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In this work, 10% Gd 3+ -doped cerium pyrophosphates (CGPs) with core-shell structure are synthesized by reacting Ce 0.9 Gd 0.1 O 2 with H 3 PO 4 in a two-step solid-state slow digestion method. Use of high P/(Ce+Gd) ratio in initial reaction mixture gives a coreshell morphology composed of crystalline CGP core and amorphous phosphate (P m O n ) shell. The amorphous phosphate helps in densification of CGP pellets during sintering, without causing the appearance of any impurity. Variation of ionic conductivity of CGPs with temperature is studied in unhumidified and humidified air conditions, and is explained on the basis of microstructure, phosphate content and proton conduction mechanism. The basic dissolution of protons occurs in the crystalline pyrophosphate phase of material bulk at the oxygen vacancies formed due to the aliovalent doping of Gd 3+ and acidic dissolution of protons occurs in the amorphous phase due to the hydrolysis of P m O n groups. Ionic conductivities of CGP samples vary in 10 −3 −10 −2 range in 90-230 • C range in humidified air and maximum conductivity obtained is 2.91 × 10 −2 S cm −1 at 190 • C, pH 2 O = 0.16 atm. The pH 2 O dependence and long term response (for 450 h) of the ionic conductivity in humidified air is analyzed for potential application as electrolyte in proton-conducting ceramic electrolyte fuel cells.Materials with high proton conductivity have shown great potential in fulfilling the food and energy requirements of the everincreasing world population in a sustainable manner, by their application as electrolytes in electrochemical devices. 1-5 Proton conducting electrolyte-based fuel cells, which convert chemical energy to electrical energy cleanly and efficiently, has presented immense potential in this regard. 6-8 However, various material-related challenges have been major impediment toward the widespread commercialization of otherwise conceptually very simple fuel cells and, more often than not, these challenges arise from the choice of electrolyte, which has been the main deciding factor for various fuel cell configurations. 9,10 Among the various proton conducting electrolyte-based fuel cells types, the polymer electrolyte membrane fuel cells (PEMFCs) and proton conducting ceramic electrolyte fuel cells (PCFCs) have been widely studied for their widespread commercialization. However, these fuel cells suffer from a number of drawbacks. PEMFCs operate optimally ∼100 • C, require precious metal catalysts for electrode reactions and, because of being porous, waste fuel by chemical shortcircuit. Similarly, PCFCs operate optimally at >600 • C and its high operating temperature results in many limitations, such as higher system manufacture and maintenance costs, high performance degradation rates, slow startup and shutdown cycles. 11 Therefore, the development of new electrolytes with high conductivity in 100-600 • C range, which can lower the operating temperature of PCFCs and do away with the constraints on the operation of PEMFCs, is highly desired for fuel cells. 7,12 A...

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The thermal and chemical expansion properties of Ba 0.5 Sr 05 Co 0.8 Fe 0.2 O 3-δ , a potential candidate for oxygen permeable membranes and cathodes in solid oxide fuel cells, were measured in the temperature range of 600 • C to 900 • C and the oxygen partial pressure (pO 2 ) range of −4 to −0.67 log pO 2 /atm. The chemical expansion increased linearly with decreasing pO 2 and with increasing temperature, as interpreted by both the volume expansion due to the increased ionic radii of the B-site cations and the increased oxygen nonstoichiometry. The chemical diffusivity of oxygen and the surface exchange kinetics were successfully extracted from the transient chemical expansion relaxation profile for a small displacement from the equilibrium. The oxygen ionic conductivity was calculated from the Nernst-Einstein equation by combining the oxygen self-diffusivity from the chemical diffusivity and the thermodynamic enhancement factor from the oxygen nonstoichiometry data in the literature.

The electrical conductivity, Seebeck coefficient, and thermal chemical expansion properties of La2NiO4+δ were measured as a function of temperature and oxygen partial pressure (pO2). The electronic conductivity was analyzed in relation to the thermoelectric power (thermopower) data to elucidate the positive deviation of the activity coefficient of hole on the basis of the delocalized electron model by using the Joyce-Dixon approximation of the Fermi-Dirac integral from the partition function in the quasi-free-particle approximation with the regular solution model. The electrical conductivity and thermopower vs oxygen activity were all explained consistently in the logical frame of the degenerate p-type conductor across the entire experimental range investigated. Like other K2NiF4–type oxides, the chemical diffusion coefficient was slightly higher than the surface exchange coefficient, suggesting the need for further work to enhance the sluggish surface exchange kinetics of oxygen. The best-estimated mobility values were in reasonable agreement with the reported values and the very weak temperature dependence of hole mobility suggested a band conduction.