Showing papers on "Cyclic voltammetry published in 2016"

In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of CuIII Oxides as Catalytically Active Species

Abstract: Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu2O, Cu(OH)2, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu(OH)2 during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a CuIII oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCuIIIO2 standard, we suggest that the chemical composition of the CuIII oxide is CuO2–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurement...

Methanol Electro-Oxidation on the Pt Surface: Revisiting the Cyclic Voltammetry Interpretation

Abstract: Methanol is a promising fuel for direct methanol fuel cells in portable devices. A deeper understanding of its electro-oxidation is needed for evaluating electrocatalytic performance and catalyst design. Here we provide an in-depth investigation of the cyclic voltammetry (CV) of methanol electro-oxidation. The oxidation peak in backward scan is shown to be unrelated to residual intermediate oxidation. The origin of the second oxidation peak (If2) is expected to the methanol oxidation on Pt–Ox. Electrochemical impedance spectroscopy coupled with CV reveals the origin of CV hysteresis to be a shift in the rate-determining step, from methanol dehydration to OH adsorption by water dissociation, induced by a change in Pt surface coverage with oxygenated species. The peak ratio between forward oxidation peak current (If) and backward oxidation peak current (Ib), which is If/Ib, is not related to the degree of CO tolerance but to the degree of oxophilicity indeed.

Functional Groups and Pore Size Distribution Do Matter to Hierarchically Porous Carbons as High-Rate-Performance Supercapacitors

Abstract: A series of nitrogen and oxygen enriched porous carbons are prepared from poly-N-phenylethanolamine (PNPEA) and polyaniline (PANI) conducting polymers through pyrolysis, chemical activation, and oxidation processes. Ar or N2-adsorption, Fourier transform infrared, and X-ray photoelectron spectroscopy are used to characterize the surface areas, pore volumes, surface chemical compositions, and oxygen and nitrogen content. Mikhail and Brunauer micropore analytical method (MP method) is successfully used to analyze the micropore size distribution of the samples. The electrochemical behavior of the samples is studied in two- and three-electrode cells. The contribution of pseudocapacitance is confirmed by cyclic voltammetry and galvanostatic tests performed in acidic (H2SO4) and basic (KOH) media. The potential drop and the equivalent series resistance value certify that the samples with wide micropore size distribution possess low interface resistances. A sample with a Brunauer–Emmett–Teller (BET) surface area...

Enhanced Pseudocapacitive Performance of α-MnO2 by Cation Preinsertion.

Abstract: Although the theoretical capacitance of MnO2 is 1370 F g–1 based on the Mn3+/Mn4+ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K+-inserted α-MnO2 (or KxMnO2) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of KxMnO2 is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K+ ions can be replaced by Na+ ions, which determines the pseudocapacitance of the electrode. The K+ or Na+ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0–1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K+-free α-MnO2 nanorod ar...

Operando Raman Spectroscopy of Amorphous Molybdenum Sulfide (MoSx) during the Electrochemical Hydrogen Evolution Reaction: Identification of Sulfur Atoms as Catalytically Active Sites for H+ Reduction

Abstract: Amorphous molybdenum sulfide (MoSx) is currently being developed as an economically viable and efficient catalyst for the electrochemical hydrogen evolution reaction (HER). An important yet unsolved problem in this ongoing effort is the identification of its catalytically active sites for proton reduction. In this work, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to investigate the catalytically active sites and structural evolution of MoSx films during HER in 1 M HClO4 electrolyte. Transformation of anodically deposited MoSx (x ≈ 3) to a structure with MoS2 composition during the cathodic sweep of a CV was demonstrated using XPS and operando Raman spectroscopy. Interestingly, a Raman peak at 2530 cm–1 was recorded at potentials relevant to H2 evolution, which we ascribed to the S–H stretching vibration of MoSx–H moieties. This assignment was corroborated by H/D isotope exchange experiments. Mo–H (or Mo–D) stretching vibrations were not observed, which...

Effects of Intentionally Incorporated Metal Cations on the Oxygen Evolution Electrocatalytic Activity of Nickel (Oxy)hydroxide in Alkaline Media

Abstract: Fe-doped Ni (oxy)hydroxide, Ni(Fe)OxHy, is one of the most-active oxygen-evolution-reaction (OER) catalysts in alkaline conditions, while Fe-free NiOxHy is a poor OER catalyst. One approach to better understand the role of Fe, and enable the design of catalysts with higher activities, is to find other cations that behave similarly and compare the common chemical features between them. Here we evaluate the effects of La, Mn, Ce, and Ti incorporation on the OER activity and redox behavior of NiOxHy in rigorously Fe-free alkaline solution using cyclic voltammetry and electrochemical quartz-crystal microgravimetry. We use X-ray photoelectron spectroscopy and time-of-flight secondary-ion-mass spectrometry to confirm that measurements are free from relevant levels of trace Fe contamination. We find that only Ce leads to increased activity in NiOxHy (about a factor of 10 enhancement), but this effect is transient, likely due to phase separation. We further find no clear correlation between activity and the nomin...

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

Abstract: Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cell's low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

An electrochemical sensor based on molecularly imprinted polypyrrole/graphene quantum dots composite for detection of bisphenol A in water samples

Abstract: Bisphenol A (BPA) is an important endocrine disrupter in environments, for which sensitive and selective detection methods are highly necessary to carry out its recognition and quantification. Here a novel electrochemical sensor was developed based on molecularly imprinted polypyrrole/graphene quantum dots (MIPPy/GQDs) composite for the detection of bisphenol A (BPA) in water samples. A MIPPy/GQDs composite layer was prepared by the electropolymerization of pyrrole on a glassy carbon electrode with BPA as a template. The MIPPy/GQDs composite could specifically recognize BPA in aqueous solutions, which resulted in the decrease of peak currents of K 3 [Fe(CN) 6 ] at the MIPPy/GQDs) modified electrode in cyclic voltammetry (CV) and differential pulse voltammetry (DPV). There was a linear relationship between BPA concentrations ranging from 0.1 μM to 50 μM and the response value ( ΔI DPV ) in DPV, with a limit of detection of 0.04 μM (S/N = 3). The sensor was applied for the detection of BPA in tap and sea water samples, with recoveries of 94.5% and 93.7%, respectively. The proposed method provides a powerful tool for rapid and sensitive detection of BPA in environmental samples.

Mechanistic Insights into the Selective Electroreduction of Carbon Dioxide to Ethylene on Cu2O-Derived Copper Catalysts

Abstract: In this work, we made a comprehensive investigation to unravel the underlying causes for the selectivity of CO2 electroreduction toward ethylene on Cu2O-derived Cu catalysts. Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, chronopotentiometry, online gas chromatography, nuclear magnetic resonance spectroscopy, and numerical simulations of local pH were used toward this end. Ten Cu2O-derived Cu films of different thicknesses and morphologies were prepared and extensively characterized. Aqueous 0.1 M KHCO3 was used as the electrolyte. We report here, for the first time, a remarkably strong correlation between the statistically relevant crystallite sizes of the Cu2O-derived Cu particles and selective CO2 electroreduction to C2H4. Specifically, as the crystallite size of the particles decreased from 41 to 18 nm, the Faradaic efficiency (FE) of C2H4 formation increased from 10 to 43%. Using cyclic voltammetry, samples with smaller particle crystallite sizes were found to...

Dechlorination of Trichloroacetic Acid Using a Noble Metal-Free Graphene-Cu Foam Electrode via Direct Cathodic Reduction and Atomic H.

Abstract: A three-dimensional graphene–copper (3D GR–Cu) foam electrode prepared by chemical vapor deposition method exhibited superior electrocatalytic activity toward the dechlorination of trichloroacetic acid (TCAA) as compared to the Cu foam electrode. The cyclic voltammetry and electrochemical impedance spectra analysis confirmed that GR accelerated the electron transfer from the cathode surface to TCAA. With the applied cathode potential of −1.2 V (vs SCE), 95.3% of TCAA (500 μg/L) was removed within 20 min at pH 6.8. TCAA dechlorination at the Cu foam electrode was enhanced at acidic pH, while a slight pH effect was observed at the GR–Cu foam electrode with a significant inhibition for Cu leaching. The electrocatalytic dechlorination of TCAA was accomplished via a combined stepwise and concerted pathway on both electrodes, whereas the concerted pathway was efficiently promoted on the GR–Cu foam electrode. The direct reduction by electrons was responsible for TCAA dechlorination at Cu foam electrode, while at...

Sensitive voltammetric sensor based on polyoxometalate/reduced graphene oxide nanomaterial: Application to the simultaneous determination of l-tyrosine and l-tryptophan

Abstract: In this report, a novel voltammetric sensor based on polyoxometalate (H 3 PW 12 O 40 , POM) functionalized reduced graphene oxide (rGO) modified glassy carbon electrode (GCE) was presented for simultaneous determination of l -tyrosine ( l -Tyr) and l -tryptophan ( l -Trp). The nanocomposites were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The electrochemical determination of l -Tyr and l -Trp has been studied using square wave voltammetry (SWV) on GCE modified with POM-rGO composite (POM-rGO/GCE). The developed method was also applied successfully for the simultaneous determination of l -Tyr and l -Trp in spiked serum sample and the linearity range of l -Tyr and l -Trp was 1.0 × 10 −11 − 1.0 × 10 −9 M with the detection limit of 2.0 × 10 −12 M.

High performance PEDOT/lignin biopolymer composites for electrochemical supercapacitors

Abstract: Developing sustainable organic electrode materials for energy storage applications is an urgent task. We present a promising candidate based on the use of lignin, the second most abundant biopolymer in nature. This polymer is combined with a conducting polymer, where lignin as a polyanion can behave both as a dopant and surfactant. The synthesis of PEDOT/Lig biocomposites by both oxidative chemical and electrochemical polymerization of EDOT in the presence of lignin sulfonate is presented. The characterization of PEDOT/Lig was performed by UV-Vis-NIR spectroscopy, FTIR infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, cyclic voltammetry and galvanostatic charge–discharge. PEDOT doped with lignin doubles the specific capacitance (170.4 F g−1) compared to reference PEDOT electrodes (80.4 F g−1). The enhanced energy storage performance is a consequence of the additional pseudocapacitance generated by the quinone moieties in lignin, which give rise to faradaic reactions. Furthermore PEDOT/Lig is a highly stable biocomposite, retaining about 83% of its electroactivity after 1000 charge/discharge cycles. These results illustrate that the redox doping strategy is a facile and straightforward approach to improve the electroactive performance of PEDOT.

Ultrahigh capacitance of amorphous nickel phosphate for asymmetric supercapacitor applications

Abstract: This article presents the effect of different calcination temperatures on the structural, morphological and capacitance of nickel phosphate (Ni3(PO4)2) as an electrode material for supercapacitor applications. Ni3(PO4)2 was synthesized via a sonochemical method followed by calcination at different temperatures (300, 600 and 900 °C, denoted as N300, N600 and N900, respectively). The phase structure and purity of Ni3(PO4)2 were confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. The surface morphologies showed that the particle size increased with increasing the calcination temperatures. The electrochemical performance of N300, N600 and N900 were investigated using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in a 1 M KOH electrolyte. It was found that N300 exhibited the maximum specific capacity of 620 C g−1 at 0.4 A g−1, which was significantly higher than N600 (46 C g−1) and N900 (14 C g−1). Here, the enhanced electrochemical performance was obtained due to the amorphous structure and augmentation of the redox active sites of the N300 particles. Additionally, the fabricated N300//activated carbon based asymmetric supercapacitor can be cycled reversibly at a cell voltage of 1.45 V. The device exhibited an energy density of 76 W h kg−1 and a power density of 599 W kg−1 with life cycles of 88.5% capacitance retention after 3000 cycles.

WO3 nanoflowers with excellent pseudo-capacitive performance and the capacitance contribution analysis

Abstract: Pseudocapacitors have attracted more and more attention during the recent years because of their high capacitance and large energy density. As a traditional pseudocapacitive material, WO3 possesses high theoretical capacitance and good conductivity among various transition metal oxides. Herein, a nanoflower-like WO3 (NFL-WO3) electrode with perfect electrochemical performance is synthesized through a facile and effective electrodeposition method. The NFL-WO3 electrode after optimization exhibits a very high areal specific capacitance (Ca) of 658 mF cm−2, which is the highest one in pure WO3 materials to the best of our knowledge and a large gravimetric specific capacitance (Cg) of 196 F g−1 at a scan rate of 10 mV s−1. Our result demonstrates the excellent balance between Ca and Cg compared with other reported WO3 based materials. After experiencing 5000 cycles, 85% of its capacitance can still be retained. Based on the full analysis of cyclic voltammetry curves and Nyquist plots, the pseudocapacitance should be the dominate contribution to the total capacitance. In addition, it can be assembled into an asymmetric supercapacitor for powering small electronics (light-emitting diode sets and mobile phone). All of the above exhibit the potential application of the NLF-WO3 electrode in energy storage devices.