Powder Microelectrode Research Articles

Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at −0.34 V (vs. standard hydrogen electrode), indicative of U60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at −0.71 V is assigned to the subsequent U60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e- (per step) reduction of U60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and −0.05 V (U(V)/U(IV)). For uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum–mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (EH-pH) diagrams for adsorbed and structural species.

Electrochemical behavior of pure graphite studied with a powder microelectrode

In this work, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques have been used to study the lithiation/delithiation of graphite using powder microelectrode in a 1.2 M LiPF6 in EC:EMC electrolyte. The advantage of using the powder microelectrode is the possibility to easily study graphite without any additive and determine electrochemical characteristics of Li-insertion. The use of cyclic voltammetry at very low scan rates allows to estimate the diffusion coefficient of lithium inside graphite. The exchange current density of graphite for several state of charge (SoC) has also been determined by EIS measurements. Moreover, the powder microelectrode is a useful tool to study the formation of the solid electrolyte interphase (SEI) on graphite either by cyclic voltammetry or EIS.

Powder microelectrode embedded with carboxyl multi-walled carbon nanotubes for sensitive and quantitative detection of nitrofuran residues

In this study, a sensitive electrochemical detection method for nitrofurazone (NF) and semicarbazide (SC) based on a powder microelectrode embedded with carboxyl multi-walled carbon nanotubes was developed.

Rapid Detection of Ascorbic Acid Based on a Dual-Electrode Sensor System Using a Powder Microelectrode Embedded with Carboxyl Multi-Walled Carbon Nanotubes.

In this paper, carboxyl groups were introduced by liquid oxidation methods onto multi-walled carbon nanotubes (MWCNTs) to improve the MWCNTs’ electrocatalytic properties. A platinum wire microelectrode (ME) was corroded using aqua regia and subsequently embedded with MWCNTs to achieve more active sites, producing a so-called powder microelectrode (PME). Compared with conventional MEs, the PME has a larger specific surface area and more active sites. When PME was used to detect ascorbic acid (AA), the AA oxidation potential shifted negatively and current peak was visibly increased. The calibration curve obtained for AA was in a range of 5.00 × 10−6~9.50 × 10−4 mol·L−1: Ipa(μA) = 3.259 × 10−2 + 1.801 × 102 C (mol·L−1) under the optimum testing conditions. Moreover, the detection and quantitation limits were confirmed at 4.89 × 10−7 mol·L−1 and 1.63 × 10−7 mol·L−1, respectively. When the fabricated PME was practically applied to detect AA, it was shown a recovery rate of 94~107% with relative standard deviation (RSD) <5%. The proposed strategy thus offers a promising, rapid, selective and low-cost approach to effective analysis of AA.

Applications of Carbon Fiber Ultra-microelectrode and Powder Microelectrode in Exploring Influences of Non-aqueous Solvents and Cathode Materials on ORR and OER†

Applications of Carbon Fiber Ultra-microelectrode and Powder Microelectrode in Exploring Influences of Non-aqueous Solvents and Cathode Materials on ORR and OER†

Two-electrode Sensor System for Rapid Detection of Sulfonamides by Applying the Nafion-Carboxyl Multiwalled Carbon Nanotubes Powder Microelectrode

Two-electrode Sensor System for Rapid Detection of Sulfonamides by Applying the Nafion-Carboxyl Multiwalled Carbon Nanotubes Powder Microelectrode

A direct electrochemical route from oxides to TiMn2 hydrogen storage alloy

This study is for investigating the direct electro-deoxidation of mixed TiO2–MnO2 powder to prepare TiMn2 alloy in molten calcium chloride. The influences of process parameters, such as sintering temperature, cell voltage, and electrolysis time, on the electrolysis process were examined to investigate the mechanism of alloy formation. The composition and morphology of the products were analyzed by XRD and SEM, respectively. The electrochemical property of TiMn2 alloy was investigated by cyclic voltammetry measurements. The results show that pure TiMn2 can be prepared by direct electrochemical reduction of mixed TiO2/MnO2 pellets at a voltage of 3.1V in molten calcium chloride of 900°C for 7h. The electro-deoxidation proceeds from the reduction of manganese oxides to Mn, which is reduced by TiO2 or CaTiO3 to form TiMn2 alloy. The cyclic voltammetry measurements using powder microelectrode show that the prepared TiMn2 alloy has good electrochemical hydrogen storage property.

Preparation of ZrMn2 hydrogen storage alloy by electro-deoxidation in molten calcium chloride

ZrMn2 alloy was electro-synthesized directly from cathode pellets compacted with powdered mixture of MnO2 and ZrO2 in molten calcium chloride. Sintering temperature, cell voltage and electrolysis time were the dominant factors that affected the characteristics of the final product. The results confirmed the formation of pure ZrMn2 alloy through the electro-deoxidation of the mixed oxide pellets at 3.1 V for 12 h in 900 °C CaCl2 melt. The X-ray diffraction (XRD) and cyclic voltammetry analysis suggested that the electro-deoxidation proceeded from the reduction of manganese oxides to Mn, followed by ZrO2 or CaZrO3 reduction on the pre-formed Mn to ZrMn2 alloy. The cyclic voltammetry measurements using powder microelectrode showed that the prepared ZrMn2 alloy has a good electrochemical hydrogen storage property.

The energetics and kinetics of uranyl reduction on pyrite, hematite, and magnetite surfaces: A powder microelectrode study

There are many studies describing the influence of parameters such as pH, pCO2, and complexing ligands on the sorption of the aqueous uranyl species onto mineral surfaces. However, few of these studies describe the reduction reaction mechanisms and the factors that influence the rate of reduction, despite the fact that the oxidation state of uranium is the most important factor controlling the mobility of uranium. In this study, the energetics and kinetics of the U(VI) reduction half-reaction on pyrite, hematite, and magnetite were investigated by electrochemical methods using a powder microelectrode (PME) as the working electrode. Anodic and cathodic peaks corresponding to the 1e- redox couple, U(VI)/U(V), were identified in cyclic voltammograms of pyrite, hematite, and magnetite at pH 4.5. A second oxidation peak, corresponding to the oxidation of U(IV), was identified and provides evidence for the formation of reduced uranium phase(s) on the mineral surfaces. In addition, uranium-containing precipitates were identified on pyrite surfaces after polarization in a PME. This study identifies the disproportionation of U(V) species on the surface as a possible rate-limiting step in the two-step U(VI) reduction mechanism: (1) charge transfer to form U(V) followed by, (2) a disproportionation reaction that forms U(IV) and U(VI). The Tafel slope (i.e., the derivative of the electrode potential with respect to log[current]) was used to evaluate electrochemical mechanisms. High Tafel slopes (>220mV/(logunit of current) on all minerals evaluated) suggest that uranyl reduction is mediated by insulating (hydr)oxide layers that are present on the semiconducting mineral surfaces. The onset potential for uranyl reduction was determined for pyrite (>+0.1V vs. Ag/AgCl), and hematite and magnetite (between−0.02 and−0.1V vs. Ag/AgCl). The onset potential values establish a baseline kinetic parameter that can be used to evaluate how solution conditions (e.g., dissolved reductants, complexing ligands, and polarizing ions) affect the kinetics of uranyl reduction.The results of this study demonstrate the potential for using PMEs to evaluate redox potentials and mechanisms for U(VI) reduction by Fe-oxides and sulfides under more complex solution conditions as well as other environmentally-relevant mineral-analyte systems. However, it should be noted that the determination of redox kinetics using Butler–Volmer theory has limitations when applied to semiconductor mineral electrodes. Charge depletion in semiconductor surface states can affect the kinetic values obtained for redox reactions on the surface. These limitations and a discussion of the flat band potential are considered in the interpretation of U redox kinetics in this study.

Electrochemical oxidation of solid Li2O2 in non-aqueous electrolyte using peroxide complexing additives for lithium–air batteries

Using the anion receptor tris(penftafluorophenyl) borane as an additive to non-aqueous electrolytes, the solubility of solid Li2O2 can be dramatically increased through the Lewis acid–base interaction between boron and peroxide. The complexed boron-peroxide ions can be electrochemically oxidized with much better kinetics than the oxidation of solid Li2O2 on a carbon powder microelectrode. This discovery could lead to a new avenue for the development of high capacity, high rate, rechargeable, Li–Air batteries.

Electroanalysis in micro- and nano-scales

The tremendous advances in nanoscience and nanoengineering have sparked the accelerated development of electroanalysis towards highly sensitive and specific detections. In particular, novel nanomaterials with unique physical and chemical properties have been synthesized for electroanalysis in micro- and nano-scales to qualitatively and/or quantitatively examine the changes of compositions and structures for scientific insights and practical applications. In this review, we mainly survey our recent works on electroanalysis in micro- and nano-scales, which covers micro- and nano-fabrication techniques, microelectrodes and powder microelectrodes, direct electrochemistry of proteins, electrochemical analysis of living cells, and electrochemical immunoassay, with emphasis on the performance improvement by engineered functional nanomaterials. These electroanalytical methods have demonstrated their ability and reliability to study fundamental science and provide practical applications. It is our intention that this review could attract increasing interest to explore new electroanalytical methods in micro- and nano-scales and further develop broad important applications for critical chemical/biological sciences.

Synthesis and electrochemical characteristics of NASICON-structured LiSn2(PO4)3 anode material for lithium-ion batteries

Li3PO4, with a Li ion conductivity of 8.62 × 10−8 S cm−1, is considered as a matrix material in this study to improve the cyclability of tin anodes. Toward this goal, a well-crystallized NASICON-structured LiSn2(PO4)3 using nano-SnO2 as a precursor has been prepared at 900 °C using a solid-state reaction. Compared to SnO2, this material exhibits better cycling performance, with a capacity of 320 mAh g−1 after 50 cycles. Additionally, the insertion/extraction mechanism of LiSn2(PO4)3 is investigated through ex-situ X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) measurements. The apparent diffusion coefficient (DLi) is studied using cyclic voltammetry (CV) experiments employing a powder microelectrode. In addition, differential scanning calorimetry (DSC) measurements are employed to investigate the thermal stability of LiSn2(PO4)3.

Microstructures and Properties of WC/Natural Zeolite Composites

WC/zeolite nanocomposites were prepared by ball-milling a mixture of ammonia metatungsten(AMT) and zeolite and then reducing the ball-milled precursor under mixed CH4/H2 atmosphere at 900℃.The effects of carbonization time on the microstructure,phase composition,and WC distribution of as-prepared nanocomposites were investigated by X-ray diffraction(XRD),high resolution transmission electron microscope(HRTEM),and scanning transmission electron microscope(STEM).The electrocatalytic activities in alkaline solutions of these nanocomposites were evaluated in a three-electrode system by a powder microelectrode method.Due to the porous structure and high surface area of zeolite,the ball-milled AMT/zeolite precursor could be rapidly reduced in a mixed CH4/H2 atmosphere and then formed WC/zeolite nanocomposites.It was found that the active formed phase mainly composed of W2C and WC which were highly dispersed and extremely small-sized.The results also showed that the WC/zeolite nanocomposite obtained by carbonizing the ball-milled precursor for 4 h exhibited the best electrocatalytic activity,because this sample had smallest-sized and highest relative concents of tungsten carbide.

Decomposition mechanism of pentlandite during electrochemical bio-oxidation process

Electrochemical measurements were carried out to elucidate decomposition mechanism of pentlandite using modified powder microelectrode with Acidithiobacillus ferrooxidans attached or without on the mineral powder surface. Cyclic voltammetry (CV) results show that at a low potential of about −0.2 V (vs SCE), the pentlandite was transformed to an intermediated phase like Fe4.5-yNi4.5-xS8-z when Fe and Ni ions were evacuated from mineral lattice; when the potential was changed from −0.2 V to 0.2 V, the unstable violarite (Fe3Ni3S4) and FeNi2S4 were formed which was accompanied by element sulfur formed on the mineral surface; when the potential increased over 0.2 V, the unstable intermediated phase decomposed entirely; at a higher potential of 0.7 V, the evacuated ferrous ion was oxidized to ferric ion. The presence of Acidithiobacillus ferrooxidans made the oxidation peak current increase with initial peak potential negatively moving, and the bacteria also contributed to the sulfur removing from mineral surface, which was demonstrated by the reduction characteristic at potential ranging from −0.75 to −0.5 V. Leaching experiments and electrochemical results show that the solution acidity increasing when pH<2 may impede the oxidation process slightly.

High-Rate Oxygen Reduction in Mixed Nonaqueous Electrolyte Containing Acetonitrile

A mixed nonaqueous electrolyte that contains acetonitrile and propylene carbonate (PC) was found to be suitable for a Li-O(2) battery with a metallic Li anode. Both the concentration and diffusion coefficient for the dissolved O(2) are significantly higher in the mixed electrolyte than those in the pure PC electrolyte. A powder microelectrode was used to investigate the O(2) solubility and diffusion coefficient. A 10 mA cm(-2) discharge rate on a gas-diffusion electrode is demonstrated by using the mixed electrolyte in a Li-O(2) cell.

Ferrous Ion Oxidation Behavior on a Bacteria Attached Power Micro Electrode

In this paper a special At.f (Acidthiobacillus ferrooxidans) modified carbon powder microelectrode was prepared, and a series of electrochemical measurements were conducted to study ferrous ion Fe2 + oxidation mechanism on the electrode. CV (cyclic voltammetry) studies show that this oxidation reaction is reversible when c (Fe2 +) is lower than 0.16mol/L. At higher speed scanning, the currents through the electrode include two parts of the micro disc current and the thin layer current, while at lower speed scanning of steady state process the current provided by thin layer could be ignored, the whole oxidation reaction was controlled by diffusion process, and the calculated diffusion coefficient D0 is about 6. 25 ×10 - 6cm2.s – 1. The transient potential stair step studies also have been conducted. The results are consistence with steady state ones although thin layer currents should be taken in to account.

Synthesis of Ultrafine Mesoporous Tungsten Carbide by High-energy Ball Milling and Its Electrocatalytic Activity for Methanol Oxidation

Ultrafine mesoporous tungsten carbide (WC) was prepared from as-synthesized mesoporous WC using high-energy ball milling treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption-desorption techniques were used to characterize the samples. Brunauer-Emmett-Teller (BET) surface areas of WC samples increased with the increasing ball milling time and kept constant at 10–11 m2·g−1 for over 9 h. The electrocatalytic properties of methanol electro-oxidation at WC powder microelectrodes were investigated by cyclic voltammetry, chronoamperometry, and quasi-steady-state polarization techniques. The results reveal that ball-milled WC exhibits higher activity for methanol electro-oxidation than as-synthesized mesoporous WC. The suitability of ball-milled WC for methanol electro-oxidation is better than platinum (Pt) micro-disk, although the current peak is not as high as the Pt micro-disk. Moreover, increasing the methanol concentration and reaction temperature promotes methanol electro-oxidation on ultrafine mesoporous WC.

Direct Evaluation of O2-Reduction-Based H2O2 Generation at Pt/C-Containing Powder Microelectrode Using Scanning Electrochemical Microscopy

The percentage of H2O2 during oxygen reduction reaction (ORR) at practical powder electrocatalysts have been evaluated using a scanning electrochemical microscopy (SECM). We employed a porous microelectrode which contains the electrocatalysts of Pt/C, Pt-Co/C, and Pt-Ru/C as the oxygen reduction electrode of the SECM. And the Pt microelectrode was used as the H2O2 detector. First, the H2O2 generation amount at Pt/Cs has been measured by changing the Pt loading amount. A higher Pt loading Pt/C possesses a higher ORR activity and generates a larger amount of H2O2. However, the percentage of H2O2 generated with respect to the ORR is the same irrespective of the Pt loading amount. Next, H2O2 generations have been drastically suppressed at the Pt-Co/C and Pt-Ru/C in the potential ranges of practical fuel cell cathode and anode, respectively. This explains that the used Pt-Co/C is effective when used as a cathode; and, the anode Pt-Ru/C enables to reduce the H2O2 generation even if O2 crossleak occurred in the practical PEFC.

High rate oxygen reduction in non-aqueous electrolytes with the addition of perfluorinated additives

The discharge rate capability of Li–air batteries is substantially increased by using perfluorinated compounds as oxygen carriers. The solubility of oxygen in a non-aqueous electrolyte can be significantly increased by the introduction of such compounds, which leads to the increase in the diffusion-limited current of oxygen reduction on the gas diffusion electrode in a Li–air battery. The perfluorinated compound is found to be stable within the electrochemical window of the electrolyte. A powder microelectrode and a rotating disk electrode were used to study the gas diffusion-limited current together with a rotating disk electrode. A 5 mA cm−2 discharge rate is demonstrated in a lab Li–O2cell.

A Comprehensive Study of Effects of Carbon Coating on Li4Ti5O12 Anode Material for Lithium-Ion Batteries

Carbon coating is an important technique used to improve the electrochemical performance of many electrode materials in lithium-ion batteries. In the present work, was selected as the probe material and a series of carbon-coated samples was prepared to discuss the effects of graphitization and thickness of the coated carbon layer on its rate capability for lithium-ion intercalation. The apparent diffusion coefficient and rate capability were studied by cyclic voltammetry experiments using a powder microelectrode. The results show that a higher carbon coating temperature could promote the graphitization of the surface carbon, thus enhancing the electronic conductivity of the electrodes while hindering the lithium-ion diffusion. The thickness of carbon layer is demonstrated to have a major influence on ion diffusion while slightly affecting the electronic conductivity. Considering both the electronic conductivity and ion diffusion in the carbon layer, an optimal condition for carbon coating to achieve a good rate capability is determined to be performed at with a thickness of ca. .