Page 326 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

302 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

whose cell voltage (emf), opposing the overall SOFC voltage, is observed as a
voltage loss contribution, i.e., concentration polarisation. Thus, oxygen partial
pressure in the cathode pores near the cathode/electrolyte is lower than that in
the air channel. The more difficult the transport of oxygen through the porous
medium, the greater the concentration polarisation at the cathode. Thus, a thick
cathode in cathode-supported cells gives rise to high concentration polarisation
even at moderate current densities. To lower concentration polarisation at high
current densities to acceptable levels, the cathode should be as thin as practically
feasible and the porosity and pore size as large as possible.
Excessive mass transfer resistance may cause a current limitation if the
reactant concentration at the reaction site becomes small. In the extreme, that
concentration may become zero (or rather, negligibly small). The current, in that
case, reaches a plateau called the limiting current for the reactant species in
question. With a number of simplifying assumptions, the limiting current
concept can be used to derive a simple one-parameter expression for the
concentration polarisation:
vC = (RT/nF))ln(l - i/&) (11)

Here i, is the limiting current for the reacting species, i.e., O2 for the cathode,
with n = 4, and H2 or CO for the anode, with n = 2 [9]. The limiting current of a
species depends on its diffusivity in the surrounding gas mixture, its partial
pressure, and the porosity, tortuosity, and thickness of the electrode. For H2 fuel,
the limiting current density can be calculated [ 151 as:
f~z 2~~2Deff(a)/(CIa) (12)
=

while for air as oxidant, the O2 limiting current density is

where P is the air pressure and I, and IC are respectively the anode and cathode
thickness. The effective diffusion coefficients are given in terms of binary
diffusion coefficients, porosities (V,,,), and tortuosities (T~,~):

Deff(c) = vcD02-N2/rc ( 144

Analogous results can be obtained with CO as fuel. Because the anode binary
diffusion coefficient, DH2-H20, is about four times that of the cathode counterpart,
D02-N2, the cathode would have a much larger concentration polarisation than
that of the anode for similar thickness, porosity, and tortuosity. Fairly thick
anodes may be used without incurring excessive voltage loss. This is one of the
reasons why anode-supported designs are preferred over cathode-supported
designs in the thin-electrolyte intermediate temperature SOFCs.