Porous Ni-YSZ Anode Research Articles

Sulfur- and Coking-Tolerant Anodes for Solid Oxide Fuel Cells

Sulfur- and Coking-tolerant Anodes for Solid Oxide Fuel Cells Zhijun Liu,1 Yucun Zhou,1 Weilin Zhang,1 Jie Hou,1 Xueyu Hu,1 Meilin Liu1, * 1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States Solid oxide fuel cells (SOFCs) have potential to be one of the most efficient systems for direct conversion of a wide variety of chemical fuels into electricity. 1-3 However, the lack of sulfur- and coking-tolerant anodes still hinders the development of direct hydrocarbon-fueled SOFCs for efficient and cost-effective operation. The conventional Ni-based anodes may suffer from carbon deposition and sulfur poisoning when they are directly exposed to sulfur-containing hydrocarbon fuels, resulting in degradation in performance or even destruction of the anodes. 4-6 In this presentation, we will report a conventional Ni-Zr0.84Y0.16O2-δ (YSZ) anode decorated with Ce and Ba-based catalyst coatings to significantly enhance the sulfur and coking tolerance. Results suggest that the enhanced sulfur tolerance is attributed to the evenly distributed Ce-based catalyst on the inner surface of the porous Ni-YSZ anode while the enhanced coking tolerance is benefited from the Ba-based catalyst layer on the top of the anode. SOFCs with the catalysts-modified Ni-YSZ anodes show minimal degradation when hydrogen with 100 ppm H2S is used as the fuel. More importantly, stable operation is demonstrated for direct utilization of sulfur-containing liquid fuels (e.g., octane and gasoline). This work demonstrates the great potential of catalyst coatings for enhancing contaminant tolerance and durability of SOFC electrodes. Z. Wang, Y. Wang, D. Qin, Y. Gu, H. Yu, S. Tao, B. Qian and Y. Chao, J. Eur. Ceram. Soc. (2022). S. Sengodan, M. Liu, T.-H. Lim, J. Shin, M. Liu and G. Kim, J. Electrochem. Soc., 161, F668 (2014). Z. Cheng, J.-H. Wang, Y. Choi, L. Yang, M. C. Lin and M. Liu, Energy Environ. Sci., 4, 4380 (2011). T. Hays, A. M. Hussain, Y.-L. Huang, D. W. McOwen and E. D. Wachsman, ACS Appl. Energy Mater., 1, 1559 (2018). Y. Chen, B. deGlee, Y. Tang, Z. Wang, B. Zhao, Y. Wei, L. Zhang, S. Yoo, K. Pei, J. H. Kim, Y. Ding, P. Hu, F. F. Tao and M. Liu, Nat. Energy, 3, 1042 (2018). L. Yang, S. Wang, K. Blinn, M. Liu, Z. Liu, Z. Cheng and M. Liu, Science, 326, 126 (2009).

An adaptive thermo-mechanical peridynamic model for crack analysis in anode-supported solid oxide fuel cell

Nickel–yttria-stabilized zirconia (Ni–YSZ) composite cermet is widely used as the conventional anode material in solid oxide fuel cells (SOFCs) for both functional and supporting layers under high-temperature operating conditions. Although with high energy conversion efficiency and multi-fuel compatibility, the cell always suffers irreversible gradual degradation and sudden deterioration, which can be attributed to mechanical damages formed during thermal cycling and long-time operation. In this study, the three-dimensional (3D) real microstructures of various porous Ni–YSZ anodes are reconstructed using the focused ion beam-scanning electron microscopy (FIB-SEM) dual-beam technique. Based on the reconstructions, a weakly coupled thermo-mechanical peridynamic (PD) model is proposed to predict the possible mechanical damages under severe conditions for different anodes associated with the single-cell design under thermal shocks. The relevant equivalent mechanical and thermal properties of the composite anodes are analyzed and correlated to the real microstructures. The work provides a trans-scale model to simulate the generation and propagation of cracks based on the microstructural characteristics, and the simulation results show great potential in providing supporting information for trans-scale optimization of SOFCs.

Electrochemical Sulfur Oxidation in Solid Oxide Fuel Cells Studied by Near Infrared Thermal Imaging and Chronocoulometry

A newly adapted electrochemical technique, chronocoulometry, was used to characterize sulfur’s effect on the performance of porous Ni-YSZ anodes in electrolyte supported, solid oxide fuel cells (SOFCs) operating with dry H2 at 600 °C, 650 °C, 700 °C and 800 °C. Chronocoulometry data together with near-infrared thermal imaging show that H2S poisoning is more complex than sulfur simply blocking electrochemically active sites. Thermal imaging supports findings that SOFC susceptibility to sulfur poisoning depends strongly on temperature with higher performance and greater sulfur tolerance at higher temperatures. Chronocoulometry data are consistent with this description. Chronocoulometry results, however, are also more nuanced and show that sulfur adsorbed to the triple phase boundary (TPB) can be electrochemically oxidized, thereby limiting performance loss that would result simply from blocked or inaccessible electrochemically active sites. Furthermore, chronocoulometry results imply an increased TPB length at higher operating temperatures and suggest that the spatial extent of a SOFC electrode’s electrochemically active region plays a significant role in electrode surface chemistry. A simple model is developed to interpret the chronocoulometry results and determine the relative amount of sulfur adsorbed to the anode’s active triple phase boundary.

Noise suppressed and bias field corrected image segmentation method for porous Ni-YSZ anode microstructure

Accurate three-phase identification from Solid Oxide Fuel Cell (SOFC) anode micrograph is challenged by both noise and intensity inhomogeneity. In this paper, a novel framework is proposed for porous Ni-YSZ cermet anode Optical Microscopy (OM) image segmentation. The proposed framework takes advantage of a statistical model in which an observed image is decomposed into two multiplicative components (bias field and true image) and one additive component (noise). A two-stage Principal Component Analysis with Local Pixel Grouping (LPG-PCA) denoising algorithm is firstly performed to suppress additive noise, it can preserve more image local structural features by modeling a pixel and its nearest neighbors as a vector variable and selecting training samples with similar contents to this variable in a local window for PCA transformation. In order to enhance the robustness to noise, uneven illumination and other outliers, a kernel metric is introduced into fuzzy clustering method embedded with bias field correction for image segmentation. The proposed method has been compared to other state-of-the-art segmentation algorithms on both simulated images and real SOFC anode OM images. Extensive experiments results have demonstrated that the proposed framework can successfully eliminate the influence of both uneven illumination and noise on real Ni/YSZ anode OM images to obtain a better three-phase identification accuracy. The high-quality segmentation results lay firm foundation for accurate microstructural parameter extraction.

Development of Cell-Protecting Methods for Carbon-Removal from Porous Ni-YSZ Anodes and Regeneration of the Cell Performance

Solid oxide fuel cells are highly efficient units, which directly convert the chemical energy of gaseous fuels into electrical energy without additional conversion steps. The overall efficiency can be increased by using heat losses occurring during the operation at high temperatures between 600–1000°C. SOFCs further offer a great fuel flexibility and compared to other fuel cell types they can internally reform hydrocarbons. Internal conversion of carbon-containing fuels over nickel as the most widely used catalyst tends to promote carbonaceous deposits on the anode surface as well as inside the anode, causing deactivation of the fuel cells. Nevertheless, undisturbed operation with conventional fuels and early detection of the cell degradation are highly important for the commercial usage of solid oxide fuel cells in industry, such as in auxiliary power units or especially for mobile application. Formation and deposition of carbonaceous species on the anode surface of solid oxide fuel cells may be prevented under specific operating conditions such as high steam/carbon ratio or high current density. The increasing amount of steam in fuel can chemically remove carbon adsorbed on the anode surface thus reducing the total amount of carbon deposits. However, this would significantly dilute the fuel thus reducing the overall power and the cell efficiency. Further possibility to avoid carbon formation is to operate the cells under load higher than the critical current density. However, increase of the steam amount or variation of the cell load is neither realistic nor advantageous for the commercial purpose. Without the need for the variation of the operating conditions, such as higher amount of steam or higher load, early detection of degradation phenomenon and applying of appropriate cell-protecting regeneration strategies can improve the operating reliability of the SOFC systems. In order to prolong the cell’s lifetime and to accelerate the market access of SOFCs, diverse approaches for carbon removal in a cell-protecting manner and regeneration of the cell performance were developed and detailed examined. They are outlined within this study. The practicability of the developed methods for industrial application was also considered. The regeneration strategies are based on application of: (1) gaseous components, for the gasification of carbon deposits, and (2) electrochemical methods, which remove carbon during the cell loading. Operation under gaseous components as hydrogen, water vapour or oxygen-enriched gas mixtures allowed complete carbon removal. Nevertheless, under operation in a fuel cell mode, only the approach that combined hydrogen and water vapour removed carbon from the anode while also protecting it from further undesired degradation. It is important to note that very high amount of water can deteriorate the cell performance and even provoke unwanted nickel oxidation, which thus must be considered. However, the allowed water vapor amount must thus me determined. Furthermore, feeding the anode with a very low amount of oxygen in a wide temperature range showed high carbon removal rates but resulted in further deterioration of the cell performance, and eventually led to mechanical degradation. Both methods for carbon removal in a cell-protecting manner and approaches that induced further microstructure degradation are presented within this study.

In-Operando Detection of Carbon Depositions and Carbon Formation Predictions for Industrial-Sized SOFCs Fueled with Synthetic Diesel Reformate

Auxiliary power units (APUs) with solid oxide fuel cells (SOFC) as a main component provide solution for high efficient and low emission energy system of tomorrow. Feeding SOFCs with conventional or alternative carbon-containing fuels brings risk of carbon formation and its deposition on the porous Ni-YSZ anode. Carbon deposition is one of the major degradation sources which reduce the cell activity, cause an irreversible cell degradation and can significantly decrease the cell’s lifetime. In order to prolong the cell’s lifetime and to ensure the undisturbed operation of SOFCs, an in-depth understanding of this degradation phenomenon is required. For that purpose, experimental and numerical investigations were performed within this study.

Development of Cell-Protecting Methods for Carbon-Removal from Porous Ni-YSZ Anodes and Regeneration of the Cell Performance

Carbon deposition is one of the major obstacles that reduce the lifetime of solid oxide fuel cells (SOFCs) and can thus significantly shorten the lifetime of auxiliary power units (APUs). Carbon deposits block the gas pores, cover the catalytic active sites, and induce the microstructure and performance degradation. In order to extend the lifetime of the cells used, effective and cell-protecting strategies for carbon gasification and restoring of the cell performance must be developed. This study scrutinizes various approaches for this purpose. They are based on hydrogen, steam, and oxygen enriched gas mixtures. Gasification strategies, which enable complete carbon removal and performance restoring in a cell-protecting manner and their practicability for industrial usage are examined and summarized in this study.

Formulation of ammonia decomposition rate in Ni-YSZ anode of solid oxide fuel cells

Ammonia decomposition rate on a conventional porous Ni-YSZ anode of solid oxide fuel cells (SOFCs) is experimentally investigated under various temperatures and gas compositions to formulate a reaction rate equation. It is confirmed that the decomposition rate depends not only on the partial pressure of ammonia but also on that of hydrogen at the common operating conditions of SOFCs, with larger hydrogen concentration significantly reducing the decomposition rate. 3D microstructure of the tested Ni-YSZ anode is analyzed using the focused ion beam and scanning electron microscope (FIB-SEM), and microstructural parameters are quantified. By using the quantified Ni–Pore interface area density, a reaction rate model for a unit reaction area in Ni-YSZ anodes is obtained. The proposed model is then applied to a simple computational fluid dynamics (CFD) model for validation. The proposed reaction rate model can be applied to a variety of numerical analysis of ammonia decomposition on Ni-YSZ anodes having different microstructures, and expected to be useful for the analysis of ammonia-fed SOFCs.

Investigation on the microstructures, mechanical and electrical properties of solid oxide fuel cells anodes fabricated by using chitosan and cold mounts powders as new pore formers

The physical properties and microstructures of supporting anodes are extremely important for the performances of solid oxide fuel cells (SOFCs). In the present work, for the first time, two new pore formers, cold mount powder and chitosan as well as graphite, activated carbon and corn starch as three well-known pore formers were studied and used for fabrication of porous Ni-YSZ anodes of solid oxide fuel cells. Morphological features and size of the pore former particles were characterized by using scanning electron microscopy (SEM) and particle size analyzer (PSA) technique and their thermal decomposition/oxidation properties were determined by thermo gravimetric analysis (TGA). The performances of the fabricated porous Ni-YSZ anodes were evaluated in terms of the resultant microstructure, mechanical strength, and electrical conductivity measurements. The strengths were measured by ball-on-ring method. Weibull statistics were used to describe the distribution of the measured strengths. The results indicated that the microstructure, mechanical strength, and electrical conductivity of the fabricated porous Ni-YSZ anodes depend directly on the type of pore former used. The results also showed that the cold mount powder and chitosan, by creating porosities with irregular shapes, produce anodes with lower fracture strength and modest electrical conductivity with respect to the other used pore formers.

Y0.08Sr0.88TiO3–CeO2 composite as a diffusion barrier layer for stainless-steel supported solid oxide fuel cell

A new diffusion barrier layer (DBL) is proposed for solid oxide fuel cells (SOFCs) supported on stainless-steel where DBL prevents inter-diffusion of atoms between anode and stainless steel (STS) support during fabrication and operation of STS-supported SOFCs. Half cells consisting of dense yttria-stabilized zirconia (YSZ) electrolyte, porous Ni-YSZ anode layer, and ferritic STS support, with or without Y0.08Sr0.88TiO3–CeO2 (YST-CeO2) composite DBL, are prepared by tape casting and co-firing at 1250 and 1350 °C, respectively, in reducing (H2) atmosphere. The porous YST-CeO2 layer (t ∼ 60 μm) blocks inter-diffusion of Fe and Ni, and captures the evaporated Cr during cell fabrication (1350 °C). The cell with DBL and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode achieved a maximum power density of ∼220 mW cm−2 which is stable at 700 °C. In order to further improve the power performance, Ni coarsening in anode during co-firing must be prevented or alternative anode which is resistive to coarsening is suggested. This study demonstrates that the new YST-CeO2 layer is a promising as a DBL for stainless-steel-supported SOFCs fabricated with co-firing process.

Strategy for Carbon Gasification from Porous Ni-YSZ Anodes of Industrial-Sized ASC-SOFCs and Effects of Carbon Growth

Auxiliary power units (APUs) running on carbon-containing fuels provide solutions for high-efficient power supply and emission reduction. In order to ensure safe operation, unwanted degradation caused by carbon depositions should be avoided or controlled. Early detection allows trigerring of counteractions and avoids rapid deterioration of the cell performance as well as mechanical degradation of the cell microstructure and it is presented here. This study also shows that carbon does not only block the active catalyst sites and porous gas channels thus deteriorating the cell performance, but massive carbon depositions lead to destruction of the lattice YSZ-structure. In order to prolong the lifetime of the investigated cells, carbon-dioxide is tested as a possible agent for gasification of deposited carbon. Supplying with CO2 gasifies carbon to carbon-monoxide, but experimental investigations show further degradation of the cell performance and a reduced methane conversion rate. A method, which represents a combination of CO2 fed to the anode and an overvoltage applied to the cell, enabled complete performance regeneration in a cell-protecting manner.

Effect of infiltrated transition metals on nickel morphology change and area-specific resistance of Ni-YSZ based SOFC anode during long-term operation

Evaporation of Ni in the form of Nickel Hydroxide (Ni(OH)2) in Solid Oxide Fuel Cell (SOFC) anodes (Ni-YSZ) is one of the major causes of anode degradation. Transition metals such as Fe, Cr and Co can act as sacrificial anodes for Ni, because of lower Gibbs Free Energy values for the formation of their corresponding volatile hydroxides as compared to Ni (OH)2. The transition metals were added to porous Ni-YSZ anode scaffold by infiltration method. Nano-sized particles were sporadically dispersed on the Ni-YSZ surface, confirmed by Scanning Electron Microscopy (SEM). X-Ray Diffraction (XRD) patterns show a very good chemical compatibility between the added metals and Ni-YSZ anodes. Symmetric cells were then prepared and the Area-Specific Resistance (ASR) was monitored at 1000 °C, with a fuel gas containing 25 vol.% H2, 75 vol.% N2, for more than 250 h . To control accelerated evaporation condition of anode, relative humidity in anode gas was fixed at 12 %. The difference in the amount of the added metal before and after long-tern test was determined by EDS analysis. Change in the grain size distribution of Ni particles and Triple Phase Boundary (TPB) density, before and after long term test were calculated by image analysis. Well-defined relations were obtained among ASR change rate determined from electrochemical measurements and grain size distribution, TPB density change rate calculated from image analysis.

Application of PVD methods to solid oxide fuel cells

a b s t r a c t In this paper, attention is paid to the application of such a method of vacuum physical vapor deposition (PVD) as magnetron sputtering for fabrication of a solid oxide fuel cell (SOFC) materials and structures. It is shown that the YSZ (yttria-stabilized zirconia) electrolyte and Ni-YSZ anode layers with required thick- ness, structure and composition can be effectively formed by PVD methods. The influence of parameters of pulsed power magnetron discharge on the deposition rate and the microstructure of the obtained YSZ electrolyte films were investigated. It is shown that the deposition rate of the oxide layers by magnetron sputtering can be significantly increased by using asymmetric bipolar power magnetrons, which creates serious prerequisites for applying this method on the industrial scale. Porous Ni-YSZ anode films were obtained by reactive co-sputtering of Ni and Zr-Y targets and subsequent reduction in the H2 atmosphere at a temperature of 800 ◦C. The Ni-YSZ films comprised small grains and pores of tens of nanometers.

Quantitative analysis methods for three-dimensional microstructure of the solid-oxide fuel cell anode

The electrochemical performance is closely related to three-dimensional microstructure of the Ni-YSZ anode. X-ray nano-tomography combined with quantitative analysis methods has been applied to non-destructively study the internal microstructure of the porous Ni-YSZ anode. In this paper, the methods for calculating some critical structural parameters, such as phase volume fraction, connectivity and active triple phase boundary (TPB) density were demonstrated. These structural parameters help us to optimize electrodes and improve the performance.

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm2 at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm−2 at 750 °C was obtained of the single cell. Short-term stability was measured as well.

Ni infiltrated YSZ anode stabilization by inducing strong metal support interaction between nickel and titania in solid oxide fuel cell under accelerated testing

Sintering of Ni particles in Ni infiltrated porous YSZ anodes and decrease in triple phase boundary is the reason for performance loss in SOFC. In the present work, the idea of strong metal support interaction (SMSI) has been used to prevent the sintering of Ni particles by introducing TiO2 as support with Ni catalyst. Electrical conductivity variation of porous YSZ matrix impregnated with Ni and Ni/TiO2 have been investigated. Single button cells (anode supported) with and without TiO2 impregnated Ni–YSZ anode were fabricated and characterized through current–voltage measurement at different loads. It is shown that the conductivity of porous Ni–YSZ anode and the performance of SOFC button cell with the same anode decreased with the increase in temperature and redox cycling at different time intervals. The power density of 12% Ni–YSZ anode was 116 mW/cm2 and it increased to 180 mW/cm2 for 12% Ni–4% TiO2–YSZ based anodes at 800 °C. This increase was interpreted by strong attachment of Ni particles on TiO2 preventing Ni coarsening during prolonged reduction in H2 at 800 °C as observed by SEM. The power density increased with further increase in Ni loading and it reached to 400 mW/cm2 for 16% Ni–4% TiO2–YSZ based anodes. The performance increases with addition of TiO2 support in Ni–YSZ based anodes corroborates with the impedance spectroscopy analyses.

Fabrication of porous Ni–YSZ anodes by PSH-PIM

This paper has investigated the fabrication process of porous Ni–YSZ anodes by the powder injection molding method, in which a powder space holder (PSH) is used. Polymethyl methacrylate (PMMA) has been used as a PSH for mixing with NiO–YSZ powders. For this study, five kinds of feedstock containing 0%, 10%, 20%, 30%, and 40% PMMA by volume were prepared. The thermoplastic binder used for the process had a fixed 35 vol.%, and the powder loads formed the remaining 65, 55, 45, 35, and 25 vol.% of the feedstock. After molding and debinding, the parts were sintered at 1,500 °C. The obtained results showed that increasing the PMMA portion of the feedstock and reducing its powder load causes the viscosity of the feedstock to decrease. The amount of shrinkage of the samples containing 0–30% PMMA showed an almost linear increase with the increase of the PMMA content, and for the samples with 40% PMMA, this increase of shrinkage was higher. The amount of porosity in the samples having 0–30% PMMA increased with the rise in the PMMA content, but in the samples containing 40% PMMA, the amount of porosity decreased, such that it was less than that of the samples with 30% PMMA. The electrical conductivity and flexural strength of all the samples were also studied in this work.

YSZ Thin Films Prepared by a Novel Powder Coating Process on Porous NiO-YSZ Cermet

Dense YSZ thin film with a thickness from 2 to 15 µm on porous Ni-YSZ anode support was fabricated by a novel powder coating process. The performance of the cell with a composite of Sm0.8Sr0.2CoO3- YSZ (40 wt.%) as cathode was examined over a temperature range from 550 to 800 °C, using H2 -3% H2O as fuel and air as oxidant. The maximum power density of the cells was over 300 mW/cm2 at 800 °C. The open circuit voltage of the cells reached 1.1 V at 750 °C, illustrating the full dense of the YSZ electrolyte. The anode-electrolyte interfacial resistances were negligible in comparison with that of cathode-electrolyte, indicating the good coherency of the YSZ electrolyte to the anode support. These results demonstrate the reliability of this novel process for thin electrolyte SOFCs.

Methane reforming kinetics within a Ni–YSZ SOFC anode support

This paper reports experimental and modeling investigations of thermal methane reforming chemistry within porous Ni–YSZ anode materials. Because the reforming chemistry is difficult to observe directly in an operating fuel cell, a specially designed experiment is developed. In the experiment a 0.75 mm-thick anode is sandwiched between two small co-flowing gas channels. One channel represents the fuel channel of a solid-oxide fuel cell (SOFC). The composition in the other channel carries the species that would be produced in an operating fuel cell by the electrochemical charge-transfer reactions in the thin three-phase regions near the interface between the anode and the dense electrolyte membrane (i.e., H 2O and CO 2). Because the anode structure is porous (and there is no dense electrolyte or cathode applied), there is convective and diffusive species flux between the two flow channels. The entire assembly is maintained at approximately 800 ° C in a furnace. The results of heterogeneous reforming kinetics are determined by using mass spectrometry to measure the species composition at the outlet of both channels. Experimental results are interpreted using a computational model that incorporates channel gas flow, porous-media transport, and elementary heterogeneous chemical kinetics. The overall objective is to develop quantitative models of non-electrochemical heterogeneous reforming chemistry within a Ni–YSZ anode.

SOFC Based on Thin-Film Electrolyte

Solid oxide fuel cells (SOFCs) with thin-film electrolyte were fabricated and tested. MOCVD technique was applied for deposition of thin YSZ electrolyte films with thickness of 4-15 μm on porous Ni-YSZ anode substrate. Volatile complexes of metals with 2,2,6,6-tetramethyl-heptanedione: Zr(thd) 4 and Y(thd) 3 were used as the precursors. Cells exhibited a high open circuit voltage of 1.08 V and power density higher than 200 mW/cm 2 at 800°C.