Presence Of Specific Adsorption Research Articles

Kinetics of Fast Electrochemical Processes Studied By Impedance Spectroscopy. Frequency Dispersion at High Frequency Range

The difficulty of the kinetics determination for very fast electrochemical reaction by using impedance spectroscopy (EIS) is related to the fact that the charge transfer resistance for this process is much smaller than the solution resistance. Moreover, for very fast reactions only a low frequency part of the spectra can be observed when potentiostat is coupled with frequency response analyzer in a classical three electrode arrangement. In such case, the high frequency part of the spectra is very often entirely covered by artifacts related to a relatively low time constant of the potentiostat. A special experimental setup is needed for such experiments, in which impedance is measured in stand-alone mode of the frequency response analyzer. The solution resistance can be minimized by using twin electrode arrangement, in which two identical electrodes are facing each other [1]. However, at high frequency part of the impedance spectra a distinct frequency dispersion occurs due to an unequal and frequency dependent current distribution. Therefore a special attention has to be devoted to control the geometry of the electrode arrangement. Unequal current distribution can be eliminated by using recessed electrodes [2]. However, even for recessed electrodes, a distinct frequency dispersion can be observed at kHz region. This effect is more pronounced in the presence of specific adsorption [3]. To date its origin has not been fully understood. In this study high frequency impedance measurements (up to 1 MHz) have been carried out in a twin electrode and hanging meniscus configurations. Recessed electrodes have been used for the first time to study the kinetics of hydrogen electrosorption at Pt(111) at high frequency range. Hydrogen absorption in palladium thin layers (50 nm) covered by platinum monolayers (1ML/Pd) have also been investigated as model systems involving diffusion processes related to hydrogen absorption. In the 1ML/Pd system, the hydrogen insertion is particularly fast [4], the resistance of the charge transfer, related to this process, is in the order of 0.02 Ωcm2. For these model electrochemical processes the frequency dispersion at high frequency rage will be discussed. Literature: [1] B. Losiewicz, R. Jurczakowski, A. Lasia, Electrochim. Acta, 80 (2012) 292-301 [2] Frateur, I.; Huang, V. M. W.; Orazem, M. E.; Pebere, N.; Tribolletd, B.; Vivier, V., Electrochim. Acta 53 (2008) 7386-7395. [3] T. Pajkossy, Z. Phys. Chem.,226 (2012) 935-943 [3] P. Połczyński, R. Jurczakowski, J. Power Sources, 2016 in press

Repulsive interactions induced by specific adsorption: Anomalous step diffusivity and inadequacy of nearest-neighbor Ising model: (part I experimental)

This is Part I of two closely related papers, where we show that the specific adsorption of anions leads to a failure of the nearest-neighbor Ising model to describe island perimeter curvatures on Au(100) electrodes in dilute KBr, HCl and H2SO4 electrolytes and the therewith derived step diffusivity vs. step orientation. This result has major consequences for theoretical studies aiming at the understanding of growth, diffusion and degradation phenomena. Part I focuses on the experimental data.As shown theoretically in detail in Part II (doi:10.1016/j.susc.2016.03.022), a set of nearest-neighbor and next-nearest-neighbor interaction energies (ϵNN, ϵNNN) can uniquely be derived from the diffusivity of steps along 〈100〉 and 〈110〉. We find strong repulsive next-nearest neighbor (NNN) interaction in KBr and HCl, whereas NNN interaction is negligibly for H2SO4. The NNN repulsive interaction energy ϵNNN therefore correlates positively with the Gibbs adsorption energy of the anions. We find furthermore that ϵNNN increases with increasing Br− and Cl− coverage.The results for ϵNNand ϵNNN are quantitatively consistent with the coverage dependence of the step line tension. We thereby establish a sound experimental base for theoretical studies on the energetics of steps in the presence of specific adsorption.

Specific Adsorption of Aspartic Acid on Iron (III) and Nickel (II) Oxides

<p>The dependences of zeta – potential and adsorption of aspartic acid on NiO and Fe<sub>2</sub>O<sub>3</sub> oxideswere investigated as a function of aspartic acid concentration, pH and adsorption time using microelectrophoresis and potentiometric titration methods. Shifting of pHIEP (the isoelectric point or IEP) towards the acid zone for Fe<sub>2</sub>O<sub>3</sub> and towards the base zone for NiO in the solutions of aspartic acid in comparison with HCl shows the presence of specific adsorption of aspartic acid anion and cation forms on the surfaces of Fe<sub>2</sub>O<sub>3</sub> and NiO oxides, respectively. The complexation process of Fe(III) ions with aspartic acid in the bulk solution and on the oxide surface was examined by spectrophotometer. It was determined that Fe(III) ions form complexes with aspartic acid in the bulk solution.</p>

Fifty years of studies of double layer effects in electrode kinetics—a personal view

Developments in the treatment of double layer effects in electrode kinetics are outlined. These include discreteness-of-charge effects in the presence of specific adsorption, solvent effects in electron transfer reactions, and effects related to the distribution of charge in polyatomic reactants. The importance of studies at single crystal electrodes is emphasized, and the development of a single crystal ultramicroelectrode described. Finally, a method of improving the estimate of the diffuse layer potential drop on the basis of Monte Carlo simulations is presented.

A practical approach to modeling the electrical double layer in the presence of specific adsorption of ions

Model calculations are presented that yield in a straightforward manner the quantitative dependence of the specific adsorption of ions at electrode surfaces on the applied electrode potential (electrode charge). Furthermore, the double-layer capacitance and the potential at the outer Helmholtz plane (ø2) are obtained. The derivation is based on Devanathan's three-capacitor model for the interfacial electric-potential distribution. A convenient correction function for the ø1 potential accounting for the discreteness-of-charge effect is derived, largely on the basis of recent work by Conway et al. The results are shown to be in very good agreement with published work by Lawrence and Parsons on the double layer between Br− electrolyte and the mercury electrode. Keywords: electrochemistry, specific adsorption, electric double layer, discreteness-of-charge effect.

Capacitance dispersion on solid electrodes: anion adsorption studies on gold single crystal electrodes

In order to explore the origin of the capacitance dispersion observed on solid electrodes, voltammetric and impedance measurements on single crystal gold electrodes have been carried out in the absence and presence of specifically adsorbed anions, mostly Br −. Evidence is presented showing that (i) in the absence of specific adsorption, the double layer on these electrodes behaves as an ideal capacitance (were the impedance approximated by a constant phase element, the exponent would be larger than 0.99); (ii) in the presence of specific adsorption, capacitance dispersion can be observed due to the sluggishness and/or diffusion control of the adsorption step, to the surface reconstruction/deconstruction and to adlayer structural transformations.

Impedance aspects of anion adsorption on gold single crystal electrodes

In order to explore the origins of the capacitance dispersion observed for single crystal electrodes, voltammetric and impedance measurements on Au(l 11) and Au(100), in the absence and the presence of specifically adsorbed anions, mostly Br −, have been carried out. Interfacial capacitances and rate coefficients characterizing the kinetics of adsorption have been evaluated from the impedance spectra. We present evidence that the double layer behaves as an ideal capacitance in the absence of specific adsorption. The approximation with a constant phase element yields an exponent larger than 0.99. The “capacitance dispersion” observed in the presence of specific adsorption is either assigned to a slow diffusion or adsorption process, or to transformations within the adlayer or the substrate surface.

Optimal electrolytic chloriding of silver ink electrodes for use in electrical impedance tomography.

The electrode-electrolyte interface impedance may be simplistically modelled by an equivalent circuit comprising a resistance, RTOTAL, in series with an empirical, constant phase angle impedance, ZCPA. This pseudo-capacitance can be thought of as representing empirically the non-faradaic, double layer capacitance in the presence of specific adsorption and surface roughness effects. RTOTAL is the sum of the lead and electrolyte resistances. Depositing a thin layer of silver chloride on silver electrodes can yield improved electrical performance characteristics (potential and impedance) when used in conjunction with a chloride gel. An electrolytically deposited AgCl layer tends to have a rough surface profile thus leading to an increase in the effective interface area. This gives rise to a decrease in RTOTAL and ZCPA, both of which are desirable. Unfortunately AgCl is a relatively poor conductor. Increasing layer thickness causes RTOTAL to increase, thus adversely affecting the inter-electrode impedance at high frequencies. Electrode systems for use in electrical impedance tomography therefore require only relatively thin layers of AgCl.

Anion absorption on the (111) face of silver

Differential capacity (admittance-impedance) measurements for a (111) face of silver in supra-pure solutions of NaF, Na 2SO 4 and NaCH 3COO are analysed to obtain data on the specific adsorption of the above-mentioned anions. It is shown that the frequency dependence of the impedance of the electrochemical interface cannot be represented by an equivalent circuit consisting of the ohmic resistance ( R OHρ) and double-layer capacitance ( C dl) connected in series in the presence of specific adsorption. The addition of a series combination of a resistance ( R ct) and capacitance ( C ad) in parallel with C dl gives a reasonable representation of the data. R ct permits the evaluation of some semiquantitative information about the adsorption kinetics.

A thermodynamic analysis of common intersection points in potentiometric titration studies of solid surfaces

Potentiometric titration curves for solid/solution interfaces often exhibit common intersection points (CIP) at different levels of supporting electrolyte and, for a fixed electrolyte level, at different concentrations of cosolvents. Both situations are discussed. For the first, explicit expressions are derived which summarise the conditions for a CIP to occur in the presence of specific adsorption and which allow formally for solution non-ideality. Attention is focussed primarily on plots of amount adsorbed vs. activity of the potential determining species rather than concentration. However, the latter case is also discussed briefly. The treatment is extended to cases where several ionic species are strongly specifically adsorbed and leads to a useful test for the occurrence of specific adsorption which is applicable in non-ideal systems. The situation where the bulk concentrations of such species are not negligible is also considered. For mixed solvents a simple expression is derived which describes the effect of solvent composition on the concentration of potential determining ions at the zero point of charge (z.p.c.). In this case CIP may arise from competition between medium effects associated with the adsorption of potential determining ions and electric fields in the diffuse double layer. The treatment is applicable to ionic crystals and to surfaces with fixed dissociable groups.

Adsorption on well-defined solid surfaces chloride adsorption on a (110) face of silver

Abstract The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak. Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.

Adsorption on well-defined solid surfaces: Bromide adsorption on a (110) face of silver

The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak. Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.

Points of zero charge in the presence of specific adsorption

In the absence of specific adsorption, the point of zero charge (pzc) of a colloid or suspension can be obtained by measuring the relative surface charge as a function of the concentration of the charge-determining ions at various concentrations of the indifferent electrolyte. These curves show a common intersection point (cip) which may be identified with the pzc. In the presence of electrolytes containing specifically adsorbing ions, also cip's are observed but they do not coincide with the pzc. The physical meaning of cip's is that at such points the Esin-Markov coefficient is zero, indicating equal compensation of the surface charge on a molecular basis by cations and anions. For the establishment of cip's in the presence of specific adsorption, the surface charge must be measured in a mixture of the specifically adsorbing electrolyte and an indifferent one. A thermodynamic theory is developed to obtain from this surface charge and from information on cip's the complete composition of the double layer at any composition of the bulk. Examples are worked out for an insoluble oxide in a mixture of a specifically adsorbing (2-1) and an indifferent (1-1) electrolyte.

Direct determination of the surface excess of specifically adsorbed ions on mercury

Gibbs' adsorption equation has been derived in the case of mixed solutions of strong electrolytes at constant molal ionic strength. Under these conditions, the specifically' adsorbed surface excess of ionic species at the mercury electrode and the charge of the diffuse, layer in presence of specific adsorption may be obtained. The main fea.Ture.of the treatment consists in the fact that neither the differential equation of Poisson—Bpltzman nor the integrated solution of Gouy-Chapman have to be introduced.