Mixed Ionic And Electronic Conducting Research Articles

(Invited) Mixed Conductors for Advanced Solid Oxide Fuel Cells

Developing innovative electrode materials with high electrocatalytic activity is essential to advance the commercial viability of solid oxide fuel cells (SOFCs). Perovskite-type metal oxides have shown great potential as electrocatalysts for the oxygen reduction reaction and fuel oxidation. Mixed ionic and electronic conducting (MIECs) perovskites are being explored as electrodes which have demonstrated excellent chemical stability and provide more electrochemically active surface area. Here, we discuss our strategies towards the rational design of compositions to develop cost-effective and high-performance electrode materials for intermediate-temperature solid oxide fuel cells. Particular attention is centred on cathode materials and the role of thermal compatibility, electrical conductivity, and chemical stability towards improving the electrochemical performance of the cathode materials has been discussed.

Ca2Fe2O5 and Ca2Co2O5 based composite cathode for IT-SOFC application

Solid Oxide Fuel Cells at intermediate operating temperatures require high catalytic activity and compatibility of cathode material with electrolyte to reduce interfacial polarisation losses. In the present work, Brownmillerite structured composite based on Ca2Fe2O5 and Ca2Co2O5 [CFO-CCO] was studied for its cathodic properties. The Mixed Ionic and Electronic conducting (MIEC) properties of this composite material improves the Oxygen Reduction Reaction (ORR), catalytic activity and interfacial polarisation resistance. The structural, morphological and elemental analysis of the composite was carried out by XRD, BET, XPS, SEM and EDX. Electrical studies revealed that the Composite Cathode [CFO-CCO] has better ionic conductivity and sufficient electronic conductivity at 800 °C. Impedance analysis of Half-cell[CFO-CCO/SDC] and Symmetric-cell [CFO-CCO/SDC/CFO-CCO] at 800 °C shows the Area Specific Resistance (ASR) of 10.45 Ω cm2 and 1.14 Ω cm2 respectively. Performance of full cell [NiO-SDC/SDC/CFO-CCO] with Samarium doped Ceria (SDC) as electrolyte yields a power density of 160 mW/cm2. Electrochemical Impedance spectroscopy (EIS) measured at open circuit voltage (OCV) shows lower polarisation loss but higher ohmic loss due to the thicker electrolyte. The CFO-CCO composite material shows promising results, attributed to its microstructure and mixed conducting properties, enabling application for Intermediate Temperature solid oxide fuel cells (SOFCs).

Crystallography Analysis on LSCF and LSM-based Perovskite Oxides

Perovskite oxide as a metal oxide offers an excellence on mixed ionic and electronic conductor (MIEC) properties which work simultaneously. The crystal structure of perovskite oxide plays the important role to achieve the superior performance. Generally, it formed a cubic which potentially distorted due to the ratio of metal ion constituents, the calcination temperature and the synthesis method used. Thus, this study presents chemical analysis of La0.7Sr0.3Co0.2Fe0.8O3-δ (LSCF) and La0.7Sr0.3MnO3-δ (LSM)-based perovskite oxides through crystallographic identification using Rietveld Refinement method (Rietica software) and material porosity analysis. The results show that the two types of perovskite oxide have different distorted crystal structures. Perovskite oxide LSCF produces hexagonal crystals and 63.98% rhombohedral dominance, while LSM remained significantly different from LSCF. LSM consist less hexagonal and 55.47% cubic structure. In addition, the pore characteristics indicate that macropores and pore volumes are more abundant in LSCF than LSM. This information can be used as a basis for the use of oxide materials as a separator of oxygen ions from the air, solid oxide fuel cells and catalyst for value-added chemical production from methane flue gas.

Identifiability of the mechanisms governing the reaction kinetics of MIEC electrodes in solid oxide cells

The oxygen reduction reaction (ORR) is the main phenomenon occurring in mixed ionic and electronic conductors (MIECs) used as air electrodes in solid oxide cells. Their optimisation requires the identification of the ORR regime, which is typically performed via electrochemical impedance spectroscopy (EIS). In this study we present a physics-based model to simulate the impedance spectra of p-type oxygen-deficient perovskite MIEC materials. The EIS response of four extreme kinetic scenarios, characterised by the rate-determining step (electron-transfer or ion-transfer) and the high/low surface coverage of adsorbed oxygen, is mechanistically interpreted for both dense films and porous electrodes. A strategy for kinetic identification is proposed based on distinctive EIS fingerprints at different oxygen partial pressures (pO2) and cathodic bias. However, distinguishing the kinetic scenarios is not free from ambiguity even in dense films since some scenarios are discriminated only via a quantitative analysis, which may be susceptible to experimental errors in real measurements, and reverse behaviours appear when combining cathodic bias with pO2 variation. More difficulties arise in porous electrodes since bulk oxygen vacancy transport interacts with the ORR response. Application of the proposed strategy using literature data for some common MIEC materials shows the typical challenges of kinetic identification when relying solely on EIS.

Studying Surface Chemistry of Mixed Conducting Perovskite Oxide Electrodes with Synchrotron-Based Soft X-rays.

A fundamental understanding of the electrochemical reactions and surface chemistry at the solid-gas interface in situ and operando is critical for electrode materials applied in electrochemical and catalytic applications. Here, the surface reactions and surface composition of a model of mixed ionic and electronic conducting (MIEC) perovskite oxide, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255), were investigated in situ using synchrotron-based near-ambient pressure (AP) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The measurements were conducted with a surface temperature of 500 °C under 1 mbar of dry oxygen and water vapor, to reflect the implementation of the materials for oxygen reduction/evolution and H2O electrolysis in the applications such as solid oxide fuel cell (SOFC) and electrolyzers. Our direct experimental results demonstrate that, rather than the transition metal (TM) cations, the surface lattice oxygen is the significant redox active species under both dry oxygen and water vapor environments. It was proven that the electron holes formed in dry oxygen have a strong oxygen character. Meanwhile, a relatively higher concentration of surface oxygen vacancies was observed on the sample measured in water vapor. We further showed that in water vapor, the adsorption and dissociation of H2O onto the perovskite surface were through forming hydroxyl groups. In addition, the concentration of Sr surface species was found to increase over time in dry oxygen due to Sr surface segregation, with the presence of oxygen holes on the surface serving as an additional driving force. Comparatively, less Sr contents were observed on the sample in water vapor, which could be due to the volatility of Sr(OH)2. A secondary phase was also observed, which exhibited an enrichment in B-site cations, particularly in Fe and relatively in Cr, and a deficiency in A-site cation, notably in La and relatively in Sr. The findings and methodology of this study allow for the quantification of surface defect chemistry and surface composition evolution, providing crucial understanding and design guidelines in the electrocatalytic activity and durability of electrodes for efficient conversions of energy and fuels.

Investigation on Reaction Mechanism in an SOFC Composite Cathode by Using Patterned Thin Film Electrodes

Introduction For the further commercial use of solid oxide fuel cells (SOFCs), operating at intermediate temperatures is demanded. When reducing the operating temperature, the degradation of cathodic activity occupies the largest part of degradation of cell performance. Therefore, to improve the performance of SOFCs at intermediate temperature, the activity of cathodic reaction has to be improved.The cathodic reaction of SOFCs proceeds not only at triple phase boundaries (TPBs) of electrode, electrolyte and gas but also at double phase boundaries (DPBs) of electrode and gas, when a mixed ionic and electronic conductor (MIEC) is used. Thus, increasing the DPB area is considered as the best way to improve the cathodic performance. However, it was reported that composite cathodes of MIEC and electrolyte particles showed better performance than simple MIEC cathodes, despite the DPB area decreased [1]. Possible reasons for such performance enhancement can be considered as (i) promotion of oxide ion diffusion and/or (ii) the increase of TPB concentration. However, the reason for the performance enhancement by making a composite cathode are not-well understood so far.In this study, we aimed to understand the cathodic reaction in a composite cathode of MIEC and electrolyte. For this purpose, we proposed new types of original model electrodes to simulate the reaction in actual porous composite electrodes. From electrochemical and the operando X-ray absorption spectroscopic measurements of the model electrodes, we tried to separately evaluate the contribution of ion diffusion and TPB reactions on the performance improvement of composite cathode. Methods When using a porous electrode, it is difficult to investigate the reaction mechanism because of its complicated microstructures. Our research group proposed so-called “patterned thin film model electrode” like Figure 1(a) as a model electrode to reproduce the reaction in a porous electrode while removing the influence of microstructure [2]. In this model electrode, the contact area of electrode and electrolyte was limited by partially inserting an insulating layer, and thus the reaction distribution was formed in the electrode as a function of the distance from the electrode/electrolyte interface.In this study, we proposed two new types of model electrodes as Figure 1(b) and 1(c) to simulate the porous composite electrode. In the patterned composite electrode (b), an electrolyte layer was inserted between the electrode and insulator. On the other hand, in the patterned composite electrode (c), a part of electrode was removed to introduce TPBs. From the difference between the model electrodes (a) and (b), the contribution of oxide ion diffusion in composite electrode was estimated. From the difference between the model electrodes (b) and (c) the contribution of the TPB reaction was estimated. In this study, we selected La0.6Sr0.4CoO3-δ (LSC64) as an MIEC cathode and Gd0.1Ce0.9O2-δ (GDC10) as an electrolyte material. Operando micro X-ray absorption spectroscopy (μ-XAS) measurement was conducted with the model electrodes to investigate the effective reaction area of the model electrode. In this measurement, a change in the oxygen chemical potential can be detected as a change of X-ray absorbance at the fixed energy near the Co K-edge accompanying a change of the oxidation state of Co in the LSC64 [3]. Thus, the reaction distribution could be evaluated by measuring the change of the X-ray absorbance due to the polarization as a function of the distance from the electrode/electrolyte interface. Results and Discussion Figure 2 shows the results of μ-XAS measurement at 973 K under 1 bar O2 with cathodic polarization. The area where the ratio of absorbance is larger than 1 corresponds to the area where the oxygen chemical potential is changed due to the polarization, i.e. the effective reaction area. The reaction area of the model electrode (b) was longer than that of the model electrode (a). This suggested oxide ion diffusion was improved by inserting the GDC layer. On the other hand, the reaction area of the model electrode (c) was longer than that of the model electrode (b). Generally, the reaction on the mixed conducting LSC64 electrode is supposed to proceed DPB reaction mainly. However, the longer reaction area of model electrode (c) indicated that the contribution of TPB reaction was also significant in the LSC64 composite electrode. As a conclusion, both promotion of oxide diffusion and the increase of TPB effective for the enhancement of the cathodic reaction in MIEC composite electrode.[1] Y. T. Kim, et al., Solid State Ionics, 309, 77-85 (2017).[2] K. Amezawa, et al., ECS transactions, 62(2), 129-135 (2015).[3] Y. Orikasa, et al., Journal of Physical Chemistry C, 115, 16433–16438 (2011). Figure 1

Multiphysics Modeling for Solid Oxide Electrolyzer Cell with Heterogenous Synthetic Microstructure

Solid oxide fuel cell (SOFC) and electrolyzer cell (SOEC) are the main focus for researchers in recent times to diminish greenhouse gas emissions and meet cleaner energy demand, as these devices have highest energy conversion rates and efficiencies [1]. The major hinderance in commercializing SOEC is due to its faster cell degradation under higher current density, compared against its SOFC counterpart. The delamination of oxygen electrode (OE) from the electrolyte (EL) is one of the major causes for the cell degradation [2].We employed a multi-physics numerical model to understand the electrochemical performance of a SOEC half-cell more precisely at a microscale. In this study, widely used mixed ionic and electronic conductor (MIEC) La1-xSrxCo1-yFeyO3-d (LSCF) and Gadolinium-doped Ceria (GDC) composite is used as the oxygen electrode and, pure ionic conductor 8 mol% Y2O3 doped ZrO2 (YSZ) is used as the electrolyte. A GDC buffer layer is tucked in between to prevent undesirable side reactions between OE and EL during long term polarization operation. Until now, a homogenous OE structure has commonly been used with effective physicochemical properties to predict the performance of SOFC/SOEC [3,4]. Only a few researchers have worked on heterogenous microstructure model [5] and simulated the electrochemical performance of the cell.Synthetic microstructure of LSCF-GDC porous electrode as shown in Figure 1 are constructed with an opensource software Dream 3D with particle size ranging from 0.45um - 1.5um. We developed in-house Matlab code for labelling domains, interfaces (2PB and 3PB) and digitally removing the unconnected phases to reduce numerical instabilities while solving the mathematical model in COMSOL Multiphysics 6.0. Mesh in these domains is constructed using Iso2Mesh toolbox [6] and translated as mphtxt file which is later imported into COMSOL. This practice reduces the computational time and memory.We employed Nernst-Planck theory for solving the transport of species like vacancy, hole and electron within LSCF inside OE. Multi-step charge transfer at two phase boundary (2PB) and triple phase boundary (3PB) interfaces for oxygen evolution reaction are formulated by Butler-Volmer expressions [4]. A physics-based impedance model under DC bias is developed to separate the electrochemical contributions from 2PB and 3PB transport pathways. The voltage-current curve during long-term polarization test is coupled with the model for validation and approximation of process parameters. Furthermore, we will simulate the crack propagation at OE-buffer layer interface in various directions and orientations to understand the failure mechanism on the synthetically constructed microstructure.

Impact of Anionic Ordering on the Iron Site Distribution and Valence States in Oxyfluoride Sr2FeO3+xF1-x (x = 0.08, 0.2) with a Layered Perovskite Network.

Sr2FeO3F, an oxyfluoride compound with an n = 1 Ruddlesden-Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO2 atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe4+ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr2FeO3.2F0.8 oxyfluoride, containing 20% of Fe4+, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr2FeO3.2F0.8 and its use in MIEC-based devices, such as solid oxide fuel cells.

Measurement of electronic conductivity in mixed ionic and electronic conductor (MIEC) via a novel embedded microcontact method

Electronic conductivity plays an important role on the performance and durability of energy devices containing mixed ionic and electronic conductors (MIEC). Over the past decades, it has been realized that the classical Hebb-Wagner(H-W) method has some critical limitations or issues that may limit its accuracy or testing feasibility, such as relatively long relaxation time towards steady state and challenges on sealing. In this study, we reported an embedded microcontact ionic blocking electrode to minimize these issues. Using this method, the activation energy in Ce0.9Gd0.1O1.95 (GDC10) are measured, respectively as, 2.51 and 1.44 eV, for n type electronic and p type hole conductivity. The values are similar to those reported in literature as 2.51 and 1.45 eV, respectively, for n and p type conduction also in GDC10 via non-embedded ionic-blocking electrode. At a refence oxygen partial pressure of 0.21 atm, the n type conductivity, σe(0), is obtained as 1.05×10−7, 2.81×10−6, 4.28×10−5 S cm−1 at 600, 700, and 800 °C, respectively, and the p type conductivity, σh(0), is obtained as 1.18×10−4, 5.86×10−4, 3.51×10−3 S cm−1 at 600, 700, and 800 °C, respectively. The new method could be an effective technique to determine electronic conduction in other MIECs.

회전경사증착 기법을 이용한 Ni-GDC 연료극이 적용된 AAO 기반의 저온작동 박막 고체산화물 연료전지의 저항 특성 연구

In this study, we fabricated thin film solid oxide fuel cells on nanoporous anodic aluminum oxide (AAO) substrate for low-temperature operation using the all-through sputtering method. To deposit up to a three-micrometer thick anode with both porosity and electrical conductivity, we used the glancing angle deposition and co-sputtering methods. For the anode materials, we used nickel gadolinium-doped-ceria (Ni-GDC) mixed ionic and electronic conductor (MIEC), which improved hydrogen oxidation reaction reactivity at the anode side. TF-SOFCs were successfully operated at 500℃, and 223.6 mW/cm² was their highest measured maximum power density. We conducted structural and electrochemical analyses to figure out cells’ unique resistant characteristics; ohmic resistance through the anode thin film and polarization resistance of reaction area near the narrowed anode pores. We found how the anode thin film thickness affects the current collecting performance and the anode reactivity, and their effects were qualitatively and quantitatively compared.

Garnet-type solid-state mixed ionic and electronic conductor

Low ion and electron conductivities of the cathode and poor cathode interface performance severely limit the application of garnet solid-state batteries. While various approaches have been developed to improve the cathode interface contact or enhance ion and electron transport, none of these methods can simultaneously achieve high interfacial electrochemical activity and chemical stability. In this work, we propose to develop a single-phase garnet-type mixed ionic and electronic conductor (MIEC) as the cathode framework, which is expected to achieve the unification of high activity and high stability at the cathode interface. Combining first-principles calculations and ultrafast sintering techniques, we have synthesized and screened a series of garnet-type MIECs by introducing transition metals into garnet SSEs. Among the garnet-type MIECs, Li43Fe3La24Zr12Ta4O96 (3Fe) not only exhibits an excellent electronic conductivity of up to 2.87 × 10−4 S cm−1 but also maintains an ionic conductivity of 4.57 × 10−5 S cm−1. The high electronic conductivity is believed to originate from the high-temperature reduction phase formed during the rapid sintering process under inert conditions. A small polaron hopping mechanism is proposed to explain the electronic conductivity based on electronic structure calculations and activation energy analysis. Since garnet-type MIECs have the same crystal structure as garnet solid-state electrolytes (SSEs) and transition metal elements similar to those of oxide cathode materials, they potentially have good cosintering stability with both electrolytes and cathodes. This work provides a new strategy to solve the cathode interface problem in garnet solid-state batteries.

High CO2-resistance Ag modified La0.8Ca0.2Fe0.94O3-δ hollow fiber membrane with a three-layer sandwich structure for oxygen separation

Cobalt-free mixed ionic and electronic conducting (MIEC) oxygen permeation membrane is a kind of promising materials for advanced air separation technology. However, its low oxygen permeability limits its practical application. In this work, we successfully prepared a type of La0.8Ca0.2Fe0.94O3-δ-0.05Ag (LCF-Ag) hollow fiber membrane with “three-layer sandwich” structure by using a combined one-step sintering and phase conversion technology. Under the same experimental conditions, the oxygen permeation flux of the membrane structure was 1.79 times that of the traditional membrane structure, and the maximum oxygen flux was achieved at 2.15 mL min−1 cm−2. The model fitting proved that the membrane with new structure can effectively reduce the resistance of bulk diffusion. The stability test of oxygen permeation flux was carried out alternately in pure He and CO2 atmosphere. The membrane displays high stability during the long-term operation of 240 h. Despite the oxygen flux had decreased slightly when CO2 was used as sweep gas due to chemical adsorption of CO2 to occupy the active sites on the membrane surface. The oxygen flux could be fully recovered once sweep gas switching to He, which indicates that the perovskite structure was not been damaged due to its good chemical stability. These results prove the feasibility of the new membrane structure for oxygen separation with CO2 resistance, which may expand their applications to membrane reactors where the presence of CO2 is not avoidable.

Ultra-Thin Lithium Silicide Interlayer for Solid-State Lithium-Metal Batteries.

All-solid-state batteries with metallic lithium (LiBCC ) anode and solid electrolyte (SE) are under active development. However, an unstable SE/LiBCC interface due to electrochemical and mechanical instabilities hinders their operation. Herein, an ultra-thin nanoporous mixed ionic and electronic conductor (MIEC) interlayer (≈3.25 µm), which regulates LiBCC deposition and stripping, serving as a 3D scaffold for Li0 ad-atom formation, LiBCC nucleation, and long-range transport of ions and electrons at SE/LiBCC interface is demonstrated. Consisting of lithium silicide and carbon nanotubes, the MIEC interlayer is thermodynamically stable against LiBCC and highly lithiophilic. Moreover, its nanopores (<100 nm) confine the deposited LiBCC to the size regime where LiBCC exhibits "smaller is much softer" size-dependent plasticity governed by diffusive deformation mechanisms. The LiBCC thus remains soft enough not to mechanically penetrate SE in contact. Upon further plating, LiBCC grows in between the current collector and the MIEC interlayer, not directly contacting the SE. As a result, a full-cell having Li3.75 Si-CNT/LiBCC foil as an anode and LiNi0.8 Co0.1 Mn0.1 O2 as a cathode displays a high specific capacity of 207.8 mAh g-1 , 92.0% initial Coulombic efficiency, 88.9% capacity retention after 200 cycles (Coulombic efficiency reaches 99.9% after tens of cycles), and excellent rate capability (76% at 5 C).

Colloidal synthesis of the mixed ionic-electronic conducting NaSbS2 nanocrystals.

Solution-based synthesis of mixed ionic and electronic conductors (MIECs) has enabled the development of novel inorganic materials with implications for a wide range of energy storage applications. However, many technologically relevant MIECs contain toxic elements (Pb) or are prepared by using traditional high-temperature solid-state synthesis. Here, we provide a simple, low-temperature and size-tunable (50-90 nm) colloidal hot injection approach for the synthesis of NaSbS2 based MIECs using widely available and non-toxic precursors. Key synthetic parameters (cationic precursor, reaction temperature, and ligand) are examined to regulate the shape and size of the NaSbS2 nanocrystals (NCs). FTIR studies revealed that ligands with carboxylate functionality are coordinated to the surface of the synthesized NaSbS2 NCs. The synthesized NaSbS2 nanocrystals have electronic and ionic conductivities of 3.31 × 10-10 (e-) and 1.9 × 10-5 (Na+) S cm-1 respectively, which are competitive with the ionic and electrical conductivities of perovskite materials generated by solid-state reactions. This research gives a mechanistic understanding and post-synthetic evaluation of parameters influencing the formation of sodium antimony chalcogenides materials.

Dual-phase Ga-containing Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe1-xGaxO3-δ oxygen transport membranes with high CO2 resistance

Here we report a new series of Ga-doped dual-phase mixed ionic and electronic conducting (MIEC) oxygen transport membranes (OTMs) with the compositions of Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe1-xGaxO3-δ (CPO-PSF1-xGxO, x = 0.025, 0.05, 0.075, 0.1, 0.15, 0.2). The structure, surface topography, oxygen permeability, and stability are studied. The optimal CPO-PSF0.95G0.05O OTM performs the highest oxygen permeation fluxes of 0.75 mL min−1 cm−2 and 0.41 mL min−1 cm−2 with He or CO2 as the sweep gas, respectively. All Ga-doped OTMs maintain excellent stability in the two different atmospheres over 120 h. Moreover, a kind of MIEC-MIEC OTM with a composition of Ce0.85Pr0.1Cu0.05O2-δ-Pr0.6Sr0.4Fe0.95Ga0.05O3-δ is obtained by Cu doping. It performs higher oxygen permeation fluxes of 1.19 mL min−1 cm−2 and 0.46 mL min−1 cm−2 under the same conditions and operates stably for 250 h under the He and CO2 cycle switching as the sweep gas. Through stability tests in a reducing atmosphere, our findings are expected to improve the studies and possible applications of OTMs in solid oxide fuel cells.

Numerical simulations of the Hebb-Wagner polarization kinetics: (I) weakly acceptor-doped perovskite oxides

The Hebb-Wagner polarization method can be used to separate the partial electronic conductivity of mixed ionic and electronic conductors (MIEC) from the partial ionic conductivity by polarizing the material between an ion-blocking and a reversible electrode using an external electric voltage. The electronic conductivity is then calculated from the steady state I-V curve where the partial current of the blocked ionic species is zero. The kinetics of the relaxation process towards the steady state and the corresponding relaxation time were, however, not investigated until now, although they are crucial to the success of the Hebb-Wagner method. In this contribution, the kinetics of the relaxation process during the Hebb-Wagner polarization of a nominally undoped perovskite-type oxide ABO3 (mixed ionic and electronic conductor) is simulated by means of numerical methods and using SrTiO3 as a model system. The simulation results show an unexpected dependence of the relaxation time of the polarization process on the applied voltage. The relaxation time first decreases until a certain voltage is reached, where it remains constant for a few voltage steps, and afterwards it increases again for higher voltage values. We show that these three different relaxation time dependencies correspond to the transition from (i) a prevailing electron hole conduction at high oxygen partial pressures to (ii) a predominant oxygen ion conduction near the p − n transition at intermediate oxygen partial pressures followed by (iii) a predominant electron conduction at low partial pressures. Additionally, the dependence on other parameters and experimental variables, such as surface exchange coefficient at the reversible electrode, acceptor concentration, temperature, and sample length, is investigated, which reveals that a slight change of one of these parameters can change the relaxation time by several orders of magnitude. The obtained insight from the numerical simulation of the electrochemical polarization process is advantageous for the experimental realization of the Hebb-Wagner polarization method, and in particular for its application to nominally undoped perovskite-type oxides, like SrTiO3.

Structural, Dielectric and Electrical Properties of Homovalent Doped SrSn1-xTixO3 (0 ≤ x ≤ 0.08) System

The samples SrSn1-xTixO3 with composition 0≤x≤0.08 have been prepared using sol-gel chemical route by sintering at 1173 K. All the samples are found to be single phase crystallized in orthorhombic structure. The dielectric properties indicate the existence of interfacial and orientation polarization in samples found to be stable up to 300 oC. Thermal dependence of electrical conductivity represents two conduction regions with activation energy (0.77-0.94) eV in region-1 and (0.19-0.27) eV in region-2 respectively. The plot of dc conductivity with hopping frequency results unit slope representing that the charge carriers remain same in both processes. DC conductivity of samples are found to be increased with Ti4+, due to reduction in polaron size. The present materials can be potentially used in thermally stable capacitor and mixed ionic and electronic conductors (MIECs) applications.

(Invited) Effect of Anode Thickness on Thin-Film Solid Oxide Fuel Cells Deposited on Nanoporous Substrates

In this study, low-temperature solid oxide fuel cells having different thin-film anode thicknesses were fabricated, and their characteristics were studied. Since the low-temperature solid oxide fuel cells (LT-SOFCs) have the advantages of a low-cost fuel cell system and a wide selection of components’ materials, many researchers have made efforts to lower the operating temperature range. Naturally, thin-film electrolyte technology has been improved to reduce the exponentially increasing ohmic resistance of oxygen conduction when the working temperature is lowered. Various substrates or even no substrate were used to fabricate thin-film solid oxide fuel cells (TF-SOFCs). However, free-standing TF-SOFCs have poor mechanical strength, and the active area was limited to tens of square micrometers. A nanoporous substrate such as Anodic aluminum oxide (AAO) has been a great platform on which TF-SOFCs were stably deposited and operated. AAO has well-arranged and straight pores from its bottom to the top but no electrical conductivity. Due to the resistance characteristics of TF-SOFCs based on AAO, the anode’s thickness and the nanostructure significantly influence the overall performance of the fuel cell. According to previous studies, the performance tends to increase as the thickness increases with well-maintained porosity in the thin-film anode. To confirm this tendency and further improve cell performance, AAO substrates with 250nm-size pores and relatively rough surfaces were used. On the AAO substrate, nickel gadolinium-doped-ceria (Ni-GDC) mixed ionic and electronic conductor (MIEC) was introduced as the anode material, which increased hydrogen oxidation reaction (HOR) reactivity at the anode. In addition, the glancing angle deposition (GLAD) of the sputtering method was applied to maintain porosity better than the conventional method. With this method, thickening the thin-film anode improved the current collecting performance while the pores were well connected to the anode-electrolyte interface. Following that, the yttria-stabilized zirconia (YSZ) electrolyte was densely deposited, and the porous deposited platinum cathode completed the entire cell. The fabricated cells were tested at 500C and electrochemically analyzed by i-V-P curves and the EIS method. Also, the anode’s nanostructures were revealed by various analyzing methods such as surface FESEM, FIB cross-sectional SEM, TEM-EDS, and 4-probe conductivity measurement. Each cell’s stable structure and the uniformity of some fuel cell components such as the electrolyte and the cathode were confirmed. We found that the electrochemical performance increases as the anode thickness increases while its porosity is well-maintained. About 550mW/cm2 was measured as the highest maximum power density, which is more than a 20% increased result than the other fabricated cells. Those investigations also enabled the anode nanostructure to be identified, such as the anode thin-film porosity, conductivity, and three-dimensional morphology. The correlation between the anode nanostructure and the fuel cell performance was discussed, and the optimized cell structure was found. This study enlarged the understanding of nanoporous substrate-based TF-SOFC manufacturing and their expandability.

On-Board Strontium Getters for Improved Solid Oxide Cell Durability

Nano-composite electrodes fabricated by introducing nano-sized mixed ionic and electronic conducting (MIEC) catalysts into partially-sintered micro-sized ionic conducting backbones have been used by many researchers to improve the electrochemical performance, and/or reduce the operating temperature, of Solid Oxide Cells (SOCs).1 Unfortunately, these infiltrated electrodes have often possessed poor electrochemical performance stability.2 This presentation will review recent results showing how pre-infiltrated ceria nano-particles3 or atomic layer deposited zirconia overcoats4 can be used as on-board Sr getters to increase the long-term stability of La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF)-infiltrated Ce0.9Gd0.1O1.95-x (GDC) SOC electrodes, without compromising initial performance. This talk will also present results showing that when combined, these surface modification techniques result in LSCF-GDC performance degradation rates that are no better than those obtained from each technique used separately. Lastly, this talk will present GDC pre-infiltration results supporting a newly-emerging consensus that Sr does not surface segregate in Sm0.5Sr0.5CoO3-x (SSC) like it does in LSCF. References D. Ding, X. X. Li, S. Y. Lai, K. Gerdes and M. L. Liu, Energy Environ. Sci., 7, 552 (2014).M. Shah, P. W. Voorhees and S. A. Barnett, Solid State Ionics, 187, 64 (2011).Y. Zhang and J. D. Nicholas, Journal of the Electrochemical Society, 168, 024522 (2021).Y. Zhang, Y. Wen, K. Huang and J. D. Nicholas, ACS Applied Energy Materials, 3, 4057 (2020).

Structural and dielectric properties of ultra-fast microwave-processed La0.3Ca0.7Fe0.7Cr0.3O3−δ ceramics

Perovskite La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 ​min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 ​°C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from δ ​= ​0.272(7) at RT to δ ​= ​0.333(5) at 900 ​°C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 ​°C for 1 ​h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible.