Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell are consistent with the chemical-diffusion and surface-exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials.

In the power-to-gas process, hydrogen, produced by water electrolysis, is used as storage for excess, fluctuating renewable electric power. Reconversion of hydrogen back to electricity with the maximum possible efficiency is one pre-requisite to render hydrogen storage technically and economically viable. Pure oxygen is a byproduct in the electrolysis of water. The use of pure oxygen as the oxidant in a polymer electrolyte fuel cell (PEFC) is a possible way of increasing the conversion efficiency of hydrogen to power, by reducing the fuel cell's cathodic kinetic overvoltage, which is the most important energy loss process in low temperature PEFCs. As we demonstrate in this work, when using pure oxygen, either high efficiencies at current densities around 1 A cm À2 are obtained or a very high power density operation (up to 1.6 W cm À2 at cell voltages above 0.62 V) can be reached, giving the technology a broad window of operation and application. The fuel cell stack durability is assessed in accelerated longterm tests of up to 2700 h. The potential of the technology is demonstrated with the realization of a complete 25 kW prototype system delivering a peak efficiency of 69% LHV (57% HHV).

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A rigorous mathematical model is developed for the complex impedance of a solid-state electrochemical cell, which is commonly used for the measurement of oxygen transport, oxygen exchange kinetics, and thermodynamic properties of nonstoichiometric mixed conducting oxides. The model leads to a simple equivalent circuit for the cell with unambiguous definition of the physical significance of its components. A method is proposed for the analysis of experimental data. The methodology thus developed is validated by comparing the experimental data measured for a well-studied perovskite (SrCo 0.5 Fe 0.5 O 3Ϫ␦ ) with the results obtained from the completely equivalent potential-step technique. In addition, various electrochemical properties of the other cell components, such as Pt electrodes and yttria-stabilized zirconia electrolyte, also obtainable from measurements, show good agreement with the available literature data. The cell design, which significantly minimizes the gas space in contact with the sample, has a clear advantage over similar relaxation cells in terms of reducing the dominating effect of the gas-phase capacitance in numerical data analysis. A possible disadvantage, however, is the large impedance of the oxygen pump at low oxygen partial pressures, which may in a similar manner obstruct deconvolution of the sample properties from the measured data.Significant attention has been focused recently on the oxygen conduction properties of certain nonstoichiometric oxides as cathodes for solid oxide fuel cells 1,2 and as membranes for highly selective oxygen separation. 3 A variant of the latter application is related to membrane reactors for syngas production. 4 In such applications, reliable information is required about the nonstoichiometry of the material, kinetics of the surface exchange reaction for oxygen, as well as bulk diffusivity of oxygen, as a function of temperature and partial pressure of oxygen. Various techniques available for measuring these properties, mostly based on the relaxation of oxygen concentration in the material after a step change at a surface, have been widely discussed. [5][6][7][8][9][10][11] We have investigated the construction and methodology in relation to a solid-state electrochemical cell, first designed and applied by Belzner et al. 5 for simultaneous measurement of the chemical diffusion coefficient and the thermodynamic enhancement factor of oxygen in manganites. 6 The methodology had to be improved for studying materials with high diffusion coefficients such as SrCo x Fe 1Ϫx O 3Ϫ␦ because of rate limitation imposed by the surface exchange reaction of oxygen. 7,8 The difficulties involved technical problems in cell construction due to reactivity of the sampleelectrolyte interface and theoretical problems related to data analysis. In particular, the difficulty in extracting the surface exchange coefficient due to interference from the impedances of cell components other than the sample has been discussed. 12 In this connection, electrochemical impedance spectroscopy ͑E...

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