Kinetics of CO/Co? and H?/H?O reactions at Ni-based and ceria-based solid-oxide-cell electrodes

By Graves, Christopher; Chatzichristodoulou, Christodoulos & Mogensen, Mogens B.
Published in Faraday Discuss. The Royal Society of Chemistry 2015

Abstract

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 Co? + 2e- [leftrightarrow] CO + o?- and H?O + 2e- [leftrightarrow] H? + o?-, for which the electrocatalytic activities of different electrodes differ considerably. The relative activities in CO/Co? and H?/H?O 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 H?/H?O than for CO/Co? 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 Ceo? (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.Comparing model and porous Ni-SZ electrodes, the ratio of electrode polarization resistance in CO/Co? vs. H?/H?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?/H?O reactions than CO/Co? 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? than for H?/H?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? and in cathodic polarization can explain why the highest reaction rate is obtained for Co? electrolysis. Large differences observed between model electrode kinetics and porous electrode kinetics are discussed.

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