Nickel Yttria-stabilized Zirconia Research Articles

Development of plasma sprayed Ni/YSZ anodes for metal supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) offer a promising technique for producing electricity by clean energy conversion through an electrochemical reaction of fuel and air. Plasma spraying could be a potential manufacturing route for commercial SOFCs, as it provides a distinct advantage especially in case of metal supported cells (MSCs) by allowing rapid processing at relatively low processing temperatures preventing thus the degradation of the metallic substrate. The objective of this work was to develop nickel/yttria stabilised zirconia (Ni/YSZ) anodes with high porosity and homogeneous phase distribution by atmospheric plasma spraying for MSCs. Various feedstock material approaches were explored in this study, both with single injection as well as separate injection of different feedstock materials , and with and without the use of pore formers to create additional porosity. The advantages and issues with each material route were investigated and discussed. It was shown that agglomerated Ni/YSZ/polyester feedstock material resulted in the best distribution of Ni and YSZ in the anode microstructure with homogeneous porosity. Subsequently, the Ni/YSZ/polyester material route with different amounts and size distributions of polyester was chosen to develop anode symmetrical cells using a commercial zirconia sheet as support for electrochemical testing. The Ni/YSZ/polyester anode powder with 10wt.% standard size polyester exhibited the best electrochemical performance. The results show that plasma spraying of the agglomerated Ni/YSZ/polyester could be a promising route to achieve high performance and rapid production anodes without using the carcinogenic nickel oxide.

A Study on Degradation of Ni-YSZ Anode Materials in Terms of Redox Stability for Solid Oxide Fuel Cells

▪ Introduction Solid oxide fuel cells (SOFCs) have lately attracted considerable attention as the source of new energy considering environmental pollution and exhaustion of underground resources. Nickel-yttria stabilized zirconia (Ni-YSZ) cermet has been used as an anode material for SOFCs for more than 20 years. Although Ni-YSZ has various advantages such as high catalytic activity for H2 oxidation and methane reforming, stability in reducing atmosphere, high electrical conductivity, it exhibits various types of degradations such as poor redox stability, sulfur poisoning, carbon deposition. Ni-YSZ anode still suffers from exposure to oxidizing condition caused by sudden interruption of fuel or sealing problem like leakage. When exposed in oxidizing atmosphere, Ni-YSZ experiences significant volume change due to the oxidation of Ni to NiO, which causes internal cracks and consequent destruction of the cell. In this study, the redox stability of Ni-YSZ cermet according to various conditions such as porosity, pore size distribution, and composition has been studied systematically. ▪ Experimental NiO-YSZ composite powders have been prepared by mixing of commercial NiO (High purity chemicals, 99.97%), 8YSZ (TZ-8YS: 8 mol% Y2O3-stabilized ZrO2, 99.9% Tosoh, Japan) and PMMA as a pore former. The NiO-YSZ bulk samples were fabricated by the typical die compaction method and sintering at 1450 °C for 4 h in air. The sintered NiO-YSZ samples were reduced at 800 °C for 3 h in H2 in order to make Ni-YSZ. The phase change was confirmed by the X-ray diffraction (XRD). The morphology of Ni-YSZ was investigated by scanning electron microscopy (SEM). Degree of oxidation (D.o.O) of Ni was calculated from thermogravimetric analysis (TGA) data. The amount of volume change of Ni powders during oxidation was measured using a helium pycnometer. The both porosity and pore size distribution of Ni-YSZ bulk samples were analyzed with mercury porosimeter. The linear length change of Ni-YZ during oxidation was measured using dilatometer. ▪ Results and Discussion If we know both temperature and time of exposure in oxidizing atmosphere, we can predict the D.o.O of Ni at a certain condition using the D.o.O equation obtained from TGA data. The percentage weight gain per specific surface area can explain the oxidation kinetics of the Ni-YSZ anode supports, which obeys a multi-stage parabolic law expressed by Kp. Kp can be expressed by Kp1 and Kp2. Kp1 and Kp2 correspond to fast oxidation reaction at early oxidation stage and slow oxidation reaction at the next oxidation stage, respectively. The volume change of Ni-YSZ caused by oxidation can be the result of two competing factors: expansion caused by NiO formation and shrinkage caused by continuous sintering of both Ni and NiO. Ni-YSZ cermet with high porosity showed higher tolerance to the cumulative length change due to providing the more relaxation space.

Characteristics of SrxY1-XTiyNi1-YO3-d anode in Humidified Methane Fuel for Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are a promising alternative energy conversion device because of high energy efficiency and fuel flexibility. Nickel/yttria-stabilized zirconia (Ni/YSZ) cermet has been widely used as an anode material in SOFCs. The Ni/YSZ anode, however, may not be appropriated for direct use of commercial hydrocarbon fuels because of carbon deposition and sulfur poisoning on the Ni phase. In this study, yttrium and nickel co-doped strontium titanium oxide SrxY1-xTiyNi1-yO3-δ (SYTN) was investigated as an alternative anode material for SOFCs. To improve the ionic conductivity of the Sr0.92Y0.08TiO3-δ (SYT) anode, Ti4+ at the B-site was substituted to Ni2+, leading to introduction of oxygen vacancies in the SYT. The stability of the SYTN anode at high temperatures and reducing atmospheric conditions was as good as the SYT anode. The SYTN exhibits excellent phase stability during the cell fabrication processes, SOFC operation, and good electrical conductivity at 700~900 oC in methane. The SYTN anode was compatible with YSZ at the SOFC operating condition. The electrochemical reactions of the SYT anode in both hydrogen and methane were limited by relatively slow reactions, such as non-charged processes including oxygen surface exchange and solid surface diffusion. To improve such poor electrochemical activities, the surface of the SYTN anode was modified by the SDC coating. The polarization resistance and the cell performance in hydrogen and methane were significantly improved by the SDC modification. The SDC-coated SYT anode cell was successfully operated with methane for more than 300 hours without any degradation of the cell performance.

3D reconstruction size effect on the quantification of solid oxide fuel cell nickel–yttria-stabilized-zirconia anode microstructural information using scanning electron microscopy-focused ion beam technique

Self-made conventional nickel–yttria-stabilized zirconia composite anodes after reduction and 500h operation were analyzed by three-dimensional microstructure reconstruction based on focused ion beam-scanning electron microscopy technique. Interfacial area, three-phases-boundary density and phase volume fraction were measured based on the three-dimensional microstructure reconstruction to quantitatively study the statistical characterization of solid oxide fuel cell nickel–yttria-stabilized-zirconia anode microstructure before and after operation. It is found that for anode operated with long time, it is necessary to increase the corresponding three-dimensional reconstruction size to suppress the influence of microstructure variation caused by Ni agglomeration in order to obtain more accurate microstructural quantification information.

Performance and long-term stability of nickel/yttria-stabilized zirconia anode-supported solid oxide fuel cell in simulated biosyngas

Here solid oxide fuel cells (SOFCs) technology and microwave-induced pyrolysis of sewage sludge (MWPSS) are combined to recover energy from waste by taking biosyngas as these medium. The biosyngas from the MWPSS with high concentration of H2 and CO is a novel fuel alternative for SOFCs to generate electricity other than the counterparts from air gasification and anaerobic digestion of biomass. The maximum power densities of the nickel/yttria-stabilized zirconia (Ni/YSZ) anode-supported SOFC in simulated biosyngas are 0.324 and 0.485 W/cm2 reaching up to 91.5 and 86.3% of the same cell in H2 at 973 and 1023 K, respectively. The gradual degradation and following stability of the cell performance are observed from an 1100 h long-term operation at 1023 K. The postmortem characterization indicates that carbon deposition on the anode surface is greatly related to the degradation. Finally, the changed thermodynamic propensity of the simulated biosyngas is attributed to be responsible for the special long-term stability analyzed by anode process models and carbon-hydrogen-oxygen ternary diagram. In summary, the results indicate that the biosyngas from the MWPSS is feasible and applicable as the medium for energy recovery from the sewage sludge by SOFCs technology in high efficiency with appropriate gas cleaning systems.

Modeling of gas transport with electrochemical reaction in nickel-yttria-stabilized zirconia anode during thermal cycling by Lattice Boltzmann method

Abstract This work reports an investigation of the impact of microstructure on the performance of solid oxide fuel cells (SOFC) composed of nickel yttria-stabilized zirconia (Ni YSZ). X-ray nano computed tomography (nano-CT) was used to obtain three-dimensional (3D) models of Ni-YSZ composite anode samples subjected to different thermal cycles. Key parameters, such as triple phase boundary (TPB) density, were calculated using 3D reconstructions. The electrochemical reaction occurring at active-TPB was modeled by the Lattice Boltzmann Method for simulation of multi-component mass transfer in porous anodes. The effect of different electrode geometries on the mass transfer and the electrochemical reaction in anodes was studied by TPB distributions measured by nano CT for samples subjected to different thermal cycles. The concentration polarization and the activation polarization were estimated respectively. The results demonstrate that a combined approach involving nano-CT experiments in conjunction with simulations of gas transport and electrochemical reactions using the Lattice Boltzmann method can be used to better understand the relationship between electrode microstructure and performance of nickel yttria-stabilized zirconia anodes.

A DFT Study of CO2-H2 Co-Adsorption on Ni/YSZ(111) for Solid Oxide Fuel Cell Applications

Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing fuels such as hydrogen, natural gas, ethanol or hydrocarbon mixtures, which highlights the capability of SOFCs as candidates for applications in stationary power generation. The main species used as fuel in the SOFC are H2 and CO which are oxidized at the anode [1]. Instead of pure H2 or CO, H2/CO mixtures can be used leading to more complicated mechanisms since other processes such as the water-gas shift reaction (CO + H2O ↔ CO2 + H2) are involved. Robust materials are needed for SOFC since the working temperature is approximately equal to 800˚C. For example, a suitable anode is Ni/YSZ (nickel – yttria stabilized zirconia) since it has a high catalytic activity, mechanical and chemical stability and compatibility with the electrolyte under working conditions [2]. It is well known that the performance of the Ni/YSZ cermet depends on the microstructure and the distribution of Ni and YSZ phases in the cermet [3]. Additionally, this performance depends on the key reactions occurring at the triple phase boundary where the gas phase, Ni particles and YSZ surface meet. Thus, it is relevant to understand, at the atomic scale, the structure of Ni/YSZ and its interaction with the gas phase, and in particular CO2 and H2. To simulate the interaction of CO2 and H2 with Ni-YSZ, we have used electronic structure calculations based on the Density Functional Theory using the Vienna Ab-initio Simulation Package (VASP). We have studied the deposition of Nin (n=4-10) clusters on the YSZ(111) followed by the adsorption of molecules on the optimised Nin/YSZ(111) structures and calculated the geometric and electronic structure at the most stable adsorption site. We have also analysed the reaction path of the reverse water-gas shift reaction (CO2 + H2 ↔ CO + H2O) on three different Nin/YSZ(111) (n = 5, 6 and 10) interfaces, considering two possible pathways intermediates: hydrocarboxyl or formate. Finally, we have observed that both CO2 and H2 prefer to adsorb on the Nin cluster rather than on the surface. On the metal cluster the CO2 molecule is activated and the H2 is dissociated upon adsorption, whereas charge is transferred from the cluster to the molecules. We have determined two intermediate states and three intermediate states for the carboxylic acid and formate intermediate pathway, respectively, on the three Nin/YSZ(111) (n = 5, 6 and 10) interfaces.

Fabrication and performance of solid oxide fuel cell anodes from core–shell structured Ni/yttria-stabilized zirconia (YSZ) powders

A core–shell structured nickel/yttria-stabilized zirconia (Ni/YSZ) powder was synthesized by the hydrazine method from an aqueous nickel chloride solution (NiCl2·6H2O) with YSZ particles, which were prepared by a hydrothermal technique. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB) was added to form the core–shell structure. Nano-sized YSZ shell particles were homogeneously coated onto the surface of the spherical Ni core particles. It was found that the addition of CTAB was crucial for the formation of the core–shell structured Ni/YSZ powder. The size of the YSZ shell particles affected the morphology of the Ni/YSZ core–shell powder. Finer YSZ shell particles led to a denser coverage of the Ni core particle. During the heat treatment of the Ni/YSZ core–shell powder in air, the Ni was partly oxidized to NiO, and a three-dimensional network structure of NiO and YSZ was obtained. The enhanced electrical conductivity in the Ni/YSZ anode because of the core–shell structured powder was considered to be due to the improved connectivity between the Ni and YSZ particles. The maximum power densities and the polarization resistances of the Ni/YSZ anode from the core–shell structured powder were 0.38, 0.57, and 0.84W/cm2 and 1.01, 0.52, and 0.25Ωcm2 at 700, 750, and 800°C, respectively.

Experimental Study of the Carbon Deposition from CH4 onto the Ni/YSZ Anode of SOFCs

AbstractA challenge in the operation of solid oxide fuel cells (SOFCs) with hydrocarbon fuels is the carbon deposition on the nickel/yttria‐stabilized zirconia (Ni/YSZ) anode. The Grabke‐type kinetic model has been proposed for the carbon formation based upon the assumption of elementary steps, which consist of a rate‐limiting dissociative chemisorption step and a stepwise dehydrogenation of the chemisorbed methyl group. This work experimentally studied the carbon formation on a SOFC Ni/YSZ anode exposed to CH4+H2 gas mixtures. Experiments were conducted with various gas compositions of CH4/H2 and temperatures in the range from 873 K to 1,123 K. The experimental results were used to determine a kinetic model that was applied to the SOFC operating environments. Based on the experimental data, the formula for the carbon formation rate that is dependent on the operating temperature and the gas compositions of CH4/H2 was established.

Methane/steam global reforming kinetics over the Ni/YSZ of planar pre-reformers for SOFC systems

A solid oxide fuel cell (SOFC) is a promising all-solid-state energy conversion device that produces electricity by electrochemically combining a fuel with an oxidant across an oxide electrolyte. To optimize SOFC performance, it is necessary to investigate the effects of the design and operating parameters on system components such as pre-reformers and afterburners. In pursuit of this, a multi-component SOFC design based on a planar stack was developed. Two specially designed planar-type steam pre-reformers (the first consisting of five layers with air heating and the other of one with electric heating) utilizing nickel yttria-stabilized zirconia (Ni/YSZ) as catalyst were built to investigate the kinetics of steam reforming with methane. Whereas experimental results on the global reaction kinetics of a Ni-based catalyst in the 5-layer planar pre-reformers within a temperature range of between 350°C and 610°C have been published, this work reports the results of reforming kinetics using a 1-layer reformer as a flow reactor in a higher temperature range (500°C–740°C), as well as on reforming kinetics in combination with anode off-gas recycling. The results confirm the proposed kinetic expression of the Arrhenius type (second order with respect to the mole fraction of methane and first order with respect to the mole fraction of water) for different steam-to-carbon ratios and also for anode off-gas recycling. The activation energy for methane steam reforming corresponds to the desorption energy of CO or H2O from the catalyst surface depending on the inlet composition of the mixture.

In situ high-temperature powder diffraction studies of solid oxide electrolyte direct carbon fuel cell materials in the presence of brown coal

Direct carbon fuel cells are an attractive alternative for conventional power generation; however, there is almost no information on the stability of conventional fuel cell materials in direct carbon fuel cell environments, in particular when exposed to realistic fuels and the contaminants contained within these fuels. Similarly, there is little information on the structure and phase assemblage of solid fuels exposed to typical environments found in direct carbon fuel cells. In this paper, we use in situ high-resolution synchrotron powder diffraction on conventional high-temperature fuel cell materials and untreated brown coal to assess the stability and reactivity of these materials at various temperatures up to 850 °C. Materials investigated include nickel metal (Ni), gadolinium-doped ceria (GDC) and yttria-stabilised zirconia (YSZ). The phase stability, crystallite size, lattice parameters and associated linear coefficient of thermal expansion were determined. The phase stability of all fuel cell materials was found to be good with no additional phase formation noted either during heating or after prolonged periods at temperature. An increase in the crystallite size was observed for both GDC and Ni. Non-linear thermal expansion observed in these materials was related to partial reduction of cerium ions (GDC) and due to a Curie point transition (Ni). A wide range of mineral phases were observed in the coal samples and these phases were found to change significantly with temperature. Mineral phases consistent with the ash composition and existing literature on Victorian brown coal were assigned to phases observed.

Understanding the phase formation and compositions of barium carbonate modified NiO-yttria stabilized zirconia for fuel cell applications

A simple modification of a conventional nickel-yttria stabilized zirconia (Ni-YSZ) anode for a solid oxide fuel cell (SOFC) by BaCO3 can enhance the coking resistance of the anode in carbon-containing fuels. In this study, X-ray diffraction, Raman spectroscopy, and thermo-gravimetric analysis are used to investigate the chemical reactions among BaCO3, NiO, and YSZ to understand the phase formation and compositions during fabrication process. Our results indicate that BaCO3 reacts readily with YSZ to produce a BaZr1−xYxO3−δ (BZY) phase at temperatures higher than 900 °C. For the mixture of BaCO3 and NiO, both BaNiO3 and BaNiO2.36 are formed at 900 °C, which are unstable at elevated temperatures, in the presence of YSZ, or under a reducing atmosphere. The results imply that a BZY/YSZ-BaO/Ni anode can be easily created by a simple heat-treatment (e.g., during cell fabrication) and a subsequent reducing process (e.g., exposure to the fuel) of a mixture consisting of BaCO3, NiO, and YSZ.

High-performance solid oxide electrolysis cell based on ScSZ/GDC (scandia-stabilized zirconia/gadolinium-doped ceria) bi-layered electrolyte and LSCF (lanthanum strontium cobalt ferrite) oxygen electrode

The work presented focuses on the development and performance evaluation of Ni-YSZ (nickel-yttria-stabilized zirconia) supported solid oxide cell with bi-layered ScSZ/GDC electrolyte structure and LSCF (lanthanum strontium cobalt ferrite) oxygen electrode in high-temperature steam and carbon dioxide electrolysis. At 800 °C, the cell exhibited a very high electrolysis current density of about −2.2 A/cm2 and −1.9 A/cm2 in steam and CO2 electrolysis, respectively. A slightly lower ASR (area specific resistance) is observed in electrolysis mode when compared to fuel-cell mode. Moreover, the ASR is increased when increasing the CO2 concentration in both modes of operation. The OCV (open circuit voltage) of SOEC with bi-layered electrolyte structure is significantly improved than the cell with single-layered GDC (gadolinium-doped ceria) electrolyte but at the cost of an increase in the ohmic resistance (Rs) of cell. The formation of (Zr, Ce)O2−x solid solution by the mutual diffusion of zirconium and cerium during co-sintering is found to be the origin of the increase in the Rs. Impedance spectra revealed that the high electrolysis performance is caused by the low polarization losses at LSCF oxygen electrode rather than the thin bi-layered ScSZ/GDC electrolyte structure.

Reactive Force-Field Development for Metal/Ceramic SOFC Anode Modeling

A solid oxide fuel cell (SOFC) system anode material is usually composed of a mixture of Ni and other oxide ceramics, such as doped-CeO2, and Sc stabilized ZrO2, due to their high activity and structural stability. Among them the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet is widely used today as an anode material of SOFC due to its good electrocatalytic property against low price. For SOFC performance, it is essential to improve the stability of the Ni/YSZ cermet anode. Combination of Ni and YSZ has been studied experimentally, but the interaction is not well understood in the boundary between Ni and YSZ. However, there is no evidence about the atomic-scale Ni/YSZ interface dynamics and chemical changes in the structure during the operation of SOFC. Unfortunately, most classical force fields are incapable of describing chemical reactions. To overcome this difficulty, reactive force field (ReaxFF) potential was developed by van Duin and co-workers. Recently, Ni/YSZ/H2 interactions are reported by ReaxFF. But in this interaction Ni/YSZ interface stability is not taken into account. As a first step we developed our own ReaxFF potential to investigate Ni/ZrO2 interface and for determining stable Ni/YSZ interface for modeling of the triple-phase-boundary in SOFCs. Big difference is found between our developed and reported ReaxFF and that is work of separation. Using our ReaxFF potential, work of separation is in excellent agreement with literature. It is found that O around Zr4+cation is reduced in case of bulk and interface regions. Although interface region exhibited less reduction compared to bulk region. More details and Ni/YSZ interface stability through O vacancy may present at conference.

Li/Na Modified Ni-SDC Anode for Methane-Fueled Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have the potential to direct use hydrocarbon fuels without pretreatment due to its high operating temperature [1]. The most common used nickel/yttria-stabilized zirconia (Ni/YSZ) anodes have showed excellent performance in H2 fuel, however, there is serious carbon deposition when hydrocarbon fuels are used. In recent years, great efforts have been devoted to develop alternative anodes to alleviate carbon deposition issue encountered in Ni-based cermets anode. However, the performance of the proposed alternative anode materials is still not satisfactory yet when hydrocarbon fuels are used. Alkali or alkaline earth oxides have been used to enhance the activity of catalysts for reforming [2] and water-gas shift reactions [3]. In this work, a facile chemically lithium/sodium insertion method has been developed to prepare Li/Na doped-NiO composites (Li/Na-NiO). The single cells were fabricated with a Li/Na doped NiO-Ce0.8Sm0.2O1.9 (SDC) composite anode screened on a 250um La0.9Sr0.1Ga0.8Mg0.2O3- d (LSGM) electrolyte and a composite cathode consisting of Sr0.95Ce0.05CoO3- d and SDC (30wt%). When CH4 was used as fuel, the peak power density of the cells with the Li and Na doped NiO anode reaches about 126 mW cm-2 and 106mW cm-2 at 800oC, respectively. Compared with that of the cell with the pristine NiO-SDC anode, the cells with Li/Na doped NiO anode material shows that it gives much higher open circuit voltage (OCV) and better long-term stability at 800oC than that of the cell with the pristine Ni-based anode. The preliminary results demonstrate that the Li/Na doped NiO anode materials are promising alternative anode for hydrocarbon-fueled solid oxide fuel cells.

Anode regeneration following carbon depositions in an industrial-sized anode supported solid oxide fuel cell operating on synthetic diesel reformate

Carbon deposition is a primary concern during operation of solid oxide fuel cells (SOFCs) fueled with carbon-containing fuels. It leads to cell degradation and thus reduces SOFC sustained operation and durability. This paper reports on an experimental investigation of carbon formation on the nickel/yttria-stabilized zirconia (Ni/YSZ) anode of an anode-supported SOFC and its regeneration. The cell was fueled with a synthetically produced diesel reformate to investigate and simulate the cell behavior under real operating conditions. For this purpose the cell was operated under load to determine the critical operating time. Rapid carbon generation, such as at open circuit voltage (OCV), can be prevented when the cell is under load. Carbon depositions were detected using scanning electron microscopy (SEM) and further analyzed by Raman spectroscopy. Industrial-size cells suitable for commercial applications were studied. This study proves the reversibility of carbon formation and the reproducibility of the regeneration process. It shows that carbon formations can be recognized and effectively, fully and cell-protecting regenerated. It indicates the excellent possibility of using SOFCs in the automotive industry as an auxiliary power unit (APU) or combined power-heat unit, operated with diesel reformate, without danger from cell degradation caused by carbon-containing fuels.

Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes

Currently, the most widely considered material for solid oxide fuel and electrolysis cells is the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet1. However, issues such as low resistance to poisoning, coking, and changes in the microstructure have pushed for the use of new materials. Mixed ionic conductors2 alleviate the shortcomings of Ni/YSZ and can achieve more efficient use of the electrode material, since a three-phase boundary is not required for reaction. Among these materials, doped ceria has shown promise, yet little is known regarding the reaction mechanism for water electrolysis. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better picture of the surface, as well as the species involved in the reaction. Unfortunately, there is still controversy regarding the nature of the rate-determining step3,4. In addition, most work has made use of electrochemical impedance spectroscope (EIS), a linear technique that cannot differentiate among rate determining scenarios. The use of nonlinear techniques such as non-linear EIS (NLEIS) has been shown to be a powerful tool that is sensitive to different reaction mechanisms5. In order to elucidate these rate limiting phenomena, porous 10% doped gadolinia-doped ceria /YSZ electrochemical cells were analyzed at 750 and 800 °C under different H2-H2O gas environments using both EIS and NLEIS. The experimental results were then analyzed using a macro-homogeneous model6 involving oxygen vacancies and the chemical potential of electrons7. Multiple modeling scenarios were considered including one-dimensional and multidimensional transport, as well as the presence or absence of surface diffusion of hydroxyl species in the surface. References Nakamura, T; Kobayashi, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Mizusaki, J; Kawada, T. Electrochemical Behaviors of Mixed Conducting Oxide Anodes for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B563-B569Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013 , 135, 11572-15579Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014 , 5, 4374Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402Adler, S. B; Lane, J. A; Steele, B. C. H. Electrode Kinetics of Porous Mixed-Conducting Oxygen Electrodes. J. Electrochem Soc. 1996, 143, 3554-3564Wang, S; Kobayashi, T; Dokiya, M; Hashimoto, T. Electronic and Ionic Conductivity of Gd-Doped Ceria. J Electrochem Soc. 2000, 147, 3606-3609

Nanostructured Ni-YSZ Anode By Atomic Layer Deposition for Solid Oxide Fuel Cells

Nickel yttria-stabilized zirconia (Ni-YSZ) is the most commonly used anode for solid oxide fuel cells (SOFCs). The electrochemical performance of Ni-YSZ anodes can be improved when the size of constituent particles is reduced. However, under SOFC operating environment, nano-sized Ni particles suffer from sintering; nanoscale Ni-YSZ anodes can undergo structure changes, which constrain the development of nanoscale Ni-YSZ anodes. In this work, nanoscale nanostructured Ni-YSZ anode was successfully fabricated by combining atomic layer deposition (ALD) and glancing angle deposition (GLAD). Prior to atomic layer deposition of YSZ, the porous Ni nanorod electrode was firstly prepared by GLAD. ALD was applied to deposit a uniform nanoscale YSZ coating layer over the nickel nanorods. The morphology of the nanostructured Ni-YSZ anode was examined with scanning electron microscopy (SEM). The surface area was increased to more than 3 times as large as plane area and the triple phase boundary (TPB) extended to the entire nickel surface. The surface composition was examined with X-ray photoelectron spectroscopy (XPS). The atomic composition of YSZ film was controlled by the ALD process and not affected by the post-deposition annealing process. ALD coating approach combined ionic conductive YSZ electrolyte phase with electrical conductive Ni phase flexibly and controlled the porosity of the nano-composite electrode just through deposition and post-deposition annealing process. Electrical conductivity measurements were performed using a 4-point probe method. The Ni-YSZ anodes maintained resistivity in the 10-4 cm·S-1 magnitude, which is sufficiently conductive as an SOFC anode. The ALD YSZ coating layer showed sintering-resistance on the nanostructured electrode with an as thin as 1.9 nm YSZ coating layer and excellent anode performance under SOFC operating conditions.

Ethylene glycol as a new sustainable fuel for solid oxide fuel cells with conventional nickel-based anodes

Abstract In this study, renewable ethylene glycol (EG) was exploited as a potential fuel for solid oxide fuel cells (SOFCs) with conventional nickel yttria-stabilized zirconia (Ni–YSZ) cermet anodes for sustainable electric power generation. Carbon deposition behaviors over Ni–YSZ anodes under different carbon-containing atmospheres such as EG, glycerol, ethanol and methane were characterized through thermodynamic prediction, oxygen-temperature programmed oxidation and SEM–EDX analysis. EG was observed to be better than acetic acid and glycerol and much better than methane and ethanol in terms of carbon deposition. A calculation of the open-circuit voltages of EG-fueled SOFCs suggested that EG is a suitable fuel for SOFCs. A maximum power output of 1200 mW cm−2 at 750 °C was obtained from a cell operating on EG-steam fuel, which is only a little lower than that from a cell based on hydrogen fuel. The cell was further operated stably on an EG-steam gas mixture for 200 h with no apparent performance degradation, carbon deposition over the anode, Ni agglomeration, or change in the morphology of the anodes. The current study confirmed the practical applicability of EG as a direct fuel for SOFCs, which may have a great effect on future energy systems.

Electrical Resistance Measurements and Microstructural Characterization of the Anode/Interconnect Contact in Simulated Anode-Side SOFC Conditions

Metallic interconnects in solid oxide fuel cell (SOFC) stacks are often in direct contact with a nickel/yttria stabilized zirconia (Ni/YSZ) cermet anode. Interdiffusion between the two components may occur at the operating temperature of 700–850°C. The alteration of chemical composition can result in phase transformation of the steel and in formation of oxides with a poor electrical conductivity in the anode. In this study, the area specific resistance (ASR) of the steel Crofer 22 APU, in contact with a Ni/YSZ anode with and without a tape casted CeO2 barrier layer was measured in simulated SOFC anode conditions at 800°C. The microstructure in the contact area was characterized using scanning electron microscopy techniques. The ASR was low for the steel in direct contact with the Ni/YSZ anode. Nickel diffusion into the steel resulted in a fine grained zone, which was identified as ferrite. The zone is austenitic at the exposure temperature but transforms to ferrite during cooling. When a CeO2 nickel diffusion barrier layer was used The ASR was considerably higher. These results imply that nickel diffusion is not only detrimental: It leads to microstructural instability but also results in a low electrical resistance of the anode/interconnect contact.