Transport through mixed proton, oxygen ion and electron/ hole conductors: Analysis of fuel cells and electrolyzer cells using Onsager equations

摘要

Steady state transport through mixed proton, oxygen ion and electron/hole conductors is examined in fuel cell, driven fuel cell, and electrolyzer modes. Oxygen ion and proton current densities and the corresponding effective permeation fluxes of H_2 and O_2 obey Onsager equations. Chemical potentials of electrically neutral species, μ_(H_2) And μ_(O_2) in the electrolyte are determined in terms of transport parameters and operating conditions. In fuel cell mode μ_(H_2) And μ_(O_2) in the electrolyte are bounded by electrode values. However, in electrolyzer and driven fuel cell modes, μ_(H_2) And μ_(O_2) in the electrolyte need not be bounded by electrode values. Under some conditions μ_(H_2) And μ_(O_2) in the electrolyte exceed electrolyte thermodynamic stability leading to device failure. The analysis shows that from the standpoint of stability, electrolytes exhibiting mixed ionic-electronic conducting properties with large interfacial electron transfer resistances are preferred over predominantly ionic conducting materials.