDF控制方法运用到电炉电极系统的控制上,其中在电极控制系统的设计方面,采用双闭环模式--速度环作为内环,电极的电流环作为外环.并对PID控制和PDF自适应控制进行了仿真,仿真结果表明PDF自适应控制优于PID控制.     

Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

Local direct and indirect reduction of electrografted aryldiazonium/gold surfaces for polymer brushes patterning

The patterning of conductive substrates by polymer brushes may be achieved by using successively scanning electrochemical microscopy (SECM) and atom transfer radical polymerization (ATRP). After the surface functionalization by a brominated aryldiazonium initiator, SECM allows the local reduction at the micrometer scale of the initiator grafted layer. Different channels sizes involved in charge transport within the initiator layers are evidenced by combining SECM, CV and observation of the aryl-grafted layer transformation. ATRP is performed on the SECM patterned substrate. Inside the pattern, the lower density of initiator decreases the polymer thickness. The pattern resolution is enhanced when the direct mode of the SECM is used instead of the mediated indirect mode.     

Controlled relative humidity storage for high toughness and strength of binderless green pellets

An equation was developed to predict fracture toughness of green powder compacts. The model combines crack tip toughness predicted by Kendall's model with crack tip shielding due to bridging of moisture meniscuses across the crack. The model predicts that crack tip shielding due to moisture should be dominant. Fracture tests on ceria green pellets verified that storing pellets at a high relative humidity (98% RH) for an extended period of time led to fracture strength more than double those stored at lower RH. However, at lower RH there is no significant increase in fracture strength with increased RH as predicted by the model. The lower strength at low RH is due to insufficient capillary and surface forces but may also be related to the lack of sufficient adsorbed moisture to form bridging meniscuses. The high green strengths achieved by storing pellets at a high RH suggest a method of strengthening green parts without adding binder.