Steady-state multiplicity in an oxygen-ion-conducting and a protonconducting solid oxide fuel cell
Mona Bavarian
Advisor: Prof. Masoud Soroush
Abstract:
Solid oxide fuel cells (SOFCs) have received considerable attention in recent
years. Efforts have been made to improve their design and operation, and to develop
mathematical models that can predict their steady-state and dynamic behavior
accurately. Steady-state multiplicity in an oxygen-ion-conducting SOFC and a protonconducting SOFC in three modes of operation, constant ohmic external load,
potentiostatic and galvanostatic, is studied using a detailed first-principles lumped
model. Steady-state multiplicity in SOFCs is a consequence of exponential increase in
the electrolyte ionic conductivity with electrolyte temperature. An increase in solid
(electrolyte) temperature reduces the electrolyte resistance with respect to oxygen
anion transport or proton transport, which in turn increases the current drawn from the
cell and accordingly increases the rate of the exothermic reactions occurring at the
electrolyte interface and therefore the electrolyte temperature. In summary, the positive
feedback between electrolyte temperature and the rate of heat generation by the
electrochemical reactions is responsible for the steady state multiplicity in SOFCs. In
the present study, the SOFC model is derived by accounting for heat and mass transfer
as well as electrochemical processes taking place inside the fuel cell. Conditions under
which the fuel cell exhibits steady state multiplicity are determined. The heat generation
term and the heat removal term are plotted in the same graph to find the intersection
points corresponding to steady states. The effects of operating conditions such as
convection heat transfer coefficient and inlet fuel and air temperatures and velocities on
the steady state multiplicity regions are studied. Depending on the operating conditions,
the cell exhibits one or three steady states. When there are three steady states, the
middle steady-state is unstable, while the other two are stable. For example, it has three
steady states (a) at low external load resistance values in constant ohmic external load
operation and (b) at low cell voltage in potentiostatic operation.