Page 256 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrode Polarisations 23 3

electrolyte (typically -10 microns), and highest in electrolyte-supported cells.
Anode-supported cells thus exhibit higher performance.

9.3 Concentration Polarisation
In fuel cells, the reacting species are gaseous; at the anode H2 (or H2 + CO), and at
the cathode 02. At the anode, H2 (or H2 + CO) must be transported from the fuel
stream, through the porous anode, to (or near) the anode/eIectrolyte interface.
Hydrogen (or H2 + CO) then reacts with oxide ions transported through the
electrolyte, at or near the anode/electrolyte interface, to formH20 (or H20 + C02),
and release electrons to the anode, for their subsequent transport to the cathode,
through the external circuit. The H20 (or H20 + C02) formed must be transported
away from the electrolyte/anode interface, through the porous anode, to the fuel
stream. This transport of H2 (H2 + CO) and H20 (H20 +C02) must be consistent
with the net current flowing through the cell, adjusted for appropriate charge
balance/mass balance parameters. In steady state, the following equality

must be obeyed, where jH2 and jco are respectively the fluxes of hydrogen and
carbon monoxide through the porous anode to the anode/electrolyte interface,
1~~0 jco2 are respectively the fluxes of water vapor and carbon dioxide
and
through the porous anode, away from the anode/electrolyte interface, ioz is the
flux of oxygen through the porous cathode, to the cat.hode/electrolyte interface,
and NA is the Avogadro’s number.
For simplicity, the following discussion is confined to pure hydrogen as the
fuel. Thus, equation (6) reduces to

Transport of gaseous species usually occurs by binary diffusion, where the
effective binary diffusivity is a function of the fundamental binary diffusivity
DH,-H~o, and microstructural parameters of the anode [3, 41. In electrode
microstructures with very small pore sizes, the possible effects of Icnudsen
diffusion, adsorption/desorption and surface diffusion may also be present. The
physical ‘resistance’ to the transport of gaseous species through the anode at a
given current density is reflected as an ‘electrical voltage loss’. This polarisation
loss is known as concentration polarisation, q&nc, and is a function of several
parameters, given as

rfone = ~(DH~-H~~, Microstructure, Partial Pressures, Current Density) (8)

where DH~-H?o is the binary H2-H20 diffusivity. It is assumed here that the effects
of Knudsen diffusion, adsorption/desorption and surface diffusion are negligible.
The increases with increasing current density, but not in a linear fashion. A