Verlag des Forschungszentrums Jülich
JUEL-3414
The electrode reactions for three different H2/H2O mixtures (P(H2) = 0.19 bar/p(H2O) = 0.05 bar;
p(H2 = 0.48 bar/p(H2O) = 0.05 bar; p(H2 = 0.48 bar/p(H2O) = 0.12 bar) were investigated as a
function of the electrode potential and the temperature. Different macrokinetics are obtained in a
low temperature region (725°C - 845°C) and a high temperature region (845°C - 950°C). At
726 °C the apparent reaction order of the H2-oxidation reaction for hydrogen, mH', is close to 0.5
and is nearly independent of the electrode potential. The apparent activation enthalpy of the H2-
oxidation rate, [Delta]HOx', is 145 kJ/mol at the rest potential and decreases linearly with increasing
anodic overpotential with a slope of -0.7 e V/V .The apparent anodic charge-transfer coefficient,
[alpha]a', in the low temperature region is approximately 0.7, independent of temperature. In the high
temperature region mH' decreases steeply from 0.6 to almost zero with increasing electrode
potential. It is concluded that a charge transfer reaction, which is most likely the oxidation of
adsorbed H atoms on the Ni surface, determines the H2-oxidation reaction rate in the low
temperature region. At higher temperatures, the results can be explained by an almost
completely covered Ni-surface, indicating a limited number of available adsorption sites. For
the H2-evolution reaction the potential and temperature dependence differs significantly from
the H2-oxidation reaction. The apparent activation energy , [Delta]HRed', is 90 kJ/mol in the low- T
region. The apparent reaction orders of the H2-evolution reaction for water, mw', at high
cathodic potentials (0.3 in the low-T region and 0.6 in the high-T region) indicate a complex
H2-evolution mechanism most likely involving adsorbed intermediates. Impedance spectra for
the H2/H2O reaction at the rest potential show three time constants which are influenced in
different ways by the temperature and the partial pressure of the reactants. The impedance
spectra are analysed using an equivalent circuit representing two reaction pathways
(adsorption path and diffusion path) in parallel to the charging of the electrochemical double
layer. A charge transfer resistance, correlated to the electrochemical oxidation of adsorbed
H atoms on the Ni-surface, dominates the total faradayic resistance in the adsoption path. The
diffusion path is tentatively attributed to the oxidation of H atoms at the Ni/YSZ two phase
interface limited by diffusion of protons or hydroxides in the YSZ-electrolyte.
The reaction of CO/CO2 mixtures on Ni-YSZ cermet electrodes was investigated as a function
of the electrode potential and the partial pressures of the reactants at 1000 °C. Time dependent
reaction rates are observed for the CO-oxidation reaction in the oxygen partial pressure range
from 3·10-13 bar to 2·10-17 bar. The electrode changes between an inactive state and several
active states for the CO/CO2 reaction. Periodical changes between different active states are
observed every 30 s and 80 s. Carbon formation on single crystal domains on the Ni surface
followed by a reconstruction of the Ni surface is discussed to explain these results. In the
active state the CO-oxidation reaction is more than one order of magnitude slower than the
H2-oxidation reaction on these kinds of Ni-YSZ cermet electrodes.
Holtappels, Peter
Die Elektrokatalyse an Nickel-Cermet Elektroden
115 S., 1997
Ni-YSZ cermets are the most commonly used electrode materials for Solid Oxide Fuel Cell
(SOFC) anodes to oxidise reformed methane, a mixture of hydrogen, water, carbon monoxide
and carbon dioxide. Ni- YSZ cermet electrodes made from a mixture of 40 vol% Ni and 60 vol%
yttria (8mol%) stabilised zirkonia (8YSZ) were investigated electrochemically in hydrogen/water
(H2/H2O) and carbon monoxide/carbon dioxide (CO/CO2) atmospheres in order to obtain an
insight into the fundamental electrode processes. Quasi steady state current voltage
measurements and impedance spectroscopy were performed in a three electrode configuration.
Annular shaped Ni-YSZ cermet electrodes, screen printed on 150µm thick YSZ-electrolyte foils
and circular shaped Ni-YSZ cermet electrodes, sprayed on specially designed YSZ-electrolyte
pellets (Risø-3 electrode pellets), were used for studying the H2/H2O reaction and the CO/CO2
reaction, respectively. Both electrode configurations are shown to ensure reliable
electrochemical single electrode measurements.
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