Anwendung der Impedanzspektroskopie für die
Untersuchung von Biogasreformat in
Brennstozellen
Fachgebiet: Energieverfahrenstechnik und
Umwandlungstechniken regenerativer Energien
von
Wendelin Homann-Walbeck
Betreuer an der TU Berlin:
Univ. Prof. Dr. rer. nat. Frank Behrendt
Betreuer am Leibniz-Institut
für Agrartechnik Potsdam-Bornim e.V.:
Dr.-Ing. Volkhard Scholz
23. November 2007
Ich erkläre hiermit, dass ich die vorliegende Arbeit selbständig verfasst und keine
anderen als die angegebenen Quellen und Hilfsmittel verwendet habe.
Berlin, den 23. November 2007
Abstract
The purpose of the experimental work reported here is to examine the use of biogas
reformat in PEM fuel cells using electrochemical impedance spectroscopy. The ex-
periments focus on the fact that biogas reformat may contain a very high fraction
of carbon dioxide compared to natural gas reformat. Carbon dioxide is believed to
adsorb on the Platinum/Ruthenium catalyst of the cell and to react according to
the reverse water-gas shift reaction. This reaction produces carbon monoxide which
poisons the catalyst and reduces the cell voltage. The fuel cell is run at 50
, 60
and 70
using a Membrane Electrode Assembly containing a Pt/Ru (0,3 mg/cm
2
)
catalyst on the anode side. The analysis is done by electrochemical impedance spec-
troscopy during 8 hours of fueling with synthetic gas containing 40 % carbon dioxide.
The results are compared to the cell performance while fueled with a gas containing
inert nitrogen instead of carbon dioxide.
The change in the cell conditions is analyzed by characteristic curves and by the
drift in the Nyquist-plot. To allow a more detailed analysis of the impedance data
a model for the cell is used and parameter optimizations are carried out both for
fault conditions with dened amounts of carbon monoxide in the anode gas and for
non-fault conditions.
In the tests with 40 % carbon dioxide the occurrence of the reverse water-gas shift
reaction can not be conrmed. Nearly no negative eects can be observed compared
to anode gas containing 40 % nitrogen and 60 % hydrogen.
At cell temperatures of 70
and 60
no dierence can be observed. At 50
one of three identical Membrane Electrode Assemblies shows a very small power loss
during 8 hours that is within the range of the general cell instabilities. This loss is
still far smaller than that caused by 10 ppm carbon monoxide on the same system.
It can be concluded that 40 % of CO
2
in the anode gas do not cause any problems
with the used Catalysts for temperatures of 60
and above. At 50
there may
be a very small negative impact leading to a voltage loss of about 5 mV. As this loss
is observed in only one of three tested membranes further experiments will have to
be done for low cell temperatures.
Loading more pages...