Page 253 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

230 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and AppIicntions

kads Rate constant for adsorption
kdes Rate constant for desorption
ked Rate constant for reduction
$, Rate constant for oxidation

9.1 Introduction
Polarisation is a voltage loss or overpotential, which is a function of current
density. It can be broken down into a number of terms, originating in various
phenomena that occur when a finite current flows in a cell. The three dominant
polarisations are: (a) ohmic polarisation or ohmic loss: (b) concentration
polarisation; and (c) activation polarisation. This chapter defines and discusses
these polarisations, and describes methods to measure them.
Solid oxide fuel cells (SOFCs) generally operate above 600°C, with the typical
operating range being from 800 to 1000°C. High temperature operation makes
it possible to use hydrocarbon fuels once they have been processed to form a
gaseous mixture of H2 and CO, with appropriate amounts of H20 and C02 present
in the fuel to prevent the deposition of solid carbon. The SOFC thus can use CO as
a fuel in addition to Ha. Even in the case of SOFC, however, the largest
component of the fuel mixture is H2. For this reason, and for the sake of
simplicity, much of the discussion in this chapter is restricted to hydrogen as the
fuel and oxygen (air) as the oxidant.
The overall reaction in an SOFC is the oxidation of Hz to formHzO, namely,
H2(gas, anode) +$02(gas, cathode) + HzO(gas, anode).

Under open circuit conditions, with electrochemical potential of oxide ions
equilibrated across the oxide-ion conducting electrolyte, a voltage difference, E,
the Nernst potential, appears between the anode and the cathode. It is related to
the net free energy change, AG, of the reaction via the following relation [1,2]
AG = -nFE = -2FE (1)

where n denotes the number of electrons participating in the reaction. The
Nernst potential, E, is the open circuit voltage, OCV, and is given in terms of the
various partial pressures by [ 1,2]

where p$2 is the partial pressure of oxygen in the cathode gas, p"H, and pE20 are
respectively the partial pressures of H2 and H20 in the anode gas, R is the gas
constant, F the Faraday constant, and T the absolute temperature.
In what follows, it is assumed that partial pressures of the various species,
namely, p$2, pS2, and &20 , are fixed just outside of the electrodes, regardless of