Mixed Conductor Research Articles

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.

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.

In Situ Functionalization of Polar Polythiophene-Based Organic Electrochemical Transistor to Interface In Vitro Models.

Organic mixed ionic-electronic conductors are promising materials for interfacing and monitoring biological systems, with the aim of overcoming current challenges based on the mismatch between biological materials and convectional inorganic conductors. The conjugated polymer/polyelectrolyte complex poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT/PSS) is, up to date, the most widely used polymer for in vitro or in vivo measurements in the field of organic bioelectronics. However, PEDOT/PSS organic electrochemical transistors (OECTs) are limited by depletion mode operation and lack chemical groups that enable synthetic modifications for biointerfacing. Recently introduced thiophene-based polymers with oligoether side chains can operate in accumulation mode, and their chemical structure can be tuned during synthesis, for example, by the introduction of hydroxylated side chains. Here, we introduce a new thiophene-based conjugated polymer, p(g42T-T)-8% OH, where 8% of the glycol side chains are functionalized with a hydroxyl group. We report for the first time the compatibility of conjugated polymers containing ethylene glycol side chains in direct contact with cells. The additional hydroxyl group allows covalent modification of the surface of polymer films, enabling fine-tuning of the surface interaction properties of p(g42T-T)-8% OH with biological materials, either hindering or promoting cell adhesion. We further use p(g42T-T)-8% OH to fabricate the OECTs and demonstrate for the first time the monitoring of epithelial barrier formation of Caco-2 cells in vitro using accumulation mode OECTs. The conjugated polymer p(g42T-T)-8% OH allows organic-electronic-based materials to be easily modified and optimized to interface and monitor biological systems.

Space-Charge-Limited Current Measurements: A Problematic Technique for Metal Halide Perovskites.

Space-charge-limited current (SCLC) measurements play a crucial role in the electrical characterization of semiconductors, particularly for metal halide perovskites. Accurate reporting and analysis of SCLC are essential for gaining meaningful insights into charge transport and defect density in these systems. Unfortunately, performing SCLC measurements on perovskites is complicated by their mixed electronic-ionic conductivity. This complexity led to SCLC data often being incorrectly analyzed using simplified models unsuitable for these materials and reported without essential information about how the measurements were performed. In light of recently published SCLC data, common challenges in using SCLC measurements on perovskite materials are addressed, and solutions are discussed in this paper. The applicability of the often-used analytical models, the overlooked issues related to the mixed ionic-electronic conductivity of perovskites, and the complexity of creating single-carrier devices are investigated using drift-diffusion simulations. Finally, guidelines for more accurate reporting and improved analysis are provided.

Enhanced protonic and oxygen-ionic conductivity in Ga-doped Nd2Ce2O7 electrolytes for intermediate-temperature solid oxide fuel cell

Developing mixed oxygen-ion and proton conductors with high ionic conductivity is crucial for advancing intermediate temperature-solid oxide fuel cell (IT-SOFC). In this study, a novel Nd2-xGaxCe2O7 (NGCO, x = 0, 0.05, 0.075, 0.10 and 0.15) ceramic is synthesized by the citric acid-nitrate sol-gel combustion process. The influence of Ga doping on crystal structure, oxygen vacancy, morphology, proton transport characteristic and electrochemical performance of this materials is systematically investigated. It is found that the NGCO exhibits the fluorite-type structure and belongs to space group Fm3¯m. NGCO shows remarkable chemical stability, resisting harmful effects from CO2, water, and wet hydrogen exposure. Adding a small amount of Ga results in high-density ceramics with enlarged grain size and reduced grain boundary density. Nd1.925Ga0.075Ce2O7 (7.5NGCO) displays outstanding conductivities, exceeding Nd2Ce2O7 (NCO) by 271.4% and 248.6% in dry air and wet hydrogen conditions. Further, an anode-supported structure fuel cell with 7.5NGCO electrolyte is constructed and evaluated. The peak power density (PPD) at 700 °C reaches 702 mW/cm2, a 96% enhancement over the fuel cells with NCO electrolyte. These findings indicate that Ga-doped NCO electrolyte holds promise as a proton-conducting electrolyte suitable for IT-SOFC.

Small-Molecule Mixed Ionic-Electronic Conductors for Efficient N-Type Electrochemical Transistors: Structure-Function Correlations.

The fundamental challenge in electron-transporting organic mixed ionic-electronic conductors (OMIECs) is simultaneous optimization of electron and ion transport. Beginning from Y6-type/U-shaped non-fullerene solar cell acceptors, we systematically synthesize and characterize molecular structures that address the aforementioned challenge, progressively introducing increasing numbers of oligoethyleneglycol (OEG; g) sidechains from 1g to 3g, affording OMIECs 1gY, 2gY, and 3gY, respectively. The crystal structure of 1gY preserves key structural features of the Yn series: a U-shaped/planar core, close π-π molecular stacking, and interlocked acceptor groups. Versus inactive Y6 and Y11, all of the new glycolated compounds exhibit mixed ion-electron transport in both conventional organic electrochemical transistor (cOECT) and vertical OECT (vOECT) architectures. Notably, 3gY with the highest OEG density achieves a high normalized transconductance of 25.29 S cm-1, an on/off current ratio of ~106, and a turn-on/off response time of 94.7/5.7 ms in vOECTs. Systematic optoelectronic, electrochemical, architectural, and crystallographic analysis explains the superior 3gY-based OECT performance in terms of denser ngY OEG content, increased crystallite dimensions with decreased long-range crystalline order, and enhanced film hydrophilicity which facilitates ion transport and efficient redox processes. Finally, we demonstrate an efficient small-molecule-based complementary inverter using 3gY vOECTs, showcasing the bioelectronic applicability of these new small-molecule OMIECs.

Preparation and motion-sensing properties of ion/electron mixed conductive hydrogels fabricated by carbon-coated sepiolite nanofibers

Hydrogels have experienced significant advancements owing to their versatile applications in biomedicine, wound care, food packaging, and smart devices. The pursuit of environmentally sustainable and scalable hydrogel production methods remains a primary research objective. This study introduces an innovative hydrogel fabrication technique involving the integration of surface-modified sepiolite nanofibers with polyacrylamide (PAM), resulting in the formation of sepiolite-based composite hydrogels (PASSC). These hydrogels exhibit both ionic and electronic conductivity, along with improved mechanical properties and rheological behavior. PASSC hydrogels demonstrate remarkable fatigue resistance, maintaining mechanical integrity over 10,000 stress cycles, thereby indicating their potential for long-term applications. Additionally, these hydrogels exhibit distinctive mixed conductivity, which enhances their strain sensitivity and efficacy in strain sensing. The ability of PASSC hydrogels to accurately detect a range of movements underscores their versatility and reliability in monitoring strain changes across diverse applications.

High Performance Organic Mixed Ionic-Electronic Polymeric Conductor with Stability to Autoclave Sterilization.

We present a series of newly developed donor-acceptor (D-A) polymers designed specifically for organic electrochemical transistors (OECTs) synthesized by a straightforward route. All polymers exhibited accumulation mode behavior in OECT devices, and tuning of the donor comonomer resulted in a three-order-of-magnitude increase in transconductance. The best polymer gFBT-g2T, exhibited normalized peak transconductance (gm,norm) of 298±10.4 S cm-1 with a corresponding product of charge-carrier mobility and volumetric capacitance, μC*, of 847 F V-1 cm-1 s-1 and a μ of 5.76 cm2 V-1 s-1, amongst the highest reported to date. Furthermore, gFBT-g2T exhibited exceptional temperature stability, maintaining the outstanding electrochemical performance even after undergoing a standard (autoclave) high pressure steam sterilization procedure. Steam treatment was also found to promote film porosity, with the formation of circular 200-400 nm voids. These results demonstrate the potential of gFBT-g2T in p-type accumulation mode OECTs, and pave the way for the use in implantable bioelectronics for medical applications.