Nickel Yttria-stabilized Zirconia Research Articles

Suitability of Sm3+-Substituted SrTiO3 as Anode Materials for Solid Oxide Fuel Cells: A Correlation between Structural and Electrical Properties

Perovskite anodes, nowadays, are used in any solid oxide fuel cell (SOFC) instead of conventional nickel/yttria-stabilized zirconia (Ni/YSZ) anodes due to their better redox and electrochemical stability. A few compositions of samarium-substituted strontium titanate perovskite, SmxSr1−xTiO3−δ (x = 0.00, 0.05, 0.10, 0.15, and 0.20), were synthesized via the citrate-nitrate auto-combustion route. The XRD patterns of these compositions confirm that the solid solubility limit of Sm in SrTiO3 is x < 0.15. The X-ray Rietveld refinement for all samples indicated the perovskite cubic structure with a P m 3 ¯ m space group at room temperature. The EDX mapping of the field emission scanning electron microscope (FESEM) micrographs of all compositions depicted a lower oxygen content in the specimens respect to the nominal value. This lower oxygen content in the samples were also confirmed via XPS study. The grain sizes of SmxSr1−xTiO3 samples were found to increase up to x = 0.10 and it decreases for the composition with x > 0.10. The AC conductivity spectra were fitted by Jonscher’s power law in the temperature range of 500–700 °C and scaled with the help of the Ghosh and Summerfield scaling model taking νH and σdc T as the scaling parameters. The scaling behaviour of the samples showed that the conduction mechanism depends on temperature at higher frequencies. Further, a study of the conduction mechanism unveiled that small polaron hopping occurred with the formation of electrons. The electrical conductivity, in the H2 atmosphere, of the Sm0.10Sr0.90TiO3 sample was found to be 2.7 × 10−1 S∙cm−1 at 650 °C, which is the highest among the other compositions. Hence, the composition Sm0.10Sr0.90TiO3 can be considered as a promising material for the application as the anode in SOFCs.

(Invited) Building Resilience into Ni-YSZ Anodes with Secondary Phases

Solid oxide fuel cells anodes are typically composed of a porous nickel-yttria stabilized zirconia (Ni-YSZ) ceramic metallic (cermet) composite. The advantages of a Ni-YSZ anode are many: the materials are inexpensive, Ni is an exceptionally effective CH bond activation catalyst and these electrodes can operate with a wide variety of fuels. However, Ni-YSZ anodes are also susceptible to material degradation and deteriorating device performance, especially when subjected to repeated reduction/oxidation cycling and/or carbon accumulation and metal dusting. Recent work in our laboratory has sought to improve the durability and resilience of Ni-YSZ anodes by adding small amounts of secondary materials to the anode stock prior to fabrication and processing. One secondary material that has proven particularly effective is aluminum titanate (Al2TiO5or ALT) During anode processing, ALT (4% by mass) reacts with Ni-YSZ to form nickel aluminate (NiAl2O4) and zirconium titanate (Zr5Ti7O24). Upon anode reduction, NiAl2O4 forms Ni particles decorated with Al2O3 nanoparticles. These secondary phases improve device performance and appear to preserve anode microstructure even after 100 hours of continuous operation. Furthermore, these secondary phases enhance anode resilience to redox cycling. We have used a variety of electrochemical and operandooptical methods to test the resilience of ALT-enhanced anodes subject to both electrochemically-induced oxidative stress and environmentally driven anode oxidation. In both cases, ALT-enhanced anodes can withstand more than twice as many redox cycles as anodes that have not had ALT added. In addition, adding ALT to the anode reduces its susceptibility to carbon accumulation when operating with hydrocarbon fuels.

Experimental Optimization of Ni-GDC Microstructure with Various Phase Fractions for Solid Oxide Fuel Cell Anode

The design of electrode microstructure is important to enhance performance and stability of solid oxide fuel cells (SOFCs). The most common material used for the fuel electrode is nickel-yttria stabilized zirconia (Ni-YSZ) composite, in which electron, ion and gas conduct in the metallic Ni phase, the ceramic YSZ phase and the pores, respectively. The electrochemical reaction therefore takes place at the triple-phase-boundary (TPB), hence the increase in TPB density improves electrode performance. Recently, the ceramic materials, which exhibits mixed ionic-electronic conductivity (MIEC) properties, are considered to be beneficial solution for the SOFC electrodes. They can greatly enhance the available reaction sites by introducing surface reactions on the double phase boundaries (DPB) between MIEC phase and pore. One of the promising materials is considered to be gadolinium doped ceria (GDC). GDC exhibits higher ionic conductivity compared to YSZ and has MIEC property in the reducing atmosphere. In the case of Ni-GDC composites, the electrochemical performance is supported by both DPB and TPB reactions. In general, the active DPB density increases with the share of GDC phase in the composite. On the other hand, the active TPB density achieves maximum value when the share of both solid phases is balanced. The optimal electrode design has to be determined by the ratio between DPB and TPB reactions and their kinetics. In the present study, electrolyte supported SOFC with Ni-Gd0.1Ce0.9O2-δ anodes with various phase fractions were fabricated and their polarization characteristics were experimentally investigated. The ratio of Ni and GDC was controlled by the initial powder composition, and the porosity was varied by isostatically pressing the anode after screen printing. Initially, NiO (AGC Seimi) and GDC (ShinEtsu) powders were mixed to achieve the target volume fractions of Ni:GDC = 70:30, 60:40, 50:50, 40:60, 30:70 vol%. The anodes were then screen-printed on the YSZ discs and calcinated at 1350oC. In order to modify the porosity of the anodes, some of the cells undergoes isostatic pressing treatment at 200 MPa for 30 minutes prior to the calcination. The LSCF (lanthanum strontium cobalt ferrite by Fuel Cell Materials) cathode with GDC (ShinEtsu) barrier layer were screen-printed and sintered on the other side of the electrolyte. Current – voltage curves and electrochemical impedance spectra were measured at 500 – 800 oC. Additionally, the anodes with optimal phase composition were discharged for 100 hrs. 3D microstructural analysis was conducted with the focused ion beam scanning electron microscopy (FIB-SEM). The microstructural parameters, such as phase volume fractions, particle intercept length, TPB and DPB densities and tortuosity factors were quantified. By evaluating the connectivity of each phase, the total and active TPB and DBP densities were calculated. Quantified microstructural parameters were correlated with the polarization characteristics. Neither the DPB reactions nor the TPB reactions could individually explain the measured polarization resistances of the composite anodes. In the case of screen-printed anode samples without isostatic pressing, the high porosity (ca. 52% for all of the samples) lead to low connectivity of the solid phases. The optimal composition of the cermet was found to be Ni:GDC = 0.4:0.6 vol%. Introducing isostatic pressing, improvements in both ohmic and polarization resistances are observed. The decrease in ohmic resistance is mainly attributed to the improved connectivity between the electrode and the electrolyte. The polarization resistance was influenced by the changes in active DPB and TPB densities. As the porosity is reduced and connectivities of the solid phases are enhanced by the isostatic pressing, not only the nominal values of active DPB and TPB densities increased, but also the ratio between them has changed. This effect is most significant for Ni:GDC = 0.3:0.7 vol% sample, which exhibits the smallest polarization resistance among all the investigated electrodes. The discharge testing showed the improved stability of the cells with the decreased porosity. It is therefore concluded that not only the ratio between Ni and GDC but also porosity is a very important parameter for the performance and stability of the anode. Figure 1

Effects of Polarization on the Microstructural Changes at the YSZ/Ni-YSZ Interface

This study examines the effect of Ni migration near the interface with the electrolyte after fuel cell (SOFC) and electrolysis mode (SOEC) operation on nickel-yttria stabilized zirconia (Ni-YSZ) electrodes. The experimental dataset consists in a series of short-stacks tested under different polarization conditions and durations. Selected zones have been imaged by 3-D FIB-SEM serial sectioning. After reconstruction and segmentation, the spatial variations of metric and topological properties have been quantified along the direction perpendicular to the interface with the YSZ electrolyte, including TPB density, volume fraction, dihedral angles and interfacial curvature. Their spatial distribution and time evolution have been analysed to advance the understanding of the driving forces that cause Ni depletion near the YSZ electrolyte.

Reverse Current Treatment of Short Stacks – Experimental Results and System Considerations

Anode degradation is one of the main factors that limit the lifetime of solid oxide cells (SOCs), and thus complicate their commercial application. For nickel/yttria-stabilized-zirconia (Ni/YSZ) anodes, one possible counter measure is re-activation through reverse current treatment (RCT). This method has been described for single cells by other researchers. In order to investigate the applicability of RCT in SOC systems, we used this treatment on anode supported short stacks. Two stacks that had been exposed to different degradation phenomena were investigated. The first stack, suffering from partial re-oxidation and a cracked cell, already showed temporary performance improvement after electrolysis operation under mild conditions. The second stack had degraded over a thermal cycle, and different from the first one showed less re-activation. RCT under dry conditions was found to cause severe degradation. Furthermore, we found that, in agreement with literature, YSZ becomes electronically conductive during the RCT, meaning that electrolysis power during RCT is not limited by the amount of available steam. We discuss the issues of this behavior for SOC systems and possible solutions.

(Invited) Ex-Situ Tracking of Microstructural Evolution in Solid Oxide Cell Electrodes with Ptychographic Nanotomography

The 3D morphology of solid oxide cell (SOC) electrode microstructure and its evolution has a strong impact on the SOC electrochemical performance during their lifetime. The morphology of the nickel network, in nickel – yttria-stabilized-zirconia (Ni-YSZ) hydrogen electrodes is particularly influenced by: their initial start-up reduction profile, high temperature annealing, high current density operation and exposure to oxidation-reduction cycles. Thus, the development of 3D microstructure characterisation techniques to quantify the 3D microstructure parameters has been an intense topic of research. What remains challenging, due to SOC operating conditions, is to track non-destructively the evolution of the same volume of electrode material with high 3D spatial resolution from before, after and during processes which alter initial SOC microstructures[1–3]. Here we summarise the advantage of X-ray ptychographic nanotomography to image Ni-YSZ electrodes before and after oxidation[4], reduction[5] and annealing[6] with voxel resolutions below 55 nm. Ptychography provides outstanding data quality containing quantitative electron density image contrast. The image quality extrapolates to phase segmentations which in turn provide 3D microstructure parameters of sufficient quality to track the details of e.g. phase fractions, interfacial areas and triple phase boundary density of the changing electrode. Further to this, the image quality enables the possibility to observe the details of structural change of individual particles and interfaces as the basis for gaining fundamental insights into the operating mechanisms. In particular we draw attention to the benefits of identifying isolated particles as models of closed systems that can provide empirical control points into physically based 3D models of electrode evolution. Examples of: redox induced YSZ backbone mechanical failure, resulting change in the Ni particle size distribution and interfacial areas (see Fig. 1a); as well as the effects of annealing on Ni and pore phase percolation on the activity of individual TPBs and the coarsening of isolated Ni particles will be summarised (see Fig. 1b). The importance of appropriate sample preparation will also be discussed. Figure 1: a) Magnified view of a region of Ni-YSZ electrode evolving from the pristine state via the oxidised state to the re-reduced state illustrating the effects volumetric expansion and contraction of Ni/NiO on the YSZ backbone. Reprinted from [4], with permission from Elsevier. b) A magnified view of the Ni network in a Ni-YSZ electrode illustrating the detachment of a Ni particle from a thinly necked region of the Ni network during annealing. Reprinted from [6], with permission from Elsevier. Kennouche D, Chen-Wiegart YK, Yakal-Kremski KJ, et al (2016) Observing the microstructural evolution of Ni-Yttria-stabilized zirconia solid oxide fuel cell anodes. Acta Mater 103:204–210. https://doi.org/10.1016/j.actamat.2015.09.055Shearing PR, Bradley RS, Gelb J, et al (2011) Using Synchrotron X-Ray Nano-CT to Characterize SOFC Electrode Microstructures in Three-Dimensions at Operating Temperature. Electrochem Solid-State Lett 14:B117. https://doi.org/10.1149/1.3615824Shearing PR, Bradley RS, Gelb J, et al (2012) Exploring microstructural changes associated with oxidation in Ni–YSZ SOFC electrodes using high resolution X-ray computed tomography. Solid State Ion 216:69–72. https://doi.org/10.1016/j.ssi.2011.10.015De Angelis S, Jørgensen PS, Esposito V, et al (2017) Ex-situ tracking solid oxide cell electrode microstructural evolution in a redox cycle by high resolution ptychographic nanotomography. J Power Sources 360:520–527. https://doi.org/10.1016/j.jpowsour.2017.06.035De Angelis S (2017) Tracking Solid Oxide Cell Microstructure Evolution by High Resolution 3D Nano-Tomography. PhD Thesis Department of Energy Conversion and Storage, Technical University of Denmark. http://orbit.dtu.dk/files/139977895/finalDef2.pdfDe Angelis S, Jørgensen PS, Tsai EHR, et al (2018) Three dimensional characterization of nickel coarsening in solid oxide cells via ex-situ ptychographic nano-tomography. J Power Sources 383:72–79. https://doi.org/10.1016/j.jpowsour.2018.02.031 Figure 1

Long-term durability of La0.75Sr0.25Cr0.5Mn0.5Oδ−3 as a fuel electrode of solid oxide electrolysis cells for co-electrolysis

Electrochemical conversion of carbon dioxide to a hydrocarbon fuel can reduce greenhouse emissions and lead to economic profits. Solid oxide electrolysis cells (SOECs) are potential candidates for decreasing CO2 emission due to their low electrical energy consumption properties for CO2 reduction. However, their fuel electrode, which is nickel-yttria stabilized zirconia (Ni-YSZ), shows critical durability issues. This study aimed to find an SOEC fuel electrode material among perovskite catalysts. The selected catalysts should satisfy the following properties: high catalytic performance in the reverse water gas shift (RWGS) reaction, low carbon deposition rate after the RWGS reaction, high electrical conductivity, and long-term durability in a co-electrolysis environment. Ni-YSZ and perovskite catalysts were fabricated. Their performance was investigated by considering the four properties. In this study, we found La0.75Sr0.25Cr0.5Mn0.5Oδ-3 (LSCM) to be a promising SOEC fuel electrode. The LSCM electrode showed comparably good catalytic performance and electrical conductivity. A 1,200 h co-electrolysis test showed that the electrode had a stable performance.

Energy storage and syngas production by switching cathode gas in nickel-yttria stabilized zirconia supported solid oxide cell

Energy storage is needed for the renewable energy power, and it is highly promising to convert CO2 to fuels utilizing the surplus renewable energy. Solid oxide cell (SOC) is an efficient energy conversation device, and it can realize energy storage by reversible operation. However, the CO2 electrolysis process is limited by the intensive energy input and low efficiency. To improve the electrolysis efficiency and realize syngas production, in this work, nickel-yttria stabilized zirconia (Ni/YSZ) supported solid oxide cell has been used for CH4 assisted CO2 electrolysis (CH4-SOEC). Moreover, a novel energy storage concept by cycling operating the cell in CH4-SOFC mode (discharge) and CH4-SOEC mode (charge) has been proposed, called CH4 assisted solid oxide cell (CH4-SOC). Besides energy storage, the CH4-SOC technology can realize syngas production as well. In the experiments, CH4 is continuously supplied to the Gd-doped ceria (GDC) modified Ni/YSZ anode, while air/CO2 are supplied to the YSZ-La0.6Sr0.4FeO3−δ (LSF)/GDC cathode in discharge/charge cycles, respectively. Experiments demonstrate that the cell presents well electrochemical performance and durability for 400 h under different operation modes. Using distribution of relaxation times (DRT) method, the rate limiting steps of the fundamental electrochemical processes in the electrodes have been identified. Moreover, thermodynamic calculations have been performed to evaluate the cell efficiency. Results suggest that the cell could realize an energy conversion efficiency of 84% in case of fuel utilization of 0.9, where the ratio of the discharge electric energy to the charge electric energy is about 0.95. Both experiments and calculations demonstrate the feasibility of technology, and using a conventional SOC is much important for the real application of it.

Ag monolayer doped by Pt atom on WC (0001) surface: A good catalyst for H2 dissociation with high sulfur tolerance

H2 dissociation barriers on Ag (111), Ag monolayer on WC (0001) (AgML/WC), Ag monolayer doped by Pt atom on WC (0001) (AgMLPt-d/WC) surface are decreasing from 1.07 to 0.03 eV. The ab initio atomistic thermodynamic data shows that at typical planar SOFC operating temperatures of 923–1073 K, under 100000-ppm pH2S/pH2, AgMLPt-d/WC can be sulfur free clean surface. Therefore, compared with traditional Nickel/Yttria-stabilized zirconia (Ni/YSZ) Solid Oxide Fuel Cells (SOFCs) anode, AgMLPt-d/WC shows high activity towards H2 dissociation and high tolerance of sulfur poisoning.

Review: Enhancement of composite anode materials for low-temperature solid oxide fuels

Solid oxide fuel cell (SOFC) technology is attractive for its high-energy efficiency and expanded fuel flexibility. It is also more environmentally benign than conventional power generation systems. Recently, increasing attention has been paid to intermediate-to-low-temperature solid oxide fuel cells, which operating at 400–800 °C. Reducing its operating temperature can render SOFC more competitive with other types of fuel cells and portable energy storage system (EES) over a range of applications (eg: transportation, portable, stationary) and more conducive for commercialization. The high-performance composite anode requirements for low operating temperature (400–600 °C) demand microstructural and chemical stability, high electronic conductivity, and good electrochemical performance. The current high-temperature anode, Ni-YSZ (nickel-yttria stabilized zirconia) is generally reported with high interfacial resistance at reduced temperatures. This review highlights several potential composite anode materials (Ni-based and Ni-free) that have been developed for low-temperature SOFCs within the past 10 years. This literature survey shows that most of these anodes still exhibit relatively high polarization resistance. Focus is also given on reducing polarization resistance to maintain the cell power density. In literature, common approaches that have been adopted to enhance the performance of anodes are (i) selecting high-performance electrolyte, (ii) exploiting nanopowder properties, and (iii) adding noble metals as electrocatalysts.

Effect of anodic current density on the spreading of infiltrated nickel nanoparticles in nickel-yttria stabilized zirconia cermet anodes

Nickel – yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells were infiltrated with nickel nanoparticles with the objective of improving its performance by increasing the density of triple phase boundaries (TPBs). However, the nanoparticles deposited on the YSZ grains are not connected to one another, so the additional TPBs are not electrochemically active. It is postulated that at highly humid conditions (>95% H2O) created by high current density, nickel nanoparticles near the anode-electrolyte interface will spread and percolate, becoming electrochemically active. To study this behavior, infiltrated nickel nanoparticles were exposed to low, high and extreme humidity conditions during testing. When exposed to low humidity, no performance improvement or changes to the nanoparticle morphology was observed. At high humidity, the cell showed performance improvement and when cooled they showed nanoparticle coarsening. Exposure to extreme humidity (≈100%) caused performance to degrade back towards that of an un-infiltrated cell and severe nickel nanoparticles coarsening was observed in the cooled cell. This study validates the hypothesis that exposure to controlled high local humidity conditions can lead to the right amount of spreading of the deposited nickel nanoparticles to achieve percolation, thereby activating their TPBs and improving cell performance.

High activity of Au monolayer doped by Pt atom on WC (0001) surface towards H2 dissociation and high tolerance of sulfur poisoning

The Nickel/Yttria-stabilized zirconia (Ni/YSZ) Solid Oxide Fuel Cells (SOFCs) anode faces the sulfur-poisoning problem. The criteria of ideal sulfur tolerant anode materials are weak atomic sulfur adsorption, high catalytic activity toward H2 dissociation and low cost. It is found that H2 dissociation barriers on Au (111), Au monolayer on WC (0001) (AuML/WC), Au monolayer doped by Pt atom on WC (0001) (AuMLPt-d/WC) surface are decreasing from 0.98 to 0.1 eV. The ab initio atomistic thermodynamic data shows that at typical SOFC operating temperatures of 923–1073 K, for any pH2S/pH2, AuMLPt-d/WC can be sulfur free clean surface. Therefore, we proposed AuMLPt-d/WC as ideal sulfur tolerant anode materials.

Molybdenum-based double perovskites A2CrMoO6−δ (A = Ca, Sr, Ba) as anode materials for solid oxide fuel cells

The development of sulfur-tolerant anode materials is vital for solid-oxide fuel cells (SOFCs) that use conventional fossil fuels as fuel source to decrease emissions and increase energy-conversion efficiency. In this work, A2CrMoO6–δ (A = Ca, Sr, Ba) double perovskite materials are synthesized and evaluated as potential SOFC anode materials to assess their electrocatalytic activity and sulfur tolerance. All samples exhibit well-matched thermal expansion coefficient and satisfactory chemical compatibility with La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) electrolyte over the measured temperature range. Among the compositions investigated, the B-site partially disordered Sr2CrMoO6–δ (SCM) double perovskite exhibits superior electrical conductivity, cycle stability, and polarization resistance on LSGM electrolyte because of its high-symmetry structure and appropriate oxygen-vacancy concentration. The relatively stable single-cell performance in 100 ppm H2S/H2 demonstrates that the behavior of SCM anode is more stable than that of nickel–yttria-stabilized zirconia. Postexposure characterization by X-ray diffraction is further conducted to evaluate SCM as potential sulfur-tolerant SOFC anode.

Differential Electrochemical Mass Spectrometry Coupled with Linear and Non-Linear Electrochemical Impedance Spectroscopy of Gadolinia-Doped Ceria: CO2 and H2o Co-Electrolysis

Recent advances in carbon capture create new opportunities for recycling CO2 into liquid fuels to store intermittent electrical energy1. One option is to co-electrolyze CO2 and H2O at high temperature using solid oxide electrolysis cells (SOECs). Although promising, the factors controlling rates of CO2 and H2O reduction in SOECs are not well understood, hampering development2,3. Traditional electrochemical techniques have difficulty resolving the mechanisms and kinetic parameters of the individual steps governing CO2 and H2O reduction. This limitation can potentially be overcome using linear and non-linear electrochemical impedance spectroscopy (EIS and NLEIS) in conjunction with dynamic measurement of gas-phase species using differential electrochemical mass spectrometry (DEMS). Regarding co-electrolysis, mixed ionic electronic conductors (MIEC) have gained interest as alternatives to nickel-yttria stabilized zirconia (Ni-YSZ) because the active region is not limited to the triple phase boundary and they are more stable in reducing environments3. The MIEC studied here, gadolinia-doped ceria (GDC), has been well characterized as an electrolyte3 and is a promising cathode in SOECs. Unlike Ni-YSZ, it does not coke in carbon environments or oxidize completely in a feed of water and CO2. We have performed NLEIS and EIS measurements of co-electrolysis on button cells composed of GDC as the working electrode with electrolyte YSZ under various temperatures and gas compositions. Gas atmospheres surrounding the cells contain mixtures of water, CO2, CO, H2, and carrier gas such as Ar or N2, used to manipulate the oxygen partial pressure of the system and thus the oxygen vacancy concentration of the surface and bulk electrode. The data we have collected on co-electrolysis of CO2 and water on GDC have been compared with model NLEIS spectra developed to include CO­2 reduction, water reduction, and reverse water gas shift reaction mechanisms and rate determining steps. A rough vacuum and low volume reactor was developed to cross-check the proposed mechanisms through measuring a gas phase response in DEMS and a micro-kinetic model was developed to predict product distributions corresponding to the mechanism that most closely matched the results.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides.

In recent years, the lowering of the operation temperature of solid oxide fuel cells (SOFCs) has attracted much attention owing to the trade-off between the best performance and the life span of SOFCs. For this challenge, new active sites on the Ni surfaces in a Nickel-Yttria-Stabilized Zirconia (Ni-YSZ) cermet anode of SOFCs have been created by deposition of trace amounts of platinum oxide (PtOx ) followed by an activation step of the anode at 1073 K in a hydrogen flow. The internal resistance (IR) free value (185 mA cm-2 at 0.8 V) observed for the single cell with an anode sputtered with a trace amount of PtOx (Pt content in anode: from 9 to 91 ppm) at 973 K is conspicuously higher than that of a similar single cell with a nonsputtered cermet anode (85 mA cm-2 ) at 0.8 V and 1073 K. Transmission electron microscopy microanalysis shows that the defect structure is formed on a partially oxidized Ni surface by active Pt species. Also, surface atomistic simulation on NiO (111) predicts the formation of Frenkel defect clusters with Pt cations, which partially cover the Ni surface. The formation of Frenkel defect clusters on the partially oxidized Ni surface (i.e., creation of new active sites for formation of water molecules) promotes the anode reaction, resulting in improvements in the anode performance of SOFC single cells at 973 K. Design of the aforementioned new active sites on Ni through sputtering of trace amounts of PtOx provides a great opportunity for "radical innovation" in the design of intermediate-temperature SOFCs.

Improving sulfur tolerance of Ni-YSZ anodes of solid oxide fuel cells by optimization of microstructure and operating conditions

In this study, we propose an improvement in the sulfur tolerance of nickel-yttria stabilized zirconia (Ni-YSZ) anodes for solid oxide fuel cells (SOFCs) by simultaneously employing optimized operating conditions and microstructural modifications. An electrolyte supported SOFC is operated at 20 ppm H2S impurity at 750 °C for 20 h degradation and 10 h recovery test. The current cycles with a higher amplitude and small pulse time during the constant current operation are beneficial for the mitigation of sulfur poisoning. The effect of humidity on the sulfur degradation of Ni-YSZ anode is also studied. The synergetic effect of microstructure modification and current cycling conditions improves the sulfur tolerance of Ni-YSZ anode. It has been found that, when an anode with a modified microstructure by infiltrated CeO2 and Yb2O3 nanoparticles is operated on 20 ppm H2S poisoned gas at 10% relative humidity and the optimum pulsed current cycling conditions, about 7 times less degradation of the SOFC performance is observed. This study shows that at lower H2S concentration, a stable operation of a SOFC with minimum degradation can be achieved with the combination of optimization of operating conditions and modification of the anode microstructure.

Effect of nano-Al2O3 addition on mechanical durability of nickel-yttria stabilized zirconia anode support of solid oxide fuel cells

For anode supported solid oxide fuel cells (SOFCs), the strength and durability of the porous cermet support during long-term operation is critical for the reliability of the cells and stacks. The effect of adding nano-Al2O3 on the long-term durability of the mechanical strength of nickel-yttria stabilized zirconia (Ni-YSZ) based SOFC anode supports was investigated in this study. The SOFC anode support materials, Ni-3YSZ (3 mol% YSZ), Ni-8YSZ (8 mol% YSZ), and nano-Al2O3 added Ni-YSZ composites, were subjected to a long-term degradation test at 900 °C and 30% humidity in an H2 environment and after 1000 h exposure, the flexural strength, phase, and microstructural changes were analyzed. The results suggest that the addition of nano-Al2O3 to Ni-3YSZ improves the long-term strength and durability of the cermet while its addition to Ni-8YSZ cermet did not show any significant changes in the strength degradation behavior of the Ni-8YSZ based anode support. Further analysis by SEM and XRD shows that the tetragonal to monoclinic phase transformation in partially stabilized zirconia was suppressed due to the addition of nano-Al2O3 in Ni-3YSZ resulting in improved long-term stability and higher mechanical strength. Replacing the Ni-8YSZ anode support with nano-Al2O3 added Ni-3YSZ might significantly reduce the material cost of SOFCs without compromising long-term strength durability, thus bringing SOFCs a step closer to commercialization.

Three-dimensional thermal stress analysis of the re-oxidized Ni-YSZ anode functional layer in solid oxide fuel cells

Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is widely used as an anode material in solid oxide fuel cells (SOFCs); however, Ni re-oxidation causes critical problems due to volume expansion, which causes high thermal stress. We fabricated a Ni-YSZ anode functional layer (AFL), which is an essential component in high-performance SOFCs, and re-oxidized it to investigate the related three-dimensional (3D) microstructural and thermo-mechanical effects. A 3D model of the re-oxidized AFL was generated using focused ion beam-scanning electron microscope (FIB-SEM) tomography. Re-oxidation of the Ni phase caused significant volumetric expansion, which was confirmed via image analysis and calculation of the volume fraction, connectivity, and two-phase boundary density. Finite element analysis (FEA) with simulated heating to 500–900 °C confirmed that the thermal stress in re-oxidized Ni-YSZ is concentrated at the boundaries between YSZ and re-oxidized NiO (nickel oxide). NiO is subjected to more stress than YSZ. Stress exceeding the fracture stress of 8 mol% YSZ appears primarily at 800 °C or higher. The stress is also more severe near the electrolyte-anode boundary than in the Ni-YSZ cermet and the YSZ regions. This may be responsible for the electrolyte membrane delamination and fracture that are observed during high-temperature operation.

Differential Electrochemical Mass Spectrometry Coupled with Linear and Non-Linear Electrochemical Impedance Spectroscopy of Gadolinia-Doped Ceria: Deconvolution of CO2 and H2o Co-Electrolysis

Recent advances in carbon capture create new opportunities for recycling CO2 into liquid fuels to store intermittent electrical energy1. One option is to co-electrolyze CO2 and H2O at high temperature using solid oxide electrolysis cells (SOECs). Although promising, the factors controlling rates of CO2 and H2O reduction in SOECs are not well understood, hampering development2,3. Traditional electrochemical techniques have difficulty resolving the mechanisms and kinetic parameters of the individual steps governing CO2 and H2O reduction. This limitation can potentially be overcome using linear and non-linear electrochemical impedance spectroscopy (EIS and NLEIS) in conjunction with dynamic measurement of gas-phase species using differential electrochemical mass spectrometry (DEMS). Regarding co-electrolysis, mixed ionic electronic conductors (MIEC) have gained interest as alternatives to nickel-yttria stabilized zirconia (Ni-YSZ) because the active region is not limited to the triple phase boundary and they are more stable in reducing environments3. The MIEC studied here, gadolinia-doped ceria (GDC), has been well characterized as an electrolyte3 and is a promising cathode in SOECs. Unlike Ni-YSZ, it does not coke in carbon environments or oxidize completely in a feed of water and CO2. We have performed NLEIS and EIS measurements of CO2 reduction, water reduction, and co-electrolysis on button cells composed of GDC as the working electrode with electrolyte YSZ under various temperatures and gas compositions. Gas atmospheres surrounding the cells contain mixtures of water, CO2, CO, H2, and carrier gas such as Ar or N2, used to manipulate the oxygen partial pressure of the system and thus the oxygen vacancy concentration of the surface and bulk electrode. The data we have collected on CO2 and water reduction on GDC have been compared with model NLEIS spectra developed in our previous work3,4 in the context of a reaction mechanism and a rate determining step. Additionally, these mechanisms were cross-checked through measuring a gas phase response in DEMS and a micro-kinetic model was developed to predict product distributions corresponding to the mechanism that most closely matched the results. References Graves, C., Ebbesen, S. D., Mogensen, M., & Lackner, K. S. (2011). Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews, 15(1), 1–23. https://doi.org/10.1016/j.rser.2010.07.014 Graves, C., Chatzichristodoulou, C., & Mogensen, M. B. (2015). Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes. Faraday Discussions, 182(0), 75–95. https://doi.org/10.1039/C5FD00048C Valdés-Espinosa, H., Stuve, E. M., & Adler, S. B. (2015). Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes. ECS Transactions, 66(2), 229–251. https://doi.org/10.1149/06602.0229ecst Witt, J., Stuve, E.M., & Adler, S.B. (2017). Modeling CO2 Electrolysis on Gadolinia Doped Ceria Porous Electrode using a 1-D Macro-Homogeneous Model and Linear and Non-Linear Electrochemical Impedance Spectroscopy. ECS Transactions (accepted and awaiting publication).

Three dimensional characterization of nickel coarsening in solid oxide cells via ex-situ ptychographic nano-tomography

Nickel coarsening is considered a significant cause of solid oxide cell (SOC) performance degradation. Therefore, understanding the morphological changes in the nickel-yttria stabilized zirconia (Ni-YSZ) fuel electrode is crucial for the wide spread usage of SOC technology. This paper reports a study of the initial 3D microstructure evolution of a SOC analyzed in the pristine state and after 3 and 8 h of annealing at 850 °C, in dry hydrogen. The analysis of the evolution of the same location of the electrode shows a substantial change of the nickel and pore network during the first 3 h of treatment, while only negligible changes are observed after 8 h. The nickel coarsening results in loss of connectivity in the nickel network, reduced nickel specific surface area and decreased total triple phase boundary density. For the condition of this experiment, nickel coarsening is shown to be predominantly curvature driven, and changes in the electrode microstructure parameters are discussed in terms of local microstructural evolution.