Page 328 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

304 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and AppIications

electrochemical reactions. These reactions are not always simple: methane fuel,
for example, in the presence of steam may undergo steam reforming upstream of
the electrode reaction sites, so the overall heat generation may be due to a
multitude of anode reactions such as
H2 + 02- ---t H20 + 2e- (1 5a)

CO+H2O -+ C02+H2 (1 5d)

The cathode reaction, on the other hand, has a single stoichiometry:
1/202 + 2e- + 02- (1 5e)

The heat source is related to the enthalpy change of the reactions, and the
free-energy change of reactions (15a) and (15b) combined with (15e)
determines the fuel cell Nernst potential. If chemical equilibrium is achieved in
the system, the fuel composition, heat generation, and Nernst potential can be
determined from thermodynamic theory. However, chemical equilibrium is
usually not attained. In such cases, fuel composition and other information
cannot be rigorously determined and must be approximated. The details of the
reaction mechanism are complicated and usually not well understood, both for
electrochemical and chemical reactions.
For example, anodic hydrogen oxidation probably involves dissociative
hydrogen adsorption on the electrocatalyst (e.g., nickel) surface, surface
diffusion of hydrogen adatoms, electron transfer under oxidation of H adatoms
by an oxide ion at an adjacent active reaction site, and desorption of the H20
molecule formed. Unstable and bulk unknown species such as OH may function
as reaction intermediates. In chemical reactions, too, intermediates may play
a role. Partial oxidation pathways for CH4 may exist: e.g., CH4+0 + CH20+H2
and CH4+0 -+ CH30H, resulting in the formation of chemicals such as CH30H or
CH2O with concomitant energy loss for the SOFC. Similarly, reaction
intermediates such as 0 adatoms or adions may play a role.
Therefore, when equilibrium cannot be plausibly assumed, apparent kinetic
parameters (effective rate constants) must be used to express the reaction
rate. The parameters that describe the electrochemical reaction rate include
the above-mentioned exchange current density, the transfer coefficient, the
activation enthalpy, and the pre-exponential factor as well as the reaction order
of the species involved. These parameters are not necessarily related to a single
rate-determining step, as is often assumed in electrochemical theory. By
investigating i-q curves as functions of electrode potential, temperature, and
concentration of the reacting species, insight may be gained into the reaction
mechanism and microscopic transport processes (such as surface diffusion) that