Mixed Conductor Research Articles

Studies of Structural, Impedance, Modulus Spectroscopy, and Magnetic Properties of Cr‐Modified SrTiO3 Perovskite Ceramic

This study investigates the effects of chromium inclusion on the structural, microstructural, frequency dependent electrical and magnetic properties of SrTiO3 perovskite. The composition of Sr1−xCrxTiO3 (SCT) ceramics with x = 0.00, 0.02, 0.04 is preapred via a high‐temperature solid‐state reaction route. Rietveld refinement of X‐ray diffraction data revealed a cubic structure with space group . All SCT samples have been analyzed for Cr incorporation at the Sr‐site using Raman and Fourier transform infrared spectroscopy. Field‐emission scanning electron microscopy studies probed a gradual increase in grain size due to the uniform distribution of dopants across sample surfaces, as confirmed by energy‐dispersive X‐ray spectroscopy analysis. Cr doping strongly influences the electrical properties of grain and grain boundary resistance. All samples undergo Arrhenius‐type transport via oxygen ion migration and mixed conduction (ionic/electron), whereas relaxation occurs through mixed‐type association () and (). The magnetic nature of Cr‐doped samples confirms room‐temperature ferromagnetism due to carrier‐associated oxygen vacancies and Ruderman‐Kittel‐Kasuya‐Yosida interaction. These findings investigate Cr doping as mixed‐ionic and electronic conductors for intermediate temperature‐solid oxide fuel cells and spintronics‐based semiconductor devices.

Understanding Electrochemical Reaction Mechanisms of All-Electrochem-Active Mg2Si Electrode in All-Solid-State Batteries.

To better understand the electrochemical reaction mechanism of the Mg2Si electrode in all-solid-state batteries (ASSBs), a rational all-electrochem-active Mg2Si electrode is first designed to minimize the inactive component-related interfacial degradation. This is due to its unique mixed conductivity properties, including a high electronic conductivity of up to 8.9 × 10-2 S cm-1 and an ionic conductivity of 9.7 × 10-5 S cm-1, which allow for fast charges transport. In ASSBs, the Mg2Si electrode exhibits a higher initial Coulombic efficiency of 83.5% at 300 mA g-1 compared with the Si electrode (72.3%). X-ray diffraction and X-ray photoelectron spectroscopy results demonstrate that intermediate products (LixMg2Si, Li-Si alloy, and Li-Mg alloy) can form when the Mg2Si electrode discharges to 0.01 V, and partial Mg2Si still does not react with lithium. This work provides valuable insights into the reaction mechanisms and guides optimization strategies for developing next-generation Si-based ASSBs.

Preparation and Properties of Fe-Based Double Perovskite Oxide as Cathode Material for Intermediate-Temperature Solid Oxide Fuel Cell

Double perovskite oxides with mixed ionic and electronic conductors (MIECs) have been widely investigated as cathode materials for solid oxide fuel cells (SOFCs). Classical Fe-based double perovskites, due to their inherent low electronic and oxygen ionic conductivity, usually exhibit poor electrocatalytic activity. The existence of various valence states of B-site ions modifies the material’s catalytic activity, indicating the possibility of the partial substitution of Fe by higher-valence ions. LaBaFe2−xMoxO5+δ (x = 0, 0.03, 0.05, 0.07, 0.1, LBFMx) is used as intermediate-temperature solid oxide fuel cell (IT-SOFC) cathode materials. At a doping concentration above 0.1, the Mo substitution enhanced the cell volume, and the lattice expansion caused the formation of the impurity phase, BaMoO4. Compared with the parent material, Mo doping can regulate the oxygen vacancy concentration and accelerate the oxygen reduction reaction process to improve the electrochemical performance, as well as having a suitable coefficient of thermal expansion and excellent electrode stability. LaBaFe1.9Mo0.1O5+δ is a promising cathode material for IT-SOFC, which shows an excellent electrochemical performance, with this being demonstrated by having the lowest polarization resistance value of 0.017 Ω·cm2 at 800 °C, and the peak power density (PPD) of anode-supported single-cell LBFM0.1|CGO|NiO+CGO reaching 599 mW·cm−2.

Tuning direct-written terahertz metadevices with organic mixed ion-electron conductors

In the past decade, organic mixed ion-electron conductors have been successfully adopted in innovative bioelectronic, neuromorphic, and electro-optical technologies, as well as in multiple energy harvesting and printed electronics applications. However, despite the intense research efforts devoted to these materials, organic mixed conductors have not yet found application in electronic/photonic devices operating in key regions of the electromagnetic spectrum, such as the microwave (>5 GHz) and terahertz (0.1-10 THz) ranges. A possible reason for this technological gap is the widespread notion that organic electronic materials are unsuitable for high-frequency applications. In this work, we demonstrate for the first time the utility of high-performance polymer mixed conductors as electro-active tuning layers in reconfigurable terahertz metasurfaces, achieving modulation performances comparable with state-of-the-art inorganic and 2D semiconductors. Through time-domain terahertz spectroscopy, we show that the large conductivity modulations of these polymers, until now probed only at very low frequencies, are effectively preserved in the terahertz range, leading to optimal metadevice reconfigurability. Finally, we leverage the unique processability of organic materials to develop fully direct-written electrically tuneable metasurfaces onto both rigid and flexible substrates, opening new opportunities for the mass-scale realization of flexible and light-weight terahertz optics with unique mechanical characteristics and environmental footprint.

Chemi-Impeditive Sensing Platform Based on Single-Walled Carbon Nanotubes.

Chemical sensing methodology based on electrochemical impedance spectroscopy (EIS) targeting analytes in aqueous samples on functionalized single-walled carbon nanotube (SWCNT) is reported. The SWCNT in contact with electrolyte shows unique impedance spectra that cannot be analyzed with classical equivalent circuit models. Inspired by the charge transport property of mixed ionic-electronic conductors, we propose an equivalent circuit based on transmission line model (TLM), by which the impedance of the CNT-electrolyte system can be analyzed to track down all the equivalent circuit parameters. By combining multiple pieces of information, which are technically immeasurable with conventional chemiresistive or chemicapacitive techniques, several analyte species responding to the sensor can be differentiated from each other. We demonstrate the "chemi-impeditive" concept on chemically modified SWCNTs for detecting perfluoroalkyl substances (PFAS) in aqueous solutions. The EIS coupled with a fluorination chemistry on SWCNT surface provides unique changes in equivalent circuit components for each PFAS, i.e., changes in CNT and solution resistances, as well as interfacial CNT-solution capacitance, through which perfluorooctanesulfonic acid, perfluorooctanoic acid, hexafluoropropylene oxide dimer acid, and perfluorobutanesulfonic acid are detected in a discriminative manner. The new impedimetric method opens up new vistas in chemical sensing in that the EIS analysis provides an additional dimension of information beyond the single resistance or capacitance typically measured by many conventional types of sensors.

Covalently Merging Ionic Liquids and Conjugated Polymers: A Molecular Design Strategy for Green Solvent‐Processable Mixed Ion–Electron Conductors Toward High‐Performing Chemical Sensors (Adv. Funct. Mater. 45/2024)

Covalently Merging Ionic Liquids and Conjugated Polymers: A Molecular Design Strategy for Green Solvent‐Processable Mixed Ion–Electron Conductors Toward High‐Performing Chemical Sensors (Adv. Funct. Mater. 45/2024)

Characterization of Oxygen Diffusion and Catalytic Behavior of Composite Materials in Solid Oxide Fuel Cells Using the Non-destructive Adler-Lane-Steele Method

Solid oxide fuel cells (SOFCs) are technologically interesting because they provide high-efficiency energy conversion via an electrochemical reaction within the cells. To bring SOFC to market, researchers must develop highly electroactive, low-cost devices that are chemically stable in both high (800 – 1000 °C) and low (600 – 800 °C) temperatures, as well as optimize oxygen reduction in electrodes. The electrochemical performance of SOFC is highly dependent on the catalytic activity (or catalyst behaviours) of the electrode materials involved in the oxygen reduction reaction due to the slower oxygen reduction kinetics at lower temperatures. Understanding the fundamental properties of oxygen self-diffusion in solid-state ionic systems is critical for developing nextgeneration electrolyte and electrode material compositions and microstructures that enable SOFCs to operate at lower temperatures more effectively, durably, and affordably. To date, the most common method for evaluating electrocatalytic performance is electrochemical impedance spectroscopy (EIS), which causes sample damage due to its set-up procedure. As a result, modelling approaches have been used to better understand the reaction mechanism, oxygen diffusion coefficient, and kinetics of SOFC electrode reactions, including the mixed ionic electronic conductor (MIEC)-based electrode. Several models have recently been developed to represent the reaction mechanisms of SOFCs, with Adler-Lane-Steele (ASL) mathematical theory believed to be suitable for predicting oxygen gas diffusion through the pores of the MIEC-based materials. Hence, the aim of this paper is to validate the area-specific resistance of composite electrodes using the ASL modeling technique rather than the standard EIS analysis. The findings are significant in contributing to the development of a new non-destructive method for analyzing the catalytic behavior of SOFC components.

Emerging mixed electronic-ionic conductivity in double perovskite (La2/3Sr1/3)2(Sn1/3Fe1/3Cu1/3)2O6-δ: Phase transitions, structural, optical and dielectric study

We report on the eco-friendly synthesis of a double perovskite compound (La2/3Sr1/3)2(Sn1/3Cu1/3Fe1/3)2O6-δ, attractive for sustainable applications due to its unique structural and conductive properties. The incorporation of Cu2+ (3d9) ions into the B sites of the perovskite structure enhanced octahedral distortion via the Jahn-Teller effect. X-ray crystallography and Rietveld structural analysis revealed a significant deviation in the octahedra of (Cu/Fe)O6 and (Sn/Fe)O6 from the ideal perovskite structure, resulting in a decreased overlap between the atomic orbitals. The material displays a high dielectric constant, evidenced by impedance measurements across varying temperatures and frequencies. The analysis highlighted a considerable influence of the grain intrinsic properties and the grain boundary dynamics on both the conduction mechanism and dielectric relaxation. Notably, the introduction of dopants significantly reduces the bandgap energy to 2.7eV, compared to the 4.3eV of SrSnO3. The bandgap reduction factor measured here is lower than that of other reported double-doped SrSnO3 compounds. At 493K, a transition in DSC measurements suggests a structural change of the conductivity from a purely electronic to an electronic-ionic conductivity. The smaller bandgap and enhanced conductivity, coupled with phase transitions lead to a mixed conductivity making this material particularly effective for the use in energy storage technologies such as supercapacitors and batteries with the aim of improving their capacity and efficiency. Additionally, due to its temperature sensitivity, it also has the potential to be used as a valuable component in sensor applications.

Pre-existing mobile Zn2+ ions in ZnZr4(PO4)6 for capacitive- and battery- type energy storage systems

Zinc-ion based energy storage systems with a pure intrinsic Zn2+ ion conductor, ZnZr4(PO4)6 (ZnZrP), and consisting of variable valence Zr sites, are fabricated for the first time. Zn2+ ions are loosely held within the voids of the Zr-phosphate framework enabling ionic conduction in the solid matrix. By combining with activated carbon (AC), the resulting ZnZrP-AC composite exhibits a high mixed ionic-electronic conductivity of 1.25 mS cm−1. Benefitting from the high conductivity, fast charge transfer and transport across the ZnZrP-AC cross-section are enabled through short diffusion pathways in the resulting hybrid supercapacitor (HSC) or Zn-ion battery (ZIB). The HSC (ZnZrP-AC/ZnCl2-γ-butyrolactone/Zn-AC) delivers a specific capacitance of 275F g−1, energy and power density maxima of 153 Wh kg−1 and 7.2 kW kg−1 and a cycling life of 10,000 cycles, with 87.2 % capacitance retention. In the ZIB configuration of ZnZrP-AC/Zn(CF3SO3)2-acetonitrile/Zn-AC, a capacity of 136 mAh/g (at 50 mA g−1), with capacity retention of 95 % after 100 cycles is achieved, with a mixed diffusive and surface-confined fast charge storage mechanism. The active materials remain largely intact after cycling confirming the durability of the cell architecture with potential for scale-up and practical applications.

Superselective Hydrogen Separation through a Mixed Conducting Graphene Oxide Membrane.

A cost-effective H2 separation method is required for the purification of gaseous mixtures containing H2. Thus, in this study, we investigate the H2 separation properties of Ce ion-doped partially reduced graphene oxide (prGO) membranes. Pt/C-catalyst-coated, dense, micrometer-thick membranes are fabricated by stacking Ce-prGO nanosheets, followed by thermal annealing. They are permeable to only H2 at room temperature. H2 permeation occurs based on mixed conduction; the H2 initially dissociates into protons and electrons at the feed side. Thereafter, they diffuse into the membrane and recombine to produce H2 at the permeate side. However, the diffusion of other molecules, such as He and CO2, is inhibited, resulting in superselective separation. The developed carbon-based mixed proton/electron conduction (MPEC) membrane can be employed for H2 capture, deuterium gas production from D2O, and organic molecule deuteration.

Electrochemical Impedance Spectroscopy Study of Ceria- and Zirconia-Based Solid Electrolytes for Application Purposes in Fuel Cells and Gas Sensors.

In this study, the structural and electrochemical properties of commercial powders of the nominal compositions Ce0.8Gd0.2O1.9, Sc0.1Ce0.01Zr0.89O1.95, and Sc0.09Yb0.01Zr0.9O1.95 were investigated. The materials are prospective candidates to be used in electrochemical devices, i.e., gas sensors and fuel cells. Based on a comparison of the EIS spectra in different atmospheres (synthetic air, 3000 ppm NH3 in argon, 10% H2 in argon), the reactions on the three-phase boundaries were proposed, as well as the conduction mechanisms of the electrolytes were described. The Ce0.8Gd0.2O1.9 material is a mixed ionic-electronic conductor, which makes it suitable for anode material in fuel cells. Moreover, it exhibits an apparent and reversible response for ammonia, indicating the possibility of usage as an NH3 gas-sensing element. In zirconia-based materials, electrical conduction is realized by oxygen ion carriers. Among them, the most promising from an applicative point of view seems to be Sc0.09Yb0.01Zr0.9O1.95, showing a high, reversible reaction with hydrogen.

Unification of insertion and supercapacitive storage concepts: Storage profiles in titania

Insertion storage in battery electrodes and supercapacitive storage are typically considered to be independent phenomena and thus are dealt with in separate scientific communities. Using tailored experiments on titanium oxide thin films of various thicknesses, we demonstrate the simultaneous occurrence of both processes. For the interpretation of the entire storage profile encompassing both contributions, the (free) energies of the charge carriers in the mixed conductor and the neighboring phase are the only materials parameters required. The experimental results enable no less than a unification of insertion and supercapacitive storage, the first being dominant for thick films, the latter for thin films or negligible electronic conductivity. Therefore, the size of the storage medium and the nature of the current collecting phases can be used to tune power density versus energy density.

Enhancing Oxygen Reduction Kinetics and Proton Transfer of La0.6Sr0.4Co0.2Fe0.8O3−δ Cathode through Pr2Ni0.5Co0.5O4−δ Impregnation for Protonic Ceramic Fuel Cells

AbstractSluggish reaction kinetics in oxygen reduction reaction (ORR) is one of the most important challenges to the development of protonic ceramic fuel cells (PCFCs). La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) exhibits high mixed ionic–electronic conductivity in traditional solid oxide fuel cells, but their slow proton transfer and ORR kinetics impedes practical applications. Herein, composite Pr2Ni0.5Co0.5O4−δ (PNC) particles composed of a perovskite PrNi0.5Co0.5O3−δ phase and a PrO2 phase are impregnated into a LSCF cathode to enhance the ORR activity and proton transfer. The polarization tested in a symmetric cell with PNC‐impregnated LSCF cathode is 0.06 Ω cm2 at 700 °C. The fuel cell with this impregnated cathode shows maximum power densities of 1857 mW cm−2 at 700 °C. Moreover, the impregnated cathode exhibits a low degradation rate in the durability test. This work not only provides a novel and practical approach to improving the performance of current cathode materials for PCFCs but also highlights the potential for enhancing the commercial viability of PCFC technology.

Linearly programmable two-dimensional halide perovskite memristor arrays for neuromorphic computing.

The exotic properties of three-dimensional halide perovskites, such as mixed ionic-electronic conductivity and feasible ion migration, have enabled them to challenge traditional memristive materials. However, the poor moisture stability and difficulty in controlling ion transport due to their polycrystalline nature have hindered their use as a neuromorphic hardware. Recently, two-dimensional (2D) halide perovskites have emerged as promising artificial synapses owing to their phase versatility, microstructural anisotropy in electrical and optoelectronic properties, and excellent moisture resistance. However, their asymmetrical and nonlinear conductance changes still limit the efficiency of training and accuracy of inference. Here we achieve highly linear and symmetrical conductance changes in Dion-Jacobson 2D perovskites. We further build a 7 × 7 crossbar array based on analogue perovskite synapses, achieving a high device yield, low variation with synaptic weight storing capability, multi-level analogue states with long retention, and moisture stability over 7 months. We explore the potential of such devices in large-scale image inference via simulations and show an accuracy within 0.08% of the theoretical limit. The excellent device performance is attributed to the elimination of gaps between inorganic layers, allowing the halide vacancies to migrate homogeneously regardless of grain boundaries. This was confirmed by first-principles calculations and experimental analysis.

Mixed-valence Sm-doped LaF3 crystals as ion-electron conductors: Crystal growth and impedance characterization

The single crystals with the composition La1−y(Sm3+1−xSm2+x)yF3−xyLa1−ySm1−x3+Smx2+yF3−xy(y = 0.04) were grown from melt by the vertical Bridgman technique. A part of the Sm3+ doping ions in LaF3 matrix is reduced to the Sm2+oxidation state due to interaction with carbon during the growth process. The crystals were studied by X-ray diffraction analysis, optical and impedance spectroscopy. The Sm-doped crystals LaF3 are single-phase, retaining the tysonite-type structure (sp. gr. P-3c1P3̄c1) and demonstrate a bipolar electrical conductivity mechanism. Both the ionic conductivity σi = 4.7 × 10−5 S/cm caused by heterovalent substitutions of La3+ for Sm2+ and the comparable electronic conductivity σe = 3 × 10−5 S/cm due to the variable oxidation states Sm2+/Sm3+ ions were detected for the grown crystals. The discovered mixed ionic-electronic conductivity of La0.96Sm3+0.004Sm2+0.036F2.964 crystals opens up a new direction for the practical application of the tysonite-type fluorides as a component of electrode materials for fluorine-ion current sources.

Cation-Dependent Mixed Ionic-Electronic Transport in a Perylenediimide Small-Molecule Semiconductor.

A rapidly growing interest in organic bioelectronic applications has spurred the development of a wide variety of organic mixed ionic-electronic conductors. While these new mixed conductors have enabled the community to interface organic electronics with biological systems and efficiently transduce biological signals (ions) into electronic signals, the current materials selection does not offer sufficient selectivity towards specific ions of biological relevance without the use of auxiliary components such as ion-selective membranes. Here, we present the molecular design of an n-type (electron-transporting) perylene diimide semiconductor material decorated with pendant oligoether groups to facilitate interactions with cations such as Na+ and K+. Using the cyclic 15-crown-5 oligoether motif, we find that the resulting mixed conductor PDI-crown displays a strong dependence on the size of the electrolyte cation when tested in an organic electrochemical transistor configuration. In stark contrast to the low current response on the order of 1 μA observed with aqueous sodium chloride, a nearly 200-fold increase in current is observed with aqueous potassium chloride. We ascribe the high selectivity to extended molecular aggregation and therefore efficient charge transport in the presence of K+ due to a favourable sandwich-like structure between two adjacent 15-crown-5 motifs and the potassium ion.

Inhibiting the current spikes within the channel layer of LiCoO2-based three-terminal synaptic transistors

Synaptic transistors, which emulate the behavior of biological synapses, play a vital role in information processing and storage in neuromorphic systems. However, the occurrence of excessive current spikes during the updating of synaptic weight poses challenges to the stability, accuracy, and power consumption of synaptic transistors. In this work, we experimentally investigate the main factors for the generation of current spikes in the three-terminal synaptic transistors that use LiCoO2 (LCO), a mixed ionic-electronic conductor, as the channel layer. Kelvin probe force microscopy and impedance testing results reveal that ion migration and adsorption at the drain–source-channel interface cause the current spikes that compromise the device's performance. By controlling the crystal orientation of the LCO channel layer to impede the in-plane migration of lithium ions, we show that the LCO channel layer with the (104) preferred orientation can effectively suppress both the peak current and power consumption in the synaptic transistors. Our study provides a unique insight into controlling the crystallographic orientation for the design of high-speed, high-robustness, and low-power consumption nano-memristor devices.

Molecular Ordering Manipulation in Fused Oligomeric Mixed Conductors for High-Performance n-Type Organic Electrochemical Transistors.

Advanced n-type organic electrochemical transistors (OECTs) play an important part in bioelectronics, facilitating the booming of complementary circuits-based biosensors. This necessitates the utilization of both n-type and p-type organic mixed ionic-electronic conductors (OMIECs) exhibiting a balanced performance. However, the observed subpar electron charge transport ability in most n-type OMIECs presents a significant challenge to the overall functionality of the circuits. In response to this issue, we achieve high-performance OMIECs by leveraging a series of fused electron-deficient monodisperse oligomers with mixed alkyl and glycol chains. Through molecular ordering manipulation by optimizing of their alkyl side chains, we attained a record-breaking OECT electron mobility of 0.62 cm2/(V s) and μC* of 63.2 F/(cm V s) for bgTNR-3DT with symmetrical alkyl chains. Notably, the bgTNR-3DT film also exhibits the highest structural ordering, smallest energetic disorder, and the lowest trap density among the series, potentially explaining its ideal charge transport property. Additionally, we demonstrate an organic inverter incorporating bgTNR-3DT OECTs with a gain above 30, showcasing the material's potential for constructing organic circuits. Our findings underscore the indispensable role of alkyl chain optimization in the evolution of prospective high performance OMIECs for constructing advanced organic complementary circuits.

Broadband Dielectric Spectroscopy: Unraveling Na+diffusion and mixed conduction in Na₂O-modified zinc phosphate glasses for electrode material applications

The fundamental understanding of electric charge (ion + polarons/electrons) transport and relaxation mechanism is essential for applications in solid state ionics. In present work, to study Na+ transport, the electrical conductivity of zinc phosphate glasses modified with Na2O has been investigated in temperature range 313K and 473K and frequency range of 100 mHz to 1 MHz. The experimental data of Nyquist plots is fitted with appropriate equivalent circuits at different temperatures which reveals presence of mixed conduction (polaronic + ionic) and Na+ diffusion (at 50 mol% Na2O) in studied glass samples. The values of frequency exponent (s) obtained from the fitting of experimental data of the real part of electrical conductivity with Almond–West equation (WAE) have been used to determine the conduction mechanism. The electric transport in studied glasses occurred via correlated barrier hopping and non-overlapping small polaron tunnelling conduction models depending on the glass composition and temperature range of investigation. Electric Modulus studies further supports the assertion of composition and temperature dependent conduction mechanisms. The activation energies determined from conductivity, electric modulus, and impedance measurements are found to be consistent, indicating a good correlation in the study.

The hierarchical structure of organic mixed ionic-electronic conductors and its evolution in water.

Polymeric organic mixed ionic-electronic conductors underpin several technologies in which their electrochemical properties are desirable. These properties, however, depend on the microstructure that develops in their aqueous operational environment. We investigated the structure of a model organic mixed ionic-electronic conductor across multiple length scales using cryogenic four-dimensional scanning transmission electron microscopy in both its dry and hydrated states. Four-dimensional scanning transmission electron microscopy allows us to identify the prevalent defects in the polymer crystalline regions and to analyse the liquid crystalline nature of the polymer. The orientation maps of the dry and hydrated polymers show that swelling-induced disorder is mostly localized in discrete regions, thereby largely preserving the liquid crystalline order. Therefore, the liquid crystalline mesostructure makes electronic transport robust to electrolyte ingress. This study demonstrates that cryogenic four-dimensional scanning transmission electron microscopy provides multiscale structural insights into complex, hierarchical structures such as polymeric organic mixed ionic-electronic conductors, even in their hydrated operating state.