Kinetics of CO/CO₂ and H₂/H₂O reactions at Ni-based and ceria-based solid-oxide-cell electrodes

Abstract

Document Version Peer reviewed version Link back to DTU Orbit Citation (APA):Graves, C. R., Chatzichristodoulou, C., & Mogensen, M. B. (2015). Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes. Faraday Discussions, 182, Comparing model and porous Ni-SZ electrodes, the ratio of electrode polarization resistance in CO/CO 2 vs. H 2 /H 2 O 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 H 2 /H 2 O reactions than CO/CO 2 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/CO 2 than for H 2 /H 2 O reactions in the absence of gas diffusion limitations. We propose that enhanced surface reduction at the CGOn/gas two phase boundary in CO/CO 2 and in cathodic polarization can explain why the highest reaction rate is obtained for CO 2 electrolysis. For all materials investigated, large differences

Evaluation of the Effect of Sulfur on the Performance of Nickel/Gadolinium‐Doped Ceria Based Solid Oxide Fuel Cell Anodes

“…sits on the surface oxygen vacancy site . Thus, surface reduction leads to an increasing number of oxygen vacancies on the surface and a higher Ce 3+ concentration . Thus, the mixed‐valence Ce 3+ /Ce 4+ is responsible for the large chemical capacitance.…”

“…[19] Thus, surfacer eduction leads to an increasing number of oxygen vacancies on the surfacea nd ah igher Ce 3+ + concentration. [49] Thus, the mixed-valence Ce 3+ + /Ce 4+ + is responsible for the large chemicalc apacitance. This capacitance, in turn, could possibly be coupled to hydrogen dissociation on Ni to explaint he lowfrequency influence of sulfur poisoning.…”

Carbon and Redox Tolerant Infiltrated Oxide Fuel-Electrodes for Solid Oxide Cells

“…, however at 750 ℃, but in the same gas atmosphere (19), where gas diffusion resistance had also been estimated and subtracted (not done for the present study). Ramos (38). Unlike the cells presented in this study, all of the mentioned cells have used Ni to enhance the electro-catalytic activity and are thus prone to carbon formation.…”

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…In the fuel environment, the activity of the MIEC/H2,H2O 2PB, as observed for CGO and other low-pO2 MIECs, considerably increases when reduced by cathodic polarisation and decreases when oxidized by anodic polarisation, corresponding to the pO2 changes 29,60,64,65 . Interestingly, MIECs are often activated by redox cycles, even those that are chemically and dimensionally stable both in air and fuel environments.…”

“…This "redox cycle" must be performed at the right conditions and with an appropriate porous electrode microstructure in order to avoid breaking the porous YSZ skeleton by the large Ni-NiO volume change 56 . On the other end of the pO2 spectrum is activation by the partial reduction of YSZ, in which brief applications of high cathodic polarisation drive the formation of a more active, nanostructured Ni/YSZ/H2,H2O 3PB [58][59][60] . Prolonged exposure to high cathodic overpotential, however, can lead to undesirable nanostructure morphologies characterized by Ni/YSZ contact loss 61 .…”