SOFC Anode Research Articles

Investigation of praseodymium and samarium co-doped ceria as an anode catalyst for DIR-SOFC fueled by biogas

The Pr and Sm co-doped ceria (with up to 20 mol.% of dopants) compounds were examined as catalytic layers on the surface of SOFC anode directly fed by biogas to increase a lifetime and the efficiency of commercially available DIR-SOFC without the usage of an external reformer.The XRD, SEM and EDX methods were used to investigate the structural properties and the composition of fabricated materials. Furthermore, the electrical properties of SOFCs with catalytic layers deposited on the Ni-YSZ anode were examined by a current density-time and current density-voltage dependence measurements in hydrogen (24 h) and biogas (90 h). Composition of the outlet gasses was in situ analysed by the FTIR-based unit.It has been found out that Ce0.9Sm0.1O2-δ and Ce0.8Pr0.05Sm0.15O2-δ catalytic layers show the highest stability over time and thus are the most attractive candidates as catalytic materials, in comparison with other investigated lanthanide-doped ceria, enhancing direct internal reforming of biogas in SOFCs.

Methane steam reforming in water-deficient conditions on a new Ni-exsolved Ruddlesden-Popper manganite: Coke formation and H2S poisoning

This research deals with the catalytic behavior of the methane steam reforming reaction over a new Ni-exsolved Ruddlesden-Popper manganite during prolonged reaction time (up to 100 h) with special focus on the possible carbon deposition and H2S poisoning. La1.5Sr1.5Mn1.5Ni0.5O7±δ material was synthesized and reduced in diluted hydrogen to induce Ni exsolution. Its catalytic behavior in long reaction times was compared to Ni-impregnated manganite and Ni/YSZ cermet. The catalytic measurements for the steam reforming reaction were carried out at 850 °C in low steam-to-carbon conditions. All materials are susceptible to H2S poisoning (50 ppm), forming undesired sulfide compounds with damaging impact on their catalytic activity. In contrast, during tests without H2S, the activity for cermet and impregnated materials drops at relatively short reaction time due to coking formation, as evidenced by TEM and TGA/MS analysis, while the behavior of new exsolved material remains stable throughout the test. This high stability of the new exsolved catalyst over a prolonged reaction time is a noticeable advantage due to its potential use as SOFC anode fed with natural gas free of H2S.

Rationally structuring proton-conducting solid oxide fuel cell anode with Ni metal catalyst and porous skeleton

In response to the urgent demand for highly active anodes for lower-temperature proton-conducting solid oxide fuel cells (H–SOFCs) and with the aim to explore the function of metal catalysts and porous skeletons, two series anode-supported BaZr0.1Ce0.7Y0.2O3-δ (BZCY)-based single cells with varying Ni catalysts and pore formers were assembled and evaluated comparably. In the exploration of Ni catalyst variable, the NBZCY65-35-20SS (65 wt% NiO in NiO-BZCY prepared with adding 20 wt% starch) anode possesses the highest performance, for the 1:1 vol ratio of Ni-BZCY could offer the maximum effective triple-phase boundary (TPB) area in the prerequisite of having abundant pores achieved with the 20 wt% pore former as a constant. In addition, both the NBZCY65-35-15SS and NBZCY65-35-25SS anode demonstrate inferior electrochemical properties separately due to the inadequate reducing gas transmission channels and reaction sites. The BZCY cell assembled with NBZCY65-35-20SS reveals an excellent performance, in which the peak power densities (PPDs) were 660, 539, 413, 272 mW cm−2 and the polarization resistances (RP) were 0.061, 0.126, 0.28, 0.652 Ω cm2 at 700, 650, 600, 550 °C, respectively. NBZCY65-35-20SS, which has both a superior TPB area and a fine porous anode skeleton, is a preferable option for anode-supported H–SOFCs. On the whole, the scientific regulations governing metal catalysis and pore-forming could be beneficial to the architecture of fine H–SOFC anode structures.

Coking resistant Ni–La0.8Sr0.2FeO3 composite anode improves the stability of syngas-fueled SOFC

An improved SOFC anode with excellent stability against carbon deposition with syngas as fuel is reported. The anode material is Ni–La0.8Sr0.2FeO3 (LSF) composite synthesized by anhydrous impregnation. After reduction in wet H2 (3% H2O), the material partially decomposes to SrLaFeO4 and exsolved Fe. The exsolved Fe forms Ni–Fe alloy with impregnated Ni. The particle size of Ni–Fe alloy is about 20–50 nm. The Ni–Fe alloy nanoparticles disperse on the surface of the La0.8Sr0.2FeO3 and SrLaFeO4 oxides. The increase of Ni content promotes the exsolution of Fe and increases the reaction sites of Ni–Fe alloy. With the increase of the Ni content, the electrical conductivity and catalytic activity are enhanced, which improves the electrochemical performance of the single cell. The cell with 10 mol.% Ni impregnated Ni-LSF as anode achieves a maximum power density of 550 mW cm−2 at 700 °C fueled with syngas. The strong interaction of the nano-sized Ni–Fe alloy with the LaxSryFeOz (La0.8Sr0.2FeO3 or SrLaFeO4) oxide substrate efficiently suppresses carbon deposition with high graphitization degree. Besides, the SrLaFeO4 phase which can accommodate interstitial oxygen facilitates the removal of the deposited carbon.

In-situ exsolved FeRu alloy nanoparticles on Ruddlesden-Popper oxides for direct hydrocarbon fuel solid oxide fuel cells

FeRu alloy (FRA) nanoparticles surface decorated Ruddlesden-Popper type layer perovskite PrSrFe1-xRuxO4+δ (RP-PSFeRu) was prepared by in-situ reduction of the cubic (Pr0.5Sr0.5)0.9Fe0.9Ru0.1O3-δ (P–PSFeRu) in H2 at 800 °C. When used as the SOFC anode material, it has excellent catalytic activity for H2 and hydrocarbon fuels. The La0.8Sr0.2Ga0.83Mg0.17O3-δ electrolyte supported SOFC single cell with RP-PSFeRu-FRA-GDC composite anode can deliver a maximum power density of 0.75 and 0.50 W cm−2 in wet H2 and C3H8 at 800 °C, respectively. Furthermore, the single cell shows a stability outputs at a constant current load of 0.5 A cm−2 in wet H2 and 0.15 A cm−2 in wet C3H8 fuels, indicating an exceptional stability and cooking resistance.

Europium-Doped Ceria Nanowires as Anode for Solid Oxide Fuel Cells

CeO2-based materials have been studied intensively as anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, pristine and europium (Eu)-doped CeO2 nanowires were comprehensively investigated as anode materials for IT-SOFCs, by a combination of theoretical predictions and experimental characterizations. The results demonstrate: (1) Oxygen vacancies can be energetically favorably introduced into the CeO2 lattice by Eu doping; (2) The lattice parameter of the ceria increases linearly with the Eu content when it varies from 0 to 35 mol.%, simultaneously resulting in a significant increase in oxygen vacancies. The concentration of oxygen vacancies reaches its maximum at a ca. 10 mol.% Eu doping level and decreases thereafter; (3) The highest oxygen ion conductivity is achieved at a Eu content of 15 mol.%; while the 10 mol.% Eu-doped CeO2 sample displays the highest catalytic activity for H2-TPR and CO oxidization reactions. The conducting and catalytic properties benefit from the expanded lattice, the large amount of oxygen vacancies, the enhanced reactivity of surface oxygen and the promoted mobility of bulk oxygen ions. These results provide an avenue toward designing and optimizing CeO2 as a promising anode for SOFCs.

Numerical Modeling of Transport Properties in Battery Electrodes

Analysis and optimization of transport properties such as the effective diffusion coefficient have a strong impact on the development of advanced batteries. Since electrodes of all batteries, from lithium-ion batteries to fuel cells, used in various applications have different porous structures, there is a need to evaluate their transport property dependence on the microstructure of the electrodes.Among the calculation methods of effective diffusion coefficient, the methods based on the simulation of the tortuosity have been gaining attraction of many scientists. Kishimoto et al. [1] evaluated the tortuosity of SOFC anode for its microstructure optimization applying a random walk method. Iwai et al. [2] studies on tortuosity factor evaluation showed less than 3% differences between random walk and lattice Boltzmann methods (LBM). Tjaden et al. [3] compared the tortuosities calculated using the fast marching image-based method with those obtained by using the diffusion cell experimental measurements. Semeykina et al. [4] developed a tortuosity-based calculation method of diffusion coefficient for optimization of catalyst texture used to macromolecule conversions applying Voronoi radical tessellation. Our study is concentrated to uncover the relationship between Voronoi parameters and transport properties since a detailed analysis of the porous structure of powder compact via Voronoi tessellation will provide the opportunity to anticipate optimal decisions for many parameters such as particle shape, size distribution, and composition.In the present research, three electrode structures were generated using CFDEM®WORKBENCH software based on the discrete element method (DEM) [5] as ternary powder compact with different size ratios. Periodic boundary conditions were set along x- and y-axes and fixed one along z-axis. Due to periodic boundaries the wall effect during particle compaction is reduced. The generation of the packing structure was carried out under gravity. Each of three compacts consists of small, medium and large spherical particles with volume fractions equal to 25 %, 25 % and 50 %, respectively, having the ratios of their diameters equal to 1:2:4 for the first compact, and 1:2:6 and 1:2:8 for the second and third compacts, respectively. The number of particles varied from roughly twenty thousand to thirty thousand. Then, the Voronoi radical tessellation was applied to the compacts to simulate the Voronoi diagram representing the pathway network of the void spaces among grains. The tesselation was carried out using the open-source software package voro++ which also output Voronoi cell parameters, i.e. cell volume, total surface area, number of faces, etc. as described by Akhmetov et al. [6]. Finally, the tortuosity was calculated using the Dijkstra algorithm on the Voronoi diagram. The Dijkstra algorithm calculates the shortest path between two specified points. Here it was applied to every pair of vertices lying on the opposite faces of the domain separately in x, y and z directions. The path length was calculated by summing the Voronoi cell edge lengths along the path. This methodology is summarized in Figure 1.The packing densities of particle compacts generated by DEM were measured as 0.651, 0.677 and 0.690 for the first, second and third compact, respectively. Voronoi tessellation provided information about the length of every cell edges and their connectivity, necessary for tortuosity calculation. Results of tortuosity simulations demonstrate that there is no significant difference between tortuosity values of three compacts in the x and y directions, while the well and medium packed compacts give higher tortuosities (approximately 1.27) than the loosely packed ones in the z-direction (1.17 in average). It can be attributed to the periodic boundary conditions along the x- and y-directions and gravity applied in the z-direction. Since the Voronoi cell metric properties such as volume and surface area vary significantly with the particle size ratio, the lengths of the cell edges will also vary in the same way. Therefore, there is a positive correlation between the Voronoi cell metric properties and the effective diffusivity in the powder compact.

High-performance Ni in-situ exsolved Ba(Ce0.9Y0.1)0.8Ni0.2O3-δ/Gd0.1Ce0.9O1.95 composite anode for SOFC with long-term stability in methane fuel

For intermediate temperature solid oxide fuel cell (IT-SOFC) using hydrocarbon gases as fuels, the carbon deposition resistance of the anode is the most critical factor affecting its long-term stability. In this study, Ba(Ce0.9Y0.1)0.8Ni0.2O3-δ (BCYN) decorated Gd0.1Ce0.9O1.95 (GDC) composite is prepared by solution impregnation as anode for SOFC. And metal Ni nanoparticles are in-situ exsolved from BCYN as catalyst for the fuel gas cracking. At 750 °C, the polarization resistance of the optimized cell with Ni-BCYN/GDC composite anode are as low as 0.085 and 0.12 Ω cm2 in H2 and CH4, respectively. The related electrolyte-supported full cell's peak power density up to 211 mW cm−2 can be achieved in CH4 at 750 °C. In the long-term polarization test (200 mA cm−2), single cell with Ni-BCYN/GDC composite anode maintains stable output for 100 h without obvious attenuation and carbon deposition.

Development of tungsten stabilized SrFe0.8W0.2O3-δ material as novel symmetrical electrode for solid oxide fuel cells

The SrFe0.8W0.2O3-δ (SFW) perovskite oxide which is prepared by a modified citrate-EDTA-nitrate combustion method is developed as an electrode material for symmetrical solid oxide fuel cell (SSOFC). The tungsten dopant can stabilize the cubic perovskite structure of SrFeO3 and increase the resistance to the reducing atmosphere, demonstrating its great potential as an alternative SOFC anode. Meanwhile, the electro-catalytic activity of SFW as SSOFC electrode is intensively studied. Area specific polarization resistances (ASRs) of SFW based on SSOFC with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte at 800 °C are measured to be as low as 0.084 and 0.20 Ω cm2 in air and H2, respectively. The combined analysis of electrochemical impedance spectra (EIS) and distribution of relaxation times (DRT) of SFW electrode process reveals that the rate-limiting step in the anodic condition is associated with the hydrogen adsorption/dissociation process at SFW surface, while in the cathodic condition is ascribed to dissociation of adsorbed oxygen and charge transfer processes. LSGM electrolyte-supported SSOFC with SFW electrode exhibits a high power density of 931 mW cm−2 and a low polarization resistance of 0.075 Ω cm2 at 850 °C when using hydrogen as fuel gas and air as the oxidant, indicating that SFW is a promising SSOFC electrode material.

A New Numerical Study Method of Thermal Stress Distribution and Tortuosity Effectiveness in an Anode Porous Electrode for a Planar Solid Oxide Fuel Cell

A fuel cell is an electro-chemical tool capable of converting chemical energy into electricity. High operating temperature of solid oxide fuel cell, between 700oC to 1000oC, causes thermal stress. Thermal stress causes gas escape, structure variability and cease operation of the SOFC before its lifetime.The purpose of the current paper is to present a method that predicts the thermal stress distribution in an anisotropic porous anode of planar SOFC. The coupled governing non-linear differential equations, heat transfer, fluid flow, mass transfer, mass continuity, and momentum are solved numerically. A code based on computational fluid dynamics (CFD), computational structural mechanics and finite element method (FEM) is developed and utilized. The code uses the generated data inside the porous anode in order to detect the temperature and the stress distribution using the Darcy’s law and the Navier-Stokes equations. The numerical results used to govern the areas of high values of stresses were higher than the yield strength of materials. The results show that a highest thermal stress occurs at lower corners of the anode. The concentrated temperature occurs at the middle of the electrolyte-anode whereas the maximum pressure occurs at the middle of the upper and lower section of the anode.

Characteristics of Sr1−xYxTi1−yRuyO3+/−δ and Ru-impregnated Sr1−xYxTiO3+/−δ perovskite catalysts as SOFC anode for methane dry reforming

Dry reforming, a technique to combine methane and carbon dioxide, has received significant attention in efforts to produce synthesis gas (carbon monoxide and hydrogen). These gases can subsequently be used for C1 chemistry applications, and even for high-temperature fuel cell devices. However, suitable electrode materials with highly active and stable properties remain to be found for efficient utilization of a C1 fuel-based fuel cell system. A promising electrode material of yttria-doped strontium titanium oxide with trace ruthenium (Sr0.92Y0.08Ti0.98Ru0.02O3+/−δ; SYTRu) was developed in this work for an efficient and stable dry-reforming catalyst and compared to a Ru-loaded SYT (Ru/SYT) catalyst. The substituted Ru in the former sample was uniformly incorporated (primarily in the atomic level) in the SYT lattice with a perovskite structure. Different Ru species were observed in this case, unlike in the Ru/SYT sample. Ru doping decreased the oxygen vacancy formation energy in calculated by density functional theory and led to a high activity for the surface oxygen. The surface oxygen donating ability to the catalyst was responsible for facilitating the dissociation and decomposition of CH4 and CO2. This led to high conversion efficiency, good syngas selectivity, and stability, along with a small amount of coke formation.

Catalytic Steam Reforming of Natural Gas over a New Ni Exsolved Ruddlesden‐Popper Manganite in SOFC Anode Conditions

AbstractThe catalytic behavior of the new Ni exsolved Ruddlesden‐Popper (RP) manganite (La1.5Sr1.5Mn1.5Ni0.5O7) for the reforming reaction was studied. The material was synthesized by the Pechini method and reduced to induce the formation of two phases: n=1 RP structure LaSrMnO4 decorated with Ni nanoparticles. Ni impregnation on (La,Sr)2MnO4 ceramic support of similar composition was also prepared for comparison. The catalytic measurements were carried out in a reduction‐reaction process with low steam to carbon content (S/C=0.15) at 700, 800 and 850 °C. The exsolved material exhibits a better performance than the impregnated for the methane steam reforming reaction, especially at 850 °C with higher conversion and H2 production rate. However, in light alkane gas mixtures (CH4−C2H6 and CH4−C3H8), the behavior is affected due to the competition between reactions and low available metallic active sites, without affecting the H2 production. The exceptional overall results considering this new material as a promising anode material in a SOFC fed with Colombian natural gas.

Study on Fabrication of Anodes for SOFCs with 3D Printing Technology

The porous structure of anodes in SOFCs is the key factor to improve its performance and the manufacturing method to control its design needs to be established. Additive manufacturing, so-called 3D printing technology can be a prospective method for the fabrication of electrodes of SOFCs. In this experiment, stereolithography, which is a form of 3D printing technology using UV-light with approximately 365nm wavelength and photo-curable resin, is applied. The UV-light that goes through pinholes enables the production of multiple cylindrical objects at once. As a result, the objects and the empty spaces between them make the porous structure. Moreover, by changing the size and the number of holes and distance between them, the fabrication of an anode with the desired microstructure can be expected.

Direct-methane solid oxide fuel cells with an in situ formed Ni–Fe alloy composite catalyst layer over Ni–YSZ anodes

Coking on Ni surfaces limits the direct application of methane-based fuels in SOFCs with Ni-cermet anodes. Loading an anodic catalytic layer with a high catalytic activity for CH4 conversion can effectively protect the Ni-based anode from coking and increase the cell durability. In this work, a Ni–Fe alloy composite catalyst was prepared by reducing perovskite La0.7Sr0.3Fe0.8Ni0.2O3-δ (LSFN) and then evaluating its catalytic activity in the partial oxidation of CH4. The catalyst was applied on a conventional Ni–8 mol.% Y-stabilized ZrO2 (YSZ) anode for methane SOFCs using two methane-containing fuels (97% CH4–3% H2O and 30% CH4–70% air). The catalyst-modified cells showed much higher performances and durability than the conventional cell using a Ni–YSZ anode, indicating the potential application for direct-methane SOFCs.

Development of Ni-based alloy catalysts to improve the sulfur poisoning resistance of Ni/YSZ anodes in SOFCs

Alloying Au into Ni surfaces provides a way to alleviate sulfur poisoning in the anode of solid oxide fuel cells.

Tailoring tantalum doping into a perovskite ferrite to obtain a highly active and stable anode for solid oxide fuel cells

Novel Ta-doped LSF perovskite anodes for SOFCs are prepared and evaluated. The LSFTa05 anode possesses high activity and stability simultaneously with appropriate Ta content. DFT calculations are performed to study Ta-doped LSF system theoretically.

What does carbon tolerant really mean? Operando vibrational studies of carbon accumulation on novel solid oxide fuel cell anodes prepared by infiltration.

Operando Raman spectroscopy and electrochemical techniques were used to examine carbon deposition on niobium doped SrTiO3 (STN) based SOFC anodes infiltrated with Ni, Co, Ce0.9Gd0.1O2 (CGO) and combinations of these materials. Cells were operated with CH4/CO2 mixtures at 750 °C. Raman data shows that carbon forms on all cells under operating conditions when Ni is present as an infiltrate. Additional experiments performed during cell cool down, and on separate material pellets (not subject to an applied potential), show that chemically labile oxygen available in the CGO infiltrate will preferentially oxidize all deposited surface carbon as temperatures drop below 700 °C. These observations highlight the benefit of CGO as a material in SOFC anodes but more importantly, the value of operando spectroscopic techniques as a tool when evaluating a material's susceptibility to carbon accumulation. Solely relying on ex situ measurements will potentially lead to false conclusions about the studied materials' ability to resist carbon and improperly inform efforts to develop mechanisms describing electrochemical oxidation and material degradation mechanisms in these high temperature energy conversion devices.

Oxygen reduction from flammable off-gas for solid oxide fuel cell so as to avoid anode deterioration

In the present study, chemical equilibrium FactSage computations with an actual flammable off-gas have been firstly done for designing a prototype O2 reducing apparatus to convert the off-gas into safe SOFC fuel and determining a thermal pretreating temperature range. O2 reducing experiments from simulated flammable off-gases as thermal pretreatments before feeding to the SOFC anode side have been carried out.O2 concentration can be reduced in proportion to the Ni surface area, reaching the minimum concentration of about 0.16 vol% from its initial value of 10.2 vol% with the Ni surface areas of 562 m2/m3 at 1073 K. H2 and CO concentrations of the objective SOFC fuel constituents are close to their respective initial concentrations, due to accelerated Boudouard reactions and reverse shift reaction. Concentration of H2 plus CO reached the maximum value at 537 m2/m3 of Ni surface area, concretely 92.8% of H2 plus CO is remaining in comparison with the initial simulated flammable off-gas.Some recommended future strategies of this thermal pretreating process to convert the flammable off-gas into safe SOFC fuel are also proposed in every part through this paper so as to supply complete solutions for any global-scale problems.

Efficient and stable conversion of oxygen-bearing low-concentration coal mine methane by the electrochemical catalysis of SOFC anode: From pollutant to clean energy

Large quantity of low-concentration coal mine methane (LC-CMM) has been directly emitted to the air, causing severe greenhouse effect and energy loss. We propose to convert the pollutant gas to clean energy by the anodic electrochemical catalysis of solid oxide fuel cells. In the electrochemical conversion of oxygen-bearing LC-CMM, the Ni-based anode shows high catalytic activity and good stability without severe carbon deposition, Ni oxidation or thermal runaway. High electrochemical performance and good discharging stability were achieved with very flexible O2/CH4 ratio and low CH4 concentration. The high electric efficiency of 38.2 % with the fuel utilization of 72 % and the stable discharging for 400 h can be realized by the stacks using the LC-CMM. Accordingly, a heterogeneous kinetics and electrochemical charge-transfer model was established to study the elementary reaction mechanism of O2 in CH4 conversion and carbon removing, also showing the theoretical thresholds of coking and Ni oxidation.

Study of (La,Sr)(Ti,Ni)O3-δ materials for symmetrical Solid Oxide Cell electrode - Part B: Conditions of Ni exsolution

The stability of La2xSr1-2xTi1-xNixO3-δ (LSTN) and La7x/4Sr1-7x/4Ti1-xNixO3-δ (25LSTN) materials in high temperature reducing conditions has been studied with a special focus on the Ni exsolution process for SOFC anode / SOEC cathode application. In a general way, LSTN and 25LSTN compounds are stable after treatments at 800 °C in air and wet reducing atmosphere. The low chemical expansion makes them compatible with a use in an electrochemical Solid Oxide Cell. Those materials are therefore useful for an application as symmetric oxide cell electrode or only hydrogen electrode. In situ Nickel exsolution is evidenced at 800 °C in Ar/H2(2%) but is limited by a slow kinetics and a higher temperature pre-reduction is preferred before a use as SOFC anode (SOEC cathode). Depending on the treatment, in situ reduction or pre-reduction (T>1000 °C), the compounds are not in the same thermodynamic equilibrium state.