I. Riess

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Characterization of adsorbed water layers on Y2O3-doped ZrO2

Chemisorption and physisorption of water from a humid atmosphere, on the oxide Y2O3-doped ZrO2 (YSZ), were investigated by conductivity measurements and thermogravimetry (TG), in the temperature range 35–700°C. Ionic conduction is indicated to take place in a thin layer of water adsorbed on the oxide. That ionic conduction is most probably proton conduction. At T<600°C, it can exceed the oxygen ion conduction within the YSZ bulk when the oxide is made of fine powder. The electrical conductivity measurements of YSZ fine powder in humid atmosphere are analyzed in terms of water coverage, defect formation and proton mobility in the water layers. Further analysis of the water coverage is done using TG measurements. The enthalpies of defect formation and migration are determined. Results of preliminary conduction measurements of Al2O3 fine powder in a humid atmosphere reveal considerable similarities to those of YSZ.

Electrochemical Characteristics of Cathodes in Solid Oxide Fuel Cells Based on Ceria Electrolytes

The ionic current-overpotential characteristics of cathodes on ceria-based electrolytes have been evaluated by galvanostatic current-interrupt measurements. The measurements were carried out on mixed conducting ceria electrolytes under fuel cell operating conditions. The effect on the oxygen reduction kinetics of the simultaneous transport of electrons and oxygen was investigated. As model substances for cathodes were La{sub 0.8}Sr{sub 0.2}MnO{sub 3}, La{sub 0.84}Sr{sub 0.16}CoO{sub 3}, Pt, Ag, and Au. Cathode reaction mechanisms as a function of the transport properties are discussed for the different cathode materials. Steady-state cathode overpotentials were interpreted using a Butler-Volmer-type equation to describe charge-transfer processes and to evaluate exchange current densities. At 700 C the exchange current density of the best cathode La{sub 0.84}Sr{sub 0.16}CoO{sub 3} was 180 mA/cm{sup 2} whereas the one for La{sub 0.8}Sr{sub 0.2}MnO{sub 3} was only 30 mA/cm{sup 2}. In both cases the authors identified charge-transfer on the cathode material as the rate-limiting step.

Mixed ionic–electronic conductors—material properties and applications

Mixed ionic–electronic conductors (MIECs) are examined first with respect to their possible applications and then with respect to their properties. The emphasis is on electrochemical cells in which a MIEC serves either as an electrode or it replaces the solid electrolyte (SE). The dependence of the I–V relations on point defects nature, concentrations and local neutrality is discussed. The question whether it is possible to introduce significant changes in the defect concentrations by doping is examined and shown to have, in many cases, a negative answer. In MIECs the partial electronic (electron/hole) and ionic currents have to be treated separately and so also the corresponding partial conductivities. Methods for determining the partial conductivities are discussed.

Specific heat and phase diagram of nonstoichiometric ceria (CeO2−x)

The phase diagram for nonstoichiometric ceria, CeO2−x, was determined from specific heat measurements in the temperature range 320–1200 K and composition range CeO2CeO1.72. Coexistence temperatures of three phases are found at 722, 736, 766, 913, and 1084 K. There is some indication for the existence of two other coexistence temperatures at 850 and at 880 K. The maximum of the miscibility gap occurs at T = 910 K and 2 − x = 1.93. The phase diagram exhibits some phases in the homologous series CenO2n−2 with n = 7, 10, 11, and two phases at 2 − x = 1.79 and 2 − x = 1.808 not belonging to this series.

A percolation treatment of high-field hopping transport

A previous percolation theory for hopping conduction is extended to high fields, and applied specifically to the Mott (1969) model (a constant density of states). The importance of nearest-neighbour pair correlations, the local chemical potential, and certain spatial correlations are discussed. The directional constraints created by the spatial correlations and by the nature of the percolation cluster are included approximately. The effect of the local chemical potential is included in an approximation similar to the mean field approximation. The theory is worked out primarily in the moderate field regime. The conductance is found to be proportional to exp (-A+eFl/kT), where exp(-A) is the low field conductance, F the electric field, and l a fraction of the characteristic low-field hopping distance rm. For three dimensions, l=0.17 rm, and for two dimensions, l=0.18 rm. Expressions are also derived for the high-field limit, and agree functionally with derivations by others.

Current-voltage relation and charge distribution in mixed ionic electronic solid conductors

Abstract Current-voltage relation and charge distribution in mixed ionic electronic solid conductors (MC) are evaluated for dc steady state conditions. Explicit dependence of the electronic and ionic currents on the applied voltage, V , are given for two models: (I) for a MC with a large and uniform ion disorder as, e.g., in δ-Bi 2 O 3 , doped CeO 2− x , stabilized and reduced ZrO 2 or α-AgI and (II) for a MC with equal concentrations of quasifree electrons and mobile ions as, e.g., in CeO 2− x . The composition of the current in model I depends on V . It is mainly electronic for V close to the polarization voltage and is mainly ionic for V → 0. In model II the current is mainly electronic. In model I the concentration of electrons, n , is not uniform when the MC is in contact with a nonunifonn surrounding. Furthermore, n and the stoichiometry depend both on V . When electrons are present dominantly in one side and holes in the other side of a MC, a p - n quasijunction is formed which depends on the applied voltage V . The charge distribution for V = 0 and for V equals the polarization voltage, is calculated.

Defect chemistry of Cu2−yO at elevated temperatures. Part II: Electrical conductivity, thermoelectric power and charged point defects

Abstract The electrical conductivity and the Seebeck coefficient of Cu 2 O were measured as a function of temperature and oxygen partial pressure. The measurements were performed between 900 K and 1300 K and between 10 −12 atm and 0.15 atm. The results indicate that the dominant electronic charge carriers are holes, although at high temperatures (⩾ 1200 K) and low oxygen partial pressure (⩽ 10 −5 atm) there is also a significant contribution of electrons to the electrical conductivity and Seebeck coefficient. The dominant ionic point defects are doubly charged oxygen interstitials O″ i , dominating at temperatures above 1150 K, and singly charged copper vacancies V′ cu , dominating at temperatures below 950 K. The values of the enthalpy and entropy for the formation of the charged defects were found. The mobility values of holes and electrons were determined in the temperature range of 1000 K ⩽ T ⩽ 1250 K. The hole mobility is 3 ⩽, v h ⩽ 6 cm 2 /Vs, and it decreases when the temperature increases. The electron mobility is higher than that of holes, with values of 150 ⩽ v e ⩽ 200 cm 2 /Vs. The variation of the Fermi level within the Cu 2 O phase as a function of oxygen partial pressure and temperature was also determined.

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

The relation between cell voltage (Vcell), applied chemical potential difference (Δμ(O2)) and cell current (It) 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 (Ii) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce0.8Sm0.2O1.9 and Ce0.8Gd0.2O1.9 electrolytes with La0.8Sr0.16CoO3 or Pt as the cathode and Ni/Ce0.9Ca0.1O1.9−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.

Perovskite cathodes for solid oxide fuel cells based on ceria electrolytes

The ionic current-overpotential characteristics of cathodes on ceria-based electrolytes have been evaluated by galvanostatic current-interruption measurements under fuel cell operating conditions. The effect of either mixed ionic electronic conductivity or high oxygen diffusivity on the oxygen reduction kinetics was investigated. La0.8Sr0.2MnO3 served as a model substance for cathodes with a high activity for oxygen reduction and a low oxygen ionic conductivity. The behaviour of La0.8Sr0.2MnO3 is compared to that of La0.84Sr0.16CoO3 which shows high activity for oxygen reduction and high oxygen diffusivity.