Thermally-Induced Dopant Segregation Effects on the Space Charge Layer and Ionic Conductivity of Nanocrystalline Gadolinia-Doped Ceria

Abstract

Grain boundaries are known to block ionic conduction across grain boundaries in oxide ion conductors due to adjacent space charge layers. Since the positively charged grain boundary core is intensified with a high local concentration of defects such as oxygen vacancies, uniform distribution of a dopant may mitigate the formation of space charge layers and enhance the ionic conductivity. To investigate the dopant segregation effect on the space charge layer and ionic conductivity, we provided thermal energy to nanocrystalline gadolinia-doped ceria (GDC) thin film by post-annealing at different temperatures of 700 • C, 900 • C, and 1100 • C. STEM-EELS analysis demonstrates strong dopant segregation and a higher Ce 3+ content near the grain boundary than in the bulk after post-annealing. The concurrent segregation of dopants and Ce 3+ ions implies that once thermal treatment is applied to nanocrystalline GDC thin films, complete space charge layers are formed while the non-thermally treated GDC film with a uniform distribution of dopants has less of a space charge effect and exhibits superior ionic conductivity.Nanocrystalline materials have attracted a great deal of attention for applications in various energy conversion and storage systems including rechargeable lithium ion batteries, oxygen/ozone gas sensors, oxygen storage systems, and solid oxide fuel cells (SOFCs). 1-5 Compared to macro-or micro-scale materials, nanocrystalline materials possess extraordinary electrical or electrochemical properties (i.e., ionic conduction or surface exchange reactions). One major cause of the unusual properties is related to the fine grain size (<100 nm), which corresponds to an extremely high grain boundary density. For instance, nanocrystalline calcium oxide-stabilized zirconia (0.14 um grain size) showed a 15 times higher specific grain boundary conductivity than microcrystalline materials (>4 um grain size). 6 In terms of the surface kinetics, a nanocrystalline interlayer (∼65 nm grain size) applied to the interface between the cathode and electrolyte of a SOFC exhibited a 5-6 times lower electrode interface resistance than a microcrystalline interlayer (∼6 um grain size). 7 Therefore, understanding the grain boundary properties is important to appropriately utilize nanocrystalline materials for energy conversion devices due to their exceptional features compared to bulk materials.Recently, oxide ion conductors (e.g., gadolinia-, samaria-, or yttriadoped ceria (GDC, SDC, or YDC)) usually with a polycrystalline structure, have been widely studied as electrolyte materials for lowtemperature SOFCs (LT-SOFCs) since they exhibit higher ionic conductivity and surface exchange rate than the most commonly used electrolyte, yttria-stabilized zirconia (YSZ), especially in the low operating temperature regime (<500 • C). 7-9 In terms of the ionic conductivity, polycrystalline acceptor-doped ceria has shown grain boundary blocking of ionic conduction originating from the space charge effect. 10-14 A high local concentration of oxygen...

Solvent‐deficient method lowers grain‐boundary resistivity of doped ceria

“…Besides, it was similar to that observed in the prior work although the density and sintering temperature utilized here are lower . The effects of thermally induced segregation on the electrical properties have been studied well . Annealing at over 900°C induces Gd 2 O 3 segregation and Ce 3+ ion accumulation near GB, which leads to the depletion of the oxygen vacancy concentration and simultaneously the increase in the height of potential barrier at GB .…”

“…[45][46][47] The effects of thermally induced segregation on the electrical properties have been studied well. [55][56][57][58] Annealing at over 900°C induces Gd 2 O 3 segregation and Ce 3+ ion accumulation near GB, which leads to the depletion of the oxygen vacancy concentration and simultaneously the increase in the height of potential barrier at GB. 55 This oxygen vacancy region (space charge layer) typically decreases the ionic conductivity.…”

“…[55][56][57][58] Annealing at over 900°C induces Gd 2 O 3 segregation and Ce 3+ ion accumulation near GB, which leads to the depletion of the oxygen vacancy concentration and simultaneously the increase in the height of potential barrier at GB. 55 This oxygen vacancy region (space charge layer) typically decreases the ionic conductivity. Similarly, segregated dopants in CeO 2 such as transition metals and rare earth oxides tend to hinder intergranular ionic transportation.…”

Rapid, cool sintering of wet processed yttria-stabilized zirconia ceramic electrolyte thin films

“…Grain boundaries react as barriers that interfere with ionic transport and the conductivity can be significantly influenced by their composition and structure. The distribution of dopants, which could be induced by segregation effects, also affects the ionic conductivity behaviour 51 , 52 . Meanwhile, the ionic conductivities of the thermally and photonic-annealed YSZ films existed in the range between the reference thin film and bulk YSZ, and had similar activation energies (E a ) to these films, as determined by the slopes of the data sets shown in Fig.…”

An Interdisciplinary View of Interfaces: Perspectives Regarding Emergent Phase Formation

“…High resolution scanning transmission electron microscopy has revealed evidence of Gd segregation in both the bulk as nanodomains as well as at the grain boundaries [27][28][29][30]. Figure 3 graphically illustrates the aggregation of M þ3 dopants substituted on M þ4 sites at the grain boundary resulting in decreased oxygen vacancy concentration at the interface.…”