Metals, electrode potentials

Fig. 2.2] Difitnbutipn of electrode potential and normalized current in a porous electrode. (a) and (b) Thin bed electrode, (c) and

A metallic electrode whose potential is a function of the concentration of M"+ in an M"+/M redox half-reaction. 

Potentiometric electrodes are divided into two classes metallic electrodes and membrane electrodes. The smaller of these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in solution thus the potential of an Ag wire is determined by the concentration of Ag+ in solution. When another species is present in solution and in equilibrium with the metal ion, then the electrode s potential will respond to the concentration of that ion. Eor example, an Ag wire in contact with a solution of Ck will respond to the concentration of Ck since the relative concentrations of Ag+ and Ck are fixed by the solubility product for AgCl. Such electrodes are called electrodes of the second kind. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

The measurement of resistance to remote earth of a metallic structure is normally carried out with a four-electrode instrument. The connections are shown in Fig. 10.52. A current / is passed between the structure and a remote electrode. The potential difference V is measured between the structure and a second remote electrode. In this way the ohmmeter records the resistance of the structure to earth, i.e. V/I. The spacing of the electrode from the structure is important and must be such that the remote potential electrode lies on the horizontal part of the resistance/distance curve, as shown in Fig. 10.52. Generally speaking, a minimum distance of 15 m from the structure is necessary for the potential electrode to lie on the flat part of the curve, with the current electrode usually at least twice the distance of the potential electrode. 

A//metal salt/anion (Type 2 electrode) Metal in contact with its sparingly soluble salt MX and a soluble salt the anion X giving an M/MX/X electrode whose potential depends on /f, of MX and the a, -. Hg/HgjClj/KCKaq.) Hg/Hg2S04/K2S04(aq.) Ag/AgCl/KCl(aq.)... 

Metal/metal oxide Metal filmed with oxide in a solution of OH giving an A//M,Oy/OH electrode whose potential is dependent on pH. Sb/Sb20j/0H- Bi/Bi20j/0H-... 

Steady-state potential comparable with Type 1 reversible electrode Metal in a solution of electrolyte in which ions are produced by a corrosion reaction in an VAf exchange that determines the potential. Zn in NaCI solution Zn in dilute HCI... 

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

For example, the reaction enthalpy for the reduction of PC proceeding at lithium amalgam to form propylene gas and lithium carbonate is estimated to be -I41kcal (molPC)-1 [149]. PC is reduced at noble-metal electrodes at potentials below 1.5 V vs. Li, and yields lithium alkyl carbonates when lithium salts are the supporting electrolytes. Reduction occurs at 0.7-0.8 V vs. Li with Bu4NC104as supporting electrolyte [150],... 

It was shown in Section I that the potential of zero charge is related to the electron work function of the electrode metal by Eq. (27) ... 

The areas bounded by solid lines correspond to regions of thermodynamic stability of certain substances that are named in the diagram. This stability is relative. The dashed line a in the diagram corresponds to the equilibrium potential of the hydrogen electrode. Metallic zinc, for which the reaction lines are below the line for the hydrogen electrode, can be oxidized while hydrogen is evolved (see Section 2.4.1). 

An important condition for potentiometry is high selectivity the electrode s potential shonld respond only to the snbstance being examined, not to other components in the solntion. This condition greatly restricts the possibilities of the version of potentiometry described here when metal electrodes are nsed as the indicator electrodes. The solntion shonld be free of ions of more electropositive metals and of the components of other redox systems (in particnlar, dissolved air). Only corrosion-resistant materials can be nsed as electrodes. It is not possible at all with this method to determine alkali or alkaline-earth metal ions in aqneons solntions. 

Among potentiometric methods of analysis that are important for ecological applications, the one most widely used is that of pH measurements with an indicator electrode whose potential is a function of the hydrogen ion concentration. More recently, ion-selective electrodes reversible to other cations such as those of heavy metals have become available. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Bradley et al.109 have combined a p-Si photocathode and homogeneous catalysts (tetraazamacrocyclic metal complexes, which had been shown to be effective catalysts for C02 reduction at an Hg electrode110) to reduce the applied cathode potential. The catalysts showed111 reversible cyclic voltammetric responses in acetonitrile at illuminated p-Si electrodes at potentials significantly more positive (ca. 0.4 V) than those required at a Pt electrode, where the p-Si used had surface states in high density and Fermi level pinning112 occurred. Electrolysis of a C02-saturated solution (acetonitrile-H20-LiC104 1 1 0.1 M) in the presence of 180 mM... 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

Fig. 10 Aviram-Ratner rectification via HOMO and LUMO. (a) A D-o-A molecule is sandwiched between two metal electrodes. MD is the electrode proximal to the donor, MA is the electrode proximal to the acceptor, is the electrode metal work function, IPD is the ionization potential of the donor, EAa is the electron affinity of the acceptor, (b) No pathway for current exists when a voltage is applied in the reverse bias direction, (c) Under a comparable voltage to (b) but in the forward bias direction, rectification results from electrons flowing from MA to LUMO to HOMO to MD...

Fig. 1 Schematic drawings of a tunnel diode, an STM, and the electronic energy diagram appropriate for both. U is the height of the potential barrier, E is the energy of the incident electron, d is the thickness of the barrier, A is approximately 1.02 A/(eV)1/2 if U and E are in electron volts and d is in angstroms, /0 is the wavefunction of the incident electron, and /d is the wavefunction after transmission through the barrier. I is the measured tunneling current, V is the applied bias, and M and M are the electrode metals...

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

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