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...
