电化学动力学Chaper 6 - Double layer.ppt

Chapter 6The properties and structures of the electric double layer at electrode/electrolyte interfacesnIt is essential to have an electrode/electrolyte interface for the electrochemical reactions to take place. Such an interface is established by direct contact of the electrode (electron conductors) with electrolyte (ion conductors). nThe electric double layer is the core of the electrode/electrolyte interface, which is formed by the charges reside in the interfacial region, and in which the interfacial potential difference is distributed. The electrode/electrolyte interfacesnAs two substances of different nature are made in contact, a charged region may be produced in each phases near the phase boundary due toØCharge transfer between two phase, e.g., the dissolution of metal: Ag  Ag+ + eØAdsorption of charged species at interfacesØOrientated arrangement of dipole(偶极)The formation of charged layers at the interfaces between two phases a b c A relatively detailed view of potentials and interfacial potential differences（电位差）nThe equi-potential nature of conductorsØThe potential difference between two points within a phase —— the work required to bring a unit positive charge from point 1 to point 2.ØØThe potential is uniform within conductorsThe potential is uniform within conductors The conductors usually contains abundance of charge carriers. When no The conductors usually contains abundance of charge carriers. When no current passes through a conducting phase, there is no net movement of current passes through a conducting phase, there is no net movement of charge carriers. This implies that the electric field at all interior points charge carriers. This implies that the electric field at all interior points within a conducting phase must be zero. Therefore, the potential difference within a conducting phase must be zero. Therefore, the potential difference between any two points in the interior of the phase must also be zero.between any two points in the interior of the phase must also be zero. 21The potential within a phase, The potential within a phase,  , is called the , is called the inner inner potential (or potential (or GalvaniGalvani potential) potential) of the phaseof the phasenThe location of the excess charges in a conducting phaseØThe Gauss law: if we enclose a volume with an imaginary surface ( a Gaussian surface), we will find that the net charge “q” inside the surface is given by an integral of the electric field over the surface:This implies that the excess charge resides in the region near the surface of the conducting phaseØØThe thermal processes will impede the compact The thermal processes will impede the compact accumulation of the excess charges strictly on accumulation of the excess charges strictly on the surface. Then a charged zone is established the surface. Then a charged zone is established near the surface, which is called a near the surface, which is called a space charge space charge region region , the thickness of which depends on the , the thickness of which depends on the bulk concentration of the charge carriers.bulk concentration of the charge carriers.nThe inner potential and outer potential of a phaseØThe value of the potential at a particular point within a phase, , is related to the work ( Wc ) required to overcome the coulombic interaction （库仑作用）between the test charge and the studying phase upon bringing the test charge from the location of zero potential into the phase. Ø“ W ” consists of two contributions: (a) W1 --- that required to bring the unit charge to the closest distance from the phase boundary where no non-coulombic interaction starts to function; (b) W2 --- that required to overcome the coulombic force when bring the test charge from outside of phase boundary into the phase. W1W2 + mWhere y is called the outer potential (or Volta potential )of the phase,Where c is called the surface potential of the phase 1m m Due to that moving the test charge into a phase always involves chemical interactions between the phase and the test charge, it is impossible to measure the surface potential independently.4.The concept of the electrochemical potential（电化学势 ）ØIn addition to the coulombic work, ( Wc) , it is also necessary to overcome the chemical interaction upon moving the test charge into the phase, the corresponding free energy change is defined as the chemical potential ( m ) of the test charge in the studying phase. Thus, the total free energy change due to moving a test charge ( z ) from the point of zero potential into a phase is: W1W2 + mis called the Electrochemical potential of the test charge in the phase（针对特定离子i）5. 5.The concept of the emersion work ( The concept of the emersion work ( WWi i ) )ØØThe work required to move one mole charged particle out of a The work required to move one mole charged particle out of a phase into t。