A numerical Kramers-Kronig transform for the calculation of dielectric relaxation losses free from Ohmic conduction losses

 A numerical Kramers–Kronig transform is described which allows the calculation of dielectric relaxation losses from dielectric constant data measured at a limited set of frequencies differing by a factor of 2. Conversion formulas for both the central frequencies and for frequencies near the edges of the experimental frequency window are derived. The approach used can be extended easily to measurement frequencies with a different logarithmic spacing. Using this conversion, relaxation and dissipative, conduction losses can be separated. In this way Ohmic conduction processes and simultaneously occurring relaxation processes like dipole or space-charge relaxations can be analysed independently. The results of some simulations and of calculations on experimental data for poly(vinyl-chloride) are used to illustrate the potentials of the ɛ′ to ɛ″ conversion. Received: 17 May 1996 Accepted: 16 August 1996     

Estimate of mass-transfer rate via Nernst model with Ohmic losses

The limiting-current technique, employed e.g. in electrodiffusion flow diagnostics, is based on an oversimplified electrochemical concept, which accounts only for the convective transport of depolarizers across a diffusion layer with constant bulk and wall concentrations. However, there are additional transport resistances that cannot be neglected: Faradaic resistance at surface of working electrode, and Ohmic losses in the bulk of electrolyte solution. Effect of these additional resistances is analyzed using the Nernst model of a two-electrode cell (no reference electrode). The Faradaic resistances due to electrode kinetics are considered for a single redox couple O + ne = R according to the Butler-Volmer electrode kinetics. The effect of Ohmic losses is accounted for, considering primary current distribution for a uniformly accessible circular electrode.     

Microflow electroorganic synthesis without supporting electrolyte

Anodic methoxylation of several organic compounds has been successfully achieved in the absence of intentionally added supporting electrolyte using an electrochemical microflow system.     

Limiting currents in potentiostatic voltammetry without supporting electrolyte

Traditionally, electrochemists study solutions of electroactive salts with excess supporting electrolyte present. The imposition of a large enough potential step on a macroelectrode in such a solution results in a limiting current that obeys Cottrell's equation, declining as t^−1/2. In the present article it is demonstrated that t^−1/2 dependence is preserved even when supporting electrolyte is absent. The limiting current however is shown to depart in magnitude from the Cottrellian prediction by a factor (greater or less than unity) that depends on the charge numbers of the salt's ions and that of the electroproduct.     

Specific interactions in mixed electrolyte solutions from solubility measurements

The solubility of thallium I chloride in a wide range of dilute electrolyte solutions is interpreted using a new specific-interaction equation for activity coefficients. This equation is shown to be consistent with the solubility data, but the Guggenheim specific-interaction equation is not. The relation of interaction coefficients to ion association is discussed.     

Estimated ionicB-coefficients from NMR measurements in aqueous electrolyte solutions

¹⁷O-NMR spin-lattice relaxation timesT ₁ of D₂O molecules were measured at 5–85°C in D₂O solutions of alkali metal halides (LiClCsCl, KBr, and KI), DCl, KOD, Ph₄PCl, NaPh₄B, and tetraalkylammonium bromides (Me₄NBrAm₄NBr) in the concentration range 0.1–1.4 mol-kg^–1 TheB-coefficients of the electrolytes obtained from the concentration dependence of relaxation ratesR ₁=1/T₁ were divided into the ionicB-coefficients by three methods: (i) the assumption ofB (K⁺)=B(Cl^–), (ii) the assumption ofB(Ph₄P⁺)=B(Ph₄B^–), and (iii) the use ofB(Br^–) obtained from a series ofB(R₄NBr). It was found that Methods (ii) and (iii) resulted in an abnormal temperature dependence of theB-coefficients of alkali metal ions and a negative values of rotational correlation times _c at lower temperatures for hydroxide and halide ions. These results suggest that the methods based on the van der Waals volume are not adequate for the ionic separation of NMRB-coefficients. From the analysis using the assumption ofB(K⁺)=B(Cl^–), it was found that D₃O⁺, OD^–, and Me₄N⁺ ions are the intermediates between structure makers and breakers, and that the hydrophobicity of phenyl groups is weaker than that of alkyl groups due to the interactions between water molecules and -electrons in phenyl groups.     

Retention measurements of nanofiltration membranes with electrolyte solutions

Retention measurements with single salt solutions of CaCl₂, NaCl and Na₂SO₄ revealed that the rejection mechanism of commercial polymeric nanofiltration membranes investigated in this study may be divided into two categories:

• 1.Membranes for which Donnan exclusion seems to play an important role.
• 2.Membranes for which retention is determined by both Donnan exclusion and size effects.

In category 1 both positively and negatively charged membranes were found.Ceramic γ-Al₂O₃ ultrafiltration membranes with a pore size of 3 nm showed a same type of salt retention behavior as the positively charged polymeric membranes.The extended Nernst–Planck equation in combination with the Donnan equilibrium has been used to model the flux-retention experiments for the salt solutions. The numerical calculations resulted in a good agreement with experimental data and acceptable values for the fixed charge densities have been determined. The effective membrane thicknesses calculated were higher than those observed by scanning electron microscopy.     

Adaptive response by an electrolyte: resilience to electron losses in a dye-sensitized porous photoanode

Photovoltage and photocurrents below theoretical limits in dye-sensitized photoelectrochemical solar energy conversion systems are usually attributed to electron loss processes such as dye–electron and electrolyte–electron recombination reactions within the porous photoanode. Whether recombination is a major loss mechanism is examined here, using a multiscale reaction–diffusion computational model to evaluate system characteristics. The dye-sensitized solar cell with an I⁻/I₃⁻ redox couple is chosen as a simple, representative model system because of the extensive information available for it. Two photoanode architectures with dye excitation frequencies spanning 1–25 s⁻¹ are examined, assuming two distinct recombination mechanisms. The simulation results show that although electrolyte–electron reactions are very efficient, they do not significantly impact photoanode performance within the system as defined. This is because the solution-phase electrolyte chemistry plays a key role in mitigating electron losses through coupled reactions that produce I⁻ within the photoanode pores, thereby cycling the electrolyte species without requiring that all electrolyte reduction reactions take place at the more distantly located cathode. This is a functionally adaptive response of the chemistry that may be partly responsible for the great success of this redox couple for dye-sensitized solar cells. The simulation results provide predictions that can be tested experimentally.

Interfacial electrolyte reactions in the pores of a photoanode consume electrons. The losses are offset by compensating solution-phase reactions that generate I⁻ locally, and promote efficient dye cycling and photocurrent generation.     

Determination of the dissociation constant and limiting equivalent conductance of a weak electrolyte from conductance measurements on the weak electrolyte

A method is described for computing the dissociation constant and equivalent conductance at infinite dilution, A_o , for a weak electrolyte from as few as two conductance measurements at different concentrations. The iterative procedure applied to acetic acid data yielded a dissociation constant of 1.80 X 10-⁵ and an average value (n = 4) of A_o of 389.5 compared with an expected value of 390.7.     

Force measurements between Teflon AF and colloidal silica particles in electrolyte solutions

The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH⁻ ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl⁻ ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH⁻ ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role.     

电化学测量中的欧姆电压降补偿问题

对工作电极与参比电极之间的溶液电阻(Ru)进行准确的欧姆电压降补偿是获取可靠的电化学实验结果的前提,但测量中该如何进行补偿尚未建立规范的操作流程.本文首先探究了工作电极与Luggin毛细管末端距离对Ru的影响.随后对比了 Autolab PGSTAT 302N、CHI系列恒电位仪的交流阻抗法与CHI系列恒电位仪所测得R...     

Metals and alloys bonded on solid polymer electrolyte for electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte

Metals and alloys bonded on the solid polymer electrolyte (SPE) Nafion® 117 were studied as both the electrodes and electrolyte for the electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte. The results indicated that SPE electrodes modified with metals such as Pt, Ni, Pb, Cu and Ag by the ion exchange chemical deposition method had a more stable structure and could provide a larger electrochemical active surface area than those prepared by other methods. The composition of reducing agents and the pH value have a significant effect on the characteristics of the prepared SPE electrodes. In this study a novel method was developed to prepare a Pt+Pb/Nafion® electrode which formed a protective Pt layer on the surface of Pb/Nafion®. The results of scanning electron microscopy further confirmed that Pt+Pb/Nafion® electrodes had obvious advantages for the electrochemical reduction of benzaldehyde. The results also revealed that the current efficiencies of benzylalcohol production at various SPE electrodes decreased in the order Pt+Pb/Nafion® > Pb/ Nafion® > Ni/Nafion® > Cu/Nafion® > Ag+Cu/Nafion® > Ag/Nafion® > Pt/Nafion®.     

Paired electrosynthesis: micro-flow cell processes with and without added electrolyte

The thin layer flow cell geometry with working and auxiliary electrodes directly facing each other, allows electrosynthetic processes to be conducted in flow-through mode. At sufficiently small cell height, the two diffusion layers of working and auxiliary electrode overlap or become ‘coupled’. As a result electro-generated acids or bases are instantaneously neutralised, products from anode and cathode can interact, and, more importantly, bulk electrolysis is possible without intentionally added electrolyte. In this preliminary report, the operation of a micro-flow cell with coupled diffusion layers is demonstrated for the one electron oxidation of ferrocene and for the two electron–two proton reduction of tetraethyl ethylenetetracarboxylate dissolved in ethanol. In proof-of-principle bulk electrolysis experiments without intentionally added electrolyte, high yields of the product, tetraethyl ethanetetracarboxylate are obtained at a nickel working electrode. It is demonstrated that a sufficient concentration of electrolyte for bulk electrolysis is generated locally and in situ between working and auxiliary electrode.     

Fibrinogen conformations and charge in electrolyte solutions derived from DLS and dynamic viscosity measurements

Hydrodynamic properties of fibrinogen molecules were theoretically calculated. Their shape was approximated by the bead model, considering the presence of flexible side chains of various length and orientation relative to the main body of the molecule. Using the bead model, and the precise many-multipole method of solving the Stokes equations, the mobility coefficients for the fibrinogen molecule were calculated for arbitrary orientations of the arms whose length was varied between 12 and 18nm. Orientation averaged hydrodynamic radii and intrinsic viscosities were also calculated by considering interactions between the side arms and the core of the fibrinogen molecule. Whereas the hydrodynamic radii changed little with the interaction magnitude, the intrinsic viscosity exhibited considerable variation from 30 to 60 for attractive and repulsive interactions, respectively. These theoretical results were used for the interpretation of experimental data derived from sedimentation and diffusion coefficient measurements as well as dynamic viscosity measurements. Optimum dimensions of the fibrinogen molecule derived in this way were the following: the contour length 84.7nm, the side arm length 18nm, and the total volume 470nm(3), which gives 16% hydration (by volume). Our calculations enabled one to distinguish various conformational states of the fibrinogen molecule, especially the expanded conformation, prevailing for pH<4 and lower ionic strength, characterized by high intrinsic viscosity of 50 and the hydrodynamic radius of 10.6nm. On the other hand, for the physiological condition, that is, pH=7.4 and the ionic strength of 0.15M NaCl, the semi-collapsed conformation dominates. It is characterized by the average angle equal to <φ>=55°, intrinsic viscosity of 35, and the hydrodynamic radius of 10nm. Additionally, the interaction energy between the arms and the body of the molecule was predicted to be -4kT units, confirming that they are oppositely charged than the central nodule. Results obtained in our work confirm an essential role of the side chains responsible for a highly anisotropic charge distribution in the fibrinogen molecule. These finding can be exploited to explain anomalous adsorption of fibrinogen on various surfaces.     

Quantitative real-time measurements of DNA hybridization with alkylated nonoxidized silicon nanowires in electrolyte solution

The quantitative, real-time detection of single-stranded oligonucleotides with silicon nanowires (SiNWs) in physiologically relevant electrolyte solution is demonstrated. Debye screening of the hybridization event is circumvented by utilizing electrostatically adsorbed primary DNA on an amine-terminated NW surface. Two surface functionalization chemistries are compared: an amine-terminated siloxane monolayer on the native SiO2 surface of the SiNW, and an amine-terminated alkyl monolayer grown directly on a hydrogen-terminated SiNW surface. The SiNWs without the native oxide exhibit improved solution-gated field-effect transistor characteristics and a significantly enhanced sensitivity to single-stranded DNA detection, with an accompanying 2 orders of magnitude improvement in the dynamic range of sensing. A model for the detection of analyte by SiNW sensors is developed and utilized to extract DNA-binding kinetic parameters. Those values are directly compared with values obtained by the standard method of surface plasmon resonance (SPR) and demonstrated to be similar. The nanowires, however, are characterized by higher detection sensitivity. The implication is that SiNWs can be utilized to quantitate the solution-phase concentration of biomolecules at low concentrations. This work also demonstrates the importance of surface chemistry for optimizing biomolecular sensing with silicon nanowires.     

Effect of inert gas flow on hydrogen underpotential deposition measurements in polymer electrolyte fuel cells

The effect of inert gas flow rate on hydrogen underpotential deposition (H_upd) measurements in polymer electrolyte fuel cells (PEFCs) was investigated using a novel experimental technique. The method combines local voltammetric measurements in PEFCs with the use of sectioned electrodes. The results give experimental proof that the high inert gas flow rate usually employed in voltammetric measurements in PEFCs at the working electrode results in high hydrogen reduction currents in both the cathodic and the anodic sweep, which hampers an accurate determination of the electrochemically active surface area (ECA). Strong spatial inhomogeneities occur at low potentials as a consequence of formation and accumulation of molecular hydrogen along the flow field. The results show that the flow of inert gas should be minimized or even stopped during a measurement to allow molecular hydrogen to accumulate at the working electrode and to provide uniform conditions along the flow field.     

The solubilization of n-pentane gas in sodium dodecyl sulfate-polyethylene glycol solutions with and without electrolyte

The solubility of n-pentane gas in aqueous solution of sodium dodecyl sulfate (SDS), SDS-0.1 wt% polyethylene oxide (PEG), SDS-0.1 wt% PEG+NaCl (0.1 mol/l), and SDS-0.1 wt% PEG+NaOH (0.1 mol/l) has been determined at 318.15 K. The concentration of SDS (m(SDS)) is up to 50 mmol/kg. The solubility increases linearly with the concentration of SDS above its critical micelle concentration (CMC) or critical aggregation concentration (CAC), indicating that micelles in the solutions solubilize the gas molecules and the solubility of n-pentane gas in the micelles is independent of the SDS concentration. It was found that the solubilization ability of micelles bound to PEG and free micelles to n-pentane gas is almost the same. The solubility of n-pentane gas in micelle phase is three magnitudes higher than that in the bulk solution. The solubilization property of SDS is changed by the addition of PEG, although the solubilizing effect of the polymer alone is not considerable. NaCl and NaOH affect the solubilization noticeably and increase the interaction strength between SDS and PEG. The standard Gibbs energies for the transfer of n-pentane gas from bulk phase to micelle phase are large negative values, indicating that the hydrocarbon gas prefers to exist in the hydrophobic interior of the micelles.     

The impact of ionic strength and background electrolyte on pH measurements in metal ion adsorption experiments

Our understanding of metal ion adsorption to clay minerals has progressed significantly over the past several decades, and theories have been promulgated to describe and predict the impacts of pH, ionic strength, and background solution composition on the extent of adsorption. Studies evaluating the effects of ionic strength on adsorption typically employ a broad range of background electrolyte concentrations. Measurement of pH in these systems can be inaccurate when pH values are measured with liquid junction pH probes calibrated with standard buffers due to changes in the liquid junction potential between standard, low ionic strength (0.05 M) buffers and high ionic strength solutions (>0.1 M). The objective of this research is to determine the extent of the error in pH values measured at high ionic strength, and to develop an approach for accurately measuring pH over a range of ionic strengths using a combined pH electrode. To achieve this objective, the adsorption of cobalt (10(-5) M) onto gibbsite (10 g/L) from various electrolyte solutions (0.01-1 M) was studied. The pH measurements were determined from calibrations with standard buffers and ionic strength corrected buffer calibrations. The results show a significant effect of the aqueous solution background electrolyte anion and ionic strength on pH measurement. The 0.5 and 1 M ionic strength metal ion adsorption edges shifted to lower pH with increasing ionic strength when pH was calibrated with standard buffers whereas no shift in the adsorption edges was observed when calibrated with ionic strength corrected buffers. Therefore, to obtain an accurate pH measurement, pH calibration should contain the same electrolyte and ionic strength as the samples.     

A new in-situ spectroelectrochemical setup for FTIR measurements in operating high temperature polymer electrolyte fuel cells

In-situ infrared spectroelectrochemical measurements have been carried out in a high temperature PEFC setup. The design of the setup allowed to acquire infrared spectra at the catalytic surface of a Pt layer on a conventional phosphoric acid doped polymer electrolyte membrane. CO has been used as the probing molecule and the spectroscopic signature of the resulting adsorbate has been recorded under technically relevant conditions.