Ni Yttria-stabilized Zirconia Research Articles

Mechanistic Insights on Direct Ammonia Solid Fuel Cell Using Ni-YSZ Anode: Experimental and Density Functional Theory Studies

While Ni-Yttria-stabilized zirconia (Ni-YSZ) cermet exhibits a great potential to realize high-efficiency ammonia to power using direct ammonia solid oxide fuel cell (DA-SOFC), the mechanisms of NH3 cracking and H2 oxidation reactions as well as direct electro-oxidation of ammonia are still unclear. In this context, with the motivation, the DA-SOFC performance of fuel electrode (Ni-YSZ) using conventional anode-supported cell (Ni-YSZ|YSZ|GDC-LSCF) is studied through microscale observation coupled with density functional theory (DFT) calculations to unravel the reaction mechanisms on Ni-YSZ(111) anode. The performance of DA-SOFC is examined using NH3, H2 and N2/H2 mixture as fuel at the temperature range of 700-800℃. In addition to XRD and SEM-EDX characterizations, the electrochemical characterization is performed using the impedance spectroscopy (EIS) measurements at three different temperatures of 700, 750 and 800 °C. Interestingly, the experimentally observed trend in Open-circuit voltage (OCV) implies that direct electro-oxidation of ammonia is the dominant reaction mechanism. Notably, polarization resistance is primarily contributed by ammonia anodic reaction and secondarily by the gas diffusion impedance. DFT results revealed that ammonia adsorbs favorably on the Ni cluster, and hydrogen follows a dissociative mode of adsorption on the Ni cluster. The activation barriers of elementary steps involved in the proposed mechanisms for fuel cracking and oxidation reactions are computed. This work is anticipated to provide valuable insights into the rational design of Ni-YSZ-based fuel electrodes. Keywords: ammonia, hydrogen, SOFC, fuel cells, DFT, reaction mechanism

Cu infiltrated Ni-YSZ cathode in CO2 (+H2) stream: Reverse water gas shift vs. CO2 electrolysis

Ni-Yttria Stabilized Zirconia (Ni-YSZ) cermet electrode is known to perform in CO2 (+H2) stream. Introducing H2 in CO2 containing streams enables thermochemical reverse water gas shift reaction (rWGS: CO2 + H2 → CO + H2O) at open circuit. Without the application of any bias, the rWGS responds positively to an increase in temperature and concentrations of CO2 and H2. Application of bias results in enhancement in CO yield above the rWGS baseline value. With bias, both CO2 and H2O electrolysis are enabled. The infiltration of Cu on the Ni-YSZ backbone results in significant improvement of the reaction kinetics and increases H2 and CO production. Impedance analysis indicates that the kinetic limitation originates from reaction steps with slower time constants with Ni{Cu}x-YSZ outperforming Ni-YSZ in this aspect. Cu infiltration suppresses particle coarsening typically observed in Ni-YSZ.

In situ exsolution nanophase decorated perovskite cathode for solid oxide electrolysis cells with efficient CO2 electrolysis performance

CO2 electrolysis using solid oxide electrolysis cells is a promising technology for CO2 utilization and conversion, which has attracted more and more attention in recent years because of its extremely high efficiency. However, traditional Ni-yttria-stabilized zirconia (Ni-YSZ) or Ni–Gd0.1Ce0.9O2−δ (Ni-GDC) metal-ceramic cathode faces many problems such as Ni agglomeration and carbon deposition during long-time operation. Herein, a perovskite oxide La0.43−xCa0.37Ti0.9Ni0.1O3−δ (LCTN, x = 0, 0.05, 0.1) with nanophase-LaVO4 exsolution was investigated as the novel cathode of solid oxide electrolysis cell (SOEC) for efficient CO2 electrolysis. The results confirm that the exsolution nanophase on LCTN surface can significantly improve the CO2 adsorption and conversion performance. For CO2 electrolysis at 1.8 V, an electrolysis current density of 1.24 A/cm2 at 800 °C can be obtained on SOEC with La0.43−xCa0.37Ti0.9Ni0.1O3−δ decorated with LaVO4 (LCTN-V0.05) cathode. Furthermore, the corresponding cell can maintain stable operation up to 100 h without apparent performance degradation. These results demonstrate that doping-induced second nanophase exsolution is a promising way to design high-performance SOEC cathodes for CO2 electrolysis.

Performance evolution analysis of solid oxide electrolysis cells operating at high current densities

Hydrogen generation by water electrolysis using solid oxide electrolysis cells (SOECs) is highly promising because of the favorable thermodynamics and kinetics. Commercial applications require SOEC operating at high current densities (ǀiǀ≥1 A·cm−2) to achieve substantial hydrogen production rates. This study demonstrates the operation of a full-size Ni–yttria-stabilized zirconia (Ni–YSZ) cell with an effective area of 16 cm2 at −2 A·cm−2 for 336 h, illustrating the feasibility of operating SOECs at a high current density. The electrochemical characteristics of the SOEC evolved during constant current electrolysis, exhibiting an activation stage with a degradation rate (DR) of 54.6 mV/100 h, followed by a rapid decline process(DR: 180.9 mV/100 h), a gentle decline period (DR: 105.5 mV/100 h), and a stable stage (DR: 11.0 mV/100 h). The contributions of individual processes to cell degradation during each process are identified using the distribution of relaxation times (DRT) and subsequent equivalent circuit model (ECM) fitting. The results suggest that the Ohmic resistance, ionic transport and charge-transfer reaction in the Ni-YSZ fuel electrode contribute the most to performance loss. Ni redistribution is regarded as the dominant degradation mechanism, as verified by detailed post-test characterization.

(Invited) Forming Multifunctional Ceramics with Different ZrO2 Polymorphs: Mixed Ion-Electron Conducting Properties of Zirconium Titanium Oxides

Infiltrating porous electrodes with solution phase precursors is a strategy employed to improve performance, catalytic activity and longevity in high temperature electrochemical devices. For example, traditional Ni yttria-stabilized zirconia (Ni-YSZ) cermet anodes used in solid oxide fuel cells (SOFCs) are susceptible to carbon accumulation and to sulfur poisoning. To counter the effects of degradation, anodes have been infiltrated with secondary materials containing Sn, Ba, Al and a host of other constituents, typically at levels of 1-5% (by mass). Processing and preparing infiltrated electrodes, however, can lead to formation of new material phases that affect surface chemistry and electrode microstructure in unexpected ways.In this work, XRD and Raman spectroscopy are used to investigate the formation of functional zirconium titanate secondary phases in Ti/Zr oxides. Different zirconia polymorphs (either pure or stabilized with small amounts of yttria (YSZ)) mechanically mixed with TiO2 and heated above 1000 ˚C in stagnant air react to form several secondary phases. For sintering temperatures > 1100 ˚C, mixtures of TiO2 and m-ZrO2 form a ZrTiO4 secondary phase while mixtures of TiO2 and 3YSZ or 8YSZ form Zr5Ti7O24. The extent of secondary phase formation is enhanced with both higher sintering temperature and yttria content. The Zr5Ti7O24 secondary phase formed from TiO2/8YSZ samples sintered at 1400 ˚C exhibits unexpected mixed ionic and electronic conductivity at 800 ˚C. These findings suggest that secondary phases intentionally integrated into electrode architectures can improve the performance of high temperature electrocatalyst systems.

Functionally Graded Infiltration Triggering Ultrahigh Electrolytic Current in Robust Reversible Solid Oxide Cells

The reversible solid oxide cell (ReSOC) offers the dual capability to store electricity as chemical fuels and convert fuels to electricity, thereby enabling carbon neutral cycling. The nickel‐cermet fuel electrode exhibits high electrocatalytic activity for redox reactions while being particularly vulnerable to carbon deposition and nickel coarsening, especially at high current densities. Herein, a novel approach involving a graded infiltration onto a dendritic Ni–YSZ (yttria‐stabilized zirconia) skeleton with Pr6O11 nanoparticles/films is presented, demonstrating remarkable electrolytic current and coke tolerance in CO–CO2 conversion. At 800 °C, the current densities achieve approximately −2.0 and +2.0 A cm−2 under 1.3 and 0.6 V, respectively, showcasing robust cycling stability switching between two operational modes within 20 cycles. Notably, the attained electrolytic current density reaches an unprecedented −3.06 A cm−2 under 1.6 V. Furthermore, the button cell runs smoothly at ultrahigh electrolytic current densities ranging from −0.5 to −3.0 A cm−2 for nearly 270 h. The mechanism of high‐performance CO2 reduction is elucidated by in/ex situ experimental characterizations and theoretical calculations. These results indicate the viability of employing Ni–YSZ‐supported ReSOC for stable renewable electricity storage at previously unattainable reaction rates, and have implications for fundamental understanding of designing robust ReSOC based on the nickel‐cermet fuel electrode.

La0.6Sr0.4MnO3-Based Fuel Electrode Materials for Solid Oxide Electrolysis Cells Operating under Steam, CO2, and Co-Electrolysis Conditions

The conventional Ni–YSZ (yttria-stabilized zirconia) fuel electrode experiences severe degradation due to Ni- agglomeration and migration away from the electrolyte. Therefore, herein, we have considered Ni free electrodes, i.e., La0.6Sr0.4MnO3-δ (LSM)-based perovskite oxides as fuel electrodes. The LSM perovskite phase transforms into a Ruddlesden–Popper LSM (RP-LSM) phase with exsolved MnOx under reducing atmospheres. The RP-LSM is mainly interesting due to its good electrical conductivity, redox stability, and acceptable electrochemical behaviour. In this work, we synthesized the LSM powder and characterized it using several methods including X-ray diffraction (XRD), thermogravimetry analyses (TGA), four-probe conductivity, and scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX). Finally, the electrolyte-supported single cells were fabricated and electrochemically characterized using AC and DC techniques under electrolysis conditions. In addition to pure LSM fuel electrodes, we have also investigated the electrochemical behaviour of LSM + YSZ (50:50) and LSM + GDC (50:50) composite fuel electrodes. The single cells containing LSM and LSM + GDC fuel electrodes show higher cell performance than LSM + YSZ. For instance, current densities of 1, 1.03, and 0.51 A·cm−2 at 1.5 V are obtained for LSM, LSM + GDC, and LSM + YSZ fuel electrodes containing single cells, respectively, with a 50% N2 and 50% H2O feed gas mixture. Moreover, the performance of the cell was also investigated under co-electrolysis with 50% CO2 and 50% H2O and under direct CO2 electrolysis conditions with 100% CO2 fuel gas.

Self-hydrating of a ceria-based catalyst enables efficient operation of solid oxide fuel cells on liquid fuels

The commercialization of solid oxide fuel cells (SOFCs) that run on liquid hydrocarbon fuels is hindered by the poor coking tolerance of the state-of-the-art anode. Among the strategies developed, modulating the reforming reaction site’s local steam/carbon ratios to enhance the coking tolerance is efficient but challenging. Here we report our rational design of a ceria-based catalyst (with a nominal composition of Ce0.95Ru0.05O2–δ, CR5O) that demonstrates remarkable tolerance to coking while maintaining excellent activity for direct utilization of liquid fuels in SOFCs. Under operating conditions, the catalyst is transformed to a partially reduced oxide frame covered with Ru nanoparticles (Ru/Ce0.95Ru0.05–xO2–δ, Ru/CR5–xO), as confirmed by experimental analyses. The Ru/CR5–xO demonstrates excellent self-hydration capability to remove the coke. When applied to the Ni-yttria-stabilized zirconia (Ni-YSZ) anode of an SOFC with liquid fuels, the catalyst enables excellent performance, achieving a peak power density of 1.010 W cm−2 without coking for ∼200 h operation (on methanol) at 750 °C. Furthermore, density functional theory calculations reveal that the high activity and coking tolerance of the Ru/CR5–xO catalyst-coated Ni-YSZ anode is attributed to the reduced energy barrier for the rate-limiting step and the formation of a COH intermediate for rapid carbon removal.

Effects of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have emerged and attracted more and more attentions due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. Two major types of microstructure evolution, nickel (Ni) migration and Ni coarsening in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have been widely reported, which have strong impacts on both cell performance and durability. Therefore, materials-driven research has focused on developing more robust fuel electrodes with good electrocatalytic ability. Utilization of Ni alloys has immersed as a promising concept to enhance the performance and robustness of conventional Ni/YSZ. In the present study, Ni-M (M = Fe, Cu) bimetallic fuel electrodes are applied to investigate the effects of transition metal elements on the morphological evolutions under SOFC and SOEC operations. Ni-Fe, Ni-Cu and pure Ni patterned fuel electrodes are sputtered on YSZ pellets. The electrochemical performance of these Ni-M bimetallic fuel electrodes decreases compared with pure Ni fuel electrode, while the performance degradation rate of Ni-Fe fuel electrodes is smaller than others. Besides, the spreading of Ni film on YSZ surface is observed for all samples under anodic polarization and such Ni migration is suppressed by Fe doping, whereas enhanced by Cu doping. On the other hand, the adhesion is weakened at the electrode / electrolyte interface for Ni-Cu and pure Ni fuel electrodes under cathodic polarization, while good adhesion at the interface is maintained for Ni-Fe, which correlates with the smaller performance degradation rate.

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are a promising electrochemical technology able to work both as energy storage devices in solid oxide electrolysis (SOEC) mode, and as power generators in solid oxide fuel cell (SOFC) mode. rSOCs implemented for CO2 electroreduction into valuable CO-rich steams through electrolysis would provide opportunities for CO2 utilization, and hence become useful tools for the reduction of greenhouse gases emissions. High-temperature co-electrolysis of CO2/H2O mixtures has recently become of interest since high conversion and energy efficiency are thermodynamically favored in addition to the reduction in cell area-specific resistance when compared to pure CO2 electrolysis [1]. However, reversible operation with CO2-containing feeds requires developing flexible, high-performing, and long-lasting materials for the rSOCsManaging issues such as coking and low redox tolerance in the state-of-the-art (SoA) Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in the rSOCs is crucial. Ni-YSZ cermet exhibits high electronic conductivity and electrochemical activity in hydrogen. Nonetheless, Ni is prone to oxidation due to operation under high steam concentrations, and rapid variations in the fuel supply. Additionally, Ni is well-known for its coke-formation tendency which becomes detrimental in CO2-rich mixtures for electrolysis [2]. Thus, ceramics with mixed ionic and electronic conductivity (MIEC) have emerged as alternative fuel electrodes thanks to the larger electrochemically active area compared to standard cermets. Among the MIEC electrodes, the SrTi0.3Fe0.7O3 (STF) perovskite has proven to give low polarization resistances especially when modification techniques such as exsolution of metal nanoparticles are employed [3]. This work focuses on the investigation of the electrocatalytic properties of exsolution-based STF electrodes operating with CO/CO2 and H2O/CO2 mixtures. The mixtures used were 50% CO2/50% CO, 25% H2O/25% CO2/25%H2/25%CO, and 45% H2O/45% CO2/10% H2. A comparison between the performance and microstructure of the studied electrode formulation is provided.Electrolyte-supported cells were manufactured via screen-printing of the electrode inks on scandia-stabilized zirconia (ScSZ) electrolytes. STF perovskite-based electrodes were used, namely: SrTi0.3Fe0.7O3 (STF), Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni), and Sr0.95(Ti0.3Fe0.63Ru0.07)O3 (STF-Ru). The latter two formulations allowed the enhancement of the perovskite structure via exsolution of catalytic nanoparticles (Ni, Co alloyed with Fe) on the oxide surface during cell operation. Each cell configuration included a gadolinium-doped ceria (GDC) buffer layer coupled with an LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O1.95) oxygen electrode. Baseline performances of cell mounting the Ni-YSZ cermet electrode were also evaluated for comparison with the perovskite alternatives, both in the short and long term. Electrochemical characterization was done via impedance spectroscopy, polarization experiments, and durability tests in potentiostatic mode. Chemical characterization was performed via X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction analyses.Compared to Ni-YSZ, significant improvement in terms of reversibility and maximum current densities of the current-voltage (I/V) curves in both SOFC and SOEC modes is found on the STF-based cells at 750°C under CO/CO2 mixtures (Figure 1B). The initial difference in the performance of the STF cells with respect to the Ni-YSZ cermet in humidified H2 (SOFC mode, Figure 1A) was overcome by using the exsolved electrode formulations. Aging experiments up to 100 h performed in alternating operation modes from SOFC (6 h) to SOEC (6 h) indicated an initial performance degradation within the first 24 h for both the cermet and perovskite-based fuel electrodes which later stabilized in time. Compared to the undoped STF formulation, improvement in performance stability was observed for the exsolved STF-Ni and STF-Ru fuel electrodes while working under the target CO/CO2 mixture. This was attributed to the combination of the STF structure benefits (low polarization resistance towards CO/CO2 and H2O/CO2 mixtures) with the boost that metallic nanoparticles provide to the electrode’s heterogeneous catalytic reactivity and overall stability over time.

Sintering of Protonic Ceramics for Tubular Solid Oxide Fuel Cells

Protonic ceramics are considered to be the most promising candidates for intermediate-temperature solid oxide fuel cells (SOFCs) owing to their high proton conductivity and competitive electrochemical performance at intermediate temperatures (typically 500–700 °C). However, the fabrication of proton-conducting SOFCs has been challenging mainly due to the poor sinterability of common proton-conducting electrolyte materials. Significantly high sintering temperatures (1500–1600 °C) and long sintering times (up to 24 h) have been reported for obtaining dense, gas-tight proton-conducting electrolytes. These sintering conditions may lead to undesirable interfacial reactions and dense electrode microstructures, thereby limiting the cell performance. The present work focuses on optimizing the co-sintering temperature of tubular SOFCs with a traditional Ni–yttria-stabilized zirconia (YSZ) anode support and proton-conducting functional layers. BaZr0.3Ce0.5Y0.2O3 (BZCY), Ni–BZCY, and BZCY–BaCo0.4Fe0.4Ce0.1Y0.1O3 were used as the electrolyte, anode functional layer, and cathode, respectively. Various approaches to lowering the co-sintering temperature and resulting cell performances will be discussed. We will also examine the thermal cycling behavior and durability of the fabricated protonic SOFCs.

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are promising electrochemical devices, able to work both in energy storage mode as solid oxide electrolyzes (SOECs), and in power generation mode as solid oxide fuel cells (SOFCs). rSOCs dedicated to high-temperature co-electrolysis of CO2/H2O mixtures are of interest since they could foster the utilization and consequent reduction of CO2 gas emissions while converting it and H2O into valuable fuels (i.e. syngas and H2). However, operation under CO2-rich feeds requires alternatives to the state-of-the-art Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in view of its low coking and redox resistance. Perovskites with mixed ionic and electronic conductivity are major alternatives, especially when the functionality of their active surface is enhanced via metal nanoparticle exsolution. This work focuses on the investigation of the electrocatalytic properties of the SrTi0.3Fe0.7O3 (STF), and exsolution Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni) perovskite-based fuel electrodes together with Ni-YSZ while operating with CO2-rich mixtures both in the short and long term.

Understanding thermal and redox cycling behaviors of flat-tube solid oxide fuel cells

The performance stability of solid oxide fuel cells (SOFCs) under thermal and redox cycles is vital for large-scale applications. In this work, we investigated the effects of thermal and redox cycles on cell performances of flat-tube Ni/yttria-stabilized zirconia (Ni/YSZ) anode-supported SOFCs. Cell performance was considerably affected by the duration of oxidation during redox cycles and the heating rate during the thermal cycles. The cell tolerated 20 short-term redox cycles (5 min oxidation) without significant performance degradation. Besides, the cell exhibited superior stability during 8 thermal cycles with a slow heating rate (4 °C min−1) to that with a fast heating rate (8 °C min−1). These results reflected that the thick anode support (2.7 mm) offered strong resistance to the shocks caused by redox and thermal cycling. Moreover, the morphological changes of the Ni phase during the redox and thermal cycling were investigated using Ni-film anode cells. Agglomeration of Ni particles and dissociation between the Ni film and the YSZ substrate were confirmed after 5 redox cycles, whereas no significant changes in Ni film emerged after 8 thermal cycles.

Study of Ni-YSZ Electrode Capacitance and Its Correlation with the Microstructure

Herein, electrochemical measurements are performed on an Ni-dense electrode in a yttria-stabilized zirconia (YSZ) electrolyte to determine the correlation between the measured capacitance ) and double-phase boundary (DBP) of Ni–YSZ. The Ni-dense electrode is prepared via pulsed-laser deposition by varying the electrode diameter (50–1000 μm), thus revealing that the value is proportional to the DPB area. This implies that the capacitance of Ni–YSZ can be used to observe the change in microstructure during operation. This result is confirmed by electrochemical measurements of the Ni–YSZ porous electrode. The impedance spectra of the Ni–YSZ porous electrode from electrochemical measurements are analyzed using a 1D transmission-line model, where the microstructure parameters are determined using scanning electron microscopy. The analysis reveals that the electrode resistance is correlated with the triple-phase boundary length as a function of r p ∝ and the electrode capacitance is linearly correlated with the DPB as a function of c p ∝ of Ni–YSZ. Thus, the results from the dense and porous electrodes are used to understand the degradation mode in Ni–YSZ (owing to microstructural changes), which are determined by their capacitance.

Highly conductive hematite electrolyte for low‐temperature solid oxide fuel cell

Hematite (HEM) is a kind of cheap natural mineral with ion transport ability. In this paper, some basic properties of HEM were studied to determine its potential as an electrolyte for low-temperature solid oxide fuel cell (SOFC). A composite (HEM-GDC) formed by Gd0.1Ce0.9O2-δ (GDC) and HEM was developed to improve the performance of the HEM. The result of the X-ray fluorescence spectrometer confirms that the main components of HEM are Fe2O3 and SiO2. The X-ray diffraction (XRD) results show that the chemical compatibilities of HEM and HEM-GDC are good with La2NiO4 + δ (LNO) and NiO-yttria-stabilizedzirconia (NiO-YSZ). Moreover, the thermal expansion coefficients (TECs) of HEM and HEM-GDC are 11.83 × 10−6 K−1 and 12.1 × 10−6 K−1, permitting that they have good physical compatibility with Ni-yttria-stabilized zirconia (Ni-YSZ) and LNO. According to the results of the differential scanning calorimetry with thermal gravimetric analysis (DSC-TGA), HEM and HEM-GDC are stable at high temperatures. The ionic conductivities of HEM and HEM-GDC are 1.07 × 10−2 S·cm−1 and 7.23 × 10−2 S·cm−1 at 450°C, which meet the requirement of the low-temperature SOFC electrolytes. In addition, electrochemical impedance spectra (EIS) tests of GDC, HEM, and HEM-GDC half-cell with LNO as cathode show that HEM-GDC performs best.

In-Operando Observations of Ni-YSZ Patterned Fuel Electrodes Under SOFC and SOEC Operations

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have being attracting more and more attention due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. In particular, migration and coarsening of nickel (Ni) in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have strong impacts on the decreases in both active three-phase boundary (TPB) and Ni network connectivity, which result in performance deterioration. One of the mechanisms proposed as an explanation of Ni migration is Ni transport via volatile Ni(OH)x species, so-called evaporation-condensation mechanism [1]. On the other hand, Jiao and Shikazono [2] proposed a model to explain Ni spreading on YSZ surface based on the variation of Ni-YSZ wettability which is modified by the oxygen adsorption. Nevertheless, further investigations are required to understand the mechanisms of Ni migration. In the present study, Ni-film patterned electrodes which can provide well defined TPB configurations are used to investigate the Ni-YSZ degradation under SOFC, SOEC and sequential SOFC and SOEC operations, i.e. reversible operation (RSOC). The edge of sputtered Ni-film contacting with open YSZ surface can be treated as active TPB during operation. Confocal laser scanning microscopy (CLSM, Keyence, VK-X 1000) is used, which enabled in-operando observation of the real-time local morphological change at active TPB [3]. The in-operando potentiostatic experiment was carried out inside a high temperature chamber which was kept at 800oC with 4% H2O-4% H2-92% N2 as fuel gas and 30% O2-70% N2 as oxidant as shown in Fig. 1(a). The correlations between the Ni morphological changes and cells’ electrochemical performances were investigated. The Ni phase dynamically spread and de-percolated only under the SOFC operation, while such a phenomenon was not observed during the SOEC operation, as shown in Figs. 1(c), (d) and (e). Ni coarsening occurred regardless of the operation modes. The competition between Ni-film spreading and breaking-up finally determined the complicated local morphological change and corresponding cell performance variation. The time variations of active TPB and current density showed good correlation as shown in Fig. 1(b). Real-time in-operando observations revealed different Ni migration under SOFC, SOEC and RSOC operations, which may provide new insights for the understanding of Ni-YSZ fuel electrode degradation.Fig. 1. (a) Schematic illustration of in-operando setup. (b) Correlation between current density and Ni morphological evolution. (c)-(e) CLSM images and 3D surface height contour maps of the yellow-frame-enclosed areas after SOFC, SOEC and RSOC modes, respectively.

Exploring the Role of Humidity, Temperature, and Mixed Ionic and Electronic Conductivity on SOFC Anode Electrocatalysis

Infiltration of nanoscale electrocatalysts into Ni/yttria-stabilized zirconia (Ni-YSZ) cermets has been shown to improve the electrochemical performance of solid oxide fuel cell (SOFC) anodes. While infiltrated electrodes in SOFCs result in improved cell performance, long-term operation leads to coarsening of the infiltrated nanoparticles and negates the short-term performance improvements. This study explores the roles of humidity, temperature, and number of cycles of nanocatalyst infiltration in improving mixed conduction within the Ni-YSZ electrode. Two mixed conduction phases were studied: Gd0.1Ce0.9O2-δ (GDC) as an infiltrant into Ni-YSZ electrodes, and Ni/transition metal doped-YSZ electrodes infiltrated with Ni. Analysis of impedance data from these cells shows improved electrochemical performance in infiltrated cells with mixed conduction compared with infiltrated cells containing purely ionic and electronic conducting phases. Improved anode performance is attributed to the availability of electronic pathways through predominantly ionic-conducting phases to connect distant Ni nanoparticles and/or Ni grains.

Remarkably Improved Durability of Ni–Co Dispersed Samaria-Doped Ceria Hydrogen Electrodes by Reversible Cycling Operation of Solid Oxide Cells

We have examined the durability of a double-layer hydrogen electrode, consisting of a samaria-doped ceria (SDC) scaffold with highly dispersed Ni–Co nanoparticles as the catalyst layer and a thin current collecting layer of Ni–yttria-stabilized zirconia (YSZ) cermet for a reversible solid oxide cell (R-SOC). When steam electrolysis was performed continuously (solid oxide electrolysis cell, SOEC) at 800 °C, a rapid, large increase in the ohmic resistance of the hydrogen electrode side was observed. In contrast, the durability of the hydrogen electrode was found to be improved remarkably by reversible cycling operation between SOEC and solid oxide fuel cell (SOFC) modes, i.e., virtually no degradation over 1200 h. This could be ascribed to a stabilization of the microstructure of the hydrogen electrode. It was also found that the durability of the oxygen electrode, which was based on a composite of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and SDC with an SDC interlayer, was also improved by the reversible cycling operation, compared with a slight degradation in the continuous SOEC operation.

Cellular Nickel-Yttria/Zirconia (Ni-YSZ) Cermet Foams: Manufacturing, Microstructure and Properties.

Open-celled ceramic composite foams were prepared from NiO and yttria-stabilized zirconia (YSZ) powders by the polymer sponge replication (Schwartzwalder) technique using the respective aqueous dispersions. Mechanically stable NiO–YSZ foams with an average porosity of 93 vol.% were obtained. After chemical reduction of the NiO phase with hydrogen, cellular Ni–YSZ cermet structures were obtained. They are characterized by an electric conductivity up to 19∙103 S∙m−1 which can be adjusted by both, the Ni volume fraction, and the sintering/reduction procedure. The NiO–YSZ ceramic foams, as well as the cellular Ni–YSZ cermets prepared therefrom, were characterized with respect to their microstructure by scanning electron microscopy, confocal Raman microscopy and X-ray diffraction with Rietveld analysis. In addition, the compressive strength, the electric conductivity and the thermal conductivity were determined. The collected data were then correlated to the sample microstructure and porosity and were also applied for modelling of the mechanical and electric properties of the bulk Ni–YSZ strut material.

Evolution of the Morphology Near Triple-Phase Boundaries in Ni–Yttria Stabilized Zirconia Electrodes Upon Cathodic Polarization

Abstract Microstructural changes in Ni–yttria stabilized zirconia (YSZ) near the YSZ electrolyte were examined by three-dimensional (3D) electron microscopy after electrolysis and fuel cell operation up to 10,700 h and 15,000 h, respectively. The depletion of Ni and three-phase boundaries (TPBs) close to the electrolyte was detected upon cathodic polarization. It corresponded to spatial variations of dihedral angles (θ) at TPBs and Ni surface curvature along the direction perpendicular to the electrolyte, which comport with electrowetting and Zener pinning theory on several aspects. θNi decreased by up to 6 deg next to the electrolyte after electrolysis but remained uniform after fuel cell operation. This is in line with predictions from electrowetting theory with capacitances measured by electrochemical impedance spectroscopy and distribution of relaxation times. The decrease in θNi was concurrent to transition toward concave Ni/pore interfacial shapes and lower genus of the Ni phase, which suggests the pinch-off of Ni ligaments following surface diffusion-controlled Rayleigh instability. The increase in absolute mean curvature near the electrolyte interface is a driving force for outward transport of Ni. The decrease in θYSZ further suggests that TPB lines relocate on YSZ surface features that provide higher Zener pinning force. In contrast, few localized contact losses between Ni and YSZ that can also occur under high cathodic polarization and trigger Ni depletion were detected. The results are expected to advance the understanding of the driving forces that cause Ni depletion near the electrolyte in electrolysis for the design of improved solid oxide cell electrode microstructures.