Ni-YSZ Anode Research Articles

Effects of water on the degradations in the Ni-YSZ anode of the direct ammonia solid oxide fuel cells

Direct ammonia solid oxide fuel cells (DA-SOFCs) are promising next-generation energy conversion devices. Despite their possibilities, the electrochemical performance of DA-SOFCs degrade severely over long-term operation. However, the degradation mechanisms are still unveiled. Here, we reveal that the degradations of DA-SOFCs are significantly initiated by the presence of the water, which is the by-product of the SOFC operation, inside the fuel electrode. We measured the stability of the DA-SOFCs by varying the fuels and water partial pressure in the fuel. Thorough surface characterizations verified that Ni becomes favorable for oxidation when ammonia is present in the water, resulting in a reduction of the catalyst reactivity for NH3 decomposition and electric conductivity. Furthermore, we conducted a density functional theory calculation and found that N* adsorbed onto Ni decomposed from NH3 facilitates the H2O decomposition and results in nickel oxidation.

Prediction of Dynamic Ni Morphology Changes in Patterned Ni-YSZ Anode with Physics-Informed Neural Networks

Abstract Nickel (Ni) film on patterned Ni- yttria-stabilized zirconia (YSZ) anode shows dynamic spreading and splitting during solid oxide fuel cell (SOFC) operation, where wettability of Ni on YSZ is greatly enhanced (Z. Jiao, N. Shikazono, J. Power Sources 396 119–123, 2018). In the present study, a physics-informed neural network (PINN) constrained by Cahn-Hilliard equation of phase field model is proposed to estimate the unknown parameters for predicting dynamic Ni movements of the patterned Ni-YSZ anode. The unknown parameters such as interface thickness and mobility are inversely inferred by PINN using top-view images obtained from the operando experiments. Obtained excess surface diffusivity values were three to four orders of magnitude larger than the values reported for surface diffusion in the literature. It is therefore considered that Ni spreading and splitting of patterned anode cannot be simply explained by surface diffusion, and other mechanisms should be introduced.

A robust tubular solid oxide fuel cell through bifunctional praseodymium oxide nanocatalyst infiltration

Due to inherent merits such as rapid start-up, easy sealing, and good thermal cycling resistance, tubular solid oxide fuel cell (SOFC) is becoming a promising alternative device for clean and efficient energy conversion; yet the lower power density is a challenging problem when compared to planar SOFC. In this study, an asymmetric infiltration of Pr6O11 nanocatalyst, a bi-functional catalyst, is innovatively proposed to enhance the electrochemical performance and prolong the cell lifespan. First, a suitable anode functional layer (AFL) based on the Ni-YSZ (yttria stabilized zirconia) anode is presented. The additional AFL results in a significantly enhanced peak power density (PPD) of 1259 mWcm−2 at 800 °C, which is 66 % higher than that of the primitive cell (756 mWcm−2). Subsequently, a new approach of asymmetric infiltration is devised in one step during the cell manufacturing procedure to simultaneously modify the anode and cathode-electrolyte interface to boost cell output power. Consequently, the nanostructure-engineered tubular cell exhibits outstanding peak power density of 1592 mWcm−2 at 800 °C, that is more than twice that of the blank cell, outperforms the majority of the Ni-YSZ supported tubular cells. Furthermore, the robust tubular cell runs on hydrogen fuels with excellent stability over more than 2000 h at a constant current density of ∼300 mAcm−2. The exceptional performance and endurance can be attributed to the rational cell design, which holds great potential for the commercialization of tubular SOFC.

Novel BaO-decorated carbon-tolerant Ni-YSZ anode fabricated by an efficient phase inversion-impregnation approach

In this study, an efficient phase inversion-impregnation approach is developed for fabricating BaO-decorated Ni-8 mol% yttria-stabilized zirconia (YSZ) anode-supported tubular solid oxide fuel cells (SOFCs) toward anti-coking. This technique involves a simple phase-inversion process, along with the saturated barium nitrate solution as coagulation bath. Experimental results show that BaO nanoislands with particle size of less than 100 nm have been successfully and uniformly introduced inside the Ni-YSZ anode. The corresponding peak power densities are determined to be 0.30 W cm−2 and 0.22 W cm−2 at 800 °C in wet hydrogen and methane fuel, respectively. The long-term stability in methane fuel with the BaO-decorated anode is significantly improved with respect to the pristine one. Density functional theory (DFT) calculations suggest that the loading of BaO nanoislands can efficiently capture and dissociate the H2O molecules to generate OH, which subsequently serves as effective carbon elimination medium. The OH diffusion from BaO to BaO/Ni interface coupling with the formation of COH is the key step involved in the carbon elimination mechanism. The as-proposed phase inversion-impregnation approach exhibits advantage in time and cost savings compared to traditional impregnation methods, providing a new route for fabricating anti-coking SOFC anodes.

Modeling and microstructural study of anode-supported solid oxide fuel cells: Experimental and thermodynamic analyses

Developing novel solid oxide fuel cells (SOFCs) with high stability running at low temperatures is an important objective in SOFC science. In the current paper, a comprehensive physics-based microstructure modeling using scanning electron microscope (SEM) image analysis was performed on several anode-support SOFCs operating at low temperatures with high stability. To bridge the gap in the literature regarding an accurate and realistic modeling, a new model was developed based on the variable fuel and air utilization factors and updated microstructure values (e.g., tortuosity, porosity, pore size, grain size). The model accuracy was verified by a thorough point-to-point validation for eight different cells with the configuration of Ni-YSZ (anode), YSZ (electrolyte), and GDC/PNO (cathode). Different temperatures, hydrogen, and air mass flow rates were used, for which an average error of less than 3% in the I-V curves was achieved. The microstructure of the cells, including cathode thickness (15–26 μm), anode-support thickness (350–460 μm), porosity (39 and 43%), grain size (1.1–1.4 μm), and pore radius (0.9–1.1 μm) were varied. Moreover, the effects of the critical operational and design parameters on the overpotential losses and cell performance were studied. The results show that a hydrogen flow rate of 43 sccm was ideal when the cell operated at 0.9 A/cm2 and 700 °C. Moreover, an average anode-support pore radius of 1.75 μm resulted in the best cell performance. It was also concluded that the electrolyte thickness has a higher effect on the cell performance compared to the cathode thickness.

Temperature-dependent chemical incompatibility between NiO-YSZ anode and alkaline earth metal oxides: Implications for surface decoration of SOFC anode

Alkaline earth metal oxides (AEMOs) have been recognized one of the most effective modifiers to alleviate coking issue of Ni-based anodes of solid oxide fuel cells (SOFCs). Nevertheless, the interaction between AEMOs and Ni-based anode matrix may also degrade the cell performance. In this work, the chemical compatibility between AMEOs and NiO-YSZ anode as well as their possible consequences were systematically investigated from both experimental and theoretical perspectives. XRD analyses reveal that the chemical compatibility of AEMOs with NiO-YSZ is temperature-dependent. The formation of alkaline earth metal (AEM) zirconates is detrimental to the oxygen ionic conductivity while the formation of AEMO-NiO solid solution suppresses the catalytic activity and electrical conductivity of Ni-YSZ anodes. The Ca, Sr and Ba enrichment in YSZ are observed, indicating the higher affinity of YSZ to AEMOs than that of NiO. Two potential strategies to preferentially deposit AEMO on Ni particles without chemical interaction are proposed. It is anticipated the findings in this work can provide guidance for designing Ni-based SOFC anode with both high-performance and anti-coking.

Electrochemical Performance of Direct Ammonia Solid Oxide Fuel Cell

Recently, as regulations on greenhouse gas emissions have been strengthened in various industries such as automobiles and ships, many studies on alternative energy are being conducted. Hydrogen is a good energy carrier and an environmentally benign fuel because the reaction product is only water or water vapor when it is burned or is used for fuel cells. However, hydrogen suffer from its low specific volume, so the storage issue is still remained unsolved.Ammonia (NH3) has a higher energy density (12.7 MJ/L) than liquid hydrogen (8.5 MJ/L). Liquid hydrogen has to be stored at cryogenic conditions of –253°C, whereas ammonia can be stored at a much less energy-intensive –33°C. In addition, ammonia is much less flammable than hydrogen although it is hazardous to handle.In this study, anode-supported SOFCs consisting of LSCF-GDC cathode, YSZ electrolyte, and Ni/YSZ cermet anode was fabricated and their electrochemical performances (Impedance spectra, J-V curve and cell voltage degradation with operation time) were investigated at 600, 650, and 700°C under ammonia and hydrogen. Regardless of the operating temperature, the power densities of ammonia-fueled SOFCs were almost similar to those of hydrogen-fueled SOFCs. This suggested that the ammonia cracking reaction effectively occurred in the Ni-YSZ anode support. However, the cell voltage at 0.2 A/cm2 was slightly decreased as a function of operating time, which means that the nickel was degraded under ammonia atmosphere. The change of an anode’s microstructure before and after the degradation test will be discussed. In addition, Ni-YSZ catalysts for ammonia cracking were fabricated and their catalytic activity was measured in terms of operating temperature and space velocity using in-line gas chromatography. The phase evolution and microstructure of catalysts were characterized by XRD, SEM and TEM.

Contrasting Intermediate-Temperature La0.7Sr0.3VO3.86-⸹ (LSV) Porous Catalyst Elevated Sulfur Tolerance Using Gaseous and Liquid Fuel Explored through Experimental and Modeling Characterization Over Time

The U.S. Army is investigating Solid Oxide Fuel Cells (SOFCs), which can produce clean, secure, and sustainable energy to replace existing power sources. Today SOFCs are known to provide high density, high efficiency power with long term stability at low cost and environmental impact, utilizing a variety of naturally sourced fuels such as hydrogen, hydrocarbons, and alcohols. This fuel flexibility makes SOFCs attractive for use with multiple applications and helps bridge the gap between current hydrogen-fueled SOFC technologies and future hydrocarbon-fueled based use. Despite this potential, many technical hurdles remain, such as thermal stress cracking and coarsening, delamination, and catalyst contamination. Catalyst contamination is of particular concern for the U.S. Army, which operates on JP-8 kerosene-based fuel via AR-70 single fuel policy, as sulfur impurities commonly found in naturally sourced hydrocarbon-based fuels, can poison the catalyst, permanently degrading SOFC system performance, even at low concentrations. As the U.S. Army begins to investigate SOFCs to enable additional capabilities like silent watch, advanced radios, and exportable power in their fleet, catalyst poisoning from sulfur presents a concerning technology gap that must be addressed, especially since it is also possible that sulfur contaminated fuel, scavenged in theater, may be used, and could contain elevated levels (>300ppm) of sulfur.While lower temperature SOFC operation is also actively being pursued to help alleviate physical material degradation issues, with the introduction of hydrocarbon-based fuels, catalyst development to reduce sulfur poisoning degradation also needs further exploration. This study continues to experimentally and theoretically investigate the response of sulfur tolerant anode catalyst La0.7Sr0.3VO3.86-⸹ (LSV) at currently targeted SOFC intermediate operating temperatures (400-600°C). We have previously investigated low and moderate hydrogen sulfide (H2S) concentrations (30ppm, 300ppm) in balance hydrogen and methane gas environments for up to 100 hours. In this work, LSV showed significant sulfur tolerance in hydrogen and methane balance gases when compared to the industry standard Ni-YSZ anode, where sulfur adsorption rates were 278-287x lower in some cases. The lowest rates occurred between 600-700°C, which is attributed to the cubic structure the material presents in this temperature range. Higher adsorption rates were observed in the monoclinic/tetragonal structure the material presents at temperatures between 400-500°C. Average adsorption rates were overall higher in methane. When compared against Ni-YSZ, which is known to accumulate sulfur from hydrogen sulfide via two-step dissociative adsorption, LSV was observed to accumulate sulfur through weak chemisorption (H2S*, 0.43 eV max) of molecular hydrogen sulfide on oxygen deficient surfaces and one-step dissociative adsorption (H2S*, 0.34 eV, max, HS*+H*, 8.34 eV, max) on oxygen sufficient surfaces. The weak chemisorption behavior on oxygen deficient surfaces is attributed to the reduced oxygen stoichiometry of the material, in which only vanadium V2+/3+ couples exist. Overall, molecular adsorption/dissociation was observed to occur near strontium defects on LaO/SrO terminations. The propensity of H2S adsorption on LSV surfaces was observed to be significantly less for oxygen deficient LSV, and significantly higher for oxygen sufficient LSV, when compared to H2S adsorption calculations on Ni (100) surfaces, which follow a two-step dissociative adsorption reaction (H2S* → HS*+H* → H*+S*), resulting in a strongly adsorbed S* species (5.96 eV, max).Under the Army’s single fuel policy, it is more likely SOFCs would be fueled using hydrogen and/or methane, which are both gases, however SOFCs in the future could also be directly fueled using liquid alcohol fuels, or as scavenged from theater. Typical small alcohol molecules (methanol or ethanol) do not contain high concentrations of sulfur; however, alcohols can be contaminated with sulfur-containing hydrocarbons. This work will investigate sulfur adsorption using methanol contaminated with 300ppm thiophene using the same operating temperature and heating duration conditions with hydrogen and methane. These results will be contrasted against the 300ppm H2S hydrogen and methane results previously reported. Figure 1

Conditional GAN for Generation of 3D SOFC Electrode Microstructure Dataset

Synthetic three-dimensional porous structures of solid oxide fuel cell anode are generated using the deep convolutional conditional generative adversarial neural network (DCCGAN). The developed network consists of a generator that produces an artificial structure dataset from random numbers, so-called latent variables, and a discriminator that judges whether the input structure dataset is real or fake. The generator and discriminator are alternately trained to improve their performance in an adversarial manner so that the generator can eventually create realistic structures indistinguishable from the real ones. The training process is performed using the real structure dataset of conventional Ni-YSZ anodes with different solid volume ratios, i.e., Ni: YSZ = 70:30, 50:50, and 30:70. In this study, additional training is performed for the generator to control the volume fraction of the generated structures. First, the values of the volume fractions are input to the generator alongside the latent variables. Then, the volume fraction loss is calculated from the difference between the volume fractions of the generated structure and the input values. Finally, the volume fraction loss is used to train the generator in addition to the ordinal training process. The trained network is validated by comparing the generated structures with the real structures in terms of microstructural parameters such as volume fraction, surface area density, and triple-phase boundary density. This comparison is conducted not only for the structures used for the training but also those that are not used for the training, i.e., a solid volume ratio of Ni: YSZ = 40:60 and 60:40. It is found from the results that the volume fractions of the generated structures are in good agreement with the specified input values to the generator. Also, other microstructural parameters quantified from the artificial microstructures are similar to those of the real structures. In addition, since the developed network does not contain fully-connected layers, and is hence categorized as a fully-convoluted network, structures with arbitrary sizes can be generated using the same generator by changing the dimension of the latent variables. The generated structures are confirmed to be free from digital artifacts such as noise, disconnectivity, and periodicity in structures.

Microstructural Changes in Ni-YSZ Electrodes Operated in Fuel Cell and Electrolysis Modes: Effect of Gas Diffusion Limitations

Lifetime performance stability is a key issue for the commercialization of solid oxide cells. Ni migration in Ni-YSZ electrode supported cells is an important degradation mechanism. Here we present the results of life tests on symmetric Ni-YSZ electrode-supported cells. These symmetric cells have similar processing and microstructure as conventional Ni-YSZ-supported cells, but the symmetric structure provides information on the Ni-YSZ electrode operating as both an anode and a cathode in the same test. Life tests of up to 1000 h were carried out at 800 ˚C in 50-50 H2-H2O at current densities of 0, 0.75, 1.00, and 1.50 A/cm2. Total applied voltage to each cell was tracked over the lifetime, and small portions of the cells were removed during the life tests to study the time-dependent changes in microstructure.Figure 1 summarizes the microstructural results from a Ni-YSZ cell operated at 1.0 A/cm2, observed at 100, 500, and 1000 h. 2D and 3D microstructure characterization was used to show the volume fractions of pore and active (electrically connected) Ni versus position. Somewhat surprisingly, the cathode microstructure remains relatively unchanged compared to a non-polarized cell, with no evidence of Ni migration or isolation. In contrast, Ni migration and isolation was observed to increase with time at the anode in polarized cells, with the thickness of the Ni deactivated region growing from ~5 mm at 500 h to ~15 mm at 1000 h for 1.0 A/cm2 and ~2 μm at 500 h and ~8 μm at 1000 h for 0.75 A/cm2. In addition, total cleavage at the anode-electrolyte interface occurs at 1.5 A/cm2 by 100 h, and at 1.0 A/cm2 by 1000 hr. The increase of porosity in the altered zones clearly shows where Ni is depleted. There is no evidence of Ni enrichment adjacent to the depleted region, as might be expected if Ni was moving via surface diffusion.Although there have been numerous recent reports of Ni migration/isolation in Ni-YSZ cathodes during electrolysis cell operation1, Ni migration has also been reported in Ni-YSZ fuel cell anodes2,3. Here we suggest that such results can be explained by a relatively high steam content in the Ni-YSZ anode functional layer. Conversely, the lack of Ni migration in the electrolysis cathode can be explained by a relatively low steam content in the cathode functional layer. To quantitatively assess the gas compositions, one-dimensional modeling of the electrochemical and gas diffusion processes of these cells was performed using a finite difference method (FDM) with modified Butler-Volmer kinetics and the dusty-gas model respectively. Using diffusivity values calculated via electrochemical impedance spectroscopy and microstructural measurements, the modeling reveals that electrochemically active Ni sees significantly different gas composition than the inlet, creating steam-rich anodes (PH2O = 0.73, 0.8, and 0.95 atm for 0.75, 1.0 and 1.5 A/cm2 respectively) and steam-depleted cathodes (PH2O = 0.27, 0.2, and 0.05 atm for 0.75, 1.0 and 1.5 A/cm2 respectively). This is in accord with results and models suggesting that Ni migration is important mainly under high steam conditions, probably due to vapor transport. Note that these effects are exacerbated by the relatively low porosity and small pore size in the present electrodes that lead to asymmetric Knudsen diffusion, wherein H2O diffuses at ~1/3 the rate of H2. These results suggest that the nature of the porosity in Ni-YSZ supports can lead to significant variations in the extent and directionality of Ni migration.Figure 1: (a, b, c) Polished cross sectional backscatter electron (BSE) images and (d,e,f) low voltage secondary electron (LV-SE) images of an electrode-supported symmetric Ni-YSZ cell at 100, 500, and 1000 h of galvanostatic operation at 1.00 A/cm2 in 50-50 H2-H2O and T = 800 ˚C. BSE images reveal a depletion of total Ni at the anode-electrolyte interface (AEI) by a net increase in porosity, and eventual cleavage at that line. LV-SE images reveal that near total deactivation of Ni occurs near the AEI as well. (g,h,i) Quantitative image analysis reveals the extent of porosity increase (via Ni depletion) and Ni deactivation at 100, 500, and 1000 hours. The regimes for both events exactly overlap, with the depletion/deactivation extending 0, 5, and 15 μm from the AEI, respectively. M. B. Mogensen et al., Fuel Cells, 21, 415–429 (2021).J. Geng et al., J. Power Sources, 495, 229792 (2021).Z. Jiao and N. Shikazono, J. Power Sources, 396, 119–123 (2018). Figure 1

Physical-chemical processes at Mo-doped ceria-yttria stabilized zirconia (YSZ) anodes in solid oxide fuel cells using syngas derived by the distribution of relaxation times method

The use of environment-friendly carbonaceous fuels (such as, biomass and synthetic fuels) to produce electricity using solid oxide fuel cells (SOFCs) has emerged as a promising alternative to the use of high-purity hydrogen, which remains as a difficult to produce and store gas. Much effort has been put in the development of SOFC anodes highly active and stable for the use of carbonaceous fuels, such as ceria-based materials, but only recently research has focused on the comprehensive analysis of the interfacial physical-chemical processes affecting their performance.This work presents an in-depth analysis of the interfacial processes occurring at a Mo-doped ceria-yttria stabilized zirconia (YSZ) cermet anode in a SOFC using humidified syngas as fuel. To this end, electrochemical impedance spectroscopy (EIS) data is generated and analysed by the distribution of relaxation times (DRT) method. In addition, gas chromatography analysis of the internal reforming products generated under open circuit voltage (OCV) conditions and post-mortem analysis of the electrode materials after polarization are discussed in support of the DRT results.It is observed that the presence of carbon monoxide (CO) in fuel, as compared to pure hydrogen, influences the anodic charge transfer processes not only due to promotion of the water-gas-shift reaction at the anode two-phase (fuel | electrode) and three-phase (fuel | electrode | electrolyte) interfaces, but also because of its direct electrooxidation at the anode three-phase interfaces. The electrooxidation of carbon monoxide presents a slightly higher activation energy than the electrooxidation of hydrogen (ca. 1.86 and 1.21 eV, respectively), which explains the slower kinetics of SOFCs when using syngas as fuel. Additionally, other oxidation processes of CO at the anode interfaces (such as, the Boudouard reaction) promote the formation of carbon deposits lacking ordered structure, which are more reactive than graphitic structures formed at Ni-YSZ anodes but still can negatively influence the electrochemical kinetics.

Development of robust YSZ thin-film electrolyte by RF sputtering and anode support design for stable IT-SOFC

Thin-film solid oxide fuel cells (TF-SOFCs) have attracted attention as a strategy for lowering the operating temperature of SOFCs. However, the porous and rough surface of the SOFC support and the shadowing effect have hindered the construction of pinhole-free thin-film electrolytes during sputtering. In this study, we report the deposition of a gas-tight YSZ thin-film electrolyte (approximately 0.8 μm) by RF sputtering on a Ni-YSZ anode support. To utilize a sputtered thin film as an electrolyte, we examined the relationship between the deposition pressure and thin-film properties and determined the optimal deposition conditions. By designing the anode functional layer (AFL) and annealing, a thin-film electrolyte is successfully deposited on the porous Ni-YSZ support without cracks and pinholes. The resulting thin-film electrolyte cell achieves a high open-circuit voltage (OCV) of approximately 1.09 V and a maximum power density of 0.446 W/cm2 at 600 °C, which implies high ionic conductivity and gas tightness. Moreover, the single cell exhibited exceptional long-term stability (140 h) despite the extremely thin and vacuum-deposited electrolyte components. This study provides guidelines for the practical application of thin films as electrolytes to lower the operating temperature of SOFCs.

Multiscale analysis of Ni-YSZ and Ni-CGO anode based SOFC degradation: From local microstructural variation to cell electrochemical performance

Nickel based fuel electrodes are widely used for commercial solid oxide fuel cells showing a high catalytic activity, despite of involving severe microstructural changes which reduce the system lifetime. Needing a detailed knowledge of such phenomena, the authors compare the behaviour of two state-of-the-art planar cells, Ni-YSZ based anode supported cell and Ni-CGO based electrolyte supported cell, working for 1000 hours under a galvanostatic operation with H2 rich feed. Following a multiscale approach, the system was analysed in terms of both global performance and local properties. Experimental observations through electrochemical characterization and microstructural analysis laid the basis for developing a physics-based model able to predict the cell operation at reference and aged status. Indeed, the kinetics was expressed as a function of microstructural features and considers the time evolution of some parameters. Ni-based electrode was identified as the first source of degradation due to Ni instability resulting in a reduction of catalytic activity and conductivity, correlated mainly to Ni particle coarsening and migration respectively. Each degradation mechanism prevailed depending on the material structure (i.e., initial particle size and distribution) and imposed working conditions (i.e., temperature, load and gas composition).

Optimizing infiltration parameters of nanostructured anode electrode in solid oxide fuel cells

This study focuses on the development and optimization of nanostructured anode electrodes for solid oxide fuel cells (SOFCs) by infiltration method. Ni (nickel) catalyst in the form of a nickel nitrate solution is infiltrated into porous YSZ (yttria stabilized zirconia) backbone fabricated by tape casting. Numerous electrolyte-supported cells are fabricated to investigate the effects of several significant infiltration fabrication parameters such as catalyst loading and infiltration sintering temperature. Conventional cell with screen printed Ni-YSZ anode is also fabricated for comparison. Electrochemical performances and microstructural properties of the cells are examined and evaluated. The best peak performance of 0.398 W/cm2 at 800 °C is obtained from the cell, which is infiltrated 9 times with 2 M solution followed by firing at 800 °C. The conventional cell, on the hand, exhibits only 0.174 W/cm2 under the same testing conditions in spite of the relatively higher Ni catalyst content in the anode. Furthermore, the optimized cell produces 0.169 W/cm2 maximum power density at 700 °C. The overall results reveal that nickel catalyst infiltration is a very effective method to improve the cell performance by providing increased number of electrochemical reaction zones within the anode electrode due to nanostructured nickel catalyst formed around the main YSZ phase.

Enhanced stability of co-reforming diesel and methanol into hydrogen-enriched gases for solid oxide fuel cell application

Steam reforming of diesel is performed over Ru/γ-Al2O3 and Ni/γ-Al2O3 catalysts. The catalysts both achieve 100% diesel conversion rate and high hydrogen selectivity at temperatures over 600 °C but Ni/γ-Al2O3 catalyst shows a poor carbon tolerance, carbon deposition has been detected after 10 h of test which is determined as encapsulating carbon. Co-reforming diesel and methanol is conducted by adding methanol into the steam-diesel mixture as a stabilizer and it successfully extends the long-term life from 120 h to 600 h at 750 °C. The reformed diesel gas (RDG) is then fed into a solid oxide fuel cell (SOFC) to investigate fuel adaptability. The cell has achieved a peak power density of 1.34 W/cm2 at 750 °C under RDG fuel, and the cell voltage has stabilized at 0.82 V for 200 h during the galvanostatic discharge of 0.4 A/cm2. No detectable carbon deposition has occurred on the surface of the Ni-YSZ anode. The results demonstrate that the reformed diesel gas provides a good fuel choice for solid oxide fuel cell application.

3D microstructures of solid oxide fuel cell Ni-YSZ anodes with carbon deposition

Carbon deposition is a critical problem for operating solid oxide fuel cells (SOFCs) with carbon containing fuels such as natural gas, biogas, etc. When steam to carbon ratio is below the critical threshold, carbon may form on the surface of nickel (Ni) catalyst. Carbon covers the active triple boundaries and deteriorates the electrochemical performance. In most severe cases, carbon may even lead to an irreversible deformation of a porous cermet. However, quantitative investigations of carbon deposition have been limited due to the difficulties in observing the microstructures. In the present study, the 3D structures of carbon deposition in the SOFC electrode are quantitatively reconstructed for the first time by a machine learning assisted image processing framework. Focused ion beam - scanning electron microscope (FIB-SEM) measurements of anode samples are conducted without resin infiltration. A U-net neural network with multiple inputs is developed to identify the unfilled pores, from which phase segmentations of raw SEM images without resin filling became possible. The mild and severe carbon deposition conditions are tested by exposing the electrodes to humidified and dry methane conditions. It is revealed from the 3D reconstruction that carbon layer formed over Ni particles and Ni pulverizes due to high temperature corrosion (metal dusting). At the same time, YSZ network deformed by the internal stress. The observed microstructural changes are in good agreement with the performance recovery after the regeneration process. The developed microstructure evaluation framework will enable quantitative investigations of carbon deposition and developments of mitigation strategies for SOFC anodes.

Nanoengineering modification of Ni-YSZ anode using in-situ solvothermal process in solid oxide fuel cells with internally reformed fuel

The conventional Ni-Y0.16Zr0.84O1.92 anode in solid oxide fuel cells fueled with hydrocarbon fuels is prone to suffer from carbon deposition. In this work, we employed an in-situ solvothermal method to decorate the Ni-Y0.16Zr0.84O1.92 anode with Gd0.1Ce0.9O1.95-δ nanoparticles to improve its resistance to carbon deposition and thus improve the electrochemical performance of the cell. This method generates small Gd0.1Ce0.9O1.95-δ particles and at the same time does not block the gas diffusion channels. The Gd0.1Ce0.9O1.95-δ-decorated anode showed enhanced long-term stability in methane with no significant performance degradation after 700 at 750 °C, whereas an untreated Ni-Y0.16Zr0.84O1.92 anode only worked for less than 8 h under the same conditions. The addition of Gd0.1Ce0.9O1.95-δ nanoparticles was found to enhance internal reforming reactions of methane by the distribution of relaxation times technique. This work demonstrated that in-situ solvothermal modification strategy can provide anodes with excellent carbon deposition resistance.

Partial oxidation of methane enabling complete coking resistance of Ni-YSZ anode supported solid oxide fuel cells

• Partial oxidation of methane ensures complete coking resistance of Ni-YSZ. • Small decrease in cell performance can be observed with the oxygen content increasing. • Enhanced carbon activity is attributed to the partial oxidation of methane. • Incorporating partial oxidation and electrochemical oxidation is an effective approach. The utilization of natural gas as fuel for Ni-YSZ anode supported solid oxide fuel cells (SOFCs) is a most promising method to accelerate the commercialization process. However, the state-of-art Ni-based anode is sensitive to the natural gas, leading to the carbon formation and cell degradation. Herein, a catalytic partial oxidation approach is proposed by introducing different amounts of oxygen in methane for improving the coking resistance of Ni-based anode. The carbon activity is reduced from 400 to 0.4 when using 15%CH 4 -5%O 2 as fuel, and the electrochemical performance is only decreased by 10 %. Considering the working condition of SOFCs, a complete coking resistance of SOFC can be obtained by introducing only 1.25% O 2 at 0.59 V. This indicates that introducing a small amount of oxygen in methane is beneficial for improving coking resistance of SOFC under working condition.

Catalytic and electrocatalytic performance of Sr(Ti0.3Fe0.7Ru0.07)O3-δ for applications in solid oxide fuel cells supplied with ethanol steam reforming mixtures

Direct utilization of alcohols in Solid Oxide Fuel Cells (SOFCs) can improve the conversion efficiency by using excess heat for internal reforming and simplify the system balance of plant. Thanks to their scarce carbon-formation tendency, perovskite oxide anodes are promising alternatives to Ni-based cermets. Here we show that Sr(Ti0.3Fe0.7Ru0.07)O3-δ (STF-Ru) exsolution anodes provide both catalytic reforming activity and electrochemical activity with steam/ethanol (S/E) mixtures at 3:1 and 2:1 M ratio between 700 and 800 °C. Electrolyte-supported scandia stabilized zirconia SOFCs with STF-Ru anodes and Sr0.95Ti0.3Fe0.7O3-δ (STF) cathodes are reasonably stable in the 3:1 S/E mixture over 20 h at 800 °C, with maximum power density of 195 mW/cm2 and 140 mW/cm2 at 0.7 V. Deactivation instead occurs with the 2:1 S/E mixture. Comparatively, cells with Ni-YSZ anodes rapidly coke and fail under the same conditions. Ethanol steam reforming experiments in a fixed bed powder reactor reveal that, above 650 °C, gas phase reactions convert ethanol to CO, hydrogen, acetaldehyde and ethylene, while STF-Ru catalyzes steam reforming. Thermogravimetric and Raman characterizations show that a moderate amount of carbon is deposited on the STF-Ru surface, which is more amorphous than the disruptive carbon which degrades Ni-YSZ.

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).