Electrolytes anionic conductors

Fee DC, Zwick SA, Ackerman JP (1983) Solid oxide fuel cell performance. In Salzano FJ (ed) Proceedings of the conference on high temperature solid oxide electrolytes anion conductors, vol 1. Brookhaven National Laboratory, Upton, New York pp 29-38... 

D.C. Fee, S A. Zwick, J.P. Ackerman, Solid Oxide Fuel Cell Performance, in Proc. Conf. High Temperature Solid Oxide Electrolytes, Anion Conductors, F.J. Salzano Ed., Brookhaven National Laboratory, Vol. 1, pp. 29-38 (1983)... 

When Wagner had recognised the mechanism of conduction in the Nernst glower, he pointed out in 1943 For fuel cells with solid electrolytes anion conductors are to be considered exclusively. From this point of view a systematic investigation of the mixed crystal systems of the type of the Nernst mass with roentgenographic and electrical methods seems to be desirable [22]. This was the start of concentrated work on solid oxide fuel cells (SOFCs). 

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

Oxygen anion conductors solid electrolyte Oxygen, chemisorbed - chemisorption of oxygen Oxygen conducting solid electrolyte - solid electrolyte... 

The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem astonishing. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. and 0 ) and cations including monovalent (e.g. H+, Fi+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. A variety of names that have been used for these materials include solid electrolytes, superionic conductors, and fast-ionic conductors. Solid electrolytes arguably provides the least misleading and broadest description for this class of materials. 

The reactions are more likely to be observed with cations in cationic conductors and with anions in anionic conductors. The Cl ions for instance are easily injected into stabilized zirconia. Such reactions are well known in silica based materials 3-85. They are somewhat similar to the traditional electrode reactions in the presence of a supporting electrolyte. 

Miura N, Yan Y, Sato M, Yao S, Nonaka S, Shimizu Y, Yamazoe N (1995) Solid state potentiometric CO sensors using anion conductor and metal carbonate. Sens Actuators B 24-25 260-265 Miura N, Lu G, Yamazoe N (2000) Progress in mixed-potential type devices based on solid electrolyte for sensing redox gases. Solid State Ionics 136-137 533-542... 

DC and AC conductivity analysis on the Mg(II) and Pb(II) electrolytes were carried out using non-blocking (Mg or Pb) and blocking electrodes. The Mg(II) electrolytes showed no evidence of Mg(II) motion and appear to be virtually pure anion conductors. The Pb(II) electrolytes appeared to be good conductors of Pb(II) as well as halide ions. An initial estimate of the transport number of Pb(II) in PbBr. (PE0)2q is 0.6-0.7 at 140 C. We must caution that these transport number measurements are preliminary estimates. It is a major undertaking to measure definitive transport numbers, and that work is not yet begun. 

Essentially pure anion conductors. This category includes polymer electrolytes with salts of small cations, which are highly polarisable, such as Mg " and Ca ". In these materials, the cationic transference number is generally lower than 0.05 in the range 100-150 °C. The ether oxygens in the polymer chains trap the cations electrostatically, and ionic conduction is due to the anions. 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

In electronic conductors, e.g., a metallic material such as a copper wire, all the current is carried by the electrons, and for such conductors N = 1 and N+ = 0. For solutions of electrolytes it is not at all easy to ascertain what fraction of the current is carried past a certain position in the electrolyte by the cations and what fraction is carried by the anions. 

This material was first synthesized in the middle 1960s by E.I. Du Pont de Nemours and Co., and was soon recognized as an outstanding ion conductor for laboratory as well as for industrial electrochemistry. The perfluorinated polymeric backbone is responsible for the good chemical and thermal stability of the polymer. Nation membrane swollen with an electrolyte solution shows high cation conductivity, whereas the transport of anions is almost entirely suppressed. This so-called permselectivity (cf. Section 6.2.1) is a characteristic advantage of Nation in comparison with classical ion-exchange polymers, in which the selective ion transport is usually not so pronounced. 

Solid electrolytes are frequently used in studies of solid compounds and solid solutions. The establishment of cell equilibrium ideally requires that the electrolyte is a pure ionic conductor of only one particular type of cation or anion. If such an ideal electrolyte is available, the activity of that species can be determined and the Gibbs energy of formation of a compound may, if an appropriate cell is constructed, be derived. A simple example is a cell for the determination of the Gibbs energy of formation of NiO ... 

The charge on the surface of an ionic conductor arises from a local excess of cations over anions or anions over cations. For example if the surface of Na-) -Al203 has a positive charge there will be an excess of Na ions in the surface of the electrolyte over the number of Na" ions which would be required to maintain electroneutrality. Likewise if the surface has a negative charge there will be an overall deficit of Na" ions compared with the number required for electroneutrality. For a metal surface the surface charge (an excess or deficit of electrons) is generally assumed to be within 10 pm of the surface. 

The conductance of an electrolyte solution characterizes the easiness of electric conduction its unit is reciprocal ohm, = siemens = S = A/V. The electric conductivity is proportional to the cross-section area and inversely proportional to the length of the conductor. The unit of conductivity is S/m. The conductivity of an electrol3de solution depends on the concentration of the ions. Molar conductivity, denoted as X, is when the concentration of the hypothetical ideal solution is 1 M = 1000 mol/m. Hence, the unit of molar conductivity is either Sm M , or using SI units, Sm mol . For nonideal solutions, X depends on concentration, and the value of X at infinite dilution is denoted by subscript "0" (such as >,+ 0, and X for cation and anion molar conductivity). The conductivity is a directly measurable property. The molar conductivity at infinite dilution may be related to the mobility as follows ... 

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