Metal-electrolyte interface ionization

If gaseous, electrochemicaUy active components of the measuring environment are not dissolved in the electrode, then the electrode process will consist of the following stages (also shown in Figure 1.18). They are adsorption-desorption of electrochem-icaUy active gaseous components on gas-electrolyte (GE) and gas-metal (GM) interfaces, ionization reaction (with electron transfer) on the metal-electrolyte (ME) and gas-electrolyte interfaces, and mass-transfer processes on all boundaries of three phases (gas-metal, gas-electrolyte, and metal-electrolyte). Furthermore, mass transfer of electrons and holes on the surface electrolyte layer may also occur. It is evident that the quantity of the current in the stationary state is equal to the quantity of the nonmetal component adsorbing on the gas-metal and gas-electrolyte surfaces as a result of ionization of this component on the ME and GE surfaces. 

Fig. 2.16 Energy diagram for a PEC cell based on a n-type semicondnctor and a metal counter electrode. The vacuum energy level is taken as a reference this is the energy of an electron in vacuum at infinite distance. The electron affinity (y) and ionization oiergy (EE) are materials constants, whereas the semiconductor work function (< s) depends on the distance to the surface. Note that a Helmholtz layer is also present at the metal/electrolyte interface...

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

Metal oxidation (ionization) at die metal-fihn interface Transport of metal cation (and/or cation vacancies) across the film Metal cation transfer into solution species (solvated, complexed) at the film-electrolyte interface. 

These diffuse through the lithium iodide via cation vacancies that form as part of the intrinsic Schottky defects in the crystals, to reach the iodine in the cathode (Fig. 2.3b). The electrons lost by the lithium metal on ionizing traverse the external circuit and arrive at the interface between the cathode and the electrolyte. Here they react with the iodine and the incoming Li+ ions to form more lithium iodide. 

Figure 1. Combined energy diagram for a regenerative photoelectrochemical cell with n-CdSe as the anode, metallic cathode and polysulfide as the electrolyte. The diagram indicates some of the charge accumulation modes that might contribute to the potential distribution at the interface. ((Qn) ionized donors (Qdt) deep traps ...

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

D. E. Yates, S. Levine, and T. W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J.C.S. Faraday I 70 1807 (1974). J. A. Davis, R. O. James, and J. O. Leckie, Surface ionization and complexation at the oxide/water interface. I Computation of electrical double layer properties in simple electrolytes, J. Colloid Interface Sci. 63 480 (1978). J. A. Davis and J. O. Leckie, Surface ionization and complexation at the oxide/water interface. II Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions, J, Colloid Interface Sci, 67 90 (1978). 3 Adsorption of anions, J. Colloid Interface Sci. 74 32 (1980). 

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