Theoretical Analysis of Solid Oxide Fuel Cells with Two‐Layer, Composite Electrolytes: Electrolyte Stability

Abstract

Theoretical analysis of solid oxide fuel cells (SOFCs) using two-layer, composite electrolytes consisting of a solid electrolyte of a significantly higher conductivity compared to zirconia (such as ceria or bismuth oxide) with a thin layer of zirconia or thoria on the fuel side is presented. Electrochemical transport in the two-layer, composite electrolytes is examined by taking both ionic and electronic fluxes into account. Similar to most electrochemical transport phenomena, it is assumed that local equilibrium prevails. An equivalent circuit approach is used to estimate the partial pressure of oxygen at the interface. It is shown that thermodynamic stability of the electrolyte (ceria or bismuth oxide) depends upon the transport characteristics of the composite electrolyte, in particular the electronic conductivity of the air-side part of the electrolyte. For example, the greater the electronic conductivity of the air-side part of the electrolyte, the greater is the interface partial pressure of oxygen and the greater is the thermodynamic stability. The analysis shows that it would be advantageous to use composite electrolytes instead of all-zirconia electrolytes, thus making low-temperature (~600-800~ SOFCs feasible. Implications of the analysis from the standpoint of the desired characteristics of SOFC components are discussed.The current SOFC technology is based on stabilized zirconia electrolytes which are stable in fuel (reducing) environment. The ionic conductivity of zirconia, however, is too low below about 950~ Consequently, most of the SOFCs are operated at -1000~ It is well known that the efficiency of a fuel cell is greater at lower temperatures. In this regard, the need for the development of solid electrolytes with substantially higher ionic conductivity compared to zirconia has been long recognized. Several researchers have demonstrated that CeO2 doped with alkaline earth or rare earth oxides exhibits ionic conductivity up to two orders of magnitude higher than zirconia at comparable temperatures (1-9). Ceria, however, exhibits a significant electronic conduction which is undesirable. Moreover, the electronic transference number increases in the presence of reducing atmospheres. Theoretical analysis of fuel cell operation with mixed conducting solid electrolytes has been conducted by Ross and Benjamin (10)

Stable and high conductivity ceria/bismuth oxide bilayer electrolytes for lower temperature solid oxide fuel cells

“…Bilayer electrolytes can be used to increase the thermodynamic stability of ESB and SDC [11][12][13]. However, in most of these investigations YSZ was used to protect these high conductivity materials from reduction by avoiding exposure of the reducing gas [11,12].…”

“…However, in most of these investigations YSZ was used to protect these high conductivity materials from reduction by avoiding exposure of the reducing gas [11,12]. In contrast, our bilayer electrolytes consist of SDC on the reducing side and ESB on the oxidizing side [13].…”

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

“…In many cases, this inconsistency often does not lead to significant problems. However, as will be discussed in this report, and has been discussed to some extent in earlier manuscripts, there are many situations in which the neglect of electronic current, however small, leads to substantial difficulties in analysis -and thus also in the interpretation of experimental results [14][15][16].…”

Modeling Degradation in Solid Oxide Electrolysis Cells

“…However, the electronic resistances cannot be mathematically infinite and thus cannot be dropped out of the analysis (and cannot be dropped out of the equivalent circuit) since this will violate local equilibrium and the chemical potential of oxygen will be indeterminate. 27,37 That is, e R is very large, but cannot be set to infinity a-priori. Note that the magnitude of the slope of V-I curve is ( ) c The positive direction of the current c. In the SOFC stack degradation model described previously, 7 the direction of the ionic current is always fixed.…”