Oxide ion transport in oxides is instrumental for enabling various energy conversion technologies including, but not limited to solid oxide fuel cells and electrolyzers, oxygen separation membranes, and chemical looping. [1][2][3][4][5] For example, in solid oxide cells (Figure 1a), oxide ion conduction is required not only in the electrolyte, which ensures the selective transport of oxide ions between electrodes, preventing the reactants from mixing directly, but also within the electrodes responsible for electrochemically breaking down the reactants into respective ions and transporting them to the electrolyte. [2,4] In chemical looping (Figure 1b), an oxygen carrier material reversibly exchanges oxygens, by reacting successively with a reducing stream (e.g., CH 4 ), during which oxide ions are supplied to form the products (e.g., syngas), and then an oxidizing stream (e.g., O 2 ), where the oxide ions are replenished (cations also undergo redox changes to preserve charge neutrality). [5,6] Oxide ion transport is typically controlled and enhanced by chemical substitution, doping, or crystal lattice engineering. Notable examples include yttria-substituted zirconia (YSZ), the "classic" ion conductor, magnesium-doped lanthanum strontium gallates (LSGM), a perovskite ion conductor with a relatively high conductivity at intermediate temperatures, and also more recently ferroelectric and hexagonal perovskite oxide structures. [4,[7][8][9][10] A more exotic form of ion transport modulation is through strain, the artificial distortion of a crystal lattice. [11][12][13][14][15][16][17] Tensile strain, for example, involves "stretching" the crystal lattice, which seemingly allows more space for ion transport and lowers the activation energy for ion migration. [11,13] Generally, strain is induced by depositing the material of interest as a thin film upon a substrate, which results in a mismatch in lattice parameters between the two crystalline phases (ε i ), causing an artificial expansion or contraction of the thin-film layer. [12,13,15] Elastic relaxation, and potentially the formation of interfacial dislocations, then confines strain and its effects to a small region near the interface, which extends to no more than 100 nm. [12,13,18] Therefore, strain effects are generally limited to thin films and the nanoscale.However, two recent studies from the fields of solid oxide cells (SOC) and chemical looping (CL) suggest that strain might be induced in macroscopic systems by dispersing nanoparticles within the oxide ion conductor matrix, i.e., as internal or "endo-particles" (Figure 1c). This would effectively introduce a dense distribution of spherical interfaces, thus propagating the interface misfit ε i throughout the volume. The SOC study prepared such a system by assembling and sintering gold (Au) nanoparticles with a double perovskite oxide matrix Pr 1.9 Ni 0.71 Cu 0.41 Ga 0.05 O 4þδ (PNCO) and measured up to %2.5-fold increase in ion conductivity for an Au to PNCO matrix content of 1-3 mol%. [19] The CL study employe...
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