Mathematical Modeling and Steady-State Analysis of a Proton-Conducting Solid Oxide Fuel Cell

Generally, there are two types of solid oxide fuel cells based on the electrolyte used in the cell [1]. The electrolyte can be an oxygen ion-conducting or a proton-conducting material. The typical oxygen ion-conducting electrolyte is yttria-stabilized zirconia (YSZ), which requires the cell to be operated at high temperatures from 800 to 1200°C to exhibit high ionic conductivity. This is due to the high activation enthalpies of their conductivity [2]. Because of their high operation temperature, SOFCs with oxygen ion-conducting electrolytes have two major drawbacks: long start-up and shut-down, and  the high cost of materials that stand the high temperatures, in the manufacture of the fuel cells [3]. By lowering the operation temperature to 600-800°C, these challenges can be addressed to some extent. One way to develop a low temperature SOFC is to utilize a proton conducting electrolyte. Perovskite-type proton conductors, such as  SrCeO₃ and BaCeO₃, are doped with low valence cations such as Y³⁺or Yb³⁺ to create oxide ion vacancies, which are required for the formation of protonic defects [2, 4].

This study deals with a solid oxide fuel cell with SrCe_0.95Yb_0.05O_3-α (SCY) electrolyte and two platinum electrodes. A mathematical model of the proton-conducting SOFC is first developed. The model captures electrochemical processes as well as the transport phenomena. The model is validated with the experimental results of Iwahara [5] obtained under isothermal conditions. The fuel cell model is then used to study the existence of multiplicity in the cell under non-isothermal conditions. Simulation results show that a multiple steady states region exists at low inlet fuel and air temperatures under non-isothermal conditions. The occurrence of ignition and extinction in the cell solid (electrolyte, anode and cathode) temperature is observed. This result is in agreement with the studies on oxygen ion-conducting SOFCs in which the existence of multiplicity is attributed to the dependence of electrolyte oxygen-ion conductivity on temperature. The results show that the region of -steady-state multiplicity disappears as the inlet fuel and air temperatures increase.

References:

[1] A. Arpornwichanop, Y. Patcharavorachot, S. Assabumrungrat, Chemical Engineering Science, 65 (2010) 581-589.

[2] K. Kreuer, Ann. Rev. Mater. Res., 33 (2003) 333-359.

[3] D. Brett, A. Atkinson, N. Brandon, S. Skinner, Chem. Soc. Rev., 37 (2008) 1568-1578.

[4] H. Matsumoto, Y. Furuya, S. Okada, T. Tanji, T. Ishihara, Science and Technology of Advanced Materials, 8 (2007) 531-535.

[5] H. Iwahara, Solid State Ion., 28 (1988) 573-578.