Interfaces in ionic devices

Abstract

Abstract. The technology of Ionics is based on the availability of materials with fast ion transport. Individual materials are, however, meaningless from a practical point of view; all applications require combinations of materials with appropriate ionic and electronic properties. This situation is similar to Electronics which requires combinations of semi-conducting or metallic conducting materials with differences in the chemical potentials of the electrons. The technology of Ionics requires interfaces between ionic and electronic conductors which generate strong electrical fields or allow to modify the field by the application of external voltages. Ions and electrons equilibrate both at these "ionic junctions". While semi-conductor junctions have commonly a width in the ~tm-range, the space charge region is several orders of magnitude smaller in the case of ionic junctions, i.e. in the nm or even sub-nm-range. The interfaces have to be chemically stable for the lifetime of the device which is difficult to achieve in view of the commonly large number of components present in both phases and the existence of mobile species with sometimes large variations in the activity of the electroactive component. Furthermore, the kinetics of transfer of ions across the interface has to be fast to allow high current densities which are required in many cases. In addition, two such interfaces are required to convert the electronic current into an ionic one and again back into an electronic current at the opposite side of the electrolyte. The development of ionic devices depends to the strongest extent on the engineering of appropriate interfaces. Examples of the role and engineering of interfaces will be presented for applicationes such as chemical sensors, electrochromic devices, fuel cells, batteries and photogalvanic solar cells.

XPS characterisation of the interface between anode and electrolyte in a proton conducting solid oxide fuel cell

“…[2,3,5] This electric field will extend into the electrolyte to a depth of typically 0.1-1 nm, [3] influencing the concentration of the charged species in this region. It is thus expected to affect the ionic transport along and across the interface and also change the electrocatalytic properties of the system.…”

“…[1] A key issue is the electrochemical impedance (overpotential) at the electrodes, which is affected by the electrical and electrocatalytic properties of the materials involved and the formation of reactive layers at the electrode-electrolyte interface. [2,3] In this sense, understanding and engineering the interfaces between the electrode and electrolyte is of crucial importance. XPS is a characterisation technique that enables the study of changes in chemistry as well as in electrostatic potentials of atoms in materials.…”

The effect of local inhomogeneities on the performance of solid ionic devices

“…In evaluating this conductivity quite often the internal field, which is related to space charge according to Maxwell's third equation, is ignored mainly due to the fact that it exhibits high values at depths not greater than a few Debye lengths. Ignoring internal field has already been pointed out as one of the factors that limit the validity of results obtained through the Hebb-Wagner technique [8]. On the other hand, consideration of the internal field has lead to results in agreement with experiments or to the elimination of disagreements between theory and experiment [6,9].…”

Development and investigation of perovskite (ABO3)-type oxides for power generation

“…In spite of some loss in this regard, this device has several major advantages [31]: Since cost is a major concern for the commercialization of ionic devices such as SOFCs, especially in competition with existing technologies, the SEA concept shows the possibility to generate ionic junctions over very narrow regimes of 1 nm up to the gm-range. …”

“…Among the alkaline earth ions occupying the A-site, Sr provides the highest conductivity compared to Ca and Ba [11]. As far as B-site substitutions are concerned, [31][32][33]. Figure 2 shows the typical propetty required for the metal oxide to be employed in SOFCs based on the SEA concept.…”