Electrochemical Oxidation of CO/H2 Mixtures on Ni and Ceria Pattern Anodes

In many proposed applications of SOFC, the syngas, primarily containing CO and H2, will be fed directly to the anode. Therefore, the understanding of electrochemical co-oxidation of CO/H2 fuel is very important for designing fuel cell systems and their long term operation. Pattern electrodes with well-defined geometrical electrode/electrolyte interface have been successfully applied as a tool to study the electrochemical oxidation mechanisms at triple phase boundary (TPB).

So far, Ni/YSZ cermet anodes have mostly been used to study CO/H2 co-oxidation and Ni pattern anodes to study elementary reaction mechanisms of CO and H2. Recently, ceria based anodes are becoming of potential interest because of their ability to suppress carbon deposition on electrodes besides other advantages of ceria over conventional Ni based anodes. Ceria has widely been used for preferential oxidation of CO in automotive industry. However, only limited experimental data is available for electrochemical oxidation CO/H2 mixtures on ceria.

In this study, electrochemical oxidation of dry CO/H2 mixtures on symmetrical Ni and ceria pattern anodes on YSZ electrolyte has been examined between 700-850 oC. Regardless of the mixed ionic and electronic (MIEC) nature of ceria, ceria patterns were prepared for comparison with Ni pattern anodes with the same geometrically defined TPB length. Dry environment was used to exclude the possible side reactions like water gas shift in case of CO and CO/H2 mixtures.

Spectra obtained from electrochemical impedance spectroscopy (EIS) of Ni and ceria anodes for different gas environments at 800 oC are shown in figure. EIS with Ni anodes revealed 2-3 times higher polarization resistance for pure CO as compared to pure H2. The addition of small quantities of H2 in CO decreased the polarization resistance drastically, indicating negligible contribution of CO oxidation for total current production.

With ceria electrodes the polarization resistance was 5-6 times higher in case of CO as compared to pure H2. For all considered CO/H2 mixtures the polarization resistance was closer to that of pure H2 indicating preferential oxidation of H2 which is contrary to the expected role of ceria catalyst for preferential oxidation of CO as observed in case of non-electrochemical oxidation.

Impedance spectra from both Ni and ceria pattern anodes were fitted to the equivalent circuit model [R(RQ)(RQ)]. In all impedance spectra with ceria patterns, a small arc at high frequency was observed as compared to main arc at low frequency. Relaxation frequency of small arc was much higher as compared to the main arc. Activation energies for both Ni and ceria anodes were also similar to that of pure hydrogen for all considered mixtures further indicating preferential oxidation of hydrogen.