Pure Ionic Conductor Research Articles

Defect Structure, Oxygen Ion Conduction, and Conducting Mechanism in Ruddlesden-Popper Sr3Zr2-xMxO7-0.5x (M = Ga, Y, In).

Ruddlesden-Popper (RP)-structured materials based on transition metals with a variable valence, such as Fe, Mn, Ni, and so on, have been well documented for their potential of being used as electrodes in solid-oxide fuel cells. However, RP materials with pure or dominant ionic conduction are rare. Here, a series of Zr-based RP materials Sr3Zr2-xMxO7-0.5x (M = Ga, Y, In) with electrical conductivity as high as 3.25 × 10-3 S cm-1 at 900 °C in air was reported, which represents the highest conductivity for the Zr-based RP materials and is comparable to that of the recently reported In-based RP oxide-ion conductors, such as NdBaInO4-based and La2BaIn2O7-based materials. Under low oxygen partial pressure (pO2), the doped samples show pure ionic conducting behaviors without n-type electronic conductivity. The defect formation energies, local structure around the oxygen vacancies, and oxide-ion-conducting mechanism of the acceptor-doped Sr3Zr2O7-based materials were studied for the first time. The results revealed a two-dimensional oxide ion migration characteristic within the perovskite slabs. This work therefore provides a good reference for developing new oxide-ion conductors in the Zr-based RP-structured materials.

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells.

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Phases, stabilities, and oxide ion conductions of multiple lanthanide and tungsten co–doped δ-Bi2O3-based materials: From low to high configuration entropy

In this work, a series of multiple lanthanide and tungsten co-doped fluorite structured (Bi2O3)0.95-x(Ho0.4Er0.4Tm0.4Yb0.4Lu0.4O3)x(WO3)0.05 (x = 0.1, 0.2, 0.3, 0.4) and (Bi2O3)0.26(Ho0.5Er0.5Tm0.5Yb0.5O3)0.48(WO3)0.26 materials were prepared by traditional solid-state reaction method. The phases, stabilities, and electrical properties of these materials were thoroughly studied. The results revealed that under high oxygen partial pressure, CO2, or inert atmospheres, all these samples had excellent long term thermal stability and chemical stability, and the samples of (Bi2O3)0.95-x(Ho0.4Er0.4Tm0.4Yb0.4Lu0.4O3)x(WO3)0.05 (x = 0.1, 0.2, 0.3, 0.4) with low or mediate configuration entropies showed pure and high oxide ion conductivities, e.g. ∼2.6 × 10−2 S/cm at 600 °C for the sample x = 0.2. Whereas, for the (Bi2O3)0.26(Ho0.5Er0.5Tm0.5Yb0.5O3)0.48(WO3)0.26 sample with high configuration entropy, pure ionic conduction occurred only within a narrow temperature range (700−750 °C) under the N2 atmosphere. Under strong reducing condition, only the high-entropy sample (Bi2O3)0.26(Ho0.5Er0.5Tm0.5Yb0.5O3)0.48(WO3)0.26 showed high phase and chemical stabilities, but would introduce strong electronic conduction rise from the reduction of the high content W6+. While for (Bi2O3)0.95-x(Ho0.4Er0.4Tm0.4Yb0.4Lu0.4O3)x(WO3)0.05 with low or mediate configuration, phase decompositions with impurities of Bi metal and HoH2 were observed under strong reducing atmosphere, accompany with obviously decreased conductivities. Therefore, we provided here a comprehensive understanding for the multiple components doped Bi2O3-based materials with configuration entropy from low to high on their phases, stabilities, and electrical properties, and suggested a new strategy for designing and stabilizing bismuth-oxide-based materials.

Chlorine-Rich Na6-xPS5-xCl1+x: A Promising Sodium Solid Electrolyte for All-Solid-State Sodium Batteries.

Developing argyrodite-type, chlorine-rich, sodium-ion, solid-state electrolytes with high conductivity is a long-term challenge that is crucial for the advancement of all-solid-state batteries (ASSBs). In this study, chlorine-rich, argyrodite-type Na6-xPS5-xCl1+x solid solutions were successfully developed with a solid solution formation range of 0 ≤ x ≤ 0.5. Na5.5PS4.5Cl1.5 (x = 0.5), displaying a highest ionic conductivity of 1.2 × 10-3 S/cm at 25 °C, which is more than a hundred times higher than that of Na6PS5Cl. Cyclic voltammetry and electrochemical impedance spectroscopy results demonstrated that the rich chlorine significantly enhanced the ionic conductivity and electrochemical stability, in addition to causing a reduction in activation energy. The Na5.5PS4.5Cl1.5 composite also showed the characteristics of a pure ionic conductor without electronic conductivity. Finally, the viability of Na5.5PS4.5Cl1.5 as a sodium electrolyte for all-solid-state sodium batteries was checked in a lab-scale ASSB, showing stable battery performance. This study not only demonstrates new composites of sodium-ionic, solid-state electrolytes with relatively high conductivity but also provides an anion-modulation strategy to enhance the ionic conductivity of argyrodite-type sodium solid-state ionic conductors.

Pressure effects on the ionic transport properties of LiNH2

A pressure-induced abnormal transition from mixed ionic and electronic conduction to pure ionic conduction was found in LiNH2. The energy required for Li+ ion migration increased with increasing pressure.

Distribution of Relaxation Times of Fuel Electrodes for Reversible Solid Oxide Cells Fabricated Under Various Conditions

Introduction Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can switch reversibly between power generation as a solid oxide fuel cell (SOFC) and hydrogen production as a solid oxide electrolysis cell (SOEC). It is important to systematically understand the electrode reaction processes and degradation factors in these two modes to tailor high-performance and durable r-SOC electrodes. The dependence of polarization resistance on cell fabrication conditions and operating conditions has been individually investigated for SOFC and SOEC by impedance measurement and distribution of relaxation times (DRT) analysis [1-4], but it is necessary to evaluate r-SOC as a whole [5]. Here in this study, we aim to systematically clarify the similarities and differences in the electrode reaction processes at the fuel electrodes of both SOFC and SOEC modes of r-SOCs, with respect to fuel electrode fabrication conditions. Experimental Several types of cells were fabricated with different fuel electrode constituent materials, thicknesses, and sintering temperatures. Three types of fuel electrode materials were used: Ni-GDC cermet, a composite of Ni and mixed ionic-electronic conductor Gd0.9Ce0.1O3 (GDC); Ni-YSZ cermet, a composite of Ni and pure ionic conductor YSZ (8 mol% Y2O3 - stabilized ZrO2); and Ni-GDC co-impregnated electrode with LST-GDC as a backbone support (LST: La0.1Sr0.9TiO3). Scandia-stabilized zirconia (ScSZ) was used as the electrolyte plate, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was used as the air electrode. GDC buffer layer was prepared between the electrolyte and the air electrode to suppress the formation of interfacial insulting layers.Electrochemical impedance measurements were performed for the fuel electrode of the fabricated cell under SOFC and SOEC modes by supplying H2-H2O mixture and varying operating temperature, fuel humidification, and current density. The DRT analysis was conducted to separate various polarization resistance components of the fuel electrode with different relaxation times. The area of each DRT peak corresponds to the resistance of each electrode reaction process. In this study, activation energy was derived based on the assumption that the electrical conductance is of an Arrhenius-type thermally activated character. Its dependence on the fuel electrode fabrication conditions was investigated. Furthermore, electrode microstructure was observed by using focused-ion beam scanning electron microscopy (FIB-SEM). Results and Discussion Figure 1 shows typical DRT peaks of the Ni-GDC cermet fuel electrode in (a) SOFC and (b) SOEC modes, measured at different temperatures. In the range of 10-1 to 106 Hz where the frequency response was measured, three DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all with a DRT peak area decreasing with increasing operating temperature. This indicates that there are at least three electrode reaction processes with different relaxation times, which are thermally active processes. Activation energies of 0.1-0.3 eV (SOFC mode) and 0.2-0.5 eV (SOEC mode) were obtained for P1, and 0.9-1.0 eV (SOFC mode) and 1.1-1.3 eV (SOEC mode) for P2. The activation energies in SOEC mode were higher than those in SOFC mode for both P1 and P2. In the presentation, the dependence of activation energies on the fuel electrode constituent materials and related electrode reaction processes will be discussed.

Predicting Fast Oxygen Ion Conductors Via Descriptors for Migration Barrier Based on Electronic Structure and Lattice Dynamics

Solid-state oxygen ion conductors are of paramount importance to solid oxide fuel cells and electrolytic cells. However, with the oxygen ion conductors proposed to date, these technologies require too high (close to 800 oC) temperatures to operate, leading to technological challenges and high costs. Therefore, it is of primary importance to design faster oxygen ion conductors at intermediate and low temperatures (below 400 oC) that can enable sustainable energy technologies to mitigate climate change. Descriptors for oxygen ion conduction are highly desirable to identify promising materials, which can be done by leveraging fast-growing statistical and machine learning approaches for screening of material databases [1]. Recently, computational tools combined with experiments have allowed researchers to gain atomistic insights and relate ion transport to features of the lattice dynamics [2] and electronic structure [3] of a given material. In this work, building upon the previous literature, we leverage both the lattice dynamics and electronic structure to predict the migration barrier for oxygen migration in pure and mixed electronic ionic conductors and search for promising oxygen ion conductors.Using density functional theory simulations, we model oxygen migration in perovskite, rutile, fluorite, and Ruddlesden-Popperoxides. We analyze oxygen ion transport in pure and mixed electronic ionic conductors and oxygen vacancies with and without charge. We study changes in the electronic structure during oxygen migration within the solid host lattice in different oxides. By analyzing trends of migration barrier and electronic density of states, we relate the migration barrier to the electronic structure of the transition state. Our results show that the migration barrier depends on the capability that the host lattice has of screening charge associated with the mobile ion in the transition state, which if increased would lower the localization of charge around the mobile ion and in turn the oxygen migration barrier. In pure ionic conductors – where no charge is associated with the mobile ion - the migration barrier is largely dependent on the M-O strength. Based on the mechanistic understanding derived by these initial analyses, we further relate oxygen ion conductivity to the lattice dynamics. We show that in pure ionic conductors, increasing migration barrier follows trends of increasing oxygen vibrational frequency based on phonon-electron coupling, which also relates the prefactor of Arrhenius law with the migration barrier. However, when trying to predict trends of the migration barrier across different crystal structures or mixed ionic conductors, no single descriptor can capture the complex physiochemistry behind oxygen migration. Thus, we leverage statistical learning tools to identify multivariate descriptors and analyze specific features of lattice moieties that improve oxygen ion conduction. The results of this work provide mechanistic understanding of oxygen ion migration and strategies to accelerate the designs of fast oxygen ion conductors.

Understanding the Physical Properties of Pr-Gd Co-Doped Ceria and Its Role As the Interlayer in Solid Oxide Cells

In solid oxide cells, a doped ceria interlayer is widely applied between the state-of-the-art yttrium-stabilized zirconia electrolyte and the oxygen electrode to mitigate thermal expansion mismatch and chemical interactions for extended durability. The conventional gadolinium doped ceria is usually a pure ionic conductor and blocks electronic currents under oxidative atmosphere. Adding electronic conduction to the interlayer may increase triple phase boundary areas, thus decrease charge transfer resistance and improve electrode stability. The addition of Pr in Gd-doped ceria results in unique physical properties such as an oxygen partial pressure (pO2) dependent ionic conductivity and electronic conduction in a wide range of pO2. In this study, Pr/Gd co-doped ceria (PGCO) is prepared with varying doping levels and its physical properties such as ionic and electronic conductivities are measured under different oxygen partial pressures and temperatures. PGCO with varying Pr/Gd doping is investigated as interlayer for solid oxide cell applications.

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.

Revolutionizing hydrogen production with LSGM-based solid oxide electrolysis cells: An innovative approach by sonic spray

Solid oxide electrolysis cells (SOECs) are considered promising systems for generating green hydrogen, which is an alternative new energy source. Yttria-stabilized zirconia (YSZ), the most common electrolyte in SOFC (solid oxide fuel cell) and SOEC (solid oxide electrolysis cell) has pure ionic conductivity, but it is limited in high temperatures. Instead, La1-xSrxGa1-yMgyO3-δ (LSGM) has emerged as an important candidate electrolyte in such cells because of its superior ionic conductivity with a value of 0.1 S/cm at 1073 K. Therefore, electrolysis cells supported by LSGM may exhibit high durability and efficiency compared to YSZ-supported cells. In this study, an LSGM electrolyte-supported SOEC was successfully fabricated using a sonic spray method, which is presented as a new manufacturing approach. The cell was optimized by controlling the spray frequency and particle size, and its performance was then characterized and electrochemically analyzed. The results showed that the LSGM-based SOEC presents high performance for H2 production. During electrochemical electrolysis at 1073 K, a current density as high as 1.15 A·cm−2 was achieved when 1.3 V was applied. This superior electrolysis performance is attributed to the high ionic conductivity of LSGM. The performance and efficiency obtained with the use of LSGM demonstrate that this electrolyte could be successfully used in SOEC applications.

Polarons and electrical leakage in BaZrO3 and BaCeO3

The proton conductors barium zirconate (${\mathrm{BaZrO}}_{3}$, or BZO), barium cerate (${\mathrm{BaCeO}}_{3}$, or BCO), and their alloys have attracted considerable interest as solid-state electrolytes in solid-oxide fuel and electrolysis cells. However, the reasons for their non-negligible electrical conductivity, which can limit their performance, are not fully understood. To address that question, we use first-principles calculations based on density functional theory to study the properties of hole and electron polarons in BZO and BCO. We confirm that hole polarons form in both materials, slightly more favorably in BCO. Electron polarons, on the other hand, are stable only in BCO and in Ce-containing alloys; in pure BZO, electron polaron states are unfavorable relative to free carriers, though they become accessible when Ce impurities are introduced. In general, doped BZO and BCO will have negligible electron concentrations, but larger concentrations of holes and hole polarons may be present. Avoiding extreme O-rich conditions and limiting dopant concentrations are key strategies for reducing significant $p$-type electrical leakage. Our results provide physical insights into the different electronic behaviors of BZO and BCO, which can be used to optimize their performance as pure ionic conductors.

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1

High Formability Bromide Solid Electrolyte with Improved Ionic Conductivity for Bulk-Type All-Solid-State Lithium–Metal Batteries

Solid electrolytes with excellent formability and high room-temperature Li-ion conductivities used in bulk-type all-solid-state lithium-metal batteries (ASSLBs) are expected to suppress the formation of Li dendrites that affects the application of metal Li anodes with high energy density. However, voids and cracks within sintered electrolytes lead to short-circuiting of ASSLBs due to the formation of Li dendrites. This work aimed to develop solid electrolytes with high formability and high room-temperature Li-ion conductivities that can suppress Li dendrites’ formation in ASSLBs. Three indicators of bulk moduli, lithium-ion migration energies, and energies above the hull were investigated: formability, ionic conductivity, and thermodynamic stability. The 109 inorganic materials containing elements of Li and Br listed in the open web-based database of the Materials Project were surveyed due to their weak Coulombic interaction with Li ions and high polarizability. LiGaBr4 with monoclinic symmetry (LGB, S.G.: P121/a1) was focused on due to its lower values for all three analyzed indicators. A ball milling mechanochemical synthesis method was adopted to synthesize LGB. It indicated that the obtained LGB possessed a monoclinic structure with a lithium ionic conduction of 2.0 × 10–5 S cm–1 (cold isostatic pressed powder) at 25 °C. The ionic conductivity was nearly three times higher than that of the conventional sintered LGB and comparable to that of a typical oxide-type electrolyte of LISICON-type Li3.55(Ge0.45Si0.10V0.45)O4 (LGSV) at 25 °C, which is appealing for applying the sintered samples for Li-metal anodes. The SEM and the distribution of relaxation times (DRT) analysis demonstrated that LGB contributed minimal grain-boundary resistance due to tight connections between the particles, while the LGSV oxides contributed an extremely large grain-boundary resistance. An ASSLB using LGB powder as an electrolyte, bare LiCoO2 as a cathode, and lithium metal as the anode exhibited the ability to function smoothly, while short-circuiting occurred at the second cycle with LGSV, demonstrating that LGB is a pure ionic conductor and may be suitable for use in ASSLB that can prevent Li dendrites with electrochemical stability.

Oxide Ion Conduction, Defect Chemistry, and the Conducting Mechanism in Ruddlesden–Popper La2–xMxBaIn2O7–0.5x (M = Ca, Sr, Ba)

Tetragonal La2BaIn2O7 belongs to the stoichiometric n = 2 members of the Ruddlesden–Popper (RP) structure, with La3+ and Ba2+ cations ordered in the rock salt and octahedral layers, respectively. Herein, a series of alkaline-earth-metal-doped materials La2–xMxBaIn2O7–0.5x (M = Ca, Sr, Ba) were prepared by the traditional solid-state reaction method. The results revealed that the substitutions of Ca/Sr/Ba for La introduced oxygen vacancies and led to good oxide ion conduction, e.g., ∼1.21 × 10–3–1.38 × 10–2 S/cm within 600–900 °C for the specimen La1.7Ba1.3In2O6.85, which was more than 2 orders of magnitude higher than the parent material. However, both the parent and acceptor-doped La2BaIn2O7 showed very limited proton conductions. The energetics of defect formation and oxygen vacancy migrations in La2BaIn2O7-based materials was studied through the static lattice and molecular dynamics (MD) atomistic simulations based on the interatomic potential approach. The defect formation energy calculations for the electrons and holes in La2BaIn2O7 rationalized the experimentally observed pure ionic conduction under low pO2 and mixed ionic and p-type electronic conductions under high pO2, respectively. The MD simulations indicated two-dimensional oxygen vacancy migration within the perovskite slab for the acceptor-doped La2BaIn2O7.

Electro-Chemical-Mechanical Coupled Modeling of Oxygen Electrodes in Solid Oxide Electrolyzer Cells

Solid oxide electrolyzer cell (SOECs), which is an electrochemical device to split water with electricity to generate hydrogen, have received widespread attention for multi-day or seasonal energy storage to integrate variable renewable energy (VRE) into the grid [1]. Even though there have been tremendous material development and performance improvement of the technology in the past decade [2], challenges remain in understanding its degradation mechanism, especially the delamination of oxygen electrode/electrolyte interface [3-5]. Developing new oxygen electrodes which could decelerate the degradation and extend the stack lifetime beyond 60,000 hours at 1.5A/cm2 would significantly drive the stack price down to $100/kW and hydrogen cost down to $2/kg [6].In the state-of-the-art SOECs, pure ionic conductor 8 mol% Y2O3 doped ZrO2 (YSZ) and Gadolinium-doped Ceria (GDC) are usually used as the dense electrolyte and the buffer layer; mixed electronic and ionic conductor (MIEC) La1-xSrxCo1-yFeyO3-d (LSCF) is widely used as the oxygen electrode [7]. Such cells still suffer from short lifetimes with current densities over 1A/cm2. To overcome the challenge, a bilayer oxygen electrode structure, consisting of a commercial LSCF-GDC porous backbone coated by a thin SrCo0.9Ta0.1O3- d (SCT10) film (denoted as LSCF-SCT bilayer design), has been proposed and demonstrated with much higher performance compared to traditional LSCF electrode (single layer design) [8]. The new oxygen electrode has an inherently fast oxygen evolution reaction (OER) electrokinetics (or high oxygen evolution rate) and can mitigate the delamination problem by minimizing the accumulation of oxygen in oxygen evoluation reaction (OER) electrode lattice and thus chemical stresses.In this study, based upon an Electro-Chemical-Mechanical coupled model, we will correlate the crack length at the oxygen electrode/electrolyte interface with the electrochemical performance of the cell, specifically the voltage-current curve (Fig.1a). We will use J-integral (Fig.1b) as the fracture criteria to evaluate the crack growth rate under different current densities and with different oxygen electrode designs. LSCF single layer design is the baseline of the study. The long-term performance improvement of LSCF-SCT bilayer design will be compared against the baseline and its degradation mechanism will be investigated. The model will also be validated by long-term overpotential testing data as a function of time under 1A/cm2. It will be used as an optimization tool to mitigate delamination and extend the cell lifetime under higher current densities. Key Words: Electrolysis, Oxygen Electrode (OE), Delamination, Chemical Expansion, J integral, Crack Growth Rate Figure 1

Recent Progress in Semiconductor-Ionic Conductor Nanomaterial as a Membrane for Low-Temperature Solid Oxide Fuel Cells.

Reducing the operating temperature of Solid Oxide Fuel Cells (SOFCs) to 300–600 °C is a great challenge for the development of SOFC. Among the extensive research and development (R&D) efforts that have been done on lowering the operating temperature of SOFCs, nanomaterials have played a critical role in improving ion transportation in electrolytes and facilitating electrochemical catalyzation of the electrodes. This work reviews recent progress in lowering the temperature of SOFCs by using semiconductor-ionic conductor nanomaterial, which is typically a composition of semiconductor and ionic conductor, as a membrane. The historical development, as well as the working mechanism of semiconductor-ionic membrane fuel cell (SIMFC), is discussed. Besides, the development in the application of nanostructured pure ionic conductors, semiconductors, and nanocomposites of semiconductors and ionic conductors as the membrane is highlighted. The method of using nano-structured semiconductor-ionic conductors as a membrane has been proved to successfully exhibit a significant enhancement in the ionic conductivity and power density of SOFCs at low temperatures and provides a new way to develop low-temperature SOFCs.

Advanced LT-SOFC Based on Reconstruction of the Energy Band Structure of the LiNi0.8Co0.15Al0.05O2–Sm0.2Ce0.8O2-δ Heterostructure for Fast Ionic Transport

Formation of a heterostructure of semiconductor materials is a promising method to develop an electrolyte with high ionic conductivity at low operational temperature of solid oxide fuel cells (LT-SOFCs). Herein, we develop various heterostructure composites by introducing a pure ionic conductor Sm0.2Ce0.8O2-δ (SDC) into a semiconductor LiNi0.8Co0.15Al0.05O2 (LNCA) for LT-SOFCs electrolyte. The morphology, crystal structure, elemental distribution, micro-structure, and oxidation states of the composite of LNCA–SDC are analyzed and studied via X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), high energy dispersive spectrometry, and X-ray photoelectron spectroscopy (XPS). Electrochemical studies found that the optimal weight ratio of 0.5 LNCA-1.5 SDC heterostructure composite exhibits relatively high ionic conductivity (0.12 S cm–1 at 520 °C), which is much higher than that of SDC. The designed composite of LNCA-SDC heterostructures with optimal weight ratio (0.5:1.5) delivers a remarkable fuel cell power output of 0.735 W cm–2 at 520 °C. The formation of the heterostructure and reconstruction of energy bands at the interface play the crucial roles in enhancing ionic conduction to improve electrochemical performance. The prepared composite heterostructure delivers a unique and insightful strategy of electrolyte in advanced LT-SOFCs.

Ag-based electrocatalysts for ethylene epoxidation

Ethylene oxide (EO) is industrially produced by epoxidation of ethylene on Ag-based heterogeneous catalysts. Electrophilic oxygen species are assumed to be active key species in the EO production. This study explores the possibility to electrochemically produced these oxygen species in a solid oxide electrochemical cell. Two series of Ag based cermet electrodes were prepared either with Yttria Stabilized Zirconia (YSZ), a pure oxygen ionic conductor, or with Gadolinia-Doped Ceria (GDC), a mixed ionic and electronic conductor. Ag/YSZ cermet layers were deposited on planar and tubular YSZ dense substrates by using a conventional chemical method while Ag/GDC cermets were coated on GDC dense pellets by reactive magnetron sputtering. We evidenced a self-redispersion of Ag in the ethylene/oxygen mixture in Ag/YSZ cermet layers leading to the formation of small Ag clusters in close interaction with YSZ which are selective for epoxidation. However, the sintering process of these small Ag clusters is too fast even at 260°C to maintain the EO selectivity. On the other hand, the selectivity towards EO and the ethylene conversion of Ag/GDC cermet coatings were found to linearly increase with the applied current, demonstrating for the first time, the direct electrooxidation of ethylene into EO.

Examination of Gadolinium Doped Ceria Mixed with Yttrium Stabilized Zirconia Mixed Ionic Electronic Conducting Solid Electrolytes for Use As Reversible High Temperature Cells

Reversible cells can be used as solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC). The solid electrolytes used for such cells frequently have pure ionic conduction with insignificant electronic conduction. A previous study (Virkar and Tao, 2015) has shown that cells made of ceria doped yttrium stabilized zirconia (YSZ), a mixed ionic electronic conducting (MIEC) material, can be made to operate at the same efficiency level and with superior stability compared to a pure ionic conductor, like pure YSZ. This work is a continuation of that study, examining the use of gadolinium doped ceria (GDC) mixed with YSZ. The MIEC cell, 8mol%GDC+YSZ, will be further tested and designed to consume fuel or water vapor, generate power or hydrogen, and release heat at the same rate as the pure ionic conducting cell, YSZ, when operating in SOFC or SOEC mode. Future testing will also determine if the GDC MIEC cell can match or exceed the durability of the ceria doped YSZ.The electrolyte layer, made of either pure 8YSZ or 8YSZ doped with 8mol%GDC, was die pressed and sintered at 1400oC (2oC/min) for five hours. On one side of the electrolyte, the anode interlayer is mixed with isopropyl alcohol and painted on the electrolyte's surface. The anode interlayer is composed of 50wt% NiO and 50wt% 8YSZ. The cell was heated to 950oC (2oC/min) for one hour. The anode functional layer, 70wt% NiO and 30wt% 8YSZ, was painted on top of the anode interlayer. The sample was then again heated to 950oC (2oC/min) for one hour to burn off any organic compounds. A cathode interlayer was then painted on the other side of the electrolyte layer. The cathode interlayer was also painted on and consists of 50wt% lanthanum strontium manganite (LSM) and 50wt% YSZ. The cell is heated to 1200oC (2oC/min) for two hours. Finally, a pure LSM layer is painted on top as the cathode current collector material and heated to 1100oC (2oC/min) for one hour to ensure good porosity. The cells were then sealed by compression-loading at the edges using glass-mica seals.Results of tests will be reported for both SOEC and SOFC modes at 800oC. Data to be reported include open-circuit voltage, area-specific resistance, corresponding ionic currents based on an applied voltage, and long-term durability of the two cells in both SOFC and SOEC modes.

Interrelationship between Extended Transport Pathways of Mixed Conducting Electrode and Delamination in Solid Oxide Electrolyzer Cells

Generating electricity using renewable energy such as solar and wind rather than fossil fuels like coal and natural gas reduces CO2 emissions and addresses climate change. However, due to the intermittency nature of renewables, highly efficient and massive scale energy storage method is the major enabler for its integration into the grid. Solid oxide electrolyzer cell (SOECs) (1), which is an electrochemical device to convert electricity into chemicals (such as hydrogen, syngas, ammonia or even methane, ethane, and ethylene), have received widespread attention. There have been tremendous improvements of the technology during the past 10 to 15 years. On the other hand, to drive the technology for commercialization, durability for long-term operation is still facing significant challenges (2). One degradation mechanism that has been widely reported is the delamination of the oxygen electrode/electrolyte interface. Pure ionic conductors such as 8 mol% Y2O3 doped ZrO2 (YSZ) and Gadolinium-doped ceria (GDC) are usually used as the dense electrolyte; mixed electronic and ionic conductors (MIEC) such as La1-xSrxMnO3-d(LSM), La1-xSrxCo1-yFeyO3-d (LSCF), Sm0.5Sr0.5CoO3 (SSC) are widely used as the oxygen electrode (3).The most popular delamination mechanism (2, 4) is attributed to the buildup of oxygen partial pressure at the LSM/YSZ interface and growth of microcracks caused by exceeding the fracture strength of the electrolyte. Another interesting explanation (5) from atomic level is that under high temperature the inter-diffusion of different cations across the LSM/YSZ interface significantly affect oxygen transport which develops pressure buildup and delamination. A mechanism (6) from a different angle has been proposed by analyzing SEM images of the LSM/YSZ interface at different stages of polarization. It is shown that the formation of nanoparticles caused by the migration or incorporation of oxygen ions from the electrolyte into the electrode, leading to the shrinkage of LSM lattice.Meanwhile, there have been modeling and experimental investigations of extended charge transport pathways from triple phase boundary (3PB) to two phase boundary (2TP) (3, 7) in MIEC electrodes, such as LSM and LSCF, as well as the multi-step charge transfer reaction (8-10) at gas/MIEC interface. It has been demonstrated that there is a transition voltage between two different transport pathways, and it can cause significant oxygen vacancy concentration variation at the electrode/electrolyte interface. In this work, we explore the correlation between the transition of transport pathways and the delamination phenomenon. We assume that large oxygen vacancy concentration variation will cause chemical-expansion-mismatch at the electrode/electrolyte interface, which leads to delamination of oxygen electrode from the electrolyte.Figure 1 shows a schematic of model domain constructed with details at the interface. A bilayer oxygen electrode structure consists of a commercial LSCF-GDC porous backbone coated by a thin SrCo0.9Ta0.1O3- d (SCT10) film. The new oxygen electrode has an inherently fast oxygen evolution reaction (OER) electrokinetics (or high oxygen evolution rate) and is capable of mitigating the delamination problem by minimizing the accumulation of oxygen in OER electrode lattice and thus chemical stresses. To understand the fundamental Electro-Chemical-Mechanical coupling mechanism and its contribution to SOEC performance degradation, we develop a rigorous Multiphysics model to explore the interplays between reaction rates and transport for two different pathways in delamination probability of the pristine and bilayer OER electrodes. The work will lead to a better understanding of the delamination mechanisms and effective mitigation plan and provide guidelines for the experimental optimization of bilayer OER electrode processing with improved electrochemical and mechanical performance. References A. Hauch, R. Küngas, P. Blennow, A. B. Hansen, J. B. Hansen, B. V. Mathiesen and M. B. Mogensen, Science, 2020, 370, eaba6118.B.-K. Park, Q. Zhang, P. W. Voorhees and S. A. Barnett, Energ Environ Sci, 2019, 12, 3053-3062.Y. Li, R. Gemmen and X. Liu, J Power Sources, 2010, 195, 3345-3358.A. V. Virkar, Int J Hydrogen Energ, 2010, 35, 9527-9543.S. N. Rashkeev and M. V. Glazoff, Int J Hydrogen Energ, 2012, 37, 1280-1291.K. Chen and S. P. Jiang, Int J Hydrogen Energ, 2011, 36, 10541-10549.M. Gong, R. S. Gemmen and X. Liu, J Power Sources, 2012, 201, 204-218.J. Fleig, Phys Chem Chem Phys, 2005, 7, 2027-2037.D. S. Mebane and M. Liu, J Solid State Electr, 2006, 10, 575-580.D. S. Mebane, Y. Liu and M. Liu, J Electrochem Soc, 2007, 154, A421. Figure 1