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Published byJulianna Rosamund Woods Modified over 9 years ago
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BASIC PRINCIPLES OF ELECTRODE PROCESSES Heterogeneous kinetics
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- - - - - - - - Charged interfaces - - - - - -
Ions and dipoles in the volume element : solutions (random distribution, isotropic) (charge Q = O) charged interface: electrode/electrolyte (anisotropic) (Q O) Metal - + Electrolyte - Electric (electrode) double layer (EDL): metals/electrolytes (Ag/Ag+) ; (Hg/NaF) ; (Hg/KI) non-metallic elements/ electrolyte (gas, glass, polymers, membranes, colloides…) liquid I/liquid II - - + + + - - + + + - - - + - - + + - - - + +
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Potential creation on interface
Heterogeneous system solid conductive phase – interface – liquid conductive phase 1. Ag (Ag+ ) Ag+ NO3- 2. Pt ( Pt-Ir, graphite) Fe2+ , Fe3+ ( SO4 2- ) spontaneous ion crossing 1. or electron transfer 2. spontaneous interface charging: 1. Ag e Ag 2. Fe e Fe2+ 0x z e Red
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Chemical potential: i = io + RT ln ai
i * = i + zF = in balance i = /z/ F = potential difference between solid phase and solution inner (Galvani) potential, immeasurable! balance stabilization electrochemical potential equality of ion i in both phases: i *(l) = i* (s) i *(l) = i zF chemical work electrical work
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Measurable cell potential EMN , made from 2 hemi-cells:
standard hydrogen electrode SHE Eo = 0 V measured electrode E = b + b = constant EMN = E = E Nernst : 1. 2. That was a balance
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From within entered potential on electrode:
charge goes through an interface impossible of charging impolarizedable interface (impolarizedable electrode, Ag / Ag+ ) charge doesn´t go through an interface possible of charging polarizedable interface (polarizedable electrode, Pt, graphite, Hg, without Hgz+ in solution)
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ELECTRODE DOUBLE LAYER (EDL)
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ELECTRODE DOUBLE LAYER (EDL)
electrostatic potential charge density σ surface tension g capacity C adsorption G Electrode Solution Electrode Electrode - +σS G + - - - + φ + - -σM + Solution - + - Solution + - + - - - + - + - - - + -
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a) capillary elevation b) stalagmometr (also DME)
surface tension g 2r F Fg b) stalagmometr (also DME) h
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differential capacity Cd
charge density σ Lippman equation capacity C differential capacity Cd integral capacity Ci adsorption G Langmuir isotherm fraction of coverage surface or maximum surface excess , adsorption coefficient activity of species i in bulk solution
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Temkin isotherm Frumkin isotherm Esin-Markov effect
g …. parameter treating the interaction energy between the adsorbed species Frumkin isotherm Esin-Markov effect The degree of specific adsorption should vary with electrolyte concentration, just as there should be a change in the point of zero charge
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ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE
electrostatic models ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE charges - points + history Electrolyte - + φ M φs x xH Cd E = 5-7 Electrode (metal) about F/cm2 xH Helmholtz Model (1879)
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- c1 > c2 + c2 c1 Gouy-Chapman Model (1910-1913) diffuse layer φ φM
ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE diffuse layer Electrolyte - + c1 > c2 φ φM x xH Cd c2 Electrode (metal) c1 φs E EZ Gouy-Chapman Model ( )
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Gouy-Chapman Model (1910-1913)
distribution of species with distance from electrode (xDL = distance characteristic of the diffuse layer) Bolztmann´s law Poisson´s equation
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diffuse layer thickness
Poisson-Bolztmann equation xDL = distance characteristic of the diffuse layer diffuse layer thickness xDL for water at 298 K is 3.04*10-8 z-1c-1/2 cm if c = 1M and z = 1, then xDL is 0.3 nm.
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ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE
Compact Layer Electrolyte - + φ φM Diffuse Layer Electrode (metal) φs xH x OHP Cd Stern Model (1924) EZ E
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far from EZ, CH CGC and so Cd ~ CH
Stern Model (1924) close to EZ, CH >> CGC and so Cd ~ CGC far from EZ, CH CGC and so Cd ~ CH separation plane between the two zones is called the outer Helmholtz plane (OHP)
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ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE
IHP - Inner Helmholtz Plane IHP OHP - Outer Helmholtz Plane OHP OHP φ IHP Diffuse Layer + - + E > EZ - - - - + E = EZ + + Electrode (metal) Electrolyte - - + E < EZ - + x - + - xH + - Cd - - + + - - - - + + EZ -E Grahame Model (1947)
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- - - - - - - - - - - - - OHP IHP + + - + - + - + - + + - + - + - + -
ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE OHP + + - IHP + + - - + - - + - + + - + - Electrolyte - - - + - + + - Electrode (metal) - + - - + - + - + + - - - - - + - - + - + + - + - - - - + - + - - Layer of oriented molecules of supporting electrolytes or solvents + - BDM Model Bockris, Devanathan, Müller Model (1963)
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CHARACTERISTIC OF ELECTRODE POTENTIAL IN DOUBLE LAYER
OHP 2 x2 x x2 2 x OHP adsorption
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Chemical Model (Damaskin and Frumkin) (Trasatti) (Parsons)
ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE Chemical Models - the electronic distribution of the atoms in the electrode ( not only electrostatic forces) - difference between (sp) metals and transition (d)metals - IHP as an electronic molecular capacitor - jellium model Classical representation φM φM The jellium model φs φs Variation of the electrostatic potentials with distance from a metallic electrode Chemical Model (Damaskin and Frumkin) (Trasatti) (Parsons)
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ELECTRODE DOUBLE LAYER STRUCTURE ON INTERFACE ELECTRODE - ELECTROLYTE
Electron spill-over at the surface of a metal according to the Jellium model.
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