Page 188 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Anodes 165

in interfacial polarisation with 55% C02 and CO as fuel. Evidence is now
emerging [36] that while the polarisation is little influenced by CO partial
pressure over a wide range, decreasing towards P(co) = 1 atm, the effect of
carbon dioxide is uniformly to reduce polarisation with increasing partial
pressure, with a reaction order of 0.5. Since oxygen partial pressure is not an
independent variable given that

a reaction order of 0.5 for COa is equivalent to 0.25 for oxygen. The adsorbed
species are then identified as oxygen - probably on oxygen vacancies near the
three-phase boundary, and CO on the metal. In [36] it was also recorded that
impedance spectra were difficult to obtain at low frequencies, due to instability.
This is in accord with the observation of high electrical noise on electrodes
exposed to CO [37]. It is tempting to consider this effect as associated with the
reversible Boudouard coking reaction. 2CO ++ C + C02, with occupation of CO
adsorption sites by carbon, followed by the reaction with C02 or electrochemical
oxidation to remove it.
When after reaction of hydrocarbon fuel with steam, both hydrogen and CO
with traces of C02 are admitted to an anode as reformate, the situation is even
more complex. The electrochemical oxidation rate of hydrogen is several
times faster than that for CO, the divergence increasing with temperature and the
water-gas shift reaction being faster than either [ 3 81. However, the concentration
of CO is a fraction of that of hydrogen, 14.9% of the total gas present when the
feedstock to the reformer has a steam to carbon ratio of 2 at 800°C. rising to
17.2% at IOOO'C. As a consequence the electrochemical depletion rates of the
two fuels are comparable, polarisation does not significantly rise, and there is no
accumulation of carbon species in the system. These laboratory results are fully
confirmed by the successful long-term operation of SOFC systems on reformate
fuel mixtures in large-scale demonstration plants.

6.9 Anodes for Direct Oxidation of Hydrocarbons

Reforming of the hydrocarban fuel does present a balance of plant requirement
impacting on investment, maintenance and overalI system efficiency, providing
an incentive to develop systems and materials capable of sustaining direct
oxidation on a fuel cell anode. It has been known for some time that at
high current densities, with steam and carbon dioxide being formed
electrochemically, and therefore with a higher P(02) over the anode, methane
can oxidise on a nickel cermet without serious carbon deposition [39]. It is
presumed that the oxidation products of the cell reform the incoming fuel,
though to maintain a current and therefore a power density above the necessary
threshold [39] may not be a practical procedure in commercial operation.
Admission of some steam with the fuel, or an autothermal process by partial
oxidation, can extend the regime of operation, particularly with a co-catalyst as