To solve the current issues of coking and redox instability related to the presence of nickel in conventional fuel electrodes in solid oxide cells (SOCs), porous Ce_0.9Gd_0.1O_1.95 (CGO) electrodes were studied using symmetric cells. These electrodes showed low polarization resistance (R_p) due to high electrocatalytic activity, but high ohmic resistance (R_s) due to low electronic conductivity. When infiltrated with Sr_0.99Fe_0.75Mo_0.25O_3-δ (SFM) aqueous precursor solution and subsequently calcined at low temperatures (<1100 °C), the electronic conductivity was enhanced, due to a mixture of phases, primarily consisting of SFM, SrFeO₃ and the highly electronically conducting SrMoO₃ impurity phase. However, R_p of the cells increased, which is likely because the infiltrated material is less electrocatalytically active and was partly blocking the reaction sites. The electrocatalytic activity could be regained by infiltrating nano-sized CGO or NiCGO (10 wt.% Ni relative to CGO) on top of the SFM, while still sustaining the high electronic conductivity. Compared with the porous CGO electrodes, the electrode performance was greatly improved; at 800 °C in H₂/H₂O 83 %/17 %, R_S of the electrodes decreased from ~0.26 Ω·cm² to close to 0 Ω·cm² and R_P was restored to 0.03 +/- 0.01 Ω·cm². Similar performance improvement was observed at 650 °C. Thus, performance comparable to, or better than, state-of-the-art fuel electrodes has been achieved. Upon exposure to a fuel gas of 50 %/50 % CO/CO₂ at 750 °C, R_s of the cells was largely unaffected while R_p increased by a factor of 2-4 compared with R_s and R_p measured in 50 %/50 % H₂/H₂O. The Ni containing cells coked when the CO/CO₂-ratio was raised above the carbon deposition equilibrium, while none of the non-nickel cells coked or showed signs of intolerance to a CO/CO₂-atmosphere (see figure). Stability towards redox cycles and variations in oxygen partial pressure of the infiltrated cells was also proven.

Figure: (a) R_S and (b) single electrode R_P of three different cell types for 15 h in CO:CO₂ 90:10 at 750 °C (above the carbon deposition threshold). The NiCGO and SFM is infiltrated into the plain CGO electrode. The inserts on the right are top-down images of the cells after testing.