Mixed conduction
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Mixed conductors, also known as mixed ion-electron conductors (MIEC), are a single-phase material that has significant
conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured f ...
ionically and electronically. 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. They are used as catalysts (for oxidation), permeation membranes,
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s, and electrodes in
batteries Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...
and
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
s, because they allow for rapidly transducing chemical signals and permeating chemical components.
Strontium titanate Strontium titanate is an oxide of strontium and titanium with the chemical formula Sr Ti O3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure. At low temperatures it approaches a ferroelectric phase ...
(), titanium dioxide (), , cerium(IV) oxide (), lithium iron phosphate (), and are examples of mixed conductors.


Introduction

MIEC materials tend to be
nonstoichiometric In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mos ...
oxides An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
, many of which have perovskite structures with rare earth metals on the A-site and
transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
on the B-site. Substituting various ions into the lattice of such an oxide can result in increased electronic conductivity through the formation of
holes A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
and introduce ionic conductivity by developing oxygen vacancies. This mechanism is known as defect theory, which states that defects like these offer additional pathways that favor fast diffusion. Other promising materials include those with pyrochlore, brownmillerite, Ruddlesden-Popper, and orthorhombic K2NiF4-type structures. 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. 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. 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.


Applications


SOFC/SOEC

Current state-of-the-art
solid oxide fuel cell A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte. A ...
s (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 An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
, 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 Lanthanum strontium cobalt ferrite (LSCF), also called lanthanum strontium cobaltite ferrite is a specific ceramic oxide derived from lanthanum cobaltite of the ferrite group. It is a phase containing lanthanum(III) oxide, strontium oxide, coba ...
(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. One of the most commonly used oxygen electrode (cathode) materials is the H-MIEC LSM-YSZ, consisting of
lanthanum strontium manganite Lanthanum strontium manganite (LSM or LSMO) is an oxide ceramic material with the general formula La1−xSrxMnO3, where ''x'' describes the doping level. It has a Perovskite (structure), perovskite-based crystal structure, which has the general f ...
(LSM) infiltrated onto a Y2O3-doped ZrO2 scaffold. 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.


See also

* Fast ion conductor *
Proton conductor A proton conductor is an electrolyte, typically a solid electrolyte, in which H+ are the primary charge carriers. Composition Acid solutions exhibit proton-conductivity, while pure proton conductors are usually dry solids. Typical materials a ...
*
Superconductivity Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...


References

{{Reflist Electrical conductors Electrochemistry Solid-state chemistry Superconductivity