Solid oxide electrochemical cells (SOCs) show great promise to become a cornerstone of a renewable energy future, based on their ability to reversibly store electrical energy as green chemical fuels and to produce electricity from fuels. The conventional nickel and yttria-stabilized zirconia composite (Ni-YSZ) fuel-electrode provides high reaction rates for the H₂O/CO₂ reduction and H₂/CO oxidation reactions, but it is often a major source of long-term performance degradation due to poisoning of reaction sites, carbon deposition, Ni mobility, and other degradation mechanisms. Alternative electrodes made of mixed ionic and electronic conducting (MIEC) oxides have been shown to avoid these issues, however they also typically show much lower electrochemical activity than that of state-of-the-art Ni-YSZ electrodes.

During the last 10 years, DTU Energy has developed MIEC electrodes with nanostructured doped ceria (NDC) electrocatalysts as the active component. As doped ceria has low electronic conductivity, the initial electrodes were comprised of a small amount of NDC coated by infiltration onto the internal surfaces of porous electron-conducting backbones made of Sr(Ti,Nb)O₃ and/or stainless steel. Although the electrochemical activity matches or even exceeds that of Ni-YSZ, insufficient long-term stability has proven to be a persistent issue with these electrodes.

Recently we have developed new NDC based electrodes with no additional electron conductor or only a small volume fraction. Nanostructure optimization, primarily by adjusting heat treatment parameters, have yielded an unprecedented electrode polarization resistance (R_p) of ~0.01 Ω cm² at 650 °C in a fuel gas mixture of H₂/H₂O 0.8/0.2 atm for a nanoporous Ce_0.9Gd_0.1O_1.95-d thin film prepared by spin-coating. Initial results show highly promising long-term stability of the electrochemical reaction rates, although providing and maintaining sufficient electronic current collection is challenging due to the low in-plane conductivity of the film. We found that adding a small amount of a better electron conductor to the film minimizes ohmic resistance and maintains electronic contact. Similarly, more conventional structured porous electrodes with NDC surfaces and only a small amount of electronic conductor were found to show high activity and negligible electronic resistivity. We have also applied the same design principles to obtain NDC based electrodes that work almost equally well as either a fuel-electrode or oxygen-electrode, using Ce_xPr_1-xO_2-d as an active surface component. High activity for both fuel and oxidant electrochemical reactions is possible because Ce_xPr_1-xO_2-d is a MIEC in both the low oxygen partial pressure (pO₂) of the fuel gas and high pO₂of the oxidant gas. Electrochemical impedance and structural analyses of these highly defective, highly active MIEC electrodes will be presented, together with the impact of current collector components and a discussion of known limitations and future outlook.

Figure: (a) Impedance spectra measured 42 h apart at 650 °C in a fuel gas mixture of H₂/H₂O 0.8/0.2 atm on a nanoporous Ce_0.9Gd_0.1O_1.95-d thin film electrode. In between the measurements, the operating temperature was kept at 550 °C to 650 °C. (b) Scanning electron micrograph with backscattered electron detector on the same type of electrode.