Tailoring the Transport Properties of Mesoporous Doped Cerium Oxide for Energy Applications

Abstract

Hard-template nanocasted mesoporous cerium oxide possesses a unique combination of thermal stability, high surface area, and short diffusion lengths for mass and gas transport, which makes it relevant for high-temperature catalysis, sensing, and electrochemical applications. Here, we present an in-depth study of a number of mesoporous doped ceria systems, and we assess their fundamental structure and functionalities by complementary transmission electron microscopy imaging and spectroscopy, electron tomography reconstructions, and electrochemical impedance spectroscopy. We employed surface chemical modifications for increasing the ionic conductivity of as-synthesized mesoporous Gd-doped ceria by 2 orders of magnitude, enabling the ionic pathway across mesoporous particles. Complementary bulk doping strategies (by the addition of Pr) result in the easy tuning of the electrical transport mechanisms converting pure ionic mesoporous ceria into a mixed ionic–electronic conductor. The results obtained here are rationalized in light of local charge accumulation and mobility effects, providing a potential tool for engineering transport properties in nanocasted ceria and similar nanostructured materials for use in energy applications in the form of functional composites, infiltrated structures, or catalytic layers.

Solid Oxide Cell Electrode Nanocomposites Fabricated by Inkjet Printing Infiltration of Ceria Scaffolds

“…Cells with a commercial CGO scaffold present similar values (1.23 ± 0.01, 1.23 ± 0.02 and 1.29 ± 0.01 eV for CGO, 5-CGO and 10-CGO cells, respectively) while a slight increase is observed employing the mesoporous scaffold (1.38 ± 0.04 eV). This difference could be caused by either the presence of small amounts of silica contamination (≈1 wt%), due to the incomplete removal of the employed mesoporous template, or dopant segregation to the interfaces, as previously reported by the authors [36,96]. Concerning ASR pol values, a clear difference in resistance scaffolds with and without infiltration is observed (due to the lack of a catalytic phase in the functional layer).…”

Understanding the Impedance of Mesoporous Oxides: Reliable Determination of the Material-Specific Conductivity

“…In consequence, the impedance of mesoporous thin films may in principle exhibit up to three contributions, representing bulk transport, transport across the grain boundaries, and the geometric current constriction effect. However, experimental impedance data of mesoporous ceramic oxide thin films typically exhibit only one semicircle in Nyquist representation. ,− , …”

“…A lattice contraction χ of 70% in the z -direction is considered to account for the oblate shape of the pores. Typically, the transport properties of mesoporous thin films are measured in in-plane geometry, i.e., perpendicular to the z -direction. − For the simulations, however, we consider all possible transport directions, i.e., the x -, y -, and z -direction. This means the voltage is applied between two homogeneous electrodes enclosing a single periodic unit volume, as shown exemplarily in Figure c for the z -direction.…”

“…This is typically justified with the size of the nanocrystallites, as a merging of the two semicircles has been reported when decreasing the grain size to the nanometer range. , Therefore, the interplay between grains, grain boundaries, and pore structure hinders a straightforward calculation of material-specific transport parameters, such as the bulk or grain boundary conductivity from the measured impedance. Mostly, only an effective conductivity of the mesoporous thin films is estimated simply by neglecting the porosity, i.e., a dense thin film is assumed. − The exclusive consideration of the electrode geometry, however, significantly underestimates the material-specific conductivity of the solid framework.…”

Design of Ordered Mesoporous CeO2–YSZ Nanocomposite Thin Films with Mixed Ionic/Electronic Conductivity via Surface Engineering

“…The regular pore arrangement enables efficient penetration of gases or liquids into the bulk, thereby improving the accessibility of active surface sites. This characteristic structure qualifies mesoporous oxides for a variety of electrochemical applications, − e.g., in the field of (photo)­electrocatalysis, as electrode materials for batteries and supercapacitors or gas sensors. Several studies have shown that mesoporous architectures are favorable for electrochemical applications, which is mostly attributed to their large specific surface area.…”