Mixed Conductor Research Articles

Activity and stability of electrochemically reduced graphene oxide films for applications requiring mixed conductivity

A protonic and electronic conductor composed of partially reduced graphene oxide (GO) has been prepared by electrochemical methods. Conductivity measurements and detailed surface analysis by XPS, FTIR, and Raman methods show that the electrochemically reduced GO (ERGO), prepared under different reduction potentials, exhibits a unique dual (protonic and electronic) conductivity which is tunable by adjusting the surface conditions. Selected removal of the oxygenated groups, evolution of C(sp2)/C(sp3) hybrid carbon structure, and surface structure of ERGO can impact the protonic and electronic transport mechanisms. The ERGO samples reported here are ideal support materials with proper ratios of protonic and electronic transport. The as-prepared ionomer-free and flexible ERGO-supported Pt (Pt/ERGO) electrodes were characterized for oxygen reduction reaction (ORR) reactivity in acidic solution. The results showed not only higher electrochemical surface area (ECSA), but also improved mass activity towards ORR compared to conventional carbon supported Pt catalysts.

PrBaFe2O6-δ-based composites as promising electrode materials for protonic ceramic electrochemical cells

PrBaFe2O6–δ (PBF) is considered as a promising material for mixed ionic-electronic conductors used in high-temperature applications. In the present work, PBF was used as a basis for two chemical modification approaches, including doping with cobalt, nickel and copper, as well as composite design with the addition of a BaCe0.8Sm0.2O3–δ phase. The single-phase and composite materials were successfully prepared and characterized in terms of their functional properties. The two approaches were found to synergistically affect the functional properties of PBF, allowing a trade-off between thermomechanical and transport properties to be achieved. As a result, the prepared 70 wt% PrBaFe1.9M0.1O6–δ (M = Co, Ni, Cu) + 30 wt% BaCe0.8Sm0.2O3–δ composites exhibit acceptable electrochemical activity in the intermediate-temperature range. The best electrochemical activity was found for the sample with M = Cu, whose polarization resistance at 700 °C reaches 1.7 Ω cm2. Therefore, the designed PBF-based composites can be considered as promising electrode materials for solid oxide fuel cell applications.

Multiphase engineering regulation of double perovskite and RP structure heterojunction interfaces for solid oxide fuel cells

In the rigorous testing environment of fuel electrodes, surface-modified and element-doped Sr2Fe1.5Mo0.5O6-δ (SFMO6) experiences RP phase decomposition, thereby significantly improving performance - an aspect often underexplored and underutilized. Therefore, the design of a multiphase structure with complementary advantages and the exploration of their interactions become crucial. In the present investigation, SFMO6, which has outstanding redox stability, high conductivity, and catalytic activity, was combined with Sr3Fe1.8Ni0.2O7-δ (SFNO7) in different proportions. SFNO7, characterized by thermodynamic stability, absence of phase transitions, and mixed electrical conductivity, serves as an anode material. XRD results reveal structural stability, with minimal exsolution in a hydrogen environment, and good chemical compatibility. TEM confirms the hetero-interface, while ECR results show a complementary role of Dchem and Kex values for the two structures. At 800 °C, a composition comprising 70% SFMO6 and 30% SFNO7 demonstrates a notably low polarization resistance (Rp) of 0.12 Ω cm2. The maximum power density (MPD) reaches 725.7 mW cm−2 at 800 °C, a 20% improvement compared to single-phase single-cell counterparts (578.9 and 574.1 mWcm−2). This research provides a valuable strategy for the preparation of complementary hetero-interface materials for SOFC anodes and supports the improvement of performance by controlling the decomposition of SFMO6 into the RP phase in a reducing atmosphere.

A Cross-linked n-Type Conjugated Polymer with Polar Side Chains Enables Ultrafast Pseudocapacitive Energy Storage.

Pseudocapacitors bridge the performance gap between batteries and electric double-layer capacitors by storing energy via a combination of fast surface/near-surface Faradaic redox processes and electrical double-layer capacitance. Organic semiconductors are an emerging class of pseudocapacitive materials that benefit from facile synthetic tunability and mixed ionic-electronic conduction. Reported examples are mostly limited to p-type (electron-donating) conjugated polymers, while n-type (electron-accepting) examples remain comparatively underexplored. This work introduces a new cross-linked n-type conjugated polymer, spiro-NDI-N, strategically designed with polar tertiary amine side chains. This molecular design aims to synergistically increase the electroactive surface area and boost ion transport for efficient ionic-electronic coupling. Spiro-NDI-N demonstrates excellent pseudocapacitive energy storage performance in pH-neutral aqueous electrolytes, with specific capacitance values of up to 532 F g-1 at 5 A g-1 and stable cycling over 5000 cycles. Moreover, it maintains a rate capability of 307 F g-1 at 350 A g-1. The superior pseudocapacitive performance of spiro-NDI-N, compared to strategically designed structural analogues lacking either the cross-linked backbone or polar side chains, validates the essential role of its molecular design elements. More broadly, the design and performance of spiro-NDI-N provide a novel strategy for developing high-performance organic pseudocapacitors.

Role of Side-Chain Free Volume on the Electrochemical Behavior of Poly(propylenedioxythiophenes).

Mixed ionic/electronic conducting polymers are versatile systems for, e.g., energy storage, heat management (exploiting electrochromism), and biosensing, all of which require electrochemical doping, i.e., the electrochemical oxidation or reduction of their macromolecular backbones. Electrochemical doping is achieved via electro-injection of charges (i.e., electronic carriers), stabilized via migration of counterions from a supporting electrolyte. Since the choice of the polymer side-chain functionalization influences electrolyte and/or ion sorption and desorption, it in turn affects redox properties, and, thus, electrochemically induced mixed conduction. However, our understanding of how side-chain versus backbone design can increase ion flow while retaining high electronic transport remains limited. Hence, heuristic design approaches have typically been followed. Herein, we consider the redox and swelling behavior of three poly(propylenedioxythiophene) derivatives, P(ProDOT)s, substituted with different side-chain motifs, and demonstrate that passive swelling is controlled by the surface polarity of P(ProDOT) films. In contrast, active swelling under operando conditions (i.e., under an applied bias) is dictated by the local side-chain free volume on the length scale of a monomer unit. Such insights deliver important design criteria toward durable soft electrochemical systems for diverse energy and biosensing platforms and new understanding into electrochemical conditioning ("break-in") in many conducting polymers.

Phase evolution, defect chemistry, and electrical properties of Na0.5Bi0.5TiO3-BiNi0.5Ti0.5O3 solid solutions

Solid solutions of Na0.5Bi0.5TiO3 (NBT) and BiNi0.5Ti0.5O3 (BNiT) were prepared by a solid-state reaction route, and their electrical properties investigated by a combination of impedance spectroscopy and electromotive force measurements to explore the possibility of developing mixed ionic-electronic conductors based on NBT. Phase analysis showed that BNiT has a large solid solution limit in NBT (60 mol% based on X-ray diffraction), and the room temperature crystal structure changes from rhombohedral to pseudo-cubic with increasing BNiT content. Neutron diffraction revealed the coexistence of rhombohedral and tetragonal phases when the BNiT content ≥ 40 mol%. Electrically, incorporation of BNiT induces p-type electronic conduction into NBT by hopping of holes between Ni2+ (NiNix) and Ni3+ (NiNi·), and therefore changes the electrical conduction mechanism systematically from predominant oxide-ion conduction to mixed ionic-electronic conduction and then to predominant p-type electronic conduction. The total conductivity of the solid solutions showed a “V-shape” variation with increasing BNiT content. Possible mechanisms for the phase evolution and the conductivity-composition relationships are discussed. Achieving high levels of ionic and electronic conductivity simultaneously in NBT by introducing elements with variable oxidation states remains challenging due to the competition between an enhanced electronic component and a suppressed ionic component. Low levels of BNiT incorporation are, however, beneficial to reducing the dielectric loss of NBT for dielectric applications.

The Trade‐Off between Transconductance and Speed for Vertical Organic Electrochemical Transistors

AbstractThe high transconductance gm of organic electrochemical transistors (OECTs) has received widespread attention and made OECTs a valid candidate for wearable sensor systems. However, the large transconductance is often accompanied by large switching time constants, τ, making the transconductance an imperfect benchmark. For a fair assessment, the ratio of transconductance to switching time constant has to be considered instead of any single parameter in isolation. One approach put forward to optimize OECTs is a vertical design, in which the channel length can be scaled into the sub‐micrometer regime. Here, a new vertical device geometry is proposed, in which the active volume of the mixed conductor is confined to a small cavity between source and drain, yielding excellent performance and reproducibility. It is shown that this approach yields optimized ratios instead of maximized transconductance only. However, this scaling is effective only in a small range of device dimensions and requires careful optimization of the device to not be limited by parasitic effects such as excess volume of the mixed conductor or parasitic series resistances. Overall, the design considerations discussed here provide new guidelines to optimize OECTs, not only for high transconductance but for operation at high frequencies as well.

Mixing Insulating Commodity Polymers with Semiconducting n-type Polymers Enables High-Performance Electrochemical Transistors.

Diluting organic semiconductors with a host insulating polymer is used to increase the electronic mobility in organic electronic devices, such as thin film transistors, while considerably reducing material costs. In contrast to organic electronics, bioelectronic devices such as the organic electrochemical transistor (OECT) rely on both electronic and ionic mobility for efficient operation, making it challenging to integrate hydrophobic polymers as the predominant blend component. This work shows that diluting the n-type conjugated polymer p(N-T) with high molecular weight polystyrene (10KDa) leads to OECTs with over three times better mobility-volumetric capacitance product (µC*) with respect to the pristine p(N-T) (from 4.3 to 13.4 F V-1 cm-1 s-1 ) while drastically decreasing the amount of conjugated polymer (six times less). This improvement in µC* is due to a dramatic increase in electronic mobility by two orders of magnitude, from 0.059 to 1.3 cm2 V-1 s-1 for p(N-T):Polystyrene 10 KDa 1:6. Moreover, devices made with this polymer blend show better stability, retaining 77% of the initial drain current after 60 minutes operation in contrast to 12% for pristine p(N-T). These results open a new generation of low-cost organic mixed ionic-electronic conductors where the bulk of the film is made by a commodity polymer.

Charge Transport in the A6B2O17 (A = Zr, Hf; B = Nb, Ta) Superstructure Series

The electrical properties of the entropy stabilized oxides: Zr6Nb2O17, Zr6Ta2O17, Hf6Nb2O17 and Hf6Ta2O17 were characterized. The results and the electrical properties of the products (i.e. ZrO2, HfO2, Nb2O5 and Ta2O5) led us to hypothesize the A6B2O17 family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A6Nb2O17 and A6Ta2O17. The results indicate that electrons are involved in conduction in A6Nb2O17 while holes play a role in conduction of A6Ta2O17. Between 900 °C–950 °C, the charge transport in the A6B2O17 system increases in Ar atmosphere. A combination of DTA/DSC and in situ high temperature X-ray diffraction was performed to identify a potential mechanism for this increase. In-situ high temperature X-ray diffraction in Ar does not show any phase transformation. Based on this, it is hypothesized that a change in the oxygen sub-lattice is the cause for the shift in high temperature conduction above 900 °C–950 °C. This could be: (i) Nb(Ta)4+- oxygen vacancy associate formation/dissociation, (ii) formation of oxygen/oxygen vacancy complexes (iii) ordering/disordering of oxygen vacancies and/or (iv) oxygen-based superstructure commensurate or incommensurate transitions. In-situ high temperature neutron diffraction up to 1050 °C is required to help elucidate the origins of this large increase in conductivity.

Exploring the Effect of V2O5 and Nb2O5 Content on the Structural, Thermal, and Electrical Characteristics of Sodium Phosphate Glasses and Glass-Ceramics.

Na-V-P-Nb-based materials have gained substantial recognition as cathode materials in high-rate sodium-ion batteries due to their unique properties and compositions, comprising both alkali and transition metal ions, which allow them to exhibit a mixed ionic-polaronic conduction mechanism. In this study, the impact of introducing two transition metal oxides, V2O5 and Nb2O5, on the thermal, (micro)structural, and electrical properties of the 35Na2O-25V2O5-(40 - x)P2O5 - xNb2O5 system is examined. The starting glass shows the highest values of DC conductivity, σDC, reaching 1.45 × 10-8 Ω-1 cm-1 at 303 K, along with a glass transition temperature, Tg, of 371 °C. The incorporation of Nb2O5 influences both σDC and Tg, resulting in non-linear trends, with the lowest values observed for the glass with x = 20 mol%. Electron paramagnetic resonance measurements and vibrational spectroscopy results suggest that the observed non-monotonic trend in σDC arises from a diminishing contribution of polaronic conductivity due to the decrease in the relative number of V4+ ions and the introduction of Nb2O5, which disrupts the predominantly mixed vanadate-phosphate network within the starting glasses, consequently impeding polaronic transport. The mechanism of electrical transport is investigated using the model-free Summerfield scaling procedure, revealing the presence of mixed ionic-polaronic conductivity in glasses where x < 10 mol%, whereas for x ≥ 10 mol%, the ionic conductivity mechanism becomes prominent. To assess the impact of the V2O5 content on the electrical transport mechanism, a comparative analysis of two analogue series with varying V2O5 content (10 and 25 mol%) is conducted to evaluate the extent of its polaronic contribution.

Materials aspects of PEDOT:PSS for neuromorphic organic electrochemical transistors

Abstract This article summarizes recent developments in organic mixed ion-electron conductors for organic electrochemical transistors (OECTs) in the field of materials science. Materials for the active layers, including crosslinkers and additives, are summarized, with a focus on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). Particular aspects of organic devices are highlighted, including their flexibility, biocompatibility, and facile fabrication processes.

Ionic Solvent Shell Drives Electroactuation in Organic Mixed Ionic-Electronic Conductors.

The conversion of electrochemical processes into mechanical deformation in organic mixed ionic-electronic conductors (OMIECs) enables artificial muscle-like actuators but is also critical for degradation processes affecting OMIEC-based devices. To provide a microscopic understanding of electroactuation, the modulated electrochemical atomic force microscopy (mEC-AFM) is introduced here as a novel in-operando characterization method for electroactive materials. The technique enables multidimensional spectroscopic investigations of local electroactuation and charge uptake giving access to the electroactuation transfer function. For poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based microelectrodes, the spectroscopic measurements are combined with multichannel mEC-AFM imaging, providing maps of local electroactuation amplitude and phase as well as surface morphology. The results demonstrate that the amplitude and timescales of electroactuation are governed by the drift motion of hydrated ions. Accordingly, slower water diffusion processes are not limiting, and the results illustrate how OMIEC microactuators can operate at sub-millisecond timescales.

Designing organic mixed conductors for electrochemical transistor applications

Designing organic mixed conductors for electrochemical transistor applications

Bridging length scales in organic mixed ionic-electronic conductors through internal strain and mesoscale dynamics.

Understanding the structural and dynamic properties of disordered systems at the mesoscale is crucial. This is particularly important in organic mixed ionic-electronic conductors (OMIECs), which undergo significant and complex structural changes when operated in an electrolyte. In this study, we investigate the mesoscale strain, reversibility and dynamics of a model OMIEC material under external electrochemical potential using operando X-ray photon correlation spectroscopy. Our results reveal that strain and structural hysteresis depend on the sample's cycling history, establishing a comprehensive kinetic sequence bridging the macroscopic and microscopic behaviours of OMIECs. Furthermore, we uncover the equilibrium and non-equilibrium dynamics of charge carriers and material-doping states, highlighting the unexpected coupling between charge carrier dynamics and mesoscale order. These findings advance our understanding of the structure-dynamics-function relationships in OMIECs, opening pathways for designing and engineering materials with improved performance and functionality in non-equilibrium states during device operation.

Influence of Electrode Potential on Oxygen Mobility Probed by Polarized Isotopic Exchange in Solid Oxide Electrolyser Cells: Insights for Electro‐Assisted Oxidation Reactions

AbstractOxygen mobility was studied by oxygen isotopic exchange on three electrodes used in Solid Oxide Electrolyser Cells under polarization (La0.8Sr0.2MnO3 (LSM), La0.6Sr0.4Co0.2Fe0.8O3‐δ (LSCF) and La2NiO4+δ (LNO)). The rate of the surface and the bulk mechanisms for oxygen mobility is depending on the type of conductivity (electronic conduction or mixed ionic and electronic conductivity). It is shown that a one oxygen atom exchange is dominant for the surface path whereas a two oxygen atoms mechanism dominates for the bulk path. The rate constant for the bulk path is much higher than the one for the surface path by two orders of magnitude. Additionally, polarized oxygen isotopic exchange revealed that electrode overvoltage increases significantly the rate constant for the surface path, whereas its impact on the bulk path is negligible.

Architected Poly(ionic liquid) Composites with Spatially Programmable Mechanical Properties and Mixed Conductivity.

Structural electrolytes present advantages over liquid varieties, which are critical to myriad applications. In particular, structural electrolytes based on polymerized ionic liquids or poly(ionic liquids) (pILs) provide wide electrochemical windows, high thermal stability, nonvolatility, and modular chemistry. However, current methods of fabricating structural electrolytes from pILs and their composites present limitations. Recent advances have been made in 3D printing pIL electrolytes, but current printing techniques limit the complexity of forms that can be achieved, as well as the ability to control mechanical properties or conductivity. We introduce a method for fabricating architected pIL composites as structural electrolytes via embedded 3D (EMB3D) printing. We present a modular design for formulating ionic liquid (IL) monomer composite inks that can be printed into sparse, lightweight, free-standing lattices with different functionalities. In addition to characterizing the rheological and mechanical behaviors of IL monomer inks and pIL lattices, we demonstrate the self-sensing capabilities of our printed structural electrolytes during cyclic compression. Finally, we use our inks and printing method to spatially program self-sensing capabilities in pIL lattices through heterogeneous architectures as well as ink compositions that provide mixed ionic-electronic conductivity. Our free-form approach to fabricating structural electrolytes in complex, 3D forms with programmable, anisotropic properties has broad potential use in next-generation sensors, soft robotics, bioelectronics, energy storage devices, and more.

Gd3+ substitution effects, electrical and AC impedance characterizations of Gd/Cr co-doped SrBi4Ti4O15 Aurivillius-phase ceramics

Gd/Cr co-doped SrBi4Ti4O15 (Sr1-xGdxBi4Ti4O15 + 0.2 wt% Cr2O3, x = 0∼0.08) Aurivillius-phase ceramics were prepared by the traditional solid-state reaction process, the influences of Gd3+ doping concentration (x) on the structures and electrical properties of SrBi4Ti4O15 were investigated, as well as the AC impedance characterizations of the ceramics were analyzed. The incorporation of Gd2O3 and Cr2O3 into SrBi4Ti4O15 induced the lattice distortion of its orthorhombic structure. The donor substitution of Gd3+ for Sr2+ at A-site improved the Curie temperature (TC) and decrease the temperature coefficient of dielectric constant (TKε), as well as change the relaxation activation energy (Earel) of SrBi4Ti4O15 through the lattice distortion caused. Between 425 °C and 700 °C, the DC conduction behaviors of the ceramics was found with a transition from the mixed conduction mechanism involved in both the single-ionized oxygen vacancy and the double-ionized one to the single mode dominated by the latter one. Above TC, the imaginary part of modulus (M″) gradually shifts towards higher frequencies with increasing temperature, indicating the emergence of a new dielectric relaxation mode related to the phase transition process of the ceramics. Owing to the co-doping effects of Gd3+ and Cr3+, the optimized composition with x = 0.03 achieved excellent electrical properties (TC = 550 °C and d33 = 30 pC/N) and good thermal stability (TKε = 4.19 × 10−3/°C between RT and 500 °C, and the lose in d33 is only 3.3% after being annealed at 500 °C for 4 h), which make it promising for stable applications below 500 °C in the high-temperature piezoelectric devices.

Li Dynamics in Mixed Ionic-Electronic Conducting Interlayer of All-Solid-State Li-metal Batteries.

Lithium-metal (Li0) anodes potentially enable all-solid-state batteries with high energy density. However, it shows incompatibility with sulfide solid-state electrolytes (SEs). One strategy is introducing an interlayer, generally made of a mixed ionic-electronic conductor (MIEC). Yet, how Li behaves within MIEC remains unknown. Herein, we investigated the Li dynamics in a graphite interlayer, a typical MIEC, by using operando neutron imaging and Raman spectroscopy. This study revealed that intercalation-extrusion-dominated mechanochemical reactions during cell assembly transform the graphite into a Li-graphite interlayer consisting of SE, Li0, and graphite-intercalation compounds. During charging, Li+ preferentially deposited at the Li-graphite|SE interface. Upon further plating, Li0-dendrites formed, inducing short circuits and the reverse migration of Li0. Modeling indicates the interface has the lowest nucleation barrier, governing lithium transport paths. Our study elucidates intricate mechano-chemo-electrochemical processes in mixed conducting interlayers. The behavior of Li+ and Li0 in the interlayer is governed by multiple competing factors.

Programmable Ferroelectricity in Hf0.5Zr0.5O2 Enabled by Oxygen Defect Engineering.

Ferroelectricity, especially the Si-compatible type recently observed in hafnia-based materials, is technologically useful for modern memory and logic applications, but it is challenging to differentiate intrinsic ferroelectric polarization from the polar phase and oxygen vacancy. Here, we report electrically controllable ferroelectricity in a Hf0.5Zr0.5O2-based heterostructure with Sr-doped LaMnO3, a mixed ionic-electronic conductor, as an electrode. Electrically reversible extraction and insertion of an oxygen vacancy into Hf0.5Zr0.5O2 are macroscopically characterized and atomically imaged in situ. Utilizing this reversible process, we achieved multilevel polarization states modulated by the electric field. Our study demonstrates the usefulness of the mixed conductor to repair, create, manipulate, and utilize advanced ferroelectric functionality. Furthermore, the programmed ferroelectric heterostructures with Si-compatible doped hafnia are desirable for the development of future ferroelectric electronics.

Boosting the cycle stability of nickel-rich ternary materials at high voltage by coating mixed conductor Li7.5La3Zr1.5Co0.5O12

Ni-rich NCM materials have the high electrochemical capacities and low costs. However, the disadvantage of poor cycle performance severely limits their large-scale use. In this work, the nanoscale ion-electron mix-conductor Li7.5La3Zr1.5Co0.5O12 was prepared by ball milling and used to directly mix LiNi0.83Co0.11Mn0.06O2. After annealing, a mixed conductive protective layer could be formed on the surface of the NCM. The coating layer can protect the structural integrity of the NCM particles during cycling. At a current density of 1C between 2.8 and 4.5 V, the modified sample exhibited a capacity of 196 mAh g−1, and the capacity retention rate was as high as 85.7 % after 200 cycles, which was a significant improvement of 23.8 % over the pristine sample. The lithium-ion diffusion coefficient is enhanced and the charge transfer resistance is reduced by the mixed conductor cladding. The modified material shows an excellent rate performance. The specific capacity at 5C was 181 mAh g−1. The proposed modification method is simple in process and suitable for large-scale applications. The method has reference significance for the modification of other cathode materials.