Transient Studies of Ni-, Cu-Based Electrocatalysts in CH₄ Solid Oxide Fuel Cell

Solid oxide fuel cells (SOFCs) have attracted much research attention because of their capability of oxidizing hydrocarbons directly to produce electricity. A key to successfully designing CH4 SOFC lies in the development of good anode electrocatalysts for CH4 electrochemical oxidation. The transient techniques combined with the current/voltage acquisition and gas monitoring system provides a unique method for studying anode electrochemical oxidation reactions. Ni/YSZ, Cu/Ce_0.8Sm_0.2O_1.9/YSZ, Cu/Ce_0.8Mg_0.2O_1.8/YSZ, and Cu/Ce_0.5Zr_0.5O₂/LSCF anode electrocatalysts were developed in this study for CH₄ SOFC.

CH₄ pulsing studies over Ni/YSZ anode indicated that (i) the reaction of D₂O/CH₄ (i.e., reforming of CH₄ with D₂O) occurs at the sites in the vicinity of the three phase boundary (i.e., the Ni-YSZ interface) where the electrochemical oxidation of HD/D₂ takes place; (ii) the production of H₂ from the dissociation of CH₄ occurred on the Ni surface sites which is far from the three phase boundary. The absence of CO₂ formation is due to that Ni/YSZ anode has been shown to exhibit a low activity for the water-gas shift reaction in 750 - 850 °C to produce CO₂. The addition of D₂O into CH₄ improved the SOFC performance with the Ni/YSZ anode. Carbon deposition over Ni/YSZ electrocatalyst exhibits a significant inhibition effect on the electrochemical oxidation reaction over SOFCs with various metal/oxide anodes. CH₄ step switch studies over the Cu/Ce_0.8Sm_0.2O_1.9/YSZ anode showed that the electrochemical oxidation of CH4 involves C-H dissociation, hydrogen oxidation, and then carbon oxidation. Lattice oxygen in ceria is more active than oxygen ions (O^2-) diffusing from cathode to react with adsorbed carbon to form CO and CO₂. The filling up of the oxygen vacancy in ceria by O^2- diffusing from cathode could produce current and occurred faster than the electrochemical oxidation reactions. The addition of D₂O into CH₄ fuel decreased the fuel cell performance, but provided the oxygen source for oxidation of hydrogen and carbon. Result suggested that the electrochemical oxidation of CH₄ over the Cu/Ce_0.8Mg_0.2O_1.8 anode catalyst is the limiting step. Coke residing over the Cu/Ce_0.5Zr_0.5O₂/LSCF anode surface can be oxidized to CO and CO₂, producing 100 mA/cm2 at 850 °C.