Ordered mesoporous metal oxides for electrochemical applications: correlation between structure, electrical properties and device performance

Ordered mesoporous metal oxides with a high specific surface area, tailored porosity and engineered interfaces are promising materials for electrochemical applications. In particular, the method of evaporation-induced self-assembly allows the formation of nanocrystalline films of controlled thickness on polar substrates. In general, mesoporous materials have the advantage of benefiting from a unique combination of structural, chemical and physical properties. This Perspective article addresses the structural characteristics and the electrical (charge-transport) properties of mesoporous metal oxides and how these affect their application in energy storage, catalysis and gas sensing

Comparing the electrical and protonic conductivity of mesoporous and nanocrystalline thin films of ceria-zirconia solid solutions

Due to the redox activity of the redox couple Ce3+/Ce4+, ceria-based solid solutions are typical mixed electronic and ionic conductors (MIECs) which are used e.g. as solid electrolytes in oxygen membranes or as electrode material in solid oxide fuel cells. CexZr1-xO2 (CZO) solid solutions not only show an increased thermal and mechanical stability compared to the corresponding binary oxides, but also exhibit an improved oxygen storage capacity making CZO a prominent material system for heterogeneous catalysis. Besides the control over composition, the defect chemistry of CZO may be optimized by nanostructuring. Here we present investigations of the electrical properties of mesoporous C0.8Z0.2O2 thin films prepared by solution phase coassembly of salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly (EISA) process. The mesoporous thin films exhibit a regular pore network with a high surface to volume ratio making them an ideal model system to study the influence of surface effects on the transport properties. Structural characterization using SEM, WAXD, XRD, XPS and Raman spectroscopy reveal the high structural quality of the thin films with 24 nm diameter pores which are surrounded by a crystalline wall structure consisting of 3 to 15 nm grains. Nanocrystalline thin films were prepared using pulsed laser deposition and characterized by SEM and XRD. Using electrochemical impedance spectroscopy, the electrical properties of the mesoporous and nanocrystalline thin films were investigated in a temperature range from room temperature to 500 °C and under different oxygen partial pressures between 1 and 10-4 bar. Measurements under varying humidity show large differences between the mesoporous and nanocrystalline thin films. While a significant increase in the conductivity is observed for the nanocrystalline thin films at temperatures below 250 °C and high humidity conditions, the mesoporous samples show no contribution of protonic conductivity. As will be discussed, these results indicate that the high surface area of the mesoporous samples has either no or very little effect on the protonic transport properties in CZO. Please click Additional Files below to see the full abstract

Transport properties of mixed ionic and electronic conductors - from bulk to nanostructure

Ceria-based materials exhibit mixed ionic and electronic conductivity due to the redox-activity of the cation (Ce4+/Ce3+) and the oxygen ion mobility in the fluorite-type lattice, which intrinsically tends to form a high concentration of Anti-Frenkel defects. Both electrons and ions migrate by an activated hopping mechanism with activation barriers of 0.4 eV for the hopping process of small polarons, 0.8 eV and up to 1.6 eV for the migration and the extrinsic formation and migration of oxygen vacancies, respectively. This leads to a p(O2)-sensitive electrical conductivity, which can be dominated by each process depending on temperature and oxygen activity. Moreover, the material is quite tolerant regarding the substitution of cations. By choosing the type and range of substitution, electrical properties can be adjusted in different ways. Therefore, ceria and its substituted analogues qualify for various applications as solid electrolytes in oxygen membranes, electrode material in solid oxide fuel cells (SOFCs) and in combination with the high oxygen storage capacity for support material in heterogeneous catalysis. The defect chemistry of ceria is already extensively investigated in literature. Thus the material is an ideal model system to study interface effects, in particular the concentration and type of electronic and ionic defects as well as their transport properties in the vicinity of interfaces, complementing the established defect chemical models for bulk material. Within this work we compare the solid solution of ceria and praseodymia (Ce1-xPrxO2-δ) with the solid solution of ceria and zirconia (Ce1-xZrxO2-δ). It is already known, that due to the redox-activity of Pr-ions the combination with praseodymia can lead to an additional transport of polarons, increasing the electronic contribution to electrical conductivity. In contrast, the combination with isovalent zirconia results in an increase of the so-called reducibility of ceria due to the size mismatch of the cations. To gain a deeper understanding of the role of these substitutions on electrochemical transport processes at interfaces, highly ordered mesoporous thin films of Ce1-xPrxO2-δ (CPO) and Ce1-xZrxO2-δ (CZO) have been investigated. The mesoporous samples have been synthesized by a sol-gel process using evaporation induced self-assembly, resulting in a regular pore structure surrounded by a closed packed, interconnected 3D architecture of nanocrystallites. The structural properties were analyzed by SEM, WAXD, XRD, XPS and Raman spectroscopy, confirming the successful synthesis of a mesoporous material of high structural quality, where the surface dominates over the bulk behaviour. The electrical properties were investigated as a function of temperature and oxygen partial pressure using electrochemical impedance spectroscopy. The comparison of the results with previous results of single crystalline samples elucidates the effect of the continuous pore structure on electrical transport properties. The CZO thin films show an unusual p(O2)-dependence at high oxygen partial pressures, which cannot be explained by standard defect chemical models. Furthermore, both mesoporous samples reveal a conductivity plateau under strongly reducing conditions, which is discussed in terms of hopping statistics and defect-defect interaction

DFT+U studies including spin-orbit coupling - a case study for f-electrons in praseodymium-doped ceria

The mixed ionic electronic conductor Ceria exhibits not only a high concentration of Anti-Frenkel defects with high mobility, resulting in ionic conductivity of oxygen ions, but also enables an additional electronic conduction mechanism in form of small polaron hopping between the f-states of the cations. This promotes the reversible exchange of oxygen with the surrounding atmosphere and thus the oxygen storage capacity of the binary oxide CeO2-δ. The material has been established as a model system to describe both ionic and electronic transport processes in bulk material to gain deeper insights into the characteristics of polaron hopping and defect-defect interactions in mixed conductors. By introducing the redox active lanthanide Praseodymium to the Ceria host lattice, both electronic and ionic conductivities are increased in temperature and oxygen partial pressure regions where pure Ceria lacks of good performance. The redox properties of Pr-ions, shifting the equilibrium from Pr4+ to Pr3+ and forming oxygen vacancies, is key to understand the additional contribution to the total electrical conductivity and the enhanced catalytic activity. So far in literature, only the effect of Pr3+-ions in the Ceria host lattice has been investigated by means of density functional theory. To complement these investigations with the impact of Pr-ions in both oxidation states, density functional theory was applied, including a Hubbard-U correction for electronic correlation in the f-states of both cations in Ce1-xPrxO2-δ. A systematic study of spin polarization, antiferromagnetic coupling and spin-orbit interaction of the unpaired 4f-electrons was performed to investigate the influence of magnetic interactions on the description of localized polarons. The preferred localization of the excess electrons on Pr- rather than Ce-ions as well as the defect formation and configuration is discussed by analyzing the resulting energy levels and densities of states of the investigated ideal and defective super cells

Influence of NCM Particle Cracking on Kinetics of Lithium-Ion Batteries with Liquid or Solid Electrolyte

In liquid electrolyte-type lithium-ion batteries, Nickel-rich NCM (Li$_{1+x }$(Ni$_{1−y−z}$Co$_{ y}$Mnz)$_{1−x}$O$_{2}$) as cathode active material allows for high discharge capacities and good material utilization, while solid-state batteries perform worse despite the past efforts in improving solid electrolyte conductivity and stability. In this work, we identify major reasons for this discrepancy by investigating the lithium transport kinetics in NCM-811 as typical Ni-rich material. During the first charge of battery half-cells, cracks form and are filled by the liquid electrolyte distributing inside the secondary particles of NCM. This drastically improves both the lithium chemical diffusion and charge transfer kinetics by increasing the electrochemically active surface area and reducing the effective particle size. Solid-state batteries are not affected by these cracks because of the mechanical rigidity of solid electrolytes. Hence, secondary particle cracking improves the initial charge and discharge kinetics of NCM in liquid electrolytes, while it degrades the corresponding kinetics in solid electrolytes. Accounting for these kinetic limitations by combining galvanostatic and potentiostatic discharge, we show that Coulombic efficiencies of about 89% at discharge capacities of about 173 mAh g$_{1+x }$NCM$^{-1}$ can be reached in solid-state battery half-cells with LiNi$_{0.8}$Co$_{0.1}$Mn$_{0.1}$O$_{2}$ as cathode active material and Li$_{6}$PS$_{5}$Cl as solid electrolyte

Elucidation of the Transport Properties of Calcium‐Doped High Entropy Rare Earth Aluminates for Solid Oxide Fuel Cell Applications

Solid oxide fuel cells (SOFCs) are paving the way to clean energy conversion,relying on efficient oxygen-ion conductors with high ionic conductivitycoupled with a negligible electronic contribution. Doped rare earth aluminatesare promising candidates for SOFC electrolytes due to their high ionicconductivity. However, they often suffer from p-type electronic conductivity atoperating temperatures above 500°C under oxidizing conditions caused bythe incorporation of oxygen into the lattice. High entropy materials are a newclass of materials conceptualized to be stable at higher temperatures due totheir high configurational entropy. Introducing this concept to rare earthaluminates can be a promising approach to stabilize the lattice by shifting thestoichiometric point of the oxides to higher oxygen activities, and thereby,reducing the p-type electronic conductivity in the relevant oxygen partialpressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3issynthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3compounds exhibit a higher ionic conductivity than most of thecorresponding Ca-doped rare earth aluminates accompanied by a reduction ofthe p-type electronic conductivity contribution typically observed underoxidizing conditions. In light of these findings, this study introduces highentropy aluminates as a promising candidate for SOFC electrolytes

Improved thermoelectric properties of nanostructured composites out of Bi1−xSbx nanoparticles and carbon phases

Thermoelectric figures of merit of ZT ≈ 0.4 at room temperature were achieved in nanostructured composite materials prepared by uniaxial pressing of Bi1−xSbx nanoparticles and 0.3 wt.% of a carbon phase. This constitutes a significant improvement of the low-temperature thermoelectric material Bi1−xSbx and strongly suggests the possibility of employing these materials in efficient thermoelectric devices working at room temperature. Interestingly, the beneficial effect of the carbon phase added to nanostructured Bi1−xSbx is the same for either carbon nanotubes or active carbon. This finding is attributed, on the one hand, to a combination of electronic band gap engineering due to nanostructuring and energy filtering due to graphene-like interlayers between Bi1−xSbx grains and, on the other hand, to modified phonon scattering at the grain boundaries and additional phonon scattering by agglomeration sites of carbon material on the μm scale

Improved thermoelectric properties of nanostructured composites out of Bi1−xSbx nanoparticles and carbon phases

Thermoelectric figures of merit of ZT ≈ 0.4 at room temperature were achieved in nanostructured composite materials prepared by uniaxial pressing of Bi1−xSbx nanoparticles and 0.3 wt.% of a carbon phase. This constitutes a significant improvement of the low-temperature thermoelectric material Bi1−xSbx and strongly suggests the possibility of employing these materials in efficient thermoelectric devices working at room temperature. Interestingly, the beneficial effect of the carbon phase added to nanostructured Bi1−xSbx is the same for either carbon nanotubes or active carbon. This finding is attributed, on the one hand, to a combination of electronic band gap engineering due to nanostructuring and energy filtering due to graphene-like interlayers between Bi1−xSbx grains and, on the other hand, to modified phonon scattering at the grain boundaries and additional phonon scattering by agglomeration sites of carbon material on the μm scale