Development of Fuel Electrodes for Reversible Solid Oxide Fuel Cell Applications
Abstract
The stability, sulfur tolerance, and electrochemical performance of Ni-YSZ (yttria-stabilized zirconia) composites and single phase La0.3M0.7Fe0.7Cr0.3O3-δ (M = Sr, Ca, (LMFCr)) perovskites, for use as fuel electrodes for reversible solid oxide fuel cell (RSOFC) applications, were investigated in detail. RSOFCs are electrochemical devices that can be operated in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. Although Ni-YSZ is the most common fuel electrode for RSOFC applications, it is prone to sulfur poisoning in ppm H2S levels at operating temperatures > 650 oC, thereby decreasing the rate of the hydrogen oxidation/reduction reaction (HOR/HER).
It is shown that, at ≤ 650 oC, exposure of Ni-YSZ to ≤ 10 ppm H2S unexpectedly enhanced the HOR activity, while at higher temperatures, conventional Ni poisoning was seen. It was concluded that the activation behaviour at ≤ 650 oC was due to the formation of a catalytic Ni-S species (e.g., Ni3S2) at the triple phase boundary where the HOR/HER occurs. The sulfur tolerance of LCFCr electrodes was also investigated in H2 + low ppm H2S, with LCFCr showing the inverse behavior of Ni/YSZ. At > 700 oC, exposure of LCFCr to ≤ 10 ppm H2S activated the HOR/HER, while deactivation was seen at ≤ 700 oC. It was concluded that exposure of LCFCr to H2S at higher temperatures led to an increase in the density of surface FeO2 terminated species, the proposed active site for the HOR/HER.
The LSFCr materials were also shown, for the first time, to be very active (comparable or better than other oxide materials (e.g., La1-xSrxCr1-yMnyO3-δ)) and stable electrocatalysts in CO/CO2 gas environments in both the SOFC and SOEC reaction directions at 600-800 oC. LSFCr was shown to be structurally stable in pure CO2 and 70% CO2:30% CO mixtures at 800 oC, with no major impurities detected. Also, extensive electrochemical characterization, based on both 2-electrode and 3-electrode full and half cell configurations, showed that LSFCr was more active as an electrocatalyst for the reduction of CO2 than for the oxidation of CO. Furthermore, it was found that the surface interaction of CO2 with LSFCr (adsorption, dissociation, electron transfer) was the slowest step in the reaction, relative to oxide ion transport processes.
Description
Keywords
Chemistry--Physical, Energy
Citation
Addo, P. (2017). Development of Fuel Electrodes for Reversible Solid Oxide Fuel Cell Applications (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/25678