Mixed Conductor Research Articles

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.

Interfacial Stabilization of Organic Electrochemical Transistors Conferred Using Polythiophene-Based Conjugated Block Copolymers with a Hydrophobic Coil Design.

The recent interest in developing low-cost, biocompatible, and lightweight bioelectronic devices has focused on organic electrochemical transistors (OECTs), which have the potential to fulfill these requirements. In this study, three types of poly(3-hexylthiophene) (P3HT)-based block copolymers (BCPs) incorporating different insulating blocks (poly(nbutyl acrylate) (PBA), polystyrene, and poly(ethylene oxide) (PEO)) were synthesized for application in OECTs. The morphological, crystallographic, and electrochemical properties of these BCPs are systematically investigated. Accordingly, P3HT-b-PBA demonstrates superior performance in the KCl-based aqueous electrolyte, with a higher product of mobility and capacitance (μC*) at 170 F s-1 cm-1 V-1 than that of the P3HT homopolymer at 58 F s-1 cm-1 V-1. P3HT-b-PBA exhibits better stability over 50 ON/OFF switching cycles than do other BCPs and P3HT homopolymers. With regard to the performance in the KPF6-based aqueous electrolyte, P3HT-b-PBA outperforms with a higher μC* of 9.2 F s-1 cm-1 V-1 than that of 8.6 F s-1 cm-1 V-1 observed from P3HT. Notably, both polymers exhibited almost no decay in device performance over 110 ON/OFF switching cycles. The strongly different performance of polymers in these two electrolytes is due to the side chain's hydrophobicity and interdigitated lamellar structures, thereby retarding the doping kinetics of the highly hydrated Cl- ions compared with the slightly hydrated PF6- ions. Concerning the improved performance of P3HT-b-PBA, this is attributed to its soft and hydrophobic backbone. Our morphological and crystallographic analyses reveal that P3HT-b-PBA experiences minimal structural disorder when swelled by the electrolyte, maintaining its original structure better than the P3HT homopolymer and the hydrophilic BCP of P3HT-b-PEO. The hydrophobic nature of P3HT-b-PBA contributes to the stability of its backbone structure, ensuring enhanced capacitance during the operation of the OECT operation. These findings provide reassurance about the stability and performance of P3HT-b-PBA in the field of OECT applications. In summary, this study represents the first exploration of P3HT-based BCPs for OECT applications and investigates their structure-performance relationships in mixed ionic-electronic conductors.

Space charge effects in mixed ionic-electronic conducting electrodes for solid-state batteries.

Mixed ionic-electronic conductors are widely used as electroactive materials in energy applications. The contact of a mixed conductor with another phase plays a crucial role in charge storage and transport in energy devices. However, the interfacial chemistry at the heterojunctions comprising mixed conductors and its interplay with the bulk chemistry remains imperative yet inadequately understood. This study addresses the fundamentals of space charge effects by exploring the equilibrium situations for contacts consisting of mixed conductors. From the perspective of defect chemistry, and by unifying the bulk and interfacial conditions with the electrochemical potential, our treatment allows for predicting the built-in potential at heterojunctions, profiling the space charge distributions, and evaluating the resulting interfacial charge storage and transport. The treatment can be related to experimental characterization, including coulometric titration, conductivity, and capacitance measurements at electrochemical interfaces in all-solid-state batteries. Besides, our treatment also highlights the significance of size and doping effects in nanocrystalline electrodes. This work provides a comprehensive framework for understanding and engineering the heterojunctions in electrochemical devices.

Small signal analysis for the characterization of organic electrochemical transistors

A method for the characterization of organic electrochemical transistors (OECTs) based on small signal analysis is presented that allows to determine the electronic mobility as a function of continuous gate potential using a standard two-channel AC potentiostat. Vector analysis in the frequency domain allows to exclude parasitic components in both ionic and electronic conduction regardless of film thickness, thus resulting in a standard deviation as low as 4%. Besides the electronic mobility, small signal analysis of OECTs also provides information about a wide range of other parameters including the conductance, transconductance, conductivity and volumetric capacitance through a single measurement. General applicability of small signal analysis is demonstrated by characterizing devices based on n-type, p-type, and ambipolar materials operating in accumulation or depletion modes. Accurate benchmarking of organic mixed ionic-electronic conductors through small signal analysis can be anticipated to guide both materials development and the design of bioelectronic devices.

A Universal Biocompatible and Multifunctional Solid Electrolyte in p-Type and n-Type Organic Electrochemical Transistors for Complementary Circuits and Bioelectronic Interfaces.

The development of soft and flexible devices for collection of bioelectrical signals is gaining momentum for wearable and implantable applications. Among these devices, organic electrochemical transistors (OECTs) stand out due to their low operating voltage and large signal amplification capable of transducing weak biological signals. While liquid electrolytes have demonstrated efficacy in OECTs, they limit its operating temperature and pose challenges for electronic packaging due to potential leakage. Conversely, solid electrolytes offer advantages such as mechanical flexibility, robustness against environmental factors, and ability to bridge the interface between rigid dry electronics systems and soft wet biological tissues. However, few systems have demonstrated generality and compatibility with a wide range of state-of-the-art organic mixed ionic-electronic conductors (OMIECs). This paper introduces a highly stretchable, flexible, biocompatible, self-healable gelatin-based solid-state electrolyte, compatible with both p- and n-type OMIEC channels while maintaining high performance and excellent stability. Furthermore, this nonvolatile electrolyte is stable up to 120°C and exhibits high ionic conductivity even in dry environment. Additionally, an OECT-based complementary inverter with a record-high normalized-gain of 228 V-1 and a corresponding ultralow static power consumption of 1 nW is demonstrated. These advancements pave the way for versatile applications ranging from bioelectronics to power-efficient implants.

Engineering Conductive Hydrogels with Tissue‐like Properties: A 3D Bioprinting and Enzymatic Polymerization Approach

Hydrogels are promising materials for medical devices interfacing with neural tissues due to their similar mechanical properties. Traditional hydrogel‐based bio‐interfaces lack sufficient electrical conductivity, relying on low ionic conductivity, which limits signal transduction distance. Conducting polymer hydrogels offer enhanced ionic and electronic conductivities and biocompatibility but often face challenges in processability and require aggressive polymerization methods. Herein, we demonstrate in situ enzymatic polymerization of π‐conjugated monomers in a hyaluronan (HA)‐based hydrogel bioink to create cell‐compatible, electrically conductive hydrogel structures. These structures were fabricated using 3D bioprinting of HA‐based bioinks loaded with conjugated monomers, followed by enzymatic polymerization via horseradish peroxidase. This process increased the hydrogels’ stiffness from about 0.6 to 1.5 kPa and modified their electroactivity. The components and polymerization process were well‐tolerated by human primary dermal fibroblasts and PC12 cells. This work presents a novel method to fabricate cytocompatible and conductive hydrogels suitable for bioprinting. These hybrid materials combine tissue‐like mechanical properties with mixed ionic and electronic conductivity, providing new ways to use electricity to influence cell behavior in a native‐like microenvironment.

Real-time information processing via volatile resistance change in scalable protonic devices

Biological neural systems operate on multiple time scales, enabling real-time interaction with their environment. However, replicating this in electronic systems is challenging, as scalable devices that operate on similar time scales, particularly from seconds to minutes, are lacking. This study addresses this gap by exploiting proton dynamics to achieve volatile resistance changes over long time scales, and developed a neuromorphic system that can predict biomedical data, such as blood glucose levels, in real time. By applying a low voltage (below 1 V) to a two-terminal device, the Pd electrode hydrogenates or dehydrogenates the mixed conductor WO3, resulting in significant changes in electronic resistance. The device is scalable due to the uniform proton distribution, leading to high impedance and minimal power consumption. Utilizing these volatile protons for short-term memory in an Echo State Network (ESN), this approach demonstrates low-power, efficient real-time information processing, paving the way for future neuromorphic computing applications.

Stable cycling of halide solid state electrolyte enabled by a dynamic layered solid electrolyte interphase between Li metal and Li3YCl4Br2

Halides are potential electrolytes for Li metal solid state batteries owing to their combination of high ionic conductivity, ductility and electrochemical stability against oxidation. However, their reactivity with the Li metal electrode may result in the formation of secondary compounds hindering their practical utility in terms of cycling performance as key indicator in battery operation. In this work, we investigate the high performance of symmetric cells with Li3YCl4Br2 halide and bare Li-metal electrode, able to withstand 1000 h of Li electrodeposition-dissolution with an overpotential as low as 46 mV. Through a comprehensive analysis employing physico-chemical and electrochemical characterizations, complemented by computational methodologies, we unravel the dynamics of the complex of the Li/halide interface and its evolution during cycling. The reactivity between Li3YCl4Br2 with metallic Li results in the reduction of the halide into LiCl, LiBr and Y metal. Surprisingly, during cycling, those secondary products from the reduction of the halide build a structured solid electrolyte interphase, containing a Y-rich electronic conductive and LiCl and LiBr ionic conductive layers. The particular chemistry and robustness of this solid electrolyte interphase exhibiting a mixed ionic and electronic conductivity appears to be responsible for the outstanding cycling stability.

Remote plasma-enhanced chemical vapor deposition of GeSn on Si (100), Si (111), sapphire, and fused silica substrates

GeSn films were simultaneously deposited on Si (100), Si (111), c-plane sapphire (Al2O3), and fused silica substrates to investigate the impact of the substrate on the resulting GeSn film. The electronic, structural, and optical properties of these films were characterized by temperature-dependent Hall-effect measurements, x-ray diffractometry, secondary ion mass spectrometry, and variable angle spectroscopic ellipsometry. All films were polycrystalline with varying degrees of texturing. The film on Si (100) contained only GeSn (100) grains, 40.4 nm in diameter. The film deposited on Si (111) contained primarily GeSn (111) grains, 36.4 nm in diameter. Both films deposited on silicon substrates were fully relaxed. The layer deposited on Al2O3 contained primarily GeSn (111) grains, 41.3 nm in diameter. The film deposited on fused silica was not textured, and the average grain size was 35.0 nm. All films contained ∼5.6 at. % Sn throughout the layer, except for the film deposited on Al2O3, which contained 7.5% Sn. The films deposited on Si (111), Al2O3, and fused silica exhibit p-type conduction over the entire temperature range, 10–325 K, while the layer deposited on the Si (100) substrate shows a mixed conduction transition from p-type at low temperature to n-type above 220 K. From ∼175 to 260 K, both holes and electrons contribute to conduction. Texturing of the GeSn film on Si (100) was the only characteristic that set this film apart from the other three films, suggesting that something related to GeSn (100) crystal orientation causes this transition from p- to n-type conduction.

Optimizing oxygen transport properties in Al-substituted La0.5Sr0.5Co0.8Fe0.2-xAlxO3-δ (x = 0–0.20) perovskites

In recent years, there has been a surge in the investigation of perovskite-type complex oxides, with a particular focus on La1-xSrxCo1-yFeyO3, renowned for their exceptional mixed ionic-electronic conducting (MIEC) properties. La0.6Sr0.4Co1-yFeyO3 compositions have prominently emerged in this research domain. The hypothesis of enhancing redox stability and mitigating oxygen variations by incorporating trivalent aluminum (Al3+) into the B-site of perovskite structures has gained attention. In this study, single-phase La0.5Sr0.5Co0.8Fe0.2-xAlxO3-δ (x = 0–0.20) perovskite oxides were synthesized using the solution combustion technique. The physicochemical properties of the synthesized materials were thoroughly characterized, including their morphology, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) analysis, oxygen nonstoichiometry, and electrical transport properties. Results revealed lattice parameter variations associated with oxygen nonstoichiometry and B-site cation size, exhibiting a decline up to x = 0.10 followed by an increase. Al-substitution significantly influenced surface morphology, oxygen nonstoichiometry, and electrical conductivity. Notably, La0.5Sr0.5Co0.8Fe0.1Al0.1O3-δ displayed lower electrical conductivity (676 S cm−1 at 300 °C) among the studied oxides. Oxygen nonstoichiometry had a significant impact on oxygen transport parameters, with these perovskites demonstrating improved chemical diffusion coefficient, Dchem (5.5 × 10-5 cm2 s−1), and the oxygen surface exchange coefficient, Kchem (2.32 × 10-5 cm s−1) at 900 °C, suggesting its potential as an oxygen transport membrane. These findings underscore the promising role of Al-substituted perovskite oxides in various advanced applications.

Ultrahigh-mobility microbial cytochrome nanowires generate power from humidity.

Mixed electronic-ionic conductors are crucial for various technologies, including harvesting power from humidity in a durable, self-sustainable, manner unrestricted by location or environment1,2. Biological proteins have been proposed as mixed conductors for 50 years3,4. Recently, Geobacter sulfurreducens pili filaments have been claimed to act as nanowires to generate power5,6. Here, we show that the power is generated by G. sulfurreducens-produced cytochrome OmcZ nanowires that show 20,000-fold higher electron conductivity than pili7. Remarkably, nanowires show ultrahigh electron and proton mobility (>0.25 cm2/Vs), owing to directional charge migration through seamlessly-stacked hemes and a charged, hydrogen-bonding surface, respectively. AC impedance spectroscopy and DC conductivity measurements using four-probe van der Pauw and back-gated field-effect-transistor devices reveal that humidity increases carrier mobility by 30,000-fold. Cooling halves the activation energy, thereby accelerating charge transport. Electrochemical measurements identify the voltage and mobilities required to switch pure electronic conduction to mixed conduction for power generation. The high aspect ratio (1:1000) and hydrophilic nanowire surface captures moisture efficiently to reduce oxygen reversibly, generating large potentials (>0.5 V) necessary to sustain high power. Our studies establish a new class of biologically-synthesized, low-cost and high-performance mixed-conductors and identify key design principles for improving power output using highly-tunable electronic and protein structures.

Influence of Microstructure on Defect Chemistry - Transport - Chemical Strain Coupling of (Pr,Ce)O2-δ Nanoparticles

Solid oxide fuel/electrolysis cells (SOFC/SOECs) are promising and environmentally-friendly technologies for meeting ever-increasing electricity and energy storage demands. Nanostructuring of SOFC/SOEC electrodes can enable improvement of the reaction kinetics and reduction of the operating temperature, owing to factors including the enlarged surface area, short diffusion lengths, and unique surface & interface properties. While the kinetic benefit is apparent, the effect of nanostructuring on other device-relevant physico-chemical properties cannot be neglected. Especially, the coefficients of chemical expansion (CCEs) are an important metric for mixed ionic-electronic conductors that can undergo changes in oxygen stoichiometry with variations in the surrounding atmosphere, temperature, and voltages during processing/operation. Chemical strains can cause cracking of monolithic parts during fabrication and delamination of electrodes from electrolytes in operation. CCEs are typically measured on bulk, polycrystalline materials with large particle or grain sizes. Therefore, it is important to understand the coupled transport and chemical expansion behavior in nano-ionic materials with large interface/surface density. This work aims to uncover the role of microstructure and nanoscale effects on the chemical strain and transport properties, through the lens of defect chemistry, with the goal of ultimately developing design principles for zero-strain materials with improved device lifetime and performance.Pr0.2Ce0.8O2- δ (PCO20) was selected as a model system and prepared as nanoparticles by a surfactant-free, low temperature co-precipitation method. (Pr,Ce)O2-δ compositions are valuable model oxide ion conductors to study, since both Pr and Ce can undergo changes of valence states under different temperature and pO2 regimes to accommodate oxygen stoichiometry excursions. Prepared nanoparticle samples were first in-situ sintered in the dilatometer at three different temperatures (600 ⁰C, 725 ⁰C, 850 ⁰C) to induce and evaluate microstructure evolution. The chemical strain and electronic/ionic conductivity were then studied on stable microstructures with different average particle sizes at lower temperatures by simultaneous in-situ dilatometry and electrochemical impedance measurement (EIS) upon changing pO2 at different isotherms (550 to 400 ⁰C). At each isotherm, all the samples expand as pO2 decreases. From the conductivity vs. pO2 measurements, the maxima at certain pO2 indicate the dominant polaron mechanism of the electronic transport, consistent with the measured conductivity activation energy. However, samples with lower sintering temperatures exhibit less pO2 dependence of the conductivity, which implies a transition in the mixed ionic/electronic conduction behavior toward higher ionic transference numbers as particle size decreases. Correspondingly, the chemical strains associated with losing oxygen monotonically decrease with the decrease of the sintering temperature, and hence the particle size. The chemical strain vs. oxygen stoichiometry dependence and corresponding CCEs were quantified as function of particle size and temperature by combining thermogravimetric analysis (TGA) with the dilatometry results and in-situ X-ray diffraction. The particle size, chemical composition, valence states, and morphology of samples after CCEs measurements were also compared using XRD, SEM, and S/TEM-EELS, with the latter indicating the expected differences between bulk and surface defect chemistry. Implications of the results for size-dependent defect chemistry and its influence on chemical strains and redox CCEs will be further discussed. Keywords: redox, chemical expansion, defect chemistry, mixed ionic electronic conduction, ceria, chemo-mechanics, nanoparticle, thermogravimetric analysis, dilatometry, impedance

Comparison of Cobalt-Based Vs. Cobalt-Free Infiltrated Nano-Catalyst Cathodes for Low Temperature Solid Oxide Cells

Tailoring inexpensive cathode materials which demonstrate high catalytic activity at lower temperature is crucial for the deployment of solid oxide cells as a next generation premier clean energy source.1 Cobalt is a crucial component and widely used in the most state-of-the-art cathodes of many types of batteries, fuel cells, and electrolysis cells.2 However, due to supply chain issues, replacement of cobalt (which is the most frequently used electrocatalyst to facilitate the oxygen reduction reaction) with other transition metals that are far more available, cheaper, and have less geopolitical, environmental, and social concerns have been explored in this work.In order to overcome the lethargic oxygen reduction reaction (ORR) kinetics, various transition metal oxide electrocatalysts (Ni, Fe, and Zn) and Co (for baseline comparison) were infiltrated on porous scaffold backbone support along with SrO and PrOx. This scalable spray coated porous scaffold of mixed ionic electronic conductor (MIEC) Gd0.1Ce0.9O2- δ (GDC) electrolyte material increases triple phase boundary for ORR reaction. SEM images shown in Figure 1(a) and 1(b) confirms nano-catalyst formation inside scaffold pores.At first, multiphase Co oxide-based nano-catalysts were tested on symmetric cells for baseline data, which demonstrated an area specific resistance (ASR) of 0.08 Ωcm2 at 650°C. Ni, Fe and Zn oxides were investigated as potential replacement electrocatalysts for CoOx because they exhibited similarly low ASR on symmetric cells in the range of 0.08 to 0.16 Ωcm2 at 650°C. To further assess their ability to perform as electrocatalysts for ORR, solutions of these transition metal nitrates were infiltrated, and heat treated to form oxides into SOFC cathode scaffolds and their performance in solid oxide fuel cell operating conditions was characterized. At optimal infiltration loading, Fe and Zn exhibited peak power density of 1.0 W/cm2 and 0.5 W/cm2 with polarization ASR of ~0.03 Ωcm2 and ~0.06 Ωcm2 respectively at 650°C as shown in Figure 1(c). Fe and Zn are considered relatively non-toxic, and are very cheap, however, further optimization will be required for enhanced performance.Conversely, Ni and Co as infiltrated nano-catalyst cathodes both showed extremely low polarization ASR of ~0.01 Ωcm2 with peak power density of 1.7 and 1.9 W/cm2 respectively at 650°C, as shown in Figure 1(c). Cobalt-free Ni based cathode demonstrated remarkably high peak power density of 1 W/cm2 and 1.5 W/cm2 at 550°C and 600°C respectively. Further exploration of cobalt-free, electrocatalytically active materials may open doors to new opportunities for systems like regenerative fuel cells that can operate both as a fuel cell and electrolysis cell with integrated system for energy conversion, and storage.3 Wachsman, E. D., & Lee, K. T. (2011). Lowering the temperature of solid oxide fuel cells. Science, 334(6058), 935– https://doi.org/10.1126/science.1204090.Olivetti, E., Ceder, G., Gaustad, G., & Fu, X. (2017). Lithium-Ion battery supply chain considerations: Analysis of potential bottlenecks in critical metals. Joule, 1(2), 229–243. https://doi.org/10.1016/j.joule.2017.08.019.Thangarasu, S., Dhanabalan, K., Roh, S., Kim, S. W., Park, K., Jung, S., Kurkuri, M. D., & Jung, H. (2017). A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments. International Journal of Hydrogen Energy, 42(7), 4415–4433. https://doi.org/10.1016/j.ijhydene.2016.10.140. Figure 1

Sr and Fe-Substituted LaMnO3 Perovskite for High-Rate Asymmetric Hybrid Supercapacitors

Perovskite has been getting significant attention in solar cells, photocatalysts, and hybrid supercapacitors. The symmetry or structural stability of ABO3-type perovskite oxides depends mainly on the size of ‘A’ and ‘B’ cations, which determines the material properties. The partial substitution of these cations may be used to tune these properties. The ionic sizes and valence states of the cations play an important role in improving the properties of perovskite. In this study, the substitution of La3+ with Sr2+ with a larger ionic radius and Mn3+ with Fe3+ with a similar ionic radius favored both the crystal symmetry and the mixed ionic-electronic conductivity of the perovskite. Electrodes based on La0.7Sr0.3Mn0.5Fe0.5O3 (LSMFO55) exhibited a faradaic behavior with a specific capacity of 330 C g-1 (92 mAh g-1) at a 12C rate, while this electrode maintained a capacity of 259 C g-1 at 240C (charge or discharge in 15s). Additionally, exohedral carbon nano-onions (CNO) were introduced as a negative electrode to design an asymmetric hybrid supercapacitor (AHS) with a widened cell voltage. The use of CNO as a negative electrode in the AHS improved the rate capability drastically compared to rGO. This device maintained a good energy density even at an extra-high charging rate (600C), owing to its outstanding rate capability. The high-rate performance of the LSMFO55//CNO AHS can be elucidated by successful fabrication with a mixed ionic-electronic conductive positive electrode and a CNO negative electrode. Tuning the electronic and ionic conductivities by cationic substitution and introducing CNO negative electrodes can provide a practical high-rate hybrid device using various perovskites.

(Invited) Nanohybrid Carbon Nanotube and P3HT Composite Semiconductor Materials for Solution Processed Optoelectronics

Carbon nanotube networks have demonstrated promise for enabling printable transistors which can be utilized as switching elements within active matrices. Such active matrices have been utilized for controlling pixels and interrogating sensor elements. A new promising technology for imaging, called an active pixel matrix, aims to integrate both the underlying transistor and sensor element within a single device, namely a phototransistor. Carbon nanotubes are poised to enable printable active pixel matrices for sensing applications, however, due to minimal absorption a secondary material needs to be added as a photo synthesizer. Additionally, the material must be sufficiently electrically coupled with the carbon nanotube network to allow for efficient carrier extraction. Our lab has previously developed techniques for crystallizing P3HT directly onto carbon nanotubes using solution-phase soft epitaxy. In this work, the solution phased technique has been adapted to functional carbon nanotube networks already deposited onto a surface.I will present the performance of a device that consists of a solution-processed carbon nanotube network, which acts as the carrier extraction layer, and a surface crystalized P3HT, which acts as the absorption element. First, I will detail the fabrication scheme and soft-epitaxial method of polymer crystallization. Next, I will overview how the P3HT crystallization affects the electronic performance of the devices, including the photo response. The device exhibits high responsivity, with a maximum responsivity value of 3 x 105. Additionally, the device exhibits a wide spectral response within the visible range, due to the exciton generation at the P3HT/CNT interface. I will also present the transient response of the device, which provides insight into the role of carrier injection/trapped states within the material.Lastly, I will discuss the potential applications for an active matrix of phototransistors, including neuromorphic computing. In these specific applications, the semiconductor can be utilized as the functional material within an Organic Electrochemical Transistor (OECT), where it exhibits behavior associated with mixed ionic/electronic conduction (MEIC). I will benchmark the performance, and discuss various methods to fabricated charge trapped switches, a key memory/computational element for future neuromorphic architectures.

(Invited) Designing Materials for Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have gained a renewed interest in the field or bioelectronics and biosensors. OECTs act as amplifiers and therefore provide high signal/noise ratio for detecting a wide range of biomolecules or physiological signals. A critical component of the OECTs is the channel material, which is an organic mixed ionic-electronic conductor (OMIEC). In simpler terms, OMIECs are (semi)conducting polymers that can transport electronic charges (electrons or holes), transport ionic charges, and display a capacitive behavior (reversible doping/dedoping process). Amongst OMIECs, the polyelectrolyte complex PEDOT:PSS has been the most widely used, likely due to its commercial availability and ease of use as a water dispersion. PEDOT:PSS, however, is a black box with a proprietary composition and does not have particularly reactive handles for functionalization. As such, it is difficult to modify its properties for specific applications in biosensing. In this talk, I will describe my team’s work in understanding PEDOT:PSS and methods for modifying it using covalent organic chemistry. Our principal approach is to control the synthesis of the PSS polymer and/or modify it with functional units by using random or block copolymerization. This approach was successfully used to achieve high performance OECT devices and new conductive materials with temperature or light-response.