Oxygen Surface Exchange Kinetics Research Articles

Multi-walled carbon nanotube wrapped rare earth tungstate nanosheet composites for selective and rapid detection of sulfadiazine

Multi-walled carbon nanotube wrapped rare earth tungstate nanosheet composites are developed for the ultrasensitive and selective electrochemical detection of sulfa drug, sulfadiazine (SAN) as they exhibit high levels of toxicity towards public health and ecosystem. Here, we synthesized Gadolinium tungstate decorated with multi-walled carbon nanotubes (Gd2WO6/MWCNT) though hydrothermal processing and characterized by XRD, SEM, EDX, TEM, and EIS measurement to investigate the phase purity, morphology, and interfacial property. Gd2WO6 is a double perovskite that has high oxygen surface exchange kinetics and the MWCNT possesses high catalytic capability and a larger surface area contributes to the efficient sensing ability. The Gd2WO6/MWCNT possessed a great electrical conductivity owing to the synergism between the Gd2WO6 and MWCNT. The synergistic effect increases the surface area, ionic and electrical conductivity which increases the current response of SAN. Under optimum conditions, the Gd2WO6/MWCNT/GCE displays a broad linear of 0.01 – 671 µM and a LOD level of 2 nM (S/N ratio: 3) with a sensitivity of 0.371 µA µM cm−1. The analytical applicability of the proposed electrode modifier leads to a promising sensor for SAN detection in human urine and water samples with appreciable results.

Realizing extraordinary bifunctional electrocatalytic performance of layered perovskite through Ba-site Gd doping toward oxygen reduction and evolution reactions

Herein we propose a Ba-site Gd doping protocol in layered perovskites, demonstrating highly active bifunctional PrBa1-xGdxCo2O5+δ electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Benefiting from enhanced oxygen surface exchange and chemical diffusion kinetics, resultant PrBa0.95Gd0.05Co2O5+δ (PBG0.05CO) exhibits an area-specific resistance (ASR) of 0.038 Ω cm2 at 700 oC, reduced by ∼46.5% relative to undoped PrBaCo2O5+δ (PBCO). When evaluated as the air electrode, the as-fabricated single coin fuel cell delivers a peak power density (PPD) of 1230 mW cm-2 at 700 oC. For the CO2-electrolysis at 750 oC, the large current density (J) of 2630 mA cm-2 is achieved in the PBG0.05CO anode-based electrolysis cell at a voltage (V) of 1.8 V. Furthermore, exceptional operating stability is realized in both ORR and OER manners. Our findings highlight an effective strategy to optimize the electrocatalytic properties of layered perovskites, endowing potential applications in energy storage and conversion aspects.

Cross-Validation of the Remarkably High Surface Oxygen Exchange Kinetics of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ: A Combined Thin-Film Mass Relaxation and Bulk Electrical Conductivity Relaxation Study.

In this work, we measure the oxygen kinetic properties of double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), a material widely used as the air electrode in solid oxide electrochemical cells, by mass relaxation (MR) and electrical conductivity relaxation (ECR) experiments. MR studies are carried out using thin films deposited on a gallium phosphate piezocrystal microbalance, and ECR studies are performed using a bulk bar sample with 97% theoretical density. Measurements are performed at 600 °C over the temperature oxygen partial pressure range from 10-4 to 0.21 atm. Despite the differences in experimental formats and surface microstructural features, the ks values extracted from the two methods are found to be in good agreement with one another. The rate constant is found to increase with oxygen partial pressure with a power law dependence, rising from 1.0 × 10-6 cm/s at 3.2 × 10-4 atm to 1.2 × 10-4 cm/s at 0.24 atm, as averaged over the oxidation and reduction directions. The rates in the oxidation direction are observed to be slightly higher than those in the reduction direction for a given pair of pO2 values, suggesting that the final pO2 value controls the overall relaxation behavior. The power law exponent describing the dependence of ks on pO2 is found to be 0.74 ± 0.01. The ECR study of the bulk sample reveals that even with a diffusion length of 1.8 mm, the relaxation process is largely free of diffusion limitations, indicating that PBSCF has the high bulk transport properties required for a double-phase boundary oxidation/reduction pathway.

Comprehensive study of O2 and H2O interaction with La0.9Sr0.1ScO3–δ oxide

In this paper we study the processes of O2 and H2O interaction with La0.9Sr0.1ScO3−δ. Using high temperature neutron powder diffraction (HT-NPD), the evolution of crystal structure and water uptake was investigated in the temperature range 25–700 °C at 21.3 kPa in O2 and O2 + D2O atmospheres. The results of HT-NPD show the influence of the water uptake by La0.9Sr0.1ScO3–δ bulk on the thermal expansion of the material. The kinetics of oxygen surface exchange was studied using pulsed isotope exchange technique. The measurements were carried out in the temperature range 300−900 °C at pO2 = 5.1, 12.2 and 21.3 kPa in dry and humid (pH2O = 2.6 kPa) conditions. For the first time the physical model describing the kinetics of oxygen surface exchange between the mixture of oxygen and water in the gas phase and a solid oxide has been suggested. The model includes following steps: oxygen dissociative adsorption, oxygen incorporation, hydrogen surface diffusion, water adsorption and dissociation. The rate-determining step of the exchange is considered. The influence of O2 humidification on the oxygen surface exchange is discussed.

Spontaneous Growth of Perovskite‐Derived Oxide over Double Perovskite Surface for Enhancing Cathodic Performance in Protonic Ceramic Fuel Cells

AbstractRobust catalytic materials with high activity and stability play important roles in energy conversion and storage devices such as protonic ceramic fuel cells (PCFCs), in which a favourable cathode should possess high oxygen ion, proton and electron triple conductivities, and superior surface oxygen exchange kinetics. Herein, a thermal‐driven self‐construction phenomenon in cation‐nonstoichiometric Ba1+xGd1‐xCo2O6‐δ is reported, accordingly developing a new type of nanocomposite, that is, double perovskite BaGdCo2O6‐δ (DP‐BGCO) anchored by perovskite‐derived BaCoO3‐δ (P‐D‐BCO) nanoparticles, which, used as the cathode of PCFCs, demonstrates low area‐specific resistances of 0.053 and 0.026 ohm cm−2 respectively at 650 and 700 °C over BaZr0.3Ce0.5Y0.1Yb0.1O3‐δ protonic electrolyte and attractive peak power densities of 0.87 (650 °C) and 1.15 W cm−2 (700 °C) with outstanding stability, much superior to the similar cell with single‐phase BCO or BGCO cathodes. The synergy between the two components brings the outstanding performance with the mixed oxygen ion and electronic conducting perovskite‐derived oxide showing superior catalytic activity for oxygen reduction reaction while the double perovskite provides good bulk protonic conductivity to enlarge reaction sites. Such selective self‐construction, well manipulated through the A‐site cation stoichiometry engineering, provides a facile way for developing new high‐performance electrocatalysts with broad application potential.

Ta/Nb doping motivating cubic phase stability and CO2 tolerance of Sr2Co1.6Fe0.4O6-δ double perovskite for high-performing SOFC cathode

Developing cathodes that are both phase-stable and tolerant to carbon dioxide (CO2) is a crucial and challenging task for solid-oxide fuel cells (SOFCs) operating at reduced temperatures. The activity of oxygen reduction reaction (ORR) in SOFC cathodes is typically influenced by factors such as geometric features and oxygen vacancies. In this study, new and robust cathodes for SOFCs, Sr2Co1.5Fe0.4Ta0.1O5+δ (SCFT) and Sr2Co1.5Fe0.4Nb0.1O5+δ (SCFN) are developed by doping a small amount of niobium (Nb) and tantalum (Ta) into Sr2Co1.6Fe0.4O5+δ (SCF) using a simple solid-state reaction technique. The effects of Nb/Ta doping on the crystal structure, oxygen vacancies, electrical conductivity, and electrochemical properties of SCFT and SCFN are investigated. The results reveal that these materials possess a fully cubic structure in the Fm-3m space group, with improved oxygen vacancy and electrical conductivity. They also exhibit good chemical compatibility with electrolytes at 1100 °C for 8 h and remain thermally stable when exposed to air and CO2 at 700 °C, without any additional phase formation. The area-specific resistance (ASR) of the SCFT cathode is found to be only 0.06 Ω cm2, whereas the SCFN cathode has an ASR of 0.10 Ω cm2. The superior performance of SCFT can be attributed to the higher oxygen vacancy, which enhances oxygen surface exchange kinetics and diffusivity compared to SCFN. When tested in a single cell using humidified H2 as the fuel and ambient air as the oxidant, the SCFT cathode achieves a power density of 656 mW cm−2 at 700 °C, while the SCFN cathode reaches 559 mW cm−2, indicating the potential of SCFT as a promising cathode material for SOFCs. The excellent performance of these materials can be attributed to their mixed electronic-ionic conduction properties, unique structural features, and high observed oxygen vacancy, which contribute to their remarkable electronic conductivity and significant ionic mobility.

Oxygen surface exchange kinetics of La1−xSrxCoO3–δ thin-films decorated with binary oxides: links between acidity, strontium doping, and reaction kinetics

Recent study shows the enhancement of oxygen exchange kinetics of oxygen electrodes using oxides selected using Smith acidity. However, what are the limits to this approach?

Facile and Low Temperature Synthesis of Nd1.8Sr0.2NiO4-δ Cathode Nanofibers for Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are usually operated at high temperatures (800-1000 °C). As the overall efficiency of SOFC is governed by thermodynamics and kinetics during operation, reducing the operating temperature to 500-650 °C causes significant increase in the electrode polarization losses especially at cathode due to high activation energy towards oxygen reduction reaction (ORR) at cathode, which eventually reduces the overall cell performance. Addressing increased polarization losses at relatively low temperatures (500-650 °C) has been the key issue and a focus of many groups over past couple of decades. Conventional cathode materials lose their ORR activity and so discarded for intermediate temperature (IT)-SOFC application. In this regard, mixed ionic-electronic conductors (MIECs) offer high thermal and chemical stability along with high oxygen diffusion and good electronic conductivity at IT [1,2]. In addition, Ruddlesden-Popper (RP) type MIECs with A2BO4 structure have been demonstrated as promising cathode for IT- SOFCs due to their unique structure, rapid oxygen surface exchange kinetics, and excellent stability in atmosphere. Particularly Nd2NiO4+δ oxides offer an excellent oxygen diffusion coupled with high surface exchange coefficients and comparable thermal expansion coefficients with those of oxygen-ion conductors (YSZ and CGO) [2]. Enhanced electronic conductivity and electrochemical performance have been demonstrated for partial replacement of Nd with Sr in our previous reports [3].Albeit, the intrinsic material properties such as electrochemical catalytic activity and electrical conductivity are of primary importance, an actual electrode performance strongly depends on its microstructure. Therefore, in addition to using high catalytic and conductive materials, achieving an extremely high-power density, thus, requires a finely controlled electrode microstructure, to expand the electrochemically active triple-phase boundary (TPB). Recently, electrospinning has been considered as an efficient, cost effective and promising method for synthesis of ceramic nanofibers. Pertinent literature suggests that SOFC with cathode nanofiber has superior/better performance (nearly doubled power density and significant reduced polarization resistance) compared to powder type electrodes. Selection of suitable cathode materials with modified microstructure (nanofibers) would facilitate more electrochemically active sites for the oxygen reduction at relatively low temperatures <650 °C.In this paper we synthesized Nd1.8Sr0.2NiO4-δ (NSNO) cathode nanofibers using electrospinning technique and compared its electrochemical performance with NSNO nanopowder. Practically, 10 wt.% Polyvinylpyrrolidone (PVP) solution was prepared by dissolving it in ethanol, to which stoichiometric amounts of Nd-nitrate, Sr-nitrate, Ni-nitrate (total weight percentage of metal salts- 3 wt%) were added, together with some deionized water. The mixture was then stirred for 10 h at room temperature on a magnetic stirrer to obtain a homogeneous sol. The sol was loaded into the electrospinning syringe. Distance between the spinneret and the collector was fixed at 25 cm and the high-voltage supply was maintained at 30 kV. The spinning rate was controlled at 5 ml h- 1 by a syringe pump. The obtained nanofibers were dried at 80 °C for 12 h, then sintered at 900 °C for 2 h in air to obtain single-phase NSNO.The obtained nanofibers were characterized using room temperature XRD, XPS, SEM and dc electrical conductivity (400-650 °C). In addition, electrochemical impedance spectroscopy studies on symmetric cells were carried out as a function of temperature (400-650 °C). Improved electrochemical performance is attributed to enhanced electrochemical active sites for ORR due to nanofibers and optimum porosity.

Characterization of B‐Site Sc‐doped La2Ni1-xScxO4+δ (x=0, 0.05, 0.10, and 0.15) perovskites as cathode materials for IT-SOFCs

Cathode materials play a crucial role in enabling the widespread adoption of intermediate-temperature solid oxide fuel cells (IT-SOFC) for commercial applications. This study utilized the sol-gel method to prepare a range of perovskite oxides, specifically Sc-doped La2Ni1-xScxO4+δ (x = 0, 0.05, 0.10, and 0.15), without the use of cobalt. The investigation focused on making a study of how the introduction of Sc doping affected the thermal stability, electrochemical properties, and structure of cathode materials. XRD, FE-SEM, and electrochemical workstations were subjected to structural characterization and performance testing of materials. The findings indicate that all the prepared materials have a typical Ruddlesden-Popper structure, and an abundance of additional oxygen can be generated in close proximity to the interstitial by incorporating Sc into the cathode of La2Ni1-xScxO4+δ to increase the oxygen vacancy and improve the catalytic activity of ORR. La2Ni0.9Sc0.1O4+δ has the highest oxygen surface exchange kinetics, the dissociative adsorption of oxygen is the rate-limiting step at the cathode. At a temperature of 700 °C, the lowest polarization resistance of La2Ni0.9Sc0.1O4+δ is 0.1298 Ω cm2, while the single cell with La2Ni0.9Sc0.1O4+δ as the cathode achieves an impressive peak power density of 0.462 W cm−2, thus demonstrating exceptional long-term stability. La2Ni0.9Sc0.1O4+δ is expected to be a potential high-performance IT-SOFC cathode material.

Preparation of Pr, Co co-doped BaFeO3–δ-based nanofiber cathode materials by electrospinning

The solid oxide fuel cell (SOFC) is considered promising technology that directly converts the chemical energy stored in the fuel into electrical energy. However, the undesired oxygen reduction reaction (ORR) activity and stability of the cathode have hindered its commercialization. In this work, a series of Pr-doped PrxBa1-xFe0.8Co0.2O3-δ (x = 0∼0.3) nanofibers and powder cathode materials with excellent performance are synthesized by electrospinning and sol-gel methods, respectively. The influence of different Pr doping amounts on the nanofiber morphology, phase structure, thermal stability and electrochemical properties of BaFe0.8Co0.2O3-δ (BFCO) nanofibers cathode materials were investigated. Compared the microstructures and electrochemical performance of Pr0.1Ba0.9Fe0.8Co0.2O3-δ nanofiber and powders prepared by different methods. The partial substitution of Pr for Ba (doping of a few of Pr) can effectively eliminate the BFCO hexagonal phase and induce the transformation to the cubic phase. The content of oxygen vacancies was significantly increased as well as the cathode ORR activity was well enhanced. In addition, we demonstrated by DRT that oxygen adsorption is the rate-limiting step at the cathode. Among them, Pr0.1Ba0.9Fe0.8Co0.2O3-δ nanofiber has the best electrochemical performance and the highest oxygen surface exchange kinetics performance. The area-specific resistance (ASR) of Pr0.1Ba0.9Fe0.8Co0.2O3-δ nanofiber is 0.1416 Ω cm2 at 700 °C. When the peak power density (PPD) of the YSZ-NiO/YSZ/GDC/PBCF1–F single cell shows 0.787 W cm−2, an anode-supported fuel cell with Pr0.1Ba0.9Fe0.8Co0.2O3-δ nanofiber demonstrates excellent stability after 100 h of long-term stability test. Electrospinning technology is an effective method to prepare high-performance cathodes, and Pr0.1Ba0.9Fe0.8Co0.2O3-δ nanofiber cathodes are expected to be high-performance cathode materials for IT-SOFCs.

Quantifying Diffusion Across Solid-Solid Interfaces in Electrochemical Cells

Enabling an increased rollout of hydrogen-based technologies requires decarbonisation of both hydrogen production and conversion. Solid Oxide Cells (SOCs), in the form of electrolysers and fuel cells, could play a significant role in achieving this, however lifetime and degradation of SOCs remain major obstacles to commercialisation. Understanding the mechanisms governing ionic transport in ceramics is important to improving the performance and durability of SOCs. Properties such as the oxygen tracer diffusivity, , can be measured by Isotopic Exchange Depth Profiling (IEDP) as developed by Kilner et al. [1].This method has, so far, primarily been used to characterise the properties of single materials; however, SOCs are multilayer devices with solid-solid interfaces that may also affect transport. Some investigations into the diffusion behaviour across multiple layers using the IEDP technique have been done in thin film samples [2][3]. Profiles obtained from these studies appear to show an abrupt concentration drop at the interface between certain materials, indicating an interface which significantly impacts the diffusion behaviour (and by extension the overall cell performance). However, no attempt was made to quantify this interface effect with a theoretical modelling approach.A finite-difference model for diffusion in a system containing multiple layers with interfaces is developed. It numerically solves Fick’s second law of diffusion with various boundary conditions. This model can be used to fit experimental data obtained from tracer diffusion SIMS data, yielding new way of quantifying interfacial resistance. A new interfacial resistance parameter, , has been defined, which quantifies the magnitude of concentration drop across any given interface and thus offers a universal way of numerically characterising resistance to diffusion.The validity of the developed method has been experimentally tested in samples containing layers of lanthanum strontium cobalt ferrite (LSCF) and gadolinium-doped ceria (GDC), a commonly used system in SOCs. Initial data from tracer diffusion experiments as seen in Figure 1 have shown the presence of a significant concentration drop at the interface of a LSCF-GDC stack which can be fitted to the numerical model developed by the authors. This approach is used to measure the changing interface properties under various ageing conditions, as well as the influence of interlayers and material selection on the interfacial resistances both in SOC and other diffusion systems.[1] J. A. Kilner, B. C. H. Steele, and L. Ilkov. Oxygen self-dffusion studies using negative-ion secondary ion mass-spectrometry (sims). Solid State Ionics, 12(MAR):89-97, 1984.[2] K. Develos-Bagarinao, H. Yokokawa, H. Kishimoto, T. Ishiyama, K. Yamaji, and T. Horita. Elucidating the origin of oxide ion blocking effects at gdc/srzr(y)o-3/ysz interfaces. Journal of Materials Chemistry A, 5(18):8733-8743, 2017.[3] Katherine Develos-Bagarinao, Haruo Kishimoto, Jeffrey De Vero, Katsuhiko Yamaji, and Teruhisa Horita. Effect of la0.6sr0.4co0.2fe0.8o3-delta microstructure on oxygen surface exchange kinetics. Solid State Ionics, 288:6-9, 2016.Fig. 1. Oxygen isotopic fraction data of an LSCF layer deposited on a GDC substrate and annealed in Oxygen-18 atmosphere at 650 C for 1h obtained using cross-sectional SIMS (left) and fit of the data using the multi-layer diffusion model developed by the authors (right). Figure 1

Exploring Stable and Selective Anode Materials for the Electrochemical Oxidative Coupling of Methane (EOCM): A Case Study of Doped Titanates

The development of next-generation solid oxide fuel cells (SOFCs) relies on integrating materials such as La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF), which has been lauded for its fast oxygen ion and electron conduction, along with its high oxygen reduction activity.[1] However, the use of LSCF has been plagued by 1) stability issues caused by surface Sr precipitation and 2) subsequent poisoning by Cr- or S- containing species.We have previously shown that the introduction of more oxidizable cations at the electrode surface can effectively improve both the stability and the oxygen exchange kinetics of the electrodes. However, the influence of these cations, as with many dopant infiltration studies, is currently not fully understood across all possible modes of electrode surface stabilization/activity enhancement. For example, we have recently discovered that at 800°C and under air atmospheres (i.e. at typical SOFC operational conditions), these Hf/Zr surface cation dopants are mobile and diffuse into the lattice of a porous LSCF electrode as evidenced by their vanishing x-ray photoelectron signals. This is accompanied nevertheless by a noticeable electrode stabilization, suggesting a sub-surface effect of the introduced dopants.Inspired by this, in this work our aim is to uncover the mechanism responsible for the stabilization of the surface oxygen exchange kinetics. Our approach involves dense thin film LSCF as model system to investigate the temperature-dependent physicochemical properties of these surface and sub-surface dopant cations. We fabricate LSCF thin films via pulsed laser deposition onto single crystalline substrates and then introduce our targeted cations at the surface via an aqueous solution infiltration. We characterize the electrochemical performance of the thin films by performing chronoamperometric and impedance spectroscopic measurements to discern the extent of the temporal electrode performance degradation using metrics such as area-specific resistance. In parallel, we aim to rationalize the observed electrochemical performance stabilization using complementary surface science techniques. We propose investigating the cation dissolution phenomenon through a combination of x-ray photoelectron and Auger electron spectroscopies, along with atomic force microscopy, scanning/transmission electron microscopy, scanning tunnelling microscopy and grazing incidence thin film x-ray diffraction. This study aims to reveal the influence of sub-surface more oxidizable cations on the extent of Sr segregation and highlights how two major factors contribute to the observed trends: Electrode fabrication conditions, for example the dopant concentration dependence (insufficient or excessive surface dopant concentrations has previously been shown to suppress the stabilization effect).[3]Electrode operation conditions, for example the temperature profile experienced by the cell (higher temperatures lead to dopant dissolution). [1] SP Jiang, Int. J. Hydrogen Energy 44 (14), 7448 (2019).[2] N Tsvetkov et al., Nature Mat. 15 (9), 1010 (2016).[3] N Tsvetkov et al., Faraday Disc. 182 (0), 257 (2015).

A novel symmetrical bi-electrode supported ceramic oxygen pump for efficient oxygen production

Based upon the solid oxide electrolysis cell (SOEC) technology, ceramic oxygen pumps (COPs) show great promise owing to their distinctive advantage of continuous and in situ production of high-purity oxygen with control easiness. Here we report on the fabrication of novel ceramic oxygen pumps by the phase-inversion tape casting, lamination and co-firing methods, featuring thin yttria-stabilized zirconia (YSZ) electrolytes supported on symmetrical La0.8Sr0.2 MnO3-δ (LSM)-YSZ electrodes with straight open pores. A current density of 0.33 A•cm−2 is obtained at 750°C and 0.6 V for the ceramic pumps to separate oxygen out of air. Impregnating nano-scale La0.6Sr0.4CoO3-δ (LSC) catalysts into both LSM-YSZ electrodes yields a sharp increase in the performance. For example, the pumping current density at 750°C and 0.6 V increases by a factor of 8 to 2.56 A•cm−2. The total interfacial polarization resistance decreases from 1.17 Ω•cm2 by a factor of 20 to 0.06 Ω•cm2. Analysis of impedance data in the homogeneous environments of air and oxygen indicates that the oxygen separation capability is predominantly limited by the surface oxygen exchange kinetics, which is substantially promoted by the added LSC catalysts. Preliminary short-term measurements show good stability in oxygen separation, indicating excellent resistance of the nano-scale LSC catalysts against coarsening at elevated temperatures. Furthermore, the present symmetrical ceramic oxygen pumps also demonstrate excellent thermal cycling durability.

Yttria-Doped Ceria Surface Modification Layer via Atomic Layer Deposition for Low-Temperature Solid Oxide Fuel Cells

Atomic layer deposition (ALD) is performed to obtain less than 1 nm thick yttria-doped ceria (YDC) layers as cathode functional layers to increase the surface oxygen incorporation rate for low-temperature solid oxide fuel cells (LT-SOFCs). Introducing a YDC surface modification layer (SML) has revealed that the optimized yttria concentration in YDC can catalyze surface oxygen exchange kinetics at the interface between the electrolyte and cathode. The YDC SML-containing fuel cell performs 1.5 times better than the pristine fuel cell; the result is an increased exchange current density at the modified surface. Moreover, a heavily doped YDC SML degrades the performance of LT-SOFCs, owing to the weakened oxygen surface kinetics due to the increased migration energy of the oxygen ions.

Understanding the influence of strain-modified oxygen vacancies and surface chemistry on the oxygen reduction reaction of epitaxial La0.8Sr0.2CoO3-δ thin films

One of the key challenges in developing high-performance electrochemical energy devices is the slow kinetics of the oxygen reduction reaction (ORR). Implementing epitaxial strain induced by a lattice mismatch between a film and a substrate has shown great potential for enhancing the ORR by increasing the concentration of oxygen vacancies. However, strain also influences the surface chemistry which also impacts the ORR. Here, we show the role of strain in concurrent changes of oxygen vacancies and surface chemistry, which in turn significantly influence the ORR activity by employing epitaxial La0.8Sr0.2Co3-δ (LSC) thin films with three different strain states. Tensile strained LSC films dramatically enhanced ORR activity up to two orders of magnitude compared to compressively strained LSC films. Among the tensile strained films, moderately tensile strained LSC films show the highest ORR activity, which is attributed to both increased concentration of oxygen vacancies and suppressed Sr segregation. Our study provides a new design strategy to enhance the ORR activity by controlling the strain state to tune the Sr segregation, oxygen vacancies, and oxygen surface exchange kinetics.

Surface oxygen exchange kinetics of mixed conducting oxides: Dilatometric vs electrical conductivity relaxation study

The surface oxygen exchange kinetics of mixed ionic-electronic conducting (MIEC) oxides play a crucial role in a variety of applications, including solid oxide fuel/electrolysis cells (SOFCs/SOECs), permeation membranes and sensors. To date, a variety of methods, including isotope exchange, electrical conductivity, optical absorptivity and thermogravimetry relaxation, and impedance spectroscopy have been used to measure the oxygen exchange coefficient of MIEC oxides. Each of these methods have their advantages and limitations, depending on the physical and chemical properties of the investigated materials and their sample dimensions and densities. Here, we demonstrate the ability to precisely measure the surface oxygen exchange coefficient (kchem) of MIEC model material Pr0.1Ce0.9O2-δ (PCO) by use of dilatometric relaxation measurements, particularly of interest for materials that exhibit larger chemical expansion coefficients. This is achieved by use of porous bulk specimens to ensure that the contribution of oxygen exchange to the overall kinetics is dominant. We demonstrate that kchem values extracted from chemical expansion relaxation measurements on PCO are nearly identical to those derived from electrical conductivity relaxation measurements. This provides the opportunity to precisely investigate the oxygen exchange and chemical expansion kinetics of a wide range of materials used in high-temperature applications, particularly where more conventional methods are difficult or inappropriate to apply.

Tuning Electrochemical Performance by Microstructural Optimization of the Nanocrystalline Functional Oxygen Electrode Layer for Solid Oxide Cells.

Further development of solid oxide fuel cell (SOFC) oxygen electrodes can be achieved through improvements in oxygen electrode design by microstructure miniaturization alongside nanomaterial implementation. In this work, improved electrochemical performance of an La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) cathode was achieved by the controlled modification of the La0.6Sr0.4CoO3-d (LSC) nanocrystalline interlayer introduced between a porous oxygen electrode and dense electrolyte. The evaluation was carried out for various LSC layer thicknesses, annealing temperatures, oxygen partial pressures, and temperatures as well as subjected to long-term stability tests and evaluated in typical operating conditions in an intermediate temperature SOFC. Electrochemical impedance spectroscopy and a distribution of relaxation times analysis were performed to reveal the rate-limiting electrochemical processes that limit the overall electrode performance. The main processes with an impact on the electrode performance were the adsorption of gaseous oxygen O2, dissociation of O2, and charge transfer-diffusion (O2-). The introduction of a nanoporous and nanocrystalline interlayer with extended electrochemically active surface area accelerates the oxygen surface exchange kinetics and oxygen ion diffusions, reducing polarization resistances. The polarization resistance of the reference LSCF was lowered by one order of magnitude from 0.77 to 0.076 Ω·cm2 at 600 °C by the deposition of a 400 nm LSC interlayer at the interface. The developed electrode tested in the anode-supported fuel cell configuration showed a higher cell performance by 20% compared to the cell with the reference electrode. The maximum power density at 700 °C reaches 675 and 820 mW·cm-2 for the reference cell and the cell with the LSC interlayer, respectively. Aging tests at 700 °C under a high load of 1 A·cm2 were performed.

Nickel Doping Manipulation towards Developing High-Performance Cathode for Proton Ceramic Fuel Cells

An ideal cathode for proton ceramic fuel cell (PCFC) should have superior oxygen reduction reaction activity, high proton conductivity, good chemical compatibility with electrolyte and sufficient stability, thus rational design of the electrode material is needed. Here, by taking advantage of the limited solubility of nickel in perovskite lattice, we propose a new dual phase cathode developed based on nickel doping manipulation strategy. We rationally design a perovskite precursor with the nominal composition of Ba(Co0.4Fe0.4Zr0.1Y0.1)0.8Ni0.2O3−δ (BCFZYN0.2). During high temperature calcination, a nanocomposite, composed of a B-site cation deficient and nickel-doped BCFZY perovskite main phase and nanosized NiO minor phase, is formed. The NiO nanoparticles effectively improve the surface oxygen exchange kinetics and the B-site cation deficiency structure enhances proton conductivity, thus leading to superior ORR activity of BCFZYN0.2. Furthermore, a low thermal expansion coefficient (15.3 × 10–6 K−1) is achieved, ensuring good thermomechanical compatibility the electrolyte. A peak power density of 860 mW cm−2 at 600 °C is obtained from the corresponding PCFC, and the cell operates stably for 200 h without any significant degradation. The proposing strategy, by providing a new opportunity for the development of highly active and durable PCFC cathodes, may accelerate the practical use of this technology.

Improved oxygen reduction reaction activity by in-situ synthesizing Sr3Fe1.8Nb0.2O7-δ coating on SrFe0.9Nb0.1O3-δ cathode via the microwave water bath heating method

The sluggish oxygen reduction kinetics remains to be the main obstacle to the development of low temperature proton ceramic fuel cells which limits its commercial application. This phenomenon has driven the exploration of the oxide electrode materials which both possess high oxygen reduction reaction activity and heterogeneous structure. In this work, mixed ionic and electronic conductor SrFe0.9Nb0.1O3-δ (SFN113) is partially transformed into the triple-phase conductor Sr3Fe1.8Nb0.2O7-δ (SFN327) by in-situ reaction with uniformly coated SrO layer using a microwave water bath heating method. The introduction of the second phase SFN327 significantly improves the oxygen surface exchange kinetics of SFN113 investigated by the electrical conductivity relaxation method. The present work not only remarkably improves the electrocatalytic reduction activity of SFN113 by the introduction of the second phase SFN327 and the construction of the heterogeneous interface, but also provides a valuable route for the further optimization of the exiting electrode materials.

Oxygen-Deficient (Ln,Sr)2NiO4- δ Nickelates for Oxygen Electrodes of Solid Oxide Fuel and Electrolysis Cells: Anisotropic Thermochemical Expansion and Thermomechanical Constraints

Perovskite-like and perovskite-related oxides based on variable-valence transition metals may exhibit substantial changes in oxygen content in the course of temperature cycling or under variation of oxygen chemical potential at elevated temperatures. This gives rise to a chemical contribution to the thermal expansion (chemical expansion/contraction or chemical strain) originating mainly from the accompanying changes in the average oxidation state of transition metal cations and, consequently, their ionic radii. Excessive dimensional changes caused by the chemical expansion impose constraints on the applicability of functional oxides at elevated temperatures and under variable redox conditions (due to thermomechanical instability or incompatibility with other materials) and complicate the processing of oxide ceramics at high temperatures. Another unfavorable phenomenon is microcracking caused by a strongly anisotropic expansion of crystal lattice, with effects on the microstructural and mechanical integrity and electrical and electrochemical properties. The present work is a representative study case that focuses on layered Ln2NiO4-based ceramic materials for oxygen electrodes of solid oxide fuel/electrolysis cells (SOFC/SOEC).Ln2NiO4+δ–based oxides (Ln = La, Pr, Nd) with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature SOFC/SOEC [1]. Acceptor-type substitution of Ln3+ by Sr2+ in Ln2NiO4+ δ is compensated by comparatively high oxygen deficiency in Sr-rich Ln2-xSrxNiO4-δ phases at elevated temperatures [2,3]. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers.Ceramic powders of Ln2-xSrxNiO4-δ (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry (Fig.1A), which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln2-xSrxNiO4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating (Fig.1B). Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes (Fig.1C) and also in variations of electrical conductivity (Fig.1D) on cycling in air for the ceramic samples sintered at 1200-1300°C.An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln2-xSrxNiO4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties. Acknowledgments: This work was supported by the FCT/MCTES/FEDER (projects CARBOSTEAM (POCI-01-0145-FEDER-032295) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), and PhD grant SFRH/BD/138773/2018).