2D Doping of Proton Conductors: BaZrO₃‐Based Heterostructures

Abstract

the major tool in this context. Even though homovalent dopants can be beneficial, it is usually heterovalent doping, which is most effective in generating charge carriers. Introducing an effectively negatively charged dopant (such as replacing a cation by a lower-valent one) will-apart from rather exotic cases-increase the concentration of all effectively positively charged point defects (cation interstitials, anion vacancies, and electron holes) and decrease the concentration of all negatively charged point defects (cation vacancies, anion interstitials, and excess electrons) as long as they are in mutual equilibrium. [1] Introducing effectively positively charged defects (e.g., higher-valent cationic substitution) will have the opposite effect. To predict to what degree this occurs, whether, e.g., the dopant concentrations are primarily ionically or electronically compensated, is a matter of defect chemistry. For the sake of precision, the homogeneous introduction of effectively charged foreign elements in the form of point defects will be termed 0D doping in the following. Here, the impurities are usually distributed randomly, and compensating counter charges are locally trapped and/or free beyond the Bjerrum length, at which electrostatic and thermal energies are equal. While the trapped carrier may be potentially mobile around the dopant, i.e., within the trapping zone (see Figure 1a,b), longrange transport requires detrapping. Figure 1 illustrates doping mechanisms of ionic conductors of various dimensionalities. Higher-dimensional doping has recently been explored as alternative doping strategy, potentially overcoming, e.g., limited solubility and charge-carrier trapping issues associated with 0D doping. It has been the availability of sophisticated thin film preparation techniques, that lets higher-dimensional doping come to the fore. If, e.g., in an epitaxial film of a layered compound a complete cationic layer is-in an atomically sharp fashion-replaced by a layer with a heterovalent cation, the whole 2D defect will be charged and the counter charge is generated in the adjacent space-charge zones. One could also think of this 2D doping situation as a 2D arrangement of charged point defects (see Figure 1c). Such an ordering constitutes perfect percolation along the layer, but may also be more easily interrupted by, e.g., impurities or stacking faults. Even more easily interrupted is transport in a 1D doping scenario (1D ordering of point defects). Importantly, in the 0D doping case, the total number (volume integrated) of free charge carriers increases with distance to the dopants, because of the 3D distribution, Ionically conducting oxide heterostructures provide ideal geometries for revealing interfacial effects in solid state ionics. Beyond that, they can provide a high density of heterointerfaces and thus allow for reliable studies of interfacial phenomena with respect to fundamental understanding as well as with respect to nanoionic applications. Substituting an entire single layer rather than random...

Inelastic Electron Tunneling Spectroscopy at High‐Temperatures

“…To provide a defined local environment for the particles under study and to minimize their interactions with the contacts of the tunnel junctions, we also employed a second approach to enhance proton conduction of BaZrO3 and take advantage of proton diffusion along interfaces in BaZrO3-BaYOx-based multilayers. [31] Strong enhancements of ion conduction at interfaces in heterostructures have already been found for several heterostructures, e.g., for CaF2-BaF2 and SrCeO3-SrZrO3 superlattices. [32,33] Therefore, the architecture of some of the tunnelling barrier was chosen to take advantage of two-dimensional doping.…”

“…However, tunnel junctions with undoped BaZrO 3 barriers showed only a minute peak structure (see Supporting Information). As the proton conductivity is much lower in BaZrO 3 than in Ba(Zr 0.8 Y 0.2 )O 3 or BaZrO 3 BaYO x BaZrO 3 , [20,22,23,24,29,30,31] this observation suggests that the observed peaks are associated with the presence of protons in the barrier.…”

“…In such a structure, a ZrO 2 layer is replaced in the growth sequence by a YO x layer, which is intercalated between BaO x layers, thereby generating oxygen vacancies along the Y-doped layers. [31] In hydrated conditions, hydroxide ions partially fill these oxygen vacancies, yielding the desired proton-conducting channels located in the middle of the barrier. For reference, we also explored tunnel junctions with barriers consisting of undoped and Y-doped BaZrO 3 layers.…”

“…[32,33] Therefore, the architecture of some of the tunnelling barrier was chosen to take advantage of two-dimensional doping. [31] The tunnel barrier comprises of a BaYOx layer embedded between BaZrO3 layers (Figure 1b). In such a structure, a ZrO2 layer is replaced in the growth sequence by a YOx layer, which is intercalated between BaOx layers, thereby generating oxygen vacancies along the Y-doped layers.…”

Novel mixed H+/e−/O2− conducting cathode material PrBa0.9K0.1Fe1.9Zn0.1O5+δ for proton-conducting solid oxide fuel cells

“…14,16 To achieve the low-temperature operation of H-SOFCs, excellent oxygen catalysis and hydration ability are of great importance. 17,18 The former could be acquired by the high oxygen vacancy concentration using heterovalent doping; 19 the latter is involved in many factors, such as the oxygen vacancy concentration, 20 electronegativity, 21 and structure symmetry. 22 The oxygen vacancy uses active sites for the uptake and transportation of protons, 20 but the proton concentration is not linear with the oxygen vacancy concentration.…”