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Chapter 2 Electrode/electrolyte interface: ----structure and properties. Electrochemical reactions are interfacial reactions , the structure and properties of electrode / electrolytic solution interface greatly influences the reaction. Influential factors:. - PowerPoint PPT Presentation

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  • Electrochemical reactions are interfacial reactions, the structure and properties of electrode / electrolytic solution interface greatly influences the reaction.Influential factors: 1) Chemistry factor: chemical composition and surface structure of the electrode: reaction mechanism 2) Electrical factor: potential distribution: activation energy of electrochemical reactionChapter 2 Electrode/electrolyte interface: ----structure and properties

  • For process involving useful work, W should be incorporated in the following thermodynamic expression. dG = -SdT + VdP + W+idni2.1 Interfacial potential and Electrode PotentialFor electrochemical system, the useful work is: W = zieUnder constant temperature and pressure, for process A B:1) Electrochemical potential

  • Electrochemical potential1) Definition:zi is the charge on species i, , the inner potential, is the potential of phase . In electrochemical system, problems should be considered using electrochemical potential instead of chemical potential.

  • 2) Properties: 1) If z = 0 (species uncharged) 2) for a pure phase at unit activity 3) for species i in equilibrium between and .3) Effect on reactions1) Reactions in a single phase: is constant, no effect2) Reactions involving two phases: a) without charge transfer: no effect b) with charge transfer: strong effect

  • 2) Inner, outer and surface potential(1) Potential in vacuum: the potential of certain point is the work done by transfer unite positive charge from infinite to this point. (Only coulomb force is concerned). - strength of electric field

  • This process can be divided into two separated steps. Electrochemical reaction can be simplified as the transfer of electron from species in solution to inner part of an electrode.(2) Potential of solid phase

  • (3) Inner, outer and surface potential

  • The total work done for moving unit charge to inner of the charged sphere is W1 + W2 = (W1+ W2) / z e0 = + The electrostatic potential within a phase termed the Galvani potential or inner potential (). If short-distance interaction, i.e., chemical interaction, is taken into consideration, the total energy change during moving unite test charge from infinite to inside the sphere:

  • innerhollow10-6~10-7infinitedistanceworkfunction

  • work function the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface or energy needed to move an electron from the Fermi energy level into vacuum. (4) Work function and surface potential

  • For two conductors contacting with each other at equilibrium, their electrochemical potential is equal. 3) Measurability of inner potential(1) potential difference

  • different metal with differentTherefore

  • Conclusion No potential difference between well contacting metals can be detectedGalvanic and voltaic potential can not be measured using voltmeter.

  • If electrons can not exchange freely among the pile, i.e., poor electrical conducting between phases.(2) Measurement of inner potential difference

  • Knowing V, can be only measured when(3) Correct connection

  • Consider the cell: Cu|Cu2+||Zn2+|Zn/CuFor homogeneous solution without liquid junction potentialthe potential between I and II depends on outer potential difference between metal and solution.(4) Analysis of real system

  • Using reference with the sameThe value of IS is unmeasurable but the change of is [ (IS )] can be measured. the exact value of unknown electrode can not be detected. absolute potential

  • Chapter 2 Electrode/electrolyte interface: structure and properties

  • 1) Transfer of electrons 2.4 origination of surface potential

  • 2) Transfer of charged species

  • AgIAgIAgI3) Unequal dissolution / ionization

  • 4) specific adsorption of ions

  • 5) orientation of dipole molecules

  • 6) Liquid-liquid interfacial charge Different transference number

  • 1), 2), 3) and 6): interphase potential4), 5) surface potential.

    1) Transfer of electron2) Transfer of charged species3) Unequal dissolution4) specific adsorption of ions5) orientation of dipole molecules6) liquid-liquid interfacial charge

  • Electric double layercapacitorHolmholtz double layer (1853)Electroneutrality: qm = -qs

  • 2.5 Ideal polarizable electrode and Ideal non-polarizable electrodeequivalent circuiti = ich + iec Charge of electric double layerElectrochemical rxnFaradaic process and non-Faradaic process

  • an electrode at which no charge transfer across the metal-solution interface occur regardless of the potential imposed by an outside source of voltage. ideal polarizable electrode no electrochemical current:i = ich

  • Virtual ideal polarizable electrodeHg electrode in KCl aqueous solution: no reaction takes place between +0.1 ~ -1.6 VK+ + 1e = K2Hg + 2Cl- - 2e- = Hg2Cl2+0.1 V-1.6 V

  • an electrode whose potential does not change upon passage of current (electrode with fixed potential)ideal non-polarizable electrode no charge current:i = iecVirtual nonpolarizable electrodeAg(s)|AgCl(s)|Cl (aq.) Ag(s) + Cl AgCl(s) + 1e

  • For measuring the electrode/electrolyte interface, which kind of electrode is preferred, ideal polarizable electrode or ideal non-polarizable electrode?

  • 2.6 interfacial structuresurface charge-dependence of surface tension: 1) Why does surface tension change with increasing of surface charge density? 2) Through which way can we notice the change of surface tension?Experimental methods: 1) electrocapillary curve measurement 2) differential capacitance measurement

  • interfaceInterphase Interfacial regionThe Gibbs adsorption isotherm When T is fixed

  • Integration givesGibbs adsorption isotherm

  • When the composition of solution keeps constantLippman equationElectrocapillary curve measurement

  • Electrocapillary curves for mercury and different electrolytes at 18 oC. Zero charge potential: 0(pzc: potential at which the electrode has zero charge)Electrocapillary curve

  • Theoretical deduction of

  • Differential capacitanceDifferential capacitance

  • capacitorThe double layer capacitance can be measured with ease using electrochemical impedance spectroscopy (EIS) through data fitting process.Measurement of interfacial capacitance

  • Integration of capacitance for charge densityDifferential capacitance curvesCd = C()

  • Dependence of differential capacitance on potential of different electrolytes. Differential capacitance curves

  • Charge density on potential

  • differential capacitance curves for an Hg electrode in NaF aqueous solution Dependence of differential capacitance on concentrationPotential-dependentConcentration-dependentMinimum capacitance at potential of zero charge (Epzc)36 F cm-2; 18 F cm-2;

  • Surface excess

  • For any electrolyteFor R.E. in equilibrium with cation

  • KF0.0-0.4-0.8-1.20.4 / Vq / Ccm-2-20-4-6246KAcKClKBrKFKAcKClKBrAnion excesscation excessSurface excess curves

  • Electrode possesses a charge density resulted from excess charge at the electrode surface (qm), this must be balanced by an excess charge in the electrolyte (-qs)2.7 Models for electric double layer1) Helmholtz model (1853)

  • q charge on electrode (in Coulomb)

  • 2) Gouy-Chappman layer (1910, 1913)Charge on the electrode is confined to surface but same is not true for the solution. Due to interplay between electrostatic forces and thermal randomizing force particularly at low concentrations, it may take a finite thickness to accumulate necessary counter charge in solution. Plane of shear

  • Boltzmann distributionPoisson equation Gouy and Chapman quantitatively described the charge stored in the diffuse layer, qd (per unit area of electrode:)

  • Integrate from x = d to x =

  • For 1:1 electrolyteFor Z:Z electrolyte

  • Experimentally, it is easier to measure the differential capacitance:Hyperbolic functions

  • For a 1:1 electrolyte at 25 oC in water, the predicted capacitance from Gouy-Chapman Theory. 1) Minimum in capacitance at the potential of zero charge 2) dependence of Cd on concentration2) Gouy-Chappman layer (1910, 1913)

  • 3) Stern double layer (1924)Combination of Helmholtz and Guoy-Chapman ModelsThe potential drop may be broken into 2:

  • Inner layer + diffuse layerThis may be seen as 2 capacitors in series: Ci: inner layer capacitanceCd: diffuse layer capacitance-given by Gouy-Chapman

  • Total capacitance (Ct) dominated by the smaller of the two.

    At low c0At high c0Cd dominantCi dominantCd Ct Ci Ct

  • Discussion: When c0 and are very smallStern equation for double layer

  • Discussion: When c0 and are very large Cd plays a role at low potential near to the p. z. c.

  • Fitting result of Stern Model.Fitting of Gouy-Chapman model to the experimental results

  • 4) Gramham Model-specific adsorption0dETriple layer Specifically adsorbed anionsHelmholtz (inner / outer) plane

  • 5) BDM fine modelBockris-Devanathan-Muller, 1963

  • If rH2O = 2.7 10-10 m, i=6di i =5-6do i =40

  • Helmholtz modelGouy-Chappman modelStern modelGraham modelBDM modelWhat experimental phenomenon can they explain? The progress of Model for electric double layer

  • 2.8 Fine structure of the e

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