Mixed conductor

Mixed conductors, also known as mixed ion-electron conductors (MIEC), are a single-phase material that has significant conduction ionically and electronically.[1][2][3] Due to the mixed conduction, a formally neutral species can transport in a solid and therefore mass storage and redistribution are enabled. Mixed conductors are well known in conjugation with high-temperature superconductivity and are able to capacitate rapid solid-state reactions.

Cerium oxide is a potent mixed conductor.[1]

They are used as catalysts (for oxidation), permeation membranes, sensors, and electrodes in batteries and fuel cells, because they allow for rapidly transducing chemical signals and permeating chemical components.[3]

Strontium titanate (SrTiO3), titanium dioxide (TiO2), (La,Ba,Sr)(Mn,Fe,Co)O3−d, La2CuO4+d, cerium(IV) oxide (CeO2), lithium iron phosphate (LiFePO4), and LiMnPO4 are examples of mixed conductors.[1]

Introduction

MIEC materials tend to be nonstoichiometric oxides, many of which have perovskite structures with rare earth metals on the A-site and transition metals on the B-site.[4] Substituting various ions into the lattice of such an oxide can result in increased electronic conductivity through the formation of holes and introduce ionic conductivity by developing oxygen vacancies.[4] This mechanism is known as defect theory, which states that defects like these offer additional pathways that favor fast diffusion.[5] Other promising materials include those with pyrochlore, brownmillerite, Ruddlesden-Popper, and orthorhombic K2NiF4-type structures.[5]

However, true (single-phase) MIECs that are compatible with other design parameters can be difficult to find, so many researchers have turned to heterogeneous MIEC materials (H-MIECs). An H-MIEC is a composite mixture of two phases: one for conducting ions, and another conducting electrons or holes.[6] These materials are desirable for the ability to tune their properties for specific applications by adjusting concentration levels to achieve optimal electron and ion transport.[7] Porous H-MIECs also incorporate a third phase in the form of pores, which allow the formation of triple phase boundaries (TPBs) between the three phases that provide high catalytic activity.[7]

Applications

SOFC/SOEC
Schematic of a solid oxide fuel cell. Note that the cathode material must conduct both oxygen ions and electrons.

Current state-of-the-art solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs) frequently incorporate electrodes made of MIEC materials. SOFCs are unique among fuel cells in that negatively charged ions (O2-) are transported from the cathode to the anode across the electrolyte, making MIEC cathode materials critical to achieving high performance. These fuel cells operate with the following oxidation-reduction reaction:

Anode Reaction: 2H2 + 2O2− → 2H2O + 4e−
Cathode Reaction: O2 + 4e− → 2O2−
Overall Cell Reaction: 2H2 + O2 → 2H2O

MIECs like lanthanum strontium cobalt ferrite (LSCF) are frequently the subject of modern fuel cell research, as they enable the reduction reaction to occur over the entire cathode surface area instead of only at the cathode/electrolyte interface.[8]

One of the most commonly used oxygen electrode (cathode) materials is the H-MIEC LSM-YSZ, consisting of lanthanum strontium manganite (LSM) infiltrated onto a Y2O3-doped ZrO2 scaffold.[9] The LSM nanoparticles are deposited on the walls of the porous YSZ scaffold to provide an electronically conductive pathway and a high density of TPBs for the reduction reaction to occur.[9]

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See also

References
  1. "Mixed conductors". Max Planck institute for solid state research. Retrieved 16 September 2016.
  2. I. Riess (2003). "Mixed ionic–electronic conductors—material properties and applications". Solid State Ionics. 157 (1–4): 1–17. doi:10.1016/S0167-2738(02)00182-0.
  3. Chia-Chin Chen; Lijun Fu; Joachim Maier (2016). "Synergistic, ultrafast mass storage and removal in artificial mixed conductors". Nature. 536 (7615): 159–164. Bibcode:2016Natur.536..159C. doi:10.1038/nature19078.
  4. Teraoka, Y. (January 1988). "Mixed ionic-electronic conductivity of La1−xSrxCo1−yFeyO3−δ perovskite-type oxides". Materials Research Bulletin. 23: 51–58. doi:10.1016/0025-5408(88)90224-3.
  5. Sunarso, Jaka (15 July 2008). "Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation". Journal of Membrane Science. 320 (1–2): 13–41. doi:10.1016/j.memsci.2008.03.074.
  6. Riess, I (February 2003). "Mixed ionic–electronic conductors—material properties and applications". Solid State Ionics. 157 (1–4): 1–17. doi:10.1016/S0167-2738(02)00182-0.
  7. Wu, Zhonglin (December 1996). "Modeling of ambipolar transport properties of composite mixed ionic-electronic conductors". Solid State Ionics. 93 (1–2): 65–84. doi:10.1016/S0167-2738(96)00521-8.
  8. Leng, Yongjun (July 2008). "Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte". International Journal of Hydrogen Energy. 33 (14): 3808–3817. doi:10.1016/j.ijhydene.2008.04.034.
  9. Sholklapper, Tal (2006). "LSM-Infiltrated Solid Oxide Fuel Cell Cathodes". Electrochemical and Solid-State Letters. 9 (8): A376–A378. doi:10.1149/1.2206011.
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