Potential Sweep Technique Research Articles

Preparation of Spherical δ-MnO2 Nanoflowers by One-Step Coprecipitation Method as Electrode Material for Supercapacitor.

Spherical δ-MnO2 nanoflower materials were synthesized via a facile one-step coprecipitation method through adjusting the molar ratio of KMnO4 to MnSO4. The influence of the molar ratio of the reactants on the crystal structure, morphology, and electrochemical performances was investigated. At a molar ratio of 3.3 for KMnO4 to MnSO4, the spherical δ-MnO2 nanoflowers composed of nanosheets with the highest specific surface area (228.0 m2 g-1) were obtained as electrode materials. In the conventional three-electrode system using 1 M Na2SO4 as an electrolyte, the specific capacitance of the spherical δ-MnO2 nanoflowers reached 172.3 F g-1 at a current density of 1 A g-1. Moreover, even after 5000 cycles at a current density of 5 A g-1, the GCD curves remained essentially unchanged, and the specific capacitance still retained 86.50% of the maximum value. The kinetics of the electrode reaction were preliminarily studied through the linear potential sweep technique to observe diffusion-controlled contribution toward total capacitance. For the spherical δ-MnO2 nanoflower electrode material, diffusion-controlled contribution accounted for 65.1% at low scan rates and still remained significant at high scan rates (100 mV s-1), indicating excellent utilization efficiency of the bulk phase. The as-fabricated asymmetric supercapacitor HFC-7//MnO2-3.3-ASC presented a prominent specific energy of 16.5 Wh kg-1 at the specific power of 450 W kg-1. Even when the specific power reached 9.0 kW kg-1, the energy density still retained 9.5 Wh kg-1.

Towards Realizing High-Voltage Aqueous Zinc Batteries: An Investigation of the Zinc Anode in Concentrated Acetate-Based Electrolytes

Zinc-based aqueous battery systems are attractive for grid storage applications due to the inherent electrolyte safety, material non-toxicity, low cost, high volumetric capacity of zinc and the low polarizability of the zinc anode in aqueous solutions. Despite these key features, they possess lower energy density when compared to lithium-ion batteries primarily due to the limited working potential range for aqueous electrolytes. In addition, zinc hosts a slew of problems such as inhomogeneous material distribution upon cycling and the formation of inactive zinc species and dendrites, all of which contribute to capacity loss and ultimately, cell failure. Water-in-salt electrolytes (WiSE) have been found to be an effective strategy to expand the electrochemical window of aqueous electrolytes for high energy density aqueous-based batteries, including zinc batteries. There is a growing amount of literature on highly concentrated acetate-based electrolytes as low-cost and non-toxic WiSE for lithium, potassium, sodium and magnesium and zinc battery systems. However, work is scarce on the nature of zinc electrochemistry, morphology and reversibility in these concentrated electrolytes at realistic current densities (> 1 mA/cm2) and higher zinc utilization. Understanding the electrochemical behavior of zinc in this electrolyte system will significantly aid in the development of a high voltage zinc aqueous battery.In this work, the zinc electrochemical performance and surface morphology is studied in highly concentrated bi-salt potassium acetate (KOAc)-zinc acetate (Zn(OAc)2) electrolytes. The impact of the high concentration on the electrochemical working window of the electrolytes is measured using potential sweep and constant-current techniques. As the acetate concentration is varied from 1 m to 32 m, the largest electrochemical window is measured at 3.33 V for 31 m KOAc+ 1 m Zn(OAc)2. Zinc half cells are cycled at current densities of 1-5 mA/cm2 to examine zinc reversibility with respect to zinc depth-of-discharge (DOD) in these electrolytes. The zinc discharge product is visualized by in-operando optical microscopy as a function of the potential and is observed to be a zinc plating/stripping phenomenon in contrast to zinc dissolution/precipitation typically observed in alkaline media. Energy dispersive X-ray spectroscopy and X-ray diffraction studies display the minor presence of zinc oxide and zinc hydroxide after repeated cycling. Cloud point measurements are performed to measure the phase stability of these electrolytes at 5-25ºC as a function of acetate concentration, which show that concentrations below (and including) 27 m KOAc + 1 m Zn(OAc)2 does not induce crystallization at these temperatures. Preliminary measurements using spectroscopic techniques such as Raman and nuclear magnetic resonance (NMR) spectroscopy are conducted to elucidate ion solvation structures as a function of acetate concentration. Overall, the current and proposed studies are fruitful in understanding the relationship between zinc anode and the concentrated acetate-based electrolytes in the pursuit of a high-voltage aqueous zinc battery.

Short-Term Strong Cathodic Polarization of Ni/YSZ and Pt/YSZ

The initial stages of YSZ reduction and formation of intermetallic phases at Ni∣YSZ interfaces on strongly cathodically polarized electrodes were studied by a number of potential sweep and impedance techniques. The measurements were carried out in the potential range −1.0 V to −3.0 V vs E°(O2) in H2/H2O at 650 and 750 °C. Below −1.7 V the cathodic current increases almost exponentially, mainly due to electronic conductivity in the YSZ. Reduction and oxidation peaks develop below and above −1.9 V, respectively. The peaks reveal a simultaneous reduction of YSZ at the Ni∣YSZ interface and reoxidation of a Ni-Zr phase. Charges calculated from the reoxidation peaks indicate a thickness of 200 nm for the layer formed by conditioning the electrode for 300 s at −2.6 V vs E°(O2). Impedance measurements show a suppressed arc decreasing with increasing polarization. Below −2.3 V vs E°(O2) a capacitive high frequency arc segment and an inductive low frequency loop develop. Both are ascribed to electronic conductivity. SEM/EDS microscopy on cross sections of samples cooled with and without polarization showed the formation of uniform and homogeneous intermetallic reaction layers which after reoxidation resulted in a two-phased nanostructures.

Resistive switching and impedance characteristics of M/TiO2−x/TiO2/M nano-ionic memristor

Employing electrochemical impedance spectroscopy (EIS), we demonstrate a novel approach towards characterizing the switching behavior of nano-ionic memristor, M/TiO2−x/TiO2/M where M = Au, fabricated using combined RF/DC magnetron sputtering techniques. The non-linear resistive switching behavior of the device has been investigated using a constant potential sweep technique to observe I-V switching profile (a pinched hysteresis loop) in which ex-situ impedance response has been measured by interrupting the potential sweep (mV s−1) over different distinct regimes (pristine device, SET and after zero-crossing). Accordingly, the obtained EIS spectra are corroborated with oxide circuit model and the corresponding numerical fittings of the impedance response reveals distinct RC frequency domains as exemplified in the equivalent circuit models obtained using the simulated impedance response and attributed to the interfacial barrier between the stoichiometric and non-stoichiometric TiO2 respectively. It is evident that the device shows various functional features as memory element owing to the transport of mobile charges in TiO2−x when the bias potential (V) is applied across the device. It is revealed that EIS studies render a new insight into much acclaimed formation and annihilation of nanofilament like dendrites (magneli phase of titania, TinO2n−1) which are found to be responsible for the change of switching states between low resistance state (LRS) and high resistance state (HRS) respectively. In order to validate the memory retention and endurance characteristics, the reproducible resistive switching is found to occur consistently over 104 s and the device endurance has been verified by toggling between HRS and LRS over 1000 cycles.

Fast Nanoscale Surface Charge Mapping with Pulsed-Potential Scanning Ion Conductance Microscopy.

A vast range of interfacial systems exhibit charge heterogeneities on the nanoscale. These differences in local surface charge density are challenging to visualize, but recent work has shown the scanning ion conductance microscope (SICM) to be a very promising tool to spatially resolve and map surface charge and topography via a hopping potential sweep technique with a single nanopipette probe, with harmonic modulation of a bias applied between quasi-reference counter electrodes in the nanopipette and bulk solution, coupled with lock-in detection. Although powerful, this is a relatively slow process, with limitations on resolution and the size of the images that can be collected. Herein, we demonstrate a new scanning routine for mapping surface charge and topography with SICM, which increases the data acquisition rate by an order of magnitude and with the potential for further gains. Furthermore, the method is simplified, eliminating the need for bias modulation lock-in detection, by utilizing a potential-pulse, chronoamperometric approach, with self-referencing calibration of the response at each pixel in the image. We demonstrate the application of this new method to both a model substrate and living PC-12 cells under physiological (high ionic strength) conditions, where charge mapping is most challenging (small Debye length). This work contributes significantly to the emergence of SICM as a multifunctional technique for simultaneously probing interfacial structure and function with nanometer resolution.

Electrochemical Reduction of CO2 on Copper Oxidized By Electrochemical Methods

The electrochemical oxidation of copper surface in alkaline solution results in the formation of different products, which are a function of scan rate and concentration of OH- ions. It has been found that, the electro-oxidation results in oxide film on copper surface consist of Cu2O, Cu(OH)2 and CuO. The scanning electron microscopy (SEM) images of copper foils oxidized by linear sweep voltammetry at different scan rate show a systematic growth of oxides on the copper surface. At low scan rate growth of oxide takes place resulting in needle like structures projecting outwards from copper surface. Further, the density of these needles of oxides decreases as the scan rate increases. A similar behaviour was also observed in the chronoamperometry experiments performed at different potential and for different time duration. The length and the density of needles increase with increase in time when the applied potential was in potential region of peak B. On the other hand, at more negative potential and low concentration OH- ions instead of needles, a layer of oxide particles were observed. The x-ray photoelectron spectroscopy (XPS) analysis of oxidized copper surface shows the presence of Cu2O, Cu(OH)2and CuO in the oxide layer and the percentage of CuO increases as the density of needles increases on the surface. It has found that the catalytic activity of the copper obtained by reducing it from its oxides is more than that of the original polycrystalline copper foil for CO2reduction. The oxidation of copper surface can be achieved either by annealing in air or oxygen, or by electrochemical oxidation. Further, electrochemical oxidation of copper foil can be performed either by potential sweep or potential step techniques. Here, the oxidation of copper surface was carried out by chronoamperometry (CA), linear sweep voltammetry (LSV) as well as cyclic voltammetry (successive oxidation and reduction cycles) in Ar saturated 0.5 M KOH solution. The copper foil was oxidized by CA at -0.1 V for 30 minutes and LSV at scan rate of 0.5 mV/s from -1.2 to 0.5 V in Ar saturated 0.5 M KOH. The five cycles of CVs were carried out at a scan rate of 0.5 mV/s in Ar saturated 0.5 M KOH between -1.2 to 0.5 V. The SEM images shows that the needle like growth of oxides gets reduced back to copper in first few minutes of the electrochemical reaction when applied potential is more negative than hydrogen evolution. These in turn have been used for electrochemical reduction of CO2. The main product of CO2 reduction in aqueous phase was found to HCOO- and the gas phase analysis revealed predominantly hydrogen molecule. Faradaic efficiency for formic acid on copper foil oxidized by CV was higher than that for copper foil oxidized by the CA and LSV. The reason this observation may be the amount of active copper produced on the polycrystalline copper from the reduction of the copper oxides. This has been verified from the amount of charge transferred during the oxidation of copper which is highest for the CVs as with each cycle the peak current of both the peaks observed during oxidation of copper increases. During oxidation of copper foil by CV in fifth cycle a total charge of ~0.70 C/cm2 charge was transferred, whereas, during LSV a total of ~0.45 C/cm2 and during CA at total of ~0.30 C/cm2 of charge was transferred. Thus indicating more oxidation during CV than LSV and CA, and hence more amount of active copper will be produced on the copper surface resulting in increased faradaic efficiency of formic acid. Figure 1

Electrooxidation of Glycerin by Potential Sweep Technique and Controlled Potential Electrolysis

Electrooxidation of Glycerin by Potential Sweep Technique and Controlled Potential Electrolysis

Modeling of polarization curves not exhibiting a Tafel region using Excel spreadsheets

This paper describes the modeling of polarization curves not exhibiting a Tafel region using Excel spreadsheets. Carbon steel in aerated NaCl unbuffered solutions was studied applying a linear potential sweep technique. Current–potential curves were obtained from linear potential sweep at a rate of 10mVs−1 in un-buffered 0.1, 0.4 and 0.9M NaCl solutions, at a constant temperature of 25°C and potential sweeps that were conducted in a potential range −1.0 to 0.4V vs. Ag/AgCl.

Hydrogen evolution reaction on spheroidal graphite cast iron with different pearlite areas in sulphuric acid solutions

The rate equation of hydrogen evolution reaction of spheroidal graphite cast iron with different pearlite area has been studied in sulphuric acid solutions at 298 K. The cathodic Tafel slope of −0·130 V/decade and the reaction order with respect to the activity of hydrogen ion of 1 are obtained by linear potential sweep technique. The rate equation of hydrogen evolution reaction does not depend on the area of pearlite. There is no difference in hydrogen evolution reaction mechanisms between pure iron and spheroidal graphite cast iron.

Gold nanowire electrodes in array: simulation study and experiments

Recent developments in nanofabrication have enabled fabrication of robust and reproducible nanoelectrodes with enhanced performance, when compared to microelectrodes. A hybrid electron beam/photolithography technique is shown that permits discrete gold nanowire electrode arrays to be routinely fabricated at reasonable cost. Fabricated devices include twelve gold nanowire working electrode arrays, an on-chip gold counter electrode and an on-chip platinum pseudo reference electrode. Using potential sweep techniques, when diffusionally independent, these nanowires exhibit measurable currents in the nanoAmpere regime and display steady-state voltammograms even at very high scan rates (5000 mV s(-1)) indicative of fast analyte mass transport to the electrode. Nanowire electrode arrays offer the potential for enhancements in electroanalysis including increased signal to noise ratio and increased sensitivity while also allowing quantitative detection at much lower concentrations. However, to achieve this goal a full understanding of the diffusion profiles existing at nanowire arrays is required. To this end, we simulate the effects of altering inter-electrode separations on analyte diffusion for a range of scan rates at nanowire electrode arrays, and perform the corresponding experiments. We show that arrays with diffusionally independent concentration profiles demonstrate superior electrochemical performance compared to arrays with overlapping diffusion profiles when employing sweep voltammetric techniques. By contrast, we show that arrays with diffusionally overlapping profiles exhibit enhanced performance when employing step voltammetric techniques.

Anodic Dissolution of Spheroidal Graphite Cast Iron with Different Pearlite Areas in Sulfuric Acid Solutions

The rate equation of anodic dissolution reaction of spheroidal graphite cast iron in sulfuric acid solutions at 298 K has been studied. The cast irons have different areas of pearlite. The anodic Tafel slope of 0.043 V decade−1and the reaction order with respect to the hydroxyl ion activity of 1 are obtained by the linear potential sweep technique. The anodic current density does not depend on the area of pearlite. There is no difference in the anodic dissolution reaction mechanisms between pure iron and spheroidal graphite cast iron. The anodic current density of the cast iron is higher than that of the pure iron.

Electrochemical Characterization of Mass Transport in Porous Electrodes

Mass transport in reticulated vitreous carbon electrodes in flow by mode has been studied with the cathodic deposition of copper as a model reaction. Two methods characterizing mass transport are being compared. The first one relies on determination of the electrorecovery kinetics under mass-transfer limiting conditions in galvanostatic mode. The second method is the determination of the limiting current by a linear potential sweep technique. This technique has been modified to allow for a more unambiguous limiting current determination. In some cases, the modified linear sweep technique can also be used to measure the influence of a gaseous reaction side products on mass transport.

The cathodic behavior of titanium: Serum effect

The cathodic behavior of titanium was investigated in inorganic buffer solutions and in fetal bovine serum through potential sweep techniques. Under cathodic polarization, the oxygen reduction and the hydrogen evolution reactions were observed. It was found that the activity of the electrode toward the electron transfer increased with decreasing the electrode surface charge. The polarization curves recorded in serum displayed the same features as those observed in inorganic buffered solutions. However, organic molecules were likely adsorbed onto the titanium surface and the adsorption reactions were potential dependant. Additionally, it was noticed that the buffer capacity of serum could be overwhelmed by the progressive interface alkalinization during cathodic polarization. Furthermore, serum affected the corrosion current of the implant materials and the effect was pH-dependent.

Electrochemical formation of viologen nano/microsized wires and tubes by potential sweep technique combined with micellar disruption method

Abstract The first bottom-up electrochemical approach for the preparation of viologen (1,1′-didodecyl-4,4′-bipyridinium salt) nano/microsized wires and tubes are described. This approach involves film formation of the viologen using the micellar disruption method followed by transformation of the film into the nano/microstructures using a potential sweep technique. The hexagonal cross-section of the wires and tubes has a 400 nm–30 μm width and >1 mm length. The formation mechanism of the nano/microstructures was discussed on the basis of the X-ray photoelectron spectroscopic analyses, FT-IR spectra, scanning electron microscopic observation, UV–vis spectra, and cyclic voltammograms before and after the transformation.

The polarization behavior of the anode in a microbial fuel cell

Microbial fuel cell (MFC) systems are unique electrochemical devices that employ the catalytic action of bacteria to drive the oxidation of organic compounds. These systems have been suggested as renewable energy sources for small remote devices; however, questions remain about how MFCs can be efficiently optimized for this purpose. Several electrochemical techniques have been employed in this study to elucidate the limiting factors in power production by MFCs. Impedance spectra were collected for the anode and cathode at their open-circuit potential (OCP) before and after all other electrochemical tests. Cell voltage–current curves were obtained using a potential sweep technique and used to determine the maximum power available from the system. Potentiodynamic polarization in two different potential regions was used to determine the exchange current for the reaction occurring at the anode at its OCP and to explore the polarization behavior of the anode and the cathode in a wide potential range. Cyclic voltammetry was used to evaluate the redox activity of the anode. These techniques used in combination showed that the microorganism Shewanella oneidensis MR-1 is solely responsible for the observed decrease of the OCP of the anode, the increased rate of oxidation of lactate, the larger cell voltage and the increased maximum power output of the MFC.

Three-phase-boundary dynamics at Pt/YSZ microelectrodes

Strong electrode activation, inductive hysteresis and non-linearity are well-known phenomena on model Pt-YSZ cathodes, and recently also a regular current fluctuation pattern have been reported. The oxygen electrode reaction with YSZ as electrolyte is studied at Pt microelectrodes prepared by electrochemical etching of platinum wire. The result is a well-defined contact area of ∼ 25 μm in diameter. Due to the small size and a favourable ratio between the three-phase-boundary (TPB) length and the contact area, microelectrodes should have an increased sensitivity to local events at the TPB compared to larger point electrodes normally applied. The electrode processes are studied by potential sweep, step and impedance techniques. Investigation of the YSZ and Pt surfaces afterwards reveals the growth of dendrite-like Pt structures from the TPB. The formation of these structures can explain the observed regular current fluctuation pattern and contributes to the explanation of the activation phenomena of the model Pt-YSZ and SOFC cathodes.

Determination of electrochemical parameters and corrosion rate for carbon steel in un-buffered sodium chloride solutions using a superposition model

Abstract Corrosion of carbon steel in un-buffered NaCl solutions was studied applying linear potential sweep technique to a rotating disk electrode. Current–potential curves were obtained from linear potential sweep at a rate of 1 mV s−1 in solution with concentrations in the range 0.02–1 M NaCl and rotation rates in the range 170–370 rad s−1, at 22 °C. Potential sweeps, which were conducted in the potential range −700 to −100 mV/SHE, were started from the cathodic limit in order to approach the measurement of corrosion under rust-free conditions. Polarization curves were analyzed with a superimposition model developed ad hoc and implemented in a computer program, which enabled determining the corrosion rate and kinetics parameters of the underlying anodic and cathodic sub-processes. The anodic sub-process, dissolution of iron, was well described in terms of a pure charge transfer controlled reaction, while the cathodic sub-process, oxygen reduction on iron, was well described in terms of mixed mass transfer and charge transfer control. Increase of electrode rotation rate increases the limiting current of oxygen reduction, which results in an enhanced corrosion rate of carbon steel. Increase of NaCl concentration has a dual effect: the limiting current of oxygen reduction decreases as a result of the influence of NaCl concentration on solution viscosity and the anodic dissolution of iron increases due to the influence of NaCl on pitting formation. However, this last mechanism predominates and a net increase in carbon steel corrosion rate is observed in this case.

High catalytic activity of nanostructured Pd thin films electrochemically deposited on polycrystalline Pt and Au substrates towards electro-oxidation of methanol

Nanostructured Pd thin films are directly formed on polycrystalline Pt and Au substrates in the absence of hard and soft templates by using a cyclic potential sweep technique, which is confirmed by both SEM observation and their unusual cyclic voltammetric characteristics in H 2SO 4 solution. Interestingly, the bimetallic electrodes obtained after the deposition of ultrathin Pd films onto Pt and Au substrates display much higher catalytic activity towards the electro-oxidation of methanol than the bulk Pt electrode. Besides, it is found that the foreign metal substrate has great influence on the electro-catalytic behavior of the Pd films.

Electrochemical Behavior of Tennantite in Chloride Solutions

Electrochemical oxidation of tennantite was investigated in this study. Cyclic voltammetry, potential sweep, and potential step techniques were employed for the study. Based on the experimental results, a reaction mechanism of internal diffusion of through a sulfur film was proposed to describe the effect of , , , and temperature. The relatively small value of charge-transfer coefficient of the reaction, which was obtained from the Tafel slope, and the irreversibility of the reaction could also be explained by this mechanism. Different sulfur allotropes with various porosities and thicknesses contribute to an unusual temperature effect on tennantite oxidation in conditions of below and a potential range of .

Influence of phosphoric acid on the grid alloys of positive plates in the lead acid battery system: A comparative study

The effect of phosphoric acid in lead acid batteries was studied with potential sweep techniques. Lead–tin (2%) and lead–calcium (0.1%) tin (0.35%) alloys were used as the grids. The influence of phosphoric acid in the presence of tin, calcium and calcium alone on the grid were examined. Phosphoric acid inhibits the formation of β-PbO 2 in the alloys.