Oxygen mobility and surface reactivity of nanoceramic materials for hydrogen energy

Solid oxide fuel cells (SOFC) are environment friendly devices for power generation with internal or external (using oxygen/hydrogen separation membranes) fuel processing. The efficiency of these devices is determined by... [ view full abstract ]

Solid oxide fuel cells (SOFC) are environment friendly devices for power generation with internal or external (using oxygen/hydrogen separation membranes) fuel processing. The efficiency of these devices is determined by functional materials’ oxygen diffusivity and surface reactivity. Thus, tailor-made design of materials for membranes and SOFC solid electrolytes and electrodes requires reliable characterization of the oxygen self-diffusion coefficient D_O and surface exchange constant k_ex. A novel technique of D_O estimation based upon the oxygen heteroexchange dynamics analysis in the temperature-programmed mode (TPIE) between the material powder and C¹⁸O₂ was developed and verified by comparison with results of traditional methods. A higher rate of the surface exchange with CO₂ versus that with O₂ solves the problem of surface exchange limitation and makes D_O calculation more accurate. For solid electrolytes (Sc+Ce-, Y- or Gd-doped zirconia with fluorite structure, Fe/Al-doped lanthanum silicates with apatite structure, etc.) a strong negative effect of composition inhomogeneity on the oxygen mobility was demonstrated. For oxides with asymmetric structures where oxygen migration occurs via cooperative mechanisms, doping (i.e., La₂Mo₂O₉  by W, Ln₂NiO₄ (Ln = La, Pr, Nd) by Sr/Ca, etc.) disrupts such a cooperation movement, so fast and slow diffusion channels were revealed by TPIE C¹⁸O₂ as two separate peaks of exchange. For cathode nanocomposites comprised of ionic conductors (Y-doped ceria) and perovskites (PrNi_1-xCo_xO₃, etc) TPIE also revealed such two channels, with D_O values and their shares depending upon the cations redistribution between domains of phases and their interface. For proton-conducting materials (La_0.99Ca_0.01NbO₄, (Nd,La)_5.5(W,Mo)O_11.25‑δ and their nanocomposites) nonuniformity of the bulk oxygen is caused by structural features (the phase composition, local variation of cation distribution neighboring extended defects, etc.).

Different parts of this work were supported by the Russian Science Foundation (Project 16-13-00112) and the budget project #АААА-А17-117041110045-9 for Boreskov Institute of Catalysis.