TY - JOUR
T1 - Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes
AU - Graves, Christopher R.
AU - Chatzichristodoulou, Christodoulos
AU - Mogensen, Mogens Bjerg
PY - 2015
Y1 - 2015
N2 - The solid oxide electrochemical cell (SOC) is an energy conversion technology that can be
operated reversibly, to efficiently convert chemical fuels to electricity (fuel cell mode) as
well as to store electricity as chemical fuels (electrolysis mode). The SOC fuel-electrode
carries out the electrochemical reactions CO2 + 2e 4 CO + O2 and H2O + 2e 4
H2 + O2, for which the electrocatalytic activities of different electrodes differ
considerably. The relative activities in CO/CO2 and H2/H2O and the nature of the
differences are not well studied, even for the most common fuel-electrode material, a
composite of nickel and yttria/scandia stabilized zirconia (Ni–SZ). Ni–SZ is known to be
more active for H2/H2O than for CO/CO2 reactions, but the reported relative activity
varies widely. Here we compare AC impedance and DC current–overpotential data
measured in the two gas environments for several different electrodes comprised of
Ni–SZ, Gd-doped CeO2 (CGO), and CGO nanoparticles coating Nb-doped SrTiO3
backbones (CGOn/STN). 2D model and 3D porous electrode geometries are employed
to investigate the influence of microstructure, gas diffusion and impurities present at
reaction sites.
Comparing model and porous Ni–SZ electrodes, the ratio of electrode polarization
resistance in CO/CO2 vs. H2/H2O decreases from 33 to 2. Experiments and modelling
suggest that the ratio decreases due to a lower concentration of impurities blocking the
three phase boundary and due to the nature of the reaction zone extension into the
porous electrode thickness. Besides showing higher activity for H2/H2O reactions than
CO/CO2 reactions, the Ni/SZ interface is more active for oxidation than reduction. On
the other hand, we find the opposite behaviour in both cases for CGOn/STN model
electrodes, reporting for the first time a higher electrocatalytic activity of CGO
nanoparticles for CO/CO2 than for H2/H2O reactions in the absence of gas diffusion
limitations. We propose that enhanced surface reduction at the CGOn/gas two phase
boundary in CO/CO2 and in cathodic polarization can explain why the highest reaction
rate is obtained for CO2 electrolysis. For all materials investigated, large differences observed between model electrode kinetics and porous electrode kinetics are modelled
and discussed.
AB - The solid oxide electrochemical cell (SOC) is an energy conversion technology that can be
operated reversibly, to efficiently convert chemical fuels to electricity (fuel cell mode) as
well as to store electricity as chemical fuels (electrolysis mode). The SOC fuel-electrode
carries out the electrochemical reactions CO2 + 2e 4 CO + O2 and H2O + 2e 4
H2 + O2, for which the electrocatalytic activities of different electrodes differ
considerably. The relative activities in CO/CO2 and H2/H2O and the nature of the
differences are not well studied, even for the most common fuel-electrode material, a
composite of nickel and yttria/scandia stabilized zirconia (Ni–SZ). Ni–SZ is known to be
more active for H2/H2O than for CO/CO2 reactions, but the reported relative activity
varies widely. Here we compare AC impedance and DC current–overpotential data
measured in the two gas environments for several different electrodes comprised of
Ni–SZ, Gd-doped CeO2 (CGO), and CGO nanoparticles coating Nb-doped SrTiO3
backbones (CGOn/STN). 2D model and 3D porous electrode geometries are employed
to investigate the influence of microstructure, gas diffusion and impurities present at
reaction sites.
Comparing model and porous Ni–SZ electrodes, the ratio of electrode polarization
resistance in CO/CO2 vs. H2/H2O decreases from 33 to 2. Experiments and modelling
suggest that the ratio decreases due to a lower concentration of impurities blocking the
three phase boundary and due to the nature of the reaction zone extension into the
porous electrode thickness. Besides showing higher activity for H2/H2O reactions than
CO/CO2 reactions, the Ni/SZ interface is more active for oxidation than reduction. On
the other hand, we find the opposite behaviour in both cases for CGOn/STN model
electrodes, reporting for the first time a higher electrocatalytic activity of CGO
nanoparticles for CO/CO2 than for H2/H2O reactions in the absence of gas diffusion
limitations. We propose that enhanced surface reduction at the CGOn/gas two phase
boundary in CO/CO2 and in cathodic polarization can explain why the highest reaction
rate is obtained for CO2 electrolysis. For all materials investigated, large differences observed between model electrode kinetics and porous electrode kinetics are modelled
and discussed.
U2 - 10.1039/c5fd00048c
DO - 10.1039/c5fd00048c
M3 - Journal article
C2 - 26284532
SN - 1359-6640
VL - 182
SP - 75
EP - 95
JO - Faraday Discussions
JF - Faraday Discussions
ER -