Air Electrode Materials Research Articles

Long-term stability of oxygen surface exchange kinetics of Pr0.8Ca0.2FeO3-δ against SO2-poisoning

The long-term stability of the oxygen exchange kinetics of the Sr- and Co-free solid oxide fuel cell (SOFC) and solid oxide electrolyser cell (SOEC) air electrode material Pr0.8Ca0.2FeO3-δ (PCF82) is investigated by in-situ dc-conductivity relaxation measurements. The chemical oxygen surface exchange coefficient kchem is determined as a function of time at 700 °C under ideal (O2-Ar atmosphere) and accelerated ageing conditions (O2-Ar with 2 ppm SO2). In pure O2-Ar PCF82 shows fast oxygen surface exchange kinetics with kchem = 6 × 10−4 cm s−1 and excellent stability for 1000 h. Due to the addition of 2 ppm SO2 to the test gas only a moderate degradation occurs with kchem decreasing to 8 × 10−5 cm s−1 during further 1000 h. Post-test analyses by scanning (transmission) electron microscopy and X-ray photoelectron spectroscopy show S-rich secondary phases in the near-surface region of the SO2-poisoned sample. These inactive phases occur in an island-like arrangement of relatively large crystals, whereas significant amounts of the surface remain unaffected. This effect explains the high stability of the oxygen exchange kinetics of PCF82 against SO2-poisoning in comparison to state-of-the art materials like La0.6Sr0.4CoO3-δ (LSC64).

Accelerated Testing of Chromium Poisoning of Sr-Containing Mixed Conducting Solid Oxide Cell Air Electrodes

A straightforward method for accelerated testing of Cr poisoning phenomena on mixed ionic and electronic conducting Solid Oxide Cell (SOC) air electrode materials was developed. Cr2O3-powder is mixed with an organic matrix and screen printed onto anode-supported button cells with (La,Sr)(Co,Fe)O3-δ (LSCF) cathode. Then, a thermal treatment was conducted to achieve the formation of a well-defined amount of SrCrO4 on the cathode surface within only a few hours. For the proof of concept, single cell measurements were done to investigate the influence of this new kind of Cr deposition. As reference, a cell poisoned via gas phase diffusion and a cell without any Cr contamination were characterized in the same manner. According to the impedance data, the polarization resistance for both cells, which were contaminated with Cr species, increased. The expected relationship between deposited amount of Cr and increase of polarization resistance was found. ICP-OES analysis proves the high reproducibility of the method as the Cr content is only a function of the Cr paste composition and almost independent of external poisoning conditions. Due to its scalability and reproducibility, this method is proposed to be a new tool for screening of cathode materials and the investigation of different stages of Cr poisoning prior to more sophisticated and time consuming investigations like stack tests.

Characterization and Optimization of La0.97Ni0.5Co0.5O3−δ-Based Air-Electrodes for Solid Oxide Cells

On the basis of previous studies of perovskites in the quasi-ternary system LaFeO3–LaCoO3–LaNiO3, LaNi0.5Co0.5O3 (LNC) is chosen as the most promising air-electrode material in the series for solid...

Nickel Foam as a New Air Electrode Material to Enhance the Performance in Rechargeable Zn-Air Batteries

Developing efficient air electrodes for improving the stability and power density of batteries plays a significant role in rechargeable Zn-air batteries. We here report a new air electrode by applying nickel foam (Ni foam) as support and the catalysts are sprayed on it. Therefore, NiCo2O4@Ni foam, FeCo2O4@Ni foam and MnCo2O4@Ni foam as new air electrodes are successfully established, which show superior stability in the discharge and charge processes and higher energy density than traditional carbon paper (CP) as supports. The improved Zn-air battery performances using NiCo2O4@Ni foam, FeCo2O4@Ni foam and MnCo2O4@Ni foam as air electrodes were attributed to the more abundant pore structure, good electrical conductivity, and high mechanical strength. Especially, the FeCo2O4@Ni foam air electrode owned the highest discharge power density of 483 mW cm-2 in a designed primary Zn-air battery, and the MnCo2O4@Ni foam air electrode owned the best stability of 648 cycles under 5 mA cm-2 current density for the rechargeable Zn-air battery.

Validation of calcium-doped neodymium nickelates as SOFC air electrode materials

The search for new electrode materials that can be applied to the solid oxide fuel cell (SOFC) technology is an active field of research. In continuing our studies of Ca-substituted layered nickelates, in this work we investigated Nd2−xCaxNiO4+δ materials synthesized via the solution assistant solid state reaction method to evaluate their prospective usage as cathode materials for the intermediate temperature SOFC. It was found that Ca doping resulted in the temperature decreasing for the Fmmm → I4/mmm transition in Nd2NiO4+δ (NNO) and the tetragonal structure stabilizing at room temperature at x ≥ 0.2 in ambient air. The coefficient of thermal expansion calculated from the linear expansion under heating and cooling was different; however there was a common tendency for it to decrease when the Ca content was increased above 5 mol% (x = 0.1). The dc four-probe measurements revealed an increase in total electrical conductivity with doping. The polarization resistance of the Nd2−xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 depended non-linearly on the Ca content with a moderate maximum at x = 0.1. Ca-doping was found to decrease the interaction of the electrode materials with the electrolyte. It resulted in the polarization resistance of the composite electrodes containing doped materials decreasing compared to the composite electrodes based on Nd2NiO4+δ. The polarization resistance of the Nd1.7Ca0.3NiO4+δ-Ce0.8Sm0.2O1.9 (NCNO3-SDC) electrode formed at 1200 °C is equal to 0.56 Ω cm2 at 700 °C in ambient air and comparable with the resistance of the NNO electrode (0.51 Ω cm2). Due to its superior stability and lower interaction with the electrolyte this electrode composition can be recommended for preferential usage in IT-SOFCs.

Exploring mixed proton/electron conducting air electrode materials in protonic electrolysis cell

In this work, we investigate and compare the performance and cell polarization resistance of Ba0.5La0.5CoO3−δ (BLC) and double perovskite oxide BaGd0.8La0.2Co2O6−δ (BGLC) anode on cathode supported protonic steam electrolysis cells using a 20 μm SrZr0.5Ce0.4Y0.1O3−δ electrolyte with Ni-SZCY541 composite as the cathode. The kinetics of protons through the bulk and across the gas electrolyte interfaces of both anode materials were also studied by direct measurement of their tracer diffusions using time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS). Cell terminal voltages of 1.74 and 1.93 V, were observed at a current density of 0.5 A cm−2 for both BLC and BGLC whereas a hydrogen evolution rate of 121.85 and 111.15 μmol cm−2 every minute was also obtained at the same current density, translating to a current efficiency of 78 and 72% respectively. Hydrogen tracer diffusion studies confirm BGLC can incorporate protons into the bulk relative to BLC even though the present steam electrolysis results show a better performance for BLC at 600 °C.

Single-Site Active Iron-Based Bifunctional Oxygen Catalyst for a Compressible and Rechargeable Zinc-Air Battery.

The exploitation of a high-efficient, low-cost, and stable non-noble-metal-based catalyst with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) simultaneously, as air electrode material for a rechargeable zinc-air battery is significantly crucial. Meanwhile, the compressible flexibility of a battery is the prerequisite of wearable or/and portable electronics. Herein, we present a strategy via single-site dispersion of an Fe-Nx species on a two-dimensional (2D) highly graphitic porous nitrogen-doped carbon layer to implement superior catalytic activity toward ORR/OER (with a half-wave potential of 0.86 V for ORR and an overpotential of 390 mV at 10 mA·cm-2 for OER) in an alkaline medium. Furthermore, an elastic polyacrylamide hydrogel based electrolyte with the capability to retain great elasticity even under a highly corrosive alkaline environment is utilized to develop a solid-state compressible and rechargeable zinc-air battery. The creatively developed battery has a low charge-discharge voltage gap (0.78 V at 5 mA·cm-2) and large power density (118 mW·cm-2). It could be compressed up to 54% strain and bent up to 90° without charge/discharge performance and output power degradation. Our results reveal that single-site dispersion of catalytic active sites on a porous support for a bifunctional oxygen catalyst as cathode integrating a specially designed elastic electrolyte is a feasible strategy for fabricating efficient compressible and rechargeable zinc-air batteries, which could enlighten the design and development of other functional electronic devices.

New, Efficient, and Reliable Air Electrode Material for Proton-Conducting Reversible Solid Oxide Cells.

Driven by the demand to minimize fluctuation in common renewable energies, reversible solid oxide cells (RSOCs) have drawn increasing attention for they can operate either as fuel cells to produce electricity or as electrolysis cells to store electricity. Unfortunately, development of proton-conducting RSOCs (P-RSOCs) faces a major challenge of poor reliability because of the high content of steam involved in air electrode reactions, which could seriously decay the lifetime of air electrode materials. In this work, a very stable and efficient air electrode, SrEu2Fe1.8Co0.2O7-δ (SEFC) with layer structure, is designed and deployed in P-RSOCs. X-ray diffraction analysis and High-angle annular dark-filed scanning transmission electron microscopy images of SEFC reveal that Sr atoms occupy the center of perovskite slabs, whereas Eu atoms arrange orderly in the rock-salt layer. Such a special structure of SEFC largely depresses its Lewis basicity and therefore its reactivity with steam. Applying the SEFC air electrode, our button switches smoothly between both fuel cell and electrolysis cell (EC) modes with no obvious degradation over a 135 h long-term test under wet H2 (∼3% H2O) and 10% H2O-air atmospheres. A record of over 230 h is achieved in the long-term stability test in the EC mode, doubling the longest test that had been previously reported. Besides good stability, SEFC demonstrates great catalytic activity toward air electrode reactions when compared with traditional La0.6Sr0.4Co0.2Fe0.8O3-δ air electrodes. This research highlights the potential of stable and efficient P-RSOCs as an important part in a sustainable new energy power system.

Three-dimensional SnS2 nanopetals for hybrid sodium-air batteries

Na-air batteries are regarded as a potential alternate to Li-air batteries due to the abundant sodium source and high theoretical energy density. However, non-aqueous Na-air battery suffers from the electrode polarization owing to the formation of insoluble discharge product, which severely limits its cyclability and performance. Herein, a high performance hybrid Na-air cell is demonstrated using a dual electrolyte (mixed aqueous and non-aqueous electrolyte) system and three dimensionally (3D) grown tin sulfide (SnS2) nanopetals based air electrode. 3D SnS2 nanopetals are synthesized by a facile solvothermal method and used as an air electrode material for hybrid Na-air battery. The vertically-grown and self-assembled ultra-thin nanosheets of 3D SnS2 nanopetals provide exposed active sites for the efficient air and electrolyte diffusion to air electrodes, resulting in high performance hybrid Na-air cell. The fabricated hybrid Na-air cell displays low overpotential gap (0.52V), high round trip efficiency (83%), high power density (300mWg−1) and good rechargeability up to 40 cycles. The proposed 3D SnS2 nanopetals as air electrodes can provide a robust platform for the future development of Na-air batteries and other energy storage devices.

Coupling cobalt-iron bimetallic nitrides and N-doped multi-walled carbon nanotubes as high-performance bifunctional catalysts for oxygen evolution and reduction reaction

Zn-air battery, as an ideal energy conversion and storage device, is always limited by the expensive and less-than-ideal air electrode materials. The coupling of outstanding oxygen evolution reaction (OER) active sites (oxides derived from Co-Fe nitrides) and superior oxygen reduction reaction (ORR) active centers (metal-N-C and graphitic N) to acquire high-performance and low-cost catalysts is an ideal solution. Herein, we have successfully combined cobalt-iron bimetallic nitrides with N-doped multi-walled carbon nanotubes (Co-Fe-N@MWCNT) as a robust bifunctional material. Benefiting from the synergistic effect between Co, Fe and MWCNTs, Co-Fe-N@MWCNT not only possesses large electrochemically active surface area and effective transport path, but also realizes the integration of superior OER and ORR active sites. Only a low overpotential (290mV) is needed to achieve a current density of 10mA cm-2 for OER and the ORR catalytic activity is close to that of the commercial Pt/C. Additionally, Co-Fe-N@MWCNT as an ideal air electrode material can also be applied in Zn-air battery, which exhibits low voltage drop and favorable stability. The voltage gap has a slight change (about 0.03V) even after 100 cycles of galvanostatic charge-discharge. More importantly, the synthetic strategy in our work may facilitate the design of more high-efficient bifunctional catalysts in various domains.

NiCo2O4 @Ni As the New Air Electrode Material to Enhance the Performance in Rechargeable Zinc–Air Batteries Application

In recent years, pollution and shortage of fossil fuels in the environment have made it a popular trend to develop portable electronic devices, use renewable energy and grid-scale energy storage in electric vehicles. Batteries have long been recognized for their capacity to convert and store electrical energy efficiently. In the present work, rechargeable metal−air batteries such as Zn-air batteries could be idealized energy storage devices which have attracted much attention compared with lithium-ion batteries ,because Zn-air batteries possess high power density, economic viability and high safety.[1-2] Active and durable electrocatalysts on the cathode side are required to catalyse oxygen reduction reaction during discharge and oxygen evolution reaction during charge for rechargeable batteries.[3] The aim of experiment is to synthesize mesoporous NiCo2O4@Ni. All the reagents used in this experiment were analytical grade and were used without further purification. In a typical synthetic procedure, a piece of nickel foam was thoroughly cleaned with 3 M HCl solution in an ultrasound bath for 30 minutes in order to remove the NiO layer on the surface and then washed by deionized water and absolute ethanol. Put it into watch glasses for drying. C4H6NiO4 • 4H2O (0.3 g), Co(NO3)2 • 6H2O (0.6 g) and hexamethylenetetramine (HMT, 0.8 g) were dissolved into a mixed solution of deionized water (30 mL) and ethanol (20 mL) under vigorous magnetic stirring. After stirring for 30 min, the as-obtained solution was transferred into a 60 mL polytetra-fluoroethylene (PTFE) (Teflon)-lined autoclave, and then use the cleaned nickel foam which has been worked to immerse into the reaction solution. The autoclave was sealed and maintained at 95 °C for 8 hours in an electric oven. After cooling to room temperature, the products on the nickel foam were washed with deionized water and absolute ethanol for several times respectively and then dried the nickel foam at 60°C overnight. Afterward, the samples were annealed at 250° C, 350° C and 450° C for 1h at a ramping rate of 1°C min-1 to transform into NiCo2O4@Ni-250, NiCo2O4@Ni-350 and NiCo2O4@Ni-450. NiCo2O4@Ni-350 is a new air electrode material which has been synthesized via a hydrothermal method. This nanomaterial displays good bifunctional ORR/OER activity, cyclic stability, low cost and high energy density in the discharge and charge process. Further studies are ongoing to improve the Zn-Air batteries performance by manipulating the structure.

Innovative Approach on Nano-Metal Particles of Electrodes for Solid Oxide Cells

Solid oxide cells (SOCs) are electrochemical systems capable of operating reversibly and therefore may serve as versatile energy converters. Electrolysis mode, electricity can be converted to fuels or other valuable chemicals, on the other hand, fuel cell mode converts efficiently fuel to electricity on demand. Due to their severe operating conditions, the ideal fuel electrode materials has not been found yet to accomplish of its requirements such as electronic and ionic conductivity, and catalytic activity with strong stability in redox condition, while several high performance air electrode materials -have been identified. For a long time, Ni-YSZ cermet has been regarded as the embodiment of this functional trinity. However, even though degradation associated with reversible operation can be mitigated, the cermet electrodes still suffered from agglomeration during redox condition. Recent electrode designs have been developed to microstructures consisting of a porous mixed ionic electronic conductor backbone decorated with metallic nanoparticles (typically Ni) by impregnation method. However, these are still hindered by the lack of cost and time-effective methods to produce both robustness and high-performance from their nanostructured electrodes. Here, a simpler alternative, will be introduced, exsolution whereby the catalytically active metal is substituted in the crystal lattice of the backbone in oxidizing conditions and exsolved on the surface as designed metal particles under reduction condition. Here we demonstrated a simple and highly effective in situ method for producing nanostructured electrodes capable of delivering high performances in both fuel cell and electrolysis mode. Both the nanostructures and corresponding electrochemical activity show no degradation over 150 hours of testing.

CNT Sheet Air Electrode for the Development of Ultra-High Cell Capacity in Lithium-Air Batteries

Lithium-air batteries (LABs) are expected to provide a cell with a much higher capacity than ever attained before, but their prototype cells present a limited areal cell capacity of no more than 10 mAh cm−2, mainly due to the limitation of their air electrodes. Here, we demonstrate the use of flexible carbon nanotube (CNT) sheets as a promising air electrode for developing ultra-high capacity in LAB cells, achieving areal cell capacities of up to 30 mAh cm−2, which is approximately 15 times higher than the capacity of cells with lithium-ion battery (LiB) technology (~2 mAh cm−2). During discharge, the CNT sheet electrode experienced enormous swelling to a thickness of a few millimeters because of the discharge product deposition of lithium peroxide (Li2O2), but the sheet was fully recovered after being fully charged. This behavior results from the CNT sheet characteristics of the flexible and fibrous conductive network and suggests that the CNT sheet is an effective air electrode material for developing a commercially available LAB cell with an ultra-high cell capacity.

Bacterial-Cellulose-Derived Carbon As Air Electrode Material for Lithium Air Secondary Batteries

Introduction Lithium air secondary batteries exhibit higher theoretical energy density than lithium ion batteries and are expected as next-generation secondary batteries. However, there remain technical challenges related to their poor cycle properties.Since the first report by K. M. Abraham et al. [1], various oxygen reduction/evolution catalyst [1-3] and electrolyte [1, 4] materials have been intensively investigated for air batteries to improve their electrochemical properties, such as their cyclability. However, there have only been a few studies on support materials for air electrodes, such as nanoporous gold [5].We consider that one of the reasons for the poor cyclability is a decrease in the electrical contact of the air electrode during Li2O2 deposition (discharge) and oxygen generation (charge), which results from large the volume change. The purpose of our work is to improve the cyclability by using a new air electrode structure and material. As an elastic support material for air electrodes, we focus on carbon fabricated from bacterial cellulose (BC), which has a continuous interconnected network of carbon. Such an elastic carbon support would prevent the large volume change of the air electrodes and improve the cycle performance. Here we report the preparation conditions for BC-derived carbon supports materials and the performance of lithium air secondary batteries incorporating them. Experimental A BC sheet (Fujicco Co., Ltd.) was frozen at -40 °C and then freeze-dried [6]. The freeze-dried BC sheet was carbonized in N2 atmosphere at 600, 900, and 1200 °C to obtain BC-derived carbon. It was cut into a circle (diameter: 5 mm) to obtain the binder-free air electrode. The lithium air secondary battery consisted of the air electrode, a lithium metal sheet, and 1.0 mol/l lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/tetraethylen glycol dimethyl ether (TEGDME) as the positive electrode, negative electrode, and electrolyte solution, respectively. The battery was assembled with an ECC-Air Cell (EL-CELL GmbH). Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm2 in an O2 atmosphere. The discharge and charge capacities were normalized by the weight of the BC-derived carbon (the air electrode). Results and discussion Figure 1 shows an SEM image of the BC-derived carbon prepared at 1200 °C. The continuous interconnected network of the carbon fibers with diameter of less than 10 nm can be seen in this image. These carbon fibers build a 3-D disordered macroporous framework. The BC-derived carbon prepared at 600 and 900 °C was confirmed to have morphologies similar to that at 1200 °C. Moreover, in a preliminary compression test, the BC-derived carbon showed a highly elastic compressibility and almost completely recovered its original volume after the compression was released.Figure 2 shows the first discharge/charge curves of the air batteries incorporating the BC-derived carbon prepared at 600, 900, and 1200 °C. The air batteries with the carbon prepared at 900 °C showed the largest discharge capacities of 8800 mAh/g and the average discharge voltage of 2.68 V. This is because it has the largest number of active sites among the carbons tested. The batteries incorporating BC-derived carbon prepared at 900 and 1200 °C showed almost the same charge capacities of about 8000 mAh/g. These charge properties have a tendency similar to the discharge properties even though the air electrodes have no catalysts. These results indicate that the BC-derived carbon can be used as support material for binder-free air electrodes. Figure 3 shows the correlation between the discharge capacities, D/G ratio, and carbonization temperature. In the Raman spectra of the carbon as shown in the inset, the peaks of D- and G-bands are derived from the edge-plane and basal-plane in the crystal structure of carbon, respectively. The D/G ratio was obtained from the results of wave analyses with the spectra in the inset. It correlates roughly with the discharge capacities as shown in Fig. 3. In particular, the battery incorporating the carbon prepared at 900 °C, which has the largest discharge capacity, has the highest D/G ratio. This indicates the importance of the number of carbon edge planes in relation to the discharge property, because the D/G ratio corresponds to a ratio of number of carbon edge planes to that of basal planes. These results indicate that the carbon edge planes would play an important role as active sites in the reaction mechanism with the air electrode.

Defect-Structure Analysis on La2(Ni,Cu)O4+ d with Reverse Monte Carlo Simulation Using Neutron and Synchrotron X-Ray Total Scatterings

Recently, it is highly expected to lower an operating temperature of the solid oxide fuel cell (SOFC). To satisfy this demand, a development of a novel air-electrode material can be considered as an important issue. Materials with high mixed oxide-ion and electronic conduction can be regarded as a promising candidate for the electrode since the materials enable to expand area for the oxygen-reduction reaction. As such materials, La2NiO4+δ with the layered perovskite structure has drawn much attention nowadays. In the oxides, interstitial oxide ions are introduced into the crystal due to a charge compensation of Ni oxidation from 2+ to 3+. This change of the defect structure leads to high mixed oxide-ion and electronic conduction and thus superior electrode property. It is also found that a partial substitution of Cu for Ni enhances electrochemical property of La2NiO4+δ [1]. Considering the mechanism for the mixed conduction mentioned above, the Cu substitution must affect the defect structure in the crystal: that is, an extra oxygen amount, a distribution and/or a local situation of the interstitial oxide ion. From such background, some previous works have carried out actively the crystal structure analysis using the Bragg profile and discussed the average position of the extra oxide ions. As is well known, however, the structure analysis (a so-called average-structure analysis) cannot give deep insight on the defect structure unfortunately. In this work, we focused on La2NiO4+δ and La2Ni0.6Cu0.4O4+δ with the layered perovskite structure, and investigated their defect structures by the reverse Monte Carlo (RMC) simulation using the Bragg profiles and the pair distribution functions (PDFs) simultaneously [2, 3]. Since PDF data include information on the atomic configurations without periodicity, we could clarify the defect structures of the samples on the basis of the atomic-configuration snapshots of the RMC simulations. La2NiO4+δ and La2Ni0.6Cu0.4O4+δ could be synthesized by a citric-acid method, according to literature [1]. The obtained products were identified by laboratorial X-ray diffraction measurements, and their metal compositions were evaluated by the inductively coupled plasma spectrometry. Average structures of the samples were determined by the Rietveld analysis (Z-Rietveld) using the neutron Bragg profiles which were recorded with iMATERIA installed at J-PARC. Neutron and synchrotron X-ray total scattering patterns were also measured with NOVA at J-PARC and BL04B2 at SPring-8, respectively. By using all the data, we performed the RMC simulations with RMCProfile for the specimens. In the analyses, we used super cells with 1152 atoms which were constructed based on the refined unit cells. The Rietveld analysis using neutron Bragg profiles confirmed that both the La2NiO4+δ and La2Ni0.6Cu0.4O4+δ had a single phase of the layered rock-salt structure with a space group of Fmmm. In order to determine distributions of the interstitial oxide ions, we analyzed atomic configurations of the samples by the RMC modeling using neutron PDF, neutron and synchrotron X-ray structure factors in addition to the neutron Bragg profile. As a result, it was found that a distance of Cu-O was longer than that of Ni-O, and the distribution of the distance became wider around Cu. Such a change in the local situation may be one of the reasons for higher oxide-ion diffusion in the Cu-substituted specimen. It was also indicated from the La-Ni and La-Cu distances that La2Ni0.6Cu0.4O4+δ had larger interstitial cavity volume in the rock-salt layer compared with La2NiO4+δ. Since the rock-salt layer forms the diffusion pathway of the interstitial oxide ion, the larger interstitial volumes in the Cu-substituted sample should contribute to better electrode property. From the analysis on cavities around interstitial oxide ions, it was suggested that an oxide-ion diffusion accompanied a significant change in the cavity volume around the mobile oxide ion in the case of La2NiO4+δ, whereas such a change was suppressed in La2Ni0.6Cu0.4O4+δ. This should be one of the reasons why the oxide-ion diffusion was enhanced by substituting Cu for Ni partially.

Origin of Compositional Instability for a-Site/B-Site Rich La0.6Sr0.4Co0.2Fe0.8O3−D (LSCF6428) Under Oxygen Potential Gradient

Currently, LaxSr1−xCoyFe1−yO3−δ (LSCF) is under intensive research as a potential air electrode material for SOFC and SOEC. As a mixed ionic and electronic conductor (MIEC) with high catalytic activity, LSCF shows high electrode performance.【1 】However, the stability of LSCF as air electrode has yet to be thoroughly verified experimentally. One of the major concerns for LSCF is the surface segregation of Sr.【2 】 This has been discussed in terms of the mismatch of the ionic radius of Sr and La.【3-5 】Another mechanism that can lead compositional instability is the kinetic demixing or kinetic decomposition. Previous work showed that the severe segregation of cobalt oxides appeared on the high-pressure side of LSCF placed under an oxygen potential gradient. The segregation tended to appear on the grain boundary lines【6 】. However, segregation was not always found in the same way, and the detailed mechanism for the segregation is not clear. In this article, the aim is to uncover the mechanism of decomposition of LSCF under oxygen potential gradient. For this purpose, we compared the A-site and B-site rich LSCF6428 materials in the surface microstructure and microchemistry arising from the cation segregation. The A-site or B-site 2% rich LSCF6428 were prepared using oxides: Co2O3, La2O3, Fe2O3 and SrO. La2O3is easy to absorb water, so it’s firstly put in the furnace at 1273k for 2 hours and then all the powder was carefully weighed in the ratio of 2% of commercial powder LSCF6428 and mixed them using a ball mill. The mixed powders were calcined at 1273 K for 10 h in air. The calcined powders were pressed into a cylindrical shape under the hydrostatic pressure of 200MPa and sintered at 1573K for 4 h. The density of the sintered pellets was higher than 97%. After the sintering, the materials were confirmed as the perovskite single phase without any obvious impurity phase by an X-ray diffraction analysis. The specimen was polished, and then a pellet was placed between two alumina tubes using gold rings as the sealing materials. Temperature was firstly brought to 1273 K, which is close to the melting temperature of gold (～1323 K), in order to seal via the pressurized gold ring. Then temperature was reduced down to the measurement temperature maintaining the gas tightness. Sintered pellets of LSCF6428 have been heat-treated with and without applying oxygen potential gradients. Their morphology and composition changes were investigated by Scanning Electron Microscope (SEM, JSM-7001F，JEOL ) and Energy Dispersive X-ray Spectroscope (EDS, INCA Energy250XT，OXFORD) and TEM. As is shown in Fig. 1, a second phase can be easily observed on the oxidation surface of LSCF6428 membrane after the experiment under p(O2) gradients at 1273K, when 1 bar oxygen was introduced to the high p(O2) side, and 10−4 of oxygen was introduced into the low p(O2) side. For A-site rich sample, strontium oxide or related compounds were observed on the high p(O2) surface according to EDX analysis. This is different from the results of B-site rich sample, which was found with cobalt-rich precipitates on the high p(O2) surface. Meanwhile, materials were not homogeneous at the low p(O2) side, with Co oxide precipitates, and noticeable changes of the morphology happened. Co segregation was observed clearly on the grain boundary of cross section for the B-site rich sample after sintering at 1573K in the air. TEM test also show similar phenomenon using the high resolution image. Reference: 1.Zehua Pan, Qinglin Liu, Lan Zhang, Xiongwen Zhang, and Siew Hwa Chana, Journal of The Electrochemical Society, 162 (12) F1316-F1323 (2015). 2. P. Iora, M. A. A. Taher, P. Chiesa, and N. P. Brandon, Int. J. Hydrogen Energy, 35, 12680 (2010). 3. W. Lee, J. W. Han, Y. Chen, Z. Cai, and B. Yildiz, J Am Chem Soc, 135, 7909 (2013). 4. H. Ding, A. V. Virkar, M. Liu, and F. Liu, Phys Chem Chem Phys, 15, 489 (2013). 5.Zhuhua Cai, Markus Kubicek, Jürgen Fleig, and Bilge Yildiz, Chem. Mater. 2012, 24, 1116−1127 6. Mi-Young Oh, Atsushi Unemoto Koji Amezawa, and Tatsuya Kawada, Journal of The Electrochemical Society, 159 (10) F659-F664 (2 Figure 1

Innovative Approach on Nano-Structuring of Electrode for Solid Oxide Cells

Solid oxide cells (SOCs) are electrochemical systems capable of operating reversibly and therefore may serve as versatile energy converters. Electrolysis mode, electricity can be converted to fuels or other valuable chemicals, on the other hand, fuel cell mode converts efficiently fuel to electricity on demand. Due to their severe operating conditions, the ideal fuel electrode materials has not been found yet to accomplish of its requirements such as electronic and ionic conductivity, and catalytic activity with strong stability in redox condition, while several high performance air electrode materials -have been identified. For a long time, Ni-YSZ cermet has been regarded as the embodiment of this functional trinity. However, even though degradation associated with reversible operation can be mitigated, the cermet electrodes still suffered from agglomeration during redox condition. Recent electrode designs have been developed to microstructures consisting of a porous mixed ionic electronic conductor backbone decorated with metallic nanoparticles (typically Ni) by impregnation method. However, these are still hindered by the lack of cost and time-effective methods to produce both robustness and high-performance from their nanostructured electrodes. Here, a simpler alternative, will be introduced, exsolution whereby the catalytically active metal is substituted in the crystal lattice of the backbone in oxidizing conditions and exsolved on the surface as designed metal particles under reduction condition. Here we demonstrated a simple and highly effective in situ method for producing nanostructured electrodes capable of delivering high performances in both fuel cell and electrolysis mode. Both the nanostructures and corresponding electrochemical activity show no degradation over 150 hours of testing.

Bioinspired Synthesis of Hierarchically Porous MoO2/Mo2C Nanocrystal Decorated N-Doped Carbon Foam for Lithium–Oxygen Batteries

The lithium oxygen (Li–O2) battery is one of the most promising technologies among various electrochemical energy storage systems. The challenge to develop a high-performance Li–O2 battery lies in exploring an air electrode with optimal porous structure and high efficient bifunctional electrocatalyst. The present work demonstrates a bioinspired synthesis route for the preparation of high performance Li–O2 air electrode materials that are made out of N-doped carbon foams decorated with heteronanostructured MoO2/Mo2C nanocrystals (MoO2/Mo2C@3D NCF). Here, recombinant proteins (ELK16-FLAG) facilitated the self-assembly of metal precursors and provided a carbon source for Mo2C formation. The as-prepared MoO2/Mo2C@3D NCF showed superior electrocatalytic activity in both oxygen evolution reaction and oxygen reduction reaction mechanisms with a high round-trip efficiency of 89.1% (2.77 V/3.11 V) at 100 mA g–1 as well as exceptional rate performances and good cyclability in Li–O2 battery. The desirable electroche...

Life cycle assessment of the manufacture and operation of solid oxide electrolyser components and stacks

Solid oxide electrolysis cells (SOEC) can be used for high-temperature electrolysis to efficiently transform electrical energy into hydrogen. To evaluate the environmental impacts of manufacturing 1 kW stack consisting of SOEC and interconnects, we performed a life cycle assessment for an early development stage of the SOEC. Cells differed in air electrode materials. Cell and interconnect production is generally responsible for most of the impacts on the environment and human health, while differences between stacks due to differing air electrode materials are low. Reducing interconnect material use was identified as most promising improvement potential regarding manufacturing. If the electrolysis process is included, most impacts per MJ hydrogen produced come from the electrolysis itself (>80%). Reducing degradation rate and increasing current density and lifetime could e.g. reduce climate change impacts by up to 20%. From an environmental perspective, emphasis should be on reducing degradation and expanding cell lifetime.

Tailoring Electrode Materials for Proton-Conducting Solid Oxide Fuel/Electrolysis Cells with Chemically Stable BaZrO3 Electrolyte

Proton-conducting oxides offer an alternative solution for solid oxide cells with its unique advantages, such as low activation energy, high ionic conductivity at intermediate temperatures (1, 2). In the community, it is generally realized that the use of chemically stable electrolyte material BaZrO3 is the most promising way for the development of proton-conducting solid oxide cells. Although the use of BaZrO3 has been achieved with desirable fuel cell performance (2), the design of its air electrode is still a challenge, especially for the proton-conducting solid oxide electrolysis cells. In this presentation, we report our recent work on the tailor of the air electrode materials for proton-conducting solid oxide cell, including the use of traditional Sr-doped LaMnO3 (LSM) cathode for BaZrO3-based fuel cells with desirable cell performance and the design of mixed proton-electronic mixing conducing air electrode for proton-conducting electrolysis cells. LSM, which is one of the most classical air electrode materials for solid oxide cells, is regarded can be only used above 700 oC due to its poor catalytic activity at intermediate temperatures (around 600 oC). In our study, we successfully employed the inkjet printing technique, enabling LSM work properly as the air electrode for proton-conducting solid oxide fuel cells with BaZrO3 electrolyte. After the optimization, the fuel cell performance reached 200 mW cm-2 at 600 oC, which is encouraging considering the air electrode material is LSM. A further improvement of fuel cell performance was achieved by using mixed electronic-oxygen conductor air electrode material, reaching the highest power output for the BaZrO3-based cells up to now. In addition to the work on fuel cells, research on the air electrode material for solid oxide electrolysis cells was also carried out. Following to our previous work using proton-electronic mixed conductor for proton-conducting fuel cells (3, 4), a similar strategy was proposed for electrolysis cells. Coupling BaZr0.5Pr0.3Y0.2O3 (BZPY30, a mixed protonic-electronic conductor) with La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the air electrode, the cell showed a dramatic electrolysis cell improvement compared with the LSCF- BaZr0.7Pr0.1Y0.2O3 (BZPY10, a pure protonic conductor). This result suggests the extension of proton conduction at the air electrode is critical for the transportation of proton at air electrode side and thus facilitates the electrochemical reactions for the proton-conducting electrolysis cells.