Topotactically transformable antiphase boundaries with enhanced ionic conductivity

Solid-state oxygen ion conductors are of paramount importance to solid oxide fuel cells and electrolytic cells. However, with the oxygen ion conductors proposed to date, these technologies require too high (close to 800 oC) temperatures to operate, leading to technological challenges and high costs. Therefore, it is of primary importance to design faster oxygen ion conductors at intermediate and low temperatures (below 400 oC) that can enable sustainable energy technologies to mitigate climate change. Descriptors for oxygen ion conduction are highly desirable to identify promising materials, which can be done by leveraging fast-growing statistical and machine learning approaches for screening of material databases [1]. Recently, computational tools combined with experiments have allowed researchers to gain atomistic insights and relate ion transport to features of the lattice dynamics [2] and electronic structure [3] of a given material. In this work, building upon the previous literature, we leverage both the lattice dynamics and electronic structure to predict the migration barrier for oxygen migration in pure and mixed electronic ionic conductors and search for promising oxygen ion conductors.Using density functional theory simulations, we model oxygen migration in perovskite, rutile, fluorite, and Ruddlesden-Popperoxides. We analyze oxygen ion transport in pure and mixed electronic ionic conductors and oxygen vacancies with and without charge. We study changes in the electronic structure during oxygen migration within the solid host lattice in different oxides. By analyzing trends of migration barrier and electronic density of states, we relate the migration barrier to the electronic structure of the transition state. Our results show that the migration barrier depends on the capability that the host lattice has of screening charge associated with the mobile ion in the transition state, which if increased would lower the localization of charge around the mobile ion and in turn the oxygen migration barrier. In pure ionic conductors – where no charge is associated with the mobile ion - the migration barrier is largely dependent on the M-O strength. Based on the mechanistic understanding derived by these initial analyses, we further relate oxygen ion conductivity to the lattice dynamics. We show that in pure ionic conductors, increasing migration barrier follows trends of increasing oxygen vibrational frequency based on phonon-electron coupling, which also relates the prefactor of Arrhenius law with the migration barrier. However, when trying to predict trends of the migration barrier across different crystal structures or mixed ionic conductors, no single descriptor can capture the complex physiochemistry behind oxygen migration. Thus, we leverage statistical learning tools to identify multivariate descriptors and analyze specific features of lattice moieties that improve oxygen ion conduction. The results of this work provide mechanistic understanding of oxygen ion migration and strategies to accelerate the designs of fast oxygen ion conductors.

Most of the ceramic ionic conductors typically conduct one ion. For example, AgI conducts Ag+ ion, Sodium β”-alumina conducts Na+ ion and Yttria-stabilized zirconia (YSZ) conducts O2- ions. Some selected materials such as alkaline earth cerates or zirconates conduct both protons and O2- ions. In such materials also, conductivity of one species is not independent of the other species. Thus, temperature and atmosphere determine the total conductivity. All of such materials are single phase materials. We have been investigating two phase materials such that one phase conducts one ion (Na+) while the other phase conducts another ion (O2-). The materials in such cases is a composite with the two phases forming an interpenetrating network. The volume ratios of the two phases can be adjusted as necessary. Since the two phases essentially constitute a mixture, conduction of one species occurring through one phase is independent of the conduction of the other species through the second phase. Yet, the condition of electroneutrality is applicable globally across multiple phases. Transport equations can then be written down in the usual manner but accounting for different volume fractions for the two phases. A mixed Na+ ion and O2- ion two phase conductor is fabricated by first making a sintered dense composite containing α-alumina and YSZ such that both phases are contiguous. YSZ and α-alumina are immiscible. This allows the fabrication of fine-grained, dense, strong composites. Subsequently, the sample is packed in Na-β”-alumina + YSZ powder and heat treated in air over a range of temperatures from 1200oC to 1400oC. During this process, α-alumina is converted into Na-β”-alumina by the incorporation of Na2O into α-alumina by coupled transport in which Na+ transports through the formed Na-β”-alumina and O2- ions transport through YSZ. The reaction front over time sweeps through the sample forming a two phase composite of Na-β”-alumina + YSZ. Such a material is now capable of conducting two ions; Na+ and O2-. Materials capable of transporting other alkali ions can be made by an ion exchange process in molten salts. For example, by ion exchanging in lithium salts, it is possible to make mixed Li-ion and O2- ion conducting materials. The oxygen ion conductivity is negligible at low temperatures. Thus, the materials at room temperature are predominantly alkali ion conductors with possible use in all solid state lithium ion and sodium ion batteries. Another mixed ion conductor was made by a similar process using a composite of Ga2O3 and YSZ. It was similarly packed in Na-β”-alumina powder and heat treated. In this case a two phase mixture containing YSZ and sodium zirconium gallate formed, the latter with a chemical formula of Na0.7Ga4.7Zr0.3O8 with a monoclinic structure. This compound is isostructural with Na0.7Ga4.7Ti0.3O8 which is known to be one dimensional sodium ion conductor. Measurement of ionic conductivity on Na0.7Ga4.7Zr0.3O8 + YSZ showed it to be a mixed Na+ ion and O2- ion conductor. Sodium ion conductivity of Na0.7Ga4.7Zr0.3O8 is much lower than that of Na-β”-alumina. The activation energy for Na+ ion transport is much lower than for O2- ion transport in YSZ. As a result, the two phase mixture is predominantly a Na+ ion conductor at low temperatures and an O2- ion conductor at elevated temperatures. Both Na-β”-alumina + YSZ and Na0.7Ga4.7Zr0.3O8 + YSZ are mixed Na+ ion and O2- ion conductors. This allows one two explore electrochemical transport of two species and examine the applicability of Goldman-Hotchkin-Katz trype of an equation, routinely used in aqueous electrochemistry, for solid-state systems. Preliminary results of experiments conducted by maintaining different chemical potentials of Na2O at the two electrodes and different chemical potentials of oxygen (by testing in a fuel cell mode) will be described. Possible degradation of Na-β”-alumina in reducing atmosphere will also be addressed. Acknowledgements: This work was supported by the National Science Foundation under Grant Number DMR-1407048.

Transport in mixed ionic electronic conductor solid oxide membranes with porous electrodes: Large pressure gradient

Despite spectacular recent advances in electronic metal-organic frameworks (MOFs) that exhibit either electronic or ionic conductivity, mixed ionic-electronic conducting (MIEC) MOFs with dual electron and ion transport capabilities are rare yet crucial for future energy storage and electrochemical applications. Here, we present a modular design strategy for MOF-based MIEC materials by combining a Li+-ion-conducting ytterbium-hexahydroxy-hexaazatrinaphthalene (Yb-HOHATN) framework based on an N-heteroaromatic ligand with electrically conductive multiwalled carbon nanotubes (CNTs), yielding Yb-HOHATN@CNT composites with impressive dual conduction properties. The Yb-HOHATN MOF features hexagonal channels decorated with Lewis basic N-sites that facilitate Li+ ion binding and transport, generating excellent ionic conductivity (7.3 × 10-5 S/cm), while the ligand π-stacks support through-space charge transfer, leading to modest electrical conductivity (9.8 × 10-8 S/cm). Solvothermal growth of Yb-HOHATN MOF on electrically conducting multiwalled carbon nanotubes (CNTs) yielded porous, crystalline Yb-HOHATN@CNT composites that displayed significantly improved electronic conductivity (5.4 × 10-3 S/cm). The Li+-doped Yb-HOHATN@CNT composite simultaneously exhibited impressive electronic and ionic conductivities (1.1 × 10-2 and 4.03 × 10-6 S/cm, respectively), emerging as an effective MIEC. This work not only presents an intriguing dual ionic-electronic conducting MOF@CNT composite but also provides a generalizable blueprint for crystalline MIECs that seamlessly combine the Li+ ion-transport capabilities of heteroatomic ligand-based MOFs with the electronic conductivity of CNTs, thereby unveiling MOF@CNT composites as a promising platform for advanced energy storage and electronic technologies.

Shining light on a mixed ionic-electronic conductor induces variations in both its electronic and ionic behaviors. While optoelectronic processes in semiconductors with negligible ionic conductivities are well understood, the role of mobile ions in photoactive mixed conductors, such as hybrid halide perovskites, is largely unexplored. Here, we propose a model addressing this problem, combining optoelectronics and optoionics. Using methylammonium lead iodide (MAPI) as a model material, we discuss the expected influence of optical bias on the charge carrier chemistry of mixed conductors under steady-state conditions. We show that changes in the concentration of ionic defects under light with respect to the dark case are a direct consequence of their coupling to electrons and holes through the component chemical potential (iodine in the case of MAPI) and the electroneutrality condition. Based on the trend in the quasi-Fermi level splitting in the mixed conductor, we emphasize implications of controlling point defect chemistry for the function and performance optimization of solar energy conversion devices, including those based on halide perovskites. Lastly, we show that, in the presence of multiple redox reactions mediating the ionic and electronic quasi-equilibrium, either positive or negative changes in the ionic defect pair chemical potential can be obtained. These findings indicate the intriguing possibility to increase or reduce ionic defect concentrations in mixed conductors through exposure to light.

Optimization of mixed conductivity through nanostructure control in WO3 thin films

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

Review of the limitation of the Hebb-Wagner polarization method for measuring partial conductivities in mixed ionic electronic conductors

An electrochemical gas sensor using and as sensing and reference electrodes, respectively, and as the electrolyte is the subject of this paper. The sensor response to gas showed a systematic deviation from the prediction of the Nernst equation at low . Based on the electromotive force (emf) measurement, the transference numbers of , a lithium-ion conductor, were estimated for different values, and the conduction domain boundary for separating n-type electronic conduction from ionic conduction was constructed. The conduction domain predicts that change in the Li activity in the sensing side of the cell drives the electrolyte to a mixed (n-type electronic and ionic) conduction region at low . Hebb-Wagner dc polarization measurements also indicate n-type electronic conduction in with a mixture of and gold as a reversible electrode. The transference numbers obtained from both the emf measurement and the Hebb-Wagner polarization measurements demonstrate that the origin of the non-Nernstian behavior of the sensor is due to the lithium mass transport from the -sensing electrode to the electrolyte, resulting in nonstoichiometry of at temperatures above .

NEW FAST Cu+ ION-CONDUCTING GLASSES IN THE CuI-Cu2O-WO3 SYSTEM —– PREPARATION, STRUCTURE, AND ELECTRICAL PROPERTIES

Examples are discussed for the electrical characterization of interfaces with solid electronic, ionic and mixed conductors by steady‐state current voltage relations and impedance spectroscopy. Polarization measurements are presented with electron and ion conducting point contacts on mixed conducting solids. Examples for the use of impedance spectroscopy concern the effect of fast grain boundary transport on diffusion impedances in mixed conductors and the impedance of electrodes on solid electrolytes. The experimental realization of in‐situ investigations with XPS and UPS are discussed for oxygen electrodes with yttria‐stabilized zirconia as solid electrolyte.

Application of HTcS materials as thin-film electrodes in solid state electrochemical systems, like solid oxide fuel cells (SOFC) and sensors, at elevated temperatures is a distinct possibility, provided that sufficient stability and electronic conductivity can be maintained. In addition, for these applications, HTcS materials need to exhibit also some oxygen ion conduction at the temperatures of operation. From the group of ceramic HTcS we have selected YBa2Cu3O7-x, 1, EuBa2Cu3O7-x,2 and La1.85Sr0.15CuO4, 3, for detailed study. These materials have been shown to exhibit variable stoichiometry in the oxygen sublattice which can be related with ionic conduction at elevated temperatures1–3.

Organic mixed ionic and electronic conductors are of significant interest for bioelectronic applications. Here, three different isoindigoid building blocks are used to obtain polymeric mixed conductors with vastly different structural and electronic properties which can be further fine-tuned through the choice of comonomer unit. This work shows how careful design of the isoindigoid scaffold can afford highly planar polymer structures with high degrees of electronic delocalization, while subtle structural modifications can control the dominant charge carrier (hole or electron) when probed in organic electrochemical transistors. A combination of experimental and computational techniques is employed to probe electrochemical, structural, and mixed ionic and electronic properties of the polymer series which in turn allows the derivation of important structure-property relations for this promising class of materials in the context of organic bioelectronics. Ultimately, these findings are used to outline robust molecular-design strategies for isoindigo-based mixed conductors that can support efficient p-type, n-type, and ambipolar transistor operation in an aqueous environment.

Rare-earth doped CeO2 materials find extensive application in high-temperature energy conversion devices such as solid oxide fuel cells and electrolyzers. However, understanding the complex relationship between structural and electrical properties, particularly concerning rare-earth ionic size and content, remains a subject of ongoing debate, with conflicting published results. In this study, we have conducted comprehensive long-range and local order structural characterization of Ce1-xLnxO2-x/2 samples (x ≤ 0.6; Ln = La, Nd, Sm, Gd, and Yb) using X-ray and neutron powder diffraction, Raman spectroscopy, and electron diffraction. The increase in the rare-earth dopant content leads to a progressive phase transformation from a disordered fluorite structure to a C-type ordered superstructure, accompanied by reduced ionic conductivity. Samples with low dopant content (x = 0.2) exhibit higher ionic conductivity in Gd3+ and Sm3+ series due to lower lattice cell distortion. Conversely, highly doped samples (x = 0.6) exhibit superior conductivity for larger rare-earth dopant cations. Thermogravimetric analysis confirms increased water uptake and proton conductivity with increasing dopant concentration, while the electronic conductivity remains relatively unaffected, resulting in reduced ionic transport numbers. These findings offer insights into the relationship between transport properties and defect-induced local distortions in rare-earth doped CeO2, suggesting the potential for developing new functional materials with mixed ionic oxide, proton, and electronic conductivity for high-temperature energy systems.

Simultaneous doping with La3+ and Y3+ for Ba2+- and Ce4+-sites in BaCeO3 and the ionic conduction