Function Of Scan Rate Research Articles

Modeling Charging Current Dynamics at Microelectrodes and their Interfaces with Electrolyte and Insulators with a Focus on Microfabricated Gold Microband Electrodes on an SU-8 Substrate

The suitability of electrochemical methods for quantitative measurements at microdevices is influenced by the relatively large electrode-insulator interface-to-electrode area ratio, greatly impacting charging dynamics due to interactions among electrolyte, conductor material, and insulator layers. The resulting charging current can overwhelm the faradaic current from redox chemistry. The device studied here features a 70 μm × 100 μm electroactive window, hosts gold coplanar microband electrodes, and is insulated by SU-8, which serves as both overlayer and substrate. The overlayer defines the electroactive length and isolates the leads of the electrodes from the sample solution. Cyclic voltammetry in 0.10 M KCl yields an unexpected, nonlinear dependence of current on scan rate, which can be explained with two empirical approaches. The first employs an equivalent circuit model, involving leakage resistance and double-layer capacitance in parallel, to address both background processes and electrode imperfections as a function of scan rate. The second associates the enhanced current to a changing-chargeable area resulting from interface irregularities. Prior publications on alternative conductor-insulator materials are benchmarked in this study. The comparison of the materials shows that the charging dynamics for devices made with SU-8 lead to more favorable electrochemical performance than for those constructed with glass, epoxy, and silicon nitride, and under certain circumstances, polyimide.

Exploring the linear relationship between potential dynamics and interfacial capacitance: implications for enhancing the turnover frequency in electrochemical water splitting

Effect of scan rate over intrinsic activity or TOF for OER, has been analysed for the very first time. It has found that increased TOF value varies as a function of scan rate which arises due to increased interfacial capacitance.

Electrochemical Performance of Lithographically-Defined Micro-Electrodes for Integration and Device Applications

Small; lithographically-defined and closely-spaced metallic features of dimensions and separation in the micrometer range are of strong interest as working and counter electrodes in compact electrochemical sensing devices. Such micro-electrode systems can be integrated with microfluidics and optical biosensors, such as surface plasmon waveguide biosensors, to enable multi-modal sensing strategies. We investigate lithographically-defined gold and platinum micro-electrodes experimentally, via cyclic voltammetry (CV) measurements obtained at various scan rates and concentrations of potassium ferricyanide as the redox species, in potassium nitrate as the supporting electrolyte. The magnitude of the double-layer capacitance is estimated using the voltammograms. Concentration curves for potassium ferricyanide are extracted from our CV measurements as a function of scan rate, and could be used as calibration curves from which an unknown concentration of potassium ferricyanide in the range of 0.5–5 mM can be determined. A blind test was done to confirm the validity of the calibration curve. The diffusion coefficient of potassium ferricyanide is also extracted from our CV measurements by fitting to the Randles–Sevcik equation (D = 4.18 × 10−10 m2/s). Our CV measurements were compared with measurements obtained using macroscopic commercial electrodes, yielding good agreement and verifying that the shape of our CV curves do not depend on micro-electrode geometry (only on area). We also compare our CV measurements with theoretical curves computed using the Butler–Volmer equation, achieving essentially perfect agreement while extracting the rate constant at zero potential for our redox species (ko = 10−6 m/s). Finally, we demonstrate the importance of burn-in to stabilize electrodes from the effects of electromigration and grain reorganization before use in CV measurements, by comparing with results obtained with as-deposited electrodes. Burn-in (or equivalently, annealing) of lithographic microelectrodes before use is of general importance to electrochemical sensing devices

Fluoride-Ion Batteries: On the Electrochemical Stability of Nanocrystalline La0.9Ba0.1F2.9 against Metal Electrodes

Over the past years, ceramic fluorine ion conductors with high ionic conductivity have stepped into the limelight of materials research, as they may act as solid-state electrolytes in fluorine-ion batteries (FIBs). A factor of utmost importance, which has been left aside so far, is the electrochemical stability of these conductors with respect to both the voltage window and the active materials used. The compatibility with different current collector materials is important as well. In the course of this study, tysonite-type LaBaF, which is one of the most important electrolyte in first-generation FIBs, was chosen as model substance to study its electrochemical stability against a series of metal electrodes viz. Pt, Au, Ni, Cu and Ag. To test anodic or cathodic degradation processes we carried out cyclic voltammetry (CV) measurements using a two-electrode set-up. We covered a voltage window ranging from −1 to 4 V, which is typical for FIBs, and investigated the change of the response of the CVs as a function of scan rate (2 mV/s to 0.1 V/s). It turned out that Cu is unstable in combination with LaBaF, even before voltage was applied. The cells with Au and Pt electrodes show reactions during the CV scans; in the case of Au the irreversible changes seen in CV are accompanied by a change in color of the electrode as investigated by light microscopy. Ag and Ni electrodes seem to suffer from contact issues which, most likely, also originate from side reactions with the electrode material. The experiments show that the choice of current collectors in future FIBs will become an important topic if we are to develop long-lasting FIBs. Most likely, protecting layers between the composite electrode material and the metal current collector have to be developed to prevent any interdiffusion or electrochemical degradation processes.

Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries.

Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolytes. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UMEs) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical-electrochemical (CE) deposition process. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate ( kc) obtained at the Pt UME is ∼10 times faster than that at the Au UME. Field desorption ionization mass spectrometry (MS) suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to larger kc. Raman spectroscopy and electrospray ionization MS data show that Ca2+ ions are strongly coordinated with tetrahydrofuran and weakly interacting with BH4- anions. Electron microscopy shows that the surface morphology of Ca electrodeposition is different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.

Development of a Cytosensor for the Detection of Fusarium Oxysporum - A Functional Approach Towards Bioanalytical Applications

The development of bio-analytical methods for monitoring microorganisms have created opportunities for applications in biosensors, bioprocess monitoring, assessment of cell signalling, analysis of drug responses, among several others. The voltammetric sensing system employed for studying the electrode behavior of the fungus Fusarium oxysporum comprised working (gold) electrode (0.2 cm2 ) platinum as counter electrode (0.2 cm2 ) and a saturated calomel as the reference, where the electrochemical response corresponded to the growth phases (lag, log, stationary and decline) of the fungus. The electrochemical method based on voltammetric response matched well with the response obtained through conventional methodology, where the dry weight of the fungus is estimated against time. The peak potential is a function of scan rate, which is one of the characteristic features of a totally irreversible electrode process. It is important to mention here that this dependence is true regardless of reversibility for any diffusing redoxactive species. The proposed electrochemical method is less cumbersome and more accurate. Furthermore, the proposed electrochemical method captures the decline phase of fungal growth, which is generally difficult using the conventional method of assessment of the growth curve. Further experiments confirm that the anodic peaks were not due to the biomass or the fungal spores and only due to the extracellular metabolites. However, at this stage it is difficult to exactly determine the metabolite or the group of metabolites that are responsible for the anodic peak. In conclusion this cytosensor is capable of accurately and rapidly quantifying fungi with Fusarium oxysporum as a model organism.

Temperature Effects on the Performance of Activated Carbon Electrochemical Cell in Aqueous and Non-Aqueous Electrolytes

The aim of this study is to observe the effects of temperature changes on the cycability and the electrochemical performance of activated carbon (AC) electrochemical cell in aqueous (K2SO4) and organic (TEABF4 in acetonitrile) electrolytes. A series of experiments have been carried out inside of the environmental chamber with temperature ranging from 273 to 313 K for aqueous electrolyte and 263 to 333 K for organic electrolyte. The AC electrochemical cells have been tested using cycling voltammetry (CV) at 25 mV/s sweep rate. This was followed by the step potential electrochemical spectroscopy (SPECS) experiment with a 25 mV potential step and 300 s equilibration time. SPECS method is based on applying a series of equal magnitude potential steps on a working electrode, with sufficient rest time to allow for equilibrium to be established for each step throughout an applied potential window. This method has been used to effectively differentiate the charge storage contributions from electrical double layer capacitance at the geometric and porous surface areas and diffusion-limited capacitance as a function of scan rate [1]. The outcome of this work indicates the systematic effects of changes in temperature on different charge storage mechanisms. It was expected that the overall electrochemical performance of AC electrochemical cell increases by increasing the temperature in both aqueous and non-aqueous electrolytes, due to increase of kinetic energy of ions in solvent. However, different charge storage mechanism behaved differently in the aqueous and non-aqueous electrolytes. Figure 1. shows the comparison for CV tests (a) in aqueous and (b) in organic electrolytes at different temperatures. It was found that, the total specific capacitance of AC electrochemical cell is increased by increasing the temperature in aqueous electrolyte. While, in non-aqueous electrolyte, the total specific capacitance decreases by increasing the temperature. Additionally, SPECS method was used to obtain the contribution of different charge storage mechanisms at different temperatures. The result indicates that the electrical double layer and diffusion limited processes behaved differently in each electrolyte. For instance the capacitance contribution from diffusion-limited processes decreases by increasing the temperature while, the electrical double layer capacitance at the geometric and porous surface areas increases by increasing the temperature in aqueous electrolyte. Finally, the specific capacitance obtained by the SPECS method was used to obtain the performance of the AC electrochemical cell in the aqueous and non-aqueous electrolytes in the form of a Ragone diagram. [1]. Marveh Forghani and Scott W. Donne, Journal of the Electrochemical Society 2018 165: A664-A673. Figure 1

Nanowire-Templated Three-Dimensional out-of-Plane Fuzzy Graphene As Thin Film Supercapacitor

Supercapacitors have gained popularity as future energy storage solutions due to their simple architecture, long cycling life, pulse power supply, and high dynamic charge propagation.1 They are generally categorized as Electrical Double Layer Capacitors (EDLCs) or pseudo-capacitors.2 EDLCs store energy as electrostatic charge on the electrodes’ surfaces; their efficiency is thus limited by the electrochemically active surface area available to the electrolyte.3 Whereas, pseudo-capacitors utilize fast and reversible faradaic processes due to electro-active species localized on the electrode surface to enhance current density.1, 4 Concentration of electro-active charged species and electro-chemical efficiency governs the efficiency of these supercapacitors. Recently, carbon-based materials (such as CNTs, graphene foams, graphene aerogels) have received research interest as promising candidates for supercapacitors due to their high electrochemical surface area, high electrical conductivity and accessible surface active sites for modification with electro-active species.5 Graphene has emerged as an ideal EDLC electrode material due to its superior electrical properties and high theoretical surface area.6 Current graphene electrodes are either pinned to a two-dimensional (2D) surface (such as graphene foams) or are tethered to a 2D substrate (such as carbon nanowalls). Graphene based hybrid-nanomaterials that leverage graphene’s outstanding surface-to-volume ratio coupled with pseudo-capacitive species for high efficiency supercapacitors have not been demonstrated to date. Here we demonstrate that direct synthesis of three-dimensional (3D) out-of-plane fuzzy graphene (3DFG) on Si nanowire (SiNW) mesh template (NT-3DFG) provides an ideal surface for functionalization with electro-active species to develop EDLC and pseudo-capacitive supercapacitors. We report fine control over 3DFG flake size and edge density by varying the partial pressure of CH4 precursor, plasma enhanced chemical vapor deposition (PECVD) synthesis process time, and PECVD synthesis temperature (Figure 1.A and B).7 Through electron microscopy (SEM and TEM) and Raman spectroscopy a distinct increase in single-to-few layer out-of-plane graphene edges is observed. High density of graphene edges presents high concentration of active sites for surface functionalization through surface doping, such as that through treatment with HNO3. Post treatment, the electrical conductivity of NT-3DFG is observed to be as high as 2400 S m-1. Leveraging on graphene’s outstanding surface-to-volume ratio, we have developed a material capable of attaining electrochemically active surface area of ca. 1017 m2 g-1. The introduction of electro-active species on this high surface area material renders specific capacitance as high as 87 ± 8 F g-1 (Figure 1.C). This value is further enhanced by decorating the template with metal oxide nanoparticles. The reported novel graphene based hybrid-nanomaterial shows great potential to be utilized in next-generation energy storage solutions. Figure 1. Characterization of NT-3DFG. (A) SEM image of NT-3DFG synthesized with increasing partial pressure of CH4 and increasing PECVD process time. (B) Effect of increasing PECVD process temperature on NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 10 min. (C) Cyclic voltammetry characterization of NT-3DFG before and after HNO3 treatment. (I) Representative cyclic voltammograms presenting double layer capacitance of NT-3DFG. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 90 min, respectively. Dashed lines represent respective samples after 2-hour HNO3 treatment. (II) Current density as a function of scan rate. Blue, yellow and red denote Au working electrode, NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 30 min and NT-3DFG synthesized under 25 mTorr partial pressure of CH4 for 90 min, respectively. Closed and open square represent samples before and after 2-hour HNO3 treatment, respectively.

Electrocatalytic Reduction of Carbon Dioxide on Nanosized Fluorine Doped Tin Oxide in the Solution of Extremely Low Supporting Electrolyte Concentration: Low Reduction Potentials

Fluorine doped tin oxide (FTO) has been prepared via the direct chemical reaction of tin oxide powders and hydrofluoric acid at room temperature. The image of FTO displays a sphere-like structure with an average diameter of 40–100 nm. The spectra of X-ray photoelectron spectroscopy (XPS) demonstrate that fluorine is doped into SnO2. A well-defined reduction peak at −0.50 V (vs SCE) is detected on the cyclic voltammogram (CV) of the nanosized FTO (n-FTO) electrode in CO2-saturated 3.6 × 10–4 μM H2SO4 solution of pH 5.5, which is strong evidence for the electrochemical reduction of CO2. This result indicates that the n-FTO electrode in such an extremely low supporting electrolyte concentration exhibits good electrocatalytic ability toward CO2 reduction under lower potentials. On the basis of reduction peak current as a function of scan rate, the reduction of CO2 is first performed via adsorption of CO2 on the n-FTO electrode surface, and then CO2 is reduced. The product solution obtained under a constant po...

Activation of a Nickel-Based Oxygen Evolution Reaction Catalyst on a Hematite Photoanode via Incorporation of Cerium for Photoelectrochemical Water Oxidation.

There has been debate on whether Ni(OH)2 is truly catalytically active for the photo/electrocatalytic oxygen evolution reaction. In this report, we synthesized a Ni(OH)2 cocatalyst on a hematite photoanode and showed that, as has been proposed in other studies, the current density varies as a function of scan rate, which arises due to a photoinduced capacitive charging effect. We discovered that this photoinduced charging of Ni2+/3+ can be overcome by mixing cerium nitrate into the Ni precursor solution. Under illumination, the NiCeOx cocatalyst on a hematite photoanode exhibited an approximately 200 mV cathodic shift in onset potential and a ∼53% enhancement in photocurrent at 1.23 V vs RHE. Material characterization by electrochemical impedance spectroscopy revealed that the Ni species create a p-n junction across the charge space region, which facilitates collection of the photogenerated holes by the cocatalyst layer, and core level X-ray photoelectron spectroscopy showed that Ce incorporated into the Ni-based cocatalyst layer may possibly induce the oxidation of the Ni species. In addition, we observed a reduction in binding energies of Ni after photoelectrochemical water splitting reactions, which suggests that the lattice oxygen of the NiCeOx is consumed in the catalytic cycle, forming oxygen vacancies. The NiCeOx cocatalyst, however, was incapable of passivating the surface recombination centers of the hematite photoanode, as indicated by the unaltered flat-band potential determined with Mott-Schottky analysis.

Characterization of Electrochemical Surface Area and Porosity of Zirconia Sensors

The platinum electrode/yttria-stabilized zirconia metal/solid electrolyte interface is used in a wide variety of sensing applications including lambda oxygen sensors, wide range oxygen sensors, HC and NOx sensors.1–3 In many cases the electrode is protected by a porous ceramic layer to shield the device from exhaust particle impingement,1 or in the case of some types of mixed potential sensors, the electrolyte itself is porous.4The electrochemically active surface area and porosity are important factors that may control the performance of all types of zirconia-based sensors. While measuring AC impedance behavior in the gas phase is possible, the high diffusivity of gases through the porous layer makes extracting diffusion parameters difficult. Mixed potential electrochemical sensors being electro-kinetic devices, are particularly sensitive to these parameters, but few techniques have been developed to measure active surface areas and characterize transport through porous layers. We have developed sensors consisting of a yttria-stabilized zirconia (YSZ) substrate coated with an insulating ceramic onto which dense electrodes of Pt, La0.8Sr0.2CrO3, or Au0.5Pd0.5alloy are screen printed. Finally, a porous YSZ electrolyte is deposited on top of the electrodes. Measuring the effective diffusivity through the porous layer and the electrochemically active surface area of the underlying electrodes is useful in optimizing the performance of the devices. The electrochemically active surface area of the Pt electrodes beneath the porous electrolyte layer was determined by immersing the sensor in aqueous H2SO4 solution and measuring the double layer capacitance or adsorption of a monolayer of H. The oxidation and reduction of [Fe(CN)6]-4/-3 observed by cyclic voltammetry (CV) acts as a probe for the effective diffusion constant through the porous layer as the kinetics of this reaction are fast and the reaction is easily reversible. The effective diffusion coefficient is obtained by fitting to the Randles-Secvik equation for the peak CV current as a function of scan rate. Figure 1 shows the CVs obtained on (a) a Pt disk and (b) the Pt sensor electrode beneath a porous YSZ electrolyte. The effective diffusion coefficient is on the order of 10-6 cm2/s for the Pt disk and is consistent with values reported in the literature.4 The diffusion coefficient is 10-10 cm2/s for the transport through the porous layer on sensor indicating significant hindrance by the YSZ. Measuring these characteristics for both sensors produced by screen printing (ESL ElectroScience) and sensors produced by additive manufacturing in our lab will guide the optimization of sensor performance characteristics. The measurements are correlated to the device sensing and AC impedance response.

Biomechanical Strengthening of the Human Cornea Induced by Nanoplatform-Based Transepithelial Riboflavin/UV-A Corneal Cross-Linking

The purpose of this study was to investigate the biomechanical stiffening effect induced by nanoplatform-based transepithelial riboflavin/UV-A cross-linking protocol using atomic force microscopy (AFM). Twelve eye bank donor human sclerocorneal tissues were investigated using a commercial atomic force microscope operated in force spectroscopy mode. Four specimens underwent transepithelial corneal cross-linking using a hypotonic solution of 0.1% riboflavin with biodegradable polymeric nanoparticles of 2-hydroxypropyl-β-cyclodextrin plus enhancers (trometamol and ethylenediaminetetraacetic acid) and UV-A irradiation with a 10 mW/cm2 device for 9 minutes. After treatment, the corneal epithelium was removed using the Amoils brush, and the Young's modulus of the most anterior stroma was quantified as a function of scan rate by AFM. The results were compared with those collected from four specimens that underwent conventional riboflavin/UV-A corneal cross-linking and four untreated specimens. The average Young's modulus of the most anterior stroma after the nanoplatform-based transepithelial and conventional riboflavin/UV-A corneal cross-linking treatments was 2.5 times (P < 0.001) and 1.7 times (P < 0.001) greater than untreated controls respectively. The anterior stromal stiffness was significantly different between the two corneal cross-linking procedures (P < 0.001). The indentation depth decreased after corneal cross-linking treatments, ranging from an average of 2.4 ± 0.3 μm in untreated samples to an average of 1.2 ± 0.1 μm and 1.8 ± 0.1 μm after nanoplatform-based transepithelial and conventional cross-linking, respectively. The present nanotechnology-based transepithelial riboflavin/UV-A corneal cross-linking was effective to improve the biomechanical strength of the most anterior stroma of the human cornea.

Multidimensional Cyclic Voltammetry Simulations of Pseudocapacitive Electrodes with a Conducting Nanorod Scaffold

This paper aims to understand the effect of nanoarchitecture on the performance of pseudocapacitive electrodes consisting of conducting scaffold coated with pseudocapacitive material. To do so, two-dimensional numerical simulations of ordered conducting nanorods coated with a thin film of pseudocapacitive material were performed. The simulations reproduced three-electrode cyclic voltammetry measurements based on a continuum model derived from first principles. Two empirical approaches commonly used experimentally to characterize the contributions of surface-controlled and diffusion-controlled charge storage mechanisms to the total current density with respect to scan rate were theoretically validated for the first time. Moreover, the areal capacitive capacitance, attributed to EDL formation, remained constant and independent of electrode dimensions, at low scan rates. However, at high scan rates, it decreased with decreasing conducting nanorod radius and increasing pseudocapacitive layer thickness due to resistive losses. By contrast, the gravimetric faradaic capacitance, due to reversible faradaic reactions, decreased continuously with increasing scan rate and pseudocapacitive layer thickness but was independent of conducting nanorod radius. Note that the total gravimetric capacitance predicted numerically featured values comparable to experimental measurements. Finally, an optimum pseudocapacitive layer thickness that maximizes total areal capacitance was identified as a function of scan rate and confirmed by scaling analysis.

Combined Numerical and Experimental Approach to Study the Effect of External SS316 Cathode on the Localized Corrosion in AA7050-T7451 Galvanically Coupled with SS316 Under Atmospheric Conditions

Connections between dissimilar metal alloys (e.g., aluminum (Al) alloy components and stainless steel (SS) fasteners) are frequently encountered in airframes exposed to an atmospheric marine environment. This exposure results in the formation of thin layer of electrolyte containing chloride species on the materials surface. The thin film electrolyte can wick into the tight crevice formed between the Al alloy component and the SS fastener when breakdown of corrosion protection coating happens. As a result, an electrochemical cell is established due to this galvanic coupling, and localized corrosion is likely aggravated in the crevice that forms. This corrosion damage can serve as a preferential site for fatigue crack formation, leading to failure of Al alloy components during operation. Developing mechanistic understanding of metal-environment interactions can provide guidelines for current protection system evaluation, as well as develop corrosion mitigation strategy during materials design stage. There are several influential environmental factors affecting atmospheric corrosion, including relative humidity (RH), water layer thickness (WL), concentration of dissolved species in the solution, temperature, etc. [2]. For localized corrosion due to galvanic coupling under atmospheric condition, it has been shown that the effect of external cathode outside the crevice formed in the galvanic coupling controls the extent of localized corrosion inside the crevice [3-4]. In the literature there is a limited amount of corrosion modeling work which focuses on atmospheric corrosion, and in those few cases that do exist, the electrochemical kinetics used in the modeling framework are mainly based on full immersion conditions, which might lose key features under atmospheric conditions. Hence atmospheric corrosion modeling work with accurate electrochemical kinetics is greatly needed to better understand the effects of the key external factors on electrochemical distributions that developed in the corrosion system involving a realistic thin film/galvanic couple configuration. In this work, a combined numerical and experimental approach is being applied to study potential and current density distributions as a function of two important variables for external cathode of SS316: water layer thickness on the external cathode as well as the size of external cathode. For the experimental work, a sintered Ag/AgCl electrode serving as a combined reference electrode (RE) and counter electrode (CE) [5] is implemented to characterize thin film electrochemical kinetics of AA7050-T7451 and SS316. A series of thin film electrolyte with thicknesses from 10m to 2000 m were used to study the effect of WL on electrochemical kinetics of testing materials as a function of scan rate and electrolyte concentration. An example of cathodic kinetics for SS316 in terms of WL is shown in Figure 1. In addition, the evolution of th ecrevice solution chemistry of AA7050 is mimicked by dissolving the alloy galvanostatically in the thin film cell before determining the anodic kinetics of AA7050 in that solution. The electrochemical kinetics of tested materials serve as boundary conditions in the modeling framework. For the computational work, the finite element method (FEM) [6] is applied to study both the effect of external cathode size and water layer thickness on potential and current density distributions along the AA7050 inside the crevice. Comparisons of potential/current density distributions with different external cathode sizes as well as water layer thicknesses under corresponding boundary conditions are used to illustrate the effects of atmospheric variables on localized corrosion damage along the crevice. Acknowledgement This work has been supported by the Office of Naval Research (ONR) Grant N00014-14-1-0012. Mr. William Nickerson, Technical Officer at Office of Naval Research is gratefully acknowledged. Reference [1] N. E. Co, and J. T. Burns, In Corrosion Research in Progress Symposium , pp. 47–51, NACE, Vancouver, Canada (2016). [2] H. Simillion, O. Dolgikh, H. Terryn, J. Deconinck, Corros. Revs., 32, 73-100 (2014). [3] Z. Y. Chen, F. Cui, and R. G. Kelly, J. Electrochem. Soc., 155, C360- C368 (2008). [4] Z. Y. Chen, and R. G. Kelly, J. Electrochem. Soc.,, 157, C69-C78 (2010). [5] P. Khullar, J. V. Badilla, and R. G. Kelly, ECS Electrochem. Lett., 4, C31-C33 (2015). [6] C. Liu, and R. G. Kelly, Electrochem. Soc. Interface, 23, 47-51 (2014). Figure 1

Direct Growth of Birnessite-Type MnO2 on Activated Carbon Cloth for a Flexible Supercapacitor

1. Introduction Carbon cloth (CC), which is composed of numerous uniform carbon fibers, has attracted significant attention in recent years as an electrode for use in flexible supercapacitors, owing to the low cost of this material, along with its unique 3D structure, high surface area, chemical stability, electrical conductivity and flexibility. However, because the active part of CC consists solely of an exfoliated carbon shell, as-purchased CC exhibits lower gravimetric capacitance (1~2 F g-1) than other carbonaceous materials, such as graphene and carbon nanotubes (100~200 F g-1). Thus, CC is effective when used as a scaffold to support highly capacitive materials. In the present study, commercial CC pieces were first treated using facile wet processes and the resulting material served as the substrate for the direct growth of a birnessite-type MnO2 film.1 During this process, the mass loading of the MnO2 was optimized in terms of the areal capacitance (capacitance normalized by the unit area of the electrode) and the electrochemical utilization. An asymmetric supercapacitor was subsequently built by assembling the optimized MnO2/CC together with activated carbon (AC)-coated CC as the positive and negative electrodes, respectively. We report the fabrication of a MnO2-modified CC electrode with optimized loading and the construction of a device with high performance and good stability. 2. Experimental MnO2 was deposited electrochemically in 2 mM MnSO4 aqueous solution containing 50 mM KCl, in which the treated and untreated CC pieces were used as the working electrodes. The CC piece with the optimized mass loading of MnO2 was used as the positive electrode together with an activated carbon-coated CC (AC/CC) as the negative electrode to build a two-electrode asymmetric cell. First, this cell was assembled in an open beaker, such that the potentials of both electrodes could be monitored with respect to a Ag/AgCl reference electrode while the voltage between the two electrodes was controlled within the range of 0 to 2.0 V. After a stable response was observed, the equilibrated electrodes were detached and re-assembled in an acrylic holder with a cellulose filter paper (pre-soaked in 0.5 M Na2SO4 electrolyte) sandwiched between the MnO2- and AC-modified CC pieces to construct a solid-state supercapacitor (The cell volume, 0.07 cm3). 3. Results and discussion A simple wet process was developed to enhance the EDLC performance of commercial CC, including oxidation followed by reduction with NaBH4. This treatment was found to enhance the areal capacitance from 4 to 78 mF cm-2. The mass loading of birnessite-type MnO2 film could be varied by controlling the delivered charge density during electrodeposition. Based on the magnitude of areal capacitance (C A) and the electrochemical utilization, the optimal MnO2 loading appeared to be approximately 4 mg cm-2. The MnO2-modified CC positive electrode with the optimized mass loading was combined with an AC-modified CC negative electrode to build an asymmetric supercapacitor operating at a cell voltage of 2.0 V, in which the potential windows of 0.73 and 1.27 V were assigned to the MnO2 and AC electrodes, respectively. The volumetric capacitance (C vol) values of each experimental cell were collected as a function of scan rate. The C vol at 2 mV s-1 increased in the order symmetric AC/CC < asymmetric MnO2/CC//AC/CC < symmetric MnO2/CC, which can largely be attributed to the capacitance of the single electrode. With increasing scan rate, the symmetric MnO2/CC cell exhibited a significantly decreased C vol due to slow kinetics. In contrast, the asymmetric cell showed a gentle decrease, equivalent to better rate-capability, indicating a contribution from the EDLC due to the AC negative electrode. The volumetric capacitance of the MnO2//AC asymmetric supercapacitor at scan rates above 20 mV s-1 was much higher than those of the MnO2 and AC symmetric supercapacitors. The asymmetric MnO2/CC//AC/CC cell exhibited high volumetric energy density (0.978 mW cm-3) and power density (0.158 W cm-3) with excellent cycling stability (94.4% after 10,000 cycles). This exceptional performance per unit volume can be attributed to the following reasons: an increase in the mass loading of MnO2 resulting from the pre-treatment of the commercial CC,an improvement in the rate capability, due to the specific structure of the electrodeposited MnO2 (birnessite) and the optimization of the loading mass based on considering the electrochemical utilization of the MnO2 on the CC, andavoidance of side reactions at the positive and negative electrodes in the asymmetric cell. Reference 1) M. Nakayama et al, “A Direct Electrochemical Route to Construct a Polymer/Manganese Oxide Layered Structure”, Inorg. Chem. , 43, 8215-8217 (2004).

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

Conduction and Reactivity in Heterogeneous-Molecular Catalysis: New Insights in Water Oxidation Catalysis by Phosphate Cobalt Oxide Films.

Cyclic voltammetry of phosphate cobalt oxide (CoPi) films catalyzing O2-evolution from water oxidation as a function of scan rate, phosphate concentration and film thickness allowed for new insights into the coupling between charge transport and catalysis. At pH = 7 and low buffer concentrations, the film is insulating below 0.8 (V vs SHE) but becomes conductive above 0.9 (V vs SHE). Between 1.0 to 1.3 (V vs SHE), the mesoporous structure of the film gives rise to a large thickness-dependent capacitance. At higher buffer concentrations, two reversible proton-coupled redox couples appear over the capacitive response with 0.94 and 1.19 (V vs SHE) pH = 7 standard potentials. The latter is, at most, very weakly catalytic and not responsible for the large catalytic current observed at higher potentials. CV-response analysis showed that the amount of redox-active cobalt-species in the film is small, less than 10% of total. The catalytic process involves a further proton-coupled-electron-transfer and is so fast that it is controlled by diffusion of phosphate, the catalyst cofactor. CV-analysis with newly derived relationships led to a combination of the catalyst standard potential with the catalytic rate constant and a lower-limit estimation of these parameters. The large currents resulting from the fast catalytic reaction result in significant potential losses related to charge transport through the film. CoPi films appear to combine molecular catalysis with semiconductor-type charge transport. This mode of heterogeneous molecular catalysis is likely to occur in many other catalytic films.

Coupling of Electrochemical Charging with Mechanical Expansion and Degradation in Nanostructured MnO2 Cathodes

The faster kinetics of nanostructured charge-storage materials are often accompanied by accelerated degradation and failure, compromising the potential advantages of nanostructure and nanoporosity. Understanding and controlling the exact mechanisms of this degradation is a complex materials characterization challenge. The recent availability of in situ and in operando techniques allows new fundamental studies of these issues, including the failure of nanoscale porosity to fully accommodate the mechanical strains of charge insertion. Here, we use atomic force microscopy in liquid electrolytes to experimentally measure the mechanical expansion of porous, nanostructured Li-ion cathodes during charge cycling. Using thin films of LixMnO2 on Pt collectors, we analyze expansion as a function of scan rate, voltage window, and porosity. Comparative measurements in aqueous and nonaqueous electrolytes confirm the role of Li-ion species in the results. The rate dependent measurements of expansion can be separated into diffusion-limited, bulk components driven by Mn3+/4+ charging and faster, surface processes, in the same way that these contributions to capacitance are normally separated. Consequently, we have extracted the electrochemical-mechanical coupling coefficients ∂V/∂Q for surface and bulk processes separately. While the bulk coupling coefficient provides an excellent match to values predicted from first-principles theory, the surface coupling is surprisingly large. In fact, surface capacitance is responsible for a majority of the mechanical expansion at high rates, even when most of this capacitance would normally be attributed to double-layer charging mechanisms. The result demonstrates the importance of Mn3+/4+ species at the electrolyte interface and the inability of these species to take advantage of the film’s nanoporosity for accommodating expansion. In other words, fast surface charging processes do not involve mechanical relaxation of the material into adjacent pores, but rather drive expansion of the solid matrix.

Electrochemical Study of Bulk and Monolayer Copper in Alkaline Solution

The formation of copper oxides and hydroxides from metallic copper and underpotentially deposited copper atoms on platinum nanoparticles, when it is subjected to a potential scan between the hydrogen evolution potential and the oxygen evolution potential in alkaline solution as a function of scan rate and the pH of the solution has been investigated. Cyclic voltammetry of bulk copper electrodes in alkaline solution reveal that oxidation proceeds in steps leading to formation of Cu2O, Cu(OH)2 and CuO on the surface as well as some soluble oxides which corroborates well with X-ray photoelectron spectroscopy (XPS) analysis. Furthermore, interesting needle like formations on copper surface was observed in a scanning electron microscope (SEM), which can explain the unusual relation between the peak current and scan rate. On the other hand, the oxidation of copper monolayer on platinum nanoparticles leads primarily to formation of Cu2O and shows the effect of nobility imparted by the underlying platinum in terms of increased potential.

Avoiding Resistance Limitations in High-Performance Transparent Supercapacitor Electrodes Based on Large-Area, High-Conductivity PEDOT:PSS Films.

PSS) conducting polymer films for use as transparent supercapacitor electrodes. To facilitate this, we provide a detailed explanation of the factors limiting the performance of such electrodes. These films have a very low optical conductivity of σop = 24 S/cm (at 550 nm), crucial for this application, and a reasonable volumetric capacitance of CV = 41 F/cm(3). Secondary doping with formic acid gives these films a DC conductivity of σDC = 936 S/cm, allowing them to perform both as a transparent conductor/current collector and transparent supercapacitor electrode. Small-area films (A ∼ 1 cm(2)) display measured areal capacitance as high as 1 mF/cm(2), even for reasonably transparent electrodes (T ∼ 80%). However, in real devices, the absolute capacitance will be maximized by increasing the device area. As such, here, we measure the electrode performance as a function of its length and width. We find that the measured areal capacitance falls dramatically with scan rate and sample length but is independent of width. We show that this is because the measured areal capacitance is limited by the electrical resistance of the electrode. We have derived an equation for the measured areal capacitance as a function of scan rate and electrode lateral dimensions that fits the data extremely well up to scan rates of ∼1000 mV/s (corresponding to charge/discharge times > 0.6 s). These results are self-consistent with independent analysis of the electrical and impedance properties of the electrodes. These results can be used to find limiting combinations of electrode length and scan rate, beyond which electrode performance falls dramatically. We use these insights to build large-area (∼100 cm(2)) supercapacitors using electrodes that are 95% transparent, providing a capacitance of ∼12 mF (at 50 mV/s), significantly higher than that of any previously reported transparent supercapacitor.