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WIREs Energy Environ.
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Electrolytes for solid oxide fuel cells

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Abstract Solid oxide fuel cells are extremely flexible energy conversion systems able to operate within a broad temperature range (500–1000°C), with a variety of fuels (from hydrogen to liquid fuels), including concepts able to be scaled to deliver power from the milliwatt to the megawatt range. The solid electrolyte, as an ionic charge carrier, is one central component that determines the operational characteristics of the fuel cell system, namely the working temperature. Design of new electrolytes includes manipulation of ionic defects concentration and mobility. Here, particular attention is given to the impact on ionic transport of point defects in various types of structures, dislocations, grain boundaries, and heterostructure interfaces. Properties derived from structural and compositional characteristics, but also from microstructural features, including recent complex engineered thin films, are reviewed. Major families of materials are compared with respect to key performance parameters. Finally, the effects of composition, structure, microstructure, and strain on ionic transport are assessed as complementary tools for future developments in solid electrolyte materials. This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Energy Research & Innovation > Science and Materials
Schematic view of SOFCs based on oxid‐on conductors (A) or protonic conductors (B). In both cases the fuel is hydrogen while air is the supplier of oxygen for the global fuel cell reaction. Electrons flow from the anode to the cathode.
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(a) Schematics of heterostructures consisting of alternating layers of one poorly conducting phase and one good ionic conductor. The conductive phase is strained at the interface region, where the conductivity is enhanced by more than one order of magnitude with respect to the nonstrained regions. The heterostructures can be engineered to maximize the volume fraction of the strained highly conductive region, by increasing the number of layers with lower thickness. (b) Prediction of the conductivity enhancement with respect to the bulk of a strained interfacial region of YSZ as a function of the mismatch factor at temperatures T1 = 300°C and T2 = 800°C (according to with ΔVm = 2.08 cm3 mol−1, and EYSZ = 175 GPa and νYSZ = 0.305 values averaged over 25 and 800°C,).
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Space‐charge potential (left scale) and defect concentration (right scale) profiles perpendicular to a positively charged grain boundary core. Values estimated with for double (z = 2) and single (z = 1) charged positive defects at temperatures T1 and T2 = T1 + 700 K. In the case of negative defects (e.g., concentration of electrons), the concentration profiles would follow the reverse trend of positive defects.
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Qualitative quick reference guide for some of the best‐known families of solid electrolytes and recently discovered materials: (a) Selected performance indicators for temperatures in the range 600–800°C. The green arrow indicates the desirable development trend, almost perpendicular to the actual performance range; (b) state‐of‐the‐art knowledge and technological application of selected electrolytes. The green area highlights mature materials. Overlapped areas can extend considerably in some cases.
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Schematic defect and related conductivity (inset) diagrams for one MO2‐type oxide with anti‐Frenkel defects, codoped with a redox stable trivalent cation (FM) and with Ce (CeM), one mixed valence cation. The first dopant is used to generate oxide‐ion vacancies (see ) and the latter to manipulate the electronic conductivity within a small pO2 domain. The total and hopping conductivities are highlighted. The interval between blue arrows indicates the limited pO2 domain where the hopping electronic conductivity prevails over the ionic conductivity.
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Schematic defect and related conductivity (inset) diagrams for one MO2‐type oxide with anti‐Frenkel defects, doped with a trivalent cation (FM) to generate oxide vacancies (VO). The blue arrow shows the large pO2 domain where the ionic conductivity prevails over the electronic conductivity, inside the region where positive oxide vacancies are created to balance the negative dopant charge ([F] = 2[V]). The highlighted total conductivity is constant within this large pO2 domain, and the material behaves as a pure electrolyte within a large fraction of this constant conductivity range.
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Schematic defect and related conductivity (inset) diagrams for one pure MO2‐type oxide with dominant anti‐Frenkel defects. The blue arrow shows the narrow pO2 domain where the ionic conductivity prevails over the electronic conductivity, inside the region where intrinsic ionic defects are dominant ([O] = [V]). The highlighted total conductivity, mostly electronic, shows a strong dependence on pO2.
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Schematic view of various types of defects relevant in ionic transport in solids: (A) oxide vacancy (cube) in one ABO3 perovskite involved in the ionic motion of one oxide ion (yellow); (B) edge dislocation as potential pathway for enhanced ionic conduction (orange ion) perpendicular to the Burgers vector (b); (C) defective grain boundaries where space charge effects determine ionic conduction.
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Schematic view of one SOFC and auxiliary systems used for fuel processing/supply, thermal management and dc/ac electrical power conversion.
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