Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage (V_cell), applied chemical potential difference (Δμ(O₂)) and cell current (I_t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I_i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 electrolytes with La_0.8Sr_0.16CoO₃ or Pt as the cathode and Ni/Ce_0.9Ca_0.1O_1.9−xor Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.     

Simple method to determine electronic and ionic components of the conductivity in mixed conductors a review

In materials that are of interest for use as solid electrolytes it is important that charge transport is predominantly related to ionic motion, with minimal minority electronic conduction. On the other hand, a number of materials are mixed ionic and electronic conductors. Several DC methods that have been developed over many years to evaluate the separate electronic and ionic components of the total charge transport in solids. These include the Tubandt DC method, the DC assymetric polarization technique sometimes called the Hebb-Wagner method, and the DC open circuit potential method. This paper presents a new method that is relatively simple, and could be advantageous in some cases. It involves the employment and proper interpretation of low amplitude variable frequency AC measurements, a general technique that is often called impedance spectroscopy. As an example, the results of the use of this method on a mixed-conducting perovskite related to the well-known LSGM, La_0.4Pr_0.4Sr_0.2In_0.8Mg_0.2O_2.8, are shown. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Radiation-induced electronic and ionic charge storage and release in sapphire

Radiation-induced thermally stimulated relaxation (TSR) processes in the reduced f -Al 2 O 3 (sapphire) crystal were investigated at 290-650 v K by means of the TS current (TSC), ionic depolarisation current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarisation of sapphire wide (～75 v K) and asymmetric ionic dipolar TSDC peak at T max , 590 v K (disorientation of the anion vacancy-related dipoles) was detected. This peak correlates with the wide TSEE peak at T max , 615 v K, the radiation-induced electrical degradation (RIED) yield rise above 550 v K ( T max , 745 v K) and the chromium emission line broadening in ruby. Above 450-500 v K the anion vacancy hopping (migration) starts. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphire (especially in vacuo near the sample surface, grain boundaries, dislocations) in various TSR (TSC, TSDC, TS heat release and bleaching) and RIED phenomena. Surface structure and impurity content, surrounding atmosphere (vacuum or air) and electric fields determine these phenomena.     

Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ), an O²⁻ conductor, TiO₂, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H⁺ conductor and also on Pd films deposited on YSZ and β″-Al₂O₃ a Na⁺ conductor. Four catalytic systems were investigated, i.e. C₂H₆ oxidation on Pt/YSZ, C₂H₄ oxidation on Pd/YSZ and Pd/β″-Al₂O₃, C₂H₄ oxidation on Pt/TiO₂ and H₂ oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Physical device models: A potential tool in investigating conductivity in ionic and mixed conductors

Extensive work has been done in the last three decades on modelling classical semiconductor devices through physical device models. The majority of these models is based on the simultaneous solution of Poisson's equation, current and continuity equations for electrons and holes using iterative techniques. The vast work done on classical semiconductor devices has been extended to include the study of the motion of charged species upon the application of external fields in insulators, ionic materials, mixed conductors or proton conductors. In this paper the methods used are discussed together with their potential in leading to an understanding of the mechanisms governing charge transport in materials exhibiting more complex conductivity than classical semiconductors. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

On the relation between the collective modes of ionic liquids and superionic conductors

A recent discussion concerning the effects of the Coulomb forces on the spectrum of hydrodynamic modes of superionic conductors is updated. It is pointed out that in general the low-frequency charge-conduction mode cannot be disentangled from the high-frequency charge oscillation mode. For moderate Coulomb effects the chargedensity fluctuation spectrum of superionic-conductors should reveal an optic quasihydrodynamic peak analogous to the one already found in ionic liquids.Association Euratom-Etat Belge     

Investigation of phase transitions of solid ionic conductors by automatic impedance spectroscopy

The first order phase transitions of the solid ionic conductors AgI, Ag₂HgI₄, Cu₂HgI₄, and CuTeBr at 420, 320, 343, and 348 K respectively, were investigated by automatic low frequency (100 Hz to 10 MHz) impedance spectroscopy. Measurements showed all four transitions covering a rather broad interval of temperatures, i.e. 3 to 5 K with AgI and CuTeBr, and about 8 K with Ag₂HgI₄ and Cu₂HgI₄, as well as possessing a hysteresis between 2.5 K (CuTeBr) and 4.4 K (AgI).     

Reducing over potential by surface mixed ionic–electronic conduction

We report I–V measurements on two distinct solid electrochemical cells which both exhibit increased conductivity at high biases. We propose that this is due to the introduction of mixed ionic–electronic conductivity into the solid electrolyte near surface region, which decreases electrode over potential significantly. The decrease is attributed to an increase in the triple phase boundary area.     

Bond and charge ordering in low-dimensional organic conductors

We review 35 years of structural studies of quasi-1D organic conductors during which the concepts of 2k_F and 4k_F BOW and CDW have been elaborated. In strongly correlated quarter filled band systems these instabilities give rise to SP, DM and CO ground states. We relate these structural features to the instabilities of the 1D electron gas. To stabilize the different ground states the nature of the electron-phonon coupling has to be considered together with the coupling of the organic stacks with the anion sublattice. New results concerning the classification of the SP phase in connection with the adiabatic or antiadiabatic phonon field and its competition with the CO are also introduced.     

Bicyclic diironcryptates with mobile guest cations: A new class of solid ionic conductors

Polycrystalline bicyclic diironcryptates with different mobile guest cations were synthesized and characterized by XRD analysis, IR spectroscopy, FAB-mass spectroscopy, elemental analysis, electrical conductivity spectroscopy and ion exchange experiments. We find strong indication that the guest cations exhibit a long-range mobility and that their mobility is governed by their size relative to the size of the cryptand molecules. On the other hand, the valence of the guest cations seems to play a minor role for the mobility. These results are discussed in comparison to the properties of conventional solid cationic conductors.     

Charge densities and charge noise in mesoscopic conductors

We introduce a hierarchy of density of states to characterize the charge distribution in a mesoscopic conductor. At the bottom of this hierarchy are the partial density of states which represent the contribution to the local density of states if both the incident and the out-going scattering channel is prescribed. The partial density of states play a prominent role in measurements with a scanning tunneling microscope on multiprobe conductors in the presence of current flow. The partial density of states determine the degree of dephasing generated by a weakly coupled voltage probe. In addition the partial density of states determine the frequency-dependent response of mesoscopic conductors in the presence of slowly oscillating voltages applied to the contacts of the sample. The partial density of states permit the formulation of a Friedel sum rule which can be applied locally. We introduce the off-diagonal elements of the partial density of states matrix to describe charge fluctuation processes. This generalization leads to a local Wigner-Smith life-time matrix.     

Raman scattering in one-dimensional ionic conductors Rb-priderites

Raman spectra of single crystals of RbAl and RbMg-priderites, which are known to be one-dimensional ionic conductors, have been measured and analyzed. In addition to the spectra due mainly to the vibrations of Ti(Al)-O or Ti(Mg)-O frameworks observed in the frequency region above 100 cm^?1, the Raman bands, which can be assigned to the vibration modes of Rb⁺ ions, have been observed in the frequency region below 100 cm^?1. Raman spectra of the solid solutions (Rb, K)-Al priderites have also been studied.     

The haven ratio in fast ionic conductors

Measurements of the Haven Ratio are conventionally performed in order to extract unique information about the diffusion mechanism in ionic conductors. In this review article, the historical background for such measurements, their present status, results, and the current theories of interpretation are discussed in detail with emphasis on the special problems found in fast ionic conductors. Likely directions for future work are also pointed out.     

Origin of constant loss in ionic conductors

We have analyzed the constant loss contribution to the ac conductivity in the frequency range 10 Hz-1 MHz and temperatures down to 8 K, for two Li ionic conductors, one crystalline (Li(0.18)La(0.61)TiO(3)) and the other glassy (61SiO(2);35Li(2)O.3Al(2)O3.P(2)O(5)). As temperature is increased a crossover is observed from a nearly constant loss to a fractional power law frequency dependence of the ac conductivity. At any fixed frequency omega, this crossover occurs at a temperature T such that omega approximately nu(0)exp(-E(m)/k(B)T), where nu(0) is the attempt frequency and E(m) is identified with the barrier for Li+ ions to leave their wells.     

Ceria based mixed conductors with adjusted electronic conductivity in the bulk and/or along grain boundaries

Ceramic samples of Ce_1 ? xPr_xO_2 ? δ (CPO) and Ce_1 ? xGd_xO_2 ? δ (CGO) were obtained by different sintering schedules, including the use of cobalt as a sintering aid, added by mixing the precursor powders with cobalt nitrate solution; this allowed one to obtain different microstructural features and to change the transport properties, with emphasis on changes in grain boundary behaviour. Cobalt plays a double effect as sintering aid and also to induce important changes of grain boundary properties. Specific changes of grain boundary properties were ascribed by de-convolution of impedance spectra.Relatively high levels of mixed conductivity could be attained by adding a lanthanide species to yield ionic transport, whereas electronic conduction was promoted by the mixed valence character of PrO_x, combined with the additional contribution of Co-rich grain boundaries. These effects can be used to tune preferential electronic conductivity at bulk or grain boundary level. Oxygen permeability and a modified e.m.f. method were used to obtain the overall ionic transport number under oxidising conditions and its dependence on processing conditions. Additions of PrO_x induce bulk electronic conduction which assumes a greater role at lower temperatures. Further enhancement of electronic conductivity is attained by effects of Co-addition. Though Co-rich grain boundaries also yield significant levels of electronic conductivity in CGO, this contribution becomes minor at intermediate temperatures, due to differences between the activation energies for electronic and ionic conduction.     

Phase relations, ionic transport and diffusion in the alloys of Cu2S-Ag2S mixed conductors

Results are reported of the phase relations in the (Ag_x-δ/2Cu_1-x)₂ system, ionic conductivity, self-diffusion and chemical diffusion coefficients for the solid solutions as a function of the composition x, degree of non-stoichiometry δ, and temperature in the range 473 – 673 K. The extensions of the homogeneity regions for single-phases are determined. Total and partial ionic conductivity values are given for copper and silver ions for the solid solutions. Measurements of the self-diffusion coefficient and the correlation factors are reported. It is shown that for solid solutions that the chemical diffusion is well described in terms of the phenomenological theory of ionic transport in mixed ionic electronic conductors.