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

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 H2 and O2 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.