PNNL-13501 - Pacific Northwest National Laboratory

Simulation and Modeling of Electrochemistry and Performance Assessment of Solid Oxide
Fuel Stacks
Mohammad A. Khaleel, Kurt P. Recknagle, Z. Lin, John E. Deibler, John E. Jaffe, Rick E. Williford, Byung K. Ahn
Study Control Number: PN00081/1488
Development of methods for calculating current density, cell voltage, and heat production in planar solid oxide fuel cells
(SOFC) stacks with various fuels is critical for coupling an electrochemical model and thermo-fluids and stress analysis.
Project Description
The objective of this project was to develop
computational methods and simulation tools for
predicting electrochemical behavior in SOFC stacks, and
to gain a fundamental understanding of the effect of
various operation conditions on the V-i relation. This
project also was performed to provide analytical insight
into the correlation between cell efficiency and fuel use.
The technical work involved developing numerical
procedures to simulate the heat generation from Joule
heating and chemical reaction, and species production and
destruction via mass balance to analyze the dependence of
the Nernst potential on temperature and pressure. Also,
the effect of having CO oxidation at the anode as an
added contribution to the total current was investigated.
Introduction
Solid oxide fuel cells (SOFCs) produce direct current
power from fuel and oxidant via an electrochemical
process. The efficiency of such power generating
methods compares favorably with conventional thermal
power generation that is limited by Carnot-type
constraints. Current SOFCs represent a highly developed,
but immature technology. One advance necessary is to
increase efficiency. Although the design and operation of
a SOFC appears simple, many of the phenomena
dominating the performance of a SOFC are complex,
competing, and poorly understood; this is especially true
of the electrochemistry. Mathematical models are
required to incorporate the known physics and behavior of
SOFC materials to predict and improve performance. It is
well established that the cell direct current voltage and
current depend on conditions that include fuel flow,
oxidant flow, pressure, temperature, and the demands of
the load circuit. These parameters affect the
electrochemical processes that ultimately determine the
generated power and cell voltage. By taking into account
1) ohmic losses, 2) concentration polarization based on
144 FY 2000 Laboratory Directed Research and Development Annual Report
the assumption of binary diffusion of gaseous species
through porous electrodes, and 3) Tafel-type activation
polarization in composite electrodes, it is shown that the
V versus i traces can be adequately described by:
V(i) = E open – iR i – b sinh -1 (i/2i 0) + (RT/4F)ln(1-i/i O2) +
(RT/2F)ln(1-i/i H2) – (RT/2F)ln[1+p 0 H2i/(p 0 H2Oi H2)]
In the above equation, Eopen is the open circuit voltage,
usually referred as Nernst potential. Ri is the area specific
resistance of the cell, b is the Tafel parameter. iO2 and iH2
are respectively the cathode- and anode-limiting current
density, p 0 H2 and p 0 H2O are respectively the partial
pressure of hydrogen and water vapor in the fuel channel,
R is the gas constant, F is the Faraday constant, and T is
the temperature.
It should be noted, however, that the above equation is
valid only for a SOFC using H2 as the input fuel. To
accommodate multifuel capacity, which is one of the
main advantages of SOFCs, modification is required.
Fuels other than pure hydrogen generally contain CO,
reacting with H2O to produce CO2, and this reaction needs
to be included in the mathematical modeling for a more
accurate electrochemistry analysis. At present, PNNL is
in the process of including the CO effect in an alreadyestablished
electrochemistry code programmed by current
authors for pure hydrogen-fed SOFCs. This effort will
soon be linked to FEA modeling using Marc to complete
interfacing the electrochemistry analysis with the thermofluids
and stress analysis.
Results and Accomplishments
Electrochemistry Modeling
An electrochemistry code was written in Fortran based on
Kim et al. (1999). Prior to the main analysis, dependence
of Nernst potential on temperature and pressure has been
examined, and the summary is listed below.