Electrochemical Cell Potentials Research Articles

Critical evaluation of CuSO4-H2O system up to solubility limit, from eutectic point to 373.15 K

• CuSO 4 -H 2 O system is modelled from freezing point to boiling point. • The behaviour of the electrolyte solution is described by Pitzer model. • Several parameter sets with different temperature dependency were evaluated. • Only four parameters with eight temperature dependent terms are required. • Model is tested against independent vapour pressure and electrochemical cell data. The solubility of copper(II) sulfate in water as a function of temperature and ionic strength is successfully modelled using Pitzer formalism. Four types of experimental data from the literature were implemented for the assessment, namely activity and osmotic coefficients, freezing point depression and solubility of CuSO 4 in water. This work proved that four Pitzer parameters with eight terms are sufficient to create a consistent thermodynamic model of the CuSO 4 -H 2 O system up to 5 mol/kg-H 2 O of CuSO 4 , from the eutectic point to 373.15 K. The whole optimization study included a critical deviation analysis to sort out less reliable experimental data. To verify the results of this work, comparisons were carried out with experimental data which were not included in the assessment, i.e. the vapour pressure of the saturated solution, thermodynamic values of the cell reaction as well as electrochemical cell potential (EMF).

Design and optimization of electrochemical cell potential for hydrogen gas production

Abstract This study deals with the optimization of best working conditions in molten melt for the production of hydrogen (H2) gas. Limited research has been carried out on how electrochemical process occurs through steam splitting via molten hydroxide. 54 combinations of cathode, anode, temperature and voltage have been investigated for the optimization of best working conditions with molten hydroxide for hydrogen gas production. All these electrochemical investigations were carried out at 225 to 300°C temperature and 1.5 to 2.5 V applied voltage values. The current efficiency of 90.5, 80.0 and 68.6% has been achieved using stainless steel anodic cell with nickel, stainless steel and platinum working cathode respectively. For nickel cathode, an increase in the current directly affected the hydrogen gas flow rate at cathode. It can be hypothesized from the noted results that increase in current is directly proportional to operating temperature and applied voltage. Higher values were noted when the applied voltages increased from 1.5 to 2.5 V at 300°C, the flow rate of hydrogen gas increased from 1.5 to 11.3 cm3 min−1, 1.0 to 13 cm3 min−1 in case of electrolysis @ stainless steel and @ graphite anode respectively. It is observed that the current efficiency of stainless steel anodic cell was higher than the graphite anodic cell. Therefore, steam splitting with the help of molten salts has shown an encouraging alternate to current methodology for H2 fuel production.

Ultrasmall Metal Nanoclusters As Bifuntional Electrocatalysts for Overall Water Splitting

Splitting water into hydrogen and oxygen is one of the key processes for renewable energy production. Electrocatalytic water splitting composed of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is considered to be a viable method due to its high efficiency and strong connectivity to energy-related electrodevices. Consequently, development of water splitting electrocatalysts is emerging field spanning from traditional Pt group metals to non-precious metal based catalysts. Ligand-protected metal nanoclusters could be one of the candidates because of their uniformity, stability and distinctive catalytic activity. Here we report electrocatalytic activity of atomically precise Mx(SR)y (M = metal, SR = thiolate) nanoclusters for water splitting reactions. HER activities were investigated by linear sweep voltammetry (LSV) and constant potential electrolysis (CPE). Metal nanoclusters exhibited distinctive HER activity with low Tafel slopes for universal pH range (pH 1 – 14). The activity could be modified by controlling the number of metals and ligands with atomic precision. OER activities of metal nanoclusters were examined by LSV and showed higher catalytic current than commercial IrO2 in alkaline media. Furthermore, metal nanoclusters were used as bifunctional water splitting (HER/OER) catalysts that showed high current density (100 mA cm-2) at a low electrochemical cell potential (2.5 V).

Fine Tuning the Performance of Multiorbital Radical Conductors by Substituent Effects.

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers Ueff, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in Ecell, the electrochemical cell potential for 2 with R = F, H and NO2. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO2 have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior, with χTIP = 6 × 10-4 emu mol-1, but its charge transport behavior, with σRT near 0.04 S cm-1 and Eact = 0.05 eV, is more consistent with a Mott insulating ground state. High pressure crystallographic measurements confirm uniform compression of the π-stacked architecture with no phase change apparent up to 8 GPa. High pressure conductivity measurements indicate that the charge gap between the Mott insulator and metallic states can be closed near 6 GPa. These results are discussed in the light of DFT band structure calculations.

Electrochemistry of Glycerol Oxidaton at Low Index Crystal Planes of Platinum

Introduction: The energy crisis related to the consumption of fossil fuels has raised concern in many countries, thus the use of biofuels is an alternative to those fuels. In Brazil, in recent years has increased biodiesel production in line with current legislation, introducing biofuels in their energy mix. Biodiesel is a renewable fuel produced from transesterification of a vegetable or animal’s oil and an alcohol (methanol or ethanol), the process produces as a co-product glycerol. Glycerol has many industrial applications in industries: cosmetics, pharmaceutical, food and others, but the large amount produced is not absorbed by the industry, which has lost its market value and in a few years could pose damage to the environment. The glycerol electrooxidation in platinum low index single crystal electrodes in alkaline medium is investigated in this work using techniques such as cyclic voltammetry and chronoamperometry. Experimental: Glycerol electrooxidation experiments were performed in a 0.1M NaOH and 0.1M glycerol solution. Electrochemical cell potential were controlled with a potentiostat (autolab). The platinum single crystals work electrodes ((111 ) , (100 ) and (110)) used were prepared by Clavillier’s method [1]. Previous to each electrochemistry experiment the work electrode was cleaned by flame annealing in an oxygen gas flame, cooled in H2/Ar atmosphere and protected with water in equilibrium with this gas mixture to prevent contamination before immersion in the electrochemical cell. Results and Discussion: Figure 1 displays the cyclic voltammograms recorded at 50 mV/s for Pt(hkl) in 0.1 M NaOH. The dotted lines are blank voltammograms profiles of Pt(111), Pt(100) and Pt(110) electrodes in the absence of glycerol in the support electrolyte which are in good agreement with previously published results. In presence of glycerol in the electrolyte, the electrochemical responses show the strongly reaction dependence to the electrode surface arrangement. As already documented for other small alcohols (methanol, ethanol, etc. ), at low potentials the surface is completed blocked by the adsorbates which are only oxidized at higher potentials ( < 0.45 V). The electrooxidation glycerol voltammograms expose the different reaction mechanisms for the three electrodes. These are not surprising results since the presence of alcohol group in each carbon in the glycerol molecule enables many possibilities of adsorbate species following by their oxidation: producing soluble products or adsorbed intermediates. It is interesting to note the hysteresis displayed for Pt(111) and Pt(110). For both surfaces, the positive and negative scans differ only by current density. The same was observed for the electrooxidation of ethanol in Pt(111) in alkaline media[2]. In the other hand, the Pt(100) shows superior electroactivity in the negative scan, characterized by a sharp peak centered at 0.65V. Although not shown in Figure 1, we investigated the successive cycles for the glycerol oxidation. We observed a continuous surface poisoning cycle after cycle evidence by the decrease of density current. This is evidence that some of the adsorbates formed during the cycles remains at the surface even at higher potentials. Some works [2,3] observed the presence of CO during the glycerol electrooxidation but there is a lack of information in relation of extensive electrolysis. Conclusion: The glycerol electrooxidation is a field of research still to be explored since the reaction complexity is evidenced in this work. The main observation is the reaction surface sensitivity which can lead to produce different kind of products for different surfaces. It is particularly important if we want to use the glycerol as feedstock to produce more valuable products.

Bubble Over-Potential During Two-Phase Alkaline Water Electrolysis

During two-phase water electrolysis production of bubbles at one or both electrodes is observed. This leads to a change in the electrolyser electrical and hydrodynamic properties. When gravity is present, the production of bubbles at the electrodes induces a macro-convection in the electrolyser. This leads to a local distribution of the bubbles determining the local gas void fraction and current density at the electrodes. The absence of gravity eliminates the natural convection and buoyancy forces and consequently the frictional forces. It is generally difficult to estimate the quantitative influence of each of these single phenomena due to strong coupling. In the present work, alkaline water electrolysis is performed. The formation of gas bubbles at the anode is observed using four cameras. The aim of this study is to establish the quantitative evolution laws for the electrochemical cell potential, the bubble diameter and population density during alkaline (NaOH) two-phase electrolysis in function of the two explored inputs current density and gravity.

Hexathiophenalenylium Cations: Syntheses, Structures, and Redox Chemistry

Preparations of two hexathiophenalenylium compounds as stable salts from the reaction of 3,4,6,7-tetrathio-9-hydroxyphenalenone with Lawesson's reagent have been reported. The presence of three disulfide linkages on the periphery of the core phenalenyl unit is confirmed by X-ray crystallographic characterizations. Electrochemical cell potentials are lower than those of related dithio- and tetrathio-bridged phenalenyl radicals, and the hexathiophenalenyl radical shows a strong electron paramagnetic resonance (EPR) signal in the solid state.

Semiquinone-Bridged Bisdithiazolyl Radicals as Neutral Radical Conductors

Semiquinone-bridged bisdithiazolyls 3 represent a new class of resonance-stabilized neutral radical for use in the design of single-component conductive materials. As such, they display electrochemical cell potentials lower than those of related pyridine-bridged bisdithiazolyls, a finding which heralds a reduced on-site Coulomb repulsion U. Crystallographic characterization of the chloro-substituted derivative 3a and its acetonitrile solvate 3a·MeCN, both of which crystallize in the polar orthorhombic space group Pna2(1), revealed the importance of intermolecular oxygen-to-sulfur (CO···SN) interactions in generating rigid, tightly packed radical π-stacks, including the structural motif found for 3a·MeCN in which radicals in neighboring π-stacks are locked into slipped-ribbon-like arrays. This architecture gives rise to strong intra- and interstack overlap and hence a large electronic bandwidth W. Variable-temperature conductivity measurements on 3a and 3a·MeCN indicated high values of σ(300 K) (>10(-3) S cm(-1)) with correspondingly low thermal activation energies E(act), reaching 0.11 eV in the case of 3a·MeCN. Overall, the strong performance of these materials as f = ½ conductors is attributed to a combination of low U and large W. Variable-temperature magnetic susceptibility measurements were performed on both 3a and 3a·MeCN. The unsolvated material 3a orders as a spin-canted antiferromagnet at 8 K, with a canting angle φ = 0.14° and a coercive field H(c) = 80 Oe at 2 K.

The Variation of Electrochemical Cell Potentials with Temperature

Electrochemical cell potentials have no simple relationship with temperature but depend on the interplay between the sign and magnitude of the isothermal temperature coefficient, dE°/dT, and on the magnitude of the reaction quotient, Q. The variations in possible responses of standard and non-standard cell potentials to changes in the temperature, topics that are not discussed in any current textbook, are described.

Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

The preparation of two bisthiadiazinyls (7, R1 = Me, Et; R2 = Cl, R3 = Ph), the first examples of a new class of resonance-stabilized heterocyclic thiazyl radical, are reported. Both radicals have been characterized in solution by EPR spectroscopy and cyclic voltammetry, which confirm highly delocalized spin distributions and low electrochemical cell potentials, features which augur well for the use of these materials as building blocks for neutral radical conductors. In the solid state, the radicals are undimerized, crystallizing in slipped pi-stack arrays which ensure the availability of electrons as potential charge carriers. However, despite these favorable electrochemical and structural properties, both materials exhibit low conductivities, with sigma(300K) < 10-7 S cm-1, a result which can be rationalized in terms of their EHT band electronic structures, which indicate that intermolecular interactions lateral to the pi-stacks are limited. The materials are thus very 1-D with low bandwidths, so that a Mott insulating state prevails. When R1 = Me, the intermolecular overlap along the pi-stacks is weak and the material is essentially paramagnetic. When R1 = Et, intermolecular pi-overlap is greater and variable-temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, the behavior of which has been modeled in terms of a molecular-field modified 1-D Heisenberg chain of S = 1/2 centers. Broken-symmetry DFT methods have been used to estimate the magnitude of individual exchange interactions within both structures.

Prototypal dithiazolodithiazolyl radicals: synthesis, structures, and transport properties.

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

Electrocatalytic Enhancement of Methanol Oxidation at Pt−WOx Nanophase Electrodes and In-Situ Observation of Hydrogen Spillover Using Electrochromism

A Pt−WOx nanophase electrode showed a considerably more enhanced electrocatalytic activity than Pt itself for methanol oxidation and exactly the reverse change in optical signal intensity with respect to electrochemical cell potential, compared with an electrochromic WOx electrode. This provides evidence for hydrogen spillover from Pt to WOx. Consequently, it was possible to directly observe the in-situ hydrogen transfer from platinum to tungsten oxide in a fuel cell electrode using electrochromism. The transfer of hydrogen ions, produced on the platinum during the electrooxidation of methanol, to the tungsten oxide ensures that the active reaction sites on the platinum remain clean, thus enhancing the electrooxidation current density.

Resonance-stabilized 1,2,3-dithiazolo-1,2,3-dithiazolyls as neutral pi-radical conductors.

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.

Characterization of Charge Storage in Redox-Active Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of thiol-derivatized alkyl ferrocenes and Zn tetraarylporphyrins on a gold surface have been characterized under open circuit conditions using fluorescence imaging microscopy and a varietyof electrochemical methods. The fluorescence of the neutral form of the Zn tetraarylporphyrin SAM and the electrode cell potential are monitored simultaneously after oxidation of the SAM and subsequent establishment of an open circuit condition. These experiments capitalize on the fact that the neutral Zn tetraarylporphyrin SAM is fluorescent whereas emission from the oxidized Zn tetraarylporphyrin SAM is quenched. Accordingly, the fluorescence recovery after disconnection from an oxidizing potential provides an independent measure of the change in oxidation state of the porphyrin SAM as a function of time after the cell is open-circuited. This measurement demonstrates that the porphyrin SAM remains oxidized for an extended period (hundreds of seconds) even though the electrochemical cell potential decays rapidly to the open circuit potential (OCP). Cyclic voltammetry was used to confirm the electrochemical properties of the redox SAMs and to determine the surface coverage of the redox-active molecules. A new method, designated open circuit potential amperometry, was used to read the charge of the oxidized SAM at the OCP after charging currents have decayed away. Additionally, open circuit potential voltammetry yields a sigmoidal response with a characteristic half-wave potential identical to that observed with cyclic voltammetry, indicative of a Nernstian process. Collectively, these studies show that the electrochemical cell discharges to the OCP quickly, that the SAM remains oxidized under these conditions, and that these methods can be used to quantitatively determine the amount of stored charge in the SAM.

Common student misconceptions in electrochemistry: Galvanic, electrolytic, and concentration cells

This study replicates, with additions, research done by Garnett and Treagust. Garnett and Treagust's interview questions for galvanic and electrolytic cells were used with modifications; concentration cell questions were asked in a similar manner. These questions were administered to 16 introductory college chemistry students after electrochemistry instruction. Student misconceptions most commonly encountered included notions that electrons flow through the salt bridge and electrolyte solutions to complete the circuit, plus and minus signs assigned to the electrodes represent net electronic charges, and water is unreactive in the electrolysis of aqueous solutions. New misconceptions identified included notions that half-cell potentials are absolute and can be used to predict the spontaneity of individual half-cells, and electrochemical cell potentials are independent of ion concentrations. Most students demonstrating misconceptions were still able to calculate cell potentials correctly, which is consistent with research suggesting that students capable of solving quantitative examination problems often lack an understanding of the underlying concepts. Probable origins of these student misconceptions were attributed to students being unaware of the relative nature of electrochemical potentials and chemistry textbooks making misleading and incorrect statements. A minor technical flaw in the Garnett and Treagust study is also addressed. J Res Sci Teach 34: 377–398, 1997.

Lithium Insertion Characteristics of CuNb 2O 6

Chemical and electrochemical lithium intercalatı́on reactions of CuNb 2O 6 with a columbite structure were examined. The chemical lithiation by n-butyllithium occurred accompanied by the formation of a new phase, Li x CuNb 2O 6 (0 ≦ x ≦ 1.9), with an apparent crystal lattice. The a axis of the new phase is a little longer (0.6 Å) than that of the parent phase. The electrochemical cell potential of Li/CuNb 2O 6 almost attained a plateau at 2.6 V against Li/Li + in the range from x = 0.1 to x = 1.0 in Li x CuNb 2O 6, corresponding to the reduction from Cu 2+ to Cu +. The reduction behavior in the range from x = 1.0 to x = 1.9 suggested the reduction from Nb 5+ to Nb 4+.