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Abstract:
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Three cobalt -based oxides operating at the Co (III ) /Co (II ) redox couple have been investigated as potential cathode materials for the intermediate -temperature solid oxide fuel cells (IT -SOFCs ) . X -ray absorption spectroscopy measurements confirmed that both the oxygen -deficient perovskite Sr[subscript 0 .7]Y[subscript 0 .3]CoO[subscript 2 .65 -delta] (SYCO ) and the double -perovskite Ba₂[Co][Bi[subscript x]Sc[subscript 0 .2]Co[subscript 1 .8 -x]][subscript O6 -delta] (x = 0 .1 and 0 .2 ) (BBSC ) contain high -spin Co (III ) in the bulk at room temperature and thus avoid the thermally driven spin -state crossover of the Co (III ) ions usually observed in other cobalt -containing perovskite oxides . Electrochemical characterizations demonstrated that both cobalt oxides operating on the Co (III ) /Co (II ) redox couple are equally catalytically active for the oxygen reduction reaction as those operating on the Co (IV ) /Co (III ) redox couple . With an LSGM electrolyte -supported single test cell and NiO+GDC as anode , the maximum power densities Pmax at 800 ºC reach 927 and 1180 mW·cm⁻² for SYCO and BBSC cathodes , respectively . The oxygen -deficient perovskites Sr[subscript 1 -x]R[subscript x]CoO[subscript 3 -delta] (R = Eu -Ho , Y , x [approximately equal] 0 .3 ) are identified as a new class of cathode materials for IT -SOFCs in this dissertation . On the other hand , the layered Ba2Co9O14 (BCO ) containing the low -spin Co (III ) at room temperature undergoes a thermally driven spin -state crossover , which has prevented it from being evaluated as the cathode of IT -SOFCs . This problem was overcome by fabrication of a 50 -50 wt . % BCO + SDC (Sm[subscript 0 .2]Ce[subscript 0 .8]O[subscript 1 .9] ) composite cathode . The addition of SDC not only improved the adhesion to the electrolyte , but also enhanced the electrocatalytic activity for the oxygen reduction reaction . The composite cathode delivers a nearly stable P[subscript max] of ~450 mW·cm -2 at 800 °C in an LSGM electrolyte -supported single test cell . In addition , the electrochemical lithium intercalation process in the monoclinic Nb12O29 was studied with a Li /Nb₁₂O₂₉ half -cell , and the results showed that it can reversibly incorporate a relatively large amount of Li -ions in the voltage window of 2 .5 -1 .0 V at a slow discharge /charge rate while retaining structural integrity . Compared with that of the bare Nb₁₂O₂₉ , samples with carbon coating show an improved rate capability . The lithium insertion mechanism into Nb₁₂O₂₉ has also been discussed in terms of sites available to the lithium ions |